Abstract
Perchlorate contamination in drinking water is an emerging concern due to its chemical persistence and its ability to disrupt thyroid function by competitively inhibiting iodide uptake in humans. Although the exact mechanisms of perchlorate formation during water treatment remain unclear, recent studies suggest that reactions between chlorine-based disinfectants (sodium/calcium hypochlorite, chlorine dioxide, chloramine, chlorine gas) and strong oxidants such as ozone may be significant sources.
This study adopted an integrated approach combining computational modelling, laboratory simulations, ion chromatography, and field monitoring to investigate perchlorate formation and stability in South African water treatment systems. Density Functional Theory (DFT) calculations (Gaussian 16 Rev. C.01; M06-2X/def2-SVP with Solvation Model based on Density (SMD), single-point corrections at def2-TZVP were carried out on reactants, intermediates, transition states, and products to determine minimum-energy pathways for perchlorate formation. Results indicated several energetically feasible ozonation routes for chlorine species, with Gibbs free energy barriers below ~30 kcal·mol⁻¹. The mechanism predicts ozone-assisted oxidation of intermediate chlorine oxyanions (chlorite and chlorate), with the overall process being exergonic and thermodynamically favourable under treatment plant conditions.
Laboratory simulations confirmed that perchlorate formation occurs when hypochlorite reacts under ozonated conditions, whereas negligible formation was observed in buffered systems at neutral pH, supporting the computational predictions.
LC/MS field analyses of raw and treated waters from Daspoort and Rietfontein showed perchlorate in most samples (63.1–63.8 ppb), while some clarifier and unchlorinated activated sludge samples had non-detectable levels. All positive detections exceeded the Maximum Contaminant Level Goal (MCLG) of 56 ppb by ~1.13–1.14×, indicating a potential public health risk. Ion chromatography (IC) analysis provided complementary quantification of perchlorate and supported the LC/MS results.
Finally, perchlorate stability tests demonstrated that perchlorate concentrations remained largely unchanged over extended periods under various pH and microbial conditions, confirming its persistence and resistance to biodegradation in aquatic environments.
Overall, the findings highlight that conventional biological and chlorination treatment processes are insufficient for perchlorate removal and can contribute to perchlorate formation during treatment, underscoring the need for targeted monitoring and advanced remediation technologies (e.g., ion exchange, bioreduction). This work provides mechanistic insights, empirical evidence, and risk assessments that inform sustainable water management, integrated catchment planning, and evidence-based policy for emerging contaminant control.