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Article

Efficient Phosphate Adsorption from Groundwater by Mn-FeOOHs

by
Mengxue Li
1,2,*,
Guanghui Sun
1,
Ziyang Chu
3,
Jing Wang
1,2 and
Yu Qiu
1
1
Experiment Centre of Environmental Science, Bengbu University, Bengbu 233000, China
2
Anhui Rural Ecological Environment Protection and Restoration Research Center, Bengbu University, Bengbu 233030, China
3
Institute of Environmental Mineral and Material, School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China
*
Author to whom correspondence should be addressed.
Water 2024, 16(16), 2294; https://doi.org/10.3390/w16162294
Submission received: 16 July 2024 / Revised: 7 August 2024 / Accepted: 13 August 2024 / Published: 14 August 2024
(This article belongs to the Section Wastewater Treatment and Reuse)
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Versions Notes

Abstract

:
Manganese co-precipitated with goethite (Mn-FeOOH) is ubiquitous within (sub-)surface environments, which are considered one of the most important sinks for phosphorus pollution management. Accordingly, various mole ratios of Mn-FeOOHs are synthesized and characterized by XRD, FE-SEM, FTIR, BET, XPS, hysteresis loop, acid–base titration and zero potential. According to XRD and FESEM images, the substitution of Mn causes subtle alterations in the microstructure and crystal structure of goethite, and the morphology of Mn-FeOOHs is transformed from needle-shaped goethite to a short-rod-shaped rough surface with increasing Mn substitution. Based on the analysis of BET and acid–base titration, the substitution of Mn into goethite significantly improved the surface area, pore volume, surface properties and active sites of goethite, thereby establishing a theoretical basis for effective subsequent adsorption. Batch experiment results show that the removal rate of phosphate decreases with the increasing solution pH, indicating that acidic groundwater conditions are more conducive to the removal of phosphate. In addition, the adsorption of phosphate on Mn-FeOOHs is independent of ionic strength, indicating that the inner-sphere surface complexation predominated their adsorption behaviors. The isotherm experiment results showed that Mn-G15 exhibits the strongest adsorption capacity for phosphate at pH 5.5 and T = 318 K, with a maximum adsorption capacity of 87.18 mg/g. These findings highlighted the effect of Mn content on the fixation of phosphate onto Mn-FeOOHs from (sub-)surface environments in pollution management.

1. Introduction

Phosphorus is a non-renewable resource and an important component in ensuring ecosystem stability [1,2]. In recent years, due to human agricultural and industrial production activities, a large amount of phosphorus has been released into water bodies, causing serious eutrophication, algal blooms, water quality degradation and ultimately leading to the destruction of ecosystems [3,4,5]. Therefore, the recovery of phosphate from water is crucial for reducing eutrophication and alleviating the crisis of phosphorus resources [6]. It is urgent to develop more effective measures to reduce phosphorus pollution and promote phosphorus cycling, and designing a more effective, economical and environmentally friendly phosphorus removal technology remains crucial.
Iron (hydroxyl) oxide is a type of iron containing inorganic compounds that are widely present in soil and rocks, mainly including goethite (α-FeOOH), orthorhombic hematite (β-FeOOH), lepidocrocite (γ-FeOOH), magnetite (Fe3O4) and ferrihydrite (Fe5HO8·4H2O) [7,8,9,10]. Among them, goethite is the most important, and its unique crystal structure and physicochemical properties make it have important application value in many fields, such as adsorbents [11,12,13], catalyst carriers [14], magnetic materials [15,16,17], etc. However, the isomorphic substitution of goethite in soil is a complex and interesting phenomenon that involves ion exchange processes in mineral lattices. Homomorphic substitution involves various metal ions such as Al~(3+), Ni~(2+), Cu~(2+), Zn~(2+), Cd~(2+), Pb~(2+), Cr~(3+), Mn~(3+), Co~(3+), etc. [11,18,19,20,21,22]. According to previous reports [23], aluminum substitution for goethite can promote its adsorption of phosphate ions, but the maximum adsorption capacity is relatively low. When the molar ratio of aluminum substitution is 12%, the maximum adsorption capacity for phosphate ions is only 2.13 mg/g. In recent years, researchers in the field of materials science have been exploring how to improve the performance of goethite through modification methods. Among them, manganese (Mn), as an important transition metal element, is widely used in the field of material modification due to its unique electronic structure and chemical properties. The introduction of manganese can not only change the crystal structure of materials but also affect their electronic distribution and chemical reactivity, thereby optimizing their physical and chemical properties [24,25,26,27]. Therefore, by introducing manganese into goethite and conducting research on manganese modified goethite, it is expected to obtain new materials with superior performance and further expand the application fields of goethite. Additionally, the research results of this article also provide a new type of material for the efficient adsorption of phosphates.
The aims of the research are to (1) explore the phase composition, evolution, surface active groups, valence state changes in elements and surface charge variations in Mn-FeOOHs using XRD, FE-SEM, FTIR, BET, VSM, XPS, zeta potential and potentiometric titration; (2) explore the effect of the lattice substitution of Mn on phosphate adsorption by Mn-FeOOHs within variable factors; (3) discuss the reaction mechanism between phosphate and Mn-FeOOHs, as well as the relationship between phosphate and active functional groups based on the FTIR and XPS results.

