Journal of Environmental Science and Health, Part A Toxic/Hazardous Substances and Environmental Engineering
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Use of functinalized adsorbents for tetracycline removal in wastewater: adsorption mechanism and comparison with activated carbon Letícia Reggiane de Carvalho Costa & Liliana Amaral Féris To cite this article: Letícia Reggiane de Carvalho Costa & Liliana Amaral Féris (2020): Use of functinalized adsorbents for tetracycline removal in wastewater: adsorption mechanism and comparison with activated carbon, Journal of Environmental Science and Health, Part A, DOI: 10.1080/10934529.2020.1827654 To link to this article: https://doi.org/10.1080/10934529.2020.1827654
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JOURNAL OF ENVIRONMENTAL SCIENCE AND HEALTH, PART A https://doi.org/10.1080/10934529.2020.1827654
Use of functinalized adsorbents for tetracycline removal in wastewater: adsorption mechanism and comparison with activated carbon Letıcia Reggiane de Carvalho Costa and Liliana Amaral Feris Department of Chemical Engineering, Federal University of Rio Grande do Sul, Porto Alegre, Brazil ABSTRACT
ARTICLE HISTORY
The present study investigated the application of functinalized adsorbents in the removal of tetracycline in water by adsorption. Activated carbon was impregnated with iron chloride, copper sulfate and hydrogen peroxide, in order to improve its adsorption properties. The solids were characterized by adsorption/desorption isotherms N2, XRD, FTIR and SEM. The influence of pH, adsorbent concentration and contact time parameters was evaluated. The results using activated carbon as adsorbent and initial TC concentration of 20 mg.L1 showed that 93% TC removal was achieved at pH 4.0, contact time of 120 minutes and 30 g.L1 adsorbent concentration. Experiments applying the functionalized solids showed a faster initial adsorption kinetics. Also, no pH adjustment was required (pH 6.0). In addiction, a minimum of 50% reduction in adsorbent mass (15, 10 and 5 g.L1 for CA–H2O2, CA–Fe and CA–Cu, respectively) was observed, compared to the untreated solid. The pseudo-second-order kinetic model was chosen as the optimal model for the study. The application of FeCl3, CuSO4 and H2O2 composites as adsorbent solids are technically feasible for the removal of tetracycline in wastewater treatment.
Received 18 May 2020 Accepted 19 September 2020
Introduction Drugs generated by domestic, commercial, rural, industrial and medical-hospital human activities, when improperly discharged, cause negative impacts on the physical, chemical and biological characteristics of the natural environment, especially water and soil, endangering health and health survival of humans.[1,2] Many of these substances, known as emerging contaminants (CE), are often found in aqueous matrices at concentrations in the range of mg.L1 and ng.L1.[3] Because they have low biodegradability, high persistence and ease of bioaccumulation, there is a high resistance of these compounds to conventional treatment processes, being difficult to remove them. In addition, the presence of residual drugs in surface water may be indicative of effluent contamination from treatment plants.[4] Among the groups in the drug category, antibiotics stand out, considered the largest drug category provided by human and veterinary medicine, for therapeutic purposes or as growth promoters.[5] Tetracycline (TC), produced from strains of microorganisms or as a semi-synthetic product, represents a broad spectrum spectrum of antibiotics.[6] Tetracycline was found in surface and groundwater and in 80% of effluent samples. Yao et al.[7] investigated the occurrence of 14 antibiotics in ground and surface water on Jianghan Plain in China. In groundwater evaluated at 10, 25 and 50 m depth, the fluoroquinolones and tetracyclines antibiotics were predominant. Zhang et al.[3] revealed that
KEYWORDS
Adsorption; emerging pollutants; impregnation; composites; granular activated carbon
tetracycline, as a popular antibiotic, may be present in the environment at concentrations up to 110 lg.L1. The product has also been found in some foods, such as eggs, milk and fish s.[8] In addition, high concentrations of this drug were found in soil and sediment matrices, indicating a tendency of accumulation, persistence and also strong sorption characteristics.[6,9] Many processes for the removal or reduction of TC from wastewater containing these micro-pollutants are reported in the literature as advanced oxidation, photocatalytic degradation, electrochemical treatments, adsorption, and so on.[10–16] Adsorption is still considered one of the most promising means for the elimination of tetracyclines from the aquatic environment due to its advantages of simple operation, low investment cost.[17] The solids used in this type of process can be functionalized according to their use, introducing specific characteristics in order to intensify the adsorption capacity of the material in the removal of certain pollutants. Some studies discuss the relationship between adsorption performance and surface treatment of adsorbent for TC removal. Among them, Li et al.[18] studied the efficiency of activated carbon as adsorbent prepared from Iris tectorum with and without modification by iron nitrate. The authors observed that the modification increased tetracycline adsorption capacity by 23% (769.2 mg.g1) compared to unmodified activated carbon (625.0 mg.g1). Likewise, the modification of activated carbon derived from brassica
CONTACT Letıcia Reggiane de Carvalho Costa [email protected] Laboratory of Separation and Unit Operations (LASOP), Department of Chemical Engineering, Federal University of Rio Grande do Sul, Ramiro Barcelos Street, 2777, 90035-007, Porto Alegre, RS, Brazil. ß 2020 Taylor & Francis Group, LLC
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L. R. D. C. COSTA AND L. A. FÉRIS
napus L. (rapeseed) stem residue by H2O2 by Tan et al.[19] improved TC adsorption by approximately 20% compared to the untreated solid (26.3–35.9 mg.g1), reaching an adsorption capacity between 32.0 and 42.5 mg.g1 at pH 9. In this context, the present work presents the preparation and use of composites based on highly dispersed activated carbon and iron chloride, copper sulfate and hydrogen peroxide in the removal of tetracycline drug in water. Adsorption tests were performed to maximize the removal of the tetracycline (TC) pollutant.
The tetracycline (TC) concentration in both the initial and the adsorbent solutions were determined by the ultraviolet (UV-visible) spectrophotometry method using the Thermo Scientific, Genesys 10S UV-Vis equipment at the TC detection wavelength, 357 nm. The conversion of absorbance to concentration was obtained by Eq. (1), with the linear coefficient obtained from the tetracycline calibration curve:
Experimental procedure
where C is the solute concentration (mg.L1); A is the absorbance at 357 nm; and a is the linear coefficient of the calibration curve. The adsorbents capacity was calculated by Eq. (2) and, the experiments were carried out in different concentrations of tetracycline (0–1000 mg.L1), keeping the amount of adsorbents constant.
Preparation of activated carbon composites The composites were prepared separately by impregnating with iron chloride (CA–Fe), copper sulfate (CA–Cu) and hydrogen peroxide (CA–H2O2) from a aqueous suspension containing commercial activated carbon (Exodus), FeCl3 (2% w/v), CuSO4 (2% w/v) and H2O2 (30% v/v) at 25 C for 24 hours. After this time, the material was filtered and the composites were oven dried for 4–6 hours at 100 C. Characterization of adsorbents The synthesized composites were characterized by X-ray diffraction (XRD) using a Bruker D2 Phaser diffractometer with angular variation of 5 –90 (2h), CoKa radiation (k ¼ 1,542 Å) and exposure velocity 0.05 2h sec1. The morphological analysis of the materials was performed by scanning electron microscopy (SEM). Physical adsorption/ desorption tests of N2 at 77 K were also performed, and the specific surface area was calculated by the BET method and the pore volume and diameter were calculated using the Barrett-Joyner-Halenda (BJH) equation. The infrared spectra (IR) of the samples were obtained from a Nicolet 6700 spectrophotometer at a wavelength range of 4000–400 cm1 at room temperature. Adsorption experiments Tetracycline was purchased from Sigma-Aldrich, showing purity 98.0%. Adsorption tests were performed in batch using 100 mL volumes of solution. Tetracycline solution (20 mg.L1) was prepared from a concentrated solution by diluting with distilled water. The diluted solution was placed in a Schott flask and allowed to stir in Wagner Shaker (MA160BP, Marconi brand) at 30 rpm, according to the methodology adopted by de Franco[20] and Haro.[21] For each test, the adsorbent was added to the solution, the pH was adjusted and the process parameters to obtain the best experimental conditions were evaluated. All experiments were performed in duplicate and the standard deviation was calculated to evaluate the dispersion of the experimental points. A comparative study with untreated activated carbon was performed under the same conditions for all tests conducted.
