Dissolution of magnetite by etidronic acid: Dependence on diprotic and monoprotic conjugates

Abstract

A mechanism of Fe3O4 dissolution by the tetraprotic etidronic acid (HEDP) proposed 40 years ago is revised here. We reproduce the previously observed activation energy and reaction order of phosphonate with a selective subset of experimental conditions, showing that diprotic HEDP- and not the previously reported triprotic HEDP- is the species ultimately limiting the mechanism by protonating magnetite oxos and desorbing Fe3+. This proposed step is consistent with a speciation diagram showing monoprotic HEDP as the spontaneous chelator of ferric iron below pH 3, in the form of a partially soluble precipitate. Hence, while the magnetite dissolution rate is increased using the tri or tetraprotic acids, lower pH promotes reprecipitation of ferric iron. This has likely resulted in at least one previously reported upper bound for magnetite solubility in HEDP (0.6Fe:1HEDP)- and any others reported under conditions of the fastest observed dissolution kinetics- to be underestimated. This restriction on yield is avoided at the cost of rate by dissolving Fe3O4 at pH 5.