Chapter 4 Adsorption of Ammonium Dinitramide (ADN) from Aqueous Solutions Part of the results from this chapter has been published: 1. G. Santhosh, S. Venkatachalam, K.N. Ninan, R. Sadhana. S. Alwan. V. Abarna. M.A. Joseph "Adsorption of ammonium dinitramide (ADN) from aqueous solutions 1. Adsorption on powdered activated charcoal" Journal of Hazardous Materials, 2003, 898, 117-126. 2. G. Santhosh, S. Venkatachalam, K.N. Ninan, R. Sadhana, "Adsorption of ammonium dinitramide (ADN) from aqueous solutions 2. Adsorption on granular activated charcoal" Adsorption Science and Technology (to be communicated).
T HIS CHAPTER details the investigations on the adsorption of ammonium dinitramide (ADN) from aqueous solutions on powdered activated charcoal (PAC) and granular activated charcoal (GAC) in order to find out an effective and cheaper method of separating ADN from aqueous solutions. The effectiveness of PAC and GAC in the selective adsorption of ADN from aqueous solutions of ADN (ADN-F) and ADN in presence of sulphate (sod2-) and nitrate (NO;) ions (ADN-PS) is examined and compared using batch and column methods. The adsorption process follows both Langmuir and Freundlich adsorption isotherms. A detailed study on the use of PAC on the adsorption of ADN is described. The adsorption isotherm parameters for the models are determined. Break-through curves for ADN-F and ADN-PS are obtained for the optimization of separation of ADN from aqueous solutions. Elution curves are generated for the desorption of ADN from PAC using hot water as eluent. The reaction rate constants are determined for the adsorption of ADN over PAC. Langmuir and Freundlich adsorption isotherm parameters for adsorption of ADN over GAC are compared with those of obtained for PAC. The reaction rate constants were determined for the adsorption of ADN on GAC.
4.1. Adsorption of ADN on Powdered Activated Charcoal (PAC) & Granular Activated Charcoal (GAC) 4.2 Introduction Much of the work is published on the synthesis of ADN and its analogues [''51. Most of the methods make use of exotic nitrating agents such as N205 or N02BF4 [51. A recent literature reports the synthesis of ADN using conventional nitrating agents such as HN03/H2S04 [61. This process results in ADN along with ammonium sulphate, ((NH4)2S04) and ammonium nitrate (AN, NH4N03) in water. The separation of ADN from aqueous solution involves precipitation, drying, extraction and recrystallization from different solvents and is a time consuming and costly process. The adsorption technique is an effective method of separation at a relatively lower cost in comparison to solvent extraction procedure for many materials. ADN can be adsorbed on a variety of adsorbents like molecular sieves, activated alumina, silica gel, activated charcoal etc., the adsorbed ADN is then eluted with a suitable eluting solvent. Powdered activated charcoal (PAC) & Granular activated charcoal (GAC) were used as adsorbent for the separation of ADN from aqueous solutions. The adsorption behaviour of ADN by PAC & GAC was investigated using Langmulr and Freundlich isotherms. The Langmuir and Freundlich isotherm constants were determined and the amount of PAC & GAC to be used for the adsorption of ADN was also worked out. Breakthrough curves were generated for predicting the column efficiency in the recovery of ADN. Adsorbed ADN was desorbed using hot water as eluent. The reaction rate constants were determined for both the system. 4.3. Experimental 4.3.1. Materials Ammonium dinitramide (ADN) was synthesized in our laboratory ['I. It is a pale yellow hygroscopic powder soluble in polar solvents including water. The compound was characterized by UV, IR and thermal methods.
