(flpHMlheHo 30. oxr o tipa 1996. penunupaxo 15. cjJe6pyapa 1997). REFERENCES. 1. M. Caballero, R. Cela , 1. A. Perez-Bustamante, Talanta 37 (1990) 275.
1. Serb. Chern. Soc. 62(6)523-530( 1997)
UDC 546.47 .815:622.765 :543.51 Original scientific paper
./SCS-24 I 6
Precipitate flotation of lead and zinc and their determination by atomic absorption spectrometry KATARINA CUNDEVA and TRAJeE STAFILOV'
lnstitut of Chemistry, Faculty ofScience, St. Cyril and Methodius University, P.0. Box 162. 91001 Skopje, Macedon ia (Received 30 October 1996, revised 15 Februar y 1997) A preconc entration flotation procedure has been developed to extend the range of con ventional atomic absorption analysis to trace concent ration of lead and zinc in natural waters. The optim al conditions for the preconcentration and separation of these metals from fre sh water by precipitate flotati on with hydrated ironifll) oxy de (Fe20 3.xH20 ) were studied. All important parameters necessary for the successful flotat ion, Iike optim al mass of collector, pH of the medium , induction time etc., were checked. Under the same conditions Pb and Zn were floated quantit ativel y (100 % for lead and 95.16% for zinc) with 30 mg Fe(II1). The possibility of the usage of atomic absorption spectrometry was determin ed of the colligend levels in fresh water samples. The quantitative determin ation of lead was carried out by electrothermal atomic absorp tion spectrometry, whil e zinc was determined by flame atomic absorpt ion spectrometry. The results obtained by atomic absorption spectro metry were compared with the results obtained by inductively coupled plasma-atomic emiss ion spectrometry. T he detection limit for lead is 0.50 ug/L and for zinc is 0.85 ug/L ,
Key words: lead, zinc , determination , precipitate flotation, atomic absorption spec trometry.
Atomic absorption spectrometry (AAS) provides high sensitivity and rapid mea surement for number of elements. However, for low ug/L level of heavy metals in fresh water, a preci se direct determination is impractical even by this method. Among the various techni ques for the pre concentration of tra ce amounts ofPb and Zn from aqueous so lutions the precipitate flotat ion is simple, rapid and permits the determination of several elements in water. I ,2 Hydrated iron(Ill) oxide (Fe 203' XH20 ) is well known as one of the most effi cient collectors of trace metals in aqueous systems. The published methods using this collector in combination with flame atom ic absorption spectrometry (FAAS ) are frequently used,I -IO but the application of this collector in combination with ele ctrothermal atomic absorption spectrometly (ETAAS ) is scarce. 11 -14 There are papers about the determ ination of lead * Correspondence
523
\
524
'CUN DEVA and STAFIL OV
and zinc using Fe203 . XH20 as collector with different tensides: c etyltrimethylam monium bromide 15 or monododecyl phosphate and monododecyldithiocarbamatel" for zinc and fatty acids 17 or sodium dodecylbenzenesulfonate 18 for lead. In this paper the method of lead and zi n c concentration with Fe203·xH20 a s the precipitating collector using sodium dodecylsulfate and sodium oleate as tensides in combination with ETAAS for lead and F AAS for zinc determination is investigated . The proposed method is simple, rapid and applicable to the separation oflead and zinc present at IJ.glL levels from a large volume of water. All the important parameters necessary for the successful precipitate flotation (optimal mass ofcollector, pH ofthe medium, flotability of calcium and magnesium as macro elements, etc.) were investigated. EXPERIMENT AL App aratus
4 The apparatus employed in this work have been described previousl/ Perkin-Elm er hollow cathode lamps were used as a source. The instrumental parameter s for AAS are give n in Table 1. T he instrum ental parameters (tempe rature and time) for lead determination by ETAAS were established by extensive testing and they are: 120°C and 30 s (for dry ing), 1300 °C and 30 s (for charrin g), 2600 °C, 5 s (for atomizing), 2650 °C, 5 s (for cleaning). Inductively coupled plasma -atomic emission spectrometry (ICP-AES) was perfomed using Varian ICP-A ES spectrometer Model Liberty 110. The flotation cell used to carry out the preconcentration was a glass cylind er (4 x 105 ern) with a sintered glass disc (poros ity No. 4) at the bottom to generate air bubbling. TAB LE 1. Optim al instrumental parameters
Wavelength Spectral slit Hollow cahode lamp curren t Oxidant/fuel gas mixture
Pb 240,7 nm 0.7 nm 30 mA
Mg 285.2 0.7 nm 6mA
Zn Ca 213.8 nm 422.7nm 2.0 nm 1.3nm J5 mA \0 mA Air/acetylene
Reagents
All reagents used were of "analytical reagent" grade except for sodium dodecylsulfate (NaDDS) and sodium oleate (NaOL). Aqueous reagents were prepared in deionized redistilled water. Stock solution of lead and zinc were made respectively from Pb(N03 )z and ZnS04 .7H20 as I mg/mL solutions. Before each investigation, standard solutions were freshly prepared by diluting these stock solutions. Iron(III) solution (30 mg/mL) was prepared by dissolving high-purity iron metal (Merck) in cone. NH03. The 0.5% solutions oftensides used were prepared by dissolving appropriate amounts ofNaDDS and NaOL in 95% ethanol.111e pH of the working solutions was regulated by 0.1 mol/L solution ofHN03and 2.5% and 10% solutions ofKOH. A saturated solution of KN03 (c = 2.78 mol/L) served as an ionic strength adjuster.
