Metals In Anoxic Sediments Research Articles

Dissolution of PbS, CuS, and ZnS in oxic waters: effects of adsorbed natural organic matter.

Metal sulfides serve as sinks of toxic heavy metals in anoxic sediments. Suspension of metal sulfides to oxic water columns may cause their oxidative dissolution, leading to the release of toxic heavy metal ions. Ubiquitous natural organic matter (NOM) could adsorb on the surfaces of metal sulfides and influence their dissolution. In this study, the dissolution of suspended PbS, CuS, and ZnS with different levels of adsorbed NOM was investigated. The rates of metal release showed the following order after normalization by the available surface areas: PbS > CuS > ZnS. The adsorbed NOM was found to inhibit the oxidative dissolution of PbS and ZnS; the degree of inhibition was enhanced by increased levels of NOM adsorption. In contrast, the dissolution of CuS was found to increase and then decrease with increased levels of NOM adsorption. These results show that adsorbed NOM can promote metal release via ligand-induced dissolution, as well as inhibit metal release by forming a barrier against oxygen and proton attacks. The relative importance of these processes is metal specific and governs the overall impacts of NOM adsorption on the dissolution of metal sulfides. The results imply that remobilization of heavy metals from contaminated sediments during intensified storm events should be carefully evaluated in terms of metals of concern and levels of organic matter adsorption.

Environmental Assessment of Trace Metals in San Simon Bay Sediments (NW Iberian Peninsula)

A gravity core (220 cm depth) was collected to investigate the geochemistry, enrichment, and pollution of trace metals in anoxic sediments from San Simon Bay, an ecosystem of high biological productivity in the northwest of Spain. A five-step sequential extraction procedure was used. The Cu, Pb, and Zn contents decreased with depth, with maximum values in the top layers. Ni and Zn were bound to pyrite fractions, while Cd and Pb were associated with the most mobile fractions. The analyzed metals were associated with the fractions bound to organic matter, mainly with the strongly bound to organic matter fraction. High Cd and Cu values were observed. The fractionation showed a high mobility for Cd (28.3–100%) and Pb (54.0–70.2%). Moreover, the pollution factor and the geoaccumulation index reflected a high contamination for Pb and a moderate contamination for Cu and Zn in the superficial layers, pointing to a possible ecotoxicological risk to organisms in San Simon Bay.

Effects of dissolved oxygen, pH, salinity and humic acid on the release of metal ions from PbS, CuS and ZnS during a simulated storm event

Metal sulfides serve as the sinks of toxic heavy metals in anoxic sediments. Once exposed to fluctuations in environmental conditions, dissolution of metal sulfides can occur and release toxic heavy metal ions into water column. In this study, we investigated the effects of dissolved oxygen (DO), pH, salinity and humic acid (HA) on the dissolution of CuS, PbS and ZnS using batch experiments with continuous aeration to simulate the re-suspension of these metal sulfides during a storm event. Experimental results indicated that oxidative dissolution of CuS, PbS and ZnS occurred in the presence of DO. The rate was the fastest for PbS, followed by CuS and ZnS. Soluble metal concentrations also increased with decreasing pH under oxic conditions. Compared to metal sulfides dissolution rates in freshwater without HA, the presence of saline conditions and HA generally accelerated the dissolution of CuS but inhibited the dissolution of PbS and ZnS. A higher saline or HA concentration, however, did not always lead to a stronger enhanced or inhibited dissolution rate, which could be a collective effect resulting from ligand-enhanced dissolution, complexation, and decreased oxygen and proton attacks due to HA adsorption on the metal sulfide surfaces.

Investigating speciation and toxicity of heavy metals in anoxic marine sediments—a case study from a mariculture bay in Southern China

The pollution of marine sediments by heavy metals is still a major concern, especially in zones affected by industry or mariculture. Toxicity of sediment heavy metal contents may be assessed using sequential extraction (SE) procedures, minding inherent constraints of such approaches. In this study, we investigated heavy metal speciation and toxicity in anoxic marine sediments in Zhelin Bay, a mariculture bay in Southern China, using an SE and acid volatile sulfur-simultaneously extracted metals (AVS-SEM) approach. Speciation of Cd, Cu, Ni, Pb, and Zn were studied by a modified SE of five fractions, adapted to separate organic and sulfidic metal fractions in anoxic sediments: F1 weak acid soluble (readily available), F2 reducible fraction, F3 organic matter-bound fraction, F4 sulfide-bound fraction, and F5 residually bound fraction. Toxicity predictions based on the sum of non-residual (NR) metal fractions from sequential extraction were compared to predictions based on AVS-SEM. Results showed that Cd, Ni, and Pb predominantly occurred in the weak acid soluble fraction (F1), residual fraction (F5), and sulfide-bound fraction (F4), respectively; Cu and Zn were mainly obtained in F4 and F5. Based on the distribution of indicator elements for metal fractions, the SEM from AVS extraction included different yields of non-residual and residual fractions besides the sulfidic fraction. Estimates for potential heavy metal toxicity based on NR metals of the SE procedure were thus based on a better-defined speciation compared to the simplistic approach of the AVS-SEM method. Based on the contents of NR metals and normalizing them by organic matter content, toxic effects are not expected for any of the sampling sites, irrespective of the presence or absence of mariculture. Using Pearson correlation analysis to identify predominant fractions influencing toxicity, we conclude that toxicity of heavy metals in anoxic sediments can be well predicted by their non-residual heavy metal contents.

