Elevated antimony (Sb) and arsenic (As) concentrations have been found in numerous rivers and streams worldwide, and are due, in large part, to mining and smelting operations. In these rivers, streambed sediments usually become severely contaminated with and serve as holding reservoirs for these heavy metals. Since these metal contaminants can be subsequently released to stream waters over time, continually contaminating the river systems, it is imperative to understand their potential mobility under various stream physicochemical conditions. This study investigated the fractionation of Sb and As following an extensive sequential extraction procedure in sediments collected from a highly contaminated semi-arid creek at various depths. Partitioning of Sb and As between the sediment and pore water was also evaluated. A batch experiment study focusing on the effects of pH, ionic strength, and background electrolyte on the release of Sb and As from contaminated streambed sediments was also undertaken, and PHREEQC was used to model the experimental results obtained. A laboratory flume experiment study was conducted to demonstrate and compare the extent of Sb and As release/upwelling from contaminated streambed sediment under representative stream pH and ionic strength conditions, and defined physical (streambed topography) conditions. A modified multiphase reactive transport model accounting for advective pumping and linear equilibrium desorption was applied to interpret the flume experiment data obtained. Sequential extraction results show that Sb and As were mainly associated with Fe oxides and the residual fraction in the sediment, which are fractions widely considered to be stable under typical water chemistry conditions found in most river systems. Depth profiles of As and Sb associations with various sediment fractions were largely constant. A general increase of As and Sb concentrations in pore water with depth was observed. Observed partition coefficients were fairly constant with depth for As and Sb, and indicate that both metals are being similarly retained by the sediment at all depths. Batch experiment results show that pH plays a critical role in Sb and As desorption, with high fractions of Sb and As desorbed at low pH (pH < 2), lower and constant release observed in the pH range from 3 – 10 for Sb and from 3 – 8 for As, and a slight increase in As release from pH 8 - 10. Flume experiment results showed similar release of Sb and As release under two pH conditions tested (pH 3.55 and 8.35). In the batch and flume experiments, ionic strength in the range encountered in most natural river systems (0.001 I – 0.01 I) had little influence on Sb and As desorption and release/upwelling from contaminated sediments. In the batch study, the effect of background electrolytes (i.e., Na, K, Ca, and Mg) on Sb and As desorption was also investigated, but did not significantly affect metal desorption. The models applied to simulate Sb and As desorption and release/upwelling from contaminated sediment captured the major release trends observed in batch and flume experiment results. This study will improve our understanding of Sb and As fractionation and partitioning in the subsurface, the effects of water chemistry conditions of pH, ionic strength, and background electrolyte on Sb and As desorption, and the release/upwelling of Sb and As from contaminated streambed sediment in river systems.
July 27, 2016
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