Cd Species Research Articles

Potential environmental risk of natural particulate cadmium and zinc in sphalerite- and smithsonite-spiked soils

Cadmium (Cd)-bearing sphalerite and smithsonite ore particles are ubiquitous in soils near metal-mining areas. Previous studies indicate that smithsonite is more readily dissolved in acidic waters and soils than sphalerite but the mobility of Cd and zinc (Zn) derived from these ores in soils is unknown. Using microcosm incubation experiments and microscopic and spectroscopic analysis, we found that the mobility of Cd and Zn derived from smithsonite is higher than from sphalerite. The mobilization rates of Cd (16.6%) and Zn (13.7%) released from smithsonite in soils after 30-day incubation experiments were higher than those from sphalerite (Cd, ~ 1.42%; Zn, ~ 0.75%). Moreover, the percentages of Cd2+ and Zn2+ in soil pore water showed a dynamic increase in smithsonite-spiked treatments but a decrease in sphalerite-spiked treatments. HRTEM-EDX-SAED analysis further indicates the occurrence of dynamic transformation of amorphous Cd and Zn species in soil pore water to crystalline ZnS and iron oxides in sphalerite-spiked soil but crystalline ZnCO3 nanoparticles were dynamically transformed to amorphous metal-bearing species in smithsonite-spiked soil. The opposite transformation trends in pore water of Zn ore-spiked soils provide new insights into the Cd environmental risks in soils affected by Zn mining.

Inhibitory Effects of Soil Amendment Coupled with Water Management on the Accumulation of Cd and Pb in Double-Cropping Rice

The bioavailability of heavy metals in soil and the physiological activities of rice determine the accumulation of heavy metals in brown rice. In this study, a field experiment was conducted in a rice paddy in which the total amount of Cd in the soil did not exceed the national standard, whereas the Cd in rice grains was at risk of overreaching in the suburbs of Guangzhou city. The bioavailability of heavy metals in the soil and the physiological barrier of rice were taken as the starting point. The early and late rice yield, brown rice heavy metal content, Cd and Pb enrichment coefficient, total soil heavy metals, soil physical and chemical properties, and soil Cd and Pb species distribution were investigated under the Si-rich amendment (JD), Ca-Mg amendment (YY), Si-rich amendment+flooding irrigation (JD+YS), and Ca-Mg amendment+flooding irrigation (YY+YS) treatments. The results showed that:① the total ω(Cd) in the soil was only 0.13 mg·kg-1 in the CK treatment. However, the average ω(Cd) in the grain of early rice reached up to 0.19 mg·kg-1. The early rice varieties (hybrid rice) had a more vital ability to accumulate Cd and total As in brown rice than that in late rice varieties (conventional rice) but a lower capacity for Pb accumulation. ② JD and YY application alone had no noticeable inhibitory effect on the accumulation of Cd and Pb in brown rice; however, JD+YS and YY+YS treatments significantly inhibited the accumulation of Cd and Pb in brown rice in both early and late rice, especially in the JD+YS treatment, which decreased the Cd and Pb accumulation by 65.8% and 68% for early rice and by 71.43% and 49.15% for late rice, respectively. The primary mechanism of JD+YS was to increase soil pH and maintain a low redox potential to promote soil Cd and Pb to be transformed from acid-soluble to a reduced state and residue state, thus decreasing Cd and Pb to migrate from the soil to the rice. At the same time, it effectively suppressed the absorption and transportation of Cd and Pb by early and late rice via the physiological barrier effect of Si nutrition and the competition for transportation channels between calcium and magnesium ions and cadmium and inhibited the accumulation of Cd and Pb in the brown rice of early and late rice. These results provide a theoretical basis for the exploration and application of the control technologies in the brown rice Cd and Pb resistance and have important practical significance for guiding the safe production in the rice-growing area in South China.

Mutations in artificial self-replicating tiles: A step toward Darwinian evolution

Artificial self-replication and exponential growth holds the promise of gaining a better understanding of fundamental processes in nature but also of evolving new materials and devices with useful properties. A system of DNA origami dimers has been shown to exhibit exponential growth and selection. Here we introduce mutation and growth advantages to study the possibility of Darwinian-like evolution. We seed and grow one dimer species, AB, from A and B monomers that doubles in each cycle. A similar species from C and D monomers can replicate at a controlled growth rate of two or four per cycle but is unseeded. Introducing a small mutation rate so that AB parents infrequently template CD offspring we show experimentally that the CD species can take over the system in approximately six generations in an advantageous environment. This demonstration opens the door to the use of evolution in materials design.

