The current study was designed to test the potential of diammonium
phosphate (DAP) in immobilizing Pb and Zn in an industrial contaminated soil. Four
phosphate treatments including various ratios of added P to total heavy metals
content corresponding to stoichiometric P to Pb ratio of chloropyromorphite mineral
were tested. The obtained results revealed that the total contents of Pb and Zn in
the topsoil were above the maximum acceptable concentrations (MAC) that reported
by USEPA- 503 regulations. In the tested soils ,the data referred that the sequential extraction procedure
employed in this study appeared to be useful for classifying Pb and Zn within six
geochemical fractions. The fractionation studies indicated that Fe &Mn oxide fraction
proved to be the most abundant for Pb (more than 41% of total Pb) , whereas the
residual form was the dominant for Zn (37%) . The sum of the relative soluble pools
implies that a great portion of these metals may be highly bioavailable . Amending the
soil with DAP converted the relative soluble pools into more stable pools. Zn was
distributed in the order; residual »oxide>carbonate>organic>exchangeable > water-
soluble. While the order for Pb was residual = oxide >organic >carbonate >
exchangeable >water -soluble. On an average basis, the mobility indexes of Pb and Zn decreased from 20.1
to 11.10 and from 28.2 to 15.44 in the untreated and phosphate- treated soil ,
respectively. The concentrations of the active portion of Pb and Zn ,expressed as
minus logarithm of ML·¹ (RC) sharply decreased as a result of soluble phosphorous
amending. The pC of Pb²+ decreased from 10.23 to 11.23 and from 7.59 to 7.75 for
Zn²+ , suggesting that DAP has potential to immobilize Pb and Zn in industrial
contaminated soil. The results of solubility diagrams of lead-phosphate minerals suggested that
choropyromorphite mineral in equilibrium with either DAP or DCPD has been
recognized as a mineral controlling Pb solubility untreated (contaminated) and treated
soil solutions, respectively. Also, the results of solubility diagrams of Zn suggested
that Zn -soil regulate the solubility of Zn in all the tested soil solutions.