Photocatalytic treatment of mixed selenocyanate and phenol streams: Process modeling, optimization, and kinetics

Elevated selenium presence in the aquatic bodies because of increased industrial practices and the resulting toxicity concerns have prompted selenium treatment. Although most studies have focused on selenite (SeO₃²⁻) and selenate (SeO₄²⁻) removal, however selenocyanate (SeCN⁻) is another Se-species whose treatment is challenging often because of a complex wastewater matrix and presence of co-pollutants such as phenol. The present study therefore investigated the efficiency of TiO₂ photocatalysis (with EDTA as a hole/h⁺ scavenger) for the treatment of mixed selenocyanate/phenol wastewater along with response surface methodology (RSM) based process optimization, determination of reaction intermediates, and kinetic modeling. The application of face-centered central composite design (CCD) RSM modeling yielded Natural Log Transformed Linear model for selenium removal, and non-transformed two-factor interaction model for phenol removal. These models were duly validated using statistical tools, and then tested employing additional experiments. The optimal process conditions for maximum selenium and phenol removal were found to be pH 4, 10 ppm selenocyanate, 5 ppm phenol, and 450 ppm EDTA. Findings from the reaction byproducts analysis indicated selenite, selenate, benzenediols, and benzeneseleninic acid as most noted intermediates. A kinetic model that predicts the changes in parent compounds selenocyanate/phenol and dominant reaction intermediates during photocatalysis is also presented.