Application of biochar in advanced oxidation processes: supportive, adsorptive, and catalytic role

Integrating adsorption and in-situ catalytic regeneration on N doped carbon aerogel for sustainable continuous-flow water treatment

Peroxymonosulfate (PMS) activation renders a promising way for in-situ regeneration of carbon-based adsorbents towards sustainable water decontamination, but the effects of structure and composition of carbon adsorbent on its adsorption and catalytic regeneration performances remains unclear. Herein, the nitrogen-doped carbon aerogels (NCAs) were prepared to couple adsorption and PMS activation in a continuous fixed-bed reactor for effective bisphenol A (BPA) removal. The nitrogen species and carbon structure of NCAs were varied by changing carbonization temperature (700 °C, 800 °C, 900 °C and 1000 °C) to investigate their correlation with the adsorption and catalytic regeneration abilities of NCAs. Results showed the PMS activation significantly boosted the adsorption capacity of NCAs and extended the breakthrough time of BPA. The optimal NCA-800/PMS system showed 1.8 times higher adsorption capacity and 37.5 times longer breakthrough time that those of NCA-800 alone. Moreover, the NCA-800/PMS system also demonstrated good adaptability across a broad pH range (3.0-12.0) and maintained high performance in real surface water matrices. Experimental and characteristic results collectively confirmed the critical roles of carbon structure and N species of NCA in adsorption and catalytic regeneration: On one hand, the intrinsic carbon defects served as the main adsorption site for BPA; on the other hand, the pyrrolic N and graphitic N promoted PMS adsorption and surface-mediated electron transfer process, while the electron-deficient C atoms adjacent to N species induced PMS oxidation into 1O2, which jointly contributed to efficient BPA degradation for in-situ regeneration of NCA.

A critical review on emerging industrial applications of chars from thermal treatment of biosolids

Thermochemical treatment is rapidly emerging as an alternative method for the management of stabilised sewage sludges (biosolids) to effectively reduce waste volume, degrade contaminants, and generate valuable products, particularly biochar and hydrochar. Biosolids-derived char has a relatively high concentration of heavy metals compared with agricultural chars but is still applied to land due to its beneficial properties and ability to retain metals. However, non-agricultural applications can provide additional economic and environmental benefits, promote sustainability and support a circular economy. This review identifies extensive non-agricultural opportunity for biosolids biochar, including adsorption, catalysis, energy storage systems, biological process enhancement, and as additives for rubber compounding and construction. Biosolids chars have received limited attention vs agricultural char, and we draw on both areas of literature, as well as evaluating differences between agricultural and biosolids chars. A key opportunity for biosolids biochar in comparison with other materials and agricultural chars is its sustainable and low-cost nature, relatively high metals content, improving catalyst properties, and ability to modify in various stages to tune it to specific applications. The specific opportunities for hydrochar have only received limited attention. Research needs to include better understanding of the benefits and limitations for specific applications, as well as adjacent drivers, including society, regulation, and market and economics.

Review on biochar as a sustainable green resource for the rehabilitation of petroleum hydrocarbon-contaminated soil

Petroleum pollution is one of the primary threats to the environment and public health. Therefore, it is essential to create new strategies and enhance current ones. The process of biological reclamation, which utilizes a biological agent to eliminate harmful substances from polluted soil, has drawn much interest. Biochars are inexpensive, environmentally beneficial carbon compounds extensively employed to remove petroleum hydrocarbons from the environment. Biochar has demonstrated an excellent capability to remediate soil pollutants because of its abundant supply of the required raw materials, sustainability, affordability, high efficacy, substantial specific surface area, and desired physical-chemical surface characteristics. This paper reviews biochar's methods, effectiveness, and possible toxic effects on the natural environment, amended biochar, and their integration with other remediating materials towards sustainable remediation of petroleum-polluted soil environments. Efforts are being undertaken to enhance the effectiveness of biochar in the hydrocarbon-based rehabilitation approach by altering its characteristics. Additionally, the adsorption, biodegradability, chemical breakdown, and regenerative facets of biochar amendment and combined usage culminated in augmenting the remedial effectiveness. Lastly, several shortcomings of the prevailing methods and prospective directions were provided to overcome the constraints in tailored biochar studies for long-term performance stability and ecological sustainability towards restoring petroleum hydrocarbon adultered soil environments.

Super capacity of ligand-engineered biochar for sorption of malachite green dye: key role of functional moieties and mesoporous structure

This study synthesized a new thiomalic acid-modified rice husk biochar (TMA-BC) as a versatile and eco-friendly sorbent. After undergoing chemical treatments, the mercerized rice husk biochar (NaOH-BC) and TMA-BC samples showed higher BET surface area values of 277.1 m2/g and 305.8 m2/g, respectively, compared to the pristine biochar (BC) sample, which had a surface area of 234.2 m2/g. In batch adsorption experiments, it was found that the highest removal efficiency for malachite green (MG) was achieved with TMA-BC, reaching 96.4%, while NaOH-BC and BC exhibited removal efficiencies of 38.6% and 27.9%, respectively, at pH 8. The engineered TMA-BC exhibited a super adsorption capacity of 104.17 mg/g for MG dye at pH 8.0 and 25 °C with a dosage of 2 g/L. The SEM, TEM, XPS, and FTIR spectroscopy analyses were performed to examine mesoporous features and successful TMA-BC carboxylic and thiol functional groups grafting on biochar. Electrostatic forces, such as π - π interactions, hydrogen bonding, and pore intrusion, were identified as key factors in the sorption of MG dye. As compared to single-solution adsorption experiments, the binary solution experiments performed at optimized dosages of undesired ions, such as humic acid, sodium dodecyl sulfate surfactant, NaCl, and NaSCN, reflected an increase in MG dye removal of 2.8%, 8.7%, 5.4%, and 12.7%, respectively, which was attributed to unique mesoporous features and grafted functional groups of TMA-BC. Furthermore, the TMA-BC showed promising reusability up to three cycles. Our study indicates that mediocre biochar modified with TMA can provide an eco-friendly and cost-effective alternative to commercially accessible adsorbents.

