Removal of Water Contaminants

Removal of pollutants from water is a serious challenge for water supplying companies and municipalities. In fact, heavy metals, organic compounds like aromatic derivatives or synthetic dyes harmful to organisms are toxic and recalcitrant pollutants, that tend to accumulate in water. It is now well established that for the wastewater treatment, adsorption is a much better process than other physical techniques because of its efficiency and economy (Dabrowski 2001).

The parental CDs are soluble in water, therefore they cannot be used directly in the adsorption of pollutants from water. For these reasons, several workers have attempted to immobilize CDs on appropriate supports, including organic, polymeric and mineral materials. Different approaches have been developed for the preparation of CD functionalized materials, like the synthesis of CD polymers (Fig. 2.6a) and the grafting or coating of CD onto insoluble matrix (Fig. 2.6b, c) (Crini 2008).

These materials present several advantages in wastewater treatment like high stability, recoverability, reutilization, low cost, sorption capacity increase (Li et al. 2010).

2.3.1.1 Cyclodextrins Coupled with Insoluble Support

Several insoluble supports (organic or inorganic matrix) have been studied to produce materials containing CDs. All these studies were demonstrated on a laboratory scale, only a few of them reported the removal of pollutants from industrial waste-water (Chen et al. 2007; Fan et al. 2003a).

The coupling between CDs and natural materials has been extensively investigated for the removal of pollutants because they are abundant, renewable and biodegradable (Crini 2005, 2006). Chitosan bearing CDs are efficient sorbents for selective extraction ofp-nonylphenol, bisphenol (Aoki et al. 2003), phenol (Li et al. 2009a) and p-nitrophenol (Tojima et al. 1999). They have also been tested for the decontamination of water containing textile dyes, with a rate of sorption and a global efficiency superior to that of the parent chitosan polymer and of the well-known CD-epichlorohydrin (CD-EP) gels (Martel et al. 2001). a-CD-linked chitosan beads were used to remove various phthalate esters from aqueous solution both in batch and continuous column tests (Chen et al. 2007) . The a-CD-linked chitosan beads could be reused 15 times in the real wastewater at least.

Cellulose is also a natural carbohydrate polymer, insoluble in water. Crini and Peindy (2006) used b-CD-carboxymethylcellulose (CD-CMC) for the removal of C.I. Basic Blue 9 (BB 9) from aqueous solutions. Bonenfant et al. (2010) have shown that CD-CMC could constitute a good adsorbent for removing nonylphenol 9 mole ethoxylate (NP9EO) from wastewater with adsorption capacity of 83-92 wt% depending on the initial NP9EO concentration in liquid phase.

The efficiency of CD derivatives grafted or coated onto silica for aromatic pollutants has been compared (Phan et al. 1999, 2000, 2002). Sorption capacities were closed to each other independently of the synthesis procedure for a given pollutant. They were affected by the same parameters e.g. contact time, CD content, structure and concentration of pollutant. The mechanism of sorption is both physical adsorption in the polymer network (for supports obtained by coating) and/or the formation of an inclusion complex between CD and guest molecules. With respect to environmental (preparation in a water medium) and financial aspects (one-step reaction, fewer and inexpensive reagents), coating seems better. Unfortunately, the low CD loading of these CD-coated (178 mmol/g) or grafted materials (159 mmol/g) limit their absorption capacities.

Bibby and Mercier (2003) developed high CD loading materials by incorporating CD groups inside the pore channels of mesoporous silica with uniform channel structure (denoted CD-HMS). Adsorption studies on CD-HMS materials have shown that their open-framework structures promote unhindered interactions of organic guests with their CD binding sites because they easily permeate the material and reach the abundant CD groups inside the adsorbent.

Sawicki and Mercier (2006) have described the efficiency of CD-silica nanocomposites for the removal of pesticides like DDT (dichlorodiphenyltrichloro-ethane) which could be removed from water in low levels typically encountered in environment.

Other siliceous materials were synthesized between CDs, aminopropyltriethox-ysilane (APTES) and tetraethoxysilane (TEOS) to remove remazol Red 3BS from aqueous solution (Asouhidou et al. 2009). Results showed a good adsorption of the dye and an efficient regeneration after repeated sorption-regeneration cycles. Aromatic compounds like phenolic derivatives were also removed from water by CD bonded silica under solid phase extraction (Fan et al. 2003a, b).

