Bioavailable Cd Concentrations Research Articles

Recalcification stabilizes cadmium but magnifies phosphorus limitation in wastewater-irrigated calcareous soil

Long-term wastewater irrigation leads to the loss of calcium carbonate (CaCO3) in the tillage layer of calcareous land, which irreversibly damages the soil's ability to retain cadmium (Cd). In this study, we selected calcareous agricultural soil irrigated with wastewater for over 50 years to examine the recalcification effects of sugar beet factory lime (SBFL) at doses of 0%, 2.5%, 5%, and 10%. We found that SBFL promoted Cd transformation in the soil from active exchangeable species to more stable carbonate-bonded and residual species, which the X-ray diffraction patterns also confirmed results that CdSO4 reduced while CdS and CaCdCO3 increased. Correspondingly, the soil bioavailable Cd concentration was significantly reduced by 65.6–84.7%. The Cd concentrations in maize roots and shoots were significantly reduced by 11.7–50.6% and 13.0−70.0%, respectively, thereby promoting maize growth. Nevertheless, SBFL also increased the proportion of plant-unavailable phosphorus (P) in Ca8–P and Ca10–P by 4.3–13.0% and 10.7−25.9%, respectively, reducing the plant-available P (Olsen P) content by 5.2–22.1%. Consequently, soil P-acquiring associated enzyme (alkaline phosphatase) activity and microbial (Proteobacteria, Bacteroidota, and Actinobacteria) community abundance significantly increased. Our findings showed that adding SBFL to wastewater-irrigated calcareous soil stabilized Cd, but exacerbated P limitation. Therefore, it is necessary to alleviate P limitations in the practice of recalcifying degraded calcareous land.

Enhancing Cd (II) immobilization with thiol-modified low-temperature pyrolysis biochar: Efficiency, mechanism, and applications

The fixation efficiency of thiol-modified lotus seedpod biochar on Cd (II) was assessed through a comprehensive investigation involving batch experiments and sediment cultivation experiments. The influence of thiol modification on the Cd (II) adsorption performance was investigated under different pyrolysis temperatures (550°C, 700°C, and 850°C). The physicochemical properties, adsorption kinetics, isotherms, and adsorption mechanisms of the modified biochar were studied. Oxidation and thiolation significantly reduced the specific surface area and total pore volume of the biochar, augmenting its hydrophilicity. High-temperature pyrolysis biochar was more suitable for oxidation modification than thiol modification. The abundant oxygen-containing functional groups in low-temperature pyrolysis biochar can serve as active sites for thiol loading. Therefore, it exhibited the highest grafting rate, the highest affinity for Cd (II), and the maximum Cd (II) adsorption capacity of 68 mg/g. Thiol-modified biochar primarily immobilized Cd (II) through chelation. Moreover, it significantly reduced the concentration of bioavailable Cd (II) in the sediment and enhanced enzymatic activity. These findings underscore the potential role of low-temperature pyrolysis thiol-modified biochar in addressing Cd (II) pollution in water and sediments.

Surface Co-application of Dolomitic Lime with Either Biochar or Compost Changes the Fractionation of Cd in the Soil and Its Uptake by Cacao Seedlings

The purpose of this study was to determine the effects of the application of compost or biochar on the mobility of soil-applied dolomite lime in the soil and its impact on Cd bioavailability, as well as its uptake by cacao seedlings. The experiment was conducted in a greenhouse for 120 days. Dolomite lime was applied superficially without incorporation, individually or in co-application with commercial compost or corn straw-derived biochar. Soil samples were collected at four depths (0–10, 10–20, 20–30, and 30–40 cm). Subsequently, pH, electrical conductivity, and dissolved organic carbon were measured. Bioavailable Cd and geochemical fractionation of Cd were measured at each soil depth. Root- and leaf-Cd concentrations in cacao seedlings were also determined. The results illustrate that the application of either compost or biochar has a pronounced impact on enhancing the mobility of dolomite lime within the soil (up to 40 cm). Notably, heightened alkalinity penetration was observed when dolomite lime and biochar were jointly applied. Concurrently applying biochar with dolomite lime led to a significant elevation in soil pH and a marked reduction in the concentration of bioavailable Cd. Furthermore, the concentration of leaf-Cd underwent a reduction exceeding 50% (± 2%) due to the application of dolomite lime and biochar. Potential mechanisms connected to Cd immobilization may encompass ion exchange and the formation of co-precipitates. Overall, the simultaneous use of dolomite lime and biochar was effective in decreasing Cd concentrations in cacao seedlings.

