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Synthesis and characterization of a mesoporous hydrous zirconium oxide used for arsenic removal from drinking water
Materials Research Bulletin 45 (2010) 142–148
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Synthesis and characterization of a mesoporous hydrous zirconium oxide used for arsenic removal from drinking water Anatoly Bortun a, Mila Bortun a, James Pardini a, Sergei A. Khainakov b, Jose´ R. Garcı´a b,* a b
MELChemicals Inc, 500 Barbertown Point Breeze Road, Flemington, NJ 08822, USA Departamento de Quı´mica Orga´nica e Inorga´nica, Universidad de Oviedo, 33006 Oviedo, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 June 2009 Received in revised form 23 September 2009 Accepted 25 September 2009 Available online 4 October 2009
Powder (20–50 mm) mesoporous hydrous zirconium oxide was prepared from a zirconium salt granular precursor. The effect of some process parameters on product morphology, porous structure and adsorption performance has been studied. The use of hydrous zirconium oxide for selective arsenic removal from drinking water is discussed. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds A. Oxides B. Chemical synthesis D. Phase equilibria
1. Introduction Inorganic ion exchangers have been studied extensively in the past decade [1–3]. Interest in such materials comes mainly from the fact of their unique selectivity to certain ions and superior thermal and radiation stability in comparison with ion-exchange resins. The majority of inorganic adsorbents do not lose adsorption properties after thermal treatment at 200–300 8C and some crystalline materials even after 600–700 8C. Moreover, in many cases thermal treatment of inorganic ion exchangers is a powerful tool for regulation of their properties, especially selectivity to certain species by changing the type of functional groups, porosity and even crystalline phase. Organic ion-exchange resins typically cannot tolerate or perform at temperatures higher than 80–150 8C. However, commercial applications for inorganic adsorbents are still rare. The main reason for this is technical difficulties in their manufacturing. Typically inorganic adsorbents are synthesized as fine powders that cannot be used in column type applications due to high pressure drop [4,5]. Different strategies have been pursued to resolve this problem and produce granular materials. Among them are extrusion and sol–gel techniques, incorporation of fine powders into organic polymers, etc. None of these approaches is universal and each of them has certain drawbacks. The one drawback common for all of them is inferior kinetics of adsorption in comparison with ion-exchange resins. Poor kinetics of exchange
* Corresponding author. E-mail address: [email protected] (J.R. Garcı´a). 0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2009.09.030
relates to the rigid, predominantly micro-porous structure of products that prevents their swelling. According to Liu et al. [6] mesoporous materials have improved kinetics of exchange. However, use of solid state reactions or organic templates/ surfactants is often not viable for large scale manufacturing. Another approach for solving these problems has been used at MELChemicals—the technique of making mesoporous inorganic adsorbents in the form of micron size powders with narrow particle size distribution has been developed [7]. Such an approach allows use of materials in thin bed type applications and enables kinetics of adsorption superior to any resin (residence time 3–10 s). Currently MELChemicals produces two grades of hydrous zirconium oxides (HZO’s) with narrow 20–30 mm particle size distribution on commercial scale: microporous 301-type and mesoporous 302-type oxide. Both materials are designed for different separation applications. The results of physical and chemical characterization of 301-HZO and 302-HZO type adsorbents, with main stress on the effect of temperature on particle size, porosity change and efficiency of arsenic sorption from aqueous solutions are summarized in this paper. 2. Experimental 2.1. Synthesis A zirconium salt granular precursor, with narrow particle size distribution (25–35 mm), has been used for making hydrous zirconium oxides. Synthesis of 301-HZO includes the preparation of aqueous slurry of the zirconium salt and its conversion into
A. Bortun et al. / Materials Research Bulletin 45 (2010) 142–148
hydroxide by slow addition of an aqueous base solution until equilibrium pH exceeds 10. The process is carried out at ambient temperature. Preparation of 302-HZO is similar to that of 301-HZO, but it includes an additional stage of slurry treatment with alkali at 90–100 8C for 1–3 h. After the conversion process is finished zirconium hydroxides are washed with deionized water to remove all anions and cations and then dried in air to targeted residual water content (40 wt% for 301-HZO and 5 wt% for 302-HZO) [7]. 2.2. Characterization The particle size distribution of zirconium precursor and all zirconium hydroxides has been determined on a Leeds + Northrup Microtrac ASVP instrument. Media packed density has been determined by using standard test method for determination of tap density of powder materials [ASTM B527-93 (2000)e1, standard test method for determination of tap density of metallic powders and compounds]. SEM micrographs were obtained using a JEOL JSM 6100 electron microscope, operating at 20 kV. N2 adsortion/desorption isotherms were measured at 77 K on the TriStar analyzer (Micromeritics Instrument Corporation, USA). Prior to measurement, all samples were outgassed at 300 8C for 2 h. The specific surface area was calculated by applying the BET equation. By analysis of the adsorption curve using the BJH calculation method, the pore size distribution was also obtained. For the IR measurements, 50 mg of HZO were pressed into a selfsupporting wafer, placed in the in situ IR cell and spectra were recorded on a Nicolet Magna II FTIR spectrometer by averaging 32 spectra at a resolution of 2 cm1. TPD-NH3 and TPD-CO2 were carried out using U-shape quartz reactor on Altamira AMI-200 instrument. A 0.15 g sample was pre-treated in a flow of helium at 500 8C for 1 h. After cooling to 110 8C, the sample was saturated with NH3 or CO2 and then purged for 1 h with helium to remove physically adsorbed species. After that the sample was heated to 650–700 8C in a flow of helium at constant rate 108 min1. NH3 or CO2 desorption was monitored with the use of TCD or mass spectroscopy. 