Characterisation by Mössbauer Spectroscopy of the Forms of Iron in Sulfide-Rich Fulvic Acid Solutions

Abstract

Variations in the nature of the bonding between iron and fulvic acid as a function of pH in sulfide-rich solutions have been investigated by ⁵⁷Fe Mössbauer spectroscopy. In all solutions iron was enriched to 95% in the ⁵⁷Fe isotope and ratios of Fe : fulvic acid were maintained at 1 : 100 on a weight basis for all measurements. When the pH was decreased below 5.0, there was a progressive change in the composition of the spectra, which contained three distinct components that were similar to those seen in the absence of sulfide; i.e., a sextet from magnetically-dilute Fe(III) and doublets from Fe(II) and Fe(III). There was, however, a higher proportion of the iron as Fe(II) in the sulfide-containing solutions and this represented the only form of iron at very low pH. Establishment of equilibria was slow as evidenced by considerable hysteresis between the compositions of the solutions with decreasing and increasing pHs. On increasing the pH, sulfide prevented the formation of magnetically dilute Fe(III) species and instead, in the range 3.5–6.0, compounds were generated with parameters consistent with sulfur being coordinated to the iron. One had parameters similar to those of pyrite and hence probably contains disulfide (S₂ ²⁻) units, whilst a second, with poorly defined magnetic hyperfine splitting, may contain monosulfide (S²⁻) units, with structures related to either pyrrhotite (Fe_1−x S) or greigite (Fe₃S₄). A third component probably corresponds to a mononuclear low spin Fe(II) complex, involving sulfur and fulvic acid in the iron coordination sphere. These species were oxygen-sensitive and decomposed to yield magnetically dilute Fe(III) complexes and Fe(III) oxyhydroxides on aeration of the solutions.