Geochemical Fate Research Articles

Surface complexation of antimony on kaolinite

Geochemical fate of antimony (Sb) – a similar oxyanion as arsenic (As) – in a variety of environment is largely unexplored. Kaolinite is an important, naturally occurring clay mineral in soils and aquifers and is known to control the fate of several contaminants via a multitude of geochemical processes, primarily adsorption. Here we report adsorption of antimony on kaolinite as a function of solution chemistry: initial antimony concentration, pH, ionic strength, and a competing anion. A surface complexation modeling (SCM) approach was undertaken to understand the potential mechanistic implications of sorption envelope data. In the SCM, a multicomponent additive approach, in which kaolinite is assumed to be a (1:1) mixture of quartz (SiOH) and gibbsite (AlOH), was tested. Results indicated that ionic strength has a minimal effect on antimony adsorption. For the lower initial antimony concentration (4.11μM), the additive model with binuclear surface complexes on quartz and gibbsite showed a better fit at pH<6, but somewhat under predicted the experimental data above pH 6. At the higher initial antimony concentration (41.1μM), the sorption envelope was of different shape than the lower load. The additive model, which considered binuclear surface complexes for quartz and gibbsite, resulted in over prediction of the adsorption data at pH>3.5. However, the additive model with binuclear surface complex on quartz and mononuclear surface complex on gibbsite showed an excellent fit of the data. Phosphate greatly influenced antimony adsorption on kaolinite at both low and high antimony loadings, indicating competition for available surface sites.

The influence of particle size and structure on the sorption and oxidation behavior of birnessite: I. Adsorption of As(V) and oxidation of As(III)

Sorption and oxidation reactions in the environment may affect substantially the mobility of redox-sensitive toxic trace elements and compounds. Investigating the environmental factors that influence these reactions is crucial in understanding and predicting the geochemical fate of these environmental species, as well as to design appropriate engineered remediation schemes. Arsenic is a widespread contaminant of concern, especially in its oxidized forms, and Mn oxide minerals are some of the major contributors to its oxidation. The goal of this work was to investigate the influence of particle size and structural differences of environmentally-relevant Mn(IV) birnessites on the adsorption of As(V) and on the oxidation of As(III). An acid birnessite of 39m2/g and a δ-MnO2 of 114m2/g were used. Both birnessites sorbed a maximum Pb(II) of 0.3 Pb/Mn, indicating a significantly larger layer cationic vacancy content for acid birnessite, and a density of reactive edge sites for both of 12sites/nm2. As(V) forms a bidentate bridging complex on singly-coordinated surface sites at the birnessite particle edges regardless of loading, pH, birnessite type, and presence of pre-sorbed metals(II). Maximum As(V) adsorption, under repulsive electrostatic pH conditions did not yield adsorption congruency behavior between both birnessites at constant pH, presumably because the increase in internal vacancy content causes negative electrostatic repulsion towards external As(V) oxyanion binding.At pH 4.5 As(III) oxidation on birnessites was fast and quantitative at As/Mn ratios of 0.3–0.33, the reaction being largely driven by the proton concentration. At pH 6 δ-MnO2 oxidized As(III) faster and to a higher extent than acid birnessite, at equal masses; but the reverse at equal total surface areas. The oxidation driving force (independently from protons) was higher at pH 6 than at pH 4.5 because of Mn(II) product removal by sorption to interlayer vacancies, which overcomes reactive surface site blockage by this species, provided sufficient vacancies are present. Metals(II) pre-sorbed on birnessites always decreased the initial stages of As(III) oxidation rates as compared to the metal(II)-free systems presumably through site blockage. But after 24h the Pb(II)-equilibrated birnessites at pH 6 reached equal and sometimes higher oxidation extents through removal of As(V) from solution, by stabilizing its (binary) adsorption to edge sites through a decrease in electrostatic repulsion by the sorbed Pb(II).This work provides useful insights on the influence of particle size and structure (vacancy content) of birnessite minerals analogous to biogenic Mn oxides relevant to the environment, especially as it pertains to reactivity towards sorption of Pb(II), Zn(II), and As(V), and to oxidation of As(III), all of which are significant processes that dictate their transport and fate in aqueous geochemical environments.

