Removal of Heavy Metals from Soil Components and Soils by Natural Chelating Agents. Part II. Soil Extraction by Sugar Acids

Aqueous solutions of the natural chelatingagents D-gluconic acid and D-glucaric acid (D[+]-saccharic acid) were tested for their ability to remove heavy metals (Cd, Cr, Cu,Ni, Pb, Zn) from a soil polluted by long-term application of sewage sludge. Batch equilibrium experiments were performed undervariation of fundamental process parameters, i.e. pH value, sugaracid concentration, batch solution volume, solid:liquid ratioand number of treatment cycles.The extractability of heavy metals was low under near-neutral andslightly basic pH conditions. It increased drastically between pH12.0 and 13.0. Pb and Cu were preferentially extracted metals.Compared with the extraction efficiency of pH adequate puresodium hydroxide solutions, the sugar acids enhanced thesolubilisation of Pb and Cr especially. The metal depletion fromsoil was the highest when applying 20 or 50 g L^-1 solutionsof the chelating agents. Under strongly basic conditions solid:liquid ratios of 1:10 or 1:20 were proofed to be advantageous.Except Ni, multi-step extraction improved the metal removalstrongly. This effect was the greatest for Cr extraction. Underoptimised conditions the following metal extraction degrees wereachieved with strongly alkaline D-gluconic acid solutions: Ni 43%%, Cr 60%%, Cd 63%%,Zn 70%%, Pb 80%%, and Cu 84%%.