Environmental Science and Pollution Research https://doi.org/10.1007/s11356-018-2186-9
RESEARCH ARTICLE
Ultrasound-assisted adsorption of phenol from aqueous solution by using spent black tea leaves Asmat Ali 1 & Muhammad Bilal 1 & Romana Khan 1 & Robina Farooq 2 & Maria Siddique 1 Received: 8 February 2018 / Accepted: 30 April 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract This study is conducted to examine the removal of phenol using spent black tea leaves (SBTL) by the process of ultrasoundassisted adsorption. The effect of different treatment processes, i.e., sonolysis, adsorption, and ultrasound-assisted adsorption, was investigated. The morphology of SBTL was studied using a scanning electron microscope (SEM), and the porous structure of the SBTL was identified before phenol was adsorbed onto the adsorbent. FTIR analysis of SBTL after adsorption showed the presence of an aliphatic band of carboxylic acids which depict degradation of the phenol molecule due to ultrasound-assisted adsorption. The experimental results showed that the hybrid process was found more effective for phenol removal (85%) as determined by a spectrophotometer. The optimum conditions of the reaction parameters were found as: phenol conc. = 25 mg L−1, pH = 3.5, time = 60 min, adsorbent dosage = 800 mg L−1, ultrasound power = 80 W, and operating temperature = 30 ± 2 °C. Chemical oxygen demand (COD) and total organic carbon (TOC) were found to be 78 and 39%, respectively. HPLC studies suggest nonselective oxidation of phenol resulting in by-products such as catechol and hydroquinone and finally carboxylic acids and CO2. In order to find reaction kinetics, different kinetic models, viz. pseudo-first- and pseudo-second-order models, were studied. The best fit to the isotherm models, i.e., Langmuir and Freundlich, was determined. It is concluded that phenol removal by the hybrid process follows the pseudo-second-order reaction kinetics and Langmuir isotherm model. In addition, thermodynamic studies revealed the nonspontaneous and exothermic nature of the phenol adsorption process. Keywords Phenol . Spent black tea leaves . Ultrasound-assisted adsorption . Ultrasonic cavitations . Waste water treatment
Introduction Phenol and its derivatives are one of the largest groups of environmental pollutants due to their wide use in paints, pesticides, cosmetics, petroleum refining, steel, dyestuff, synthetic resins, by-products of agricultural chemicals, paper and pulp mills, tanning, fiberboard production, and pharmaceutical industries, and the subsequent untreated wastewater of these industries leads to freshwater resource contamination (Entezari et al. 2003; Santos et al. 2006; Anku et al. 2017).
Responsible editor: Guilherme L. Dotto * Maria Siddique
[email protected] 1
Department of Environmental Sciences, COMSATS Institute of Information Technology, Abbottabad, Pakistan
2
Department of Chemical Engineering, COMSATS Institute of Information Technology, Lahore, Pakistan
The risk associated to this contaminant is related to its persistence in natural environment even at a low concentration of 0.005 mg L−1. It may cause toxic effects to human and aquatic organisms such as mouth irritation, diarrhea, excretion of dark urine, conjunctional swelling, vision problems, eye irritation, corneal whitening, and finally, blindness. Furthermore, anorexia, dermal rash, dysphasia, gastrointestinal disturbance, hepatic tenderness, nervous disorder, paralysis, and cancer have also been reported (Aksu and Bulbul 1999; Aksu 2005; Yousef et al. 2011). Phenol removal has always attained the interest of researchers using different conventional and advanced techniques (Kanekar et al. 1998; Gogate and Pandit 2004; Pardeshi and Patil 2008; Mozia 2010; Demarche et al. 2012; Zheng et al. 2013; Villegas et al. 2016). The application of the adsorption method has gained prominent importance from the point of economic considerations especially in a developing country scenario. For being cost-effective, commercial activated carbon is the most widely used adsorbent; however, despite its efficiency and versatility, it is expensive and its regeneration is
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challenging (Hamdaoui and Naffrechoux 2009). This fact has prompted a growing interest in the production of low-cost alternatives to activated carbon to keep the cost under control (Viraraghavan and de Maria Alfaro 1998; Din et al. 2009; Lin and Juang 2009; Kumar and Min 2011; Nie and Xie 2011; Amin et al. 2012). Recently, the interest on biomaterials and especially spent tea leaves (available in different varieties, i.e., black or green) has been growing, and some attractive results have been obtained in the adsorption of some pollutants (Kazmi et al. 2013). Black tea is basically the dried and processed leaves of a plant called Camellia sinensis L. It is largely consumed around the globe and is considered the second most popular beverage in the world after water. It is estimated that somewhere between 18 and 20 billion cups of tea are consumed daily. Therefore, spent tea- an abundantly available solid waste can be used as a nonconventional adsorbent. The better adsorption can be