2. Materials and Methods

2.1. Chemical Reagents

The potassium dihydrogen phosphate, potassium antimony tartrate, ascorbic acid, ammonium molybdate, manganese nitrate, potassium hydroxide, sodium hydroxide, iron nitrate and sodium chloride used in this study are all purchased from China National Pharmaceutical Group Chemical Reagent Co., Ltd. (Shanghai, China). In addition, the hydrochloric acid, sulfuric acid and nitric acid are all purchased from China National Pharmaceutical Group Chemical Reagent Co., Ltd. (Shanghai, China). All the chemicals are analytically pure and used without any further purification.

2.2. Synthesis of Mn-FeOOH

The goethite and Mn-FeOOHs used in this study are synthesized using the co-precipitation method [28,29]. Briefly, we prepared 1 mol/L ferric nitrate solution and 0.5 mol/L manganese nitrate solution separately and mixed the two solutions evenly according to a fixed ratio. Subsequently, we added a certain amount of deionized water and KOH solution in proportion to ensure the pH of the solution reaches 13. After completing the above steps, we placed the mixture in a 70 °C oven and aged for 11 days. Then, the suspension was centrifuged and washed 6 times with deionized water, and the remaining solid was dried at 60 °C. According to the amount of Mn content, the samples are sequentially recorded as G-0, Mn-G6, and Mn-G15. The specific proportion scheme is shown in Table S1 in Supporting Information (SI).

2.3. Characterization

The crystalline phases of Mn-FeOOHs are characterized by a SmartLabSE X-ray diffractometer (Rigaku, Japan). The XRD diffraction patterns of adsorbents are recorded ranging from 10 to 70°, operated with Cu-Kα irradiation in reflection mode at 40 kV and 30 mA. The morphologies of Mn-FeOOHs are investigated by a Thermo Scientific ESCALAB Xi+ microscope (Thermo Fisher, Waltham, MA, USA). The surface functional groups of Mn-FeOOHs are characterized using a Tristar3020 FTIR spectrometer (Thermo Nicolet, Waltham, MA, USA) at room temperature. The SBET of Mn-FeOOHs are determined using nitrogen gas adsorption at 77 K under atmospheric pressure on a VERTEX80 surface area and pore size analyzer. The magnetic properties of Mn-FeOOHs are measured by a LakeShore7404 Vibrating Sample Magnetometer (LakeShore, Carson, CA, USA). The X-ray photoelectron spectroscopy (XPS) analysis is carried out using a Thermo Scientific ESCALAB Xi+ diffractometer. The surface-active site concentrations of Mn-FeOOHs are examined using the acid–base titration method on a Metrohm 888 potentiometer (Metrohm, Herisau, Switzerland). The zeta potentials of Mn-FeOOHs are tested by a Marvin Zetasizernan, with a pH range of 2–11, while the solid–liquid ratio remained consistent with the adsorption experimental conditions.

2.4. Batch Experiments

The phosphate adsorption measurements on Mn-FeOOHs are conducted by batch experiments. The pH of the suspensions is adjusted using 0.01–0.5 mol/L HNO3/NaOH solutions. To ensure that the adsorption experiment reaches equilibrium, the tubes are then shaken for 24 h. After that, the suspension is subjected to solid–liquid separation using a high-speed centrifuge. Subsequently, the concentration of phosphate is tested by a spectrophotometer (722S, Shanghai Jinghua, Shanghai). More details on the experimental operations are attached to SI.

