Residual Fraction Of Cd Research Articles

Evaluation of the effects of mandarin (Citrus reticulate) by-products containing citric acid on immobilization of cadmium in arable soils

In a pilot study, we observed cadmium (Cd) immobilization with citric acid (CA) and suggested that mandarin by-products (MB), which contain CA at ca. 1.65%, can be used as soil amendments that reduce Cd bioavailability. In the present study, we (1) elucidated mechanisms of Cd immobilization by CA and (2) evaluated the use of MB as a soil amendment for Cd immobilization. In Experiment 1, CA was mixed with Cd contaminated soil at 0 and 3.5 mmol kg−1. We then added MB to Cd-spiked soil at 10, 20, and 40 g kg−1. Addition of CA decreased F2 (surface adsorbed Cd fraction) contents by 2.64 mg kg−1 compared with the control but was associated with increases in Cd fractions F1 (bioavailable Cd fraction) and F5 (residual Cd fraction) of 1.04 and 1.49 mg kg−1, respectively. Addition of CA enhanced the concentration of fraction F5, likely reflecting Cd precipitation from soil solutions with increased HCO3− concentrations. However, although this treatment immobilized Cd, it also led to increasing residual and bioavailable Cd fractions. Unlike CA treatments, MB increased non-bioavailable Cd fractions without increasing the bioavailable Cd fraction. Moreover, at 40 g kg−1, MB decreased F1 contents by 8% compared with the control, but increased F2, F3, and F5 contents by 3.6%, 0.7%, and 4.5%, respectively. Cd may be immobilized by MB through HxCO3− mediated precipitation as CdCO3 following decomposition of CA and concomitant increases in the negative charge of soil due to the organic matter in MB. MB also improved the chemical properties of soils, with increased nutrient concentrations and cation exchange capacities.

Characterization of phosphorus engineered biochar and its impact on immobilization of Cd and Pb from smelting contaminated soils

Biochar has been used as an amendment to immobilize toxic metals (TMs) and increase nutrient content in the contaminated soil. In this study, Ca(H2PO4)2-engineered swine manure biochar (PSMB) and swine manure biochar (SMB) were prepared and investigated to evaluate their effects on the availability of toxic metals (Cd and Pb) and soil nutrient content in Fengxian (FX)- and Kunming (KM)-contaminated soils through incubation experiment. The soil samples were collected at 0–20 cm depth from two contaminated farmlands close to abandoned Pb smelters, which are located in FX and KM. Swine manure was obtained from the rural hog lot in Yangling, China. Each biochar (SMB and PSMB) was mixed with the air-dried soil samples (FX and KM) at a dosage of 1% and 3% w/w, respectively. Then, a portion of 3 kg of the mixture was placed in plastic pots and incubated for 30 days in a greenhouse. After the soil incubation experiment, pot experiments were performed in a greenhouse of Northwest A&F University, Yangling, China. Adding PSMB at a rate of 3% maximally decreased the concentrations of DTPA-extractable Cd (34.02 and 47.73%) and Pb (18.70 and 24.58%) in FX and KM soil. The BCR data revealed that the percentage of acid-soluble Cd and Pb fraction in FX and KM soils remarkably decreased whereas the percentages of residual Cd fraction and reducible Pb fraction in FX and KM soils significantly increased in all the biochar-amended soils compared with control. To confirm these results, a pot experiment was conducted to investigate the effects of amendments on growth and uptake of toxic metals in Chinese cabbage (Brassica rapa L. spp. pekinensis). The results showed that the addition of PSMB3% significantly (P < 0.05) increased the biomass of plants and reduced the absorption of toxic metals in plant. The results revealed that the dry biomass increased and the absorption of toxic metals decreased in plants. All treatments of biochar were beneficial and application of PSMB 3% was most effective in enhancing plant biomass and reducing the accumulation of toxic metals in the plants.