C¼
Qe ¼
A a
ðCi Ce ÞV m
(1)
(2)
where Ci is the initial drug concentration (mg.L1), Ce the equilibrium drug concentration (mg.L1), V the volume of solution (L) and m the mass of solid (mg). The tetracycline removal efficiency was evaluated by pH, contact time and solid dosage parameters by the adsorption process. In addition, all experiments were performed at room temperature. Error correction caused by possible leakage of salts and hydrogen peroxide in solution, due to their hydrophilic character, was performed by reading the solution before and after adsorption, discounting the effect of the interferents. Determination of solution pH For solution pH study, the contact time was maintained for 30 minutes for a constant mass of 1 g of composite. Experiments were performed varying the solution pH (2–10) and adjusting it when necessary using 0.1 M HCl or NaOH solutions. Determination of contact time The contact time study was performed keeping the concentration of sodium constant, with the pH of the being mixture adjusted to the value where the highest removal rate occurred in the previous test. Sorption times ranging from 5 to 300 minutes were evaluated, and the best contact time was the one with the highest removal of the drug studied. Determination of adsorbent concentration To evaluate the effect of solid concentration on tetracycline adsorption, experiments were carried of using different concentrations of composites, ranging from 0.5 to 20 g.L1. The pH and adsorption time used were considered optimum in in previous sections.
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Table 1. Surface areas and porosity characteristics of CA, CA–Fe, CA–Cu, CA–H2O2, CA–Fe–TC, CA–Cu–TC and CA–H2O2–TC. Parameters Solid CA CA–Fe CA–Cu CA–H2O2 CA–Fe–TC CA–Cu–TC CA–H2O2–TC
2
1 a
SBET (m .g ) 508.1 555.8 533.5 581.8 553.8 532.1 536.0
SMES (m .g1)b 285.1 335.8 314.2 356.9 328.2 318.6 301.2 2
VMES (cm3.g1)c 0.10 0.12 0.11 0.13 0.11 0.12 0.11
TMD (Å)d 4.8 4.8 4.8 4.8 4.8 4.8 4.8
a
BET surface area, mesoporous surface area, mesoporous volume, d average pore size. b c
Figure 1. N2 adsorption/desorption isotherms and pore distribution to materials before and after TC adsorption.
Results and discussions Study of the tetracycline sorption mechanism Adsorption kinetics was evaluated after fitting the experimental data to the kinetic models of pseudo-first and pseudo-second order, in non-linear form, described in Eqs. (3) and (4), respectively. dqt ¼ k1 ðq 1 q t Þ dt
(3)
dqt ¼ k2 ðq2 qt Þ2 dt
(4)
where q1,2 and qt are the adsorbed quantities (mg.g1) at equilibrium and at time t, respectively, k1 is the adsorption rate constant (min1), k2 is the adsorption rate constant (g .mg1.min1) et is the time in minutes. For the isothermal study, the two models examined in this study were Langmuir (Eq. 5) and Freundlich (Eq. 6). These models were used in their non-linear form, even that linear methods are still seen in much of the scientific literature. Q¼
qm KL Ce 1 þ KL Ce
Q ¼ KF Ce=n 1
(5)
(6)
where, Q is the amount adsorbed at equilibrium (mg.g1), Ce is the concentration of the drug at equilibrium (mg.L1), qm is the maximum adsorption capacity (mg.g1); KL the Langmuir constant related to the affinity between the adsorbent and adsorbate (L.mg1); KF is the Freundlich constant related to the relative adsorption capacity (mg.g1) and n the heterogeneity factor (g.L1). In kinetic and isothermal analyzes, the parameters of each model were determined by non-linear regression analysis using the Microsoft Excel resolution tool. The effectiveness of the models with the experimental equilibrium data was detected by the values of the coefficient of determination (R2).
Characterization of adsorbents Figure 1 shows the textural characterization of activated carbon and composites. Additionally, Table 1 shows the specific values obtained by this characterization step. From Figure 1 and the results shown in Table 1, it can be seen that impregnation of the adsorbent by iron chloride, copper sulfate and hydrogen peroxide causes an increase in the specific surface area of the material compared to the untreated solid (CA, 508.1 m2.g1), which suggests a low compromise of the pores present in activated charcoal used as support for the composites. All materials have characteristics of complex materials containing micro and mesopores in their structure and, consequently, present variations in hysteresis. According to the classification of the International Union of Pure and Applied Chemistry (IUPAC), the curves obtained for the analyzed solids show a behavior similar to type I isotherms, indicating an adsorption in micropores. According to the empirical classification of hysteresis curves, it can be said that hysteresis is of the H4 type, representative of materials with ordered and uniform pores. The BET area and pore volume for CA–H2O2 (581.8 m2.g1 and 356.9 cm3.g1, respectively) was larger than in the other samples, while pore diameter was similar for all. In fact, analyzing the graph of pore distribution of the materials, shown in Figure 1, it can be seen that the composites before TC adsorption have a profile very similar to activated carbon with slight decrease of adsorbed N2. Additionally, the pore diameter indicates that the adsorbent solids were mainly microporous. After adsorption, a slight decrease in specific surface area and volume of mesopores is noted, probably caused by the filling of the active sites with the TC molecule. For metalimpregnated solids (CA–Fe and CA–Cu), this decrease was not so significant, so it is believed that this mild decrease in surface area and mesopore volume is due to merely superficial interactions between the adsorbent and tetracycline due to the antibiotic-metal complex formed between the two materials. In addition, evacuation prior to BET measurements can remove most of the adsorbed TC, due to the lack of chemical bonding in the adsorption mechanism. Figure 2 shows the SEM images of all materials.
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L. R. D. C. COSTA AND L. A. FÉRIS
Figure 2. SEM images of solids before and after tetracycline adsorption. 30 k magnification and 50 lm scale for all images. Samples based on FeCl3 (a), CuSO4 (b) and H2O2 (c).
In general, the morphology of the solid materials analyzed by SEM showed particles with irregular grains. It is also observed that there is a greater presence of agglomerate on the surface of the samples, which confers a larger specific surface area, contributing to the adsorption. Physical changes can be observed in the adsorbent materials after adsorption compared to the solid initially with fully filled and prominently swollen pores. This indicates that the drug is adsorbed to both the functional groups present on the surface and/or interior of the pores. The vibrational spectra of the solids are shown in Figure 3, before and after the adsorption of TC. In it, it is revealed that the three strongly absorbent peaks at about 3500–3000, 1400 and 1100 cm1 are respectively assigned to the OH, CH and CO stretch frequency to all samples. The difference
observed between the spectra in the region of 3000 and 3500 cm1 is associated with the presence of OH groups, related to KBr humidity, and to the CH2 and CH groups of the aliquot. In the case of composites, although this first band is evident, it was smooth and better defined, which may be related to the moisture loss of the adsorbent in question. The spectrum at 1650 cm1 CA is the result of C ¼ C skeletal vibrations in aromatic rings.[22] A reduction in this spectrum is noted after solid functionalization, probably caused by bond breakage. In addition, analyzing the spectrum after the adsorption of TC in all adsorbents, it is noted that, possibly, this is also the function group responsible for the drug adsorption phenomenon, together with the OH, CH and CO groups (peaks 1400–1100 cm1 ).
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Figure 3. FTIR spectra of solids before and after tetracycline adsorption compared to conventional granular activated carbon.
Compared with CA, the intensities of the modified solids bands (CA–H2O2, CA–Fe and CA–Cu) were weaker, which may suggest that impregnating solutions interact or overlap with the CA surface groups. In addition, peak changes indicate that the surface chemical status of the corresponding groups changes after the impregnation process. The XRD profiles of adsorbents before and after tetracycline adsorption are illustrated in Figure 4 and are compared to untreated activated carbon. All materials exhibited reflections at 2h angles near 25 and 45 , corresponding to the carbon and graphite carbon planes respectively, characteristic of amorphous materials.[23] Compared to non-impregnated activated carbon (CA), a difference in the intensity of these characteristic peaks was observed, regardless of the material used, both in the impregnation phase and in the adsorption step by TC. Results show that the impregnation with iron chloride, copper sulfate and hydrogen peroxide did not compromise the structural characteristics of the adsorbent support material. However, it is important to highlight that highly dispersed materials with small particle size, as presented in this work, may alter the surface morphology of the sample, which justifies the difference in absorption intensity due to the higher content of surface mixtures.[24] Adsorption tests Determination of solution pH In this study, natural pH values were 6.7, 3.8, 4.8 and 5.5 for the adsorbents CA, CA–Fe, CA–Cu and CA–H2O2, respectively. The initial TC solution had pH 7.5. Figure 5 shows the effect of pH on the adsorption assay performed with activated carbon and the three composites from tetracycline synthetic solution. Over the whole pH range (2–10), the adsorbed amount of TC was higher under acid or neutral conditions. This pH effect is closely associated with the character of TC. The molecule of this antibiotic has an zwitterion point between pH 4 and 6, behaving as cation (H3Lþ) when the pH of the
Figure 4. Composite diffractograms before and after tetracycline adsorption compared to untreated granular activated carbon. Samples based on FeCl3 (a), CuSO4 (b) and H2O2 (c).
solution is below this value, and as anions (HL and L2) when the pH of the solution is above the pH same.[25] According to the distribution of TC chemical species, their solutions are quite stable at neutral or weakly acidic pH values, as can be seen in Figure 6.[26] In general, the removal of TC in acidic media is more efficient and is attributed to the cation exchange mechanism.[27] This fact was recorded by Khanday and Hameed,[28] He et al.[29] and Lian et al.[30] The cited authors related the adsorption of TC on coal, mainly with electrostatic interactions and p-p interactions. Among the characteristics, it is important to consider the zero charge point (pHpcz) of the adsorbents. In this study,
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L. R. D. C. COSTA AND L. A. FÉRIS
Figure 5. Effect of pH on tetracycline adsorption assay using composites as adsorbent. Conditions: dosage of sorbent 10 g.L1, contact time 30 minutes, 20 mg.L1 tetracycline in 100 mL solution.
