Adsorption, recovery, and regeneration of Cd by magnetic phosphate nanoparticles

Abstract

Adsorption plays an important role in removing cadmium (Cd²⁺) from water, and magnetic adsorbents are increasingly being used due to their ease of separation and recovery. Magnetic Fe₃O₄–coated hydroxyapatite (HAP) nanoparticles (nHAP-Fe₃O₄) were developed by co-precipitation and then used for the removal of Cd²⁺ from water. The properties of these nanoparticles were characterized by transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and magnetization curves. Experiments were conducted to investigate the effects of adsorption and mechanisms. Results illustrated that kinetic data were well fitted by a pseudo-second-order model. The adsorption capacity of nHAP-Fe₃O₄ was 62.14 mg/g. The mechanisms for the adsorption of Cd²⁺ on nHAP-Fe₃O₄ included rapid surface adsorption, intraparticle diffusion, and internal particle bonding, with the ion exchange with Ca²⁺ and chemical complexation being the most dominant. The regeneration efficiency and recovery rate of nHAP-Fe₃O₄ eluted by EDTA-Na₂ after the fifth cycle were 63.04% and 40.2%, respectively. Results revealed that the feasibility of nHAP-Fe₃O₄ as an adsorbent of Cd²⁺ and its environmental friendliness make it an ideal focus for future research.

Introduction

With the development of industry and mining, metallurgy, electroplating, printing, and dye manufacturing, cadmium (Cd²⁺), as a potential heavy metal pollutant, contaminates water bodies (Qian 2007; Yang et al. 2016a) and thus harms plants, animals, and human health because of its difficult removal in the environment (Heitmann et al. 2014). Cd²⁺ is a heavy metal with high mobility, and it easily causes a series of hazards when it is enriched in living organisms (Dey et al. 2016). Cd²⁺ becomes selectively enriched in organs such as the kidney and liver after entering human or animal bodies through the digestive and respiratory tracts, ultimately affecting normal metabolic activities and causing chronic Cd²⁺ poisoning (Ke et al. 2015). Unlike organic pollutants, heavy metals are often difficult to degrade under natural conditions after entering water bodies, and they possess strong migration properties (Wan et al. 2016; Rahmanian et al. 2018). Therefore, the removal of Cd²⁺ has become an important part of the current research. Some techniques, including chemical precipitation, ion exchange, and adsorption (Cheng et al. 2014), are used to remove Cd²⁺ from wastewater. Among these methods, adsorption is widely used due to its simple operation and economic advantages. The research on Cd²⁺ adsorption materials in wastewater is mainly focused on carbon materials (Ling et al. 2017; Liang et al. 2014), iron manganese oxides (Karami 2013; Khraisheh et al. 2004), nano-ferromagnetic materials (Su et al. 2014; Zendehdel et al. 2018), clay minerals (Abollino et al. 2003; Basualto et al. 2017), biomass materials (Schiewer and Patil 2008; Ajmal et al. 2003) and their modifications, and composite materials.

Apatite mineral has been widely studied because of its low cost and its reaction with a variety of metals to form heavy metal phosphates. As an apatite mineral, hydroxyapatite (HAP) is an important component of human teeth and bones, and it has good biological activity and biocompatibility. The special crystal structure of HAP makes it effective in ion exchange and in the removal of heavy metal ions from wastewater (Yamamura et al. 2018). Nano-hydroxyapatite (nHAP) has a small particle size and a large specific surface area and thus has a stronger adsorption capacity than ordinary apatite; as a result, it has attracted much attention in recent years (Guo et al. 2017). Yang et al. (2016b) prepared HAP with poor crystallinity for adsorbing copper in solutions. Guo et al. (2018) prepared an attapulgite HAP composite via a co-precipitation method with attapulgite as the matrix. They found that the maximum adsorption capacity of Cd²⁺ in the solution can reach 1.99 mmol/g. A hydroxyapatite/calcium silicate hydrate prepared by a phosphate recovery method was performed by Zhang et al. (2018b). They found that calcium silicate hydrate and hydroxyapatite/calcium silicate hydrate were effective for heavy metal retention but that the adsorption capacity of the latter was greater due to the presence of phosphate. However, these adsorbents used in existing studies are inconvenient to separate.

