Zn²⁺ removal from the aqueous environment using a polydopamine/hydroxyapatite/Fe₃O₄ magnetic composite under ultrasonic waves

Abstract

In this study, an easily magnetically recoverable polydopamine (PDA)-modified hydroxyapatite (HAp)/Fe₃O₄ magnetic composite (HAp/Fe₃O₄/PDA) was suitably synthesized to exploit its adsorption capacity to remove Zn²⁺ from aqueous solution, and its structural properties were thoroughly examined using different analytical techniques. The effect of multiple parameters like pH, ultrasonic power, ultrasonic time, adsorbent dose, and initial Zn²⁺ concentration on the adsorption efficiency was assessed using RSM-CCD. According to the acquired results, by increasing the adsorbent quantity, ultrasonic power, ultrasonic time, and pH, the Zn²⁺ adsorption efficiency increased and the interaction between the variables of ultrasonic power/Zn²⁺ concentration, pH/Zn²⁺ concentration, pH/absorbent dose, and ultrasonic time/adsorbent dose has a vital role in the Zn²⁺ adsorption. The uptake process of Zn²⁺ onto PDA/HAp/Fe₃O₄ followed Freundlich and pseudo-second order kinetic models. The maximum capacity of Zn²⁺ adsorption (q_m) obtained by PDA/HAp/Fe₃O₄, HAp/Fe₃O₄, and HAp was determined as 46.37 mg g⁻¹, 40.07 mg g⁻¹, and 37.57 mg g⁻¹, respectively. Due to its good performance and recoverability (ten times), the HAp/Fe₃O₄/PDA magnetic composite can be proposed as a good candidate to eliminate Zn²⁺ ions from a water solution.

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Introduction

The economic development and the rapid growth of agricultural industries have primarily contributed to the production of wastewater containing organic and inorganic pollutants, which when discharged into the environment can cause serious problems for human health, water resources, and other living organisms.^1–3 Among the organic and inorganic pollutants, dyes, drugs, and heavy metals have received paramount attention due to their high environmental persistence and toxicity.^3,4 Recently, the removal of heavy metals from water matrices has become an important concern and research topic because of the strong impact that this class of pollutants has on human health, and the environment.⁵ Heavy metals find application in various industrial sectors, such as the metal plating, electroplating, mining, leather, and fertilizer industries, and, as a consequence, every year directly or indirectly a large number of these pollutants, mainly zinc, copper, nickel, arsenic, lead, cadmium, chromium, mercury, and cobalt, enter the environment through industrial wastewater discharge.^6–8 It has been demonstrated that toxic metals can enter the human body through food chains and their accumulation has been linked to the onset of various diseases and illnesses such as neurological disorders, various cancers, imbalances in hormones, obesity, abortion, asthma, infertility, and death.⁹ The zinc ion is one of the most employed metals in the industrial field. Even though it is an important and essential micronutrient for life, in high concentrations it can lead to depression, lethargy, increased thirst, neurological disorders, diarrhoea, anaemia, kidney failure, and low immunity.^10,11 Owing to its hazardous nature for public health, the maximum Zn concentration in seawater is 40 μg L⁻¹ and in freshwater is 45–500 μg L⁻¹, strongly correlated to the water hardness.¹² Therefore, it is evident that improper discharge of toxic metal ions into the water bodies is an important issue and their removal from aqueous systems before releasing them to the environment is crucial.

So far, various techniques like electrochemical methods, coagulation, ion exchange, chemical precipitation, membrane filtration, biological treatment, and adsorption have been used to eliminate metals from wastewaters.^13,14 Most of these approaches show drawbacks, such as high operating costs, sensitive operating status, low efficiency, and secondary sludge production,¹⁵ which has limited their use on an industrial scale. In particular, if compared to the other processes, the adsorption method is an effective way for pollutants abatement from wastewater,¹⁶ thanks to its simplicity, low cost, high performance and efficiency, low selectivity, and no production of secondary sludge.^17–19 In addition to the mentioned advantages, when the content of metal ions in industrial wastewater is in the range of 10–100 mg L⁻¹ or the concentration of ions of discharged metals is less than 1 mg L⁻¹, the adsorption method is effective.³

In the last years for this kind of application, various solid sorbents have been investigated, such as Tamarix hispida activated carbon,²⁰ Aspergillus flavus biomass,²¹ magnetic MnFe₂O₄,²² and nano bentonite.²³ Among them, due to its unique properties, such as low cost, insolubility in water (K_ps ≈ 10⁻⁵⁹), the high adsorption capacity of heavy metals, availability, suitable thermal and chemical stability, calcium hydroxyapatite (HAp) has been the main focus of numerous studies.^24,25 Thanks to the presence of calcium, hydroxyl, and phosphate ions in the crystalline structure, it can effectively sorb heavy metals through ion exchange or electrostatic interactions.²⁶ Although HAp has exhibited evident and significant advantages, its constituent particles are prone to aggregation and clump formation, causing a decrease in their active surface and dispersion in aqueous media and finally reducing their performance towards pollutants removal.²⁷ Moreover, HAp is characterized by poor mechanical and high abrasive properties that can be improved by combining this material with synthetic polymers, biopolymers, and nanoparticles. HAp increases crystallization, morphology, dimensional anisotropy and improves the adsorption capacity of the composite.²⁸ In addition to HAp, polydopamine (PDA) is another interesting material that has recently attracted the attention of the scientific community in particular in the field of nanotechnology, materials chemistry, and medicine and has been used in lithium-ion batteries, tissue engineering, bioimaging, biosensors, etc.^29,30 PDA is very sticky and belongs to the category of catecholamines, which has a benzene ring in its structure and functional groups such as amines, imines, and phenols, which make this polymer function properly in removing heavy metals through electrostatic interactions.³¹ Adding iron-containing compounds like Fe₃O₄ to adsorbents can create magnetic properties and use less force to separate them from the refined solution.³²

