Metal distribution and biological diversity of crusts in paddy fields polluted with different levels of cadmium

Disclosing the key role of Fe/As/Cu in community co-occurrence and microbial recruitment in metallurgical ruins

Mining activities have led to the persistent presence of substantial heavy metals at metallurgical sites. However, the impact of long-term and complex heavy metal pollution in metallurgical ruins on the structure and spatial shift of microbiome remains unclear. In this study, we focused on various types of metallurgical sites to uncover the occurrence of heavy metals in abandoned mines and the response patterns of microbial communities. The results indicate that mining activities have caused severe exceedances of multiple heavy metals, with AsBio, CuBio, and FeBio being the primary factors affecting community structure and function. Co-occurrence network analyses suggest that several genera, including Ellin6515, Cupriavidus, Acidobacteria genus RB41, Vicinamibacteraceae, Blastococcus, and Sphingomonas, may play significant roles in the synergistic metabolism of communities responding to Fe-Cu-As stress. Although random dispersal contributed to community migration, null models emphasized that variable selection predominates in the spatial turnover of community composition. Additionally, metagenomic prediction (PICRUSt2) identified key genes involved in stress and detoxification strategies of heavy metals. The composite heavy metal stress strengthened the relationship between network structure and the potential function of the community, along with critical ecosystem functions. Our findings demonstrated that microbial interactions were crucial for ecosystem management and the ecological consequences of heavy metal pollution remediation.

Ecological responses and functional significance of paddy crust in the southern Chinese environment

Paddy Crusts (PC) play a pivotal role in the migration and transformation of heavy metals within paddy ecosystems, situated at the critical intersection of air, water, and soil. This study focused on PC samples from heavy metal-contaminated rice paddies in six southern Chinese provinces. It's the first time we've screened and quantified the impact of nutrition, physicochemical properties, and heavy metals on bacterial diversity in PC. Our results highlight the significant influence of zinc, total nitrogen, and soil manganese on bacterial diversity. Using structural equation models, we identified the pathways through which these three types of environmental factors shape bacterial diversity. Heavy metal indicators and physical and chemical indicators exerted a direct negative effect on bacterial diversity in PC, while nutritional indicators had a direct and significant positive effect on bacterial diversity. Variance partitioning analysis revealed heavy metals had the most significant impact, accounting for 7.77% of the total effect. Moreover, the influence of heavy metals on bacterial diversity increased as diversity decreased, ranging from 3.81% to 42.09%. To remediate specific heavy metal pollution, our proposed method involves cultivating indigenous bacteria by controlling these environmental factors, based on an analysis of the interplay among bacterial diversity, environmental variables, and heavy metal bioconcentration factors. These findings enhance our understanding of PC and provide insights into rice field heavy metal pollution mitigation.

Biogenic manganese oxides promote metal(loid) remediation by shaping microbial communities in biological aqua crust

Biological aqua crust (biogenic aqua crust-BAC) is a potentially sustainable solution for metal(loid) bioremediation in global water using solar energy. However, the key geochemical factors and underlying mechanisms shaping microbial communities in BAC remain poorly understood. The current study aimed at determining the in situ metal(loid) distribution and the key geochemical factors related to microbial community structure and metal(loid)-related genes in BAC of a representative Pb/Zn tailing pond. Here we showed that abundant metal(loid)s (e.g. Pb, As) were co-distributed with Mn/Fe-rich minerals (e.g. biogenic Mn oxide, FeOOH) in BAC. Biogenic Mn oxide (i.e. Mn) was the most dominant factor in shaping microbial community structure in BAC and source tailings. Along with the fact that keystone species (e.g. Burkholderiales, Haliscomenobacter) have the potential to promote Mn ion oxidization and particle agglomeration, as well as Mn is highly associated with metal(loid)-related genes, especially genes related to As redox (e.g. arsC, aoxA), and Cd transport (e.g. zipB), biogenic Mn oxides thus effectively enhance metal(loid) remediation by accelerating the formation of organo-mineral aggregates in biofilm-rich BAC system. Our study indicated that biogenic Mn oxides may play essential roles in facilitating in situ metal(loid) bioremediation in BAC of mine drainage.

