Leachability, availability and bioaccessibility of Cu and Cd in a contaminated soil treated with apatite, lime and charcoal: A five-year field experiment

Cement and zeolite stabilization/solidification of heavy metal-contaminated sediments: 841-Day leaching characteristics, mechanisms, and microstructure

The long-term effectiveness of stabilized/solidified sediments (S/S sediments) is increasingly attracting attention. This study investigated the long-term leaching characteristics and mechanisms of S/S sediment through an 841-day tank leaching test, considering the influence of cement content, curing time, and zeolite. The results indicate significant correlations among pH, heavy metals, TN, NH3-N, and COD. The specimens with 6 % cement cured for 30 days (C6(30)) demonstrated considerable heavy metal stabilization, with stabilization rates for Cr, Ni, Cu, Zn, As, and Pb reaching 99.81 %, 99.06 %, 98.93 %, 99.61 %, 97.58 %, and 99.97 %, respectively. Compared to C6(30), partial replacement of cement with 10 % zeolite (C5 + Z0.5(30)) not only more effectively stabilized heavy metals except As, but also reduced the release of COD and NH3-N by 4.23 % and 10.04 %, respectively. However, there was a risk of TN, NH3-N, and COD exceeding permissible limits during long-term leaching. Microscopic analysis results suggested that hydration products and low porosity contributed to stabilization of heavy metals. Leaching mechanisms was revealed that surface wash-off controls the leaching of Cr and Pb, while diffusion controls the leaching of Ni, Cu, Zn, As, COD, TN, and NH3-N. Considering stabilization performance, cost and carbon emissions, C5 + Z0.5(30) is an effective strategy for reducing long-term environmental risks of S/S sediments.

In-situ remediation of cadmium contamination in paddy fields: from rhizosphere soil to rice kernel

Cadmium (Cd) has become an important heavy metal pollutant because of its strong migration and high toxicity. The industrial production process aggravated the Cd pollution in rice fields. Human exposure to Cd through rice can cause kidney damage, emphysema, and various cardiovascular and metabolic diseases, posing a grave threat to health. As modern technology develops, the Cd accumulation model in rice and in-situ remediation of Cd pollution in cornfields have been extensively studied and applied, so it is necessary to sort out and summarize them systematically. Therefore, this paper reviewed the primary in-situ methods for addressing heavy metal contamination in rice paddies, including chemical remediation (inorganic-organic fertilizer remediation, nanomaterials, and composite remediation), biological remediation (phytoremediation and microbial remediation), and crop management remediation technologies. The factors that affect Cd transformation in soil and Cd migration in crops, the advantages and disadvantages of remediation techniques, remediation mechanisms, and the long-term stability of remediation were discussed. The shortcomings and future research directions of in situ remediation strategies for heavily polluted paddy fields and genetic improvement strategies for low-cadmium rice varieties were critically proposed. To sum up, this review aims to enhance understanding and serve as a reference for the appropriate selection and advancement of remediation technologies for rice fields contaminated with heavy metals.

Characteristics of three organic fertilizers and their influence on the mobility of cadmium and arsenic in a soil-rice (Oryza sativa L.) system

The properties of different organic fertilizers and their potential for stabilizing toxic metals(loids) in soil have not been fully investigated. This study characterized and evaluated three organic fertilizers from different raw materials. The mushroom residue organic fertilizer (MO) had higher C, H, and O contents and more functional groups (-OH, C-H, and C = O). Its application significantly increased pH (1.00 ~ 1.32 units), organic matter (OM) content (26.58 ~ 69.11%), and cation exchange capacity (CEC) (31.52 ~ 39.91%) of soil. MO treatments can simultaneously reduce the bioavailable TCLP-Cd and TCLP-As in soil, solving the difficulties of remediating the combined Cd and As pollution. MO treatments inhibited the migration of Cd and As from soil to plant, promoting plant growth. Redundancy analysis (RDA) revealed that metal(loid) variations in plants were related to soil properties (40.09%) and TCLP-Cd/As (44.74%). Furthermore, the toxic metals(loids) risk assessment for all organic fertilizers was at safe levels. This study provided a valuable reference for choosing organic fertilizers and presented a novel option for the "producing while remediating" of farmlands with low pollution.

Layered double hydroxides for simultaneous and long-term immobilization of metal(loid)s in soil under simulated aging

Soil contamination by toxic metals and metalloids poses a grave threat to food security and human well-being. Immobilization serves as an effective method for the remediation of soils contaminated by metal(loid)s. Nevertheless, the ability of soil amendments for simultaneous immobilization of cations and oxyanions, and the long-term effectiveness of immobilization need substantial improvements. In this study, we used a series of layered double hydroxides (LDHs), including Mg-Al LDH and Ca-Al LDH fabricated from pure chemicals, and one waste-derived LDH synthesized using granulated ground blast furnace slag (GGBS), for the immobilization of Cu, Zn, As, and Sb in a historically contaminated soil obscured from a mining-affected region. The LDHs were first subjected to iron (Fe) modification to enhance their short-term immobilization performances toward metal(loid)s. Furthermore, the long-term effectiveness of Fe-modified LDHs was examined via two sets of experiments, including column experiments simulating 2-year water leaching, and accelerated aging experiments simulating 100-year proton attack. It was observed that Fe-modified LDHs, either made from pure chemicals or GGBS, demonstrated promising long-term immobilization performances toward metal(loid)s. Results from this study are encouraging for the future use of LDHs for simultaneous and long-term immobilization of metal(loid)s in soil.

Alteration of soil pH induced by submerging/drainage and application of peanut straw biochar and its impact on Cd(II) availability in an acidic soil to indica-japonica rice varieties

The effects of soil pH variations induced by submergence/drainage and biochar application on soil cadmium (Cd) availability to different rice (Oryza sativa L.) varieties are not well understood. This study aims to investigate the possible reasons for available Cd(II) reduction in paddy soil as influenced by biochar and to determine Cd(II) absorption and translocation rates in different parts of various rice varieties. A pot experiment in a greenhouse using four japonica and four indica rice varieties was conducted in Cd(II) contaminated paddy soil with peanut straw biochar. The results indicated that the submerging led to an increase in soil pH due to the consumption of protons (H+) by the reduction reactions of iron/manganese (Fe/Mn) oxides and sulfate (SO42-) and thus the decrease in soil available Cd(II) contents. However, the drainage decreased soil pH due to the release of protons during the oxidation of Fe2+, Mn2+, and S2- and thus the increase in soil available Cd(II) contents. Application of the biochar increased soil pH during soil submerging and inhibited the decline in soil pH during soil drainage, and thus decreased soil available Cd(II) contents under both submerging and drainage conditions. The indica rice varieties absorbed more Cd(II) in their roots and accumulated higher amounts of Cd(II) in their shoots and grains than the japonica rice varieties. The Cd(II) sensitive varieties exhibited a greater absorption and translocation rate of Cd(II) compared to the tolerant varieties of both indica and japonica rice. Biochar inhibited the absorption and accumulation of Cd(II) in the rice varieties, which ultimately lowered the Cd(II) contents in rice grains below the national food safety limit (0.2 mg kg-1). Overall, planting japonica rice varieties in Cd(II) polluted paddy soils combined with the use of biochar can effectively reduce Cd(II) content in rice grains which protects human health against Cd(II) toxicity.

Ecological consequences of biochar and hydrochar amendments in soil: assessing environmental impacts and influences

Anthropogenic activities have caused irreversible consequences on our planet, including climate change and environmental pollution. Nevertheless, reducing greenhouse gas (GHG) emissions and capturing carbon can mitigate global warming. Biochar and hydrochar are increasingly used for soil remediation due to their stable adsorption qualities. As soil amendments, these materials improve soil quality and reduce water loss, prevent cracking and shrinkage, and interact with microbial communities, resulting in a promising treatment method for reducing gas emissions from the top layer of soil. However, during long-term studies, contradictory results were found, suggesting that higher biochar application rates led to higher soil CO2 effluxes, biodiversity loss, an increase in invasive species, and changes in nutrient cycling. Hydrochar, generated through hydrothermal carbonization, might be less stable when introduced into the soil, which could lead to heightened GHG emissions due to quicker carbon breakdown and increased microbial activity. On the other hand, biochar, created via pyrolysis, demonstrates stability and can beneficially impact GHG emissions. Biochar could be the preferred red option for carbon sequestration purposes, while hydrochar might be more advantageous for use as a gas adsorbent. This review paper highlights the ecological impact of long-term applications of biochar and hydrochar in soil. In general, using these materials as soil amendments helps establish a sustainable pool of organic carbon, decreasing atmospheric GHG concentration and mitigating the impacts of climate change.

