Modeling of Calcium Hydroxide Transport in a Volcanic-ash Soil using HP1 code.

Construction sludge is usually dehydrated with a coagulant material such as lime hydrate for recycling the sludge as a soil foundation. A well-buffered volcanic-ash soil is often used at the bottom of the landfill site to minimize adverse effects of high pH solutions for the surrounding area from the dehydrated sludge. The soil buffering capacity is evaluated with a variable charge model describing the pH dependent charges. Variable cation exchange capacity (CEC) is modeled with a dissociation of hydrogen from the hydroxyl groups at the edge of soil minerals. Variable anion exchange capacity (AEC) is also described with the attachment of hydrogen ions to the surface hydroxyl groups. A titration experiment is conducted for a Kanto loam soil (Japanese volcanic-ash soil) by adding acid and alkaline solutions to a soil-water suspension. Parameter values for the variable charge model are determined based on the titration curve using the PHREEQC geochemical database code. Furthermore, a leaching experiment of calcium hydroxide solutions is carried out for the Kanto loam soil. Cation and anion concentrations including soil solution pH are evaluated with a numerical code HP1, which couples the water flow and solute transport code HYDRUS-1D and the PHREEQC code.