Magnetic nanoparticles have attracted considerable interest in the research on environmental remediation due to their large surface area, easy separation, and convenient recovery. Bare magnetite nanoparticles are likely oxidized in air, and the stability of the magnetite in the environment can be increased by modifications on its surface. For example, a calcium-based magnetic biomass carbon was prepared by biochar and magnetic materials and then used to adsorb Cd²⁺ in an aqueous solution; the adsorption amounts were 6.34 and 10.07 mg/g (Wu et al. 2018), respectively. Humic acid–coated magnetite nanoparticles have also been used as adsorbents to remove heavy metals, such as copper (Cu), Cd²⁺, and mercury (Liu et al. 2015). Phosphate, a natural environmentally friendly material, can be combined with magnetite to increase its stability. In addition, recent research has indicated that phosphate can form insoluble salts with various heavy metals and that it is widely used in environmental restoration (Beesley and Marmiroli 2011; Bachoua et al. 2016).

In this study, a nanomagnetic HAP (nHAP-Fe₃O₄) prepared by co-precipitation (Zhuzhou 2007) with hydroxyapatite, iron salt, and ferrous salt was developed for the adsorption and removal of Cd²⁺ from the water. The physical and chemical characterization of nHAP-Fe₃O₄ were investigated, and the removal effect and mechanism of Cd²⁺ by nHAP-Fe₃O₄ in the solution were studied to provide a scientific basis for the repair of Cd²⁺ in magnetic composite materials.

Materials and method

Materials and preparation of nHAP-Fe₃O₄

The nHAP (particle size 60 nm), FeCl₂·4H₂O, FeCl₃·6H₂O, CdCl₂·2.5H₂O, and ammonium water used were all of the analytical grade.

The bare and nHAP-coated Fe₃O₄ magnetic nanoparticles were prepared by co-precipitation. Briefly, 1.020 g of FeCl₂·4H₂O was dissolved in 50 mL of pure water and then added into 100 mL of pure water containing 2.770 g of FeCl₃·6H₂O under vigorous mechanical stirring. Then, the solution was heated to 80 °C, and ammonium water was added until its pH reached approximately 8. The black precipitate produced instantly was the Fe₃O₄ nanoparticle. A certain amount of nHAP was ultrasonically dispersed in pure water and was then added to the newly prepared Fe₃O₄ solution after heating to 80 °C. The solution was stirred continuously for 2 h, and the whole reaction temperature was controlled at 75 ± 5 °C under N₂ protection. The precipitate was then separated using a magnet after the reaction and washed with ethanol and pure water to neutral pH. The obtained product was nHAP-Fe₃O₄, which was then freeze-dried and ground for use.

Characterization of nanoparticles

The crystalline structures of the bare and nHAP-Fe₃O₄ magnetic nanoparticles were characterized by X-ray powder diffraction (BRUKER D8 ADVANCE, Germany), operated on Co Kα radiation, with a 2θ scan range from 10 to 90° and a rate of 6°/min. The infrared spectrometer (Nicolet IS10, USA) was used to obtain the functional group of the material, and KBr was used for compression. A transmission electron microscope (TEM) (JEM-1200EX, Japan) was used to investigate the morphological structure of the material, and TEM images were obtained using 120 kV acceleration voltage. The samples were prepared by dispersing a certain number of particles in deionized water with an ultrasonic bath. Then, a few drops of the suspended liquid were transferred to a 400-mesh Cu grid. After the instrument ran automatically for 40 min, the Cu mesh with samples was inserted into the sample chamber to observe the sample morphology. The magnetic properties of the samples were recorded using the liquid-free multifunction vibrating sample magnetometer system (Versalab, Quantum, USA). Analysis was performed using X-ray photoelectron spectroscopy (XPS, Thermo Escalab 250Xi, USA), and the spectra were collected with monochrome Al Ka (hv = 1486.6 eV) operated at 150 W and 650-μm beam spot. Charge correction was done with contaminated carbon C1s = 284.8 eV.s.

Adsorption and regeneration experiment

In a typical adsorption procedure, 1000 mg/L CdCl₂ is prepared for use. In this study, 0.100 g of the as-prepared nanoparticles was then added into 50 mL of the mixed solution containing varying initial concentrations of Cd²⁺. The shaking time was 1–1440 min. The pH of the mixture was adjusted to different values using HCl and NaOH. Then, the mixture was stirred for a certain time at 200 r/min and 25 °C. Thereafter, the magnetic nanoparticles with adsorbed heavy metals were separated from the mixture using a permanent handheld magnet. The amount of adsorbed Cd²⁺ was measured with an atomic absorption spectrophotometer. The adsorption capacity of the nanoparticles was determined using the following formula:

$$ Q\mathrm{e}=\frac{\left({C}_0-{C}_t\right)\times \mathrm{V}}{m} $$

(1)

where Q_e (mg/g) is the amount of Cd²⁺ adsorbed at equilibrium; C₀ (mg/L) and C_t (mg/L) are the initial concentration and equilibrium concentration of the mixed solution, respectively; V (mL) is the volume of the solution, and m (mg) is the mass of the adsorbent. All experiments were repeated thrice, and the average results were reported.

