The removal performance and mechanisms of tetracycline over Mn-rich limonite

Naturally occurring Mn-rich limonite mainly composed of goethite and manganese oxides was used to remove tetracycline (TC) from the aqueous solution. The effects of dosage, initial solution pH, temperature, and coexisting anions on TC removal were investigated. Results showed that 95% of TC (30.0 mg·L−1) was removed in a wide pH range of 3.0–9.0 by limonite with high specific surface area (145.0 m2·g−1) and mesoporous structure. The presence of Cl−, NO3−, and SO42− in the studied concentration range did not influence TC removal efficiency significantly, while PO43− inhibited the adsorption of TC over limonite due to the competition with TC for active sites. Integrated with the FT-IR analysis, electrostatic interaction and complexation were proved to be the adsorption mechanisms of TC by limonite. The quenching experiments and ESR analysis revealed that singlet oxygen (1O2) also was involved in TC degradation. In addition, limonite displayed an efficient recycling performance and stability after four cycles. This study revealed that the Mn-rich limonite was a promising adsorbent for TC removal from aqueous solutions and promoted the application of natural mineral material in the environmental field.


Introduction
Tetracyclines (TCs) as a broad-spectrum antibiotic are used in the pharmaceutical industry and veterinary medicine (Sarmah et al. 2006;Wammer et al. 2011). However, a large amount of TCs is directly excreted into the environment in the form of parent compounds or metabolites due to their incomplete absorption and metabolism in bodies (Cheng et al. 2016;Zhou et al. 2017). These residual antibiotics in water and soil induced the generation of antibiotics resistance genes (ARGs) (Daghrir and Drogui 2013;Liu et al. 2018;Rodriguez-Mozaz et al. 2015), which aroused increasing concerns about their potentially risks to public health. To date, various methods of treating TC-containing wastewater have been applied, such as adsorption (Ji et al. 2011), photocatalytic degradation , biodegradation (Yang et al. 2016), and advanced oxidation process (Tian et al. 2018). Adsorption was widely applied in the treatment of wastewater containing antibiotics because of its advantages of low cost, simple operation, and no secondary pollution.
Metal oxide minerals in soils, such as hematite (Fe 2 O 3 ), magnetite (Fe 3 O 4 ), goethite (α-FeOOH), and manganese oxides (MnO 2 ), play essential roles in the transformation of antibiotics (Hafner and Parikh 2020;. Goethite with high specific surface area and large surface hydroxyl contents is considered as an effective adsorbent for antibiotics. Wu et al. (2019b) reported that TC molecules were adsorbed by goethite through complexation and electrostatic interaction. Carrasquillo et al. (2008) found that oxytetracycline was more likely to be immobilized on the surface of goethite than norfloxacin, because two adjacent hydroxyl groups on the oxytetracycline molecule facilitated surface complexation. In addition, manganese oxides have been proved to exhibit excellent oxidation capacities to antibiotics. Analysis of TC transformation products indicated that MnO 2 facilitated C-ring isomerization and further oxidized phenol diketones and tricarbonyl groups, leading to the interposition of oxygen on the two labile sites (Chen and Huang 2011).  found that δ-MnO 2 could oxidize chlortetracycline to form less toxic intermediate products. Encouraged by the above research, the application of ferric and manganese binary oxide (FMBO) for the efficient removal of pollutants aroused widespread concerns. Joshi et al. (2017) and Qiu et al. (2020) synthesized FMBO to remove organic pollutants and As(III) based on the adsorption properties of FeOOH and the oxidation properties of MnO 2 . The development of FMBO provided new insights into the treatment of wastewater containing TC.
Previous studies discovered a naturally occurring Mn-rich limonite ore mainly composed of goethite and manganese oxides . The Mn-rich limonite is a natural nano-mineral with nano-acicular morphology and rich porous structure, which exhibited great potential application in environmental remediation (Chen et al. 2017). In this work, the naturally occurring Mn-rich limonite was used for TC removal from aqueous solutions. The main objectives are (1) to explore the removal performance of TC by limonite; (2) to test the effects of dosage, pH, temperature, and coexisting anions on TC removal; and (3) to investigate the removal mechanisms of TC by limonite.

