Competitive adsorption behaviour and mechanisms of cadmium, nickel and ammonium from aqueous solution by fresh and ageing rice straw biochars

Organic particles and high pH in food waste anaerobic digestate enhanced NH4+ adsorption on wood-derived biochar

Biogas residues (i.e., digestate) are rich in NH4+ that has great agricultural value but environmental risk if not recycled. Biochar can be an effective adsorbent retaining NH4+ from digestate. However, it remains unclear how the unique composition of digestate affects the capacity and mechanisms of NH4+ adsorption on biochar. This study examined the mechanisms and driving factors of NH4+ recovery from digestate containing different molecular-weight organic particles by using wood-derived biochar with or without H2O2 modification. Four solutions were prepared, including pure NH4+, synthetic NH4+ with multiple cations mimicking digestate solution, supernatant of digestate with small organic particles and dissolved organic matter, and digestate mixture containing supernatant and large organic particles. The results showed that compared with pure NH4+ solution, the adsorbed NH4+ was 42% lower in the synthetic NH4+ solution with multiple cations but was 2.2 time higher in the supernatant of digestate on two biochars following 48-h adsorption. Modified biochar did not change NH4+ adsorption in pure NH4+ solution despite higher specific surface area than raw biochar, but it increased the adsorption of NH4+ in digestate solutions with high pH (e.g., 4.03 vs. 3.37 mg N g-1 for modified and raw biochar, respectively, in the supernatant of digestate). Compared with the supernatant, the large organic particles in digestate mixture significantly but slightly decreased NH4+ adsorption on modified but not raw biochar. The desorption rate of NH4+ on the biochar was up to 74%-100%, and it was not supressed by the adsorption of organic particles in digestate. The findings here demonstrate the dominant role of electrostatic attraction in NH4+ adsorption, the important role of high pH and organic particles in digestate in facilitating NH4+ adsorption on biochar, and the suitability of the wood-derived biochar in recovering NH4+ from digestate and releasing N for agricultural application.

A systematic review of biochar aging and the potential eco-environmental risk in heavy metal contaminated soil

Biochar is widely accepted as a green and effective amendment for remediating heavy metals (HMs) contaminated soil, but its long-term efficiency and safety changes with biochar aging in fields. Currently, some reviews have qualitatively summarized biochar aging methods and mechanisms, aging-induced changes in biochar properties, and often ignored the potential eco-environmental risk during biochar aging process. Therefore, this review systematically summarizes the study methods of biochar aging, quantitatively compares the effects of different biochar aging process on its properties, and discusses the potential eco-environmental risk due to biochar aging in HMs contaminated soil. At present, various artificial aging methods (physical aging, chemical aging and biological aging) rather than natural field aging have been applied to study the changes of biochar's properties. Generally, biochar aging increases specific surface area (SSA), pore volume (PV), surface oxygen-containing functional group (OFGs) and O content, while decreases pH, ash, H, C and N content. Chemical aging method has a greater effect on the properties of biochar than other aging methods. In addition, biochar aging may lead to HMs remobilization and produce new types of pollutants, such as polycyclic aromatic hydrocarbons (PAHs), environmentally persistent free radicals (EPFRs) and colloidal/nano biochar particles, which consequently bring secondary eco-environmental risk. Finally, future research directions are suggested to establish a more accurate assessment method and model on biochar aging behavior and evaluate the environmental safety of aged biochar, in order to promote its wider application for remediating HMs contaminated soil.

Immobilization mechanisms of Sr(II), Ni(II), and Cd(II) on glomalin-related soil protein in mangrove sediments at the microscopic scale

Glomalin-related soil protein (GRSP) is a significant component in the sequestration of heavy metal in soils, but its mechanisms for metal adsorption are poorly known. This study combined spectroscopic data with molecular docking simulations to reveal metal adsorption onto GRSP's surface functional groups at the molecular level. The EXAFS combined with FTIR and XPS analyses indicated that the adsorption of Cd(II), Sr(II), and Ni(II) by GRSP occurred mainly through the coordination of -OH and -COOH groups with the metal. The -COOH and -OH groups bound to the metal as electron donors and the electron density of the oxygen atom decreased, suggesting that electrostatic attraction might be involved in the adsorption process. Two-dimensional correlation spectroscopy revealed that preferential adsorption occurred on GRSP for the metal in sequential order of -COOH groups followed by -OH groups. The presence of the Ni-C shell in the Ni EXAFS spectrum suggested that Ni formed organometallic complexes with the GRSP surface. However, Sr-C and Cd-C were absent in the second shell of the Sr and Cd spectra, which was attributed to the adsorption of Sr and Cd ions with large hydration ion radius by GRSP to form outer-sphere complexes. Through molecular docking simulations, negatively charged residues such as ASP151 and ASP472 in GRSP were found to provide electrostatic attraction and ligand combination for the metal adsorption, which was consistent with the spectroscopic analyses. Overall, these findings provided new insights into the interaction mechanisms between GRSP and metals, which will help deepen our understanding of the ecological functions of GRSP in metal sequestration.

Silica-magnesium coupling in lignin-based biochar: A promising remediation for composite heavy metal pollution in environment

Lignin hydrothermal silica-carbon material served as a backbone for MgCl2 activation to prepare lignin-based silicon/magnesia biochar (ALB/Si-Mg) for Cd2+, Pb2+, Cu2+, and Zn2+ removal from water and soil environment. Characterization studies revealed a 1017.71-fold increase in the specific surface area of ALB/Si-Mg compared to the original lignin biochar (ALB), producing abundant oxygen functional groups (OC-O, Si-O, Mg-O), and mineral matter (Mg2SiO4 and MgO). Crucially, batch adsorption experiments demonstrated that the adsorption capacity of ALB/Si-Mg for Cd2+, Pb2+, Cu2+, and Zn2+ was 848.17, 665.07, 151.84, and 245.78 mg/g, which were 29.09-140.45 times of the ALB. Soil remediation experiments showed that applying ALB/Si-Mg increased soil effective silicon (109.04%-450.2%) and soil exchangeable magnesium (276.41%-878.66%), enhanced plant photosynthesis, and notably reduced the bioavailability of heavy metals in soil as well as the content of heavy metals in Pakchoi, thereby promoting Pakchoi growth and development. The presence of oxygen-containing functional groups on ALB/Si-Mg, along with Mg2SiO4 and MgO nanoparticles, enhanced the adsorption capacity for heavy metals through the promotion of heavy metal precipitation, ion exchange, and complexation mechanisms. This study establishes the groundwork for the coupling of silica and magnesium elements in biochar and the remediation of composite heavy metal environmental pollution.

A critical review of biochar as an environmental functional material in soil ecosystems for migration and transformation mechanisms and ecological risk assessment

At present, biochar has a large application potential in soil amelioration, pollution remediation, carbon sequestration and emission reduction, and research on the effect of biochar on soil ecology and environment has made positive progress. However, under natural and anthropogenic perturbations, biochar may undergo a series of environmental behaviors such as migratory transformation, mineralization and decomposition, and synergistic transport, thus posing certain potential risks. This paper outlines the multi-interfacial migration pathway of biochar in "air-soil-plant-animal-water", and analyzes the migration process and mechanism at different interfaces during the preparation, transportation and application of biochar. The two stages of the biochar mineralization process (mineralization of easily degradable aliphatic carbon components in the early stage and mineralization of relatively stable aromatic carbon components in the later stage) were described, the self-influencing factors and external environmental factors of biochar mineralization were analyzed, and the mineral stabilization mechanism and positive/negative excitation effects of biochar into the soil were elucidated. The proximity between field natural and artificially simulated aging of biochar were analyzed, and the change of its properties showed a trend of biological aging > chemical aging > physical aging > natural aging, and in order to improve the simulation and prediction, the artificially simulated aging party needs to be changed from a qualitative method to a quantitative method. The technical advantages, application scope and potential drawbacks of different biochar modification methods were compared, and biological modification can create new materials with enhanced environmental application. The stability performance of modified biochar was compared, indicating that raw materials, pyrolysis temperature and modification method were the key factors affecting the stability of biochar. The potential risks to the soil environment from different pollutants carried by biochar were summarized, the levels of pollutants released from biochar in the soil environment were highlighted, and a comprehensive selection of ecological risk assessment methods was suggested in terms of evaluation requirements, data acquisition and operation difficulty. Dynamic tracing of migration decomposition behavior, long-term assessment of pollution remediation effects, and directional design of modified composite biochar materials were proposed as scientific issues worthy of focused attention. The results can provide a certain reference basis for the theoretical research and technological development of biochar.

