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Ind. Eng. Chem. Res. 2007, 46, 4982-4990
Adsorption of Aromatic Compounds on Activated Carbons from Lignin: Equilibrium and Thermodynamic Study Luis M. Cotoruelo,* Marı´a D. Marque´ s, Jose´ Rodrı´guez-Mirasol, and Toma´ s Cordero Departamento de Ingenierı´a Quı´mica, UniVersidad de Ma´ laga, 29071 Ma´ laga, Espan˜ a
Juan J. Rodrı´guez Ingenierı´a Quı´mica, UniVersidad Auto´ noma de Madrid, 28049 Madrid, Espan˜ a
The adsorption processes of several aromatic chemicals onto activated carbons (ACs) from aqueous solutions have been studied. The activated carbons were prepared from eucalyptus kraft lignin with CO2 at 1073 K activation temperature. The surface properties of the ACs play a significant role when these materials are used for adsorption from liquid phases. The physicochemical properties and the surface chemical structure of the ACs were studied by means of N2 adsorption experiments, elemental analysis, XPS, and TPD. The XPS and TPD spectra of ACs suggest the presence of aromatic rings and oxygenated functional groups in the surface material. Benzene, nitrobenzene, aniline, p-nitroaniline, toluene, and p-nitrotoluene as aromatic adsorbates have been used in the present study. Adsorption isotherms data were fitted to the Freundlich and Fritz-Schlu¨nder equations. We have developed the thermodynamic study from the equilibrium data. The values of ∆H, ∆G, and ∆S were calculated, and these indicate that the process is exothermic in nature in all the examined cases. Introduction Activated carbons (ACs) are materials widely used for the elimination of a great variety of polluting agents, regarding their high adsorption capacity. One of the main applications of these materials is their use as adsorbents in the wastewater treatment to eliminate dissolved species such as dyes, chlorinated or nitrated compounds, phenols, and aromatic surfactants, among other substances.1-4 Toluene is frequently found in industrial effluents such as those of the paint manufacture, and it is also found in the polluted subterranean waters from the dripping of the gasoline tanks. Toluene adsorption can be carried out with activated carbon.5 Activated carbon can also be used in the potable water treatment, especially to control its color, taste, and smell.6 Systematic pollution of natural waters due to industrial and agricultural effluents has a special incidence. The contamination does not always take place directly with dragged substances; rather, the damage can be made by degradation products of certain substances that, in origin, could be considered innocuous. The adsorption with activated carbons is an important separation technology that can be used in environmental engineering. To provide a design of activated carbon adsorption processes, equilibrium and kinetic data are required. Equilibrium data describe the capacity of the activated carbon for the adsorption of adsorbates. In general, the adsorption involves the accumulation of molecules from a liquid phase onto the external and internal surface area of the adsorbent. This surface phenomenon is a compilation of interactions among the liquid-phase solvent, the adsorbate, and the adsorbent. Thermodynamic parameters of adsorption such as free energy, enthalpy, and entropy can illustrate these interactions. Activated carbons are produced from a variety of carbonaceous materials from mineral or vegetable origin. Anthracite, bituminous coals, coke, peat, wood, coconut, or almond shells, * To whom correspondence should be addressed. E-mail: lcot@ uma.es. Phone: +34 952 13 20 37. Fax: +34 952 13 20 38.
