*2.5. Agricultural Waste*

A great deal of solid agricultural waste is generated each year, presenting a major challenge for disposal processes, as it creates a foul odor and attracts pests, resulting in health issues for living beings. Some of the most commonly generated types of agricultural waste include rice husks [138], rice straw [139], wheat husks and straw, coconut shells [140], sugarcane bagasse [141], corn cobs, and almond shells. These types of agricultural waste are rich sources of carbon, silica, calcium, and other trace elements like Fe, Al, etc. Even today, only a small fraction of such agricultural waste is applied as a fuel, additive, and filler in the construction industry. A major fraction of agricultural waste is disposed of using three main techniques, thermal treatment, landfill, and decomposition, and these have been reported to negatively affect the environment [26]. For example, the thermal treatment of waste results in the release of numerous noxious gases like CO2, CO, Cn Hm, SOx, NOx, ash, etc., many of which are classified as greenhouse gases. As a result, many scholars have raised objections over combustion as a method of agricultural waste disposal.

Agricultural waste such as sugarcane bagasse and rice husks constitute a rich source of silica. Both rice husks and sugarcane bagasse can act as potential candidates for silica extraction. The silica synthesized from such wastes can provide an alternative, renewable source, minimizing the pollution resulting from such materials. Meanwhile, coconut shell husks are a rich source of calcium [26]. The possibility of recovering of such valuable minerals from waste not only provides alternative precursor materials, it also provides an environmentally friendly, cost-effective approach to the problem. The use of such agricultural waste also minimizes environmental pollution. All of these waste products are biological materials; thus, they are rich sources of carbons that can be used for the synthesis of activated carbons or biochars.

There are certain plants that possess a high accumulation of silica in their leaves, stems, fruits, etc. The silica is taken up from the soil by the roots of the plants and distributed to the other plant parts. In plants, silica is mainly present as silicon. Silica can potentially be found in the solid waste used extensively in the industrial sector. Two factors, namely, the availability and quantity of silicon in the soil, affect silica deposition in agricultural residues [142]. Several plants consist of silica, includingwheat, rice, sunflowers, corn, and bamboo [143]. In general, silica can be extracted from leaves, stems, and other parts of a plant with a yield of between 0.1 and 10 wt%. On the other hand, the quantity of silica

in agricultural residues is dependent on the season, species, maturity, and geographical characteristics of the given farm.

#### 2.5.1. Rice Husks (RH) and the Recovery of Silica

Rice husks are agricultural waste products that are produced during processing. During the hulling process, rice husks are obtained, which are mainly used as fodder for cattle. Rice husks and straw are major agricultural products, with an annual global production as high as one million tons. These waste products are common in rice-producing countries like India, Vietnam, and Japan. RH are rich is silica, but at the same time they also contain organic compounds that may interfere with the final purity of the silica. Therefore, RH have to first be calcinated at high temperature (400–1000 ◦C) in a muffle furnace. Furthermore, they have to be washed with phosphoric acid, which will remove the organic content. Once the RH ash is free from organic content, it is dried and treated with 4–16 M NaOH, along with 90–95 ◦C, for 60–90 min along with continuous stirring. NaOH will react with NaOH and will form sodium silicate, which will be further treated with 1–2 N HCl, resulting in silica gel. This silica gel is further washed and dried to obtain pure silica or nanosilica. Some authors have also reported calcination at 400–600 ◦C for 2–6 h in order to obtain nanosilica with the desired shaped [144].

#### 2.5.2. Sugarcane Bagasse and the Recovery of Silica

Similarly, sugarcane bagasse is also one of the major byproducts of the sugarcane industry, and is produced after the extraction of sugarcane juice. Sugarcane bagasse is a rich source of carbohydrates and other minerals besides silica. It is also produced in huge quantities in sugar-producing countries. Sugarcane bagasse (SB) is considered to be a non-biodegradable solid material, and primarily consists of crystalline silica [145]. A major problem in the sugarcane industry is how to dispose of SB. At present, it is used in the production of ceramic tiles, soil fertilizers, and fodder in some parts of the world [146]. In comparison with other agricultural residues, sugarcane bagasse ash (SBA) possesses a very high quantity of silica. Table 2 presents the complete elemental composition of SBA. Several parameters affect the quantity of silica extractable from bagasse, including the nature of the soil, the surrounding environment, the harvesting process, and the time of harvesting.

