INTRODUCTION
Research background
The rising demand for synthetic dyes in industries like textiles, plastics, and paper printing has led to significant environmental concerns, including increased dye pollution and the release of harmful chemicals into wastewater Brilliant green (BG) dye, commonly used in these sectors, poses serious health risks such as eye burns, skin irritation, and respiratory issues Consequently, effective treatment of dyestuff wastewater is essential Various techniques, including biodegradation, coagulation, reverse osmosis, and adsorption, are employed to remove dye compounds from water, with adsorption being the most efficient method However, the widespread application of activated carbon is hindered by its high cost.
Hydrothermal carbonization (HTC) is an efficient heat treatment method that converts raw materials into valuable products, primarily hydrochar and bio-oil, with hydrochar comprising 40-70% of the output Operating at lower temperatures (180-350 °C) than traditional methods, HTC eliminates the need for energy-intensive drying, making it a cost-effective option Compared to conventional pyrolysis, HTC offers significant advantages, including reduced energy consumption, higher yields, and lower emissions, which has piqued the interest of researchers exploring hydrochar as a sustainable alternative to fossil fuels Recent studies have focused on producing activated carbon from agro-waste, which is economical due to its abundance and renewability Among these, okara stands out as a promising material, as its annual production leads to environmental concerns due to rapid decomposition, prompting research into its recycling, particularly as an additive in snacks.
Okara, despite being limited in its application as human food due to its high fiber content, offers significant potential for dye removal Its composition of crude fiber, including cellulose, hemicellulose, and lignin, enables it to effectively engage in various mechanisms for eliminating dyes from wastewater.
This study focuses on developing an innovative adsorbent using okara to effectively remove BG dye from wastewater, aligning with waste control principles Utilizing okara not only provides a cost-effective solution for wastewater treatment but also enhances economic value by reducing disposal costs and addressing dye pollution Additionally, the pressing issue of energy demand is highlighted, as the global reliance on fossil fuels continues to grow while reserves diminish rapidly, raising concerns about the sustainability of energy supply and demand.
In light of the urgent need for affordable and alternative renewable energy sources, hydrothermal carbonization (HTC) has emerged as a promising method for generating high-energy products from agricultural by-products Future research should focus on exploring the combustion behavior of hydrochar derived from agrowaste to maximize its potential as a sustainable energy solution.
Research objectives
This study aims to produce hydrochar from okara through hydrothermal carbonization (HTC) technology for the effective removal of Brilliant Green (BG) dye from water, while also examining the fuel properties of the resulting hydrochar The specific objectives include assessing the dye removal efficiency and evaluating the energy potential of the hydrochar.
• Optimization of the okara-derived hydrochar fabrication process
• Selection of the most effective activation method to enhance the BG dye adsorption of the fabricated okara
• Evaluation of the physicochemical and fuel properties of the pristine hydrochar and activated hydrochar
• Investigation of the adsorptive removal of BG dye from aqueous solutions using the fabricated hydrochar n
LITERATURE REVIEW
Dye pollution in the world and in Vietnam
2.1.1 Environmental concerns of dyes in the world
Dyes are essential in various industries, including textiles, paper printing, and cosmetics, with over 10,000 types produced annually, amounting to more than 70 million tons of synthetic dyes (Ali, 2010; Feng et al., 2017) The textile industry is a major contributor to water pollution, accounting for 20% of industrial water contaminants, as it consumes vast amounts of water in dyeing processes (Niinimọki et al., 2020) Approximately 280,000 tons of dye waste enter wastewater each year due to inefficient practices, leading to severe environmental and health risks when untreated wastewater is discharged into water bodies This pollution reduces dissolved oxygen levels and light penetration, harming aquatic ecosystems and diminishing aesthetic quality, as many dyes are detectable at concentrations as low as 1 mg/L (Ali, 2010).
2.1.2 Dye treatment technologies in the world
Current dye removal methods are classified into three main categories: physical methods, oxidizing methods and biological methods
Physical dye removal methods include coagulation & flocculation, adsorption and n
Coagulation is a method used to remove dispersed dyes in wastewater by altering the characteristics of suspended particles, causing them to agglomerate; however, it has drawbacks such as high chemical usage and increased sludge volume (Nguyen and Juang, 2013) Filtration techniques, including reverse osmosis and ultrafiltration, effectively eliminate dyes and allow for their recovery and reuse, but they come with high membrane costs and maintenance needs Adsorption is a cost-effective wastewater treatment method that relies on the physical and chemical properties of the selected material, with activated carbon being a common adsorbent for dyes Nonetheless, the high cost and challenging regeneration of activated carbon have led to a trend towards using bio-based adsorbents as alternatives.
Oxidation is a chemical method for breaking down dyes, classified into advanced oxidation processes (AOP) and chemical oxidation AOP generates highly reactive hydroxyl radicals, while chemical oxidation employs oxidants like ozone (O3) and hydrogen peroxide (H2O2) to form non-selective hydroxyl radicals at high pH levels These radicals effectively disrupt the conjugated double bonds and complex aromatic rings of dye pigments, leading to the formation of smaller, non-pigmented molecules that reduce wastewater color A key advantage of ozonation is that it does not increase wastewater volume or create sludge, although it can be costly Additionally, combining UV light with H2O2 also degrades dyes without generating sludge and minimizes odors, as UV triggers the decomposition of H2O2 into hydroxyl radicals for effective dye oxidation.
