Release Of Trace Metals Research Articles

Accumulation processes of trace metals into Arctic sea ice: distribution of Fe, Mn and Cd associated with ice structure

Biogeochemical cycling of trace metals in sea ice is important to the productivity of the Arctic Ocean. Unfortunately, the processes by which trace metals accumulate into sea ice are poorly understood. To gain a clearer understanding of the mechanisms behind trace metal accumulation, dissolved (D, <0.2 μm), and labile particulate (LP = Total Dissolvable – Dissolved) iron (Fe), manganese (Mn), and cadmium (Cd) concentrations were compared to the structure observed in sea ice. Samples were pre-concentrated via solid-phase extraction on NOBIAS Chelate PA-1 resin and analyzed on a Graphite Furnace Atomic Absorption Spectrometer. Using photographic analysis for the percentage of pore microstructure and δ18O analysis, sea ice structure was determined to be snow ice, granular ice (frazil ice), mixed ice (granular and columnar ice) and columnar ice. Salinity and nutrients were low, indicating brine drainage and multi-year ice. High trace metal concentrations in snow ice indicated meteoric snow was a source of trace metals to sea ice. High concentrations of LPFe in granular ice indicated entrainment of suspended particulate trace metals by frazil ice during the formation of the granular ice structure. Whereas the high concentrations of DFe and DMn in granular ice may have been due to reduction from LPFe and LPMn after particle entrainment, indicating chemical transformation processes. Low dissolved and labile particulate trace metal concentrations in mixed and columnar ice indicated a release due to brine drainage. Our study clearly indicates that the differences observed in trace metals among sea ice structures, showed that sea ice formation, chemical reduction and brine release were the processes driving trace metal accumulation and release in the Arctic sea ice.

Cu, Pb and Fe release from sulfide-containing tailings in seawater: Results from laboratory simulation of submarine tailings disposal

Metal release from the deposition of sulfide-containing tailings in seawater was investigated using a batch reaction experiment inside a temperature and dissolved oxygen-controlled chamber. Two hundred grams of tailings from a porphyry Cu-Au and a sediment-hosted Cu deposit were submerged in 1.8 L synthetic seawater. The sulfides present in the porphyry Cu-Au tailings are pyrite (FeS2), chalcopyrite (CuFeS2) and bornite (Cu5FeS4) while in the sediment-hosted Cu tailings are bornite, chalcocite (Cu2S) and covellite (CuS). Galena occurs as a minor sulfide in both tailings. Pore water and overlying seawater samples were collected and analyzed for pH, redox potential and trace metals (Cu, Pb and Fe) concentration. Results show that there is very low Cu (10–40 μg/L), Pb (1–10 μg/L) and Fe (5–50 μg/L) released into solution throughout the course of 87 days. Long-term trace metal release from tailings in seawater is therefore theorized to be low and is a slow process.

Importance of Hydrothermal Vents in Scavenging Removal of 230Th in the Nansen Basin

AbstractIn this study we present dissolved and particulate 230Th and 232Th results, as well as particulate 234Th data, obtained as part of the GEOTRACES central Arctic Ocean sections GN04 (2015) and IPY11 (2007). Samples were analyzed following GEOTRACES methods and compared to previous results from 1991. We observe significant decreases in 230Th concentrations in the deep waters of the Nansen Basin. We ascribe this nonsteady state removal process to a variable release and scavenging of trace metals near an ultraslow spreading ridge. This finding demonstrates that hydrothermal scavenging in the deep‐sea may vary on annual time scales and highlights the importance of repeated GEOTRACES sections.

