Removal and recovery of chromium (VI) from industrial wastewater and contaminated groundwater/soil using micro-sized magnetic hydrogel

Abstract

The effectiveness of a magnetic polymeric adsorbent, namely magnetic hydrogel, which was developed and synthesized for Cr(VI) removal and recovery from contaminated waters in various environments, such as industrial wastewater, groundwater and soil, was studied through batch and column tests. Two types of hydrogel were synthesized and compared for Cr(VI) removal performance. In addition, a prototype-scale magnetic separation unit for Cr(VI)-bearing industrial wastewater treatment using magnetic hydrogel was developed and tested to assess its treatment efficiency. The two hydrogels, type 1 and type 2, with different functional groups, trimethylamine on quaternary ammonium and dimethylethoxyamine on quaternary ammonium, were prepared and characterized. Both hydrogels showed fast Cr(VI) removal kinetics, reaching equilibrium in 5 min, and achieving a high removal capacity of around 200 mg/g. The Cr(VI) removal capacity was not significantly affected by the pH value due to the presence of the positively charged trimethyl ammonium functional group. The adsorbed Cr(VI) was easily recovered through regeneration with NaCl solution. Both type 1 and type 2 hydrogels showed higher adsorption selectivity towards Cr(VI) than other anions. To facilitate the separation of the hydrogel, magnetic nanoparticles can be introduced into the hydrogel. However, considering the chemical modification process for the type 2 hydrogel synthesis would affect the stability of magnetic nanoparticles, the type 1 hydrogel was selected for magnetic hydrogel preparation by embedding magnetic nanoparticles in the polymeric matrix. Batch study on the magnetic hydrogel showed that the magnetic nanoparticles did not affect the Cr(VI) removal and recovery performance. To mimic in-situ groundwater remediation using a column study, it was found that Cr(VI) has a higher preference for being adsorbed onto the type 1 hydrogel than sulphate, bicarbonate ions and humic acid (HA) co-present in the solution. However, the adsorbed HA reduced the Cr(VI) removal capacity of the hydrogel, especially after regeneration of the adsorbents, probably due to the blockage of adsorption sites by the HA. The regeneration and reusability study showed that the effectiveness of the hydrogel remained almost unchanged for six cycles, with an average of 93% recovery of the adsorbed Cr(VI) in each cycle. An industrial wastewater treatment prototype, including adsorption and magnetic separation units, was developed to investigate the feasibility of the application of the magnetic hydrogel for industrial wastewater treatment. Both permanent magnetic and electromagnetic separation units were designed to provide a magnetic field along the pathway, with a zigzag pathway feature for maximizing the chance of capturing magnetic hydrogel. The separation efficiency for the magnetic hydrogel was above 97% and the Cr(VI) removal efficiency was maintained at above 96% throughout the 20 cycles of treatment, while the regeneration efficiency gradually decreased in each cycle because Cr(VI) gradually accumulated on the magnetic hydrogel. In sum, the findings of this study show that the type 1 magnetic hydrogel can be applied for effective Cr(VI) removal and recovery from industrial wastewater and contaminated groundwater or soil. The hydrogel can also be regenerated and magnetically separated, and thus reused for continuous cycles of treatment.

Date of Award	2013
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology