Relation of acid-volatile sulfides (AVS) with metals in sediments from eutrophicated estuaries: Is it limited by metal-to-AVS ratios?

Purpose

The role of acid-volatile sulfide (AVS) as a factor affecting (or reflecting the environmental conditions influencing) the behavior of metals has been evidenced in anoxic sediments. However, sediment quality studies tend not to consider any potential AVS role when sum of simultaneously extracted metal (ΣSEM) concentrations exceed AVS concentrations, restricting the application of the AVS model to predict toxicity, rather than coupling this model (when applicable) with other possible interpretations of metal–AVS relations.

Materials and methods

The relations between total organic carbon (TOC), AVS, and simultaneously extracted Fe, Cd, Cu, Ni, Pb, and Zn were investigated in sediment cores from two eutrophicated estuaries in Guanabara Bay (SE Brazil) in order to contribute to our understanding about metal–AVS relations in conditions of variable ΣSEM and AVS levels, due to gradients of eutrophication and metal contamination.

Results and discussion

Correlation analyses indicated a more important role of TOC, AVS, and Fe in the mechanisms affecting trace metal distribution in the less eutrophicated estuary. This suggests that AVS was a more important binding phase, or at least a better proxy for biogeochemical conditions affecting metal distribution, when it exceeds the sum of simultaneously extracted Cd, Cu, Ni, Pb, and Zn concentrations (ΣSEM). However, this potential role of AVS in anoxic estuarine sediments should not be discarded before evaluating individual metals–AVS relations even when ΣSEM levels exceed AVS levels, since the biogeochemical controls on the behavior of individual metals may be also related to metal–AVS associations in this situation (as indicated for Cu and Ni in the more eutrophicated estuary). The same is suggested for all AVS-based approaches, such as the (ΣSEM???AVS)/f _OC (organic carbon normalization of excess ΣSEM), since (ΣSEM???AVS)/f _OC values were mostly positive and significantly higher in the more eutrophicated estuary.

Conclusions

Although the importance of metal relations to AVS in evaluating individual metal behavior in anoxic estuarine sediments may not be restricted to situations in which AVS levels exceed ΣSEM levels (as observed for Ni and Cu in the present study), metal–AVS relations were apparently favored in this situation. Approaches in this way are recommended for future research, coupling the possibilities of metal behavior interpretations (and often predictions) allowed by AVS models.     

Mobilization Potential of Heavy Metals: A Comparison Between River and Lake Sediments

Toxic metals introduced into aquatic environments by human activities accumulation in sediments. A common notion is that the association of metals with acid volatile sulfides (AVS) affords a mechanism for partitioning metals from water to solid phase, thereby reducing biological availability. However, variation in environmental conditions can mobilize the sediment-bound metal and result in adverse environmental impacts. The AVS levels and the effect of AVS on the fate of Cu, Cd, Zn, Ni in sediments in the the Changjiang River, a suboxic river with sandy bottom sediment and the Donghu Lake, a anoxic lake with muddy sediment in China, were compared through aeration, static adsorption and release experiments in laboratory. Sips isotherm equation, kinetic equation and grade ion exchange theory were used to describe the heavy metal adsorb and release process. The results showed that AVS level in the lake sediment are higher than that of the river. Heavy metals in the overlying water can transfer to sediments incessantly as long as the sediment remains undisturbed. The metal release process is mainly related to AVS oxidation in lake sediment while also related to Org-C and Fe–Mn oxyhydroxide oxidation in river sediment. The effect of sulfides on Zn and Ni is high, followed by Cd, and Cu is easy bound to Org-C. AVS plays a major role in controlling metals activity in lake sediment and its presence increase the adsorption capacity both of the lake and river sediments.     

