PNNL

Abstract

In addressing groundwater contamination, several remediation technologies are widely employed, including ion exchange (IX) resins and other adsorbents in pump-and-treat (P&T) facilities. However, many sites do not have the complex nature of the Hanford Site – a legacy production site that is part of the U.S. Department of Energy nuclear weapons complex. The Hanford Site covers an area of nearly 1,500 km2 and has various inorganic and organic contaminants. One contaminant of concern in the Hanford Site’s Central Plateau is cyanide, particularly in the recalcitrant form of ferrocyanide. While ferrocyanide is often considered a stable complex, it can release free cyanide under certain conditions and presents challenges when testing for cyanide as a total cyanide analysis cannot differentiate between complexed and free cyanide. The Hanford Site already uses P&T to mitigate groundwater contamination across the site, notably removing radionuclides technetium-99 and uranium from groundwater at the 200 West Area (200W) P&T facility and chromium at the five 100 Area P&T facilities. The 200W P&T facility specifically uses two IX resins, Dowex 21K to remove uranium and Purolite A532E to remove technetium-99. To evaluate the potential of IX resin and other adsorbents for ferrocyanide treatment at the Hanford Site, this study aimed to determine which commercially available IX resins/sorbents are likely to take up the most ferrocyanide under batch conditions (high liquid to solid ratio) using synthetic groundwater (SGW) of a similar composition to that at the 200W P&T facility. This work did not include evaluation involving Tc, U, or Cr, it did include common groundwater anions and nitrate, which is also a contaminant of concern. In addition to the targeted treatment of ferrocyanide Dowex 21K and A532E were investigated as they may incidentally uptake ferrocyanide during P&T activities. Both resins were able to uptake ferrocyanide at a concentration of 1.5 g/L, with Dowex 21K taking up more (nearly 100%) than Purolite A532E (approximately 50%). Both resins also took up nitrate under testing conditions. In addition to the competitive nature of anion uptake by the resins already in use at the 200W P&T facility, various other commercially available resins were also evaluated and were shown to uptake ferrocyanide along with nitrate. At a concentration of 1.5 g/L ferrocyanide, almost all tested commercially available resins removed all ferrocyanide from the system within 72 hours. However, when exposed to a concentration of excessively high ferrocyanide, one resin – Purolite A500 – outperformed the others. At the higher concentration (10.5 g/L ferrocyanide), only one of the resins (A532E) tested took up any nitrate, suggesting that A532E has a high affinity for nitrate despite the high concentration gradient for ferrocyanide uptake. These findings emphasize the need for site-specific testing to optimize treatment technologies for ferrocyanide removal at the 200W P&T facility. Purolite A500 emerged as the best performing IX resin for ferrocyanide removal, with SBG1 and Dowex 21K also demonstrating significant uptake. While lower concentrations of ferrocyanide were effectively removed by most IX resins, this process simultaneously interacts with anions of interest (e.g., TcO4-) and groundwater constituents. Dowex 21K showed high uptake of ferrocyanide and nitrate, while A532E exhibited moderate performance, indicating that increased concentrations of influent cyanide could disrupt treatment efficiency due to competition. To address these knowledge gaps, future testing with one-dimensional flow columns of Dowex 21K, A532E, and Purolite A500 would further constrain cyanide uptake and its influence on contaminant removal efficiency, along with the addition of other contaminants of concern (e.g., TcO4-).