*Article* **Biogenic Nanosilica Synthesis Employing Agro-Waste Rice Straw and Its Application Study in Photocatalytic Degradation of Cationic Dye**

**Garima Singh <sup>1</sup> , Hossein Beidaghy Dizaji 2,3 , Hariprasad Puttuswamy <sup>1</sup> and Satyawati Sharma 1,\***


**Abstract:** The current study aims towards a holistic utilization of agro-waste rice straw (RS) to synthesize nanosilica (SiNPs) employing the sol–gel method. The effect of ashing temperature was evaluated for the synthesis process. X-ray diffraction demonstrated a broad spectrum at 21.22◦ for SiNPs obtained using RSA-600, signifying its amorphous nature, whereas crystalline SiNPs were synthesized using RSA-900. The EDX and FTIR spectra confirmed the significant peaks of Si and O for amorphous SiNPs, confirming their purity over crystalline SiNPs. FE-SEM and TEM micrographs indicated the spheroid morphology of the SiNPs with an average size of 27.47 nm (amorphous SiNPs) and 52.79 nm (crystalline SiNPs). Amorphous SiNPs possessed a high surface area of 226.11 m2/g over crystalline SiNPs (84.45 m2/g). The results obtained attest that the amorphous SiNPs possessed better attributes than crystalline SiNPs, omitting the need to incorporate high temperature. Photocatalytic degradation of methylene blue using SiNPs reflected that 66.26% of the dye was degraded in the first 10 min. The degradation study showed first-order kinetics with a half-life of 6.79 min. The cost-effective and environmentally friendly process offers a sustainable route to meet the increasing demand for SiNPs in industrial sectors. The study proposes a sustainable solution to stubble burning, intending towards zero waste generation, bioeconomy, and achieving the Sustainable Development Goals (SDGs), namely SDG 13(Climate Action), SDG 3(Good health and well-being), SDG 7(use of crop residues in industrial sectors) and SDG 8 (employment generation).

**Keywords:** rice straw; ash; nanosilica; methylene blue; zero waste generation; decolorization; SDGs

#### **1. Introduction**

Rice straw (RS), lignocellulosic biomass, is a very common agro-waste generated in the agriculture system after the post-harvesting of rice. Incorporating the crop and harvesting method, approximately 40–60% of residual biomass comprises RS [1,2]. RS is a stiff, voluminous lignocellulosic biomass with significant silica (SiO2) deposits, for which the level of biogenic silica can reach up to 82% on a dry weight basis. The complex structure limits the usability of RS. Being a quick, easy and cheap process, most farmers opt for open field-burning as the most preferred approach to dispose of RS in agricultural fields [3,4]. The burning of farm waste causes the ghastly pollution of soil and water at the regional scale. This practice also adversely reduces the nutrient composition in the soil. The elemental carbon, nitrogen and sulphur become completely burnt and subsequently emit hazardous gases such as methane, nitrogen oxide and ammonia, causing austere atmospheric pollution. These gases also contribute and further add up to the existing ozone pollution. Burning releases fine particles which are known to aggravate chronic heart and lung diseases [3].

**Citation:** Singh, G.; Dizaji, H.B.; Puttuswamy, H.; Sharma, S. Biogenic Nanosilica Synthesis Employing Agro-Waste Rice Straw and Its Application Study in Photocatalytic Degradation of Cationic Dye. *Sustainability* **2022**, *14*, 539. https:// doi.org/10.3390/su14010539

Academic Editors: Marc A. Rosen and Antoni Sánchez

Received: 25 November 2021 Accepted: 28 December 2021 Published: 4 January 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Rice plants accumulate Si by polymerizing water-soluble silicic acid (H4SiO4) absorbed from the soil into insoluble polysilicic acids, precipitated as amorphous silica and deposited on the plant cell's exterior [5,6]. Si is deposited in plants primarily as phytoliths, which consist of amorphous hydrated silica. These beneficial attributes and the rich Si content of RS make it a suitable alternate source of biogenic nano-silica [7]. The Si can be extracted from RS by ashing at a temperature beyond 400 ◦C. However, temperature above 700 ◦C leads to the production of crystalline Si such as cristobalite and tridymite, with limited applications, possessing higher risks of silicosis [8–10].

In recent years, many efforts have been made to synthesize silica nanoparticles (SiNPs) from various preparatory materials, precisely chemical and natural sources. Different approaches such as the sol−gel process, chemical precipitation method, microemulsion processing, plasma synthesis, chemical vapor deposition, combustion in a diffusion flame and hydrothermal treatment have been employed for preparing Si-NPs [11,12]. Among these, the sol−gel process, also known as the "Stöber method" is a relatively modest and low-cost process [13,14]. It is worth highlighting that the chemical route is not only expensive, but it also adds to the list of pollutants, therefore, adapting a green route is the need of the hour for a healthy and safe environment [15].

The scientific community has successfully utilized the potential of nanotechnology to develop different products and materials at the nanoscale for societal welfare [12,16]. Silica is an important inorganic material with a panoramic range of applications in the textile industry, automobile industry, biology, medicine, adsorbents, drug delivery system, etc. [11,12,17]; additionally, it also holds an advantage over conventional precursors owing to its abundancy as well as being a cheap substrate [18].

SiNPs holds potential application in various sectors due to its high surface area and reactivity in broad areas [12,13,19,20]. These versatile properties enhance their potentials for developing biosensors and biomarkers, holding application in the detection of plateletderived microparticles and the identification of leukemia cells [21]. The combination of SiNPs with super absorbent polymers helps in assuaging the plastic shrinkage. Additionally, the unique ability of SiNPs to exhibit the nucleation effect and pozzolanic activity leads to a decrease in the setting time and the mitigation of calcium leaching losses for cement-based materials [22,23]. The suitability of SiNPs as fillers in nanosilica composites has also been investigated by Salimian et al. [24]. The unique ability of SiNPs also tends to enhance its catalytic and photocatalytic applicability for removing an organophosphate pesticide, elimination of heavy metals from wastewater, the treatment of textile effluents and dye decolorization [23,25].

Of the various pollutants reported, methylene blue is a prominent blue cationic thiazine dye used widely in textile, paper and wood industries. It is documented that intense exposure to this dye leads to release of aromatic amines with severe environmental and health hazards. In previous years, chemical-based products have been designed to degrade toxic pollutants effectively; yet again, the persistent and non-degradable nature with its known tendency to bioaccumulate serves well as another potential environmental health hazard, demanding an adequate replacement [26,27].

In this regard, nanomaterials have emerged as an influential factor in removing organic pollutants due to their excellent high surface area and adsorption capacity. Exploring the potential of naturally synthesized nanomaterials holds potential utility. With reference to the proposed methods, although silica is inert for many reactions, it shows noticeable catalytic activities under ultraviolet irradiation below ~390 nm, e.g., photo-oxidation of CO, photo-metathesis of propene, photo-epoxidation of propene and silica-based photocatalysts such as silica-alumina, silica-alumina-titania and gold-coated SiO<sup>2</sup> with practical photocatalytic activities possessing a significant utility in photodegradation of toxic products [27–29]. The utilization of agro-waste-derived SiNPs in the degradation of cationic dye serves as an excellent example of two birds with one stone, on one end promoting a natural route towards the treatment of the toxic effluents while on the other proposing an

excellent alternative towards the minimization of chemical-based routes for the treatment of organic pollutants.

A dearth of literature highlights the difference observed in the characterization of SiNPs when RS ashing is performed at 900 ◦C vs. the ashing temperature of 550 to 600 ◦C. To this end, a comparative analysis of amorphous and crystalline SiNPs synthesized using the sol–gel method was conducted in the current study to empirically attest the amorphous SiNPs as a preferential choice over crystalline SiNPs. The optimal SiNPS were further explored for a cost-effective route for the removal of the toxic pollutant methylene blue. The study offers insights to researchers and stakeholders towards a sustainable route for the synthesis of SiNPs and its application in the degradation of the toxic cationic dye methylene blue. The increase in the utility of silica-rich RS can serve as an integral factor in avoidance of stubble burning and exploring its utility will provide new ventures towards a suitable replacement to chemical route adopted to eliminate the toxic dyes in industries, thus playing an integral role towards bioeconomy as well as to the safe and healthy environment. The study overall is an attempt to meet the Sustainable Development Goals (SDGs). An alternative to stubble burning will assist in targeting SDG 13 (Climate Action), SDG 3 (Good health and well-being), SDG 7 (Use of crop residues in industrial sectors) and SDG 8 (Employment generation), highlight the aim of the study undertaken [30].

