Effects of Post-Pyrolysis Air Oxidation of Biomass Chars on Adsorption of Neutral and Ionizable Compounds

Effect of different production methods on physicochemical properties and adsorption capacities of biochar from sewage sludge and kitchen waste: Mechanism and correlation analysis

Different pyrolysis methods, parameters and feedstocks result in biochars with different properties, structures and removal capacities for heavy metals. However, the role of each property on adsorption capacity and corresponding causal relationships remain unclear. Here, we investigated various physicochemical properties of biochar produced via three different methods and two different feedstocks to clarify influences of biomass sources and pyrolysis processes on biochar properties and its heavy metal adsorption performance. Experimental results showed biochars were more aromatic and contained more functional groups after hydrothermal carbonization, while they had developed pores and higher surface areas produced by anaerobic pyrolysis. The inclusion of oxygen resulted in more complete carbonization and higher CEC biochar. Different biochar properties resulted in different adsorption capacities. Biochar produced by aerobic calcination showed higher adsorption efficiency for Cu and Pb. Correlation analysis proved that pH, cation exchange capacity and degree of carbonization positively affected adsorption, while organic matter content and aromaticity were unfavorable for adsorption. Microstructure and components determined biochar macroscopic properties and ultimate adsorption efficiency for metal ions. This study identifies the degree of correlation and pathways of each property on adsorption, which provides guidance for targeted modification of biochar to enhance its performance in heavy metal removal.

Influence of adsorption sites of biochar on its adsorption performance for sulfamethoxazole

In this study, the effects of various types of key adsorption sites on biochar were investigated on its adsorption capacity for sulfamethoxazole (SMX). The biochar obtained by carbonization of corncob at 800 °C (named CC800) was applied to the adsorption of SMX in aqueous environment. The adsorption of SMX by CC800 exhibited a "Three-stage downward adsorption ladder" characteristic in the whole pH range, which was attributed to the different mechanisms corresponding to different adsorption sites of CC800. The organic solvent method and heat treatment method restored the adsorption sites of CC800 after saturated adsorption. And the results revealed that the pore structure and aromatic structure under acidic conditions, and surface functional groups and pore structure under alkaline conditions were confirmed to be key SMX adsorption sites. The adsorption energies of each adsorption mechanism were calculated by density functional theory (DFT), and their order was (-)CAHB (-COO^-) > π⁺-π EDA interaction > (-)CAHB (-O^-) > pore filling mechanism > π-π EDA interaction. Based on the above studies, the adsorption performance of biochar to SMX can be improved by targeted modification of its micropore structure, surface functional groups, and aromatic structures.

Agricultural waste to real worth biochar as a sustainable material for supercapacitor

Biochar made from agricultural waste is gaining more attention in energy field due to its sustainability, low cost, apart from having high supercapacitance performance. Also, it has a wide range of environmental applications, including wastewater treatment, upgrading soil fertility, contaminant immobilization, and in situ carbon sequestration. The existing thermo-chemical methodologies for converting agricultural waste into a sustainable material i.e. biochar and the role of activation agents in enhancing the performance of these materials were critically analyzed and discussed. An overview of recent trends in agricultural waste-derived biochar for supercapacitor electrodes is highlighted in this review that emphasizes green circular economy for encouraging net-zero utility of agriculture waste biomass. The roles of various newly prepared "green" electrolytes in reducing the negative consequences of supercapacitor is also reviewed. The trashing of agricultural waste and the depletion of energy supplies has become a global concern, hurting the world's ecosystem and economy through pollution and a fuel crisis and hence the concept of a green circular economic model is also highlighted.

Influence of thermal air oxidation on the chemical composition and uranium binding property of intrinsic dissolved organic matter from biochar

In this work, uranium (U(VI)) binding characteristics of the intrinsic dissolved organic matters (DOM) from the biochars prepared under thermal air oxidation (TAO) and non-TAO conditions were studied using synchronous fluorescence spectra (SFS) and Fourier transform infrared (FTIR) in conjunction with the general two-dimensional correlation spectroscopy (2D-COS), heterospectral 2D-COS and moving-window (MW) 2D-COS. The chemical compositions of the intrinsic DOMs from biochars with/without TAO were investigated by Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS). Results showed that the preferential binding of U(VI) to functional groups followed the order: 937 (carboxyl γ_C-OH), 981 (carboxyl γ_C-OH), 1511 (aromatic v_C = C), 1108 (esters or ethers v_C-O), 1282 (esters or carboxyl v_C-O), 1698 (saturated carboxylic acid or ketone v_C = O) cm^-1 for biochar DOM after TAO (OB600), and 937 (carboxyl γ_C-OH), 1484 (lipids δ_C-H or phenolic v_C-O), 1201 (esters or carboxyl v_C-O), 1112 (esters or ethers v_C-O), 1706 (saturated aldehyde, carboxylic acid or ketone v_C = O), 1060 (phenolic, esters or ethers v_C-O), 1014 (phenolic, esters or ethers v_C-O) cm^-1 for the pristine biochar (B600). Fulvic-like substances at 375 nm in the biochar DOM showed a preferential binding with U(VI) after TAO, while humic-like substances played a more critical role in the U(VI) complexation with biochar DOM obtained from non-TAO condition. The results also indicated that TAO increased the content of fluorescent DOM and the chemical stability of DOM-U(VI) complexes. The FT-ICR MS results showed an increase in the relative abundance of protein-like, carbohydrates-like, tannins-like, unsaturated hydrocarbons, and condensed aromatic structure and a decrease in the relative abundance of lipid-like and lignin-like after TAO. Consequently, although biochar after TAO had a much poorer content of intrinsic DOM, its intrinsic DOM showed a much higher capacity in U(VI) precipitation. Therefore, the TAO substantially changed the chemical composition, binding property and environmental behavior of intrinsic DOM from biochar.

Enhanced adsorption capacity of activated carbon over thermal oxidation treatment for methylene blue removal: kinetics, equilibrium, thermodynamic, and reusability studies

Activated carbon (AC) is an effective and inexpensive adsorbent material for dye removal, but it cannot always be used repeatedly. Furthermore, the adsorbed dyes with toxicity usually remain on its surface. In this study, a thermal air oxidation process was used to modify the surface of AC and decompose adsorbed methylene blue (MB). The behavior of this process on spent AC was investigated using TGA-DTA, while the degradation of MB before and after the regeneration process was analyzed using a carbon, hydrogen, nitrogen, sulfur (CHNS) analyzer. It was discovered that thermal air oxidation could promote the formation of oxygenated functional groups on AC produced from steam-activated carbon coconut shell (SACCS), which when treated at 350 °C (denoted as SACCS-350), demonstrated an adsorption capacity 2.8 times higher than the non-air-oxidized AC (SACCS). The key parameters for the MB adsorption of SACCS and SACCS-350, such as kinetics, equilibrium, and thermodynamics, were compared. Moreover, the SACCS-350 could be reused at least 3 times for the adsorption of MB. Based on these results, thermal air oxidation treatment could successfully improve the adsorption performance of AC and regenerate spent AC through a reasonable and environmentally friendly process compared to other regeneration methods.

