Simultaneous Treatment of Dioxins and Heavy Metals in Tagonoura Harbor Sediment by Two-Step Pyrolysis

This paper describes the simultaneous treatment of PCDD/PCDF and toxic heavy metals contaminated sediment by pyrolysis with emphasis on complete removal and destruction of PCDD/PCDF in Tagonoura harbor sediment during a two-step pyrolysis process. Pyrolysis was conducted at different temperatures and N ₂ gas flow rates. Results showed that almost all PCDD/PCDF (99.998%) can be removed from the sediment by treatment at 800°C for 30 minutes in single-step pyrolysis; however, PCDD/PCDF remained in the gas phase without destruction. Analysis also revealed that some PCDD/PCDF formation occurred during the single-step pyrolysis. However, by the use of two-step pyrolysis, where off-gases from the first furnace are heated at 1000°C, PCDD/PCDF in the final gas effluent was dramatically reduced by a thermal degradation mechanism. Stabilization of toxic heavy metals was also observed. Specifically, their leaching activity drastically decreased with increasing pyrolysis temperature. Significant metal volatilization did not occur. Though the low temperature pyrolysis was effective in removing PCDD/PCDF, the leachability of toxic heavy metals such as Cr and Ni was higher that of 800°C; however, lower than untreated. Two-step pyrolysis at 800°C for 30 minutes in the single-step and 1000°C in the two-step pyrolyzer would be effective for the treatment of sediments contaminated with PCDD/PCDF, toxic heavy metals and organic matters simultaneously.     

水体沉积物中酸可挥发性硫化物(AVS)研究进展

水体沉积物中酸可挥发性硫化物 (AVS)是总硫含量中活性最高的部分 ,是沉积物中有毒重金属的重要结合形态 ,它的含量在很大程度上影响着沉积物重金属的生物有效性 ,从而作为沉积物中有毒重金属环境污染评价的一个重要指标 ;就十多年来水体沉积物中酸可挥发性硫化物 (AVS)的研究进行了综述。概述了 AVS的测定方法及其影响因素 ;探讨了水体沉积物中 AVS含量时空变化的规律 ;同时就目前“同时可提取重金属”(SEM)与 AVS摩尔浓度比值和水体沉积物重金属生物毒性关系的研究进行了概括和分析。     

The effect of heavy metal speciation in sediment on bioavailability to tubificid worms

The bioavailability of heavy metals in sediment to freshwater tubificid worms was compared with measures of chemical extractability using a sequential extraction procedure. In order to provide a range of test sediments of different quality, various mineral phases were prepared, in which the metals were spiked by adsorption or coprecipitation and these were then mixed with a bulk base sediment in known proportions. Results indicated good correlation between worm metal burden and metal mobilised from the sediments in the first (exchangeable) sequential extraction step for Cd, Cu and Pb. Of the other metals tested, Zn levels in the worms were found to be constant, suggesting regulation, and Ni uptake was too small for accurate measurement. In general, metals spiked to the sediment directly, or adsorbed on the clay mineral phase were found to be much more available than those bound to sewage sludge, carbonate or hydrous ferric oxide phases.     

Model for instream regulation of radioisotopes and heavy metals in riverine waters subjected to a uranium mill discharge

In the Athabaska sandstone region of northern Saskatchewan, mining and milling of uranium-bearing ore is subjecting, and will continue to subject, surface waters to inputs of soluble heavy metals and uranium and thorium decay series radioisotopes. A mathematical simulation model was developed to assess the role of riverine sediments in regulating soluble concentrations of heavy metals and radioisotopes released to the environment through treated mining and milling process effluents. Specific elements studied included As, Ni, Pb, U, and Ra-226. The model considered that diffusion into sediment porewater and adsorption by sediment particulates from sediment porewater were the two dominant mechanisms regulating sediment loading. Experiments indicated that the equilibrium adsorption behaviour of the heavy metals and radioisotopes studied could be adequately explained using a linear adsorption isotherm, over the range of concentrations expected in the environment. Experimentally determined linear isotherm slopes ranged from 200 ml g⁻¹ to 21 000 ml gl. Diffusion into the effective depth of the sediments (20 cm) was modelled using a quadratic (second order) driving force and a corresponding mass transfer coefficient. The model coefficients were calibrated using field and laboratory data. Results of the modelling suggested that the sediments have a small effect on instream concentrations during the active phases of mining and milling. The sediments were predicted to accumulate sufficient adsorbed mass, during the operational phase of mining and milling, to act as a distributed instream source after completion of milling activities. The significance of this post-operational source was a function of the initial effluent loading, elapsed time and site specific river characteristics.     

