Competitive sorption and availability of coexisting heavy metals in mining-contaminated soil: Contrasting effects of mesquite and fishbone biochars

Adsorption and distribution of heavy metals in aquatic environments: The role of colloids and effects of environmental factors

The study investigated the distributions of heavy metals (Cd, Cr, Cu, Mn, and Pb) between dissolved fraction (<0.7 µm) and particles (>0.7 µm) during the adsorption process. The dissolved fraction was further separated into truly dissolved (<3 kDa) and colloidal (3 kDa-0.7 µm) fractions. Significant metal adsorption occurred on the colloids, resulting in their aggregation into particles, which in turn influenced the particle adsorption kinetics. Colloids could either accelerate or inhibit the transformation of metal ions into particulates, depending on their stability. Competitive metals for colloids (Pb and Cr) were more susceptible to the effects of colloids than other elements. DOM was the predominant environmental factor influencing colloid behavior. The XDLVO theory showed that DOM enhanced the negative charge of colloids and made the colloid surface more hydrophilic, inhibiting the aggregation of colloids. DOM resulted in substantial increases in the concentrations of colloidal Pb and Cr from 0.31 μg/L and 4.58 μg/L to 20.52 μg/L and 43.51 μg/L, respectively, whereas the increment for less competitive metals (Cd and Mn) was smaller. These findings suggest that the distribution of heavy metals is influenced not only by adsorption from particles and ions but also by the complex dynamics of colloids.

Rice straw biochar and lime regulate the availability of heavy metals by managing colloid-associated- but dissolved-heavy metals

Heavy metal (HM) pollution has extensively spread in agricultural soils, posing potential threats to food safety and human health. Biochar and lime are two amendments used to remediate the soils contaminated with HMs. However, colloids have been shown to increase the mobility of HMs in paddy soils. Nevertheless, limited investigations have been made into the impact of biochar and lime on the formation of colloid-associated (colloidal) HMs in paddy soils. In this study, column and microcosm incubation experiments were conducted to examine how biochar and lime affected the availability of HMs (arsenic, cadmium, copper, iron, manganese, lead, and zinc) in different layers of paddy soils. The results revealed that biochar significantly inhibited the formation of colloidal HMs in the soil flooding phase, whereas the lime increased the colloidal HMs. These colloids containing HMs were identified as poorly dissolved metal sulfides. When the soil was drained, colloidal HMs transformed into dissolved forms, thereby improving the availability of HMs. Biochar decreased HM availability by reducing colloidal- but dissolved- HMs, whereas lime had the opposite effect. Hence, biochar demonstrated a stable and reliable remediation ability to decrease HM availability in paddy soil during flooding and drainage processes. In conclusion, this study highlighted that biochar efficiently reduced HM availability by mitigating the formation of colloidal HMs during flooding and their transformation into dissolved HMs during drainage in paddy soils.

Contrasting effect of pristine, ball-milled and Fe-Mn modified bone biochars on dendroremediation potential of Salix jiangsuensis "172" for cadmium- and zinc-contaminated soil

Bone biochar (BC) has a high capacity for the immobilization of potentially toxic elements (PTEs); however, its effect on dendroremediation efficiency remains unclear. Therefore, this study aimed to determine the effects of various concentrations (0, 0.5, 1, and 2 wt%) of BC, ball-milled BC (MBC), and Fe-Mn oxide-modified BC (FMBC) on soil properties, plant growth, and metal accumulation in Salix jiangsuensis "172" (SJ-172) grown in cadmium (Cd)- and zinc (Zn)-contaminated soil. BC and MBC promoted the photosynthetic rate, mineral element absorption, and plant growth of SJ-172, whereas FMBC inhibited the growth of SJ-172. Different biochars greatly influenced the concentrations of Cd and Zn in tissues of SJ-172. BC and MBC elevated the Cd levels, whereas FMBC decreased the Cd content in the leaves, stems, and cuttings of SJ-172. Unlikely, BC, MBC and FMBC show no evident change to the Zn concentration in the aboveground tissues of SJ-172, while decreased root Cd and Zn content compared with the control. MBC, at a 2.0% application rate, significantly increased the translocation factors of Cd (55.0%) and Zn (40.87%), whereas BC and FMBC demonstrated no significant effects compared with the control (P > 0.05). Moreover, 2.0% BC and MBC increased Cd and Zn accumulation in SJ-172 by 28.40 and 41.14, and 25.89 and 36.16%, respectively, whereas 2.0% FMBC reduced Cd and Zn accumulation by 53.20% and 13.18 %, respectively, compared with the control. The phytoremediation potential of SJ-172 for Cd- and Zn-contaminated soils was enhanced by MBC and BC, whereas it was lowered by FMBC compared to the control. These results provide novel insights for the application of fast-growing trees assisted by biochar amendments in the dendroremediation of severely PTEs-contaminated soil.

Effect of biochar application on rice, wheat, and corn seedlings in hydroponic culture

In recent years, biochar has attracted considerable attention for soil quality improvement and carbon sequestration due to its unique physicochemical properties. However, the mechanism by which biochar application negatively affects the growth of crop seedlings has not been fully investigated. In this study, a hydroponic experiment was conducted to evaluate the response of rice, wheat, and corn seedlings to biochar application (CK, 0 g/L; BC1, 0.5 g/L; and BC2, 1.0 g/L). Compared with the CK treatment, the BC1 and BC2 treatments decreased the fresh shoot and root weights of rice and corn seedlings (P < 0.05), but there was no significant effect on wheat seedlings (P > 0.05). For the contents of nutrient elements in seedlings, both BC1 and BC2 treatments hindered the roots from absorbing Fe and Cu and increased the uptake of Ca and Mn. Compared with the CK treatment, the translocation factor (TF) values of Ca, Mn, and Zn were significantly decreased especially in rice seedlings (35.3%-36.8%, 68.7%-76.5%, and 29.8%-22.0%, respectively) under the BC1 and BC2 treatments, while only Mn was significantly decreased in wheat and corn seedlings (P < 0.05). Transmission electron microscope (TEM) analysis of root cross-sections showed that nano-sized biochar particles (10∼23 nm) were found in the root cells under BC2 treatment conditions. Our findings reveal that a large amount of biochar application can reduce nutrient absorption and translocation, and hinder rice, wheat, and corn seedlings, particularly rice seedling, in hydroponic system.

