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BIO Magazine - Purification of Water by Hybrid Materials based on Functional Dendritic Polymers Δεκέμβριος 2015
Δεκέμβριος 2015 No38

BIO Environment

Purification of Water by Hybrid Materials based on Functional Dendritic Polymers
Purification of Water by Hybrid Materials based on Functional Dendritic Polymers

 

Abstract

Dendritic polymers, depending on the nature of their nanocavities, encapsulate a diversity of organic and inorganic compounds. Encapsulation inside the nanocavities of dendritic polymers has also been applied for pollutants dissolved in water leading in this manner to its purification. For this process filtering devices were formed by non-covalent or covalent binding of functional dendrimers or hyperbranched polymers on the pore surfaces of their ceramic materials. The strategies proved quite effective and convenient to operate for water purification, since the pollutants are reduced at the level of a few ppb. Also these processes can be scaled-up for the industrial production of ultra pure water, employing primarily the cost-effective hyperbranched polymers. 

 

Introduction

Dendritic polymers[1-3] exhibit a tree-like architecture primarily consisting of symmetrical dendrimers and random hyperbranched polymers. Dendrimers are prepared under tedious experimental conditions being highly branched symmetrical macromolecules of nanosized dimensions. They have well-defined molecular weight, consisting of a central core, repeating units and terminal functional groups. On the other hand, hyperbranched polymers, although sharing analogous structural features with dendrimers they are non-symmetrical and poly-dispersed. Importantly, the latter are of lower cost compared to dendrimers since they are prepared under facile experimental procedures. The main structural characteristics of both  polymers is the existence of nanocavities in the interior of which various molecules can be encapsulated and  also the exhibition of a significant number of groups on their external surface which can easily be functionalized .

Taking advantage of the property of dendritic polymers to encapsulate various  compounds, encapsulation of  bioactive compounds has led to drug delivery systems.[4-5] while the incorporation of metal ions and subsequent reduction to metal nanoclusters afforded catalytic systems.[6] In analogous manner, encapsulation of  water contaminants inside the nanocavities leads to novel water purification systems. The microenvironment of nanocavities dictates the nature of encapsulated pollutants. The mode of attachment of the external  groups of dendritic polymers on glass or ceramic filters leads to two types of hybrid materials, i.e: a. Hybrid materials formed by non-covalent binding of functional dendritic polymers to the ceramic devices and b. Hybrid materials made by covalent attachment of functional dendritic polymers to the ceramic filtering devices.

 

Purification of Water Employing Hybrid Materials Based on Alkylated Dendritic Polymers 

 To render encapsulation of pollutants a practically useful process for water purification, dendritic polymers have to be firmly attached on the ceramic filters thus avoiding their leaching and transfer in the water under treatment. This was achieved through long alkyl-chain functionalization of dendritic polymers. 

As a proof-of-concept study[7] it was conducted alkylation of diaminobutane poly(propylene imine) dendrimers of the generations bearing 32 and 64 surface primary amino groups, i.e. DAB32, and DAB64. They were alkylated by interacting with long n-alkyl-chain isocyanates affording water insoluble alkylurea dendritic polymers, Figure1. Solutions of alkylated dendrimers formed film on  glass surface containers or on the pores of ceramic filters.

 

Encapsulation efficiency of contaminants inside cavities of octyl and octadecyl DAB32 and DAB64 dendrimeric derivatives, established that the length of the aliphatic chains and the generation of the dendrimers only slightly affect the inclusion rate. Alkylated DAB64 polymers incorporate a slightly higher percentage of pyrene although the adsorption rate remains the same. Also, the inclusion of phenanthrene is slightly affected by the length of the aliphatic chains while octadecyl substituted dendrimers show a higher adsorption percentage. Furthermore, alkylated derivatives of DAB32 exhibit a lower inclusion rate, the entrapment of fluoranthene requiring a longer period of time than the other polycyclic aromatic hydrocarbons (PAHs).

Based on these results, one can safely assume that inclusion of molecules is determined by the dynamic character of the dendritic chains which can embrace solutes of various sizes and shapes. These PAHs were solubilized inside dendrimers because they are slightly soluble in water and therefore they are at a high-energy state. Thus, their inclusion in the lipophilic environment of the dendrimers is a thermodynamically favorable process. Additionally, the encapsulation is further amplified by the formation of charge-transfer complexes between PAHs  and tertiary amino groups.[8]

The inclusion formation constants, K, were of the order of 108 M-1 for pyrene and fluoranthene [7], i.e., many orders of magnitude higher than those reported for activated carbon (1.4x104 - 3.4x105 M-1)[9] or cyclodextrins (10 -103 M-1)[10] and comparable to those for cyclodextrin polymers (1x108 - 5x109).[10] A chemical reaction is considered irreversible when K is of the order of 2x109 M-1,[10] and therefore the formation of the complex in the solid film should be considered as practically irreversible. The inclusion formation constant of phenanthrene is much smaller, i.e. of the order of 1 x 106 M-1. This was attributed to the higher solubility of phenanthrene in watercomparedto fluoranthene or pyrene. This molecule is therefore in a more favorable energy state in water and formation of the inclusion complex is less favored.

