Formaldehyde Removal by Activated Carbon Impregnated with KMnO4
Formaldehyde is a high toxicity pollutant listed as a Group 1 carcinogen. The formaldehyde in indoor air mainly comes from composite boards, adhesives, paints, disinfectants, and incomplete combustion of fuels of building decoration materials. Common methods to purify formaldehyde include ventilation, physical adsorption, chemical reaction, and photocatalytic oxidation. Among these methods, the ventilation method is restricted by air quality and ventilation conditions; the chemical reaction method faces the safety problem of chemical reagents; the application of the photocatalytic oxidation method is restricted by low utilization efficiency of visible light and its destructive effect on furniture. Meanwhile, activated carbon adsorption has become the most widely used air purification method because of its simplicity to use and low cost.
Activated carbon has a large specific surface area due to its porosity and shows good performance in adsorbing formaldehyde. However, the adsorption of unmodified activated carbon is generally physical adsorption, and the adsorbed formaldehyde is often desorbed later. In order to strengthen the adsorption capacity and stability, the activated carbon is usually impregnated with other chemicals for modification to improve its chemical adsorption performance. Potassium permanganate (KMnO4) is an ideal formaldehyde adsorbent that has strong oxidizing properties and can react with formaldehyde to generate carbon dioxide and water. For activated carbon impregnated with KMnO4, the main factors affecting its adsorption performance of formaldehyde are the changes in the activated carbon surface structure, such as surface area, pore size, pore volume. And because KMnO4 directly reacts with formaldehyde, the impregnation rate of potassium permanganate is the main factor that influences the chemical adsorption of the modified activated carbon. Besides, it is found that relative humidity can also have an important effect on the adsorption of formaldehyde.
From the perspective of environmental protection as well as cost reduction, after the activated carbon impregnated with KMnO4 is saturated, in order to reduce the waste of a large amount of activated carbon, it can be re-impregnated with KMnO4 for reuse in adsorbing formaldehyde.
Material preparation process
Weigh a certain amount of extruded activated carbon for pretreatment. Put the activated carbon in a container and put it in a drying cabinet with a temperature of about 90°C. Dry for 8 hours until the quality of the activated carbon does not change. Prepare 100 mL potassium permanganate solutions with different mass percentages (considering the consumption of potassium permanganate in the impregnation process, the actual concentration of the prepared solutions is slightly greater than the calculated concentration). Mix the activated carbon and solution evenly, then place the mixture in an ultrasonic vibrator and vibrate at 25°C for 1-2 hours. After that, place the mixture in the drying cabinet with a temperature of 80-90°C and dry for 8 hours to obtain modified activated carbon with different impregnation rates.
Activated carbon surface structure
The surface of activated carbon without KMnO4 impregnated is relatively smooth, and its pore shapes are mostly elliptical. Whereas those of activated carbon with KMnO4 impregnated are eroded, rough and uneven. As the impregnation rate increases, the surface area, average pore diameter, and pore volume of activated carbon all decrease to varying degrees. Among these, the specific surface area, the average pore size and the total pore volume of activated carbon with a 20% impregnation rate are 11% less, 12.8% less, 22% less, respectively, than those with a 0% rate. This is because potassium permanganate is relatively fully loaded in the pore structure of activated carbon, blocking part of the structure. The pore volume of mesopores and micropores decreased significantly, indicating that the mesopores and micropores are the main impregnation sites of potassium permanganate in activated carbon.
Effect of impregnation rate on formaldehyde adsorption
The test results show that the adsorption capacity of activated carbon with potassium permanganate impregnation rates of 0%, 5%, 10%, and 20% for formaldehyde adsorption is 16.30 mg/g, 18.56 mg/g, 57.88 mg/g, 73.94 mg/g, and the time to reach saturation is 150 min, 190 min, 310 min, 360 min, respectively. The adsorption capacity of the four carbon materials all decreases with time. The maximum adsorption rates corresponding to the four materials are 27.6%, 31.2%, 46.0%, and 55.2%, respectively. Compared with unmodified activated carbon, the average adsorption attenuation rates of those with 5%, 10%, and 20% rates are 1.9% less, 10.9% less, and 16.8% less, respectively. This shows that the adsorption continuity of carbon with 10% and 20% impregnation rates are relatively better. This conclusion can also be drawn from the saturation time of the four materials.
Regarding activated carbon with an even higher impregnation rate, when the rate of KMnO4 is 24%, the formaldehyde adsorption performance is the best, with formaldehyde adsorption capacity of 90.23 mg/g. If the impregnation rate continues to increase, the adsorption capacity will gradually decrease. The main reason is that at an impregnation rate higher than 24%, the uniformity of the dispersion of potassium permanganate decreases, which reduces its contact with formaldehyde molecules and reduces chemical adsorption. In addition, too many potassium permanganate molecules block the pore structure of activated carbon, and the physical adsorption of activated carbon decreased significantly.
Effect of relative humidity (RH) on formaldehyde adsorption
The adsorption performance of activated carbon materials with a 10% KMnO4 impregnation rate is tested with the relative humidity (RH) of 20%, 50%, and 80%. The result shows that when RH is 20%, the carbon material has the largest adsorption capacity, the longest adsorption saturation time, and the smallest average adsorption attenuation rate for formaldehyde. The formaldehyde adsorption performance of modified activated carbon decreasing as the relative humidity increases is because more and more water molecules are occupied on the surface of the modified activated carbon. At the same time, during the catalytic oxidation of formaldehyde with KMnO4, CO2 and H2O are produced. Too many water molecules affect the cyclic adsorption process of formaldehyde molecules.
Effect of repeated impregnation times on formaldehyde adsorption
The initial adsorption capacity and adsorption saturation time of activated carbon with a 10% impregnation rate of KMnO4 for formaldehyde are 57.88 mg/g and 310 min. Those of the first re-impregnated carbon are 50.20 mg/g and 280 min, and those of the second re-impregnated carbon are 36.57 mg/g and 210 min. From the data, it can be seen that the adsorption performance of the modified activated carbon material significantly decreases after the second re-impregnation. But it still shows a good adsorption capacity for formaldehyde compared with unmodified ones.