Impacts of biochar materials on copper speciation, bioavailability, and toxicity in chromated copper arsenate polluted soil

Trace element polluted soils pose risks to human and environmental health. Biochar can decrease trace element bioavailability in soils, but their resulting ability to reduce soil toxicity may vary significantly depending on feedstocks used, pyrolysis conditions, and the target pollutants. Chromated copper arsenate (CCA) polluted sites are common, but only very few types of biochar have been tested for these sites. Hence, we tested fourteen well-characterized biochar materials for their ability to bind Cu and reduce toxicity in a CCA polluted soil in a 56-day experiment. Biochar (1%, wt/wt) increased plant (wheat, Triticum aestivum L.) shoot and root growth by 6-58% and 0-73%, reduced soil toxicity to Arthrobacter globiformis by 7-55%, decreased bioavailable Cu (Pseudomonas fluorescens bioreporter) by 5-65%, and decreased free Cu2+ ion activities by 27-89%. The A. globiformis solid-contact test constituted a sensitive ecotoxicological endpoint and deserves further attention for assessment of soil quality. Oil seed rape straw biochar generally performed better than other tested biochar materials. Biochar performance was positively correlated with its high cation exchange capacity, multiple surface functional groups, and high nitrogen and phosphorus content. Our results pave the way for future selection of feedstocks for creation of modified biochar materials with optimal performance in CCA polluted soil.

Effects of physical aging processes on the bioavailability of heavy metals in contaminated site soil amended with chicken manure and wheat straw biochars

The physicochemical properties of biochars undergo slow changes in soils due to the natural aging processes, which influences their interaction with heavy metals. The effects of aging on immobilization of co-existing heavy metals in contaminated soils amended with fecal and plant biochars possessing contrasting properties remain unclear. This study investigated the effects of wet-dry and freeze-thaw aging on the bioavailability (extractable by 0.01 M CaCl₂) and chemical fractionation of Cd and Pb in a contaminated site soil amended with 2.5% (w/w) chicken manure (CM) biochar and wheat straw (WS) biochar. Compared to that in the unamended soil, the contents of bioavailable Cd and Pb in CM biochar-amended soil decreased by 18.0% and 30.8%, respectively, after 60 wet-dry cycles, and by 16.9% and 52.5%, respectively, after 60 freeze-thaw cycles. CM biochar, which contained significant levels of phosphates and carbonates, effectively reduced the bioavailability of Cd and Pb and transformed them from the labile chemical fractions to the more stable ones in the soil during the accelerated aging processes, mainly through precipitation and complexation. In contrast, WS biochar failed to immobilize Cd in the co-contaminated soil in both aging regimes, and was only effective at immobilizing Pb under freeze-thaw aging. The changes in the immobilization of co-existing Cd and Pb in the contaminated soil resulted from aging-induced increase in oxygenated functional groups on biochar surface, destruction of the biochar's porous structure, and release of dissolved organic carbon from the aged biochar and soil. These findings could help guide the selection of suitable biochars for simultaneous immobilization of multiple heavy metals in co-contaminated soil under changing environmental conditions (e.g., rainfall, and freezing and thawing of soils).

Comparative efficacy of Parthenium hysterophorus (L.) derived biochar and iron doped zinc oxide nanoparticle on heavy metals (HMs) mobility and its uptake by Triticum aestivum (L.) in chromite mining contaminated soils

In this study we investigated the efficacy of a novel material parthenium weed (Parthenium hysterophorus L.) biochar (PBC), iron doped zinc oxide nanoparticles (nFe-ZnO), and biochar modified with nFe-ZnO (Fe-ZnO@BC) to adsorb heavy metals (HMs) and reduce their uptake by wheat (Triticum aestivum L.) in a highly chromite mining contaminated soil. The co-application of the applied soil conditioners exhibited a positive effect on the immobilization and restricted the HMs uptake below their threshold levels in shoot content of wheat. The maximum adsorption capacity was because of large surface area, cation exchange capacity, surface precipitation, and complexation of the soil conditioners. The scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) showed porous smooth structure of parthenium weed derived biochar that helped in HMs adsorption, increase the efficiency of soil fertilizers and nutrients retention which help in the enhancement soil condition. Under different application rates the highest translocation factor (TFHMs) was obtained at 2 g nFe-ZnO rate followed the descending order: Mn > Cr > Cu > Ni > Pb. The overall TFHMs was found <1.0 indicating that low content of HMs accumulation in roots from soil slight transferred to shoot, thus satisfying the remediation requirements.

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.

Artificial aging induced changes in biochar,s properties and Cd2+ adsorption behaviors

Fresh biochar has been widely applied to the remediation of heavy metals in soil by its property of adsorption, but the changes in its physicochemical properties and in situ adsorption performance over time cannot be ignored. In this study, the sorption of Cd²⁺ by corn straw biochars (CB) and municipal sludge biochars (SB) produced at 350 °C and 650 °C before and after H₂O₂ oxidation, and dry-wet and freeze-thaw aging were investigated using batch sorption experiments. The changes of physicochemical properties of biochar before and after aging were analyzed by various characterization methods. Based on these results, the impact of aging on the Cd²⁺ adsorption behavior could be clarified, which showed that CB650 was able to display the highest adsorption capacity in fresh biochars. Aging treatments reduced the ash content and pH value of CB, and significantly diminished the adsorption performance of Cd²⁺. These changes indicated that precipitation was a critical factor in the adsorption of Cd²⁺ on CB. The adsorption capacity of SB was enhanced after H₂O₂ oxidation, but weakened after dry-wet and freeze-thaw aging. This was closely related to the increase or decrease in the content of oxygen-containing functional groups, which in turn enhanced or inhibited its ability to compound with heavy metals. These results are of great significance for evaluating its long-term application prospects in the natural environment.

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.