Amelioration of cadmium stress by supplementation of melatonin and ZnO-nanoparticles through physiochemical adjustments in Brassica oleracea var. capitata

The Brassicaceae family, including the globally consumed Brassica oleracea var. capitata, faces multiple stressors like heavy metal toxicity, impacting their growth and quality. These stressors significantly impact plant growth, productivity, and overall quality. Particular nanoparticles and phytohormones play a pivotal role in regulating plant responses to such abiotic stressors. In the current study, the utilization of zinc oxide nanoparticles (ZnO-NPs) and melatonin (MT) was explored to enhance the resilience of Brassica oleracea var. capitata to cadmium (Cd) stress. The nanoparticles were synthesized in an environmentally friendly manner using Clinopodium vulgare L. leaf extract. Microscopic and spectroscopic techniques were employed to characterize the ZnO-NPs. Treatments were chosen as different concentrations of nanoparticles i.e., NP1 = 25 mg L−1 ZnO, NP2 = 50 mg L−1 ZnO, NP3 = 100 mg L−1 ZnO, MT = 200 µM, and NP0 (distilled water). The results revealed that ZnO-NPs showed a concentration-dependent effect on B. oleracea growth parameters and chlorophyll content, with the foliar application of MT further augmenting the impact. ZnO-NP treatments effectively reduced Cd concentrations while concurrently elevating zinc (Zn) levels in shoots and roots. The addition of exogenous MT further potentiated these effects. Notably, the treatments with 50 and 100 mg L−1 ZnO-NPs individually resulted in a growth increase of 38 % and 40 %, respectively, along with corresponding Zn content elevations of 20.1 % and 24 % compared to the control. The 100 mg L−1 ZnO-NPs treatment extended shoot length by 63.5 % and enhanced Zn content by 51 % compared to the control. ZnO-NPs and MT elevated total Cd absorption in shoots and roots, reducing post-harvest bioavailable Cd concentration in soil compared to the control. In conclusion, the synergy of ZnO-NPs and melatonin holds promise as a viable strategy to both counteract Cd toxicity and promote Zn bio-fortification in crops, presenting implications for healthier produce in contaminated soils.

Cerium oxide nanoparticles alleviates stress in wheat grown on Cd contaminated alkaline soil

The cadmium contamination of soil is an alarming issue worldwide and among various mitigation strategies, nanotechnology mediated management of Cd contamination has become a well-accepted approach. The Cerium Oxide Nanoparticles (CeO2-NPs) are widely being explored for their novel works in Agro-Industry and Environment, including stress mitigation in crops. Very little work is reported regarding role of CeO2-NPs in management of Cd contamination in cereal crops like wheat. Present work was planned to check efficacy of CeO2-NPs in Cd stress mitigation of wheat under alkaline calcareous soil conditions. In this experiment, 4 sets of Cd contamination (Uncontaminated control-UCC, 10, 20, and 30 mg Cd per kg soil) and 5 sets of CeO2-NPs NPs (0, 200, 400, 600, and 1000 mg NP per kg soil) were applied in pots following completely randomized design (CRD) and wheat crop was grown. The growth, physiology, yield and Cd and Ce accumulation by wheat root, shoot and grain was monitored. The maximum Cd spiking level (30 mg kg−1) was found to be most toxic for plant growth. The results showed that the nanoparticles were overall beneficial for wheat growth and maximum level (1000 mg kg−1) being the most significant one under all Cd spiking sets. In Cd-30 sets, 1000 mg kg−1 NPs application resulted in decreased soil bioavailable Cd concentration (49.63% decrease compared to 30 mg kg−1 Cd spiked sets termed as Cd-30 Control), decreased Cd accumulation in all three tissues: root (58.36% decrease), shoot (52.30% decrease) and grain (55.56% decrease) while increased root dry weight (62.14%), shoot dry weight (89.32%), total grain yield (80.08%) and improved plant physiology with respect to Cd-30 control. Nanoparticles application substantially increased wheat root, shoot and grain Ce concentrations as well. The further prospects of these nanoparticles in relation to various biotic and abiotic stresses are advised to be explored.

