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Ind. Eng. Chem. Res. 2004, 43, 1472-1477
Comparative Study of the Solubility of the Crystalline Layered Silicates r-Na2Si2O5 and δ-Na2Si2O5 and the Amorphous Silicate Na2Si2O5 Antonio de Lucas, Lourdes Rodrı´guez, Paula Sa´ nchez, Manuel Carmona, Pedro Romero, and Justo Lobato* Facultad de Ciencias Quı´micas, Departamento de Ingenierı´a Quı´mica, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain
The solubility of a crystalline layered silicate, δ-Na2Si2O5, has been evaluated and compared with that of a crystalline alpha phase (R-Na2Si2O5) and an amorphous silicate of the same composition. Experiments were carried out at 293, 313, and 333 K. It was found that on raising the temperature from 293 to 333 K, a large increase in solubility occurred for all silicates studied, whereas increasing the time from 30 to 180 min had a less marked effect. Interactions between the silicates and deionized and tap water are discussed. The solubility in tap water was determined taking into account the alkaline earth metal (Ca2+ and Mg2+) binding capacity of the solid phases. A model aimed at understanding the reaction of δ-Na2Si2O5 in deionized water has been proposed on the basis of species identified in solution (Na+, Si4+). A good reproduction was achieved between the amount of NaOH predicted by the model and the pH measured in situ at each temperature studied. 1. Introduction Detergents of the future will depend on the evolution of household appliances, substrates, and consumer needs. In addition, the environmental constraints, which may well become more stringent, will also play on important role - particularly in terms of formulations. Raw materials will have to be more environmentally friendly, with criteria concerning biodegradability and renewability becoming more prominent. One of the main ingredients in detergents is builders. In a detergent the builder has to fulfill several functions. The most important function is to remove the calcium and magnesium ions from the tap water. Other functions of builders include the ability to supply and buffer the alkalinity of the wash liquor. In the field of detergent formulations, the established builder is sodium tripolyphosphate (STPP). However, the addition of STPP to detergent formulations is no longer widespread because of the contribution of phosphates to the eutrophication of waterways.1-4 There are a number of alternatives to phosphates and one of these is zeolites. Unfortunately, however, this material remains as a solid in the wash liquor and wastewater until it is removed with sludge in a wastewater treatment plant. Zeolites, particularly zeolite A, have proven to be suitable replacements for STPP because they are ecologically sound water softeners. Despite the fact that zeolite A possesses a good ion exchange capacity for Ca2+ ions, it has a lower ion exchange capacity for the Mg2+ ion.5,6 For this reason other zeolites, such as zeolite 13X,7,8 zeolite P,9 or clinoptilolite10 have been studied for use in detergent formulations. Crystalline layered sodium silicate has recently been included as a new multifunctional builder. This new * To whom correspondence should be addressed. Tel: +34 926 29 53 00. Fax: +34 926 29 53 18. E-mail: Justo.Lobato@ uclm.es.
builder essentially consists of the δ phase sodium disilicate Na2Si2O5, which has a polymeric layered bidimensional crystal structure in addition to small R and β and amorphous sodium disilicate contents as impurities.11 The substance-specific properties of δ layered crystalline silicates are essentially based on the absence of structural water, the exchangeability of intermediate layer sodium, and solubility. These properties mean that layered silicates are efficient in removing water hardness, provide suitable washing ability, enhance the performance of surfactants and bleachers, have a corrosion-inhibiting action, and can be used in formulations of both liquid and highly compact detergents. Moreover, these silicates are inert from an ecological point of view and can be mixed with any other builder. Few papers in the literature have focused on the properties and washing characteristics of these silicates12-14 and extensive studies regarding their synthesis have not been performed.15-20 Sodium tripolyphosphate, zeolite, and δ-Na2Si2O5 differ in their water-softening mechanism and behavior in wastewater. δ-Na2Si2O5 is soluble in the wastewater and forms an extremely dilute waterglass solution. The high solubility of δ-Na2Si2O5 in wastewater means that this material makes a very low contribution to sludge formation in the treatment plant. With regard to ecological concerns, zeolite and δ-Na2Si2O5 have clear advantages because they are designed to be phosphate substitutes.13 Zeolite is insoluble in water and so δ phase sodium disilicate is a good alternative to sodium triphosphate. The trend toward more compact powder detergents puts increasing demands on the detergent builder, and the layered disilicate δ-Na2Si2O5 meets these demands because it combines high performance per unit weight with a high degree of multifunctionality.21 An in-depth study into the solubility of the new builder δ-Na2Si2O5, used as a substitute for phosphates in detergent for-
