F. Prieto, E. Barrado, M. Vega, and L. Deban. University Autonoma del Estado de Idalgo, Mexico;. University Vagliadolida, Spain. Received May 15, 2000.
Russian Journal of Applied Chemistry, Vol. 74, No. 8, 2001, pp. 132131324. Translated from Zhurnal Prikladnoi Khimii, Vol. 74, No. 8, 2001, pp. 128531289. Original Russian Text Copyright C 2001 by Prieto, Barrado, Vega, Deban.
ÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ
ENVIRONMENTAL PROBLEMS ÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ OF CHEMISTRY AND TECHNOLOGY
Measurement of Electrical Conductivity of Wastewater for Fast Determination of Metal Ion Concentration F. Prieto, E. Barrado, M. Vega, and L. Deban University Autonoma del Estado de Idalgo, Mexico; University Vagliadolida, Spain Received May 15, 2000
-
Abstract The possibility of determining the content of 15 toxic elements in wastewater from their electrical conductivity was studied in order to evaluate the amount of iron(II) ions required for precipitation of these toxic elements.
Treatment of wastewater to remove metal ions requires data on their total content. Convenient express method for determining the solution mineralization and concentration of metal ions is based on measuring the electrical conductivity of solutions. To determine the weight fraction of certain particles in solution, the total content of solid substance, and the content of ionized solid substance, the electrical conductivity (mS cm31) is multiplied by an empirical factor varying from 0.55 to 0.99, depending on the nature of dissolved components and temperature [1]. Electrical conductivity measurements are often used to determine the salinity of natural and waste waters [2]. For this purpose, various empirical equations have been obtained and salinity scales developed [3], which can be used for a wide variety of samples. For example, scales of the seawater salinity [4] including a scale for low-salinity solutions [5] have been developed. Highly efficient treatment of wastewater containing metal ions is based on addition of Fe(II) ions to a solution under study in alkali medium in the presence of oxidants, which results in precipitation of metal ions in the form of ferrites possessing magnetic properties [6, 7]. It has been found that the highest purification efficiency and the greatest magnetic moments of the precipitates are attained when the weight concentration of Fe(II) is 15 times the total concentration of the metal ions to be removed [8, 9]. Thus, evaluation of the total content of Fe(II) that should be added to wastewater to maintain the optimal 15 : 1 ratio requires that the concentration of all metal ions in solution should be determined. However, common anal-
ytical procedures used in this case are rather laborious. Thus, development of a new fast and cheap procedure for determining metal ions in wastewater, based on measurement of its certain physicochemical properties, is rather urgent. Electrical measurements are rather simple, and the electrical conductivity correlates with the concentration of metal ions in solution. The aim of this study was to determine the empirical dependence between the electrical conductivity and the concentration of metal ions in solution and thus to evaluate the total content of metal ions and, consequently, the amount of Fe(II) necessary for in situ treatment of wastewater via precipitation of metal ferrites. Previously, it has been found that compounds able to form complexes with metal ions affect the precipitation of the corresponding metal ferrites from solutions [10]. Moreover, complex formation changes the charge and size of metal-containing ions in a solution and thus affects its electrical conductivity. Hence, evaluating the effect of complex formation on how the electrical conductivity depends on the content of metal ions in solution, with the aim to determine the general empirical correlation, is equally urgent. EXPERIMENTAL To determine the specific electrical conductivity c, the electrolyte solution is placed in a measurement cell (length L and cross-section A). Then the resistance R of this cell is determined and the electrical conductivity is calculated using the following expression
1070-4272/01/7408-1321 $25.00 C 2001 MAIK
c=
L/A ÄÄÄÄ . R
[Nauka/Interperiodica]
(1)
1322
PRIETO et al.
Although the electrical conductivity is measured in the SI system in s m31, results are mostly given in mS cm31 because of the low conductivities of the systems in question. The size of the measurement cell is fixed, and for a given cell the L/A ratio (cell constant Kc) is invariable. This constant is determined experimentally by measuring R for a solution with known c:
c=
Kc /R.
(2)
The electrical conductance G is a characteristic not only of the electrolyte solution with given concentration and temperature, but also of the cell geometry: G = 1/R.