#### **2. Materials and Methods**

#### *2.1. Collection of Raw Material and Rice Straw Ash Preparation*

RS was collected from the Khatauli village of Uttar Pradesh, India. Before any treatment, the piled agro-residue was washed thoroughly using distilled water and then dried at 105 ◦C. Dried RS was chopped into small pieces and pulverized in a supermass collider (Masuko Sangyo Co. Ltd., Kawaguchi, Japan). For uniform size powdered straw, the pulverized material was passed through a 20-mesh screen. The ground RS powder was re-washed to remove any dust particles and dried in a hot air oven at 60 ◦C. Finally, dried and cleaned straw powder was burned to ashes using a muffle furnace, maintaining the furnace temperature at 600 ◦C and 900 ◦C for 4 h leading to grey and white ash production, respectively [12]. The ashes obtained at two different temperatures are denoted hereafter as RSA-600 and RSA-900, used to synthesize SiNPs.

#### *2.2. Nano Silica Extraction from Rice Straw Ash*

A combined method for extracting nanosilica from RS was performed based on the methodology provided by Bahrami et al. [10] and Kalapathy et al. [31]. A detailed methodology followed is outlined in Figure 1.

#### *2.3. Characterization of Rice Straw Ash and Nanosilica Powders*

#### 2.3.1. X-ray Diffraction (XRD)

The amorphous and crystalline nature of calcinated RSAs and synthesized SiNPs were determined using XRD X'Pert Pro (PANalytical The Netherlands). The samples were flattened in the sample container using a glass slide. Radical scans of scattering angle (2θ) vs. intensity scans were recorded from 5 to 100◦ with CuKα radiation of 1.54 Å.

#### 2.3.2. Fourier Transform Infrared Spectroscopy (FTIR)

The functional bonds present in the RSA and SiNPs were studied using an FTIR spectrophotometer (Perkin-Elmer1600). The absorbance of the dried sample was measured in the spectral range of 4000–500 cm−<sup>1</sup> for 128 scans at a speed of 16 cm s−<sup>1</sup> . The spectral obtained was compared with the commercial nanosilica based on a literature review [4].

**Figure 1.** Synthesis of SiNPs using RSA combusted at 600 ◦C for amorphous particles and at 900 ◦C for crystalline particles.

2.3.3. Field Emission Scanning Electron Microscope-Energy Dispersive Spectroscopy (FE-SEM-EDX)

Microstructure and surface characteristics of the substrates (Raw RS, RSA, and SiNPs) were observed using FE-SEM-EDX (Field Emission Scanning Electron Microscope with Oxford-EDX system IE 250 X Max 80, The Netherlands). The dehydrated sample was mounted on the carbon tape. Gold sputtering was performed under vacuum 120 s with an acquisition time of 2 min, beam accelerating voltage of 10 kV at beam aperture (30 mm), with a working distance of 10 mm and probe current of 3 <sup>×</sup> <sup>10</sup>−<sup>10</sup> A. Mean EDX count rate was kept as 1600 ± 200 cps.

#### 2.3.4. Transmission Electron Microscopy (TEM)

TEM analyses were performed by dissolving 5 mg of samples (RSA and SiNPs) in 50 mL of double distilled water and kept for 30 min ultrasonication. Next, 10 µL of the suspension (0.005% *w/w*) was mounted on carbon-coated copper grids. The shape and size of the samples prepared were characterized by TEM (JEOL JEM-1400) at an accelerating voltage of 100 kV. The diameter and size distribution of synthesized SiNPs was calculated using ImageJ software.

#### 2.3.5. Surface Area and Porosity

A Brunauer–Emmett–Teller (BET) of the make (Micromeritics ASAP 2010, USA) was used to analyze the surface area of synthesized SiNPs at 77 K in N<sup>2</sup> atmosphere. The pore size distribution of the catalyst was calculated from the Barret–Joyner–Halenda (BJH) method using the adsorption data at relative pressure P/P<sup>0</sup> – 0.990.

#### 2.3.6. Photocatalytic Degradation of Methylene Blue Dye

The photocatalytic effect of different concentrations of the optimum synthesized SiNPs (0.2–0.5 g/L) on methylene blue (100 ppm) at alkaline pH 11.0 was investigated following the protocols of Saleh and Dijaja [29] and Aly and Elhamid [26] with slight modifications. The photocatalytic experiment was conducted in a glass beaker equipped with continuous stirring under exposure to ultraviolet light (Philips 30 W, two tubes). Different suspensions were swirled in the dark for 30 min before irradiation to obtain a colloidal solution. The beakers were placed at a 15 cm distance from the light source. The samples were extracted every 2 h and centrifuged at 10,000 rpm for 5 min, and the study was conducted for 2 h. The absorbance of the solution was determined using a BioTek Epoch 2 microplate spectrophotometer at 630 nm (*λ*max), corresponding to the maximum absorption of methylene blue. The dye removal efficiency percentage was calculated as (A*0*-A*t*)/A*<sup>0</sup>* × 100, where A*<sup>0</sup>* and A*<sup>t</sup>* are the initial and final dye concentrations at time *t*, respectively.

#### **3. Results**

#### *3.1. X-ray Diffraction Analysis*

The XRD analysis of RSA-600 depicted a broad hump between 20.52◦–22.71◦ , indicating the material to be amorphous (Figure 2A), possessing the ability for adsorption; whereas RSA-900 indicated sharp peaks between 20◦and 30◦ (Figure 2B), documented to decrease the available surface area, thus restricting the adsorption potential [8].

**Figure 2.** XRD pattern (**A**) RS ash obtained at 600 ◦C (RSA-600). (**B**) RS ash obtained at 900 ◦C (RSA-900). (**C**) SiNPs synthesized using RS ash obtained at 600 ◦C. (**D**) SiNPs synthesized using RS ash obtained at 900 ◦C.

The synthesis process of SiNPs carried out using RSA-600 depicted a broad peak centered at 22◦ ; the absence of any other peaks confirmed the SiNPs to be of amorphous nature (Figure 2C). SiNPs synthesized from RSA-900 depicted the presence of crystalline components between 20◦–30◦ with a crystallinity index of 65%, calculated according to the methodology of Mendes et al. [32]. However, various other peaks signified the impurity of the synthesized product, as shown in Figure 2D. The results are in coherence with the findings of several other researchers, where the diffraction peaks at 2θ angles between 20◦ and 30◦ is documented to be the characteristic peak of silica [6,33].

#### *3.2. EDX Analysis*

The EDX profiling of RSA-600 showed Si and O's presence and various other minor elements such as Mg, K, Na, Al, S, Fe and Ca, shown in Figure 3A. The profiling study of white RSA-900 also showed various elements along with Si and O, with a low K content, as shown in Figure 3B. The amorphous SiNPs confirmed the presence of Si and O and the absence of any other impurities, thus validating the purity of the product obtained as depicted (Figure 3C). The crystalline SiNPs denoted small peaks for Na and S, which could be due to traces of residual elements left in the process of washing (Figure 3D).

**Figure 3.** EDX analysis (**A**) RS ash obtained at 600 ◦C (RSA-600) (**B**) RS ash obtained at 900 ◦C (RSA-900) (**C**) SiNPs synthesized using RS ash obtained at 600 ◦C (**D**) SiNPs synthesized using RS ash obtained at 900 ◦C.

#### *3.3. Fourier Transform Infrared Analysis*

The Fourier transform IR analysis samples were recorded in the spectrum range of 4000–500 cm−<sup>1</sup> . FTIR of RSAs, presented in Figure 4, showed significant bands at 794 cm−<sup>1</sup> , 1109 cm−<sup>1</sup> corresponding to the symmetric and asymmetric stretching vibration of the Si−O−Si bond [6,12]. The peak observed for RSA-600 at 991 cm−<sup>1</sup> signified the Si−OH bond's bending vibration that diminished completely for RSA-900 [34]. The bonds 1629 cm−<sup>1</sup> and 3448 cm−<sup>1</sup> indicated the bending and stretching vibration of the H−OH bond. The results are indicated in (Figure 4A,B). FTIR analyses of synthesized SiNPs showed broadband ranged between 3000–3500 cm−<sup>1</sup> that indicated the presence of silanol group (Si−OH) bonding. The little band in the region of 1633 cm−<sup>1</sup> corresponded to the bending vibrations of H−O−H (water molecules). The dominant peak at 1097 cm−<sup>1</sup> was due to the asymmetric vibration of the Si−O−Si bond. The band at 789 cm−<sup>1</sup> corresponded to Si−O−Si bond stretching [13,35], as shown in Figure 4C,D. A comparison of the spectral values obtained, and literature reports are briefly outlined in Table 1.

