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Sorption using natural materials such as clay minerals is generally regarded as a suitable method for wastewater treatment. Equilibrium nickel (Ni) sorption on Ca-bentonite was determined in solutions with initial Ni concentrations ranging from 0–100 mg L−1. The effect of contact time and temperature on Ni sorption was also investigated. The equilibrium capacity of the bentonite for Ni removal was 2.93, 3.91, and 6.00 mg g−1at 278, 298, and 318 K, respectively, showing sorption enhancement with rising temperature. The results obtained from the kinetic tests indicated that major Ni removal occurred within the first 2 h, but the equilibrium condition was not attained before 24 h. Negative changes in Gibbs free energy (ΔG°) of sorption indicated that Ni sorption on Ca-bentonite was a spontaneous reaction. The positive values of both ΔH° and ΔS° suggest the endothermic nature of the Ni sorption and increased randomness at the solid–solution interface during the Ni chemisorption on the Ca-bentonite, respectively. In conclusion, Ca-bentonite is a potential sorbent for removal of Ni from wastewaters with the most favorable removal behavior at higher temperatures. [ABSTRACT FROM PUBLISHER]

Lead (Pb) solubility is commonly limited by dissolution–precipitation reactions of secondary mineral phases in contaminated soils and water. In the research described here, Pb solubility and free Pb2+ ion activities were measured following the precipitation of Pb minerals from aqueous solutions containing sulfate or carbonate in a 1:5 mole ratio in the absence and presence of phosphate over the pH range 4.0–9.0. Using X-ray diffraction and Fourier-transform infrared spectroscopic analysis, we identified anglesite formed in sulfate-containing solutions at low pH. At higher pH, Pb carbonate and carbonate-sulfate minerals, hydrocerussite and leadhillite, were formed in preference to anglesite. Precipitates formed in the Pb-carbonate systems over the pH range of 6 to 9 were composed of cerussite and hydrocerussite, with the latter favored only at the highest pH investigated. The addition of phosphate into the Pb-sulfate and Pb-carbonate systems resulted in the precipitation of Pb3(PO4)2 and structurally related pyromorphite minerals and prevented Pb sulfate and carbonate mineral formation. Phosphate increased the efficiency of Pb removal from solution and decreased free Pb2+ ion activity, causing over 99.9% of Pb to be precipitated. Free Pb2+ ion activities measured using the ion-selective electrode revealed lower values than predicted from thermodynamic constants, indicating that the precipitated minerals may have lower KSP values than generally reported in thermodynamic databases. Conversely, dissolved Pb was frequently greater than predicted based on a speciation model using accepted thermodynamic constants for Pb ion-pair formation in solution. The tendency of the thermodynamic models to underestimate Pb solubility while overestimating free Pb2+ activity in these systems, at least in the higher pH range, indicates that soluble Pb ion-pair formation constants and KSP values need correction in the models. [ABSTRACT FROM AUTHOR]