Modelling the effects of ageing on Cd,Zn, Ni and Cu solubility in soils using an assemblage model

Ageing reactions can reduce trace metal solubility and can explain natural attenuation of contaminated soils. We modelled ageing reactions in soil with an assemblage model that considers slow reactions in Fe‐oxyhydroxides and reversible sorption on organic matter and clay minerals. Metal adsorption kinetics on Fe‐oxyhydroxides was obtained from data with synthetic oxyhydroxides. Metal solubility and isotopic exchangeability data were obtained from 28 soils amended with Ni, Zn, Cu and Cd metal salts and monitored for 850 days. The assemblage model was constructed in WHAM 6.0 and used soil properties and dissolved organic matter as input data. The model was first validated to predict dissolved metal concentrations, based on the concentration of isotopic exchangeable metals. The model overestimated metal solubility without parameter adjustment by mean factors of 4–7, and successful fits were obtained by increasing the specific surface area of Fe‐oxyhydroxides from measured values of synthetic systems to a value of 600 m² g^?1 recommended by other authors. The effect of ageing on the isotopic exchangeable metal fraction was subsequently modelled starting from the predicted fraction of metals present on Fe‐oxyhydroxides immediately after soil spiking. The observed isotopic exchangeable metal fractions of Ni, Zn and Cd agreed reasonably well with predicted values. The model predicts that ageing reactions are more pronounced at higher pH because metal sorption is increasingly directed to oxyhydroxide surfaces with increasing soil pH. Modelling fixation of Cu requires more information on fixation of that metal in organic matter.     

Sorption Behaviors of Sodium Dodecylbenzene Sulfonate (SDBS) on Marine Sediments

The sorption behavior of sodium dodecylbenzene sulfonate (SDBS), an anionic surfactant, on marine sediments was systematically investigated. The experimental results showed that 100 min was required for the adsorption equilibrium. For the sediments treated by HCl and H₂O, sorption behavior of SDBS was fit with linear isotherm very well over the concentration range studied at 298 K. The sorption occurred primarily due to partition function of hydrophobic chains into organic carbon of sediments. Sorption of SDBS on H₂O₂-treated sediments was satisfactorily fit with Freundlich isotherm model and mainly through surface function of clay minerals in the sediment. The sorption was favorably influenced by the increased salinity, deceased pH and decreased temperature of seawater.     

Potential contaminant release from agricultural soil and dredged sediment following managed realignment

Purpose

Laboratory experiments were conducted to examine the potential for metal (Cu, Ni and Zn) and herbicide (simazine, atrazine and diuron) release from agricultural soil and dredged sediment in managed realignment sites following tidal inundation.

Materials and methods

Column microcosm and batch sorption experiments were carried out at low (5?practical salinity units, psu) and high (20?psu) salinity to evaluate the changes in the partitioning of metals and herbicides between the soil/sediment and the aqueous phase, and the release of metals and herbicides from soil/sediment to the overlying water column.

Results and discussion

For both the metals and herbicides, the highest contaminant loads were released from the sediment within the first 24?h of inundation suggesting that any negative impacts to overlying water quality in a managed realignment scheme will be relatively short term following tidal inundation of soil and sediment. The release of metals was found to be dependent on a combination of salinity effects and the strength of binding of the metals to the soil and sediment. In the case of the herbicides, salinity impacted on their release. Particulate organic carbon was found to control the binding and release of the herbicides, highlighting the importance of assessing soil and sediment organic matter content when planning managed realignment sites.

Conclusions

Our research demonstrates that metals and herbicides may be released from contaminated sediments and agricultural soils during initial periods of flooding by seawater in managed realignment sites.     

The effect of waste water treatment on river metal concentrations: removal or enrichment?

Purpose

Discharge of untreated domestic and industrial waste in many European rivers resulted in low oxygen concentrations and contamination with trace metals, often concentrated in sediments. Under these anoxic conditions, the formation of insoluble metal sulfides is known to reduce metal availability. Nowadays, implementation of waste water treatment plants results in increasing surface water oxygen concentrations. Under these conditions, sediments can be turned from a trace metal sink into a trace metal source.

