The role of arbuscular mycorrhiza on change of heavy metal speciation in rhizosphere of maize in wastewater irrigated agriculture soil

To understand the roles of mycorrhiza in metal speciation in the rhizosphere and the impact on increasing host plant tolerance against excessive heavy metals in soil, maize(Zea mays L.) inoculated with arbuscular mycorrhizal fungus(Glomus mosseae) was cultivated in heavy metal contaminated soil. Speciations of copper, zinc and lead in the soil were analyzed with the technique of sequential extraction. The results showed that,in comparison to the bolked soil, the exchangeable copper increased from 26% to 43% in non-infected and AM-infected rhizoshpere respectively; while other speciation (organic, carbonate and Fe-Mn oxide copper) remained constant and the organic bound zinc and lead also increased but the exchangeable zinc and lead were undetectable. The organic bound copper, zinc and lead were higher by 15%, 40% and 20%, respectively, in the rhizosphere of arbuscular mycorrhiza infected maize in comparison to the non-infected maize. The results might indicate that mycorrhiza could protect its host plants from the phytotoxicity of excessive copper, zinc and lead by changing the speciation from bio-available to the non-bio-available form. The fact that copper and zinc accumulation in the roots and shoots of mycorrhia infected plants were significantly lower than those in the non-infected plants might also suggest that mycorrhiza efficiently restricted excessive copper and zinc absorptions into the host plants. Compared to the non-infected seedlings, the lead content of infected seedlings was 60% higher in shoots. This might illustrate that mycorrhiza have a different mechanism for protecting its host from excessive lead phytotoxicity by chelating lead in the shoots.

Microbial activity related to N cycling in the rhizosphere of maize stressed by heavy metals

A greenhouse experiment was carried out to compare differences in potential activities of ammonification, nitrification and denitrification in rhizosphere and bulk soil in a heavy-metal-stressed system. Exchangeable fractions of Cd, Cu and Cr were all higher in the rhizosphere of maize than in bulk soil. Results showed that the mineralization of N in soil was stimulated by low concentration of Cd. Addition of Cd at low levels stimulated the ammonifying and nitrifying activity in soil, while inhibitory influences were shown at high levels. Nitrifying bacteria was proved to be the most sensitive one, whilst the effect on denitrifying bacteria was very limited. Comparing Cd, Cu and Cr(VI) at 20 mg/kg soil, Cd was the most effective inhibitor of ammonification and denitrification, while Cr(VI) had the strongest inhibitory influence on nitrifying activity. Root exudates played important roles on the different exchangeable metal fractions and bacterial activities between rhizosphere and non-rhizosphere. Nitrate was the main form of mineral N in soil, as well as the main form of N absorbed by plants, but the formation and relative absorption of ammonium were promoted in response to high Cd exposure.

Effects of Heavy Metals on Ammonification, Nitrification and Denitrification in Maize Rhizosphere

The ammonification, nitrification and denitrification in maize rhizosphere of alluvial soils were compared with those in the bulk soil after exposure to different kinds of heavy metals. The addition of cadmium at low levels (2 mg kg-1 and 5 mg kg-1) could stimulate the ammonification and nitrification in the soils, while inhibition influences were found at high levels of Cd addition (10 mg kg-1 and 20 mg kg-1). The relationship between microbial activity and cadmium concentration varied with the kind of microorganisms. The nitrifying bacteria were more sensitive to cadmium pollution than the ammonifying bacteria. When Cd(II), Cu(II) and Cr(VI) were compared at the same addition concentration of 20 mg kg-1 soil, Cd(II) was the most effective inhibitor of ammonification and denitrification among the three investigated heavy metals, and Cr(VI) had the most strong inhibitory influence on the nitrifying bacteria. The microbial activities in rhizosphere were higher than those in the bulk soil for most of the treatments. Cr(VI) proved to be the most effective in enhancing the microbial activities in rhizosphere, and this could be caused by the positive reduction of Cr(VI) to Cr(III) in rhizosphere, and the relatively sufficient existence of organic matter which intensified the adsorption of the metal. It seemed that the rhizosphere had some mitigation effect on heavy metal toxicity.

