Environ. Sci. Technol. 1997, 31, 745-749
Analytical Evaluation of Immobilization of Heavy Metals in Cement Matrices ANDRZEJ BOBROWSKI,* MAREK GAWLICKI, AND JAN MAŁOLEPSZY Department of Material Science and Ceramics, University of Mining and Metallurgy, Al. Mickiewicza 30, 30-059 Krako´w, Poland
The method of Cd, Pb, Zn, and Cr immobilization in alkaliactivated slag cement mortars and Portland cement mortars are evaluated. It has been found that differential pulse anodic stripping voltammetry enables the determination of trace amounts (from 0.1 to 400 µg/L) of Cd, Pb, and Zn in extract solutions obtained as a result of leaching the metals from cementitious materials. The heavy metals were determined by anodic stripping voltammetry after the UV mineralization of water extracts performed to destroy organic surfactants and sulfide present in the solutions. To determine the chromium amounts in the extracts (20 µg/ L-200 mg/L), several polarographic, adsorptive stripping voltammetric, and spectrophotometric methods were tested. The spectrophotometric method with diphenylcarbazide has been selected as it possesses the required sensitivity and enables the speciation studies. Precision and accuracy data of the applied methods are presented along with comparative studies using AAS and ICP AES analysis of the extracts. It has been found that the proposed method enables the immobilization of 1% addition of Cd, Pb, Zn, and Cr(VI) to the cement matrices with the final efficiency of 99.9%, 99.9%, 99.9%, and 98.8-99.6%, respectively.
Introduction Deposition of wastes containing heavy metals compounds involves certain definite procedures, the aim of which is to prevent the migration of these metals outside the deposition site. It is a troublesome task, and it often requires the immobilization of these metals in a definite matrix. Materials that may be successfully utilized to produce matrices of such a type are the pastes of different cementitious materials such as ordinary Portland cement (OPC), blended cements, as well as slag-alkaline and slag-ash-alkaline binders. Alkali-activated slag cements (AASC) are cementitious materials obtained as a result of hydration of pulverized blast furnace slag in Na2CO3, NaOH, or sodium metasilicate solutions. The basic products of hydration are the CSH phases, calcite and hydrogarnets. Application of these binding materials guarantees that the mechanical strength of the obtained concretes is higher that of standard Portland cement concretes. Moreover, AASC concretes exhibit great resistance to corrosion. All these cementitious materials are characterized by a high pH value of pastes prepared from them, low solubility of their hydration products, and very well-developed specific surface and microporosity. The basic product of hydration of all the mentioned cementitious materials, which determines their practical properties, is * Corresponding author e-mail:
[email protected]; fax: +(48)(12)331593.
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1997 American Chemical Society
semicrystalline calcium hydrosilicates, usually denoted by the symbol CSH. With many unquestionably similar properties, pastes prepared from the particular cementitious materials show a number of essential differences. This refers particularly to the chemical and phase compositions of the hydration products, the rate of the chemical reactions occurring in the cementitious material-water system, and the dynamics of the increase of the mechanical strength of the pastes and consequently of concretes prepared from them. The mechanisms of the immobilization of the particular metals in the products of hydration of the above-mentioned cementitious materials as well as their influence on the progress of hydration and the practical properties of the binders are not as yet sufficiently well-known. It is obvious that this influence depends not only on the element itself but also on the degree of its oxidation. Glasser (1) distinguishes two types of the immobilization of heavy metals in cement pastes: physical immobilization, associated first of all with the microporosity of the CSH phase and consisting of the adsorption of ions and particles on the surface of this phase; and chemical immobilization, taking place as a result of the formation and precipitation in the pastes of compounds of low solubility products. Under conditions existing in the pastes, i.e., high pH, these are most often the hydroxides of heavy metals (2, 3). More complex phenomena are observed in the case of amphoteric elements. Here we must mention first zinc, which in a strongly alkaline medium forms zincates and chromium(VI), occurring in the pastes in anionic form (4). Beside CSH, an important role in the immobilization of heavy metals is also played by other products of the hydration of cementitious compounds. In the case of Portland cement pastes, these are the hydroaluminates and calcium hydrosulfoaluminates, and in the case of alkali-activated slag cement pastes, these are the zeolites and hydrogarnets. One must also mention the phenomenon of isomorphism, which leads as a consequence to various types of substitutions both in the structure of the CSH phase and in other hydration products (5). The basic method of evaluating the efficiency of heavy metals immobilization in pastes, mortars, and concretes is the metal ions extraction during the leaching tests. The conditions of testing may vary, and their selection should be determined by the actual conditions in which the examined wastes will be deposited (6). The most popular and most often used leaching methods are the static TANK methods, which consist in placing monolithic samples made of the paste, mortar, or concrete in a tightly closed plastic container with distilled water or a weak solution of nitric acid (pH ) 4). This method allows us to carry out the investigations at arbitrary time intervals. The leaching after a definite time period or when only made up to volume medium may be exchanged after taking samples of the solution for investigation. Another popular method of leaching heavy metals from mortars pastes and concretes is the DEV-S4 method (7). It consists of shaking with water, under some definite conditions, suitably prepared chips of the material to obtain a clear filtrate in which the contents of the selected elements as well as its pH and conductivity are determined. In many laboratories, different modifications of these methods are used, or a completely new technological approach, consisting, for example, of forcing water at high pressure through an appropriately shaped sample is used. These methods, however, require the use of complicated apparatus and create great problems in the course of the investigations; the obtained results are often not reproducible.
