Adsorption and desorption behaviors of HPEI and thermoresponsive HPEI based gels on anionic and cationic dyes

In this work, two hyperbranched polyethyleneimine (HPEI) based gels were prepared and employed as adsorbents for the removal of dyes. HPEI-MBA gel was obtained by a cross-linking reaction between HPEI and N, N'-methylene-bis-acrylamide (MBA). And HPEI-MBA gel was further treated with isobutyric anhydride to form thermoresponsive HPEI-MBA-IBAm gel having important application in aspect of desorption of dyes. The structure and morphology of gels were characterized by FTIR, XPS, SEM and BET. Many different affecting factors for adsorption capacity of gel, such as contact time, pH, concentration of dyes and adsorptive selectivity, were investigated. The HPEI-MBA gel demonstrated a satisfactory adsorption capacity for methyl orange (MO) and xylenol orange (XO) as high as 1552.76 mg/g and 3312.06 mg/g, respectively. Interestingly, the HPEI-MBA gels had a considerable adsorption capacity for mixed system of anionic and cationic dyes by regulating the value of pH. In the MO/methylene blue (MB) mixture system, the adsorption capacities of gels for MO and MB were all higher than that of single-dye system, and in the MO/basic fuchsin (BF) mixture, the adsorption capacity for MO and BF can be improved under neutral condition. In the MO adsorption process, the adsorption/desorption kinetic of HPEI-MBA and HPEI-MBA-IBAm gels exhibited a good fit to the pseudo-second-order model. Meaningfully, the adsorption efficiency of HPEI-MBA gel for MO was still as high as 93.52% after 11 cycles of adsorption/desorption. More importantly, we developed a general and facile method to achieve dye desorption by adjusting temperature which relies on the temperature-sensitive properties of HPEI-MBA-IBAm. Therefore, the HPEI-MBA gels and HPEI-MBA-IBAm gels, as a new recycle adsorbent, could have comprehensive application in the removal of dyes from wastewater.