Figure 6. Tetracycline ionization balance as a function of solution pH.
solids CA, CA–Cu, Ca–Fe and Ca–H2O2 showed pHpcz values of 7.5, 5.5, 5.4 and 5.5, respectively. Note that the conventional solid (CA) is slightly alkaline (7.47). At this value, the material acts as a buffer solution. Thus, when in a solution the pH < pHpcz of the adsorbent, the surface is positively charged which favors the anion adsorption, but if in the solution the pH > pHpcz of the adsorbent then the surface is negatively charged, thus favoring the adsorption of cations.[31] After chemical functionalization, it is possible to verify that the adsorbents showed a greater tendency to acidity. This was due to the introduction of more acidic functional groups on the surface of the coal and perhaps to a possible decomposition of the elements present in it.[32] From Figure 5, it can be seen that for CA there was an increase in TC adsorption capacity with the increase of acid degree of the solution. This behavior may be due to the cationic character of TC at lower pH and also by the pHpcz of the adsorbent. As the pH increases, the negative charge density of activated carbon increases and electrostatic repulsion plays a major role between activated charcoal surface charges and the anionic character of TC, causing a decrease in adsorption capacity. For this solid, the pH chosen as the most appropriate was pH 4, which presented a higher percentage of TC removal compared to the others (54%). A similar trend was found by Saygili and Guzel[15] studying the adsorption of TC in activated carbon obtained from tomato industrial processing residues. The authors observed
Figure 7. Adsorption capacity according to contact time. Conditions: Natural pH for composites, pH 4.0 for CA, 10 g.L1 CA, CA–Cu, CA–Fe and CA–H2O2 adsorbent dosage, 20 mg.L1 tetracycline in 100 mL solution.
that as the pH of the solution increased, the adsorbent gradually gained more adsorption capacity to the maximum adsorption point (around the isoelectric point of TC). In contrast, the additional increase in this pH led to a slight decrease in the adsorption capacity of adsorbent solids. The adsorption capacity of TC remains constant throughout the pH range evaluated for CA–H2O2. This result can be explained by the acidification of the medium by hydrogen peroxide present in the activated carbon pores, making the pH variation more difficult to occur. In addition, electrostatic interactions between the adsorbent surface and tetracycline may have been disadvantaged. For this reason, the character of the adsorbate and the available binding sites are important for adsorption. For CA–Fe and CA–Cu composites, TC adsorption is much higher than CA and CA–H2O2 adsorption. The impregnation of the surface with copper and iron introduces more acidic functional groups in the activated carbon, forming several intramolecular hydrogen bonds through cation chelation and, being able to provide more active surface sites for adsorption, increasing its capacity. Also, as mentioned in item 3.1, these two composites have a larger surface area and volume of mesoporous compared to the other adsorbents. From the behavior of the TC molecule in relation to the pH and adsorbent solids used, even though the highest percentages of removal for tetracycline occurred near the isoelectric point of the molecule, it was considered as optimum pH for continuity of the study, the natural pH of the molecules. solutions for all composites, where it was possible to obtain 61%, 49% and 26% removal percentages for CA–Cu, CA–Fe and CA–H2O2. Thus, these composites stand out in relation to activated charcoal without treatment, since it was not necessary to perform pH adjustment to obtain good TC removal percentages. Determination of contact time Figure 7 shows the influence of contact time on the percentage of tetracycline removal from solution. According to these results, by applying CA–Cu adsorbent, the removal increased rapidly in the first 5 minutes and subsequently stabilized over time. The same can be observed for
JOURNAL OF ENVIRONMENTAL SCIENCE AND HEALTH, PART A
Figure 8. Adsorption capacity by varying the concentration of the adsorbent solid. Conditions: Natural pH for composites, contact time of 120 min CA–Cu, 200 min CA–Fe and 210 min CA–H2O2, 20 mg L1 tetracycline in 100 mL of solution.
CA–Fe adsorbent. The rapid adsorption of tetracycline on the solid surface of these two composites indicates a higher affinity of these solids for tetracycline. Metal ions in the adsorbent have been reported to primarily promote the removal of antibiotic adsorption through the cationic bonding bridge.[33] According to Wang et al.,[34] tetracycline can be bonded to metal ions to form an antibiotic-metal complex due to its electron donor groups, and can also form antibiotic-metal-adsorbent complexes by chelation with the adsorbent surface functional groups. The impregnation of the solid with hydrogen peroxide results in an increase of oxygen-containing functional groups on its surface, providing the formation of hydrogen bonds.[19,35] Note that CA–H2O2 adsorbent showed slightly different kinetics, with slower initial adsorption, probably caused by lower affinity for the compound. The same behavior was observed for untreated activated charcoal. Based on the obtained data, 120 min was defined as the optimum contact time between adsorbent-adsorbate for CA and CA–Cu (removal of 55.4 and 75.8%, respectively). For CA–Fe and CA–H2O2, the time chosen was 200 and 210 min respectively, reaching removals of 76.2% and 61.8%. Under these conditions, a TC adsorption capacity of 1.15, 1.39, 1.37 and 1.10 mg.g1 was obtained for CA, CA–Cu, CA–Fe and CA–H2O2, respectively. Importantly, the tradeoff between maximum removal capacity and the time required to determine the best operating conditions must be taken into account. In all cases, after gradual pore filling, removal efficiency did not show such a significant increase. Ahmed and Theydan[36] evaluated the removal of Ciprofloxacin and Norfloxacin with an activated charcoal obtained from Albizia seeds. The authors observed that from 30 minutes after removal of almost 80% for the compounds, this removal efficiency did not increase significantly in later times. The same was reported by Pacheco[37] in the removal of Diclofenac and Ibuprofen by adsorption on anionic clays. Making a direct comparison with untreated activated carbon, considering the contact time of 120 minutes, CA–Cu and CA–Fe had a higher adsorption capacity. This is believed to have occurred thanks to the role of the bridge that is played by the metal ions present on the surface of
7
Figure 9. Adsorption capacity by varying the concentration of the adsorbent solid. Conditions: pH 4.0, 120 min CA contact time, 20 mg L1 tetracycline in 100 mL solution.
the solids, providing a better affinity of the adsorbent with tetracycline. In turn, CA–H2O2, despite having a behavior lower than CA at the same time evaluated, obtained a significant TC adsorption capacity at longer times. Determination of adsorbent concentration The effect of adsorbent concentration on tetracycline adsorption is shown in Figure 8. It can be seen that there is an increase in tetracycline removal efficiency of 20 to 80% with the amount of solid increase from 1 to 20 g.L1. Probably this is due to the increase in the specific amount of sorbent which, consequently, increases the number of available sites on the surface of the coal for adsorption. The adsorbent concentration of 15 g.L1 was selected as optimium for CA–H2O2, culminating in a removal percentage of 78.02%. While for CA–Cu and CA–Fe, 5 and 10 g.L1 were selected respectively, with removals around 75.2%. In all cases, the lowest concentration among those with the best result was chosen. This choice was based on the cost associated with synthesis for this type of material and also on the lower generation of waste, making it interesting to select the lowest possible dosage as the ideal operating point. The Husk[38] adsorption study on the effect of adsorbent dosing for tetracycline removal using melon peel activated charcoal showed that the removal increased from 50 to 80% with the adsorbent dose from 0.2 to 1.0 g. Figure 8 also shows that as the adsorbent concentration increases in the system, while the removal of TC gradually increases. However, after a given concentration, the percentage of removal becomes constant, caused by the saturation of the materials. In general, the CA–Cu adsorbent needs less mass to saturate (5 g.L1), probably for the highest exchange affinity. Considering its electronic structure, Cu contains empty orbitals that can accept electrons in the tetracycline molecule. This shows that the interactions are superficial, agreeing with the ion exchange results. Furthermore, it has been reported in the literature that this compound could synergistically improve the adsorption and removal of tetracycline from aqueous solutions.[34,39,40] Figure 9 shows the behavior of the adsorbent concentration for untreated activated charcoal by way of comparison.
