Solid–Water Partitioning and Speciation of Trace Metal Micronutrients in Wetland Soils and Stream Sediments

Abstract

The solid-phase speciation of metals plays a crucial role in determining their bioavailability in aquatic ecosystems. The interactions of metals with metal (hydr)oxides, sulfides, clays, and organic matter govern their availability in subsurface environments. Our study combined solid−water partitioning experiments under anaerobic conditions (97% N 2 and 3% H 2 ) for 24 h and synchrotron-based spectroscopy to determine the factors controlling copper, nickel, and zinc availability in soils and sediments from three different natural aquatic systems. While soils and sediments from all three systems strongly bound added trace metals, there were substantial differences in trace metal partitioning trends between different sites, especially for Cu. There was no distinct correlation between trace metal partitioning and the total organic matter, iron, and sulfur contents of the samples. X-ray absorption spectroscopy indicated that the speciation of the freshly added metals taken up by the solids differs substantially from the speciation of the metals originally present in unamended samples. Cu sulfides dominated speciation at low loadings (1 μmol Cu/g solid), whereas complexation to thiol groups and formation of metallic Cu governed speciation at high loadings (10 μmol Cu/g solid). For Ni and Zn, adsorption to mineral surfaces and organic matter governed their speciation in materials from most sites at low (1 μmol Zn/g solid) and high (10 μmol Zn/g solid) loadings. Under the evaluated time scale (24 h), the background speciation of metals in natural aquatic systems is a poor predictor of the speciation and lability of metals introduced from anthropogenic or natural processes. Our findings imply that geochemical processes controlling trace metal speciation may vary considerably with metal loading in different natural systems.