Environ. Sci. Technol. 2007, 41, 5466-5470
Concentration of Soluble and Nonsoluble Zinc-Based Impurities by Unidirectional Freezing: Basis for a Method of Sludges Treatment GUILLAUME GAY* AND AZA AZOUNI Laboratoire des Mate´riaux et des Structures du Ge´nie Civil (U.M.R. 113), Cite´ Descartes, 2 alle´e Kepler, 77420 Champs-sur-Marne, France
papers (9, 13-14) and will not be explained again in this paper. Moreover, solidification presents another interest in terms of cost: water freezing is less energy-consuming than water evaporation, with a factor of almost 7. In this paper, we apply these mechanisms of forced migration by freezing to models of polluted sludges, which consist of dilute clayey suspensions contaminated by a heavy metal (zinc). We expect to reject both soluble impurities (dissolved zinc, e.g., Zn2+, and complexed zinc, e.g., Zn(OH)+) and nonsoluble impurities (precipitated zinc, e.g., Zn(OH)2 or ZnCO3, and adsorbed zinc on clayey particles surface) from the frozen region of the sludges in order to purify this region and to concentrate impurities in the non-frozen zone.
2. Materials and Methods Heavy metals are known to be significant pollutants in sludges produced by wastewater treatment. A new technique to make the metallic contaminants migrate is to submit such sludges to a slow freezing in order to purify the frozen region and to concentrate impurities in the non-frozen zone. In this paper, we apply this method to model contaminated sludges consisting of clayey suspensions charged with a heavy metal (zinc). The effect of some physicochemical parameters (zinc and clay concentrations, pH) on the effectiveness of the method is experimentally investigated. The pH is the most relevant parameter and the highest purification rate (∼80%) is obtained for pH ∼9.
1. Introduction Sludges as residues of wastewater treatment are produced in increasing quantities. They are characterized by high water content, and may be strongly contaminated by heavy metals like zinc (1-2). Metallic pollutants may occur as dissolved forms (ionic, complexed, ...) and/or as non-soluble forms (precipitated, adsorbed on colloidal surfaces, ...) (3-5). All current techniques for treatment of such sludges present some disadvantages (effectiveness restricted to some species and/or to small concentrations, long-time and energyconsuming processes, high costs). It is therefore of great interest to investigate a new technique which is able to make the maximum of heavy metals migrate, whatever their species, under satisfying environmental and economic conditions. Freezing has been applied in several industrial domains where separation of components from their original medium was desired. For instance, solutes separation was carried out in metallurgy to purify metallic alloys (6-9) and in analytical chemistry to concentrate solutes from dilute solutions to make their determination easier (10-11). Particles separation has also been achieved, for example, in food-processing to dewater fruit juices or milk and in medicine to preserve biological tissues (12). These industrial applications are based on fundamental mechanisms of solutes and particles rejection during a liquid-solid phase transition (13). Indeed, during the solidification of a liquid suspension containing solutes and second-phase particles, the advancing solid-liquid interface can at a low rate reject the solutes and repulse the particles, so that under certain experimental conditions the solid phase is purified. Theoretical phenomena are described in previous * Corresponding author e-mail address: guillaume.gay@ ineris.fr; present address: INERIS, Parc Technologique Alata, 60550 Verneuil-en-Halatte, France; fax: + 33-3-44556556. 5466
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2.1 Experimental Setup. The experimental setup consisted of the cell, the cryogenic system, the temperature regulator, and the cell-filling device (Figure 1). The experimental cell was made of a Plexiglas parallelepiped limited at the top and at the bottom by two hollow brass plates. Plexiglas was chosen because it is a transparent medium, a good heat insulator, and its machining is easy. Brass was chosen for its high thermal conductivity. The cell was 80 mm long, 60 mm high, and 20 mm wide (internal dimensions). The Plexiglas walls were 10 mm thick. The two brass plates were hollow so that coolants could easily flow through them. The temperatures at the bottom and at the top of the cell depended on the imposed cooling temperatures of coolants. The actual temperatures at the bottom Tb and the top Tt of the cell were measured with thermocouples T (copper-nickel-copper), with accuracy better than 0.05 °C. To avoid bubbles during the filling of the cell, we used a three-way valve that connected the cell, a tank containing the suspension, and a vacuum pump via a vacuum flask. 