Fertilizer and activated carbon production by hydrothermal carbonization of digestate
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In this study, the efficiency of recycling phosphate from digestate through hydrothermal carbonization and subsequent production of an activated carbon with high surface area was investigated. Phosphate fractionation was conducted to understand the phosphate existing forms in hydrochars and the recovery mechanism. Hydrothermal carbonization was conducted in a 250 mL batch reactor at three different temperatures (190, 220, and 250 °C), through this process, more than 92.6% of digestate phosphate was recovered into the hydrochars, mostly in the form of precipitated calcium phosphate. Through leaching the hydrochars with sulfuric acid, 88.9 to 94.3% of the total digestate phosphate was recovered in the acid solution. Untreated and acid-leached hydrochars were subjected to a chemical activation process with KOH. The surface area of the products was comparable to commercial activated carbon and was composed mainly of micropores. The activated carbon generated from acid-leached hydrochar showed higher microporosity (up to 1762 m2 g−1) and better adsorption capacity (656.7 to 788.0 mg methylene blue per gram activated carbon).
KeywordsDigestate Hydrothermal carbonization Acid leaching Phosphate recovery Activated carbon
The authors sincerely appreciate Ms. D. Neumann-Walter, Mr. K. Hermann, and Mr. A. Lautenbach from the Institute of Catalysis Research and Technology (Karlsruhe Institute of Technology, Germany), for conducting the N2 isotherms, ICP-OES, and phosphate measurement, respectively. Thanks also go to Ms. M. Wunderlich for her assistant in the lab and the Bauer family for providing the digestate. The authors acknowledge the scholarship provided by China Scholarship Council.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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