Efficiency of lime, biochar, Fe containing biochar and composite amendments for Cd and Pb immobilization in a co-contaminated alluvial soil

Enhancing the phytoextraction efficiency of heavy metals in acidic and alkaline soils by Sedum alfredii Hance: A study on the synergistic effect of plant growth regulator and plant growth-promoting bacteria

Plant growth regulators (PGR) and plant growth-promoting bacteria (PGPB) have the potential in phytoremediation of heavy metals (HMs) contaminated soils. However, their sole application may not yield the optimal results, thus necessitating the combined application. The present study aimed to enhance the phytoremediation efficiency of Sedum alfredii Hance (S. alfredii) in acidic and alkaline soils through the combination of PGR (Brassinolide, BR) and PGPB (Pseudomonas fluorescens, P. fluorescens). The combination of BR and P. fluorescens (BRB treatment) effectively increased the removal efficiency of S. alfredii for Cd, Pb, and Zn by 355.2 and 155.3 %, 470.1 and 128.9 %, and 408.4 and 209.6 %, in acidic and alkaline soils, respectively. Moreover, BRB treatment led to a substantial increase in photosynthetic pigments contents and antioxidant enzymes activities, resulting in a remarkable increase in biomass (86.71 and 47.22 %) and dry mass (101.49 and 42.29 %) of plants grown in acidic and alkaline soils, respectively. Similarly, BRB treatment significantly elevated the Cd (109.4 and 71.36 %), Pb (174.9 and 48.03 %), and Zn levels (142.8 and 104.3 %) in S. alfredii shoots, along with cumulative accumulation of Cd (122.7 and 79.47 %), Pb (183.8 and 60.49 %), and Zn (150.7 and 117.9 %), respectively. In addition, the BRB treatment lowered the soil pH and DTPA-HMs contents, while augmenting soil enzymatic activities, thereby contributing soil microecology and facilitating the HMs absorption and translocation by S. alfredii to over-ground tissues. Furthermore, the evaluation of microbial community structure in phyllosphere and rhizosphere after remediation revealed the shift in microbial abundance. The combined treatment altered the principal effects on S. alfredii HMs accumulation from bacterial diversity to the soil HMs availability. In summary, our findings demonstrated that synergistic application of BR and P. fluorescens represents a viable approach to strengthen the phytoextraction efficacy of S. alfredii in varying soils.

Mechanism of clay mineral modified biochar simultaneously immobilizes heavy metals and reduces soil carbon emissions

Heavy metal pollution in farmland soil has become increasingly severe, and multi-element composite pollution has brought enormous harm to human production and life. Environmental changes in cold regions (such as freeze-thaw cycles and dry-wet alternations) may increase the potential physiological toxicity of heavy metals and exacerbate pollution risks. In order to reveal the effectiveness of sepiolite modified biochar in the remediation of the soil contaminated with lead (Pb), cadmium (Cd), and chromium (Cr), the rice husk biochar pyrolyzed at 500 and 800 °C were selected for remediation treatment (denoted as BC500 and BC800). Meanwhile, different proportions of sepiolite were used for modification (biochar: sepiolite = 1: 0.5 and 1: 1), denoted as MBC500/MBC800 and HBC500/HBC800, respectively. The results showed that modified biochar with sepiolite can effectively improve the immobilization of heavy metals. Under natural conservation condition, the amount of diethylenetriaminepentaacetic acid (DTPA) extractable Pb in BC500, MBC500, and HBC500 decreased by 5.95, 12.39, and 13.55%, respectively, compared to CK. Freeze-thaw cycles and dry-wet alternations activated soil heavy metals, while modified biochar increased adsorption sites and oxygen-containing functional groups under aging conditions, inhibiting the fractions transformation of heavy metals. Furthermore, freeze-thaw cycles promoted the decomposition and mineralization of soil organic carbon (SOC), while sepiolite hindered the release of active carbon through ion exchange and adsorption complexation. Among them, and the soil dissolved organic carbon (DOC) content in HBC800 decreased by 49.39% compared to BC800. Additionally, the high-temperature pyrolyzed biochar (BC800) enhanced the porosity richness and alkalinity of material, which effectively inhibited the migration and transformation of heavy metals compared to BC500, and reduced the decomposition of soil DOC.

A novel Biochar-PGPB strategy for simultaneous soil remediation and safe vegetable production

There is currently increasing pressure on agriculture to simultaneously remediate soil and ensure safe agricultural production. In this study, we investigate the potential of a novel combination of biochar and plant growth-promoting bacteria (PGPB) as a promising approach. Two types of biochar, corn stover and rice husk-derived, were used in combination with a PGPB strain, Bacillus sp. PGP5, to remediate Cd and Pb co-contaminated soil and enhance lettuce performance. The contaminated soil was pre-incubated with biochar prior to PGP5 inoculation. The combined application of biochar and PGPB reduced the diethylenetriaminepentaacetic acid (DTPA) -extractable Cd and Pb concentrations in the soil by 46.45%-55.96% and 42.08%-44.83%, respectively. Additionally, this combined application increased lettuce yield by 23.37%-65.39% and decreased Cd and Pb concentrations in the edible parts of the lettuce by 57.39%-68.04% and 13.57%-32.50%. The combined application showed a better promotion on lettuce growth by facilitating chlorophyll synthesis and reducing oxidative stress. These demonstrated a synergistic effect between biochar and PGPB. Furthermore, our study elucidated the specific role of the biochar-PGPB combination in soil microbial communities. Biochar application promoted the survival of PGP5 in the soil. The impact of biochar or PGPB on microbial communities was found to be most significant in the early stage, while the development of plants had a greater influence on rhizosphere microbial communities in later stage. Plants showed a tendency to recruit plant-associated microbes, such as Cyanobacteria, to facilitate growth processes. Notably, the combined application of biochar and PGPB expedited the assembly of microbial communities, enabling them more closely with the rhizosphere microbial communities in late stage of plant development and thus enhancing their effects on promoting plant growth. This study highlights the "accelerating" advantage of the biochar-PGPB combination in the assembly of rhizosphere microbiomes and offers a new strategy for simultaneous soil remediation and safe agricultural production.

Human health risks of heavy metal(loid)s mediated through crop ingestion in a coal mining area in Eastern China

The heavy metal(loid)s (HMs) in soils can be accumulated by crops grown, which is accompanied by crop ingestion into the human body and then causes harm to human health. Hence, the health risks posed by HMs in three crops for different populations were assessed using Health risk assessment (HRA) model coupled with Monte Carlo simulation. Results revealed that Zn had the highest concentration among three crops; while Ni was the main polluting element in maize and soybean, and As in rice. Non-carcinogenic risk for all populations through rice ingestion was at an "unacceptable" level, and teenagers suffered higher risk than adults and children. All populations through ingestion of three crops might suffer Carcinogenic risk, with the similar order of Total carcinogenic risk (TCR): TCRAdults > TCRTeenagers > TCRChildren. As and Ni were identified as priority control HMs in this study area due to their high contribution rates to health risks. According to the HRA results, the human health risk was associated with crop varieties, HM species, and age groups. Our findings suggest that only limiting the Maximum allowable intake rate is not sufficient to prevent health risks caused by crop HMs, thus more risk precautions are needed.

Effect of biochars on the immobilization and form of Cadmium (Cd) in simulated Cd deposition of iron rich soils

Atmospheric deposition of Cd poses a serious threat to ecosystem security. Biochar is widely used for polluted soil remediation, however, whether biochar already applied to the soil can reduce the hazards of newly deposited Cd remains to be studied. Thus, an indoor cultural experiment and static adsorption method were conducted to study the isothermal and kinetic adsorption processes of three types of biochar (rice husk, rubber wood, and tobacco stem biochars) on Cd in iron rich soils and the effect of biochar on the morphological distribution of Cd in the soil and the soil pH. The results showed that the soil with biochar in our study could quickly fix "the new deposited Cd" in the soil in 3 h with the maximum adsorption capacity in rubber wood biochar-treated sample (3227.34 mg/kg）. The addition of all three biochar treatments significantly increased the soil pH and reduced the soil exchange state Cd content, with a 13.69-17.32% increase in the pH and a 13.22-54.39% reduction in the exchange state Cd content when contrasted with the control, which could promote those Cd converting into unavailable Cd (carbonate-bound form Cd, Fe-Mn oxide-bound form Cd, or residual form Cd) for crops. In summary, the addition of three kinds of biochar treatments could effectively reduce the ecological and environmental risk of soil that was contaminated by Cd and could provide a reliable theoretical basis for the effect of biochar on the improvement of the quality of soil that is contaminated by heavy metals.

Biotoxicity attenuation and the underlying physicochemical mechanism of biochar aged under simulated natural environmental conditions

Biochar (BC), with the benefits of enhancing soil fertility, absorbing heavy metals, carbon sequestration, and mitigating the greenhouse effect, has been extensively used for soil remediation. However, the long-term changes in the biotoxicity of BC under complex environmental conditions, which are the key factors influencing the sustainable application of BC in soil, are still unclear. Herein, the biotoxicity of BC aged with various processes, including dry‒wet cycle (DW) aging, freeze‒thaw cycle (FT) aging, ultraviolet irradiation (UV) aging, and low molecular weight organic acid (OA) aging, was systematically investigated by Escherichia coli (E. coli) culture experiments. The toxicity attenuation rate (%·week-1) was proposed to more concisely and clearly compare the influence of different aging methods on BC toxicity. The results indicated that after 5 weeks of aging, the toxicity attenuation rate during the four aging modes followed the order OA aging > FT aging > UV aging > DW aging. BC was nontoxic after 1 week of OA aging, 4 weeks of FT aging, 7 weeks of UV aging, and 14 weeks of DW aging. Spectroscopic characterizations revealed that humic acids in the dissolved organic matter of BC were the main reason for the biotoxicity. In addition, the attenuation of environmentally persistent free radicals on BC during aging was also an important factor for reducing environmental toxicity. This work provides insight into the detoxification mechanism of the BC aging process under ordinary environmental conditions and guidance for the safe application of BC in soil.

