**2.1. Biosorption and bioaccumulation**

Biosorption is a physicochemical process that utilizes the mechanisms of absorption, adsorption, ion exchange, surface complexing, and precipitation processes. It is a spontaneous process independent of the metabolism of microorganisms. In biotechnology, it is used to separate inorganic and organic substances from the solution using biosorbents. Biosorption is an important process also in protecting the environment.

Biosorption is defined as the passive adsorption of toxic substances by dead, inactive or biologically derived materials. Biosorption is a consequence of several metabolic processes independent of the cell membrane, the mechanisms responsible for the absorption of the pollutant vary according to the type of applied biomass.

Bioaccumulation is defined as the phenomenon occurring in living organisms. More specifically, bioaccumulation is defined as the absorption of toxic contaminants by living cells or organisms. Compounds are passively or actively transported into cells, accumulated inside them, and they also enter the metabolic cycle through the cell membranes. Bioaccumulation is therefore often dependent on cell metabolisms.

Both bioaccumulation and biosorption have certain advantages and disadvantages. In general, the use of living organisms is not suitable for continuous water purification processes from highly toxic organic/inorganic contaminants. If the concentration of the toxic substance is too high or the process step takes a long-time period, the accumulated substance quantity may reach partition equilibrium, or saturation. Due to the high accumulated pollutant concentration the metabolism of the organism will be disturbed and death may occur. This scenario can be avoided by using inactive, dead biomass. Moreover, if the sorption process is reversible, compounds may be desorbed back to the treated water if the concentration drops. To avoid desorption, a high sorption capacity has to be provided. This is not always feasible in processes applying living cells, because of various restrictions such as requirements of nutrients, aeration, maximum cell density, and so on. This is why we devote more attention to biosorption than bioaccumulation.
