Biogeochemistry of Metals and Arsenic in Environmental Systems II Posters
Presiding: K Makris, University of Texas at San Antonio; S Sharma, University of Texas at San Antonio
B33B-01 1330h
Trophic transfer of cadmium from a freshwater oligocheate Lumbriculus variegates to predatory stoneflies (Perlodidae and Perlidae).
In trace metal-contaminated streams, insects accumulate metals from both aqueous and dietary sources. Previously, we demonstrated that aquatic insect species differ tremendously in their rates of dissolved metal accumulation in addition to metal efflux rates (after dissolved exposures). We have also observed large inter-specific differences in how dissolved metals are partitioned subcellularly, detoxified and stored. Our current work examines dietary metal exposures in predatory stonefly species. Specifically, we are exploring the extent to which stonefly species differ in their assimilation efficiencies of cadmium from their prey, and how widely their post-assimilation efflux rates vary. We also are comparing subcellular distributions of Cd from aqueous versus dietary sources, and are exploring these differences in a phylogenetic context, working with several species in both the Perlidae and Perlodidae. To date, consistencies have been observed among four perlid genera in terms of metal sub-cellular distributions. Cadmium is beter detoxified in the perlodids we have examined, than in the perlids. Ongoing work is being conducted to determine if Cd handling is consistent among different genera of these two families. We also ask whether generalizations about species' sensitivities at broad taxonomic levels are appropriate for heavy metal pollution.
B33B-02 1330h
Environmental Biomonitoring of Cr and As in Shallow Groundwater: Do Red Oak Trees Preserve Long-Term Records of Contaminant Loading?
The use of dendrochemical analysis has been shown to be a valuable, although controversial, tool in monitoring historical trends in trace metal deposition and mobilization in groundwater and sediments. Neutron activation analysis (NAA) is one method that has been used to determine annual dendrochemical patterns in tree rings. The use of NAA may also provide a practical tool for revealing sub-annual differences in metal concentrations between earlywood and latewood. In a variety of geochemical settings, Cr and As can be mobile in the groundwater-root environment and are subsequently taken up by trees and stored in xylem tissues specifically associated with groundwater transport. For the purposes of determining historical patterns in Cr and As bioavailability at a Woburn, MA, superfund site along the Aberjona River, Quercus rubra (red oak) sectioned tree rings were analyzed. Sub-annual dendrochemical analyses were used to identify different As and Cr loading pathways in oak stem wood. A sixty-year record of [As] and [Cr] in stem wood was obtained, and results suggest seasonally dependent correlations with Aberjona River flow and with pumping rates for a municipal well in close proximity to the sampling location. These two hydrological pathways likely dominate in providing a flux of dissolved As and Cr into oak stem wood.
B33B-03 1330h
Advances in Dating of Desert Varnish by Portable X-Ray Fluorescence Spectroscopy
In prior studies we have estimated ages of desert varnish (DV) from the mass per area of Mn and Fe on flat rock surfaces, compared to a measured average accumulation rate across the US Southwest. To extend the technique to non-horizontal surfaces, we compared the apparent ages of horizontal and vertical surfaces of petroglyphs of the same type in the same area, i.e., glyphs that most likely are coeval. The source of the material in desert varnish is from the fall of dust, but other important factors in the formation of the varnish include moisture and exposure to sunlight, which may regulate the bacterial activity responsible for deposition of Mn and Fe. Fall of dust on a non-horizontal surface is proportional to the projected horizontal area, i.e., to cos A, the angle of the surface from the horizontal. For two stylistically coeval glyph panels that we measured, one at 30° to the horizontal and one at 60°, the projected areas scale as 1.0 : 0.866 : 0.5, where 1.0 is a horizontal surface. These ratios suggest that the "true" age of the glyphs could be calculated from a measurement on a sloping surface if the only operative factor was the accumulation of dust. For the 30 and 60 degreee panel pair, 0.866/0.5 = 1.73, which is DV thickness ratio predicted by orientation. But the ratio of the measured ages in years, i.e., the measured actual thicknesses, is 9250/8400 = 1.10. Since these ratios differ, it is apparent that, as expected, factors other than the accumulation of dust on the surface of the rock also are important. Nonetheless, the ratios are at least reasonably close. Taking many measurements of this type may prove more efficient than trying to model the effects of orientation on sunlight, moisture retention, dust accumulation, bacterial activity, etc. We anticipate that building an empirical data base of this type will eventually allow better estimation of the age of glyphs on any sloping rock. Research supported by US Bureau of Reclamation Grant Agreement No. 02-FG-30-0028.
