Application of the colloidal stability of TiO₂ particles for recovery and reuse in solar photocatalysis

TiO₂-catalyst suspensions work efficiently in photocatalysis for wastewater treatment. Nevertheless, once photocatalysis is complete, separation of the catalyst from solution becomes the main problem. Catalyst recovery has been enhanced through charge neutralisation and coagulation with electrolytes at lab and pilot-plant scale (40L) to evaluate the potential for its separation after photocatalytic degradation of pollutants. Zeta-potential analysis showed that the isoelectric point (IEP) of TiO₂ suspensions is near pH 7. Settling rates and hydrodynamic diameter of TiO₂ particles are maximum at the IEP. However, suspensions are stable at different pH. TiO₂ was reused in solar photocatalysis pilot-plant (40L) for treatment of tetrachloroethylene (C₂Cl₄) comparing two procedures: reuse of the entire suspension after destruction of the organics without separation of the catalyst, and reuse of the catalyst after it had settled to the bottom and clear water had been removed. Photocatalytic efficiency worsens with successive runs when catalyst and water are reused without separation, whereas, when TiO₂ is separated, the photocatalyst is not deactivated.