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

The solid-phase speciation of metals plays a crucialrole in determiningtheir bioavailability in aquatic ecosystems. The interactions of metalswith metal (hydr)oxides, sulfides, clays, and organic matter governtheir availability in subsurface environments. Our study combinedsolid-water partitioning experiments under anaerobic conditions(97% N-2 and 3% H-2) for 24 h and synchrotron-basedspectroscopy to determine the factors controlling copper, nickel,and zinc availability in soils and sediments from three differentnatural aquatic systems. While soils and sediments from all threesystems strongly bound added trace metals, there were substantialdifferences in trace metal partitioning trends between different sites,especially for Cu. There was no distinct correlation between tracemetal partitioning and the total organic matter, iron, and sulfurcontents of the samples. X-ray absorption spectroscopy indicated thatthe speciation of the freshly added metals taken up by the solidsdiffers substantially from the speciation of the metals originallypresent in unamended samples. Cu sulfides dominated speciation atlow loadings (1 mu mol Cu/g solid), whereas complexation to thiolgroups and formation of metallic Cu governed speciation at high loadings(10 mu mol Cu/g solid). For Ni and Zn, adsorption to mineral surfacesand organic matter governed their speciation in materials from mostsites at low (1 mu mol Zn/g solid) and high (10 mu mol Zn/gsolid) loadings. Under the evaluated time scale (24 h), the backgroundspeciation of metals in natural aquatic systems is a poor predictorof the speciation and lability of metals introduced from anthropogenicor natural processes. Our findings imply that geochemical processescontrolling trace metal speciation may vary considerably with metalloading in different natural systems.