Reliability and stability of immobilization remediation of Cd polluted soils using sepiolite under pot and field trials

Accumulation and bioavailability of heavy metals in an acid soil and their uptake by paddy rice under continuous application of chicken and swine manure

Heavy metal contamination of agricultural soils is a global concern, as it can cause the accumulation of heavy metals in food. In this study, a field experiment was carried out to investigate the effect of the continuous application of chicken or swine manure on the Pb, Cd, Cr and As bioavailability, fractionation, and accumulation in soil and uptake by rice plants. Results showed that chicken or swine manure significantly reduced the Cd and Pb contents in rice grain by 7.8-79.3% and 7.2-59.4%, respectively, with increasing application rates and number of years; the exchangeable Cd and Pb fractions, and the diethylenetriaminepentaacetic acid (DTPA)-extractable Cd and Pb in the soil were also decreased. Furthermore, the application of chicken or swine manure substantially increased the DTPA-extractable As and exchangeable As fractions in the soil but had limited effect on As accumulation in rice grain. No significant differences in the bioavailability in soil nor accumulation in the rice grain were found for Cr between the treatments. Therefore, livestock manure can be used as soil amendments to decrease Cd and Pb accumulation in rice grains, nevertheless, the potential risk of metal accumulation in soils caused by livestock manure application should be considered.

Bioavailability of cadmium to celery (Apium graveolens L.) grown in acidic and Cd-contaminated greenhouse soil as affected by the application of hydroxyapatite with different particle sizes

Cadmium contamination in greenhouse vegetable fields greatly limited the sustainable production especially of leafy vegetables. Hydroxyapatite (HAP), as a common soil amendment, has been widely used in the remediation of Cd-contaminated soils, while its remediation efficiency greatly depends on its particle sizes. In this study, a rhizobag pot experiment was conducted to investigate the effects of HAP (<60 nm, <12 μm and <80 μm) on bioavailability of Cd to celery grown in acidic and slightly Cd-contaminated greenhouse soil. The results suggested that HAP with the largest particle size (<80 μm) had the best effectiveness in reducing Cd uptake especially by the edible part of celery. Specifically, the increase in HAP (<80 μm) addition from 0.5% to 3% prominently reduced Cd concentrations in celery shoot by 19.6%-76.8% as compared with the untreated group. Also, adding HAP (<80 μm) especially at 3% significantly decreased translocation factor (TF) of Cd from celery root to shoot by 30.6% and reduced bioconcentration factor (BCF) of Cd from rhizosphere soil to celery shoot by 76.4%. These were predominantly associated with the significantly increased soil pH and the subsequently decreased soil CaCl₂-Cd concentration after adding HAP (<80 μm). Overall, although rhizosphere soil pH was the key factor in controlling Cd uptake by edible celery and regulating BCF and TF of Cd, insignificant root-induced acidification had limited effect on the immobilization efficiency of Cd by HAP (<80 μm). In conclusion, HAP (<80 μm) has good potential for the remediation of Cd-contaminated greenhouse soils.

The effect of sepiolite application on rice Cd uptake - A two-year field study in Southern China

Sepiolite (SEP) is a clay mineral with great potential to stabilize soil heavy metals. A two-year field experiment was conducted to explore the optimum use of SEP to immobilize soil Cd and to promote the consumption safety of rice grown in a typical paddy field in Southern China. SEP was applied once or twice over the two-year study at three levels (0.1, 0.5, and 1%, w/w) before rice planting. The results showed that SEP effectively reduced rice grain Cd concentrations by 47-49% in the first year and by 44-50% in the second year due to the residue effect. Application of SEP for two consecutive years reduced the rice grain Cd concentration by up to 75%, achieving a safe level (<0.2 mg kg^-1). SEP also reduced Zn concentrations in rice grains (by 6-10%), while the Cd/Zn ratios of rice grains were decreased by 24-72% over the two years, implying it was much safer for consumption. SEP significantly increased the soil pH (0.9-1.8 units) and available phosphorus, and it reduced the soil available Cd (by 20-95%) and Zn concentrations (by 30-99%). In brief, SEP effectively stabilized soil Cd and reduced uptake by rice; the effect was dose-dependent and 0.5% (w/w) was optimum in the present study. The main mechanism of SEP to stabilize soil Cd is the increase in soil pH analogous to liming. This study shows that SEP application can be an efficient way to remediate Cd contaminated rice paddies and fulfill the goal of safe production of rice and thereby reduce the health risks associated with consuming rice.

