${\mathrm{Fe}}^{3+}$-NTA-Catalyzed Homogenous Fenton-Like Degradation of Trichloroethylene in Groundwater at Natural pH ($\sim 8.0$): Efficacy, By-Products, and ${\mathrm{H}}_2{\mathrm{O}}_2$ Utilization

The ferric-nitrilotriacetate (${\mathrm{Fe}}^{3+}\text{-}\mathrm{NTA}$)-catalyzed homogenous Fenton-like reaction (i.e., ${\mathrm{Fe}}^{3+}\text{-}\mathrm{NTA}/{\mathrm{H}}_2{\mathrm{O}}_2$) has received increasing interest for environmental applications, but its potential in degrading trichloroethylene (TCE) in groundwater still is not well known. The results of this study showed that ${\mathrm{Fe}}^{3+}\text{-}\mathrm{NTA}/{\mathrm{H}}_2{\mathrm{O}}_2$ was more effective than other ${\mathrm{Fe}}^{3+}\text{-}\mathrm{chelate}/{\mathrm{H}}_2{\mathrm{O}}_2$ processes (e.g., including ethylenediaminetetraacetate, ethylenediamine-$N,N^{\prime }$-disuccinate, citrate, malonate, and tartrate) in degrading TCE in groundwater at natural pH ($\sim 8.0$). The effects of important parameters including the ratio of $\mathrm{NTA}:{\mathrm{Fe}}^{3+}$, and the dosages of ${\mathrm{Fe}}^{3+}\text{-}\mathrm{NTA}$ and ${\mathrm{H}}_2{\mathrm{O}}_2$ were investigated. Greater than 98% degradation efficiency of TCE in actual groundwater was obtained after 60 min of reaction time under the given conditions of $\mathrm{NTA}:{\mathrm{Fe}}^{3+}=1.5:1$, $0.1\mathrm{mM}{\mathrm{Fe}}^{3+}\text{-}\mathrm{NTA}$, $5\mathrm{mM}{\mathrm{H}}_2{\mathrm{O}}_2$, and pH 8.0. Hydroxyl radical (${\mathrm{HO}}^{•}$) was responsible for the degradation of TCE, and the by-products were identified as formic acid and chloride ion (${\mathrm{Cl}}^{-}$). Greater than 90% degradation efficiency of TCE in a sand column was achieved under dynamic conditions [$0.5\mathrm{mM}{\mathrm{Fe}}^{3+}\text{-}\mathrm{NTA}$ ($1:1.5$), $4\mathrm{mM}{\mathrm{H}}_2{\mathrm{O}}_2$, and hydraulic retention time of 4 h], resulting in stoichiometric ratios of $\mathrm{\Delta }{\mathrm{Cl}}^{-}:\mathrm{\Delta }\mathrm{TCE}$ and $\mathrm{\Delta }\mathrm{TCE}:\mathrm{\Delta }{\mathrm{H}}_2{\mathrm{O}}_2$ of $\sim 3:1$ and 4%, respectively. Compared with iron mineral (e.g., magnetite)-catalyzed Fenton-like processes, ${\mathrm{Fe}}^{3+}\text{-}\mathrm{NTA}/{\mathrm{H}}_2{\mathrm{O}}_2$ led to much smaller consumption of ${\mathrm{H}}_2{\mathrm{O}}_2$ (at least 1 order of magnitude smaller). In addition, the buffering capacity of bicarbonate anions (${\mathrm{HCO}}_3^{-}$) in groundwater had a positive effect on the degradation of TCE by ${\mathrm{Fe}}^{3+}\text{-}\mathrm{NTA}/{\mathrm{H}}_2{\mathrm{O}}_2$.