Superfund Research Program
Mobilization of Arsenic in Sediments
Background: The toxicity of arsenic in sediments is determined by the concentration of bioavailable arsenic in the pore water. It sounds simple. But understanding the mechanisms that drive the cycles between arsenate [As(III)], arsenite [As(v)], dimethylarsenate (DMA) and monomethylarsenic acid (MMA) and predicting the extent to which arsenic species are released from sediments to the overlaying water have posed significant challenges.
For example, according to chemical thermodynamics, As(V) is expected to dominate in oxic surface waters and As(III) is expected to dominate in anoxic bottom waters and in sediments. However, the exact opposite is observed with As(III) and DMA present in the oxic surface waters and elevated concentrations of As(V) in bottom waters. As part of the New York University School of Medicine SBRP, scientists led by Dr. Dominic Di Toro are examining the fate of arsenic in aquatic environments.
Advances: Dr. Di Toro’s group analyzed batch studies to investigate the reduction and methylation of arsenic by algae. The researchers found that the reduction of arsenate in algal cells is regulated by the availability of phosphate (P). Under P-limited conditions, which exist in the summer, algae take up As(V), methylate it to MMA and DMA and excrete it. Under non P-limited conditions, which occur in the early stages of algal blooms, algae up-regulate the PO4 transport system. As(V) is taken up by the PO4 transport system and once inside the cell is rapidly reduced to As(III). Methylation is slower than reduction, so As(III) builds up in the cell. Algal excretion of As(III) leads to the increase in As(III) concentrations in the body of water. This information will help us predict the relative quantities of arsenite and arsenate, which is very important in performing risk assessments. The researchers incorporated arsenic reduction and methylation into a coupled water column-sediment model for arsenic cycling in lakes. Their publication includes the downloadable Visual Basic model code and model documentation.
In related work, Dr. Di Toro’s laboratory has focused on the formulation of a reactive-transport model that can describe the mechanism of oxidation of As(III) in simplified iron-rich sediment systems. In sediments with an oxic overlying water column, divalent iron [Fe(II)] diffuses from the anoxic sediment layer to the oxic-anoxic interface within the sediment where it is rapidly oxidized to Fe(III). Dr. Di Toro examined the proposal that radical species generated during the oxidation of Fe(II) by molecular oxygen are capable of facilitating As(III) oxidation over a wide range of pH values in the absence of light. The researchers used a laboratory sand column to examine the influence of transport and redox kinetics on the extent of arsenic oxidation and the potential release of arsenic to the overlying water. They modeled the column experiment modeled using the Tableau Input Coupled Kinetic Equilibrium Transport (TICKET) model. On the whole, the researchers found good agreement between the experimental and computational data. Most importantly, both the experimental data and the model data indicate that As(III) is being oxidized to As(V) within the sediment. The dominance of As(V) in the porewater of the upper sediment layers and As(III) in the porewater of the bottom layers (indicated in both the model output and experimental data) agrees with the typical speciation pattern observed in natural systems. The model agrees reasonably well with experiment data for arsenic releases to the overlying water. All this suggests that the Fe(II)-catalyzed As(III) oxidation mechanism is capable of explaining the oxidation of As(III) in iron-rich sediment systems.
Significance: Arsenite is more toxic than arsenate, partly because it enters cells more readily. Therefore, the ability to predict the relative quantities of arsenite and arsenate is critical to a comprehensive analysis of the risk to environmental and human health posed by arsenic-contaminated sediments.
The coupled water column-sediment model can be used as part of a risk assessment of arsenic to make the speciation calculations and quantify the relative risk. The link between the reduction of arsenate and the availability of phosphate tells us that the extent of formation of the more toxic arsenite species is related to the extent of nutrient enrichment and suggests that both the risk from arsenite and the problems of excessive algal blooms can both be mitigated by controlling the discharge of phosphate. This is a totally unexpected benefit of controlling nutrient discharges.
The results of experimental and modeling work indicate that radical species generated during the oxidation of Fe(II) by molecular oxygen are capable of facilitating As(III) oxidation and explain As(III) oxidation in sediments. This work also shows that transport in sediments plays a vital role in increasing the extent of As(III) oxidation and efficiency of the Fe(II) catalysis.
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To learn more about this research, please refer to the following sources:
- Bisceglia KJ, Rader KJ, Carbonaro RF, Farley KJ, Mahony JD, Di Toro DM. 2005. Iron(II)-catalyzed oxidation of arsenic(III) in a sediment column. Environ Sci Technol 39(23):9217-9222. PMID:16382945
- Dombrowski PM, Long W, Farley KJ, Mahony JD, Capitani JF, Di Toro DM. 2005. Thermodynamic analysis of arsenic methylation. Environ Sci Technol 39(7):2169-2176. PMID:15871252
- Hellweger FL, Farley KJ, Lall U, Di Toro DM. 2004. Greedy algae reduce arsenate. Limnology and Oceanography 48(6):2275-2288.
- Hellweger FL, Lall U. 2004. Modeling the effect of algal dynamics on arsenic speciation in Lake Biwa. Environ Sci Technol 38(24):6716-6723. PMID:15669332
- Rader KJ, Dombrowski PM, Farley KJ, Mahony JD, Di Toro DM. 2004. Effect of thioarsenite formation on arsenic(III) toxicity. Environ Toxicol Chem 23(7):1649-1654. PMID:15230317
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