1. Introduction
The current USA Environmental Protection Agency (EPA) effluent limitations guidelines and standards for the Steam Electric Power Generating Point Source Category’s rule published in November 2015 mandates the limitation of toxic metals and other chemical compounds from coal-fired power plants [
1]. On an annual basis, the rule incrementally reduces the amount of toxic metals, nutrients, and other pollutants that steam electric power plants are allowed to discharge (no more than 1.4 billion pounds or 635 million kg per year) and reduces water withdrawal by 57 billion gallons or 216 billion liters per year [
1].
Currently, a Flue Gas Desulfurization (FGD) system is used to remove sulfur dioxide (SO2) from flue gas produced from coal-fired power plants. In the FGD system, limestone reacts with SO2 and produces gypsum. The process also removes mercury (Hg), selenium (Se), arsenic (As) and other elements and compounds. An FGD-wastewater treatment (WWT) system also uses various chemicals, such as organosulfides for mercury, to remove solids (e.g., CaSO4, Se, Hg, As, etc.,) from FGD wastewater. Chemicals required for the FGD wastewater treatment are stoichiometrically based on coal and limestone chemistry, various operational variables (e.g., cycle of absorber concentration, competing reactions, etc.,) and the numerical discharge limit regulated by the EPA. The plant operators periodically collect samples at various stages of the FGD wastewater process and measure numerous chemical species with hand-held kits that are later sent to commercial laboratory for further analysis. This entire process can be considered time consuming, costly, and tedious. Despite these factors, the level of detection and precision needed to detect contaminants at the designated limits require analysis by a commercial lab. The turnaround time on this analysis method may be weeks, making it difficult to control the process in a timely and efficient manner. This monitoring methodology is not acceptable for ensuring that discharge from the wastewater treatment plant is within current guidelines.
An alternate option which utilities may elect in the projected EPA rule for 2023 is even stricter, where the allowable discharges of mercury, selenium, nitrates, and nitrites are significantly lower [
1]. The state-of-the-art FGD wastewater treatment technologies (e.g., CH2M Hill’s FGD wastewater treatment) will need to adapt new technologies to meet the discharge limits. Overall, real-time sensing of FGD wastewater during processing is needed to enable operators to make process adjustments in a timely manner for an acceptable, consistent discharge quality.
Therefore, a real-time robust sensor module that detects water quality at various stages in the FGD wastewater treatment plant units is necessary to keep in compliance with the regulatory discharge limits. Ion selective electrode-based sensor-arrays provide a convenient solution to these problems as they target the species of interest in the FGD wastewater treatment process and provide real-time monitoring.
This study evaluated several commercially available ion selective electrodes for their use in FGD wastewater systems. The goal of this study was to determine if ion selective electrodes (ISEs) were compatible with the general composition of FGD wastewater and if an accurate analytical methodology for FGD wastewater characterization could be constructed from both model solutions and actual FGD wastewater samples.
As of 2015, more than 66 companies are manufacturing water related sensors in the world, but only a handful of them market ISE, which utilize special membranes that allow for the measurement of specific ions. An ISE (with its own internal reference electrode) is immersed in an aqueous solution containing the ions to be measured, together with a separate, external reference electrode. The external reference can be separate or incorporated in the body of the ISE to form a Combination Electrode. The electrochemical circuit is completed by connecting the electrodes to a sensitive voltmeter using special low-noise cables and connectors. A potential difference is developed as the selected ions diffuse onto the ISE membrane surface and interact with the molecules imbedded in the membrane. Most ISEs use a polyvinylchloride (PVC) based membrane, but crystalline and other special membranes do exist.
Sensor distributors such as NexSens Technology (Fairborn, OH, USA), Thermo Fisher Scientific (Waltham, MA, USA), Fondriest Environmental (Fairborn, OH, USA), NICO2000 (London, UK), etc., market ISE electrodes for various metals and ions e.g., nitrates, chlorides, mercury, etc. The ISE electrodes can be calibrated for high-to-low concentration. The main advantage of the ISE is the flexibility to calibrate and monitor very high (mg/L) to very low concentration (ng/L). However, ISE are sensitive to the environment, especially operating temperature and pH. In some cases, a one-degree Celsius increase of water temperature may lead towards 1.5% detection error. Most of the ISE are calibrated with individual water solution and not tested with different atmospheres. FGD wastewater quality varies dramatically in different locations in the same FGD wastewater treatment plant. Off-the-shelf made ISE sensors are suitable for clean, room temperature water, whereas the FGD water is contaminated with various chemicals and temperature can be much higher than room temperature (76–125 °F/24–52 °C).
Previous studies have investigated the use and suitability of ISEs with various media, and have found that ISEs are typically cheaper and quicker for mass data acquisition compared to traditional methods such as inductively coupled plasma (ICP) analysis [
2,
3]. However, certain complications have occurred, such as the ionic strength of the undiluted solution either damaging the electrode membranes or causing poor quality in readings. Therefore, ISEs must be evaluated for use with FGD wastewater to determine if they would be a more viable option than standard procedures currently in use.
