云南滇池沉积物中重金属的形态分布特征

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.     

磷改良剂对中度和高度含碳酸盐的菜园土壤中Cd和Pb环境有效性和植物有效性的影响

The behaviour of metals mainly depends on soil p H, carbonate contents and contamination level, which should be considered for the management of contaminated soils. In this study, kitchen garden topsoils(0–25 cm) were sampled from the area around three smelters in France, with different Cd and Pb contamination levels. Effect of a phosphate amendment(a mixture of diammonium phosphate and hydroxyapatite) on the environmental availability and phytoavailability of Cd and Pb was evaluated by different chemical extractions and cultivating lettuce(Lactuca sativa L.), respectively. Changes in the distribution of Cd and Pb were found in most contaminated soils after phosphate amendment. An increase of Cd and Pb in the residual phase was highlighted in almost all carbonated contaminated soils, whereas a decrease of Pb in the exchangeable, water and acid-soluble phase was observed in most contaminated soils with the lowest carbonate contents. The concentrations of extractable Cd and Pb using calcium chloride and acetic and citric acids generally decreased after the soil amendment. Lettuces grown on amended soils were acceptable for human consumption as regard to Pb concentration. In contrast, some lettuces were unacceptable for human consumption, since the concentrations of Cd in the leaves were higher than the European legislation limit. Surprisingly, in carbonated soils with very low concentration of Cd, the Cd concentrations in lettuce reached up to the European legislation limit, making the lettuce unacceptable for human consumption.Our study highlighted the fact that the total metal concentration in soils does not always allow to predict the metal accumulation in the edible parts of vegetables in order to make a judgement about their acceptability or unacceptability for human consumption.     

广东大宝山矿区土壤重金属污染

Soil contamination in the vicinity of the Dabaoshan Mine, Guangdong Province, China, was studied through determi- nation of total concentrations and chemical speciation of the toxic metals, Cu, Zn, Cd, and Pb, using inductively coupled plasma mass spectrometry. The results showed that over the past decades, the environmental pollution was caused by a combination of Cu, Zn, Cd, and Pb, with tailings and acid mine drainage being the main pollution sources affecting soils. Significantly higher levels （P ≤ 0.05） of Cu, Zn, Cd, and Pb were found in the tailings as compared with paddy, garden, and control soils, with averages of 1486, 2516, 6.42, and 429 mg kg^-1, respectively. These metals were continuously dispersed downstream from the tallings and waste waters, and therefore their concentrations in the paddy soils were as high as 567, 1 140, 2.48, and 191 mg kg^-1, respectively, being significantly higher （P ≤ 0.05） as compared with those in the garden soils. The results of sequential extraction of the above metals from all the soil types showed that the residual fraction was the dominant form. However, the amounts of metals that were bound to Fe-Mn oxides and organic matter were relatively higher than those bound to carbonates or those that existed in exchangeable forms. As metals could be transformed from an inert state to an active state, the potential environmental risk due to these metals would increase with time.     

对矿区和冶炼区土壤重金属污染进行化学和矿物学特征分析

The binding of metallic contaminants （Pb, Cd, and Zn） and As on soil constituents was studied on four highly contaxninated alluvial soil profiles from the mining/smelting district of Pribram （Czech Republic） using a combination of mineralogical and chemical methods. Sequential extraction analysis （SEA） was supplemented by mineralogical investigation of both bulk samples and heavy mineral fractions using X-ray diffraction analysis （XRD） and scanning electron microscopy with an energy dispersive X-ray spectrometer （SEM/EDS）. The mineralogy of Fe and Mn oxides was studied by voltammetry of microparticles （VMP） and diffuse reflectance spectrometry （DRS）. Zinc and Pb were predominantly bound in the reducible fraction attributed to Fe oxides and Mn oxides （mainly birnessite, Na4Mn14O27.9H2O）, which were detected in soils by XRD and SEM/EDS. In contrast, Cd was the most mobile contaminant and was predominantly present in the exchangeable fraction. Arsenic was bound to the residual and reducible fractions （corresponding to Fe oxides or to unidentified Fe-Pb arsenates）. SEM/EDS observations indicate the predominant affinity of Pb for Mn oxides, and to a lesser extent, for Fe oxides. Thus, a more suitable SEA procedure should be used for these mining-affected soils to distinguish between the contaminant fraction bound to Mn oxides and Fe oxides.     

