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Sorptive removal of salicylic acid and ibuprofen from aqueous solutions using pine wood fast pyrolysis biochar
Accepted Manuscript Sorptive removal of salicylic acid and ibuprofen from aqueous solutions using pine wood fast pyrolysis biochar Matthew Essandoh, Bidhya Kunwar, Charles U. Pittman Jr., Dinesh Mohan, Todd Mlsna PII: DOI: Reference:
S1385-8947(14)01616-7 http://dx.doi.org/10.1016/j.cej.2014.12.006 CEJ 13003
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
3 September 2014 1 December 2014 2 December 2014
Please cite this article as: M. Essandoh, B. Kunwar, C.U. Pittman Jr., D. Mohan, T. Mlsna, Sorptive removal of salicylic acid and ibuprofen from aqueous solutions using pine wood fast pyrolysis biochar, Chemical Engineering Journal (2014), doi: http://dx.doi.org/10.1016/j.cej.2014.12.006
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Sorptive removal of salicylic acid and ibuprofen from aqueous solutions using pine wood fast pyrolysis biochar Matthew Essandoha, Bidhya Kunwara, Charles U. Pittman Jr.a, Dinesh Mohanb, Todd Mlsnaa,* a
Department of Chemistry, Mississippi State University, Mississippi State, MS 39763, USA
b
School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067, India
ABSTRACT Pine wood biochar, prepared at 698 K with a residence time of 20-30 s in an auger-fed reactor, was used as a 3-dimensional adsorbent to remove salicylic acid and ibuprofen from aqueous solutions. This biochar was characterized by FT-IR spectroscopy, scanning electron microscopy, transmission electron microscopy, surface area determination, and zero point charge measurements. Batch sorption studies were carried out at pH values from 2 to 10, adsorbate concentrations from 25 mg/L to 100 mg/L and temperatures from 298 to 318 K. The adsorption of both adsorbates was highest at low pH values, dropped as pH increased and then exhibited a second increase related to the pKa of these carboxylic acid adsorbates. This was followed by a further drop at high pH. Conjugate acid/base equilibria of the adsorbates and the phenolic hydroxyl and carboxylic acid biochar sites versus pH dominated the mechanism. Sorption followed pseudo-second order kinetics. Sorption was evaluated from 298 to 318 K using the Freundlich, Langmuir, Redlich–Peterson, Toth, Sips, and Radke-Prausnitz adsorption isotherm models. Langmuir adsorption capacities for both salicylic acid and ibuprofen were 22.70 and 10.74 mg/g, respectively. This low surface area pinewood biochar (1.35 m2/g) can adsorb far more adsorbate compared to commercial activated carbons per unit of measured surface area. Methanol stripping achieved 93 and 88 % desorption of salicylic acid and ibuprofen, respectively, from the spent biochar, and 76 and 72 % of the initial salicylic acid and ibuprofen adsorption capacity, respectively, remained after four full capacity equilibrium recycles.
KEYWORDS: ibuprofen, salicylic acid, biochar, adsorption, solution pH.
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1. INTRODUCTION Wastewater pollution is an urgent environmental concern. The negative effects of pollution on both humans and the environment have become a subject of intense discussion. Pharmaceutical pollutants are increasingly important and require immediate attention [1]. Pharmaceutical products enter the aquatic environment through municipal waste water treatment, plant effluent, animal excreta, direct discharge of pharmaceuticals into water bodies, and septic tank leakage [2]. Negative effects on humans and animals include the suppression of embryonic cell growth in humans [3], the incapacitation of liver and gills in fish [4], and the inhibition of Gram-positive bacteria [5]. Despite public concerns about the negative effects that pharmaceuticals have on aquatic ecosystems, there are currently no federal laws defining the amount of these chemicals permitted in waste water streams or in drinking water. Acetylsalicylate is a widely sold “over the counter” drug. Deacetylation of acetylsalycylate produces salicylic acid 1 and its two metabolites (ortho-hydroxyhippuric acid and the hydroxylated metabolite gentisic acid) [6]. Salicylic acid has been found in industrial influent samples in concentrations up to 54 ug/L [7]. Ibuprofen 2 is a non-steroidal anti-inflammatory drug with a wide global consumption. It is produced on an industrial scale as a racemate which is generally excreted in the form of conjugates such as 2-[4-(2-hydroxy-2-methylpropyl)phenyl]propanoic acid, 2-[4-(2-carboxypropyl)phenyl]propanoic acid, and 2-phenylpropanoic acid. Ibuprofen is known to have endocrine disruptive activity [8]. Aqueous solutions of these pharmaceutical pollutants have been treated by physical, biochemical and chemical processes [9]. However, these techniques can be expensive. Thus, low cost methods to remove pharmaceutical contaminants from aqueous solution will be well embraced. Low cost adsorbents can be employed to remove recalcitrant compounds from aqueous solution inexpensively and can be effective for removing both organic and inorganic contaminants [10]. The objective of this work is to establish if pine wood biochar can be used to successfully remove ibuprofen and salicylic acid from water. This pinewood biochar is produced by the fast pyrolysis 2
of pine chips in an auger-fed fast pyrolysis reactor used to make bio-oil [11]. Hence, it is available as a byproduct of bio-oil [12], an emerging renewable liquid fuel. If these types of biofuels are eventually produced in large amounts, biochars will be widely available in many locations. Biochars have fuel value (as a coal substitute) or can be used for carbon sequestration or soil conditioners [13, 14]. A value-added application as a low cost widely applicable adsorbent [10, 15] is desirable and could increase the value of this biofuel byproduct. This research investigates the adsorption behavior of salicylic acid and ibuprofen at different concentrations, pH values, and temperatures.
2. EXPERIMENT 2.1. Chemicals and equipment All chemicals used were either GR or AR-grades. They were purchased from Sigma Aldrich (Saint Louis, MO) unless otherwise specified. Salicylic acid and ibuprofen sodium salt were purchased as white solids with pKa’s of 2.97 and 4.91, respectively. Salicylic acid’s solubility in distilled water is 2 g/L at 293 K, while that of ibuprofen’s sodium salt is 100 g/L. However, the solubility of ibuprofen in water is only 21 mg/L in distilled water.
2.2. Preparation of pine wood biochar The biochar used for adsorption experiments was produced as a byproduct of bio-oil produced by fast pyrolysis of pine wood chips in an auger-fed reactor at a temperature of 698 K using a 3
feed rate of 1-2.5 kg/h [11]. The residence time of the pine wood chips in the hotzone was 20-30 s and the approximate gas residence time in the reactor was 2 s. The chips were preheated to between 383 – 393 K, and a proprietary heat transfer method was used to speed the temperature rise to 698 K in the hotzone. The powdered biochar was collected from the system by capturing it in an enclosed pot after moving past the hotzone. After subsequent removal from this container at room temperature, it was washed several times with distilled water to remove salt impurities and water-soluble organic residuals. Biochar was sieved to a particle size distribution of 100 to 600 microns (0.1 to 0.6 mm), and moisture was removed by heating to 383 K for 12 h. The adsorbent was then stored in a closed vessel at 338 K for several days until needed.
2.3. Char characterization 2.3.1. FT-IR The infrared spectrum of powdered biochar was obtained using a Thermo Nicolet 6700 FTIR spectrometer. A total of 64 scans were taken from 4000 to 500 cm-1 with a resolution of 4 cm-1. 2.3.2. Scanning Electron Microscopy (SEM) A JEOL JSM-6500F FE-SEM operated at 5 kV was used to examine the biochar’s surface morphology. The sample was coated on a carbon stub attached to carbon tape and then sputtered-coated under argon with a 5 nm layer of platinum. The coated sample was then mounted into a sample holder for SEM analysis. 2.3.3. Transmission Electron Microscopy (TEM) The biochar was examined using a JEOL model 2100 TEM operated at 80 kV. A small amount of adsorbent was mixed with 100 % ethanol, sonicated for about 4 min, and allowed to stand for 24 h. A drop of this suspension was applied to a carbon film on 300 mesh copper grid and allowed to dry in air before TEM analysis.
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2.3.4. Surface area measurements and elemental analysis The biochar’s BET surface area (1.35 m2/g) was determined using a Gemini 2375 V 1.00 surface area analyzer. Biochar elemental analysis, using a CE 440 elemental analyzer (Exeter Analytical, Inc.), found 73.14 % C, 3.27 % H, 0.32 % N with the remaining percentage being 23.27 %. Ash content was 3.98 % using ASTM D1506-99 method. Thus, this char’s oxygen content was ~ 19 %. 2.3.5. Zero Point Charge Determination Zero Point Charge was determined using 0.01 M NaCl aqueous solutions with pH values ranging from 2 to 10 at pH intervals of 2. About 10 mL was added to 0.1 g of the biochar and the mixtures were swirled for 48 h. pH was adjusted using either 0.1 N HCl or 0.1 N NaOH solutions. The supernatant was decanted and its pH measured using an ORION model 210 pH meter. The point of zero charge was obtained by plotting supernatant pH verses the initial solution pH.
2.4. Adsorption Studies The adsorbate concentrations were varied from 25-100 mg/L for kinetic studies. Kinetic studies for ibuprofen and salicylic acid adsorption were carried out at pH values of 3 and 2.5, respectively, at 298, 308, and 318 K. A known amount of biochar was added to 25 mL solutions containing different adsorbate concentrations in 40 ml amber glass vials. Samples were then swirled at 200 rpm for 16 h. After equilibration, the samples were filtered through Whatman No. 1 filter paper. The amount of adsorbate remaining in the filtrate was determined at their λmax wavelengths of 220 nm for ibuprofen and 298 nm for salicylic acid. Samples were analyzed in duplicate and their average absorbances used. The amount of adsorbate removed per gram of adsorbant was obtained by: =
(1)
5
where is the amount of adsorbate (mg) removed per g of adsorbent, and are the initial and equilibrium adsorbate concentrations (mg/L) in solution, V is the solution volume (L), and M is the biochar weight (g). 2.5. Regeneration procedure The pinewood biochar (4 g/L) was equilibrated with an aqueous solution of the adsorbate (100 mg/L) at pH 2 for ibuprofen and 3 for salicylic acid in a temperature controlled shaker at 318 K. The experiments were conducted separately for salicylic acid and ibuprofen. The spent biochar was then washed twice with 20 ml of methanol (regenerant) and the filtrate analyzed by UV-vis spectrophotometry to quantify the adsorbate in the filtrate. The adsorption/regeneration procedure was repeated using the regenerated biochar in each recycle to determine the recyclability of the biochar. The char was recycled four times.
