H32C-01 10:20h
Geochemical and Hydrogeological Controls on Arsenic in Sand-and-Gravel Aquifers in Central Illinois, USA
A total of 176 wells in sand-and-gravel aquifers in central Illinois were sampled for arsenic and other chemical parameters. These results were combined with archived and published data from several hundred well samples to characterize the spatial distribution of arsenic and determine the potential geochemical controls on its solubility and mobility. There was considerable spatial variability in the arsenic concentrations, and arsenic was not correlated with aquifer depth. There did not appear to be any arsenic "plumes" at the scale of the sampling. Arsenic solubility appeared to be controlled by oxidation-reduction conditions, especially the presence of organic matter. Geochemical conditions in the aquifers are typically reducing, but only in the most reducing waters does arsenic accumulate in solution. In wells in which sulfate was present, arsenic concentrations were low or below the detection limit (0.5 g/L). Elevated arsenic concentrations were only found in wells where sulfate was absent or at low concentrations, indicating post-sulfate-reducing conditions. Methane was detected and organic carbon and ammonium concentrations were generally elevated in these wells. Iron is common in the aquifer sediments, and iron reduction appears to be occurring throughout the aquifers. Arsenic is likely released from the solid phase as iron oxide is reduced.
H32C-02 10:35h
Controls on Arsenic Concentrations in Ground Water Near Lake Geneva, Wisconsin
Approximately 15% of wells open to Quaternary glacial and shallow bedrock aquifers near the town of Lake Geneva in southeastern Wisconsin have As concentrations above the U.S. Environmental Protection Agency standard of 10 ug/l. Comparison of groundwater As concentrations with well construction records indicates that As concentrations are highest in wells open to the deep Quaternary and shallow Silurian sediments; wells open to a shallow Quaternary aquifer are not significantly impacted by As. Geochemical analysis of core samples from a single borehole near Lake Geneva shows solid-phase As concentrations in the 2-20 mg/kg range throughout the entire thickness (310 ft) of Quaternary sediments. Direct identification of mineralogic As sources in such sediments is difficult; isolation of As-rich phases is problematic due to the heterogeneous mineralogy and relatively low As concentrations. X-ray diffraction is of limited use for identifying As sources in these core samples because As-bearing minerals such as (hydr)oxides are typically poorly crystalline and present at weight percents below the detection limit. More sophisticated spectroscopic techniques such as, infrared, Raman, and X-ray adsorption spectroscopy, have the potential to provide more detailed information about the solid-phase associations of As in aquifer sediments. However, these techniques require access to expensive instrumentation and analyses of heterogeneous natural samples can be difficult to interpret. As an alternative to direct identification of As sources, we used a variety of indirect methods to develop and test hypotheses regarding As sources and processes controlling As mobility in these heterogeneous aquifer sediments. Groundwater sampling, chemical extractions, batch experiments, aquifer testing, and flow modelling provided evidence about the controls on As concentrations in groundwater in the study area. The results of these techniques suggest that geologic, hydrogeologic, and geochemical factors combine to create reducing conditions in the deep Quaternary and shallow Silurian sediments where As is mobilized via reductive dissolution of (hydr)oxide minerals.
