The Investigation of the Adsorption of Methylene Blue from Water by Torrefied Biomass

Abstract

This research investigates the adsorption potential of four types of adsorbents produced from agro-industrial waste (grape pomace—GP, tree pruning—TP, sugarcane bagasse—SB, and eucalyptus sawdust—ES) for the uptake of thiazine dye methylene blue (MB) from aqueous solution. A kinetic model based on a hybrid-order rate equation was fitted to experimental data. The result showed that BGP-300 presented the highest mass yield (58.84%) and energy yield (69.56%), followed by BTP-300 > BES-300 > BSB-300. Adsorption studies showed that BGP-300 had a better performance in the uptake of MB, with a removal efficiency (R_e) of 96.5% and adsorption capacity at equilibrium (q_e) of 9.3 mg g⁻¹, followed by tree pruning biochar (BTP-300), with an R_e of 65.0% and q_e of 5.3 mg g⁻¹. Meanwhile, eucalyptus sawdust (BES-300) and sugarcane bagasse (BSB-300) biochar did not facilitate any significant removal of MB. Adsorption kinetics is best described by a second-order rate with R² varying from 0.75 to 0.96. Desorption studies show a low concentration released to the solution, indicating that adsorption may occur physically and chemically. Therefore, this research provides comprehensive insights into the adsorption characteristics of different biochars, emphasizing the potential of torrefied materials BGP-300 and BTP-300 as effective for MB uptake from aqueous solution.

1. Introduction

Increasing environmental challenges, along with rising fuel costs and diminishing oil reserves, have generated a need to develop sustainable solutions to manage waste and recover bioresources for conversion into value-added products [1]. In this context, biomass has recently gained significant interest in the sustainable production of fuels and chemicals owing to its abundance, renewability, chemical composition, and sustainable characteristics [2]. Worldwide, 181.5 billion tons of lignin-rich biomass, such as wood and crop waste, are generated annually, demonstrating the resource’s availability [2].

Most thermochemical conversion technologies (e.g., gasification, rapid pyrolysis, and microwave pyrolysis) are carried out at higher temperatures (>300 °C) and require higher initial energy input [3]. In contrast, low-temperature torrefaction technology can produce biochar (carbon-rich porous material) with high yield and low energy intake. Compared to raw biomass, torrefaction biochar has a higher calorific value, carbon content, hydrophobicity, and grindability [2,3]. Additionally, the torrefaction condensate is useful as a pesticide, herbicide, and pest prevention for plant protection [4].

Torrefaction is a mild pyrolysis process that is applicable for producing solid biofuel, commonly known as hydrochar or biochar. The process involves treating biomass at relatively low temperatures (200–300 °C) for less than 30 min in an O₂-free environment (often using inert gas) under atmospheric pressure. During torrefaction, the feedstock undergoes various transformation reactions, such as polymerization, decomposition, and carbonization [2]. The weight loss of biomass is around 30% due to the degradation of hemicellulose content in the temperature range of 200–300 °C [2,5]. This enhancement is beneficial for long-term storage or long-distance transportation.

Originally, torrefied biomass was thought to be useful for energy purposes. However, recent studies have demonstrated the use of biochar as an effective adsorbent of contaminants from water [2,3,6,7,8,9,10]. Its porous structure and significant specific surface area provide an ideal environment for the adsorption and fixation of these contaminants, thereby reducing their adverse effects on aquatic ecosystems. Biochar has fewer pores than biochar produced at higher temperatures, but it includes more oxygen-containing functional groups [3]. Additionally, the sources of feedstock used for biochar production are extensive and complex, and it is known to influence the quality of the biochar produced. Recent studies have demonstrated the use of diverse torrefied feedstock in the removal of contaminants from aqueous solutions, such as the use of kiwi branches [9] and bamboo shoot shell [7] biochar for the removal of hexavalent chromium; bamboo [8] biochar for the removal of lead; rapeseed [11], seaweed [11], white wood [11], camellia shell [12], and microalgal [6] biochar for the removal of colorant methylene blue (MB); and microalgal [6] biochar for the removal of the colorant Congo red.