Application of biological soil crusts for efficient cadmium removal from acidic mine wastewater

Utilizing an acid-resistant biological soil crust (BSC) species that we discovered, we developed a device capable of efficiently removing cadmium (Cd) from mine wastewater with varying levels of acidity. Our research has demonstrated that this particular BSC species adapts to acidic environments by regulating the balance of fatty acids and acid-resistant enzymes. At a Cd concentration of 5 mg/L, the BSC grew well. When the initial Cd concentration was 2 mg/L, and the flow rate was set at 1 mL/min (at pH levels of 3, 4, and 5), BSC had a high removal rate of Cd, and the removal rate increased with the increase of pH (from 90% to 97%). Chemisorption is the primary removal mechanism in the initial stage, where the functional groups and minerals on the surface of the BSC play a significant role. In addition, BSC also adapts to Cd stress by changing bacterial community structure. It was discovered through infrared spectroscopy and two-dimensional correlation analysis that hydrophilic groups, specifically phosphate and carboxyl groups, exhibited the highest reactivity during the Cd binding process. Protein secondary structure analysis confirmed that as the pH increased, the adsorption capacity of the BSC increased; making biofilm formation easier. This study presents a novel approach for the treatment of acidic wastewater.

The dissolution characteristics of cadmium containing birnessite produced from paddy crusts

The cadmium (Cd) accumulates in birnessite as it forms on the surface of paddy crusts (PC). The stability of Cd-containing birnessite is influenced by environmental factors, and destabilized birnessite releases dissolved Cd. We report the effects of pH, oxalic acid, and light on the dissolution of Cd-containing birnessite. We found that at pH 4.0, with light and 0.20 mol/L oxalic acid, the ratio of dissolved Cd and manganese (Mn) peaked after 24 h at 2978.0 μg/g and 326.8 mg/g, respectively. The three environmental factors affected the dissolution of Cd-containing birnessite in the following order: pH > oxalic acid > light. During dissolution process, Cd and Mn did not dissolve simultaneously, and the dissolved Cd/Mn ratio in the solution was significantly lower than that of the pristine mineral (33.5 × 10-3). Compared with Mn, Cd dissolution was inhibited by strong acidity (pH 4.0-5.0), and the dissolved Cd/Mn ratio was 5-10 × 10-3. Mild acidity (pH 6.0) was weakly inhibitory, with a Cd/Mn ratio of 6-15 × 10-3. In an alkaline (pH 8.0) oxalate environment, light illumination inhibited Cd dissolution, and the Cd/Mn ratio decreased over time due to the stability of the products formed by oxalate and carbonate, with Cd being more stable than those formed by Mn. Our findings would provide insights into the migration and transformation of PC-associated Cd in paddy fields.

Sasa argenteostriata - A potential plant for phytostabilization remediation of lead-zinc tailing-contaminated soil