Antimony sorption at gibbsite–water interface

Antimony (Sb) is extensively used in flame retardants, lead-acid batteries, solder, cable coverings, ammunition, fireworks, ceramic and porcelain glazes and semiconductors. However, the geochemical fate of antimony (Sb) remained largely unexplored. Among the different Sb species, Sb (V) is the dominant form in the soil environment in a very wide redox range. Although earlier studies have examined the fate of Sb in the presence of iron oxides such as goethite and hematite, few studies till date reported the interaction of Sb (V) with gibbsite, a common soil Al-oxide mineral. The objective of this study was to understand the sorption behavior of Sb (V) on gibbsite as a function of various solution properties such as pH, ionic strength ( I), and initial Sb concentrations, and to interpret the sorption-edge data using a surface complexation model. A batch sorption study with 20 g L −1 gibbsite was conducted using initial Sb concentrations range of 2.03–16.43 μM, pH values between 2 and 10, and ionic strengths ( I) between 0.001 and 0.1 M. The results suggest that Sb (V) sorbs strongly to the gibbsite surface, possibly via inner-sphere type mechanism with the formation of a binuclear monodentate surface complex. Weak I effect was noticed in sorption-edge data or in the isotherm data at a low surface coverage. Sorption of Sb (V) on gibbsite was highest in the pH range of 2–4, and negligible at pH 10. Our results suggest that gibbsite will likely play an important role in immobilizing Sb (V) in the soil environment.

Geochemical Landscape Analysis: Development and Application to the Risk Assessment of Acid Mine Drainage. A Case Study in Central Sweden

Acid mine drainage containing toxic contaminants is a major cause of landscape degradation at numerous historic mine sites in Europe. Risk assessment of acid mine drainage and related polluted lands requires an approach that is able to study the complexity of pollution emissions and impacted landscapes. The objective of this paper is to link geochemical contaminant fate modelling and landscape analysis for the risk assessment of acid mine drainage along the source–pathway–receptor chain. A simple geochemical landscape analysis tool is developed using landscape ecology spatial analysis and geochemical modelling methods. A case study is presented for the analysis of geochemical landscapes in central Sweden. Results show that the method can be used efficiently for the risk assessment of toxic mine contaminants in the complex wetland landscape in the study area.

Reduction of Se(IV) in Boom Clay: XAS Solid Phase Speciation

The geochemical fate of selenium is of key importance for today's society due to its role as a highly toxic essential micronutrient and as a significant component of high level radioactive waste (HLRW) originating from the operation of nuclear reactors. Understanding and prediction of the long-term behavior of Se in natural environments requires identification of the in situ speciation of selenium. This article describes an XAS-based investigation into the solid phase speciation of Se upon interaction of Se(IV) with Boom Clay, a reducing, complex sediment selected as model host rock for clay-based deep geological disposal of HLRW in Belgium and Europe. Using a combination of long-term batch sorption experiments, linear combination XANES analysis and ITFA-based EXAFS analysis allowed for the first time to identify Se0 as the dominant solid phase speciation of Se in Boom Clay systems equilibrated with Se(IV).

The role of Al-Goethites on arsenate mobility

The geochemical fates of Fe and As are so closely correlated that methods of As removal from contaminated water are in general based on the high affinity of this metalloid for Fe (hydr)oxides. Dissimilatory Fe reducing bacteria, however, play a fundamental role in catalysing the redox transformations that ultimately control the mobility of As in anoxic environments. The potential of Al-goethites in adsorbing As(V) compared with hematite, goethite, ferrihydrite, and gibbsite, and the stability of As retained by the Fe compounds under anoxic conditions were investigated in this study. The (hydr)oxides were synthesised, and adsorption isotherms and As(V) adsorption maxima at different pH were measured. Arsenic loaded samples were anaerobically incubated in the presence of Shewanella putrefaciens, and periodically sampled to evaluate the contents of soluble As and Fe. The As(V) adsorption maxima decreased in the following order: Fh > AlGt 13 > AlGt 20 > AlGt 23 > Gb > Hm > Gt. In terms of surface area, Gb, Gt, and Hm showed higher As(V) loading capacity than Fh, suggesting available reactive sites not fully occupied by arsenate on Fh. The same hypothesis can be considered for Al-goethites, as they showed even lower arsenate loading capacity per surface area. The presence of structural Al in the goethites enhanced considerably the As uptake capacity and stability under reducing conditions. Therefore, the Al-goethites showed good potential as adsorbents to remove As from water. S. putrefaciens cells were able to utilise both noncrystalline and crystalline Fe (hydr)oxides as electron acceptors, releasing As into solution. Al-goethites showed a decrease in Fe and As mobilisation as structural Al increased.