Among the contaminants, dyes stand out as they are emerging contaminants, i.e., a large group of chemicals discovered in the environment due to regular human activities such as domestic, agricultural, and industrial processes [13]. Dye contamination in the wastewater of textile industries is a crucial environmental concern. A variety of dyes and additives are widely used in industrial applications, including textiles, food, paint, paper, printing, carpet, rubber, leather treatment, wool, and personal care products [3]. It is estimated that over 10,000 varieties of both natural and synthetic dyes are produced annually, with a global output of over 7 × 10⁵ tons per year [3]. The chemical complexity, stability, and recalcitrance of untreated dye effluents make them one of the leading causes of breathing difficulties, irregular heartbeats, skin rashes, dizziness, cancer, and other allergic reactions [14,15,16]. Dyes present in water sources can pose challenges for conventional water treatment processes, as they may not be effectively removed by common treatment methods. Therefore, necessary treatments are required to ensure the safe discharge of wastewater into the environment.

Thus far, limited literature on the adsorption of dye removal using agro-industrial waste derived from the torrefaction conversion process is available. Therefore, this study aims to address such a research gap by evaluating the removal of emerging contaminants using four different types of torrefied agro-industrial waste (grape pomace, tree pruning, sugarcane bagasse, and eucalyptus sawdust) by the batch adsorption of cationic colorant methylene blue (MB), one of the major thiazine dyes.

2. Materials and Methods

2.1. Biomass

Four different types of feedstocks were used to produce biochar: grape bagasse, tree pruning, sugarcane bagasse, and eucalyptus sawdust.

The grape pomace used in this study is from a winery in the northeastern region of Brazil. The grape pomace residue was stored in polyethylene bags and transported under refrigeration to prevent sample fermentation. As received, the material was air-dried and homogenized.

The tree pruning residues were collected from the pruning of mango (Mangifera indica) and pitanga (Eugenia uniflora) trees on the Lorena School of Engineering campus, University of São Paulo. The pruning residues were sun-dried and ground in a knife mill with a 9 mm sieve.

Sugarcane bagasse was obtained from a sugar and alcohol plant in the southeastern region of Brazil. It was provided wet, air-dried, and homogenized for this work.

The eucalyptus sawdust was supplied by a sawmill in the Paraíba Valley Region, in the interior of the state of São Paulo, already dry and powdered.

2.2. Biochar Production

The dry torrefaction, thermochemical conversion, known as mild pyrolysis, was carried out on a laboratory scale in a reactor with a processing capacity of 350 g per batch; this quantity varied according to the bulk density of the biomass. The raw waste was dried at 105 °C for 24 h before the process to standardize the samples (

Table 1. Illustrative comparison of the feedstocks and biochar obtained from the torrefaction of grape pomace, tree pruning, sugarcane bagasse, and eucalyptus sawdust and the biochar sieved for adsorption studies.

Feedstock	Biochar	Biochar Sieved (0.6–1.2 mm)
Grape pomace
Tree pruning
Sugarcane bagasse
Eucalyptus sawdust

). The reaction occurred in an inert nitrogen atmosphere at 300 °C (the most severe temperature of the torrefaction process [14,15,16]), with a heating rate of 5 °C/min and a residence time of 60 min. The biochar produced was named after the feedstock used in the process, followed by the temperature of the thermochemical process: biochar grape pomace (BGP-300), biochar tree pruning (BTP-300), biochar sugarcane bagasse (BSB-300), and biochar eucalyptus sawdust (BES-300) (Table 1).

2.3. Physical and Chemical Properties of Biochar and Feedstocks

The mass yield (Y_M) and energy yield (Y_E) were obtained according to Bridgeman et al. (2008) [17], using Equations (1) and (2).

$$Y_M=\left({\displaystyle \frac{m_{biochar}}{m_{feedstock}}}\right)\times 100\%$$

(1)

$$Y_E=Y_M\left({\displaystyle \frac{{HHV}_{biochar}}{{HHV}_{feedstock}}}\right)\times 100\%$$

(2)

The moisture content (MC) (ASTM D3173 [18]), ash (ASTM D3174 [19]), volatile matter (VM), and fixed carbon (FC) (ASTM D3175 [20]) are part of the immediate analysis that determines the characteristics of the biochar.