Batch experiments
Batch experiments were initiated by mixing 40.0 mg limonite with 50.0 mL of 30.0 mg·L −1 TC solutions in a 100.0 mL tube. Then, the tube containing suspension was placed onto a shaker at a constant temperature of 298 ± 2 K. At pre-set intervals, 1.0 mL sample was extracted and filtered by a 0.22 μm PES for measurement. The initial pH of TC solutions was adjusted to 5.0 using HCl and NaOH. The effects of dosage and pH values on TC removal were carried out within the range of 0.8 to 2.0 g·L −1 and 3.0-11.0, respectively. The effects of temperature (283 K, 293 K, and 303 K) and co-existing anions (SO 4 2− , Cl − , PO 4 3− , and NO 3 − ) with various concentration (1.0-10.0 mM) on TC removal were also explored.
The TC adsorption amount (q e ) was given in the following equation: where C 0 and C e (mg·L −1 ) are the TC concentrations before and after reaction, respectively. V (L) and m (g) represent the volume of TC solution and the mass of limonite adsorbent, respectively.

Analytical methods
TC concentration was analyzed by ultrahigh performance liquid chromatography (UHPLC, U3000, Thermo, America) equipped with a C18 column (4.6 × 250 mm) at a UV wavelength of 357 nm. The mobile phase was 45/35/20 (v/v/v) of oxalate/acetonitrile/methanol at a flow rate of 1.0 mL·min −1 at 30 ℃. A total organic carbon analyzer (TOC, Jena, Germany) and atomic absorption spectrometer (AAS, WYS2200, WAYEE Technology Co., Ltd) were used to measure the concentration of residual organic carbon and dissolved metal ions, respectively. HPLC-MS (ACQUITY UPLC LCT Premier XE, Water, USA) detected the intermediates produced during TC degradation. The reactive oxygen species involved in reaction were detected by the ESR system (JESFA200, JEOL, Japan) with TEMP as the spin trapping agent. Electrochemical experiments were carried out by CHI-660E electrochemical workstation (CHI-660E, China).

Results and discussion
Characterization of limonite Figure 1 shows the XRD patterns of limonite before and after reaction. Four dispersed reflections at 21.2°, 33.2°, 36.7°, and 53.2° were indexed to the (110), (130), (111), and (221) planes of goethite (JCPDS 81-464) due to the low crystallinity. Besides, a reflection at 26.7° was accordant with quartz (JCPDS 85-794). The absence of manganese oxides in the XRD spectra could be ascribed to the high dispersion degree of manganese oxides. Compared with the raw, the intensity and position of the goethite reflections in used limonite remained unchanged. The TEM images ( Fig. 2a) shows that limonite particles were arranged in short rod-shaped aggregates, which was the typical (021), (101) plane of goethite ( Fig. 2b) (Paul et al. 2014). Besides, the (210) plane was identified as ramsdellite. The appearance of Mn in the mapping image ( Fig. 2e) and XPS spectra of limonite ( Fig. S1) indicated the existence of manganese oxides in limonite. Besides, the intensity of Mn 2p spectra changed after reaction, indicating that manganese oxides participated in TC removal. On the basis of the above results, it could be concluded that the Mn-rich limonite mainly comprised of goethite, manganese oxides, and quartz. According to Fig. S2, the N 2 adsorption/desorption isotherms of limonite could be classified as a type IV isotherm, indicating the mesoporous structures of limonite (Hayati et al. 2020). The specific surface area (145.0 m 2 ·g −1 ), total pore volume (0.254 cm 3 ·g −1 ), and average pore size (35.05 nm) suggest that limonite has the advantage of being an effective adsorbent (Rahimi et al. 2015).

