Cocoa Shells as Adsorbent for Metal Recovery from Acid Effluent

Cocoa shells are commonly used in the horticulture field. This inexpensive substrate was studied for metal removal from acidic effluents. Batch adsorption tests in shake flasks revealed that cocoa shells were particularlyefficient for lead removal. More than 90% of lead could be removed from a mono-metallic solution containing 51.8 mg Pb/L (250 μM Pb) using 20 g/L of cocoa shells. Langmuir isotherm indicated that cocoa shells have a maximum lead uptake of 7.56 mglg (36.5 μmol/g) at pH = 2.0. Adsorption tests were also successfully completed with three types of heavily contaminated acid effluents: a multi-element synthetic solution and effluents produced during sewage sludge and soil decontamination. These tests have shown that the presence of other metals and organic matter only slightly decreases the lead removal by cocoa shells. After adsorption, metals could be eluted using a diluted acid solution (0.5 N) and the cocoa shells could be reused for many adsorption/desorption cycles.