Trace metal behavior during in-situ iron removal tests in Leuven, Belgium

Subsurface iron removal (SIR) is an in-situ technique to lower the iron content of extracted groundwater. Through cyclic injection of oxygenated water ferrous iron oxidises and precipitates as iron hydroxide in a zone surrounding the extraction well, enhancing the sorptive capacity of the aquifer. During subsequent pumping phases, groundwater traverses the oxidation zone and ferrous iron sorbs to available and newly formed exchange and sorption sites, thereby retarding the breakthrough of dissolved iron. The process is well-understood in regards to the retardation of iron. Less well understood, however, is the behavior of a number of trace metals and metalloids during SIR operations, foremost arsenic (As). In this study, we analyse major and minor ion and trace metal concentrations from a number of SIR tests in a sand aquifer near Leuven, Belgium. We use reactive transport modelling to evaluate conceptual models of trace metal release and arrest. The test data, underpinned by model results, show that metal release, namely arsenic and barium (Ba), occurs through the oxidation of trace amounts of sulphide minerals during the injection phase. Sorption through cation exchange retards Ba while complexation lowers dissolved As concentrations. Arsenic is mobilized again during the pumping phase through varying phosphate concentrations in the native groundwater, despite available sorption surfaces, while Ba remains adsorbed. Concentrations, however, remain below WHO guideline values for As and Ba (10 μg/l and 0.7 mg/l), respectively. The developed conceptual model of As fate reveals a high propensity for As mobility during SIR due to desorption reactions and delivers an explanation as to why many SIR operations fail to show substantial As removal, despite efficient iron removal. Other monitored trace elements showed no mobilisation, including Zn, Al, Cd, Cr, Cu, F, Hg, Ni, Pb, Sb and Se.

Trace metal enrichment during the Industrial Period recorded across an altitudinal transect in the Southern Central Pyrenees

The study of three lacustrine sedimentary archives along an altitudinal transect in the Southern Central Pyrenees - lakes Estanya, Basa and Marboré- has provided a unique record of changes in anthropogenic trace metal concentrations over the last six centuries in NE Iberian Peninsula. Although site-specific processes influence metals enrichments in each lacustrine system, significant enrichments of Hg and Pb and minor to moderate enrichments of Cu, Cd, and Zn with respect to baseline (Pre-industrial) concentrations highlight intensive release of anthropogenic trace metals with the advent of the Industrial Revolution leading to maximum values during the 20th century. The largest trace metal pollution occurred between 1840s and 1920s CE mainly derived from the increasing demand of ore resources in Southern Europe during the Industrialization. A second, less distinct pollution phase occurred between 1950s and 1990s, associated with the “Great Acceleration” and increased trace metal emissions related to road-transport, use of fertilizers in agriculture and the global boost of the Chemical Industry. Enrichment of mercury during the Industrial Period correlates well with Hg production in Spanish Almadén mines and global emission inventories. Local mining in the Pyrenees and regional smelting activities in Spain and Southern France may explain the enrichment of lead (and associated by-products cadmium and zinc) during the first pollution phase while the use of leaded gasoline since the mid-20th century drives the higher Pb enrichment factor found till the late 20th century. This investigation demonstrates that environmental regulations controlling emissions of hazardous metals during the last decades have greatly contributed to a significant reduction of these anthropogenic trace metals enrichments in natural ecosystems although they still double pre-industrial levels. This study also exemplifies the different sensitiveness of lacustrine systems to record past atmospheric pollution phases and highlights the need of multi-archive studies to conduct regional (rather than local) pollution reconstructions.

Remediation of metal-contaminated marine sediments using active capping with limestone, steel slag, and activated carbon: a laboratory experiment

ABSTRACTThe objectives of this study are to assess the effectiveness of limestone (LS), steel slag (SS), and activated carbon (AC) as capping materials to sequester trace metals including As, Cd, Cr, Cu, Ni, Pb, and Zn in heavily contaminated marine sediments and to minimize the release of these metals into the water column. A flat flow tank was filled with 10 mm of capping material, contaminated sediments, and seawater, and the metal concentrations were monitored over 32 d. After completion of the flow tank experiments, the sediments below the capping material were sampled and were sequentially extracted. SS effectively reduced the As, Cr, Cu, Ni, Pb, and particularly Cd elution from the contaminated sediments to the overlying seawater. Adsorption and surface precipitation were the key mechanisms for interrupting the release of cationic trace metals by SS. LS was appropriate for interrupting the release of only Cu and Pb with high hydrolysis reaction constants. AC capping could interrupt the release of Cr, Cu, Ni, and particularly Zn from the sediments by binding with the metals via electrostatic interaction. The results obtained from the sequential extraction revealed that LS capping is appropriate for stabilizing Zn, whereas AC is appropriate for Cd and Pb. LS, SS, and AC can be applied effectively for remediation of sediments contaminated by trace metals because it interrupts their release and stabilizes the trace metals in the sediments.

