Facile one-pot synthesis of urchin-like Fe–Mn binary oxide nanoparticles for effective adsorption of Cd(II) from water

Abstract

The development of efficient and low-cost adsorbent is critical for water treatment, but still presents great challenges. Herein, we report the synthesis of three-dimensional (3D) hierarchical nanostructured adsorbent, urchin-like Fe–Mn binary oxides (UFMBO), by simple heating without any template/surfactant. The surface morphology, crystalline and pore structure were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and nitrogen adsorption–desorption isotherm, respectively. Results revealed that the UFMBO had a 3D hierarchical nanostructure with a high specific surface area of 142 m² g⁻¹, which was conducive to pollutant adsorption and adsorbent separation. Cd(II) removal using the UFMBO was evaluated by batch adsorption experiments. The adsorption equilibrium was established within 3 h, and the adsorption process was better described by pseudo second-order kinetics model. The adsorption isotherm data fitted well to Langmuir model, and the maximum adsorption capacity was 74.76 mg g⁻¹ at pH 6.0. Influence by ionic strength on the adsorption was significant, implying that Cd(II) may form outer-sphere complexes on the adsorbent surface. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) analysis suggested hydroxyl group played an important role in Cd(II) uptake. The highly effective 3D UFMBO adsorbent can be easily separated and regenerated, demonstrating its great potential in cadmium removal from contaminated water source.

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Introduction

Heavy metal pollution is a major environmental issue in many countries due to the high toxicity of heavy metals even at trace concentration.¹ Cadmium, well known as an extremely toxic heavy metal, can cause many life-threatening diseases, such as renal disturbances, lung insufficiency, bone lesions, cancer, anemia, itai-itai disease and weight loss.² To minimize the possible health risks, World Health Organization (WHO) has set a stringent limit of 5 μg L⁻¹ Cd(II) in drinking water.² The main sources of Cd(II) pollution can be attributed to waste effluents from electroplating, battery, mining, pigments, smelting, alloy manufacturing and nuclear industries.³ The concentration in these waste effluents can be as high as 100 mg L⁻¹, thus requiring productive pre-treatment before discharging to environment.⁴

Nowadays, various methods have been developed for the removal of Cd(II), such as precipitation, ion exchange, membrane filtration, solvent extraction and adsorption.⁵ Among these techniques, adsorption, which is simple, economical and environment-friendly, has become one of the most promising and widespread applied methods. As adsorbent is the key to the adsorption process, many researches have been focused to develop high performance adsorbents. Owing to their efficient adsorption performance, low cost, environment-friendly nature, as well as readily available raw materials, iron oxide nanoparticles have attracted many attentions.⁶ However, separation of zero dimensional (0D) iron oxide nanoparticles from an aqueous medium often requires centrifugation, which is rather tedious and expensive.⁷ In addition, 0D nanomaterials often tend to aggregate, while aggregation could lead to a low specific surface area and subsequently hamper the molecular diffusion of the adsorbates.⁸ Recently, efforts have been made to fabricate three dimensional (3D) hierarchical nanomaterials for wastewater treatment.^9–12 Compared to conventional 0D nanoparticles, 3D hierarchical nanomaterials possess an overall micron-sized structure, which can be easily separated from the aqueous solution and recycled for further use.^10,12 Besides, 3D nanomaterials also exhibit large specific surface area and own abundant active sites because of their hierarchical structure. Hence, 3D hierarchical nanomaterials, which are able to obtain high adsorption capacity and fast adsorption kinetics, are promising to be employed in the practical adsorption application. However, current methods for 3D hierarchical nanomaterials fabrication usually suffer from tedious processes and hard/soft template-requirement.¹³

Compared to the pure compounds, hybrid or mixed composites often have superior properties, such as improved adsorption affinity to heavy metals or enhanced electrical capacitance.^14–18 For example, the incorporation of manganese oxides into iron oxides can increase the adsorption affinity and capacity for heavy metals by 5–10 times.¹⁶ Several groups have reported the fabrication of 0D Fe–Mn binary oxide nanomaterials for the removal of different toxicants from aqueous solution.^19–23 However, their tiny sizes, these 0D nanomaterials are difficult to be separated from the aqueous solution. In addition, 0D nanoparticles tend to aggregate in the water, leading to a low specific surface area and low adsorption capacity, which increases the cost of water treatment remarkably. Unfortunately, few studies have been done on 3D hierarchical iron manganese binary oxides but rather focused on 3D iron oxide nanomaterials alone. To the best of our knowledge, there have been rare works reported on one-pot synthesis of 3D hierarchical Fe–Mn binary oxide nanomaterials without any template/surfactant and the application for the adsorptive removal of heavy metals from water.

The aims of this study were to develop a simple, economical and environment-friendly method for fabricating Fe–Mn binary oxide nanoparticles with 3D hierarchical structure, and to evaluate the as-synthesized 3D urchin-like Fe–Mn binary oxides (UFMBO) for Cd(II) removal via batch adsorption experiments with different process parameters, such as solution pH, contact time, ionic strength, and initial Cd(II) concentration.

Experimental

Chemical reagents

All chemicals were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), except Cd(NO₃)₂·4H₂O with purity of 99.0%, which was obtained from Ting Xin New Chemical Reagent Factory (Shanghai, China). A stock Cd(II) solution (1000 mg L⁻¹) was prepared by dissolving adequate amount of Cd(NO₃)₂·4H₂O in deionized (DI) water, and all working solutions were prepared by diluting the Cd(II) stock solution with deionized water. Solution pH was adjusted with either HNO₃ or NaOH solution.

Preparation of the adsorbent

UFMBO was prepared via a one-pot method. In brief, 6.25 g of ferrous sulfate heptahydrate (FeSO₄·7H₂O) and 1.19 g of potassium permanganate (KMnO₄) were separately dissolved in 100 mL of deionized water. The prepared KMnO₄ solution was heated to boil, and then FeSO₄ solution was gradually added into the KMnO₄ solution under vigorous magnetic stirring. 13 mL of 5 mol L⁻¹ NaOH solution was then added dropwise to the boiling mixture. After that, the mixed solution was cooled down to 25 °C, and the precipitates were collected and washed several times with deionized water to remove the impurities. After washing, the product was dried in an oven at 80 °C to a constant weight. Finally, the dried product was crushed and stored in a desiccator for further use.

Characterization

Surface morphology and element composition of the UFMBO were examined by a field emission scanning electron microscopy (FESEM, Hitachi S-4800, Japan) and transmission electron microscopy (TEM, Philips Tecnai F30, USA).

X-ray diffraction (XRD) patterns were collected on a PANalytical X' Pert PRO X-ray diffractometer (Almelo, The Netherlands) with a Ni filter, Cu Kα radiation source, and angular variation of 10–80° operated at a tube voltage of 40 kV and a tube current of 40 mA. The sample was thoroughly dried and grinded in an agate mortar before testing.

Nitrogen adsorption–desorption isotherms were conducted at 77 K with a Micromeritics ASAP2020M+C surface area and porosity analyzer (GA, USA). The specific surface area of the UFMBO was calculated according to the Brunauer–Emmett–Teller (BET) method. The pore size distribution was derived from the desorption branch of the isotherm with the Barrett–Joyner–Halenda (BJH) model.

The point of zero charge pH (pH_PZC) of UFMBO was determined by an immersion technique. In brief, a number of aqueous solutions (20 mL) were firstly prepared with a final background electrolyte (NaNO₃) concentration of 0.1 M. NaNO₃ solutions were then adjusted to different initial pH values of 4, 5, 6, 7, 8, 9 or 10 using 0.1 M HNO₃ or 0.1 M NaOH solution. After that, 0.01 g of the UFMBO were added to each solution, and the mixtures were equilibrated at 25 °C. The change in pH during equilibration, ΔpH (difference between the equilibrium and initial pH), was then plotted against the initial pH of the NaNO₃ solution. The initial pH at which ΔpH equaled to zero was taken as pH_PZC.