The reaction mixture containing ammonium sulphate (3900 mg/l) and ammonium nitrate (3000 mg/l) along with ADN (600 mg/l) was the process solution (ADN-PS), for the adsorption experiments employing PAC. Activated charcoal powder (Sarabhai M. Chemicals Limited, Mumbai, India) with a specific surface area (BET method) of 400-450 m2g-' and a particle size (Fischer method) of 17km was dried at 110 "C for 10-12 hrs. and stored in a desiccator prior to use. The process solution (ADN-PS) containing ammonium sulphate (4100 mg~.') and ammonium nitrate (3200 mg~.') along with ammonium dinitramide (500 mg~.') is used for the adsorption experiments using GAC. Granular activated charcoal ( SRL Limited, Mumbai) of particle size 3-5mm was dried at 120 C for 10-12h and stored in a desiccator prior to use. 4.3.2. Instruments A CARY 5e UV-VIS-NIR spectrometer was used for the measurement of ADN concentration before and after adsorption. 4.3.3. Adsorption Experiments Adsorption experiments using PAC were conducted by batch and continuous methods at room temperature (30 "C). Batch adsorption experiments were carried out by stirring 3 g of PAC with 100 ml of an aqueous solution of ADN-F or ADN-PS of the desired concentration (0.01 M to 0.1M) in different glass-stoppered erlenmeyer flasks using a magnetic stirrer for predetermined time intervals (30-60 min) till equilibrium was achieved. Column adsorption experiments were carried out on a 20 mm
diameter glass column with a sintered disc at the bottom by placing 4 g of PAC and 100 ml solution of ADN-F or ADN-PS of varying concentrations (0.01 M to 0.1 M) at a constant flow rate of 3 mllmin, and the solution after adsorption was collected for different periods of time (30 min intervals). Batch experiments using GAC were conducted at room temperature (30 C). Experiments were carried out by stirring 6g of GAC with 100ml of an aqueous solution of ADN-F or ADN-PS of the desired concentration (0.01-0.1M) in amber coloured stoppered flasks using a magnetic stirrer for predetermined time intervals (30-400min) till equilibrium was achieved. The equilibrium concentration, Ce of the solution was determined by UV spectrophotometer by taking weighed quantities of aliquots of the collected solution. The concentration of ADN adsorbed was obtained by calculating the difference of the concentration of ADN in solution before and after adsorption using the experimentally determined molar extinction co-efficient E284 5258 ~mo~-"cm-' ['I. 4.4. Results and Discussion 4.4.1. Effect of Adsorption Time for PAC & GAC The amount of ADN adsorbed per unit weight of adsorbent (PAC) as a function of time is given in Figure. 4.1. The quantity of ADN adsorbed increas'es with the increase of the adsorption time for ADN-F and ADN-PS. However, it remains constant after an equilibrium time of 175 min for ADN-F and 200 min for ADN-PS, which indicates that the adsorption tends toward saturation at the above-mentioned time.
im 100-0- -0-ADN+ (640 mgl) 1-1 - 1. 1. 1 - I. so 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 & 4 0 0 Time (mn) Figure 4.1 : The amount of ADN adsorbed per unit weight of PAC as a function of time with 49 of adsorbent at 30 C The amount of ADN adsorbed per unit weight of adsorbent (GAC) as a function of time is given in Figure 4.2. l,. l * l. l. l. l. l.,. l. l. ~ 0 50 100150200m300350400450500 Time (mn) Figure 4.2: Amount of ADN adsorbed per unit weight of GAC as a function of time with 6g of adsorbent at 30%
The quantity of ADN adsorbed increases with the increase of the adsorption time for ADN-F and ADN-PS. It remains constant after an equilibrium time of 350min for ADN-F and 370min for ADN-PS, which indicates that the adsorption tends toward saturation at the abovementioned time. Comparing the equilibrium time taken for the adsorption of ADN on PAC & GAC, the time taken for equilibrium for GAC is almost twice that of PAC, indicating that the equilibrium is achieved fast in the case of adsorption of ADN on PAC. 4.4.2. Adsorption Isotherms Various isotherm models are available for expressing the adsorption procesi The adsorption isotherm for the adsorbed ADN on PAC & GAC can be analyzed by Langmuir and Freundlich isotherms. 4.4.2.1. Langmuir Isotherm The linear representation of the Langmuir isotherm [Io1 can be expressed as in Equation 4.1. where Ce is the equilibrium concentration of ADN in solution (mgll), Ye is the milligrams of adsorbed ADN per gram of adsorbent (mglg), Q is the maximum amount of adsorbed ADN per gram of adsorbent (mglg) and b is the Langmuir constant (Llmg). Thus, a plot of C,/Ye versus Ce should yield a straight line with a slope of 1IQ and an intercept of 11Qb from which the values of Q and b can be readily obtained. Figure. 4.3 shows the plot of Ce/Y, versus C,, which is a straight line with relatively good correlation coefficient (Table 4.1). All the data correctly fit the Langmuir relation and indicating that the adsorption of ADN from aqueous solution on PAC follows the monolayer adsorption.
Figure 4.3: Langrnuir plots for the adsorption of ADN on PAC at 30 C The Langrnuir constants obtained from Figure. 4.3 are given in Table 4.1. The Langrnuir equilibrium constant K = Qb was also measured and given in Table 4.1. As it is seen in Table 4.1, the amount of ADN adsorbed per gram of charcoal is higher for ADN-PS compared to ADN-F performed in batch, and the amount of ADN adsorbed is lower for ADN-PS compared to ADN-F performed in column. Since Langmuir theory is restricted to cases where only one layer of molecules can be adsorbed at the surface ["I, the adsorption of ADN from aqueous solutions on PAC follows monolayer adsorption as evident from the observed data fit in Langmuir isotherm.