**Table 1.** Infrared bands observed in ashes (RSA-600, RSA-900), SiNPs (amorphous and crystalline).


RSA 600—Rice straw ash obtained at 600 ◦C; RSA 900—Rice straw ash obtained at 900 ◦C; SiNPs—Silica nanoparticles.

**Figure 4.** FTIR analysis (**A**) RS ash obtained at 600 ◦C (RSA-600). (**B**) RS ash obtained at 900 ◦C (RSA-900). (**C**) SiNPs synthesized using RS-600. (**D**) SiNPs synthesized using RS-900.

#### *3.4. Morphology Studies*

FE-SEM analysis of RS showed a stable, well-defined structure (Figure 5A). The microscopic analysis of RS ash (RSA-600 and RSA-900) showed a homogeneous distribution of dumbbell-shaped phytoliths commonly referred to as silica bodies over the entire surface (Figure 5B,C). The TEM analysis of RSAs showed the agglomeration of small circular bodies over the entire surface (Figure 5D,E).

**Figure 5.** (**A**) FESEM micrographs of raw RS at 500 X. (**B**) Microscopic images of RSA-600 at 40X. (**C**) Microscopic images of RSA-900 D at 40 X. (**D**) TEM micrographs of RSA-600 at 120 kV. (**E**) TEM micrographs of RSA-900 at 120 kV. (**F**) FE-SEM micrographs of amorphous SiNPs at 10 kX. (**G**) FE-SEM micrographs of crystalline SiNPs at 10 kX. (**H**) TEM micrographs of amorphous SiNPs at 120 kV. (**I**) TEM micrographs of crystalline SiNPs at 120 kV.

FE-SEM analysis of amorphous SiNPs synthesized by RSA-600 depicted the particles of spheroid morphology with loose aggregates compared to crystalline SiNPs synthesized

by RSA-900 (Figure 5F,G). The formation of aggregates could be attributed to the gel-like property of the hydrated silica and its high surface area [7,11,12]. TEM analysis revealed the amorphous SiNPs to be spherical, with the average particle size of 27.47 nm possessing little agglomeration. In contrast, crystalline SiNPs possessed average particle size of 52.79 nm with high agglomeration (Figure 5H,I). The results obtained can be well attested by the findings of Bahrami et al. [10] and Lu et al. [6] where it was postulated that high temperature leads to the formation of crystalline silica, keeping the Si bonds intact.

#### *3.5. Surface Area and Porosity Studies*

BET analysis revealed that amorphous SiNPs possessed a specific surface area (SSA) of 226.811 m2/g with an average pore volume of 1.144 cm3/g, whereas the crystalline SiNPs had a BET SSA of 84.45 m2/g with a pore volume of 0.497 cm3/g. Nitrogen adsorptiondesorption isotherm for the amorphous and crystalline SiNPs are shown in Figure 6A,B. The results were in line with the findings of Beidaghy Dizaji et al. [37] who documented that the porosity of silica-rich ashes diminish once the crystallinity fraction is higher than 10 wt.%. Didamony et al. [38] reported a surface area of 160 m2/g from SiNPs extracted using sodium silicate solution, while Yuvakumar et al. [39] reported the amorphous synthesized SiNPs with a surface area of 274 m2/g and an average pore diameter of 1.46 nm.

**Figure 6.** Nitrogen adsorption-desorption isotherms of (**A**) amorphous SiNPs (**B**) crystalline SiNPs.

The results attested the findings that the amorphous SiNPs had significantly better attributes when compared to the crystalline SiNPs, thus increasing its utility in industrial sectors.

#### *3.6. Decolorization of Cationic Dye Methylene Blue Using Amorphous SiNPs*

The photocatalytic degradation studies tested the effect of different concentrations of SiNPs on a constant concentration of methylene blue dye (100 ppm). The studies reflected that the dye was efficiently degraded by 66.26% within the first 10 min by the SiNPs at 50 ppm concentration. Degradation of the dye at the lowest SiNPs concentration of 10 ppm did not reveal any observable degradation pattern. It showed a similar trend as to the degradation process without SiNPs (Figure 7A). This could be possibly explained owing to the availability of fewer adsorption sites of SiNPs at a relatively high concentration of dye. Decolorization of ~100% was achieved within the initial 30 min of the study for the dye treated with SiNPs.

**Figure 7.** (**A**) Decolorization of methylene blue (MB) under the effect of UVC irradiation and different concentrations of SiNPs (**B**) Schematic representation of photocatalytic mechanism of methylene blue degradation by SiNPs (**C**) First-order kinetics plot of dye degradation, denoted by ( **. . . .**) with SiNPs and by (**—–**) without SiNPs.

The result obtained could be well correlated to the effect of UV-C irradiation that assisted in the induction of the direct photolysis on the dye. Moreover, as the UV-C photons have a shorter penetration potential through photocatalyst particles, the possibility of electron-hole recombination is minimized due to shorter travel distances, leading to higher photocatalytic activity [27,29,40]. The high surface area of amorphous SiNPs accelerated the degradation process. A comprehensive detail of dye decolorization documented by numerous researchers using SiNPs is highlighted in Table 2.

*Sustainability* **2022**, *14*, 539


#### 3.6.1. Mechanism of Photocatalytic Degradation of Methylene Blue Using SiNPs

The mechanism of photocatalytic activity can be explained owing to the ability of SiNPs to be photoexcited under UV irradiation. This phenomenon can be supported by the charge transfer from Si−O bonding orbital to 2p non-bonding orbital of non-bridging oxygen. Interestingly, as observed in FTIR studies, the presence of Si−O and Si−OH groups imparts a negative charge on the silica surface, thus offering the SiNPs to serve as an excellent medium for adsorbent cationic dyes [43,46].

On striking the surface of SiO<sup>2</sup> by UV light, an electron transfer occurs from the valence band to the conduction band, generating a positive hole in the valence band (vb) and a negative hole in the conduction band (cb), leading to the formation of active photocatalytic centers on the surface of SiNPs (Equation (1)). The v<sup>b</sup> hole further interacts with chemisorbed H2O molecules to form OH radicals that successively attacks dye molecules (Equation (2)). The generation of heat in this process could be ascribed due to the combination of ecb <sup>−</sup> and hvb <sup>+</sup> on the particle's surface. A plausible cause for dye decolorization can be attributed to the hydroxyl attack and conduction of the experiment at high pH, increasing OH− groups on the silica surface, leading to an acceleration of dye degradation process [25,47].

A diagrammatic sketch of the mechanism is shown in Figure 7B.

$$\text{SiO}\_2 + \text{h}v \rightarrow \text{e}\_{\text{cb}}{}^- + \text{h}\_{\text{vb}}{}^+ \tag{1}$$

$$\rm{HO}\_2 + \rm{OH}^- + \rm{h}\_{vb} \rm{}^- \rightarrow \rm{OH} \tag{2}$$

The ecb <sup>−</sup> and hvb + recombine on the particle's surface within nanoseconds, and the generated energy becomes dissipated in the form of heat. ecb − further reacts with the acceptor dissolved O<sup>2</sup> and is transformed to a super oxide radical anion (O<sup>2</sup> ·−), leading to the further growth of O2H molecules (3):

$$\rm O\_2 + e\_{cb} \rm \rm \rightarrow O\_2^{\cdot - \cdot} + (H^+ + \rm \rm OH) \rightarrow HO\_2 + OH^- \tag{3}$$

hvb + interacts with the donor −OH and ·O2H forming ·OH radical that attacks the MB in the following manner:

$$\rm{HO\_2^- + OH^- + h\_{vb}^- \to \rm{^\cdot OH}}\tag{4}$$

The governing factor monitoring the efficiency of SiNPs is the amount of ·OH radicals generated. Since the hydroxyl groups on the SiNPs surface are attached to the silicon atom, they are termed as silanols. The OH groups present in the silanols can preferentially complex particular chemicals or metal ions, imparting functionality to SiNPs [48]. It is worth highlighting that shifting the pH towards the alkaline range led to a dramatic boost in dye degradation. Henceforth, varying the pH value can significantly impact the interactions of various compounds with silanols. Subsequently, any factor that contributes to the generation of ·OH radicals lead to an enhancement in the photocatalytic degradation process of methylene blue.

#### 3.6.2. Kinetic Study of Dye Degradation

Kinetic studies were performed for the optimal concentration of SiNPs (50 ppm) that aided in the complete degradation of the methylene blue dye compared to the dye treated under UVC. The effect of SiNPs on dye decolorization indicated first-order kinetics. The linear form of the first-order rate equation is denoted by Equation (5), and the half-life (t0.5) of dye decolorization was calculated using Equation (6) [47]:

$$
\ln \mathcal{D}\_{\text{Ab}} = -\text{kt} + \ln \mathcal{D}\_{\text{A0}} \tag{5}
$$

$$\mathbf{t}\_{0.5} = \mathbf{0}.693/\mathbf{k} \tag{6}$$

where DAb is the dye absorbance at different incubation times, k is decolorization rate constant, DA0 is the initial absorbance of the dye, and t0.5 is the time required to decolorize 50% dye.

