Systematic comparison of sono-synthesized Ce-, La- and Ho-doped ZnO nanoparticles and using the optimum catalyst in a visible light assisted continuous sono-photocatalytic membrane reactor

•Systematic design and sonochemical synthesis of La-, Ho- and Ce-ZnO nanoparticles.•Evaluation and optimization of the effects of the synthesis operational variables.•Favorable antibacterial and sono-photocatalytic behavior of the optimum catalyst.•High performance of the catalyst in continuous sono-photocatalysis/membrane process. In the present work, La-ZnO, Ho-ZnO and Ce-ZnO nanoparticles were synthesized by sonochemical method to use as catalysts in visible light photocatalysis, sono-photocatalysis and visible light sono-photocatalysis/membrane separation (SPMS) processes for degradation of an organic pollutant. The effect of doping source, mass ratio of doping source to the precursor of ZnO synthesis, pH, sonication temperature and time, and calcination temperature and time was investigated in visible light photocatalytic activity of the prepared lanthanides-doped ZnO nanoparticles using Taguchi design. The optimum conditions for the nanoparticles synthesis were obtained at 8 wt% of cerium nitrate, pH 10, sonication time for 1 h at 60 °C and calcination for 3 h at 300 °C. FE-SEM, EDS, XRD, PL and DRS analyzes were used to identify the characteristics of Ce-ZnO as the optimum catalyst. The Ce-ZnO nanoparticles were used to remove Reactive Orange 29 (RO29) solution via sono-photocatalysis process under the visible light irradiation. The effect of initial pH of solution, catalyst dosage, light intensity (power of applied lamp(s)), initial concentrations of inorganic salts such as Na2CO3, NaCl and Na2SO4 on the decolorization efficiency was investigated. Finally, a continuous flow visible light SPMS reactor was used in the presence of Ce-ZnO catalyst and polypropylene hollow fiber membrane for treatment of dye solution. In the best conditions of SPMS reactor, 97.84% of dye removal was achieved. GC-Mass, COD and TOC analyses were used to approve degradation and mineralization of RO29 using the SPMS process. Moreover, the prepared Ce-ZnO nanocomposite was shown the favorable antibacterial behavior against positive and negative bacteria.