A study on heavy metal partitioning in sediments from Poyang Lake in China

The partitioning of heavy metals in sediment from Poyang Lake, the largest freshwater lake in China, was studied. The majority of the heavy metals, copper, lead and zinc, were found to be bound to the organic matter and iron oxide phases. The distribution of the metals among the different geochemical phases in sediments was controlled by the abundance of the geochemical phases. An equilibrium adsorption model developed by Oakley et al. (1981) and Davies-Colley et al. (1984) was applied to predict the partitioning of copper among different geochemical phases in the sediments of Poyang Lake. The conditional equilibrium constants (slopes of the linear portions of the adsorption isotherms) were determined using an artificial water-sediment system at various pH's. This model was used to describe the heavy metal partitioning in sediment samples from Poyang Lake and the predicted results were consistent with those measured in the laboratory.     

Bioleaching of Heavy Metal Polluted Sediment: Influence of Sediment Properties (Part 2)

A remediation process for heavy metal polluted sediment has previously been developed, in which the heavy metals are removed from the sediment by solid‐bed bioleaching using sulfuric acid as a leaching agent arising from added elemental sulfur (S⁰). This process has been engineered with Weiße Elster River sediment (dredged near Leipzig, Germany), as an example. Here, six heavy metal polluted sediments originating from various bodies of water in Germany were subjected to bioleaching to evaluate the applicability of the developed process on sediment of different nature: each sediment was mixed with 2 % S⁰, suspended in water and then leached under identical conditions. The buffer characteristics of each sediment were mainly governed by its carbonate and Ca content, i.e., by its geological background, the redox potential and oxidation state depended on its pre‐treatment (e.g., on land disposal), while the pH value was influenced by both. The added S⁰ was quickly oxidized by the indigenous microbes even in slightly alkaline sediment. The microbially generated H₂SO₄ accumulated in the aqueous phase and was in part precipitated as gypsum. Significant acidification and heavy metal solubilization only occurred with sediment poor in buffer substances. With the exception of one sediment, the behavior in bioleaching correlated well with the behavior in titration with H₂SO₄. Since the content in carbonate seemed to be the most important factor deciding on the leachability of a sediment, oxic Weiße Elster River sediment was mixed with 2 % S⁰ and 0 to 100 g/kg of ground limestone to simulate various buffer capacities, suspended in water and then leached. The lime did not inhibit microbial S⁰ oxidation but generated a delay in acidification due to neutralization of formed H₂SO₄, where the pH only started to decrease when the lime was completely consumed. The more lime the sediment contained, the longer this lag period lasted, and the higher the pH and the lower the fraction of the solubilized heavy metals finally was. Since Cu requires stronger acidic conditions for its solubilization, it responded more sensitively to lime addition than Zn, Ni, and Cd. Heavy metal polluted sediment containing large amounts of carbonate may, in principle, also be remediated by bioleaching, but metal solubilization requires excessive amounts of the leaching agent and is thus uneconomical.     