重金属迁移与土壤性质的关系

Cu, Zn, Pb and Hg runoff from yellow limestone soil and purple soils and the relationships between the mobility of the heavy metals and the soil characteristics were studied in laboratory using a rainfall simulator. The results showed that the concentrations of soluble Zn in surface runoff were significantly negatively correlated with the contents of < 0.002 mm particles and CEC of the soils, indicating that Zn was mostly adsorbed by clays in the soils. The contents of Cu and Hg in surface runoff were positively related to their contents in the soils. The amounts of Cu, Zn, Pb and Hg removed by surface runoff were influenced by the amounts of soil and water losses and their contents in the soils, and were closely related to the contents of soil particles 1~0.02 mm in size.     

中国原油污染区重金属空间分布模式及其潜在来源研究

Twenty-two soil samples were collected at the subregional scale （50 km2） around Gudao Town, a typical oil-producing region of the Shengli Oilfield in the Yellow River Delta, China to determine the spatial distribution patterns and potential sources of heavy metals in soils of crude oil-polluted regions. The concentrations of total petroleum hydrocaxbons （TPHs） and heavy metals as well as other soil properties were determined and the enrichment factor values were calculated for the heavy metals measured. Principal component analysis （PCA） and cluster analysis （CA） were used to estimate potential sources contributing to the concentrations of heavy metals in the soils. The results revealed that the soils were slightly alkaline （pH = 7.33-8.05） and high in salinity （1.43-41.30 g kg-1）, TPHs （0.51 28.40 g kg-1） and organic matter （1.74-31.50 g kg-1）. The mean concentrations of the measured heavy metals Cu, Zn, Pb, Cd, Cr, Ni and V were 18.4, 78.2, 20.8, 0.19, 56.6, 26.3 and 62.1 mg kg-1, respectively. Although the concentrations of all the metals measured in this study were not high enough to exceed the national control standards, there was a significant enrichment of Cd in the study area and Zn and Ni were in the category of deficiency to minimal enrichment. The spatial distribution patterns of Cu, Cr, Ni and V were similar and partially affected by oil exploitation and petroleum hydrocarbon spills. Potential sources of Cr, Ni, V and Cu in the soils were both natural sources and petroleum hydrocarbon spills, while Zn, Pb and Cd were probably from anthropogenic sources such as farming activities and traffic.     

施用污泥后石灰性土壤中铜、锌、镉的植物有效性

The toxicity of trace elements (TEs), such as copper (Cu), zinc (Zn), and cadmium (Cd), often restrict land application of sewage sludge (SS) and there was little information about soil-plant transfer of TEs in SS from field experiments in China. In this study pot and field experiments were carried out for 2 years to investigate the phytoavailability of TEs in calcareous soils amended with SS. The results of the pot experiment showed that the phytoavailability of Zn and Cu in the SS was equal to 53.4%-80.9% and 54.8%-91.1% of corresponding water-soluble metal salts, respectively. The results from the field experiment showed that the contents of total Zn, Cu, and Cd in the soils increased linearly with SS application rates. With increasing SS application rates, the contents of Zn and Cu in the wheat grains initially increased and then reached a plateau, while there was no significant change of Cd content in the maize grains. The bioconcentration factors of the metals in the grains of wheat and maize were found to be in the order of Zn > Cu > Cd, but for the straw the order was Cd > Cu > Zn. It was also found that wheat grains could accumulate more metals compared with maize grains. The results will be helpful in developing the critical loads of sewage sludge applied to calcareous soils.     