Biochar inhibited hydrogen radical-induced Cd bioavailability in a paddy soil

Herein, hydrogen (H·) radical was observed as a new pathway to produce hydroxyl (OH·) radicals that promoted cadmium sulfide (CdS) dissolution and thus Cd solubility in paddy soils. In soil incubation experiments, the bioavailable Cd concentrations in flooded paddy soils were increased by 8.44 % as the soil was aerated for 3d. For the first time, the H· radical was observed in aerated soil sludge. The association of CdS dissolution with free radicals was thereafter confirmed in an electrolysis experiment. Both H· and OH· radicals in electrolyzed water were confirmed by the electron paramagnetic resonance analysis. In the system with CdS, water electrolysis increased soluble Cd2+ concentration by 60.92 times, which was compromised by 43.2 % when the radical scavenger was introduced. This confirmed the free radicals can lead to oxidative dissolution of CdS. The H· radical was generated in systems with fulvic acid or catechol irradiated by ultraviolet lights, indicating soil organic carbon could be an important precursor for H· and OH· radicals. Biochar application decreased soil DTPA-Cd by 22–56 % invoking mechanisms besides adsorption. First, biochar quenched radicals and reduced CdS dissolution by 23.6 % in electrolyzed water in which -C-OH of biochar was oxidized to CO. Second, biochar boosted Fe/S-reducing bacteria and thus compromised CdS dissolution, as affirmed by a reversal correlation between soil available Fe2+ and DTPA-Cd concentrations. A similar phenomenon occurred in Shewanella oneidensis MR-1-inoculated soils. This study provided new insights into the bioavailability of Cd and offered feasible measures to remediate Cd-contaminated paddy soils with biochars.

Relative Bioavailability of Cadmium in Rice: Assessment, Modeling, and Application for Risk Assessment.

Rice consumption is the primary route of cadmium (Cd) exposure to the populations with rice as the staple food. To accurately assess the potential health risks of Cd exposure via rice consumption, determination of Cd relative bioavailability (RBA) in rice is necessary. However, large variations exist in Cd-RBA, hindering the application of source-specific Cd-RBA values to different rice samples. In this study, we collected 14 rice samples from Cd contaminated areas and determined both rice compositions and Cd-RBA using in vivo mouse bioassay. Total Cd concentration varied from 0.19 to 2.54 mg/kg in the 14 rice samples, while Cd-RBA in rice ranged from 42.10% to 76.29%. Cadmium-RBA in rice correlated positively with calcium (Ca) (R = 0.76) and amylose content (R = 0.75) but negatively with the concentrations of sulfur (R = -0.85), phosphorus (R = -0.73), phytic acid (R = -0.68), and crude protein (R = -0.53). Cd-RBA in rice can be predicted by Ca and phytic acid concentrations in a regression model (R2 = 0.80). Based on the total and bioavailable Cd concentrations in rice, weekly dietary Cd intake for adults was estimated to be 4.84-64.88 and 2.04-42.29 μg/kg bw/week, respectively. This work demonstrates the possibility of Cd-RBA prediction based on rice compositions and provides valuable suggestions for health risk assessment with consideration of Cd-RBA.

Polypropylene microplastics affect the distribution and bioavailability of cadmium by changing soil components during soil aging.

Compared with the widespread and serious heavy metal contamination in soils, microplastic pollution has gained attention only recently. Little is known about how microplastics affect the distribution of heavy metals in soils, especially across soil components level. In this study, a 180-day soil aging experiment and soil density fractionation were performed to investigate the effect of polypropylene (PP) microplastics on the binding behavior of cadmium (Cd) to solid components, i.e. particulate organic matter, organo-mineral complexes (OMC), and mineral. Results showed addition of 2-10% microplastics in soils induced the decomposition of OMC fraction by 10.88-23.10%. Compared to the control, the content of dissolved organic carbon increased, and pH, humic substances, and soil organic matter decreased with microplastics. After 180d of aging, the content of Cd in OMC fraction increased by 17.92%, while microplastics made Cd contents decline by 10.01-19.75%. The impacts strongly depended on the dose and surface characteristic of microplastics. Overall, PP microplastics increased the concentration of bioavailable Cd in soils via decreasing soil retention of Cd by the OMC fraction. These findings based on the solid components level will provide a new perspective for understanding microplastics effects on soil systems and pollutants.

