Amphiphilic layered silicate clay for the efficient removal of organic pollutants from water

Abstract

An amphiphilic smectite clay, interlayered by regularly alternating galleries of organic [C₁₆H₃₃N(C₄H₉)₃ ⁺] and inorganic (Li⁺ ) exchange cations, is shown to be especially efficient for the removal of a chlorinated aromatic pollutant (2,4-dichlorophenol) from water in comparison to the fully-exchanged hydrophobic form of the same clay.

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Green Context

Selective removal of aromatics from water is of prime importance in the clean-up of contaminated water. It is also of great importance in the development of water-based organic chemistry, an important area of research in Green Chemistry, but one where the water solvent is never completely free of organic contamination after the reaction. The research here relates to the generation of composites based on clays, made hydrophobic by surfactant molecules alternating with Li ions from layer to layer, which form an environment capable of highly selective adsorption of organics from water. These relatively cheap and accessible materials show great potential in the removal of organic molecules from water.

DJM

Introduction

Smectite clay minerals have a substantial capacity for the sorption of cationic organic contaminants from aqueous solution (ca. 100 meq g⁻¹).¹ The binding of cationic organic species occurs through ion exchange of the alkali metal and alkaline earth cations initially present in the gallery region between the silicate layers. In contrast, the sorption of non-ionic organics by native smectic minerals is generally restricted to only the external surfaces of the clay tactoids.² The sorptive capacity towards non-ionic organics can be greatly improved, however, if the clay interlayers are modified by ion exchange with cationic surfactants that render the gallery regions lipophilic.^3,4. Long-chain alkylammonium ion exchange forms of smectite clays, for instance, afford especially lipophillic galleries.⁵ Organic pollutants readily penetrate the onium ion interlayers and become physically adsorbed through van der Waals interactions with the hydrophobic segments of the exchange cations and the siloxane surfaces of the clay.^6–8 In this way the onium ion clay functions as a solid state partitioning medium for the removal of the pollutant from solution.¹

The sorptive properties of organo cation exchange forms of smectite clays also can be tailored to adsorb specific organics through a judicious choice of the organic exchange cation.⁷ This selectivity makes organo clays even more attractive for the removal of specific contaminants (e.g., chlorinated organics) from water. However, most organo clays are exceptionally hydrophobic and cannot be wetted by an aqueous phase. This extreme hydrophobicity makes it impractical to use most organo clays for the adsorption of organic contaminants from water, because the clays rapidly segregate at the water–air interface. If an organo clay is used in packed bed form, severe channeling of the bed typically is encountered.

In the present work we describe the use of a new type of amphiphilic layered silicate clay for the removal of organic pollutants from aqueous solution. These recently reported clays are hybrid heterostructures containing regularly alternating galleries of organic [e.g., C₁₆H₃₃N(C₄H₉)₃ ⁺] and inorganic (e.g., Li⁺) exchange cations.^9,10 The segregation of the organic and inorganic cations into separate galleries, a behavior reminiscent of staging in graphite intercalation compounds,¹¹ allows for the desired amphiphilic properties.

Results and discussion

Fig. 1 schematically illustrates a 1∶1 mixed ion smectite clay heterostructure with organic onium ions and inorganic cations segregated in alternate galleries. The segregation of the ions is driven by differences in the solvation properties of the two very different types of cations. The inorganic galleries of the heterostructure are hydrophilic and readily solvated by water, whereas the organic galleries are lipophilic and avoid hydration. This difference in the polarities of the galleries makes it possible to disperse the heterostructure in either a polar or a non-polar solvent. When the heterostructure is wetted by water, for example, the inorganic galleries become fully hydrated and exfoliate into the aqueous phase. However, as is also illustrated in Fig. 1, the lipophilic organic galleries remain sandwiched between pairs of silicate layers in the exfoliated state. These dispersed packets of organophilic galleries are capable of adsorbing low concentrations of organic solutes from solution, while at the same time being dispersed in the aqueous phase. This wetting property is in contrast to the extreme hydrophobic character of a fully exchanged clay, wherein every gallery is interlayered with alkyl ammonium ions and the clay is completely non-dispersible in water.

Fig. 1 A schematic illustration of the segregation of organic and inorganic cations in the galleries of a 1∶1 mixed ion smectite clay heterostructure and the dispersion of the amphiphilic clay in water. Note that the inorganic galleries are easily wetted and exfoliated through hydration of the inorganic cations, whereas the lipophilic organic galleries remain sandwiched between pairs of silicate layers.

