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Abstract. This paper reports the removal of nickel ions from aqueous solutions using carboxymethyl cellulose-graft-poly(acrylic acid) CMC-g-PAA hydrogel as an adsorbent. The hydrogel was prepared through graft copolymerization of acrylic acid monomer onto a carboxymethyl cellulose backbone in the presence of N,N'-methylenebisacrylamide as a crosslinking-agent using benzoyl peroxide initiator system. Isotherm for the adsorption of nickel on CMC-g-PAA hydrogel was developed and the equilibrium data fitted to the Langmuir and Freundlich isotherm models. The result showed that at pH of 5 and shaker speed of 100 rpm, Freundlich model was better fitted than Langmuir model, of which R 2 = 0.999 and R 2 = 0.935, respectively. The constant of Langmuir model was KL value = 0.003 and qmax = 1000 mg/g, whereas the constant of Freundlich model n = 1.2 and KF = 4.614. The CMC-g-PAA hydrogel before and after Ni(II) adsorption were characterized using scanning electron microscopy (SEM) and Fourier transform infrared (FTIR). IntroductionEnvironmental pollution by toxic metals arises from industries, agricultural effluents and waste disposal from various sources. Many industries such as metal plating facilities, mining operations and tanneries discharge waste containing heavy metal ions.As pollutants, heavy metals were intensively studied due to their significance from the point of view of persistence and toxicity. These toxic metals can cause accumulative poisoning, cancer, and brain damage when present at above the tolerance levels. The agencies for the environmental monitoring have set permissible limits for heavy metals levels in drinking water because of their harmful effects. The removal and rapid decontamination of heavy metals such as Cd, Pb, Cu, Ni, Hg become very important for the environmental remediation [1]. The World Health Organization has recommended strict controls on the percentages of various heavy metals in effluent waters [2].Hydrogels are water-swollen crosslinked network of hydrophilic polymer. Due to the hydrophilic groups in their backbone, hydrogels can imbibe large quantities of water and swells. The degree of hydrogel swelling is related to the degree of crosslinker, ionic strength of the solution, temperature, pH, and the presence and extent of hydrophilic groups such as -OH, -COOH, -NH2, -CONH2, and SO3H. These functional groups in the hydrogel networks can be utilized for the toxic metal ion removal. Hydrogels can also be modified with new functionalities or prepared as composites with clays and with other materials to increase the metal ion absorption capacities [3][4][5][6].

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Summary: Batch experiments of adsorption of heavy metals cations from aqueous solution by using carboxymethyl cellulose-graft-poly(acrylic acid)/monmorilonite (CMC-g-PAA/MT) hydrogel composite were carried out. The parameter such as different pH and the presence of other ions as competitor were studied to observe their effects on the adsorption capacity of the hydrogel composite. The results showed that the adsorption of metal ions by the hydrogels was higly pH dependent, and maximum adsorption was obtained at pH 5.0. At this pH, the adsorption capacities of hydrogels for the Pb 2þ and Zn 2þ ions were 146.19 mg/g and 286.67 mg/g, respectively. The hydrogels can be regenerated after releasing metal ions by washing with low pH solution and reused two times. Based on the scanning electron microscope (SEM) images and Fourier transform infrared spectroscopy (FTIR) spectra before and after adsorption by the hydrogels revealed that the complex formation between heavy metal ions and carboxyl groups of the hydrogels was the main adsorption mechanism.

Summary The grafted copolymer CMC‐g‐PAA has been prepared by graft copolymerization reaction using 10% w/w carboxymethyl cellulose (CMC), acrylic acid (AA), 2.5% w/w benzoyl peroxide (BPO) as initiator, 3.75% w/w methylenbisacrylamide (MBA) as crosslinking agent in distilled water at 30, 50, 70, and 90 °C during reaction periods of 30 to 300 min and composited with 10% w/w montmorillonite for 5 h.The grafted copolymer CMC‐g‐PAA and homopolymer were dried in the oven at 40 °C, then washed with distilled water and aceton. After that the hydrogel material was dried, and subjected to Soxhlet extraction with dioxane to separate the homopolymer, poly(acrylic acid), from the graft copolymer, CMC‐g‐PAA. The water absorption capacities and grafting values of grafted CMC were also determined. The graft copolymer CMC‐g‐PAA obtained at 30 °C has high conversion rate of AA (87.6%). The maximum grafting yield (99.181%) and the lowest of homopolymer formation (0.819%) were obtained at 50 °C. It was observed that polyacrylic acid‐grafted CMC produced at 90 °C had the highest water retention capacity (55.03%). The graft copolymer was characterized by infra‐red spectroscopy to determine the structure of the graft chains.