Prediction of metal precipitates in tannery sludge leachate based on thermodynamic calculations

Thermodynamic calculations were performed in order to predict the formation of metal precipitates during the tannery sludge leaching. Gibbs free energ y DeltaG=RTln(Q(1)/K-t) of precipitation reaction was used to examine the f ormation of precipitates during the leaching process. The values of activit y equilibrium constant (K-a) of various precipitation reactions were adopte d from the literature. The K-a values were corrected to obtain the correspo nding values of concentration product (Q,) and concentration equilibrium co nstant (K-a) using the activity coefficients (gamma) of the ions. The activ ity coefficients (gamma) of the ions was calculated using Davies equation L n gamma (1)=-1.172Z,(2)((I-0.5/(1+I-0.5))-0.3I) (aqueous solution, 25 degre esC, latm, 1>0.3 mol.l(-1)). The values of ionic strength (I) at different sludge solids concentration and leaching pH were obtained by measuring the concentration of all ionic species in the leachate. The thermodynamic calcu lations indicated that the possible metal precipitates formed during the le aching process were (am)Cr(OH)(y) (am)CrPO4, (am)Fe(OH) alpha -FeOOH(goethi te), FePO4-2H(2)O, (am)AlPO4. 2H(2)O(2) CaSO(4)2H(2)O(gypsum). The solubili ty of these precipitates was found to decrease with the increase in the ion ic strength (or sludge solids concentration). The results of computations w ere supported by the experimental observations.