Natural and Surfactant-Modified Zeolite for the Removal of Pollutants (Mainly Inorganic) From Natural Waters and Wastewaters

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NATURAL AND SURFACTANTMODIFIED ZEOLITE FOR THE REMOVAL OF POLLUTANTS (MAINLY INORGANIC) FROM NATURAL WATERS AND WASTEWATERS

23 N.S. Dionisiou, T. Matsi

Aristotle University of Thessaloniki, Thessaloniki, Greece

CHAPTER OUTLINE 23.1 23.2 23.3 23.4 23.5

Introduction ...............................................................................................................................591 Structure, Properties, and Sources of Natural Zeolite ................................................................... 592 Applications of Natural Zeolite for Environmental Purposes...........................................................593 Surface Modification of Natural Zeolite........................................................................................ 596 Applications of Surfactant-Modified Zeolite for Environmental Purposes ........................................598 23.5.1 Removal of Inorganic Anionic Pollutants.................................................................599 23.5.2 Removal of Inorganic Cationic Pollutants................................................................601 23.5.3 Removal of Organic Pollutants ...............................................................................602 23.6 Conclusions ...............................................................................................................................603 References ..........................................................................................................................................603

23.1 INTRODUCTION Natural zeolites are hydrated aluminosilicate minerals with high cation exchange capacity (CEC) owing to extended isomorphic substitution of Si4þ by Al3þ in their tetrahedra and large specific surface area, among other properties. Based on their high CEC, zeolites can serve as cation exchangers and thus as sorbents for cations, but their ability to sorb anions is negligible (Mumpton, 1999; Widiastuti et al., 2008; Wang and Peng, 2010). However, their sorption capacity for anions can substantially increase upon modifying their surface, which results in a change in the unsatisfied charge of their external surface from negative to positive. For the surface modification of zeolites, certain surfactants can be used and the produced zeolite is called a surfactant-modified zeolite (SMZ). SMZ can serve as sorbent for both anions because of the positive charge of its external surface, but also for cations, mainly owing to the negative charge of its interior (Wang and Peng, 2010; Bowman, 2003). Environmental Materials and Waste. http://dx.doi.org/10.1016/B978-0-12-803837-6.00023-8 Copyright © 2016 Elsevier Inc. All rights reserved.

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The objective of this chapter was to compile and evaluate existing research and knowledge concerning natural zeolite and SMZ with the perspective of using them to remove inorganic pollutants mainly from natural waters and wastewaters. Specifically, the structure, properties, and sources of natural zeolites and their ability to retain cations are reported briefly. Emphasis is given to the surface modification of natural zeolites by cationic surfactants and the ability of SMZ to sorb anions. In addition, SMZ sorption capacity for cations and organic pollutants is reported.

23.2 STRUCTURE, PROPERTIES, AND SOURCES OF NATURAL ZEOLITE Natural zeolites are crystalline, tectosilicate minerals composed of silica and aluminum tetrahedra linked to each other through shared oxygen atoms. Because of the latter, zeolites have a ring-style, open, three-dimensional, infinite crystal structure consisting of interconnected cavities and channels (Fig. 23.1). Because of extended isomorphic substitution of Si4þ by Al3þ in tetrahedra, a large unsatisfied negative charge is produced at the zeolites’ lattice. The negative charge is balanced by cations, eg, Caþ2, Mgþ2, Kþ, Naþ, NH4 þ , etc., which are retained by electrostatic bonds and forces onto the zeolites’ surface or into their crystal structure. Many of the zeolites’ counter-cations (ie, exchangeable cations) are weakly held and thus are free to exchange with others in solution (Widiastuti et al., 2008; Wang and Peng, 2010). The structural cavities and entry channels leading into them are large enough to retain hydrated inorganic cations selectively, as well as certain organic molecules and water molecules (which can be easily lost upon heating at high temperatures but also easily regained at room temperatures). Because of their high selectivity, zeolites can be used as catalysts. Furthermore, upon removal of water molecules from zeolites, small substances can pass through entry channels, whereas large substances are excluded; in this sense, zeolites can be used as chemical sieves (Mumpton, 1999; Widiastuti et al., 2008). Certain properties of zeolites are reported in Table 23.1. The CEC of zeolites is based on their unsatisfied negative charge and consequently is strongly related to the extent of isomorphic substitution of Si4þ by Al3þ. Natural zeolites have values of CEC usually ranging from 200 to 400 cmolc kg1, almost twice the CEC of bentonite clay. Unlike most noncrystalline ion exchangers, eg, organic resins and aluminosilicate gels (mislabeled in the trade as “zeolites”), zeolites selectively sorb cations. Because of their hydration spheres, certain cations cannot approach

FIGURE 23.1 Open three-dimensional structure of natural zeolite. From Ackley et al. (1992).