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

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

Disinfection and removal performance for Escherichia coli, toxic heavy metals and arsenic by wood vinegar-modified zeolite

Zeolite has traditionally been used to remediate wastewater and soil. The present study shows a new method for natural zeolite (NZ) modification with wood vinegar (WV). The optimal conditions for NZ modification with WV were determined, and the adsorption capacities towards lead (Pb), cadmium (Cd) and arsenic (As), antimicrobial activities against Escherichia coli, heavy metal(loid) fraction and characterizations of selected modified zeolites (MZs) were also investigated. The results indicate that 50-fold dilution of WV, 5 g of NZ dosage, 105 °C of drying temperature, 4 h and 95 °C of water bath are preferred for NZ modification with WV. The WV+NaOH-MZ exhibited the best performance in heavy metal removal and the most powerful antimicrobial activity among all the zeolites. The sequence of WV+NaOH-MZ for the maximum single metal(loid) adsorption capacities was Pb (48.67 mg/g) >Cd (23.67 mg/g) > As (0.024 mg/g). The WV+NaOH and WV modifications also can increase the stabilities of heavy metals in the MZs. The residual fractions of single Pb and Cd in WV+NaOH-MZ and WV-MZ were 50%, 55%, 34% and 30%, respectively. The pore size of WV+NaOH-MZ (11.73 nm) was bigger than that of NZ or WV-MZ. Additionally, the proportion of clinoptilolite in WV+NaOH-MZ was also higher than other zeolites. The surfaces of WV+alkali-MZs were rougher than that of NZ. Considering the low cost and environmental risk of WV, this work provides some useful information for management of agricultural and industrial residues, environment and food safety.

Enhancing phytoextraction of potentially toxic elements in a polluted floodplain soil using sulfur-impregnated organoclay

Enhancing metals phytoextraction using gentile mobilizing agents might be an appropriate approach to increase the phytoextraction efficiency and to shorten the phytoremediation duration. The effect of sulfur-impregnated organoclay (SIOC) on the redistribution of potentially toxic elements (PTEs) among their geochemical fractions in soils and their plant uptake has not yet been studied. Therefore, our aim is to investigate the role of different SIOC application doses (1%, 3% and 5%) on operationally defined geochemical fractions (soluble+exchangeable; bound to carbonate; manganese oxide; organic matter; sulfide; poorly- and well-crystalline Fe oxide; and residual fraction) of Cd, Cr, Cu, Ni, Pb, and Zn, and their accumulation by pea (Pisum sativum) and corn (Zea mays) in a greenhouse pot experiment using a polluted floodplain soil. The SIOC caused a significant decrease in soil pH, and an increase in organic carbon and total sulfur content in the soil. The addition of SIOC increased significantly the soluble+exchangeable fraction and bioavailability of the metals. The SIOC leads to a transformation of the residual, organic, and Fe-Mn oxide fractions of Cd, Cu, Ni, and Zn to the soluble+exchangeable fraction. The SIOC addition increased the potential mobile (non-residual) fraction of Cr and Pb. The SIOC increased the sulfide fraction of Cr, Ni, and Zn, while it decreased the same fraction for Cd, Cu, and Pb. The effect of SIOC on the redistribution of metal fractions increased with enhancing application dosages. Pea accumulated more metals than corn with greater accumulation in the roots than shoots. Application of the higher dose of SIOC promoted the metals accumulation by roots and their translocation to shoots of pea and corn. Our results suggest the potential suitability of SIOC for enhancing the phytomanagement of PTEs polluted soils and reducing the environmental risk of these pollutants.

Remediation of Cd- and Pb- contaminated clay soils through combined freeze-thaw and soil washing.

