Utilization of Digestate from Agricultural and Food Waste for the Production of Biochar Used to Remove Methylene Blue

Abstract

The treatment of waste or by-products from the agri-food industry in agricultural biogas plants results in the production of biogas. After anaerobic digestion, digestate remains and is often used for soil fertilization. The solid digestate (SD) can also be used for the production of biochar, a material with specific properties and many applications. Such a model of operation fits perfectly into the concept of a circular economy, because the waste material can be used to produce an adsorbent that can be used to treat industrial wastewater. The research assumption of this study was to investigate selected properties of biochar prepared at variable temperatures in the pyrolysis process from solid. The potential of biochar for methylene blue (MB) sorption was also initially investigated in terms of biochar’s suitability for immobilizing metals in degraded soils. Biochar was produced at temperatures between 400–900 °C, with a temperature gradient of 50 °C. The efficiency of the production was in the range of 51–40% and decreased with the increasing temperature. The rising process temperature was also accompanied by a decrease in the nitrogen and hydrogen content of biochar. The produced biochar had an alkaline pH ranging from 11.40 to 12.69 and it increased as the temperature increased. The rise in the pyrolysis temperature effected a significant increase in the specific surface area BET (Brunauer–Emmett–Teller) in the case of biochar BSD750, BSD800, and BSD850 (BSD—biochar solid digestate). When analyzing the C content of individual biochar variants, there was no clear downward or upward trend, just as in the case of TOC (total organic carbon) value for the produced biochar. The greatest potential for removing MB (methylene blue) from solutions was demonstrated by biochar produced at 650–900 °C. The ability to remove MB rose along with the production temperature of the biochar.