A NEW APPROACH TO TOTAL ORGANIC CARBON ANALYSIS USING CLOSED-LOOP PHOTOCATALYTIC OXIDATION

Transcription

1 A NEW APPROACH TO TOTAL ORGANIC CARBON ANALYSIS USING CLOSEDLOOP PHOTOCATALYTIC OXIDATION KAREN COOPER PRODUCT MANAGER, APPLICATIONS ANATEL CORPORATION P.O. BOX 477 LOVELAND, CO Presented at ISA Analytical Division Symposium, Denver, CO April 2002 and ISA National in Chicago, October 2002 KEYWORDS TOC, Photocatalytic Oxidation, NDIR; Dynamic Endpoint Detection; Titanium dioxide ABSTRACT TOC is the acronym for total organic carbon. The first TOC methods were developed to correlate information obtained from chemical oxygen demand (COD) and biochemical oxygen demand (BOD) tests in drinking and wastewater. Today new government regulations are making TOC analysis a standard test for all industry water types. This paper discusses the principles of operation of common TOC methods and outlines a new methodology for TOC analysis using closedloop photocatalytic oxidation. The new TOC methodology includes a titanium dioxide slurry and a 400 nm light source for the oxidation process. The closedloop TOC system incorporates a closedloop design to eliminate the need for carrier gases and uses what is called dynamic endpoint detection, where all of carbon in the sample is oxidized to completion. The reaction is measured using a nondispersive infrared (NDIR) detector. This new technology can directly measure TOC from a single sample eliminating the loss of purgable organics and results in comparable recoveries of various organic compounds when compared to the combustion TOC method. This new methodology can be used for the same applications where the traditional TOC methods have been employed.

2 INTRODUCTION TOC is the acronym for total organic carbon. The first TOC methods were developed to correlate information obtained from chemical oxygen demand (COD) and biochemical oxygen demand (BOD) tests in drinking and wastewater. The TOC methods were designed to be more efficient than the COD and BOD tests, which required the use of hazardous chemicals and multiple days to complete. Today new government regulations are making TOC analysis a standard test for all industry water types. The Environmental Protection Agency (EPA) has instituted rules to monitor municipal drinking water systems 1. The main reason for measuring TOC in drinking water is that chlorine, the primary drinking water disinfectant, has the potential to react with some organic compounds in the water to form chlorinated hydrocarbons which have been associated with carcinogenic activity. Drinking water is the feedwater for most water purification systems. Monitoring this feedwater is critical to maintaining the overall efficiency of any water purification process. The water s characteristics such as hardness, particulates, bacterial levels, conductivity, and TOC levels significantly influence downstream processing. The power generation industry recognizes organic carbon as a significant corrosion contributor. Some carbon compounds found in water are a source of corrosive acids that can reduce the life of boilers, reactors and turbine blades. Highpurity water is necessary for continuous operation of power facilities. TOC levels in ultrapure water such as that used in the semiconductor industry have been found to be a major contributor to increased product defects 2. Today, device geometry reductions accompanied by increases in circuit densities are imposing challenging demands on the purity of water used. Semiconductor manufacturers must monitor TOC levels in all stages of the water purification, most importantly at the pointofuse. Since even the slightest change in TOC levels can affect production yield, the industry has incorporated online TOC monitoring to provide trend information throughout the entire water purification process 3. Today, TOC levels in semiconductor process water are being controlled to below 1 ppb. Finally, and most recently, TOC regulations are now a part of the pharmaceutical industry. The United States Pharmacopeia (USP) has made TOC analysis a standard test for production of Purified Water and Water For Injection 4. Measurement of TOC is a direct reflection on the quality of the water being produced and can have a significant impact on the manufacturing process of drug products. This paper discusses the principles of operation of common TOC methods and outlines a new methodology for TOC analysis using closedloop photocatalytic oxidation. COMMON METHODS FOR MEASURING TOC TOC methods share the same basic chemistry of converting all the carbonaceous material in a given sample to a form that is more readily measures, such as CO 2. The oxidation and detection technologies vary based on the specific analysis method, and each technique is appropriately applied to different TOC concentration levels.