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L. R. D. C. COSTA AND L. A. FÉRIS
Figure 10. Experimental kinetic curves with pseudo-first and second order adjustments for tetracycline removal. Table 2. Parameters of the kinetic models for the adsorption of tetracycline. Pseudo-first order CA
qexp¼ 1.28 mg.g1
CA–Cu qexp¼ 1.44 mg.g1 CA–Fe qexp¼ 1.29 mg.g1 CA–H2O2 qexp¼ 1.09 mg.g1
Qe (mg.g1) K1 (min1) R2 v2 Qe (mg.g1) K1 (min1) R2 v2 Qe (mg.g1) K1 (min1) R2 v2 Qe (mg.g1) K1 (min1) R2 v2
1.24 0.030 0.981 0.003 1.38 0.286 0.992 0.001 1.24 0.075 0.973 0.003 1.06 0.017 0.980 0.004
Pseudo-second order K2
K2
K2
K2
Qe (mg.g1) (g.mg1.min1) R2 v2 Qe (mg.g1) (g.mg1.min1) R2 v2 Qe (mg.g1) (g.mg1.min1) R2 v2 Qe (mg.g1) (g.mg1.min1) R2 v2
1.39 0.106 0.997 0.000 1.41 2.058 0.993 0.001 1.31 0.279 0.995 0.001 1.30 0.038 0.991 0.002
It is noticed that with the increase of the adsorbent solid concentration in the system, the removal of the studied drug gradually increases until, from a given solid concentration, the percentage of removal became approximately constant, stabilizing with a solid concentration 30 g.L1 adsorbent. At this concentration, the removal reaches a maximum value of 92.7% with a residual TC concentration of 1.45 mg.L1. The adsorption performance of CA–Fe, CA–Cu and CA–H2O2 shown in Figure 8 indicates that the amount of adsorbed tetracycline does not depend on the specific surface area of the adsorbent. CA–Cu presented the highest tetracycline uptake, despite the smaller surface area compared to the others. Compared to activated carbon, the composites provided a reduction of at least 50% adsorbent mass
for tetracycline adsorption. This large difference suggests that tetracycline removal depends on the nature of the sites rather than the amount of sites available. Therefore, the surface modification of the adsorbent solid must be taken into account. Study of the tetracycline sorption mechanism Figure 10 shows the experimental kinetic curve for TC sorption in (a) CA, (b) CA–Cu, (c) CA–Fe e (d) CA–H2O2, as well as the pseudo-first and second order adjustments (Eqs. 3 and 4). The data fits well into the pseudo-second order model for the four solids. The second order equation in relation to the available fraction of active sites, assumes the dissociation adsorbed on the adsorbent surface. From the adjustment, the values of the initial sorption rate, qexp, the reaction rate constant, k, and the amount of TC absorbed at equilibrium, qe (Table 2), were obtained. The loading with hydrogen peroxide is similar to the conventional solid, with a contact time of at least 175 minutes. The kinetic curve reflects a fast sorption for solids modified by iron chloride and copper sulfate, with about 40 to 60% of the drug being absorbed in the first 5 minutes, suggesting that sorption occurs preferably through the retention of the compound by the active groups present on the surface of the adsorbent solid and not by diffusion. Finally, these results demonstrated that the sorption of TC in the adsorbents is possibly controlled by the chemisorption mechanism, involving valence forces through the sharing or exchange of electrons between the adsorbent and the adsorbate.[41]
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Table 3. Parameters of the isothermal models for the adsorption of tetracycline. CA–H2O2
CA
L F
Qm KL R2 KF N R2
CA–Fe
CA–Cu
25 C
35 C
45 C
25 C
35 C
45 C
25 C
35 C
45 C
25 C
35 C
45 C
22,89 0,003 0,988 0,66 2,12 0,959
24,90 0,004 0,999 0,82 2,20 0,959
29,58 0,006 0,991 1,50 2,39 0,963
34,50 0,008 0,990 1,81 2,30 0,896
49,11 0,022 0,999 5,40 2,97 0,922
56,78 0,025 0,994 7,45 3,28 0,921
45,82 0,015 0,992 3,56 2,50 0,950
65,45 0,011 0,999 3,93 2,38 0,922
71,57 0,019 0,995 5,20 2,44 0,921
41,47 0,006 0,997 1,72 2,20 0,967
61,61 0,010 0,999 3,89 2,53 0,91
72,93 0,010 0,985 6,62 2,92 0,894
L ¼ Langmuir; F ¼ Freundlich; (mg.g1); (L.mg1); (mg.g1).(L.mg1)1/n). Table 4. Comparison of the maximum adsorption of TC (qm) with several adsorbents declared in literature. Adsorbent Activated carbon from melon peel HCl-modified zeolite Bamboo charcoal CA Biochar derived from rice straw Biochar impregnated with H2O2 CA–H2O2 Cu-immobilized alginate CA–Fe CA–Cu Nanosheet-layered double hydroxide Pistachio shell powder coated with ZnO nanoparticles
qm (mg.g1)
Reference
16.0 20.4 23.5 26.3 32.0 42.5 52.0 53.3 62.2 64.1 98.0 98.7
[38] [44] [45] This study [11] [19] This study [46] This study This study [47] [48]
Li et al.[18] in the preparation of activated carbons from Iris tectorum using ferric nitrate to remove tetracycline from aqueous solutions found that the pseudo-second order model combines with the experimental data with higher R2 values (0.997) for both the pure solid and then of impregnated. In addition, the authors approached that the chemical model was based on the assumption that the rate limits were affected through the sharing of electrons or covalent forces between adsorbent and adsorbed.[42] Likewise, Saygili and Guzel[15] when explaining the kinetics of TC adsorption on activated carbon from tomato residues by chemical activation of ZnCl2, concluded that, as in the present study, the R2 values of the pseudo-second order model (0.989–0.996) were better compared to those of the pseudo-first order (0.971–0.985). Table 3 shows the results of the isothermal parameters obtained through non-linear analysis, for TC adsorption in the four adsorbents used in the present study, at 25, 35 and 45 C temperatures. The results related to Freundlich showed a large deviation from the data, demonstrating a lesser correlation with the equilibrium data at the end of the curve, considering that his model predicts the continuous increase of the adsorbed quantity, which is not observed in the experimental data. For the case under study, it is clearly identified that adsorption in all cases follows the Langmuir model, assuming that initially the chemisorption occurs superficially, with the distribution of the tetracycline molecules along the plane of the adsorbent solids.[43] As previously mentioned, the adsorbent adsorption capacity was calculated according to Eq. (2), varying the tetracycline concentration and keeping the amount of adsorbent constant. From the results, we have the sequence of maximum adsorption capacity for tetracycline among the adsorbents follows the order: CA–Cu
(64,12 mg.g1) > CA–Fe (62,15 mg.g1) > CA–H2O2 (51,96 mg.g1) > CA (26,27 mg.g1). Table 4 compared qm for TC in several adsorbents mentioned in previous studies. Based on the results presented in this table, the impregnated solids exhibited a favorable performance for the elimination of TC molecules in the adsorption treatment process when compared to other adsorbents. Similar analyze was performed by Mohammed et al.[48,49] for simultaneous adsorption of TC and other drugs in pistachio shell powder coated with zinc oxide nanoparticles.
Conclusion In general, all composites prepared in the present work showed high emergent pollutant removal capacity. Compared to untreated activated carbon, the composites stand out, since pH adjustment was not necessary, had a faster initial kinetics and provided a reduction of at least 50% of adsorbent mass (15, 10 and 5 g.L1 for CA–H2O2, CA–Fe and CA–Cu). Among them, the best result was obtained with CA–Cu solid, with removal above 75%, with contact time of 120 min and solid concentration of 5 g.L1. The proposed mechanism for the sorption of TC in adsorbents is that of chemisorption, referring to the kinetic model of pseudo-second order. Additionally, no compromise of the structural characteristics of the adsorbent support material was observed during the XRD, FTIR and SEM characterization of the adsorbents before and after tetracycline removal.
