Biogeochemistry of Metals and Arsenic in Environmental Systems I
Presiding: K Makris, University of Texas at San Antonio; S Sharma, University of Texas at San Antonio
B31A-01 08:30h
Arsenite Sorption by Drinking-Water Treatment Residuals: Redox Effects
Arsenic (As) is a major human carcinogen and could pose a serious human health risk at concentrations as low as 50 ppb in drinking water. Elevated As concentrations in soils currently used for residential purposes (located on former agricultural lands amended with arsenical pesticides) have increased the possibility of human contact with soil-As. Studies have shown that As bioavailability in the environment is primarily a function of its chemical speciation, which depends upon the redox potential. Arsenic toxicity and carcinogenicity to living organisms is primarily due to exposure to the reduced species of As - arsenite, i.e., As(III), rather than the oxidized species - arsenate, i.e., As(V); the mobility of As(III) is much higher than As(V). One of the most promising methods to decrease the mobility of arsenite in the soil-water system is promoting its retention onto amorphous Fe/Al hydroxides. Drinking-Water Treatment Residuals (WTRs) are an inexpensive source of such Fe/Al hydroxides, which can be land-applied following the USEPA-regulated biosolids application rules. The WTRs are byproducts of drinking-water purification processes and generally contain sediment, organic carbon, and Al/Fe hydroxides. The hydroxides are typically amorphous and have tremendous affinity for oxyanions (e.g., arsenate). Preliminary work showed that WTRs are characterized by large internal surface area and porosity that partly explains their high affinity for As(V). The current study examines the potential of two WTRs (Fe-based and Al-based) to adsorb arsenite from solution. We hypothesize that As(III) adsorption onto the Fe-based WTR (whose stability is highly redox-sensitive) would be vastly different from the adsorption of As(III) onto the redox-insensitive Al-based WTR. Our main objective is to characterize As(III) sorption by both Fe- and Al-based WTRs by changing critical factors, such as the solid:solution ratio, contact time, and initial As(III) load. Results from this study are expected to identify the optimal conditions for As(III) sorption onto WTRs as a function of solution pH and redox potential. Potential desorption of the retained As will be assessed in batch studies using phosphate as the competing ligand.
B31A-02 08:45h
Bioavailability Of Arsenic In Arsenical Pesticide-Amended Soils: Preliminary Greenhouse Study
Long-term application of arsenical pesticides in agricultural lands has resulted in high levels of arsenic (As). Conversion of former agricultural lands to residential areas has resulted in increased human contact with soil As. Soil ingestion from incidental hand-to-mouth activity by children is now a very important issue in assessing human health risk associated with exposure to arsenical pesticide-applied former agricultural soils. Human health risk from direct exposure to soil As via hand to mouth action is restricted only to those fractions of As in the soil that are available to the human gastrointestinal system. Thus this study aimed at addressing the issue of soil variability on As bioavailability as a function of soil physiochemical properties in a dynamic interaction between soils, water and plants and pesticides. In the current greenhouse study two soils with drastically different chemical characteristics w.r.t As reactivity (Immokalee-low As retention potential and Millhopper-high As retention potential) and one pesticide (sodium arsenate) were used. Soils were amended with sodium arsenate at two rates representing the high and low ends of As contamination, generally representative of Superfunds site conditions: 675 and 1500 mg/kg As. Rice (Oryza sativa) was used as the test crop. Sequential digestion to estimate in-vitro As in the stomach phase and the intestinal phase was employed on soils sampled at 4 times: 0-time, after 3 mo, 6 mo and 9 mo of soil-pesticide equilibration. In-vitro bioavailability experiments were also performed with the same soils in order to obtain an estimate of the amount of As that would be absorbed to the intestinal linings in simulated systems. Following the greenhouse study, selective in-vivo bioavailability studies using As-contaminated soils will be conducted on male and female mice to correlate in-vitro results with the in-vivo data. Treatments will consist of a soil group (As in soil), a positive control group (only As) and a negative control group (no soil, no As). Results from the in-vitro and in-vivo studies will help understand the effects of soil properties on As bioavailability. Keywords: Bioavailability, pesticide, soil, arsenic, greenhouse.
B31A-03 09:00h
Competitive Adsorption-Desorption Kinetics of Arsenate and Phosphate in Soils
Phosphate competition on arsenate sorption can increase the mobility of toxic arsenic in soils and aquifers, leading to possible contamination of groundwater. Rate-limited adsorption-desorption of arsenate [As(V)] and phosphate (P) as well as their complex interaction makes the prediction of the fate and transport of arsenic in soil and water environment rather complex. In this study, kinetics of competitive adsorption-desorption of arsenate and phosphate was investigated in batch systems by introducing mixed As(V) and P solution at different concentration ratios to three soils having different properties, sampling at different reaction times, following by successive desorptions. Our results indicated that As(V) and P adsorption on soils were highly time-dependent, with decreased sorption rate with increasing reaction time. Rates and amounts of As(V) adsorption on soils were significantly reduced as increasing P concentrations presented. The three soils tested in our experiments exhibited different sorption affinity to As(V) and P, suggesting that specific sorption for As(V) or P by soils might play a significant role in their interaction.