(3)
The molar electrolytical conductivity L (S m mol31) is a measure of the capability of a given electrolyte to carry electric current: 2
L = c/C 0 103 , 3
(4)
where c = c(solution) 3 c(solvent) (S m31) and C is the electrolyte molar concentration (M). The molar electrical conductivity of electrolyte is determined by the contributions of all the present ions:
L = Sn l , i i
(5)
where li is the ionic molar conductivity (S m2 mol31) and ni is the number of ions (cations and anions) formed by one electrolyte molecule. The ionic molar conductivity gives quantitative information on the contribution of a given ion type to the solution conductivity. Its value depends on the total concentration of ions (Sz i c i) in the solution and grows with its dilution. With increasing concentration of a strong electrolyte, c grows because a greater amount of ions participate in the current transport. In weak electrolytes, c also grows with increasing concentration, but L decreases because of the decreasing dissociation. At very high ion concentrations, L may decrease because of ion pair formation and increasing solution viscosity. The electrical conductivity depends on temperature. With increasing temperature, the Brownian motion becomes more pronounced, leading to a rise in c. The accuracy of the c measurement is determined by the stability of temperature in the course of the tests. The temperature coefficient a of electrolyte conductivity can be expressed as follows: dc/dt a = ÄÄÄÄ c
(6)
and thus
c =c i
25[1
+
a(t 3 25)].
(7)
The content of metal ions in solution was determined by atomic-emission spectrometry with inductively coupled plasma (AES-ICP) on a Philips 7000 atomic-emission inductively coupled plasma spectrometer (Philips, Netherlands). The electrical conductivity was measured with an accuracy of no less than 0.20% in the entire conductivity range, using a Crison 522 conductometric unit (Crison, Spain) with a platinum cell (Kc 1.280 cm31). The solution pH was measured with a Crison micro-pH 2002 unit (Crison, Spain) equipped with a combination glass electrode and a temperature compensation probe. In all the tests, analytically pure chemical reagents were used. The test solutions were prepared using deionized water (conductivity 17.2 mS cm31). The electrical conductivity of water was subtracted from all experimental results. Nineteen wastewater samples were collected at laboratories of the Analytical Chemistry Department of the University Vagliodolida within one academic year. The ion concentrations of 15 toxic elements [As(V), Ba(II), Cd(II), Co(II), Cu(II), Cr(VI), Fe(III), Hg(II), Mn(II), Mo(II), Ni(II), Pb(II), Sr(II), V(V), and Zn(II)] were determined by AEP-ICP method using solutions diluted to an appropriate degree. The measurements were performed 3 times and the average concentrations of the above elements were calculated. To determine the electrical conductivity, an aliquot (1 ml) of acidic wastewater was diluted to 50 ml with deionized water, and its pH was adjusted to 3.0 with sodium hydroxide. The measurements were carried out at a constant temperature of 20.0 + 0.2oC. The tests were performed 5 times for each solution under these conditions. The average electrical conductivities and corresponding rms errors for 19 samples in question (5 parallel tests) and the total concentrations of metal ions (3 parallel tests for each of 15 elements) are listed in the table. The total concentration of metal ions was calculated as a sum of concentrations of 15 elements studied and its rms error was evaluated. Then, using experimental concentrations, the total content of metal ions in diluted solutions was determined and the concentration dependence of the electrical conductivity was plotted (Fig. 1). For the samples in question this dependence is linear and is described by the following equation obtained by leastsquares procedure (R2 = 0.9970):
c=
25.104C + 909.85.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY
Vol. 74
(8) No. 8
2001
MEASUREMENT OF ELECTRICAL CONDUCTIVITY OF WASTEWATER