3. RESULTS AND DISCUSSION 3.1. Char Characterization The FTIR spectra of the adsorbent before and after adsorption experiments are shown in Figure S1 (supplementary materials). All spectra exhibit OH, C=C, C-O, C-H bond stretching at 3400, 1600, 1200 and 800 cm-1, respectively. The C=O stretch in the 1750 cm-1 region was slightly stronger in char samples with adsorbed ibuprofen (Figure S1b) and salicylic acid (Figure S1c) compared to the virgin biochar (Figure S1a). Salicylic acid and ibuprofen both have sp2 C-O single bond stretching peaks at ~ 1200 cm-1. The biochar/adsorbate spectra look very similar to that of the biochar alone, primarily because of the low surface adsorbate concentration present. The point of zero charge (pHpzc) is the pH at which the net charge on the surface is zero. This was found to be ~ 2 for the pinewood biochar. When pHpzc< pH of the solution, the biochar’s surface
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will be negatively charged. When the pHpzc> pH of the solution, the surface of the biochar will be positively charged. Biochar TEM images are shown in Figure S2 (supplementary materials). The lighter portions of these images indicate that electrons are passing through the carbon particles while the darker portion is due to carbon particle electron scattering. SEM images of the biochar are in shown in Figure 1. The structures clearly indicate that the biochar has a macroporous surface texture. The wood cell morphology present in the pine wood chips is still clearly observed after pyrolysis. Biochar compositions are known to vary significantly. For example ash contents (11.09 %) and elemental compositions have been reported for oak wood biochar [15], surface areas of 20.9 and 7.0 m2/g for fast and slow pyrolysis corn stover biochar have appeared [16], and different physico-chemical properties for rice husks, olive pomace, orange waste and compost biochars are known [17]. Each biochar’s properties will depend on many factors, including differences in feedstock composition, pyrolysis residence time and temperature, particle size, and pyrolysis heating rate [18].
3.2. Sorption Studies 3.2.1 Effect of solution pH on adsorbate adsorption Solution pH plays a crucial role in ibuprofen and salicylic acid adsorption from aqueous solution. The percent removal of both ibuprofen and salicylic acid is higher in acidic solutions than in the basic region (Figure 2). Guedidi et al. [19] observed a similar decrease in ibuprofen adsorption on activated carbon cloths as the solution pH increases. During adsorption at pH = 2.5 for salicylic acid and 3 for ibuprofen, the biochar’s point of zero charge is lower than the solution pH, so the biochar’s surface becomes negatively charged. The surface net negative charge density at these pH values will be very low. Also, the pKa of both the ibuprofen (4.91) and the salicylic acid (2.98) are greater than these low solution
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pH values (2.5 and 3) so these pollutants exist primarily as their conjugate acid (R-COOH) forms. Electrostatic repulsion is minor at these solution pHs, promoting adsorption by attractive H-bonds and other interactions.
3.2.2. Mechanism for the adsorption of salicylic acid and ibuprofen unto pinewood biochar At higher pH, (pH greater than the pKa of the adsorbates), both adsorbates are increasingly present as their carboxylate anions (R-COO-). Simultaneously, the biochar becomes increasingly negatively charged as pH rises (pH>pzc), causing increased electrostatic repulsion of adsorbate carboxylate anions and decreasing the adsorption capacity. This trend is seen from pH 2 or 3 through 10. However, both adsorbates show a region of intermediate pH values where adsorption increases, maximizes and then continues to drop as pH increases. These adsorptions peak at about pH 8 for ibuprofen and 6 for salicylic acid, where both adsorbates are 99.9 % in their carboxylate anion forms (each approximately 3 pH units above their pKa). For example, at pH values above 8, almost all salicylic acid (pKa = 2.98) exists as RCOO- and the surface of the biochar will have a substantial negative charge density. Thus, little salicylic acid can adsorb. However, at pH 6, the biochar’s surface phenolic hydroxyls are not ionized and they can Hbond to both salicylic acid and its carboxylate anion. This is the region where the peak in adsorption of salicylic acid occurs. Ibuprofen is two pKa units weaker an acid than salicylic acid and its adsorption verses pH plot is displaced by two pH units to higher pH. Ibuprofen’s adsorption peaks at ~ pH 8. This is most likely due to the unionized surface phenolic groups acting as H-bond donors to the carboxylate anion of ibuprofen. These biochars contain carboxylic acid, hydroxyl (both phenolic and aliphatic), lactone, ether and ketone functions, both on surfaces and within the solid content. At 19 % oxygen content, water
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can diffuse into the solid regions of the biochar and swell this solid. Adsorption is not limited to the small surface areas. This is obvious from both Table 1 (salicylic acid) and Table 2 (ibuprofen). For example, the capacity of salicylic acid on pine biochar (surface area = 1.35 m2/g) is 22.70 mg/g at 318 K or 16.82 mg/m2. This can be compared to Filtrasorb F400 (0.33 mg/m2), Sephabeads SP206 (0.08 mg/m2), GAC 830 (0.02 mg/m2), and HJ-G02 (0.26 mg/m2). Similar comparisons for ibuprofen can be made. It is clear that only the surfaces of the biochar would not take up such large amounts of the adsorbates. Pinewood biochar adsorbs as a 3-D adsorbent and not only at its outer and exposed pore surfaces. The functional groups responsible for adsorption were never generated by separate reactions with oxygen or oxidizing agents as in the case of activated carbons. This oxygen is derived from the oxygen present in the precursor wood’s cellulose, hemicellulose, and lignin content. Thus, oxygen functions exist everywhere in the biochar and not mostly on solid pore surfaces as with activated carbons. Earlier we have shown how fast pyrolysis pinewood biochar adsorbed by imbibing water and adsorbate within its solid structure [15]. This interesting behavior was augmented by demonstrating that porosity exists in the char that was actually not available to gas adsorption (N2-BET) when the char was dry. Closed pores can open during swelling, providing additional sites for adsorption. Hence, both micro and macro pores exist in the char that might have been isolated or closed when dry. However, during immersion the adsorbate is able to permeate into these pores. Also, solid nonporous charred biomass regions containing 10-20 wt. % oxygen will swell, imbibing both water and adsorbates. Thus, some adsorbates can be adsorbed within the 3-D solid structure of the char. This swelling behavior causes the char to adsorb far more adsorbate per measured unit surface area when compared to activated carbons as shown in Table 1. This is greater than 50 times more adsorption per measured surface area, clearly implicating a 3-D adsorption process in the biochar.
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The char’s point of zero charge is ~ 2 and carboxylic acids and phenols contribute to this acidic character [20]. Thus, at pH values above 2, as the pH increases, the surface acid groups ionize, negatively charging the surface. The adsorbates can exist in their acid or conjugate base forms as mentioned before. The pH dependence of these adsorbates’ equilibria and that of protonation/deprotonation equilibria of the biochar’s carboxylic acid and phenolic groups in combination give rise to the adsorption vs pH curves seen in Figure 2. Surface attractive interactions include van der Waals dispersion forces, permanent dipole-dipole attractions, π-π interactions as well as a variety of types of hydrogen bonding. The genesis of the surface basicity of carbons is still a subject of intense discussion and is not well understood [20]. The mechanism of adsorption must vary with pH. The adsorbates attraction to the surface changes with pH, and we believe, these changes in attractions are dominated by H-bonding changes. At very high pH, anion repulsions of the adsorbates’ conjugate bases (carboxylate anions) by negatively charged surface carboxylate and phenolate anions dominate. Some types of attractive H-bonding interactions are shown in Figure 3 along with charge repulsions that become dominant at high pH. The surface negative charge density is a complex function of pH because surface carboxylic acids, phenols, and aliphatic hydroxyls ionize at different pH ranges and their concentrations also differ. This complex situation is compounded because the analytes are subject to their own pH-dependent carboxylic acid/carboxylate equilibria. The interaction of all these features combine to contribute to the observed adsorption verses pH behavior, illustrated in Figure 2.
3.2.3. Kinetic Studies The removal of both ibuprofen and salicylic acid from solution was followed verses time using a biochar concentration of 4 g/L. Equilibrium was reached after approximately 10 h. All subsequent
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kinetic studies were done using a 16 h contact time to be certain that equilibrium has been achieved. The effect of initial adsorbate concentrations from 25 – 100 mg/L was studied using 25 ml of the adsorbate solution, an agitation speed of 200 rpm, a 16 h equilibration time and a solution pH of 2.5 for salicylic acid and 3 for ibuprofen. A significant increase in the adsorption capacity occurred for both adsorbates as the initial adsorbate concentration was raised. This is clearly illustrated in Figure 4 and this observation is consistent with other studies using activated bleaching earth [21]. The effect of temperature on both ibuprofen and salicylic acid adsorptions, which were run until equilibrium was reached, was studied at 298, 308 and 318 K. The amount adsorbed verses time was similar between 298 to 318 K for ibuprofen (Figure S3a). This parallels a previous study [22] where the ibuprofen capacity on activated carbon was unaffected between 298 to 313 K. Conversely, the amount of salicyclic acid adsorbed (Figure S3b) increased slightly as the temperature rose from 298 to 318 K, suggesting that the process is endothermic.
3.3. Adsorption kinetic models 3.3.1. Pseudo-first order kinetic model Lagergen’s pseudo-first order equation [23] for describing liquid-solid adsorption is given by
= −
(2)
− = − .
(3)
A plot of log ( - ) against t should be linear where is the intercept and is the slope, respectively (Equation 3 terms are defined in Table 4). Fitting ibuprofen and salicylic acid adsorption data to equation (3) gave regression coefficients ranging from 0.782 to 0.987 for ibuprofen and 0.940 to 0.990 for salicylic
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acid. The plots of these fits can be found in the supplementary material (Figure S4). The pseudo-first order parameters and its regression coefficients are found in Table S1.
3.3.2. Pseudo-second order kinetic model The pseudo-second order models are given by equations 4 and 5. Equation (5) [24] was also fitted to the experimental data.
= −
=
+
(4) (5)
A plot of t/ against t should be linear. The pseudo-second order model fits the experimental data well, giving regression coefficients of 0.980 or greater for ibuprofen and 0.990 or higher for salicylic acid. Fittings are plotted in Figure 5. The experimental and calculated and values using equation 5 for adsorbate concentrations from 25 to 100 mg/L are shown at 298, 308 and 318 K in Table 3 for both ibuprofen and salicylic acid. The calculated and values were obtained from the slope and intercept, respectively. The experimental and calculated values agree very well which indicates that ibuprofen and salicyclic acid adsorptions follow a pseudo-second order model. Pseudo-first and second order models are summarized in Table 4.