Biological methods for dye removal leverage the adaptability of microorganisms and their secreted enzymes, either directly or in free form The efficiency of dye removal is influenced by factors such as the organic load in relation to microbial load, temperature, and oxygen concentration These methods can be categorized into aerobic and anaerobic techniques Key advantages include environmental friendliness, economic benefits, reduced sludge production, and the generation of non-hazardous metabolites or complete mineralization (Navin et al., 2018).
Potential and challenges of agrowaste
2.2.1 Generation and disposal of agrowaste in Vietnam
2.2.1.1 Generation of agrowaste in Vietnam
Agricultural waste, generated during farming activities, primarily consists of by-products such as rice husks, sawdust, bagasse, and soybean residue In 2010, Vietnam's agricultural landscape included approximately 7.47 million hectares of rice, 1.1 million hectares of maize, and various other crops, resulting in an estimated 61.43 million tons of agricultural by-products If not managed properly, these wastes can negatively impact the environment through decomposition, misuse, or burning However, with appropriate treatment, organic crop waste can be transformed into valuable resources that promote environmental sustainability and enhance economic efficiency.
2.2.1.2 Disposal of agrowaste in Vietnam
Due to its high nutritional value, the ability to provide large calories and fiber content, if appropriate technologies are applied, agricultural by-products will become valuable n
Currently, only about 10% of agricultural by-products are utilized as fuel for household cooking and brick kilns, while 5% are used as industrial fuels, like rice husks and bagasse, for heating in boilers and drying systems Additionally, 3% is repurposed for animal feed, flavoring, and soil fertilizer Alarmingly, over 80% of agricultural by-products remain unused, leading to significant environmental issues as they are often discharged into canals, ditches, and rivers, contributing to pollution and obstructing water flow (Tin., 2017) To combat this, various technologies are being implemented in Vietnam to reduce pollution and promote sustainable use of agricultural by-products.
Composting is an effective method for transforming organic waste into valuable fertilizer through natural aerobic decomposition by microorganisms This process enhances soil quality by increasing humus content and improving its physicochemical properties, making it beneficial for plant growth at a reasonable cost However, composting is influenced by climatic conditions, and the multi-stage decomposition process can lead to unpleasant odors and an unsightly appearance.
This method involves the production of biochar from agricultural by-products through high-temperature pyrolysis, where both the temperature and material type significantly influence the yield and properties of the biochar As the pyrolysis temperature rises, the yield of coal and concentrated liquids decreases; for instance, at 280°C, coal yield ranges from 30-50%, dropping to 20-30% at 850°C Common feedstocks for biochar production include shrubs, waste wood, crop residues, animal waste, and organic waste from agricultural processing However, a notable drawback of this method is the lack of homogeneity in the resulting biochar product.
Serving and processing animal feed
In order to reduce environmental pollution in livestock production and create a source n
Utilizing agricultural waste products like rice, corn, soybeans, and corn residues as animal feed can significantly reduce costs while maximizing profits The integration of these by-products in livestock feed is becoming increasingly common in the agriculture sector.
Rice straw is increasingly utilized in cattle rearing for both plowing and breeding, serving as an excellent fiber source when combined with puree in dairy farming and beef cattle fattening While it is rich in potassium, rice straw lacks calcium absorption, necessitating the addition of easily digestible calcium sources in cattle diets Currently, composting rice straw with 4-5% urea enhances its digestibility, increasing its energy value from 4.74 MJ to 5.49 MJ/kg dry matter (Cai, 2002) Other agricultural byproducts such as bagasse, rice bran, wine residue, and okara are also used in livestock feeding; however, the reuse of these materials remains limited due to inadequate agricultural waste management and planning, leading to significant environmental pollution in many rural areas.
2.2.2 Sources, current use and disposal of okara
Okara, a by-product of tofu and soy milk production, is generated in significant quantities worldwide, with approximately 1.2 kg produced from every 1 kg of dry soybeans Major producers include Japan, which generates around 800,000 tons, Korea with about 310,000 tons, and China leading the way with an impressive 2.8 million tons annually from the tofu industry.
Dried okara is a nutrient-rich byproduct, containing approximately 50% fiber, 25% protein, and 10% lipid It also includes beneficial compounds such as isoflavones (genistein and daidzein), lignans, phytosterols, coumestans, saponins, and phytates, which contribute to its antioxidant properties and potential in preventing cardiovascular diseases and various cancers Given its impressive nutritional profile, okara can be effectively reused or recycled to recover these valuable components.
2.2.2.2 Current use and disposal of okara
Okara has been used as a food for many years in China and Japan It can be used in wet, n
Okara, a byproduct of soy milk production, is a versatile ingredient that can enhance the nutritional value of various food products, including meat and baked goods, due to its high fiber and protein content Its oil and moisture binding properties make it a cost-effective option for increasing yield in meat products Research indicates that incorporating okara at an optimal level of 5% can improve the shelf life of chocolate chip cookies and prevent structural issues in frozen cheese ravioli without compromising flavor or texture Despite its benefits, the use of okara in food manufacturing remains limited, as most large-scale soy milk producers do not utilize or sell it for food purposes, resulting in a relatively small amount being repurposed compared to what is produced.
