Pilot‐Scale Removal of Total and Hexavalent Chromium From Groundwater Using Stannous Chloride

Abstract

Alternative processes for hexavalent chromium (Cr(VI)) removal from drinking water continue to be of interest for utilities despite the existence of several established technologies. Stannous chloride (SnCl 2 ) can reduce Cr(VI) to trivalent chromium, but research has been limited, especially related to the filterability of total chromium (Cr(T)) following reduction. At the pilot scale, SnCl 2 was tested over a range of doses in three ground-waters with naturally occurring Cr(VI) concentrations ranging from 0.020 to 0.090 mg/L. Stannous chloride was found to be effective as a reductant at doses <2 mg/L and contact times <5 min. A tin-to-chromium molar dose ratio of 4 was sufficient for reducing Cr(VI) to below 0.010 mg/L. Cartridge filters were unable to practically remove Cr(T) following reduction, but a standard-design sand filter was able to remove Cr(T) to <0.010 mg/L.

Stannous chloride reduction–filtration for hexavalent and total chromium removal from groundwater

“…A groundwater well in central Oklahoma with a naturally occurring and relatively high Cr(VI) concentration was chosen for this study, the same water designated as OK in Kennedy et al (). As shown in Table , the Cr(T) concentration, predominately present as Cr(VI), averaged 0.090 ± 0.005 mg/L (average ± standard deviation) over the course of the study, nearly nine times the previously rescinded California Cr(VI) MCL (State Water Resources Control Board, ) and close to exceeding the USEPA Cr(T) MCL.…”

Full‐scale demonstration testing of hexavalent chromium reduction via stannous chloride application

“…Pilot testing of RCF with stannous as an alternative reductant, in lieu of ferrous, resulted in complete Cr removal and extended filter run time, with stannous resulting in more than double the unit filter run volume than that of ferrous (Gorman, Plummer, Samson, Townsend, & Seidel, 2019). Stannous pilot testing conducted by Kennedy et al (2018) on Cr(VI)-reduced waters also demonstrated Cr removal with sand filtration, although little or no Cr removal occurred with depth or pleated cartridge filtration. Kennedy et al (2018) used a stannous stock solution made from reagent-grade SnCl 2 with diluted deionized water that resulted in a pH of~2, which is in the pH range where stannic hydroxide can form and precipitate out of solution (Al-Hinai, Al-Hinai, & Dutta, 2014).…”

“…Stannous pilot testing conducted by Kennedy et al (2018) on Cr(VI)-reduced waters also demonstrated Cr removal with sand filtration, although little or no Cr removal occurred with depth or pleated cartridge filtration. Kennedy et al (2018) used a stannous stock solution made from reagent-grade SnCl 2 with diluted deionized water that resulted in a pH of~2, which is in the pH range where stannic hydroxide can form and precipitate out of solution (Al-Hinai, Al-Hinai, & Dutta, 2014). Throughout their testing, stability issues were observed, and the diluted stock was regularly remade because, as the stock age increased, its Cr(VI) reduction potential decreased, it gained a yellow color, and turbidity increased, indicating that Sn(II) was oxidized in air-saturated water and precipitated out of solution.…”

“…Stannous stock solution testing conducted at the end of the full-scale demonstration by spiking Sn from the 50% stock solution into base-preserved deionized water ( Figure 5) demonstrated no dissolution or stability issues after the 10-week testing period. The application of the undiluted stannous stock solution and use of the base preservation method throughout the demonstration resulted in no stannous stability challenges, such as those observed in the Kennedy et al (2018) pilot testing.…”

“…In the presence of 1 mg/L chlorine residual, only 1 μg/L (6%) Cr reoxidation occurred over the 5‐day sampling period (Gorman et al, ). Additional bench testing of Cr(VI) reduction via stannous application for three waters also demonstrated complete reduction of Cr(VI) by the first measurement at 6 min and partial reoxidation to Cr(VI) in the presence of chlorine (Kennedy, Korak, Flint, Hoffman, & Arias‐Paic, ).…”