Biochar-based materials as remediation strategy in petroleum hydrocarbon-contaminated soil and water: Performances, mechanisms, and environmental impact

Petroleum contamination is considered as a major risk to the health of humans and environment. Biochars as low-cost and eco-friendly carbon materials, have been widely used for the removal of petroleum hydrocarbon in the environment. The purpose of this paper is to review the performance, mechanisms, and potential environmental toxicity of biochar, modified biochar and its integration use with other materials in petroleum contaminated soil and water. Specifically, the use of biochar in oil-contaminated water and soil as well as the factors that could influence the removal ability of biochar were systematically evaluated. In addition, the modification and integrated use of biochar for improving the removal efficiency were summarized from the aspects of sorption, biodegradation, chemical degradation, and reusability. Moreover, the functional impacts and associated ecotoxicity of pristine and modified biochars in various environments were demonstrated. Finally, some shortcoming of current approaches, and future research needs were provided for the future direction and challenges of modified biochar research. Overall, this paper gain insight into biochar application in petroleum remediation from the perspectives of performance enhancement and environmental sustainability.

Renewable Carbonaceous Materials from Biomass in Catalytic Processes: A Review

This review paper delves into the diverse ways in which carbonaceous resources, sourced from renewable and sustainable origins, can be used in catalytic processes. Renewable carbonaceous materials that come from biomass-derived and waste feedstocks are key to developing more sustainable processes by replacing traditional carbon-based materials. By examining the potential of these renewable carbonaceous materials, this review aims to shed light on their significance in fostering environmentally conscious and sustainable practices within the realm of catalysis. The more important applications identified are biofuel production, tar removal, chemical production, photocatalytic systems, microbial fuel cell electrodes, and oxidation applications. Regarding biofuel production, biochar-supported catalysts have proved to be able to achieve biodiesel production with yields exceeding 70%. Furthermore, hydrochars and activated carbons derived from diverse biomass sources have demonstrated significant tar removal efficiency. For instance, rice husk char exhibited an increased BET surface area from 2.2 m2/g to 141 m2/g after pyrolysis at 600 °C, showcasing its effectiveness in adsorbing phenol and light aromatic hydrocarbons. Concerning chemical production and the oxidation of alcohols, the influence of biochar quantity and pre-calcination temperature on catalytic performance has been proven, achieving selectivity toward benzaldehyde exceeding 70%.

Recent progress in TiO2-biochar-based photocatalysts for water contaminants treatment: strategies to improve photocatalytic performance

Toxic organic pollutants in wastewater have seriously damaged human health and ecosystems. Photocatalytic degradation is a potential and efficient tactic for wastewater treatment. Among the entire carbon family, biochar has been developed for the adsorption of pollutants due to its large specific surface area, porous skeleton structure, and abundant surface functional groups. Hence, combining adsorption and photocatalytic decomposition, TiO2-biochar photocatalysts have received considerable attention and have been extensively studied. Owing to biochar's adsorption, more active sites and strong interactions between contaminants and photocatalysts can be achieved. The synergistic effect of biochar and TiO2 nanomaterials substantially improves the photocatalytic capacity for pollutant degradation. TiO2-biochar composites have numerous attractive properties and advantages, culminating in infinite applications. This review discusses the characteristics and preparation techniques of biochar, presents in situ and ex situ synthesis approaches of TiO2-biochar nanocomposites, explains the benefits of TiO2-biochar-based compounds for photocatalytic degradation, and emphasizes the strategies for enhancing the photocatalytic efficiency of TiO2-biochar-based photocatalysts. Finally, the main difficulties and future advancements of TiO2-biochar-based photocatalysis are highlighted. The review gives an exhaustive overview of recent progress in TiO2-biochar-based photocatalysts for organic contaminants removal and is expected to encourage the development of robust TiO2-biochar-based photocatalysts for sewage remediation and other environmentally friendly uses.

Rice husk-based pyrogenic carbonaceous material efficiently promoted peroxymonosulfate activation toward the non-radical pathway for the degradation of pharmaceuticals in water

Pristine pyrogenic carbonaceous material (BRH) obtained from rice husk and modified with FeCl3 (BRH-FeCl3) were prepared and explored as carbocatalysts for the activation of peroxymonosulfate (PMS) to degrade a model pharmaceutical (acetaminophen, ACE) in water. The BRH-FeCl3/PMS system removed the pharmaceutical faster than the BRH/PMS. This is explained because in BRH-FeCl3, compared to BRH, the modification (iron played a role as a structuring agent mainly) increased the average pore diameter and the presence of functional groups such as -COO-, -Si-O-, or oxygen vacancies, which allowed to remove the pollutant through an adsorption process and significant carbocatalytic degradation. BRH-FeCl3 was reusable during four cycles and had a higher efficiency for activating PMS than another inorganic peroxide (peroxydisulfate, PDS). The effects of BRH-FeCl3 and PMS concentrations were evaluated and optimized through an experimental design, maximizing the ACE degradation. In the optimized system, a non-radical pathway (i.e., the action of singlet oxygen, from the interaction of PMS with defects and/or -COO-/-Si-O- moieties on the BRH-FeCl3) was found. The BRH-FeCl3/PMS system generated only one primary degradation product that was more susceptible to biodegradation and less active against living organisms than ACE. Also, the BRH-FeCl3/PMS system induced partial removals of chemical oxygen demand and dissolved organic carbon. Furthermore, the carbocatalytic system eliminated ACE in a wide pH range and in simulated urine, having a low-moderate electric energy consumption, indicating the feasibility of the carbocatalytic process to treat water polluted with pharmaceuticals.

From Waste to Resource: Valorization of Lignocellulosic Agri-Food Residues through Engineered Hydrochar and Biochar for Environmental and Clean Energy Applications-A Comprehensive Review

Agri-food residues or by-products have increased their contribution to the global tally of unsustainably generated waste. These residues, characterized by their inherent physicochemical properties and rich in lignocellulosic composition, are progressively being recognized as valuable products that align with the principles of zero waste and circular economy advocated for by different government entities. Consequently, they are utilized as raw materials in other industrial sectors, such as the notable case of environmental remediation. This review highlights the substantial potential of thermochemical valorized agri-food residues, transformed into biochar and hydrochar, as versatile adsorbents in wastewater treatment and as promising alternatives in various environmental and energy-related applications. These materials, with their enhanced properties achieved through tailored engineering techniques, offer competent solutions with cost-effective and satisfactory results in applications in various environmental contexts such as removing pollutants from wastewater or green energy generation. This sustainable approach not only addresses environmental concerns but also paves the way for a more eco-friendly and resource-efficient future, making it an exciting prospect for diverse applications.

