The potential role of biochar in the removal of organic and microbial contaminants from potable and reuse water: A review

Risk Evaluation of Pyrolyzed Biochar from Multiple Wastes

This paper aims at demonstrating the significance of biochar risk evaluation and reviewing risk evaluation from the aspects of pyrolysis process, feedstock, and sources of hazards in biochar and their potential effects and the methods used in risk evaluation. Feedstock properties and the resultant biochar produced at different pyrolysis process influence their chemical, physical, and structural properties, which are vital in understanding the functionality of biochar. Biochar use has been linked to some risks in soil application such as biochar being toxic, facilitating GHGs emission, suppression of the effectiveness of pesticides, and effects on soil microbes. These potential risks originate from feedstock, contaminated feedstock, and pyrolysis conditions that favor the creation of characteristics and functional groups of this nature. These toxic compounds formed pose a threat to human health through the food chain. Determination of toxicity levels is a first step in the risk management of toxic biochar. Various sorption methods of biochar utilized low-cost adsorbents, engineered surface functional groups, and nZVI modified biochars. The mechanisms of organic compound removal was through sorption, enhanced sorption, modified biochar, postpyrolysis thermal air oxidation and that of PFRs degradation was through activation, photoactive functional groups, magnetization, and hydrothermal synthesis. Emissions of GHGs in soils amended with biochar emanated through physical and biotic mediated mechanisms. BCNs have a significance in reducing the health quotient indices for PTEs risk contamination by suppressing cancer risk arising from consumption of contaminated food. The degree of environmental risk assessment of HM pollution in biomass and biochars has been determined by using potential ecological risk index and RAC while organic contaminant degradation by EPFRs was considered when assessing the environmental roles of biochar in regulating the fate of contaminants removal. The magnitude of technologies’ net benefit must be considered in relation to the associated risks.

Adsorption of perfluoroalkyl and polyfluoroalkyl substances (PFASs) from aqueous solution - A review

Perfluoroalkyl and polyfluoroalkyl substances (PFASs) have gained increasingly global attention in recent years. Due to their unique amphiphilic properties and stability, PFASs are recognized as highly persistent, toxic, and environmentally bioaccumulative. Among several physicochemical technologies, adsorption has been extensively used and proved to be an effective method for removing PFASs from aqueous environment. In this review article, the technical feasibility of the use of different adsorbents, such as activated carbon, ion exchange resins, minerals, molecularly imprinted polymer (MIP), carbon nanotubes (CNTs), and a wide range of potentially low-cost biosorbents, for PFASs removal from water or wastewater is critically reviewed. The evaluation and comparison of their PFASs sorption behavior in terms of kinetics and isotherms is presented. The mechanisms involved in PFASs adsorption processes, such as diffusion, electrostatic interaction, hydrophobic interaction, ion exchange and hydrogen bond, are discussed. The effects of the parameters variability on sorption process are highlighted. Based on the literature reviewed, a few recommendations for future research on PFASs adsorption are also elaborated. Capsule: The adsorption behavior and mechanisms of perfluoroalkyl and polyfluoroalkyl substances (PFASs) on various adsorbents are reviewed.

Regeneration of porous electrospun membranes embedding alumina nanoparticles saturated with minocycline by UV radiation

A regeneration method for porous electrospun membranes embedding alumina nanoparticles saturated with minocycline was investigated by UV-LED system. The percentage of adsorption capacities before and after regeneration were used to evaluate regeneration efficiency. The PVDF and PVDF-Al₂O₃ fiber mats were prepared by electrospinning technique. Scanning electron microscope (SEM), transmission electron microscope (TEM), energy-dispersive X-ray spectroscopy (EDS) analyses directly confirmed that Al₂O₃ nanoparticles were generally exposed to the surface of PVDF-Al₂O₃ fiber mats. Among them, PVDF-Al₂O₃ 10% fiber mats can effectively adsorb minocycline (remove efficiency >97% in 18 h) with first-order rate constant k = 2.253 ± 0.331 h^-1. The sorption capacity can still keep 81% after five sorption/UV-regeneration circulations. Two successional stages may exist during regeneration: (i) transfer of minocycline from the surface of PVDF-Al₂O₃ fibers to the DI water, followed by the (ii) decomposition of this compound in aqueous solution by direct and indirect photolysis to yield the intermediate species. The desorption capacity and desorption percentage were 4.39 mg g^-1 and 23.30% respectively. The regeneration yields were further enhanced to 94.20% by UV radiation. Minocycline was effectively degraded to intermediate products by direct and indirect photolysis, further degraded into CO₂, H₂O, and NO_x by UV-generated ozone during regeneration. The results indicated that UV radiation was an effective method of regenerating PVDF-Al₂O₃ fiber mats with low energy requirements. The photochemical byproducts and the reaction sites during regeneration were also determined and recognized.

Road to full bioconversion of biowaste to biochemicals centering on chain elongation: A mini review

Production of biochemicals from waste streams has been attracting increasing worldwide interest to achieve climate protection goals. Chain elongation (CE) for production of medium-chain carboxylic acids (MCCAs, especially caproate, enanthate and caprylate) from diverse biowaste has emerged as a potential economic and environmental technology for a sustainable society. The present mini review summarizes the research utilizing various synthetic or real waste-derived substrates available for MCCA production. Additionally, the microbial characteristics of the CE process are surveyed and discussed. Considering that a large proportion of recalcitrantly biodegradable biowaste and residues cannot be further utilized by CE systems and remain to be treated and disposed, we propose here a loop concept of bioconversion of biowaste to MCCAs making full use of the biowaste with zero emission. This could make possible an alternative technology for synthesis of value-added products from a wide range of biowaste, or even non-biodegradable waste (such as, plastics and rubbers). Meanwhile, the remaining scientific questions, unsolved problems, application potential and possible developments for this technology are discussed.

