A method for reducing and dechlorinating a composite surfactant-coupled sulfide micron zero-valent iron system

Abstract

The present invention discloses a method for reducing dechlorinated chlorohydrocarbons by coupling a compound surfactant with a sulfide micron zero-valent iron system, and belongs to the technical field of in-situ chemical remediation of water bodies. The present invention uses a compound surfactant HTAB and Tween80 coupled with a sulfide micron zero-valent iron material as a reducing agent to reduce chlorinated hydrocarbons in water bodies. Under the effective regulation of the compound surfactant, the sulfide micron zero-valent iron is promoted to efficiently reduce carbon tetrachloride. The compound surfactant coupled with the sulfide micron zero-valent iron is superior in reusability to the HTAB coupled with the sulfide micron zero-valent iron system, and has a higher removal rate for chlorinated hydrocarbons (carbon tetrachloride). The surfactant compound system reduces the amount and cost of the cationic surfactant HTAB without causing secondary pollution to the environment. In addition, the method provided by the present invention has a simple preparation process, a fast reaction rate, a strong ability to remove a variety of chlorinated hydrocarbon pollutants, a wide applicable pH range, and is more suitable for practical applications.

Description

Technical Field

[0001] The invention relates to a method for reducing and dechlorinating a composite surfactant-coupled sulfide micron zero-valent iron system, belonging to the technical field of in-situ chemical remediation of water environments. Background Art

[0002] Chlorinated hydrocarbons are widely used in industry, agriculture, and manufacturing as dry-cleaning solvents, metal degreasers, and refrigerants. However, their excessive use and improper discharge lead to water and soil pollution. Because most chlorinated hydrocarbons are difficult to degrade, bioaccumulate, and migrate over long distances, untreated they pose long-term risks to the ecological environment and human health. Carbon tetrachloride, chloroform, trichloroethylene, and tetrachloroethylene have been included on my country's "blacklist" of 68 priority pollutants.

[0003] Currently, the commonly used remediation technologies for chlorinated hydrocarbons in water bodies are mainly divided into three categories: physical, biological, and chemical. Microbial degradation usually utilizes the growth and metabolic processes of anaerobic or aerobic microorganisms to degrade chlorinated hydrocarbons, but the slow degradation rate and long remediation cycle of microbial remediation technology limit its application scenarios. Chemical oxidation includes photocatalytic degradation and advanced oxidation. Photocatalytic degradation mostly requires ultraviolet light irradiation, while visible light catalysts have been slow to develop and difficult to apply on a large scale. Advanced oxidation technology uses strong oxidizing free radicals to quickly mineralize pollutants. Although it can quickly and efficiently degrade chlorinated organic pollutants, the catalysts are extremely expensive and difficult to separate and reuse.

[0004] In contrast, chemical reduction remediation technology using micro-nano-sized zero-valent iron as a reducing agent is widely used in sites contaminated with chlorinated organic pollutants, aromatic compounds, and heavy metals due to its readily available raw materials, low cost, and strong electron-donating capacity. To address the susceptibility of micro-nano-sized zero-valent iron to aging, rapid activity loss, and poor mobility during use, a sulfide-modified zero-valent iron technology has been developed. However, due to the inefficient contact between hydrophobic chlorinated hydrocarbons and the sulfide-modified zero-valent iron, the sulfide-modified zero-valent iron particles aggregate in water, hindering the dechlorination effect. To increase the contact efficiency between the chlorinated hydrocarbons and the sulfide-modified zero-valent iron, and to stabilize its mobility and electron-donating capacity in water, the present invention provides a method for dechlorinating chlorinated hydrocarbons using a composite surfactant coupled with sulfide-modified micronized zero-valent iron. Summary of the Invention

[0005] In response to the above-mentioned shortcomings of the existing sulfide zero-valent iron chemical reduction remediation technology, the present invention provides a method for removing chlorinated organic pollutants in water bodies by coupling a composite surfactant with sulfide micron zero-valent iron material, which has a simple preparation process, a fast reaction rate, a strong ability to remove complex pollutants, biodegradable reagents, and no secondary pollution.