Okara, a byproduct of soy milk production, is primarily utilized as animal feed due to its high protein and non-fibrous carbohydrate content, making it a nutrient-rich option Additionally, its cost-effectiveness compared to soybean meal enhances its appeal for animal nutrition To effectively use okara in animal feed, it must be thoroughly dried and pelletized, facilitating easy transportation and recycling into feed formulations (Li et al., 2012).
Okara is rich in carbohydrates, proteins and other nutrients, making it a useful substrate for microbial fermentation
Fermented okara serves as a nutritious human food source, as the fermentation process enhances its soluble fiber, protein, amino acids, and isoflavones while reducing phytic acid, which hinders nutrient absorption This improved nutritional profile and processing capability makes fermented okara a valuable ingredient According to Lu et al (2007), the optimal conditions for solid-state fermentation of okara using Mucor include 75% initial moisture content, 1% NaCl, and a temperature of 28 °C over a two-day period, resulting in increased protein enzyme activity in the final product.
The crude protein content of okara increased significantly from 19.76% to 22.96%, while the amino acid nitrogen content rose from 0.26% to 1.45% (Lu et al., 2007) Furthermore, okara serves as an effective fermentation medium for natto production and as a substrate for various applications.
9 fermentation such as in the production of iturin A, alcohol and citric acid.
Agrowaste thermal conversion technologies
Hydrothermal carbonization (HTC) is a thermochemical process that transforms organic biomass into a carbon-rich solid known as hydrochar This process involves submerging feed material in water at temperatures between 180 - 350°C and pressures of 2 – 6 MPa for durations of 5 - 240 minutes The main products of HTC include solid hydrochar, liquid, and a small amount of gas, primarily CO2, with hydrochar yielding 40-70% mass The reaction pressure is typically automatic, aligning with the saturated vapor pressure of water at the given temperature Elevated temperatures enhance the hydrolysis and cleavage of lignocellulosic biomass, improving hydrochar's fuel properties such as fuel ratio and calorific value Additionally, dehydration and decarboxylation reactions lead to significant aromatization and the presence of oxygen-containing functional groups on the hydrochar surface, which enhances its compatibility with water and increases soil water holding capacity when used as a soil improver Notably, wet biomass can be utilized directly in the HTC process, contributing to energy savings.
The HTC method offers significant advantages, including operation at lower temperatures than incineration and pyrolysis, and it does not require feedstock pre-drying This process occurs in a solvent environment, leading to a higher yield of coal with more organic compounds dissolved in water Additionally, the gas products generated, particularly CO2, are produced in smaller quantities compared to other conversion methods due to limited oxygen exposure in the reactor Notably, the chemical structure of coal produced through HTC closely resembles that of natural coal (Medick et al., 2017).
10 of this unique property of hydrochar, it is a potential material to replace fossil fuels in the future
The main drawback of the HTC process is the absence of crucial reaction kinetic data, such as reaction curves and mass transfer information, which are vital for optimizing processes and designing reaction kinetics Furthermore, the separation of liquid and solid phases not only increases operational costs but also diminishes the overall yield of the product.
In the HTC process, water serves as a safer alternative to toxic and corrosive solvents, acting as a non-polar solvent that enhances the solubility of organic compounds, including biomass At elevated temperatures and pressures, water exhibits significant ionization, dissociating into hydroxide (OH-) and hydronium (H3O+) ions, which contribute to its acidity and basicity (Marcus, 1999) The use of subcritical water initiates hydrolysis, effectively reducing the activation energy required for the decomposition of cellulose and hemicellulose, resulting in a rapid formation of water-soluble products (Bobleter, 1994).
Temperature plays a crucial role in the HTC process, significantly influencing the characteristics of hydrochar and the ionic reactions in the subcritical region Higher temperatures facilitate water penetration into the porous biomass, promoting further degradation As temperature rises, it enhances hydrolysis while simultaneously driving dehydration, decarboxylation, and condensation reactions Additionally, variations in temperature impact the elemental composition of hydrochar, with an increase from 230 to 250 °C resulting in a decreased O/C and H/C ratio, alongside a higher degree of aromaticization.
1994) The energy content and thermal stability of hydrochar were also significantly improved with increasing reaction temperature
The yield of hydrochar is significantly influenced by reaction time, with longer residence times and higher temperatures resulting in decreased hydrochar yields, while shorter residence times enhance yields For lignocellulosic materials, hydrochar formation relies on residence time, as soluble monomers require extensive polymerization; reducing retention time leads to fewer condensation products with higher O/C and H/C atom ratios due to limited hydrolysis and polymerization Additionally, increased residence time improves the structural and morphological properties of hydrochar by releasing more volatiles and promoting carbonization Two types of char are produced: primary char, the solid remainder of biomass, and secondary char, formed through condensation and depolymerization Prolonged residence time primarily results in the formation of secondary chars, which ultimately reduces bio-oil conversion and yield.
The HTC process involves a complex endothermic reaction characterized by dehydration and decarboxylation, as noted by Berge et al (2011) Key reactions in hydrothermal carbonization include hydrolysis, dehydration, decarboxylation, condensation, polymerization, and aromatization.
Hydrolysis involves the reaction of water with cellulose or hemicellulose, breaking ester and ether bonds to yield various products During biomass heating, water molecules cleave to form hydronium ions (H3O+), which facilitate hydrolysis As temperature rises, biomolecules hydrolyze into intermediates like oligomers and glucose, which further decompose into organic acids such as acetic, lactic, and levulinic acid, resulting in a decrease in pH The process also leads to the formation of 5-HMF Hemicellulose hydrolysis begins above 180°C, while cellulose hydrolysis occurs above 230°C This physical and chemical process is crucial for dehydration and oxygen removal.