Phytoremediation is an effective way to remediate metal-contaminated soils. During phytoremediation, plants immobilize heavy metals through the roots to reduce the mobility, toxicity and dispersal of the metals, and the changes in the activity of the roots are often accompanied by changes in the rhizosphere ecosystems, in which rhizobacteria are essential components and interact with roots to maintain the stability of the rhizosphere ecosystem and improve soil health. In this study, the phytoremediation potential of Sasa argenteostriata (Regel) E.G. Camu and the response of rhizobacteria were revealed with different levels of lead-zinc tailing contamination (Pb, Zn, and Cd concentrations of 1197.53, 3243.40, and 185.44 mg/kg for M1 and 2301.71, 6087.95, and 364.00 mg/kg for M2, respectively). The BCF of Sasa argenteostriata increased with increasing soil pollution, and the BCFPb, BCFZn, and BCFCd were 0.19, 0.27, and 0.08, respectively, under the M2 treatment; in contrast, the TF decreased with increasing soil pollution, and the TFPb, TFZn, and TFCd were 0.39, 0.85, and 0.07, respectively, under the M1 treatment. The mobility of Pb in the rhizosphere was higher than that of Zn and Cd, and the percentage of residual (Res) Zn and Cd in the rhizosphere increased, while the acid-soluble (Aci) Pb was significantly higher, leading to obvious uptake of Pb by the roots. Correlation analysis showed that Sasa argenteostriata affected the rhizobacterial community by changing the rhizosphere soil pH, the contents of organic matter and NRFM, and bacteria such as Proteobacteria and MND1, which are highly resistant to heavy metals (HMs), became the dominant species in the community. Further PICRUSt2 analysis showed that reducing metal transport across the membranes and increasing the efficiency of cellular reproduction were the main metabolic mechanisms of bacterial tolerance to HMs. Overall, the roots of Sasa argenteostriata were able to immobilize more heavy metals in PbZn tailing-contaminated soil, reducing the toxicity of HMs in the soil, and then influencing the rhizobacteria to change the community structure and metabolism mechanism to adapt to the HM-contaminated environment, and the soil fertility was increased, which together promoted the health and stability of the soil. This study is the first to illustrate the phytoremediation potential and response of the rhizobacterial community of Sasa argenteostriata under multimetal contamination of PbZn tailings. The results of the study provide some guidance for the practice of lead-zinc tailing-phytoremediation and soil health.

Fertilizer-induced manganese oxide formation enhances cadmium removal by paddy crusts from irrigation water

Fertilization is a crucial agrological measure for agricultural production that can significantly impact the removal of Cd from irrigation water by paddy crusts (PC). In this study, laboratory and field experiments were conducted to investigate the impact of fertilization at low, medium, and high concentrations on the accumulation of Cadmium (Cd) in PC and the underlying mechanisms involved. The results showed that only low fertilizer concentration could promote the removal of Cd by PC, which reduced the Cd concentration in irrigation water from 19.52 μg/L to 5.35 μg/L. Conversely, medium and high fertilizer concentrations reduced the accumulation of Cd by PC. After fertilizer addition, the proportion of Fe-Mn oxidizable-Cd in PC reached 55 % (with low concentration of fertilizer treatment). The application of low concentration of fertilizer was found to stimulate the growth of filamentous green algae, leading to a significant increase in the relative abundance of sphingomonadaceae (by 1.39 %) and comamonadaceae (by 1.29 %). The XRD, SEM and correlation analysis show that a large amount of manganese oxide is formed on the surface of PC, which increases the fixation of Cd. These findings provide a new perspective for the remediation of heavy metal contamination in paddy fields.

Cadmium accumulation in rice grains is mitigated by duckweed-like hydrophyte through adsorption and increased ammonia nitrogen

Although increasing attention has been paid to agronomic measures for reducing the heavy metal load in rice grain, the effects of duckweed-paddy co-cropping technology on the accumulation of cadmium (Cd) in rice grains remain unclear. To investigate its specific effects on Cd accumulation in paddy fields, three types of duckweed-like hydrophyte (DH), Azolla imbricata, Spirodela polyrrhiza, and Lemna minor were chosen for study. Their use resulted in a reduction of Cd content in rice grains from 0.40 mg/kg to <0.20 mg/kg, with A. imbricata yielding the best results (0.15 mg/kg). The three types of DH reduced the available Cd content in the soil by 10 % to 35 % after the paddy tillering stage. The reduction of available Cd content was attributed to the absorption, high pH, and increase of relative abundance of special bacteria of immobilizing Cd. In addition, DH could regulate soil nitrogen leading to ammonium nitrogen increased from 75 mg/kg to 100 mg/kg, while nitrate nitrogen decreased from 0.55 to 0.1-0.3 mg/kg. The increase of ammonium nitrogen content might induce the low Cd transfer ability in rice plant and then low Cd content in rice grain. This study demonstrated that DH has a good effect on the reduction of the Cd concentration in rice grains. Consequently, duckweed-paddy co-cropping technology offers a potential solution to heavy metal pollution and agricultural non-point source pollution, as it not only reduces Cd levels in rice plants, but also fixes nitrogen, reducing the need for nitrogen application.

