treatment of wastewater from the lead‐zinc ore processing industry

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This article was downloaded by: [University of Cambridge] On: 08 October 2014, At: 13:35 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Journal of Environmental Science and Health . Part A: Environmental Science and Engineering and Toxicology: Toxic/ Hazardous Substances and Environmental Engineering Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lesa19 Treatment of wastewater from the leadzinc ore processing industry M. Panayotova a & J. Fritsch b a Dept of Chemistry , University of Mining&Geology , Sofia, 1756, Bulgaria b Dept of Physical Technics , Fachhochschule Ravensburg Weingarten , im Töbele, Weingarten, D88241, Germany Published online: 15 Dec 2008. To cite this article: M. Panayotova & J. Fritsch (1996) Treatment of wastewater from the leadzinc ore processing industry, Journal of Environmental Science and Health . Part A: Environmental Science and Engineering and Toxicology: Toxic/Hazardous Substances and Environmental Engineering, 31:9, 2155-2165, DOI: 10.1080/10934529609376483 To link to this article: http://dx.doi.org/10.1080/10934529609376483 PLEASE SCROLL DOWN FOR ARTICLE

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Page 1: Treatment of wastewater from the lead‐zinc ore processing industry

This article was downloaded by: [University of Cambridge]On: 08 October 2014, At: 13:35Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number:1072954 Registered office: Mortimer House, 37-41 Mortimer Street,London W1T 3JH, UK

Journal of EnvironmentalScience and Health .Part A: EnvironmentalScience and Engineeringand Toxicology: Toxic/Hazardous Substancesand EnvironmentalEngineeringPublication details, including instructionsfor authors and subscription information:http://www.tandfonline.com/loi/lesa19

Treatment of wastewaterfrom the lead‐zinc oreprocessing industryM. Panayotova a & J. Fritsch ba Dept of Chemistry , University ofMining&Geology , Sofia, 1756, Bulgariab Dept of Physical Technics ,Fachhochschule Ravensburg ‐ Weingarten ,im Töbele, Weingarten, D‐88241, GermanyPublished online: 15 Dec 2008.

To cite this article: M. Panayotova & J. Fritsch (1996) Treatment ofwastewater from the lead‐zinc ore processing industry, Journal ofEnvironmental Science and Health . Part A: Environmental Science andEngineering and Toxicology: Toxic/Hazardous Substances and EnvironmentalEngineering, 31:9, 2155-2165, DOI: 10.1080/10934529609376483

To link to this article: http://dx.doi.org/10.1080/10934529609376483

PLEASE SCROLL DOWN FOR ARTICLE

Page 2: Treatment of wastewater from the lead‐zinc ore processing industry

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Page 3: Treatment of wastewater from the lead‐zinc ore processing industry

J. ENVIRON. SCI. HEALTH, A31(9), 2155-2165 (1996)

T R E A T M E N T OF W A S T E W A T E R

F R O M T H E LEAD-ZINC O R E

P R O C E S S I N G I N D U S T R Y

Key words: electrocoagulation, electrochemical treatment, wastewater, heavy

metal ions

M. Panayotova *, J. Fritsch**

*University of Mining&Geology, Dept of Chemistry, 1756 Sofia, Bulgaria;**Fachhochschule Ravensburg - Weingarten, Deptof Physical Technics,

im Töbele, D-88241 Weingarten, Germany

ABSTRACT

Two different methods for treating an acid wastewater from lead-zinc

processing industry are compared. The results obtained show that chemical

treatment (neutralization, alkalization and precipitation by means of NaOH

addition) is a better method than electrochemical treatment with Fe electrodes for

the heavy metal ions' diminution, in the case when the initial heavy metal

concentrations are relatively law.

INTRODUCTION

Two different ways of treatment of wastewater from the lead-zinc ore

processing industry are compared. The main characteristics of the water are given

2155

Copyright © 1996 by Marcel Dekker, Inc.

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2156 PANAYOTOVA AND FRITSCH

in Table 1. The parameters that are to be reached1 in order to use the treated water

for irrigation purposes are also listed there.

