Dissolved Solids

  Total dissolved solids TDS represents the total amount of salts in the water.

 Measured by weighing the salts remaining after oven drying a water sample.

 Therefore it is a gravimetric property

 TDS enters the water by dissolution

 TDS comes out of the water by precipitation

Precipitation and Dissolution

 Dissolution: Minerals enter into water

 Precipitation: Minerals are deposited out of water

Effect of pH on TDS

 Some solids such as carbonates, oxides and hydroxides tend to dissolve under acidic conditions (pH < 7)

 By the same token, these solids tend to precipitate out of the water under basic conditions pH >7)

 Example. Calculate the pH of a saturated Mg(OH)2 solution.

Solution

The equilibrium reaction for the dissolution of magnesium hydorxide (Brucite) is,

pKsp = 11.15.

This reaction can be expressed mathematically using the Law of Mass Action,

By definition all solids equal 1.0. Therefore,

Electroneutrality Condition (ENC) requires that [H+] + 2[Mg2+] = [OH-]. However, since a base is added, it is anticipated that [H+] << [OH-]. Therefore ENC can be simplified to,

, or

Substitution into the Law of Mass Action equation leads to:

Solving for the hydroxide concentration:

Therefore pOH = -log(2.42x10-4) = 3.62, and pH = 14 - 3.62 = 10.38.

 Example. An electroplating operation discharges 1.0 L/s of wastewater into a river. The concentration of Cr3+ in the industrial effluent is 10 mg/L and the pH is 1.0. The NPDES permit to discharge allows this industry to discharge Cr3+ at concentrations below 0.62 mg/L in the pH range between 6.0 and 9.0.

Determine:
    1. The minimum pH of the wastewater required to meet the Cr3+ permit. Note: Cr3+ precipitates as Cr(OH)3. Use Ksp from the Solubility of Oxide and Hydroxide Table.
    2. Determine if the pH found before is within the acceptable range.
    3. Determine final operation pH and the amount of neutralizing base (Quick lime, CaO) in Kg/mo.

Solution

1. Chromium precipitation approach

  pKsp = 29.8

By definition all solids equal 1.0. Therefore,

Where Cr3+ is set by the NPDES limit ( molecular weights):

Solving for (OH-):

pOH = -log(1.10x10-9) = 8.96

pH = 14 - pOH

pH = 5.04 (not OK). Note: this value is outside the NPDES permit range (6 - 9).

2. Therefore the NPDES range rules. Use pH = 6.0 for the acid neutralization program.

3. The pH has to be changed from an initial 1.0 to a final 6.0. Therefore the change in H+ is:

This change in proton concentration is created by the addition of an equal amount of hydroxide ions from the base. Neutralization of the acid occurs as follows:

Dissolution of quick lime, CaO generates the hydroxides needed to neutralize the acid:

Therefore each mol of CaO produces two hydroxide ions. The concentration of CaO required to neutralize the acid is ( molecular weights):

The chemical requirements are:

 Solubility of solids of strong acids (chlorides, fluorides, sulfates, etc.) is not affected by pH. Their solubility is only affected by the presence of the conjugate base (Cl-, F-, SO42-, etc.)

 Example. Ira T. Dement, the operator at the local water treatment plant has gone insane. He has decided to poison the community by overfeeding the treated water with fluoride (Note: the SDWA requires a minimum of 1.0 mg/L F-, while the MCL = 4.0 mg/L, therefore fluoride is added to the water if the natural concentration is below 1.0 mg/L. Fluoride in concentrations above the MCL are in violation of the SDWA, but not necessarily lethal. F- would have to be above 20 mg/L to cause some health damage). Determine if he can cause any damage to the unaware population.

Hints:

Solution:

Ksp = 3.0x10-11

Where

  ( molecular weights):

Conclusion: Most of the fluoride would precipitate out in the storage tank and distribution system. Less than the SDWA's MCL would be available in soluble form. Therefore Ira's efforts would be futile. However, there is potential danger in communities where the water hardness is low, or in plants where that practice softening.

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Updated: Aug. 11, 98