Using sequential extraction and DGT techniques to assess the efficacy of plant- and manure-derived hydrochar and pyrochar for alleviating the bioavailability of Cd in soils

Washing antimony and arsenic from agricultural soil with eco-friendly organic acids and the relevant bioavailability assessment

Antimony (Sb) and arsenic (As) contamination in agricultural soil poses human health risks through agricultural products. Soil washing with degradable low molecular weight organic acids (LMWOAs) is an eco-friendly strategy to remediate agricultural soils. In this study, three eco-friendly LMWOAs, oxalic acid (OA), tartaric acid (TA), and citric acid (CA), were used to treat Sb and As co-contaminated agricultural soil from Xikuangshan mine area. The OA, TA, and CA washed out 18.4, 16.8, and 26.6% of Sb and 15.3, 19.9, and 23.8% of As from the agricultural soil, with CA being the most efficient reagent for the soil washing. These organic acids also led to pH decline and macronutrients losses. Fraction analysis using a sequential extraction procedure showed that the three organic acids targeted and decreased the specifically sorbed (F2) (by 19.3-37.6% and 2.41-23.5%), amorphous iron oxide associated (F3) (by 49.1-61.2% and 51.2-70.2%), and crystallized iron oxide associated (F4) (by 12.3-26.0% and 26.1-29.1%) Sb and As. The leachability of Sb and As, as well as their concentrations and bioconcentration factor (BCF) in vegetables reduced due to the soil washing. It demonstrated that the bioavailability of both the elements was decreased by the organic acids washing. The concentrations of Sb and As in typical vegetable species cultivated in CA washed soil were less than the threshold value for consumption safety, while those in OA and TA washed soils were still higher than the value, suggesting that only CA is a potential washing reagent in soil washing for Sb- and As-contaminated agricultural soil.

Organic amendments perform better than inorganic amendments in reducing the absorption and accumulation of cadmium in lettuce

The main purpose of applying organic or inorganic amendments is to guarantee crop safe production in heavy metal contaminated soil. However, previous studies showed that the effects of organic or inorganic composite amendments on the cadmium (Cd) concentration of lettuce (Lactuca sativa var. ramosa Hort) were inconsistent. Accordingly, a sixty-day pot experiment was carried out to examine the impacts of the inorganic materials (lime, L and zeolite, Z), organic materials (biochar, B and compost, C), and their combination on the immobilization of Cd in soil and its uptake by lettuce. The objective was to identify the most suitable soil amendment combination that promotes safe lettuce production. The results revealed that the combined application of BC, LZC, and LBC significantly increased the plant height by 11.09-28.04% and fresh weight by 183.47-207.67%. This improvement can be attributed to enhanced soil quality, such as increased dissolved organic carbon (DOC) by 70.19-80.42%, soil respiration (SR) by 29.04-38.46%, and soil microbial carbon content (SMC) by 36.94-46.63%. Compared to inorganic fertilizers and their combination with organic amendments, organic amendments had a significant impact on reducing shoot Cd concentration by 33.93%-56.55%, while increasing the activity of catalase by 138.87-186.86%. And soil available Cd measured by diffusive gradients in thin-films (DGT-Cd) decreased 24.73-88.13% in all treatments. Correlation analysis showed that plant Cd concentration was significantly correlated with soil pH, SR, cation exchange capacity (CEC), DOC and SMC. These results demonstrated that organic amendments, especially the combination of biochar and compost, have greater potential than inorganic amendments and inorganic-organic combinations for realizing safe production of lettuce and improving soil quality in the Cd moderately contaminated acid farmland.

Mercapto-based palygorskite modified soil micro-biology and reduced the uptake of heavy metals by Salvia miltiorrhiza in cadmium and lead co-contaminated soil

Salvia miltiorrhiza is an important traditional Chinese medicinal and edible plant that can easily accumulate excessive cadmium (Cd) and lead (Pb) from contaminated soils. The soil contaminated with heavy metals severely threatened the quality of S. miltiorrhiza products. In this study, we investigated the effects of mercapto-based palygorskite (MPAL), a new passivation amendment, on restraining the uptake of Cd and Pb by S. miltiorrhiza, and the impact on soil micro-ecology. Results showed that the application of MPAL prominently enhanced the biomass and antioxidant enzyme activities of S. miltiorrhiza. With the treatment of 4% MPAL, the Cd and Pb contents in the roots were significantly decreased by 81.42% and 69.09%, respectively. The active ingredients of S. miltiorrhiza, including Danshensu, Cryptotanshinone, Tanshinone I and Tanshinone II were remarkedly increased by 1899.46%, 5838.64%, 54.23% and 200.78%, respectively. In addition, MPAL decreased the bio-availability of Cd and Pb by speciation transformation, which simultaneously boosted the activities of cellulase and sucrase. The application of MPAL also improved the bacterial community composition. These findings revealed that the application of MPAL regulated the soil micro-ecology, positively modified the growth and obstructed the Cd and Pb accumulation in S. miltiorrhiza.

