Preparation of rice straw-derived biochar for efficient cadmium removal by modification of oxygen-containing functional groups

Modeling and Optimization of Biochar Based Adsorbent Derived from Kenaf Using Response Surface Methodology on Adsorption of Cd2+

Cadmium is one of the most hazardous metals in the environment, even when present at very low concentrations. This study reports the systematic development of Kenaf fiber biochar as an adsorbent for the removal of cadmium (Cd) (II) ions from water. The adsorbent development was aided by an optimization tool. Activated biochar was prepared using the physicochemical activation method, consisting of pre-impregnation with NaOH and nitrogen (N2) pyrolysis. The influence of the preparation parameters—namely, chemical impregnation (NaOH: KF), pyrolysis temperature, and pyrolysis time on biochar yield, removal rate, and the adsorption capacity of Cd (II) ions—was investigated. From the experimental data, some quadratic correlation models were developed according to the central composite design. All models demonstrated a good fit with the experimental data. The experimental results revealed that the pyrolysis temperature and heating time were the main factors that affected the yield of biochar and had a positive effect on the Cd (II) ions’ removal rate and adsorption capacity. The impregnation ratio also showed a positive effect on the specific surface area of the biochar, removal rate, and adsorption capacity of cadmium, with a negligible effect on the biochar yield. The optimal biochar-based adsorbent was obtained under the following conditions: 550 °C of pyrolysis temperature, 180 min of heating time, and a 1:1 NaOH impregnation ratio. The optimum adsorbent showed 28.60% biochar yield, 69.82% Cd (II) ions removal, 23.48 mg/g of adsorption capacity, and 160.44 m2/g of biochar-specific area.

Recent Developments in Understanding Biochar’s Physical–Chemistry

Biochar is a porous material obtained by biomass thermal degradation in oxygen-starved conditions. It is nowadays applied in many fields. For instance, it is used to synthesize new materials for environmental remediation, catalysis, animal feeding, adsorbent for smells, etc. In the last decades, biochar has been applied also to soils due to its beneficial effects on soil structure, pH, soil organic carbon content, and stability, and, therefore, soil fertility. In addition, this carbonaceous material shows high chemical stability. Once applied to soil it maintains its nature for centuries. Consequently, it can be considered a sink to store atmospheric carbon dioxide in soils, thereby mitigating the effects of global climatic changes. The literature contains plenty of papers dealing with biochar’s environmental effects. However, a discrepancy exists between studies dealing with biochar applications and those dealing with the physical-chemistry behind biochar behavior. On the one hand, the impression is that most of the papers where biochar is tested in soils are based on trial-and-error procedures. Sometimes these give positive results, sometimes not. Consequently, it appears that the scientific world is divided into two factions: either supporters or detractors. On the other hand, studies dealing with biochar’s physical-chemistry do not appear helpful in settling the factions’ problem. This review paper aims at collecting all the information on physical-chemistry of biochar and to use it to explain biochar’s role in different fields of application.

Combined effects of rice straw-derived biochar and water management on transformation of chromium and its uptake by rice in contaminated soils

Chromium (Cr) pollution in soil is a global problem owing to its wide industrial use. The mobility, toxicity, and crop uptake of Cr depends on its valence state. Cr(VI) is highly mobile and toxic whereas Cr(III) is generally considered immobile and less toxic. We performed a pot experiment to investigate the combined effects of rice straw-derived biochar and water management on transformation of Cr and its uptake by rice in contaminated soils. The main plots had water management treatments of alternating wetting and drying (AWD) and continuous flooding (CF), and the subplots had three levels of straw biochar (0, 5, and 10 g kg^-1). The results showed that water management and the addition of biochar had a significant effect on the dynamics of soil redox potential (Eh), pH, dissolved organic carbon (DOC), and Fe(II) concentration. As these parameters are important factors affecting Cr transformation in paddy soils, the dynamics of the Cr(III) and Cr(VI) concentrations were clearly different under different treatments. The highest reduction of Cr(VI) was observed in the treatment with CF water management in combination with 10 g kg^-1 of biochar amendment, which resulted in a 62% reduction of Cr(VI) to Cr(III) in soil. The alterations in the oxidation state of Cr greatly affected its accumulation in the rice grains. The CF combined with 10 g kg^-1 of biochar treatment, caused the Cr concentration in rice grains to be 66.2% lower compared with that of the unamended control under AWD water management. Possibly owing to the reduction in phytotoxic effects of Cr(VI), the combined treatment showed an improvement in rice grain weight. In conclusion, the combination of 10 g kg^-1 of biochar amendment and CF water management may potentially be used in Cr-contaminated soil to mitigate the impacts of Cr contamination on rice production.

