How can the small amount of organic matter in wastewater be measured accurately?
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This post was last edited by lihaiyong826 on 2017-8-21 at 15:10. Hello, everyone. There is a project in which the wastewater contains about 5% organic substances (alcohols such as methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol, and octanol). The goal is to recover these alcohols and to separate the main ones among them (methanol, ethanol, and propanol). This raw material sample contains a lot of water; how can we accurately measure the contents of water and various alcohols? After distillation to remove some of the water later on, the water content in the alcohol is around 30–40%. What analytical methods would be suitable for conducting an accurate analysis at this point?
For this sample, I personally think chromatography is the best method – it’s convenient and fast. 1. If you do not need to analyze the water content and only want to measure the levels of various alcohols, it is recommended to use a capillary column, with a FID detector, and a split-injection system. 2. If you also need to analyze the water content, then a packed column should be used, along with a TCD detector; this is because only the TCD detector is capable of detecting water. As for the separation performance of packed columns for alcohols, I am not sure. Due to my limited skills, this is all I can help you with. Regarding the specifications and models of chromatography columns, you can consult the manufacturers that produce chromatography equipment; they will propose a testing plan based on your requirements
For the detection of alcohols, gas chromatography is recommended as a method for reference; the columns and other components can be selected based on existing conditions. For water, the Karl Fischer method is still advised. Please feel free to correct me if I’m wrong. I) Principle: Detection is achieved by utilizing the chemical ionization of different alcohols in a hydrogen flame, with quantification being performed by comparing the peak height to standards. Detection limit: n-propanol, n-butanol 0.2 ng ； Isopentanol, n-pentanol 0.15 ng ； Butyl alcohol, isobutyl alcohol: 0.22 ng. (II) Reagents (1) Carrier GDX-102 (60–80 mesh), for gas chromatography. (2) Methanol, chromatographically pure. (3) n-Propanol, chromatographically pure. (4) Butan-2-ol, chromatographically pure. (5) Isobutanol, chromatographically pure. (6) n-Butanol, chromatographically pure. (7) Isopentanol, chromatographically pure. (8) Ethyl acetate, chromatographically pure. (9) To determine the alcohol content without methanol or fusel oils, 0.5 µl of the sample should be used; no peak peaks should appear. (10) For the standard solutions, 600 mg each of methanol, n-propanol, sec-butanol, isobutanol, n-butanol, and isopentanol, along with 800 mg of ethyl acetate, were accurately weighed. These were washed into a 100 mL volumetric flask using a small amount of water, diluted to the mark with water, and stored in the refrigerator. (11) Standard solution: Transfer 10.0 mL of the standard solution into a 100 mL volumetric flask, add a certain amount of ethanol that has been treated as described in (9), bring the volume to the mark to achieve an ethanol concentration of 60%, and then dilute with water to the mark. This solution is stored in the refrigerator for later use (or prepared according to the sensitivity of the instrument). (III) Instruments (1) Gas chromatograph: equipped with a hydrogen flame ionization detector. (2) Micro-syringe for injection. (IV) Analysis steps 1. Chromatography reference conditions: (1) Column length: 2 m, inner diameter: 4 mm; glass or stainless steel column. (2) Fixed phase GDX-102, 60–80 mesh. (3) Gasification chamber temperature: 190°C. (4) Detector temperature: 190°C. (5) Column temperature: 170°C. (6) The flow rate of the carrier gas (N2) is 40 mL/min. (7) Hydrogen (H2) flow rate: 40 mL/min. (8) Air flow rate: 450 mL/min. (9) Injection volume: 0.5µL. 2. Qualitative analysis: Qualification is carried out based on the retention time of each component. 0.50 µL of each of the standard solution and the sample solution is taken, and their retention times are measured; qualitative analysis is performed by comparing the peak times of the sample with those of the standard. 3. Quantification: Add 0.5 µL of the standard solution to prepare a chromatogram, and measure the peak heights of each component separately. 0.5 µL of the sample is injected to generate a chromatogram; the peak heights are measured separately and compared with those of the standard peaks for calculation.
1. Currently, FID chromatographic analysis is also being used; there might be some issue somewhere, resulting in highly inaccurate analysis. With such a high water content, is GC analysis using FID reliable? 2. TCD responds to water, but it is not widely used these days and is therefore rarely seen. I used a moisture meter to measure the water content, and it’s still too high; I’m really stuck.
Based on what you’ve said, here are my personal opinions; please correct me if I’m wrong. First, a high water content has no impact on the FID detector. I have conducted analyses of methanol in wastewater, and in such cases it’s mostly water, so there is no effect either. Is the analysis you mentioned inaccurate due to poor repeatability? Are there large differences in the results of parallel samples? If that’s the case, I suggest you look for the reasons from the following aspects: 1. Is the sample injection done manually? If so, errors are likely to occur. 2. Check whether the liner of the injection port is clean. 3. Should the chromatography column be aged to see? II. Regarding the issue of moisture you mentioned, I have a method of my own that I hope will be helpful to you. Since you need to measure the moisture content, and if that level is high, using a moisture detector can be detrimental to the performance of the instrument over time, as well as result in waste of reagents. Well, you can simply dilute your sample using another reagent that contains almost no water. Since you mentioned earlier that you need to recover alcoholic substances, you could try using the high-purity alcohols obtained from that recovery process to dilute the sample; this way, the water content will be lower (you’ll need to determine the specific dilution ratio on your own). The diluted sample should then work fine with a water content analyzer. Of course, you’ll need to take into account the water content of the high-purity alcohols used for dilution when making calculations
If the analysis is subject to large deviations due to too low concentrations, pre-treatment using polymer materials for adsorption and enrichment, followed by elution with organic reagents, before proceeding with chromatographic analysis, can be considered. But it’s more complicated to determine the adsorption and elution rates.