Factors influencing hydraulic conductivity and metal retention capacity of geosynthetic clay liners exposed to acid rock drainage

Evaluating the factors that affect hydraulic conductivity and metal retention in geosynthetic clay liners (GCLs) becomes crucial in order to decide whether or not GCLs can be used as a barrier material in rock containments with potential for acid rock drainage (ARD) generation. This study presents a state-of-the-art review of the factors that impact the hydraulic conductivity and heavy metal retention on GCLs: effect of the type of bentonite inside GCLs, prehydration, confining pressure, pH, metal concentration, and type of metal ion in ARDs. ARDs affect the hydraulic performance of GCLs because of their characteristic low pH (usually lower than pH 3) and presence of different heavy metals and concentrations (especially the presence of divalent and trivalent metals). The retention of metals present in ARD through GCLs are governed by the pH (higher retention at pH higher than 3.5), the metals that are present in the ARD as well as the buffering capacity of the bentonite (determined by the cation exchange capacity of bentonite). Ion exchange constitutes the main mechanism for metal uptake, but other mechanisms also take place. Secondary sorption on ferrihydrite or gypsum (formed through precipitation of metals present in ARD) of metalloids such as As, which cannot be sorbed by bentonite directly, may also occur.