Experimental and theoretical investigations of photolytic and photocatalysed degradations of crystal violet dye (CVD) in water by oyster shells derived CaO nanoparticles (CaONP)

Crystal violet dye (CVD) is one of the most stable and toxic dyes, whose adverse consequencus in the aquatic and hence other components of the environment are well documented. Consequently, th remediation of water contaminated by this dye, has several advantages to industries that discharge dye rich-wastes to the aquatic environment. In this study, advantages of using nanoparticles that are ecofriendly and cost effective was implemented by converting CaCO₃ in oyster shells to calcium oxide nanoparticles (CaONP). The synthesized CaONP were characterized using UV–visible, XRD, FTIR, SEM, EDX, DLS and XPS techniques.The XRD profile indicated peaks typical to CaONP with principal peak observed at 2θ= 34.19° and crystalline size equal to 27.70 nm. Information deduced from EDX and XPS also confirmed the composition and electron releasing profile of the CaONP. The DLS measurement indicated the average diameter of the CaONP as 50.24 nm, which confirms its fitness as a mesoporous nanoparticles. The application of the CaONP as a catalyst for the photodegradation of CVD in water showed a strong dependency on some physicochemical parameters but showed an optimum efficiency of 99 % at initial dye concentration of 50 ppm, catalyst dosage of 1.5 g and pH of 11. The degradation kinetics showed a good degree of fitness for the Langmuir-Hinshelwood, modified Freundlich, first order and parabolic diffusion models. The theoretically evaluated bandgap (≈ 4.4 eV) for the nanoparticles agrees with the experimental value and indicates that the material is a semiconductor that absorb in the UV region. The observation of the XPS of the CaONP before and after photocatalysis indicated that electrons were consumed during the photodegradation process. Also, the conduction band potential of the CaONP ($E_{Cond}^{\left(p\right)}=-1.7eV)$ was found to be more negative than that of $O_2/O_2^{-}$ (-0.282 eV), which indicated that the photodegradation reaction is limited by the ability of electrons to reduce the O₂ in the dye solution to superoxide $O_2^{-}$.