12 hydroxyl group is a chemical dehydration process thus it reduces the H/C and O/C ratios
In addition, the biomass was significantly carbonized leading to a significant reduction in the O/C ratio At temperatures above 230 o C, the decomposition of the carboxyl group takes place
During polymerization, unstable intermediate monomers like 5-HMF and aldehydes, formed during hydrolysis, undergo aldol-condensation and intermolecular dehydration to create polymer chains Additionally, the linear structure of cellulose is crosslinked, resembling the polymerization process of lignin Notably, lignin fragments can polymerize within minutes at a temperature of 300°C.
Lignin, characterized by its stable aromatic rings, increases in percentage as reactor temperature and residence time rise, relative to roughage Additionally, the linear carbohydrate chains found in hemicellulose and cellulose can be easily converted into lignin
Other minor mechanisms that can occur under the HTC process include demethylation, pyrolytic reactions, fischer–tropsch reactions, transformation reactions and secondary char formation
Pyrolysis is a thermochemical process that converts organic biomass into valuable products by heating raw materials at high temperatures (300 to 650 °C) in the absence of oxygen This process generates three primary outputs: biochar, bio-oil, and gases such as CO2, CO, H2, and CH4 Pyrolysis can be categorized into slow, intermediate, and fast stages based on factors like reaction temperature, retention time, and heating rate Notably, slow pyrolysis is favored for biochar production due to its higher solid yield of 25-35% This method involves heating at 300-650 °C with a low heating rate and extended retention time.
The physicochemical properties and yield of biochar are significantly influenced by key parameters such as reaction temperature, time, initial humidity, heating rate, and pressure Specifically, a lower reaction temperature combined with a slower heating rate tends to produce a higher solid yield, while increasing the reaction temperature and heating rate generally leads to a reduced solid yield.
13 yield and in addition it affects the surface area and heating value (HHV) and carbon content (Karaosmanoğlu et al., 1999)
Incineration is a waste management technology that converts waste into heat and energy, significantly decreasing the volume of solid waste sent to landfills While it provides heat for various industries, incineration poses environmental challenges, particularly air pollution, due to the release of harmful exhaust gases containing dioxins and heavy metals like cadmium, mercury, lead, copper, chromium, and zinc Despite its benefits in waste reduction and energy recovery, this method generates substantial fly ash and hazardous chemicals that can adversely affect the ecosystem and human health.
Application of agrowaste-derived hydrochars
The HTC process effectively converts biomass into high-quality activated carbon, serving as a viable coal substitute for energy production This transformation increases the C/O ratio by altering cellulose and hemicellulose, which in turn enhances the higher heating value (HHV) of the resulting solid product Hydrochar derived from pellets boasts a calorific value comparable to lignite coal, reaching up to 21.74 MJ/kg Furthermore, the removal of hemicellulose improves the hydrophobic properties of hydrochar, reducing its hygroscopicity and facilitating combustion Consequently, hydrochar exhibits coal-like characteristics, positioning it as a promising alternative to coal in the energy sector.
Hydrochar serves as an affordable adsorbent for pollutant removal in water treatment The adsorption capacity of hydrochar is influenced by the production conditions and the characteristics of the initial biomass Typically, primary hydrochar exhibits a limited surface area and pore volume, leading to decreased adsorption effectiveness.
The presence of oxygen-rich functional groups on the surface of hydrochar enhances its capacity to adsorb positively charged pollutants, while its effectiveness against negatively charged pollutants is diminished Consequently, activating hydrochar through modification could enhance its properties for more efficient contaminant removal.
Hydrochar derived from plants typically has low nutrient content, making it an effective additive to soil that enhances fertilizer efficiency by minimizing nutrient loss through surface runoff The hydrochar's surface pores absorb nutrients, which are gradually released into the soil, allowing plants to access them over time (Yao et al., 2013; Fang et al., 2018) Research indicates that incorporating hydrochar into soil can improve various physiological properties, including water retention, stable agglomeration, pH levels, cation and anion exchange, and the availability of extractable nutrients.
When biomass is converted to solid coal and introduced into the soil, the process is known as carbon sequestration However, some studies suggest that hydrochar has a low
Hydrochar exhibits significant carbon sequestration potential due to its higher H/C and O/C ratios compared to biochar, making it more susceptible to microbial decomposition This rapid degradation can facilitate the transformation of microorganisms before achieving long-term carbon storage The effectiveness of carbon sequestration is influenced by the interactions between dissolved carbon and microbial activity (Malghani et al., 2013; Ramke and Blohse, 2009).
The interaction between hydrochar and soil properties, including mineral surfaces and texture, significantly influences the effectiveness of carbon amendments Current research suggests that hydrochar has limited utility for carbon sequestration due to its low stability in soil Consequently, additional studies are essential to enhance the application of hydrochar in soil for improved carbon absorption and stability.
MATERIALS AND METHODS
Materials
In this experiment, Brilliant Green (C27H34N2O4S, molecular weight = 482.65 g) was utilized as a dye pollutant A standard stock solution of 1000 mg/l was created by dissolving the appropriate amount of Brilliant Green dye in deionised water Subsequently, working dye solutions at various concentrations were prepared by diluting the standard solution with deionised water The pH levels of these working dye solutions were adjusted using 0.1M HCl and 0.1M NaOH.
