Activated Carbon Mitigates Mercury and Methylmercury Bioavailability in Contaminated Sediments

Abstract

There are few available in situ remediation options for Hg contaminated sediments, short of capping. Here we present the first tests of activated carbon and other sorbents as potential in situ amendments for remediation of mercury and methylmercury (MeHg), using a study design that combined 2 L sediment/water microcosms with 14 day bioaccumulation assays. Our key end points were pore water concentrations, and bioaccumulation of total Hg and MeHg by a deposit-feeding oligochaete Lumbriculus variegatus. Four amendments were tested: an activated carbon (AC); CETCO Organoclay MRM (MRM); Thiol-SAMMS (TS), a thiol-functionalized mesoporous silica; and AMBERSEP GT74, an ion-exchange resin. Amendments were tested in four separate microcosm assays using Hg-contaminated sediments from two freshwater and two estuarine sites. AC and TS amendments, added at 2-7% of the dry weight of sediments significantly reduced both MeHg concentrations in pore waters, relative to unamended controls (by 45-95%) and bioaccumulation of MeHg by Lumbriculus (by between 30 and 90%). Both amendments had only small impacts on microcosm surface water, sediment and pore water chemistry, with the exception of significant reductions in pore water dissolved organic matter. The effectiveness of amendments in reducing bioaccumulation was well-correlated with their effectiveness in increasing sediment:water partitioning, especially of MeHg. Sediments with low native sediment:water MeHg partition coefficients were most effectively treated. Thus, in situ sediment sorbent amendments may be able to reduce the risk of biotic Hg and MeHg uptake in contaminated sediments, and subsequent contamination of food webs.

Critical review of mercury sediment quality values for the protection of benthic invertebrates

“…Numerous other factors affect SQV derivation by confounding the relationships between chemicals of interest and toxic effects. First, the considerable variation in bioavailability among sediments poses a challenge to any approach that attempts to predict toxic effects based on bulk sediment chemical concentration alone, including co‐occurrence approaches . Mercury geochemistry has been extensively studied with respect to the potential for net methylmercury formation , yet a comprehensive understanding remains elusive .…”

“…As has been generally observed for other metals and organic compounds in sediment, the EqP approach for mercury assumes bioavailability is directly related to the freely dissolved concentration of mercury in sediment porewater, because this measurement best represents the portions of sediment associated chemicals that can readily diffuse into dermal, respiratory, or digestive tissues in contact with sediment . The processes affecting concentrations of bioavailable mercury in sediment porewater are controlled by sorption of mercury to sediment solid and mineral phases, which in turn is greatly affected by sediment chemical and physical conditions such as pH, redox potential, and presence of sulfate, sulfides, iron, and organic matter . Exposure and fate of mercury in sediment and sediment porewater also are greatly influenced by the form of mercury .…”

“…However, more research is justifiable to explore the role organic carbon may play in affecting mercury bioavailability and toxicity to invertebrates. For example, controlled bioavailability and toxicity studies in which carbon is added to sediment would aid in evaluating the utility of organic carbon normalization.…”

Persistent reductions in the bioavailability of PCBs at a tidally inundated Phragmites australis marsh amended with activated carbon

“…Trends in K d were evaluated with reference to a simple partitioning model based on the assumption that organic carbon was the dominant binding phase for PCBs, where f OC is the fraction of sediment comprising organic carbon and K OC is the organic carbon–water equilibrium partitioning constant for each congener or coeluting group. In the absence of a directly measured value, f OC was set at 11% by multiplying the average measured loss‐on‐ignition in the plots (31%) by a correction factor of 0.36 (Costello et al ; Gilmour et al ). The K OC values for all congeners were derived from a model for sorption of hydrophobic chemicals to sediment (Karickhoff et al ): …”

Effects of two sorbents applied to mercury-contaminated river sediments on bioaccumulation in and detrital processing by Hyalella azteca

“…Additionally, the initial sorbent particle size seems critical (Table 1), while the contact time did not substantially influence the absolute Hg accumulation in the sorbent-added treatments. Hence, both sorbent types have the capacity to reduce for a longer time the transfer of Hg and associated effects along the aquatic food chain (Gilmour et al 2013;Kraus et al 2014). But, the finer particles arising from Sedimite ® amendment used during the first experiment depressed detrital processing (Figs.…”