engineered against the target contaminant in a number of ways, for instance pretreatment or activation of adsorbent or developing a hybrid technique such as adsorption facilitated by sonochemistry. Ultrasonics is found efficient for removing a variety of pollutants in wastewater with less cost and environmentally safe byproducts (Fu et al. 2007). The process involves the formation, growth, and subsequent collapse of bubbles which results in the formation of large amplitudes of energy at a particular location (Gogate 2002). The cavitation mechanism generates convection in the media via microstreaming, microturbulence, acoustic waves, and microjets. Further, it increases mass transfer rate by reducing diffusion resistance (Zheng et al. 2005). These effects have been found to enhance adsorption processes. Few recent reports indicated a synergic effect of ultrasound and adsorption process using a range of pollutants and adsorbents (Djelloul and Hasseine 2013; Asghari et al. 2014; Asfaram et al. 2015). The use of ultrasound assists in the adsorption process and was found effective to control the concentration of pollutants. Furthermore, the process was found to be an economically viable alternative to conventional techniques in terms of reduced chemical utilization and associated environmental impacts. The present study is focused to investigate the enhancement effects on phenol removal by hybridizing adsorption with the ultrasound irradiation process. The influences of certain factors, such as pH, time, adsorbent, adsorbate dosage, temperature, and ultrasonic power for removal of phenol, are investigated. Further, the adsorption isotherms, kinetics, and thermodynamics have also been studied to describe the mechanism of adsorption.
Material and methods General remarks Phenol was purchased from Sigma-Aldrich (CAS No. 108-95-2). Reagents used in chemical oxygen demand
(COD) and total organic carbon (TOC) analysis are HCl, NaOH, H3PO4, and H2SO4 having high purity and were purchased from Merck (Germany) and used as received. Black spent tea leaves (Lipton brand) were collected from COMSATS Institute of Information Technology, Abbottabad, Pakistan.
Preparation of adsorbent The spent black tea leaves were boiled and washed with distilled water until the filtrate appeared colorless. The resulting solid material was dried in an oven at 60 °C for 24 h. The dried material was ground and sieved to particles < 500 μm. Chemical activation and carbonization was performed by placing the adsorbent in 2.5 M of ortho-phosphoric acid in a ratio of 1:3 for 24 h. Afterwards, the material was placed in a muffle furnace under a continuous flow of nitrogen for 4 h, till 400 °C. The activated carbon was rinsed with distilled water till neutral pH and then oven dried at 110 °C for 24 h. The prepared material was stored in polyethylene bags until use.
Characterization The surface morphology of the SBTL was investigated by a scanning electron microscope (SEM) instrument (Model-JSM 5910, JEOL, Japan). Functional groups of adsorbate and SBTL adsorbent before and after phenol loading were analyzed by Fourier transform infrared (FTIR) spectroscopy (Alpha Bruker, Germany, spectral range between 400 and 4000 cm−1).
Adsorption activity Phenol solution preparation A stock solution of 1000 mg L−1 of phenol was prepared. From the stock solution, the standard phenol solutions of 5, 10, 25, 50, 75, and 100 mg L−1 were prepared. The calibration curve was set from standard concentrations of phenol at maximum wavelength = 269 nm.
Analytical procedure The pH of the samples was monitored using a digital pH meter (PHS-38W Microprocessor). An ultrasonic bath (Cleaner model U Tech Products, Inc., USA) was used for sonolysis and ultrasound-assisted adsorption studies (35 kHz). The ultrasonic bath was calibrated by the calorimetric method (Siddique et al. 2014a, b). The spectrum was taken with a double-beam UV/VIS spectrophotometer (T80 + PG Instruments, UK). The absorbance values of the supernatants were measured using a detection wavelength of 269 nm. COD
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was analyzed by using the colorimetric method after digestion of the sample in a COD digester (Model TR320, Merck Spectroquant) according to the standard method (Federation 2005). TOC of the samples was determined by TOC analyzer (Shimadzu, model TOC-V CSH) operated at 680 °C furnace temperature and 20 mL sample injection. For degradation studies, samples were periodically withdrawn and immediately filtered using a plastic syringe filter with pore size of 0.45 μm. The filtered samples were analyzed by high-performance liquid chromatography (HPLC, Shimadzu LC-20AP, Shimadzu Corporation, Kyoto, Japan) installed with Shim-Pack GIST C18 chromatographic column (150 × 4.6 mm). A mixture of methanol and HPLC-graded water (60:40, v/v) was used as the mobile phase at a flow rate of 1 mL/min. A volume of 20 μL samples was injected into the sampling system and analyzed with a UV-Vis detector (SPD20AV Prominence, 200–700 nm) at a wavelength of 269 nm. In this method, the detection inferior limit for phenol was observed as 0.1 mg L−1 and the retention time of phenol was found to be 6.3 min.