10.1021/ie0303909 CCC: $27.50 © 2004 American Chemical Society Published on Web 02/13/2004
Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004 1473
mulations, has been performed and is reported here. The results are compared with those of a crystalline alpha phase (R-Na2Si2O5) and the amorphous silicate Na2Si2O5 used in the industrial synthesis of δ-Na2Si2O5. The amorphous disilicate removes the water-hardening ions by formation of a voluminous alkaline earth metal silicate precipitate in which the polyvalent cations are mainly bound by the δ crystalline substance through an ion exchange process.12 The aim of the work described here was to determine the influence of temperature on the solubility of different silicates in deionized or tap water. Furthermore, based on the investigations on the solubility of δ-Na2Si2O5, this work resulted in a theoretical model that allows us to understand the different species formed during the hydrolysis process of δ-Na2Si2O5. The results described here could also help to identify the correct detergent dosage per wash, a factor that would contribute to decreasing the environmental impact of the wash process. 2. Experimental Section The experimental setup consisted of several Pyrex glass flasks (100 mL each) that were hermetically sealed and vigorously stirred. The flasks contained the deionized water solutions and the solid. The flasks were submerged in a temperature-controlled thermostatic bath. The temperature was kept constant within (0.5 K. One set of experiments was conducted with deionized water, and another set was conducted with tap water to make the conditions more similar to those of the washing process. All experiments were carried out at 293, 313, or 333 K in the aforementioned bath. The amount of solid used was 0.3 g in 100 mL and this represents a typical detergent dosage. After different times the mixtures were filtered, and the ionic solutions were analyzed by inductively coupled plasma atomic emission spectroscopy (ICPAES) at 251 nm (Si4+) and 589 nm (Na+) or by flame atomic absorption spectroscopy at 589 nm (Na+). The solubility of δ-Na2Si2O5 in deionized water solutions was obtained from sodium ions for different times and temperature. The evolution of pH with time was monitored using a pH meter (Crison 2002) connected to a computer and run through a data acquisition program developed in our department. The crystalline disilicates were synthesized in our laboratory.22 The amorphous disilicate was supplied by IQE . The mean size of all silicates was 175 µm (range 100-250 µm). The deionized water was conventionally treated in our laboratories (conductivity less than 1 µS/cm). The tap water from our region had the following properties: calcium and magnesium concentrations were 50 and 17 ppm, respectively (19.5 °F), conductivity was 384 µS/cm, and the pH was 6.8. 3. Results and Discussion 3.1 Kinetic Study of Solubility in Deionized Water. The δ-Na2Si2O5, alpha, and amorphous-phase solubilities in deionized water were determined at 293, 313, and 333 K. The temperatures studied here encompass the usual ranges found in the washing process. Figure 1 shows the influence of temperature and time
Figure 1. Solubility of silicates in deionized water at 293, 313, and 333 K: (a) δ-phase crystalline silicate; (b) R-phase crystalline silicate; (c) amorphous silicate.
on the solubility of the materials in question. It can be seen that the solubilities of the three solid phases used increase with increasing temperature. On the other hand, the effect of time is less marked because after 30 and 60 min the solubility was generally constant for δ-Na2Si2O5 and R-Na2Si2O5, whereas the amorphous phase solubility was found to increase with increasing time over the range studied. The solubilities reached for the two crystalline phases were similar, and the amorphous silicate had lower solubility values even at the highest temperature and time. This phenomenon is due to the differences in the structural build-up of the materials. In particular, the crystalline silicate units break down under the conditions in question. 3.2 Kinetic Study of Solubility in Tap Water. The δ-Na2Si2O5, alpha, and amorphous phase solubilities in tap water were determined at 293, 313, and 333 K. Figure 2 shows the influence of temperature and time on the solubility. The amounts of Ca2+ and Mg2+ in tap water before and after solubilization were quantified. It can be seen that the solubility of all three solid phases increases with increasing temperature. In each case the solubility was determined by taking into account the difference between the total sodium ions in solution and the amount of sodium exchanged by the alkaline earth metal (Ca2+ and Mg2+) binding of the solid phases.
