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Equilibrium, kinetics and thermodynamics study of phenols

Activated carbon was prepared from lignocellulosic
material (Eucalyptus Globulus labill seed) by
chemical activation with ZnCl2 at two different concentrations
(10 and 25 % m/v) named ACS25 and ACS10. The
textural characteristics of the activated carbons (ACs) were
determined by N2 adsorption isotherms; these exhibit
B.E.T. surface areas of 250 and 300 m2 g-1 for ACS25 and
ACS10, respectively, with micropore volume contents of
0.140 and 0.125 cm3 g-1 in the same order. In addition, the
FTIR and Boehm methods were conducted for the chemical
characterisation of ACs, where many groups with basic
character were found, which favours the adsorption of
phenols. The prepared carbonaceous adsorbents were used
in the adsorption of wide pollutants monosubstituted phenol
derivatives: phenol, 4-nitrophenol and 4-chlorophenol.
The effect of temperature on the thermodynamics, kinetic
and equilibrium of phenols adsorption on ACs was thoroughly
examined. The adsorption kinetics adjusted properly
for a pseudo-second-order kinetic model. However, the
Elovich model (chemisorption) confirms that phenols
adsorption did not occur via the sharing of electrons
between the phenolic ring and basal plane of ACs because
is not properly adjusted, so the process is given by
physisorption. The thermodynamic parameters [i.e. Gibbs
free energy change (DG), enthalpy change (DH) and
entropy change (DS)] were also evaluated.

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Equilibrium, kinetics and thermodynamics study of phenols

  1. 1. Equilibrium, kinetics and thermodynamics study of phenols adsorption onto activated carbon obtained from lignocellulosic material (Eucalyptus Globulus labill seed) Nelson Giovanny Rinco´n-Silva1 • Juan C. Moreno-Piraja´n1 • Liliana Giraldo2 Received: 30 August 2015 / Revised: 11 November 2015 / Accepted: 20 November 2015 Ó Springer Science+Business Media New York 2015 Abstract Activated carbon was prepared from lignocel- lulosic material (Eucalyptus Globulus labill seed) by chemical activation with ZnCl2 at two different concen- trations (10 and 25 % m/v) named ACS25 and ACS10. The textural characteristics of the activated carbons (ACs) were determined by N2 adsorption isotherms; these exhibit B.E.T. surface areas of 250 and 300 m2 g-1 for ACS25 and ACS10, respectively, with micropore volume contents of 0.140 and 0.125 cm3 g-1 in the same order. In addition, the FTIR and Boehm methods were conducted for the chemi- cal characterisation of ACs, where many groups with basic character were found, which favours the adsorption of phenols. The prepared carbonaceous adsorbents were used in the adsorption of wide pollutants monosubstituted phe- nol derivatives: phenol, 4-nitrophenol and 4-chlorophenol. The effect of temperature on the thermodynamics, kinetic and equilibrium of phenols adsorption on ACs was thor- oughly examined. The adsorption kinetics adjusted prop- erly for a pseudo-second-order kinetic model. However, the Elovich model (chemisorption) confirms that phenols adsorption did not occur via the sharing of electrons between the phenolic ring and basal plane of ACs because is not properly adjusted, so the process is given by physisorption. The thermodynamic parameters [i.e. Gibbs free energy change (DG°), enthalpy change (DH°) and entropy change (DS°)] were also evaluated. The overall adsorption process was exothermic and spontaneous in nature. The values found in the thermodynamic study, confirm that the adsorption process corresponds to a clearly physical process. Keywords Activated carbon Á Kinetic Á Thermodynamics Á 4-Chlorophneol Á Elovich equation Abbreviations Qt Amount of adsorbate adsorbed at time t (mg g-1 ) Kf The pseudo-first-order rate constant (h-1 ), t Time Ks The pseudo-first-order rate constant (g gm-1 min-1 ) a Desorption constant in Elovich Equation b Initial adsorption rate in Elovich Equation kid The intraparticle diffusion rate constant (mg g-1 h-1/2 ) I Thickness of the boundary layer in intraparticle model of Weber and Morris k Rate constant of adsorption of Dumwald–Warner model (min-1 ) F Fractional attainment of equilibrium in the Dumwald-Warner model kfd Film diffusion rate coefficient in the Dumwald– Wagner mode Qmax Maximum phenol uptake in Langmuir model kL Constant in Langmuir model that denoted the energy of adsorption and affinity of the binding sites (L mg-1 ) kf Constant in Freundlich model giving an indication of adsorption capacity [mg g-1 (L mg-1 ) n ] & Liliana Giraldo 1 Departamento de Quı´mica, Facultad de Ciencias, Grupo de Investigacio´n de So´lidos Porosos y Calorimetrı´a, Universidad de los Andes, Bogota´, Colombia 2 Departamento de Quı´mica, Facultad de Ciencias, Universidad Nacional de Colombia, Bogota´, Colombia 123 Adsorption DOI 10.1007/s10450-015-9724-2
  2. 2. n Constant in Freundlich model giving an indication of adsorption intensity QmDRK Amount adsorbed of solute on the monolayer in Dubinin–Radusckevisch–Kanager model Cs Concentrations of equilibrium saturation in Dubinin–Radusckevisch–Kanager model ES Is related to the energy characteristic of the process in Dubinin–Radusckevisch–Kanager model DG0 Gibbs free energy change of adsorption process R R is the gas constant (8.314 J mol-1 K-1 ) Ko K0 is the apparent equilibrium constant, in this study the Langmuir constant was used DS0 Entropy change of adsorption process DH0 Enthalpy change of adsorption process T Temperature in Kelvin A Arrhenius factor Ea Arrhenius activation energy of adsorption 1 Introduction Water pollution is one of the most undesirable environ- mental problems in the world and requires solutions (Rain- bown 2002). This is caused by the introduction of substances which may be non-toxic but affect biological cycles at high concentrations, in the case of substances such as nitrates, phosphates and some organic compounds, and secondly the introduction of toxic substances, such as heavy metals, hydrocarbons, pesticides, phenols, etc. (Blanchard et al. 1984; Dabrowski et al. 2005; Qing-Song et al. 2010). The latter are considered priority pollutants due to their toxicity in living organisms, even at low concentrations (Kumar et al. 2007). Phenolic wastewater is generated from chemicals, pharmaceuticals, papermaking, rubber, wood, dye, and pesticide industries and are highly toxic and harmful. In addition, phenols are considered priority pollutants by the EPA since they are not only carcinogenic but also cause an unpleasant taste and odour, even at low concentrations (Ahmaruzzaman and Sharma 2005). Various technologies have been developed to treat phenolic wastewater. Among them, adsorption is an effective technology for the removal of phenols from wastewater (Qing-Song et al. 2010; Rincon-Silva et al. 2015). Activated carbon (AC) is an extremely versatile carbonaceous material that is widely used as an adsorbent and catalyst support in industries (Rodrı´guez-Reinoso and Linares-Solano 1989, Rodriguez-Reinoso 2007). More- over, its large surface area and micropore volume content, favourable pore size distribution, surface chemistry including