Cadmium Speciation Distribution Responses to Soil Properties and Soil Microbes of Plow Layer and Plow Pan Soils in Cadmium-Contaminated Paddy Fields.

Cadmium (Cd) speciation ratio in arable land determines the Cd exposure risk and Cd uptake in crops. However, the driving mechanisms of Cd speciation change on the vertical scale of paddy fields remain poorly understood. In this study, the effects of plow layer and plow pan on Cd speciation distribution were investigated in a long-term Cd-contaminated rice ecosystem. The Cd accumulative effect within rice grain was enhanced with high levels of activated Cd speciation ratios in soils. Activated Cd speciation ratios were higher in plow layer soils, while stabilized Cd speciation ratios were elevated in plow pan soils. Soil physicochemical properties and soil microbes synergistically affected the Cd speciation changes in different ways between the two soil layers. Soil pH and organic elements in plow layer environment directly hindered the transformation of stabilized Cd speciation, while in plow pan environment, soil pH and organic elements indirectly decreased activated Cd speciation ratios and resulted in the accumulation of stabilized Cd speciation via regulating the predominant bacterial taxa. This study will improve our understanding of how soil environments regulate Cd speciation distributions in rice ecosystems and help to seek effective remediation methods of Cd-contaminated paddy fields to reduce the Cd accumulation in rice.

Organic/inorganic amendments for the remediation of a red paddy soil artificially contaminated with different cadmium levels: Leaching, speciation, and phytoavailability tests

In the present study, the viability of using manure (M), lime (L), and sepiolite (S) alone and in combinations (M/L, M/S, and M/L/S) was evaluated for the remediation of a red paddy soil artificially contaminated with three levels of cadmium (Cd- 0.6, 1, and 2 mg kg−1 soil). Experiments were performed in columns (to evaluate Cd leaching) and pots by growing rice plants (to study Cd accumulation in plants). Before their application, the tested amendments were thoroughly characterized using SEM, EDS and FT-IR spectroscopy. The leaching experiment indicates that the application of L or M/L significantly improved the pH of soil leachate collected at different time intervals. However, the use of M/L/S was found better in decreasing the Cd contents in collected leachate. The use of M/L efficiently decreased the DTPA metal extraction (0.19, 0.41, and 0.55 mg kg−1) as compared to the CK (0.35, 0.63, and 1.13 mg kg−1, respectively). The Cd speciation results depicted a 33% decrease in exchangeable Cd with M/L/S treatment when compared with control (55%). Moreover, the M/L/S treatment was more efficient in lowering the Cd phytoavailability and subsequent accumulation in rice grains (0.05, 0.09, and 0.08 mg kg−1). These findings demonstrate that the use of composite amendments is categorically effective as an in-situ remediation tool to decrease Cd leaching and availability in diverse contaminations.

Chemical and algological composition of the snow cover at the mouth of the Onega river (White Sea basin)

Abstract In the study, the content and speciation of Mn, Pb, Cd, Zn, Cr, Ni, Co, Cu, and Fe in the snow cover at the mouth of the Onega river (White Sea basin) at the end of the winter periods in 2018, 2019 and 2020 were determined. Winter 2019 year was the snowiest, as the maximum values of the snow cover depth and water equivalent were almost two times higher than in all other years. The total content of suspended matter in the snow cover was 0.2–5.5 μg/L. Increased concentrations of suspended solids were identified near highways. Mn, Cr and Ni were present in the snow cover mainly in their dissolved form, while Fe, Pb and Co were mostly contained as solids. The algological composition of the snow cover was also studied.

Speciation, transportation, and pathways of cadmium in soil-rice systems: A review on the environmental implications and remediation approaches for food safety.