Geochemical characterization, source, and fate of methane and hydrogen sulfide at the Belmont Learning Center, Los Angeles

This article examines environmental concerns at the site of a Los Angeles, California, high school known as Belmont Learning Center (BLC) during its construction. A part of the site on which the school was to be built is a largely depleted oil field in the center of Los Angeles, close to a heavily populated area. Located nearby are another high school and an elementary school, each constructed more than 70 yr ago atop the same oil field. Prior to and during construction at the BLC site, which began in 1990, methane (CH4) and hydrogen sulfide (H2S) were detected in deep soil borings and monitoring wells. After the building construction began, a stratigraphic offset was also detected and interpreted as a fault line, which could be traced beneath one of the nearly completed buildings. This resulted in a decision to demolish that building and an adjacent one and to temporarily suspend all construction activity. Additional analytical measurements showed that the highest concentrations of both CH4 (5% atmospheric concentration) and H2S (100 ppmv) occurred only in the deeper samples, and that only traces of these environmentally hazardous gases were present at or near the soil surface. In this article, we use carbon, hydrogen, and sulfur stable isotope data, as well as 14C measurements to describe the sources and natural processes of CH4 and H2S removal. The interpretations offered in this article will hopefully be useful in assisting environmental professionals and government officials who evaluate hazardous soil gas concentrations to make decisions during the process of issuing or denying construction permits.

Free and bound polybrominated diphenyl ethers and tetrabromobisphenol A in freshwater sediments

Both free and bound fractions of polybrominated diphenyl ethers (PBDEs) and tetrabromobisphenol A (TBBPA) in sediment were examined. The free fraction was operationally defined as the fraction of each target compound extracted with a solvent mixture of hexane/acetone by Soxhlet extraction, while the bound fraction was the portion released from the sediments by alkaline hydrolysis following the solvent extractions. Tri- to hepta-BDEs and TBBPA were detected in the bound fraction. Bound PBDEs accounted for 0.2–2.8% of the total PBDEs and bound TBBPA accounted for 6.5–48% of the total TBBPA, in surface sediments. In sediment cores, these percentages ranged from 1.8% to 42% for bound PBDEs and 22–92% for bound TBBPA. The percentages of bound materials showed an increasing trend with the sediment depth. Sequestration of PBDEs and TBBPA within sediments might be an important geochemical fate of these brominated flame retardants.

Exploring Amino Acid Side Chain Decomposition Using Enzymatic Digestion and HPLC-MS: Combined Lysine Transformations in Chlorinated Waters

Characterizing the transformations of polypeptides is important across a broad range of scientific disciplines. As polypeptides are an important constituent of dissolved organic matter within seawater and freshwater, it is important to understand their (bio)geochemical fate. Oxidants, formed in blood as part of the immunological response or applied to waters for disinfection, react with polypeptides to form transformation products that may exert toxicity. An analytical method was developed to characterize and quantify modifications to the side chains of amino acid residues within polypeptides. Enzymatic digestion of polypeptides using Pronase E, a protease cocktail, proved preferable to common strong acid digestion techniques, because the circumneutral pH conditions employed during enzymatic digestion prevent artifacts arising from extreme pH conditions. Lysine nitrile, one of the predicted transformation products of lysine residues within polypeptides, was destroyed during strong acid digestion but not enzymatic digestion. Due to the potential variability in enzymatic digestion efficiencies, the liberation of a mass-labeled leucine monomer from an octapeptide spiked standard was employed as a measure of complete digestion efficiency for each sample and enabled quantification of modified amino acid residues within polypeptides. A multivariate statistical analysis was conducted to evaluate the influence on digestion efficiency of Pronase E loadings, salinity, natural organic matter concentration, and pH across the range of conditions relevant to blood, seawater, and concentrated freshwaters and disinfected drinking/recreational waters. At Pronase E loadings of 10 mg, the analysis indicated that digestion efficiencies ranged from 25 to 55% over the range of conditions expected for typical drinking waters concentrated from 1 L to 10 mL. The analytical method was applied to triplicate 1 L samples of a chlorinated tap water and a chlorinated indoor pool water. For the tap water, the digestion efficiency was 47.2% (+/-11.1% relative standard deviation), and the lysine nitrile concentration was 104.6 ng/L (+/-6.8 ng/L standard deviation). For the pool water, the digestion efficiency was 23% (+/-15% relative standard deviation); although the occurrence of lysine nitrile was verified, matrix effects must be overcome for quantification.

