Coffee waste biochars: characterization and zinc adsorption from aqueous solution

Abstract

The final disposal of organic wastes has become a major challenge with increasing industrialization and population growth. Coffee wastes can be converted into biochar and provide economic and environmental benefits, such as heavy metal remediation in different media. We produced biochars at 700 °C from spent coffee grounds and coffee parchment, characterized them physically and chemically and evaluated their Zn adsorption capacity from aqueous solution. Batch adsorption experiments were performed with six Zn concentrations using four replicates. The desorption process was performed sequentially with a pH 4.9 buffer acetic acid solution. Langmuir and Freundlich isotherms were fitted to the adsorption data using non-linear models. Results show that both biochars are alkaline and have high values of point of zero charge (PZC) (9.2 and 7.7 for coffee parchment and spent coffee grounds biochar). These characteristics indicate precipitation as the main mechanism of Zn immobilization and may have contributed to the low adsorption capacity obtained (0.056 and 0.792 mg g-1 for spent coffee ground and coffee parchment biochar, respectively). Although the biochars have shown low adsorption capacity, they bound Zn strongly and the adsorption process is not easily reversed.