1474 Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004 Table 1. Values of the Constants Obtained from the Solubility Equation for Different Types of Water Used species
solution
A
B
r2
δ-Na2Si2O5 R-Na2Si2O5 δ-Na2Si2O5 R-Na2Si2O5
deionized water deionized water tap water tap water
421.2 648.84 539.91 559.71
4.636 5.3532 4.8945 5.0631
0.978 0.998 0.96 0.975
silicates at different temperatures were fitted to eq 123 and the constants obtained are shown in Table 1. The solubility data (C∞) for the amorphous silicate were not fitted to eq 1 because complete solubility was not achieved during the time interval studied.
Log C∞i ) -
A +B T
(1)
where C∞i is the solubility of species i in solution (ppm), i represents each silicate studied, T is the absolute temperature in Kelvin, and A and B are constants. 3.3 Hydrolysis Reactions in Deionized Water. At present, R-Na2Si2O5 does not have any known industrial application, but δ-Na2Si2O5 is used as a multifunctional builder in nonphosphate detergents. For this reason only the hydrolysis reactions of δ-Na2Si2O5 are covered in this section. Data published in the literature14 indicate that δ-Na2Si2O5 rapidly releases sodium into solution. It therefore seems reasonable to propose that the following reaction takes place on contact with an aqueous solution with the formation of amorphous silica.
δ - Na2Si2O5(S) + H2O f SiO2(S) + NaOH(aq) (2) Amorphous silica is known to be partly water soluble, and data published in the literature24,25 indicate that the solubility of SiO2 reaches a minimum of 100-130 ppm between pH 7 and 8. The solubility then increases significantly above pH 9 to reach 400 ppm at pH 10 between 292 and 298 K. In aqueous solution, silicon exhibits a 4-fold coordination of oxygen and forms species generally referred to as silicates (H4SiO4). It therefore seems reasonable to propose that the following reaction occurs on contact with an aqueous solution:
SiO2(S) + 2H2O T H4SiO4(aq) Figure 2. Solubility of silicates in tap water at 293, 313, and 333 K: (a) δ-phase crystalline silicate; (b) R-phase crystalline silicate; (c) amorphous silicate.
The solubility of δ-Na2Si2O5 in deionized water is higher than its solubility in tap water, which is thought to be a consequence of the binding capacity of δ-Na2Si2O5. It can be seen that the solubility of the R phase in tap water is similar to that in deionized water at the three temperatures investigated. This phenomenon can be explained by the fact that this solid has the lowest retention capacity for the Ca2+ and Mg2+ ions.22 Hydrolysis of the amorphous phase occurs to a lesser extent in tap water than in deionized water at 293 and 313 K, but is similar at 333 K. This situation is due to the increase in the retention capacity, caused by precipitation, as the temperature decreases. It can be observed that the differences in the hydrolysis for both types of water became less marked as the temperature increased. The solubility data (C8) for the different crystalline
(3)
Orthosilicic acid is understood to be a four protonic, weak acid, and, depending on the causticity of the solution, one or more protons can be released. In more alkaline media the charges carried by the silicate species retard the mutual reactions and depolymerization is favored under certain conditions.26 The general reaction of orthosilicic acid in alkaline solution is as follows:
H4SiO4(aq) + NaOH(aq) f NaH3SiO4(aq)
(4)
3.3.1 Reaction Schemes. Prediction of the δ phase sodium disilicate reaction in deionized water requires a knowledge of the kinetics and the equilibrium constants for the three temperatures studied. Sodium and silicon were determined at 293, 313, and 333 K in deionized water. Figure 3 shows the sodium and silicon concentrations generated in the hydrolysis of δ-Na2Si2O5 along with the evolution of these species with time and temperature. The decrease in δ-silicate in deionized water can be represented by the empirical expression proposed by
Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004 1475
Figure 4. Comparison of model (solid lines) and experimental concentrations (points) for δ-Na2Si2O5 and silicon species at 293 K. Table 2. Values of Kinetics Constants for Different Reactions in Deionized Water at 293, 313, and 333 K
Figure 3. Concentrations in deionized water at 293, 313, and 333 K: (a) Na+ ions; (b) Si4+ ions.