2.3. Adsorption properties The following conditions have been used for column experiments. 1.00 g of 301- or 302-HZO has been put into a glass column with 8 mm inner diameter. Two types of challenge solutions have been used. Phosphate-containing challenge water had composition: 0.001 M NaH2PO4 and 0.01 M Na2SO4. NSF53-challenge water had composition: [Ca2+] = 40 mg/L; [Mg2+] = 12 mg/L; [SO42] = 50 mg/L; [N-NO3] = 2 mg/L; [F] = 1 mg/L; [SiO2] = 20 mg/L; [As(V)] = 0.05 mg/L and pH 8.5 [NSF/ ANSI 53–2002, Addendum 1.0–2002]. Challenge solutions have been passed through the adsorbent bed with a constant flow rate of 360 mL/h. Water samples have been collected every 100–200 bed volumes. Phosphate and arsenic content in them have been analyzed using ICP or AAS with graphite furnace, respectively. Arsenic adsorption capacity has been found from corresponding breakthrough curves by surface integration method. 3. Results and discussion The reaction of zirconium salt precursor with alkali (NaOH, pH > 10) in the process of making 301- and 302-HZO results in gradual substitution of salt anions with hydroxyl groups and, eventually, in formation of a zirconium hydroxide phase. Moreover, at relatively high pH (>10) a partial (3–10%) conversion of Zr– OH groups into Zr–ONa+ takes place also. The additional step of alkaline digestion of hydrous zirconium oxide at 90–100 8C in the case of 302-HZO significantly facilitates ageing processes by
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increasing the rate of condensation and olation reactions on the surface and in the bulk of oxide. Considering that such ageing takes place when some of the reactive functional Zr–OH groups are ‘‘protected’’ due to conversion into Zr–ONa form so they can not participate in the condensation reactions, this creates a certain effect on both porosity and surface chemistry of treated products. In the case of surface chemistry a certain change in functional group distribution takes place for 302-HZO in comparison with non-treated zirconium hydroxide. Three types of hydroxyl groups can be present on the surface of HZO [8–10]:
Mono-bridged groups are considered to be the most protruded from the surface and least sterically hindered, while di-bridged groups are located almost in the plane of the surface and tribridged to some extent below the surface plane. According to Pauling’s law, and taking into consideration that zirconium charge is 4+ and zirconium coordination number is 8, these hydroxyl groups should bear different charges. Namely, the mono-bridged Zr–OH group is negatively charged (d1/2) while the di-bridged is neutral and the tri-bridged is positively charged carrying d1/2+. The oxygen atom in the BBZr3–OH group has a saturated environment and has no possibility for a complexing reaction with ions. The only reaction with the BBZr3–OH group is a proton release. On the other hand deprotonation of the mono-bridged BBZr–OH group is strongly suppressed due to the high effective negative charge on the oxygen atom. As a result, one can expect that predominantly di-bridged Zr–OH groups should undergo partial conversion into sodium form at pH 11.5. This suggests that the two other types of functional groups and remaining non-substituted di-bridged groups are free for participation in condensation reactions. Considering steric factors, the main contribution in making new covalent bonds, connecting particles together, should be the mono-bridged groups. This is in good agreement with IR data for both oxides (Fig. 1). It is clearly seen that hydroxyl coverage of alkaline digested material consists predominantly of tri-bridged groups (3600–3700 cm1) with a lesser amount of di-bridged (3700–3750 cm1) and terminal groups (3750–3800 cm1). In the case of non-treated 301-HZO, the surface of zirconium hydroxide evacuated at 120 8C is predominantly covered with tri-bridged groups and depleted of di-bridged and terminal groups. The difference in surface chemistry has been confirmed also by checking acid-base properties of zirconium hydroxides with the use of temperature programmed desorption of NH3 and CO2 (Figs. 2 and 3). As seen, normalized TPD-NH3 curves for 301-HZO and 302-HZO have a similar shape. Release of adsorbed ammonia begins at 150– 160 8C, it reaches a maximum at 210–250 8C with following gradual decrease until the process ends at 500–600 8C (Fig. 2a). Deconvolution of TPD-NH3 peaks (Fig. 3) has shown that both materials have two types of ‘‘weak’’ acid adsorption sites (200– 300 8C) and three types of ‘‘strong’’ acid sites (300–600 8C) with surprisingly similar peak surface area ratios 2:1, respectively. This suggests that both materials have similar types of functional hydroxyl groups responsible for their acidic properties. The main difference between samples is that the total acidity (amount of desorbed NH3) of 302-HZO (0.276 mmol NH3/g) is 3.3 times higher than that for 301-HZO (0.084 mmol NH3/g). This could be a result of much higher thermal stability of alkaline treated material leading to a higher population of Zr–OH groups on the surface. TPD-CO2 profiles for zirconium hydroxides are shown in Fig. 2b. The strongest desorption peak for both hydroxides is in the temperature range from 150 to 450–470 8C. This broad peak can be de-convoluted into three peaks representing weak basic
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Fig. 1. IR spectra of: (a) 302-HZO sample evacuated at 120 or 250 8C, and (b) 301HZO sample evacuated at 120 8C.