Effects of harvest time and desalination of feedstock on Spartina alterniflora biochar and its efficiency for Cd2+ removal from aqueous solution

Cadmium (Cd²⁺), as the primary contaminant in Chinese soils, is dangerous to human health and ecological security. Invasive plant Spartina alterniflora in Chinese coastal wetlands presents a promising feedstock for biochar, which is an efficient adsorbent for heavy metal removal. S. alterniflora harvested in summer, autumn and winter were pyrolyzed to produce biochars. We analyzed the effects of harvest time and desalination of feedstock on biochar properties and Cd²⁺ adsorption capacity in aqueous solution. Biochars were characterized by pH probe, elemental analyzer, SEM, BJH, BET, and FTIR, and the Cd²⁺ concentrations were measured using AAS. Except pH (9.85-10.95) and nitrogen contents (0.71-1.59%), other biochar properties had no linear correlations with harvest time. Biochars produced from feedstock harvested in autumn had the highest carbon contents (73.25%) and lowest functional groups diversity (CC and -CH_x). The pH and carbon contents (64.44-73.25%) were increased by desalination treatment. The surface area (0.48-2.27 m²/g), total pore volume (0.0015-0.0055 mL/g), mesopore volume (0.0015-0.0052 mL/g), and Cd²⁺ adsorption capacities (16.29-32.34 mg/g) were affected by desalination treatment, and the effects varied with harvest time. Biochars produced from desalted feedstock harvested in summer and untreated feedstock harvested in winter showed higher surface area, porosity, and Cd²⁺ adsorption capacity. Moderate salt contents (1.5-3.0% in chloride content) in feedstock promote the formation of biochars with higher surface area and porosity.

Modification on biochars for applications: A research update

Biochars are the solid product of biomass under pyrolysis or gasification treatment, whose wholesale prices are lower than commercial activated carbons and other fine materials now in use. The employment of biochars as a renewable resource for field applications, if feasible, would gain apparent economic niche. Modification using physical or chemical protocol to revise the surface properties of biochar for reaching enhanced performances of target application has attracted great research interests. This article provided an overview of biochar application, particularly with the respect to the use of modified biochar as preferred soil amendment, adsorbent, electrochemical material, anaerobic digestion promotor, and catalyst. Based on literature works the current research trends and the prospects and research needs were outlined.

Preparation and Characterization of MgO-Modified Rice Straw Biochars

Rice straw is a common agricultural waste. In order to increase the added value of rice straw and improve the performance of rice straw biochar. MgO-modified biochar (MRBC) was prepared from rice straw at different temperatures, pyrolysis time and MgCl₂ concentrations. The microstructure, chemical and crystal structure were studied using X-ray diffraction (XRD), a Scanning Electron Microscope (SEM), Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption desorption isotherms and Elementary Analysis (EA). The results showed that the pyrolysis temperature had significant influence on the structure and physicochemical property of MRBCs. MRBC-2 h has the richest microporous structure while MRBC-2 m has the richest mesoporous structure. The specific surface area (from 9.663 to 250.66 m²/g) and pore volume (from 0.042 to 0.158 cm³/g) of MRBCs increased as temperature rose from 300 to 600 °C. However, it was observed MgCl₂ concentrations and pyrolysis time had no significant influence on pore structure of MRBCs. As pyrolysis temperature increased, pH increased and more oxygen-containing functional groups and mineral salts were formed, while MgO-modified yield, volatile matter, total content of hydrogen, oxygen, nitrogen, porosity and average pore diameter decreased. In addition, MRBCs formed at high temperature showed high C content with a low O/C and H/C ratios.