Efficient Removal of Nickel from Wastewater Using Copper Sulfate-Ammonia Complex Modified Activated Carbon: Adsorption Performance and Mechanism

The necessity to eliminate nickel (Ni) from wastewater stems from its environmental and health hazards. To enhance the Ni adsorption capacity, this research applied a copper sulfate-ammonia complex (tetraamminecopper (II) sulfate monohydrate, [Cu(NH3)4]SO4·H2O) as a modifying agent for a Phragmites australis-based activated carbon preparation. The physiochemical properties of powdered activated carbon (PAC) and a modified form ([Cu(NH3)4]-PAC) were examined by measuring their surface areas, analyzing their elemental composition, and using Boehm's titration method. Batch experiments were conducted to investigate the impact of various factors, such as Ni(II) concentration, contact time, pH, and ionic strength, on its substance adsorption capabilities. Additionally, the adsorption mechanisms of Ni(II) onto activated carbon were elucidated via Fourier-transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The findings indicated that modified activated carbon ([Cu(NH3)4]-PAC) exhibited a lower surface area and total volume than the original activated carbon (PAC). The modification of PAC enhanced its surface's relative oxygen and nitrogen content, indicating the incorporation of functional groups containing these elements. Furthermore, the modified activated carbon, [Cu(NH3)4]-PAC, exhibited superior adsorption capacity relative to unmodified PAC. Both adsorbents' adsorption behaviors conformed to the Langmuir model and the pseudo-second-order kinetics model. The Ni(II) removal efficiency of PAC and [Cu(NH3)4]-PAC diminished progressively with rising ionic strength. Modified activated carbon [Cu(NH3)4]-PAC demonstrated notable pH buffering and adaptability. The adsorption mechanism for Ni(II) on activated carbon involves surface complexation, cation exchange, and electrostatic interaction. This research presents a cost-efficient preparation technique for preparing activated carbon with enhanced Ni(II) removal capabilities from wastewater and elucidates its underlying adsorption mechanisms.

Respective evolution of soil and biochar on competitive adsorption mechanisms for Cd(II), Ni(II), and Cu(II) after 2-year natural ageing

To reveal the respective evolution of soil and biochar on competitive heavy metal adsorption mechanisms after natural ageing, three soils and two biochars were tested in this study. The soil-biochar interlayer samples were buried in the field for 0.5, 1, and 2 years, for which competitive adsorption characteristics and mechanisms of soils and biochars in four systems (Cd, Cd+Ni, Cd+Cu, and Cd+Ni+Cu) were investigated. Results showed that physicochemical properties, adsorption capacity and mechanisms of soils and biochars all changed the most in the first 0.5 years. The properties and adsorption capacity of biochars gradually weakened with the ageing time, meanwhile, those of soils gradually enhanced. After co-ageing with acidic soil for 0.5 years, the Cd(II) adsorption capacity of modified biochar decreased by 86.59% in the ternary system; meanwhile, that of acidic soil increased by 65.52%. The contributions of mineral mechanisms decreased significantly, while non-mineral mechanisms were slightly affected by ageing. This study highlighted that when using biochar to remediate heavy metal-contaminated soils, biochar should be applied at least half a year in advance before planting crops so that biochar can fully contact and react with the soil.

Effective adsorption of cadmium and nickel ions in mono and bicomponent systems using eco-friendly adsorbents prepared from peanut shells

The work investigates the potential of peanut shells, an abundant agro-industrial waste, to serve as an adsorbent precursor for the effective and simple treatment of effluents loaded with cadmium and nickel ions. Among the adsorbents prepared, carbonized peanut shell (CCarb), due to its higher adsorption capacity, proved to be the most effective compared to carbonized and activated peanut shell (CATQ). The carbonization process led to structural changes, which resulted in an increase in surface area (around 6 times more in CATQ) and pore volume (around 3 times more in CATQ). Even so, the amount of H+ acid sites due to acid activation produced unfavorable effects for adsorption. Hydroxyl, carboxyl and carbonyl groups were identified on the adsorbent surface which presented favorable charges for metal adsorption. This improvement propels the carbonized variant to the forefront, demonstrating the highest adsorption capacity and reaching equilibrium in less than 90 and 60 min for cadmium and nickel ions, respectively. In both monocomponent and bicomponent systems concentrations greater than 40 ppm signify an increase in adsorption capacity for Ni2+. The experimental data best fit the Freundlich model, showing maximum adsorption capacities of 17.04 mg g-1 for cadmium and 31.28 mg g-1 for nickel. Despite the antagonistic effect observed in the bicomponent system, this study concludes by underlining the promise of activated carbon from peanut shells to harmonize technical and environmental concerns.

Conversion of locally available materials to biochar and activated carbon for drinking water treatment

For environmental sustainability and to achieve sustainable development goals (SDGs), drinking water treatment must be done at a reasonable cost with minimal environmental impact. Therefore, treating contaminated drinking water requires materials and approaches that are inexpensive, produced locally, and effortlessly. Hence, locally available materials and their derivatives, such as biochar (BC) and activated carbon (AC) were investigated thoroughly. Several researchers and their findings show that the application of locally accessible materials and their derivatives are capable of the adsorptive removal of organic and inorganic contaminants from drinking water. The application of locally available materials such as lignocellulosic materials/waste and its thermo-chemically derived products, including BC and AC were found effective in the treatment of contaminated drinking water. Thus, this review aims to thoroughly examine the latest developments in the use of locally accessible feedstocks for tailoring BC and AC, as well as their features and applications in the treatment of drinking water. We attempted to explain facts related to the potential mechanisms of BC and AC, such as complexation, co-precipitation, electrostatic interaction, and ion exchange to treat water, thereby achieving a risk-free remediation approach to polluted water. Additionally, this research offers guidance on creating efficient household treatment units based on the health risks associated with customized adsorbents and cost-benefit analyses. Lastly, this review work discusses the current obstacles for using locally accessible materials and their thermo-chemically produced by-products to purify drinking water, as well as the necessity for technological interventions.

Adsorption effect and mechanism of Cd(II) by different phosphorus-enriched biochars

The resource utilization of agricultural and forestry waste, especially the high-value transformation of low-grade phosphate rock and derivatives, is an important way to achieve sustainable development. This study focuses on the impregnation and co-pyrolysis of rice straw (RS) with fused calcium magnesium phosphate (FMP), FMP modified with citric acid (CA-FMP), and calcium dihydrogen phosphate (MCP) to produce three phosphorous-enriched biochars (PBC). The Cd(II) removal efficiency of biochars before and after phosphorus modification was investigated, along with the adsorption mechanism and contribution of biochars modified with different phosphorus sources to Cd(II) adsorption. The result indicated that CA-FMP and MCP could be more uniformly loaded onto biochar, effectively increasing the specific surface area (SSA) and total pore volume. The adsorption of Cd(II) onto PBC followed a mono-layer chemisorption process accompanied by intraparticle diffusion. The adsorption of Cd(II) by PBC involved ion exchange, mineral precipitation, complexation with oxygen-containing functional groups (OFGs), cation-π interaction, electrostatic interaction, and physical adsorption. Ion exchange was identified as the primary adsorption mechanism for Cd(II) by BC and FBC (51.53% and 53.15% respectively), while mineral precipitation played a major role in the adsorption of Cd(II) by CBC and MBC (51.10% and 47.98% respectively). Moreover, CBC and MBC significantly enhanced the adsorption capacity of Cd(II), with maximum adsorption amounts of 128.1 and 111.5 mg g-1 respectively.