Effect of pH-Dependent Homo/Heteronuclear CAHB on Adsorption and Desorption Behaviors of Ionizable Organic Compounds on Carbonaceous Materials

Herein, the adsorption/desorption behaviors of benzoic acid (BA) and phthalic acid (PA) on three functionalized carbon nanotubes (CNTs) at various pH were investigated, and the charge-assisted H-bond (CAHB) was verified by DFT and FTIR analyses to play a key role. The results indicated that the adsorption order of BA and PA on CNTs was different from K_ow of that at pH 2.0, 4.0, and 7.0 caused by the CAHB interaction. The strength of homonuclear CAHB (≥78.96 kJ·mol^-1) formed by BA/PA on oxidized CNTs is stronger than that of heteronuclear CAHB formed between BA/PA and amino-functionalized CNTs (≤51.66 kJ·mol^-1). Compared with the heteronuclear CAHB (Hysteresis index, HI ≥ 1.47), the stronger homonuclear CAHB leads to clearly desorption hysteresis (HI ≥ 3.51). Additionally, the contribution of homonuclear CAHB (≥52.70%) was also greater than that of heteronuclear CAHB (≤45.79%) at pH 7.0. These conclusions were further confirmed by FTIR and DFT calculation, and the crucial evidence of CAHB formation in FTIR was found. The highlight of this work is the identification of the importance and difference of pH-dependent homonuclear/heteronuclear CAHB on the adsorption and desorption behaviors of ionizable organic compounds on carbonaceous materials, which can provide a deeper understanding for the removal of ionizable organic compounds by designed carbonaceous materials.

Decipher the molecular descriptors and mechanisms controlling sulfonamide adsorption onto mesoporous carbon: Density functional theory calculation and partial least-squares path modeling

Mesoporous carbons (MCs) exhibit excellent removal efficiencies to various organic chemicals. However, how the properties of chemicals influence the adsorption mechanisms and further determine their adsorption onto MCs are poorly understood. We investigated the adsorption of 22 sulfonamides (SAs) onto four MCs, and further uncovered the major molecular descriptors and adsorption mechanisms influencing the adsorption by density functional theory (DFT) and partial least-squares path modeling (PLS-PM). The results revealed that the excess molar refraction (E), McGowan's molar volume (V), energy of the highest occupied molecular orbital (E_HOMO), hardness (H), and most positive net charge on carbon atom (Q_c⁺) were identified as the indirect factors affecting the distribution coefficient (logK_D), by influencing the BE(π-π), BE(H), and logK_ow. BE(π-π) and logK_ow displayed significant direct impacts on logK_D (p < 0.05), while BE(H) showed insignificant direct influences on logK_D (p > 0.05). The PLS-PM results indicate the main driving forces for SAs adsorption including π-π interactions, hydrophobic effects, and hydrogen bonding. This study provides a new perspective on revealing the adsorption mechanisms, and the identified factors can be used to develop the quantitative model to further predict the adsorption of SAs onto MCs.

Novel agrotechnological intervention for soil amendment through areca nut husk biochar in conjunction with vetiver grass

Soil quality management through effective utilization of agricultural residue is the cynosure of intense global research. Therefore, we have explored the pyrolytic conversion of a locally available agricultural residue, the areca nut husk (AH), into biochar (BC) as a sustainable option towards residue management. The AH was carbonized at 250-400 °C, and residence times of 30-90 min. Subsequent detailed analysis revealed areca nut husk biochar (AHBC) formed at 250 °C with 60 min residence time, had the highest soil organic matter yield index (SOMYI), the lowest H/C and O/C ratio, and an average particle size of 1191.6 nm. Further characterization exposed the highly porous structure of prepared AHBC with oxygenated functional groups attached to its surface. The application of AHBC in conjunction with vetiver (Chrysopogon zizanioides L.) was used as a novel agrotechnological approach to assess soil quality improvement. Various doses of AHBC (5 t ha^-1, 10 t ha^-1, and 15 t ha^-1) were applied in the experimental soils, and the principal component analysis (PCA) revealed that the 15 t ha^-1 dose was optimum for the growth of the vetiver. AHBC amendment in soil resulted in increase of plant height and relative water content. This could be attributed to the increase in organic carbon, cation exchange capacity, and nutrients in the soil. Application of AHBC along with vetiver could be a simple, yet effective option, for sustainable agricultural residue and soil management.

Adsorption of polar and ionic organic compounds on activated carbon: Surface chemistry matters

Persistent and mobile organic compounds (PMOCs) are often detected micropollutants in the water cycle, thereby challenging the conventional wastewater and drinking water treatment techniques. Carbon-based adsorbents are often less effective or even unable to remove this class of pollutants. Understanding of PMOC adsorption mechanisms is urgently needed for advanced treatment of PMOC-contaminated water. Here, we investigated the effect of surface modifications of activated carbon felts (ACFs) on the adsorption of six selected PMOCs carrying polar or ionic groups. Among three ACFs, defunctionalized ACF bearing net positive surface charge at neutral pH provides the most versatile sorption efficiency for all studied PMOC types representing neutral, anionic and cationic compounds. Ion exchange capacity giving quantitative information of sorbent surface charges at specified pH is recognized as a frequently underestimated key property for evaluating adsorbents aiming at PMOC adsorption. A most recently developed prediction tool for Freundlich parameters in PMOC adsorption was applied and the prediction results are compared to the experimental data. The comparison demonstrates the so far underestimated importance of the sorbent surface chemistry for PMOC adsorption affinity and capacity. PMOC adsorption mechanisms were additionally investigated by adsorption experiments at various temperatures, pH values and electrolyte concentrations. Exothermic sorption was observed for all sorbate-sorbent pairs. Adsorption is improved for ionic PMOCs on AC carrying sites of the same charge (positive or negative) at increased electrolyte concentration, while not affected for neutral PMOCs unless strong electron donor-acceptor yet weak non-Coulombic interactions exist. Our findings will allow for better design and targeted application of activated carbon-based sorbents in water treatment facilities.