Appraisal on the role of passive sampling for more integrative frameworks on the environmental risk assessment of contaminants

Over time multiple lines of research have been integrated as important components of evidence for assessing the ecological quality status of water bodies within the framework of Environmental Risk Assessment (ERA) approaches. One of the most used integrative approaches is the triad which combines, based on the weight-of-evidence, three lines of research, the chemical (to identify what is causing the effect), the ecological (to identify the effects at the ecosystem level) and the ecotoxicological (to ascertain the causes of ecological damage), with the agreement between the different lines of risk evidence increasing the confidence in the management decisions. Although the triad approach has proven greatly strategic in ERA processes, new assessment (and monitoring) integrative and effective tools are most welcome. In this regard, the present study is an appraisal on the boost that passive sampling, by allowing to increase information reliability, can give within each of the triad lines of evidence, for more integrative ERA frameworks. In parallel to this appraisal, examples of works that used passive samplers within the triad are presented providing support for the use of these devices in a complementary form to generate holistic information for ERA and ease the process of decision-making.

Do tillage systems affect the cadmium threshold in farmland soil for environmental quality standard setting?

Tillage systems may change the cadmium (Cd) threshold of farmland soil. However, there have been few studies on this topic. Therefore, this study aimed to explore the influence of tillage systems on Cd threshold. The study conducted 2-year field experiments under different tillage systems (early rice-fallow, early rice-late rice and early rice-vegetable) at three typical Cd-polluted sites in China. The species sensitivity distribution (SSD) method was used to construct the SSD curves for the calculation of the Cd threshold by analyzing the experimental data. The sensitivity analysis results based on the SSD curves revealed that the sensitivities to Cd in rice varieties under the same tillage system were substantially different but almost the same under different tillage systems. These results can help select rice varieties with low Cd sensitivity for crop safety. Different tillage systems at the same site varied in their influence on Cd threshold values. Cd threshold values under early rice-late rice (e.g., 0.27, 0.28 mg/kg in Xiangtan City) and early rice-vegetable (e.g., 0.26, 0.31 mg/kg in Xiangtan City) tillage systems were roughly lower than that under the early rice-fallow tillage system (e.g., 0.33, 0.35 mg/kg in Xiangtan City). Notably, the influence of tillage systems resulted in Cd threshold values being generally lower than the Cd risk screening values of the current Chinese soil environmental quality standard. Analysis of the influence of different tillage systems on the Cd threshold is beneficial for the optimization of farmland soil environmental quality standards.

Predictive and estimation model of Cd, Ni, and Zn bioaccumulations in maize based on diffusive gradients in thin films

Consumption of maize contaminated with heavy metals such as cadmium, nickel, and zinc threaten human health. For situ measuring the bioavailability of heavy metals, the diffusive gradients in thin films (DGT) is superior to other traditional methods. It is also important to find a method for predicting heavy metal enrichment in maize based on the DGT method. In this study, field surveys were conducted in the main maize producing areas of Tianjin, China. Heavy metal concentrations in maize grains were predicted by coupling DGT with traditional extraction methods. The results show that coupling DGT with soil solution can significantly improve prediction accuracy (Cd-R² = 0.908, Ni-R² = 0.903, and Zn-R² = 0.904). This indicated that DGT and soil solution were feasible predictors of heavy metal concentration in maize. The DGT induced fluxes in soil/sediment (DIFS) model was used to simulate the uptake process of heavy metals by DGT, and better reveal the desorption processes of heavy metals in soils. DIFS-based desorption processes were employed to characterize the resupply ability of heavy metals in soils. The coupling of DGT and DIFS parameters provided the best prediction accuracy in this study (Cd-R² = 0.920, Ni-R² = 0.928, and Zn-R² = 0.908). Predictions are slightly weaker for Zn than for Cd and Ni (Cd-P < 0.01, Ni-P < 0.01, and Zn-P < 0.05). The reason is that the average resupply type of Cd and Ni in soil is partially sustained while Zn is resupplied via diffusion only. The desorption rate k_-1 can excellently improve the prediction accuracy of DGT, which avoids the disadvantage that soil solution does not consider desorption. The coupling of DGT and DIFS parameters provides an accurate and reliable method for predicting heavy metal enrichment in maize.

Long-term immobilization of cadmium and lead with biochar in frozen-thawed soils of farmland in China

The problem of potentially toxic elements (PTEs) in farmland is a key issue in global pollution prevention and control and has an important impact on environmental safety, human health, and sustainable agricultural development. Based on the climate background of high-latitude cold regions, this study simulated freeze-thaw cycles through indoor tests. Different initial conditions, such as biochar application rates (0%, 1%, 2%) and different initial soil moisture contents (15%, 20%, 25%), were set to explore the morphological changes in cadmium (Cd) and lead (Pb) in soil and the response relationship to the changes in soil physicochemical properties. The results indicate that soil pH decreases during freeze-thaw cycles, and soil alkalinity increases with increasing biochar content. Freeze-thaw cycles caused the total amount of PTEs to have a U-shaped distribution, and the amount of PTEs in the soluble (SOL) and reducible (RED) fraction increased by 0.28-56.19%. Biochar reduced the amount of Cd and Pb migration in the soil, and an increase in soil moisture content reduced the availability of Cd and Pb in the soil. Freezing and thawing damaged the soil structure, and biochar reduced the fractionation of small particle aggregates by enhancing the stability of soil aggregates, thereby reducing the soil's ability to adsorb Cd and Pb. In summary, for farmland soil remediation and pollution control, the application of biochar has a certain ability to optimize soil properties. Considering the distribution of PTEs in the soil and the physicochemical properties of the soil, the application of 1% biochar to soil with a 20% moisture content is optimal for regulating seasonally frozen soil remediation.