Competitive adsorption behaviors and mechanisms of Cd, Ni, and Cu by biochar when coexisting with microplastics under single, binary, and ternary systems

In this study, the effects of coexistence with microplastics and co-ageing with the soil on adsorption behaviors and mechanisms of biochar for heavy metals were investigated. Adsorption experiments of Cd, Ni, and Cu by microplastics, biochar, and their combination were conducted in single, binary, and ternary systems. The results indicated that the heavy metal adsorption by microplastics was ranked as Ni > Cd > Cu, which increased with decreasing particle size, and the adsorption capacity of microplastics was enhanced after dry-wet and freeze-thaw ageing. Biochar preferentially adsorbed Cd in the single system, while the maximum adsorption of Cu was observed in the binary and ternary systems due to the minimizing impact of competition on the Cu adsorption by biochar. The heavy metal adsorption by the combination of microplastics and biochar was less than that by single biochar, and the smaller the particle size of microplastics, the greater the negative effects on heavy metal adsorption. Coexistence with microplastics reduced Cd adsorption of biochar by 0.72 %-50.35 %, Ni adsorption by 1.17 %-30.43 %, and Cu adsorption by 5.78 %-47.88 %, respectively. Moreover, coexistence with microplastics exacerbated the adverse impacts of competition on biochar adsorption for heavy metals. The contribution percentages of biochar mineral mechanisms for heavy metal adsorption were ranked as Cu > Cd > Ni. When coexisting with microplastics or after ageing, the mineral mechanisms of heavy metal adsorption by biochar significantly decreased. This study investigated the competitive adsorption behaviors and mechanisms of heavy metals by biochar when coexisting with microplastics, which highlighted that the application of biochar for the remediation of heavy metal pollution should be concerned with the impacts of microplastics.

Conversion of carbon black recovered from waste tires into activated carbon via chemical/microwave methods for efficient removal of heavy metal ions from wastewater

In this research study, recovered carbon black (rCB) was obtained via pyrolysis of waste tires. The obtained rCB was then converted into activated carbon species through both chemical treatment and microwave coupled with chemical treatment as a two-step activation process. The activated carbon obtained from chemical activation was denoted as C-AC, while that obtained from exposure to microwave followed by chemical activation was labeled as MC-AC. These two structures were consequently introduced as sorbents for the removal of cadmium ions from an aqueous solution. The structural characteristics of the introduced adsorbents were confirmed using various techniques, namely X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, and energy-dispersive X-ray (EDX) spectroscopy. Additionally, textual features of these adsorbents were acquired via both scanning electron microscopy (SEM) and N2 adsorption-desorption BET surface area analyses. These two structures were then introduced for Cd ion adsorption under different operating conditions. Particularly, the effect of pH, contact time, adsorbent dose, and metal ion concentration on the efficiency of adsorption was investigated. The 1maximum adsorption capacity was detected at a pH value of 5.0, a contact time of 30 min, a sorbent dose of 0.4 g L-1, and an initial metal concentration of 50 mg L-1 using MC-AC, which exhibited nearly double the sorption capacity detected for C-AC. Kinetic studies indicated that the process of Cd(ii) adsorption is perfectly described and fitted by the pseudo-second-order model. However, adsorption isotherms for the two adsorbents were found to match the Langmuir model, referring to the occurrence of uniform monolayer adsorption for the metal ions. Thermodynamic analysis demonstrated that the adsorption process was spontaneous and endothermic.

Single and Competitive Adsorption of Naphthalene, Phenanthrene, and Pyrene on Polystyrene Nanofibers

In this investigation, polystyrene (PS) nanofibers were prepared by electrospinning for the adsorption of naphthalene (Nap), phenanthrene (Phe), and pyrene (Pyr) from an aqueous solution. Surface morphology and physicochemical characteristics of PS nanofibers were analyzed using Fourier transform infrared spectroscopy (FT-IR) and point-of-zero-charge calorimetry (pHpzc). The effects of pH, ion concentration, and temperature on the adsorption were also investigated. The adsorption mechanism of target pollutants on PS nanofibers was investigated by a batch adsorption method. The adsorption kinetic studies showed that the adsorption of the three polycyclic aromatic hydrocarbons (PAHs) on PS nanofibers conformed to the pseudo-second-order kinetic model in both single and ternary systems. Meanwhile, in a single system, the three PAHs adsorbed on nanofibers were controlled by both intraparticle diffusion and liquid film diffusion, whereas the adsorption of Nap in a ternary system was controlled mainly by intraparticle diffusion, and the adsorption of Phe and Pyr was controlled mainly by liquid film diffusion. The isotherm data indicated that the Freundlich model performed better than the Langmuir model for the adsorptions of Nap, Phe, and Pyr on PS nanofibers in both the single system and the ternary system. Due to competitive adsorption, the adsorption capacities of Nap and Pyr on PS nanofibers decreased from 105.816 and 19.098 mg g-1 in the single system to 23.626 and 12.126 mg g-1 in the ternary system, but the adsorption of Phe was not affected. π-π interactions and pore filling may be jointly involved in the adsorption of PAHs on PS nanofibers.

Comprehensive review on recent production trends and applications of biochar for greener environment

The suitability of biochar as a supplement for environmental restoration varies significantly based on the type of feedstocks used and the parameters of the pyrolysis process. This study comprehensively examines several aspects of biochar's potential benefits, its capacity to enhance crop yields, improve nutrient availability, support the co-composting, water restoration and enhance overall usage efficiency. The supporting mechanistic evidence for these claims is also evaluated. Additionally, the analysis identifies various gaps in research and proposes potential directions for further exploration to enhance the understanding of biochar application. As a mutually advantageous approach, the integration of biochar into agricultural contexts not only contributes to environmental restoration but also advances ecological sustainability. The in-depth review underscores the diverse suitability of biochar as a supplement for environmental restoration, contingent upon the specific feedstock sources and pyrolysis conditions used. However, concerns have been raised regarding potential impacts on human health within agricultural sectors.

Cadmium and lead removal by Mg/Fe bimetallic oxide-loaded sludge-derived biochar: batch adsorption, kinetics, and mechanism

Biochar is a valuable adsorbent for the removal of heavy metals from water, and it is important to explore ways to increase its heavy metal adsorption capacity. In this study, Mg/Fe bimetallic oxide was loaded onto sewage sludge-derived biochar to enhance its heavy metal adsorption capacity. Batch adsorption experiments for the removal of Pb(II) and Cd(II) were performed to evaluate the removal efficiency of Mg/Fe layer bimetallic oxide-loaded sludge-derived biochar ((Mg/Fe)LDO-ASB). The physicochemical properties of (Mg/Fe)LDO-ASB and corresponding adsorption mechanisms were studied. The maximum adsorption capacities of (Mg/Fe)LDO-ASB for Pb(II) and Cd(II), which were calculated by isotherm model, were 408.31 and 270.41 mg/g, respectively. Adsorption kinetics and isotherms analysis showed that the dominant adsorption process of Pb(II) and Cd(II) uptake by (Mg/Fe)LDO-ASB was spontaneous chemisorption and heterogeneous multilayer adsorption, and film diffusion was the rate-limiting step. SEM-EDS, FTIR, XRD, and XPS analyses revealed that the Pb and Cd adsorption processes of (Mg/Fe)LDO-ASB involved oxygen-containing functional group complexation, mineral precipitation, electron-π-metal interactions, and ion exchange. The order of their contribution was as follows: mineral precipitation (Pb: 87.92% and Cd: 79.91%) > ion exchange (Pb: 9.84% and Cd: 16.45%) > metal-π interaction (Pb: 0.85% and Cd: 0.73%) > oxygen-containing functional group complexation (Pb: 1.39% and Cd: 2.91%). Mineral precipitation was the main adsorption mechanism, and ion exchange played a crucial role in Pb and Cd adsorption.