Tuning oxygenated functional groups on biochar for water pollution control: A critical review

Biochar has attracted increasing attention in water pollution control, attributed to its various merits, e.g., tunable physico-chemical properties. The oxygenated functional groups (OFGs) on biochar are key active sites for removing pollutants from water through interfacial adsorption/redox reaction. However, there is still a lack of comprehensive knowledge and perspective on tuning OFGs on biochar for enhanced performance in water pollution control. Here, this review highlighted the mechanisms of biochar OFGs in water pollution control, analyzed the strategies and mechanisms for tuning OFGs on biochar, and investigated the performances of biochars with tuned OFGs in removing inorganic/organic pollutants via adsorption/redox reactions. Specifically, strategies for tuning OFGs on biochar are far more than the well-recognized ex-situ oxidation of pristine biochar. These strategies include in-situ low temperature preservation of hydroxyl and carboxyl, in-/ex-situ oxidation of biochar, and in-/ex-situ grafting of carboxyl on biochar via cycloaddition/acylation reaction. The resultant biochars showed enhanced performances in adsorption (mainly mediated by hydroxyl, carboxyl and ketone through surface complexation, H-bonding, and electrostatic attraction) and redox reaction (mainly mediated by redox-active hydroxyl and ketone). Finally, this review presented future directions on developing biochar with specially tuned surface OFGs as a sustainable high-performance adsorbent/carbocatalyst for water pollution control.

Physicochemical Changes in Biomass Chars by Thermal Oxidation or Ambient Weathering and Their Impacts on Sorption of a Hydrophobic and a Cationic Compound

This study examined conditions that mimic oxidative processes of biomass chars during formation and weathering in the environment. A maple char prepared at the single heat treatment temperature of 500 °C for 2 h was exposed to different thermal oxidation conditions or accelerated oxidative aging conditions prior to sorption of naphthalene or the dication paraquat. Strong chemical oxidation (SCO) was included for comparison. Thermal oxidation caused micropore reaming, with ambient oxidation and SCO much less so. All oxidative treatments incorporated O, acidity, and cation exchange capacity (CEC). Thermal incorporation of O was a function of headspace O₂ concentration and reached a maximum at 350 °C due to the opposing process of burn-off. The CEC was linearly correlated with O/C, but the positive intercept together with nuclear magnetic resonance data signifies that, compared to O groups derived by anoxic pyrolysis, O acquired through oxidation by thermal or ambient routes contributes more to the CEC. Thermal oxidation increased the naphthalene sorption coefficient, the characteristic energy of sorption, and the uptake rate due to pore reaming. By contrast, ambient oxidation (and SCO) suppressed naphthalene sorption by creating a more hydrophilic surface. Paraquat sorption capacity was predicted by an equation that includes a CEC² term due to bidentate interaction with pairs of charges, predominating over monodentate interaction, plus a term for the capacity of naphthalene as a reference representing nonspecific driving forces.

Co-transport and retention of zwitterionic ciprofloxacin with nano-biochar in saturated porous media: Impact of oxidized aging

While biochar (BC) is used for contaminant remediation (i.e. antibiotics) in the field, geochemical aging can alter its chemical structure, releasing nano-sized BC (NBC, sizes ranging from approximately 200 nm to 500 nm), and further influence the environmental behaviour of antibiotics affiliated with BC. In this study, we comprehensively examined the sorption behaviour of NBCs with and without aging toward ciprofloxacin (CIP), their aggregation performance, and transport behaviour in porous media. The results showed that aging improved the oxygen-containing groups within the NBCs and made their surfaces more negatively charged. The thermodynamic enhancements of specific interactions (i.e. π-π interaction or Coulombic force) with CIP resulted in the enhancement of slow sorption (from 60-64% to 40-58%) and a higher normalised sorption capacity (Q_e). The aggregation of NBCs was affected by changes in individual specific interactions and interfacial forces between the NBCs before and after CIP sorption. Further, aging could enhance the transport of NBCs both in the absence and presence of CIP. In addition to the interaction with the quartz sand surface, the contributions of aggregation and chemical heterogeneity caused by rebalanced specific interactions with CIP, may explain the observed transport behaviours of the aged NBCs in porous media. Additionally, the presence of NBCs, regardless of aging, suppressed the transport of CIP. Thus, mechanisms such as increased sorption sites due to aggregation and competitive sorption between NBCs and CIP, rather than the contribution of co-transport from NBCs, might play an important role in determining the fate of CIP in the natural environment.

Effect of granular activated carbon and other porous materials on thermal decomposition of per- and polyfluoroalkyl substances: Mechanisms and implications for water purification

Thermal treatment is routinely used to reactivate the spent granular activated carbon (GAC) from water purification facilities. It is also an integral part of sewage sludge treatment and municipal solid waste management. This study presents a detailed investigation of the fate of per- and polyfluoroalkyl substances (PFAS) and one PFAS alternative (GenX) in thermal processes, focusing on the effect of GAC. We demonstrate that the thermolysis of perfluoroalkyl carboxylic acids (PFCAs), including perfluorooctanoic acid (PFOA), and GenX can occur at temperatures of 150‒200 °C. Three temperature zones were discovered for PFOA, including a stable and nonvolatile zone (≤90 °C), a phase-transfer and thermal decomposition zone (90‒400 °C), and a fast decomposition zone (≥400 °C). The thermal decomposition began with the homolysis of a C‒C bond next to the carboxyl group of PFCAs, which formed unstable perfluoroalkyl radicals. Dual decomposition pathways seem to exist. The addition of a highly porous adsorbent, such as GAC or a copolymer resin, compressed the intermediate sublimation zone of PFCAs, changed their thermal decomposition pathways, and increased the decomposition rate constant by up to 150-fold at 250 °C. The results indicate that the observed thermal decomposition acceleration was linked to the adsorption of gas-phase PFCA molecules on GAC. The presence of non-activated charcoals/biochars with a low affinity for PFOA did not accelerate its thermal decomposition, suggesting that the π electron-rich, polyaromatic surface of charcoal/GAC played an insignificant role compared to the adsorbent's porosity. Overall, the results indicate that (1) substantial decomposition of PFCAs and GenX during conventional thermal GAC/sludge/waste treatment is very likely, and (2) the presence or addition of GAC or other highly porous materials can accelerate thermal PFAS decomposition and alter decomposition pathways.

Coupling experiments with calculations to understand the thermodynamics evolution for the sorption of zwitterionic ciprofloxacin on oxidizing-aged pyrogenic chars in the aquatic system

Oxidized aging due to the long-term exposure can significantly alter the sorption of pyrogenic chars (i.e., biochar, BC) towards antibiotics, which determined their fates in natural environments. In this study, the sorption of ciprofloxacin (CIP) on the oxidizing-aged BCs was studied linking the experimental thermodynamics and theoretical calculations. Results revealed that Q₀ of CIP negatively correlated with their average site energies (E_m), while pore-normalized Q₀ on aged BCs were 2-6 folds higher than fresh BCs. From competitive sorption, it is proposed that the transformation of CIP^± to CIP⁺ occurred and the π⁺-π electron donor-acceptor interaction and Coulombic attraction onto the aged BCs played a critical role. These two specific interactions with CIP were thermodynamically improved when aging degree increased and favored the free energies (Δ_aG) of sorption by 2-5 kJ mol^-1. Based on the identified relationship between experimental Δ_OA-ΔG⁰ with E_a through DFT calculations, the contributions of the specific interactions to antibiotic sorption on aged BCs were quantified. This study provided an in-depth understanding of how the aging process affects the sorption of zwitterionic antibiotics on BCs and also possibilities to predict the fate of antibiotics in the presence of BCs over a long-term period.