Accumulation and potential sources of heavy metals in soils of the Hetao area,Inner Mongolia,China

Soil contamination by heavy metals is a problem in agricultural irrigation systems.To assess the accumulation and sources of heavy metals in the Yongji irrigation district of the Hetao area,Inner Mongolia,China,195 soil samples from 39 sites(0–100 cm)were collected,and Zn,Cu,Pb,Cr,and Cd concentrations were analyzed.The mean concentrations were 107.17,32.48,12.31,53.53,and 0.22 mg kg-1,respectively,with no significant differences between soil depths(P>0.05).Concentrations of Zn,Cu,and Cd were higher than the background levels,with moderate accumulation;the contamination factor(CF)values were 1.9,1.7,and 1.9,respectively,and the geoaccumulation index(Igeo)was>0.Concentrations of Pb and Cr were lower than,or close to,the background levels(CF<1,Igeo<0),indicating that they originated from a natural source.The monomial potential ecological risk index(Eri)for Zn,Cu,Pb,and Cr was low;Eri for Cd was 55.73,implying a moderate risk.The grade of potential ecological risk index of the five heavy metals(RI)was low,declining from south to north.The studied soils were contaminated with Zn,Cu,and Cd;principal component(PC)analysis implicated the enrichment of Cd and partial Cu(high loading in PC 2)was related to agricultural activities;Zn and partial Cu,closely associated with PC 3,may have originated from irrigation water from the Yellow River.Future agricultural development should focus on fertilizer and pesticide application and the quality of irrigation water.     

Effect of Synergism and Antagonism between Metals on Toxicity in Soils

Synergism and antagonism of cadmium(Cd),copper (Cu) and selenium (Se) to biological toxicities in red soil,yellow brown soil and black soil were evaluated by MICROTOX method.The relation between forms of the tested metals in soil and the synergism or antagonism between them was also studied.Results showed that owing to the difference of soil chemical properties,toxicity of these metals in soils was different.In red soil with acid reaction and low in cation exchange capacity,antagonism occurred significantly between metals when they coexisted at high concentrations,while synergism occurred only under low concentrations.It is indicated that in red soil,toxicity of metals affected by synergism or antagonism depends on concentration of the metals present.For yellow brown soil and black soil with larger cation exchange capacity and lower exchangeable aluminium(Al),no toxicity of metals was observed even if metals were added to soil in high concentrations.Synergism and antagonism between Cd,Cu and Se were controlled by the forms of metals present.The amount of water-soluble metals was the most important factor in determining synergism and antagonism. In this paper,comparisons of synergism and antagkonism between metals in soils and in water solutions were made.There occurred the synergism of metal toxicity in water solutions when the concentration of coexisting metals was high.This is just opposite to the case in soils.     

杭州市城市土壤中重金属、磷和其它元素的特征

Health implications of inhaling and/or ingesting dust particles with high concentrations of heavy metals from urban soils are a subject of intense concern. Understanding the geochemistry of these metals is key to their effective management. Total concentrations of heavy metals, phosphorus (P) and 8 other elements from topsoil samples collected at 82 locations in Hangzhou City were measured to: a) assess their distribution in urban environments; and b) understand their differentiation as related to land use. Metal mobility was also studied using a three-step sequential chemical fractionation procedure. About 8.5%, 1.2%, 3.6%, 11.0% and 30.3% of the soil samples had Cd, Cr, Cu, Pb, and Zn concentrations, respectively, above their allowable limits for public and private green areas and residential use. However, in commercial and industrial areas, most samples had metal concentrations below their allowable limits. Statistical analyses revealed that the 16 measured elements in urban soils could be divided into four groups based on natural or anthropic sources using a hierarchical cluster analysis. Additionally, Cu, Pb, and P showed similar spatial distributions with significant pollution in commercial zones, suggesting vehicle traffic or commercial activities as dominant pollutant sources. Also, Cd, Co, Cr, Ni, Zn, Mn and Fe had the highest concentrations in industrial locations, signifying that industrial activities were the main sources of these seven metals. Moreover, the data highlighted land-use as a major influence on heavy metal concentrations and forms found in topsoils with large proportions of soil Cd, Co, Cr, and Ni found in residual fractions and soil Cu, Pb and Zn mainly as extractable fractions.     