Simultaneous in-situ remediation and fertilization of Cd-contaminated weak-alkaline farmland for wheat production

In-situ remediation of heavy metal-contaminated farmland mainly focuses on acidic soil, however, weak-alkaline farmland widely exists in north China. Meanwhile, fertilization is usually ignored, but it may influence remediation efficiency as well as grain production. In this paper, field experiments were carried out to investigate in-situ simultaneous remediation and fertilization of Cd-contaminated weak-alkaline soil by microbial agent mixed with fulvic acid (MFA), wheat straw biochar, sepiolite and their mixture. Results showed that addition of these conditioners decreased the soil available Cd by 39.86%-71.33% and the wheat Cd by 41.94%-87.10%. The decrease order of soil available Cd followed sepiolite > mixture > biochar > MFA, while the decrease order of wheat Cd was mixture > sepiolite > biochar > MFA. With addition of mixture, the wheat Cd reduced to 0.08 mg/kg, lower than the Cd limit of 0.1 mg/kg in Contaminant Limit in Food of National Food Safety Standards (GB2762-2017), and the highest wheat yield reached 7590 kg/hm². The MFA had significant effects on improvement of soil organic matters, nutrients and rhizosphere microbes; the biochar was prominent in improving soil organic matters, inhibiting wheat Cd and soil available Cd; the sepiolite had obvious advantages in reducing wheat Cd and soil available Cd; and the mixture had a more balanced effect on soil remediation and fertilization. Correlation study showed that soil available Cd significantly affected the uptake of Cd by wheat, and wheat yield was significantly positively correlated with soil organic matters, available N. Therefore, reducing soil available Cd, increasing soil organic matters and nutrients are the keys to simultaneous remediation and fertilization of Cd-contaminated weak-alkaline soil for wheat production.

The stabilizing mechanism of cadmium in contaminated soil using green synthesized iron oxide nanoparticles under long-term incubation

Despite numerous studies having been conducted on the stabilization of heavy metal contaminated soil, our understanding of the mechanisms involved remains limited. Here green synthesized iron oxide nanoparticles (GION) were applied to stabilize cadmium (Cd) in a contaminated soil. GION not only stabilized soil Cd, but also improved soil properties within one year of incubation. After GION application both the exchangeable and carbonate bound Cd fractions decreased by 14.2-83.5% and 18.3-85.8% respectively, and most of the Cd was translocated to the residual Cd fraction. The application of GION also strongly altered soil bacterial communities. In GION treatments, the abundance of Gemmatimonadetes, Proteobacteria, and Saccharibacteria increased which led to a shift in the dominant bacterial genera from Bacillus to Candidatus koribacter. The variation in bacteria confirmed the restoration of the contaminated soil. The most abundant bacterial genus and species found in GION treatments were related to (i) plant derived biomass decomposition; (ii) ammoxidation and denitrification; and (iii) Fe oxidation. GION application may enhance the formation of larger soil aggregates with anaerobic centers and coprecipitation coupled Fe (II) oxidization, ammoxidation and nitrite reduction followed by Fe mineral ripening may be involved in Cd stabilization. The predominant stabilization mechanism was thus coprecipitation-ripening-stabilization.

Inhibition of Cd accumulation in winter wheat (Triticum aestivum L.) grown in alkaline soil using mercapto-modified attapulgite

Cd contamination in agricultural soils threatens the safety of agricultural products and poses human health risk via food chain. However, the remediation of Cd polluted alkaline soils has not drawn the public concern, and the corresponding efficient amendments that can reduce Cd accumulation in crop grains are relatively few. In current study, mercapto-modified attapulgite (MA in abbreviation) was selected as the amendment to conduct winter wheat (Triticum aestivum L.) cultivation pot experiment to investigate the effect of MA on Cd accumulation in winter wheat and Cd bioavailability in alkaline soil. MA had no adverse impact on the normal growth of winter wheat but could inhibit Cd accumulation in wheat grain of both cultivars grown in alkaline soil with a maximum reduction of 75%, while pH-regulating amendment sepiolite had no reduction effect. In the term of soil chemistry, MA could decrease the zeta potential of soil particles and enhance the sorption amount of Cd on soil particles, resulted in the increase of Fe-Mn-oxides bounded Cd fraction in alkaline soil. The enhanced sorption effect combined with complexation effect of MA itself, made the exchangeable and bioavailable Cd concentrations in the soil decrease. In the term of plant uptake, MA could inhibit the uptake of Cd via roots from the soil, and hinder Cd transfer from roots to grains. MA had environmental friendliness and capability in the aspect of soil pH, effective cation exchange capacity and available micronutrients in the soil. The high performance of MA in inhabitation of Cd in winter wheat revealed that it was an efficient immobilization agent with great application potential for Cd-polluted alkaline soil.

