A synergistic effect of reductant and complexant is observed in the dissolution of goethite by dithionite and citrate or EDTA. The rate data are interpreted using the surface complexation approach to describe the interface of the reacting oxide. Adsorption of both S2O42− (D) and complexant (L) generates three surface complexes that define the dissolution behavior: ≡ Fe-D, ≡ Fe-L, and dimeric surface complexes. The initial rate increases at lower pH values because of increased surface complexation conditional formation constants. At pH values below 4, however, the fast decomposition of S2O42− gives rise to a rapid depletion of reductant, and total dissolution is not observed. It is shown that for best analytical results in soil analysis, EDTA is a better complexant than citrate; the iron extracted in one dithionite-EDTA treatment at pH 5–6, under N2 at 315 K is not increased by increasing the number of extractions, and is equivalent to the total extractable iron found by previous procedures.

Understanding the reactions taking place in the hematite-oxalic acid system is important in order to clean iron oxides from filters and to remove iron from mineral concentrates. Previous studies reported the formation of an unwanted solid phase during this process. The objective of the current work, therefore, was to visualize and rationalize the iron dissolution steps taking place in the hematite–oxalic acid reaction by combining density functional theory (DFT) calculations and experimental data. The results of DFT calculations indicated that a precipitate was formed in this reaction; XRD analysis of the solid phase after the dissolution experiment revealed the formation of humboldtine as the precipitate. The attachment of oxalate on the hematite surface and the reduction of Fe(III) to Fe(II) were key steps for humboldtine formation. Both simulations and the experimental results showed that greater oxalic acid concentrations yielded more precipitate, suggesting a simple and novel route to synthesize humboldtine, a material which is relevant to the demand for clean energy.