Comparative study of the adsorption of acetaminophen on activated carbons in simulated gastric fluid
© Rey-Mafull et al.; licensee Springer. 2014
Received: 8 June 2013
Accepted: 14 September 2013
Published: 24 January 2014
Samples of commercial activated carbons (AC) obtained from different sources: Norit E Supra USP, Norit B Test EUR, and ML (Baracoa, Cuba) were investigated. The adsorption of acetaminophen, Co = 2500 mg/L, occured in simulated gastric fluid (SGF) at pH 1.2 in contact with activated carbon for 4 h at 310 K in water bath with stirring. Residual acetaminophen was monitored by UV visible. The results were converted to scale adsorption isotherms using alternative models: Langmuir TI and TII, Freundlich, Dubinin-Radushkevich (DR) and Temkin. Linearized forms of the characteristic parameters were obtained in each case. The models that best fit the experimental data were Langmuir TI and Temkin with R2 ≥0.98. The regression best fits followed the sequence: Langmuir TI = Temkin > DR > LangmuirTII > Freundlich. The microporosity determined by adsorption of CO2 at 273 K with a single term DR regression presented R2 > 0.98. The adsorption of acetaminophen may occur in specific sites and also in the basal region. It was determined that the adsorption process of acetaminophen on AC in SGF is spontaneous (ΔG <0) and exothermic (−ΔHads.). Moreover, the area occupied by the acetaminophen molecule was calculated with a relative error from 7.8 to 50%.
KeywordsActivated carbon Acetaminophen Adsorption Isotherms Simulated gastric fluid
Activated carbon (AC) can be applied orally as an antidote to different intoxications. Several studies, both in vitro and in vivo have demonstrated the capacity of activated carbon to adsorb numerous toxic compounds (Neuvonen & Olkkola 1989; Alaspaa et al. 2000; Ho et al. 1989; Pond 1986; McGoodwin & Schaeffer 2000; Cooper et al. 2005; Hoegberg et al. 2003; Modi et al. 1994; Hoegberg et al. 2002; El-Kemary et al. 2011). The 1-15acetaminophen (N- acetyl- p- aminophenol) is a drug with analgesic properties, without clinically significant anti-inflammatory properties. It acts by inhibiting prostaglandin synthesis, cellular mediators responsible for the onset of pain. It also has antipyretic effects. It is available usually in the form of capsules, tablets, suppositories, and drops for oral administration. It is a common ingredient in a variety of products against cold and flu.
Its low price and widespread availability have resulted in frequent cases of overdose. In the indicated doses, acetaminophen presents no effect on the gastric mucosa, blood clotting or kidneys, but the liver might be severely affected.
Adsorption capacity of AC depends on the nature of the adsorbent (pore structure, functional groups, ash content) as well as the nature of the adsorbate (functional groups, polarity, molecular size and weight). The type of precursor and the process of activation determine basic properties of AC such as surface area and pore size distribution. The ACs has strong heterogeneous surfaces, in both geometrical and chemical character. The geometrical heterogeneity is the result of differences in the size and shape of pores as well as pits, and vacancies. Chemical heterogeneity is associated to different functional groups at a surface (mainly oxygen) and to various surface contaminants. Both heterogeneities contribute to unique adsorption properties of activated carbons (Neuvonen & Olkkola 1989; Alaspaa et al. 2000; Ho et al. 1989; Pond 1986; McGoodwin & Schaeffer 2000; Cooper et al. 2005; Hoegberg et al. 2003; Modi et al. 1994; Hoegberg et al. 2002; El-Kemary et al. 2011; American Academy of Clinical Toxicology (AACT); European Association of Poisons Centres and Clinical Toxicologists (EAPCCT) 1999; Bryant et al. 2003; Neuvonen et al. 1984; Neuvonen 1982; Neuvonen & Olkkola 1984; Yamamoto et al. 2007). The purpose of this study is to investigate the efficacy of AC to remove acetaminophen dissolved in simulated gastric fluid (SGF).