The treatment methods compared are: a) usual chemical treatment with

NaOH in order to precipitate the heavy metal ions2"4 ; b) electrochemical treatment

frequently referred to in the literature514 as electrocoagulation. According to

authors2"4 in this process the treatment is performed by an electric current that

is passed through iron (or aluminium electrodes) immersed in the wastewater. As a

result heavy metal hydroxides coprecipitate or adsorb on the iron hydroxides

formed be electrochemicalry dissolved iron (aluminium).

Also, an attempt was made to find out the actual physico-chemical processes

connected with the decrease — in heavy metal concentration during the

electrocoagulation.

EXPERIMENTAL

The chemical treatment was carried out following descriptions given inw: 0,1

NaOH was added to obtain solutions of pH 9 and pH 10,5 respectively. According

to our preliminary calculations these are the pH values at which most of the heavy

metal ions are decreased in concentration (due to their precipitation as hydroxides)

to 0,1 and 0,01 respectively of their initial figures. Alternatively, the alkalization

was carried out by adding 0,1 N NajCO3. The pH value obtained as a maximum

was 9,95 and at pH>9 the system behaved like buffered. The precipitation was

performed as follows:

-Stirring (600 rev/min.) the water while adding alkalis.

-Stirring another 30 min. after reaching the desired pH value.

-Leaving the precipitate to settle down for 2 hours.

-Taking samples for analysis from the clear water above the precipitate.

-After the precipitation the clear water phase was neutralized to pH 8,5 with 0,1 N

HC1 in order to reach proper conditions for discharge. The results are given in

Table 2.

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TREATMENT OF WASTEWATER 2157

Water Parameters

TABLE 1.

Parameter

t,°C

pHae, mS/cmCa2+, ppmMg2+, ppmFe3+, ppmMn2+, ppmZn2+, ppmPb 2 + , ppmNj2+, ppmCd2+, ppmCu2+, ppmAs5+, ppm

Si, ppmSO42", ppm

Cl", ppmNO?"» ppmPOr5-, ppm

PermanganateConsumption, ppmDischarge, m3/day

Values in thewastewater

13,0

2,43,38156,575,49,27,563,75,521,010,470,300,300,7075049017

<0,500,88

432

Values to be reached for waterusing for irrigation1

Not more than 3°C higher thanaverage seasonal temperature

6,0-8,51,3

-

1,50,35,0

0,050,2

0,010,1

0,0150

300300

10, like N2,030

-

The electrocoagulation was carried out under variation of the quantity of

dissolved iron. This procedure was chosen because of the presumed major role of

Fe hydroxides 7>9>1U3'14 in the water purification processes. The experimental

conditions (see Table 3), the design of the Fe electrodes, etc., were chosen on the

basis of descriptions given elsewhere7"9'11'12. Experiments at lower pH values (Nrs.4

and 7), as well as at much higher applied voltage (Nr.5) were performed in order to

obtain results useful for clarifying the processes going on at the electrodes and in

the solution. Samples for analysis were taken after 3 hours from the clear water

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2158 PANAYOTOVA AND FRITSCH

TABLE 2.Results from Chemical Treatment

ExperimentsParameters

Quantity of addedalkalynl/1 waste

waterFinal pH*

pH of soin, afterprecip.

ae after precip.,mS/cm

Fe2+,ppm**Mn2+, ppmZn^+, ppmPb2 + , ppmNi^+, ppmCd2+,ppmCu2 + , ppm

Na , ppm

K+, ppmMg^+, ppmCa i + , ppm

SO42->Ppm

Cl-****,ppm

NOV, ppmQuantity of O,1N

HCl, added to reachpH 8,5, ml/1 waste

waterPrecipitate^nlA

a) 2 h aftertreatment

b) 3 h aftertreatment

Precipitation with 0,1 NaOH

103

10,5110,15

2,90

0,120,140,770,0540,040,010

<0,004-118

-64,5

44,84

-131,0790

507,623,5

1,25

85

75

87

9,018,31

2,91

0,161,520,920,0270,120,0240,004-120

-65,0

55,5

-135,3760

477,224,0

-

62

53

Precip. with 0,1 NNa2CC>3

248

9,959,77

3,28

0,221,281,57

0,1600,110,022

<0,004-119

-65

-38,6-115,2

770

492,824,0

40,0

4 5 * . .