Immobilization of cadmium in polluted soils by phytogenic iron oxide nanoparticles

While phytogenic nanomaterials have been successfully used to remove heavy metals in wastewater, the potential to successfully use such materials to immobilize heavy metals in soils is still unclear. In this study, phytogenic iron oxide nanoparticles (PION) were used to immobilize cadmium (Cd) in six soils. Amendment with PION effectively immobilized Cd, with a concomitant increase in the concentrations of iron oxides, soil pH and dissolved organic carbon (DOC) under both oxic and anoxic conditions. However, observed changes in soil properties and Cd fractions were different under oxic and anoxic conditions. After PION application, the exchangeable Cd fraction decreased by up to 91 and 69%, while the carbonate bound Cd fraction decreased by up to 61 and 75%, under oxic and anoxic conditions, respectively. Pearson correlation analysis revealed that under both oxic and anoxic conditions, Cd fractions were significantly and positively correlated with free iron oxide content and pH, where free iron oxide content was positively correlated with amorphous iron oxide, DOC and pH. The Cd immobilization mechanisms potentially involved either (1) formation of insoluble hydroxides at elevated pH; (2) participation of biomolecules released from PION in ligand complexation with Cd and (3) co-precipitated of Cd during the formation of iron oxides. This study provided new insights into the potential effects of PION applications for practical Cd immobilization in contaminated soils.

Effects of soil amendments applied on cadmium availability, soil enzyme activity, and plant uptake in contaminated purple soil

Soil Cd pollution resulting from human activities has become a serious food safety concern. This study was to investigate the impacts of eight soil amendments including a humic acid material (XZ), three organic-inorganic combined materials (FY, PX and PLY) and four inorganic materials (FS, LIME, PLL and PL) on soil Cd availability, enzyme activity, and the uptake by vegetables under incubation, greenhouse and field conditions. Results showed that soil exchangeable Cd (EX-Cd) concentrations under four amendments (FY, XZ, PX, FS) significantly decreased by 12.10-13.59% after 20d treatments, and both PX and PLY resulted in reduced soil EX-Cd fraction while increased fractions of Fe-Mn oxides (OX-Cd), organic matter (OM-Cd) and residual (Res-Cd) in the Cd-contaminated purple soil. PX significantly reduced soil EX-Cd and carbonate-bound (CB-Cd) fractions by 4.4% and 11.4%, and decreased vegetable Cd uptake by 38.8% and 49.1% in greenhouse and field experiments, respectively. Moreover, PX elevated the activities of soil catalase, urease and saccharase by 15.7%, 53.6% and 48.2% in pot soil, which were ~4, ~5 and ~14 times higher those in field soil, respectively. This research demonstrated that PX could be an effective soil amendment to reduce the Cd health and ecological riskthe 1s in the Cd-contaminated purple soil.

Field experiment on the effects of sepiolite and biochar on the remediation of Cd- and Pb-polluted farmlands around a Pb-Zn mine in Yunnan Province, China

The effects of sepiolite and biochar on the contents of available nutrients (N, P, and K); the chemical forms and available contents of Cd and Pb in soils; the biomass and growth of maize; and the contents of nutrients, Cd, and Pb in maize were studied in situ in Cd- and Pb-polluted farmlands around the Lanping Pb-Zn mine in Yunnan Province, China. Results demonstrated that sepiolite did not influence the contents of available nutrients in soils, although it significantly increased the pH value and decreased available Cd (CaCl₂-extractable and exchangeable) contents and exchangeable and reducible Pb. Moreover, sepiolite increased the biomass in the aboveground part of maize, resulting in the reduction of Cd contents in maize plants and grains by 25.6-47.5%. Meanwhile, the biochar increased the contents of available nutrients in soils and decreased the contents of exchangeable Pb in soils and biomass in the aboveground part of maize plants and grains; decreased the Cd contents in maize stems and grains by 26.7% and 24.6%, respectively; and decreased the Pb content in roots by 16.2%. However, neither sepiolite nor biochar had considerable influence on the Pb content in maize grains. According to a correlation analysis, soil pH has extremely significant negative correlations with available Cd content in soils, which in turn have extremely significant positive correlation with the Cd content in maize plants and grains. These results revealed that sepiolite increases soil pH and decreases Cd bioavailability in farmland soils around the Pb-Zn mine. Furthermore, biochar increases the contents of available nutrients in farmland soils and the maize yield. Sepiolite and biochar both decrease the contents and transfer coefficients of Cd in maize plants and grains and are, thus, applicable to the immobilization remediation of Cd-polluted farmlands.

Effect of Liming with Various Water Regimes on Both Immobilization of Cadmium and Improvement of Bacterial Communities in Contaminated Paddy: A Field Experiment

