Algal softening followed by ozonation: The fate of persistent micropollutants and natural organic matter in groundwater

[Display omitted] •Softening followed by ozonation were applied to remove carbamazepine and diclofenac.•The oxidation of those chemicals with O3 was enhanced at elevated pH values.•This study used microalgae to soften groundwater via photosynthesis in the light.•The softening of groundwater greatly removed radical-scavenging carbonate species.•The softening also mitigated the quenching of OH radicals by natural organic matter. A proof-of-concept study evaluates the performance of a novel strategy using photosynthetic microorganisms to soften groundwater instead of using caustic chemicals. The microalga Scenedesmus quadricauda was used to increase the pH of the groundwater via natural photosynthesis. This work applied softening as a pretreatment to ozonation of hard groundwater and mainly focused on investigating the multiple effects of algal softening on the degradation of persistent micropollutants upon subsequent ozonation. The algae-induced alkaline conditions (pH > 10) were favorable to catalyze the formation of OH radicals directly from O3 molecules. Moreover, algal softening removed the strong radical-scavenging carbonate species (HCO3− and CO32−) to a much greater extent than that achieved by chemical softening, which was attributed to the combination of mineral carbonation and metabolic CO2 reduction. The fate of the natural organic matter (NOM) was characterized with spectroscopy, chromatography, and bioassay, which indicates that algal treatment decomposed the NOM to be less susceptible to attack by OH radicals. Consequently, the ozonation of alkaline groundwater achieved a better removal of the micropollutant residues in groundwater. Carbamazepine and diclofenac were used as model chemicals of persistent groundwater contaminants and were almost completely removed with an addition of 1.25 mg O3 L−1 (0.63 mg-O3 mg-C−1).