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Speciation of heavy metals exerts important controls on chemical behavior, solubility, toxicity, adsorption and even physical properties of solutions (Ure and Davidson 2002). In the present study, speciation and mineral–solid phase of Cd, Ni, and Pb in solution of soil prolonged irrigated with sewage effluent were predicted using GEOCHEM model (Sposito and Mattigod (1980) and constructed stability diagrams (Sposito ,1989 and Lindsay ,1979).The present data indicate that total contents of all tested metals have increased dramatically as a result of sludge amending, particularly, for Cd. In virgin soil solution, the results of speciation indicated that, free species M was the prevalent form (60.35%) followed by carbonate species MCO3aq (20.12%), MSO4 (5.93%), MHCO3 (5.56%), MOH(4.21%) and MCL(4.21%), while in sludge – soil solution the sequence was; M-DOC (41.92%), followed by MHPO4 species (26.74%), M( 20.07%) , MHCO3(4.22%), MSO4 aq (3.49%) and MCO3 aq (2.02%). It is clear from the results that amending soil with sludge decreases active portion (M) contribution to total speciation of the tested metals. In virgin soil, the majority of both Cd and Ni was (M) species. It amounted (79.03%) and (83.8%) , respectively . While MCO3aq was the prominent for pb (55.86%). In sludgesoil, Pb was almost exclusively in organically complexed forms (88.54%), while the majority species of Ni was M (38.56%) and for Cd, CdHPO4aq (58.72%). The data also showed that in sludgesoil the activities (Ml ) of both Pb and Ni decreased by 26% and 72%, respectively, compared to their corresponding values in virgin soil. Meanwhile, the activity of Cd increased. These findings suggesting that amending soil with sludge probably inhibit activities of some metals, while promote activity of others. The data showed that the values of activities of Cd in virgin soil was closed to the solubility of CdCO3 (log CO2=-2.52) and CdSO4-2Cd (OH)2 at (log So4=-2) , while in sludge soil ,the value of Cd was under saturated with respect the constructed Cdminerals. This suggested that other possible solid phase of cadmium may be controlling its activity in that soil .Also , the data suggested that activity of Ni in virgin soil is governed by Ni3(PO4)2 in equilibrium by gypsite. While in sludge-soil, value of Ni was controlled by Ni3 (PO4)2 in equilibrium with Caphosphate mineral at (log CO2= -4.52). Also, the results indicated that Pb3(CO3)2(OH)2was the possible solid phase which may be controlled the activity of Pb , in both virgin and sludge soil. Keywords; sewage effluent, heavy metals, speciation, stability diagrams

Predicted Speciation and Mineral Solid Phases of some Heavy Metals in Sludge - amended Soil