Improving Testing Laboratory Collaboration
Posted on Thursday, January 30, 2020
Understanding your relationship with your testing laboratory is essential to the success of your finishing operation. Your testing lab partner is only as good as the information and instruction they receive with testing materials. Because false positives are possible with even newer pretreatment alternatives, finishers should work closely with their chemical suppliers to ensure testing accuracy regarding effluent discharge.
Zirconium-based conversion coatings can be an excellent alternative to conventional phosphate pretreatment technology with many advantages. Compared with traditional iron and zinc phosphate processes, zirconium oxide coatings produce less sludge, often contain low or no phosphate, and can be used at lower temperatures. This results in cost savings from lower energy usage, reduced water consumption, and reduced waste treatment and disposal requirements. Some zirconium treatments can be considered non-hazardous or non-regulated by many publicly owned treatment works (POTWs) and therefore can be discharged without treatment costs. However, local regulations generally require approvals before allowing metal finishers to discharge directly to the municipal sewer.
Discharge permits vary by location and local municipal authorities should always be contacted to obtain effluent limitations which describe a list of required tests, discharge limitations, as well as testing frequency. Most require heavy metal testing at minimum, though other tests may also be necessary. In order to obtain approval, a representative wastewater sample must be analyzed at a certified laboratory. The most common method of analysis is inductively coupled plasma atomic emission spectroscopy (ICP-AES) using EPA method 200.7. This analytical technique produces excited atoms and ions by ionization in an electromagnetic field. Each element emits detectable amounts of light at a characteristic wavelength. Concentration is directly proportional to the intensity of the light emitted. Though each element has a signature wavelength, spectral interferences are possible and must be considered.
During spectral interference, light emission from other elements can contribute to the apparent intensity of an element of interest. This phenomenon results in falsely high concentrations of the element in question. Spectral interference can occur due to direct spectral line overlaps (elements emitting at the same wavelength), or when wavelengths are close (possibly only 1 nm apart). When signature wavelengths are close, it is important to consider broadening of intense spectral lines as well as light emissions from elements at high concentrations. Corrections and alternatives should be investigated when encountering either type of interference.
For example, zinc and nickel have a direct spectral line overlap at 213.86 nm. Without knowing the elements in the matrix, an analyst may mistakenly assume the sample contains nickel, when it really only contains zinc. In this case, an alternate wavelength should be selected for nickel, such as the spectral line at 231.60 nm. The same phenomenon holds true for zirconium and silver, where current users of zirconium technology experience “false positive” test results for silver. Silver, a commonly tested metal in wastewater, emits light at 328.07 nm. Zirconium emits light at 327 nm. Because the amount of zirconium in the wastewater sample will be fairly high in comparison to heavy metals, the peak will be broad and may flood the region. Instead of having a well- defined Gaussian curve, the peak may have a tail. Computer programming may incorrectly identify this response as silver and report a concentration higher than exists in the sample matrix. Additional scrutiny is required and the alternate wavelength for silver at 338.39 nm should be considered instead. However, as with all samples analyzed by ICP-AES, care should be taken to ensure the detection limit for the wavelength is within the range of the effluent limitation.
Alternatively, there are other methods to confirm the concentration of silver in a wastewater sample that are not subject to spectral interferences (though other interferences are possible). Inductively coupled plasma mass spectrometry (ICP-MS) builds upon the concept of ICP-AES. After excited atoms and ions are produced, they are separated by their mass-to-charge ratios. Silver and zirconium are easily separated and identified by this method. Another analysis technique that can be used to confirm results is flame atomic absorption spectroscopy (AAS). Flame AAS uses flame to convert metal ions in solution to their atomic state. All ground-state metals absorb light at a specific wavelength. When the sample is exposed to the correct wavelength for a given metal, the amount of light absorbed is measured and used to calculate concentration.
While alternative analysis techniques are great ways to confirm the concentration of silver in zirconium wastewater samples, they may not be readily available in a standard wastewater analysis laboratory that relies on ICP- AES. In this case, the best course of action is to work with the testing facility so that they understand elements that may be typically found in your waste stream. This allows the analyst to anticipate the use of alternate wavelengths and appropriate dilutions of the sample to avoid flooding, broad peaks, and shoulders which can all impact the concentration reported by the instrument. If analysis of the waste stream has consistently passed for silver but fails suddenly and unexpectedly, it is worth having a discussion with the laboratory to understand if analysis methods were recently changed. Ask the laboratory to report the wavelengths used to analyze a particular element and check for common interferences listed on EPA test method 200.7.
Zirconium pretreatment allows for a reduction in water usage and waste disposal, but it is also possible to close the loop and eliminate the need for effluent discharge. With a system of complementary products, rinse counterflows, and an evaporator, the need to discharge wastewater is avoided. This process is referred to as zero discharge.
Communication is key to a partnership with the laboratory. When choosing a new laboratory for wastewater discharge, work with a certified laboratory and discuss your testing needs. The facility may already be familiar with common interferences and have experience analyzing wastewater containing zirconium.
Kim Kontra is chemist III, Surface Finishing for Quaker Houghton.