In the present study, we propose an innovative PDA/HAp/Fe₃O₄ magnetic composite based on hydroxyapatite produced in our laboratory from a biological source (chicken bone) characterized by outstanding ability to remove Zn²⁺ ions from an aqueous solution. HAp/Fe₃O₄/PDA was extensively characterized by various techniques. To assess the influence of the reaction parameters in the adsorption process, like pH, temperature, time, Zn²⁺ ions concentration, and adsorbent mass, the Response Surface Method-Central Composite Design (RSM-CCD) was used. Different models were utilized to explore the equilibrium, kinetic and thermodynamic behaviour of the adsorption process and the type of process was examined physically, chemically, spontaneously, or non-spontaneously. Finally, the effect of co-existing ions and phenomena of adsorption/desorption were also investigated.

Experimental section

Chemicals and materials

Iron(II) chloride tetrahydride (FeCl₂·4H₂O), iron(III) chloride hexahydride (FeCl₃·6H₂O), HCl, NaOH, ethanol, zinc nitrate tetrahydride (Zn(NO₃)₂·4H₂O), and dopamine hydrochloride was purchased from Merck Co. with high purity. To prepare the initial solutions for the Zn²⁺ adsorption process, the stock solution was diluted using distilled water. To regulate the pH of the desired aqueous solutions, NaOH and HCl solutions (0.1–1 M) were added dropwise. Chicken bones were obtained from local restaurants and used as a primary source for the production of hydroxyapatite.

Synthesis of PDA/HAp/Fe₃O₄ composite

HAp production was prepared using the calcination process and chicken bones as a suitable primary source according to the previously described method.³³ 50 g of the chicken bones were washed with distilled water and dried for 24 h at 105 °C. The dried bones were first exposed to a direct flame to become fully carbonized and then, the carbonized bones were calcinated at 1000 °C for 4 h. The change in color of the bones from black to white indicates HAp formation. This latter was powdered using a ball mill for 24 h and granulated using sieve no. 25 (ASTM E11) and then stored at room temperature.

The HAp/Fe₃O₄ magnetic composite was synthesized using a co-precipitation method previously described.³⁴ To modify HAp/Fe₃O₄ magnetic composite by polydopamine coating, 1 g of HAp/Fe₃O₄ magnetic composite was added to 100 mL of distilled water at pH 8.5 and subjected to an ultrasonic wave for 30 min. At the end of this time, 0.7 g of dopamine hydrochloride was added and stored at room temperature for 24 h under stirring. The PDA-modified composite particles were isolated from the aqueous solution using a magnet and washed several times using distilled water, and ethanol and then dried for 24 h at 50 °C. The synthesized composite was powdered using a ball mill and stored at ambient temperature.

Batch adsorption study using RSM-CCD

The interactions and effects of influential parameters on the Zn²⁺ ions adsorption by PDA/HAp/Fe₃O₄ magnetic composite were studied in batch mode in the presence of ultrasonic process (35 kHz) using RSM-CCD methodology. The effect of independent parameters, like pH (2–6), ultrasonic power (100–300 W), Zn²⁺ ions (10–30 mg L⁻¹), composite dose (0.4–1.2 g L⁻¹), and time (10–50 min) was investigated at five levels (−α, −1, 0, 1, +α). Table 1 shows more details for the independent parameters. Based on the number of parameters (n) and their levels, the experiments number was included 2ⁿ + 2n + 4 = 46, where the number 4 is the repetitions number in the central points. An acetylene–air fuel flame atomic absorption spectrometer (Varian AA240FS) was used to measure the concentration of Zn²⁺ ions remaining in the aqueous solution. During the adsorption tests, the adsorption percentages (R%) of Zn²⁺ ions and elimination capacity (q_e) were determined from eqn (1) and (2), respectively:Here C_i and C_e (mg L⁻¹) denote the initial and equilibrium contents of Zn²⁺ ions, V reflects the volume of used solution (L), and m denotes the sorbent weight (g).

Table 1 The variables and coding levels

Independent variables	Coded	Unit	Factorial and center level			Axial level
			Low (−1)	Center (0)	High (+1)	Lowest (−α)	Highest (+α)
Power of ultrasound	X1	°C	150	200	250	100	300
pH	X2	—	3	4	5	2	6
Content Zn^2+	X3	mg L^−1	15	20	25	10	30
Sonication time	X4	min	20	30	40	10	50
Adsorbent dosage	X5	g L^−1	0.6	0.8	1	0.4	1.2

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To analyze the obtained experimental data based on the suggestion of Design-Expert software, a 2-degree polynomial model was applied. This polynomial model shows the link between the independent variables (X₁ to X₅) and the dependent variable (Y) as described by eqn (3):

Here, Y implies the desired response, β₀ implies the model constant-coefficient, β_i implies the linear coefficients, β_ij implies the interaction coefficients, β_ii implies the quadratic coefficients, ε reflects the residual time, and X_i and X_j are the independent variables that are known in each laboratory step. The obtained data were processed using Design-Expert 11.0 software and the analysis of variance (ANOVA) was examined to acquire the link between the process and response variables. The authenticity of the polynomial model was determined by assessing the correlation coefficient (R²) and its statistical significance by the value of the F-test.

Desorption studies

For the desorption studies, 1.2 g L⁻¹ of PDA/HAp/Fe₃O₄ magnetic composite, HAp/Fe₃O₄ magnetic composite, and HAp were added to aqueous solutions containing Zn²⁺ ion (10 mg L⁻¹) in optimal laboratory conditions, and an ultrasonic process was performed. After the adsorption experiments, Zn²⁺ ions remaining in the aqueous solution, as well as the percentage of adsorbed Zn²⁺ ions were determined by atomic absorption spectroscopy, as reported above. Then, each adsorbent was put in 50 mL of 0.75 M HCl solution and magnetically stirred for 2 h. Finally, the adsorbents were removed from the reaction media and the metal content of the solutions was determined. Each material was regenerated and reused 10 stages and its adsorption/desorption ability was investigated.

