Sample pretreatment method for measuring contents of chromium and cadmium elements in recycled mineral oil by ICP-OES

Abstract

The disclosed sample pretreatment method for determining the content of chromium and cadmium elements in recycled mineral oil through ICP-OES comprises the following steps: S1, accurately weighing 0.2-0.5 g of a recycled mineral oil sample, adding 3-7mL of concentrated nitric acid with the concentration of 98 wt% or above into the sample, then heating to 70-80 DEG C, dropwise adding 3-7mL of hydrogen peroxide with the concentration of 25-35 wt% into the sample, keeping stirring in the dropwise adding process, and keeping stirring for 0.5-1 hour; after dropwise adding, heating the sample below the boiling point until no obvious bubbles emerge; s2, the sample obtained in the step S1 is heated to 103-107 DEG C and subjected to heat preservation for 2-3 h, after heat preservation is finished, the temperature continues to rise to 750-850 DEG C, and sealed calcination is conducted for 40-80 min at the temperature; s3, after calcination is finished, digestion and acid removal are carried out in the mode that concentrated nitric acid is added into calcination residues for multiple times and heating digestion is carried out for multiple times, and finally a pretreated sample with the liquid smaller than 0.5 mL is obtained. According to the pretreatment method, the loss of chromium and cadmium elements in the sample is reduced, calcining residues can be well digested, and the accuracy of subsequent determination results of the contents of the chromium and cadmium elements through ICP-OES is effectively improved.

Description

Technical Field

[0001] The invention relates to a sample pretreatment method, in particular to a sample pretreatment method for determining the content of chromium and cadmium elements in recovered mineral oil by ICP-OES. Background Art

[0002] With the gradual strengthening of people's environmental awareness and the rapid development of the environmental protection industry, the disposal methods of hazardous wastes have gradually transitioned from the previous reduction and harmlessness to resource utilization in recent years. Waste mineral oil is extremely harmful to the human body because it contains heavy metals such as cadmium, chromium, lead, and arsenic. Therefore, it was listed as a hazardous waste in the "National List of Hazardous Wastes" (Huanfa

[1998] No. 89). The latest National List of Hazardous Wastes (2021 Edition) has made a detailed classification and description of HW08 waste mineral oil and mineral oil-containing waste. In the early stage of the reduction and harmless disposal of hazardous wastes, most of the waste mineral oil was directly incinerated as fuel, which caused great waste. In recent years, as countries have gradually increased their attention to the resource utilization of waste mineral oil, the technology for the resource utilization of waste mineral oil has become more perfect.

[0003] In the process of recycling waste mineral oil, it is necessary to detect heavy metals in the recycled mineral oil. At present, heavy metal detection is mainly carried out using ICP-OES. When testing recycled mineral oil, due to its wide sources, complex composition and unclear properties, it is necessary to pre-treat the recycled mineral oil samples before testing to ensure the accuracy of the results of ICP-OES determination.

[0004] Existing pretreatment methods for determining chromium and cadmium in recovered mineral oil usually include nitric acid digestion and concentrated nitric acid digestion after calcination. Since the recovered mineral oil contains a large amount of high-molecular organic matter and a relatively large number of light components, and is flammable, when using conventional nitric acid digestion or calcination digestion for pretreatment, the digestion is either incomplete or causes the loss of chromium and cadmium in the sample, thereby affecting the accuracy of the subsequent chromium and cadmium content determination results by ICP-OES. Summary of the invention

[0005] In view of the existing sample pretreatment method described in the background technology, when it is applied to recovered mineral oil, there is a problem that the accuracy of the determination results of the chromium and cadmium content is not ideal. The present application provides a sample pretreatment method for determining the chromium and cadmium content in the recovered mineral oil by ICP-OES, which comprises the following steps:

[0006] S1. Accurately weigh 0.2-0.5g of the recovered mineral oil sample, add 3-7mL of concentrated nitric acid with a concentration of more than 98wt% to the sample, then heat to 70-80°C, and dropwise add 3-7mL of 25-35wt% hydrogen peroxide to the sample, stirring is maintained during the addition process, and after the addition is completed, heat the sample below the boiling point until no obvious bubbles emerge;

[0007] S2. The sample obtained in step S1 is heated to 103-107°C and kept at this temperature for 2-3 hours. After the end of the heat preservation, the temperature is further raised to 750-850°C, and sealed and calcined at this temperature for 1 hour;

[0008] S3. After the sample calcination in step S2 is completed, concentrated nitric acid is added to the calcination residue several times and heated for digestion to remove the acid, and finally a pretreated sample with a liquid content of less than 0.5 mL is obtained.