3. Results

3.1. Characterization

The FE-SEM and mapping of Mn-FeOOHs before and after adsorption of phosphate are illustrated in Figure 1. Figure 1A presents a picture of G-0, characterized by a smooth, needle-like morphology with an average size of approximately 400 nm. Upon Mn doping (Figure 1B,C), the G-0 surface becomes rough with the appearance of granular crystals, and as the Mn-doping level increases, the number of surface particles also rises. Mn-doping has altered the crystal morphology of goethite. Figure 1D shows the image of G-Mn15 after adsorbing phosphorus. The energy spectrum analysis results in Figure 1E reveal the presence of the Mn element on the structure of G-Mn15-P, confirming that Mn has successfully substituted the iron in G-0. Additionally, the presence of phosphorus indicates that G-Mn15 exhibits an adsorption effect on phosphates. Figure 1F–J present the mapping images of G-Mn15 after phosphate adsorption. Distribution analyses of O, Fe, Mn, and P elements reveal that the distribution area of Mn closely overlaps with that of Fe, further substantiating the successful incorporation of Mn into the G-0 crystals [30,31].
Figure 2A shows the XRD patterns of Mn-FeOOHs, the reflections at 2θ = 21.16, 33.25, 34.62, 36.64, 39.98, 41.07, 53.19, 58.92 and 61.36° of G-0, Mn-G6 and Mn-G15 are attributed to the crystal planes of (101), (301), (210), (111), (211), (401), (212), (601) and (020) of goethite [11,15,32], according to the standard reference pattern [ICSD-071809] [29]. The XRD pattern shows that as the amount of manganese isomorphic substitution increases, the peak intensities of the (101) and (111) crystal planes notably diminish, with corresponding reductions in the peak intensities of other crystal planes. These observations suggest that Mn ions have likely integrated into the goethite structure through isomorphic substitution, leading to alterations in specific crystal planes of goethite [18,33].
Figure 2B exhibits the FTIR spectrum of Mn-FeOOHs; the stretching vibrations in the range of 3500~3000 cm−1 are due to the absorption peaks of dissociative hydroxyl (−OH) and hydrated hydroxyl (−OH2) groups in goethite [34,35]. The peak around 908 cm−1 is characteristic of goethite, corresponding to the structure Fe-OH-Fe [36]. The peak at 794 cm−1 is due to out-of-plane bending (γOH) [11]. The peak at 610 cm−1 is associated with the Fe-O/Mn-O lattice vibrations [37,38]. The comparisons of FT-IR spectra with various amounts of isomorphic substitution reveal substantial differences in certain peaks for goethite with varying Mn substitution levels. As illustrated, with increasing Mn lattice and substitution amounts, the hydrated hydroxyl peak (-OH) peaks at 3126 cm−1 significantly weakened and shifted leftward, suggesting that Mn substitution impacts the surface free hydroxyl groups of goethite. Similarly, the peak intensities of Fe-OH-Fe, out-of-plane bending (γOH) and Fe-O/Mn-O diminished as the Mn substitution increased. Additionally, Figure 2B displays the FTIR of Mn-FeOOHs after adsorbing phosphate. Compared to the pre-adsorption spectrum, a new characteristic peak appeared in the infrared spectrum after adsorption within the range of 1100–1300 cm−1, corresponding to the stretching vibration peaks of P=O and P-OFe/Mn [23]. The peaks intensified with the increasing Mn substitution, indicating that Mn’s lattice substitution boosts the adsorption capability of goethite for phosphates, with the Fe-O, -OH and Mn-O bonds contributing to the reaction [29].
The results of the acid–base titration of Mn-FeOOHs are shown in Figure 2C. Figure 2C shows the Gran plots for the titration data of Mn-FeOOHs. The Gran equation is as follows [6,22]:
pH < 7: G1 = (V0 + VHCl + VNaOH) × 10−pH × 100
pH > 7: G2 = (V0 + VHCl + VNaOH) × 10−(13.8−pH) ×100
