Novel pH-sensitive nanocomposite hydrogels based on polyvinyl imidazole -graphene quantum dots for adsorption of congo red and methyl orange from aqueous solution

Recently, hydrogel adsorbents have attracted special attention for dye removal because of their three dimensional network structure and high swollen property, allowing the dye molecules to be easily absorbed in hydrogels1. Color removal from effluent is one of the most difficult requirement faced by the textile finishing, dye manufacturing, pulp and paper industries. The discharge of these to the river stream without proper treatment causes irreparable damage to the crops and living beings, both aquatic and terrestrial.Dyes are classified in three broad categories: (a) anionic: direct, acid and reactive dyes; (b) cationic: all basic dyes and (c) nonionic: dispersed dyes. Congo red (CR) is an example of diazo dyes and is prepared by coupling tetrazotised benzidine with two molecules of napthionic acid. Methyl orange (MO) has been widely applied in the textile, printing, paper, food, research laboratory and pharmaceutical industries2. The pH sensitive hydrogels are extensively used in different fields such as medicine, pharmacy, agriculture, food industry and controlled drug delivery etc., but the removal of toxic dyes using pH sensitive hydrogel are rarely reported3. In this work, a series of pH-sensitive nanocomposite hydrogels (PVI/GQDs) based on polyvinyl imidazole (PVI) were synthesized using an imidazolium-based dicationic IL (DIL) as a crosslinker and N-doped graphene quantum dots (NGQDs) as a nanofiller with different feeding amounts. The obtained hydrogels containing 0.5 and 8% of NGQDs showed the removal efficiency above 95% for CR and MO with concentration of 100-600 ppm at pH=7, respectively. The synthesized PVI/GQDs hydrogels were characterized by FT-IR, TGA, XRD and SEM.

Keywords:

Nanocomposite hydrogels , N-Doped graphene quantum dot , Polyvinyl imidazole , Dye adsorption.