[0006] The technical solution of the present invention:

[0007] The present invention provides a method for removing chlorinated hydrocarbons in water based on a compound surfactant coupled with a sulfide micron zero-valent iron system. The method comprises: adding a compound surfactant coupled with a sulfide micron zero-valent iron material to a water body containing chlorinated hydrocarbons to reduce the chlorinated hydrocarbons in the water body; wherein the compound surfactant comprises cetyltrimethylammonium bromide (HTAB) and Tween80.

[0008] It is further defined that the concentrations of the HTAB and Tween 80 complex system are 0.08 g / L and 0.03 g / L, respectively.

[0009] Further, the S / Fe molar ratio of the sulfided micron zero-valent iron is 0.168. Based on existing ball milling methods for preparing sulfided micron zero-valent iron, the sulfided micron zero-valent iron in the present invention is prepared by ball milling sublimed sulfur powder and 400-mesh micron zero-valent iron at 500 rpm for 6 hours. During the ball milling process, 25 zirconia balls with a diameter of 10 mm and 150 zirconia balls with a diameter of 5 mm are used. The prepared sulfided micron zero-valent iron is freeze-dried for 24 hours and then stored in a nitrogen-filled glove box.

[0010] It is further defined that the preparation of the compound surfactant-coupled sulfide micron zero-valent iron material is to add 0.3 g of sulfide micron zero-valent iron material to a 100 mL serum bottle containing HTAB and Tween80 at concentrations of 0.08 g / L and 0.03 g / L, respectively, and oscillate and balance at a speed of 150 r / min in a constant temperature shaking incubator for 1 hour.

[0011] It is further defined that the concentration of the compound surfactant coupled sulfide micron zero-valent iron material is 3.0 g / L.

[0012] It is further specified that the chlorinated hydrocarbon pollutant contained in the water body is carbon tetrachloride, with an initial concentration of 20 mg / L.

[0013] It is further defined that the degradation temperature is 20°C-25°C, the time is 1h-48h, and the oscillation rate is 150r / min.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] (1) The present invention uses zero-valent iron with a particle size of 400 mesh as the sulfurized modification object to prepare sulfurized micron zero-valent iron. Although both micron-scale zero-valent iron and nano-scale zero-valent iron have excellent properties for reducing chlorinated hydrocarbons, since the degradation of pollutants by zero-valent iron is a surface area reaction, the specific surface area normalized reaction rate constant of the degradation of chlorinated hydrocarbons by using micron-scale zero-valent iron is of the same order of magnitude as that of nano-scale zero-valent iron. Under the same conditions, micron-scale zero-valent iron corrodes more slowly and has lower sensitivity to side reactions. In addition, micron-scale zero-valent iron has the advantages of lower cost, simpler raw material acquisition, and less agglomeration during use, good dispersion, and more uniform contact with pollutants. Therefore, it can be considered that micron-scale zero-valent iron is more suitable for field applications.

[0016] (2) The present invention uses sulfur to modify the micron zero-valent iron. After sulfurization, the surface of the micron zero-valent iron forms FeS, FeS2, Fe x S y Iron sulfides such as iron sulfides make the surface rougher and the specific surface area larger, increasing the hydrophobicity of the micronized zero-valent iron, enhancing the adsorption of hydrophobic organic matter, and increasing the surface reaction rate of the zero-valent iron. Iron sulfides themselves are also relatively strong reducing agents, capable of reducing a variety of organic pollutants such as trichloroethylene and carbon tetrachloride. The combined reaction of iron sulfides and zero-valent iron can increase the degradation rate of organic chlorinated pollutants. At the same time, iron sulfides FeS and FeS2 are excellent conductive materials. Due to the presence of delocalized electrons between the iron sulfide layers, they can act as electron conductors to promote electron transfer from the interior of the zero-valent iron to the surface, increasing its conductivity while improving the reduction rate of organic pollutants.