Long-term effects of compound passivator coupled with silicon fertilizer on the reduction of cadmium and arsenic accumulation in rice and health risk evaluation

Cadmium (Cd) and arsenic (As) are precedence-controlled contaminants in paddy soils, that can easily accumulate in rice grains. Limestone and sepiolite (LS) compound passivator can obviously reduce Cd uptake in rice, whereas Si fertilizer can effectively decrease rice As uptake. Here, the synergistic effects of the LS compound passivator coupled with Si fertilizer (LSCS) on the soil pH and availability of Si, Cd, and As, as well as rice grain Cd and As accumulation and its health risk were studied based on a 3-year consecutive field experiment. The results showed that the LSCS performed the best in terms of synchronously decreasing soil Cd and As availability and rice Cd and As uptake. In the LSCS treatments, soil pH gradually decreased with the rice-planting season, while soil available Cd and As contents gradually increased, suggesting that the influence of LSCS on Cd and As availability gradually weakened with rice cultivation. Nonetheless, the contents of Cd and inorganic As (i-As) in rice grains treated with LSCS were slightly affected by cultivation but were significantly lower than the single treatments of LS compound passivator or Si fertilizer. According to the Cd and As limit standards in food (GB2762-2022), the Cd and i-As content in rice grains can be lowered below the standard by using the 4500 kg/hm2 LS compound passivator coupled with 90 kg/hm2 Si fertilizer in soil and spraying 0.4 g/L Si fertilizer on rice leaves for at least three years. Furthermore, health risk evaluation revealed that LSCS treatments significantly reduced the estimated daily intake, annual excess lifetime cancer risk, and hazard quotient of Cd and i-As in rice grains. These findings suggest that LSCS could be a viable approach for reducing Cd and As accumulation in rice grains and lowering the potential health risks associated with rice.

Utilization of phosphoric acid-modified biochar to reduce vanadium leaching potential and bioavailability in soil

Remediating vanadium (V) polluted soil has garnered widespread attention over the past decade. Yet, few research projects have investigated the stabilization of soil V using modified biochar, so the effects and interacting mechanisms between soil properties and modified biochar for V immobilization and stabilization remain unclear. Hence, this gap is addressed by determining the leaching behavior and mechanisms of soil V on different dosages of phosphoric acid (H3PO4) impregnated biochar (MLBC, 0.5%-4%). The applicability and durability in soil V immobilization was investigated under acid precipitation. The MLBC effect on V bioavailability and mobility was assessed first by CaCl2, Toxicity Characteristic Leaching Procedure (TCLP) and Synthetic Precipitation Leaching Procedure (SPLP) extractions in different periods. The V concentrations significantly reduced in CaCl2, TCLP, and SPLP extract with MLBC at each dosage (30 d), while slight to significant increase in SPLP and TCLP extract V was recorded in a long-term incubation (90 d). Column leaching test further demonstrated the high durability of 4% MLBC in V stabilization under continuous acid exposure. Compared to the control (no-biochar), the accumulated V content in the leaching solution significantly decreased in MLBC-amended soil. Acid soluble fraction of V showed significant negative correlation with both soil organic matter (SOM) and available P, which was positively correlated with pH, suggested that pH, available P and SOM were key factors affecting the bioavailability of V in soil. Moreover, combining with the characterization results of MLBC and amended soil, the results revealed that H3PO4 modified biochar played a vital role on V immobilization and soil improvement by forming electrostatic adsorption, ion exchange, redox reaction or complexation with the increase of functional groups. These revealed an efficient and steady development of soil quality and treatment for soil V contamination, under MLBC operation to soil polluted with exogenous V.

Impacts of atmospheric deposition on the heavy metal mobilization and bioavailability in soils amended by lime

Atmospheric deposition is an important source of heavy metal in agricultural soils, but there is limited research on the mobility of these metals in soil and their impact on soil amendment. Here, we performed a dust incubation experiment in soils in the laboratory and a factorial transplant experiment at three field sites with a gradient of atmospheric deposition to examine the impacts of atmospherically deposited heavy metals (Cu, Cd, and Pb) on the mobility and bioavailability in soils with and without lime applications. Results showed that the atmospherically deposited heavy metals showed high mobility and were primarily presented in the soluble ionic fractions in the wet part and acid-exchangeable and reducible fractions in the dry part of atmospheric deposition. Atmospheric dust addition caused the 2p3/2 and 2p1/2 electrons of Cu atoms in uncontaminated soils to transition the 3d vacant states, resulting in similar copper absorption peaks as atmospheric particles by the observation of X-ray absorption near-edge spectroscopy (XANES). In the field, atmospheric deposition can only increase the mobile fractions in the surface soils, but not in the deeper layers. However, the deposition can increase the soluble and diffusive gradients in thin films (DGT)-measured bioavailable fractions in profile along with the soil depth. Lime applications cannot significantly reduce the mobile fractions of heavy metals in the surface soils exposed to atmospheric deposition, but significantly reduce the heavy metal concentrations in soil solutions and the DGT-measured bioavailable concentrations, particularly in the deeper layer (6-10 cm). The major implication is that atmospherically deposited heavy metals can significantly increase their bioavailable concentrations in the plough horizon of soil and constrain the effects of soil amendments on heavy metal immobilization, thereby increasing the risks of crop uptake.

The Immobilization Mechanism of Inorganic Amendments on Cu and Cd in Polluted Paddy Soil in Short/Long Term

This study investigated the impact of soil colloidal characteristics on the transfer patterns of different Cu and Cd speciation in contaminated soil treated with three different amendments: lime (L), zero-valent iron (ZVI), and attapulgite (ATP). It seeks to clarify the activation hazards and aging processes of these modifications on Cu and Cd. Compared with the control (CK), the available Cu concentrations treated with amendments reduced in the short term (6 months) by 96.49%, 5.54%, and 89.78%, respectively, and Cd declined by 55.43%, 32.31%, and 93.80%, respectively. Over a 12-year period, there was no significant change in the immobile effect with L, while Cu and Cd fell by 19.06% and 40.65% with ZVI and by 7.63% and 40.78% with ATP. Short- and long-term increases in the readily reducible iron and manganese oxide fraction of Cu and Cd were accompanied by a considerable rise in the concentrations of amorphous iron oxide in the soil and colloid after amendment treatment. This suggested that Cu and Cd were immobilized and stabilized in part by amorphous iron oxide.

Unraveling the aging dynamics in the simultaneous immobilization of soil metal(loid)s using oxides

Immobilization represents the most extensively utilized technique for the remediation of soils contaminated by heavy metals and metalloids. However, it is crucial to acknowledge that contaminants are not removed during this process, thereby leaving room for potential mobilization over time. Currently, our comprehension of the temporal variations in immobilization efficacy, specifically in relation to amendments suitable for industrial sites, remains very limited. To address this knowledge gap, our research delved into the aging characteristics of diverse oxides, hydroxides, and hydroxy-oxides (collectively referred to as oxides) for the simultaneous immobilization of arsenic (As), cadmium (Cd), and antimony (Sb) in soils procured from 16 contaminated industrial sites. Our findings unveiled that Ca-oxides initially showed excellent immobilization performance for As and Sb within 7 days but experienced substantial mobilization by up to 71 and 13 times within 1 year, respectively. In contrast, the efficacy of Cd immobilization by Ca-oxides was enhanced with the passage of time. Fe- and Mg-oxides, which primarily operate through encapsulation or surface complexation, exhibited steady immobilization performances over time. This reliable and commendable immobilization effect was observed across distinct soils characterized by varying physicochemical properties, including pH, texture, CEC, TOC, and EC, underscoring the suitability of such amendments for immobilizing metal(loid)s in diverse soil types. MgO, in particular, displayed even superior immobilization performance over time, owing primarily to gradual hydration and physical entrapment effects. Remarkably, Mg-Al LDHs emerged as the most effective candidate for the simultaneous immobilization of As, Cd, and Sb. The results obtained from this study furnish valuable data for future investigations on the immobilization of metals and metalloids in industrial soils. They enable the projection of immobilization performance and offer practical guidance in selecting suitable amendments for the immobilization of metal(loid)s.

Application of apatite particles for remediation of contaminated soil and groundwater: A review and perspectives

With rapid industrial development and population growth, the pollution of soil and groundwater has become a critical concern all over the world. Yet, remediation of contaminated soil and water remains a major challenge. In recent years, apatite has gained a surging interest in environmental remediation because of its high treatment efficiency, low cost, and environmental benignity. This review summarizes recent advances in: (1) natural apatite of phosphate ores and biological source; (2) synthesis of engineered apatite particles (including stabilized or surface-modified apatite nanoparticles); (3) treatment effectiveness of apatite towards various environmental pollutants in soil and groundwater, including heavy metals (e.g., Pb, Zn, Cu, Cd, and Ni), inorganic anions (e.g., As oxyanions and F-), radionuclides (e.g., thorium (Th), strontium (Sr), and uranium (U)), and organic pollutants (e.g., antibiotics, dyes, and pesticides); and (4) the removal and/or interaction mechanisms of apatite towards the different contaminants. Lastly, the knowledge or technology gaps are identified and future research needs are proposed.