Okara was sourced from a household tofu production facility located on Yen Hoa Street in the Cau Giay district of Hanoi It was washed with distilled water to eliminate impurities and subsequently dried in an oven at 105°C until a consistent weight was achieved The dried okara was then blended and sieved to a particle size of 150 µm.
Experiment setup and equipment
Hydrothermal carbonization (HTC) of okara was conducted using lab-scale Teflon-lined stainless steel autoclave reactors in a furnace, with varying conditions including temperatures of 180, 220, and 260 °C, reaction times of 3, 6, and 9 hours, and solid-to-liquid ratios of 1, 3, 5, and 7 g/30 mL After the treatment, the autoclaves were swiftly cooled with tap water, and the solid product was separated from the liquid through vacuum filtration The resulting solid was then dried at 105 °C for 24 hours to produce raw hydrochar (RH).
The raw hydrochars (RH) 5 g were activated using different methods as follows:
• Method 1: Mixing with 200 ml of 1M NaOH solution (AH1) n
• Method 2: Mixing with 200 ml of 1M NaOH solution followed by heating at 700 o C, for 30 min, at the heating rate of 5 o C/min in the supply of N2 (AH2)
• Method 3: Heating at 700 o C, for 30 min, at the heating rate of 5 o C/min in the supply of N2 (AH3)
• Method 4: Heating at 700 o C, for 30 min, at the heating rate of 5 o C/min in the supply of N2 followed by mixing with 200 ml of 1M NaOH solution (AH4)
Using filtration, the solid products were collected, then washed with 0.1M NaOH and 0.1M HNO3 to obtain pH value of 7-8 Then, the modifed hydrochars dried at 105 o C for 24h
The morphology of raw hydrochar (RH) and NaOH-modified hydrochar (AH2) was analyzed using scanning electron microscopy (SEM), specifically with the Tabletop Microscopes TM4000 Plus The specific surface areas of these materials were determined through the Brunauer-Emmett-Teller (BET) method utilizing the NOVAtouch LX-BET Additionally, the chemical functional groups present in the hydrochars were identified via Fourier transform infrared (FTIR) spectroscopy using the JASCO Asia Portal FT/IR-4600.
3.2.4 Fuel properties of raw and activated hydrochars
High heating value (HHV) of RH, AH1, AH2, AH3 and AH4 were measured using Parr
3.2.5 BG dye adsorption by the selected activated hydrochar
To determine the adsorption isotherms, batch adsorption experiments were performed in a set of 250 ml beakers containing 50 ml of BG with different initial concentrations
The adsorption capacity of the BG dye by AH2 was assessed through batch experiments conducted with varying dye concentrations (5, 20, 40, 60, 80, and 100 mg/L) at a pH of 7 and a dosage of 0.25 g/L The mixtures were agitated at 120 rpm for 4.5 hours to reach equilibrium, with each experiment being duplicated The concentration of BG dye was measured using a UV/VIS spectrophotometer (S2150 UV, Unico) at its maximum wavelength (λmax), allowing for the determination of the dye's adsorption capacity based on the difference between initial and final concentrations in the aqueous solution.
624 nm The amount of dye retained per unit mass of the adsorbent and the percent n
17 removal (%R) of dye were calculated using the following equations:
The adsorption capacity (qe) is expressed in mg/g and is determined by the initial concentration (Ci) of the BG dye in the aqueous solution (mg/l), the equilibrium concentration (Ce) of the BG dye in the solution (mg/l), the volume of the solution (V in liters), and the mass of the adsorbent used (m in grams).
Point of zero charge (pH zpc )
The pHzpc is a crucial parameter for characterizing adsorbents, indicating the pH at which the surface charge is neutral When the pH is below pHzpc, the adsorbent surface carries a positive charge, while a pH above this point results in a negative charge In this study, pHzpc was determined by adding 0.0125 g of adsorbent to 50 ml of 0.1M KNO3 solution, adjusting the initial pH from 3 to 10 using 0.1M HCl and 0.1M NaOH After stirring the mixture for 24 hours and filtering it, the pH values of the remaining solutions were measured The pHPZC was then calculated from the pHi curve, plotting pHf against pHi, where pHi and pHf represent the initial and final pH values, respectively.
The effects of pH on the BG dye uptake by hydrochars were investigated in the pH range of 3-10 The pH was adjusted using 0.1M NaOH and 0.1M HCl The concentration of
The initial concentration of BG was 20 mg/l, with an adsorbent dose of 0.25 g/l The solution was agitated for 4.5 hours at a speed of 120 rpm, followed by filtration The remaining BG concentration in the solution was measured using a UV-VIS spectrophotometer at a wavelength of 624 nm.
Adsorption isotherms offer valuable insights into the concentration and amount of pollutants adsorbed onto surfaces These equilibrium isotherms are essential for understanding the interaction dynamics between solutes and adsorbents, highlighting how various factors influence adsorption behavior.
The Langmuir adsorption model describes monolayer adsorption of the pollution onto a homogeneous adsorbent surface This model can also be used to determine the maximum capacity of the adsorbent:
The Langmuir isotherm model is represented by the equation \( q_e = \frac{q_m k_L C_e}{1 + k_L C_e} \), where \( C_e \) denotes the equilibrium concentration of the dye in the liquid phase (mg/l), \( q_e \) is the quantity of dye adsorbed at equilibrium (mg/g), \( q_m \) signifies the maximum adsorption capacity (mg/g), and \( k_L \) is the Langmuir constant (l/mg).