Effect of fulvic acid coating on biochar surface structure and sorption properties towards 4-chlorophenol

Fulvic acid (FA) in soil ubiquitously affected the long-term benefits of biochars as soil amendments. The sorption of ionizable organic pollutants on biochars was complicated by FA because of the presence of ionic groups. To investigate the effect of FA coating on the interaction between biochars and 4-chlorophenol (4-CP), sorption isotherms at pH 4.0 and 12.0 and pH-sorption edge curves were generated. The biochars derived from platane wood were pyrolyzed at 300°C, 500°C and 700°C, resulting in low-, medium- and high-temperature biochars, respectively. The FA coating increased the surface area of the low-temperature biochar but decreased those of the medium- and high-temperature biochars. After coating biochars with FA, the aromaticity and the surface charge of the biochars decreased, but the content of oxygen-containing functional groups, especially carboxyl groups, on the biochar surfaces increased. The results from the sorption isotherm and pH-sorption edge curves showed that FA coating inhibited the sorption under alkaline conditions but did not change the sorption under acidic conditions, which indicated that coating by FA not only occupied the sorption sites but also strengthened the hydrogen bonding between 4-CP and biochars. As determined by characterization of biochars before and after coating, the number of carboxyl groups on the biochar surface increased after coating, which participated in hydrogen bonding with the hydroxyl groups of non-dissociated 4-CP. This research indicated that FA in the soil influenced the interaction between biochar with 4-CP, and the influences varied mainly due to the formation of hydrogen bonds at different pH values. This study could help us better understand the environmental behavior of biochar after aging, and provided a reference for sustainable utilization of biochar.

Removal of pharmaceutical compounds, artificial sweeteners, and perfluoroalkyl substances from water using a passive treatment system containing zero-valent iron and biochar

Emerging contaminants are widely detected and persistent in environmental waters. Advanced oxidation processes are among the most effective methods for removing emerging contaminants from water; however, high energy consumption greatly increases the operating costs and limits large-scale applications. In this study, a passive treatment system consisting of four columns packed with mixtures of silica sand, zero-valent iron (ZVI), biochar (BC), and a mixture of (ZVI + BC) were evaluated for simultaneous removal of eight pharmaceuticals, four artificial sweeteners, and two perfluoroalkyl substances (PFASs) from water. Overall, the passive treatment system was more effective for removing target pharmaceuticals (almost complete removal) than artificial sweeteners and PFASs (partial removal). Columns ZVI, BC, and (ZVI + BC) exhibited similarly effective removal (>97%) of target pharmaceuticals, including carbamazepine, caffeine, sulfamethoxazole, 3,4-methylenedioxyamphetamine, 3,4-methylenedioxymethamphetamine, ibuprofen, gemfibrozil, and naproxen, from ~9 to <0.25 μg L^-1; pharmaceuticals were more rapidly removed by Columns ZVI and (ZVI + BC) than Column BC, except for ibuprofen. Column ZVI was more effective for removing artificial sweeteners acesulfame-K and sucralose than Columns BC and (ZVI + BC); however, BC exhibited relatively greater removal of saccharin than ZVI and (ZVI + BC). Acesulfame-K and saccharin (~110 μg L^-1) were partially removed in the treatment columns. Cyclamate was not removed in any of the columns. However, >76% of input sucralose (~110 μg L^-1) was removed in the three treatment columns. Reactive medium BC alone was more effective for removing target PFASs than ZVI and (ZVI + BC). Input perfluorooctanoic acid (PFOA) (~45 μg L^-1) was partially removed in the columns containing BC but not ZVI alone. Between 10 and 80% of input perfluorooctane sulfonic acid (PFOS) (24 ̶ 90 μg L^-1) was removed in Column ZVI; greater removals (57 ̶ 99%) were observed in Columns BC and (ZVI + BC).

New carbon/ZnO/Li2O nanocomposites with enhanced photocatalytic activity

Our study was focused on the synthesis of photocatalytic materials for the degradation of organic dyes based on the valorization of biomass resources. The biochar resulted from pyrolysis process of cherry pits wastes was activated by CO₂ flow. Activated and inactivated carbon was used to obtain carbon-based photocatalysts impregnated with different zinc salt precursors. The activation of carbon had no significant influence on the photodegradation process. The doping procedure used Li₂CO₃ and Zn(CH₃COO)₂ of different concentrations to impregnate the biochar. The carbon-ZnO-Li₂O based nanomaterials were analysed by TEM and SEM, while the presence of hexagonal wurtzite ZnO was investigated by XRD. The solid samples were analysed by PL at 360 nm excitation fixed wavelength to correlate their morphology with the optical and photocatalytic properties. The presence of Li atoms led to photocatalytic activities of the doped ZnO similar to the undoped ZnO obtained at higher concentrations of zinc acetate precursor.