Cadmium (Cd) in paddy soil is one of the most harmful potentially toxic elements threatening human health. In order to study the effect of lime combined with intermittent and flooding conditions on the soil pH, Cd availability and its accumulation in tissues at the tillering, filling and maturity stages of rice, as well as enzyme activity and the microbial community in contaminated soil, a field experiment was conducted. The results showed that liming under flooding conditions is a more suitable strategy for in situ remediation of Cd-contaminated paddy soil than intermittent conditions. The availability of Cd in soils was closely related to the duration of flooding. Liming was an effective way at reducing available Cd in flooding soil because it promotes the transformation of Cd in soil from acid-extractable to reducible fraction or residual fraction during the reproductive growth period of rice. Compared with control, after liming, the concentration of Cd in brown rice was reduced by 34.9% under intermittent condition while reduced by 55.8% under flooding condition. Meanwhile, phosphatase, urease, and invertase activities in soil increased by 116.7%, 61.4% and 28.8%, and 41.3%, 46.5% and 20.8%, respectively. The high urease activity in tested soils could be used to assess soil recovery with liming for the remediation of contaminated soil. Soil microbial diversity was determined by the activities of soil acid phosphatase, urease and available Cd by redundancy analysis (RDA). The results indicated that the problem of Cd-contaminated paddy soil could achieve risk control of agricultural planting by chemical treatment such as lime, combined with various water regimes.

In-Situ Remediation of Cadmium and Atrazine Contaminated Acid Red Soil of South China Using Sepiolite and Biochar

The in-situ immobilization effect of sepiolite and biochar on the compound pollution of cadmium (Cd) and atrazine in acid red soil of south China was studied. Results showed that in a certain concentration range, sepiolite and biochar can improve the germination rate, ensure normal growth and increase biomass of pakchoi, reduce the content of Cd in edible parts of pakchoi in different degrees and meet international standards, and restrain absorption of atrazine in pakchoi completely. Sepiolite was more effective than biochar for the immobilization of Cd, while biochar was more effective than sepiolite for the immobilization of atrazine. Combined application of sepiolite and biochar in soil contaminated by Cd and atrazine was better than individual applications of sepiolite or biochar. The remediation method applied in this research can effectively repair the acid red soil with combined pollution of Cd and atrazine in south China, while also providing ideas for in-situ remediation of farmland soil with combined pollution of pesticides and metals.

An overview of field-scale studies on remediation of soil contaminated with heavy metals and metalloids: Technical progress over the last decade

Soil contamination by heavy metals and metalloids has been a major concern to human health and environmental quality. While many remediation technologies have been tested at the bench scale, there have been only limited reports at the field scale. This paper aimed to provide a comprehensive overview on the field applications of various soil remediation technologies performed over the last decade or so. Under the general categories of physical, chemical, and biological approaches, ten remediation techniques were critically reviewed. The technical feasibility and economic effectiveness were evaluated, and the pros and cons were appraised. In addition, attention was placed to the environmental impacts of the remediation practices and long-term stability of the contaminants, which should be taken into account in the establishment of remediation goals and environmental criteria. Moreover, key knowledge gaps and practical challenges are identified.

Stability of immobilization remediation of several amendments on cadmium contaminated soils as affected by simulated soil acidification

Chemical immobilization is a practical approach to remediate heavy metal contamination in agricultural soils. However, the potential remobilization risks of immobilized metals are a major environmental concern, especially in acid rain zones. In the present study, changes in the immobilization efficiency of several amendments as affected by simulated soil acidification were investigated to evaluate the immobilization remediation stability of several amendments on two cadmium (Cd) contaminated soils. Amendments (hydrated lime, hydroxyapatite and biochar) effectively immobilized Cd, except for organic fertilizer, and their immobilizations were strongly decreased by the simulated soil acidification. The ratio of changes in CaCl₂-extractable Cd: pH (△CaCl₂-Cd/△pH) can represent the Cd remobilization risk of different amended soils. Hydroxyapatite and biochar had a stronger durable immobilizing effect than did hydrated lime, particularly in soil with a lower pH buffering capacity, which was further confirmed by the Cd concentration and accumulation in lettuce. These results can be attributed to that hydroxyapatite and biochar transformed greater proportions of exchangeable Cd to other more stable fractions than lime. After 48 weeks of incubation, in soil with a lower pH buffering capacity, the immobilization efficiencies of lime, hydroxyapatite, biochar and organic fertilizer in the deionized water group (pH 6.5) were 71.7%, 52.7%, 38.6% and 23.9%, respectively, and changed to 19.1%, 33.6%, 26.5% and 5.0%, respectively, in the simulated acid rain group (pH 2.5). The present study provides a simple method to preliminarily estimate the immobilization efficiency of amendments and predict their stability in acid rain regions before large-scale field application. In addition, hydrated lime is recommended to be combined with other acid-stable amendments (such as hydroxyapatite or biochar) to remediate heavy metal-contaminated agricultural soils in acid precipitation zones.

