Evaluation of the effect of humic acids on the reductive elution of fe from Fe 2 O 3 in a saline, seawater-like medium

Abstract

A fertilizer composed of steelmaking slag and compost, which would be expected to supply dissolved Fe to the sea, was evaluated for use in a seaweed-bed restoration technique. The Fe species in steelmaking slag is present in the form of insoluble Fe(III)-oxides, but could be eluted by seawater via reducing reactions. Humic acid (HA) would contribute to such a reductive Fe elution because it can act as an electron donor/acceptor. In this study, the effect of HAs with added ascorbic acid (ASC) on the reductive elution of Fe from Fe 2 O 3 into a saline medium (pH 8, I = 0.7 as NaCl) was evaluated by a laboratory-based method. After a 3-day incubation, approximately 3 μM of Fe(II) was eluted in the presence of 5 mM of ASC. The effect of reductive Fe elution was clearly enhanced by the co-presence of low concentrations of HA, due to the electron shuttling function of HA. When 5 mM ASC was used in conjunction with 5 mg of L −1 HA, which was derived from hard-wood bark compost, about 5 μM of Fe(II) was eluted. The function was attributed to the structural features of HAs, polyaromatic compounds, N-content and highly substituted aromatic compounds, all of which could enhance the reductive Fe elution process. The present study also provided a simple method to evaluate the efficiency of HA on the reductive Fe elution.