The influence of photoperiod and organic carbon levels in parabens removal from wastewater by Chlorella vulgaris

Parabens are emerging contaminants due to their abundant use as preservatives and inefficient treatment in wastewater (WW) treatment plants. To overcome the limitations of WW treatment plants in removing parabens, microalgae-based bioremediation has aroused great interest as an effective and sustainable process. Nevertheless, several factors affect the WW bioremediation capacity, which must be studied to achieve an effective biological treatment. The main objective of the present work was to evaluate the effects of photoperiod and WW composition, specifically organic carbon concentration (sourced as glucose), on the ability of the microalga Chlorella vulgaris to remove methylparaben (MetP). For that, two photoperiods (12/12 h light/dark and 24 h light) were studied and the composition of synthetic WW (SWW) was manipulated to have a glucose concentration of 0, 3, 30, or 300 mg/L. It was observed that the photoperiod significantly affects the bioremediation process. For a 12/12 h photoperiod, MetP was not removed. Removal was effective for a 24 h photoperiod, with percentages of MetP removal over 88% for glucose concentrations lower than 30 mg/L, decreasing for higher glucose levels. MetP photodegradation was negligible. For the 24 h photoperiod, it was possible to verify that higher glucose concentrations decreased microalga growth, with a decrease in the specific growth rate and the production of photosynthetic pigments. Furthermore, with the increase of the glucose concentration, the MetP constant rate of degradation decreased and its half-life time increased, taking longer to degrade the contaminant (through diauxic growth). Also, C. vulgaris exhibited strong growth ability and removed over 80% of nitrogen and phosphorous, unaffected by the presence of MetP and proportional to glucose levels, underscoring its potential for treating WW contaminated with high concentrations of parabens.