where G1 and G2 are the Gran equations at the acidic and alkaline sides, respectively [39]; V0 (mL) is the initial volume of suspension; VHCl (mL) is the total volume of HCl solution added during the operation. Similarly, VNaOH (mL) is the volume of NaOH added during the process [40]. The hydroxide ions introduced into the Mn-FeOOH suspension are discernible in the Gran plot. The addition of NaOH only neutralized the H+ in the suspension before Veb1 and then complexed between Veb1 and Veb2, ending at Veb2. After Veb2, NaOH is only used to regulate the pH of the suspension. As illustrated in Figure 2C, the two equivalent points for Veb1 and Veb2 can be determined via linear regression analysis of the Gran plot [39]. Surprisingly, the change in the abscissa difference between the Veb1 and Veb2 values of Mn-FeOOHs is positively correlated with an increase in Mn content, indicating that Mn-FeOOHs have different capabilities. Therefore, the maximum abscissa difference in Mn-G15 indicates the strongest load capabilities. This result provides a theoretical basis for the subsequent high adsorption performance. Additionally, as determined from the acid–base titration data, the surface-active site concentrations(C(SOH)) of G-0, Mn-G6 and Mn-G15 are 0.22, 0.41 and 0.73 mmol/g, respectively. The gradually increasing surface-active site concentrations suggest that the Mn lattice substitution has altered the surface properties of goethite.
Figure 2D displays the zeta potential of Mn-FeOOHs, which were measured in solutions with various pH levels. The pHPZC for G-0, Mn-G6 and Mn-G15 are 7.39, 6.27 and 5.65, respectively. The analysis results indicate that with the increase in Mn substitution, the zeta potential gradually decreases. The zeta potential of Mn-FeOOHs is closely related to its surface charge in different pH solutions. When the pH of the solution is below the zeta potential, the material surface carries a positive charge, and vice versa.
To investigate the effect of Mn substitution for goethite on its magnetic properties, the hysteresis loop of Mn-FeOOHs was tested at room temperature, and the results are shown in Figure 3A. Goethite exhibits minimal magnetism, which is reasonable. With Mn lattice substitution, the saturation magnetization of Mn-G6 and Mn-G15 reaches 2.46 and 6.47 emu/g, respectively. This suggests that Mn substitution in goethite amplifies its magnetic hysteresis properties.
Specific surface area analysis was performed on goethite with varying amounts of Mn isomorphic substitution, with the results presented in Table 1. The analysis revealed that the specific surface area of goethite without isomorphic substitution was 7.32 m2/g, while that of goethite with 6% and 15% isomorphic substitution was 43.86 and 77.18 m2/g, respectively. The results clearly indicate that the isomorphic substitution of Mn significantly influences the specific surface area. Moreover, the pore volume is also influenced by the isomorphic substitution of Mn, generally following the same trend as changes in BET. The pore volume of G-0, Mn-G6 and Mn-G15 is 0.042, 0.34 and 0.40 cm3/g, respectively. Figure 3B presents the N2 adsorption–desorption curves for Mn-FeOOHs. The figure clearly shows that the adsorption isotherms of Mn-FeOOHs fall into type IV among the six types of isotherms [18]. In terms of pore size (Figure 3C), the Mn-FeOOHs samples predominantly exhibit mesopores, ranging roughly from 2−50 nm.
Figure 3D presents an examination of the total XPS spectra for Mn-FeOOHs. All samples exhibited characteristic peaks attributed to O 1s and Fe 2p at binding energies of 529.90 and 712.27 eV, respectively [15]. Mn-G15 displayed a new characteristic peak at a binding energy of 640.66 eV, assigned to Mn 2p, confirming the incorporation of Mn into the goethite crystal structure [29]. Moreover, following phosphate adsorption, P 2p peaks are observed in both G-0 and Mn-G15, indicating that phosphate was successfully adsorbed onto Mn-FeOOHs.