Challenges and opportunities for biochar to promote circular economy and carbon neutrality

Biochar, derived from unused biomass, is widely considered for its potential to deal with climate change problems. Global interest in biochar is attributed to its ability to sequester carbon in soil and to remediate aquatic environment from water pollution. As soil conditioner and/or adsorbent, biochar offers opportunity through a circular economy (CE) paradigm. While energy transition continues, progress toward low-emissions materials accelerates their advance towards net-zero emissions. However, none of existing works addresses CE-based biochar management to achieve carbon neutrality. To reflect its novelty, this work provides a critical overview of challenges and opportunities for biochar to promote CE and carbon neutrality. This article also offers seminal perspectives about strengthening biomass management through CE and resource recovery paradigms, while exploring how the unused biomass can promote net zero emissions in its applications. By consolidating scattered knowledge in the body of literature into one place, this work uncovers new research directions to close the loops by implementing the circularity of biomass resources in various fields. It is conclusive from a literature survey of 113 articles (2003-2023) that biomass conversion into biochar can promote net zero emissions and CE in the framework of the UN Sustainable Development Goals (SDGs). Depending on their physico-chemical properties, biochar can become a suitable feedstock for CE. Biochar application as soil enrichment offsets 12% of CO₂ emissions by land use annually. Adding biochar to soil can improve its health and agricultural productivity, while minimizing about 1/8 of CO₂ emissions. Biochar can also sequester CO₂ in the long-term and prevent the release of carbon back into the atmosphere after its decomposition. This practice could sequester 2.5 gigatons (Gt) of CO₂ annually. With the global biochar market reaching USD 368.85 million by 2028, this work facilitates biochar with its versatile characteristics to promote carbon neutrality and CE applications.

Applications of biochar in sulfate radical-based advanced oxidation processes for the removal of pharmaceuticals and personal care products

Recently, biochar (BC) has been increasingly used as a catalyst for the degradation of 'emerging pollutants' (EPs). Pharmaceuticals and personal care products (PPCPs), which come under 'EPs', can be harmful to the aquatic ecosystem despite being present in very low concentrations (ng/L-μg/L). Advanced oxidation processes (AOPs), which produce sulfate radical (SR-AOPs), show a great potential to degrade PPCPs effectively from wastewater. It is mainly due to the higher stability, long half-lives and better non-selectivity of SO₄^{• -} compared with AOPs with •OH generation. Furthermore, research focus is now given on AOPs coupled with BC-supported catalyst to enhance the degradation of PPCPs because of quicker generation of radicals (•OH, SO₄^•-) by the activation of persulfate (PS) and peroxymonosulfate (PMS). This article sheds light on the catalytic ability of BC after its physical and chemical modifications such as acid/alkali treatment and metal doping. The role of persistent free radicals (PFRs) in the BC for effective removal of PPCPs has been elaborated. Its potential applications in synthetic as well as real wastewater have also been discussed.

Non-metal activated peroxydisulfate by straw biochar for tetracycline hydrochloride oxidative degradation: catalytic activity and mechanism

In this study, stalk biochar (BC) was prepared by a high-temperature pyrolysis process and used as a non-metallic catalyst to activate peroxydisulfate (PDS) to degrade tetracycline hydrochloride (TCH). Various characterization results showed that BC had a hollow tubular structure, irregular folds, and important active sites such as oxygen-containing functional groups. Under the optimal reaction conditions, the degradation rate of TCH reached 98.1% within 120 min. In addition, the degradation performance was satisfactory and similar under acidic and near neutral pH, and higher temperature promoted the degradation of TCH. The SO₄·^-, ·OH, and ¹O₂ generated by PDS activation were reactive oxygen species (ROS), which degraded TCH through free radical/non-radical synergistic pathways. Quenching experiments proved that the generated SO₄·^- and ·OH might be the dominant reactive oxygen species (ROS) during the oxidative reaction. The research results will provide a theoretical basis for the application of PDS activated by non-metallic catalysts in the remediation of tetracycline antibiotics pollution.

Degradation of organic pollutants from water by biochar-assisted advanced oxidation processes: Mechanisms and applications

Biochar has shown large potential in environmental remediation because of its low cost, large specific surface area, porosity, and high conductivity. Biochar-assisted advanced oxidation processes (BC-AOPs) have recently attracted increasing attention to the remediation of organic pollutants from water. However, the effects of biochar properties on catalytic performance need to be further explored. There are still controversial and knowledge gaps in the reaction mechanisms of BC-AOPs, and regeneration methods of biochar catalysts are lacking. Therefore, it is necessary to systematically review the latest research progress of BC-AOPs in the treatment of organic pollutants in water. In this review, first of all, the effects of biochar properties on catalytic activity are summarized. The biochar properties can be optimized by changing the feedstocks, preparation conditions, and modification methods. Secondly, the catalytic active sites and degradation mechanisms are explored in different BC-AOPs. Different influencing factors on the degradation process are analyzed. Then, the applications of BC-AOPs in environmental remediation and regeneration methods of different biochar catalysts are summarized. Finally, the development prospects and challenges of biochar catalysts in environmental remediation are put forward, and some suggestions for future development are proposed.

Hydrothermal recycling of carbon absorbents loaded with emerging wastewater contaminants

Adsorption using carbon materials is one of the most efficient techniques for removal of emerging contaminants such as pharmaceuticals from wastewater. However, high costs are a major hurdle for their large-scale application in areas currently under economic constraints. While most research focuses on decreasing the adsorbent price by increasing its capacity, treatment costs for exhausted adsorbents and their respective end-of-life scenarios are often neglected. Here, we assessed a novel technique for recycling of exhausted activated biochars based on hydrothermal treatment at temperatures of 160-320 °C. While a treatment temperature of 280 °C was sufficient to fully degrade all 10 evaluated pharmaceuticals in solution, when adsorbed on activated biochars certain compounds were shielded and could not be fully decomposed even at the highest treatment temperature tested. However, the use of engineered biochar doped with Fe-species successfully increased the treatment efficiency, resulting in full degradation of all 10 parent compounds at 320 °C. The proposed recycling technique showed a high carbon retention in biochar with only minor losses, making the treatment a viable candidate for environmentally sound recycling of biochars. Recycled biochars displayed potentially beneficial structural changes ranging from an increased mesoporosity to additional oxygen bearing functional groups, providing synergies for subsequent applications as part of a sequential biochar system.

