Langmuir 1996, 12, 3195-3200
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Chemical and Structural Transformation of Aggregated Al13 Polycations, Promoted by Salicylate Ligand E. Molis,† F. Thomas,*,† J. Y. Bottero,‡ O. Barre`s,† and A. Masion‡ Laboratoire Environnement et Mine´ ralurgie, URA 235, CNRS-INPL, BP 40, F-54501 Vandoeuvre Cedex, France, and Laboratoire des Ge´ osciences de l’Environnement, URA 132, CNRS, Universite´ Aix-Marseille III, BP 80, F-13762 Les Milles Cedex, France Received November 6, 1995. In Final Form: March 8, 1996X The chemical and structural consequences of the interaction between aggregates made of Al13 polycations and salicylate ligand were examined using adsorption isotherms, electrophoresis, infrared spectroscopy, Fraunhofer light diffraction, and static light scattering spectroscopy. Drastic changes were observed for a pseudo equilibrium ligand concentration equimolar to Al13. Below this concentration, salicylate forms monodentate complexes with the charged aluminum sites, decreasing the electrostatic repulsion between Al13 particles. A densification of the aggregates is evident by an increase of the apparent fractal dimension from 1.8 to 2.9. No dissolution was observed in this concentration range. Above this ligand concentration, excess adsorption of salicylate causes charge reversal, and dissolution of the aggregates, in the form of soluble complexes and small clusters was observed. The consequences were a size reduction and loosening of the aggregates. In aquatic media, such radical in situ changes are able to strongly influence the transport of colloids and associated species.
Introduction Aggregated Al13 is the component of some aluminumbased coagulants used in the water treatment process for primary clarification. It is produced by a two-step hydrolysis of aluminum chloride. The first step is a slow hydrolysis to an OH/Al ratio ranging from 2.2, which yields almost pure Al13 polycations solutions, to 2.8, which yields aggregates of fractal dimension close to 1.8. The second step is a fast hydrolysis and dilution of this suspension in the aquatic medium at natural pH (7 to 9), which results in the formation of millimetric flocs. The molecular and colloidal chemistry of the aluminum species formed in these conditions has been intensively studied and is well established.1,2 These systems are used in the present study as a model to study the reactivity of naturally occurring amorphous aluminum colloids, assuming similar formation pathways. Aluminum is released from soil minerals to freshwaters as a result of acidification due to natural or anthropic acidic inputs. Its fate as phytotoxic mono- or oligomers or as colloidal precipitates is governed by the physicochemical conditions encountered locally, i.e., pH, mineral ions, and organic matter. The aluminum speciation in solution and within the precipitates, as a result of aluminum hydrolysis in the presence of complexing organic ligands, has been established previously. The main assessments were that organic ligands hinder the formation of Al13, and depolymerize Al13 according to their affinity for the aluminum mono- and oligomers, so that the precipitates contain almost exclusively oligomeric, complexed aluminum.3-5 Studies concerning long-term evolution (up to 15 years) of precipitation products of aluminum in the presence of organic ligands have shown †
Laboratoire Environnement et Mine´ralurgie. Laboratoire des Ge´osciences de l’Environnement. X Abstract published in Advance ACS Abstracts, June 1, 1996. ‡
(1) Bottero, J. Y.; Marchal, J. P; Poirier, J. E.; Cases, J. M. Bull. Soc. Chim. Fr. 1982, No. 11-12. (2) Bottero, J. Y.; Axelos, M. D.; Tchoubar, D.; Cases, J. M.; Fripiat, J. J.; Fiessinger, F. J. Colloid Interface Sci. 1987, 117, 47. (3) Thomas, F.; Masion, A.; Bottero, J. Y.; Gene´vier, F. Environ. Sci. Technol. 1991, 25, 1553. (4) Thomas, F.; Masion, A.; Bottero, J. Y.; Rouiller, J.; Montigny, F.; Gene´vrier, F. Environ. Sci. Technol. 1993, 27, 2511. (5) Masion, A.; Thomas, F.; Bottero, J. Y.; Tekely, P.; Tchoubar, D. J. Non-Cryst. Solids 1994, 171, 191.