adsorption sites (<300 8C) and medium strong basic sites (300– 450 8C). The second and third desorption peaks of smaller intensities are at 520 and 600 8C for 301-HZO and at 480 and 590 8C for 302-HZO, respectively. These peaks are related to CO2 bound on strong basic adsorption sites and, possibly, due to release of some remaining chemisorbed CO2 not completely removed from media during pre-treatment at 500 8C. Total amount of basic sites on 302-HZO (0.292 mmol/g) is significantly higher than on 301-HZO (0.167 mmol/g), which is in agreement with higher thermal stability of 302-HZO. Interestingly, the ratio between weak and strong basic sites (2:1) is very similar to the ratio between weak and strong acid sites. This suggests that the same functional zirconium hydroxyl groups depending on conditions can be responsible for acidic or basic properties. Mild alkaline treatment at elevated temperature does not significantly change the product’s morphology in comparison with precursor or non-treated 301-HZO (Fig. 4). Hydrous zirconium oxide retains a ‘‘raspberry-type’’ structure consisting of dozens of 0.5–3 mm spheres fused together to form larger 20– 40 mm aggregates. Such agglomerates are rigid and possess surprisingly high mechanical resistance to attrition. At the same time precursor conversion into hydroxide form for both 301-HZO and 302-HZO results in 10–15% shrinkage of particle’s size (Fig. 5). Narrow particle size distribution and lack of fines (particles less than 5–10 mm) allows use of hydrous zirconium oxides of 301- and 302-type in thin bed applications (layer depths less than 5–10 cm) without creating significant pressure drop
Fig. 2. TPD-NH3 (a) and TPD-CO2 (b) profiles of 301-HZO and 302 HZO samples.
(pressure drop is less than 1.0–1.5 kg/cm2) and ensuring quick kinetics of exchange. According to powder XRD data, conversion of zirconium salt into both 301- and 302-HZO materials does not result in the product’s crystallization and both materials remain amorphous. An amorphous structure is favorable for maintaining the integrity of hydrous zirconium oxide agglomerates, but as a rule, amorphous products have lower thermal stability than crystalline ones and, what is the most important, inferior adsorption properties with respect to drying conditions. The majority of amorphous hydrous zirconium oxide based adsorbents are microporous and they are most active when freshly prepared [11]. Typically they lose significantly ion-exchange capacity during storage and, especially, after drying at elevated temperature. The most probable reason for this could be a collapse of microporous structure during drying which makes adsorption sites in the bulk of material not accessible for ion exchange. For example, freshly prepared amorphous 301HZO with 43% of physically bound water shows high affinity towards phosphate ion and is able to remove it from over 3000 bed volumes of solution, which gives capacity before breakthrough 100 mgP/g (Fig. 6). However, drying of the media results in a significant drop of adsorption capacity to 2000 bed volumes (or 60 mgP/g) at 30%—level of water and to only 100 bed volumes (or 3 mgP/g) at 20% water content. On contrary, adsorption capacity of 302-HZO is not sensitive to residual water content in the structure and even increases with water loss. 302-HZO shows the best performance when completely dry (purifies 4000 bed
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Fig. 4. SEM images of: (a) zirconium salt precursor and (b) 302-HZO sample (scale 10 mm). Fig. 3. De-convolution of TPD-NH3 profiles of 301-HZO (a) and 302-HZO (b) samples.
volumes of challenge solution, which gives capacity 120 mgP/g). Such behavior of 302-HZO is similar to what can be expected from crystalline zirconium hydroxides that are known as thermally stable adsorbents. Considering this it was interesting to check the transformations that take place within the media during thermal treatment, namely, how calcination effects particle size, porosity and adsorption performance. As seen in Fig. 7, particles of both tested hydrous zirconium oxides undergo certain shrinking with increase in calcination temperature. This suggests that zirconium hydroxide agglomerates are rather loosely packed and have some void space inside. The shrinkage from 100 to 550 8C totals 15% for 301-HZO and 20% for 302-HZO. In both cases almost 90% of the decrease in a particle’s size takes place in the temperature range 100–400 8C. In this temperature range both zirconium hydroxides are amorphous. At 430–440 8C zirconium hydroxides undergo crystallization and after 400–450 8C particles size shrinking practically stops. Change in particle size affects the media packed density (PD), which is one of the most important parameters for designing adsorbers/ cartridges for specific applications. Zirconium hydroxide packed densities as a function of calcination temperature are shown in Fig. 8. First, it should be mentioned that PD of the 301-HZO is substantially higher than that of 302-HZO. This can be related to differences in their pore structure that will be shown below.
However, in both cases density/temperature curves have similar shape and there is a trend of density increase with increase in calcination temperature. A gradual increase in packed density takes place in the temperature range from 100 to 300 8C, which is followed by a sharp increase in PD at 300–400 8C. From 400 to
Fig. 5. Particle size distribution of zirconium salt precursor and 302-HZO sample.
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A. Bortun et al. / Materials Research Bulletin 45 (2010) 142–148
Fig. 6. Effect of drying (in parentheses, residual water content) on phosphate removal by reference 301- and 302-HZO samples.
500 8C packed density remains almost constant with another sharp increase in PD after 500 8C. The shapes of density/temperature curves correspond well to changes in hydrous zirconium oxides particle shrinkage with the only exception at 500–550 8C sharp increase in packed density is not accompanied by particles shrinking. Total increase in packed density from 100 to 550 8C
Fig. 8. Packed density as a function of calcination temperature for 301- and 302HZO samples.