1∶1 Heterostructured smectite clays can be formed spontaneously upon mixing equal molar quantities of the inorganic and organic ion exchanged forms of the parent end member clays.^9,10 The redistribution of organic and inorganic ions into a mixed ion heterostructure is favored when the size of the head group on the organic cation is sufficiently large to occupy the area corresponding to one unit of charge on the clay surface.^9,10 Synthetic fluorohectorite with a layer charge density of ca. 1.2 e⁻ per O₂₀F₄ unit cell is well suited for heterostructure formation with readily available quaternary ammonium ions and alkali metal ions. Heterostructured clays can also be formed by the direct addition of a half an exchange equivalent of alkyl ammonium ions to a suspension of the inorganic exchange form of the clay.

We first compare the adsorption properties of a 1∶1 mixed ion clay heterostructure and a fully exchanged (homoionic) organo clay for the removal of an organic pollutant from water under equilibrium conditions. The isotherms shown in Fig. 2 were obtained for the adsorption of 2,4-dichlorophenol from water by a homoionic C₁₆H₃₃NBu₃⁺ exchanged fluorohectorite (circles) and a 1:1 C₁₆H₃₃NBu₃⁺∶Li⁺ mixed ion heterostructured fluorohectorite clay (squares). The binding capacities of the two clays are normalized in terms of mmol 2,4-dichlorophenol adsorbed per gram of alkylammonium ion bound to the clay phase. Both clays exhibit a type I isotherm characteristic of strong adsorbate–adsorbent interactions.¹² Note that the normalized adsorption capacities are essentially equivalent. This means dichlorophenol adsorption occurs almost exclusively in the lipophilic organic galleries of both the heterostructured clay and the fully exchanged homoionic clay. Little or no adsorptive capacity can be assigned to the Li⁺-exchanged surfaces of the heterostructure.

Fig. 2 Adsorption isotherms (25 °C) for the removal of 2,4-dichlorophenol from a vigorously stirred aqueous solution by homoionic C₁₆H₃₃NBu₃⁺ exchanged fluorohectorite (circles) and a 1∶1 C₁₆H₃₃NBu₃⁺ and Li⁺ mixed ion fluorohectorite heterostructure (squares). The clay suspensions were equilibrated for five days prior to being analyzed for 2,4-dichlorophenol uptake by the clays.

Although the normalized adsorption properties of the heterostructured clay and the fully exchanged homoionic organo clay are equivalent under equilibrium conditions, the advantage of the heterostructured clay becomes readily apparent when the adsorption of 2,4-dichlorophenol is followed as a function of time. Fig. 3 illustrates the time dependence for the adsorption of 2,4-dichlorophenol from water by homoionic C₁₆H₃₃NBu₃⁺ exchanged fluorohectorite (circles) and by a 1∶1 C₁₆H₃₃NBu₃⁺ and Li⁺ mixed ion heterostructured fluorohectorite clay (squares). Both clays were added to standing (unstirred) 2,4-dichlorophenol solutions in amounts containing equal quantities of onium exchange ions. The 2,4-dichlorophenol concentration reaches its final equilibrium concentration almost instantaneously when the mixed ion heterostructured clay is added to the 2,4-dichlorophenol solution. In contrast, the 2,4-dichlorophenol concentration decreases very slowly when homoionic C₁₆H₃₃NBu₃⁺ exchanged fluorohectorite clay is added to the solution. The final equilibrium concentration was not attained even after 25 h.

Fig. 3 Change in 2,4-dichlorophenol concentration with time when the solute is adsorbed under static conditions from water solution by a homoionic C₁₆H₃₃NBu₃⁺ fluorohectorite clay (circles) and a 1∶1 C₁₆H₃₃NBu₃⁺∶Li⁺ mixed ion fluorohectorite clay heterostructure (squares). The air dried clays were added to standing (unstirred) 2,4-dichlorophenol solutions at 25 °C.