The same conclusion was obtained when the Gibbs free energies (ΔG0 = -RTlnK) were calculated. It becomes evident that the absorption of the contaminant inside nanocavities is a spontaneous process with a driving force, ΔG0, between -8 and -11 kcal/mol, in contrast to other water purification procedures such as reverse osmosis or filtration which require energy to drive the process to completion.

The loading capacities of the alkylated dendrimeric derivatives have maximum capacities, ranging from 6 to 67 mg of pollutant per g of dendrimer depending mainly on polycyclic aromatics. Although, the inclusion of these aromatics is a non-reversible process in water, it is reversible in non-polar organic solvents. The binding constants between the dendrimeric polymers and the pollutants in hexane are 11.2, 11.1 and 10.9 M-1G0 ≈ 1.4 kcal/mol) for pyrene, phenanthrene and fluoranthene respectively. Therefore, films originating from these functional dendrimeric materials were readily regenerated when treated by the appropriate non-polar solvent. This parameter contributes to lowering the cost of the purification process.

This investigation was extended.[11]  employing a variety of commercially available, low-cost, hyperbranched polymers. Thus alkylated derivatives of poly(ethylene imine), PEI (Mn=5000), polyglycerol, PG (Mn=5000), and hyperbranched polyesteramides, i.e., S1200, H1500, PS2550 and SL1520, (Figure 2) were also used for water purification. The structural diversity of these polymers eventually affected their encapsulation efficiency and loading capacity. 

 

 

For PEI alkylated dodecyl- and octadecylurea derivatives the length of the aliphatic chains affect considerably the inclusion rate in contrast to symmetric dendrimeric analogues. Derivatives with shorter chains exhibit faster absorption rates. The symmetrical structure of dendrimers favors fast absorption for all the investigated pollutants. Thus, alkylated dendrimer DAB64 exhibits faster absorption rates compared to non-symmetric alkylated hyperbranched polymers functionalized with an aliphatic chain of 18 carbon atoms. Encapsulation is therefore a topochemically-affected process.

The alkylated derivatives of the hyperbranched polymers with glass transition temperatures below room temperature, exhibit in general, faster absorption rates than those exhibiting more rigid ones. Indeed DAB64 and PEI as well as S1200 have low Tg values (–84oC for DAB64, -25oC for PEI, 20oC for S1200 and SL1520) while H1500 and PS2550 are in a glassy state at room temperature (Tg >40oC for H1500 and >70oC for PS2550). Thus the flexibility of the dendritic scaffold favors the formation of adaptable cavities that can be tuned according to the size and shape of the pollutant favoring encapsulation of diversified pollutants.

The molecular nature of the dendritic scaffolds has also a marked influence on  absorption kinetics. Comparing octadecyl hyperbranched derivatives of H1500 and PS2550, the presence of aromatic rings inside the cavities of the second one is significantly affecting absorption rate, which is apparently attributed to the higher chemical affinity to polyaromatic pollutants. On the other hand, poly(ethylene glycol) hyperbranched derivative, PG, which exhibits a rather hydrophilic environment leads to low absorption kinetics and inclusion formation constants. The inclusion formation constantsK, were of the order of 108-106 M-1 for pyrene and 107-106 M-1 for fluoranthene. The inclusion formation constant K of phenanthrene is much lower,  of the order of 106-105 M-1. The same conclusion was reached with the Gibbs free energies (ΔG0 = -RT lnK). It is evident that the absorption of the contaminant inside the film is a spontaneous process with a driving force, ΔG0, between –7.5 and -11 kcal/mol.

Furthermore for the hyperbranched polymers, Kpyrene>Kfluoranthene>Kphenanthrene . This is in accordance with the order of their decreasing solubility in water, establishing that the driving force behind the inclusion process is the Gibbs free energy obtained for stabilizing the PAHs  inside the lipophilic interior. 

