Sorption of Metals from Low-Iron Mine Drainage Using Agricultural Waste Materials

In this study, agricultural wastes including banana peels, coconut coir, sheep and goat manure, plantain peels, and bamboo stalks were tested for the equilibrium removal of Cd, Cu, Pb, Zn, and Hg from mine water at pH 4. Trials were also conducted at pH 7 and 8 to determine pH controls on metal removal and suggest removal mechanism (i.e. sorption vs. precipitation). Materials were dried at 100 °C and ground prior to use. An artificial mine water mixture was created to simulate a mine drainage dominated by non-iron metals and sulfate at pH 4, 7, and 8. Mine water was mixed with varying masses of each sorbent, shaken, and filtered before analysis. Further testing was conducted on the most effective materials. At pH 7, banana peels, sheep manure, and coconut coir removed the largest proportion of the target metals from solution achieving greater than 80–90% removal depending on metal and sorbent. At pH 4, 0.1 g of sheep dung removed 0.18, 1.43, and 1.54 mg/g respectively, for Cd, Cu, and Zn. 0.1 g of coconut coir removed Cd and Cu of 0.18 and 1.51 mg/g, respectively. Banana peel was effective for removing Pb and Hg at both 0.003 and 0.01 g/mL treatments achieving adsorption efficiencies of 94.7% and 96.9%, respectively, for Pb and 93% and 93.5%, respectively, for Hg. The pH at the point of zero charge (pH-PZC) for banana peel, coconut coir, and sheep dung was determined to be 5.8 ± 0.3, 5.4 ± 0.7, and 7.2 ± 0.1, respectively.