Materials characterization

The structure of the HAp and magnetic composites was characterized by a FT-IR spectroscopy (PerkinElmer system 2000). In the case of PDA/HAp/Fe₃O₄ magnetic composite, the FT-IR analysis was carried out pre- and post- zinc adsorption X-ray diffraction analyses were carried out by a D8 Focus diffractometer (Bruker Corporation, Germany) working with Cu Kα radiation (1.5405 Å) and at 40 kV × 20 mA nominal X-ray power. Porosity and surface area analyses were measured according BET method (ASAP 2460, Micromeritics Corporation, United States). For X-ray photoelectron spectroscopy (XPS) analyses a K-alpha 1063XPS apparatus (Thermo Fisher Scientific, USA) was used. The magnetic behavior of the samples was analyzed by a vibrating sample magnetometer (Lakeshore, model 7410). The micro-structure, adsorbent size, and surface morphology were characterized by scanning electron microscope (SEM, REGULUS8230, Hitachi, Japan) and transmission electron microscope (TEM, HT7700, Hitachi, Japan). Atomic force microscopy (AFM, Dimension Icon) was applied to evaluate the surface roughness of adsorbents samples.

Results and discussion

Characterization of adsorbents and composites

Fig. 1a depicts the FT-IR spectra of HAp from chicken bones, HAp/Fe₃O₄, and HAp/Fe₃O₄/PDA magnetic composites were investigated before and after Zn²⁺ ions removal. Concerning the FT-IR spectra of the materials before the metal adsorption, in the HAp spectrum, the wavenumbers at 1046 cm⁻ and 566 cm⁻¹ respectively can be linked to the asymmetric vibrations of P–O and O–P–O.³⁵ Moreover the band at 1695 cm⁻¹ and that at 1462 cm⁻¹ are due to the presence of –OH and C–O functional groups, as previously reported.³⁶ After modification of HAp by Fe₃O₄ nanoparticles (HAp/Fe₃O₄), the previously described bands showed a slight shift because of the interactions between Fe₃O₄ nanoparticles and functional groups of the HAp structure. For example, for HAp/Fe₃O₄ composite the vibration of 566 cm⁻¹ in the HAp structure moved to 567 cm⁻¹, indicating the interaction of the O–P–O functional group with Fe₃O₄ nanoparticles. It is worth noting that the typical absorption peak of Fe₃O₄ at 570 cm⁻¹³⁴ overlaps with the O–P–O vibration band. In the FT-IR spectrum of PDA/HAp/Fe₃O₄ magnetic composite, additional low-intensity peaks appeared in the span of 1400–1510 cm⁻¹, which are due to C–C and C–N vibrations in the PDA structure.³⁷ Moreover, C–H vibrations can be responsible for the low-vibration peak observed at 2922 cm⁻¹. After the Zn²⁺ ions adsorption process, the vibration intensity linked to PDA/HAp/Fe₃O₄ functional groups changed, probably because of the interactions with the zinc ions. The change in the vibrations intensity of and the position of the functional groups in HAp/Fe₃O₄/PDA after the adsorption was not very extensive, which shows that the adsorption of zinc ions using the desired composite is physically.

Fig. 1 (a) FT-IR and (b) VSM images of HAp, HAp/Fe₃O₄ and PDA/HAp/Fe₃O₄/magnetic composites before and after Zn²⁺ adsorption process, (c) XRD patterns of all the samples, and (d) DLS graphic of PDA/HAp/Fe₃O₄ magnetic composite.

To magnetically separate and reuse the adsorbents, they must have the appropriate magnetic properties.³⁸ For this aim, the magnetic properties of Fe₃O₄, HAp/Fe₃O₄, and PDA/HAp/Fe₃O₄ magnetic composites were investigated pre- and post-Zn²⁺ adsorption. The results are reported in Fig. 1b. The value of magnetic saturation (M_s) for Fe₃O₄, HAp/Fe₃O₄, and PDA/HAp/Fe₃O₄ magnetic composites pre- and post- Zn²⁺ adsorption was determined as 84.283 emu g⁻¹, 30.305 emu g⁻¹, 24.014 emu g⁻¹, and 22.571 emu g⁻¹, respectively. The diminish in the level of magnetic saturation in the HAp/Fe₃O₄ and PDA/HAp/Fe₃O₄ samples can be due to the decrease in the amount of magnetic material, the non-magnetic nature of the matrix material, and the composite coating.⁴ The presence of non-magnetic particles such as HAp and PDA in the structure of Fe₃O₄ reduces the magnetic torque.³⁹ As a result, the amount of magnetic saturation decreases, confirming the qualitative composition of HAp/Fe₃O₄ and PDA/HAp/Fe₃O₄. Although the magnetic saturation of both HAp/Fe₃O₄ and PDA/HAp/Fe₃O₄ magnetic composites decreased in comparison to Fe₃O₄, it is sufficient for magnetic separation from aqueous solutions. In addition, these materials show paramagnetic behavior due to negligible coercivity (H_c) and remanent magnetization (M_r).⁴⁰ Therefore, they can be easily isolated from liquid medium using a magnetic field (magnet) and reused.

