Metals are classified as lustrous and malleable electrical conductors that are located mainly on the left side of the periodic table. Many industries around the world, such as metal plating and mining companies, produce wastewater contaminated with heavy metals such as lead and copper which they dispose of into the environment (Seniūnaitė, Vaiškūnaitė, Bolutienė 2014). This is a significant problem as, while some metals are required in trace amounts in the body, heavy metal poisoning can cause the illness or even death of sensitive organisms and people if this wastewater is consumed. High copper concentrations in the body can cause kidney damage, nausea, diarrhea and vomiting, and can be highly toxic to wildlife even at low concentrations (Gautam et al 2014). While there are some options for removing heavy metals from water, like precipitation and filtration, these options are often expensive. Thus, cost-efficient methods are needed so that the processing and removal of heavy metal from water supplies can become more economically viable (Bailey et al, 1999). Due to this, various studies have taken place to find substances that can act as sufficient biosorbents, which are natural substances such as activated charcoal that can remove metals from solution.
As of 2019, the annual global coffee consumption reached 170.94 million 60 kg bags of coffee (Shahbandeh, 2020). For every one of these 60 kg bags, about 1920 kg of waste is also generated (Hawken, 2000). This is equivalent to about 360000 kilotons of coffee grounds being produced every year, which is usually disposed of in landfills. Rather than wasting a possibly useful resource, it would only make sense to use these coffee grounds as a cost-effective method of cleaning water contaminated by metals. Not only would this reduce the risk to wildlife, but it would also be beneficial to the environment by reducing pollution. This report aims to investigate the effects of particle size of spent coffee grounds on their ability to adsorb copper ions, as this variable has not been thoroughly explored thus far. Therefore, this experiment will assist in determining whether this method is a viable option for future use.
Method
To create a calibration curve, 10 mLs of 5 different concentrations of copper solution (0, 0.5, 1, 2, 5, and 10 ppm) were each collected and dispensed into vials. Then 10 mLs of deionised water, 5 mL of alizarin red and 5 mL of ammonium acetate solution were added to each vial and gently agitated. A cuvette was rinsed with the solution that it will be filled with, then filled with the solution to the line and placed it into the spectrophotometer. Once the reading had been taken for that solution, the cuvette was rinsed with the next solution and this process was repeated until all of them had been measured in the spectrophotometer. Once all results were obtained from each sample, they were plotted on a graph to make a calibration curve.
Once each of the 4 samples of 0.1g of coffee grounds had been ground for their respective times (0, 1, 2 and 5 mins) in a mortar, 30 mL of a 5 ppm copper solution was transferred into a centrifuge tube and 15 mL of ammonium acetate buffer solution was added to the tube. The tube was then agitated gently, then 10 mL of the solution was added to a new sample vial along with 5 mL of alizarin red solution. A sample of this solution was added to a clean cuvette and its absorbance was measured in the spectrophotometer and recorded. This was done another 2 times, with 3 readings taken for each replicate. Four centrifuge tubes were then collected and 10 mL of the solution was added to each, along with 0.1 g of coffee grounds of each different size. The tubes were stirred for approximately 10 seconds before being allowed to rest for 10 mins. After resting, the tubes were placed in the centrifuge at 5000 rpm for 5 mins. The tubes were removed from the centrifuge and the solutions were pipetted into vials with 2.5 mL of Alizarin red. The absorbance for each solution was recorded on a spectrophotometer at 510 nm per the steps described in the calibration curve method (i.e. the cuvette was rinsed with each solution before being filled).
Discussion
The aim of this experiment was successfully met, as an obvious relationship between particle size and percent copper adsorption was shown. Through the results obtained in this experiment, it was determined that a decrease in the size of coffee grounds corresponded with an increase of copper ions adsorbed. This can be attributed to the fact that grinding the coffee grounds to create smaller particles also creates more surface area for the copper ions to adsorb to, a concept which is supported by other studies of the adsorption capabilities of various materials (Bernd, Müller, 2010). The results obtained were consistent, with no obvious outliers and very little deviation, which indicates they are accurate. However, they do have some aspect of unreliability, as the size measurement for the particle size of coffee grounds is not very reproducible. Results could vary even if the coffee grinding times are followed exactly, as the size of the particles will also depend on the force used when grinding the coffee. This limitation could be amended by using a microscope to measure the grounds, or by using filters of different sizes to roughly categorise them.
These results are supported by the results found in the paper 'Coffee grounds as an adsorbent for copper and lead removal form aqueous solutions'. In this study, the results showed that coffee ground particles smaller than 200 µm adsorbed more copper ions than particles larger than 200 µm. However, it is difficult to accurately compare these results to the ones obtained in this experiment, as the coffee grounds were not measured in particle size, but rather by the amount of time they were ground. This is a significant limitation in the experiment which makes it somewhat unreliable and could be an area to improve on in future experiments.
This experiment lays good groundwork for further experiments to be conducted to investigate the limitations of the impact reduction of particle size has on the amount of copper ions adsorbed. More accurate measurements could be taken of the particle size using a microscope, and the size of the particles could be continually reduced until it no longer made a difference in percent copper adsorption. This could establish the most efficient way to treat water with coffee grounds by expanding the dataset and increasing the number of dependent variables, which has potential for industry use in purifying water (Utomo, Hunter 2006). This method has the potential to be used for other types of metal such as lead and cadmium, however, this would require further testing.
This study helps to fill a gap in research into this topic, specifically the effect of surface area on copper ion adsorption. While there are many existing studies into the capabilities of coffee grounds in removing metals from water, very few focus on the effects of particle size, and those that do (Seniūnaitė, Vaiškūnaitė, Bolutienė 2014) do not go into much depth. Studying this topic further could open future pathways for research and has potential to be used as an industry resource for how companies can reasonably reduce their contribution to environmental pollution without it being at the expense of their earnings.
Conclusion
The aim of the experiment discussed above was to investigate the effect of decreasing particle size on the adsorptive capabilities of coffee grounds, specifically in reference to copper ions. It was hypothesised that smaller coffee particles would adsorb more copper ions, which was supported by the results. Coffee that had been ground for 5 minutes were found to be the most effective, with 75.87% copper removal on average, which supports the possible use of coffee grounds as a cost-effective option for the removal of copper ions from water. Although there were some limitations in the experiment, the effect of various particle size was clear, and these results could be used as groundwork for future studies that wish to go into more depth. The possible implications of these findings are that they could be used by industries that produce heavy metal waste as a cheap way of purifying contaminated water before it is disposed of in the environment. Not only would this save money, but it would reduce the amount of waste produced by coffee companies, which in turn would be better for the environment.