Performance of integrated ferrate–polyaluminum chloride coagulation as a treatment technology for removing freshwater humic substances

Abstract

Ferrate-based technologies can play versatile roles in water treatment because of their potential for in situ production and because they do not form any harmful by-products. We compared the oxidative performance of Fe(VI) generated by an electrochemical process, with H2O2-UV irradiation (a standard oxidation process) for removing Suwannee River natural organic matter. It took only 5 min for Fe(VI) (Fe: 1.67 × 10−4 M) to reduce the fluorescence intensity of a humic-like fluorophore by 36% of the original value; in contrast, it took 120 min of irradiation using H2O2-UV ([OH] ∼ 1.8 × 10−13 M) to remove 68% of the original value. In addition to the short reaction time, ferrate can also form aggregates that can remove turbidity and adsorb organics and other contaminants present in water. Simultaneous addition of ferrate and polyaluminum chloride (PACl) to unfiltered natural water displayed the most efficient reduction of UVA254, primarily under acidic conditions. Ferrate pre-oxidation followed by PACl coagulation was the most effective process for reducing turbidity and chromaticity, because of the effects arising from the coagulation of ferrate that resulted in Fe(III)(s) species. Ferrate pre-oxidation generated low-molecular-weight UVA254-absorbing organics, whose dissolved organic matter (DOM) peak at 1250 Da was removed by PACl coagulation. Neither the initial pH nor the process order significantly affected the removal of organic carbon by the integrated ferrate-PACl process.