The logarithm plot of dye concentration vs. treatment time with SiNPs exhibited a rate constant (k) of 0.102 min−<sup>1</sup> and t0.5 of 6.79 min. In contrast, the untreated dye exhibited a rate constant of 0.044 min−<sup>1</sup> and t0.5 of 15.75 min, reported for the 30 min study. The results obtained thus positively attest to the enhanced effect in dye degradation due to incorporation of the SiNPs as shown in Figure 7C.

#### 3.6.3. FTIR Studies of Methylene Blue Decolorization

The methylene blue molecular structure transformation was further evaluated by the FTIR analysis of the treated and untreated samples. UV-Vis spectra illustrated the presence of aromatics along with conjugates of N–S heterocycle group and phenothiazine structure [29]. The results in Figure 8 showed that the broad peak at 3451 cm−<sup>1</sup> was determined as the O−H stretching vibration of water molecules. It could be seen that the peak corresponding to C−H absorption of benzene ring occurred at 2975 cm−<sup>1</sup> . The peak at 2105 cm−<sup>1</sup> denoted the stretching vibration peak of the methyl group. The C=C framework corresponding to benzene ring vibration and the C=N stretching vibration was found at 1646 cm−<sup>1</sup> . The absorption peak at 1420 cm−<sup>1</sup> was related to another typical vibration in methyl bending. Other prominent peaks were in the vibration of C=O and -C-C at 1206 cm−<sup>1</sup> and 991 cm−<sup>1</sup> . It was noticed that the intensity of absorption peaks for C=N and O−H had significant decrease in C=C aromatic stretch in the treated sample, indicating a change in chemical composition. This could be possibly attributed to the breakdown of the N−S heterocyclic compound during the degradation process [25,26,49]. The characteristic peak of benzene and C−H bending vibration of aromatic C−H declined substantially, indicating the variation in chemical compositions of phenyl groups.

**Figure 8.** FTIR spectrum of methylene blue about its treatment with SiNPs.

It henceforth can be deduced from the spectra that the adsorption of methylene blue on the surface of SiNPs led to a remarkable change in infrared bands intensities, retaining its positions.

Stanley [26], Salimi [40] and Singh [43] hypothesized that the conjugate structure of N−S heterocyclic underwent variations in chemical composition, and consequently the aromatic ring was oxidized to open the ring leading to degradation of the dye molecules during the photocatalysis reaction. The result obtained provides new insight into potential waste usage and utilizing the low-cost synthesized SiNPs for dye degradation.

#### **4. Conclusions**

The study revealed that RS combusted at 600 ◦C served as an ideal condition for synthesizing amorphous SiNPs. The findings and characterization postulated that the amorphous SiNPs served better attributes when compared to crystalline SiNPs, establishing that all the researchers/stakeholders working in this domain can note that lower temperature offers a more sustainable route in product synthesis. This first-hand analysis provided the particulars pertaining to temperature's crucial role and effect on the characterization and properties of SiNPS. The efficacy of the green synthetic route for amorphous SiNPs holding potential applications in different sectors was accessed in the field of wastewater and textile effluents for degradation of the toxic dye methylene blue. Additionally, incorporating microwave, sonication, conjugation of substrates as laccase, surface modification of SiNPs can further be attempted towards the dye degradation process. However, the study has much to offer in terms of optimizing the synthesis process via integrating surfactants, catalyst, residence time, etc., to enhance the surface area and pore size of SiNPs; that will further assist in the degradation process with a minimal dose of SiNPs and enhancing its utility as an adsorbent in different industrial sectors. Contemplating the abundance of agro-waste RS worldwide, the study establishes a background in converting the waste to a value-added product, providing a comprehensive and viable sustainable resolution towards stubble burning to achieve the SDGs 3, 7, 8 and 13.

**Author Contributions:** Conceptualization, S.S. and H.B.D.; methodology, G.S. and H.B.D.; software, G.S.; validation, H.B.D. and S.S.; formal analysis, G.S. and H.B.D.; investigation, G.S. and H.B.D.; resources, H.B.D. and S.S.; data curation, G.S.; writing—original draft preparation, G.S.; writing review and editing, H.B.D. and S.S.; visualization, H.P. and S.S.; supervision H.P. and S.S.; project administration, H.B.D. and S.S.; funding acquisition, H.B.D. and S.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** The APC was funded by the Thermochemical Conversion department of DBFZ Deutsches Biomasseforschungszentrum gemeinnützige GmbH.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data that support the findings of this study are available upon request from the authors.

**Acknowledgments:** The authors are thankful to the Indian Institute of Technology, New Delhi, and DBFZ Deutsches Biomasse for schungszentrumgemeinnützige GmbH, Leipzig, Germany, for providing the research funds and financial support.

**Conflicts of Interest:** The authors declare no conflict of interest.

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## *Article* **Characteristics of Smoldering on Moist Rice Husk for Silica Production**

**Shengtai Yan <sup>1</sup> , Dezheng Yin <sup>1</sup> , Fang He 1,\*, Junmeng Cai <sup>2</sup> , Thomas Schliermann <sup>3</sup> and Frank Behrendt 1,4**


**Abstract:** In order to assess the possibility of silica production via smoldering of moist rice husk, experiments of washed (moist) rice husk (7 kg with moisture content of 51%) in a newly designed smoldering apparatus was performed. The temperature inside the fuel bed during smoldering was recorded, and characteristics of ash were analyzed. Results showed that the highest temperature in the middle of the naturally piled fuel bed was about 560.0 ◦C, lower than those in most of combustors. Some volatiles from the lower part of the fuel bed adhere to its upper ash during piled smoldering. Silica content and specific surface area of ash from smoldering of washed (moist) rice husk were 86.4% and 84.9 m2/g, respectively. Compared to our experiments, they are close to smoldering of unwashed rice husk (89.0%, 67.7 m2/g); different from muffle furnace burning (600 ◦C, 2 h) of washed (93.4%, 164.9 m2/g) and un-washed (90.2%, 45.7 m2/g) rice husk. The specific surface area is higher than those from most industrial methods (from 11.4 to 39.3 m2/g). After some improvements, the smoldering process has great potential in mass product of high quality silica directly from moist rice husk.

**Keywords:** smoldering; rice husk; high moisture content; silica; specific surface area

#### **1. Introduction**

Smoldering is slow, low-temperature, and flameless burning of porous fuels, which is an important and complex phenomenon [1,2]. The application of it in the field of waste-to-energy conversion such as sludge treatment [3], recovery of resources from waste streams [4], and biomass energy conversion [5] has attracted lots of attention in recent years. The main advantages are its low temperature of the solid phase [6] and self-sustainability in a fuel bed with high moisture content (75–80 wt.%) [7]. From an environmental point of view, these characteristics avoid the ash-related slagging/corrosion [8], making nutrients recovery easy via recycling of ash directly to farms [9] and reducing the pollution of solid waste. As to energy consumption, it makes the complete burning of moist solid waste possible [10], reducing the energy consumption for drying fuel.

Rice husk is a typical biomass waste [11], accounting for 14–25% of the grain's overall mass [12]. In 2021, approximately 150 million tons of rice husk were produced around the world, with China contributing approximately 40 million tons. Nowadays most rice husk is directly buried or open burned [13] due to its low nutritive value for humans compared with rice grain and rice bran [14]. Direct burying results in soil pollution, because of its slow decomposition owing to its hard surface resulting from its high silicon and high lignin content [15]. Open burning leads to air pollution because of the release of fine dust and incomplete combustion gases of CO, NOx, CH4, poly-cyclic hydrocarbons (PAH) and soot [16].

**Citation:** Yan, S.; Yin, D.; He, F.; Cai, J.; Schliermann, T.; Behrendt, F. Characteristics of Smoldering on Moist Rice Husk for Silica Production. *Sustainability* **2022**, *14*, 317. https:// doi.org/10.3390/su14010317

Academic Editor: Alberto-Jesus Perea-Moreno

Received: 2 December 2021 Accepted: 24 December 2021 Published: 29 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

There is great potential in producing silica from rice husk due to its high content of amorphous silica (around 18–23% [17]) and ash (around 85–95% [18]). Silica is an important inorganic material and is widely used in various fields such as fertilizer, insulator, adsorbent and catalyst [19]. It is characterized by high mechanical strength, good chemical stability, high-temperature resistance, easy dispersion in solvents, etc [20,21]. With the widespread application of silica, a variety of methods have been adopted to produce it, such as precipitation, plasma synthesis, chemical vapor deposition, micro emulsion processing, combustion synthesis and hydrothermal technique [22,23]. At present, most popular mass producing methods are precipitation from alkaline silicates and hydrothermal treatment of sand with lye [24]. However, both methods are expensive, intensive energy input, and environmentally harmful due to the production of dust, nitrogen and sulfur oxides, etc in the process of obtaining silica [25,26].

