Extraction Testing of Powdered Materials
By Rick Andrew
Certain media and other materials used in the manufacturing of water treatment equipment may exist in powdered form prior to being incorporated into their final form for POU/POE water treatment. As such, manufacturers of these powdered materials are interested in establishing conformance of their products to the material safety requirements NSF/ANSI Drinking Water Treatment Unit (DWTU) standards by conducting extraction testing on them.
Although it might seem trivial to expose powdered materials to water per the extraction test methodology in the standards to determine whether contaminants leach from them, it actually is not. Many powdered materials are hydrophobic, meaning that they repel water. So it is difficult to wet them. Because of the small particle size and potential hydrophobic properties of the material, it can be challenging to flow water through a column of a powdered material under pressure. Also, it can be difficult to separate them from the water once exposed. Lack of separation makes it very difficult to analyze the water for contaminants because remaining powdered material in the water can interfere with the analysis. All of this adds up to challenges in evaluating powdered materials for extraction of potential contaminants.
In cases of certain powdered materials, such as with activated carbon, it was common practice under the standards for POU applications to test granular carbon of the same type as opposed to the powdered carbon itself. The larger particle size of the granular carbon makes it much easier to expose the carbon to the water and then separate the water from the carbon, compared to testing powdered carbon. This practice, however, is not applicable to all types of powdered materials, some of which may not be available in larger particle sizes. This practice also may not be completely representative of testing the powdered material itself. In light of these challenges and limitations surrounding testing of powdered materials, the standards were recently updated to include a clear and consistent method for testing them.
Extraction testing of powdered materials
An update to the standards now provides a new extraction testing exposure method for powdered materials finer than 100 mesh, which is a particle size of about 0.006 inches or 0.15 mm. The test involves using a laboratory flask for the exposure of the material to the extraction water and adding a 200-gram sample of powdered material combined with one liter of extraction testing water. The flask with the material sample and water is shaken vigorously for one minute and allowed to settle for 24 hours. After the settling/exposure period has passed, the water is poured off, without pouring off any of the powdered material that has settled or is settling.
A minimum of 600 mL is required to be poured off to be used for analysis. This minimum volume assures that there will be enough resulting water volume to conduct all of the required analysis for potential contaminants that may be leaching from the powdered material sample. Depending on how well the powdered material settles, it may be necessary to use more than one flask with 200 grams of powdered material and one of extraction testing water to be able to pour off a total of 600 mL from all flasks of material and water used for the test.
After pouring off a total of at least 600 mL, the flask(s) is/are refilled with the same volume of water that was poured off. By doing so, the ratio of powdered material sample to extraction water is maintained. The flask(s) is/are then shaken vigorously for one minute and allowed to settle for 24 hours. Water is once again poured off, without pouring off any of the powdered material. The flask(s) is/are once again refilled with the same volume of water that was poured off and then shaken vigorously for one minute then allowed to settle one final time for 24 hours. Water is poured off one last time, again without pouring off any of the powdered material. The volumes of water poured off from each flask for each of the three successive pour-offs is composited into one sample for analysis for potential contaminants.
Analysis for potential contaminants
As with all extraction testing under the standards, the water that was exposed to the sample of the powdered material is analyzed for potential contaminants. These potential contaminants are chemicals that could be related to raw ingredients of the material, solvents used in the manufacturing of the material, monomers of polymers, chemicals related to processing of the material, etc. It is even possible that natural materials can be contaminated from heavy metals that occur in nature. The standards require that sophisticated analytical equipment must be used to assure that these types of chemical contaminants are not leaching at levels of toxicological significance. Different analytical equipment is required for various potential contaminants, with metals being analyzed with different technology from organic contaminants, and so on. In order to establish conformance to the standard, the complete results of all the various analyses must be assessed to assure that any detected contaminants are below acceptable levels based on health risks associated with them.
Standards contributing to public health
Through development of clearly described, repeatable and reproducible testing methods, standards such as the NSF/ANSI DWTU standards are important tools used to protect public health. Manufacturers can understand the requirements and make sure they are met as they develop new products, as well as checking the quality of products they currently manufacture. Regulators can rest assured that products conforming to these standards are going to be consistent with laws and codes. And end users can be assured that conforming products have been assessed according to transparent and highly rigorous testing requirements to ensure that they are safe for use with drinking water.
By including this updated method for evaluating powdered materials, there is now a more clear and consistent approach to help manufacturers of these products as well as users of powdered materials evaluate them for conformance to the NSF/ANSI DWT Standards, which supports efforts to maintain quality of materials and throughout the supply chain.
About the author
Rick Andrew is NSF’s Director of Global Business Development–Water Systems. Previously, he served as General Manager of NSF’s Drinking Water Treatment Units (POU/POE), ERS (Protocols) and Biosafety Cabinetry Programs. Andrew has a Bachelor’s Degree in chemistry and an MBA from the University of Michigan. He can be reached at (800) NSF-MARK or email: Andrew@nsf.org