Characterization of Bio-Adsorbents Produced by Hydrothermal Carbonization of Corn Stover: Application on the Adsorption of Acetic Acid from Aqueous Solutions

Abstract

In this work, the influence of temperature on textural, morphological, and crystalline characterization of bio-adsorbents produced by hydrothermal carbonization (HTC) of corn stover was systematically investigated. HTC was conducted at 175, 200, 225, and 250 °C, 240 min, heating rate of 2.0 °C/min, and biomass-to-H₂O proportion of 1:10, using a reactor of 18.927 L. The textural, morphological, crystalline, and elemental characterization of hydro-chars was analyzed by TG/DTG/DTA, SEM, EDX, XRD, BET, and elemental analysis. With increasing process temperature, the carbon content increased and that of oxygen and hydrogen diminished, as indicated by elemental analysis (C, N, H, and S). TG/DTG analysis showed that higher temperatures favor the thermal stability of hydro-chars. The hydro-char obtained at 250 °C presented the highest thermal stability. SEM images of hydro-chars obtained at 175 and 200 °C indicated a rigid and well-organized fiber structure, demonstrating that temperature had almost no effect on the biomass structure. On the other hand, SEM images of hydro-chars obtained at 225 and 250 °C indicated that hydro-char structure consists of agglomerated micro-spheres and heterogeneous structures with nonuniform geometry (fragmentation), indicating that cellulose and hemi-cellulose were decomposed. EDX analysis showed that carbon content of hydro-chars increases and that of oxygen diminish, as process temperature increases. The diffractograms (XRD) identified the occurrence of peaks of higher intensity of graphite (C) as the temperature increased, as well as a decrease of peaks intensity for crystalline cellulose, demonstrating that higher temperatures favor the formation of crystalline-phase graphite (C). The BET analysis showed 4.35 m²/g surface area, pore volume of 0.0186 cm³/g, and average pore width of 17.08 μm. The solid phase product (bio-adsorbent) obtained by hydrothermal processing of corn stover at 250 °C, 240 min, and biomass/H₂O proportion of 1:10, was activated chemically with 2.0 M NaOH and 2.0 M HCl solutions to investigate the adsorption of CH₃COOH. The influence of initial acetic acid concentrations (1.0, 2.0, 3.0, and 4.0 mg/mL) was investigated. The kinetics of adsorption were investigated at different times (30, 60, 120, 240, 480, and 960 s). The adsorption isotherms showed that chemically activated hydro-chars were able to recover acetic acid from aqueous solutions. In addition, activation of hydro-char with NaOH was more effective than that with HCl.

1. Introduction

Hydro-char is porous carbonaceous material with reactive, functionalized/aromatic surfaces [1]. These morphological and textural properties make hydrochar a potential adsorbent to remove/recover chemical contaminants from process water [1,2,3,4,5], until the process aqueous phase produced by hydrothermal carbonization of biomass [6]. Hydrochars differ from biochars due to its lower aromaticity, consisting of mostly alkyl moieties [7]. In recent years, the literature reports the application of hydro-chars as bio-adsorbents to selectively remove organic and inorganic compounds, as well as heavy metal, that is, the adsorption/sorption of aqueous contaminants onto hydro-chars [1,2,3,4,5,6,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51].

Hydro-chars have been applied as adsorbents due to their capacity to remove and or selectively absorb polar and non-polar organic compounds, including acetic acid [6], bisphenol A, 17α-ethinyl estradiol and phenanthrene [22], 2-naphthol [11], pyrene [22]; phenolic compounds (phenol, guaiacol, vanillyl alcohol, and resorcinol) [31]; dyes, such as methylene blue [8,9,23,24,26,27,29,36,38,39,40,41,42,43,44,45,46,47], methyl orange [36,49], methylene green [48], and Congo red [11]; herbicides, such as fluridone, norflurazon [18], and isoproturon [19]; pharmaceuticals, such as triclosan, estrone, carbamazepine, acetaminophen [12], sulfamethoxazole, diclofenac, bezafibrate, carbamazepine, atrazine [16,51], tetracycline [20], diclofenac sodium, salicylic acid, and flurbiprofen [21]; alkaline (K⁺, Na⁺) and alkaline earth metals (Mg²⁺, Ca²⁺) [31]; heavy metals, such as lead [2,3,4,10,15,23,24,25,28,30,32,42], cadmium(II) [2,4,5,10,13,24,32,38], uranium(VI) [1,35], antimony [13], copper(II) [4,5,10,14,17,24,39,50], nickel [4], zinc [10], and chromium(II) [33]; and fertilizers such as phosphate [23,34], orthophosphate [50], and ammonium [34].

The chemical activation of hydro-chars was carried out by either modifying the reaction media composition (H₂0 + modifier) or the solid phase reaction products (hydro-char + modifier) and has been intensively investigated in recent years [4,8,9,10,21,26,27,28,29,30,32,33,35,36,38,39,40,41,42,43,45,46,47,48,49,50,51]. The physical activation of hydrochars has also been investigated [24]. Changes in the mesoporous and surface properties of hydro-chars by modifying the reaction medium composition include the addition of H₃PO₄ (phosphoric acid) [30], CH₂ = CH-COOH (acrylic acid) [29,32], terminal amino hyper-branched polymer solutions [33], C₂H₂(CO)₂O (maleic anhydride) followed by deprotonation of carboxyl groups with NaHCO₃ solution [35], ZnCl₂ (zinc chloride) [41], NaOH [48], HCl, NaOH, and NaCl [51]. In addition, hydro-char mesoporous and surface properties have been chemically activated with H₂O₂ [4,25], NaOH [6,8,9,26,27,29], KOH [10,28,47,50], H₃PO₄ [21], polyaminocarboxylated modified hydro-char [39], O₃/NaCl [42], HNO₃ [43], and until etherification, amination and protonation reaction [49].

