Extraction of Heavy Metals from Sewage Sludge with Phosphoric Acid

On basis of chemical extraction technology, H₃PO₄ has been used to remove heavy metals from the industrial sewage sludge. At the laboratory scale, heavy metals Zn, Pb, Cd, Ni, Cr and Cu were extracted synchronously from the sewage sludge. The results showed that H₃PO₄ concentrations had a significant effect on the extraction efficiency of heavy metals. The extraction efficiency increased gradually with H₃PO₄ concentrations increasing, and 5% H₃PO₄ was the optimal condition with the extraction efficiency of six heavy metals for Zn 79.71%, Pb 99.1%, Cd 87.15%, Ni 70.57%, Cr 78.01%, and Cu 60.91%, respectively. The species distribution of heavy metals in sewage sludge played an important influence on the extraction of heavy metals. From the analysis of species distribution of heavy metals before and after extraction, it was found that the extraction of heavy metals except for Pb came mainly from four fractions that were, exchangeable, bounding to carbonates, bounding to iron and manganese oxides, and bounding to organic matter. However, the residual fraction of Pb can almost be extracted completely. It was presumed that the extraction of heavy metals was mainly controlled by three aspects: the effect of H₃O⁺, the coordination reaction, and the precipitation reaction. On the synergistic effect of H₃O⁺, the coordination reaction predominated in the process of extraction compared with the precipitation reaction.