Procedureforflotation The recommended procedure is for clear and uncontamin ated fresh water. The investigated sam ples were tap water from the city of Skopje and water from the spring St. Pantelejmon from the Skopj e neighbourchod. The water samples were not filtered. To prevent the possible hydrolytic precipirtation of some mineral salts a few millilitres ofconc . HN03 had to be added to I L of natural water. The pH had to be ca. 2.8-3. A wate r sample (I L) was placed in a beaker (1 L). After add ing 6 mL of a saturated solution of KN03 and I mL of a Y mg/mL Fe(N03h solution , the pH was carefull y adju sted to 8.0-8.5 by
\
525
LEAD AND Z INC ATOM IC ABSORPTION SPECTROMETRY
KOH solution F.5% and J 0%). The system with the red-brown precipitate was stirred for 10 min (induction time ). Then, I mL NaDDS and I mL NaOL alcoholic solution were added and the contents of the beaker transferred into the cell with a small portion of O. I mol/L NH4N03 sol ution. Air (50 mL/min) was passed from the perforated bottom of the cell for 2-3 min. Then, a glass pipette-tube was immersed into the cell through the foam layer and the water phase was sucked off. Hot 4 mol/L HN0 3 solution was added to the cell to destroy the scum. The solutio n was sucked off and collec ted in a volumetric tlask (25 mL). 111e cell and the pipette-tube were washed with 4 mol/L HN03 solution. The tla sk was filled to the mark with the same solution and the sample was ready for AAS measuremen t. To obtain the optimal experimental parameters, like collector mass, medium pH, induction time, for lead and zinc precipitate flotation, as well as to obtain the flotability of calcium and magnesium, standard solutions of these analytes were treated by the recommended procedure and before being tested by FAAS. The Pb conte nt in the final floated solutions was determined by ETAAS, wh ile the Zn concentration in the finaJ floated solutions was determined by FAAS. RESULTS AND DISCUSSION '
Collector mass
The effect of co llector mass on the Pb and Zn rec overies wa s inves tigated as a functio n of the amount of iron(III ) added as a constitut ive element of collec tor used. A se ries of flotation with Fe20 3.xH 20 were performe d by the addi tion of different iron (III) mas s concentrations (y(Fe) = 2.5-100 mg/L) to the working so lutions at a constant pH (8.5) and ionic streng th (Ic = 0.02 mol/L). T he co llector pre c ipitates were obtained from water soluti ons (l L) con tainin g SO ug Pb and Zn respectively. After flotatio n separation the sublates we re dissolving a 4 mol/L R N0 3 solution and collected in 25 m L volume tric flasks. Th us, the final flo ated so lutions have a co ncentration of 2 ug/rnl. . The exp erimental data are g iven in Tab le II. As can be seen from the data presented in Table ll, lead and zinc were floated suc cessfully with 30 mg Fe at pH 8.5 and an ionic strength of 0.02 mo l/ L. The recovery for lead was l OO.O% and for zinc 95.2% . TABLE II. The values of the Pb and Zn flotation recovery dependence on the iron(III) mass. Coprecipi tation with Fe203.xH20 at a constant pH and ionic strength (l) y(Fe), rng/L
t.. mol/L
pH
2.5 5.0 10 20
0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02
85 8.5 8.5
30 40 60 80 100
8.5 8.5 8.5 8.5 8.5 8.5
R(%)
y(Pb)12 ug/ml, 8 1.4 85.7 90.7 94.7 100.0 100.0 100.0 100.0 100.0
"((Zn)/2 ug/rnl,
77.8 86 .0 88.2 92.2 95 .2 96.0 98 .2 98.2
100.0
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526
CUNDEVA and STAFILOV
Influence ofpH
The pH interval of the working medium, within which colligends Pb and Zn could be successfully separated, was determined from the aspect of the collector stability. The influence of the medium pH of the Pb and Zn flotation recoveries were studied using a series of solutions (1 L) containing 25 and 50 ug of colligend mass (the final flotated solutions have a concentration of 1 and 2l-Lg/mL). The influence of pH was investigated within the range of 3 to 10 pH. The ionic strength (0.02 mol/L) and the iron(III) mass added (30 mg) were always constant.
