Enhancement of 2-chlorophenol photocatalytic degradation in the presence Co2+-doped ZnO nanoparticles under direct solar radiation

The performance of Co²⁺-doped ZnO nanoparticles, prepared using the sol–gel method, for 2-chlorophenol degradation under direct solar radiation was investigated. Various parameters were investigated during the degradation process, namely solar intensity, Co²⁺ ion concentration, loading concentrations of Co²⁺-doped ZnO, and pH. The photocatalytic degradation efficiency increased when the initial concentration of 2-chlorophenol decreased; the optimum concentration was 50 mg/L under similar experimental conditions. Moreover, optimum values, established on a sunny day, were 0.75 wt% of Co²⁺, a 1 g/L loading concentration, and a pH of 6.0, respectively. The highest degradation efficiency observed was 95 %, after only 90 min of solar light irradiation. The mechanism of visible photocatalytic degradation using Co²⁺-doped ZnO was explained as a strong electronic interaction between Co²⁺, Co³⁺ and ZnO, and a promotion in the charge separation, which enhanced the degradation performance. The fragmentation of 2-chlorophenol under the optimal conditions was investigated using HPLC, comparing standards of all intermediate compounds. The pathway of the fragmentation was proposed as involving hydroxyhydroquinone, catechol, and phenol formation, which were then converted to non-toxic compounds such as oxalic acid and acetic acid with further decomposition to CO₂ and H₂O.