The Study Of The Flocculation Recovery And Reuse Of The Nano TiO₂Photocatalyst In Suspension System

Semiconductor photocatalysts, mainly titania, is considered to be one of the mostpromising green material for treating organic pollutants, because of its non-toxic, innocuity,non-corrosiveness, high catalytic activity, high oxidative power, recycled, without secondarypollutionand and the ability of mineralizing the organic pollutant into water (H2O) andinorganic ions completely. The main form of the research and application of TiO2consist ofsuspension system and the load system. Suspension type of TiO2has higher photocatalyticefficiency due to its large specific surface area and good dispersibility but with the problem ofhard recovery. However, direct emissions not only cause secondary pollution, but alsoresulting the processing costs rise rapidly. The photocatalytic efficiency of TiO2loaded onglass, ceramics and other lump materials and powder materials is decreases considerably,although it solves the problem of hard recovery. Flocculation precipitation is one of theimportant operating units in commonly used water treatment, with the characteristics of Wideapplication and low cost. Flocculation objects is mainly the abhorred liquid sol and suspendedparticles formed by the insoluble substances in water, while TiO2usually exists in colloidalstate in the water. So this study attempts to use flocculant to make TiO2particles flocculationsedimentation, and then to use again.Four kinds of flocculant such as polymeric aluminium(PAC), Polymeric FerricSulfate(PFS), polyacrylamide(PAM) and polyethyleneimine(PEI)are selected to research theeffect of “flocculating sedimentation-reuse” on the P25TiO2goods. The best flocculantdosage and optimum pH value were determined respectively through the evaluation ofturbidity. The TiO2alum floc received under the optimum flocculation conditions wasdispersed by ultrasonic, and then was used to carry out the experiment of photocatalyticdegradation of phenol. The life of the photocatalyst was inspected by10times repeatedexperiments. Designed the contrast text of the adsorption of phenol between the TiO2and theflocculant to indicate the flocculant adsorption function of phenol. Analyzing thePhotocatalyst particle size change both in the process of flocculation and after ultrasonictreatment to investigate the effect of flocculant on the TiO2Photocatalyst particle size.Choosing the best effect of flocculant through “flocculating sedimentation-reuse” experiment, then evaluating the economic value of the flocculation recovery and reuse ofTiO2photocatalyst by accounting the dosage of the flocculant in10times repeatedexperiments and the dosage of the TiO2photocatalyst in the same number of photocatalyticdegradation experiment. The TiO2/SiO2photocatalysts with negative charge was synthesizedby hydrothermal method, and choosing four kinds of cationic flocculant such as polymericaluminium(PAC), Polymeric Ferric Sulfate(PFS), cationic polyacrylamide(CPAM) andpolyethyleneimine(PEI) to research the effect of “flocculating sedimentation-reuse” on it.Through research, this paper obtained the following conclusions:1．The TiO2/SiO2photocatalyst shows better photocatalytic activity than the pure TiO2prepared under the same condition. Both the TiO2/SiO2photocatalyst and the pure TiO2werecharacterized by XRD, TEM, FTIR, Zeta. The results show that a moderate amount of SiO2inhibits the growth of the grain size of TiO2, and the calcining process causes a certain degreeof reunion for photocatalyst particles, and the Ti-O-Si bonds are formed in the TiO2/SiO2photocatalyst, the particles surface of TiO2/SiO2photocatalyst in suspension system carrysnegative charge.2．It can be obtained through investigating the effect of flocculant dosage on theflocculation sedimentation that both inorganic flocculant and organic flocculant have theoptimum dosage, and the flocculation effect is bad when the dosage is less than or greaterthan the optimum dosage. For P25, organic flocculants showed better function of flocculationsedimentation and the effect of PAM is the best due to the turbidity can be reached to1NTU.While the inorganic flocculants have the best flocculation sedimentation effect for theTiO2/SiO2photocatalyst and the turbidity can be reached to0NTU.3．It can be found through investigating the effect of pH value change on theflocculation sedimentation that there is no link between the optimum pH value of theflocculant and the intrinsic characteristics of the particles, and different types ofpolyacrylamide has different optimum pH value. PAC and PFS showed better flocculationeffect for photocatalyst when the pH value under the neutral and alkaline condition and theoptimum pH value for PEI is neutral. The range of the optimum pH value for PAM andCPAM are4～7,7～9, respectively. 4．Analyzing the particle size change of two kinds of Photocatalyst both differentmoments in the process of flocculation and after ultrasonic treatment, the results show thatadding the flocculant has a great influence on size distribution of the photocatalyst. A idealparticle size distribution can be obtained by ultrasonic.5．The result of contrast text of the adsorption shows that there is almost no adsorptionof phenol both photocatalyst itself and flocculant.6．The photocatalyst was flocculated at the condition of the best flocculant dosage andoptimum pH value, and then was used to carry out the experiment of photocatalyticdegradation of phenol, and repeat10times to inspect the life of the photocatalyst. Thedegradation rate of phenol shows that after10times “flocculating sedimentation-reuse”experiments, the photocatalytic degradation rate of phenol has remained at more than95%when organic flocculant was used, while the degradation rate of phenol changed greatly whenit was inorganic flocculant. The degradation rate of phenol decreases fastly with theincreasing of the number of flocculation. The photocatalytic degradation rate of phenol after10times “flocculating sedimentation-reuse”of the P25TiO2and the TiO2/SiO2by PFS were25.9%,10%, respectively, and by PAC its were10.7%,52%, respectively.7．Through10times “flocculating sedimentation-reuse” experiments, getting a kind offlocculant which making the best service life of photocatalyst, and then accounting itseconomic value. The results indicate that using PAM and CPAM to carry out the “flocculatingsedimentation-reuse” experiments of the photocatalyst can maintain the advantages of highcatalytic efficiency of the suspension type photocatalytic system. Through calculation, theycan save cost89.9%compared with recycling none and have a good application prospect.8．The same kind of flocculant shows different flocculating settling effect for thephotocatalyst with different surface properties.