The higher heating value (HHV) was calculated according to Parikh (2005) [21] from the immediate analysis of the material, according to Equation (3).

$$HHV (\mathrm{M}\mathrm{J} {\mathrm{k}\mathrm{g}}^{-1})=0.3536\times FC+0.1559\times VM-0.0078\times Ash$$

(3)

2.4. Preparation of Adsorbent and MB Solution

For the experimental procedures, the samples were disintegrated with a knife mill and sieved to achieve a target size of particles 0.6–1.2 mm in diameter (Table 1).

The MB solution was prepared in the laboratory prior to adsorption analysis using distilled water and analytical-grade MB (Synth^®). The natural pH of the solution was determined to be 6.0. The absorbance was determined using UV–VIS (model K37-UVVIS from KASVI, São José dos Pinhais, Brazil) and a quartz container with 10.0 mm optical path at 665 nm wavelength.

2.5. Adsorption Studies

Batch tests were performed on an orbital shaker to determine the influence of time, initial dye concentration during sorption studies, and adsorbent type (BGP-300, BTP-300, BSB-300, and BES-300). The tests were carried out at room temperature (30 °C), using 125 mL Erlenmeyer flasks with 50 mL of MB solution (40 and 120 mg L⁻¹, real wastewater MB concentration [22], and a drastic scenario considering more concentrated wastewater, respectively) and 0.5 g of adsorbent (previously reported as an ideal portion of adsorbent and solution volume [23]). The system was shaken on an orbital shaker at a 100 rpm frequency, and aliquots were taken at 0, 1, 5, 15, 30, 60, 90, 120, 300, 480, and 1440 min for concentration determination. At 1440 min, equilibrium was reached.

For the concentration determination, an aliquot was taken from the Erlenmeyer flasks for MB absorbance determination using UV-VIS (model K37-UVVIS from KASVI) and a quartz cuvette with an optical path length of 10.0 mm.

The removal efficiency ($R\%$) and adsorption capacity at time t, $q_t$ (mg g⁻¹), were calculated using Equations (4) and (5), respectively.

$$R\%={\displaystyle \frac{100\times (C_0-C_t)}{C_0}}$$

(4)

$$q_t={\displaystyle \frac{V\times (C_0-C_t)}{m}}$$

(5)

where $C_0$ and $C_t$ are the initial dye concentration (mg L⁻¹) and dye concentration at time t (mg L⁻¹), respectively; $V$ is the volume of the dye solution (L); and $m$ is the biochar mass (g). When t = 1440 min, the concentration, adsorption capacity, and removal efficiency at equilibrium, $C_e$, $q_e$, and $R_e$, were obtained, respectively.

2.6. Kinetic Modeling

The adsorption kinetics considered in this work consisted of a hybrid-order rate equation, as described by Liu and Shen (2008) [24].

$${\displaystyle \frac{d{\theta }_t}{dt}}=k_1\left({\theta }_e-{\theta }_t\right)+k_2{\left({\theta }_e-{\theta }_t\right)}^2$$

(6)

$${\theta }_t={\displaystyle \frac{q_t}{q_{max}}}$$

(7)

where ${\theta }_t$ is the coverage fraction at time t; $q_t$ is the adsorption capacity at time t (mg g⁻¹); $q_{max}$ is the maximum adsorption capacity (mg g⁻¹); $k_1$ is the pseudo-first-order rate constant (s⁻¹); and $k_2$ is the pseudo-second-order rate constant (s⁻¹).

The following assumptions were considered in the model:

• Homogeneous liquid phase;
• Isothermal process;
• No reactions occur.

Equation (6) was numerically solved in Scilab by the algorithm ode.

2.7. Desorption Studies

The adsorbents resulting from the adsorption studies were filtered and air-dried for desorption studies. Desorption was conducted in 125 mL Erlenmeyer flasks, with 50 mL of distilled water with adsorbent (0.50 g ± 0.02 g). The system was kept under agitation for 24 h, at room temperature (30 °C) and with a shaking speed of 100 rpm [25]. Absorbance was determined using UV–VIS (model K37-UVVIS from KASVI), as described previously. Results of the desorption studies were analyzed by dye concentration in the solution.

3. Results and Discussions

3.1. Solid Yield and Energy Properties of Biochar

The biochar exhibited solid yields ranging from 58.84 to 44.10% and energy yields from 69.56 to 64.15%, in the sequence BGP-300 > BTP-300 > BES-300 > BSB-300, and a higher calorific value on average of 23.66 MJ kg⁻¹, with BSB-300 and BES-300 presenting the highest HHVs of 24.27 MJ kg⁻¹ and 24.32 MJ kg⁻¹, respectively (Figure 1).

