Addition of MnO2 in synthesis of nano-rod erdite promoted tetracycline adsorption

Erdite is a rare sulphide mineral found in mafic and alkaline rocks. Only weakly crystallised fibrous erdite has been artificially synthesised via evaporation or the hydrothermal method, and the process generally requires 1–3 days and large amounts of energy to complete. In this study, well-crystallised erdite nanorods were produced within 3 h by using MnO2 as an auxiliary reagent in a one-step hydrothermal method. Results showed that erdite could synthesised in nanorod form with a diameter of approximately 200 nm and lengths of 0.5–3 μm by adding MnO2; moreover, the crystals grew with increasing MnO2 addition. Without MnO2, erdite particles were generated in irregular form. The capacity of the erdite nanorods for tetracycline (TC) adsorption was 2613.3 mg/g, which is higher than those of irregular erdite and other reported adsorbents. The major adsorption mechanism of the crystals involves a coordinating reaction between the −NH2 group of TC and the hydroxyl group of Fe oxyhydroxide produced from erdite hydrolysis. To the best of our knowledge, this study is the first to synthesise erdite nanorods and use them in TC adsorption. Erdite nanorods may be developed as a new material in the treatment of TC-containing wastewater.


Results and Discussion
The morphology and crystallography of the erdite particles were recorded by Scanning electron microscope (SEM) and X-ray powder diffraction (XRD) . Figure 1(A) shows that, without MnO 2 , the prepared P0 presents as irregular flake-shaped flocs. After MnO 2 addition, nanorod particles with a diameter of approximately 200 nm and lengths of 0.5-2 μm are formed ( Fig. 1(B)). The weak diffraction peaks of P0 (Fig. 2 P0) at 2θ = 12.7°, 16.5°,  When the Mn/Fe molar ratio was increased from 0.05 to 0.2, crystallisation was improved, and the length of the nanorod crystals increased to 3 μm ( Fig. 1(C)). Moreover, the predominant orientation of the erdite phase of P0.2 along the (110) reflection intensified (Fig. 2) in comparison with that of P0.05. This finding suggests that excess MnO 2 promotes microcrystal growth along the (110) reflection to produce the long erdite nanorods of P0.2. A peak at 2θ = 23.1°, which belongs to S 8 (JCPDS 78-1888), was observed in all three spectra, thereby indicating that the polysulphide S 8 is generated in the formation of erdite.
When the Mn/Fe ratio was increased to 1, a mixture of nanorods ( Fig. 3(A)) and octahedral and irregular particles was obtained ( Fig. 3(B)). The nanorod particles showed a morphology similar to that of P0.2 and was attributed to well-crystallised erdite (Fig. 4); the octahedral and irregular particles were considered to belong to γ-MnS and polysulphide S 8 (Fig. 4), respectively. This result demonstrates that excess MnO 2 at a molar ratio of 1 maintains the morphology of erdite nanorods but increases the content and size of γ-MnS crystals.
The valences of S, Fe and Mn on the surface of the erdite particles were determined by X-ray photoelectron spectrometry (XPS). The S 2p spectra in Fig. 5(A) reveal four major S 2p3/2 peaks at 160.2, 161.3, 163.2 and 167.5 eV; these peaks belong to structural S 2 in erdite, S 2− , polysulphide and SO 4 2− , respectively. Deconvolution of the S 2p envelope indicates that the relative area of the S 2− peaks is 26.5% in P0 but only 15.6% in P0.2. By contrast, the relative area of structural S 2 peaks in erdite increased from 5.2% in P0 to 44.3% in P0.2. This result  indicates that S 2− is involved in the formation of erdite. The Fe 2p 3/2 spectra of P0 in Fig. 5(B) show a weak peak at 707.8 eV, which could be assigned to Fe 3+ -S in erdite; this peak intensified in P0.05 and P0.2. The results indicate that erdite generation is enhanced by addition of MnO 2 , consistent with the XRD results (Fig. 2). The Mn 2p spectra showed a peak with a binding energy of 640.4 eV and a satellite peak 5.1 eV ahead of the binding energy ( Fig. 5(C)), which may be assigned to structural Mn 2+ in MnS 33,34 . These results reveal that reduction of MnO 2 by Na 2 S, along with the generation of MnS, occurs in P0.05 and P0.2.
