Carbonate and Chloride Scavenging in S₂O₈^2-/Fe²⁺/Citric Acid Oxidation of PCE

Contamination of the subsurface by persistent organic contaminants remains a significant problem, even after decades of research on remediation technologies (Watts et al. 1999; Watts, Teel 2005). First, approaches focused on excavation, pump and treat via activated carbon, bioremediation, and natural attenuation. In the 1990s the first report on in situ chemical oxidation were published. Modified Fenton's reagent was the first technique investigated and used in full scale. Shortly after ozone and permanganate came into use. In the past few years persulfate has provided yet another option. Besides using the oxidation techniques in situ, they can be used on site for treating contaminated groundwater. Whether the oxidation techniques are applied in situ or on site, they face some general problems, for example scavenger ions.

Radicals produced in oxidations systems react rapidly with other chemical species in solution at near diffusion controlled rates (Neta et al. 1977, Haag, Yao 1992, Buxton et al. 1988). Besides contaminant degradation these reactions include non-productive reactions with other compounds and radicals resulting in reduced reaction efficiency and effectiveness, often referred to as scavenging reactions. Scavenger ion is one of the most common factors limiting the oxidation efficiency. Especially chloride and (bi)carbonate have the potential to impact pathway, kinetics, and efficiency of oxidation reactions both as radical scavengers and as metal complexing agents (Valentine, Wang 1998; Lipczynskakochany, Sprah & Harms 1995; Beltran et al. 1998; De Laat, Le & Legube 2004). Furthermore, chloride and (bi)carbonate can also form radicals on their own (Buxton et al. 1988; Liang, Wang & Mohanty 2006; Yu, Barker 2003). However, the reactivity of these radicals and organic contaminants are not widely investigated or well understood, but it is assumed that they have an overall negative impact on performance (Liang, Wang & Mohanty 2006; Huang, Couttenye & Hoag 2002). High concentrations of chloride have also been reported to result in formation of halogenated by-products (Aiken 1992).

The present work is based on issues from the largest contaminated site in Denmark, Kærgård Plantation. The contamination is located in 6 different hotspots and the plume covers approximately 700,000 m². Two hotspot areas containing a total of 62,000 kg PCE and 26,000 kg total hydrocarbons are to be treated within the next couple of years, most likely with chemical oxidation. Large amounts of sulfonamides, barbiturates, phenols, mercury, and lithium are also present. However, very high concentrations of chloride (up to 4,600 mg/l) and bicarbonate (up to 6,900 mg/l) have been observed in the groundwater. pH is about 6.

In order to evaluate the impact of the high concentration of scavengers, different batch experiments have been performed in the laboratory. Since scavenging in hydrogen peroxide systems is well documented, focus has been on persulfate oxidation of PCE with different activations methods (mainly peroxide and iron). Series of batches were prepared with different types and concentrations of scavengers to determine the impact. Concentrations of PCE were followed in time and degradation kinetics were identified and compared with other studies of scavengers in hydrogen peroxide and persulfate oxidation systems.

This poster will present the results from the laboratory work and discuss the influence of high concentrations of chloride and (bi)carbonate in oxidations systems with focus on persulfate activated by peroxide and iron.