Droplet fractionation of hexavalent chromium from bubbles bursting at liquid surfaces of chromic acid solutions

The aim of this study was to evaluate the effects of electrolyte concentration and gassing rate (in terms of gas flow rate) on the droplet fractionation of Cr(VI) from bubbles bursting at liquid surfaces of chromic acid solutions. An experimental system was used to generate gas bubbles in solutions, and the droplets thus generated were measured by an aerodynamic particle sizer (APS). A Marple personal impactor was used simultaneously to collect droplet samples for chemical analysis. Ratios of the Cr(VI) concentration in droplets to that in the bulk solution were calculated to evaluate the droplet fractionation effect. The results showed that the concentration ratios had a range of about 5 to less than unity for the experimental conditions used in this study, indicating that the droplets can be either enriched or depleted with Cr(VI) relative to the bulk solution. The concentration ratio decreased exponentially with increasing electrolyte concentration, but decreased approximately linearly with increasing gas flow rate. In addition, surface tension also played an important role in chemical fractionation by droplet formation from bursting bubbles. The results should have applications in assessing the health risk of chromium electroplating plants where chromium-containing droplets are generated as a result of electrochemical reactions and gassing.