[0017] (3) The surfactant used in the present invention has good biodegradability. Surfactants are now mostly used in the research of surface modification of nanomaterials and water and soil remediation. Because chlorinated organic pollutants have low water solubility, they are easily adsorbed on soil and sediments in actual contaminated sites. Although the introduction of sulfur element can greatly improve the selective chemical reaction rate of micron zero-valent iron on pollutants, sulfide micron zero-valent iron cannot degrade chlorinated organic pollutants adsorbed on solids and sediments. Therefore, in the actual application process, it is very important to improve the contact efficiency between sulfide zero-valent iron and chlorinated organic pollutants. Surfactants are amphiphilic chemical substances that contain both hydrophilic groups and hydrophobic groups. When the surfactant concentration is greater than or equal to the critical micelle concentration, the surfactant will form micelles with a hydrophilic surface and a hydrophobic core, in which the hydrophobic groups of the surfactant molecules are connected to the inner micelles and the hydrophilic end is connected to the water phase. At this time, the hydrophobic groups of the chlorinated organic pollutants are connected to the core of the micelle formed by the surfactant to form a hydrophobic core, which increases the solubility of the chlorinated organic pollutants in water by reducing the surface tension. At the same time, surfactants can provide additional electrostatic repulsion and steric hindrance to inhibit the agglomeration of zero-valent iron particles, improve their dispersibility and stability in the suspension, and extend their service life.

[0018] Furthermore, the present invention adopts a cationic surfactant cetyltrimethylammonium bromide (HTAB) and a non-ionic surfactant Tween 80 in a compounded form. Compared with the use of cetyltrimethylammonium bromide or Tween 80 alone, it has the following advantages: First, the cationic surfactant cetyltrimethylammonium bromide and the non-ionic surfactant Tween-80 are used in a compounded form. Through the interaction between the polar polyoxyethylene group of the non-ionic surfactant and the ionic group of the cationic surfactant, the critical gel concentration of the cationic surfactant can be reduced, the dosage of the two surfactants is reduced, and the cost of use is reduced; second, the cationic surfactant cetyltrimethylammonium bromide and the non-ionic surfactant Tween 80 are used in a compounded form, which overcomes the problem of large adsorption loss and high concentration requirement when the cationic surfactant is used alone, which leads to the generation of biological toxicity. The present invention utilizes a combination of two surfactants to increase the dispersibility and service life of sulfided micronized zero-valent iron in water. After ten cycles, it can also achieve efficient removal of chlorinated hydrocarbons (such as carbon tetrachloride) from water. This solves the problem of surface passivation during long-term use of existing zero-valent iron and sulfided zero-valent iron materials, which reduces the removal rate and degradation rate of chlorinated organic pollutants (such as carbon tetrachloride). Furthermore, even at very low surfactant concentrations, the present invention can ensure excellent removal efficiency, significantly reducing treatment costs. Furthermore, the present invention is applicable over a wide pH range, uses highly biodegradable reagents, and does not cause secondary pollution. Furthermore, the present invention has the advantages of a simple preparation process and a relatively fast reaction rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a graph showing the efficiency of carbon tetrachloride degradation by sulfided micronized zero-valent iron with different molar ratios prepared in Example 1;

[0020] Figure 2 This is a graph showing the effect of different addition amounts of sulfided micron zero-valent iron with an S / Fe molar ratio of 0.168 on the degradation rate of carbon tetrachloride;

[0021] Figure 3 This is a graph showing the effect of different concentrations of HTAB coupled with S-mZVI on the degradation rate of carbon tetrachloride in Example 3;

[0022] Figure 4 This is a graph showing the effect of different concentrations of Tween80 coupled with S-mZVI on the degradation rate of carbon tetrachloride in Example 4;

[0023] Figure 5 This is a graph showing the effect of the complex coupling of HTAB and Tween80 on the degradation rate of carbon tetrachloride in Example 5;

[0024] Figure 6 This is a graph showing the effect of the composite surfactant coupled with S-mZVI on the reusability of carbon tetrachloride degradation in Example 6;

[0025] Figure 7 This is a diagram showing the effect of S-mZVI coupled with HTAB and Tween80 on the reduction and dechlorination of different chlorinated hydrocarbons in Example 7;

[0026] Figure 8 This is a graph showing the effect of different concentrations of Trition-100 coupled with S-mZVI on the degradation rate of carbon tetrachloride in Comparative Example 1;

[0027] Figure 9 This is a graph showing the effect of the HTAB and Trition-100 compound ratio coupled with S-mZVI on the degradation rate of carbon tetrachloride in Comparative Example 2;

[0028] Figure 10 This is a diagram showing the effect of HTAB coupling S-mZVI on the reusability of carbon tetrachloride degradation in Comparative Example 3. DETAILED DESCRIPTION

[0029] The experimental methods used in the following examples are conventional methods unless otherwise specified. The materials, reagents, methods, and instruments used are conventional in the art and can be obtained commercially by those skilled in the art unless otherwise specified.