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Use of functinalized adsorbents for tetracycline removal in wastewater: adsorption mechanism and comparison with activated carbon Letícia Reggiane de Carvalho Costa & Liliana Amaral Féris To cite this article: Letícia Reggiane de Carvalho Costa & Liliana Amaral Féris (2020): Use of functinalized adsorbents for tetracycline removal in wastewater: adsorption mechanism and comparison with activated carbon, Journal of Environmental Science and Health, Part A, DOI: 10.1080/10934529.2020.1827654 To link to this article: https://doi.org/10.1080/10934529.2020.1827654
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JOURNAL OF ENVIRONMENTAL SCIENCE AND HEALTH, PART A https://doi.org/10.1080/10934529.2020.1827654
Use of functinalized adsorbents for tetracycline removal in wastewater: adsorption mechanism and comparison with activated carbon Letıcia Reggiane de Carvalho Costa and Liliana Amaral Feris Department of Chemical Engineering, Federal University of Rio Grande do Sul, Porto Alegre, Brazil ABSTRACT
ARTICLE HISTORY
The present study investigated the application of functinalized adsorbents in the removal of tetracycline in water by adsorption. Activated carbon was impregnated with iron chloride, copper sulfate and hydrogen peroxide, in order to improve its adsorption properties. The solids were characterized by adsorption/desorption isotherms N2, XRD, FTIR and SEM. The influence of pH, adsorbent concentration and contact time parameters was evaluated. The results using activated carbon as adsorbent and initial TC concentration of 20 mg.L1 showed that 93% TC removal was achieved at pH 4.0, contact time of 120 minutes and 30 g.L1 adsorbent concentration. Experiments applying the functionalized solids showed a faster initial adsorption kinetics. Also, no pH adjustment was required (pH 6.0). In addiction, a minimum of 50% reduction in adsorbent mass (15, 10 and 5 g.L1 for CA–H2O2, CA–Fe and CA–Cu, respectively) was observed, compared to the untreated solid. The pseudo-second-order kinetic model was chosen as the optimal model for the study. The application of FeCl3, CuSO4 and H2O2 composites as adsorbent solids are technically feasible for the removal of tetracycline in wastewater treatment.
Received 18 May 2020 Accepted 19 September 2020
Introduction Drugs generated by domestic, commercial, rural, industrial and medical-hospital human activities, when improperly discharged, cause negative impacts on the physical, chemical and biological characteristics of the natural environment, especially water and soil, endangering health and health survival of humans.[1,2] Many of these substances, known as emerging contaminants (CE), are often found in aqueous matrices at concentrations in the range of mg.L1 and ng.L1.[3] Because they have low biodegradability, high persistence and ease of bioaccumulation, there is a high resistance of these compounds to conventional treatment processes, being difficult to remove them. In addition, the presence of residual drugs in surface water may be indicative of effluent contamination from treatment plants.[4] Among the groups in the drug category, antibiotics stand out, considered the largest drug category provided by human and veterinary medicine, for therapeutic purposes or as growth promoters.[5] Tetracycline (TC), produced from strains of microorganisms or as a semi-synthetic product, represents a broad spectrum spectrum of antibiotics.[6] Tetracycline was found in surface and groundwater and in 80% of effluent samples. Yao et al.[7] investigated the occurrence of 14 antibiotics in ground and surface water on Jianghan Plain in China. In groundwater evaluated at 10, 25 and 50 m depth, the fluoroquinolones and tetracyclines antibiotics were predominant. Zhang et al.[3] revealed that
KEYWORDS
Adsorption; emerging pollutants; impregnation; composites; granular activated carbon
tetracycline, as a popular antibiotic, may be present in the environment at concentrations up to 110 lg.L1. The product has also been found in some foods, such as eggs, milk and fish s.[8] In addition, high concentrations of this drug were found in soil and sediment matrices, indicating a tendency of accumulation, persistence and also strong sorption characteristics.[6,9] Many processes for the removal or reduction of TC from wastewater containing these micro-pollutants are reported in the literature as advanced oxidation, photocatalytic degradation, electrochemical treatments, adsorption, and so on.[10–16] Adsorption is still considered one of the most promising means for the elimination of tetracyclines from the aquatic environment due to its advantages of simple operation, low investment cost.[17] The solids used in this type of process can be functionalized according to their use, introducing specific characteristics in order to intensify the adsorption capacity of the material in the removal of certain pollutants. Some studies discuss the relationship between adsorption performance and surface treatment of adsorbent for TC removal. Among them, Li et al.[18] studied the efficiency of activated carbon as adsorbent prepared from Iris tectorum with and without modification by iron nitrate. The authors observed that the modification increased tetracycline adsorption capacity by 23% (769.2 mg.g1) compared to unmodified activated carbon (625.0 mg.g1). Likewise, the modification of activated carbon derived from brassica
CONTACT Letıcia Reggiane de Carvalho Costa [email protected] Laboratory of Separation and Unit Operations (LASOP), Department of Chemical Engineering, Federal University of Rio Grande do Sul, Ramiro Barcelos Street, 2777, 90035-007, Porto Alegre, RS, Brazil. ß 2020 Taylor & Francis Group, LLC
2
L. R. D. C. COSTA AND L. A. FÉRIS
napus L. (rapeseed) stem residue by H2O2 by Tan et al.[19] improved TC adsorption by approximately 20% compared to the untreated solid (26.3–35.9 mg.g1), reaching an adsorption capacity between 32.0 and 42.5 mg.g1 at pH 9. In this context, the present work presents the preparation and use of composites based on highly dispersed activated carbon and iron chloride, copper sulfate and hydrogen peroxide in the removal of tetracycline drug in water. Adsorption tests were performed to maximize the removal of the tetracycline (TC) pollutant.
The tetracycline (TC) concentration in both the initial and the adsorbent solutions were determined by the ultraviolet (UV-visible) spectrophotometry method using the Thermo Scientific, Genesys 10S UV-Vis equipment at the TC detection wavelength, 357 nm. The conversion of absorbance to concentration was obtained by Eq. (1), with the linear coefficient obtained from the tetracycline calibration curve:
Experimental procedure
where C is the solute concentration (mg.L1); A is the absorbance at 357 nm; and a is the linear coefficient of the calibration curve. The adsorbents capacity was calculated by Eq. (2) and, the experiments were carried out in different concentrations of tetracycline (0–1000 mg.L1), keeping the amount of adsorbents constant.
Preparation of activated carbon composites The composites were prepared separately by impregnating with iron chloride (CA–Fe), copper sulfate (CA–Cu) and hydrogen peroxide (CA–H2O2) from a aqueous suspension containing commercial activated carbon (Exodus), FeCl3 (2% w/v), CuSO4 (2% w/v) and H2O2 (30% v/v) at 25 C for 24 hours. After this time, the material was filtered and the composites were oven dried for 4–6 hours at 100 C. Characterization of adsorbents The synthesized composites were characterized by X-ray diffraction (XRD) using a Bruker D2 Phaser diffractometer with angular variation of 5 –90 (2h), CoKa radiation (k ¼ 1,542 Å) and exposure velocity 0.05 2h sec1. The morphological analysis of the materials was performed by scanning electron microscopy (SEM). Physical adsorption/ desorption tests of N2 at 77 K were also performed, and the specific surface area was calculated by the BET method and the pore volume and diameter were calculated using the Barrett-Joyner-Halenda (BJH) equation. The infrared spectra (IR) of the samples were obtained from a Nicolet 6700 spectrophotometer at a wavelength range of 4000–400 cm1 at room temperature. Adsorption experiments Tetracycline was purchased from Sigma-Aldrich, showing purity 98.0%. Adsorption tests were performed in batch using 100 mL volumes of solution. Tetracycline solution (20 mg.L1) was prepared from a concentrated solution by diluting with distilled water. The diluted solution was placed in a Schott flask and allowed to stir in Wagner Shaker (MA160BP, Marconi brand) at 30 rpm, according to the methodology adopted by de Franco[20] and Haro.[21] For each test, the adsorbent was added to the solution, the pH was adjusted and the process parameters to obtain the best experimental conditions were evaluated. All experiments were performed in duplicate and the standard deviation was calculated to evaluate the dispersion of the experimental points. A comparative study with untreated activated carbon was performed under the same conditions for all tests conducted.