The solid yield of biochar is affected by the temperature, followed by the time of the thermochemical process [26]. The higher the temperature and residence time, the lower the solid yield of the biochar obtained, due to the degradation of carbohydrates. The samples studied at the same temperature and time presented different mass yields, due to differences in their compositions (cellulose, hemicellulose, and lignin), with hemicellulose being the fraction that undergoes the most degradation, followed by part of the cellulose [26].

The ultimate analysis and heat value of torrefied samples are shown in

Table 2. Comparison of ultimate analysis and heating value of feedstock and biochar.

Sample	Moisture Content (%)	Volatile Matter (%)	Fixed Carbon (%)	Ash Content (%)	HHV (MJ kg−1)
GP raw	8.59	70.36	23.82	5.81	19.35
BGP-300	5.24	44.91	45.10	9.98	22.87
TP raw	10.11	75.36	20.70	3.94	19.04
BTP-300	4.33	45.74	45.61	8.64	23.19
SB raw	6.32	75.61	14.08	10.30	16.69
BSB-300	2.02	38.17	52.03	9.79	24.27
ES raw	8.46	84.91	14.99	0.09	18.54
BES-300	3.47	55.12	44.50	0.38	24.33

. Dry torrefaction converts biomass into biochar with lower moisture content and volatile material compared to the raw sample. This is due to the reduction in hydroxyl groups, making the material hydrophobic and increasing the carbon content and calorific value [10]. The fixed carbon content of the biochar ranged from 44.50 to 52.03%. BSB-300 presented the highest fixed carbon concentration, 52.03%, which is 3.7 times more than raw SB. In addition, the ash content of the biochar was higher in BGP-300 and BSB-300, at 9.98 and 9.79%, respectively. This accumulation of ash content in the biochar is due to the degradation of the biomass.

3.2. Adsorption Studies

Figure 2 displays the effect of initial dye concentration (40 and 120 mg L⁻¹) and different biochars on the adsorption efficiency and adsorption capacity. Increasing the initial concentration of MB resulted in a reduction in the R_e and an increase in the q_e, whereas reducing particle diameter resulted in an increase of R_e and q_e.

The impact of initial concentration is determined by the direct relationship between the initial dye concentration and the number of adsorbing sites on the surface of the adsorbent. The highest removal efficiency was obtained using samples BGP-300 (96.5%) and BTP-300 (65.0%) and an initial dye concentration of 40 mg L⁻¹. Therefore, these samples were selected for kinetic modeling. Samples BSB-300 and BES-300 did not facilitate the significant removal efficiency of MB. For the tested concentrations, removal efficiency and adsorption capacity obeyed the following order for the biochar tested: BGP-300 > BTP-300 > BSB-300 ≈ BES-300. The absorbance spectra of MB solution after adsorption studies using BGP-300 and BTP-300 are shown in Figure 3.

While studies reporting the use of biochar as an adsorbent for water contaminants are emerging rapidly in the literature, the use of low-temperature agro-industrial waste biochar for the adsorption of MB is scarce. For instance, no study using BGP for MB adsorption was found. However, recent studies have demonstrated the potential of this biochar for the removal of lead [27] and cymoxanil [28]. In these studies, the adsorption capacity for BGP produced at low temperatures (300–350 °C) yielded an adsorption capacity of approximately 80–160 mg g⁻¹, whereas BTP has been applied for the removal of colorants, such as the use of acacia wood for the removal of MB (81.20 mg g⁻¹) [29] and maple wood for the removal of methyl orange and rhodamine B (54.2 mg g⁻¹) [30]. However, the biochars in these studies were produced with wood (not including leaves, as in this study) and produced at higher temperatures (500–550 °C), increasing the energy input and cost of biochar.

3.3. Kinetic Model

The model predictions were compared to the experimental data acquired in the present study. The results are shown in Figure 4.

Figure 4 shows that the hybrid-order adsorption model provides good predictions for the cases studied herein. The R² values obtained from these fittings range from 0.89 to 0.96 for all runs, except for BGP-300 in the solution containing 120 mg L⁻¹ MB, where R² = 0.75. Specifically for this run, the experimental profile exhibits a steeper adsorption slope, which is associated with the high adsorption capacity of this material, further enhanced by the concentrated medium used in this condition.

