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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.50 n.3 Concepción sep. 2005

http://dx.doi.org/10.4067/S0717-97072005000300004 

 

J. Chil. Chem. Soc., 50, N° 3 (2005), págs: 547-551

 

A RAPID ESTIMATION OF METAL CONTENTS IN WASTEWATER TREATMENT FOR CONDUCTIVITY MEASUREMENTS

 

Francisco Prieto García1*, Enrique Barrado Esteban2, Marisol Vega2 and Luis Debán2

1Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Hidalgo, Ctra. Pachuca-Tulancingo, Km. 4.5, 42076, Pachuca, Hidalgo, Mexico. *prietog@uaeh.reduaeh.mx
2Departamento de Química Analítica, Facultad de Ciencias, Universidad de Valladolid, 47005 Valladolid, Spain.


ABSTRACT

An efficient wastewater treatment for metal removal is based on the precipitation of metal ferrites by addition of iron(II) in alkaline and oxidizing conditions. The maximum efficiency is achieved for an iron(II)/total metal ratio of 15:1 (w:w) (Barrado et al., 1996a) which implies that the total metal concentration must be known. It has been established a linear empirical correlation (R2=0.997) between wastewater conductivity and the total metal concentration that allows a rapid estimation of the amount of iron(II) to be added for metal precipitation with no need of time-consuming metal analysis. The empirical correlation obtained overestimates the total metal concentration, and therefore the added iron(II), thus assuring maximum efficiency of metal removal.

The occurrence of metal complexing substances modifies the wastewater ionic composition decreasing the conductivity measurements and the estimated metal concentration. However, it has been demonstrated that the estimated amount of iron(II) is still in excess to assure maximum metal precipitation.

Key words: wastewater treatment, conductivity, metal removal, ferrite


INTRODUCTION

Conductivity measurements can be used to establish the mineralization degree of a solution, to determine the effect of ionic concentration and evaluate its variation, and to calculate the amount of dissolved solids. Some conductivity measurements are given in terms of total dissolved solids, total ionized solids, or salinity, and are meant to estimate the mass fraction of certain species in solution. In this case, the conductivity value (mScm-1) is multiplied by an empirical factor that can range from 0.55 to 0.95, depending on the temperature and the soluble components of the solution (APHA-AWWA-WPCF, 1985). Also, conductivity measurements have classically been applied to establish the salinity of natural waters and wastewater (Lewis, 1978). With this purpose, different empirical relationships or salinity scales have been developed (Lewis, 1980), and applied to a variety of samples. An example is the salinity scale for seawater (Bradshaw and Schleicher, 1989), which has been modified to extend to low salinity solutions (Hill et al., 1986).

A highly efficient procedure for the decontamination of metal-containing wastewater (Katsura et al., 1977; Tamaura et al.; 1991) is based on the addition of Fe(II) to the solution in alkaline and oxidizing conditions, which causes precipitation of the metallic ions as metal ferrites with magnetic properties. It has been demonstrated that maximum purification efficiency and highest magnetic properties of the precipitates are achieved when the Fe(II) concentration is fifteen-fold (w:w) the sum of the concentrations of the metals to be removed (Barrado et al., 1996a and 1996b). Therefore, to estimate the amount of Fe(II) to be added to a given wastewater to achieve the optimum 15:1 ratio, the concentrations of all metals in the solution must be determined, which is time-consuming and inoperative. Thus, the interest is centered in finding out a fast and inexpensive method that is able to correlate any measurable physico-chemical property of the wastewater with its contents in dissolved metals. Conductivity measurements are easy to implement as a routine method and can be related to the concentration of ions in solution. The aim of this work is to obtain an empirical correlation between the conductivity of a solution and its contents in metallic ions that allows to approximately estimate the total metal concentration and, therefore, the amount of Fe(II) to be added to the solution to be purified by the in-situ ferrite process.

It has been demonstrated that the occurrence of metal-complexing substances affects in some extent the precipitation of metals from wastewater as ferrite compounds (Barrado et al., 1998). Besides, these complexation reactions result in a variation of the charge and size of the dissolved metal species, and hence in its conductivity, thus leading to a variation of the conductivity of the solution. This effect must be therefore evaluated in order to obtain an empirical correlation of general application between conductivity and total metal concentration.

EXPERIMENTAL

Fundamentals

The ability of an electrolyte solution at a specific concentration and temperature to conduct electricity is the electrolytic conductivity or specific conductance, c. It is measured by placing an electrolyte solution in a measurement cell of length L and cross-sectional area A. The resistance R in this cell is then measured, and c calculated by:

(1)

The SI unit of c is Sm-1, which is equal to W-1m-1; because of the low values commonly determined, most c measurements are given in mScm-1.

