Hydrogels Based on Polyacrylamide and Functionalized Carbon Nanomaterials for Adsorption of a Cationic Dye

Abstract

Malachite green (MG) is used for the dyeing of cotton, paper, and jute, among other materials, and presents acute toxicity to a wide range of aquatic and terrestrial animals. A polyacrylamide (PA) hydrogel modified with a low content of oxygenated and aminated carbon nanomaterials may be a suitable candidate to adsorb MG from wastewater. Herein, graphene oxide and carbon nanotubes (CNTs) were incorporated during the in situ polymerization of PA. The hydrogels were characterized through thermogravimetry, differential scanning calorimetry and infrared spectroscopy as well as imagined by scanning electron microscopy. The swelling and adsorption capacity were investigated to explore the influences of different carbon dimensionalities (1D and 2D), zeta potentials and nanofiller concentrations on the adsorption behavior of the hydrogel. There was an increase of approximately 1500% in the adsorption capacity after 24 h of exposure of a hydrogel with graphene oxide (GO) at 0.25 wt% with respect to the neat PA. Aminated graphene produced similar gains in the adsorption capacity of the GO-hydrogel, although it presents a positive zeta potential that is the opposite of that of GO. The modified CNTs showed smaller gains in the adsorption capacity, reaching a maximum 400% increase with respect to the PA hydrogel. The main factors that seem to affect the adsorption capacity were the dimensionality and degree of functionalization. The graphene oxide-based nanofillers were 3 to 4 times more functionalized than the nanotubes. The adsorption results were adjusted with a pseudo-second order kinetic model that allowed a complementary discussion about the nature of the physico-chemical effects on the process of MG adsorption in the hydrogel nanocomposites.