The water process streams by hydrothermal carbonization of lignocellulosic materials is a complex mixture containing aromatic-ring compounds (furfural, HMF, phenols, cresols, catechol, and guaiacol) [52,53,54], carboxylic acids (formic acid, acetic acid, propionic acid, and lactic acid) [52,53,54], alcohols (methanol and ethanol) [52,53], and sugars [54], with high concentrations of volatiles carboxylic acids [52,53,54], as well as hazardous substances such as HMF (hydromethylfurfural), thus making its reuse, even as washing water not possible, so that application of separation processes is necessary to recover organic and inorganic compounds. In addition, Machado et al. [52] stated that complex chemical composition of water process streams by HTC poses a hard separation task, because of the huge differences in chemical structure, as well as thermal and physical properties such as boiling point. In fact, many of those compounds present boiling points higher than H₂O, so that application of separation processes (except adsorption [6,31]), such as distillation and evaporation are not effective [52].

Despite some studies on the adsorption/sorption of organic compounds within hydro-char produced by hydrothermal carbonization of biomass, activated chemically with NaOH [6,8,9,26,27,29], only a few have investigated the uptake of organic compounds from aqueous process streams from hydrothermal carbonization/liquefaction [6,31], as summarized synthetically below. In addition, until now, no study has investigated the adsorption of acetic acid in hydro-char activated chemically with HCl.

Machado et al. [6], studied the adsorption of acetic acid form H₂O solutions on hydro-chars produced by hydrothermal carbonization of corn stover at 200, 225, and 250 °C, 240 min, heating rate of 2 °C/min, biomass/H₂O proportion of 1:10, using a reactor of 18.927 L, in batch mode. The hydro-chars were characterized by SEM, EDX, and XRD [6]. The hydro-char obtained at 250 °C, was activated chemically with a 2.0 M NaOH solution [6]. The influence of initial CH₃COOH concentrations on the adsorption kinetic was investigated [6]. The adsorption kinetics was investigated at 30, 60, 120, 240, 480, and 960 s [6]. The adsorption isotherms showed that chemically activated hydro-chars were able to recover acetic acid from aqueous solutions [6].

Sanette et al. [31], investigated the adsorption of AAEMs (alkaline and alkaline earth metals) (Ca²⁺, K⁺, Na⁺, and Mg²⁺), as well as phenolic compounds (guaiacol, phenol, vanillyl alcohol, and resorcinol) present in process water streams obtained by hydrothermal carbonization of a model (synthetic) mixture that mimics the organic fraction of municipal solid waste at 300 °C, 15 min, feedstock/H₂O proportion of 1:1, using a stainless steel reactor of 945 mL, on chemically activated hydro-char with KOH solutions of concentrations between 0.5 and 2.5 mol/L. For AAEM the order of adsorption was Ca²⁺ > K⁺ > Na⁺ > Mg²⁺ [31]. The synthetic hydro-char was able to uptake 100% of guaiacol, phenol, and resorcinol and 61% of vanillyl alcohol present in process water streams [31]. The multilayer adsorption of phenolic compounds was correlated using the Dubinin-Radushkevich isotherm, showing equilibrium adsorbent-phase loadings of 68.7 mg/g and 50.3 mg/g for vanillyl alcohol and resorcinol, respectively [31]. On the other hand, Henry’s law best correlated the adsorption of AAEMs [31].

In this work, the influence of process temperature on the textural, morphological, and crystalline characterization of hydro-chars produced by hydrothermal carbonization of corn stover at 175, 200, 225, and 250 °C, 240 min, heating rate of 2.0 °C/min, and biomass/H₂O proportion of 1:10, using a reactor of 18.927 L, was investigated systematically. In addition, this work also investigated the influence of alkali (NaOH) and acid (HCl) pre-treatment over the hydro-char produced at 250 °C, on the adsorption/sorption of acetic acid (CH₃COOH) on hydro-char by analyzing the effect of initial concentration of acetic acid (CH₃COOH), as well as acetic acid solution-to-adsorbent ratio on the adsorption kinetics, adsorption equilibrium, and sorption equilibrium capacity in laboratory scale, batch mode.

2. Materials and Methods

2.1. Methodology

Figure 1 outlines the methodology as a rational scheme of ideas, methods, and procedures to produce the bio-adsorbent. The chemically activated adsorbent was applied to uptake acetic acid from aqueous solutions. Initially, the corn stover was collected. Afterwards, it was submitted to pretreatments of drying, grinding and sieving. The hydrothermal carbonization was carried out in batch mode, closed system, and pilot scale, as described elsewhere [52]. The hydro-chars were characterized by TG/DTG/DTA, SEM/EDX, BET, and XRD. The hydro-char were obtained by processing of corn stover with hot compressed H₂O at 250 °C, chemically activated with NaOH and HCl. The adsorption of acetic acid (CH₃COOH) on hydro-char was investigated by analyzing the adsorption kinetics, adsorption equilibrium, and sorption equilibrium capacity in laboratory scale, batch mode.

2.2. Materials, Pre-Treatment, and Physicochemical Characterization of Corn Stover

The corn stover was supplied by ATB-Bornin [52]. The residues were pretreated by drying, grinding, and sieving, as described elsewhere [52]. Afterwards, the residues were physicochemically characterized for dry matter, organic matter, ash, and elemental analysis, while oxygen was computed by difference, as described elsewhere [52].

2.3. Experimental Apparatus and Procedures

Hydrothermal Carbonization

The experiments were performed at 175, 200, 225, and 250 °C, 240 min, and biomass/H₂O proportion of 1:10, using a reactor of 18.927 L, and the apparatus and procedures described elsewhere [52].

