Evaluation of applicability of filling materials in permeable reactive barrier (PRB) system to remediate groundwater contaminated with Cd and Pb at open solid waste dump sites

This study aimed to identify appropriate filling materials for a permeable reactive barrier (PRB) system to treat groundwater contaminated with trace metals in the vicinity of solid waste landfills in Sri Lanka. Mixtures of alluvial loamy soil, coconut shell biochar, and laterite clay brick in different proportions were tested as easily-available PRB adsorbents. A series of adsorption and desorption experiments were carried out to investigate the effects of initial concentration, pH, ionic strength, and multiple competitive trace elements on Cd and Pb adsorption onto the tested adsorbents. In addition, hydraulic conductivities (Ks) of the tested adsorbents under different compaction levels were measured to examine a suitable packing condition for the PRB system. Results showed that the Langmuir model performed well for fitting Cd and Pb adsorption isotherms and maximum adsorption capacities (Qm) for Pb (2.1-15.3 mg/g) became higher than those for Cd (0.8-6.8 mg/g). All tested adsorbents showed low leaching of adsorbed metals with high hysteresis indices in desorption studies. In the multiple trace element solution, the existence of other trace metals (Cu, Zn, Ni) had a significant effect on Cd adsorption but less on Pb adsorption. The three mixed adsorbents had no dependency on the initial pH and ionic strength of the solution, while the single material showed a low dependency in both Cd and Pb adsorption. The inclusion of brick was effective to improve the hydraulic property and measured Ks values for the 75% brick mixed materials resulted of >10−4 cm/s at high compaction levels (Dr = 90% and 100%). Three mixed materials can be strongly recommended as a PRB filling material to treat landfill leachate based on their reactivity and hydraulic properties.