Contaminant containment for sustainable remediation of persistent contaminants in soil and groundwater

Contaminant containment measures are often necessary to prevent or minimize offsite movement of contaminated materials for disposal or other purposes when they can be buried or left in place due to extensive subsurface contamination. These measures can include physical, chemical, and biological technologies such as impermeable and permeable barriers, stabilization and solidification, and phytostabilization. Contaminant containment is advantageous because it can stop contaminant plumes from migrating further and allow for pollutant reduction at sites where the source is inaccessible or cannot be removed. Moreover, unlike other options, contaminant containment measures do not require the excavation of contaminated substrates. However, contaminant containment measures require regular inspections to monitor for contaminant mobilization and migration. This review critically evaluates the sources of persistent contaminants, the different approaches to contaminant remediation, and the various physical-chemical-biological processes of contaminant containment. Additionally, the review provides case studies of contaminant containment operations under real or simulated field conditions. In summary, contaminant containment measures are essential for preventing further contamination and reducing risks to public health and the environment. While periodic monitoring is necessary, the benefits of contaminant containment make it a valuable remediation option when other methods are not feasible.

Empirical and Mechanistic Modeling of Release Kinetics of Heavy Metals and Their Chemical Distribution in the Rhizosphere and Non-rhizosphere Soils Under Vegetable Cultivation

Biochemical processes in the rhizosphere affect the availability and distribution of heavy metals (HMs) in various forms. Rhizosphere soil (RS) and non-rhizosphere soil (NRS) samples were collected from 10 fields under tarragon (Artemisia dracunculus L.) cultivation to investigate the release kinetics and distribution of HMs including cadmium (Cd), cobalt (Co), copper (Cu), iron (Fe), and zinc (Zn) in five fractions. The cumulative amounts of Cu and Fe released after 88 h were in the following ranges, respectively: 1.31-2.76 and 3.24-6.35 mg kg^-1 in RS and 1.41-2.72 and 3.15-5.27 mg kg^-1 in NRS. The parabolic diffusion and pseudo-second-order equations provided the best fit to the release kinetics data of Cu and Fe, respectively. The cation exchange model (CEM) based on Gaines-Thomas selectivity coefficients implemented in the PHREEQC program could well simulate the release of Cu and Fe suggesting that cation exchange was the dominant mechanism in the release of Fe and Cu from soils by 0.01 M CaCl₂. Cadmium was predominantly found in fraction F2, while other HMs were mainly present in fraction F5. According to the risk assessment code, there was a very high risk for Cd, a medium risk for Co and Cu, a very low risk for Fe, and a low risk for Zn. Correlation analysis showed that soil physicochemical properties were effective in the distribution and transformation of HMs. Significant positive correlations between five fractions indicated that different forms of HMs can potentially transform into each other.

Remediation of Cu and As contaminated water and soil utilizing biochar supported layered double hydroxide: Mechanisms and soil environment altering

Preparing materials for simultaneous remediation of anionic and cationic heavy metals contamination has always been the focus of research. Herein a biochar supported FeMnMg layered double hydroxide (LDH) composites (LB) for simultaneous remediation of copper and arsenic contamination in water and soil has been assembled by a facile co-precipitation approach. Both adsorption isotherm and kinetics studies of heavy metals removal by LB were applied to look into the adsorption performance of adsorbents in water. Moreover, the adsorption mechanisms of Cu and As by LB were investigated, showing that Cu in aqueous solution was removed by the isomorphic substitution, precipitation and electrostatic adsorption while As was removed by complexation. In addition, the availability of Cu and As in the soil incubation experiments was reduced by 35.54%-63.00% and 8.39%-29.04%, respectively by using LB. Meanwhile, the addition of LB increased the activities of urease and sucrase by 93.78%-374.35% and 84.35%-520.04%, respectively, of which 1% of the dosage was the best. A phenomenon was found that the richness and structure of microbial community became vigorous within 1% dosage of LB, which indirectly enhanced the passivation and stabilization of heavy metals. These results indicated that the soil environment was significantly improved by LB. This research demonstrates that LB would be an imaginably forceful material for the remediation of anionic and cationic heavy metals in contaminated water and soil.

Spatial and Temporal Variations of Heavy Metals' Bioavailability in Soils Regulated by a Combined Material of Calcium Sulfate and Ferric Oxide

Heavy metal pollution in soils threatens food safety and human health. Calcium sulfate and ferric oxide are commonly used to immobilize heavy metals in soils. However, the spatial and temporal variations of the heavy metals' bioavailability in soils regulated by a combined material of calcium sulfate and ferric oxide (CSF) remain unclear. In this work, two soil column experiments were conducted to investigate the spatial and temporal variations of CSF immobilized Cd, Pb, and As. In the horizontal soil column, the results showed that CSF's immobilization range for Cd increased over time, and adding CSF in the center of the soil column decreased the concentrations of bioavailable Cd significantly, up to 8 cm away by day 100. The CSF immobilization effect on Pb and As only existed in the center of the soil column. The CSF's immobilization depths for Cd and Pb in the vertical soil column increased over time and extended to 20 cm deep by day 100. However, the CSF's immobilization depths for As only extended to between 5 and 10 cm deep after 100 days of incubation. Overall, the results from this study can serve as a guide to determine the CSF application frequency and spacing distance for the in-situ immobilization of heavy metals in soils.

Effects of a novel Cd passivation approach on soil Cd availability, plant uptake, and microbial activity in weakly alkaline soils

Heavy metal pollution, including that caused by cadmium (Cd), is a matter of increasing concern. Although in situ passivation remediation has been widely used to treat heavy metal-polluted soils, most studies have focused on acidic soils, while studies on alkaline soil conditions are scarce. In this study, the effects of biochar (BC), phosphate rock powder (PRP), and humic acid (HA) on Cd²⁺ adsorption were examined alone and in combination to choose an appropriate Cd passivation approach for weakly alkaline soils. Additionally, the combined impact of passivation on Cd availability, plant Cd absorption, plant physiology indexes, and soil microbial community was elucidated. BC had a higher Cd adsorption capacity and removal rate than those of PRP and HA. Moreover, HA and PRP enhanced the adsorption capacity of BC. A combination of biochar and humic acid (BHA), and biochar and phosphate rock powder (BPRP) significantly affected soil Cd passivation. BHA and BPRP decreased the plant Cd content and soil Cd-DTPA (diethylenetriaminepentaacetic acid) by 31.36 %, 20.80 %, 38.19 %, and 41.26 %, respectively; however, they increased the fresh weight by 65.64-71.48 % respectively, and dry weight by 62.41-71.35 %, respectively. Notably, only BPRP increased the node and root tip number in wheat. Total protein (TP) content increased in BHA and BPRP, with BHA showing lower TP than BPRP. BHA and BPRP showed a reduction in glutathione (GSH), malondialdehyde (MDA), H₂O₂, and peroxidase (POD); BHA showed a significantly lower GSH than BPRP. Additionally, BHA and BPRP increased soil sucrase, alkaline phosphatase, and urease activities, with BPRP showing considerably higher enzyme activity than BHA. Both BHA and BPRP increased the number of soil bacteria, altered the community composition, and critical metabolic pathways. The results demonstrated that BPRP could be used as a highly effective, novel passivation technique for the remediation of Cd-contaminated soil.

Heavy metal stabilization remediation in polluted soils with stabilizing materials: a review

The remediation of soil contaminated by heavy metals has long been a concern of academics. This is due to the fact that heavy metals discharged into the environment as a result of natural and anthropogenic activities may have detrimental consequences for human health, the ecological environment, the economy, and society. Metal stabilization has received considerable attention and has shown to be a promising soil remediation option among the several techniques for the remediation of heavy metal-contaminated soils. This review discusses various stabilizing materials, including inorganic materials like clay minerals, phosphorus-containing materials, calcium silicon materials, metals, and metal oxides, as well as organic materials like manure, municipal solid waste, and biochar, for the remediation of heavy metal-contaminated soils. Through diverse remediation processes such as adsorption, complexation, precipitation, and redox reactions, these additives efficiently limit the biological effectiveness of heavy metals in soils. It should also be emphasized that the effectiveness of metal stabilization is influenced by soil pH, organic matter content, amendment type and dosage, heavy metal species and contamination level, and plant variety. Furthermore, a comprehensive overview of the methods for evaluating the effectiveness of heavy metal stabilization based on soil physicochemical properties, heavy metal morphology, and bioactivity has also been provided. At the same time, it is critical to assess the stability and timeliness of the heavy metals' long-term remedial effect. Finally, the priority should be on developing novel, efficient, environmentally friendly, and economically feasible stabilizing agents, as well as establishing a systematic assessment method and criteria for analyzing their long-term effects.