Experimental procedure Experiments of phenol removal were carried out using sonolysis, adsorption, and ultrasound-assisted adsorption. The experimental setup for sonolysis and ultrasound-assisted adsorption was carried out in a Perspex vessel immersed in the digital ultrasonic bath. In the sonolysis experiment, 25 mg L−1 (50 mL working volume) of phenol concentration was used for the optimization of different parameters. The initial pH of the sample was set using dilute sodium hydroxide and hydrochloric acid (1 M). Batch adsorption experiments were performed using a known amount of activated spent black tea leaves in 50 mL of phenol solution of selected concentration in an orbital shaker at a continuous agitation rate of 220 rpm at 30 °C. After desired contact time, samples were filtered using 40-μm filter paper, and the residual concentration of phenol in the filtrate was measured. The effect of operational parameters for sonolysis and adsorption experiments, i.e., contact time (0– 180 min), pH (2–11), adsorbent dose (200–800 mg L−1), initial phenol concentrations (5–100 mg L−1), ultrasonic power (40–100 W) at 35 kHz frequency, and temperature (25– 45 °C), was tested and optimized. The optimized parameters—phenol concentration of 25 mg L−1, adsorbent dose of 800 mg L−1, pH 3.5, contact time of 60 min, room temperature of 30 ± 2 °C, and ultrasound power of 80 W having 35 kHz frequency with 50 mL working volume were used for the ultrasonic assisted adsorption experiments. The filtrate was analyzed for its absorbance value at specific wavelength using the UV/Visible spectrophotometer. The experiments for sonolysis, adsorption, and ultrasound-assisted adsorption were carried out in triplicates for reproducibility of results.
The amount of phenol removal was calculated using the following equation: Removal efficiency ð%Þ ¼ ðC i −C f Þ=Ci*100
ð1Þ
Where Ci and Cf are the initial and final concentrations (mg L−1) of phenol. TOC percentage removal efficiency of the samples was calculated using Eq. 2. A similar equation was used to calculate the COD of the samples. TOC ð%Þ ¼ ðTOC initial−TOC final=TOC initial*100Þ ð2Þ
Results and discussion Adsorbent (SBTL) characterization In order to characterize the surface morphology of the adsorbent, SEM analysis was performed. The SEM micrographs are shown in Fig. 1 (a) before and (b) after ultrasoundassisted adsorption of phenol. It was observed that the adsorbent had a diversified and coarse porous surface composed of small particles and fibrous bonds. These variable size pores ensured that the adsorbent provided a good surface medium for adsorption of phenol molecules into the improved SBTL. Figure 1b clearly demonstrates that the cavities at the adsorbent surface were filled that could be ascribed to filling with adsorbate. This trend in morphology was also observed by using chitin as an adsorbent (Karthik and Meenakshi 2015). In order to understand the interaction between phenol and SBTL, FTIR analyses were performed as shown in Fig. 2. The FTIR spectra of pure phenol showed a broad band due to stretching vibration of O–H at 3222 cm−1, while the band at 3045 cm−1 can be assigned to C–H stretching bands of aromatic compound. The C=C stretching bands were observed at the regions of 1593, 1499, and 1472 cm−1 which can be assigned to a series of weak combination and overtone bands of the benzene ring. Likewise, phenol C–O stretching bands appeared in the 1300–1000 cm−1 region. Some characteristic bands of C–H in-plane bending appeared in the region of 1023–616 cm−1. An FTIR spectrum of treated SBTL before adsorption of phenol does not show the peaks of functional groups such as – OH (~ 3400 cm−1) and aliphatic C–H (~ 2950–2900 cm−1). This can be explained by the chemical activation and carbonization of the tea leaves as a result of which some structural and/or chemical modifications occur. The peak in the region at about 1650–1000 cm−1 can be due to C=O or C=C groups and C–O stretching (Zuorro et al. 2013). The SBTL after adsorption shows a shift of bands from the aromatic to the aliphatic region. The appearance of bands in the region of 2956 and 2923 cm−1 represents O–H stretching of carboxylic acids. A corresponding keto group C=O stretch
Environ Sci Pollut Res Fig. 1 SEM image of SBTL before (a) and after (b) ultrasound-assisted adsorption process
(a)
(b)
band was observed at 1667 cm−1 which indicates that phenol degradation took place into catechol. Similar degradation peaks and route were also observed by Umamaheswari and Rajaram (2014).