The XRD patterns of HAp/Fe₃O₄, Fe₃O₄, and PDA/HAp/Fe₃O₄ magnetic composites allowed to investigate the structural properties of these materials, and the results are illustrated in Fig. 1c. For the Fe₃O₄ magnetic sample, peaks at a diffraction angle of 18.28, 30.14, 35.43, 43.1, 53.47, 56.98, and 63.04 2θ (°) were observed, which correspond to the crystal plates of (111), (220), (311), (400), (442), (511) and (440) respectively in pure Fe₃O₄ nanoparticles with inverted spinel structure according to the JCPDS card number 19-0629.⁴¹ Besides, peaks in the diffraction angle range of 2θ equal to 10–80° were observed in the HAp structure, which represents different crystal plates in the HAp structure (ICDD file no. 01-079-8093).⁴² The produced HAp had a hexagonal structure.^43,44 After modification of HAp by Fe₃O₄ and PDA, new vibrations related to Fe₃O₄ and HAp crystalline plates have been successfully observed in the structure of the samples, which shows that the inverted spinel structure of Fe₃O₄ and the hexagonal HAp had a good interaction, and have been successfully incorporated into the composites structure.

The DLS analyses were performed to determine the particle size of PDA/HAp/Fe₃O₄ and the acquired finding are depicted in Fig. 1d. 90.1% and 9.9% of PDA/HAp/Fe₃O₄ magnetic nanocomposite have a particle size of 103.8 nm and 610 nm respectively and the polydispersity index (PDI) was 14.78.

N₂ physical adsorption tests were performed to calculate specific surface area, pore-volume, and pore diameter of Fe₃O₄, HAp/Fe₃O_4, and PDA/HAp/Fe₃O₄ magnetic composites (Fig. 2). N₂ adsorption–desorption isotherms for all the three samples followed the type IV isotherms and had a residual ring at a relative pressure of P/P₀ in the range of 0.5–1, which shows monolayer adsorption⁴⁵ and the produced materials are mesoporous.⁴⁶ As the pore diameter is in the range of 2–50 nm all three types of materials have a mesoporous structure according to the IUPAC standard.⁴⁷ The specific active surface for Fe₃O₄, HAp/Fe₃O_4, and PDA/HAp/Fe₃O₄ magnetic composites was computed to be 72.148 m² g⁻¹, 53.294 m² g⁻¹, and 37.536 m² g⁻¹ respectively. As the results show, specific areas of HAp/Fe₃O₄ and PDA/HAp/Fe₃O₄ are lower than Fe₃O₄ nanoparticles, which can be due to coverage of some pores and cavities in by PDA and Fe₃O₄ nanoparticles. It should also be noted that with the addition of HAp and PDA in the structure, the pore volume of Fe₃O₄ was reduced, which confirms the good placement and interaction between the components.

Fig. 2 BET isotherms and BJH pore-size distribution of Fe₃O₄, HAp/Fe₃O_4, and PDA/HAp/Fe₃O₄ samples.

SEM and Map-EDX methods were used to investigate the morphology and interaction between HAp, Fe₃O_4, and PDA in the composites structure (Fig. S1†). The results showed that HAp synthesized from chicken bones has a hexagonal morphology with a Ca/P w/w ratio of 0.95 (Fig. S1a–c†). After the modification of HAp by Fe₃O₄, different sizes particles (spherical morphology) appeared on the surface of HAp, related to the presence of magnetic particles (Fig. S1d†), as confirmed by Map-EDX (Fig. S1e and f†). After coating the surface of HAp/Fe₃O₄ magnetic composite with PDA, the density and accumulation of particles on the composite surface increased, which could be due to the coating of PDA on HAp/Fe₃O₄ magnetic composite (Fig. S1g†). Map-EDX test was carried out to confirm the presence of PDA on the surface of HAp/Fe₃O₄ magnetic composite, which showed the presence of N and C elements (Fig. S1h and i†). The observation of N and C elements in the composite structure confirms the existence of PDA in the composite structure and, as a consequence, that the PDA/HAp/Fe₃O₄ magnetic composite has been successfully synthesized.

Two-dimensional and three-dimensional analyzes by AFM and TEM techniques were carried out to deepen the study on morphology and structure, of these materials. The AFM images (Fig. S2a†) showed that after the addition of Fe₃O₄ nanoparticles and PDA in the structure of HAp, and the formation of the desired magnetic composites, significant changes occur on the HAp surface. Based on the results, the surface roughness of HAp, HAp/Fe₃O₄, and PDA/HAp/Fe₃O₄ samples was measured as 102.693 nm, 65.877 nm, and 65.589 nm, respectively. The reduction in roughness values indicates that Fe₃O₄ nanoparticles and PDA were successfully incorporated on the HAp surface and the desired magnetic composites have been successfully synthesized.

The TEM investigations showed that the produced HAp from chicken bones has a rod shape (Fig. S2b†). According to the results obtained by SEM and XRD investigations, after the addition of Fe₃O₄ magnetic nanoparticles on the HAp surface, the appearance of new particles with different sizes and morphology was observed, indicating the presence of Fe₃O₄ particles on the HAp structure (Fig. S2c†). In the TEM image of PDA/HAp/Fe₃O₄ magnetic composite, the particle aggregation increased which could be due to the formation of polydopamine around the HAp/Fe₃O₄ particles (Fig. S2d†), which is consistent with the SEM results. Particle aggregation and darkness after polydopamine formation around HAp/Fe₃O₄ particles have been attributed to polydopamine formation around particles in some previous studies.⁴⁸