2.4. Adsorption of CH₃COOH

The adsorption kinetic of acetic acid into hydro-char produced by hydrothermal carbonization of corn stover at 250 °C, 240 min, biomass/H₂O proportion of 1:10, in pilot scale, chemically activated with NaOH and HCl, was investigated systematically. The uptake of CH₃COOH can be investigated by determining the acetic acid solution initial concentration ${\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}\left(0\right)$ and the concentrations of CH₃COOH in aqueous solutions at time (τ) ${\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}\left(\mathsf{\tau }\right)$, until the time reaches its minimum value at (∞), that is, the acetic acid concentration in aqueous phase at equilibrium with hydro-char ${\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}\left(\infty \right)$ [6].

In cases, where only one organic acid specie is solvated in water (model solution), it is possible to determine the concentration of an organic acid in water by computing the acid value, as the proportion acidity/concentration of acetic acid $({\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}$) is constant, given by Equation (1).

Applying a steady-state mole balanced, closed, and conservative system for the adsorption analysis (acetic acid aqueous solution (liquid phase) + hydro-char (solid phase)), the quantity of moles of acetic acid in water at the beginning ${\mathrm{n}}_0^{\mathrm{L}}$ is equal to quantity of moles of acetic acid in water at time (τ) ${\mathrm{n}}_{\mathsf{\tau }}^{\mathrm{L}}$ and quantity of moles of acetic acid uptake into hydro-char at time (τ) ${\mathrm{n}}_{\mathsf{\tau }}^{\mathrm{S}}$ defined by Equation (2). Dividing Equation (1) by solution volume V yields Equation (3), where ${\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}\left(0\right)$ is the initial acetic acid solution concentration, ${\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}\left(\mathsf{\tau }\right)$ the acetic acid solution concentration at time (τ), and ${\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{S}}\left(\mathsf{\tau }\right)$ the acetic acid concentration in hydro-char at time (τ).

The acetic acid concentration in hydro-char at time (τ) is given by Equation (4). By substituting the relation given by Equation (1) in Equation (4), it is possible to compute, indirectly, the acetic acid concentration in hydro-char at time (τ), by the difference between the initial solution acid value ${\mathrm{I}}_{\mathrm{CHeCOOH}}^{\mathrm{L}}\left(0\right)$ and the solution acid value at time (τ), ${\mathrm{I}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}\left(\mathsf{\tau }\right).$ When acetic acid concentration in water reaches equilibrium, that is, ${\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}\left(\infty \right)={\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{*},{ \mathrm{I}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}\left(\infty \right)={\mathrm{I}}_{\mathrm{CH}3\mathrm{COOH}}^{*}$, the acetic acid concentration into hydro-char reaches its maximum at equilibrium ${\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{S}}\left(\infty \right)={ \mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{*}$, and the difference ${\mathrm{I}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}\left(0\right)-{\mathrm{I}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}\left(\infty \right)$, Equation (5), reaches its maximum.

$${\mathrm{I}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}\propto {\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}},{\mathrm{I}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}={\mathrm{K}*\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}$$

(1)

$${\mathrm{n}}_0^{\mathrm{L}}={\mathrm{n}}_{\mathsf{\tau }}^{\mathrm{L}}+{\mathrm{n}}_{\mathsf{\tau }}^{\mathrm{S}}$$

(2)

$${\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}\left(0\right)={\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}\left(\mathsf{\tau }\right)+{\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{S}}\left(\mathsf{\tau }\right)$$

(3)

$${\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{S}}\left(\mathsf{\tau }\right)={\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}\left(0\right)-{\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}\left(\mathsf{\tau }\right),\mathrm{I}\left(\mathsf{\tau }\right)={\mathrm{I}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}\left(0\right)-{\mathrm{I}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}\left(\mathsf{\tau }\right)$$

(4)

$${\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{S}}\left(\infty \right)={\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{*},{\mathrm{I}}^{*}={\mathrm{I}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}\left(0\right)-{\mathrm{I}}_{\mathrm{CH}3\mathrm{COOH}}^{\mathrm{L}}\left(\infty \right)$$

(5)

A first order kinetic was applied to describe the adsorption process, expressed as a dimensionless acidity, given by Equation (6).

$$\mathrm{I}\left(\mathsf{\tau }\right)={\mathrm{I}}_{\mathrm{CHeCOOH}}^{\mathrm{L}}\left(0\right)*\left[1-\exp \left(-\mathrm{K}*\mathsf{\tau }\right)\right]$$

(6)

where $\mathrm{K}$, is the adsorption kinetic constant.

2.4.1. Adsorption Isotherm of CH₃COOH

The Langmuir isotherm, given by Equation (7), was applied to analyze the adsorption equilibrium data of acetic acid in aqueous solutions within a porous solid matrix (adsorbent), where ${\chi }_{\mathrm{CH}3\mathrm{COOH}}^{*}$ is the equilibrium adsorbent-phase concentration of CH₃COOH, ${\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{*}$, is the equilibrium aqueous-phase concentration of acetic acid, and ${\mathrm{K}}_0, {\chi }_{\mathrm{Max}}$, the adsorption equilibrium constant and the saturation adsorption loading, respectively.

$${\mathsf{\chi }}_{\mathrm{CH}3\mathrm{COOH}}^{*}=\frac{{\mathrm{K}}_0{*\mathsf{\chi }}_{\mathrm{Max}}{*\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{*}}{\left(1+{\mathrm{K}}_0{*\mathrm{C}}_{\mathrm{CH}3\mathrm{COOH}}^{*}\right)}$$

(7)

2.4.2. Adsorption Apparatus and Procedures

The adsorption apparatus and procedures of CH₃COOH on hydro-char activated with NaOH (2.0 M) and HCl (2.0 M), was described in detail by Machado et al. [6].

