Precipitation of calcium sulfate dihydrate in the presence of fulvic acid and magnesium ion

The present work focused on the reduction of the calcium and sulfate ions in a membrane permeate generated from a newly developed two-stage of tight ultrafiltration (TUF) for recovering humic substance from leachate nanofiltration (NF) concentrate by a precipitation process of calcium sulfate dihydrate (gypsum). The operating parameters (temperature and pH), mechanism of precipitating behaviors, and kinetics of this process in the presence of fulvic acid (FA) and magnesium ion simultaneously were investigated. It was found that Ca2+ removal increased with increasing temperature. The decline in pH, especially towards low pH range of 2-3 around the point of zero charge of gypsum, showed a significant effect on gypsum precipitation. Adsorption of FA onto gypsum surface had an important effect on the precipitating behavior of gypsum whose mechanism might be mainly weak interactions of hydrogen bonding and hydrophobic effect at low pH, while electrostatic interactions became dominant at neutral pH. In contrast, retardation effect of Mg2+ ion on gypsum precipitation was negligible at pH 2.5. The soluble calcium removal during precipitation process corresponded with first order rate equation. The results indicated that the optimal operating parameters were found at 70 degrees C and around pH 2.5, which can be applied to the scale-up and industrial design of the precipitation process of gypsum for removing the calcium and sulfate ions in the presence of FA and magnesium ion.