Post-treatment of paint industry effluents by filtration using Andropogon biochar (Andropogon gayanus Kunth cv. Planaltina)

This study evaluates the filtration potential of the biomass obtained from Andropogon grass (Andropogon gayanus Kunth cv. Planaltina) that was converted to biochar by pyrolysis. The biochar is used in filtration systems for the post-treatment of paint industry effluents. The biomass is characterized by elemental analysis (CHSN-O), determination of specific compounds (cellulose/hemicellulose/lignin), FTIR, and SEM. The produced biochar is characterized by SEM, TGA, and surface area analysis. The efficiency of the filters is evaluated by the following parameters: color, turbidity, removal of total solids (suspended and sedimentable), chemical oxygen demand (COD), and metals (Al, Cu, Zn, Co, Cd, and Cr(VI)). Over 99% removal of aluminum, cadmium, and hexavalent chromium is achieved. Moreover, almost 100% of COD and solids are removed, whereas turbidity and color are reduced by over 90%.

Similarities and differences in adsorption mechanism of dichlorvos and pymetrozine insecticides with coconut fiber biowaste sorbent

In this study, the similarities and differences of the adsorption mechanisms between dichlorvos and pymetrozine and coconut fiber biowaste sorbent (CF-BWS) were investigated. CF-BWS was produced using the slow pyrolysis process at 600 °C for 4 h. HCl acid modification was used to improve the specific surface area. The properties of CF-BWS were analyzed by SEM, FT-IR, BET, and pHpzc. The adsorption kinetics of dichlorvos and pymetrozine on the CF-BWS were well explained by the pseudo-second-order model. The adsorption isotherms for both insecticides were followed the Langmuir isotherm. The difference in molecular structures and surface chemistry caused the difference in adsorption mechanisms of both insecticides. The pore-filling and the hydrophobic interactions were the key mechanisms for both insecticide adsorptions. However, the π-π electron donor-acceptor interaction played the major role in the pymetrozine adsorption but hardly impacted on the adsorption of dichlorvos. The hydrogen bonding mechanism was pronounced in the pymetrozine adsorption, but it had little influence on the dichlorvos adsorption. The CF-BWS is exhibited as an excellent material for the removal of both pollutants and has high potential to be used further as the adsorbent in water treatment process.

Enhanced ciprofloxacin removal by sludge-derived biochar: Effect of humic acid

Humic acid (HA) is ubiquitous in the environment, and its effect on the adsorption of pollutants onto biochar (BC) has been widely investigated. However, the catalytic degradation of organic contaminants induced by persistent free radicals (PFRs) in BC is not negligible. In this study, two different source of external HA, dissolved HA and coated HA, was employed to assess the effect of HA on the degradation of organic contaminant ciprofloxacin (CIP) by PFRs in BC. Results showed that relatively low concentration of dissolved HA promoted the removal of CIP from 59.78% (control) to 66.70% (5 mg/L dissolved HA) by BC, but higher concentration of dissolved HA inhibited. By contrast, coated HA altered the characteristics of BC and had negative impact on organics removal. The removal of CIP by BC in the presence of HA did not have a great difference at pH 6-9, and increasing dissolved oxygen promoted CIP degradation. In addition, the contribution of CIP removal led by adsorption was 47.61%, 41.62% and 47.84% respectively for BC, BC + dissolved HA and 1% HA coated BC. Correspondingly, the contribution of CIP removal led by •OH induced from PFRs in BC was respectively 21.88%, 35.58% and 25.11%, and the contribution led by PFRs themselves oxidation was 30.51%, 22.80% and 27.05%, respectively. The calculated contributions of adsorption and degradation to CIP removal demonstrated that the contribution of degradation to CIP removal led by •OH increased, while the contribution led by PFRs themselves oxidation was inhibited in the presence of dissolved and coated HA.

Biochar reduces volatile organic compounds generated during chicken manure composting

The efficiency of biochar for reducing the levels of volatile organic compounds (VOC) was investigated in a composting mixture containing 90% poultry manure and 10% straw (with and without 3% biochar addition) at three different stages of the process. The use of a low application rate of biochar reduced the concentration of VOC during the thermophilic phase. Biochar significantly reduced the levels of nitrogen volatile compounds, which are the most abundant VOC family, originated from microbial transformation of the N-compounds originally present in manure. The most efficient VOC reduction was observed in oxygenated volatile compounds (ketones, phenols and organic acids), which are intermediates of organic matter degradation, whereas there was no effect on other VOC families (aliphatic, aromatic and terpenes). These results suggest the importance of not only the sorption capacity of biochar but also its impact in the composting progress as main drivers for VOC reduction.

Biochar-supported nZVI (nZVI/BC) for contaminant removal from soil and water: A critical review

The promising characteristics of nanoscale zero-valent iron (nZVI) have not been fully exploited owing to intrinsic limitations. Carbon-enriched biochar (BC) has been widely used to overcome the limitations of nZVI and improve its reaction with environmental pollutants. This work reviews the preparation of nZVI/BC nanocomposites; the effects of BC as a supporting matrix on the nZVI crystallite size, dispersion, and oxidation and electron transfer capacity; and its interaction mechanisms with contaminants. The literature review suggests that the properties and preparation conditions of BC (e.g., pore structure, functional groups, feedstock composition, and pyrogenic temperature) play important roles in the manipulation of nZVI properties. This review discusses the interactions of nZVI/BC composites with heavy metals, nitrates, and organic compounds in soil and water. Overall, BC contributes to the removal of contaminants because it can attenuate contaminants on the surface of nZVI/BC; it also enhances electron transfer from nZVI to target contaminants owing to its good electrical conductivity and improves the crystallite size and dispersion of nZVI. This review is intended to provide insights into methods of optimizing nZVI/BC synthesis and maximizing the efficiency of nZVI in environmental cleanup.

