Organic Oxidation and Manganese and Aluminum Mobilization in Forest Soils

A kinetic model is presented that describes the initial and rapid redox processes between polyhydroxyphenolic acid and soil or manganese oxide suspensions. The rate constants derived by the model were similar in magnitude in both suspensions for the organic reductants studied. Polyhydroxyphenolic acids with para- and ortho-OH groups were rapidly oxidized by manganese oxides with spectral evidence suggesting that the reaction leads to polymeric humic products by way of benzoquinone derivatives. In contrast, polyhydroxyphenolic acids with meta-oriented phenolic-OH groups were not oxidized by soil or by manganese oxide suspensions within the 120-min reaction period, presumably because these compounds were not capable of being oxidized to benzoquinone intermediates. The oxidative process was also accompanied by formation of Mn²⁺ both from soil and from manganese oxide suspensions. The rate of Mn²⁺ formation by gallic acid from Challenge A horizon soil suspensions containing a total of 3.6 × 10⁻⁴ mol L⁻¹ Mn content was comparable to the rate from birnessite suspensions containing a total of 2.5 × 10⁻⁴ mol L⁻¹ Mn content. The appearance of Mn²⁺ in soil suspensions within the reaction period was attributed mainly to reduction rather than clelation by the organic reductants or their products of oxidation. Dissolution of soil Mn also occurred by 2,5-dihydroxybenzoic, and 3,4-dihydroxybenzoic acids, but not by 2,6-dihydroxybenzoic and 3,5-dihydroxybenzoic acids. Humic products formed from oxidation by Mn were quite effective chelators of Al. The rates of Al dissolution by humic products from Blodgett A horizon soil in suspensions were comparable to the rates in the original organic acids, and were 3 to 6 times faster in dissolving Al than mineral acid at comparable pH levels. The parallel behavior in reactivities of both soil and manganese oxide suspensions towards polyhydroxyphenolic acids suggests that manganese oxides are being reduced in both systems.