2.5. Morphological, Crystalline, and Textural Characterization of Hydro-Chars

The morphological, crystalline, and textural characterization of hydro-chars obtained by hydrothermal processing of corn stover at 175, 200, 225, and 250 °C, 240 min, and biomass/H₂O proportion of 1:10, using a reactor of 18.927 L, was performed by SEM, EDX, and XRD (the equipment and procedures were described elsewhere [55,56]) as well as by thermo-gravimetric analysis (TG/DTG/DTA) and BET [56].

Thermo-Gravimetric Analysis (TG/DTG/DTA)

The weight loss of hydro-chars obtained by hydrothermal processing of corn stover at 175, 200, 225, and 250 °C, 240 min, and biomass/H₂O proportion of 1:10, using a reactor of 18.927 L, was analyzed by TG/DTG/DTA, and the equipment and procedures were described elsewhere [57].

3. Results

3.1. Morphological, Crystalline, and Textural Characterization of Hydro-Chars

3.1.1. Thermo-Gravimetric Analysis (TG/DTG/DTA)

Sittisun et al. [58], studied the thermal degradation of corn stover between 25 and 900 °C, heating rates of 10, 20, and 50 °C/min. The authors reported for heating rates of 10 °C/min, a mass loss of approximately 92 (wt.%), between 25 and 510 °C.

In the temperature region 510–900 °C, the mass loss was constant, and the remaining solid phase was composed of ash. The DTG curve shows three different thermal degradation steps. In the first one, between 25 and 167 °C, a mass loss of 8.5 (wt.%) was reported, representing the recovery of moisture within the solid phase. Afterwards, in the temperature region 167–368 °C, a mass loss of 56.59 (wt.%) occurred, mainly due to release of volatile compounds by the degradation reactions of cellulose and hemi-cellulose, described chemically by degradation of chemical functions containing oxygen, including hydroxyl, carbonyl, and carboxyl groups [58,59]. In the last step, between 368 and 514 °C, a mass loss of 26.50 (wt.%) occurred, associated with the thermal degradation and/or combustion of lignin, as well as residual char compounds, produced at the second step.

Figure 2, Figure 3 and Figure 4 describe the TG, DTG, and DTA analysis of corn stover after hydrothermal processing at 175, 200, 225, and 250 °C, 240 min, and biomass/H₂O proportion of 1:10, respectively, using a reactor of 18.927 L, between 25 and 800 °C, 10 °C/min, under N₂ atmosphere.

One observes, for the thermal degradation of corn stover after hydrothermal processing at 175 °C, the occurrence of 03 reaction steps. In the first one, between 25 and 150 °C, a mass loss of approximately 3.0–4.0 (wt.%) occurred, due to the release of moisture. It is also probable that degradation of volatile compounds selectively adsorbed into the pores of solid phase products has taken place, including alcohols (methanol, ethanol), carboxylic acids (acetic acid, propanoic acid) [6], and aldehydes (HMF). In the second degradation step, between 150 and 530 °C, a mass loss of approximately 79–80 (wt.%) occurred.

The mass balance by hydrothermal carbonization of corn stover at 175 °C shows that approximately 63.0 (wt.%) of initial biomass still remains as solid phase reaction products [52]. In addition, the low concentration of lignin-derived reaction compounds such as phenols and guajacol in the aqueous phase indicates that only a small portion of lignin has been thermally degraded [52]. Contrarily, the high concentration of cellulose/hemi-cellulose-derived reaction compounds in the aqueous phase including furfural, HMF, and acetic acids is a sign that higher amounts of cellulose/hemi-cellulose compared to lignin have been thermally degraded [52].

Since corn stover consist basically by cellulose + hemi-cellulose (69.0%), lignin (20.0%), ash (8.0%) [60,61,62], and small amounts of soluble substances [61], the TG/DTG analysis shows that almost all cellulose + hemi-cellulose and soluble substances in the aqueous phase have been thermally degraded, as well as approximately the haft of lignin, thus remaining within the solid phase reaction products the inorganic matter and lignin. In the last step, between 530 and 800 °C, a mass loss of approximately 11.0–12.0 (wt.%) occurred, associated with the thermal degradation and/or combustion of lignin. The ash content of 8.0 (wt.%) is according to that reported in the literature [60,61,62].

The thermal degradation of corn stover after hydrothermal processing at 200 °C shows the occurrence of 03 reaction steps. In the first one, between 25 and 110 °C, a mass loss of approximately 2.0 (wt.%) occurred, because of humidity removal, as well as the degradation of low molecular weight compounds (volatile compounds) adsorbed into the pores of solid phase products such as methanol. [6,52]. In the second degradation step, between 110 and 520 °C, a mass loss of approximately 62.0 (wt.%) was observed. The mass balance by hydrothermal carbonization of corn stover at 200 °C shows that approximately 57.4 (wt.%) of initial biomass still remains as solid phase reaction products [52]. Although, the concentrations of lignin-derived reaction compounds in process water such as phenols and guajacol has increased between 175 and 200 °C [52], still a small portion of lignin has been thermally degraded. The high concentration of cellulose/hemi-cellulose-derived reaction compounds in the aqueous phase including furfural, HMF, and acetic acids show that high amount of cellulose/hemi-cellulose has been thermally degraded [52], remaining within the solid phase reaction products the inorganic matter and lignin. In the third step, between 520 and 800 °C, a mass loss of approximately 32.0 (wt.%) occurred, associated with the thermal degradation and/or combustion of lignin. The ash content of 4.0 (wt.%) is according to that reported in the literature [60,61,62].

