J. MATER. CHEM., 1994, 4(7), 1143-1147 1143 Modified Zeolites for the Removal of Calcium and Magnesium from Hard Water Raymond Le Van Mao,* Ngoc Thanh Vu, Shuyong Xiao and Arlene Ramsaran Catalysis Laboratory and Laboratories for Inorganic Materials, Department of Chemistry and Biochemistry, Concordia University, 7455 de Maisonneuve West, Montreal, Quebec, Canada H3G I M8 The cation-exchange capacity (cec) and the pore size of zeolite materials are the two key factors which influence the removal of calcium and magnesium in aqueous solution. Na-X and Na-Y zeolites have been modified by selective extraction of Si from the zeolite framework using sodium carbonate. At ambient temperature, the modified Na-X zeolite was more efficient than the Na-A commercially used in detergency, for both Ca and Mg ions. Polyphosphates are very efficient as sequestering detergent builders.' The role of phosphates is to remove calcium and magnesium from wash waters in order to favour the removing, solubilizing, emulsifying and suspending action of surfactants. However, the massive discharge of phosphates, although relatively safe for humans and non-toxic to aquatic life,2 results in the phenomena of eutrophication observed in lakes and streams. Because of this harmful effect to the environment, phosphate-free detergents have recently been developed. This has been achieved by using alternative builders including Today, zeolites appear to be the preferred phos- phate substitute in powder detergents and detergent producers are taking advantage of zeolites' other beneficial functions (effective in removing manganese and iron ions that can stain fabrics; very cost-effective package when coupled with sili- cates).6 Although zeolite Na-A is known to be very effective as an insoluble ion-exchange builder for the removal of Ca2+, its removal efficiency for magnesium is particularly at ambient temperature owing to the low rate of e~change.~ Zeolite X exchanges magnesium rapidly; however, it exhibits a lower total ion-exchange capacity. Mixtures of zeolites A and X are claimed to have synergistic effects in the ion exchange of calcium and magnesium ions in hard water.* The objectives of this work were as follows: (a) to study the selectivity of Ca and Mg ion removal by using A, X and Y zeolites at temperatures ranging from 25 to 65"C, and to measure the respective initial rates of ion exchange; and (b) to study the ion-exchange properties of X and Y zeolites from which some silicon sites have been selectively removed.' Experimental Preparation of Hard Water 0.588 g of CaC12.2H20 and 0.493 g of MgS04.7H20 were dissolved in 2 dm3 of distilled water. The ion concentrations of the resultant hard water were Ca2+ 80.2 ppm and Mg2+ 24.2 ppm. Preparation of the Zeolite Materials Parent zeolites in powder form, Na-A, Na-X and Na-Y, were purchased from U.O.P. Na-X( Mod) and Na-Y (Mod) were prepared by selective removal of Si from the corresponding zeolites, using sodium carbonate (SC) in aqueous solution.' Following SC treatment and washing with distilled water, they were dried overnight at 120 "C. Zeolite Characterization Characterization techniques included the determination of (i) the chemical composition by atomic absorption spec- trometry; (ii) the structure and the degree of crystallinity by X-ray powder diffraction; (iii) the BET and Langmuir surface areas by adsorption of nitrogen using the Micromeretics Model ASAP 2000; (iv) the micropore size distribution by adsorption of argon at 87 K, using the Micromeretics Model 2000 M and the data interpretation method of Horvath-Kawazoe (HK)." In the latter case, bv using parent zeolites of known structures, we have determined that suitable values for the interaction parameter (11') to be used to calculate the micropores of zeolite materials are 4.60 x erg cm4 for the Y-type zeolites and 5.20 x erg cm4 for the X-type zeolite. The IP value determined by Horvath-Kazawoe for microporous carbons is 5.89 x erg cm4 (ref. 10) and that proposed by Micromeretics Corp. for zeolites is 3.19 x erg cm4." It is worth noting that the new values of the interaction param- eter give more accurate results than the