Sediment toxicity and heavy metals in the Kattegat and Skaggerak

Superficial (0 to 2 cm) sediments were sampled from 62 sites in Kattegat and Skagerrak during autumn 1989 and spring 1990, tested for toxicity to Daphnia magna and Nitocra spinipes (Crustacea) and analyzed for heavy metals (Cd, Cr, Cu, Hg, N, Pb, Zn), nutrients (N and P) and organic carbon. Whole sediment toxicity to Nitocra spinipes, expressed as 96-h LC50, ranged from 1.8 to > > 32 percent sediment (wet wt), which is equivalent to 0.63 to 53 percent dry wt. Sediment total metal concentrations (mg kg^-1 dry wt) ranged from 0.01 to 0.32 for Cd, 8 to 57 for Cr, 3 to 40 for Cu, 0.03 to 0.86 for Hg, 3 to 43 for Ni, 6 to 37 for Pb and 21 to 156 for Zn. Analyzed concentrations of heavy metals were tested for correlation with whole sediment toxicity normalized to dry wt, and significant correlations (Spearman p<0.05) were found for Cd, Cr, Cu, Hg, and Ni. However, the analyzed concentrations of these metals were below the spiked sediment toxicity of these heavy metals to N. spinipes, except for Cr and Zn for which analyzed maximum concentrations approached the 96-h spiked sediment LC50s. There was no improvement in correlation between the sum of heavy metal concentrations normalized to their spiked toxic concentrations (Toxic Unit approach) and the whole sediment toxicity. Calculated heavy-metal-derived toxicity based on toxic units and whole sediment toxicity ranged from 0.1 to 24 (mean value 2.3 and SD 4.2). Theoretically, a value of 1.0 would explain whole sediment toxicity from measured metal concentrations using this approach. Thus, in spite of the fact that the total concentrations of the heavy metals were sufficient to cause toxicity based on an additive model for most of these sediments, the observed toxicity of the sediments from Kattegat and Skagerrak could not exclusively be explained by the concentrations of heavy metals, except for Cr and Zn at their maximum concentrations. Therefore, other pollutants than these heavy metals must also be considered as possible sediment toxicants.     

海洋沉积物中重金属对底栖无脊椎动物的生物有效性

海洋沉积物是重金属的重要贮库,而海洋底栖无脊椎动物主要从沉积物中摄取重金属,这些被摄取的重金属能够通过食物链进行传递,进而影响到人类健康。本文总结了近些年来在海洋沉积物中重金属对底栖无脊椎动物生物有效性方面的研究进展,包括海洋底栖无脊椎动物对重金属的吸收途径、沉积物地球化学性质和底栖无脊椎动物生理等生物因素对沉积物中重金属生物有效性的影响。在此基础上,展望了未来研究重点,主要包括近海富营养化对沉积物中重金属生物有效性的影响,海洋底栖无脊椎动物消化道中的物理消化过程对沉积物中重金属生物有效性的影响,海洋底栖无脊椎动物整个生活史过程中沉积物中重金属生物有效性的变化等。     

铜锈环棱螺对沉积物中重金属的生物积累及其与重金属赋存形态的关系

以铜锈环棱螺（Bellamya aeruginosa）为测试生物,采用28 d沉积物生物积累试验研究铜锈环棱螺对污染河流沉积物中重金属的生物积累,并探讨其与重金属赋存形态的关系.结果表明：铜锈环棱螺肝胰脏对Cd、Pb、Cu、Cr、Zn和Mn均具有较强的积累作用.不同重金属的积累量存在较大差别,Zn的积累量最多,占重金属总积累量的84.32%±4.36%,其次为Cu,占7.67%±2.84%;Pb、Cr和Mn的比例相对较少,分别为3.62%±1.84%、2.22%±1.03%和1.33%±0.15%;Cd所占比例最少,为0.83%±0.53%.肝胰脏中重金属元素之间的相关性均不显著.肝胰脏金属污染指数与沉积物污染综合指数具有显著的正相关关系,铜锈环棱螺可以作为沉积物重金属污染的监测生物.不同沉积物Cd、Cr、Zn和Mn的生物-沉积物积累因子（BSAF）具有较大的差异,Cu和Pb的BSAF比较稳定.Cd的生物积累与沉积物中Cd的可交换的与酸可溶态及可氧化态显著相关;Pb的生物积累与Pb的可还原态显著相关;Cu的生物积累与Cu的可氧化态显著相关;Mn的生物积累与Mn的可交换的与酸可溶态和可还原态显著相关;Cr和Mn的生物积累与其不同形态和总量均不相关.BSAF不宜作为衡量铜锈环棱螺对沉积物中重金属生物积累能力的指标.     