Remediation performance and mechanisms of Cu and Cd contaminated water and soil using Mn/Al-layered double oxide-loaded biochar

The combined pollution of heavy metals is ubiquitous worldwide. Mn/Al-layered double oxide-loaded crab shells biochar (LDO/BC) was prepared, so as to remediate the combined pollution of Cd and Cu in soil and water. The pristine and used LDO/BC were characterized and the results revealed that the layered double oxide was successfully loaded on crab shells biochar (BC) and metal element Ca in crab shells was beneficial to the formation of more regular layered and flake structure. The maximal adsorption capacity (Q_m) of LDO/BC for aqueous Cu²⁺ and Cd²⁺ was 66.23 and 73.47 mg/g, respectively. LDO/BC and BC were used to remediate e-waste-contaminated soil for the first time and exhibited highly efficient performance. The extraction amount of Cu and Cd in the contaminated soil by diethylene triamine penta-acetic acid (DTPA) after treating with 5% LDO/BC was significantly reduced from 819.84 to 205.95 mg/kg (with passivation rate 74.8%) and 8.46 to 4.16 mg/kg (with passivation rate 50.8%), respectively, inferring that the bioavailability of heavy metals declined remarkably. The experimental result also suggested that after remediation by LDO/BC the exchangeable and weak acid soluble Cu and Cd in soil translated to reducible, residual and oxidizable fraction which are more stable state. Precipitation, complexation and ion exchange were proposed as the possible mechanisms for Cd and Cu removal. In general, these experiment results indicate that LDO/BC can be a potentially effective reagent for remediation of heavy metal contaminated water and soil.

Application of humic acid and hydroxyapatite in Cd-contaminated alkaline maize cropland: A field trial

Soil quality is critical to the quality and safety of agricultural products, and remediation of heavy metal contaminated soils is an urgent task to be implemented. This study applied hydroxyapatite (HAP) and humic acid (HA) as remediation materials to Cd-contaminated alkaline cropland. Data on soil pH, electrical conductivity (EC), cation exchange capacity (CEC), soil organic matter (SOM), diethylenetriamine pentaacetic acid (DTPA) extraction, and improved BCR sequential extraction were obtained for different periods. The joint application of HAP and HA enhanced the soil's buffering capacity. During the experiment, treatment groups CK, H1, H2, H3, and H4 showed changes in pH of 0.29, 0.28, 0.21, 0.24, and 0.32, respectively, and changes in the conductivity of 341.4, 183.0, 133.1, 104.6 and 320.2 μS/cm. Soil organic matter had a positive effect on soil's effective phosphorus content. HAP and HA both reduced the BCF_grain/soil of Cd for the maize, but the impact of HA was more substantial (20.19 % reduction compared to CK). HA increased the yield of maize by 44.20 %. The combination of HA and HAP was recommended.

Whole-Process Risk Management of Soil Amendments for Remediation of Heavy Metals in Agricultural Soil-A Review

Reducing the mobility and bioavailability of heavy metals in soils by adding exogenous materials is a technology for remediating soils contaminated with heavy metals. Unlike industrial sites, the use of such techniques in agricultural soils requires consideration of not only reducing the mobility of heavy metals but also avoiding adverse effects on soil fertility and the growth of plants. Due to the uncertainty of the stability of amendments applied to agricultural soil, the application of amendments in farmland soil is controversial. This article reviewed the field studies in which amendments were used to immobilize heavy metals, and identified the potential environmental impacts of all aspects of soil amendment usage, including production and processing, transportation, storage, application to soil, long-term stability, and plant absorption. Results of the study indicated that after identifying the environmental risks of the whole process of the application of improvers in agricultural fields, it is necessary to classify the risks according to their characteristics, and design differentiated risk control measures for the safe application of this type of technology.

Trace Element Contents in Petrol-Contaminated Soil Following the Application of Compost and Mineral Materials

The global use of petroleum hydrocarbons as raw materials and an energy source in industry results in serious environmental, health, and ecological problems. Consequently, there is growing interest in the development of technologies for the rehabilitation of contaminated areas. This study was undertaken in order to determine the effect of different phytostabilising materials (compost, bentonite, and CaO) on the trace element content in soil contaminated with unleaded petroleum 95 (0, 2.5, 5, and 10 cm³ kg⁻¹ of soil). The doses of petroleum applied to the soil were based on the previously conducted preliminary experiment. The highest petroleum dose (10 cm³ kg⁻¹ of soil) significantly reduced the chromium, zinc, and cobalt contents in the soil. Petroleum increased the cadmium, lead, nickel, and copper contents in the soil. The materials used for phytostabilisation (compost, bentonite, calcium oxide) had a significant effect on the trace element content in the soil. The application of mineral materials (bentonite and calcium oxide) was more effective than the application of compost, compared to the control series (without soil amendments) as they reduced the contents of cadmium, chromium, nickel, and cobalt in the soil to the greatest extent. The reduction effect of bentonite and calcium oxide on the content of these trace elements in the soil was stronger than compost.

Effects of pH variations caused by redox reactions and pH buffering capacity on Cd(II) speciation in paddy soils during submerging/draining alternation

Incubation experiments were conducted to investigate the influencing factors of pH variation in different paddy soils during submerging/draining alternation and the relationship between pH buffering capacity (pHBC) and Cd speciation in ten paddy soils developed from different parent materials (including 8 acid paddy soils and 2 alkaline paddy soils). The soil pHBC and the changes in soil pH, Eh, Fe2+, Mn2+, SO42- and Cd speciation were determined. The results showed that there was a significant positive correlation between cation exchange capacity (CEC) and pHBC of these paddy soils, indicating that soil CEC is a key factor affecting the pHBC of paddy soils. The contribution of Fe(III) oxide reduction to H+ consumption is far greater than the reduction of Mn(IV)/Mn(III) oxides and SO42- during the submerging. For example, the contribution of the reduction of manganese oxides, SO42- and iron oxides to H+ consumption in the paddy soils from Anthrosol at 15 d submerging was 1.2%, 11.6% and 87.2%, respectively. This confirms that the reduction of Fe(III) oxides plays a leading role in increasing soil pH. Importantly, we noticed that during submerging, soil pH was increased and resulted in the content of available Cd in soils being reduced. This was due to the transformation of Cd to less active forms. Also, there was a significant positive correlation between the change rate of available Cd, the percentage of acid extractable Cd and pH variation. This suggests that the variation in soil pH was responsible for the transformation of Cd speciation. In addition, the change rate of available Cd and the percentage of acid extractable Cd concentration were significantly negatively correlated with soil pHBC. The soil with higher pHBC experienced less pH change, and thus the change rate of available Cd and the percentage of acid extractable Cd concentration were less for the soil. The results of this study can provide a basis for the remediation of Cd-contaminated acidic paddy soils.

Aging features of metal(loid)s in biochar-amended soil: Effects of biochar type and aging method

Soil contamination with toxic metals and metalloids has become a major threat to global food security. Among various immobilization agents that can stabilize toxic metal(loid)s effectively, biochar is promising due to its ability to restore soil health. Yet the aging characteristics of biochar following its amendment in soil remain poorly explored. Therefore, this study used standard biochars to depict their aging effects on remediation of metal(loid)-contaminated soil. A total of 2304 observations were made, including 6 biochar feedstocks (rice husk, soft wood, oilseed rape straw, miscanthus straw, sewage sludge and wheat straw), 2 pyrolysis temperatures (550 °C, 700 °C), 8 metal(loid)s (Mn, Ni, Cu, Zn, As, Cd, Sb, Pb), 4 aging methods (natural aging, freeze-thaw cycling, wet-dry cycling, chemical oxidation with H₂O₂), and 6 sampling intervals. Sewage sludge biochars exhibited the highest resistance to both artificial and natural aging, which may be related to the abundant oxygen-containing functional groups that favor metal complexation, and poorly-developed pore structures that limit the access of natural aging forces. A distinct relationship between ash and temperature was observed, where for high-ash biochars, an increase in pyrolysis temperature indicated lower resistance to aging, while for low-ash biochars, elevated pyrolysis temperature led to higher resistance. The aging behaviors of Cu and Sb were quite similar, which were both highly susceptible to chemical oxidation-induced dissolved organic carbon (DOC) release. Wet-dry cycling and freeze-thaw cycling revealed aging patterns that were similar to those of naturally aged soils as confirmed by cluster analysis. Lab aging data were then compared with existing biochar field aging results. Contrasting long-term immobilization performances were found in different studies, which were attributed to various causes associated with both biochar property and climate. The results of this study provide fresh insights into the long-term risks in the management of metal(loid)-contaminated agricultural soils.