empirical correction factor that we used to obtain the average values of thc zeolite pores in our previous work.g Moreover, for the measurements of the BET and Langmuir surface areas and the micropore size, an outgassing temperature of 150 "C was used in order not to favour any advanced 'healing' process as observed in ref. 9. Ion-exchange Tests Prior to ion-exchange testing, the zeolites were dehydrated overnight at 120 "C. The ion-exchange tests were performed in the apparatus as shown in Fig. 1. With such an experimental set-up, we wanted to reproduce the working conditions for FcEl Fig. 1 Experimental set-up for the Ca and Mg ion removal from hard water using zeolite powders. R = distilled water rmeservoir; P = peristatic pump; FC = fraction collector; TT = test-tubes; F =ion- exchange flask; WB = water bath; and HP = hot-plate Downloaded by Syracuse University on 14/05/2013 09:17:08. Published on 01 January 1994 on http://pubs.rsc.org | doi:10.1039/JM9940401143 View Article Online / Journal Homepage / Table of Contents for this issue
Transcript
J. MATER. CHEM., 1994, 4(7), 1143-1147 1143
Modified Zeolites for the Removal of Calcium and Magnesium from Hard Water
Raymond Le Van Mao,* Ngoc Thanh Vu, Shuyong Xiao and Arlene Ramsaran Catalysis Laboratory and Laboratories for Inorganic Materials, Department of Chemistry and Biochemistry, Concordia University, 7455 de Maisonneuve West, Montreal, Quebec, Canada H3G I M8
The cation-exchange capacity (cec) and the pore size of zeolite materials are the two key factors which influence the removal of calcium and magnesium in aqueous solution. Na-X and Na-Y zeolites have been modified by selective extraction of Si from the zeolite framework using sodium carbonate. At ambient temperature, the modified Na-X zeolite was more efficient than the Na-A commercially used in detergency, for both Ca and Mg ions.
Polyphosphates are very efficient as sequestering detergent builders.' The role of phosphates is to remove calcium and magnesium from wash waters in order to favour the removing, solubilizing, emulsifying and suspending action of surfactants. However, the massive discharge of phosphates, although relatively safe for humans and non-toxic to aquatic life,2 results in the phenomena of eutrophication observed in lakes and streams. Because of this harmful effect to the environment, phosphate-free detergents have recently been developed. This has been achieved by using alternative builders including
Today, zeolites appear to be the preferred phos- phate substitute in powder detergents and detergent producers are taking advantage of zeolites' other beneficial functions (effective in removing manganese and iron ions that can stain fabrics; very cost-effective package when coupled with sili- cates).6 Although zeolite Na-A is known to be very effective as an insoluble ion-exchange builder for the removal of Ca2+, its removal efficiency for magnesium is particularly at ambient temperature owing to the low rate of e~change .~ Zeolite X exchanges magnesium rapidly; however, it exhibits a lower total ion-exchange capacity. Mixtures of zeolites A and X are claimed to have synergistic effects in the ion exchange of calcium and magnesium ions in hard water.*
The objectives of this work were as follows: (a) to study the selectivity of Ca and Mg ion removal by using A, X and Y zeolites at temperatures ranging from 25 to 65"C, and to measure the respective initial rates of ion exchange; and (b) to study the ion-exchange properties of X and Y zeolites from which some silicon sites have been selectively removed.'
Experimental Preparation of Hard Water
0.588 g of CaC12.2H20 and 0.493 g of MgS04.7H20 were dissolved in 2 dm3 of distilled water. The ion concentrations of the resultant hard water were Ca2+ 80.2 ppm and Mg2+ 24.2 ppm.
Preparation of the Zeolite Materials
Parent zeolites in powder form, Na-A, Na-X and Na-Y, were purchased from U.O.P.
Na-X( Mod) and Na-Y (Mod) were prepared by selective removal of Si from the corresponding zeolites, using sodium carbonate (SC) in aqueous solution.' Following SC treatment and washing with distilled water, they were dried overnight at 120 "C.