the oxygen functional groups, the degree of polarity and the active surface area lend AC as an appro- priate adsorbent for a variety of environmental applica- tions, i.e. the removal of organic materials, and the purification and storage of gases and organic compounds from aqueous solution. The adsorption efficiency of AC relies strongly on its special surface and structural char- acteristics (Rodrı´guez-Reinoso and Linares-Solano 1989; Dabrowski et al. 2005; Rodriguez-Reinoso 2007). Activated carbon is mainly produced by thermal and chemical activation (Rodrı´guez-Reinoso and Linares- Solano 1989; Sun and Jian 2010). Thermal or physical activation involves the primary carbonisation of a car- bonaceous precursor (below 700 °C) followed by activa- tion of the obtained char with oxidising gases such as air, CO2 or steam at high temperature in the range of 700–1000 °C. Chemical activation consists of the impregnation of raw material with chemical agents such as ZnCl2, H3PO4 or KOH followed by carbonisation at tem- peratures between 400 and 800 °C under a N2 atmosphere (Rodrı´guez-Reinoso and Linares-Solano 1989). An enor- mous range of lignocellulosic materials including rice husk, corn cobs, fruit stone, almond shell, coconut shell, sugar cane bagasse, palm shell, pistachio-nut shell and cotton stalk have been used as activated carbon precursors (Li et al. 2008; Li et al. 2010; Chandra et al. 2009; Nor et al. 2013). Eucalyptus Globulus labil is other kind of lignocellulosic material which has a reasonably high con- tent of carbon, utilized also as raw material for AC (Tan- credi et al. 2004; Mojica-Sanchez et al. 2012; Rincon Silva et al. 2014). The objective of this work is study the capacity of adsorption of derivatives phenolics monosubstituted: phe- nol, 4-clorophenol and 4-nithrophenol (phenol, 4-CP and 4-NP, respectively) on activated carbons prepared from Eucalyptus shell by chemical activation. Additionally, an extensive kinetic study was conducted to evaluate the efficiency of the adsorption process. The pseudo-first-order and pseudo-second-order models were used to correlate the adsorption kinetics data of phenols onto AC, the kinetic as well as the diffusion parameters were evaluated. Thermo- dynamics studies have also been performed to understand the process of removal of the selected phenols on ACs. 2 Materials and methods 2.1 Preparation of activated carbon In this research, the lignocellulosic materials were treated by impregnation with zinc chloride at low concentrations in order to obtain microporous activated carbons at low costs. This type of activation leads to ACs with high porosity, and although the distribution of pore size is largely determined Adsorption 123
  3. 3. by the precursor, the amount of zinc chloride used also influences the porosity of the final product (Khalilia et al. 2000; Azevedo et al. 2007). The suggested mechanism for activation using zinc chloride can be summarised as follows: during impregna- tion, the chemical reagent is introduced into the interior of the precursor particles and causes some hydrolysis reactions which are seen in a weight loss, in the output volatile mate- rial, in the weakening of the structure and the increase in elasticity, the chemical also causes the swelling of particles. The two processes are more evident with increasing con- centrations of zinc chloride. During heat treatment, zinc chloride prevents the formation of volatiles thus increasing process yield. During the impregnation and carbonisation at low ratios impregnation occurs minimal weight loss, since the amount of zinc chloride may be distributed uniformly throughout the precursor with a large dispersion in the interior of the particles resulting of activated carbons after extensive washing with uniform microporosity and low macroporosity. At higher impregnation ratios, hydrolysis and swelling are accentuated, the zinc chloride may not be distributed uniformly within the particles, and although the total pore volume increases, the pore size distribution is more heterogeneous, with meso- and macroporosity becoming more important (Khalilia et al. 2000; Azevedo et al. 2007). Thus, activated carbon was prepared by mixing of the eucalyptus shell (ranging in size from 2.5 to 3.5 mm) at two different concentrations of ZnCl2 (10 and 25 m/v), which will be called ACS10 and ACS25. The impregnation ratio of ZnCl2 and eucalyptus shell was 2.0/1.0; this pro- cess was performed for 12 h. After mixing both together, the salt impregnated material was dried overnight at 90 °C, then a weighed sample was carbonised in a quartz tube furnace at temperature of 550 °C, holding time of 2 h and heating rate of 5 °C min-1 . After carbonisation, the sample was cooled down to room temperature in a flow of nitrogen and then removed from the reactor. In order to remove impurities in the synthesised ACs, it was dispersed in distilled water at 80 °C. After that, the sample was washed sequentially with hot and cold distilled water until the wash water reached a pH of 6-7 (Qing-Song et al. 2010; Rincon- Silva et al. 2015). 2.2 Characterisation of the ACs samples The textural characterisation of the prepared ACs included the surface area, the extent of micro- and mesoporosity was conducted using N2 adsorption/desorption at 77 K using a computer system Autosorb 3B, Quantachrome Co. The specific surface areas were calculated from the N2 adsorption isotherm with Brunauer, Emmet and Teller equation (B.E.T.) at the relative pressure in the range of 0.001–0.3 bar. The total pore volume was determined from the amount of N2 adsorbed at P/P0 0.99. The volume of micropores was estimated using the Dubinin–Radushke- vich (D.R.) method (Qing-Song et al. 2010; Mojica-San- chez et al. 2012; Anisuzzaman et al. 2014) The selective method for determining of the total acid and basic sites on the carbon surface was employed (Boehm 1994, 2002; Lopez-Ramon et al. 1999). The pH at point of zero charge (pHPZC) was determined using the titration method of mass using a CG840B Schott pH meter (Babic et al. 1999; Mojica-Sanchez et al. 2012). The surface functional groups of the ACs samples were also detected by Fourier Transform Infrared (FTIR) spec- troscope (FTIR—Nicolet Impact 410) using a potassium bromide (KBr) pellet prepared by mixing 0.033 % of dried AC sample in KBr. The spectra were recorded between 4000 and 400 cm-1 (Pakulaa et al. 2005; Saka 2012). 2.3 Adsorption kinetics Adsorption kinetic experiments were carried out using a shaker water bath. Kinetic experiments were carried out by agitating 50 mL of solution of phenols 100 mg of ACs at a constant agitation speed, 20 °C and natural pH, because measuring the pH of the solution showed that this was below the pKa of phenols. Therefore, the molecular form of each phenol of the ionic form predominates, favouring the adsorption process due to electrostatic interactions (Mor- eno-Castilla et al. 1995; Moreno-Castilla 2004; Qing-Song et al. 2010; Rincon-Silva et al. 2015). Agitation was per- formed for 120 min, which is more than sufficient time to reach equilibrium at a constant stirring speed of 120 rpm. Preliminary experiments had shown that the effect of separation time on the adsorbed amount of phenol was negligible. Two millilitres of samples was drawn at suit- able time intervals. The samples were then centrifuged at 500 rpm and the omitted concentration in the supernatant solution was analysed using UV–VIS spectrophotometry with Milton Roy Co, Spectronic Genesys, equipment by monitoring the absorbance changes at a wavelength of maximum absorbance: 269, 280 and 319 nm for phenol, 4-CP and 4-NP, respectively (Tseng et al. 2010; Qing-Song et al. 2010). Each experiment continued until equilibrium conditions were reached when no further decrease in the phenol concentration was measured; then, we proceeded to plot the adsorption capacity of phenols on activated car- bons versus time (Kumar et al. 2007; Kunwar et al. 2008; Tseng et al. 2010; Qing-Song et al. 2010). 2.4 Adsorption isotherms of phenol, 4-nitrophenol and 4-chlorophenol The equilibrium isotherm of phenols adsorption on ACs was determined by performing an adsorption test in Adsorption 123