Remobilization of trace metals during laboratory resuspension of contaminated sediments from a dam reservoir

Dam reservoirs sometimes need to be fully drawn down, an operation that induces sediment resuspension. In the case of contaminated sediments, such resuspension may promote the remobilization of trace metals from the solid to the dissolved phase, threatening the water quality. The objective of this study is to evaluate the kinetic release and removal processes of trace metals during the resuspension of polluted sediments from a dam reservoir. Total contents of trace elements were analyzed in the surface sediments collected in the dam. Ascorbate and sequential extractions were used to assess the trace metal fractionation. Aliquots of wet sediments were resuspended during a week using a 3-L glass reactor. Three solid/solution ratios (2.7, 5.0, and 9.4 g L−1) were tested. The pH, Eh, dissolved oxygen, and temperature were continuously measured, and water samples were collected through a small pipe inserted into the reactor. Evolution of dissolved organic carbon, alkalinity, major ions, and trace element concentrations were measured in the water over the resuspension experiments. Another resuspension experiment was carried out to determine the quantity of elements that could be adsorbed onto the glass wall of the reactor. High levels of As, Cd, Pb, and Zn were measured in the reservoir sediments, compared to the quality guidelines. The temporal evolutions of the dissolved elements were similar during the resuspension experiments, regardless of the mass of sediment used. Depending on the elements, different trends can be highlighted: (1) Cd, Cr, Cu, Ni, V, and Zn were not remobilized; (2) Al, Mo, As, Ba, and Sr continuously increased during the experiments; and (3) Mn, Co, Fe, and Pb increased at the beginning of the experiments and then were removed from the dissolved phase. Co was likely adsorbed onto Mn oxyhydroxides, whereas Pb appeared to be regulated by the Fe oxyhydroxides. The results from sequential extractions did not allow to relate these groups to specific distributions in the solid phase. The greater risk of water quality degradation for this reservoir would be posed by the remobilization of As, which showed the greatest and the most prolonged release during resuspension, because it could not be removed onto particles due to its speciation. Dissolved As reached a maximum of 30 μg L−1 at the conclusion of the experiments, a concentration above the drinking water standard but well below the acute toxicity levels reported in the literature for fishes.

Long-term leaching of nutrients and contaminants from wood combustion ashes

With increasing amounts of woody biomass being combusted for energy purposes worldwide, more wood ash is being generated and needs management. As an alternative to landfilling, residues may be utilised for liming and fertilising purposes on forest soils. Comprehensive evaluations of long-term leaching from these residues are needed in order to assess potential environmental impacts associated with their utilisation. Two Danish wood ash samples, one fly ash and one mixed ash (a combination of fly ash and bottom ash), were evaluated in long-term percolation column tests (up to L/S ∼2000 L/kg), in order to quantify the release of major, minor and trace metal(loid)s. While columns of three different lengths were used, the leaching of individual elements could be described as a function of the L/S ratio – irrespective of the column length. At L/S 1000 L/kg, the cumulative releases of K, S, Na, Ca and Rb were at 40–100% of their respective solid contents, followed by Ba, Cr, Sb, Sr and V at 15–40% and Al, Mg, Zn, Cd, Co, Fe, Pb, Tl, Mn and P at < 5%. Speciation calculations indicated that (i) the observed concentrations of Ca, Mg, Al, Ba, Si and sulphate from both ash types could be described through the dissolution/precipitation of a limited set of minerals and that (ii) leaching of silicates should be included in long-term assessment of alkalinity release from wood ashes. Non-equilibrium conditions were indicated by flow interruptions. However, the presence of non-equilibrium did not have significant effect on the calculated cumulative releases at high L/S ratios. Based on the assessment of cumulative releases at L/S 10 L/kg and L/S 1000 L/kg it is concluded that low L/S-based data may not provide sufficient background for prediction of long-term release from wood ash, in particular for Ba, Cr, Sb and V, and less critically also for As, Cd, Cu, Mo and Ni.