Potential of Cedrella fissilis bark as an adsorbent for the removal of red 97 dye from aqueous effluents

Cedar bark (Cedrella fissilis), a waste from wood processing, was evaluated as an adsorbent for the removal of red 97 dye from effluents. The material exhibited an amorphous structure, irregular surface, and was mainly composed of lignin and holocellulose. The adsorption was favored at pH 2.0. The general order model was most suitable for describing the experimental kinetic data, being the equilibrium reached in around 30 min. The isotherm experiments were better described by the Langmuir model. The maximum adsorption capacity was 422.87 mg g^-1 at 328 K. The values of standard Gibbs free energy change (ΔG⁰) were from - 21 to - 26 kJ mol^-1, indicating a spontaneous and favorable process. The enthalpy change (ΔH⁰) was 18.98 kJ mol^-1, indicating an endothermic process. From the fixed bed adsorption experiment, an inclined breakthrough curve was found, with a mass transfer zone of 5.36 cm and a breakthrough time of 329 min. Cedar bark was able to treat a simulated effluent attaining color removal of 86.6%. These findings indicated that cedar bark has the potential to be applied as a low-cost adsorbent for the treatment of colored effluents in batch and continuous adsorption systems.

Biochar versus bone char for a sustainable inorganic arsenic mitigation in water: What needs to be done in future research?

Arsenic (As) is an emerging contaminant on a global scale posing threat to environmental and human health. The relatively brief history of the applications of biochar and bone char has mapped the endeavors to remove As from water to a considerable extent. This critical review attempts to provide a comprehensive overview for the first time on the potential of bio- and bone-char in the immobilization of inorganic As in water. It seeks to offer a rational assessment of what is existing and what needs to be done in future research as an implication for As toxicity of human health risks through acute and chronic exposure to As contaminated water. Bio- and bone-char are recognized as promising alternatives to activated carbon due to their lower production and activation cost. The surface modification via chemical methods has been adopted to improve the adsorption capacity for anionic As species. Surface complexation, ion exchange, precipitation and electrostatic interactions are the main mechanisms involved in the adsorption of As onto the char surface. However, arsenic-bio-bone char interactions along with their chemical bonding for the removal of As in aqueous solution is still a subject of debate. Hence, the proposed mechanisms need to be scrutinized further using advanced analytical techniques such as synchrotron-based X-ray. Moving this technology from laboratory phase to field scale applications is an urgent necessity in order to establish a sustainable As mitigation in drinking water on a global scale.

Adsorption of selected organic micro-pollutants on sewage sludge biochar

In this study, biochar was produced from three differently treated sewage sludge biomasses, in three pyrolytic temperatures, 300 °C, 500 °C and 700 °C, under continuous N₂ supply. The produced samples were physicochemically characterized and their initial metal concentration, along with metal leaching potential, were investigated. Moreover, the application of the biochar samples as adsorbents for the removal of seven emerging organic micro-pollutants from table water and treated wastewater matrices was investigated. The results showed that even though the biochar samples were not especially enriched in terms of physicochemical characterization, they were effective as adsorptive materials in the respective experiments. Pollutant removal was in the range of 67-99% for the table water experiments, while the removal for the wastewater experiments was 35-97%. The results of this study indicate that sewage sludge biochar has the potential to be an effective, low-cost adsorbent, providing, at the same time, a viable and environmentally friendly solution concerning the difficult task of sludge management.

The adsorption, regeneration and engineering applications of biochar for removal organic pollutants: A review

In recent years, with the continuous development of industry and agriculture, the content of organic pollutants in the environment has been increasing, which has caused serious pollution to the environment. Adsorption has proven to be an effective and economically viable method of removing organic contaminants. Since biochar has many advantages such as various types of raw materials, low cost, and recyclability, it can achieve the effect of turning waste into treasure when used for environmental treatment. This paper summarizes the source and production of biochar, points out its research status in the removal of organic pollutants, expounds its adsorption mechanism on organic pollutants, introduces the relevant adsorption parameters, summarizes its regeneration methods, studies its application of engineering, and finally analyses of benefits and describes the development prospects.

Accelerated Microbial Reduction of Azo Dye by Using Biochar from Iron-Rich-Biomass Pyrolysis

Biochar is widely used in the environmental-protection field. This study presents the first investigation of the mechanism of biochar prepared using iron (Fe)-rich biomass and its impact on the reductive removals of Orange G dye by Shewanella oneidensis MR-1. The results show that biochars significantly accelerated electron transfer from cells to Orange G and thus stimulated reductive removal rate to 72–97%. Both the conductive domains and the charging and discharging of surface functional groups in biochars played crucial roles in the microbial reduction of Orange G to aniline. A high Fe content of the precursor significantly enhanced the conductor performance of the produced biochar and thus enabled the biochar to have a higher reductive removal rate of Orange G (97%) compared to the biochar prepared using low-Fe precursor (75%), but did not promote the charging and discharging capacity of the produced biochar. This study can prompt the search for natural biomass with high Fe content to confer the produced biochar with wide-ranging applications in stimulating the microbial reduction of redox-active pollutants.

