Disagreements over various unanswered questions about contribution of the adsorption process and functional groups on dye adsorption still exist. The main aim of this research was to evaluate the contributions of physical hole structure, hydrogen bond, and electrostatic attraction on dye adsorption. Three ideal representatives, namely, a sponge with porous structure, P(AM) containing -CONH2 groups, and P(AANa/AM) containing -COONa groups, were chosen to evaluate the above contributions. The methylene blue (MB) removal rates of these three products were compared through individual experiments. The results revealed that physical hole structure did not play a role in decreasing dye concentration. Hydrogen bond existed in dye adsorption but did not remarkably reduce dye concentration. The excellent removal results of P(AANa/AM) demonstrated that electrostatic attraction was critical in enriching dye contaminants from the solution into solid adsorbent. The results could provide insights into the dye adsorption mechanisms for further research.

No abstract

To address the poor removal rate of low-concentration dye adsorption, the soaking pretreatment of hydrogel adsorbent in distilled water was suggested. In contrast to the conventional method of directly adding dried particles into dye solution, the purposed procedure involved adding an adsorbent into distilled water before transferring it into the dye solution. A hydrogel adsorbent based on P(AANa/AM) containing COONa groups was fabricated as an example for the hypothesis. The maximum adsorption capacity of P(AANa/AM) hydrogel adsorbent onto MB was 2455.45 mg/g under an initial MB concentration of 1000 mg/L. After two rounds of soaking pretreatment in distilled water, the clearance rate of 20 mg/L MB increased from 97.79% to 99.40%. Considering the hydrophilicity and ionization of COONa group, a high concentration of Na + was released into solution. Hydrogel soaked multiple times in fresh distilled water induced the continuous release of Na + into distilled water, although the swelling equilibrium was already achieved in the first soak. In dye adsorption and desorption experiments, the water absorption behavior was observed. Overall, this work revealed that the soaking pretreatment of hydrogel adsorbent in distilled water can enhance the removal rate of low concentration dye through adsorption.