Super-adsorbent hydrogel for removal of methylene blue dye from aqueous solution

Adsorptive removal of dyes from industrial effluent has attracted intensive interest in the treatment of water pollution. Although we have witnessed great development in adsorbents for treating dyeing waste water, super-adsorbent hydrogels still cannot meet the high requirement for huge adsorption capacities towards various dyes. Traditional uncross-linked poly(acrylic acid) (PAA) adsorbents possess a low adsorption capacity towards dyes due to the poor three-dimensional structure in the bulk, which has a negative effect on the exposure of adsorptive sites and the penetration of water within the internal channels of the adsorbent. Thus, other monomers or chemical cross-linkers are introduced to design a well-cross-linked PAA-based super-adsorbent hydrogel with the intention of improving its adsorptive behavior. However, the above-mentioned additives may negatively affect the adsorptive properties of composite adsorbents based on the PAA hydrogel. To the best of our knowledge, adsorption capacities of PAA-based super-adsorbent hydrogels for organic dyes reaching more than 2000 mg g−1 have rarely been reported as yet. Therefore, it is urgent to explore super-adsorbent hydrogels with high adsorptive properties to alleviate dye-based water pollution. In this work, a novel poly(acrylic acid) (PAA)-based super-adsorbent nanocomposite hydrogel (NC gel) acting as an effective dye adsorbent is prepared via free radical in situ polymerization of acrylic acid by employing non-aggregated calcium hydroxide (Ca(OH)2) nano-spherulites (CNSs) with a diameter less than 5 nm as cross-linkers. The high water penetration of our super-adsorbent NC gel with a high swelling ratio (500 times) allows the internal adsorption sites to be fully exposed to methylene blue (MB). Thus, the adsorption capacity is as high as 2100 mg g−1 for MB under near neutral pH conditions due to the exposure of a large amount of active sites. This is the first time that it has been reported that a maximum adsorption of more than 2000 mg g−1 for MB has been obtained using a PAA-based super-adsorbent NC gel. In addition, the adsorption behavior of our NC gel matches well with the pseudo-second-order model and Langmuir adsorption isotherm model. This work confirms that CNSs can create a well-cross-linked structure of a PAA hydrogel without the aid of any other cross-linker and further confirms that the PAA-based super-adsorbent NC gel has a potential application in the removal of organic dyes from dyeing waste water.