Materials and methods

In an ex situ experiment with metal contaminated sediment, we investigated the effect of surface water aeration on sediment metal sulfide (acid volatile sulfides (AVS)) concentrations and sediment metal release to the surface water. These results were compared with long-term field data, where surface water oxygen and metal concentrations, before and after the implementation of a waste water treatment plant, were compared.

Results and discussion

Aeration of surface water in the experimental setup resulted in a decrease of sediment AVS concentrations due to sulfide oxidation. Metals, known to precipitate with these sulfides, became more mobile and increasing dissolved metal (arsenic (As), cadmium (Cd), copper (Cu)) concentrations in the surface water were observed. Contrary to As, Cd, or Cu, manganese (Mn) surface water concentrations decreased in the aerated treatment. Mn ions will precipitate and accumulate in the sediment as Mn oxides under the oxic conditions. Field data, however, demonstrated a decrease of all total metal surface water concentrations with increasing oxygen concentrations following the implementation of the waste water treatment plant.

Conclusions

The gradual decrease in surface water metal concentrations in the river before the treatment started and the removal of metals in the waste water treatment process could not be countered by an increase in metal flux from the sediment as observed in the experiment.     

Characterisation, Availability, and Risk Assessment of the Metals in Sediment after Aging

The behavior of metals in sediments after their disposal to land has important implications for the environmental management. The sediment from the Carska Bara (Serbia) was polluted with adequate metal salts in order to reach severe contamination based on the pseudo-total metal content of Pb, Cd, Ni, Zn, Cu, and Cr according to the corresponding Dutch standards and Canadian guidelines. The toxicity and fate of the metal in sediment depend on its chemical form, and therefore, quantification of the different forms of a metal is more meaningful than the estimation of its total concentration. In this study, fractionation of metals in sediment has been investigated to determine its speciation and ecotoxic potential, as well as evaluation of metal potential toxicity based on the simultaneously extracted metals (SEMs) and acid volatile sulfides (AVSs) analysis at the beginning of the experiment and after 5?weeks of sediment aging. The investigations suggest that Cd, Pb, and Zn have a tendency to associate with labile fraction, the most mobile and most dangerous fraction for the environment. Risk assessment code revealed their high risk. Copper and chromium showed low to medium risk to the aquatic environment. Nickel showed no risk to the aquatic environment. This was the case at the beginning and after 5?weeks of aging. Aging yielded an increased mobility of all metals based on the increased proportion in mobile fractions. The ??[SEM _i ]/[AVS] ratio was found to be >1 both at the beginning and after 5?weeks of aging, with the ratio showing an increase with time. This ratio indicates the potential availability/toxicity and, according to the US EPA criteria, the samples belong to the group with probable negative effect. If particular metals are considered, only the ??[SEM _i ]/[AVS] ratio for zinc was >1 at the beginning. After 5?weeks, the ratio was >1 for zinc, lead, and copper. Comparison of the results of sequential extraction and the results of SEM and AVS analysis showed good agreement for lead and zinc.     

Partitioning of Trace Metals in Suspended Sediments from Huanghe and Changjiang Rivers in Eastern China

Assessing metal contamination of sediments requires knowledge of the geochemical partitioning of trace metals at the sediment-water interface. Under controlled laboratory conditions, sequential extraction was conducted to determine the associations of metals (Cd, Cr, and Zn) and radiotracers (¹⁰⁹Cd, ⁵¹Cr, and ⁶⁵Zn) with various geochemical phases and the different partitioning and mobility of metals for two types of surface sediments collected from the Huanghe and Changjiang Rivers in Eastern China. The residual phase was the major phase for stable metal binding, indicating that these sediments had little subjection to recent anthropogenic influences. Fe–Mn oxides were the next important binding phases for metals. The partitioning of metals in various geochemical phases as a function of the duration of the radiolabeling was also examined. Trace metals transferred among the different geochemical phases over the 30 days radiolabeling period, particularly between the carbonate and Fe–Mn oxides phases. The freshwater-sediment distribution coefficients (K _d) of three metals were investigated in batch experiments using the radiotracer technique. The decreasing K _d with increasing metal concentration(from 0.5 to 200 μg L^-1) may be explained by competitive adsorption. The metal K _d in sediments from the Changjiang River was greater than those from the Huanghe River, presumably because of the higher Fe/Mn and organic carbon contents in Changjiang River sediment. The K _d decreased with increasing total suspended solid load from 3 to 500 mg L^-1, and was Cr > Zn > Cd. For Cd and Zn, increasing the pH from 5 to 8 resulted in an increase in K _d due to the reduced H⁺ competition and increasing sorptionpotential. However, the K _d for Cr in the sediments from both rivers showed no relationship with pH, presumably becauseof the complexity of the Cr species and environmental behavior.     