[0030] HTAB and Tween80 were produced by Shanghai Aladdin Biochemical Technology Co., Ltd.

[0031] Example 1:

[0032] Sublimed sulfur powder and 400-mesh micron zero-valent iron (ZVI) were mixed in a 100-mL stainless steel pot in a planetary ball mill at ratios of 0, 0.056, 0.112, 0.168, 0.224, and 0.336, respectively. Milling was performed using zirconia balls (25 10-mm diameter ZVI balls and 150 5-mm diameter ZVI balls) at a speed of 500 rpm and room temperature for 6 hours. The resulting ZVI powder was freeze-dried for 6 hours and then stored in a nitrogen-filled glove box. The resulting materials were designated Q-0, Q-0.056, Q-0.112, Q-0.168, Q-0.224, and Q-0.336, respectively.

[0033] The above-prepared sulfide micron zero-valent iron was used to degrade carbon tetrachloride with an initial concentration of 20 mg / L, wherein the amount of sulfide micron zero-valent iron added was 0.6 g. The efficiency of carbon tetrachloride degradation is shown in the figure. Figure 1 As shown by Figure 1It can be seen that the sulfide micron zero-valent iron prepared by mechanical ball milling can completely degrade carbon tetrachloride within 3 hours, and its reduction dechlorination efficiency is good. It can also be determined that the modified material with a sulfur-iron molar ratio of 0.168 has the fastest carbon tetrachloride degradation rate and the best performance.

[0034] Example 2:

[0035] 0.075g, 0.15g, 0.3g, 0.45g and 0.6g of Q-0.168 were added to a carbon tetrachloride aqueous solution (100mL) with an initial concentration of 20mg / L, and the mixed reaction system was placed in a constant temperature shaker for shaking treatment. The temperature of the constant temperature incubator was set to 25°C, the speed was 150r / min, and the reaction time was 4h to complete the removal of carbon tetrachloride in the water. The efficiency of reducing carbon tetrachloride is as follows Figure 2 As shown by Figure 2 It can be seen that with the increase of material dosage, the reduction and dechlorination reaction rate of Q-0.168 accelerates, but when the material dosage is ≥0.3g, the substrate is degraded within 2h, so the material dosage in the reaction system is determined to be 0.3g.

[0036] Example 3:

[0037] The concentrations of hexadecyltrimethylammonium bromide solutions were prepared as 0g / L (control group - S-mZVI), 0.08g / L, 0.16g / L, 0.20g / L, 0.40g / L and 0.80g / L, respectively. 0.3g of sulfide micron zero-valent iron with an S / Fe molar ratio of 0.168 was added to the solution, and the mixture was shaken and balanced in a constant temperature shaker for 1h. The temperature of the constant temperature incubator was set to 25°C and the speed was 150r / min. After equilibrium, carbon tetrachloride was added to the system with an initial concentration of 20mg / L for reduction dechlorination experiments. The reaction time was 3h, and the removal of carbon tetrachloride in the water was completed. The effect of sulfide micron zero-valent iron on the degradation of carbon tetrachloride was investigated, and the results are as follows. Figure 3 As shown in the figure, compared with the uncoupled cetyltrimethylammonium bromide, the reduction and dechlorination reaction rates of the sulfide micron zero-valent iron material were increased by 2.83, 3.85, 3.90, 3.41 and 2.51 times when the cetyltrimethylammonium bromide was coupled with the sulfide micron zero-valent iron at concentrations of 0.08g / L, 0.16g / L, 0.20g / L, 0.40g / L and 0.80g / L, respectively. This shows that cetyltrimethylammonium bromide can promote the reduction and dechlorination of sulfide micron zero-valent iron. As the concentration increases, the promotion effect decreases, and the optimal concentration is 0.20g / L.

