Valorization of Kitchen Waste into Functional Biochar: Progress in Synthesis, Characterization, and Water Remediation Potential

Abstract

The continuous increase in urbanization and global population has led to the generation of a substantial amount of kitchen waste, posing severe environmental and disposal challenges. The utilization of kitchen waste as organic biomass for biochar production offers a promising, sustainable, and cost-effective solution. This review comprehensively analyzes the recent developments in the transformation of kitchen waste into biochar. Moreover, the current study involves various synthesis techniques, the physicochemical characteristics of biochar, and its applications in soil and water remediation. Afterwards, the experimental parameters and feedstock types are critically evaluated in terms of their key characteristics for biochar. Moreover, the current study highlights the effectiveness of kitchen waste-derived biochar (KWBC) in decomposing organic pollutants, heavy metals, and pharmaceutical pollutants from contaminated environments. Additionally, the mechanisms of adsorption, ion exchange, complexation, and redox interactions are thoroughly illustrated to evaluate the pollutant removal pathways. At the end of the study, experimental parameters such as pH, dosage, contact time, and initial pollutant concentration are discussed, which play the main role in enhancing the adsorption capacity of biochar. Finally, this review outlines current limitations and proposes future directions for optimizing biochar performance and promoting its large-scale application in sustainable environmental management.

1. Introduction

The rapid population growth, industrialization, and food consumption habits have contributed to a continuous increase in kitchen waste, emerging as a significant global environmental issue [1,2,3]. Kitchen waste is considered as a form of food waste and primarily includes household organic residues such as fruit and vegetable peels, skins, stems, and other discarded parts [4,5]. It also commonly contains eggshells, spoiled food, and leftover meals generated during food preparation and consumption [6]. In addition, municipal solid waste often includes food scraps, further contributing to the kitchen waste load [7]. The major problem with kitchen waste is that it cannot be effectively incinerated to reduce its volume and mass due to its high moisture content [8]. Additionally, the high water content present in kitchen waste makes the transportation of this type of waste more challenging. Due to the continuous increase in population, food consumption habits have intensified, resulting in a corresponding rise in kitchen waste [9,10]. Another problem that arises with modernization and industrialization is the release of large amount of organic contaminants, heavy metals, and dyes into the environment, creating several challenges to the need for clean drinking water [2,11]. Therefore, there is an urgent need to mitigate these contaminants from the environment, as they are considered primary pollutants. Kitchen waste, on the other hand, is considered a secondary pollutant and is a rich biomass resource that can be transformed into biochar. The contamination of soil and water is a major problem that poses risks to environmental as well as human health [12,13]. Due to a large amount of chemicals released from the industries, water and soil get contaminated, which affects the environment badly. Moreover, a large amount of food waste is generated in India per day due to an increase in heavy demand for food. This waste has been utilized during the preparation of biochar for removal of contaminants. In the past, contaminants were removed using chemical methods, which again caused pollution into the environment due to usage of toxic chemicals. There are various physical, chemical, biological, and thermal methods, including reverse osmosis and ultrafiltration methods, have been utilized for the removal of emerging contaminants, which are categorized based on the type of pollutants [14]. These methods have their disadvantages which limit their applicability. To overcome these challenges, adsorption is the most commonly utilized process nowadays due to its high efficiency, broad applicability, eco-friendliness, and low energy requirement [15,16]. Adsorption is used more than any other method of remediation since it is efficient, cost-effective, and eco-friendly [17]. Among the different adsorbents, biochar exhibits promising results owing to their high surface area, tunable porosity, and large number of available functional groups, which makes it highly effective for pollutant removal. To prevent further environmental damage, the utilization of kitchen waste for pollutant removal has gained significant attention. To address these issues, kitchen waste is now being utilized as a precursor for biochar preparation, which offers a sustainable and effective solution. Biochar is a well-known carbon-rich material produced by the pyrolysis of biomass under controlled atmospheric conditions [18,19]. It is considered a carbon-rich porous material obtained through thermal decomposition at different temperatures [20]. Biochar has a high surface area, tunable porosity, and functional groups, making it effective for the adsorption of various pollutants [21]. Several studies have investigated the use of kitchen waste as a natural precursor for the removal of contaminants from the environment [22,23,24,25]. However, the majority of research has been conducted on the agricultural waste-, wood waste-, and electronic waste-based feedstock for biochar production [26,27,28]. In comparison, the potential of kitchen waste-derived biochar remains underutilized and less systematically reviewed. Several comprehensive reviews demonstrated the use of food waste and kitchen waste for the production of biochar. For instance, the study by Pradhan et al. systematically reviewed the production of food waste-derived biochar and their applications in soil water remediation [29]. The study by Yu et al. provides valuable insights into catalytic potential. Authors provide an overview of applications of KWBC for adsorption, energy materials, and soil amendment [7]. In another study, Rex et al. propose an integrated approach for the conversion of kitchen waste. The study evaluates the technological readiness level of biochar production by combining greenhouse solar drying with microwave pyrolysis [30]. In contrast, our review narrows the focus specifically to KWBC; the present review specifically emphasizes the role of KWBC in soil amendment and water remediation, with particular attention to modification strategies and interactions with diverse pollutants. Although research on biochar for environmental remediation is gradually increasing, there is limited discussion on the role of physicochemical modifications and the interaction of KWBC with specific environmental pollutants such as pesticides, heavy metals, and dyes. Therefore, this review provides a comprehensive overview of the recent advancements in converting kitchen waste into biochar, focusing on synthesis methods, physicochemical properties, and environmental applications. It further discusses the mechanisms of pollutant removal and highlights the challenges and prospects of KWBC in environmental remediation. This review aims to fulfill the gap by critically analyzing the latest developments in synthesis strategies, structural properties, and pollutant removal mechanisms. By addressing these aspects, this review paper offers valuable insights for researchers seeking sustainable approaches for waste valorization and pollution control.

