The objective of this work was to investigate the reaction stoichiometry, k
inetics, and mechanism for Cr(VI) reduction by hydrogen sulfide in the aque
ous phase. Batch experiments with excess [Cr(VI)] over [H2S](inverted perpe
ndicular) indicated that the molar amount of sulfide required for the reduc
tion of 1 M Cr(VI) was 1.5, suggesting the following stoichiometry: 2CrO(4)
(2-) + 3H(2)S + 4H(+) --> 2Cr(OH)(3)(s) + 3S(s) + 2H(2)O. Further study wit
h transmission electron microscopy (TEM) and energy-dispersive X-ray spectr
oscopy (EDS) confirmed that chromium hydroxide and elemental sulfur were th
e stable products. The kinetics of Cr(VI) reduction by hydrogen sulfide was
measured under various initial concentrations of Cr(VI) and sulfide as wel
l as pH values controlled by HEPES, phosphate, and berate buffers. Results
showed that the overall reaction was second-order, i.e., first-order with r
espect to Cr(VI):and first-order to sulfide. The reaction rate increased as
pH was decreased, and the pH dependence correlated well with the fraction
of fully protonated sulfide (H2S) in the pH range of 6.5-10. The nature of
buffers did not influence the reaction rate significantly in the homogeneou
s system. The reaction kinetics could be interpreted by a three-step mechan
ism: formation of an inner-sphere chromate-sulfide intermediate complex (((
H2O4CrS)-S-VI)(2-)), intramolecular electron transfer to form Cr(IV) specie
s, and subsequent fast reactions leading to Cr(III).