This study identifies the electrochemical and solution chemical controls on the production of Fe(II,III) (hydr)oxides formed by the electrolysis of Fe(0) metal, also knows as Fe(0) electrocoagulation. EXAFS spectroscopy and X-ray diffraction were used to characterize the solids produced as a function of: i) applied current, which corresponded to iron(II) production rates of 30–300 μM min−1, ii) pH and iii) background electrolyte. Two systems were investigated where: i) the dissolved oxygen (O2) concentration was maintained at 0.1, 0.3 and 3.0 mg L−1 and ii) the O2 drifted in response to varied Fe(II) addition rates. A narrow range of O2 separated the domains for Fe(II,III) and Fe(III) (hydr)oxide formation. At O2 ≥ 0.3 mg L−1, Fe(III) solids dominated, while Fe(II,III) (hydr)oxides were the principal phases at O2 = 0.1 mg L−1. The highest fraction of Fe(II,III) (hydr)oxides formed in the O2 drift experiments at the highest Fe(II) production rate, i.e. 300 μM min−1. The background electrolyte determined the type of Fe(II,III) (hydr)oxide that formed: NaCl solutions favored magnetite and NaHCO3 solutions favored carbonate green rust. Our results are consistent with an Fe(II,III) (hydr)oxide formation pathway where Fe(II) addition after O2 depletion leads to rapid (<10 min) transformation of precursory Fe(III) precipitates.
- Programområde 2: Vandressourcer