Real wastewater features a complex composition, which may interfere with the adsorption performance of nHAP-Fe₃O₄. The effects of coexisting cations were studied using Na⁺, Mg²⁺, and Cu²⁺ as model cations by adding NaCl, MgCl₂, and CuCl₂. The ionic strength levels were set to 0, 20, 40, and 80 meq/L, with the concentration of Cd²⁺ set at 100 mg/L. Other conditions were the same as those in a typical adsorption procedure.

The recovery of the as-prepared nanoparticles was evaluated by regeneration processes using 0.01 mol/L HCl and 0.01 mol/L EDTA-Na₂, respectively. Briefly, 0.100 g of the as-prepared nanoparticles was added into 50 mL of the Cd²⁺ solution with a concentration of 250 mg/L. The resulting mixture was shaken for 3 h to reach the adsorption equilibrium. Then, the spent nanoparticles were eluted with 50 mL of 0.01 mol/L HCl and 0.01 mol/L EDTA-Na₂ for 3 h. After regeneration, the recovered materials were separated, washed with deionized water, and then used for the next adsorption experiments. The procedure was repeated for five consecutive cycles. The adsorption capacities of the nanoparticles were calculated by Formula (1).

The regeneration efficiency (%RE) of each cycle was calculated as follows (Ahmad et al. 2018):

$$ \%\mathrm{RE}=\frac{Q_{e,i}}{Q_e}\times 100\% $$

(2)

where Q_e is the adsorption capacity of nHAP-Fe₃O₄. In addition, another 0.100 g of nanoparticles was subjected to the above procedure of adsorption and desorption, dried in a vacuum oven at 60 °C, and then weighed after each desorption. The recovery rate of adsorbent (%Ra) was calculated by the following formula:

$$ \%\mathrm{Ra}=\frac{m_i}{m_0}\times 100\% $$

(3)

where m_i is the dry weight of adsorbent after each cycle and m₀ is the initial mass of the adsorbent.

Results and discussion

Basic properties

Through a pre-experiment, 1 g of Fe₃O₄ was found to load up to 1.14 g of nHAP, and the ratio of the raw materials of nHAP to Fe₃O₄ was 1:1. Therefore, this ratio was used in the successive tests. The basic physicochemical properties of the prepared materials are shown in Table 1. Nearly no free nanoparticles were observed in the solution through a magnetic separation after shaking, indicating that nHAP and Fe₃O₄ were tightly bound together and could not be separated by simple physical methods.

Table 1 Basic physicochemical properties of nanoparticles in this study

The leaching of sorbent components into treated water is unfavorable to the environment. Deionized water; 0.1 mol/L CaCl₂; 0.01, 0.1, 0.5, 1, and 5 mol/L HCl; and 2 mol/L NaOH were used as the matrix to discuss the leaching property of the nanoparticles (Liu et al. 2015). Table 2 shows the leaching of Fe, P, and Ca in the different solutions of nHAP-Fe₃O₄. The nHAP-Fe₃O₄ dispersed by deionized water exhibited a free concentration of 0.13 mg/L phosphorus and nearly zero iron ions in equilibrium, whereas the concentration of free iron ions in the aqueous dispersion of Fe₃O₄ nanoparticles was 0.93 mg/L. This result indicated that the phosphate coating markedly improved the stability of the magnetic nanoparticles and reduced the release of nanomaterials into the environment. In the 0.01 and 0.1 mol/L HCl matrices, the leaching of Fe, P, and Ca from nHAP-Fe₃O₄ into the solution phase was below 10%, 45%, and 52%, respectively; when the concentration was increased to 0.5 mol/L, the leaching of Fe increased to 85.92%, and the phosphate was completely dissolved. The nHAP-Fe₃O₄ was completely leached out in 5 mol/L HCl. The leaching of Fe and P in pure water was found to be similar to that in 0.1 mol/L CaCl₂. This result indicated that the two solutions exerted similar effects on the stability of nHAP-Fe₃O₄. As the nanoparticles were prepared in basic solutions, the leaching values of Fe, P, and Ca in 2 mol/L NaOH was 1.76%, 0.22%, and 2.15%, respectively. Overall, the nHAP-Fe₃O₄ was stable in pure water, as well as in salt, weak acid, and basic solutions.