Effect of humic and calcareous substance amendments on the availability of cadmium in paddy soil and its accumulation in rice

Humic substances (HS) are widely known as important components in soil and significantly affect the mobility of metals due to their large surface area and abundant organic functional groups. Calcareous substances (CSs) are also commonly used as robust and cost-effective amendments for increasing the pH of acidic soils and decreasing the mobility of metals in soils. In this study, we developed a new remediation scheme for cadmium (Cd)-contaminated soil remediation by coupling HS and CS. The results showed that regardless of the addition of fulvic acid (FA), all the CS-containing treatments significantly increased the soil pH by 0.32-0.60, and the concentration of bioavailable Cd decreased in the moderately (field experiment soil, maximum 62%) and highly (pot experiment soil, maximum 57%) Cd-contaminated soils. The Cd content in rice (Oryza sativa L.) tissues significantly decreased after all the treatments. The bioaccumulation factors (BAFs) decreased by over 50% in the roots, stems, leaves and husks in all treatments, while the translocation factors (TFs) only significantly decreased in the highly contaminated soil. Among all treatments, the two HS+CS treatments (FA+CaCO₃ and FA+CaO) had the greatest effect on decreasing the concentration of bioavailable Cd in soil and Cd in brown rice grains. The suggested mechanism for the effectiveness of coupled HS and CS was that CS first mitigated the pH and precipitated Cd, followed by a complexation effect between HS and Cd. Although the Cd in rice grains in both cases was higher than the standard limit, HS+CS remediation can be advocated as a robust, simple and cost-effective scheme for Cd remediation if the additive dose is slightly increased, as this approach can simultaneously improve the pH of acidic soil and adsorb Cd in soil.

Influence of biochar and microorganism co-application on stabilization of cadmium (Cd) and improved maize growth in Cd-contaminated soil

Cadmium (Cd) is one the leading environmental contaminants. The Cd toxicity and its potential stabilization strategies have been investigated in the recent years. However, the combined effects of biochar and microorganisms on the adsorption of Cd and maize plant physiology, still remained unclear. Therefore, this experiment was conducted to evaluate the combined effects of biochar (BC) pyrolyzed from (maize-straw, cow-manure, and poultry-manure, and microorganisms [Trichoderma harzianum (fungus) and Bacillus subtilis (bacteria)], on plant nutrient uptake under various Cd-stress levels (0, 10, and 30 ppm). The highest level of Cd stress (30 ppm) caused the highest reduction in maize plant biomass, intercellular CO₂, transpiration rate, water use efficiency, stomatal conductance, and photosynthesis rate as compared to control Cd₀ (0 ppm). The sole application of BC and microorganisms significantly improved plant growth, intercellular CO₂, transpiration rate, water use efficiency, stomatal conductance, and photosynthesis rate and caused a significant reduction in root and shoot Cd. However, the co-application of BC and microorganisms was more effective than the sole applications. In this regard, the highest improvement in plant growth and carbon assimilation, and highest reduction in root and shoot Cd was recorded from co-application of cow-manure and combined inoculation of Trichoderma harzianum (fungus) + Bacillus subtilis (bacteria) under Cd stress. However, due to the aging factor and biochar leaching alkalinity, the effectiveness of biochar in removing Cd may diminish over time, necessitating long-term experiments to improve understanding of biochar and microbial efficiency for specific bioremediation aims.

Aging reduces the bioavailability of copper and cadmium in soil immobilized by biochars with various concentrations of endogenous metals

Biochar is widely used for environmental remediation. However, the effects of aging on the bioavailability of trace metals in biochar-amended soil remain largely unknown, especially for the biochars with various concentrations of endogenous metals. In this study, three biochars marked as BB, MB, and HB were produced from the straws of Pennisetum sinese grown in background soil, moderately-, and highly-polluted soils by trace metals, respectively. We distinguished the effects of dry-wet (DW) and freeze-thaw (FT) aging on the bioavailability of copper (Cu) and cadmium (Cd) from soil particles, the biochar interior, and the surface of biochar. The adsorption capacities of Cu²⁺ and Cd²⁺ followed the order of BB > MB > HB. DW and FT aging both increased the adsorption capacity of Cu²⁺, but decreased that of Cd²⁺ in the three biochars, resulting in a reduction in Cu bioavailability and increase in Cd bioavailability in the biochars after the saturated adsorption of Cu²⁺ and Cd²⁺. The incorporation of the three biochars decreased Cu bioavailability compared to the control after incubation for 30d, while the addition of MB increased Cd bioavailability. DW and FT aging decreased Cu bioavailability in biochar-amended soil by decreasing the bioavailability of Cu adsorbed on the biochar surface and immobilized by soil particles. Meanwhile, aging decreased Cd bioavailability by decreasing the bioavailability of Cd immobilized by soil particles. Overall, environmental risk would be increased by the application of biochars with high endogenous Cd. The major implications are that biochar dosage and environmental risk should be carefully assessed before large-scale, continuous application, especially for biochars containing high contents of endogenous trace metals.

Biosorption of Copper in Swine Manure Using Aspergillus and Yeast: Characterization and Its Microbial Diversity Study

Dietary copper supplementation in the feed of piglets generally exceeds 250-800 mg/kg, where a higher quantity (>250 mg/kg) can promote growth and improve feed conversion. Despite the reported positive effects, 90% of copper is excreted and can accumulate and pollute the soil. Data indicate that fungi have a biosorptive capacity for copper. Thus, the objectives of the present experiment were to study the effects of adding different strains of fungi on the biosorptive capacity for copper in swine manure and to evaluate potential effects on microbiota profiles. Aspergillus niger (AN), Aspergillus oryzae (AO), and Saccharomyces cerevisiae (SC) were selected, and each added 0.4% into swine manure, which contain 250 mg/kg of copper. The incubations lasted for 29 days, and biosorption parameters were analyzed on the 8th (D8), 15th (D15), 22nd (D22), and 29th (D29) day. Results showed that after biosorption, temperature was 18.47-18.77°C; pH was 6.33-6.91; and content of aflatoxin B1, ochratoxin A, and deoxynivalenol were low. In addition, residual copper concentration with AN was the lowest on D15, D22, and D29. The copper biosorption rate was also highest with AN, averaging 84.85% on D29. Biosorption values for AO reached 81.12% and for SC were lower than 80%. Illumina sequencing of 16S and ITS rRNA gene revealed that fungal treatments reduced the diversity and richness of fungal abundance, but had no effect on bacterial abundance. Unknown_Marinilabiliaceae, Proteiniphilum, Tissierella, and Curvibacter were the dominant bacteria, while Aspergillus and Trichoderma were the dominant fungi. However, the added strain of S. cerevisiae was observed to be lower than the dominant fungi, which contained less than 0.05%. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment predicted via PICRUSt2 that there were bacterial genes potentially related to various aspects of metabolism and environmental information processing. Overall, data indicated that Aspergillus can provide microbial materials for adsorption of copper.

Bottom Ash Modification via Sintering Process for Its Use as a Potential Heavy Metal Adsorbent: Sorption Kinetics and Mechanism

Heavy metal pollution in the environment is a critical issue, engendering ecosystem deterioration and adverse effects on human health. The main objective of this study was to evaluate heavy metal adsorbents by modifying industrial byproducts. The bottom ash was sintered and evaluated for Cd and Pb sorption. Three adsorbents (bottom ash, sintered bottom ash (SBA), and SBA mixed with microorganisms (SBMA)) were tested to evaluate the sorption kinetics and mechanism using a lab-scale batch experiment. The results showed that the highest sorption efficiency was observed for Cd (98.16%) and Pb (98.41%) with 10% SBA. The pseudo-second-order kinetic model (R² > 0.99) represented the sorption kinetics better than the pseudo-first-order kinetic model for the SBA and SBMA, indicating that chemical precipitation could be the dominant sorption mechanism. This result is supported by X-ray photoelectron spectroscopy analysis, demonstrating that -OH, -CO₃, -O, and -S complexation was formed at the surface of the sintered materials as Cd(OH)₂ and CdCO₃ for Cd and PbO, and PbS for Pb. Overall, SBA could be utilized for heavy metal sorption. Further research is necessary to enhance the sorption capacity and longevity of modified industrial byproducts.