To improve the washing efficiency for low-permeability clay, the method of freeze-thaw synergistic chemical washing was put forward to study the cadmium (Cd)-and lead (Pb)- real-site contaminated soils as the research object and ethylene diaminete traacetic acid (EDTA) was selected as the eluent to conduct a soil column washing test of heavy metal-contaminated clay soil. Results show that repeated freeze-thaw of soil can destroy the original structure of soil particles, help the eluent fully interact with pollutants, and thus improve washing efficiency. Cd was showed the best removal effect after supplying 0.1 mol/L EDTA twice and deionized water thrice by freeze-thaw and washing, and the removal rate reached 88.07%. Pb was also showed the better removal effect after three times of 0.1 mol/L EDTA and four times of deionized water through freeze-thaw and washing, and the removal rate reached 37.78%. Combined freeze-thaw and chemical washing can effectively remove the reducible and residual fractions of Cd and Pb in the clay soils. Moreover, freeze-thaw synergistic chemical washing can remove Cd and Pb at low liquid/soil ratios. The conclusions obtained in the study are of important theoretical value and technical support for the similar engineering.

Assessment of Dumpsite Soils in Mangrove Forest at Eagle Island, Nigeria: It’s Effect on Potential Bioavailability of Heavy Metals in the Environment

Heavy metals can be absorbed by plants resulting to contamination of other organisms in the food chain. This study was intended to determine heavy metals in soil, their mobility factor and impact on flora and fauna. To determine bioavailability of metal ions in soil chemical speciation and mobility factor indices were calculated. The level of Fe, Pb, Zn and Cd in readily available forms were assessed in dumpsite soils within mangrove forest in the Eagle Island and compared with control (i.e., relatively undisturbed soil). The aim of the study was to evaluate the impact and contribution of municipal waste discharge on alteration of soil quality and bioavailability of metals within the soil matrix. Samples of soil were collected in triplicate from five locations across the dump area while the control point was established at a less impacted area. Sampling was done in November 2018. Concentration of metal ion/species was analysed using GBC Avanta PM A6600 atomic absorption spectrophotometer. Soils of the sampling area were acidic with pH values ranging between 4.55 and 5.74. The most important heavy metal fractions in the dumpsite soils were; Fe (residual fraction, 53.75%), Pb (residual fraction, 42.58%), Zn (Fe-Mn oxide fraction, 46.85%) and Cd (carbonate bound fraction, 37.77%). However, the less impacted soil was predominantly affiliated to the residual fractions of Fe (68.75%), Pb (54.86%), Zn (37.45%) and Cd (51.51%). Heavy metal mobility factor indices reflected the order: (Cd>Pb>Zn>Fe) for soils of both the solid waste dumpsite and control areas. Despite the prevalence of heavy metals to the inert fractions, the significant affiliation of Cd to the readily mobile fractions of waste dump soils may suggest its release to have come from toxic constituents such as petroleum products that are associated with municipal wastes.

Role of phosphoric acid in the bioavailability of potentially toxic elements in hydrochars produced by hydrothermal carbonisation of sewage sludge

The effect of phosphoric acid addition to the feed-water on the speciation and transformation behaviour of potentially toxic elements (PTEs) in the hydrothermal carbonisation (HTC) of sewage sludge was explored. Over 70% of each of the PTEs (As, Cd, Cr, Cu, Mn, Ni, Pb and Zn) was in the directly bioavailable and potentially bioavailable fraction in the raw sludge, and especially Cu and Zn at 97.5 and 98.6%, respectively. Through the HTC process the directly bioavailable and potentially bioavailable fractions of PTEs in the sludge hydrochar clearly decreased, and the residual fraction in the hydrochar showed an observable increase. Further stabilisation of PTEs in hydrochar occurred during HTC with the addition of phosphoric acid solution to the feed-water. As the concentration of phosphoric acid in the feed-water increased the percentages of the residual fraction of Cd, Cr, Ni, Pb and Zn in hydrochars each exceeded 80%, but different PTEs behaved differently with increasing phosphate molar ratio in the feed-water. When the molar ratio of phosphate was 15%, the percentages of the residual fractions of Cd, Mn and Zn reached their maximum values in accordance with the changing trend in aromaticity of the hydrochar. Moreover, a large number of phosphate mineral crystals effectively occluded the PTEs in hydrochar. In conclusion, the addition of phosphoric acid to the feed-water during HTC further deactivated PTEs leading to a substantial decline in the potential environmental risk associated with the land application of the sewage sludge.