Trace metal release after minimally-invasive repair of pectus excavatum.

BackgroundSeveral studies have shown a high incidence of metal allergy after minimally-invasive repair of pectus excavatum (MIRPE). We postulated that MIRPE is associated with a significant release of trace metal ions, possibly causing the allergic symptoms.MethodsWe evaluated the concentration with chromium, cobalt and nickel in blood, urine and tissue in patients prior to MIRPE and in patients who underwent an explantation of the stainless-steel bar(s) after three years.ResultsOur study group consisted of 20 patients (mean age 19 years) who had bar explantation and our control group included 20 patients (mean age 16 years) prior to MIRPE. At the time of bar removal we detected significantly elevated concentrations of chromium and nickel in the tissue compared to patients prior to the procedure (p<0,001). We also found a significant increase in the levels of chromium in urine and nickel in blood in patients three years post MIRPE (p<0,001). Four patients temporarily developed symptoms of metal allergy, all had elevated metal values in blood and urine at explantation.ConclusionsMinimally-invasive repair of pectus excavatum can lead to a significant trace metal exposure.

Pollution characteristics and source identification of trace metals in riparian soils of Miyun Reservoir, China

The South-to-North Water Diversion Project, one of China's largest water diversion projects, has aroused widespread concerns about its potential ecological impacts, especially the potential release of trace metals from shoreline soils into Miyun Reservoir (MYR). Here, riparian soil samples from three elevations and four types of land use were collected. Soil particle size distributions, contents and chemical fractionations of trace metals and lead (Pb) isotopic compositions were analyzed. Results showed that soil texture was basically similar in four types of land use, being mainly composed of sand, with minor portions of clay and silt, while recreational land contained more abundant chromium (Cr), copper (Cu), zinc (Zn) and cadmium (Cd), suggesting a possible anthropogenic source for this soil pollution. The potential ecological risk assessment revealed considerable contamination of recreational land, with Cd being the predominant contaminant. Chemical fractionations showed that Cu, arsenic (As), Pb and Cd had potential release risks. Additionally, the 206Pb/207Pb and 208Pb/207Pb values of soils were similar to those of coal combustion. By combining principal component analysis (PCA) with Pb isotopic results, coal combustion was identified as the major anthropogenic source of Zn, Cr, Cu, Cd and Pb. Moreover, isotope ratios of Pb fell in the scope of aerosols, indicating that atmospheric deposition may be the primary input pathway of anthropogenic Zn, Cr, Cu, Cd and Pb. Therefore, controlling coal combustion should be a priority to reduce effectively the introduction of additional Zn, Cu, Cd, and Pb to the area in the future.

Iron(II)-Catalyzed Iron Atom Exchange and Mineralogical Changes in Iron-rich Organic Freshwater Flocs: An Iron Isotope Tracer Study.