Cavitated Charcoal-An Innovative Method for Affecting the Biochemical Properties of Soil

Thermal biomass transformation products are considered to be one of the best materials for improving soil properties. The aim of the study was to assess the effect of charcoal after cavitation on the chemical and biochemical properties of soil. The study was carried out with a 10% aqueous charcoal mixture that was introduced into loamy sand and clay at rates of 1.76%, 3.5%, 7.0%, and 14.0%. The effect of the application of cavitated charcoal was tested on Sorghum saccharatum (L.). Soil and plant material was collected to determine chemical and biochemical properties. The application of cavitated charcoal reduced the acidification of both soils. The highest rate (14.0%) of cavitated charcoal increased the content of soil total carbon (C_Tot) by 197% in the loamy sand compared to C_Tot in the control treatments, 19% for clay soil, respectively. The application of cavitated charcoal did not significantly change the total content of heavy metals. Regardless of the element and the soil used, the application of cavitated charcoal reduced the content of the CaCl₂-extracted forms of heavy metals. Following the application of cavitated charcoal, the loamy sand soil presented an even lower content of the most mobile forms of the studied elements. It should also be noted that regardless of the soil texture, mobile forms of the elements decreased with the increased cavitated charcoal rate. The results of dehydrogenase and urease activity indicated the low metabolic activity of the microbial population in the soils, especially with the relatively high rates (7.0% and 14.0%) of cavitated charcoal. However, the cavitated charcoal used in the study showed a significant, positive effect on the amount of biomass S. saccharatum (L.), and its application significantly reduced the heavy metal content in the biomass of S. saccharatum (L.).

Assessment of Aqueous Extraction Methods on Extractable Organic Matter and Hydrophobic/Hydrophilic Fractions of Virgin Forest Soils

The assessment of water-extractable organic matter using an autoclave can provide useful information on physical, chemical, and biological changes within the soil. The present study used virgin forest soils from Chini Forest Reserve, Langkawi Island, and Kenyir Forest Reserve (Malaysia), extracted using different extraction methods. The dissolved organic carbon (DOC), total dissolved nitrogen (TDN), total dissolved phosphorus (TDP), and ammonium-nitrate content were higher in the autoclave treatments, up to 3.0, 1.3, 1.2, and 1.4 times more than by natural extraction (extracted for 24 h at room temperature). Overall, the highest extractable DOC, TDN, TDP, ammonium and nitrate could be seen under autoclaved conditions 121 °C 2×, up to 146.74 mg C/L, 8.97 mg N/L, 0.23 mg P/L, 5.43 mg N mg/L and 3.47 N mg/L, respectively. The soil extracts became slightly acidic with a higher temperature and longer duration. Similar trends were observed in the humic and nonhumic substances, where different types of soil extract treatments influenced the concentrations of the fractions. Different soil extraction methods can provide further details, thus widening the application of soil extracts, especially in microbes.

Assisted phytostabilization of Pb-spiked soils amended with charcoal and banana compost and vegetated with Ricinus communis L. (Castor bean)

A greenhouse experiment was performed to elucidate the potency of Prosopis juliflora charcoal (PJC) and banana waste compost (BWC) to improve soil fertility and enhance plant growth rate. Plantlets of Ricinus communis were grown in 0, 400, and 800 mg kg^-1 Pb-spiked soil ameliorated with P. juliflora charcoal and banana waste compost at 0, 5%, and 10% (w/w) for 60 days. PJC and BWC significantly (p < 0.05) increased plant growth parameters, that is, number of leaves, node number, plant height, and leaf diameter and reduced oxidative stress manifested by the lesser production of proline, hydrogen peroxide (H₂O₂), and malondialdehyde (MDA) with respect to control plants. Soil usage of PJC at 10% decreased the Pb accumulation by 61%, whereas BWC decreased Pb concentration in roots by 56% concerning control. Field emission scanning electron microscope (FE-SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) showed high macro and microspores on the surface of charcoal while banana compost showed significant raise in the nutrient content (N, P, K, Zn, Ca, Fe, and Mg). Thermogravimetric (TG) and Fourier-transform infrared spectroscopy (FTIR) analysis of banana compost showed enhanced molar convolution of carbohydrate composites and nitrogen content. These findings pave a clear understanding that PJC and BWC are recalcitrant for Pb phytotoxicity and can also be used as nutrient-rich composites for increased crop production.

In-situ immobilization of cadmium-polluted upland soil: A ten-year field study

In-situ immobilization is an effective and economically viable strategy for remediation of soil extensively polluted with heavy metals. The long-term sustainability is critical for the remediation practice. In the present study, a ten-year experiment was performed in a Cd-polluted agricultural field to evaluate the long-term stability of lime, silicon fertilizer (SF), fused calcium magnesium phosphate fertilizer (FCMP), bone charcoal, steel slag, and blast furnace slag with one-off application. All amendments had no significant effect on biomass but significantly reduced Cd uptake by Artemisia selengensis at higher dose. Among them, SF and FCMP applied at 1% could reduce Cd uptake by more than 40% to meet the Chinese maximum permissible limit for Cd content in food products (50 μg kg^-1). These amendments stimulated high Cd immobilization by increasing the soil pH and decreasing the soil acid-extractable Cd content, which were closely associated with Cd uptake. In addition, the two amendments altered the soil microbial structure and stimulated metabolism pathways, including amino acid, carbohydrate, and lipid metabolism, which are beneficial for soil function and quality. The results proved that SF and FCMP at 1% are stable and ecologically safe amendments, suitable for long-term Cd immobilization, and provide a strategy to mitigate the risk of food product contamination in heavy-metal-polluted soil.

Analysis of the long-term effectiveness of biochar immobilization remediation on heavy metal contaminated soil and the potential environmental factors weakening the remediation effect: A review

Currently, the research and application of biochar in the remediation of heavy metal contaminated soil has become a hotspot, especially regarding the remediation of agricultural land. Biochar has been proved to be effective in reducing the content of available heavy metals in the soil as well as the heavy metals in plants. However, the long-term effectiveness of biochar immobilization has not been widely studied. In this review, retrospective search was carried out on the published literature results concerning remediation effects of biochar on different areas of heavy metal contaminated soil in the recent years, its application in field remediation (several years), and some potential abiotic and biotic factors that may weaken the immobilization effects of biochar. This results indicate that: (1) biochar is widely used in the remediation of heavy metal contaminated soil in different areas and has excellent immobilization effect. (2) Most of the research demonstrate that the immobilization effect of biochar is effective for 2-3 years or according to few results even for 5 years. However, there have been various reports claiming that the immobilization effect of biochar decreases with time. (3) Abiotic factors such as acid rain, flooded environment, changes in soil condition (pH, redox and dissolved organic matter) and changes in biochar (Cl^- and alkali leaching) can significantly weaken the immobilization effect of biochar. (4) Biotic factors such as plant roots, earthworms and soil microorganisms can also significantly reduce the immobilization effect of biochar. Therefore, field experiments having longer time span with biochar need to be further carried out, and the developmental research of modified biochar with a more stable immobilization effect also needs further attention.

Evaluation of hydroxyapatite derived from flue gas desulphurization gypsum on simultaneous immobilization of lead and cadmium in contaminated soil

Flue gas desulphurization gypsum (FGD) is a major solid waste in coal-fired energy plants, and the appropriate reuse of this resources is still a major challenge. In this study, the feasibility of FGD as a calcium source to produce hydroxyapatite (FGD-HAP) for the immobilization of lead (Pb) and cadmium (Cd) in spiked soil was investigated. The effects of FGD and FGD-HAP on soil properties and redistribution, bioaccessibility and plant uptake of Pb and Cd were examined. Results showed that application of FGD and FGD-HAP could significantly improve the enzymes activities of contaminated soils, but the effectiveness was more pronounced with FGD-HAP. Addition of only 1% FGD-HAP could effectively reduce bioavailable Pb and Cd concentration in soil as measured by CaCl₂ extraction by 60.6% and 65.4%, respectively. On the other hand, plant available Pb and Cd could significantly decrease by 93.8% and 73.2% after amendment of 5% FGD-HAP. Significant changes in the micro-scale distribution of heavy metals before and after FGD-HAP treatment demonstrated that while heavy metals were predominantly associated with iron/manganese oxides in untreated soil, high correlation between heavy metals and phosphorus/sulfur was observed in FGD-HAP treated soil. In addition, results of the leaching tests showed that incorporation of FGD-HAP enhanced the retention capacity of heavy metals in soil, indicating that application of FGD-HAP could diminish the environmental risk of leachable heavy metals to groundwater. Overall, this study highlighted the potential value of FGD-HAP as a low-cost and high-efficient amendment for remediation of Pb and Cd contaminated soils.

Cadmium mobility in three contaminated soils amended with different additives as evaluated by dynamic flow-through experiments

As arable land has become an important sink for cadmium (Cd), soil is being recognized as a major source of metals to the food chain. It becomes, therefore, essential to investigate metal mobility in contaminated soils and to identify suitable remediation strategies. For this, immobilization of Cd was evaluated in contaminated stagnic anthrosol: S1, gleysol: S2 and fluvisol: S3 under flow through conditions. Ten treatments including control were tested alone or in composite form firstly at natural Cd contents (0.58-0.69 mg kg^-1). Here, T2 (lime), T5 (biochar) and T10 (composite amendment) were found better in reducing the Cd concentration in the soils' leachates, so, their efficacy was further investigated in the same soils of higher Cd contents (1 and 2 mg kg^-1 imposed by soil spiking). Amendments significantly reduced the leachate metal contents especially in 1 mg kg^-1 spiked soils. Characterization of T2, T5 and T10 revealed their structural transformations in all the studied soil types, while active functional groups e.g. C-O, CO, O-H, Si-O-Si, ester and alcoholic groups were notably involved in Cd precipitation or adsorption on amendments surface. Variations in Cd speciation in these soils exhibited the exchange of Cd to more stable fractions with tested amendments. These continuous-flow experiments confirmed the strong efficiency of T2, T5 and T10 in reducing the Cd concentration in the leachate of three soils. This study has strong implications in understanding the role of different amendments in controlling the fate, leaching behavior and immobilization of Cd in diverse soil types.