for 301-HZO is 85% and it becomes even more significant for 302HZO (95%). Comparative data on the porous structure of 301- and 302-HZO as a function of calcination temperature are summarized in Table 1, and typical N2/77 K adsorption/desorption isotherms for 302-HZO are presented in Fig. 9. As seen 302-HZO has significantly higher surface area, pore volume and average pore diameter than the reference material when it is dried at 100 8C and retain them on a much higher level when calcined (up to 500 8C). This indicates that 302-HZO is much more thermally stable than 301-HZO and that this is the result of introducing an additional stage of mild alkaline digestion during preparation. Similar improvement of hydrous zirconium oxide thermal stability after alkaline digestion has been reported earlier by Chuah et al. [12]. However, it should be noted, that in the Chuah case, the increase in thermal stability was the result of product crystallization [13] and significant doping (up to 5%) with silica due to silicate leaching from glass reactor [14]. 302HZO is amorphous and the silica contamination level is below 50 ppm. Surface area (SA) for 302-HZO is high (300 m2/g) and remains almost unchanged up to 300 8C. The sharp decrease of SA in the temperature range 300–400 8C coincides with particle shrinkage and an increase in packed density. Surface area of 302 calcined at 500 8C is almost 4.5 times lower than that of material dried at 100 8C. Surprisingly, pore volume of material does not change significantly with increase in drying temperature (pore volume even increases by 10–20% with temperature increase to 300 8C and only begins to decrease after 300 8C). Surface area for 301-HZO is lower than that of 302-HZO and gradually decreases (almost by 7 times) with increase in drying temperature. A similar trend has been found for changes in pore volume of 301-HZO. Table 1 Textural data (SA, specific surface area; PV, pore volume; PD, average pore diameter) for 301- and 302-HZO samples, stabilized in air at high temperature. Temperature (8C)
Fig. 7. Particle size as a function of calcination temperature for: (a) 301-HZO and (b) 302-HZO samples.
100 200 300 350 400 450 500
301-HZO
302-HZO
SA (m2/g)
PV (cm3/g)
PD (nm)
SA (m2/g)
PV (cm3/g)
PD (nm)
266 185 135 97 65 58 37
0.095 0.100 0.085 0.080 0.055 0.050 0.035
2.1 2.2 2.7 3.6 4.3 5.1 6.1
306 302 292 225 123 116 69
0.200 0.224 0.246 0.205 0.191 0.184 0.174
2.6 2.8 3.4 4.2 6.1 7.6 9.5
A. Bortun et al. / Materials Research Bulletin 45 (2010) 142–148
Fig. 9. N2/77 K adsorption–desorption isotherms for 302-HZO sample as a function of calcination temperature.
Hydrous zirconium oxide capacities for arsenic(V) removal from NSF-53-type challenge water as a function of drying temperatures are shown in Figs. 10 and 11. In a good agreement with the above mentioned statement on strong dependency of 301-HZO on residual water content, this adsorbent has a low
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Fig. 11. Arsenic sorption capacities of 301- and 302-HZO samples as a function of drying temperature. Capacities are presented as amount of arsenic adsorbed per unit of media surface area.
capacity for arsenic (10 mg As/cm3 or 7 mg As/g) after drying at 100 8C. Capacity changes not significantly with increase in calcination temperature from 100 to 400 8C, while some increase in arsenic uptake has been observed in the temperature range 400– 550 8C. It is interesting that 301-type HZO arsenic capacity presented as amount of As(V) taken per media volume becomes even higher after calcination at 450–500 8C in comparison with product dried at 100 8C. On the contrary, 302-HZO, which is not sensitive to residual water content, shows much higher adsorption capacity for arsenic. Material dried at 100 8C adsorbs 150– 160 mg As/g, which is 15–25 times more than 301-HZO. Increase in calcination temperature up to 350–400 8C results in gradual decrease (up to 1.4–2.4 times) of arsenic uptake, which is seen more clear in the case when adsorption is presented as As uptake per weight of media. Considering that in this temperature range surface area (and pore volume) of 302-HZO remain almost constant, these changes in capacity cannot be related to pore collapse resulting in poor accessibility of adsorption sites in the bulk. This effect rather relates to decrease in total amount of functional Zr–OH groups due to continuous condensation reactions (Zr–OH + HO–Zr ! Zr–O–Zr + H2O). The decrease of 302-HZO adsorption capacity on As(V) after drying at 350–400 8C, presented as arsenic uptake per unit of media surface (Fig. 11), strongly supports this suggestion. Interestingly, calcination of 302-type oxide at higher temperatures (>350 8C) results in an increase in media arsenic capacity similar to what has been observed for 301-HZO. Adsorption capacity of 302-HZO after drying at 400 8C calculated per media volume is comparable to that of material dried at 100 8C, whereas capacity per unit of surface area (Fig. 11) becomes significantly higher (almost in 3 times) than that of non-calcined product. Taking into account the significant decrease of 302-HZO surface area and the continuous hydroxyl group condensation that takes place in this temperature range, the observed phenomena can be attributed to changes in types of remaining Zr–OH groups on the surface, namely, an increase in population of the most reactive hydroxyl groups with high affinity towards arsenic. 4. Conclusions
Fig. 10. Arsenic sorption capacities of HZO samples as a function of drying temperature: (a) 301-HZO and (b) 302-HZO samples. Capacities are presented as amount of arsenic adsorbed per weight or per volume of material.
The hydrous zirconium oxide presented in this paper is a mesoporous material that differs significantly from other commercially available zirconium hydroxide products: it is more basic, exhibits higher affinity towards arsenic(V), show higher kinetics of
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ion-exchange and improved thermal stability that allows retention of adsorption properties even after treatment at 500–550 8C. Acknowledgments We express our thanks for financial support from the Spanish Ministerio de Educacio´n y Ciencia (MAT2006-01997 and Factorı´a de Cristalizacio´n–Consolider Ingenio 2010) and the Gobierno del Principado de Asturias (PCTI 2006-2009). References [1] A. Clearfield, Solvent Extract. Ion Exch. 18 (2000) 655.