Sequestration of strontium, nickel, and cadmium on glomalin-related soil protein: Interfacial behaviors and ecological functions

Glomalin-related soil protein (GRSP) is a widespread recalcitrant soil protein complex that promotes the immobilization of metals in soils. Herein, we combined indoor simulation and field investigation to reveal the interfacial behaviors and ecological functions of GRSP to the three typical metals (Sr(II), Ni(II), and Cd(II)). The kinetic and isotherm data suggested that GRSP had a strong ability to adsorb the metals, which was closely related to the Hard-Soft-Acid-Base theory and the film diffusion mechanisms. Regarding environmental factors, the higher solution pH was beneficial to the adsorption of the metals onto GRSP, while the adsorption capacity decreased at lower or higher salinity due to the salting-out and Na⁺ competition effects. Moreover, Sr(II), Ni(II), and Cd(II) showed competitive adsorption onto GRSP, which was associated with the spatial site resistance effect. By comparing the retention factors of seven natural and artificial particles, GRSP had elevated distribution coefficients in high metal concentration, while its retention factors showed a relatively lower decrease, suggesting that GRSP had excellent buffer performance for a potential metal pollution emergency. Through the continental-scale coastal regions investigation, GRSP sequestered 1.05-3.11 μmol/g Ni, 0.31-1.49 μmol/g Sr, and 0.01-0.06 μmol/g Cd with 0.54-0.91 % of the sediment mass, demonstrating its strong ability to adsorb the metals. Therefore, we advocate that GRSP, as a recalcitrant protein complex, can be considered an effective tool for buffering capacity of metal pollution and environmental capacity within coastal wetlands.

Competitive adsorption and immobilization of Cd, Ni, and Cu by biochar in unsaturated soils under single-, binary-, and ternary-metal systems

This study investigated the competitive adsorption and immobilization of cadmium (Cd), nickel (Ni), and copper (Cu) by biochar in unsaturated soils under single-, binary-, and ternary-metal systems. The results showed that the immobilization effects by the soil itself were in the order of Cu > Ni > Cd, and the adsorption capacities of freshly contaminated heavy metals by biochar were in the order of Cd > Ni > Cu in unsaturated soils. The adsorption and immobilization of Cd by biochars in soils was weakened by competition more in the ternary-metal system than that in the binary-metal system; the competition with Cu caused a more significant weakening effect than that with Ni. For Cd and Ni, nonmineral mechanisms preferentially adsorbed and immobilized Cd and Ni compared to mineral mechanisms, but the contributions of the mineral mechanisms to the adsorption gradually increased and became dominant with increasing concentrations (at average percentages of 62.59%-83.30% and 41.38%-74.29%, respectively). However, for Cu, the contributions of the nonmineral mechanisms to Cu adsorption were always dominant (average percentages of 60.92%-74.87%) and gradually increased with increasing concentrations. This study highlighted that the types of heavy metals and coexistence should be focused when remediating heavy metal contamination in soils.

NaHCO3 activated sludge-derived biochar by KMnO4 modification for Cd(II) removal from aqueous solutions

The surface flat pristine biochar provides limited adsorption sites for Cd(II) adsorption. To address this issue, a novel sludge-derived biochar (MNBC) was prepared by NaHCO₃ activation and KMnO₄ modification. The batch adsorption experiments illustrated that the maximum adsorption capacity of MNBC was twice that of pristine biochar and reached equilibrium more quickly. The pseudo-second order and Langmuir model were more suitable for analyzing the Cd(II) adsorption process on MNBC. Na⁺, K⁺, Mg²⁺, Ca²⁺, Cl^- and NO^-₃ had no effect on the Cd(II) removal. Cu²⁺ and Pb²⁺ inhibited the Cd(II) removal, while PO^3-₄ and humic acid (HA) promoted it. After 5 repeated experiments, the Cd(II) removal efficiency on MNBC was 90.24%. The Cd(II) removal efficiency of MNBC in different actual water bodies was over 98%. Furthermore, MNBC owned excellent Cd(II) adsorption performance in fixed bed experiments, and the effective treatment capacity was 450 BV. The co-precipitation, complexation, ion exchange and Cd(II)-π interaction were involved in Cd(II) removal mechanism. XPS analysis showed that NaHCO₃ activation and KMnO₄ modification enhanced the complexation ability of MNBC to Cd(II). The results suggested that MNBC can be used as an effective adsorbent for treating of Cd-contaminated wastewater.

Quantitative transport and immobilization of cadmium in saturated-unsaturated soils with the combined application of biochar and organic fertilizer

In this study, cadmium (Cd) transport and immobilization on passivators (biochar, organic fertilizer) and soils under saturated-unsaturated conditions were independently analyzed. The results showed that the Cd adsorption capacities of biochar and organic fertilizer were comparable in acidic soils. But in alkaline soils, the Cd adsorption capacity of organic fertilizer was significantly larger than that of biochar. In acidic soils, passivators effectively immobilized Cd, and the total net effects were in the order: combination (44.05-58.13%) > 3% biochar (31.96-46.88%) > 3% organic fertilizer (28.78-41.82%). In alkaline soils, all treatments had negative effects on Cd immobilization. For acidic soils, the immobilization of Cd was mainly attributed to the passivators, and the positive contribution percentages of relatively stable Cd increase by passivators were 81.05-100%, while those by soils were 0-18.95%. For alkaline soils, after the treatments of passivators, although a considerable amount of Cd was immobilized inside the passivator, Cd was activated more inside the soil. Therefore, it is noteworthy that soil conditions must be fully considered when applying biochar and organic fertilizers for Cd remediation.

Waste to catalyst: Role of agricultural waste in water and wastewater treatment

Presently, owing to the rapid development of industrialization and urbanization activities, a huge quantity of wastewater is generated that contain toxic chemical and heavy metals, imposing higher environmental jeopardies and affecting the life of living well-being and the economy of the counties, if not treated appropriately. Subsequently, the advancement in sustainable cost-effective wastewater treatment technology has attracted more attention from policymakers, legislators, and scientific communities. Therefore, the current review intends to highlight the recent development and applications of biochars and/or green nanoparticles (NPs) produced from agricultural waste via green routes in removing the refractory pollutants from water and wastewater. This review also highlights the contemporary application and mechanism of biochar-supported advanced oxidation processes (AOPs) for the removal of organic pollutants in water and wastewater. Although, the fabrication and application of agriculture waste-derived biochar and NPs are considered a greener approach, nevertheless, before scaling up production and application, its toxicological and life-cycle challenges must be taken into account. Furthermore, future efforts should be carried out towards process engineering to enhance the performance of green catalysts to improve the economy of the process.

Contribution of components in natural soil to Cd and Pb competitive adsorption: Semi-quantitative to quantitative analysis

Cadmium (Cd) and lead (Pb) are two of the most common elements found in contaminated sites. The behavior of specific metals in the soil may be affected by other metals because of the competition for adsorption sites. In this study, adsorption experiments after chemical extraction, multi-surface models, and advanced spectroscopy technology were jointly used to explain the adsorption mechanism of Cd and Pb and to determine the contribution of each component in the competitive system. The results show that pH is the key factor in determining the contribution of soil components to metal adsorption. Soil organic matter (SOM) is the dominant adsorbent for both Cd and Pb. Clay minerals play an adsorption role at low pH, whereas Fe/Al oxides adsorb metals primarily in the high pH range. Further, the competitive effect of Pb on Cd occurred primarily on SOM rather than on clay minerals. When the Pb concentration increased from 0 to 500 mg/L, the adsorption of Cd on SOM decreased by 132.0 mg/kg, whereas it decreased only by 1.9 mg/kg on clay minerals. Therefore, the competitive effect of Pb on Cd cannot be ignored in soils with high organic matter content.

Effects of modification and co-aging with soils on Cd(II) adsorption behaviors and quantitative mechanisms by biochar

In this study, original and two KMnO₄-modified rice straw biochars (pre- and postmodification) were prepared, which were all pyrolysed at 400 °C. Premodified biochar had the largest Cd adsorption capacity, strongest acid and solute buffering capacity, which benefited from the increase of carbonate content, specific surface area, and the emergence of Mn(II) and MnO_x through modification. Original and premodified biochars were then conducted four types of aging process by an improved three-layer mesh method, namely, aging without soil and co-aging with acid (pH = 5.00), neutral (pH = 7.00), and alkaline (pH = 8.30) soils. The adsorption capacities of modified biochar were always larger than those of original biochar after aging processes. After four aging processes, Cd(II) adsorption capacities were basically in the order of aged biochar without soil > biochar co-aged with alkaline soil > biochar co-aged with neutral soil > biochar co-aged with acid soil, and KMnO₄-modified biochar was always better than original biochar after co-aging with soils. The dominant adsorption mechanism of original and premodified biochars (fresh and aged) for Cd(II) was all the precipitation and adsorption with minerals (accounted for 58.55 ~ 85.55%). In this study, we highlighted that biochar remediation for Cd should be evaluated by co-aging with soil instead of aging without soil participation.