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Table 23.1 Certain Properties of Natural Zeolite Property

Common Values

Channels Cavities Thermal stability Cation exchange capacity

0.22e0.80 nm 0.66e1.18 nm 500e1000 C 400 cmolc kg1

the zeolites’ framework, whereas certain others are generally more tightly held by zeolites and are more selectively exchanged than other cations. For example, although clinoptilolite has a relatively small CEC (225 cmolc kg1), it exhibits cation selectivity which was found to follow the decreasing order: Csþ > Rbþ > Kþ > NH4 þ > Ba2þ > Sr2þ > Naþ > Ca2þ > Fe2þ > Al3þ > Mg2þ > Liþ (Mumpton, 1999). About 50 different kinds of natural zeolites are known, and more than 150 have been synthesized for different applications (Ozaydin et al., 2006). The most common natural zeolites are analcime, chabasite, clinoptilolite, erionite, ferrierite, heulandite, laumontite, mordenite, phillipsite, and stilbite; clinoptilolite is the most studied and used (Wang and Peng, 2010; Ozaydin et al., 2006). Natural zeolites have been found in the United States, Bulgaria, Russia (Mumpton, 1999), Australia, Armenia, Canada, Chile, China, Cuba, Georgia, Italy, New Zealand, Slovakia, Romania (Wang and Peng, 2010), Brazil (Campos and Buchler, 2007), Croatia (Curkovic et al., 1997), Greece (Misaelides et al., 1995; Perraki and Orfanoudaki, 2004), Hungary, the United Kingdom (Widiastuti et al., 2008), Iran (Faghihian et al., 1999), Japan (Takagi, 1978), Mexico (Llanes-Monter et al., 2007), Serbia (Lemic et al., 2006), Turkey (Ozaydin et al., 2006), Ukraine (Sprynskyy et al., 2006), and elsewhere. Because of their structure and the derived chemical properties, such as increased CEC, dehydration and rehydration, sorption capacity, catalytic role, and biological activity, but also because of certain physical properties such as a large specific surface area, natural zeolites have numerous applications in various fields. Owing to their unique cation exchange behavior, zeolites have been extensively recommended for environmental applications such as purification and softening of natural waters, wastewater treatment, and rehabilitation of soils polluted with heavy metals and radionuclides (Mumpton, 1999).

23.3 APPLICATIONS OF NATURAL ZEOLITE FOR ENVIRONMENTAL PURPOSES Surface and groundwater, irrigation and drinking water, and wastewater (industrial, municipal, agricultural) may contain cations more or less hazardous for human beings, animals, and plants and in general for the environment. Because of the previously mentioned properties, especially the high CEC, zeolites can exchange their harmless counter-cations with hazardous cations (especially those of heavy metals), retaining and thus removing them from water and aqueous media. Numerous studies with respect to the ability of natural zeolites to retain cationic pollutants are reported in the literature, and selected studies are reviewed in the current section.

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Natural zeolites can efficiently and selectively remove several heavy metals from solution. This ability is related, among other parameters, to the crystal size and the kind of counter-cations of zeolites, the hydration ionic radii and concentration of metals in solution, the solution pH, and the contact time and temperature (Llanes-Monter et al., 2007; Sprynskyy et al., 2006). Zeolites’ affinity for heavy metals, especially of clinoptilolite for Pb2þ, has been demonstrated by several studies. It is reported that clinoptilolite’s affinity for certain heavy metals follows the decreasing order: Pb2þ > Cd2þ > Csþ > Cu2þ > Co2þ > Cr3þ > Zn2þ > Ni2þ > Hg2þ (Zamzow et al., 1990). The efficiency of the same kind of zeolite to retain Pb2þ, Cd2þ, Cu2þ, and Ni2þ follows the decreasing order: Pb2þ > Cu2þ > Cd2þ > Ni2þ and increases when both zeolite’s particle size and metal concentration in solution decreases (Sprynskyy et al., 2006). In another study of Pb2þ, Cu2þ, Fe3þ, and Cr3þ retention by clinoptilolite, the mineral’s selectivity for Pb2þ was the maximum (Inglezakis et al., 2004). Similar findings are reported regarding Pb2þ retention compared with Ba2þ, Cd2þ, Ni2þ, Sr2þ, and Csþ (Faghihian et al., 1999). Zeolites retained Pb2þ more selectively than Cd2þ, although retention of both metals decreased drastically upon an increase in their concentrations in solution (Curkovic et al., 1997). In other studies, clinoptilolite showed strong affinity for Pb2þ and large amounts of the metal were retained by the mineral regardless of the kind and concentration of competing ions in solution (Zamzow et al., 1990; Bailey et al., 1999). Furthermore, a natural zeolitic tuff containing 50% clinoptilolite retained Pb2þ, Zn2þ, and Cu2þ to a higher degree at a lower concentrations of metals in solution and its maximum retention capacity for Pb2þ but also for Cu2þ was double than that for Zn2þ (Peric et al., 2004). The ability of natural zeolites to retain these heavy metals has also been studied by other researchers (Cincotti et al., 2006; Oter and Akcay, 2007). In addition, treatment of wastewater of an abandoned copper mine with clinoptilolite reduced Cu2þ, Zn2þ, and Fe3þ below drinking water standards (Zamzow et al., 1990) and its selectivity for certain heavy metals followed the decreasing order: Co2þ > Cu2þ > Zn2þ > Mn2þ (Erdem et al., 2004). From the literature, it seems that zeolites retain heavy metals mainly by electrostatic forces after exchange with their counter-cations. However, precipitation and specific sorption as additional retention mechanisms cannot be excluded. It is reported that clinoptilolite removed Pb2þ efficiently from solution at pH 2e5, which was attributed to Pb2þ retention by the mineral via a cation exchange process at all pH values, especially the most acidic. However, Pb(OH)2 precipitation onto zeolite at pH 5 could not be excluded (Llanes-Monter et al., 2007). Except for Pb2þ, the cation exchange process is reported as the main mechanism of Cd2þ (Curkovic et al., 1997), Zn2þ, and Cu2þ retention (Peric et al., 2004), although precipitation and sorption are also mentioned as other possible mechanisms in the latter case. Raw and pretreated (with HCl and NH4Cl) clinoptilolite were proven to be effective means for Pb2þ, Cd2þ, Cu2þ, and Ni2þ retention (Sprynskyy et al., 2006). The retention mechanism for the cation exchange process is accomplished in three stages. During the first 30 min, metals are retained on the microcrystals’ surface of clinoptilolite. Then, for a certain period of time, desorption is prevalent over cation exchange. Finally, retention of the metals occurs in the microcrystals’ interior at a slow rate. Almost 25% of Pb2þ, 40% of Cd2þ and Cu2þ and 90% of Ni2þ are retained during the first stage of the process (Sprynskyy et al., 2006). Several researchers connected Naþ enrichment of natural zeolites with enhancement of their ability to remove heavy metals from solution (Curkovic et al., 1997; Faghihian et al., 1999; Llanes-Monter et al., 2007; Zamzow et al., 1990; Inglezakis et al., 2004). Probably the main reasons

