A liquid-liquid microreactor for the intensification of hexavalent chromium removal from wastewaters

Abstract

There is a lively interest to improve hexavalent chromium removal from aqueous solutions since it is highly toxic and can be found in many industrial wastewaters. In this work, a novel slug flow microreactor (SFMR) was used as a new-generation contactor to intensify the Cr (VI) extraction. The applied SFMR was a T-shaped microreactor with a circular microchannel. The microchannel wall wettability profile was altered using a microchannel made of alternate PTFE and stainless steel parts, leading to a significant hydrodynamic change along with slug coalescence and elongation. At low total flow rates, the wall wettability alteration enhanced the Cr (VI) extraction; however, it reduced the extraction at the high total flow rates. The observed results were discussed in details and the liable mechanisms were unveiled. The influence of the microreactor size was also studied, showing the remarkable enhancement of the chromium removal by reducing the microreactor diameter. The time required to reach equilibrium was significantly reduced to even almost 3 s by the reduction of the microreactor size, which was 100–1000-folds shorter than that reported for the conventional contacting systems. The silica nanoparticles were then employed to achieve a higher chromium extraction. Hydrophobic silica nanoparticles led to an increase of the Cr (VI) removal; however, hydrophilic nanoparticles showed a negligible influence. Finally, an empirical correlation was proposed to predict mass transfer.