Chemical Forms Of Heavy Metals Research Articles

Research Progress in the Analysis of Chemical Forms of Mercury in Traditional Chinese Medicine

A comprehensive evaluation of the chemical forms of heavy metals, rather than their total amount of elements, is the basis of the scientific and objective evaluation of safety in heavy metals. As mercury, the most toxic heavy metal in the environment (such as in water, soil, and air), accumulates in medicinal plants, chemical forms of mercury in medicinal plants and their preparation need to be clearly understood. This study aims to summarize the chemical status of mercury in bulk Chinese medicinal herbs and traditional Chinese medicine preparations and to discuss research methods for their analysis. Further, widespread high-performance liquid chromatography–inductively coupled plasma mass spectrometry, advanced synchrotron X-ray absorption fine structure spectroscopy, and X-ray fluorescence were used for the in situ analysis of the chemical state of mercury. The results were then analyzed to improve the analytical methods and evaluation standards for the chemical state of mercury in Chinese herbal medicines.

Assessment of heavy metals mobilization in road-deposited sediments induced by COVID-19 disinfection.

Road-deposited sediments (RDS) on urban impervious surfaces are important carriers of heavy metals (HMs), and can contribute to urban runoff pollution. With the outbreak of COVID-19, chlorinated disinfectants (CDs) have been extensively sprayed on these surfaces. This practice may have a superposed or priming effect on HMs contaminants in RDS, yet this remains unknown. This study examined the effects of seven CDs concentration gradients (0, 250, 500, 1000, 2000, 5000, 60,000mg/L) on the leaching and chemical forms of HMs (Cd, Cr, Ni, Pb, and Zn) in seven particle size fractions (<44, 44-63, 63-105, 105-149, 149-250, 250-450, 450-1000μm). The results showed that CDs can promote the leaching of HMs in RDS, at the recommended CDs dose (2000mg/L), except for Pb, the leaching amounts increased by 21.8%-237.2% compared with the untreated RDS. The alteration in the leaching were primarily attributed to the redistribution of chemical forms of HMs in RDS, specifically, the acid-extractable fractions percentage increased by 0.23%-24.39%, and the reducible fractions percentages decreased by 3.21%-38.35%. The lower oxidation-reduction potential (ORP) and alkalinity of CDs as strong oxidants were responsible for the redistribution of forms. The leaching and chemical forms of HMs vary among different particle sizes, but in any case, finer particle sizes (< 105μm) still dominate their contribution. The current control measure of street sweeping is ineffective in removing these particles. These findings will facilitate the development of strategies for controlling urban diffuse pollution from RDS during the pandemic. Finally, this study suggests potential directions for future research.

Effects of Biochar and Apatite on Chemical Forms of Lead and Zinc in Multi-Metal-Contaminated Soil after Incubation: A Comparison of Peanut Shell and Corn Cob Biochar

Heavy metal pollution in soils caused by mining activities is a severe issue worldwide. It is necessary to find a suitable approach to mitigate heavy metal-contaminated soil. Yet little is known about how soil amendments affect the chemical forms of heavy metals. Biochar produced from peanut shells (PSB300) and corn cob (CCB300) at 300 °C, and apatite (AP) were applied at various ratios to investigate their ability to adsorb lead (Pb) and zinc (Zn) in contaminated soil. The Pb and Zn’s chemical fractions were analyzed utilizing Tessier’s sequential extraction procedure and quantified using inductively coupled plasma mass spectroscopy. The one-month amendment incubation of biochar and AP could significantly diminish Pb and Zn’s exchangeable fractions, and CCB300 showed a slightly better effect on declining the exchangeable fractions of Pb and Zn than PSB300, which might be attributed to the higher values of OC and EC of CCB300 than those of PSB300. Moreover, the amendments could also transform the exchangeable fractions of Pb and Zn into stable fractions, resulting in immobility in natural conditions. Thus, PSB300 and CCB300 and the mixture of biochar/apatite could be hopeful amendments for immobilizing heavy metals in heavy metal multi-metal-contaminated field soil.