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for this are both the low charge and the large hydration ionic radius of Naþ. Because of these properties, Naþ is expected to be weakly bound to zeolite, and thus it can be more easily exchanged with heavy metals in solution than other common exchangeable cations of zeolite, ie, Ca2þ, Mg2þ, Kþ, and NH4 þ . However, an undesirable increase in pH in aqueous medium at a strongly alkaline range cannot be excluded in the case of zeolite’s enrichment with Naþ. Similarly, an undesirable decrease in solution pH at a strongly acidic range cannot be excluded in the case of zeolite’s treatment with HCl, as previously mentioned. Except for heavy metals, several studies report the ability of natural zeolites to remove NH4 þ from aqueous systems, the most common cation in polluted natural waters and wastewaters that negatively affects human and animal health, aquatic organisms, and eutrophication in rivers and lakes. As for heavy metals, also in this case the most studied kind of natural zeolite is clinoptilolite. Clinoptilolite’s capacity for NH4 þ is about 5e10 times higher than that of sepiolite (Bernal and Lopez-Real, 1993) and greater than that of bentonite clay (Rozic et al., 2000). Ammonium retention was unaffected by the clinoptilolite’s particle size but it increased with an increase in the surface charge density of the mineral, which was attributed to the increase in readily available exchange sites Consequently, the cation exchange process was considered to be the main mechanism of NH4 þ retention by zeolite (Bernal and Lopez-Real, 1993). Similar findings were reported by other researchers, who also found that NH4 þ removal by clinoptilolite was completed within the first 15 min of equilibration time and depended on the solution pH at the range 5.5e7.6 (Ashrafizadeh et al., 2008), but it decreased at concentrations higher than 100 mg NH4 þ L1 in solution (Rozic et al., 2000; Ashrafizadeh et al., 2008). Similarly to heavy metals, Naþ treatment of clinoptilolite can increase its retention capacity for NH4 þ because of its higher affinity for NH4 þ over Naþ. Zeolite’s affinity for alkali and alkaline earth metals and NH4 þ is reported to follow the decreasing order: Rbþ > Kþ > NH4 þ > Ba2þ > Naþ > Ca2þ > Liþ (Wang and Peng, 2010). In addition, NH4 þ exchange selectivity for zeolite’s counter-cations followed the decreasing order: Naþ > Ca2þ > Kþ (Sprynskyy et al., 2005) and although a competitive effect was evident between NH4 þ and other cations in solution, eg, Ca2þ, Mg2þ, and Kþ, selectivity of Na-rich zeolite for NH4 þ was the highest (Cooney et al., 1999). However, other researchers reported a reduction in NH4 þ retention by zeolite in the presence of Naþ in solution (Nguyen and Tanner, 1998). Zeolite was studied as a means for NH4 þ removal from different kinds of wastewater. Clinoptilolite and mordenite showed considerable ability in removing NH4 þ from domestic (dairy and piggery) wastewater. This ability depended on the mineral’s particle size, the contact time between zeolite and wastewater, and the wastewater loading flow rate and composition (Nguyen and Tanner, 1998). Similarly to the latter finding, results of a study of zeolite’s ability to retain NH4 þ from leakage water of waste dumps containing high amounts of NH4 þ (820 mg L1) and organic substances (1033 mg C L1) indicated that organic molecules present in wastewaters may reduce zeolite’s capacity for NH4 þ owing to their sorption onto the mineral (Farkas et al., 2005). In a study of NH4 þ removal from wastewater by zeolite conducted with columns, it was found that the maximum capacity of zeolite for NH4 þ was observed at pH 4 or after acid pretreatment of the mineral (Sarioglu, 2005). In addition to this, natural zeolites have the ability to retain radionuclides, especially those of Cs, from aqueous medium. Clinoptilolite selectively removed 137Cs, 90Sr, 60Co, and 110mAg from liquid radioactive waste, especially the two former isotopes (Osmanlioglu, 2006). Three natural zeolites (clinoptilolite, chabazite, and mordenite) were tested as sorbents for 137Cs. All the three zeolites