One observes, for the thermal degradation of corn stover after hydrothermal processing at 225 °C, the occurrence of 03 reaction steps. In the first one, between 25 and 130 °C, a mass loss of approximately 2.0 (wt.%) occurred, because of humidity removal, as well as the degradation of low molecular weight compounds (volatile compounds) adsorbed into the pores of solid phase products including alcohols (methanol, ethanol), volatile carboxylic acids (acetic acid, propanoic acid) [6], and aldehydes (HMF). In the second degradation step, between 130 and 530 °C, a mass loss of approximately 92.0 (wt.%) occurred. The mass balance by hydrothermal carbonization of corn stover at 225 °C shows that approximately 41.0 (wt.%) of initial biomass still remains as solid phase reaction products [52]. The high concentration of lignin-derived reaction compounds in process water such as phenols and guajacol, as well as cellulose/hemi-cellulose-derived reaction chemicals dissolved in process water such as carboxylic acid, show that most of lignin and cellulose/hemi-cellulose have been thermally degraded [52], remaining within the solid phase reaction products the inorganic matter. In the third step, between 530 and 800 °C, a mass loss of approximately 3.0 (wt.%) occurred, associated with the thermal degradation and/or combustion of lignin. The ash content of 3.0 (wt.%) is according to that reported in the literature [60,61,62].

The thermal degradation of corn stover after hydrothermal processing at 250 °C shows the occurrence of 04 reaction steps. In the first one, between 25 and 110 °C, a mass loss of approximately 1.0 (wt.%) occurred, due to the release of moisture. In the second degradation step, between 110 and 270 °C, a mass loss of approximately 9.0 (wt.%) has been observed. In the third degradation step, between 270 and 430 °C, a mass loss of approximately 15.0 (wt.%) occurred. In the last degradation step, between 430 and 800 °C, a mass loss of approximately 25.0 (wt.%) occurred. The mass balance by hydrothermal carbonization of corn stover at 250 °C shows that approximately 35.8 (wt.%) of initial biomass still remains as solid phase reaction products [52]. The high concentration of cellulose/hemi-cellulose-derived reaction chemicals in process water such as acetic acid, as well as lignin-derived reaction compounds solvated in process water such as guajacol and phenols, show that high amounts of cellulose/hemi-cellulose and lignin have been thermally degraded [52], being the solid phase reaction products a carbon rich material. Finally, the results illustrated in Figure 2 and Figure 3 are in accord to a similar study described in the literature, for the thermal analysis of corn stover, by Mohammed et al. [63].

Finally, one observes in Figure 3, for the hydro-chars obtained at 175 and 200 °C, that highest mass loss are associated with the occurrence of endothermic peaks at 355.2 and 362.9 °C, respectively. This behavior, was also observed in Figure 4 by the presence of exothermic peaks around 360 °C. For the hydro-chars obtained at 225 and 250 °C, the endothermic peaks occurred at 397 and 434 °C, respectively, showing that the higher the temperature the higher the thermal stability of solid reaction products. In addition, the hydro-char obtained at 250 °C presented the highest thermal stability compared to the hydro-chars obtained at 175, 200, and 225 °C, as it presented the lowest mass loss of 50.51% (wt.) between the temperatures of 144 and 768.5 °C.

3.1.2. SEM Analysis

The microscopic analyses of hydro-chars obtained at 175, 200, 225, and 250 °C, 240 min, and biomass/H₂O proportion of 1:10, using a reactor of 18.927 L, are illustrated in Figure 5a,b and Figure 6a,b, respectively. The SEM image in Figure 5a indicates a rigid and well-organized fiber structure, demonstrating that temperature had almost no effect on the vegetal structure, as it largely retained the original morphological microscopic characteristics. The results are in agreement to analogous investigations on the effect of process temperature over the morphology of hydro-chars produced by corn stover [63], and corn straw [64].

The SEM image illustrated in Figure 5b shows that disaggregate, amorphous, and heterogeneous structures with nonuniform geometry dominated, demonstrating that temperature had caused the appearance of micropores with average diameter of 3.00 μm, indicating that the lignocellulose material started to decompose. However, process temperature still had little effect on the vegetal structure, as it largely retained the original microscopic characteristics. The results are in agreement to similar investigations described by Xing et al. [64], and Mohammed et al. [63].

The SEM image in Figure 6a indicates that hydro-char structure consists of agglomerated micro-spheres and heterogeneous structures with irregular shapes (fragmentation), showing particle size averaging 0.5 μm as depicted in detail by Figures S1 and S2 in Supplementary Materials. The agglomerated micro-spheres and fragmentation indicates that cellulose and hemi-cellulose were decomposed, as reported by Xing et al. [64], demonstrating that temperature had generated significant alterations on the morphological structure of corn stover by destructing the plant cell walls [6]. Xing et al. [64] reported the occurrence of micro-spheres by the hydrothermal carbonization of corn straw at 230 °C, 30 min, and biomass/H₂O proportion of 1:8.

One observes in Figure 6b an aggregate amorphous solid phase consisting of micro-spheres and heterogeneous structures with nonuniform geometry (fragmentation), showing that original vegetal surface structure had been drastic changed, and was the carbonization grade higher compared to the SEM image at 225 °C. In fact, process temperature had generated significant alterations on the morphological structure of corn stover by destructing the plant cell structure. The agglomerated micro-spheres and fragmentation indicates that cellulose and hemi-cellulose were decomposed. Xing et al. [64] reported the occurrence of a similar structure by the hydrothermal carbonization of corn straw at 260 °C, 30 min, and biomass/H2O proportion of 1:8.

3.1.3. EDX Analysis

Table 1. Percentages in mass and atomic mass of hydro-chars, obtained by hydrothermal carbonization of corn stover at 175, 200, 225, and 250 °C, 240 min, biomass/H2O proportion of 1:10, using a reactor of 18.927 L, at the point marked by EDX technique [6].