40

* - All data for pH and « are referred to 20°C;** - All data concerning ions are for clear water above the precipitate;• • • - The precipitation was not absolutely completed;*•*• - Concentration, obtained after the neutralization with 0,1 N HCl.

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TREATMENT OF WASTEWATER 2159

TABLE 3.Results from the Electrocoagulation

Noexp.Parameter

UA1

u,mT, [mini

Calculated*dis.Fe2+,mg/lwastewater

PH startpHmf)pH«nl,,

Precipitate,ml/1

^soln»mS/cm

Fe i + ,ppm

Mn2+, ppmZnz + , ppm

Pb^1"»**,ppm

Ni^+, ppm

ppmCu^+, ppmNa+, ppmK+, ppm

Ca^+, ppmMg^+, ppmSO^- 'PP m

Cj-,ppm

1

0,151,651026

7,208,207,3060

2,78

1,34

4,45,84

0,110

0,250,071

0,010«122,1«66,0»139,8

56,9788

455,2

2

0,302,611052

7,328,998,3565

2,73

1,32

1,460,87

0,062

0,120,024

0,004«120,5«65,0»131,3

55,3798

457,3

3

0,603,7010104

7,229,108,5485

2,72

1,20

1,220,45

0,027

0,110,024

0,004»121,0»66,2

»136,057,1806

455,9

4

0,302,101052

2,392,652,66

-

3,42

75

7,4-62,7

4,85

1,060,56

0,24»120,2»75,1«150,1»67,7836

490,6

5

1,438250

7,269,218,6579**

2,72

0,85(0,42)1,140,55

0,040

0,120,021

0,004»121,0»67,4

»130,456,1111

441,0

6

0,151,93078 •

7,248,918,356 3 «

2,76

3,10(0,60)1,741,50

(0,53)0,102

0,150,028

0,004»120,5»72,6«137,7

55,7810

443,8

7

0,302,951052

5,016,235,26

0,043«

2,77

10,41(2,8)6,5

»51,3(7,13)0,200

0,900,49

0,13120,8«69,7»142,2«63,5828

458,7

* The quantity of dissolved iron was calculated using Faraday's law, assuming 100 % yield. Irondissolution due to its corrosion was not taken into consideration (even at solution's pH 2.4 in thebeginning of the treatment);** Unclear solution above the precipitate in the Imhof s cone;( ) Data for the same water above the precipitate, but filtered, given only when considerably differentfrom the data for nonfillered water,*** Pb^+ concentration in the beginning of the experiments no. 1, 2, 3 - 5,50 ppm;*•** Cd^1" concentration in the beginning 0,56 ppm (for exps. no. 4*7), 0,40 ppm (for exps. no. 1+3).

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Page 8: Treatment of wastewater from the lead‐zinc ore processing industry

2160 PANAYOTOVA AND FRITSCH

above the precipitate settled. The figures of pH, oxidation-reduction potential Eh

and conductivity as (referred to 20°C) of these solutions are denoted as pH,^ ,

Ehsota and œsoI]1 in Table 3.

All experiments were carried out with a model water prepared by dissolution

of the appropriate metal salts in deionised water. The same concentration figures as

for the real water, released by a Bulgarian lead-zinc processing industry (Table 1)

were obtained. Heavy metal ions whose concentrations were <5 ppm were added

immediately before the experiments. All chemicals used were of p.a.grade,

produced by Merck and Fluka.

The analysis of heavy metal ions was made with AAS (GBS-903, Scientific

equipment-Australia and Unicam 939-Netherlands), ICP-AES (Spectroanalytical

instruments Germany), HMASV (VA Controller E 608, VA Stand 663, Polarecord

626 Metrom Ltd.-Switzerland) and Spectrophotometry (Nanocolor-R-100D,

Macherey-Nagel GmbH& CoKG - Germany). For the analysis of other cations

ICP-AES and an Ion-sensitive electrode (Orion-Research) were applied. The other

parameters were determined with the aid of conventional methods15. X-ray analysis

(Debye-Sherrer method and X-ray defractionmeter) was applied to identify the

final products of the iron dissolution and consequent oxidation.