  4. 4. 100 mL flasks, where 50 mL of phenols solutions with different initial concentrations (50–1500 mg L-1 ) were placed in each flask. Then, 100 mg of each of the prepared activated carbon was added to each flask and kept in shaker of 120 rpm at 20 °C for 72 h to reach equilibrium. The equilibrium concentration of phenolic compounds was determined with respect to calibration curves, at wave- lengths (kmax) (Dabrowski et al. 2005; Qing-Song et al. 2010; Rincon-Silva et al. 2015). Data amount Qe adsorbed at equilibrium (mg g-1 ) were calculated from the following equation: Qe ¼ V Ã C0 À Ce m ð1Þ where Co and Ce are the initial and equilibrium phenol concentration in mg L-1 , V is the volume of solution (L) and m is the mass of ACs (g) (Qing-Song et al. 2010; Rincon-Silva et al. 2015). 2.5 Thermodynamic analysis A thermodynamic study of adsorption process of phenols on ACs to estimate the feasibility of the adsorption process was performed. Therefore, the enthalpy of adsorption was determined by the isosteric method which is known as the standard differential enthalpy of adsorption, where exper- imental adsorption isotherms were used at different tem- peratures (20, 30 and 40 °C) with the procedure described in Sect. 2.4 (Humpola et al. 2013; Rincon-Silva et al. 2015; Ashraf et al. 2014). This method is useful for estimating the enthalpy dif- ferential adsorption, under the constraint of constant frac- tion of coating of the surface adsorption. Initially, for a system in thermodynamic equilibrium, the following fun- damental equation is satisfied (Stoeckli et al. 1995; Douillard 1996; Arias et al. 2009): DG ads ¼ ÀRTLnkeq ¼ DH ads À TDS ads ð2Þ where Keq is the equilibrium constant dynamic, chemical equilibrium type during the adsorption process. Thus, the constant kL of the Langmuir isotherm (see Eq. (11) in Sect. 3.4), can be directly related to the equilibrium con- stant of Eq. 2 in the vicinity of the boundary of Henry law (Douillard 1996; Arias et al. 2009). Incorporating the Langmuir isotherm in this equation, we obtain: DG ads ¼ ÀRTIn h ð1 À hÞ 1 p ¼ DH ads À TDS ads ð3Þ As a result, it is possible to obtain the equation of Van´t Hoff, which can be used to calculate the values of entropy and enthalpy. Additionally, the equilibrium conditions could be used to obtain the free energy of the adsorption process (Stoeckli et al. 1995; Douillard 1996; Arias et al. 2009; Ashraf et al. 2014). The Gibbs free energy change (DG°) values can discern whether a process is spontaneous or not; negative values of DG° imply a spontaneous process. The enthalpy change (DH°) provides information about the exothermic or endothermic nature of the process and differentiates between physical and chemical adsorption process. The entropy change (DS°) predicts the magnitude of the chan- ges on the adsorbent surface, allowing the randomness of the adsorbate-adsorbent interface to be evaluated (Hum- pola et al. 2013; Rincon-Silva et al. 2014; Ashraf et al. 2014). 3 Results and discussion 3.1 Characterisation of activated carbons 3.1.1 Textural and chemical properties The surface area of samples was calculated by B.E.T. equation. Figure 1 shows the nitrogen adsorption isotherms for the samples ACS25 and ACS10, demonstrating that activated carbons obtained fit to Langmuir isotherm or Type I related with the IUPAC classification (Martinez 1988; Rodrı´guez-Reinoso and Linares-Solano 1989; Lovera 2003). A greater volume was adsorbed at low rel- ative pressures, characteristic of microporous adsorbents, and the next part of the isotherm is not completely linear, indicating the presence of a larger pore size generated by the activation type (Martinez 1988; Lovera 2003). In the adsorption isotherms for the carbon ACS10, it can be observed that the curve displayed a small hysteresis loop, indicating the presence of mesopore volume (Rodrı´guez- Reinoso and Linares-Solano 1989; Lovera 2003). Finally, it 0.0 0.2 0.4 0.6 0.8 1.0 0 20 40 60 80 100 120 Relative Pressure (P/P0 ) Vadsorbed(cm3 g-1 STP) ACS10 ACS25 Fig. 1 Nitrogen adsorption isotherms at 77 K for activated carbons Adsorption 123
  5. 5. is evident that the ACS25 sample adsorbs greater volumes of nitrogen, exceeding 100 cm3 g-1 ; this happens because the increase in concentration of ZnCl2 favours the devel- opment of porosity and apparent surface area, because zinc chloride is a dehydrating agent, which produces the greater removal of water molecules from the lignocellulosic matrix, which increases porosity development. Also, due to the increase in zinc chloride atoms on ACS25 sample, more pores will be developed when the temperature of carbonisation and washing the material remove particles of this activating agent. Table 1 shows the apparent surface area calculated by the B.E.T. method, the micropore volume content, meso- pore and total pore volume which were calculated by the D.R. method, where the characteristic energy also was determined. The results show changes in textural charac- teristics of carbonaceous materials. It was observed that for sample ACS25, the apparent surface area was 300 m2 g-1 and for sample ACS10, it was 250 m2 g-1 , demonstrating similar values. Results of micropore volume obtained by the equation D.R. showed a similar tendency to the car- bonaceous samples, in which the micropore volume was 0.140 and 0.125 cm3 g-1 for ACS25 and ACS10, respec- tively. On the other hand, the volume of mesopores pre- sented low values at both samples. However, an increased content of mesopores for ACS10 carbon is observed. Additionally, the average pore diameter was determined and reported in Table 1, which shows the larger diameter pore (1,820 nm) for the sample with the higher surface area and greater micropore volume content, i.e. the activated carbon ACS25. In general, the textural properties presented in Table 1 are best developed for ACS25 sample, which happens because with increasing concentration of zinc chloride the high development of porosity is evidenced and a higher surface area is apparent. As reported in other studies of lignocellulosic materials activated by zinc chloride (Khalilia et al. 2000; Azevedo et al. 