Mitigation of the mobilization and accumulation of toxic metal(loid)s in ryegrass using sodium sulfide

Remediation of soils contaminated with toxic metal(loid)s (TMs) and mitigation of the associated ecological and human health risks are of great concern. Sodium sulfide (Na2S) can be used as an amendment for the immobilization of TMs in contaminated soils; however, the effects of Na2S on the leachability, bioavailability, and uptake of TMs in highly-contaminated soils under field conditions have not been investigated yet. This is the first field-scale research study investigating the effect of Na2S application on soils with Hg, Pb and Cu contents 70-to-7000-fold higher than background values and also polluted with As, Cd, Ni, and Zn. An ex situ remediation project including soil replacement, immobilization with Na2S, and safe landfilling was conducted at Daiziying and Anle (China) with soils contaminated with As, Cd, Cu, Hg, Ni, Pb and Zn. Notably, Na2S application significantly lowered the sulfuric-nitric acid leachable TMs below the limits defined by Chinese regulations. There was also a significant reduction in the DTPA-extractable TMs in the two studied sites up to 85.9 % for Hg, 71.4 % for Cu, 71.9 % for Pb, 48.1 % for Cd, 37.1 % for Zn, 34.3 % for Ni, and 15.7 % for As compared to the untreated controls. Moreover, Na2S treatment decreased the shoot TM contents in the last harvest to levels lower than the TM regulation limits concerning fodder crops, and decreased the TM root-to-shoot translocation, compared to the untreated control sites. We conclude that Na2S has great potential to remediate soils heavily tainted with TMs and mitigate the associated ecological and human health risks.

The influence of cysteine in transformation of Cd fractionation and microbial community structure and functional profile in contaminated paddy soil

Remediating cadmium (Cd) contaminated paddy soil is vital for agroecology, food safety, and human health. Soil washing is more feasible to reduce remediation method due to its high efficiency. However, green, low-cost and more efficient washing agents are still required. In this study, we investigated the ability of cysteine as a washing agent for soil washing to remove Cd from contaminated paddy soil. Through a batch experiment, we evaluated the removal efficiency of cysteine as a washing agent by comparing their removal rate with that of a microbial inoculant and sulphuric acid as other washing agents. The transformation of Cd fractionation and microbial community structure and functional profile in paddy soils after cysteine leaching was studied by using sequential extraction and high-throughput sequencing. Results showed that cysteine had better efficiency in the removal of Cd from paddy soil in comparison to sulphuric acid and the microbial inoculant, and could achieve a maximum removal rate of 97 % Cd in paddy soil. Cysteine decreased the proportion of Cd in the exchangeable fraction, carbonate bound fraction, iron and manganese bound fraction, and organic matter bound fraction and was best for the removal of the residual fraction, which contributed to its higher Cd removal ability. Considering the economic benefits of the reagents used, cysteine was shown to be economically feasible for use as a leaching agent. In addition, cysteine could significantly increase the relative abundance of Thermochromatium, Sideroxydans, Streptacidiphilus, and Frankia which promoted the nitrogen and sulfur metabolism in the paddy soil. In summary, this study revealed that cysteine was readily available, cheap, non-toxic, highly efficient, and even has fertilizing properties, making it eco-friendly and ideal for remediation of Cd-contaminated paddy soils. Besides, the health of paddy soils would also benefit from cysteine's promotion of microbial nitrogen and sulfur metabolism.

Coal slime as a good modifier for the restoration of copper tailings with improved soil properties and microbial function

In recent years, the solid wastes from the coal industry have been widely used as soil amendments. Nevertheless, the impact of utilizing coal slime for copper tailing restoration in terms of plant growth, physicochemical characteristics of the tailing soil, and microbial succession remains uncertain.Herein, the coal slime was employed as a modifier into copper tailings. Their effect on the growth and physiological response of Ryegrass, and the soil physicochemical properties as well as the bacterial community structure were investigated. The results indicated that after a 30-day of restoration, the addition of coal slime at a ratio of 40% enhanced plant growth, with a 21.69% rise in chlorophyll content, and a 62.44% increase in peroxidase activity. The addition of 40% coal slime also increased the content of nutrient elements in copper tailings. Following a 20-day period of restoration, the concentrations of available copper and available zinc in the modified tailings decreased by 39.6% and 48.51%, respectively, with 40% of coal slime added. In the meantime, there was an observed augmentation in the species diversity of the bacterial community in the modified tailings. The alterations in both community structure and function were primarily influenced by variations in pH value, available nitrogen, phosphorus, potassium, and available copper. The addition of 40% coal slime makes the physicochemical properties and microbial community evolution of copper tailings reach a balance point. The utilization of coal slime has the potential to enhance the physicochemical characteristics of tailings and promote the proliferation of microbial communities, hence facilitating the soil evolution of two distinct solid waste materials. Consequently, the application of coal slime in the restoration of heavy metal tailings is a viable approach, offering both cost-effectiveness and efficacy as an enhancer.

Evaluation of the effectiveness of amendments derived from vermicompost combined with modified shell powder on Cd immobilization in Cd-contaminated soil by multiscale experiments

The widespread heavy metal contamination of agricultural soils poses an enormous challenge to food safety. To evaluate the Cd immobilization potential of vermicompost combined with modified shell powder (VMSP) on Cd-contaminated soil, batch adsorption tests and field experiments were conducted. First, the Cd²⁺ removal characteristics and adsorption mechanisms of vermicompost (V), vermicompost combined with shell powder (VSP), and VMSP in an aqueous solution were investigated by batch tests. Then, 3 kg·m² V, VSP, and VMSP doses were applied to Cd-contaminated farmland soils as soil amendments to plant green garlic (Allium sativum L.) and investigate their Cd immobilization effects in Cd-contaminated soils. Batch adsorption tests showed that VMSP was most effective for Cd²⁺ removal, with adsorption rates as high as 85.7-99.79% and desorption rates of approximately 1.25-1.34%. Combining further characterization analysis of VMSP, it was demonstrated that the adsorption mechanism of Cd²⁺ was monolayer chemisorption, mainly involving the complexation reaction of Cd²⁺ with organic functional groups and the precipitation reaction of Cd²⁺ with mineral elements. The field experiment showed that adding V, VSP, and VMSP effectively inhibited the enrichment of Cd in green garlic, and the Cd content was reduced by 42.18%, 46.88%, and 68.75%, respectively. However, only the Cd content of green garlic treated with VMSP was lower than the national standard for food safety in China (Cd≤ 0.2 mg·kg^-1). V, VSP, and VMSP additions improved soil fertility and reduced Cd bioavailability in the soil by 15.5%, 18.9%, and 36.3%, respectively. In addition, V, VSP, and VMSP addition increased bacterial diversity and improved bacterial communities and functions in the soil by improving basic soil properties and reducing Cd-related toxicity. The results indicated that VMSP is a promising amendment for Cd immobilization in Cd-contaminated farmland soils.

Understanding the cadmium passivation and nitrogen mineralization of aminated lignin in soil

Aminated lignin (AL) was prepared and first applied to remediation of cadmium (Cd) pollution in soil. Meanwhile, the nitrogen mineralization characteristics of AL in soil and its effect on soil physicochemical properties were elucidated by soil incubation experiment. The results showed that the Cd availability was dramatically lowered in soil by adding the AL. The DTPA-extractable Cd content of AL treatments was considerably reduced by 40.7-71.4 %. The soil pH (5.77-7.01) and absolute value of zeta potential (30.7-34.7 mV) enhanced simultaneously as the AL additions increased. The content of soil organic matter (SOM) (99.0-264.0 %) and total nitrogen (95.9-301.3 %) were gradually enhanced due to high C (63.31 %) and N (9.69 %) content in AL. Moreover, AL significantly elevated the content of mineral nitrogen (77.2-142.4 %) and available nitrogen (95.5-301.7 %). The first-order kinetic equation of soil nitrogen mineralization revealed that AL greatly enhanced nitrogen mineralization potential (84.7-143.9 %) and reduced environmental pollution by lowering the loss of soil inorganic nitrogen. AL could effectively reduce the availability of Cd through direct (self-adsorption) and indirect effects (improvement of soil pH, SOM and reduction of soil zeta potential), thereby achieving passivation of Cd in soil. In short, this work will develop a novel approach and technical support for soil heavy metal remediation, which is of great significance for improving the sustainable development of agricultural production.