Effects of Bacillus subtilis and nanohydroxyapatite on the metal accumulation and microbial diversity of rapeseed (Brassica campestris L.) for the remediation of cadmium-contaminated soil

This study investigated the effects of the co-application of Bacillus subtilis and nanohydroxyapatite (NHAP) on plant growth, soil cadmium (Cd) dynamics, and the microbiological characteristics (such as enzyme activity and bacterial species richness) of the rhizosphere soil. Rapeseed was used as a model plant in pot experiments. Different concentrations of B. subtilis and 0.5% NHAP were applied alone and in combination to Cd-contaminated soil. The Cd contents in soils and plants as well as the rhizospheric microorganism diversity were assessed. The addition of B. subtilis or NHAP alone increased the soil Cd content and decreased the plant Cd content, while their co-application more effectively increased the soil and plant Cd contents than either treatment alone. B. subtilis and NHAP reduced the plant Cd content by 43.15-57.04% compared with that in the control. Rhizosphere community richness and bacterial diversity were significantly increased after co-application of B. subtilis and NHAP. Co-application of B. subtilis and NHAP effectively promoted rapeseed growth and improved Cd-contaminated soil remediation.

Effect of nanohydroxyapatite on cadmium leaching and environmental risks under simulated acid rain

Cadmium (Cd) contamination in soil is a global environmental pollution issue. Nanohydroxyapatite (NHAP) has been used in soil remediation to immobilize cadmium in contaminated soils. However, the effect of acid rain on the export of cadmium from topsoil and its behavior in deep soil and leachate is unclear. In this study, column experiments and development of parsimonious model were performed to estimate Cd leaching behavior from topsoil and environmental risk of groundwater after 0.5% NHAP remediation. Four leaching events were performed and total Cd, different fractions of Cd determined by sequential extraction procedure and pH were determined for each leaching. The results show that with the export of Cd in topsoil, the total Cd concentration in soil at different depths had the following vertical distributions: 0-5 cm > 5-10 cm > 10-15 cm > 15-20 cm. NHAP treatment increased the soil pH and decreased Cd leaching loss by 56.45% compared to the control, and the results fit the parabolic diffusion model. With sequential extraction it was observed that NHAP application increased the residual fraction of cadmium in soil. After leaching, there was a positive correlation between soil pH and Cd concentration with regards to the exchangeable, reducible, oxidizable and residual Cd fractions. The parabolic diffusion model showed that Cd-contaminated soil with NHAP remediation is harmless to humans after sufficient remediation duration, whereas the resultant concentrations from the CK treatment could be toxic. The results indicate that nanohydroxyapatite could significantly reduce the bioavailability of cadmium and the environmental risk. However, the release of Ca and P from the dissolution of NHAP should be carefully studied as this will impact the mobilization of Cd or colloid Cd, and high leaching of P may result in P-induced eutrophication risk.

Amendment of vanadium-contaminated soil with soil conditioners: A study based on pot experiments with canola plants (Brassica campestris L.)

We performed pot experiments with canola plants (Brassica campestris L.) to evaluate the effect of eight soil conditioners on the amendment of vanadium (V)-contaminated soil based on analysis of the growth of canola plants and the uptake, bioaccumulation, and translocation of heavy metals. Tested soil conditioners included polyacrylamide (PAM), sepiolite, humic acid (HAC), peat, sludge compost (SC), bentonite, lime, and fly ash. Results from the analysis of the growth of canola plants and the analysis of variance showed that the best soil conditioners for V-contaminated soil were 0.05-0.1 wt% PAM, 1 wt% peat, 1 wt% HAC, and 1 wt% SC; moderately effective soil conditioners included sepiolite and lime. The best combination of soil conditioners was 0.1 wt% PAM, 1 wt% HAC, and 0.15 wt% lime, in addition of 1% ZVI, which increased the biomass and height of canola plants by 1.18-fold and 59.49%, respectively. We conclude that the best combination of soil conditioners determined from this study is promising for mitigating V contamination in soil.

Phytostabilization potential of ornamental plants grown in soil contaminated with cadmium

In a greenhouse experiment, five ornamental plants, Osmanthus fragrans (OF), Ligustrum vicaryi L. (LV), Cinnamomum camphora (CC), Loropetalum chinense var. rubrum (LC), and Euonymus japonicas cv. Aureo-mar (EJ), were studied for the ability to phytostabilization for Cd-contaminated soil. The results showed that these five ornamental plants can grow normally when the soil Cd content is less than 24.6 mg·kg^-1. Cd was mainly deposited in the roots of OF, LV, LC and EJ which have grown in Cd-contaminated soils, and the maximum Cd contents reached 15.76, 19.09, 20.59 and 32.91 mg·kg^-1, respectively. For CC, Cd was mainly distributed in the shoots and the maximum Cd content in stems and leaves were 12.5 and 10.71 mg·kg^-1, however, the total amount of Cd in stems and leaves was similar with the other ornamental plants. The enzymatic activities in Cd-contaminated soil were benefited from the five tested ornamental plants remediation. Soil urease and sucrase activities were improved, while dehydrogenase activity was depressed. Meanwhile, the soil microbial community was slightly influenced when soil Cd content is less than 24.6 mg·kg^-1 under five ornamental plants remediation. The results further suggested that ornamental plants could be promising candidates for phytostabilization of Cd-contaminated soil.