The impact of river sand mining on remobilization of lead and cadmium in sediments - A case study of the Jialing River

Due to the fast pace of urbanization worldwide, industrial sand mining activities have imposed great pressure on the environment, and consequently, these activities have led to serious environmental problems in aquatic ecosystems. However, the current understanding of the effect of sand mining on heavy metal remobilization in river sediments remains incomplete. The present study employed sediment quality guidelines (SQGs) and the sequential extraction (SE) and diffusive gradients in thin films (DGT) techniques to comprehensively investigate the effect of sand mining on the remobilization process of heavy metals in the aquatic system of the Jialing River. The SQGs results indicated that stations (S1 to S4) with sand mining disturbance exhibited Pb and Cd accumulation in surface sediments. Both C_total-Pb (61.78-122.04 mg·kg^-1) and C_total-Cd (0.85-3.96 mg·kg^-1) were higher than CSQGI (60 mg·kg^-1 for Pb and 0.5 mg·kg^-1 for Cd) and TEC (35.8 mg·kg^-1 for Pb and 0.99 mg·kg^-1 for Cd) limitation in most of sand mining stations. Pb and Cd were mainly bounded in the acid-soluble/exchangeable fraction (F1) and oxidizable fraction (F3) of the surface sediments. At the four stations with sand mining disturbance, about 5-10 folds of DGT-labile Pb and Cd were released in deep sediments (-9 to -12 cm), and Pb and Cd exhibited a transport trend from the sediments into the overlying water, while the above phenomenon was not observed at the two stations without sand mining activities. Correlation analysis revealed that DGT-labile Pb and Cd were suitably correlated with the F1 and F3 fractions, indicating that the acid-soluble/exchangeable and oxidizable fractions were the main sources leading to Pb and Cd remobilization in the sediments. A potential mechanism explanation may be that (1) intense sediment stirring could result in remobilization of the weakly bound fraction, which is related to the contribution of the F1 fraction, and (2) Cd/Pb experienced a corelease process with sulfur due to O₂ introduction (elevation of the dissolved oxygen level) attributed to sediment evacuation, which is related to the contribution of the F3 fraction. The above results suggested that sand mining in the Jialing River should be paid high attention to prevent heavy metal pollution in aquatic ecosystem.

Effect of dissolved organic matter on the bioavailability and toxicity of cadmium in zebrafish larvae: Determination based on toxicokinetic-toxicodynamic processes

The presence of dissolved organic matter (DOM) strongly influences the bioavailability of metals in aquatic environments; however, the association between the binding activities and the concentrations of DOM compositions is not well documented, leading to uncertainties in metal toxicity assessment. We creatively quantify the mitigation and acceleration effects of DOM compositions on cadmium (Cd) bioaccumulation and toxicity in zebrafish larvae using abiotic ligand (ABLs) and biotic ligand (BLs) in a toxicokinetic-toxicodynamic (TK-TD) model. The BL-TK-TD model could accurately predict the protective effect of fulvic acid while overestimating the complexing capacity of citric acid. The model also could successfully simulate the protective effects of native DOM in most cases from 32 natural water bodies in China. The observed LC50 values of Cd showed a peak effect for the native DOM fraction comprising hydrophilic acidic contents (3.55 ± 0.44 mg L ^- 1) in natural water from 32 sites. The BL-TK-TD model provides practically useful information to identify the effect of different DOM compositions on metal bioavailability and toxicity in aquatic environments and guides future water management policies aimed at controlling aquatic heavy metal pollution.

Facilitating maintenance of stormwater ponds: comparison of analytical methods for determination of metal pollution

Stormwater ponds are widely used for controlling runoff quality through the sedimentation of particles and associated pollutants. Their maintenance requires regular removal and disposal of accumulated material. This necessitates an assessment of material hazardousness, including potential hazard due to its contamination by metals. Here we analyze 32 stormwater pond sediment samples from 17 facilities using several chemical analysis methods (total extraction, sequential extraction, diffusive gradients in thin-films DGT, and pore water extraction) in order to consider the complementarity and comparability of the different approaches. No clear relationship was found between analyses that have the potential to measure similar metal fractions (DGT and either fraction 1 of the sequential extraction (adsorbed and exchangeable metals and carbonates) or pore water concentrations). Loss on ignition (LOI) had a significant positive correlation with an indicator of the environmental risk developed in this paper (∑ranks) that incorporates different metals, speciations, and environmental endpoints. Large variations in metal levels were observed between ponds. As clustering was limited between the different analyses, a comprehensive analysis of different parameters is still needed to fully understand metal speciation and bioavailability.