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
A.B. Yaroslavtsev, Uspekhi Khimii 66 (1997) 641. N.Z. Misak, Adv. Colloid Interface Sci. 51 (1994) 29. A.I. Bortun, V.V. Strelko, Stud. Surf. Sci. Catal. 80 (1993) 59. I.Z. Zhuravlev, V.A. Kanibolotsky, V.V. Strelko, A.I. Bortun, L.I. Bortun, S.A. Khainakov, A. Clearfield, Solvent Extract. Ion Exch. 17 (1999) 635. H. Liu, X. Sun, C. Yin, C. Hu, J. Hazard. Mater. 151 (2008) 616. A.I. Bortun, C.J. Butler, Hydrous zirconium oxide, hydrous hafnium oxide and method of making same, US Patent 7,252,767 (2007). A.A. Tsyganenko, V.N. Filimonov, J. Mol. Struct. 19 (1973) 579. M. Bensitel, V. Moravek, J. Lamotte, O. Saur, J.C. Lavalley, Spectrochim. Acta 43A (1987) 1487. G. Cerrato, S. Bordiga, S. Barbera, C. Morterra, Surf. Sci. 50 (1997) 50. C.B. Amphlett, Inorganic Ion Exchangers, Elsevier, New York, 1964. G.K. Chuah, S. Jaenicke, S.A. Cheong, K.S. Chan, Appl. Catal. A: Gen. 145 (1996) 267. G.K. Chuah, Catal. Today 49 (1999) 131. S.F. Yin, B.Q. Xu, ChemPhysChem 4 (2003) 277.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Synthesis and characterization of a mesoporous hydrous zirconium oxide used for arsenic removal from drinking water Anatoly Bortun a, Mila Bortun a, James Pardini a, Sergei A. Khainakov b, Jose´ R. Garcı´a b,* a b
MELChemicals Inc, 500 Barbertown Point Breeze Road, Flemington, NJ 08822, USA Departamento de Quı´mica Orga´nica e Inorga´nica, Universidad de Oviedo, 33006 Oviedo, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 June 2009 Received in revised form 23 September 2009 Accepted 25 September 2009 Available online 4 October 2009
Powder (20–50 mm) mesoporous hydrous zirconium oxide was prepared from a zirconium salt granular precursor. The effect of some process parameters on product morphology, porous structure and adsorption performance has been studied. The use of hydrous zirconium oxide for selective arsenic removal from drinking water is discussed. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds A. Oxides B. Chemical synthesis D. Phase equilibria
1. Introduction Inorganic ion exchangers have been studied extensively in the past decade [1–3]. Interest in such materials comes mainly from the fact of their unique selectivity to certain ions and superior thermal and radiation stability in comparison with ion-exchange resins. The majority of inorganic adsorbents do not lose adsorption properties after thermal treatment at 200–300 8C and some crystalline materials even after 600–700 8C. Moreover, in many cases thermal treatment of inorganic ion exchangers is a powerful tool for regulation of their properties, especially selectivity to certain species by changing the type of functional groups, porosity and even crystalline phase. Organic ion-exchange resins typically cannot tolerate or perform at temperatures higher than 80–150 8C. However, commercial applications for inorganic adsorbents are still rare. The main reason for this is technical difficulties in their manufacturing. Typically inorganic adsorbents are synthesized as fine powders that cannot be used in column type applications due to high pressure drop [4,5]. Different strategies have been pursued to resolve this problem and produce granular materials. Among them are extrusion and sol–gel techniques, incorporation of fine powders into organic polymers, etc. None of these approaches is universal and each of them has certain drawbacks. The one drawback common for all of them is inferior kinetics of adsorption in comparison with ion-exchange resins. Poor kinetics of exchange
* Corresponding author. E-mail address: [email protected] (J.R. Garcı´a). 0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2009.09.030
relates to the rigid, predominantly micro-porous structure of products that prevents their swelling. According to Liu et al. [6] mesoporous materials have improved kinetics of exchange. However, use of solid state reactions or organic templates/ surfactants is often not viable for large scale manufacturing. Another approach for solving these problems has been used at MELChemicals—the technique of making mesoporous inorganic adsorbents in the form of micron size powders with narrow particle size distribution has been developed [7]. Such an approach allows use of materials in thin bed type applications and enables kinetics of adsorption superior to any resin (residence time 3–10 s). Currently MELChemicals produces two grades of hydrous zirconium oxides (HZO’s) with narrow 20–30 mm particle size distribution on commercial scale: microporous 301-type and mesoporous 302-type oxide. Both materials are designed for different separation applications. The results of physical and chemical characterization of 301-HZO and 302-HZO type adsorbents, with main stress on the effect of temperature on particle size, porosity change and efficiency of arsenic sorption from aqueous solutions are summarized in this paper. 2. Experimental 2.1. Synthesis A zirconium salt granular precursor, with narrow particle size distribution (25–35 mm), has been used for making hydrous zirconium oxides. Synthesis of 301-HZO includes the preparation of aqueous slurry of the zirconium salt and its conversion into
A. Bortun et al. / Materials Research Bulletin 45 (2010) 142–148
hydroxide by slow addition of an aqueous base solution until equilibrium pH exceeds 10. The process is carried out at ambient temperature. Preparation of 302-HZO is similar to that of 301-HZO, but it includes an additional stage of slurry treatment with alkali at 90–100 8C for 1–3 h. After the conversion process is finished zirconium hydroxides are washed with deionized water to remove all anions and cations and then dried in air to targeted residual water content (40 wt% for 301-HZO and 5 wt% for 302-HZO) [7]. 