Aspergillus niger-mediated release of phosphates from fish bone char reduces Pb phytoavailability in Pb-acid batteries polluted soil, and accumulation in fenugreek

Soil receiving discharges from Pb-acid batteries dismantling and restoring units (PBS) can have a high concentration of phytoavailable Pb. Reducing Pb phytoavailability in PBS can decline Pb uptake in food crops and minimize the risks to humans and the environment. This pot study aimed to reduce the concentration of phytoavailable Pb in PBS through Aspergillus niger (A. niger)-mediated release of PO₄^3- from fish bone [Apatite II (APII)] products. The PBS (Pb = 639 mg kg^-1 soil) was amended with APII powder (APII-P), APII char (APII-C), and A. niger inoculum as separate doses, and combining A. niger with APII-P (APII-P + A. niger) and APII-C (APII-C + A. niger). The effects of these treatments on reducing the phytoavailability of Pb in PBS and its uptake in fenugreek were examined. Additionally, enzymatic activities and microbial biomass carbon (MBC) in the PBS and the indices of plant physiology, nutrition, and antioxidant defense machinery were scoped. Results revealed that the APII-C + A. niger treatment was the most efficient one. Compared to the control, it significantly reduced the Pb phytoavailability (DTPA-extractable Pb fraction) in soil and its uptake in plant shoots, roots, and grain, up to 61%, 83%, 74%, and 92%. The grain produced under APII-C + A. niger were safe for human consumption as Pb concentration in grain was 4.01 mg kg^-1 DW, remaining within the permissible limit set by WHO/FAO (2007). The APII-C + A. niger treatment also improved soil pH, EC, CEC, MBC, available P content and enzymatic activities, and the fenugreek quality parameters. A. niger played a significant role in solubilizing PO₄^3- from APII-C, which reacted with Pb and formed insoluble Pb-phosphates, thereby reducing Pb phytoavailability in PBS and its uptake in plants. This study suggests APII-C + A. niger can remediate Pb-polluted soils via reducing Pb phytoavailability in them.

One stone two birds: Bone char as a cost-effective material for stabilizing multiple heavy metals in soil and promoting crop growth

Remediation of farmland soils contaminated with high levels of multiple heavy metals near PbZn smeltery is still a great challenge. It is of great significance to find cost-effective green remediation technologies for stabilization of multiple heavy metals in soil and reduce metal accumulation in crops with ensured yield. In this study, we demonstrated that bone char (BC) is an effective heavy metal stabilizer which can substantially increase residual fractions of heavy metals and reduce metal accumulation in pea (Pisum sativum) with its enhanced growth. We chose the soils contaminated with high levels of Pb, Zn, Cu and Cd near the Baiyin PbZn smeltery as the tested soil. After 2 months of BC application, the relative mobile fractions (non-residual fractions) of Cu, Zn, Pb and Cd in the contaminated soil decreased while the residual fraction increased significantly. The leachability of Cu, Zn, Pb and Cd decreased by 91.2%, 38.6%, 67.6% and 54.3%, respectively compared with the control. BC application remarkably promoted pea growth and reduced accumulation of heavy metals in shoots. The mechanisms for stabilization of multiple heavy metals BC include ion exchange, surface complexation and subsequent mineralization, accompanied with release of Ca and phosphate. The immobilization of heavy metals led to their reduced toxicity to plant, and thus increased pea growth. The results show that BC is a cost-effective and sustainable heavy metal stabilizer with phosphate fertilization function. It can simultaneously immobilize multiple heavy metals in soil and facilitate crop production.

Chitosan-Modified Biochars to Advance Research on Heavy Metal Ion Removal: Roles, Mechanism and Perspectives

The chitosan-modified biochars BC-CS 1-1, BC-CS 2-1 and BC-CS 4-1 were subjected to the synthetic application of biochar from agriculture waste and chitosan for the adsorption of Cu(II), Cd(II), Zn(II), Co(II) and Pb(II) ions from aqueous media. The results displayed a heterogeneous, well-developed surface. Additionally, the surface functional groups carboxyl, hydroxyl and phenol, determining the sorption mechanism and confirming the thermal stability of the materials, were present. The sorption evaluation was carried out as a function of the sorbent dose, pH, phase contact time, initial concentration of the solution and temperature. The maximum value of q_t for Pb(II)-BC-CS 4-1, 32.23 mg/g (C₀ 200 mg/L, mass 0.1 g, pH 5, 360 min), was identified. Nitric acid was applied for the sorbent regeneration with a yield of 99.13% for Pb(II)-BC-CS 2-1. The produced sorbents can be used for the decontamination of water by means of the cost-effective and high-performance method.

Assessing the impact of different salinities on the desorption of Cd, Cu and Zn in soils with combined pollution

Exploring the relationships between heavy metal release and salinity can help address the problems of combination of toxic heavy metals and salinization in contaminated soils. Therefore, in this study, the release characteristics of heavy metals (Cd, Cu and Zn) under different salt types and mass concentrations were investigated through batch desorption experiments. Spearman's correlation analysis was performed to assess the effects of typical physicochemical properties on metal release under salt stress. The results indicated that the types and concentration gradients of salt had notably different impacts on the release of different metals; specifically, there were significant impacts for Cd but slight impacts for Cu and Zn. MgCl₂ and CaCl₂ had more obvious promoting effects on Cd release, followed by a salt mixture and NaCl, and this pattern was similar for Zn release. Most salts could slightly restrain the release of Cu, except for MgCl₂ and Na₂SO₄, which had slight promoting effects. Moreover, low levels of CaCl₂ could effectively restrain the release of Cu. The results showed that the release capacities of metals followed the order of Cd > Cu > Zn, possibly attributed to the competitive adsorption among cations. Ferromanganese oxides in the soil favored the release of Cd and Zn under salt treatments, and redox potential was an important factor affecting Cu release. The results also suggested that the background values of metals could affect metal release, but the effects were varied under different salt treatments for Zn. The reason for this may be that the addition of different salts changed the effects of certain soil properties on the metal release. Overall, this study can serve as an important reference for controlling heavy metal pollution in soils in salinization and coastal areas.

