A magnetically-separable Fe3O4 nanoparticle surface grafted with polyacrylic acid for chromium(iii) removal from tannery effluents

Abstract

Chromium(III) contamination in tannery effluents continues to represent a significant challenge towards sustainable development of the global leather industry. Despite various magnetite-supported adsorbents for chromium removal previously reported, few of them were specifically designed to address trivalent chromium and the complexity of pollutants in tannery effluents. In the present study, polyacrylic acid, capable of adsorbing chromium(III) by coordination, was grafted from the surface of Fe₃O₄ nanoparticles via the bridging function of a silane coupling agent to produce a magnetically-separable nanoadsorbent (PAA@VTES@Fe₃O₄) for chromium(III) remediation from tannery effluents. The structure and morphologies of the nanoadsorbent were systematically characterized by scanning electron microscopy, low-temperature nitrogen adsorption/desorption experiments, energy dispersive X-ray spectroscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, total organic carbon content and X-ray diffraction analysis. With a saturated magnetization of 46.8 emu g⁻¹, PAA@VTES@Fe₃O₄ could be separated from water within 5 min by using a magnet. Adsorption experiments showed that chromium(III) adsorption on PAA@VTES@Fe₃O₄ was pH- and adsorbent dose-dependent; adsorption equilibrium could be reached in 2.5 h, resulting in a chromium(III) removal percentage of more than 90% under optimized conditions. The adsorption kinetics were best described by pseudo-first order and pseudo-second order models, while the isotherm data were found to agree well with both Langmuir and Freundlich models. Based on the fitting results, it was found that PAA@VTES@Fe₃O₄ exhibited high adsorption capability relative to many other adsorbents previously suggested as efficient for chromium(III) removal from effluents. In addition, a thermodynamic study revealed that the adsorption process was endothermic and spontaneous in nature. By chemical desorption, PAA@VTES@Fe₃O₄ could be regenerated and repeatedly used for five cycles without significantly compromising the adsorption capacity. Even in the presence of large amounts of decomposing organic matter, hair, lime, sulphide and organic nitrogen, PAA@VTES@Fe₃O₄ was still capable of removing more than 90% of chromium(III) from tannery effluents, exhibiting a high potential for practical application.