3.2. Adsorption Kinetics

Figure 4A illustrates the adsorption kinetics of phosphate ions by Mn-FeOOHs. The figure shows that the phosphate adsorption by G-0 rapidly rose within the first 5 h of the reaction, from 39% to 59%, then gradually increased until adsorption saturation was reached, with a maximum adsorption of 70%. The phosphate adsorption by Mn-G6 and Mn-G15 increased sharply within 0.5 h, with adsorption from 58% and 62% to 68% and 70%, respectively. Subsequently, the adsorption improved steadily, reaching adsorption saturation within 2 h of the reaction, with maximum adsorption of 71% and 76%, respectively.
To delve deeper into the adsorption mechanism of phosphate on Mn-FeOOHs, kinetic models are employed to simulate the adsorption data (Figure 4B–D), with the pertinent kinetic equations provided in the SI. In contrast to the pseudo-first-order kinetic model (R2 < 0.9848), the pseudo-second-order kinetic model provides a superior fit for the adsorption kinetics of phosphate on Mn-FeOOHs (R2 > 0.9985), suggesting that the chemical adsorption of phosphate on Mn-FeOOHs constitutes a rate-limiting step [41,42]. Additionally, the intraparticle diffusion model is further used to analyze the rate-limiting step of adsorption. According to Figure 4D, the fitting curves of the kinetic data for each sample are divided into three stages. Based on the calculation results, all fitted lines did not pass through the origin, confirming that the intraparticle diffusion model is not the sole rate-limiting step in the adsorption process [35]. Comparing the correlation coefficients of the three models, it can be concluded that the pseudo-second-order kinetic model is the most suitable for the adsorption of phosphate on Mn-FeOOHs. Detailed parameters are presented in Table 2 and Table 3.

3.3. Effect of pH and Ionic Strength

Figure 4E,F illustrate the effect of solution pH on batch experiments. As depicted, the pH of the solution significantly affects the adsorption results. At a pH of approximately 3.0, G-0 and Mn-G15 achieve the highest removal rates for phosphate ions, at 90% and 95%, respectively. Subsequently, the removal rates decrease as the pH of the solution increases, suggesting that acidic conditions are more favorable for phosphate ion removal. As previously mentioned, the zero-point charges of G-0 and Mn-G15 are 7.39 and 5.65, respectively, meaning that the surfaces of G-0 and Mn-G15 carry positive charges when the solution pH is below 3.0, and vice versa. Moreover, the literature reports [38,43] indicate that phosphate in solutions of different pH primarily exists in the forms of PO43−, HPO42− and H2PO4−, all bearing negative charges. Thus, at pH < 3.0, electrostatic attraction occurs between the positively charged G-0 and Mn-G15 and the negatively charged phosphate, enhancing phosphate adsorption by Mn-FeOOHs [44]. When the pH value increases from 3.0 to 6.0, the phosphate adsorption remains high at approximately 85%, indicating that the key factor in the adsorption of phosphate by Mn-FeOOHs is not electrostatic attraction but complexation reaction [6]. As the alkalinity of the solution steadily increases, the negative charges on the surfaces of G-0 and Mn-G15 repel the negatively charged phosphate ions, resulting in a reduced adsorption of phosphate [6]. Additionally, under alkaline conditions, excessive -OH groups compete strongly with negatively charged phosphate, further reducing the adsorption of phosphate by Mn-FeOOHs.
Additionally, the ionic strength of the solution is a significant factor influencing the interaction between phosphate ions and adsorbents. Consequently, the influence of 0.001~0.01 mol/L NaCl solution on adsorption experiments was discussed separately in the article. As shown in Figure 4E,F, the ion strength has a minimal impact on the adsorption experiment results of phosphate, indicating that the adsorption of phosphate on Mn-FeOOHs is attributed to the internal sphere complexation [21,45,46].

