Coffee Waste Biochar: a Widely Available and Low-cost Biomass for Producing Carbonaceous Water Treatment Adsorbents

Abstract

Coffee is one of the world’s most traded commodities grown in about 80 countries and widely consumed. Accordingly, a high quantity of spent coffee grounds (SCGs) produced from coffee beverage preparation is generated and disposed globally. The high availability and low cost of SCG allow its valorization to obtain a valuable product that could be used as an alternative adsorbent for water treatment applications. This study used dried SCG to produce biochar by pyrolyzing the biomass at three different temperatures: 300, 450, and 600 °C. Pyrolysis was conducted at a slow heating rate of 5 °C.min-1 for 30 min. SCG biomass and biochar were characterized for various properties that would describe the biomass changes during pyrolysis and the effectiveness of biochar for water treatment applications. The SCG biomass was acidic in nature with a pH of 5.54, whereas biochar was mildly alkaline with a pH of 7.5 to 10.6, increasing with pyrolysis temperature. The electrical conductivity of SCG biochar at 600 °C (2,278 µS.cm-1) was higher than that of biomass (550 µS.cm-1). This indicates SCG biochar could have promising ion-exchange capacity. SCG biochar produced at 450 °C has the most negative zeta potential of -57.33 mV, compared to the least negative measure of -30.73 mV for SCG biomass. A negative zeta potential has a strong affinity for positively charged cations and this was confirmed through cation exchange capacity (CEC) measurements. The highest CEC was 29 cmolc.kg-1 for 450 °C SCG biochar, a 58 % increase over that for the SCG biomass. SCG biochar contained 63 % to 88 % of C content for pyrolysis temperatures of 300 to 600 °C, respectively, whereas the SCG biomass had a C content of 43 %. With increasing temperature, the fixed carbon (FC) content of biochar increases and reached 69 % at 600 °C, while in biomass, the FC content was only 13 %. The large change in C content as SCG biomass is pyrolysed indicates large changes in structure and therefore tunability. SCG could be a promising carbonaceous adsorbent to remove organic and inorganic pollutants from polluted water.