In freshwater wetlands, organic flocs are often found enriched in trace metal(loid)s associated with poorly crystalline Fe(III)-(oxyhydr)oxides. Under reducing conditions, flocs may become exposed to aqueous Fe(II), triggering Fe(II)-catalyzed mineral transformations and trace metal(loid) release. In this study, pure ferrihydrite, a synthetic ferrihydrite-polygalacturonic acid coprecipitate (16.7 wt % C), and As- (1280 and 1230 mg/kg) and organic matter (OM)-rich (18.1 and 21.8 wt % C) freshwater flocs dominated by ferrihydrite and nanocrystalline lepidocrocite were reacted with an isotopically enriched 57Fe(II) solution (0.1 or 1.0 mM Fe(II)) at pH 5.5 and 7. Using a combination of wet chemistry, Fe isotope analysis, X-ray absorption spectroscopy (XAS), 57Fe Mössbauer spectroscopy and X-ray diffraction, we followed the Fe atom exchange kinetics and secondary mineral formation over 1 week. When reacted with Fe(II) at pH 7, pure ferrihydrite exhibited rapid Fe atom exchange at both Fe(II) concentrations, reaching 76 and 89% atom exchange in experiments with 0.1 and 1 mM Fe(II), respectively. XAS data revealed that it transformed into goethite (21%) at the lower Fe(II) concentration and into lepidocrocite (73%) and goethite (27%) at the higher Fe(II) concentration. Despite smaller Fe mineral particles in the coprecipitate and flocs as compared to pure ferrihydrite (inferred from Mössbauer-derived blocking temperatures), these samples showed reduced Fe atom exchange (9-30% at pH 7) and inhibited secondary mineral formation. No release of As was recorded for Fe(II)-reacted flocs. Our findings indicate that carbohydrate-rich OM in flocs stabilizes poorly crystalline Fe minerals against Fe(II)-catalyzed transformation by surface-site blockage and/or organic Fe(II) complexation. This hinders the extent of Fe atom exchange at mineral surfaces and secondary mineral formation, which may consequently impair Fe(II)-activated trace metal(loid) release. Thus, under short-term Fe(III)-reducing conditions facilitating the fast attainment of solid-solution equilibria (e.g., in stagnant waters), Fe-rich freshwater flocs are expected to remain an effective sink for trace elements.

Oxalate-Promoted Trace Metal Release from Crystalline Iron Oxides under Aerobic Conditions

Iron oxides are common in soils and sediments and bind trace metals, such as nickel, through adsorption and structural incorporation. Recent work has shown that incorporated metals can be released during recrystallization catalyzed by aqueous Fe(II), which is generally stable only under anoxic conditions. This paper investigates the effects of the organic acid oxalate on the fate of nickel substituted into goethite and hematite to evaluate potential nonredox pathways of iron oxide recrystallization. Nickel-substituted hematite and goethite were synthesized and then reacted with 1 mM oxalate at pH 3–7 for ≤15 days. At all pH values, the presence of oxalate led to substantial nonstoichiometric release of nickel into solution compared to oxalate-free systems, with the extent of release increasing with decreasing pH. The rate of nickel release also increased with increasing oxalate surface coverage, and nonstoichiometric release of nickel was observed even under conditions producing no net mineral dissolution...

Trace metal release during wood pyrolysis

The use of a differential thermal analyzer with an on-line inductively coupled plasma mass spectrometer allows continuous and near-synchronous analysis of mass loss and elemental release patterns during pyrolysis of biomass and other feedstock. Applied to a mixed-conifer softwood, the results show that the released elements can be divided into those that are dominantly released during decomposition (250–475°C) and those that are dominantly released at high temperature (900–1300°C), thus reflecting their different roles in the organic matrix. Production and analysis of a 950°C pyrolysis wood ash allows the bulk partition between flue gas (including entrained particles) and solid char to be evaluated, mass balance to be demonstrated, and a semi-quantitative calibration to be suggested. Our exploratory experiment demonstrates that the present approach can provide reliable insight and semi-quantitative modeling tools useful for elucidating the role and behavior of many elements during thermal treatment of biomass.

Emissions and ash behavior in a 500 kW pellet boiler operated with various blends of woody biomass and peat

Due to the increasing utilization of wood-based raw materials, there is growing interest in the use of alternative biomass raw materials for biomass pellet production. However, the utilization of these new raw materials may significantly increase particle emissions from small boilers, which can cause adverse effects on human health and the environment. In this study, the effects of biomass raw materials on particle and gaseous emissions and ash behavior were studied in a real-world operating 500kW reciprocating grate boiler. The studied fuels were pellets, which consisted of softwood stem, bark, peat and various mixtures of these materials. The emission factors of the fine particles (PM1) ranged between 11 and 26mg/MJ, which were primarily determined by alkali metal release from the fuel. The co-firing of peat with wood significantly decreased PM1 emissions. This decrease was observed because of the high silicate content in peat, which led to alkali silicate formation during co-combustion and, consequently, the retention of alkali metals in the bottom ash. In contrast, the release of trace metals into the fine particle fraction was independent of the fuel ash composition with the exception of Cu. The emissions of As, Cd, Pb, Tl, and Zn were linearly dependent on their concentrations in the fuel. The highest release rates, which were between 40% and 60%, were observed for zinc and lead. Zinc formed the majority of the total PM1 heavy metal emissions, and zinc emissions were especially high in bark combustion. On the basis of these results, fuel composition based indexes were derived for predicting particle emissions. The results highlight the importance of both the fuel alkali metal content and alkali metal/silicate ratio in the formation of fine particles with fuels that consist of woody biomass and peat.