Effects of dosage and solution pH
The influence of dosage on TC removal was systematically investigated (Fig. 3a). In the first stage (60 min), TC was quickly adsorbed by limonite with abundant adsorption sites. Subsequently, an evident decrease in the adsorption efficiency of TC was observed. The inhibitory adsorption of TC could be ascribed to the reduction of available surface binding sites of limonite as the adsorption proceeded. On the other hand, the decreased amount of TC molecules in solution reduced the collision chance with limonite, decreasing the removal efficiency. The TC removal significantly improved from 58 to 96% after 540 min with the dosage increased from 0.2 to 0.8 g·L −1 . The higher dosage of limonite led to more active sites resulting in the improvement of TC removal. When the dosage further increased from 0.8 to 2.0 g·L −1 , the removal efficiency of TC was unchanged, which could be on account of the mass transfer resistance (Li et al. 2021a). Therefore, 0.8 g·L −1 was selected as an optimum dosage for subsequent experiments.
Solution pH is a key factor controlling the TC removal since it affected the surface electric charge of limonite and the existing state of TC molecular. According to the literatures, the pKa 1 , pKa 2 , and pKa 3 of TC were 3.3, 7.7, and 9.7, respectively Wang et al. 2020b). As shown in Fig. S3, TC molecules were fully protonated and existed as cation (TC 3 + ) at pH < 3.3. When 3.3 < pH < 7.7, the zwitterionic (TC 2 ± ) existed due to release of the enolic proton at C3, whereas the hydroxyl proton at C12 was dissociated to form anionic (TC − ) when 9.7 > pH > 7.7. At higher pH (> 9.7), the dimethylamino group at C4 was deprotonated, resulting the formation of anionic (TC 2− ). Figure 3b shows that the pH pzc of limonite was 4.5, indicating that the limonite surface was negatively charged when solution pH was over 4.5.
As shown in Fig. 3c, TC could be effectively removed by limonite in the range of 3.0 to 9.0 and the highest TC removal (95% after 540 min treatment) was achieved at pH = 5.0. This was attributed to the lowest electrostatic repulsion between limonite and TC 2 ± . The high reduction potential of manganese oxides at weakly acidic conditions also promoted TC removal (Lin et al. 2009). With the initial solution pH further increased from 5.0 to 11.0, the TC removal significantly decreased by 70%. The phenomenon could be ascribed to the electrostatic effect in the adsorption. When pH increased from 5.0 to 11.0, the effect of electrostatic repulsion was enhanced between the limonite with negative surface charge and TC − or TC 2− . As a result, the adsorption process of TC molecular over limonite was inhibited. On the other hand, the increase of pH suppressed the reduction of Mn 4+ to Mn 2+ as shown in Eq. (2) (Lin et al. 2009), which weaken the oxidizability of MnO 2 . For example, the reduction potential of MnO 2 decreased from 0.99 to 0.76 V when pH increased from 4.0 to 8.0 (Zhang and Huang 2003). Thus, the decrease in limonite oxidizability at higher pH was another vital factor responsible for the lower TC removal.

Effect of temperature
As shown in Fig. 3d, it could be observed that TC removal improved with an increase in temperature from 283 to 303 K, indicating that the removal of TC over limonite was an endothermic reaction (Gu et al. 2021). As the temperature increased, the driving force between the adsorbent and solution became more extensive, which promoted the adsorption of TC (Wang et al. 2020a).

Effects of coexisting anions
Natural water existed various inorganic anions including nitrate (NO 3 − ), phosphate (PO 4 3− ), sulfate (SO 4 2− ) and chloride (Cl − ), which may affect TC removal. Therefore, the interfering of inorganic anions with concentrations ranging from 1.0 to 10.0 mM on TC removal was investigated. Figure 4 depicts that the no reduction in TC removal was observed in the presence of Cl − , NO 3 − , and SO 4 2− with various concentrations, indicating that the adsorption of TC by limonite might mainly interact through the inner-sphere surface complexation (Zhao et al. 2014). Differently, the TC removal decreased with the addition of PO 4 3− . In detail, when the PO 4 3− concentration was 1.0 mM, the TC removal decreased from 92 to 87% after 540 min. It was reported that PO 4 3− could be adsorbed by goethite through electrostatic attraction and inner-sphere surface complexation (Ajmal et al. 2018;Liu et al. 2012b). Therefore, the adsorption sites over the surface of limonite were occupied by PO 4 3− leading to the decrease of TC removal. However, the increased PO 4 3− concentration has a negligible effect on TC removal, perhaps due to the formation of PO 4 3− bridging between TC and limonite.