When Na 2 S is added to the liquid phase, it hydrolyses to HS − and OH − , resulting in increases in pH. When the pH of the solution is greater than 13, Fe 3+ is hydrolysed to Fe(OH) 4 − 35 . HS − is diffused into the structure of Fe(OH) 4 − , and OH − is replaced by HS − with the formation of Fe(OH) 3 HS − (Eq. (1)). The two newly formed Fe(OH) 3 HS − species are subsequently polymerised with the release of two water molecules (Eq. (2)). The polymerisation reaction continues in the presence of adequate Fe(OH) 4 − , with the polymer chain (FeS 2 ) n n− being the final product (Eq. (3)). The free charges on the (FeS 2 ) n n− chain are neutralised by Na + 36 , and free channels within these chains are occupied by water molecules 37 . Therefore, well-crystalised erdite consisting of long linear (FeS 2 ) n n− chains, with each Fe ion tetrahedrally surrounded by four sulphur atoms 38 , is generated.
(2) www.nature.com/scientificreports www.nature.com/scientificreports/ (3) In Eq. (3), R 1 and R 2 represent the growing polymeric species of the (FeS 2 ) − unit. During erdite formation, replacement of OH − and polymerisation of (FeS 2 ) n n− (Eqs (1-3) occur spontaneously and rapidly 3 , and the formed (FeS 2 ) n n− chain is stable in structure, even after calcination at approximately 500 °C for 12 h 39 . This finding indicates that the redox reaction of Fe-S in erdite is considerably inhibited.
Dissolved oxygen in the liquid phase reacts with HS − to form elemental sulphide and OH − as the byproduct (Eq. 4). Elemental sulphur is a key intermediate 10,40 and could react with HS − to form polysulphide S 8 via Eqs (5, 6) 41 . Polysulphide S 8 has low solubility in solution 10 and forms in the colloidal state at high pH 17,41 , followed by precipitating from the aqueous solution 42 . Oxidation of HS − to sulphate also occurs under oxygen-rich conditions via Eq. (7) 43 but is inhibited by the exhaustion of dissolved oxygen during the hydrothermal process. Luther et al., using a frontier molecular orbital model, postulated the formation of elemental S as an intermediate in the H 2 S and H 2 O 2 reactions and found the formation of polysulphide S 8 as the major end product as well as a small amount of sulphate as a minor product 43,44 .
Addition of MnO 2 accelerates the oxidation of HS − via the following reactions. HS − is rapidly oxidised to elemental sulphide with the generation of MnOOH and OH − via Eq. (8) 45 . Thus, the reaction of HS − and dissolved oxygen is catalysed by the insoluble MnOOH formed under alkaline conditions 41,45 , which promotes the formation of polysulphide S 8 via Eqs (5,6).
Therefore, a large amount of OH − , which promotes the formation of Fe(OH) 4 − and polymerisation of (FeS 2 ) n n− chains, is produced as a result of the three reactions (Eqs (1, 4 and 8)). When the dissolved oxygen is exhausted, the structural Mn in MnOOH is readily reduced to Mn 2+ by HS − 46 with the generation of MnS. Without MnO 2 , oxidation of HS − is slow, leading to the low production of OH − and Fe(OH) 4 − , which inhibits erdite formation.
The adsorption capacity of the erdite nanorods for TC was investigated because the latter is a common pollutant in pharmaceutical wastewater 19,29 . Four of the most widely established isotherm models, namely, the Langmuir (Eq. (9)), Freundlich (Eq. (10)), Dubinin-Radushkevich (D-R) (Eq. (11)) and Temkin (Eq. (12)) isotherms, were used to simulate the experimental data; these models can be expressed as follows: where C e is the equilibrium concentration of TC in the solution (mg/L), q e is the equilibrium adsorption capacity (mg/g), K L and q m are the Langmuir isotherm constant and maximum adsorption capacity (mg/g), respectively, K F and b F are the Freundlich constants, K DR is the D-R isotherm constant and A and B are the Temkin isotherm constants.
The linear fittings of the four isotherm models are shown in Fig. 6. The parameters of each isotherm model were calculated with their correlation coefficients (R 2 ) and the results are shown in Table 1. The fitting of the linearised Langmuir model to the experimental data was better than those of the Freundlich, D-R and Temkin isotherms. Thus, the Langmuir isotherm provides the best description of the experimental data.