2. Characteristics of Kitchen Waste and Its Potential for Biochar Production

Kitchen waste has high organic content and is a readily available and underutilized biomass resource for biochar production [31]. Kitchen waste includes peels, leftover food, vegetable scraps, and spoiled items [32]. Nowadays, kitchen waste is utilized frequently, which not only addresses the main issue of solid waste management but also contributes to a greener step towards circular economy. Decomposed kitchen waste affects human health as it leads to carcinogenic diseases and can also affect the nervous system, kidneys, as well as liver [12]. Furthermore, during landfilling practices, the emission of harmful pollutants such as sulphur dioxide and other harmful gases are released [33]. Figure 1 demonstrates the primary sources of kitchen waste sources and their conventional disposal pathways. The study by Vianna and Polan highlighted the risks associated with the unmanaged risk. Authors reported that people who are living near the dump yards face risks during childbirth [34]. These complications arise due to the exposure of harmful vapors from dump yards and contamination in the nearby sites of living area [34]. The newborn child faces the risk of lower weight, birth defect rates, miscarriage, and other complications. This highlights the importance of utilizing kitchen waste in controlled valorization processes. Researchers nowadays focus on these kinds of issues and are taking steps toward the utilization of waste to reduce the load on landfilling. Food waste, which largely overlaps with kitchen waste, contains various components such as cellulose, hemicellulose, and fats, which require specialized treatment methods. In the past, landfilling and incineration were common practices of managing food waste, which caused various disadvantages. Landfilling causes damage to human, animal, as well as environmental health [35]. At landfilling sites, animals eat waste food, which worsens their condition. Also, incineration near plant areas causes damage to them and is costly. Moreover, the burning of food waste releases toxic pollutants and heavy metals. Therefore, recycling of kitchen/food waste is an effective way towards a cleaner environment. A thermochemical process, pyrolysis, is an effective method for managing the waste [29]. It reduces a large amount of waste volume by using them as feedstock. Pyrolysis provides flexibility in operating parameters to prepare materials according to needs [29]. Studies have demonstrated that kitchen waste serves not only as an abundant and low-cost feedstock but also as a strategic input for sustainable environmental management.

General Synthesis Methods of Biochar

The physicochemical properties of biochar depend on the type of feedstock utilized during the preparation of biochar. Although in the current study, the focus is mainly on KWBC, it is important to first understand the general synthesis method of biochar. During preparation of biochar, various thermal and non-thermal techniques are involved, including hydrothermal carbonization, slow pyrolysis, microwave-assisted pyrolysis, and co-pyrolysis which are also applied when kitchen waste is used as natural precursor. The physical and chemical properties of biochar are closely related to the type of process utilized for its synthesis, as well as the nature of the feedstock. Different types of biomass feedstocks are treated with specific thermal processes and production techniques, which significantly influence the final characteristics of biochar. Consequently, the performance of biochar in various applications depends on both the production method and the feedstock used. The thermochemical processes employed in biochar synthesis are broadly classified as pyrolysis, hydrothermal treatment, chemical activation, gasification, and others [36,37,38,39,40].

Table 1. General synthesis techniques for the preparation of biochar for environmental remediation applications with their process conditions and advantages.

Sr. No.	Synthesis Technique	Heating Source	Biomass	Process Temperature	Residence Time	Pyrolysis Atmosphere	Activator Type	Key Features	Application/Relevance to KWBC	Targeted Pollutant	Reference
1.	Slow pyrolysis	Thermal, microwave	Kitchen waste, citrus peel fruit waste, peanut shell, cotton stalk	~400 °C	~2–4 h	Limited O2/N2	-	Optimization of temperature, energy efficiency, scalability	Solid biofuel; citrus peel as KWBC precursor	Organic pollutants, heavy metals	[50,51,52]
2.	Fast pyrolysis	Thermal	Crop residue, litchi seeds	~500 °C	~2 s	Inert gas N2	-	High bio-oil yield	Water remediation	Organic compounds	[53,54]
3.	Microwave-assisted pyrolysis	Microwave	Kitchen food waste, sugarcane bagasse, rice husk	~550–700 °C	~5–20 min	N2	-	Highly porous structure obtained, rapid heating	Dye removal, catalysis; common kitchen waste feedstock	Antibiotics, dyes, heavy metals	[55,56,57]
4.	Co-pyrolysis	Thermal	Sewage sludge, walnut shell	350–600 °C	30 min–2 h	N2	-	Synergistic effect, porosity	Soil and water remediation	Heavy metals, hydrocarbons	[58,59]
5.	Hydrothermal carbonization	Autogenous	Kitchen waste	~180–250 °C	1–12 h	Autogenous	-	Enhancement in functional groups, hydrochar	Adsorption, soil improvement; kitchen waste utilization	Heavy metals, others	[60,61]
6.	Flash carbonization	Direct combustion	Lignocellulosic biomass, Albizia odoratissima	~300–600 °C	~30 min	Air/O2	-	Instant ignition, gas rich	Environmental use, energy	Organics, pathogens	[62,63]
7.	Chemical activation	Thermal + chemical agents	Rice straw, orange peel	~500–900 °C	30 min–2 h	N2/CO2	Chemical (KOH, H3PO4, ZnCl2)	Surface area enhancement	Catalysis, adsorption	Heavy metals, dyes	[39,64,65]
8.	Template-assisted synthesis	Thermal + template removal	Food waste	400–800 °C	30 min–2 h	N2	Chemical	Tunable properties	Wastewater treatment; KWBC application	Pharmaceuticals, dyes	[66,67,68]
9.	Ball milling	Mechanical	Wood chips	~30–60 °C	1–48 h	-	-	Increased functionalities	Soil remediation	Metal, organics	[69,70]
10.	Torrefaction	Thermal	Sweet sorghum bagasse, peanut shell, soyabean straw, barley straw	~300 °C	~10–60 min	N2/Limited O2	-	Carbonization at low temperature	Solid fuel, energy	Organics, CO2	[71,72,73]
11.	Gasification	thermal	Whiteoak, pinewood, woodchips	~900 °C	~20 s	Steam/air	-	Syngas production	Soil amendment, carbon sequestration	CO, VOCs	[74,75]

provides a general overview of these synthesis methods, including studies that utilize kitchen waste as feedstock as well as other studies using agricultural biomass for comparative analysis. The inclusion of heating source, synthesis technique, biomass type, residence time, and pyrolysis atmosphere, as well as key features and targeted pollutants provide valuable insights into how different types of biomasses require specific parameters for particular applications. For instance, trends observed in porosity of structure, surface area development, and carbon content in agricultural residue biochar can provide valuable insights for optimizing the properties of KWBC. Biochar produced using different pyrolysis temperatures exhibits different properties due to the utilization of various feedstock and synthesis processes [41,42]. Moreover, biochar is a well-known carbon-rich material; the research conducted in the past reveals that as the pyrolysis temperature increases, the carbon content in biochar also increases [43]. Consequently, with an increase in carbon content, the hydrogen and oxygen contents decrease, which is attributed to a higher degree of carbonization [44]. In support of this, studies demonstrate that weak chemical bonds are present in the biochar structure, which start to break with a rise in temperature from 300 to 500 °C [45,46,47]. During this process, gases such as oxygen and hydrogen are released, resulting in the observed loss of these elements. Although higher temperatures typically promote the formation of more stable, carbon-rich biochar structures, it is important to recognize that the synthesis temperature itself plays a crucial role in determining these transformations [48]. Specifically, an increase in pyrolysis temperature enhances the porosity of the material and increases its surface area only up to a certain limit. Beyond the temperature 1000–1100 °C, graphitization occurs which reduces its specific surface area. Moreover, this decreases overall biochar yield and increases the energy costs. Since surface area is directly linked to the effectiveness of biochar in remediation applications, the temperature is considered a key factor during biochar preparation [49]. Therefore, controlling synthesis temperature is not only important for optimizing the surface area and porosity but also for balancing the cost, yield, and retention of functional groups. It was found that optimization is significant for KWBC due to heterogeneity of feedstock.

