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 chelating agents 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 under variation of fundamental process parameters, i.e. pH value, sugar acid concentration, batch solution volume, solid:liquid ratio and number of treatment cycles. The extractability of heavy metals was low under near-neutral and slightly basic pH conditions. It increased drastically between pH 12.0 and 13.0. Pb and Cu were preferentially extracted metals. Compared with the extraction efficiency of pH adequate pure sodium hydroxide solutions, the sugar acids enhanced the solubilisation of Pb and Cr especially. The metal depletion from soil was the highest when applying 20 or 50 g L-1 solutions of 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 removal strongly. This effect was the greatest for Cr extraction. Under optimised conditions the following metal extraction degrees were achieved with strongly alkaline D-gluconic acid solutions: Ni 43%, Cr 60%, Cd 63%, Zn 70%, Pb 80%, and Cu 84%.