Gypsum or calcium sulfate is a naturally existing mineral. The white crystalline material has found use in a myriad of applications. However, when gypsum is buried in a C&D landfill together other construction waste materials, it is decomposed to form the toxic hydrogen sulfide gas through fermentation by specialized sulfate-reducing bacteria.
Hydrogen Sulfide Analysis
Since the human odor threshold of determination of hydrogen sulfide is 8ppb, many local and state regulatory agencies have limited the amount of detectable hydrogen sulfide, especially in areas nearby to a C&D landfill.
It is necessary to have instruments capable of detecting hydrogen sulfide at ppb concentration levels. One such instrument is the Jerome® 605 Hydrogen Sulfide Gas Analyzer (Figure 1). This portable gold-film sensing instrument is employed to monitor C&D sites for the ‘rotten egg’ odor of hydrogen sulfide.
Figure 1. The Jerome® 605 Hydrogen Sulfide Gas Analyzer.
Gypsum Purity Analysis
Many different wet chemical methods are available to determine gypsum purity. The use of a rapid loss-on-drying instrument enables determining gypsum purity quickly and reliably. One such instrument widely used for determining gypsum purity is the Computrac® MAX® 5000XL (Figure 2).
Figure 2. Computrac® MAX® 5000XL Rapid Loss-on-Drying Analyzer.
This rapid loss-on- drying analyzer can heat samples to 600°C, while allowing the analysis to start at room temperature. These capabilities make it a valuable instrument to analyze gypsum for free and bound moisture. In addition, measurements can be performed in real time during analysis with optimized testing criteria.
Besides simultaneous determination of free and bound moisture, the MAX® 5000XL can provide the gypsum purity results following the analysis, thus avoiding human calculation errors. This article discusses the correlation between gypsum purity and hydrogen sulfide concentration by combining both methods of analysis.
Experimental Procedure and Results
Gypsum Purity Measurement
Since a slow temperature ramp can be performed with the MAX® 5000XL, different levels of hydration are allowed to evolve from the sample of pure synthetic gypsum dihydrate (CaSO4•2H2O) (Figure 3). The red line represents the rate (%/min) of weight loss during the temperature scan at a rate of 10°C/min.
Figure 3. Gypsum temperature scan.
The evolution of the bulk of free moisture is observed at roughly 80°C, while the evolution of the bound moisture is at 240°C. Using the MAX® 5000XL, two separate but connected tests can be performed on the same sample.
In the triplicate run of pure synthetic gypsum, the average moisture determined from the second bound moisture peak was to be 20.84%, representing a gypsum purity of 99.6%. However, this purity value is not an indication of the actual composition of gypsum ended up in a landfill.
In this experiment, carbon source present in a C&D landfill was simulated by adding and homogenizing 10% calcium citrate with the 99.6% synthetic gypsum powder. The resulting product was used as the control stock in this analysis.
For the ‘control,’ 100 g of the control stock was added with 10 mL of sterile media A in a 500 mL Erlenmeyer vacuum flask. After displacing the head space of the vial with pure nitrogen, the vial was securely capped to provide a control vessel, which has all theoretical elements for anaerobic fermentation of gypsum except active sulfate-reducing agents.
The same control stock was used to generate two other vessels (O2 and N2) but with 10 mL of incubated ‘media A’ soil extract. The O2 vessel consisted of all fermentation components including, sulfate-reducing agents, but in oxygen-rich environment. Conversely, the N2 vessel consisted of all fermentation components but in nitrogen environment.
In between the analyses, all these vessels were maintained at 45°C in the incubator and re-purged with nitrogen if necessary when the sample was taken. From each vessel, roughly 7-8g was removed out of the approximately 100g sample for loss-on- drying analysis on the MAX® 5000XL running at 240°C.
Percent bound moisture and percent gypsum purity are presented in Figures 4 and 5, respectively, under these variable conditions for day 1, 5 and 10. It was observed that pure gypsum and the ‘Control’ remained unaltered even after 10 days.
Conversely, sulfate purity was decreased over time in other two vessels containing active sulfate-reducing agents, especially in the N2 vessel.
Figure 4. Percent bound moisture of gypsum.
Figure 5. Percent purity of gypsum.
Measurement of Hydrogen Sulfide Concentration
The headspace of each vessel was measured periodically using the Jerome® 605. Two-hour ramp testing was carried out using the same Jerome® 605 during a 10-hour period on all three conditions in ‘Day 1’ of incubation. After setting the Jerome® 605 to ‘auto’ range, all data points were collected in triplicate. Figure 6 presents the day one results.
Figure 6. Hydrogen sulfide generation - day one.
Figure 7 presents the results over a five-day period, involving sampling once per day. It is evident that the hydrogen sulfide concentration was increased considerably over time. Hydrogen sulfide generation might plateau during day 5 to 10. Further analysis is required to explain the trend.
Figure 7. Hydrogen sulfide generation.
The mean values for hydrogen sulfide gas sampling and gypsum purity for Day 1, 5, and 10 are summarized the following table:
Under anaerobic conditions, the gypsum purity decreased from 101.05% to 82.47% over time. At the same time, there was a significant increase in the hydrogen sulfide concentration over time, showing the inverse trend between hydrogen sulfide concentration and gypsum purity during anaerobic fermentation. The total % moisture at day 10 is presented in Figure 8.
Figure 8. Total % moisture (free and bound) at day 10.
The combination of gypsum purity analysis using the MAX® 5000XL and hydrogen sulfide analysis using the Jerome® 605 demonstrated the inverse relationship between gypsum purity and hydrogen sulfide concentration. This trend was observed at different levels of oxygen, but significant under anaerobic conditions
The study results presented in this article were for a fermentation process of 10 days. Further analysis is required to determine the impact of longer fermentation periods. Future studies may examine other types of gypsum, including hemihydrates and anhydrous, as well as other construction debris such as wall board.
It was not possible to measure the total hydrogen sulfide concentration at each time point because of a large headspace volume in the vial. Future studies may involve vessels with small headspace volume in order to more accurately measure the concentration of hydrogen sulfide.
This information has been sourced, reviewed and adapted from materials provided by AMETEK Brookfield Arizona
For more information on this source, please visit AMETEK Brookfield Arizona