Competitive adsorption, immobilization, and desorption risks of Cd, Ni, and Cu in saturated-unsaturated soils by biochar under combined aging

This study investigated saturated-unsaturated soils, which were closer to the actual field conditions than traditional batch and column experiments with large water-soil ratios. The competitive adsorption, immobilization, and desorption of Cd, Ni, and Cu in soils treated with original and KMnO₄-modified biochars were investigated under combined aging. Moreover, the employment of a three-layer mesh method enabled the independent analysis of heavy metals on biochar and soil during aging. The results showed that the order of biochar adsorption capacities was Cd > Cu > Ni in tested soils, and competing with Ni and Cu enhanced the Cd adsorption on biochars. Cd desorbed most with the CaCl₂ solution while Ni and Cu desorbed most with citric acid. Modified biochar had improved immobilization effects compared to original biochar, and maintained the most stable remediation effects. The maximum variations in the stable Cd fraction during aging were 7.21%, 13.26%, and 14.71% for modified biochar, original biochar, and CK, respectively. However, for Ni and Cu, the biochar application reduced the residual fraction and increased desorption by citric acid. However, the stable fractions of Ni and Cu remained dominant, accounting for 83.28-97.85% and 86.31-98.96%, respectively.

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.

Biochar as a potential strategy for remediation of contaminated mining soils: Mechanisms, applications, and future perspectives

Soil heavy metal contamination caused by mining activities is a global issue. These heavy metals can be enriched in plants and animals through the food chain, and eventually transferred to the human system and threatening public health. Biochar, as an environmentally friendly soil remediation agent, can effectively immobilize heavy metals in soil. However, most researchers concern more about the remediation effect and mechanism of biochar for industrial and agricultural contaminated soil, while related reviews focusing on mining soil remediation are limited. Furthermore, the remediation effect of soil in mining areas is affected by many factors, such as physicochemical properties of biochar, pyrolysis conditions, soil conditions, mining environment and application method, which can lead to great differences in the remediation effect of biochar in diverse mining areas. Therefore, it is necessary to systematically unravel the relevant knowledge of biochar remediation, which can also provide a guide for future studies on biochar remediation of contaminated soils in mining areas. The present paper first reviews the negative effects of mining activities on soil and the advantages of biochar relative to other remediation methods, followed by the mechanism and influencing factors of biochar on reducing heavy metal migration and bioavailability in mining soil were systematically summarized. Finally, the main research directions and development trends in the future are pointed out, and suggestions for future development are proposed.

Integrated approaches to mitigate threats from emerging potentially toxic elements: A way forward for sustainable environmental management

Potentially toxic elements (PTEs) such as toxic metal (loid)s and other emerging hazardous contaminants, exist in the environment and poses a serious threat. A large amount of wastewater containing PTEs such as cadmium, chromium, copper, nickel, arsenic, lead, zinc, etc. Release from industries during production process. Besides these, chemical-based fertilizers used in soils during crop production have become one of the crucial sources of PTEs. Various techniques are being employed for the mitigation of PTEs like chemical precipitation, ion exchange, coagulation, activated carbon, adsorption, membrane filtration, and bioremediation. Among these mitigation strategies, biological processes such as bioremediation, phytoremediation etc. Are extensively used, as they are economic have high-efficiency rate and are eco-friendly. This review intends to provide information on PTEs contamination through various sources; along with the toxicity of metal (loid)s with respect to their patterns of transmission and risks in the changing environment. Various remediation methods for the management of these pollutants along with their techno-economic perspective are also summarized in this review.

Pre-magnetic bamboo biochar cross-linked CaMgAl layered double-hydroxide composite: High-efficiency removal of As(III) and Cd(II) from aqueous solutions and insight into the mechanism of simultaneous purification

Trivalent arsenic (As(III)) and divalent cadmium (Cd(II)) contamination in water environment is an urgent issue because of their most toxic physicochemical properties. Herein, the simultaneous purification of As(III) and Cd(II) from aqueous solution was achieved by use of a pre-magnetic Fe modified bamboo biochar that cross-linked CaMgAl layered double-hydroxide composite (Fe-BC@LDH). In a binary system, adsorption equilibrium of As(III) and Cd(II) onto specific sorbent Fe-BC@LDH was reached within 100 and 10 min of contact time under anaerobic conditions, respectively, and the maximum adsorption capacities of As(III) and Cd(II) by Fe-BC@LDH were respectively calculated to be ⁓265.3 and ⁓320.7 mg/g at pH 4.5 and 5- and 14-times than that of unmodified biochar. Moreover, adsorption in a competitive or single system, the sorbent displayed a greater preference for Cd(II). Importantly, the removal of As(III) and Cd(II) onto the composite was more favorable in a binary system due to formation of ternary FeOCdAs bonding configuration as well as the redox transformation of As(III) to As(V), inner-sphere complexation of MOAs/Cd (MFe, Ca, Mg, Al), electrostatic attraction, and co-precipitation of scorodite and hydroxy‑iron‑cadmium. Furthermore, the nanocomposite was still highly efficient after 5 adsorption cycles. This study demonstrated that the synthesized cost-effective Fe-BC@LDH is a promising candidate for the simultaneous separation of As(III) and Cd(II) from wastewater.

Evaluation of the Immobilization of Coexisting Heavy Metal Ions of Pb2+, Cd2+ , and Zn2+ from Water by Dairy Manure-Derived Biochar: Performance and Reusability

Heavy metals including Cd, Pb, and Zn are prevalent stormwater and groundwater contaminants derived from natural and human activities, and there is a lack of cost-effective treatment for their removal. Recently, biochar has been increasingly recognized as a promising low-cost sorbent that can be used to remediate heavy metal contaminated water. This study evaluates the immobilization/release performance of dairy manure-derived biochar (DM-BC) as a sustainable material for competitive removal of coexisting heavy metal ions from water and explains the underlying mechanism for regeneration/reusability of biochar. Results showed that the metal ions exhibited competitive removal in the order of Pb2+ ≫ Zn2+ > Cd2+. The pH played a decisive role in influencing metal ion speciation affecting the electrostatic attraction/repulsion and surface complexation. Higher pH led to greater removal for Pb2+ and Cd2+, whereas Zn2+ showed maximum removal at pH ≈ 7.5. Diffuse reflectance infrared spectroscopy, scanning electron microscopy, and X-ray diffraction confirmed the interactions and precipitation reactions of oxygen-containing functional groups (e.g., ─OH, C O 3 2 - , and Si─O) as key participants in metal immobilization. Langmuir, Freundlich, and Redlich-Peterson isotherm modeling data showed varied and unique results depending on the metal ion and concentration. The removal kinetics and model fitting showed that the three steps of intraparticle diffusion might be more representative for describing the immobilization processes of metal ions on the external surface and internal pores. In the flow-through columns, DM-BC effectively retained the mixed metal ions of Cd2+, Pb2+, and Zn2+ showing 100% removal for the duration of the column run over three cycles of regeneration and reuse.

