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Heavy metal sorption and hydraulic conductivity studies using three types of bentonite admixes Li, Franky

Abstract

This study investigated the sorption and migration behaviour of heavy metals permeating through clay barriers, the effect of heavy metals on clay barrier hydraulic conductivity, and the use of Forest soil and Spruce bark as potential clay barrier materials. Bentonite, Forest soil, and Spruce bark were submitted to Batch adsorption testing to determine sorption capacities and investigate competition effects associated with multi-heavy-metal sorption. The heavy metal solutions consisted of single, binary, and ternary combinations of lead (Pb), copper (Cu), and (Cd) ranging from 0 - 1000 mg I A According to sorption capacities, the heavy metals ranked as follows: Forest soil > bentonite = Spruce bark. A n equation based on the Freundlich equation was developed for describing multi-heavy-metal sorption, and expressed the following ranking in terms of sorption competitiveness: Pb> Cu> Cd. A bentonite admix (100:8 ratio of sand:bentonite), Forest soil admix (100:7:1 ratio of sand:bentonite:Forest soil), and Spruce bark admix (100:7:1 ratio of sand:bentonite:Spruce bark) were submitted to Leaching cell testing to investigate the heavy metal compatibility of clay barriers. The admixes were compacted into rigidwalled cells and permeated with 500 mg L⁻¹ solutions of Pb, Cu, and Cd. The hydraulic conductivities of heavy metal leachates were ≈ 2 orders of magnitude greater than the blank (0.01 M calcium nitrate) leachate. The Forest soil admix ranked the best in terms of heavy metal retention capacity, and breakthrough points. After each Leaching cell test, the admix samples were extruded and submitted to Selective Sequential Extractions (SSEs). The SSE determined the distribution of heavy metals amongst five soil components: cation exchangeable, carbonates, Fe and Mn oxides and hydroxides, organic matter, and the residual. The addition of Forest soil, and Spruce bark in clay barrier mixes promoted stronger heavy metal fixation. In addition, the SSE results showed a linear relation between the sorption characteristics and sorption concentrations of the admixes. The Leaching cell and SSE results showed that heavy metals followed unpredictable pathways, and caused significant short-circuiting. In terms of relative mobility, Cd was ≈1.5 times more mobile than Cu, and ≈ 4 times more mobile than Pb, respectively.

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