A method for determining hexavalent chromium in soil and sediment

Abstract

The invention relates to a method for determining hexavalent chromium in soil and sediments. Aiming at the problem that the existing test method for determining hexavalent chromium in soil and sediments has defects, a new alkaline leaching solution and a signal enhancer are adopted, and an ultrasonic heating method is used to solve the problems that a large amount of flocculent precipitation generated in the early stage is difficult to separate when separating hexavalent chromium from trivalent chromium in soil and sediments, liquid droplets splash during heating, detection signals are low, batch determination efficiency is low, and the like, and the content of hexavalent chromium in soil and sediments can be accurately determined.

Description

Technical Field

[0001] The invention belongs to the technical field of determination of inorganic metal elements in soil and sediments, and particularly relates to a method for determining hexavalent chromium in soil and sediments. Background Art

[0002] Chromium is one of the important elements that pollute the environment and affect human health. Chromium in different valence states will produce different physiological effects. Hexavalent chromium is very toxic due to its oxidizing properties and high permeability to the skin and is identified as a carcinogen. In recent years, hexavalent chromium pollution incidents have occurred frequently, causing more and more hexavalent chromium to enter the soil and water environment, and chromium pollution has become increasingly prominent.

[0003] However, in actual operation, it was found that this method had serious problems such as cumbersome extraction steps, the large amount of flocculent precipitate generated in the early stage was difficult to separate, the polyethylene film was easy to break during the heating process, the detection signal was low, and the batch measurement efficiency was low, which caused different degrees of deviation in the measurement results. Summary of the invention

[0004] In view of the above problems, the present invention provides a method for determining hexavalent chromium in soil and sediments. The determination method is an improvement and upgrade of the national standard method. By selecting and optimizing the extraction solution, optimizing the extraction method, and optimizing the separation method, the efficiency and quality of processing batch samples are further improved. The method adopts a new alkaline leaching solution and a signal enhancer, an ultrasonic heating method, and a new determination process, which solves the problems of the difficulty in separating a large amount of flocculent precipitates generated in the early stage when separating hexavalent chromium from trivalent chromium in soil and sediments, splashing of droplets during heating, easy breakage of polyethylene membranes, low detection signals, and low batch determination efficiency. The method can accurately determine the content of hexavalent chromium in soil and sediments.

[0005] In order to achieve the above-mentioned object of the invention, the technical solution of the present invention is:

[0006] A method for determining hexavalent chromium in soil and sediment comprises the following steps:

[0007] 1) Weigh 5.0 g of air-dried, crushed, ground, and sieved soil sample with a 100-mesh sieve, place the soil sample in a 100 mL screw-capped plastic centrifuge tube, add 50 mL of alkaline extract, and shake well;

[0008] 2) Add 0.5 mL of potassium dihydrogen phosphate-potassium dihydrogen phosphate buffer solution to the centrifuge tube and shake well;

[0009] 3) Cover the centrifuge tube with a lid but do not tighten it completely, insert it into the centrifuge tube rack and put it into the ultrasound machine, then add 98±2℃ hot water to the ultrasound machine, turn on the ultrasound machine for 60 minutes, and turn on the heating function of the ultrasound machine at a heating temperature of 98±2℃;

[0010] 4) After the ultrasound is finished, take the centrifuge tube rack out of the ultrasound machine and cool it to room temperature. Tighten the centrifuge tube cap and place it in a centrifuge for centrifugal separation at a speed of 3000 r / min for 6 minutes.

[0011] 5) After centrifugation, take out the centrifuge tube, pour the supernatant into a 150mL beaker, add 1.0g of anhydrous sodium sulfate, stir to dissolve; then add 1 drop of phenolphthalein solution, adjust the pH value of the solution with concentrated nitric acid until the solution turns light pink, transfer the solution to a 100mL volumetric flask after cooling, dilute to the scale with water, shake well, and test.

[0012] 6) Drawing of standard working curve:

[0013] Add 0.00mL, 2.00mL, 4.00mL, 6.00mL, 8.00mL, 10.00mL, 15.00mL, and 20.00mL of the hexavalent chromium standard working solution to eight 100mL screw-capped empty plastic centrifuge tubes, respectively, and then add 50mL of the alkaline extract, and centrifuge according to step 3), step 4), and step 5) to obtain a standard working curve solution;

[0014] The absorbance value of the standard working curve solution is determined by flame atomic absorption spectrophotometry, and a standard working curve of hexavalent chromium concentration versus absorbance is drawn;

[0015] 7) Sample determination:

[0016] The absorbance of the sample solution was determined by flame atomic absorption spectrophotometry, and the concentration of the sample solution was found from the standard working curve, and the content of hexavalent chromium in the soil sample was further calculated;

[0017] 8) Make a blank for the whole procedure with the sample to obtain a blank sample and perform the measurement.