Immobilization of metal(loid)s from acid mine drainage by biological soil crusts through biomineralization

Acid mine drainage is harmful to the environment. Bioremediation based on biological soil crusts (BSCs) can be used as a new method to alleviate metal pollution in acid mine drainage. In this study, we found that BSCs can survive in a strongly acidic environment (pH = 3.28) and have a high metal(loid)s accumulation ability. The algae of genera Fragilaria, Klebsormidium, Cymbella, Melosira, Microcystacea, and Planctonema a're the main components of BSCs. These organisms in the BSCs regulated fatty acids and produced acid-resistant enzymes. The bioconcentration factors for As, Cd, Pb, Zn, and Cu were as high as 16,000, 200, 50, 26, and 400, respectively. The concentration of As and Cd in acid mine drainage decreased from 7.1 μg and 350 μg/L to 1.9 μg and 110 μg/L, respectively. In total, 56% of As, 73% of Cd, 88% of Pb, 85% of Zn, and 92% of Cu were present in BSCs as residual or mineral-bound forms. The XRD results (e.g., quarartz and phyllosilicates), SEM results (e.g., phylosilicates and diatom shells) and correlation results show that these metal(loid)s are immobilized by Cymbella (diatoms) during the deposition of silica in the acidic environment. Furthermore, adsorption and co-precipitation are other ways that metal(loid)s could have been bound. These findings provide new insights into the removal of metals (loid) in acidic water.

Cultivar-specific response of rhizosphere bacterial community to uptake of cadmium and mineral elements in rice (Oryza sativa L.)

Toxic metal-contaminated farmland from Cadmium (Cd) can enhance the accumulation of Cd and impair the absorption of mineral elements in brown rice. Although several studies have been conducted on Cd exposure on rice, little has been reported on the relationship between Cd and mineral elements in brown rice and the regulatory mechanism of rhizosphere microorganisms during element uptake. Thus, a field study was undertaken to screen japonica rice cultivars with low Cd and high mineral elements levels, analyze the quantitative relationship between Cd and seven mineral elements, and investigate the cultivar-specific response of rice rhizosphere bacterial communities to differences in Cd and mineral uptake in japonica rice. Results showed that Huaidao-9 and Xudao-7 had low Cd absorption and high amounts of mineral nutrient elements (Fe, Zn, Mg, and Ca, LCHM group), whereas Zhongdao-1 and Xinkedao-31 showed opposite accumulation characteristics (HCLM group). Stepwise regression analysis showed that zinc, iron, and potassium are the key minerals that affect Cd accumulation in japonica rice and zinc was the most important factor, accounting for 68.99 %. The accumulation of Cd and mineral elements is potentially associated with rhizosphere soil bacteria. Taxa enriched in the LCHM rhizosphere (phyla Acidobacteriota and MBNT15) indicated the high nutrient characteristics of the soil and reduced activity of Cd in soil. The HCLM rhizosphere was highly colonized by metal-activating bacteria (Actinobacteria), lignin-degrading bacteria (Actinobacteria and Chlorofexi), and bacteria scavenging nutrients and trace elements (Anaerolinea and Ketobacter). Moreover, the differences in the uptake of Cd and mineral elements affected predicted functions of microbial communities, including sulfur oxidation and sulfur derivative formation, human or plant pathogen, and functions related to the iron oxidation and nitrate reduction. The results indicate a potential association of Cd and mineral elements uptake and accumulation with rhizosphere bacteria in rice, thus providing theoretical basis and a new perspective on the maintenance of rice security and high quality simultaneously.