2.2 Characteristics of Materials. Among all the existing forms of zinc salts, zinc nitrate was chosen: it dissolves completely, and NO3- ions react very little. Zinc nitrate Zn(NO3)2, 6 H2O contained some traces of metallic impurities ( 1. The experimental runs were carried out with three ranges of values of the parameter σ: 10, 1.0, or 0.1. Values of σ are reported in Table 2. 2.3 Experimental Procedure. Various experimental conditions were tested to choose the optimal setup: thermal parameters influenced the freezing speed, which influenced the efficiency of zinc purification. These experiments are
described in previous papers (13, 14) and the optimal conditions are presented in the present paper. When the experimental cell was filled with the polluted sample, thick walls of expanded polystyrene thermally insulated it. Before applying a negative temperature at the bottom of the cell, the sample was kept at a uniform temperature near 0 °C for 12 h in order to reach equilibrium. Freezing started when the temperature at the bottom of the cell was equal to -2.00 ( 0.05 °C, and the temperature at the top of the cell was equal to 0.10 ( 0.05 °C. The average velocity of the upward freezing front was around 1 µm s-1 (13). Such a setup did not allow us to follow the temporal dynamic evolution of the concentrations in the liquid and the solid phases. As it was necessary to analyze both the solid and the liquid phases, only the third of the sample (almost 32 × 103 mm3) was frozen. One run lasted about 6 h. Then, the solidified and non-solidified phases were carefully separated, and zinc and Na-montmorillonite concentrations were measured in each phase. In all cases, one-third of the sample was frozen, so that all the following results are expressed for the same solidified volume. Zinc was extracted from the Na-montmorillonite particles by acid attack, and its concentration was measured by atomic absorption spectrophotometry. By this method, only the total concentration of zinc was evaluated, without identifying the amount of each speciation. The zinc concentration was determined with a precision of about 1%. Na-montmorillonite concentration was evaluated with an uncertainty between 1% and 2% by titration with methylene blue, which is a precise method when only a small quantity of clay is concerned (14-16). All results for zinc are expressed in terms of a purification rate, or efficiency, . The efficiency of zinc separation is the initial difference between the zinc concentration C zinc in the final S initial solution and the zinc concentration C zinc in the final solid-phase divided by the initial zinc concentration:
)
initial final C zinc - SC zinc initial C zinc
Since small particles of Na-montmorillonite (with or without adsorbed zinc) may be pushed ahead of the freezing front, we also introduce a dimensionless ratio to evaluate the repulsion rate τ of Na-montmorillonite particles:
τ)
S final C initial clay - C clay
C initial clay
These definitions facilitate comparison from one run to another. When ) 0, zinc is completely entrapped in the solid phase. For > 0, at least a part of zinc is removed from the solidified phase. Total purification is attained for ) 1. The same considerations are also valid for τ and Namontmorillonite particles.
3. Results and Discussion 3.1 Effect of the pH on Efficiency. The aim of the first set of runs was to identify the behavior of zinc species in the absence of Na-montmorillonite when a saline solution was submitted to unidirectional freezing. Table 1 gives efficiencies of zinc separation for various values of initial pH and zinc concentrations. These results are also represented in Figure 3. At pH ) 6 and for initial zinc concentrations of the order of 10-2 mol kg-1, zinc was totally dissolved and partially rejected (21.8% e e30.5% ). At pH ) 8.5, more than 99.5% of zinc was precipitated as hydroxides and partially repulsed (37.9% e e39.0% ). Thus, the freezing front was able to reject dissolved zinc as well as zinc hydroxide Zn(OH)2, which VOL. 41, NO. 15, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Predominant species of zinc in function of pH, in the absence of clay particles (with total zinc concentration equal to 0.01 mol L-1, EH ) 0.5 V, and t ) 25 °C).
