Soaking pretreatment of hydrogel adsorbent in distilled water to enhance efficiency of adsorption onto low concentration dye

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.