Cls06-2

PIERO M. ARMENANTENJIT

Precipitation of HeavyMetals from Wastewaters


Typical Heavy Metals Found inIndustrial Wastewaters

Heavy metals (as soluble ions) are common contaminantsof industrial wastewaters. Because of their toxicity theyare typically removed prior to wastewater discharge. Themost common heavy metal contaminants are:

• Arsenic • Mercury

• Barium • Nickel

• Cadmium • Selenium

• Chromium • Silver

• Copper • Zinc

• Lead


Precipitation of Heavy Metals• The heavy metal contents of wastewaters can

be effectively removed to acceptable levels byprecipitating the metal in an insoluble form.

• Heavy metals are typically precipitated fromwastewater as:

◊ hydroxides

◊ sulfides or sometime sulfates

◊ carbonates

• Metal co-precipitation during flocculation withiron or aluminum salts is also possible forsome metals (e.g., arsenic).


Precipitation of Heavy Metals asHydroxides

• Precipitation by hydroxide formation is themost common heavy metal precipitationmethod. The precipitation typically follows thereaction:

( )M nOH M OHnn

+ −+ ⇔

• Many heavy metals are amphoteric. Thereforetheir solubility reaches a minimum at a specificpH (different for each metal).


Precipitation of Heavy Metals asHydroxides (continued)

• The addition of caustic materials is used toraise the pH. The most common caustics are:

◊ sodium hydroxide (NaOH)

◊ calcium hydroxide (Ca(OH)2; lime)

• Sodium hydroxide is more expensive thanlime.

• Lime has the advantage of also acting as acoagulant during the precipitation/settlingprocess, whereas NaOH does not.


Precipitation of Heavy Metals asHydroxides (continued)

• Lime must be produced on site (via slaking)from CaO (quicklime), and the resulting slurrycontinuously stirred to prevent cake formation.

• Lime may react with sulfates in the wastewaterproducing a hard precipitate and resulting inscaling formation.


Solubility of Metal Hydroxidesas a Function of pH

6 7 8 9 10 11 12

pH

0.0001

0.001

0.01

0.1

1

10

100S

olub

ility

(mg/

L)

CuNi

Cd

Ag

Zn

Pb

After EPA, 1983


Wastewater Pretreatment Prior toMetal Precipitation

• If the wastewater contains complexing agentsthat can keep the metals in solution andprevent them from precipitating thecomplexing agent must be destroyed prior tometal precipitation as hydroxides.

• This is especially true for the case of cyanidesalts because many heavy metals form strongcomplexes with cyanide.

• Cyanide is typically removed via oxidation withchlorine under alkaline conditions.


Solubility of Metals Sulfidesas a Function of pH

2 4 6 8 10 12 14

pH

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102

Sol

ubili

ty (m

g/L)

CuNi

Cd

Ag

Zn

Pb

After EPA, 1983


Precipitation of Heavy Metals asSulfides

• Metal sulfides are typically very insoluble.Therefore metals can be precipitated by addingsulfide ions (S-2).

• Metal sulfides have much lower solubilitiesthan the corresponding metal hydroxides, thusallowing lower residual metal concentrationsin the treated wastewater.

Solubility of Metals Sulfidesas a Function of pH

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102

Sol

ubili

ty (m

g/L)

CuNi

Cd

Ag

Zn

Pb


Precipitation of Heavy Metals asSulfides (continued)

• Hydrogen sulfide (H2S) is a weak acid anddissociates in two steps according to:

H2S ⇔ H++HS-

HS- ⇔ H++S-2

• Sulfide salts undergo hydrolysis, partiallyreforming the undissociated acid:

S-2 + H2O ⇔ HS- + OH-

HS- + H2O ⇔ H2S + OH-



• S-2 is not the dominating species until the pHis very high (~14). Hence, by adding a sulfidesalt an equilibrium is generated resulting inonly the partial formation of the S-2 ionsactually used in precipitation.