Water quality and periphyton functional response to input of dissolved manure-derived hydrochars (DHCs)

Dissolved organic matter (DOM) plays a critical role in the global carbon cycle and provides food and energy for aquatic organisms. Recently, hydrochar, as a solid carbonaceous substance derived from hydrothermal carbonization, has been increasingly used as a soil amendment. Upon entering the soil, dissolved components (DHCs) were released from hydrochar as exogenous DOM, finally entering the aquatic ecosystems by runoff, which participates in environmental geochemical processes. However, relevant reports revealing the response of the aquatic ecosystem to the input of DHCs remain insufficiently elucidated. For the first time, the fundamental features of DHCs and their influence on water quality and aquatic biological function were investigated in this study. DHCs at 260 °C (DHC260) had lower yields, a greater [C/N], worse biodegradability, and larger humic acid relative amounts than did DHCs at 180 °C (DHC180). The DHC structural alterations in periphyton-incubated aquatic ecosystems suggested that protein substances were more easily degraded or assimilated by periphyton, especially for DHC180, with rates of decrease of 34.5-63.5%. The increased chemical oxygen demand (COD) degradation in the DHC260 treatments was most likely due to humic acid substances with higher COD equivalents. Furthermore, DHC260 caused phosphorus to accumulate in periphyton, reducing aquatic phosphorus concentration. Notably, the abundances of Flavobacteria and Cyanobacteria associated with water blooms increased 12.7-25.5- and 1.3-8.3-fold, respectively; consequently, the promotional impact of DHCs on algal blooms should be considered. This result extends the nonnegligible role of DHCs in aquatic ecosystems and underlines the need to regulate the hydrochar application process.

Clay-hydrochar composites return to cadmium contaminated paddy soil: Reduced Cd accumulation in rice seed and affected soil microbiome

Excess cadmium (Cd) in rice precipitated by Cd contamination in paddy soils is a global human health threat and rational response is urgently needed. In this study, attapulgite-modified hydrochar (CA) and the montmorillonite-modified hydrochar (CM) were utilized in Cd-contaminated paddy soils at 0.5% (w/w) and 1% (w/w) application rates to investigate the effects of these clay-hydrochar composites on rice growth and soil Cd availability. The results show that the utility of CA and CM resulted in a significant increase in rice yield, especially at 1% application rate, which extended rice yield by 46.7-50.0% compared to 0.5% application rate. This is related to the Cd fixation and nutrient sequestration of the acidic functional groups on the surface of CA and CM. Additionally, CA-1% and CM-1% reduced the Cd concentration in rice seeds by 26.9-28.1% relative to the control. Notably, CA-1% showed the capacity to passivate Cd at the early stage of rice transplanting, lowering the proportion of Cd in the ion exchange state by 41.6% compared to the control, and this passivation effect persisted until the late stage of transplanted rice. The soil microbial community consequences showed that CA and CM did not significantly change the horizontal composition of the soil bacterial phylum and species diversity, indicating that CA and CM had excessive soil microbial adaptability. Moreover, results of correlation and Canonical Correspondence Analysis confirm that microbial genera responded significantly to the soil Cd morphologies, revealing the importance of CA and CM in the remediation of Cd-contaminated soils by influencing microorganisms. Our findings provide clay-hydrochar composites as a low-cost approach to effectively mitigate soil Cd contamination and improve the security and quality of rice.

Assessment of Cd bioavailability using chemical extraction methods, DGT, and biological indicators in soils with different aging times

Total cadmium (Cd) cannot be used to accurately assess the ecological risk of Cd pollution in soil. Currently there is no universally recognized method to evaluate Cd bioavailability in soil. In this study, chemical extraction methods, diffusive gradients in thin films (DGT) and bioindicator methods were used to evaluate Cd bioavailability in soils with the same properties but different aging times. Results indicate that aging decreased the Cd bioavailability in soil and its toxicity to barley. This was primarily due to a decrease in the proportion of ion-exchangeable Cd. Correlation analyses were conducted on the Cd bioavailable content obtained via the soil extraction methods and the toxicity effect of barley. Results showed that the order of the minimum value of the linear regression determination coefficient (R²) of chemical extraction methods and DGT was as follows: DGT-Cd (0.7385, p < 0.05) > total Cd (0.6931, p < 0.05) > acetic acid-Cd (0.6078) > ion-exchangeable Cd (0.5933) > DTPA-Cd (0.5842) > CaCl₂-Cd (0.4980) > water-soluble Cd (0.4602). The order of minimum value of R² of biological indicators of barley was integrated biomarker response (IBR) (0.8501, p < 0.01) > length (0.6492) > dry weight (0.6320) > fresh weight (0.4980) > Cd concentration (0.4602). The root is more suitable for indicating the plant uptake and accumulation of Cd in soil. Meanwhile, the shoot can effectively evaluate the toxic effect of Cd stress on plants. DGT is more suitable to reflect Cd bioavailability to barley compared to chemical extraction methods, Furthermore, it can be used to evaluate stable polluted soil with longer aging time. In the study of the bioavailability of heavy metals in soil, IBR can be used as a reliable reference index to contribute to the comprehensive evaluation of metal bioavailability in addition to considering plant uptake.