Adsorption mechanism of dichlorvos onto coconut fibre biochar: the significant dependence of H-bonding and the pore-filling mechanism

The adsorption mechanism of dichlorvos onto coconut fibre biochar (CFB) was investigated by the batch adsorption technique. Coconut fibre waste material was synthesised at 600 °C for 4 h under oxygen-limited conditions. The biochar was modified by HCl acid to enhance the specific surface area and porosity. The characteristics of the biochar were analysed by scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) specific surface area, and Fourier transform-infrared (FT-IR). The results showed that the BET specific surface area of biochar was 402.4 m²/g. Experimental data presented a good fit to Langmuir isotherm and the pseudo-second-order model. Langmuir isotherm illustrated that monolayer adsorption of dichlorvos occurred on the surface of CFB, with a maximum adsorption capacity of 90.9 mg/g. The diffusion model confirmed that the liquid film diffusion was the rate-limiting step, and the major diffusion mechanism of dichlorvos onto biochar. The BET result after dichlorvos adsorption demonstrated that pore-filling occurred and occupied 58.27%. The pore-filling and chemical interactions, performed important roles in the adsorption of dichlorvos onto CFB. Chemical adsorption is comprised of two interactions, which are hydrophobic and H-bonding, but the prime is H-bonding. CFB is a very potential material for the removal of dichlorvos and environmental pollutants.

Fe0/H2O Filtration Systems for Decentralized Safe Drinking Water: Where to from Here?

Inadequate access to safe drinking water is one of the most pervasive problems currently afflicting the developing world. Scientists and engineers are called to present affordable but efficient solutions, particularly applicable to small communities. Filtration systems based on metallic iron (Fe0) are discussed in the literature as one such viable solution, whether as a stand-alone system or as a complement to slow sand filters (SSFs). Fe0 filters can also be improved by incorporating biochar to form Fe0-biochar filtration systems with potentially higher contaminant removal efficiencies than those based on Fe0 or biochar alone. These three low-cost and chemical-free systems (Fe0, biochar, SSFs) have the potential to provide universal access to safe drinking water. However, a well-structured systematic research is needed to design robust and efficient water treatment systems based on these affordable filter materials. This communication highlights the technology being developed to use Fe0-based systems for decentralized safe drinking water provision. Future research directions for the design of the next generation Fe0-based systems are highlighted. It is shown that Fe0 enhances the efficiency of SSFs, while biochar has the potential to alleviate the loss of porosity and uncertainties arising from the non-linear kinetics of iron corrosion. Fe0-based systems are an affordable and applicable technology for small communities in low-income countries, which could contribute to attaining self-reliance in clean water supply and universal public health.

Developments in biochar application for pesticide remediation: Current knowledge and future research directions

The indiscriminate use of pesticides due to modern agricultural practices has received special attention from the scientific community to address the persistence, recalcitrance and multi-faceted toxicity of several pesticides. Pesticides are hazardous/toxic and can accumulate easily into non-target organisms including humans and other life forms. Several studies have been performed to investigate the effect of biochar addition for pesticide remediation. This review provides a comprehensive information on biochar amendment for the remediation of persistent organic pollutants such as pesticides. The types of pesticides and their hazards to life forms are briefly introduced before detailing biochar production, its characteristics and applications. Biochar addition in pesticide polluted environment offers the following advantages: (a) increases the soil water holding capacity, (b) improves aeration conditions in soil, and (c) provides habitat for the growth of microorganisms, thereby facilitating microbial community for metabolic activities and pesticide degradation. This paper also provides an up-to-date review on remediation of pesticides using biochar, the knowledge gaps and the future research directions in this field to evaluate the effect of biochar addition on agricultural and environmental performances.

Assessment of Physicochemical and Nutritional Characteristics of Waste Mushroom Substrate Biochar under Various Pyrolysis Temperatures and Times

The prime objective of biochar production is to contribute to nutrients recycling, reducing waste and converting useful bio-wastes into carbon rich products in the environment. The present study was intended to systematically evaluate the effect of pyrolysis conditions and characteristics of feedstock influencing the generation of biochar. The study revealed the nutritional importance of waste mushroom substrate (WMS) biochar which may elevate soil nutritional status and soil quality. The results showed that the yields and properties of WMS biochar depended principally on the applied temperature where pyrolysis at higher temperatures, that is, 600 °C and 700 °C produced biochar having high ash, P and K contents. Moreover, numerous useful macro and micro nutrients such as Ca, Mg, Fe and Zn were observed to positively correlate with the increase in temperature. The WMS biochar in our study is highly alkaline which can be used to rectify acidic soil pH. Overall our results suggest that WMS biochar being a rich source of nutrients can be the best remedy to maintain and further enhance the soil nutritional status. Thus by interpreting biochar feedstock characteristics and pyrolysis conditions, the regulation of tailored WMS biochar manufacturing and application in soil can be facilitated.

Phenoxyacid pesticide adsorption on activated carbon - Equilibrium and kinetics

The adsorption of herbicides belonging to the group of halogenated phenoxyacids on the activated carbon was studied. They are differentiated in terms of quantity and type of functional groups (such as chloride, bromide, fluoride) and their position on an aromatic ring. The experimental equilibrium data were analyzed using adsorption isotherm equations taking into account energetic heterogeneity of the adsorption systems. The calculated concentration profiles from the kinetic data were discussed applying two diffusion models, MOE, f-MOE and multi-exponential equations. The dependences between the properties of adsorbates, adsorption uptake and rate were analyzed. The adsorption affinity of pesticides was correlated with adsorbate hydrophobicity, character of functional group, molecular structure. The applicability of kinetic models and equations was investigated; the assumptions of the models were analyzed with regard to consistency with adsorption mechanism. Similarity of adsorption mechanism was found for all adsorbates confirmed by similarity of kinetic curves and corresponding distributions of rate coefficients. The differences in kinetic profiles were attributed to differentiation of herbicide's molecules - number and type of functional groups and their positions on aromatic ring.