Biochar as a Green Sorbent for Remediation of Polluted Soils and Associated Toxicity Risks: A Critical Review

Soil contamination with organic contaminants and various heavy metals has become a global environmental concern. Biochar application for the remediation of polluted soils may render a novel solution to soil contamination issues. However, the complexity of the decontaminating mechanisms and the real environment significantly influences the preparation and large-scale application of biochar for soil ramification. This review paper highlights the utilization of biochar in immobilizing and eliminating the heavy metals and organic pollutants from contaminated soils and factors affecting the remediation efficacy of biochar. Furthermore, the risks related to biochar application in unpolluted agricultural soils are also debated. Biochar production conditions (pyrolysis temperature, feedstock type, and residence time) and the application rate greatly influence the biochar performance in remediating the contaminated soils. Biochars prepared at high temperatures (800 °C) contained more porosity and specific surface area, thus offering more adsorption potential. The redox and electrostatic adsorption contributed more to the adsorption of oxyanions, whereas ion exchange, complexation, and precipitation were mainly involved in the adsorption of cations. Volatile organic compounds (VOCs), dioxins, and polycyclic aromatic hydrocarbons (PAHs) produced during biochar pyrolysis induce negative impacts on soil alga, microbes, and plants. A careful selection of unpolluted feedstock and its compatibility with carbonization technology having suitable operating conditions is essential to avoid these impurities. It would help to prepare a specific biochar with desired features to target a particular pollutant at a specific site. This review provided explicit knowledge for developing a cost-effective, environment-friendly specific biochar, which could be used to decontaminate targeted polluted soils at a large scale. Furthermore, future study directions are also described to ensure a sustainable and safe application of biochar as a soil improver for the reclamation of polluted soils.

Silver nanoparticles strengthen Zea mays against toxic metal-related phytotoxicity via enhanced metal phytostabilization and improved antioxidant responses

This study investigated the phytostabilization and plant-promoting abilities of silver nanoparticles (AgNPs). Twelve Zea mays seeds were planted in water and AgNPs (10, 15 and 20 mg mL-1) irrigated soil for 21 days on soil containing 0.32 ± 0.01, 3.77 ± 0.03, 3.64 ± 0.02, 69.91 ± 9.44 and 13.17 ± 0.11 mg kg-1 of As, Cr, Pb, Mn and Cu, respectively. In soil treated with AgNPs, the metal contents were reduced by 75%, 69%, 62%, 86%, and 76%. The different AgNPs concentrations significantly reduced accumulation of As, Cr, Pb, Mn, and Cu in Z. mays roots by 80%, 40%, 79%, 57%, and 70%, respectively. There were also reductions in shoots by 100%, 76%, 85%, 64%, and 80%. Translocation factor, bio-extraction factor and bioconcentration factor demonstrated a phytoremediation mechanism based on phytostabilization. Shoots, roots, and vigor index improved by 4%, 16%, and 9%, respectively in Z. mays grown with AgNPs. Also, AgNPs increased antioxidant activity, carotenoids, chlorophyll a and chlorophyll b by 9%, 56%, 64%, and 63%, respectively, while decreasing malondialdehyde contents in Z. mays by 35.67%. This study discovered that AgNPs improved the phytostabilization of toxic metals while also contributing to Z. mays' health-promoting properties.

Effect of amendments on bioavailability of cadmium in soil-rice system: a field experiment study

The field experiment study investigated the effect of lime (L), manure compost (M), combination of lime and manure (LM), and combinations of lime with four kinds of passivators (LP1, LP2, LP3, and LP4) on the bioavailability of cadmium (Cd) in soil and Cd accumulation in rice plants. These four passivating products were composed of organic and inorganic compounds such as silicon-sulfhydryl group, CaO, SiO₂, and so on. The results indicated that the application of these amendments improved soil pH, organic matter content, and cation exchange capacity (CEC) by 0.19-0.73 unit, 0.6-8.2%, and 5.7-38.9%, respectively; meanwhile, decreased soil acid-extractable Cd by 4.0-13.9% compared with before remediation. Alleviating Cd stress to rice also resulted in a significant increase in rice grains yield, whereas the LP4 showed an increment of 15.8-27.6%. Among these amendments, LP4 had a relatively high effectiveness, it promoted the decrease of extractable Cd by 13.9% and the increase of residual Cd by 8.1%; meanwhile, the bioconcentration factor of rice grain in LP4 decreased by 71.3%. The high pH, CEC, and rich functional groups in amendments might cause soil Cd transform from mobile fraction to residual fraction, and the cation ions in amendments also competed with Cd ions due to the antagonism. Taken all of these effects, the amendments alleviated Cd pollution in soil-rice system, decreasing Cd migration from soil to grain. In future, the long-term field experiment will need to be done for verify the long-term effect of soil amendments.

Synergistic mechanism and application of microbially induced carbonate precipitation (MICP) and inorganic additives for passivation of heavy metals in copper-nickel tailings

Tailings are one of the largest quantities of hazardous waste in the world, and their treatment is difficult and expensive. In this work, a new, low-cost technique coupling microbially induced carbonate precipitation (MICP) and inorganic additives was proposed, optimized, and applied. The results showed that CaO was the best additive among the six tested, with an optimum dosage of 5%. A 90-day experiment indicated that the MICP-CaO coupled technique was highly effective for all the concerned heavy metals (Cu, Ni, Pb, and Cr) in the Cu-Ni tailings. During the stabilization period (20-90 days), the passivation rates were stable at 78.8 ± 2.9% (Cu), 78.1 ± 1.0% (Ni), 89.2 ± 1.0% (Pb), and 97.8 ± 0.5% (Cr), 2%-866% higher than the single technique of either MICP or CaO. Multiple analyses demonstrated that the synergistic effect of MICP and CaO produced a large amount of calcite (1.5% of the tailings). This calcite cemented the tailings particles, sequestrated heavy metal ions into the lattices, and played a key role in heavy metal passivation. Moreover, CaO and MICP improved the strength and compactness of solidified body, respectively. This work demonstrates the feasibility of the MICP-CaO coupled technique in tailings solidification, which can be applied in practical projects.

Deciphering soil amendments and actinomycetes for remediation of cadmium (Cd) contaminated farmland

Soil heavy metal pollution is one of the most serious environmental problems in China, especially cadmium (Cd), which has the most extensive contaminated soil coverage. Therefore, more economical and efficient remediation methods and measures are needed to control soil Cd contamination. In this study, different amendments (biochar (B), organic fertilizer (F), lime (L)) and actinomycetes (A) inoculants were applied to Cd contaminated farmland to explore their effects on wheat growth. Compared with Control, all treatments except A treatment were able to significantly increase the underground parts dry mass of wheat, with the highest increase of 57.19 %. The results showed that the B treatment significantly increased the plant height of wheat by 3.45 %. All treatments increased wheat SOD activity and chlorophyll content and reduced the MDA, which contributes to wheat stress resistance under Cd contamination. F, L and AF treatments can significantly reduce the Cd content in wheat above- and underground parts by up to 56.39 %. Soil amendments can modify the physical and chemical properties of the soil, which in turn affects the absorption of Cd by wheat. Moreover, the addition of soil amendments significantly affects the composition and structure of the rhizospheric soil bacterial community at the wheat jointing stage. The application of organic fertilizer increases the richness and diversity of the bacterial community, while lime makes it significantly decreases it. T-test and microbiome co-occurrence networks show that actinomycetes could not only effectively colonize in local soil, but also effectively enhance the complexity and stability of the rhizosphere microbial community. Considering the practical impact of different treatments on wheat, soil microorganisms, economic benefits and restoration of soil Cd contamination, the application of organic fertilizer and actinomycetes in Cd contaminated soil is a more ideal remediation strategy. This conclusion can be further verified by studying larger repair regions and longer consecutive repair cycles to gain insight into the repair mechanism.

Potential of granular complexes of lime and montmorillonite for stabilizing soil cadmium and the underlying mechanisms

Cadmium (Cd) contaminated soils were widely remediated by alkaline materials in powder, while the effects of granular materials are still unknown. This study was conducted to prepare granular materials based on hydrated lime and montmorillonite with ratios of 1:1, 1:2, and 1:3 (LM1, LM2, and LM3); their effects and mechanisms on stabilizing Cd in hydroponic, pot, and field conditions were further explored. The results showed that powdery materials caused intense pH elevations within 30-60 min and dissolved-Cd reductions within 8-100 min. However, granular materials significantly delayed these effects; the highest solution pH and lowest dissolved-Cd occurred after 250 min. The LM1 granules induced a much higher reduction of dissolved-Cd (99.8%) than that in the LM2 (53.6%) and LM3 granules (14.3%) due to the generation of more cadmium carbonate precipitates. Additionally, the soil pH gradually decreased after an intense elevation induced by powdery materials, but the LM1 granules maintained the soil pH at approximately 7.0, resulting in a lower level of CaCl₂-extractable Cd (0.03 mg kg^-1) than the LM1 powder (0.22 mg kg^-1) after 30 d of cultivation. Similar to lime powder, a small spatial variation (Std. of 3.45) of DGT (diffusive gradient in thin films) extractable Cd in soil profile was observed in the LM1 granules, revealing a homogeneous stabilization effect induced by the LM1 granules. Accordingly, the LM1 granules induced a higher reduction in brown rice Cd (50.9%) than that in the LM1 powders (35.1%). Thus, the granular material of hydrated lime and montmorillonite (1:1) h the potential to replace lime powder in the remediation of Cd-contaminated field.

