Partitioning of mercury onto suspended sedimentsin estuaries

Abstract

Radiochemical partitioning experiments using ²⁰³Hg have been undertaken with mixtures of river, seawater and sediment samples taken from three geochemically contrasting UK estuaries: the Plym, Beaulieu and Mersey. Species of dissolved Hg were determined using reversed-phase C₁₈ chelating columns and particulate species were determined by sequential leaching with 1 M NH₄OAc and 1 M HCl. Mercury had a high particle reactivity with partition coefficients, K_Ds, ranging from 10⁴ to 5 × 10⁵ ml g⁻¹, depending on salinity, the chemical composition of the end-member waters, and on the physico-chemical characteristics of the sediment. Dissolved organic matter present in the waters (humic substances and/or anthropogenic compounds) was found to be the main factor governing the forms of dissolved Hg and their reactivity. From the spiked ²⁰³Hg, up to 95% of the dissolved metal was retained on the C₁₈ columns for the Mersey waters, whereas this fraction was <60% in the Plym and Beaulieu waters. Quasi-irreversible adsorption of Hg onto particles from each estuary was observed over a time-scale of a few hours and <20% of total particulate Hg was released by the sequential leach. In this paper, physico-chemical processes are proposed to explain the estuarine behaviour of Hg and the results are discussed in terms of Hg availability in estuarine systems.