Spent coffee grains biochar activation and its effects on the characteristics of the final products to be used as sorbents or catalysts support

Coffee is considered the most widely spread beverage and annually, approximately, 11 billion tons are produced all around the world. Raw spent coffee residue (Raw-SCG), biochar coffee residue (SCG), and the activated biochar with distilled H₂O (W-SCG), H₂SO₄ (SCG-S), H₃PO₄ (SCG-P), and NaOH (SCG-ALK) were fully characterized for their surface area, density, ash, surface topography, surface functional groups and suspension pH. These are parameters that could affect sorption capacity and catalyst properties. In this study, Raw-SCG, obtained from a coffee shop in the campus of Patras University, was pyrolyzed at 850^oC in a custom-made ceramic saggar box. BET surface area of biochar consists a basic characteristic that controls its nutrient and pollutant sorption capacity. SCGs produced at 850^oC have high specific surface and micropore area and at the same time, low external surface area. The t-plot for the SCG disclosed that the activation with H₃PO₄ leads to high specific surface (921 m²/g) and micropore (626 m²/g) area compared to other SCG samples. Simultaneously, the activation of SCG increases the pore size of biochar and the highest pore size was observed for the SCG-ALK (37 Å) compared to activated SCG-S (34 Å) and SCG-P (34 Å). The Raw-SCG has slightly acidic nature (pH 5.5) than the biochar SCG (pH 10.6) which has the most alkaline nature. For the activated SCG biochars, SCG-S (pH 4.6) is more acidic than SCG-P (pH 5.2). Furthermore, W-SCG biochar (pH 9.1) is more alkaline than SCG-ALK (pH 8.8). A plethora of possible explanations can be given for the alkaline nature of biochar, such as the removal of acidic groups, but this is still under investigation. Moreover, the five samples present a peak at 1050 cm^-1 that corresponds to (C-O) bond and a weak peak at 3450 cm^-1 that reveals O-H groups. For the Raw-SCG, a carboxyl stretching mode (C=O) demonstrate a weak peak at 1740 cm^-1; the sharp peaks at 2830 and 2950 cm^-1 are related to aliphatic C-H₂ bending. The presence of a shallow peak at 2450 cm^-1 corresponds to CO₂ bond for the SCG and W-SCG. It is very important to mention that the peaks of W-SCG and SCG-ALK are similar. The results indicates that biochars SCG produced at 850^oC are acidic in nature, they are highly porous materials with micropores corresponding to the majority of pore volume and the activated SCG-P shows a rapid increase of micropore area compared to other activated and washed biochars (SCG-S, SCG-ALK, W-SCG). 

 

 

 

Acknowledgment

We acknowledge support of this work by the project “Research Infrastructure on Food Bioprocessing Development and Innovation Exploitation – Food Innovation RI” (MIS 5027222), which is implemented under the Action “Reinforcement of the Research and Innovation Infrastructure”, funded by the Operational Programme "Competitiveness, Entrepreneurship and Innovation" (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).