2007). Likewise, as shown in Fig. 1 and Table 1, the microp- orosity can be attributed to concentrations of activated agent in samples, which favours the development of microporosity (Rodrı´guez-Reinoso and Linares-Solano 1989; Lovera 2003). The results of total surface groups are also shown in Table 2. The carbons immersed in HCl, determine the total amount of basic sites, indicating that sample ACS25 had the largest concentration with 0.238 meq g-1 , and sample ACS10 presented the lowest concentration of basic sites with a value of 0.178 meq g-1 . Moreover, immersion in NaOH carbons can determine the concentration of total acid sites, finding that ACS25 sample contained the highest concentration of total acid sites with 0.092 meq g-1 , while the other sample had a value of 0.057 meq g-1 . The obtained carbons had a higher content of basic groups and pH values in the relatively neutral zero point of charge, with values between 6.98 and 6.91, which favours adsorption of phenols derivatives. This is because if the samples have a greater amount of acidic groups, they are located on the edges of graphene layers, withdrawing the electron density of electrons p, leading to a weaker inter- action between the p electrons of the aromatic ring of the phenol and graphene layers, which reduces the adsorptive capacity (Moreno-Castilla et al. 1995; Moreno-Castilla 2004). In order to detect the functionality present in ACs, adsorption in the infrared (IR) region takes place (4000–400 cm-1 ) due to the rotational and vibrational movement of the molecular groups and chemical bond of a molecule. The FT-IR spectra were obtained to evaluate qualitatively the chemical structures of ACs. Figure 2 Shows the FT-IR spectrum of ACs, which indicated various surface functional groups. The broad band at around 3500 cm-1 is typically attributed to the hydroxyl group of phenol, alcohol, and carboxylic acid. The relatively intense band at about 1200 cm-1 observed in the samples is attributed to C–O–C stretching in ethers. In the FT-IR spectra, the peaks observed at 1577 and 1586 cm-1 can be attributed to C = O stretching in ketones. Moreover, the region of the spectrum of 2220 cm-1 is attributed to alkyne group (C:C) (Saka 2012). The bands observed from 700 to 750 correspond to stretching by the presence of the C–Cl group, due to the activating agent used. Finally, to appreciate the bands corresponding to specific surface chemical groups, no specific differences were evidenced by variation in the concentration of the activating agent (Pakułaa et al. 2005). Table 1 Textural and chemical properties of the activated carbons Sample AB.E.T. (m2 g-1 ) Model D.R. Chemistry properties Vlpore (cm3 g-1 ) Vmesopore (cm3 g-1 ) Vtotal (cm3 g-1 ) E0 (KJ mol-1 ) Average pore diameter (nm) Acidity (meq g-1 ) Basicity (meq g-1 ) pHPCC ACS25 300 0.140 0.007 0.146 10.946 1.510 0.092 0.238 6.98 ACS10 250 0.125 0.015 0.140 8.354 1.820 0.057 0.178 6.91 Adsorption 123
  6. 6. Table2Pseudo-firstorder,pseudo-secondorderandchemisorptionmodelconstantsandcorrelationcoefficientsforphenolsadsorptionontoACs PhenolsCarbonC0(mgL-1 )Qeexp (mgg-1 ) Pseudo-firstorderPseudo-secondorderElovichkineticmodel Qe(mgg-1 )Kf*10-2 (h-1 )R2 DQ(%)Qe(mgg-1 )b(g-1 h-1 )R2 DQ(%)a(mgg-1 h-1 ) b(gh-1 )R2 DQ(%) PhenolACS25104.6743.6002.3030.9150.0954.7501.9610.9540.0540.1096.5790.8371.452 4019.02517.8591.6120.8560.34220.8590.0970.9680.0950.6472.9410.8000.895 7036.52635.4731.3820.9270.16240.4730.6090.9790.0090.7940.9230.8660.963 10045.05539.4664.6060.9680.09346.4660.0720.9480.0281.4080.5510.8812.624 ACS10103.6014.4174.9300.9080.2562.41714.0130.9880.0310.1165.6500.9382.658 4020.20815.5971.3820.9070.42716.59713.1530.9790.2560.3951.7570.8830.896 7031.40030.8971.1520.8560.22732.8970.0680.9880.0560.5121.5060.8342.365 10041.95836.9321.1520.9280.12846.9322.9860.9790.0251.0560.8910.8703.365 4-NPACS25106.3443.0763.6850.9950.1746.0761.9550.9880.0431.5263.3670.9883.541 4022.93520.5408.9820.9940.35124.5402.7060.9980.1741.2501.1220.9812.658 7037.56339.2359.9820.9270.13038.6523.6090.9270.3511.2352.9410.8800.985 10056.62451.26311.9820.9380.08668.2144.6090.9890.0031.2100.9230.8601.023 ACS10105.5372.0543.2240.9950.7814.0544.0340.9880.0040.7884.4440.9332.365 4029.64718.2962.7640.9960.38822.2966.2300.9690.7811.1281.1530.9771.365 7038.52636.1233.7360.9250.14433.2419.2300.9390.3881.1592.0540.8792.984 10068.23547.1424.7649.4040.08260.63514.0340.9880.0052.1482.6540.8784.365 4-CPACS25107.7903.1682.7640.9870.4217.1682.3400.9890.0050.1005.2080.8453.654 4023.09419.8772.3030.9750.18228.8771.6120.9690.4210.5170.9420.9171.365 7043.30235.6701.3820.9260.05548.6700.8420.9890.0090.7450.6540.9040.895 10071.95857.2571.1520.9650.04569.2570.6360.9780.0051.2290.4000.9151.354 ACS10106.4654.5382.5330.9040.1226.53810.5690.9890.0200.1076.3290.9213.564 4021.70922.3301.3820.9240.18825.3301.7780.9580.1220.3241.5550.9222.698 7049.61739.1771.3820.9950.06641.1770.6520.9780.10916.7900.0600.9114.658 10078.26859.0301.1520.9650.04769.0300.4930.9890.0140.9900.4690.9025.654 Adsorption 123
  7. 7. 3.2 Adsorption kinetics The study of adsorption kinetics is important because the rate of adsorption (which is one of the criteria for efficiency of adsorbent) and also the mechanism of adsorption can be concluded from kinetic studies (Ho and McKay 2000; Ho 2006b; Wu et al. 2009; Tseng et al. 2010; Boparai et al. 2011; Aljeboree et al. 2014). The relationship between contact time and phenol adsorption onto ACs at different initial phenol concentra- tions (10.0 to 100 mg L-1 ) is shown in Fig. 3. This graph is obtained by plotting the absorption capacity versus time t in the time required to reach equilibrium system in hours, proving the variation of the amount of adsorbed (Q) as a function of time. The rate of adsorption for phenols is high at initial times of adsorption (Azizian 2008; Plazinski and Plazinska 2012). For phenols, most of the adsorption takes place within 300 min which indicate that the rate of phe- nols adsorption by ACs is high (Figure to 4-CP and 4-NP not was shown here, but had a similar behaviour). Figure 3 indicates that while the adsorption of phenol was quite rapid initially, the rate of adsorption became slower with the time and reached a constant value (equilibrium time). Additionally, it showed that the speed increased at lower concentrations. The initial faster rate may be due to the availability of the uncovered surface area of the adsorbents (Ho and McKay 1999; Ho 2006a; Manasi and Rajesh 2014). In order to analyse the adsorption kinetics of mono- substituted phenols by ACs the pseudo-first-order, pseudo- second order, Elovich equation, Dumwald–Wagner equa- tion and intra-particle diffusion model were tested (Leyva- Ramos and Geankoplis 1994; Qiu et al. 2009; Feng-Chin et al. 2009; Siminiceanu et al. 2010; Gihan and El-Khaiary 2010; Plazinski et al. 2013) The result of fitting is listed in Tables 2 and 3. A simple kinetic analysis of adsorption (pseudo-first- order equation or Lagergren equation) is in the form: Qt ¼ Qe 1 À expðkf tÞ Â Ã ð4Þ Where Qt is the amount of adsorbate adsorbed at time t (mg g-1 ), Qe is the adsorption capacity in the equilibrium (mg g-1 ), kf is the pseudo-first-order rate