Effect of pH on transport of Pb²⁺, Mn²⁺, Zn²⁺ and Ni²⁺ through lateritic soil: Column experiments and transport modeling

This study investigated the effects of pH on the transport of Pb²⁺, Mn²⁺, Zn²⁺ and Ni²⁺ through lateritic soil columns. Model results by fitting the symmetric breakthrough curves (BTCs) of bromide (Br⁻) with CXTFIT model suggested that physical non-equilibrium processes were absent in the columns. The heavy metal BTCs were, however, asymmetrical and exhibited a tailing phenomenon, indicating the presence of chemical non-equilibrium processes in the columns. The retardation factors of Pb²⁺ were the largest of the four metal ions at both pH 4.0 (33.3) and pH 5.0 (35.4). The use of Langmuir isotherm parameters from batch studies with HYDRUS-1D did not predict the BTCs well. Rather the two-site model (TSM) described the heavy metal BTCs better than the equilibrium linear/nonlinear Langmuir model. The fraction of instantaneous sorption sites (f) of all four metal ions on the lateritic soil was consistently about 30%-44% of the total sorption sites.