Homogeneous ferrous iron oxidation in a pilot-scale electrocoagulation system treating municipal wastewater: a model validation and simulation study

Abstract

During an iron-electrocoagulation (Fe-EC) process, floc formation is essential for achieving high contaminants removal. Thus, the complete oxidation of the Fe2+ dosed as coagulant is a critical step for ferric oxides flocs formation. Since the fluctuation in the quality of the influent wastewater affects the kinetics of Fe2+ oxidation, the estimation of optimal operating conditions (i.e. the retention time, dissolved oxygen (DO) concentration, etc.) for high Fe2+ oxidation is required. In this study, the kinetics of Fe2+ oxidation was simulated using PHREEQC software by theoretically optimizing, validating and improving the previously published kinetic models. During model simulation, the process parameters were varied from low to high ranges: Fe2+ dosage (10–100 mg/L) and retention times under the influence of changing pH (7.5–8.2), temperature (12–22 °C), alkalinity (5–10 mEq/L) and initial DO (8.6–10.5 mg/L). Fe2+ oxidation rate was more affected by pH variations in the influent than by temperature variations. A pH increase (+0.4 to +1.7 pH units) was observed due to the low wastewater alkalinity, promoting high Fe2+ oxidation rates. To ensure optimum Fe2+ oxidation levels (≥98%), a minimum retention time of 20 minutes was estimated. Finally, the residual DO concentration should be >3.5 mg/L to avoid a decrease in the oxidation rate. This study contributes to the ongoing research in the field of physico-chemical wastewater treatment with EC by establishing the optimal process parameters required for system optimization and process scalability. HIGHLIGHTS PHREEQC software was used to model Fe2+ oxidation kinetics in a Fe-EC system.; The formation of OH- during the EC process increases the Fe2+ oxidation rate.; Low oxygen content is a critical parameter for EC applications in wastewater.; The residual DO should be >3.5 mg/L to avoid a decrease in the oxidation rate.; A minimum retention time of 20 minutes is need for achieving ≥98% of Fe2+ oxidation.;

Keywords

• chemical kinetics
• homogeneous ferrous iron oxidation
• iron electrocoagulation
• municipal wastewater treatment