Sorption and photodegradation of tylosin and sulfamethazine by humic acid-coated goethite

Abstract

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.

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1. Introduction

Tylosin (TYL) and sulfamethazine (SMT) were widely used in the aquiculture and livestock industries as the typical antibiotic and have drawn growing attention. These antibiotics entered into the environmental matrices by direct runoff and excretion as unmetabolized drugs or active metabolites and degradation products.¹ Recently, it has been reported that TYL and SMT were detected in surface water, wastewater and soils.² The occurrence of cumulated TYL and SMT in the environment could induce genetic exchange, increase the resistance of bacteria against drugs and subsequently threaten human health.^3,4 Sorption and degradation were important processes that controlled the transport, fate, bioavailability and ecotoxicological risk of TYL and SMT in the environment, thus a thorough understanding of sorption and degradation are of central importance for predicting mobility and availability of TYL and SMT in soils.⁵

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.

2. Materials and methods

2.1. Materials and preparation

Tylosin tartrate (purity > 95%) and sulfamethazine (purity > 99%) were purchased from Sigma-Aldrich Corporation (St Louis, MO). Acetonitrile and formic acid (HPLC grade, Merck Chemicals Co. AQ5) were used as received. Pure water was prepared by a Milli-Q® water machine (Millipore Co., Guangzhou, China). All the other chemicals were of analytical reagent grade and used without further purification.

TYL and SMT, like most antibiotics, are ionic compounds. TYL is a weak base with a pK_a 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 pK_a 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.

2.2. Preparation and characterization of goethite and HA–goethite complexes

Goethite was synthesized according to the method introduced by Brigante et al.¹⁶ In brief, 5 M KOH was added to 0.5 M Fe(NO₃)₃ until a red colloid was generated. The resulting ferrihydrite dispersion was aged at 60 °C in a capped Teflon container for 60 h and then it was washed with double distilled deionized (DDI) water until the supernatant reached a pH close to the point of zero charge. Furthermore, the dispersion was freeze-dried to obtain a dry powder.

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.

2.3. Sorption procedure

The sorption experiments were conducted using a batch equilibrium technique. TYL and SMT were mixed at high concentration in methanol before being added to the background solution. The background solution contained 0.003 M NaN₃ to minimize bioactivity and 0.01 M KNO₃ to adjust ionic strength. A predetermined amount of goethite and HA–goethite complexes were filled with the initial aqueous solution in completely mixed batch reactor (CMBR) systems with Teflon gaskets and mixed for sorption equilibrium on a shaker at 150 rpm. After sorption experiments, the screw cap vials were centrifuged at 4000 rpm for 30 min, and 1 mL of supernatant was transferred to a pre-weighed 1.5 mL amber glass vial for chemical analyses. Each concentration level, including blanks, was run in parallel three times. KOH or HNO₃ solutions were used for pH adjustment.

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.

2.4. Sorption models

The equilibrium sorption data was fitted using Henry (eqn (1)), Langmuir (eqn (2)) and Freundlich (eqn (3)) models:¹⁸

q_e = k_dc_e (1)

q_e = k_fC_eⁿ (3)

where C_e (mg L⁻¹) and q_e (mg kg⁻¹) are the equilibrium concentration of TYL and SMT in the liquid phase and solid phase, respectively, and k_d (L kg⁻¹) is the distribution coefficient of solute between soil and water. b and q_m are the Langmuir constants, which are related to the sorption bonding energy and the maximum sorption capacity; k_f (μg g⁻¹) (mg L⁻¹)⁻¹ is the capacity affinity parameter and n (dimensionless) is the exponential parameter. Parameters were estimated by nonlinear regression weighted by the dependent variable.

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:

After integration with the initial condition q_t = 0 at t = 0, eqn (4) can be obtained.

The pseudo-second-order model is given as follows:

where q_e and q_t are the amounts of sorption TYL and SMT at equilibrium and time t, respectively. k₁ (h⁻¹) and k₂ (g μg⁻¹ h⁻¹) are the sorption rate constant of pseudo-first-order and pseudo-second-order sorption rates, respectively. The rate constants k₁ and k₂ can be derived from linear regressions based on experiment results.

2.5. Photodegradation study

The photodegradation experiments were performed in a slurry reactor containing a solution of TYL and SMT (200 mL, 10 mg L⁻¹) and the goethite and HA–goethite complexes. A Xe lamp was used as a sunlight source. Prior to irradiation, the suspension was kept in the dark under strong magnetic stirring for 12 h to ensure the establishment of a sorption/desorption equilibrium. At given time intervals, aliquots of about 2 mL were collected from the suspension and centrifuged immediately and then 1 mL of supernatant was transferred to a pre-weight 1.5 mL amber glass vial for chemical analyses.

2.6. Chemical analysis

The concentrations of TYL and SMT in an aqueous solution were measured by reverse-phase high-performance liquid chromatography (Agilent 1200) with a C₁₈ column (5 μm, 4.6 × 250 mm; Agilent) and a diode array UV detector (wavelength at 290 nm for TYL and 264 nm for SMT). The mobile phase (at a flow rate of 0.5 mL min⁻¹) for TYL was a mixture of acetonitrile (35%) and an aqueous solution (65%) containing 0.01 mol L⁻¹ KH₂PO₄ (pH = 2.0) but for SMT it was a mixture of acetonitrile and formic acid solution (0.05% v/v) at a volumetric ratio of 60 : 40 with a flow rate of 1 mL min⁻¹. The injection volume was 20 μL. External standards of TYL and SMT (0.1–100 mg L⁻¹) were employed to establish a linear calibration curve and the sample concentrations were calculated from its integrated peak areas. The solid phase concentrations were calculated based on the mass balance of the solute between the two phases. The concentration of H₂O₂ was determined using a colorimetric method developed by Ou et al.,¹⁹ and concentration of Fe(II) was determined using the o-phenanthroline method by a spectrophotometer (Jasco UV-550, Japan).