The adequacy of empirical equation (8) for determining the concentration of metal ions in wastewater from its electrical conductivity was analyzed using a model sample containing 15 above elements present in all 19 wastewater samples (the total average concentration of metal ions was 3232 mg l31, see table). The model solution was prepared by dissolving the required amounts of Cd(II), Co(II), Cr(III), Cu(II), Fe(III), Hg(II), Ni(II), Pb(II), Sr(II), and Zn(II) nitrates, Ba(II) and Mn(II) chlorides, and sodium molybdates, vanadates, and arsenates in deionized water. Then 5 aliquots of the model solution were diluted (1 : 50), the solution pH was adjusted to 3.0 using sodium hydroxide, and the electrical conductivity of the resulting solutions was measured (n = 5). The average conductivity was 2866 mS cm31. Substitution of this value in Eq. (8) gives the total concentration of metal ions of 77.9 mg l31, which corresponds to the concentration of the initial model solution (3895 mg l31). Thus, the deviation from the average concentration (3232 mg l31) is 20%. This error is rather large but positive, and thus Eq. (8) can be used to calculate the amount of Fe(II) which should be added to the wastewater to remove no less than 99% of heavy metals [Fe(II) concentration should exceed the total concentration of these metals by a factor of 15]. This equation guarantees that an excess amount of Fe(II) is added to the solution, sufficient for effective precipitation of metal ions. Oxidants and organic compounds, which can interact with metal ions and thus hinder their precipitation as ferrites because of competitive reactions modifying the species in solutions, can strongly affect the efficiency of wastewater treatment to remove metal ions via formation of ferrites [8, 10]. It would be expected that presence of these compounds also affects the electrical conductivity and thus the feasibility of using its values to determine the total concentration of metal ions and the amount of Fe(II) required for effective wastewater treatment. Previously [10], we simulated the effect of complexing organic compounds for the example of ethylenediaminetetraacetate ions (EDTA), which were taken because they form stable complexes with metal ions and can be analyzed as a model of the other natural ligands with similar complexing properties. To study the effect of organic ligands on the electrical conductivity of a wastewater containing metal ions, aliquots of the model solution were diluted (1 : 50), and appropriate amounts of EDTA disodium salt (from 1038 to 0.5 M) were added. The solution pH was adjusted to 3.0 with NaOH, and the electrical conductivity was measured. RUSSIAN JOURNAL OF APPLIED CHEMISTRY
Vol. 74
1323
Electrical conductivity (n = 5) and total concentration of metal ions (n = 3) for wastewater samples diluted in 1 : 50 ratio
ÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ , S cm31 ³ [Men+ ], mg l31 Sample ÃÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄ no. ³ average ³ rms ³ average ³ rms ³ value ³ error ³ value ³ error ÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄ 1 ³ 1058 ³ 9 ³ 445 ³ 5 2 ³ 2005 ³ 11 ³ 2356 ³ 20 3 ³ 2933 ³ 10 ³ 4072 ³ 33 4 ³ 4210 ³ 12 ³ 6575 ³ 30 5 ³ 3485 ³ 16 ³ 5050 ³ 33 6 ³ 4048 ³ 16 ³ 6245 ³ 30 7 ³ 2188 ³ 13 ³ 2541 ³ 16 8 ³ 1598 ³ 8 ³ 1253 ³ 8 9 ³ 2593 ³ 16 ³ 3296 ³ 19 10 ³ 2118 ³ 13 ³ 2356 ³ 18 11 ³ 1670 ³ 9 ³ 1401 ³ 11 12 ³ 1105 ³ 8 ³ 451 ³ 6 13 ³ 2455 ³ 16 ³ 2973 ³ 11 14 ³ 2220 ³ 15 ³ 2608 ³ 10 15 ³ 3443 ³ 16 ³ 5034 ³ 21 16 ³ 3663 ³ 16 ³ 5284 ³ 24 17 ³ 3933 ³ 16 ³ 6009 ³ 32 18 ³ 2368 ³ 16 ³ 2879 ³ 10 19 ³ 1350 ³ 11 ³ 574 ³ 8 ÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄ
c m
S
The logarithmic dependence of the electrical conductivity of the wastewater (n = 5) on the EDTA concentration is shown in Fig. 2. As seen, the electrical conductivity is nearly constant at EDTA concentrations lower than 5 0 1035 M, then it sharply decreases and becomes constant at concentrations higher than 1032 M. This fact can be accounted for as follows. At pH 3, EDTA participates in the competitive reactions with protons (H3EDTA3 and H2EDTA23 are predominant) and thus the concentration of EDTA required for complexation increases. At low EDTA concentrations, only metal ions forming the strongest chelates with the organic ligand participate in complexation and the electrical conductivity remains nearly constant because the ionic composition of the solutions changes insignificantly. At higher EDTA concentrations (>1032 M), almost all metal ions participate in complexation and, as a result, further increase in the ligand concentration does not affect the conductivity, because EDTA is a weak electrolyte only slightly changing the ionic composition of the solution. At the maximum EDTA concentration studied (0.5 M), the electrical conductivity was 2631 mS cm31, which is 8% smaller than that in the absence of the complexing additive. The total concentration of metal ions in the initial model solution, calculated from No. 8
2001
1324
PRIETO et al.