Activated bio-chars derived from rice husk via one- and two-step KOH-catalyzed pyrolysis for phenol adsorption

The activated bio-chars (AB) were successfully synthesized from rice husk by one- and two-step KOH-catalyzed pyrolysis. The two-step pyrolysis can produce the high yields of AB compared to the one-step pyrolysis. Moreover, the yield of AB decreased with the increase of the mass ratio of KOH and char, which had a significant effect on the development of the surface area and porosity of carbon. In particular, the AB derived from the two-step pyrolysis at 750°C (mass ratio of KOH and char was 3) had the highest specific surface area (S_BET=2138m²/g) with many micro-porous structures, which was favored for the phenol adsorption. The maximum adsorption capacity of AB2-3-750 reached 201mg/g because of its excellent surface porosity property. The phenol can be efficiently removed from water by only several minutes. The Langmuir model defined well the adsorption isotherm with a high correlation coefficient value, indicating a monolayer adsorption behavior. And the adsorption process defined well with the pseudo-second-order model. The phenol molecules passed into the internal surface via the liquid-film controlled diffusion, so the behavior of phenol adsorption onto the AB was predominantly controlled via the chemisorption. Furthermore, the functional groups on the outer surfaces of AB can attract the phenol molecules onto the internal surfaces via "π-π dispersion interaction" and "donor-acceptor effect".

Calcium-rich biochar from crab shell: An unexpected super adsorbent for dye removal

Adsorption is the common-used method to remove dyes from wastewater, and many efforts have been made to develop low-cost but excellent adsorbents. Here, calcium-rich biochar (CRB) as a low-cost adsorbent was directly prepared from crab shell via a simple pyrolysis process without any modification. Batch adsorption results suggested that CRB was among the dye adsorbents with highest adsorption capacities and fastest adsorption rate. Specifically, it showed high adsorption capacities of 12,502 and 20,317 mg/g for cationic malachite green and anionic Congo red, respectively. The adsorption equilibrium for Congo red onto CRB could be achieved as short as 2 min. Furthermore, the dye adsorption mechanism for CRB, as investigated by zeta potential and FTIR spectra, could be attributed to electrostatic attraction, hydrogen bonding and π-π interaction. Finally, this study suggested that, attributed to its cheap source, simple synthesis process and excellent adsorption performance, CRB was promising in dye removal.

Enhancement of ciprofloxacin sorption on chitosan/biochar hydrogel beads

Biochar is effective in water treatment but it is hard to retrieve or separate biochar powder from aqueous solutions. In this study, the removal of ciprofloxacin from aqueous solutions was investigated using chitosan/biochar hydrogel beads (CBHB). The results showed that the adsorption rate was almost independent of the temperature and occurred at the homogeneous sites of adsorbent thus obeying the Langmuir model. The equilibrium time was varying for different initial concentrations and found to be 48 h for maximum one. The maximum sorption was found to be >76 mg/g of adsorbent out of 160 mg/L as initial concentration. Adsorption obeyed the second-order mechanism with leading role of intra-particle diffusion and outer diffusion. Adsorption capacity decreased from 34.90 mg/g to 15.77 mg/g in the presence of 0.01 N Na₃PO₄ whereas other electrolytes such as NaCl, Na₂SO₄, NaNO₃ with same concentration did not affect the sorption capacity. However, increased concentration of NaCl reduced the sorption capacity to some extent. CBHB showed a mixed mechanism by removing CIP through π-π electron donor-acceptor (EDA) interaction, hydrogen bonding and hydrophobic interaction. The reformation of CBHB with methanol and ethanol instead of water decreased its sorption capacity to 32.69 mg/g and 29.29 mg/g. Adsorption decreased by little after every regeneration of CBHB and was still >64 ± 0.68% (25.73 mg/g) after 6th regeneration. The efficacy of CBHB for CIP removal proved that CBHB is an economical and sustainable adsorbent.

The environmental characteristics and applications of biochar

The environmental deterioration is in a grave situation, and it is urgent to restore the environment. Biochar is a carbon-rich pyrolysis product of feedstock, which has aroused extensive concern due to its broad application potential for getting rid of pollutants and rehabilitating environment. This review generalizes three aspects on biochar, including production and properties, applications and mechanisms, and its modifications. Firstly, the production and characteristics have been summarized, because the practical applications of biochar are highly related to the special characteristics of biochar. Secondly, this paper outlines the latest applications of biochar for environmental remediation, and further provides a critical review on the application mechanisms in environmental restoration. Thirdly, the modification methods and applications of modified biochar are summarized, and all of the ways can be classified into two types: pretreatment of feedstock and modification of primitive biochar. Furthermore, the possible improvements and outlooks of applying biochar in environmental remediation are proposed. This review provides useful information for the application of biochar in environmental restoration.

Humic substances, their microbial interactions and effects on biological transformations of organic pollutants in water and soil: A review

Depicted as large polymers by the traditional model, humic substances (HS) tend to be considered resistant to biodegradation. However, HS should be regarded as supramolecular associations of rather small molecules. There is evidence that they can be degraded not only by aerobic but also by anaerobic bacteria. HS presence alters biological transformations of organic pollutants in water and soil. HS, including humin, have a great potential for an application in aerobic and anaerobic wastewater treatment as well as in bioremediation. Black carbon materials, including char (biochar) and activated carbon (AC), long recognized effective sorbents, have been recently discovered to act as effective redox mediators (RM), which may significantly accelerate degradation of organic pollutants in a way similar to HS. Humic-like coating on the biochar surface has been identified. Explanation of mechanisms and possibility of applications of black carbon materials have only started to be explored. Results of many original and review papers, presented and discussed in this article, show an enormous potential for an interesting, multidisciplinary research as well as for a development of new, green technologies for biological wastewater treatment and bioremediation. Future research areas have been suggested.