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FIGURE 1. Examples of DP ASV voltammetric curves of extracts (pH ) 2) obtained after leaching the OPC mortars (curve 1) and AASC mortars (curves 2-4): 1, AASC mortar with addition of fly ash; 2, OPC mortars doped with 1% Cd; 3 and 4, AASC mortars doped with Cd (curve 3) and Pb (curve 4). Curves 3-3′′ and 4-4′′ present the determination of Cd and Pb by the standard addition method. The standard addition is equal to 20 µg/L Cd (curves 3′ and 3′′) and 100 µg/L Pb (curves 4′ and 4′′). Conditions (accumulation time and potential; tacc, Eacc): 1, tacc ) 200 s, Eacc ) -0.8 V; 1′, tacc ) 60 s, Eacc -1.2 V; 2, tacc ) 30 s, Eacc -1.2 V; 3, tacc ) 30 s, Eacc -0.8 V; 4, tacc ) 20 s, Eacc -0.6 V. The concentration of metal ions in leaching solutions obtained as a result of leaching, depending on the amount and kind of the immobilized metals, equals from 0.1 µg/L to some hundreds µg/L Cd, Pb, Zn, and Cu, and in the case of chromium equals from 20 µg/L to some hundreds mg/L Cr(VI). In order to determine such concentrations, it is necessary to apply sensitive instrumental methods such as atomic absorption spectrometry (AAS), inductively coupled plasma atomic emission spectrometry (ICP AES) or voltammetry. The multi-element ICP AES and the AAS methods require the use of expensive apparatus, not always available in an industrial laboratory; moreover, the sensitivity of the ICP AES (8) method and that of flame AAS method is not always sufficient to determine the concentration of metal ions on the 0.1-10 µg/L level. The flameless AAS (GFAAS) method has the appropriate sensitivity; however, the presence of great amounts of sodium and calcium ions in the leaching solutions may pose some problems with accurate determination of toxic metals traces by this method. The anodic stripping voltammetric method (ASV), used in many countries as the standard method for the determination of the traces of some heavy metals in waters and liquid wastes (9, 10), is free from these limitations and is perfectly suitable for Cd, Pb, Zn, and Cu determination (11, 12) in the presence of an excess of sodium, calcium, magnesium, and aluminum ions. The aim of this work is the evalution of the degree of Cd, Pb, Zn, and Cr immobilization in AASC and OPC mortars and the development of analytical methods and the conditions of their application for the determination of immobilized metals in leaching solutions of various cement matrices. The effect of the immobilized metals on the basic property of the investigated mortars, i.e., the setting time, determined by Vicat’s method, is also examined.
