Adapting Near-Ultraviolet Colorimetry for Long-Term In Situ Monitoring of Hexavalent Chromium in Groundwater Aquifers
Year of Award
Master of Science (MS)
Department or School/College
Environmental Studies Program
Len Broberg, Stephen Hall
Ultraviolet, Hexavalent Chromium, pH, Sensor, Groundwater, Hanford
University of Montana
This paper briefly describes sources of hexavalent chromium [Cr(VI)] and the risk it poses to human health and the environment; current methods used to regulate, monitor, and measure Cr(VI); the basic design of a submersible, direct-reading sensor in development for long-term monitoring of Cr(VI) concentration in natural waters; and the means developed to correct sensor readings for two common analytical interferences, turbidity and pH, that could lead to an incorrect measurement of Cr(VI). The principal purposes of this study are to analyze the current methods used to compensate for sample turbidity, to develop methods to compensate for sample pH, and to investigate a method for estimation of pH by measuring sample absorbance at two wavelengths.
Cr(VI) in humans and some animals is carcinogenic, and in acute exposures, can result in respiratory strain, gastrointestinal, and neurological effects. Chronic exposure can increase the risk of respiratory cancer when inhaled and can cause liver, kidney, and blood cell damage when ingested.
Current methods for monitoring and measuring Cr(VI) in groundwater are labor intensive, therefore costly and infrequent. The sensor under development at Freestone Environmental Services, Inc. produces a near-continuous data flow and is fully submersible for long-term deployment in groundwater monitoring wells. The sensor is based on the principle of absorption spectrophotometry where an ultraviolet (UV) light beam, at a specific wavelength (371 nm), is passed through a water sample toward a photoelectric sensor and is partially absorbed by Cr(VI).
Turbidity interferes with the UV light beam by scattering the light in random directions, thus mimicking absorption. To compensate for turbidity, a second photoelectric sensor measures the intensity of the scattered light, and the result is used to calculate the corrected absorption.
The effect of pH on the Cr(VI) measurement is more complex than the turbidity effect because there are two different species, chromate and hydrogen chromate, that can exist in aqueous solutions above pH 3.5 and at lower concentrations than 1,000 µg/L. Both the sensor’s UV light source and chromate’s peak absorbance is at the UV wavelength of 371 nm. Absorption by hydrogen chromate is relatively weak at that wavelength. Chromate is dominant relative to hydrogen chromate in solutions with a pH equal to or higher than pH 7.8. If the aqueous environment has a pH between pH 3.5 and pH 7.8, the ratio of chromate to hydrogen chromate can be calculated from pH. The calculated ratio is used to temporarily correct the sensor calibration by adjusting net molar absorptivity.
Hydrogen chromate’s peak absorbance is at 349 nm. Theoretically, if chromate and hydrogen chromate are independently measured, the results could be interpreted as pH. Solutions containing different concentrations of Cr(VI) at pH values ranging from pH 3 to pH 9.2 were measured in a spectrophotometer at both the 349 nm and 371 nm wavelength. Results show that if the relative strengths of both the 349 nm and the 371 nm peaks are measured, it is possible to calculate pH. The practicality of this method for estimating sample pH has yet to be established.
Carter, Janine, "Adapting Near-Ultraviolet Colorimetry for Long-Term In Situ Monitoring of Hexavalent Chromium in Groundwater Aquifers" (2016). Graduate Student Theses, Dissertations, & Professional Papers. 10681.
© Copyright 2016 Janine Carter