3 COMBUSTION OXIDATION The combustion method measures total carbon (TC). It requires samples injection by syringe into a hightemperature furnace with a platinum or cobalt catalyst. This process oxidizes all of the carbon materials present to CO 2. The CO 2 is swept into a nondispersive infrared (NDIR) detector by a carrier gas usually nitrogen for final measurement. The amount of CO 2 measured is directly proportional to the amount organics present in the original sample. A variation of this method employs a stream splitter, which directs equal parts of the sample to two furnaces at different temperatures. One furnace at 150º C measures the total inorganic carbon (TIC) present and the other at 950º C measures the total carbon (TC) present. The final total organic carbon (TOC) amount is then calculated by: TOC = TC TIC (1) WET OXIDATION Wet oxidation involves adding an acid to the water sample to reduce the ph to approximately 2 or 3. At this low ph, any inorganic carbon that is present is liberated as CO 2 into a nitrogen carrier gas and is directly measured by an NDIR detector. Any remaining carbon in the sample is assumed to be TOC. A persulfate oxidant is added to the sample, and in the presence of heat and/or UV radiation, the remaining carbon is oxidized to CO 2. The amount of CO 2 generated is then measured by the NDIR to determine the amount of TOC. CONDUCTIVITY Conductivity based TOC methods oxidize the TOC that is present to CO 2 using UV radiation typically in the presence of a titanium oxide catalyst. The CO 2 that is produced redissolves into water as bicarbonate (HCO 3 ) and hydrogen ions (H ). These ions increase the conductivity of the water. A conductivity meter or probe is then used to measure the change in conductivity of the solution. The change in conductivity is proportional to the amount of TOC present in the water. As mentioned previously, the measurement of TOC in water is important to many different industries. The methods chosen to analyze for TOC will depend on a number of variables. The water source to be measured is a major factor in considering which TOC method to use. Certain types of wastewater and raw water contain both soluble and insoluble organic material in high concentrations (> 100 ppm C). Since Combustion Oxidation oxidizes all carbon materials present by the use of a high temperature furnace, this method is suitable for measuring insoluble organics found in wastewater and raw water. Drinking water and other purified water sources typically contain soluble TOC at levels less than 10 ppm C. The TOC in this water can easily be measured by Wet Oxidation. Measurement of TOC in high purity water and ultrapure water used in the Pharmaceutical and Semiconductor industries respectively, requires a more sensitive method like the Conductivity based method. TOC levels in these waters range from less than 500 ppb for pharmaceutical high purity water to less than 1 ppb for semiconductor ultrapure water.

4 NEW TOC METHODOLOGY The goal in developing a new TOC methodology was to design an analysis system that was easier to operate and used less hazardous chemicals and accessories than the current Combustion and Wet Oxidation TOC methods. Combustion TOC methods require high temperature furnaces and frequent replacement of the catalyst. Wet Oxidation TOC methods require the use UV light, and hazardous oxidizing chemicals like sodium persulfate. Both methods require a constant supply of a CO 2 free carrier gas, and are designed as flowthrough systems (see Figure 1). Vent Detector AutoInjector Gas Flow Control Gas treatment module Gas Source Reactor Combustion furnace or Persulfate chamber Dehumidifier FIGURE 1: SCHEMATIC OF A FLOWTHROUGH SYSTEM Flowthrough systems inject the sample into the reactor along with any reagents. The oxidation process is allowed to proceed and the CO 2 produced is transported by the carrier gas to the NDIR detector for TOC measurement. The problem with flowthough systems is that the oxidation time in the reactor has to be fixed. Since neither the quantity or species of carbon in the sample is known, the preset oxidation time is just an approximation of what is needed to ensure a complete reaction. If the time is too short, the TOC results could be incorrect due to incomplete oxidation of the sample. A more rational approach to ensure complete oxidation is to use a closedloop design 5 (see Figure 2).

5 Pump Detector Reactor AutoInjector Gas treatment module FIGURE 2: SCHEMATIC OF A CLOSEDLOOP DESIGN Foe the closedloop system, the loop is initially open to allow atmospheric air to be pumped into the loop. This eliminates the need for a carrier gas. After the loop is closed, the background CO 2 in the loop is used as the baseline. An injection is made through the autoinjector into the reactor. The oxidation process begins. As the oxidation proceeds there is an increase in CO 2 in the loop. The CO 2 is continuously circulated through the loop. The concentration of CO 2 will continue to increase until the sample has been completely oxidized. The time it takes for complete oxidation will depend on the type and concentration of carbon species being analyzed. There is no fixed oxidation time for the closedloop system. The oxidation process continues until all of the carbon in the species has been oxidized. The closedloop TOC system uses what is called dynamic endpoint detection. An example of the process is shown in Figure 3. The system will establish a baseline CO 2 level. It will then monitor the oxidation until there is no further increase in CO 2. The resulting TOC concentration is calculated from the initial baseline CO 2 level to the final measured CO 2 level. The system is then vented to reestablish a baseline for the next sample. This reaction to completion is significant since many water samples contain components not easily converted to CO 2. Final CO 2 concentration Result calculated from CO 2 concentration Injection Oxidation Initial CO 2 concentration FIGURE 3: CLOSEDLOOP DYNAMIC ENDPOINT DETECTION