Zeolite Characterization
Characterization techniques included the determination of (i) the chemical composition by atomic absorption spec- trometry; (ii) the structure and the degree of crystallinity by X-ray powder diffraction; (iii) the BET and Langmuir surface areas by adsorption of nitrogen using the Micromeretics Model ASAP 2000; (iv) the micropore size distribution by adsorption of argon at 87 K, using the Micromeretics Model 2000 M and the data interpretation method of Horvath-Kawazoe (HK)." In the latter case, bv using parent zeolites of known structures, we have determined that suitable values for the interaction parameter (11') to be used to calculate the micropores of zeolite materials are 4.60 x erg cm4 for the Y-type zeolites and 5.20 x erg cm4 for the X-type zeolite. The IP value determined by Horvath-Kazawoe for microporous carbons is 5.89 x erg cm4 (ref. 10) and that proposed by Micromeretics Corp. for zeolites is 3.19 x erg cm4." It is worth noting that the new values of the interaction param- eter give more accurate results than the empirical correction factor that we used to obtain the average values of thc zeolite pores in our previous work.g Moreover, for the measurements of the BET and Langmuir surface areas and the micropore size, an outgassing temperature of 150 "C was used in order not to favour any advanced 'healing' process as observed in ref. 9.
Ion-exchange Tests
Prior to ion-exchange testing, the zeolites were dehydrated overnight at 120 "C. The ion-exchange tests were performed in the apparatus as shown in Fig. 1. With such an experimental set-up, we wanted to reproduce the working conditions for
FcEl Fig. 1 Experimental set-up for the Ca and Mg ion removal from hard water using zeolite powders. R = distilled water rmeservoir; P = peristatic pump; FC = fraction collector; TT = test-tubes; F =ion- exchange flask; WB = water bath; and HP = hot-plate
Dow
nloa
ded
by S
yrac
use
Uni
vers
ity o
n 14
/05/
2013
09:
17:0
8.
Publ
ishe
d on
01
Janu
ary
1994
on
http
://pu
bs.r
sc.o
rg |
doi:1
0.10
39/J
M99
4040
1143
View Article Online / Journal Homepage / Table of Contents for this issue
detergent powders in washing machines. These conditions are quite different from those of the usual ion-exchange experi- ments carried out on chromatography-type columns packed with zeolite-based pellets or beads.
The hard water (700 ml) was placed in a flask containing a strong magnetic stirring bar. The flask was placed in a water bath which was heated at a constant temperature by a hot-plate equipped with a magnetic stirrer which ensured a strong and constant stirring action throughout the experiment. Homogenization of the exchange medium was one of the key factors for the data reproducibility. When the solution reached the desired temperature, 1.0 g of the zeolite sample was rapidly poured into the flask. At that moment, the time was taken as zero. Every minute, a solution sample (2 ml) was taken from the flask using a fraction collector, while a reservoir kept adding water to compensate for the volume lost. The experi- ment was stopped after 16 min, which corresponds approxi- mately to the residence time of the detergent in the washing machine.
The concentrations of Ca and Mg ions remaining in each sample were determined by atomic absorption and finally given in ppm.
Data Computation
The results reported in this paper are average values of at least two results exhibiting differences not exceeding 3 YO.
The concentrations, CCa, of Ca2+ and CMgl of Mg2+ were plotted against time. A polynomial function (limited to degree 2, for physically meaningful purposes) was used for curve fitting:
c,, = y = a + bt + C t 2
The calcium ion removal was expressed as:
IRc,(%)= [CL, - C;,]/CLa x 100
where CL, and C;, were the initial and final concentrations (expressed in ppm) of Ca2+. C;, was calculated from the curve by replacing t in the previous equation by 16 (min). Magnesium ion removal (IRMg) was determined in the same way.
The total ion removal was expressed as:
IR,,, (equiv. based YO) = [ 1 -(C;, + CLg)/(CLa + C&J] x 100
where all the concentrations are expressed in ion equivalents. The rate of calcium removal (in ppm min-l) is equal to the
first derivative of the function y:
rCa = dy/dt = b + 2ct
The initial rate of calcium removal was determined by taking t=O. So, [r,,]'=b (in ppm min-I). The rate and the initial
rate of magnesium removal (rMg and [rMg]') were determined in the same way.
