Preparation of Biomass-based Mesoporous Carbon with Higher Nitrogen-/Oxygen-chelating Adsorption for Cu(II) Through Microwave Pre-Pyrolysis

This method can help answer key questions in the utilization of biomass and the wastewater remediation field, such as preparation of modified biomass-based carbon for removal of heavy metals in wastewater. The main advantage of this technique is that the microwave pyrolysis benefits the subsequential modification process to simultaneously introduce more nitrogen and oxygen functional groups of carbon. To begin, rinse the bagasse with deionized water, and put the samples in a drying oven at 100 degrees Celsius for 10 hours.

Crush the dried bagasse with a grinder. Then, sieve the powder through a 50-mesh sieve. Now, place 30 grams of fine bagasse powder into a 15-weight-percent phosphoric acid solution in a one-to-one weight ratio for 24 hours.

Dry the mixture in an oven at 105 degrees Celsius for six hours. Collect the resulting product as the precursor for bagasse-based activated carbon, or BAC. Now, put 15 grams of the precursor in a microwave oven with a 2.45-gigahertz frequency.

Set the power of the microwave oven at 900 watts to pyrolyze the sample for 22 minutes. Ensure a nitrogen flow rate of 20 milliliters per minute with a rotor flow meter. The air inlet of the rotor flow meter is connected to a nitrogen cylinder using a hose, while the outlet is connected to the air inlet of the microwave oven.

After allowing the resultant carbon to cool to room temperature in nitrogen, triturate and collect the carbon sample in a beaker. Now, add 300 milliliters of 0.1-molar hydrochloric acid. Stir the mixture using a magnetic stirrer at 200 rpm for more than 12 hours at room temperature.

Filter the carbon by filter paper with vacuum filtration. Then, rinse the sample with deionized water until the pH value of the wash water is greater than six. Dry the microwave-pyrolyzed bagasse-based activated carbon, or MBAC, in a vacuum drying oven at 105 degrees Celsius for 24 hours.

Mix 50 milliliters of concentrated sulfuric acid and 50 milliliters of concentrated nitric acid in a beaker at zero degrees Celsius. Then, add 10 grams of MBAC to the mixed solution. Use a magnetic stirrer to stir the mixture for 120 minutes at 200 rpm.

Filter the nitrified MBAC by filter paper with vacuum filtration. Wash the carbon with deionized water until the wash water reaches pH six. Then, dry the washed carbon in a drying oven at 90 degrees Celsius for 24 hours.

In a three-necked flask, add 5.05 grams of the resulting product, 50 milliliters of deionized water, and 20 milliliters of 15-molar ammonium solution. Stir this mixture for 15 minutes with a magnetic stirrer at 200 rpm. Then, add 28 grams of sodium dithionite, and leave the mixture stirring at room temperature for 20 hours.

After 20 hours, fit a reflux condenser to the flask, and warm the mixture up to 100 degrees Celsius using an oil bath. Add 120 milliliters of 2.9-molar acetic acid to the flask. Then, allow the mixture to stir for five hours with a magnetic stirrer under reflux.

Remove the oil bath to allow the solution to cool down to room temperature. Filter the carbon sample, and wash it with deionized water until the solution pH is greater than six. Dry the modified MBAC at 90 degrees Celsius, and denote it as MBAC-nitrogen.

To perform structural characterization through nitrogen adsorption and desorption isotherms, first weigh an empty sample tube. Add approximately 0.15 grams of the carbon sample to the sample tube. Degas the sample at 110 degrees Celsius for five hours in a vacuum.

Then, weigh the sample tube containing carbon, and calculate the weight of the carbon sample. Install the sample tube into the test area of the surface area and porosimetry analyzer using liquid nitrogen to measure it at minus 196 degrees Celsius. To perform the chemical characterization using Fourier transform infrared spectroscopy, first check the temperature and hygrometer.

The temperature should be 16 to 25 degrees Celsius and the relative humidity 20%to 50%Remove the desiccant and dust cover in the sample storehouse. Dry the carbon sample and potassium bromide at 110 degrees Celsius for four hours to avoid the effect of water on the spectrum. Then, mix the carbon sample with potassium bromide, and use a press mechanism to prepare the test sample.

Place the sample in the test area, and set the parameters of the software. Then, save the spectra, and take out the sample before processing the spectra. To perform the copper ion adsorption experiments, first adjust the pH of copper sulfate solutions to pH five using 0.1-molar nitric acid and 0.1-molar sodium hydroxide solutions.

Then, place 0.05 grams of adsorbent in each of the conical flasks containing 25 milliliters of the pH-adjusted copper sulfate solutions. Fit lids on the conical flasks, and put them in a thermostatic orbital shaker with a stirring rate of 150 rpm at five degrees Celsius, 25 degrees Celsius, and then 45 degrees Celsius for 240 minutes at each temperature. Use 0.22-micron membrane filters to separate the adsorbents from the solution.

Finally, use flame atomic absorption spectrophotometry to determine the copper concentration of the filtrate. Structural characteristics and elemental compositions of all samples are shown here. Microwave pyrolysis and modification contribute to a smaller specific surface area and smaller total pore volume but a greater nitrogen and oxygen content.

FTIR spectra show that the modified carbon materials have obtained distinct nitrogen/oxygen functional groups, and the microwave-pyrolyzed carbon gets more. The effect of pH on copper ion adsorption by all samples is shown here. MBAC-nitrogen presents a better copper ion adsorption than EBAC-nitrogen, although MBAC-nitrogen has a lower surface area and pore volume, owing to more abundant nitrogen/oxygen surface groups.

In this model, the mechanism for copper ion adsorption by modified carbon is proposed. In this reaction process, the chemical adsorption mainly involves ion exchange and complexing. While attempt to use a Western approach to prepare biomass by based on mesoporous carbon, was better physicochemical properties by microwave pyrolysis.

It's important to determine the optimum experimental conditions considering the effect of the impregnation ratio, pyrolysis time, and the microwave oven power. Following this procedure, other modification methods that can effectively introduce more functional groups of the carbon can be performed in order to overcome shortcomings, like the decrease of specific surface area and the total pore volume. After its development, this technique paved the way for researchers in the field of functionalized nanomaterial to explore rapid preparation of high-adsorptive carbon from biomass for wastewater remediation.

Don't forget that working with concentrated sulfuric acid and concentrated nitric acid can be extremely hazardous, and precautions such as protective spectacles should always be taken while performing this procedure.