XPS analyses were performed to investigate the chemical composition, PDA/HAp/Fe₃O₄ magnetic composite in the energy range of 0–1300 eV. The acquired findings of XPS test (Fig. 3) revealed that the surface of the material contains only Ca, O, P, N, Fe, and C, which is consistent with the EDX results. More in detail, the peaks at 132.84, 284.21, 347.21, 531.32, 712.06, and 399.87 eV can be corresponded to the binding energies of P 2p, C 1s, Ca 2p, O 1s, Fe 2p, and N 1s, respectively (Fig. 3a).⁴⁹ Moreover, the high-energy resolution XPS spectrum of Fe 2p was resolved in two single peaks (Fig. 3b), at 710.68 eV and 723.98 eV respectively, characteristic of Fe 2p_3/2 and Fe 2p_1/2, characteristic of Fe³⁺ (oxide).⁵⁰ The high energy resolution spectrum of the N 1s region (Fig. 3c) shows a single peak at 399.87 eV corresponding to the amine groups present in the polydopamine structure, confirming that polydopamine has been successfully incorporated into the magnetic composite structure.⁵¹ The high energy resolution Ca 2p spectrum was fitted with peaks (Fig. 3d) at 346.78 eV and 350.28 eV respectively, characteristic of Ca 2p_3/2 and Ca 2p_1/2 and that can be linked to Ca bonds typical of HAp and to calcium-binding with carbonate due to carbon absorbed from the air. The P 2p spectrum was deconvoluted into peaks at 132.58 eV, 132.78 eV, and 133.48 eV respectively (Fig. 3e), which are justified by the P 2p peaks of phosphate in the HAp structure.⁵² The binding energy in the range 525–545 eV was examined to scan the peaks of O 1s (Fig. 3f). As the results show, the O 1s spectrum has peaked in the binding energy range of 529.48 eV, 530.58 eV, and 532.38 eV, which can be categorized into the hydroxyl oxygen (hydroxyl attached to the metal), oxygen absorbed from the surface (water), and network oxygen, which has been confirmed by the literature.^50,53 The peak of C 1s (284.38 eV) in the PDA/HAp/Fe₃O₄ magnetic composite (Fig. 3g) can be linked to C–C or C=C bonds in the polydopamine structure.⁵⁴

Fig. 3 (a) XPS survey in the range 0–1300 eV and (b–g) the XPS spectra of Fe 2p, N 1s, Ca 2p, P 2p, O 1s, and C 1s of the PDA/HAp/Fe₃O₄ sample.

Statistical analysis of variance (ANOVA)

The response surface methodology was utilized to reduce the number of tests, to investigate the impact of parameters on the response (% adsorption), and to optimize the effective factors in the adsorption process. This method has enabled researchers to obtain acceptable and sufficient statistical results and information with fewer experiments. In this study, the RSM-CCD method was examined for 5 variables in five levels (Table 1). The results of the experimental and predicted data for the variables are given in Table S1.† Based on the experimental data, the efficiency of the Zn²⁺ ion adsorption process in the desired laboratory conditions is in the range of 62.25% to 98.68%. The second-order polynomial equation of the used CCD method to investigate the appropriate response between the removal efficiency and the critical parameters is described in eqn (4).

Zn adsorption = 97.51 − 0.174X₁ + 11.87X₂ − 3.60X₃ − 1.08X₄ + 13.62X₅ − 0.004X₁X₃ + 0.156X₂X₃ − 2.73X₂X₅ + 0.412X₄X₅ + 0.0007X₁² − 0.894X₂² + 0.083X₃² + 0.017X₄² (4)

According to this equation, the effect of interaction between variables affecting the Zn²⁺ ions adsorption. The effect of parameters with unfavorable interaction (P value > 0.05) was not reported. The findings of model coefficients and analysis of variance (ANOVA) for the Zn²⁺ ions adsorption process using the PDA/HAp/Fe₃O₄ magnetic composite are shown in Table S2.† The importance of each variable on the uptake efficiency is determined based on the values of F and P, and the larger and smaller the values of F and P, respectively, the more important is the effect of the variable on the efficiency of the adsorption process is. Based on the results, the values of F and P for the model were determined as 138.61 and P < 0.0001, respectively, showing that the model has very high accuracy and has a confidence level of more than 95% for experimental data.⁵⁵ Also, the ANOVA results showed that the interaction between the variables of ultrasonic power/Zn²⁺ concentration, pH/Zn²⁺ concentration, pH/adsorbent dose, and ultrasonic time/adsorbent dose has a vital role in the Zn²⁺ ions removal (according to the values of F and P). In addition to the values of F and P, to assess the capability of the second-order polynomial model, R² and Adj-R² were used, which were determined as 0.9826 and 0.9755 for the model, respectively. Since the value of parameter R² is close to 1, the model has a high ability to describe the adsorption of Zn²⁺ (%) as a function of independent variables,⁵⁶ and all variables are effective and important in examining the quadratic polynomial model for the Zn²⁺ ions adsorption.⁵⁷ The lack-of-fit value for the Zn²⁺ ions adsorption was determined to be greater than 0.05 using the PDA/HAp/Fe₃O₄ magnetic composite (P-value = 0.4258), which indicates the desirability and good predictability of the model. Based on the lack-of-fit value, the second-order formula has a enough potential to model the interaction between variables.⁵⁸ In addition, the low value of CV shows the reliability of the prediction model and shows that the predicted values are slightly different from the experimental data.⁵⁹

In the present study, two-dimensional (2D) contour design, three-dimensional (3D) surfaces, and the effect of each variable were coded separately to assess the behavior of variables on the Zn²⁺ ions elimination using the PDA/HAp/Fe₃O₄ magnetic composite (Fig. 4). Based on eqn (4), first-order responses (X₁, X₂, X₃, X₄, and X₅) and two-factor responses (X₁X₃, X₂X₃, X₂X₅, X₄X₅) have positive effects on the adsorption efficiency of Zn²⁺ ions, while other two-factor responses have negative effects on the decontamination process. Therefore, showing the effect of two-factor responses that had negative effects on the adsorption process has been omitted, and 3D levels are shown only for two-factor responses that have had positive effects.

Fig. 4 2D contour design, 3D surfaces, and the effect of one-dimensional variables on the removal efficiency of Zn²⁺ using PDA/HAp/Fe₃O₄ magnetic composite.