Butler27 for crystallization processes:
dm R(CA - C∞)n dt
(5)
where n is greater than 1, m is the mass of the crystalline solid, CA and C∞ are the concentrations of crystalline solid at given times and at the saturation conditions (solubility), respectively. The values of C∞ for δ-Na2Si2O5 at each temperature were obtained using eq 1. The overall reaction scheme is outlined as follows:
rA )
dCA ) -K0(CA - C*)n dt
(6)
dCB ) 2K0(CA - C*)n - K1CB + K2CD (7) dt dCC rC ) ) 2K0(CA - C*)n - K3CDCC (8) dt dCD ) K1CB - K2CD - K3CDCC (9) rD ) dt dCE ) K3CDCC (10) rE ) dt
rB )
where C* ) CAo - C∞.
T (K)
K0 (L2.2‚min-1‚mol-2.2)
K1 (min-1)
K2 (min-1)
K3 (L‚mol-1‚min-1)
293 313 333
60000 100000 180000
46.2 915 184000
1200 4120 6150
114 835 7120
The Rosenbrock method28 was used to numerically integrate the stiff set of ordinary differential equations due to its stability and the low number of integration steps required to achieve a satisfactory solution. The nonlinear system was fitted using Marquardt’s algorithm.29 For this purpose a Fortran 6.0 application was developed to solve this model. Figure 4 shows the experimental concentrations of Na+ and Si4+ and their respective fitted data using the model. These concentrations correspond with the Na+ and Si4+ ions of the species in aqueous solutions (NaOH, H4SiO4, and NaH3SiO4). Kinetics constants (K0, K1, K2, and K3) were obtained by fitting the experimental data with the model at the respective temperature. The reaction exponent (n) obtained for δ-Na2Si2O5 hydrolysis was constant but not fixed. The kinetics parameters are summarized in Table 2. It can be observed that as the temperature increases all kinetic constants also increase. However, the increase with temperature of the relation K1/K2 for the reversible reaction 3 is remarkable. This behavior indicates that as the temperature increases, reaction 3b tends to be irreversible - as can be observed in Figure 3. Thus, at 333 K the Si4+ species is formed rapidly after only short times. In addition, the activation energy and preexponential factors for the three reactions could be evaluated from the kinetics constants, K, and the temperature. Assuming a linear variation of lnK with 1/T, a negative slope is obtained. These activation energy values and preexponential factors are given in Table 3 along with the correlation coefficients. Figure 5 shows the concentrations of the species’ as predicted by model in deionized water. The validity of the model was checked by making an additional comparison between the pH measured experimentally in situ and the concentration of NaOH obtained by the model. The results of this comparison are shown in Figure 6. The lowest pH value obtained at 333 K occurs because reaction 3 tends to be irreversible, as indicated above, and therefore the highest concentration of the acid H4SiO4 is formed and then
1476 Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004
Figure 5. δ-Na2Si2O5, SiO2, NaOH, H4SiO4, and NaH3SiO4 predicted concentrations from the model in deionized water at 293 K.