Emission Characteristics, Speciation, and Potential Environmental Risks of Heavy Metals from Coal-Fired Boilers: A Review

Coal-fired boilers, including coal-fired power plants (CFPPs) and coal-fired industrial boilers (CFIBs), are an important area for achieving sustainability globally as they are one of the globally important sources of anthropogenic emissions of heavy metals (HMs) due to huge amount of coal consumption. To date, the investigation of atmospheric emission characteristics, speciation, and potential environmental risks of HMs from coal-fired boilers has received widespread attention and achieved significant progress. To characterise the emissions of HMs from coal-fired boilers, research is currently being carried out in the areas of (1) studying the release of HMs from coal combustion processes, (2) developing emission factors and emission inventories, and (3) revealing the cross-media partitioning of HMs between different output streams. Research on the chemical forms of HMs in waste from coal-fired boiler is currently focused on chemical valence and speciation components. The sequential chemical extraction method is currently the most widely used method for investigating the chemical fractionations of HMs in wastes from coal-fired boilers. Studies indicate that different HM elements display differentiated characteristics of speciation in waste from coal-fired boilers. Early studies on potential environmental risk and ecological risk caused by HMs are usually based on actual monitoring values of HMs in the target environmental media. The risk assessment code method and the leaching toxicity method are the most widely used method to study the potential environmental risk of HMs in waste from coal-fired boilers. With the implementation of global carbon emission reduction strategies, the scale of coal-fired boilers and air pollution control technologies are bound to change in the future. Therefore, as an important component of global efforts to achieve sustainable development, more research is needed in the future to improve the accuracy of emission inventories, reveal the mechanisms of HM chemical transformation, and establish methods for potential environmental risk assessment at regional scales.

The Use of biochar in the Remediation of Pb, Cd, and Cu-Contaminated Soils. The Impact of biochar Feedstock and Preparation Conditions on Its Remediation Capacity

Soil constitutes an important part of terrestrial ecosystems, prone to be adversely impacted by human activities. During the last decades, several methods have been developed aiming at its remediation, including the use of biochar as a soil amendment. In the present work, we have assessed the reduction of Pb, Cd, and Cu soil concentrations as a function of the mixing ratio of biochar added to soil, as well as the source of biochar employed. Furthermore, we have investigated the effects of biochar addition relating to the chemical forms of heavy metals (HMs) related to their bioavailability and mobility. The concentrations of HMs were determined by the BCR (European Community Bureau of Reference) sequential extraction procedure before and after biochar addition to the soils. Five types of biochar were used, obtained as by-products of sugarcane bagasse (Β), bamboo (ΒΒ), rice straw (RSB), garden waste (GB), and paulownia (PB) treatment, respectively. Biochar derived from sugarcane (B) reduced the availability of metals, as it decreased their concentration in the acid extractable fraction, by 40.5, 66.6, and 50% for Pb, Cd, and Cu, respectively. In addition, (B) application increased the residual fraction of Cu and Pb by 9% and 24.8%, respectively. Biochar derived from garden residues (GB) and paulownia plant (PB) dramatically increased the residual fraction of Cd over 97%, minimizing its availability. Sugarcane-derived biochar appeared to significantly increase Cu and Pb residual fraction concentrations and decrease available Cd concentration. Similar changes are caused by the types of biochar in the following order: biochar from sugarcane &gt; paulownia &gt; garden wastes &gt; bamboo &gt; rice straw. The redistribution of HM concentrations causes a significant improvement of environmental quality in polluted soils, as it limits the mobility and availability of toxic metals to the soil ecosystem. The use of biochar is a low-cost and eco-friendly method for the remediation of contaminated with HMs soils in the framework of a circular economy.

Effects of Free Amino Acids, Metallothionein, and Chemical Forms of Heavy Metals on the Accumulation and Detoxification of Cadmium and Chromium in Chinese Goldthread (Coptis chinensis Franch.)

