Xuetao Guoa,
Jing Zhanga,
Jianhua Gea,
Chen Yang*b,
Zhi Dangb,
Shaomin Liua and
Liangmin Gaoa
aSchool of Earth and Environment, Anhui University of Science and Technology, Huainan 232001, China
bCollege of Environment and Energy, South China University of Technology, Guangzhou, 510006, China. E-mail: cyanggz@scut.edu.cn; Fax: +86-20-39380508; Tel: +86-20-87110198
First published on 10th November 2015
Humic acid and mineral oxides are simultaneously present in soils and can form organomineral complexes. These complexes could influence the transport and fate of antibiotics in the environment. The objective of this study was to investigate the sorption and photodegradation of TYL and SMT on these complexes. The results showed that HA tended to interact with goethite via hydrophobic and π–π interactions. The sorption capacity and sorption rate gradually increased with the increasing concentrations of HA and the equilibrium time for TYL and SMT was 7 h and 24 h, respectively, on HA–goethite complexes. The sorption isotherms of TYL were more nonlinear and SMT were less nonlinear on HA–goethite complexes with the increasing concentrations of HA, which implies a more heterogeneous distribution of the sorption sites for TYL and more rigid and porous structures developed for SMT. The photodegradation of TYL and SMT by HA–goethite complexes increased with increasing the concentration of HA. An iron redox cycle couple should be a common phenomenon in the system, since both Fe(III) and HA are ubiquitous in the natural environment. The influence of HA and goethite on the fate of antibiotics in the environment is worth noting. This study is helpful in understanding the potential of toxic organic pollutants migration and transformation in the natural environment.
Although numerous experiments have focused on the sorption of TYL and SMT onto soils or soil components, the full mechanism of sorption is still not completely understood.1,6,7 Humic substances bound strongly to metal oxide and hydroxide particles, and such associations occur in the solid phase of soils and sediments.8 It is well known that ion binding to oxides is somewhat dependent on the electrostatic potential profile in the vicinity of the surface and this potential profile would be strongly affected by the presence of adsorbed humic substances.9 Goethite (α-FeOOH) is one of the most common and stable crystalline iron oxide in sediments and natural systems.10 This mineral has a relatively high surface area and high reactivity, which could be suitable for sorption and deactivation of pesticides, nutrients, and hazardous compounds in natural conditions and might greatly affect the distribution and transport of contaminants in the environment.7 Previous studies have shown that sorption of TYL and SMT on goethite is strongly dependent on pH and ionic strength and has been suggested to be due to the favourable interaction between TYL/SMT and the positively charged surface of the iron oxides.3,7,11,12 Goethite was also found to be associated with organic matter in soils. This mutual interaction could modify the individual reactivity of both organic matter and the mineral surface affecting the cycle of the various chemical species present in soil.11 Some researchers suggested that coated HA could significantly promote sulfonamide sorption in comparison to mineral particles, which was explained by the specific interactions between sulfonamide and organic functional groups.13
Except for the sorption of contaminants, goethite also decomposes or catalyzes the decomposition of some contaminants in the absence and presence of hydrogen peroxide and/or UV radiation.14,15 Recently, Han et al. first reported that aqueous goethite can generate singlet oxygen and hydroxyl radical under room light, and aeration conditions investigated using spin trapping electron paramagnetic resonance and H2O2 can improve the generation of both reactive species.14 A similar result could be found in the report indicating goethite surfaces catalysed a Fenton-like reaction responsible for the decolorizing of azo dye Orange G.15 However, the treatment of antibiotic by goethite has not been documented, and little information has been obtained about the transformation of antibiotics in natural environment systems.
In this study, the aims were to investigate TYL and SMT sorption and degradation by organo-goethite complexes and the possible effect of sorption parameters on its degradation. For this purpose, the study was conducted in three parts: (1) investigating the use of goethite and well-characterized organo-goethite complexes, which should provide better understanding of mechanisms controlling TYL and SMT sorption and their influence on its degradation; (2) the sorption of TYL and SMT by goethite with their complexes with different concentrations of HA; (3) the degradation of TYL and SMT in the presence of goethite with their complexes with different concentrations of HA.
TYL and SMT, like most antibiotics, are ionic compounds. TYL is a weak base with a pKa of 7.1. In acidic conditions, there might be ionic bonds formed between protonated TYL and anionic components of soil and manure matrices. SMT is an amphoteric compound with pKa values at 2.28 and 7.42. The net charges of SMT in different pH conditions would be more complicated and lead to heterogeneous sorption activities between SMT and solid phase.
