Sulfide Precipitation in Wastewater at Short Timescales

Abatement of sulfides in sewer systems using iron salts is a widely used strategy. When dosing at the end of a pumping main, the reaction kinetics of sulfide precipitation becomes important. Traditionally the reaction has been assumed to be rapid or even instantaneous. This work shows that this is not the case for sulfide precipitation by ferric iron. Instead, the reaction time was found to be on a timescale where it must be considered when performing end-of-pipe treatment. For real wastewaters at pH 7, a stoichiometric ratio around 14 mol Fe(II) (mol S(-II))(-1) was obtained after 1.5 s, while the ratio dropped to about 5 mol Fe(II) (mol S(-II))(-1) after 30 s. Equilibrium calculations yielded a theoretic ratio of 2 mol Fe(II) (mol S(-II))(-1), indicating that the process had not equilibrated within the span of the experiment. Correspondingly, the highest sulfide conversion only reached 60%. These findings differed significantly from what has been demonstrated in previous studies and what is attained from theoretical equilibrium conditions.