Page 258 - Materials Chemistry, Second Edition

Technologies for Treatment of Heavy Metal–Contaminated Groundwater 239

11.5.6.3 Electro-Dialysis
Electro-dialysis is a sort of film detachment treatment in which the ionized species
in the aqueous solution pass following the use of an electric potential. The anions
present in the solution move toward the anode, while the cations move toward
the cathode, crossing the diversely planned membranes. The process efficiency
is advanced by increasing voltage and temperature; nonetheless, it diminishes at
higher flow rates. Most commonly, electro-dialysis innovation uses ion exchange
membranes to isolate metal particles dispersed in wastewater. Cation and anion
exchange membranes can be used to accomplish copper removal efficiency of
94%–97%, and the performance can be upgraded using high particle exchange for
the removal of the selected metal.

11.5.7 elecTro-Winning
Electro-winning, also called electroextraction, is the electrode deposition of metals
from their minerals that have been dissolved in solution or melted. Electro-winning is
one of the numerous innovations used to remove metals from process water streams.
This procedure uses power to pass a current through a fluid metal-bearing solution
containing a cathode plate and an insoluble anode. Charged metallic particles stick
to the oppositely charged cathode, leaving a metal deposit that can be stripped and
recovered. A recognizable disadvantage was that consumption could become a seri-
ous restriction, whereby cathodes would often have to be replaced.
Electro-winning is generally used as part of the mining and metallurgical mechan-
ical operations for pile filtering and corrosive mine seepage. It is likewise used as
part of the metal reclamation, gadget, and electrical businesses for the removal and
recovery of metals. Metals such as Ag, Au, Cd, Co, Cr, Ni, Pb, Sn, and Zn occurring
in effluents can be recovered by electro-winning using insoluble anodes.

11.5.8 ceMenTaTion
Cementation is used as a general term to depict the procedure whereby a metal is
precipitated from a solution of its salts by another electropositive metal with uncon-
strained electrochemical reduction to its essential metallic state, accompanied by
the oxidation of a sacrificial metal, for the recovery of more costly and more valu-
able dispersed metal species in fluid solutions. Cementation is a metal-substitution
treatment in which a solution containing the dispersed metallic ion(s) interacts with
a more dynamic metal, for example, iron. Cementation is, in a way, another precipi-
tation technique, suggesting an electrochemical system in which a metal having a
higher oxidation potential goes into solution; for example, the oxidation of metallic
iron, Fe(0), to ferrous Fe(II), to replace a metal having a lower oxidation poten-
tial. Copper is mainly isolated by cementation alongside other valuable metals, for
example, Ag, Au, and Pb, and also As, Cd, Ga, Pb, Sb, and Sn can be recovered in
this way.
The reaction for copper and iron is

Cu 2+ + Fe° → Cu° + Fe 2+ (11.7)