Flocculation Kinetics of TiO2-enzyme Microfloccules

Abstract

TiO2-enzyme microfloccules were prepared by using titanium dioxide nanoparticles as immobilizing carriers, three kinds of polyacrylamide (nonionic, cationic and anionic polyacrylamide) as flocculants and papain as a model of en- zyme. The effects of pH values, dosage and types of polyacrylamide (PAM) on the flocculation and sedimentation behaviour of TiO2-enzyme microfloccules were investigated. The SEM, EDS and particle size analyzer were used to characterize the morphology of TiO2-enzyme microfloccules. The results showed that an effective flocculation was formed among TiO2, pa- pain and different types of PAM flocculant by hydrogen bonding interactions, electrostatic attractions, adsorption bridging action, etc. It was noted that the settling rate of TiO2-enzyme microfloccules, turbidity of the supernatant, floc size and com- pactness of resulting floccules were highly dependent on the PAM dosage. For three kinds of PAM, similar trends of floccu- lation kinetics were observed, a general increase in settling rates were relevant to decrease in turbidity. Attributed to high settling rates, strong flocculation with big size and stable floc occurred with the function of optimum PAM concentrations. But the optimum dosages were different. When the concentration was in the range of 75 mgL -1 to 175 mgL -1 , nonionic PAM (nPAM) displayed the best flocculant performance in all kinds of PAM with a rapid settling rate and large floc size. Moreover, under excessive dosage condition, breakup of floc then occurred. Flocculation kinetics of TiO2-enzyme micro- floccules also could be effectively controlled by changing the pH value of reaction system. Compared nPAM with cationic PAM (cPAM), the microfloccules by using nPAM displayed a high stability and compactness in a wide range of pH values. The settling rate and floc size by using cPAM showed a strong dependence on pH values. It indicated that the immobilized enzyme size could be regulated by the PAM dosage and pH value according to the enzyme structure and properties. Such porous and flexible microstructure was expected to provide the free space as much as possible for the access of substrate molecules to enzyme. Keywords flocculation kinetics; settling rate; particle size distribution; nano TiO2; morphology