Ultrasonic power was tested in the conditions of 100–300 W and 35 kHz and the results showed that with increasing ultrasonic power, process efficiency increased. Increasing the zinc decontamination by increasing the ultrasonic power can be justified by the collision of Zn²⁺ ions with the composite surface, creating active sites on the adsorbent surface and strengthening the bond between the adsorbate and the solid surface. Same findings have been reported in previous studies, and it has been shown that with increasing ultrasonic power, process efficiency has increased.^60,61

Based on the results given in Fig. 4, with increasing the contact time, the zinc adsorption increased. This can be justified by sufficient time to absorb zinc.⁶²

By determining an optimal pH, it is possible to minimize the hydrolysis of the metal ion or cause the formation of stable complexes of the metal ion with the adsorbent surface.⁶³ Investigation of the pH effect on the Zn²⁺ ions adsorption showed that by increasing pH in the range of 2 to 6, the process efficiency growth, which could be due to the reduction of competition between H₃O⁺ ion and zinc ions in water to occupy active sites. Attaining the low decontamination at acidic pH can also be due to the protonation of functional groups at the adsorbent surface, mainly amine and hydroxyl groups, thus preventing the adsorption of Zn²⁺ ions due to the repulsive electrostatic forces, and finally reducing the uptake efficiency.⁶⁴

As the results show, by increasing the Zn²⁺ ions, the decontamination, and the performance of the PDA/HAp/Fe₃O₄ magnetic composite decreased. The high adsorption efficiency at the low initial concentrations of Zn²⁺ ions can be linked to the accessibility of surface area for the metal ions, while at high concentrations the adsorption efficiency reduces because the active sites are saturated rapidly causing a repulsive electrostatic force between the located metal ions on the composite surface and water.

As the 2D contour design and 3D surfaces have shown, by increasing the composite mass, the removal Zn²⁺ ions from the water is increased, which can be corresponded to the provision of sufficient levels of unsaturated sites to adsorb Zn²⁺ ions. Therefore, according to the obtained findings, the maximum decontamination was obtained at the adsorbate content of 10 mg L⁻¹ and the composite mass of 1.2 g L⁻¹.

Kinetic studies

The kinetic models of pseudo-first-order (PFO), pseudo-second-order (PSO), Elovich, and intraparticle diffusion have been used (eqn (5)–(7)).

PEO:

q_t = q_e(1 − e^−k₁t) (5)

PSO:Elovich:Intra diffusion particle:where, q_t and q_e are the adsorption capacity of Zn²⁺ per gram of dry adsorbent (mg g⁻¹), k₁ constant adsorption rate (min⁻¹), k₂ constant quasi-quadratic kinetic model rate (g mg⁻¹ min⁻¹), α is the initial absorbance (mg g⁻¹ min⁻¹), β is the desorption constant (g mg⁻¹), I is corresponded to the effects of the boundary layer, and k_i is the constant rate of intraparticle diffusion.

Fig. S3† shows the nonlinear relationship of kinetic models for the Zn²⁺ elimination using PDA/HAp/Fe₃O₄, HAp/Fe₃O₄, and HAp. The constants and variables obtained from the kinetic studies are listed in Table 2. The experimental data of the Zn²⁺ ions removal using the designed adsorbent follows the PSO kinetic model due to the higher correlation coefficient. In addition, the determined value of q_e.cal for the PSO is higher than the calculated value of q_e.cal using the PFO model and has little difference with q_e.exp. The experimental data follows PSO, and this shows that chemical reactions are also effective in the Zn²⁺ ions adsorption.⁶⁵ The value of the determined α parameter showed that the magnetic composite PDA/HAp/Fe₃O₄ has a greater tendency to adsorb Zn²⁺ ions compared to other adsorbents.

Table 2 Parameters of kinetic models for the Zn²⁺ ions adsorption

Kinetic model	Adsorbent
	HAp	HAp/Fe3O4	HAp/Fe3O4/PDA
Pseudo-first order
qe.cal	7.835	7.983	8.113
KP1st	0.1247	0.218	0.2441
R^2	0.9074	0.9411	0.9104
RMSE	0.4195	0.2203	0.2442
Pseudo-second order
qe.cal	8.906	8.646	8.71
KP2st	0.01964	0.04389	0.05168
qe.exp	7.947	8.151	8.285
R^2	0.954	0.9905	0.9914
RMSE	0.2958	0.0884	0.0757
Elovich equation
α (mg g^−1 min)	5.416	84.33	218
β (g mg^−1)	0.6331	0.9997	1.11
R^2	0.9308	0.8877	0.8933
RMSE	0.3626	0.3043	0.2677

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To further investigate the kinetic behavior of zinc ion adsorption by the studied adsorbents, the intraparticle diffusion model was used. This model has the ability to determine the effective mechanisms and controlling stages in the adsorption process. Fig. S4† shows the linear regression of the intraparticle diffusion model for the Zn²⁺ adsorption process using adsorbents produced from hydroxyapatite. In the intraparticle diffusion model, if the value of I = 0 (intercept), the adsorption process is mainly controlled by the intraparticle diffusion mechanism. If I ≠ 0, the internal and external intrusion of particles dominates the whole process.⁶⁶ As the results show, for the adsorption of Zn²⁺ using the adsorbents, the value of ‘I’ was determined ≠0, which indicates that in the Zn²⁺ adsorption, the internal and external diffusion of particles are effective. The adsorption process of Zn²⁺ using the desired adsorbents from the aqueous solution is divided into two stages, the first stage is faster than the second one (see Fig. S4†). The first stage of the adsorption process is due to the adsorption of Zn²⁺ on the adsorbent surface and the second stage is a gradual adsorption step due to the adsorption of Zn²⁺ ions in the adsorbent layers.¹⁹ According to the data in Table 3, the value of I for Zn²⁺ adsorption using PDA/HAp/Fe₃O₄ is higher compared to other adsorbents, which indicates that the limiting phase for Zn²⁺ adsorption using PDA/HAp/Fe₃O₄ has a greater contribution.⁶⁷

