Reactive Barriers for Containment of Metals-Contaminated Dredged Materials: Diffusion Studies

摘要

Heavy metals are a prevalent and tenacious contaminant in many sediments and dredged materials. Traditionally these contaminated sediments are moved to secure locations and capped with clean sediment materials in a confined aquatic disposal (CAD) cell. This technique relies upon the tortuosity and reactivity of the overlying clean sediments to inhibit metal diffusion. The use of reactive barriers in sediment caps offers the opportunity to actively adsorb and precipitate contaminants as they migrate from contaminated sediments, thus improving overall cap performance. Apatite minerals, such as fluorapatite [Ca_5(PO_4)_3F)], were evaluated as reactive barriers in a series of laboratory-scale diffusion experiments that took place over a period of 600 days. It was hypothesized that surface adsorption and precipitation would effectively bind diffusing heavy metals, preventing them from migrating past the reactive barrier. Diffusion models were used to determine barrier effects on effective diffusion coefficients (D_e). Decreases in diffusivity are attributed to adsorption/precipitation reactions in the presence of the apatite barriers. Reductions in average diffusivity for the most effective phosphate barrier (Florida apatite), when compared to a conservative control, were 91 % for Pb, 99.8% for Zn, 37% for Cr, and 57% for Cu. After diffusion, mineralogical analysis revealed several of the metal phosphate minerals in the reactive barrier material were highly stable apatite and tertiary metal phosphates including Pb_4O(PO_4)_2, ZnCr_(0.85)(PO_4)_2·xH_2O, Pb_3Cr(PO_4)_3, Pb_3Fe(PO_4)_3, Cd_5(PO_4)_3OH, and CaZn_2(PO_4)_2·H_2O. The presence of such metal phosphate minerals further indicates that Pb, Cd, and Zn had adsorbed to the reactive barrier mineral surfaces and reprecipitated as metal phosphates. The use of reactive barrier sediment caps reduces the environmental concerns associated with sediment disposal and may allow for beneficial reuse of dredged materials.