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showed appreciable capacities for 137Cs; that of chabazite was the highest. In all cases, 137Cs retention was unaffected by solution pH but significantly decreased in the presence of Kþ in solution (Borai et al., 2009). The radioactive nuclides 134Cs, 85Sr, and 131I were added to river and seawater to prepare artificially contaminated liquid samples, and their retention by natural zeolite (mordenite) but also by bentonite clay and activated carbon was investigated (Kubota et al., 2013). In the river water samples, mordenite successfully removed only 134Cs and 85Sr at levels comparable to those of bentonite clay, whereas activated carbon partly removed only 134Cs and 131I. In the seawater samples, only 134Cs was removed by mordenite and bentonite clay to a lesser degree than the river water samples, whereas 85Sr and 131I were hardly removed by all three substances (Kubota et al., 2013).

23.4 SURFACE MODIFICATION OF NATURAL ZEOLITE As mentioned, zeolites have a permanent negative charge caused by isomorphic substitution of Si4þ by Al3þ in the tetrahedra of their crystal lattice. Because of the negative charge, zeolites can retain cations but their affinity for anions is negligible. Their ability to retain anions, and thus to remove them from natural waters and wastewaters, can substantially increase by altering their surface charge from negative to positive, via sorption of certain substances onto their external surface. Such substances are Al2(SO4)3 (Campos and Buchler, 2007), long-chain primary amines such as oleylamine (Vujakovic et al., 2000), and cationic surfactants, usually quaternary ammonium salts with chloride or bromide. Such surfactants are tetraethylammonium (Sullivan et al., 1998), hexadecyltrimethylammonium (HDTMA) (Sullivan et al., 1998; Haggerty and Bowman, 1994; Schulze-Makuch et al., 2003), ethylhexadecyldimethylammonium (Campos et al., 2007), dodecyltrimethyammonium, octyltrimethyammonium (Li et al., 2002), benzyldimethyltetradecylammonium (Kuleyin, 2007), benzyltrimethylammonium, stearyldimethylbenzylammonium (Lemic et al., 2006), dodecypyridinium (Li, 2006), cetylpyridinium (Zhan et al., 2011), and 4-methylpyridinium (Widiastuti et al., 2008). Cationic surfactants contain both hydrophobic and hydrophilic groups in their chemical formula, and because of the latter groups they are water-soluble substances. Their molecules consist of a positively charged head group, usually quaternary ammonium, and a counter-anion, commonly Cl or Br. In addition, their molecules are larger than the diameter of zeolite’s channels, and thus they cannot penetrate into the interior (internal surface) of zeolite. However, they can be sorbed onto the external surface of zeolite, altering in this way the external charge of the mineral from negative to positive. The strength and stability of surfactant’s retention onto zeolite’s surface increase as the length of the surfactant’s carbon chain increases. Probably for this reason, HDTMA salts are commonly used for surface modification of natural zeolites (Li et al., 1998a). At very low concentrations of surfactant in aqueous solution, its molecules exist as monomers. Under this condition, the molecules can be sorbed onto zeolite’s surface at a monolayer. When the surfactant’s concentration becomes greater than a critical value (critical micelle concentration), the molecules tend to associate with each other to form micelles in addition to monomers in solution. At this point, the surfactant’s molecules can be sorbed onto zeolite by forming a bilayer; the first layer is retained by electrostatic forces and the second layer, by hydrophobic interactions (eg, van der Waals forces), with the positive charge of the second layer (ie, the positively charged heads of the surfactant’s molecules) being neutralized by its counter-anions (Fig. 23.2) (Li et al., 1998a).

Surfactant modification of zeolite’s external surface. From Chutia et al. (2009).