Short-term application of chicken manure under different nitrogen rates alters structure and co-occurrence pattern but not diversity of soil microbial community in wheat field

Manure application is an effective way to improve the utilization efficiency of organic resources and alleviate the adverse effects of long-term application of chemical fertilizers. However, the impact of applying manure under different nitrogen rates on soil microbial community in wheat field remains unclear. Treatments with and without chicken manure application under three nitrogen rates (N 135, 180 and 225 kg⋅hm-2) were set in wheat field. Soil organic carbon, available nutrients, and abundance, diversity, structure and co-occurrence pattern of soil microbial community at wheat maturity were investigated. Compared with no manure application, chicken manure application increased the soil organic carbon and available phosphorus, while the effects on soil mineral nitrogen and available potassium varied with different nitrogen rates. Chicken manure application significantly increased soil bacterial abundance under the nitrogen fertilization of 135 and 225 kg⋅hm-2, increased soil fungal abundance under the nitrogen fertilization of 135 kg⋅hm-2, but decreased soil fungal abundance under the nitrogen fertilization of 180 and 225 kg⋅hm-2 (P < 0.05). There was no significant difference in alpha diversity indices of soil microbial communities between treatments with and without chicken manure application under different nitrogen rates (P > 0.05). Chicken manure application and its interaction with nitrogen rate significantly changed soil bacterial and fungal community structures (P < 0.05). There were significantly different taxa of soil microbial communities between treatments with and without chicken manure application. Chicken manure application reduced the ecological network complexity of soil bacterial community and increased that of soil fungal community. In summary, the responses of soil available nutrients and microbial abundance to applying chicken manure varied with different nitrogen rates. One growing season application of chicken manure was sufficient to alter the soil microbial community structure, composition and co-occurrence pattern, whereas not significantly affected soil microbial community diversity.

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.

Biological soil crusts alleviate the stress of arsenic on rice germination and the underlying immobilization mechanisms

The high concentration of arsenic (As) in paddy soil has seriously threatened the growth of rice and human food safety. Biological soil crusts (BSC), which are ubiquitous in paddy fields, have been shown a high ability to capture trace metal elements. In the present study, we investigated the effectiveness and mechanism of BSC for immobilizing As, and tested their potential to alleviate the stress of As on rice germination. It is found that BSC can remove 77.8% of arsenic in solution with 3.5 mg L^-1 initial As concentration. The As content in BSC reached 514.5 mg kg^-1 after 216 h exposure, and the entrapped As was mainly distributed in BSC as a non-EDTA-exchangeable fraction, which might be intracellularly accumulated. Proteobacteria and Bacteroidetes were the dominant phyla in BSC after being exposed to As, playing a significant role in tolerating As and As biogeochemical cycling. The presence of BSC notably promoted the germination rate (18.3%) and dry biomass (103.4%) of rice seeds under 3.5 mg L^-1 As stress while reducing the As content in plant roots (8.2-34.3%) and shoots (8.7-47.6%). These findings demonstrate that BSC have a great entrapping effect on As and highlight the importance of BSC in alleviating the stress on rice germination by As, providing a potential nature-based and low-cost strategy to decontaminate paddy fields polluted with As.

Lime-Phosphorus Fertilizer Efficiently Reduces the Cd Content of Rice: Physicochemical Property and Biological Community Structure in Cd-Polluted Paddy Soil

Due to the biomagnifying effect in the food chains, heavy metals will cause serious harm to the food produced in paddy soil, and then threaten human health. The remediation of soil heavy metals by the addition of amendments is a common method. However, the combination of the two amendments has been less studied and its effect is unknown. In this study, we investigated the effects of different concentrations of a lime and calcium-magnesium phosphate (CMP) amendments metal availability and paddy soil bacteria biodiversity. The experiment proves that the addition of 0.5 and 1.0‰ amendment can effectively reduce cadmium (Cd) availability and the cadmium content in rice to be below 0.2 mg/kg, meeting the national food safety level. The results demonstrate that increasing pH and phosphorous (P) in soil were two important factors decreasing available cadmium. Furthermore, biodiversity analysis of the treated soil showed that the amendment increased biodiversity. Proteobacteria and Chloroflex were the most abundant bacteria at the phylum level, followed by Acidobacterium and Nitrospirae. The abundance of Bacterodietes-vadinHA17, Syntrophaceae, and Thiobacillus increased as phosphorous increased. Cadmium passivation might induce those species.