• Sulfide precipitation is always conductedunder alkaline conditions to promote sulfideion formation.

• Sulfide precipitation is rather insensitive to thepresence of chelating agents.



• Sulfide precipitates tend to form colloidalparticles requiring the addition of coagulantsto settle. However, sulfide sludges are easierto dewater than hydroxide sludges.

• Sulfide sludges are more prone to oxidationresulting in resolubilization of the metals assulfates.

• Evolution of H2S is a potential hazard if the pHis not carefully maintained in the alkalineregion.


Precipitation of Heavy Metals asCarbonates

• Some metals (lead, cadmium, nickel) forminsoluble carbonates that can be used incarbonate precipitation.

• Some wastewaters already contain enoughcarbonates to allow precipitation to occur.Alternatively, inorganic carbonates (e.g.,Na2CO3) can be added.


Precipitation of Heavy Metals asCarbonates (continued)

• Carbonate precipitation takes place only ifcarbonate ions (CO3

-2) are present. Freecarbonate ions are present only if the pH ishigh. A caustic is often added to raise the pH.

• High pH’s also promote the precipitation of themetals as hydroxides. Hence, carbonateprecipitation is often a co-precipitation.


Precipitation of Heavy Metals asCarbonates (continued)

• pH values above 10 promote the formation ofmetal hydroxy complexes that can increase themetal solubility and reduce the precipitationeffectiveness.

• Carbonate precipitates settle and can bedewatered more easily than the correspondinghydroxide precipitates.


Arsenic in Industrial WastewatersArsenic is present in the wastewaters of a numberof industries producing:

• metallurgical products

• glassware and ceramic

• tannery products

• dye stuff

• pesticides

• synthetic chemicals

• petroleum refinery products


Precipitation of Arsenic fromIndustrial Wastewaters

• Arsenic can be removed by precipitation assulfide, through the addition of sodium sulfideor hydrogen sulfide to the wastewater. Theeffluent concentration is 0.05 ppm

• Arsenic can also be removed by co-precipitation with FeCl3 when a Fe(OH)3 floc isformed. The effluent concentration is 0.005ppm


Barium in Industrial WastewatersBarium is present in the wastewaters of a numberof industries producing:


• glassware and ceramic

• dye stuff

• explosives

• rubber products


Precipitation of Barium fromIndustrial Wastewaters

• Barium can be removed by precipitation assulfate, by adding any sulfate ion source

• The solubility of barium sulfate is 1.4 ppm

• Even lower residual barium concentrations(0.5 ppm) can be obtained using an excess ofsulfate ions

Note: 1 ppm = 1 mg/L


Cadmium in Industrial WastewatersCadmium is present in the wastewaters of anumber of industries producing:


• ceramics

• electroplated products

• photographic products

• pigments

• textiles

• synthetic chemicals


Precipitation of Cadmium fromIndustrial Wastewaters

• Cadmium can be removed by precipitation ashydroxide at pH ranging from 8 (solubility: 1ppm) to 11 (solubility: 0.05 ppm)

• Cadmium can be removed by precipitation assulfide. The effluent concentration is 0.05 ppm

• Cadmium can also be removed by co-precipitation at pH 6.5 with FeCl3 when aFe(OH)3 floc is formed. The effluentconcentration is 0.008 ppm


Precipitation of Cadmium fromIndustrial Wastewaters

• Cadmium can also be removed by precipitationas carbonate. The pH required in this case isbetween 7.5 and 8.5. The effluentconcentration is comparable to that obtainedthrough hydroxide precipitation at high pH

• Cyanides interfere with any of these processesand must be removed prior to cadmiumprecipitation


Chromium in Industrial WastewatersChromium is present in the wastewaters of anumber of industries producing:

• steel manufacturing

• chrome plated products

• tannery products

• dye stuff

• paints


Precipitation of Chromium fromIndustrial Wastewaters

Chromium is typically precipitated in two steps:

• hexavalent chromium (Cr+6) is reduced totrivalent chromium (Cr+3). Compounds such asFeSO4, Na2S2O5 (sodium bisulfite) or SO2

(sulfur dioxide) are used as reducing agents.The reaction is conducted at low pH (< 3)

• trivalent chromium is precipitated as Cr(OH)3.Lime is typically used for the precipitationreaction. The effluent concentration is 0.2 ppmat pH 7.5


Precipitation of Chromium fromIndustrial Wastewaters

The reactions involved in chromium precipitation are:Reduction reaction (at pH < 3):

( )H Cr O SO Cr SO H O2 2 7 2 2 4 3 23+ → +

(i.e., 1.85 ppm SO2/ppm Cr) or, alternatively:

( ) ( )H Cr O FeSO H SO

Cr SO Fe SO H O2 2 7 4 2 4

2 4 3 2 4 3 2

6

3 7

+ + →+ +

Precipitation reaction (at pH of 8 to 9):( ) ( ) ( )Cr SO Ca OH Cr OH CaSO2 4 3 2 3 43 2 3+ → ↓+

(i.e., 2.13 ppm Ca(OH)2/ppm Cr)


Copper in Industrial WastewatersCopper is present in the wastewaters of a numberof industries producing:

• Chemicals using copper salts

• Chemicals using copper catalyst

• Metal processing products

• Metal plated products


Precipitation of Copper fromIndustrial Wastewaters

• Copper can be removed by precipitation ashydroxide at pH ranging from 9 to 10.3(solubility: 0.01 ppm as cupric oxide)

• Copper can be removed by precipitation assulfide at pH 8.5. The resulting effluentconcentration is 0.01 - 0.02 ppm

• The presence of cyanide or ammonia mayinterfere with copper precipitation. In such acase activated carbon can be used to removecopper cyanide


Lead in Industrial WastewatersLead is present in the wastewaters of a number ofindustries producing:

• batteries

• pigments

• printing products


Precipitation of Lead from IndustrialWastewaters

• Lead can be removed by precipitation ashydroxide (lime) at pH 11.5. The effluentconcentration is 0.02 to 0.2 ppm

• Lead can be removed by precipitation assulfide at pH 7.5 to 8.5

• Lead can also be removed by precipitation ascarbonate. The pH required in this case isbetween 7.5 and 8.5. The effluentconcentration is comparable to that obtainedthrough hydroxide precipitation at high pH


Mercury in Industrial WastewatersMercury is present in the wastewaters of anumber of industries producing:

• chlor-alkali

• explosive

• electronic products

• pesticides

• petrochemical products


Precipitation of Mercury fromIndustrial Wastewaters

• Mercury can be removed by precipitation assulfide, through the addition of sodium sulfideor hydrogen sulfide to the wastewater. Theeffluent concentration is 0.01 ppm

• Mercury can be removed by co-precipitationwith alum. The effluent concentration is 0.001to 0.01 ppm

• Mercury can be removed by co-precipitationwith FeCl3 when a Fe(OH)3 floc is formed. Theeffluent concentration is 0.0005 to 0.005 ppm


Nickel in Industrial WastewatersNickel is present in the wastewaters of a numberof industries producing:

• Metal products (e.g., aircrafts)

• Steel

• Chemicals


Precipitation of Nickel from IndustrialWastewaters

• Nickel can be removed by precipitation ashydroxide at pH ranging from 10 to 11(solubility: 0.12 ppm)

• Nickel can be also be removed by precipitationas sulfate or carbonate

• The presence of cyanide may interfere withnickel precipitation


Metal Complexation• Many transition metals (e.g., Cd, Co, Cu, Fe,

Hg, Ni, Zn) can form complexes with a numberof different ligands such as hydroxides,sulfides, chlorides, cyanides, EDTA(ethylenediaminetetraacetic acid).

