In this research, a combined electrocoagulation (EC) and electroflotation (EF) process was developed and applied to the treatment of two industrial wastewaters: F^- containing wastewater from semiconductor industry and Cr⁶⁺ containing wastewater from electroplating industry. The system primarily consisted of an EC cell and an EF cell. The function of the EC cell is to produce ions reacting with and precipitating the pollutants. The EF cell generates many tiny bubbles that attach to and float the flocs produced in EC cell leaving a treated water stream to flow out of the system....[ Read more ]

In this research, a combined electrocoagulation (EC) and electroflotation (EF) process was developed and applied to the treatment of two industrial wastewaters: F^- containing wastewater from semiconductor industry and Cr⁶⁺ containing wastewater from electroplating industry. The system primarily consisted of an EC cell and an EF cell. The function of the EC cell is to produce ions reacting with and precipitating the pollutants. The EF cell generates many tiny bubbles that attach to and float the flocs produced in EC cell leaving a treated water stream to flow out of the system.

Operating conditions such as influent pH, retention time, charge loading, and the presence of other ions were studied. Aluminum and iron plates were used as electrodes in the EC while an in-house invented anode material was used in the EF cell. The sludge produced from the process was characterized using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and Dynamic Secondary Ion Mass Spectrometry XPS.

In the F^- removal experiments, weakly acidic influent condition is favored. Ca²⁺ is helpful in precipitating F^- and reducing the residual F^- concentration. A solution of 50 mg/L Fe³⁺ or Mg²⁺ was also beneficial to fluoride removal. Anions give a negative effect on fluorine removal. According to the characterization of sludge produced in the process, the mechanism of defluorination process is chemical adsorption of F^- on the polymer network of AI_m(OH)_n. The energy consumption in such a process is less than 1.8 kWh/m³ water. The comparison between electrocoagulation-sedimentation and combined electrocoagulation-electroflotation process showed that the combined method was quite simple and easy to implement.

In the Cr⁶⁺ removal process, acidic condition is favored for Cr⁶⁺ to be reduced to Cr³⁺, while a neutral pH is preferred in the coagulation process. The optimum charge loading is 2.48 Faradays/m³ water. Too low or too high a charge loading will result in an increase of residual Cr concentration. If more than 10 mg/L aluminum ions are pre-mixed into the solution, the residual Cr concentration can be reduced below 0.5 mg/L, the national discharge standard of China, without any filtration process or surfactant addition. This result represents a significant progress in the application of EC in Cr⁶⁺ removal.