From Tech Trends November 1995

"Total Hydrocarbon" Measurements Cause Confusion

Canada's Bio-Reactor Project turned up an unexpected "by-product" when it set out to assess the rate of remediation of flare pitwastes. The finding should provide some sound guidance for methodology to quantify bioremediation success.

During bioremediation of the flare pit waste, a serious analytical problem was identified. The first results on hydrocarbon contents in the various treatments after four and seven months surprisingly indicated that the losses of oil were much higher than predicted by either chemical composition or the lab treatability study. As a result a critical evaluation was made of the standard methodology for extraction and quantification of hydrocarbons in soil-like wastes. This study, involving two independent methods for measuring TEH, indicates that the first losses were overestimated and that actual losses were much less than estimated by the original method. Therefore, selection of analytical method is crucial; and, remediation operators may be seriously misled by results obtained by inappropriate methods. The analytical method should be picked based on the knowledge of the specific hydrocarbon material to be tested. Carbon number scans can be used to measure hydrocarbon fraction that is present.

Currently, all regulatory agencies and developers of remediation technologies rely on analytical results generated by laboratories specialized in chemical analysis. Precise and accurate methods are required to meet key criteria for hydrocarbon residuals set by regulators or to assess the success of a remediation strategy. Analysts use numerous methods; and, the results of comparisons of these methods may be made under the assumption that all of them measure the same pool of hydrocarbons. The determination of TEH requires two separate steps: (1) extraction from the solid waste and (2) quantification of the material extracted. Each of these steps is subject to errors or shortcomings and extraction efficiency cannot be assessed without a "good" detection/quantification method -- hence the importance of the Canadian study of TEH procedures.

To quantify TEH, the study chose two methods routinely used by analytical service labs, but which are distinctly different: the gravimetric method (extractable material dried and weighed) and the Gas Chromatographic (GC) method (extractables separated based on mean boiling point).

The gravimetric quantification measures heavy hydrocarbons, with light hydrocarbons lost as volatiles. The GC method measures light hydrocarbons and cannot detect very heavy hydrocarbons. Both methods would give similar results only when there is no heavy hydrocarbon fraction and when no volatiles are present in the light hydrocarbon fraction. Since gasoline, jet fuel, diesel fuel, crude oil and oily sludges never satisfy these conditions, results using the two detection methods are guaranteed to be different. Most importantly, assessing the biotreatability of a waste using one method or the other will lead to different conclusions, as the light fraction will be lost and the heavy fraction conserved.

Since the choice of organic solvent used to generate the extract may influence the TEH estimate, the flare pit waste was extracted with standard solvents and quantified by both methods. The solvent extractions yielded a complete mixture of heavy and light hydrocarbons. Thus, the method of extracting the flare pit waste had little effect in this TEH estimate.

However, the quantification method had a dramatic effect on the TEH estimate. Using dichloromethane (DCM) extraction solvent, extractable hydrocarbons were 5.9% with gravimetric detection method and 4.0% with GC. .Using the toluene solvent, extractable hydrocarbons were 5.9% with gravimetric and 2.9% with GC. With the super-critical fluid (CO2) solvent, extractable hydrocarbons were 5.8% with gravimetric and 3.0% with GC. In certain cases, different extractions will remove different fractions.

The conclusion of the Bio-Reactor methods study is that exactly the same method should be used throughout a remediation sequence. Further, for reporting routine analyses, the term TPH should be reserved exclusively for methods that truly estimate total hydrocarbons. For methods which estimate some unknown fraction, terms such as "DCM extractable or GC detectable TEH" should be adopted to reflect more accurately what is being measured.

For more information, call Lin Callow of Gulf Canada Resources, Ltd., who is the Bio-Reactor Project's Project Manager, at 403-233-3924.

Material for this article is based on information in the "BIO-REACTOR PROJECT Newsletter," Issue 3, August 1995, published by the Canadian Association of Petroleum Producers.