Frequent Questions

Is SW-846 Method 3050B appropriate for the digestion of coal ash?

We are trying to determine whether EPA 3050B is appropriate for the digestion of coal ash. Section 1.2 of the method 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.  If absolute total digestion is required use Method 3052."

Since coal ash could contain silicas, would EPA 3050B be an appropriate digestion method for all of the metals listed in section 1.1 of EPA 6010C?  Could you also provide a more detailed explanation of Section 1.2 and the statement above?


Method 3050B is based on a very strong acid digestion that leaches elements from the sample that could become environmentally available.  The premise is if this strong acid leach cannot remove the element from the sample and bring it into solution, then there is no further worry about the sample from an environmental standpoint.  If the element is associated with the silicate matrix, which is not broken down in the environment, then the element cannot be mobilized and it is not of environmental concern.

In order to release elements contained in the silicate matrix of the soil or coal ash, the silicate matrix must be destroyed.  This is done with hydrofluoric acid (HF).  Method 3052 employs HF to destroy the silicate matrix, along with the strong mineral acids in a high pressure microwave vessel.  As such, it completely destroys the sample and releases all elements.  It is considered an absolute total digestion.  Such numbers might be needed in case of material balance studies, or comparison to other types of true total analyses, but they are not generally necessary from an environmental release standpoint.

If the study goal is to evaluate the coal-ash contaminated soil or remediated soil to see if it releases elements of environmental concern, then Method 3050B would be appropriate.  If the study goal is to determine whether all the ash has been removed, an element or multiple elements that are unique to the ash and not the soil must be identified.  The remediated soil should then be digested by Method 3050B to see if the ash marker elements are still being released.  If they are under a particular threshold, the soil could be considered remediated.

Other Category: 3000 Series



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