Cadmium removal from aqueous solution by blended bamboo sawdust/rice-husk biochar; optimization of influencing parameters

This study attempted to investigate the adsorptive potential of blended bamboo (Oxytenanthera abyssinica) sawdust/rice husk (BSD/RH) at a ratio of 1:1 carbonized at 400 °C for the sorption of cadmium ions from synthetic solutions in batch mode. The Box-Behnken Design in response surface methodology (RSM) was used to achieve the best removal efficiency and adsorption capacity of the biochar. With a p-value of 0.0001, the initial Cd2+ concentration and adsorbent dose were discovered to be the most significant parameters controlling the adsorption capacity and removal efficiency of Cd2+ from the solution. At a pH of 8.95, ionic strength of 0.020 mol/L KNO3, a contact time of 15 min, an initial concentration of 200 mg/L, and an adsorbent dose of 0.5 g, the optimum Cd2+ removal and adsorption capacity of 99.97% and 358.65 mg/g, respectively, were obtained. The optimized conditions were later used to determine the removal efficiency and adsorption capacity of pristine biochars of rice husk and bamboo sawdust, which were found to be 79.8% and 83.7%, respectively. This finding indicates the potential for using biosorbent derived from blended feedstock materials to remove heavy metals such as cadmium.

Adsorption Study of Continuous Heavy Metal Ions (Pb2+, Cd2+, Ni2+) Removal Using Cocoa (Theobroma cacao L.) Pod Husks

The serious toxicological effects of heavy metal ions in aquatic ecosystems have motivated the search for alternatives to reduce contamination of water sources from industrial wastewater. In this work, continuous adsorption of nickel, cadmium, and lead was assessed using a packed bed column filled with Cocoa (Theobroma cacao L.) pod husks widely available in the northern region of Colombia. The physicochemical characterization of the agricultural biomass was performed to quantify its chemical composition by bromatological, FT-IR, and energy-dispersive X-ray spectroscopy (EDS). The breakthrough curves were constructed for all heavy metal ions with bed depth of 4 and 7.5 cm, taking aliquots at 10, 30, 60, 90, 120, 150, 180, 210, 240, and 270 min. Moreover, experimental data were fitted to adsorption models in continuous mode to predict adsorptive performance (Adams−Bohart, Thomas, and Yoon−Nelson). For the FT-IR analysis of biomass before and after adsorption, the most representative bands occur around 3200−3900 cm−1 attributed to the presence of hydroxyl groups, showing the destruction of the peaks of lignocellulosic materials. The breakthrough curves revealed that for a 7.5 cm bed, adsorption performance reported the following order of promising results: Pb2+ > Ni2+ > Cd2+; while for a 4 cm bed, Pb2+ > Ni2+. The mechanism of adsorption of the evaluated metals onto cocoa pod husk was attributed to cationic exchange and microprecipitation due to the presence of Ca, K, and Si in the structure of the bio-adsorbent. Finally, the continuous adsorption was modeled under the mathematical expressions of Adams−Bohart, Thomas, and Yoon−Nelson reporting good fitting with correlation coefficient above 0.95.

Efficient removal of cadmium by salts modified-biochar: Performance assessment, theoretical calculation, and quantitative mechanism analysis

Modified biochar is a feasible adsorbent to solve cadmium pollution in water. However, few studies could elucidate the mechanism of cadmium adsorption by biochar from a molecular perspective. Furthermore, traditional modification methods are costly and have the risk of secondary contamination. Hence, several environmentally friendly sodium salts were used to modify the water chestnut shell-based biochar and employ it in the Cd²⁺ adsorption in this work. The modification of sodium salt could effectively improve the specific surface area and aromaticity of biochar. Na₃PO₄ modified biochar exhibited the highest Cd²⁺ adsorption capacity (112.78 mg/g). The adsorption of Cd²⁺ onto biochar was an endothermic, monolayer, chemisorption process accompanied by intraparticle diffusion. Microscopically, the enhancement of aromatization after modification made Cd²⁺ more likely to interact with the regions rich in π electrons and lone pair electrons. This study provided a new research perspective and application guidance for heavy metal adsorption on biochar.

Effect of oxidative aging of biochar on relative distribution of competitive adsorption mechanism of Cd2+ and Pb2

In this study, aged biochar (CCB350 and CCB650) were obtained from pyrolysis of corn stalk biochar (CB350 and CB650) at the degree of 350 °C and 650 °C by artificial oxidation with hydrogen peroxide (H₂O₂). Also, the mechanism of Pb²⁺ and Cd²⁺ on fresh and aged biochars was analyzed qualitatively and quantitatively by batch adsorption experiments combined with characterization. The adsorption isotherm results showed that aging treatment decreased the adsorption capacity of Pb²⁺ and Cd²⁺ and inhibited the competitive adsorption behavior of heavy metals. In the single-metal system, precipitation and cation exchange were considered as the main adsorption mechanisms for CB350 and CB650, with a ratio of 40.07–48.23% and 38.04–57.19%, respectively. Competition between Pb²⁺ and Cd²⁺ increased the relative contribution of mineral precipitation, but decreased the contribution of cation exchange mechanism. Aging resulted in the rise of the contribution of surface complexation to the adsorption of Pb²⁺ and Cd²⁺ on biochars, especially in low-temperature biochars, but weakened the contribution of mineral precipitation to the adsorption. Further, the contribution of other adsorption mechanisms was significantly enhanced for high-temperature aged biochars. These results are important to evaluate its long-term application prospects in the natural environment.

Biochars and activated carbons as adsorbents of inorganic and organic compounds from multicomponent systems - A review

Biochars are obtained by biomass pyrolysis, whereas activated carbon is a biochar that has undergone chemical or physical activation. Owing to the large surface area and easy surface modification both solids are widely applied as adsorbents. They are low-costs materials, they could be regenerated and their disposal is not troublesome. Adsorption of heavy metals, dyes, pharmaceuticals on the surface of biochars and activated carbons, from simple systems of adsorbate containing only one compound, are described extensively in the literature. The present paper provides an overview of reports on adsorption of inorganic and organic compounds onto these two types of adsorbents from the mixed adsorbate systems. The described adsorbate systems have been divided into those consisting of: two or more inorganic ions, two or more organic compounds and both of them (inorganic and organic ones). The research of this type is carried out much less frequently due to the more complicated description of interactions in the mixed adsorbate systems.

Enhancing Cd(II) adsorption on rice straw biochar by modification of iron and manganese oxides

Metal oxide-modified biochar showed excellent adsorption performance in wastewater treatment. Iron nitrate and potassium permanganate were oxidative modifiers through which oxygen-containing groups and iron-manganese oxides could be introduced into biochar. In this study, iron-manganese (Fe-Mn) oxide-modified biochar (BC-FM) was synthesized using rice straw biochar, and the adsorption process, removal effect, and the mechanism of cadmium (Cd) adsorption on BC-FM in wastewater treatment were explored through batch adsorption experiments and characterization (SEM, BET, FTIR, XRD, and XPS). Adsorption kinetics showed that the maximum adsorption capacity of BC-FM for Cd(II) was 120.77 mg/g at 298 K, which was approximately 1.5-10 times the amount of adsorption capacity for Cd(II) by potassium-modified or manganese-modified biochar as mentioned in the literature. The Cd(II) adsorption of BC-FM was well fit by the pseudo-second-order adsorption and Langmuir models, and it was a spontaneous and endothermic process. Adsorption was mainly controlled via a chemical adsorption mechanism. Moreover, BC-FM could maintain a Cd removal rate of approximately 50% even when reused three times. Cd(II) capture by BC-FM was facilitated by coprecipitation, surface complexation, electrostatic attraction, and cation-π interaction. Additionally, the loaded Fe-Mn oxides also played an important role in the removal of Cd(II) by redox reaction and ion exchange in BC-FM. The results suggested that BC-FM could be used as an efficient adsorbent for treating Cd-contaminated wastewater.

