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Southern Methodist University School of Engineering and Applied Science SMU SSH 8321 SMU ME 5315 NTU HW 741-N Treatment Technology I - Physical and Chemical Methods April 11, 2000 Dr. Roger Dickey
IV. PHASE AND SPECIES TRANSFORMATIONS (A) pH Neutralization Wastewater containing excessive acidity (low pH) or alkalinity (high pH) must often be neutralized.
Neutralization typically involves pH adjustment into the allowable range for discharge to the environment which is a pH of 6.0 to 9.0.
Also, some treatment processes require pH adjustment for optimum performance. Examples include chemical precipitation and air stripping. Neutralization involves the common acid-base reactions.
Common reagent chemicals used for neutralization include: Acids - H2SO4 (sulfuric acid, oil of vitriol) HCl (hydrochloric acid, muriatic acid) CO2 (carbon dioxide)
Bases - NaOH (sodium hydroxide, caustic soda) Ca(OH)2 (calcium hydroxide, hydrated lime) CaO (calcium oxide, quick lime) CaCO3 (calcium carbonate) Na2CO3 (sodium carbonate, soda ash) NaHCO3 (sodium bicarbonate) NH4OH (ammonium hydroxide, aqua ammonia) Mg(OH)2 (magnesium hydroxide)
Some chemical wastes have neutralizing capacity - • Waste pickle liquor from the steel industry is a source of acidity • Lime-soda sludge from municipal water supply softening processes is a source of alkalinity.
Typical pH Neutralization System: Q Q Flow Meter pH Cell Rapid Mix Tank pH Signal Q Signal Chemical Storage Tank pH Controller Pump Speed Signal Metering Pump
Design is usually based on simple bench-scale titration of a wastewater sample to evaluate the chemical dosage required. Consider the following hypothetical examples:
Acidic Waste 14 12 10 Acceptable pH = 6 to 9 pH 8 6 4 2 Base Added (meq/l) CB CB = base dosage required (meq/l or mg/l as CaCO3) to reach target pH
Alkaline Waste 14 12 10 Acceptable pH = 6 to 9 pH 8 6 4 2 CA Acid Added (meq/l) CA = acid dosage required (meq/l or mg/l as CaCO3) to reach target pH
Neutralization is considered complete when the target pH has been achieved. For the example of a waste containing a strong acid being neutralized with a strong base at a dosage of CB, the required amount of a given strong base stock solution is given by,
where, VBS = volume of base stock solution [L3] QBS =volumetric flow rate of base stock solution[L3/T] CB = base dosage required (meq/l or mg/l as CaCO3) V= volume of waste neutralized [L3] Q = volumetric flow rate of waste neutralized [L3/T] CBS = base stock solution concentration (meq/l or mg/l as CaCO3)
The equation is simply rewritten in terms of VAS and QAS and the acid dosage, CA, to determine the required amount of a strong acid reagent to neutralize a waste containing a strong base:
where, VAS = volume of acid stock solution [L3] QAS =volumetric flow rate of acid stock solution[L3/T] CA = acid dosage required (meq/l or mg/l as CaCO3) V= volume of waste neutralized [L3] Q = volumetric flow rate of waste neutralized [L3/T] CAS = acid stock solution concentration (meq/l or mg/l as CaCO3)
Additional Practical pH Neutralization Concepts - (1) If very precise control is required for effluent pH, then two pH neutralization tanks in series are sometimes used:
Chemical Feed System pH Range 6 to 9 pH = 7.0 Q Q Coarse pH Adjustment Fine pH Adjustment
(2) Neutralization of strong acids with strong bases, or strong bases with strong acids, leaves no pH buffering capacity in the effluent. Downstream addition of a very small quantity of strong acid or strong base can cause the pH to swing wildly from the neutral pH. Consider the previous example titration curve,
Acidic Waste 14 12 10 Acceptable pH = 6 to 9 pH 8 6 4 2 Base Added (meq/l) CB
If maintaining significant buffering capacity in the effluent is desirable, then certain neutralizing chemicals are preferred: (i) When neutralizing bases, Buffering agent - CO2 Buffering is achieved by the resulting carbonate system equilibrium. Design of the neutralization system must allow for precipitation of CaCO3.
(ii) When neutralizing acids, Buffering Agents Design must NaHCO3 Carbonate allow for Na2CO3 buffering liberation CaCO3 system of CO2 gas Mg(OH)2 pseudo- buffering agent
(3) Self-neutralization - Individual wastewater streams of differing pH can be blended to provide for self-neutralization to minimize the need for chemical addition.
(B) Chemical Precipitation • Chemical precipitation is commonly used for treatment of wastes that contain dissolved heavy metals. The metals most commonly requiring treatment include: • arsenic (As) • barium (Ba) • cadium (Cd)
copper (Cu) • chromium (Cr) • lead (Pb) • mercury (Hg) • nickel (Ni) • selenium (Se) • silver (Ag) • thallium (Th) • zinc (Zn)
Precipitation is based on altering the chemical equilibrium of the waste to exceed the solubility product constant for the metal species. After precipitation, the metal salt solid particles are removed by the common solids-liquid separation processes.