3. Physicochemical Properties of Kitchen Waste-Derived Biochar

KWBC exhibits various physicochemical properties depending on its synthesis methods, temperature conditions, biomass composition, and post-modification processes. The key characteristics of biochar involve surface area, porosity, elemental composition, presence of functional groups, surface charge, zeta potential, and thermal stability—each analyzed using different characterization techniques [76,77]. These properties can be tailored to meet specific requirements for targeted environmental applications. Various studies confirmed that these characteristics are influenced by the feedstock type, temperature conditions, and post-synthesis modification processes [78,79]. For instance, Xie et al. [36] demonstrated that with a rise in temperature from 300 °C to 500 °C, the surface area and porosity of KWBC increased due to the release of volatile organic matter and development of micropores. Moreover, this enhancement occurs only up to 600 °C, beyond this temperature range, the surface area starts decreasing due to pore collapse. This shows that higher temperatures do not always result in enhancement of biochar performance. The study also revealed that a rise in temperature results in an increase in carbon content and a simultaneous decrease in H and O content, which is mainly due to the release of gases during carbonization. With high temperature, thermally stable biochar with high aromaticity is formed. This limits the surface functionality and interaction with polar contaminants. Thus, optimization varies the stability of structure and functionality depending on the specific application. Also, Ning et al. reported that phosphorus modification enhanced the surface area of KWBC from 13.11 m²/g to 15.33 m²/g (P-KBC), thereby increasing the active sites available on the adsorbent surface for enhancement in adsorption performance [80]. The decrease in the O/C ratio in P-KWBC was observed, which indicates improved stability and stronger binding capability, while a low H/C ratio confirmed a higher degree of aromaticity. The optimization of zeta potential further showed that P-modified KWBC achieved a stronger negative charge in the pH range 3–8 due to deprotonation of functional groups. P-modification further enhanced negative potential, improving electrostatic adsorption. This reveals the importance of modification to target specific contaminants. In another study by Meilani et al. [81], the authors prepared Al-modified KWBC which revealed the surface area enhancement of prepared composite had a rise in temperature from 4.50 m²/g to 20.95 m²/g. The crystalline structure of AlF₃·3H₂O and NaF formation was confirmed using XPS in Al-FWB. Moreover, the formation of inner-sphere complexation was identified by the presence of Al–F bonding. These modifications enhance the performance of biochar but introduce environmental risks of metal leaching. Therefore, there is an urgent need to work on the long-term stability of the material. In another study, Xu et al. also reported that surface area of KWBC increases between 300 and 500 °C due to thermal decomposition, which promotes the formation of pore structure [50]. At 600 °C, pore collapse reduced surface area. Therefore, tailoring the physicochemical properties of KWBC through controlled pyrolysis conditions and surface modification is essential to enhance the adsorption performance for environmental remediation applications. Overall, these studies highlight that synthesis conditions and modifications enhance the physicochemical properties of KWBC. Future work will not only focus on achieving high adsorption efficacy but will also work on the metal leaching and pore collapse risks.

Table 2. Physicochemical properties of kitchen waste-derived biochar.

Sr. No.	Feedstock Type	Pyrolysis Temperature(°C)	Surface Area (m2/g)	Zeta Potential	Functional Groups Identified	Performance	Reference
1.	Kitchen waste	500	2.57	-	–OH, –COOH	High stability	[82]
2.	Food waste	300	68.555	-	–OH, –COOH	Synergistic effects of the ultrasound and magnetic biochar	[83]
3.	Kitchen waste + P	500	15.33	Negative (pH 3–8)	Phosphorus-related groups	Stable due to lower O/C and H/C ratios	[80]
4.	Kitchen waste	300–500	-	-	–OH, C=O	With temp. aromaticity increases	[36]
5.	Al-modified KWBC	315	20.95	Negatively charged	Al–F bonds	XPS confirmed inner-sphere bonding	[81]

is prepared based on the above-cited studies to illustrate the key physicochemical characteristics of KWBC.

4. Applications of Biochar Beads in Soil and Water Remediation

The continuous increase in population poses a significant burden on waste management systems, as they generate large quantities of kitchen waste every day. This type of organic waste is often difficult to degrade naturally because of the presence of high water content and, if not managed properly, poses serious environmental risks. Traditionally, landfilling, like disposable methods, contributes to the accumulation of waste in the ground, which can harm animals and reduce soil fertility due to leachate contamination and nutrient imbalance. Similarly, incineration releases harmful gases, contributing to air pollution and greenhouse gas emissions. Therefore, there is an urgent need to adopt sustainable strategies for kitchen waste utilization. Among the various methods of disposing of kitchen waste, converting it into biochar has emerged as a promising solution. Biochar prepared using kitchen waste addresses disposal challenges and also serves as an effective material for environmental remediation. Biochar has shown high potential in improving soil health through contaminant immobilization and nutrient enrichment, as well as in purifying polluted water. Thus, the valorization of kitchen waste into biochar represents a viable pathway toward sustainable environmental management.

