Biochar is a green material obtained as by-product from thermochemical processes producing bio-crude from biomass such as pyrolysis and hydrothermal liquefaction. Biochar owns very interesting characterisics: it presents very high surface area with well developped porosity. It represents thus one of the best candidate to replace activated carbon in water remediation treatments. Activated carbon is, until now, the most used adsorbent in water treatments but its production from coal or biomass is still very expensive.
Goal of this project is to use the biochar produced as by-product of hydrothermal liquefaction of oak wood as adsorbent to remove organic pollutant from water. The project aims to overcome one of the big issues linked to the use of biochar for water remediation that is the difficulty of separation of powdered biochar from the acqueous solution.
To reach this objective, in the hydrothermal liquefaxction reactor we will produce in one step a magnetic biochar, easily to be recovered. The magnetic biochar comes from the use of metal Fe as catalyst in the HTL of oak wood, to enhance the bio-crude yield and quality. At the end of the reaction in fact four products are obtained: a bio-crude that is the target product of the process, a water phase with a very high carbon load, a gaseous phase rich in CO2 and a solid residue composed by the biochar and the exhausted catalyst. When Fe is used two advantages on the biochar side should be acquired; biochar has higher surface since the biomass is converted more into liquid products and it becomes magnetic.
The project will be centered on optimization of the operative condistions to be used to produce high quality bio-crude togheter with a biochar having the already mentioned characteristic. Furthermore we will analyze the efficiency of the pollutant removal process from water using two model compounds, phenol and furfural focusing on the efficiency of biochar recovering from water.
The use of biochar as adsorbent for water pollutants is a well-known altrenative to the use of activated carbon. In fact, many studies are present on the potentiality of the biochar to remove heavy metals and organic species from water. However, the separation of the powdered biochar is still a challenge. The biochar is now commonly separated by centrifugation and filtration, these steps limit the application of biochar in large-scale processes. Furthermore during these separation procedures the resolubilization of the adsorbed pollutants may occur, decreasing the efficiency of the treatment [1]. To this end the use of magnetic biochar can be an effective alternative; in this way the solid can be easily removed from water by magnetic separation.
The production of magnetic biochar is not trivial, the majority of the processes includes two steps or high temperatures and furthermore, the biochar cannot be considered a by-product but it must be produced for the scope.
In this project we propose to produce magnetic biochar in one step in the hydrothermal liquefaction reactor. In the hydrothermal liquefaction process (HTL) biochar is a by-product; the reaction is conducted at medium temperature around 300 °C and in presence of a catalyst. The use of metal Fe as a catalyst in HTL leads to an increase of bio-crude yield and quality. In fact, Fe in the HTL conditions is oxidized by water forming Fe oxides and H2, the H2 is needed to stabilize and then hydrogenate the fragments which are produced from biomass decomposition.
The biochar in HTL is produced mainly by condensation and polymerization of the biomass heavy molecular weight decomposition fragments. When Fe particles are present into the reactor they act as nucleation kernels for the char formation. In this way the biochar results to be linked to the Fe which in turn is protected by the biochar to a complete oxidation. In fact, at the end of the process we found that metal Fe is only partially oxidized at Fe3O4 which is still magnetic [2].
The project combines, thus, the production of a high quality bio-crude with the valorization of a by-product, the biochar. The use of HTL biochar for waste water treatments is quite new and for the best of our knowledge it is the first time that the magnetization of biochar is the secondary target of a process and is made in only one single step.
References
[1]Yunqiang, Y. et al. Bioresource Technology, 2020, vol. 298
[2] de Caprariis, B. et al. Journal of Applied and Analytical Pyrolysis, 2019, vol. 139