Adsorption
As mentioned above, TC was efficiently removed by limonite under various conditions. The electrostatic interaction and inner-sphere surface complexation have been proved to participate in the adsorption of TC. To further shed light on the adsorption mechanism between limonite and TC, the FT-IR spectra of the limonite before and after TC adsorption were analyzed and depicted in Fig. 5. For limonite, the peaks at 793 and 890 cm −1 were assigned to O-H bending vibration of Fe-OH in and out of the plane, respectively (Oliveira et al. 2008). The main characteristic peaks of TC were concentrated at the range of 1200-1700 cm −1 , and the bands at 1523, 1616, 1582, and 1456 cm −1 could be assigned to the stretching vibration of NH 2 amide and ketone C = O stretching in ring A, ketone C = O in ring C, and C-C skeletons, respectively (Wu et al. 2019a;Zhang et al. 2015). Specifically, the intensity of O-H bending band out of the plane decreased, indicating that TC complexed with limonite by replacing -OH of limonite (Liu et al. 2012b). Meanwhile, TC-adsorbed limonite showed that the presence of C-C stretching of TC, NH 2 amide of ring A, and C = O of rings A suggests that TC molecules were adsorbed on limonite successfully (Wu et al. 2019b). Figure 6a displays TOC and TC removal efficiencies by limonite. After treatment 540 min, 74% of TOC was less than the removal efficiency of TC (92%). This phenomenon indicated that 18% of TC was degraded to small molecular organic in solution . The HPLC spectra of TC (Fig. 6b) shows that a new peak was emerged during the degradation process, which could be regarded as generated intermediate. Furthermore, the mass spectra (Fig. S4) displays a protonated form of TC with m/z value of 445 and representative intermediate with m/z value of 415. In addition, Fig. 6c shows that the Mn components gradually dissolved during the whole reaction, indicating that the reduction of MnO 2 was responsible for the degradation of TC ). On the contrary, no dissolution of Mn ions was observed in deionized water with the same pH during the whole reaction process (Fig. 6d). Therefore, it was speculated that TC participated in the process of electron transfer and mediated the degradation of TC. Differently, no dissolution of Fe ions was observed in water and TC solution.

Degradation
The interaction between oxygen and MnO 2 has been evidenced to influence the degradation (Chien et al. 2009;Mahamallik et al. 2015). To explore the effect of oxygen on TC degradation, the nitrogen and air were continuously purged into the solution to create anoxic and aerobic environment with dissolved oxygen concentration of 0.1 and 7.9 mg·L −1 , respectively. The concentration of dissolved oxygen in control group was 7.0 mg·L −1 . Figure 7a shows that the TC removal efficiency increased from 74 to 85% at 60 min of reaction after air poured compared with control group while the TC removal efficiency was decreased  by 7% under anoxic conditions. Therefore, oxygen played a vital role in the oxidation process. In the previous reports, singlet oxygen ( 1 O 2 ) and hydroxyl radical (·OH) might be generated in the interaction between manganese oxides and oxygen (Pham et al. 2020;Zhang et al. 2011). To identify the reactive oxygen species in the reaction, TBA and FFA were acted as scavengers of ·OH and 1 O 2 , respectively (TBA, k ·OH = 4.8-7.6 × 10 8 L·mol −1 ·s −1 ; FFA, k 1O2 = 1.2 × 10 8 L·mol −1 ·s −1 ) (Fan et al. 2019;Li et al. 2021b). In Fig. 7b, the removal of TC decreased from 93 to 87% with an introduction of FFA while the slight inhibition role of TBA was observed. Furthermore, ESR coupled with TEMP as a specific spin trapping agent was used to detect the 1 O 2 . As shown in Fig. S5, the characteristic signals of the three-line spectrum of 1 O 2 were observed (Chu et al. 2021), indicating that 1 O 2 was involved in the TC degradation.
The redox reaction mediated by pollutant as electron donor has been widely accepted for the oxidation mechanism of pollutants (Joshi et al. 2017;Kamagate et al. 2020;Zhang et al. 2021b). In this work, no Mn ions leaching was observed in the absence of TC, which confirmed that redox reaction occurred in TC solution with TC as an electron donor. To further verify the existence of redox reaction, linear sweep voltammetry (LSV) was tested. As shown in Fig. 7c, under the same pH and electrolyte concentration, the current response of limonite in TC solution was stronger than that of deionized water, illustrating that electron transfer was promoted in limonite-TC system . Besides, a recent study confirmed that Mn 3+ contributed electrons to adsorbed O 2 to produce superoxide radicals (·O-2) (Zhang et al. 2021a), and ·O-2 could act as the precursor to form 1 O 2 . Therefore, it could be speculated that both TC and Mn 3+ played the role of electron donors mediated the production of 1 O 2 to degrade TC. A continuous interaction was occurred between Mn 3+ / Mn 4+ and O 2 , leading to the production of 1 O 2 to degrade TC molecules.