4.1. Water Remediation

A large portion of the population lacks access to clean drinking water. Over the past few years, growing water stress has shifted societal perspectives of wastewater from viewing contaminated water as waste to recognizing it as a valuable resource. In this context, biochar has emerged as an effective option owing to its unique properties relevant to wastewater treatment techniques. Water is contaminated by heavy metals, pharmaceutical residues, dyes, and other emerging contaminants, which are necessary to be degraded. The use of KWBC for wastewater treatment has shown promising results owing to its unique properties, including high surface area, functional groups, and its potential for further modification. Several studies have been evaluated to explore the potential of KWBC to decontaminate wastewater. For instance, Moureen et al. work on biochar production using KW for the removal of Pb²⁺, Cr²⁺, and Cd²⁺ ions by varying temperatures from 350 °C, 450 °C, and 550 °C, respectively [84]. In contrast, the study revealed that with temperature rise, biochar yield decreases from 18.4% to 14.31%; simultaneously, ash content increases from 39.87% to 42.05%, and the biochar pH becomes more alkaline. The study reported that biochar prepared using different temperatures effectively removes all the targeted heavy metals, but the biochar prepared using 550 °C temperature shows high removal capacity. Xu et al. [50] worked on the utilization of kitchen waste for biochar production and optimized the pyrolysis temperature to evaluate its performance. The increase in pH of KWBC is associated with an increase in its ash content. Additionally, a rise in pyrolysis temperature results in a reduction in the H/C and N/C ratios. The study reveals that the biochar exhibited Cd(II) adsorption capacities of 18.2 mg/g at 300 °C, 23.6 mg/g at 400 °C, 46.5 mg/g at 500 °C, and 49.0 mg/g at 600 °C, indicating the influence of temperature on the biochar performance. The biochar produced at 500 °C was found to be the most efficient for the removal of Cd(II) ions. The study considered temperature as an important parameter due to the occurrence of variation in the specific surface area of biochar. The studies have shown that within the temperature range of 300–500 °C, the surface area of KWBC tends to increase with increasing temperature. The improvement in the specific surface area of biochar is attributed to decomposition of organic matter, which further decomposes and is released in the form of gas. The study also observed that after 600 °C, the biochar area decreases. At high temperatures, the pore structure of biochar collapses, resulting in a decrease in specific surface area. The biochar prepared using pyrolysis at 500 °C exhibits a porous structure and a high surface area. Also, in the study by Feng et al., they prepared MgO-coated biochar derived from tea waste for the removal of phosphorus in wastewater treatment [85]. The study utilized the template elimination method for the preparation of tea waste-derived biochar composite. The results revealed that the adsorption efficacy of 192.8 mg g⁻¹ was achieved using a solution of pH 9, which was fermented liquid, and the removal efficacy of 58.80 mg g⁻¹ was achieved using a solution of pH 7. It was clearly observed that pH plays a crucial role in the removal rate, as pH is directly proportional to the removal efficacy of phosphorus solution. Under acidic conditions from pH of solution 3 to 4, the removal efficiency increases from 85.55% to 96.83%. Simultaneously, the removal efficiency of fermentation liquid increases from 95.00% to 99.52% between pH 3 to 9. This mainly happened due to the pH effect on surface properties of phosphate species and biochar. This indicates that modification of operation pH condition is important to achieve maximum removal rate. Moreover, this fact raises concerns towards the real-world applications in which wastewater fluctuates the pH level. In regard to this, future research should focus on the performance of KWBC in real wastewater instead of work in laboratory-controlled conditions. Another study by Feng et al. states that an increase in ionic strength reduces the removal efficiency from 98.53% to 93.01%. On the other hand, in the fermentation liquid, no change is observed even after increasing the ionic strength of the solution [85]. It was found that the electrostatic attraction, ligand exchange, and precipitation mechanisms play the main role in understanding the adsorption of phosphorus from prepared and fermented liquid, which exhibits the main role of the ligand exchange mechanism in their study. Moreover, the stability of the prepared composite was found to be stable for up to five cycles. On the other hand, Xing et al. prepared biofilm-attached biochar using food waste as a precursor for the removal of cadmium (Cd) and lead (Pb) from wastewater [86]. In this study, biochar was modified by growing biofilm onto the biochar to enhance the pore size and adsorption performance of metal ions. The authors utilized 30 mL of 0.1 M HCl for the desorption of cadmium ions. The adsorption capacity of the adsorbent remained stable up to seven cycles. The biofilm-attached biochar was grown again before each cycle. The significance of biochar attached with biofilm revealed higher adsorption capacity potential over pristine biochar of Cd²⁺ and Pb²⁺ ions. The performance of BAB was due to a rise in metal ion concentration, which is attributed to the fact that metal availability in the solution is a key controlling factor for sorption. Also, with rise in pH value of solution, the adsorption efficiency also increases. Also, Niu et al. reported the production of KWBC for the degradation of methyl orange (MO) and tetracycline (TC) from wastewater [11]. In this study, biochar was synthesized via thermal treatment of kitchen waste at 400 °C and used to modify BiOCl and BiOBr via a solvothermal method, resulting in KBC/BiOCl and KBC/BiOBr composites. The BiOX materials exhibited a flower-like morphology composed of 50 nm nanosheets with diameters of 3–5 μm. To remove the MO and TC, a photocatalytic composite 0.15KBC/BiOX has been utilized. The prepared BiOCl/BiOBr nanostructures enhance the photocatalytic activity, and KWBC acts as a supporting material to improve the stability and active sites availability in nanocomposites. In the study by Jae-Hun Chu et al., biochar was prepared using food waste as a natural precursor for the degradation of methylene blue (MB) and methyl orange (MO) [83]. The study utilized a pre-pyrolysis process for the synthesis of biochar, and iron oxide was also utilized to improve the efficiency of biochar. The magnetic biochar containing maghemite was used as a catalyst for the Fenton-like processes to remove MO and MB dyes. The removal efficiency was maintained by up to 84% for methylene blue; on the other hand, for methyl orange, it was reduced to 10%. Utilizing the US-assisted H₂O₂–MC composite, 90% and 100% of MB was removed after 30 and 60 min, respectively. In contrast, after 180 min, almost 95% of MO was removed. The study by Meilani et al. prepared biochar derived from food waste modified with aluminum to improve the degradation potential of fluoride ions [81]. At the pH value of 7.1, the adsorption capacity reached 123.4 mg/g, resulting in high removal efficacy up to 91.4%. Also, the adsorbent surface area increases up to 20.95 m²/g with an increase in temperature. These studies highlight the potential of KWBC and its modified forms for the removal of wide variety of pollutants. However, a major issue with these studies is that most studies have been performed only at lab scale under controlled laboratory conditions, which would not be applicable for real wastewater systems. To truly assess the scalability and applicability of KWBC, future research should focus on testing it in real wastewater with complex pollutant mixtures and fluctuating pH levels.

4.2. Soil Remediation

Soil stability, carbon sequestration, improvement of soil structure, enhancement of microbial activity, as well as adsorption and ion-exchange mechanisms are some of the fundamental characteristics that make biochar effective for soil remediation. Numerous studies have reported biochar production using various types of kitchen waste to improve the soil quality. The utilization of KWBC has shown promising results in enhancing soil properties. It possesses good water-holding capacity and is rich in essential nutrients. Furthermore, studies have demonstrated that biochar prepared from kitchen waste can effectively immobilize toxic metals, which decrease mobility and reduce their availability for plant uptake. In addition, it promotes soil aggregation and reduces soil compaction, allowing better root penetration and plant growth. Various researchers have explored the potential of KWBC for soil remediation. For instance, Kumar et al. utilized biochar for arsenic removal for soil remediation. The biochar was prepared using pyrolysis at varying temperatures from 300 °C to 700 °C [82]. In this study, biochar was prepared using three different precursors, including rice, wheat straw, and kitchen waste, to compare the adsorption performance towards arsenic. The prepared biochar showed a surface area of 15.8 m² g⁻¹ for wheat straw-derived biochar, 12.5 m² g⁻¹ for rice straw-derived biochar, and the surface area of 2.57 m² g⁻¹ for kitchen waste-derived biochar. The study exhibits that maximum removal of arsenic was achieved using 8 mg L⁻¹ of KWBC prepared at 500 °C temperature with a contact time of 60 min. In another study, Xu et al., in 2020, prepared biochar using different precursors, including (i) kitchen waste (KW), (ii) corn straw, and (iii) peanut hulls, to observe the immobilization of Pb and Cd in the contaminated soil [87]. The authors conducted batch experiments to evaluate the ability of KWBC to immobilize Cd and Pb in contaminated soil. The study observed that all the prepared biochars increased the pH of the soil, which further reduced the mobility of Pb and Cd metals. When kitchen waste-derived biochar was used, it reduced Pb by 71.01% and Cd by 22.61%. Similarly, corn straw-derived biochar reduced Pb by 64.35% and Cd by 18.54%. Also, when the Peanut hull-derived biochar was used, it reduced Pb by 64.35% and Cd by 18.54%. This demonstrates that Pb and Cd ions can be significantly reduced by using prepared biochar. The concentration of biochar is directly proportional to the reduction in Cd and Pb in plant parts. As the study reveals, when using 60 mg/kg of biochar, Cd in roots decreased up to 97.68%, and Pb in stems decreased by 96.64%. Among all the prepared biochar, the KWB showed the highest immobilization performance due to its favorable physicochemical properties.