3.4. Adsorption isotherm models Isotherms are used to describe adsorption processes, typically the amount adsorbed on the surface at a constant temperature. The pinewood biochar adsorption was evaluated using the two parameter Langmuir, Freundlich, and the three parameter Redlich-Peterson, Sips, Radke-Prausnitz, and Toth isotherms. Isotherm adsorption data was collected at three different temperatures (298, 308, 318 K) 12
for salicylic acid, and one temperature (308 K) for ibuprofen (ibuprofen data was shown to be athermic). A summary of the isotherm models used in this study and references to these models are shown in Table 4. The isotherm fits to the experimental data and fitted values of the model parameters are shown in Figure 6 and Table 5, respectively. The absorbance of the ibuprofen solutions remain essentially unchanged above 100 mg/L – most likely because precipitation occurs above this concentration. This precipitation, caused by the low solubility of ibuprofen, resulted in a poor fit isotherm for ibuprofen (Figure 6d). Ibuprofen precipitation was observed in some solutions at concentrations from 100 to 200 mg/L. The exact solubility is not known and some induced ibuprofen precipitation may occur on the char if the solution is supersaturated or if the pH at the surface is modified. The experimental data gave excellent fits (R2> 0.96) to the Sips, Redlich-Peterson, and Toth isotherms, all three of which are three parameter models. Previous studies of phenol and Ni2+ on aerobic activated sludge [25] have also found the three parameter empirical equation (e.g. RedlichPeterson isotherm) to be more suitable for describing adsorption systems compared to two parameter empirical equations (e.g. Langmuir and Freundlich isotherm).
In our current studies, the Sips and
Redlich-Peterson fits were best. These are shown in Figure 6 along with the Langmuir fitting for salicylic acid. Ibuprofen’s fits to these models are all poor (Figure 6d). Fitting isotherms and obtaining their corresponding parameters can be useful when designing columns for scaling of adsorption processes [26]. The adsorption curves are typically used to determine the efficiency of industrial adsorbents employed in purification processes and are used to identify the equilibrium between adsorbate and adsorbent [27]. However, trying to identify mechanistic details from isotherm fittings is often unwarranted. Linear and nonlinear models give different correlation coefficients and often different results violate error variance and normality assumptions of standard least squares [28]. This leads to bias of the adsorption data. When various models give good R2 values,
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selecting mechanistic features from one model verses another is likely naive. Furthermore, experimental error will effect different models differently. Thus, we present fittings in Table 5, but do not try to infer physical and mechanistic details from the fittings at this point.
3.5. Regeneration of the spent biochar Methanol was selected as a stripping solvent for both salicylic acid and ibuprofen because both of these adsorbates are highly soluble in methanol and this solvent is easily removed from biochar using water. Methanol was previously used to successfully strip acidic 2,4,6-trinitrophenol from almond shell char [29]. Figure 7, plots the amounts of salicylic acid and ibuprofen adsorbed through four sequential cycles. Methanol was effective at desorbing the adsorbates from the biochar achieving 93 and 88 % desorption of salicylic acid and ibuprofen, respectively, from biochar which had been equilibrated with each of these adsorbates to full capacity. The adsorption capacity was restored to 76 and 72 % of its initial capacity in the fourth recycle for salicylic acid and ibuprofen, respectively. This level of recovery and reusability of the raw pinewood biochar adsorbent after four cycles is an added advantage to its low cost.
4. CONCLUSIONS Fast pyrolysis pinewood biochar was characterized and this biochar successfully adsorbed ibuprofen and salicylic acid from aqueous solutions. Adsorption of both ibuprofen and salicylic acid followed pseudo-second order kinetics. Adsorption was favored at low pH values because the char’s pHpzc is ~ 2 relative to the pKa’s of ibuprofen (4.91) and salicylic acid (2.98). Adsorption capacities were 22.70 and 10.74 mg/g at 318 and 308 K, for salicylic acid and ibuprofen, respectively. Despite low surface area, the biochar (~19 % O) appears to adsorb by imbibing water and adsorbate within its solid structure.
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Methanol was able to achieve 93 and 88 % desorption of salicylic acid and ibuprofen, respectively, from the spent biochar. Additionally, 76 and 72 % of the biochars initial adsorption capacity for salicylic acid and ibuprofen, respectively, remained after four full capacity equilibrium recycles.
AUTHOR INFORMATION Corresponding Author * (Tel: 662- 325-6744; fax: 662-325-1618; email:[email protected] Notes The authors declare no competing financial interest.
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B. Kunwar,T. Mlsna, Upgrading bio-oil with a combination of synthesis gas and alcohol, Energy & Fuels 58 (2013) 90-92. M. Rondon, J. Lehmann, J. Ramirez, M. Hurtado, Biological nitrogen fixation by common beans (Phaseolus vulgaris L.) increases with bio-char additions, Biol. Fertility Soils 43 (2007) 699-708. D. A. Laird, The Charcoal Vision: A Win-Win-Win Scenario for Simultaneously Producing Bioenergy, Permanently Sequestering Carbon, while Improving Soil and Water Quality., Agron. J. 100 (2008) 178-181. D. Mohan, S. Rajput, V. K. Singh, P. H. Steele, C. U. Pittman Jr., Modeling and evaluation of chromium remediation from water using low cost bio-char, a green adsorbent, J. Hazard. Mater. 188 (2011) 319-333. C. E. Brewer, K. Schmidt-Rohr, J. A. Satrio, R. C. Brown, Characterization of biochar from fast pyrolysis and gasification systems, Environ. Prog. Sustain. 28 (2009) 386-396. F.-M. Pellera, et al., Adsorption of Cu(II) ions from aqueous solutions on biochars prepared from agricultural by-products, J. Environ. Manage. 96 (2012) 35-42. H. Sun, W. C. Hockaday, C. A. Masiello, K. Zygourakis, Multiple Controls on the Chemical and Physical Structure of Biochars, Ind. Eng. Chem. Res. 51 (2012) 3587-3597. H. Guedidi, L. Reinert, Y. Soneda, N. Bellakhal, L. Duclaux, Adsorption of ibuprofen from aqueous solution on chemically surface-modified activated carbon cloths, Arabian J. Chem. (In Press). H. P. Boehm, Surface oxides on carbon and their analysis: a critical assessment, Carbon 40 (2002) 145-149. W. T. Tsai, C. Y. Chang, C. H. Ing, C. F. Chang, Adsorption of acid dyes from aqueous solution on activated bleaching earth, J. Colloid Interface Sci. 275 (2004) 72-78. A. S. Mestre, J. Pires, J. M. F. Nogueira, A. P. Carvalho, Activated carbons for the adsorption of ibuprofen, Carbon 45 (2007) 1979-1988. S. Lagergren, Zur theorie der sogenannten adsorption geloster stoffe, K. Sven. Vetenskapsalkad. Handl. 24 (1898) 1-39. Y.-S. Ho, Review of second-order models for adsorption systems, J. Hazard. Mater. 136 (2006) 681-689. Z. Aksu,D. Akpinar, Modelling of simultaneous biosorption of phenol and nickel(II) onto dried aerobic activated sludge, Sep. Purif. Technol. 21 (2000) 87-99. R. S. Summers, Strategies and Technologies for Meeting SDWA Requirements, Technomic Publishing Company Inc., Lancaster,1993. J. M. Montgomery, Water treatment principles and design, Wiley-Interscience, New York,1985. K. Y. Foo, B. H. Hameed, Insights into the modeling of adsorption isotherm systems, Chem. Eng. J. 156 (2010) 2-10. D. Mohan, A. Sarswat, V. K. Singh, M. Alexandre-Franco, C. U. Pittman, Jr., Development of magnetic activated carbon from almond shells for trinitrophenol removal from water, Chem. Eng. J. 172 (2011) 1111-1125. M. Otero, C. A. Grande, A. E. Rodrigues, Adsorption of salicylic acid onto polymeric adsorbents and activated charcoal, React. Funct. Polym. 60 (2004) 203-213. R. G. Combarros, I. Rosas, A. G. Lavin, M. Rendueles, M. Diaz, Influence of biofilm on activated carbon on the adsorption and biodegradation of salicylic acid in wastewater, Water Air Soil Pollut. 225 (2014) 1858. J. Huang, G. Wang, K. Huang, Enhanced adsorption of salicylic acid onto a β-naphthol-modified hypercross-linked poly(styrene-co-divinylbenzene) resin from aqueous solution, Chem. Eng. J. 168 (2011) 715721. F. Liu, M. Xia, Z. Fei, J. Chen, A. Li, Adsorption selectivity of salicylic acid and 5-sulfosalicylic acid onto hypercrosslinked polymeric adsorbents, Front. Environ. Sci. Eng. 1 (2007) 73-78. A. S. Mestre, et al., Waste-derived activated carbons for removal of ibuprofen from solution: Role of surface chemistry and pore structure, Bioresour. Technol. 100 (2009) 1720-1726. R. Baccar, M. Sarra , J. Bouzid, M. Feki, P. Blanquez, Removal of pharmaceutical compounds by activated carbon prepared from agricultural by-product, Chem. Eng. J. 211-212 (2012) 310-317.
16
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Figure Captions
Figure 1. SEM images of the pyrolyzed pine wood biochar. Figure 2. Effect of solution pH on ibuprofen and salicylic acid adsorption. Experiments employed 0.05 g of the biochar and 50 mg/L of the adsorbate. Figure 3.Various H-bonding and charge-charge repulsive interactions. Both adsorbate and surface functions can serve as H-donors and H-acceptors. Figure 4. Adsorption at 318 K at different ibuprofen [pH=3, amount of biochar was 4 g/L] and salicylic acid [pH=2.5, amount of biochar was 4 g/L] concentrations. Figure 5. Pseudo-second order plots at 308 K for salicylic acid [pH=2.5, biochar amount was 4 g/L] and ibuprofen [pH=3, biochar amount was 4 g/L] at adsorbate concentrations of 25, 50, and 100 mg/L. Figure 6. Adsorption isotherms for salicylic acid (a-f) and ibuprofen (g) at different temperatures. Experimental conditions [adsorbent concentration was 4 g/L, pH 2.5 and 3 for salicylic acid and ibuprofen, respectively]. Figure 7. Biochar recycling using an adsorbent dose of 4 g/L and adsorbate concentration of 100 mg/L, at a pH of 2.5 for salicylic acid and pH 3 for ibuprofen at 318 K. Methanol (20 ml x 2) was used as a stripping agent.
17
Table 1. Comparison of pinewood fast pyrolysis biochar’s salicylic acid adsorption capacity to those reported for high surface area adsorbents Adsorbents
Filtrasorb F400 Sephabeads SP207 Sephabeads SP206 Filtrasorb 400 GAC 830 HJ-G02
Hypercrosslinked polymeric adsorbent Hyprecrosslinked polymeric adsorbent with amine modified beads Pinewood biochar
Temp.
