Broad-spectrum dye removal in textile wastewater by selective liquid–liquid extraction and adsorption

The textiles sector is a leading source of wastewater pollution, largely due to its numerous wet processing stages, including pretreatment, dyeing, washing, finishing, coating, and printing. Among these, the dyeing step is particularly critical, generating the highest volume of effluents and contributing significantly to their toxicity, as it often discharges unreacted dyes and auxiliary substances. Current treatment technologies for wastewaters containing dyes span biological (e.g., enzymatic or microbial degradation), oxidative (including chemical and advanced oxidation processes), and physical methods (such as coagulation, absorption, and adsorption). However, only physical techniques —specifically adsorption and liquid–liquid extraction— enable complete recovery of all key components: the solvent or adsorbent, the residual dye, and the treated water, facilitating a circular and sustainable treatment loop. In this context, both Deep Eutectic Solvents (DESs) and biodegradable adsorbents offer promising, low-impact, and cost-effective platforms for dye removal. This work investigates two such complementary strategies: the extraction of various dye classes using custom-designed DESs, and the adsorption of dyes via modified biopolymer-based materials.

Liquid–liquid extraction systems based on DESs formed from natural components like Thymol, Camphor, Menthol, and Decanoic Acid showed high extraction efficiencies (up to 80%) for various dye classes, with solvents characterized using NMR and DSC. In parallel, chitosan beads doped with Choline Chloride–Urea DES—with and without magnetic FeO nanoparticles—were synthesized and characterized via SEM and FTIR. These adsorbents demonstrated strong decolorization capacity, with adsorption behavior fitting Langmuir isotherms and pseudo-second-order kinetics, indicative of monolayer adsorption dominated by electrostatic interactions. Together, these results highlight the versatility of both liquid–liquid equilibrium and adsorption processes, offering efficient and recoverable solutions for removing a wide range of dyes from textile wastewater.