Terrestrial sedimentary pyrites as a potential source of trace metal release to groundwater – A case study from the Emsland, Germany

Pyrite is a common minor constituent of terrestrial freshwater sediments and a sink for trace elements. Different amounts and morphological types (framboids and euhedral crystals) of sedimentary pyrites were found in the heavy mineral fraction of cores obtained from several drillholes located in the Emsland region, NW Germany. Their trace element contents were investigated to assess their potential for groundwater contamination after oxidation, e.g. induced by dewatering or autotrophic denitrification. Nickel, arsenic and cadmium were found in significant concentrations in pyrite. Geochemical modeling showed that elevated trace metal concentrations in groundwater, potentially exceeding drinking water standards, should preferentially occur in a less than 1 m thick zone situated around the depth of the redoxcline, where nitrate is reduced by pyrite. This was confirmed by depth-specific groundwater sampling in the Emsland and by previously published studies. The absolute concentration of released trace metals depends on their content in the pyrite but also strongly on the nitrate load of groundwater.

CO2 seawater acidification by CCS-simulated leakage: Kinetic modelling of Zn, Pb, Cd, Ni, Cr, Cu and As release from contaminated estuarine sediment using pH-static leaching tests

A modified pH-dependent leaching test with continuous pH control that employed CO2 to acidify a seawater-sediment mixture is used to address Zn, Pb, Cd, Ni, Cr, Cu and As release from contaminated estuarine sediments under the influence of acidification processes. Long-term (480h) leaching experiments at pH values of 7.0, 6.5 and 6.0 are performed. The different evolutionary patterns of the redox potential and Fe release at pH=6 with respect to the other pH values shows the need to assess the influence of the initial Fe content in seawater upon elemental release. Hence, assays at pH=6.0 are conducted using natural seawater with Fe concentrations between 9.02 and 153μg/L. A set of in-series reactions for trace elements, Fe and other ions associated with Fe is proposed to model a Fe/multi-ion-dependent mechanism for trace metal release. The maximum concentration of each contaminant that can be released from the sediment and the kinetic parameters of the proposed model are completed for the studied pH values, for good consistency between the experimental and simulated mobilisation of each studied element.

Evaluation of the Use of Sea Sand, Crushed Concrete, and Bentonite to Stabilize Trace Metals and to Interrupt Their Release from Contaminated Marine Sediments

The aim of this study is to assess the effectiveness of sea sand (SS), crushed concrete (CC), and bentonite (BN) as a capping material to block the release of trace metals (As, Cd, Cr, Cu, Ni, Pb, and Zn) from heavily contaminated marine sediments, and to stabilize/solidify these trace metals in the sediments. The efficiency of SS, CC, and BN for blocking trace metals was evaluated in a flat flow tank in which a 1-cm-thick layer of capping materials was placed above the contaminated sediments. Trace metals were released into the overlying water from contaminated sediments in the following decreasing order: Cd > As > Zn > Ni > Pb > Cr > Cu. Electronegativity, electrostatic force, or metal hydrolysis significantly influenced the mobility of trace metals in sediments. The elution of Cu, Ni, and Cd from contaminated sediments was effectively reduced by CC, especially in case of Cd elution. SS was appropriate for interrupting Cr and Zn release, and BN was appropriate for interrupting Pb release. A sequential extraction study demonstrated that SS capping is appropriate for stabilizing As, Cu, and Ni; CC capping for Zn; and BN capping for Pb. SS, CC, and BN can be applied effectively for remediating the contaminated sediments.