The maximum adsorption capacity of erdite increased significantly from 675.7 mg/g for P0 to 2446.7 mg/g for P0.05 and then to 2613.3 mg/g for P0.2; these results indicate that MnO 2 is important to the adsorption capacity of erdite for TC. MnO 2 is a common auxiliary reagent and widely used to promote the performance of adsorbents toward various contaminant (e.g. TC) 31,47 . For instance, Song et al. reported that graphene loaded with 40% nanorod MnO 2 exhibits a maximum TC adsorption capacity (q m ) of 1789 mg/g and that the observed adsorption (2019) 9:16906 | https://doi.org/10.1038/s41598-019-53420-x www.nature.com/scientificreports www.nature.com/scientificreports/ capacity remains constant with increasing MnO 2 loadig from 40% to 60% 31 . In our study, P0.2 showed a desirable TC q m of 2613.3 mg/g, which is higher than those of P0, 40% MnO 2 -loaded graphene 31 , carbon materials (maximum of 1340.8 mg/g) 30 and zeolite materials (800 mg/g) 19 . P0.2 also showed a TC removal rate higher than  www.nature.com/scientificreports www.nature.com/scientificreports/ those of other flocculants (Fig. 7(A)), such as polymeric aluminium chloride (PAC) and polymeric ferric sulphate (PFS). Similar to these flocculants, used P0.2 has an amorphous form and shows a lower TC removal rate in comparison with that of P0.2 ( Fig. 7(B)); this finding suggests that the reusability of erbite particles is undesirable. After adsorption, the solution pH slightly increased to approximately 7.4, but the concentrations of Fe, Mn, sulphate and sulphide did not change obviously. Hence, no secondary pollutant was released to the solution during adsorption.
P0 and P0.2 were characterised by XRD and XPS after adsorption to investigate the adsorption mechanism of TC by erdite. The diffraction peaks of erdite intensified from P0 to P0.2, as shown in Fig. 3, but disappeared after TC adsorption, leaving only S 8 diffraction peaks (Fig. 8). Thus, only the XPS peaks of polysulphide and sulphate are recorded in Fig. 9(A). The Fe 2p XPS spectra ( Fig. 9(B)) shows the absence of the typical peak of Fe(III) -S in erdite at 707.8 eV and the formation of a new peak at 709.7 eV, which is attributed to Fe oxyhydroxide from erdite hydrolysis. These findings indicate that the erdite in P0 and P0.2 is completely hydrolysed with the formation of weakly crystallised Fe oxyhydroxide. The Mn 2p3/2 peak with a binding energy of 641.7 eV (Fig. 9(C)) agrees with the Mn 3+ peak in MnOOH reported in other studies [48][49][50] , thus suggesting that the hydrolysis of MnS in P0.2 is followed by oxidation during TC adsorption 45 . The N 1 s XPS spectra (Fig. 9(D)) of P0 and P0.2 show two peaks at 399.5 and 401.4 eV, which belong to the N atoms of -NH 3 + and -NH-of TC, respectively, after adsorption. No peak was observed in the N1s spectra of P0 and P0.2 before adsorption, thus suggesting that the adsorption of TC occurs on the hydrolysed species of P0 and P0.2.  (1) (Fig. 10, step 1) to yield Fe oxyhydroxide as the final product because Fe(OH) 4 − is unstable and rapidly dehydroxylated at pH < 11.  www.nature.com/scientificreports www.nature.com/scientificreports/ No S 2− -containing compounds are observed in the XPS S 2p spectra (Fig. 9(A)), which suggests that the concentration of surface S 2− is low. Thus, the redox reaction of the Fe-S bond in erdite is subordinate in comparison with the hydrolysis of erdite. MnS is similarly hydrolysed, and the Mn 2+ obtained is oxidised to MnOOH in the presence of dissolved oxygen 51 , which oxidises HS − at ambient condition (Fig. 10, step 2) 45 .
Addition of MnO 2 during erdite synthesis enhanced TC adsorption. Well-crystallised erdite (P0.2) could be generated by addition of MnO 2 , which is subsequently hydrolysed to generate more Fe oxyhydroxide and MnOOH ( Fig. 10(B,C)). Both products have abundant surface hydroxyl groups 48,52 , which serve as coordination sites for TC adsorption (Fig. 10, step 3). In the liquid phase, TC forms an amphoteric ion 30 , which is readily adsorbed by Fe oxyhydroxide and MnOOH 29 . The -NH 2 group in the side chain of TC links to H in the hydroxyl group of Fe oxyhydroxide and MnOOH to form the -NH 3 + group, resulting in TC adsorption. As erdite hydrolysis continues, polymerisation of two adjacent Fe oxyhydroxides occurs with the release of a water molecule, resulting in the formation of irregular aggregates (Fig. 10, step 4). The uniform P0.2 nanorod particles www.nature.com/scientificreports www.nature.com/scientificreports/ are apparently smaller than the P0 flocs ( Fig. 1(A,C)), but the hydrolysis product of erdite in P0.2 (i.e. Fe oxyhydroxide) shows better TC adsorption in comparison with that of the P0 flocs.
