Page 252 - Materials Chemistry, Second Edition

Technologies for Treatment of Heavy Metal–Contaminated Groundwater 233

Some metal salts are insoluble in water, which is promoted when a counter-anion is
included. Despite the fact that the procedure is expensive, its productivity is influenced
by low pH and the proximity of different salts (particles). The procedure requires large
amounts of different chemicals, which at long last prompts the period of a high–water
content sludge, which is costly to transfer. Precipitation with lime or bisulfide, or par-
ticle exchange, is non-specific and is insufficient for the removal of metal particles at
low concentration. The most commonly used precipitation method is hydroxide treat-
ment because of its relative straightforwardness, ease of precipitant (lime), and sim-
plicity of programmed pH control. The solubilities of the different metal hydroxides
are limited at pH in the range of 8.0 to 11.0 (Alvarez-Ayuso et al., 2003).
Mixed precipitation procedures are suggested when fluids with a rather high
salt content must be dealt with. As a result of wide experience over many years
in the treatment of radioactive fluid waste by substance precipitation, the pro-
vision of equipment should not represent any irregular issues. Expenditure is,
however, an indicator that should be painstakingly considered due to the broad
use of an extensive variety of concentrated synthetic reagents over a wide range
of pH values.
Concerning process costs, there is, right now, an inclination to replace dissipation
with different systems (compound or new processes) beyond what many would con-
sider possible, and for the most part, when huge volumes of fluid radioactive wastes
must be dealt with. Synthetic precipitation is viewed as the least expensive process
(20–50 times more affordable than dissipation). The costs of particle exchange treat-
ment are situated between those for dissipation and substance precipitation.

11.5.2 ion excHange
Ion exchange is another technique used effectively in the business for the removal of
heavy metals from effluents. Despite the fact that it is moderately costly when con-
trasted with alternate strategies, it can accomplish parts per billion levels of purifica-
tion while taking care of a generally expansive volume. It is a reversible compound
reaction in which the removal of heavy metals is accomplished by the exchange of
particles on a bed for those in wastewater. At the point when the resin is saturated,
it must be recovered with an acid or alkaline medium to remove the metal particles
from the resin bed. The regenerated salt water is lower in volume and higher in
concentration than the initial wastewater; however, these metals should then be sat-
isfactorily treated or recovered.
Ion exchange, in the form of a particle exchanger, is highly suitable for exchang-
ing either cations or anions from the surrounding materials. The systems normally
used for ion exchange are synthetic organic ion exchange resins (Alyuz and Veli,
2009). Generally, resins are fabricated in a spherical, stress-free shape to counter
physical degradation. They are stable at high temperatures and over a wide pH range.
Ion exchange resins, which are totally insoluble in many fluids and natural systems,
comprise a cross-connected polymer lattice to which charged residues are attached
by covalent bonding. There is an assortment of resins for particular applications with
different metals. Synthetic organic resins are most regularly used because of their
capacity to be made for particular applications.