Heavy metal speciation and leaching behaviors in cement based solidified/stabilized waste materials

A circuit board printing factory sludge containing high concentrations of c opper, zinc and lead was stabilized and solidified (S/S) with different por tions of ordinary Portland cement (OPC) and pulverized fly ash (PFA). The c hemical speciation and leaching behavior of heavy metals in these cement-ba sed waste materials were studied by different sequential extraction procedu res, standard toxicity characteristic leaching procedure (TCLP) and progres sive TCLP tests. The sequential extraction results showed that more than 80 % of Cu, Pb and Zn were associated with Fraction 2 (weak acid soluble, extr acted with 1 M NaOAc at pH 5.0 with a solid to liquid ratio of 1:60). This indicated that the heavy metals could exist in the S/S matrix as metal hydr ated phases or metal hydroxides precipitating on the surface of calcium sil icate hydrates (C-S-H), PFA and sludge particles. The progressive TCLP test results and MINTEQA2 calculation also showed the importance of Cu and Zn o xides during the leaching process. The leaching, behaviors of these metals in the S/S waste materials were mainly controlled by the alkaline nature an d acid buffering capacity of the S/S matrix. During the progressive TCLP te sts, the alkaline conditions and acid buffering capacity of the matrix decr eased with the dissolution of calcium hydroxide and (C-S-H), therefore, the leaching of heavy metals in the S/S waste materials increased. The leachin g of heavy metals in the S/S materials can be considered as a pH dependent and corresponding metal hydroxide solubility controlled process. (C) 2001 E lsevier Science B.V. All rights reserved.