2.2. Characterization The particle size distribution of zirconium precursor and all zirconium hydroxides has been determined on a Leeds + Northrup Microtrac ASVP instrument. Media packed density has been determined by using standard test method for determination of tap density of powder materials [ASTM B527-93 (2000)e1, standard test method for determination of tap density of metallic powders and compounds]. SEM micrographs were obtained using a JEOL JSM 6100 electron microscope, operating at 20 kV. N2 adsortion/desorption isotherms were measured at 77 K on the TriStar analyzer (Micromeritics Instrument Corporation, USA). Prior to measurement, all samples were outgassed at 300 8C for 2 h. The specific surface area was calculated by applying the BET equation. By analysis of the adsorption curve using the BJH calculation method, the pore size distribution was also obtained. For the IR measurements, 50 mg of HZO were pressed into a selfsupporting wafer, placed in the in situ IR cell and spectra were recorded on a Nicolet Magna II FTIR spectrometer by averaging 32 spectra at a resolution of 2 cm1. TPD-NH3 and TPD-CO2 were carried out using U-shape quartz reactor on Altamira AMI-200 instrument. A 0.15 g sample was pre-treated in a flow of helium at 500 8C for 1 h. After cooling to 110 8C, the sample was saturated with NH3 or CO2 and then purged for 1 h with helium to remove physically adsorbed species. After that the sample was heated to 650–700 8C in a flow of helium at constant rate 108 min1. NH3 or CO2 desorption was monitored with the use of TCD or mass spectroscopy. 2.3. Adsorption properties The following conditions have been used for column experiments. 1.00 g of 301- or 302-HZO has been put into a glass column with 8 mm inner diameter. Two types of challenge solutions have been used. Phosphate-containing challenge water had composition: 0.001 M NaH2PO4 and 0.01 M Na2SO4. NSF53-challenge water had composition: [Ca2+] = 40 mg/L; [Mg2+] = 12 mg/L; [SO42] = 50 mg/L; [N-NO3] = 2 mg/L; [F] = 1 mg/L; [SiO2] = 20 mg/L; [As(V)] = 0.05 mg/L and pH 8.5 [NSF/ ANSI 53–2002, Addendum 1.0–2002]. Challenge solutions have been passed through the adsorbent bed with a constant flow rate of 360 mL/h. Water samples have been collected every 100–200 bed volumes. Phosphate and arsenic content in them have been analyzed using ICP or AAS with graphite furnace, respectively. Arsenic adsorption capacity has been found from corresponding breakthrough curves by surface integration method. 3. Results and discussion The reaction of zirconium salt precursor with alkali (NaOH, pH > 10) in the process of making 301- and 302-HZO results in gradual substitution of salt anions with hydroxyl groups and, eventually, in formation of a zirconium hydroxide phase. Moreover, at relatively high pH (>10) a partial (3–10%) conversion of Zr– OH groups into Zr–ONa+ takes place also. The additional step of alkaline digestion of hydrous zirconium oxide at 90–100 8C in the case of 302-HZO significantly facilitates ageing processes by
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increasing the rate of condensation and olation reactions on the surface and in the bulk of oxide. Considering that such ageing takes place when some of the reactive functional Zr–OH groups are ‘‘protected’’ due to conversion into Zr–ONa form so they can not participate in the condensation reactions, this creates a certain effect on both porosity and surface chemistry of treated products. In the case of surface chemistry a certain change in functional group distribution takes place for 302-HZO in comparison with non-treated zirconium hydroxide. Three types of hydroxyl groups can be present on the surface of HZO [8–10]:
Mono-bridged groups are considered to be the most protruded from the surface and least sterically hindered, while di-bridged groups are located almost in the plane of the surface and tribridged to some extent below the surface plane. According to Pauling’s law, and taking into consideration that zirconium charge is 4+ and zirconium coordination number is 8, these hydroxyl groups should bear different charges. Namely, the mono-bridged Zr–OH group is negatively charged (d1/2) while the di-bridged is neutral and the tri-bridged is positively charged carrying d1/2+. The oxygen atom in the BBZr3–OH group has a saturated environment and has no possibility for a complexing reaction with ions. The only reaction with the BBZr3–OH group is a proton release. On the other hand deprotonation of the mono-bridged BBZr–OH group is strongly suppressed due to the high effective negative charge on the oxygen atom. As a result, one can expect that predominantly di-bridged Zr–OH groups should undergo partial conversion into sodium form at pH 11.5. This suggests that the two other types of functional groups and remaining non-substituted di-bridged groups are free for participation in condensation reactions. Considering steric factors, the main contribution in making new covalent bonds, connecting particles together, should be the mono-bridged groups. This is in good agreement with IR data for both oxides (Fig. 1). It is clearly seen that hydroxyl coverage of alkaline digested material consists predominantly of tri-bridged groups (3600–3700 cm1) with a lesser amount of di-bridged (3700–3750 cm1) and terminal groups (3750–3800 cm1). In the case of non-treated 301-HZO, the surface of zirconium hydroxide evacuated at 120 8C is predominantly covered with tri-bridged groups and depleted of di-bridged and terminal groups. The difference in surface chemistry has been confirmed also by checking acid-base properties of zirconium hydroxides with the use of temperature programmed desorption of NH3 and CO2 (Figs. 2 and 3). As seen, normalized TPD-NH3 curves for 301-HZO and 302-HZO have a similar shape. Release of adsorbed ammonia begins at 150– 160 8C, it reaches a maximum at 210–250 8C with following gradual decrease until the process ends at 500–600 8C (Fig. 2a). Deconvolution of TPD-NH3 peaks (Fig. 3) has shown that both materials have two types of ‘‘weak’’ acid adsorption sites (200– 300 8C) and three types of ‘‘strong’’ acid sites (300–600 8C) with surprisingly similar peak surface area ratios 2:1, respectively. This suggests that both materials have similar types of functional hydroxyl groups responsible for their acidic properties. The main difference between samples is that the total acidity (amount of desorbed NH3) of 302-HZO (0.276 mmol NH3/g) is 3.3 times higher than that for 301-HZO (0.084 mmol NH3/g). This could be a result of much higher thermal stability of alkaline treated material leading to a higher population of Zr–OH groups on the surface. TPD-CO2 profiles for zirconium hydroxides are shown in Fig. 2b. The strongest desorption peak for both hydroxides is in the temperature range from 150 to 450–470 8C. This broad peak can be de-convoluted into three peaks representing weak basic
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Fig. 1. IR spectra of: (a) 302-HZO sample evacuated at 120 or 250 8C, and (b) 301HZO sample evacuated at 120 8C.