23.4 SURFACE MODIFICATION OF NATURAL ZEOLITE

FIGURE 23.2

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In addition to the critical micelle concentration, surfactant’s concentration in relation to the CEC of zeolite’s external surface (ECEC) controls also the formation of a monolayer or a bilayer. Specifically, large surfactant molecules such as HDTMA salts are sorbed only onto the external surface of zeolite, and consequently the capacity of the mineral to retain and exchange the surfactant depends on its ECEC. When the surfactant’s concentration is below or equivalent to the zeolite’s ECEC, the positively charged head groups of the surfactant’s molecules can substitute the counter-cations of the zeolite’s surface selectively, forming a monolayer. At concentrations higher than the ECEC of the zeolite, the surfactant’s molecules may also be sorbed on the organic monolayer, in this way forming a bilayer via hydrophobic interactions between the organic tails of the two layers. The bilayer can be patchy or complete when in the latter case the surfactant’s concentration is twice the zeolite’s ECEC (Bowman, 2003; Sullivan et al., 1998). However, results of stability studies of SMZ question the bilayer formation. It was shown that upon addition of HDTMA-Br to zeolite at rates equal to 100% and 200% of its ECEC, HDTMA-Br was irreversibly bound to zeolite only in the former case (Haggerty and Bowman, 1994). In either case of patchy or complete bilayer formation, the external charge of SMZ becomes positive and the mineral obtains anion exchange capacity. Furthermore, the surfactant’s bilayer is a solvent-like medium into which organic compounds can be dissolved (Bowman, 2003; Sullivan et al., 1998) and SMZ is stable in water and concentrated chemical solutions. Based on thermodynamics measurements, clinoptilolite, modified by surfactants whose concentrations were higher than the critical micelle concentration and the mineral’s ECEC, was found to be the most stable (Sullivan et al., 1998). Except for the critical micelle concentration and the relative concentration of surfactant to zeolite’s ECEC, counter-anions of the surfactant’s molecules also affect the kind of layer. HDTMA-Br and HDTMA-Cl can be sorbed onto zeolite’s surface in a complete bilayer, whereas HDTMA-HSO4 can be sorbed only onto an incomplete one (Li et al., 1998b). After its surface modification, the resultant SMZ is capable of simultaneous sorption from water and other aqueous systems of: (1) anions (by electrostatic forces with the positively charged heads of the surfactant’s molecules onto the external surface of SMZ, ie, anion exchange with the counter-anions of the surfactant), (2) cations (by electrostatic forces with the negatively charged interior of SMZ or the remaining unsatisfied by the surfactant’s molecules negative charge of the SMZ’s external surface, ie. in either case, cation exchange with the zeolite’s cations), and (3) organic molecules (by hydrophobic attraction with the surfactant’s carbon tails onto the external surface of SMZ). In addition, the cost of the zeolite’s surface modification is relatively low and makes SMZ a viable alternative sorbent for different pollutants with respect to other reactive substances, such as activated C and ion-exchange resins (Bowman, 2003).

23.5 APPLICATIONS OF SURFACTANT-MODIFIED ZEOLITE FOR ENVIRONMENTAL PURPOSES As was previously mentioned, SMZ is capable of sorbing three different kinds of pollutants: anions (mainly by an anion-exchange process), cations (mainly by a cation-exchange process) and organic compounds (by hydrophobic attraction). Consequently, SMZ becomes a material that could be used effectively to remove anionic but also cationic inorganic pollutants and organic contaminants from drinking and irrigation water, other aqueous media, soil solution, and

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wastewater. According to the literature, the most studied kind of SMZ is clinoptilolite modified by HDTMA-Br or HDTMA-Cl.