Accumulative phases for heavy metals in limnic sediments

Data from mechanical concentrates of recent sediments indicate that clay minerals, clay-rich aggregates and heavy minerals are the major carriers of heavy metals in detrital sediment fractions. Hydrous Fe/Mn oxides and carbonates and sulfides, in their specific environments, are the predominant accumulative phases for heavy metals in autochthonous fractions. Sequential chemical extraction techniques permit the estimation of characteristic heavy metal bonding forms: exchangeable metal cations, easily reducible, moderately reducible, organic and residual metal fractions, whereby both diagenetic processes and the potential availability of toxic compounds can be studied. The data from lakes affected by acid precipitation indicate that zinc, cobalt and nickel are mainly released from the easily reducible sediment fractions and cadmium from organic phases. In contrast at pH 4.4, neither lead nor copper seem to be remobilized to any significant extent. Immobilization by carbonate precipitation seems to provide an effective mechanism for the reduction of dissolved inputs 9f metals such as zinc and cadmium in pH-buffered, hard water systems.     

From geochemical background determination to pollution assessment of heavy metals in sediments and soils

Establishing geochemical background concentrations to distinguish the natural background from anthropogenic concentrations of heavy metals in sediments and soils is necessary to develop guidelines for environmental legislation. Due to the fact that the background concentrations strongly depend on geological characteristics such as mineral composition, grain size distribution and organic matter content, several normalization methods have been developed. Empirical (geochemical), theoretical (statistical) and integrated methods (combining both empirical and theoretical methods) are the main approaches described in literature for determination of geochemical background concentrations. In this review paper, the different approaches as well as the main normalization methods for heavy metal concentrations in sediments and soils will be discussed. Both geochemical background concentrations and added risk level (maximum permissible addition) should be taken into account for setting up legal threshold limits. Moreover, different approaches to evaluate the pollution status of heavy metals in sediments and soils, from Sediment/Soil Quality Guidelines to quantitative indices (Geo-accumulation Index-I_geo, Enrichment Factor-EF, Pollution Load Index-PLI and Risk assessment Code-RAC) will be presented. Although guidelines to establish whether a sediment or soil is polluted or not are generally only related to total metal concentrations, the available/reactive pool i.e., availability/reactivity of metals should be taken into account for sediment/soil pollution assessment.     

Bioavailability and Concentration of Heavy Metals in the Sediments and Leaves of Grey Mangrove, <Emphasis Type="BoldItalic">Avicennia marina</Emphasis> (Forsk.) Vierh,in Sirik Azini Creek,Iran

The concentration and bioavailability of Ni, Cu, Cd, Zn, and Pb in the sediments and leaves of grey mangrove, Avicennia marina, were studied throughout Sirik Azini creek (Iran) with a view to determine heavy metals bioavailability, and two methods were used. Results show that Zn and Ni had the highest concentrations in the sediments, while Cd and Cu were found to have the lowest concentrations in the sediments. Compared to the mean concentrations of heavy metals in sedimentary rock (shales), Zn and Cu showed lower concentration, possibly indicating that the origin of these heavy metals is natural. A geo-accumulation index (I _geo) was used to determine the degree of contamination in the sediments. I _geo values for Zn, Cu, Pb, and Ni showed that there is no pollution from these metals in the study area. As heavy metal concentrations in leaves were higher than the bioavailable fraction of metals in sediments, it follows that bioconcentration factors (leaf/bioavailable sediment) for some metals were higher than 1.     

Diagenesis of Metals Chemically Complexed to Bacteria: Laboratory Formation of Metal Phosphates, Sulfides, and Organic Condensates in Artificial Sediments

Cells of Bacillus subtilis, when suspended in a 5mM metal solution, bind metals tenaciously to their cell walls. These metal-loaded cells, when mixed with a synthetic sediment and put under laboratory conditions to simulate low-temperature sediment diagenesis, nucleate the formation of a mixed assemblage of crystalline metal phosphates, metal sulfides, and polymeric, metal-complexed, organic residues. The sequential series of diagenetic events leading to the formation of authigenic mineral phases was followed by transmission electron microscopy and energy-dispersive X-ray analysis. The minerals quartz (SiO₂) and calcite (CaCO₃) were employed in the synthetic sediment. Crystalline magnetite (Fe₂O₃) and elemental sulfur were added as redox buffering agents to ensure anoxic conditions. Quartz and magnetite appeared unreactive throughout the experimental conditions. Elemental sulfur interacted with the metal-loaded cells, affected both the eventual chemistry and crystal habit of the metal phosphates, and formed a variety of crystalline metal sulfides. Calcite raised the pH of the fluid phase of the sediment, which influenced phosphate mineralization and inhibited metal sulfide genesis.     