Humic acids (solid granule, particle size is 0.4–0.6 μm) used throughout this investigation was obtained from JuFeng Chemical Corporation, Shanghai, China. The elemental composition of HA is 52.37% C, 3.57% H, 36.12% O, and 1.80% N.
Primary stock solutions of TYL and SMT at 1000 mg L−1 were prepared with pure water and stored at 4 °C for a maximum of 1 month. The working solutions were prepared by diluting 0.01 M KNO3 stock solution.
The HA–goethite complexes were synthesized according to Yang and Xing17 with minor modification. Briefly, 5 g goethite and was mixed with 1 L HA solution in a bottle and shaken for 2 days, after which the suspensions were centrifuged at 3500g for 30 min. The precipitated materials were freeze-dried, ground, and stored for future experiments. HA solution was obtained by dissolving solid HA with 0.1 M NaOH and adjusting to pH 5. HA concentrations in the solution were 50, 100, 200, and 500 mg L−1.
The general characterization of the synthesized goethite and HA–goethite complexes was performed by X-ray diffraction (XRD), FT-IR, SEM, Raman spectroscopy and N2-BET. XRD patterns were obtained using a Bruker D8 X-ray diffractometer with Cu-Kα X-ray source (λ = 0.15418 nm). FT-IR spectra were obtained with a Nicolet FT-IR Nexus 470 Spectrophotometer. The samples were dried under vacuum until constant weight was achieved and diluted with KBr powder (1%) before the FT-IR spectra were obtained. The morphologies were recorded with a scanning electron microscope (SEM, JEOL JSM-6510LV), which was operated at an acceleration voltage of 10 kV. Raman spectroscopy (Jobin Yvon T64000) was used to further characterize goethite and HA–goethite complexes. The N2-BET adsorption at 77 K was measured with a Quantachrome Nova 1200e instrument. Each sample was degassed under vacuum at 30 °C for 60 min prior to analysis. The point of zero charge (PZC) of goethite was measured by potentiometric titrations at three KCl concentrations.
Kinetic studies of TYL and SMT sorption on goethite and HA–goethite complexes were carried out from aqueous solutions with a certain concentration and pH. A fixed volume of the aliquot was withdrawn at designated time points while the reactors were run continuously. In order to investigate the influences of temperature, the shaker was adjusted at the desired temperature.
qe = kdce | (1) |
![]() | (2) |
qe = kfCen | (3) |
To investigate the potential rate-controlling steps involved in the sorption of TYL and SMT on goethite and HA–goethite complexes, a pseudo-first-order model and pseudo-second-order kinetic model were employed to fit the data.7
The pseudo-first-order rate expression is generally expressed as follows:
![]() | (4) |
After integration with the initial condition qt = 0 at t = 0, eqn (4) can be obtained.
![]() | (5) |
The pseudo-second-order model is given as follows:
![]() | (6) |
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Fig. 1 TEM image of goethite (a), SEM images of goethite (b), 100 ppm HA–goethite complexes (c) and 500 ppm HA–goethite complexes (d). |
The XRD patterns of goethite and HA–goethite complexes are shown in Fig. 2. All the peaks in the pattern were labeled and indexed to a tetragonal goethite phase (JCPDS file no. 34-1266), indicating that goethite was synthesized without any detectable impurity.3 After combining goethite and HA at different concentrations of HA, all the peaks of as-prepared goethite were still observed, implying that HA–goethite complexes were retained in the composition. This result indicates that the crystal structure of goethite was not changed after modification with HA.7,10,20
The surface chemistry of goethite and HA–goethite complexes were studied using FTIR spectroscopy. Fig. 3 shows the Fe–O stretches of HA–goethite complexes at 1630 cm−1, indicating the hydroxyl group interacting with the goethite surface.21 The band at 3400 cm−1 corresponds to the OH stretching of the hydroxyl surface groups that weakens as the concentrations of HA is increased.7 The band at 3125 cm−1 that possibly corresponds to the bulk OH stretching were stronger with increasing HA concentration.3 The band at 1376 cm−1 is most likely due to the –CH2 scissoring. For the bare goethite materials, however, weak CO stretches were observed, and no C–O stretches were found, suggesting the binding of HA to goethite. It is generally believed that the binding of HA to goethite surface is mainly through ligand exchange.21,22
Further information on the structure of goethite and HA–goethite complexes was obtained using Raman spectroscopy. Each of the spectra shown in Fig. 4 exhibits peaks at 200–500 cm−1 and 500–1800 cm−1, which could be assigned to the Raman Fe–O and symmetric –OH intermolecular stretching vibrations, respectively.23 These results confirm that the addition of HA has no effects on the crystal structure of goethite, in agreement with the XRD analysis. The peaks are observed at around 572, 1257 and 1511 cm−1 corresponding to the CC, Fe–O–C and C
O stretching vibrations, which confirmed the surface structure changes of goethite. The shifts of aromatic C
C and C
O bands are indicative of hydrophobic and π–π interactions between HA and goethite.23
The zeta potentials of goethite and HA–goethite complexes were measured at various pH values and are shown in Fig. 5. The pHPZC of HA–goethite complexes decreased since the coated HA had abundant carboxylic acid groups. The zeta potential of gray humic acid is negatively charged in the range of pH 2–11.21 The low pHPZC indicates that the HA–goethite complexes were negatively charged in the entire environmentally relevant acidity range (pH 3–9), which prohibited the aggregation of HA–goethite complexes and benefited the sorption of positively charged substances.
