A new approach to increasing the efficiency of low-pH Fe-electrocoagulation applications

Incomplete oxidation of Fe(II) species released from the anode to Fe(III) may impede iron electrocoagulation processes conducted under low dissolved oxygen and/or pH<7 conditions, accompanied by the typically high buffering capacity of wastewater. This paper introduces a new approach to overcome this drawback by applying a second electrochemical cell (Ti/RuO₂ anode and Ti cathode) to be operated in parallel to the electrocoagulation cell. The second unit oxidizes Cl^- ions invariably present in the water to HOCl, which is capable of oxidizing Fe(II) species at a high rate, irrespective of pH or O_2(aq) concentration. An electrolytic cell with a Ti/RuO₂ anode and Ti cathode was shown to successively operate in parallel to a sacrificial electrocoagulation cell (Fe anode and Ti cathode) to attain complete Fe(II) conversion to Fe(III) under low-pH conditions, in which, in the absence of the 2nd cell, unwanted Fe(II) species would have dominated the dissolved iron species. Current efficiency for Cl₂ production was 12.4% and 45.7% at 200 and 1000mg Cl/l, respectively. Under three practical conditions (pH 6, [Cl^-]=200mg/l; pH 6, [Cl^-]=400mg/l; pH 5, [Cl^-]=600mg/l) the power demand of the combined system was 25.29, 12.7 and 8.1kWh/kg Fe(III)_produced, respectively, suggesting that the presented approach is competitive at [Cl^-]>~600mg/l.