Effect of contact time The effect of contact time on the percentage removal of phenol was determined by ultrasound and adsorption processes using 25 mg L−1of initial phenol concentration at pH 7.5 for a reaction time of 0–180 min at 30 ± 2 °C temperature. For the sonolysis experiments, an ultrasonic power of 80 W was used, and for the adsorption experiments, an adsorbent dose of 800 mg L−1 was used. The results are shown in Fig. 3. In both techniques, it was found that initially the rate of phenol removal is high, and maximum removal takes place at a contact time of 60 min. In the sonolysis experiments, a maximum of 11% removal of phenol was observed at 60 min and later it becomes consistent. As expected, phenol due to its physicochemical properties remains in bulk rather than vapor phase
during cavitations (Maleki et al. 2007). In bulk solutions, the hydroxyl radicals are generated, recombined, and thus become unavailable to attack the phenol molecules. Hence, low efficiency is observed. Further, after the initial period of sonication whatever dissolved gas is present in the solution, it is degassed that could result in the decreased generation of ˙OH radicals as discussed in previous studies (Siddique et al. 2014a, b). In the adsorption experiments, a similar trend was found. Maximum adsorption was achieved rendering to a large number of available sorption sites on the surface of the adsorbent, and equilibrium was established in a contact time of 60 min. Further, increase in time up to 180 min did not affect the equilibrium and overall phenol adsorption was 48% (Ingole and Lataye 2015).
Effect of pH The effect of pH was investigated for sonolysis and adsorption techniques and is shown in Fig. 4. It was observed that generally both techniques favor phenol removal at acidic pH
120
60 100
50
% removal
%T
80
60
40 sonolysis 30
adsorpon
20
40 Phenol SBTL before adsorption SBTL after adsorption
20
10 0 0
0 500
1000
1500
2000
2500
3000
3500
wave number (cm-1)
Fig. 2 FTIR spectra of phenol, SBTL before adsorption, and SBTL after adsorption of phenol
20
40
60
80
100
120
Time (min)
140
160
180
200
Fig. 3 The effect of contact time on phenol removal by sonolysis and batch adsorption. Experimental conditions: phenol conc. = 25 mg L−1, pH = 7.5, ultrasonic power = 80 W, adsorbent dose = 800 mg L −1, temperature = 30 ± 2 °C
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Initial phenol concentration The effect of initial phenol concentration (5–100 mg L−1) at optimum conditions was investigated for sonolysis and adsorption processes, and it is observed that the extent of removal decreased with the increase in initial phenol concentrations (Fig. 5). The increase in phenol concentration from 5 to 100 mg L−1 decreased the removal from 17 to 2% in sonolysis and from 85 to 41% in the adsorption process. As the initial phenol concentration increased, the rate of phenol removal decreased (Maleki et al. 2007). The possible reasons for the sonolysis methods may be as follows: (i) higher concentrations of phenol increase the number of phenol molecules, but not the ˙OH radical concentration, so the removal rate becomes slower; (ii) the cavities and ˙OH approached saturation with the increase in initial phenol concentration; and (iii) there is a possibility for the generation of more inorganic anions with the increase in concentration which may compete with organic species for the reaction with ˙OH radicals (Siddique et al. 2014a, b). During the adsorption process, it was observed that the increase in phenol initial concentration decreases the phenol removal rate, which might be due to the accumulation of phenol particles on the surface of the adsorbent. At low concentration, the ratio of surface active sites increases, and as the concentration increases, these sites become saturated and thus affect the removal efficiency. Hence, a high concentration of phenol results in less availability of active sites on the adsorbent surface (Liu et al. 2010).
