High Tech Upcycling: From Spent Coffee Grounds to Activated Carbon Via Plasma Jet

     Activated carbon is excellent for absorbing unwanted matter from various sources from home applications, to industrial pollutant absorption. Carbon filters are used for air and water purification and can be found in home reverse-osmosis water filtration systems, as well as pre-filters in air purifiers and respirators. Activated carbon can even be found in many medicine cabinets since it can be taken orally to help treat diarrhea and certain instances of poisoning. It’s even used to decaffeinate coffee, coincidentally enough. On an industrial level, it plays a huge role in cleaning pollutants from the environment, and is regarded as somewhat inexpensive in relation to other methods of pollution cleanup. Activated carbon is a beneficial substance which has widespread applications, and is in high demand.

One substance that is in high supply is spent coffee grounds. I don’t have to go into detail about the massive amount of coffee consumed worldwide for you to imagine the massive amount of spent grounds resulting from this, even if only considering businesses alone. Not only are these spent grounds so abundant, but thanks to the studies conducted by the Korea Institute of Machinery and Materials, Korea University of Science and Technology, and Korea Research Institute of Chemical technology, by Hongjae Kang, Seongil Choi, Jin Hee Lee, Kwan-Tae Kim, Young-Hoon Song, and Dae Hoon Lee, we now know that spent coffee grounds are also feasible materials for carbonization and activation to become activated carbon. 

The way carbonization and activation of spent coffee ground waste is accomplished is by exposure to a plasma jet. Two different types, actually; the first plasma jet carbonizes the grounds to remove impurities and increase the level of carbon, while the second activates the carbon, which creates the aforementioned porosity and subsequent increase in surface area. Both of these processes are accomplished by processing the grounds at high temperatures and inert gasses.  This is the difference between the carbonization plasma jet and the activation plasma jet; the carbonization jet utilizes nitrogen and the activation jet utilizes carbon dioxide. 

Now, you may be wondering about what activation is or even how carbon works as a filter.  The biggest factor in how activated carbon works so well for absorbing matter is due to its potential to be extremely porous on a granular level, which results in extremely high surface area per grain. These pores are what absorb the matter that is to be filtered from the air, water, or other intended sources, and depending on how the carbon is activated, the carbon my possess microporosity, which would absorb smaller particles of matter, or macroporosity, which allows it to absorb larger particles of matter. This can be controlled during the activation process, where various outputs of plasma, gases and their flow rates have an impact on the pore formation in the coffee-ground-carbon.

So how is all this done?  A reactor is used where plasma is injected into one port into a chamber where the spent grounds are located, along with a rotating blade for tossing the grounds inside of the reactor for more even exposure to the plasma, as pictured in the diagram below (Fig.1).  During the first of two major cycles,  the reactor is fitted with nitrogen gas to push the plasma while the blade tosses the contents to ensure more even processing. This carbonizes the coffee grounds, A.K.A. turns them into carbon. However, this does not activate the carbon. After carbonization, the now carbonized material is washed with a 9 wt% HCl solution for one hour to remove more impurities and facilitate micropore development, washed with distilled water, and fully dried. The next step requires swapping the nitrogen gas for carbon dioxide and repeating the process in the reactor which, depending upon the voltage, temperature, output of gas, and duration, can produce larger or smaller pore structure in the carbon. This process of producing pores is the activation process.

Fig. 1. Schematic of a plasma jet reactor used for the treatment of coffee ground waste (batch-type reactor).  (Kang, et al., 2020)

The results of these tests were concluded to be feasible, and further experimentation is taking place to optimize the process. Since there’s reason to believe that higher efficiency in the processes can be achieved at lower electrical output for the plasma jets, thereby lowering the operating cost, this is one area in which further research is required. 

The potential is high for this extreme upcycling technique to have a net positive impact on the environment by not only utilizing massive amounts of consumer waste in the form of coffee grounds, but also the impact it will have on initiatives to remove pollutants from the environment by creating a more efficient, practically effective, and cost effective method for producing activated carbon. Plus, using spent coffee grounds to decaffeinate coffee is quite cool in a meta sort of way.

Work Cited

Hongjae Kang, Seongil Choi, Jin Hee Lee, Kwan-Tae Kim, Young-Hoon Song, Dae Hoon Lee,

Plasma jet assisted carbonization and activation of coffee ground waste,

Environment International, Volume 145, 2020, 106113, ISSN 0160-4120,

https://doi.org/10.1016/j.envint.2020.106113

(https://www.sciencedirect.com/science/article/pii/S0160412020320687)