Modeling of chemical armoring in a permeable reactive barrier (PRB) in acid sulfate soil (ASS) terrain
The acidification of coastal waterways resulting from the oxidation of sulfidic minerals in acid sulfate soil (ASS) is a geo-environmental problem worldwide causing loss of biodiversity and agricultural productivity plus corrosion of concrete and steel infrastructure by acidic drainage. A permeable reactive barrier (PRB) of recycled concrete was installed on the Shoalhaven Floodplain, southeast New South Wales (NSW), Australia as an innovative and cost-effective technology for passive treatment of acidic groundwater in ASS terrain. A long-term laboratory column experiment simulating acidic groundwater flow through the PRB examined the acid neutralization behavior of recycled concrete and its potential to remove dissolved aluminium (Al) and iron (Fe). The recycled concrete treated the acidic groundwater through the predominant reactions of (Stage 1) bicarbonate buffering and (Stage 2) precipitation/dissolution of Al and Fe oxy/hydroxides, resulting in near-neutral effluent pH and ∼95% removal of Al and Fe. However, armoring of the recycled concrete aggregates by secondary mineral precipitation (Al- and Fe-bearing minerals) reduced the acid neutralization capacity (ANC) of the concrete by ∼50%. The challenge of this study was to develop a model including geochemical reaction kinetics, supported by laboratory testing, as a step in determining PRB longevity. Saturation indices for the precipitated minerals were calculated using numerical codes, PHREEQC and MINTEQ. Geochemical algorithms were developed to calculate changes in the quantity of minerals precipitated/dissoluted over time. For Stage 1, pseudo-first order reaction rates proportional to the reactive material surface and H+ concentration were assumed. The kinetics of mineral precipitation/dissolution (Stage 2) was assumed to follow transition state theory. Changes in porosity and hydraulic conductivity within the column were modeled by non-linear geohydraulic flow analysis. Once validated with results obtained from the column experiments, this approach can be directly applied to the field PRB using appropriate scale factors.