施污土壤与污泥中Cu、Pb、Cd、Zn的形态分布

污泥中的重金属元素是限制其大规模农田利用的重要因素。施污土壤和污泥中重金属的形态研究可以用来评价土壤中重金属的生物有效性以及它们在土壤中的移动性。用修正BCR三步连续提取法进行分步提取研究了污水污泥和施污后的西红柿地土壤中Cu、Pb、Cd、Zn的形态分布状况。施用污泥堆肥10t hm-2后的土壤中Cu、Pb、Cd、Zn的全量与各种形态含量无明显增加,Cu、Pb、Zn含量远低于国家土壤环境质量标准。土壤中Cu的各种形态分布关系是:残渣态>可还原态=可氧化态>可交换态和弱酸溶解态,Cu在土壤中的存在是以最稳定的残渣态为主。堆肥污泥与干化污泥相比,残渣态Cu的比例明显增加。土壤中Pb的各种形态分布关系是以残渣态和可还原态为主,但可氧化态的分布比例最小。土壤中Cd的可交换态、可还原态和残渣态各占据相等的含量,但可氧化态Cd的含量几乎为零。Zn在土壤中的各种形态分布关系是:可交换态和弱酸溶解态>可氧化态>可还原态>残渣态,Zn在土壤中的存在是以最易迁移的可交换态和弱酸溶解态为主。这些金属元素在土壤中的相对稳定性顺序为:Cu>Pb>Cd>Zn。Zn在土壤中的移动性要远高于Cu。     

Cadmium,Lead, Copper,Zinc, and Nickel in Lettuce and Dry Beans as Related to Mehlich-3 Extraction in Three Brazilian Latossols

Lettuce (Lactuca sativa L.) and dry beans (Phaseolus vulgaris L.) were grown in three Brazilian Red-Yellow Latossols (Oxisols) in greenhouse conditions with cadmium (Cd), lead (Pb), copper (Cu), zinc (Zn), and nickel (Ni) applied to soils in treatments arranged as a randomized complete block design. Plant metals were analyzed in lettuce shoots and dry beans roots, stems, leaves, and seeds. After plant growth, soil samples from the pots were extracted with Mehlich-3 (M-3) for metal availability evaluation. The release of Ni in the M-3 extraction was dependent on the soil exchangeable aluminum (Al^{3 +}). Mehlich-3 was efficient for determination of availability of Cd, Pb, Cu, Zn, and Ni for dry beans and availability of Cd and Ni for lettuce. The dry bean leaves Cd, Pb, Cu, Zn, and Ni were highly correlated with their recovering from soils with M-3. The same was observed for Cd and Ni in lettuce shoots and the M-3 recovered metals from soils.     

尾矿库尾砂及周边农田土壤重金属形态分布及其生物有效性

采用BCR（community bureau of reference）连续提取法对大宝山矿山槽对坑尾矿库尾砂和周边农田土壤重金属Cd、Pb、Cu和Zn的形态分布及其生物有效性进行了分析。结果表明,尾砂中Cd、Pb、Cu和Zn残渣态占绝对优势,占其总量的百分数均在85%以上。农田土壤中Cd、Cu和Zn都以残渣态为主,分别占其总量的60%、60%和90%以上,Pb以残渣态和可还原态为主,占其总量的93.44%。农田土壤重金属有效性较尾砂大,尾砂和农田土壤重金属生物有效性均以Pb为最高。     

Contents and chemical forms of heavy metals in school and roadside topsoils and road-surface dust of Beijing

Purpose

The concentration of human activities in urban systems generally leads to urban environmental contamination. Beijing is one of ancient and biggest cities on the world. However, information is limited on Beijing’s soil contamination, especially for roadside and campus soils. Thus, the aims of this study were to investigate the contents and chemical forms of toxic heavy metals Cd, Cr, Cu, Ni, Pb, and Zn in the road-surface dust, roadside soils, and school campus soils of Beijing. In addition, enrichment and spatial variation of these toxic heavy metals in the soils and dust were assessed.

