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| In this work three colloidal material types were prepared by hydrothermal carbonization (HTC) and subsequent pyrolysis – two carbonaceous adsorbents and one reactive Fe/C composite. The process parameters were closely investigated and adjusted for generating particles with the desired textural and colloidal properties. The latter are essential for the planned application in in-situ groundwater remediation where stable suspensions are needed for injection into aquifers. The two carbonaceous adsorbents were synthesized by HTC of sucrose with an optional subsequent pyrolysis step. The addition of the stabilizing agent carboxymethyl cellulose (CMC) during the HTC of sucrose optimized the mass yield of carbonaceous spheres (CS) with a narrow size distribution around 1 μm. Due to their hydrophilic surface and negative surface charges these ‘raw’ CS exhibit an extraordinary suspension stability which makes them suitable for in-situ groundwater remediation. Sorption coefficients of up to 105 L kg-1 were observed for hydrophobic substances such as phenanthrene in environmental concentrations. The parameters for pyrolysis of the CS were investigated closely and suitable conditions were found in order to maintain the aqueous dispersibility of the activated CS (aCS) also after the pyrolysis step. Under all applied pyrolysis conditions a low heating rate of about 1 K min-1 was crucial in order to minimize inter-particulate aggregation processes and to ensure sufficient dispersibility of the aCS in water. The defunctionalization in combination with tuning the mean pore diameter upon adding 3 vol.-% steam to the N2-atmosphere during the pyrolysis led to high adsorption coefficients of up to 106.5 L kg -1 even for the anionic contaminant perfluorooctanoic acid (PFOA) at an equilibrium concentration of 20 μg L-1 PFOA. However the ageing of the aCS surface during storage over one month in water with abundant oxygen significantly impaired the adsorption of PFOA. For the generation of a material that is sorptive as well as reactive mechanically stable Fe/C composites with the desired colloidal properties were hydrothermally synthesized. In this process ferrous gluconate proved as most promising precursor due to the complexation of Fe2+ by gluconate. It was shown that the morphology and speciation of the synthesized Fe/C composite enable reductive dechlorination of chloroform with specific activities (normalized to the concentration of zero-valent iron (ZVI) equivalents) that are comparable to nanoscale ZVI. X-ray diffraction (XRD) analyses revealed the presence of Fe3C in addition to pristine Fe0 in the crystalline phase of the composite. It is assumed that Fe3C additionally generates reactive species (e- and H*) needed for the reductive dechlorination. The synthesis process parameters derived in this work can be seen as a basis for future studies where mass yield and material properties can be optimized further according to specific classes of pollutants. Alternative substrates for the production of CS e.g. derived from regional waste sources can be explored within the developed scopes in order to improve the sustainability of the synthesis. |
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