As you can see here, the chemical reaction caused by exposing the dye to the biocomposite fibers began occurring very rapidly. Over the course of five minutes, 99% of the dye color had been removed.
Also, because the manganese particles were trapped in the cellulose cavities and cannot escape, the fibers could be used over and over again to remove dye. After eight uses, the fibers are were still able to remove 98% of the dye.
[1]"Indigo", DePual University, http://facweb.cs.depaul.edu/sgrais/indigo.htm
[2] Suslick, Kenneth S. (February 1989). The Chemical Effects of Ultrasound. Scientific American. pp.62-68
Creating Clarity
The numbers following "t=" correspond to the number of minutes since beginning the experiment.
Sonicated fique fibers
SEM image of fique-MnO2 biocomposite.
Fibers change color depending on
MnO2 concentration
Hinestroza and his colleagues created a “bio-composite” by filling the nano-sized hollow spaces in natural fique fibers with nanoparticles of manganese oxide. They then used this fiber biocomposite to break down indigo carmine dye in water.
MnO2 , or Manganese Dioxide, is a chemical compound commonly used in the alkaline batteries that you put in your flashlight or smoke detector. Manganese oxide nanoparticles have been used before to purify water polluted with difficult-to-break-down organic chemicals, like natural dyes. However, using a biocomposite of nanoparticles and natural fibers is a completely new technique that Hinestroza and his colleagues originated.
The dye they used in their experiments, indigo carmine, has a chemical makeup similar to the indigo-based dyes used at textile mills that make denim. But unlike pure indigo, indigo carmine can dissolve in water, which makes it easier to use in lab experiments.[1]
The scientists used a scanning electron microscope (SEM) to study the structure of the fique fiber at the nano-scale. As Hinestoza explains, natural fibers may appear very similar to one another, however, if we look at them under very high magnification, we can see their unique characteristics. Fique fibers have nano-sized spaces in their cellulose structure, and Hinestroza and his colleagues were inspired to use these spaces to grow the manganese oxide particles inside.
SEM Image of Cellulose in Raw Fique Fibers
The scientists began by placing the raw fique fibers in ultra-filtered water and exposing them to ultra-sonic waves. The process of “sonicating” the fibers loosens and eliminates any organic impurities left over from the fiber harvesting process, to produce as clean of a fiber surface as possible.
Once the fibers dried, they placed them in an acid solution, rinsed them in distilled water, and then placed them in a stronger basic solution . This process made the cellulose in the fibers develop a strong positive charge, which meant that the manganese oxide would be attracted to it.
Next, they soaked the positively-charged fibers in a manganese solution and sonicated them once again. At this point, something really amazing happened! The ultra-sonic waves caused acoustic cavitation, in which the sonic waves caused tiny bubbles to form and collapse in the water, which started a process of reducing the manganese ions into metal oxide nanoparticles.[2] Then, these nanoparticles of manganese oxide traveled into the nano-sized spaces in the cellulose and actually began to grow inside them.
Once they created the biocomposite of fique and manganese dioxide, the scientists used two independent scientific methods to measure the amount of manganese particles trapped inside the fibers. The purpose of using two different methods to measure the same thing is for a scientist to be sure that what he or she observes with one method is “real” and can be verified independently with another method. One of these methods was to look at the fiber at extremely high magnification to directly observe the nanoparticles inside the cavities. The other was to measure the color change of the fibers when they were sonicated in the manganese solution. The more concentrated the manganese solution was, the darker the fibers woud become as they “soaked up” the manganese oxide.
Finally, the scientists placed the treated fique fibers into the indigo carmine solution, stirred the solution constantly, and observed how well the manganese oxide in the fibers was able to react with the dye and degrade it. The process of dye degradation can easily be seen just by looking at how the color of the water changed. But, in order to sensitively measure this process, the scientists monitored the amount of light that the water absorbed. The less light it absorbed, the clearer the water was. At the same time, they measured the chemical make-up of the solution to identify the chemical products that were produced as the dye began breaking down.