Effect of ultrasonic power In this study, the experiments were conducted with various ultrasonic powers ranging from 40 to 100 W under optimized
70 sonolysis
% removal
60
adsorpon
50 40 30 20 10 0 1
2
3
4
5
6 7 pH
8
9
10
11
12
Fig. 4 pH effect by sonolysis and adsorption processes. Experimental conditions: phenol conc. = 25 mg L−1, adsorbent dose = 800 mg L−1, ultrasonic power = 80 W, temperature = 30 ± 2 °C, time = 60 min
parameters. The results are shown in Fig. 6. It can be seen that the removal efficiency increased as ultrasonic power increased from 40 to 80 W and maximum removal was observed at 80 W after 60 min. The increase in ultrasonic power would increase the mixing intensity owing to turbulence generated by the collapse of the cavitation bubbles (Siddique et al. 2014a, b). However, further increase in ultrasonic power from 80 to 100 W did not lead to an increase in phenol removal after 60 min of reaction time. This was due to the reason that when a chemical reaction takes place, an optimum acoustic power exists that provides maximum reaction rate. At higher ultrasonic power, the numbers of bubbles generated are increased. Then, they act as a barrier for further transfer of acoustic energy through the whole liquid (Siddique et al. 2014a, b).
Effect of adsorbent dosage The effect of adsorbent dosage on phenol removal was investigated under optimum conditions of phenol conc. (25 mg L−1), 90
% removal
compared to basic. The degradation of phenol by sonolysis strongly depends on the pH of the solution. The pKa value for phenol is 9.95 in water which means it is in phenolate form at pH 10. Its phenolate form is hydrophobic and remains trapped among water bubbles and can only react with ˙OH radicals generated due to water dissociation. At lower pH, the phenol molecule can penetrate into the water bubble cavities and subject to thermal cleavage along with ˙OH radicals outside the bubbles (Okouchi et al. 1994; Wu et al. 2001). The pH was found to be one of the most influential parameters of adsorption studies. At low pH, the surface of the SBTL adsorbent is positively charged, while it is negatively charged under basic conditions. In acidic condition at pH 2–6, strong attractive forces exist between the adsorbent and phenol molecule (Aksu 2005; Kazmi et al. 2013). Thus, a significant amount (62%) of phenol is removed by adsorption. At pH > 7, the dissociated phenolate experiences repulsion from the negatively charged surface of the adsorbent. Thus, only 40% phenol removal is observed at pH 11.
80
sonolysis
70
adsorpon
60 50 40 30 20 10 0 0
10
20
30
40
50
60
70
80
90
100
Initial phenol concentration (mg L-1)
Fig. 5 Effect of initial phenol concentration on percentage removal by sonolysis and adsorption processes. Experimental conditions: pH = 3.5, adsorbent dose = 800 mg L−1, ultrasonic power = 80 W, temperature = 30 ± 2 °C, time = 60 min
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20 sonolysis
90
16
80
14
70 % removal
% removal
18
12 10 8
60 50 40
6
30
4
20
2
10
0 40
60 80 US Power (W)
0
100
Fig. 6 The effect of ultrasonic power on sonolysis process. Experimental conditions: phenol conc. = 25 mg L−1, pH = 3.5, temperature = 30 ± 2 °C, time = 60 min
pH (3.5), temperature (30 ± 2 °C), and time (60 min). Figure 7 shows that in the first 60 min, the phenol removal was found remarkable due to the accessibility of enormous binding sites on SBTL. About 48% phenol is removed by fixing the operational parameters such as phenol concentration, adsorbent dose, and pH, and equilibrium is achieved in 60 min at room temperature. The ortho-phosphoric acid activation of the adsorbent enhanced the number of pores (Ingole and Lataye 2015) which improved the SBTL adsorption. It is clear from the results that the increase in adsorbent masses enhanced the percentage adsorption of phenol. The optimum dose for phenol was 800 mg L−1 which provided more attachment sites for adsorbate particles (Nagda et al. 2007; Liu et al. 2010; Sarker and Fakhruddin 2017).
Ultrasound-assisted adsorption
ultrasound assisted adsorpon
5
10
20
30 40 Time (min)
50
60
Fig. 8 Removal of phenol using ultrasound-assisted adsorption by spent black tea leaves. Reaction conditions: time = 60 min, phenol conc. = 25 mg L−1, pH = 3.5, dose = 800 mg L−1,US power = 80 W, temperature = 30 ± 2 °C
adsorbent processes under optimum operational conditions, i.e., phenol concentration of 25 mg L−1, 50 mL volume, 800 mg L−1 adsorbent dose, 3.5 pH, and 60 min contact time at room temperature of 30 ± 2 °C. The combined techniques resulted in high adsorption efficiencies as compared to individual processes (Fig. 8) (Asghari et al. 2014; Asfaram et al. 2015). It is obvious from the principle of cavitation that the ultrasound process results in convection of the solution medium through shock waves, microturbulence, microstreaming, and microjetting. The extreme temperature and pressure conditions in the bubble result in the generation of radicals due to dissociation of entrapped vapor molecules. With fragmentation of the bubble at the point of maximum compression, these radicals are released into the medium where they induce and accelerate chemical reactions, which are well-known as sonochemical reactions.