constant (h-1 ), and t is the contact time (Rudzinski and Plazinski 2006; Qiu et al. 2009). A pseudo-second-order equation based on adsorption equilibrium capacity may be expressed in the equation: Qt ¼ ksQ2 et 1 þ ksQet ð5Þ where ks is the pseudo-first-order rate constant (g gm-1 - min-1 ) and t is the contact time (Qiu et al. 2009). The applicability of the kinetic model to describe the adsorption process was further validated by the normalised standard deviation DQ (%), which is defined as (Qiu et al. 2009; Valderrama et al. 2010): DQ ¼ 100 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P ðQexp À Qcal=QexpÞ Â Ã2 N À 1 s8 : 9 = ; ð6Þ The Elovich equation has been applied satisfactorily to some chemisorption processes and has been found to cover a wide range of slow adsorption rates. Moreover, this describes the adsorption of adsorbate by solid adsorbents in aqueous medium. The same equation is often valid for 3600 3000 2400 1800 1200 600 60 70 80 90 100 C=C C=O C-O-COH- 1586 700 1200 1577 2220 3500 Transmittance%T Wavenumber cm-1 ACS10 ACS25 Fig. 2 FT-IR spectra for the ACs derivate from Eucalyptus shell activated with ZnCl2 0 1 2 3 4 5 6 7 8 9 10 0 10 20 30 40 50 ACS25 10 mg L-1 ACS10 10 mg L-1 ACS25 40 mg L-1 ACS10 40 mg L-1 ACS25 70 mg L-1 ACS10 70 mg L-1 ACS25 100 mg L-1 ACS10 100 mg L-1 Qt (mgg-1 ) Time (h) Fig. 3 Kinetics of phenol adsorption at different concentrations onto ACs Adsorption 123
  8. 8. systems in which the adsorbing surface is heterogeneous, and is formulated as (Qiu et al. 2009; Feng-Chin et al. 2009): Qt ¼ 1 b LnðabÞ þ 1 b LnðtÞ ð7Þ The intra-particle diffusion model based on the theory proposed by Weber and Morris was used to identify the diffusion mechanism (Gihan and El-Khaiary 2010). According to this theory, the adsorbate uptake Qt varies almost proportionally with the square root of the contact time, t‘ rather than t, Eq. (7) (Qiu et al. 2009; Gihan and El-Khaiary 2010): Qt ¼ kid ffiffi t p þ I ð8Þ where I is the intercept and kid (mg g-1 h-1/2 ) is the intraparticle diffusion rate constant. The Dumwald–Warner model is another intraparticle diffusion model, which is written as (Acharya et al. 2009; Qiu et al. 2009; Siminiceanu et al. 2010; Gihan and El- Khaiary 2010; Qing-Song et al. 2010; Theydana and Ahmed 2012): F ¼ Qt Qe ¼ 1 À 6 p2 X1 n¼1 1 n2 expðÀn2 ktÞ ð9Þ where k (min-1 ) is the rate constant of adsorption. Equa- tion (9) can be simplified as: Inð1 À FÞ ¼ Àkfdt ð10Þ Where F is the fractional attainment of equilibrium (F = Qt/Qe) and kfd (min-1 ) is the film diffusion rate coefficient. The Dumwald–Wagner model proved to be a reasonable model for different kinds of adsorption systems (Qiu et al. 2009; Siminiceanu et al. 2010). The results of the kinetic data of adsorptions of phenol, 4-NP and 4-CP at different initial concentrations are given in Tables 2 and 3. In the pseudo-first order model kf and Qe, were calculated using the slope and intercept of plots of Log (Qe - Qt) versus t (Fig. 4; Table 2). It shows that the Table 3 Intraparticle diffusion and liquid film diffusion model constants and correlation coefficients for phenols adsorption onto ACs Phenols Carbon C0 (mg L-1 ) Intraparticle diffusion Liquid film diffusion kid (mg g-1 h0.5 ) I R2 DQ (%) Qe (mg g-1 ) kfd (h-1 ) R2 DQ (%) Phenol ACS25 10 0.094 0.042 0.979 0.854 3.546 0.009 0.993 1.526 40 0.487 0.365 0.978 1.562 18.5621 0.009 0.998 2.651 70 0.733 0.552 0.989 3.541 40.621 0.010 0.990 2.865 100 1.221 0.806 0.998 2.654 40.896 0.404 0.982 3.365 ACS10 10 0.099 0.045 0.998 2.456 3.456 0.010 0.978 2.365 40 0.380 0.304 0.998 1.365 12.365 0.010 0.985 0.954 70 0.456 0.332 0.989 2.365 25.658 0.008 0.993 1.984 100 0.768 0.338 0.998 2.321 40.998 0.010 0.971 2.654 4-NP ACS25 10 0.196 0.428 0.899 2.654 5.654 0.019 0.863 3.256 40 0.262 1.419 0.959 2.365 20.652 0.066 0.988 3.651 70 0.362 2.019 0.979 2.123 36.654 0.009 0.928 2.365 100 0.982 2.119 0.979 2.415 62.365 0.046 0.938 2.654 ACS10 10 0.140 0.34 0.819 1.025 3.658 0.023 0.801 1.254 40 0.502 0.514 0.899 2.125 23.654 0.043 0.883 2.365 70 0.620 0.564 0.979 2.451 35.568 0.078 0.873 2.854 100 0.730 0.614 0.979 3.562 65.651 0.033 0.889 4.654 4-CP ACS25 10 0.216 0.101 0.989 3.214 6.854 0.011 0.998 3.254 40 0.700 0.362 0.999 3.245 25.658 0.014 0.985 3.652 70 1.034 0.584 0.998 3.354 46.365 0.012 0.990 4.562 100 2.122 0.988 0.988 3.654 60.652 0.016 0.965 4.658 ACS10 10 0.295 0.042 0.997 2.124 4.658 0.011 0.984 2.654 40 0.425 0.197 0.997 2.365 20.654 0.011 0.974 2.854 70 0.629 0.497 0.918 2.854 40.654 0.011 0.989 3.654 100 1.303 0.969 0.978 2.654 62.687 0.013 0.972 3.854 Adsorption 123
  9. 9. correlation coefficients (R2 ) for the pseudo-first order kinetic model fit are far from 1.00 and the standard devi- ation values are high compared with the pseudo-second- order. Moreover, a low correlation was also observed between Qe exp and Qe calculated of model. Thereby, the pseudo-first-order model isn’t a suitable equation to describe the adsorption kinetics of phenols on the ACs (Qiu et al. 2009; Siminiceanu et al. 2010). In the pseudo-second order adsorption parameters Qe and ks in Eq. (3) were determined by plotting t/Qt and t (Fig. 5; Table 2). The values of Qe calculated of model are close from the experimental value, the R2 values derived from the second-order kinetic model were rela- tively high in comparison with the Pseudo-first order model. Therefore this model fit the adsorption process of phenol onto ACs. As shown in Table 2, phenol and 4-CP demonstrate similar adsorption kinetics, while other phenols exhibit slower initial adsorption rates. For phenol, it should be noted that its adsorption driving force is weaker due to a relatively lower ultimate uptake; in fact, its adsorption kinetics is remarkable, as suggested by the second-order rate index. For 4-NP slower adsorption rates should be ascribed to the steric effects, i.e., the adsorbate molecules have difficulties in moving within pores with size not large enough. Adsorption kinetics is more sensitive to the steric effects, which demonstrates their influences during the adsorption process. The similar adsorption kinetics of phenol and 4-CP indicate that their adsorption process is not hindered to an appreciable extent, suggesting that steric effects are negligible if the molecular dimensions are below some limits (Qiu et al. 2009; Siminiceanu et al. 2010). In the Elovich model, a and b were calculated from the slope and intercept of the plot of Qt vs In t and the results are present in Table 2 (figure not shown here). The values show that the correlation coefficients were not satisfactory for most of the cases, which indicated that the Elovich model is not appropriate for the description of phenol, 4-CP and 4-NP adsorption by ACs. Therefore, phenols adsorp- tion on the ACs does not follow chemisorption models. Thus, suggested phenols adsorption does not occur via the sharing of electrons between the phenolic ring and basal plane of ACs (Qiu et al. 2009; Wu et al. 2009; Ahmaruz- zaman and Laxmi 2010; Al-Khateeb et al. 2014). 