Reaction of Aquatic Humic Substances with Manganese and Chromium

Ultraviolet-visible and e.s.r. spectroscopies have been used to investigate the reactions of humic substances, extracted from swamp waters, with manganese(VII) and with chromium(VI). Potassium permanganate reacts quickly with humic substances to form manganese(II) derivatives. Phenolic groups are probably involved as a similar reaction occurs with simple phenols such as phenol and catechol. Manganese(II) interacts directly with carboxylate anionic groups in humic substances. E.s.r. spectroscopy shows that manganese(II) forms outer-sphere complexes with humic substances. E.s.r. spectra show that chromium(VI) is reduced to chromium(V) and then to chromium(III) in the presence of humic substances. The rate reduction of chromium(VI) depends on the concentration of humic substances and on pH, temperature, and u.v.-visible radiation. At pH 3 the rate of formation of chromium(V) is about 100 times faster than the rate of its reduction. At neutral pH and at room temperature the chromium(V) species is stable for a long period.