The Impact of Biochar Addition on Nutrient Leaching and Soil Properties from Tillage Soil Amended with Pig Manure

Poses dam in the Seine River estuary acts as receptacle of water drain-offs from highly urbanized and industrialized catchment area; therefore, this water is highly contaminated by trace metals. Most trace elements are mainly bound to particulate matter and are incorporated rapidly into the sediments. Scavenging of these metals in the sediments can be reversible due to several perturbations so as sediments also act as a source of pollutants for the overlying water. For instance, natural events (tide, flood, storm) and anthropogenic processes (water management actions) can cause disturbance of sediments and subsequent remobilization of pollutants to the water column, thereby posing a potential threat for aquatic organisms. The purpose of this study was to evaluate the mobility of trace metals by different methods in the Seine estuary sediments. The surface sediment sampled at Poses dam was characterized by high pollution level of Cd, Cu, Zn, and Pb. The estimation of metal bioavailability through ratio ΣSEM/AVS (simultaneously extracted metals/acid volatile sulfides) indicates a potential bioavailability of trace metals. The chemical partitioning using the European Community of Bureau of Reference sequential extraction method revealed that over 85, 82, and 80 % of the total Cd, Zn, and Pb, respectively, were found to be associated with the exchangeable and reducible fractions of the sediment. Another approach used consists in the quantification of dissolved metals released by sediment resuspension experiments in laboratory under controlled conditions. The results indicated that metals are released rapidly from sediment with a sharp peak at the beginning of the experiment, followed by a fast coprecipitation and/or adsorption processes on the suspended particles. Also, the Cd, Pb, and Ni mobility is higher compared to that of the other metals.     

Observations and modelling of the reactions of 10 metals with goethite: adsorption and diffusion processes

A sample of goethite was mixed for periods which ranged from 2 hours to 8 weeks with solutions of dilute nitrate salts of Pb, Hg, Cd, Zn, Cu, Ni, Co, Mn, Cr and Al. The amount of sorption after each period was measured for an appropriate pH range for each metal. The sorption behaviour was characterized both by using characteristics of the sorption curves such as the pH at which half of the added metal was sorbed (pH₅₀) and by fitting a model in which sorption was mainly characterized by an affinity constant and by a diffusion constant. Initial sorption, whether characterized by the pH₅₀ or by the affinity constant, was closely correlated with the appropriate dissociation constants of the metals. The greater the affinity of the metals for hydroxide ions, the greater their affinity for the goethite surface. The metals differed in the rate at which they continued to react with the goethite. Lead had the slowest continuing reaction, cobalt the fastest. The continuing reaction was due to diffusion into the particles. It was characterized by the fitted diffusion constant and by the change with time in the pH₅₀. For seven of the eight divalent metals, these were correlated with the ionic radius of the metals: the larger the radius, the slower the reaction. For Al and Cr, rates were slower than would be expected from the ionic radii and we suggest this shows that these ions react as the larger M(OH)²⁺ ions. The behaviour of Ni was consistent with oxidation of the surface species and diffusion of Ni(OH)²⁺ ions. The continuing reaction was similar to that observed when metal ions react with soils and suggests that their reaction with iron oxides is important in soils. The results also show that studies in which sorption is measured at only one period of reaction are incomplete and the application of equilibrium models to such results is misleading.     

