The search for elements to help eliminate heavy metals, gases, and toxic materials, among other contaminants, in rivers, lakes and water streams as well as in the air, lead back to nature. Products considered as waste have turned into an optimal resource to obtain activated charcoal, a solid porous material with great adsorption capabilities and make it a great purifying agent.
In Colombia, coffee grounds, sugarcane bagasse, orange peels and corn and coconut husks have been used in several studies carried out by Universidad Nacional de Colombia (UNal) researchers, with the objective of offering more sustainable and economic alternatives to the impact provoked in water sources by contaminating industrial manufacturers.
One of the most recent contributions is that of UNal-Bogotá Chemical Sciences Ph.D. candidate Ana María Carvajal, who has gone a little further: She modified the activated charcoal of coconut husks to increase its adsorption capacity. Then she tested its efficiency in compounds with great contaminating capability such as 2,4-dinitrophenol or otherwise known as picric acid used in that manufacturing of explosives, and 4-nitrophenol, produced in oil fractioning, rubber, paint, and plastic manufacturing.
Furthermore, the improved charcoal was also tested with hydrocarbons such as pentane, hexane, heptane, octane, and, nonane, volatile organic compounds in gasoline and that contribute to air pollution, mainly in urban areas.
“Due to the ample range of oil derivatives, it was not possible to assess the pollution involved from the exploitation stage to the obtainment of other organic phenol compounds. These compounds are hard to extract using simple treatments as they remain in the water impacting its taste and producing damage to the ecosystem,” said Carvajal.
The Estudio Nacional del Agua (National Water Study), published in 2015 by the Hydrology, Meteorology and Environmental Studies Institute (IDEAM, for its Spanish acronym) highlights that “the quality of the water, as expressed in the contaminating load of biodegradable, no biodegradable, nutrients, heavy metals and mercury impacts close to 150 municipalities in cities such as Bogotá, Medellín, Cali, Barranquilla, Cartagena, Cúcuta, Villavicencio, Manizales, and Bucaramanga. Furthermore, the non-biodegradable organic matter is estimated at 918,670 tons a year, being Bogotá, Cali, Medellín, and Cartagena the main contributors.”
Carvajal explains that to transform coconut husks into activated charcoal it is necessary to pass the material through a nitrogen current at a temperature of between 400° and 900° C or through a carbon dioxide current (CO2), to produce greater porosity.
“This type of solid has graphene structures (aromatic rings) in layers. On the borders of each layer, there are nitrogenized and oxygenized compounds, which modify the surface chemistry of the activated charcoal and the graphene layers produce the porosity,” adds Carvajal.
These paths are categorized according to size: ultra-porous is less than 0.7 nanometers; micro-porous, which have greater adsorption, measure 20 nanometers; and macro-porous and meso-porous have larger cavities.
To increase the adsorption capability of the coconut charcoal they carried out several modifications to the material surface. The first consisted of impregnating it with a concentrated solution of nitric acid to increase the amount of oxygen.
As opposed to other studies, with this study, impregnation was done at room temperature (18° C), as the acid is a powerful oxidizing agent which could affect the porous structure of the charcoal. Phosphoric acid was used for the second modification, also at room temperature, in order to create polyphosphate chains on the surface.
Carvajal says that to get a product with a greater amount of nitrogen-based compounds –as nitrogen has electrons which could boost the adsorption of compounds– they used charcoal impregnated with nitric acid and then achieved an “improved” version with a solution of ammonium hydroxide.
The three tests were carried out to increase the content of functional oxygenized and nitrogenized groups. Furthermore, they performed three modification to diminish the amount of these groups, through carbonizations at 800°, 900°, and 1000° C of the original coconut activated charcoal sample.
Once the material was obtained the proceeded to prepare the phenolic (in liquid phase) and hydrocarbon (in gas phase) compounds at different concentrations. Later they applied the model known as “adsorption isotherms”, at temperatures of 10°, 20°, and 30° C, with the purpose of assessing the adsorption capability and determine the amount of energy involved in the process.
They discovered that in 4-nitrophenol and 2,4-dinitrophenol the carbonizations increased the adsorption capacity of the activated charcoal between 10 and 25%, while for volatile hydrocarbons it was between 23 and 44 %.
Since 2005 the demand for activated charcoal in Colombia has been between 900 and 1,300 tons a year, a majority of which has been supplied by imports. While the country does have an activated charcoal producing plant (Carbón Activado Sulfoquímica S.A.), its production does not satisfy the growing demand.
Using coconut husk, to increase the production of activated carbon and contribute to decontamination could boost this market in Colombia as well as help preserve our natural water resources.
These findings show how alternations to the micro-porous structure of activated charcoal influence the adsorption of determining compounds.
“This helps us to become cognizant of what type of modifications should be done according to the contaminant to be removed from water sources or air to obtain greater performance and carry out an improved purification process,” concluded Carvajal.
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