X-ray photoelectron spectroscopy (XPS) spectra were collected using a PHI Quantum-2000 electron spectrometer (Ulvac-Phi, Japan) with 150 W monochromatized Al Kα radiation (1486.6 eV).

The chemical structure analysis was conducted by a Fourier transform infrared spectroscopy (FTIR, iS10, Thermo, USA) with a transmission mode.

Batch adsorption

For batch adsorption experiments, UFMBO with a dosage of 0.5 g L⁻¹ were firstly added to a 100 mL flask containing 20 mL of Cd(II) solution with a concentration of 50 mg L⁻¹ and pH 6.0, unless otherwise stated. The mixture was then shaken at 200 rpm and 25 °C for 24 h to ensure adequate time for adsorption equilibrium. After that, the aqueous solution was filtered through 0.22 μm Millipore membrane filter for the measurement of residual Cd(II) concentration.

In the pH effect study, the pH value of mixture was adjusted to and maintained at 2 to 6 by 0.1 M HNO₃ or 0.1 M NaOH solution. The experiments were conducted in triplicate to ensure the reliability of the results.

In the kinetics experiments, 0.1 g of UFMBO were dosed to 200 mL of 50 mg L⁻¹ Cd(II) solution. 1.3 mL aliquot was withdrawn at predetermined time intervals. Cd(II) concentrations of all samples were measured, and the adsorption capacity q_t (mg g⁻¹) at a specific time t was calculated by eqn (1):

where C_i and C_t are the concentrations of Cd(II) (mg L⁻¹) initially and at time t, respectively; V (L) is the volume of the solution and M (g) is the weight of the adsorbent.

The adsorption isotherm studies were performed with initial Cd(II) concentrations from 10 to 100 mg L⁻¹. The adsorption capacity q_e (mg g⁻¹) was calculated by eqn (2):

where C_e is the equilibrium concentration of Cd(II) (mg L⁻¹).

In the ionic strength effect study, NaNO₃ with concentration ranging from 0 to 1 M was used as a common electrolyte background. The experiment was conducted in triplicate to ensure the reliability of the results.

To evaluate the reusability of UFMBO, regeneration was also investigated. First, UFMBO was added to 50 mg L⁻¹ Cd(II) solution and shaken for 24 h to ensure adsorption equilibrium was reached. The adsorbent was then separated from the Cd(II) solution via facile sedimentation in less than 2 h. In the desorption cycle, the separated UFMBO were shaken in 1 mM HNO₃ solution for 4 h, and then washed with DI water to neutralize the composite and dried in an oven for 8 h. Finally, the regenerated UFMBO were used for adsorption and desorption in the succeeding cycles.

The UFMBO were also assessed for its adsorption capability for trace Cd(II). Different amounts of adsorbent were added to a series of 100 mL flask containing 20 mL of Cd(II) solution. The initial concentration of Cd(II) was 100 μg L⁻¹, and the solution pH was kept at 6.0. Other procedures were the same as described in the pH effect study.

Analytical methods

Cd(II) concentrations were determined by atomic adsorption spectroscopy (AAs, Thermo Elemental M6, USA) or Agilent 7500cx inductively coupled plasma mass spectrometry (ICP-MS, CA, USA). The required dilution was done with 1% HNO₃ according to the instrument requirement.

The leaching of Fe and Mn from the UFMBO after pH_PZC determination and regeneration studies was measured by Optima 7000DV inductively coupled plasma optical emission spectroscopy (ICP-OES, PerkinElmer, USA).