Because the dimensions of the cell are usually rigidly fixed, L/A is a constant for a given cell and is called the cell constant, Kcell. In practice, Kcell is determined experimentally by measurement of R for a solution of known c.

(2)

A property of an electrolyte solution at a specific concentration and temperature in a specific cell is the conductance, G.

(3)

Whereas c is an intrinsic property of a solution, G depends on the cell in which the solution is measured.

The ability of individual electrolytes to conduct electric current is given by the molar conductivity, L (S·m2·mol-1):

(4)

where (S m-1) and C is the molar concentration (mol l-1). The molar conductivity of an electrolyte results from the contributions of all ions in the electrolyte:

(5)

where li is the ionic molar conductivity (Sm2mol-1), and vi is the number of ions (cations and anions) produced by one molecule of electrolyte. The ionic molar conductivity gives quantitative information about the contribution of each ionic species to the conductance of the solution. Its value is somewhat dependent on the total ionic concentration ( ) of the solution and increases with increasing dilution. As the concentration of a strong electrolyte increases, c increases because more ions are present to conduct electricity. For weak electrolytes, c increases with the concentration, but L decreases because of decreased dissociation. At very high concentration of ions, L may decrease due to formation of ion pairs and to increase in solution viscosity.

Conductivity depends on temperature. As thermal energy increases, Brownian motion increases, increasing c. The uncertainty of the measurement of c terms depends on the accuracy and stability of the temperature of measurement. The temperature coefficient, a, of an electrolyte may be given as:

(6)

and

(7)

Instrumental and Reagents

Dissolved metals were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) using a sequential ICP-AES spectrometer Philips PU 7000 (Philips, Holland).

Conductivity measurements were carried out with a precision better than 0.20% for the conductivity range used, using a Crison 522-conductimeter (Crison, Spain) equipped with a platinum cell (Kcell=1.280 cm-1).

A Crison micro-pH 2002 pH-meter (Crison, Spain) was used for pH measurements in conjunction with a combined glass electrode and a temperature probe for temperature compensation.

Analytical grade reagents were used throughout the experiment. Deionized water of conductivity 17.2 mS·cm-1 was used for preparation of solutions and sample dilution, and this value was subtracted from all conductivity measurements presented in this work.

Procedures

Nineteen wastewater samples originating from training Laboratories at the Department of Analytical Chemistry of the University of Valladolid were collected during one academic year. Volumes up to 50 l of wastewater heavily polluted with toxic elements are generated from each session (4 h) of practicals on inorganic chemistry and volumetric analysis carried out by students (ca. 60 per session) in the second year of Chemistry. The concentrations of fifteen toxic metals (AsV, BaII, CdII, CoII, CuII, CrVI, FeIII, HgII, MnII, MoVI, NiII, PbII, SrII, VV and ZnII) occurring in these residues were determined by inductively-coupled plasma-atomic emission spectroscopy (ICP-AES) on samples diluted as necessary. Three replicate measurements were carried out, and mean concentrations of these elements were calculated.

For conductivity determinations, 1 ml of the acidic wastewater was diluted to 50 ml with deionized water, the pH adjusted to 3.0 by addition of sodium hydroxide, and the temperature vessel thermostated at 20.0±0.2C. Five replicate measurements of each solution were done in these conditions.

RESULTS AND DISCUSSION

Table 1 displays mean and standard deviation of conductivity (5 replicate measurements) and total metal concentration (3 replicates for each of the 15 metals) for the 19 samples analyzed. Total metal concentration was calculated as the sum of the concentrations of the 15 elements analyzed, and its standard deviation was calculated as the square root of the sum of their variances. From these concentrations, the total metal contents in the fifty-fold diluted samples were recalculated and plotted vs. the conductivity values (see Fig. 1). It can be seen the linear correlation existing between conductivity and total metal contents in these wastewater samples. A linear regression of these data by the least squares method led to the following equation relating wastewater conductivity to total metal concentration (R2=0.9970): (8)


FIGURE 1. Dependence of conductivity on the total metal concentration of wastewater heavily contaminated with 15 metals. Wastewater samples diluted 1:50.