23.5.1 REMOVAL OF INORGANIC ANIONIC POLLUTANTS The main environmental application of SMZ is based on its ability to sorb, retain, and thus remove anionic pollutants from natural waters and wastewaters. Such anions are mainly toxic chromates and arsenates, and the phosphates and nitrates, which are responsible for eutrophication (especially the former) and enrichment (especially the latter) of natural waters. All of them are oxy-anions; the main part of this subsection summarizes research into their sorption onto SMZ. In addition, SMZ sorption capacity for other oxy-anions (selenates, perchlorates, sulfates, and borates) is mentioned. In a study of chromates removal from a hexavalent chromium solution by SMZ (96% mordenite), the maximum sorption capacity of SMZ was 93% and 67% in the case of using HDTMA-HSO4 and ethylhexadecyldimethylammonium for zeolite’s modification, respectively (Campos et al., 2007). SMZ modified with cetylpyridinium was proven to be a superior sorbent for chromates to SMZ modified with ethylhexadecyldimethylammonium (Bailey et al., 1999). Certain researchers reported that SMZ modified by HDTMA-Br had significant sorption capacity for chromates, but also for nitrates and sulfates (Li et al., 1998a). This capacity was highest when the initial HDTMA-Br concentration was much greater than the critical micelle concentration and also the HDTMA-Br loading was twice the zeolite’s ECEC, ensuring in this way the formation of a complete bilayer on zeolite’s surface (Li et al., 1998a). Based on this, and because chromates sorption was inhibited in the presence of nitrates and sulfates, it was concluded that oxy-anions sorption onto SMZ is primarily a surface anion-exchange process (Li et al., 1998a), which is in agreement with other researchers’ conclusions (Li, 2006; Li and Hong, 2009). However, a different mechanism of oxy-anions sorption onto SMZ is also reported in the literature (Haggerty and Bowman, 1994). SMZ substantially reduced the concentration of chromates, but also of selenates and sulfates, in aqueous solutions, when the ECEC of zeolite had been fully satisfied by HDTMA-Br (ie, formation of a monolayer). It was concluded that the most likely mechanism of anions sorption by SMZ is surface precipitation of an HDTMAeanion complex (Haggerty and Bowman, 1994). Especially for chromates, their sorption on SMZ decreased as the solution pH and ionic strength increased (Li, 2004). Based on the consideration that the mechanism of chromates sorption on SMZ is a surface anion-exchange process, this decrease was attributed to increased competition between the chromates and the background electrolyte anions for the same sorption sites (Li, 2004). Sorption of chromates by SMZ modified with HDTMA-Br was investigated with column experiments. Although SMZ efficiently sorbed chromates, a slow and persistence release of HDTMA-Br but also of chromates was evident throughout the experiments. It was concluded that a change in the surfactant surface configuration from a bilayer to a monolayer formation had occurred which resulted in the loss of the SMZ ability to sorb and thus immobilize chromates (Li, 2006; Li and Hong, 2009). The latter was confirmed by the results of a subsequent column experiment which showed that SMZ had lost almost 90% of its sorption capacity for chromates. In addition, chromates’ sorption was related to the size of SMZ’s aggregations: the smaller the size, the higher was chromates’ sorption (Li and Hong, 2009).

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In general, arsenic sorption on SMZ depends on the zeolitic material, the arsenic chemical species, the chemical properties of aqueous medium, as well as the characteristics of SMZ (Macedo-Miranda and Olguin, 2007). It was found that sorption capacity for arsenates of SMZ modified with HDTMAHSO4 was higher than that of SMZ modified with ethylhexadecyldimethylammonium and both SMZs exhibited a higher capacity than that of natural zeolite modified with Al2(SO4)3 (Campos and Buchler, 2007). SMZ (clinoptilolite modified with HDTMA-Br) sorption capacity for arsenates significantly increased when the loading level of the surfactant onto zeolite exceeded the monolayer coverage; this was attributed to anion exchange of the SMZ’s Br with the As5þ ionic species of solution, which was H2 AsO4  and HAsO4 2 at common pH values (Li et al., 2007). In addition, increased arsenite sorption on the particular SMZ was found, although it was less than that of arsenates. This was attributed to arsenite retention on SMZ by mechanisms other than the anion-exchange process, such as surface complexation, because As3þ species at common pH values is neutral H3AsO3. The ionic strength of solution significantly reduced only the arsenate sorption, probably because of competition of other anions with arsenates for the same exchange sites of SMZ. However, in both cases of As5þ and As3þ species, solution pH had a slight effect on their sorption (Li et al., 2007). Others researchers observed a significant effect of solution pH on arsenate sorption onto SMZ and concluded that HAsO4 2 were sorbed more than H2 AsO4  (Mendoza-Barron et al., 2010). Inconclusive findings are reported with respect to the reversibility of arsenate sorption on SMZ (Mendoza-Barron et al., 2010; Chutia et al., 2009). Furthermore, mordernite modified by HDTMA-Br was more efficient than clinoptilolite modified by the same surfactant for reducing arsenic concentrations in water below the World Health Organization guideline value of 10 mg L1 (Chutia et al., 2009). Research on the use of SMZ for phosphates removal from waters was also been carried out. Almost 50% of H2 PO4  was sorbed by SMZ modified with oleylamine during the first 10 min of equilibration (Vujakovic et al., 2000). In another study, SMZ modified by cetylpyridinium chloride removed phosphates at 50e90%, depending on the concentration of the surfactant used to produce SMZ, the initial concentration of phosphates, the contact time, and the initial pH (Widiastuti et al., 2008). In a study of P sorption-desorption on SMZ with batch experiments, SMZ exhibited high sorption capacity for P, which was comparable to a kaoliniteegoethite mixture and kaolinite clay. SMZ sorption capacity increased with pH, and although it decreased with temperature and in the presence of a relative high concentration of nitrates, it remained appreciable in all cases (Dionisiou et al., 2013a). In addition, SMZ retained the greatest amount of added P, and P concentrations desorbed from SMZ ranged at levels below than those above which environmental risks are likely. The possibility of using P-laden SMZ as a slow-release P fertilizer after its use for removing P from waters was also mentioned (Dionisiou et al., 2013a). Nitrates sorption on SMZ has been adequately investigated. The influence of various parameters of the solution on nitrates sorption onto SMZ was investigated with batch sorption experiments (Schick et al., 2010). The equilibration time was short (0.5e1 h), the final percentage of nitrates removal from the solution was higher than 80%, the presence of competing anions at equivalent concentrations did not significantly reduce nitrates sorption onto SMZ, and leaching of the cationic surfactant from the SMZ was negligible. Especially for anions, their affinity for SMZ followed the increasing order: Cl << HCO3   SO4 2 < NO3  (Schick et al., 2010). Results of both batch and column sorption and desorption experiments confirmed the high sorption ability of SMZ for nitrates (Li, 2003; Schick et al., 2011).