Adsorption of Hg(II) in an Aqueous Solution by Activated Carbon Prepared from Rice Husk Using KOH Activation

With the development of industry, the discharge of wastewater containing mercury ions posed a serious threat to human health. Using biomass waste as an adsorbent to treat wastewater containing mercury ions was a better way due to its positive impacts on the environment and resource saving. In this research, activated carbon (AC) was prepared from rice husk (RH) by the KOH chemical activation method. The characterization results of scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), Fourier transform infrared (FTIR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) showed that rice husk-activated carbon (RHAC) had good pore structure and oxygen-containing functional groups. The influences of contact time, initial concentration of Hg(II), adsorbent dosage, pH, and ionic strength on mercury ion removal were investigated. The Langmuir model was most suitable for the adsorption isotherm of RHAC, and its maximum adsorption capacity for Hg(II) was 55.87 mg/g. RHAC still had a high removal capacity for Hg(II) after five regeneration cycles. RHAC had excellent removal efficiency for mercury ion wastewater. At the same time, RH could be used as a nonpolluting and outstanding characteristic adsorbent material.

Efficient removal of Cd(II) from aqueous solution by pinecone biochar: Sorption performance and governing mechanisms

Cadmium (Cd) is one of the most harmful and widespread environmental pollutants. Despite decades-long research efforts, the remediation of water contaminated by Cd has remained a significant challenge. A novel carbon material, pinecone biochar, was previously hypothesized to be a promising adsorbent for Cd, while so far, it has received little attention. This study evaluated the sorption capacity of pinecone biochar through isotherm experiments. Based on Langmuir model, the adsorption maximum for Cd(II) was up to 92.7 mg g^-1. The mechanism of Cd(II) adsorption on pinecone biochar was also explored through both thermodynamic and kinetics adsorption experiments, as well as both solution and solid-phase microstructure characterization. The solid-solution partitioning behaviour of Cd(II) fitted best with the Tόth model while the adsorption process followed a pseudo-second-order rate, suggesting that the Cd(II) adsorption on the pinecone biochar was mainly a chemisorption process. Microstructure characteristics and mechanism analysis further suggested that coprecipitation and surface complexation were the main mechanisms of Cd adsorption by biochar. Coprecipitation occurred mainly through the forms of Cd(OH)₂ and CdCO₃. Our results demonstrated that pinecone biochar was an efficient adsorbent which holds a huge potential for Cd(II) removal from aqueous solution.

Superefficient Removal of Heavy Metals from Wastewater by Mg-Loaded Biochars: Adsorption Characteristics and Removal Mechanisms

Six types of biochar (BSB, CSB, FSB, CFSB, MSB, and TSB) were prepared from different raw materials by loading magnesium ions (Mg²⁺) via an impregnation process. The adsorption kinetics and thermodynamics of heavy metals at high concentrations were analyzed. The adsorption mechanisms were investigated by zeta potential, scanning electron microscopy-energy-dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and inductively coupled plasma-atomic absorption spectroscopy analyses. The adsorption of heavy metals by BSB, CSB, FSB, CFSB, MSB, and TSB conformed to the Langmuir model and PS-order. The maximum theoretical saturation adsorption capacities for Cd(II), Cu(II), and Pb(II) were 333.33, 238.10, 75.19, 96.15, 66.23, and 185.19 mg·g^-1; 370.37, 294.12, 111.11, 169.49, 84.75, and 217.39 mg·g^-1; and 302.58, 200.00, 61.73, 90.91, 54.47, and 166.67 mg·g^-1, respectively. According to the analysis of the contribution of adsorption, the adsorption process was mainly controlled by cation-π interactions, ion exchange, mineral precipitation, and functional group interactions. Biochars contain ash, functional groups and load a large number of Mg²⁺, which can form complexes with metal ions and perform strong ion exchange; therefore, mineral precipitation and cation exchange played dominant roles in the adsorption process. The prepared Mg-loaded biochars presented in this research showed excellent adsorption properties for heavy metals and have great potential for practical application; in particular, BSB had the strongest adsorption capacity for the three heavy metal ions.

