Removal of Heavy Metal Ions by Poly(vinyl alcohol) and Carboxymethyl Cellulose Composite Hydrogels Prepared by a Freeze–Thaw Method

Abstract

Environmentally friendly composites made of poly(vinyl alcohol) and carboxymethyl cellulose (PVA/CMC) hydrogels were proposed. The PVA/CMC hydrogels were prepared by freeze–thaw processes and applied to adsorb heavy metal ions including Ag+, Ni2+, Cu2+, and Zn2+. The phase separation of polymer solutions during freezing stages led to the formation of crystallites and resulted in insoluble hydrogels. The PVA/CMC hydrogels were effectively acquired, confirmed by at least 71% of insoluble gels found in the gel fraction. In addition, the gel fraction, physical properties, and swelling ratios were clearly modulated by the compositions of PVA and CMC among the samples of pure PVA, P2C1 (containing two-thirds of PVA and one-third of CMC), P1C1, and P1C2 hydrogels. For pure PVA hydrogels, the swelling ratio was 416%, while the P1C2 hydrogels exhibited a much higher swelling ratio (1437%). For the metal adsorption, the experimental results indicated that the P2C1 hydrogels presented not only the highest adsorption capacity toward Ag+ (8.4 mg per gram of hydrogel) but also the highest selectivity toward Ag+ in the mixed solutions. Interestingly, the experimental results revealed clearly that the adsorbed metal ions were further reduced on the P2C1 hydrogels. In summary, an insoluble hydrogel composed of PVA/CMC with excellent capacity of adsorbing metal ions was prepared in an economic and energy-saving manner. The prepared hydrogels showed potential applications for removal of heavy metal ions and wastewater treatment.