[0038] Example 4:

[0039] Prepare Tween80 solutions with concentrations of 0g / L (control group - S-mZVI), 0.01g / L, 0.05g / L, 0.10g / L, 0.50g / L, and 1.00g / L, respectively. Add 0.3g of sulfide micron zero-valent iron with an S / Fe molar ratio of 0.168 to the solution, and shake and balance in a constant temperature shaker for 1h. The temperature of the constant temperature incubator is set to 25°C and the speed is 150r / min. After equilibrium, carbon tetrachloride is added to the system with an initial concentration of 20mg / L for reduction dechlorination experiments. The reaction time is 3h, and the removal of carbon tetrachloride in the water is completed. Investigate its effect on the degradation of carbon tetrachloride by sulfide micron zero-valent iron, and the results are as follows Figure 4 As shown in the figure, the reduction and dechlorination rate of Tween80 coupled with sulfide zero-valent micro-iron decreased, but when the Tween80 coupling concentration was 0.05g / L, the reduction and dechlorination rate was close to that of the control group.

[0040] Example 5:

[0041] Prepare mixed solutions of hexadecyltrimethylammonium bromide and Tween80 with concentration ratios of 0.06g / L:0.035g / L, 0.08g / L:0.03g / L, 0.1g / L:0.025g / L, 0.12g / L:0.02g / L, and 0.14g / L:0.015g / L, add 0.3g of sulfide micron zero-valent iron with a S / Fe molar ratio of 0.168 to the solution, and shake and balance in a constant temperature shaker for 1h, wherein the temperature of the constant temperature incubator is set to 25°C and the speed is 150r / min. After equilibrium, carbon tetrachloride is added to the system with an initial concentration of 20mg / L for reduction dechlorination experiment. The reaction time is 3h, and the removal of carbon tetrachloride in the water is completed. The results are as follows Figure 5 As shown in the figure, when the concentrations of HTAB and Tween80 are 0.08g / L:0.03g / L, the compound system greatly reduces the amount of HTAB used, reduces costs, and avoids secondary pollution to the environment due to excessive HTAB; the compound use of Tween80 and HTAB has a better solubilizing effect on carbon tetrachloride than using HTAB alone.

[0042] Example 6:

[0043] A mixed solution with HTAB and Tween80 concentrations of 0.08 g / L and 0.03 g / L was prepared, 0.3 g of sulfide micron zero-valent iron with an S / Fe molar ratio of 0.168 was added to the solution, and the mixture was shaken and balanced in a constant temperature shaker for 1 hour. The temperature of the constant temperature incubator was set to 25°C and the rotation speed was 150 r / min. After equilibrium, carbon tetrachloride was added to the system with an initial carbon tetrachloride concentration of 20 mg / L for a cyclic reduction of carbon tetrachloride experiment. Samples were taken at 0h, 0.25h, 0.5h, 0.75h, 1.0h, 1.5h, and 2.0h in each cycle, and the experiment was repeated ten times. The experimental results are as follows: Figure 6 As shown in the figure, it can be seen that the carbon tetrachloride can be completely degraded after ten cycles, the use effect is good, and the actual utilization value is high.

[0044] Example 7:

[0045] Four mixed solutions of HTAB and Tween80 were prepared with concentrations of 0.08 g / L and 0.03 g / L, respectively. 0.3 g of sulfide micronized zero-valent iron with a S / Fe molar ratio of 0.168 was added to the solution and the mixture was shaken and balanced in a constant temperature shaker for 1 hour. The temperature of the constant temperature incubator was set at 25°C and the rotation speed was 150 r / min. After equilibrium, tetrachloroethylene, trichloroethylene, trichloroethane, and dichloroethane were added to the system respectively, with an initial concentration of 20 mg / L to conduct reduction and dechlorination experiments of different chlorinated hydrocarbons. The results are shown in Figure 2. Figure 7 As shown in the results, the composite surfactant coupled S-mZVI system has a good reduction and dechlorination effect on different chlorinated hydrocarbons, and the coupling system has a wide range of applications.

[0046] Comparative Example 1:

[0047] The difference between this comparative example and Example 4 is that Trition-100 is used to replace Tween80. The specific configuration concentrations are 0g / L (control group-S-mZVI), 0.01g / L, 0.05g / L, 0.10g / L, 1.00g / L and 5.00g / L of Trition-100 solution, respectively. 0.3g of sulfide micron zero-valent iron with an S / Fe molar ratio of 0.168 is added to the solution, and the solution is shaken and balanced in a constant temperature shaker for 1 hour, wherein the temperature of the constant temperature incubator is set to 25°C and the speed is 150r / min. After equilibrium, carbon tetrachloride is added to the system with an initial concentration of 20mg / L of carbon tetrachloride for reduction dechlorination experiment. The effect of non-ionic surfactant Trition-100 coupling sulfide micron zero-valent iron on the degradation of carbon tetrachloride is explored, and the results are as follows Figure 8 As shown in the figure, the reductive dechlorination of S-mZVI coupled with Trition-100 was inhibited, but the reductive dechlorination at a Trition-100 concentration of 0.10 g / L was close to that of the control group.