The ultrasound-assisted adsorption study was further conducted for the removal of phenol by combining sonolysis and 90 80
80 adsorpon
70
70 Removal efficiency (%)
% removal
60 50 40 30 20 10 0 50
100
200 400 600 800 Adsorbent dosage (mg L-1 )
1000
Fig. 7 The effect of adsorption dosage on phenol removal. Experimental conditions: phenol conc. = 25 mg L−1, pH = 3.5, temperature = 30 ± 2 °C, time = 60 min
60 50 40 30 20 10 0 US
SBTL
US+SBTL
Fig. 9 Comparison of phenol removal using ultrasound, adsorption, and ultrasound-assisted adsorption
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45
(b)
40
70
35
60
Removal efficiency (%)
Removal efficiency (%)
(a)
30 25 20 15 10
50 40 30 20 10
5
0
0
US
SBTL
US+SBTL
US
SBTL
US+SBTL
Fig. 10 Removal of TOC (a) and COD (b) by US, SBTL, and US+SBTL
Comparison of US, SBTL, and US+SBTL Comparison studies were conducted by using 25 mg L−1 initial concentration of phenol for sonolysis, adsorption, and ultrasound-assisted adsorption processes. For the sonolysis experiment, the optimized parameters such as US power of 80 W, pH value of 3.5, and contact time of 60 min at room temperature of 30 ± 2 °C were used. During the adsorption experiment, the parameters were also optimized, i.e., 60 min of contact time, pH 3.5, and adsorbent dose of 800 mg L−1 were used for the removal of phenol at room temperature of 30 ± 2 °C. In the ultrasound-assisted adsorption, the aforesaid optimum conditions of parameters were also used. A comparison between ultrasound, adsorption, and ultrasound-assisted adsorption is shown in Fig. 9. Ultrasound-assisted adsorption
Fig. 11 The HPLC spectra of phenol degradation by ultrasound-assisted absorption process: A 15 min, B 30 min, C 45 min, and D 60 min. Experimental conditions: phenol conc. = 25 mg L−1, ultrasonic power = 80 W, adsorbent dose = 800 mg L−1, temperature = 30 ± 2 °C
shows better removal efficiency than the sonolysis and adsorption processes.
Reduction in total organic carbon and chemical oxygen demand It was found that total organic carbon and chemical oxygen demand removal in ultrasonic-assisted adsorption was more than that of individual sonolysis and adsorption as shown in Fig. 10. The reduction in the concentration of TOC and COD is a confirmation of phenol elimination. In aqueous solution, TOC (a) and COD (b) are due to the presence of organic compounds. The use of a coupled technique for the removal of TOC and COD was found moderately additional. This suggests that ultrasound has a great synergistic effect on the
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enhanced phenol adsorption by SBTL and that the collapse of the cavitation bubble exerts a large mechanical stress on the surface of the adsorbent, and previous researchers reported the same conditions (Landi et al. 2010).
Identification of phenol degradation intermediates In the present study, the degradation mechanism of phenol is proposed based on the detection of intermediates and end products by HPLC results (Fig. 11). It was found that the first 15-min treatment resulted in the disappearance of phenol molecule with decreased peak areas of phenol residues. It can be suggested that the oxidation route of phenol occurs via nonselective hydroxyl radicals that attack the phenyl ring leading to the generation of catechol and hydroquinone, respectively, in aqueous phase during the 15 and 30 min of the phenol degradation process. Further oxidation of the dihydroxylbenzenes then occurs to produce benzoquinones and then aliphatic intermediates in the reaction mixture. These findings were also supported by the FTIR spectra of SBTL after adsorption where peaks assigned to carboxylic acids were observed. These observations suggest that the phenol ring was cleaved forming organic compounds that are ultimately oxidized to CO2 and H2O during 60 min of treatment (Fig. 12). These results were also supported by a decrease in TOC. Based on these observations, it can be proposed that the oxidative decomposition of phenol results principally by its interaction with the ˙OH radicals produced during the cavitational collapse. Literature supports the formation of three principal intermediate species, i.e., hydroquinone, benzoquinone, and catechol via sonochemical oxidation route of phenol (Serpone et al. 1992). Similarly, other oxidation processes such as photo-oxidation and photocatalytic oxidation of phenol also resulted in the formation of the abovementioned intermediates due to the hydroxyl radical attack with the formation of organic acids as a final product (Wysocka et al. 2018).