RESULTS AND DISCUSSION

The results are shown in Tables 2, 3 and Fig.l.

It was found that the desired low heavy metal concentrations can be obtained

only by neutralization and alkalization of the treated water to pH 10,5-10,6.

Considering solubility products2 of heavy metal hydroxides, carbonates, basic

chlorides and sulfates, pH values of the waters and the concentration of chloride

and sulfate ions before and after the treatment, it was estimated that heavy metal

ions are precipitating mainly in the form of hydroxides. For Pb2+ the lowest

concentration was found at pH about 9-9,2 in agreement with2'13. This can be

explained by the different solubility of the Pb(OH)2 formed at pH 7 and pH 102.

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Page 9: Treatment of wastewater from the lead‐zinc ore processing industry

TREATMENT OF WASTEWATER 2161

2 3 4 5 6 7 8 9 1 0 p H

FIG.l. Dependence of heavy metal concentrations in the conditioned waters(pC^1* = -lgCfcfc1*) on pH values of the wastewater at the end of treatment:v - Pb; ^ - Zn; ^ - Cu; D - Cd; o - Ni; • - Mn; black signs - values obtained afterneutralization and precipitation; white signs - values obtained after electrochemicaltreatment.

As results of the electrocoagulation experiments it was found that: a) there is

no relation between the decrease in the concentration of heavy metals and the

amount of iron electrochemically dissolved and brought into the water (respectively

the quantity of the iron hydroxides formed); b) there is a strong dependence of the

decrease in the heavy metal concentrations on pH values of the treated water. The

heavy metal quantities obtained in waste water conditioned by electrocoagulation

lay on one curve with the quantities found after chemical precipitation, showing the

relation between heavy metal concentrations and pH values independently of the

way to reach it (Fig.l); c) electrocoagulation starting at pH values lower than 7

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Page 10: Treatment of wastewater from the lead‐zinc ore processing industry

2162 PANAYOTOVA AND FRITSCH

does not lead to a significant decrease of heavy metal concentrations in the treated

water (experiments 4 and 7); d) application of a much higher voltage does not

result in considerably lower heavy metal concentrations in the conditioned water

(experiment 5).

The observation of: a) an increased pH value of the treated wastewater; b)

the amount of heavy metal ions in the precipitate formed by electrocoagulation; c)

gas bubbles around the electrodes (much more pronounced around the cathode and

at lower pH values or higher voltages applied) lead us to the following suggestion

for the main processes taking place during the electrocoagulation with Fe

electrodes:

a) The hydrogen evolution (2HJO++2e--->H2+2H2O) is the predominant cathodic

process at lower pH values. For a treatment starting at pH~7 the main cathodic

process could be a diffusion controlled reduction of the O2 available near the

cathode surface (O^Hîfi+Ae-—>4OH") or water reduction (2H2O+ 2e' ---> H2W

+ OHT).

b) The major anodic process is the Fe dissolution (Fe—>Fe2++ 2e-). Due to the

existing (low-) oxidation conditions in the treated water' next oxidation of Fe2+ in

the solution takes place and leads to Fe(OH)3 and mesomorphic or crystal

hydrolepidocrocite formation. Anodic oxidation of Cl" and an evolution of

environmentally harmful Cl2(g) cannot be excluded totalh/.(A small loss of Cl* was

found, it was higher at higher voltages applied; CT were not found adsorbed on the

precipitate formed).

c) Unbalanced water electrolysis (21^0—>2Hj+O2) with predominant Hj

evolution and nearly absent O2 evolution seems very probable.

d) The decrease of heavy metal concentrations is mainly due to the increased

quantity of OH- ions in the treated water which leads to a heavy metal precipitation

as hydroxides.

Thus, what is called electrocoagulation in literature7"12 is better described as a

1This statement is justified by the pH and Eh values of the clear solution, seeTable 3;

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TREATMENT OF WASTEWATER 2163

chemical precipitation process, whereby in this case the OH" ions are formed by

means of the iron electrodes. Since a relation between the quantity of iron

hydroxides formed and the decrease of heavy metal content is missing an

explanation of the electrocoagulation by adsorption processes can be excluded.