Elimination of Chromium (VI) and Nickel (II) Ions in a Packed Column Using Oil Palm Bagasse and Yam Peels

The single-component adsorption of chromium (VI) and nickel (II) on oil palm bagasse (OPB) and yam peels (YP) in a packed bed column was explored and improved using a central 22-star T composite design. The temperature, bed height, and particle size were evaluated, and the optimized response variable was the removal efficiency. The remaining concentration of heavy metals in solution was determined by Ultraviolet–Visible and Atomic Absorption Spectroscopy. It was found that bioadsorbents have a porous structure, with the presence of functional groups such as hydroxyl, carboxyl, and amino, which favor adsorption processes, and that the adsorption mechanisms controlling the process is cation exchange, precipitation, and complexation on the exposed surface of the biomaterials. In the adsorption trials, removal percentages higher than 87% were obtained in all cases, showing better results in the removal of Cr(VI), and that particle size is the most influential factor. Maximum Cr(VI) capacities of 111.45 mg g−1 and 50.12 mg g−1 were achieved on OPB and YP, respectively, while for nickel values of 103.49 mg g−1 and 30.04 mg g−1 were obtained. From the adjustment of the breakthrough curve to the models, it was determined that the model best able to adjust the data was the Thomas model, and the thermodynamic parameters of Cr(VI) and Ni(II) removal suggest that the process on YP is endothermic, while on OPB it is exothermic. In both biomaterials, the process is controlled by spontaneous chemisorption with a great affinity of the active centers for the ions.

Effects of competitive adsorption with Ni(II) and Cu(II) on the adsorption of Cd(II) by modified biochar co-aged with acidic soil

To investigate the effects of competitive adsorption with Ni(II) and Cu(II) on the adsorption of Cd(II) by modified biochar co-aged with acidic soil, four biochars were employed in this study, namely original biochar, KMnO₄-modified biochar and two aged biochars which co-aged with an acidic soil using above biochars under freeze-thaw cycling and dry-wet cycling for 54 days simulating 6 years of natural aging. The results showed that biochar adsorption capacities of three heavy metal ions were in the order of Cd(II) > Cu(II) > Ni(II) in the single system while Cu(II) > Cd(II) > Ni(II) in binary and ternary systems. Modification improved biochar adsorption capacity of Cd(II), but competitive adsorption with Ni(II) and Cu(II) weakened the improvement of modification on adsorption performance of modified biochar in binary and ternary systems. The Q_MBC/Q_BC of Cd(II) (Q_MBC and Q_BC are the adsorption capacities of heavy metals by modified and original biochars) decreased from 231.57% (single system) to 216.67%∼219.41% (binary system) and further decreased to 207.74% (ternary system). Co-aging with soil weakened the adsorption capacities of biochars for Cd(II), even worse, competition aggravated this negative effect of co-aging. The Q_AMBC/Q_MBC of Cd(II) (Q_AMBC is the adsorption capacities of heavy metals by aged modified biochar) decreased from 65.41% (single system) to 14.43%∼19.46% (binary and ternary systems). Therefore, the impact of competition should be fully considered when evaluating Cd long-term remediation effects of modified biochar in Cd polluted soils accompanied with other heavy metals.

Adsorption properties and influencing factors of Cu(II) on polystyrene and polyethylene terephthalate microplastics in seawater

As an emerging contamination in the ocean, microplastics can act as effective vectors of pollutants, the ecological risks caused by the combined pollution of microplastics and other pollutants have attracted growing attention. In this work, Copper (Cu(II)) was chosen as the classic pollutant, polystyrene (PS) and polyethylene terephthalate (PET) pellets were used as the typical marine microplastics, the adsorption performance of Cu(II) on PS and PET beads was investigated by adsorption kinetics and isotherm experiments, and other influencing conditions, such as pH, salinity, coexisting heavy metals ions and aging treatment, were evaluated. The results indicated that the adsorption behavior of Cu(II) on PS and PET was spontaneous and endothermic in the simulated seawater environment, and the batch experimental data can be effectively described by pseudo-second-order model and Freundlich isothermal model. Besides, the adsorption capacity of microplastics for Cu(II) was the best at pH 7, the change of salinity had no obvious effect on the adsorption in the natural marine environment. Moreover, co-existence of lead (Pb(II)) exhibited evident impacts on Cu(II) sorption onto PS and PET, which confirmed the adsorption competition effect between them. Additionally, high temperature aging treatment of microplastics in different environments for different duration time could obviously affect the properties of microplastics. It was found that the microplastics after being exposed to high temperature environment in the air for 168 h showed relatively stronger adsorption amount for Cu(II). In summary, these findings suggested that electrostatic interaction and distributed diffusion mechanisms may be the main mechanisms of adsorption, while no new functional groups were generated after the adsorption, indicating that physisorption may dominate the adsorption performance of PS and PET pellets for Cu(II). This study provides supplementary insights into the role of microplastics as carriers of heavy metals in the marine environment.

Roles of soluble minerals in Cd sorption onto rice straw biochar

Transforming to biochar provides an environmentally friendly approach for crop residue reutilization, which are usually applied as sorbent for heavy metal removal. As typical silicon-rich material, the specific sorptive mechanisms of rice straw derived biochar (RSBC) are concerned, especially at the low concentration range which is more environmentally relevant. In the present study, Cd sorption onto RSBCs at the concentration of ≤ 5 mg/L was investigated. The sorptive capacity was positively correlated with the pyrolytic temperature of the biochar and the environmental pH value. Water soluble minerals of the RSBCs played the dominant roles in Cd sorption, contributing 29.2%, 62.5% and 82.9% of the total sorption for RSBCs derived under 300°C, 500°C and 700°C, respectively. Increased number of cations, dominantly K⁺, were exchanged during the sorption. Coprecipitation with cations and carbonates may also be contributive to the sorption. The dissolution of silicon-containing minerals was found to be declined during sorption, suggesting its involvement in the sorption process, possibly through precipitation. Whilst, the sparingly soluble silicate crystals may impose ignorable role in the sorption. Complexation with organic groups is only a minor mechanism in Cd sorption, compared to the much more dominant roles of the inorganic ashes.

Biochar-supported starch/chitosan-stabilized nano-iron sulfide composites for the removal of lead ions and nitrogen from aqueous solutions

Novel materials that nano-FeS and starch (or chitosan) loaded on peanut shells biochar(Starch-FeS@PSB and Chitosan-FeS@PSB) were prepared and applied for removal of Pb(II) and nitrogen(NO₃-N and NH₄-N) in wastewater. It showed that Starch-FeS@PSB and Chitosan-FeS@PSB had excellent absorptive effects compared with PSB. The maximum adsorption capacity of Pb(II) by Starch-FeS@PSB and Chitosan-FeS@PSB reached 91.74 mg/g, 98.04 mg/g, respectively. Absorption of Pb(II) by Starch-FeS@PSB and Chitosan-FeS@PSB were controlled by monolayer chemisorption. Mechanism studies showed that complexation, electrostatic attraction, REDOX and physical absorption happened on the adsorbent surface. In addition, the maximum adsorption capacity of NO₃-N and NH₄-N by Starch-FeS@PSB and Chitosan-FeS@PSB reached 16.89 mg/g, 15.65 mg/g, and 18.45 mg/g, 18.28 mg/g, respectively. Absorption of N by Starch-FeS@PSB and Chitosan-FeS@PSB were controlled by multilayer chemisorption. Mechanism studies showed that complexation, electrostatic attraction and physical absorption happened on the adsorbent surface. Starch-FeS@PSB and Chitosan-FeS@PSB can be utilized in Pb(II) and N wastewater treatment.

