Back­ground

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 process

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.

Orig­i­nal article: Indus­trielle Anwen­dun­gen von Bio­massekar­bon­isat­en. Adsorp­tion­s­mit­tel (Indus­tri­al appli­ca­tions of biomass car­bon­ates. Adsor­bents)
Author: Kathrin Weber
Pub­lished in: Peter Quicker, Kathrin Weber (Hrsg): Biokohle. Her­stel­lung, Eigen­schaften und Ver­wen­dung von Bio­massekar­bon­isat­en. Springer Verlag 2016, p. 277–279