[0009] Preferably, in step S1, 0.3 g of the recovered mineral oil sample is accurately weighed into a beaker, 5 mL of concentrated nitric acid with a concentration of more than 98 wt% is added to the sample in the beaker, and then heated to 75°C, 5 mL of 30 wt% hydrogen peroxide is added dropwise to the sample in the beaker, and stirring is maintained during the addition process. After the addition is completed, the sample in the beaker is heated below the boiling point until no obvious bubbles emerge.

[0010] Preferably, in step S2, the sample obtained in step S1 is transferred to a crucible and placed in a muffle furnace, first heated to 105°C in the muffle furnace and kept warm for 2.5 hours, then heated to 800°C after the insulation is completed, and sealed and calcined for 1 hour at this temperature with the lid closed.

[0011] Preferably, in step S3, after the sample calcination in step S2 is completed, the crucible is taken out from the muffle furnace and cooled to room temperature, the remaining residue in the crucible is transferred to a beaker, and the calcination residue is digested and acid is driven out by adding concentrated nitric acid to the calcination residue multiple times and heating and digesting multiple times, and finally a pretreated sample with a liquid content of less than 0.5 mL is obtained in the beaker.

[0012] Preferably, the digestion and acid removal is carried out according to the following process: 2 ml of 98 wt% concentrated nitric acid is added to the calcination residue in the beaker, and then placed in a digital constant temperature magnetic stirrer for heating and digestion. The heating and digestion temperature is 170° C. After heating and digestion until the liquid is less than 0.5 ml, 1 ml of 98 wt% concentrated nitric acid is added, and the above steps are repeated several times.

[0013] Through the above technical solution, the present invention at least includes the following beneficial effects:

[0014] The sample pretreatment method for ICP-OES determination of chromium and cadmium content in recovered mineral oil described in the present application has been proved through experiments that its precision for detecting chromium and cadmium content in recovered mineral oil samples meets the requirements; in the sample pretreatment process, hydrogen peroxide is first used to allow the light components in the recovered mineral oil to escape, then the oil is subjected to high-temperature calcination, and finally digestion and acid removal are performed to allow the light components in the recovered mineral oil to escape, thereby avoiding the generation of open flames and black smoke during the calcination process, thereby reducing the loss of chromium and cadmium elements, and the calcination residues can also be better digested, thereby effectively improving the accuracy of the subsequent chromium and cadmium content determination results by ICP-OES. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is the temperature control curve of Example 1 of the present application;

[0016] Figure 2 is the standard curve of chromium element and the linear regression equation;

[0017] Figure 3 is the standard curve of cadmium element and the linear regression equation. DETAILED DESCRIPTION

[0018] The technical solution of the invention will be clearly and completely described below through embodiments, and the application of the present invention is not limited to the embodiments given;

[0019] And two comparative examples are used to illustrate the superiority of the technical solution of the present invention. The samples pretreated in each embodiment and each comparative example are all from the same waste mineral oil recovered by our company.

[0020] Example 1

[0021] A sample pretreatment method for ICP-OES determination of chromium and cadmium content in recovered mineral oil comprises the following steps:

[0022] S1. Accurately weigh 0.3g of the recovered mineral oil sample into a beaker, add 5mL of concentrated nitric acid with a concentration of more than 98wt% to the sample in the beaker, then heat to 75°C, and drop 5mL of 30wt% hydrogen peroxide to the sample in the beaker. Stirring must be maintained during the addition process. After the addition is completed, heat the sample in the beaker below the boiling point until no obvious bubbles emerge.

[0023] S2. Transfer the sample obtained in step S1 to a crucible and place it in a muffle furnace. First, heat it to 105°C in the muffle furnace and keep it warm for 2.5 hours. After the insulation is completed, continue to heat it to 800°C and calcine it at this temperature with the lid closed for 1 hour.