Table 2 Leaching of Fe, P, and Ca in different solutions of nHAP-Fe₃O₄

Characterization of Fe₃O₄ and nHAP-Fe₃O₄

Figure 1 shows the TEM images of the as-prepared Fe₃O₄ (a) and nHAP-Fe₃O₄ (b). The Fe₃O₄ magnetic nanoparticles (a) were spherical with typical sizes of 5–25 nm, and their distribution was relatively uniform. Figure 1 b shows the TEM images of the as-prepared nHAP-Fe₃O₄. nHAP was rod-shaped, typically 170 nm long and 35 nm thick, and had Fe₃O₄ attached on its surface. Similar observations were reported by Piar and Pakade (2015). Meanwhile, no obvious change was observed in the morphology and size of Fe₃O₄, and the combination of Fe₃O₄ caused the magnetic property of the as-prepared nanoparticles.

Fig. 1

TEM images of Fe₃O₄a and nHAP-Fe₃O₄b

The results of the wide-angle X-ray diffraction are shown in Fig. 2. The broad peaks at diffraction angles of 35.1, 41.4, 50.5, 63.0, 67.4, and 74.3, which were characteristic of the (220), (311), (400), (422), (511), and (440) crystal planes of the Jade PDF card (JCPDS 88-0315), respectively (Farokhi et al. 2018), were distinguished in the bare Fe₃O₄ and nHAP-Fe₃O₄. These peaks were of standard anti-spinel crystal form. The characteristic peaks of HAP (Fig. 2b) were noted at 37.0, 38.3, and 30.1, which corresponded to (JCPDS 72-1243) (211), (300), and (002) crystal planes (Núñez et al. 2014), respectively. Compared with that of the bare Fe₃O₄, the characteristic peak intensity of nHAP-Fe₃O₄ was significantly reduced, and no other diffraction peaks were observed, except for the peaks of Fe₃O₄ and HAP. This result indicated that no new substances were formed during the material synthesis. This finding also revealed that the combination of phosphate and Fe₃O₄ was mainly by electrostatic interaction and intermolecular force.

Spectroscopic analysis showed that the nHAP was successfully coated on the surface of Fe₃O₄ (

Fig. 2

X-ray diffraction of Fe₃O₄a and nHAP-Fe₃O₄b

Fig. 3). The Fe–O stretch of nHAP-Fe₃O₄ was found at 569 cm⁻¹ Ma et al. (2018b), and the absorption peaks at 3435 and 1630 cm⁻¹ were attributed to the surface adsorption water (Viswanathan and Meenakshi 2008; Ahmad et al. 2019), as well as the hydroxyl stretching and bending vibration on the composite materials. The bands at 961 and 1034–1125 cm⁻¹ were due to the PO₄³⁻ and P–O vibrations (Wei et al. 2017), and the band at 602 cm⁻¹ was attributed to the characteristic peak of HAP (Nabavinia et al. 2019). Meanwhile, for the bare Fe₃O₄, no bands at 600–1200 cm⁻¹ were observed, and the Fe–O vibration peak in the composite material was obviously weakened, also indicating the existence of nHAP on Fe₃O₄. For nHAP-Fe₃O₄, the peak at Fe–O was slightly offset in comparison with the case of the bare Fe₃O₄ most likely due to the influence of the nHAP.

Fig. 3

IR Spectrum of Fe₃O₄a and nHAP-Fe₃O₄b

Figure 4 shows the magnetization curves of Fe₃O₄ (a) and nHAP-Fe₃O₄ (b). No hysteresis effect was observed on the two materials, indicating that the materials prepared were super-paramagnetic. The saturation magnetizations were 68.1 and 34.1 emu/g for Fe₃O₄ and nHAP-Fe₃O₄, respectively. The saturation magnetization of nHAP-Fe₃O₄ decreased, indicating that nHAP was successfully coated on the surface of Fe₃O₄. A saturation value of 16.3 emu/g was reported to be sufficient for magnetic separation with a conventional magnet (Ma et al. 2005). In the solution test, the materials were completely separated from the solution by an external magnetic field.