Eddy, Inc. 2003; Hussain Gardazi et al. 2016). In this process, the contaminant does not migrate to the surface plane. The following Langmuir isotherm equation is used to model the phenol adsorption data by SBTL: 1 1 1 1 ¼ þ ð3Þ qe qmax qmax K ads C e qe is the amount of phenol adsorbed at equilibrium per gram of SBTL adsorbent, qmax is the maximum adsorption capacity of the adsorbent, Ce is the adsorbate concentration in solution, and Kads is the equilibrium constant. RL is the dimensionless
OH
OH
OH
OH OH OH OH OH
OH O
Adsorption isotherms The mathematical models are used to compare the contaminant adsorption strength and to design the adsorption process effectively. The adsorption mechanism of phenol by SBTL was explored at initial phenol concentrations of 10, 25, 50, and 100 mg L−1. The data was used for isotherm studies, while all the other parameters like adsorbent dose (800 mg L−1), particle size (420 μm), temperature (30 ± 2 °C), pH (3.5), and agitation speed (220 rpm) were kept constant. Two isotherm models, i.e., Langmuir and Freundlich, were used for the analysis of phenol adsorption data by SBTL. Langmuir (1918) assumes that the contaminant adsorption by the adsorbent follows monolayer adsorption onto active sites with homogeneous energy distribution (Metcalf, and
O OH Organic acids OH CO2, H2O Fig. 12 Possible pathway for phenol degradation during the ultrasoundassisted adsorption process
Environ Sci Pollut Res Fig. 13 Data fitness to isotherm models by SBTL
70
0.15 Langmuir isotherm
Freundlich isotherm
56
0.09
y = 0.0809x + 0.0245 R² = 0.9888
qe (mgg-1)
1/qe (mg g-1)
0.12
0.06
y = 10.546 x 0.3763 R² = 0.9441
42 28 14
0.03
0
0 0
0.5
1
1.5
0
20
40
RL ¼
1 1 þ ðqmax bÞC 0
ð4Þ
The value of RL indicates the isotherm shape to be linear (RL = 1), favorable (0 < RL < 1), unfavorable (RL > 1), and irreversible adsorption (R L = 0) (Haroon et al. 2016; Gardazi et al. 2017). The Freundlich model is applicable to heterogeneous systems with nonuniform distribution of adsorption affinities over a heterogeneous surface. This model assumes the contaminant adsorption as multilayers on the surface of the adsorbent. The Freundlich adsorption isotherm is given by qe ¼ K f C e 1=n
ð5Þ
Where Kf is the Freundlich isotherm constant, Ce and qe indicate the residual phenol in the aqueous phase and phenol adsorbed by SBTL, and n represents the heterogeneity of the adsorbent. Data fitness to both isotherm models (Fig. 13) revealed that the adoption of phenol by SBTL occurred through single-layer adsorption and follows the assumption of Langmuir with R2 value of 0.9888 compared to the multilayer assumption of the Freundlich isotherm with poor R2 value of 0.9441. Moreover, a close agreement between experimental (47.15 mg/g) and Langmuir maximum adsorption (40.8 mg/g) was observed that confirmed the data fitness to be Langmuir isotherm. The value of RL was calculated as 0.057, which indicates the SBTL is a favorable adsorbent for phenol removal from waste streams.
Adsorption kinetics
80
chemisorption controls the phenol adsorption and involves exchange/sharing of electrons between SBTL and phenol (Latinwo and Agarry 2015). This model is expressed as follows: t 1 1 ¼ þ t qt k 2 q2e qe
ð6Þ
where k2 (g mg−1 min−1) is the pseudo-second-order rate constant, qe (mg g−1) represents the adsorption capacity, and qt is the amount of adsorbed phenol at time t (min). k2 (g mg−1 min−1) was calculated from the slope and intercept of the plot t/qt versus t. Figure 14 shows that coefficients of correlation (R2) of the pseudo-second-order model at 10 mg L−1 (R2 = 0.9917), 25 mg L−1 (R2 = 0.96), 50 mg L−1 (R2 = 0.992), and 100 mg L−1 (R2 = 0.993) reflect the best fit to phenol adsorption by SBTL. This indicates that chemisorption is largely involved in the removal of phenol by SBTL. This also confirms that phenol removal follows the Langmuir assumption of monolayer adsorption by SBTL. However, the decrease in k2 was observed (0.007, 0.005, 0.001 g mg−1 min−1) with rising concentration of phenol, i.e., 25, 50, and 100 mg L−1, respectively, which showed that chemisorption is not the sole adsorption-limiting factor.