Results and discussion

Characterization of UFMBO

Fig. 1a gives the low magnification FESEM image of the as-synthesized UFMBO. As shown, UFMBO are composed of many uniform urchin-like nanoparticles with an average diameter of ∼450 nm. The high magnification FESEM image (Fig. 1a inset) clearly shows that dozens of nanorods with approximately 20 nm in diameter are connected to each other and forms urchin-like 3D hierarchical structure. TEM image (Fig. 1b) reveals that the architecture is built from a spherical core and numerous nanorods which grow radially from the core. EDX analysis (Fig. 1c) demonstrates that both Fe and Mn are distributed on the surface, and the molar ratio of Fe/Mn on surface is about 3 : 1, which matches well with the amount of precursors used in the synthesis process.

Fig. 1 (a) FESEM (inset: magnified image), (b) TEM, (c) EDX surface analysis, and (d) XRD pattern of UFMBO.

The phase composition and crystalline structure of the synthesized UFMBO were identified by XRD (Fig. 1d). The diffraction peaks could be assigned to pure orthorhombic phase of FeOOH (JCPDS 29-713).²⁴ However, no obvious diffraction crystalline peak of manganese oxides is detected, suggesting that most of manganese oxides are likely to exist in amorphous form in the composite.²⁵

Nitrogen adsorption and desorption isotherm measurements were conducted to further investigate the internal pore structure, and measure the specific surface area of UFMBO. The obtained nitrogen adsorption isotherm demonstrates a typical type IV isotherm (IUPAC nomenclature) with H3 hysteresis loop, indicating a characteristic distribution of slit-shaped mesopores.²⁶ Based on the BET calculation, UFMBO have a specific surface area of 142 m² g⁻¹, which is much higher than that of a previously reported Fe–Mn hybrid nanomaterial (59 m² g⁻¹),¹⁶ and a pore volume of 0.35 cm³ g⁻¹, respectively. The high specific surface area and large pore volume make UFMBO a promising candidate for Cd(II) adsorption. The corresponding pore size distribution calculated from the desorption branch of nitrogen isotherm is presented in the inset of Fig. 2. The average pore size of the UFMBO was determined to be 9.0 nm.

Fig. 2 Nitrogen adsorption and desorption isotherms of the UFMBO with corresponding pore size distributions (inset) calculated by BJH method.

Effect of pH

Solution pH is one of the most important factors in the adsorption process, which may affect the surface charge property and degree of ionization of the adsorbent. Furthermore, the change in solution pH can also affect the speciation of the adsorbate, Cd(II), in water. The distribution of cadmium species as a function of solution pH was determined by MINEQL+ 4.6 as shown in Fig. S1.† It was found that Cd(II) can be present in water in different forms, such as Cd²⁺, Cd(OH)⁺, Cd(OH)² (s), etc. Noteworthy, Cd(II) was the predominant species in the aqueous solution when pH < 6.8.²⁷ When pH increased from 6.8, Cd(II) started to precipitate from the solution in the form of Cd(OH)₂. When pH > 8, the prevailing species in the system was Cd(OH)₂. Thus, pH effect study was conducted at pH below 6.0 to avoid unnecessary disturbance by Cd(II) precipitation.

The adsorption capability of UFMBO varies with pH as shown in Fig. 3a. When pH increased, the amount of Cd(II) adsorbed on UFMBO also increased, and the maximum adsorption was obtained at pH 6. To further analyze the adsorption behavior, pH_PZC of UFMBO was determined and found to be around 7.8 (Fig. 3b), indicating that the adsorbent was positively charged throughout the pH effect study. Hence, at low pH, stronger electrostatic repulsion between Cd(II) and the positively charged UFMBO could inhibit the Cd(II) adsorption to a greater extent. When solution pH increased, the electrostatic repulsion strength became weaker and the adsorption of Cd(II) on UFMBO surface was improved, which was consistent with the trend shown in Fig. 3a. According to the analyses above, besides the electrostatic interaction, complexation and ion exchange may occur during the adsorption,^28,29 which allowed UFMBO to adsorb Cd(II) although the two both carried positive charges.