Cadmium (Cd) contamination in paddy fields is a serious health concern because of its high toxicity and widespread pollution. Recently, much progress has been made in elucidating the mechanisms involved in Cd uptake, transport, and transformation from paddy soils to rice grains, aiming to mitigate the associated health risk; however, these topics have not been critically reviewed to date. Here, we summarized and reviewed the (1) geochemical distribution and speciation of Cd in soil-rice systems, (2) mobilization, uptake, and transport of Cd from soil to rice grains and the associated health risks, (3) pathways and transformation mechanisms of Cd from soil to rice grains, (4) transporters involved in reducing Cd uptake, transport, and accumulation in rice plants, (5) factors governing Cd bioavailability in paddy, and (6) comparison of remediation approaches for mitigating the environmental and health risks of Cd contamination in paddy fields. Briefly, this review presents the state of the art about the fate of Cd in paddy fields and its transport from soil to grains, contributing to a better understanding of the environmental hazards of Cd in rice ecosystems. Challenges and perspectives for controlling Cd risks in rice are thus raised. The summarized findings in this review may help to develop innovative and applicable methods for controlling Cd accumulation in rice grains and sustainably manage Cd-contaminated paddy fields.

Influence of nitrogen forms, pH, and water levels on cadmium speciation and characteristics of cadmium uptake by rapeseed.

Rapeseed (Brassica napus L.) is an ideal crop for remediation in cadmium (Cd)-contaminated soil in farmland. The main objective of this study was focused on the combined effects of four nitrogen forms (urea, ammonium nitrogen, nitrate nitrogen, ammonium nitrate fertilizer), four pH levels (5, 6, 7, 8), and three water levels (low water, middle water, high water) on Cd speciation and characteristics of Cd uptake by rapeseed. A pot experiment was conducted at the Xindu Experimental Park in Sichuan Province, China. Experimental results indicated that the interaction effects of pH and nitrogen forms, three factors on Cd speciation (except organic-bound Cd and exchangeable Cd), were significant and the interaction effects of pH and nitrogen forms on Cd uptake by rapeseed also was significant only under the condition of planting rapeseed. The higher the water level was or the lower the pH value was, the better the repair effect rapeseed to Cd was. High water significantly increased the stem Cd content by 11.89% and 29.55% through significantly increasing the content of exchangeable Cd by 23.40% and 52.63%, respectively, compared with middle water and low water as planting rapeseed, and pH 5 significantly increased the stem Cd content, total Cd enrichment coefficients, bio-availability coefficient, and Cd removal rate by 24.45~40.33%, 49.45~76.62%, 60.00~166.67%, and 16.67~26.00%, respectively, through significantly increasing the content of exchangeable Cd by 37.78~113.79%, compared with pH 7 and pH 8 as the significant decreasing of carbonate-bound Cd, bound to Fe-Mn oxide, and content of organic-bound Cd and residual Cd. Ammonium nitrate also significantly increased total Cd enrichment coefficients, bio-availability coefficient, and Cd removal rate by 45.63~138.10%, 21.05~109.09%, and 40.00~77.50%, respectively, compared with other three nitrogen forms as good growth and Cd resistance of rapeseed. Path analysis structural equation modeling revealed that content of exchangeable Cd and residual Cd had significant and direct path coefficients with variances in stem Cd content of rapeseed. Combined with the safety of edible oil, the best management practices for optimal remediation efficiency of rapeseed to Cd-contaminated soil were ammonium nitrate fertilizer, pH = 5, and high water.

Cadmium uptake and transport processes in rice revealed by stable isotope fractionation and Cd-related gene expression

