Designing smart triple-network cationic cryogels with outstanding efficiency and selectivity for deep cleaning of phosphate

Abstract

Phosphate is one of the most difficult pollutants in the wastewaters asking for deep cleaning before discharge into the environment. To address this stringent need, novel composite sponges consisting of triple-cationic networks (TN) with cross-linked chitosan (CS) as the first network, cross-linked low molar mass branched poly(ethylene imine) (b-PEI) as the second network, and either b-PEI (TN1) or poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) (TN2) as the third network were fabricated in this work by ice-templating technique as advanced sorbents equipped not only with enhanced performances in the reversible capture of phosphate but also with a high mechanical strength. The structural and morphological characterization and the main factors influencing the sorption performances for phosphate anions of the TN sponges were evaluated in detail. The maximum sorption capacity of TN sorbents was reached at pH 4. The equilibrium data and the sorption kinetics were well described by the Sips isotherm and pseudo-second-order kinetic model, respectively. The equilibrium constant given by the Sips isotherm was used for the evaluation of the thermodynamic parameters. The sorption of phosphate onto the TN composite cryogels was endothermic and spontaneous. The presence of PDMAEMA facilitated desorption of phosphate when the temperature was in the range 30–35 °C. Hydrated iron oxide (HFO) nanoparticles were generated into the pores of cryogels to enhance the selectivity for phosphate anions. The pH effect on the sorption capacity, the influence of interfering ions and the analysis of the FTIR spectra, and desorption conditions demonstrated that the sorption mechanism of phosphate was complex consisting of the Coulomb attraction and inner sphere complexation. A great stability and recyclability were found for TN cryogels up to five sorption/desorption cycles.