Adsorption of Dimethyl Phthalate on Marine Sediments

Experiments were performed to investigate the sorption behaviors of dimethyl phthalate on marine sediments. The sorption of dimethyl phthalate on marine sediments reached equilibrium within 10 h. The sorption behavior of dimethyl phthalate on HCl-treated and untreated sediments accorded well with the linear sorption isotherm. The sorption occurred primarily via partition function of organic carbon of marine sediments. The sorption behavior of dimethyl phthalate on H₂O₂-treated sediment was nonlinear and conformed to Freundlich isotherm. Sorption of dimethyl phthalate on H₂O₂-treated sediment was chiefly through surface function of clay in marine sediments. Salinity of seawater had an important effect on the sorption of dimethyl phthalate. As the salinity of seawater increased, both the partitioning coefficients K _d and empirical constant K would increase.     

Predicting the solubility of Cd,Cu, Pb and Zn in uncontaminated Croatian soils under different land uses by applying established regression models

Soil pH, soil organic matter (SOM), dissolved organic carbon (DOC) and total trace metal concentration (M_(tot)) control the solubility of metals in the soil. Several regression models have included these soil chemical variables for the prediction of metal solubility and free metal ion (FMI) concentrations in contaminated soils. We hypothesize that models developed on contaminated soils (after optimization of the coefficients) can be used on samples from uncontaminated sites. Soil samples were collected from unpolluted agricultural and forest soils located in Eastern Croatia and extracted with water to determine the concentrations of Cd, Cu, Pb and Zn. We used these data to test the applicability of three regression models on existing conditions under different land uses. The same predictors issued in the three models and the same regression coefficients were utilized in the present study. The results showed a good correlation between the observed and predicted values of metal solubility. However, the models overestimate the total solution concentration (M_(sol)) and the concentrations of free metal ions (FMI) in solution, and therefore the same regression coefficients were optimized to fit our own observations. This was found to be very successful. The results showed that pH and DOC played a very important role in controlling metal solubility, while SOM and CEC were somewhat less significant. The impact of total soil concentration of metals (M_(tot)) was rather minor. However, we feel that to carry out good predictions of M_(sol) and FMI, the M_(tot) is needed in such regression models.     

Extraction of copper,lead, zinc or cadmium ions sorbed on calcium carbonate

The amount of sorbed metal ion released from CaC0₃ by 16 different extractants was found to vary with the chemical nature of the solution and the metal ion involved. In general, acid solutions dissolved a high proportion of both substrate and Cu, Ph, Cd coatings; complexing agents dissolved the same coatings but left most of the calcite; and competing cations (e.g. NH₄ ⁺, Ca²⁺) displaced primarily chemisorbed Cd and Cu. In Zn studies, little metal ion was retrieved by any extractant due to the limited solubility of the coatings formed at pH < 7.7. The diverse behavior observed in the sorption studies has been interpreted in terms of solubility and absorption equilibria. The pH of the CaC0₃ suspensions was high enough to precipitate all added Pb as hydroxy species, and excess Cu tended to precipitate at pH > 6.4 if one increased the soluble carbonate level (e.g. by adding acid). Unlike Cd and Cu, Zn was not chemisorbed; it formed sparingly soluble compounds such as ZnC0₃.2Zn(OH)₂, with excess coming out as Zn(OH)₂ at pH > 7.7. The significance of the results in respect to the mobility of metal ions in calcareous soils, and the evaluation of available levels, has been considered.     