Influence of Aged Biochar Modified by Cd2+ on Soil Properties and Microbial Community

Biochar is a promising addition for cadmium-contaminated soil in-situ remediation, but its surface properties change after aging, cadmium adsorption is not well-documented, and subsequent environmental effects are still unknown. In this study, wood-derived (Eucalyptus saligna Sm.) biochar was pre-treated to simulate aging and the cadmium sorption process. We then analyzed the resulting physicochemical characteristics. We conducted comparative incubation studies on three age stages of biochar under cadmium adsorption or no cadmium adsorption and then measured soil properties and microbial communities after incubation. Biochar addition raised soil organic carbon (SOC), and aging significantly increased C/N ratios. Aged biochar promoted higher microbial abundance. Aged biochar treatments possessed different microflora with more gram-positive bacteria, significantly altering gram-positive/gram-negative bacteria ratios. Aging significantly increased the oxygen-containing functional groups (OCFGs) and surface area (SA) of biochar. Thus, aged biochar adsorbed more cadmium. Cadmium-binding biochar increased the proportion of gram-negative bacteria and decreased the proportions of gram-positive bacteria and fungi. Similar patterns in phospholipid fatty acids (PLFAs) across adsorption treatments indicated that changes in microbial communities due to the effects of cadmium were confined. The results reveal that biochar aging altered microbial community structure and function more than cadmium binding.

Phosphorus retention using iron (II/III) modified biochar in saline-alkaline soils: Adsorption, column and field tests

Iron oxide-biochar composites have been widely used in removal of phosphate from water, however, their effects on phosphorus retention and decrease leaching are unclear in saline-alkaline soils. We utilized rice straw-derived biochar modified with ferrous chloride (Fe(II)) and ferric chloride (Fe(III)) to study the potential mechanisms of phosphorus retention and leaching under field conditions. Results showed that the Fe(II) biochar exhibited superior phosphate adsorption capacity (39.2 mg g^-1) over the unmodified. In addition, Fe(III) biochar was relatively insensitive to pH and competed anions. This might be due to iron in Fe(II) biochar that exists primarily in an amorphous state as FeOOH, which enhanced its ability to adsorb phosphate because it has high isoelectric points. Crystals of Fe₂PO₅ and (PO₃)₃ were formed in the Fe(II) and Fe(III) biochars, respectively. Electrostatic attraction and ligand exchange contributed to phosphate adsorption. In the column leaching experiments, all treatments were found to significantly increase the phosphorus content in 0-20 cm soil compared to Control, especially with Fe(II) biochar amendment. Fe(II) biochar decreased leaching by 86.4%. In the field experiments, Fe(II) and Fe(III) biochar increased the available phosphorus by 78.6% and 90.3%, respectively. Overall, application of iron modified biochar to saline-alkaline soils promoted phosphorus adsorption and decreased leaching.

Enhanced removal of aqueous Cr(VI) by a green synthesized nanoscale zero-valent iron supported on oak wood biochar