Producing silica from thermochemical conversion of rice husk has received considerable attention due to its economic and environmental advantages [27]. As to lab-scale production, Dizaji et al. [28] prepared silica by burning raw rice husk and pretreated rice husk (water washing at 50 ◦C for 2 h) in a muffle furnace at 600 ◦C for 4 h. The specific surface area was around 45.0 and 240.0 m2/g, respectively. Abu Bakar et al. [29] prepared silica by burning rice husk (unleached/acid-leached) in a muffle furnace (600 ◦C for 2 h). The purity of silica from unleached and acid-leached rice husk was 95.8 and 99.6 wt.% (XRF results), respectively, and specific surface area was 116.0 and 218.0 m2/g, respectively. Almeida et al. [30] prepared a mixture of silica and carbon by pyrolysis of raw rice husk in a tubular furnace. The obtained silica was black, in a mixture of amorphous and crystalline, with purity of 81.6 wt.% and specific surface area of 114.0 m2/g. Schliermann et al. [31] obtained ashes produced from water washed (50 ◦C for 2 h) rice husk using ÖKOTHERM® furnaces. The ashes are post-treated with acid and then thermally treated at 650 ◦C using a muffle furnace. The specific surface area of silica is about 150–200 m2/g. As to industrial production, Fernandes et al. [32] investigated characteristics of ash from burning rice husk in a grate furnace, a fluidized bed, and a suspension/entrained combustor. The silica content in these three types of ash was 90.0, 96.7, 93.6 wt.%, and the specific surface area was 39.3, 11.4, 26.7 m2/g, respectively. The specific surface area of silica from mass production tends to be lower than that prepared in a laboratory, which may be related to none pretreatment of rice husk and the high burning temperature in industrial combustors. It is recorded that high combustion temperature results in the transformation of amorphous silica to crystalline material [33].

Pretreatment of rice husk is an effective way to increase the purity and specific surface area of silica and typical pretreatments for rice husk are acid-leaching and waterwashing [29,34,35]. Moisture content of the treated rice husk is normally high. According to our pre-experiments, moisture content of rice husk is around 50% after washing. The moist rice husk is not suitable to be burned directly in a normal combustor (the moisture content in a fluidized bed combustor needs to be <35% [36], for a suspension burner <15% [37]). Smoldering might be a good choice for thermochemical conversion of moist rice husk directly to silica. Yet, to the best of our knowledge there are no experiments using smoldering in literature.

The objective of this study is to assess the possibility of silica production via smoldering of rice husk with high moisture content omitting a drying step before thermochemical conversion. A smoldering apparatus was designed and smoldering experiment of rice husk with high moisture content was performed. Temperature inside a fuel bed was recorded and characteristics of ash (silica content, specific surface area and mass loss characteristic) were analyzed.

#### **2. Materials and Methods**

#### *2.1. Material*

Rice husk was collected from rural southern China in 2021. Two types of rice husk (washed/unwashed) were chosen as raw material in the experiment. The washing process

is the following: 3.5 kg of rice husk was put into a bucket (*φ*42 cm diameter × 40 cm height), then the bucket was filled with tap water (mass ration of water to rice husk: 10:1). The mixture was stirred for 10 min at ambient temperature. After being immersed for 12 h, rice husk was taken out and leaked above a screen in air for 1 h. The proximate and ultimate analysis of washed and unwashed rice husk were performed three times and the results were shown in Table 1. height), then the bucket was filled with tap water (mass ration of water to rice husk: 10:1). The mixture was stirred for 10 min at ambient temperature. After being immersed for 12 h, rice husk was taken out and leaked above a screen in air for 1 h. The proximate and ultimate analysis of washed and unwashed rice husk were performed three times and the results were shown in Table 1. **Table 1.** Proximate and elemental analysis of the rice husk. Oxygen is calculated by difference (C +

**Table 1.** Proximate and elemental analysis of the rice husk. Oxygen is calculated by difference (C + H + O + N + S + Ash = 100 wt.%, dry basis). H + O + N + S + Ash = 100 wt.%, dry basis). **Proximate Analysis (Arrival Basis, wt.%) Elemental Analysis (Dry Basis, wt.%)**


is the following: 3.5 kg of rice husk was put into a bucket (Ф42 cm diameter × 40 cm

#### *2.2. Experimental Set-Up* A batch smoldering apparatus consisting of three parts (a smoldering chamber, a gas

*Sustainability* **2022**, *13*, x FOR PEER REVIEW 3 of 13

A batch smoldering apparatus consisting of three parts (a smoldering chamber, a gas burning chamber and a heat exchanger) was designed, as shown in Figure 1. The smoldering chamber is rectangular (inner size 60 × 30 × 30 cm) with an insulation layer (4 cm) outside of its inner wall. An air inlet (*φ*5 cm) and a flue gas outlet (*φ*5 cm) are on the left and the right wall, respectively. A hole (*φ*30 cm) with door on the top wall is used for material feeding and ash removal. The gas burning chamber is a square cavity (30 × 30 × 30 cm) with an insulation layer (4 cm) outside of its inner wall. Its inlet (*φ*5 cm) on the left wall is connected with the flue gas outlet of the smoldering chamber. A quartz glass tube inside the burner is used to introduce the incomplete combustion flue gas from the smoldering chamber to the bottom of the burner, then the gas is ignited using an igniter. The heat exchanger is a cylinder with a diameter of 30 and a height of 100 cm. The heat is transferred from high temperature flue gas through three tubes (diameter of 5 and a height of 40 cm) to the water outside them. Rice husk is smoldered in the smoldering chamber, smoke is burned out in the gas burning chamber, and flue gas is discharged into the air after cooled in the heat exchanger. burning chamber and a heat exchanger) was designed, as shown in Figure 1. The smoldering chamber is rectangular (inner size 60 × 30 × 30 cm) with an insulation layer (4 cm) outside of its inner wall. An air inlet (Ф5 cm) and a flue gas outlet (Ф5 cm) are on the left and the right wall, respectively. A hole (Ф30 cm) with door on the top wall is used for material feeding and ash removal. The gas burning chamber is a square cavity (30 × 30 × 30 cm) with an insulation layer (4 cm) outside of its inner wall. Its inlet (Ф5 cm) on the left wall is connected with the flue gas outlet of the smoldering chamber. A quartz glass tube inside the burner is used to introduce the incomplete combustion flue gas from the smoldering chamber to the bottom of the burner, then the gas is ignited using an igniter. The heat exchanger is a cylinder with a diameter of 30 and a height of 100 cm. The heat is transferred from high temperature flue gas through three tubes (diameter of 5 and a height of 40 cm) to the water outside them. Rice husk is smoldered in the smoldering chamber, smoke is burned out in the gas burning chamber, and flue gas is discharged into the air after cooled in the heat exchanger.

**Figure 1.** Photo and schematic of the smoldering apparatus**. Figure 1.** Photo and schematic of the smoldering apparatus.

#### *2.3. Ash Preparation and Treatment 2.3. Ash Preparation and Treatment*

kg/m<sup>3</sup>

2.3.1. Ash Preparation from Rice Husk 2.3.1. Ash Preparation from Rice Husk

For comparison, four types of ash―from washed/unwashed rice husk via smoldering and muffle furnace burning―were prepared. The washed and unwashed rice husk (3.5 kg of raw materials) were put naturally piled into the smoldering chamber. The height of the fuel beds was around 24 and 20 cm, and the bulk density were about 160 and 100 , respectively. Then, it was ignited with a block of solid alcohol at the air inlet. Two For comparison, four types of ash—from washed/unwashed rice husk via smoldering and muffle furnace burning—were prepared. The washed and unwashed rice husk (3.5 kg of raw materials) were put naturally piled into the smoldering chamber. The height of the fuel beds was around 24 and 20 cm, and the bulk density were about 160 and 100 kg/m<sup>3</sup> , respectively. Then, it was ignited with a block of solid alcohol at the air inlet. Two thermocouples (*φ*1 mm, KMTXL-040-G) were put into the bottom and middle (10 cm from the bottom) layer of the fuel bed to monitor the change of temperature inside the fuel bed, and data were recorded every 60 s. It was found that a thin layer of upper ash was black due to the low temperature and the other part of ash was homogeneous after extinguishing and cooling of the fuel bed. A vacuum cleaner was used to remove the black ash on the upper layer and 200 g rice husk ash was taken out from the center of the piled residue every time.

