WATER TREATMENT
QCM components
Sequestration , the complexation of a metal ion such that the ion does not retain its original reactive properties , can be another important function of treatment additives , particularly of many polymers and phosphonates . Unlike threshold inhibition , it does not specify stoichiometry or specific functionality . Some phosphonate or polymer additives commonly used for mineral scale control can sequester
Figure 2 - Example of blank QCM experiment
ions like calcium , magnesium and barium , preventing them from forming insoluble complexes with compounds such as carbonate and sulfate .
A chelate is a coordination compound in which a central metal ion such as Ca 2
+ is attached by coordinate links to two or more nonmetal atoms in the same molecule , called ligands . Thus , a chelating agent is an additive that links to a metal ion at two or more points within the agent molecule .
In practice , polymers such as polycarboxylates and sulfonated copolymers and phosphonates act as chelating agents with most multivalent ions , due to the multiple anionic binding sites within the molecules . In common usage , chelation further implies a more permanent or substantive relationship between the ion and the ligand , and refers to stoichiometric relationships between them .
Stabilisation may refer to two distinct mechanisms . In colloidal stabilisation , precipitation in a fluid occurs , but the polymer additive prevents agglomeration of particles of < 1 µ g in size . These are stabilised via electrostatic interactions with the polymer and remain suspended throughout the water phase . Colloidal stabilisation can fail due to physical or chemical changes in the fluid that result in particulate agglomeration beyond 1 µ g and bulk settling of the precipitate .
The alternate usage of ‘ stabilisation ’ is as a synonym for sequestration , where a coordination complex
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