Metal Remobilisation during Resuspension of Anoxic Contaminated Sediment: Short-Term Laboratory Study

The sediments of the Tagus estuary North Channel arecharacterised by high concentrations of trace metals andmonosulphides. During dredging operations Cd, Cu and Pb wereanalysed in water and suspended sediments collected 50 to 100 m around the dredging point. Concentrations in bothfractions fluctuated randomly: 2–3 fold for Cd and Cu and 10 for Pb. Since sampling in the dredging point reflectsintegration of rapid chemical reactions, a short-termlaboratory experiment was conducted to follow the geochemicalalterations occurring in the highest turbidity sites. Theexperiment was monitored as a function of time over a period of4 hr in short time intervals. Dissolved oxygen, pH, E_H,AVS, SO₄ ^2-, Cl^- and metals were monitored in theslurry samples. Iron, Mn, Cd, Pb and Cu were determined in thedissolved fraction (<0.45 μm), in the reactive solid phaseand in the total fraction. Resuspension resulted in asignificant release of Fe, Mn, Cd, Cu and Pb from the solids.Following the release Pb and Cu were almost totally scavengedin the 4 hr by the newly precipitated Fe oxyhydroxides, while more than 50% of the mobilised Cd remained in the dissolved fraction. The less efficient removal of Cd from solution implies a prolonged availability of this metal in the environment.     

Predicting Metal Uptake and Risk to the Human Food Chain from Leaf Vegetables Grown on Soils Amended by Long-Term Application of Sewage Sludge

The success of risk assessment of metal contaminated soils depends on how precisely one can predict the bio-availability of metals in soil and transfer to the human food chain. In the present investigation, we tested several formulations of the ‘free-ion activity model (FIAM)’ to predict uptake of Cd, Zn and Cu by perpetual spinach (Beta vulgaris, Cicla) grown on a range of soils amended with sewage sludge. The model was parameterised using data measured on samples of pore water extracted by centrifugation and with porous Rhizon samplers installed within the rhizosphere of the growing plants. Free ion activities (M²⁺) were estimated following speciation of solution data using version 6 of the ‘Windermere Humic Aqueous Model (WHAM-VI). For all three metals, the best formulation of the FIAM appeared to require only one hypothetical root sorption site without competition from protons. Values of (M²⁺) could also be predicted satisfactorily from a pH-dependent Freundlich relation. Thus, from a combined FIAM–Freundlich relation and population dietary information, it was possible to estimate risk (hazard quotients) to consumers from very simple soil measurements: extractable metal content (0.05 M EDTA (Zn and Cu) or 1 M CaCl₂ (Cd)), soil humus content and pH. The role of increased soil organic matter content and soil pH, in reducing risk to consumers, is illustrated for Cd in a hypothetical soil at the current UK statutory Cd limit for sludge application to agricultural land.     

Sulfur isotope fractionation and sequential extraction to assess metal contamination on lake and river sediments

Purpose

The present study investigated lake and river sediments affected by metals from an acid mine drainage (AMD) from a former uranium mine. The role of bacterial sulfate reduction in the immobilization of contaminants was evaluated, and the analyses of acid volatile sulfide (AVS) and sequential extraction were performed. Consequently, the potential mobility and bioavailability of contaminants were established.

Materials and methods

Sulfur isotopic fractionation (δ³⁴S), AVS, and sequential extraction procedure were used to assess the sulfate bacterial reduction and the availability of contaminants in the environment at six sampling stations.

Results and discussion

The δ³⁴S indicated that bacterial reduction is a key process in the natural attenuation of contamination in the Águas Claras reservoir, precipitating metal sulfides. According to the USEPA criteria, adverse biological effects are expected for sample S1 (inside the reservoir) which is likely to be toxic, while for sediment S4 (in the river), the toxicity is uncertain. The other samples were classified as non-toxic, likely because of the decreased solubility of zinc sulfide. A decrease in the concentration of the contaminants downstream of the reservoir was observed. The predominance of U (0.4 %) in the labile fraction and the elevated concentrations of Zn (0.5 %) and Mn (0.7 %) in the sediments inside the reservoir raises concerns regarding the availability of these contaminants in the environment.