Coating magnetite alters the mechanisms and site energy for sulfonamide antibiotic sorption on biochar

Magnetite-coating biochar (MBC) is a promising remediator for antibiotic contamination. Accurate models describing the sorption affinity are required to better understand the role of minerals. In this study, the presence of magnetite led to the improvements of oxygen-containing groups (i.e. C˭O) and regulation of π-systems within BC. Based on Dubinin-Ashtakhov (DA) model, the differences of site energy (E_m) and sorption heterogeneity (σ_e*) led to the variances between sorption capacities of sulfonamides (SAs). The positive correlations between E_m and the oxygen content or pore volume of MBCs indicated that π-π interactions, H-bonding, and pore-filling may act as the high energy sites. Moreover, σ_e* was related to the distribution of magnetite on BC and their porosities. These results suggested that compared to BCs, the coating minerals improved the π-interaction assisted H-bonding and proton configuration of antibiotic when sorbing on MBC. The negative correlations between the E_m of different SAs with their molecular sizes and solubilities resulted from steric effects and competition with water, which further confirmed the proposed high energy sites on MBCs. This study provided the insightful information of site energy distribution and understanding of fate and transport of organic pollutants on BC when the iron minerals were embedded or coated.

Air activation of charcoal monoliths for capacitive energy storage

Charcoal monoliths derived from waste wood were activated with air for the application of electrochemical capacitor electrodes and an insight was given into the activation mechanism. The mild air activation is effective and pollution-free compared to the common chemical activation using KOH etc. for the preparation of crack-free carbon monoliths. The activation process was controlled by altering the activation temperature and time, and their effects on the nanostructure of charcoal monoliths were studied. As the activation temperature or time increased, air eroded the defective surface of charcoal layer-by-layer, with the oxygen atoms being introduced by chemisorption and oxidation reactions and removed by dehydration and decomposition reactions. Meanwhile, micro-pores were produced. The electrode activated at 300 °C for 1 h, with a specific surface area of 567 m² g⁻¹ and a high micro-porosity of 86%, exhibited a specific capacitance of 203 F g⁻¹ and 35.5 F cm⁻³. Moreover, it presented a higher total capacitance of 3.6 F cm⁻² than most reported pellet electrodes. These findings give a reasonable picture of the air activation process and are instructive to prepare activated carbon monoliths under an oxidizing environment.

Charcoal monoliths derived from waste wood were activated with air for the application of electrochemical capacitor electrodes and an insight was given into the activation mechanism.

Sorption mechanisms of antibiotic sulfamethazine (SMT) on magnetite-coated biochar: pH-dependence and redox transformation

Sorption of sulfonamides (SAs) on magnetite-coated biochar (MBC) is a promising approach for the remediation of antibiotic contaminants, due to its extended adsorption capacity and irreversibility. However, the actual sorption mechanisms of SAs on MBC remain unclear and the gap in knowledge hinders understanding of the fate of SAs in soils or sediments. In this study, various MBCs were prepared under different pyrolysis temperatures, with batch sorption experiments conducted using SMT as the model pollutant. Results of a two-compartment kinetic model demonstrated that aromatic components of MBCs dominated slow-sorption mechanisms, whereas the embedded magnetite further accelerated fast-sorption due to H-bonding. Modification of BC with magnetite improved the distribution coefficient (K_d) and isotherm linearity of SMT. Multi-parameter model results indicated that the pH-dependence of SMT sorption on BCs and MBCs occurred via a dominant mechanism of π-bond assisted H-bonding. Compared to pristine BCs, the change in pH-dependent sorption characteristics of SMT on MBC results from the regulation of π-bonding and proton configuration. Simultaneous transformation of SMT to sulfate ions on BCs or MBCs was also observed. The degradation of SMT occurred because of persistent free radicals (PFRs) on BCs or the inherent redox of iron minerals on MBCs. However, the small fraction of SMT transformed on BCs or MBCs was not found to result in overestimation of SMT sorption. This study presents the critical mechanisms of SMT sorption on pyrochars and provides novel understanding of the fate of SMT on carbonaceous materials during practical remediation applications.

Chemical Speciation, Plant Uptake, and Toxicity of Heavy Metals in Agricultural Soils

Heavy metals in agricultural soils exist in diverse dissolved (free cations and complexed species of positive, neutral, or negative charges), particulate (sorbed, structural, and coprecipitated), and colloidal (micro- and nanometer-sized particles) species. The fate of different heavy metal species is controlled by the master variables: pH (solubility), ionic strength (activity and charge-shielding), and dissolved organic carbon (complexation). In the rhizosphere, chemical speciation controls toxicokinetics (uptake and transport of metals by plants) while toxicodynamics (interaction between the plant and absorbed species) drives the toxicity outcome. Based on the critical review, the authors recommend omics and data mining techniques to link discrete knowledge bases from the speciation dynamics, soil microbiome, and plant transporter/gene expression relevant to homeostasis conditions of modern agriculture. Such efforts could offer a disruptive application tool to improve and sustain plant tolerance, food safety, and environmental quality.

Formulation of Biochar-Based Phosphorus Fertilizer and Its Impact on Both Soil Properties and Chickpea Growth Performance

There is no alternative to phosphorus (P) in agriculture as it is second most important plant nutrient after nitrogen. Mineral P fertilizers are derived from rock phosphate (RP) which is finite, non-renewable and geographically restricted to a few countries, thus its shortage likely affects agriculture in near future as the world population is growing at a greater pace. This could increase P inputs in agriculture in order to meet rising food demands which may result in the depletion of RP reserves. Furthermore, P losses from farmlands in case of mineral P fertilizers also demands the sustainable use of P not only because of its finite resources but also the environmental concerns associated with P fertilization such as eutrophication. The present study was designed to formulate biochar-based P fertilizer that would help in the sustainable use of P fertilizer. Biochar(s) were prepared using wheat straw at 350–400 °C pyrolytic temperature followed by enrichment with Di-ammonium phosphate (DAP) taking into account all possible combination of DAP to biochar on the w/w basis (0:100, 25:75, 50:50, 75:25 and 100:0). Enrichment was carried out using two different methods i.e., phosphorus enriched biochar (PEB1) by hot method and cold method (PEB2). An incubation experiment was performed to assess the impact of each biochar on selected properties of soil. The treatments were organized in factorial arrangement under complete randomized design (CRD) with three replications. Both the amendments were applied at rate of 1% of dry soil on a w/w basis. A significant increase in soil extractable P and total nitrogen (N) was recorded for the ratio 50:50 as compared to control as well of rest of treatments. Similarly, high organic contents were found for both PEB1 and PEB2 at the ratio 50:50. An incubation experiment was followed by pot trial using 50:50 for both PEB1 and PEB2 and split doses of recommended P were applied (0%, 25%, 50% and 100%) with a control under CRD with three replications using chickpea as test crop. Both PEB1 and PEB2 with 50% P have significantly improved crop growth, yield, nodulation, and plant physiological and chemical parameters as compared to a recommended dose of P alone. The result may imply that the integration of P-enriched biochar and chemical fertilizer could be an effective approach to improve chickpea production and soil properties.