B31A-04 09:15h
Investigating the Biogeochemical Response of The Benthic Boundary Layer to Erosion and Deposition Events Using a 1-D Coupled Sediment Entrainment-Biogeochemistry Model
The entrainment of cohesive sediments involves coupled sediment-bottom boundary layer processes forced by waves, currents and complex physical, biological, and chemical processes (e.g. consolidation, bioturbation/bioirrigation, geochemistry) that occur within the seafloor sediments. Transition metal contaminants are significantly affected by the redox environment of the surrounding sediments. Therefore, an ability to predict their transport and chemical alteration is necessary for environmental remediation. This study uses a system of coupled models to simulate the 1-D vertical entrainment of heterogeneous coastal sediments and the biogeochemistry of the benthic boundary layer sediments as an initial attempt to create such a predictive tool. An ideal case, forced only by tides and waves, is examined in order to understand the sensitivities of the sediment profiles to the wave-tide forcing and the bottom sediment biogeochemistry to erosion/deposition events generated by the sediment entrainment. In particular, the spatial and temporal changes of the redox species NH4 and SO4 in the pore water of the bottom sediments are analyzed. In addition, the sensitivity of the suspended sediment profile to changes in seafloor properties (% of cohesive sediments, grain size, erosion rate parameters) is investigated. The model is calibrated using suspended sediment concentration data collected at Hunters Point Shipyard (HPS) in San Francisco Bay, California. Realistic sea surface elevation and current velocities, generated using the 3-D baroclinic NCOM hydrodynamic model, are used by the 1-D sediment model to determine the suspended sediment profiles at two locations in the northern and southern regions of HPS. The parameters of the sediment entrainment model are then adjusted to match the measured values of suspended sediment concentration and the response of the biogeochemistry is examined over the entire month of January 2004.
B31A-05 09:30h
Differential Solubilization of As and Se in a Drained and Urbanized Wetland
There is concern over elevated As and Se in surface and ground waters of the arid western United States due to their detrimental impacts on wildlife and human health. While much research has examined trace element mobility on a catchment scale, this project provides a specific case study of the hydrologic regime and groundwater biogeochemistry of a catchment bearing the effects of urbanization. Land use changes paired with the draining of a large historic swamp in the central area of the San Diego Creek - Newport Bay catchment have caused mobilization of As and Se which had accumulated in the former wetland. Major and trace ion geochemical analysis was completed on groundwater and soil samples from the catchment. Results indicate that arsenic and selenium had accumulated within the historic swamp region. Also, the data suggest that two areas, one more - and the other less - regularly inundated, impact the spatial distribution of groundwater As and Se concentrations. Though previous data suggests metal sulfide oxidation as the source of contemporary As and Se concentrations in groundwater, the highest concentrations of As and Se could not be associated with a metal sulfide oxidation mechanism. Additionally, As and Se concentrations, though spatially correlated, are not mechanistically linked. The data indicate competitive ligand exchange between As and Si and the flushing of vadose zone Se as the key processes solubilizing these trace elements. Furthermore, because wetland biogeochemistry is a reflection of certain hydrologic conditions that may be difficult to ever duplicate, wetland mitigation strategies need to incorporate trace element biogeochemistry into their design. This research was conducted while the primary author was at the University of California, Riverside from 2002-2004. The author is now an employee of the United States Environmental Protection Agency.
B31A-06 09:45h
Biogeochemistry of Metals in Periodic Cicada
Metal concentrations were measured in three species of 17-year periodic cicadas (Magicicada spp.) to determine the bioavailability of metals from both uncontaminated and lead-arsenate-pesticide contaminated soils and evaluate whether these metal concentrations might threaten wildlife. Collections were made in Clarke and Frederick Counties, Virginia and Berkeley and Jefferson Counties, West Virginia during Brood X emergence in May and June 2004. Periodic cicadas emerge synchronously at high density after 13 or 17 years of underground development, feeding on xylem fluids, and molt into their adult form leaving a keratin exoskeleton shell. They are an important food source for birds and animals during emergence events, and influence nutrient cycles in woodland settings. Soil concentrations at the collection sites vary over one order of magnitude for Co, Cu, Fe, Hg, Mn, Mo, Se, and Zn and over two orders of magnitude for As, Au, and Pb. The concentration levels of metals in adult periodic cicadas do not pose a dietary threat to birds and other wildlife that preferentially feed upon cicadas during emergence events. The adult cicadas contain concentrations of metals similar to, or less than, other invertebrates, such as earthworms. Average adult cicada body concentrations for As, Cu, Hg, Pb, and Zn are 3, 64, 0.015, 0.4, and 160 mg/Kg (dry weight), respectively. Much of the cicada nymph body load of metals is partitioned into the molt exoskeleton. Elements, such as Al, Fe, and Pb, are strongly enriched in the exoskeleton relative to the adult body; Cu and Zn are enriched in bodies. Concentrations of Fe, Co, and Pb, when normalized to inert soil constituents such as aluminum and cerium, are similar between the molt exoskeleton and their host soil, implying that passive assimilation through prolonged soil contact (adhesion or adsorption) may control these metal concentrations. Normalized concentrations of bioessential elements, such as S, P, K, Ca, Mn, Cu, Zn, and Mo, and chalcophile (sulfur-loving) elements, such as As, Se, and Au, show strong enrichment in cicada tissues relative to soil, implying selective absorption by xylem fluids and/or cicada nymph during development. Chalcophile elements, such as As and heavy metals, accumulate in keratin-rich tissues and may bind to sulfhydryl groups. Metal concentrations in exoskeleton show a positive correlation with soil metal concentrations. Metal concentrations in adult bodies do not correlate with soil chemistry, but bioessential elements S, Mn, Fe, and Zn show differences by sex and Cu and Zn by species.