Elucidation of Contaminant-Induced Toxic Responses in the Biota of Lake Vembanad,Kerala, India

This study investigates the spatial distribution of various organochlorine insecticides (OCIs) and heavy metals in both sediment and a bivalve, Villorita cyprinoides var. Cochinensis, collected from three pristine areas of Lake Vembanad. The detected concentrations of OCIs in the bivalve and sediment are in the order DDT > Heptachlor > BHC and BHC > DDD > heptachlor, respectively. However the DDT concentration (28.5 ng/g) in the bivalve at Chambakkara exceeded the limit suggested by the U.S. Geological Survey's National Water-Quality Assessment Program (NWQAP) and indicated an anthropogenic input source for the parent DDT. The maximum potential contamination indices for Zn and Ni in the bivalve were 267.8 mg/kg and 22.3 mg/kg, respectively. The Enrichment Factor (EF) and geoaccumulation index (I_geo) values of sediment indicated that Cd showed high pollution status and the Biota Sediment Accumulation Factor value of heavy metals suggested that Zn, Cu, and Cd were bioaccumulated in the bivalve's tissue. The concentrations of Zn and Cd were above the threshold levels and regarded as harmful according to the international standard for metals in mollusc compiled by the Food and Agriculture Organization. Therefore, they create great health risk for mussel consumers.     

Chemical speciation and risks of heavy metals in sediment of urban wetlands in southeastern China

Dredging, remediation, and other management of sediment in polluted urban wetlands require the information of environmental risks associated with heavy metals (HMs). In this study, sediment samples collected from three typical wetlands in the urban area of Wenzhou City, China, were analyzed to evaluate their risks posed by five HMs. Sediments from an industrial area stored higher Cr and Cu but lower Cd and Pb than those from the residential and agricultural areas. The assessment by the pollution load index method indicated that all of the three study wetlands were polluted. An ecological risk index approach identified low risk for wetlands in forested and residential areas but high risk for wetlands in an industrial area as well as in mixed residential and agricultural area, which was also confirmed by the geo-accumulation index (I_geo) method. Furthermore, the ratio of the secondary phase and primary phase and risk assessment code methods recognized that Cd poses a high pollution risk. The results indicated that sediment pollution by HMs in urban wetlands is possibly a widespread problem in China and needs great attention.     

Effect of pulsed exposure to heavy metals (copper and cadmium) on some population variables of <Emphasis Type="Italic">Brachionus calyciflorus</Emphasis> Pallas (Rotifera: Brachionidae: Monogononta)

Heavy metals are widely recognized as potential toxic agents to zooplankton, yet experiments are usually performed with a continuous exposure to the metal being analyzed. Here we describe experiments that examined the influence of pulsed exposure of the heavy metals copper and cadmium to a parthenogenetic population of the planktonic rotifer Brachionus calyciflorus. Our protocol called for exposure durations of 3, 6, 12, and 24 h to either copper (as CuSO₄) at concentrations of 0.0375, 0.075, 0.15 mg l⁻¹ or cadmium (as CdCl₂) at concentration of 0.025, 0.05, 0.1 mg l⁻¹. Control animals were treated in similar ways but did not receive exposure to heavy metals. Four end points were used to evaluate the outcome of exposure: population growth (r), body size, egg ratio, and egg hatching percent. Increase in heavy metal concentration and exposure time had an adverse influence on the population growth of B. calyciflorus. However, while the response of B. calyciflorus was similar for both heavy metals, the magnitude of the impact of cadmium was more severe. Population growth varied depending on which heavy metal was tested, as well as its concentration and the duration of exposure (r = 0.11–0.28 day⁻¹). There was a significant reduction in lorica size of B. calyciflorus subjected to different exposure times and concentrations of both Cd and Cu. Egg ratios were inversely related to population density in controls and in treatments involving Cu, but not for Cd. While nearly 100% of eggs hatched in the control treatments, egg hatching in experimental treatments containing Cu, were reduced (range = 16–41%) depending on the exposure time and the concentration. Guest editors: S. S. S. Sarma, R. D. Gulati, R. L. Wallace, S. Nandini, H. J. Dumont & R. Rico-Martínez Advances in Rotifer Research     

