The great opportunity of agricultural waste: keys to purify air and water in an environmentally friendly way through activated carbon
Colombian households are well acquainted with activated charcoal, an ingredient present in soaps used for washing dishes and even in others marketed for cleaning the body.
Well, the agricultural residues left by Colombian crops have the potential to become a carbonaceous solid, and even activated carbon, when exposed to temperatures of around 350 °C. We tell you about the benefits of this technique.
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Activated carbon and its effects against pollution
For Colombian producers there are alternatives capable of mitigating the environmental effects of industrialization based on organic residues from the cultivation of fruits and vegetables that are becoming more and more desirable in international markets.
When agricultural waste is subjected to high temperatures, it can be converted into a solid composed of approximately 85% carbon. If this solid is properly activated, it becomes activated carbon. The latter has compounds that facilitate the capture and retention of metals that pollute water resources and even carbon dioxide (CO2) present in the air.
This was explained to the News Agency of the National University of Colombia (UNAL) by Marlon Fabián Córdoba Ramírez, PhD in Engineering – Energy Systems from the National University of Medellín, who has joined forces to study the use of agricultural waste and the potential of activated carbon.
An opportunity in agricultural waste
It is well known that both seasonal and permanent crops -coffee, sugar cane, corn and rice, among others- emit organic residues that could be used to purify the air.
This accumulation of waste, “which includes grass clippings, bagasse, husks, etc., […] can be transformed – through thermochemical and biological processes – into products with high added value with which environmental problems can be mitigated and more nature-friendly energies can be produced, in a way that decreases the use of fossil fuels”, explains Córdoba.
Traditionally, the technique applied in these cases consists of subjecting the biomass to a slow pyrolysis process. In this process, the biomass is exposed to temperatures between 350 and 700 °C, and the resulting solid is converted into activated carbon.
Córdoba points out that this, despite being the most widely used technique, is not the only one, because in his studies he also applied a different structure that includes a previous process known as torrefaction.
With this technique, the biomass, i.e., the agricultural residues to be used, are subjected to a temperature that must not exceed 300 °C initially. After this first stage comes the slow pyrolysis, a phase in which the biomass is exposed to temperatures of up to 700 °C, and finally the activation of the carbonaceous solid occurs, so that it becomes activated carbon.
By using the latter technique, the results improved considerably with respect to those obtained with the former method. The evidence showed an increase in the surface area of biochar, which is the soil improver formed from biomass, from 80 to 110 m2/gram to an impressive 700 m2/gram.
“In addition, in activated carbon we managed to have areas of 2,300 m2/gram, a fact that is usually achieved by chemical activation, but we did it simply by incorporating temperatures and longer action times (torrefaction plus slow pyrolysis)”, explains Córdoba.
The treatment of agricultural residues is aimed at obtaining activated carbon with a higher degree of porosity in order to enhance the scope of the active ingredient in terms of decontamination. In other words, when the solid is more porous, its compounds trap more harmful chemicals and retain them better.
Studies have shown that for each gram of activated carbon, an average of 95 milligrams of CO2 can be encapsulated.
The technique that proved to be most effective could simplify production processes involving the treatment of agricultural waste and the chemical pretreatment of certain substances.
In addition, the carbonaceous solid obtained from the process, and the activated carbon derived from it, allows for optimal levels of carbon in the soil, making it a more productive soil capable of retaining more nutrients.
“Likewise, it can have positive implications in the adsorption of hydrogen sulfide, which is a polluting and very acidic gas, a product of industrial processes”, says Cordoba.
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