The study by Zhu et al. illustrated the preparation of Layered Double Hydroxide (LDH)-modified biochar using kitchen waste fermentation residue for soil decontamination [88]. LDH is a well-known nanomaterial with high adsorption capacity and structural stability. The study utilized a solvent-free co-precipitation method to produce a functionalized (Mg/Fe) biochar composite. Furthermore, the Mg/Fe-LBC possessed a specific surface area of 61.98 m²/g along with a highly porous structure. Biochar acted as a carrier and showed great potential for soil remediation. In this study, over 90 days, the concentration of Cd in the soil was reduced by 77.68% due to a synergistic mechanism. Such long-term immobilization performance shows promising results, but field-scale studies under varying soil conditions are still lacking. Therefore, moving from laboratory-scale findings to practical soil remediation requires further field validation.

Table 3. Summary of various kitchen waste-based materials, their synthesis methods, modifications, and applications for the degradation of organic contaminants, dyes, and metal ions from contaminated water and soil.

Sr. No.	Material	Synthesis Method	Pollutants	Catalyst Dosage (g/L)	pH	Mechanism	Adsorption Capacity (mg/g)	Degradation Time	Reference
1.	Kitchen waste-derived biochar	Pyrolysis	Pb(II)	-	-	Complexation and precipitation	257.95 mg/g	240 min	[80]
2.	MBC600	Pyrolysis	Cu2+, Pb2+ and Zn2+	5 g/L	-	Ion exchange and complexation	8925.5 mg/g, 32,177.6 mg/g, 5652.4 mg/g	60 min for Zn2+	[89]
3.	Aluminum-modified food waste biochar	Pyrolysis	Fluoride	3.33 g/L	7.1	-	123.4 mg/g	39 min	[81]
4.	KWB500	Pyrolysis	Cd (II)	1.7 g/L	6	Precipitation, complexation, ion exchange	46.5 mg/g	-	[50]
5.	H3PO4-modified tea branch biochar	Pyrolysis	Cd2+ and Pb2+	2 g/L	6	Complexation and precipitation	98.25 mg/g, 127.5 mg/g	90, 60 min	[90]
6.	Biofilm-attached biochar	Pyrolysis	Cd, Pb	-	6	Electrostatic adsorption, complexation	-	120 min	[86]
7.	Magnetic biochar containing maghemite (γ-Fe2O3/biochar)	Heterogeneous sono-Fenton-like process	Methylene blue, methyl orange	2 g/L	7	Fenton-like reaction, ultrasonic cavitation	-	60 min	[83]
8.	KBC/BiOX(X = Br, Cl)	Solvothermal+ ultrasonication	Methyl orange, tetracycline	-	-	Charge separation and transfer, reactive oxygen species, light absorption	-	20 min (MO), 60 min (TC) for 0.15KBC/BiOBr; 35 min (MO), 60 min (TC) for 0.15KBC/BiOCl	[11]
10.	Tea waste biochar	Carbonization	Phosphate	-	9	Ligand exchange, precipitation	192.8 mg/g	720 min	[85]
11.	KWB-500	Pyrolysis	Arsenic	0.008 g/L	6.5–7.0	Chemisorption, physisorption, diffusion, and ion-exchange soil	11.3 mg/g	60 min	[82]
12.	Food waste-derived biochar	Pyrolysis	Pb2+, Cr2+, Cd2+	2.5 g/L	8	-	-	120 min	[84]
13.	Kitchen waste-derived biochar	Pyrolysis	Cd2+, Pb2+	0.060 g/L	9.91	Complexation	-	-	[87]

summarizes recent studies demonstrating the application of KWBC and its modified forms to decontaminate the water and soil, along with associated synthesis methods, operating conditions, and removal mechanisms.

5. Mechanism of Pollutant Removal in Water and Soil Remediation

The removal of various contaminants from wastewater using kitchen waste-derived biochar involves a combination of chemical and physical mechanisms. The removal mechanisms vary according to the type of pollutants and surface properties of biochar. Kitchen waste-derived biochar involves ion exchange, electrostatic interaction, complexation, precipitation, physisorption, and chemisorption mechanisms. Moreover, studies utilized advanced modification methods like biofilm attachment, hybrid composite formation, and metal doping to enhance the adsorption and degradation efficiency towards dyes, heavy metals, and other organic contaminants. A literature survey has been performed to find out the various mechanisms involved in the study. For instance, the study by Niu et al. investigated photocatalytic mechanisms and determined photogenerated holes (h+), hydroxyl radicals (•OH), and superoxide radicals as the main reactive species working in the degradation process [11]. The active species are identified using scavenger experiments in which the quenchers are as follows: EDTA-2Na, which quenches H+; IPA, which quenches hydroxyl radicals; and BQ, which quenches •O₂⁻. Moreover, KBC facilitates effective charge separation, which also enhances proton (H⁺) activity. Under visible light irradiation, photogenerated electrons are excited from the valence band to the conduction band of BiOX, leaving holes (h⁺) in the valence band. The charge transfer and photocatalytic degradation mechanism under visible light irradiation has been shown in Figure 2a. Under visible light, photocatalytic reactions occurred, which are as follows:

• KBC/BiOX + hv → KBC/BiOX* (e⁻ + h⁺)
• h⁺ + H₂O → •OH+ H+
• Methyl orange or tetracycline + H+ → CO₂ + H₂O + intermediates
• Methyl orange or tetracycline + •OH → CO₂ + H₂O + degradation products

Also, Ning et al. prepared KWBC modified with phosphorus to decontaminate lead-contaminated soil [80]. The usage of KW as a natural precursor offers an abundance of K⁺ and Na⁺, which promotes the mesoporous structure of biochar with an increase in pores of sizes varying from 2 to 5 nm in the P-KBC. The specific surface area of P-modified biochar increases up to 15.33 m² g⁻¹ from 13.11 m² g⁻¹. Due to an increase in surface area, binding sites increase on the adsorbent surface, which enhances adsorption capacity. After the biochar was mixed with lead-contaminated soil, the distribution of lead in different chemical forms in soil was evaluated using ANOVA analysis. The elemental analysis showed a decrease in the O/C ratio in P-KBC, which indicates higher stability and strong binding with metals. The low H/C ratio exhibits more aromatic character, which improves the stability of the material during soil remediation. As illustrated in Figure 2b, the influence of biochar on soil water retention is significant.

