Pollutants from soil, water and gases can be removed with biochar. According to Weber (Industrielle Anwendungen von Biomassekarbonisaten. Adsorptionsmittel, in: Quicker/Weber 2016, Biokohle), these can be organic pollutants such as PAHs, antibiotics and other pharmaceutical residues, pesticides, herbicides and fungicides as well as heavy metals or other residues from industrial processes. The efficiency of the separation (adsorption) depends on the properties of the biochar, the pollutant and its environment.
Adsorption is the adhesion of a liquid or gaseous substance to the surface of a solid. This process is either physical or chemical. In the case of physical adhesions of a substance, the buildup occurs due to molecular surface forces (van-der-Waals forces). In the case of chemical deposits, the pollutant and the biochar undergo a chemical reaction on the surface, and the bond is usually stronger.
Materials with a porous structure are used to achieve the highest possible adsorption performance. Pores are divided into micro, meso and macro pores and together they are responsible for a very large surface area. According to Weber, the density and thermal conductivity of industrial adsorbents is decisive in addition to the surface and the pore volume. To achieve a pore structure with a large surface, a so-called ‘activation’ is carried out, which leads to an expansion and enlargement of the pore structure and thus also of the inner surface.
Fields of application
The capabilities of carbonaceous adsorbents, such as activated carbon, are already being used successfully in several areas:
- Gas and air purification: A typical application is the separation of mercury from flue gases after combustion. As a rule, activated carbon is used for this purpose.
- Water treatment: Activated carbon is used, for example, in water treatment in sewage treatment plants as so-called 4th cleaning stage. Micropollutants (microplastics) are eliminated with the help of activated carbon filters.
- Soil remediation: To this end, adsorbents are introduced into the soil where they bind pollutants such as heavy metals and prevent leaching into the groundwater. However, the floor must still be removed for actual cleaning.
- Detoxification in medicine: Toxins can be bound in the stomach immediately after taking activated carbon, and then excreted. Not only that, substances that circulate between liver, intestine and gall bladder and have already entered the bloodstream can also be re-adsorbed with the aid of activated charcoal.
Activation of biochar
As in other areas, according to Weber, pyrolytically produced biomass carbonates (biochar) can replace and supplement fossil resources during deposition. The biochar is often used as an adsorbent without prior carbonisation. Activation similar to fossil coal is possible, however. This can be done in two ways. During physical activation, the biomass is first carbonised and then activated by the addition of steam or carbon dioxide. During chemical activation, the biomass is treated with a chemical substance (e.g. NaOH, H3PO4 or KOH) and then carbonised. Chemical activation is a one-step process and compared to physical activation it usually takes place at lower pyrolysis temperatures.