Materials and methods

Topsoil samples were collected from the schools and roadside adjacent to main ring roads, and dust samples were collected from the surface of the main ring roads of Beijing. These samples were analyzed for total contents and chemical forms of Cd, Cr, Cu, Ni, Pb, Sc, Zn, Al, and Fe. Enrichment factors (EFs, relative to the background content) were calculated to evaluate the effect of human activities on the toxic heavy metals in soils.

Results and discussion

Heavy metal contents in the road dust ranged from 0.16 to 0.80, 52.2 to 180.7, 18.4 to 182.8, 11.9 to 47.4, 23.0 to 268.3, and 85.7 to 980.9 mg kg^?1 for Cd, Cr, Cu, Ni, Pb, and Zn, respectively. In the roadside soil and school soil, Cd, Cr, Cu, Ni, Pb, and Zn contents ranged from 0.13 to 0.42, 46.1 to 82.4, 22.7 to 71.6, 20.7 to 29.2, 23.2 to 180.7, and 64.5 to 217.3 mg kg^?1, respectively. The average EF values of these metals were significantly higher in the dust than in the soils. In addition, the average EF values of Cd, Cu, Pb, and Zn in the soils near second ring road were significantly higher than those near third, fourth, and fifth ring roads. Anthropogenic Cd, Pb, and Zn were mainly bound to the carbonates and soil organic matter, while anthropogenic Cu was mainly bound to oxides. The mobility and bioavailability of these metals in the urban soils of Beijing generally decreased in the following order: Cd?>?Zn?>?Pb?>?Cu?>?Ni?>?Cr; while in the dust, they decreased in the following order: Zn, Cu, and Cd?>?Pb?>?Ni?>?Cr.

Conclusions

Both EF and chemical forms documented that Cr and Ni in the soils and dust mainly originated from native sources, while Cd, Cu, Pb, and Zn partially originated from anthropogenic sources. In overall, Beijing’s road dust was significantly contaminated by Cd and Cu and moderately contaminated by Cr, Pb, and Zn, while Beijing’s roadside soil and school soil were moderately contaminated by Cd and Pb. However, the maximal hazard quotients (HQs) for individual Cd, Cr, Cu, Ni, Pb, and Zn and comprehensive hazard index (HI) of these metals in the dust and soil were less than 1, indicating that the heavy metals in the dust and soil generally do not pose potential health effects to children, sensitive population.     

包头市铜厂周边土壤中重金属垂直分布特征与形态分析

为了解包头市铜厂周边地区土壤剖面中重金属污染状况,采用火焰原子吸收分光光度法和Tessier连续提取法,对土壤中6种重金属（Cu,Zn,Mn,Ni,Pb和Cd）的垂直分布特征、形态及潜在生物可利用性进行了分析。结果表明:研究区土壤剖面各层土壤中6种重金属含量均超过内蒙古土壤背景值,Cu,Pb和Cd为主要污染物。随采样深度的增加,Cu,Zn,Pb和Mn的含量呈现下降趋势,且由相关性系数可知重金属Cu,Zn和Pb可能有相同人为或自然污染源;土壤剖面中6种重金属均主要以残渣态存在,含量均在50%以上,对生物危害较小;潜在生物可利用性分析结果为:Cu（32.61%） > Mn（31.85%） > Ni（24.90%） > Zn（16.60%） > Cd（15.23%） > Pb（14.87%）,Cu和Mn的潜在生物可利用性较大,其次为Ni,Zn,Cd和Pb潜在生物可利用性较小。     

Total content and extractable fraction of cadmium,cobalt, copper,nickel, lead,and zinc in calcareous orchard soils

