Enhanced Photocatalytic Removal of Cyanotoxins by Al-Doped ZnO Nanoparticles with Visible-LED Irradiation

Abstract

The ZnO-based visible-LED photocatalytic degradation and mineralization of two typical cyanotoxins, microcystin-LR (MC-LR), and anatoxin-A were examined. Al-doped ZnO nanoparticle photocatalysts, in Al:Zn ratios between 0 and 5 at.%, were prepared via sol-gel method and exhaustively characterized by X-ray diffraction, transmission electron microscopy, UV-vis diffuse reflectance spectroscopy, photoluminescence spectroscopy, and nitrogen adsorption-desorption isotherms. With both cyanotoxins, increasing the Al content enhances the degradation kinetics, hence the use of nanoparticles with 5 at.% Al content (A5ZO). The dosage affected both cyanotoxins similarly, and the photocatalytic degradation kinetics improved with photocatalyst concentrations between 0.5 and 1.0 g L^-1. Nevertheless, the pH study revealed that the chemical state of a species decisively facilitates the mutual interaction of cyanotoxin and photocatalysts. A5ZO nanoparticles achieved better outcomes than other photocatalysts to date, and after 180 min, the mineralization of anatoxin-A was virtually complete in weak alkaline medium, whereas only 45% of MC-LR was in neutral conditions. Moreover, photocatalyst reusability is clear for anatoxin-A, but it is adversely affected for MC-LR.

Keywords: LEDs; ZnO-doped nanoparticles; anatoxin-(a); cyanotoxins; microcystin-LR; photocatalysis; visible light.

Figure 1

X-ray diffraction patterns of (a) A0ZO, (b) A1ZO, (c) A3ZO, and (d) A5ZO nanopowders with Williamson-Hall plots (inset).

Figure 2

Transmission electron microscopic images and grain sizes distribution of (a) A0ZO, (b) A1ZO, (c) A3ZO, and (d) A5ZO nanopowders. Scale bar: 50 nm.

Figure 3

(a) UV-Vis DRS absorption spectra in diffuse reflectance spectroscopy and (b) photoluminescence spectra of ZnO and Al-doped ZnO nanoparticles at room temperature.

Figure 4

Photocatalytic activity of ZnO and AZO nanoparticle photocatalysts (photocatalyst dosage = 0.5 g L⁻¹) in the photocatalytic degradation of (a) microcystin-LR (MC-LR) and (b) anatoxin-A under visible light-emitting diode (LED) irradiation at pH 7.0 and 20 °C. Effect of A5ZO photocatalyst dosage on the photocatalytic degradation of (c) MC-LR and (d) anatoxin-A under visible LED irradiation at pH 7.0 and 20 °C. Effect of pH of A5ZO photocatalyst (0.5 g L⁻¹) on the photocatalytic degradation of (e) MC-LR and (f) anatoxin-A under visible LED irradiation at 20 °C. Error bars indicate standard deviations of the four replicated experiments.

Figure 5

Mineralization (180 min) as a function of pH for AZO nanoparticle photocatalysts (photocatalyst dosage = 0.5 g L⁻¹) of (a) microcystin-LR and (b) anatoxin-A under visible-light-emitting-diode irradiation at 20 °C. Error bars indicate standard deviations of the four replicated experiments.

Figure 6

(a) Degradation of MC-LR at pH 7.0 and anatoxin-A at pH 9.0, in five consecutive reusability experiments with A5ZO nanoparticle photocatalysts (photocatalyst dosage = 0.5 g L⁻¹), after 80 min of visible light-emitting diode (LED) irradiation at 20 °C. (b) Time-dependent concentration of Zn(II) ions from A5ZO at different pH levels during 40 h of visible-LED irradiation at 20 °C. Error bars indicate standard deviations of the four replicated experiments.

Scheme 1

Synthesis of AZO nanoparticles by the sol-gel method.