Fig. 4

Magnetization curves of Fe₃O₄a, nHAP-Fe₃O₄b, and its recover procedure

Sorption study

Sorption kinetics

The sorption dynamics of Cd²⁺ to nHAP, nHAP-Fe₃O₄, and Fe₃O₄ were evaluated by adding 0.1 g of the as-obtained nanoparticles into 50 mL of a mixed solution containing 50, 100, and 500 mg/L Cd²⁺ at room temperature. The adsorption time was 1, 2, 3, 5, 10, 20, 30, 45, 60, 120, 360, 720, and 1440 min. The results are shown in Fig. 5. The adsorption capacities of the nanoparticles were determined with a quick step continuing up to 2 h, followed by a slow one up to the equilibrium state. The equilibrium adsorption capacities of nHAP, nHAP-Fe₃O₄, and Fe₃O₄ were 12.00, 11.20, and 7.20 mg/g at Cd²⁺ concentration of 50 mg/L; 32.10, 27.05, and 10.15 mg/g at Cd²⁺ concentration of 100 mg/L; and 46.20, 53.35, and 11.43 mg/g at Cd²⁺ concentration of 500 mg/L, respectively. The result indicated that the adsorption amount of nHAP was higher than that of the other two nanoparticles in a low concentration of Cd²⁺. However, when the Cd²⁺ concentration was high, the adsorption of nHAP was lower than that of nHAP-Fe₃O₄.

Fig. 5

Adsorption capacity of Cd²⁺ in 50, 100, and 500 mg/L by nHAP, nHAP-Fe₃O₄, and Fe₃O₄ at different times

The pseudo-first-order kinetic model (4) and pseudo-second-order kinetic model (5) were used to fit the adsorption data. The results are shown in Table 3. The data of nHAP and nHAP-Fe₃O₄ fitted the pseudo-second-order model well, with an excellent correlation coefficient of R² > 0.9, which implied the occurrence of chemisorption during the adsorption process (Ahmad et al. 2018). Meanwhile, the adsorption process of Fe₃O₄ could be described by the pseudo-first-order kinetic model with a correlation coefficient of 0.79–0.98. The above results indicated that the adsorption sites were mainly provided by the nHAP during the adsorption process. For the nHAP and nHAP-Fe₃O₄, the calculated h was large, and t_0.5 was small, indicating that the initial adsorption rate was fast and that the time to reach adsorption equilibrium was short. This finding is likely due to the large specific surface area that can provide many active binding sites for Cd²⁺. Therefore, at the beginning of adsorption, Cd²⁺ was rapidly bound to the active sites on the surface due to the large concentration of Cd²⁺ in the solution while the adsorption rate gradually slowed down because of the decreasing number of active sites and Cd²⁺ concentration until equilibrium was reached (Zheng et al. 2010). Moreover, the theoretical equilibrium adsorption amount was equal to the practical value Ma et al. (2018a).

$$ \ln \left({Q}_{\mathrm{e}}-{Q}_{\mathrm{t}}\right)=\ln \left({Q}_{\mathrm{e}}\right)-{k}_1t $$

(4)

$$ \frac{t}{Q_t}=\frac{1}{Q_e}t+\frac{1}{k_2{Q_e}^2} $$

(5)

Table 3 The kinetic model parameters for the adsorption of Cd²⁺ on nHAP, nHAP-Fe₃O₄, and Fe₃O₄ (C₀ 50 mg/L, 100 mg/L, 500 mg/L)

where Q_e (mg/g) and Q_t (mg/g) are the amounts of Cd²⁺ adsorbed at equilibrium and at any time, respectively; and k₁ (min⁻¹) and k₂ (g/(mg·min)) are the rate constants of the pseudo-first-order kinetics and pseudo-second-order kinetics, respectively.

Adsorption isotherm

The adsorption capacity of the as-prepared nHAP-Fe₃O₄ to Cd²⁺ was measured at various initial concentrations (0, 10, 20, 40, 60, 80, 100, 200, 400, and 600 mg/L). The results (Fig. 6) showed that the adsorption capacity of Cd²⁺ increased with the increase of the equilibrium concentration within 24 h. The maximum adsorption capacities of the nHAP-Fe₃O₄ for Cd²⁺ were evaluated using the adsorption isotherms as follows:

$$ Q\mathrm{e}=\frac{k_1{Q}_{max}{C}_e}{1+{k}_1{C}_e} $$

(6)

$$ Q\mathrm{e}={k_f{C}_e}^{\frac{1}{n}} $$

(7)

where Q_e (mg/g) and Q_max (mg/g) represent the equilibrium adsorption capacity and maximum adsorption capacity of the nanoparticles, respectively; k₁ is the Langmuir constant related to the affinity of the adsorbent; C_e is the equilibrium concentration of Cd²⁺; and k_f represents the Freundlich adsorption constant.