Combined apatite, biochar, and organic fertilizer application for heavy metal co-contaminated soil remediation reduces heavy metal transport and alters soil microbial community structure

Soil amendments are used extensively to remediate soils contaminated with heavy metals. However, the effects of soil amendments on heavy metal bioavailability, plant yield, and bacterial community structure in tropical farmland soils remain largely unknown. In the present study, seaweed organic fertilizer (S), apatite (A), biochar (B), and seaweed organic fertilizer-apatite-biochar mixtures (SAB) were applied at different rates to assess their influence on cadmium (Cd), lead (Pb), and chromium (Cr) bioavailability in contaminated farmland soils, using different component ratios and doses in maize field plots, and maize yield. Effects on soil bacterial community structure were also evaluated based on high-throughput sequencing. Following addition of 2 % S + A + B combined amendment at a ratio of 1:0.5:1.5 (2%S₁A_0.5B_1.5), soil pH and electrical conductivity (EC) were elevated, and bioavailable Cd, Pb, and Cr concentrations were reduced in potted soils, leading to higher heavy metal immobilization. Under field conditions, soil pH, EC, organic matter, ammonium‑nitrogen, available phosphorus, available potassium, and crop productivity were all increased considerably, whereas soil Cd and Cr bioavailability were lower in the combined amendment treatments than in the control treatments. Particularly, application of a 2.49 t·ha^-1 combined amendment (0.83 t·ha^-1 S + 0.41 t·ha^-1A + 1.25 t·ha^-1B,1:0.5:1.5) decreased Cd, Pb, and Cr concentrations in maize grain by 68.9 %, 68.9 %, and 65.7 %, respectively. Species abundance and evenness in bacterial communities increased in field soils subjected to combined amendments, with shifts in community structure and function mostly driven by changes in soil pH, organic matter content, and nutrient availability. Overall, the results suggest that 1.5%S₁A_0.5B_1.5 is the optimal treatment for remediating heavy metal co-contaminated soil, and thereby, improving maize yield and quality. Combined organic and inorganic amendments achieve high remediation efficiency, mainly by improving chemical properties, reducing heavy metal bioavailability, and altering bacterial community structure and function in heavy metal contaminated farmland soils.

Effect of biochar amendment on metal mobility, phytotoxicity, soil enzymes, and metal-uptakes by wheat (Triticum aestivum) in contaminated soils

The use of low-cost substances such as biochar could be a sustainable approach to reduce the mobility, accumulation, and toxic impact of heavy metals in crop systems. This study investigates the effect of biochar amendment on heavy metal (Cr, Cd, Cu, Pb, Ni, Zn, Mg and Fe) mobility, bioaccumulation factor (BAF), plant (wheat) metal-uptake, plant oxidative stress, and soil enzymatic profile in contaminated industrial soil. Biochar was obtained from slow pyrolysis of Lantana (LBC), and Parthenium (PBC) biomass, and applied at 3% rates in contaminated soils for wheat crop study under a greenhouse experimental setup. Results show in comparison with control setups, low mobility of Cr (14.15-16.35%), Cd (7.17-15.24%), Cu (9.81-12.97%), Pb (7.99-15.23%), Ni (1.52-2.38%), Zn (10.47-14.42%), Mg (48.85-52.89%), and Fe (19.13-19.90%) contents in soils. The heavy metal uptake rates were 63.08% (Cr), 78.07% (Cd), 74.61% (Cu), 78.11% (Pb), 75.73% (Ni), 69.71% (Zn), 28.78% (Mg), and 49.26% (Fe) lower in biochar amendments, compared with the control treatments. Similarly, the biochar amended treatments exhibited low oxidative stress in wheat plants than control setups. In addition, soil enzymes (dehydrogenase, β-glucosidase, alkaline phosphatase, and urease) alleviated in biochar amended soils indicating reduced toxicity of metals in experimental soils. In summary, this study indicates that biochar amendment in contaminated soils not only improves plant growth but also lowers the rates of soil and plant toxicity and metal bioavailability as well.

Corncob biochar combined with Bacillus subtilis to reduce Cd availability in low Cd-contaminated soil

Soil contamination by heavy metals such as Cd can pose a risk to the environment and human health. However, Cd is difficult to immobilize at low concentration levels in soil. Individually, Bacillus subtilis and biochar have been shown to be inefficient at immobilizing Cd in soil. In this study, corncob biochar was generated at different pyrolysis temperatures (300 °C-550 °C), and the Cd immobilization efficiency and performance of corncob biochar loaded with B. subtilis (CB@B) and corncob biochar alone (CB) were evaluated in solutions and in soil. The characterization (SEM and FTIR) of CB generated at different pyrolysis temperatures and CB generated at different pyrolysis temperatures in CB@B (300 °C-550 °C) indicated that a superior pore structure and abundant O-functional groups were obtained at a pyrolysis temperature of 400 °C for both CB@B and CB. The X-ray diffraction and X-ray photoelectron spectroscopy results indicate that the formation of Cd compounds was associated with the positive combined biosorption effect of the bacteria and biochar, electronic adsorption, activity of the O-functional groups (C[double bond, length as m-dash]O, COOH, OH, and Si-O-Si), and complexation between extracellular substances and Cd²⁺. Adsorption experiments were conducted in a solution to assess the effects of various operating parameters such as the time, pH, and adsorbent dose. The 400 °C-CB@B and 400 °C-CB samples achieved the largest reductions in the Cd concentration at 81.21% and 5.70%, respectively. Then, CaCl₂ extraction experiments were conducted in soil, and using 0.25%-CB@B, a 55.21% decrease was realized in the Cd concentration after 56 days and a 16.71% increase was realized in soil pH to 8.38. No significant difference was observed in the CB-treated groups, among which 1.0%-CB achieved the largest reduction of 26.08% after 56 days and a 3.20% increase in the soil pH to 7.41. The Tessier sequential extraction method obtained similar trends. Overall, 400 °C-CB@B demonstrated outstanding immobilization efficiency and durability, indicating that it provided a safe and nutrient-rich habitat for B. subtilis to realize a synergistic effect for Cd immobilization.

Inconsistent effects of a composite soil amendment on cadmium accumulation and consumption risk of 14 vegetables

Organic and inorganic mixtures can be developed as immobilizing agents that could reduce heavy metal accumulation in crops and contribute to food safety. Here, inorganic materials (lime, L; zeolite, Z; and sepiolite, S) and organic materials (biochar, B, and compost, C) were selectively mixed to produce six composite soil amendments (LZBC, LSBC, LZC, LZB, LSC, and LSB). Given the fact that LZBC showed the best performance in decreasing soil Cd availability in the incubation experiment, it was further applied in the field condition with 14 vegetables as the test crops to investigate its effects on crop safety production in polluted greenhouse. The results showed that LZBC addition elevated rhizosphere soil pH by 0.1-2.0 units and reduced soil Cd availability by 1.85-37.99%. Both the biomass and the yields of edible parts of all vegetables were improved by LZBC addition. However, LZBC addition differently affected Cd accumulation in edible parts of the experimental vegetables, with the observation that Cd contents were significantly reduced in Allium fistulosum L., Amaranthus tricolor L., and Coriandrum sativum Linn., but increased in the three species of Lactuca sativa. Further health risk assessment showed that LZBC application significantly decreased daily intake of metal (DIM), health risk index (HRI), and target hazard quotient (THQ) for Cd in Allium fistulosum L., Amaranthus tricolor L., and Coriandrum sativum Linn., whereas increased all the indexes in Lactuca sativa. Our results showed that the effect of a composite amendment on Cd accumulation in different vegetables could be divergent and species-dependent, which suggested that it is essential to conduct a pre-experiment to verify applicable species for a specific soil amendment designed for heavy metal immobilization.

Immobilization of Cd and Pb in soil facilitated by magnetic biochar: metal speciation and microbial community evolution

The preparation of magnetic biochar from sewage sludge and rice straw for heavy metal contaminated soil remediation has greater application prospects, but its remediation mechanism was rarely considered by combining soil physicochemical properties with microbial community. In this study, the effects of magnetic sewage sludge biochar (SSB) and rice straw biochar (RSB) on Cd and Pb immobilization in paddy soil were compared and analyzed by 60-day soil incubation experiments. The results illustrated that DTPA-Cd and DTPA-Pb were reduced by 51.53% (43.07%) and 53.57% (50.47%), while the percentage of residual fraction of the BCR procedure was enhanced by 31.27% (30.78%) of Cd and 27.25% (23.22%) of Pb in the SSB (RSB) treatment, respectively. Fe was detected on both SSB and RSB surfaces, but SSB had rougher and a larger specific surface area compared to RSB. The addition of SSB and RSB in paddy soil increased soil pH and TOC content, and affected the diversity and species of soil microbial community. Compared with the CK group, the relative abundance of Proteobacteria, Bacteroidota, and Lysobacter decreased, and the relative abundance of Actinobacteriota, Pontibacter, and Alkaliphilus increased with SSB and RSB treatments, all of which reflected the bioavailability of Cd and Pb reduction.

