Abstract
Hydrochars from hydrothermal carbonization of different biowaste materials (dried dandelion, sawdust, coconut shell powder) formed in the presence of aqueous salt solutions were compared to those obtained by the common method in pure water. Hydrochars with increased carbon contents, pore volume and surface areas were specifically obtained from coconut shell powder in the presence of zinc chloride. Compositional and structural changes within the hydrochar products caused by the process conditions and/or the additive were characterized by solid state 13C NMR spectroscopy, proving that cellulose and, in particular, lignin units in the biomass are more easily attacked in the presence of the salt. Under saline conditions, a distinct particle break-up led to the creation of mesoporosity, as observable from hysteresis loops in nitrogen adsorption isotherms, which were indicative of the presence of pores with diameters of about 3 to 10 nm. The obtained hydrochars were still rich in functional groups which, together with the mesoporosity, indicates the compounds have a high potential for pollutant removal. This was documented by adsorption capacities for the methylene blue and methyl orange dyes, which exceeded the values obtained for other hydrochar-based adsorbers. A subsequent physical activation of the mesoporous hydrochars in steam at different temperatures and times resulted in a further drastic increase in the surface areas, of up to about 750 m2/g; however, this increase is mainly due to micropore formation coupled with a loss of surface functionality. Consequently, the adsorption capacity for the quite large dyes does not provide any further benefit, but the uptake of smaller gas molecules is favored.
Highlights
Hydrothermal carbonization (HTC) provides a green and sustainable process, allowing the production of a variety of carbonaceous materials starting from cheap natural precursors, such as plant biomass, manure or digestates, under comparatively mild hydrothermal conditions [1]
Comparison of the carbon content as obtained by the elemental analysis of the raw materials and hydrochars (Table 1) shows that the HTC process leads to an increase in the carbon content by 4 to 11 wt.%, depending on the raw material used
The presence of a mo Fourier transform infrared (FTIR) spectra of the coconut shell powder, the related hydrochars from HTC with phology is salt, a good prerequisite forare the introduction porosity water or with and the activated chars shown in Figure 3.ofThe samples into from these coconut shell powder chosen as representative examples because all with three tested hydrochars fromwere dried dandelion could only be obtained yields of 3 biomasses and obtained hydrochars showed similar trends regarding their surface funcderlining the low stability and high inorganic content of this substrate
Summary
Hydrothermal carbonization (HTC) provides a green and sustainable process, allowing the production of a variety of carbonaceous materials starting from cheap natural precursors, such as plant biomass, manure or digestates, under comparatively mild hydrothermal conditions [1]. The obtained hydrochars from HTC have received increasing interest in various relevant fields of the chemical industry, such as for catalysis, water purification, energy storage and CO2 sequestration [7,8] Due to their low degree of porosity and small internal surface areas of less than 10 m2 /g [9], the hydrochars mainly serve as precursors instead of competitive substitutes to the energy expensive activated carbons received by pyrolysis at high temperatures [10,11,12,13]. With a eutectic mixture of lithium chloride and zinc chloride in the presence of water, a high surface area for carbons from glucose was obtained (673 m2 /g) Due to their extreme structural complexity and diversity, natural raw lignocellulosic biomasses have not received much attention as substrates for HTC conversion to porous hydrochars. In a subsequent process we further treated the hydrochars by means of a physical activation with steam at different activation temperatures to elucidate the potential and drawbacks of this treatment
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