Modeling the Effect of pH and Salinity on Biogeochemical Reactions and Metal Behavior in Sediment

A mathematical model is developed to investigate the effect of pH and salinity fluctuation on biogeochemical reactions and metals' behavior in sediments. The model includes one-dimensional vertical advective and diffusive transport of species, serial reductions of electron acceptors, and precipitation/dissolution of species, acid–base chemistry, and metal sorption to sediments. The model was tested using data obtained from laboratory microcosm experiments which exposed metal (Cd, Zn) contaminated sediment to alternating fresh and salty overlying water. The model successfully reproduces the contrasting metal's release behavior and the vertical profiles of pH, Cl−, SO4 2−, Mn and Fe in porewater and the acid volatile sulfides (AVS) and simultaneously extracted metals (SEM) in sediments. The model showed that FeOOH(s) was the dominant sorption phase controlling the solubility of the metals at the surficial sediments while AVS controlled the solubility of the metals in anoxic sediments. The model also showed that the release of the metals to overlying water was controlled by the oxidation of metal sulfides in a very thin layer of oxic sediments (2–3 mm). The proposed model can be useful in managing metal contaminated sediments where pH and salinity are fluctuating by assessing the underlying biogeochemical processes and metals' behavior.

Minor element distribution in iron disulfides in coal: A geochemical review

Electron beam microanalysis of coal samples in U.S. Geological Survey (USGS) labs confirms that As is the most abundant minor constituent in Fe disulfides in coal and that Se, Ni, and other minor constituents are present less commonly and at lower concentrations than those for As. In nearly all cases, Hg occurs in Fe disulfides in coal at concentrations below detection by electron beam instruments. Its presence is shown by laser ablation ICP-MS, by selective leaching studies of bulk coal, and by correlation with Fe disulfide proxies such as total Fe and pyritic sulfur. Multiple generations of Fe disulfides are present in coal. These commonly show grain-to-grain and within-grain minor- or trace element compositional variation that is a function of the early diagenetic, coalification, and post-coalification history of the coal. Framboidal pyrite is almost always the earliest Fe disulfide generation, as shown by overgrowths of later Fe disulfides which may include pyrite or marcasite. Cleat- (or vein) pyrite (or marcasite) is typically the latest Fe disulfide generation, as shown by cross-cutting relations. Cleat pyrite forms by fluid migration within a coal basin and consequently may be enriched in elements such as As by deposition from compaction-driven fluids, metal enriched basinal brines or hydrothermal fluids. In some cases, framboidal pyrite shows preferential Ni enrichment with respect to co-occurring pyrite forms. This is consistent with bacterial complexing of metals in anoxic sediments and derivation of framboidal pyrite from greigite (Fe3S4), an Fe monosulfide precursor to framboidal pyrite having the thio-spinel structure which accommodates transition metals. Elements such as As, Se, and Sb substitute for S in the pyrite structure whereas metals, including transition metals, Hg and Pb, are thought to substitute for Fe. Understanding the distribution of minor and trace elements in Fe disulfides in coal has important implications for their availability to the environment through coal mining and use, as well as for potential reduction by coal preparation, and for delineating diagenetic compositional changes throughout and after coal formation.

Stressed by Heavy Metal

Microbiology When an ecosystem is stressed, do the more tolerant species benefit or do all species decline? In natural microbial communities, examples of both responses occur, but comparisons across field sites and experiments are difficult. In a relatively controlled natural setting, Gough and Stahl studied the stress response to metals in anoxic sediments of a eutrophic lake that are loaded with high but variable levels of metal contaminants from a nearby zinc smelter. By using a broad sequencing technique to identify dominant microbial sequences at the domain level, they found that across a range of metal concentrations, the communities were quite similar, and except in a few instances, the abundance and diversity of bacteria, archaea, and eukaryotes were uncorrelated with metal concentrations. In a notable exception within the archaeal fraction, cold-adapted Crenarchaeota were strongly correlated with high levels of zinc, arsenic, and copper. Although little is known about these freshwater anaerobes—even their metabolic role in the microbial communities is unclear—they appear to have a competitive advantage in their response to metal stress over other archaeal clades, including more common methanogens. ISME J. 4 , 10.1038/ismej.2010.132 (2010).