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that the crystallization of the Al(OH)3 polymorphs is dramatically hindered and retarded and that the final products are amorphous, according to the complexing power of the ligands.6-9 In the present work, we examine the short-term (3 days) structural transformations of aggregated Al13 polycations interacting in suspension with organic ligands at near neutral pH. This system was intended to simulate the fate of the highly reactive unstable amorphous aluminum precipitates. Salicylate was chosen as a simplified model of naturally occurring organic acids and as a ligand able to bind metal ions fairly strongly, especially hard ions such as Al3+, to form stable complexes.10 Experiments were conducted with aggregated Al13 floc suspensions to which salicylate was added at various molar concentrations. Salicylate uptake by the flocs and the resulting aluminum dissolution were followed over 3 days. More detailed analyses were conducted moderately on aged samples, electrophoretic mobility, light diffraction, and static light scattering measurements, in order to follow the size evolution and structural transformations of the aggregates. Experimental Section Materials. All chemicals used in this study were of analytical grade. All solutions and suspensions were prepared using deionized, 0.22 µm filtered water. Al13 solutions at OH/Al molar ratio 2.2 were first prepared by slowly adding 100 mL of freshly prepared 0.44 M NaOH to 100 mL of 0.2 M AlCl3‚6 H2O. The final pH of this solution is 4.5. During preparation, the solution was vigourosly stirred and the rate of NaOH addition was slow (2 mL/min), in order to avoid local oversaturation of hydroxyl ions. The resulting solution contains 90-95% Al13 polycation.11 The stock solutions were prepared (6) Vialante, A.; Huang, P. M. Soil. Sci. Soc. Am. J. 1984, 48, 1193. (7) Violante, A.; Huang, P. M. Clays Clay Miner. 1985, 33, 181. (8) Violante, A.; Huang, P. M. Clays Clay Miner. 1992, 40, 462. (9) Violante, A.; Gianfreda, L.; Violante, P. Clays Cay Miner. 1993, 41, 353. (10) Rakotonarivo, E.; Tondre, C.; Bottero, J. Y.; Mallevialle, J. Water Res. 1989, 23, 137. (11) Bottero, J. Y.; Cases, J. M.; Fiessinger, F. J. Phys.Chem. 1980, 84, 2933.
© 1996 American Chemical Society
3196 Langmuir, Vol. 12, No. 13, 1996
weekly. Precipitated aggregates of Al13 were produced by fast neutralization of the initial Al13 solution. NaOH (1 M) was added to Al13 solutions at 10-2 M total Al, in order to reach a neutral pH (7.0-7.5). The pH remained then stable at 7.0 ( 0.2. The resulting precipitates contain aggregates of Al13 with a fractal dimension of 1.86, as determined by small angle x-ray scattering.2 Precipitated Al13 suspensions were mixed with sodium salicylate solutions prepared at pH 7.5, in order to obtain a final aluminum concentration of 7.5 × 10-3 M and salicylate concentrations ranging from 10-5 to 10-1 M. Contact times varying from 5 min to 3 days were studied in a preliminary kinetic study. Analytical Methods. Chemical Interactions. In order to quantify the salicylate complexed to the aggregates after a contact time ranging from 5 min to 3 days, the residual salicylate concentration of the supernatant after a 20 min, 30000g centrifugation was analyzed by UV absorbance at 295 nm using a UV-visible spectrometer (Shimadzu). The adsorbed quantity Qa, expressed in moles of salicylate/mole of Al13, is
Qa )
Figure 1. Adsorption of salicylate at 3.75 × 10-3 M on aggregated Al13 as a function of time.
