REMOVAL OF ANIONIC CONTAMINANTS BY ENVIRONMENTAL-FRIENDLY ENGINEERED MATERIALS

Abstract

In this study, two novel polymeric materials, chitosan-based polymeric sorbent (CTS-MG) and cellulose-based adsorptive membrane (RC-MG), were synthesized for an efficient sorption of boron and arsenic. Surface initiators were initially immobilized on crosslinked chitosan (CCTS) through the interaction between surface hydroxyl and amine groups and 2-bromoisobutyrate bromide. Glycidyl methacrylate (GMA) polymer brushes were grafted to the surface-initiated chitosan at ambient temperature. Epoxide functional groups of the grafted poly(glycidyl methacrylate) (PGMA) further reacted with NMDG to create the affinity binding sites for boron. The SEM and BET studies revealed that chemical modification resulted in a rough surface and porous structure compared to raw chitosan. Boron uptake capacity of CTS-MG was significantly enhanced after surface modification. At the optimum neutral pH, the maximum sorption capacity was as high as 2.84 mmol/g, much higher than those of commercial boron selective resins and many other synthesized sorbents. Almost 90 % of boron sorption occurred within 8 h and the experimental data was well fitted by intraparticle diffusion model. Existence of sodium chloride and sodium nitrate had little effect on boron removal, implying formation of inner-sphere surface complexes on the sorbent. Furthermore, simulated seawater was employed to evaluate the actual application of the sorbent. The final boron concentration of less than 0.5 mg/L is achievable at the dose of sorbent > 1.2 g/L, of which the removal efficiency is above 90 %. The interaction between boron and surface functional groups on the novel sorbent was also explored by FT-IR and XPS. The oxygen in the form of secondary alcohol played an important role in boron sorption, and the tetrahedral B complexes were finally present on the surface of novel sorbent. Laboratory experiments were also carried out to investigate sorption behaviors of inorganic arsenic as well as methylated arsenic species, monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) onto CTS-MG. Surface modification of crosslinked chitosan was found to increase the sorption capabilities and affinities of arsenic species. Arsenic sorption was highly pH-dependent and the maximum sorption capacities were 69.28 mg/g for arsenate, 15.4 mg/g for MMA and 7.1 mg/g for DMA. Affinity of these three species to CTS-MG decreased in the sequence of arsenate > MMA > DMA. Most of the arsenic was rapidly adsorbed in the first 5 h. Uptake of MMA and DMA was sensitive to the existence of background electrolytes, suggesting that both inorganic and organic arsenic species formed outer-sphere complexes on the surface. Presence of natural organic matter was unfavorable for arsenic removal; simultaneous uptake of organic contaminants such as humic acid was observed in this study as well. Effects of co-existing anions in natural water on arsenic removal decreased in the order of sulfate > phosphate > fluoride. Spectroscopic analyses also verified the successful attachment of both inorganic and organic arsenic species, which was mainly due to tertiary amine and polyhydroxyl functions on the surface of CTS-MG. Cellulose-based adsorptive membrane was also prepared for efficient boron removal through a three-step surface modification of regenerated cellulose (RC) membranes. Results show that designed functional groups have been successfully grafted onto RC substrates, and surface functionalization contributes to higher boron binding capability. The optimal pH for boron sorption falls in a wide range from 4 to 8. Under neutral pH condition, the maximum sorption capacity of the modified membrane is determined to be 0.75 mmol/g, which is comparable with those of commercial resins. Studies of electrolytes influence indicate the formation of inner-sphere surface complexes on the membrane surface.