3.3 Diffusion kinetic model A detailed understanding of adsorption mechanisms facil- itates the determination of the rate-limiting step. This information can then be used to optimise the design of adsorbents and adsorption conditions. The overall rate of adsorption can be described by the following three steps: (i) film or surface diffusion where the sorbate is transported from the bulk solution to the external surface of sorbent (ii) intraparticle or pore diffusion, where sorbate molecules move into the interior of sorbent particles, and (iii) adsorption on the interior sites of the sorbent (Theydana and Ahmed 2012). Since the adsorption step is very rapid, it is assumed that it does not influence the overall kinetics. The overall rate of adsorption process, therefore, will be 0 1 2 3 4 5 6 7 8 9 10 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Log(Qe -Qt ) ACS25 10 mg L-1 ACS10 10 mg L-1 ACS25 40 mg L-1 ACS10 40 mg L-1 ACS25 70 mg L-1 ACS10 70 mg L-1 ACS25 100 mg L-1 ACS10 100 mg L-1 Time (h) Fig. 4 Pseudo-first-order kinetic model fit for phenol adsorption at different concentrations onto ACs 0 1 2 3 4 5 6 7 8 9 10 0 15 30 45 60 Time (h) t/Qt (hg/mg) ACS25 10 mg L-1 ACS10 10 mg L-1 ACS25 40 mg L-1 ACS10 40 mg L-1 ACS25 70 mg L-1 ACS10 70 mg L-1 ACS25 100 mg L-1 ACS10 100 mg L-1 Fig. 5 Pseudo-second-order kinetic model fit for phenol adsorption at different concentrations onto ACs Adsorption 123
  10. 10. controlled by either surface diffusion or intraparticle dif- fusion (Gihan and El-Khaiary 2010; Theydana and Ahmed 2012). The Weber–Morris intraparticle diffusion model has often been used to determine whether intraparticle diffu- sion is the rate-limiting step. According to this model, a plot of Qt versus t0.5 should be linear if intraparticle dif- fusion is involved in the adsorption process and if the plot passes through the origin then intraparticle diffusion is the sole rate-limiting step. It has also been suggested that in instances when Qt versus t0.5 is multilinear two or more steps govern the adsorption process, the multilinearity of this plot for adsorption on activated carbon suggests that adsorption occurred in three phases. The initial steeper section represents surface or film diffusion, the second linear section represents a gradual adsorption stage where intraparticle or pore diffusion is rate-limiting and the third section is final equilibrium stage. In Fig. 6, the adjustment graph of the intraparticle diffusion model for phenol is shown (Figures for 4-CP and 4-NP are not shown here), as the plot passes through the origin, intraparticle diffusion could be rate-limiting step for most phenol concentrations studied; however, for some concentrations of phenol (100 mg L-1 ), in both carbons, there were three processes controlling the adsorption rate but only one was rate lim- iting in any particular time range. The intraparticle diffu- sion rate constant kid was calculated from the slope linear section (Fig. 6; Table 3). The value of the intercept I in the second section provides information related to the thick- ness of the boundary layer. Larger intercepts suggest that surface diffusion has a larger role as the rate-limiting step (Gihan and El-Khaiary 2010; Theydana and Ahmed 2012; Ocampo-Pere´z and Leyva-Ramos 2013). In the liquid film diffusion models, a linear plot of In(1 - F) versus t with zero intercept suggests that the kinetics of the adsorption process is controlled by diffusion through the liquid film (this Figure is not shown here). Application of the liquid film diffusion model to the adsorption of phenol, 4-CP and 4-NP by ACs did not converge, and the regression coefficient values were very low; however, the standard deviation was higher, as shown in Table 3. This indicates that the liquid film diffusion was not the rate-determining step (Acharya et Al 2009; Gihan and El-Khaiary 2010; Qing-Song et al. 2010; Theydana and Ahmed 2012; Ocampo-Pere´z and Leyva-Ramos 2013). Based on the results presented in Table 3, it is clear that the mechanism of interaction between the phenolic com- pounds and the ACs is somewhat complex. The application of the intra-particle diffusion model, and liquid film dif- fusion model, on the experimental data yielded different straight lines but only in intraparticle model the line passing through the origin, which indicates some degree of boundary layer control. This further show that the intra- particle diffusion and liquid film diffusion are not the only rate-controlling step, but other processes may also control the rate and mechanism of adsorption (Acharya et Al 2009; Gihan and El-Khaiary 2010; Theydana and Ahmed 2012; Ocampo-Pere´z and Leyva-Ramos 2013). 3.4 Adsorption isotherms The adsorption isotherm can describe the distribution of phenol between solid phase and the solution at a certain temperature when the equilibrium was reached. Figure 7 shows the experimental adsorption isotherms for phenol, 4-CP and 4-NP, for the two samples, which depicts the phenol adsorption capacity (Qe expressed in mg per gram of activated carbon retained). It shows that phenol adsorption behaviour follows the Freundlich isotherm, because the mass of adsorbed phenol in a wide range of concentrations as considered in this work does not became asymptotic at high concentrations; the same behaviour applied for the adsorption of 4-NP by ACS10. Although in some isotherms, e.g. phenol adsorption on ACS10, it behaves according to the Langmuir model, since the mass of phenol remains constant when the phenol concentration at equilibrium is greater than 400 mg L-1 . Compared with phenol, substituted phenols showed greater intensity of adsorption and adsorption capacity given in the following order: 4-CP [ 4-NP [ phenol. As phenol has smaller molecular size than substituted phenols, these results imply that only a small part of the micropores is filled in phenol, adsorption and the micropore filling phenomenon is more evident for the substituted phenols (Moreno-Castilla et al. 1 2 3 4 0 10 20 30 40 Qt (mgg-1 ) Time0.5 (h0.5 ) ACS25 10 mg L-1 ACS10 10 mg L-1 ACS25 40 mg L-1 ACS10 40 mg L-1 ACS25 70 mg L-1 ACS10 70 mg L-1 ACS25 100 mg L-1 ACS10 100 mg L-1 Fig. 6 Intraparticle diffusion plots for phenols adsorption at different concentrations onto ACs Adsorption 123