Predicting soil cadmium uptake by plants in a tailings reservoir during 48-year vegetation restoration

Plant uptake can reduce soil cadmium (Cd) pollution, while how to exactly predict plant Cd uptake in industrial or mining areas during vegetation restoration remains unexplored. Taking Heteropogon contortus as the object plant, we predicted plant Cd uptake in the Majiatian tailings reservoir during 48-year vegetation restoration by the methods of soil total Cd, DGT (diffusive gradients in thin films technique) and acetic acid (HAc) extraction. Meanwhile, we explored the effects of soil properties on the accuracy of the prediction. Total Cd concentrations in the soils exhibited a better prediction of plant Cd uptake relative to the methods of HAc extraction and DGT. However, the DGT method effectively predicted plant Cd uptake at low Cd supply (lower than 0.42 μg/L), probably because of the dominant diffusion limitation by plants. The prediction of plant Cd uptake by HAc extraction was improved when combined with soil pH. Our results indicate that with increasing external Cd inputs during the vegetation restoration, soil total Cd and traditional extraction method in combination with soil properties are effective ways to predict plant Cd uptake, especially when the Cd fractions cannot be measured by DGT. However, the DGT method works once plant Cd uptake dominated by diffusion limitation despite the interference in soil properties.

Differences and mechanism of dynamic changes of Cd activity regulated by polymorphous sulfur in paddy soil

Sulfur (S) can decrease the bioavailability of Cd in paddy soil, and therefore reduce Cd uptake by rice. However, the dynamic influence mechanism of different forms of sulfur on the bioavailability of cadmium in soil has not been systematically studied. In this study, we used Diffusive gradients in thin films (DGT) technology and soil pore water sampling technology to investigate the effects of different types of S application on the bioavailability of Cd. The three forms of S are elemental sulfur (S⁰), sodium sulfate (SO₄^2-), and mercapto-grafted palygorskite (MP), which have been treated in six ways according to CK, SL, SH, SO₄²L, SO₄²H, MP (L and H are the low and high levels of treatments). The results showed that soluble and labile Cd concentration was quickly fixed after flooding but activated after rice transplantation. Both MP and SL treatments increased the content of dissolved organic carbon (DOC) and significantly reduced the accumulation of Cd in roots and grains. The Cd content of roots treated with MP was only 0.50 mg kg^-1, which was 77.6% lower than the Cd content in CK (2.22 mg kg^-1). S promotes the reduction and dissolution of Fe, and the formation of FeS/FeS₂ is also one of the reasons for the low bioavailability of Cd, while the SH treatment has a greater effect on lowering pH, so the effect of fixing Cd is not obvious. In addition, SO₄^2- treatment delays the soil reduction process, so it cannot effectively fix Cd. At the stage of rice maturity, dissolved Cd increased in S treatment, but MP treatment did not have this phenomenon. The possible reason was the consumption of DOC or the oxidation of CdS which caused Cd to enter the soil solution. Taken together, Fe-S play a critical role in controlling the mobilization of Cd in paddy systems.

Microbial induced carbonate precipitation contributes to the fates of Cd and Se in Cd-contaminated seleniferous soils

Bioremediation based on microbial induced carbonate precipitation (MICP) was conducted in Cd-contaminated seleniferous soils with objective to investigate effects of MICP on the fates of Cd and Se in soils. Results showed that soil indigenous microorganisms could induce MICP process to stabilize Cd and mobilize Se without inputting exogenous urease-producing strain. After remediation, soluble Cd (SOL-Cd) and exchangeable Cd (EXC-Cd) concentrations were decreased respectively by 59.8% and 9.4%, the labile Cd measured by the diffusive gradients in thin-films technique (DGT) was decreased by 14.2%. The MICP stabilized Cd mainly by increasing soil pH and co-precipitating Cd during the formation of calcium carbonate. Compared with chemical extraction method, DGT technique performs better in reflecting Cd bioavailability in soils remediated with MICP since this technique could eliminate the interference of Ca²⁺. The increase in pH resulted in Se conversion from nonlabile fraction to soluble and exchangeable fractions, thus improving Se bioavailability. And Se in soil solution could adsorb to or co-precipitate with the insoluble calcium carbonate during MICP, which would partly weaken Se bioavailability. Taken together, MICP had positive effects on the migration of Se. In conclusion, MICP could stabilize Cd and improve Se availability simultaneously in Cd-contaminated seleniferous soils.