2. Materials and Methods
2.1. Equipment and Reagents
A Thermo Fisher Scientific Orion™ VersaStarPro™ 40b2 (Thermo Fisher, Waltham, MA, USA) was used for potential difference and temperature measurements. All electrodes are from NICO2000 (London, UK). Nitrate (ELIT 8021), nitrite (ELIT 8071), calcium (ELIT 8041) and chloride (ELIT 8261) ISEs were used with a lithium acetate reference electrode. The nitrate, nitrite, and calcium electrodes use a PVC based membrane, while the chloride electrode has a crystalline membrane. Sodium nitrite and sodium nitrate of granular reagent grade (Fisher Scientific, >98% pure) and anhydrous reagent grade calcium chloride (Fisher Scientific, >96% pure) was used for all experiments.
2.2. FGD Wastewater Samples
Samples of FGD wastewater were provided by American Electric Power (AEP). The samples were taken from five locations in the FGD wastewater treatment process. Two, 1-gallon (4 L) samples were taken from each location (the inlet purge tank, primary clarifier, equalization tank, secondary clarifier, and effluent stream). These samples were stored at 20 °C in polytetrafluoroethylene (PTFE) lined containers.
2.3. Induced Coupled Plasma-Optical Emission Spectroscopy (ICP-OES)
Cations in the liquid phase of the samples were analyzed using a Thermo iCAP 6000 ICP (Thermo Fisher, Waltham, MA, USA). Solid products were prepared using microwave acid digestion (Mars 6 microwave) with nitric acid (1 mg solids per 10 mL 85% nitric acid) at 200 °C followed by analysis on the same ICP. Both types of samples were filtered using Whatman 0.45-micron filters. Samples were analyzed at multiple factors of dilution (typically 10, 100, 1000, and 10,000). Data points that fell within the prepared calibration curve were averaged.
2.4. Uv-Vis Measurement Methdology for Determination of Nitrate and Nitrite Concentrations
Nitrate and Nitrite concentrations were determined using Hach test kits. Each sample was completed in triplicate to ensure accurate readings. Ion chromatography, the ideal option for measuring anions at high precision, was not used due to the high levels of contaminants in the FGDWW samples.
2.5. Experimental Methods
Experiments where created to determine the relationship between the potential difference of the electrodes and the concentration of nitrate, nitrite, calcium and chloride in an aqueous solution. A starting solution of 250 mL deionized (DI) water and a concentrated solution (ranging from 0.001 to 1 molar to target different concentration ranges) of preferred molarity for one of the selected ions was prepared for each test. The solutions were stirred and heated when necessary. Tests where preformed at 20, 30, and 50 °C. 1 mL of the concentrated solution was added into the DI water and mixed for 5 min to create the starting solution. The ISE of the selected ion, reference electrode and thermocouple were rinsed with DI water before being placed in the starting solution. After 5–10 min (to allow for the electrodes to stabilize) 5 measurements of the electrodes were taken, to increase reliability of the results. Next, 1 mL of the concentrated solution was added to the starting solution. This was repeated every 5 min until there was a minimal change in the measurements, this typically resulted after 25 or so additions, which is expected for this semi-batch style additive process. The final concentration of test solutions would be approximately 1/10th that of the concentrated solution. While the wait time for measurements is far greater than the response time, it ensured that accurate measurements were being taken.
A similar method was used to evaluate the effect of ion strength on electrode measurements. A starting solution was prepared using the methods previously described. A non-interfering ion was then added to the solution and the change in measurement was recorded after the system gave a constant measurement. Sodium chloride, calcium chloride, and sodium nitrate were used to increase the ionic strength of the solution. This method of interference evaluation is similar to those done in studies evaluating novel membranes for use in future analytical techniques [
4,
5]. The range of experimental values are shown in
Table 1.
The electrodes were calibrated using standard solutions of calcum chloride for the calcium and chloride ISE’s, and sodium nitrate and sodium nitrite for the nitrate and nitrite electrodes respectively. These standard solutions were verified using the previously described ICP methods. For verification of accurate measurements, standard soltuions were used that spanned the intended test range. For example, a test of the calcium ISE that ranged from 0.0001 to 0.001 M would be calibrated at 0.00001, 0.0005, and 0.002 M before the experiments. These values were checked to previous measurements to ensure that the elctrodes were not damaged and had not experieneced significant performance degradation since the previous experiments. Two reference electrodes were used during this calibration process to verify that the reference electrode was functioning properly as well.
For the determining drift over time, measurements for calcium, chloride, nitrate, and nitrite ISEs were taken in a continuous flow-through system using Nico2000 ELIT 6-Channel Flow-through Monitor (MCC-MON-6f) for 8, 16, and 24 h.
Figure 1 shows the set-up of this experimental system. The purpose of this set-up was to determine what corrections are needed to account for electrode drift over time. Additionally, different methods of electrode restorations were evaluated to determine how often cleaning is needed, and how it should be done.