Green nanoscale zero iron (nZVI) on an oak wood biochar support was prepared from tea polyphenol (TP-nZVI-OB), and applied to the removal of hexavalent chromium (Cr(VI)) from aqueous solution. The effects of experimental parameters on the Cr(VI) removal were evaluated by varying the Fe/C mass ratio, contact time, initial pH, and initial Cr(VI) concentration. The Cr(VI) removal performance of the TP-nZVI-OB was optimized at an Fe/C mass ratio of 2:1. The initial pH significantly affected the Cr(VI) removal, and 99.9% of the Cr(VI) was eliminated at pH 2.0. The kinetic data were well fitted to a pseudo-second order model, indicating that Cr(VI) removal was dominated by chemisorption. The successful TP-nZVI-OB synthesis and effective Cr(VI) removal mechanisms were confirmed by multiple techniques. The reaction between Cr(VI) and TP-nZVI-OB (2:1) involved multiple processes (sorption, reduction and co-precipitation), clarifying that TP-nZVI-OB is a potentially superior composite for Cr(VI) treatment of contaminated aqueous solution.

Electro-membrane extraction of cadmium(II) by bis(2-ethylhexyl) phosphate/kerosene/polyvinyl chloride polymer inclusion membrane

The development of the electroplating and battery industries has increased the environmental problems and the needs for resource recovery of Cd(II). In this study, the Electro-membrane extraction (EME) behaviour of Cd(II) was investigated by using polymer inclusion membrane with bis(2-ethylhexyl) phosphate as carrier and polyvinyl chloride as base polymer(PD-PIM) at 0-80 V. Results showed that the EME of Cd(II) by PD-PIM can be obtained in the feed phase with pH 3-8 and stripping phase of dilute acid. Voltage is the main factor to increase the mass transfer rate of Cd(II). The applied electric field reduced the mass transfer activation energy of Cd(II) by PD-PIM and weakened the mass transfer interference of Cd(II) on the background material of the feed phase. After using kerosene-stabilised PD-PIM for operation at pH5, 60 V for 120 h, Cd(II) in the 1 L solution reduced from 15 mg/L to 0.08 mg/L, and the enrichment factor was 9.79.

Characterization and carbon mineralization of biochars produced from different animal manures and plant residues

Renewing carbon and re-establishing it again in the soil is one of the valuable means to cope with climate change. There are many technologies for carbon apprehension and storage, but the most important one gaining attention is biochar technology. So, to carbonize and return different biological materials back to the farmland, a comprehensive study was proposed to characterize and evaluate the carbon (C) mineralization of biochars produced from different animal manures and crop straws. Six types of biochars were prepared from animal manures (poultry litter, swine and cattle manures) and crop straws (rice, soybean, and corn straws). The biochars were analyzed for chemical characteristics (elemental variables, thermal decomposition, cation exchange capacity, pH, electrical conductivity, specific surface area, and surface functional groups) and an incubation experiment was conducted to evaluate C mineralization from soil biochar mixture. Biochars produced from crop straws resulted to have more C as compared to the biochars produced from animal manures. Concentration of nitrogen was low, while P, K, Ca, and Mg were found reasonably higher in all biochars except swine manure biochar. The plant-derived biochars presented lower CO₂ emissions when incorporated to soil at 1 and 2% of C. Varying but all the biochars prepared represented an alkaline pH. Biochars prepared from the crop straws resulted to have more C, alkaline in nature, high CEC, low CO₂ emissions, can sequester C and more suitable to enhance the soil fertility in comparison to biochars produced from other sources.

Key factors and microscopic mechanisms controlling adsorption of cadmium by surface oxidized and aminated biochars

Modified biochar has great potential for adsorbing cadmium (Cd) in the aquatic environment, but the micro-immobilization mechanisms, driven by surface modifications, remain unclear. There has been no attempt to determine the key adsorption factors by integrating the numerous physiochemical indicators. In this study, surface oxidized biochar (OPBC) and surface aminated biochar (APBC) were prepared from porous biochar (PBC), and the Cd adsorption mechanisms by the modified biochars at the molecular and electronic scales were investigated. The adsorption capacity of APBC and OPBC for Cd was 23.54 and 19.04 mg g^-1, respectively, which was about three times higher than that of PBC. Macroscopically, physicochemical adsorption and intraparticle diffusion dominated the Cd adsorption, and surface properties, such as functional groups, were identified as key factors controlling adsorption. Microscopically, the adsorption of Cd mainly occurred in regions rich in π electrons, lone pair electrons and electron donor groups. The interaction between carboxyl and Cd dominated the adsorption performance of OPBC, while the Cd²⁺-π interaction was weakened by increasing the π electron electrostatic potential of aromatic rings. The lone pair electrons of the amino groups dominated the complexation of APBC with Cd, and the π electron electrostatic potential was almost unaffected.