Calcareous Materials Effectively Reduce the Accumulation of Cd in Potatoes in Acidic Cadmium-Contaminated Farmland Soils in Mining Areas

The in situ chemical immobilization method reduces the activity of heavy metals in soil by adding chemical amendments. It is widely used in farmland soil with moderate and mild heavy metal pollution due to its high efficiency and economy. However, the effects of different materials depend heavily on environmental factors such as soil texture, properties, and pollution levels. Under the influence of lead-zinc ore smelting and soil acidification, Cd is enriched and highly activated in the soils of northwestern Guizhou, China. Potato is an important economic crop in this region, and its absorption of Cd depends on the availability of Cd in the soil and the distribution of Cd within the plant. In this study, pot experiments were used to compare the effects of lime (LM), apatite (AP), calcite (CA), sepiolite (SP), bentonite (BN), and biochar (BC) on Cd accumulation in potatoes. The results showed that the application of LM (0.4%), AP (1.4%), and CA (0.4%) had a positive effect on soil pH and cations, and that they effectively reduced the availability of Cd in the soil. In contrast, the application of SP, BN, and BC had no significant effect on the soil properties and Cd availability. LM, AP, and CA treatment strongly reduced Cd accumulation in the potato tubers by controlling the total 'flux' of Cd into the potato plants. In contrast, the application of SP and BN promoted the migration of Cd from the root to the shoot, while the effect of BC varied by potato genotype. Overall, calcareous materials (LM, CA, and AP) were more applicable in the remediation of Cd-contaminated soils in the study area.

Quantitative evaluation of the synergistic effect of biochar and plants on immobilization of Pb

Biochar and plant cooperation in remediation of heavy metal contaminated soil is effective and important, but there still have knowledge gaps of synergistic effect between the two and the synergistic pathway has not been clarified. We prepared the Enteromorpha prolifera biochar at 400 °C and 600 °C (denoted as BC400 and BC600). The Pb fractions changes in soil and Pb toxicity in Brassica juncea were investigated by adding 30 g kg^-1 biochar to soil containing 1200 mg kg^-1 Pb in a pot experiment. There was a significant synergistic effect between biochar and plants on Pb immobilization in soil, according to the "E > 0" of Pb fractions in the interaction equation. Pb immobilization rates of biochar-plant treatments (BJBC4 and BJBC6) were 12.47% and 11.38% higher than biochar treatment (BC4, BC6), and 17.66% and 16.28% plant treatment (BJ). BJBC4 had a better immobilization effect than BJBC6. Biochar alleviated the phytotoxicity of Pb by increasing the antioxidant enzymes activities of plants. These results indicated two synergistic pathways: (1) The high pH and oxygen-containing functional groups of biochar could immobilize Pb through ion exchange, precipitation, or complexation. (2) Biochar enhanced the activity of the antioxidant enzyme system in plants thus improving the Pb tolerance of plants. Statistical analysis methods such as the partial least squares path modeling (PLS-PM) also confirmed the pathways. In a word, clear synergistic effects and pathways could guide the application of biochar and plants in Pb-contaminated soil.

The potential effectiveness of mixed bacteria-loaded biochar/activated carbon to remediate Cd, Pb co-contaminated soil and improve the performance of pakchoi plants

Cadmium (Cd) and lead (Pb) are toxic heavy metals that cause severe soil pollution and pose health risks to humans. It is urgent to develop feasible strategies for Pb and Cd remediation. In this study, a bacteria consortium (Enterobacter asburiae G3, Enterobacter tabaci I12 and Klebsiella variicola J2 in a 1:3:3 proportion) with optimal Cd, Pb adsorption ability was constructed and immobilized on biochar (BC)/activated carbon (AC) via physisorption and sodium alginate encapsulation. The effects of mixed bacteria-loaded BC/AC on Cd and Pb remediation were investigated. The results indicated that their application reduced the DTPA-extractable Cd, Pb in soil by 22.05%-55.84% and 31.64%-48.13%, respectively. The residual Pb, Cd were increased while the exchangeable fractions were decreased. Soil urease, catalase and phosphatase activities were enhanced and soil bacterial community was improved, indicating a soil quality improvement. Consequently, the biomass of pakchoi plants was significantly increased. Cd and Pb in the shoots of pakchoi plants were decreased by 28.68%-51.01% and 24.18%-52.87%, respectively. Collectively, the bacteria-loaded BC/AC showed superior performance than free bacteria, BC and AC alone. Our study may provide a better understanding of the development of green and sustainable materials for remediation of heavy metal by the combination of BC/AC and functional bacteria.

Immobilization of soil Cd by sulfhydryl grafted palygorskite in wheat-rice rotation mode: A field-scale investigation

The safe utilization of heavy metal contaminated farmland has attracted extensive attention of the whole society, and there is an urgent need to develop novel high-efficiency amendments. To clarify the actual remediation effect and potential for large-scale application of sulfhydryl grafted palygorskite (SGP) in Cd polluted soil in wheat-rice rotation mode, a field-scale experiment was conducted. SGP at the dosages of 0.5 g/kg-2.0 g/kg could reduce gain Cd contents by 27.15-59.05% and 16.16-79.47% for wheat and rice, respectively. The maximal decreases of soil available Cd figured out by DTPA extraction in wheat and rice season were 58.18% and 33.67%, respectively. The immobilization ratio for Cd was much more than that of trace elements, including Fe, Mn, Cu, and Zn, Ni. SGP showed an effective immobilization rate for soil Cd under the interference of many elements in the soil, pointing to the targeting and selectivity of its high-efficiency immobilization. It had no lifting effect on soil pH but decreased zeta potentials of soil particles. The sorption of Cd²⁺ on SGP amended soil could be fitted by the second-order kinetic model and Langmuir isotherm, and the changes of thermodynamic parameters showed SGP strengthened the fixation. SGP made the biological accumulation coefficient and transfer factor of rice grain drop dramatically but had no noticeable effect on these parameters of winter wheat, indicating different botanical responses. SGP as a novel immobilization amendment may provide an efficient and sustainable solution for the remediation of contaminated soil in wheat-rice rotation mode in field-scale.

A review of pristine and modified biochar immobilizing typical heavy metals in soil: Applications and challenges

In recent years, the application of biochar in the remediation of heavy metals (HMs) contaminated soil has received tremendous attention globally. We reviewed the latest research on the immobilization of soil HMs by biochar almost in the last 5 years (until 2021). The methods, effects and mechanisms of biochar and modified biochar on the immobilization of typical HMs in soil have been systematically summarized. In general, the HMs contaminating the soil can be categorized into two groups, the oxy-anionic HMs (As and Cr) and the cationic HMs (Pb, Cd, etc.). Reduction and precipitation of oxy-anionic HMs by biochar/modified biochar are the dominant mechanism for reducing HMs toxicity. Pristine biochar can effectively immobilize cationic HMs. The commonly applied modification method is to add substances that can precipitate HMs to the biochar. In addition, we assessed the risks of biochar applications. For instance, biochar may cause the leaching of certain HMs; biochar aging; co-transportation of biochar nanoparticles with HMs. Future work should focus on the artificial/intelligent design of biochar to make it suitable for remediation of multiple HMs contaminated soil.

Comparing the effects of calcium and magnesium ions on accumulation and translocation of cadmium in rice

Rice (Oryza sativa L.) is one of China's most important food crops, and it is considered the primary source of human exposure to cadmium (Cd) pollution. Adding calcium (Ca) and magnesium (Mg) to the plant nutrient solutions reduces the accumulation of Cd in the rice, but under the same condition, which one has the better effect remains unclear. Thus, hydroponic experiments were performed to compare the effects of Ca and Mg ions with concentration gradients (0.10, 0.25, and 0.50 g/L, respectively) on the absorption, distribution, and translocation of Cd in rice. The Cd contents of roots, stems, leaves, panicles, husks, and grains in different growth stages were determined. The results revealed that the supplementation of both Ca and Mg influenced the Cd accumulation and translocation in rice tissues. The Cd concentrations of different patterns were in the following order: roots > stems > leaves ≈ panicles ≈ husks > grains. Both of Ca and Mg had an apparent antagonism with Cd in different parts of the rice plant, and the antagonism was more obvious in the high Cd stress treatments. With the addition of 0.1 g/L Ca²⁺ and Mg²⁺ ions, the grain Cd contents increased, while the application of 0.25 and 0.5 g/L Ca²⁺ and Mg²⁺ ions reduced grains Cd by 19.08-38.99%, with the average value of 26.75%. Under the same concentrations, the grain Cd content of Ca treatments was lower than that of Mg treatments by 8.74%. In the Ca (Mg)-deficient and Ca (Mg)-sufficient conditions, the husks and panicles accumulated Cd to hinder Cd translocation, respectively. Altogether, the results of this study indicated that Ca had a greater effect for decreasing rice Cd accumulation and translocation than Mg, and the panicle and husk were the important parts for reducing Cd translocation to grain, and these might be a focal point for the future research. It was possible to plant and grow rice in Cd-polluted soil and that the accumulation and translocation of Cd in rice plants could be reduced by optimizing soil nutrient elements.

Oyster shell amendment reduces cadmium and lead availability and uptake by rice in contaminated paddy soil

Contamination with cadmium (Cd) and lead (Pb) in rice and paddy soil threatens food safety and human health. This study determined the effects of oyster shell amendment (0, 6, and 12 g kg^-1) on the mobility and uptake of Cd and Pb by two rice plants (ZY18, japonica and DL5, indica) in contaminated paddy soil. Oyster shell amendment significantly increased the pH of soil and pore water, and decreased the DTPA-extractable Cd concentration in soil, but not the Cd concentration in pore water. Furthermore, the DTPA-extractable Pb concentration in rhizosphere soil was not significantly influenced by oyster shell addition. Application of oyster shell reduced the Pb concentration in pore water and Pb uptake by both cultivars, but excessive application (12 g kg^-1) resulted in Cd accumulation in tissues for ZY18. Furthermore, oyster shell addition significantly increased the dissolved organic carbon, calcium, and magnesium concentrations in soil pore water, and decreased glutathione and phytochelatin levels in roots, all of which alleviated heavy metal toxicity and improved rice growth. These results demonstrate that amendment with the appropriate amount of oyster shell, combined with rice cultivar type, could simultaneously reduce Cd and Pb accumulation in rice grown in heavy metal-contaminated soil.