7 10 mg/L
6
25 mg/L
y = 0.1125x + 0.4811 R² = 0.9917
50 mg/L
5 t/qt (min g/mg)
constant (separation factor) that was measured for best-fit Langmuir and is generally expressed as:
60
ce (mgL-1)
1/Ce (mg L-1)
100 mg/L
4 y = 0.0453x + 0.2699 R² = 0.997 y = 0.0338x + 0.2268 R² = 0.9673 y = 0.0197x + 0.1965 R² = 0.9627
3 2 1 0
The pseudo-second-order kinetic model (Haroon et al. 2017) was used to explore the mechanism and rate of phenol adsorption onto the surface of SBTL. This model assumes that
0
10
20
30 Time (min)
40
50
60
Fig. 14 Pseudo-second-order fitness to phenol removal by SBTL at various initial concentrations of phenol
Environ Sci Pollut Res Table 1
Thermodynamic parameters of SBTL for phenol adsorption −1
ΔH (kJ mol )
− 27.87
−1
ΔS (kJ mol )
− 0.12
−1
ΔG (kJ mol )
2
R
298 K
308 K
328 K
6.56
7.72
10.02
0.9987
Adsorption thermodynamics Temperature dependence of the SBTL adsorbent for phenol removal was studied at various temperatures, i.e., 298, 308, and 328 K. The parameters of thermodynamics such as Gibbs free energy (ΔG), entropy (ΔS), and enthalpy (ΔH) were calculated from the slope and intercept of the Van’t Hoff plot (lnKd versus l/T). The following equations were used to obtain the aforesaid thermodynamic parameters (Gardazi et al. 2017). Kd ¼
qe Ce
ΔS ΔH lnK d ¼ þ plot ðlnK d ∼1=T Þ R RT ΔG ¼ ΔH−T ΔS
ð7Þ ð8Þ ð9Þ
Kd is the distribution coefficient, R is the universal gas constant (8.314 J/mol K), and T is the temperature (K). The ΔG values were found to be positive, i.e., 6.56, 7.72, and 10.02 kJ mol−1 at different temperatures of 298, 308, and 328 K, respectively (Table 1). ΔG (+ve) displayed the nonspontaneous nature of the phenol adsorption by the SBTL adsorbent. ΔS (− 0.12 kJ mol−1) confirmed less randomness at the interface. The ΔH of the system was observed as − 27.87 kJ mol−1, which revealed the exothermic nature of the phenol adsorption process.
Conclusion This study showed the first results on the effect of combined ultrasound and adsorption process on phenol removal using SBTL. The usage of SBTL provided a good source of activated carbon with great surface area. The ultrasound-assisted adsorption method showed better percentage removal efficiency for phenol compared to individual methods. The maximum removal efficiency was 85% in 60 min, along with the optimization of influential parameters, i.e., 3.5 pH, 800 mg L−1 adsorbent dose, 80 W ultrasonic power, and 30 °C temperature. The removal of TOC and COD was 78 and 39%, respectively, by this technique showing consistency of phenol removal. The results obtained are good enough to produce effective and rapid treatment techniques. Both FTIR and HPLC studies confirm the presence of catechol, hydroquinone, and carboxylic acid suggesting nonselective
oxidation of phenol. The adsorption isotherm models (Langmuir and Freundlich) were plotted that showed the data obtained from the phenol adsorption mechanism follows the Langmuir isotherm model. The adsorption kinetic behavior followed pseudo-second-order kinetics with a very good correlation coefficient, which shows that the basic phenomenon involved was chemisorption. The values of ΔG°, ΔH°, and ΔS° indicated the nonspontaneous and exothermic nature of the adsorption process. This study suggests that the use of black spent tea leaves as adsorbent would effectively accompany the process of sonolysis for rapid removal of phenol. The designed process is efficient and cost-effective.
Compliance with ethical standards Competing interests The authors declare that they have no competing interests.
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