Porous carbons derived from hydrothermally treated biogas digestate Maria Bernardo a,⇑ , Catalina Rodriguez Correa b , Yvonne Ringelspacher b , Gero C. Becker b , Nuno Lapa a , Isabel Fonseca a , Isabel A.A.C. Esteves a,⇑ , Andrea Kruse b,⇑ a LAQV-REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal b University of Hohenheim, Institute of Agricultural Engineering, Department of Conversion Technologies of Biobased Resources, Garbenstrasse 9, 70599 Stuttgart, Germany article info Article history: Received 18 September 2019 Revised 4 February 2020 Accepted 10 February 2020 Keywords: Biogas production Biomass digestate Hydrothermal carbonization Nanoporous carbons Phosphate recovery from water abstract Porous carbons from digestate-derived hydrochar were produced, characterized and their performance to reclaim phosphate from water was evaluated as a preliminary approach to demonstrate their practical application. In a first step, the digestate was converted into hydrochars through hydrothermal carboniza- tion by using two different pH conditions: 8.3 (native conditions) and 3.0 (addition of H 2 SO 4 ). The result- ing hydrochars did not present significant differences. Consecutively, the hydrochars were activated with KOH to produce activated carbons with enhanced textural properties. The resulting porous carbons pre- sented marked differences: the AC native presented a lower ash content (20.3 wt%) and a higher surface area (S BET = 1106 m 2 /g) when compared with the AC-H 2 SO 4 (ash content = 43.7 wt% S BET = 503 m 2 /g). Phosphorus, as phosphate, is a resource present in significative amount in wastewater, causing serious problems of eutrophication. Therefore, the performance of the porous carbons samples to recover phos- phate – P(PO 4 3À ) – from water was evaluated through exploitation assays that included kinetic studies. The lumped model presented a good fitting to the kinetic data and the obtained uptake capacities were the same for both carbons, 12 mg P(PO 4 3À )/g carbon. Despite the poorer textural properties of AC-H 2 SO 4 , this carbon was richer in Ca, Al, Fe, K, and Mg cations which promoted the formation of mineral com- plexes with phosphate anions. The results obtained in this work are promising for the future develop- ment of P(PO 4 3À ) enriched carbons that can be used thereafter as biofertilizers in soil amendment applications. Ó 2020 Elsevier Ltd. All rights reserved. 1. Introduction Biogas is an important source for renewable heat, electricity and fuel generation. A large variety of biomass and biowastes, for instance agricultural or forestry bioresources, sewage sludge, municipal solid wastes (MSW), industrial and household wastes, can be used as raw materials for biogas production (Mao et al., 2015). Biogas is produced by anaerobic digestion of biomass, at which an additional residual product, referred to as digestate, is formed. The potential of biogas production is emphasized by the increasing number of biogas plants installed in Europe (Association, 2018). Consequently, the large amounts of digestate produced must be disposed, or alternatively used to avoid con- straints in the development of biogas sustainable industries. Until now, the digestate is mainly used as organic fertilizer and soil con- ditioner, after proper treatment (Eder, 2014). It provides a good source of available nutrients (especially nitrogen and phosphorus) and has a positive effect on soil biological properties. However, its use in agriculture is restricted, since most EU member states regu- late the quality and specifications of digestate through waste laws or fertilizer legislation regarding, for instance, hygiene and biodegradability parameters. Additionally, an improper use of the digestate can lead to ecological problems due to, for example, heavy metal ions (particularly of Cu and Zn), persistent organic pollutants, and spreading of pathogens through soil and water (Eder, 2014). Therefore, new opportunities for digestate valoriza- tion are emerging (Monlau et al., 2015) such as hydrothermal car- bonization (HTC) (Funke, 2015; Garlapalli et al., 2016; Mumme et al., 2011; Reza et al., 2014). HTC is an exothermic process during which organic matter is decomposed. This process results in a lignite-like char (hydrochar) with a higher carbon and lower hydrogen and oxygen contents than the respective feedstock. Generally, any organic material can be used as feedstock (Funke and Ziegler, 2010). HTC is per- formed in the presence of water at moderated temperatures (typ- ically in a range of 180–250 °C) under subcritical conditions. The high-water content of the digestate does not play a significant role in terms of energy balance (Wang et al., 2018). Since the feed must https://doi.org/10.1016/j.wasman.2020.02.011 0956-053X/Ó 2020 Elsevier Ltd. All rights reserved. ⇑ Corresponding authors. E-mail addresses: maria.b@fct.unl.pt (M. Bernardo), i.esteves@fct.unl.pt (I.A.A.C. Esteves), Andrea_Kruse@uni-hohenheim.de (A. Kruse). Waste Management 105 (2020) 170–179 Contents lists available at ScienceDirect Waste Management journal homepage: www.elsevier.com/locate/wasman