TABLE 2. Experiments on Na-montmorillonite Suspensions Charged with Zinc: Initial Conditions and Results initial concentration initial pH
σ
zinc (mol kg-1)
3.0 3.6 5.0 6.4 7.0 9.0 9.4 10.0 7.8 8.1 8.7 10.3
10.1 7.3 8.5 7.8 7.0 7.4 7.7 9.7 1.5 0.8 1.1 0.1
0.95 × 10-2 0.78 × 10-2 0.85 × 10-2 0.86 × 10-2 0.82 × 10-2 0.87 × 10-2 0.74 × 10-2 0.99 × 10-2 1.02 × 10-2 0.10 × 10-2 0.11 × 10-2 0.11 × 10-2
final concentration in the solid phase
clay (g kg-1)
zinc (mol kg-1)
clay (g kg-1)
E
τ
1.63 1.85 1.75 1.91 2.06 2.05 1.67 1.77 10.3 1.78 1.81 12.8
0.97 × 10-2 0.73 × 10-2 0.72 × 10-2 0.65 × 10-2 0.45 × 10-2 0.14 × 10-2 0.18 × 10-2 0.78 × 10-2 0.90 × 10-2 0.07 × 10-2 0.04 × 10-2 0.11 × 10-2
1.88 2.17 2.49 1.05 1.00 0.56 0.63 1.17 4.77 0.15 0.12 12.1
-2.4% 7.0% 15.6% 23.9% 45.0% 83.5% 75.0% 21.4% 11.8% 29.4% 67.8% 2.6%
-15.1% -17.1% -41.9% 44.6% 51.4% 72.8% 62.0% 33.8% 53.7% 91.7% 93.3% 5.8%
occurred in the form of small non-miscible particles. The efficiency depends strongly on the zinc speciation, which depends also on the pH. In the presence of Na-montmorillonite, the distribution of zinc species versus pH becomes complex, and will also certainly influence . Table 2 summarizes the initial and final conditions of the experiments relative to Na-montmorillonite suspensions charged with zinc. In Figure 3, and τ are displayed versus pH for 7 e σ e10. The efficiency of zinc separation is almost zero at pH ∼3, then it increases to reach a maximum value at pH ∼9. The repulsion rate of Na-montmorillonite τ is negative at pH < 5.5 and even goes through a minimum at 4 e pH e 5. At 6 e pH e 10, τ presents the same profile as . Except at pH ) 8.5, efficiencies of zinc separation are similar with or without Na-montmorillonite (see Figure 3). This could be explained by the minor influence of Namontmorillonite for some ranges of pH values, e.g., for 5 e pH e 6. As assumed below, Na-montmorillonite would significantly influence zinc speciation for very acid pH (pH ∼ 3) and for basic pH (pH > 7) as well. For the range of moderate acid pH, zinc species are supposed to be almost the same (mainly dissolved zinc) with and without Namontmorillonite, and then efficiency of zinc separation is of the same order of magnitude (between 22% and 30%). To explain the variations of versus pH in the presence of Na-montmorillonite, in addition to the pH effect on the 5468
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distribution of zinc species, we have to consider the influence of the pH on the colloidal behavior of Na-montmorillonite suspensions, and of the ion exchange phenomena between zinc and the binding sites of Na-montmorillonite surface. Regarding the colloidal behavior, the potential energy ΨDC for a double layer interaction between two clay particles quantifies the repulsive force between the particles, and then it appears as an indicator of the stability of a clayey suspension. The potential energy ΨDC for a double layer interaction is proportional to the square of the ζ-potential (17), and consequently depends on the pH of the suspension like the ζ-potential. Figure 4 illustrates the expected influence of the pH on the ζ-potential and the potential energy ΨDC for a double layer interaction. At pH ) 3.0, and τ are negative. This results from the sedimentation of some zinc species and some Na-montmorillonite particles. Therefore, the final concentration in the solid phase is higher than the initial one. At pH < pHIEP, the ζ-potential of Na-montmorillonite particles is slightly positive (17), and consequently zinc cations Zn2+ should not be adsorbed by the particles surface (4-5, 18). In this assumption, should be nearly the same as the efficiencies obtained for similar zinc concentrations in the absence of Na-montmorillonite, i.e., 22% e e30% (Table 1). Therefore, zinc cations are partly adsorbed since ) -2.4% at pH ) 3.0. This adsorption is likely due to the excess of zinc compared to the amount of binding sites as σ ) 10.1. Thus, the future
FIGURE 3. Efficiencies E of zinc separation in the absence (white squares 0) and in the presence (black squares 9) of Namontmorillonite, and repulsion rates τ for clay particles (black circles B) versus pH for 7 eσ e10. The lines are only used to guide the eye.