Combined effects of zinc oxide nanoparticles and melatonin on wheat growth, chlorophyll contents, cadmium (Cd) and zinc uptake under Cd stress

Zinc oxide nanoparticles (ZnONPs) and melatonin (MT) have been known to regulate heavy metal toxicities in plants in some studies, the effect of their combined use on cadmium (Cd) uptake by wheat (Triticum aestivum L.) and underlying mechanisms is largely unknown. Thus, plant growth, uptake and translocation of Cd mediated by soil applied ZnONPs and foliar applied MT were investigated in wheat grown in Cd polluted soil under ambient conditions. The results depicted that ZnONPs stimulated the growth, chlorophyll contents, and yield of wheat in a dose additive way and this effect was further increased with foliar application of MT. 100 mg/kg of ZnONPs alone enhanced the grain yield by 60.5 % and this increase was about 177.5 % under combined ZnONPs and 100 μM MT treatment. ZnONPs treatments decreased Cd concentration whereas increased zinc (Zn) concentrations in shoots, roots, husks and grains and the effect was further increased with exogenous MT combined with NPs in a dose-additive way. 50 and 100 mg/kg ZnONPs treatments alone decreased grain Cd by 6.5 %, and 20 % and increased the Zn concentration by 20.1 % and 24 % than control. 100 mg/kg ZnONPs +100 μM MT treatment decreased the grain Cd by 63.5 % and increased grain Zn by 51 % than control treatment. Total Cd uptake (tissues biomass × Cd concentration in respective tissues) in shoots, roots, husks and grains increased with ZnONPs alone or combined with MT than control whereas soil post-harvest bioavailable Cd concentration decreased with treatments than control. The Cd reduction in grains was due to increase in biomass and Zn concentration thereby decreasing bioavailable Cd in soil and its accumulation in plants. This study suggested that combined use of ZnONPs and MT may provide new approaches for minimizing Cd and biofortification of Zn in edible parts of plants.

Dissolved organic matter-assisted phytoremediation potential of cotton for Cd-contaminated soil: a relationship between dosage and phytoremediation efficiency.

Dissolved organic matter (DOM) is a novel Cd-contaminated soils amendment for phytoremediation. However, the phytoremediation efficiency for different DOM doses has been insufficiently investigated. In this study, we investigated the effect of five DOM doses (v/w, 0%, 1%, 2%, 4% and 8%) on the phytoremediation efficiency of cotton in Cd-contaminated soil through pot experiment. The results showed that bioavailable Cd concentrations and organic matter in the soil increased with the increased of DOM dosage. The DOM dose increased the chlorophyll content, photosynthesis, and the total biomass of cotton. In addition, the DOM application increased the Cd content in cotton roots and changed the Cd uptake in cotton shoots, increasing shoot Cd extraction efficiency by 8.53-20%. Simultaneously, soil Cd phytoextraction efficiency significantly increased. Furthermore, applying a 1% DOM dose resulted in safeguarding fibre biomass and maximising the efficiency of shoot extraction. Redundancy analysis showed that the Mn content in leaves is critical for increasing cotton biomass, anti-oxidation competence and phytoremediation efficiency under 1% DOM dose. In conclusion, DOM enhanced cotton remediation in Cd-contaminated soils and applying DOM at 1% was a suitable choice for Cd-contaminated soils.

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

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

Detection of Bioavailable Cadmium by Double-Color Fluorescence Based on a Dual-Sensing Bioreporter System.