Multiple processes are involved in Cd transfer in rice plants, including root uptake, xylem loading, and immobilization. These processes can be mediated by membrane transporters and can alter Cd speciation by binding Cd to different organic ligands. However, it remains unclear which processes control Cd transport in rice in response to different watering conditions in soil. Herein, Cd isotope fractionation and Cd-related gene expression were employed to investigate the key regulatory mechanisms during uptake, root-to-shoot, and stem-to-leaf transport of Cd in rice grown in pot experiments with Cd-contaminated soil under flooded and non-flooded conditions, respectively. The results showed that soil flooding decreased the Cd concentration in soil porewater and, thereby, Cd uptake and transport in rice. Cd isotopes fractionated negatively from soil porewater to the whole rice (flooded: ∆114/110Cdrice-porewater = −0.15‰, non-flooded: ∆114/110Cdrice-porewater = −0.39‰), suggesting that Cd transporters preferentially absorbed light Cd isotopes. The non-flooded treatment revealed an upregulated expression of OsNRAMP1 and OsNRAMP5 genes compared to the flooded treatment, which may partially contribute to its more pronounced porewater-to-rice fractionation. Cd isotopes fractionated positively from roots to shoots under flooded conditions (∆114/110Cdshoot-root = 0.19‰). However, a reverse direction of fractionation was observed under non-flooded conditions (∆114/110Cdshoot-root = −0.67‰), which was associated with the substantial upregulation of CAL1 in roots, facilitating xylem loading of Cd-CAL1 complexes with lighter isotopes. After being transported to the shoots, the majority of Cd were detained in stems (44%–55%), which were strongly enriched in lighter isotopes than in the leaves (∆114/110Cdleaf-stem = 0.77 to 1.01‰). Besides the Cd-CAL1 transported from the roots, the expression of OsPCS1 and OsHMA3 in the stems could also favor the enrichment of Cd-PCs with lighter isotopes, leaving heavier isotopes to be transported to the leaves. The higher expression levels of OsMT1e in older leaves than in younger leaves implied that Cd immobilization via binding to metallothioneins like OsMT1e may favor the enrichment of lighter isotopes in older leaves. The non-flooded treatment showed lighter Cd isotopes in younger leaves than the flooded treatment, suggesting that more Cd-CAL1 in the stems and Cd-PCs in the older leaves might be transported to the younger leaves under non-flooded conditions. Our results demonstrate that isotopically light Cd can be preferentially transported from roots to shoots when more Cd is absorbed by rice under non-flooded conditions, and isotope fractionation signature together with gene expression quantification has the potential to provide a better understanding of the key processes regulating Cd transfer in rice.

Establishing a weighted methodology for human health risk assessment of cadmium based on its equilibrium speciation in groundwater

A weighted human health risk evaluation methodology that integrated fuzzy-synthetic evaluation and chemical equilibrium simulation was proposed. The method was applied to study the potential toxicity and morphological characteristics of cadmium (Cd) species in groundwater. The concentrations and activities of 15 Cd (II) ion species were conducted by Visual MINTEQ for optimizing average daily dose through ingestion pathway in the health risk model. Both carcinogenic and non-carcinogenic health risk of Cd (II) species for adults and children were conducted. The health risk assessment revealed that metal species of Cd2+ and CdCl + posed carcinogenic risk to adults. The carcinogenic risk of Cd2+ with the value 8.03E-04 and 5.43E-04 for adults and children demonstrated high carcinogenicity, respectively. Non-carcinogenic risk values for Cd (II) ion species were all less than 1 except for Cd2+. It was noteworthy that the non-carcinogenic risk index of Cd2+ and CdCl+ were slightly higher than those after weight assignment. Besides, the investigation indicated that conventional risk assessment may overestimate or underestimate risk levels and the health risk of Cd (II) species principally depended on their metal speciation associated affecting factor of pH in groundwater. The Nemero index for Cd (II) species showed a slight contamination accounted for a percentage of 13.33% of the sampling areas. Therefore, the methodology identified Cd2+ and CdCl + as the targeted species for contamination through the assessment. This strategy provided a solution of correlation degree optimization for groundwater health risk assessment and a reference for the administration to manage groundwater pollution.

Cadmium isotope fractionation during leaching with nitrilotriacetic acid

This study aimed to determine the leaching mechanism of cadmium (Cd) from contaminated soil by nitrilotriacetic acid (NTA). 114Cd/110Cd isotope technology was adopted as the main characterization method, and experiments were conducted with a typical high-Cd soil to determine the speciation distribution of Cd, isotopic composition, and changes in bonding environment before and after leaching. The speciation distribution of Cd in the soil before leaching in order from most to least was exchangeable Cd, iron‑manganese oxide Cd, residual Cd, carbonate-bound Cd, and organic fraction Cd. The removal efficiency of exchangeable Cd was the highest, followed by iron‑manganese oxide Cd and carbonate-bound Cd. Residual Cd and organic fraction Cd content did not significantly change. Cd(NTA)− and Cd(NTA)24− were the major species of Cd in the leaching reagent, which was enriched in the heavy isotope 114Cd and had a strong bonding environment. The isotope fractionation coefficient α decreased as the proportion of Cd remaining in the soil decreased. Cd speciation distribution had an important influence on the 114Cd/110Cd fractionation degree in the leaching process. The Cd isotope composition in soil (from −0.12‰ ~ 0.04‰ to −0.35‰ ~ −0.18‰ without buffer and − 0.28‰ ~ −0.09‰ with buffer) can reflect the change of Cd speciation distribution, which determines the characteristics of the bonding environment and, in turn, determines the efficiency of NTA leaching to remove soil Cd. As such, isotope fractionation characteristics can be a significant theoretical support related to NTA leaching Cd contaminated soil.