Materials and methods
Norit B (NB) Test EUR (Germany) and Norit E (NE) Supra USP (Holland) were taken as references. The activated carbon ML was supplied by the Baracoa Activated Carbon Plant (Cuba) and purified by acid/basic treatment (Rey-Mafull et al. 2007; Rey-Mafull et al. 2010). The particle sizes correspond to 100% < 250 μm. All carbons follow the requirements of the standard actived charcoal according to the United States Pharmacopeia (USP30-NF25, 2007).
Simulated gastric fluid (SGF)
The SGF was prepared according to USP 30 as follows: 2 g NaCl were dissolved in 7 mL of concentrated HCl and filled up to 1 L with distilled water free of CO2 and simultaneously adjusting the pH of the solution to 1.2. Acetaminophen was added to the SGF solution reaching a concentration 2500 mg/L. The calibration curve of acetaminophen in SGF was performed using a UV/VIS spectrophotometer (Ultrospec 2100 pro from Amersham Biosciences). The optical density of all samples was determined with maximum absorbance at λmax = 245 nm in the zone of Lambert Beer transmittance. The calibration curve was adjusted using the linear or quadratic regression analysis. Each experiment was performed by triplicate.
Batch equilibrium experiments and analytical method
where C 0 (mg/mL) is the initial concentration (t = 0), C e (mg/ mL) is the equilibrium concentration (t = 4 h), M is the mass of carbon (g) and V is the volume of the solution (L).
Adsorption isotherms CO2 at 273 K
Isotherms and their linearized expressions
ΔG = − RTln [KL]
lnqe = lnKF + n− 1lnCe
lnqe = lnqmax − Dϵ2
E0 = [2D]− 0.5
qe = BlnKTK + BlnCe
Functional groups identification (FTIR)
FTIR spectra for different activated carbon samples (4000–400 cm-1) were recorded on a FTIR spectrophotometer (Nicolet 50X), using KBr pellets containing 0.1 wt% carbon. Those pellets were dried for 8 h at 100°C before the spectra were recorded.
Results and discussion
Textural properties of the carbons
Textural characterization by adsorption of CO 2 at 273 K
Norit E Supra USP (NE)
Norit B Test EUR (NB)
Baracoa, Cuba (ML)
Identification of surface functional groups
Characteristic parameters of models
qm = 840
qm = 1315.8
KF = 685
qmax = 685
b = 17.5
KLI = 3.6
KLII = 2
n = 1.6
D = 0.018
KTK = 82
qm = 555
qm = 356
KF = 503
qmax = 458
b = 29.6
KLI = 6
KLII = 19
n = 1.9
D = 0.016
KTK = 202
qm = 769
qm = 1462
KF = 605
qmax = 632
b = 7.2
KLI = 3.3
KLII = 1
n = 1.7
D = 0.04
KTK = 27
Free energy change Gibbs (ΔG) calculated by the Langmuir and Temkin equations and characteristic energy (E) calculated by the Dubinin Radushkevich’s equation
Inumerous works have been conducted in order to elucidate the mechanism of adsorption of many molecules on different adsorbents (Passé et al. 2009; Behnamfard & Salarirad 2009; Richard et al. 2009; Xin et al. 2011; Ahmad & Rahman 2011; Moreno et al. 2000; Moreno 2004; Pradhan & Sandle 1999; Terzyk et al. 2003). Those publications reveal that adsorption of organic molecules from dilute aqueous solutions on carbon-based materials is a complex interaction between electrostatic and non-electrostatic forces. Moreover, both interactions depend on the characteristics of the adsorbent and adsorbate, as well as on chemical properties of the solution. It was observed that the adsorption capacity is negatively influenced by the presence of basic surface groups. Terzyk et al. 2003 reported a decrease of acetaminophen maximal adsorption capacity as the total amount of surface basic groups and carbonyls increases on ACs. It was postulated that the acetaminophen molecule interacts by the OH- group with carbon basic surfaces, and the repulsion effect occurs between the CO group of this molecule and similar groups attached to the surface (Yamamoto et al. 2007; Terzyk et al. 2003; Terzyk 2002).
The Langmuir adsorption model describes monolayer adsorption of adsorbate onto a homogeneous adsorbent surface. Moreover, there is negligible interaction between the adsorbed molecules and adsorption sites having uniform energies. The Langmuir isotherm accounts for surface-coverage by balancing the relative rates of uptake and release, the former being proportional to the fraction of the surface is open, while the latter is proportional to the fraction that is covered. The equilibrium constant for those rates is K (L/mg), which also corresponds to the Henry’s law coefficient. When the fluid concentration is very high, a monolayer forms on the adsorbent surface, having a loading of q max .(mg/g).