H32C-03 10:50h
Arsenic Mobilization Through Microbial Bioreduction of Ferrihydrite Nanoparticles
Under anaerobic conditions Fe(III)-reducing microorganisms can couple the reduction of solid phase Fe(III) (hydr)oxides with the oxidation of organic carbon. Nutrients and trace metals, such as arsenic, associated with Fe(III) hydroxides may be mobilized through microbially-mediated surface reduction. Although arsenic mobilization has been attributed to mineral surface reduction in a variety of pristine and contaminated environments, minimal information exists on the mechanisms causing this arsenic mobilization. Understanding of the fundamental biochemical and physicochemical processes involved in these mobilization mechanisms is still limited, and has been complicated by the often contradictory and interchangeable terminology used in the literature to describe them. We studied arsenic mobilization mechanisms using a series of controlled microcosm experiments containing aggregated arsenic-bearing ferrihydrite nanoparticles and an Fe(III)-reducing microorganism, Geobacter metallireducens. The phase distribution of iron and arsenic was determined through filtration and ultracentrifugation techniques. Experimental results showed that in the biotic trials, approximately 10 percent of the Fe(III) was reduced to Fe(II) by microbial activity, which remained associated with ferrihydrite surfaces. Biotic activity resulted in changes in nanoparticle surface potential and caused deflocculation of nanoparticle aggregates. Deflocculated nanoparticles were able to pass through a 0.2 micron filter and could only be removed from solution by ultracentrifugation. Arsenic mobilized over time in the biotic trials was found to be exclusively associated with the nanoparticles; 98 percent of arsenic that passed through a 0.2 micron filter was removed from solution by ultracentrifugation. None of these changes were observed in abiotic controls. Because arsenic contamination of natural waters due to mobilization from mineral surfaces is a significant route of human arsenic exposure worldwide, improved understanding of the biologically-mediated mechanisms that partition arsenic between solid and solution phases is required for development of effective treatment and remediation strategies.
H32C-04 11:05h
The Effect of Reductive Mineral Dissolution and Porewater Chemistry on Arsenic Mobilization
The mobilization of As from sediments into porewater has been observed at Haiwee Reservoir (Olancha, CA, USA), where a large-scale treatment of drinking water supply to Los Angeles has resulted in an accumulation of iron- and arsenic-rich sediments. Arsenic is associated with amorphous iron oxide phases and is partially mobilized into the porewater at depth. In order to study the mechanisms of arsenic mobilization {\it in situ}, a gel probe has been constructed based on previous work. Porewater samples are collected using polyacrylamide gel slabs held in a Plexiglas ladder probe. The probe is inserted into the sediments and allowed to equilibrate with the porewater. Upon removal, the gels are re-equilibrated in acid, and As, P, Mn, and Fe are measured. Several probes were deployed in the field in October 2003 and cores were collected at the same time and locations for sequential extraction. Substantial variability was observed among the five sets of probes and cores. However, there were some general observations common to all sample sets. Dissolved concentrations of As, Fe, Mn, and P were found to increase at depth, with a strong correlation between As and Fe. Although Mn and P were also mobilized into the porewater, their concentrations with depth did not follow the same pattern as those of As. In the solid phases, As and Fe concentrations were strongly correlated and 55-95% of the As was released during extraction with 1M NaH$_{2}$PO$_{4}$, suggesting that As is primarily associated with the solid phase by adsorption. The amount of As, Fe, and Mn in the sediments decrease with depth, in contrast to their concentrations in porewaters, which increase with depth. These results are indicative of reductive dissolution of iron oxides in the sediment. It is likely that porewater composition influences the fate of As released into the porewater by reductive dissolution of Fe oxides. These effects are examined using modified gel probes in which hydrous ferric oxide (HFO), an amorphous Fe oxyhyroxide, is embedded in the gel slabs. By loading the probe with both clear and HFO gels, it is possible to simultaneously measure porewater composition and sorption behavior. Preliminary studies indicate that 24 hours is necessary to reach equilibrium with the sediment porewater when HFO gels are present, possibly due to sediment re-supply and/or sorption kinetics. Based on laboratory sediment microcosms, porewater composition may limit arsenic resorption onto fresh sorption sites.