About 3 g of washed and unwashed rice husk were put into an ash tray (6 × 3 × 2 cm) respectively, heated in a muffle furnace from ambient temperature to 600 ◦C at 10 ◦C /min and holding for 2 h. Air entered the muffle furnace through a 2–3 cm gap between the furnace door and the wall. Then the ash was taken out and cooled to ambient temperature in a desiccator for later use.

#### 2.3.2. Grinding and Drying of Ash

For homogeneity in the subsequent measurements, the four types of ash were separately crushed to powder with a particle size <120 mesh using an agate mortar. After that, these powders were dried in an oven at 105 ◦C for 24 h.

#### *2.4. Thermal and Physical Characterization of Ash*

Mass loss characteristic was analyzed using a simultaneous thermogravimetric analysis (TGA DSC1, Mettler TOLEDO). To avoid corrosion to the instrument, a pair of crucibles (inner: alumina 50 µL, outer: platinum 70 µL) were used in experiments. About 8 mg of ash was put into the inner crucible and heated from 50 to 950 ◦C at a heating rate of 10 ◦C /min. An air flow rate of 200 mL/min was used as reactive gas and 20 mL/min of N2 was used as protective gas. Each experiment was repeated three times to check reproducibility.

The contents of silica and other elements in ash were triplicate and measured by Wavelength Dispersive X-Ray Fluorescence Spectrometer (ZXS100e, Rigaku Corporation) at room temperature. It should be noted that contents are only reliable for elements with atomic weight ≥23 [38]. Because of the data overflow of the results of carbon and boron, the results of them were deleted before calculation of the oxides' content.

Specific surface area was evaluated according to the Brunnauer, Emmett and Teller (BET) method and based on the nitrogen adsorption of the material at 77K. It was determined in the pressure range of p/p<sup>0</sup> = 0.05–0.3 [28], where p is the system pressure, and p<sup>0</sup> is the initial pressure (1 bar in this experiment). The measurement was conducted in a surface area analyzer (ASAP 2460, Micromeritics).

#### **3. Results and Discussion**

#### *3.1. Characteristics of Smoldering Process*

#### 3.1.1. Temperature inside Fuel Bed

Temperature inside the washed (moist) and unwashed fuel bed is shown in Figure 2. The temperature history at one spot of the batch fuel bed can be divided into two stages—drying and oxidation. At the drying stage, the temperature first increases and then stabilizes at a temperature of about 60 ◦C. This is similar to the temperature of smoldering pine bark particle [39] and sewage sludge [4], which is different from the temperature (around 100 ◦C) of smoldering corn stalk powder [40] and corn flour [6]. Supplement experiments show this temperature is always around 60 ◦C in natural piled rice husk. In our experiments of smoldering branches, there is even no obvious plat temperature at the preheating period, which is similar to smoldering of unwashed rice husk. It implies temperature in the fuel bed at drying stage might be related to the porosity inside the fuel bed, materials, particle size, air flow, etc. The detailed analysis of this will be left for future work. At the oxidation stage, the temperature first increases rapidly and then increases at a stable rate. It drops quickly at the end stage of oxidation. For both cases the highest temperatures are around 560.0 ◦C, being much lower than those in most combustors (>700 ◦C) [41,42]. The low temperature is favorable to maintain the amorphous state of the silica [43].

[43].

longer duration.

tively)*.*

**Figure 2.** History of temperature of middle and bottom layer of washed (**a**) and unwashed (**b**) fuel bed (red and green pentacles for the maximum temperatures of middle and bottom layers, respec-**Figure 2.** History of temperature of middle and bottom layer of washed (**a**) and unwashed (**b**) fuel bed (red and green pentacles for the maximum temperatures of middle and bottom layers, respectively).

temperatures are around 560.0 °C, being much lower than those in most combustors (>700 °C) [41,42]. The low temperature is favorable to maintain the amorphous state of the silica

Comparing the temperature development of the washed (moist) fuel bed with the unwashed fuel bed, the former has a longer duration of drying stage. The lower the part of the zone in the fuel bed, the longer the drying stage lasts. At oxidation stage, the duration of temperature > 400 °C of the moist fuel bed is shorter than for the unwashed fuel bed. This happens due to more heat generated in the process of smoldering is used to dry the moist rice husk and heat transfer rate to dry fuel is bigger than those for unwashed rice husk. The oxidation duration is also affected by bulk density of the fuel bed. In our supplementary experiments, smoldering of unwashed rice husk with bulk density of 170 kg/m<sup>3</sup> was performed in a small apparatus. It was found that the maximum temperature becomes higher (around 600 °C) than for naturally piled rice husk. The bigger bulk density decreases the porosity inside the fuel bed, reducing thermal dispersions [39]. Besides, another possible reason is that the dwell time of gaseous species is extended resulting in

3.1.2. Absorption of Volatiles by the Upper Ash A one-dimensional simplified illustration on temperature field of the whole fuel bed after formation of a thin layer of ash at top surface is shown in Figure 3. It was drawn according to the history of temperature inside the fuel bed, our previous experiments [40] and the characteristic temperature profile in a forward smoldering system in the literature [4]. Due to the longer drying time of the moist fuel bed than an unwashed one shown in Figure 2, the former has a thinner layer of high temperature area (reaction zone) than the latter after a short time, as illustrated in the curve of Figure 3. The amount of the unreacted rice husk (without pyrolysis) is proportional to the marked area of the left side. During their devolatilization, part of the volatiles can be absorbed by the upper ash due to its low temperature. As a result, the ash of the moist fuel bed has a higher tendency to absorb Comparing the temperature development of the washed (moist) fuel bed with the unwashed fuel bed, the former has a longer duration of drying stage. The lower the part of the zone in the fuel bed, the longer the drying stage lasts. At oxidation stage, the duration of temperature > 400 ◦C of the moist fuel bed is shorter than for the unwashed fuel bed. This happens due to more heat generated in the process of smoldering is used to dry the moist rice husk and heat transfer rate to dry fuel is bigger than those for unwashed rice husk. The oxidation duration is also affected by bulk density of the fuel bed. In our supplementary experiments, smoldering of unwashed rice husk with bulk density of 170 kg/m<sup>3</sup> was performed in a small apparatus. It was found that the maximum temperature becomes higher (around 600 ◦C) than for naturally piled rice husk. The bigger bulk density decreases the porosity inside the fuel bed, reducing thermal dispersions [39]. Besides, another possible reason is that the dwell time of gaseous species is extended resulting in longer duration.

#### volatiles from its lower part than the ash of fuel with less moisture. 3.1.2. Absorption of Volatiles by the Upper Ash

A one-dimensional simplified illustration on temperature field of the whole fuel bed after formation of a thin layer of ash at top surface is shown in Figure 3. It was drawn according to the history of temperature inside the fuel bed, our previous experiments [40] and the characteristic temperature profile in a forward smoldering system in the literature [4]. Due to the longer drying time of the moist fuel bed than an unwashed one shown in Figure 2, the former has a thinner layer of high temperature area (reaction zone) than the latter after a short time, as illustrated in the curve of Figure 3. The amount of the unreacted rice husk (without pyrolysis) is proportional to the marked area of the left side. During their devolatilization, part of the volatiles can be absorbed by the upper ash due to its low temperature. As a result, the ash of the moist fuel bed has a higher tendency to absorb volatiles from its lower part than the ash of fuel with less moisture.

