Activated carbons from hydrothermal carbonization of municipal solid waste

1. Introduction – In recent years, the global production of municipal solid waste (MSW) has increased. A significant amount of the waste that is not suitable for recycling is incinerated or landfilled, raising serious environmental issues. This has promoted the investigation of more sustainable technologies for waste exploitation and conversion towards valuable materials. Hydrothermal carbonization (HTC) has received considerable attention for its potential to process heterogeneous organic wastes. Operating under aqueous conditions at moderate temperatures and autogenous pressure, HTC yields a carbonaceous solid product known as hydrochar. The condensed carbon structure makes hydrochar attractive as a feedstock to produce materials such as activated carbons [1]. In the present study the potential of HTC to valorize a waste stream from mechanical biological treatment of mixed MSW, specifically the under-sieve fraction, which is currently landfilled, was explored. Activated carbons were prepared through HTC followed by chemical activation, and tested for the removal of pollutants, namely Benzene, Toluene, Ethylbenzene, and Xylenes (BTEX), from aqueous phases. The investigation was performed through the Design of Experiment (DoE) - Response Surface Methodology (RSM) approach, in order to assess the optimal process conditions to achieve the desired product properties. 2. Experimental – The under-sieve fraction from the mechanical treatment of mixed MSW was provided by Scapigliato S.r.l., a waste management company located in Tuscany (Italy). HTC tests were conducted in a laboratory-scale stainless-steel reactor. Activated carbons were prepared by chemical activation of hydrochar (HC) using potassium hydroxide (KOH). Tests were performed in a fixed bed tubular reactor at different activation temperatures (500-700 °C) and impregnation ratios KOH/HC (0.5–1.5 wt/wt), according to a randomized design matrix obtained by DoE-RSM. Batch adsorption experiments were carried out using BTEX solutions in tap water with the activated carbons produced. 3. Results and Discussion – The obtained results demonstrated the feasibility of HTC for the waste feedstock considered. The joint effect of process parameters (temperature, time, and solid load) on the yield and properties of the hydrochar produced was evaluated. Predictive models were developed by RSM analysis, and the optimum conditions for maximizing the carbon yield were identified and selected to produce hydrochar for activated carbons preparation: 190 °C reaction temperature, 60 min reaction time, and 5 wt % solid load. The chemical activation stage led to the development of a porous structure, as evident by scanning electron microscopy (SEM) analysis (Image 1). RSM analysis allowed to investigate the effects of activation temperature and KOH/HC impregnation ratio on activated carbons morphology and adsorption capacity. Specific surface areas reaching up to 587 m2/g were achieved with the highest activation temperature and impregnation ratio explored. Notably, the maximum BTEX removal efficiency, approximately 85 %, was attained under mild activation conditions (600 °C) (Image 2). 4. Conclusions – The results evidenced that hydrothermal carbonization followed by chemical activation is a promising way to convert waste into valuable adsorbents suitable for contaminants removal from water. 5. References [1] E. Stefanelli, S. Vitolo, N. Di Fidio, M. Puccini, J. Environ. Manag., 345, (2023) p. 118887.