Table 4.1: Langmuir constants from Equation 4.1 at 30 C System Langmuir Constants t Q (mglg) b (Umg) K=Qb (Ug) - ADN-F (Batch) 63.3 0.00062 0.0392 ADN-PS (Batch) 119.0 0.00029 0.0345 ADN-F (Column) I 105.3 0.00103 0.1084 Correlation coefficient (r) 0.9884 0.9739 0.9894 ADN-PS (Column) 82.0 0.0030 0.2455 0.9949 Figure. 4.4 shows the plot of Ce/Ye versus Ce for the batch adsorption of ADN on GAC, which is a straight line with relatively good correlation coefficient (Table 4.2), showing that all the data correctly fit the Langmuir relation and indicating that the adsorption of ADN from aqueous solution on GAC also follows monolayer adsorption. Figure 4.4: Langrnuir plots for the adsorption of ADN on GAC at 30 C Ce
The Langmuir constants obtained from Figure 4.4 are given in Table 4.2. The equilibrium constant 'K' was also calculated and given in Table 4.2. The adsorption of ADN from aqueous solutions on GAC follows monolayer adsorption as evident from the observed data fit in Langmuir isotherm. As it is seen from Table 4.2 the amount of ADN adsorbed per gram of GAC is higher for ADN-F than ADN-PS. Table 4.2: Langmuir constants for batch adsorption of ADN on GAC at 30% System ADN-F (Batch) ADN-PS (Batch) Langmuir Constants Q (mglg) b (Umg) K=Qb (L lg) 47.92 0.00021 0.0101 35.92 0.00076 0.0273 Correlation coefficient (r) 0.9907 0.9995 Comparison of Table 4.1 & 4.2 reveals that PAC has a better adsorption capacity than GAC. The amount of ADN adsorbed (Q) is much lower on GAC than PAC. 4.4.2.2. Freundlich Isotherm The Freundlich adsorption equation [I2] is expressed as given in Equation 4.2. or the linearised form of the equation where P and lln are empirical constants (Freundlich parameters), the values of which are equal to the intercept and slope of the plot of log Ye versus log C,. Freundlich plots for the adsorption of ADN on PAC are shown in Figure. 4.5.
The adsorption of ADN on powdered activated charcoal was found to correspond with the Freundlich adsorption isotherm. The Freundlich constants deduced from the straight lines in Figure. 4.5 are given in Table 4.3 along with correlation coefficients. Figure 4.5: Freundlich plots for the adsorption of ADN on PAC at 30 C. Table 4.3: Freundlich constants from Equation 4.3 for adsorption of ADN on PAC at 30 C System ADN-F (Batch) ADN-PS (Batch) ADN-F ( Column) ADN-PS (Column) Freundlich Constants log P n 0.15 1.96 0.18 1.81 0.60 2.67 0.93 3.56 Correlation coefficient (r) 0.9996 0.9941 0.9993 0.9981
For all the experiments, the exponent 'n' is greater than one, which indicates good adsorption of ADN on PAC. The adsorption of ADN from aqueous solutions on PAC follows both Freundlich and Langmuir isotherm as these two fits well with good correlation coefficient (Table 4.1 and 4.3). Freundlich plots for the batch adsorption of ADN on GAC are shown in Figure. 4.6. log C, Figure 4.6: Freundlich plots for the adsorption of ADN on GAC at 30 C The adsorption of ADN on GAC was found to correspond with the Freundlich adsorption isotherm. The Freundlich constants obtained from the straight lines in Figure 4.6 are given in Table 4.4 along with correlation coefficients. For all the experiments, the exponent 'n' is greater than 1, which indicates good adsorption of ADN on GAC. The adsorption of ADN on GAC fits well for both Langmuir and Freundlich isotherms.