Evaluating biochar and its modifications for the removal of ammonium, nitrate, and phosphate in water

Removal of nitrogen (N) and phosphorus (P) from water through the use of various sorbents is often considered an economically viable way for supplementing conventional methods. Biochar has been widely studied for its potential adsorption capabilities for soluble N and P, but the performance of different types of biochars can vary widely. In this review, we summarized the adsorption capacities of biochars in removing N (NH₄-N and NO₃-N) and P (PO₄-P) based on the reported data, and discussed the possible mechanisms and influencing factors. In general, the NH₄-N adsorption capacity of unmodified biochars is relatively low, at levels of less than 20 mg/g. This adsorption is mainly via ion exchange and/or interactions with oxygen-containing functional groups on biochar surfaces. The affinity is even lower for NO₃-N, because of electrostatic repulsion by negatively charged biochar surfaces. Precipitation of PO₄-P by metals/metal oxides in biochar is the primary mechanism for PO₄-P removal. Biochars modified by metals have a significantly higher capacity to remove NH₄-N, NO₃-N, and PO₄-P than unmodified biochar, due to the change in surface charge and the increase in metal oxides on the biochar surface. Ambient conditions in the aqueous phase, including temperature, pH, and co-existing ions, can significantly alter the adsorption of N and P by biochars, indicating the importance of optimal processing parameters for N and P removal. However, the release of endogenous N and P from biochar to water can impede its performance, and the presence of competing ions in water poses practical challenges for the use of biochar for nutrient removal. This review demonstrates that progress is needed to improve the performance of biochars and overcome challenges before the widespread field application of biochar for N and P removal is realized.

Wood-based activated biochar to eliminate organic micropollutants from biologically treated wastewater

Implementing advanced wastewater treatment (WWT) to eliminate organic micropollutants (OMPs) is a necessary step to protect vulnerable freshwater ecosystems and water resources. To this end, sorption of OMP by activated carbon (AC) is one viable technology among others. However, conventional AC production based on fossil precursor materials causes environmental pollution, including considerable emissions of greenhouse gases. In this study, we produced activated biochar (AB) from wood and woody residues by physical activation and evaluated their capability to eliminate OMPs in treated wastewater. Activated biochar produced under optimized conditions sorbed 15 model OMPs, of which most were dissociated at circumneutral pH, to the same or higher extent than commercial AC used as a reference. While wood quality played a minor role, the dosage of the activation agent was the main parameter controlling the capacity of ABs to eliminate OMP. Our results highlight the possibility for local production of AB from local wood or woody residues as a strategy to improve WWT avoiding negative side effects of conventional AC production.

Thermal oxidation activation of hydrochar for tetracycline adsorption: the role of oxygen concentration and temperature

Poplar hydrochar (RHC) was activated by thermal oxidation (TA-O) in air at 300 °C (O300) and in air + N2 (0.5% O2) at 500 and 700 °C (O500 and O700), respectively, and in N2 at 300-700 °C (N300-N700) as control. Samples characterized by various methods were used to analyze their effect on tetracycline adsorption. The results showed that TA-O greatly increased adsorption capacity qe, 100 (mg·g-1, C0 = 100 mg·L-1) from 6.29 for RHC to 33.32, 96.23 and 60.90 for O300, O500 and O700, respectively. The O300 increased carboxyl and aromaticity whereas little influenced on porosity. The O500, with the highest SBET and Smicro, enhanced adsorption probably by micropore filling and π-π interactions. The O700 fused micropore into mesopore but decreased the SBET, Smicro and qe, 100. Thus, thermal oxidation at 500 °C and 0.5% O2 is recommended for hydrochar activation to absorb tetracycline.

Biochar as simultaneous shelter, adsorbent, pH buffer, and substrate of Pseudomonas citronellolis to promote biodegradation of high concentrations of phenol in wastewater

Microbial degradation is an elimination method for removal of organic contaminants from soil and water. However, the main factor limiting its practical application is high bacterial sensitivity to environmental factors such as pH, toxicity, and mass transfer. In this study, biochar was produced pyrolytically from peanut shells at 350 °C, 550 °C, and 750 °C (referred to as BC350, BC550, and BC750, respectively) and their promotion on phenol biodegradation in wastewater by the bacterium Pseudomonas citronellolis was investigated. Higher initial phenol concentration (>400 mg L^-1) showed obvious inhibition on biodegradation with the removal efficiencies being less than 46%, and even the bacterium failed to survive at the phenol concentrations of higher than 1000 mg L^-1. With biochar incorporated, the removal efficiencies of phenol increased from below 46% to up to 99% at the initial concentrations of 400-1200 mg L^-1. Immobilization of strains in biochar by calcium alginate further increased the microbial tolerance to high concentrations of phenol (i.e., 63% removal at 1200 mg L^-1). Scanning electron microscopy demonstrated that biochar acted as shelter to support the bacterium to struggle with extreme conditions. The initial adsorption of phenol by biochar alleviated the initial toxicity of phenol to bacterium and the subsequent gradual desorption controlled the bioavailability of phenol. In this regard, BC350 showed a comparable sorption capacity with BC550 and BC750, while a higher desorption potential than them, thus balanced better the toxicity and bioavailability of phenol to microbes. Alkalinity of BC550 and BC750 played important roles in rescuing the microbes from being damaged by pH shock via neutralizing the fast generation of acidic intermediates. The extractable organic substances in BC350 could be consumed by bacterium as substrates, which was confirmed by incubating the strains in water-extractable solution. Results of this study indicate that incorporation of microbes with biochar could promote the biodegradation of high concentration organic wastewater.

Post-engineering of biochar via thermal air treatment for highly efficient promotion of uranium(VI) adsorption

Biochar from pyrolysis/gasification is relatively poor in oxygen-containing groups and low in micro/mesoporosity, which constrains its adsorption performance. Here, thermal air treatment (TAT) at a mild condition (300 °C in air) was applied to oxygenate the surfaces of various biochars and modify their pore structures for the promotion of their uranium (U(VI)) adsorptions. Results showed that TAT had a high product yield (>76%), increased the O contents, O/C ratios and O-containing groups in biochars, and substantially developed the micro/mesoporosities of biochars. Batch adsorption results showed that TAT remarkably improved U(VI) adsorption capacities of various biochars. Specifically, the maximum U(VI) adsorption capacities of ash-poor corn cob biochar and ash-rich sewage sludge biochar were increased by 137% to 163 mg/g and 23% to 97 mg/g, respectively. Thus, TAT might be a promising strategy to engineer various biochars for adsorptive applications.

