The manifold role of heavy metal ions (Cu(II), Cd(II), and Zn(II)) in the solar photocatalytic and sonophotocatalytic treatment of water containing phenol

Abstract The efficiency of ⋅OH-based advanced oxidation processes can be strongly affected by the copresence of organic pollutants and heavy metals in the aqueous effluent. This chapter studied the impact of heavy metal ions (HMIs), namely, Cu(II), Cd(II), Zn(II), and Fe(II) on the TiO2-photocatalytic (UV/TiO2) and sonophotocatalytic (ultrasound [US]/UV/TiO2) degradation of phenol in aqueous solutions. Experiments were conducted in forced batch loop system connecting a Suntest solar photochemical reactor (830 W/m2) with a standing wave ultrasonic reactor operating at 600 kHz and 70 W. The effect of each heavy metal was investigated at two concentrations, 1 and 100 μM, i.e., low and high concentration, for a single concentration of phenol of 100 μM. The HMIs present different behaviors, depending on their concentrations. A low-concentration, 1 μM, of HMI significantly increased the removal rate of phenol by both photocatalysis and sonophotocatalysis. At higher concentrations of HMIs, 100 μM, the photocatalytic performance was slightly reduced, while a significant decrease in the phenol removal was recorded for sonophotocatalysis. The rapid photoreduction of HMIs through electron (e−CB) tapping at the catalyst surface implies that more oxidation of H2O/OH− or phenol occurred in holes (h+VB), thus generating a higher level of ⋅OH in the solution to degrade phenol quickly. The rate of this redox processes induced by HMIs slowed down at higher HMI level, 100 μM, where a higher amount of the photoreduced metal could be disposed on the TiO2 surface and could deactivate the photocatalytic process. Consequently, the removal of both HMIs and phenol could be efficiently achieved by using simulated solar photocatalysis and sonophotocatalysis.