## 4 Conclusions  
This study represents the first comparative investigation of carbon isotope fractionation during KMnO\({}_{4}\) degradation of TCE under varying concentrations of KMnO\({}_{4}\), TCE and HA across a pH range of 2 to 12 and in the presence of 0.2 M inorganic ions (Cl\({}^{-}\), HCO\({}_{3}\)\({}^{-}\), NO\({}_{3}\)\({}^{-}\), and SO\({}_{4}\)\({}^{2-}\)) in phosphate buffer. 1. The molar ratio n(KMnO\({}_{4}\))/n(TCE) has a subtle yet definitive effect on \(\varepsilon\) values, which should not be overlooked during isotopic interpretation. We observed that \(\varepsilon\) values are higher when the molar ratio n(KMnO\({}_{4}\))/n(TCE) exceeds 2, compared to ratios below 2. This effect may be attributed to insufficient KMnO\({}_{4}\) (n(KMnO\({}_{4}\))/n(TCE) \(<\) 2) being entirely consumed by the excess TCE, generating a substantial amount of MnO\({}_{2}\) colloids that further react with residual TCE. Moreover, under severely limited conditions (n(KMnO\({}_{4}\))/n(TCE) \(<\) 1), KMnO\({}_{4}\) primarily cleaves the carbon-carbon double bond of TCE, resulting in the formation of carboxylic acid intermediates. These findings differ from earlier studies, which reported no influence of the n(KMnO\({}_{4}\))/n(TCE) ratio on isotope fractionation. 2. A linear relationship was observed between pH and the corresponding \(\varepsilon\) values, exhibiting a trend opposite to E\({}_{0}\) as pH increased. This suggested that E\({}_{0}\) and TCE degradation intermediates are likely key factors influencing the observed isotope effects during KMnO\({}_{4}\) oxidation. 3. The addition of SO\({}_{4}\)\({}^{2-}\) and NO\({}_{3}\)\({}^{-}\) slightly influenced TCE carbon isotope fractionation, likely due to pH reductions during the reaction, which impart mild oxidizing characteristics to these anions under acidic conditions. Increasing initial Cl\({}^{-}\) concentration from 0 to 2 M also led to elevated \(\varepsilon\) values, potentially due to the salting-out effect that reduces TCE solubility in the aqueous phase. 4. High concentration of HA (50-250 mM) significantly influenced carbon isotope fractionation during TCE degradation, likely due to competition between HA functional groups and TCE for KMnO\({}_{4}\) consumption. Figure 6: (**a**) Effect of HA initial concentration on the oxidative degradation of TCE in the KMnO\({}_{4}\) reaction. (**b**) The variation of Rayleigh plots of TCE with respect to HA initial concentration. (**c**) The variation of \(\varepsilon\) values of KMnO\({}_{4}\) degradation of TCE at HA initial concentration of 0 mM to 50 mM. Experimental boundary conditions: [TCE]\({}_{0}\) = 75 mg/L and [KMnO\({}_{4}\)]\({}_{0}\) = 250 mg/L at initial suspension pH 8. Furthermore, evaluating ISCO performance at field sites may involve considerable uncertainty if the influences of n(KMnO\({}_{4}\))/n(TCE), inorganic ions, HA, and pH on the \(\varepsilon\) values are disregarded. Therefore, appropriate \(\varepsilon\) values must be carefully selected for quantitative analysis, and further research is needed to identify the limitations of applying stable carbon isotope analysis at the field scale. Y.D. : writing--review and editing, writing--original draft, visualization, and formal analysis. Y.W. : validation and resources. L.X. : methodology and resources. G.U. : writing--review and editing. Y.G. : visualization and resources. Z.E. : visualization and resources. J.L. : visualization and investigation. P.L. : visualization and investigation. C.L. : conceptualization, supervision, funding acquisition, and writing--review and editing. Q.F. : supervision, investigation, and writing--review and editing. All authors have read and agreed to the published version of the manuscript. This study is funded by the CAS Pioneer Hundred Talents Program (E229080101). Data Availability Statement: Data will be made available on request. The authors thank [PERSON] and [PERSON] for their help with the isotope analyses.