Geochemical distribution, fractionation, and sources of heavy metals in dammed-river sediments: the Longjiang River, Southern China

In the present study, six sediment cores were collected from six river-dammed reservoirs to reveal the geochemical distribution of heavy metals(As, Cd, Pb, Sb,and Zn) in the Longjiang River, South China, which is highly impacted by nonferrous metal mining and smelting activities. The sediments were geochemically characterized, combining geochemical analysis, sequential extractions, and210 Pb chronology. The results indicated that the river sediments were severely polluted by heavy metals in the order of Cd [ Zn & Pb & Sb [ As. These heavy metals generally exhibited relatively low enrichment in the upstream sediments because of the limited anthropogenic impact, but their abundances drastically increased in the midstream sediments due to local smelting activities. In downstream sediments, the heavy metal concentrations(except for Cd) decreased, owing to the effect of dam interception and detrital inputs. Cadmium levels tended to increase in downstream sediments, which were attributed to the intensive discharge of Cd during the pollution event in 2012. The sedimentary records were traced back to1985, and a significant decrease of heavy metal enrichments could be found after the year 2000, suggesting the enhancement of environmental management in this period.The statistical results indicated that local metal smelting and mining activities were the main anthropogenic contributors for the enrichment of heavy metals in the dammed-river sediments. High enrichment factor and nonresidual fractions of heavy metals in local sediments may pose a direct threat to aquatic organisms. Cd presents significant danger because of its extreme enrichment and high labile fractions.

Geochemical fractionation and potential release behaviour of heavy metals in lead–zinc smelting soils

The lack of understanding of heavy metal speciation and solubility control mechanisms in smelting soils limits the effective pollution control.In this study smelting soils were investigated by an advanced mineralogical analysis(AMICS),leaching tests and thermodynamic modelling.The aims were to identify the partitioning and release behaviour of Pb,Zn,Cd and As.The integration of multiple techniques was necessary and displayed coherent results.In addition to the residual fraction,Pb and Zn were predominantly associated with reducible fractions,and As primarily existed as the crystalline iron oxide-bound fractions.AMICS quantitative analysis further confirmed that Fe oxyhydroxides were the common dominant phase for As,Cd,Pb and Zn.In addition,a metal arsenate(paulmooreite)was an important mineral host for Pb and As.The pH-stat leaching indicted that the release of Pb,Zn and Cd increased towards low pH values while release of As increased towards high p H values.The separate leaching schemes were associated with the geochemical behaviour under the control of minerals and were confirmed by thermodynamic modelling.PHREEQC calculations suggested that the formation of arsenate minerals(schultenite,mimetite and koritnigite)and the binding to Fe oxyhydroxides synchronously controlled the release of Pb,Zn,Cd and As.Our results emphasized the governing role of Fe oxyhydroxides and secondary insoluble minerals in natural attenuation of heavy metals,which provides a novelty strategy for the stabilization of multi-metals in smelting sites.

Integration of chemical fractionation, Mosbauer spectrometry, and magnetic methods for identification of Fe phases bonding heavy metals in street dust

Street dust is one of the most important carriers of heavy metals(HMs)originating from natural and anthropogenic sources.The main purpose of the work was to identify which of Fe-bearing phases bind HMs in street dust.Magnetic parameters of the Fe-bearing components,mainly magnetically strong iron oxides,are used to assess the level of HM pollution.Chemical sequential extraction combined with magnetic methods(magnetic susceptibility,magnetization,remanent magnetization)allowed determining the metal-bearing fractions and identifying the iron forms that aremostly associated with traffic-related HMs.The use of Mossbauer spectrometry(MS)supplemented bymagnetic methods(thermomagnetic curves and psarameters of hysteresis loops)enabled precise identification and characterization of iron-containing minerals.The classification of HMs into five chemical fractions differing in mobility and bioaccessibility revealed that iron is most abundant(over 95%)in the residual fraction followed by the reducible fraction.HMs were present in reducible fraction in the following order:Pb>Zn>Mn>Cr>Ni>Fe>Cu,while they bound to the residual fraction in the following order:Fe>Ni>Cr>Mn>Pb>Cu>Zn.The signature of the anthropogenic origin of street dust is the presence of strongly nonstoichiometric and defected grains of magnetite and their porous surface.Magnetite also occurs as an admixture with maghemite,and with a significant proportion of hematite.A distinctive feature of street dust is the presence of metallic iron and iron carbides.Magnetic methods are efficient in the screening test to determine the level ofHMpollution,while MS helps to identify the iron-bearing minerals through the detection of iron.