Metal speciation in sulphidic sediments: A new method based on oxidation kinetics modelling in the presence of EDTA

The solid phase partitioning of metals (Zn, Cu and Pb) was determined in four anoxic, metal polluted sediments by investigating at pH 8 the 1 day oxidation kinetics of the metal sulphide phases present in the sediments in a background solution containing excess EDTA. A mathematical model consisting of a combination of two pseudo-first order reactions was used to fit the metal release data as a function of oxidation time. The model permitted to fractionate the trace metals in a ‘quickly-oxidizable’ and a ‘slowly-oxidizable’ fraction, which could be assigned to two different trace metal pools, respectively (1) FeS minerals (e.g. amorphous FeS, mackinawite) and (2) discrete trace metal sulphide phases. The sum of the fractions associated with these sulphide pools was taken as an approximation for the sulphide-associated fraction of the trace metals and coincided (for the case of Zn and Pb) with the sulphide-associated fraction derived from the analysis of acid volatile sulphide (AVS) and simultaneously extracted metals (SEM). Oxidation kinetics modelling allowed also determining the sulphide-associated fraction of a broad range of trace metals (as demonstrated for Cu) more accurately than the AVS/SEM method, which suffers from non-efficient extraction of a number of trace metal sulphides. A correction was made for the determination of the sulphide-associated fraction by subtracting the trace metal fraction dissolved after 1 day under anoxic conditions in the background EDTA solution. The combination of (1) one day oxidation kinetics modelling and (2) correction for the 1 day anoxic EDTA-soluble fraction is a suitable method to determine accurately the true sulphide speciation of trace metals in anoxic sediments.

Chemical binding of heavy metals in anoxic river sediments

Acid volatile sulfides (AVS) in sediments are available for binding with divalent cationic metals through the formation of insoluble metal–sulfide complexes, thereby controlling the metal bioavailability and subsequent toxicity to benthic biocommunities. However, when the molar concentrations of simultaneously extracted metals (SEM) were greater than AVS, the unexpectedly low or nondetectable levels of metal in pore water could also be found. Thus, except AVS, additional binding phases in sediments were supposed to provide the binding sites for SEM. The aims of this study are to realize the spatial distribution of AVS, SEM, and other binding phases of heavy metals in anoxic sediments of the Ell-Ren river and to elucidate what may be the main additional binding phases except AVS in the anoxic river sediments. By comparing the spatial distributions of SEM/AVS ratio with various binding phases in extremely anoxic sediments (redox potential was between −115 and −208 mV), both organic matter and carbonates could be considered to be the main additional binding phases of SEM other than AVS. In addition, AVS appeared to have the priority to bind with SEM. By comparing the binding phases of heavy metals before and after AVS extraction, it could be found that Fe-oxides could also be considered to be the main additional binding phase associated with Zn in slightly anoxic sediments (redox potential was between −50 and −130 mV), while organic matter with Cu being the next.

Selective chemical extraction and grainsize normalisation for environmental assessment of anoxic sediments: validation of an integrated procedure

An integrated selective extraction and size normalisation procedure for use in metal partitioning and diagenetic studies of anoxic sediments and soils is presented. Data obtained by this procedure can readily be combined with other primary data (e.g. sulfur concentrations, carbonate concentrations, cation exchange capacity, etc.) and derived parameters (e.g. degrees of pyritisation and sulfidisation) that enhance interpretation of the behaviour of trace metals in anoxic sediments. Achieving size normalisation during extraction, allows direct comparison of sediments from dissimilar sedimentary environments, and simplifies assessment of the processes that determine whether a sediment is a source of or a sink for trace metals. Aspects of a study of trace metals in sediments from the Brisbane River estuary, Australia, are used to illustrate applications of the integrated procedure.

