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 K_SP 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 K_SP values need correction in the models.

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⁻¹ at 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.