[0018] Furthermore, the step 3) is performed in a fume hood, and the power of the ultrasound machine is greater than or equal to 100W.

[0019] Furthermore, the alkaline extract of step 1) is prepared by weighing 6 g NH4C1 and dissolving it in water, adding 414 mL ammonia water, diluting it to 1 L with water, and storing it in a polyethylene bottle.

[0020] Furthermore, the preparation method of the dipotassium hydrogen phosphate-potassium dihydrogen phosphate buffer solution in step 2) is: weigh 87.1g of dipotassium hydrogen phosphate and 68.0g of potassium dihydrogen phosphate, dissolve them in water, and dilute them with water to 1L.

[0021] Furthermore, the preparation method of the phenolphthalein solution in step 5) is: weigh 0.1 g of phenolphthalein, dissolve it in a small amount of 90% ethanol solution, and transfer it to a 100 ml volumetric flask after the phenolphthalein is dissolved.

[0022] Furthermore, the preparation method of the hexavalent chromium standard working solution is: aspirate 50mL of the hexavalent chromium standard stock solution and place it in a 500mL volumetric flask, dilute it to the scale line with water, shake it well, and 1mL of the hexavalent chromium standard working solution contains 0.01mg of hexavalent chromium, which is prepared before use.

[0023] Furthermore, the preparation method of the hexavalent chromium standard stock solution is: weigh 0.2829 g of potassium dichromate dried at 105° C. for 2 hours, dissolve it in water, transfer it to a 1000 mL volumetric flask, dilute it to the scale with water, shake it well, and 1 mL of this solution contains 0.1 mg of hexavalent chromium.

[0024] Beneficial technical effects of the present invention:

[0025] 1. The present invention adopts a new alkaline extraction solution. Compared with the national standard method, it not only avoids the artificial generation of a large amount of flocculent precipitation, which causes the cumbersome separation and filtration in the subsequent steps; it also avoids the interference of high concentration of sodium ions and coexisting elements, which causes the problem of reduced analytical sensitivity during atomic absorption spectrometry; it can accurately, quickly and stably determine hexavalent chromium in soil and sediment, with a small standard deviation of the test results and a good spike recovery rate.

[0026] 2. The present invention adopts direct leaching in a 100mL screw-capped plastic centrifuge tube and then centrifugal separation, which not only avoids the loss of solution caused by repeated solution transfer, but also avoids the problems of droplet splashing during heating, easy breakage of polyethylene film, and reduced extraction rate, while greatly improving the efficiency of batch sample determination.

[0027] 3. The present invention adopts an ultrasonic-heating method to make the particle size of soil and sediment samples smaller and more dispersed, increase the contact area between the sample particles and the alkaline leaching solution, and ensure the efficiency of leaching hexavalent chromium from soil and sediment particles.

[0028] 4. The present invention adds anhydrous sodium sulfate as a signal enhancer, which greatly enhances the signal intensity of the flame atomic absorption spectrometry and ensures the sensitivity and accuracy of the determination method.

[0029] 5. In step 5), the present invention uses phenolphthalein solution as an acid-base indicator, so that the titration endpoint is clearer when adjusting the pH value of the solution. Specific embodiments

[0030] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments; based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. Example

[0031] This example adopts a method of adding a standard hexavalent chromium solution to a blank soil sample and a national standard material for analysis of hexavalent chromium in soil GBW07583. A hexavalent chromium standard solution is added to the sample, and a trivalent chromium standard solution is added as interference. The sample is extracted according to the steps specified in this method, and the volume is fixed to 100 mL. The hexavalent chromium content is determined by flame atomic absorption, and the recovery rate of the standard addition is calculated and the separation effect of hexavalent chromium and trivalent chromium is tested.

[0032] The specific implementation steps are as follows:

[0033] Step 1: Weigh 5.0000g of a sample containing hexavalent chromium and place it in a 100mL screw-capped plastic centrifuge tube, add a certain amount of hexavalent chromium standard solution, shake well and let it stand for adsorption for half an hour, then add a specified amount of trivalent chromium standard solution as interference, immediately add 50mL of alkaline extract to the centrifuge tube and shake well;

[0034] The preparation method of the hexavalent chromium standard working solution is as follows: take 50mL of the hexavalent chromium standard stock solution and place it in a 500mL volumetric flask, dilute it to the scale line with water, and shake it well. 1mL of the hexavalent chromium standard working solution contains 0.01mg of hexavalent chromium, and it is prepared before use;

[0035] The preparation method of the hexavalent chromium standard stock solution is as follows: weigh 0.2829 g of potassium dichromate dried at 105°C for 2 hours, dissolve it in water, transfer it to a 1000 mL volumetric flask, add water to dilute to the scale, shake well, and 1 mL of this solution contains 0.1 mg of hexavalent chromium.