FIGURE 4. Expected variations of the ζ-potential (solid line) and the potential energy ΨDC for double layer interaction (dotted line) versus pH. of the adsorbed zinc cations is linked to the future of the Na-montmorillonite particles. According to the DLVO (Derjaguin, Landau, Verwey, and Overbeek) theory, the clay particles with a weak absolute value of the ζ-potential form some aggregates (19), which deposit or are entrapped during the propagation of the freezing front. The negative value of τ ) -15.1% at pH ) 3.0 confirms the weak dissociation of Na-montmorillonite particles. In the range 4.3 e pH e 6.5, Figure 3 shows that increases continuously when τ increases sharply with pH. Efficiency of zinc separation is quite similar with and without Namontmorillonite. Indeed, in this range of pH, the major part of zinc can be dissolved, and then the presence of Namontmorillonite has only minor influence. To explain the behavior of the repulsion rate τ of Na-montmorillonite particles, we have to consider the decrease of H+ concentration which weakens the ionic strength of the suspension. Consequently, after the DLVO theory, Na-montmorillonite particles dissociate gradually (19) and are more easily repulsed by the freezing front. At 7.0 e pH e 10.0, and τ present a peak with an optimum at pH ) 9.0. The peak of is likely correlated with the predominance of one completely rejected species of zinc in this range of pH. As the behavior of zinc and Na-montmorillonite seems to be strongly dependent on each other, we assume that this predominant species results from bonds between zinc and Na-montmorillonite particles. Although this hypothesis should be validated by further analyses, it is
FIGURE 5. Influence of σ on the efficiency E for zinc (square symbols) and on the repulsion rate τ for clay particles (circle symbols) at 7.0 e pH e 10.3 (black symbols for 7 eσ e10, white symbols for 0.8 eσ e1.5, and gray symbols for σ ) 0.1). The lines are only used to guide the eye. already in good agreement with other experiments: indeed, when pH increases, the amount of the dissolved zinc decreases in favor of the zinc adsorbed on the Na-montmorillonite particles surface (4-5, 18). 3.2 Effect of Concentrations on Efficiency. In the absence of Na-montmorillonite, the rejection of zinc was more pronounced in less concentrated solutions: 21.8% e e30.5% for an initial zinc concentration of 0.85 × 10-2 mol kg-1, and ) 72.3% for an initial zinc concentration of 0.09 × 10-2 mol kg-1, at 5.5 e pH e 6.0 for which zinc was totally dissolved. Theoretically, should not be influenced by the concentration of dissolved zinc, since the ratio of the concentrations in the solid and the liquid phases at the interface is not affected by such small concentrations (9). Nevertheless, it is reasonable to attribute this concentration effect to supercooling (20). Indeed, during solidification, the presence of a solute favors dendritic growth at the solid-liquid interface. At the microscopic scale, dendrites form at this interface and lead to the entrapment of small volumes of liquid with a considerable zinc concentration (21). Thus, the higher the initial zinc concentration is, the more dissolved zinc is captured between the dendrites, and the more the zinc segregation and the efficiency decrease. Concerning the experiments performed on Na-montmorillonite suspensions charged with zinc, Figure 5 shows the variations of the efficiency for zinc and the repulsion rate τ of Na-montmorillonite versus pH, for three different orders of magnitude of σ. For 0.8 eσ e1.5, and τ increase with pH as observed for 7 eσ e10 (Figure 5), but lower than for 7 eσ e10, and τ higher than for7 eσ e10. In this range of pH, we assumed in Section 3.1 the existence of a surface precipitate for7 eσ e10. For 0.8 eσ e1.5, there are statistically fewer bonds between zinc precipitate and Na-montmorillonite particles than for 7 eσ e10 at equivalent pH, and therefore zinc occurs proportionally as a surface precipitate less for 0.8 eσ e1.5 than for 7 eσ e10. Therefore, must be lower for 0.8 eσ e1.5 than for 7 eσ e10, since the surface precipitate is likely the best-rejected speciation. This is in good agreement with Figure 5. Moreover, the ionic strength is lower for 0.8 eσ e1.5 than for 7 eσ e10, and consequently the Na-montmorillonite particles are more dissociated and form smaller aggregates, which explains the increase of τ for 0.8 eσ e1.5 in comparison with the case 7 eσ e10. One run was carried out with σ ) 0.1 at pH ) 10.3: ) 2.6% and τ ) 5.8% (Table 2). These low values of and τ are likely due to the high concentration of Na-montmorillonite which leads to the formation of aggregates that can hardly be repulsed by the freezing front. For σ ) 0.1, the amount VOL. 41, NO. 15, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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of zinc is so small that it does not influence the behavior of the Na-montmorillonite particles. In fact, for non-charged Na-montmorillonite suspensions with similar pH and concentrations, we obtained τ ) 3.5% (14, 22), which is of the same order of magnitude as τ ) 5.8%. 3.3 Feasibility of Purification by Freezing. In this paper, we make it evident that treatment by freezing of a model of sludge contaminated by heavy metals is efficient. The experimental investigation points out the crucial influence of the pH on the effectiveness of this method. On a large domain of pH, the purification rate for zinc increases with pH to reach an optimum of more than 80% at basic pH ∼ 9. Our purpose now is to extend this experimental work to a porous pack saturated by the contaminated clayey suspensions studied here. This can be considered as a model of soils polluted by heavy metals; preliminary results are promising (23). A first step would be to determine precisely the predominant species of the heavy metals in the presence of clayey particles, in order to evaluate the species that are repulsed the most by any unidirectional freezing. Another perspective would be to conceive an industrial prototype at a larger scale to confirm the feasibility of such a wastewater treatment method and to evaluate the time and energy required. Preliminary data are available: an experimental device using radial freezing with stirring effects allowed efficient purification of a 0.25 L volume of a model sludge in 40 min (24). We hope that an industrial prototype at a larger scale will improve this duration and at the same time the energy efficiency, for example by using countercurrent heat exchangers.
