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Algal softening followed by ozonation: the fate of persistent micropollutants and natural organic matter in groundwater
Citation:
Kim, H., T. Timmes, H. Ryu, H. Yang, H. Yoon, AND S. Kim. Algal softening followed by ozonation: the fate of persistent micropollutants and natural organic matter in groundwater. JOURNAL OF HAZARDOUS MATERIALS. Elsevier Science Ltd, New York, NY, 402:123480, (2021). https://doi.org/10.1016/j.jhazmat.2020.123480
Impact/Purpose:
The aim of this study was to investigate a novel strategy for improving the performance of ozonation in regards to persistent micropollutant removal in order to achieve the following goals during the treatment of groundwater: (1) effective removal of the carbonate species (as a strong radical scavenger) with an alternative method of softening groundwater using photosynthetic bio-agents; (2) enhanced oxidation of groundwater by catalyzing the production of highly reactive OH radicals under alkaline conditions, where the indirect reaction dominates; (3) substantial reduction in the aromaticity of NOM to mitigate the quenching of OH radicals with NOM during ozonation of alkaline groundwater. In this study, softening was done in combination with subsequent ozonation for the robust treatment of hard groundwater in which a certain level of persistent micropollutants and humic-like organic compounds were present. Two different methods were used for groundwater softening using natural bio-agents and caustic chemicals, and each of those softening was followed by ozonation. The performance was evaluated in regard to the removal of DOC, UV254, and micropollutants (CBZ and DCF) in addition to the variations in well-known indicators of the organic aromaticity (SUVA) and biodegradability (AOC). The fate of NOM was also characterized in this study using a suite of innovative analytical tools. This work is distinctive from previous investigations in the following ways: (1) we identified the effectiveness and applicability of an alternative softening strategy using living algal components (as a renewable bio-agents) which are capable of self-repair and reproduction, rather than adding caustic chemicals which results in a high level of total dissolved salt in the produced water; (2) the robustness of persistent micropollutant removal was demonstrated through softening followed by ozonation, unlike in most previous investigations where ozonation is conducted ahead of softening; (3) a suite of advanced characterization techniques were applied (e.g., spectroscopy, chromatography, and bioassay) to better elucidate the variation in composition and functional properties of NOM through groundwater treatment and to provide further insight into the contribution and role of microalgae in the treatment of NOM-laden groundwater.
Description:
A proof-of-concept study was conducted to evaluate the feasibility of utilizing photosynthetic microorganisms for groundwater softening rather than caustic chemicals. A microalga Scenedesmus quadricauda was used to increase the pH of groundwater via natural photosynthesis. This work was designed to apply softening as a pretreatment to ozonation of hard groundwater and mainly focused on investigating the multiple beneficial effects of algal softening on the degradation of persistent micropollutants upon subsequent ozonation. The algae-induced alkaline conditions (pH >10) were favorable for the formation of OH radicals directly from O3 molecules. Moreover, algal softening was more efficient in the removal of strong radical-scavenging carbonate species (HCO3− and CO32−) than chemical softening, which was attributed to the combination of mineral carbonation and metabolic CO2 reduction. The natural organic matter (NOM) was characterized with spectroscopy, chromatography, and bioassay, and indicated that algal treatment decomposed NOM to a form less susceptible to OH radicals attack. Consequently, the ozonation of alkaline groundwater achieved a better removal of micropollutant residues from the groundwater. Carbamazepine and diclofenac were used as model chemicals of persistent groundwater contaminants and were almost completely removed with addition of 1.25 mg O3/L (0.63 mg-O3/mg-C).
URLs/Downloads:
DOI: Algal softening followed by ozonation: the fate of persistent micropollutants and natural organic matter in groundwater
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