Deep insight into the effect of bicarbonate on pollutant abatement in the UV/chlorine process

The UV/chlorine process has gained significant attention from researchers and practitioners and is considered as a viable alternative to the UV/H2O2 process. HCO3–, widely spread in natural water, was traditionally perceived to act as a quencher in advanced oxidation processes (AOPs), converting less selective radicals into the more selective CO3·–. Unlike other radical precursors (e.g., persulfate and H2O2), HOCl rapidly reacts with CO3·–, resulting in the formation of ClO·. Consequently, HCO3– introduces complexity to the radical chemistry within the UV/chlorine process. This study comprehensively investigated the role of HCO3– in the UV/chlorine process with regards to pollutant degradation, employing 1,4-dimethoxybenzene (DMOB) as the target contaminant. Adding 2 mM of HCO3− in the UV/chlorine system significantly amplifies the generation of ClO· by ∼ 2 times at pH 7.0, thereby markedly expediting the degradation of DMOB. The reaction rate constant of CO3·− with free chlorine was measured to be 8.82 × 107 M−1 s−1 at pH 7.0, and the rate depends on pH and the species of chlorine. The influence of HCO3− on radical distribution within the UV/chlorine process is also contingent upon other solution conditions, including pH and chlorine doses. Furthermore, this study demonstrates that the HCO3−-mediated radical conversion remains relatively unaffected by background constituents. The experimental data on pollutant degradation via the UV/chlorine process in pure water can be extrapolated to estimate the treatment efficiency in actual water scenarios. These findings enhance our understanding of the intricacies of UV/chlorine chemistry in practical water treatment, thus advancing its potential as an emerging AOP for specialized applications.