Table 3 Constants and variables determined for the intraparticle diffusion model

Intraparticle diffusion	Adsorbent
	HAp	HAp/Fe3O4	HAp/Fe3O4/PDA
Ki,1 (mg g^−1 min^−1/2)	1.1262	0.8264	0.745
I1 (mg g^−1)	1.8558	3.9887	4.5379
R^2	0.9847	0.9388	0.9551
Ki,2 (mg g^−1 min^−1/2)	0.1013	0.0514	0.0386
I2 (mg g^−1)	7.1991	7.7661	7.9992
R^2	0.8311	0.8324	0.84

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Isotherm studies

In the current study, the nature of Zn²⁺ metal ions adsorption using PDA/HAp/Fe₃O₄ magnetic composite, HAp/Fe₃O₄ magnetic composite, and HAp employing Langmuir, Freundlich, and Dubinin–Radeskovich (DR) isotherm models were evaluated.⁶⁸ The desired figures and the determined parameters using the isotherm models are illustrated in Fig. S5† and Table 4, respectively. The experimental data of the Zn²⁺ adsorption process using magnetic composite (PDA/HAp/Fe₃O₄ and HAp/Fe₃O₄) and HAp follow the Freundlich and Langmuir isotherm, respectively. The maximum adsorption capacity (q_m) using PDA/HAp/Fe₃O₄ magnetic composite, HAp/Fe₃O₄ magnetic composite, and HAp was determined as 46.37 mg g⁻¹, 40.07 mg g⁻¹, and 37.57 mg g⁻¹, respectively. The amendment of HAp using Fe₃O₄ nanoparticles and PDA has increased the adsorption capacity, which is in line with the experimental findings. In addition, according to Table S3,† the adsorption capacity of the adsorbents in this study has a good rank compared to the adsorbents studied for the removal of zinc metal. The amount of adsorption energy (K_L) for the Zn²⁺ ions adsorption process using PDA/HAp/Fe₃O₄ magnetic composite, HAp/Fe₃O₄ magnetic composite, and HAp was determined as 0.3687 L mg⁻¹, 0.5537 L mg⁻¹, and 0.5746 L mg⁻¹, respectively. The PDA/HAp/Fe₃O₄ magnetic composite has higher adsorption energy compared to others. Also, the zinc adsorption was desirable as the R_L values were calculated in the range of 0–1. The value of the n parameter determined by the Freundlich model for the Zn²⁺ ions adsorption process by all adsorbents was computed >1, thus, the zinc elimination process is physical. Also, the K_f value for the adsorption process using HAp/Fe₃O₄/PDA magnetic composite is higher compared to other adsorbents, which indicates that the adsorption bond created between Zn²⁺ ions and the surface of PDA/HAp/Fe₃O₄ magnetic composite powder is higher and stronger compared to others.⁶⁹ The mean value of average energy (E) for the decontamination process using all the composites/adsorbents was determined less than 8 kJ mol⁻¹, which re-affirms the physical nature of the zinc removal,⁷⁰ as well as the A_T and b_T parameters showing that the interactions between the composites/adsorbent surface and Zn²⁺ are weak and, as a consequence, that the adsorption process may be physical.⁷¹

Table 4 Variables of isotherms for Zn²⁺ ions adsorption process using the produced adsorbents

Models	Parameters	Adsorbent
		HAp	HAp/Fe3O4	HAp/Fe3O4/PDA
Langmuir	qm (mg g^−1)	37.57	40.07	46.37
	KL (L mg^−1)	0.3687	0.5537	0.5746
	R^2	0.9583	0.9176	0.8222
	RL	0.026–0.213	0.017–0.152	0.018–0.157
	RMSE	2.29	3.677	6.531
Freundlich	N	3.534	3.688	3.48
	Kf (mg g^−1 (L mg^−1)^−1/n)	13.21	15.85	18.15
	R^2	0.9509	0.9866	0.9928
	RMSE	2.485	1.482	1.314
Dubinin–Radushkevich (D–R)	E (kJ mol^−1)	0.995	1.675	1.758
	qm (mg g^−1)	31.28	32.57	36.79
	β × 10^−7 (mol^2 J^−2)	5.045	1.782	1.618
	R^2	0.8102	0.7029	0.5883
	RMSE	4.885	6.98	9.939
Temkin	bT (kJ mol^−1)	0.36	0.372	0.382
	AT (L g^−1)	5.266	11.43	27.46
	R^2	0.9831	0.9881	0.9162
	RMSE	1.458	1.397	4.484

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Effect of foreign ions and reusability of the adsorbent

Industrial wastewater usually has different anions and cations due to the passage of different leachate fields and agricultural lands, which can affect the ability of adsorbents in the decontamination process.⁷² Sodium (Na⁺), calcium (Ca²⁺), and magnesium (Mg²⁺) ions are the common cations existing in water and wastewater,⁷³ therefore in the present study, the impact of these ions on the Zn adsorption was investigated in the concentration of 0.05–0.5 M (Fig. S6a–c†). Based on the results, by increasing foreign ions molarity, the Zn²⁺ adsorption decreased for all three types of adsorbents. This decrease may be due to the competition of Na⁺, Ca²⁺, and Mg²⁺ with Zn²⁺ for the accessible sites on the adsorbent. As a result, the zinc decontamination process has decreased.