3.4. Adsorption Isotherms

Figure 5 investigates the effects of initial pollutant concentration and reaction temperature on adsorption performance. As depicted in Figure 5A–C, the adsorption capability of Mn-FeOOHs for phosphate correlates positively with both the concentration of phosphate and reaction temperature. This correlation arises as increasing initial concentrations bolster the mass transfer drive, leading to greater phosphate adsorption on the active sites of Mn-FeOOHs. Furthermore, as Figure 5A–C demonstrate, the adsorption capacity of Mn-G15 for phosphate is markedly superior to that of G-0. As the reaction temperature increases from 288K to 318 K, the adsorption capacity of Mn-G15 for phosphate rises from approximately 65 mg/g to 79 mg/g. The result further demonstrates that Mn lattice substitution positively influences phosphate removal, corroborating the findings from the preceding analysis.
Additionally, to quantify the adsorption performance of Mn-FeOOHs for phosphate and more fully understand the specific adsorption characteristics of the adsorbent, three commonly used isotherm models of Langmuir [47], Freundlich [48], and Dubinin–Radushkevich (D-R) [49] are applied to fit the isotherm data(Figure 5D–F and Figure 6). Detailed descriptions can be found in SI. As shown in Table 4, compared with the Freundlich model (R2 < 0.0846), the Langmuir model (R2 > 0.9743) is more suitable for the adsorption of phosphate on Mn-FeOOH. Compared with the results calculated by the D-R model, the maximum adsorption capacity calculated by the Langmuir model is more closely aligned with the experimental values. At T = 318 K and pH 5.5, the maximum adsorption capacity for phosphate on Mn-G15, as calculated by the Langmuir model, is 87.18 mg/g (82.56 mg/g at 288 K and 85.54 mg/g at 303 K, respectively). Fitting results from the three isotherm models indicate that the adsorption of phosphate on the surface of Mn-FeOOHs is uniform and monolayer adsorption [48].
As shown in Table 5, the maximum adsorption capacity of Mn-G15 for phosphate is significantly higher than that of flue gas desulfurization gypsum [50], MgCl2-alginate modified biochar [51], lanthanum-modified zeolites [44], lanthanum-incorporated porous zeolite [52] and α-(Al, Fe)OOH [23] but lower than that of magnesium oxide/biochar for phosphate [42], indicating that Mn-FeOOHs have potential and value in the management of phosphorus-containing wastewater.

3.5. XPS Analysis

The high-resolution spectrum of Fe 2p is depicted in Figure 7A, with peaks at 710.70 eV and 724.59 eV corresponding to Fe 2p3/2 and Fe 2p1/2, respectively [29,53]. Comparing the peak intensities of G-0 and G-0-P, the peak value of Fe 2p in Mn-G15-P is significantly reduced. This finding aligns with the trends observed in the characteristic XRD peaks, further confirming that Mn has successfully substituted the Fe element in goethite. To validate this hypothesis, the high-resolution XPS spectra of Mn 2p are further analyzed, as illustrated in Figure 7B. The XPS spectra of G-0 and G-0-P exhibit characteristic peaks of Mn 2p, whereas the high-resolution spectra of Mn-G15-P display double peaks corresponding to Mn 2p3/2 and Mn 2p1/2 at binding energies of 641.69 eV and 653.37 eV, respectively [11,23,28]. This finding conclusively validates the aforementioned hypothesis.
The high-resolution XPS spectrum of P 2p is shown in Figure 7C, and P is not detected on G-0. However, after adsorbing phosphate, a clear characteristic peak appeared on the surface of G-0 at 133.0 eV, which belongs to P-O [38]. For Mn-G15, the peak intensity of P-O bonds is further enhanced, indicating that Mn-FeOOHs has a promoting effect on the adsorption of phosphate. The high-resolution XPS spectrum of O 1s is illustrated in Figure 7D. The double peaks appearing at 529.36 eV and 530.59 eV are attributed to Fe-O and M(Fe/Mn)-O, respectively [28,30,31,38]. Compared with G-0 and G-0-P, the characteristic peak intensity on the surface of Mn-G15-P significantly decreases. Based on the analysis of the results in Figure 7C, the adsorption of phosphate by Mn-FeOOHs is closely related to oxygen-containing functional groups. Therefore, as the adsorption amount of phosphate increases, the consumption of oxygen-containing functional groups increases, leading to a gradual decrease in the characteristic peak of O 1s.

4. Conclusions

This article prepared Mn-FeOOHs and studied their adsorption properties and reaction mechanism for phosphates. According to the characterization results, the substitution of Mn in goethite not only affects the crystal structure and increases the surface area of goethite but also changes the surface properties of goethite, increasing the concentration of active sites on the surface of goethite from 0.22 to 0.73 mmol/g. The adsorption experiment results illustrate that acidic conditions are more conducive to the removal of phosphate, while under alkaline conditions, electrostatic repulsion and competitive adsorption lead to lower removal rates of phosphate ions. In addition, substituting goethite with lattice Mn has a promoting effect on the removal of phosphate, which is consistent with the high-resolution XPS analysis results of the P element. The phosphate adsorption on Mn-FeOOHs follows the Langmuir model and pseudo-second-order model, which implies that chemical adsorption plays a dominant role, and the adsorption of phosphate by Mn-FeOOHs belongs to single-layer adsorption.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w16162294/s1. Table S1. The synthesis scheme of Mn FeOOHs; Figure S1. Ion distribution of phosphate in different pH solution.