C¼
Qe ¼
A a
ðCi Ce ÞV m
(1)
(2)
where Ci is the initial drug concentration (mg.L1), Ce the equilibrium drug concentration (mg.L1), V the volume of solution (L) and m the mass of solid (mg). The tetracycline removal efficiency was evaluated by pH, contact time and solid dosage parameters by the adsorption process. In addition, all experiments were performed at room temperature. Error correction caused by possible leakage of salts and hydrogen peroxide in solution, due to their hydrophilic character, was performed by reading the solution before and after adsorption, discounting the effect of the interferents. Determination of solution pH For solution pH study, the contact time was maintained for 30 minutes for a constant mass of 1 g of composite. Experiments were performed varying the solution pH (2–10) and adjusting it when necessary using 0.1 M HCl or NaOH solutions. Determination of contact time The contact time study was performed keeping the concentration of sodium constant, with the pH of the being mixture adjusted to the value where the highest removal rate occurred in the previous test. Sorption times ranging from 5 to 300 minutes were evaluated, and the best contact time was the one with the highest removal of the drug studied. Determination of adsorbent concentration To evaluate the effect of solid concentration on tetracycline adsorption, experiments were carried of using different concentrations of composites, ranging from 0.5 to 20 g.L1. The pH and adsorption time used were considered optimum in in previous sections.
JOURNAL OF ENVIRONMENTAL SCIENCE AND HEALTH, PART A
3
Table 1. Surface areas and porosity characteristics of CA, CA–Fe, CA–Cu, CA–H2O2, CA–Fe–TC, CA–Cu–TC and CA–H2O2–TC. Parameters Solid CA CA–Fe CA–Cu CA–H2O2 CA–Fe–TC CA–Cu–TC CA–H2O2–TC
2
1 a
SBET (m .g ) 508.1 555.8 533.5 581.8 553.8 532.1 536.0
SMES (m .g1)b 285.1 335.8 314.2 356.9 328.2 318.6 301.2 2
VMES (cm3.g1)c 0.10 0.12 0.11 0.13 0.11 0.12 0.11
TMD (Å)d 4.8 4.8 4.8 4.8 4.8 4.8 4.8
a
BET surface area, mesoporous surface area, mesoporous volume, d average pore size. b c
Figure 1. N2 adsorption/desorption isotherms and pore distribution to materials before and after TC adsorption.
Results and discussions Study of the tetracycline sorption mechanism Adsorption kinetics was evaluated after fitting the experimental data to the kinetic models of pseudo-first and pseudo-second order, in non-linear form, described in Eqs. (3) and (4), respectively. dqt ¼ k1 ðq 1 q t Þ dt
(3)
dqt ¼ k2 ðq2 qt Þ2 dt
(4)
where q1,2 and qt are the adsorbed quantities (mg.g1) at equilibrium and at time t, respectively, k1 is the adsorption rate constant (min1), k2 is the adsorption rate constant (g .mg1.min1) et is the time in minutes. For the isothermal study, the two models examined in this study were Langmuir (Eq. 5) and Freundlich (Eq. 6). These models were used in their non-linear form, even that linear methods are still seen in much of the scientific literature. Q¼
qm KL Ce 1 þ KL Ce
Q ¼ KF Ce=n 1
(5)
(6)
where, Q is the amount adsorbed at equilibrium (mg.g1), Ce is the concentration of the drug at equilibrium (mg.L1), qm is the maximum adsorption capacity (mg.g1); KL the Langmuir constant related to the affinity between the adsorbent and adsorbate (L.mg1); KF is the Freundlich constant related to the relative adsorption capacity (mg.g1) and n the heterogeneity factor (g.L1). In kinetic and isothermal analyzes, the parameters of each model were determined by non-linear regression analysis using the Microsoft Excel resolution tool. The effectiveness of the models with the experimental equilibrium data was detected by the values of the coefficient of determination (R2).
Characterization of adsorbents Figure 1 shows the textural characterization of activated carbon and composites. Additionally, Table 1 shows the specific values obtained by this characterization step. From Figure 1 and the results shown in Table 1, it can be seen that impregnation of the adsorbent by iron chloride, copper sulfate and hydrogen peroxide causes an increase in the specific surface area of the material compared to the untreated solid (CA, 508.1 m2.g1), which suggests a low compromise of the pores present in activated charcoal used as support for the composites. All materials have characteristics of complex materials containing micro and mesopores in their structure and, consequently, present variations in hysteresis. According to the classification of the International Union of Pure and Applied Chemistry (IUPAC), the curves obtained for the analyzed solids show a behavior similar to type I isotherms, indicating an adsorption in micropores. According to the empirical classification of hysteresis curves, it can be said that hysteresis is of the H4 type, representative of materials with ordered and uniform pores. The BET area and pore volume for CA–H2O2 (581.8 m2.g1 and 356.9 cm3.g1, respectively) was larger than in the other samples, while pore diameter was similar for all. In fact, analyzing the graph of pore distribution of the materials, shown in Figure 1, it can be seen that the composites before TC adsorption have a profile very similar to activated carbon with slight decrease of adsorbed N2. Additionally, the pore diameter indicates that the adsorbent solids were mainly microporous. After adsorption, a slight decrease in specific surface area and volume of mesopores is noted, probably caused by the filling of the active sites with the TC molecule. For metalimpregnated solids (CA–Fe and CA–Cu), this decrease was not so significant, so it is believed that this mild decrease in surface area and mesopore volume is due to merely superficial interactions between the adsorbent and tetracycline due to the antibiotic-metal complex formed between the two materials. In addition, evacuation prior to BET measurements can remove most of the adsorbed TC, due to the lack of chemical bonding in the adsorption mechanism. Figure 2 shows the SEM images of all materials.
4
L. R. D. C. COSTA AND L. A. FÉRIS
Figure 2. SEM images of solids before and after tetracycline adsorption. 30 k magnification and 50 lm scale for all images. Samples based on FeCl3 (a), CuSO4 (b) and H2O2 (c).
In general, the morphology of the solid materials analyzed by SEM showed particles with irregular grains. It is also observed that there is a greater presence of agglomerate on the surface of the samples, which confers a larger specific surface area, contributing to the adsorption. Physical changes can be observed in the adsorbent materials after adsorption compared to the solid initially with fully filled and prominently swollen pores. This indicates that the drug is adsorbed to both the functional groups present on the surface and/or interior of the pores. The vibrational spectra of the solids are shown in Figure 3, before and after the adsorption of TC. In it, it is revealed that the three strongly absorbent peaks at about 3500–3000, 1400 and 1100 cm1 are respectively assigned to the OH, CH and CO stretch frequency to all samples. The difference
observed between the spectra in the region of 3000 and 3500 cm1 is associated with the presence of OH groups, related to KBr humidity, and to the CH2 and CH groups of the aliquot. In the case of composites, although this first band is evident, it was smooth and better defined, which may be related to the moisture loss of the adsorbent in question. The spectrum at 1650 cm1 CA is the result of C ¼ C skeletal vibrations in aromatic rings.[22] A reduction in this spectrum is noted after solid functionalization, probably caused by bond breakage. In addition, analyzing the spectrum after the adsorption of TC in all adsorbents, it is noted that, possibly, this is also the function group responsible for the drug adsorption phenomenon, together with the OH, CH and CO groups (peaks 1400–1100 cm1 ).
JOURNAL OF ENVIRONMENTAL SCIENCE AND HEALTH, PART A
5
Figure 3. FTIR spectra of solids before and after tetracycline adsorption compared to conventional granular activated carbon.
Compared with CA, the intensities of the modified solids bands (CA–H2O2, CA–Fe and CA–Cu) were weaker, which may suggest that impregnating solutions interact or overlap with the CA surface groups. In addition, peak changes indicate that the surface chemical status of the corresponding groups changes after the impregnation process. The XRD profiles of adsorbents before and after tetracycline adsorption are illustrated in Figure 4 and are compared to untreated activated carbon. All materials exhibited reflections at 2h angles near 25 and 45 , corresponding to the carbon and graphite carbon planes respectively, characteristic of amorphous materials.[23] Compared to non-impregnated activated carbon (CA), a difference in the intensity of these characteristic peaks was observed, regardless of the material used, both in the impregnation phase and in the adsorption step by TC. Results show that the impregnation with iron chloride, copper sulfate and hydrogen peroxide did not compromise the structural characteristics of the adsorbent support material. However, it is important to highlight that highly dispersed materials with small particle size, as presented in this work, may alter the surface morphology of the sample, which justifies the difference in absorption intensity due to the higher content of surface mixtures.[24] Adsorption tests Determination of solution pH In this study, natural pH values were 6.7, 3.8, 4.8 and 5.5 for the adsorbents CA, CA–Fe, CA–Cu and CA–H2O2, respectively. The initial TC solution had pH 7.5. Figure 5 shows the effect of pH on the adsorption assay performed with activated carbon and the three composites from tetracycline synthetic solution. Over the whole pH range (2–10), the adsorbed amount of TC was higher under acid or neutral conditions. This pH effect is closely associated with the character of TC. The molecule of this antibiotic has an zwitterion point between pH 4 and 6, behaving as cation (H3Lþ) when the pH of the
Figure 4. Composite diffractograms before and after tetracycline adsorption compared to untreated granular activated carbon. Samples based on FeCl3 (a), CuSO4 (b) and H2O2 (c).
solution is below this value, and as anions (HL and L2) when the pH of the solution is above the pH same.[25] According to the distribution of TC chemical species, their solutions are quite stable at neutral or weakly acidic pH values, as can be seen in Figure 6.[26] In general, the removal of TC in acidic media is more efficient and is attributed to the cation exchange mechanism.[27] This fact was recorded by Khanday and Hameed,[28] He et al.[29] and Lian et al.[30] The cited authors related the adsorption of TC on coal, mainly with electrostatic interactions and p-p interactions. Among the characteristics, it is important to consider the zero charge point (pHpcz) of the adsorbents. In this study,
6
L. R. D. C. COSTA AND L. A. FÉRIS
Figure 5. Effect of pH on tetracycline adsorption assay using composites as adsorbent. Conditions: dosage of sorbent 10 g.L1, contact time 30 minutes, 20 mg.L1 tetracycline in 100 mL solution.