**Figure 3.** Schematic of spatial temperature distributions of washed (**a**) and unwashed (**b**) fuel bed*.* **Figure 3.** Schematic of spatial temperature distributions of washed (**a**) and unwashed (**b**) fuel bed. **Figure 3.** Schematic of spatial temperature distributions of washed (**a**) and unwashed (**b**) fuel bed*.*

#### *3.2. Physical Properties and Mass Loss Characteristic of Rice Husk Ash 3.2. Physical Properties and Mass Loss Characteristic of Rice Husk Ash 3.2. Physical Properties and Mass Loss Characteristic of Rice Husk Ash* 3.2.1. Physical Properties of Ash

#### 3.2.1. Physical Properties of Ash 3.2.1. Physical Properties of Ash

Photos of ash before and after grinding from washed and unwashed smoldering as well as washed and unwashed burning are shown in Figure 4. These ashes are gray, soft, and almost retains the shape of rice husk itself. The whitest ash stems from washed burning, followed by unwashed burning. As for the other two ashes, the difference in the whiteness is negligible. The main reason is that temperature in smoldering (560 °C) is significantly lower than in muffle furnace (600 °C) and the duration of this temperature in smoldering is shorter. Other possible reasons are the removal of impurities like dust by washing or absorption of volatiles by the upper ash. The higher the whiteness, the higher silica content in ash [44]. It is worth mentioning that there are always some black particles in the ash. These might be the rice husk with incomplete combustion. Bridge forming in the fuel bed that makes the cooling of the related particle faster than the dense piled Photos of ash before and after grinding from washed and unwashed smoldering as well as washed and unwashed burning are shown in Figure 4. These ashes are gray, soft, and almost retains the shape of rice husk itself. The whitest ash stems from washed burning, followed by unwashed burning. As for the other two ashes, the difference in the whiteness is negligible. The main reason is that temperature in smoldering (560 ◦C) is significantly lower than in muffle furnace (600 ◦C) and the duration of this temperature in smoldering is shorter. Other possible reasons are the removal of impurities like dust by washing or absorption of volatiles by the upper ash. The higher the whiteness, the higher silica content in ash [44]. It is worth mentioning that there are always some black particles in the ash. These might be the rice husk with incomplete combustion. Bridge forming in the fuel bed that makes the cooling of the related particle faster than the dense piled should be the reason. Photos of ash before and after grinding from washed and unwashed smoldering as well as washed and unwashed burning are shown in Figure 4. These ashes are gray, soft, and almost retains the shape of rice husk itself. The whitest ash stems from washed burning, followed by unwashed burning. As for the other two ashes, the difference in the whiteness is negligible. The main reason is that temperature in smoldering (560 °C) is significantly lower than in muffle furnace (600 °C) and the duration of this temperature in smoldering is shorter. Other possible reasons are the removal of impurities like dust by washing or absorption of volatiles by the upper ash. The higher the whiteness, the higher silica content in ash [44]. It is worth mentioning that there are always some black particles in the ash. These might be the rice husk with incomplete combustion. Bridge forming in the fuel bed that makes the cooling of the related particle faster than the dense piled should be the reason.

**Figure 4.** Photos of four types of rice husk ash (circles show black particles in ash)*.* **Figure 4.** Photos of four types of rice husk ash (circles show black particles in ash).

**Figure 4.** Photos of four types of rice husk ash (circles show black particles in ash)*.*

3.2.2. Mass Loss Characteristic of Rice Husk Ash

#### 3.2.2. Mass Loss Characteristic of Rice Husk Ash

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Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of four types of ash (washed and unwashed smoldering, washed and unwashed burning) are shown in Figure 5. It is seen from TG data that total mass-loss of four ashes is <5%. The mass loss from 50 to 950 ◦C for the above four types of ash are 4.2 wt.%, 3.1 wt.%, 2.5 wt.%, and 2.3 wt.%, respectively. The lower the combustion temperature and oxidation duration, the higher total mass loss. Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of four types of ash (washed and unwashed smoldering, washed and unwashed burning) are shown in Figure 5. It is seen from TG data that total mass-loss of four ashes is <5%. The mass loss from 50 to 950 °C for the above four types of ash are 4.2 wt.%, 3.1 wt.%, 2.5 wt.%, and 2.3 wt.%, respectively. The lower the combustion temperature and oxidation duration, the higher total mass loss.

**Figure 5.** TG & DTG curves of four types of ash*.* **Figure 5.** TG & DTG curves of four types of ash.

As shown in the DTG curves, there are three stages of mass loss: drying, oxidation, and combustion of residual carbon. Mass loss at each stage is shown in Table 2. At the drying stage (<200 °C), more water and longer drying time are there for burning ash than those for smoldering ash. This implies that the absorption of condensed materials stated in Section 3.1.2 decreases the capability of moisture absorption of ash. At the oxidation stage (200–700 °C), more mass is lost at lower temperatures for the two smoldering ashes than those for the burning ashes. For smoldering ashes, especially for smoldering of moist rice husk, mass loss occurs in the range 400–560 °C. In theory, the mass in this range is burned out during smoldering due to the maximum temperature inside the fuel bed is around 560 °C. The mass loss in this range indicates the oxidation of volatiles and carbon. At the stage of combustion of residual carbon (>700 °C), the mass loss of ash from smoldering is higher compared with burning ash due to more carbon in ash. The reason for the formation of residual carbon is that the melted silica obstructs the transport of oxygen to carbon [45]. At a higher temperature, the residual carbon can be burned out. Besides, the mass loss at this stage might also relate to the evaporation of KCl [38], the decomposition As shown in the DTG curves, there are three stages of mass loss: drying, oxidation, and combustion of residual carbon. Mass loss at each stage is shown in Table 2. At the drying stage (<200 ◦C), more water and longer drying time are there for burning ash than those for smoldering ash. This implies that the absorption of condensed materials stated in Section 3.1.2 decreases the capability of moisture absorption of ash. At the oxidation stage (200–700 ◦C), more mass is lost at lower temperatures for the two smoldering ashes than those for the burning ashes. For smoldering ashes, especially for smoldering of moist rice husk, mass loss occurs in the range 400–560 ◦C. In theory, the mass in this range is burned out during smoldering due to the maximum temperature inside the fuel bed is around 560 ◦C. The mass loss in this range indicates the oxidation of volatiles and carbon. At the stage of combustion of residual carbon (>700 ◦C), the mass loss of ash from smoldering is higher compared with burning ash due to more carbon in ash. The reason for the formation of residual carbon is that the melted silica obstructs the transport of oxygen to carbon [45]. At a higher temperature, the residual carbon can be burned out. Besides, the mass loss at this stage might also relate to the evaporation of KCl [38], the decomposition of carbonates [42].



Washed burning 0.9 ± 0.1 1.3 ± 0.1 0.3 ± 0.1 *3.3. Silica Content in Rice Husk Ash*

Unwashed burning 0.8 ± 0.2 1.3 ± 0.1 0.2 ± 0.1 3.3.1. Reproducibility and Reliability of XRF Measurement

*3.3. Silica Content in Rice Husk Ash* 3.3.1. Reproducibility and Reliability of XRF Measurement Main compositions (>0.5%) from triplicate XRF measurements of ash produced by smoldering of washed rice husk are shown in Table 3. It is seen that the reproducibility is good (relative error < 10%). As to reliability, if there are elements with atomic weights <23,

Main compositions (>0.5%) from triplicate XRF measurements of ash produced by

the absolute contents of element are not reliable as pointed in Section 2.4. For smoldering ash, residue carbon and absorbed volatiles affect this measured elemental content.

**Table 3.** Contents of main elements in ash from triplicate measurements (wt.%).


#### 3.3.2. Content of Silica and Other Main Compositions in Rice Husk Ash

The contents of SiO<sup>2</sup> and others main compositions in the four types of ash are listed in Table 4. The main component is SiO<sup>2</sup> and the contents of it in all ashes is >85%. The SiO<sup>2</sup> content in descending order is washed burning (93.4%), unwashed burning (90.2%), unwashed smoldering (89.0%), and washed smoldering (86.4%). Contents of other compositions in descending order of the content are K2O, CaO, SO3, P2O5, MgO, Cl, Fe2O3, Al2O<sup>3</sup> in smoldering ash, and this order holds for most elements in other ashes.

**Table 4.** Main compositions in 4 types of ash.


#### 3.3.3. Effect of Production Method on Silica Content

The purity of silica in ash is affected by three main factors: absorption of volatiles by upper ash, combustion temperature and pretreatment of rice husk. Volatiles adhering to the surface of upper-ash smoldering decreases the SiO<sup>2</sup> content. Incomplete burn out of solid organics at low combustion temperature also decreases SiO<sup>2</sup> content in ash. The pretreatment way of washing can remove some water soluble inorganics, such as K, Cl and dust [46,47]. The removal of water soluble inorganics can increase the silica content [35]. According to Table 4, the measured silica content in ash of washed smoldering is similar to or lower than that in unwashed smoldering, while the content of water-soluble ions (Ca, S, P, Mg, Cl, Fe, Al) is higher than that in unwashed smoldering. The lower silica content for washed smoldering is related to the shorter oxidation duration in the moist fuel bed, which results in incomplete combustion of rice husk. The higher content of water-soluble ions might relate to the inaccuracy of XRF measurement as mentioned in Section 3.3.1. The removing of carbon results in an increase of those water-soluble ions contents as a percentage of the whole ash.