A critical review on biochar-based catalysts for the abatement of toxic pollutants from water via advanced oxidation processes (AOPs)

Proper waste disposal is a key towards sustainable development. Wastewater treatment is delineated by the application of efficient, economic and novel catalysts. Biochar is derived from the thermochemical conversion of biomass or any carbonaceous materials and is considered as one of the most eco-friendly substitute for activated carbon. Owing to its large surface area, porosity, crystallinity and active functional groups, the biochar-based catalysts has been extensively applied for the abatement of toxic pollutants from wastewater streams. While most of the reviews focus on the adsorptive properties of the biochar, this review critically analyses the recent development of biochar-based catalysts in the field of advanced oxidation processes (Fenton-like systems, photocatalytic and sonocatalytic systems). The presence of persistent free radicals and oxygen-containing functional groups renders biochar to act as catalyst. The mechanisms accompanying catalytic performance of biochar-based catalysts have also been reviewed. However, the research in this area is quite at an initial phase, and many advancements schemes are essential prior to scale-up and commercialization. Future researches should be devoted to more efficient and rigorous understanding of the structural properties of biochar to engineer the catalytic degradation of targeted pollutants in wastewater treatment.

Enhanced degradation of DDT using a novel iron-assisted hydrochar catalyst combined with peroxymonosulfate: Experiment and mechanism analysis

Poplar wood (PW) hydrochar modified by iron (Fe@HC) was prepared greenly by one-step hydrothermal method. The adsorption and degradation performance of DDT was investigated in a heterogeneous advanced oxidation system (Fe@HC/PMS) formed by Fe@HC collaborated with peroxymonosulfate (PMS). The effects of Fe@HC dosage, PMS dosage and DDT initial concentration were quantitatively analyzed. The results showed that DDT removal efficiency can reach to 88.62% in 240 min under optimal conditions (4 g/L Fe@HC, 10 mM PMS, 0.5 mg/L DDT, 5.5 pH₀) in Fe@HC/PMS system. Furthermore, Fe@HC/PMS system exhibited high degradation rate and TOC removal efficiency for the removal of various organic contaminants. The influence mechanisms of Fe@HC/PMS system on DDT adsorption and degradation were proposed based on electron paramagnetic resonance (EPR) testing analysis and radical quenching experiments. Based on the mechanism analysis, the influence of Fe@HC/PMS on DDT removal efficiency can be concluded in the order: Active substance indirect degradation (60.95%) > Fe@HC direct degradation (10.13%) > Fe@HC adsorption (17.54%). Among active substance indirect degradation, SO₄^•-, ^•OH, O₂^•- and ¹O₂ occupied 27.56%, 15.74%, 5.33% and 12.32%, respectively. Moreover, DDT degradation intermediates were detected by a gas chromatography-mass spectrometer (GC-MS) to predict DDT degradation pathways. This study provided a green progress for the reuse of biomass resources and a new way for the enhanced degradation of DDT.

Resource utilization of agricultural waste: Converting peanut shell into an efficient catalyst in persulfate activation for degradation of organic pollutant

Agricultural waste was characterized by large quantity and low degree of resource utilization. The peanut shell waste was converted into value-added biochar to alleviate the pollution of dyeing wastewater, which caters to the concept of resource recovery and sustainable utilization. In this work, peroxydisulfate (PDS) could be efficiently activated by biochar obtained by pyrolysis at 700 °C (BC) and Acid Orange 7 (AO7) was rapidly eliminated with 96% removal ratio in 10 min. Meanwhile, BC catalyst performed good stability and reusability. In addition, remarkable removal performance within 40 min (>94%) could be achieved in a wide range of pH (3.0-11.0). Through series characterizations, it was found that 700 °C was the critical pyrolysis temperature to prepare material with excellent property mainly attributing to large specific surface area (SSA), followed by high defect structure and rich C-O. It was speculated that radical pathway mainly especially surface-bounded radicals (SO₄^•-、•OH、O₂^-•) worked in the degradation of AO7. Specifically, abundant and typical oxygen-containing functional groups (OFGs) and defect structure catalytic sites of BC enhanced PDS activation. In addition, various radicals participated the whole degradation processes, such as the cleavage of azo bond (-NN-), hydroxylation, deamination and desulfurization.

The application of transition metal-modified biochar in sulfate radical based advanced oxidation processes

Sulfate radical (SO₄^•-) based advanced oxidation processes (SR-AOPs) is a very important chemical oxidation technology for the degradation of recalcitrant organic pollutants in water and has been well developed. Recently, transition metals or their oxides-modified biochar has been widely used as the catalyst to catalyze peroxymonosulfate (PMS) and peroxydisulfate (PS) in SR-AOPs due to their outstanding properties (e.g., large surface area, high stability, abound catalytic sites, and diversity of material design, etc.). These composite materials not only combine the respective beneficial characteristics of biochar and transition metals (or their oxides) but also often present synergistic effects between the components. In this review, we present the synthesis of different types of transition metal (or metal oxides)/biochar-based catalysts and their application in SR-AOPs. The catalytic mechanism, including the generation process of free radicals and other reaction pathways on the surface of the catalyst were also carefully discussed. Particular attention has been paid to the synergistic effects between the components that result in enhanced catalytic performance. At the end of this review, the future development prospects of this technology are proposed.

Efficient Activation of Peroxymonosulfate by Biochar-Loaded Zero-Valent Copper for Enrofloxacin Degradation: Singlet Oxygen-Dominated Oxidation Process

The removal of contaminants of emerging concern (CECs) has become a hot research topic in the field of environmental engineering in recent years. In this work, a simple pyrolysis method was designed to prepare a high-performance biochar-loaded zero-valent copper (CuC) material for the catalytic degradation of antibiotics ENR by PMS. The results showed that 10 mg/L of ENR was completely removed within 30 min at an initial pH of 3, CuC 0.3 g/L, and PMS 2 mmol/L. Further studies confirmed that the reactive oxygen species (ROS) involved in ENR degradation are ·OH, SO4-·, ¹O₂, and O2-. Among them, ¹O₂ played a major role in degradation, whereas O2-· played a key role in the indirect generation of ¹O₂. On the one hand, CuC adsorbed and activated PMS to generate ·OH, SO4-· and O2-·. O2-· was unstable and reacted rapidly with H₂O and ·OH to generate large amounts of ¹O₂. On the other hand, both the self-decomposition of PMS and direct activation of PMS by C=O on biochar also generated ¹O₂. Five byproducts were generated during degradation and eventually mineralized to CO₂, H₂O, NO3-, and F^-. This study provides a facile strategy and new insights into the biochar-loaded zero-valent transition-metal-catalyzed PMS degradation of CECs.