Sperm nuclear transformations consist of enlargement and condensation coordinate with stages of meiotic maturation in fertilized Spisula solidissima oocytes

Rates of sperm nuclear expansion were measured and correlated with processing of the maternal chromatin in synchronous populations of fertilized surf clam (Spisula solidissima) oocytes fixed at regular intervals following insemination and stained with the DNA fluorochrome Hoechst 33342. Sperm nuclei expanded in four distinct phases each temporally coordinate with events of meiotic maturation: germinal vesicle stage (phase A), germinal vesicle breakdown (phase B), polar body formation (phase C), and female pronuclear development (phase D). Sperm nuclei were essentially unchanged during phase A (rate = 0.1 micron2/min, enlarged during phases B (rate = 8.2 microns2/min) and D (rate = 6.2 microns2/min), and condensed during phase C (rate = -1.9 micron2/min). Sperm nuclear enlargement during phase D was significantly less in polyspermic and polygynic zygotes. The effects of various treatments (temperature, microtubule disruption, pH alterations, and metabolic and protein synthesis inhibitions) which perturbed sperm nuclear enlargement and meiotic processing of the maternal chromatin indicated that the two processes are coupled and may be linked by common regulatory agents.     

Nutrients and heavy metal contamination of plants and sediments in Futian mangrove forest

An ecological survey was carried out to determine the levels of nutrients and heavy metals in the sediments and leaf tissues of two dominant mangrove plant species, Kandelia candel and Aegiceras corniculatum, in Futian mangrove forest, Shenzhen, the People's Republic of China. The spatial and seasonal variations of these elements were also investigated. The results show that there was no major difference between two sampling sites 150 m apart. In both sites, the sediment concentrations of total and NH₄ ⁺-N, total and extractable P, total and extractable K, total organic carbon were consistently higher in the landward locations and decreased gradually towards the sea. The sediment sample collected at the seaward edge of the mangrove plant community had the lowest levels of nutrient and organic matter. The vertical variations (from the land to the sea) of sediment heavy metals were less obvious and no particular trend could be identified. Extremely high contents of Cu, Cd, Pb, Cr and Zn were found at certain locations, suggesting the occurrence of some local contamination. The mean total metal concentrations in sediments decreased in the order Mn > Zn > Cu > Cr = Pb > Cd for the sample locations. Most of the heavy metals were not in a bioavailable form as the concentrations of extractable metals were relatively low (< 1% of total metals). Pb, Cr and Cd were not detected in leaf samples. Leaf C, N, P and K contents were similar between the two species and no significant difference was found among locations, although A. corniculatum seemed to have lower Mn concentrations than K. candel. With reference to temporal variations, no significant difference in sediment concentrations of some nutrients and metals was found between the spring and autumn seasons.     

Bioleaching of Heavy Metal Polluted Sediment: Kinetics of Leaching and Microbial Sulfur Oxidation