Chinese goldthread (Coptis chinensis Franch.) is an important medicinal plant worldwide. However, its clinical safety is negatively impacted by high cadmium (Cd) and chromium (Cr) levels. Understanding the accumulation pattern and detoxification mechanism of heavy metals, such as Cd and Cr, is important for regulating their transportation and reducing their accumulation in plants. Free amino acids (FAAs), metallothioneins, and chemical forms of heavy metals are closely associated with heavy metal accumulation and detoxification mechanisms. In the present study, we evaluated the uptake of Cd and Cr in Chinese goldthread and characterized the changes in FAA levels, CcMT2, a metallothionein gene, expression under Cd and Cr stress, and Cr chemical forms in various tissues of Chinese goldthread. The majority of Cd and Cr was retained in the roots; this might be the tolerance strategy utilized by Chinese goldthread to protect the aboveground photosynthetic tissues. A chemical continuous extraction method was utilized to analyze the chemical forms of Cr, and the results suggested that Cr was primarily fixed by oxalate in all tissues, and after Cr treatment, Cr tended to combine with phosphate to form an insoluble complex that reduced the toxicity and bioavailability of Cr in Chinese goldthread. The accumulation of Cd and Cr was significantly positively correlated with valine, histidine, threonine, tyrosine, and serine levels and significantly negatively correlated with glutamic acid and alanine levels. The variation in proline levels under Cd stress was different than that under Cr stress. FAAs might affect the accumulation of and resistance to Cd and Cr through various detoxification strategies, such as chelation with heavy metals ions and conversion into the required products. Furthermore, CcMT2 was isolated for the first time and found to be dominantly expressed in leaves. CcMT2 was induced under Cd and Cr stress in all plant tissues, indicating its role in the accumulation of and tolerance to Cd and Cr.

Evaluation of heavy metal pollution risk in surface sediment of the South Lagoon of Tunis by a sequential extraction procedure

In this study, the total concentrations and chemical forms of heavy metals (Fe, Zn, Pb, Cu, Cr, Cd, and Ni) in surface sediments of the South Lagoon of Tunis located in northeast Tunisia were investigated. Multiple geochemical indices were applied to assess the potential environmental risks. The South Lagoon is a valuable regional resource for fisheries, tourism and aquaculture. Total metal concentrations exhibited significant spatial variation attributed to the principal water circulation direction (east to west). The chemical speciation of Fe, Cr and Ni shows that they were mostly related to the residual fraction (Fe 34.8%, Cr 37.4% and Ni 37.9%), while Pb, Zn, Cu, and Cd were mostly related to the non-residual fraction (labile fraction Pb 89.4%, Zn 26.1%, Cu 71.8% and Cd 84.3%). Pb, Zn, Cu and Cr are of high potential bioavailability. The highest pollution was found on the west side of the lagoon according to the calculated global contamination factors. Besides, individual contamination factors, Pb followed by Zn and Cu, posed the highest risk of contamination. This study shows that, even after dredging, the persistence of low concentrations of some contaminants may cause environmental problems in certain physico-chemical conditions.

Does combined heavy metal stress enhance iron plaque formation and heavy metal bioaccumulation in Kandelia obovata?

Iron plaque (IP) has significant impact on rhizosphere environment, and affects the uptake and translocation of heavy metals in plants. However, information on IP formation and heavy metal bioaccumulation for mangrove plants under combined heavy metal stress is currently limited. Based on a 120-day pot experiment, this study explored the temporal changes of IP formation on root surface of Kandelia obovata, and heavy metal bioaccumulations under combined heavy metal stress of Cu, Pb and Zn. The results showed that IP formations on root tip, middle and basal roots were improved with combined heavy metal stress and treatment time (root tip: 0.49−0.61 mg g−1 to 2.55–4.77 mg g−1; middle root: 0.27−0.31 mg g−1 to 3.01–4.22 mg g−1; basal root: 0.16−0.23 mg g−1 to 2.24–4.10 mg g−1). IP formation was accompanied with improved depositions of Cu and Pb on root surface, regardless of root tip, middle and basal roots. IP and roots deposited large amounts of Cu (24.97–69.52 μg g-1, 9.39–154.75 μg g−1), Pb (8.35–84.53 μg g-1, 12.29–191.49 μg g-1) and Zn (85.71–139.19 μg g-1, 51.07–195.65 μg g−1), inhibiting their translocations to aboveground leaves. Based on subcellular distribution and chemical forms of heavy metals in roots, the improved depositions of phosphate-integrated Cu (6.77 %–25.13 %), oxalate-integrated Pb (34.89 %–57.88 %), as well as pectate- and protein-integrated Zn (14.22 %–35.41 %) were detected in root cell vacuoles. In conclusion, the combined heavy metal stress enhanced IP formation and heavy metals deposition (especially for Cu and Pb) on root surface, and improved their vacuolar sequestration and chelation in roots of K. obovata.

