Removal of PCE/TCE from groundwater by peroxydisulfate activated with citric acid chelated ferrous iron at 13 °C

Chlorinated organic compounds (CVOCs), such as tetrachloroethene (PCE) and trichloroethene (TCE) belong to the most abundant groundwater contaminants across the world. Over the last 15 years, in situ chemical oxidation (ISCO) using peroxydisulfate (PDS) activated by citric acid chelated ferrous ion (Fe(II)-CA) has been proposed as a suitable method for CVOCs removal from groundwater. However, published studies examining Fe(II)-CA activated PDS systems under laboratory conditions (T ≈ 20 °C) and in demineralized water have not provided a picture of the desired behavior of Fe(II)-CA activated PDS systems for CVOCs degradation at a lower temperature (T ≈ 13 °C) that are typical for shallow groundwater of larger areas of Europe and North America and in the presence of typical groundwater constituents, such as chlorides and bicarbonates. To resolve this knowledge gap, a series of batch experiments were conducted to investigate PCE, TCE and combined PCE/TCE degradation performance in Fe(II)-CA (fixed molar ratio Fe(II)/CA = 2/3) activated and unactivated PDS systems based on their PDS/Fe(II) initial molar ratios (1/2.5; 1/1; 2.5/1; 5/1; 7.5/1; 10/1; 15/10). The experiments were conducted in artificially prepared groundwater (0.5 mmol⋅L−1 CaCl2 ⋅H2O, 0.5 mmol⋅L−1 MgCl2 ⋅H2O, 0.5 mmol⋅L−1 NaHCO3 and 0.5 mmol⋅L−1 KHCO3) at 13 °C. The results indicated that the most efficient PDS/Fe(II) initial molar ratios in terms of CVOCs removal efficiency and reaction stoichiometry efficiency (RSE) were 10/1 for PCE, and 15/1 for both TCE and combined PCE/TCE. Surprisingly, the efficiency of unactivated PDS systems was comparable to Fe(II)-CA activated PDS systems. The results therefore suggest that unactivated PDS could be an appropriate option for field applications and should not be disregarded within feasibility studies at given thermal and hydro-chemical conditions. Overall, conclusions made in this study deepened the knowledge of ISCO using PDS at lower than laboratory temperatures and in the presence of typical groundwater constituents, therefore, could be exploited by remediation practitioners as well as in future research work.