Mobility tests made with distilled water acidified to pH 3,5 by HNO3 addition

(modeling an acid rain) showed that: a) no heavy metals can be mobilized from the

chemically formed precipitate; b) Zn and Mn are partially mobilized from the

electrochemically formed precipitate.

An economical assessment2 of costs for both types of treatment (for the

water with parameters given in Table 1), assuming continuous work of the lead-zinc

industry - 360 days/annum has been done. It has not shown very significant

difference in the total and specific costs for the water conditioning by means of

both treatments.

CONCLUSIONS

For the investigated type of wastewater low heavy metal concentrations can

be successfully and economically reached only by alkalization (to pH~10,5) and

consequent precipitation. The electrochemical treatment with Fe electrodes is not

suitable because the heavy metal concentrations obtained are not low enough. The

mechanism of heavy metal concentration decrease by electrocoagulation was found

to be mainly the precipitation as hydroxides by means of electrochemically

increased OH" quantity in the conditioned water.

REFERENCES

1. Bulgarian State Standard, no. 7, State Newspaper no. 96/1986, (in Bulg.).

2 T h e prices for market available treatment plants, other equipment and chemicalswere taken from their producers. Costs for the waste disposal as well as values ofthe sinking fund and capital interest rates were inquired by the authorities incharge. German prices of electricity and working force were taken.

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2164 PANAYOTOVA AND FRITSCH

2. Hartiger L. Handbuch der Abwasser- und Recyclingtechnik, Munchen-Wien:Carl Hanser Verlag, 1991.

3. Graf R., Hartinger L., Lohmeyer S., Schwering H. Abwassertechnik in derProduktion (Verminderung, Behandlung, Ruckgewinnung), WEKA Fachverlagfur technische Fuhrungskrafte GmbH, Stand Juni, 1993.

4. Stefanov J., Kim K. Reduction of the Leachability of Heavy Metals in AcidMine Drainage. J. Environ. Sci. and Health, part A 1994; A 29: 371.

5. Conway B. Electrochemical Approaches to Small-scale Wastewater Purification,Proc. Electrochem. Soc. (Water Purification by Photocatarytic,Photoelectrochemical and Electrochemical Processes), 1994; 94-19:10.

6. Scott K. Industrial Wastewater Treatment: an Electrochemical Perspective,Symp. Pap. Inst. Chem. Eng., North West Branch, 1992; 3 (Integr. Pollut. ControlClean. Technol.): 601.

7. Tsatchev Ts. Wastewater Treatment, 2-nd part: Industrial WastewaterTreatment, Sofia: Jusautor, 1992, (in Bulg.).

8. Kulskii L., Grenuk V., Savchuk O. Electrochemistry of Wastewater Treatment,Kiev: Technika, 1987, (in Russ.).

9. Brewster M., Passmore R. Use of Electrochemical Iron Generation forRemoving Heavy Metals from Contaminated Groundwater. Environ. Prog. 1994;13:143.

10. Dohse D., Dold A., Czeska B. Electrochemical Wastewater Treatment.Metalloberflaeche 1995; 49: 365.

11. Chernova O., Kurdyumov G., Vasheikina T., Samsonov A. Coprecipitation ofHeavy Metals with Ferric Oxide Hydrate in Wastewater Treatment. Tsvetn. Met.(Moscow) 1992; 9: 30, (in Russ.).

12. Gladysheva A., Spaskaya N., Vorobeva L., Noskov J. ElectrocoagulationPurification of Wastewater of the Nonferrous Metallurgical Plant. Tsvetn. Met.(Moscow) 1992; 2: 33, (in Russ.).

13. Samuel D., Osman D., Chemistry of Water Treatment, Boston - London -Sidney -Wellington-Durban- Toronto: Butterworths, 1991.

14. Manzione M., Merrill D., Me Learn M., Chow W. Meeting Stringent MetalsRemoval Requirements with Iron Adsorption / Coprecipitation. Proc. Ind. WasteConf. 44th. 1989 (Publ. 1990): 335.

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TREATMENT OF WASTEWATER 2165

15. Velikov B., Hydrochemistry, Sofia: Ministry of Education, 1986, (in Bulg.).

Received: April 8, 1996Accepted: May 21, 1996

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