Contaminated soil amendment by diatomite: chemical fractions of zinc, lead, copper and cadmium

High concentration of labile metals in soil directly affects soil quality, water health and human safety. Decreasing mobility of metals, especially in contaminated soils, by cost-effective amendments may alleviate environmental problems. Natural diatomite was investigated to immobilize toxic elements of zinc, lead, copper and cadmium in a contaminated calcareous soil. The diatomite was characterized using X-ray fluorescence spectrometry and scanning electron microscopy. Contaminated soil was incubated with 0, 2 and 5 weight -% of diatomite at 25 °C for 8 weeks. Tessier sequential extraction method was also used as a suitable method for identification of chemical forms of heavy metals; metal stability index and mobility factor were calculated. Results showed that application of diatomite in soil significantly decreased the metals in the exchangeable fraction and increased them in the residual fraction. The exchangeable metal fractions decreased by 66–88%, and residual fractions increased by 18–94% in the 5 weight -% of diatomite treatment after 8-week incubation. With an increase in the amount of diatomite applied and the incubation time, metal mobility factor values significantly decreased and soil pH values increased. Application of diatomite caused the heavy metals redistribution toward more stable forms and leads to an increase in stability index values. In conclusion, in situ immobilization of heavy metals by application of diatomite, as a low-cost amendment, had a significant potential to stabilize metals in the contaminated calcareous soil.

Identification of the Chemical Forms of Heavy Metals in Municipal Sewage Sludge as a Critical Element of Ecological Risk Assessment in Terms of Its Agricultural or Natural Use.

The present study aimed to demonstrate that identification of the chemical forms of heavy metals in sewage sludge produced in municipal Wastewater Treatment Plants (WWTPs) is a critical element of ecological risk assessment, especially in terms of its agricultural or natural use. The concentrations of seven heavy metals (Cd, Cr, Cu, Ni, Pb, Zn and Hg) were determined using inductively coupled plasma optical spectrometry (ICP-OES) and cold vapor atomic absorption spectrometry (CV-AAS). The chemical forms of heavy metals were analyzed in accordance with the sequential extraction method proposed by the Community Bureau of Reference (BCR). Sludge samples were collected at the five municipal WWTPs located in the largest industrial area in the country, i.e., the Upper Silesian Industrial Region (southern Poland, central Europe). The ecological risk was assessed by calculating the Potential Ecological Risk Factor (ER), Risk Index (RI), Risk Assessment Code (RAC), Individual Contamination Factor (ICF), Global Risk Index (GRI) as well as the author’s indices, i.e., Individual Ecological Risk (IER) and Global Ecological Risk (GER). To demonstrate the differences between the level of ecological risk posed by the different heavy metals, sludge samples were collected at two specific points of the processing line. Considering the chemical forms of heavy metals, the highest ecological risk was posed by Zn, Cd and Ni, while in the case of their total concentrations, by Cd and Hg. The obtained results confirm that quantitative determination of the total content of heavy metals in sewage sludge is not a sufficient criterion in assessment of the ecological risk that these elements pose to the natural environment and living organisms. Moreover, multivariate statistical analysis revealed a significant correlation between the concentrations of heavy metals, which indicates that they plausibly originate from the same source of pollution.