[0024] S3. After the sample calcination in step S2 is completed, the crucible is taken out from the muffle furnace and cooled to room temperature, the remaining residue in the crucible is transferred to a beaker, and the acid is removed by adding concentrated nitric acid to the calcination residue multiple times and heating and digesting multiple times. The specific digestion and acid removal process is as follows: add 2 ml of 98wt% concentrated nitric acid to the calcination residue in the beaker, and then place it in a digital constant temperature magnetic stirrer for heating and digestion. The heating and digestion temperature is 170°C. After heating and digestion until the liquid is less than 0.5 mL, continue to add 1 mL of 98wt% concentrated nitric acid, repeat the above steps several times, until finally less than 0.5 mL of liquid is obtained in the beaker. Pretreated sample after the pretreatment.

[0025] Example 2

[0026] A sample pretreatment method for ICP-OES determination of chromium and cadmium content in recovered mineral oil comprises the following steps:

[0027] S1. Accurately weigh 0.2g of the recovered mineral oil sample into a beaker, add 3mL of concentrated nitric acid with a concentration of more than 98wt% to the sample in the beaker, then heat to 70°C, and drop 3mL of 35wt% hydrogen peroxide to the sample in the beaker. Stirring must be maintained during the addition process. After the addition is completed, heat the sample in the beaker below the boiling point until no obvious bubbles emerge.

[0028] S2. The sample obtained in step S1 was transferred to a crucible and placed in a muffle furnace. The crucible was first heated to 103°C in the muffle furnace and kept warm for 3 hours. After the insulation was completed, the temperature was continued to be raised to 750°C. At this temperature, the crucible was sealed and calcined for 80 minutes in a closed state.

[0029] S3. After the sample calcination in step S2 is completed, the crucible is taken out from the muffle furnace and cooled to room temperature, the remaining residue in the crucible is transferred to a beaker, and the acid is removed by adding concentrated nitric acid to the calcination residue multiple times and heating and digesting multiple times. The specific digestion and acid removal process is as follows: add 2 ml of 98wt% concentrated nitric acid to the calcination residue in the beaker, and then place it in a digital constant temperature magnetic stirrer for heating and digestion. The heating and digestion temperature is 170°C. After heating and digestion until the liquid is less than 0.5 mL, continue to add 1 mL of 98wt% concentrated nitric acid, repeat the above steps several times, until finally less than 0.5 mL of liquid is obtained in the beaker. Pretreated sample after the pretreatment.

[0030] Example 3

[0031] A sample pretreatment method for ICP-OES determination of chromium and cadmium content in recovered mineral oil comprises the following steps:

[0032] S1. Accurately weigh 0.5g of the recovered mineral oil sample into a beaker, add 7mL of concentrated nitric acid with a concentration of more than 98wt% to the sample in the beaker, then heat to 80°C, and drop 7mL of 25wt% hydrogen peroxide to the sample in the beaker. Stirring must be maintained during the addition process. After the addition is completed, heat the sample in the beaker below the boiling point until no obvious bubbles emerge.

[0033] S2. The sample obtained in step S1 was transferred to a crucible and placed in a muffle furnace. The crucible was first heated to 107°C in the muffle furnace and kept warm for 2 hours. After the insulation was completed, the temperature was continued to be raised to 850°C. At this temperature, the crucible was sealed and calcined for 40 minutes with the lid closed.

[0034] S3. After the sample calcination in step S2 is completed, the crucible is taken out from the muffle furnace and cooled to room temperature, the remaining residue in the crucible is transferred to a beaker, and the acid is removed by adding concentrated nitric acid to the calcination residue multiple times and heating and digesting multiple times. The specific digestion and acid removal process is as follows: add 2 ml of 98wt% concentrated nitric acid to the calcination residue in the beaker, and then place it in a digital constant temperature magnetic stirrer for heating and digestion. The heating and digestion temperature is 170°C. After heating and digestion until the liquid is less than 0.5 mL, continue to add 1 mL of 98wt% concentrated nitric acid, repeat the above steps several times, until finally less than 0.5 mL of liquid is obtained in the beaker. Pretreated sample after the pretreatment.

[0035] Comparative Example 1

[0036] This comparative example uses conventional nitric acid digestion method to pretreat the recovered mineral oil sample, and the specific steps are as follows:

[0037] First, accurately weigh 0.3g of the recovered mineral oil sample into a beaker, then add 3ml of concentrated nitric acid with a concentration of more than 98wt%, place the beaker with the recovered mineral oil and nitric acid in a digital constant temperature magnetic stirrer for heating and digestion at a temperature of 165°C, and continue to add 2ml of nitric acid after heating and digestion until the liquid is less than 0.5ml, and repeat the above steps until the liquid in the beaker is less than 0.5ml. The liquid is the pretreated sample.

