A Proposed Approach to Measure Oil Removal Efficiency by Surface Washing Agents in the Laboratory

Over 50 surface washing agents (SWAs) are listed on the U.S. National Oil and Hazardous Substances Pollution Contingency Plan Product Schedule (NCPPS). Currently, listing does not require testing to measure SWA product effectiveness in removing weathered oil from substrates. The U.S. Environmental Protection Agency (USEPA) Office of Research and Development is conducting research to develop a testing protocol to determine the influence of different parameters on SWA performance, which involves artificially weathering Alaska North Slope oil on stainless-steel discs and adding SWA products to lift the oil from the substrate. Gravimetric analysis is used to determine the amount of oil removed by the SWA and remaining on the substrate.

In this study, five SWAs listed on the NCPPS were evaluated and showed widely varying effectiveness. The repeatability (S^r) and reproducibility of the protocol was evaluated with the five SWA products and a control (no-product-added) on three separate days. Efficient products exhibited higher protocol variability (S_R of 9.2-15.8%), while inefficient products and controls showed lower variability (S_R of 22-4.4%) across the three test days. The h-statistic, which describes the deviation of each trial from the overall mean for any given product, was used to confirm statistical similarity of the three trials. Results suggest that the protocol was repeatable and reproducible for the products investigated and conditions used. Further investigations of protocol parameters, such as mix time, turbulence, contact time, salinity, SWA to oil ratio (SOR), and temperature, will be conducted to further differentiate SWA product performance. Such investigation is necessary to develop a robust efficacy screening test that is appropriate for the wide range of SWA products on the NCPPS.