as well as other lignocellulosic residues, are frequently used. The raw material is generally amorphous, and the porous structure is achieved during the activation process, which creates a great internal surface. The structure of the activated carbon can be explained as a network of defective carbon layer planes, cross-linked by aliphatic bridging groups. Every carbon atom at the edge of the planes exhibits a high available surface activity. The surface area, dimensions, and distribution of pores depend on the precursor and the operation conditions, carbonization, and activation. The adsorption capacity and the rate of the adsorption depend on the internal surface area and the distributions of pore size and shape, but they also are influenced by the surface chemistry of the active carbon. Surface functional groups are formed during activation by reaction of free radicals on the carbon surface with heteroatoms such as oxygen or nitrogen. These functional groups can be carboxylic, phenols, hydroxyl, carbonyl, or peroxides, among others.7,8 In previous works, we have reported our results about the preparation of activated carbons by physical9,10 and chemical11 activation from eucalyptus kraft lignin and other lignocellulosics.12,13 The objective of this work is to study the adsorption of several organic compounds with environmental concern onto activated carbons prepared by physical activation of kraft lignin and to collect the equilibrium and thermodynamic data. Benzene, nitrobenzene, aniline, p-nitroaniline, toluene, and p-nitrotoluene have been used as adsorbates. These organic compounds have been chosen regarding their potential damage to the aquatic environment and because of their great industrial importance derived from their numerous uses as raw or intermediate materials in chemical and petrochemical syntheses. For instance, they are involved in commercial detergents manufacture, dyes preparation, and synthetic polymers production, as well as the agrochemicals, solvents, explosives, and drugs businesses, in general.
10.1021/ie061415h CCC: $37.00 © 2007 American Chemical Society Published on Web 06/13/2007
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Experimental Section The activated carbons used in this work have been prepared in our laboratory according to the procedure described in a previous work.9 Eucalyptus kraft lignin was supplied by the Empresa Nacional de Celulosas as obtained by acid precipitation of kraft black liquors. The lignin sample was carbonized under N2 atmosphere in a laboratory horizontal tubular furnace. The heating rate was 10 K/min until 623 K, a temperature that was held for 2 h. The carbonized product obtained was washed with 1% H2SO4 aqueous solution to diminish its final ash content ( 50 nm) volume. Ultimate analyses of activated carbons were carried out in a Perkin-Elmer (model 2400 CHN) analyzer, and the surface chemistry analysis was carried out by means of X-ray photoelectron spectroscopy (XPS) and by temperature-programmed desorption (TPD). XPS analyses of the samples were obtained using a 5700C model Physical Electronics apparatus with Mg KR radiation (1253.6 eV). For the analysis of the XPS peaks, the C1s peak position was set at 284.5 eV and used as the reference to position the other peaks.14 TPD profiles were obtained with a custom quartz tubular reactor placed inside an electrical furnace. The samples were heated from room temperature up to 900 °C at a heating rate of 5 °C/min in a helium flow (200 cm3 STP/min). The amounts of CO and CO2 desorbed from the samples were monitored with a ndir-analyzer Siemens (model Ultramat 22) apparatus. The organic compounds used in the present study as adsorbates were as follows: benzene (B) supplied by Probus; nitrobenzene (NB), p-nitroaniline (p-NA), and p-nitrotoluene (p-NT) supplied by Aldrich; and aniline (A) and toluene (T) supplied by Panreac, in the highest purity available. The equilibrium tests have been carried out in an orbital incubator (Gallenkamp, model INR-250) at 150 rpm equivalent stirring rate. Adsorbates solutions were prepared with different concentrations: benzene (15-250 mg/L); nitrobenzene (30500 mg/L); aniline (30-500 mg/L); p-nitroaniline (15-250 mg/ L); toluene (12-200 mg/L); and p-nitrotoluene (15-250 mg/ L). Typically, 100 mL samples of these solutions were put in contact, in stoppered flasks, with a dose of 100 mg/L of activated carbon. Temperatures for the different experiments varied between 278 and 308 K. The contact time varied between 1 and 2 weeks, time enough to reach the equilibrium adsorption. Adsorbates equilibrium concentrations were determined by UV