H32C-05 11:20h
Removal of Arsenic(III) from Groundwater with Nano Scale Zero-Valent Iron
Arsenite (As(III)) is a highly toxic, soluble species that is a naturally occurring groundwater contaminant of environmental concern. There is a need for detailed information about the natural geochemical cycling of As(III), including the fundamental chemical mechanisms of the reactions of As(III) with a variety of surfaces, both natural and engineered. In this paper we focus on the development of ultra-fine, synthetic nanoscale zero-valent iron (nano-Fe(0)) material as both a potential candidate for As(III) remediation and a high surface area model compound to study the remediation of groundwater containing As(III) with larger Fe(0) particles. A variety of techniques were used including SEM, AFM, XRD, and X-ray absorption spectroscopy (XAS) to characterize particle size, surface morphology, corrosion layers formed, and As(III)-nano-Fe(0) surface complexation chemistry. Results from AFM showed particle size ranged from 1-120 nm. XRD and SEM results revealed that nano-Fe(0) gradually converted to magnetite/maghemite corrosion products mixed with lepidocrocite over 60 d. Arsenic(III) batch adsorption kinetics were rapid following a pseudo-first-order rate expression with observed reaction rate constants (kobs) of up to 1.3 per min (at varying Fe(0) densities). These values are about 1000 times higher than kobs literature values for As(III) adsorption on micron size Fe(0). Results from laser light scattering (electrophoretic mobility) and XAS confirmed that inner-sphere surface complexation occurred on nano-Fe(0) corrosion products. In addition, oxidation of As(III) to As(V) was evident in batch experiments. Addition of 10 mM anions (bicarbonate, sulfate, nitrate, and arsenate) had no effect on the uptake of As(III) whereas 10 mM silicic acid and phosphate reduced the uptake of As(III) from 99.9% to 44.9 and 66.3%, respectively. Our results suggest that nano-Fe(0) is an appropriate material for further investigation of the feasibility of using Fe(0) for As(III) remediation.
H32C-06 11:35h
Oxidation Kinetics of Arsenic(III) by Aquifer Material
Laboratory experiments were conducted to study the kinetics of As(III) oxidation by aquifer material collected from the USGS research site on Cape Cod, Massachusetts, USA. Aquifer material consisted of coarse sand and gravel but gravel-size material (greater than 2 mm) with greater than 90 percent quartz. The chemical properties were, however, controlled by coatings on grain surfaces dominated by iron and aluminum oxides and silicates. Five different solid samples with similar specific surface areas (0.6 to 0.9 m2/g) and reductively extractable Fe contents (18 to 26 umoles/m2), but with varying total Mn contents (0.5 to 3.5 umol/m2) were used. Both dissolved and surface-bound As(III) and As(V) concentrations were measured with time up to 250 hr. The As(III) oxidation rate correlated with the solid Mn content. The rate of oxidation increased with increasing As(III) initial concentration and increasing solid Mn content, and decreased somewhat with increasing pH (4 to 7). Under all conditions, dissolved As(V) concentrations were very low. A mathematical model was developed to simulate the kinetics and extent of arsenic speciation and transformation by aquifer material. The model included rate-limited adsorption of As(III) onto both oxidative and non-oxidative sites, rate-limited oxidation of As(III), and equilibrium adsorption of As(V). Rate constants for As(III) adsorption and oxidation, and equilibrium constants for As(V) adsorption were the same for all sediments samples. The observed As(III) oxidation rate here is consistent with previous observations of As(III) oxidation over short transport distances during field-scale transport experiments. The model developed here may be incorporated into groundwater transport models to predict As speciation and transport in chemically heterogeneous systems.
