Chemical precipitation in water and wastewater treatment is the transformation of dissolved elements in water into solid particles.
Chemical precipitation is a technique for removing ionic components from water by adding counter-ions to lower solubility.
It is typically used to remove metallic cations, but it is also utilized to remove anions like fluoride, cyanide, and phosphate, as well as organic molecules such as the precipitation of phenols and aromatic amines by enzymes and detergents, and oily emulsions by barium chloride.
Major precipitation processes include water softening and stabilization, heavy metal removal and phosphate removal.
Water softening involves the removal of divalent cationic species, primarily calcium and magnesium ions.
Heavy metal removal is most commonly used in the metal plating business, where soluble salts of cadmium, chromium, copper, nickel, lead, zinc and other elements must be removed and perhaps recovered.
Phosphate removal from wastewater is used to protect receiving surface waters from eutrophication (plant growth stimulated by nutrient addition).
Competing processes for ion removal include ion exchange, electron precipitation, and reverse osmosis.
The disadvantages of these processes relative to chemical precipitation are higher capital costs and, in the case of the latter two, higher energy costs for operation.
Their advantage is that all these processes are better adapted to metal recovery and recycling than chemical precipitation is. Chemical precipitation has the advantage of low capital cost and simple operation.
Its major disadvantages are its operating costs from the chemical expense and the cost of disposing of the precipitated sludge that is produced.
The majority of metals are precipitated as hydroxides; however, sulfide and carbonate precipitation are also used.
In some circumstances, the chemical species to be eliminated must first be oxidized or reduced to a valence that can then be directly precipitated.
Phosphate can be removed by precipitation as iron or aluminum salts, and fluorine can be removed using calcium chloride.
Precipitation should be separated from coagulation and flocculation processes.
Coagulation is the aggregation of finely divided non-settleable solid particles, particularly colloids, into bigger particles via the instability of the electric double layer.
Flocculation is the production of bigger particles by forming bridges between coagulated particles via the adsorption of large polymer molecules and other factors.
Coagulation and flocculation, which frequently occur together, produce particles that can be removed using sedimentation or filtration.
Coagulation and flocculation occur after and are associated with the precipitation processes, which are commonly used in waste treatment.
Description of Chemical precipitation process
Precipitation is a chemical unit process in which undesirable soluble metallic ions and certain anions are removed from water or wastewater by conversion to an insoluble form.
It is a commonly used treatment technique for the removal of heavy metals, phosphorus and hardness.
The procedure involves the alteration of the ionic equilibrium to produce insoluble precipitates that can be easily removed by sedimentation.
Chemical precipitation is always followed by a solids separation operation that may include coagulation and/or sedimentation, or filtration to remove the precipitates.
The process can be preceded by chemical reduction to change the characteristics of the metal ions to a form that can be precipitated.
Chemical precipitation
The chemical equilibrium relationship in precipitation that affects the solubility of the component(s) can be achieved by a variety of means.
One or a combination of the following processes induces precipitation reactions in a water environment.
Hydroxide Precipitation
Heavy metal ions dissolved in water can be chemically precipitated as hydroxide and removed physically through sedimentation or filtering.
The process uses an alkaline agent to raise the pH of the water which causes the solubility of metal ions to decrease and thus precipitate out of the solvent.
The optimum pH at which metallic hydroxides are least soluble varies with the type of metal ion.
A simple form of the hydroxide precipitation reaction may be written as:
M2+ + 2(OH)- = M(OH)2
The product formed is an insoluble metal hydroxide.
If the pH is below the optimum of precipitation, a soluble metal complex will form:
M2+ + OH− = M(OH)+
Hydroxide precipitation is also affected by the presence of organic radicals that can form chelates and mask the typical precipitation reaction:
M2+ + OH− + nR = M(R)n OH+
Reagents commonly used to affect hydroxide precipitation include alkaline compounds such as lime or caustic soda (sodium hydroxide).
Lime in the form of quicklime or un-slaked lime, CaO, and hydrated lime, Ca (OH)2, can be used.
Lime is generally made into wet suspensions or slurries before introduction into the treatment system.
The precise steps involved in converting lime from the dry to the wet stage will vary according to the size of the operation and the type and form of lime used.
In the smallest plants, bagged hydrated lime is often charged manually into a batch-mixing tank with the resulting “milk of lime” (or slurry) being fed using a solution feeder to the treatment process.
Where bulk hydrated lime is used, some type of dry feeder charges the lime continuously to either a batch or continuous mixer. A solution feeder transfers lime to the point of application.
With bulk quicklime, a dry feeder is also used to charge a slaking device, where the oxides are converted to hydroxides, producing a paste or slurry.
The slurry is then further diluted to milk-of-lime before being fed by gravity or pumping into the process.
Dry feeders can be of the volumetric or gravimetric type.
Caustic soda, in the form of 6–20% aqueous solution, is fed directly to the treatment system and does not require any dispensing and mixing equipment.