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As for the other oxy-anions previously mentioned, the increase in nitrates sorption corresponded to the increase in HDTMA-Br loading onto zeolite (100%, 150%, and 200% of zeolite’s ECEC), which indicated the formation of a surfactant bilayer and of nitrates retention by anion exchange. The sorbed nitrates were desorbed instantaneously but slowly from SMZ, under different ionic strength conditions, probably owing to competition with other anions of the background electrolyte solution (Li, 2003). Based on the slow rate of nitrate release from the SMZ, as well as the results of a pot experiment in which SMZ loaded with nitrates was used as N fertilizer for corn, it was concluded that SMZ could be used as a slow-release fertilizer for NO3-N after its use for nitrates removal from aqueous systems (Li, 2003). Similar findings with respect to the formation of a surfactant bilayer, the effect of solution ionic strength, and the mechanism of nitrates retention by SMZ are also reported by other researchers who investigated zeolite modified with cetylpyridinium bromide. They also reported that the presence of chlorides, sulfates, or bicarbonates in solution only slightly reduced SMZ efficiency for nitrates (Zhan et al., 2011). In addition, it was reported that SMZ can be easily and fully regenerated, and consequently it could be used continuously to purify aqueous systems enriched with nitrates by using two series of SMZloaded columns. However, contrary to the batch experiments previously mentioned, continuous leaching of HDTMA from SMZ was evident during column experiments and a reduction in HDTMA concentrations in the effluent was achieved by subsequent filtration through an activated carbon bed (Schick et al., 2011). As far as other anions are concerned, SMZ effectively sorbed selenates, sulfates (Haggerty and Bowman, 1994; Li et al., 1998a), and perchlorates (Zhang and Pathan, 2007; Zhang et al., 2007). Particularly for perchlorates, their affinity for SMZ was higher than that of other anions in solution and followed the increasing order: Cl < HCO3  < SO4 2 < NO3  << ClO4  (Zhang et al., 2007). Boron sorption on SMZ was studied at different pH values, at a preliminary level (Dionisiou et al., 2013b). In all pH values the relative amounts of sorbed B were much lower than other anions reported in the literature. This was expected for the strongly acid and neutral pH, where B species is the neutral H3BO3, whereas it was unexpected for the strongly alkaline pH, where BðOHÞ4  are formed at appreciable amounts. In addition to the low sorption capacity of SMZ, B concentration in the equilibrium solution was above the critical limit of 3 mg L1 in irrigation waters with respect to the risk of B phytotoxicity (Dionisiou et al., 2013b).

23.5.2 REMOVAL OF INORGANIC CATIONIC POLLUTANTS Except for anions, SMZ has the ability to retain cations, mainly because of the negative charge of its interior, and to a much smaller degree because of certain negatively charged sites of its external surface, which remained unmodified. Consequently, SMZ could also be used as a sorbent for cationic heavy metals from waters. In a study of Cd2þ sorption to natural zeolite and SMZ, both materials were effective sorbents of 2þ Cd . The natural zeolite was slightly superior to SMZ (Cortes-Martinez et al., 2004). However, in another study of Sr2þ and Pb2þ sorption by both kinds of zeolite, Sr2þ was strongly retained by the natural zeolite but not by the SMZ (Bowman et al., 2000). This was attributed to the saturation of the SMZ’s external exchange sites by the surfactant’s molecules and to the low penetration of Sr2þ into the SMZ’s interior. Contrary to Sr2þ, Pb2þ sorption by natural zeolite and SMZ was similar. This difference in SMZ’s sorption capacity for the two cations was attributed to different retention

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mechanisms. Pb2þ may be bound to the remaining positively charged external surfaces of SMZ by mechanisms in addition to electrostatic attraction, such as precipitation onto the exterior or retention by the interior of SMZ (Bowman et al., 2000). The influence of quaternary ammonium surfactants (HDTMA, dodecytrimethyammonium, and octyltrimethyammonium) on the retention of Csþ, Sr2þ, Pb2þ, Zn2þ, and La3þ by a modified clinoptilolite was investigated (Li et al., 2002). Generally, the SMZ retention capacity for the cations decreased compared with clinoptilolite, probably because the exchange sites were blocked by the surfactant’s molecules. However, Csþ and Pb2þ retention by the SMZ was higher than for the other cations, probably because of selective sorption of Csþ and specific sorption of Pb2þ, in addition to the cation-exchange process. Upon an increase in the surfactant’s carbon chain length, metal desorption from clinoptilolite increased, especially that of Sr2þ and La3þ, because of competition of the surfactants’ molecules for the same exchange sites (Li et al., 2002).