34S tracer study of pollutant sulfate behaviour in a lowland peatland

Field experiments were carried out in order to assess the practicality and application of 34SO42− as a tracer of the physical and geochemical fate of aerially derived sulfur in peat. Six enclosures were isolated in a lowland peat with high historical acid sulfate inputs at Thorne Moors, UK, and treated with regular additions of 99.9% pure 34SO42− for 12 months. The total 34S sulfate addition resulted in negligible change to the sulfate concentration, but allowed unequivocal change to the isotopic composition of sulfate inputs relative to pre-experiment control data set. Migration and biogeochemical transformations of the 34S tracer were monitored via depth-specific sampling of surface and pore-waters every 6 weeks, and sacrificial sampling of solid peat at 12, 24, and 48 week intervals. Tracer incorporation into the various sulfur forms within the surface and pore-waters, vegetation, organic and inorganic fractions of the peat was apparent through strong positive deviation of δ34S from natural values (in comparison with 18 months control data set for the same site). Consistency within enclosures is good and a detailed model of sulfur cycling within each enclosure can be established but natural variability in the control data and differences between replicate enclosures prevents more quantitative assessment. The 34S tracer was initially rapidly removed from surface waters. The majority of uptake was by living vegetation (5.7–33% of tracer added, mean 17.6%), or through transformation to the organic fraction of the upper peat (25 cm) after rapid bacterial reduction of sulfate to sulfide. Despite penetration of 34S labelled sulfate to deeper pore-waters over time, there was no significant reduction to sulfide or subsequent incorporation into organic or inorganic fraction at these depths (>25 cm); organic and inorganic sulfur, and pore-water sulfide retained their initial unlabelled isotopic compositions. This limitation on sulfur cycling at relatively shallow depth may be attributed to diminished labile organic matter inhibiting the activity of sulfate reducing bacteria or poisoning of sulfate reducers by high dissolved sulfide, after long-term sulfur pollution of this ecosystem.

Crop Effects on Closed System Element Cycling for Human Life Support in Space

ABSTRACT Nutrient recycling in a space-based Bioregenerative Life Support System (BLSS) will require an understanding of nutrient dosage effects on crop production, plant tissue partitioning, and geochemical fates within crop systems. Sodium (Na+), fluoride (F−), and iodide (I−) are found in human waste streams. These elements were examined using crops in hydroponic systems. Lettuce, radish, spinach, and beet were used to study Na+ uptake and tolerance. Spinach, lettuce, and radish growth were inhibited at 8.0 × 10−2 M Na+ compared to the control. Beet growth improved at 2.0 and 4.0 × 10−2 M Na+ compared to the control. Rice plants were used to study F− and I− uptake and tolerance. Rice growth was inhibited at 5.0 × 10−4 M F− and at 5.0 × 10−6 M I−. Solution redox and sorption reactions were predicted with the aid of a chemical equilibrium model. A simulation model was used to predict element fates.

