Sulfur creates more than nasty odors. Exposure to low levels (0-10ppm) of hydrogen sulfide (H2S) causes irritation of eyes, nose, and throat; higher levels can cause headaches, dizziness, nausea and vomiting, coughing, and breathing difficulty. H2S and numerous other odor-causing compounds are generated as a result of organic material decomposition.
In the case of the Muddy River in Boston, it is a concern of many area residents, schools and hospitals, where the massive remediation of the Muddy River is being undertaken. The river is part of Boston’s “Emerald Necklace,” the 1,100 acre chain of parks linked by parkways and waterways, built in the late 1800s.
In the century since the river’s course was redirected, it has deteriorated with inflow of toxic heavy metals, decaying vegetation and other pollutants. Although some amount of sulfur is naturally present in river sediments, the deterioration of the river’s ecosystem has created the potential for the release of a significant quantity of sulfur in the form of H2S gas during the dredging operation. In order to estimate the quantity of H2S potentially emitted, it was essential to first determine the quantity of sulfur in the sediments.
An investigation to evaluate the levels of sulfur at this site was initiated by Simmons College to help in selecting a remediation strategy that minimizes volatile sulfur emissions during the sediment dredging operations. Previously, sulfur has been considered too light an element to be detected at levels of interest by handheld x-ray fluorescence (XRF) analyzers. The innovative Thermo Scientific Niton XRF analyzer with helium purge has made it possible to perform an analysis of sulfur and other light elements in the field. Experiments were conducted to determine the accuracy and precision of Sulfur measurements in soils and sediments using the handheld Niton XRF analyzer.
Assistant Professor Michael Berger and student Ling Zou of Simmons College sampled the area collecting soil and sediment from various locations to establish a baseline of materials for analysis. Thermo Fisher Scientific provided technical assistance and support, including the pro-bono loan of the instruments used to screen samples for levels of sulfur in river sediment. Samples ranged from sandy with low levels of organic matter to others with high levels of organic matter and petroleum hydrocarbons.
Careful sample preparation is key to obtaining accurate results for light-elements with XRF analyzers, and requires removal of organic debris and stones. For the most accurate and precise work, the samples must be dried, ground and sieved to achieve a high degree of homogeneity. The samples can be dried in an oven overnight at 60 degrees C. Alternatively, samples could be air dried by leaving them on a paper towel in an interior environment. It is recommended that the sample then be ground to pass through a 120 mesh sieve (125 µm); either a mortar and pestle or a ball mill can be used to complete this process.
In the field it is often not possible to dry the sample; the study showed that useful results can still be obtained if the highest precision and accuracy are not required. In order to determine the effect of sample water content on the sulfur reading, organic-free water was added in proportions to the prepared samples. (See figure 1)
This case study compared the sulfur analyses obtained by using a handheld XRF analyzer (Thermo Scientific Niton XLt898 He) with the analyses of similar samples using the oxygen bomb method followed by sulfate analysis. Samples with certified sulfur concentrations were used for calibration. Sediment samples were run with and without the helium purge in the XRF in order to determine the degree of improvement in the signal detection attributable to helium’s elimination of scattering by atmospheric gases.
The Niton XLt898 He system has a sealed measurement head which is purged with pure helium at 70 mL/min to remove air from the x-ray analysis path. This allows the light element X-rays to pass through and reach the X-ray detector. The analyzer has factory calibrations for various common applications using certified reference material. The factory calibration can be modified, if necessary, by the user to improve the accuracy for a particular type of material being analyzed. The analyzer uses a 40 KeV miniaturized X-ray tube and can quantify elements from magnesium through uranium.
Standard sediment samples, with known and certified composition were used for calibration of the Niton XLt898 He. Data for all experiments consisted of counts (of X-ray fluorescence) detected per second. Total acquisition times were kept constant, nominally at 240 seconds.
An empirical calibration curve was determined over a range of 0 to 3.0 percent S. The measured S fluorescence intensity was plotted against the certified S concentrations for standard sediment samples. Excellent correlation, R ² =0.9967, was achieved using a linear curve fit.
The Niton XLt898He results were in good agreement with certified standard reference materials values and ICP AES results. R² exceeded 0.9 for sulfur, lead, and most other elements. The concentrations of total sulfur ranged from 0.3% to 2% while lead in the sediment varies from 0.01% to 0.53% depending upon location.
A handheld XRF analyzer was shown to be a versatile and easy to use analytical method for a number of metals and non-metals. Use of the helium purge extends the utility of the device by permitting the analysis, not only of the heavy metals, but elements that could not previously be analyzed in the field with a handheld unit. The detection limit of S for handheld XRF analyzers had previously been established at approximately 1 percent, but new helium-purge capabilities have effectively reduced that number to 0.03 percent.