Fig. 6

Adsorption isotherm of Cd²⁺ by nHAP-Fe₃O₄ and Fe₃O₄

The isotherm parameters obtained from the models are listed in Table 4. The adsorption data fitted well with the Langmuir model, with the correlation coefficient (R²) being 0.9778, which indicated that the adsorption process was a uniform adsorption of monolayers (Langmuir 1918). The adsorption capacity according to the Langmuir model was 62.14 mg/g, and the nHAP-Fe₃O₄ prepared in this study demonstrated a higher adsorption capacity for Cd²⁺ than did the materials reported in other studies (Table 5). The Freundlich fitting parameter n > 1 indicated that the adsorption of Cd²⁺ by nHAP-Fe₃O₄ was easy (Tang et al. 2018).

Table 4 Isotherm constants for Cd²⁺ adsorption on nHAP-Fe₃O₄ and Fe₃O₄

Table 5 Adsorption of Cd²⁺ by different adsorbents

Effects of pH

The solution pH not only affects the Cd²⁺ adsorption capacity of nanoparticles but also adjusts the pH of the equilibrium liquid. It can also indirectly affect the adsorption of heavy metal ions by the material. The adsorption capacities of the nanoparticles for Cd²⁺ at pH values of 2, 2.5, 3, 4, 5, 6, and 7 in this study are shown in Fig. 7. The pH was found to greatly affect the adsorption of Cd²⁺. The adsorption capacity of Cd²⁺ by the nHAP-Fe₃O₄ nanomaterials increased rapidly with the increase of pH from 2 to 3, and it became gentle with the increase of pH from 3 to 7. The pH of the zero charge point (pH_PZC) for nHAP-Fe₃O₄ was 3.08 (Table 1), which was obtained by plotting the charges at pH 2, 4, 6, 8, and 10. The charges were measured by a laser particle size analyzer (Zetasizer Nano ZS, Malvern, UK). Nanoparticles are negatively charged when pH > pH_PZC. Electrostatic attraction occurs with Cd²⁺, which is positively charged, and as the pH increases, the zeta potential increases, and the adsorption capacity of Cd²⁺ becomes strong. In this study, the nHAP-Fe₃O₄ was positively charged when the pH of the material < pH_PZC while some Cd²⁺ became adsorbed. This result indicated the occurrence of other mechanisms, such as ion exchange and chelation reaction.

Fig. 7

The adsorption of Cd²⁺ by nHAP-Fe₃O₄ and Fe₃O₄ at different pH values

The charge on the surface of HAP was negative, thereby generating an electrostatic attraction with Cd²⁺ during the adsorption process. Cd²⁺ may gradually hydrolyze to Cd (OH)₂, Cd(OH)₃⁻, and Cd(OH)₄²⁻ during the gradual increase of pH acidity to alkalinity; this condition decreased the electrostatic attraction between the nanoparticles and Cd²⁺, hence the slow increase in adsorption Zhang et al. (2018a).

The pH of the equilibrium solution is shown in Table 6. The pH of the solution increased when its pH was in the range of 2–6 but decreased when it was 7, which is close to 6.5. The solution pH values of 2.5 and 3 demonstrated the highest increase, indicating that the nanoparticles have a certain buffer capacity for the solution pH that can neutralize H⁺ and OH⁻ in the solution.

Table 6 The pH of equilibrium solution after treating 50 mL of water with 0.1 g nHAP-Fe₃O₄ and Fe₃O₄

Effects of coexisting cationics

The influence of commonly coexisting ions on the adsorption of Cd²⁺ was studied using Na⁺, Mg²⁺, and Cu²⁺ as model cations (Fig. 8). The results showed that obstacles occurred on the adsorption of Cd²⁺ in the existence of cations, and the adsorption capacity decreased with the increase of ionic strength, indicating that a competitive adsorption relationship existed between the cations and Cd²⁺. The effects of the three ions on the adsorption process of Cd²⁺ were noted in the order Cu²⁺ > Mg²⁺ > Na⁺, with a decreasing adsorption of Cd²⁺ from 44.24 mg/g to 6.59 mg/g. Even at low concentrations, the adsorption amount of Cd²⁺ was obviously decreased possibly because Cu²⁺ precipitated on the surface of the nanoparticles and some of the adsorption sites were masked (Qian 2007). Moreover, previous reports showed that the affinity of iron oxide and phosphate for heavy metals such as Cu²⁺ and Cd²⁺ is higher than that for Na⁺ (Zhou et al. 2017). The radius of Na⁺ is smaller than those of the other two cations, and therefore, the influence of Na⁺ on Cd²⁺ adsorption is less obvious than that of Mg²⁺and Cu²⁺.