Fractionation of heavy metals in shallow marine sediments from Jinzhou Bay,China

This work investigated the distribution and speciation of Cd, Cu, Pb, Fe and Mn in the shallow sediments of Jinzhou Bay, Northeast China, which has been heavily contaminated by nonferrous smelting activities. The concentrations of Cd, Cu and Pb in sediments were found to be 100, 13 and 7 times, respectively, being higher than the national guideline （GB 18668-2002）. Sequential extraction test showed that 39%-61% of Cd were exchangeable fractions, indicating that Cd in the sediments posed a high risk to local environments. While Cu and Pb were at moderate risk levels. According to the relationships between percentage of metal speciation and total metal concentration, it was concluded that the distributions of Cd, Cu and Pb in some geochemical fractions were dynamic in the process of pollutants migration and the stability of metals in sediments of Jinzhou Bay decreased in the order of Pb 〉 Cu 〉 Cd.

Fractionation of Heavy Metals in Soils as Affected by Soil Types and Metal Load Quantity

Two series of soil subsamples, by spiking copper (Cu), lead (Pb), zinc (Zn) and cadmium (Cd) in an orthogonal design, were prepared using red soil and brown soil, respectively. The results indicated that heavy metal fractions in these soil subsamples depended not only on soil types, but also on metal loading quantity as well as on interactions among metals in soil. Lead and Cu in red soil appeared mostly in weakly specifically adsorbed (WSA), Fe and Mn oxides bound (OX), and residual (RES) fractions. Zinc existed in all fractions except organic bound one, and Cd was major in water soluble plus exchangeable (SE) one. Different from the results of red soil, Pb and Cu was present in brown soil in all fractions except organic one, but over 75% of Zn and 90% of Cd existed only in SE fraction. Meanwhile, SE fraction for any metal in red soil was lower than that in brown soil and WSA and OX fractions were higher. It is in agreement with low cation exchange capacity and large amounts of metal oxides included in red soil. Metal fractions in soil, especially for water soluble plus exchangeable one, were obviously influenced by other coexisting metals. The SE fraction of heavy metals increased with increasing loading amounts of metals in red soil but not obviously in brown soil, which suggest that metal availability be easily affected by their total amounts spiked in red soil. In addition, more metals in red soil were extracted with 0.20 mol L-1 NH4Cl (pH 5.40) than that with 1.0 mol L-1 Mg(NO3)2 (pH 7.0), but the reverse happened in brown soil, implicating significantly different mechanisms of metal desorption from red soil and brown soil.

Fractionation of Heavy Metals in Sediments from Dianchi Lake,China

Fractionation of heavy metals in sediments can help in understanding potential hazards of heavy metals. The present study analyzed total concentrations and fractions of selected heavy metals （Cd, Cr, Cu, Pb, and Zn） in surface sediments from Dianchi Lake, Yunnan Province, China, as well as factors that may affect distributions of the various heavy metal fractions. Total concentrations of the heavy metals decreased in the order Zn 〉 Cu 〉 Pb 〉 Cr 〉 Cd. These heavy metals, except Cr, were much higher than their background levels, indicating that Dianchi Lake was polluted by Cd, Zn, Pb, and Cu. Cadmium occurred mainly as the non-residual fraction （sum of the HOAc-soluble, reducible, and oxidizable fractions） （97.6%）, and Zn （55.7%） was also predominantly found in the non-residual fraction. In contrast, most of the Cr （88.5%）, Pb （81.8%）, and Cu （59.2%） occurred in the residual fraction. Correlation analysis showed that total heavy metal concentrations, organic matter and reducible Fe were the main factors affecting the distributions of the various heavy metal fractions. In the Walhai section of Dianchi Lake （comprising 97% of the lake area）, the concentrations of Cd, Zn, Pb, and Cu in the non-residual fraction were significantly lower （P ≤ 0.01 or 0.05） than those of the Caohal section （3% of the lake area）. This indicated that potential heavy metal hazards in the Caohai section were greater than the Waihai section.