Cd and Pb immobilisation with iron oxide/lignin composite and the bacterial community response in soil

Iron oxide is a natural mineral that generally exists in the form of iron oxide-organic complexes (Fe-OM) in soil. Lignin is a naturally occurring polymer that is considered to be an important part of soil carbon cycling. In this study we prepared a composite material (MGE) with iron oxide and lignin based on the Fe-OM present in the soil. MGE was then applied to remediate Cd and Pb in contaminated soil. The results show that DTPA-Cd and DTPA-Pb levels were reduced by 58.87% and 78.09%, respectively. The bacterial community diversity index decreased in the iron oxide (GE) group, but a slight increase was observed in the MGE group. In terms of species composition in the MGE group, the abundance of Proteobacteria, Gemmatimonadota and Acidobacteriota increased, while the abundance of Bacteroidota, Actinobacteriota and Firmicutes decreased. The outcome in the GE group was the opposite. In the MGE group, HCl-Fe²⁺, HCl-Fe³⁺, and pH were significantly higher than in the other groups, indicating that MGE stimulated the growth of iron-reducing bacteria (FeRB) and promoted iron redox reactions. Iron oxide could be reduced to Fe²⁺ due to the activity of FeRB, and then Fe²⁺ would be oxidised and hydrolysed, which led to an increase in soil pH. Secondary minerals were formed during this process. With the oxidation of Fe²⁺ and the formation of secondary minerals, Cd and Pb could be stabilised in the oxides and were not easily released through a co-precipitation mechanism.

Toxic chromium treatment induce amino-assisted electrostatic adsorption for the synthesis of highly dispersed chromium catalyst

Removal of toxic Cr (VI) from aqueous solutions using silicon-based adsorbents has been widely investigated. Meanwhile, contradictory between highly dispersed active Cr species and high Cr loading over commercial Cr-based catalyst was inevitable. In this work, amino-assisted electrostatic adsorption from toxic Cr (VI) treatment was developed to prepare highly dispersed Cr oxides catalysts supported on MCM-41. The Cr loading was as high as 15 wt%, and structure characters of the catalysts were well-reserved. As a result, electrostatic adsorption and subsequent complexation from negatively charged Cr (VI) species and positively charged ammonium groups made a positive contribution to the appearance of highly dispersed mono Cr species, which gave rise to improved non-oxidative propane dehydrogenation (PDH) activity. In contrast, the agglomeration of Cr species and lower PDH activity were observed on the sample synthesized using the traditional wet impregnation method. Besides, the transformation of Cr (VI) to active Cr (III) sites over the catalyst was proved by the designed in-situ H₂-TPR, ex-situ UV-vis and Raman spectra results. This procedure reflects a new avenue of green chemistry, which can recycle waste Cr adsorbents as efficient PDH catalysts.

Does sand mining affect the remobilization of copper and zinc in sediments? - A case study of the Jialing River (China)

It is generally accepted that the sand mining industry causes severe destruction in river basin environments. In this study, six sediment cores were collected, and sequential extraction was applied in conjunction with the diffusive gradients in the thin films (DGT) technique to explore the effect of sand mining on the remobilization of Cu and Zn in the sediments. The results showed that Cu and Zn were mainly bound in the residual fraction in the sediments. C_DGT-Cu/Zn in the sediments presented obvious increasing trends at the bottom (-9 to -12 cm) at the four sites that experienced sand mining and a decreasing trend at the sites with no sand mining disturbance. Cu and Zn also tended to be transported from the sediments to the overlying water at the four sand mining sites. A correlation analysis found that F1 and F3 correlated well with C_DGT-Cu/Zn, indicating that the water/exchangeable fraction and oxidized fraction were the main fractions that led to increases in DGT-labile Cu and Zn in the sediments. Further analysis found that the introduction of oxygen (O₂) was the main reason for the simultaneous release of sulfur (S), Cu and Zn in the sediments, as indicated by the "dark area" of AgI gel appearing at the same position as the "hot spot area" of Chelex gel. Two main sand mining effects on the release of Cu and Zn were hypothesized: (1) intense sand disturbance leads to the transfer of the water/exchangeable fraction (F1) to the DGT-labile fraction and (2) O₂ introduction promotes the reaction of stable sulfide (F3), thus transferring it to the DGT-labile fraction. The above results indicated that the sand mining industry should be paid much attention in the Jialing River, as it can obviously cause labile Cu and Zn release into the water.

Impact of Biochar on Soil Properties, Pore Water Properties, and Available Cadmium

Some effects of biochar on soil properties (such as pore water DOC) are not very clear. The changes of soil properties [cation exchange capacity (CEC)], pore water properties [pH, dissolved organic carbon (DOC), and Cd concentration (C_PW-Cd)], Cd concentration measured by diffusive gradients in thin films (C_DGT-Cd), and available Cd content (Cd in weak acid extractable state and reducible state, C_BCR-Cd) determined by the BCR sequential extraction procedure over time after biochar addition were studied by soil incubation and potted corn experiments with five soils from a mining area. The results showed increases of 20.3%-64.6% in CEC and 0.34-1.02 in pH (both p < 0.05) in the soil incubation after adding biochar. The DOC concentration was reduced by 8.2%-33.2% (p < 0.05). C_PW-Cd, C_DGT-Cd, and C_BCR-Cd decreased by 14.2%-47.2%, 15.3%-47.9%, and 22.3%-61.4%, respectively. During the corn cultivation phase, CEC increased by 5.1%-29.0%, and DOC concentration decreased by 10.4%-41.3% (p < 0.05). C_PW-Cd, C_DGT-Cd, and C_BCR-Cd decreased by 5.9%-22.4%, 7.2%-25.1%, and 10.5%-64.8%, respectively. Biochar effectively increased the biomass of corn roots and reduced the concentration of Cd in the roots. Biochar altered the properties of soil and pore water, reduced the bioavailability of Cd in soil, and mitigated the harm to corn caused by Cd.