Fig. 3 (a) Cd(II) adsorption capacities of UFMBO and residual Fe and Mn in solution with varying pH. Error bars represent the standard deviation of triplicate experiments. (b) Determination of pH_PZC based on immersion technique. Experimental condition: initial Cd(II) concentration = 50 mg L⁻¹, adsorbent dosage = 0.5 g L⁻¹, contact time = 24 h, temperature = 25 °C.

Possible leaching is an important evaluation criterion to assess the feasibility of adsorbents in practical use. Thus, the leaching of Fe and Mn at different pH was measured in the pH impact study as well. As shown in Fig. 3a, the residual Fe and Mn ion concentration in the aqueous solution was rather high at extreme low pH. However, when solution pH was higher than 3, the residual concentrations of Fe and Mn were almost negligible. Hence, UFMBO can be used to treat Cd contaminated wastewater.

Adsorption kinetics

To determine the time necessary to reach adsorption equilibrium and elucidate the mechanism of the adsorption process, adsorption kinetic experiment of Cd(II) on the UFMBO was carried out. As shown in Fig. 4, the adsorption process could be divided into two stages. In the initial stage, the adsorption process was very fast, and over 87% of equilibrium adsorption capacity (q_e) was achieved within 40 min. In the following stage, the adsorption slowed down and over 96% of q_e was reached within 2 h contact time. To ensure complete adsorption equilibrium, contact time of 24 h was chosen for all batch adsorption experiments in this study.

Fig. 4 Adsorption kinetics of Cd(II) on the UFMBO. Error bars represent the standard deviation of triplicate experiments. Experimental conditions: initial Cd(II) concentration = 50 mg L⁻¹, adsorbent dosage = 0.5 g L⁻¹, solution pH = 6.0, temperature = 25 °C.

The experimental data were fitted by two commonly used kinetic models, pseudo-first-order kinetic model and pseudo-second-order kinetic model.³⁰ The results obtained were displayed in Fig. 4.

The expression of pseudo-first-order kinetic model is given by eqn (3):

q_t = q_e(1 − e^−k₁t) (3)

where k₁ (min⁻¹) is the rate constant of pseudo-first-order kinetic model.

The expression of pseudo-second-order kinetic model is given by eqn (4):

where k₂ (g mg⁻¹ min⁻¹) is the rate constant of pseudo-second-order kinetic model.

The kinetics experimental data were better fitted to pseudo-second-order model (Fig. 4), with a higher correlation coefficient (R²) of 0.9471, indicating the adsorption of Cd(II) on UFMBO was likely to be a chemisorption process.³¹

Adsorption isotherm

The adsorption isotherm experiment was conducted to determine the maximum adsorption capacity of the UFMBO. The obtained experimental data together with the fitted curves by Langmuir and Freundlich model were shown in Fig. 5.³²

Fig. 5 Adsorption isotherm of Cd(II) on the UFMBO. Error bars represent the standard deviation of triplicate experiments. Experimental conditions: adsorbent dosage = 0.5 g L⁻¹, solution pH = 6.0, contact time = 24 h, temperature = 25 °C.

The expression of Langmuir isotherm model is as follows:

where q_m (mg g⁻¹) and q_e (mg g⁻¹) are the maximum and equilibrium adsorption capacity of Cd(II), respectively. C_e (mg L⁻¹) is the concentration of Cd(II) at adsorption equilibrium. K_L (L mg⁻¹) is the constant that related to the energy of adsorption.

The Freundlich isotherm model is expressed as following:

q_e = K_FC^1/n_e (6)

where q_e (mg g⁻¹) is the equilibrium adsorption capacity of Cd(II), C_e (mg L⁻¹) is the equilibrium concentration of Cd(II). K_F (L mg⁻¹) and n are the Freundlich constant related to the adsorption capacity and adsorption intensity of Cd(II), respectively.