Distribution characteristics and controlling factors of typical heavy metals in Huanghe River estuary,China

The geochemical characteristics and potential controlling factors of colloidal Zn,Cd,and Pb in Huanghe(Yellow)River estuary(HRE),China were investigated.The three metals were highly variable over a range of spatiotemporal scales,comprehensively forced by various physical and biological processes.Total dissolved Zn,Cd,and Pb varied from 200.1 to 321.7,2.6 to 4.1,and 0.5 to 1.0 nmol/L,respectively.Only one near-estuarine station of Zn had contamination factor values>1,which indicate the lower contaminant levels.Five dissolved species of Zn,Cd and Pb were fractionated,namely<1 kDa,1-3 kDa,3-10 kDa,10-100 kDa,and 100 kDa-0.45μm.The<1 kDa truly dissolved phase was the main fraction of the three dissolved metals(50%-62%),while the 100-kDa-0.45-μm high molecular weight colloidal fraction was dominant in their respective colloidal phase.Territorial input and sediment acted as important sources of strong ligands and natural colloids for the HRE water system.<3-kDa Zn and Pb were susceptible to the dissolved oxygen,the behaviors of colloidal Zn and 3-10-kDa Pb were related to dissolved organic carbon(DOC).However,no significant correlation between each dissolved fraction of Cd and salinity,pH,temperature,colloidal organic carbon,and DOC was found in this study.Overall,these findings,completed by the evaluation of the dissolved species of Zn,Cd,and Pb at 10 sites over the river-sea mixing zone,provided new insights into the colloidal heterogeneity that affect metals geochemical features,migration and fate in estuaries.

Distribution,fractionation,and contamination assessment of heavy metals in offshore surface sediments from western Xiamen Bay,China

Surface sediment samples were collected at 21 offshore sites in western Xiamen Bay, Southeast China. Total concentrations of Li, V, Cr, Co, Ni, Cu, Zn, St, Mn, Pb, Ba, Fe, and Ti were determined by inductively coupled plasma-optical emission spectrometry; Hg was determined by atomic fluorescence spectrometry. A modified BCR sequential extraction procedure was used to extract fractions of the above elements. Concentrations of Pb, Cr, and Hg at most sites met the primary standard criteria of Marine Sediment Quality except site S12 for Pb and S7 for Cr, while concentrations of Zn at 17 sites and Cu at seven sites exceeded the criteria. The mean concentration of Hg was three times higher than the background, with a possible source being the Jiulong River. Fe, Ti, Ba, Co, V, and Li dominated the residual phase, mainly from terrestrial input. Ni, Cr, Pb, and Hg in the non-residual phase varied largely between sites. Sr, Mn, Cu, and Zn were mainly in the non- residual fraction. Most sites showed considerable ecological risk; exceptions were site S7 （very high） and sites S10, S11, and S14 （moderate）. Cu showed moderate-to-high pollution and Pb exhibited no-to-low pollution, while other metals had a non-pollution status according to their ratios of secondary phase to primary phase （RSP）. Results of two assessment methods showed moderate pollution and a very high ecological risk for Cu, Zn, Ni, and Cr at site S7, which might be due to the local sewage treatment plant.

