One of the services that FMC’s Environmental Solutions Team offers to its customers is an estimation of the oxidant demand for the amount of Klozur^® Persulfate required to achieve the clean-up targets for a given contaminated site. In this month’s Peroxygen Talk, an explanation of the process and assumptions that FMC makes in determining the oxidant demand is given.

Estimating the Amount of Klozur Persulfate Needed for a Site

    Stoichiometric calculations can be used as a non-empirical method for determining the amount of Klozur persulfate needed to effectively treat a site and are utilized in FMC’s proprietary Klozur Calculator. The oxidant demand estimate generated from the calculator contains a number of assumptions, which are described below, and FMC strongly recommends using such an estimate as a first pass estimation tool only. 
    The next level of refining the oxidant demand is the determination of the persulfate soil oxidant demand (SOD) for the site. This laboratory test measures the amount of persulfate lost through oxidation on non-target soil organic species and naturally occurring reduced metals. This test is critical in assuring that the oxidant is not under-dosed for the specific site conditions. Further discussion on SOD will follow below. 
    Subsequent to the SOD test, a treatability study is recommended in which the actual decomposition of the contaminants of concern is monitored as a function of time as well as the persulfate dosage within the actual site soils and groundwater. Treatability studies are extremely useful in verifying proper dosing and efficacy of the persulfate for a given set of contaminants, especially for contaminants for which there is not a historic oxidation efficacy data set. Most treatability studies are run from one to three weeks. However, persulfate under many activation systems will remain present in the subsurface for a period of one to three months. As a result, a treatability study run for just a couple of weeks may not be fully indicative of the persulfate efficiency, as a significant amount of persulfate may still remain at the end of the test period. As an example, if at the end of a two week study only 60% of the contaminant may have been destroyed, this may at first look like poor persulfate performance if the clean-up goal was 90%. However, if only 30% of the persulfate was utilized in these two weeks, then the persulfate efficacy is actually sufficient, as there is still a significant amount of oxidant still available for oxidative destruction of the contaminant. It is thus recommend that the Persulfate Efficiency Number (PEN) be determined, as the PEN will provide dosing and efficacy estimation regardless of the length of the study. The PEN is defined as:

PEN = % contaminant destroyed / % persulfate used

% contaminant destroyed = ([contaminant] _initial – [contaminant] _final) / [contaminant] _initial

% persulfate used = ([persulfate] _initial – [persulfate] _final) / [persulfate] _initial

A PEN > 1 indicates that the persulfate is dosed properly and the contaminant is being destroyed at a sufficient rate. A PEN in great excess of 1 may indicate an over-dosing of the persulfate, and a PEN less than 1 indicates an under-dosing of the persulfate.
    Because treatability studies are often performed in glass jars with water to soil ratios in excess of 2 to 1, with some degree of repetitive shaking, these studies do not tend to mimic actual site conditions very well. As a result, enhanced testing prior to full scale field application may be required. Such testing may include column testing and actual field pilot scale testing. Field pilot scale testing can be very critical in determining the persulfate radius of influence and anisotropy in the oxidant distribution that may occur due to site heterogeneity and ground water flow, especially in “difficult” lithological and hydrogeological regimes. Such analyses are important for proper well spacing design and injection parameters, all necessary to insure adequate contact time between the oxidant and the contaminants.
    While testing performed prior to full scale field application will cost money, it can also increase the probability of a successful site remediation with activated persulfate. As an example:

Test Step	General Cost*	Probability of Successful Site Remediation
Klozur Demand Calculation	Free from FMC	Lowest level of probability of success
SOD testing	$600 / soil sample – FMC labs	Increased level of success probability
Treatability testing	$6000 - $25,000 or more, depending on complexity	Higher level of success probability
Field pilot testing	$25,000 + , depending on complexity	Highest level of success probability

*typical values, not meant to be guarantees of pricing

Elimination of one or more of these steps may decrease the probably of reaching the targeted clean-up goals for the site.

Persulfate Stoichiometric Demand (Klozur Calculator)

    Several pieces of information are needed for FMC to estimate the oxidant demand for a site. These include: contaminant soil and groundwater concentrations, treatment area, vertical treatment interval, and porosity. Often, contaminant soil concentrations are not known, especially if a soil core has not been obtained and assayed. As a result, a significant amount of contaminant mass may be unaccounted for if this is not measured and reported. One can take various partitioning coefficients, such as oil / water partitioning values, Freundlich isotherm data and others to estimate the amount of sorbed or soil-bound contaminant based on the groundwater concentrations. However, FMC’s calculations do not specifically take into account these partitioning coefficients, but can be incorporated at the customer’s request. In general we recommend directly measuring the soil contaminant concentrations if it is believed these are significant. In addition, if non-aqueous phase liquid (NAPL) is present, a good quantification of the NAPL volume is critical. 
    The amount of contaminant present then can be calculated from the treatment volume (treatment area x vertical treatment interval) and the contaminant concentrations. Groundwater volume can be ascertained from the treatment area and the porosity. If porosity of the soil is not known, FMC typically will assume 30%, except for fractured bedrock, where we will use a porosity of 5%. Total soil weight is determined from the treatment volume and the density of the soil. FMC assumes a soil density of 3000 lb / cy (1780 kg / m³), unless otherwise provided.
    Once the amount of contaminant is determined, the corresponding amount of Klozur persulfate can be calculated using stoichiometric ratios from the relevant chemical equations. For this purpose, it is assumed that the oxidation event occurs through a “two-electron direct oxidation” process:

S₂O₈^-2 + 2H⁺ + 2e^- --> 2HSO₄^-

This is a generalized assumption, as activated persulfate oxidations propagate through a variety of radical steps, including sulfate radicals, hydroxyl radicals, organic radicals, and potentially additional oxidative species, such as hydroperoxide and superoxide. As an example:

SO₄·^- + e^- --> SO₄^-2

However, the propagation of these reactions is influenced by a variety of factors, including temperature, concentration, activation chemistry, ionic strength, contaminant species, soil mineralogy, etc. As a result, determining the “stoichiometry” of the radical reactions is extremely complex, and thus the simplification of the two-electron direct transfer stoichiometry is used. It must be pointed out that such a simplification may lead to an over or under estimation of the true oxidant demand, but empirical data over the years suggest that this approach is adequate.
    As an example, the stoichiometry for the reaction of persulfate with benzene would be:

15 S₂O₈^-2 + C₆H₆ + 12 H₂O --> 6 CO₂ + 30 HSO₄^-

Thus, to completely mineralize one mole of benzene, fifteen moles of persulfate are needed. In more practical terms, 45.8 lbs of persulfate per lb of benzene is required for full mineralization. In comparison, the stoichiometry for the reaction of persulfate with PCE is:

2 S₂O₈^-2 + C₂Cl₄ + 4 H₂O --> 2 CO₂ + 4 HSO₄^- + 4 HCl

requiring 2 moles of persulfate to fully mineralize one mole of PCE, or 2.9 lbs of persulfate per lb of PCE. Chlorinated compounds require less oxidant to mineralize than petroleum compounds due to the presence of chlorine, which cause the adjacent carbons to be in a higher oxidation state than a typical carbon in a petroleum compound which is often bound to hydrogen.
    FMC provides the amount of Klozur persulfate needed to fully mineralize the contaminants of concern, which might be in excess of the amount needed to reach the site treatment goals. For sites with very heavy contamination, 100 + mg / kg as an example, the amount of oxidant needed for full mineralization may appear to be very high, especially for petroleum hydrocarbons. Oxidant demand can be determined for less stringent clean-up targets per customer request. However, if full mineralization is not assumed, there is potential to have long term daughter product formation. As an example, in the oxidation pathway to full mineralization, acetone is often a daughter product formed by the oxidation of aromatic hydrocarbons, and being in a highly oxidized state, often takes longer to covert to carbon dioxide. If the persulfate is dosed below the full mineralization value, acetone production may occur. FMC recommends a treatability study at this point to confirm the dosing, as in these cases the amount of oxidant may be overestimated, and persulfate may be prematurely ruled out as a viable treatment option. Alternatively, persulfate chemical oxidation can be combined with other remedial approaches, such as air sparging, as an economical approach to heavily contaminated sites.

Incorporation of SOD

    Unless the persulfate soil oxidant demand is measured for the specific site soils, the FMC calculations assume a persulfate soil oxidant demand of 1 g persulfate per kg of soil. This is the average SOD for most soils. This is a significant assumption, as in many cases the SOD may be the over-riding oxidant demand for a given site, and can thus be the major cost factor in the use of activated persulfate. The April 2007 Peroxygen Talk discusses in detail the impact of SOD on the total persulfate oxidant demand, and it is highly recommended that this be measured prior to application in the field. (Please contact FMC for back issues of Peroxygen Talk.)
    Activated persulfate has the lowest soil oxidant demand of all the oxidants currently utilized for chemical oxidation remediation. Recently, other vendors have suggested “effective” or “estimated” soil oxidant demand to reduce the overall apparent SOD impact from their oxidants. To a large degree, the current SOD test methods, which rely on “beaker” tests, are not truly representative of actual field conditions. However, FMC does recommend that the SOD be measured rather than estimated, because significant oxidant under-dosing may occur if the SOD is not treated conservatively. Under-dosing of the oxidant due to lack of SOD measurement or use of estimates may lead to a lack of success in meeting clean-up criteria or potential re-bounding of the contaminant as a result of incomplete oxidative destruction. As a result, FMC strongly recommends and incorporating a measured SOD into any ISCO estimations. One area for potential research is the development of a low cost, column-based SOD methodology. By employing a column versus a beaker, groundwater to soil ratios can be utilized that are more reflective of actual site conditions. In addition the flow through a soil-packed column, while not ideal, will have better correlation to field application than beaker SOD tests.