The following sequence of treatment processes is typically employed: • chemical precipitation • coagulation • flocculation • clarification • filtration
Because metals are mass conservative, precipitation serves as a concentration process and the resulting sludge must be disposed of as hazardous waste. The effectiveness of chemical precipitation is influenced by the factors important in any chemical equilibrium:
(1) pH The solubility of metal salts varies with pH. For example, metal hydroxides usually have a pH of minimum solubility. Above this pH, the solubility increases with increasing pH. Similarly, below this pH, the solubility increases with decreasing pH. (See Figure 7.2.2, p. 7.23 in the Freeman textbook)
(2) Ionic Strength Increasing ionic strength lowers the “chemical activity” of the metal ion in solution thereby increasing the solubility of the solid metal salt.
(3) Complexation The presence of some organic ligands binds metals in complexes that interfere with their precipitation. Cyanide ion (CN-), an inorganic ligand, is present in many metal plating wastes and is highly effective at keeping metals in solution.
(4) Temperature Except for metal carbonates, the solubility of metal salts increase with temperature.
A number of anions are used as reagents for metal precipitation, but hydroxide (OH-) and sulfide (S2-) are the most common. Both OH- and S2- yield very low soluble metal equilibrium concentrations after precipitation.
Approximately two to three times the stoichiometric amount of precipitant chemical is usually needed for efficient precipitation because: • Precipitation reactions are non-specific for multivalent metal cations and non-toxic metals such as Ca2+ and Mg2+ are removed from solution along with the toxic metals.
Suspended and colloidal solids may also be removed by precipitation treatment. • Complexation and/or the common ion effect may increase the solubility of metal salts requiring higher precipitant dosages to drive the equilibrium to acceptably low dissolved metal concentrations in accordance with the law of mass action.
Chemical kinetic limitations must sometimes be overcome, i.e., the reaction rate may need to be increased for cost-effective treatment by increasing the precipitant chemical dosage in accordance with the law of mass action.
In summary, large quantities of sludge containing toxic heavy metal salts are typically produced by precipitation treatment. The mass of solids produced is often much greater than that predicted by stoichiometry. Dewatered sludge cake from precipitation treatment is often delivered to a hazardous waste landfill for ultimate disposal.
Jar tests are commonly used to develop design • parameter values: • optimum pH • optimum precipitant dosage • mixing requirements • settling properties of the precipitate solids • sludge quantity
Hydroxide Precipitation - Either caustic soda (NaOH) and lime (Ca(OH)2) is commonly used as the chemical reagent: Effluent concentrations < 1.0 mg/l M2+ are achievable for most metals.
The solubility of metal hydroxides vary with pH and the pH of minimum of solubility varies from one metal to the next. For example:
pH of Minimum Metal Solubility Metal Solubility (mg/l of M2+) Cd 11.2 0.002 Ni 10.2 0.001 Pb 9.3 7.0 Zn 9.2 0.09 Cu 8.9 0.0006
Hydroxide precipitation is the most common precipitation method and is frequently used for removal of Cd, Cu, Cr, Ni, and Zn. Hexavalent chromium (Cr6+) salts are highly soluble. Therefore, pretreatment for reduction of Cr6+ to trivalent chromium (Cr3+) is necessary for precipitation of chromium hydroxide (Cr(OH)3) which is only slightly soluble.
Sulfide Precipitation - • Common reagents used for precipitation of heavy metals as sulfide salts include: • sodium sulfide (Na2S) • sodium hydrosulfide (NaHS) • ferrous sulfide (FeS)
The solubility of the sulfide salt is typically lower than the solubility of the hydroxide salt for most metals. However, the potential for evolution of highly toxic hydrogen sulfide (H2S) gas during treatment reduces the popularity of sulfide precipitation. Sulfide precipitation is common for lead removal.
Other Precipitation Chemicals - (i) Carbonate Precipitation - Using soda ash (Na2CO3), for example, Carbonate (CO32-) salts are much more soluble than the OH- and S2- salts for most metals. Carbonate precipitation is sometimes used for Cd and Pb removal.
(ii) Sulfate Precipitation - Sulfate precipitation is sometimes used for barium (Ba2+) removal:
(iii) Chloride Precipitation - Chloride ion (Cl-) is sometimes used for silver (Ag+) precipitation:
(iv) NaBH4 Reduction and Precipitation - Sodium borohydride (NaBH4) can be used for chemical reduction and precipitation of the elemental metal - sodium borate
NaBH4 reduction and precipitation has some promise for recovery of valuable metals. NaBH4 is produced in liquid form as a 12% solution in 40% NaOH.
Pretreatment Prior to Precipitation Reactions - • (i) Cr6+ Reduction to Cr3+ - • Chemical reducing agents include: • sulfur dioxide gas (SO2) • sodium bisulfite (NaHSO3) • ferrous sulfate (FeSO4)