Adsorption of organic contaminants of emerging concern using microalgae-derived hydrochars

This study explored the adsorption capacity of hydrochars derived from a strain of microalgae biomass native to northern Sweden for contaminants of emerging concern (CECs) such as caffeine, chloramphenicol, trimethoprim, carbamazepine, bisphenol A, diclofenac, and triclosan. The findings indicate that the surface functionality of the microalgae-derived hydrochars - a blend of alkane/alkene and aromatic structures, coupled with different oxygen-containing functional groups (hydroxyl, carboxyl, and lactone) - significantly influenced the adsorption of the contaminants. The alkane/alkene and aromatic structures increased with increasing hydrothermal treatment temperature, while the oxygen- and nitrogen-containing groups diminished. Bisphenol A and triclosan, which were the compounds with the highest distribution coefficients, displayed improved adsorption on the hydrochars. The study measured peak adsorption values for the hydrochars processed at 180 °C, which achieved adsorption levels of 25.8 mg g^{- 1} for bisphenol A and 58.8 mg g^{- 1} for triclosan. The hydrochars produced using lower carbonisation temperatures (180 and 220 °C) exhibited enhanced adsorption of positively charged molecules such as trimethoprim, which was attributed to the increased presence of negatively charged oxygen-containing functional groups. Contrastingly, negatively charged molecules such as diclofenac and chloramphenicol demonstrated either low adsorption (2.5 mg g^{- 1} for chloramphenicol on hydrochar prepared at 180 °C) or no adsorption (diclofenac) due to repulsion by the negatively charged functional groups on the surface of the hydrochars.