Optimization analysis of separation conditions of washed water containing a lead complex and characterization of the precipitation products

Presently, the large amount of industrial leaded wastewater creates a great challenge to both environmental governance and wastewater recycling. Lead complexes in washed water must be removed mostly before the washed water can be recycled. This paper reports the mechanism and factors of removing Pb complexes in simulated washed water by the sulfide precipitation method. The reaction time, sodium sulfide dosage, pH, and polymeric aluminum chloride (PAC) dosage were analyzed and the optimal conditions were explored. The composition of the reaction products was also verified by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Additionally, the kinetics of the precipitation reaction of sodium sulfide and Pb-EDTA were studied. These results showed that the Pb removal efficiency reached 91.7% under the optimal process conditions which were as follows: the dosages of Na2S and PAC were 188 mg/L (Na2S/Pb2+ molar ratio of 5: 1) and 30 mg/L, respectively, the reaction time was 40 min, and the pH was 9. It was demonstrated using SEM and XRD that the reaction product in the separation process was PbS and the precipitation process was fitted to the following first-order reaction kinetics equation: C-t = 89.1e(-0.1047t) + 10.1 (R-2 = 0.9929; C-t is Pb concentration at reaction time t).