The Freundlich isotherm model represents heterogeneous surface energy systems and is expressed by the following equation:
The equation \( nlnCe + lnqf \) (Eq.4) illustrates the relationship between the equilibrium concentration in the liquid phase (Ce, in mg/l) and the adsorption capacity (qe, in mg/g) In this equation, \( kf \) denotes the Freundlich constant, which indicates the adsorption capacity, while \( n \) represents the empirical parameter that reflects the energetic heterogeneity of the adsorption sites.
The pseudo-first-order kinetic model can be described as follows: log(𝑞 𝑒 − 𝑞 𝑡 ) = 𝑙𝑜𝑔𝑞 𝑒 − 𝑘 1 𝑡
In the equation 2.303 (Eq.5), qe represents the quantity of dye adsorbed per unit mass of the adsorbent at equilibrium, while qt indicates the amount adsorbed at a specific time t, measured in mg/g Additionally, k1 is defined as the rate constant, expressed in min^-1.
The pseudo-second order can be described as follows: n
𝑞 𝑒 𝑡 (Eq.6) where k2 is the rate constant (min −1 ), qe and qt are the dye adsorption capacities (mg/g) at the equilibrium and t time, respectively.
Statical analysis
Batch experiments were repeated (N = 2) and data represent the mean value Data on
BG adsorption and final solution concentration were fitted to Langmuir and Freundlich isotherm models using nonlinear regression analysis n
RESULTS AND DISCUSSION
Factors influencing the fabrication of okara-derived hydrochars
Figure 4.1 Effect of the temperature on okara-derived hydrochar fabrication
The properties of hydrochar, including higher heating value (HHV), yield, and adsorption capacity for BG dye, are influenced by factors such as temperature, contact time, and the biomass/water ratio Research shows that as hydrothermal carbonization (HTC) temperature rises from 180 to 260 °C, hydrochar yields decrease significantly from 79% to 55.7% This decline is attributed to the hydrolysis and decomposition of hemicellulose, cellulose, and some lignin, with hemicellulose beginning to transform at 180 °C and cellulose undergoing substantial decomposition at higher temperatures.
Heating subcritical water at 200°C enhances the dissolution of organic compounds, which results in a decrease in mass yield Agro-waste is rich in oxygen-containing functional groups such as hydroxyl, phenolic, carbonyl, and carboxylic, contributing to a negatively charged surface on hydrochar This characteristic is advantageous for various applications.
Removal efficiency (%) Adsorption capacity (mg/g) n
The adsorption of cationic dyes, such as BG dye, is influenced by electrostatic interactions, but increasing hydrothermal carbonization (HTC) temperatures can lead to processes like hydrolysis, dehydration, decarboxylation, and aromatization These processes diminish the number of oxygen-rich functional groups on the adsorbent surface, resulting in a decreased removal efficiency of cationic pollutants Specifically, the BG adsorption capacity of hydrochars was observed to decline from 17.5 mg/g at 180°C to 15.2 mg/g at 260°C.
Figure 4.2 Effect of the contact time on okara-derived hydrochar fabrication
In the HTC process, contact time significantly influences the properties of hydrochars, affecting their reactivity and product distribution Longer residence times can lead to a reduction in oxygen functional groups through intermolecular dehydration and aldol condensation, resulting in the formation of more stable oxygen groups like ethers or quinones However, the content of these functional groups is also contingent on biomass concentration; extended residence times may enhance oxygen functional group levels due to increased reaction extent and the formation of secondary char Consequently, even at lower temperatures, prolonged residence times can promote the polymerization of aromatic clusters, ultimately elevating the overall quality of the hydrochars.
Removal efficiency (%) Adsorption capacity (mg/g) n
Increasing the content of oxygen functional groups on hydrochar surfaces enhances their negative charge, improving the removal capacity for cationic pollutants (Jain et al., 2016) Results show that as residence time increased from 1 to 9 hours, the adsorption capacity for BG dye improved from 17.6 to 20 mg/g, and removal efficiency rose from 76.6% to 87.5% No significant difference in BG removal efficiencies was observed between 6 and 9 hours, leading to the selection of a 6-hour contact time as optimal for energy savings in the hydrothermal carbonization (HTC) process.
Figure 4.3 Effect of the okara: water ratio on okara-derived fabrication
The density of functional groups is also affected by the okara/water ratio It was found that the BG dye removal efficacy and adsorption capacity increased significantly from
The removal efficiencies of BG dye ranged from 74% to 78%, with adsorption capacities between 17.67 mg/g and 18.63 mg/g as the ratio of okara to water increased from 1 to 7 g per 30 ml Notably, there were minimal differences in dye removal efficiencies and adsorption capacities at the 3 g and 5 g ratios per 30 ml of water Additionally, increasing the reactant-to-water ratio can result in polymerization occurring at shorter residence times, which hinders the complete hydrolysis of most reactants.
Removal efficiency (%) Adsorption capacity (mg/g) n
Higher substrate concentrations lead to fewer condensed products and increased O/C and H/C atomic ratios due to incomplete hydrolysis, as noted by Sevilla et al (2009) Adjustments to the okara/water ratio and residence time are essential for specific applications A higher okara/water ratio enhances the formation of oxygen functional groups at 180°C, as hydrolysis occurs more rapidly with increased okara Consequently, a ratio of 5 g of okara per 30 ml of water is identified as optimal for achieving a higher mass yield.