Method and mechanism of chromium removal from soil: a systematic review

Heavy metal pollution has increasingly affected human life, and the treatment of heavy metal pollution, especially chromium pollution, is still a major problem in the field of environmental governance. As a commonly used industrial metal, chromium can easily enter the environment with improperly treated industrial waste or wastewater, then pollute soil and water sources, and eventually accumulate in the human body through the food chain. Many countries and regions in the world are threatened by soil chromium pollution, resulting in the occurrence of cancer and a variety of metabolic diseases. However, as a serious threat to agriculture, food, and human health. Notwithstanding, there are limited latest and systematic review on the removal methods, mechanisms, and effects of soil chromium pollution in recent years. Hence, this article outlines some of the methods and mechanisms for the removal of chromium in soil, including physical, chemical, biological, and biochar methods, which provide a reference for the treatment and research on soil chromium pollution drawn from existing publications.

Removal of potentially toxic elements from contaminated soil and water using bone char compared to plant- and bone-derived biochars: A review

Conversion of hazardous waste materials to value-added products is of great interest from both agro-environmental and economic points of view. Bone char (BC) has been used for the removal of potentially toxic elements (PTEs) from contaminated water, however, its potential BC for the immobilization of PTEs in contaminated water and soil compared to bone (BBC)- and plant (PBC)-derived biochars has not been reviewed yet. This review presents an elaboration for the potentials of BC for the remediation of PTEs-contaminated water and soil in comparison with PBC and BBC. This work critically reviews the preparation and characterization of BC, BBC, and PBC and their PTEs removal efficiency from water and soils. The mechanisms of PTE removal by BC, BBC, and PBC are also discussed in relation to their physicochemical characteristics. The review demonstrates the key opportunities for using bone waste as feedstock for producing BC and BBC as promising low-cost and effective materials for the remediation of PTEs-contaminated water and soils and also elucidates the possible combinations of BC and BBC aiming to effectively immobilize PTEs in water and soils.

Phosphorus-modified biochar cross-linked Mg-Al layered double-hydroxide stabilizer reduced U and Pb uptake by Indian mustard (Brassica juncea L.) in uranium contaminated soil

The decommissioning of uranium tailings (UMT) is usually accompanied by uranium (U) contamination in soil, which poses a serious threat to human health and ecological security. Therefore, the remediation of uranium pollution in soil is imminent from ecological and environmental points of view. In recent years, the use of biochar stabilizers to repair uranium tailings (UMT) soil has become a research hotspot. In this study, a novel phosphorus-modified bamboo biochar (PBC) cross-linked Mg-Al layered double-hydroxide composite (PBC@LDH) was prepared. The hyperaccumulator plant Indian mustard (Brassica juncea L.) was selected as the test plant for outdoor pot experiments, and the stabilizers were added to the UMT soil at the dosage ratio of 15 g kg-1, which verified the bioconcentrate and translocate of U and associated heavy metal Pb in the UMT soil by Indian mustard after stabilizer remediated. The results shown that, after 50 days of growth, compared with the untreated sample (CK), the Indian mustard in PBC@LDH treatment possessed a better growth and its biomass weight of whole plant was increased by 52.7%. Meanwhile, the bioconcentration factors (BF) of U and Pb for PBC@LDH treatment were significantly decreased by 73.4% and 34.2%, respectively; and the translocation factors (TF) were also commendable reduced by 15.1% and 2.4%, respectively. Furthermore, the Tessier available forms of U and Pb in rhizosphere soil showed a remarkably decrease compared with CK, which reached by 55.97% and 14.1% after PBC@LDH stabilization, respectively. Complexation, precipitation, and reduction of functional groups released by PBC@LDH with U and Pb described the immobilization mechanisms of biochar stabilizer preventing U and Pb enrichment in Indian mustard. As well as, the formation of U-containing vesicles was prevented by the precipitation of -OH functional groups with free U and Pb ions around the cell tissue fluids and vascular bundle structure of plant roots, thereby reducing the migration risk of toxic heavy metals to above-ground parts. In conclusion, this research demonstrates that the PBC@LDH stabilizer offers a potentially effective amendment for the remediation of U contaminated soil.

Biochar promotes arsenic (As) immobilization in contaminated soils and alleviates the As-toxicity in soybean (Glycine max (L.) Merr.)

Pot experiments were carried out to examine the biochar application and its alleviating effect on arsenic (As) toxicity in soybean plants. The data showed that As inhibits the growth indices and it increased with enhanced As-concentration in the substrate. The growth indices declined by more than 40% and the osmolyte concentration, photosynthetic pigments and antioxidant enzymes were decreased significantly among As-stressed plants. However, biochar application effectively mitigated the inhibitory effects of As on the soybean growth and the mitigation effect of treatment is more prevalent to the plants subjected to higher As-treatment. Biochar significantly reduced the As-uptake as revealed by the translocation factor (<1), indicating more As is restrained in the roots. The reduction in the total chlorophyll and carotenoid content was found less in the As-treated soybean plants upon biochar application. Similarly, the osmolytes comprising proline, sugar and protein increased upon application of biochar. The biomarkers viz., membrane stability index (MSI), hydrogen peroxide and malondialdehyde (MDA) content significantly decreased at higher As-levels upon biochar application as was also supported by the heatmap analysis. Moreover, the antioxidative enzymes also showed a significant increase upon addition of biochar. Our data showed that biochar amendment effectively alleviates the As-stress by enhancing the sorption of As in the substrate thus, significantly declining the As concentration in plant leaves, and thus the results of the current study depicting the role of biochar as a promising, cost-effective and eco-friendly amendment to decontaminate the As-polluted soils.