Various isotherms are performed to find out the mechanisms involved during the removal/detection of contaminants. These mathematical models are generally used to find out the relationship between the amount of adsorbate adsorbed on the adsorbent surface. In the study of Abhishek Kumar et al., several isotherm models, including the Langmuir isotherm model, the Freundlich model, the Dubinin–Radushkevich model, and the Temkin isotherm has been performed [82]. The study reveals that the Langmuir adsorption isotherm fits best for all the prepared samples. The adsorption studies are useful to know about the interaction that occurs between the surface of adsorbent, which can be determined by adsorption isotherms, adsorption kinetics, and adsorption thermodynamics. There are three kinetics models; (i) pseudo-first-order, (ii) pseudo-second-order, and (iii) Weber–Morris models, which describe the adsorption potential in removal of arsenic using biochar. The study demonstrates that the adsorption of arsenic follows the pseudo-second-order kinetic model, which suggests that the adsorption process is governed by chemisorption mechanisms. Likewise, Kumar et al. studied the adsorption of arsenic onto the biochar surface, which was associated with the pseudo-second-order model owing to electrostatic interactions and ion exchange, as shown in Figure 3b [82]. The characterization of prepared material reveals the chemisorption mechanism followed during the process. The adsorption studies are categorized into isotherms, kinetics, and thermodynamics. The adsorption studies are well known to provide a mechanistic view of how pollutants interact with adsorbent materials. The relationship between the amount of adsorbate captured by the adsorbent has been evaluated by adsorption isotherms. Moreover, kinetics models assess the rate of adsorption and identify the surface adsorption or pore-diffusion-like rate-controlling steps. Also, thermodynamic parameters play a key role in determining the adsorption process. The parameters like Gibbs free energy indicate the spontaneity of reaction, enthalpy evaluates any heat change, and entropy determines the disorder associated with the process. These parameters are essential to determine the adsorption mechanism.

The study by Xu et al. in 2021 demonstrated that adsorption of Cd(II) ions occurs mainly through an ion-exchange mechanism [50]. The removal of Cd(II) ions is influenced by processes such as complexation, π–electrons coordination, exchange with cations like Na⁺, K⁺, Ca²⁺, and precipitation. Likewise, Feng et al. reported that the enhancement in phosphorus adsorption mainly happens due to the influence of pH, which affects the surface properties and the phosphate species [85]. The adsorption from the synthetic solution takes place through mechanisms like electrostatic interactions, ligand substitution, and surface precipitation. In contrast, when interacting with the fermentation liquids, the ligand exchange is the predominant pathway for adsorption. The mechanistic representation of MgO-coated tea waste biochar is illustrated in Figure 3a.

In the study of Zhu et al., the remediation process involved surface complexation, adsorption, precipitation, and cation–π bond conjugation [88]. Additionally, the study by Xing et al. reveals that electrostatic adsorption and complexation are the primary mechanisms followed during the removal process [86]. Moreover, the adsorption behavior of biochar-based materials occurs through multiple phases. Initially, a two-stage adsorption process was observed, with rapid adsorption occurring within the first 120 min, likely attributed to monolayer adsorption. This was followed by a slower phase in which chemical interactions occur between the active and adsorbate on the surface of the adsorbent. The increased pH improves the uptake capacity of Cd²⁺ and Pb²⁺ ions owing to the deprotonation of surface functional groups such as carboxyl and phenolic oxygen, which act as active binding sites. The study reveals that surface interactions in the modified biochar composite are temperature-sensitive. As the adsorption of unmodified biochar increases with a rise in temperature, it slightly reduces the capacity of BAB above 28 °C. The kinetic analysis revealed that the adsorption process followed the pseudo-second-order model, indicating chemisorption as the rate-limiting mechanism, governed by chemical binding and intraparticle diffusion. IR spectroscopic analysis provided mechanistic insights, showing that functional groups like methylene groups and hydroxyl groups actively participated in binding both metal ions. Additionally, in the case of Cd²⁺ ions, participation of groups like C–O–C, N–H, C–N, and C=O was detected. This indicates that while Pb²⁺ may dominate in competitive ion scenarios, Cd²⁺ can still be efficiently removed at lower Pb²⁺ concentrations due to its interaction with multiple functional groups. The study by Chu et al. revealed an increase in removal efficiency with an increase in H₂O₂ concentration and ultrasonic (US) power [83]. The reaction occurred due to the synergistic effect of US and MC on the decomposition of H₂O₂ into •OH radicals in the US–H₂O₂–MC system, illustrated as follows:

H₂O₂ + US → 2HO•

Fe²⁺ + H₂O₂ → Fe³⁺ + HO• + HO⁻

Fe³⁺ + H₂O₂ → Fe²⁺ + HO₂• + H⁺

Fe³⁺ + HO₂• → Fe²⁺ + O₂ + H⁺

Organic dyes + HO• → Degraded products

This mechanism shows how the Fenton-like process and ultrasound treatment accelerate the generation of reactive radicals responsible for dye degradation. Additionally, the study concluded that at low pH values (up to pH 3) for MO, the removal efficiency was maintained, as carbonate and bicarbonate ions convert into carbonic acid, which is less reactive with hydroxyl radicals. Also, in the study by Ning et al., the surface charge property of the prepared material using zeta potential indicated that both the pristine biochar and phosphorus-modified biochar exhibit negative potential on the material surface in the pH range of 3–8 [80]. The negative potential indicates the presence of negatively charged sites on the adsorbent surface, primarily resulting from the deprotonation of surface functional groups. Moreover, P-modified KBC shows higher potential than pristine KBC, which was observed by absolute zeta values under the same pH condition. This generally happens because the phosphorus group makes the adsorbent surface negatively charged, which enhances electrostatic repulsion. The immobilization of Pb²⁺ in contaminated soils follows synergistic mechanisms including complexation, electrostatic attraction, and mineral precipitation, with surface complexation helping in the removal of contaminants from soil. Also, in the study of Meilani et al., the mechanism of fluoride removal using aluminum-modified food waste biochar (Al-FWB) has been investigated via X-ray photoelectron spectroscopy (XPS), which confirmed the formation of characteristic peaks [81]. Specifically, peaks corresponding to AlF₃·3H₂O and NaF were indicated by F1s peaks at 686.3 eV and 684.5 eV, respectively. Moreover, the removal of fluoride is not governed by electrostatic attraction, as both the adsorbent surface and fluoride ions are negatively charged. Additionally, outer-sphere complexation is not involved in the process, since it is independent of both pH and ionic strength. Instead, the removal of fluoride ions occurs through precipitation with aluminum species such as AlF₃·3H₂O and inner-sphere complexation. Furthermore, the study reveals that bare biochar exhibits a low adsorption rate of 0.62 mg/g, indicating that aluminum plays a key role in the binding of fluoride. The surface adsorption is significantly enhanced due to the formation of Al-fluoride complexes, attributed to the high aluminum content in Al-FWB. In contrast to the prepared composite, bare biochar demonstrates a low potential for fluoride removal, primarily because its oxygen-containing functional groups possess lone pairs that are not suitable for fluoride adsorption. The effective adsorption requires the protonation of oxygenated functional groups, which is possible only at pH levels below 2.5. The study shows that the fluoride adsorption onto the surface of Al-FWB follows the pseudo-second-order kinetic model. According to this model, chemical bonding is the main process followed during the mechanism, as it involves the exchange of electrons between the Al-modified biochar and fluoride ions. In this study, a strong bonding formed between the Al–F bonds as compared to the weak van der Waals forces of interaction. Furthermore, the utilization of biochar plays a major role in soil remediation derived using kitchen waste. The synergistic interactions among the soil, microbes, plants, and biochar play a key role in the enhancement of plant growth and productivity. Figure 4a illustrates multifunctional amendment of biochar that alters the physicochemical properties of soil, which supports nutrient improvement and water availability in plants. Also, the study by Xu et al. illustrates the mechanistic representation of immobilization of Cd and Pb using biochar and its influence on Cd and Pb uptake in swamp cabbage is shown in Figure 4b [87]. Owing to the porous structure of biochar, a conductive microhabitat for plant growth has been created, which therefore exhibits a positive response towards the soil that supports sustainable plant growth under various experimental conditions.

6. Experimental Parameters

The potential of kitchen waste-derived biochar in removing contaminants from water and soil is significantly influenced by various experimental parameters. Parameters such as pH, temperature, initial concentration, adsorbent dosage, and contact time play an important role in improving the adsorption capacity and removal efficiency. Understanding these parameters is essential for scaling up the studies for industrial applications. This section discusses the impact of these parameters as reported in recent studies to provide insights into their role in enhancing the performance of kitchen waste-derived biochar for environmental remediation.

pH: The pH of the solution is an important parameter that plays a crucial role in the sorption process. Additionally, it highly influences the adsorption capacity of metal ions. In contrast, previous studies have thoroughly investigated the changes that occur at high or low pH values. Consequently, most of the studies reveal that the sorption of heavy metals decreases with an increase in pH value. For instance, in the study of Moureen et al., the pH of the solution varied from 2 to 5. The adsorption performance was noticed at low and high pH values [84]. The study reveals that (i) at low pH value, concentration of H⁺ ions is high, which tends to decrease in adsorption rate, and (ii) at high pH values, all the protons become deprotonated and thus the adsorption performance increases from pH of 2 to 8. In contrast, the decrease in adsorption rate is attributed to the fact that at low pH values, all the active sites are occupied by protons. In a nutshell, the biochar’s potential for adsorption of pollutants is highly influenced by the solution pH. In this study, KWB has a higher pH, which leads to high ash contents of alkali and alkaline earth metals. With the addition of Na, K, Ca, and Mg present in the kitchen residues, the functional group rearrangement increases in pH value. The increase in pH reduces the competition for the adsorption of H⁺, which releases binding sites. The overall performance of biochar transforms Cd and Pb into stable forms, which reduces the metal uptake by swamp cabbage and improves the quality of the soil. It was reported that at higher pH values, the carbonates, metal oxides, and organometallic compounds formed more easily (2020) [87]. The study by Meilani et al. demonstrates the effect of pH on removal of fluoride ions by optimizing the pH range from 3 to 11, using a concentration of 300 mg/L for 24 h with continuous shaking [81]. The study achieved an adsorption capacity of 70.86 mg/g within the pH range of 5 to 11. In contrast, within the pH range of 3 to 5, an increase in adsorption capacity was observed, which can be attributed to the formation of hydrofluoric acid. Since it is weakly ionized, it is less likely to interact with the adsorbent surface. In another, the pH of the solution played an important role in determining the efficiency of Fenton-like processes. The study optimized the pH of the solution from 3 to 11, observing that with an increase in pH from 3 to 11, the removal efficiency of MO declined from 100% to 95% due to reduced iron solubility and the oxidative potential of hydroxyl radicals. On the other hand, for MB, pH did not significantly affect removal efficiency, as it was reduced continuously under all pH conditions [83].

Temperature: The authors studied the temperature effect during the removal of arsenic using prepared biochar. In this study, the removal rate started decreasing with a rise in temperature beyond 25 °C. Initially, an increase in adsorption was observed with an increase in temperature, then the adsorption efficacy decreased with a further rise in temperature. This mainly happens due to weakened interactions that occurred from damage in binding sites. The maximum removal rate achieved at 25 °C for WSB500 which was 83.7 ± 0.52%, 80.1 ± 0.09% for RSB500, and 75.4 ± 0.16% for KWB500, respectively [82]. The study by Kataya et al. prepared biochar from kitchen waste for the removal of methyl orange and methylene blue from the wastewater [91]. The authors found that dye removal efficiency is highly dependent on pyrolysis temperature. The biochar prepared using higher temperatures facilitated a hydrophobic nature, which favored the adsorption of non-polar dyes through π–π and hydrophobic interactions with the loss of oxygen-containing groups. In contrast, low temperature biochar was less aromatic and retained more –OH and –COOH groups, where hydrogen bonding and electrostatic interactions facilitated the removal of polar organic dyes. As discussed in previous section, Xu et al. reported an increase in ash content and pH with a rise in pyrolysis temperature from 300 to 600 °C but observed decrease in yield and H/C and N/C ratios [50]. The authors varied the temperature and observed the maximum removal at 500 °C due to increased surface area and pore development. However, further increasing the temperature to 600 °C resulted in decreased adsorption efficiency due to pore collapse. Moreover, other studies show that the highest removal rate was achieved at higher pyrolysis temperatures and longer heating times due to increased surface area, porosity, and the formation of aromatic carbon structures. These conditions enhanced the adsorption efficiency of biochar [92,93]. In conclusion, temperature plays critical role in determining the adsorption performance of KWBC with varying conditions balancing the stability of structure and surface area development.

Initial concentration: The initial concentration of any pollutant also affects the adsorption potential of biochar. To illustrate, Kumar et al. investigated the effect of the initial concentration of arsenic (As) on adsorption performance of prepared biochar by varying its concentration. The concentration of As was varied from 0.5 to 14 mg L⁻¹ to observe the maximum removal rate of KWBC [82]. Furthermore, the removal efficiency of KWBC at temperature 500 °C was found to increase as the initial concentration of arsenic increased. The maximum adsorption capacity of KWBC was 78.5 ± 0.22%, which was attained at 8 mg L⁻¹, respectively.

Adsorbent dosage: Several studies have investigated the effect of adsorbent dosage by varying the amount of biochar in adsorption experiments to determine the optimal amount of adsorbent to achieve the maximum removal rate. For instance, the study by Chu et al. evaluates the removal efficiency at different concentrations under sonocatalytic conditions [83]. Initially, the concentration was increased from 0.1 to 10 g/L using 200 mM H₂O₂ with a catalyst concentration. An increase in catalyst dosage led to a significant enhancement in the removal efficiency of methylene blue (MB), increasing from 65% at 0.1 g/L to 96% at 0.5 g/L, respectively. The study reveals that complete degradation of both dyes was obtained at a dosage of 10 g/L of maghemite for methylene blue. Moreover, the adsorbent dose is optimized by varying it at different levels while keeping other parameters constant. In addition, the authors reported improvement in adsorption rate with an increase in biochar dosage. The removal rate of 75.3 ± 0.11% was achieved at a 5% biochar dosage, clearly indicating that any further increase in adsorbent dosage resulted in a decrease in removal efficiency. Also, it was observed that if the adsorbent dosage was increased further, the removal efficiency decreased. As the adsorbent dosage increases, a reduction in adsorption capacity is also observed, which is attributed to the reduction in availability of active sites at higher dosages [82]. In another study, the study optimized the adsorbent dosage to observe the better performance of the adsorbent towards Pb²⁺, Cr³⁺, and Cd²⁺ ions. In this study, the authors varied the range from 1 to 4 g L⁻¹ of adsorbent material. While increasing the adsorbent dosage, it was seen that there was no increase in adsorption up to 2.5 g L⁻¹. Moreover, the study reveals that with an increase in adsorbent dosage, surface area increases, which enhances the adsorption process. After a certain point, saturation occurs between the binding sites present on the surface of the adsorbent. A sudden increase in adsorption rate was observed at 2.5 g L⁻¹. Afterwards, with a further increase in adsorbent dosage beyond 2.5 g L⁻¹, a reduction in adsorption rate was observed, which was mainly due to a decrease in surface area. Also, due to the higher dosage, agglomeration occurs, which leads to a reduction in active sites at higher dosages [84]. The adsorbent dosage was optimized to determine the point of highest fluoride removal. As the adsorbent dosage increased from 1.7 to 6.7 g/L, the degradation potential improved from 44.37% to 91.42%. However, further increase up to 13.3 g/L, resulting in a decline in efficiency due to saturation of active sites [81].

Contact Time: Contact time is a key parameter that significantly influences the adsorption and removal efficiency of pollutants. For instance, the study by Kumar et al. determines the effect of contact time by varying it from 15 to 240 min [82]. Initially, an increase in removal efficiency was observed with increasing contact time up to 60 min for WSB500 and KWB500. At the stage of 60 min, the equilibrium is reached as all binding sites are fully occupied on the biochar surface. Further increase in contact time does not affect adsorption efficiency. The highest adsorption capacity was observed for KWB500, which was 2.46 mg g⁻¹ in 60 min, then for the WSB500, which was 1.79 mg g⁻¹ in 60 min. The biochar derived using rice straw at a temperature of 500 °C showed the lowest adsorption rate of 1.78 mg g⁻¹, achieved at 90 min. In another study, the authors optimized time intervals from 30 to 210 min. The findings suggest that the adsorption rate increases with an increase in contact time up to 120 min. A slight decrease in removal efficiency was observed after 120 min due to the saturation that occurs on the biochar surface [84].

The KWBC show significant adsorption performance over other biochar. The study by Xu (2020) et al. prepared biochar using wastes (like kitchen, corn straw, and peanut hulls) via pyrolysis [87]. The study observed that KWBC exhibits better immobilization performance than the biochar derived using CSB and PHB. This is because kitchen residues contain more alkaline metals, and the ion-exchange ability of kitchen waste-derived biochar is higher. The regeneration of biochar is an important factor for environmental remediation. The study by Xing et al. demonstrated that ion exchange and precipitation contribute to approximately 40% of metal ions during the first cycle [86]. The adsorption rate for biochar is maintained and starts decreasing up to 64% for Cd and 80% for Pb²⁺ ions after the seventh cycle. In contrast, for biofilm-assisted biochar, the adsorption was maintained across all cycles, which was attributed to the chemical complexation involving the coordination of π–electrons from functional groups. Moreover, due to hydrochloric acid exposure, some of the biofilms were damaged but adsorbed up to 95% of metal ions.

7. Conclusions and Future Perspectives

Biochar derived from kitchen waste offers a sustainable solution for addressing the problems associated with kitchen waste disposal and for decontaminating water and soil. Kitchen waste is important to utilize as a natural precursor because its degradation is challenging due to the high water content. This review highlights the significance of utilizing KWBC and explores various synthesis methods for its production. Biochar prepared using pyrolysis, hydrothermal carbonization, microwave-assisted treatment, and ball milling exhibits different key characteristics tailored to specific applications. Moreover, the feedstock type and production method highly influence the properties of biochar. To enhance the adsorption efficiency of biochar, various studies have modified its characteristics through targeted alterations. Moreover, previous research has thoroughly discussed the application of biochar in soil and water remediation, along with their mechanistic insights. The issues of soil infertility and lack of access to clean drinking water are two major problems addressed in this study. Biochar prepared from kitchen waste offers a promising solution to improve soil water retention capacity. Its function as an adsorbent is strongly influenced by experimental parameters such as pH, adsorbent dosage, temperature, contact time, and initial contaminant concentration. The improved adsorption efficiency of the prepared adsorbent is highly dependent on optimizing these parameters. Despite the numerous advantages of using kitchen waste-derived biochar, there are also several limitations. The heterogeneous nature of kitchen waste affects the reproducibility and consistency of biochar properties. In addition, several challenges arise when scaling up the production process. So far, this method has only been successful at the laboratory scale. However, to realize its full environmental benefits, the application of kitchen waste-derived biochar must be expanded to the industrial level. The regeneration of the prepared material is another critical factor, essential for ensuring both cost-effectiveness and environmental safety. These factors must be carefully considered to overcome current challenges.

To address these issues, future research should focus on developing standardized protocols for feedstock handling and tailoring the functionalization of biochar for specific contaminants. Additionally, life-cycle assessments and studies on the long-term environmental impact of these materials are essential to ensure their safety. There is an urgent need to establish regulatory guidelines and policy support to promote the commercial viability and environmental integration of KWBC technologies. Kitchen waste-derived biochar (KWBC) holds significant potential as a practical tool for environmental remediation and circular waste management.