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 energy deltaG=RTln(Qc/Kc) of precipitation reaction was used to examine the formation of precipitates during the leaching process. The values of activity equilibrium constant (Ka) of various precipitation reactions were adopted from the literature. The Ka values were corrected to obtain the corresponding values of concentration product (Qc) and concentration equilibrium constant (Kc) using the activity coefficients (gamma) of the ions. The activity coefficients (gamma) of the ions was calculated using Davies equation Lngammai=-1.172Zi2((I0.5/(1+I0.5))-0.3I) (aqueous solution, 25 degrees C, 1 atm, I>0.3 mol x 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 calculations indicated that the possible metal precipitates formed during the leaching process were (am)Cr(OH)3, (am)CrPO4, (am)Fe(OH)3, alpha-FeOOH(goethite), FePO4.2H2O, (am)AIPO4-2H2O, CaSO4.2H2O(gypsum). The solubility of these precipitates was found to decrease with the increase in the ionic strength (or sludge solids concentration). The results of computations were supported by the experimental observations.
Shen
S B
SB
Institut National de la Recherche Scientifique (INRS-Eau), Université du Québec, Sainte-Foy, Canada.
Tyagi
R D
RD
Blais
J F
JF
D011232:Chemical Precipitation; D005544:Forecasting; D007220:Industrial Waste; D008420:Manufactured Materials; D008670:Metals / Q000737:chemistry*; D008962:Models, Theoretical; D012037:Refuse Disposal; D013816:Thermodynamics
D007220:Industrial Waste; D008670:Metals