Persistent free radicals in carbon-based materials on transformation of refractory organic contaminants (ROCs) in water: A critical review

With the increased concentrations and kinds of refractory organic contaminants (ROCs) in aquatic environments, many previous reviews systematically summarized the applications of carbon-based materials in the adsorption and catalytic degradation of ROCs for their economically viable and environmentally friendly behavior. Interestingly, recent studies indicated that carbon-based materials in natural environment can also mediate the transformation of ROCs directly or indirectly due to their abundant persistent free radicals (PFRs). Understanding the formation mechanisms of PFRs in carbo-based materials and their interactions with ROCs is essential to develop their further applications in environment remediation. However, there is no comprehensive review so far about the direct and indirect removal of ROCs mediated by PFRs in amorphous, porous and crystalline carbon-based materials. The review aims to evaluate the formation mechanisms of PFRs in carbon-based materials synthesized through pyrolysis and hydrothermal carbonization processes. The influence of synthesis conditions (temperature and time) and carbon sources on the types as well as the concentrations of PFRs in carbon-based materials are also discussed. In particular, the effects of metals on the concentrations and types of PFRs in carbon-based materials are highlighted because they are considered as the catalysts for the formation of PFRs. The formation mechanisms of reactive species and the further transformation mechanisms of ROCs are briefly summarized, and the surface properties of carbon-based materials including surface area, types and number of functional groups, etc. are found to be the key parameters controlling their activities. However, due to diversity and complexity of carbon-based materials, the exact relationships between the activities of carbon-based materials and PFRs are still uncertain. Finally, the existing problems and current challenges for the ROCs transformation with carbon-based materials are also pointed out.

Plenty of room for carbon on the ground: Potential applications of biochar for stormwater treatment

Low impact development (LID) systems are increasingly used to manage stormwater, but they have limited capacity to treat stormwater-a resource to supplement existing water supply in water-stressed urban areas. To enhance their pollutant removal capacity, infiltration-based LID systems can be augmented with natural or engineered geomedia that meet the following criteria: they should be economical, readily available, and have capacity to remove a wide range of stormwater pollutants in conditions expected during intermittent infiltration of stormwater. Biochar, a carbonaceous porous co-product of waste biomass pyrolysis/gasification, meets all these criteria. Biochar can adsorb pollutants, improve water-retention capacity of soil, retain and slowly release nutrients for plant uptake, and help sustain microbiota in soil and plants atop; all these attributes could help improve removal of contaminants in stormwater treatment systems. This article discusses contaminant removal mechanisms by biochar, summarizes specific biochar properties that enhance targeted contaminants removal from stormwater, and identifies challenges and opportunities to retrofit biochar in LID to optimize stormwater treatment.

Biochars change the sorption and degradation of thiacloprid in soil: Insights into chemical and biological mechanisms

One interest of using biochar as soil amendment is to reduce pesticide adverse effects. In this paper, the sorption and degradation of thiacloprid (THI) in a black soil amended by various biochars were systematically investigated, and the mechanisms therein were explored by analyzing the changes in soil physicochemical properties, degrading enzymes and genes and microorganism community. Biochar amendment increased THI sorption in soil, which was associated with an increase in organic carbon and surface area and a decrease in H/C. Amendments of 300-PT (pyrolyzing temperature) biochar promoted the biodegradation of THI by increasing the microbe abundance and improving nitrile hydratase (NHase) activity. In contrast, 500- and 700-PT biochar amendments inhibited biodegradation by reducing THI availability and changing NHase activity and THI-degradative nth gene abundance, and instead promoted chemical degradation mainly through elevated pH, active groups on mineral surface and generation of •OH and other free radicals. Furthermore, THI shifted the soil microbial community, stimulated the NHase activity and elevated nth gene abundance. Biochar amendments also changed soil bacterial community by modulating soil pH, dissolved organic matter and nitrogen and phosphorus levels, which further influenced THI biodegradation. Therefore, the impact of biochars on the fate of a pesticide in soil depends greatly on their type and properties, which should be comprehensively examined when applying biochar to soil.

Predicting Cu and Zn sorption capacity of biochar from feedstock C/N ratio and pyrolysis temperature

Biochars have been proposed for remediation of metal-contaminated water due to their low cost, high surface area and high sorption capacity for metals. However, there is a lack of understanding over how feedstock material and pyrolysis conditions contribute to the metal sorption capacity of biochar. We produced biochars from 10 different organic materials by pyrolysing at 450 °C and a further 10 biochars from cedar wood by pyrolysing at 50 °C intervals (250-700 °C). Batch sorption experiments were conducted to derive the maximum Cu and Zn sorption capacity of each biochar. The results revealed an exponential relationship between Cu and Zn sorption capacity and the feedstock C/N ratio and a sigmoidal relationship between the pyrolysis temperature and the maximum Cu and Zn sorption capacity. FTIR analysis revealed that as temperature increased, the abundance of functional groups reduced. We conclude that the high sorption capacity of high temperature biochars is due to an electrostatic attraction between positively charged Cu and Zn ions and delocalised pi-electrons on the greater surface area of these biochars. These findings demonstrate a method for predicting the maximum sorption capacity of a biochar based on the feedstock C/N ratio and the pyrolysis temperature.

