Generating colloidal Fe/C composites via hydrothermal carbonization – A critical study

Abstract

The increasing need for water remediation due to water scarcity and the accumulation of persistent pollutants in water cycles requires innovative materials and their more sustainable synthesis processes. The aim of this work was therefore to develop an approach for the bottom-up synthesis of colloidal Fe/C composites. It was tested whether the composites fulfill the main criteria for use as injectable adsorbents and reducing agents for the removal of chlorinated pollutants in in-situ groundwater remediation. After screening different Fe and C precursors, the bottom-up synthesis of a particulate and reactive Fe/C composite was developed via hydrothermal carbonization (HTC) of mixtures of ferrous and sodium gluconate in a one-pot HTC and subsequent carbothermal reduction at 800 °C. The produced particles show good dispersibility with particle diameters in the range of d10 ≥ 2 µm, d50 ≈ 11 µm and d90 ≤ 40 µm in aqueous suspensions. By varying the molar ratio of Fe to gluconate in the HTC process, the final content of zero-valent iron equivalents (ZVI) in the composite could be adjusted between (24 ± 12) and (49 ± 15) wt.-%. The chemical reactivity of the composite was tested by reductive dechlorination of chloroform (CF) as a test reaction. The characterization of the composites before and after reductive dechlorination with X-ray diffraction (XRD) shows that not only pristine Fe0 but also Fe3C is active in the reductive dechlorination reaction. Palladization of the composite material shifts the product selectivity from dichloromethane as the main product to the non-chlorinated products methane (up to 77 mol-%) and ethane (up to 9 mol-%) without any external H2 feed. The dispersiblity and intrinsic reactivity of the synthesized composites meet the requirements for application in water remediation where contaminants need to be retained and degraded, e.g. for groundwater remediation or protection by the in-situ generation of permeable barriers using modern injection technologies.