Covalently bound Eosin Y on chitosan as a removable photosensitizer for ranitidine degradation

Developing visible-light-responsive materials that enable pollutant removal is essential for advancing greener water treatment technologies. In this work, a chitosan-Eosin Y conjugate (CTS-EOY) was synthesized through carbodiimide coupling to produce a biodegradable and low-toxicity photosensitizer that operates homogeneously in aqueous media. Importantly, the biopolymeric nature of CTS-EOY allows its recovery from the reaction medium after treatment, addressing key limitations associated with freely dissolved dyes. The conjugate preserves the characteristic optical properties of Eosin Y while exhibiting modified excited-state behavior associated with its restricted environment within the biopolymer matrix. Ranitidine (RAN), a widely detected pharmaceutical and precursor of N-nitrosodimethylamine, was selected as a model contaminant to evaluate the environmental performance of the material. Under visible-light irradiation (525 nm), CTS-EOY achieved nearly 90% RAN removal within 2 h. Mechanistic studies demonstrated that degradation proceeds predominantly through singlet-oxygen-mediated pathways, with secondary contributions from triplet-substrate interactions, rather than through hydroxyl-radical-driven oxidation typical of advanced oxidation processes. LC-QToF/MS analysis identified five major transformation products, enabling the proposal of an environmentally relevant degradation route involving both Type I and Type II photosensitized pathways. Overall, this work highlights CTS-EOY as a sustainable biopolymer-supported photosensitizer for visible-light degradation of pharmaceutical pollutants combining efficient photodegradation with low toxicity and post-treatment recoverability, offering an energy-efficient alternative to conventional oxidation approaches.