Biochar - An effective additive for improving quality and reducing ecological risk of compost: A global meta-analysis

Biochar is considered as a promising additive with multi-benefits to compost production. However, how the biochar properties and composting conditions affect the composting process and quality and ecological risk of compost is still unclear. In the present study, we conducted a global meta-analysis based on 876 observations from 84 studies. Overall, regardless of biochar properties and composting conditions, biochar addition could significantly increase the pH (5.90%), germination index (26.6%), contents of nitrate nitrogen (56.6%), total nitrogen (9.50%), and total potassium (10.1%), and degree of polymerization (29.4%) while decrease the electrical conductivity (-5.70%), contents of ammonium nitrogen (-33.7%), bioavailable zinc (-22.9%), and bioavailable copper (-38.6%), and emissions of ammonia (-44.2%), nitrous oxide (-68.4%), and methane (-61.7%). Other compost indicators, including the carbon to nitrogen ratio and total phosphorus content, were found to be insignificantly affected by biochar addition. The responses of tested compost indicators affected by the biochar properties and composting conditions were further explored, based on which the addition of straw biochars at a rate of 10-15% was recommended due to its greater potential to improve quality of compost and reduce its ecological risk. Combining the results of linear regression analysis and structural equation model, the increase in compost pH caused by biochar addition was identified as the key mechanism for the increased nutrient content and decreased heavy metal bioavailability. These results could guide us to choose suitable kinds of biochar or develop engineered biochars with specific functionality to realize an optimal compost production mode.

Insights into selective adsorption mechanism of copper and zinc ions onto biogas residue-based adsorbent: Theoretical calculation and electronegativity difference

Modified biomass-based adsorption technique has attracted much attention in heavy metal ions removal, but selective adsorption behavior and mechanism of heavy metal ions adsorption onto biosorbent still need to be further clarified. Herein, a carboxylated biogas residue (BR-COOH) was prepared to remove the Cu²⁺ and Zn²⁺ from single/binary heavy metal ions solution and explore selective adsorption mechanism. The results exhibited that the adsorption capacities of BR-COOH for Cu²⁺ was higher than that for Zn²⁺ obviously, whether in the single or binary heavy metal ions solution. Meanwhile, the introduced carboxy groups were identified as the main sites for metal ions adsorption. Density functional theory (DFT) calculation results exhibited that the adsorption energy of Cu²⁺ (-0.51 eV) onto BR-COOH was lower than that of Zn²⁺ (-0.47 eV), indicating that the Cu²⁺ adsorbed on BR-COOH was more stable than Zn²⁺. Moreover, the metal ions adsorption capacity of BR-COOH was positively correlated with their electronegativity, which was due to that the metal ions with stronger electronegativity was more easily interacted with the negatively charged oxygen in carboxyl groups. The same results were also verified in the control experiment conducted with two other biosorbents. Therefore, the work provided a new and in-depth insight into selective adsorption of metal ions onto carboxylated biosorbent.

Lignocellulosic Materials Used as Biosorbents for the Capture of Nickel (II) in Aqueous Solution

Four lignocellulosic materials (walnut shell, chestnut shell, pine wood and burnt pine wood) were analyzed as biosorbents to remove nickel ions in aqueous solution. The optimal pH condition was determined. Due to this, a range of different pHs (3.0 to 7.5) was tested. The adsorption isotherms and kinetics were established. To plot Langmuir and Freundlich isotherms, batch adsorption tests were made with variable nickel concentrations (5 to 200 mg L−1). The pseudo-first order, pseudo-second order, Elovich and intraparticle diffusion models were used to describe the kinetics, batch adsorption tests were carried out with 25 mg L−1 of nickel solution and agitation time varied from 10 to 1440 min. The specific surface area of the different materials was between 3.97 and 4.85 m2g−1 with the exception for wood with 1.74 m2g−1. The pore size was 26.54 nm for wood and varied between 5.40 and 7.33 nm for the remaining materials. The diffractograms analysis showed that all the lignocellulosic materials presented some crystalline domains with the exception of burnt pine wood which was completely amorphous. The best pH was found to be around 5.0. At this pH the adsorption was higher for chestnut shells, walnut shells, burnt pine wood and wood, respectively. All samples fitted the Langmuir model well, with R2 of 0.994 to 0.998. The sorption kinetics was well described by the pseudo-second order equation with R2 between 0.996 and 1.00. No significative differences on the surface of the materials before and after adsorption could be observed by SEM. Finally, all materials tested were able to remove nickel ions in aqueous solution.

A review on adsorptive separation of toxic metals from aquatic system using biochar produced from agro-waste

Water is a basic and significant asset for living beings. Water assets are progressively diminishing due to huge populace development, industrial activities, urbanization and rural exercises. Few heavy metals include zinc, copper, lead, nickel, cadmium and so forth can easily transfer into the water system either direct or indirect activities of electroplating, mining, tannery, painting, fertilizer industries and so forth. The different treatment techniques have been utilized to eliminate the heavy metals from aquatic system, which includes coagulation/flocculation, precipitation, membrane filtration, oxidation, flotation, ion exchange, photo catalysis and adsorption. The adsorption technique is a better option than other techniques because it can eliminate heavy metals even at lower metal ions concentration, simplicity and better regeneration behavior. Agricultural wastes are low-cost biosorbent and typically containing cellulose have the ability to absorb a variety of contaminants. It is important to note that almost all agro wastes are no longer used in their original form but are instead processed in a variety of techniques to improve the adsorption capacity of the substance. The wide range of adsorption capacities for agro waste materials were observed and almost more than 99% removal of toxic pollutants from aquatic systems were achieved using modified agro-waste materials. The present review aims at the water pollution due to heavy metals, as well as various heavy metal removal treatment procedures. The primary objectives of this research is to include an overview of adsorption and various agriculture based adsorbents and its comparison in heavy metal removal.

Biochar-cadmium retention and its effects after aging with Hydrogen Peroxide (H2O2)

Cadmium (Cd) is a highly toxic heavy metal that can become available to the environment from a variety of sources. The thermal transformation of organic residues into biochar can be a sustainable way to reduce cadmium environmental availability and, at the same time, a waste management solution. We studied sixteen biochars in two versions: unaged and aged with hydrogen peroxide (H2O2), regarding their Cd retention capacity. Feedstocks used included softwood biochar (SWB), almond shell (ASB), walnut shell (WSB), sewage sludge (SSB), and coconut shell (CSB); production temperatures varied from 450 to 900 °C. The objectives of this research were to understand the role of biochar properties on Cd adsorption rates and to evaluate how properties and adsorption rates vary as a function of H2O2 aging. Feedstock played a more important role than production temperature in determining biochar properties. Cd-adsorption capacity ranged from 0.67 to 415.67 mg/g, and the biochars that adsorbed the most Cd were SSB 700, SWB 800 - i, CSB 600 - m2, ASB 500-1, CSB 600 - m3, WSB 900, and CSB 600. The properties that best explained this variation in Cd retention were ash, sulfur, nitrogen and carbon content. Variation in oxygen content, cation exchange capacity and surface area had less impact of Cd adsorption. The H2O2 aging caused oxygen content to increase in all biochars, but the increase in Cd retention was not significant for the majority of the biochars and aging even reduced the Cd retention in some. Our results may help design biochars with maximized sites for Cd adsorption.

Interactive effects of biochar and mussel shell activated concoctions on immobilization of nickel and their amelioration on the growth of rapeseed in contaminated aged soil

Mussel shell (MS) and biochar (BC) are commonly used for the remediation of metal contaminated soil. However, less research has been focused to examine the efficacy of their combinations to reduce metal toxicity in crop plants. This study was therefore conducted to investigate the effects of BC, MS and their activated concoctions on the soil properties, enzyme activities and nickel (Ni) immobilization in aged Ni contaminated soil. Moreover, the growth, photosynthetic pigments and anti-oxidative machnery of Brassica napus plants has also been investigated in order to determine amendments efficiency in reducing soil Ni toxicity for plants. The results showed that the application of Ni adversely affected soil health and trigged stress responses by inducing oxidative stress in B. napus. However, the incorporation of amendments reduced the bioavailability of Ni, and the concoctions of BC and MS showed promising results in the immobilization of Ni. Among various combinations of BC and MS, treatment with BC + MS (3:1) significantly reduced Ni uptake, decreased reactive oxygen species (ROS) and enhanced antioxidant defense of B. napus plants. Results showed that amendment's combinations stimulated the transcriptional levels of ROS scavenging enzymes and suppressed the expression level of Ni transporters. The morphological and physical characterization techniques (i.e. SEM, BET, EDS, FTIR and X-ray diffraction analyses) showed that amendment's combinations had relatively higher Ni adsorption capacity, indicating that BC and MS concoctions are efficient immobilizing agents for minimizing Ni availability, preventing oxidative toxicity and promoting growth and biomass production in rapeseed plants under metal stress conditions.

