Selective Salt Recovery from Reverse Osmosis Brine - University of ...

Table 5: Solubility Constants for Common Precipitates
Solid pKSO
CaCO3
CaSO4
MgCO3
8.48
4.36
7.46
MgSO4·7H2O 2.13
There are two cases for which the solubility of a mineral is a function of pH: 1) if
one of the ions reacting to form the precipitate is H + or OH - , 2) if one of the ions is a
weak acid or base, such as carbonate.
Precipitation may not occur instantaneously even if the SI indicates that a
particular salt is supersaturated. The rate of precipitation depends on reaction kinetics,
mass transfer, and available surface area for reaction. For crystals to form in suspension,
small particles must exist in the suspension for crystals to grow around. Tiny crystals
will eventually form spontaneously as ion concentrations increases, but this could occur
at concentrations much above those specified by solubility constant. Providing tiny
particles or crystals for this purpose is called seeding and is common practice in
precipitation processes[26].
Complexes forming in solution may affect solubility. Complexes are dissolved
species in waters consisting of a metal and a ligand. Ligands are ions or molecules that
form bonds with the central metal atom. These bonds may be electrostatic, covalent, or
other types such as van der Waals, dipole-dipole, etc. Metal ligand complexes also exist
in equilibrium with the free ions and precipitated salt in the system[14]. The mass of ions
contained in these complexes at equilibrium cannot be neglected when considering
solubility.
1.221 Precipitation in Combination with RO
Precipitation has been used in water treatment for more than a hundred years, but
more recently, as RO treatment has become widespread, it has been optimized to pretreat
water to limit membrane fouling. Membrane fouling generally takes one of four forms,
depending on the nature of the foulant: colloidal, organic, biological, or scaling [27].
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