Email Us       |      Client Login

Hexavalent Chromium Extraction Method Criticized – An Interesting Double Standard

For many environmental investigations, heavy metals are frequently determined to be constituents of concern that require remedial action. For solid samples, the US EPA has published metals digestion methods, which act to transfer metals from the solid phase to a liquid acid-digest phase for instrumental analysis. The acid digestion processes also bring metals (or other trace elements) of interest to an ionic form, which is required for the dissolution during introduction to the analytical instrument. Historically, questions have arisen with regard to the efficiency of the metals digestion process and how that process realistically simulates environmental exposure. While many environmental professionals believe that US EPA Method 3050 is the “gold standard’ for digesting solid samples and represents a total metals analysis, Method 3050 is operationally defined as a “total leachable” digestion method. In fact, Section 1.2 of Method 3050 states:

“This method is not a total digestion technique for most samples. It is a very strong acid digestion that will dissolve almost all elements that could become “environmentally available.” By design, elements bound in silicate structures are not normally dissolved by this procedure as they are not usually mobile in the environment.”

Effectively, this citation states that this digestion method yields metals results that are ‘environmentally available’ and does not include metals that are bound in silicate structures, as those forms of metals are not usually mobile in the environment.

Several authors from the United States Geological Survey (USGS) recently published a paper entitled, “Modifications to EPA Method 3060A to Improve Extraction of Cr(VI) from Chromium Ore Processing Residue-(COPR) Contaminated Soils”. The authors state that US EPA Method 3060A, as written, does not adequately extract Cr(VI) from solid COPR samples. Substantial method modifications including the use of polytetrafluoroethylene vessels, intensive sample drying and grinding, an increased extraction fluid to sample ratio, and a 48-hour extraction time resulted in a maximum release of
1274 mg kg–1 Cr(VI) in a National Institute of Standards and Technology- (NIST-)certified Cr(VI) reference sample, which is more than double the certified Cr(VI) value of 551 mg kg–1 Cr(VI).

These USGS modifications were clearly designed to maximize the concentrations of extracted Cr(VI), to the point where they yielded more than double the NIST-certified Cr(VI) value compared to the method as written. These modifications would appear to include concentrations that are bound in silicate structures, and would not be expected to become mobilized in the environment except in geologic time frames. From a geologic perspective the absolute metals concentration in soils and sediments may be of interest, but from an environmental perspective, longstanding policy has held that metals which are tightly bound in minerals are not relevant. It is scientifically reasonable
to presume that if Method 3050 were subjected to the same extreme modifications listed above, substantially higher concentrations of many other metals would also be observed. Such modifications alter the operational definition of these digestion methods, and a question arises as to the prospect of substantially higher concentrations that do not bear relevance to the US EPA’s intent of determining “environmentally available” concentrations. So why would there be a double standard with respect to this one particular metal? Given the limited resources available for remediation, it would seem more protective to focus those resources on environmentally relevant components.


The Standard – December 2017