**Table 2.** Elemental composition of sugarcane bagasse ash (SBA).


Drummond et al. [147] reported silica extraction from different preparations of SBA by performing natural burning (SBA-NB) and laboratory burning (SBA-LP) at 700 ◦C for two hours using muffle furnace, which was followed by alkaline extraction. The experiments revealed that silica extraction of about 94.47% was achieved using natural burning, and 96.93% of the silica was obtained using laboratory burning. In another project, Harish et al. [148] attempted to recover silica from SBA and silica fumes as low-cost precursors for the synthesis of silica gel using a sodium hydroxide-based alkali treatment method. Norsuraya et al. [149] reported the synthesis of Santa Barbara Amorphous-15 from SBA. XRF revealed that raw sugarcane ash contained 53.10% silica, while acid-treated ash contained 88.13% silica.

Channoy et al. [141] conducted a study aiming to synthesize silica gel from SBA by treating the ash with 1.5, 2, and 2.5 N NaOH. The particle size of the obtained silica gel varied, with values of 120, 100, and 80 nm, respectively, as confirmed by SEM. SEM revealed that as the concentration of NaOH increased, the particle size decreased. Rovani et al. [150] made use of SBA waste to synthesize highly pure silica with a high adsorption capacity. The synthesized silica nanoparticles were characterized by FTIR, SEM, TEM, XRD, ICP, etc., and it was found that the particle size was 20 nm and the purity was about 88%.

To obtain silica from SBA, several steps are needed: washing of the collected SB, shredding it into small pieces, pyrolysis at high temperature, alkali treatment of ash, acidic treatment of the sodium silicate leachate, formation of the silica gel, washing, precipitation, and calcination.

### 2.5.3. Coconut Shells

Coconut (Cocos nucifera) is the major plantation crop of coastal countries such as African countries [151]. A large number of coconuts are produced around the world, and these are mainly used for food and cosmetics. The soft part of the fruit is consumed, while the coir and shell are left behind as waste. Coir is used to make mattresses, while the shell is mainly disposed of into the environment. However, in African countries, coconut waste is conventionally used as a source of fuel and applied in burning processes [152]. Nowadays, with recent research advances, coconut shell waste can be used as a source of activated carbon and/or other value-added minerals. Coconut shell has high silica content and can act as a potential candidate as a source of silica and activated carbon. The use of coconut shell as a source of both silica and activated carbon presents cost-effective green synthesis strategy. Coconut shell can act as another substance for the synthesis of silica from renewable sources. Roughly 33–35% of a coconut is composed of husk, playing the role of the mesocarp of the fruit. Nowadays, coconut husk is used as a favorable source of fuel for coconut processing and domestic fuel, and as a fiber source for the production of mats, ropes, etc. [153]. The extraction of silica from agricultural wastes involves three sequential steps: acid leaching, mixing alkaline treatment, and precipitation with acid [154]. A schematic diagram is shown in Figure 8. The chemical composition of coconut shell is given below in Table 3. It mainly contains calcium oxide, followed by silica, along with traces of Al, K, Fe, and P, in the form of either oxides or chlorides.

#### Synthesis of Carbon Nanotubes from Coconut Shell Husk Ash

Carbon nanotubes have gained considerable attention from scientific fields of study such as medicine, drug delivery, photo dynamic therapy, and environmental cleanup. They have numerous advantages over other nanoparticles due to their high mechanical properties, high tensile strength, high electrical conductivity, high aspect ratio, high thermal conductivity, and ultra-light weight. Generally, these nanotubes are synthesized using the physical vapor deposition method or the chemical vapor deposition method; both of these methods are costly and energy intensive. As a result, the synthesis of carbon nanotubes from carbon-rich waste materials like coconut shell husk ash is an economical and environmentally friendly approach. There are several approaches in which coconut shell has been used for the synthesis of carbon nanotubes.