Total concentrations and different fractions of heavy metals in sewage sludge from Guangzhou, China

Dewatered municipal sludge samples were collected from five municipal wastewater treatment plants （WWTPs） and one industrial WWTP in Guangzhou, China. A number of agricultural parameters and total metal concentrations in the sludge were determined. Metal speciation was also studied. The results showed that sewage sludge had high organic carbon, and was rich in such nutrients as N and P. The concentrations of Mn, Zn, and Cu were the highest, followed by Ni, Pb, and Cr, Cd had the lowest concentration. In addition, the concentrations of the aforementioned heavy metals in the sludge samples were higher than those recorded in the background data for crop soils. With the exception of Cu and Cd from site S1, and Ni from sites S1, $2, and $5, all other metal concentrations conformed to permissible levels prescribed by the national application standard of acid soil in China （GB 18918--2002）. The results of the BCR sequential extraction showed that the concentrations of Mn and Zn were predominant in acid-soluble/exchangeable and reducible fractions. Cu was principally distributed in oxidizable and residual fractions, whereas Cr was present in oxidizable and residual fractions, Pb was found in the state of residual fractions, and the distribution of Ni and Cd did not show significant characteristics.

Risk assessment of heavy metal contaminated soil in the vicinity of a lead/zinc mine

Heavy metal contamination of soils through anthropogenic activities is a widespread and serious problem confronting scientists and regulators throughout the world. In this study we investigated the distribution, chemical species and availability of lead, zinc, cadmium and copper in nine surface（0 to 20 cm） soils from near an abandoned lead/zinc mine tailings located in Shaoxing, Zhejiang, China. Total heavy metal contents ranged from 5271 to 16369 mg/kg for Pb, 387 to 1221 mg/kg for Zn, 3.0 to 9.3 mg/kg for Cd and 65 to 206 mg/kg for Cu. In general, all heavy metals exceeded China National Standards for Soil Environmental Quality of Heavy Metals by a factor of 3-65 times. Comparison of the heavy metal concentrations（Pb, Zn, Cd and Cu） with clay content revealed a strongly significant relationship while significant relationship（ P 〈 0.001 ） was also obtained between Cd ＋ Zn and Pb ＋ Cu. Solid phase speciation of the soils using Tessier procedure showed that the heavy metals were distributed in the order： residual 〉〉 organically complexed-Fe-Mn oxides occluded 〉 carbonate bound 〉 exchangeable 〉 water soluble. In the organic matter fraction, the ratio of Pb（29.1% ） to its total concentration in the soils was higher than those of Zn（4.70% ）, Cd（3.16% ） and Cu（9.50% ）. The percentages of the water soluble and the exchangeable fractions of Pb（1.80% ） and Cd（2.74% ） were markedly greater than those of Zn（0.10% ） and Cu（0.15% ）, suggesting that Pb and Cd are relatively more mobile and hence more toxic in the contaminated soils. Strongly significant relationships between H20-Pb, H20-Zn and H20-Cu, strong positive correlations between H20-Pb, H20-Zn, H20-Cu and organic matter in soil were found. The content of H20-Pb, H20-Zn, H20-Cu was negatively correlated with pH values. The similar negative relationships between pH values and exchangeable heavy metals were also recorded. It is suggested that increasing soil pH or liming the soil could decrease bioavailability of heavy metals in the soil.

Heavy metals in Changjiang estuarine and offshore sediments:responding to human activities

The Changjiang （Yangtze） estuarine and offshore sediments were analyzed for total heavy metals concentrations and chemical fractions. Distributions of heavy metals show typical banded diffusion pattern, with high concentrations near the river mouth and following a decreasing trend in the offshore direction. According to chemical fractions, Fe/Mn oxide fraction is the major non-residual fraction in the Changjiang estuarine and offshore sediments, Higher percentage of non-residual fraction of Pb implies that, the industrial contaminations transported via the atmosphere and river input, may affect the non-residual fraction of heavy metals. Over past fifteen years, the concentration of Pb normalizing to A1 presents significant increasing trend, corresponding to the effect of human activities. By comparison of heavy metals fractions in 2003 to 2006, it has been realized that increasing water and sediment may cause a higher percentage non-residual fraction of Cu in the southern part of offshore muddy sediments.