Mechanistic insights into carbo-catalyzed persulfate treatment for simultaneous degradation of cationic and anionic dye in multicomponent mixture using plastic waste-derived carbon

Upcycling waste into value-added products for utilization in wastewater abatements has been explored in a number of treatment technologies. One such waste, single-use plastic, which poses significant adverse environmental and economic impact, has been chosen and converted into graphitic carbon to reduce the waste burden sustainably and economically. The sorptive and catalytic performance of synthesized plastic waste-derived carbon (PWC) was evaluated using brilliant green (BG) and eosin yellow (EY) as target pollutants. The adsorption capacity of PWC was very low for BG (7.41 mg/g) and EY (4.93 mg/g). The coupling of PWC with peroxymonosulfate (PMS) promoted dye degradation. Complete degradation of the dye, with ~61% reduction in TOC and ~95% reduction in toxicity, was achieved by oxidative treatment (initial concentration: 10 mg/L). The functionalities of PWC facilitated better electron transfer to PMS for its effective activation, which led to the production of SO₄^•- and OH^•. The quenching study confirmed that the degradation of dyes was primarily due to SO₄^•-. Additionally, the pathways of dye degradation were proposed based on the intermediates identified. Thus, this study established the high potential of PWC as a metal-free catalyst in PMS activation for the abatement of organic pollutants.

A comprehensive review on persulfate activation treatment of wastewater

With increasingly serious environmental pollution and the production of various wastewater, water pollutants have posed a serious threat to human health and the ecological environment. The advanced oxidation process (AOP), represented by the persulfate (PS) oxidation process, has attracted increasing attention because of its economic, practical, safety and stability characteristics, opening up new ideas in the fields of wastewater treatment and environmental protection. However, PS does not easily react with organic pollutants and usually needs to be activated to produce oxidizing active substances such as sulfate radicals (SO₄^-) and hydroxyl radicals (OH) to degrade them. This paper summarizes the research progress of PS activation methods in the field of wastewater treatment, such as physical activation (e.g., thermal, ultrasonic, hydrodynamic cavitation, electromagnetic radiation activation and discharge plasma), chemical activation (e.g., alkaline, electrochemistry and catalyst) and the combination of the different methods, putting forward the advantages, disadvantages and influencing factors of various activation methods, discussing the possible activation mechanisms, and pointing out future development directions.

Insights into the removal of polystyrene nanoplastics using the contaminated corncob-derived mesoporous biochar from mining area

Nanoplastic has become a prominent threat to the aquatic ecosystem, and the cost-effective technologies for controlling that are still insufficient. The aim of this study is to use contaminated corncobs collected in mining area to prepare functional mesoporous biochar (MBC) and to investigate its ability to remove polystyrene nanoplastics (PSNPs) from water. The adsorption of PSNPs by MBC could be better described by the Sips isotherm and followed the second-order kinetics, with the theoretical maximum adsorption capacity of MBC for PSNPs was 56.02 mg·g^-1. Then the PSNPs adsorbed on MBC could be hydrothermally degraded and the biochar could be simultaneously regenerated. The ability was affected by various factors, including oxygen-containing functional groups, metallic components, superoxide radicals and holes. The degradation products were dominated as low-molecule-weight oligomers and the main possible pathways involved scission, hydrolysis and radical reaction. The findings highlight the great potential of biochar prepared using contaminated biowaste in mining area to remove the nanoplastic pollutants in the aqueous environment.

The effect of persistent free radicals in sludge derived biochar on p-chlorophenol removal

Sewage sludge pyrolysis can effectively dispose of sludge and obtain sludge-derived biochar (SDBC) as an adsorbent for pollutant removal. Recently, persistent free radicals (PFRs), which have also been detected in many types of biochar, have attracted considerable attention for organic pollutant degradation. Sludge collected from a sewage treatment plant was pyrolyzed into SDBC, which contained a large amount of PFRs, and the resulting SDBC was then applied for the removal of p-chlorophenol. An SDBC dosage of 5 g L^-1 was applied for treating 5 mg L^-1 of p-chlorophenol; the highest removal efficiency of 90% was achieved at pH 3, and 22% of the initial p-chlorophenol was degraded by the SDBC. Hydroxyl free radicals were observed and contributed to the degradation of p-chlorophenol. The spent SDBC was reused five times after regeneration through the desorption of adsorbed p-chlorophenol. The p-chlorophenol removal efficiency remained constant, but the degradation decreased with increasing reuse cycles, suggesting that the p-chlorophenol degradation efficiency was positively correlated with the intensity of PFRs on SDBC. Further modification of the SDBC sample in HNO₃ or NaOH increased the amount of PFRs, and consequently, the degradation of p-chlorophenol under low oxygen conditions, further confirming the crucial role of PFRs in p-chlorophenol degradation. This study provides insights into the application of SDBC, a promising material, for contaminant abatement.

Adsorption of Arsenic on Fe-Modified Biochar and Monitoring Using Spectral Induced Polarization

This work demonstrates the potential of Fe-modified biochar for the treatment of arsenic (As) simulated wastewater and the monitoring of adsorption in real-time. Specifically, we propose the utilization of date-palm leaves for the production of biochar, further modified with Fe in order to improve its adsorption function against inorganic pollutants, such as As. Both the original biochar and the Fe-modified biochar were used for adsorption of As in laboratory batch and column experiments. The monitoring of the biochar(s) performance and As treatment was also enhanced by using the spectral induced polarization (SIP) method, offering real-time monitoring, in addition to standard chemical monitoring. Both the original and the Fe-modified biochar achieved high removal rates with Fe-modified biochar achieving up to 98% removal of As compared to the 17% by sand only (control). In addition, a correlation was found between post-adsorption measurements and SIP measurements.