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.

Oxygen-rich biochar from torrefaction: A versatile adsorbent for water pollution control

Compared to pyrochar (PC), little is known about the capability of torrefaction char (TC) in water pollution control. In this study, the physicochemical properties of TC and PC, and their adsorption performances for uranium (U(VI)) and methylene blue (MB) were investigated. Results showed that TC was higher in oxygen content, and richer in oxygen-containing functional groups. The maximum U(VI) and MB adsorption capacities were increased from 56.21 and 192.67 mg/g for PC, respectively, to >100 and >350 mg/g for TC, respectively, indicating that TC was much more efficient than PC. Furthermore, torrefaction atmosphere affected the adsorption performance of resulting TC. For example, TC from N₂ was more efficient in MB adsorption, while TC from air was more efficient in U(VI) adsorption. Thus, attributed to the lower processing temperature, simpler preparation route, and higher adsorption capacity, TC could be a competent candidate for water pollution control.

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.

Understanding Activation Effects on Low-Temperature Biochar for Optimization of Herbicide Sorption

Activation treatments are often used as a means of increasing a biochar’s sorption capacity for agrochemical compounds but can also provide valuable insight into sorption mechanisms. This work investigates the effects of H2O2 activation on a low-temperature (350 °C) grape wood biochar, evaluates subsequent changes to the removal efficiency (RE) of cyhalofop and clomazone, and elucidates potential sorption mechanisms. Activation by H2O2 decreased the biochar pH, ash content, and C content. Additionally, the biochar O content and surface area increased following activation, and Fourier transform infrared spectroscopy (FTIR) data suggested a slight increase in surface O groups and a decrease in aliphatic C. Cyhalofop RE significantly increased following activation, while clomazone RE was unchanged. The increased sorption of cyhalofop was attributed to pH effects and charge-based interactions with biochar O moieties. Results from this study suggest that H2O2 activation treatments on low-temperature biochars may improve the removal of organic acid herbicides but are of little value in optimizing the removal of polar, non-ionizable herbicides.

Mechanisms and quantification of adsorption of three anti-inflammatory pharmaceuticals onto goethite with/without surface-bound organic acids

Nowadays non-steroidal anti-inflammatory drugs (NSAIDs) are often detected in surface water and groundwater. In this study, effects of environmental factors, i.e., solution pH, ionic strength, temperature and surface-bound organic acids, on bonding of three typical NSAIDs (ketoprofen, naproxen and diclofenac) onto goethite were systematically investigated. Column chromatography, batch experiments, attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy and surface complexation modeling were used to probe the adsorption mechanisms. Bonding of three NSAIDs onto goethite was totally reversible, ionic strength-dependent and endothermic (adsorption enthalpy 2.86-9.75 kJ/mol). These evidences supported H-bonding mechanism, which was further explained by ATR-FTIR observation and a triple planes model. Surface-bound organic acids (phthalic acid, trimellitic acid and pyromellitic acid) by inner-sphere complexation with goethite were hard to be desorbed. Surface-bound phthalic acid increased the uptake of NSAIDs but surface-bound trimellitic acid and pyromellitic acid reduced their adsorption. The reason is that the adsorbed phthalic acid can result in a more hydrophobic surface while adsorbed trimellitic acid and pyromellitic acid increased the surface negative charge and polarity. Finally, adsorption of NSAIDs onto goethite with/without surface-bound organic acids was well described by a free energy model, in which contributions of interactions (e.g., H-bonding and van der Waals) were evaluated.

Carbonization and ball milling on the enhancement of Pb(II) adsorption by wheat straw: Competitive effects of ion exchange and precipitation

Straw biomass is a promising adsorbent for the removal of heavy metals. To improve its Pb(II) adsorption capacity and elucidate competition of adsorption mechanisms (e.g., ion exchange and precipitation), the Pb(II) adsorption mechanisms for wheat straw (WS-CK), wheat straw-biochar (WS-BC), and ball-milled wheat straw-biochar (WS-BC + BM) samples were investigated in detail by EDX, XRD, and FTIR. The results implied that the Pb(II) adsorption capacities at an adsorbent dosage of 0.2 g/L onto WS-CK, WS-BC, and WS-BC + BM were 46.33, 119.55, and 134.68 mg/g, respectively. This indicates that carbonization and ball milling are efficient techniques for improving the adsorption capacity of Pb(II) onto wheat straw, as WS-BC and WS-BC + BM exhibited adsorption capacities comparable to other commonly used bioadsorbents. Carbonization contributed significantly to precipitation (e.g., PbCO₃ and Pb₃(CO₃)₂(OH)₂). Furthermore, competition existed between ion exchange and precipitation during the Pb(II) adsorption process. With relative lower adsorbent dosages, carbonization and ball milling enhanced ion exchange capacity.

Production of bioadsorbent from phosphoric acid pretreated palm kernel shell and coconut shell by two-stage continuous physical activation via N2 and air

In the present study, agricultural biomass-palm kernel shell (PKS) and coconut shell (CS)-was used to produce high porosity bioadsorbent using two-stage continuous physical activation method with different gas carrier (air and N2) in each stage. The activation temperature was set constant at 600, 700, 800 or 900°C for both activation stages with the heating rate of 3°C min-1. Two parameters, the gas carrier and activation temperature, were determined as the significant factors on the adsorption properties of bioadsorbent. BET, SEM, FTIR, TGA, CHNS/O and ash content were used to elucidate the developed bioadsorbent prepared from PKS and CS and its capacity towards the adsorption of methylene blue and iodine. The novel process of two-stage continuous physical activation method was able to expose mesopores and micropores that were previously covered/clogged in nature, and simultaneously create new pores. The synthesized bioadsorbents showed that the surface area (PKS: 456.47 m2 g-1, CS: 479.17 m2 g-1), pore size (PKS: 0.63 nm, CS: 0.62 nm) and pore volume (PKS: 0.13 cm3 g-1, CS: 0.15 cm3 g-1) were significantly higher than that of non-treated bioadsorbent. The surface morphology of the raw materials and synthesized bioadsorbent were accessed by SEM. Furthermore, the novel process meets the recent industrial adsorbent requirements such as low activation temperature, high fixed carbon content, high yield, high adsorption properties and high surface area, which are the key factors for large-scale production of bioadsorbent and its usage.