Characterization and Interpretation of Cd (II) Adsorption by Different Modified Rice Straws under Contrasting Conditions

Rice straw can adsorb Cd(II) from wastewater, and modification of rice straw may improve its adsorption efficiency. The rice straw powder (Sp) from the direct pulverization of rice straw was used as the control, the rice straw ash (Sa), biochar (Sa), and modified rice straw (Ms) were prepared by ashing, pyrolysis and citric acid modification, respectively, and all of them were examined as adsorbents for Cd(II) in this study. Batch adsorption experiments were adopted to systematically compare the adsorption capacities of rice straw materials prepared with different modification methods for Cd(II) from aqueous solution under different levels of initial Cd(II) concentration (0–800 mg·L⁻¹), temperature (298, 308, and 318 K), contact time (0–1440 min), pH value (2–10), and ionic strength (0–0.6 mol·L⁻¹). The results indicated that the modification method affected the adsorption of Cd(II) by changing the specific surface area (SSA), Si content, surface morphology, and O-containing functional group of rice straw. Compared with Sp, Ms held more surface O–H, aliphatic and aromatic groups, while Sa had more phenolic, C–O (or C–O–C), and Si–O groups, and Sb held more C–O (or C–O–C) and Si–O groups; besides, Sa, Sb, and Ms had larger SSA than Sp. Adsorption capacity of the four adsorbents for Cd(II) increased and gradually became saturated with the increase in the initial Cd(II) concentration (0–800 mg·L⁻¹). The adsorption capacity of Cd(II) was significantly higher at 318 K than 298 K and 308 K, regardless of the adsorbent type. Sa had the largest SSA (192.38 m²·g⁻¹) and the largest adsorption capacity for Cd(II). When the initial Cd²⁺ concentration was at 800 mg·L⁻¹, the Cd(II) adsorption amount reached as high as 68.7 mg·g⁻¹ with Sa at 318 K. However, the SSA of Sp was only 1.83 m²·g⁻¹, and it had the least adsorption capacity for Cd(II). Only the adsorption of Cd(II) upon Sb at 298 K was spontaneous, and surprisingly, all other adsorptions were nonspontaneous. These adsorptions were all chemical, and were favorable, exothermic and order-increasing processes. The pseudo-second-order model showed a strong fit to the kinetics of Cd(II) adsorption by the four adsorbents. The adsorption capacities of Cd(II) by the adsorbents were less at low pH, and all were enhanced with the increase of initial pH value (2–10) in the solution. The inhibiting effect on Cd(II) adsorption due to the increase in ionic strength was greater with Sa, Sb, and Ms than that under Sp. The rice straw ash prepared by ashing unexpectedly had greater adsorption capacity for Cd(II) than the biochar and citric acid modified rice straw. The optimum condition for Cd(II) adsorption was established as the temperature of 318 K, initial Cd(II) concentration of 800 mg·L⁻¹, contact time of 240 min, and no Na(I) interference regardless of absorbent. In conclusion, rice straw ash shows the greatest potential of being applied to paddy fields for the remediation of Cd(II) pollution so as to reduce the risk of Cd(II) enrichment in rice grains and straws.