B33B-04 1330h
Effect of Solution Properties on Arsenic Adsorption by Drinking Water Treatment Residuals
Arsenic (As) is a ubiquitous element in the environment. Higher levels of As in soils may result from various anthropogenic sources such as use of arsenical pesticides, fertilizers, wood preservatives, smelter wastes, and coal combustion. This is of great environmental and human health concern due to the high toxicity and proven carcinogenicity of several arsenical species. Thus there is a need for developing cost effective technologies capable of lowering bioavailable As concentrations in soils to environmentally acceptable levels. In-situ immobilization of metals using inexpensive amendments such as minerals (apatite, zeolite, or clay minerals) or waste by-products (steel shot, beringite, and iron-rich biosolids) to reduce bioavailability is an inexpensive alternative to the more expensive ex-situ remediation methods. One such emerging in-situ technique is the application of drinking water treatment residuals (WTRs). WTRs can be classified as a byproduct of drinking water treatment plants and are generally composed of amorphous Fe/Al oxides, activated C and cationic polymers. WTRs possess amorphous structure and generally have high positive charge. Because As is chemically similar to phosphorus, the oxyanions As (V) and As (III) may have the potential of being retained by the WTRs. Thus, it is hypothesized that WTRs retain As irreversibly, thereby reducing As biavailability. As mobility of arsenic is controlled by adsorption reactions, knowledge of adsorption of As by WTRs is of primary relevance. Although the overall rate of adsorption is dependent on numerous factors, review of the literature indicates that competing ions in solution play an important role in the overall retention of As; however, little work has been conducted to identify which ions provide the most competition. As arsenic adsorption appears to be influenced by the variable pH-dependent charges developed on the soil particle surfaces, the effect of pH is also of critical importance. Hence, the purpose of the present study is to investigate the effect of solution properties, such as pH, ionic strength and competing ions on the adsorption of As by WTRs and WTR amended soils. Three types of WTRs are being used, namely Fe- WTR, Al- WTR and Ca-WTR. Effect of pH is being studied by varying the pH values between 3 and 9. The solid/solution ratio has been fixed at 1:5 and a 24 h equilibration has been chosen based on the results of earlier adsorption experiments. Furthermore, As adsorption will be studied in presence of potentially competing ions such as phosphate, sulfate, and selenate. Keywords: Adsorption, water treatment residuals, oxyanions, in-situ remediation, Arsenic
B33B-05 1330h
Arsenic Adsorption and Desorption by Drinking Water Treatment Residuals: Incubation Studies
Arsenic (As) has been used for a long time in agricultural practices, primarily to control pests and noxious weeds. In many cases, the indiscriminate usage of toxic arsenical compounds has left a legacy of contaminated soils. Recent awareness of the toxicity of As at much lower concentrations than previously deemed to be dangerous has led to increased interest in the environmental chemistry of As. The immediate challenge, as perceived by various regulatory bodies is to develop a cost-effective, reliable and environmentally sound approach to cleaning up such contaminated soils. In-situ immobilization technologies are an attractive alternative to conventional remediation methods. One of the most interesting of these in-situ techniques is the use of Water Treatment Residuals (WTRs). The WTRs are by-products of drinking water purification processes and generally contain sediments, organic carbon, and Al/Fe oxides. The oxides are typically amorphous (with very high specific surface area) and have tremendous affinity for oxyanions (e.g., arsenate), due to their high positive surface charge. Recent studies conducted by our group have suggested that WTRs retain As and decrease arsenic mobility. However, a better understanding of As adsorption/desorption by WTRs is necessary for effective implementation of appropriate in-situ remedial strategies. Hence, the present study examines the potential use of WTRs (Al-WTR and Fe-WTR) as adsorbents for the removal of arsenate in solutions. Furthermore, it investigates the extent of desorption of the pre-adsorbed arsenate onto the WTR surfaces. Effects of various key parameters, such as solid solution ratio, equilibration time and arsenic concentration were examined to achieve the optimized conditions for arsenate adsorption. Preliminary batch adsorption experiments showed the optimum equilibration time to be 24 h and the solid/solution ratio to be 1:5 for arsenate adsorption. Sorption data has been evaluated using both Langmuir and Freundlich adsorption models; however, the regression coefficients (at 95% confidence interval) demonstrate that the Freundlich model provides better fit to the experimental data in the majority of the cases. Following adsorption, arsenate desorption was investigated using 7500 mg/kg phosphate. A significant amount of As (99%) remained bound to the WTRs even though the phosphate load was equal to the maximum initial As load (7500 mg/kg). This indicates that adsorption of As on WTRs is typically irreversible and, therefore, the WTRs are good prospects for in-situ As fixation. Keywords: Arsenate, water treatment residuals, adsorption, desorption, phosphate.
B33B-06 1330h
Effect Of Soil Properties On The Geochemical Speciation Of Arsenic In Contaminated Soils: A Greenhouse Study
Land-applied arsenical pesticides have contributed elevated soil arsenic (As) levels. Many baseline risk assessments As-contaminated sites assume that all As present in the soil is bioavailable, thereby potentially overestimating the actual health risk. However, risk from As exposure is associated only with those forms of As that are potentially extractable by the human gastrointestinal juices. It has been demonstrated that As may exist in several geochemical forms depending on soil chemical properties, which may or may not be bioavailable. The current study aims at addressing the issue of soil variability on As bioavailability as a function of soil physico-chemical properties in a greenhouse setting involving dynamic interactions between soil, water and plants. Four different soils were chosen based on their potential differences with respect to As reactivity: Immokalee, an acid sand with low extractable Fe/Al, having minimal arsenic retention capacity; Millhopper, an acid sandy loam with high extractable Fe/Al oxides; Pahokee Muck soil with 85% soil organic matter (SOM) as well as high Fe/Al content; and Orelia soil with high clay and Fe/Al content. Soils were amended with sodium arsenate (675 and 1500 mg/Kg). Rice (Oryza sativa) was used as the test crop. A sequential extraction scheme was employed to identify the geochemical forms of As in soils (soluble, exchangeable, organic, Fe/Al-bound, Ca/Mg-bound, residual) immediately after spiking; after 3 mo; and after 6 mo of equilibration time. Concentrations of these As forms were correlated with the in-vitro bioavailable As fractions to identify those As fractions that are most likely to be bioavailable. Results from this study showed that there was little to no plant growth in the contaminated soils. Sequential extractions of the soil indicated that arsenic is strongly adsorbed onto soil amorphous iron/aluminum oxides, and the degree of arsenic retention is a direct function of equilibration time.