Cadmium (Cd) is carcinogenic to humans and can accumulate in the liver, kidneys, and bones. There is widespread presence of cadmium in the environment as a consequence of anthropogenic activities. It is important to detect cadmium in the environment to prevent further exposure to humans. Previous whole-cell biosensor designs were focused on single-sensing constructs but have had difficulty in distinguishing cadmium from other metal ions such as lead (Pb) and mercury (Hg). We developed a dual-sensing bacterial bioreporter system to detect bioavailable cadmium by employing CadC and CadR as separate metal sensory elements and eGFP and mCherry as fluorescent reporters in one genetic construct. The capability of this dual-sensing biosensor was proved to simultaneously detect bioavailable cadmium and its toxic effects using two sets of sensing systems while still maintaining similar specificity and sensitivity of respective signal-sensing biosensors. The productions of double-color fluorescence were directly proportional to the exposure concentration of cadmium, thereby serving as an effective quantitative biosensor to detect bioavailable cadmium. This novel dual-sensing biosensor was then validated to respond to Cd(II) spiked in environmental water samples. This is the first report of the development of a novel dual-sensing, whole-cell biosensor for simultaneous detection of bioavailable cadmium. The application of two biosensing modules provides versatile biosensing signals and improved performance that can make a significant impact on monitoring high concentration of bioavailable Cd(II) in environmental water to reduce human exposure to the harmful effects of cadmium.

Assessing the Risk of Total and Available Potentially Toxic Elements in Agricultural Soil in Typical Mining Areas in Xiangjiang River Basin, Hunan Province

In this study, soil and rice samples from 85 sites in six cities in Hunan Province were analyzed for Cu, Zn, Pb, Cd, Hg, Mn, and Co (total and bioavailable concentrations for soil) in July 2014. The results indicated that the total concentrations of Cu, Zn, Pb, Cd, and Hg in soil had increased significantly compared with the 1980s, and were correlated with their bioavailable concentrations in soil positively. The total concentrations of Cd and Co in soil were correlated with those in rice. Bioavailable concentrations of Cd, Mn, Co, Pb, and Cu represented 64.4%, 33.2%, 12.0%, 11.6%, and 6.1% of the total soil concentrations, respectively. The bioavailable concentrations of Cd and Co in soil had a extremely significant (p < 0.01) positive correlation with those in rice, suggesting that bioavailable concentrations was a better indicator for soil potentially toxic elements contamination. The pH values had a significant influence on the bioavailability of Cd and Cu and the amounts taken up by rice. The Cd contamination in 27.0% rice samples exceeded World Health Organization recommended thresholds. The results added basic pollution distribution data, further revealing the relationships of metals in soil and crops and would offer great help to the metallic pollution control in these areas.

Biochar-based fertilizer enhanced Cd immobilization and soil quality in soil-rice system

Cadmium (Cd) contamination poses a serious problem in paddy soils because of its high health risk through soil-food chain transfer. To evaluate the effect of biochar-based fertilizer on Cd uptake, soil and rice quality, biochar amendment at rates from 0 to 15 t/hm2 was conducted in Cd polluted paddy soils. For successive two rice seasons, biochar treatments greatly reduced rice Cd and soil bioavailable Cd content. Furthermore, the concentration of bioavailable Cd decreased accordingly with the increase of biochar content. When soil properties, such as pH and soil organic carbon (SOC) were significantly improved, grow indexes of rice, especially for the late rice, would be partly improved. Most of the bioavailable Cd was immobilized in root, only a small partition of Cd was transferred to brown rice. The main stage for Cd accumulation was from heading stage to harvest stage.

Effects of different biochars on physicochemical properties and immobilization of potentially toxic elements in soil - A geostatistical approach

The impact of different biochars (BCs) on the physicochemical properties and immobilization of potentially toxic elements (PTEs) in contaminated soil irrigated with industrial wastewater for the last three decades was studied. Furthermore, the efficacy of applied BCs in reducing geostatistical risks was also evaluated. For this purpose, BCs were prepared from green waste (Cynodon dactylon L.) for the first time at different pyrolysis temperature (400 °C, 600 °C and 800 °C), and amended the contaminated soil in pots with two different ratios of 2% and 5% (w/w) under controlled conditions. The BCs amended soil samples were analyzed after five months (equivalent to the life span of a wheat crop). The physicochemical impacts of applied BCs on the soil showed that the acidic soil was changed to basic. A tremendous increase in water holding capacity, cation exchange capacity, dissolved organic carbon, carbon, phosphorus and potassium contents was observed. The PTEs concentrations and geostatistical risks were significantly (p ≤ 0.05) decreased by all the BCs. Among them, BC prepared at 800 °C and applied at a ratio of 5% was showed the best effects by reducing the bioavailable concentrations of Cd, Pb, Cr, Ni, Cu, Mn, Fe, As, Co and Zn in 88%, 87%, 78%, 76%, 69%, 65%, 64%, 63%, 46% and 21%, respectively. Similarly, significant (p ≤ 0.05) reductions in geoaccumulation index, enrichment factor, contamination factor, and ecological risk were recorded. Therefore, BC prepared at 800 °C and applied at a ratio of 5% is recommended for soil remediation.