Anuar et al. [153] reported the synthesis of silica nanoparticles from coconut shell ash. In their study, the coconut husks were burned under different temperatures, and were analyzed using XRF to identify their elemental composition. The composition of silica varies from 8–11% in the husk ash. The husks and silica were characterized by SEM-EDS, XRD for confirmation and to determine the optoelectronic properties of the silica. In another project, Sivasubramanian and Sravanthi (2015) attempted to synthesize silica nanoparticles from coconut shell ash using the NaOH-based alkali treatment method. Ash was treated with 2.5 N sodium hydroxide to obtain sodium silicate. Finally, silica was formed by treatment with HCl, and was confirmed by SEM-EDS, FTIR, TEM, and XRD [155]. Melati and Hidayati [156] reported the synthesis of multi-walled carbon nanotubes from coconut shell in two steps. First, the coconut shell was activated by treating it at 500–600 ◦C, and it was subsequently converted into carbon nanotubes by applying pyrolysis and a

wet CVD process. The characterization of the nanotubes revealed the properties of the MWCNTs. It was used for the detection of cancer in mammalian cell lines.

**Figure 8.** General method for the synthesis of silica from coconut shell waste.


**Table 3.** Elemental composition of coconut husk ash (CHA).

In the study conducted by Adewumi et al. [157], carbon nanospheres were synthesized from low-cost coconut fibers in three sequential steps: pyrolyzation, physical activation, and ethanol vapor treatment. The analysis of the samples revealed that the spherical-shaped particles had a diameter of 30–150 nm. Hakim et al. [158] applied an easy, environmentally

friendly approach called the one-step water-assisted (quenching) synthesis method to obtain carbon nanotubes using coconut shell husk ash. The chemical and physical properties of the carbon nanotubes were analyzed using sophisticated instruments, and it was found that the average diameter was 123 nm; the nanotubes were finally applied for the remediation of Pb2+ ions from wastewater. Araga and Sharma [158] synthesized PECV-assisted multiwalled carbon nanotubes (MWCNTs) over coconut shell-derived charcoal pyrolyzed at 900 ◦C in a process with only a single step. They used the mineral content in the source material as the catalyst for carbon nanotube (CNT) growth.

#### Corn Cobs as a Source of Activated Carbon

Maize, or corn (*Zea mays*), is a popular cereal crop cultivated in many parts of the world [159]. During the processing and production of corn, several waste products are generated, including corn cobs and corn husks [160]. Corn cob waste is a rich source of carbon, and can act as a potential candidate for reparing carbon with ultra-high specific surface area [161]. On the other hand, there are various waste materials that can be applied as activated carbon sources, for instance, date and palm waste, coconut shell waste, corn stalks, and corn cobs. Corn cob is most preferable source of activated carbon, as it is produced in huge amounts around the world. Corn cobs are agricultural waste materials produced in abundance around the world. After extracting the corn, the major fraction of the corn is disposed of as waste, although it is actually a rich source of various minerals. The cob is mainly made up of carbons; therefore, it can be used as a source of biochars or activated carbon.

Activated carbon (AC) refers to carbonaceous materials that possess an internal surface area (that is extremely developed) as well as porosity [162]. The large surface area (an area between 250 m2/g and 2000 m2/g) offers a significant ability to adsorb chemicals from liquids/gases, and permit application as a versatile adsorbent under various conditions. AC has been widely used to produce adsorbents, supporting materials, textiles, fabrics, animal foods, etc. [163]. AC is known to be an effective material as a result of its low density, well-developed porosity, accessibility, chemical stability, and low cost [164]. In the last few years, a major emphasis has been placed on the development of AC with ultrahigh specific surface area from both renewable and non-renewable sources using chemical approaches or with the use of chemical vapor deposition (CVD). In the process of water treatment, a substantial amount of AC is used for the purpose of removing organic and other compounds that could change the water odor and taste [165].