6 To eliminate the use of hazardous chemicals and accessories like high temperature furnaces and UV lamps, this new TOC methodology uses a photocatalytic oxidation process that includes a titanium dioxide slurry and a 400 nm lamp. The sample and a slurry of titanium dioxide are injected into the reactor. The solution is exposed to a 400 nm near UV light. When exposed to the 400 nm light, the titanium dioxide particles undergo electron transitions that generate positive holes on the particle surface. In an aqueous suspension the positive holes on the surface of the titanium dioxide interact with water to produce hydroxyl radicals. The hydroxyl radicals act as the oxidizing agent to oxidize the organic compounds that are present in the water sample. A representation of the oxidation process is shown in Figure 4. H 2 0 H 2 0 H H H 2 0 H 2 0 FIGURE 4: MECHANISM OF PHOTOCATALYTIC OXIDATION USING TITANIUM DIOXIDE All of the organic compounds are rapidly oxidized to carbon dioxide, acid, base and water as shown in the example equation: 400 nm light/tio 2 C 6 H 5 X 7O 2 6CO 2 HX H 2 O (2) The CO 2 that is produced is continually pumped through the closedloop system and its concentration is measured by the NDIR detector until the oxidation process is complete. The major advantage of this closedloop photocatalytic system over the combustion and wet oxidation flow through systems, is the ability to measure total organic carbon directly. All other TOC systems must measure TOC indirectly. Because of the flowthrough design, these systems must make two separate measurements to determine a TOC value. First a measurement is made for total inorganic carbon (TIC) and then a measurement for total carbon (TC). The flowthrough systems then calculate a TOC value by subtracting the two measurements, where: TOC = TCTIC (3)

7 The flowthrough systems result in a greater possibility of inaccuracies in TOC values because there are two separate measurements both with fixed oxidation times. These systems also have longer sample analysis times. For the closedloop system both TIC and TOC can be measured in a single sample analysis. This system being completely closed also eliminates the loss of purgable organic carbon (POC) during the TIC analysis, which has been a serious problem with the flowthrough systems. A water sample is injected into the reactor and is combined with the titanium dioxide suspension that is ph adjusted to 3. At ph=3 any inorganic carbon present in the sample is converted to CO 2 and measured. The change in CO 2 concentration in the closedloop is monitored until an endpoint is detected. The lamp is switched on to initiate the oxidation of all the organic carbon in the sample. The change in CO 2 is again continuously measured until an endpoint is detected. The actual oxidation process is depicted in Figure 5. At the end of the oxidation process the system is vented to reestablish the baseline for the next sample. NDIR RESPONSE TOC TIC TC TIME FIGURE 5: C OMPLETE OXIDATION USING THE CLOSEDLOOP SYSTEM WITH A SINGLE SAMPLE ANALYSIS

8 TOC DATA COMPARISON Table I lists a variety of compounds that were analyzed using the closedloop photocatalytic oxidation technology compared to the combustion technology. The results show that in almost all cases the closedloop photocatalytic oxidation technology produced better percent recoveries than the combustion technique. TABLE I: CLOSEDLOOP PHOTOCATALYTIC OXIDATION VERSUS COMBUSTION Organic Theoretical closedloop photocatalytic oxidation Combustion Compound (ppm C) Result (ppm C) % Recovery Result (ppm C) % Recovery MeOH EtOH Methyl1Butanol Formaldehyde D()Glucose D()Fructose D()Xylose Mannitol Formic Acid Acetic Acid Propionic Acid nbutyric Acid transcinnamic Acid Phthalic Acid Salicylic Acid Fumaric Acid DLMalic Acid Tartaric Acid Citric Acid Urea ptoluidine Glycine DLAlanine Hippuric Acid LGlutamic Acid mcresol Phenol CONCLUSION A new TOC methodology has been developed which incorporates a closedloop design to eliminate the need for carrier gases and to ensure complete oxidation of measured organic compounds. Reaction to completion is very significant since many water samples contain components not easily converted to CO2. There are no fixed analysis times, which can lead to less accurate TOC measurements, and the TOC is measured directly eliminating the loss of POC. The photocatalytic oxidation process uses nonhazardous components such as titanium dioxide and a 400 nm lamp. This new technology can directly measure TOC from a single sample and results in comparable recoveries of various organic compounds when compared to the combustion method. This new TOC methodology can be used for the same applications where the traditional TOC methods have been employed.

9 REFERENCES 1. United States Environmental Protection Agency s Office of Ground Water and Drinking Water; Microbial and Disinfection Byproduct Rules: Stage 1 D/DBP Rule, EPA 815F980014, December 1998 ( 2. P. McConnelle et al. Water Quality Improvements and VLSI Defect Density ; Semiconductor International; August Poirier, S. and R. McIntosh, The Application of Online TOC Monitoring in Semiconductor and Power Generation Plants, Ultrapure Water; July/August <643> Total Organic Carbon ; United States Pharmacopeia National Fomulary (USPNF)24, Fourth Supplement, June 2001; p AnaTOC Total Organic Carbon Analyzer; Anatel Corporation, P.O. Box 477 Loveland, CO 80539;