Preparation of chitosan/polycaprolactam nanofibrous filter paper and its greatly enhanced chromium(VI) adsorption

Chitosan/polycaprolactam (PA6) nanofibrous filter paper (NFP) was prepared by electrospinning technique. Characterization with Fourier transformation infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM) indicated that there was weak interaction (hydrogen bonds) between PA6 and chitosan, and the average diameter of the composite nanofibers decreased as the chitosan content was increased. The maximum adsorption capacity of Cr(VI) on the NFP via filtration (114.7   mg   g −1 ) was much higher than that via static adsorption (81.7   mg   g −1 ) and shaking adsorption (80.7   mg   g −1 ). Because the Cr(VI)-containing solution was forced to flow through the whole membrane in filtration, the chelation sites inside the membrane could be exploited, leading to the enhancement of the Cr(VI) adsorption. The hydrogen bonds between chitosan and PA6 could enhance the mechanical strength and the acid fastness of the NFP. Therefore, the NFP desorbed by H 2 SO 4 can be well reinstated, and the adsorption capacity of Cr(VI) decreased slightly (only 17.9%) after three filtration cycles. On the basis of X-ray photoelectron spectroscopy (XPS) analysis, the possible mechanism for the Cr(VI) removal was that Cr(VI) was initially adsorbed on the NFP due to the electrostatic attraction, and subsequently reduced to the less toxic form Cr(III) by the amino groups of chitosan.