• Metal complexes (e.g., Na2Cd(CN)4, sodiumcadmiocyanide) can increase the solubility of ametal far beyond what the solubility of themetal is (e.g., as hydroxide).


Precipitation of Complexed Metals• Precipitation of complexed metals is typically

impossible until the metal complex isdestroyed or the equilibrium is shifted byadding massive amounts of the precipitant.

• Oxidation is typically the most commonmethod to destroy metal complexes.

• Oxidants such as chlorine, hypochlorite, andozone are often used for this purpose.

• After the complexing agent is destroyed themetals are precipitated as usual.


Cyanide Removal Via AlkalineChlorination

• Cyanides are compounds that need to beremoved prior to wastewater discharge.

• Furthermore, cyanides often form metalcomplexes that must be destroyed before themetals are precipitated.

• Wastewaters containing cyanides (up to 1000mg/L) are often treated with sodiumhypochlorite (NaClO) or chlorine gas mixedwith sodium hydroxide (to form sodiumhypochlorite) in a two-step process.


Cyanide Removal Via AlkalineChlorination (continued)

• The first step is the conversion of cyanide(CN-) to cyanate ion (OCN-) in a two-reactionsequence at a pH above 10 (reaction time: 30min. to 2 h):

CN H O ClO CNCl OH− − −+ + ⇔ +2 2

CNCl ClO OCN Cl H O+ ⇔ + +− − −2

• The second step is the oxidation of cyanatewith hypochlorite at pH 8.5 to increase thekinetics (reaction time: 10 min. to 1 h):

2 3 2 3 22 2 2OCN ClO H O CO N Cl OH− − − −+ + ⇔ + + +


Treatment Plant for Wastewaters Containing Heavy Metals

Acid

Sulfur Dioxide

Chrome Wastewater

Caustic

Cyanide Wastewater

Chlorine

Acid Wastewater

Alkaline Wastewater

Cyanide Oxidation

Chrome Reduction

Equalization and Partial

Neutralization

Acid

Caustic

NeutralizationPrecipitation

CoagulationFlocculation

Coagulant

Clarification

SludgeThickening

TreatedWastewater

WastewaterRecycle

Sludge toFinal Disposal

Filtration

After EPA, Environmental Regulations and Technology: The Electroplating Industry, 1985


Additional Information and Examples onMetal Precipitation

Additional information and examples can be found in thefollowing references:

• Corbitt, R. A. 1990, The Standard Handbook ofEnvironmental Engineering, McGraw-Hill, New York, pp.6.92; 9.25.

• Eckenfelder, W. W., Jr., 1989, Industrial Water PollutionControl, McGraw-Hill, New York, pp. 98-110

• Freeman, H. M. (ed.), 1989, Standard Handbook ofHazardous Waste Treatment and Disposal, McGraw-Hill,New York, pp. 7.21-7.31.


Additional Information and Examples onMetal Precipitation

• Haas, C. N. and Vamos, R. J., 1995, Hazardous andIndustrial Waste Treatment, Prentice Hall, Englewood Cliffs,NJ, pp. 147-152.

• Sundstrom, D. W. and Klei, H. E., 1979, WastewaterTreatment, Prentice Hall, Englewood Cliffs, NJ, pp. 327-330.

• Wentz, C. W., 1995, Hazardous Waste Management, SecondEdition, McGraw-Hill, New York, pp. 155-157.

Filename: Cls06-21.docDirectory: D:\TEMPTemplate: C:\WINWORD\OVERHEAD.DOTTitle:Subject:Author: Piero M. ArmenanteKeywords:Comments:Creation Date: 02/21/97 9:12 AMChange Number: 49Last Saved On: 03/02/97 6:14 PMLast Saved By: Piero M. ArmenanteTotal Editing Time: 280 MinutesLast Printed On: 08/31/99 11:21 AMAs of Last Complete Printing

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Cls06-2

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industrial wastewaters cadmium

industrial wastewaterschromium

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alkaline conditions

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