9558
Ind. Eng. Chem. Res. 2008, 47, 9558–9565
Adsorption and Competitive Adsorption on Zeolites of Nitrophenol Compounds Present in Wastewater Bachar Koubaissy, Guy Joly, and Patrick Magnoux* UniVersite´ de Poitiers, Faculte´ des Sciences, Laboratoire de Catalyse en Chimie Organique, UMR CNRS 6503, 40 AVenue du Recteur Pineau, 86022 Poitiers Cedex, France
This work investigates the removal of various nitrophenolic compounds (ortho-nitrophenol (ONP), paranitrophenol (PNP), meta-nitrophenol (MNP), and 2,4-dinitrophenol (2,4-DNP)) from aqueous solution using hydrophobic FAU zeolites. The adsorption equilibrium of nitrophenols from aqueous solutions by FAU can be well-described using the Fowler-Guggenheim equation. The relative affinity of nitrophenols toward the FAU is dependent on the pH solution and on the pollutant solubility in water. Their sorption capacity is in the following order: ONP > 2.4-DNP > PNP > MNP. In binary mixtures, the most important parameter that governs the adsorption in zeolites seems to be the solubility of pollutants in water. Thus, the less-soluble compound (in this case, ONP) was adsorbed more easily than the other components present in the binary mixture. Finally, hydrophobic FAU zeolite seems to be an efficient adsorbent; it is able to be easily regenerated under air or by solvent leaching, through retention of these initial adsorption properties. Introduction Organic pollution in industrial waste streams is becoming a growing environmental concern. Numerous methods have been used for the removal of pollutants in effluents, including ozonation, ozonation/UV or H2O2/UV, filtration, and adsorption processes into activated carbon.1-5 Great research efforts on adsorption processes and adsorbent materials for the removing of organic pollutants from waste streams have been developed. Activated carbon is the most widely used adsorbent for water treatment; however, regeneration is difficult and expensive.6 Therefore, inorganic materials, such as synthetic zeolites, have been widely investigated, to design efficient and recyclable adsorbents. However, the application of synthetic zeolites for the removal of pollutants from wastewater has rarely been reported; only some papers can be found.7-14 It has been shown that, to adsorb pollutants selectively from water, zeolite adsorbents must be hydrophobic (i.e., possess a high Si/Al ratio).9,14 The objective of this work was to investigate the adsorptive properties of hydrophobic zeolite adsorbents for the removal of nitrophenol compounds from aqueous solutions. Adsorption was conducted in batch and flow apparatuses, and the effect of the pH solution was studied, as well as the effect of the nature of the nitrophenol pollutants (monoisomers and dinitrophenol). Furthermore, the adsorption was studied for single-component nitrophenol as well as when nitrophenols were in binary mixtures. Lastly, the regeneration and reuse of zeolite also was presented. 2. Experimental Section 2.1. Zeolite. FAU zeolite (Si/Al ratio ) 100) and silicalite (MFI, Si/Al ) 500) were supplied by Zeolyst International. The BEA zeolite (Si/Al ) ∞) was kindly synthesized by the Laboratoire des Mate´riaux Mine´raux (UMR CNRS 7016 Mulhouse, France). HFAU zeolites are characterized by the presence of one type of large cage (supercages), 13 Å in diameter and in ball form, * To whom correspondence should be addressed. Tel.: +33(0)549453498. Fax: +33(0)549453779. E-mail address: Patrick.magnoux@ univ-poitiers.fr.
accessible through a 12-ring window with a free aperture of 7.4 Å.15 These large cages are connected and a tridimensional straight channel is finally formed by the succession of these cages. Zeolite BEA (Beta) has a three-dimensional intersecting channel system: two mutually perpendicular straight channels, each with a cross section of 6.6 Å × 6.7 Å, and a sinusoidal channel, with a cross section of 5.6 Å × 5.6 Å.15 This tortuous channel system is formed by the intersection of the two main channels. The channel intersections of BEA zeolite generate cavities whose sizes are on the order of 12-13 Å.15 Zeolite MFI (ZSM-5 or silicalite with a high Si/Al ratio) is a medium-pore zeolite, presenting a three-dimensional interconnected channel system with 10-membered openings (with dimensions of 5.1 Å × 5.5 Å and 5.3 Å × 5.6 Å),15 and the size of the channel intersections is ∼8.5-9.0 Å. Nitrogen adsorption measurements were performed at a temperature of -196 °C, using a gas adsorption system (Micromeritics, Model ASAP 2000). The characteristics of zeolite samples are given in Table 1. It was concluded that highSi/Al zeolites can be considered to be hydrophobic materials.9 2.2. Adsorption. Nitrophenols were obtained from Aldrich (98% purity), and their characteristics are summarized in Table 2. The nitrophenol solutions were prepared in the concentration range of 1-500 mg/L in distilled water; the pH of the solution was adjusted using 1 M HCl or NaOH solutions. Adsorption experiments were performed using a batch equilibration technique and a flow apparatus. For each equilibration technique, 100 mg of the adsorbent was added to 200 mL of solution (e.g., nitrophenol concentrations of 3, 7.5, 15, 30, 70, 100, and 500 mg/L) and stirred for 24 h at 25 °C in a batch experiment. For adsorption in a flow apparatus, 0.5 g of adsorbent was packed in a stainless steel column with an internal diameter of 4.6 mm and a length of 100 mm. This reactor was maintained at temperature with a thermostatic bath. Pollutant solutions (niTable 1. Characterization of the Zeolite Samples zeolite
