Photocatalytic degradation technology is an important tool for treating trichloroethylene (TCE) pollution in water bodies. While previous studies have focused on catalyst optimization and degradation kinetics for trichloroethylene (TCE) photocatalysis, the systematic impact of environmental conditions on carbon isotope effects (ε) and their mechanistic implications remains poorly understood. This limits the reliability of quantitative isotope analyses in field applications. We conducted a series of laboratory experiments on the photocatalytic degradation of TCE to investigate the kinetic and isotopic effects under different conditions. Increasing the concentration of TCE, anions (NO₃⁻ and HCO₃⁻), and cations (Mg²⁺ and Ca²⁺) decreased the photocatalytic degradation of TCE. O₂ will increase the degradation efficiency of TCE. The dose required to achieve maximum photocatalytic efficiency varies for different types of catalysts, which needs to be determined on a case-by-case basis. TCE photocatalytic degradation had a small carbon isotope effect (ε = −2.0 ± 0.2‰ to −3.2 ± 0.5‰), which was slightly affected by the catalyst dosage and species (TiO₂ and ZnO), and concentrations of TCE, O₂, and inorganic ions (NO₃⁻, HCO₃⁻, Mg²⁺, and Ca²⁺). The ε values are stable and reproducible and relatively insensitive to our selected environmental factors in this study, which can reduce the uncertainty of applying stable carbon isotope enrichment factors to quantify the photocatalytic reaction for remediation of TCE contaminated sites.

Stable isotope analysis is a powerful tool for inferring and quantifying transformation processes, but its effectiveness relies on understanding the magnitude and variability of isotopic fractionation associated with specific reactions. Potassium permanganate (KMnO₄) is widely used as an efficient oxidant for the degradation of trichloroethylene (TCE); however, the influence of environmental factors on the isotope fractionation during this process remains unclear. In this study, compound-specific isotope analysis (CSIA) was conducted to investigate the variability in carbon isotope effects during the KMnO₄-mediated degradation of TCE under varying conditions, including initial concentrations of KMnO₄ and TCE, the presence of humic acid (HA), pH levels, and inorganic ions. The results showed that the overall carbon isotope enrichment factors (ε) of TCE ranged from −26.5 ± 0.5‰ to −22.8 ± 0.9‰, indicating relatively small variations across conditions. At low KMnO₄/TCE molar ratio (n(KMnO₄)/n(TCE)), incomplete oxidation and/or MnO₂-mediated oxidation of TCE likely resulted in smaller ε. For dense, non-aqueous phase liquid (DNAPL) TCE, which represents extremely high concentrations, the ε value was −13.0 ± 1.7‰ during KMnO₄ oxidation. This may be attributed to the slow dissolution of isotopically light TCE from the DNAPL phase, altering the δ¹³C signature of the reacted TCE and resulting in a significantly larger ε value than observed for dissolved-phase TCE oxidation. The ε values increased with rising pH, probably due to the decrease in oxidation potential (E₀) of KMnO₄ from pH ~2 to ~12, as well as the emergence of different degradation pathways and intermediates under varying pH conditions. Both SO₄²⁻ and NO₃⁻ slightly influenced the ε values, potentially due to the formation of H₂SO₄ and HNO₃ at lower pH, which may act as auxiliary oxidants and contribute to TCE degradation. A high concentration (50 mM) of HA led to a decrease in ε values, likely due to competitive interactions between HA and TCE for KMnO₄, which reduced the effective oxidation of TCE. Overall, the carbon isotope enrichment factors for KMnO₄-mediated TCE degradation are relatively stable, although certain environmental conditions can exert minor influences. These findings highlight the need for caution when applying quantitative assessment based on CSIA for KMnO₄ oxidation of TCE.