Table 4.4: Freundlich constants for batch adsorption of ADN on GAC at 30 C - System ADN-F (Batch) ADN-PS (Batch) Freundlich Constants log P n 1.OBI0 1.50 0.0533 2.43 Correlation coefficient (r) 0.9939 0.9894 4.4.3. Results of Column Adsorption Model for the adsorption of ADN over PAC A reported theoretical model was used in the present study for measuring the change in ADN concentration at the column exit. In a column, the fraction of ADN that is being adsorbed is denoted as 'A' and the fraction of that is remaining in the aqueous solution and passing through the stationary adsorbent is denoted as 'P'. It is assumed that the rate of decrease in the adsorption fraction (A) is proportional to A and P as shown in Equation (4.4 and 4.5). It is noted in Equation 4.5 that P = I-A. Equation 4.5 can be integrated with an initiql condition of A = A, at t = t to get Equation 4.5a. substituting P = I-A, Equation 4.5a becomes Equation 4.5b.
defining t, as the adsorption time, as denoted by 2, when P=0.5 (one-half of the adsorption capacity) Equation 4.5b becomes 4.6. The detailed derivation of Equation 4.7 is reported elsewhere [I3]. The derivation of Equation 4.7 is based on the definition that 50% breakthrough of the adsorption process occurs at time 2. Due to the sigmoid nature of the breakthrough curve the adsorbent bed should be completely saturated at 22. ADN fraction (P) passing through the adsorbent column is equal to CICo, where C is the ADN concentration of the exiting aqueous solution at time t and C, is the inlet ADN concentration. Figure 4.7 shows the plots of adsorption time t versus ln[c/(co-c)] for ADN-F and ADN-PS on PAC. It is a straight line with 2 as intercept and Ilk as slope respectively. k and r (model parameters) thus determined are used to construct the breakthrough curve using Equation 4.7. Reasonable linear fit of the data was obtained and the model parameters of the breakthrough curves derived from Figure 4.7 are listed in Table 4.5.
Figure 4.7: Linear plots of time, t vs. In[CI(C,-C)] for ADN-F and ADN-PS The breakthrough curves for ADN-F and ADN-PS were constructed using the model parameters s and k listed in Table 4.5 and are given in Figure. 4.8. The calculated breakthrough time for 0.1 mgll of ADN exiting from the column is 75 min for ADN-F and 69 min for ADN-PS. The presence of nitrate and sulphate ions have little effect on the adsorption of ADN on PAC, this is evident from the close breakthrough times observed for ADN-F and ADN-PS. Table 4.5: Model parameters of the breakthrough curves from Figure 4.7. System ADN-F ADN-PS k (Ilmin) 0.1 027 0.1140 s (min) 97 88
Figure 4.8: Predicted breakthrough curves for ADN-F and ADN-PS on PAC with a flow rate of 3ml min" and an initial ADN concentration of 6500 mg L-I. 4.4.4. Desorption of ADN from PAC Desorption studies were performed on ADN-PS adsorbed on PAC using a column at 30 C by placing 49 of PAC and an ADN-PS solution containing 6500 mg/l of ADN. The solution passing through the column was collected and refed 2 to 3 times. The exit solution was analysed by UV and the amount of ADN adsorbed in the column was calculated. The adsorbed ADN was then eluted with hot water (50 C) at a flow rate of 3 ml/min. A plot of the amount of ADN in eluted samples at different time intervals was made and is shown in Figure. 4.9. In a typical run, the amount of ADN adsorbed on PAC for an initial concentration of 6500 mgll is 4300 mg, which is about 66%. The amount of ADN recovered using hot water as eluent is 3850 mg, which is about 89%. These results show higher efficiency of the removal and recovery of ADN on PAC using hot water as
eluent. Ion chromatographic analysis of the eluent shows 60 ppm nitrate and 0 ppm sulphate ions. Hence PAC is proved to be efficient in the adsorption of ADN and allows recovery of ADN with hot water. Time (nin) Figure 4.9: Desorption of ADN from PAC with hot water as a function of time with a flow rate of 3.5 ml min-'. 4.4.5. Determination of Adsorption Rate Constants for PAC and GAC The adsorption rate constants for the adsorption of ADN on PAC & GAC were calculated based on the adsorption rate equation [14*151 given in Equation 4.8. where, adsorption time is 't', adsorption rate constant is 'K', and F=qdq,, 'q; is amount adsorbed at time t, 'q,' is amount adsorbed at equilibrium.
A plot of -In(l-F) versus time t gives a straight line for both ADN-F and ADN-PS. The slope of the curve is equal to the adsorption rate constant 'K'. The plot of -In(l-F) versus t for PAC is given in Figure 4.10. Figure 4.10: Rate constant plots for the adsorption of ADN on PAC The calculated rate constants along with correlation coefficients for ADN-F and ADN-PS for adsorption on PAC are given in Table 4.6. The rate constant for ADN-PS is higher than that of ADN-F showing that the adsorption of ADN from a mixture of sulphate and nitrate ion is better on PAC. Table 4.6: Calculated rate constants for adsorption of ADN on PAC System ADN-F ADN-PS Rate constant (K, min-') 1.49 x 10.~ 2.46 x 10.' Correlation coefficient (r) 0.991 1 0.9821 The adsorption rate constants for the adsorption of ADN on GAC were calculated based on the adsorption rate equation given in 4.8.