In situ immobilisation of toxic metals in soil using Maifan stone and illite/smectite clay

Clay minerals have been proposed as amendments for remediating metal-contaminated soils owing to their abundant reserves, high performance, simplicity of use and low cost. Two novel clay minerals, Maifan stone and illite/smectite clay, were examined in the in situ immobilisation of soil metals. The application of 0.5% Maifan stone or illite/smectite clay to field soils significantly decreased the fractions of diethylenetriaminepentaacetic acid (DTPA)-extractable Cd, Ni, Cr, Zn, Cu and Pb. Furthermore, reductions of 35.4% and 7.0% in the DTPA-extractable fraction of Cd were obtained with the Maifan stone and illite/smectite clay treatments, respectively, which also significantly reduced the uptake of Cd, Ni, Cr, Zn, Cu and Pb in the edible parts of Brassica rapa subspecies pekinensis, Brassica campestris and Spinacia oleracea. Quantitatively, the Maifan stone treatment reduced the metal uptake in B. rapa ssp. Pekinensis, B. campestris and S. oleracea from 11.6% to 62.2%, 4.6% to 41.8% and 11.3% to 58.2%, respectively, whereas illite/smectite clay produced reductions of 8.5% to 62.8% and 4.2% to 37.6% in the metal uptake in B. rapa ssp. Pekinensis and B. campestris, respectively. Therefore, both Maifan stone and illite/smectite clay are promising amendments for contaminated soil remediation.

Effects of combined amendments on crop yield and cadmium uptake in two cadmium contaminated soils under rice-wheat rotation

Soil cadmium (Cd) contamination in China has become a serious concern due to its high toxicity to human health through food chains. A pot experiment was conducted to investigate the effects of hydrated lime (L), hydroxyapatite (H) and organic fertilizer (F) alone or in combination to remedy a mild (DY) and a moderate (YX) Cd contaminated agricultural soil under rice-wheat rotation. Results showed that crops grain yield and Cd concentration, soil pH, CaCl₂ extractable Cd and Cd speciation were markedly affected by the amendments. In both cropping seasons, hydrated lime and hydroxyapatite significantly immobilized soil Cd, and hydroxyapatite, organic fertilizer significantly increased grain yield. Hydrated lime mainly increased soil carbonates bound Cd fractions resulted from 16.7% to 36.2% and from 16.8% to 28.3%, and hydroxyapatite increased Fe/Mn oxides Cd fractions from 19.3% to 33.4% and from 31.4% to 42.1% in the DY and YX soils, respectively; while organic fertilizer slightly increased soil exchangeable and organic matter bound Cd fractions. Besides, combined amendments contain alkaline materials and organic materials have the potential to decrease grain Cd and increase grain yield simultaneously. Therefore, in view of the effects of amendments on grain yield and Cd concentration, the cost as well as the potential benefits expected, combined amendments like hydrated lime + organic fertilizer, hydrated lime + hydroxyapatite + organic fertilizer are recommended in practical application. Mechanisms of Cd immobilization affected by amendments are mainly attributed to the changes in soil Cd availability and crops root uptake rather than internal translocation in plants.

Reduction of Cd accumulation in pak choi (Brassica chinensis L.) in consecutive growing seasons using mercapto-grafted palygorskite

MP significantly reduced Cd accumulation in the shoots of pak choi in comparison to the control, in consecutive growing seasons, and the minimum Cd contents were less than the maximum permitted level proposed by the Codex Alimentarius Commission.

Remediation mechanisms for Cd-contaminated soil using natural sepiolite at the field scale

Remediation of heavy metal polluted agricultural soil is essential for human health and ecological safety and remediation mechanisms at the microscopic level are vital for their large-scale utilization. In this study, natural sepiolite was employed as an immobilization agent for in situ field-scale remediation of Cd-contaminated paddy soil and the remediation mechanisms were investigated in terms of soil chemistry and plant physiology. Natural sepiolite had a significant immobilization effect for bioavailable Cd contents in paddy soil, and consequently could lower the Cd concentrations of brown rice, husk, straw, and roots of rice plants by 54.7-73.7%, 44.0-62.5%, 26.5-67.2%, and 36.7-46.7%, respectively. Regarding soil chemistry, natural sepiolite increased the soil pH values and shifted the zeta potentials of soil particles to be more negative, enhancing the fixation or sorption of Cd on soil particles, and resulted in the reduction of HCl and DTPA extractable Cd concentrations in paddy soil. Natural sepiolite neither enhanced nor inhibited iron plaques on the rice root surface, but did change the chemical environments of Fe and S in rice root. Natural sepiolite improved the activities of antioxidant enzymes and enhanced the total antioxidant capacity to alleviate the stress of Cd. It also promotes the synthesis of GSH and NPT to complete the detoxification. In general, the remediation mechanisms of natural sepiolite for the Cd pollutant in paddy soil could be summarized as the collective effects of soil chemistry and plant physiology.

