Valorization of Spend Coffee Grounds Using Koh and Concentrated Leachate as Activating Agents in Slow Pyrolysis at 600ºC – a Comparative Analysis of Char Properties

The coffee industry and landfill leachate treatment generate residual waste streams, known as spent coffee grounds (SCG) and landfill leachate membrane concentrate (LLMC). Current practices for managing these residues, including open burning, incineration, and landfilling as a final disposal method, represent a waste of resources and pose a challenge to sustainability. Due to the high pollution potential of solid waste SCG and LLMC, cost-effective management solutions are urgently needed. The present research investigates the slow pyrolysis of SCG using potassium hydroxide (KOH) (weight ratio of 1:1) and LLMC residue (weight ratio of 1:1) as activating agents. The high content of alkali and alkaline earth metals in LLMC could promote the activation of the resulting char and improve the quality of the carbon-based material produced in pyrolysis. The use of LLMC as an activating agent could be a sustainable alternative for valorizing SCG and landfill wastes, potentially replacing ingredients such as steam, CO2, and chemical additives used on an industrial scale. The SCG had a low specific surface area (4.5 m2 g-1), contrasting with the notable surface areas observed in both activated chars. In particular, the KOH-activated char exhibited a higher surface area than the LLMC-activated char, measuring 1,960 m2 g-1 compared to 1,138 m2 g-1 – a difference of about 72%. On the other hand, the combustion enthalpy of the LLMC-activated material was estimated at 22.04 MJ kg-1. The combustion enthalpy of LLMC-activated char was about 21.7% and 19.8% higher than that of SCG and KOH-activated chars, which had values of 18.11 and 18.40 MJ kg-1, respectively. Our findings confirm that pyrolysis of SCG with KOH produces a microporous material with a high specific surface area. In contrast, the resulting LLMC-activated char demonstrates a higher value of combustion enthalpy. This work showed that both activated chars had superior energetic and morphological properties compared to the non-activated char made from SCG biomass. Among the activating agents, KOH led to better performance in terms of char yield and morphological properties. Meanwhile, utilizing LLMC residue as an activating agent highlights its potential for converting landfill waste into high-value material.