Acknowledgments We are very grateful to C. Grambin-Lapeyre (Centre de Ge´ologie de l’Inge´nieur, Marne-la-Valle´e, France) for providing access to her atomic absorption spectrophotometer.
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Acta Metall. 1953, 1, 428-437. (7) Pfann, W. G. Zone Melting; John Wiley & Sons: New York, 1958. (8) Tiller, W. A. In The Art and Science of Growing Crystals; Gilman, J. J., Ed.; John Wiley & Sons: New York, 1963; ch. 15, pp 276312. (9) Chalmers, B. Principles of Solidification; John Wiley & Sons: New York, 1964. (10) Shapiro, J. Freezing out: a safe technique for concentration of dilute solutions. Science 1961, 133, 2063-2064. (11) Baker, R. A. Trace organic contaminant concentration by freezing: -I- low inorganic aqueous solutions. Water Res. 1967, 1, 61-77. (12) Otero, L.; Martino, M.; Zaritzky, N.; Carrasco, J. A. High pressure assisted freezing in biological products. In Proceedings of the International Conference on Permafrost and Actions of Natural or Artificial Cooling; International Institute of Refrigeration: Paris, 1998; pp 249-252. (13) Gay, G.; Azouni, M. A. Forced migration of nonsoluble and soluble metallic pollutants ahead of a liquid-solid interface during unidirectional freezing of dilute clayey suspensions. Cryst. Growth Des. 2002, 2 (2), 135-140. (14) Gay, G. Application du froid artificiel au traitement des boues et des sols pollue´s par des me´taux lourds; the´orie et expe´riences a` petite e´chelle sur des milieux mode`les. Ph.D. Thesis, Ecole Nationale des Ponts et Chausse´es, Marne-la-Valle´e, France, 2001. (15) Hang, P. T.; Brindley, G. W. Methylene blue absorption by clay minerals; determination of surface areas and cations exchange capacities. Clays Clay Miner. 1970, 18, 203-212. (16) Kahr, G.; Madsen, F. T. Determination of the cation exchange capacity and the surface area of bentonite, illite and kaolinite by methylene blue. Appl. Clay Sci. 1995, 9, 327-336. (17) Hunter, R. J. Zeta Potential in Colloid Science; Principles and Applications; Academic Press: London, 1981. (18) Sposito, G. The Chemistry of Soils; Oxford University Press: New York, 1989. (19) Overbeek, J. T. G. Volume 1 Irreversible Systems. In Colloid Science; Kruyt, H. R., Ed.; Elsevier: Amsterdam, 1952. (20) Rutter, J. W.; Chalmers, B. A prismatic substructure formed during solidification of metals. Can. J. Phys. 1953, 31, 15-39. (21) Shirai, Y.; Wakisaka, M.; Miyawaki, O.; Sakashita, S. Effect of seed ice on formation of tube ice with high purity for a freeze wastewater treatment system with a bubble-flow circulator. Water Res. 1999, 33 (5), 1325-1329. (22) Gay, G.; Azouni, M. A. Remediation by artificial cooling of dilute clay suspensions contaminated by heavy metals. Polar Rec. 2001, 37 (202), 257-263. (23) Gay, G.; Azouni, M. A. Experimental study of the redistribution of heavy metals contaminants in coarse-grained soils by unidirectional freezing. Cold Regions Sci. Technol. 2003, 37, 151157. (24) Gay, G.; Lorain, O.; Azouni, M. A.; Aurelle, Y. Wastewater treatment by radial freezing with stirring effects. Water Res. 2003, 37, 2520-2524.
Received for review October 23, 2006. Revised manuscript received May 22, 2007. Accepted May 29, 2007. ES0625374