Chemical Speciation and Potential Mobility of Heavy Metals in the Soils of Onitsha South Local Government Area Anambra Nigeria

The understanding of chemical forms of heavy metals and how they associate with themselves is very essential in studying the potential mobility and risk assessment of heavy metals in soils. This study determined the spatial distribution and mobility of Mn, Ni, Zn, Cr, Cu, Pb and Fe in the soil samples of Onitsha South Local Government Area in South East Nigeria. The soil samples collected were examined for the heavy metal contamination using five-step sequential extraction method. The concentrations of the extracted metals were determined using Atomic Absorption Spectrophotometer (AAS). The range of heavy metals extracted fractions for each of five fractions in percentages were as follows; exchangeables (0.0-3.5%), bound to carbonates (0.0-4.49%), bound to Fe-Mn oxides (0.03-4.14%), bound to organic matter (0.03-3.38%) and residual (0.0-86.70%) in wet season; exchangeables (0.0-3.94%), bound to carbonates (0.0-4.80%), bound to Fe-Mn oxides (0.09-5.19%), bound to organic matter (0.09-3.53%) and residual (0.0-126.00%) in dry season. Available heavy metals in the soil studied had mean values 0.52, 0.76, 1.21, 1.24, 1.40, 9.08 and 96.37 all in mgkg-1 for Pb, Cu, Zn, Ni, Mn, Cr, and Fe respectively for wet season while the mean values in dry season were 0.36, 1.21, 1.60, 1.82, 2.45, 11.54 and 143.54 all in mgkg-1 for Pb, Cu, Mn, Ni, Zn, Cr and Fe respectively. Mobility factor were 0.22-47.90% in the two seasons, levels of heavy metals in available forms differ significantly though majority of the extracted toxic metals are found in oxides and the residual fractions and as such may not pose environmental risk due to their relatively low availability and mobility factor. That notwithstanding, soils from studied area should be carefully monitored to prevent the release of these metals due to redox reactions which may make them available to plants through absorption.

Cadmium Chemical Forms in Two Calcareous Soils Treated with Different Levels of Incubation Time and Moisture Regimes

Sequential extraction is used effectively to determine the chemical forms of heavy metals. Since few studies have been conducted in calcareous soils of Iran, the current research has been carried out to evaluate the effect of moisture regimes and incubation time on the chemical forms of cadmium in two calcareous soils. Treatments included three levels of cadmium (0, 30 and 60 mg/kg of soil as CdSo4), three incubation times (2, 4 and 8 weeks), two soils (clay and sandy clay loam) and two moisture regimes (Water-logged and Field capacity). The experiment was performed as factorial on the basis of randomized complete blocks design with two replications. At 2, 4, and 8 weeks after treatment, the Tessier et al. (1979) sequential extraction method was applied to measure the cadmium concentration in water soluble-exchangeable (WsEx.), iron and manganese oxide (Fe-MnOx.), carbonate (Car.), organic matter (Om.) and residual forms (Res.). The results showed depending on the soil texture, 68 to 72 percent of the cadmium was in water soluble-exchangeable and iron-manganese oxide forms. Flooding condition decreased the concentration of cadmium in water soluble + exchangeable form and increased the concentration of cadmium in the forms of Fe-Mn oxides, carbonate and organic matter. Concentration of cadmium in residual form was higher in clay soil. In sandy clay loam soil, water soluble-exchangeable, carbonate and organic matter forms were higher than clay soil. The iron-manganese oxides form showed no significant difference in two soils at field capacity regime. In flooding conditions, the concentration of the water soluble-exchangeable form decreased and the concentration of other forms increased. In contrast to the oxidizing conditions at the reduction conditions, no significant difference was observed in the residual and organic matter forms in two soils.