enon is due to the alpha phase having the lowest retention capacity for the Ca2+ and Mg2+ ions. The results obtained in this study show that δ layered crystalline disilicate is water-soluble and therefore does not remain in wastewater as a solid. This material therefore contributes very little to sludge formation in wastewater treatment plants, and also provides and buffers the alkalinity in the wash liquor. The model is able to reproduce satisfactorily the hydrolysis of δ-Na2Si2O5 in deionized water at any temperature. The model also allows evolution of the different species during the hydrolysis process to be obtained and provides information concerning the influence of δ-Na2Si2O5 on the variation of pH in a stirrer reactor. The new builder, δ-Na2Si2O5, is more soluble than zeolite 4A, the builder currently used in detergents as a substitute for phosphates. These facts, along with other important characteristics of δ-phase sodium disilicate such as alkalinity supply or good ion-exchange capacity for Ca2+ and Mg2+ ions, make this material suitable for use as a builder or co-builder in nonphosphate detergents. Acknowledgment We thank Industrias Quı´micas del Ebro for its financial support (I+D Contact with the University of Castilla-La Mancha, UCLM). Nomenclature
Figure 6. Comparison of model (solid lines) and in situ experimental pH (points) at 293, 313, and 333 K. Table 3. Values for the Activation Energy (EA) and Pre-Exponential Factor (A) for Different Reactions in Deionized Water and Correlation Coefficients (r2) kinetics constants
A (min-1)
EA (J/mol)
r2
K0 K1 K2 K3
5.378E+08 1.730E+31 1.246E+09 8.950E+16
2.224E+04 1.671E+05 3.349E+04 8.375E+04
0.996 0.980 0.969 0.998
neutralized by the NaOH. It can be concluded that this model is reliable in providing an insight into the behavior of δ silicate in desionized water. 4. Conclusions Crystalline silicates show solubility values higher than those reached by amorphous silicate in deionized water due to the structural differences in the silicates over the time interval studied. It has been found that increasing the temperature favors the solubility of both crystalline phases. The solubility of the δ phase in deionized water is higher than its solubility in tap water because of the δ-Na2Si2O5 retention capacity. The solubility of the alpha phase in tap water is similar to its solubility in deionized water for the three temperatures studied. This phenom-
n ) reaction exponent of δ-Na2Si2O5 hydrolysis reaction. C∞ ) δ-Na2Si2O5 solubility. C* ) δ-Na2Si2O5 remaining concentration after reaching saturation. K0 ) kinetic constant for hydrolysis of δ-Na2Si2O5. K1 ) kinetic constant - direct for SiO2 reaction. K2 ) kinetic constant - inverse for SiO2 reaction. K3 ) kinetic constant for reaction between NaOH and H4SiO4. CAo ) δ-Na2Si2O5 initial concentration. CA ) δ-Na2Si2O5 concentration with time. CB ) SiO2 concentration with time. CC ) NaOH concentration with time. CD ) H4SiO4 concentration with time. CE ) NaH3SiO4 concentration with time.
Literature Cited (1) Forsberg, C. Which policies can stop large scale eutrophication? Water. Sci. Technol. 1998, 37, 193. (2) Harremoe¨s, P. The Challenge of Managing water and Material Balances in relation to Eutrophication. Water. Sci. Technol. 1998, 37, 9. (3) Leaf, S. S.; Chatterjee, R. Developing a Strategy on Eutrophication. Water Sci. Technol. 1999, 39, 307. (4) Reynolds, C. S.; Davies, P. S. Sources and bioavailability of phosphorus fractions in freshwaters: a British perspective. Biol. Rev. 2001, 76, 27. (5) Costa, E.; Lucas, A.; Uguina, M. A.; Ruiz, J. C. Synthesis of 4A Zeolite from Calcined Kaolins for Use in Detergents. Ind. Eng. Chem. Res. 1988, 27, 1291. (6) Nieuwenhuizen, M. S.; Ebaid, A. H. E. F.; Van Duin, M.; Kieboom, A. P. G.; Van Bekkum, H. Cation Exchange in the System Ca(II) or Mg(II)/complexing agent/zeolite NaA: Equilibria and Kinetics. Tenside Deterg. 1984, 21, 221. (7) Lucas, A.; Uguina, M. A.; Covian, I.; Rodrı´guez, L. Synthesis of 13X Zeolite from Calcined Kaolins and Sodium Silicate for Use in Detergents. Ind. Eng. Chem. Res. 1992, 32, 2134.
