A five-year performance review of field-scale, slow-release permanganate candles with recommendations for second-generation improvements

Abstract

In 2009, we identified a TCE plume at an abandoned landfill that was located in a low permeable silty-clay aquifer. To treat the TCE, we manufactured slow-release potassium permanganate cylinders (oxidant candles) that had diameters of either 5.1 or 7.6 cm and were 91.4 cm long. In 2010, we compared two methods of candle installation by inserting equal masses of the oxidant candles (7.6-cm vs 5.1-cm dia). The 5.1-cm dia candles were inserted with direct-push rods while the 7.6-cm candles were housed in screens and lowered into 10 permanent wells. Since installation, the 7.6-cm oxidant candles have been refurbished approximately once per year by gently scraping off surface oxides. In 2012, we reported initial results; in this paper, we provide a 5-yr performance review since installation. Temporal sampling shows oxidant candles placed in wells have steadily reduced migrating TCE concentrations. Moreover, these candles still maintain an inner core of oxidant that has yet to contribute to the dissolution front and should provide several more years of service. Oxidant candles inserted by direct-push have stopped reducing TCE concentrations because a MnO2 scale developed on the outside of the candles. To counteract oxide scaling, we fabricated a second generation of oxidant candles that contain sodium hexametaphosphate. Laboratory experiments (batch and flow-through) show that these second-generation permanganate candles have better release characteristics and are less prone to oxide scaling. This improvement should reduce the need to perform maintenance on candles placed in wells and provide greater longevity for candles inserted by direct-push.

Keywords: Chlorinated solvents; Permeable reactive barrier; Second-generation permanganate candles; Slow-release oxidants; TCE.

Figure 1

Field plot of the permeable reactive barrier (PRB) of oxidant candles and monitoring wells.

Figure 2

A. Photograph of 7.6-cm. dia. permanganate candles in screen; B. Candle broken in half to show dissolution layer of oxidant candle; C. Removing outside layer with wood planer; D. Refurbished candle.

Figure 3

A. Top view photograph of 5.1-cm diam. permanganate candle taken right after recovery in field; B. Photograph of candle with water droplet placed on oxide layer; C. Side view of candle sliced open showing thickness of oxide layer (shown in A and B) over undissolved permanganate candle.

Figure 4

Effect of SHMP on cumulative permanganate released in flow-through experiments. A. H₂O influent; B. TCE (1 mg L⁻¹) influent

Figure 5

Effect of SHMP on permanganate released into H₂O from oxidant candles when stored in A. H₂O or B. TCE (>100 mg L⁻¹) for 7-d intervals.