treatment of wastewater from the lead‐zinc ore processing industry
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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
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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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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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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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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
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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.
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8. Kulskii L., Grenuk V., Savchuk O. Electrochemistry of Wastewater Treatment,Kiev: Technika, 1987, (in Russ.).
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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.
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Received: April 8, 1996Accepted: May 21, 1996
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