Figure 6. Tetracycline ionization balance as a function of solution pH.
solids CA, CA–Cu, Ca–Fe and Ca–H2O2 showed pHpcz values of 7.5, 5.5, 5.4 and 5.5, respectively. Note that the conventional solid (CA) is slightly alkaline (7.47). At this value, the material acts as a buffer solution. Thus, when in a solution the pH < pHpcz of the adsorbent, the surface is positively charged which favors the anion adsorption, but if in the solution the pH > pHpcz of the adsorbent then the surface is negatively charged, thus favoring the adsorption of cations.[31] After chemical functionalization, it is possible to verify that the adsorbents showed a greater tendency to acidity. This was due to the introduction of more acidic functional groups on the surface of the coal and perhaps to a possible decomposition of the elements present in it.[32] From Figure 5, it can be seen that for CA there was an increase in TC adsorption capacity with the increase of acid degree of the solution. This behavior may be due to the cationic character of TC at lower pH and also by the pHpcz of the adsorbent. As the pH increases, the negative charge density of activated carbon increases and electrostatic repulsion plays a major role between activated charcoal surface charges and the anionic character of TC, causing a decrease in adsorption capacity. For this solid, the pH chosen as the most appropriate was pH 4, which presented a higher percentage of TC removal compared to the others (54%). A similar trend was found by Saygili and Guzel[15] studying the adsorption of TC in activated carbon obtained from tomato industrial processing residues. The authors observed
Figure 7. Adsorption capacity according to contact time. Conditions: Natural pH for composites, pH 4.0 for CA, 10 g.L1 CA, CA–Cu, CA–Fe and CA–H2O2 adsorbent dosage, 20 mg.L1 tetracycline in 100 mL solution.
that as the pH of the solution increased, the adsorbent gradually gained more adsorption capacity to the maximum adsorption point (around the isoelectric point of TC). In contrast, the additional increase in this pH led to a slight decrease in the adsorption capacity of adsorbent solids. The adsorption capacity of TC remains constant throughout the pH range evaluated for CA–H2O2. This result can be explained by the acidification of the medium by hydrogen peroxide present in the activated carbon pores, making the pH variation more difficult to occur. In addition, electrostatic interactions between the adsorbent surface and tetracycline may have been disadvantaged. For this reason, the character of the adsorbate and the available binding sites are important for adsorption. For CA–Fe and CA–Cu composites, TC adsorption is much higher than CA and CA–H2O2 adsorption. The impregnation of the surface with copper and iron introduces more acidic functional groups in the activated carbon, forming several intramolecular hydrogen bonds through cation chelation and, being able to provide more active surface sites for adsorption, increasing its capacity. Also, as mentioned in item 3.1, these two composites have a larger surface area and volume of mesoporous compared to the other adsorbents. From the behavior of the TC molecule in relation to the pH and adsorbent solids used, even though the highest percentages of removal for tetracycline occurred near the isoelectric point of the molecule, it was considered as optimum pH for continuity of the study, the natural pH of the molecules. solutions for all composites, where it was possible to obtain 61%, 49% and 26% removal percentages for CA–Cu, CA–Fe and CA–H2O2. Thus, these composites stand out in relation to activated charcoal without treatment, since it was not necessary to perform pH adjustment to obtain good TC removal percentages. Determination of contact time Figure 7 shows the influence of contact time on the percentage of tetracycline removal from solution. According to these results, by applying CA–Cu adsorbent, the removal increased rapidly in the first 5 minutes and subsequently stabilized over time. The same can be observed for
JOURNAL OF ENVIRONMENTAL SCIENCE AND HEALTH, PART A
Figure 8. Adsorption capacity by varying the concentration of the adsorbent solid. Conditions: Natural pH for composites, contact time of 120 min CA–Cu, 200 min CA–Fe and 210 min CA–H2O2, 20 mg L1 tetracycline in 100 mL of solution.
CA–Fe adsorbent. The rapid adsorption of tetracycline on the solid surface of these two composites indicates a higher affinity of these solids for tetracycline. Metal ions in the adsorbent have been reported to primarily promote the removal of antibiotic adsorption through the cationic bonding bridge.[33] According to Wang et al.,[34] tetracycline can be bonded to metal ions to form an antibiotic-metal complex due to its electron donor groups, and can also form antibiotic-metal-adsorbent complexes by chelation with the adsorbent surface functional groups. The impregnation of the solid with hydrogen peroxide results in an increase of oxygen-containing functional groups on its surface, providing the formation of hydrogen bonds.[19,35] Note that CA–H2O2 adsorbent showed slightly different kinetics, with slower initial adsorption, probably caused by lower affinity for the compound. The same behavior was observed for untreated activated charcoal. Based on the obtained data, 120 min was defined as the optimum contact time between adsorbent-adsorbate for CA and CA–Cu (removal of 55.4 and 75.8%, respectively). For CA–Fe and CA–H2O2, the time chosen was 200 and 210 min respectively, reaching removals of 76.2% and 61.8%. Under these conditions, a TC adsorption capacity of 1.15, 1.39, 1.37 and 1.10 mg.g1 was obtained for CA, CA–Cu, CA–Fe and CA–H2O2, respectively. Importantly, the tradeoff between maximum removal capacity and the time required to determine the best operating conditions must be taken into account. In all cases, after gradual pore filling, removal efficiency did not show such a significant increase. Ahmed and Theydan[36] evaluated the removal of Ciprofloxacin and Norfloxacin with an activated charcoal obtained from Albizia seeds. The authors observed that from 30 minutes after removal of almost 80% for the compounds, this removal efficiency did not increase significantly in later times. The same was reported by Pacheco[37] in the removal of Diclofenac and Ibuprofen by adsorption on anionic clays. Making a direct comparison with untreated activated carbon, considering the contact time of 120 minutes, CA–Cu and CA–Fe had a higher adsorption capacity. This is believed to have occurred thanks to the role of the bridge that is played by the metal ions present on the surface of
7
Figure 9. Adsorption capacity by varying the concentration of the adsorbent solid. Conditions: pH 4.0, 120 min CA contact time, 20 mg L1 tetracycline in 100 mL solution.
the solids, providing a better affinity of the adsorbent with tetracycline. In turn, CA–H2O2, despite having a behavior lower than CA at the same time evaluated, obtained a significant TC adsorption capacity at longer times. Determination of adsorbent concentration The effect of adsorbent concentration on tetracycline adsorption is shown in Figure 8. It can be seen that there is an increase in tetracycline removal efficiency of 20 to 80% with the amount of solid increase from 1 to 20 g.L1. Probably this is due to the increase in the specific amount of sorbent which, consequently, increases the number of available sites on the surface of the coal for adsorption. The adsorbent concentration of 15 g.L1 was selected as optimium for CA–H2O2, culminating in a removal percentage of 78.02%. While for CA–Cu and CA–Fe, 5 and 10 g.L1 were selected respectively, with removals around 75.2%. In all cases, the lowest concentration among those with the best result was chosen. This choice was based on the cost associated with synthesis for this type of material and also on the lower generation of waste, making it interesting to select the lowest possible dosage as the ideal operating point. The Husk[38] adsorption study on the effect of adsorbent dosing for tetracycline removal using melon peel activated charcoal showed that the removal increased from 50 to 80% with the adsorbent dose from 0.2 to 1.0 g. Figure 8 also shows that as the adsorbent concentration increases in the system, while the removal of TC gradually increases. However, after a given concentration, the percentage of removal becomes constant, caused by the saturation of the materials. In general, the CA–Cu adsorbent needs less mass to saturate (5 g.L1), probably for the highest exchange affinity. Considering its electronic structure, Cu contains empty orbitals that can accept electrons in the tetracycline molecule. This shows that the interactions are superficial, agreeing with the ion exchange results. Furthermore, it has been reported in the literature that this compound could synergistically improve the adsorption and removal of tetracycline from aqueous solutions.[34,39,40] Figure 9 shows the behavior of the adsorbent concentration for untreated activated charcoal by way of comparison.