Attapulgite and processed oyster shell powder effectively reduce cadmium accumulation in grains of rice growing in a contaminated acidic paddy field

Heavy-metal contamination is widespread in agricultural soils worldwide, especially paddy soils contaminated by Cd. Amendment-induced immobilization of heavy metals is an attractive and effective technique, provided that cost-effective materials are used. This field experiment compared three alkaline passivators (attapulgite, processed oyster shell powder, and mixed soil conditioner) at a rate of 2.25 t ha−1 for their effectiveness in decreasing Cd bioavailability in soils and accumulation in rice plants in a paddy field contaminated by Cd (0.38 Cd mg kg−1). The utilization of attapulgite and processed oyster shell powder decreased labile fractions but increased stable fractions of Cd in soils through ion exchange, precipitation and complexation. The addition of attapulgite decreased the concentration of bioavailable Cd in both bulk and rhizosphere soils, whereas the amendment of processed oyster shell powder decreased it only in bulk soil. The Cd accumulation in rice plants correlated significantly with acid-soluble and residual Cd fractions in the rhizosphere soil but not in the bulk soil. The addition of attapulgite and processed oyster shell powder decreased Cd accumulation in rice grains from 0.26 mg kg−1 to 0.14 and 0.19 mg kg−1, respectively, meeting the National Food Safety Standard (< 0.20 mg kg−1). However, the mixed soil conditioner did not decrease the Cd accumulation in rice shoots or grains. This study demonstrated that attapulgite and processed oyster shell powder were economic agents in reducing Cd accumulation in rice grains.

Comprehensive assessment of heavy metal pollution and ecological risk in lake sediment by combining total concentration and chemical partitioning.

Total concentration and chemical partitioning of heavy metals are commonly used in environmental quality assessment; however, their comparability and comprehensive application are far less discussed. Herein, bioavailability, pollution and eco-risk of As, Cd, Cr, Cu, Ni, Pb and Zn in surface sediments of Erhai Lake were evaluated referring to multiple indices following the experimental methods of complete digestion, optimized Community Bureau of Reference (BCR) and 1.0M HCl extractions. Results of bioavailability for most metals were similar and comparable from BCR and HCl extractions. While bioavailable concentrations of Cd and Pb from HCl extraction were significantly (p<0.01) lower than those from BCR extraction, indicating BCR extraction is more efficient. Results of enrichment factor (EF) and concentration enrichment ratio (CER) suggested that Cd was the highest polluted element followed by As, Pb and Zn, whereas Cr, Cu and Ni were mainly natural in origin. Similar concentrations of anthropogenic As from EF and CER assessments indicated anthropogenic As mainly existed in bioavailable form. However, anthropogenic Cd, Pb and Zn existed in both bioavailable and residue forms, resulting in the underestimation of anthropogenic metals by the CER assessment. The sediment quality guidelines (SQGs), potential ecological risk index (Er) and risk assessment code (RAC) showed inconsistent eco-risks for each of the metals except Cd. Combining pollution level and chemical partitioning with SQGs, Er and RAC assessments, high eco-risk of Cd, moderate eco-risk of As and Pb, and low eco-risk of Cr, Cu, Ni and Zn were graded. Our study highlights the limitation of single index and the necessity of integrating multiple indices following total concentration and chemical partitioning in metal pollution and eco-risk assessments.