The adsorption of Cd(II) was better fitted to the Langmuir model, implying that the adsorption process of Cd(II) on the surface of UFMBO was monolayer and the adsorption of Cd(II) ions had equal activation energy.³³ The maximum adsorption capacity according to Langmuir isotherm model was 74.76 mg g⁻¹.

A comparison of q_m of UFMBO with other adsorbents reported in the literature is also presented (Table 1). Compared to other synthesized Cd-targeted adsorbents, the UFMBO have a relative high adsorption capacity. This is probably due to the doping of manganese oxides, which effectively improved the adsorption affinity and capacity for Cd(II).^16,17 A further improvement in the adsorption capacity of UFMBO is likely due to the urchin-like 3D hierarchical structure, which significantly increased the specific surface area and adsorption sites for Cd(II).^9–12

Table 1 Comparison of maximum Cd(II) adsorption capacities of various adsorbents^a

Adsorbents	Experimental conditions			qm (mg g^−1)	Ref.
	Dosage (g L^−1)	pH	T (°C)
a Olive stone activated carbon = OSAC, lipopeptides modified Na-montmorillonite = LPSSF/Na-MMT, t. s. = this study.
Chitin	2.0	6.5	25	14.0	34
Chitosan/activated carbon	6.0	6.0	25	52.6	35
OSAC	1.0	6.0	30	11.7	36
LPSSF/Na-MMT (1 : 50)	2.0	5.0	25	62.1	37
Polyamine/Fe3O4	2.0	6.0	20	71.3	38
Orange peel–Fe2O3	0.2	7.0	25	71.4	29
Shellac-coated iron oxide	0.4	8.0	25	18.8	39
Cyclodextrin/Fe3O4	12.0	5.5	25	27.7	40
MnO2-coated Fe3O4	1.0	6.3	25	53.2	17
Mn-doped iron oxide	0.1	7.0	25	35.0	16
UFMBO	0.5	6.0	25	74.7	t. s.

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To further evaluate the thermodynamic feasibility of the Cd(II) adsorption process on the UFMBO, effect of temperature on the Cd(II) adsorption isotherm was investigated at 283, 298 and 313 K, and the corresponding thermodynamic constants, including standard free energy change (ΔG), enthalpy change (ΔH), and entropy change (ΔS) were calculated.

The standard free energy change in the adsorption process could be calculated using the below equation:

ΔG = −RT ln K_d (7)

where K_d (L g⁻¹) is the Langmuir constant.

The changes of enthalpy and entropy of Cd(II) adsorption were obtained from the equation as follows:

where T is the temperature in K, and R = 8.314 J (mol K)⁻¹. ΔH and ΔS were obtained from the slope and intercept of a plot of ln K_d versus 1/T. The thermodynamic parameters of Cd(II) adsorption were summarized in Table S1.†

The obtained results showed that the adsorption was enhanced with increase in temperature as shown in Fig. S2.† The negative ΔG value and positive ΔS value indicated that the adsorption was spontaneous. The absolute value of ΔG increased with the increase of temperature, implying that higher temperature facilitated the adsorption of Cd(II) on UFMBO.⁴¹ The value of ΔH was positive suggesting that the reaction was endothermic.