Co-pyrolysis of corn stover with industrial coal ash for in situ efficient remediation of heavy metals in multi-polluted soil

Coal ash incorporated biochar (CA/BC) composite was prepared by co-pyrolysis of agricultural residue and industrial coal ash and applied for remediation of soils polluted by lead (Pb) and cadmium (Cd). The results showed that immobilization efficiency of CA/BC for heavy metals (HMs) was significantly enhanced by 77.1 % (Pb) and 42.7 % (Cd) compared to pristine biochar (BC), and this was mainly due to the increased pH value, surface functionality and surface negative charge. By the introduction of 5 % CA/BC, the polluted soils showed the highest reduction of leaching toxicity by 67.9 % (Pb) and 49.7 % (Cd), respectively. The chemical speciation of Pb and Cd in soils was changed remarkably and the reduced bioavailable Pb and Cd were mainly transformed from acid-soluble fraction into the most stable form of residual fraction. The mechanism study showed that surface precipitation, complexation, cation exchange and cation-π interaction of CA/BC mainly contributed to heavy metals (HMs) immobilization. The pot experiments further confirmed that incorporation of 5 % CA/BC effectively reduced plant Pb and Cd accumulation by 81 % and 62.5 % respectively, and significantly promoted the plant growth of paddy rice by 3.1, 2.2 and 2.0 times in terms of root, stem length and dry mass parameters. The present study offered a cost-effective and green method to prepare soil amendment with great potential for remediation of soils polluted by HMs and realized the value-added utilization of waste agricultural residue and industrial coal ash.

A porous biochar supported nanoscale zero-valent iron material highly efficient for the simultaneous remediation of cadmium and lead contaminated soil

Risk associated with heavy metals in soil has been received widespread attention. In this study, a porous biochar supported nanoscale zero-valent iron (BC-nZVI) was applied to immobilize cadmium (Cd) and lead (Pb) in clayey soil. Experiment results indicated that the immobilization of Cd or Pb by BC-nZVI process was better than that of BC or nZVI process, and about 80% of heavy metals immobilization was obtained in BC-nZVI process. Addition of BC-nZVI could increase soil pH and organic matter (SOM). Cd or Pb immobilization was inhibited with coexisting organic compound 2,4-dichlorophenol (2,4-DCP), but 2,4-DCP could be removed in a simultaneous manner with Cd or Pb immobilization at low concentration levels. Simultaneous immobilization of Cd and Pb was achieved in BC-nZVI process, and both Cd and Pb availability significantly decreased. Stable Cd species inculding Cd(OH)2, CdCO3 and CdO were formed, whereas stable Pb species such as PbCO3, PbO and Pb(OH)2 were produced with BC-nZVI treatment. Simultaneous immobilization mechanism of Cd and Pb in soil by BC-nZVI was thereby proposed. This study well demonstrates that BC-nZVI has been emerged as a potential technology for the remediation of multiple heavy metals in soil.

Modelling evaluation of key cadmium transformation processes in acid paddy soil under alternating redox conditions

The intermittent flooding conditions in acid paddy field can substantially affect the phytoavailability of cadmium (Cd), but the underlying mechanisms controlling Cd transformation are still poorly understood, and furthermore the corresponding models for predict the Cd phytoavailability during pre-harvest drainage phase are still lacked. In this study, Cd-spiked paddy soil was incubated under anaerobic condition for 40 days and then aerobic condition for 15 days. The transformation of Cd speciation and the redox cycling of Fe/N/S was investigated. During anaerobic incubation, the dissolved and exchangeable Cd rapidly decreased, which were mainly attributed to enhanced adsorption on the surface of organic matter with the increasing soil pH. Moreover, the reductive dissolution of Fe oxides induced the release of soil colloids, which result in increasing the surface sites to immobilize Cd. In addition, the formation of Cd sulfide contributed to slight decrease the availability of Cd. However, when oxygen was re-introduced into the system, the rapid decrease of soil pH and surface sites resulted in the release of adsorption on soil surfaces, and oxidative dissolution of Cd sulfide also contributed to the increase of Cd availability. The kinetic models were established based on the key elementary reactions for soil biogeochemical processes on the transformation of Cd fractions. The kinetic models were applied to calculate the potential available Cd in acid paddy soils samples (pH ≤ 6.0) from the field, and moreover, the dissolved Cd was estimated from the surface complexation model (SCM) by incorporating the changes of soil pH. The concentration of model-predicted dissolved Cd during drainage phase had a stronger correlation with the Cd content in rice grain than that of chemical extraction methods. Our results demonstrated that it was feasible to use the kinetic models and SCM to predict Cd phytoavailability during drainage phase of acid paddy field.