Biochar-assisted Fenton-like oxidation of benzo[a]pyrene-contaminated soil

In the present study, the biochar derived from sunflower husks was used as a mediator in the heterogeneous Fenton process. The physical and chemical characteristics were studied in terms of specific surface area, elemental contents, surface morphology, surface functional groups, thermal stability, and X-ray crystallography. The main aim was to evaluate the effectiveness of biochar in a heterogeneous Fenton process catalyzed by hematite toward the degradation of benzo[a]pyrene (BaP) in Haplic Chernozem. The Fenton-like reaction was performed at a pH of 7.8 without pH adjustment in chernozem soil. The effects of operating parameters, such as hematite dosage and H₂O₂ concentrations, were investigated with respect to the removal efficiency of BaP. The overall degradation of 65% was observed at the optimized conditions where 2 mg g^-1 hematite and 1.25 M H₂O₂ corresponded to the H₂O₂ to Fe ratio of 22:1. Moreover, the biochar amendment showed an increment in the removal efficiency and promotion in the growth of spring barley (Hordeum sativum distichum). The BaP removal was reached 75 and 95% after 2.5 and 5% w/w addition of biochar, respectively. The results suggested that the Fenton-like reaction's effectiveness would be greatly enhanced by the ability of biochar for activation of H₂O₂ and ejection of the electron to reduce Fe(III) to Fe(II). Finally, the presence of biochar could enhance the soil physicochemical properties, as evidenced by the better growth of Hordeum sativum distichum compared to the soil without biochar. These promising results open up new opportunities toward the application of a modified Fenton reaction with biochar for remediating BaP-polluted soils.

Co-oxidative removal of arsenite and tetracycline based on a heterogeneous Fenton-like reaction using iron nanoparticles-impregnated biochar

A highly efficient, eco-friendly and relatively low-cost catalyst is necessary to tackle bottlenecks in the treatment of industrial wastewater laden with heavy metals and antibiotic such as livestock farm and biogas liquids. This study investigated co-oxidative removal of arsenite (As(III)) and tetracycline (TC) by iron nanoparticles (Fe NP)-impregnated carbons based on heterogeneous Fenton-like reactions. The composites included Fe NP@biochar (BC), Fe NP@hydrochar (HC), and Fe NP@HC-derived pyrolysis char (HDPC). The functions of N and S atoms and the loading mass of the Fe NP in the Fe NP@BC in heterogeneous Fenton-like reactions were studied. To sustain its cost-effectiveness, the spent Fe NP@BC was regenerated using NaOH. Among the composites, the Fe NP@BC achieved an almost complete removal of As(III) and TC under optimized conditions (1.0 g/L of dose; 10 mM H₂O₂; pH 6; 4 h of reaction; As(III): 50 μM; TC: 50 μM). The co-oxidative removal of As(III) and TC by the Fe NP@ BC was controlled by the synergistic interactions between the Fe NPs and the active N and S sites of the BC for generating reactive oxygen species (ROS). After four consecutive regeneration cycles, about 61 and 95% of As(III) and TC removal were attained. This implies that the spent carbocatalyst still has reasonable catalytic activities for reuse. Overall, this suggests that adding technological values to unused biochar as a carbocatalyst like Fe NP@BC was promising for co-oxidative removal of As(III) and TC from contaminated water.

Combining geophysics and material science for environmental remediation: Real-time monitoring of Fe-biochar arsenic wastewater treatment

In a column set-up, Fe modified biochar produced from date palm leaves was used to remove As (1 mg L^-1) from a laboratory-prepared wastewater. The wastewater treatment process was monitored in real-time by spectral induced polarization (SIP), over a wide range of frequencies (0.01-1000 Hz). Both 5 and 10% biochar-amended columns achieved As removal exceeding 98%. The SIP parameters appear to be sensitive on As removal processes, with the recorded trend following the conventional geochemical monitoring, while offering higher temporal resolution.

Insights into the mechanism of hydrogen peroxide activation with biochar produced from anaerobically digested residues at different pyrolysis temperatures for the degradation of BTEXS

The disposal of large amounts of biogas residue from anaerobically digested waste is a burden on environment protection. Porous biochars (BCs) were synthesized from biogas residue at three pyrolysis temperatures (300 °C, 550 °C, and 800 °C) and used to catalyze H₂O₂ for the degradation of benzene, toluene, ethylbenzene, xylene isomers (ortho, para, and meta), and styrene (BTEXS) to develop a new use for biogas residues. The prepared BCs were characterized through scanning electron microscopy, Brunauer-Emmett-Teller method, Fourier transform infrared spectrometry, and X-ray photoelectron spectroscopy. Results showed that BC800/H₂O₂ had the highest BTEXS degradation performance over 6 h. The degradation kinetic data were most consistent with the pseudo-second-order model. The different catalytic effect of the three BCs pyrolyzed at different temperatures were attributed to the dominant active sites (C-O/C-OH/C=C/C=O groups, pyridinic N, and graphitic N) that induced the production of reactive oxygen species (ROS). ROS-quenching experiments indicated that the degradation of BTEXS by BC300/H₂O₂, BC550/H₂O₂, and BC800/H₂O₂ involved ∙OH, ∙O₂^-, and ¹O₂. ∙OH was the dominant ROS in BC300/H₂O₂ and BC550/H₂O₂, and ¹O₂ was the dominant ROS in BC800/H₂O₂. Our findings provided new insight into the different catalytic mechanisms for BC production at different pyrolysis temperatures and demonstrated that a porous BC catalyst with high utilization value could be prepared from biogas residue and could hold considerable potential for application in BTEXS treatment in the future.

Characterization and Sodium Cations Sorption Capacity of Chemically Modified Biochars Produced from Agricultural and Forestry Wastes

Excessive amounts of sodium cations (Na⁺) in water is an important limiting factor to reuse poor quality water in agriculture or industry, and recently, much attention has been paid to developing cost-effective and easily available water desalination technology that is not limited to natural resources. Biochar seems to be a promising solution for reducing high loads of inorganic contaminant from water and soil solution, and due to the high availability of biomass in agriculture and forestry, its production for these purposes may become beneficial. In the present research, wheat straw, sunflower husk, and pine-chip biochars produced at 250, 450 and 550 °C under simple torrefaction/pyrolysis conditions were chemically modified with ethanol or HCl to determine the effect of these activations on Na sorption capacity from aqueous solution. Biochar sorption property measurements, such as specific surface area, cation exchange capacity, content of base cations in exchangeable forms, and structural changes of biochar surface, were performed by FTIR and EPR spectrometry to study the effect of material chemical activation. The sorption capacity of biochars and activated carbons was investigated by performing batch sorption experiments, and adsorption isotherms were tested with Langmuir's and Freundlich's models. The results showed that biochar activation had significant effects on the sorption characteristics of Na⁺, increasing its capacity (even 10-folds) and inducing the mechanism of ion exchange between biochar and saline solution, especially when ethanol activation was applied. The findings of this study show that biochar produced through torrefaction with ethanol activation requires lower energy demand and carbon footprint and, therefore, is a promising method for studying material applications for environmental and industrial purposes.

