Evaluating hydrophobic bacteria as potential adsorbents for estrogen removal from wastewater

Abstract

Estrogens are emerging contaminants classified as endocrine disrupting compounds (EDC). They interrupt ecological systems by altering the sex phenotype of the aquatic life, even at very low concentrations (part per trillion). Estrogens are hydrophobic compounds, which suggests that other hydrophobic materials and microorganisms could act as potential adsorbents. Consequently, several hydrophobic bacteria were evaluated as estrogen adsorbents as they are a sustainable resource, reproduce easily, and comprise high biodiversity. The aim was to isolate an array of hydrophobic bacterial strains that could adsorb the estrogens to a level that had no health and ecological implication. The relationship between the bacterial hydrophobicity and their performance in removing estrogens was investigated. Commercial strains of Rhodococcus erythropolis were adapted with n-hexadecane and hexane growth substrates, and new bacterial species were isolated from the diesel-contaminated soil through a soil-enrichment process. During the adaptation process using n-hexadecane substrate, R. erythropolis DSM311 showed a 6.70 % increase in cell surface hydrophobicity (CSH) compared to the parent strain. Six new strains were isolated, including the Tsukamurella sp. SD2-1, which showed the highest hydrophobicity at 91.33 %. Preliminary studies showed SD2-1 strain had the highest estrone removal efficiency (63.00 %) of the tested strains. A bacterial growth phase hydrophobicity study revealed that the stationary phase SD2-1 cells (collected at day 10) had higher CSH and produced higher estrone removal (95.30 %) compared to the exponential phase cells (collected at day 5). Longer incubation times (i.e. 15 and 20 days) gave no significant improvement to the CSH and estrone removal performance. Overall, the adaptation and soil-enrichment process produced bacterial strains with higher hydrophobicity and estrone adsorption capacity than both Escherichia coli (control) and commercial strains (R. erythropolis), removing estrone down to 1.78 – 9.31 ng.L-1 , very near to the predicted no-effect (PNEC) concentration of 3.6 ng.L-1 for estrone reported by the European Commission.