Adsorptive interaction of mercury ions with silica chemically modified by b-CD was demonstrated as well by Belyakova (Belyakova et al. 2008a, b). It has been established that isotherm of mercury(II) adsorption on the b-CD-containing silica is given by isotherm of Langmuir type.

More exotic supports were also synthesized. Salipira et al. (2007, 2008) have polymerized CDs and carbon nanotubes. Even if carbon nanotubes are expensive, the synthesis needs a small percentage of it. These supports were promising because they match the properties of CDs and carbon nanotubes for absorption. They absorb trichloroethylene (TCE) from water at concentration loads of part per million.

Shao et al. (2010) have followed the same idea but with a different procedure. They synthesized multiwalled carbon nanotubes grafted with P-CD by N2 plasma induced grafting technique, an environmentally friendly technique, and applied the new material for the removal of polychlorinated biphenyls from aqueous solutions.

Cationic exchange textiles have been developed to remove heavy metal (Pb, Cd, Zn, Ni) from water (Ducoroy et al. 2008). Compared to other supports, they are less expensive and non-toxic.

Ceramic membranes impregnated with CDs polymers removed polycyclic aromatic hydrocarbons (up to 99%), monocyclic aromatic hydrocarbons (up to 93%), trihalogen methanes (up to 81%), pesticides (up to 43%) and methyl-tert-butyl ether (up to 46%) (Allabashi et al. 2007). Filters based on CDs were able to remove a wide range of pollutants. They have a lot of advantages like chemical inertia, long working life and thermal stability.

Table 2.2 summarizes some examples of the supports used to anchor CDs with the associated pollutants.

Table 2.2 Examples of supports used to anchor cyclodextrins and associated pollutants

Support

Pollutant

Reference

Organic compounds Chitosan

Cellulose Silica

Carbon nanotubes Ceramic membranes

Heavy metals Silica

Phenol, phenolic derivatives

Textile dyes Phtalate esters Textile dyes Phenolic derivatives Phenol, phenolic derivatives

Acetic acid Textile dyes

Chlorinated pesticides (DDT) p-nitrophenol, TCE Polychlorinated biphenyls Aromatic hydrocarbons, trihalogen methanes, pesticides

Hg2+

Cationic exchange textiles Pb2+, Cd2+, Ni2+, Zn2+

(2003), Li et al. (2009a) Martel et al. (2001) Chen et al. (2007) Crini and Peindy (2006) Bonenfant et al. (2010) Phan et al. (2000, 2002), Bibby and Mercier (2003), Fan et al. (2003a, b) Phan et al. (2000, 2002) Phan et al. (2000), Asouhidou et al. (2009)

Sawicki and Mercier (2006) Salipira et al. (2007) Shao et al. (2010) Allabashi et al. (2007)

Generally, the sorption mechanism combines the inclusion complex formation resulting from the presence of CD but also from physico-chemical interactions. These interactions depend on the chemical structure of the pollutant, the type of sorbent and the contact time (static or dynamic). All these new adsorbents showed a good stability, satisfactory recoveries and low detection limits. They tended to be useful in the production of environmentally friendly materials.

2.3.1.2 Cyclodextrin Polymers

Numerous works involving the synthesis of insoluble CD polymers using bifunc-tional cross-linkers have been reported. Various linkers have been utilized such as epichlorohydrin (EP), toluene-2,4-diisocyanate (TDI) and hexamethylene diisocya-nate (HMDI) in the formation of these nanoporous CD polymers (Li and Ma 1999; Crini and Morcellet 2002; Crini 2003 ; Mhlanga et al. 2007; Trotta and Cavalli 2009). The most thoroughly studied polymerization is the cross-linking reaction with epichlorohydrin (Murai et al. 1997; Crini 2003, 2008; Crini et al. 2007; Romo et al. 2008; Li et al. 2010). Many studies were completed on laboratory made hand water; only one of them was done with industrial water (Yamasaki et al. 2006).

Crini studied the kinetic and the equilibrium on the removal of dyes (Crini 2003, 2008; Crini et al. 2007) ; The mechanism of adsorption was demonstrated as both physical adsorption and hydrogen bonding. The adsorbent exhibited high sorption capacities between 30 and 54 mg of dye per gram of polymer.