Effect of ionic strength

Wastewater containing Cd(II) is usually high in salinity, meaning rich in ions which may significantly affect the uptake of Cd(II). Therefore, it is important to evaluate the influence of ionic strength on the adsorption of Cd(II) by UFMBO. On the other hand, a background ionic strength dependence study is a useful method to investigate the underlying adsorption mechanism. As shown in Fig. S3,† Cd(II) adsorption decreased when the ionic strength increased. Quantitatively, increasing the background electrolyte concentration from 0.001 to 1.0 M NaNO₃, the corresponding decrease in percent uptake was occurred from 94.6% to 63.3%. It is probably because that the increase in the ionic strength of solution will influence the activity coefficient of metal ions, and hinder the complexation tendency of Cd(II) with the hydroxyl functional groups on UFMBO surfaces.²⁹ As a consequence, Cd(II) adsorption on UFMBO was mainly through outer-sphere adsorption, where an increasing solution ionic strength would decrease the adsorption of nonspecifically adsorbed ions on the adsorbent.⁴²

Adsorption of trace Cd(II)

As cadmium often exists at μg L⁻¹ level in environmental water bodies rather than mg L⁻¹ level as in industrial wastewater, the removal efficiency of UFMBO for trace level of Cd(II) was also studied. In this study, the initial Cd(II) concentration was chosen as 100 μg L⁻¹. At least 98.5% of Cd(II) were removed by UFMBO when the adsorbent dosage was higher than 0.05 g L⁻¹ (Fig. S4†). The residual Cd(II) in the solution was less than 1.5 μg L⁻¹, which is way below 5 μg L⁻¹, the maximum allowable concentration of cadmium in drinking water according to WHO guideline. The high efficiency of UFMBO for the adsorption of trace Cd(II) demonstrates that the great possibility in advanced drinking water treatment, such as household drinking water purification system. However, more thorough studies should be carried out to evaluate its feasibility in practical applications.

Regeneration of UFMBO

The reusability of adsorbent is one of the most important criteria to evaluate its performance and applicability in real practice. Taking into consideration that UFMBO exhibited a poor adsorption capacity and acceptable Fe and Mn dissolution at pH 3 (Fig. 3a), acid treatment is likely to be a suitable approach for the regeneration of UFMBO. Hence, 1 mM HNO₃ solution was chosen as eluent for the regeneration of the Cd(II) saturated UFMBO. Fig. 6a showed the successive adsorption–desorption cycles of Cd(II) using the same batch of UFMBO. Compared to the fresh UFMBO, the adsorption capacity of the regenerated UFMBO was decreased by 25%. The decrease in adsorption capacity may be attributed to the incomplete desorption of Cd(II) from the surface of UFMBO.¹⁷ In the subsequent cycles, the adsorption capacity remained almost unchanged, suggesting that the performance of UFMBO was stable after the first desorption.

Fig. 6 (a) Cd(II) removal by UFMBO over five successive adsorption–desorption cycles; FESEM images of (b) fresh UFMBO, and (c) the regenerated UFMBO.

The amount of Fe and Mn leached from UFMBO into the solution phase was measured and found to be less than 1%, indicating that the dissolution of Fe and Mn was negligible and a satisfactory stability of UFMBO under the stated experimental conditions. FESEM images of fresh and regenerated UFMBO after five successive adsorption–desorption cycles depicted that the regenerated UFMBO still retained 3D urchin-like hierarchical nanostructure (Fig. 6b and c). It is noteworthy that the nanostructured UFMBO can be easily recovered from water by simple sedimentation owing to its relative large volume and no presence of surfactant. Thus, it is concluded that the UFMBO can be easily regenerated and reused, promising its long-term use in water purification.

Adsorption mechanism analysis

XPS analysis of UFMBO before and after Cd(II) adsorption was carried out to investigate the interaction between Cd(II) and UFMBO during the adsorption process. The wide survey scan of XPS spectra in the range of 0–800 eV is shown in Fig. 7a. Compared to the fresh adsorbent, two small peaks at binding energy of about 405 and 411 eV are observed for the used adsorbent, indicating Cd(II) was adsorbed on the UFMBO.

Fig. 7 (a) Wide-scan XPS spectra, (b) high resolution O 1s XPS spectra of fresh and Cd-loaded UFMBO.