Amino modification of rice straw-derived biochar for enhancing its cadmium (II) ions adsorption from water

To enhance the adsorption capacity of Cd²⁺, -NH₂ groups were introduced into the rice straw-derived biochar surface by combining nitrification and amination. The batch and continuous Cd²⁺ adsorption experiments were performed to determine the role of -NH₂ groups on the surface of biochar. The physical and chemical characteristics were analyzed for comparison. The results indicated that the adsorption capacity of the modified biochar (BC-NH₂) was boosted by 72.1%. The results of continuous adsorption experiments in fixed bed columns showed that the penetration time of BC-NH₂ was three times that of original biochar. The adsorption of Cd²⁺ by BC-NH₂ is a spontaneous endothermic chemical reaction, which was obtained by combing sorption kinetics, isotherms and thermodynamic analysis. The Cd²⁺ adsorption was mainly the complexation between -NH₂ group on biochar surface and Cd²⁺ in solution. Finally, a possible interaction mechanism between Cd²⁺ and BC-NH₂ was proposed.

Surface modification and characterization of carbonaceous adsorbents for the efficient removal of oil pollutants

In this study, the impact of different oxidizing agents on the structural integrity of activated carbon (AC) and multiwalled carbon nanotubes (MWCNTs) was studied for the removal of BTX from aqueous solution. Seven different combinations of green oxidizing agents (mild organic acids) in conjugation with NaOCl (basic oxidizing agent) were used. The modified adsorbents were analyzed by Brunauer, Emmett, and Teller (BET) surface area analyzer, Fourier transform infrared spectroscopy (FTIR), Boehm titration, Raman spectroscopy, thermal gravimetric analysis (TGA), x-ray diffraction (XRD), zeta potential, and variable pressure field emission scanning electron microscope (VPFESEM). The results suggested that the carbonaceous sorbents modified with combination of citric acid tartaric acid, malic acid and salicylic acid (CTMS-I) showed increased surface area (O-AC: 871.67 m²/g, O-MWCNTs: 336.37 m²/g) and total pore volume (O-AC: 0.59 cm³/g, O-MWCNTs: 0.04 cm³/g), with the significantly improved thermal stability. Preliminary batch adsorption experiments conducted using the present prepared O-AC and O-MWCNTs, showed an improved performance towards the adsorption of BTX, compared with other available reported adsorbents in the literature.

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.

Mechanisms and reutilization of modified biochar used for removal of heavy metals from wastewater: A review

Heavy metals (HMs) pose serious threat to both human and environmental health and therefore, effective and low-cost techniques to remove HMs are urgently required. Because HMs are difficult to be biodegraded and transformed, adsorption is a most promising treatment method in recent times. Biochar (BC), a low-cost and sustainable adsorbent material, has recently attracted much research attention due to its broad application prospects. While BC has many merits, it has a lower HMs adsorption efficiency than traditional activated carbon, limiting its practical applications. Furthermore, the HMs retained by BC are difficult to be desorbed, making the used sorbent material hazardous wastes if not well disposed of under natural conditions. Therefore, it is critical to seek effective surface modifications for BC, to improve its ability to HMs removal ability and the recyclability of BC loaded with HMs. This review represents and evaluates the reported modification methods for BC, the corresponding HMs removal mechanisms and the potential for reutilization of BC loaded with HMs. This review provides a basis for the effective practical application of BC in the treatment of HMs containing wastewater.

Mesoporous activated carbon from starch for superior rapid pesticides removal

Pesticides contamination of water has caused considerable concern due to the potential hazard to human health. For the first time, mesoporous activated carbon from starch (ACS) was applied to remove pesticides from water. ACS could remove 11 pesticides rapidly (shake five times). The adsorption rates of ACS (>80%) for the 11 pesticides were higher than those of other adsorbents, including commercial activated carbon (AC), graphitised carbon black (GCB), C18, and primary secondary amine adsorbent (PSA). The mechanisms of the adsorption process for pyraclostrobin were also investigated. The pseudo-second-order model could better describe the adsorption for pyraclostrobin (R² = 0.99950). Langmuir model gave the best fit for the isotherm data (R² = 0.99899). Our findings demonstrate that oxygen-containing functional groups, N atom and π-bonding network of benzene promoted the adsorption. The adsorption efficiency of the ACS for 11 pesticides was still over 80% after five cycles.