Yilmaz et al. (2010) have also described the use of CD polymers for dye removal. In this case, 4,4; -methylene-bis-phenyldiisocyanate (MDI) and HMDI were preferred to EP. The polymers showed high affinity towards azo dyes (Evans Blue, Chicago Sky Blue, direct violent 51, methyl orange and Congo Red) and aromatic amines (Ozmen and Yilmaz 2007; Ozmen et al. 2008; Yilmaz et al. 2007, 2010).

Several studies have been published so far concerning the sorption of phenol or phenol derivatives (Mamba et al. 2007; Romo et al. 2008; Li et al. 2010). The efficiency of crosslinked CDs towards phenol as low as 0.8 ppm has been demonstrated (Yamasaki et al. 2006, 2008). Romo et al. (2008) have shown that the extraction efficiency of b-CD polymers is comparable to other conventional sorbents. Abay et al. (2005) described the suspension polymerization method to made microsphere of CDs. They could manage the diameter of their beads and trap phenolic compounds.

Nanospheres of CDs exhibit also a high ability to absorb aromatic organic molecules like phenol and toluene (Baruch-Teblum et al. 2010). Nanospheres were synthesized under miniemulsion polymerization. They can be used under various forms like granular solids, powders, and thin films. The same procedure was also employed for styrene and chlorinated volatile organic compounds (Kammona et al. 2008).

It has recently been reported that, by reacting CDs with suitable cross-linking agents, a novel nanostructured material consisting of hyper-cross-linked CDs can be obtained; these are known as nanosponges (Trotta and Tumiatti 2003) . Polluting agents, like heavy metals and many organic substances can easily be removed from water with these nanosponges with performances many times superior to that of active carbon. Furthermore, the properties of the nanosponges can easily be restored

Table 2.3 Examples of pollutants trapped by cyclodextrin polymers

Pollutant

Cross-linking agent

Reference

Organic compounds Textile dyes

Phenol and phenolic derivatives

Benzene derivatives

Aromatic amines Ionic surfactants Halogenated compounds Naphthalene derivatives Heavy metals Cr6+

Epichlorohydrin Diisocyanate

Epichlorohydrin

Diisocyanate

Succinyl chloride

Epichlorohydrin

Diisocyanate

Diisocyanate

Epichlorohydrin

Meta-diisopropylbenzene

Epichlorohydrin

Diisocyanate

Crini (2003, 2008), Crini et al. (2007) Ozmen and Yilmaz (2007), Yilmaz et al.

(2007, 2010); Ozmen et al. (2008) Yu et al. (2003), Romo et al. (2008), Li et al. (2010) Mamba et al. (2007), Romo et al. (2008), Yamasaki et al. (2008), Mahlambi et al. (2009), Baruch-Teblum et al. (2010) Romo et al. (2008) Yu et al. (2003) Baruch-Teblum et al. (2010) Yilmaz et al. (2010) Murai et al. (1997) Kammona et al. (2008) Orprecio and Evans (2003)

Mahlambi et al. (2009)

simply by washing them with ethanol, changing the pH, modifying the ionic strength or by mild warming (Trotta and Cavalli 2009) .

Other typical aromatic compounds were also investigated like naphthalene (Orprecio and Evans 2003), benzene derivatives under solid phase extraction attempts (Yu et al. 2003) or ionic surfactants (Murai et al. 1997).

Another type of polymers based on b-CD-ionic liquid polyurethanes has shown good ability for heavy metal and phenol derivatives removal (Mahlambi et al. 2009). These polymers were insoluble in water and organic solvents and show a high percentage of removal (up to 80% for organic pollutant and up to 100% for Cri+). Table 2.3 summarizes the different family of studied pollutants associated with the cross-linking agent used in the studies.

The adsorption mechanism was not dependent on the nature of the linker. The proposed adsorption mechanism involved several kind of interactions: physical adsorption in the polymer network, hydrogen bonding and formation of an inclusion complex due to the CD molecules through host-guest interaction. The CD molecules played the most important role in the mechanism. The polymers could be used as beads or powders. In addition, the adsorbent can be easily regenerated by extraction with solvents. The complexing abilities remain unchanged after many regeneration cycles.

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