Results and Discussion From the data of Table 1 and Fig. 2-4, the following can be seen. (i) The total ion removal increases with increasing temperature. (ii) At ambient temperature, the total ion removal depends strongly on the Si : A1 ratio, and thus, on the cec of the zeolite, resulting in the following sequence of effectiveness: Na-A > Na-X > Na-Y. However, there was a strong improve- ment in the IR,,, for the Na-X zeolite at higher temperature. In particular, at 6 5 T , the following sequence was obtained: Na-A = Na-X > Na-Y. (iii) At ambient temperature, the Ca ion removal exhibited the following sequence: Na-A>>Na-X>Na-Y, which is more or less in agreement with the sequence of the cec values. However, the sequence for Mg ion removal was as follows: Na-A<< Na-X < Na-Y. This might be related to the effects of zeolite pore size as shown in the following section. Nevertheless, the Na-A zeolite became more effective in the elimination of the Mg ions at higher temperatures, mostly at 65 "C. (iv) The values deter- mined for the initial rate confirmed the trends previously observed (Table 2). It is interesting to note that (a) at ambient temperature, the initial rates of Ca and Mg removal deter- mined for the large pore-sized zeolites X and Y were much
0 4 8 12 16 tlmin
Fig. 2 Graphs of the concentrations of Ca (a) and Mg (b) versus the time of contact with the solution, t. Data obtained with the parent Na-A zeolite. 0, T = 2 5 T ; V, T=45"C; 0 , T=65'C
Table 1 Ion-removal and some physicochemical properties of the Na-A, Na-X and Na-Y zeolites studied
Na-A Na-X Na---Y
Si : A1 ceca pore size/nmb BET/m2 g-' Langmuir/m2 g -
"cec.: cation exchange capacity, expressed in mequiv. g-', assuming that all A1 sites correspond to cation exchange sites. bAccording to ref. 12. 'Measured on the Na-A zeolite.
Fig.3 Graphs of the concentrations of Ca (a) and Mg (b) versus the time of contact with the solution, t. Data obtained with the parent Na-X zeolite (same footnotes as in Fig. 2)
0 4 a 12 16
60 I 1
20
tlmin
Fig. 4 Graphs of the concentrations of Ca (a) and Mg (b) versus the time of contact with the solution, t. Data obtained with the parent Na-Y zeolite (same footnotes as in Fig. 2)
higher than those determined for the small pore-sized zeolite Na-A; (b) the initial rate for the removal of Mg ions did not change very significantly with temperature when the Na-X and the Na-Y zeolites were used, there was an important variation for such a parameter when the Na-A zeolite was used. This suggests that the ion-exchange rate with Mg ions was strongly dependent on the diffusion rate of these ions through the narrow pores of the Na-A zeolite (Table 1). Such
diffusion limitations for the Na-A zeolite, at least at ambient temperature, were due to the strong solvation tendency of Mg ions, which could form bulky complex ions with water mol- e c u l e ~ . ~ ~ However, at higher temperatures, there was ;L signifi- cant improvement in the diffusion rates of the Mg2+ (and to a less extent, Ca2+) through the pore system of Na-A, which finally resulted in an improved performance for ion removal. On the other hand, since the Na-X and Na-Y zeolites had the same open structure but different cecs, they exhibited significantly different values for the total-ion removal (Table 1).
Fig. 5 shows the concentrations of Ca and Mg ions remain- ing in solution versus the time of contact, t, using the modified Na-X zeolite at ambient temperature. At first, it appeared that the Na-X(Mod) had almost the same Ca ion removal capacity as the Na-A zeolite (Tables 1 and 3). Howeker, with the new zeolite, the Ca elimination was much faster at the beginning of the process [at ambient temperature and t= 4 min, C,, = 30 ppm and 45 ppm for the Na-X(Mod) and the Na-A zeolites, respectively, see Fig. 2(a) and 51.
