Spent coffee grounds as a green source of highly active carbon sorbents

The coffee production amounted to approximately 159 million of 60 kg bags worldwide in 2012-2013, with the consequent formation of a similar quantity of residues and waste during the whole material life cycle, from the harvesting to the brewing. Furthermore, the consumption demand is increasing year by year. According with the directive 2008/98/EC, the waste production should be minimized through reuse and recycle, reducing the burden of waste management and disposal. This approach is useful to gain a circular economy and it could be improved by political incentives and by increasing the economic value of the waste. Based on these perspectives, the research project is focused on the conversion of spent coffee grounds, coming from different brands, into highly active porous sorbents by mean of pyrolysis. The treatment process was tested at different experimental conditions to obtain the best yield in terms of BET and adsorption capacity. Moreover, the pyrolysis was performed changing one parameter time by time, in order to understand the influence in the process of the temperature, the typology of inert and the related flow rate, the reagents’ dosage and the holding time. The initial coffee mass and the temperature ramp (10 °C/min) were kept constant for each experiment. At the end of the thermal treatment, each sample was washed with the same amount of deionized water until neutral pH and dried in oven at 105 °C. The obtained activated carbon was characterized by different microscopy and spectrophotometric techniques (i.e. ESEM, Raman, XRD, XPS, solid 13C NMR) together with the BET analysis, with the purpose to determine its physical and chemical nature. Finally, the adsorption capacity of each sample was evaluated for different pigments: methylene blue (cationic), erythrosine-b (anionic) and bromothymol blue (non-ionic). Their initial concentration and the amount of activated carbon used in the test were arbitrarily set. A fixed volume of pigments’ solution (100 ml) underwent adsorption and the concentration changing in time was evaluated with a UV-VIS spectrophotometer. Two different commercial activated carbons were analyzed as reference. They are commonly used for the treatment of potable water and wastewater, in particular for the removal of mobile persistent organic contaminants.