Adsorption and desorption of nitrous oxide by raw and thermally air-oxidized chars

Addition of biomass chars (biochar) to soil reportedly suppresses emissions of nitrous oxide (N₂O), a potent greenhouse and ozone-depleting gas, but the causes and endurance of the effect are unclear. To determine whether adsorption may play a role, adsorption isotherms of N₂O were constructed at 273 K on outgassed samples of anoxically-prepared wood-derived chars (300-700 °C) and on a subset briefly reheated in air at 400 °C. Sorption by the chars was greater and more reversible than sorption by soils or soil mineral phases. Adsorption by chars increased with pyrolysis temperature and upon post-pyrolysis air oxidation. The Langmuir maximum capacity correlates well with the CO₂-determined (but not N₂-B.E.T.-determined) surface area. At environmentally realistic partial pressures in soil, N₂O adsorption correlates with CO₂ adsorption, and is found to predominate in the micropores (<1.5 nm), especially ultramicropores (<0.7 nm). Neither adsorption nor adsorption reversibility was affected by coating the char with soil organic matter extract. It is concluded that char added at levels above 1% in soil would act as a strong and reversible sink for N₂O, and could be responsible for the temporary nature of N₂O emission suppression observed in some cases.

Charge-assisted hydrogen bonding as a cohesive force in soil organic matter: water solubility enhancement by addition of simple carboxylic acids

Weak bonds between molecular segments and between separate molecules of natural organic matter (OM) govern OM solubility, adsorption, supramolecular association in solution, and complexation with metal ions and oxides. We tested the hypothesis that especially strong hydrogen bonds, known as (negative) charge-assisted hydrogen bonds, (-)CAHB, contribute significantly to OM cohesion and play a role in the water solubility of solid-phase OM. The (-)CAHB, exemplified by structures such as (-CO2HO2C-)- and (-CO2HO-)-, may form between weak acids with similar proton affinity, and is shorter, more covalent, and much stronger than ordinary hydrogen bonds. Using a high-organic reference soil, we show that (-)CAHBs within the solid OM phase (intra-OM) are disrupted by solutions of aliphatic and aromatic acids, resulting in enhanced solubility of OM. The aromatic acids included naturally occurring plant exudate compounds. At constant pH and ionic strength, OM solubility increased with added organic acid concentration and molecular weight. Polar compounds incapable of forming (-)CAHBs, such as alkanols, acetonitrile, and dimethyl sulfoxide, were ineffective. Solubility enhancement showed behavior consistent with (-)CAHB theory and published observations-namely, (i) that formate is more effective than acetate due to its tendency to form stronger (-)CAHBs; (ii) that solubility enhancement peaks at pH ∼5-6, where the product of the concentration of the interacting carboxylate ions reaches a maximum; and (iii) that elution of acetate or formate through soil columns releases hydroxide ion, consistent with formation of (-)CAHBs between added acid and free weak acid groups on the solid OM. The results support the hypothesis that the (-)CAHB contributes to the cohesion of OM in the solid state.

Water clusters contributed to molecular interactions of ionizable organic pollutants with aromatized biochar via π-PAHB: Sorption experiments and DFT calculations

Molecular interactions between biochars and ionizable organic pollutants (IOPs) are of great concern in natural environments, however the role of water clusters on the biochar surface remain unclear. The pH-dependent adsorption of aniline, phenol, 2-chlorophenol, 3-chlorophenol, 4-chlorophenol, 4-methylphenol and 4-nitrophenol onto bamboo wood derived biochar (BW700) as a model was conducted to identify conventional and novel interaction mechanisms between aromatized surface and IOPs. The dissociation constant (pK_a,surface) of surface functional groups of BW700 was characterized by acid-base titration and Zeta potential measurements. The pH-dependent adsorption behavior depended on the pK_a,IOP of IOPs and also related to the pK_a,surface of biochar surface. An obvious peak of adsorption coefficients (K_d) in the range of solution pH was shaped at pH_peak = (pK_a,IOP + pK_a,surface)/2, which cannot be well explained by the conventional mechanisms such as hydrophobic effects, π-π interaction, electrostatic attractions, and hydrogen-binding. The contribution of ice-like adlayer (water clusters) on aromatic surface as H-acceptors is proposed for the first time to the adsorption peak of IOP as H-donors at pH_peak. Density functional theory (DFT) calculations provided a possible structure of the complex combined with ice-like adlayer and aromatic substrate of BW700, and indicated that the adsorbing peak resulted from the multiple π-bond and polarization assisted H-bond (π-PAHB) interactions. Three distinct properties of π-PAHB were given, based on multiple π-bond, hydrophobicity-dependence and pH sensitivity. This novel mechanism extends the definition of H-bonds for better understanding the molecular interactions of IOP with carbonaceous materials and their environmental fate.

Thermal treatment of biochar in the air/nitrogen atmosphere for developed mesoporosity and enhanced adsorption to tetracycline

For the purpose of producing carbons with developed mesoporosity, a wood biochar was thermally treated at 600-800 °C in the air/nitrogen atmosphere. The mesopore development was observed when the air flux increased to 50-90 mL/min, and the carbon product having high mesopore surface area (316 m²/g) and mesopore pore volume (0.284 cm³/g) was produced at the treatment temperature of 700 °C. The mesopores were developed mainly in the temperature holding stage of thermal treatment, with size mainly ranged from 20 to 60 Å. The carbons' adsorption to the antibiotic tetracycline was enhanced by 5.5-9.2 folds when the air/nitrogen mixture was used instead of nitrogen atmosphere in thermal treatment, and the enhanced adsorption is positively related to the mesopore development. In general, this research provides a facile way to produce carbons with developed mesoporosity, so as to improve their adsorption to bulky organic molecules.

Enhanced biochars can match activated carbon performance in sediments with high native bioavailability and low final porewater PCB concentrations

A bench scale study was conducted to evaluate the effectiveness of in situ amendments to reduce the bioavailability of pollutants in sediments from a site impacted with polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) and cadmium. The amendments tested included fine and coarse coal-based activated carbons (AC), an enhanced pinewood derived biochar (EPB), organoclay, and coke dosed at 5% of sediment dry weight. Strong reductions in total PCB porewater concentrations were observed in sediments amended with the fine AC (94.9-99.5%) and EPB (99.6-99.8%). More modest reductions were observed for the coarse AC, organoclay, and coke. Strong reductions in porewater PCB concentrations were reflected in reductions in total PCB bioaccumulation in fresh water oligochaetes for both the fine AC (91.9-96.0%) and EPB (96.1-96.3%). Total PAH porewater concentrations were also greatly reduced by the fine AC (>96.1%) and EPB (>97.8%) treatments. EPB matched or slightly outperformed the fine AC throughout the study, despite sorption data indicating a much stronger affinity of PCBs for the fine AC. Modeling EPB and fine AC effectiveness on other sediments confirmed the high effectiveness of the EPB was due to the very low final porewater concentrations and differences in the native bioavailability between sediments. However, low bulk density and poor settling characteristics make biochars difficult to apply in an aquatic setting. Neither the EPB nor the fine AC amendments were able to significantly reduce Cd bioavailability.