(Ci - Ce)V n
where n is the number of moles of aggregated Al13, Ci and Ce are the initial and residual pseudoequilibrium concentrations of salicylate, respectively, and V is the bulk volume. The nature of the Al/ligand bond was investigated by diffuse reflectance FTIR spectroscopy using a Bruker IFS 88 spectrometer, on air-dried centrifugates ground with KBr. The evolution of the surface charge of the aggregates was followed via the ζ potential measured by electrophoresis using a Pen Kem 501 Laser Zee Meter. Released Aluminum. The concentration of aluminum released in solution was quantified on the supernatants by using a Perkin-Elmer 1100 atomic absorption spectrometer. Prior to analysis, the samples were acidified with 1 M HCl. In order to identify the colloidal or soluble nature of the released aluminum, aggregated aluminum-salicylate mixtures were dialyzed during 3 days against a NaCl solution at equal ionic strength (0.3 M). Two different porous dialysis membranes (Spectrapor) were used: 500 and 3500 Da. Stucture of the Aggregates. The size distribution of the aggregates was measured by Fraunhofer light diffraction using a Malvern 2600C granulometer in the 1-600 µm size range. Quasi-elastic light scattering (QELS) was used to characterize the internal structure of the aggregates, since it gives access to the position correlations between particles, and consequently to the apparent fractal dimension of the aggregates. Typically, a laser beam (632 nm) of intensity I0 is directed onto a sample and the scattered intensity is measured as a function of an angle θ to the incident direction. The incident and scattered beam are characterized by the momentum transfer Q
Q)
Molis et al.
4πη θ sin λ 2
()
where λ is the wavelength in the vacuum and η is the refractive index of the medium. The measurements were performed using the Malvern PCS 100 system, at angles ranging from 20 to 120° (2° steps) and for 5 s at each angle. The apparent fractal dimension Dfap was derived from the plot (log I vs log Q)
Figure 2. Percentage of aluminum released in solution as a function of time and ligand concentration [L]: total aluminum, 7.5 × 10-3 M; 1, [L] ) 0 M; 2, [L] ) 3.75 × 10-3 M; 3, [L] ) 3.75 × 10-2 M; 4, [L] ) 3.75 × 10-1 M.
since
I ) A Q-Df where A is a constant.23 Results and Discussion Adsorption and Dissolution Kinetics. Kinetic measurements with 3.75 × 10-3 M salicylate added to 7.5 × 10-3 M precipitated aluminum showed that retention of salicylate on the aggregates at pH ) 7.0 was completed after 6 h (Figure 1). When higher salicylate concentrations were added, aluminum was released in solution (Figure 2). The rate of aluminum release was very high during the first hour and then decreased, until the stability was reached after 1 day. Similarly, dissolution rates decreasing with time have been described by Lin and Benjamin12 on ferrihydrite dissolved by tripolyphosphates. To check if nonsoluble aluminum may also be released, the released species were categorized according to their size by centrifugation and dialysis. Centrifugation was used to separate the smallest aluminum clusters from the aggregates; the largest size of particles in the supernatant is given by the sedimentation equation:
Dp2 )
( )
Rmax 18µ ln Rmin ω t ∆ρ 2
with Dp2 the size of the particles (cm2), µ the viscosity (cP), ∆F the differential density of the particles (≈0.5), ω the angular speed (rad‚s-1), t the centrifugation time (s), and Rmax /Rmin ≈ 2 (maximum and minimum radii of the rotor). For 30000g centrifugation this size is close to 10 nm and could correspond to small Al13 clusters that fall within (12) Lin, C. F.; Benjamin, M. M. Environ. Sci. Technol. 1990, 24, 126.
Transformation of Aggregated Al13 Polycations
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Table 1. Speciation of Aluminum Released from the Aggregates in the Presence of Various Amounts of Salicylate, and Passing a 500 Da and a 3500 Da Dialysis Membrane released Al (% of total Al) salicylate,
mol‚L-1
0 3.75 × 10-3 3.75 × 10-2 3.75 × 10-1
total
3 × 10-3 M seems totally different from that at ligand concentration