Fate of Arsenic in Swine Waste from Concentrated Animal Feeding Operations

Swine diets are often supplemented by organoarsenicals, such as 3-nitro-4-hydroxyphenylarsonic acid (roxarsone) to treat animal diseases and promote growth. Recent work reported roxarsone degradation under anaerobic conditions in poultry litter, but no such data exist for swine wastes typically stored in lagoons nearby concentrated animal feeding operations (CAFOs). The objectives of this study were to: (i) characterize a suite of swine wastes collected from 19 randomly selected CAFOs for soluble arsenate [As(V)], arsenite [As(III)], dimethylarsenic acid (DMA), monomethylarsonic acid (MMA), 3-amino-4-hydroxyphenylarsonic acid (3-HPPA), p-arsanilic acid, and roxarsone, and (ii) determine the geochemical fate of roxarsone in storage lagoons nearby CAFOs. Swine waste suspensions were spiked with roxarsone and incubated under dark/light and aerobic/anaerobic conditions to monitor roxarsone degradation kinetics. Arsenic speciation analysis using liquid chromatography and inductively coupled plasma mass spectrometry (LC-ICPMS) illustrated the prevalence of As(V) in swine waste suspensions. Roxarsone underwent degradation to either organoarsenicals (3-HPPA) or As(V) and a number of unidentified metabolites. Roxarsone degradation occurred under anaerobic conditions for suspensions low in solids content, but suspensions higher in solids content facilitated roxarsone degradation under both anaerobic and aerobic conditions. Increased solids content enhanced roxarsone degradation kinetics under aerobic conditions. According to current waste storage and sampling practices, arsenic in swine wastes stored in lagoons has been overlooked as a possible environmental health issue.

Sequestration of Nonylphenol in Sediment from Bohai Bay, North China

The ubiquity of nonylphenol (NP) in aquatic environments has been well documented, and the long-term fate of NP in sediments is a concern from the viewpoint of risk assessment due to its endocrine-disrupting effects. This paper reports on the assessment of long-term fate of NP in marine sediments by determining extractable and nonextractable fractions of NP in surface sediments and a sediment core from Bohai Bay, North China. The extractable fraction was operationally defined as the fraction of NP that was extracted with a solvent mixture of methanol/methylene chloride, and the nonextractable fraction was the portion of NP that can be released from the sediments by alkaline hydrolyzing after the initial solvent extraction. The total concentrations (extractable and nonextractable) of NP were 3.4-34.3 ng/g dw in the surface sediments and 2.2-17.7 ng/g dw in the sediment core. Depending on the sedimentation time, the percentage of nonextractable NP relative to the total NP in the core ranged from 38 to 99%. Based on the relationship between the percentage of nonextractable NP in sediments and sedimentation time, the sequestration rate of NP in the sediment core from Bohai Bay was estimated to be 0.94% x a(-1). These results have important implications in understanding the geochemical fate of NP in sediments.

Modelling the geochemical fate and transport of wastewater-derived phosphorus in contrasting groundwater systems

A 1D reactive transport model (RTM) is used to obtain a mechanistic understanding of the fate of phosphorus (P) in the saturated zone of two contrasting aquifer systems. We use the field data from two oxic, electron donor-poor, wastewater-impacted, sandy Canadian aquifers, (Cambridge and Muskoka sites) as an example of a calcareous and non-calcareous groundwater system, respectively, to validate our reaction network. After approximately 10 years of wastewater infiltration, P is effectively attenuated within the first 10 m downgradient of the source mainly through fast sorption onto calcite and Fe oxides. Slow, kinetic sorption contributes further to P removal, while precipitation of phosphate minerals (strengite, hydroxyapatite) is quantitatively unimportant in the saturated zone. Nitrogen (N) dynamics are also considered, but nitrate behaves essentially as a conservative tracer in both systems. The model-predicted advancement of the P plume upon continued wastewater discharge at the calcareous site is in line with field observations. Model results suggest that, upon removal of the wastewater source, the P plume at both sites will persist for at least 20 years, owing to desorption of P from aquifer solids and the slow rate of P mineral precipitation. Sensitivity analyses for the non-calcareous scenario (Muskoka) illustrate the importance of the sorption capacity of the aquifer solids for P in modulating groundwater N:P ratios in oxic groundwater. The model simulations predict the breakthrough of groundwater with high P concentrations and low N:P ratios after 17 years at 20 m from the source for an aquifer with low sorption capacity (< 0.02% w/w Fe(OH) 3). In this type of system, denitrification plays a minor role in lowering the N:P ratios because it is limited by the availability of labile dissolved organic matter.

