A mechanism of Fe3O4 dissolution by the tetraprotic 1-Hydroxyethane(1,1-diylbisphosphonic acid (HEDP) proposed 40 years ago is revised here. We reproduce the previously observed activation energy and reaction order of phosphonate, but using a more selective subset of experimental conditions which collectively point to diprotic HEDP, and not the previously reported triprotic HEDP, as the predominant species driving the mechanism. While the rate of dissolution can be increased using the tri or tetraprotic acids, this increased acidity promotes the reprecipitation of iron(III) as a hydrated and weakly acidic HEDP complex. 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 kinetics- to be underestimated from its true value (1Fe:1HEDP), which is obtained here consistently at higher pH and over a much wider range of concentrations (0.005-0.1M) than previously demonstrated.

Magnetite dissolution in etidronic acid: A tradeoff of speed and yield

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