adsorption sites (<300 8C) and medium strong basic sites (300– 450 8C). The second and third desorption peaks of smaller intensities are at 520 and 600 8C for 301-HZO and at 480 and 590 8C for 302-HZO, respectively. These peaks are related to CO2 bound on strong basic adsorption sites and, possibly, due to release of some remaining chemisorbed CO2 not completely removed from media during pre-treatment at 500 8C. Total amount of basic sites on 302-HZO (0.292 mmol/g) is significantly higher than on 301-HZO (0.167 mmol/g), which is in agreement with higher thermal stability of 302-HZO. Interestingly, the ratio between weak and strong basic sites (2:1) is very similar to the ratio between weak and strong acid sites. This suggests that the same functional zirconium hydroxyl groups depending on conditions can be responsible for acidic or basic properties. Mild alkaline treatment at elevated temperature does not significantly change the product’s morphology in comparison with precursor or non-treated 301-HZO (Fig. 4). Hydrous zirconium oxide retains a ‘‘raspberry-type’’ structure consisting of dozens of 0.5–3 mm spheres fused together to form larger 20– 40 mm aggregates. Such agglomerates are rigid and possess surprisingly high mechanical resistance to attrition. At the same time precursor conversion into hydroxide form for both 301-HZO and 302-HZO results in 10–15% shrinkage of particle’s size (Fig. 5). Narrow particle size distribution and lack of fines (particles less than 5–10 mm) allows use of hydrous zirconium oxides of 301- and 302-type in thin bed applications (layer depths less than 5–10 cm) without creating significant pressure drop
Fig. 2. TPD-NH3 (a) and TPD-CO2 (b) profiles of 301-HZO and 302 HZO samples.
(pressure drop is less than 1.0–1.5 kg/cm2) and ensuring quick kinetics of exchange. According to powder XRD data, conversion of zirconium salt into both 301- and 302-HZO materials does not result in the product’s crystallization and both materials remain amorphous. An amorphous structure is favorable for maintaining the integrity of hydrous zirconium oxide agglomerates, but as a rule, amorphous products have lower thermal stability than crystalline ones and, what is the most important, inferior adsorption properties with respect to drying conditions. The majority of amorphous hydrous zirconium oxide based adsorbents are microporous and they are most active when freshly prepared [11]. Typically they lose significantly ion-exchange capacity during storage and, especially, after drying at elevated temperature. The most probable reason for this could be a collapse of microporous structure during drying which makes adsorption sites in the bulk of material not accessible for ion exchange. For example, freshly prepared amorphous 301HZO with 43% of physically bound water shows high affinity towards phosphate ion and is able to remove it from over 3000 bed volumes of solution, which gives capacity before breakthrough 100 mgP/g (Fig. 6). However, drying of the media results in a significant drop of adsorption capacity to 2000 bed volumes (or 60 mgP/g) at 30%—level of water and to only 100 bed volumes (or 3 mgP/g) at 20% water content. On contrary, adsorption capacity of 302-HZO is not sensitive to residual water content in the structure and even increases with water loss. 302-HZO shows the best performance when completely dry (purifies 4000 bed
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Fig. 4. SEM images of: (a) zirconium salt precursor and (b) 302-HZO sample (scale 10 mm). Fig. 3. De-convolution of TPD-NH3 profiles of 301-HZO (a) and 302-HZO (b) samples.
volumes of challenge solution, which gives capacity 120 mgP/g). Such behavior of 302-HZO is similar to what can be expected from crystalline zirconium hydroxides that are known as thermally stable adsorbents. Considering this it was interesting to check the transformations that take place within the media during thermal treatment, namely, how calcination effects particle size, porosity and adsorption performance. As seen in Fig. 7, particles of both tested hydrous zirconium oxides undergo certain shrinking with increase in calcination temperature. This suggests that zirconium hydroxide agglomerates are rather loosely packed and have some void space inside. The shrinkage from 100 to 550 8C totals 15% for 301-HZO and 20% for 302-HZO. In both cases almost 90% of the decrease in a particle’s size takes place in the temperature range 100–400 8C. In this temperature range both zirconium hydroxides are amorphous. At 430–440 8C zirconium hydroxides undergo crystallization and after 400–450 8C particles size shrinking practically stops. Change in particle size affects the media packed density (PD), which is one of the most important parameters for designing adsorbers/ cartridges for specific applications. Zirconium hydroxide packed densities as a function of calcination temperature are shown in Fig. 8. First, it should be mentioned that PD of the 301-HZO is substantially higher than that of 302-HZO. This can be related to differences in their pore structure that will be shown below.
However, in both cases density/temperature curves have similar shape and there is a trend of density increase with increase in calcination temperature. A gradual increase in packed density takes place in the temperature range from 100 to 300 8C, which is followed by a sharp increase in PD at 300–400 8C. From 400 to
Fig. 5. Particle size distribution of zirconium salt precursor and 302-HZO sample.
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Fig. 6. Effect of drying (in parentheses, residual water content) on phosphate removal by reference 301- and 302-HZO samples.