Compounded chelating agent derived from fruit residue extracts effectively enhances Cd phytoextraction by Sedum alfredii

Chelating agent is known as the enhancer for metal phytoextraction; however, there is still a lack of efficient and environmentally sustainable chelators. Here, lemon residue extraction (LRE), prepared from 11 kinds of fruit wastes, was combined with N, N-bis (carboxymethyl) glutamic acid (GLDA), and tea saponin (T.S.) for the compounded plant-derived chelator (CPC), and their influences on Cd phytoextraction by the hyperaccumulator Sedum alfredii was evaluated. Among these fruits, the lemon residue extracted the most significant amount of Cd from the soil. The most effective CPC was at the volume ratio of three agents being 15:4:1 (LRE:GLDA:T.S.). Compared with the deionized water, the solubility of three Cd minerals was increased by 85–256 times, and Cd speciation was substantially altered after CPC application. In the pot experiment, CPC addition caused evident increases in plant shoot biomass, Cd phytoextraction efficiency, and organic matter content compared with EDTA and nitrilotriacetic acid (NTA) application. CPC induced fewer changes in bacterial community composition compared with EDTA and had no pronounced influence on microbial biomass carbon and bacterial α-diversity, suggesting CPC had a subtle impact on the microbiological environments. Our study provides a theoretical base for the reutilization of fruit wastes and the development of environmental-friendly chelator that assists Cd phytoextraction.

Revealing the Surface Structure of CdSe Nanocrystals by Dynamic Nuclear Polarization-Enhanced 77Se and 113Cd Solid-State NMR Spectroscopy.

Dynamic nuclear polarization (DNP) solid-state NMR (SSNMR) spectroscopy was used to obtain detailed surface structures of zinc blende CdSe nanocrystals (NCs) with plate or spheroidal morphologies which are capped by carboxylic acid ligands. 1D 113Cd and 77Se cross-polarization magic angle spinning (CPMAS) NMR spectra revealed distinct signals from Cd and Se atoms on the surface of the NCs, and those residing in bulk-like environments, below the surface. 113Cd cross-polarization magic-angle-turning (CP-MAT) experiments identified CdSe3O, CdSe2O2, and CdSeO3 Cd coordination environments on the surface of the NCs, where the oxygen atoms are presumably from coordinated carboxylate ligands. The sensitivity gain from DNP enabled natural isotopic abundance 2D homonuclear 113Cd-113Cd and 77Se-77Se and heteronuclear 113Cd-77Se scalar correlation solid-state NMR experiments which revealed the connectivity of the Cd and Se atoms. Importantly, 77Se{113Cd} scalar heteronuclear multiple quantum coherence (J-HMQC) experiments were used to selectively measure one-bond 77Se-113Cd scalar coupling constants (1J(77Se, 113Cd)). With knowledge of 1J(77Se, 113Cd), heteronuclear 77Se{113Cd} spin echo (J-resolved) NMR experiments were used to determine the number of Cd atoms bonded to Se atoms and vice versa. The J-resolved experiments directly confirmed that major Cd and Se surface species have CdSe2O2 and SeCd4 stoichiometries, respectively. Considering the crystal structure of zinc blende CdSe and the similarity of the solid-state NMR data for the platelets and spheroids, we conclude that the surface of the spheroidal CdSe NCs is primarily composed of {100} facets. The methods outlined here will generally be applicable to obtain detailed surface structures of various main group semiconductor nanoparticles.