Ind. Eng. Chem. Res., Vol. 43, No. 6, 2004 1477 (8) Lucas, A.; Uguina, M. A.; Covian, I.; Rodrı´guez, L. Use of Spanish Natural Clays as Additional Silica Sources to Synthesize 13X Zeolite from Kaolin. Ind. Eng. Chem. Res. 1993, 32, 1645. (9) Upadek, H.; Kottowitz, B.; Schreck, B. Zeolite und neuartige Silicate als Waschmittelrohstoffe. Tenside, Surfactants, Deterg. 1996, 33, 385. (10) C¸ ulfaz, M.; Sarac¸ oglu, N. E. Clinoptilolite Zeolite as a Builder in nonphosphate Detergents. J. Environ. Sci. Health. Part A: Toxic/Hazard. Subst. Environ. Eng. 1999, 34, 1619. (11) Rieck, H.-P. A New Promising Chapter in the Sodium Silicate Story. New Horizons. In AOCS/CSMA Detergent Industry Conference; Coffey, R. Ed.; FMC Corp.: Princeton, NJ, 1995; Chapter 3. (12) Wilkens, J. Structure-property relationships of sodium silicates. Tenside, Surfactants, Deterg. 1995, 32, 476. (13) Bauer, H.; Schimmel, G.; Ju¨rges, P. The Evolution of Detergent Builders from Phosphates to Zeolites to Silicates. Tenside, Surfactants, Deterg. 1999, 36, 225. (14) Coker, E. N.; Rees, L. V. C. Solubility and Water-softening Properties of a Crystalline Layered Sodium Silicate, SKS-6. J. Mater. Chem. 1993, 3, 523. (15) Schirmer, W.; Hachgenei, J.; Gunter, J.; Wichelhans, W. Phosphate-free Dishwashing Detergents Containing Sodium Disilicate. DE 4102743, 1992. (16) Delwel, F.; Thennissen, J. P.; Osinge, T. J.; Vranchen, J. M. Manufacture of Alkali Metal Silicates. EP 0526978 A2, 1992. (17) Lee, J. M.; Suh, J. K.; Jeong, S. Y.; Jin, H. K.; Park, B. K.; Park, C. H.; Park, J. H.; Kim, J. H.; Lim, C. W. A Process for Preparing Crystalline Sodium Disilicate Having a Layered Structure. EP 0745559 Al, 1996. (18) Lucas, A.; Rodrı´guez, L.; Sa´nchez, P.; Lobato, J. Synthesis of Crystalline Layered Sodium Silicate from Amorphous Silicate for Use in Detergents. Ind. Eng. Chem. Res. 2000, 39, 1249. (19) Lucas, A.; Rodrı´guez, L.; Lobato, J.; Sa´nchez, P. Effect of the Particle Size of Starting Materials on the Synthesis of
Crystalline Layered Sodium Silicate for Use in Detergents. Ind. Eng. Chem. Res. 2001, 40, 2580. (20) Lucas, A.; Rodrı´guez, L.; Lobato, J.; Sa´nchez, P. Synthesis of Crystalline δ-Na2Si2O5 from Sodium Silicate Solutions for use as a Builder in Detergents. Chem. Eng. Sci. 2002, 57, 479. (21) Bauer, H.; Schimmel, G. New aspects in the use of the builder silicate SKS-6. Tenside, Surfactants, Deterg. 1997, 34, 425. (22) Lobato, J. Sı´ntesis de Silicatos So´dicos Cristalinos Laminares. Ph.D. Thesis. University of Castilla-La Mancha, Spain, 2000. (23) Verma, M. Revised Quartz solubility temperature dependence equation along the water-vapor saturation curve. In Proceedings of the World Geothermal Congress 2000; Kyushu-Tohoku, Japan, May 28 - June 10, 2000. (24) Alexander, G. B.; Heston, W. M.; Iler, R. K. J. Phys. Chem. 1954, 58, 153. (25) Cherkinskii, Y. S.; Knyaz’kova, I. S. Dokl. Akad. Nauk SSSR 1971, 198, 358, (Engl. Transl., p 415). (26) Gasteiger, H. A.; Frederick, W. J.; Streisel, R. C. Solubility of Aluminosilicates Solutions and a Thermodynamic Equilibrium Model. Ind. Eng. Chem. 1992, 31, 1183. (27) Coulson, J. M.; Richardson, J. F., Eds. Ingenierı´a Quı´mica (operaciones ba´ sicas); Reverte´: Barcelona, 1988. (28) Press, W.; Teukolsky, S.; Vetterling, W.; Flannery, B. Numerical Fortran Recipes in Fortran, 2nd ed.; Cambridge University Press: New York, 1992. (29) Marquardt, D. W. An Algorithm for least-squares estimation of nonlinear parameters. J. Soc. Ind. Appl. Math. 1963, 2, 431.
Received for review May 6, 2003 Revised manuscript received October 20, 2003 Accepted November 5, 2003 IE0303909
