Environmental Engineering Reference
In-Depth Information
aiming at cost effective and sustainable removal or selective recovery of heavy
metals from industrial effluents [5, 6, 7, 8].
6.2 Existing Technologies for Heavy Metal Removal
A number of approaches have been developed or suggested for the treatment of
industrial effluents in order to meet mandatory discharge standards. The most
commonly used techniques are precipitation, adsorption, ion exchange, reverse
osmosis, and ion flotation. A brief overview of these existing technologies is
given here.
6.2.1 Precipitation
Precipitation is a well-known process capable of removing heavy metals from
aqueous solution [9]. For example, by the addition of sodium hydroxide or lime,
the solution pH is raised to a regime exceeding the solubility of metal hydro-
xides, causing the precipitation of metal hydroxides and lowering the concen-
tration of metal ions in solution. This method is effective for the removal of
large quantities of metals from contaminated water and is extensively used in
industry because of its simplicity. The problems associated with precipitation
process are slow solid-liquid separation, low density of solids, and the ultimate
disposal of the voluminous sludge which often contains a high content of water.
The inappropriate disposal of unstable precipitates may cause secondary con-
tamination of water because metal ions can be leached out from the sludge,
returning to the aqueous environment. In addition, a polishing step is required
for most precipitation processes in order to achieve low residual levels of metal
ions in the processed water. Furthermore, precipitation is a costly method
without the offset of producing secondary resources. There are only a few
metals that can precipitate to form a valuable solid product, such as gypsum
for the construction industry.
6.2.2 Adsorption
Adsorption process is based on the adsorption of soluble contaminants in
solution onto a solid adsorbent. The widely used material of adsorption is
activated carbon though sandstone, fly ash, clay, and other surface reactive
adsorbents are often used in wastewater treatment. This method is capable of
removing most toxic species, including Cu
2+
,Cr
4+
,Pb
2+
,Hg
2+
, and Zn
2+
.
Since most adsorption processes are preformed in a column packed with
adsorbents, a prefiltration step is needed for most industrial applications in
order to remove finely divided solids which may, otherwise, clog the channels