Tsai et al. [166] reported the synthesis of AC from corn cob waste through treatment with different physical and chemical activators such as NaOH, KOH, K2CO3, and CO2. They studied the effect of impregnation time, impregnation ratio, activation temperature, and soaking time on carbon dioxide. The surface area of the AC was analyzed using the BET analyzer. The total pore volume and BET surface area were roughly 1.0 cm3/g and 2000 m2/g, respectively. The findings of this research demonstrate that corn cob activation with KOH/K2CO3 and CO2 is able to appropriately prepare large-surface-area ACs. Furthermore, Sai et al. [167] attempted to activate corn cob using potassium salts, and subsequently gasified it with carbon dioxide. The obtained AC had a large surface area, as measured by BET. In the study carried out by Kazmierczak et al. [168], AC was developed from corn cob by activating it using chemical and physical methods. They further studied the sorption properties of the activated carbon. The final product consisted of microporous activated carbon with a high surface area, varying from m 337 to 1213 m2/g, and showing diverse acid–base characteristics on the surface [169,170]. It was also assessed for the adsorption of different materials from the aqueous solutions. Figure 9 shows the schematic diagram for the formation of activated carbon from corn cob.

**Figure 9.** Schematic diagram for the formation of activated carbon from corn cob.

#### **3. Domestic Waste**

Domestic waste refers to the materials produced in houses, mainly kitchens. There are several wastes produced in houses that offer potential for the recovery of value-added minerals. Calcium carbonate is a highly important particle that is used in every aspect of our life, as well as in a number of different industries [171]. Calcium carbonates are alkaline earth materials [172] that are present in our environment and which are widely applied in industries like papers [173], paints [174], coating agents, cosmetics [175,176], pharmaceuticals and medicine [177], agriculture, automobiles and textiles [178], and reinforcements, fillers, bio-nanocomposites and bio-ceramics in dentistry [178,179].

Calcium carbonates have excellent properties, including biodegradability, biocompatibility, pH sensitivity, safety, and cost-effectiveness, and they exhibit polymorphicity [179]. They are very lightweight; thus, they can be used for making lightweight materials. Calcium carbonate exists in three polymorphs in nature, i.e., calcite, aragonite, and vaterite [180]. All these three polymorphs vary with respect to their thermodynamic stability and morphology. Among them, calcite is thermodynamically the most stable, while aragonite shows an intermediate stability, and vaterite (μ-CaCO3) is the least stable polymorph, due to which it is less commonly present in nature. Vaterite can be rapidly transformed into the aragonite and calcite in aqueous solution. It has been experimentally proved that vaterite can transform to aragonite in 60 min at 60 ◦C and to calcite in 24 h at room temperature [181]. While calcite is rhombohedral in structure, aragonite is spindle- or needle-shaped, while vaterite is octahedral [182]. There are several waste materials that are rich sources of calcium oxide and calcium carbonates, including incense sticks, eggshells, cockle shell waste [183], and gypsum waste; however, the following sections only discuss domestic waste, i.e., incense stick ash and eggshell waste.

### *3.1. Incense Stick Ash (ISA)*

Incense stick ash is one of the most unexplored products, and are generally produced at religious places, i.e., temples, churches, mosques, etc. The burning of incense sticks is a ritual practiced in every religion. However, in South-Asian countries and zones, e.g., China, Taiwan, India, and Japan, large amounts of incense sticks are consumed [184,185]. Incense sticks are cylindrical in shape, and are fragranced and intended to be burned in order to purify the air [186]. After the burning of incense sticks, the ash is left behind in the form of a residue, which is mainly disposed of in rivers and other water bodies, as can be frequently observed in India, where this practice is considered to be holy. Moreover, due to its sacred value in Hinduism, incense is even applied on the forehead or eaten as Prasad. These incense sticks mainly contain calcium oxide, silica, ferrous, alumina, rutile, Mg, and traces of oxides of K, Na, and Mn. The sticks contain 45–60% calcium oxides or carbonates, 10–20% silica, and 5–7% Mg, with less than 5% consisting of other materials. The disposal of incense stick ash into water pollutes it by increasing the concentration of Ca and Mg, ultimately increasing the water hardness. These two elements also play a role in increasing the pH of water by forming precipitates of hydroxides in it. Due to the high calcium content, incense stick ash can act as a potential source material for the recovery of calcium oxides and carbonates. The complete schematic diagram for the synthesis of calcium carbonate from incense stick ash is given below in Figure 10.