This behavior may be influenced by an error, as microparticles of the adsorbent could have been carried along with the sample, leading to additional adsorption between sample withdrawal and analysis, thereby increasing q_t. Since this error is time-dependent (affecting the x-axis), R² may not accurately reflect the goodness of fit for BGP-300 in the 120 mg L⁻¹ MB solution.

In all cases, the equilibrium data lie above the model predictions, particularly at higher initial MB concentrations. Additionally, up to 420 min, the experimental profile appears close to the equilibrium plateau. Beyond this point, the adsorption rate increases slightly until the final measurement, especially for the 120 mg L⁻¹ MB cases. This phenomenon can be attributed to the hydrophobicity of biochar [2,3], which leads to its gradual immersion in the MB solution during the process, as it was initially introduced in a dry state. This gradual immersion likely results in a variable contact surface area over time, increasing as the process progresses.

Table 3. Kinetic parameters of adsorption of MB onto BGP-300 and BTP-300.

MB Initial Concentration	BGP-300		BTP-300
	40 mg L−1	120 mg L−1	40 mg L−1	120 mg L−1
$k_1$ (min−1)	1.43 × 10−5	1.24 × 10−4	3.32 × 10−4	1.14 × 10−3
$k_2$ (min−1)	1.24 × 10−2	4.42 × 10−3	4.59 × 10−3	1.91 × 10−3
R2	0.90	0.75	0.96	0.89

The final point of each experimental profile was considered q_e, with q_max = q_e.

shows that the second-order kinetic constant ($k_2$) prevails over the first-order constant ($k_1$). As the initial MB concentration increases from 40 to 120 mg L⁻¹, $k_1$ increases, while $k_2$ decreases for both materials. These modeling results highlight the significance of each kinetic order’s contribution to the adsorption process. In other words, the hybrid-order model proves useful by eliminating the need to apply different modeling approaches for each adsorbate concentration.

Recent studies have reported biochar production from grape pomace under a temperature range of 300 to 750 °C for the adsorption of lead [27] and cymoxanil [28]. In these studies, it was verified that lower temperatures (300–550 °C) yielded a more efficient biochar for adsorption, probably due to the presence of hydrophilic polar functional groups (hydroxyl, carbonyl, and carboxyl groups) on the surface of the adsorbent [28]. Additionally, pseudo-second-order kinetics best described the adsorption process of lead and cymoxanil onto grape pomace biochar, as verified with MB in this study. However, no studies using grape pomace biochar for the adsorption of MB were found in the literature.

Various theoretical models have been employed to investigate the primary adsorption mechanisms [31]. The interaction between water contaminants and biochar generally occurs in two distinct stages: surface adsorption and interstitial adsorption. In the first stage, molecules of the sorbate move from the aqueous solution to the surface of the biosorbent material. Once within the boundary layer surrounding the biosorbent, contaminants (whether molecules or ions) adhere to the active sites on the surface and are subsequently removed from the aqueous solution [31]. This type of adsorption is typically driven by dipole interactions, hydrogen bonding, or Van der Waals forces [31].

3.4. Desorption Studies

Desorption studies are shown in Figure 5. The desorption of MB increased with higher initial concentration; the desorption rate is an indicator that the adsorption process may occur physically. From the tested materials, BGP-300 presented a higher removal efficiency and lower rates of desorption, an ideal combination if the material were to be used as an adsorbent and reused post-adsorption.

4. Conclusions

This study investigated the adsorption capability of four different types of agro-industrial waste biochar.

BGP-300 produced in dry torrefaction showed a higher solids yield of 58.84% and energy yield of 69.56%, while the studied residues showed a higher heating value on average of 23.66 MJ kg⁻¹. Adsorption studies reveal that initial concentration, time, and biochar feedstock affected the removal efficiency and adsorption capacity.

Removal efficiency and adsorption capacity obeyed the following order for the biochar tested: BGP-300 > BTP-300 > BSB-300 ≈ BES-300.

In conclusion, the BGP-300 and BTP-300 tested can be used for the removal of MB from aqueous solutions and the promotion of a circular economy.