Author Contributions

Experimental plan conception, M.L.; experimental guidance, G.S.; characterization analysis, Z.C.; literature research, J.W.; data processing, Y.Q.; draft writing, Y.Q.; writing—review and revision, M.L.; data supervision, J.W.; financial support, M.L. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the Key project of Bengbu University Natural Science Research (No. 2023ZR04zd), Key project of Anhui Province University Natural Science (No. 2023AH052943), Project of Bengbu University Natural Science Research (No. 2024YYX64QD), Project of industry-university-research (No. 2023340306001064).

Data Availability Statement

The data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Water 16 02294 g001
Figure 1. FE-SEM ((A): G-0; (B): Mn-G6; (C): Mn-G15; (D): Mn-G15-P; (E): EDS of Mn-G15-P) and Mapping ((F): Mn-G15-P; (G): O element; (H): Mn element; (I): P element; (J): Fe element) of Mn-FeOOHs before and after adsorption of phosphate.
Figure 1. FE-SEM ((A): G-0; (B): Mn-G6; (C): Mn-G15; (D): Mn-G15-P; (E): EDS of Mn-G15-P) and Mapping ((F): Mn-G15-P; (G): O element; (H): Mn element; (I): P element; (J): Fe element) of Mn-FeOOHs before and after adsorption of phosphate.
Water 16 02294 g001
Water 16 02294 g002
Figure 2. (A): XRD (G-Goethite); (B): FTIR; (C): Gran’s plot to obtain Veb1 and Veb; (D): Zeta Potential.
Figure 2. (A): XRD (G-Goethite); (B): FTIR; (C): Gran’s plot to obtain Veb1 and Veb; (D): Zeta Potential.
Water 16 02294 g002
Water 16 02294 g003
Figure 3. (A): hysteresis loop; (B): N2 adsorption–desorption curve; (C): pore size distribution; (D): total XPS spectra.
Figure 3. (A): hysteresis loop; (B): N2 adsorption–desorption curve; (C): pore size distribution; (D): total XPS spectra.
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Water 16 02294 g004
Figure 4. (A): Adsorption Kinetics; (B): pseudo−second−order kinetic model; (C): pseudo−first−order kinetic model; (D): intraparticle diffusion model; (E): Ionic strength and pH influence of phosphate adsorption on G-0; (F): Ionic strength and pH influence of phosphate adsorption on Mn-G15. CP = 10.0 mg/L, temperature = 288 K, pH = 3.0−10.0, solid–liquid ratio = 1.0 g/L, ionic strength = 0.001−0.1 mol/L NaCl, reaction time = 24 h.
Figure 4. (A): Adsorption Kinetics; (B): pseudo−second−order kinetic model; (C): pseudo−first−order kinetic model; (D): intraparticle diffusion model; (E): Ionic strength and pH influence of phosphate adsorption on G-0; (F): Ionic strength and pH influence of phosphate adsorption on Mn-G15. CP = 10.0 mg/L, temperature = 288 K, pH = 3.0−10.0, solid–liquid ratio = 1.0 g/L, ionic strength = 0.001−0.1 mol/L NaCl, reaction time = 24 h.
Water 16 02294 g004
Water 16 02294 g005
Figure 5. (A–C): Adsorption isotherms; (D–F): Langmuir model. CP = 5.0~100.0 mg/L, temperature = 288~318 K, pH = 5.5, solid–liquid ratio = 1.0 g/L, ionic strength = 0.01 mol/L NaCl, reaction time = 24 h.
Figure 5. (A–C): Adsorption isotherms; (D–F): Langmuir model. CP = 5.0~100.0 mg/L, temperature = 288~318 K, pH = 5.5, solid–liquid ratio = 1.0 g/L, ionic strength = 0.01 mol/L NaCl, reaction time = 24 h.
Water 16 02294 g005
Water 16 02294 g006
Figure 6. D-R model of phosphate adsorption on Mn-FeOOHs.
Figure 6. D-R model of phosphate adsorption on Mn-FeOOHs.