100 90
80
TO 80
50
40
30,"+
20
I
10
I
+.... +
OL-......I..-.-'------'-_-'------'------'-----'_-L..----'----'-_'--....I..-.-'------'-_.l....-...J 2.5 3
3.5
4
4.5
1I 5.5
6
6.1I
7
7.1I 8
8.5
9
9.5
10 10.5
pH Fig. 1. The dependence of the Pb flotation recovery on the medium pH (0- ug/ml.; +-
2~g/mL).
Figure 1 illustrates the significant effect of pH on the Pb recoveries 100.0% within the range opf 6 to 10. Figure 2 illustrates the significant effect of pH on the
tOO
90 80
70
80
60
40
30
20
.f
to
Fig. 2. The dependence 01 "Ie Zn flotation recovery on the medium pH (*- I ug/rnl.; +- 2 ug/ml.),
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527
LEAD AN D ZI NC ATO MIC ABSORPTIO N SPECT ROM ETRY
Zn recoveries, 96.2-1000% within the narrower pH interval of 8.5 to 10. For the successfull simultaneous preconcentration of both colligends (lead and zinc) pH of 8.5 was chosen at the working pH. Induction time
The time necessary for the incorporation of the colligend in the collector precipitates is termed the induction time (r). The results of the investigations of the relat ion between Pb and Zn recoveries and 't are given in T able 111. From the results, it can be concluded that the separation of colligends was qu antitative over a range of 5-20 minutes. In practice, an induction time of 10 minutes was used . TABLE III. The influence of the induction time on lead and zinc flotation recoveries
r/m in
5
\0
15
20
\ ug/ml,
R(%)
99 .7
96.9
96.9
94.0
y(Zn)
1:/min
5
10
15
20
IflglmL
R(%)
95.8
100.7
97 . \
98 .2
y(Pb)
Flotability of calcium and magn esium TABLE IV. Flotability of calcium and magnesium Flotabili ty of calcium
Sample of water
y(Fe) gIL- I rna
pH
Rasce (t.w.") St. Pantelejmon'')
30 30
8.5 8.5
Before flotation y(Ca)/mgIL-1
After flotation
\00 .9 73.0
2.73 3.14
y(Ca)/mg L-
I
R % 2.7 4.3
Flotability ofmagnesiu m
Sample of water Rasce (T .W.)
Before flotation
y(Fe) mg/L" "
pH
y(Mg)/mg L-
30
8.5
6.30 7.42
1
After flotation y(Mg)/mg L-
0.49
I
R (%) 7.7
St. Patelejmon 8.5 0.52 30 'Tap waterfrom the city of Skopje (a spring of Rasce) bWater from the spring of St. Pautelejmon
7.0
Natural w aters alw ays contain calcium and ma gn es iu m as ma croelements. Th erefore it was necessa ry to investigate their behaviour dur ing the flotati on . If during the pr econcentration of the microelements, simultaneous procencentration of macroelements occurs, the matr ix in the final solution 'might be very complex. Then AAS determination of colligends (Pb 2+ and Zn 2+) will become more compli cated and diffi cult. For this purpose, the concentrations of Ca 2 + and Mg 2+ were detenninated in the natural water samples before the flotation, as well as in the fin al con centrated solutions. The water sam ples w ere floated under the same conditions as for Pb 2+ and Zn 2+ (30 mg Fe at pH 8.5 and ionic strength 0.0 2 mollL). The values of Ca 2+ and Mg 2+ recoveries presented in Table IV, show that Ca 2 + and M g 2+ flotabilty under these experimental ' cond itions is insignificant. They can not be
\
528
(; UNDEVA and STAFILOV
floated and are left in the processed water phase. The concentrations of calcium and magnesium in the final solutions concentrated by flotation are low , and so thei r influence on lead and zinc absorbance is negli gible. 19,20 Detection limit
To evaluate the dete ction limit of the method, ten successive blank measure ments we re made. The dete ction limit (Ld) was est imated as three values of the standard deviation - s (Table V). TABLE V. St andard deviation (s) , relat ive standard deviation (s.) and detection limit (Ld) of Pb (determined by ET AAS) and Zn (determined by FA AS)
