Soil Manganese Oxides and Trace Metals

Abstract

Trace metal accumulation 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 trace metal interactions with Mn in bulk soil. We investigated competitive effects of trace metal pretreatment on Cr oxidation in well‐aerated, high‐Mn soils, as well as accumulation of added Pb, Co, and Cu by soil Mn using microfocused synchrotron x‐ray fluorescence (μSXRF). Short‐term equilibrations of divalent Mn, Co, Pb, Cu, and Ni with soil samples weighed to contain equivalent amounts of NH2OH·HCl‐extractable Mn resulted in substantial interference in Cr oxidation, confirming metal interactions with Cr‐oxidizing sites on Mn oxide surfaces. Interference by Cu and Pb was greatest in samples likely to be low in competing sorbents, that is, low‐pH soils and smaller samples of higher Mn soils, respectively. Strongest interference in Cr oxidation resulted from Mn and Co addition, suggesting specific affinity for soil Mn oxide surfaces, and was greatest in high‐pH, high‐Mn‐valence soils previously shown to have the greatest Cr oxidation capacity. Nickel showed the weakest effect. Micro‐SXRF scans revealed substantial spatial correlation of soil Mn with added Co and Pb, but Pb microdistribution was equally correlated with soil Fe. Only modest overlap of soil Mn with added Cu was observed. Our data suggest that specific affinity of Pb, Cu, and Ni for soil Mn oxides was weaker than that of Mn and Co. Higher Mn oxide valence may enhance sorption and subsequent oxidation of these oxidizable metals.