[0038] Comparative Example 2

[0039] This comparative example uses a conventional calcination digestion method to pretreat the recovered mineral oil sample, and the specific steps are as follows:

[0040] Accurately weigh 0.3g of recycled waste mineral oil into a crucible that has been dried to constant weight, place the crucible containing the recycled mineral oil in a muffle furnace, and keep it at 105°C for 2.5h. After the insulation, continue to heat it to 600°C and calcine it at 600°C for 1h. After the calcination is completed and cooled to room temperature, transfer the remaining residue in the crucible to a beaker, and after digestion and acid removal, obtain a pre-treated sample with a liquid content of less than 0.5mL.

[0041] The recovered mineral oil samples after pretreatment in the above embodiments and comparative examples were subjected to the standard curve method using a full-spectrum direct-reading inductively coupled plasma emission spectrometer (ICP-OES) produced by Shanghai Meixi Instrument Co., Ltd. The chromium and cadmium content in the samples was quantitatively determined, and the precision of the test results of the pretreated samples was determined by the spiked recovery method.

[0042] Preparation of ICP multi-element standard solution: accurately pipette 5.00ml, 2.00ml, 1.00ml of chromium and cadmium standard solution (concentration 100mg / L) into a 100ml volumetric flask respectively and dilute to obtain chromium and cadmium standard solutions A (5mg / L), B (2mg / L), and C (1mg / L); accurately pipette 10ml of chromium and cadmium standard solution A (5mg / L) into a 100ml volumetric flask and dilute to obtain chromium and cadmium standard solution D (0.5mg / L); accurately pipette 10ml of chromium and cadmium standard solution B (5mg / L) into a 100ml volumetric flask and dilute to obtain chromium and cadmium standard solution E (0.5mg / L); pipette 100ml of dilute nitric acid (concentration 1%) into a 100ml volumetric flask and dilute to obtain a blank sample.

[0043] The test results of the prepared cadmium and chromium element standard solutions are shown in Table 1 below. The drawn chromium and cadmium element standard curves and the established linear regression equations are shown in Figure 2 and Figure 3 shown.

[0044] Table 1:

[0045]

[0046] Detection of chromium and cadmium in recovered mineral oil samples after pretreatment by different methods:

[0047] First, the recovered mineral oil samples were pre-treated in each embodiment and each comparative example, respectively transferred to a 50 mL volumetric flask with 1 wt % dilute nitric acid, and then diluted to the scale with 1 wt % dilute nitric acid and allowed to stand for use;

[0048] ICP-OES was used to quantitatively determine the contents of chromium and cadmium in the samples by the standard curve method. The test results are shown in Table 2 below;

[0049] The precision of the test results was determined by the spike recovery method. The effects of different pretreatment methods on the spike recovery rate of cadmium are shown in Table 3 below, and the spike amount is 5 mg / L; the effects of different pretreatment methods on the spike recovery rate of chromium are shown in Table 4, and the spike amount is 10 mg / L.

[0050] Table 2:

[0051]

[0052] Table 3:

[0053]

[0054]

[0055] Table 4:

[0056]

[0057] The analysis of the chromium and cadmium content of the recovered mineral oil samples after different pretreatments is as follows:

[0058] It can be seen from the cadmium spike recovery rate and RSD values ​​in Table 3 that the pretreatment methods of each embodiment and each comparative example meet the requirements for the precision of detecting the cadmium content in the recovered mineral oil samples. It can be seen from the chromium spike recovery rate and RSD values ​​in Table 4 that the pretreatment methods of each embodiment and each comparative example meet the requirements for the precision of detecting the chromium content in the recovered mineral oil samples.

[0059] On the premise that the precision of the test results meets the requirements, it can be seen from Table 2 that the three embodiments of pretreating the recovered mineral oil samples using the technical solution of the present application, the detected chromium and cadmium content are greater than the conventional comparative example 1 (nitric acid digestion pretreatment) and comparative example 2 (calcination digestion pretreatment), with higher accuracy.

[0060] During the sample pretreatment using conventional nitric acid digestion, since the recovered mineral oil contains a large amount of high molecular weight organic matter, there are still obvious oil stains that cannot be digested at the end of the digestion, resulting in a low content test result through ICP-OES. The RSD of the cadmium element test result is as high as 14.69%, which can also indirectly prove that the digestion during pretreatment is not thorough.