Fig. 8

The adsorption of Cd²⁺ by nHAP-Fe₃O₄ and Fe₃O₄ at different cation strengths

Adsorption mechanism

The above results showed that the adsorption mechanisms of heavy metal ions by magnetic hydroxyapatite might involve physical adsorption, ion exchange, and chemical complexation. To explore the internal factors affecting adsorption and the adsorption mechanism on nHAP-Fe₃O₄, we used the particle diffusion model and carried out XPS analysis. For many adsorptions, intraparticle diffusion is the control step of the adsorption rate, and the presence or absence of this process can be determined by the following formula:

$$ {Q}_{\mathrm{t}}={K}_{\mathrm{P}i}{t}^{0.5}+{C}_i $$

(8)

where Kp_i (mg/(g min^0.5)) is the intraparticle diffusion rate constant; C_i is the boundary layer thickness, that is, serious particle diffusion blocking equates to a great boundary effect; and i represents the different adsorption stages.

The results with Q_t as the ordinate and t^0.5 as the abscissa are shown in Fig. 9. The fitting curve exhibited a multilevel linear relationship. The first stage is the initial stage of adsorption with a rapidly rising surface adsorption; the second stage continued for a long time, and the intraparticle diffusion served as the limiting step of the adsorption rate (Lei et al. 2018). The K_p1 values of both materials were greater than K_p2, and C₂ was greater than C₁ (Table 7), indicating that at the beginning of the adsorption, the adsorption rate of Cd²⁺ was fast using the nanoparticles because of their large specific surface area. Then, as the adsorption progressed, the adsorption capacity of the particles gradually decreased. None of the fit curves passed through the origin, indicating that intraparticle diffusion was not the only step in controlling the adsorption rate (Adebisi et al. 2017).

Fig. 9

Intraparticle diffusion curve for Cd²⁺ adsorption on nHAP-Fe₃O₄ and Fe₃O₄

Table 7 Intraparticle diffusion model constants and correlation coefficients for Cd²⁺ adsorption on nHAP-Fe₃O₄ and Fe₃O₄

The XPS analysis of nHAP-Fe₃O₄ before and after adsorption of Cd²⁺ is shown in Fig. 10. Compared with nHAP-Fe₃O₄ before adsorption, Cd²⁺ was present on the surface of the nHAP-Fe₃O₄ particles adsorbed by Cd²⁺, and no change in chemical valence occurred. The peak shape and position of Fe2p before (curve I) and after (curve II) the reaction with Cd²⁺ demonstrated virtually no change; however, the intensity was changed, indicating that a strong relationship existed between Fe and Cd²⁺ and the formation of complexes (Chen et al. 2017). The peak shape of P2p at 133.5 eV after the reaction with Cd²⁺ was significantly enhanced, indicating that part of the phosphate participated in Cd²⁺ adsorption by forming complexes or via chemical bonding (Guivar et al. 2016). The analysis of O²⁻ showed that the BEs of Fe–O, P–O, and M–OH were denoted by O1s = 530.2, 531.7, and 533.3 eV, respectively; the peak at 533.3 eV was significantly weakened, whereas that at 530.2 eV was enhanced, indicating that hydroxyl and Fe participated in the adsorption process (Yang et al. 2016a; Heitmann et al. 2014).

Fig. 10

XPS spectra of nHAP-Fe₃O₄ before (I) and after (II) adsorption with Cd²⁺

The percentage of Ca²⁺ decreased from 6.8 to 5.9% after adsorption, and the amount of adsorbed Cd²⁺ accounted for 1.6% of the total amount of material, indicating the existence of an ion exchange process. Notably, the percentage of O decreased from 91.9 to 75.9%, and the percentage of P increased from 5.1 to 5.8% because of the reaction of phosphate.

