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Negra, Christine

Plant and Soil Science

2004

Ph. D.

Due to their high degree of reactivity, Mn oxides in soil systems may exert a greater influence on trace metal chemistry than that suggested by their relatively low abundance. In particular, Mn is the only known oxidizer of trivalent Cr in soils. Soil properties that influence the Cr oxidizing capacity of Mn oxides were investigated in eight well-aerated, high-Mn soils. Total and easily reducible Mn abundance were quantified by extraction with 1.5 M NH2OH.HCl and 0.02 M hydroquinone. Relative Mn oxidation state in soil samples was determined by x -ray absorption near edge structure spectroscopy (Mn-XANES) main edge energy position. Soils ranged between 0.14 and 1.27% NH2OH.HCl-extractable Mn, between 4.4 and 7.2 pH, and between 9.0 and 27.2% carbon. The Mn-XANES spectra showed that most of the study soils had a high Mn(IV):Mn(III) ratio with edge energy position intermediate to that of a synthetic birnessite and a synthetic pyrolusite. In these high-Mn soils, Mn-XANES edge energy was positively correlated with soil pH, suggesting a linear increase in the Mn(IV):Mn(III) ratio of soil oxides, over the normal range of soil pH. Soils with more total reducible Mn generally demonstrated greater net Cr(VI) production, but this pattern was moderated by soil pH and relative Mn oxidation state.
Soils with more total reducible Mn generally demonstrated greater net Cr(VI) production, but this pattern was moderated by soil pH and relative Mn oxidation state. High-Mn soils with low pH and Mn oxidation state were weaker Cr oxidizers than their Mn abundance would suggest. This work provides evidence that greater Mn abundance and greater tetravalence in soil Mn oxides enhances Cr oxidation. Preferential accumulation of trace metals by Mn in synthetic oxides and soil nodules has been attributed to specific adsorption and oxidation at Mn oxide surfaces, yet little is known about the interactions of trace metals with Mn in the bulk soil. Using the same high-Mn soils, the competitive effects of trace metal addition on Cr oxidation were investigated, as was the accumulation of added Pb, Co, and Cu by soil Mn using microscanning x -ray fluorescence (uSXRF). Short-term (15 min.) equilibrations of divalent Mn, Co, Pb, Cu, and Ni with soil samples containing 5 mg NH2OH-HCl-extractable Mn, resulted in substantial interference in Cr oxidation, confirming that these metals interact with Cr-oxidizing sites on Mn oxide surfaces. Interference in Cr oxidation did not follow the commonly accepted order of metal affinity for Mn oxides (Pb>Cu>Mn>Co>Ni), except for Ni which did show the weakest effect. Pb interference in Cr oxidation was much stronger in the highest-Mn soils, for which much smaller samples were used, and Cu interference was much stronger in the lowest-pH soil, both conditions in which the abundance of potential competing sorbent sites is greatly reduce.
Addition of Mn and Co resulted in the highest interference in Cr oxidation, probably due to specific affinity for Mn oxide surfaces even in the presence of competing soil sorbents, and was greatest in high-pH, high-Mn-valence soils which had been shown to have the highest Cr oxidation capacity. Micro-SXRF scans revealed substantial spatial correlation of soil Mn with added Co and Pb, but the microdistribution of Pb was equally well correlated with soil Fe. Only modest overlap of soil Mn with added Cu was 6bserved. These data suggest that the specific affinity of Pb, Cu, and Ni for soil Mn oxides was weaker than that of Mn and Co which probably reacted oxidatively with soil Mn. Higher Mn oxide valence may enhance sorption and oxidation of these oxidizable metals.