Heavy Metals and PAHs in Sewage Sludge from Twelve Wastewater Treatment Plants in Zhejiang Province

Objective To investigate the heavy metals （HMs） and polycyclic aromatic hydrocarbons （PAHs） in sludge of twelve wastewater treatment plants （WWTPs） in Zhejiang province of China, and to assess their potential for land application. Methods Sludge was collected from 12 WWTPs within the province. GC-MS and AAS were used to measure PAHs and HMs contents in sludge. Results Concentrations of HMs in most of the sludge samples were below the regulatory limits for the sludge to be used in agriculture in China with the exception of Zn in 2 sludge samples and Cd in 1 sample. All 16 PAHs, targeted by the USEPA agency, were found in the sludge from the twelve plants with a total concentration ranging from 33.73 mg kg^-1 to 82.58 mg kg^-1 （dry weight, d.w.）. The levels of Σ9 PAHs varied from 13.87 mg kg^-1 to 61.86 mg kg^-1 （d.w.） in the sludge, far exceeding the limitation value recommended by the Europe Union. The concentration and composition of PAHs in sewage sludge varied and depended mainly on the quantity and type of industrial wastewater accepted by the WWTPs. A significant relationship between the proportion of industrial wastewater received by WWTPs and the total content of 16 PAHs in the sludge was observed. Conclusion PAHs have become one of the primary pollutants in sludge of Zhejiang WWTPs instead of HMs. It is, therefore, essential to reduce the contents of PAHs before the sludge can be used in agriculture through proper treatment.

MSWI Fly Ash Based Novel Solidification/Stabilization Matrices for Heavy Metals

The possibilities of MSWI fly ash as a major constituent of novel solidification/stabilization matrices for secure landfill were investigated by mixing MSWI fly ash with rich aluminum components, which was added as bauxite cement or metakaolinite instead, to form Friedel and Ettringite phases with high fixing capacities for heavy metals. The physical properties, heavy metals-fixing capacity, mineral phases and its vibration bands in the novel matrices were characterized by compressive strength, TCLP（toxic characteristic leaching procedure）, XRD （x-ray diffraction） , DTG （derivative thermogravimetry）, and FTIR （fourier transform infrared spectroscopy）, respectively. The Tessier＇s five-step sequential extraction procedure was used to analyze the fractions of chemical speciation for Pb, Cd and Zn ions. The experimental results indicate that Friedel-Ettringite based novel solidification/stabilization matrices can incorporate Pb, Cd and Zn ions effectively by physical encapsulation and chemical fixation, and it exhibits a great potential in co-landfill treatment of MSWI fly ash with some heavy metals-bearing hazardous wastes.

Effects of organic acids on heavy metal release or immobilization in contaminated soil

In order to explicit the environmental activity of heavy metals affected by different organic acids in soil,a batch incubation experiment was explored to investigate the influence of high relative molecular mass organic acid(HMWOA)(humic acid and fulvic acid)and low relative molecular mass organic acid(LMWOA)(threonic acid and oxalic acid)on the release or immobilization of Pb,Cu and Cd in soils.Results showed that LMWOA,especially threonic acid,had a good performance in the release of Pb,Cu and Cd from soils,and decrease in the fractions of HOAc-extractable,reducible and oxidable Pb,Cu and Cd.Conversely,HMWOA,especially humic acid,decreased the release of Pb,Cu and Cd,while it increased the fractions of HOAc-extractable,reducible and oxidable Pb,Cu and Cd,indicating that HMWOA can immobilize heavy metals.The release of Pb,Cu and Cd caused by LMWOA was attributed to the dissociation of soil organic matter and amorphous iron oxides since the total organic carbon and the water-soluble iron increased.The immobilization of Pb,Cu and Cd by HMWOA was attributed to the adsorption onto HMWOA followed by amorphous iron since HMWOA resulted in a significant decrease of zeta potential and an increase of amorphous iron oxide.It can be concluded that LMWOA has a potential application in soil washing remediation,while HMWOA can be used in the immobilization remediation for heavy metals contaminated soils.