The treatment chemicals may be added to a flash mixer or rapid-mix tank, or directly to the sedimentation device.
Because metal hydroxides tend to be colloidal, coagulation agents may also be added to facilitate settling.
Sulfide Precipitation
To precipitate heavy metal ions as insoluble metal sulfides, both “soluble” sulfides such as hydrogen sulfide or sodium sulfide and “insoluble” sulfides such as ferrous sulfide can be utilized.
Sodium sulfide and sodium bisulfite are the two chemicals commonly used, with the choice between these two precipitation agents being strictly an economic one.
Metal sulfides have lower solubilities than hydroxides in the alkaline pH range and also tend to have low solubilities at or below the neutral pH value.
The basic principle of sulfide treatment technology is similar to that of hydroxide precipitation.
Sulfide is added to precipitate the metals as metal sulfides and the sludge formed is separated from the solution by gravity settling or filtration.
Several steps enter into the process of sulfide precipitation:
1. Preparation of sodium sulfide.
Although there is often an abundant supply of this product from by-product sources, it can also be made by reduction of sodium sulfate.
The process involves an energy loss in the partial oxidation of carbon (such as that contained in coal) as
follows:
Na2SO4 4C= Na2S + 4CO2 (gas)
Sodium sulfate + carbon = metallic sulfide + carbon dioxide
2. Excess sodium sulfide precipitates the pollutant metal (M) in the waste stream.
Na2S + MSO4 = MS precipitate + Na2SO4
3. Physical separation of the metal sulfide in thickeners or clarifiers, with reducing conditions maintained by excess sulfide ions.
4. Oxidation of excess sulfide by aeration:
Na2S + 2O2 = Na2SO4
Sodium sulfide + oxygen = sodium sulfate
Because of the toxicity of both the sulfide ion and hydrogen sulfide gas, sulfide precipitation may necessitate both pre-and post-treatment, as well as careful reagent addition control.
To decrease the generation of toxic hydrogen sulfide gas, the pH of the water is raised to between 7 and 8.
The pH adjustment can be done at the same time as the sulfide treatment, or by adding a solution containing both sodium sulfide and a strong base (such as caustic soda).
Posttreatment includes aeration or chemical oxidation to eliminate excess sulfide, a hazardous toxin.
A recently developed and patented process to eliminate the potential hazard of excess sulfide in the effluent and the formation of gaseous hydrogen sulfide uses ferrous sulfide as the sulfide source.
Fresh ferrous sulfide is prepared by adding sodium sulfide to ferrous sulfate.
The ferrous sulfide slurry formed is added to water to supply sufficient sulfide ions to precipitate metal sulfides, which have lower solubility than ferrous sulfide.
Typical reactions are:
FeS + Cu2+ = CuS Fe2+
Ferrous sulfide + copper ion = insoluble copper sulfide + iron ion
FeS + Ni (OH)2= Fe (OH)2 + NiS
Ferrous sulfide + nickel hydroxide = ferrous hydroxide + insoluble nickel sulfide.
A detention time of 10–15 min is sufficient to allow the reaction to go to completion.
Ferrous sulfide is a relatively insoluble chemical as well.
Thus, the sulfide ion concentration is constrained by the solubility of ferrous sulfide, which is around 0.02 mg/L, as well as the inherent difficulties associated with typical sulfide precipitation, which are minimized.
Cyanide Precipitation
Although cyanide precipitation is a method for treating cyanide in wastewater, it does not eliminate the cyanide molecule, which remains in the produced sludge.
According to reports, when exposed to sunshine, the cyanide complexes can degrade and create free cyanide.
As a result, the sludge generated by this treatment procedure must be disposed of with care.
The addition of zinc sulfate or ferrous sulfate, which creates zinc ferrocyanide or Ferro and ferrocyanide complexes, can precipitate and settle cyanide from wastewater.
Cyanide forms exceptionally stable cyanide complexes in the presence of iron.
Carbonate Precipitation
Carbonate precipitation may be used to remove metals either by direct precipitation using a carbonate reagent such as calcium carbonate or by converting hydroxides into carbonates using carbon dioxide.
The solubility of most metal carbonates is intermediate between hydroxide and sulfide solubilities; in addition, carbonates form easily filtered precipitates.
Coprecipitation
In coprecipitation, materials that cannot be removed from the solution effectively by direct precipitation are removed by incorporating them into particles of another precipitate, which is separated by settling, filtration, or flotation.
References
[1] S. C. Atlow, Biotechnol. Bioeng. 26, 599 (1984).
[2] B. Gomulka and E. Gomolka, Effluent Water Treat. J. (G.B.) 24, 119 (1985).
[3] D. Biver and A. Degols, Tech. de l’ Eau (Fr.), 428/429, 31, (1982); (abstr) WRC Info., 10,83-0524 (1983).
[4] V. K. La Ver, J. Colloid Science l9, 291–293 (1964).