Hydro-Chars
175 °C				200 °C			225 °C			250 °C
Chemical Elements	Mass (wt.%)	Atomic Mass (wt.%)	SD	Mass (wt.%)	Atomic Mass (wt.%)	SD	Mass (wt.%)	Atomic Mass (wt.%)	SD	Mass (wt.%)	Atomic Mass (wt.%)	SD
C	60.69	67.56	0.503	71.98	77.43	0.818	73.89	79.17	0.654	76.04	81.05	0.631
O	38.57	32.24	0.504	27.83	22.47	0.819	25.73	20.70	0.656	23.59	18.87	0.632
Si	0.136	0.065	0.020	0.194	0.089	0.043	-	-	-	-	-	-
Ca	0.181	0.061	0.018	-	-	-	0.374	0.120	0.032	-	-	-
Zn	-	-	-	-	-	-	-	-	-	0.366	0.072	0.065
Cu	0.210	0.044	0.042	-	-	-	-	-	-	-	-	-

SD = standard deviation.

shows the EDX (energy dispersive X-ray spectroscopy) analysis of hydro-chars. The samples were analyzed at five different points. By increasing the temperature, the carbon content increased and that of oxygen diminished. By comparing the results for the carbon content in Table 1 and those described in Supplementary Table S1, one observes that the EDX technique exhibited higher carbon contents but similar oxygen contents. This was probably due to limitations of the technique, as it was not adequate to identify/recognize elements such as nitrogen and hydrogen. In addition, calculation of oxygen and carbon contents, described in Table S1, was computed by subtracting the ash content [52]. In addition, Ca, Zn, Cu, Mo, Na, and P, as well as Si were identified in almost all the points marked by EDX. The inorganics identified in hydro-chars by EDX in hydro-chars are in agreement to those identified in corn stover after drying at 105 °C [65]. Finally, Table 1 shows an increase on carbon content (carbonization) with increasing process temperature.

3.1.4. XRD Analysis

The XRD (X-ray diffraction) data of hydro-chars obtained by hydrothermal processing of corn stover at 175, 200, 225, and 250 °C, 240 min, and biomass/H₂O proportion of 1:10, using a reactor of 18.927 L, is illustrated in Figure 7a,b and Figure 8a,b, respectively.

The diffractogram of corn stover obtained by hydrothermal carbonization at 175 °C, is illustrated in Figure 7a. It shows the presence of two crystalline phases—graphite (C) with a peak of higher intensity (100%) on the position 2θ: 26.47, and crystalline cellulose with a peak of higher intensity (12.95%) on the position 2θ: 22.20. The peak on the position 2θ: 22.20 is characteristics of crystalline cellulose structure, as described by Regmi et al. [5] and Kang et al. [66], who identified diffraction peaks on the positions 2θ: 15.3 and 22.3 [5,66].

At 200 °C, X-ray-diffraction data identified two crystalline phases—crystalline cellulose with a peak of higher intensity (100%) on the positions 2θ: 20.68, 15.53 (79%), and 21.96 (90.07%), because of amorphous cellulose matrix degradation and the exposure of the crystalline cellulose [57], and graphite (C) with a peak of medium intensity (59.38%) on the position 2θ: 26.45, as shown in Figure 7b.

The XRD at 225 °C identified two crystalline phases—graphite (C) with a peak of medium intensity (53.75%) on the position 2θ: 26.57, and crystalline cellulose with peaks of high and low intensity on the positions 2θ: 22.47 (100%) and 49.94 (49.86%), as well as three peaks of medium and low intensity on the positions 2θ: 20.73 (65.99%) and 49.94 (49.86%), as shown in Figure 8a.

The diffractograms (XRD) identified the occurrence of peaks of higher intensity of graphite (C) as the temperature increased, as well as a decrease of peaks intensity for crystalline cellulose, demonstrating that higher temperatures favor the formation of crystalline-phase graphite (C), being in agreement to the results illustrated in Table 1.

3.1.5. BET

Figure 9 shows the BET analysis of hydro-char obtained by HTC of corn stover at 250 °C, 240 min, and biomass/H₂O of 1:10, using a reactor of 18.927 L. The N₂ capacity increased as the relation (P/P₀) increased, showing a maximum capacity of approximately 12 cm²/g as the relative pressure approached 1.0. The density of hydro-char was 2.10 g/cm³, and the surface area measured by relative pressure (P/P₀ = 0.201) was 4.02 m²/g, while the surface area was 4.3498 m²/g. The pore volume measured by reduced pressure (P/P₀ = 0.988) was 0.01857 cm³/g. The average pore width was 17.079 μm.

3.2. Adsorption Kinetics of Acetic Acid (CH₃COOH) on Hydro-Char

One of the great disadvantages of hydrothermal processing of lignocellulose-rich materials, such as biomass, is the occurrence of hazardous lignin and cellulose-derived reaction products including phenols, furfural, and hydroxymethylfurfural in the aqueous phase [52,53,54,55,67,68]. In addition, the high concentration of carboxylic acids (acetic acid, propionic acid, etc.), in process water confers its high acidity [52,53,54,55,68,69]. From this perspective, the hydro-char obtained by HTC at 250 °C, 240 min, biomass/H₂O proportion of 1:10, was chemically activated with alkali (2.0 M NaOH) and acid (2.0 M HCl) solutions in order to investigate the its capacity to selectively uptake (adsorb) acetic acid from model aqueous solutions (1.0, 2.0, 3.0, and 4.0 mg/mL), the major carboxylic acid within hydrothermal carbonization process water streams.

3.2.1. Bio-Adsorbent Activation with NaOH

Influence of Acetic Acid Concentration

Figure 10 illustrates the influence of acetic acid initial solution concentration (2.0, 3.0, and 4.0 mg mL⁻¹) on the adsorption kinetics within hydro-char produced by HTC at 250 °C, 240 min, biomass/H₂O proportion of 1:10, alkali-activated (2.0 M NaOH), 0.1 g_Adsorbent/10 mL_CH3COOH. The adsorption kinetic is rapid and equilibrium is reached between 240 and 480 s. The activation of hydro-char with a strong alkali such as NaOH causes significant changes in the surface and porous (micropores, mesopores, and macropores) structure [8,9]. The acetic acid molecules solvated in H₂O are selectively caught by the negative-charged active sites, which is according to Liang et al. [69], who reported that adsorption process of CH₃COOH on carbon microspheres, prepared by hydrothermal carbonization of starch, was mainly due to the porous (micropores, mesopores, and macropores) structure transformations caused by destruction of hydrophilic oxygen functional groups. In addition, Liang et al. [69] proved that pore structure determines the adsorption capacity and diffusion rate and the process is physical.