8
L. R. D. C. COSTA AND L. A. FÉRIS
Figure 10. Experimental kinetic curves with pseudo-first and second order adjustments for tetracycline removal. Table 2. Parameters of the kinetic models for the adsorption of tetracycline. Pseudo-first order CA
qexp¼ 1.28 mg.g1
CA–Cu qexp¼ 1.44 mg.g1 CA–Fe qexp¼ 1.29 mg.g1 CA–H2O2 qexp¼ 1.09 mg.g1
Qe (mg.g1) K1 (min1) R2 v2 Qe (mg.g1) K1 (min1) R2 v2 Qe (mg.g1) K1 (min1) R2 v2 Qe (mg.g1) K1 (min1) R2 v2
1.24 0.030 0.981 0.003 1.38 0.286 0.992 0.001 1.24 0.075 0.973 0.003 1.06 0.017 0.980 0.004
Pseudo-second order K2
K2
K2
K2
Qe (mg.g1) (g.mg1.min1) R2 v2 Qe (mg.g1) (g.mg1.min1) R2 v2 Qe (mg.g1) (g.mg1.min1) R2 v2 Qe (mg.g1) (g.mg1.min1) R2 v2
1.39 0.106 0.997 0.000 1.41 2.058 0.993 0.001 1.31 0.279 0.995 0.001 1.30 0.038 0.991 0.002
It is noticed that with the increase of the adsorbent solid concentration in the system, the removal of the studied drug gradually increases until, from a given solid concentration, the percentage of removal became approximately constant, stabilizing with a solid concentration 30 g.L1 adsorbent. At this concentration, the removal reaches a maximum value of 92.7% with a residual TC concentration of 1.45 mg.L1. The adsorption performance of CA–Fe, CA–Cu and CA–H2O2 shown in Figure 8 indicates that the amount of adsorbed tetracycline does not depend on the specific surface area of the adsorbent. CA–Cu presented the highest tetracycline uptake, despite the smaller surface area compared to the others. Compared to activated carbon, the composites provided a reduction of at least 50% adsorbent mass
for tetracycline adsorption. This large difference suggests that tetracycline removal depends on the nature of the sites rather than the amount of sites available. Therefore, the surface modification of the adsorbent solid must be taken into account. Study of the tetracycline sorption mechanism Figure 10 shows the experimental kinetic curve for TC sorption in (a) CA, (b) CA–Cu, (c) CA–Fe e (d) CA–H2O2, as well as the pseudo-first and second order adjustments (Eqs. 3 and 4). The data fits well into the pseudo-second order model for the four solids. The second order equation in relation to the available fraction of active sites, assumes the dissociation adsorbed on the adsorbent surface. From the adjustment, the values of the initial sorption rate, qexp, the reaction rate constant, k, and the amount of TC absorbed at equilibrium, qe (Table 2), were obtained. The loading with hydrogen peroxide is similar to the conventional solid, with a contact time of at least 175 minutes. The kinetic curve reflects a fast sorption for solids modified by iron chloride and copper sulfate, with about 40 to 60% of the drug being absorbed in the first 5 minutes, suggesting that sorption occurs preferably through the retention of the compound by the active groups present on the surface of the adsorbent solid and not by diffusion. Finally, these results demonstrated that the sorption of TC in the adsorbents is possibly controlled by the chemisorption mechanism, involving valence forces through the sharing or exchange of electrons between the adsorbent and the adsorbate.[41]
JOURNAL OF ENVIRONMENTAL SCIENCE AND HEALTH, PART A
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Table 3. Parameters of the isothermal models for the adsorption of tetracycline. CA–H2O2
CA
L F
Qm KL R2 KF N R2
CA–Fe
CA–Cu
25 C
35 C
45 C
25 C
35 C
45 C
25 C
35 C
45 C
25 C
35 C
45 C
22,89 0,003 0,988 0,66 2,12 0,959
24,90 0,004 0,999 0,82 2,20 0,959
29,58 0,006 0,991 1,50 2,39 0,963
34,50 0,008 0,990 1,81 2,30 0,896
49,11 0,022 0,999 5,40 2,97 0,922
56,78 0,025 0,994 7,45 3,28 0,921
45,82 0,015 0,992 3,56 2,50 0,950
65,45 0,011 0,999 3,93 2,38 0,922
71,57 0,019 0,995 5,20 2,44 0,921
41,47 0,006 0,997 1,72 2,20 0,967
61,61 0,010 0,999 3,89 2,53 0,91
72,93 0,010 0,985 6,62 2,92 0,894
L ¼ Langmuir; F ¼ Freundlich; (mg.g1); (L.mg1); (mg.g1).(L.mg1)1/n). Table 4. Comparison of the maximum adsorption of TC (qm) with several adsorbents declared in literature. Adsorbent Activated carbon from melon peel HCl-modified zeolite Bamboo charcoal CA Biochar derived from rice straw Biochar impregnated with H2O2 CA–H2O2 Cu-immobilized alginate CA–Fe CA–Cu Nanosheet-layered double hydroxide Pistachio shell powder coated with ZnO nanoparticles
qm (mg.g1)
Reference
16.0 20.4 23.5 26.3 32.0 42.5 52.0 53.3 62.2 64.1 98.0 98.7
[38] [44] [45] This study [11] [19] This study [46] This study This study [47] [48]
Li et al.[18] in the preparation of activated carbons from Iris tectorum using ferric nitrate to remove tetracycline from aqueous solutions found that the pseudo-second order model combines with the experimental data with higher R2 values (0.997) for both the pure solid and then of impregnated. In addition, the authors approached that the chemical model was based on the assumption that the rate limits were affected through the sharing of electrons or covalent forces between adsorbent and adsorbed.[42] Likewise, Saygili and Guzel[15] when explaining the kinetics of TC adsorption on activated carbon from tomato residues by chemical activation of ZnCl2, concluded that, as in the present study, the R2 values of the pseudo-second order model (0.989–0.996) were better compared to those of the pseudo-first order (0.971–0.985). Table 3 shows the results of the isothermal parameters obtained through non-linear analysis, for TC adsorption in the four adsorbents used in the present study, at 25, 35 and 45 C temperatures. The results related to Freundlich showed a large deviation from the data, demonstrating a lesser correlation with the equilibrium data at the end of the curve, considering that his model predicts the continuous increase of the adsorbed quantity, which is not observed in the experimental data. For the case under study, it is clearly identified that adsorption in all cases follows the Langmuir model, assuming that initially the chemisorption occurs superficially, with the distribution of the tetracycline molecules along the plane of the adsorbent solids.[43] As previously mentioned, the adsorbent adsorption capacity was calculated according to Eq. (2), varying the tetracycline concentration and keeping the amount of adsorbent constant. From the results, we have the sequence of maximum adsorption capacity for tetracycline among the adsorbents follows the order: CA–Cu
(64,12 mg.g1) > CA–Fe (62,15 mg.g1) > CA–H2O2 (51,96 mg.g1) > CA (26,27 mg.g1). Table 4 compared qm for TC in several adsorbents mentioned in previous studies. Based on the results presented in this table, the impregnated solids exhibited a favorable performance for the elimination of TC molecules in the adsorption treatment process when compared to other adsorbents. Similar analyze was performed by Mohammed et al.[48,49] for simultaneous adsorption of TC and other drugs in pistachio shell powder coated with zinc oxide nanoparticles.
Conclusion In general, all composites prepared in the present work showed high emergent pollutant removal capacity. Compared to untreated activated carbon, the composites stand out, since pH adjustment was not necessary, had a faster initial kinetics and provided a reduction of at least 50% of adsorbent mass (15, 10 and 5 g.L1 for CA–H2O2, CA–Fe and CA–Cu). Among them, the best result was obtained with CA–Cu solid, with removal above 75%, with contact time of 120 min and solid concentration of 5 g.L1. The proposed mechanism for the sorption of TC in adsorbents is that of chemisorption, referring to the kinetic model of pseudo-second order. Additionally, no compromise of the structural characteristics of the adsorbent support material was observed during the XRD, FTIR and SEM characterization of the adsorbents before and after tetracycline removal.
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