23.5.3 REMOVAL OF ORGANIC POLLUTANTS Apart from inorganic anionic and cationic pollutants, SMZ has also the ability to sorb and thus to remove organic compounds (polar and nonpolar) from aqueous systems. This ability is probably the result of hydrophobic attraction of the organic pollutants (especially nonpolar) to surfactant molecules attached to SMZ. However, retention of the polar organics by electrostatic attraction also cannot be excluded. In a study of perchloroethylene sorption on SMZ, perchloroethylene was most efficiently sorbed by the SMZ, in the case of a monolayer formation of the surfactant onto zeolite’s surface. At higher surfactant loading levels, perchloroethylene sorption was limited (Li and Bowman, 1998). It was concluded that the perchloroethylene was retained in the organic phase formed by the surfactant onto the zeolite’s surface. In a similar study, perchloroethylene concentrations in contaminated water were reduced by two orders of magnitude owing to sorption by SMZ (Bowman, 2003). Sorption of benzene and its ionizable products, phenol and aniline, on a clinoptilolite-rich zeolitic tuff modified with HDTMA-Br was also studied (Li et al., 2000). The results showed that SMZ was effective in sorbing ionizable organic compounds as well as the hydrophobic nonionic. Similarly, SMZ efficiently sorbed volatile organic compounds benzene, toluene, ethylbenzene, and xylene (Bowman, 2003; Ranck et al., 2005; Altare et al., 2007) and the ionizable phenol and p-chlorophenol (Kuleyin, 2007); p-chlorophenol had a higher degree of sorption than phenol, probably because of the more hydrophobic nature of p-chlorophenol. SMZ modified with stearyldimethylbenzylammonium chloride was an effective sorbent for the pesticides atrazine, lindane, and diazinone, especially when a monolayer surfactant cover had formed on its surface (Lemic et al., 2006). The sorption ability of the same SMZ for polycyclic aromatic hydrocarbons (PAHs), ie, fluorene, phenanthrene, benz[a]anthracene, fluoranthene, and pyrene, was studied. SMZ with monolayer coverage was the most effective for sorption of PAHs, with the more hydrophobic PAHs replacing the less hydrophobic compounds on the SMZ surface (Lemic et al., 2007). Furthermore, SMZ was an efficient sorbent for reactive azo dyes, because of their retention by electrostatic attraction on the surfactant’s bilayer formed on zeolite’s surface (Benkli et al., 2005). The same was reported for clinoptilolite-heulanditeerich tuff modified with octadecyldimethylbenzyl ammonium as a sorbent for ochratoxin A (Dakovic et al., 2003). SMZ removed fulvic acids from solution. The optimum HDTMA loading level was 120% of the zeolite’s ECEC, at which 98% of fulvic acids were sorbed by the SMZ. The sorption

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of fulvic acids increased in the presence of Ca2þ and Mg2þ, whereas it decreased in the presence of other organic substances such as phenols (Wang et al., 2006). In addition, SMZ totally removed bacteria (Escherichia coli) and viruses from sewage effluents and groundwater (Bowman, 2003; Schulze-Makuch et al., 2003).

23.6 CONCLUSIONS Natural zeolite has a high ability to retain heavy metals (especially Pb2þ), ammonium, and radionuclides (especially Csþ); consequently, it can be used effectively as a sorbent to decontaminate natural waters and wastewater. However, after using natural zeolite for water decontamination, the fate of the loaded zeolite with heavy metals and/or radionuclides needs further investigation. The same stands for NH4 þ -loaded zeolite regarding its efficiency as slow-release N fertilizer, and the financial feasibility of such use. SMZ has a high sorption capacity for anionic inorganic pollutants (specifically oxy-anions). Its sorption capacity for organic compounds and certain cations (eg, Pb2þ) is also appreciable. However, the sorption mechanism in relation to the arrangement of the surfactant molecules on zeolite’s surface (ie, monolayer, patchy, or complete bilayer) and the stability of the arrangement need further clarification. In addition, multiple-component (inorganic anions and cations and organic compounds) sorption studies are rare in the literature. As for natural zeolite, the fate of SMZ loaded with anions (ie, regeneration and reuse) after it is used for water decontamination needs further investigation. The same stands for the possibility of using SMZ loaded with nitrates and/or phosphates as slow-release N and/or P fertilizer. However, in that case, the potential adverse environmental impacts of released surfactant molecules from SMZ after mixing with soil should be taken seriously.

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