Figure 1. 77 K N2 adsorption-desorption isotherms of the activated carbons. Table 1. Surface Areas and Pore Volumes of the Activated Carbons ABET (m2/g) As (m2 /g) burnoff (%) Vmicrop N2 (cm3/g) Vmicrop CO2 (cm3/g) Vmesop (cm3/g) Vmacrop (cm3/g) Vtotal (cm3/g)
AC23
AC41
AC60
753 46 23.1 0.315 0.187 0.043 0.046 0.404
847 162 41.1 0.333 0.245 0.301 0.096 0.730
1200 246 60.3 0.379 0.298 0.491 0.144 1.014
Table 2. Ultimate Analyses of the Lignin and Carbonized and Activated Carbons (%) on dry basis
C
H
S
N
O
lignin carbonized AC23 AC41 AC60
56.5 97.0 96.2 95.4 94.7
4.30 0.96 0.77 0.70 0.68
1.10 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
25.7 2.04 3.03 3.90 4.62
spectroscopy at the following wavelengths: B, 200 nm; NB, 268 nm; A, 280 nm; p-NA, 379 nm; T, 206 nm; and p-NT, 284 nm. A UV-visible (Varian, model Cary 1E) spectrophotometer was used for the analysis. The compounds used as adsorbates have negligible tendency to ionization/dissociation. The pHs of the solutions were those corresponding to the distilled water used (6.8-7.2) in all the cases, with no measurable variations due to the solute concentrations. In addition, tests carried out to determine the ACs pH values (ASTM D3838-80) showed no acid or base neutralizing capacity. Results and Discussion Characterization of the Activated Carbons. The physical and chemical characterizations of the activated carbons are necessary to correlate their structure and surface chemical properties with the different adsorption behaviors of the adsorbates. Figure 1 shows 77 K N2 adsorption-desorption isotherms of the activated carbons. We observed a variation in the shape of the isotherm and an increase of adsorbed N2 volume at low relative pressures ( NB > B > p-NT > p-NA > A. In relation to the solubility and the chemical nature of the functional groups in the aromatic ring, the behaviors observed are the following:
(a) Low soluble compounds are preferably adsorbed; i.e., toluene. (b) In spite of the high nitrobenzene solubility, it is easily adsorbed. This is because the nitro group causes the aromatic ring to act like an electron acceptor, which confers good affinity toward the activated carbon surface. (c) Aniline can produce dipole-dipole interactions with water in solution, as a consequence of its high solubility. In addition, the NH2 group behaves like an electron donor, which confers a low affinity to the aromatic ring toward the adsorbent surface. (d) Finally, toluene is better adsorbed than p-nitrotoluene; both have similar solubilites, but the smaller size and molecular weight of toluene compensate the effect of the NO2 group in p-nitrotoluene. The electron acceptor effect caused by the NO2 group is partially compensated by the inductive effect of the CH3 group in the p-nitrotoluene. Acknowledgment The authors acknowledge the Spanish DGICYT-MEC (Project PPQ2003-07160) and the Junta Andalucı´a (TEP-184) for financial support. Nomenclature a ) adsorbate activity in solution ABET ) apparent surface area of AC (m2/g) AC ) activated carbon As ) external area of AC (m2 /g) Co ) initial concentration of adsorbate in the fluid phase (mmol/L) Ce ) equilibrium concentration of adsorbate in the fluid phase (mmol/L) d ) pore width (nm) g ) mass of adsorbent (g) K ) adsorption equilibrium constant (L/mmol) KF, n ) empirical coefficients in Freundlich equation KFS, P, f, b ) empirical coefficients in Fritz-Schlu¨nder equation Ko ) frequency factor in Van’t Hoff equation (L/mmol) K′o ) Ko modified as eq 11 (L/mmol) M ) maximum capacity of adsorption (mmol/g) N ) number of moles of adsorbed solute qe ) equilibrium concentration of the adsorbate in the solid phase (mmol/g) R ) ideal gas constant (1.987 cal/(mol K)) T ) temperature (K) Vmicrop. ) micropore volume (cm3/g) Vmesop. ) mesopore volume (cm3/g) Vmacrop. ) macropore volume (cm3/g) Vtotal ) total pore volume (cm3/g) w ) adsorbent dose (g/L) ∆G ) molar adsorption free energy change (kJ/mol) ∆G′ ) adsorption free energy change (kJ) ∆G′′ ) adsorption free energy change per mass unit of adsorbent (kJ/g) ∆H ) molar adsorption enthalpy change (kJ/mol) ∆S ) molar adsorption entropy change (J/(mol K)) χ2 ) average quadratic deviation Adsorbates A ) aniline B ) benzene p-NA ) p-nitroaniline NB ) nitrobenzene
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ReceiVed for reView November 4, 2006 ReVised manuscript receiVed April 23, 2007 Accepted May 10, 2007 IE061415H