Experimental Section Apparatus. The voltammetric equipment consisted of a Pulse Polarograph PP-04 (Unitra Telpod, Krako´w), a static mercury drop electrode SMDE-2 (Laboratorni Pristroje, Prague) applied in the hanging mercury drop electrode mode, a Pt wire as an auxiliary electrode, and a silver-silver chloride (3 M KCl) reference electrode. The voltammetric curves were recorded
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with XYT recorder Model MF-8051 (Bioanalytical System, West Lafayette). The UV digestor was from Mineral, Warszawa. The digital spectrophotometer was Spekol 11 with 1-cm cuvette, and the spectrophotometer was Speckord (Carl Zeiss Jena) with 5-cm cuvette. According to standard procedure used in the cement industry (13), the standard Vicat needle was applied for measuring the setting time. The term “setting time” refers to the initial and the final setting. The initial setting is the interval between the gauging and partial loss of plasticity, whereas the final setting is the time required for the gauged cement to acquire sufficient firmness to resist a certain definite pressure (13). All solutions were deoxygenated with argon for at least 5 min prior to commencement of the voltammetric determination, and flow of argon was maintained over the solution during the measurement. After each standard addition into the voltammetric cell, the solution was deaerated for 30 s. Reagents. Hydrochloric acid, sulfuric acid, sodium acetate, sodium hydroxide (all of them Suprapur Merck), hydrogen peroxide 30%, diphenylcarbazide, 0.25% acetone solution, silver nitrate 0.1 M solution (all of them were of analytical reagent grade, POCH Gliwice), standard solutions of Cd, Pb, Zn, Cu, Cr(VI), and Cr(III) 1 g/L, Titrisol standards (Merck) were used. Water used to dilute the reagents was purified by reverse osmosis (Milli-Ro) and deionization by means of Millipore GmbH (Austria). After both leaching tests (TANK and DEV-S4), the extracts were filtered and acidified with HCl to pH ) 2. Procedure. (A) Digestion of Dissolved Organic Matter (DOM) in Extracts by UV Irradiation. Before determination of Pb, Cd, Zn, and Cu, the DOM in the extracts were decomposed by UV irradiation in a commercial UV digestor (10, 14). A total of 40 mL of the acidified extract was placed in a quartz test tube, mixed with 40 µL of 30% H2O2, and irradiated by a 125-W mercury lamp for 3 h. (B) Voltammetric Determination of Heavy Metals in Extracts of Different Kinds of Mortars. The ASV method enabled both the determination of a single element and a simultaneous determination of Zn, Cd, Pb, and Cu (Figure 1). Simultaneous determination of Zn, Pb, Cd, and Cu ions in
TABLE 1. Heavy Metals Concentrations in Some Extracts of Ordinary Portland Cement Mortars (OPCM) and Alkali-Activated Slag Cement Mortars (AASCM) Determined by DP ASV and ICP AES Methods heavy metals concentrations in some extractsa (µg/L) Cd
a
Pb
Zn
method
OPCM
AASCM
OPCM
AASCM
OPCM
AASCM
DPASV ICPAES
9.7 (0.8) 7.7
70.1 (2.8) 74
58.4 (2.8) 51
328 (11.5) 291 334b
24.3 (0.9) 24
174 (7.5) 181
Mean value of triplicate determination with standard deviation given in the parentheses. b Value determined by FAAS method.
TABLE 2. Results of Simulteneous Determination of Heavy Metals in Extracts of Pulverized Fuel Ash Concretes (DSV-S4 Test) heavy metals concentration in extractsa (mg/L) Cd
Pb
Zn
Cu
kind of extract
DPASV
GFAAS
DPASV
GFAAS
DPASV
GFAAS
DPASV
GFAAS
pulverized fuel ash cellular concrete Portland cement pulverized fuel ash concrete legal max concn in I class purity inland water (19)
0.29
0.24
8.4
8.8
36.3
32.1
10.2
11.6
0.18
0.18
4.2
3.3
38.1
33.5
9.2
8.1
a
5
50
200
50
Mean of duplicate determination.