Combined Strategies to Prompt the Biological Reduction of Chlorinated Aliphatic Hydrocarbons: New Sustainable Options for Bioremediation Application

Groundwater remediation is one of the main objectives to minimize environmental impacts and health risks. Chlorinated aliphatic hydrocarbons contamination is prevalent and presents particularly challenging scenarios to manage with a single strategy. Different technologies can manage contamination sources and plumes, although they are usually energy-intensive processes. Interesting alternatives involve in-situ bioremediation strategies, which allow the chlorinated contaminant to be converted into non-toxic compounds by indigenous microbial activity. Despite several advantages offered by the bioremediation approaches, some limitations, like the relatively low reaction rates and the difficulty in the management and control of the microbial activity, can affect the effectiveness of a bioremediation approach. However, those issues can be addressed through coupling different strategies to increase the efficiency of the bioremediation strategy. This mini review describes different strategies to induce the reduction dechlorination reaction by the utilization of innovative strategies, which include the increase or the reduction of contaminant mobility as well as the use of innovative strategies of the reductive power supply. Subsequently, three future approaches for a greener and more sustainable intervention are proposed. In particular, two bio-based materials from renewable resources are intended as alternative, long-lasting electron-donor sources (e.g., polyhydroxyalkanoates from mixed microbial cultures) and a low-cost adsorbent (e.g., biochar from bio-waste). Finally, attention is drawn to novel bio-electrochemical systems that use electric current to stimulate biological reactions.

Determination of Instinct Components of Biomass on the Generation of Persistent Free Radicals (PFRs) as Critical Redox Sites in Pyrogenic Chars for Persulfate Activation

Persulfate (PS) activation on biochar (BC) is a promising technology for degrading the aqueous organic contaminants. However, the complexity of activation mechanisms and components in biomass that used to produce BC makes it difficult to predict the performance of PS activation. In this study, we employed eight sludges as the representative biomass that contained absolutely different organic or inorganic components. Results showed that the elemental composition, surface properties, and structures of the sludge-derived BCs (SBCs) clearly depended on the inherent components in the sludges. The intensities of persistent free radicals (PFRs) in the electron paramagnetic resonance (EPR) correlated positively with N-containing content of sludges as electron shuttle, but negatively with the metal content as electron acceptor. Linking with PFRs as crucial sites of triggering a radical reaction, a poly-parameter relationship of predicting PS activation for organic degradation using the sludge components was established (k_obs,PN = 0.004 × C_protein + 0.16 × C_M^-0.895 -0.118). However, for the PS activation on those SBCs without PFRs, this redox process only relied on the sorption or conductivity-related characteristics, not correlating with the content of intrinsic components in biomass but with pyrolysis temperatures. This study provided insightful information of predicting the remediation efficiency of PS activation on BCs and further understanding the fate of contaminants and stoichiometric efficiency of oxidants in a field application.

Occurrence, influence and removal strategies of mycotoxins, antibiotics and microplastics in anaerobic digestion treating food waste and co-digestive biosolids: A critical review

Food waste anaerobic digestion (FWAD) can be assisted with the co-digestion of manures, agricultural waste, and sewage sludge. Nevertheless, contaminants like mycotoxins, antibiotics, and microplastics (MPs) could be introduced and negatively affect the AD system. Over 180 literatures involved the occurrence, influence and removal strategies of these three types of pollutants in AD were summarized in this review. Aflatoxin B1(AFB1) as the most concerned mycotoxins were poorly degraded and brought about inhibitions in short-term. Considering methanogenesis inhibition and occurrence concentration, the risk of oxytetracycline and norfloxacin were identified as priority among antibiotics. Leaching toxic additives from MPs could be responsible for the AD inhibition, while their materials and sizes could also prolong the acidification and methanation processes in FWAD. Strategies of bioaugmentation technologies and bioreactors to enhance the removal were suggested. Perspectives were provided for a better understanding of the fates of reviewed contaminants and their elimination in FWAD systems.

Burgeoning prospects of biochar and its composite in persulfate-advanced oxidation process

In the last decade, more and more refractory organic contaminants with severe health risks have been detected in the aquatic ecosystem. Sulfate radical (SO₄^·-)-based advanced oxidation process (SR-AOP) is recognized as an efficient approach for the removal of organic contaminants. Biochar (BC) and its composites (BCs) have been applied into SR-AOP for the double advantages of adsorption and catalytic ability. This paper gives systematic emphasis to the development and progress of biochar and its composites as catalyst in persulfate-advanced oxidation process. Synthetic techniques including the directed pyrolysis of mixed materials and post-immersed method are discussed. The physicochemical properties of biochar (such as surface area, surface functional groups, defect structure and persistent free radicals, etc.) that affect persulfate activation are provided. Then, emphasis is placed on the crucial role of biochar in affecting the catalytic property of BCs including stabilizing nanoparticles, expanding the surface area, increasing active sites and regulating electron transfer reactions. Integrating mechanistic insights and different biochar-based catalysts highlight the understanding of persulfate activation and catalytic degradation. Possible challenges are finally proposed in the fundamental research and practically scaled-up application.

UV-Vis LED-assisted photo-Fenton process for mineralization of losartan and hydrochlorothiazide: optimization using desirability function

This study presents the results obtained for the optimization of the mineralization of losartan (LOS) and hydrochlorothiazide (HCTZ) using the photo-Fenton process with a UV-Vis LED. Experimental design optimization employing a Doehlert matrix and a global desirability function enabled simultaneous evaluation of multiple responses, with factor fitting providing the best conditions that maximized the mineralization efficiency: Fe²⁺ at 10 mg L^-1 and H₂O₂ at 100 mg L^-1. High rates of mineralization of LOS and HCTZ were obtained, with dissolved organic carbon (DOC); removal of almost 75% after 90 min was observed for both pharmaceuticals. The kinetic model showed that the mineralization followed two regimes in the first minutes, with a fast progression followed by slower activity. The energy consumption calculated for mineralization of LOS and HCTZ at a concentration of 20 mg L^-1 using the UV-Vis LED-assisted photo-Fenton process, at 60 min, was 130 kWh m^-3. The desirability function provides a useful tool for finding optimal experimental conditions for the treatment of effluents with different characteristics. The UV-Vis LED was shown to be a good light source in the photo-Fenton process.