(K) 293 293 293 293 293 288 298 308 318 283 303 323 303 323
pH
a
3.5 3.5 a 3.5 a 11-12 11-12 2.8-3.5 a 2.8-3.5 a 2.8-3.5 a 2.8-3.5 a 2.8-3.5 a 2.8-3.5 a 2.8-3.5 a 2.8-3.5 a 2.8-3.5 a
Surface area
Adsorption capacity
(m2/g) 1050 627 556 1050 950-1050 770
(mg/g) 351 81.6 45.2 31.4 17.5 210.1 209.6 200.4 192.7 320 264 256 303 270
934
Adsorption per unit surface area (mg/m2) 0.33 0.13 0.08 0.03 0.02 0.27 0.27 0.26 0.25 0.34 0.28 0.27 0.32 0.29
Reference number
[30] [30] [30] [31] [31] [32] [32] [32] [32] [33] [33] [33] [33] [33]
298 2.5 1.35 7.56 5.60 This study 308 2.5 1.35 16.84 12.47 318 2.5 1.35 22.70 16.81 a These calculated pH values were calculated using salicylic acid Ka values and concentration. b The pinewood biochar adsorbs from 21.5 to 64.6 times as much salicylic acid per unit of measured surface area as HJ-602, provide one illustration of this biochar as a 3-dimensional adsorbent.
18
Table 2. Comparison of pinewood fast pyrolysis biochar’s ibuprofen adsorption capacity to those reported for high surface area adsorbents Adsorbents
Temp. (K)
pH
Chemically activated cork powder
303
4
Surface area (m2/g) 891
Adsorption capacity (mg/g) 145.2
Adsorption per surface area(mg/m2) 0.16
Reference number
Physically activated cork powder
303
4
1060
378.1
0.36
[34]
Physically activated PET
303
4
1426
266.6
0.19
[34]
[34]
Physically activated wood
303
4
899
291.9
0.33
[34]
Activated carbon from Artemesia vulgaris
298
4.19
358
16.94
0.05
[35]
Activated carbon from olive-waste cakes
298
2
793
12.6
0.02
[36]
Chemically activated cork waste
298
2-4
891
139.2
0.02
[22]
308
2
145.2
0.16
[22]
153.2
0.17
[22]
393.4
0.37
[22] [22]
Chemically and steam activated cork waste
Pinewood biochar
313
2
298
2
308
2
378.1
0.36
313
2
416.7
0.39
308
3
1060
1.35
a
10.74
7.96
[22] a
This study
The pinewood biochar adsorbs 159 times as much ibuprofen per unit of measured surface area as activated carbon from Artemesia vulgaris, indicating that this biochar is a 3-dimensional adsorbent rather than a predominantly surface adsorbent.
19
Table 3. Pseudo-second order kinetic parameters for ibuprofen, 2, and salicylic acid, 1, adsorptions. Pseudo-second order parameters for 2 Temp.
qe, exp.
qe, calc.
k2
(K)
Initial conc. (mg/L)
(mg/g)
(mg/g)
(gmg-1h-1)
298
25
3.99
4.26
0.24
308
318
Pseudo-second order parameters for 1 2
R
0.997
qe, exp.
qe, calc.
k2
(mg/g)
(mg/g)
(gmg-1h-1)
5.90
6.06
0.36
R2
0.999
50
7.31
9.71
0.07
0.986
7.35
7.69
0.21
0.999
100
12.25
12.82
0.09
0.992
8.08
8.62
0.10
0.992
25
3.27
3.34
0.47
0.996
6.15
6.29
0.41
0.999
50
7.09
9.71
0.11
0.994
7.73
8.00
0.15
0.996
100
10.09
10.42
0.18
0.999
8.37
8.85
0.09
0.990
25
3.48
3.64
0.33
0.997
6.19
6.33
0.43
0.999
50 100
7.80 10.10
8.00 11.11
0.06 0.06
0.980 0.995
7.93 9.39
8.20 10.00
0.23 0.08
0.999 0.996
20
Table 4. Summary of isotherm and kinetic models used in this study. Models name
Equation
Langmuir
Freundlich
qe =
q$ =
Redlich-Peterson
q$ =
Toth
Pseudo-first order Pseudo-second order
Q 0bCe 1 + bCe
q e = K F Ce
Sips or LangmuirFreundlich
Radke-Prausnitz
Parameters
K &' C$ )*+ 1 + a &' C$ )*+ K ./ C$ 1 + a ./ C$ 012
q$ =
q$ =
1/ n
a b C$ 0 a + bC$ 04 K 5 C$
1 + B5 C$ 07 8 5 9
− = − .:;:
< =>
=
? = @
<
+=
@
Reference number
Isotherm Models Qo-monolayer adsorption capacity (mg/g), b-constant related to net enthalpy of adsorption (b ∝ e-∆H/RT), qe-solute amount adsorbed per unit weight (mg/g), Ce-solute equilibrium concentration (mg/L). The Langmuir isotherm assumes a homogenous surface, monolayer coverage and no interaction of the adsorbate with neighboring sites. KF-constant indicative of the relative adsorption capacity of adsorbent (mg/g), 1/n- a constant indicative of the intensity of the adsorption, qeadsorption capacity (mg/g), Ce-equilibrium concentration of solute (mg/L). This isotherm is used in the low to intermediate adsorbate concentration range. KLF, aLF and nLF are the Sips constants. qe-solute amount adsorbed per unit weight (mg/g), Ce is equilibrium concentration (mg/L). This Sips isotherm will resemble Langmuir and Freundlich isotherms at high and low adsorbate concentrations, respectively. KRP, aRP and βRP are Redlich-Peterson constants and the exponent, β, lies between 0 and 1. qe- solute amount adsorbed per unit weight (mg/g) and Ce is equilibrium concentration (mg/L). a, b, and β are Radke-Prausnitz constants. qe- solute amount adsorbed per unit weight (mg/g) and Ce is equilibrium concentration (mg/L). This is a three-parameter empirical equation and was used initially to fit organic solvents over a broad concentration range. KT, BT, and βT are Toth constants. qe-adsorption capacity (mg/g), Ceequilibrium concentration (mg/L). This model was proposed from the potential theory. This model is well known to presume a quasi-Gaussian energy distribution. Kinetic models k1(h-1) is the first order adsorption rate constant, qe is the amount adsorbed at equilibrium and qt is the amount adsorbed at time “t”. k2(g.mg-1 h-1) is the second order rate constant, qe is the amount adsorbed at equilibrium, qt is the amount adsorbed at time “t” and k2qe2 represents the initial sorption rate.
21
[37]
[38]
[39]
[40]
[41]
[42]
[23] [24]
Table 5. Freundlich, Langmuir, Redlich-Peterson, Sips, Radke-Prausnitz and Toth isotherm parameters for salicylic acid, 1, and ibuprofen, 2, removal on pinewood biochar. Isotherm parametersa
Salicylic Acid
Ibuprofen
298 K 308 K Freundlich KF (mg/g) 3.60 2.48 1/n 0.30 0.31 R2 0.9483 0.9037 Langmuir Q0 (mg/g) 7.56 16.84 b 0.01 0.02 R2 0.9085 0.9559 Redlich-Peterson KRP (l/g) 3489.2 0.24 4523.06 0.004 aRP(l/mg)βRP 0.64 1.18 βRP 2 R 0.9581 0.9614 Sips KLF (l/g) 76.76 16.95 a aLF(l/mg) LF 0.000004 0.0202 nLF 0.37 1.07 R2 0.9576 0.9563 Radke-Prausnitz a 420934.61 8843554.05 b 0.77 2.48 β 0.35 0.31 R2 0.9581 0.9037 Toth KT 116.40 15.60 BT 0.80 0.15 βT 7.60 0.69 R2 0.9524 0.9582 a Terms in this column are defined under section 3.4
22
318 K
308 K
0.77 0.35 0.9581
0.54 0.59 0.8160
22.70 0.24 0.9793
10.74 0.22 0.79
0.59 0.03 0.9743 0.9774
5.56 9.60 0.42 0.8157
24.15 0.02 0.86 0.9807
90.37 0.0002 0.62 0.8144
2539153.09 3.60 0.30 0.9483
-
24.24 0.30 1.22 0.9800
87.13 0.01 3.45 0.8094
Figure 1. SEM images of the pyrolyzed pine wood biochar.
23
Figure 2. Effect of solution pH on ibuprofen and salicylic acid adsorption. Experiments employed 0.05 g of the biochar and 50 mg/L of the adsorbate.
24
O C O
H
surface carboxylic acid donor and acceptor
O O
H
H
O
O H
O
H
O
O H
O C O
H
H O
phenolic surface H-donor
O
phenolic surface H-acceptor
O
surface carboxylate function H-acceptor
O
O
surface phenol as H-donor
O
O C O
O
O
O
O
O
negative surface repulsions of adsorbate carboxylate (dominate at high pH)
negative surface repulsions of adsorbate carboxylate (dominate at high pH)
Figure 3.Various H-bonding and charge-charge repulsive interactions. Both adsorbate and surface functions can serve as H-donors and H-acceptors.
25
Figure 4. Adsorption at 318 K at different ibuprofen [pH=3, amount of biochar was 4 g/L] and salicylic acid [pH=2.5, amount of biochar was 4 g/L] concentrations.
26
Figure 5. Pseudo-second order plots at 308 K for salicylic acid [pH=2.5, biochar amount was 4 g/L] and ibuprofen [pH=3, biochar amount was 4 g/L] at adsorbate concentrations of 25, 50, and 100 mg/L.
27
Figure 6. Adsorption isotherms (a-c) for salicylic acid, 1, at 289, 308, and 318 K. Figure 6 d, shows the overlapping and poorly fit adsorption isotherms for ibuprofen, 2, at 308 K. Experimental conditions [adsorbent concentration was 4 g/L, pH 2.5 and 3 for salicylic acid and ibuprofen, respectively].
28
Figure 7. Biochar recycling using an adsorbent dose of 4 g/L and an adsorbate concentration of 100 mg/L, at a pH of 2.5 for salicylic acid and pH 3 for ibuprofen at 318 K. Methanol (20 ml x 2) was used as a stripping agent.
29
SUPPLEMENTARY MATERALS
Sorptive removal of salicylic acid and ibuprofen from aqueous solution using pine wood fast pyrolysis biochar
Matthew Essandoha, Bidhya Kunwara, Charles U. Pittman Jr.a, Dinesh Mohanb, Todd Mlsnaa,*
a
Department of Chemistry, Mississippi State University, Mississippi State,
MS 39763, USA
b
School of Environmental Sciences, Jawaharlal Nehru University, New Delhi
110067, India
30
Figure S1. FTIR spectra of virgin pine wood biochar [a], pine wood biochar-ibuprofen sample at pH 3 [b], pine wood biochar-salicylic acid sample at pH 2.5 [c]. Solutions (25 mL) containing 100 mg/L of adsorbate, 800 mg/L of biochar for salicylic acid, 400 mg/L of biochar for ibuprofen were used for the FTIR analyses after adsorption.