Loading capacities of the alkylated hyperbranched polymers have maximum capacities ranging from  6 to 54 mg of pollutant per g of polymer depending on the PAH. Furthermore, thin films loaded with pyrene were regenerated with acetonitrile and the calculated negative ΔGo, ranging from -1.7 to -2.6 kcal/mol indicate that regeneration is a spontaneous exothermic process. As expected, the polymers with the most hydrophilic cavities exhibited the smallest ΔGo values.

Further work [12] resulted in the development of a continuous process permitting removal of organic pollutants employing a simple filtration step, which can be easily scaled-up and applied in a diversity of pollutants. Thus titanium dioxide ceramic multichannel tubes (porosity 37 %, median pore diameter 8 μm were employed ), Figure 3, which were impregnated with octyl- and octadecylurea DAB and PEI derivatives, and used for purification of water contaminated with PAHs,  trihalogen methanes (THMs), monoaromatic hydrocarbons (BTX) and pesticides (simazine). It has been established that for the retention of these more polar pollutants employing these hybrid filter modules, it was required increased impregnation percentage and adequate contact time between the alkylated dendritic polymers and polluted water. The latter was achieved by low flow rates, increased length of the ceramic modules or alternatively by recirculation of polluted water. 

 

A recent study [13] optimized the type of hybrid hyperbranched polymer/ceramic filters. Alumina ceramic filters with different porosities and mean pore sizes were impregnated with n-octylurea PEI dissolved in chloroform. The main conclusion derived from this study is that polymer impregnation protocol must be experimentally defined for each ceramic membrane, which is dependent on its pore structure characteristics.

 

Water Purification Employing Hybrid Materials Based on Silylated Dendritic Polymers 

In this strategy dendritic polymers are bound on the surface of ceramic pores by interacting silylated dendritic polymers with their surface hydroxyl groups. Dendritic polymers will become completely non-leachable from the surface of the filters. Based on the work of Dvornic [14]  on organosilicon dendritic polymers and their nanostructured networks, analogous polymers were prepared. [15]   Amino groups of DAB32 or PEI were interacted with 3-(triethoxysilyl) propyl isocyanate. The triethoxysilyl derivatives obtained were hydrolyzed to silanols which polycondensate either intramolecularly or intermolecularly, Figure 4, affording the dendritic silanol networks DAB-Si and PEI-Si. These silanols  condensate with the hydroxyl groups which  are present on the surface of activated ceramics, forming a chemically bound film, Figure 5. Titanium dioxide ceramic filters were  treated with triethoxysilyl derivatives affording a nanostructured dendrimeric network covalently bound on the ceramic filter.

 

 

Encapsulation efficiency of these networks was performed by inclusion kinetic experiments [15] using a thin film method as the one employed the alkylated dendrimers. These networks have the capacity to absorb polycyclic aromatic hydrocarbons from water. Specifically, PEI-Si network removes 92% of pyrene and 70% of phenanthrene from water after 2 hours. For the same period of time DAB-Si siloxane network removes 80% of pyrene and 48% of phenanthrene. However the absorption of β-naphthol is significantly slower and equilibrium is reached after several days. Lower absorption is due to its higher water solubility compared to pyrene and phenanthrene which does not favor its absorption inside  dendrimers nanocavities. The inclusion formation constants were of the order of, 107 M-1 for pyrene, 106 M-1 for phenanthrene and 105 M-1 for β-naphthol. These are correlated with water solubility of pollutants whereas it is only weakly correlated with the type of dendritic architecture.

Experiments were performed employing ceramic filters which were impregnated with silylated dendrimeric and hyperbranched derivatives and cured at 90oC for 50 hours. These hybrid materials were used for continuous filtration of water containing pyrene, phenanthrene or β-naphthol. Comparing these results with non-treated filters it is evident that the impregnation with dendritic networks largely enhances the absorption efficiency of polycyclic aromatic hydrocarbons. Tube filters contaminated with pyrene, phenanthrene and β-naphthol were successfully regenerated with acetonitrile with gentle heating (< 50 oC). After regeneration, repeated inclusion of contaminants proceeds with no apparent change in the binding efficiency.

The overall performance of DAB-Si network for PAHs absorption is inferior to that of PEI-Si network especially at high flow rates (4-7 ml/min). The fully developed symmetrical structure of DAB32 and the complete functionalization of the amino groups by the introduction of ethoxysilanes results, after their hydrolysis, to high concentration of silanol groups at the external surface. The latter favor the  formation of many intermolecular siloxane bridges following curing. Network becomes comparatively more rigid to the one originating from the hyperbranched polymer, reducing the inclusion rate of bulky molecules. It should be noted that the rigidity of the polymeric structure affects only the inclusion rate and not the inclusion constant. It was therefore concluded that although the driving force for the inclusion process is the Gibbs free energy required for stabilizing the compounds inside the cavities, the size and shape of the absorbed molecules should not be disregarded since  they affect kinetics.