unit-cell formula
BEA MFI FAU
Si64O128 H0.19Al0.19Si95.62O192 H1.9Al1.9Si190.1O384
3 Si/Al pore volume (cm /g) NAl ratio micropores mesopores (mmol/g)
∞ 500 100
10.1021/ie8001777 CCC: $40.75 2008 American Chemical Society Published on Web 11/12/2008
0.197 0.220 0.285
0.043 0.056
0 0.9 4.4
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9559 Table 2. Characterization of Pollutants structure
molecular formula abbreviation molecular weight (g/mol) solubility in water (g/L) pKa size (Å) dipole moment (D)
o-C6H5NO3 ONP 139.1 1.26 7.17 8.1 3.74
p-C6H5NO3 PNP 139.1 12.6 7.15 6.7 5.7
trophenol concentration of 500 mg/L) were pumped through the column using a Gilson Model 307 pump, which allowed the release of a constant flow rate of 2 mL/min. Whatever the process, samples were collected periodically and analyzed using a high-performance liquid chromatography (HPLC) system (Varian Model Prostar chromatograph) that was equipped with a reverse phase column (Chromospher Pesticides) and an UV detector (λ ) 254 nm, Model 340) with a mobile phase that was comprised of 70% methanol and 30% water (flow rate of 1 mL/min). The amount of adsorbed compounds was determined using the equation m(Q) ) V(C0 - C)
2θW θ exp (linearized) (2) KC ) 1-θ RT C(1 - θ) 2θW ln ) -ln K + (3) θ RT where K is the equilibrium constant for adsorption of the adsorbate on an active site (given in units of L/mol), C the concentration at equilibrium adsorption (given in units of mol/ L), W the empirical interaction energy between two molecules adsorbed on nearest neighboring sites (given in units of J/mol), R the ideal gas constant (R ) 8.314 J mol-1 K-1), T the thermodynamic temperature (in Kelvin), and θ the fractional coverage of the surface. This model16 is based on the fact that the energy of interaction is constant and independent of the recovery θ and, consequently, the number and distribution of adsorbed molecules. 2.3.2. Flow Reactor. The shapes of the obtained breakthrough curves were simulated using the theoretical model of Wolborska.17-19
[
( )
( ) ( ) ()
ln
βC0 C β h ) tC0 Qads u
(4)
where C0 is the initial concentration of pollutant (given in units of mol/g), Qads the concentration of pollutant in the solid (given
2,4-C6H4N2O5 DNP 184 5.4 4.07 8.1 4.8
in units of mol/g), β the external mass-transfer kinetic coefficient (given in units of min-1), h the height of the adsorbent bed (in centimeters), and u the flow rate of a pollutant solution (given in units of cm/min). The values of the coefficient β, which is determined from the breakthrough curve and calculated from the Worlborska model, are identical for the same height of adsorbent bed and size of the grains of adsorbent; however, in our case, variation of the coefficient β was observed, and this represents the internal transfer (interaction between the adsorbate and the adsorbent, as well as the diffusivity of the adsorbed molecules). Here, β is given as
(1)
where m is the mass of zeolite (in grams), V the volume of solution (in liters), C the concentration after adsorption (given in units of mg/L), C0 the initial concentration (mg/L), and Q the amount adsorbed (given in units of mg/g of zeolite). 2.3. Adsorption Model. 2.3.1. Batch Reactor. The Langmuir and Freundlich models were used but did not perform well, with regard to modeling isotherms; the Fowler-Guggenheim model,16 which is based on thermodynamic statistics and exhibits some interaction between the particles adsorbed, was used in this study. The different theoretical curves were plotted from this model, following the equation
( ) ]
m-C6H5NO3 MNP 139.1 13.5 8.4 8.1 5.1
β)
aQads C0
(5)
where a represents the slope of the curve (the interaction strength between the adsorbate and the adsorbent), Qads the quantity of pollutant adsorbed at the equilibrium, and C0 the initial pollutant concentration. By analogy with the moments’ theory, the final part of the breakthrough curve (C/C0 > 0.5) is represented by a symmetrical part of the initial curve described by the Wolborska model. To describe the final portion of the curve, we therefore use the following equation: C 1 {2 exp(at1⁄2) - exp[a(2t1⁄2 - t)]} ) C0 exp(b)
(6)
where t1/2 corresponds to the average time of stay, which is defined as the time at which C/C0 ) 0.5 and b ) (β/u)h. For competitive adsorption, the breakthrough of nondesorbed compound has a similar shape, compared to a compound absorbed without competition. To describe the breakthrough curve of desorbed compounds, we propose the following model:
[ ] [ ( ) [( ) ] { }
C2 ) Cm
( )
βaC2,0 a2,0 βaC2,0 1 + 2 exp t βh βah a2,0 1⁄2 exp exp u 0
