Field Application of a Rapid Spectrophotometric Method for Determination of Persulfate in Soil Colin J. Cunningham 1 *, Vanessa Pitschi 2 , Peter Anderson 1 , D. A. Barry 2 , Colin Patterson 1 , Tanya A. Peshkur 1 1 Scottish Environmental Technology Network (SETN), Faculty of Engineering, University of Strathclyde, Glasgow, United Kingdom, 2 Laboratoire de technologie e ´ cologique, Institut des sciences et technologies de l’environnement, Faculte ´ Environnement Naturel, Architectural et Construit (ENAC), E ´ cole Polytechnique Fe ´de ´ral de Lausanne (EPFL), Lausanne, Switzerland Abstract Remediation of hydrocarbon contaminated soils can be performed both in situ and ex situ using chemical oxidants such as sodium persulfate. Standard methods for quantifying persulfate require either centrifugation or prolonged settling times. An optimized soil extraction procedure was developed for persulfate involving simple water extraction using a modified disposable syringe. This allows considerable saving of time and removes the need for centrifugation. The extraction time was reduced to only 5 min compared to 15 min for the standard approach. A comparison of the two approaches demonstrated that each provides comparable results. Comparisons were made using high (93 g kg 21 soil) and low (9.3 g kg 21 soil) additions of sodium persulfate to a petroleum hydrocarbon-contaminated soil, as well as sand spiked with diesel. Recoveries of 9561% and 96610% were observed with the higher application rate in the contaminated soil and spiked sand, respectively. Corresponding recoveries of 8665% and 117619% were measured for the lower application rate. Results were obtained in only 25 min and the method is well suited to batch analyses. In addition, it is suitable for application in a small field laboratory or even a mobile, vehicle-based system, as it requires minimal equipment and reagents. Citation: Cunningham CJ, Pitschi V, Anderson P, Barry DA, Patterson C, et al. (2013) Field Application of a Rapid Spectrophotometric Method for Determination of Persulfate in Soil. PLoS ONE 8(6): e65106. doi:10.1371/journal.pone.0065106 Editor: Vishal Shah, Dowling College, United States of America Received September 6, 2012; Accepted April 24, 2013; Published June 11, 2013 Copyright: ß 2013 Cunningham et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The work was funded by Contaminated Land Assessment and Remediation Research Centre (CLARRC). The funders undertook the study design, data collection and analysis, decision to publish, and preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: colin.cunningham@strath.ac.uk Introduction Chemical oxidation is most commonly applied as an in situ remediation technology but may also be applied ex situ to contaminated soils [1–3]. Common oxidants include persulfates, permanganates and peroxides [1]. Persulfate salts such as sodium persulfate (Na 2 S 2 O 8 ) dissociate to the persulfate anion S 2 O 8 22 , which has a standard oxidation potential (Eu) of 2.01 V. Activation by heat or ferrous iron generates a stronger oxidant in the form of sulfate radicals (SO 4 2 N) with Eu = 2.6 V. It is advantageous to monitor the oxidant concentration at the time of application and during remediation to optimize the treatment process. Liang et al. [4] reviewed available methods for determination of persulfate in aqueous samples and presented a rapid spectrophotometric method that produced results after approximately 20 min. This represented a considerable time saving over previous techniques described by Shuiundu et al. [5] and Huang et al. [6]. The sodium persulfate CHEMets H field test for aqueous samples (CHEMetrics Inc., Virginia, USA) uses the reaction between persulfate and ferrous thiocyanate to produce a color change that can be read using a comparator. The manufacturers highlight the possibility of interferences from hydrogen peroxide or ferrous ions, which are commonly used as activators of persulfate, as well as from cupric ions. Although the method is very rapid, the range is limited to a maximum concentration of 70 mg l 21 , and like all comparator methods is subjective as it depends on the judgment of the operator. The addition of an efficient extraction step to the rapid spectropho- tometric method of Liang et al. [4] would provide a valuable tool for assessing persulfate concentrations in soils. Extraction of water-soluble ions such as persulfate from soils can be achieved using a variety of methods including ultrasound or mechanical shaking [4]. Ultrasound is unsuitable for this application as it enhances degradation of persulfate [7]. Mechan- ical shaking is a simple robust method of extraction that is easily deployed in a field laboratory. However, centrifugation of samples following extraction to separate the soil from the aqueous persulfate-containing solution adds considerably to the overall time and cost of determination. Ex situ chemical oxidation treatments are often performed in slurry-based systems. Reducing the excess moisture content in the system to the point where chemical oxidation is still viable yields a soil that is more handleable. The authors are unaware of any previous work using sodium persulfate in non-slurry-based systems. In this study, we describe the development of an optimized soil extraction procedure for persulfate that makes use of a modified disposable syringe for both extraction and separation of soil solids, thus removing the need for centrifugation. The use of syringes in soil testing has been described for soil sorption tests [8,9] but have not been used for a combined field extraction and separation step as part of a spectrophotometric assay. The extraction time was PLOS ONE | www.plosone.org 1 June 2013 | Volume 8 | Issue 6 | e65106