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Fig. 6 Sorption kinetics of TYL and SMT on goethite and HA–goethite complexes (equilibrium pH for TYL and SMT were 3.5; temperature = 25 °C; I = 0.01 M KNO3). |
From the sorption results, two kinetic models were generated to assess the kinetic characteristics of TYL and SMT sorption on goethite and HA–goethite complexes. Table 1 shows the parameters of the simulated sorption kinetics models. The results prove that the pseudo-second-order analysis could better explain the sorption processes of TYL and SMT on goethite and HA–goethite complexes with a correlation coefficient consistently R2 > 0.998. It was obvious that the sorption rate (k1 and k2) for TYL and SMT increased as the concentrations of HA increased and the sorption capacity was also increased with the increasing concentrations of HA. The initial sorption rate may be attributed to chemical and/or hydrogen (H) bonding between TYL/SMT and the surface hydroxyls of goethite.24 As shown in Table 1, the rate constant (k2) for TYL was larger than SMT under the same conditions, indicating that the sorption of TYL on goethite and HA–goethite complexes was a faster process and this can be proven by the equilibrium time. This may be due to the differences in the sorption force for TYL and SMT.7,25,26
Conditions | Pseudo-first-order | Pseudo-second-order | |||
---|---|---|---|---|---|
k1 (min−1) | R2 | k2 (g mg−1 min−1) | R2 | ||
TYL | 0 ppm HA | 0.028 | 0.753 | 0.687 | 0.999 |
50 ppm HA | 0.058 | 0.846 | 1.432 | 0.999 | |
100 ppm HA | 0.063 | 0.810 | 1.501 | 0.999 | |
200 ppm HA | 0.068 | 0.894 | 1.586 | 0.999 | |
500 ppm HA | 0.073 | 0.874 | 1.621 | 0.999 | |
SMT | 0 ppm HA | 0.211 | 0.991 | 0.258 | 0.998 |
50 ppm HA | 0.323 | 0.973 | 0.513 | 0.999 | |
100 ppm HA | 0.376 | 0.896 | 0.586 | 0.999 | |
200 ppm HA | 0.443 | 0.912 | 0.632 | 0.999 | |
500 ppm HA | 0.496 | 0.908 | 0.685 | 0.999 |
![]() | ||
Fig. 7 Sorption isotherms of TYL and SMT on goethite and HA–goethite complexes (equilibrium pH for TYL and SMT were 3.5; temperature = 25 °C; I = 0.01 M KNO3; equilibrium time of 24 h). |
Conditions | Henry model | Freundlich model | Langmuir model | ||||||
---|---|---|---|---|---|---|---|---|---|
kd (L kg−1) | R2 | n | kf (μg g−1) (mg L−1)−n | R2 | qe (mg kg−1) | b | R2 | ||
TYL | FeOOH | 10.7 | 0.721 | 0.16 | 418 | 0.959 | 698 | 3.38 | 0.951 |
FeOOH + 50 ppm HA | 218.2 | 0.807 | 0.51 | 869 | 0.946 | 2034 | 3.85 | 0.962 | |
FeOOH + 100 ppm HA | 272.3 | 0.829 | 0.57 | 992 | 0.968 | 2298 | 4.28 | 0.971 | |
FeOOH + 200 ppm HA | 312.3 | 0.845 | 0.62 | 1135 | 0.984 | 2435 | 4.62 | 0.959 | |
FeOOH + 500 ppm HA | 372.5 | 0.884 | 0.67 | 1246 | 0.979 | 2678 | 4.89 | 0.987 | |
SMT | FeOOH | 1.02 | 0.964 | 0.84 | 4.87 | 0.985 | 19.2 | 0.12 | 0.948 |
FeOOH + 50 ppm HA | 4.19 | 0.812 | 0.71 | 16.6 | 0.957 | 112 | 0.21 | 0.945 | |
FeOOH + 100 ppm HA | 4.68 | 0.837 | 0.66 | 23.2 | 0.963 | 127 | 0.25 | 0.962 | |
FeOOH + 200 ppm HA | 5.18 | 0.894 | 0.61 | 28.7 | 0.978 | 149 | 0.36 | 0.978 | |
FeOOH + 500 ppm HA | 5.52 | 0.835 | 0.58 | 37.9 | 0.987 | 173 | 0.41 | 0.949 |
For original goethite, the isotherms of TYL were more nonlinear as suggested by the low linearity index (n = 0.16). As the concentrations of HA increased from 50 ppm to 500 ppm, the n values (0.51–0.67) increased, implying a more heterogeneous distribution of the sorption sites for TYL. In contrast, sorption of SMT on goethite and HA–goethite complexes was generally less nonlinear than that of TYL with decreased n values (0.84–0.58), implying that more rigid and porous structures were developed, which is consistent with the increased surface C content.27,28
