Evaluation of the Effect of Humic Acids on the Reductive Elution of Fe from Fe₂O₃ 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 Fe₂O₃ 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.

Effect of humic acids on the reductive elution of Fe with ASC. Diamond, cube and triangle plots represent the presence of HHA, SHA and PHA (0–50 mg L⁻¹), respectively. Filled and open plots, which are average values (S.D., n = 3), represent the presence (5 mM) and absence of ASC, respectively. The corresponding values of the absence of Fe₂O₃ were subtracted from the original values.