Changes in Copper Speciation and Geochemical Fate in Freshwaters Following Sewage Discharges

The main factors determining the geochemical fate of copper in urban freshwaters affected by raw sewage discharges were investigated in this work. Water samples from the Irai and Iguacu rivers were collected monthly during a 1-year period at two points located upstream and downstream from the city of Curitiba, in Brazil. Results revealed that raw sewage discharges from the heavily urbanized area caused an enhancement of humic-coated suspended solids in the Iguacu River. In these waters copper is predominantly associated with the humic-coated particles whereas in the Irai River copper was found primarily in the aqueous phase. The transfer of copper from the aqueous to the solid phase changed its physical speciation along the watercourse. Thus, aspects related to the overall transport of trace metals in watercourses become an important issue to be considered in further studies concerning the effect of sewage discharges on the geochemical speciation and fate of trace metals in urban rivers.

Influence of pH and ionic strength on removal processes of a sedimentary humic acid in a suspension of vermiculite

Understanding the interactions of humic acids and clay minerals is important for modeling the geochemical fate and transport of nutrients and pollutants in soil and natural waters upon changes of environmental parameters such as ionic strength and pH. This paper describes the adsorption/precipitation properties of a sedimentary humic acid under different conditions of pH and ionic strength in presence of the clay mineral vermiculite. The clay suspension was prepared to have a distribution of grain size composed mainly of particles <63 μm, which is believed to play a major role on transport and fate of adsorbed materials in aquatic environments. The pHs studied were 5.0, 6.0 and 7.0 (±0.1 pH units), in ionic medium of 0.002 and 0.020 mol l−1 KNO3, using initial HA concentrations from 0 to 100 mg l−1. The maximum removal of HA from solution occurred at pH 5.0 in medium of 0.020 mol l−1 KNO3. This removal is mostly due to precipitation, which occurred at similar rates in both ionic strength, so that the additional HA removal at higher ionic strength (0.020 mol l−1 KNO3) was attributed to adsorption. Adsorption processes intensified at higher ionic strength suggest the occurrence of chemical interactions between the interfaces since under these conditions the electrostatic interactions are minimized. A similar effect was observed at pH 6.0, but in this case adsorption prevailed as the removal mechanism in medium of 0.020 mol l−1 KNO3. At pH 7, adsorption is the predominant process removing HA from solution, with increase of adsorption upon decreasing the ionic strength, suggesting that electrostatic attractions play the major role in the adsorption mechanism under these conditions. Comparison of the results obtained using a re-precipitated commercial HA with low N content suggested that the presence of positively charged amine groups increase the adsorption and decrease desorption.

Zinc speciation in a smelter-contaminated soil profile using bulk and microspectroscopic techniques.

A soil profile contaminated as a result of Zn smelting operations from the historic Palmerton, PA smelting facility was characterized using X-ray absorption fine structure spectroscopy (XAFS) and X-ray diffraction (XRD) as bulk techniques, coupled with electron microprobe (EM), and microfocused XAFS as microscopic techniques to determine the chemical forms of Zn and elucidate its geochemical fate. The black, organic matter-rich topsoil contained 6200 mg/ kg Zn and was strongly acidic (pH 3.2). Bulk XAFS revealed that about 2/3 of Zn was bound in franklinite and 1/3 bound in sphalerite. Both minerals may have been aerially deposited from the smelter operation. Microspectroscopy detected also minor amounts of Zn adsorbed to Fe and Mn (hydr)oxides as inner-sphere sorption complexes, which may have formed after weathering of the Zn minerals. About 10% of the total Zn in this sample could be easily leached. In contrast, the yellowish, loamy subsoil contained less Zn (890 mg/kg) and had a higher pH of 3.9. XAFS revealed that Zn was mostly bound to Al-groups and to a lesser extent to Fe and Mn (hydr)oxides. Minor amounts of outer-sphere complexes or organic matter-bound Zn species could also be detected. About 70% of the total Zn content could be easily leached, indicating that outersphere sorption complexes have been underestimated and/ or inner-sphere sorption complexes are weak due to the low pH. The Zn forms in the subsoil most likely derive from weathering of the Zn minerals in the topsoil. Due to the lack of minerals incorporating Zn and due to the low pH, the availability of Zn in the subsoil is as high as in the topsoil, while the total concentration is almost 1 order of magnitude smaller.