Oxidative transformation of iron monosulfides and pyrite in estuarine sediments: Implications for trace metals mobilisation

Iron monosulfides are the initial iron sulfide minerals that form under reducing conditions in organic-rich sediments. Frequently referred as monosulfidic black ooze (MBO), these sediments exists in a range of anoxic systems including estuaries, coastal wetlands and permeable reactive barriers. The objective of this study was to investigate the transformation of solid phase sulfur, iron fractions and trace metals mobilisation in organic-rich hypersulfidic sediments during dredging. Two sediments from geographically contrasting sites in the Peel-Harvey Estuary were collected and subjected to oxidation through resuspension over 14 days. During oxidation, redox potential rapidly and continuously increased, although minimal change in pH was observed in both sediments. The majority of FeS was oxidised within 48 h. Although not as dynamic as FeS, unusually high rates of FeS2 oxidation were measured in both sediments at circumneutral pH, with between 39 and 58% of FeS2 oxidised over 14 days. The rapid oxidation of FeS2 may be attributed to the presence of nano-size FeS2 crystals (≈550–860 nm) with a high surface area. Before resuspension, solid bound Fe(total) was most abundant as measured by HCl-extractable Fe(II), followed by organic bound Fe(total) and oxide bound Fe(total). There was a marked decrease in these three fractions in both sediments during resuspension, with an increase in Fe(III) fraction. No significant release of trace metals was observed during resuspension of sulfidic sediments. However, disturbance to these estuarine sediments increases Fe(III) formation and further deteriorates the environment through smothering biological surfaces, deteriorating food sources and the quality of benthic habitats.

Initial results of batch reactor experiments to determine sulphide oxidation rates and trace metal release under seafloor conditions

"Initial results of batch reactor experiments to determine sulphide oxidation rates and trace metal release under seafloor conditions." Applied Earth Science, 125(2), pp. 88–89

Sulfidization of Organic Freshwater Flocs from a Minerotrophic Peatland: Speciation Changes of Iron, Sulfur, and Arsenic.

Iron-rich organic flocs are frequently observed in surface waters of wetlands and show a high affinity for trace metal(loid)s. Under low-flow stream conditions, flocs may settle, become buried, and eventually be subjected to reducing conditions facilitating trace metal(loid) release. In this study, we reacted freshwater flocs (704-1280 mg As/kg) from a minerotrophic peatland (Gola di Lago, Switzerland) with sulfide (5.2 mM, S(-II)spike/Fe = 0.75-1.62 mol/mol) at neutral pH and studied the speciation changes of Fe, S, and As at 25 ± 1 °C over 1 week through a combination of synchrotron X-ray techniques and wet-chemical analyses. Sulfidization of floc ferrihydrite and nanocrystalline lepidocrocite caused the rapid formation of mackinawite (52-81% of Fesolid at day 7) as well as solid-phase associated S(0) and polysulfides. Ferrihydrite was preferentially reduced over lepidocrocite, although neoformation of lepidocrocite from ferrihydrite could not be excluded. Sulfide-reacted flocs contained primarily arsenate (47-72%) which preferentially adsorbed to Fe(III)-(oxyhydr)oxides, despite abundant mackinawite precipitation. At higher S(-II)spike/Fe molar ratios (≥1.0), the formation of an orpiment-like phase accounted for up to 35% of solid-phase As. Despite Fe and As sulfide precipitation and the presence of residual Fe(III)-(oxyhydr)oxides, mobilization of As was recorded in all samples (Asaq = 0.45-7.0 μM at 7 days). Aqueous As speciation analyses documented the formation of thioarsenates contributing up to 33% of Asaq. Our findings show that freshwater flocs from the Gola di Lago peatland may become a source of As under sulfate-reducing conditions and emphasize the pivotal role Fe-rich organic freshwater flocs play in trace metal(loid) cycling in S-rich wetlands characterized by oscillating redox conditions.