Bio-based sulfur copolymers from unsaturated oils: comparative study of safflower and linseed oils for Reactive Black...

Reactive textile dyes, particularly azo-based compounds such as Reactive Black 5 (RB5), are among the most persistent and toxic micropollutants in industrial wastewater, resisting conventional biological and chemical treatment due to their stable aromatic structure and strong water solubility. While numerous adsorbents have been explored, the development of materials that are simultaneously low-cost, biodegradable, thermally stable, and derived entirely from renewable feedstocks remains an unmet challenge. Elemental sulfur, an abundant industrial by-product with unsaturated vegetable oils, offers a promising route to bio-based, sulfur-rich adsorbents. In this study, four sulfur-oil copolymers were synthesized by inverse vulcanization of elemental sulfur with safflower oil (S-S) and linseed oil (S-L), with and without sodium chloride (NaCl) as a salt-templating porogen to produce porous variants S-S-N and S-L-N. Fourier transform infrared spectroscopy confirmed successful S-C bond formation and consumption of olefinic double bonds in oil, while thermogravimetric analysis demonstrated that NaCl-induced porosity and higher oil unsaturation synergistically enhanced thermal stability, with S-L-N exhibiting the highest decomposition temperature (551 °C). A scanning electron microscope observed a change in the surface morphology between porous and non-porous copolymers. Porous copolymers showed up to 37% weight loss in six-week soil burial tests, confirming biodegradability, and exhibited stronger antimicrobial activity against Staphylococcus aureus and Escherichia coli compared to their non-porous counterparts. Batch adsorption for RB5 removal showed that S-L-N achieved the highest removal efficiency of 64% under optimized conditions (pH 7.5, 35 °C, 50 mg dosage), while non-porous S-L removed only 8%, demonstrating that porosity unlocks the adsorption potential of chemically active but physically inaccessible sulfur sites. Kinetic modeling confirmed that all four copolymers follow the pseudo-second-order (PSO) model (R² = 0.9999-1.000), with rate constants k2 following the order S-S (0.1689) > S-L-N (0.1261) > S-S-N (0.1166) > S-L (0.0504 g·mg^-1·min^-1), confirming chemisorption as the dominant rate-controlling mechanism across all systems. Equilibrium data were best described by the Freundlich model (R² = 0.973-0.996) for all copolymers. This work demonstrates that systematically tuning oil unsaturation and NaCl-induced porosity provides a simple, scalable strategy to design sulfur-based copolymers with programmable adsorption kinetics, surface heterogeneity, and removal efficiency, offering a sustainable, biodegradable, and thermally stable alternative for the remediation of dye-contaminated wastewater.