Study On The Regulation Of Aluminum Species And Structure In Electrocoagulation Process And The Defluoridation By Electrocoagulation-Ultrafiltration Process

Electrocoagulation(EC) and membrane separations are water treatment technologies that are extensively researched and applied in recent years. This study aims to further explore the mechanism of removing pollutants by EC process and the combination with membrane separation. The distribution of aluminum species in EC process was studied by means of electrolytic aluminum, and evaluated the formation of Al13, the effects of pH value, current density(j) and applied voltage on the speciation of aluminum, and the corresponding floc characteristics. Similarly, the removal of natural organic matter(NOM) by EC was investigated, and studied the removing fluoride in drinking water by electrocoagulation-ultrafiltration(EC-UF) and the membrane fouling. The results were outlined as follows:1. In EC process, especially the OH- constantly generated from metal cathode and the unbanlanced pH distribution around the interface of metal-solution may promote the hydrolysispolymerization of Al3+ ion. The predominant Al species shifted from monomer toward medium polymer then to larger polymer or Al(OH)(s) with prolonging EC time. Al13 polymer can in-situ form in EC process.2. pH 5~7 was an ideal pH range for the formation of Al13 polymer during EC process. Lowering current density and voltage favored the formation of Al13 polymer. The increase of pH, current density and voltage could accelerate the hydrolysis-polymerization process of Al3+ that released from anode, which resulted in the relative content of Ala, Alb significantly reduced, increasing the relative content of Alc.3. The pH value and current density had important influence on the characteristics of hydroxyl polymeric aluminum flocs. During the electrolysis process, in acidic and neutral conditions, the pH value increased at first and then tended to be stable; under alkaline condition, pH decreased first, after 2 min it also increased and then tended to be stable. The Zeta potential had a short ascent stage, and then began to decline, and finally tended to be stable. The increase of current density, pH increased faster, and Zeta potential droped faster.4. The pH value and current density also had important effect on the removal of NOM and turbidity in EC process. In the weak acid condition, with the current density of 10 A/m2, Zeta potential reached near the isoelectric point at 10 min, at which the stability of NOM and colloid was lowest, most easily destabilized and aggregated. The particle sizes achieved to be stable, and formed relatively uniform, porous, determinate density floc clusters at this time, with the good characteristics of high strength, large specific surface area, high adsorption capacity.5. In the continuous running, the fluoride can be largely removed by the EC-UF process. The removal rate of fluoride was up to 80% with optimizing the current density of 30 A/m2, hydraulic retention time of 20 min and initial pH of 6.0-7.0. It was observed that the membrane fouling was mainly caused by the particulates of the aluminum coagulants in electrolytic process and fluoride ions.