500 8C packed density remains almost constant with another sharp increase in PD after 500 8C. The shapes of density/temperature curves correspond well to changes in hydrous zirconium oxides particle shrinkage with the only exception at 500–550 8C sharp increase in packed density is not accompanied by particles shrinking. Total increase in packed density from 100 to 550 8C
Fig. 8. Packed density as a function of calcination temperature for 301- and 302HZO samples.
for 301-HZO is 85% and it becomes even more significant for 302HZO (95%). Comparative data on the porous structure of 301- and 302-HZO as a function of calcination temperature are summarized in Table 1, and typical N2/77 K adsorption/desorption isotherms for 302-HZO are presented in Fig. 9. As seen 302-HZO has significantly higher surface area, pore volume and average pore diameter than the reference material when it is dried at 100 8C and retain them on a much higher level when calcined (up to 500 8C). This indicates that 302-HZO is much more thermally stable than 301-HZO and that this is the result of introducing an additional stage of mild alkaline digestion during preparation. Similar improvement of hydrous zirconium oxide thermal stability after alkaline digestion has been reported earlier by Chuah et al. [12]. However, it should be noted, that in the Chuah case, the increase in thermal stability was the result of product crystallization [13] and significant doping (up to 5%) with silica due to silicate leaching from glass reactor [14]. 302HZO is amorphous and the silica contamination level is below 50 ppm. Surface area (SA) for 302-HZO is high (300 m2/g) and remains almost unchanged up to 300 8C. The sharp decrease of SA in the temperature range 300–400 8C coincides with particle shrinkage and an increase in packed density. Surface area of 302 calcined at 500 8C is almost 4.5 times lower than that of material dried at 100 8C. Surprisingly, pore volume of material does not change significantly with increase in drying temperature (pore volume even increases by 10–20% with temperature increase to 300 8C and only begins to decrease after 300 8C). Surface area for 301-HZO is lower than that of 302-HZO and gradually decreases (almost by 7 times) with increase in drying temperature. A similar trend has been found for changes in pore volume of 301-HZO. Table 1 Textural data (SA, specific surface area; PV, pore volume; PD, average pore diameter) for 301- and 302-HZO samples, stabilized in air at high temperature. Temperature (8C)
Fig. 7. Particle size as a function of calcination temperature for: (a) 301-HZO and (b) 302-HZO samples.
100 200 300 350 400 450 500
301-HZO
302-HZO
SA (m2/g)
PV (cm3/g)
PD (nm)
SA (m2/g)
PV (cm3/g)
PD (nm)
266 185 135 97 65 58 37
0.095 0.100 0.085 0.080 0.055 0.050 0.035
2.1 2.2 2.7 3.6 4.3 5.1 6.1
306 302 292 225 123 116 69
0.200 0.224 0.246 0.205 0.191 0.184 0.174
2.6 2.8 3.4 4.2 6.1 7.6 9.5
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Fig. 9. N2/77 K adsorption–desorption isotherms for 302-HZO sample as a function of calcination temperature.
Hydrous zirconium oxide capacities for arsenic(V) removal from NSF-53-type challenge water as a function of drying temperatures are shown in Figs. 10 and 11. In a good agreement with the above mentioned statement on strong dependency of 301-HZO on residual water content, this adsorbent has a low
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Fig. 11. Arsenic sorption capacities of 301- and 302-HZO samples as a function of drying temperature. Capacities are presented as amount of arsenic adsorbed per unit of media surface area.
capacity for arsenic (10 mg As/cm3 or 7 mg As/g) after drying at 100 8C. Capacity changes not significantly with increase in calcination temperature from 100 to 400 8C, while some increase in arsenic uptake has been observed in the temperature range 400– 550 8C. It is interesting that 301-type HZO arsenic capacity presented as amount of As(V) taken per media volume becomes even higher after calcination at 450–500 8C in comparison with product dried at 100 8C. On the contrary, 302-HZO, which is not sensitive to residual water content, shows much higher adsorption capacity for arsenic. Material dried at 100 8C adsorbs 150– 160 mg As/g, which is 15–25 times more than 301-HZO. Increase in calcination temperature up to 350–400 8C results in gradual decrease (up to 1.4–2.4 times) of arsenic uptake, which is seen more clear in the case when adsorption is presented as As uptake per weight of media. Considering that in this temperature range surface area (and pore volume) of 302-HZO remain almost constant, these changes in capacity cannot be related to pore collapse resulting in poor accessibility of adsorption sites in the bulk. This effect rather relates to decrease in total amount of functional Zr–OH groups due to continuous condensation reactions (Zr–OH + HO–Zr ! Zr–O–Zr + H2O). The decrease of 302-HZO adsorption capacity on As(V) after drying at 350–400 8C, presented as arsenic uptake per unit of media surface (Fig. 11), strongly supports this suggestion. Interestingly, calcination of 302-type oxide at higher temperatures (>350 8C) results in an increase in media arsenic capacity similar to what has been observed for 301-HZO. Adsorption capacity of 302-HZO after drying at 400 8C calculated per media volume is comparable to that of material dried at 100 8C, whereas capacity per unit of surface area (Fig. 11) becomes significantly higher (almost in 3 times) than that of non-calcined product. Taking into account the significant decrease of 302-HZO surface area and the continuous hydroxyl group condensation that takes place in this temperature range, the observed phenomena can be attributed to changes in types of remaining Zr–OH groups on the surface, namely, an increase in population of the most reactive hydroxyl groups with high affinity towards arsenic. 4. Conclusions
Fig. 10. Arsenic sorption capacities of HZO samples as a function of drying temperature: (a) 301-HZO and (b) 302-HZO samples. Capacities are presented as amount of arsenic adsorbed per weight or per volume of material.
The hydrous zirconium oxide presented in this paper is a mesoporous material that differs significantly from other commercially available zirconium hydroxide products: it is more basic, exhibits higher affinity towards arsenic(V), show higher kinetics of
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ion-exchange and improved thermal stability that allows retention of adsorption properties even after treatment at 500–550 8C. Acknowledgments We express our thanks for financial support from the Spanish Ministerio de Educacio´n y Ciencia (MAT2006-01997 and Factorı´a de Cristalizacio´n–Consolider Ingenio 2010) and the Gobierno del Principado de Asturias (PCTI 2006-2009). References [1] A. Clearfield, Solvent Extract. Ion Exch. 18 (2000) 655.
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