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.

Plant extracts were produced from Camellia sinesis (Green Tea) and Punica granatum (pomegranate), and the total concentration of polyphenols was measured in terms of equivalent concentration of Gallic acid by using the Folin-Ciocalteu method. Zero Valent Iron nanoparticles (nZVIs) were synthesized in a semi-batch reactor by mixing a pre-specified volume of plant extract or Gallic Acid solution with an aqueous solution of iron sulfate heptahydrate (FeSO4·7H2O). To monitor the kinetics of nZVI synthesis, the transient responses of solution pH and redox potential (Eh) were recorded with two probes adequately connected with a data acquisition card. The nanoparticles were characterized by a variety of techniques: Dynamic Light Scattering (DLS), ζ-potential, Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectroscopy, Transmission Electron Microscopy (TEM). A kinetic parametric model, based on two parallel single electron transfer (SET) and hydrogen atom transfer (HAT) reactions, was suggested to quantify the dynamics of ferrous ions reduction to zero valence, and its parameters were estimated for each experimental system by matching the transient response of pH. The temporal changes of redox potential during nZVI synthesis were indicative of the reaction progress and agreed with the numerical predictions in semi-quantitative basis. The numerical model enabled us to track the temporal variation of the concentration of iron and polyphenol species, and calculate the yield of ZVI synthesis. The reactivity of nZVIs was assessed by measuring their capacity to reduce hexavalent chromium Cr (VI) in aqueous solutions prepared from potassium dichromate (K2Cr2O7).

In spite of the progress achieved on the photo-catalytic treatment of water streams, there is still a gap of knowledge on the optimization of the performance of continuous-flow photo-reactors. Zinc-oxide (ZnO) nanoparticles were immobilized on Duranit (80% silica + 20% alumina) inert balls with dip-coating and thermal annealing. The immobilized ZnO nanoparticles were characterized by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD), ultraviolet-visible (UV-Vis) spectroscopy, and Raman spectroscopy. To assess the stability and photocatalytic capacity of immobilized ZnO, degradation tests of phenol were performed in batch mode in a 22 W UV-oven with an emission peak at 375 nm by varying the temperature, the initial phenol concentration, and the ratio of photocatalyst mass to initial phenol mass. Continuous flow tests were conducted on two types of annular photo-reactors, made of poly(methyl)methacrylate (PMMA) and stainless steel (STST), equipped with a 6 W UV-lamp with emission at 375 nm, packed with ZnO-coated Duranit beads. Experiments were conducted by recirculating the phenol solution between the annular space of reactor and an external tank and varying the flow rate and the liquid volume in the tank. A one-dimensional dynamic mathematical model was developed by combining reactive with mass-transfer processes and used to estimate the overall reaction kinetic constant with inverse modeling. The results revealed that the ZnO losses might be discernible in batch mode due to the intense stirring caused by the bubbles of injected air, while an insignificant loss of ZnO mass occurs under continuous flow conditions, even after several cycles of reuse; the order of the overall phenol photodegradation reaction is lower than unity; the pseudo-1st order kinetic constant scales positively with the ratio of photocatalyst mass to the initial phenol mass and Peclet number.