The work was further extended [16] employing silylated polymers, i.e. DAB, PEI and PG, which impregnated three ceramic substrates namely, TiO2, Al2O3 and SiC of varying porosity and median pore size diameter. These coated ceramic filters were employed for purification of water with a continuous filtration process. A variety of pollutants was tested including polycyclic aromatic hydrocarbons, monocyclic aromatic hydrocarbons, trihalogen methanes, pesticides and methyl-tert-butyl ether. Impregnated ceramic membranes exhibited superior pollutant absorption compared to non-impregnated analogs. Furthermore, increasing contact time between the pollutant and silylated dendritic coating through reducing water flow rate and/or using of ceramic substrates with smaller pore size and higher porosity improved water purification efficiency.

In connection with the described hybrid materials based on silylated dendritic polymers, analogous materials, sharing similar structural characteristics have been developed, which potentially could be used for the same purpose. Thus SiO2-Poly(amidoamine) dendrimer hybrids [17] and attapulgite grafted hyperbranched aliphatic polyester[18] were prepared. These hybrids exhibited high metal ions complexing capacity as has been shown for Cu2+, Hg2+, Zn2+ and Cd2+.

 

Conclusions

The diversity of nanocavities of dendritic polymers as far as the size, polarity and interaction ability are concerned, and the property of external groups of these polymers to be conveniently functionalized, constitute the basis for the development of novel filtering devices for effectively encapsulating pollutants dissolved in water. Thus the hybrid materials obtained from ceramics and the adsorbed dendrimers or hyperbranched  polymers have proved equally effective for water purification. The results obtained at the laboratory scale, as recently reviewed, [19,20] are quite promising  since remaining pollutants in water are at a  few  ppb level. It is anticipated that in the near future this strategy will be scaled-up for industrial production of ultra pure water and would be based  primarily on hyperbranched polymers [20,21]  due to their facile  production and consequently low cost. 

 

References

 [1]  Fréchet JMJ &. Tomalia DA Dendrimers and other dendritic polymers. 1st edition, J. Wiley & Sons, Ltd., Chichester 2001 and references cited therein.

[2]  Newkome GR et al, Dendrimers and dendrons. Concepts, syntheses, perspectives. 1st edition, Wiley-VCH, Weinheim 2001 and references cited therein.

[3]  Lee CC et al, Nature Biotechnol. 2005; 23: 1517-1526.

 [4] D’Emanuele, A & Attwood,  D, Adv. Drug Delivery Rev. 2005; 57: 2147-.2162.

 [5] Paleos, CM et al, Mol. Pharm. 2007; 4: 169-.188

 [6] Zhao, M &  Crooks, RM, Adv. Mater. 1999; 11: 217.-220.

 [7] ] Arkas, M et al, Chem. Mater. 2003; 15: 2844-2847

 [8]. Baars, MWPL et al, Chem. Commun. 1997; 20:1959-1960. 

[9]. Ma, DQLM, Filtration Sep. 1999; 36: 26-.32.

[10] Ma, DQLM, Chem. Mater. 1999; 11: 872-874.

[11] Arkas, M et al,  J. Appl. Polym. Sci.  2005; 97: 2299-.2395.

[12]  Arkas, M et al, Environ. Sci. Technol. 2006; 40: 2771-2777.

[13] Tsetsekou, A et al, J. Membr. Sci. 2008; 311: 128-135.

[14]  Dvornic, PR, J. Polym. Sci., Part A: Polym. Chem. 2006; 44: 2755-2773.

[15] Arkas, M et al, Chem. Mater, 2005; 17: 3439-3444.

[16] Allabasi, R et al, Water Res. 2007; 41: 476-486.

[17] Ruckenstein, E & Yin, W. J. Polym. Sci., Part A: Polym. Chem. 2000; 38:1443-1449.

[18] Liu, P & Wang, JM. Hazard. Mater.2007;149:75-79

[19] Savage, N & Diallo, MS, Journal of Nanoparticle Research, 2005;7:331-342.

[20] Paleos, CM, Macromol. Mater. Eng. 2010; 295: 883-898.

[21] Irfan, M, Ind. Eng. Chem. Res. 2010; 49:1169-1196.

 

 

 

Constantinos M. Paleos NCSR "Democritos"


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