To develop a highly efficient visible light-induced and conveniently recyclable photocatalyst, in this study, a ternary magnetic ZnO/Fe3O4/g-C3N4 composite photocatalyst was synthesized for the photodegradation of Monas dye. The structure and optical performance of the composite photocatalyst were characterized using X-ray diffraction (XRD), transmission electron microscopye (TEM), energy dispersive spectroscopy (EDS), photoluminescence (PL) spectra, ultraviolet–visible diffuse reflection, and photo-electrochemistry. The photocatalytic activities of the prepared ZnO/Fe3O4/g-C3N4 nanocomposites were notably improved, and they were significantly higher than those of pure g-C3N4 and ZnO. Given the presence of the heterojunction between the interfaces of g-C3N4 and ZnO, the higher response to visible light and separation efficiency of the photo-induced electrons and holes enhanced the photocatalytic activities of the ZnO/Fe3O4/g-C3N4 nanocomposites. The stability experiment revealed that ZnO/Fe3O4/g-C3N4-50% demonstrates a relatively higher photocatalytic activity after 5 recycles. The degradation efficiency of MO, AYR, and OG over ZnO/Fe3O4/g-C3N4-50% were 97.87%, 98.05%, and 83.35%, respectively, which was due to the number of dye molecules adsorbed on the photocatalyst and the structure of the azo dye molecule. Azo dyes could be effectively and rapidly photodegraded by the obtained photocatalyst. Therefore, the environment-friendly photocatalyst could be widely applied to the treatment of dye contaminated wastewater.

Pepper seedling wilt disease is the main cause of crop yield reduction. Biocontrol agents are widely used to control plant diseases caused by pathogenic fungi and activate plant defense systems. Our preliminary work showed that Bacillus subtilis SL-44 played a significant role in the reduction of wilt disease severity on pepper plants. To evaluate biological control mechanism of B. subtilis SL-44 on wilt disease caused by Rhizoctonia solani, the activities of the related enzymes were detected in the pepper seedling with different treatment in this study. Fluorescence microscopy combined with different dyes showed that B. subtilis SL-44 induced a large amount of active oxygen and callose accumulation in pepper leaves. The defense-related enzyme activities in pepper were improved significantly when treated with B. subtilis SL-44, including peroxidase, catalase, superoxide dismutase, polyphenol oxidase, and phenylalanine ammonia lyase. The activity of chitinase and β-1,3-glucanase in B. subtilis SL-44-treated pepper was also enhanced. Furthermore, the expression level of pepper-resistance gene CaPIN II was significantly increased in B. subtilis SL-44 treatment. Besides, B. subtilis SL-44 filtrate led to the death of the pathogenic fungus by fracturing the mycelia and leaking of the cell contents. Surfactin, iturin, and fengycin were found in B. subtilis SL-44 crude extracts, which could be effective antifungal compounds against R. solani. The results suggest that B. subtilis SL-44 could not only activate induced systemic resistance of pepper seedling against wilt disease caused by R. solani by jasmonic acid-dependent signaling pathway but also produce antifungal compounds to inhibit or even damage the mycelium growth of R. solani. The findings of this study provide novel guidance in plant protection development.