Combing phosphorus-modified hydrochar and zeolite prepared from coal gangue for highly effective immobilization of heavy metals in coal-mining contaminated soil

Considering the adverse effects of heavy metals (HMs) on agriculture soil, in-situ immobilization has been paid great attention worldwide. P-modified biochar/hydrochar along with synthetic zeolite for efficient HMs immobilization in contaminated soil becomes a promising choice. In this study, H₃PO₄-modified hydrochar (BPH) derived from banana peels, and Na-X zeolite (ZL) prepared from coal gangue was applied individually and synergistically (1%BPH, 2%BPH 1%ZL, 2%ZL, and 1%BPH+1%ZL) to remediate a farmland soil polluted by Cd, Cu, and Pb near the coal-mining area. Compared with the mono-application of these two amendments, their combination significantly improved the soil organic carbon (SOC), electric conductivity (EC), and dehydrogenase activity. Besides, the addition of 1%CLH+1%ZL remarkably reduced the Cd, Cu, and Pb bioavailability by 67.01%, 57.01%, and 78.72%, respectively, in the soil after 100 d incubation by transforming these metals to more stable forms. The order of the HMs immobilization capacity for these two amendments was as follows: Pb > Cu > Cd. Moreover, the dominated immobilization mechanism of their synergistic application was that BPH could immobilize HMs by precipitation, complexation, and π-π electron-donor-acceptor interaction. The precipitation and complexation blocked the surface pores of BPH. The sustained release of phosphorus groups and radicals was prevented. This obstacle was possibly alleviated by adding ZL. Besides, the formation of cationic bridging, the enhancement of soil properties, and the physical adsorption of these amendments were also conducive to HMs immobilization in soil. This work indicated that co-application of BPH and ZL possibly was an excellent choice for immobilizing HMs in soil.

The sorption and short-term immobilization of lead and cadmium by nano-hydroxyapatite/biochar in aqueous solution and soil

In this study, the composite materials using different ratios of biochar (BC) to nano-hydroxyapatite (nHAP) were prepared for the remediation of lead (Pb) and cadmium (Cd) contaminated water and soil. The sorption and the immobilization experiments indicated a higher sorption capacity and immobilization efficiency of Pb compared to those of Cd. The characteristics of XRD, FTIR, SEM, and XPS manifested that dissolution-precipitation, cation exchange, complexation, and cation-π interaction were the main four mechanisms for the sorption of Pb²⁺ and Cd²⁺ using composite material PC1 (nHAP/BC = 1/1). From semi-quantitative analysis, the mineral effect accounted for the majority of the immobilization of Pb and Cd. Due to obvious Pb-precipitates in the sorbed material, dissolution-precipitation primarily affected the sorption of Pb using PC1, while the immobilization of Cd was mainly attributable to cation exchange. Such results corresponded to the stable Pb-precipitates and unstable Cd-compounds in soil, among which the latter was prone to be released into the environment. The sorption capacity in aqueous solutions and the immobilization efficiencies in the soil for both Pb and Cd increased with the addition of nHAP, which were linearly correlated to the nHAP proportion in the composite materials. In future practical applications, the percentages of composite materials can be designed according to the specific pollutant concentration. This study sheds light on the explicit immobilization mechanisms for Pb and Cd in aqueous solutions to better understand their behaviors in the soil remediated by relevant materials.

Sorption–Desorption Behavior of Doxycycline in Soil–Manure Systems Amended with Mesquite Wood Waste Biochar

Elevated levels of doxycycline (DC) have been detected in the environment due to its extensive utilization as a veterinary antibiotic. Sorption–desorption behavior of DC in soil affects its transport, transformation, and availability in the environment. Thus, sorption–desorption behavior of DC was explored in three soils (S1, S2, and S3) after manure application with and without mesquite wood-waste-derived biochar (BC) pyrolyzed at 600 °C. Sorption batch trials demonstrated the highest DC sorption in soil S1 as compared to S2 and S3, either alone or in combination with manure or manure + BC. Chemical sorption and pore diffusion were involved in DC sorption, as indicated by the kinetic models. Soil S1 with manure + BC exhibited the highest Langmuir model predicted sorption capacity (18.930 mg g−1) compared with the other two soils. DC sorption capacity of soils was increased by 5.0–6.5-fold with the addition of manure, and 10–13-fold with BC application in a soil–manure system. In desorption trials, manure application resulted in 67%, 40%, and 41% increment in DC desorption in soil S1, S2, and S3, respectively, compared to the respective soils without manure application. In contrast, BC application reduced DC desorption by 73%, 66%, and 65%, in S1, S2, and S3, respectively, compared to the soils without any amendment. The highest DC sorption after BC application could be due to H bonding, π–π EDA interactions, and diffusion into the pores of BC. Hence, mesquite wood-waste-derived BC can effectively be used to enhance DC retention in contaminated soil to ensure a sustainable ecosystem.

Magnetic zeolitic imidazolate frameworks composite as an efficient adsorbent for arsenic removal from aqueous solution

In this study, magnetic zeolitic imidazolate frameworks (ZIF-8) was prepared by a one-step method, where its evolution involved the coprecipitation reactions concomitant with the self-assembly reactions. Structural characterizations indicated that magnetic ZIF-8 showed irregular polyhedral morphology with a large specific surface area (696.5 m²/g) and saturation magnetization (4.31 emu/g). The as-prepared magnetic ZIF-8 enhanced the adsorption performance of As(III) and As(V), compared with bare Fe₃O₄. The pseudo second-order kinetic model (R² = 0.9627 and 0.9893 for As(III) and As(V), respectively) and the Langmuir model (R² = 0.9441 for As(III) and 0.9851 for As(V)) can fit the adsorption process well, confirming the nature of single-layer homogeneous chemisorption. The adsorption capacity was 30.87 and 17.51 mg/g, and their corresponding values of PC were 2.664 and 1.286 L/g, for As(III) and As(V), respectively. Solution pH showed an adverse effect on As(V) adsorption whereas no obvious effect on As(III). The ionic strength and coexisting ions had not obvious influence on adsorption of As(III) and As(V). The adsorption mechanism was explored and discussed based on the detailed spectroscopy analysis. This adsorbent can be recovered magnetically after use, which is promising for the practical application.

