Selection of a natural sorbent to remove toxic metals from acidic leachate produced during soil decontamination

The purpose of this study was to examine the efficiency with which some natural adsorbents remove heavy metals, especially lead, from very acidic leachate produced during soil decontamination by a chemical leaching process using hydrochloric acid. Sorption assays were done in shaken flasks in the presence of cocoa shells, cedar bark, pine bark, spruce bark, vermiculite and volcanic rocks. Cocoa shells were the most efficient sorbent with a maximal capacity of fixation (qmax) of 2.60 mg Pb/g measured during assays conducted with an acidic soil leachate (initial pH=1.59 and [Pb]i=45.4 mg/L). Cedar bark can also be used for metal removal in very acidic solutions but are less efficient than cocoa shells. This study also demonstrated that the presence of various metals and cations in the acidic leachate of soil induced a slight reduction in lead uptake by cocoa shells and cedar bark. Thus, a qmax value of 3.35 mg Pb/g was measured during assays conducted with a synthetic lead solution (initial pH=1.73 and [Pb]i=42.3 mg/L). Kinetic measurements of lead removal by cocoa shells have revealed that sorption equilibrium was obtained after approximately 4 h of contact. Sorption experiments done with various granulometric fractions of cocoa shells have shown that this material does not need to be crushed to be efficient. Further research is necessary in order to develop a suitable method for soil decontamination by acidic leaching and metal recovery by sorption on cocoa shells.