Kinetics of sulfide precipitation with ferrous and ferric iron in wastewater

Traditionally, sulfide abatement has been done by adding e.g. ferrous or ferric iron salts to the start of sewer force mains. Iron dosage must hence correspond to an estimate of how much sulfide will form while the wastewater stays in the main, which is not straightforward. Adding iron salts at the end of the main has the advantage that the exact amount of sulfide to precipitate, in principle, can be known. A drawback is that the reaction time is short compared to start-of-pipe treatment. Sulfide precipitation rates and the concentration of sulfide left after the process had run to completion were measured using an online hydrogen sulfide probe to resolve the fast precipitation reaction. Experiments were conducted in anaerobic wastewaters spiked with sulfide, and carried out under different pH conditions and with various iron-to-sulfide ratios. Sulfide precipitation rates were demonstrated to be faster with Fe(III) than with Fe(II). Experiments furthermore showed that for Fe(III), pH was the controlling parameter, whereas for Fe(II) both pH and the iron-to-sulfide ratio were important. Proposed are model equations to predict variables for the rate equations, which can be adopted by practitioners dealing with sulfide abatement.