#### *3.2. Eggshell Waste*

Eggshells are another waste product produced by houses and industry, and are still classified as a byproduct of the poultry industry. Approximately one million tons of eggshell are generated per year globally. Currently, eggshell waste is dumped into landfills. Such practices may lead to the deterioration of agricultural land [187]. This waste is mainly made up of calcium oxides that are meant to provide safety to growing chicks. Due to their high calcium content (composition shown in Table 4), eggshells can be used as a potential alternative source of calcium oxide or carbonates [188,189].

The recovery of calcium oxides and carbonates from incense stick ash and poultry is easy, economical, and environmental friendly. Recovery from such wastes focuses on the use of renewable sources without affecting our natural resources. Consequently, it also minimizes pollution in the form of solid waste. Hassan et al. [190,191] reported the synthesis of CaCO3 NPs from chicken eggshell waste, which involved the following steps: cleaning and size reduction of eggshells, followed by surface modification using the sonochemical method to achieve enhanced dispersion [190,191]. In another study, Hariharan et al. [192] attempted to perform the synthesis of calcite nanoparticles from eggshell waste using gelatin. Chicken eggshells were used to obtain the calcite polymorph of calcium carbonate using gelatin by means of the precipitation method. Nanocalcite was confirmed using FTIR, XRD, UV-Visible spectroscopy, and SEM. The particles were identified as calcite polymorphs with a particle size of 25 nm. The results obtained in the FTIR experiments confirmed the creation of calcite, with characteristic absorption bands being observed at 712, 876, and 1410 cm−1, corresponding to the bending and stretching vibrations of CO3 <sup>2</sup><sup>−</sup> ions. A comparison was also made between the obtained results and calcium carbonate synthesized with no gelatin.

Calcium oxides are currently produced at commercial levels from calcium-based precursor materials like calcium nitrate [193], calcium hydroxide [194], calcium sulphate, and calcium chloride [193] using techniques such as thermal decomposition [194], microwave irradiation [195], sol–gel [196], co-precipitation [197,198], hydrogen plasma–metal reaction [199], and sonochemical synthesis [191,200]. However, the use of the abovementioned calcium precursors for the synthesis of calcium oxide nanoparticles using the techniques described above makes the whole process costly and energy intensive, and also requires the use of hazardous chemicals. As a result, for the synthesis of calcium oxide nanoparticles, there is a need to switch to green methods. Some of the most common calcium-rich waste materials are cockle shells [201], eggshells [202], gypsum waste, and

incense stick ash. Among these, incense stick ash is the most underestimated calcium-rich waste material, and is produced abundantly at religious places and in houses. Such materials can act as potential substitutes for various calcium precursors used at commercial level [203,204].

**Figure 10.** Flow chart for the synthesis of CaCO3 from incense stick ash, adopted from Yadav et al., open access journal Applied Sciences, 2021 [129].