Humic substances as a washing agent for Cd-contaminated soils

Cost-effective and eco-friendly washing agents are in demand for Cd contaminated soils. Here, we used leonardite-derived humic substances to wash different types of Cd-contaminated soils, namely, a silty loam (Soil 1), a silty clay loam (Soil 2), and a sandy loam (Soil 3). Washing conditions were investigated for their effects on Cd removal efficiency. Cadmium removal was enhanced by a high humic substance concentration, long washing time, near neutral pH, and large solution/soil ratio. Based on the tradeoff between efficiency and cost, an optimum working condition was established as follows: humic substance concentration (3150 mg C/L), solution pH (6.0), washing time (2 h) and a washing solution/soil ratio (5). A single washing removed 0.55 mg Cd/kg from Soil 1 (1.33 mg Cd/kg), 2.32 mg Cd/kg from Soil 2 (6.57 mg Cd/kg), and 1.97 mg Cd/kg from Soil 3 (2.63 mg Cd/kg). Cd in effluents was effectively treated by adding a small dose of calcium hydroxide, reducing its concentration below the discharge limit of 0.1 mg/L in China. Being cost-effective and safe, humic substances have a great potential to replace common washing agents for the remediation of Cd-contaminated soils. Besides being environmentally benign, humic substances can improve soil physical, chemical, and biological properties.

Combination of microbial oxidation and biogenic schwertmannite immobilization: A potential remediation for highly arsenic-contaminated soil

Here, a novel strategy that combines microbial oxidation by As(III)-oxidizing bacterium and biogenic schwertmannite (Bio-SCH) immobilization was first proposed and applied for treating the highly arsenic-contaminated soil. Brevibacterium sp. YZ-1 isolated from a highly As-contaminated soil was used to oxidize As(III) in contaminated soils. Under optimum culture condition for microbial oxidation, 92.3% of water-soluble As(III) and 84.4% of NaHCO₃-extractable As(III) in soils were removed. Bio-SCH synthesized through the oxidation of ferrous sulfate by Acidithiobacillus ferrooxidans immobilize As(V) in the contaminated soil effectively. Consequently, the combination of microbial oxidation and Bio-SCH immobilization performed better in treating the highly As-contaminated soil with immobilization efficiencies of 99.3% and 82.6% for water-soluble and NaHCO₃-extractable total As, respectively. Thus, the combination can be considered as a green remediation strategy for developing a novel and valuable solution for As-contaminated soils.

Suppressive effects of thermal-treated oyster shells on cadmium and copper translocation in maize plants

The effect of varied concentrations of thermal-treated oyster shells (TOS) on the suppression of cadmium (Cd) and copper (Cu) uptake and translocation into the shoots of maize plants was examined. Maize plants were grown in Cd- and Cu-contaminated Andosol for 70 days. The concentration of mobile Cd (extracted with 1 M NH₄NO₃) decreased with increasing TOS applications, whereas an increase in the concentration of mobile Cu in soil resulted from cumulative TOS additions. The addition of 2% TOS had no prohibitive effects on Cd uptake in maize shoots, but the 4 and 8% TOS treatments decreased Cd accumulation in shoots by 41 and 59%, respectively. The possible mechanisms underlying Cd suppression in maize shoots were the enhanced Cd adsorption caused by pH-induced increases in the negative charge of the soil and the antagonistic effects of Ca resulting from competition for exchange sites at the root surface. Cu accumulation in maize shoots increased by 34, 51, and 53% with the addition of 2, 4, and 8% TOS, respectively, but this increase was not observed for Cd accumulation. These results suggested that, in multi-metal-contaminated soils, attention should be paid to the potential mobility of target metals and the pH of the contaminated soil. From a plant physiological perspective, contaminated soils slightly reduced photosynthetic performance. However, the addition of TOS to the soil at levels higher than 4% substantially decreased photosynthetic performance, indicating that CaO-based suppressants at critical loads might damage the net photosynthetic rates of sensitive maize plants.

Distance-dependent varieties of microbial community structure and metabolic functions in the rhizosphere of Sedum alfredii Hance during phytoextraction of a cadmium-contaminated soil