#### *3.4. BET Specific Surface Area*

#### 3.4.1. Specific Surface Area of the Four Types of Rice Husk Ash

The BET specific surface area of ash produced in this study (washed and unwashed smoldering, washed and unwashed burning) and in literature is shown from Figure 6. Two characteristics can be seen from this data of this study: (1) The specific surface area of ash prepared from washed rice husk is higher than that from unwashed rice husk; (2) for the ash of prepared from washed rice husk, the specific surface area is lower when smoldering is used compared with burning in the muffle furnace. However, the situation is opposite for the ash prepared from unwashed rice husk.

Figure 6. Specific surface area of silica produced by different methods. (Fernandes et al. [32]: nopretreatment rice husk burned in a fluidized bed combustor; Dizaji et al. [28]: washed rice husk (50 ºC tap water for 2 h) burned in a muffle furnace (600 ºC for 4 h); Huang et al. [48]: citric acid-leached rice husk **Figure 6.** Specific surface area of silica produced by different methods. ([32]: no-pretreatment rice husk burned in a fluidized bed combustor; [28]: washed rice husk (50 ◦C tap water for 2 h) burned in a muffle furnace (600 ◦C for 4 h); [48]: citric acid-leached rice husk (1 wt.% at 80 ◦C for 3 h) burned in a muffle furnace (700 ◦C for 2 h)).

#### (1 wt.% at 80 ºC for 3 h) burned in a muffle furnace (700 ºC for 2 h)). 3.4.2. Factors of Specific Surface Area

It can be seen from the above characteristics that the specific surface area is affected by pretreatment conditions and combustion temperature. Pretreatment, such as washing, removes part of potassium. This decreases the formation of eutectic from interaction of K and Si. The decrease in amount of the eutectic partly avoids the transformation from amorphous silica to crystalline via melting in eutectic and condensing in cooling stage, and consequently increases the specific surface area [49]. Pretreatment of washing also removes soil particles which normally have lower specific surface area than amorphous silica.

As to temperature, low temperature avoids sintering/eutectic melting of the mixed components in ash of rice husk and is beneficial for silica to maintain its amorphous state and high specific surface area [50]. There is a combination effect of the two factors. It is very hard to get high-specific-surface-area ash from burning original rice husk at a high temperature (>700 ◦C).

3.4.3. Comparison of Specific Surface Area in This Study with Those of Silica Prepared Using Methods in Literature

The specific surface area of ash produced by smoldering of washed rice husk is 84.9 m2/g, which is lower than those prepared in the laboratory (99.2–293.9 m2/g) [13,29,51], but higher than those produced in the industry (11.4–39.3 m2/g) [32,52], as shown in Figure 6. In the laboratory, rice husk is normally washed or leached using water and acid to remove alkali and alkaline earth metals, such as the experiments performed by Dizaji [28] and Huang [48]. In the industry, no-pretreatment rice husk is burned directly in combustors. The high temperature (>700 °C) of most combustor is not suitable to produce silica with high specific surface area.

#### *3.5. Supplementary Experiment of Smoldering Air-Dried Rice Husk after Washing*

To decrease the absorption of volatiles by upper ash, a supplementary smoldering experiment using air dried rice husk after washing was performed in the experimental set-up. It was found that the silica content is 93.5%, and specific surface area is 145.9 m2/g. Both the purity and specific surface are significantly higher than those (86.4%, 84.9 m2/g) from moist rice husk. It shows that drying before smoldering does increase both silica content and specific surface area.

#### **4. Potential of Mass Product of Silica from Smoldering of Rice Husk 4. Potential of Mass Product of Silica from Smoldering of Rice Husk**

*4.1. Measures to Increase Silica Content and Specific Surface Area 4.1. Measures to Increase Silica Content and Specific Surface Area*

As mentioned before, the purity and specific surface area of silica produced from smoldering of rice husk are affected by pretreatment of the raw material, the volatile absorption of ash in reactor, and the solid temperature and dwell time in the fuel bed. According to the above experience in smoldering, two measures are proposed here to improve silica properties. The first is to develop a lateral continuously smoldering involving a dry stage apparatus. The second is to supply a little amount of air to improve fuel bed temperature slightly. As mentioned before, the purity and specific surface area of silica produced from smoldering of rice husk are affected by pretreatment of the raw material, the volatile absorption of ash in reactor, and the solid temperature and dwell time in the fuel bed. According to the above experience in smoldering, two measures are proposed here to improve silica properties. The first is to develop a lateral continuously smoldering involving a dry stage apparatus. The second is to supply a little amount of air to improve fuel bed temperature slightly.

#### 4.1.1. Lateral Continuously Smoldering Involving a Dry Stage of Rice Husk 4.1.1. Lateral Continuously Smoldering Involving a Dry Stage of Rice Husk

A continuously lateral propagation smoldering is proposed here as shown in Figure 7. The smoldering of rice husk can be divided into three stages using a grate: drying, pyrolysis and oxidation. Due to its lateral propagation, the volatiles generated in the pyrolysis stage are discharged into the gas burning chamber directly and burned out, which cannot adhere to the ash in oxidation stage. The heat generated by oxidation can be used for the drying of rice husk. The condensate water generated in the process of drying can be collected and used for the washing of rice husk. A continuously lateral propagation smoldering is proposed here as shown in Figure 7. The smoldering of rice husk can be divided into three stages using a grate: drying, pyrolysis and oxidation. Due to its lateral propagation, the volatiles generated in the pyrolysis stage are discharged into the gas burning chamber directly and burned out, which cannot adhere to the ash in oxidation stage. The heat generated by oxidation can be used for the drying of rice husk. The condensate water generated in the process of drying can be collected and used for the washing of rice husk.

**Figure 7.** Schematic diagram of lateral continuously smoldering. **Figure 7.** Schematic diagram of lateral continuously smoldering.

#### 4.1.2. The Supply of Air at the Oxidation Stage 4.1.2. The Supply of Air at the Oxidation Stage

To increase the combustion temperature, a small amount of air can be provided in the stage of oxidation. As mentioned before, temperature of 700 °C can increase the purity and specific surface area. The highest temperature of the current piled smoldering is around 600 °C, which can be increased by supplying a small amount of air. To increase the combustion temperature, a small amount of air can be provided in the stage of oxidation. As mentioned before, temperature of 700 ◦C can increase the purity and specific surface area. The highest temperature of the current piled smoldering is around 600 ◦C, which can be increased by supplying a small amount of air.

#### *4.2. Feasibility of Mass Product of Silica from Smoldering of Rice Husk 4.2. Feasibility of Mass Product of Silica from Smoldering of Rice Husk*

Although silica prepared in the laboratory from pretreated rice husk has a high silica content and high specific surface area, considerable energy is necessary for removal of the water from moist rice husk. Besides, a lot of waste acid and lye is produced during pretreatment and post-treatment, which is harmful to the environment. As to a traditional combustor, the high temperature of the solid during combustion is disadvantageous in maintaining a high specific surface area of silica. Besides, most combustors are not suitable Although silica prepared in the laboratory from pretreated rice husk has a high silica content and high specific surface area, considerable energy is necessary for removal of the water from moist rice husk. Besides, a lot of waste acid and lye is produced during pretreatment and post-treatment, which is harmful to the environment. As to a traditional combustor, the high temperature of the solid during combustion is disadvantageous in maintaining a high specific surface area of silica. Besides, most combustors are not suitable for burning rice husk with high moisture content.

for burning rice husk with high moisture content. Smoldering is characterized by a low temperature in the fuel bed and is self-sustained with moist content. Our experiments show that the highest silica content and specific surface area of ash is from smoldering of air-dried-washed rice husk. It indicates that after Smoldering is characterized by a low temperature in the fuel bed and is self-sustained with moist content. Our experiments show that the highest silica content and specific surface area of ash is from smoldering of air-dried-washed rice husk. It indicates that after some improvement of our experimental set-up, it has great potential in the mass production of high-quality silica directly from moist rice husk.

#### **5. Conclusions**

Smoldering experiments with moist rice husk were performed in a self-designed apparatus to check its possibility for silica production. Temperature inside the fuel bed and silica content/specific surface area/mass-loss-characteristics of ash were analyzed. The main conclusions are:


**Author Contributions:** Conceptualization, F.H.; methodology, F.H. and J.C.; software, S.Y. and F.H.; investigation, T.S.; data curation, T.S.; writing—original draft preparation, T.S.; writing—review and editing, D.Y., F.H., J.C., T.S. and F.B.; supervision, F.H.; funding acquisition, F.H. and F.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by [National Natural Science Foundation of China] grant number [51676115] and [Sino-German Center for Research Promotion] grant number [M-0183].

**Institutional Review Board Statement:** Exclude this statement.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are available upon request.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