The study found that a higher okara-to-water ratio, along with elevated temperature and extended residence time, enhances the formation of oxygen functional groups on hydrochar surfaces Consequently, the optimal conditions for hydrochar fabrication were determined to be an okara/water ratio of 5g:30ml, a holding time of 6 hours, and a temperature of 180°C.
Effect of modification on the fuel and adsorption properties of hydrochars
Effect of modification methods on the adsorption properties of hydrochars
Figure 4.4 Effects of activation methods on BG adsorption
This article compares four distinct activation methods—chemical, physical, chemical-physical, and physical-chemical—to determine the most effective approach for enhancing cationic BG dye adsorption Each activation method exhibits unique characteristics that influence the adsorption process.
AH1 AH2 AH3 AH4 RH
Removal efficiency (%) Adsorption capacity (mg/g) n
The study demonstrates that the combined chemical-physical activation method using NaOH significantly enhances the properties of okara-derived hydrochar By employing NaOH, the activation process increases the oxygen-rich functional groups and expands the pore structure at a temperature of 700 °C, resulting in a greater surface area and an increase in mesopores This enhancement allows for improved adsorption capacity of BG molecules, with the hydrochar activated through this method achieving a remarkable BG adsorption capacity of 37.2 mg/g, outperforming hydrochars activated by other techniques Thus, the combined activation approach is identified as the most effective for maximizing BG adsorption in hydrochar materials.
Effect of modification methods on the fuel properties of hydrochars
Table 4.1 Effects of activation methods on the solid fuel properties of okara-derived hydrochar
Table 4.2 Comparing the HHV of AH2 with other materials
Materials HHV ((MJ/kg) References
Palm shell 16.3 Nizamuddin et al (2016)
Longan shell 16.56 Guo et al (2016)
Corn stalk 17.79 Guo et al (2016)
Okara-derived hydrochar activated with NaOH and 700 o C
AH2 hydrochar demonstrated a higher higher heating value (HHV) than other hydrochars derived from agro-residues, and it is comparable to conventional fuel materials This suggests that hydrochar produced from okara has significant potential as a biofuel.
Characterization of hydrochars
The characterization of pristine hydrochar (RH), intermediate activated hydrochar (AH1), and selected activated hydrochar (AH2) revealed distinct differences in their morphologies and physicochemical properties SEM images indicated that RH and AH1 had dense, rough surfaces with minimal porosity, while AH2 exhibited a porous structure with uniform holes and high porosity Following NaOH treatment, both the BET surface area and pore volume of AH1 decreased compared to RH However, AH2 showed an increased presence of oxygen-containing functional groups, as confirmed by FTIR analysis, which enhances its potential for BG adsorption The mean pore diameters were measured at 213 Å for RH, 1262 Å for AH1, and 61.5141 Å for AH2, with corresponding BET surface areas of 4.3840 for RH, 1.2899 for AH1, and significantly higher for AH2.
4.2504 m²/g, respectively The mesoporous structures were found to exist in all three hydrochars with the mesoporous diameters ranged from 20 to 500 Å
The FTIR spectra of RH and AH2 reveal the presence of hydroxyl groups (OH) on their surfaces, indicating that sodium is electrostatically attached to these groups The analysis shows absorption bands at 3379.64 to 3925.39 cm-1 corresponding to the OH group, along with C–H stretching vibrations at 2926.45 cm-1, and C≡C alkyne features at 2317.05 cm-1 Additionally, C=O stretching vibrations are noted at 1647.88 cm-1, and aromatic C=C peaks at 1539.88 cm-1 The C-O vibrations from cellulose and hemicelluloses are found between 1159.97 and 1058.73 cm-1 Notably, pretreatment with NaOH increased the oxygen functional groups on AH2 compared to RH, resulting in a higher surface area for AH2 post-treatment, enhancing its BG removal efficiency However, the surface areas of AH1 and AH2 remained lower than that of RH, indicating that BG sorption was not solely dependent on surface area, as NaOH treatment did not increase RH's surface area but did enhance its oxygen functional groups.
Figure 4.5 SEM results of RH n
Figure 4.6 SEM results of AH1
Figure 4.7 SEM results of AH2 n
The fuel properties of the selected activated hydrochar
The Higher Heating Value (HHV) is a key parameter for assessing the fuel characteristics of solid materials, indicating the heat released during combustion Research indicates that the HHV of okara-derived hydrochar increases with higher hydrothermal carbonization (HTC) temperatures, reaching a peak of 21.64 MJ/kg at 260 °C, which surpasses the HHV of bamboo-derived hydrochar at 20.3 MJ/kg Although the HHV of okara-derived hydrochar is slightly lower than that of certain fossil fuels, such as peat (22.67 MJ/kg) and methanol (22.69 MJ/kg), it demonstrates potential for enhancement through optimized HTC temperatures and activation processes.
BG dye adsorption behaviors of the selected activated hydrochar
4.5.1.1 Effect of pH pH is a factor that greatly affects the adsorption process The influence of pH was n
The pHzpc value of the selected activated hydrochar (AH2) was measured at 7.4, as illustrated in Fig 4.5.1 The removal percentage and adsorption capacity of BG dye by AH2 increased from pH 3 to 7, peaking at 90.7% removal efficiency and an adsorption capacity of 70.59 mg/g at pH 7, before decreasing at higher pH levels, as shown in Fig 4.5.2.
