Andrew N. Quicksall

Geomorphic controls on groundwater arsenic distribution in the Mekong River Delta, Cambodia

Natural arsenic contamination of groundwater is a severe public health crisis affecting nearly 60 million people in South Asia and Southeast Asia alone. Contamination is spatially heterogeneous and results from the coupled microbial decomposition of organic matter and reductive dissolution of arsenic-bearing iron minerals. Here we demonstrate that elevated arsenic concentrations arise in regions of recent organic matter deposition and thus are controlled by fluvial geomorphic processes. Arsenic contamination is best expressed within recent geomorphic features such as docked islands, scroll bars, and avulsions. Within these features, the deposition of rapidly buried reactive organic matter facilitates microbial iron reduction and arsenic release. Ultimately, the organic matter supply is exhausted and the conditions necessary for soluble arsenic to persist diminish.

Size-Dependent Pb Sorption to Nanohematite in the Presence and Absence of a Microbial Siderophore

This study focused on the effects of particle size (40, 8.6, and 3.6 nm) and the presence of the microbial ligand desferrioxamine B (DFOB) on Pb(II) sorption to hematite, based on sorption edge experiments (i.e., sorption as a function of pH). Effects of hematite nanoparticle size on sorption edges, when plotted either as sorption density or as % Pb uptake, depended on whether the experiments were normalized to account for differences in specific surface area within the reaction vessels or postnormalized after the fact. Accounting for specific surface area within reaction vessels is needed to maintain comparable ratios of sorbate to sorbent surface sites. When normalized for BET specific surface area (A(s,BET)) within the reaction vessels, the Pb(II) sorption edge shifted ∼0.5 pH units to the left for <10 nm hematite particles, but maximum sorption density (at pH ≥ 6) was unaffected by particle size. DFOB had little or no effect on Pb(II) sorption to <10 nm particles, but DFOB decreased Pb(II) sorption to the 40 nm particles at pH ≥ 6 by ∼20%. Hematite (nano)particle size thus exerts subtle effects on Pb(II) sorption, but the effects may be more pronounced in the presence of a metal complexing agent.