The usefulness of this empirical correlation to determine the metal contents in wastewater from conductivity measurements was assessed by measuring the conductivity of a synthetic wastewater containing the 15 elements present in the samples at the average concentrations found in the 19 samples collected (average total metal concentration, 3232 mg·l-1, from Table 1). This solution was prepared by dissolving in deionized water the adequate amounts of the respective nitrate salt (CdII, CoII, CrIII, CuII, FeIII, HgII, NiII, PbII, SrII, ZnII), chloride salt (BaII, MnII), and of molibdate, vanadate and arseniate in the sodium form. Five aliquots of this synthetic wastewater were then diluted fifty-fold with deionized water, the pH adjusted to 3 by addition of the necessary amount of sodium hydroxide, and the conductivity measured (n=5), obtaining a mean value of 2866 mS·cm-1. Replacing this value in equation (8), a total metal concentration of 77.9 mg·l-1 is estimated, which corresponds to a concentration in the undiluted solution of 3895 mg·l-1. This means a deviation of +20% from the expected mean concentration of 3232 mg·l-1. This deviation, although high, is positive, and can thus be assumed for the calculation of the concentration of Fe(II) that must be added to the wastewater solution to achieve an efficiency >99% on the removal of heavy metals (15 times higher than the total concentration of metals to be removed), as it guarantees that an excess of Fe(II) is added, and therefore the metals are efficiently removed.


The efficiency of metal removal from wastewater by ferrite formation can be influenced by the occurrence of substances in solution, such as oxidants or metal-binding organic matter, which hinder the precipitation of metals as ferrite compounds due to competitive reactions that modify the species present in solution (Barrado et al., 1996a; Barrado et al., 1998). It is expected therefore that the occurrence of these substances will also affect the conductivity measurements, and thus their ability to estimate the total metal concentration in solution and the necessary amount of Fe(II) to efficiently purify the wastewater.

In a previous work (Barrado et al., 1998), the effect of complexing organic matter was modeled with ethylenediaminetetraacetic acid (EDTA), which was selected as it forms very stable metal complexes and can simulate other naturally occurring ligands with similar binding properties. To investigate the effect of complexing substances on the conductivity of metal-containing wastewater, identical aliquots of the synthetic wastewater described above were diluted 1:50 and the adequate amount of EDTA (as its disodium salt) was added to obtain concentrations ranging from 10-8 M to 0.5 M. The solution pH was then adjusted to 3 by addition of sodium hydroxide, and the conductivity measured.

A plot of the wastewater conductivity (mean of 5 repeats) versus log [EDTA] is shown in Fig. 2. It can be seen that the conductivity is approximately constant for EDTA concentrations below 5¥10-5 M, and then quickly decreases to stabilize again at EDTA concentration higher than 10-2 M. This could be explained considering that, at the solution pH used (pH 3), EDTA has competitive reactions with protons (the predominant species are H3Y- and H2Y2-), and therefore the EDTA concentration necessary to achieve metal complexation increases. For low EDTA concentrations, only metals forming very stable quelates are complexed, and then the solution conductivity remains nearly constant as the ionic composition of the solution varies only slightly. At high EDTA concentrations (>10-2 M), most metals are complexed and a further increase in EDTA concentration does not produce an appreciable increase in conductivity since EDTA is a weak electrolyte and therefore does not modify significantly the ionic composition.


FIGURE 2. Effect of EDTA concentration on the conductivity of a synthetic wastewater containing 15 elements (total metal concentration, 3232 mg l-1) diluted 1:50.

(c values are the mean of 5 repeats).

At the maximum EDTA concentration tested (0.5 M), the conductivity measured was 2631 mS cm-1, thus causing a reduction of 8% respect to the conductivity in the absence of complexing organic matter. From the empirical equation (8), the estimated total metal concentration in the undiluted wastewater results in 3428 mg l-1, which is still 6% higher than the actual concentration (3232 mg l-1). As the concentration of complexing organic matter in wastewater of similar origin is much lower, an overestimation of total metal concentration, and hence of Fe(II) to be added for metal removal by ferrite formation, is always expected, thus assuring the maximum efficiency of the purification process.

To assess the validity of the empirical correlation between wastewater conductivity and metal contents, a new sample containing the same elements (total metal concentration, 1990 mg l-1) was collected from the training laboratories and divided into two aliquots, which were diluted fifty-fold for conductivity measurement. Organic matter as EDTA at a concentration of 5¥10-2 M (10-3 M after dilution) was added to one aliquot. The metal contents estimated from the empirical correlation depicted in equation (8) for the solutions with and without complexing organic matter were 2231 and 2365 mg l-1, respectively. From these estimations, the amount of Fe(II) to be added was calculated according to the Fe/total metal ratio optimized in a previous work (15:1) for maximum efficiency of metal removal by ferrite precipitation (Barrado et al., 1996a). The purification process was then carried out in the optimized experimental conditions (Barrado et al., 1996a, 1998), and the metals remaining in the solution after ferrite formation were determined by ICP-AES in order to evaluate the efficiency of the procedure. In both solutions, the amount of metals removed from the solution was higher than 99%, thus confirming the validity of the conductivity measurements to estimate the necessary amount of iron for metal removal by ferrite precipitation.

 

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