The term R L indicates the shape of the isotherm as follows. When the parameters, R L >1 (unfavorable isotherm), R L = 1 (linear isotherm), 0 < R L < 1 (favorable isotherm). In all cases R L (3.99 × 10 -4 ) expresses that the isotherm has a favorable behavior.
The Dubinin- Radushkevich equation: assumes that the amount adsorbed corresponding to any adsorbate concentration is a Gaussian function of the Polanyi potential The development of this theory is based on the concept of the characteristic curve and Polanyi adsorption potential (∂ E/∂ T = 0) is applied to describe the adsorption in micropores. A linear plot of ln q e against ϵ 2 (kJ/mol)2 would give the value of q max (mg/g) and D (mol2kJ-2), from the intercept and slope. The calculated value of E (kJ/mol) is shown in the Tables 3 to 4.
Estimated calculation of the value of specific surface area for acetaminophen molecule taking as reference the value 60.2 Å 2 proposed by Terzyk et al. (2012)
Relative error (%)
Relative error (%)
Relative error (%)
The effectiveness of the degree of compaction of the molecule of acetaminophen on the surface of the activated carbon is related to the optimal distribution of the sites of adsorption to the maximum extent of packaging of this molecule. The determination of the area of the molecule of acetaminophen , in their non-ionized state, in SGF shows a better orientation on the surface of activated carbon, in an order of priority NB > NE > ML. The cause could be a better distribution of the active sites of adsorption, in both textural and functional plane respectively. However, with regard to the relative error show models in the estimated calculation, the order is as follows: Langmuir > Freundlich > DR. But in all cases the relative error is greater than 10%.
The Temkin model considered the effects of some indirect adsorbent/adsorbate interactions on adsorption isotherms. As a result of adsorbent/adsorbate interactions, the heat of adsorption of all the molecules in the layer would decrease linearly with coverage. This is a correction of Langmuir equation, and introduces the influence of temperature on the adsorption. In this case, b (J/mol) is the Temkim constant related to heat of sorption and KTK is the Temkin isotherm constant (L/g). The variation of adsorption energy b is positive for all the studied AC, Table 3, which indicates that the adsorption reaction is exothermic (−ΔHads) (Hameed et al. 2008).
The empirical Freundlich model is based upon the assumption of multilayer formation of adsorbate on the heterogeneous solid surface of the adsorbent and assumes that the stronger binding sites are occupied first and that the binding strength decreases with the increasing degree of site occupation. The values Q max and 1/n are Freundlich constants related to adsorption capacity and intensity of adsorption, respectively. The lower fractional values of 1/n [0 < (1/n) <1] indicate that weak adsorptive forces are effective on the surface of activated carbon. Values of n > 1 represent a favorable adsorption condition, suitable for highly heterogeneous surfaces. In this study, the values found for n were between 1.6 and 1.9 which prove that the adsorption is favorable and process could be physical in nature (Behnamfard & Salarirad 2009; Yan et al. 2008).
The results reported in this study show that activated carbons ML can be envisaged as alternative adsorbents for acetaminophen removal from SGF. ML did not differ substantially from commercial standards NB and NE. The CO2 isotherms indicate that ML is a microporous material with a texture similar to the NE patterns. From the comparison of three activated carbons with different nature, it is clear that both the microporous structure and the surface chemistry have a critical role in defining the adsorption capacities.
It was determined that the adsorption process of acetaminophen on activated carbon in SGF is spontaneous (ΔG <0). As seen in most cases. the models fit very well with the data analyzed, depending on the type of activated carbon. The adsorption of acetaminophen may occur in specific sites and also in the basal region. Although it is well established that the oxidation process is performed by the oxygen surface groups, previous results show that not only the amount but also the nature of these oxygen groups and distribution becomes crucial. The estimated calculation of the area of the section the acetaminophen molecule transversal demonstrates that affirmation.
The authors thank to Programa de Estudantes-Convênio de Pós-Graduação (PEC/PG), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (Capes), Brazil.
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