High-efficiency electro-catalytic performance of green dill biochar cathode and its application in electro-Fenton process for the degradation of pollutants

Biochar (BC) is a kind of carbon-rich, renewable and low-cost material, which can be prepared from various organic materials.

Relationship of the physicochemical properties of novel ZnO/biochar composites to their efficiencies in the degradation of sulfamethoxazole and methyl orange

Three different composites were produced, based on zinc oxide and biochar (ZnO/biochar), varying the type of biomass (Salvinia molesta: SM; exhausted husk of black wattle: EH; and sugarcane bagasse: SB), with pyrolysis under mild conditions at 350 and 450 °C. Evaluation was made of the capacities of the composites for photocatalytic degradation of sulfamethoxazole antibiotic (SMX) and methyl orange dye (MO). The properties of the prepared composites were influenced by the biomass source, with larger crystallite size (SB), lower band gap energy (SM), higher specific surface area (SB), and larger pore size (SM) resulting in higher photocatalytic efficiency. Good degradation results were obtained using these innovative photocatalysts prepared at low temperatures, when compared to ZnO/biochar materials reported in previous studies. The best degradation capacities were obtained for the composites produced at 450 °C from SB and SM, with 99.3 and 97% degradation of SMX after 45 min, and 90.8 and 88.3% degradation of MO after 120 min, respectively.

The Role of Carbonate in Catalytic Oxidations

CO₂, HCO₃^-, and CO₃^2- are present in all aqueous media at pH > 4 if no major effort is made to remove them. Usually the presence of CO₂/HCO₃^-/CO₃^2- is either forgotten or considered only as a buffer or proton transfer catalyst. Results obtained in the last decades point out that carbonates are key participants in a variety of oxidation processes. This was first attributed to the formation of carbonate anion radicals via the reaction OH^• + CO₃^2- → CO₃^•- + OH^-. However, recent studies point out that the involvement of carbonates in oxidation processes is more fundamental. Thus, the presence of HCO₃^-/CO₃^2- changes the mechanisms of Fenton and Fenton-like reactions to yield CO₃^•- directly even at very low HCO₃^-/CO₃^2- concentrations. CO₃^•- is a considerably weaker oxidizing agent than the hydroxyl radical and therefore a considerably more selective oxidizing agent. This requires reconsideration of the sources of oxidative stress in biological systems and might explain the selective damage induced during oxidative stress. The lower oxidation potential of CO₃^•- probably also explains why not all pollutants are eliminated in many advanced oxidation technologies and requires rethinking of the optimal choice of the technologies applied. The role of percarbonate in Fenton-like processes and in advanced oxidation processes is discussed and has to be re-evaluated. Carbonate as a ligand stabilizes transition metal complexes in uncommon high oxidation states. These high-valent complexes are intermediates in electrochemical water oxidation processes that are of importance in the development of new water splitting technologies. HCO₃^- and CO₃^2- are also very good hole scavengers in photochemical processes of semiconductors and may thus become key participants in the development of new processes for solar energy conversion. In this Account, an attempt to correlate these observations with the properties of carbonates is made. Clearly, further studies are essential to fully uncover the potential of HCO₃^-/CO₃^2- in desired oxidation processes.

Study of polarized activated carbon filters as simultaneous adsorbent and 3D-type electrode materials for electro-Fenton reactors

Electro-Fenton (EF) based water treatment processes using activated carbon (AC) packed beds constitute an attractive approach for the development of competitive degradation technology of persistent pollutants in aqueous effluents. In this work, the results of a study aimed to assess the effect on the EF performance of different parameters of the reactor's operation are presented. By means of a factorial experimental design, the influence of the AC source (lignitic or vegetal), AC acid pre-treatment, particle size distribution and the amount of Fe loaded resin in the reactor were analyzed. From the resulting data it was found that the most influential parameter in the EF performance of the reactor is the AC source. Modest effects were observed for AC acid pre-treatment, which limits Fe ion adsorption on the AC substrate. The use of a wide particle distribution of AC particles was also found to improve inter-particle electrical contact, thus favoring the electrochemical processes that take place inside the reactor. An investigation on the effect of the amount of Fe in the reactor as well as its distribution dynamics, also revealed that an excess of Fe ions in the reactor decreases the EF performance of the system since Fe ions efficiently adsorb on the AC substrate, particularly in non- acid treated samples. The best operation conditions consisted on using un-meshed vegetable AC, without acid pretreatment in an EF reactor loaded with 0.25 g of Fe, which allowed to reach full color removal of bright blue FCP model dye in 70 min.

Green immobilization of toxic metals using alkaline enhanced rice husk biochar: Effects of pyrolysis temperature and KOH concentration

Biochar is a "green" material that has been widely used in environmental applications for its capability to remove or immobilize contaminants in different environmental media (i.e. soil, water and air) and mitigate climate change. In this study, the feasibility of using KOH enhanced biochar for soil Cd and Pb stabilization was investigated, and the effects of pyrolysis temperature and alkaline concentrations for modification were explored. Field-emission scanning electron microscopy (FESEM), N₂ adsorption-desorption, and Fourier Transform Infrared Spectroscopy (FTIR) analyses were conducted to reveal the influence on biochar physiochemical properties. The immobilization performances were examined through Toxicity Characteristics Leaching Procedure (TCLP), and Response Surface Methodology (RSM) was adopted to visualize the results from leaching tests. The stabilization mechanisms of alkaline enhanced biochars were investigated using Time of Flight Secondary Ion Mass Spectroscopy (TOF-SIMS), Tessier sequential extraction method and X-ray diffraction (XRD) analyses. The results indicated that rice husk biochar pyrolyzed at a relatively low temperature (i.e., 300 °C) and activated by moderate alkaline concentrations (i.e., 1 M or 3 M KOH) rendered optimum stabilization performance. KOH activation was a double-edged sword, with high alkaline concentrations destroying biochar's cell structures. Moreover, the integration of TOF-SIMS, XRD and sequential leaching method shed lights on the underlying mechanisms involved in metal stabilization. Surface complexation between toxic metals and oxygen-containing functional groups rather than liming or precipitation was proven to be the fundamental stabilization mechanism.