Free and Bound Benzotriazoles in Marine and Freshwater Sediments

To investigate how some anthropogenic compounds are sequestered in sediments, the authors examined the free and bound fractions of six different substituted benzotriazoles (BZTs) in sediment cores from the Pawtuxet River and Narragansett Bay. The free fraction was operationally defined as the fraction of BZTs that was removed with several organic solvent extractions, and the bound fraction was that portion of BZTs removed by solvent extraction after saponifying the sediment residue remaining from the initial solvent extractions. The total concentrations (free and bound) of BZTs were as large as 10 mg g{sup {minus}1} in the riverine core and 0.05 mg g{sup {minus}1} in the estuarine core. The percent bound of the BZTs ranged from 0 to 9% of the total and varied with each compound, sediment depth, and location. BZTs that had alkyl substitution at the 3{prime} position were less likely to be found in the bound fraction than compounds that did not have this substitution. On the basis of these results, it appears that these compounds may be chemically associated with the sediments, and this association is hindered by alkyl substituents on some of the BZTs. These results have important implications in understanding the bioavailability and geochemical fate ofmore » organic contaminants in sediments as well as the acid reactions of sedimentary organic matter.« less

Trace Metal Cycling in the Surface Waters of Lake Erie: Linking Ecological and Geochemical Fates

The highly productive plankton that comprise the microbial food web are ideally suited for trace metal scavenging, the removal of particle-reactive trace metals from the aqueous phase. Scavenging is considered to be the key factor controlling the concentrations of trace metals in the surface waters of large lakes during thermal stratification. Observed characteristics of the various plankton size fractions in the microbial food web (ability to scavenge trace metals from the dissolved phase; potential to regenerate these metals back into the dissolved phase; population dynamics) were incorporated into a dynamic microbial food web model of trace metal cycling in the surface waters of Lake Erie. The model yields estimates of epilimnetic trace metal residence times under the assumption of steady-state conditions: Cs = 352 d, Cd = 19 d, Zn = 23 d, Gd = 45 d. These trace metal residence times were considerably greater than the residence times predicted when microzooplankton grazing activity was eliminated from the model simulations (Cs, +46%; Cd, +58%; Zn, +53%; Gd, +80%). The increase of residence time by microzooplankton grazing is attributed to the trace metal regeneration that results from incomplete assimilation by the grazer of metal previously scavenged by the prey item. The results illustrate the important influence of the microbial food web activities on the geochemical fates of trace metals in the pelagic surface waters of large lakes during thermal stratification.

Scavenging of 137Cs, 109Cd, 65Zn, and 153Gd by Plankton of the Microbial Food Web in Pelagic Lake Erie Surface Waters

Inorganic dissolved 137Cs, 109Cd, 65Zn and 153Gd were added, at concentrations well below their respective solubility limits, to screened water (< 210 μm) sampled from the pelagic epilimnion of Lake Erie during the summers of 1994 and 1995. The hypotheses tested were that scavenging occurs in all of the ecologically significant size fractions that comprise the microbial food web (picoplankton, 0.2 to 2 μm; nanoplankton, 2 to 20 μm; microplankton, 20 to 210 μm), and that scavenging by plankton is directly related to the respective particle-reactivity of the elements (Gd > Zn > Cd » Cs). Size-selective filtration at intervals over periods of 22 to 30 h established that picoplankton and nanoplankton were the dominant scavenging phases in this environment. Scavenging of 153Gd, 65Zn, and 109Cd by plankton was more similar than predicted on the basis of their anticipated particle-reactivity and 137Cs was weakly scavenged. Except for the picoplankton, 65Zn was the element most readily scavenged by the plankton size fractions. High accumulation of 109Cd in the picoplankton may reflect the sorption of this element by calcite associated with autotrophic picoplankton. These experiments quantify the partitioning of trace metals within the plankton of the microbial food web (0.2 to 210 μm), a dynamic community of particles that dominates the seston in the pelagic surface waters of Lake Erie during thermal stratification, and suggest that plankton dynamics should be considered in predictions of the geochemical fate of trace metals in this environment.