The absorption of acetic acid molecules occurs because of a concentration difference between the solution and hydro-char surface and internal porous, micro-porous, and macro-porous structure, as well as the appearance of dipole–dipole electrostatic attraction forces between the negative charged sites within the porous and the dissociated H⁺ of R-COOH (H⁺ + R-COO⁻). The adsorption kinetic data of acetic acid on hydro-char was correlated with a first order model, exhibiting root-mean-square error (r²) between 0.969 and 0.999, as shown in

Table 2. Regression parameters by adsorption kinetic of CH₃COOH solutions (2.0, 3.0, and 4.0 mg mL⁻¹), expressed as dimensionless acid value, on NaOH (2.0 M)-activated hydro-char produced by hydrothermal processing of corn stover at 250 °C, 240 min, and biomass/H₂O proportion of 1:10, 0.1 g_Adsorbent/10 mL_CH3COOH.

Parameters	CCH3COOH (mg/mL)
	2	3	4
Pseudo-first-order
${\mathrm{I}}_{\mathrm{CHeCOOH}}^{\mathrm{L}}\left(0\right)$ (-)	0.3694	0.2361	0.1497
κ (s−1)	0.0479	0.0545	0.0468
r2	0.969	0.999	0.988

, which is in agreement with the results described by Machado et al. [6]. It can be also observed that dimensionless acid value I(τ), increases with decreasing acetic acid solutions concentrations (2.0, 3.0, and 4.0 mg mL⁻¹), that is, the lower the acetic acid solution concentration, the higher the adsorption efficiency.

The influence of acetic acid initial solution concentration (1.0, and 2.0 mg mL⁻¹) on the adsorption kinetics within hydro-char obtained by hydrothermal processing of corn stover at 250 °C, 240 min, biomass/H₂O proportion of 1:10, chemically activated with NaOH (2.0 M), 0.2 g_Adsorbent/10 mL_CH3COOH, is illustrated in Figure 11. One can observe that dimensionless acid value I(τ), increases with decreasing CH₃COOH concentrations (1.0, and 2.0 mg/mL), that is, the lower the acetic acid solution concentration, the higher the adsorption efficiency. In addition, by comparing Figure 10 and Figure 11, it is easy to observe that higher adsorbent-to-solution ratios favors the adsorption efficiency, due to an increase on the surface area. The adsorption kinetic data of acetic acid on hydro-char was correlated with a first order model, exhibiting root-mean-square error (r²) between 0.969 and 0.982, as shown in

Table 3. Regression parameters by adsorption kinetic of CH₃COOH solutions (1.0 and 2.0 mg mL⁻¹), expressed as dimensionless acid value, on NaOH (2.0 M)-activated hydro-char produced by hydrothermal processing of corn stover at 250 °C, 240 min, and biomass/H₂O proportion of 1:10, 0.2 g_Adsorbent/10 mL_CH3COOH.

Parameters	CCH3COOH (mg/mL)
	1	2
Pseudo-first-order
${\mathrm{I}}_{\mathrm{CHeCOOH}}^{\mathrm{L}}\left(0\right)$ (-)	0.4689	0.3694
κ (s−1)	0.0305	0.0479
r2	0.982	0.969

.

Influence of Adsorbent-to-Solution Ratio

The effect of hydro-char/solution ratio on the adsorption kinetics of acetic acid solution (2.0 mg mL⁻¹) within NaOH (2.0 M)-activated hydro-char obtained by hydrothermal processing of corn stover at 250 °C, 240 min, biomass/H₂O proportion of 1:10, is illustrated in Figure 12. The adsorption performance increased with increasing hydro-char/solution ratio increase as the number of active sites on surface and porous (micropores, mesopores, and macropores) structure increased, that is, by increasing the hydro-char/solution ratio, the surface area increased. The adsorption kinetic data of acetic acid on hydro-char was correlated with a first order model, exhibiting root-mean-square error (r²) between 0.973 and 0.982, as shown in

Table 4. Regression parameters by the effect of hydro-char/solution ratio (0.1 g_Adsorbent/10 mL_CH3COOH, 0.2 g_Adsorbent/10 mL_CH3COOH) on adsorption kinetic of CH₃COOH, expressed as dimensionless acid value I(τ), on NaOH (2.0 M)-activated hydro-char produced by hydrothermal processing of corn stover at 250 °C, 240 min, and biomass/H₂O proportion of 1:10.

Parameters	CCH3COOH (mg/mL)
	2	2
Pseudo-first-order	0.1 gAdsorbent/10 mLCH3COOH	0.2 gAdsorbent/10 mLCH3COOH
${\mathrm{I}}_{\mathrm{CHeCOOH}}^{\mathrm{L}}\left(0\right)$ (-)	0.4689	0.6889
κ (s−1)	0.0305	0.0430
r2	0.982	0.973

.

3.2.2. Bio-Adsorbent Activation with HCl

Influence of Hydrochloric Acid Concentration

The influence of acetic acid initial solution concentration (1.0, 2.0, 3.0, and 4.0 mg mL⁻¹) on the adsorption kinetics within HCl (2.0 M)-activated hydro-char obtained by on hydro-char obtained by hydrothermal processing of corn stover at 250 °C, 240 min, biomass/H₂O proportion of 1:10, 0.2 g_Adsorbent/10 mL_CH3COOH, is shown in Figure 13.