Abstract

The levels and availability of six heavy metals cadmium (Cd), cobalt (Co), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) in calcareous orchard soils were measured in an investigation of a method for the determination of total content and extractable fraction of these metals by atomic absorption spectrometry. The digestion step was performed comparatively using two different types of heating. Cadmium, Cu, Ni, Pb, and Zn were determined by flame atomic absorption spectrometry (F‐AAS) and Cd was determined by electrothermal or graphite furnace atomic absorption spectrometry (GF‐AAS) using the method of standard addition. The detection limits, precision values, and accuracy (recovery assays) show the reliability of the method. The conditions for the determination of the extractable fraction of the metals were also optimized. Extraction time for Cd and Zn was studied, and the application of two hours of shaking was selected. The tested method was applied to real samples corresponding to highly calcareous orchard soils characteristic of the Mediterranean area. Only Zn, Cu and Pb showed high levels in their extractable forms, whereas Cd and Co had highest levels in their total contents. An important bio‐availability was observed in all the studied metals, but mainly for Cd and Pb.     

The transfer of heavy metals to barley plants from soils amended with sewage sludge with different heavy metal burdens

Purpose

Our main aim objective was to evaluate the transfer of Cd, Cr, Cu, Ni, Pb and Zn to barley (Hordeum vulgare) grown in various soils previously amended with two sewage sludges containing different concentrations of heavy metals. This allowed us to examine the transfer of heavv metals to barley roots and shoots and the occurrence of restriction mechanisms as function of soil type and for different heavy metal concentration scenarios.

Material and methods

A greenhouse experiment was performed to evaluate the transfer of heavy metals to barley grown in 36 agricultural soils from different parts of Spain previously amended with a single dose (equivalent to 50 t dry weight ha^?1) of two sewage sludges with contrasting levels of heavy metals (common and spiked sludge: CS and SS).

Results and discussion

In soils amended with CS, heavy metals were transferred to roots in the order (mean values of the bio-concentration ratio in roots, BCF_Roots, in brackets): Cu (2.4)?~?Ni (2.3)?>?Cd (2.1)?>?Zn (1.8)?>?Cr (0.7)?~?Pb (0.6); similar values were found for the soils amended with SS. The mean values of the soil-to-shoot ratio were: Cd (0.44)?~?Zn (0.39)?~?Cu (0.39)?>?Cr (0.20)?>?Ni (0.09)?>?Pb (0.01) for CS-amended soils; Zn (0.24)?>?Cu (0.15)?~?Cd (0.14)?>?Ni (0.05)?~?Cr (0.03)?>?Pb (0.006) for SS-amended soils. Heavy metals were transferred from roots to shoots in the following order (mean values of the ratio concentration of heavy metals in shoots to roots in brackets): Cr (0.33)?>?Zn (0.24)?~?Cd (0.22)?>?Cu (0.19)?>?Ni (0.04)?>?Pb (0.02) for CS-amended soils; Zn (0.14)?>?Cd (0.09)?~?Cu (0.08)?>?Cr (0.05)?>?Ni (0.02)?~?Pb (0.010) for SS-amended soils.

Conclusions

Soils weakly restricted the mobility of heavy metals to roots, plant physiology restricted the transfer of heavy metals from roots to shoots, observing further restriction at high heavy metal loadings, and the transfer of Cd, Cu and Zn from soils to shoots was greater than for Cr, Ni and Pb. Stepwise multiple linear regressions revealed that soils with high sand content allowed greater soil-plant transfer of Cr, Cu, Pb and Zn. For Cd and Ni, soils with low pH and soil organic C, respectively, posed the highest risk.     

Heavy metal distribution in different soil aggregate size classes from restored brackish marsh,oil exploitation zone,and tidal mud flat of the Yellow River Delta

Purpose

Heavy metal distribution in soils is affected by soil aggregate fractionation. This study aimed to demons trate the aggregate-associated heavy metal concentrations and fractionations in “sandy,” “normal,” and “mud” soils from the restored brackish tidal marsh, oil exploitation zone, and tidal mudflat of the Yellow River Delta (YRD), China.

