Removal of heavy metals from acid soil leachate using cocoa shells in a batch counter-current sorption process

Soil washing is a widely used approach for metal-polluted sites decontamination. Metals solubilized during soil washing have to be extracted from the resulting soil leachates. Cocoa shells (CS) have been identified as a very efficient natural sorbent to remove Pb and other metals from acid soil leachates (ASL). A counter-current sorption process (CCSP) has been defined to reduce the CS requirement for ASL treatment. The aim of this study was to evaluate the batch process efficiency at laboratory scale. The ASL (pHi=2.0) used in this study was initially contaminated by Pb (27.1±2.9 mg/L), Cu (2.93±0.27 mg/L) and Zn (17.1±0.9 mg/L). Three to five short time sorption steps (contact time=1 h) using 10 g CS per liter of ASL allowed a reduction of the cocoa shells quantity required for the ASL treatment by a factor of 2 to 4, in comparison to a conventional single adsorption step process. In addition, the results show that the CCSP increases the Pb measured concentration sorbed from 1060 mg/kg for a one-step process to an average value of 2730±220 mg/kg for a five-step process. This CCSP can probably be efficiently used with other natural sorbents for the treatment of different types of metal-contaminated effluents.