Zinc oxide (ZnO) photocatalysts were synthesized by sol–gel method using zinc acetate as precursor for degradation of azo dyes under UV irradiation. The resultant samples were characterized by different techniques, such as XRD, SEM, and EDX. The influence of preparation conditions such as calcination temperature and composite ratio on the degradation of methyl orange (MO) was investigated. ZnO prepared with a composite ratio of 4:1 and calcination temperature of 400 °C exhibited 99.70% removal rate for MO. The effect of operation parameters on the degradation was also studied. Results showed that the removal rate of azo dyes increased with the increased dosage of catalyst and decreased initial concentration of azo dyes and the acidic condition is favorable for degradation. Furthermore, the kinetics and scavengers of the reactive species during the degradation were also investigated. It was found that the degradation of azo dyes fitted the first-order kinetics and superoxide ions were the main species. The proposed photocatalyst can efficiently and rapidly degrade azo dyes; thus, this economical and environment-friendly photocatalyst can be applied to the treatment of wastewater contaminated with synthetic dyes.

A strong pyro-catalytic dye degradation with an ultrahigh degradation efficiency (>99%) in hydrothermally synthesized pyroelectric BiFeO3 nanoparticles was achieved under a room-temperature cold-hot alternating excitation (between 27 °C to 38 °C). The pyro-catalysis originated from a combination of the pyroelectric effect and the electrochemical oxidation-reduction reaction. The intermediate products (hydroxyl radicals and superoxide radicals) of pyro-electro-catalysis were observed. Pyro-catalysis provides a highly efficient and reusable dye wastewater decomposition technology through utilizing environmental day-night temperature variation.

To remove polycyclic aromatic hydrocarbons effectively, coal-based activated carbon (CAC) was produced by various microwave power modification. Original and modified CAC (MCACs) were characterized by N2 adsorption method, scanning electron microscopy, X-ray photoelectron spectroscopy, Boehm method, and point of zero charge determination. Their adsorption behavior of naphthalene was also investigated. Adsorption equilibrium isotherms, adsorption kinetics, and thermodynamics of naphthalene adsorption on CAC and MCACs were generated. Microwave modification enhanced the basic surface groups of MCACs, extended their Brunauer-Emmett-Teller surface area and pore volume, and varied their distribution of surface oxygen groups. Surface area, pore volume, and functional groups of MCACs were positively related to their naphthalene-adsorption capacity.The adsorption equilibrium of naphthalene on CAC and MCACs needed only 40 min, and this adsorption was fast. Adsorption isotherms revealed that the Freundlich model was applicable to the adsorption process. The adsorption kinetics of naphthalene onto adsorbents was described by pseudo-second-order kinetic model. Naphthalene adsorption was found to a spontaneous and exothermal adsorption process. All these results showed that microwave radiation was an efficient and rapid method of modifying activated carbons. Moreover, MCACs was a promising low-cost and fast adsorbent that can be used to remove naphthalene from aqueous solutions.

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