Researchers from Anhui Agricultural University (China) converted corncob (a by-product of the agricultural industry) into a material which can remove pollutants from wastewaters.
Scientists can make the material with a simple and cheap process; it also has magnetic properties, meaning that it’s possible to easily separate it from the wastewater, just by applying a magnetic field (i.e. using a magnet).
Waste Production and Valorization
One of the problems facing our society today is the production of higher and higher amounts of waste; in the European Union area, for instance, the quantity of waste produced in 2010 was 2.5 billion tonnes, 25 % more than in 2006.
The agricultural and food industries, in particular, are amongst the sectors with the highest by-products and waste generation.
The disposal of such huge amounts of waste can be a problem and a cost for the whole society. To address this, both researchers and commercial companies have been looking into ways to valorize these by-products/wastes, by converting them into more valuable compounds.
Activated carbon is a form of carbon with a highly porous structure and, therefore, very large surface area. This carbonaceous material can be used for the treatment of polluted/contaminated waters or gaseous emissions. Due to the porous structure, the material can adsorb some pollutants on its surface, and hence remove them from water, the atmosphere or gas mixtures.
The majority of activated carbon is from materials such as coal or wood (e.g., charcoal); some researchers have also used waste animal bones for this purpose.
In recent years, however, several researchers studied the way to manufacture activated carbon from renewable biomasses, including by-products of the food industry.
Novel Process Using Corncob
Researchers from the Anhui Agricultural University (Hefei, China) developed a new process to convert corncob (agriculture by-product) into a carbonaceous material, which can be used for pollutant removal.
They published their results in Bioresource Technology in April 2015. Further to adsorbing pollutants, the material also has magnetic properties.
To convert the corncob into magnetic adsorbent, the researchers used a two-step process. First, they treated the corncob with a hydrothermal process; in fact they placed the agriculture residue, in the form of powder, into a sealed autoclave, with water and zinc chloride (ZnCl2). They heated the system at 180 oC and left the reaction going for 6 hours. In this way, they prepared the powdery carbonaceous precursor (CP).
In the second step, they suspended the CP in a solution containing iron chloride (FeCl3), sodium hydroxide and water; the suspension was then treated in the autoclave at 180 oC for 6 hours, in a process similar to that described above.
The treatment with an iron-containing salt led to the formation of iron oxide (Fe3O4), which has magnetic properties. Therefore, the whole adsorbent material became magnetic (magnetic carbonaceous adsorbent, MCA).
Both steps are performed at relatively low temperature; the process, therefore, is not expensive and easily scalable to industrial level.
Testing the Material
To test the material, the scientists monitored the removal of different dyes. They chose these molecules since they are quite toxic and are major pollutants in waste waters; moreover, due to their colors, the researchers can easily monitor the removal progress.
They chose 4 cationic dyes (dyes with positive charge) and 2 anionic ones (dyes with a negative charge). More specifically methyl violet, malachite green, rhodamine B and methylene blue as cationic, and methyl orange and congo red as anionic.
In the adsorbing tests, they placed the MCA in a solution containing one of dyes and monitored the change in the concentration of the dyes with time.
Results showed that MCA was able to adsorb all the dyes; the efficiency of the adsorption process, however, was different depending on the dye. In fact, the carbonaceous material worked better with the cationic dyes, in particular with methylene blue and methyl violet.
Removing the Adsorbent from the Solution
One of the key steps in the use of solid adsorbents for water treatment is their separation from the contaminated waters.
In this case, however, this step is very simple, due to the magnetic properties of the material. In fact, the adsorbent was easily separated and removed from the solution with the use of a magnet; this is the main advantage of having a magnetic adsorbent, and which makes it highly suitable for larger scale applications.
The scientists made several repeated tests on the same MCA powder, to check whether it could have been used for more than one adsorbing cycle. The results showed that the efficiency of MCA decreased very little with use. In the case of methylene blue, for instance, a 100 % removal was seen in the first adsorbing cycle; after 6 cycles, the removal was still higher than 95 %, hence just slightly lower.
Agricultural Waste for Environmental Remediation
As environmental pollution is becoming an increasing problem, it is urgent to find ways to address this. The use of agricultural by-products for environmental remediation is an ideal solution, since it also solves the problem of the waste accumulation.
The results described here show that it is possible to convert corncob into an effective pollutant adsorbent with magnetic properties, with a simple and economic process.
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