Sustainability assessment and carbon budget of chemical stabilization based multi-objective remediation of Cd contaminated paddy field

The feasibility of chemical stabilization-based strategy for extensive field application is under debate due to lacking a proper framework for its sustainability assessment during its life cycle. Herein, a comprehensive framework consisting of crop production, soil quality, and carbon footprint was constructed for assessing agricultural land remediation based on a two-year paddy field trial. Results show that between the two representative agents, biochar scenario substantially benefits for environmental, social, and agricultural sustainability, because of its more positive impacts on human health and ecosystem, public acceptance, soil reproductive, and rice yield. A notably higher sustainability score of 80.7 for biochar scenario than that of 47.0 for lime is found, in spite of the economical sustainability of lime. The net ecosystem carbon budget of the biochar scenario exhibits an unprecedentedly positive value of 17.8 t CO₂-eq ha^-1, which can finely contribute to a positive carbon budget during remediation. Our finding demonstrates that biochar strategy enables a multi-objective achievement of soil quality - crop production - carbon budget during agricultural land remediation. This study provides new insights into sustainability assessment for restoring agricultural land for safe crop production and synergizing with carbon neutral plan.

Technical solutions for minimizing wheat grain cadmium: A field study in North China

The minimization of Cd pollution in wheat is urgently needed in many parts of the world. Thus, the aims of the present study were to evaluate the feasibility of popular technologies (i.e., soil amendment and low-Cd wheat cultivar) at sites with different Cd risk levels (high and low) and to propose a risk-based strategy for safe grain production. At a high-Cd site, wheat variety JM22 yielded significantly lower grain Cd than SX828, regardless of soil amendment (biochar, sepiolite, and microbial agent YZ1). Neither biochar nor sepiolite amendment reduced grain Cd, DTPA-Cd, or bioconcentration factors, possibly due to low dosage. Metagenomic sequencing and quantitative PCR showed that YZ1 colonization had little effect on rhizospheric fungal community structure and could not be sustained through winter. At a low-Cd site, significantly lower grain Cd was observed in JM22, LX99, and JM262, which could be used as low-Cd cultivars in the study area. Interestingly, the grain Cd of JM22 was linearly correlated with soil Cd (R² = 0.84), which allowed the inference of a soil Cd threshold of 1.55 mg·kg^-1, below which JM22 alone was capable of producing safe grain. Cost-benefit analysis also indicated that the use of low-Cd cultivars is promising for pollution control. This study provides viable technical solutions for minimizing the grain Cd of wheat grown in northern China.

Sustainable biochar: A facile strategy for soil and environmental restoration, energy generation, mitigation of global climate change and circular bioeconomy

The increasing agro-demands with the burgeoning population lead to the accumulation of lignocellulosic residues. The practice of burning agri-residues has consequences viz. Release of soot and smoke, nutrient depletion, loss of soil microbial diversity, air pollution and hazardous effects on human health. The utilization of agricultural waste as biomass to synthesize biochar and biofuels, is the pertinent approach for attaining sustainable development goals. Biochar contributes in the improvement of soil properties, carbon sequestration, reducing greenhouse gases (GHG) emission, removal of organic and heavy metal pollutants, production of biofuels, synthesis of useful chemicals and building cementitious materials. The biochar characteristics including surface area, porosity and functional groups vary with the type of biomass consumed in pyrolysis and the control of parameters during the process. The major adsorption mechanisms of biochar involve physical-adsorption, ion-exchange interactions, electrostatic attraction, surface complexation and precipitation. The recent trend of engineered biochar can enhance its surface properties, pH buffering capacity and presence of desired functional groups. This review focuses on the contribution of biochar in attaining sustainable development goals. Hence, it provides a thorough understanding of biochar's importance in enhancing soil productivity, bioremediation of environmental pollutants, carbon negative concretes, mitigation of climate change and generation of bioenergy that amplifies circular bioeconomy, and concomitantly facilitates the fulfilment of the United Nation Sustainable Development Goals. The application of biochar as seen is primarily targeting four important SDGs including clean water and sanitation (SGD6), affordable and clean energy (SDG7), responsible consumption and production (SDG12) and climate action (SDG13).

Cadmium Phytotoxicity, Tolerance, and Advanced Remediation Approaches in Agricultural Soils; A Comprehensive Review

Cadmium (Cd) is a major environmental contaminant due to its widespread industrial use. Cd contamination of soil and water is rather classical but has emerged as a recent problem. Cd toxicity causes a range of damages to plants ranging from germination to yield suppression. Plant physiological functions, i.e., water interactions, essential mineral uptake, and photosynthesis, are also harmed by Cd. Plants have also shown metabolic changes because of Cd exposure either as direct impact on enzymes or other metabolites, or because of its propensity to produce reactive oxygen species, which can induce oxidative stress. In recent years, there has been increased interest in the potential of plants with ability to accumulate or stabilize Cd compounds for bioremediation of Cd pollution. Here, we critically review the chemistry of Cd and its dynamics in soil and the rhizosphere, toxic effects on plant growth, and yield formation. To conserve the environment and resources, chemical/biological remediation processes for Cd and their efficacy have been summarized in this review. Modulation of plant growth regulators such as cytokinins, ethylene, gibberellins, auxins, abscisic acid, polyamines, jasmonic acid, brassinosteroids, and nitric oxide has been highlighted. Development of plant genotypes with restricted Cd uptake and reduced accumulation in edible portions by conventional and marker-assisted breeding are also presented. In this regard, use of molecular techniques including identification of QTLs, CRISPR/Cas9, and functional genomics to enhance the adverse impacts of Cd in plants may be quite helpful. The review's results should aid in the development of novel and suitable solutions for limiting Cd bioavailability and toxicity, as well as the long-term management of Cd-polluted soils, therefore reducing environmental and human health hazards.

Organic/inorganic amendments for the remediation of a red paddy soil artificially contaminated with different cadmium levels: Leaching, speciation, and phytoavailability tests

In the present study, the viability of using manure (M), lime (L), and sepiolite (S) alone and in combinations (M/L, M/S, and M/L/S) was evaluated for the remediation of a red paddy soil artificially contaminated with three levels of cadmium (Cd- 0.6, 1, and 2 mg kg^-1 soil). Experiments were performed in columns (to evaluate Cd leaching) and pots by growing rice plants (to study Cd accumulation in plants). Before their application, the tested amendments were thoroughly characterized using SEM, EDS and FT-IR spectroscopy. The leaching experiment indicates that the application of L or M/L significantly improved the pH of soil leachate collected at different time intervals. However, the use of M/L/S was found better in decreasing the Cd contents in collected leachate. The use of M/L efficiently decreased the DTPA metal extraction (0.19, 0.41, and 0.55 mg kg^-1) as compared to the CK (0.35, 0.63, and 1.13 mg kg^-1, respectively). The Cd speciation results depicted a 33% decrease in exchangeable Cd with M/L/S treatment when compared with control (55%). Moreover, the M/L/S treatment was more efficient in lowering the Cd phytoavailability and subsequent accumulation in rice grains (0.05, 0.09, and 0.08 mg kg^-1). These findings demonstrate that the use of composite amendments is categorically effective as an in-situ remediation tool to decrease Cd leaching and availability in diverse contaminations.

Cd diminution through microbial mediated degraded lignocellulose maize straw: Batch adsorption and bioavailability trails

Lignocellulose degraded maize straw (LMS) was prepared with the interaction of soil-indigenous microorganisms and further deployed to attenuate the Cd contamination in polluted soil. The Lignocellulose degrading ratio was determined and results revealed the significant degradation of cellulose, hemicellulose and lignin by 33.03, 26.7 and 15.97% respectively as compared to pristine maize straw (PS). Moreover, LMS was also categorized through FE-SEM, FTIR, BET analysis, elemental analysis and XPS technique and the analytical results indicated that lignocellulose structure in maize straw was successfully degraded and was involved in metal-ion complexation. Batch sorption trials revealed that Cd²⁺ sorption onto LMS was explained well by Langmuir isotherm and pseudo-second-order kinetic model. The LMS showed maximum adsorption capacities (9.84 mg g^-1) for Cd²⁺ as compared to PS (3.30 mg g^-1). Moreover, the soil incubation trials (60 days) depicted the availability of Cd decreased by 11.03 and 34.7% with PS and LMS application respectively. The addition of LMS significantly decreased the exchangeable fractions of Cd and ensued an increase in organic matter and Fe-Mn oxides bound fractions. This work clarified the LMS as a promising amendment for effective remediation of Cd-contaminated matrices.