The rapid uptake of 2,4-dichlorophenol by the heterostructured clay is concomitant with the almost instantaneous dispersion of the clay in the solution phase. The partitioning function of each organic interlayer is fully realized in the water–dispersed state. Consequently, the 2,4-dichlorophenol concentration decreases almost immediately upon the addition of the air-dried heterostructured clay. In contrast, the homoionic C₁₆H₃₃NBu₃⁺ exchanged organo clay rapidly segregates at the water–air interface, depleting dichlorophenol primarily near the surface of the solution. Consequently, there initially an abrupt drop in the dichlorophenol concentration, but further access to the lipophilic galleries of the segregated homoionic organo clay is greatly limited by diffusion. As a result, the 2,4-dichlorophenol concentration decreases at a significantly reduced rate.

Amphiphilic organic–inorganic clay heterostructures can be formed from smectite clays in combination with a variety of other inorganic cations and surfactant cations containing large head groups. For instance, adsorption properties similar to those illustrated in Fig. 3 were observed for 1∶1 heterostructures formed from Na⁺ as the inorganic cation and dodecyltributylammonium as the cationic surfactant. Thus, the concept of designing amphiphilic clay heterostructures for the efficient removal of organic solutes from bodies of contaminated water or waste streams is quite general. Also, although the heterostructured clay is dispersible in water, it also can be easily recovered by filtration or, alternatively, by allowing the clay suspension to settle over time and decanting off the supernatant aqueous phase. The ability to filter the suspended particles or to remove them through settling and decanting greatly improves the potential use of these materials for environmental cleanup and pollution prevention.

Experimental

A synthetic fluorohectorite, Li_1.12·[Mg_4.88Li_1.12]Si₈ O₂₀F₄ (Corning, Inc.), denoted as Li⁺-FH, was used as the starting smectite clay. The cation exchange capacity (CEC) of fluorohectorite, as measured by the ammonia selective electrode method,¹³ was 1.21 meq g⁻¹. A portion of the Li⁺-FH was converted to a homoionic clay by ion exchange reaction with C₁₆H₃₃NBu₃⁺ ions.¹⁰ A 10% excess of onium ions was used in the exchange reaction to ensure the complete displacement of the Li⁺ ions from the galleries. The 1∶1 heterostructured mixed cation exchanged form of fluorohectorite was prepared by a simple one-step/one-pot procedure wherein half an exchange equivalent of C₁₆H₃₃NBu₃⁺ ions was added to a Li⁺-FH suspension. Because the displacement of the inorganic cations by the onium ions was quantitative, the amount of [C₁₆H₃₃NBu₃⁺]Br⁻ surfactant added corresponded to the fraction of the onium cations present in the final mixed ion product. In a typical synthesis, 0.30 g of Li⁺-FH clay was first dispersed in ca. 100 ml of deionized water and then the surfactant was slowly added as the bromide salt. The reaction mixtures were stirred for at least 48 h at ambient temperature. Each product was washed free of excess salt and air-dried. All exchanged forms of fluorohectorite clay were checked for phase purity by XRD.^9,10

Adsorption isotherms (25 °C) were obtained for the adsorption of 2,4-dichlorophenol from water onto homoionic C₁₆H₃₃NBu₃⁺ exchanged fluorohectorite and a 1∶1 C₁₆H₃₃NBu₃⁺ and Li⁺ mixed ion heterostructured fluorohectorite clay. The adsorption isotherms were obtained by weighing known quantities of the air-dried clays into sealable polypropylene bottles. Next, precise quantities of a standard 2,4-dichlorophenol solution were added to the clay so that the amount of 2,4-dichlorophenol added was in the range 1.0–50 μmol per 100 ml of solution. The bottles were agitated on a shaker table for at least five days and then centrifuged to remove the clay. Next, the residual 2,4-dichlorophenol in solution was extracted into hexane from 5 ml of the supernatant. The 2,4-dichlorophenol concentration was then determined from the optical density of 2,4-dichlorophenol at 284 or 292 nm using an IBM 9430 UV-VIS spectrophotometer.

The change in 2,4-dichlorophenol concentration with time was followed using a static (unstirred) adsorption procedure. Air-dried powders of homoionic C₁₆H₃₃NBu₃⁺ exchanged fluorohectorite and air-dried 1∶1 C₁₆H₃₃NBu₃⁺∶Li⁺ mixed ion heterostructured fluorohectorite were sprinkled onto the surface of a 500 ml standing solution of 2,4-dichlorophenol in water (26.78 μmol/100 ml). A 5 ml sample was taken every hour from the center of the solution and the sample was extracted with hexane to determine the 2,4-dichlorophenol concentration.

Acknowledgment

The support of this research through NIEHS Grant ES 04911C is gratefully acknowledged.

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