Regulation of heavy metals accumulated by Acorus calamus L. in constructed wetland through different nitrogen forms

Improving accumulation of heavy metals (HMs) by plants is an important pathway for constructed wetland (CW) to alleviate the environmental risks caused by their release. This study aims to regulate HMs (Cr, Ni, Cu, Zn, and Cd) accumulated by Acorus calamus L. in the sandy substrate CW with different nitrogen forms, including ammonia (NH₄⁺), nitrate (NO₃‾), and NH₄⁺/NO₃‾ (1:1) in synthetic tailwaters. In general, the removal efficiency of HMs by CW could reach 92.4% under the initial concentrations below 500 μg/L. Accumulation percentages of HMs in the shoots and roots of plants in CW with NH₄⁺ and NH₄⁺/NO₃‾ influents increased by 52-395% and 15-101%, respectively, when compared with that of NO₃‾ treatment. Influents with NH₄⁺ promoted plant growth of Acorus calamus L. and metabolic functions, such as carbohydrate metabolism/amino acid metabolism, related to HMs mobilization of rhizosphere bacterial communities, which might induce more organic acids and amino acids secreted by plants and microbes during their metabolic processes. These are the main reasons for the enhancive mobilization of HMs from their precipitation fractions and their uptake by plants in CW with NH₄⁺ treatments. Moreover, the enhancement of organics secreted from plants and microbes also led to the high denitrification efficiency and nitrogen removal in CW. Overall, this study could provide a feasible method for the enhancive accumulation of HMs by wetland plants via the regulation water treatment process to appropriately increase NH₄⁺ for CW.

Nitrogen fertilizer management affects remobilization of the immobilized cadmium in soil and its accumulation in crop tissues

Immobilization of soil cadmium (Cd) has been the strategy mostly used in remediation of Cd-contaminated arable soil. However, Cd might be remobilized after the immobilization process through the acid-soluble and complexation effects. Development of agronomic management technologies to prevent soil Cd remobilization after the immobilization process was an important pathway to control the food safety of agricultural products in soils with the immobilized Cd. In this study, the ammonia (NH4+-N) and nitrate (NO3--N) forms with concentrations of 60, 90, and 150 mg-N kg-1 soil were performed for evaluating their effects on Cd remobilization with planted or unplanted treatments and Cd accumulation in tissues of edible amaranth (Liuye). With an initial soil palygorskite-bound fraction Cd concentration of 0.6 mg kg-1, bioavailable Cd in rhizosphere soils and Cd in crop shoots respectively increased from 11.4 to 20.6 μg kg-1 (dry soil weight) and 6.92 to 14.92 mg kg-1 (dry plant weight) in planted NH4+-N treatments, while significantly lower concentrations of bioavailable Cd in rhizosphere soils and Cd in crop tissues were observed with planted NO3--N treatments. Compared with that of planted NO3--N treatments, decreasing pH value (i.e., 7.64 to 7.18) induced by root proton efflux during the absorption of NH4+-N, enhancive organic/amino acid (oxalic acid, lactic acid, L-proline, and so on) secretion from roots, and increasing abundance of bacteria distributed in phyla Proteobacteria, Cyanobacteria, and Bacteroidetes with Cd mobilization ability in rhizosphere soils were the main reasons found in this study for the higher Cd remobilization in soils and Cd accumulation in crop under NH4+-N treatments. Moreover, the direct effect of NH4+-N on remobilization of immobilized Cd by upregulating the expression abundances of genes associated with pyruvate metabolism and amino acids metabolism was more significant than that of NO3--N. In summary, the use of NO3--N as preferred N fertilizer was more efficient to ensure the food safety of agricultural products than that of NH4+-N in Cd-contaminated arable soil after immobilization process.