Influence of polyethylene-microplastic on environmental behaviors of metals in soil

Microplastics (MPs) in terrestrial ecosystems have attracted increasing attention all over the world. The adsorption-desorption behavior and bioavailability of metals in soil would affect its toxicity to organisms. However, the influences of MPs on adsorption-desorption behavior between metals and soil as well as bioavailability of metals in soils are scarcely investigated. Herein, different percentage (0, 0.1%, 1%, 10%) of polyethylene-microplastic (PE-MP) were thoroughly mixed into the soil to investigate the impacts of PE-MP on adsorption-desorption and bioavailability of metals (Zn²⁺, Pb²⁺) in the soil. A series of characterization were carried out to determine the change of PE-MP before and after adsorption to investigate the mechanisms. When MP100 (average size: 129 μm) content in soil increased to 10%, the adsorption capacities of soil with Pb²⁺ and Zn²⁺ were 3.73 and 4.56 mg/g, respectively, which were significantly (p < 0.05) lower than that of pure soil. When MP300 (average size: 293 μm) content in soil increased to 10%, the extraction fraction of Zn²⁺ and Pb²⁺ from soil by diethylenetriaminepentaacetic acid reached 12.35% and 23.96%, respectively, which were significantly (p < 0.05) higher than that of pure soil, indicating high concentration (10%) of MPs in soil would decrease the adsorption capability of soil to metals and increase the mobility of metals in terrestrial environment. However, when MPs content in soil was 0.1%, the extraction fraction of Zn²⁺ and Pb²⁺ showed no significant difference with that of pure soil, indicating that actual MPs in soil is unlikely to bring significant influence on metal bioavailability.

Plastic shed production systems: The migration of heavy metals from soil to vegetables and human health risk assessment

Plastic shed production system (PSPS) provide abundant vegetable products for human consumption. Comprehensive and accurate heavy metal (HM) risk assessment of soil and vegetable under plastic sheds is crucial for human health. Pollution assessment, bioavailability and mobility evaluation and health risk assessment of Cd, Cr, Cu, Zn Ni, Pb, and As were performed in a presentative Plastic shed production system. The concentrations of the Cd, Cu and Zn exceeded their background value. Positive I_geo values suggested that soil under plastic sheds was widely contaminated with Cd. The bioavailability of heavy metals in soils was evaluated using DTPA extraction and DGT methods. The results of both methods demonstrated that Cd, Cu, and Zn have high bioavailability, especially Cd. Analogically, the results of mobility assignment based on DIFS showed that Cd has a high migration risk due to the large available pool. Based on specific cultivation and management patterns of plastic shed production system, pH reduction and salt and nutrient accumulation may increase the heavy metals migration risk in soil under plastic sheds, while a high organic matter content may reduce the heavy metals migration risk. The average concentrations of Cd, Cr, Cu, Zn, Ni, Pb, and As in vegetables were 0.023, 0.226, 0.654, 2.984, 0.329, 0.041, and 0.010 mg/kg, respectively. All samples were well below the threshold. The order of target hazard quotient of different heavy metals caused by vegetable consumption was Cd > Cr > As > Cu, Ni, Pb, Zn, and the average total hazard index value was below 1, which demonstrated that risk of vegetable consumption in the study area. However, due to its high concentration and transfer coefficient in spinach, Cd might pose a health risk to humans, which requires special attention. In this study, Cd caused a significant issue than other HMs, whether pollution level, health risk and migration risk. DGT and DIFS can be used as an effective evaluation tool in the research of controlling heavy metals migration in soil-crop systems.

Fir bark modified by Phanerodontia chrysosporium: A low-cost amendment for cd-contaminated water and agricultural soil

In this study, a modified fir barks (MFB) was prepared by mixing fir barks (FB) and white-rot fungi (Phanerodontia chrysosporium) under aerobic fermentation. The potential of MFB for Cd²⁺ adsorption was investigated by batch experiments combined with kinetic, isotherm, and thermodynamics analyses. The results revealed that the modification greatly increased the porous structures on the surfaces of fir barks and the surface area of MFB was much higher than that of FB. As a result, the adsorption capacity of Cd²⁺ on MFB (17.4 mg g^-1) was more than two times higher than that on FB (7.2 mg g^-1), and the adsorption of Cd²⁺ on MFB was controlled by physisorption and chemisorption. The immobilization of Cd by MFB in a contaminated agricultural soil was also investigated. The effect of MFB on the bioavailability of Cd was investigated using a leaching test (the European standard EN 12457-2) combined with a typical sequential extraction procedure (the community bureau of reference, BCR). The experimental results showed that the Cd leachability was reduced by 71% when the added MFB dosage was 30 mg g^-1. Besides, the MFB amendment could transform Cd from unstable geochemical fractions into more stable fractions. In total, the MFB, as a chemical-free and eco-friendly material, could be potentially employed for in-situ remediation of Cd-contaminated agricultural soils.