  11. 11. 1995; Dabrowski et al. 2005; Derylo-Marczewska et al. 2010; Qing-Song et al. 2010; Rincon-Silva et al. 2015). 3.5 Equilibrium adsorption of phenols The experimental data of the adsorption isotherms were fitted to models Langmuir, Freundlich and Dubinin– Raduskevich–Kaganer (D.R.K.), which are presented in Table 3. Each isotherm model was expressed by relative certain constants which characterised the surface properties and indicated adsorption capacity of this material (Kumar et al. 2007; Derylo-Marczewska et al. 2010; Qing-Song et al. 2010; Rincon-Silva et al. 2015). The Langmuir model proposes that monolayer sorption occurs on the solid surface with identical homogeneous sites. It also suggests that no further adsorption takes place once the active sites are covered with phenols molecules. The saturated monolayer isotherm is presented by the fol- lowing equation: Qe ¼ QmaxkLCe 1 þ kLCe ð11Þ where Ce is the concentration of phenol at equilibrium in solution (mg L-1 ); Qe is unit equilibrium adsorption capacity; Qmax is the maximum phenol uptake, giving the information about adsorption capacity for a complete monolayer (mg g-1 ); and KL is a constant denoted the energy of adsorption and affinity of the binding sites (L mg-1 ) (Qing-Song et al. 2010; Okelo and Odebunmi 2010; Rincon-Silva et al. 2015). Freundlich isotherm is an empirical model assuming that the distribution of the heat on the adsorbent surface is non- uniform, namely a heterogeneous adsorption. The equation is stated as follows (Kumar et al. 2007; Qing-Song et al. 2010; Okelo and Odebunmi 2010; Rincon-Silva et al. 2014): Qe ¼ kf ðCeÞ1=n ð12Þ where n and Kf [mg g-1 (L mg-1 )n ] are both the Fre- undlich constants giving an indication of adsorption intensity and capacity, respectively. The degree of non- linearity between solution concentration and adsorption is n dependent as follows: if the value of n is equal to unity, the adsorption is linear; if the value is below to unity, this implies that adsorption process is chemical; if the value is above unity adsorption is a favourable physical process (Mourao et al. 2006; Kumar et al. 2007; Qing-Song et al. 2010; Rincon-Silva et al. 2014). The Dubinin–Radusckevisch–Kanager model is repre- sented in Eq. (13) Qe ¼ QmDRK exp À RTIn Cs=Ce À Á ES !n ð13Þ where QmDRK represents the amount adsorbed of solute on the monolayer, Ce and Cs are the concentrations of equi- librium saturation and adsorbate, respectively. ES is related to the energy characteristic of the process, and n relates to the heterogeneity variations of the microporous adsorbents. The parameters n and Es are in principle responsible of surface heterogeneity for adsorbate-adsorbent system (Mourao et al. 2006; Rincon-Silva et al. 2015). The coefficients of determination (R2 ) and isotherm parameters from nonlinear regressive method were listed in Table 4. When the Langmuir model is applied for experimental adsorption isotherms, it is observed that the value of Qmax was greater for ACS25 with values of 55.566, 137.005 and 200.004 mg g-1 for phenol 4-nitrophenol and 4-chlorophenol respectively, the sample ACS10 also had the same order in the adsorption capacity. The adsorption was greater to 4-CP followed by 4-NP and finally phenol. In general, ACS25 carbon present the best able to adsorption of phenols compounds and this succeed because this is the activated carbon with the highest apparent sur- face area (300 m2 g-1 ) and the largest volume of microp- ores (0.140 cm3 g-1 ), favouring the ability to adsorption of phenolic compounds on this adsorbent. In Freundlich analysis, the kf was higher for the sample ACS25, in adsorption of 4-chlorophenol with a magnitude of 29.818 mg1-1/n L1/n g-1 , which is associated with monolayer adsorption capacity and textural properties of this sample. The value of 1/n is a measure of heterogeneity of the surface, in which value close to 0 indicates a heterogeneous surface. When the value of 1/n is less than 1 it is said that the adsorption process is favourable, as 0 400 800 1200 0 40 80 120 160 200 ACS10 phenol CS25 phenol ACS10 4-NP ACS25 4-NP ACS10 4-CP ACS25 4-CP Qe (mgg-1 ) Ce (mg L-1 ) Fig. 7 Experimental adsorption isotherms of phenols compounds at 20 °C Adsorption 123
  12. 12. happened in this study. However, there was a greater value in adsorption favouring 4-chlorophenol. In the D.R.K. model, the maximum quantity of QmDRK was on ACS25 for all adsorbates in the order 4-chlorophenol [ 4-nitrophenol [ phenol with values 155.544, 122.836 and 48.525 mg g-1 respectively; it was noted that there was a correlation of values of adsorption in monolayer between the two models. Value Es is the char- acteristic energy of adsorption process, and it can be observed that value ES was higher for the sample ACS25, an aspect that is also related with the amount adsorbed on the monolayer. Additionally, its increase was directly proportional to values of QmDRK and Qmax. On the other hand, it was observed that ES values were higher for the phenol compared with 4-nitrophenol, because this value is directly related to the solubility of phenol, and this com- pound had the highest solubility with a value of 93 g L-1 , compared with the value 1.7 g L-1 of 4-nitrophenol. The values of Es in the adsorption process of phenol were similar of the adsorption process of 4-chlorophenol, this is also explained by solubility, since 4-chlorophenol is also significantly soluble in water, and finally, this is the one with greater adsorption. Therefore, high values are also related to this parameter. Again, the activated carbon ACS25 is the solid that has greater adsorption capacity in D.R.K model, which happens for the same reason as the Langmuir model, that is, the textural properties favoured in this solid compared with the other carbon ACS10, in relation to the highest amount of used concentration of zinc chloride. According to the results presented in Table 4, the acti- vated carbon with zinc chloride to 25 % m/V is more efficient in the adsorption of phenolic compounds, relative to the sample activated to 10 % m/V, since it is the higher adsorption capacity presented in adjusting models of Langmuir and D.R.K. This happens because, as explained above, this sample is the one with higher development of textural properties. Likewise, ACS25 is the one with higher content of basic groups which favours adsorption, because these do not withdraw electron density, which does not weaken the interaction between the electrons of the aro- matic ring and of the graphene layers of coal (Moreno- Castilla et al. 1995). Similarly it is clear in Table 4 that the best fit of the data is for the Freundlich model because its correlation coefficient is very close to unity, Furthermore, this is confirmed because it has the lowest percentage of deviation (0.025–0.365 %), so it is assumed that the adsorption occur on adsorbents with the energetically heterogeneous surfaces (Kumar et al. 2007; Okelo and Odebunmi 2010; Qing-Song et al. 2010). Finally, another important aspect to consider in the adsorption process is the pH of the solution, because this also interacts with the solid charge, and indicates the concentration of phenolic compound in solution. From the diagram of phenol speciation it is known that a high pro- portion of protonated species at low pH values and pres- ence of species deprotonated at high pH values. The pH experimental values of phenols isotherms in this study were between 6.56 and 7.86. With these data and the dis- tribution of species, it can be demonstrated that the process was conducted for phenol protonated species, i.e. they are not dissociated, and the process favours the dispersion forces, because if solutions were in basic medium, they would have decreased the adsorption due to electrostatic repulsion between the negatively charged surface and the anions phenolates and between each other (Kumar et al. 2007; Qing-Song et al. 2010; Rincon-Silva et al. 2015). This behaviour is evidenced by the pKa value of phenol (9.89) which is greater than the data obtained from experimental solution pH, which proves that the adsorbed species were in their protonated form (Kumar et al. 2007; Qing-Song et al. 2010; Rincon-Silva et al. 2015). 