The ion exchange process was explored by measuring the concentration of Ca²⁺ released during the adsorption at initial Cd²⁺ concentrations of 0, 50, 100, 200, 400, and 600 mg/L and a material dosage of 2 g/L. The adsorption data were calculated, as shown in Table 8. The regression analysis showed that the data of the two groups presented highly significant correlation (P < 0.05). The release of Ca²⁺ from nHAP-Fe₃O₄ was 0.0546 mmol when the initial concentration of Cd²⁺ was 0 mg/L. Therefore, the ion exchange between Cd²⁺ and Ca²⁺ after subtracting the release of Ca²⁺ from the material was as follows (R² = 0.9382):

$$ y=0.5828x-0.0012 $$

(9)

where y (mmol) represents the amount of Ca²⁺ which was exchanged by Cd²⁺, and x (mmol) represents the amount of Cd²⁺ which was adsorbed by nHAP-Fe₃O₄ in a 50-mL solution. The slope of the formula indicated that the amount of Ca²⁺ substituted by per millimolar Cd²⁺ was 0.5828 mmol, indicating that ion exchange occupied an important part of the adsorption process. However, the substitution relationship between Cd²⁺ and Ca²⁺ was not the theoretical 1:1 measurement relationship, indicating the existence of other mechanisms in the adsorption process.

Table 8 Stoichiometric equivalent between Cd²⁺ and Ca²⁺ in the adsorption process of a 50-mL solution

Combining the above results reveals that the adsorption mechanism of Cd²⁺ by nHAP-Fe₃O₄ can be ion exchange and chemical complexation, accompanied by surface physical adsorption. Cd²⁺ diffused to the surface of the adsorbent and then became adsorbed into the active site. The specific surface area of nHAP-Fe₃O₄ had nearly no change after the loading of the nHAP, but the adsorption amount after loading was greatly improved, indicating that surface physical adsorption was not dominant in the adsorption process. The same was true for the influence factors of adsorption.

Adsorption–regeneration cycles of Cd²⁺

As shown in Figs. 11 and 12, the regeneration efficiency and recovery rate of nHAP-Fe₃O₄ decreased with increasing number of cycles. The regeneration efficiencies of nHAP-Fe₃O₄ eluted by HCl after the fifth regeneration cycle and by EDTA-Na₂ were approximately 55.33% and 63.04%, respectively. The recovery rates of the adsorbent eluted by HCl and EDTA-Na₂ were approximately 50% after two cycles. After the fifth cycle, the recovery rates of the adsorbent were 42% and 40.2%. These results suggested that the recovery rate of nHAP-Fe₃O₄ eluted by EDTA-Na₂ was low and that the regeneration efficiency was better than that of HCl. These results may be due to EDTA-Na₂ being a chelating agent with metal ion; hence, it can form a complex with Cd²⁺, as well as chelates with Fe²⁺ and Fe³⁺. Hence, the material was dissolved, indirectly causing the Cd²⁺ adsorbed on the material to be desorbed. The regenerations of Cd²⁺ by different adsorbents are shown in Table 9. The desorption of Cd²⁺ adsorbed by nHAP-Fe₃O₄ was lower than that reported probably because the desorption agents, as well as the elements of desorption, were different.

Fig. 11

Regeneration efficiency of nHAP-Fe₃O₄ eluted by HCl and EDTA-Na₂ from batch experiments for five consecutive cycles at 250 mg/L Cd²⁺

Fig. 12

Recovery rate of nHAP-Fe₃O₄ eluted by HCl and EDTA-Na₂ from batch experiments for five consecutive cycles at 250 mg/L Cd²⁺

Table 9 Adsorption and regeneration of Cd²⁺ by different adsorbents

Conclusion

HAP-Fe₃O₄ nanocomposite was fabricated by co-precipitation and then compared with Fe₃O₄ nanoparticles. The as-prepared nanocomposite exhibited certain enhancement in material stability. TEM, XRD, FTIR, and magnetic analysis showed that nHAP and Fe₃O₄ were successfully combined and had a good magnetic property for easy separation. The kinetic study showed that the adsorption process achieved equilibrium within 2 h and followed a pseudo-second-order reaction. Adsorption isotherm data fitted the Langmuir model well with an adsorption capacity of 62.14 mg/g. The adsorption mechanisms of Cd²⁺ by nHAP-Fe₃O₄ included rapid surface adsorption, intraparticle diffusion, and internal particle bonding. And the ion exchange of Ca²⁺ and chemical complexation occupied a dominant position. The adsorption amount increased with increasing pH from 2 to 7, and the coexisting ions exhibited great influence on the adsorption capacity, especially Cu²⁺. HCl and EDTA-Na₂ were efficient eluents used for the desorption of metal ions. After the fifth cycle, the recovery rates of nHAP-Fe₃O₄ were 42% and 40.2%, and the regeneration efficiencies were 55.33% and 63.04%, respectively. These eluents can facilitate the reuse of materials with good regeneration.