Response surface methodology optimization for sorption of malachite green dye on sugarcane bagasse biochar and evaluating the residual dye for phyto and cytogenotoxicity

In the present study, sorption and detoxification of malachite green (MG) dye was executed using biochar resulting after pyrolysis of agro-industrial waste at 400, 600 and 800 °C. Maximum sorption of MG dye (3000 mg/L) was observed on the sugarcane bagasse biochar (SCB) prepared at 800 °C. The interactive effects of different factors like dye concentration, time, pH and temperature on sorption of MG dye were investigated using response surface methodology (RSM). Optimum MG dye concentration, contact time, temperature and pH predicted through Box-Behnken based RSM model were 3000 mg/L MG dye, 51.89 min, 60 °C and 7.5, respectively. ANOVA analysis displayed the non-significant lack of fit value (0.4566), whereas, the predicted correlation coefficient values (R² 0.8494) were reasonably in agreement with the adjusted value (R² 0.9363) demonstrating highly significant model for MG dye sorption. The applicability of this model was also checked through F- test (30.39) with lower probability (0.0001) value. Furthermore, the characterization of SCB was performed using fourier transform infra-red spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscope (SEM), Brunauer-Emmett-Teller surfaces (BET), total organic carbon (TOC) and atomic absorption spectroscopy (AAS). Phyto-toxicity and cytogenotoxicity studies showed successful removal of MG dye using SCB. In addition, the batch sorption studies for reutilization of SCB revealed that the SCB was effective in removal of MG for five repeated cycles. This technology would be effective for treating the toxic textile effluent released from the textile industries.

Waste-based alternative adsorbents for the remediation of pharmaceutical contaminated waters: Has a step forward already been taken?

When adsorption is considered for water treatment, commercial activated carbon is usually the chosen adsorbent for the removal of pollutants from the aqueous phase, particularly pharmaceuticals. In order to decrease costs and save natural resources, attempts have been made to use wastes as raw materials for the production of alternative carbon adsorbents. This approach intends to increase efficiency, cost-effectiveness, and also to propose an alternative and sustainable way for the valorization/management of residues. This review aims to provide an overview on waste-based adsorbents used on pharmaceuticals' adsorption. Experimental facts related to the adsorption behaviour of each adsorbent/pharmaceutical pair and some key factors were addressed. Also, research gaps that subsist in this research area, as well as future needs, were identified. Simultaneously, this review aims to clarify the current status of the research on pharmaceuticals' adsorption by waste-based adsorbents in order to recognize if the right direction is being taken.

Ni2+ and Co2+ adsorption using Tectona grandis biochar: kinetics, equilibrium and desorption studies

In this research, the potential of a relatively new adsorbent prepared from Tectona grandis leaves by pyrolysis for heavy metal removal from aqueous solution was studied. Adsorption behavior of the pyrolytic biochar was investigated with respect to Ni²⁺ and Co²⁺ removal with its affinity examined through batch studies and the mechanism elucidated using different empirical isotherm and kinetic models. A significantly higher efficiency of 92.46% and 91.21% was achieved at a weakly acidic pH of 6 and 5, dose of 3 g L^-1 and 2 g L^-1 for Ni²⁺ and Co²⁺ removal, respectively. Pseudo-second-order kinetics and Langmuir isotherm model best represented the adsorption process for both Ni²⁺ and Co²⁺. Thermodynamic analysis proved the endothermic and spontaneous nature of the process. Desorption studies revealed hydrochloric acid to have a high potential toward eluting the adsorbed metal ions. The well-organized microporous structure, the significant surface area value along with the presence of relative functional groups together with its high adsorption capacity for Ni²⁺ and Co²⁺, revealed the significant adsorptive potential of biochar of teak leaves powder for metal ion removal.

Agronomic benefits of biochar as a soil amendment after its use as waste water filtration medium

In many water-scarce countries, waste water is used for irrigation which poses a health risk to farmers and consumers. At the same time, it delivers nutrients to the farming systems. In this study, we tested the hypotheses that biochar can be used as a filter medium for waste water treatment to reduce pathogen loads. At the same time, the biochar is becoming enriched with nutrients and therefore can act as a fertilizer for soil amendment. We used biochar as a filter medium for the filtration of raw waste water and compared the agronomic effects of this "filterchar" (FC) and the untreated biochar (BC) in a greenhouse pot trial on spring wheat biomass production on an acidic sandy soil from Niger. The biochar filter showed the same removal of pathogens as a common sand filter (1.4 log units on average). We did not observe a nutrient accumulation in FC compared to untreated BC. Instead, P, Mg and K were reduced during filtration while N content remained unchanged. Nevertheless, higher biomass (Triticum L. Spp.) production in BC (+72%) and FC (+37%) treatments (20 t ha^-1), compared with the unamended control, were found. There were no significant differences in aboveground biomass production between BC and FC. Soil available P content was increased by BC (+106%) and FC (+52%) application. Besides, mineral nitrogen content was reduced in BC treated soil and to a lesser extent when FC was used. This may be explained by reduced sorption affinity for mineral nitrogen compounds on FC surfaces. Although the nutrients provided by FC decreased, due to leaching in the filter, it still yielded higher biomass than the unamended control.