Dichromate and arsenate anion removal by PEI microgel, cryogel, and bulkgel

Abstract

Among the various metal ions, chromate (Cr(VI)) and arsenate (As(V)) are the two most hazardous toxic ion species and are found in nature in the form of dichromate and arsenate anions. In this investigation, polyethyleneimine (PEI)-based hydrogels were prepared with microgel, cryogel, and bulkgel morphologies employing glycerol diglycidyl ether as a crosslinker. The prepared PEI-based hydrogels were used to remove dichromate and arsenate anions from aqueous media. PEI microgel, cryogel, and bulkgels weighing 50 mg of each adsorbed 84.7 ± 0.8, 76.5 ± 5.2, and 108.9 ± 2.4 mg.g-1 of dichromate anions and 15.9 ± 0.7, 45.4 ± 1.9, and 79.2 ± 11.6 mg.g-1 of arsenate anions in 30, 120, and 240 min, respectively. The dichromate and arsenate anions adsorption of PEI-based hydrogels were found to fit to the pseudo-second-order kinetic, and nonlinear Langmuir isotherm models, respectively with higher R2 values. The highest distribution coefficient (Kd) value of PEI-based hydrogels for dichromate adsorption was obtained on the cryogel forms of PEI as 1.89 ± 0.05. Likewise, the highest Kd value for the arsenate adsorption was calculated on PEI-based bulkgels as 0.46 ± 0.01. The thermodynamic parameters of PEI based hydrogels in the adsorption of dichromate and arsenate anions e.g., ΔG (all negative (except for the arsenate adsorption by PEI microgels), and ΔH that is around 2-10 kJ.mol-1, and ΔS what is around 0.01-0.02 kJ.mol-1. K-1 were calculated. The reusability studies showed that PEI-based hydrogels can be used for at least 5 consecutive adsorption-desorption cycles with almost 70 % anion removal efficiency after the fifth cycle.