Distribution characteristics of cadmium and lead in particle size fractions of farmland soils in a lead\u2013zinc mine area in Southwest China

BackgroundCd and Pb were the main pollution elements in Pb–Zn mining areas. Several studies have focused on heavy metal pollution and risk assessment in the Huize Pb–Zn mining area. In the present study, the soil aggregate composition, chemical properties, contents and chemical fractions of Cd and Pb were analysed in soils at depths of 0–60 cm in the Huize Pb–Zn mining area. Agricultural soils were sampled at depths of 0–20, 20–40 and 40–60 cm surrounding the Pb–Zn mine. The particle size fractions were determined by the aggregate composition of macro-aggregates (0.25–2 mm), micro-aggregates (0.053–0.25 mm), silt (0.002–0.053 mm) and colloid (< 0.002 mm).ResultsThere was a large proportion of the macro-aggregate (52.4%) and less of the colloid (1.8%). With a decrease in particle size, the pH, organic matter contents, cation exchange capacity, and free iron oxide increased in the soil aggregates. Although the total and available Cd and Pb contents were highest in the colloid, the macro-aggregate maximally contributed to the total Cd (48.18%), total Pb (53.48%) and available Pb (47.42%) content, and the colloid maximally contributed to the available Cd (47.21%) in soils. The Cd and Pb existed primarily as residual fractions, followed by Fe–Mn oxide fraction and the exchangeable fraction in soil aggregates. There were higher distribution factors for the residual and exchangeable fractions of both Cd and Pb in the colloid (DF > 33). Furthermore, there were close correlations between the Cd and Pb fractions and the pH and the phosphorus content and between the exchangeable Pb content and the content of organic matter, cation exchange capacity, and free iron oxide in soil aggregates.ConclusionCd and Pb existed mainly in the micro-aggregate and colloid and that their chemical fractions were closely related to the chemical properties in soil aggregates of the polluted farmland.

Comparative study on effects of four energy plants growth on chemical fractions of heavy metals and activity of soil enzymes in copper mine tailings

ABSTRACTFour gramineous energy plants, Miscanthus sacchariflorus, M. floridulus, Phragmites australis, and Arundo donax were grown on copper tailings in the field for four years. Their phytoremediation potential was examined in terms of their effects on the fractions of heavy metals and soil enzyme activities. Results showed that plantation of these four gramineous plants has improved the proportion of organic material (OM)-binding fraction of heavy metals in copper tailings as a whole, and reduced the proportion of exchangeable and residual fractions. In particular, M. sacchariflorus growth improved significantly the proportion of the OM-binding fractions of Cu (1.73 times), Cd (1.71 times), Zn (1.18 times), and Pb (3.14 times) (P < 0.05) and reduced markedly the residual fractions of Cu (64.45%), Cd (82.38%), Zn (61.43%), and Pb (73.41%) (P < 0.05). Except for A. donax, the growth of other three energy plants improved the activity of phosphatase, urease and dehydrogenase in copper tailings to some extent. In particular, the activity of soil phosphatase and urease in planted tailings differed significantly from that of control (P < 0.05). The effect of M. sacchariflorus growth on soil enzyme was the highest, followed by P. australis, M. floridulus, and A. donax. The content of each heavy metal fraction in soil was correlated with soil enzyme activities, especially the content of OM-binding fraction, which correlated significantly with the activities of phosphatase, urease and dehydrogenase in soil. According to the effects of four gramineous plants growth on activity of soil enzymes and fractions of heavy metals, M. sacchariflorus had the optimal effects for phytoremediation. Therefore, M. sacchariflorus was a candidate plant with great potential for the revegetation of heavy metal tailings.