31
Figure S2. TEM image of the pyrolyzed pine wood biochar
32
Figure S3. Adsorption verses temperature at adsorbate concentrations of 50 mg/L. Ibuprofen [pH=3, amount of biochar was 4 g/L] and salicylic acid [pH=2.5, amount of biochar was 4 g/L].
33
Figure S4. Pseudo-first order plots for ibuprofen [pH=3, amount of biochar was 4 g/L] and salicylic acid [pH=2.5, amount of biochar was 4 g/L] adsorption at 308 K for adsorbate concentrations from 25 to 100 mg/L.
34
Table S1. Pseudo-first order kinetic parameters for ibuprofen, 2, and salicylic acid, 1, adsorptions Pseudo-first order parameters for 2 Temp. (K) 298
25
3.99
3.20
0.42
0.782
5.90
2.07
0.31
0.990
318
qe, calc.
k1
(mg/g)
(mg/g)
(gmg-1h-1)
R
qe, exp.
qe, calc.
k1
(mg/g)
(mg/g)
(gmg-1h-1)
R2
Initial conc. (mg/L)
308
qe, exp.
Pseudo-first order parameters for 1 2
50
7.31
4.18
0.23
0.837
7.35
3.50
0.34
0.975
100
12.25
6.03
0.28
0.911
8.08
4.90
0.26
0.952
25
3.27
1.10
0.19
0.941
6.15
1.72
0.27
0.988
50
7.09
4.97
0.34
0.985
7.73
3.36
0.21
0.983
100
10.09
3.95
0.31
0.987
8.37
4.68
0.22
0.940
25
3.48
1.71
0.26
0.977
6.19
1.99
0.34
0.966
50
7.80
5.55
0.22
0.942
7.93
3.00
0.28
0.971
100
10.10
8.11
0.37
0.980
9.39
5.68
0.25
0.966
35
.
HIGHLIGHTS Pinewood biochar was used to remove two pharmaceutical compounds Both carboxylic acid adsorbates have pH-dependent equilibria between their acid and carboxylate anion forms Adsorption was not limited to the small surface area Sorption followed pseudo-second order kinetics with regression of coefficients of 0.98 or greater
36
S1385-8947(14)01616-7 http://dx.doi.org/10.1016/j.cej.2014.12.006 CEJ 13003
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
3 September 2014 1 December 2014 2 December 2014
Please cite this article as: M. Essandoh, B. Kunwar, C.U. Pittman Jr., D. Mohan, T. Mlsna, Sorptive removal of salicylic acid and ibuprofen from aqueous solutions using pine wood fast pyrolysis biochar, Chemical Engineering Journal (2014), doi: http://dx.doi.org/10.1016/j.cej.2014.12.006
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Sorptive removal of salicylic acid and ibuprofen from aqueous solutions using pine wood fast pyrolysis biochar Matthew Essandoha, Bidhya Kunwara, Charles U. Pittman Jr.a, Dinesh Mohanb, Todd Mlsnaa,* a
Department of Chemistry, Mississippi State University, Mississippi State, MS 39763, USA
b
School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067, India
ABSTRACT Pine wood biochar, prepared at 698 K with a residence time of 20-30 s in an auger-fed reactor, was used as a 3-dimensional adsorbent to remove salicylic acid and ibuprofen from aqueous solutions. This biochar was characterized by FT-IR spectroscopy, scanning electron microscopy, transmission electron microscopy, surface area determination, and zero point charge measurements. Batch sorption studies were carried out at pH values from 2 to 10, adsorbate concentrations from 25 mg/L to 100 mg/L and temperatures from 298 to 318 K. The adsorption of both adsorbates was highest at low pH values, dropped as pH increased and then exhibited a second increase related to the pKa of these carboxylic acid adsorbates. This was followed by a further drop at high pH. Conjugate acid/base equilibria of the adsorbates and the phenolic hydroxyl and carboxylic acid biochar sites versus pH dominated the mechanism. Sorption followed pseudo-second order kinetics. Sorption was evaluated from 298 to 318 K using the Freundlich, Langmuir, Redlich–Peterson, Toth, Sips, and Radke-Prausnitz adsorption isotherm models. Langmuir adsorption capacities for both salicylic acid and ibuprofen were 22.70 and 10.74 mg/g, respectively. This low surface area pinewood biochar (1.35 m2/g) can adsorb far more adsorbate compared to commercial activated carbons per unit of measured surface area. Methanol stripping achieved 93 and 88 % desorption of salicylic acid and ibuprofen, respectively, from the spent biochar, and 76 and 72 % of the initial salicylic acid and ibuprofen adsorption capacity, respectively, remained after four full capacity equilibrium recycles.
KEYWORDS: ibuprofen, salicylic acid, biochar, adsorption, solution pH.
1
1. INTRODUCTION Wastewater pollution is an urgent environmental concern. The negative effects of pollution on both humans and the environment have become a subject of intense discussion. Pharmaceutical pollutants are increasingly important and require immediate attention [1]. Pharmaceutical products enter the aquatic environment through municipal waste water treatment, plant effluent, animal excreta, direct discharge of pharmaceuticals into water bodies, and septic tank leakage [2]. Negative effects on humans and animals include the suppression of embryonic cell growth in humans [3], the incapacitation of liver and gills in fish [4], and the inhibition of Gram-positive bacteria [5]. Despite public concerns about the negative effects that pharmaceuticals have on aquatic ecosystems, there are currently no federal laws defining the amount of these chemicals permitted in waste water streams or in drinking water. Acetylsalicylate is a widely sold “over the counter” drug. Deacetylation of acetylsalycylate produces salicylic acid 1 and its two metabolites (ortho-hydroxyhippuric acid and the hydroxylated metabolite gentisic acid) [6]. Salicylic acid has been found in industrial influent samples in concentrations up to 54 ug/L [7]. Ibuprofen 2 is a non-steroidal anti-inflammatory drug with a wide global consumption. It is produced on an industrial scale as a racemate which is generally excreted in the form of conjugates such as 2-[4-(2-hydroxy-2-methylpropyl)phenyl]propanoic acid, 2-[4-(2-carboxypropyl)phenyl]propanoic acid, and 2-phenylpropanoic acid. Ibuprofen is known to have endocrine disruptive activity [8]. Aqueous solutions of these pharmaceutical pollutants have been treated by physical, biochemical and chemical processes [9]. However, these techniques can be expensive. Thus, low cost methods to remove pharmaceutical contaminants from aqueous solution will be well embraced. Low cost adsorbents can be employed to remove recalcitrant compounds from aqueous solution inexpensively and can be effective for removing both organic and inorganic contaminants [10]. The objective of this work is to establish if pine wood biochar can be used to successfully remove ibuprofen and salicylic acid from water. This pinewood biochar is produced by the fast pyrolysis 2
of pine chips in an auger-fed fast pyrolysis reactor used to make bio-oil [11]. Hence, it is available as a byproduct of bio-oil [12], an emerging renewable liquid fuel. If these types of biofuels are eventually produced in large amounts, biochars will be widely available in many locations. Biochars have fuel value (as a coal substitute) or can be used for carbon sequestration or soil conditioners [13, 14]. A value-added application as a low cost widely applicable adsorbent [10, 15] is desirable and could increase the value of this biofuel byproduct. This research investigates the adsorption behavior of salicylic acid and ibuprofen at different concentrations, pH values, and temperatures.
2. EXPERIMENT 2.1. Chemicals and equipment All chemicals used were either GR or AR-grades. They were purchased from Sigma Aldrich (Saint Louis, MO) unless otherwise specified. Salicylic acid and ibuprofen sodium salt were purchased as white solids with pKa’s of 2.97 and 4.91, respectively. Salicylic acid’s solubility in distilled water is 2 g/L at 293 K, while that of ibuprofen’s sodium salt is 100 g/L. However, the solubility of ibuprofen in water is only 21 mg/L in distilled water.
2.2. Preparation of pine wood biochar The biochar used for adsorption experiments was produced as a byproduct of bio-oil produced by fast pyrolysis of pine wood chips in an auger-fed reactor at a temperature of 698 K using a 3
feed rate of 1-2.5 kg/h [11]. The residence time of the pine wood chips in the hotzone was 20-30 s and the approximate gas residence time in the reactor was 2 s. The chips were preheated to between 383 – 393 K, and a proprietary heat transfer method was used to speed the temperature rise to 698 K in the hotzone. The powdered biochar was collected from the system by capturing it in an enclosed pot after moving past the hotzone. After subsequent removal from this container at room temperature, it was washed several times with distilled water to remove salt impurities and water-soluble organic residuals. Biochar was sieved to a particle size distribution of 100 to 600 microns (0.1 to 0.6 mm), and moisture was removed by heating to 383 K for 12 h. The adsorbent was then stored in a closed vessel at 338 K for several days until needed.
2.3. Char characterization 2.3.1. FT-IR The infrared spectrum of powdered biochar was obtained using a Thermo Nicolet 6700 FTIR spectrometer. A total of 64 scans were taken from 4000 to 500 cm-1 with a resolution of 4 cm-1. 2.3.2. Scanning Electron Microscopy (SEM) A JEOL JSM-6500F FE-SEM operated at 5 kV was used to examine the biochar’s surface morphology. The sample was coated on a carbon stub attached to carbon tape and then sputtered-coated under argon with a 5 nm layer of platinum. The coated sample was then mounted into a sample holder for SEM analysis. 2.3.3. Transmission Electron Microscopy (TEM) The biochar was examined using a JEOL model 2100 TEM operated at 80 kV. A small amount of adsorbent was mixed with 100 % ethanol, sonicated for about 4 min, and allowed to stand for 24 h. A drop of this suspension was applied to a carbon film on 300 mesh copper grid and allowed to dry in air before TEM analysis.
4
2.3.4. Surface area measurements and elemental analysis The biochar’s BET surface area (1.35 m2/g) was determined using a Gemini 2375 V 1.00 surface area analyzer. Biochar elemental analysis, using a CE 440 elemental analyzer (Exeter Analytical, Inc.), found 73.14 % C, 3.27 % H, 0.32 % N with the remaining percentage being 23.27 %. Ash content was 3.98 % using ASTM D1506-99 method. Thus, this char’s oxygen content was ~ 19 %. 2.3.5. Zero Point Charge Determination Zero Point Charge was determined using 0.01 M NaCl aqueous solutions with pH values ranging from 2 to 10 at pH intervals of 2. About 10 mL was added to 0.1 g of the biochar and the mixtures were swirled for 48 h. pH was adjusted using either 0.1 N HCl or 0.1 N NaOH solutions. The supernatant was decanted and its pH measured using an ORION model 210 pH meter. The point of zero charge was obtained by plotting supernatant pH verses the initial solution pH.
