Application of a Mixture of Waste Concrete and Rice Husks for Acidic Mine Drainage Treatment and Reactive Transport Modelling of the Coating-Controlled Neutralization

Waste concrete (WC) has attracted attention for the passive treatment of acid mine drainage (AMD); however, its neutralization performance is often limited by the formation of a surface coating. Field-scale demonstration has shown that co-installation of rice husks (RH) can mitigate this deterioration, although the underlying mechanism remains unclear. In this study, AMD neutralization using WC and RH was investigated through dissolution and neutralization experiments. Dissolution experiments with blank AMD showed that WC was the primary source of alkalinity through portlandite dissolution, whereas RH was the main source of dissolved silicon via the alkaline dissolution of phytogenic amorphous silica. In the WC-only column, the effluent pH began to decrease after ≈ 12 h, accompanied by Zn and Cu breakthrough, whereas the WC+RH system maintained an alkaline pH of ≈ 11.5 for 24 h. SEM–EDS observations indicated that surface precipitates formed a continuous coating on WC in the WC-only system, whereas precipitation was spatially distributed between WC and RH in the mixed system. A one-dimensional reactive transport model incorporating coating-dependent reaction attenuation yielded RMSE values of 0.257–0.517 for pH across the four column systems and captured the observed differences in temporal neutralization behavior. These results indicate that surface coating formation, rather than depletion of the bulk alkalinity source, was responsible for the observed deterioration in WC neutralization performance, and that RH delayed this deterioration by redistributing precipitation between WC and RH.