TYL and SMT sorption by HA–goethite complexes were higher than goethite (Fig. 7). Li et al. reported that nanoparticle aggregates were dispersed after HA coating29 and thus the available surface area increased for phenanthrene sorption. According to our measurements of surface areas, goethite aggregate status was not significantly increased after HA coating. Thus, the size effect on TYL and SMT sorption could be excluded. Considering the higher sorption of both chemicals on HA than on goethite, the increased sorption of HA–goethite complexes in comparison to goethite particles may be mostly attributed to the adsorbed HA.
TYL and SMT sorption on sorbents is highly affected by their speciation.3 TYL and SMT have various species in an aqueous solution that depend on solution pH. In this study, more than 98% of TYL molecules (pKa = 7.1) were positively charged in the test pH (3.5 ± 0.2), which resulted in the electrostatic repulsion decreasing when the concentration of HA increased from 50 ppm to 500 ppm. In comparison, the SMT molecule (pKa2 = 7.23) exists as a neutral species at the test pH, and it could be more strongly adsorbed on the hydrophobic carbon surface of HA. Similar results were reported by Peng et al.25 that nano iron oxides coated HA can increase ofloxacin and norfloxacin sorption.
FeOOH + HA → Fe(III) − HA | (7) |
HA + hv → HA* | (8) |
HA* + O2 → products + O2˙−/HO2˙ | (9) |
O2˙−/HO2˙ → H2O2 | (10) |
Fe(III) − HA + hv → HA˙+ + Fe(II) | (11) |
Fe(II) + O2 → Fe(III) + O2˙−/HO2˙ | (12) |
Fe(II) + H2O2 → Fe(III) + ˙OH + OH− | (13) |
TYL/SMT + ˙OH → products | (14) |
![]() | ||
Fig. 8 Photodegradation of TYL and SMT by goethite, HA and HA–goethite complexes (the pH for TYL and SMT were 3.5; temperature = 25 °C; I = 0.01 M KNO3). |
Therefore, the concentration of Fe(II) and H2O2 photoformed in the solutions during TYL and SMT photodegradation was measured (Fig. 9). As shown in Fig. 9, the concentrations of Fe(II) and H2O2 were increased with the increasing concentrations of HA from 50 ppm to 500 ppm, indicating that more oxidants (e.g. ˙OH, formed by reaction of Fe(II) with H2O2) could be generated in the presence of both Fe(III) and HA, thus accelerating the photodegradation of TYL and SMT.31
![]() | ||
Fig. 9 The corresponding concentrations of H2O2 and Fe(II) photoformed in different concentrations of HA. |
On the basis of the abovementioned discussion, a possible reaction mechanism in the presence of Fe(III)–HA complex is proposed in Fig. 10. When HA and goethite coexist in a solution, HA might react with iron species followed by the formation of iron–HA complexes, and photochemical reactions of these complexes take place by electron transfer from HA to the Fe(III), which could produce Fe(II) and consume HA. Then, the reaction of the free humic radical with O2 leads to O2˙−/HO2˙ formation, and H2O2 is the product of O2˙−/HO2˙ dismutation. Ultimately, the simultaneous and rapid photoformation of Fe(II) and H2O2 in the irradiated Fe(III)–HA system leads to ˙OH formation. However, there are numerous concurrent processes in the systems, including the competing reactions of free HA radical (HA˙+) with O2 and Fe(III) species, and the ˙OH quenched by both TYL/SMT and HA. The net result is an iron redox cycle in which HA as well as oxygen are consumed, ROS are generated and reacted, and the degradation of TYL/SMT is accelerated.19,31
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