Inter-annual reduction in rice Cd and its eco-environmental controls in 6-year biannual mineral amendment in subtropical double-rice cropping ecosystems

The alkaline mineral amendment is a practical means of alleviating Cd concentration in rice grain (Cd_R) in the short-term; however, the long-term remediation effect of mineral amendment on the Cd_R and the eco-environmental controls remains unknown. Here a mineral (Si-Ca-Mg) amendment, calcined primarily from molybdenum tailings and dolomite, was applied biannually over 6 years (12 seasons) to acidic and moderately Cd-contaminated double-rice cropping ecosystems. This study investigated the inter-annual variation of Cd in the rice-soil ecosystem and the eco-environmental controls in subtropical rice ecosystems. Cd_R was reduced by 50%-86% following mineral amendment. The within-year reduction in Cd_R was similar between early rice (50%-86%, mean of 68%) and late rice (68%-85%, mean of 74%), leading to Cd_R in all early rice and in 83% of late rice samples below the upper limit (0.2 mg kg^-1) of the China National Food Safety Standards. In contrast, the inter-annual reduction in Cd_R was moderately variable, showing a greater Cd_R reduction in the later 3 years (73%-86%) than in the former 3 years (54%-79%). Three years continuous mineral amendment was required to guarantee the safety rice production. The concentrations of DTPA-extractable and exchangeable Cd fractions in soil were reduced, while the concentration of oxides-bound Cd was increased. In addition, the soil pH, concentrations of Olsen-P and exchangeable Ca and Mg were elevated. These imply a lower apparent phytoavailability of Cd in the soil following mineral amendment. An empirical model of the 3-variable using soil DTPA-Cd, soil Olsen-P, and a climatic factor (precipitation) effectively predicted temporal changes in Cd_R. Our study demonstrates that Cd phytoavailability in soil (indexed by DTPA-extractable Cd) and climatic factors (e.g., temperature and precipitation) may directly/indirectly control the inter-annual reduction in Cd_R following mineral amendment in slightly and moderately Cd-contaminated paddy ecosystems.

The effects of a combined amendment on growth, cadmium adsorption by five fruit vegetables, and soil fertility in contaminated greenhouse under rotation system

Cadmium (Cd) exposure is related to a multitude of adverse health outcomes because food crops grown on Cd-polluted soil are widely consumed by the public. The present study investigates the different application techniques of a combined amendment (lime + zeolite + biochar + compost, LZBC) for soil Cd immobilization effect on growth performance, Cd uptake by the second season crops, and soil quality in greenhouse vegetable production (GVP) under a rotation system. Five fruit vegetables were cultivated as the second season crop in the same plots which have been used for pakchoi as the first season crop (with or without LZBC application). The results indicated that LZBC with the consecutive application (T3) promoted crops biomass and fruit yield the most, followed by LZBC with the second crop application (T2) and LZBC with the first crop application (T1). LZBC application showed increasing rhizosphere soil pH and improvement in soil fertility of all crops including available nitrogen, available phosphorus, available potassium, organic matter, and cation exchange capacity. LZBC had positive influences on soluble sugar, soluble protein, and vitamin C in edible parts of 5 vegetables. Cd contents in fruit, shoot, and root of eggplant, pimento, cowpea, and tomato except cucumber were reduced by adding LZBC. As for the economic performance, T3 had the highest output/input ratio in general. Overall, these results demonstrated that T3 was dramatically more effective for minimizing health risk, increasing production, and facilitating sustainable utilization of soil under the Cd-contaminated GVP system.

Effects of Soil Amendments on Soil Pb Bioavailability and Pb Absorption by a low-Pb Accumulator Kumquat Grown in Two Types of Pb-Contaminated Soils

A pot experiment was conducted to investigate the effects of 0.5% and 1% alkaline, clay mineral and phosphorus amendments, as well as 2% and 5% organic amendments, on lead (Pb) soil bioavailability and Pb absorption by the low-Pb accumulator kumquat (Citrus japonica Thunb.) 'Cuipi' in two typical Pb-contaminated soils, Jiyuan and Yangshuo, from northern and southern China, respectively. The diethylenetriaminepentaacetic acid-extractable Pb soil concentration and Pb accumulation in kumquat significantly decreased with amendment additions. High amendment doses had greater inhibitory effects than low doses but no significant effects on the kumquat's biomass in the two typical soils. Alkaline, but not clay mineral, amendments greatly increased the soil pH level. Organic amendments effectively reduced Pb accumulation owing to their strong adsorptive capacities. Thus, using organic amendments combined with a low-Pb accumulator kumquat forms a suitable farming practice for producing safe fruit in the two common types of Pb-contaminated soils in China.

Co-pyrolysis of corn stover with industrial coal ash for in situ efficient remediation of heavy metals in multi-polluted soil

Coal ash incorporated biochar (CA/BC) composite was prepared by co-pyrolysis of agricultural residue and industrial coal ash and applied for remediation of soils polluted by lead (Pb) and cadmium (Cd). The results showed that immobilization efficiency of CA/BC for heavy metals (HMs) was significantly enhanced by 77.1 % (Pb) and 42.7 % (Cd) compared to pristine biochar (BC), and this was mainly due to the increased pH value, surface functionality and surface negative charge. By the introduction of 5 % CA/BC, the polluted soils showed the highest reduction of leaching toxicity by 67.9 % (Pb) and 49.7 % (Cd), respectively. The chemical speciation of Pb and Cd in soils was changed remarkably and the reduced bioavailable Pb and Cd were mainly transformed from acid-soluble fraction into the most stable form of residual fraction. The mechanism study showed that surface precipitation, complexation, cation exchange and cation-π interaction of CA/BC mainly contributed to heavy metals (HMs) immobilization. The pot experiments further confirmed that incorporation of 5 % CA/BC effectively reduced plant Pb and Cd accumulation by 81 % and 62.5 % respectively, and significantly promoted the plant growth of paddy rice by 3.1, 2.2 and 2.0 times in terms of root, stem length and dry mass parameters. The present study offered a cost-effective and green method to prepare soil amendment with great potential for remediation of soils polluted by HMs and realized the value-added utilization of waste agricultural residue and industrial coal ash.

Immobilization of cadmium and lead using phosphorus-rich animal-derived and iron-modified plant-derived biochars under dynamic redox conditions in a paddy soil

Functionalized biochar has gained extensive interests as a sustainable amendment for an effective remediation of paddy soils contaminated with heavy metals (HMs). We examined the efficiency of pig carcass-derived biochar (P-rich biochar, total P = 8.3%) and pristine (raw biochar, total Fe = 0.76%) and Fe-modified (Fe-rich biochar, total Fe = 5.5%) green waste-derived biochars for the immobilization of cadmium (Cd) and lead (Pb) in a paddy soil under pre-defined redox conditions (Eh, from -400 to +300 mV). Average concentrations (μg L^-1) of dissolved Cd increased under reducing conditions up to 10.9 in the control soil, and decreased under oxidizing conditions to below the detection limit (LDL = 2.7) in the raw and Fe-rich biochar treated soils. Application of the raw biochar decreased the concentrations of dissolved Cd by 43-59% under Eh ≤ -100 mV, compared to the non-treated control, which was more effective than the Fe-rich biochar (31-59%) and the P-rich biochar (8-19%). The immobilization of Cd under low Eh might be due to its precipitation with sulfide (S^2-), whereas its immobilization under high Eh might be due to the associated increase of pH. Concentrations (μg L^-1) of Pb ranged from 29.4 to 198.2 under reducing conditions, and decreased to LDL (12.5) under oxidizing conditions. The P-rich biochar was more effective in immobilizing Pb than the raw and Fe-rich biochars, particularly under Eh ≤ 0 mV (55-82%), which might be due to the retention of Pb by phosphates. The raw and Fe-rich biochars immobilized Pb under low Eh (≤ -300 mV), but both biochars, particularly the Fe-rich biochar mobilized Pb under Eh higher than -200 mV, especially at +100 mV, due to the decrease of pH at this point (pH = 6.0 to 6.5). These results improved our understanding of using P-rich and Fe-rich functionalized biochars for the immobilization of Cd and Pb in a paddy soil under stepwise redox changes. The amendment of P-rich pig carcass-derived biochar to paddy soils could be a promising approach for mitigating the risk of Pb for human health and the environment. The raw and Fe-rich green waste-derived biochars can be used for immobilizing Cd and mitigating its risk in paddy soils under both reducing and oxidizing conditions.

A review: Application of tourmaline in environmental fields

Heavy metals and organic pollutants could pose long-term threats to the ecosystem and human health, so it is urgent for us to find a friendly and efficient material to remove pollutants in environment. Since tourmaline is widely distributed in natural environment and has many excellent physical and chemical properties including radiating far infrared energy, permanently releasing negative ions, producing an electrostatic field, releasing rare microelements, and stimulating the growth and metabolism of microorganisms and plants, tourmaline had been conducted to alleviate environmental pollution. This review summarizes the application of tourmaline in aqueous solutions and soil polluted by heavy metals and organic pollutants, the factors that affect the removal of pollutants by tourmaline and the removal mechanisms. In addition, to ensure the safe use of tourmaline, this review also elaborates the environment risks of tourmaline through its toxicity indexes to soil and plant.

