A novel lignin hydrogel supported nZVI for efficient removal of Cr(VI)

A novel hydrogel-supported nanoscale zero-valent iron (nZVI) composite (nZVI@LH) was synthesized by ion exchange and in-situ reduction. The removal efficiency was tested, and the mechanism was also explored. The nZVI@LH at the precursor Fe(II) ion concentration of 0.1 mol/L presented an enhanced Cr(VI) removal capacity of 310.86 mg/g Fe⁰ at pH 5.3, which was 11.6 times more than that of the pure nZVI. The removal efficiency of the composite at pH 2.1 was more than double compared with alkaline or neutral conditions. Scanning electron microscopy (SEM) suggested that the nZVI particles were uniformly immobilized in the lignin hydrogel. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) provided evidence supporting the removal mechanism. According to the XPS results, the high removal capacity of the composite was attributed to chemical reduction/precipitation (69.7%), surface sorption (19.7%), and swelling uptake (10.6%). The pseudo-first-order reduction kinetics and pseudo-second-order kinetic model were employed to simulate the kinetic data, which supported the mechanism that chemical reduction and surface sorption could simultaneously remove Cr(VI). The electron acceptor and electron donor affected the reaction rate, and the presence of humic acid significantly inhibited the reaction. The present study demonstrated that lignin hydrogel acted as a carrier to prevent aggregation of nZVI particles. nZVI particles loaded on lignin hydrogel showed high reactivity and high degree of utilization compared with bare-nZVI. These results exhibited the great potential of nZVI@LH in practical water treatment due to its high activity.