Remediation of heavy metal polluted sediment through bioleaching using elemental sulfur (S⁰) as the leaching agent can be regarded as a two‐step process: firstly, the microbial oxidation of the added S⁰ to sulfuric acid and, secondly, the reaction of the produced acid with the sediment. Here, both subprocesses were studied in detail independently: oxidized river sediment was either suspended in sulfuric acid of various strengths, or mixed with various amounts of finely ground S⁰ powder (diameter of the S⁰ particles between 1 and 175 μm with a Rosin‐Rammler‐Sperling‐Bennet (RRSB) distribution and an average diameter of 35 μm) and suspended in water. The leaching process was observed by repeated analysis of the suspension concerning pH, soluble sulfate and metals, and remaining S⁰. In the case of abiotic leaching with H₂SO₄, the reaction between the acid and the sediment resulted in a gradual increase in pH and a solubilization of sediment‐borne heavy metals which required some time; 80 % of the finally solubilized heavy metals was dissolved after 1 h, 90 % after 10 h, and 100 % after 100 h. In the case of bioleaching, the rate of S⁰ oxidation was maximal at the beginning, gradually diminished with time, and was proportional to the initial amount of S⁰. Due to its very low solubility in water, S⁰ is oxidized in a surface reaction catalyzed by attached bacteria. The oxidation let the particles shrink, their surface became smaller and, thus, the S⁰ oxidation rate gradually decreased. The shrinking rate was time‐invariant and, at 30 °C, amounted to 0.5 μm/day (or 100 μg/cm²/day). Within 21 days, 90 % of the applied S⁰ was oxidized. Three models with a different degree of complexity have been developed that describe this S⁰ oxidation, assuming S⁰ particles of uniform size (I), using a measured particle size distribution (II), or applying an adapted RRSB distribution (III). Model I deviated slightly from the measured data but was easy to handle, Model II fitted the measured data best but its simulation was complicated, and Model III was intermediate. The amount of soluble sulfate was smaller than the amount of H₂SO₄ added or microbially generated as the H₂SO₄ reacted with the sediment to form in part poorly soluble sulfates. A model has been developed that describes the pH and the soluble sulfate and metals at equilibrium, depending on the amount of H₂SO₄ applied or microbially generated, and that is based on the condition of electrical neutrality, a global metal/proton exchange reaction, and a sulfate‐fixation reaction. In suspension, bioleaching with S⁰ required considerably more time than abiotic leaching with H₂SO₄, but the final pH and metal solubilization were identical when equimolar amounts of leaching agents were applied.     

Distribution of heavy metals and their chemical speciation in sediments from the Abelardo L. Rodríguez Dam,Sonora, México

Abstract

The purpose of this study was to investigate the distribution of total and bioavailable metals (Cd, Cu, Cr, Fe, Mn, Pb and Zn) in sediments of the Abelardo L. Rodríguez (ALR) dam located in the eastern part of the city of Hermosillo, Sonora, Mexico. Seventy two sediment samples were collected in 2009 during four sampling campaigns in February (spring), May (summer), September (end of summer) and December (winter) in five different areas within the dam surface (Zones I, II, III, IV and V), including the Gate Station. Determination of heavy metals was carried out by flame atomic absorption spectrophotometry (FAAS). The results indicate high levels of total heavy metals in the following order: Fe>Mn>Zn>Pb>Cu>Cr>Cd. This is indicative of the impact of human activities located in areas surrounding the reservoir, specifically urban and industrial. The distribution and state of accumulation of trace metals in the sediment is largely dominated by the residual and Fe/Mn oxides geochemical phases. Fraction I (exchangeable) also presented high concentrations of metals (Cu, Fe, Mn and Zn). From enrichment factor analysis, the study area is prevalently enriched in Cd, Cu and Pb in Zones I, II, III, IV and Gate Station. This indicates that the sediments are impacted by anthropogenic activities such as downloads, domestic and industrial wastewater. Geoaccumulation index (Igeo) indicates that Zones I, II, III, IV and V (including Gate Station) do not show contamination by Cr, Fe, Mn and Zn. However, there is a moderate to heavy contamination by Cd, Cu and Pb (Igeo: 2–4) in all areas of study. The comparison between the results obtained with the sediment quality criteria (LEL and SEL), indicate that Zones I, II, III, IV, V and Gate Station, are contaminated with Cd, Cu and Pb, and severely contaminated by Fe.

The elevated levels of heavy metals detected in the sediment of the ALR Dam require special attention, since in the exchangeable fraction, the metals are specifically adsorbed on the sediment and can be released when the ionic composition of water changes. However, additional studies are required in this reservoir on the chemistry and toxicology of metals for a full assessment of potential risks posed to biota and man.