3.6 Thermodynamic study The thermodynamic behaviours for adsorption of phenol, 4-CP and 4-NP on ACs were investigated. The thermody- namic parameters such as Gibbs free energy change (DG°), enthalpy change (DH°) and entropy change (DS°) were calculated using the following equations: Table 4 Parameters of the models, Freundlich, Langmuir and D.R.K. in the adsorption of phenols compounds on carbon samples obtained Phenol Carbon Langmuir Freundlich Dubinin–Raduskevich–Kaganer Qma´x (mg g-1 ) KL (L mg-1 ) R2 % Dev kf (mg1-1/n L1/ n *g-1 ) 1/n R2 % Dev QmDRK (mg g-1 ) ES (kJ mol-1 ) R2 % Dev Phenol ACS10 27.035 0.010 0.964 1.524 3.735 0.285 0.997 0.025 28.504 18.804 0.994 2.365 ACS25 55.566 0.010 0.985 0.958 2.852 0.444 0.985 0.124 48.525 19.448 0.985 1.895 4-NP ACS10 54.954 0.010 0.978 0.745 5.596 0.315 0.997 0.365 45.384 9.376 0.986 3.541 ACS25 137.005 0.030 0.996 1.214 21.984 0.296 0.996 0.018 122.836 12.578 0.855 4.654 4-CP ACS10 125.008 0.030 0.975 1.523 28.447 0.221 0.997 0.087 101.606 22.364 0.935 2.365 ACS25 200.004 0.030 0.984 2.587 29.818 0.282 0.998 0.124 155.544 20.205 0.934 1.365 Adsorption 123
  13. 13. DG0 ¼ ÀRTInK0 ð14Þ where K0 is the apparent equilibrium constant [In this case, is the equilibrium constant of Langmuir model kL, as has already been explained in Sect. 2.5), R is the gas constant (8.314 J mol-1 K-1 )], and T is absolute temperatures (K) (Qing-Song et al. 2010; Al-Khateeb et al. 2014; Rin- con-Silva et al. 2015). The enthalpy change (DH°) of adsorption and entropy change (DS°) of adsorption were calculated from adsorp- tion data at different temperatures using the Van’t Hoff Eq. (9) as follows (Kumar et al. 2007; Rodriguez et al. 2009; Qing-Song et al. 2010; Al-Khateeb et al. 2014): InK0 ¼ DS0 R À DH0 RT ð15Þ Other thermodynamic quantity is Ea (Arrhenius activa- tion energy), which can be calculated from the relationship from the pseudo-second order rate constant of phenols adsorption, which is expressed as a function of temperature (Ashraf et al. 2014): InkS ¼ LnA Ea RT ð16Þ where Ea is the Arrhenius activation energy of adsorption, A is the Arrhenius factor, R is the gas constant is equal to 8.314 J mol-1 K-1 and T is the operating temperature. A linear plot of Ln ks vs 1/T for the adsorption of phenols onto ACs was constructed to generate the activation energy from the slope (-Ea/R). The magnitude of activation energy gives an idea about the type of adsorption, which is mainly physical or chemical. Low activation energies (5–40 kJ mol-1 ) are characteristics for physisorption, while higher activation energies (40–800 kJ mol-1 ) sug- gest chemisorptions (Stoeckli et al. 1995; Kumar et al. 2007; Qing-Song et al. 2010; Rincon-Silva et al. 2015). The thermodynamic parameters of the system: Gibbs free energy change (DG°), enthalpy change (DH°) and entropy change (DS°) are presented in Table 5. The free energy values at temperatures 20, 30 and 40 °C in the adsorption of phenol; 4-NP and 4-CP were negative in all cases, showing the spontaneous nature in the process. That is to say, for three phenols, it was shown that the adsorption process on ACs was a spontaneous process, and the decrease of DG° values with the increase of temperature indicated that the adsorption became less favourable at higher temperatures. It is also shown that free energy values increases with the addition of nitro or chlorine groups to phenol. The maximum value of DG° for adsorption at 20 °C was on ACS25 for the three phenols, data directly related to the adsorption capacity of this carbon. In this study the free energy values indicated that the adsorption process occurs by physisorption (Stoeckli and Centeno 1997; Kumar et al. 2007; Qing-Song et al. 2010; Boparai et al. 2011; Rincon-Silva et al. 2015). DH° values are negative for all process on ACs, showing the exothermic nature of the adsorption process. Enthalpy change values are between -6.032 and -30.352 kJ mol-1 ; therefore, it follows a behaviour of physisorption in the process. DS° values were also reported; positive values indicated the entropy of the system increased during the adsorption, as in most cases. However, it should also be noted that the entropy of the universe (including the system and the surroundings) might increase because the adsorp- tion reaction was not an isolated process. Additionally, negative cases of DS° are also given, suggesting a decreased randomness at the solid–liquid interface during the adsorption process, as was observed in the adsorption of 4-NP and 4-CP with values of -10.231 and -5.583 J mol-1 , respectively, on ACS25. Finally, the values of Arrhenius activation energy are presented in Table 4 also, the values indicated that the adsorption pro- cess has a low potential barrier and also confirms the physisorption process, since these are in the range 6.852–14.564 kJ mol-1 (Huang and Gao 2007; Kumar et al. 2007; Qing-Song et al. 2010; Rincon-Silva et al. 2015). 4 Conclusions Eucalyptus shell, a waste solid from trees, was successfully utilised to synthesise activated carbon, a low cost alterna- tive adsorbent for the removal of phenols (phenol, 4-chlorophenol and 4-nithrophenol). The samples obtained present apparent surface area values of 250 and 300 m2 g-1 and micropore volume between 0.007 and 0.015 cm3 . The micropore and mesopore distribution is appropriate for carrying out efficient adsorption processes, especially in aqueous phenols. Also, the adsorption iso- therms of phenols on ACs were studied and modelled using three isotherm models. Freundlich model gives the best fitting for the adsorption isotherms in most cases, while Langmuir model is reasonably applicable in all cases. The values for the adsorption capacity were between 27.035 and 200.004 mg g-1 . In kinetic studies, ACs show high adsorption rate. The pseudo-second-order model gives satisfactory fitting, and the intraparticle diffusion model describes the adsorption process well. Steric effects on adsorption kinetics were found for 4-NP, due to the limi- tation of the pore structure and the retardation of the adsorbed molecules. It is proposed that the steric effects are notable on the adsorption kinetics. The classification of the kinetic models according to the adsorption study is: pseudo-first-order [ pseudo-second-order [ intra-particle- diffuse [ chemisorption. Adsorption 123
  14. 14. The thermodynamic study demonstrates the spontaneous and exothermic nature of the adsorption process due to negative values of both free energy change and enthalpy change, the increased entropy change with the increased substitution degree was observed; based on thermodynamic parameters, the adsorption is primarily physical in nature. Finally, the uptakes were observed at pH pKa which indicates that the adsorbed phenols species were in their protonated form which improves the adsorption process on ACs. Acknowledgments The authors wish to thank the Master Agree- ment established between the ‘Universidad de los Andes’ and the ‘Universidad Nacional de Colombia’ and the Memorandum of Understanding entered into by the Departments of Chemistry of both Universities. 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