The vital function of humic acid with different molecular weight in controlling Cd and Pb bioavailability and toxicity to earthworm (Eisenia fetida) in soil

Humic acid (HA) plays vital roles in regulating the environmental behaviors of metals and thus their toxicity to biota. However, the inner relation between metal bioavailability to soil organisms and the presence of HA with different molecular weight (Mw) is not well documented. In this study, we separated HAs into four fractions with Mw range of 5-30k Da, and discussed their ability to alleviating the toxicity of Cd and Pb to earthworm. The bioaccumulation capacities (Cmax) increased in order of: UF1<UF2<UF3<UF4, which is in line with the variations of bioavailable concentrations of Cd and Pb in soil. Variations of Mw and binding capacities of HA determine the accumulation behavior in soil solution. The unsatisfactory of biotic ligand model fitting and the differences in fractions of the total biotic ligand sites (f) in earthworm bound by Cd and Pb suggested that only free species of Cd could be considered as biological available to earthworm, while the Pb–HAs complexes have potential ability to interact with earthworm membrane. Antioxidant enzymes are effective biomarkers, and HA with lower Mw play more important roles in restricting the toxicity of soil Cd and Pb to earthworm. These results reveal the different mechanism for HA controlling metal bioavailability between Cd and Pb in soil environment.

Migration and transformation of cadmium in rice - soil under different nitrogen sources in polymetallic sulfide mining areas

We conducted pot experiments to assess the bioavailability of cadmium (Cd) in contaminated rhizosphere soil and accumulation in rice organs in response to nitrogen (N) supply ((NH4)2SO4, NH4NO3, NH4Cl). The results showed that the concentration of bioavailable Cd in rice rhizosphere soil was (NH4)2SO4 treatment > NH4Cl treatment > NH4NO3 treatment at the same level of N application and growth period; the Cd concentration in rice roots was (NH4)2SO4 treatment > NH4NO3 treatment > NH4Cl treatment; and the Cd concentration in rice straw was NH4NO3 treatment > NH4Cl. The Cd concentration in rice roots, straws, and seeds at the maturity stage was (NH4)2SO4 treatment > NH4Cl treatment. With the same N fertilizer, excessive N promoted Cd accumulation in rice at later growth stages. This suggested that sulfate (SO42−) influenced Cd concentration in rice. NH4Cl application maintained a low Cd level in different rice organs with the same N level. This confirmed that NH4Cl is a safe N source for rice planting in polymetallic sulfide mining areas. The study concludes that appropriate NH4Cl levels for Cd-contaminated paddy soil with high-S-content could obtain rice grains with Cd concentrations below the food safety standards (0.2 or 0.4 mg·kg−1).

Improving cadmium mobilization by phosphate-solubilizing bacteria via regulating organic acids metabolism with potassium.

Organic acids secreted by phosphorus-solubilizing bacteria (PSB) is one of the main biological metabolites with cadmium (Cd) mobilization capacity in the conversion of insoluble precipitate forms to bioavailable forms in contaminated soil. However, the fluctuating concentrations of nutrient elements caused by agricultural activities may result in the substantial variances of carbohydrate metabolism of microorganisms involved in Cd remediation, it is therefore essential to study how metabolic strategies, especially for organic acids, affected by the environmentally friendly fertilizers, such as potassium (K). In this study, adding K+ (KCl) concentrations from 0.0 to 100.0mg/L in medium clearly accelerated Cd mobilization from 15.9 to 35.9mg/L via inducing the secretion of tartaric acid, 3-hydroxybutyrate, fumaric and succinic acids, increased by 10.0-, 7.5-, 4.3- and 4.1-fold changes, respectively. Current data revealed that the significant differences of metabolic pathways and genes expressions with the varied K+ concentrations included: ⅰ) K+ induces a substantial up-regulation in metabolic pathway of pyruvic acid to oxaloacetate and tartaric acids; ⅱ) the varied expression of genes involved in encoding enzymes of tricarboxylic acid cycle result in the up-regulated fumaric acid, succinic acid and 3-hydroxybutyrate; ⅲ) the expression of genes related enzyme cysteine and glutamate metabolism processes promoted with the increasing bioavailable Cd concentrations. Besides, P-type ATPase activity increased with K+ levels, indicating that H+ efflux and medium acidification were strengthened. In general, an appropriate enhancement of K based fertilizer is an effective manner for soil Cd remediation via the regulation of organic acids metabolism and H+ secretion of PSB.