Conclusions

The main environmental impact appears to be concentrated in the Águas Claras reservoir, whereas the Antas creek does not seem to be affected by the AMD process. Although the bacterial sulfate reduction is effective in its production of sulfides capable of immobilizing the contaminants, the presence of Zn and U in the labile and reducible fraction is a matter of concern due to its long-term bioavailability. Thus, continuous monitoring of the redox potential of the waters and sediments, mainly in the reservoir, is recommended in order to assess and possibly prevent later dissolution of sequestered contaminants.

     

Influence of pH on the release and chemical fractionation of heavy metals in sediment from a suburban drainage stream in an arid mine-based oasis

Purpose

The objectives of this study were to explore the influences of pH on the release of Cu, Zn, Cd, Pb, Ni, and Cr in sediments derived from the upstream, middle, and downstream reaches of Dongdagou stream in Gansu Province, Northwest China, and to examine the fractionation changes of heavy metals in the sediments after reaching their release equilibrium under different pH conditions.

Materials and methods

Sediment samples were obtained using a stainless steel grab sampler to collect the uppermost 10 cm of sediment from the channel bed. The pH-dependent release experiment was conducted in the solid-to-liquid ratio of 1:20 at different pH values (2, 4, 6, 8, 10, and 12) at room temperature. The total Cu, Zn, Cd, Pb, Ni, and Cr concentrations in the sediments were digested using an acid digestion mixture (HNO₃ + HF + HClO₄) in an open system. Metal fractionation of selected sediments was obtained using the Tessier sequential extraction procedure. Heavy metal concentrations in the samples were determined using atomic absorption spectrophotometry.

Results and discussion

The mean concentrations of heavy metals in sediments decreased in the following order: Zn (1676.67 mg kg^?1) > Pb (528.65 mg kg^?1) > Cu (391.34 mg kg^?1) > Cr (53.48 mg kg^?1) > Ni (34.27 mg kg^?1) > Cd (11.53 mg kg^?1). Overall, the solubility of Cu, Zn, Cd, Pb, and Ni decreased with increasing pH, and they were strongly released at pH 2. Moreover, the solubility of Cr increased with increasing pH, and its release was highest at pH 12. After reaching the release equilibrium of heavy metals under different pH conditions, the percentages of organic Cu, Zn, Cd, and Fe-Mn oxyhydroxide Pb decreased, compared to their initial fractions. The residual fractions of Ni and Cr were dominant, regardless of pH.

Conclusions

The average concentrations of Cu, Zn, Cd, and Pb in sediments were highly elevated compared with the soil background values in Gansu Province, China. The results of this pH-dependent release experiment showed that the release behaviors of Cu, Zn, Pb, and Cr followed an asymmetric V-shaped pattern, whereas Cd and Ni followed an irregular L-shaped pattern. The changes in the release of heavy metals in sediments were related to their redistribution between chemical fractionations.

     

Sorption of trace cadmium on clay minerals and river sediments: effects of pH and Cd(II) concentrations in a synthetic river water medium

The sorption of Cd(II) on illite was studied at initial Cd concentrations from 1 to 1000 μg 1^?1, with a constant solid phase concentration of 0.5 g 1^?1. Percentage sorbed at equilibrium increased sharply from pH 6.5 to 9.0, with a shift toward increased adsorption at lower initial concentrations, indicating specific, non-equivalent sites. Freundlich log-log plots were linear with a slope of 0.83. Susquehanna River sediments (silt/clay fraction) showed similar behavior, sorbing Cd somewhat more strongly than illite, as did mont-morillonite, while kaolinite sorbed Cd less strongly. Sorption on illite was not altered appreciably by removing the Fe- or Mn-oxide coating with dithionite, by removing organic matter with H₂O₂, or by using a carbonate-free medium. Equilibration time, desorption at lower pH's, and the effect of citrate in shifting the sorption to higher pH's were also studied. The data are discussed in terms of Cd speciation equilibria and possible sorption mechanisms.     