Adsorption kinetics and isotherms
To explore the adsorption behavior between the limonite and TC, the pseudo-first-order and pseudo-second-order models were used to fit the TC removal process (FFA was added to exclude the effect of degradation). As presented in Fig. S6ab and Table. S2, the pseudo-second-order model with the higher R 2 value (R 2 = 0.997) was more suitable to ascribe the TC adsorption behavior. The calculated equilibrium adsorption capacity (q ecal = 34.7 mg·g −1 ) was closer to the experimental data (q eexp = 33.0 mg·g −1 ), demonstrating the adsorption process was much affected by chemisorption (Ghobadi et al. 2018). The Langmuir and Freundlich adsorption isotherms models also were used to investigate the adsorption process between TC and limonite. The results and fitted data are shown in Fig. S6c-d and Table S3, respectively. It was evident that the Freundlich isotherm model with the higher R 2 value of 0.997 was a better model to fit the adsorption of TC over limonite. This implied that the adsorption process of TC over limonite was multi-layered (Ma et al. 2019). The maximum adsorption capacity of limonite obtained from the Freundlich isotherm model was 80.0 mg·g −1 . Table 1 lists the comparative maximum adsorption ability between the Mn-rich limonite and other previously reported adsorbents. Although the experimental conditions were different, one should be noted that the q m of limonite was relatively encouraging compared with other adsorbents such as MgAl layered double oxide ). This finding demonstrated that the Mn-rich limonite was a promising adsorbent for TC removal from aqueous solution.
On the basis of the above studies and discussion, it is rational to propose the removal mechanism of TC by limonite. Firstly, TC molecules were adsorbed by limonite through electrostatic interaction and surface complexation. Subsequently, Mn 3+ / Mn 4+ was reduced to Mn 2+ /Mn 3+ by accepting electrons derived from TC. Finally, Mn 2+ /Mn 3+ produced by the above reduction and the Mn 2+ /Mn 3+ derived from limonite contributed electrons to O 2 to generate 1 O 2 , degrading adsorbed TC molecules to other intermediate products:

Synergistic effects of goethite and manganese oxides
The synergistic effects of goethite and manganese oxides in the removal of TC by Mn-rich limonite were evaluated by comparing the removal of TC by goethite and MnO 2 . Goethite was synthesized according to previous research (Liu et al. 2012a). Figure S7a shows that the reflections of MnO 2 were consistent with that of pyrolusite (JCPDS 81-2261). Figure S7b reveals that over 90% TC removals were achieved in the presence of the Mn-rich limonite, which was more effective than goethite and MnO 2 with 56% and 74% TC removal, respectively. Based on the comparison results, it could be derived that the synergistic effects between goethite and manganese oxides favored the removal of TC.

Regeneration of limonite
The recyclability of limonite is another essential indicator for its application. Figure 7d shows that limonite displayed a gradual decrease in the removal of TC in recycling experiments. The low reused capacity of limonite might be ascribed to the decline of available surface sites with the occupation of TC molecules. On the other hand, the leaching of Mn ions could be another reason (Fig. 6c). To regenerate the used limonite, the peroxymonosulfate (PMS) solution was used to oxidize adsorbed TC on limonite. The treated limonite was washed with deionized water and dried for the next recycling experiment. Obviously, a good recycling performance of treated limonite could be observed with a negligible decrease of TC removal after four runs. Therefore, the Mn-rich limonite exhibited excellent regenerated recyclability and low-cost advantages, which was useful for the enrichment and centralized treatment of antibiotics in wastewater.

Conclusion
In this study, the removal performance and mechanisms of TC by Mn-rich limonite were investigated. The TC adsorption behavior of limonite was fitted well by the pseudo-second-order model and Freundlich isotherm model. Results revealed that both electrostatic interaction and complexation participated in the adsorption of TC. In addition, the analysis of TOC, metal ions dissolution, HPLC-MS, quenching experiment, and XPS indicated that TC was degraded by  manganese oxides to intermediates during the removal process. The 1 O 2 was formed during the redox reaction and responsible for TC degradation. Based on the above discussions, it could be concluded that TC was removed by electrostatic interaction and surface complexation with goethite and oxidation by manganese oxides. These discussions were helpful for the exploration of the Mn-rich limonite as a promising material to treat antibiotics-contaminated wastewater.