Simultaneous electrochemical removal of oxalic acid and oil emulsions using a serpentine aluminum tube anode in chloride medium

Oxalic acid and oil co-occur in metal-finishing, descaling and tannery effluents, yet their simultaneous removal by electrocoagulation (EC) has, to our knowledge, not been reported. EC was performed on a synthetic wastewater in a cell using a vertical aluminum serpentine-tube anode (two perpendicular arrays, designed to double as a potential heat exchanger) between two screen cathodes. Effects of pH (2–9), NaCl (1–4 wt%), current density (10.1–36.29 mA/cm 2 ), initial concentrations (oil 250–2000 ppm; oxalic acid 500–5000 ppm), temperature (25–45 °C) and stirring (0–1500 rpm) were examined over 60 min. Under the best conditions tested (pH 7, 2 wt% NaCl, 25 °C, 36.29 mA/cm 2 ; 250 ppm oil, 500 ppm oxalic acid), removal reached 98% and 60%, respectively. Oil removal saturated above ~20 mA/cm 2 , whereas oxalic acid removal rose steadily but was limited by soluble Al–oxalate complexes. Heating to 45 °C lowered oil removal (96→74%) but slightly raised oxalic acid removal (32→40%); stirring had little effect. Specific energy consumption rose with current density (oil: ~6–30 kWh/kg) and fell with pollutant concentration (oxalic acid: 82–12 kWh/kg). Control experiments revealed slight negative effect of oxalic acid on oil removal but synergy for oxalic acid in presence of oil. Kinetic analysis confirmed pseudo-first-order removal for both pollutants. The results are consistent with a competition between coagulation and Al–oxalate complexation and support EC as pretreatment for concentrated oxalic acid–oil wastewaters.