Electrochemical coagulation for oily water demulsification

Motor oil and industrial surfactants were used to prepare synthetic oily water. When stabilized with surfactant, the turbidity shows a positive correlation with the oil content in the emulsion with a correlation coefficient of 1.78 FAU/(mg/L). The oil-water emulsion was tested with gravity separation, centrifugation, flotation, and electrochemical coagulation for the effectiveness of each process on demulsification. Electrochemical coagulation was then selected for further study. A dc voltage was applied to the electrodes, dissolving ferrous ions (Fe(II) at the anode and forming hydrogen (H-2) gas and hydroxyl (OH-) ions at the surface of the cathode. A few 100 mg/L of sodium chloride (NaCl) were added to the solution to provide ionic conductivity and to prevent passivation of the iron electrode. The ferrous ions were oxidized into ferric ions (Fe(III)), destabilizing the emulsion. The coalesced oil droplets were adsorbed into the highly dispersed and reactive ferric hydroxide (Fe(OH)(3)) coagulant. The oil-rich sludge that was generated in the operation was then floated to the surface forming a blanket that was removed by skimming. This complicated mechanism was completed in 4 min. When the reactor was operated for this amount of time, a measure of 165.8 ma/L of ferric ions was generated. The treatment reduced the turbidity of the emulsion from 1800 to 60 FAU. Beyond the 4-min treatment period, the addition of more iron to the system resulted in turbidity change at a much slower rate. When the operation occurred continuously with a current of 2 A and a throughput of 320 mL/min, the turbidity of the electrically treated effluent was less than 14 FAU, the detection limit of the Hach DR/4000 Spectrophotometer. (c) 2006 Elsevier B.V. All rights reserved.