One of the influential factors in reducing the operative costs of the adsorption method is the adsorbent synthesis costs. Reuse of adsorbents, in addition to reducing costs, can also save on the use of primary sources for the generation of adsorbents. For this purpose, the adsorption/desorption process of Zn²⁺ ion was investigated employing synthesized HAp based adsorbents up to 10 cycles (Fig. S6b†). After each cycle, the adsorbent was properly regenerated, as described in Section 2.4. The acquired findings revealed that with elevating the number of adsorption/desorption steps, the ability of adsorbents to remove Zn²⁺ ions decreased. Decreasing the adsorption efficiency by increasing the stages of adsorption/desorption can be linked to various factors like damage and/or modification of the structure, as well as adsorbent sites saturation.⁷⁴

Mechanism of Zn²⁺ adsorption process

To describe the adsorption of metal ions from aqueous solution using hydroxyapatite adsorbents (HAP) various possible mechanisms such as ion exchange, decomposition–deposition, and formation of surface complexes have been suggested. Recently, the decomposition–deposition mechanism has received much attention from researchers. The possible mechanism of this process is described as follows:

Dissolution:

Ca₁₀(PO₄)₆(OH)₂ + 14H⁺ → 10Ca²⁺ + 6H₂PO₄⁻ + 2H₂O (9)

Precipitation:

10M²⁺ + 6H₂PO₄⁻ + 2H₂O → 14H⁺ + M₁₀(PO₄)₆(OH)₂ (10)

It is assumed that the mechanism of Zn²⁺ adsorption using PDA/HAp/Fe₃O₄ nanocomposite has three steps: (1) diffusion of metal ions (M²⁺) from the aqueous solution to the surface of the calcium hydroxyapatite matrix; (2) adsorption of Zn²⁺ ions to the adsorptive sites; and (3) replacement of Zn²⁺ with Ca²⁺ in hydroxyapatite.²⁶

In addition to hydroxyapatite, Fe₃O₄ nanoparticles can also be effective in the adsorption of Zn²⁺, the process schematic of which is shown in Fig. 5. In aqueous systems, the surface of iron oxide is covered with FeOH groups. The hydroxyl group present on the surface of iron nanoparticles can have negative (FeO⁻) and positive (FeOH₂⁺) charges at pHs above or below pH_ZPC, respectively. Iron nanoparticles contain Fe²⁺ which can be hydrolyzed at different pHs to form FeOH⁺, and Fe(OH)₃⁻. Thus, an electrostatic absorption force is created between the ionic species of the metal Zn²⁺ and the surface of the iron nanoparticles. In previous studies, the pH_ZPC of Fe₃O₄ nanoparticles has been reported to be 7.4, which is almost neutral.²⁶ According to Fig. 5, with increasing the initial pH, the adsorption efficiency of Zn²⁺ using PDA/HAp/Fe₃O₄ increased. It shows the driving force increased between the adsorbent surface and the Zn²⁺ ion. As the pH increases, the surface charges of FeOH₂⁺ in the composite are converted to FeO⁻ and the electrostatic attraction between Zn²⁺ and FeO⁻ increases (Fig. 5).

Fig. 5 Schematic of Zn²⁺ (M²⁺) adsorption process using Fe₃O₄ nanoparticles in PDA/HAp/Fe₃O₄ nanocomposite.

PDA/HAp/Fe₃O₄ also contains polydopamine, which can affect the adsorption process. In the structure of polydopamine, there are –OH and NH groups that can be converted to different states at different pHs. The different forms of the –OH group in the polydopamine are similar to the functional group in the iron oxide nanoparticles, which affects the adsorption of Zn²⁺. In general, it can be mentioned that the process of adsorption of metal ions with PDA is happened through chelation and coordination.⁷⁵ The interaction between PDA and Zn²⁺ ions is depicted in Fig. 6.

Fig. 6 Schematic of Zn²⁺ adsorption using PDA in PDA/HAp/Fe₃O₄ nanocomposite structure.

Conclusions

Modification of raw minerals can increase their adsorption capacity and performance in the adsorption process. Herein, HAp generated from chicken bones was modified using Fe₃O₄ nanoparticles and PDA and used as a promising composite to decontaminate Zn²⁺ from a liquid media. The surface and structural characteristics of the composites were explored by various techniques, which showed that Fe₃O₄ nanoparticles and PDA have been successfully placed in the HAp structure and have good interaction with each other. The magnetic saturation value for Fe₃O₄, HAp/Fe₃O₄ magnetic composite, and PDA/HAp/Fe₃O₄ magnetic composite was 84.283 emu g⁻¹, 30.305 emu g⁻¹, and 24.014 emu g⁻¹, respectively, which depicted that the composites are easily separated from the aqueous solution and can be reused. RSM-CCD was used to study the effect of operative parameters on the removal efficiency and also the interaction between them. The results of ANOVA revealed that the model has a good potential to evaluate the interaction between the parameters on the decontamination efficiency of Zn²⁺ ions. The results showed that the equilibrium and kinetic behavior follow the Freundlich and pseudo-second-order models, respectively. The n and E factors showed that the zinc adsorption is physical and desirable. Adsorption/desorption capacity of PDA/HAp/Fe₃O₄ magnetic composite showed that by elevating the adsorption/desorption stages, the ability and performance of the adsorbent decreases. Therefore, based on the surface and structural properties, the performance and adsorption ability of Zn²⁺ ions, PDA/HAp/Fe₃O₄ magnetic composite can be suggested as a promising composite for the zinc ions removal from water matrixes.

Author contributions

R. Foroutan: formal analysis, methodology. S. J. Peighambardoust: funding acquisition, methodology. S. Hemmati: methodology, conceptualization, A. Ahmadi: writing – original draft. E. Falletta: writing – review& editing, methodology. B. Ramavandi: supervision, conceptualization, writing – review& editing. C. L. Bianchi: supervision, conceptualization.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank to the Bushehr University of Medical Sciences, Iran for technical supports to conduct this work. This research was undertaken, in part, thanks to funding from Velux Stiftung Foundation through the project 1381 SUNFLOAT–Water decontamination by sunlight-driven floating photocatalytic systems.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra04583k

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