Sequential Ozonation Processes for Enhanced Degradation of Aromatic Sulfonic Compounds in Water

Aromatic sulfonic compounds (ASCs), widely used in textile industry for synthesizing azo dyes, are commonly detected in aquatic environments, where they pose potential ecological risks. This study proposes a sequential ozonation system to degrade two ASCs: 4-phenol sulfonic acid (4-PSA) and 2-naphthalene sulfonic acid (2-NSA). The sequential treatment consists of two stages: first, conventional ozonation is applied to partially degrade the target compounds and produce intermediate byproducts, with ozonation time of 20 or 40 min. In the second stage, a commercial metal oxide (NiO or CuO) is used as catalyst with ozone to further degrade the intermediates generated in the first stage. The sequential approached achieved nearly complete mineralization of both ASCs, significantly surpassing the total organic carbon (TOC) removal (around 61%), when the catalyst was added at the start of ozonation and maintained for 2 h. The success of sequential system is attributed to the reactive oxygen species (ROS) generated by the catalyst, which mostly eliminate byproducts formed during conventional ozonation. Contrary to single catalytic ozonation, where ROS are generated since the beginning and act on both intermediates and target compounds, reducing the TOC removal around 33%. Additionally, sulphate ions are released during the ASCs elimination, which can inhibit the catalytic activity by blocking the active sites; the sequential process mitigates this effect by minimizing catalyst exposure to sulphate ions. FTIR and XPS analyses confirmed the presence of these ions on the catalyst surfaces. Therefore, sequential ozonation reduces the metal leaching and offers high ASCs mineralization efficiency compared to conventional ozonation processes, and thus, could have a potential use for treating ASCs contaminated effluents.

Graphical Abstract