Role of soil rhizobacteria in phytoremediation of heavy metal contaminated soils

Heavy metal pollution of soil is a significant environmental problem and has its negative impact on human health and agriculture. Rhizosphere, as an important interface of soil and plant, plays a significant role in phytoremediation of contaminated soil by heavy metals, in which, microbial populations are known to affect heavy metal mobility and availability to the plant through release of chelating agents, acidification, phosphate solubilization and redox changes, and therefore, have potential to enhance phytoremediation processes. Phytoremediation strategies with appropriate heavy metal-adapted rhizobacteria have re-ceived more and more attention. This article paper reviews some recent advances in effect and significance of rhizobacteria in phytoremediation of heavy metal contaminated soils. There is also a need to improve our understanding of the mechanisms in-volved in the transfer and mobilization of heavy metals by rhizobacteria and to conduct research on the selection of microbial isolates from rhizosphere of plants growing on heavy metal contaminated soils for specific restoration programmes.

Research on the ecological risk of heavy metals in the soil around a Pb–Zn mine in the Huize County, China

The soil pollution by heavy metals was characterized in the mine tailings and surrounding soils of an old Pb–Zn mine in Huize County. Three hundred and ninety-six samples of agricultural and non-agricultural soil were analyzed for the total metal concentration by acid digestion and sixty-eight selected soil samples were used to determine the chemical fractionation of heavy metals with the modified BCR sequential extraction method. The pollution index of the heavy metals indices for As, Cd, Cr, Cu,Hg, Pb and Zn of non-agricultural soil and agricultural soils in the study area indicated the spreading of heavy metal pollution. Sequential extraction showed that most of Cd existed in an exchangeable form(31.2 %). The available content of Pb, Cu and Zn was mainly distributed in acid extractable fractions and Fe/Mn oxide fractions(27.9, 30 and 27.2 %), and Hg, As and Cr were mainly associated with residual fractions(90.4, 72.9 and 76.8 %). The risks of heavy metals were also evaluated by the risk assessment code(RAC) and potential ecological risk index, respectively. The results of RAC showed a medium and high risk of Cd(45.6 and 54.4 %), medium risk of Zn(100 %), low and medium risk of Cu(41.2 and 58.8 %), largely no risk of Hg(97.1 %), and mainly low risk of As and Pb(92.6and 91.8 %). The range of the potential ecological risk of soil was 58.2–1839.3, revealing a considerably high ecological risk in the study area, most likely related to acid mine drainage and the mining complexes located in the area. The results can be used during the ecological risk screening stage, in conjunction with the total concentrations and metal fractionation values, to better estimate ecological risk.

Effects of Heavy Metal Contamination on Microbial Biomass and Community Structure in Soils

Zinc smelting near Magu Town, Hezhang County, Guizhou Province, resulted in vegetation destruction and the accumulation of heavy metals, to varying extent, in adjacent soils, where up to 162.2- 877.9 mg·kg -1 Zn, 37.24- 305.6 mg·kg -1 Pb, and 0.50- 16.43 mg·kg -1 Cd, were detected. These values greatly exceed the background levels of these elements in soils. The concentrations of heavy metals (particularly Pb, Zn) were positively correlated with the contents of Fe 2O 3 and Al 2O 3 in the soils, showing that Fe and Al oxides play an important role in retaining heavy metals. Chemical fractionation indicates that Pb and Zn were associated mainly with Fe and Mn oxides and minerals, whereas Cd was dominated by exchangeable form. Microbial biomass in the soils was relatively low, in the range of 57.00- 388.0μg C·g -1, and was negatively correlated with heavy metal concentrations in the soils. The correlation coefficient of microbial biomass C to Zn concentrations in the soils was as high as -0.778 (p< 0.01), indicating that the heavy metal contamination has toxic effects on microorganisms in soil. The results of Biolog measurements demonstrated that there were no significant changes in microbial community structure in the heavy metal contaminated soils. Gene fragments were similar to one another after the DNA was extracted from soil microbes and experienced PCR (polymerase chain reaction) and DGGE (denaturing gradient gel electrophoresis) reactions. These results indicated that light heavy-metal pollution may not result in any change in soil microbial community structure.