the extracts was performed in 0.01 M HCl after accumulation of the metals on the HMDE at the potentials of -1.2 V, whereas a separate determination of Zn was done in acetic buffer (pH ) 4.5) after addition of sodium acetate into digested extracts. The accumulation step was performed in the stirring solutions. For developing the voltammetric curves, the differential pulse (DP) mode with ∆Ep ) 50 mV and scan range v ) 16 mV/s was applied. The accumulation time was from 20 to 200 s depending on the concentration of the element determined in the analyzed solution. The concentration of the analyzed metals was determined by the standard addition method (Figure 1). Cr(VI) was determined by fast linear scan polarography (oscillopolarography) and normal pulse polarography in a supporting electrolyte of 1 M NaOH (15, 16), by differential pulse polarography in a supporting electrolyte of 0.01 M DTPA and 0.5 M KNO3 (17) or of 0.01 M DTPA, 0.5 M KNO3, and 0.04 M CH3COOH (18), and by catalytic adsorptive stripping voltammetry in 0.01 M DTPA, 0.5 M KNO3, and 0.04 M CH3COOH (18) (tacc ) 30 s, Eacc ) -1.0 V). (C) Spectrophotometric Determination of Cr(VI) and total Cr. Cr(VI) in leaching solutions was determined after adding the 1.0 mL of sulfuric acid (1 + 9) and 1 mL of 0.25% diphenylcarbazide to the appropriately diluted extract sample and making up to 50 mL with water. In order to determine total chromium 1.0 mL of sulfuric acid (1 + 9) was added to appropriately diluted extract solutions, and Cr(III) was oxidized to Cr(VI) by adding 0.5 g of ammonium persufate and 1 mL of 0.1 M of silver nitrate. The solution was kept boiling for 20 min to complete oxidation of Cr(III) to Cr(VI) and to decompose the excess of persulfate. After the solution was cooled, it was transferred to a 50 mL volumetric flask, 1 mL of diphenylcarbazide solution was added, and the solution was diluted to 50 mL with water. The Cr(VI) concentration was determined from calibration curve A ) f(c) prepared for a series of standard solutions of Cr(VI) within a concentration range of 0.05-1.6 mg/L.
Results and Discussion The organic substances present in the AASCM extracts (7.7 mg/L DOM detected) chiefly in the form of surfactants as well as small amounts of sulfides found in the extracts did not permit a direct determination of metals by the ASV or by
the polarographic method. After photolytic decomposition of DOM, well-shaped voltammograms are obtained using the DPASV method, which enables the determination of Cd, Pb, Zn, and Cu in the whole range of concentrations occurring in the extracts. The DPASV method with the electrolytic preconcentration of the elements on the HMDE enables the trace metals analysis with low detection limit. For example, to determine the contents of Cd and Pb in the blank (Milli Q/Milli Ro water acidified with HCl to pH ) 2), the accumulation time of 500 s was necessary to obtain measurable peaks of both elements on voltammograms. Quantification of Cd and Pb concentrations in the blank gave the values of 0.012 µg/L Cd and 0.08 µg/L Pb. However, in the applied conventional laboratory conditions, the Zn content in blank was significantly higher and amounted to 0.99 µg/L. In the analysis of Cd, Pb, and Zn in the leaching solutions, the shorter accumulation times of 500 s were applied. The 3σ detection limit was calculated from repeated DPASV determinations (n ) 20) of low level of 0.1 µg/L Cd added to the blank as well as of low concentration of Pb and Zn present in the blank. The limit of detection was 0.01µg/L Cd (RSD 3.9%, 0.01 µg/L Pb (RSD 3.4%) using the accumulation time of 200 s and it was 0.08 µg/L Zn (RSD 3.0%) using the accumulation time of 100 s. Tables 1 and 2 list selected results of the analysis of metals in the extracts together with the standard deviations calculated for DPASV method (Table 1) and with a comparison of the results with those obtained by the ICP AES method or the GAAS method (Table 2). The comparison shows that in the case of the determination of the lowest Cd, Pb, and Cu concentrations (0.1-10 µg/L), a good agreement of the results between the DPASV and the GFAAS methods is observed. For higher contents of the examined elements, above 10 µg/L, a good agreement between the results obtained by the DPASV and by the ICP AES methods was also observed. The alternative methods of ICP AES or FAAS have a detection limit for Cd, Pb, and Zn considerably higher than DPASV, and both methods are not always sensitive enough for an analysis of the leaching solutions. Voltammetric procedure does not require the use of expensive equipment as is the case with spectrometric methods. In case of the immobilization of such heavy metals as Ni and Co in the mortars, another modification of the stripping voltammetry, i.e., adsorptive stripping voltammetry (20-22),
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TABLE 3. Results of Chromium Determination in Extracts of Various Mortarsa (mg/L) analytical method spectrophotometry with DPC type of extract
total Cr
AASC mortars doped with Cr(VI)
synthetic Portland cement mortars doped with Cr(VI) synthetic Portland cement mortars doped with Cr(III) OPC mortars doped with Cr(VI)
Cr(VI)
1.75
ICP AES total Cr 1.86
51.3 0.36 25.3 100.6 122.7
53.5 0.42 25.8 102.2 129.5 (123.9)b
109.6 115.0 (106.2)b 120.1 125.5 (123.3)b 0.122