Characterization of naturally aged cement-solidified MSWI fly ash

Solidification/stabilization (S/S) is the most common treatment for municipal solid waste incineration fly ash (MSWI-FA), and is widely applied in developed countries but has a history barely longer than 10 years in China. However, our understanding of the physicochemical characteristics of the solidified FA body after long-term natural aging is comparatively poor. Focusing on cement-solidified FA that was naturally aged for 6 years (hereafter referred to as FA-6), the physicochemical characteristics including elemental composition, mineral composition, microstructure, thermogravimetry, distribution of heavy metals in mineral phases, and leaching characteristics of inorganic salts (Na, K, Ca), anions (Cl and SO4) and heavy metals (Cd, Cr, Cu, Pb, Zn) were investigated in this study. By combining pH-dependent leaching results with the geochemical model LeachXS, the chemical forms of heavy metals in the FA solid phase was determined. The main conclusion was as follows: (1) soluble salts of FA-6 decreased by more than 92% compared with fresh FA. (2) In FA-6, the proportions of Pb, Cd and Zn in the non-mineral phase were 100%, 100% and 58%, respectively, which may cause potential environmental risk of heavy metal release. The leaching concentration of Pb was 4007.37 μg/L according to compliance batch test of HJ300, which was far higher than the landfill requirement of 250 μg/L. (3) The controlling phase for Pb in FA-6 was Pb5(PO4)3Cl (pH 2–12) and Pb2(OH)3Cl (pH > 12). (4) Carbonates, hydrous Fe oxides (HFO) and dissolved organic carbon (DOC) in FA-6 also affected the phase-controlled leaching of heavy metals. The carbonate fraction partly controlled the leaching of Cd, Cu and Zn. For example, smithsonite (ZnCO3) controlled the release of Zn (pH 2–13). Adsorption to solid humic acid (SHA) controlled the Cr leaching at pH < 7 and the Cu leaching except pH > 12.

Leaching behavior of V, Pb, Cd, Cr, and As from stone coal waste rock with different particle sizes

This paper investigates the leaching behavior of heavy metals (V, Pb, Cd, Cr, and As) from stone coal waste rocks with various particle sizes using dynamic leaching experiments. The results show that the dissolved concentrations of V and As initially increased and then slightly decreased as time progressed and that the dissolved concentrations of Pb, Cd, and Cr were high in the early stage before decreasing. The particle size of the stone coal waste rocks strongly influenced the heavy metal concentration in the leaching solutions. The effects of the particle size of the stone coal waste rocks on the dissolved concentrations of V, Pb, and As decreased in the order fine fraction > medium fraction > coarse fraction, and the effects of particle size on the dissolved concentrations of Cr and Cd decreased in the order medium fraction > coarse fraction > fine fraction and coarse fraction > medium fraction > fine fraction, respectively. The quantities of heavy metals dissolved from the stone coal waste rock with fine particle sizes were observed to decrease in the order V (17104.36 μg/kg) > As (609.41 μg/kg) > Pb (469.24 μg/kg) > Cr (56.35 μg/kg) > Cd (27.52 μg/kg), and the dissolution rates decreased in the order As (2.96%) > Pb (0.93%) > V (0.35%) > Cd (0.25%) > Cr (0.01%). The specific surface area, pore size of the stone coal waste rocks, and chemical forms of heavy metals also influenced the release of heavy metals from the stone coal waste rocks. Kinetic analysis showed that the dissolution of heavy metals fundamentally agreed with the rate equation controlled by the shrinking core model. The results of this study are expected to serve as a reference for the evaluation of heavy metals contamination from stone coal waste rocks.

Concentrations and chemical forms of heavy metals in the bulk atmospheric deposition of Beijing, China

This study aimed to investigate concentrations and mobility of selected metals (As, Cd, Cr, Co, Cu, Mn, Ni, Pb and Zn) contained in the bulk atmospheric deposition of Beijing, China, between March 2007 and February 2008, and to evaluate the metal deposition fluxes. Beijing endured heavy atmospheric deposited metal pollution with a deposition flux of 149.28kgkm-2year-1 from nine metals studied, and a significant increasing trend was noted from metals studied during the past 23years. High enrichment factors (EFs) of As, Zn, Pb, Cd, and Cu manifested their major anthropogenic sources. Among which, Cd and Pb require a consistent monitoring and to be strictly controlled regarding their high mobility and ecotoxicity. Furthermore, a declined metal concentration was observed in the bulk atmospheric deposition during spring, and long-transported crustal particles sourced from East Asian dust emission belt can reasonably account for this phenomenon.

Distribution characteristics of heavy metal(loid)s in aggregates of different size fractions along contaminated paddy soil profile.