**Table 4.** The chemical composition of chicken eggshells.

Tangboriboon et al. [205] synthesized calcium oxide nanoparticles from duck eggshell waste using the calcination technique and analyzed their properties. The duck eggshells and the calcined eggshells were analyzed using FTIR, STA, XRD, XRF, TEM, BET, a particle size analyzer, and an impedance analyzer. The microscopy revealed the good dispersion quality of the calcium oxide nanoparticles, which had a spherical shape, with a ceramic yield of 53%. In another study, Jirimali et al. [206] reported the synthesis of calcium oxide and hydroxyapaptite using eggshell for the development of LLDPE Polymer Nanocomposite [206]. Mohadi et al. [202] synthesized the calcium oxide nanoparticles from the chicken eggshell waste by calcination of the shells at different temperatures ranging from 600 to 1000 ◦C. The synthesized calcium oxide nanoparticles were analyzed using FTIR, SEM-EDS, XRD, and BET. The calcium oxides were porous in nature, with pore sizes of 6.6 nm, meaning they could be classified as mesoporous, and with a surface area of 68 m2/g [202].

A typical TEM image of calcium oxide nanoparticles obtained from eggshell waste is shown in Figure 11 [204].

**Figure 11.** TEM image of calcium oxide nanoparticles obtained from eggshell waste adapted from (Render et al. [204]).

There are several reports of the synthesis of composite materials and fertilizers from plastic waste; however, research conducted in this field is very limited. A lot of work needs to be performed in this field in future, since plastics constitute a major global problem.

#### **4. Conclusions**

Waste management mainly addresses two subjects: resource recovery and final disposal. Individuals worldwide earn revenue from each stage, through the recovery of reusable materials and, to a lesser extent, the conversion of waste to energy. Turning waste into wealth not only makes sensible environmental sense, but also turns "trash" into "cash". Agricultural waste materials are mainly organic in nature; they are biodegradable, and can be used for the development of carbon-based materials and activated carbon. The burning of agricultural waste leads to the pollution of the air. In recent decades, waste management and technology awareness programs have successfully transformed hazardous materials (e.g., CFA and red mud) into useable value-added minerals. Both of these materials have been applied in the fields of ceramics, construction materials, and metallurgy, and have proved to be highly valuable. Poultry waste and domestic waste have been also been found to be important precursors of carbon and calcium oxides. The ash from incense sticks is among the emerging domestic byproducts produced at religious places, and have proved to be a valuable source of calcium oxide. All of these waste byproducts have gained importance with the advent and increasing significance of renewable energy sources. The recovery of minerals from such wastes is an economical and environmentally friendly method. Such materials act as alternative precursors, reducing the burden on industry. The use of such wastes reduces pollution at minimum cost, while developing materials and generating revenue.

**Author Contributions:** Conceptualization, G.G., S.M. and N.G.; Data curation, N.C., G.G., S.I. and S.M.; methodology, N.C., N.G. and V.T.; validation, K.K.Y., S.I., B.-H.J. and S.M.; formal analysis, V.K.Y., N.G. and A.A.; resources, K.K.Y., B.-H.J. and A.A.; writing—original draft preparation, V.K.Y., N.C. and K.K.Y.; writing—review and editing, V.K.Y., V.T., G.G., S.M. and A.A.; supervision, V.K.Y., S.I. and B.-H.J.; project administration V.K.Y., N.C., K.K.Y. and A.A.; Funding acquisition, B.-H.J., S.I., V.T. and A.A.; Investigation, V.K.Y. and V.T.; Software's, G.G., N.G. and B.-H.J.; Visualization, K.K.Y. and B.-H.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors gratefully acknowledge the Deanship of Scientific Research, King Khalid University (KKU), Abha-Asir, Kingdom of Saudi Arabia for funding this research work under the grant number RGP.2/58/42. This study was supported by the Korea Environment Industry & Technology Institute (KEITI) through Subsurface Environment Management (SEM) Projects, funded by Korea Ministry of Environment (MOE)(No.2020002480007).

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors gratefully acknowledge the Deanship of Scientific Research, King Khalid University (KKU), Abha-Asir, Kingdom of Saudi Arabia for funding this research work under the grant number RGP.2/58/42. This study was supported by the Korea Environment Industry & Technology Institute (KEITI) through Subsurface Environment Management (SEM) Projects, funded by Korea Ministry of Environment (MOE)(No.2020002480007).

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

#### **References**