The adsorption kinetic was rapid and equilibrium was reached around 240 s. The activation of hydro-char with a strong acid such as HCl causes significant changes and/or damages on the surface and porous (micropores, mesopores, and macropores) structure [51]. The acetic acid molecules solvated/dissociated in water are not efficiently captured by the positive charged active sites. The absorption of acetic acid molecules is due basically to a concentration difference between bulk solution and hydro-char surface and porous (micropores, mesopores, and macropores) structure, as the positive-charged sites on the surface and within the internal porous structure contribute to the appearance of electrostatic repulsion forces between the dissociated H⁺ of R-COOH (H⁺ + R-COO⁻) and the positive charged sites. This is according to Liang et al. [69], who proved that pore structure determines the adsorbent capacity and that process is diffusion-rate controlled. The adsorption kinetic data of acetic acid on hydro-char was correlated with a first order model, exhibiting root-mean-square error (r²) between 0.967 and 0.997, as shown in

Table 5. Regression parameters by adsorption kinetic of CH₃COOH solutions (1.0, 2.0, 3.0, and 4.0 mg mL⁻¹), expressed as dimensionless acid value I(t), on HCl (2.0 M) hydro-char produced by HTC of corn stover at 250 °C, 240 min, and biomass/H₂O proportion of 1:10, 0.1 g_Adsorbent/10 mL_CH3COOH.

Parameters	CCH3COOH (mg/mL)
	1	2	3	4
Pseudo-first-order
${\mathrm{I}}_{\mathrm{CHeCOOH}}^{\mathrm{L}}\left(0\right)$ (-)	-	0.1241	0.0908	0.0941
κ (s−1)	-	0.0156	0.0563	0.0101
r2	0.969	0.998	0.978	0.967

.

The results presented in Figure 10 and Figure 13 show that NaOH (2.0 M) chemically activated hydro-char is not only more efficient than that activated with HCl (2.0 M), but also presents the highest adsorption performance and capacity.

3.2.3. Adsorption Equilibrium Isotherms of Acetic Acid (CH₃COOH) on Hydro-Char

The Langmuir isotherm was applied to correlate the equilibrium adsorption data of CH₃COOH on alkali (2.0 M NaOH)- and acid (2.0 M HCl)-activated hydro-char produced by hydrothermal processing of corn stover at 250 °C, 240 min, biomass/H2O proportion of 1:10, 0.1 g_Adsorbent/10 mL_CH3COOH, as illustrated by Figure 14.

The adsorption isotherm of acetic acid on NaOH- and HCl-activated hydro-char was correlated with the Langmuir model, exhibiting root-mean-square errors (r²) of 0.994 and 0.989, respectively. The equilibrium adsorbent-phase concentration of CH₃COOH for NaOH- and HCl-activated hydro-chars were approximately 650 and 575 mg/g, respectively. The bio-adsorbent (hydro-chars) equilibrium loadings were in agreement with the adsorption of MB (methylene blue) from aqueous solution on NaOH-activated hydro-chars produced by HTC of factory-rejected tea and palm date seeds, as reported by Islam et al. [8,9], correlated using a pseudo-second-order model and the adsorption isotherms by the Langmuir [8], and the Freundlich models [9], respectively. For the adsorption kinetic of MB (methylene blue) on NaOH-activated hydro-chars produced by HTC of from factory-rejected tea and palm date seeds, maximum adsorption loadings of 487.4 mg/g at 30 °C [8], and 612.1, 464.3, and 410.0 mg/g at 30, 40, and 50 °C [9], were reported, respectively. Finally, the adsorption of CH₃COOH on carbon microspheres synthesized by hydrothermal carbonization was investigated by Liang et al. [69], reporting equilibrium adsorbent-phase concentration of CH₃COOH of 260 mg/g at 25 °C.

4. Conclusions

TG/DTG analysis showed, for the hydro-chars obtained at 175 and 200 °C, that highest mass loss was associated with the occurrence of endothermic peaks at 355.2 and 362.9 °C, respectively. This was also confirmed by DTA analysis by the presence of exothermic peaks around 360 °C. For the hydro-chars obtained at 225 and 250 °C, the endothermic peaks occurred at 397 and 434 °C, respectively, showing that the higher the temperature the higher the thermal stability of solid reaction products. The hydro-char obtained at 250 °C presented the highest thermal stability compared to the hydro-chars obtained at 175, 200, and 225 °C, as it presented the lowest mass loss of 50.51% (wt.) between the temperatures 144 and 768.5 °C.

SEM images indicate a rigid and well-organized fiber structure by hydrothermal processing of corn stover at 175 and 200 °C, demonstrating that temperature had almost no effect on the vegetal structure, as it largely retained the original morphological microscopic characteristics. On the other hand, SEM images of hydro-char produced by hydrothermal carbonization of corn stover at 225 and 250 °C indicate that hydro-char structure consisted of agglomerated micro-spheres and heterogeneous structures with irregular shapes (fragmentation). The agglomerated micro-spheres and fragmentation indicates that cellulose and hemi-cellulose were decomposed, as reported by Xing et al. [62], demonstrating that temperature had generated significant alterations on the morphological structure of corn stover by destructing the plant cell structure/walls [6].

As observed by EDX analysis, by increasing the temperature, the carbon content increased and that of oxygen diminished. The diffractograms (XRD) identified the occurrence of peaks of higher intensity of graphite (C) as the temperature increased, demonstrating that higher temperatures favors the formation of crystalline-phase graphite (C).

The analysis of acetic acid adsorption kinetics data, the main volatile carboxylic acid identified in the hydrothermal carbonization liquid phase, showed that NaOH (2.0 M)-activated hydro-char obtained by hydrothermal processing of corn stover at 250 °C, 240 min, biomass/H₂O proportion of 1:10, presented the highest adsorption performance and capacity. The chemically activated hydro-chars were selective to uptake of CH₃COOH, demonstrating that enriching/recovery of CH₃COOH from HTC process water streams is possible.