Soil aggregates exert a significant influence on the retention and availability of heavy metal(loid)s in soil. In this study, the concentration distribution and chemical forms of heavy metals (Cu, Zn, Cd, Pb, and Hg) and a metalloid (As) in different aggregate-sized fractions (2-0.25, 0.25-0.05, 0.05-0.002, and < 0.002mm) along the profile (0-1, 1-5, 5-15, and 15-25cm) of a contaminated paddy field were investigated. The results showed that the values of pH, free Fe oxides (Fed), bulk density, and catalase activity gradually increased, whereas the soil organic matter (SOM), cation exchange capacity (CEC), electrical conductivity (EC), microbial biomass carbon (MBC), and urease activity decreased with depth. Long-term heavy metal pollution might impact the catalase activity but showed no obvious influence on the urease activity. Additionally, there was a notable difference in physicochemical properties among the soil aggregates of various particle sizes. The 2-0.25-mm fraction aggregates contained more organic matter, whereas the highest values of CEC and Fed were observed in the < 0.002-mm fraction. The concentrations of all six heavy metals/metalloid decreased with depth. In different layers, Cu and Cd showed the highest concentrations in the 2-0.25mm-fraction, followed by the < 0.002-mm fraction, whereas the highest concentrations of Zn, Pb, and As appeared in the < 0.002-mm fraction. No obvious distribution regularity was observed for Hg among the aggregates. All of the metal(loid)s had lower activity in the deeper soil layers, except for Hg. Furthermore, Cu and Cd displayed more stable forms in the < 0.002-mm fraction aggregates.

Subcellular Distribution and Chemical Forms of Heavy Metals in Three Types of Compositae Plants from Lead-Zinc Tailings Area

Field investigation on the content of heavy metals in soils and three types of widely distributing compositae plants(Artemisia lavandulaefolia, Ageratum conyzoides L., Crassocephalum crepidioides) in lead-zinc tailings farmland of Yangshuo, Guangxi Zhuang Autonomous Region was carried out, and the differential centrifugation technique and sequential chemical extraction method were used to study the subcellular distribution and chemical forms of heavy metals in these plants. The results indicated that the soil in the tailings farmland was highly contaminated by Cd, Pb and Zn, and their concentrations were 37.7, 5.7 and 8.9 times higher than their respective values of national standard for soil environment quality(GradeⅡ). The contents of Cd, Pb and Zn in the analyzed plants exceeded the normal ranges. Ageratum conyzoides L. and Crassocephalum crepidioides showed strong capability in tolerance, accumulation and transport of Cd, and they could be used as pioneer plants for Cd-phytoremediation in study area and some related areas. In addition, Cd concentrations in the stem and leave of Crassocephalum crepidioides were 159.6 mg·kg-1 and 219.5 mg·kg-1, respectively, which exceeded the threshold of Cd hyperaccumulator. Thus, Crassocephalum crepidioides can be regarded as a Cd-hyperaccumulator. Majority of total Cd, Pb and Zn were found in soluble fraction and cell wall in the plants, while only a small quantity were distributed in organelles. As for chemical forms distribution, Cd, Zn and Pb predominated NaCl-, HAC-, and HCl-extractable forms both in roots and shoots of the plants. Therefore, cell wall binding, vacuolar compartmentalization and distribution mainly in lower active chemical forms were supposed to be the main mechanisms for tolerance to heavy metals in the study plants. A difference of Cd subcellular distribution and its chemical forms in the three Compositae plants was observed. Compared with the shoot of Artemisia lavandulaefolia, more Cd was located in the vacuolus and cellular soluble parts in Ageratum conyzoides L. and Crassocephalum crepidioides. Moreover, the proportion of active chemical Cd in the shoots was lower than that in the roots. These results showed that the subcellular distribution and chemical forms of Cd were related to the plant species,and also indicated Ageratum conyzoides L. and Crassocephalum crepidioides were likely to possess a higher tolerance and accumulation of Cd.

