Tuning the crosslink structure of cationic hydrogel for enhanced chromium (VI) removal: The covalent and electrostatic co-crosslinked effects and adsorption mechanism

Elevated chromium(VI) (Cr(IV)) concentrations in drinking water or ground water can pose a public health concern. In this study, layered double hydroxides-isethionate (LDH-ise) assisted covalent and electrostatic co-crosslinked cationic hydrogel (CH-LDH-ise) was prepared as a potential adsorbent for Cr(VI) removal from aqueous solution. The scanning electron microscopy and X-ray diffraction results indicated that the LDH-ise sheets were exfoliated and crosslinked with the 3-(Acryloylamino) propyltrimethylaminium chloride (APTMACL) polymer chains. The adsorption of Cr(VI) onto the CH-LDH-ise was highly pH-dependent, and the removal efficiency by CH-LDH-ise composites was much higher than the CH (no crosslinker). The adsorption kinetic followed a pseudo-second-order, and reached equilibrium within 2 h. The corresponding maximum Cr (VI) adsorption capacity of CH-LDH-ise was 408.4 mg/g at the equilibrium concentration of 364.4 mg/L. The enhanced adsorption of Cr(VI) was also achieved in the presence of competing anions (including Cl-, NO3-, SO42-, HCO3-, SiO44-, and HA). Moreover, the in situ regeneration/recycle results implied the column adsorption capacity of CH-LDH-ise was 2250 bed volumes as the concentration of Cr(VI) decreased from 2.5 mg/L to 0.1 mg/L (The US drinking water standard for total chromium) at empty bed contact time = 6 min, and slightly decreased in the second recycle. The combination analysis of in situ Fourier transform infrared, in situ Raman, X-ray photoelectron spectra, and electron paramagnetic resonance revealed that the Cr(VI) capture mechanism was dominated by the electrostatic interactions and hydrogen bonding effect - the sources of the -N+(CH3)(3) and -NH groups in CH-LDH-ise, respectively.