Insights on free radical oxidation and in-situ coagulation in PMS/Fe(II) process for the removal of algogenic organic matter precursors

Abstract

Algogenic organic matter (AOM) is unwelcome in raw water as it is a major precursor of carcinogenic disinfection byproducts (DBPs). Unfortunately, conventional coagulation is ineffective against AOM, raising the need for a better alternative for AOM and AOM-derived-DBP removal. This study examined the effectiveness and underlying mechanisms of a novel hybrid oxidation-coagulation PMS/Fe(II) process for effective removal of AOM-DBP precursors. Under optimal conditions, only PMS/Fe(II) or combined PMS/Fe(II) with Al/Fe coagulation removed significant amounts of dissolved organic carbon (DOC) (45–52%) and 50–65% AOM-fluorescent components. These values were much better than conventional Al/Fe coagulations (e.g. 15–19% DOC and < 40% fluorescent components at [Fe3+] = [Al3+] = 100 μM, pH 5.5–6). AOM-derived DBPs were thus significantly eliminated (>80% of total DBP levels) after PMS/Fe(II) related processes, which markedly outperformed Al/Fe coagulations (of only 20–30%). PMS/Fe(II) presented its strongest oxidative ability at pH 4 (with [SO4•−] = 2.16 × 10−12 and [•OH] = 1.26 × 10−12 M), rather than at pH 7 or 9. Acidic conditions rapidly increased oxidized-carbon contents (e.g. %ketone + %carbonyl increased from 10% to 25%) in AOM structure after PMS/Fe(II) treatment. By contrast, pH 7 and 9 were the favorable conditions for in-situ coagulation with faster floc aggregation and growth rates than pH 4. Regardless, PMS/Fe(II) process performed best in effectively removing AOM-DBP precursor only under neutral pH of 6–7 by gaining the synergistic benefits of free radical oxidation and in-situ coagulation. Our findings are a significant contribution to filling the knowledge gap in the study of PMS/Fe(II) and highlight the possibilities of this novel hybrid process as an alternative to conventional coagulation for sustainable water treatment.