Characterization of Biochars Produced by Co-Pyrolysis of Hami Melon (Cantaloupes) Straw Mixed with Polypropylene and Their Adsorption Properties of Cadmium

Reuse of waste from Hami melon (cantaloupes) straws (HS) mingled with polypropylene (PP) ropes is necessary and beneficial to mitigate environmental pollution. The objective of this study was to investigate the characteristics and mechanisms of Cd2+ adsorption on biochars produced by co-pyrolysis of HS-PP with various mixing ratios. N2-sorption, scanning electron microscopy (SEM), energy dispersive X-ray spectrometer (EDS), elemental analysis, Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermal gravity, and differential thermal gravity (TG/DTG) were applied to evaluate the physicochemical properties of materials. Batch adsorption experiments were carried out for investigating the effects of initial pH, Cd2+ concentration, and adsorption time. It was found that the Langmuir and pseudo-second-order models fitted best for the experimental data, indicating the dominant adsorption of co-pyrolysis biochars is via monolayer adsorption. Biochar derived at 4/1 mixing ratio of HS/PP by weight percentage had the highest adsorption capacity of 108.91 mg·g-1. Based on adsorption isotherm and kinetic analysis in combined with EDS, FTIR, and XRD analysis, it was concluded that the main adsorption mechanism of co-pyrolysis biochar involved the surface adsorption, cation exchange, complexation of Cd2+ with surface functional groups, and chemical precipitation. This study also demonstrates that agricultural wastes to biochar is a sustainable way to circular economy.

Organic and inorganic amendments for the remediation of nickel contaminated soil and its improvement on Brassica napus growth and oxidative defense

In-situ stabilization has been considered an effective way to remediate metal contaminated soil. Thus, pot experiments were undertaken to investigate the effectiveness of multiple stabilization agents such as biochar (BC), mussel shell (MS), zeolite (ZE) and limestone (LS) on the immobilization of Ni, physicochemical features and enzyme activities in polluted soil. Results showed that the sole application of Ni adversely affected the rapeseed growth, photosynthetic pigments, and antioxidative defense. However, the addition of amendments to the contaminated soil significantly reduced Ni bioavailability. The XRD analysis confirmed the formation of Ni related ligands and FTIR showed the presence of hydroxyl, carboxyl and sulfur functional groups, as well as complexation and adsorption of Ni on amendments. Among multiple amendments, biochar significantly enhanced plant biomass attributes and total chlorophyll content. Moreover, addition of amendments also strengthened the antioxidant defense by decreasing Ni induced oxidative stress (H₂O₂ and O₂^.-), increased macronutrient availability, reduced Ni uptake and improved soil health. The qPCR analysis showed that the Ni transporters were significantly suppressed by amendments, which is correlated with the lower accumulation of Ni in rapeseed. The present study showed that immobilizing agents, especially biochar, is an effective amendment to immobilize Ni in soil, which restricts its entry into the food chain.

Evolution of redox activity of biochar during interaction with soil minerals: Effect on the electron donating and mediating capacities for Cr(VI) reduction

Biochar in soil is susceptible to natural aging along with soil minerals, which might alter its electrochemical properties and redox reactions with contaminants. In this study, soluble mineral salts (FeCl₃, MnCl₂, AlCl₃, CaCl₂) and clay mineral (kaolinite) were selected to investigate the impact of co-aging with soil minerals on the redox activity of peanut-shell biochar for Cr(VI) reduction. Natural aging for 3-month induced oxidation of biochar with the decrease of reducing moieties, i.e., ‒C‒OH from 26.8-43.7% to 18.4-24.1%. Co-aging with minerals except for Mn(II) further decreased the proportion of ‒C‒OH to 6.94-22.2% because of the interaction between mineral ions and biochar, resulting in the formation of mineral-biochar complex and new minerals, e.g. β-FeOOH. Due to its reductivity, Mn(II) presented the least decrease or even slight increase of ‒C‒OH while itself was oxidized to Mn(III) and Mn(IV). The decline of ‒C‒OH caused the decrease of Cr(VI) reduction rate constant from 2.18 to 2.47 × 10^-2 h^-1 for original biochars to 0.71-1.95 × 10^-2 h^-1 for aged ones, of which co-aging with Fe(III) showed the lowest reduction rate constant among all minerals. The electron mediating capacity of biochar also decreased after aging alone or co-aging with Al, Ca, and kaolinite, while co-aging with Fe(III) and Mn(II) facilitated the electron transfer process, increasing the rate constant by 219.3-1237% due to electron mediation through valence transformation of Fe(III)-Fe(II) and Mn(II)-Mn(III). Given the abundance of soil minerals, it was essential to consider this crucial factor for redox reactions when applying biochar for soil remediation.

The mechanism transformation of ramie biochar's cadmium adsorption by aging

Aging is inevitable when biochar uses for remediate Cadmium (Cd) pollution, but the variation of adsorption mechanism remains unclear. This study uses ramie residue to prepare fresh biochar, and adopt it with acidification and oxidation to simulate the aging process. The difference of physicochemical properties between fresh and aged biochar are studied through microstructure. Then, two kinds of biochar are making adsorption experiments in Cd solution for analyzing their adsorption mechanism. The results show that, both chemisorption and physisorption are exist, chemisorption and physisorption is the predominant way of fresh biochar and aged biochar respectively. Cation exchange is important but weaker in aged biochar than fresh biochar. Carboxyl plays a leading role in complexation of fresh biochar and hydroxy in aged biochar. Coprecipitation and cation-π mechanism impair apparently in aged biochar. This study indicates that aging change ramie biochar's main adsorption mechanism and the primary chemisorption way.

Preparation of magnetic hydrochar derived from iron-rich Phytolacca acinosa Roxb. for Cd removal

Considering that hyperaccumulators can accumulate high concentrations of iron salt, they can successfully obtain magnetic hydrochar from iron-rich hyperaccumulators. In this study, iron-rich biomass was obtained by irrigating Phytolacca acinosa Roxb. using iron salt. Magnetic nano-Fe₃O₄ hydrochar was prepared from iron-rich Phytolacca acinosa Roxb. via hydrothermal carbonization to remove Cd. The characterization results showed that the synthesized magnetic nanoparticles had an average size of 2.62 ± 0.56 nm and N elements were doped into magnetic nano-Fe₃O₄ hydrochar with abundant oxygenic groups. Cd adsorption on magnetic nano-Fe₃O₄ hydrochar was better fitted using the Langmuir isotherm and the pseudo-second-order kinetic model. The maximum adsorption capacity was 246.6 mg g^-1 of Cd. The research confirmed that Cd adsorption was controlled by multiple mechanisms from the jar test, transmission electron microscopy mapping, scanning electron microscopy-energy dispersive X-ray spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. CdCO₃ crystals can be formed after adsorption, indicating that surface precipitation played an important role in Cd adsorption. The abundance of O atoms and the doping of N atoms on the hydrochar surface were conducive to Cd adsorption, indicating that the mechanisms were related to surface complexation and electrostatic attraction. In addition, the significant decrease in Na⁺ content after Cd adsorption illustrated that ion exchange had a non-negligible effect on Cd adsorption. This study not only provides a strategy for preparing magnetic nano-Fe₃O₄ hydrochar derived from iron-rich plants but also verifies multiple Cd adsorption mechanisms using magnetic nano-Fe₃O₄ hydrochar.