As discussed in Chapter 10, the uncontrolled release of metal ions poses a threat to the environment. An alternative removal scheme is cementation, whereby redox activity takes place between a metal substrate, M), and the ions of a more noble metal, M2 (i.e., a metal with a more positive standard potential that needs to be removed from the solution), due to a thermodynamically spontaneous reaction.

In general, a cementation process can be described by the following equation (for simplicity, two divalent metals are considered here):

In this way, N is the sacrificial metal, and M is the cemented metal (that is, the one to be removed). Besides the spontaneous nature of the process, other advantages include its relative simplicity, ease of control, and the possible recovery of valuable metals. Challenges to this process involve the possible presence of chelating agents, mass transfer limitations, and the introduction of new species into the medium. Some examples of applications of cementation processes include:

• removal of polluting metal ions from aqueous effluents

• purification steps in metallurgical operations

• recovery of spent metals from different chemical operations

The present experiment is an example of this last application. However, in environmental cleanup applications one has to analyze the possible impact of the new ions added to the aqueous system.

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