Conventional precipitation methods of removing heavy metals rely on
formation of insoluble precipitates using chemical reagents.
The sludges formed must be separated by settling or filtration.
There are serious drawbacks:
The time for precipitation is long.
Insoluble metal hydroxides or sulfides may be difficult to remove.
Resulting sludges have to be concentrated prior to their disposal.
● Sludge disposal represents an enormous problem :
● Toxic metal sludges are invariably classified as ‘hazardous
● Recovery of metals from sludges is usually not feasible.
Major problem: Precipitation/separation approach has difficulties in meeting progressively
more stringent wastewater criteria.
A contemporary high-performance process using synthetic resins
usually packed into a flow-through column. The resin binds metals
from solution until it is saturated.
Then it the column must be taken out of operation and the desorption-washing
procedure follows directly in the column. As the resin gets regenerated
for another use it releases the metal in the concentrated form in the wash
Deployment of expensive ion exchange resins for large volumes of contaminated
wastewater has been economically rather unsustainable.
Biosorbents are natural and cheap ion exchangers which operate on exactly
the same principle, using the same standard process equipment: columns,
pumps, valves and pipes.
The process uses membrane stacks and high pressure. The
metal gets concentrated on one side of the membrane, while purified water
ends up on the other. Membranes are very expensive and easily foul
up. Elevated pressure is also an operating drawback.
This high-performance system is an expensive alternative treatment
method not feasible for wastewater treatment in most cases.
Metal recovery is possible only from concentrated enough solutions.
From preconcentrated solutions metals are recovered by "electro-winning"
Metals are plated from the solution by electric current applied through
Electro-winning is a standard conventional technology used 'for
profit' by specialized operators who re-sell the recovered metals.
They collect suitably pre-concentrated metal solutions for this type of