What’s the difference and does it matter?
Soil oxidant demand (SOD) measures how much oxidant is consumed by soil, while oxidant persistence describes how long the oxidant will last. The distinction is important because it may determine the approach one takes to estimate the amount of oxidant required for in situ chemical oxidation (ISCO).
Oxidants used in ISCO react with soil components such as reduced minerals or natural organic matter, as well as with chemicals of concern (COCs). This amount of oxidant is finite and is termed the soil oxidant demand or SOD. (Note: If SOD is measured on impacted soil, then the SOD value may include oxidant consumed due to COCs. If SOD is measured on un-impacted soil it is often referred to as ‘natural oxidant demand’ or ‘NOD’.)
Most oxidants naturally decompose, though the rate of decomposition depends upon the oxidant (Table 1) and properties of the soil. For example, permanganate decomposes very slowly once SOD is met (Figure 1), while iron-activated persulfate decomposes slowly if the soil buffers well, but rapidly if it does not.
If an oxidant decomposes very slowly, the oxidant requirement for ISCO can be calculated from the measured SOD and the mass of COCs that must be treated. This approach is described for alkaline activated persulfate in “How to Use Bench Test Data” , but also applies to permanganate, un-activated persulfate. The approach can also be used to estimate the minimum requirement for ozone, which has a distinct SOD, but which decomposes quickly.
For oxidants such as activated persulfate, whose rate of decomposition may be site-specific, measurement of the oxidant’s persistence is recommended. Figure 2 shows the persistence of persulfate when activated by high pH for two sites. For Site C, persulfate decomposition was initially fast, then slowed. The initial loss is considered the SOD and the persulfate requirement can be calculated as described above. In contrast, persulfate decomposed steadily for Site D, making identification of an appropriate “SOD” difficult. Knowledge of the approximate rate of COC destruction relative to the rate of oxidant decomposition is therefore necessary to determine the persulfate requirement.
Table 1. Relative Rates of Oxidant Decomposition
Figure 1. Potassium permanganate persistence in the presence of site soil. Most permanganate is consumed within the first day.
Figure 2. Persulfate persistence in the presence of site soil (high pH activation). For Site C, persulfate is initially consumed quickly, then more slowly once the SOD is met. For Site D, the rate of persulfate decomposition is steady throughout the test.