A Poly(Acrylic Acid)-Based Hydrogel Crosslinked with Hydroxypropylcellulose as a Clarifying Agent in Nickel(II) Solutions.

Poly(acrylic acid) (PAA) and hydroxypropylcellulose (HPC) hydrogels were synthesized in the absence of a crosslinker. Chemical crosslinking between PAA and HPC was demonstrated through free radical polymerization by a precipitation reaction in acetone as the solvent. These hydrogels exhibited smaller swelling ratios (1 to 5 g H2O/g) than homo PAA hydrogels synthesized in water as the solvent. They were swollen in a 0.1 M NaOH solution and subsequently used to remove Ni2+ ions from aqueous solutions with concentrations ranging from 1000 to 4000 ppm. The absorption capacity of these hydrogels ranged from 91 to 340 mg of Ni2+/g in a rapid 1 h process, and from 122 to 435 mg of Ni2+/g in a 24 h process, demonstrating an improvement in Ni2+ absorption compared to previously reported hydrogels. The colored 1000 and 2000 ppm Ni2+ solutions became clear after treatment, while the PAA-HPC hydrogels turned green due to the uptake of Ni2+ ions, which were partially chelated by carboxylate groups as nickel polyacrylate and partially precipitated as Ni(OH)2, resulting in an average absorption efficiency of 80%. The hydrogel was able to release the absorbed Ni2+ upon immersion in an HCl solution, with an average release percentage of 76.4%, indicating its potential for reuse. These findings support the use of PAA-HPC hydrogels for cleaning Ni2+-polluted water. The cost of producing 1 g of these hydrogels in laboratory conditions is approximately 0.2 USD.