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Water 16 02294 g007
Figure 7. XPS spectra of phosphate adsorption on Mn-FeOOHs ((A): Fe 2p; (B): Mn 2p; (C): P 2p; (D): O 1s).
Figure 7. XPS spectra of phosphate adsorption on Mn-FeOOHs ((A): Fe 2p; (B): Mn 2p; (C): P 2p; (D): O 1s).
Water 16 02294 g007
Table 2. Adsorption kinetics model parameters.
Table 2. Adsorption kinetics model parameters.
Pseudo-First-OrderPseudo-Second-Order
Qe
(mg∙g−1)		K1
(g∙(mg·min)−1)		R2Qe
(mg∙g−1)		K1
(g∙(mg·min)−1)		R2
G-02.80.270.98486.630.320.9985
Mn-G60.770.180.94406.560.300.9999
Mn-G150.340.200.63817.001.360.9998
Table 3. Intraparticle diffusion model parameters.
Table 3. Intraparticle diffusion model parameters.
Intraparticle Diffusion Model
K1
(g∙(mg·min)−1)		R2C
First stage0.570.97541.63
G-0Second stage0.880.97222.76
Third stage0.461.00003.60
First stage0.280.87701.09
Mn-G6Second stage5.180.911131.30
Third stage0.441.00003.65
First stage0.420.97872.01
Mn-G15Second stage5.480.826234.53
Third stage0.531.00003.39
Table 4. Langmuir, Freundlich and D-R models of phosphate adsorption on Mn-FeOOHs.
Table 4. Langmuir, Freundlich and D-R models of phosphate adsorption on Mn-FeOOHs.
LangmuirFreundlichD-R
T(K)SampleQm
(mg·g−1)		KL
(L·mg−1)		R2KF
((mg·g−1)/(mg·L)−n)		1/nR2β (mol2·J−2)Qm
(mg·g−1)		R2
288G-078.860.090.99822.170.790.95327.8914.110.6768
Mn-G1582.560.160.99554.980.620.98462.5017.300.8237
303G-082.780.060.99725.380.650.93252.4417.900.6658
Mn-G1585.540.120.99659.310.600.78413.8734.810.9853
318G-083.060.050.97227.460.600.88692.8523.810.9153
Mn-G1587.180.020.974311.380.570.76852.3032.790.7932
Table 5. Comparison of adsorption capacity (mg·g−1) for phosphate on Mn-FeOOHs with other absorbents.
Table 5. Comparison of adsorption capacity (mg·g−1) for phosphate on Mn-FeOOHs with other absorbents.
AdsorbentsAdsorption ConditionsQmax (mg/g)Refs.
flue gas desulfurization gypsumT = 298 K, pH = 7.0 22.54[50]
MgCl2-alginate-modified biocharT = 301 K, pH = 3.046.56[51]
lanthanum-modified zeolitesT = 298 K, pH = 5.058.20[44]
lanthanum-incorporated porous zeoliteT = 313 K, pH = 6.017.20[52]
α-(Al, Fe)OOHT = 298 K, pH = 5.55.96[23]
magnesium oxide/biocharT = 296 K, pH = 4.0121.25[42]
Mn-G15T = 288 K, pH = 5.582.56This work
Table 1. The selective parameters of Mn-FeOOHs.
Table 1. The selective parameters of Mn-FeOOHs.
AdsorbentsSBET (m2/g)Pore Volume
(cm3/g)		C(SOH)
(mmol/g)		pHPZC
G-07.320.040.227.39
Mn-G643.860.340.416.27
Mn-G1577.180.400.735.65
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Li, M.; Sun, G.; Chu, Z.; Wang, J.; Qiu, Y. Efficient Phosphate Adsorption from Groundwater by Mn-FeOOHs. Water 2024, 16, 2294. https://doi.org/10.3390/w16162294

AMA Style

Li M, Sun G, Chu Z, Wang J, Qiu Y. Efficient Phosphate Adsorption from Groundwater by Mn-FeOOHs. Water. 2024; 16(16):2294. https://doi.org/10.3390/w16162294

Chicago/Turabian Style

Li, Mengxue, Guanghui Sun, Ziyang Chu, Jing Wang, and Yu Qiu. 2024. "Efficient Phosphate Adsorption from Groundwater by Mn-FeOOHs" Water 16, no. 16: 2294. https://doi.org/10.3390/w16162294

APA Style

Li, M., Sun, G., Chu, Z., Wang, J., & Qiu, Y. (2024). Efficient Phosphate Adsorption from Groundwater by Mn-FeOOHs. Water, 16(16), 2294. https://doi.org/10.3390/w16162294

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