A plot of -In(l-F) versus time t gives a straight line for both ADN-F and ADN-PS. From the slope of the curve the adsorption rate constant K was calculated. The plot of -In(l-F) versus t is given in Figure 4.1 1. Time (mn) Figure 4.1 1 : Rate constant plots for the adsorption of ADN on GAC The calculated rate constants along with correlation coefficients for ADN-F and ADN-PS for adsorption on GAC are given in Table 4.7. The rate constant for ADN-PS is slightly higher than that of ADN-F showing that the adsorption of ADN from ADN-PS is better on GAG. Table 4.7: Calculated rate constants for adsorption of ADN on GAC System ADN-F ADN-PS Rate constant (K, min-') 7.72 x IO" 8.05 x Correlation coefficient (r) 0.9916 0.9859 Comparing the rate constant values obtained for ADN-F & ADN-PS in Table 4.6 & 4.7, the adsorption of ADN is better on PAC than on GAC as evident from the higher rate constant values obtained for the adsorption on PAC.
4.5. Conclusions The separation of ADN from aqueous solutions was evaluated in batch and continuous methods, using powdered activated charcoal (PAC) and granular activated charcoal (GAC). The equilibrium concentrations were established for ADN-F and ADN-PS. Langmuir and Freundlich adsorption isotherms were generated for these systems and the equilibrium concentrations were determined. Adsorptive capacities observed were 63.3, 119.0, 105.3, 82.0 mg of ADN per gram of PAC for ADN-F (batch), ADN-PS (batch), ADN-F (column) and ADN-PS (column) respectively. The results revealed that monolayer adsorption isotherms were sufficient to describe the equilibrium adsorption of ADN from aqueous solutions. Theoretical column adsorption model was applied for the prediction of ADN concentration in the exit aqueous solution. The determination of the two model parameters T and k helps in establishing the complete breakthrough curve and is a convenient measure for determining the accurate breakthrough times for separation of ADN from ADN-PS. The calculated rate constant was higher for ADN-PS than for ADN-F. Elution of ADN was achieved by using hot water as eluent. The adsorption of ADN from aqueous solutions was evaluated in batch method using GAC. The equilibrium concentrations were established for ADN-F and ADN-PS. The equilibrium concentrations were determined from Langmuir and Freundlich isotherms for these systems. Adsorptive capacities observed were 35.92, 47.92 mg of ADN per gram of GAC for ADN-PS and ADN-F respectively. The results on the adsorption of ADN on GAC show that monolayer adsorption isotherms describe the equilibrium adsorption of ADN from aqueous solutions. Adsorption rate constants were determined for ADN-F and ADN-PS; the value is higher for ADN-PS than for ADN-F.
4.6. References Schrnitt R.J, Bottaro J.C, Penwell P.E, Bornberger D.C, U.S. Patents 51 98204, 5254324,1993. Schrnitt R.J, Bottaro J.C, Penwell P.E, Bornberger D.C, U.S. Patent 5316749,1994. Schrnitt R.J, Bottaro J.C, Penwell P.E. Bornberger D.C, U.S. Patent 5415852,1995. Suzuki S, Miazaki S, Hatano H. Shiino K, Onda T, U.S. Patent 5659080,1997. Bottaro J.C, Penwell P.E, Schrnitt R.J, J. Am. Chem. Soc., 1997, 1 19,9405-941 0. Langlet Abraham, Ostrnark Henric, Wingborg Niklas. U.S. Patent 5976483,1999. Santhosh G, Venkatachalarn S, Kanakavel M, Ninan K.N, Ind. J. Chem. Technol., 2002, 9, 223-226 Weber W.J, Mattews A.P, AIChE, 1976, 73, 91.Ruthven D.M. Principles of Adsorption and Adsorption processes, John Wiley, New York, 1984. Langrnuir I, J. Am. Chem. Soc., 1918,40,1361. Shoemaker D.P, Garland C.W, Experiments in Physical Chemistry, Mc Graw-Hill, New York, 1967. Freundlich H, Colloid and Capillary Chemistry, Matheun, London, 1926. Sheng H. Lin, Cheng P. Huang, J. Haz. Mater., 2001, 884,217-228. Rarnazan Coskun, Mustafa Yigitoglu, Mehmet Sacak. J.App1. Polym. SCi., 2000, 75, 766-772. Lu Y, Wu C, Lin W.P, Tang L.Y, Zeng H.M, J. Appl. Polym.Sci., 1994, 22,l.