Sequestration of Cadmium Using Stabilized Iron-based Nanoparticles and Removal of Per- and Polyfluoroalkyl Substances Using Ion Exchange Resins and Photo-regenerable Adsorbent

Abstract

Over years, I have been involved in several research projects that are highly cross-disciplinary. As such, the dissertation encompasses sequestration of cadmium (Cd) in contaminated water and soil using novel stabilized nanoparticles and enhanced adsorption and photodegradation of per- and polyfluoroalkyl substances (PFAS) from water and landfill leachate using a new photo-regenerable adsorbent. Cd is one of the most detected toxic heavy metals in the environment. In this work, we prepared a class of stabilized ferrous sulfide (FeS) nanoparticles using sodium carboxymethyl cellulose (CMC), starch, or carboxymethyl starch (CMS), as a stabilizer. The CMC-stabilized FeS nanoparticles (CMC-FeS) showed a faster adsorption rate and higher adsorption affinity and capacity than CMS- or starch-stabilized FeS nanoparticles. CMC-FeS were able to rapidly remove 93% of 1 mg/L Cd from water within 4 h at a dosage of 100 mg/L, and the material showed a maximum Langmuir sorption capacity of 497.5 mg/g at pH 7.0. In addition, CMC-FeS can be delivered into contaminated soil to facilitate in situ immobilization of Cd in soil. When the 58.3 mg/kg Cd-laden soil was amended with 100 mg/L CMC-FeS, the equilibrium Cd leachability based on the toxicity characteristic leaching procedure (TCLP) was reduced by 88.4%. When a CMC-FeS suspension (100 mg/L) was passed through a Cd-laden soil column at an empty bed contact time of 52.7 min, the water-leachable Cd was reduced by 98.2% after 55 pore volumes (PVs) of the treatment. PFAS have been widely detected in aquatic systems. Yet, cost-effective technologies have been lacking due to the unique molecular structures and chemical stability of PFAS. While ion exchange resins and activated carbon have been used to remove PFAS from water, they bear with some key limitations, including 1) conventional adsorbents are less effective for short-chain PFAS, and 2) the regeneration of spent adsorbents is difficult and costly. To address these pressing issues, we tested the effectiveness of three commercial anion exchange resins and one polymeric ligand exchanger (DOW 3N-Cu) for removal of perfluorobutanoic acid (PFBA), as a model short-chain PFAS. While the commercial strong-base anion exchange resin (IRA900) showed the best removal rate and capacity for PFBA at 100 mg/L, the weak-base resin (DOW-66) and DOW 3N-Cu offered flexibility in regeneration. A mixture of 1% NaCl and 40% CH3OH solution was found effective for regenerating all four materials (including IRA958), with ~90% of the capacity recovered. Moreover, we employed a new adsorptive photocatalyst (Fe/TNTs@AC) to treat multiple PFAS in landfill leachate. At a dosage of 10 g/L, Fe/TNTs@AC effectively removed >95% of 13 PFAS from a model municipal landfill leachate. The effective adsorption concentrates the PFAS on a small volume of Fe/TNTs@AC, and the concentrated PFAS were then photodegraded under UV, which also regenerates the material. Following the photo-regeneration, Fe/TNTs@AC can be reused. The “concentrate-&-destroy” strategy appears promising to treat low-concentrations of PFAS in landfill leachate which has a strong matrix.