Effect of different vegetation on copper accumulation of copper-mine abandoned land in tongling, China

Mining activity and abandoned mine land are one of the major sources of heavy metal pollution. Thus, ecological rehabilitation of abandoned mine lands is crucial to control heavy metal pollution. This research aims to explore the influencing factors and effects of different vegetation on copper (Cu) accumulation and soil amelioration. In this study, the abandoned land of Tongguanshan Cu mine in Tongling city, Anhui province, China, was chosen as the test area, and nine sampling points were established. Samples of soil and plants were collected from each plot, and the impacts of Cu pollution on soil enzymes and other features were analyzed, as well as the correlation between Cu accumulation of different plants and soil properties. The results showed that Cu content of soil in the Tongguanshan area varied greatly with the depth of the soil profile. Moreover, Cu in the soil can inhibit soil enzyme activities; and the correlation coefficients of total soil Cu with urease and catalase were -0.83 and -0.73, respectively. Clearly, the accumulation of Cu in plants was positively correlated with Cu content in soil. It was found that Pueraria lobata had the best remediation effect on soil Cu pollution in a short period of time. Hence the preliminary tests clearly indicate that phytoremediation in abandoned mine lands can not only reduce heavy metal pollution, but also enhance soil nutrition and enzyme activity, helping to ameliorate degraded land and promote regional socioeconomic sustainable development.

Removal of Cd from aqueous solution by chitosan coated MgO-biochar and its in-situ remediation of Cd-contaminated soil

In this study, magnesium oxide biochar-chitosan composite (MgO-BCR-W) prepared through modification with MgCl₂ and chitosan was investigated as an adsorbent for the removal of Cd from the aquatic and soil environment. Kinetic and thermodynamics revealed that the Cd(II) adsorption onto MgO-BCR-W was well fitted by pseudo-second-order and the Langmuir adsorption isotherm. The adsorption capacities of rice husk biochar (BCR) and MgO-BCR-W for Cd(II) reached 11.09 mg/g and 59.66 mg/g, respectively. Attractively, the computed values of R_L ranged between 0 and 1, suggesting that the adsorption of Cd(II) onto MgO-BCR-W is favourable. Characterisations of the adsorbents revealed that the synergistic effect of surface complexation and precipitation mechanisms played a major role in the removal of Cd. In soil incubation experiment, the addition of MgO-BCR-W at the level of 2% was most effective in Cd stabilization compared to the control, which reduced the content of bioavailable Cd by 22.32%. Furthermore, it reduced the acid extractable Cd by 24.77%, while increased the residual Cd content by 22.24%. The results demonstrated that MgO-BCR-W could be used as an effective and eco-friendly adsorbent for Cd remediation in both water and soil environment.

Biochar for cadmium pollution mitigation and stress resistance in tobacco growth

Pot experiments were conducted to investigate the influence of biochar addition and the mechanisms that alleviate Cd stress in the growth of tobacco plant. Cadmium showed an inhibitory effect on tobacco growth at different post-transplantation times, and this increased with the increase in soil Cd concentration. The growth index decreased by more than 10%, and the photosynthetic pigment and photosynthetic characteristics of the tobacco leaf were significantly reduced, and the antioxidant enzyme activity was enhanced. Application of biochar effectively alleviated the inhibitory effect of Cd on tobacco growth, and the alleviation effect of treatments is more significant to the plants with a higher Cd concentration. The contents of chlorophyll a, chlorophyll b, and carotenoids in the leaves of tobacco plants treated with biochar increased by 9.99%, 12.58%, and 10.32%, respectively, after 60 days of transplantation. The photosynthetic characteristics index of the net photosynthetic rate increased by 11.48%, stomatal conductance increased by 11.44%, and intercellular carbon dioxide concentration decreased to 0.92. Based on the treatments, during the growth period, the antioxidant enzyme activities of tobacco leaves comprising catalase, peroxidase, superoxide dismutase, and malondialdehyde increased by 7.62%, 10.41%, 10.58%, and 12.57%, respectively, after the application of biochar. Our results show that biochar containing functional groups can effectively reduce the effect of Cd stress by intensifying the adsorption or passivation of Cd in the soil, thereby, significantly reducing the Cd content in plant leaves, and providing a theoretical basis and method to alleviate soil Cd pollution and effect soil remediation.

Sustainable improvement of soil health utilizing biochar and arbuscular mycorrhizal fungi: A review

Conservation of soil health and crop productivity is the central theme for sustainable agriculture practices. It is unrealistic to expect that the burgeoning crop production demands will be met by a soil ecosystem that is increasingly unhealthy and constrained. Therefore, the present review is focused on soil amendment techniques, using biochar in combination with arbuscular mycorrhizal fungi (AMF), which is an indispensable biotic component that maintains plant-soil continuum. Globally significant progress has been made in elucidating the physical and chemical properties of biochar; along with its role in carbon sequestration. Similarly, research advances on AMF include its evolutionary background, functions, and vital roles in the soil ecosystem. The present review deliberates on the premise that biochar and AMF have the potential to become cardinal to management of agro-ecosystems. The wider perspectives of various agronomical and environmental backgrounds are discussed. The present state of knowledge, different aspects and limitations of combined biochar and AMF applications (BC + AMF), mechanisms of interaction between biochar and AMF, effects on plant growth, challenges and future opportunities of BC + AMF applications are critically reviewed. Given the severely constrained nature of soil health, the roles of BC + AMF in agriculture, bioremediation and ecology have also been examined. In spite of the potential benefits, the functionality and dynamics of BC + AMF in soil are far from being fully elucidated.