With the modified zeolites, the total-ion removal was sig- nificantly increased, owing to an important increase of the cec (Tables 1 and 3). In particular, the Na-X(Mod) behaved even better than the Na-A zeolite in terms of IR,,, and IRMg (almost the same IR,,). This was due to the fact that the Na-X(Mod) had the same cec as the Na-A zeolite but a larger pore size. This conclusion is in agreement with that of Kuhl and Sherry13 who observed that their low-silica type X zeolite had the same Ca-removing property as the Na-A zeolite and a much higher efficiency for Mg elimination. Moreover, our modified Na-X zeolite exhibited init la1 rates of removal for both Ca and Mg which were significantly higher than those of the Na-A zeolite at ambient temperatures (Tables 2 and 3). Therefore, the present results provided (indirect) experimental evidence that all the A1 species of the Na-X( Mod) zeolite had the tetrahedral configuration and thus generated effective ion-exchanging sites, as already shown by means of 27Al nuclear magnetic resonance spectr~scopy.~ On the other hand, the modified Na-Y zeolite exhibited the same total ion removal capacity as the Na-X zeolite because of an almost equal cec (Tables 1 and 3).
It is also worth noting that the slight decrease in the IRMg was due to the slight pore narrowing of the X and 7.' zeolite
0 4 8 12 tlmin
Fig. 5 Graphs of the concentrations of Ca (Cca) and Mg (Ck,,) versus the time of contact with the solution, t. Data obtained with the modified Na-X zeolite at 25 "C
aAverage value, as determined by using the HK method." 'Activated at 550 "C overnight prior to the micropore size determination.
pores upon treatment with sodium carbonate (Tables 1 and 3). Particularly interesting was the fairly good symmetry and sharpness of the pore size distribution peaks of the parent zeolites and the modified zeolites (Fig. 6 and 7) in spite of a noticeable pore narrowing. This suggests that the new mate- rials had pore systems which were as homogeneous as the original ones.
With the new and more accurate method for the determi- nation of micropore size, it was possible to show the following. (i) There was a significant change in the shape of the peaks of pore-size distribution for the Na-X( Mod) and Na-Y (Mod) zeolites upon activation at higher temperature (up to 550"C, see Fig. 7), which did not occur with the corresponding parent zeolites. This suggests some rearrangement within the frame- works of the modified zeolites. However, the average values of these micropore sizes did not vary significantly (Table 3). (ii) The Na-X(Mod) and Na-Y(Mod) had the same basic FAU structure (through X-ray powder diffractiong). However, they exhibited average pore sizes which were smaller than the 0.74nm value for the parent Na-X and Na-Y zeolites. The decrease in such values depended on the extent of the Si removal. l4
Conclusions
We have shown that: (i) at ambient temperature, the total ion removal increased with decreasing Si : A1 ratio of the zeolite; (ii) the rate of Mg removal of the Na-A zeolite which was low at ambient temperature because of diffusion problems within the narrow pores, improved substantially at higher temperature; this indicates that the Na-A zeolite was very
r
DIA
Fig. 6 Micropore size distribution (pore volume, V versus pore diameter, D) of the parent Na-X (a) and the modified Na-X (b) samples
4 6 8 10
4 6 8 10
. . 4 6 8 10
DIA
Fig. 7 Micropore size distribution uersus pore diameter (see Fig. 5 ) : (a) parent Na-Y; (b) Na-Y(Mod), dried at 150 'C: (c) Na Y(Mod), activated at 550 "C overnight
effective for water softening under warm conditions of wash- ing; (iii) sodium carbonate treatment of Na-X and Na-Y zeolites enhanced significantly the overall ion-exchange properties, and mostly the Ca ion removal. Particularly inter- esting was the case of Na-X(Mod) which appeared to be more effective, at ambient temperature, than the Na-A zeolite.
The development of these new materials which can be used as detergent builders, is in line with the continuous search for lower-temperature performing products in the detergent industry.
We would like to thank the NSERC of Canada and Quebec's Action Structurante Program for their financial support.
References
1
2
S. E. Manahan, in Environmental Chemistry, Lewis, Michigan, 1991, p. 59. S. J. Ainsworth, Chem. Eng. News, Jan. 20, 1992, 27.