Erdite shows unique characteristics in acidic or neutral pH solution and could generate Fe oxyhydroxide, which plays a key role in TC adsorption after its hydrolysis. The product could also adsorb various other pollutants, such as trace heavy metals 53 , dissolved organic matter 54 and bacteria 55 ; such properties confer this novel material with promising potential for application to the adsorption of heavy metals and organics from wastewater. Similar to FeCl 3 , during erdite production, solid waste containing Fe oxides, such as goethite, hematite and magnetite, could be reduced by Na 2 S 56 . These wastes ubiquitously exist in groundwater treatment sludge 57,58 , red mud and fly ash 59 and could significantly decrease the cost of erdite production. Overall, our results demonstrate an alternative strategy for recycling Fe-containing solid waste for low-cost erdite production. Future studies could investigate the synthesis of erdite from recycled Fe-containing waste and test the effectiveness of the resulting material in wastewater treatment.

Materials and Methods
Synthesis of erdite nanorods. Erdite nanorods were hydrothermally synthesised via the following steps ( Fig. 11). In brief, 1.63 g of FeCl 3 ·6H 2 O was dissolved in 30 mL of deionised water and then added with MnO 2 powder under magnetic stirring. After stirring for 10 min at 200 rpm, the suspension produced was transferred to a 50 mL Teflon vessel and added with 7.2 g of Na 2 S.9H 2 O. The vessel was then sealed and hydrothermally treated at 160 °C for 3 h before cooling down to room temperature. The deposits at the bottom of the vessel were collected, washed five times with 30 mL of deionised water, and then vacuum dried at 50 °C overnight. During erdite synthesis, the Mn/Fe molar ratio was varied from 0 to 0.05 to 0.2, and the obtained erdite particles were denoted as P0, P0.05 and P0.2, respectively. TC adsorption by the erdite nanorods. TC, a typical antibiotic found at high levels (above 920 mg/L) in pharmaceutical wastewater 12 , was employed to assess the adsorption performance of the synthesised erdite nanorods. A stock solution of 2000 mg/L TC was prepared, and its pH was adjusted to 5 by adding 1.44 mol/L HCl and 1.32 mol/L NaOH. A series of dilute solutions was prepared from the stock by addition of deionised water. Each dilution (20 mL) was mixed with 0.01 g of the erdite particles in an Erlenmeyer flask. The flasks were sealed with parafilm and shaken at 200 rpm in an incubator (THZ-98A, Yiheng, Shanghai, China) at room temperature. After 24 h, the flasks were taken from the incubator, and the erdite particles were separated by agitation at 5500 rpm for 5 min. The TC concentration in the supernatant was determined by high-performance liquid chromatography (LC-16, Shimadzu, Japan) using a mobile phase composed of 0.1% phosphoric acid and methanol at a ratio of 60:40 (v/v). The absorbance at 268 nm was read by using a UV detector, and the retention time was approximately 5 min. The adsorption capacity (q e , mg/g) of erdite was calculated by using the following equation: where C 0 and C e are the initial and equilibrium concentrations of TC (mg/L), respectively, V is the solution volume (L), and m is the particle dosage (g).
Characterisation of the erdite nanorods. The morphology of the erdite particles was determined by field-emission scanning electron microscopy (SEM, FEI Co., USA). The X-ray diffraction (XRD) patterns of the erdite particles were analysed by a diffractometer (RAPID-S, Rigaku, Japan) with Cu-Kα radiation in the 2θ range of 10°-40°. X-ray photoelectron spectroscopy (XPS) spectra were obtained using an X-ray photoelectron spectrometer (VG-ADES, England) operated at 150 W with monochromatised Al-Kα X-rays (hν = 1486.6 eV). The base pressure in the analytical chamber was ≈10 −9 mbar. Narrow-region electron spectra were acquired with an analyser pass energy of 20 eV. Binding energies were calibrated against that of the C1s peak (284.6 eV). The fitting curves were obtained using Thermo Avantage software (version 5.976, Thermo Scientific, USA) with a Shirley background and a Gaussian-Lorentzian peak model.

conclusion
Erdite nanorods were synthesised via a facile hydrothermal method using MnO 2 as an auxiliary reactant. Addition of MnO 2 considerably promoted the formation of erdite nanorods through the efficient generation of NaOH from the disintegration of Na 2 S. The generated erdite nanorods exhibited a TC adsorption capacity higher than those of previously reported adsorbents. The formation of stable -NH 3 + groups between the hydroxyl group of Fe oxyhydroxide obtained from erdite hydrolysis and the -NH 2 group in the TC side chain is the major TC adsorption mechanism of erdite.