2.4. Adsorption Studies The adsorbate concentrations were varied from 25-100 mg/L for kinetic studies. Kinetic studies for ibuprofen and salicylic acid adsorption were carried out at pH values of 3 and 2.5, respectively, at 298, 308, and 318 K. A known amount of biochar was added to 25 mL solutions containing different adsorbate concentrations in 40 ml amber glass vials. Samples were then swirled at 200 rpm for 16 h. After equilibration, the samples were filtered through Whatman No. 1 filter paper. The amount of adsorbate remaining in the filtrate was determined at their λmax wavelengths of 220 nm for ibuprofen and 298 nm for salicylic acid. Samples were analyzed in duplicate and their average absorbances used. The amount of adsorbate removed per gram of adsorbant was obtained by: =
(1)
5
where is the amount of adsorbate (mg) removed per g of adsorbent, and are the initial and equilibrium adsorbate concentrations (mg/L) in solution, V is the solution volume (L), and M is the biochar weight (g). 2.5. Regeneration procedure The pinewood biochar (4 g/L) was equilibrated with an aqueous solution of the adsorbate (100 mg/L) at pH 2 for ibuprofen and 3 for salicylic acid in a temperature controlled shaker at 318 K. The experiments were conducted separately for salicylic acid and ibuprofen. The spent biochar was then washed twice with 20 ml of methanol (regenerant) and the filtrate analyzed by UV-vis spectrophotometry to quantify the adsorbate in the filtrate. The adsorption/regeneration procedure was repeated using the regenerated biochar in each recycle to determine the recyclability of the biochar. The char was recycled four times.
3. RESULTS AND DISCUSSION 3.1. Char Characterization The FTIR spectra of the adsorbent before and after adsorption experiments are shown in Figure S1 (supplementary materials). All spectra exhibit OH, C=C, C-O, C-H bond stretching at 3400, 1600, 1200 and 800 cm-1, respectively. The C=O stretch in the 1750 cm-1 region was slightly stronger in char samples with adsorbed ibuprofen (Figure S1b) and salicylic acid (Figure S1c) compared to the virgin biochar (Figure S1a). Salicylic acid and ibuprofen both have sp2 C-O single bond stretching peaks at ~ 1200 cm-1. The biochar/adsorbate spectra look very similar to that of the biochar alone, primarily because of the low surface adsorbate concentration present. The point of zero charge (pHpzc) is the pH at which the net charge on the surface is zero. This was found to be ~ 2 for the pinewood biochar. When pHpzc< pH of the solution, the biochar’s surface
6
will be negatively charged. When the pHpzc> pH of the solution, the surface of the biochar will be positively charged. Biochar TEM images are shown in Figure S2 (supplementary materials). The lighter portions of these images indicate that electrons are passing through the carbon particles while the darker portion is due to carbon particle electron scattering. SEM images of the biochar are in shown in Figure 1. The structures clearly indicate that the biochar has a macroporous surface texture. The wood cell morphology present in the pine wood chips is still clearly observed after pyrolysis. Biochar compositions are known to vary significantly. For example ash contents (11.09 %) and elemental compositions have been reported for oak wood biochar [15], surface areas of 20.9 and 7.0 m2/g for fast and slow pyrolysis corn stover biochar have appeared [16], and different physico-chemical properties for rice husks, olive pomace, orange waste and compost biochars are known [17]. Each biochar’s properties will depend on many factors, including differences in feedstock composition, pyrolysis residence time and temperature, particle size, and pyrolysis heating rate [18].
3.2. Sorption Studies 3.2.1 Effect of solution pH on adsorbate adsorption Solution pH plays a crucial role in ibuprofen and salicylic acid adsorption from aqueous solution. The percent removal of both ibuprofen and salicylic acid is higher in acidic solutions than in the basic region (Figure 2). Guedidi et al. [19] observed a similar decrease in ibuprofen adsorption on activated carbon cloths as the solution pH increases. During adsorption at pH = 2.5 for salicylic acid and 3 for ibuprofen, the biochar’s point of zero charge is lower than the solution pH, so the biochar’s surface becomes negatively charged. The surface net negative charge density at these pH values will be very low. Also, the pKa of both the ibuprofen (4.91) and the salicylic acid (2.98) are greater than these low solution
7
pH values (2.5 and 3) so these pollutants exist primarily as their conjugate acid (R-COOH) forms. Electrostatic repulsion is minor at these solution pHs, promoting adsorption by attractive H-bonds and other interactions.
3.2.2. Mechanism for the adsorption of salicylic acid and ibuprofen unto pinewood biochar At higher pH, (pH greater than the pKa of the adsorbates), both adsorbates are increasingly present as their carboxylate anions (R-COO-). Simultaneously, the biochar becomes increasingly negatively charged as pH rises (pH>pzc), causing increased electrostatic repulsion of adsorbate carboxylate anions and decreasing the adsorption capacity. This trend is seen from pH 2 or 3 through 10. However, both adsorbates show a region of intermediate pH values where adsorption increases, maximizes and then continues to drop as pH increases. These adsorptions peak at about pH 8 for ibuprofen and 6 for salicylic acid, where both adsorbates are 99.9 % in their carboxylate anion forms (each approximately 3 pH units above their pKa). For example, at pH values above 8, almost all salicylic acid (pKa = 2.98) exists as RCOO- and the surface of the biochar will have a substantial negative charge density. Thus, little salicylic acid can adsorb. However, at pH 6, the biochar’s surface phenolic hydroxyls are not ionized and they can Hbond to both salicylic acid and its carboxylate anion. This is the region where the peak in adsorption of salicylic acid occurs. Ibuprofen is two pKa units weaker an acid than salicylic acid and its adsorption verses pH plot is displaced by two pH units to higher pH. Ibuprofen’s adsorption peaks at ~ pH 8. This is most likely due to the unionized surface phenolic groups acting as H-bond donors to the carboxylate anion of ibuprofen. These biochars contain carboxylic acid, hydroxyl (both phenolic and aliphatic), lactone, ether and ketone functions, both on surfaces and within the solid content. At 19 % oxygen content, water
8
can diffuse into the solid regions of the biochar and swell this solid. Adsorption is not limited to the small surface areas. This is obvious from both Table 1 (salicylic acid) and Table 2 (ibuprofen). For example, the capacity of salicylic acid on pine biochar (surface area = 1.35 m2/g) is 22.70 mg/g at 318 K or 16.82 mg/m2. This can be compared to Filtrasorb F400 (0.33 mg/m2), Sephabeads SP206 (0.08 mg/m2), GAC 830 (0.02 mg/m2), and HJ-G02 (0.26 mg/m2). Similar comparisons for ibuprofen can be made. It is clear that only the surfaces of the biochar would not take up such large amounts of the adsorbates. Pinewood biochar adsorbs as a 3-D adsorbent and not only at its outer and exposed pore surfaces. The functional groups responsible for adsorption were never generated by separate reactions with oxygen or oxidizing agents as in the case of activated carbons. This oxygen is derived from the oxygen present in the precursor wood’s cellulose, hemicellulose, and lignin content. Thus, oxygen functions exist everywhere in the biochar and not mostly on solid pore surfaces as with activated carbons. Earlier we have shown how fast pyrolysis pinewood biochar adsorbed by imbibing water and adsorbate within its solid structure [15]. This interesting behavior was augmented by demonstrating that porosity exists in the char that was actually not available to gas adsorption (N2-BET) when the char was dry. Closed pores can open during swelling, providing additional sites for adsorption. Hence, both micro and macro pores exist in the char that might have been isolated or closed when dry. However, during immersion the adsorbate is able to permeate into these pores. Also, solid nonporous charred biomass regions containing 10-20 wt. % oxygen will swell, imbibing both water and adsorbates. Thus, some adsorbates can be adsorbed within the 3-D solid structure of the char. This swelling behavior causes the char to adsorb far more adsorbate per measured unit surface area when compared to activated carbons as shown in Table 1. This is greater than 50 times more adsorption per measured surface area, clearly implicating a 3-D adsorption process in the biochar.
9
The char’s point of zero charge is ~ 2 and carboxylic acids and phenols contribute to this acidic character [20]. Thus, at pH values above 2, as the pH increases, the surface acid groups ionize, negatively charging the surface. The adsorbates can exist in their acid or conjugate base forms as mentioned before. The pH dependence of these adsorbates’ equilibria and that of protonation/deprotonation equilibria of the biochar’s carboxylic acid and phenolic groups in combination give rise to the adsorption vs pH curves seen in Figure 2. Surface attractive interactions include van der Waals dispersion forces, permanent dipole-dipole attractions, π-π interactions as well as a variety of types of hydrogen bonding. The genesis of the surface basicity of carbons is still a subject of intense discussion and is not well understood [20]. The mechanism of adsorption must vary with pH. The adsorbates attraction to the surface changes with pH, and we believe, these changes in attractions are dominated by H-bonding changes. At very high pH, anion repulsions of the adsorbates’ conjugate bases (carboxylate anions) by negatively charged surface carboxylate and phenolate anions dominate. Some types of attractive H-bonding interactions are shown in Figure 3 along with charge repulsions that become dominant at high pH. The surface negative charge density is a complex function of pH because surface carboxylic acids, phenols, and aliphatic hydroxyls ionize at different pH ranges and their concentrations also differ. This complex situation is compounded because the analytes are subject to their own pH-dependent carboxylic acid/carboxylate equilibria. The interaction of all these features combine to contribute to the observed adsorption verses pH behavior, illustrated in Figure 2.
3.2.3. Kinetic Studies The removal of both ibuprofen and salicylic acid from solution was followed verses time using a biochar concentration of 4 g/L. Equilibrium was reached after approximately 10 h. All subsequent
10
kinetic studies were done using a 16 h contact time to be certain that equilibrium has been achieved. The effect of initial adsorbate concentrations from 25 – 100 mg/L was studied using 25 ml of the adsorbate solution, an agitation speed of 200 rpm, a 16 h equilibration time and a solution pH of 2.5 for salicylic acid and 3 for ibuprofen. A significant increase in the adsorption capacity occurred for both adsorbates as the initial adsorbate concentration was raised. This is clearly illustrated in Figure 4 and this observation is consistent with other studies using activated bleaching earth [21]. The effect of temperature on both ibuprofen and salicylic acid adsorptions, which were run until equilibrium was reached, was studied at 298, 308 and 318 K. The amount adsorbed verses time was similar between 298 to 318 K for ibuprofen (Figure S3a). This parallels a previous study [22] where the ibuprofen capacity on activated carbon was unaffected between 298 to 313 K. Conversely, the amount of salicyclic acid adsorbed (Figure S3b) increased slightly as the temperature rose from 298 to 318 K, suggesting that the process is endothermic.
