Grape stalk application for caffeine removal through adsorption

- The use of grape stalks as adsorbent for wastewater treatment is proposed.
- Adsorption of caffeine by activated grape stalks is efficient.
- Thermal activation proved to improve the grape stalks specific surface area.
- Alternative use for grape stalk contributes to solid waste minimization.

Concern over emerging pollutants presence in water resources is growing, justifying the search for alternative or additional techniques to those applied in conventional water treatment processes. Use of adsorption with agricultural wastes directly as adsorbents or as precursors for activated carbon synthesis is a viable method, combining removal efficiency, low cost and biodegradability of the material applied. This study investigated the employability of grape stalk, a waste from grape industrialization process without effective use, in caffeine removal from aqueous solution. Grape stalk was used in three different forms: raw with only grain size adjustment (GS), modified by phosphoric acid action (MGS) and as activated carbon (GSAC). In the first two cases parameters pH, residence time and adsorbent concentration were varied in order to find optimum batch process conditions. For GSAC, on the other hand, caffeine removal percentages were high even for the least amount of adsorbent that could be measured with acceptable accuracy, which did not justify concentration parameter variation. Better adsorption capacities were observed in acidic solutions, with optimum pH values being considered as 2.0 for GS and MGS and 4.0 for GSAC. Optimum residence time and adsorbent concentration were 40 min and 25 g L−1 (GS), 30 min and 15 g L−1 (MGS) and 30 min (GSAC). Moreover, equilibrium was evaluated through adsorption isotherms construction, which were best represented by Sips model, displaying determination coefficients R2 equal to 0.994, 0.999 and 0.987 and maximum adsorption capacities equal to 89.2, 129.6 and 916.7 mg g−1. Adsorbents particular characteristics such as specific surface areas and micropore volumes were also determined, resulting in 6.23, 4.21 and 1099.86 m2 g−1 and 0.003, 0.002 and 0.568 cm3 g−1 for GS, MGS and GSAC, respectively.