Potential interference of organic acids and ferrous iron in the interpretation of Fe and Mn indicators of reduction in soil

Abstract

Removal of manganese (Mn) and iron (Fe) oxide coatings from indicators of reduction in soil (IRIS) is considered a function of microbial reduction of these elements under sufficiently reducing conditions, but there may be situations where removal is abiotic. We examined whether abiotic removal of Mn and Fe oxide coatings may be facilitated by organic acids and whether loss of Mn can take place via chemical reduction by ferrous iron (Fe²⁺) in vitro. Loss of color from Mn-IRIS in citric acid solution was highest at 1 and 10 mM regardless of pH, with no loss observed at lower concentrations. Effects of citric acid on Fe-IRIS were also limited to 1 and 10 mM, but color loss only occurred at pH 4.5 and the rate and extent of loss were lower than for Mn-IRIS. Color loss from Mn-IRIS in oxalic acid was highest at 1 and 10 mM and increased with decreasing pH. Effects of oxalic acid on loss of color from Fe-IRIS were limited to 1 and 10 mM. Loss of color from Mn-IRIS was limited to 10 mM salicylic acid and pH 4.5, with no effect on Fe-IRIS regardless of concentration and pH. Only the two highest concentrations of Fe²⁺ (50 and 100 mM) resulted in reduction of the Mn-IRIS coating and subsequent deposition of an Fe oxide coating. Organic acids and pH have a differential effect on color loss from Mn- and Fe-IRIS, with only high concentrations of organic acids at low pH possibly interfering with interpretation.