The high resolution XPS spectrum of O 1s of the virgin UFMBO can be deconvoluted into two peaks with binding energy of 530.2 and 531.7 eV (Fig. 7b), which can be attributed to oxygen in the metal oxide lattice (Fe–O and Mn–O in this study) and in the surface hydroxyl groups (H–O), respectively.^43,44 After the adsorption of Cd(II), the O 1s peak in the metal oxide lattice shifted from 530.2 eV to 529.9 eV, and the O 1s peak in the surface hydroxyl group shifted slightly down by 0.1 eV to 531.6 eV. The decrease in the binding energy of oxygen indicated formation of Cd–O bond in the adsorption process. Similar adsorption phenomena were reported by Cao et al. and Wang et al.^43,45,46 The relative contents of oxygen containing groups in UFMBO before and after Cd(II) adsorption were also calculated and summarized in Table 2. After Cd(II) adsorption, the relative content of O in the H–O decreased significantly, while the proportion of O in the metal oxides lattice increased obviously, suggesting an likely complexation between surface hydroxyl groups and Cd(II) during the adsorption process.^36,38

Table 2 Binding energy and relative content of O in UFMBO

Sample	Proposed components	Binding energy (eV)	Relative content (%)
Fresh UFMBO	Metal oxide	530.2	38.1
	H–O	531.7	61.9
Cd loaded UFMBO	Metal oxide	529.9	45.2
	H–O	531.6	54.8

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In order to further verify this guess, the interaction between Cd(II) and UFMBO was investigated by FTIR analysis as shown in Fig. S5.† The peak at wave number of 3425 cm⁻¹ is associated with the stretching vibration of hydroxyl groups on UFMBO surface.^29,43,47 After Cd(II) adsorption, the adsorption peak of hydroxyl groups shifted to wave number of 3422 cm⁻¹ and the intensity decreased, which indicated the occurrence of interaction between hydroxyl groups on UFMBO surface and Cd(II) in aqueous solution.⁴³

On the basis of aforementioned experimental results and spectroscopic analyses, a plausible adsorption mechanism was proposed. In the adsorption process, Cd(II) was adsorbed onto the UFMBO, and then formed complexation with hydroxyl groups by ion exchange. On the other hand, the solution pH was lower than the pH_ZPC (7.8) of UFMBO, hence some hydroxyl groups on the adsorbent surface would be positively charged due to protonation, which would inhibit the Cd(II) adsorption due to electrostatic repulsion.^21,29 In summary, electrostatic interaction, ion exchange and complexation were involved in the uptake of Cd(II) onto UFMBO.

Conclusions

UFMBO with 3D hierarchical nanostructure was successfully synthesized via a simple, cost-effective, environment-friendly and template/surfactant-free method, while the performance for Cd(II) adsorption was evaluated. The adsorption process was fairly fast, and the adsorption kinetics could be well fitted to pseudo-second-order kinetic model. The adsorption capacity of Cd(II) increased with an increment in pH in the studied pH range. The adsorption equilibrium data were well described by Langmuir isotherm model with a maximum adsorption capacity of about 74.76 mg g⁻¹ at pH 6.0. The adsorption of Cd(II) was dependent on the background electrolyte concentration, implying that Cd(II) formed outer-sphere complexes on UFMBO surface. As demonstrated by XPS and FTIR analysis, the interaction between surface hydroxyl groups and Cd(II) contributed heavily during the adsorption process. Furthermore, UFMBO could be easily regenerated by acid elution. Owing to the facile preparation method, excellent removal efficiency, and good regeneration performance, UFMBO can be a promising cost-effective adsorbent for the treatment of cadmium contaminated water.

Acknowledgements

The authors acknowledge the financial support received from the ​National Natural Science Foundation of China (51578525, 5153000136, and 21507124), the Hundred Talents Program of Chinese Academy of Sciences, and the Science and Technology Innovation and Collaboration Team Project of the Chinese Academy of Sciences.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21030a

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