Effect of bioturbation on metal‐sulfide oxidation in surficial freshwater sediments

AbstractRecent studies have demonstrated the role of acid‐volatile sulfide (AVS) in controlling the bioavailability of several cationic metals in anoxic sediments. However, metal‐sulfide complexes can be relatively labile with respect to oxidation associated with factors such as seasonal changes in rates of oxidation/production of AVS. Another potentially important mechanism of AVS oxidation in surficial sediments is bioturbation. We used different densities of the burrowing oligochaete Lumbriculus variegatus in a series of laboratory experiments to evaluate the effect of bioturbation on oxidation of AVS and subsequent bioavailability of cadmium and zinc spiked into freshwater sediments. Metal bioavailability was determined directly by bioaccumulation in the test organisms and indirectly through analysis of interstitial (pore) water metal concentrations. In our studies, horizon‐specific sediment analyses were conducted to assess spatial differences in AVS and pore‐water metal concentrations specifically related to organism activity. Burrowing activity of the oligochaete significantly reduced AVS concentrations in surficial sediments in a density‐dependent manner and resulted in elevated interstitial water concentrations of cadmium but not zinc. Concentrations of cadmium in pore water from deeper horizons (below the zone of active burrowing) were consistently lower than those in the surficial sediments. The bioaccumulation of cadmium and zinc by L. variegatus was reflective of pore‐water concentrations of the two metals, i.e., there was significant accumulation of cadmium, but not zinc, by the oligochaetes. Overall, our results indicate that bioturbation can enhance the bioavailability of some cationic metals in surficial sediments, via oxidation of AVS, and demonstrate the importance of analyzing surficial sediments when assessing bioavailability of metals in sediments.

Effect of aeration of sediment on cadmium binding

AbstractAcid‐volatile sulfide (AVS) has been shown to be the dominant phase reacting with metals in anoxic sediments. The AVS in sediment decreases upon resuspension due to storms and dredging, and in winter when the rate of aeration processes exceeds that of the formation of sulfide. We conducted a series of lab aeration experiments in batch reactors to investigate the effects of aeration of sediment on the sulfide content of sediment and on the partitioning of cadmium, a model toxic metal, to the sediment. Aeration of sediment results in rapid decrease of the AVS. We studied the sediment characteristics for aeration periods of approximately a month. During this time, the concentrations of dissolved metals increased by 200 to 400% or more, relative to the concentrations present at the beginning of the test. The concentration of metal associated with AVS and with pyrite decreased. During the aeration, there are increases in the concentrations of hydrous iron and manganese oxides, and these materials become increasingly more important in the binding of cadmium. Following the aeration, >50% of the cadmium was associated with the extractable iron and manganese components of the sediment. Overall, the binding capacity of the sediments for cadmium decreased after aeration.

EFFECT OF AERATION OF SEDIMENT ON CADMIUM BINDING

Acid-volatile sulfide (AVS) has been shown to be the dominant phase reacting with metals in anoxic sediments. The AVS in sediment decreases upon resuspension due to storms and dredging, and in winter when the rate of aeration processes exceeds that of the formation of sulfide. We conducted a series of lab aeration experiments in batch reactors to investigate the effects of aeration of sediment on the sulfide content of sediment and on the partitioning of cadmium, a model toxic metal, to the sediment. Aeration of sediment results in rapid decrease of the AVS. We studied the sediment characteristics for aeration periods of approximately a month. During this time, the concentrations of dissolved metals increased by 200 to 400% or more, relative to the concentrations present at the beginning of the test. The concentration of metal associated with AVS and with pyrite decreased. During the aeration, there are increases in the concentrations of hydrous iron and manganese oxides, and these materials become increasingly more important in the binding of cadmium. Following the aeration, >50% of the cadmium was associated with the extractable iron and manganese components of the sediment. Overall, the binding capacity of the sediments for cadmium decreased after aeration.

Pyritization of metals in anoxic sediments : Morse, J.W. (Department of Oceanography, Texas A and M University, College Station, TX 77834, USA)

Pyritization of metals in anoxic sediments : Morse, J.W. (Department of Oceanography, Texas A and M University, College Station, TX 77834, USA)

Metal associations in anoxic sediments and changes following upland disposal†

The ecological effects of heavy metals in contaminated sediments are more determined by the chemical form and reactivity than by the level of accumulation. Dredging of anoxic sediments and disposal on land is attended by changes of redox conditions. Under oxidizing conditions some controlling solid compounds may change gradually thus changing the solubility of certain metals. Chemical extraction experiments for estimating characteristic association forms of heavy metals in anoxic sediments were carried out, both under presence and absence of air during the analytical procedure. Drying of the sediment decreases the proportion of the sulfidic metal fractions to a stronger degree, and oxidized Cd and Zn are found in the most available, exchangeable fraction. With respect to long‐term effects acidification of poorly buffered sludges after disposal on land is probably the most important factor affecting metal associations and mobility. For many metal examples a linear relationship has been found between decreasing pH values and increasing dissolved metal concentrations. To quantify these relationships and for better comparison of samples a simple test procedure is proposed which is based on pH differences before and after addition of acid.