Development and Characterization of Lignin-Based Hydrogels for Efficient Adsorption of Diclofenac From Aqueous Environment

Hydrogels are interesting materials with potential applications in the treatment of water contaminated by organic and inorganic toxic compounds. Among the different monomers/polymers commonly used, the natural polymer lignin is undoubtedly an attractive candidate due to its biocompatibility, biodegradability, low toxicity, and availability in high quantities as the main by-product of the pulp industry. In the present work, the synthesis and characterization of a lignin-based hydrogel are described and tested in the adsorption of the nonsteroidal anti-inflammatory drug, diclofenac, one of the so-called emerging contaminants. Due to the anionic nature of diclofenac currently used in pharmaceutical preparations, a cationic functionality is included in the polymeric backbone. The obtained hydrogel is characterized by a porous structure, thermal stability, and an elastic behavior more pronounced than the viscous one. It has a high swelling capacity and is able to efficiently remove diclofenac in batch mode, following a pseudo-second order kinetic, and adsorption could be well described by Langmuir and Sips isothermal models. For the first time, diclofenac removal by a lignin-based hydrogel is also carried out in a packed-bed column with a maximum capacity of ca. 50 mg/g, and different theoretical models are used to fit the experimental data.