Remediation of Soil Polluted with Cd in a Postmining Area Using Thiourea-Modified Biochar

Cadmium presence in soil is considered a significant threat to human health. Biochar is recognized as an effective method to immobilize Cd ions in different soils. However, obtaining effective and viable biochar to remove elevated Cd from postmining soil remains a challenge. More modifiers need to be explored to improve biochar remediation capacity. In this investigation, pot experiments were conducted to study the effects of poplar-bark biochar (PBC600) and thiourea-modified poplar-bark biochar (TPBC600) on Cd speciation and availability, as well as on soil properties. Our results showed that the addition of biochar had a significant influence on soil properties. In the presence of TPBC600, the acid-soluble and reducible Cd fractions were transformed into oxidizable and residual Cd fractions. This process effectively reduced Cd bioavailability in the soil system. Compared to PBC600, TPBC600 was more effective in improving soil pH, electrical conductivity (EC), organic matter (SOM), total nitrogen (TN), ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3−-N), available potassium (AK), available phosphorus (AP), and available sulfur (AS). However, this improvement diminished as incubation time increased. Results of Pearson correlation analysis, multivariate linear regression analysis, and principal component analysis showed that soil pH and available phosphorus played key roles in reducing the available cadmium in soil. Therefore, TPBC600 was shown to be an effective modifier that could be used in the remediation of soil polluted with Cd.

Sulfur-modified biochar as a soil amendment to stabilize mercury pollution: An accelerated simulation of long-term aging effects

The stability of mercury (Hg) contamination in soil environments can change over time. This has implications for agricultural sites under long-term management after in situ treatment involving soil amendments. In this study, rice husk biochar (RHB) and sulfur modified rice husk biochar (SRHB) were synthesized and applied (dosage = 5% dry wt.) to a Hg polluted agricultural soil collected from Guizhou province, Southern China (soil total Hg content = 28.3 mg/kg; C = 2%; and, S = 0.1%). The long-term stabilization effectiveness of the soil treatments was evaluated by a combined approach involving: (i) accelerated aging for 104 simulated years; (ii) soil extraction as a proxy for plant uptake; and, (iii) sequential extraction to identify Hg fractions. The SRHB amendment raised the soil's total S content by approximately an order of magnitude (to 0.9%), which remained at a generally constant level throughout the simulation. The initial pH levels for the untreated and treated soils were alkaline and remained between 7.0 and 7.5 for the first 50 years of simulated aging, before decreasing as the simulation time increased further. The pH of the SRHB treated soils did not drop below that of untreated soils during the simulation. Soil extraction tests with 0.1 M HCl solution indicated that RHB and SRHB treatments could effectively immobilize the Hg in soil for at least 50 and 75 simulated years, respectively. At simulated year 50, the amount of Hg extracted from RHB and SRHB treated soils was <200 ng/L and <100 ng/L, respectively. Thus, showing SRHB to be a particularly promising remedial option. The soil Hg was mostly associated with the stable sequential extraction fractions (F3-5). By the end of the simulation, the F5 fraction for SRHB and RHB treated soils reduced by 44.6%, and 42.0%, respectively, whereas the F4 fraction increased by >400% in both cases. In summary, SRHB may provide long-lasting Hg stabilization at contaminated sites. Therefore, further research toward the development of this stabilization technology is warranted.

Simultaneous reduction of arsenic and cadmium bioavailability in agriculture soil and their accumulation in Brassica chinensis L. by using minerals

In situ immobilization of heavy metal cations in contaminated soil using natural minerals is an attractive remediation technique. However, little research has focused on the remediation of arsenic (As) and cadmium (Cd) co-contaminated. In this work, three different crystal structures and chemical compositions minerals, zeolite; bentonite; and dolomite, were applied to simultaneously reduce the uptake of As and Cd in Brassica chinensis L., and the mechanism on reducing As and Cd bioavailability in soil were also investigated. The results showed that the three minerals decreased the bioavailability of As and Cd and restrained their uptake by Brassica chinensis L. with the order followed bentonite > zeolite > dolomite. Particularly, bentonite decreased the exchangeable As and Cd by 4.05% and 32.5% and the concentrations of As and Cd in shoots of Brassica chinensis L. by 36.2% and 64.6%, as compared with the controls. Moreover, with the addition of minerals increased, the dry biomass of Brassica chinensis L. and the rhizosphere microbial functional diversity increased significantly, and the highest biomass increased by 289% at 4.0% addition of bentonite. Correlation analysis indicated that the uptake of As and Cd was positive with the available Cd and As in soil, and was negative with soil pH and available N. Furthermore, the Scanning Electron Microscopy-Energy Dispersive Spectroscopy and Fourier Transform Infrared Spectroscopy analysis illustrated the interaction between minerals and Cd mainly involved ion-exchange and adsorption, while As was mainly immobilized by calcium and magnesium through forming precipitation. In conclusion, this present study implied that the bentonite can be recommended as the more effective amendment to immobilize metal (loid)s in soil and thereby reduce the exposure risk of metal (loid)s associated with grains consumption.

Natural fluorapatite dissolution kinetics and Mn2+ and Cr3+ metal removal from sulfate fluids at 35 °C

In light of the consequences of global warming and population growth, access to safe drinking water becomes an ever greater challenge, in particular in low to middle income countries in arid regions. Moreover, mining which may cause acid mine drainage and heavy metal contamination puts further pressure on management of limited water resources. Hence, the development of cost effective water treatment methods is critical. Here, using batch reactor experiments we investigate the kinetics and mechanisms behind divalent Mn and trivalent Cr removal from sulfate fluids using natural fluorapatite at 35 °C. The results show that the fluorapatite dissolution rate depends on fluid pH, and that dissolution is the dominant mechanism in fluids with pH below 4. Apatite can thus serve as remediation to neutralize acidic fluids. Fluid pH of 4-6 triggers a dissolution-precipitation mechanism, in some cases following upon a dissolution-only period, with the formation of a metal phosphate. In these experiments, Cr removal is two to ten times faster than Mn removal given similar solution pH. The results demonstrate that natural apatite represents a promising, cost effective material for use in passive remediation of mining-induced contamination of soils and groundwater in arid regions.

Stabilization process and potential of agro-industrial waste on Pb-Contaminated soil around Pb-Zn mining

Sawdust wastes were used as precursors to prepare adsorbents by combustion and pyrolysis for experimental and mechanism studies and determine the potential of biomass extracted from agro-industrial residues for Pb-polluted soil remediation. Pot experiments were conducted on contaminated soils near Pb-Zn mining with sawdust ash (SA) and sawdust biochar (SB) in different proportions and dosage ratios. Studies have indicated that the application of biomass materials can enhance the adsorption, complexation and precipitation of Pb cations in soil and reduce the mobility of Pb. The concentrations of SPLP-Pb and DTPA-extractable Pb in amended soils were the lowest under 1% 1:2 and 5% 1:1 treatment, respectively. Results of fraction extraction and XANES analysis showed that the materials change the main forms of Pb in soil. Moreover, the binding behavior of Pb with organic matter increases the proportion of Pb (Ac)₂, leading to the transformation of high toxicity Pb-compounds into precipitates and complexes. The remediation methods of 2% 1:2 and 5% 1:2 were better than those of other methods in stabilizing Pb in soil. This study indicated that heat-treated sawdust can be used for Pb-polluted soil remediation, which is a type of environmental remediation measure with considerable ecological potential.

Remediation of cadmium and lead polluted soil using thiol-modified biochar

Thiol-modified rice straw biochar (RS) was prepared by an esterification reaction with β-mercaptoethanol and used for the remediation of Cd and Pb polluted soils. Modified biochar was characterized through elemental analysis, BET analysis, FE-SEM, FT-IR and XPS. These analytical results revealed that thiol groups were successfully grafted onto the surface of the biochar and were involved in metal ion complexation. Batch sorption experiments indicated that Cd²⁺ and Pb²⁺ sorption onto RS described well by a pseudo second order kinetic model and a Langmuir isotherm. The maximum adsorption capacities for Cd²⁺ and Pb²⁺, in the single-metal systems, were 45.1 and 61.4 mg g^-1, respectively. In the binary-metal systems, RS selectively adsorbed Cd²⁺ over Pb²⁺. Cd²⁺ and Pb²⁺ were removed mainly through surface complexation. In the soil incubation experiments (28 days), RS reduced the available Cd by 34.8-39.2 %; while, RS reduced the available Pb by 8.6 %-11.1 %. This research demonstrates RS as a potentially effective amendment for the remediation of heavy metal polluted soils.

