Use of immobilized zinc oxide photocatalysts for wastewater treatment: Application to methylene blue degradation

Abstract

Zinc-oxide (ZnO) nanoparticles were immobilized on borosilicate glass spheres with treatment of zinc acetate dihydrate (ZAC) solution, and calcination at 400-450 C. The isotherm and kinetic parameters of methylene blue (MB) sorption on ZnO-coated glass beads were estimated from batch sorption tests in darkness. An ultraviolet (UV)-lamp with nominal power of 6 W and emission peak at 375 nm was used as a light source. Batch experiments of MB photodegradation were performed in a double-wall glass reactor. Continuous flow tests were conducted in a poly(methyl methacrylate) (PMMA) annular photoreactor by recirculating the MB solution between it and a feed tank. All results were interpreted with dynamic models based on a two-step process consisting of MB adsorption/desorption, and adsorbed MB surface reaction. The intense stirring caused by air bubbling or aqueous phase flow triggers the detachment of catalyst mass from substrate, the partial renewal of immobilized catalyst surface exposed to UV-radiation, and the enhancement of the apparent maximum MB sorption capacity by total ZnO mass. The fast MB adsorption leads to an almost constant concentration of adsorbed MB at equilibrium, while the surface photocatalytic reaction is the MB degradation ratecontrolling step. Comparable values of the kinetic constant of surface reaction are estimated from batch and continuous flow tests, for repeated cycles of photocatalysis.

Photocatalytic Degradation of Dissolved Phenol by Immobilized Zinc Oxide Nanoparticles: Batch Studies, Continuous Flow Experiments, and Numerical Modeling

“…Two fixed-bed annular reactors were used to assess the performance of photocatalysts under continuous flow conditions: one made of poly(methyl) methacrylate (PMMA, Figure 3 [ 19 ]) and another one made of stainless steel (STST, Figure 4 ). The same UV lamp (375 nm Phillips 6 W, Amsterdam, The Netherlands) was placed vertically along the central axis of each reactor and protected by a plastic or glass cover ( Figure 4 b).…”

“…It is well-known that the wall of PMMA tube, depending on its thickness, absorbs a significant fraction of the UV-radiant flux (80–90%) that has not been absorbed by photocatalyst [ 19 ]. On the other hand, the metallic surface of STST reactor is expected to reflect much more efficiently the UV-radiation reaching to it.…”

“…The overall rate of the photocatalytic reaction, regarded as homogeneous process, can be described by pseudo-first order kinetics of the form: where is the volume of the annular space of reactor ( Figure 4 b), and is the kinetic constant (s −1 ). The hydrodynamic dispersion coefficient is given by the following [ 19 ]: where is the molecular diffusion coefficient, is the longitudinal dispersion length, and is the electrical formation factor. If is the initial phenol concentration, then, using the dimensionless variables, , , , and , we get the following system of dimensionless parametric equations which are coupled with the initial conditions and boundary conditions The following set of dimensionless parameters are included, explicitly or implicitly, into Equations (5) and (6) [ 19 ]:…”

“…However, under continuous flow conditions, it would be necessary to separate the photocatalyst particles from the solution, before reusing them, which is a cost-expensive and energy-intensive step. Alternatively, the photocatalyst particles could be immobilized on various types of substrates, with some examples: clay [ 14 ], foams [ 15 , 16 , 17 ], graphene oxides [ 18 ], and borosilicate spheres [ 19 ]. The main disadvantage of such an approach is the reduction of photocatalytic efficiency, due to the decrease of the specific surface area, and mass transfer limitations [ 20 , 21 ].…”

“…The aqueous solution of phenol recirculates between each photoreactor and an external vessel able to store variable volume of treated solution. A one-dimensional model, combining the convective flow and hydrodynamic dispersion with overall photodegradation processes, is used for the inverse modelling of experiments and estimation of the kinetic reaction constant as a function of all pertinent parameters [ 19 ].…”