Calcium Peroxide-Enhanced Fe-Based Zeolite for Remediation of MTBE-Contaminated Groundwater: Application in Slow-Release Oxidant Systems

The widespread use of methyl tert-butyl ether (MTBE) in gasoline has led to persistent groundwater contamination. Recently, permeable reactive barriers (PRBs) have emerged as a key in situ remediation technology for treating with such contamination. Various reactive media have been employed to intercept and degrade pollutants, but their short half-lives, instability, and poor dispersion in heterogeneous geological conditions limit their effectiveness and further applications. In this study, we designed a double-column PRB system incorporating slow-release oxidants (PLA@CP-CA) and Fe-based zeolite as heterogeneous Fenton catalysts. Calcium peroxide (CaO₂) was selected as the oxidant instead of hydrogen peroxide (H₂O₂) due to its higher stability, ease of transport, and prolonged release, facilitating sustained pollutant degradation. However, the direct use of CaO₂ raises groundwater pH, so citric acid (CA) was employed as a buffer to regulate pH during CaO₂ dissolution, thereby enhancing the efficiency of the heterogeneous Fenton process. Electron paramagnetic resonance (EPR) spectroscopy and radical trapping experiments confirmed that multiple reactive species, including ·OH, O 2 – ·, and ¹O₂, contributed synergistically to MTBE degradation. To mitigate calcium peroxide leakage, polylactic acid (PLA) was used to encapsulate the oxidant (PLA@CP-CA) under anhydrous conditions. The Box–Behnken design (BBD) in response surface methodology (RSM) optimized the PLA@CP-CA formulation, achieving an optimal H₂O₂ release capacity of 3.22 mmol/g with a mass ratio of PLA/CP/CA at 0.67:1.77:1.17. Approximately 97% of MTBE was remediated within 192 hours using the double-column PRB, demonstrating the efficacy of the controlled oxidant release from PLA@CP-CA in promoting MTBE degradation. This heterogeneous Fenton system, combining PLA@CP-CA with Fe-S-1, offers a promising approach for in situ remediation in MTBE-contaminated environments.