[0036] The alkaline extract was prepared by weighing 6 g NH4C1 and dissolving it in water, adding 414 mL of aqueous ammonia, diluting it to 1 L with water, and storing it in a polyethylene bottle.

[0037] Step 2: Add 0.5 mL of potassium dihydrogen phosphate-potassium dihydrogen phosphate buffer solution to the centrifuge tube and shake well;

[0038] The preparation method of dipotassium hydrogen phosphate-potassium dihydrogen phosphate buffer solution is as follows: weigh 87.1g of dipotassium hydrogen phosphate and 68.0g of potassium dihydrogen phosphate, dissolve them in water, and dilute them with water to 1L.

[0039] Step 3: Cover the centrifuge tube with a lid but do not completely tighten it, insert it into the centrifuge tube rack and put it into the ultrasonic machine, then add 98±2℃ hot water into the ultrasonic machine, turn on the ultrasonic machine for ultrasonication, and the ultrasonication time is 60 minutes. At the same time, turn on the heating function of the ultrasonic machine, the heating temperature is 98±2℃, and the ultrasonic power is greater than or equal to 100W. This step is performed in a fume hood;

[0040] Step 4: After the ultrasonic treatment, the centrifuge tube rack is taken out of the ultrasonic machine and cooled to room temperature. The centrifuge tube cap is tightened and placed in a centrifuge for centrifugal separation at a speed of 3000 r / min for 6 min.

[0041] Step 5. After centrifugation, take out the centrifuge tube, pour the supernatant into a 150mL beaker, add 1.0g of anhydrous sodium sulfate, stir to dissolve; then add 1 drop of phenolphthalein solution, adjust the pH value of the solution to light pink with concentrated nitric acid, transfer the solution to a 100mL volumetric flask after cooling, dilute to the scale with water, shake well, and test;

[0042] The preparation method of phenolphthalein solution is as follows: weigh 0.1g of phenolphthalein, dissolve it in a small amount of 90% ethanol solution, and after the phenolphthalein is dissolved, transfer it to a 100ml volumetric flask.

[0043] Step 6: Drawing of standard working curve:

[0044] To eight 100mL screw-capped plastic centrifuge tubes, add 0.00mL, 2.00mL, 4.00mL, 6.00mL, 8.00mL, 10.00mL, 15.00mL, and 20.00mL of the hexavalent chromium standard working solution, respectively, and then add 50mL of the alkaline extract, and then perform centrifugation according to steps 3, 4, and 5 to obtain a standard working curve solution. Determine the absorbance of the standard curve solution by flame atomic absorption spectrophotometry, and draw a standard working curve of hexavalent chromium concentration versus absorbance.

[0045] Step 7, sample determination:

[0046] The absorbance value of the sample solution was determined by flame atomic absorption spectrophotometry, the concentration of the sample solution was found from the standard working curve, and the content of hexavalent chromium in the soil sample was further calculated.

[0047] Step eight, make a blank for the whole procedure with the sample, obtain a blank sample, and perform the measurement.

[0048] The standard working curve of this experiment is shown in Table 1.

[0049] Table 1 Hexavalent chromium standard working curve

[0050]

[0051] Table 2 Recovery of hexavalent chromium in soil samples

[0052] Blank - 1 Blank - 2 Blank - 3 Blank - 4 GBW07583 GBW07583 GBW07583 Chromium (VI) content mg / kg 0.0 0.0 0.0 0.0 3.6 3.6 3.6 The amount of chromium (VI) in the solution ug 0.0 0.0 0.0 0.0 18.0 18.0 18.0 Chromium (VI) added ug 0.0 20.0 50.0 100.0 20.0 50.0 100.0 Chromium (III) added ug 100.0 100.0 100.0 100.0 100.0 100.0 100.0 Chromium (VI) determination results ug 0.0 19.1 48.9 96.8 35.8 66.9 118.9 Recovery rate% 95.5 97.8 96.8 94.2 98.4 100.8

[0053] The results showed that the recoveries of hexavalent chromium at different spiked concentration levels were between 94.2% and 100.8%; the addition of trivalent chromium did not affect the determination of hexavalent chromium, indicating that this method can fully separate trivalent chromium and hexavalent chromium. This method is suitable for the analysis of hexavalent chromium in soil and sediment.