Plumbojarosite formation in contaminated soil to mitigate childhood exposure to lead, arsenic and antimony

In this study, a novel method for lead (Pb) immobilization was developed in contaminated soils using iron (III) (Fe³⁺) in conjunction with 0.05 M H₂SO₄. During method optimization, a range of microwave treatment times, solid to solution ratios, and Fe₂(SO₄)₃/H₂SO₄ concentrations were assessed using a mining/smelting impacted soil (BHK2, Pb: 3031 mg/kg), followed by treatment of additional Pb contaminated soils (PP, Pb: 1506 mg/kg, G10, Pb: 2454 mg/kg and SoFC-1, Pb: 6340 mg/kg) using the optimized method. Pb bioaccessibility was assessed using USEPA Method 1340, with Pb speciation determined by X-ray Absorption (XAS) spectroscopy. Treatment efficacy was also validated using an in vivo mouse assay, where Pb accumulation in femur, kidney and liver was assessed to confirm in vitro bioaccessibility outcomes. Results showed that Pb bioaccessibility could be reduced by 77.4-97.0% following treatment of soil with Fe₂(SO₄)₃ (0.4-1.0 M), H₂SO₄ (0.05 M) at 150 °C for 60 min in a closed microwave system. Results of bioavailability assessment demonstrated treatment effect ratio of 0.06-0.07 in femur, 0.06-0.27 in kidney and 0.06-0.11 in liver (bioavailability reduction between 73% and 93%). Formation of plumbojarosite in treated soils was confirmed by XAS analysis.

Effect of organosilicone and mineral silicon fertilizers on chemical forms of cadmium and lead in soil and their accumulation in rice

Cadmium (Cd) and lead (Pb) pollution in soil and their accumulation in edible parts possess a worldwide eco-environmental and health risk, especially in developing countries. Recently, organosilicone fertilizer (OSiF) has been reported to reduce uptake of heavy metals, but the effectiveness has not been verified and its associated mechanisms are not fully understood. This work investigated whether and how OSiF and mineral silicon fertilizer (MSiF) affect mitigation of Cd and Pb stress in rice (Oryza sativa). Both soil incubation and pot experiments were conducted to assess the effect of OSiF and MSiF on bioavailability of Cd and Pb in soil and their accumulation in rice. Additionally, a hydroponic experiment was conducted to study whether Si in rice can alleviate Cd stress. We found that both Si fertilizers could increase soil pH, induce the transformation of the acid soluble and reducible fractions of Cd and Pb to the oxidizable and residual fractions in soil, decreasing their bioavailability and the uptake of Cd and Pb in rice. However, Si in OSiF was not phyto-available, but Si in MSiF was available since available Si in soil and Si in plant increased in MSiF treatments but not in OSiF treatments. Meanwhile, rice grain yields significantly increased and the Cd and Pb content of brown rice reduced in MSiF treatments but not in OSiF treatments. In addition, Si was found to be able to alleviate Cd stress by improving the antioxidant capacity of rice. These results suggested that the decreased Cd and Pb accumulation in OSiF-treated rice was due to Cd and Pb immobilization in soil simply with pH increase, but in MSiF-treated rice Cd and Pb immobilization in soil (ex planta effect) and Si-conferred inhibitory effect of root-to-shoot Cd and Pb transport (in planta effect) contribute to the lower accumulation in rice.

Early warning on risk development in compound lead and cadmium contaminated sites

Based on the transformation among metal fractions defined by the Tessier sequential extraction procedure and integrated risk information assessed by delayed geochemical hazard (DGH) methodology, including development paths and their burst probabilities, trigger conditions, and the contribution of each metal to risk development, an approach was proposed to provide an early warning on risk development in metal compound-contaminated sites and tested in a lead and cadmium-contaminated site. Risk assessment indicated that the site was at a high to extremely high ecological risk. DGH analysis revealed that the transformation from the fraction bound to carbonate and organic matter to the exchangeable fraction was dominant in the development of either single or combined lead and cadmium risk, which was triggered by soil acidification and the continuous decline of soil organic matter; risk development might have occurred in 6.52-80.4% of the case site with burst probabilities of 6.52-80.4%, 8.70-39.1% and 8.70-80.4% for lead risk, cadmium risk and combined lead-cadmium risk, respectively; with the dominant role of lead, the two metals overall accelerated the development of their compound risk by changing each other's DGH paths. The proposed DGH-based approach is promising for early warning on risk development in compound contaminated sites.

Comparative effects of Tagetes patula L. extraction, mercapto-palygorskite immobilisation, and the combination thereof on Cd accumulation by wheat in Cd-contaminated soil

Phytoextraction and in situ immobilisation are two of the most commonly used remediation techniques for Cd-contaminated farmland. In theory, phytoextraction followed by immobilisation can reduce the total Cd and available Cd contents of the soil, making it suitable for the remediation of heavily Cd-contaminated alkaline soil. However, the real remediation efficiency is uncertain, and it is also unknown whether phytoextraction affects subsequent wheat Cd accumulation. In this study, two seasonal pot experiments were conducted to determine the effects of S,S-ethylenediamine disuccinic acid (EDDS)-assisted Tagetes patula L. (T. patula) extraction, mercapto-palygorskite (MPAL) immobilisation, and the combination thereof on subsequent Cd accumulation in wheat. EDDS application significantly increased the Cd content in the subsequent-soil solution, but the EDDS-activated Cd could not be absorbed by wheat roots. T. patula extraction decreased the subsequent soil pH value by 0.1-0.2 pH units, increased the available Cd content by 0.19 mg/kg, but had no effect on subsequent wheat Cd accumulation. The Cd absorption capacity of wheat roots and the Cd translocation capacity of wheat stems to grains of high-Cd wheat were higher than that of low-Cd wheat cultivar. The application of MPAL had no effect on soil pH value, but significantly decreased soil available Cd and exchangeable Cd contents by 17.78-36.76% and 21.13-52.63%; it also increased the Fe/Mn oxide-bound Cd fraction by 14.02-64.00%. MPAL application decreased the wheat grain Cd concentrations from 0.51 to 0.13 mg/kg (high-Cd wheat) and 0.35 to 0.05 mg/kg (low-Cd wheat), but had no negative effect on Fe, Mn, Cu, and Zn elements. Compared with the single MPAL application treatments, the combination treatments had no inhibition effect on Cd accumulation in wheat. MPAL is an efficient amendment, and considering the remediation efficiency, stability, and time of these methods, the combination of MPAL application with a low-Cd accumulation wheat cultivar represents a suitable proposal to ensure the safe production of wheat in Cd-contaminated alkaline soil.

Cadmium accumulation in rice straws and derived biochars as affected by metal exposure, soil types and rice genotypes

Straw residues, as one of the conservation farming practices, are being strongly encouraged in China, including some cadmium-polluted areas. Nowadays, a portion of this plant residue is promoted to be removed for reducing excess metal(loid) in the soil and to be used for bioenergy production. Nevertheless, the possible influences of contaminated straw or the burial of its derived biochars on Cd accumulation in soil and data based on health risk assessment associated with different status and extent of soil contamination were relatively unknown. Thus it is important to provide a more systematical understanding of contaminated straw burial at specific contamination zones, which may provide useful guidance for straw utilization. In this study, we harvested two genotypes of rice straw from 6 contaminated levels among three soil types to comprehensively study the total Cd contents in straws and its derived biochars and correlate the sets of straw characteristics and Cd contents in three different contamination zones. The total Cd concentration in straws grew at a steady rate relatively with increasing soil Cd contamination levels, compared to those in biochars which performed more fluctuate due to the strong burning. According to correlational analyses, three-way ANOVA showed that the moisture, ash, volatile and fixed carbon were all significantly affected by straw Cd_Total contents (p < 0.001). Such relationships were attributed to guide straw removal portions for gasification. Meanwhile, there was a significant correlation between straw Cd concentration and soil types (p < 0.001), confirming that it might be worth determining soil remediation by straw removal according to site-specific farmland conditions. This work will help to assess efforts toward predicting Cd concentration in the paddy soils related to kinds of contamination status and would also give useful guidance to make sustainable management strategies for crop straws in polluted regions.Novelty statement This work provided data on how much rice straw is needed to remove to ensure the minimal amount to control soil contamination and reduce costs according to site-specific conditions and soil Cd contamination status. It also explains the correlations between straw characteristics related to bioenergy use and soil conditions which would give guidance to balance using crop straw for increased bioenergy production and the need to also protect, preserve, and enhance soil resources.

Effects of Long-Term Freeze-Thaw Cycles on the Properties of Stabilized/Solidified Lead-Zinc-Cadmium Composite-Contaminated Soil

Lead, zinc, and cadmium were used to prepare a composite-contaminated soil to replicate common situations, in which soil is usually simultaneously contaminated by multiple metals. To examine the long-term durability of stabilized/solidified (S/S) contaminated soil, specimens were subjected to a series of freeze–thaw (F-T) cycles, up to ninety times (one day per cycle), prior to testing. Triaxial compression tests, soil column leaching tests, and X-ray diffraction analysis were then employed to study the mechanical properties, environmental influences, and micro-mechanisms of the S/S lead-zinc-cadmium composite-contaminated soils after long-term F-T. The results showed that triaxial compressive strength increases within three F-T cycles, then decreases before slightly increasing or stabilizing after thirty F-T cycles. The stage of decreased cohesion thus occurs between three and fourteen F-T cycles, with variation in other factors similar to that of the triaxial compressive strength. The cohesion mainly increases between three and seven cycles. The soil column leaching test showed that the permeability of soil is more than four times higher than that of soil not subject to freeze–thaw cycles after ninety F-T cycles. XRD tests further revealed that the chemical composition of S/S contaminated soil and the occurrence of each heavy metal (HM) remained unchanged under F-T treatment.