[0048] Comparative Example 2:

[0049] The difference between this comparative example and Example 5 is that the concentration ratio of hexadecyltrimethylammonium bromide and Trition-100 is 0.06g / L:0.07g / L, 0.08g / L:0.06g / L, 0.10g / L:0.05g / L, 0.12g / L:0.04g / L, 0.14g / L:0.03g / L, mixed solution, 0.3g of sulfide micron zero-valent iron with an S / Fe molar ratio of 0.168 is added to the solution, and the mixture is shaken and balanced in a constant temperature shaker for 1 hour, wherein the temperature of the constant temperature incubator is set to 25°C and the speed is 150r / min. After equilibrium, carbon tetrachloride is added to the system with an initial concentration of 20mg / L for reduction dechlorination experiment. The reaction time is 3h, and the removal of carbon tetrachloride in the water is completed. The results are as shown Figure 9 As shown in the figure, it can be seen that in the Trition-100 compounded hexadecyltrimethylammonium bromide coupled sulfide micron zero-valent iron system, as the HTAB ratio increases, the reduction dechlorination rate of the compound system accelerates, but the effect is lower than the reduction dechlorination when HTAB is used alone, and the compound system cannot reduce the HTAB dosage.

[0050] Comparative Example 3:

[0051] The difference between this comparative example and Example 6 is that a 0.20 g / L HTAB solution is prepared, 0.3 g of sulfide micron zero-valent iron with an S / Fe molar ratio of 0.168 is added to the solution, and the mixture is shaken and balanced in a constant temperature shaker for 1 hour, wherein the temperature of the constant temperature incubator is set to 25°C and the rotation speed is 150 r / min. After equilibrium, carbon tetrachloride is added to the system with an initial carbon tetrachloride concentration of 20 mg / L for a cyclic reduction of carbon tetrachloride experiment. Samples are taken at 0h, 0.25h, 0.5h, 0.75h, 1.0h, 1.5h, and 2.0h in each cycle, and the experiment is repeated ten times. The experimental results are shown as follows: Figure 10 As shown in the figure, it can be seen that with the increase of the number of cycles, the reduction dechlorination reaction rate of the HTAB coupled S-mZVI system gradually decreases, and the removal rate of carbon tetrachloride is lower than that of the HTAB and Tween80 composite coupled S-mZVI system.

[0052] The above description is merely a preferred embodiment of the present invention. Given that those skilled in the art can make appropriate changes and modifications to the above embodiments, the present invention is not limited to the specific embodiments described above. Some modifications and changes to the present invention should also fall within the scope of protection of the claims of the present invention.

Claims

1. A method for reducing and dechlorinating by coupling a composite surfactant with a sulfide micron zero-valent iron system, characterized in that: The compound surfactant is coupled with sulfide micron zero-valent iron to achieve the reduction and dechlorination of chlorinated hydrocarbons in water; Compound surfactants include HTAB and Tween 80; The composite surfactant-coupled sulfide micron zero-valent iron material was prepared by adding 0.3 g of sulfide micron zero-valent iron material to a 100 mL serum bottle containing HTAB and Tween 80 at concentrations of 0.08 g / L and 0.03 g / L, respectively, and shaking and balancing for 1 h.

2. The method according to claim 1, characterized in that The dosage of the compound surfactant coupled with the sulfide micron zero-valent iron material is 3.0 g / L.

3. The method according to claim 1, characterized in that The chlorinated organic pollutants contained in the water body are carbon tetrachloride, tetrachloroethylene, trichloroethylene, trichloroethane, and dichloroethane, with an initial concentration of 20 mg / L.

4. The method according to claim 1, wherein The degradation temperature is 20℃-25℃, the time is 1 h-48 h, and the oscillation rate is 150 r / min.

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