3.3. Adsorption kinetic models 3.3.1. Pseudo-first order kinetic model Lagergen’s pseudo-first order equation [23] for describing liquid-solid adsorption is given by
= −
(2)
− = − .
(3)
A plot of log ( - ) against t should be linear where is the intercept and is the slope, respectively (Equation 3 terms are defined in Table 4). Fitting ibuprofen and salicylic acid adsorption data to equation (3) gave regression coefficients ranging from 0.782 to 0.987 for ibuprofen and 0.940 to 0.990 for salicylic
11
acid. The plots of these fits can be found in the supplementary material (Figure S4). The pseudo-first order parameters and its regression coefficients are found in Table S1.
3.3.2. Pseudo-second order kinetic model The pseudo-second order models are given by equations 4 and 5. Equation (5) [24] was also fitted to the experimental data.
= −
=
+
(4) (5)
A plot of t/ against t should be linear. The pseudo-second order model fits the experimental data well, giving regression coefficients of 0.980 or greater for ibuprofen and 0.990 or higher for salicylic acid. Fittings are plotted in Figure 5. The experimental and calculated and values using equation 5 for adsorbate concentrations from 25 to 100 mg/L are shown at 298, 308 and 318 K in Table 3 for both ibuprofen and salicylic acid. The calculated and values were obtained from the slope and intercept, respectively. The experimental and calculated values agree very well which indicates that ibuprofen and salicyclic acid adsorptions follow a pseudo-second order model. Pseudo-first and second order models are summarized in Table 4.
3.4. Adsorption isotherm models Isotherms are used to describe adsorption processes, typically the amount adsorbed on the surface at a constant temperature. The pinewood biochar adsorption was evaluated using the two parameter Langmuir, Freundlich, and the three parameter Redlich-Peterson, Sips, Radke-Prausnitz, and Toth isotherms. Isotherm adsorption data was collected at three different temperatures (298, 308, 318 K) 12
for salicylic acid, and one temperature (308 K) for ibuprofen (ibuprofen data was shown to be athermic). A summary of the isotherm models used in this study and references to these models are shown in Table 4. The isotherm fits to the experimental data and fitted values of the model parameters are shown in Figure 6 and Table 5, respectively. The absorbance of the ibuprofen solutions remain essentially unchanged above 100 mg/L – most likely because precipitation occurs above this concentration. This precipitation, caused by the low solubility of ibuprofen, resulted in a poor fit isotherm for ibuprofen (Figure 6d). Ibuprofen precipitation was observed in some solutions at concentrations from 100 to 200 mg/L. The exact solubility is not known and some induced ibuprofen precipitation may occur on the char if the solution is supersaturated or if the pH at the surface is modified. The experimental data gave excellent fits (R2> 0.96) to the Sips, Redlich-Peterson, and Toth isotherms, all three of which are three parameter models. Previous studies of phenol and Ni2+ on aerobic activated sludge [25] have also found the three parameter empirical equation (e.g. RedlichPeterson isotherm) to be more suitable for describing adsorption systems compared to two parameter empirical equations (e.g. Langmuir and Freundlich isotherm).
In our current studies, the Sips and
Redlich-Peterson fits were best. These are shown in Figure 6 along with the Langmuir fitting for salicylic acid. Ibuprofen’s fits to these models are all poor (Figure 6d). Fitting isotherms and obtaining their corresponding parameters can be useful when designing columns for scaling of adsorption processes [26]. The adsorption curves are typically used to determine the efficiency of industrial adsorbents employed in purification processes and are used to identify the equilibrium between adsorbate and adsorbent [27]. However, trying to identify mechanistic details from isotherm fittings is often unwarranted. Linear and nonlinear models give different correlation coefficients and often different results violate error variance and normality assumptions of standard least squares [28]. This leads to bias of the adsorption data. When various models give good R2 values,
13
selecting mechanistic features from one model verses another is likely naive. Furthermore, experimental error will effect different models differently. Thus, we present fittings in Table 5, but do not try to infer physical and mechanistic details from the fittings at this point.
3.5. Regeneration of the spent biochar Methanol was selected as a stripping solvent for both salicylic acid and ibuprofen because both of these adsorbates are highly soluble in methanol and this solvent is easily removed from biochar using water. Methanol was previously used to successfully strip acidic 2,4,6-trinitrophenol from almond shell char [29]. Figure 7, plots the amounts of salicylic acid and ibuprofen adsorbed through four sequential cycles. Methanol was effective at desorbing the adsorbates from the biochar achieving 93 and 88 % desorption of salicylic acid and ibuprofen, respectively, from biochar which had been equilibrated with each of these adsorbates to full capacity. The adsorption capacity was restored to 76 and 72 % of its initial capacity in the fourth recycle for salicylic acid and ibuprofen, respectively. This level of recovery and reusability of the raw pinewood biochar adsorbent after four cycles is an added advantage to its low cost.
4. CONCLUSIONS Fast pyrolysis pinewood biochar was characterized and this biochar successfully adsorbed ibuprofen and salicylic acid from aqueous solutions. Adsorption of both ibuprofen and salicylic acid followed pseudo-second order kinetics. Adsorption was favored at low pH values because the char’s pHpzc is ~ 2 relative to the pKa’s of ibuprofen (4.91) and salicylic acid (2.98). Adsorption capacities were 22.70 and 10.74 mg/g at 318 and 308 K, for salicylic acid and ibuprofen, respectively. Despite low surface area, the biochar (~19 % O) appears to adsorb by imbibing water and adsorbate within its solid structure.
14
Methanol was able to achieve 93 and 88 % desorption of salicylic acid and ibuprofen, respectively, from the spent biochar. Additionally, 76 and 72 % of the biochars initial adsorption capacity for salicylic acid and ibuprofen, respectively, remained after four full capacity equilibrium recycles.
AUTHOR INFORMATION Corresponding Author * (Tel: 662- 325-6744; fax: 662-325-1618; email:[email protected] Notes The authors declare no competing financial interest.
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Figure Captions
Figure 1. SEM images of the pyrolyzed pine wood biochar. Figure 2. Effect of solution pH on ibuprofen and salicylic acid adsorption. Experiments employed 0.05 g of the biochar and 50 mg/L of the adsorbate. Figure 3.Various H-bonding and charge-charge repulsive interactions. Both adsorbate and surface functions can serve as H-donors and H-acceptors. Figure 4. Adsorption at 318 K at different ibuprofen [pH=3, amount of biochar was 4 g/L] and salicylic acid [pH=2.5, amount of biochar was 4 g/L] concentrations. Figure 5. Pseudo-second order plots at 308 K for salicylic acid [pH=2.5, biochar amount was 4 g/L] and ibuprofen [pH=3, biochar amount was 4 g/L] at adsorbate concentrations of 25, 50, and 100 mg/L. Figure 6. Adsorption isotherms for salicylic acid (a-f) and ibuprofen (g) at different temperatures. Experimental conditions [adsorbent concentration was 4 g/L, pH 2.5 and 3 for salicylic acid and ibuprofen, respectively]. Figure 7. Biochar recycling using an adsorbent dose of 4 g/L and adsorbate concentration of 100 mg/L, at a pH of 2.5 for salicylic acid and pH 3 for ibuprofen at 318 K. Methanol (20 ml x 2) was used as a stripping agent.
17
Table 1. Comparison of pinewood fast pyrolysis biochar’s salicylic acid adsorption capacity to those reported for high surface area adsorbents Adsorbents
Filtrasorb F400 Sephabeads SP207 Sephabeads SP206 Filtrasorb 400 GAC 830 HJ-G02
Hypercrosslinked polymeric adsorbent Hyprecrosslinked polymeric adsorbent with amine modified beads Pinewood biochar
Temp.
(K) 293 293 293 293 293 288 298 308 318 283 303 323 303 323
pH
a
3.5 3.5 a 3.5 a 11-12 11-12 2.8-3.5 a 2.8-3.5 a 2.8-3.5 a 2.8-3.5 a 2.8-3.5 a 2.8-3.5 a 2.8-3.5 a 2.8-3.5 a 2.8-3.5 a
Surface area
Adsorption capacity
(m2/g) 1050 627 556 1050 950-1050 770
(mg/g) 351 81.6 45.2 31.4 17.5 210.1 209.6 200.4 192.7 320 264 256 303 270
934
Adsorption per unit surface area (mg/m2) 0.33 0.13 0.08 0.03 0.02 0.27 0.27 0.26 0.25 0.34 0.28 0.27 0.32 0.29
Reference number
[30] [30] [30] [31] [31] [32] [32] [32] [32] [33] [33] [33] [33] [33]
298 2.5 1.35 7.56 5.60 This study 308 2.5 1.35 16.84 12.47 318 2.5 1.35 22.70 16.81 a These calculated pH values were calculated using salicylic acid Ka values and concentration. b The pinewood biochar adsorbs from 21.5 to 64.6 times as much salicylic acid per unit of measured surface area as HJ-602, provide one illustration of this biochar as a 3-dimensional adsorbent.
18
Table 2. Comparison of pinewood fast pyrolysis biochar’s ibuprofen adsorption capacity to those reported for high surface area adsorbents Adsorbents
Temp. (K)
pH
Chemically activated cork powder
303
4
Surface area (m2/g) 891
Adsorption capacity (mg/g) 145.2
Adsorption per surface area(mg/m2) 0.16
Reference number
Physically activated cork powder
303
4
1060
378.1
0.36
[34]
Physically activated PET
303
4
1426
266.6
0.19
[34]
[34]
Physically activated wood
303
4
899
291.9
0.33
[34]
Activated carbon from Artemesia vulgaris
298
4.19
358
16.94
0.05
[35]
Activated carbon from olive-waste cakes
298
2
793
12.6
0.02
[36]
Chemically activated cork waste
298
2-4
891
139.2
0.02
[22]
308
2
145.2
0.16
[22]
153.2
0.17
[22]
393.4
0.37
[22] [22]
Chemically and steam activated cork waste
Pinewood biochar
313
2
298
2
308
2
378.1
0.36
313
2
416.7
0.39
308
3
1060
1.35
a
10.74
7.96
[22] a
This study
The pinewood biochar adsorbs 159 times as much ibuprofen per unit of measured surface area as activated carbon from Artemesia vulgaris, indicating that this biochar is a 3-dimensional adsorbent rather than a predominantly surface adsorbent.