The removal percentage of BG at lower pH values can be attributed to the positively charged surface of AH2 when pH is below the pHzpc, which creates a repulsive force against the cations of the BG dye.
Figure 4.9 Point of zero charge (pHpzc) for the selected hydrochar (AH2)
Figure 4.10 Effect of pH on BG adsorption by AH2
The study examined the impact of varying doses of AH2 on BG adsorption, ranging from 0.25 to 1 g/l Results showed a decrease in adsorption capacity from 73.8 mg/g at 0.25 g/l to 19.2 mg/g at 1 g/l, while the BG pollutant removal rate increased from 95.57% to 99.70% as the AH2 dose rose Notably, the highest BG removal percentage was observed at the lower dose of 0.25 g/l, leading to its selection as the optimal dose for subsequent BG adsorption experiments.
Figure 4.11 Effect of AH2 dose on BG adsorption
This study utilized Langmuir and Freundlich isotherm models to analyze the equilibrium distribution of the adsorbent BG dye between liquid and solid phases The isotherm parameters detailing BG adsorption onto AH2 are provided in Table 4.3.
Removal efficiency (%) Adsorption capacity (mg/g) n
Table 4.3 Langmuir and Freundlich isotherm parameters for BG adsorption on AH2
Langmuir isotherm Freundlich isotherm q max
Figure 4.12 Langmuir adsorption isotherm curve for BG adsorption on AH2
Figure 4.13 BG adsorption isotherms on AH2 y = 0.0041x + 0.0018 R² = 0.9982
Figure 4.14 Freundlich adsorption isotherm curve for BG adsorption on AH2
Table 4.4 Comparing qmax of AH2 with those of other hydrochars
Hydrochars Adsorption capacity (mg/g) Target pollutant References
Almond shell power 58.13 Brilliant green 47
NaOH treated saw dust 58.48 Brilliant green 48
White rice husk ash 85.56 Brilliant green 49
Cellulose modified with metaphosphoric acid
Betel Nut Husk 429.6 Methylene blue 46
The linear adsorption isotherms illustrated in Figures 4.12 and 4.14, along with the parameters from Table 4.3, indicate that the adsorption of BG onto AH2 is best described by the Langmuir model, which has a higher R2 value of 0.998 compared to Freundlich's 0.93 This suggests a monolayer adsorption process, with a maximum BG adsorption capacity (qmax) of 555.56 mg/g for AH2, significantly outperforming other bio-adsorbents used for BG removal by nearly tenfold Consequently, the superior qmax of AH2 as a hydrochar for BG adsorption highlights hydrothermal carbonization (HTC) as a promising method for fabricating effective BG adsorbents.
The adsorption kinetics of BG onto AH2 were analyzed using pseudo-first-order and pseudo-second-order kinetic models, with the corresponding parameters presented in Table 4.5.
Table 4.5 Pseudo-first-order and pseudo-second-order kinetic parameters for BG adsorption onto AH2
Pseudo-first order model Pseudo-second-order model q e.cal
Figure 4.15 Pseudo-first-order and Pseudo-second-order kinetic curves for BG adsorption on AH2
The Pseudo-second-order kinetic model, with a correlation coefficient (R²) of 0.99, outperformed the Pseudo-first-order kinetic model, which had an R² of 0.98 This indicates that the Pseudo-second-order kinetics model is more effective in accurately describing the kinetic data for BG adsorption on AH2.
Kinetic curve of adsorption BG
Time (min) first order model second-order-model n
CONCLUSION AND RECOMMENDATION
Conclusions
Among investigated agro-wastes, okara was a potential for hydrochar production
The ideal conditions for producing hydrochar from okara involve a 5 g to 30 ml ratio of okara to water, a hydrothermal carbonization (HTC) contact time of 6 hours, and a temperature of 180°C.
The combined chemical-physical modification method, involving mixing with NaOH solution and heating at 700 °C, significantly enhances the BG dye sorption capacity of okara-derived hydrochar This improvement is due to an increase in pore size and a higher density of oxygen-containing functional groups on the surface of the activated hydrochar (AH2).
The highest higher heating value (HHV) of okara-derived hydrochar was achieved at 21.64 MJ/kg through a combined chemical-physical modification process involving mixing with NaOH solution and heating at 700 °C This HHV surpasses that of other agro-waste derived hydrochars and is only slightly lower than conventional fuel materials, suggesting that the activated okara-derived hydrochar (AH2) holds promise as a bio-inspired fuel alternative.
The maximum adsorption capacity (qmax) of Brilliant Green (BG) dye by AH2 was found to be 555.56 mg/g, surpassing the performance of other biosorbents The adsorption process was rapid, achieving equilibrium within 4.5 hours Additionally, the isotherm and kinetic data for BG adsorption on AH2 were well-represented by the Langmuir isotherm model, indicating effective dye removal.
=0.99) and Pseudo-second-order kinetic model (R 2 = 0.99).
Recommendations
Future studies should focus on the use of activated okara-derived hydrochar (AH2) for effectively removing BG dye from actual textile wastewater Additionally, research should investigate the desorption and regeneration capabilities of BG dye from the spent hydrochar Furthermore, the application of this hydrochar in column mode and at a pilot scale warrants exploration to assess its practical viability.
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