Reaction of Aquatic Humic Substances with Manganese and Chromium

Abstract

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.

Interaction study between Ca2+ and humic acids in brine media

“…In [8] the concentration of the phenol group was reported to be 2.26 ± 0.72 (pK a unknown). Our results compare well with values (pK a values given at α HA = 0.5) of humic acids extracted from soils or waters [2,[9][10][11][12][13][14] or purchased from other companies (Wako [15], Fluka [16] and between 2.0 and 3.0 meq/g for the weaker sites (with pK a values from 5.0 to 6.4). Measurements of phenolic groups have not been reported as frequently but the capacity values for the phenolic groups range from 0.8 to 4.3 meq/g, while their pK a values vary from 8.9 ± 0.1 to 9.0 ± 0.2.…”

Contribution of Metal Species to the Heterogeneous Photocatalytic Degradation of Natural Organic Matter

“…It was reported that chromate esters show characteristic absorption bands around 350-400 nm region of the spectrum and the most drastic band at 365 nm wavelength was ascribed to Cr(VI) oxy esters in the literature [19,20]. This distinct absorption band is similar to that reported by Lu et al, in the reaction of dichromate with humic substances which is known to be associated with the formation of Cr(VI) complex with diol, carboxylic acid, and polyphenol groups in humic substances [21].…”

“…As reported by Gamble et al, hydrated manganese(II) ions and humic substances might form outer-sphere complexes in which manganese(II) ions remain partly hydrated at binding sites of humic acid [22]. It was also reported that manganese(II) interacts directly with carboxylate anionic groups in humic substances forming a weak complex with a log K value of 2.0 (±0.1) [21]. However, the complexation of manganese with humic acid could not be observed with UV-vis spectroscopy under the specified experimental conditions.…”

“…Considering the stepwise reaction of chromium(VI) with humic acid, the first step constitutes the formation of chromium(VI) complex with diol, carboxylic acid, and polyphenol groups in humic substances [21]. The next step is a redox process occurring under acidic conditions, in which humic substances are detected to reduce chromium(VI) to chromium(V) as an intermediate, and then to chromium(III) which is complexed by humic substances [28].…”

Redox and complexation chemistry of the CrVI/CrV-d-glucaric acid system

“…It is well known that uptake of chromates by plants produce long lived oxo-Cr V species that could afford dangerous effects to ecological cycles. 13 In soils, the presence of humic and fulvic acids contribute to the reduction of chromates generat-ing oxo-Cr V species 14 that can be absorbed by plants in contaminated soils. Low frequency EPR studies of plants contaminated with chromates have demonstrated that oxo-Cr V species are found predominantly in roots.…”