H32C-07 11:50h
Predicting Arsenate Adsorption by Soils Using Soil Chemical Parameters in the Constant Capacitance Model
Prediction of arsenate, As(V), adsorption and transport in soils requires detailed studies of As(V) adsorption and subsequent determination of model parameters. Arsenate adsorption on 49 soil samples belonging to six different soil orders was investigated as a function of solution pH (3-10). The set of soils consisted of two subgroups: one from the Midwestern U.S. and one primarily from the southwestern U.S. For most soils, As(V) adsorption increased with increasing solution pH, reached a maximum around pH 6-7, and decreased with further increases in solution pH. The constant capacitance model, a chemical surface complexation model, was well able to describe As(V) adsorption on the soil samples as a function of solution pH by simultaneously optimizing three As(V) surface complexation constants. The ability to describe As(V) adsorption as a function of pH represents an advancement over the Langmuir and Freundlich adsorption isotherm approaches. A general regression model was developed for predicting soil As(V) surface complexation constants from easily measured soil chemical characteristics using the As(V) adsorption data for 44 of the soils. These chemical properties were: cation exchange capacity (CEC), surface area (SA), inorganic carbon content (IOC), organic carbon content (OC), and iron oxide content (Fe). A preliminary analysis determined that the mean surface complexation constant values for the two soil subgroups were statistically different. For this reason, while the regression model equations for each soil subgroup contained common intercepts and ln(CEC) terms, the ln(IOC), ln(OC), ln(Fe), and ln(SA) terms were different. The constant capacitance model was able to predict As(V) adsorption on most of the 44 soils using the As(V) surface complexation constants predicted from the regression equations. The prediction equations were used to obtain values for As(V) surface complexation constants for the remaining five soils that had not been used to obtain the general regression model. This provided a completely independent evaluation of the ability of the constant capacitance model to describe As(V) adsorption. The model was able to accurately predict As(V) adsorption on three soils, qualitatively predict As(V) adsorption on one soil, and unable to prediction As(V) adsorption on one soil. Incorporation of these regression prediction equations into chemical speciation-transport models will allow simulation of soil solution As(V) concentrations under diverse environmental and agricultural management conditions without requiring soil specific adsorption data and subsequent parameter optimization.
H32C-08 12:05h
Kinetic Modeling of Arsenic Cycling by a Freshwater Cyanobacterium as Influenced by N:P Ratios: A Potential Biologic Control in an Iron-Limited Drainage Basin
Elevated As levels are common in South Texas surface waters, where As is derived from the natural weathering of geogenic sources and a byproduct of historical uranium mining. The impacted surface waters of the Nueces River drainage basin supply Lake Corpus Christi (LCC), a major drinking water reservoir for the Corpus Christi area. The soils and sediments of the Nueces River drainage basin generally have low levels of reactive iron (average concentration of 2780 mg/kg), limiting the control of iron oxyhydroxides on As geochemistry and bioavailability. Given these conditions, biologic cycling of As may have a large influence on As fate and transport in LCC. Sediment cores from LCC show evidence for cyanobacterial blooms after reservoir formation based upon stable isotopes, total organic matter and specific elemental correlations. While algae have been shown to accumulate and reduce inorganic As(V), few studies have reported biologic cycling of As by cyanobacteria. Therefore, As(V) uptake, accumulation, reduction, and excretion in a 1.0 $\mu$M As(V) solution by the freshwater cyanobacterium, {\it Anabaena sp.} Strain PCC 7120, was measured over time as a function of low, middle and high N:P ratios (1.2, 12, 120) to determine nutrient effects on As cycling by the cyanobacterium. Total As(V) reduction was observed in all three conditions upon completion of the ten-day experiment. Maximum As(V) reduction rates ranged from (0.013 mmol g C$^{-1}$ day$^{-1}$) in the low N:P solution to (0.398 mmol g C$^{-1}$ day$^{-1}$) in the high N:P solution. Increased cell biomass in the low N:P ratio solution compensated for the low maximum reduction rate to allow total As(V) reduction. Kinetic equations commonly used to model algal-nutrient interactions were utilized in modeling the current data. The Michaelis-Menten enzyme saturation equation modified with a competitive inhibition term adequately modeled As(III) excretion in the high and middle N:P ratio test conditions. The low N:P test condition further required a growth term to adequately model As(III) excretion by the cyanobacterium. The impact of N:P ratios on As reduction rates implies that N:P cycling can be coupled to As biogeochemistry in surface waters through the action of phytoplankton.