Health risks of heavy metal exposure and microbial contamination through consumption of vegetables irrigated with treated wastewater at Dubai, UAE

The shortage of fresh water is a major problem throughout the world, but the situation is worst in the arid and semiarid regions. Therefore, reuse of nonconventional water resources such as treated wastewater (TWW) is a common practice to irrigate field crops, vegetables, and forestry sectors. The present study was conducted to evaluate the significant impact of different heavy metals such as copper (Cu), iron (Fe), chromium (Cr), and zinc (Zn) on the soil and leafy, root, and fruit vegetables following irrigation with TWW through subsurface drip irrigation. Our results indicate that iron (Fe) was highest in lettuce followed by spinach, and Zn and Cr were second and third most abundant element in the different vegetables. Eggplant and radish showed the lowest concentrations of various heavy metals. A significant difference was observed in transfer factor (TF) among vegetables, and highest TF_soil-veg was observed for Fe in lettuce and the lowest for Cr in eggplant. Estimated daily intake (EDI) was the lowest in adults and highest in children. Target hazard quotient (THQ) of Cu, Zn, and Fe being < 1.0 appears relatively safe in all the tested vegetables. Risk index (RI) values showed that heavy metals were lower than 1.0 and hence lower risk for human. The combined HI values for Cu, Zn, Cd, Cr, and Pb were substaintionaly higher 12.8 and 9.21 after consumption of lettuce and carrot. So, consumption of these vegetables should be avoided after irrigation with TWW. Spinach exhibited maximum total coliform loading, while ecological risk was negligible due to sandy nature of soil type. Health risks to human could be reduced through proper selection of suitable vegetables, time of maturity, and consumed organs (leaf, fruit, or root part). Appropriate should be followed to decontaminate the microbial load in order to avoid any risks to human health (both adults and children).

Biochar applied to soil under wastewater irrigation remained environmentally viable for the second season of potato cultivation

The environmental effectiveness of plantain peel biochar in the second season of its application to soil was studied using outdoor lysimeters (0.45 m diameter x 1.0 m height) packed with sandy soil, cultivated with potatoes (Solanum tuberosum) and irrigated with wastewater. Biochar (1% w/w) was amended in the soil one-time in the first season. For two seasons, the biochar improved the soil properties, immobilized the heavy metals in the soil, and reduced their uptake by the crop. The CEC of the biochar-amended soil (WW + B) for example, as compared to the unamended treatment (WW-B), was significantly higher (p<0.05; >65%) for both seasons due to higher pH which controls the availability of cations in soils, influencing their CECs. The soil sampled in the second season showed accumulation of all the heavy metals in the topsoil, while only Zn, Pb and Fe moved to the 0.1 m depth. The Fourier transform infra-red spectra of the soil and soil-biochar mix were similar and suggested that oxygen-containing functional groups were partly responsible for binding the heavy metals. The heavy metals translocated to all the potato parts (flesh, peel, root, stem and leaves). The concentrations of the heavy metals in potato parts under freshwater were lower than those under wastewater irrigated condition. After the second season of being in the soil, biochar significantly reduced (p < 0.05) the concentrations of Cd, Cu, Cr, Pb and Zn in the edible flesh suggesting that biochar immobilized wastewater-laden heavy metals in soil and reduced their uptake in potatoes for at least two seasons.

Soil amendments for immobilization of potentially toxic elements in contaminated soils: A critical review

Soil contamination by potentially toxic elements (PTEs) has led to adverse environmental impacts. In this review, we discussed remediation of PTEs contaminated soils through immobilization techniques using different soil amendments with respect to type of element, soil, and amendment, immobilization efficiency, underlying mechanisms, and field applicability. Soil amendments such as manure, compost, biochar, clay minerals, phosphate compounds, coal fly ash, and liming materials are widely used as immobilizing agents for PTEs. Among these soil amendments, biochar has attracted increased interest over the past few years because of its promising surface properties. Integrated application of appropriate amendments is also recommended to maximize their use efficiency. These amendments can reduce PTE bioavailability in soils through diverse mechanisms such as precipitation, complexation, redox reactions, ion exchange, and electrostatic interaction. However, soil properties such as soil pH, and clay, sesquioxides and organic matter content, and processes, such as sorption/desorption and redox processes, are the key factors governing the amendments' efficacy for PTEs immobilization in soils. Selecting proper immobilizing agents can yield cost-effective remediation techniques and fulfill green and sustainable remediation principles. Furthermore, long-term stability of immobilized PTE compounds and the environmental impacts and cost effectiveness of the amendments should be considered before application.

Ex situ evaluation of the effects of biochars on environmental and toxicological availabilities of metals and polycyclic aromatic hydrocarbons

The present study experimented five biochars, one made from wood (400 °C, 12 h) and four made from miscanthus cultivated on contaminated soils (temperature 400/600 °C, duration 45/90 min). They were used as amendments at a 2% application rate on soil, cultivated or not cultivated with ryegrass, contaminated with (i) metals (Cd, Pb, and Zn), (ii) eight polycyclic aromatic hydrocarbons (PAHs), and (iii) a mix of metals and PAHs. The objectives were (i) to compare the effectiveness of the five biochars on soil parameters and pollutant availability and (ii) to determine the influence of soil multicontamination and ryegrass cultivation on biochar effectiveness. The results showed that biochar application did not necessarily lead to lower pollutant extractability and metal bioaccessibility. However, differences were highlighted between the biochars. The miscanthus biochars produced at 600 °C (BM600) showed higher effectiveness at decreasing metal extractability than the miscanthus biochars produced at 400 °C (BM400) due to its better sorption characteristics. In addition, ryegrass cultivation did not impact pollutant availability but modified metal bioaccessibility, especially for the soil amended with the BM600 and the woody biochar. Moreover, the presence of PAHs also negatively impacted the metal bioaccessibility in the soil amended with the BM600, and, on the contrary, positively impacted it in the soil amended with the BM400. Complementary studies are therefore necessary to understand the mechanisms involved, particularly in a context where soils requiring remediation operations are often multicontaminated and vegetated.

Effects of Soil Properties and Land Use Types on the Bioaccessibility of Cd, Pb, Cr, and Cu in Dongguan City, China

In order to determine the potential heavy metal contamination in soil across Dongguan City, 124 soil samples from seven land use types were collected, four heavy metals (Cd, Pb, Cr, and Cu) were analyzed. Total Cd, Cr, and Cu contents were significantly higher than the background values for Guangdong Province. Lead bioaccessibility in urban green land was lower than that in industrial and abandoned districts. The bioaccessibility of heavy metals was affected by total metal concentrations, soil properties, and land use types. The results showed that there was a negative correlation between the bioaccessibility of heavy metals (except for Cu) and their total concentrations. Soil pH and organic matter were the main factors affecting the bioaccessibility of Cd, Cr, Pb, and Cu in most land use types. Furthermore, sand, P, and clay also affected Pb, Cr, and Cu bioaccessibility. With the exception of the industrial zone periphery and urban green land, the bioaccessibility of heavy metals was mainly affected by clay.

Low-cost alkaline substances for the chemical stabilization of cadmium-contaminated soils

Owing to poor waste management in zinc mining, toxic heavy metals, particularly cadmium, are released and contaminate the surrounding agricultural areas. Waterlogging, which is a common practice in rice vegetation, creates anaerobic conditions that result in the conversion of organic matter into acetic acid and the reducing phase. This accelerates the release of cadmium into the water, where it is absorbed into the cells of rice. Chemical stabilization methods can be used to treat cadmium-contaminated soil by reacting an alkaline substance with acetic acid and increasing the soil pH for cadmium immobilization. However, to date, few studies using limestone dust and corncob fly ash have been conducted, and no studies have focused on the neutralization of the produced acetic acid in the anaerobic zone of the soil. This study aims to determine the optimum conditions for cadmium stabilization using different types of low-cost alkaline substances (lime, limestone dust, and corncob fly ash). The effects of alkaline amount, soil moisture content, and reaction time on soil stabilization were investigated. Lime was the most suitable for stabilization among the tested alkaline substances, and increasing the amount of lime can effectively reduce the amount of exchangeable cadmium. At 25% w/w of lime/soil, the exchangeable cadmium can be reduced from 29.3 to 7.8 mg kg^-1. The stabilization efficiencies of limestone dust and corncob fly ash were much lower. The statistical analysis shows that the amount of alkaline substance is the main factor affecting the stabilization performance at a 95% confidence limit for all tested alkalines.