Chemical speciation and distribution of potentially toxic elements in soilless cultivation of cucumber with sewage sludge biochar addition

Potentially toxic elements in municipal sewage sludge can be effectively immobilized during biochar production via pyrolysis. However, the bioavailability of these elements when biochar is applied in soilless cultivation to improve substrate quality has yet to be sufficiently established. In this study, we investigated the chemical speciation and cucumber plant uptake of potentially toxic elements in soilless cultivation when the growth substrate was amended with sewage sludge biochar (0, 5, 10, 15, and 20 wt%). It was found that the addition of 10 wt% biochar was optimal with respect to obtaining a high cucumber biomass and achieving low environmental risk considering the occurrence of hormesis. When the substrate was amended with 10 wt% biochar, cucumber fruit contained lower concentrations of As, Cr, and Zn and smaller bioavailable fractions of As, Cd, Cr, Ni, Cu, and Zn compared with the fruit of control plants, thereby meeting national safety requirements (standard GB 2762-2012, China). Most of the As and Cd taken up by cucumbers accumulated in the leaves and fruit, whereas Cr was found primarily in the roots, and most Ni, Cu, and Zn was detected in the fruit. Importantly, only small proportions of the potentially toxic elements in biochar were taken up by cucumber plants (As: 0.0075%; Cd: 0.038%; Ni: 0.0064%; Cu: 0.0016%; and Zn: 0.0015%). Given that the As, Cd, Ni, and Zn speciation in sewage sludge biochar was effectively immobilized after cultivation, the findings of this study indicate that sewage sludge biochar is a suitable substrate amendment in terms of the risk posed by potentially toxic elements.

Phosphate removal by ZIF-8@MWCNT hybrids in presence of effluent organic matter: Adsorbent structure, wastewater quality, and DFT analysis

Recently, zeolitic imidazolate framework-8 (ZIF8) and its derivatives have been applied in aqueous contaminant removal. Herein, three types of ZIF8@carbon nanotube (CNT) hybrids harvesting different pore structures and chemical bonding information are utilized for phosphate removal in the typical wastewater of activated sludge system (SW) and partial nitrification-denitrification treatment system (PND). Effluent organic matter (EfOM) is found to compete with phosphate for adsorption sites on adsorbents, resulting in reducing adsorptive capacities for phosphate, and the negative effect trend to become severer with increasing EfOM concentrations. Thus adverse impact are highly to be relieved by using ZIF8@CNT-2 (hybrids with CNT dosage of 120 mg) with novel structure design, the hybrid of which harvests the highest phosphate removal of 92.8-100%, the largest Partition coefficient (PC) of 9119.05 mg g^-1 μM with initial concentration of 0.96 mg L^-1, pH independence in the range from 4 to 10. Analyses of the XPS characterization and first-principles calculations demonstrate the dominant interactions of Zn-O-P and H-bond during phosphate adsorption process by ZIF8@CNT hybrids. Such interactions are suppressed in presence of EfOM by weakening the above-stated binding energy at different adsorption sites according to first-principles simulation, resulting in declined phosphate adsorption capacity. In this regard, the less sensitivity to co-existing EfOM of ZIF8@CNT-2 may be due to the increased P=O, Zn-O-P and P-OH and the strengthened tolerance of nanostructure. These results suggest the promising enhanced phosphate removal in presence of EfOM could be obtained by specifically designing adsorbent structure.

Biochar assisted phytoremediation and biomass disposal in heavy metal contaminated mine soils: a review

Mining activities causes heavy metal pollution and adversely affect the ecological safety and human well-being. Phytoremediation-biochar synergy can effectively remediate mine spoils contaminated with heavy metals (HM). A review which focuses exclusively on the application of biochar assisted phytoremediation in HM contaminated mine spoil is lacking. Mechanisms of metal immobilization by biochar, potential plants and contaminated biomass disposal methods has also been reviewed. Availability of biochar feedstock and production conditions, optimization of application rate, application techniques, selection of suitable hyperaccumulators and cost optimization of bulk biochar production are the key to a successful biochar-based HM remediation of mine tailings and coalmine spoil. Presently, herbs and shrubs are mostly used as phytoremediators, use of woody trees would encourage a long-term metal sequestration which would reduce the cost of biomass disposal. Also, use of non-edible plants would prevent the plants from entering the food chain. For a holistic biochar-phytoremediation technique, incineration and pyrolysis can effectively dispose contaminated biomass. From the economical viewpoint, the environment cost-benefit analysis should be considered before considering the feasibility of a technology.HighlightsMass scale in-situ biochar production and economics are keys issues.Biochar assisted phytoremediation for HM contaminated mine spoils.Long term studies using woody biomass needs attention.Disposal of contaminated biomass by pyrolysis method.

Specific Adsorption of Heavy Metals in Soils: Individual and Competitive Experiments

The partitioning between the soil solid phase and the soil solution determines the mobility of pollutants like heavy metals. If nonspecific sorption takes place, the reactions are easily reversible and heavy metals are released to soil solution increasing the probability of leaching through soil profile. Mobility and leaching are also favoured if other metals are in the system and competition for specific adsorption sites takes place. In this study, desorption equilibrium experiments were conducted after adsorption ones. The specific adsorption was evaluated through the amounts of the still adsorbed Cu, Pb, Cr, Ni and Zn after desorption experiments in ten different soils. In addition, competition adsorption and desorption binary experiments were conducted for evaluating the metal competition in three of the soils. Pb and Cu are the metals adsorbed and retained in higher amounts in all the studied soils. In slightly neutral soils, Cr is retained in lesser amounts while in acidic soils Zn is the metal less retained. Results showed that despite the high and variable amounts of organic matter in the soils, soil pH is the most important variable in neutral soils. In acidic soils, soil properties different than pH play important roles and specific sorption of Pb is related to the cationic exchange capacity of the soils while that of Zn to the clay content. Instead, the release of Cu during desorption experiments is probably due to the more soluble organic fraction of the soils. The individual retention of Cu, Zn, Ni and Pb is higher than when they are in competition, except if Cr is present. In this case, the amount of those four metals and that of Cr increased. Therefore, the presence of Cr together with cationic heavy metals favoured the adsorption of those metals in multi-metal polluted areas. Specific adsorption is also important during competition as soil affinities increase during competition experiments.