Sub-millimeter distribution of labile trace element fluxes in the rhizosphere explains differential effects of soil liming on cadmium and zinc uptake in maize

Trace element concentrations in the rhizosphere were quantified to better understand why soil liming often fails to reduce cadmium (Cd) uptake by plants. Maize seedlings were grown on a soil with natural background levels of Cd and zinc (Zn). Soil liming increased soil pH from 4.9 to 6.5 and lowered the soil solution free ion activities by factor 7 (Cd) and 9 (Zn). In contrast, shoot Cd concentrations were unaffected by liming while shoot Zn concentrations were lowered by factor 1.9. Mapping of labile soil trace elements using diffusive gradients in thin films (DGT) in combination with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) revealed an almost complete depletion of Cd in the rhizosphere in all soil treatments, showing that Cd uptake is controlled by diffusion. The flux of Cd from soil to the DGT, with direct contact between the soil and the binding gel, was unaffected by liming whereas it decreased by factor 3 for Zn, closely mimicking the contrasting effects of liming on Cd and Zn bioavailability. This evidence, combined with additional flux data of freshly spiked Cd and Zn isotopes in soil and with modelling, suggests that the diffusive transport of Cd in unsaturated soil is more strongly controlled by the labile adsorbed metal concentration than by its concentration in solution. This is less the case for Zn because of its inherently slower desorption compared to Cd.

Assessment of earthworm activity on Cu, Cd, Pb and Zn bioavailability in contaminated soils using biota to soil accumulation factor and DTPA extraction

The study aims to investigate effect of earthworm activity on metal bioavailability in soils using their BSAF-metals. Based on a microcosmic laboratory experiment, epigeic species Amynthas corticis (A. corticis) and endogeic species Amynthas robustus (A. robustus) were cultured in two types of soils contaminated by Cd, Zn, Pb and Cu for 120 days. Earthworm characteristics (i.e. numbers, biomass and BSAF), soil properties (i.e. pH, organic C and N contents along with their components such as mineralization and microbial masses) and DTPA extracted metals in soil were determined. After the incubation, the biomass and survival numbers of both earthworm species decreased significantly (P < 0.05). The accumulation of Cd, Zn and Pb in earthworm tissues and BSAF-metals were earthworm species dependent. According to two-way ANOVA, BSAF-Pb clearly showed the effect of different species of earthworms while BSAF-Cu indicated an interactive effect of earthworms and soil type. Earthworms changed soil properties significantly, especially for mineralized C (C_min), dissolved N (N_dis) and pH (P < 0.05). Earthworm activity increase DTPA extracted Zn and Cu, and the effect of A. robustus were stronger than for A. corticis. Redundancy analysis (RDA) showed that BSAF-Cu and BSAF-Pb contributed for respectively 51.9% and 51.7% of soil properties and DTPA metal changes, indicating that the effects of BSAF-Cu and BSAF-Pb on soil properties and on metal bioavailability in soil were similar. BSAF-Cu, indicating the interactive effect of earthworms and soil, accounted for 38.5% and 45.1% of soil properties and soil metal bioavailability changes. BSAF-Pb, representing the effect of earthworm species, accounted for 13.3% and 6.6% of soil property and soil metal bioavailability variations. Stepwise regression indicated that earthworm might change soil properties through their activities and interactions with soil, and hence increase heavy metal bioavailability. It suggested that BSAF is an important indicator for evaluating the effect of earthworm activity on soil metal bioavailability and designing remediation strategies.

Bioavailability and mobility of copper and cadmium in polluted soil after phytostabilization using different plants aided by limestone

Phytostabilization aided by soil amendments has been advocated in areas contaminated by trace metals. In this study, the effects of indigenous weed (Setaria pumila), energy plant (Pennisetum sinese), cadmium (Cd)-hyperaccumulator (Sedum plumbizincicola), and copper (Cu)-tolerant plant (Elsholtzia splendens) on the bioavailability and mobility of Cu and Cd in polluted soil were evaluated after phytostabilization aided by limestone (0.1% wt) over four years. The four plants combined with limestone significantly increased soil pH and decreased Cu and Cd fractions extracted by NH₄OAc and diffusive gradients in thin films (DGT) than the untreated soils, respectively. P. sinese treatments decreased DGT-extractable Cu and Cd by 52.1% and 40.5% than S. pumila treatments, respectively. S. plumbizincicola and E. splendens treatments increased acid-soluble fraction of Cu and decreased residual fraction of Cu compared with S. pumila treatments. P. sinese treatments had the lowest phytotoxicity (inhibitoryrates, superoxide dismutase, peroxidase and catalase activities) among all treatments. Moreover, EDTA kinetic extraction showed that S. plumbizincicola and E. splendens treatments increased the mobility of Cu and Cd by increasing labile and less labile fractions of Cu and Cd compared with P. sinese treatments. Present results suggest that P. sinese is recommended as the remediation plant for phytoremediation aided by amendments.