3. Results and discussion

3.1. Characterization of the goethite and HA–goethite complexes

The goethite and HA–goethite complexes were examined by TEM and SEM at high magnification as shown in Fig. 1. The particles of goethite were needle shaped with a 50–100 nm width and 0.5–1 μm length (Fig. 1a and b). The surface morphology of goethite changed significantly after HA immobilization. The image of HA–goethite complexes displays an irregular surface (Fig. 1c and d). From this image, we can say that the HA molecules have been immobilized onto the surface of goethite after the contact of goethite with HA solution. The specific surface area of goethite and HA–goethite complexes was 34.11, 33.28, 35.27, 37.12, and 36.78 m² g⁻¹ based on N₂-BET sorption with HA concentrations of 50 ppm, 100 ppm, 200 ppm and 500 ppm.

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

Fig. 2 XRD patterns of goethite and HA–goethite complexes.

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 C=O 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

Fig. 3 FTIR spectrum of goethite and HA–goethite complexes.

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 C=C, 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

Fig. 4 Raman spectra of goethite and HA–goethite complexes.

The zeta potentials of goethite and HA–goethite complexes were measured at various pH values and are shown in Fig. 5. The pH_PZC 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 pH_PZC 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.

Fig. 5 Zeta potentials of goethite and HA–goethite complexes.

3.2. Sorption kinetics of TYL and SMT on goethite and HA–goethite complexes

The effect of time for TYL and SMT sorption onto goethite and HA–goethite complexes is illustrated in Fig. 6. It was found that the sorption increased with increasing contact time and nearly 90% of the sorption capacity for TYL and SMT were accomplished within the first 2 h and 7 h. Then, the sorption gradually increased at a much slower rate and became almost constant nearly after 7 h and 24 h for TYL and SMT, which represent the time at which equilibrium was reached.

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

Table 1 The pseudo-first-order and pseudo-second-order sorption models constants of TYL and SMT on goethite and HA–goethite complexes

Conditions		Pseudo-first-order		Pseudo-second-order
		k1 (min^−1)	R^2	k2 (g mg^−1 min^−1)	R^2
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

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3.3. Sorption isotherms of TYL and SMT on goethite and HA–goethite complexes

Sorption isotherms of TYL and SMT on goethite and HA–goethite complexes are presented in Fig. 7. The sorption data were fitted to the linear, Freundlich and Langmuir isotherms. The sorption data could be fitted well by Freundlich and Langmuir models, as indicated by the high regression coefficient (R² > 0.940). The fitting parameters are listed in Table 2.

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).

Table 2 List of TYL and SMT sorption isotherm parameters on goethite and HA–goethite complexes

Conditions		Henry model			Freundlich model		Langmuir model
		kd (L kg^−1)	R^2	n	kf (μg g^−1) (mg L^−1)^−n	R^2	qe (mg kg^−1)	b	R^2
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

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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 (pK_a = 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 (pK_a2 = 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.

3.4. Photodegradation of TYL and SMT by goethite and HA–goethite complexes

Fig. 8 shows the photodegradation of TYL and SMT by goethite, HA and HA–goethite complexes at pH 3.5. At an irradiation time of 120 min, the photodegradation of TYL and SMT was observed and the photodegradation rate increased with the increasing concentrations of HA from 50 ppm to 500 ppm. Apparently, the HA–goethite complex presented better photoinductive activity than goethite. It has been reported that in sunlit surface waters containing natural organic matter, photochemical reactions can result in the rapid formation of both Fe(II) through LMCT reactions of Fe(III)–organic complexes and H₂O₂ mainly through the reduction of oxygen by photo-excited organic substances.^19,30 The related main reactions occurring in systems containing HA and Fe(III) are as follows:

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

Fig. 10 The iron cycling and main reactions in Fe(III)–HA complexes systems.

4. Conclusion

Our results presented an investigation of HA and goethite interaction and their effects on TYL and SMT sorption and photodegradation, which was significant in the natural environment as HA abundantly coexists on surfaces with iron and antibiotics. The characteristic results showed goethite binding of HA through ligand exchange and there were the structure changes of goethite after complexing with HA. The sorption and photodegradation of TYL/SMT in soils was obviously influenced by goethite and HA. The present study shows that HA–goethite complexes have a high sorption capacity and sorption rate for TYL and SMT, probably due to the heterogeneous surface of HA. The photodegradation of TYL and SMT by HA–goethite complexes increased with the increased concentrations of HA. An iron redox cycle couple should be a common phenomenon in the system since both Fe(III) and HA were ubiquitous in the natural environment. This study is helpful in understanding the potential of toxic organic pollutants migration and transformation in natural environments.

Acknowledgements

The study was financially supported by the China National Science Fund Program (No. 41503095, 41173104), the Natural Science Foundation of Universities of Anhui Province (KJ2015A016), the PhD Fund of Anhui University of Science and Technology (ZY540) and the Key Science Foundation for Young Teachers of Anhui University of Science and Technology (QN201507).

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