EVALUATING THE EXTENT AND RATE OF HG SORPTION BY ENGINEERED SORBENTS DEPLOYED IN BANK SOILS AND SEDIMENTS OF A HG-CONTAMINATED CREEK

Mercury (Hg) is a global pollutant and a threat to environmental and human health. Hg contamination in the East Fork Poplar Creek (EFPC) is one of the impacts of the cold war. Significant amounts of Hg were lost to soil and rock on the Oak Ridge Reservation, where EFPC originates resulting in widespread Hg contamination downstream. Sorbent amendments are considered to reduce the flux of Hg from banks soils and sediments into creek thereby reducing the bioavailability for the transformation of highly toxic methylmercury by microorganisms. In this study, three sorbent materials - activated carbon, biochar and Organoclay (organically modified phyllosilicate), and quartz sand as a negative control were placed inside nylon mesh bags to prevent loss and placed in creek sediments and bank soils in three creek sediment sites and two creek bank locations with different levels of Hg contamination. These sorbent coupons were deployed at the field sites for time periods of 1, 3, 6 and up to 12 months.

The amount of adsorbed total Hg varied highly between replicate samples at each location. This variability can be attributed to local heterogeneities and stratification of Hg concentrations in bank soils and creek sediments. However, the relative fraction of sorbed Hg is comparable for each sorbent deployed at different locations. The Hg sorption kinetics were fitted to a pseudo-first-order rate equation to obtain the adsorption rate k_t and the adsorption density at equilibrium (Q_max). Across all field sites and conditions, we observed the highest Q_max in biochar (1,051 – 2,549 ng/g) followed by activated carbon (628 – 2,231 ng/g). The k_t of biochar and activated carbon were comparable and ranged from 0.38 – 0.54 mo^-_­¹ and 0.1 – 0.43 mo^-1, respectively. Organoclay showed lower adsorption rates ranging from 0.03 – 0.20 mo^-1. A larger variability in the adsorption rates was observed for quartz sand ranging from 0.03 – 1.3 mo^-1, which is attributed to its low sorption capacity (150 – 600 ng/g). Interestingly, although background Hg concentrations varied over two orders of magnitude between field sites, they had little impact on the observed Q_max for all sorbents. Sorbent coupons deployed in bank soils showed lower Q_max values, which suggests that higher pore water contents enhance partitioning of Hg to the sorbent.