Fractionation of heavy metals in sediments and assessment of their availability risk: A case study in the northwestern of Persian Gulf

Sequential extraction procedure was applied to determine the chemical forms of heavy metals in sediments in order to assess their mobility and availability in the aquatic environment. The mean concentrations of Cr, Ni, Pb, Cd, Cu and Zn in exchangeable and carbonate fractions were 14.9, 30.9, 17, 0.37, 4.4 and 14.4mg/kg, respectively. Based on the sediments quality guidelines (SQGs), the adverse biological effects caused by Ni, can occur frequently across the study area. Risk assessment code (RAC) suggested that the Cd has the highest level of environmental risk compared with other studied metals. Modified risk assessment code (mRAC) demonstrated a high potential adverse effect in the many of sampling sites. According to the fractionation results, the concentration of Ni in exchangeable and carbonate fractions was very considerable and more than other metals, hence, the potential risk of nickel release can be much more than other investigated metals.

Relative abundance of chemical forms of Cu(II) and Cd(II) on soybean roots as influenced by pH, cations and organic acids.

Little information is available on chemical forms of heavy metals on integrate plant roots. KNO3 (1 M), 0.05M EDTA at pH6 and 0.01 M HCl were used sequentially to extract the exchangeable, complexed and precipitated forms of Cu(II) and Cd(II) from soybean roots and then to investigate chemical form distribution of Cu(II) and Cd(II) on soybean roots. Cu(II) and Cd(II) adsorbed on soybean roots were mainly exchangeable form, followed by complexed form, while their precipitated forms were very low under acidic conditions. Soybean roots had a higher adsorption affinity to Cu(II) than Cd(II), leading to higher toxic of Cu(II) than Cd(II). An increase in solution pH increased negative charge on soybean and thus increased exchangeable Cu(II) and Cd(II) on the roots. Ca2+, Mg2+ and NH4+ reduced exchangeable Cu(II) and Cd(II) levels on soybean roots and these cations showed greater effects on Cd(II) than Cu(II) due to greater adsorption affinity of the roots to Cu(II) than Cd(II). L-malic and citric acids decreased exchangeable and complexed Cu(II) on soybean roots. In conclusion, Cu(II) and Cd(II) mainly existed as exchangeable and complexed forms on soybean roots. Ca2+ and Mg2+ cations and citric and L-malic acids can potentially alleviate Cu(II) and Cd(II) toxicity to plants.

Chemical fraction, leachability, and bioaccessibility of heavy metals in contaminated soils, Northeast China.

Heavy metals in urban soils may pose risks to both urban environment and human health. However, only a fraction of heavy metals in soil is mobile and/or bioavailable for plant uptake and human ingestion. This study evaluates the chemical fraction and potential mobility and bioaccessibility of heavy metals (Cd, Cr, Cu, Pb, and Zn) in the contaminated urban topsoils from steel-industrial city (Anshan), Northeastern China. Chemical forms of heavy metals in soils are determined using Tessier sequential extraction technique. The toxicity characteristic leaching procedure (TCLP), ethylenediaminetetraacetic acid (EDTA), and US Pharmacopeia methodology (USPM) are used to determine the operationally defined potentially mobile and bioaccessible metal fractions, respectively. Sequential extraction results show that Cd has the highest percentage of exchangeable form, whereas Cr primarily exists in residual form. The non-residual fraction of heavy metals increases in the order of Cr < Cu < Pb < Zn < Cd. The leachability of heavy metals evaluated by TCLP test indicates that Cd, Zn, Cu, and Pb have much higher mobile than Cr. The bioavailability of heavy metals determined by EDTA extraction decreases in the order of Pb > Cu ≅ Zn > Cd > Cr. The order of bioaccessibility determined by USPM extraction is Pb = Cu > Zn > Cd > Cr. The Cr exhibits the lowest leachability and bioaccessibility among the investigated metals. The Pb has the highest bioaccessibility, indicating higher potential hazard for the human health. There are significant relationships between the EDTA- and USPM-extractable metals (Cd, Cu, Pb, and Zn) and the sum of first three steps of sequential extraction. Highly significant correlation is found between amounts of EDTA-extractable Cd, Cu, Pb, and Zn and USPM-extractable metals. The result suggests that EDTA extraction can be helpful to estimate the bioaccessibility of heavy metals for human ingestion. Introduction of mobile and human bioaccessible concentrations into risk assessments can give more realistic implications for urban environmental management.