The recovery of microbial community and activities is crucial to the remediation of contaminated soils. Distance-dependent variations of microbial community composition and metabolic characteristics in the rhizospheric soil of hyperaccumulator during phytoextraction are poorly understood. A 12-month phytoextraction experiment with Sedum alfredii in a Cd-contaminated soil was conducted. A pre-stratified rhizobox was used for separating sub-layer rhizospheric (0-2, 2-4, 4-6, 6-8, 8-10 mm from the root mat)/bulk soils. Soil microbial structure and function were analyzed by phospholipid fatty acid (PLFA) and MicroResp™ methods. The concentrations of total and specified PLFA biomarkers and the utilization rates for the 14 substrates (organic carbon) in the 0-2-mm sub-layer rhizospheric soil were significantly increased, as well as decreased with the increase in the distance from the root mat. Microbial structure measured by the ratios of different groups of PLFAs such as fungal/bacterial, monounsaturated/saturated, ratios of Gram-positive to Gram-negative (GP/GN) bacterial, and cyclopropyl/monoenoic precursors and 19:0 cyclo/18:1ω7c were significantly changed in the 0-2-mm soil. The PLFA contents and substrate utilization rates were negatively correlated with pH and total, acid-soluble, and reducible fractions of Cd, while positively correlated with labile carbon. The dynamics of microbial community were likely due to root exudates and Cd uptake by S. alfredii. This study revealed the stimulations and gradient changes of rhizosphere microbial community through phytoextraction, as reduced Cd concentration, pH, and increased labile carbons are due to the microbial community responses.

Thermal activation of serpentine for adsorption of cadmium

Thermal activated serpentine with high adsorption capacity for heavy metals was prepared. The batch experiment studies were conducted to evaluate the adsorption performance of Cd²⁺ in aqueous solution using thermal activated serpentine as adsorbent. These samples before and after adsorption were characterized by XRD, FT-IR, SEM, XPS, and N₂ adsorption-desorption at low temperature. It was found that serpentine with layered structure transformed to forsterite with amorphous structure after thermal treatment at over 700°C, while the surface area of the samples was increased with activated temperature and the serpentine activated at 700°C (S-700) presented the largest surface area. The pH of solution after adsorption was increased in different degrees due to hydrolysis of MgO in serpentine, resulting in enhancing adsorption of Cd²⁺. The S-700 exhibited the maximum equilibrium adsorption capacity (15.21mg/g), which was 2 times more than pristine serpentine. Langmuir isotherm was proved to describe the equilibrium adsorption data better than Freundlich isotherm and pseudo second order kinetics model could fit the adsorption kinetics processes well. Based on the results of characterization with XPS and XRD, the adsorption mechanisms could be explained as primarily formation of CdCO₃ and Cd(OH)₂ precipitation on the surface of serpentine.

The large-scale process of microbial carbonate precipitation for nickel remediation from an industrial soil

Microbial carbonate precipitation is known as an efficient process for the remediation of heavy metals from contaminated soils. In the present study, a urease positive bacterial isolate, identified as Bacillus cereus NS4 through 16S rDNA sequencing, was utilized on a large scale to remove nickel from industrial soil contaminated by the battery industry. The soil was highly contaminated with an initial total nickel concentration of approximately 900 mg kg^-1. The soluble-exchangeable fraction was reduced to 38 mg kg^-1 after treatment. The primary objective of metal stabilization was achieved by reducing the bioavailability through immobilizing the nickel in the urease-driven carbonate precipitation. The nickel removal in the soils contributed to the transformation of nickel from mobile species into stable biominerals identified as calcite, vaterite, aragonite and nickelous carbonate when analyzed under XRD. It was proven that during precipitation of calcite, Ni²⁺ with an ion radius close to Ca²⁺ was incorporated into the CaCO₃ crystal. The biominerals were also characterized by using SEM-EDS to observe the crystal shape and Raman-FTIR spectroscopy to predict responsible bonding during bioremediation with respect to Ni immobilization. The electronic structure and chemical-state information of the detected elements during MICP bioremediation process was studied by XPS. This is the first study in which microbial carbonate precipitation was used for the large-scale remediation of metal-contaminated industrial soil.

Immobilization of Cd in river sediments by sodium alginate modified nanoscale zero-valent iron: Impact on enzyme activities and microbial community diversity

This paper investigated how sodium alginate (SA)-modified nanoscale zero-valent iron (NZVI), play a constructive role in the remediation of cadmium (Cd) contaminated river sediments. The changes of the fraction of Cd, enzyme activities (urease, catalase, dehydrogenase) and bacterial community structures with the treatment by SNZVI were observed. The sequential extraction experiments demonstrated that most mobile fractions of Cd were transformed into residues (the maximum residual percentage of Cd increases from 15.49% to 57.28% after 30 days of incubation at 0.1 wt% SA), with the decrease of bioavailability of Cd. Exclusive of dehydrogenase, the activities of the other two enzymes tested were enhanced with the increase of incubation time, which indicated that dehydrogenase might be inhibited by ferric ions formed from SNZVI whereas no obvious inhibition was found for other enzymes. Polymerase chain reaction denaturing gradient gel electrophoresis (PCR-DGGE) analyses were used for the detection of microbial community changes, and the results showed that SNZVI and NZVI could increase bacterial taxa and improve bacterial abundance. All the experimental findings of this study provide new insights into the potential consequences of SNZVI treatments on the metal Cd immobilization in contaminated river sediments.