Effects of iron-ore mining and processing on metal bioavailability in a tropical coastal lagoon

Background, aim, and scope

In water systems, water quality and geochemical properties of sediments determine the speciation of trace metals, metal transport, and sediment–water exchange, influencing metal availability and its potential effects on biota. Studies from temperate climates have shown that iron-ore mining and tailing wastewaters, besides being a source of trace metals, usually show high levels of dissolved ions and particulate suspended matter, thus having the potential of indirectly changing metal bioavailability. For the first time in the tropics, we identified the effects of iron-ore mining and processing on metal bioavailability in a coastal lagoon. With an extensive sampling scheme, we investigated the potential sources of metals; the links among metal levels in water, sediments, and invertebrates; and the contrasting effects on metal speciation and bioavailability.

Methodology

The metals Fe, Mn, Al, Cr, Zn, Cu, Ni, Pb, Cd, Hg, and As were measured in water, sediments (surface and profiles), and invertebrates from Mãe-Bá Lagoon and in the sites directly influenced by the mining operations (tailing dams and nearby rivers). In addition, samples from two other lagoons, considered pristine, were analyzed. The study area is located in the southeast of Brazil (Iron Quadrangle Region and a coastal area of Espírito Santo State). General water characteristics included pH, dissolved organic carbon, alkalinity, and anion composition. Water metal speciation was assessed by a speciation model (Chemical Equilibria in Aquatic Systems). Grain-size distribution, organic carbon, carbonate, and acid volatile sulfide (AVS) were determined in sediments. Statistical methods included comparison of means by Mann–Whitney test, ordination and correlation analyses, and analysis of regression for geochemical normalization of metals with grain size.

Results and discussion

The dissolved metal concentrations, the total metal levels in sediments, and the normalization based on the fine sediment fraction showed that the mining operations constitute potential sources of Fe, Mn, Cr, Cu, Ni, Pb, As, and Hg to Mãe-Bá Lagoon. However, trace metal availability was reduced because of increased pH, hardness, and sulfide content (356 μmol/g) in the sites influenced by the mining. The lagoon showed similar water chemistry as in the mining sites, with metal bioavailability further decreased by the presence of dissolved organic carbon and chloride. Although AVS levels in the lagoon were low (0.48–56 μmol/g), metal bioavailability was reduced because of the presence of organic matter. Metal levels in invertebrates confirmed the predicted low metal bioavailability in Mãe-Bá Lagoon. The lagoon was considered moderately contaminated only by Hg and As.

Conclusions

The iron-ore mining and processing studied here constitute potential sources of metal pollution into the tropical lagoon. Contrary to expectations, however, it also contributes to reducing the overall metal bioavailability in the lagoon.

Recommendations and perspectives

These findings are believed to be useful for evaluating metal exposure in a more integrated way, identifying not only the sources of pollution but also how they can affect the components involved in metal speciation and bioavailability in water systems, leading to new insights.     

Retention of Ni,Zn and Cd by Si-associated goethite

Synthetic goethite used to study the effects of reaction time and temperature on the pH-dependent sorption of Ni, Zn and Cd was associated with amorphous silica. Ni interacted with dissolved Si and formed a Ni/Si precipitate on the goethite surface. Individual metals added at a concentration of 0.5 μmol g^?1 and sorbed during a reaction period of 504 hours (21 days) at 35°C were extracted by 0.7 M HNO₃ for 14 days. At the end of this period 11,28 and 40 percent of Ni, Cd and Zn, respectively, were not extracted whereas 20 percent of the total Fe content of the goethite and 39 percent of the associated Si were dissolved. During the sorption process metals became immobilized in the goethite particles. This effect can be related to a diffusion of metal ions into micropores. A total mobilization of sorbed metals can only be achieved by a complete dissolution of the goethite. The strong fixation of Ni, Zn and Cd by goethite suggests that additions of this Fe oxide could be used to ameliorate highly contaminated sludges or soils.