Thermal air oxidation changes surface and adsorptive properties of black carbon (char/biochar)

In this study, we systematically investigated the effects of thermal air oxidation on the properties of biomass-derived black carbon (BC) made at carbonization temperatures (HTTs) of 300-700°C. BC produced by including air in the carbonization step was found to have a low surface area and underdeveloped pore structure. Substantial changes of BC were observed after post-pyrolysis thermal air oxidation (PPAO). Well-carbonized BC samples made anoxically at relatively high HTTs (600 and 700°C) showed, after PPAO, significant increases in N₂ BET surface area (SA) (up to 700 times), porosity (<60Å) (up to 95 times), and adsorptivity (up to 120 times) of neutral organic species including two triazine herbicides and one natural estrogen. Partially carbonized BC made at a lower HTT (300 or 400°C) showed moderate increases in these properties after PPAO, but a large increase in the intensity of Fourier transform infrared spectroscopy bands corresponding to various oxygen-containing functional groups. Well-carbonized BC samples, on the other hand, were deficient in surface oxygen functionality even after the PPAO treatment. Adsorption of the test organic compounds on BC generally trended with BET SA when it was less than 300m²/g, but BET SA was poorly predictive of adsorption when it was greater than 300m²/g. Overall, our results suggest that thermal reactions between molecular oxygen and BC 1) increase surface oxygen functionality more effectively for low-HTT than for high-HTT BC samples; 2) increase SA and porosity (<60Å) especially for high-HTT BC samples; and 3) create new adsorption sites and/or relieve steric restriction of organic molecules to micropores, thereby enhancing the adsorptivity of BC. These results will prove useful not only for understanding the fate of environmental BC but also in devising strategies for improving the practical performance of the engineered form of BC (i.e., biochar).

Sorption of ionizable and ionic organic compounds to biochar, activated carbon and other carbonaceous materials

The sorption of ionic and ionizable organic compounds (IOCs) (e.g., pharmaceuticals and pesticides) on carbonaceous materials plays an important role in governing the fate, transport and bioavailability of IOCs. The paradigms previously established for the sorption of neutral organic compounds do not always apply to IOCs and the importance of accounting for the particular sorption behavior of IOCs is being increasingly recognized. This review presents the current state of knowledge and summarizes the recent advances on the sorption of IOCs to carbonaceous sorbents. A broad range of sorbents were considered to evaluate the possibility to read across between fields of research that are often considered in isolation (e.g., carbon nanotubes, graphene, biochar, and activated carbon). Mechanisms relevant to IOCs sorption on carbonaceous sorbents are discussed and critically evaluated, with special attention being given to emerging sorption mechanisms including low-barrier, charge-assisted hydrogen bonds and cation-π assisted π-π interactions. The key role played by some environmental factors is also discussed, with a particular focus on pH and ionic strength. Overall the review reveals significant advances in our understanding of the interactions between IOCs and carbonaceous sorbents. In addition, knowledge gaps are identified and priorities for future research are suggested.

Structural Transformation of Biochar Black Carbon by C60 Superstructure: Environmental Implications

Pyrogenic carbon is widespread in soil due to wildfires, soot deposition, and intentional amendment of pyrolyzed waste biomass (biochar). Interactions between engineered carbon nanoparticles and natural pyrogenic carbon (char) are unknown. This study first employed transmission electron microscopy (TEM) and X-ray diffraction (XRD) to interpret the superstructure composing aqueous fullerene C₆₀ nanoparticles prepared by prolonged stirring of commercial fullerite in water (nC₆₀-stir). The nC₆₀-stir was a superstructure composed of face-centered cubic (fcc) close-packing of near-spherical C₆₀ superatoms. The nC₆₀-stir superstructure (≈100 nm) reproducibly disintegrated pecan shell biochar pellets (2 mm) made at 700 °C into a stable and homogeneous aqueous colloidal (<100 nm) suspension. The amorphous carbon structure of biochar was preserved after the disintegration, which only occurred above the weight ratio of 30,000 biochar to nC₆₀-stir. Favorable hydrophobic surface interactions between nC₆₀-stir and 700 °C biochar likely disrupted van der Waals forces holding together the amorphous carbon units of biochar and C₆₀ packing in the nC₆₀ superstructure.

Activity and Reactivity of Pyrogenic Carbonaceous Matter toward Organic Compounds

Pyrogenic carbonaceous matter (PCM) includes environmental black carbon (fossil fuel soot, biomass char), engineered carbons (biochar, activated carbon), and related materials like graphene and nanotubes. These materials contact organic pollutants due to their widespread presence in the environment or through their use in various engineering applications. This review covers recent advances in our understanding of adsorption and chemical reactions mediated by PCM and the links between these processes. It also covers adsorptive processes previously receiving little attention and ignored in models such as steric constraints, physicochemical effects of confinement in nanopores, π interactions of aromatic compounds with polyaromatic surfaces, and very strong hydrogen bonding of ionizable compounds with surface functional groups. Although previous research has regarded carbons merely as passive sorbents, recent studies show that PCM can promote chemical reactions of sorbed contaminants at ordinary temperature, including long-range electron conduction between molecules and between microbes and molecules, local redox reactions between molecules, and hydrolysis. PCM may itself contain redox-active functional groups that are capable of oxidizing or reducing organic compounds and of generating reactive oxygen species (ROS) from oxygen, peroxides, or ozone. Amorphous carbons contain persistent free radicals that may play a role in observed redox reactions and ROS generation. Reactions mediated by PCM can impact the biogeochemical fate of pollutants and lead to useful strategies for remediation.

Reduction of Bromate by Cobalt-Impregnated Biochar Fabricated via Pyrolysis of Lignin Using CO2 as a Reaction Medium

In this study, pyrolysis of lignin impregnated with cobalt (Co) was conducted to fabricate a Co-biochar (i.e., Co/lignin biochar) for use as a catalyst for bromate (BrO₃^-) reduction. Carbon dioxide (CO₂) was employed as a reaction medium in the pyrolysis to induce desired effects associated with CO₂; (1) the enhanced thermal cracking of volatile organic compounds (VOCs) evolved from the thermal degradation of biomass, and (2) the direct reaction between CO₂ and VOCs, which resulted in the enhanced generation of syngas (i.e., H₂ and CO). This study placed main emphases on three parts: (1) the role of impregnated Co in pyrolysis of lignin in the presence of CO₂, (2) the characterization of Co/lignin biochar, and (3) evaluation of catalytic capability of Co-lignin biochar in BrO₃^- reduction. The findings from the pyrolysis experiments strongly evidenced that the desired CO₂ effects were strengthened due to catalytic effect of impregnated Co in lignin. For example, the enhanced generation of syngas from pyrolysis of Coimpregnated lignin in CO₂ was more significant than the case without Co impregnation. Moreover, pyrolysis of Coimpregnated lignin in CO₂ led to production of biochar of which surface area (599 m² g^-1) is nearly 100 times greater than the biochar produced in N₂ (6.6 m² g^-1). Co/lignin biochar produced in CO₂ also showed a great performance in catalyzing BrO₃^- reduction as compared to the biochar produced in N₂.