Investigations of Diclofenac Sorption on Intact and Modified Chlorella vulgaris Biomass with pH-Switchable Desorption

The growing interest in sustainable and structurally diverse sorbent materials has intensified the search for effective biosorbents that can complement or replace conventional adsorbents. This work presents the potential use of Chlorella vulgaris dried biomass and its modifications (ultrasound-treated, lipid-extracted, and combined forms) for diclofenac (DCF) sorption from aqueous solutions. It was demonstrated that sorption efficiency significantly depends on the solution’s pH. Lowering the pH from 6 to 2 increases the sorption from 5% to 68%, while 99% desorption occurred at pH 9. The adsorption isotherms for intact biomass and after lipid extraction (CV-E2) are best described by the Langmuir and Freundlich models; for ultrasonically treated biomass (CV-E1) by the Temkin model; and for ultrasound-assisted solvent extraction (CV-E3) by the Dubinin–Radushkevich model. These findings demonstrate that cellular lipids and particle characteristics critically govern sorption mechanisms, highlighting dried Chlorella biomass as a structurally and chemically tunable biosorbent. Importantly, the key sorption experiments were performed under strongly acidic conditions (pH 2), which differ from typical wastewater or surface water matrices. Therefore, the presented results should be regarded as a proof of concept illustrating the mechanistic potential of dried Chlorella biomass as a tunable sorptive material, with prospective relevance for separation science and laboratory-scale analytical applications rather than direct environmental remediation.