Page 708 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Precipitation 663

ambient waters. Along with the traditional concern with BOD, 21.2.1.1 Solubility Law
however, other issues began to emerge, as stream standards Consider a particular reaction, that is, the dissolution of the
became more stringent to include a wide variety of contam- solid, Mg(OH) 2 :
inants, with restoration and preservation of stream ecology
emerging as a principle goal. These issues and a host of others
Mg(OH) 2 ! Mg 2þ þ 2OH (21:1)
were addressed in the 1972 Clean Water Act, for example,
PL92500 and its subsequent amendments. Control of toxic
The equilibrium statement is
pollutants, which included metals, was included. As noted,
precipitation is a favored method of metals removal, for 2
¼ [Mg ] [OH ] (21:2)
2þ
K Mg(OH) 2
example, by hydroxide, carbonate, sulfide, etc.
Biosolids (sludge) have always been a major issue in ¼ 5:6 10 12 (21:3)
wastewater treatment. Ocean disposal had been common for
coastal cities, through the 1960s, which was always contro- in which
versial. With the Clean Water Act this changed and land is the solubility product of magnesium
K Mg(OH) 2
disposal became favored. Because crops are usually grown hydroxide
or are a potential for any land, heavy metals in the biosolids [Mg ] is the molar concentration of magnesium ion
2þ
had to be limited. Since industrial wastes have been the (mol=L)
sources of most heavy metals, pretreatment was mandated. [OH ] is the molar concentration of hydroxide ion

All of this evolved through the 1980s and into the 1990s, (mol=L)
which resulted in biosolids with a very low fraction of 5.6 10 12 is the numerical value of solubility product of
metals, suitable for application to land which could grow magnesium hydroxide (Table 21.1)
selected crops.
As a note, removal of metals involves management The solubility product is the same as any other equilibrium
strategies, such as reduction in metals flux, substitutions constant except that the activity of the solid precipitate, the
in industrial processes, separations within industrial pro- unstated denominator, equals one. The concentrations of
cesses, recovery and reuse, final disposal in secured landfills, Mg 2þ and OH are at ‘‘saturation’’ with respect to the solid.

etc. (Patterson, 1990, pp. 27–42). This addresses only ‘‘end- If the pH is changed, [OH ] changes and the [Mg ] changes
2þ

of-the-pipe treatment.’’ in accordance with Equation 21.2. If the concentrations of
Mg 2þ and OH exceed the solubility product, that is, if
2
, then precipitation occurs; if less,
2þ
21.2 PRECIPITATION THEORY [Mg ][OH ] > K Mg(OH) 2 2
2þ
that is, if [Mg ][OH ] < K Mg(OH) 2 , then the solid,
Key variables in the precipitation process are (Kemmer, 1979, Mg(OH) 2 , dissolves. Precipitation requires nuclei, which are
pp. 10–12) (1) solubility product, (2) temperature, (3) particle present in most waters.
charge, and (4) time. The solubility product is temperature
dependent, and the time has to do with the reaction kinetics 21.2.1.2 Application of Solubility Law
and the rate of mixing. Particle size increases if the chemical To remove an undesired cation from solution, an anion may
reaction occurs on particles that have been formed previously be added so that the solubility product of the precipitate
(Kemmer, 1979, pp. 10–11). product is exceeded. For example, if magnesium is to be
The formationof precipitate occurs in three stages (Patterson removed, one may raise the pH; pH ¼ 11 is common, such
2
et al. 1990, p. 95): nucleation, crystal growth, and aging. that the solubility product, [Mg ][OH ] > K Mg(OH) 2 ¼
2þ
Most often, the nucleation occurs on preexisting particles of 5.6 10 12 is exceeded. Example 21.1 shows how to calculate
various types. Crystal growth occurs in two phases: (1) move- the residual concentration of Mg 2þ when the pH is raised to
ment of solute to the crystal=water interface by advection and pH ¼ 11.
diffusion, and (2) adsorption of the solute onto the solid surface
and incorporation into the crystal lattice.
Example 21.1 Mg 2þ Concentration after Raising
pH to 11
21.2.1 EQUILIBRIA
Given
The equilibrium between concentrations of particular ions in Magnesium ion, Mg , is a hardness component in a
2þ
solution and the solid precipitate is expressed in terms of the water and is to be removed by precipitation; its concen-
solubility product. If a slightly soluble species is in equilib- tration in the ambient raw water is C(Mg ) ¼ 22 mg=Las
2þ
rium with a precipitate, for example, PbCl 2 , there will be CaCO 3 . The pH is to be raised to pH ¼ 11 (in a reactor).
some undissociated PbCl 2 molecules and PbCl ions; usually, Required

however, these are ignored and only Pb 2þ are considered Estimate the concentration of Mg 2þ residual, that is,
(Silberberg, 1996, p. 803). [Mg ] solution.
2þ