19
Table 3. Pseudo-second order kinetic parameters for ibuprofen, 2, and salicylic acid, 1, adsorptions. Pseudo-second order parameters for 2 Temp.
qe, exp.
qe, calc.
k2
(K)
Initial conc. (mg/L)
(mg/g)
(mg/g)
(gmg-1h-1)
298
25
3.99
4.26
0.24
308
318
Pseudo-second order parameters for 1 2
R
0.997
qe, exp.
qe, calc.
k2
(mg/g)
(mg/g)
(gmg-1h-1)
5.90
6.06
0.36
R2
0.999
50
7.31
9.71
0.07
0.986
7.35
7.69
0.21
0.999
100
12.25
12.82
0.09
0.992
8.08
8.62
0.10
0.992
25
3.27
3.34
0.47
0.996
6.15
6.29
0.41
0.999
50
7.09
9.71
0.11
0.994
7.73
8.00
0.15
0.996
100
10.09
10.42
0.18
0.999
8.37
8.85
0.09
0.990
25
3.48
3.64
0.33
0.997
6.19
6.33
0.43
0.999
50 100
7.80 10.10
8.00 11.11
0.06 0.06
0.980 0.995
7.93 9.39
8.20 10.00
0.23 0.08
0.999 0.996
20
Table 4. Summary of isotherm and kinetic models used in this study. Models name
Equation
Langmuir
Freundlich
qe =
q$ =
Redlich-Peterson
q$ =
Toth
Pseudo-first order Pseudo-second order
Q 0bCe 1 + bCe
q e = K F Ce
Sips or LangmuirFreundlich
Radke-Prausnitz
Parameters
K &' C$ )*+ 1 + a &' C$ )*+ K ./ C$ 1 + a ./ C$ 012
q$ =
q$ =
1/ n
a b C$ 0 a + bC$ 04 K 5 C$
1 + B5 C$ 07 8 5 9
− = − .:;:
< =>
=
? = @
<
+=
@
Reference number
Isotherm Models Qo-monolayer adsorption capacity (mg/g), b-constant related to net enthalpy of adsorption (b ∝ e-∆H/RT), qe-solute amount adsorbed per unit weight (mg/g), Ce-solute equilibrium concentration (mg/L). The Langmuir isotherm assumes a homogenous surface, monolayer coverage and no interaction of the adsorbate with neighboring sites. KF-constant indicative of the relative adsorption capacity of adsorbent (mg/g), 1/n- a constant indicative of the intensity of the adsorption, qeadsorption capacity (mg/g), Ce-equilibrium concentration of solute (mg/L). This isotherm is used in the low to intermediate adsorbate concentration range. KLF, aLF and nLF are the Sips constants. qe-solute amount adsorbed per unit weight (mg/g), Ce is equilibrium concentration (mg/L). This Sips isotherm will resemble Langmuir and Freundlich isotherms at high and low adsorbate concentrations, respectively. KRP, aRP and βRP are Redlich-Peterson constants and the exponent, β, lies between 0 and 1. qe- solute amount adsorbed per unit weight (mg/g) and Ce is equilibrium concentration (mg/L). a, b, and β are Radke-Prausnitz constants. qe- solute amount adsorbed per unit weight (mg/g) and Ce is equilibrium concentration (mg/L). This is a three-parameter empirical equation and was used initially to fit organic solvents over a broad concentration range. KT, BT, and βT are Toth constants. qe-adsorption capacity (mg/g), Ceequilibrium concentration (mg/L). This model was proposed from the potential theory. This model is well known to presume a quasi-Gaussian energy distribution. Kinetic models k1(h-1) is the first order adsorption rate constant, qe is the amount adsorbed at equilibrium and qt is the amount adsorbed at time “t”. k2(g.mg-1 h-1) is the second order rate constant, qe is the amount adsorbed at equilibrium, qt is the amount adsorbed at time “t” and k2qe2 represents the initial sorption rate.
21
[37]
[38]
[39]
[40]
[41]
[42]
[23] [24]
Table 5. Freundlich, Langmuir, Redlich-Peterson, Sips, Radke-Prausnitz and Toth isotherm parameters for salicylic acid, 1, and ibuprofen, 2, removal on pinewood biochar. Isotherm parametersa
Salicylic Acid
Ibuprofen
298 K 308 K Freundlich KF (mg/g) 3.60 2.48 1/n 0.30 0.31 R2 0.9483 0.9037 Langmuir Q0 (mg/g) 7.56 16.84 b 0.01 0.02 R2 0.9085 0.9559 Redlich-Peterson KRP (l/g) 3489.2 0.24 4523.06 0.004 aRP(l/mg)βRP 0.64 1.18 βRP 2 R 0.9581 0.9614 Sips KLF (l/g) 76.76 16.95 a aLF(l/mg) LF 0.000004 0.0202 nLF 0.37 1.07 R2 0.9576 0.9563 Radke-Prausnitz a 420934.61 8843554.05 b 0.77 2.48 β 0.35 0.31 R2 0.9581 0.9037 Toth KT 116.40 15.60 BT 0.80 0.15 βT 7.60 0.69 R2 0.9524 0.9582 a Terms in this column are defined under section 3.4
22
318 K
308 K
0.77 0.35 0.9581
0.54 0.59 0.8160
22.70 0.24 0.9793
10.74 0.22 0.79
0.59 0.03 0.9743 0.9774
5.56 9.60 0.42 0.8157
24.15 0.02 0.86 0.9807
90.37 0.0002 0.62 0.8144
2539153.09 3.60 0.30 0.9483
-
24.24 0.30 1.22 0.9800
87.13 0.01 3.45 0.8094
Figure 1. SEM images of the pyrolyzed pine wood biochar.
23
Figure 2. Effect of solution pH on ibuprofen and salicylic acid adsorption. Experiments employed 0.05 g of the biochar and 50 mg/L of the adsorbate.
24
O C O
H
surface carboxylic acid donor and acceptor
O O
H
H
O
O H
O
H
O
O H
O C O
H
H O
phenolic surface H-donor
O
phenolic surface H-acceptor
O
surface carboxylate function H-acceptor
O
O
surface phenol as H-donor
O
O C O
O
O
O
O
O
negative surface repulsions of adsorbate carboxylate (dominate at high pH)
negative surface repulsions of adsorbate carboxylate (dominate at high pH)
Figure 3.Various H-bonding and charge-charge repulsive interactions. Both adsorbate and surface functions can serve as H-donors and H-acceptors.
25
Figure 4. Adsorption at 318 K at different ibuprofen [pH=3, amount of biochar was 4 g/L] and salicylic acid [pH=2.5, amount of biochar was 4 g/L] concentrations.
26
Figure 5. Pseudo-second order plots at 308 K for salicylic acid [pH=2.5, biochar amount was 4 g/L] and ibuprofen [pH=3, biochar amount was 4 g/L] at adsorbate concentrations of 25, 50, and 100 mg/L.
27
Figure 6. Adsorption isotherms (a-c) for salicylic acid, 1, at 289, 308, and 318 K. Figure 6 d, shows the overlapping and poorly fit adsorption isotherms for ibuprofen, 2, at 308 K. Experimental conditions [adsorbent concentration was 4 g/L, pH 2.5 and 3 for salicylic acid and ibuprofen, respectively].
28
Figure 7. Biochar recycling using an adsorbent dose of 4 g/L and an adsorbate concentration of 100 mg/L, at a pH of 2.5 for salicylic acid and pH 3 for ibuprofen at 318 K. Methanol (20 ml x 2) was used as a stripping agent.
29
SUPPLEMENTARY MATERALS
Sorptive removal of salicylic acid and ibuprofen from aqueous solution using pine wood fast pyrolysis biochar
Matthew Essandoha, Bidhya Kunwara, Charles U. Pittman Jr.a, Dinesh Mohanb, Todd Mlsnaa,*
a
Department of Chemistry, Mississippi State University, Mississippi State,
MS 39763, USA
b
School of Environmental Sciences, Jawaharlal Nehru University, New Delhi
110067, India
30
Figure S1. FTIR spectra of virgin pine wood biochar [a], pine wood biochar-ibuprofen sample at pH 3 [b], pine wood biochar-salicylic acid sample at pH 2.5 [c]. Solutions (25 mL) containing 100 mg/L of adsorbate, 800 mg/L of biochar for salicylic acid, 400 mg/L of biochar for ibuprofen were used for the FTIR analyses after adsorption.
31
Figure S2. TEM image of the pyrolyzed pine wood biochar
32
Figure S3. Adsorption verses temperature at adsorbate concentrations of 50 mg/L. Ibuprofen [pH=3, amount of biochar was 4 g/L] and salicylic acid [pH=2.5, amount of biochar was 4 g/L].
33
Figure S4. Pseudo-first order plots for ibuprofen [pH=3, amount of biochar was 4 g/L] and salicylic acid [pH=2.5, amount of biochar was 4 g/L] adsorption at 308 K for adsorbate concentrations from 25 to 100 mg/L.
34
Table S1. Pseudo-first order kinetic parameters for ibuprofen, 2, and salicylic acid, 1, adsorptions Pseudo-first order parameters for 2 Temp. (K) 298
25
3.99
3.20
0.42
0.782
5.90
2.07
0.31
0.990
318
qe, calc.
k1
(mg/g)
(mg/g)
(gmg-1h-1)
R
qe, exp.
qe, calc.
k1
(mg/g)
(mg/g)
(gmg-1h-1)
R2
Initial conc. (mg/L)
308
qe, exp.
Pseudo-first order parameters for 1 2
50
7.31
4.18
0.23
0.837
7.35
3.50
0.34
0.975
100
12.25
6.03
0.28
0.911
8.08
4.90
0.26
0.952
25
3.27
1.10
0.19
0.941
6.15
1.72
0.27
0.988
50
7.09
4.97
0.34
0.985
7.73
3.36
0.21
0.983
100
10.09
3.95
0.31
0.987
8.37
4.68
0.22
0.940
25
3.48
1.71
0.26
0.977
6.19
1.99
0.34
0.966
50
7.80
5.55
0.22
0.942
7.93
3.00
0.28
0.971
100
10.10
8.11
0.37
0.980
9.39
5.68
0.25
0.966
35
.
HIGHLIGHTS Pinewood biochar was used to remove two pharmaceutical compounds Both carboxylic acid adsorbates have pH-dependent equilibria between their acid and carboxylate anion forms Adsorption was not limited to the small surface area Sorption followed pseudo-second order kinetics with regression of coefficients of 0.98 or greater
36