H2S release rate strongly affects particle size and settling performance of metal sulfides in acidic wastewater: The role of homogeneous and heterogeneous nucleation.

Sulfide precipitation is an extensively used method to precipitate metal and arsenic from acidic wastewater, whereas the tiny and negatively-charged metal sulfides with poor settling performance are generated. The factors and mechanisms that influence particle size and settling performance remain unclear. Herein, the effects of sulfuration factors, e.g., reagent dosage, acidity and H2S release rate on the particle size and settling performance of metal sulfides were investigated, and involved mechanisms were systematically revealed. The results showed that the reagent dosage and acidity had a limited effect on particle size and settling performance while the H2S release rate played a critical role. Under homogeneous conditions, the decrease in H2S release rate, which can reduce the initial supersaturation and supply the sustainable supersaturation, increased the particle size of metal sulfides generated using Na2S solution. Under heterogeneous conditions, the decrease in H2S release rate further increased the particle size of metal sulfides generated using low-solubility CaS/FeS and further improved settling performance, in which heterogeneous nucleation played a crucial role besides supersaturation. The developed dissolution-diffusion-growth model qualitatively explained the negative relationship between H2S release rate and particle growth. This work provides implications for improving the settling performance of metal sulfides in acidic wastewater.