Effects of exogenous sulfur on growth and Cd uptake in Chinese cabbage (Brassica campestris spp. pekinensis) in Cd-contaminated soil.

Soil pollution with heavy metals has many adverse effects on ecosystem health as well as food security. A pot experiment was performed to investigate the effects of different valence states of exogenous sulfur (S) on the uptake of cadmium (Cd) in Chinese cabbage in Cd-contaminated soil. The results showed that S significantly promoted plant growth in Chinese cabbage, with the following order of magnitude for the different S treatments: sodium sulfite (Na2SO3) > sodium sulfate (Na2SO4) > powdered sulfur (S0). Additionally, enzyme activity and the content of reductive substances in the leaves markedly increased, while malondialdehyde content significantly decreased; hence, S observably enhanced the ability of Chinese cabbage to tolerate Cd stress. S0 significantly reduced soil pH, thus increasing the mobility and bioavailability of Cd in the soil, while Na2SO3 increased soil pH, and Na2SO4 had no effect on soil pH. The acid-soluble and oxidizable fractions of Cd in soil increased with the S0 treatment. The applied Na2SO3 and Na2SO4 both increased the residual fraction of Cd in the soil, but they reduced the amount of the acid-extractable, reducible, and oxidizable Cd. The results showed that compared with S0, the Na2SO3 and Na2SO4 treatments decreased the acid-extractable Cd concentrations by 6.3 and 4%, respectively, in the most contaminated soil. In conclusion, the influence of S on the bioavailability and speciation of Cd varied not only with the soil Cd content but also with the application rate and S valence state.

Volatility and partitioning of Cd and Pb during sewage sludge thermal conversion

In this paper, the thermal characteristics of sewage sludge and the transformation behavior of Pb and Cd during the thermal conversion process were addressed. The incineration process and pyrolysis process of the sewage sludge were investigated by thermogravimetric analysis. The results indicated that the thermal conversion process of the sewage sludge could be divided into three stages and the presence of oxygen could accelerate the decomposition of the sewage sludge. Furthermore, the effects of thermal conditions on the concentration ratio of Cd and Pb and their species partitioning in the residual char and ash were investigated. For the pyrolysis process, the maximum concentration ratio of Cd reached 41.64% at 500 °C and the lowest one 2.92% at 700 °C. Contrary, the concentration ratio of Pb remained above 93% as the temperature increased. Thus, the suitable temperature for the sewage sludge pyrolysis was below 500 °C. For the incineration process, the incineration temperature had great influence on the concentration ratio of Cd and Pb. When the incineration temperature increased from 700 °C to 900 °C, the concentration ratio of Cd decreased drastically from 99.32% to 10.96%. The maximum concentration ratio for Pb (95.31%) was reached at 800 °C. Besides, the lowest concentration ratio of Cd and Pb were obtained at a residence time of 30 min. The partitioning analyses of the Cd and Pb contained in the ash showed that the residence time had little effect on the partitioning of Cd and Pb, and the residual fractions of Cd and Pb were both above 90%. It was concluded that Cd and Pb were properly stabilized in the ash. Thus, Cd and Pb in the ash were difficult to be released into the environment and to cause secondary pollution.

Influence of potassium hydroxide activation on characteristics and environmental risk of heavy metals in chars derived from municipal sewage sludge

To investigate the influence of KOH activation on characteristics and environmental risk of heavy metals in chars, sludge was pyrolyzed with varying amount of KOH. The analyzation of characteristics and potential ecological risk evaluation of heavy metals were conducted by surface area analyzer, FTIR, XRD and BCR sequential extraction. The activated chars have higher surface area and lower content of silica compared to those without being activated. The activation of KOH promoted residual fraction of Cd, meanwhile, Zinc, Cr, Ni and Mn were converted to relatively unstable fractions (F2 and F3). The results of risk assessment indicated that the potential ecological risk level of Cd was reduced in activated chars, while risk level of Zn, Cr, Ni and Mn were increased after pyrolysis with KOH activation. The potential ecological risk of heavy metals in activated chars was further declined, and the risk level transformed from moderate to low.

