Use of zinc oxide-immobilise-montmorillonite containing nanocomposite hydrogels in dye adsorption

The various types of toxic dyes found in the wastewater of industries such as textiles, rubber, paper, leather, plastics and cosmetics are regarded as one of the biggest environmental problems. Removal of such contaminant is very important for both human health and environmental issues. Within this context, hydrogels, which are obtained by cross-linking hydrophilic polymers, are described as water-insoluble but water-swellable networks. Hydrogels are used as adsorbents in removal of dye from waste water. In this work, cross linked poly (acrylamide) (PAAm) nanocomposite hydrogels which contains nanosized zinc oxide (ZnO) metal oxide immobilized montmorillonite clay (ZnO-MMT) were synthesized for dye adsorption from aqueous solutions. Within the scope of the study, nanocomposite hydrogels were synthesized by the in situ free radical polymerization reaction with N, N-methylene bis-acrylamide (BAAm) crosslinker and initiator redox pair ammonium persulfate-sodium metabisulphite. Thus, a triple nanocomposite hydrogel was prepared by combining PAAm hydrogel, MMT clay, and ZnO metal oxide in a single material. The use of nanocomposite hydrogels that contains ZnO-MMT in different percentages in the removal of bromothymol blue from aqueous solutions was investigated. The results obtained from the adsorption tests are related to the morphological structures of the hydrogels and the distribution of clay layer in the nanocomposite hydrogel. The compressive mechanical strengths of neat PAAm and nanocomposite hydrogels in maximum swollen forms were also investigated. Mechanical strength, modulus and toughness values of nanocomposite hydrogels are increased compared to neat PAAm hydrogel. PAAm nanocomposite with 0.25 wt% ZnO-MMT adsorbs 1760% and 125% more dye, respectively, compared to neat ZnO-MMT clay and neat PAAm hydrogel, for a pH:6 and 30 ppm dye solution. The adsorption capacity value (k) obtained from the adsorption isotherms was found to be 300% higher for the same nanocomposite hydrogel adsorbent than the neat PAAm hydrogel. This result is based on the formation of much smaller and numerous pore structures in the nanocomposite hydrogels obtained with ZnO-MMT. This pore structure determined from secondary interactions caused by clay. In addition, it is possible that hydroxyl functional groups that are formed as a result of hydrolysis on the surface of ZnO metal and MMT clay surface may contribute to the adsorption of dye.