Designing Cellulose-Based Adsorbents For Water Remediation

Abstract

Communities around the world are working to remove contaminants from freshwater sources. Adsorption is the primary technique utilized for remediation, but conventional adsorbents require long contact times to effectively remove pollutants. To overcome this limitation, we developed adsorbents using functionalized cellulose, a renewable, biodegradable, and inexpensive starting material, that rapidly remove pollutants from water via electrostatic interactions. More specifically, we demonstrated that functionalized cellulose fibers remove charged dyes and poly-/perfluoroalkyl substances (PFASs) from water in seconds. In Chapter 1, we summarize adsorption techniques used for capturing water contaminants, with a focus on the limitations of currently used adsorbents. We highlight the advantages of cellulose-based materials and our motivation to utilize electrostatic interactions to adsorb two prevalent pollutants, dyes and PFASs. Chapter 2 reports efforts designing localized cellulose-based hydrogels for rapidly removing methylene blue (MB), a cationic dye, from water. Specifically, we showed that anionic sulfated cellulose nanofibers (SCNFs) and quaternized hydroxyethyl cellulose ethoxylate (QHECE) form localized hydrogels and adsorb >90% of MB within 30 s of mixing. Adsorption was electrostatically driven and adsorption capacities for MB were greater than other cellulose-based hydrogels reported in the literature. This work showed that rapidly forming, localized hydrogels are promising adsorbents and may be useful as flocculating agents in water remediation. Toxic PFASs are infiltrating freshwater supplies, but current adsorbents are incapable of rapidly removing these contaminants from water. Chapter 3 demonstrated that cationic wood pulp (WP) fibers are an effective alternative adsorbent. Specifically, we showed that quaternized WPs (QWPs) adsorbed >90% of perfluorooctanesulfonic acid (PFOS) and >80% of perfluorooctanoic acid (PFOA) within 30 s of contact time at environmentally relevant concentrations. QWPs had adsorption capacities for PFOS and PFOA that rival conventional adsorbents. Furthermore, we found that adsorption was inhibited with humic acid present, but environmentally relevant solution pHs and salt concentrations had no effects on adsorption. These results indicated that WP is promising for removing PFASs from water. Given the effectiveness of gels and WPs for adsorbing dyes and PFASs in previous chapters, Chapter 4 describes initial efforts adsorbing an anionic dye with WP-based hydrogels and fibers. We revealed that cationic QWPs and anionic SWPs formed localized hydrogels and adsorbed methyl orange (MO), an anionic dye, within seconds of mixing. MO adsorption was electrostatically driven, and SWP-QWP crosslinking reduced dye removal efficiency. Additionally, QWP fibers alone adsorbed more MO than hydrogels, suggesting that cationic fibers are promising for capturing anionic dyes. Further studies evaluating adsorbent properties such as adsorption capacity are needed before concluding whether gels or fibers are more effective for capturing dye. Finally, we detailed initial work synthesizing WP with primary amine groups for future use as an adsorbent. Future studies will show the utility of using these materials for removing dyes from water. Chapter 5 summarizes each chapter and presents strategies for increasing the versatility of cellulose fiber adsorbents. Specifically, we intend to functionalize cellulose with groups complementary to other pollutants to expand the scope of contaminants that can be adsorbed. Additionally, adsorption will be performed in flow-based systems to evaluate functionalized cellulose fibers’ potential in large scale water treatment operations. Overall, our work designing cellulose-based adsorbents that rapidly capture pollutants is encouraging for advancing water treatment systems and will motivate researchers to develop improved adsorbents using renewable materials.