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

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

Ceramsite preparation from sea sludge with sewage sludge biochar and its environmental risk assessment

Ceramsite were produced from sea sludge (SS) by adding different percentage of sewage sludge biochar (SSB). The characteristics of ceramsite including micrograph and elementary composition were analyzed. In addition, the heavy metals (HMs) fractions, leaching behaviour and potential environmental risk were also investigated. The microstructure of the ceramsite was slit pores and the main elements of the ceramsite were Si, Al and O. The residual fraction (F4) of Cu, Cr and Cd in ceramsite with 100% SS (SS100) reached the maximum (100%, 99% and 100%, respectively), while F4 of Zn and Ni in ceramsite with 80% SS and 20% SSB (SS80) reached the top value of 99.5% and 98%. Moreover, the HMs of feedstock can be immobilized after sintering as ceramsite and the leached amounts of HMs in all ceramsite were much lower than that stated by GB 5085.3-2007. Furthermore, ceramsite preparation from sea sludge with sewage sludge biochar will not bring HMs contamination and potential ecological risk.

Using nanoparticles from water treatment residuals to reduce the mobility and phytoavailability of Cd and Pb in biosolid-amended soils

Heavy metal pollution in soils amended with biosolids has been a serious problem worldwide for clean food production. Laboratory and greenhouse experiments were performed to assess the impact of water treatment residual nanoparticles (nWTRs), at different application rates (0.1, 0.2 and 0.3%), on immobilization and phytoavailability of Cd and Pb to canola (Brassica napus L.) plants in soils amended with biosolids spiked with three different rates of Cd or Pb. Application of nWTRs significantly increased the residual fractions of Cd and Pb in metal-spiked biosolid-amended soil and thereby increased the immobilization of Cd and Pb in the amended soil. The greatest immobilization of Cd and Pb was exhibited at an application rate of 0.3% nWTRs. In addition, the application of nanoparticles to the biosolid-amended soil significantly increased canola grain yield and significantly decreased Cd and Pb phytoavailability due to immobilization of Cd and Pb in the contaminated soil. The results demonstrate, for the first time, the capability of nanoscale WTRs in stabilizing heavy metals in contaminated soils and restoring degraded agricultural land.

Contrasting effects of alkaline amendments on the bioavailability and uptake of Cd in rice plants in a Cd-contaminated acid paddy soil.

Reducing cadmium (Cd) concentrations in rice grains is important for food safety, particularly in acid paddy fields in South China where the soils have been previously contaminated with Cd. A field experiment was conducted to evaluate the effects of four alkaline amendments, i.e., lime, compost, biochar, and carbide slag on soil bioavailability and uptake of Cd in plants of two rice cultivars (Oryza sativa L.) in a Cd-contaminated acid paddy soil. The addition of these amendments significantly decreased the concentrations of CaCl2-extractable Cd by 13-41%. Cd in the acid-soluble fraction was decreased in these amended soils while it increased in the residual fraction. The amendments also decreased the uptake of Cd in the plants at the tillering and mature growth stages. The concentrations of Cd in plant tissues at maturity were in the order: root > shoot > bran > polished rice > husk. The amendment of carbide slag decreased Cd concentration in rice grains the most, followed by lime, biochar, and compost. The increases in soil pH and the decreases in the acid-soluble fraction of Cd (F1-Cd) indicated that these amendments can directly transform the highly availability fraction of Cd to a more stable fraction (residual Cd fraction) in soils. Furthermore, the Cd concentrations in polished rice grains of the two rice cultivars used were reduced by 66-67% by treatment with carbide slag. Our study suggests that carbide slag has a great potential to reduce the bioavailability and uptake of Cd in rice plants in Cd-contaminated acid paddy field soils.