Interaction mechanism between cadmium species and SiO2 of municipal solid waste incineration fly ash: Effect of HCl

Cadmium (Cd) pollutants from the incineration of municipal solid waste (MSW) has drawn considerable attention due to their high volatility and biological toxicity. Current experimental results have revealed the strong interaction between SiO2 and Cd species, which provides an insight for the removal of Cd species, but the detailed mechanism is still unclear. Thus, with the density functional theory (DFT) calculations, the interaction mechanisms of Cd species over the SiO2(001) surface were investigated, considering the effect of HCl in MSW incineration flue gas. It is found that the adsorption manners of Cd0 and CdO are chemisorption over the SiO2 surface and the active sites are O sites, while the interaction between CdCl2 and the surface is dominated by weak chemisorption. HCl plays a dual role in the interaction of Cd0 and CdO with the surface. The introduction of HCl can promote the fixation of CdO and Cd0, but also promote their chlorination into CdCl2 whose adsorption on SiO2 is too weak to be captured. The chlorination of Cd0 can be achieved by two-step and one-step routes, both following the Eley-Rideal mechanism. Whereas, CdO can only be chloridized through a two-step pathway, in which the first step follows the Eley-Rideal mechanism and the second step follows the Langmuir-Hinshelwood mechanism. Compared with Cd0, the chlorination of CdO has a higher reaction energy barrier and is more difficult to proceed. Therefore, CdO is the most effortless cadmium pollutant for SiO2 to control.

Chemical speciation and ecological risk assessment of Cd, Pb and As in sediments: a case study in the Xijiang River basin, China

Chemical speciation and ecological risk assessment of Cd, Pb and As in sediments: a case study in the Xijiang River basin, China

Effect of biochar amendment on mobility and plant uptake of Zn, Pb and Cd in contaminated soil

This study aimed to assess the effect of rice straw biochar application as a soil amendment on the mobility, availability, speciation and plant uptake of Zn, Pb and Cd in contaminated soil. A pot experiment with maize (Zea mays L.) was conducted using different rates 0, 1, 2, and 5% (w/w) of rice straw biochar. The soil pore water properties; pH, EC, and DOC concentration, the dissolved metal concentrations in soil pore water as well as plant metals uptake were determined at the end of the experiment. The BCR sequential extraction procedure was adopted to determine the effect of biochar on speciation and partitioning of the studied metals.Results showed that the application of biochar is significantly increased the plant shoots biomass by 94.5% with 5% biochar rates compared to untreated soil. Similarly, the soil pore water properties pH, EC, and DOC concentration were also increased with biochar addition compared to untreated soil. The dissolved metal concentrations were decreased in soil pore water with the increasing of biochar rates by 92%, 81.5%, and 90% for Zn, Pb and Cd, respectively at 5% biochar rate. In the same trend, the plant metals uptake reduced significantly with the increasing of biochar dose. Compared to untreated soil, the BCR sequential extraction showed that the biochar addition induced the transformation of the exchangeable metal fractions to oxidizable and residual fractions. These results confirmed the ability of rice straw biochar to immobilize the studied metals and therefore reducing their bioavailability and their uptake by plant.

Nitrate Increases Cadmium Accumulation in Sweet Sorghum for Improving Phytoextraction Efficiency Rather Than Ammonium.

Sweet sorghum has potential for phytoextraction of cadmium (Cd) owning to its large biomass and relatively high Cd tolerance. Nitrogen affects both growth and Cd concentrations in plants. However, different forms of nitrogen effects on Cd accumulation in sweet sorghum to improve efficiency of Cd phytoremediation is still elusive. In this study, nitrate substantially promoted both dry weight and Cd concentrations in leaves, stems + sheaths and roots of sweet sorghum when compared with ammonium. As a result, Cd accumulation in nitrate-supplied sweet sorghum was around 3.7-fold of that in ammonium-supplied plants under unbuffered pH condition, while the fold was about 2.2 under buffered pH condition. We speculated pH values and Cd species in the growth medium to some extent contributed to increased Cd accumulation as affected by nitrate. Net photosynthesis rate and Fv/Fm of nitrate-treated plants under Cd stress were higher than that of ammonium-treated plants when the pH was unbuffered. Responses of antioxidant capacity in roots to Cd stress with nitrate application were stronger than that with ammonium supplementation. Taken together, nitrate is more suitable than ammonium for Cd phytoextraction by using sweet sorghum, which is able to enhance at least double efficiency of phytoextraction.