[0054] Although the present invention has been illustrated and described with specific embodiments, it should be appreciated that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Those skilled in the art should understand that the technical solutions described in the above embodiments may be modified, or some or all of the technical features thereof may be replaced by equivalents without departing from the spirit and scope of the present invention. However, these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention. Therefore, this means that all such replacements and modifications within the scope of the present invention are included in the appended claims.

[0055] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for determining hexavalent chromium in soil and sediment, characterized in that: The following steps are involved: 1) Weigh 5.0 g of air-dried, crushed, ground, and sieved soil sample with a 100-mesh sieve, place the soil sample in a 100 mL screw-capped plastic centrifuge tube, add 50 mL of alkaline extract, and shake well; 2) Add 0.5 mL of potassium dihydrogen phosphate-potassium dihydrogen phosphate buffer solution to the centrifuge tube and shake well; 3) Cover the centrifuge tube with a lid but do not tighten it completely, insert it into the centrifuge tube rack and put it into the ultrasound machine, then add 98±2℃ hot water to the ultrasound machine, turn on the ultrasound machine for 60 minutes, and turn on the heating function of the ultrasound machine at a heating temperature of 98±2℃; 4) After the ultrasound is finished, take the centrifuge tube rack out of the ultrasound machine and cool it to room temperature. Tighten the centrifuge tube cap and place it in a centrifuge for centrifugal separation at a speed of 3000 r / min for 6 minutes. 5) After centrifugation, take out the centrifuge tube, pour the supernatant into a 150mL beaker, add 1.0g of anhydrous sodium sulfate, stir to dissolve; then add 1 drop of phenolphthalein solution, adjust the pH value of the solution with concentrated nitric acid until the solution turns light pink, transfer the solution to a 100mL volumetric flask after cooling, dilute to the scale with water, shake well, and wait for testing; 6) Drawing of standard working curve: Add 0.00mL, 2.00mL, 4.00mL, 6.00mL, 8.00mL, 10.00mL, 15.00mL, and 20.00mL of the hexavalent chromium standard working solution to eight 100mL screw-capped empty plastic centrifuge tubes, respectively, and then add 50mL of the alkaline extract, and centrifuge according to step 3), step 4), and step 5) to obtain a standard working curve solution; The absorbance value of the standard working curve solution is determined by flame atomic absorption spectrophotometry, and a standard working curve of hexavalent chromium concentration versus absorbance is drawn; 7) Sample determination: The absorbance of the sample solution was determined by flame atomic absorption spectrophotometry, and the concentration of the sample solution was found from the standard working curve, and the content of hexavalent chromium in the soil sample was further calculated; 8) Make a blank for the whole procedure along with the sample to obtain a blank sample and perform the measurement; Wherein, the step 3) is carried out in a fume hood, and the power of the ultrasonic machine is greater than or equal to 100W; the preparation method of the alkaline extract in the step 1) is: weigh 6g NH4C1 and dissolve it in water, add 414mL ammonia water, dilute it with water to 1L, and store it in a polyethylene bottle.

2. The method for determining hexavalent chromium in soil and sediment according to claim 1, characterized in that: The preparation method of the dipotassium hydrogen phosphate-potassium dihydrogen phosphate buffer solution in step 2) is as follows: 87.1 g of dipotassium hydrogen phosphate and 68.0 g of potassium dihydrogen phosphate are weighed and dissolved in water, and then diluted with water to a constant volume of 1 L.

3. The method for determining hexavalent chromium in soil and sediment according to claim 1, characterized in that: The preparation method of the phenolphthalein solution in step 5) is as follows: weigh 0.1 g of phenolphthalein, dissolve it in a small amount of 90% ethanol solution, and transfer it to a 100 ml volumetric flask after the phenolphthalein is dissolved.

4. The method for determining hexavalent chromium in soil and sediment according to claim 1, characterized in that: The preparation method of the hexavalent chromium standard working solution is as follows: 50 mL of the hexavalent chromium standard stock solution is taken and placed in a 500 mL volumetric flask, diluted to the scale line with water, and shaken well. 1 mL of the hexavalent chromium standard working solution contains 0.01 mg of hexavalent chromium, and the solution is prepared before use.

5. The method for determining hexavalent chromium in soil and sediment according to claim 4, characterized in that: The preparation method of the hexavalent chromium standard stock solution is as follows: weigh 0.2829 g of potassium dichromate dried at 105° C. for 2 hours, dissolve it in water, transfer it to a 1000 mL volumetric flask, add water to dilute to the scale, shake well, and 1 mL of this solution contains 0.1 mg of hexavalent chromium.

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