Effects of strong reductive process on transformation of heavy metals in protected vegetable soil

The application of sewage and manure in protected vegetable cultivation can induce the occurrence of heavy metals contamination. The present research studied the transformation of heavy metals (Cd, Cu, Pb and Zn) by incubating contaminated protected soil with maize straw and then leaching. The results showed that soil pH was significantly decreased, being more evident in maize straw treatment; soil Eh dropped quickly below -280 mV. Maize straw treatment promoted the activation of Cd, Cu, Pb and Zn from soil, and the total percent of oxidizable fraction and residual fraction of Cd, Cu, Pb and Zn declined at 9th day; the amount of Cd, Cu, Pb and Zn in soil reduced 18.1%, 19.0%, 16.1% and 15.7% at 15th day, respectively. Compared to control, maize straw treatment could increase the concentrations of dissolved Cd and Zn, but Cu decreased. The concentration of colloidal-bound Cd and Pb increased, Cu decreased and no significant change occurred in Zn in maize straw treatment. Strong reductive approach could activate heavy metals in protected vegetable soil, increase the risk of heavy metals accumulation in vegetables, and possibly cause water pollution accompanied with soil water mobilization.

Assessment of potential bioavailability of heavy metals in the sediments of land-freshwater interfaces by diffusive gradients in thin films

It is important to understand the potential bioavailability of heavy metals in the sediments of land-freshwater interfaces (between terrestrial and aquatic ecosystems). Therefore, we evaluated the pollution of Cr, Ni, Cu, Zn, As, and Cd in land-freshwater interface sediments/soils according to total concentrations, and used sequential extraction method to measure different fractions of Cd. Then, the diffusive gradients in thin films (DGT) technique was employed to study the potential bioavailability of metals. Results showed that the concentrations of exchangeable and weak acid soluble fraction and oxidizable fraction were higher in ecotone area, and the values of reducible fraction and residual fraction were higher in deep water area. There existed significant positive correlations between [Cd]-DGT (concentration of Cd measured by DGT) and EXC-Cd (exchangeable and weak acid soluble fraction of Cd) (r2 = 0.65), but the significant negative correlation was found between [Cd]-DGT and RES-Cd (Residual fraction of Cd) (r2 = 0.52). DGT technique is a feasible method to measure potential bioavailability of heavy metals for risk assessment in the sediments/soils of land-freshwater interfaces.

Compost and sulfur affect the mobilization and phyto-availability of Cd and Ni to sorghum and barnyard grass in a spiked fluvial soil

Soil reclamation via additives can cause contradictory effects on the mobilization of toxic elements in soils under dry and wet conditions. Therefore, our aim was to investigate the impact of compost and sulfur in two rates (1.25 and 2.5%) on fractionation, mobilization, and phyto-availability of cadmium (Cd) and nickel (Ni) to sorghum (dry soil) and barnyard grass (wet soil) in a fluvial soil spiked with 25mg Cd or 200mg Ni/kg soil. Compost decreased the solubility and mobilization of Cd (especially in dry soil) and Ni (in both soils). Sulfur increased the solubility of Cd (31% in dry soil-49% in wet soil) and Ni (4.6% in wet soil-8.7% in dry soil). Sulfur altered the carbonate fraction of Cd to the soluble fraction and the residual fraction of Cd and Ni to the non-residual fraction. Compost decreased Cd and increased Ni in sorghum, but enhanced Cd and degraded Ni in grass. Sulfur increased Cd and Ni in both plants, and the increasing rate of Cd was higher in grass than in sorghum, while Ni was higher in sorghum than in grass. These results suggest that compost can be used as an immobilizing agent for Cd in the dry soil and Ni in the wet soil; however, it might be used as mobilizing agent for Cd in the wet soil and Ni in the dry soil. Sulfur (with rate 2.5%) can be used for enhancing the phyto-extraction of Cd and Ni (especially Cd) from contaminated alkaline soils.