Then the treatment was performed under the optimized conditions [8, 10], and its efficiency was evaluated from the content of metal ions (AES-ICP method) remaining in the solution after treatment. In both cases the purification efficiency was greater than 99%, which confirms the possibility of using the electrical conductivity measurements to determine the amount of iron(II) required for wastewater treatment to remove metal ions in the form of their ferrites. Fig. 1. Electrical conductivity c of model wastewater vs. the total concentration of metal ions C in wastewater strongly contaminated with ions of 15 heavy metals. Samples diluted to 1 : 50 ratio; the same for Fig. 2.
CONCLUSIONS (1) A linear dependence between the electrical conductivity and the total content of metal ions in solutions was established. (2) Addition of EDTA forming complexes with metal ions decreases the electrical conductivity, but evaluation of the total content of metal ions and thus of the amount of iron(II) ions from the electrical conductivity of wastewater remains acceptable for practical use. ACKNOWLEDGMENTS
Fig. 2. Electrical conductivity c of model wastewater containing ions of 15 heavy metals vs. EDTA concentration.
is 3428 mg l31, which is 6% greater than the real concentration (3232 mg l31). Since the possible concentration of organic ligands in wastewater is significantly smaller, such calculations give the overestimated total concentration of metal ions and thus of the amount of Fe(II) ions required for their recovery as metal ferrites, which guarantees the maximal efficiency of the treatment process. To evaluate the applicability of the above empirical expression relating the electrical conductivity to the total concentration of metal ions in wastewater, a new sample containing the same ions (1990 mg l31) was taken in the academic laboratories. This sample was divided in two aliquots, which were diluted in 1 : 50 ratio for conductivity measurements. Then EDTA was added to one aliquot (5 0 1032 M or 1033 M after dilution), and the conductivity of these samples was measured. The total content of metal ions in solutions with organic ligand and without it were calculated by means of Eq. (8) to be 2231 and 2365 mg ml31. These results were used to determine the amount of Fe(II) required for water treatment, taking into account the optimal Fe/total metal content ratio of 15 : 1 [8].
The work was financially supported by C.I.C.Y.T (project no. AMB94-0938). F. Prieto is grateful to ITEGMA for the financial support and AECI for the grant received after backing his dissertation. REFERENCES 1. APHA-AWWA-WPCF: Standard Methods for the Examination of Water and Wastewater, United States: Am. Health Assoc., 16th ed., 1985, pp. 76 80. 2. Lewis, E.L., J. Geophys. Res., 1978, vol. 83, pp. 466 470. 3. Lewis, E.L., IEEE J. Oceanic Eng., 1980, vol. 5, pp. 3 11. 4. Bradshaw, A.L. and Schleicher, K.E., IEEE J. Oceanic Eng., 1980, vol. 5, pp. 50 56. 5. Hill, K.D., Dauphinee, T.M., and Woods, D.J., IEEE J. Oceanic Eng., 1986, vol. 11, pp. 109 113. 6. Katsura, T., Tamaura, Y., and Terada, H., Ind. Water, 1977, vol. 233, pp. 16 21. 7. Tamaura, Y., Katsura, T., Rojarayanont, S., et al., Water Sci. Technol., 1991, vol. 23, pp. 1893 1900. 8. Barrado, E., Vega, M., Pardo, R., et al., Water Res., Ser. A, 1996, vol. 30, pp. 2309 2314. 9. Barrado, E., Vega, M., Pardo, R., et al., J. Anal. Lett., Ser. B, 1996, vol. 29, pp. 613 633. 10. Barrado, E., Prieto, F., Vega, M., and FernandezPolanco, F., Water Res., Ser. A, 1998, vol. 32, pp. 3055 3062.
3
3
3
3
3
3
3
3
3
3
RUSSIAN JOURNAL OF APPLIED CHEMISTRY
Vol. 74
No. 8
2001