Remediation Techniques for Cadmium-Contaminated Dredged River Sediments after Land Disposal

This paper examines the remediation techniques of cadmium (Cd)-contaminated dredged river sediments after land disposal in a city in East China. Three remediation techniques, including stabilization, soil leaching, and phytoremediation, are compared by analyzing the performance of the techniques for Cd-contaminated soil remediation. The experimental results showed that the stabilization technique reduced the leaching rate of soil Cd from 33.3% to 14.3%, thus effectively reducing the biological toxicity of environmental Cd, but the total amount of Cd in soil did not decrease. Leaching soil with citric acid and oxalic acid achieved Cd removal rates of 90.1% and 92.4%, respectively. Compared with these two remediation techniques, phytoremediation was more efficient and easier to implement and had less secondary pollution, but it took more time, usually several years. In this study, these three remediation techniques were analyzed and discussed from technical, economic, and environmental safety perspectives by comprehensively considering the current status and future plans of the study site. Soil leaching was found to be the best technique for timely treatment of Cd contamination in dredged river sediments after land disposal.

Biological aqua crust mitigates metal(loid) pollution and the underlying immobilization mechanisms

Biocrust-mediated in situ bioremediation could be an alternative strategy to mitigate metal(loid) pollution in aquatic habitats. To better understand the roles of biocrusts in regulating the fate of metal(loid)s, we examined the morphology, composition and structure of biological aqua crusts (BAC) developed in the mine drainage of a representative Pb/Zn tailing pond, and tested their effectiveness for immobilizing typical metal(loid)s. Unlike terrestrial biocrusts, BAC results from an assembly of compounds produced by the strong microbial activity and mineral compounds present in the aquatic environment. The BAC exhibited a unique flexible, spongy and porous structure with a specific surface area of 12-22 m² g^-1, and was able to effectively concentrate various metal(loid)s (e.g. Cd, 0.26-0.60 g kg^-1; Pb, 0.52-0.66 g kg^-1; As, 10.4-24.3 g kg^-1). The concentrations of metal(loid)s (e.g. Cd and As) in the BAC were even three to seven times higher than those in the source tailings, and more than 98% of immobilized metal(loid)s were present as the highly stable non-EDTA-exchangeable fraction. Adsorption on the well distributed micro-particles of the clay minerals (e.g. kaolinite) and the organic matters (2.0-2.7 wt.%) were found to be the major mechanisms for BAC to bind metal cations, whereas adsorption and coprecipitation on Fe/Mn oxide (e.g. FeOOH), was proposed to be the dominant pathway for accumulating metal(loid)s, especially As. The decrease in aqueous concentrations of the metal(loid)s along the drainage could be attributed in part to the scavenging effects of the BAC. These findings therefore provide new insights into the possible and efficient strategy for metal(loid) removal from water bodies, and highlighted the important role of BAC as a nature-based solution to benefit the bioremediation of mining area.

Paddy periphyton reduced cadmium accumulation in rice (Oryza sativa) by removing and immobilizing cadmium from the water-soil interface

Periphyton plays a significant role in heavy metal transfer in wetlands, but its contribution to cadmium (Cd) bioavailability in paddy fields remains largely unexplored. The main aim of this study was to investigate the effect of periphyton on Cd behavior in paddy fields. Periphyton significantly decreased Cd concentrations in paddy waters. Non-invasive micro-test technology analyses indicated that periphyton can absorb Cd from water with a maximum Cd²⁺ influx rate of 394 pmol cm^-2 s^-1 and periphyton intrusion significantly increased soil Cd concentrations. However, soil Cd bioavailability declined significantly due to soil pH increase and soil redox potential (Eh) decrease induced by periphyton. With periphyton, more Cd was adsorbed and immobilized on organic matter, carbonates, and iron and manganese oxides in soil. Consequently, Cd content in rice decreased significantly. These findings give insights into Cd biogeochemistry in paddy fields with periphyton, and may provide a novel strategy for reducing Cd accumulation in rice.