Materials and methods

Soil samples were sieved into the aggregates of >2, 0.25–2, 0.053–0.25, and <0.053 mm to determine the concentrations of exchangeable (F1), carbonate-bound (F2), reducible (F3), organic-bound (F4), and residual fraction (F5) of Cd, Cr, Cu, Ni, Pb, and Zn.

Results and discussion

The 0.25–2 mm aggregates presented the highest concentrations but the lowest mass loadings (4.23–12.18 %) for most metal fractions due to low percentages of 0.25–2 mm aggregates (1.85–3.12 %) in soils. Aggregates <0.053 mm took majority mass loadings of metals in sandy and normal soils (62.04–86.95 %). Most soil aggregates had residual Cr, Cu, Ni, Zn, and reducible Cd, Pb dominated in the total Cd, Cr, Cu, Ni, Pb, and Zn concentrations. Sandy soil contained relatively high F4, especially of Cu (F4) in 0.25–2 mm aggregates (10.22 mg kg^?1), which may relate to significantly high organic carbon contents (23.92 g kg^?1, P?<?0.05). Normal soil had the highest total concentrations of metals, especially of Cu, Ni, and Pb, which was attributed to the high F3 and F5 in the <0.053 mm aggregates. Although mud soil showed low total concentrations of heavy metals, the relatively high concentrations of bioavailable Cd and Cu resulted from the relatively high Cd (F2) and Cu (F2) in the >2 mm aggregates indicated contribution of carbonates to soil aggregation and metal adsorption in tidal mud flat.

Conclusions

Soil type and aggregate distribution were important factors controlling heavy metal concentration and fractionation in YRD wetland soil. Compared with mud soil, normal soil contained increased concentrations of F5 and F3 of metals in the 0.053–0.25 mm aggregate, and sandy soil contained increased concentrations of bioavailable and total Cr, Ni, and Zn with great contribution of mass loadings in the <0.053 mm aggregate. The results of this study suggested that oil exploitation and wetland restoration activities may influence the retention characteristics of heavy metals in tidal soils through variation of soil type and aggregate fractions.

     

Long-Term Transformation and Redistribution of Potentially Toxic Heavy Metals in Arid-Zone Soils: II. Incubation at the Field Capacity Moisture Content

Solid-phase transformation of added Cd, Cu, Cr, Ni, Pb and Zn, in two arid-zone soils incubated in the field capacity moisture regime for one year, were studied. The heavy metals were fractionated into six empirically defined fractions using a selective sequential dissolution (SSD) protocol optimized for arid-zone soils. Each of these fractions was named based on the major soil component targeted for dissolution during the specific SSD step, but it is not assumed that they are mineralogically and chemically totally specific. The transformations of the metals in the two soils incubated at the field capacity regime were compared with those at the moisture saturation regime (Han and Banin, 1997). An initial fast stage of transformation of the soluble metals from the exchangeable (EXC) fraction to the less labile fractions (the carbonate (CARB) fraction for Cd, Pb, Zn, Ni and Cu, and the organic matter (OM) fraction for Cr, and to some extent Cu and Ni) occurred during the fractionation and within one hour after addition. This was followed by a second stage, involving long-term transformation processes of all metals: added Cd was transferred from the EXC into the CARB fraction; added Cr was transferred from the CARB to the OM fraction and Pb was transferred very slowly to the easily reducible oxide (ERO) fraction. Added Cu, Ni and Zn were transferred from the EXC and CARB fractions into the ERO fraction and to some extent OM and RO fractions. In Part I of this series, we reported that during incubation in the saturated moisture regime, Zn and Ni were transferred mainly into the RO and OM fractions. Cadmium, Cr and Pb underwent the same transformation pathways during the slow long-term process, with slightly different rates, in both water regimes. At low levels of addition, the incubated soils moved over one year towards a distribution similar to that of the native soil. At higher levels, the soils still remained removed from the quasi-equilibrium which characterized the native soil, even at the end of one year of incubation.