[0061] Since the recovered mineral oil itself is flammable and contains light components, when the conventional calcination digestion method is used for sample pretreatment, obvious black smoke and open flames are generated during the calcination process, which leads to the loss of chromium and cadmium elements in the test samples, and the accuracy of the subsequent content determination results by ICP-OES is reduced. It can also be seen from the RSD of the calcination digestion method in the table that the discrete degree between the multiple measurement results is large, the test results fluctuate greatly, and the accuracy of the measurement results is affected;

[0062] The sample pretreatment method for ICP-OES determination of chromium and cadmium content in recovered mineral oil described in the present application, during the sample pretreatment process, firstly uses hydrogen peroxide to make the light components in the recovered mineral oil escape, then calcines it at high temperature, and finally digests and removes the acid, so that the light components in the recovered mineral oil can escape, which can avoid the generation of open flames and black smoke during the calcination process, reduce the loss of chromium and cadmium elements, and the calcination residue can also be better digested, thereby effectively improving the accuracy of the subsequent chromium and cadmium content determination results by ICP-OES.

[0063] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Based on the present invention and the above description, relevant personnel can make various changes and modifications without departing from the technical idea of ​​the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A sample pretreatment method for determining the content of chromium and cadmium in recovered mineral oil by ICP-OES, characterized in that: The following steps are involved: S1. Accurately weigh 0.2-0.5g of the recovered mineral oil sample, add 3-7mL of concentrated nitric acid with a concentration of more than 98wt% to the sample, then heat to 70-80°C, and dropwise add 3-7mL of 25-35wt% hydrogen peroxide to the sample, stirring is maintained during the addition process, and after the addition is completed, heat the sample below the boiling point until no obvious bubbles emerge; S2. The sample obtained in step S1 is heated to 103-107°C and kept at this temperature for 2-3h. After the end of the heat preservation, the temperature is further raised to 750-850°C, and sealed and calcined at this temperature for 40-80min; S3. After the sample calcination in step S2 is completed, concentrated nitric acid is added to the calcination residue several times and heated for digestion to remove the acid, and finally a pretreated sample with a liquid content of less than 0.5 mL is obtained.

2. The sample pretreatment method for determining the chromium and cadmium content in recovered mineral oil by ICP-OES according to claim 1, characterized in that: In step S1, 0.3 g of the recovered mineral oil sample is accurately weighed into a beaker, 5 mL of concentrated nitric acid with a concentration of more than 98 wt% is added to the sample in the beaker, and then heated to 75°C, 5 mL of 30 wt% hydrogen peroxide is added dropwise to the sample in the beaker, and stirring is maintained during the addition process. After the addition is completed, the sample in the beaker is heated below the boiling point until no obvious bubbles emerge.

3. The sample pretreatment method for determining the chromium and cadmium content in recovered mineral oil by ICP-OES according to claim 2, characterized in that: In step S2, the sample obtained in step S1 is transferred to a crucible and placed in a muffle furnace. It is first heated to 105°C in the muffle furnace and kept warm for 2.5 hours. After the insulation is completed, the temperature is continued to be raised to 800°C, and it is sealed and calcined for 1 hour at this temperature with the lid closed.

4. The sample pretreatment method for determining the chromium and cadmium content in recovered mineral oil by ICP-OES according to claim 3, characterized in that: In step S3, after the sample calcination in step S2 is completed, the crucible is taken out from the muffle furnace and cooled to room temperature, the remaining residue in the crucible is transferred to a beaker, and the calcination residue is digested and acid is removed by adding concentrated nitric acid to the calcination residue multiple times and heating and digesting multiple times, and finally a pretreated sample with a liquid content of less than 0.5 mL is obtained in the beaker.

5. The sample pretreatment method for determining the chromium and cadmium content in recovered mineral oil by ICP-OES according to claim 4, characterized in that: The digestion and acid removal are carried out according to the following process: 2 ml of 98 wt % concentrated nitric acid is added to the calcination residue in the beaker, and then placed in a digital constant temperature magnetic stirrer for heating and digestion. The temperature of the heating and digestion is 170° C. After the liquid is less than 0.5 ml, 1 ml of 98 wt % concentrated nitric acid is added, and the above steps are repeated several times.

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