s,(%)
Element Pb
0.15
2n
0.28
6 .5 7.9
0.5 0
0.8 5
Analysis a/natural water
Th e applicability of the proposed procedure has been .verified by the AAS analysi s of natural w ater samples by the method of standard additions. For this purp ose, known amounts of Pb and Zn were added to I L aliquo ts of tap and spring water samples whi ch we re than floated. ET AAS is used for lead determination , Since the co ncentrations of zinc in investigated natu ral water samples were relt ively higher, FAA S was applied as instrumental determination method for this elem ent. Th e reco veries of 93.4 to 104.2% for lead (Table VI) and of 96.9 to 103.5% for zinc (Ta ble VII ) show that the preconcentration and sep aration of these colligends are satisfactory . Th e results obtained by ET AAS were compa red with the res ults obtained by ICP -A ES determinations; the samples were concentrated by evapora tion (from a volume of 1000 m L to 25 mL) of tap and lake wat er, resp ec tively (Ta bles VI and VII). TA BLE VI. Results of the det ermination of lead in natural w ater with ET AAS usin g the meth od of standard addi tions ETAAS Water sample
(t.w. •) 20.26 DHo
Estima ted ug/l. Pb
Fou nd Jlg/L Pb
2.50 6.25
4.62 8.37
2.12 4.60 7.82
Ra ~ce
pH = 7.08 Sv, PanteJejmon
Found Jlg/L Pb
R (%)
2.50 99.6 93.4
< 0.50
14.9 DH°"* pH = 7.6
ICP-EAS
Added ug/L Pb
2.50
2.50
2.6 1
< 0.75
104.2
' ) t.w, - tap.water:.") DHo - German degree of water hardness
\
529
LEAD AND ZINC ATOMIC ABS ORPTION SPECTROMETRY
TABLE VII . Results of the determination of zinc in natural water with FAAS using the method of stand ard add ition s
Water sample
Rasce (t.w.) 20.26 DHo pH = 7.08 S v. Pantelejmon 14.9 DHo pH = 7.6
Added ~gIL Zn
FA AS Estim ated Found ~glL Zn ~glL Zn 146.6
ICP-EAS Found ug/L Zn 150.0
R (%)
2.50 6.25
149. JO 152.80
147.4 152.1
98.9 99.5
2.50 6.25
12.90 16.65
10.4 12.50 17.23
96.9 103.5
9.9
CONCL USIO N
The results presented in this pap er pro ves that the enri chment ofl ead and zinc in the trace concentration from fresh water samples can be success fully performed by a flotation technique with Fe20 3.xHZO, as collector in combination with ETAAS for lead and FAAS for zinc determination. ETAAS can be applied as instrumental method for zinc determination, if its concentration levels in the natural w ater samples are far lower than those in the investigated samples in this wo rk. Lead and zinc can be preconcentrated sepa rately or simultanously in mixtures by preliminary selection and corre lation of their optima l experime ntal parameter s (the pH , ion ic strength, mass of Fez03.xHzO, indu ction time). T he presence of calcium and magne sium do not interfere.
V13[J01l
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9]00] Cuoii]e, Ma ..co onnja
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Il penno -xerr je no cr yn ax aa epnOTaL\lJOH O KOHL\eHTpHCaJ-he .rpa roea o rrosa 11 L\HHKa y onpebunarse aTOMCKOM arrCOprrL\HOHOM crrex-rpoeier pa josr. Hcrnrr amr cy OllTHMa nHH ycnosa xonuenrp ncaisa H cen ap au nje OBHXMeTana U3 cn aTKUX BOAa
rrpHpOAHHM BOAaMa II lhH XOBO
Tan m KHOM rpnora n aj o« XJIApaTUcaHHM Fe(III) OKCUAOM (Fe203.xH20 ). Y TBpl) eHJoI cy nap a MeTpu nOTp e6HH aa KBaHTUTaTu BHy cprroranajy, xao WTO cy
orrnoranaa Maca
KOJlCKTOpa , pH
cpenuue, ape ne HHIlYKl\uje UTA. O nOBO lJ l\lJHK cy KBaHTUTaTUBHO epJIOTlJp aHH non HCTUM yCJIOBU Ma
(100% sa OJIOBO U 95,16% aa l.\U,HK) ca 30 mg Fe( III) . Komre rrrpauaja HCIDITlIBa HllX onpebaeau.e
en eraenara y CnaTKlJM BOAaMa ycrro ars as a u360 p arou asa u aje. KBaHTuTaTu BHo
orr o aa H3BpweHo .ie y eneK TpOTepMtJ
