Many industrial processes, such as mining, metal plating, or
pigment and battery manufacturing, result in the release of heavy
metals to aquatic ecosystems. Heavy metals are toxic pollutants,
which can accumulate in living tissues causing various diseases and
disorders. Removal of toxic contaminants from wastewaters is one of
the most important environmental issues. Since all heavy metals are
non-biodegradable, they must be removed from the polluted streams
for the environmental quality standards to be met.
Many physicochemical methods have been developed for the
removal of heavy metals from aqueous solutions, such as extraction,
ion exchange, chemical precipitation and membrane separation
processes. These methods have several disadvantages like high
operating costs, low selectivity, incomplete removal, and production
of large quantities of wastes. Conventional methods are limited by
technical and economical barriers, especially when concentration of
metals in the wastewater is low (under 100 ppm). Development of
efficient and low-cost separation processes is therefore of utmost
importance.
Another popular method for the removal of heavy metals from
aqueous solutions is adsorption. Parameters taken into consideration
when choosing an appropriate adsorbent are mainly the sorption
capability, regeneration ability, kinetic parameters, price and market
availability. Maximum sorption capability is the most important
parameter that characterizes each sorbent. It is the maximum amount
of the adsorbed substance available for the uptake per sorbent unit
mass or unit volume (usually in mg/g or meq/g). The sorption
capability is determined experimentally at constant temperature, and
the results are presented as isotherms. Sorption capability is a very
important parameter that allows for estimation of process costs, since
the determined isotherms allow to predict the amount of sorbent
required for effective sorption. Ability to regenerate the sorbent is
very important in cyclic processes, especially when expensive and
selective sorbents are used. Kinetic parameters allow to determine the
rate of the sorption process. The choice of the sorbent is determined
by its price and market availability. Prices of sorbents fall within a
wide range. Waste materials from various industrial processes and
biological materials—biosorbents—are also used. Biosorption is the
natural capability of the biomass to immobilize dissolved components,
e.g. heavy metal ions, on its surface. Biomass is composed
mostly of polysaccharides, proteins and fats, and has many functional
groups able to bind heavy metal ions.