English

• 0.   Introduction

  With the rapid development of industry, a large amount of heavy metals have been discharged into the environment, causing damage to the ecosystem, agricultural activities, and human health through the food chain^[1-9]. What should be especially focused on is that industrial wastewater can enter rivers and lakes, resulting in a heavy metal pollution rate as high as 80%. It was estimated that the overall heavy metal pollutants in China about ten years ago amounted to 34 000 tons^[10]; and in particular, the most serious Cd pollution emerged in the Yangtze River basin, while the Cd content in the Yellow River basin was about 17% higher than the relevant standard. Being a branch of the Yellow River, the Weihe River was reported to contain very high amounts of Pb and Cd (surpassing the corresponding standard by 100% and 86%), and the urban rivers were said to have Pb and Cd with contents of 25% and 18% above the standards^[11].

  The issue of heavy metal pollution has been highly focused on, and it is urgent to perform the remediation of heavy metal‐polluted water bodies^[12-17]. Now, chemical, physical, and biological methods are available for remediating heavy metal‐contaminated soil and water. Among them, the chemical method is significant due to its rapid remedying velocity and high amending efficiency^[18-22]. It is well known that the key to chemical remediation lies in the amendment. Of various amendments, nanomaterials are highly interesting in terms of the remediation of heavy metal‐contaminated waters due to their small particle size, large amount of surface active sites, and high specific surface area, which account for their excellent adsorption capability^[23-36]. For example, Xu et al. found that nanoscale zero‐valent iron (ZVI, 0.08 g·L^-1), a novel metallic iron with good stability, was able to quickly remove Cr(Ⅵ) with a mass concentration of 34 mg·L^-1. When the level of ZVI increased from 0.04 to 0.12 g·L^-1, the reduction rate of Cr(Ⅵ) rose from 24% to 90%^[37]. Ding et al. found that sulphite‐activated nanoscale MnFe₂O₄ (MnFe₂O₄/S(Ⅳ)) could oxidize As(Ⅲ) in neutral solution to As􀃰, during which the oxidation process was dominant over the adsorption one; and the adsorption capacity of MnFe₂O₄/S􀃯 towards As(Ⅲ) was 26.257 mg·g^{-1 [38]}. Liu et al. reported that nanoscale FePO₄ with a small size of 8.4 nm was capable of reducing the elution rate of Cu from soil: the Cu mass concentration decreased from 1.74‐13.33 mg·L^-1 to 0.23‐2.55 mg·L^-1 after 56‐day of remediation while the bioavailability of Cu dropped by 54%‐69%^[39]. In the review on the adsorption of As, Cr, Pb, and Hg by various iron‐based minerals, Waychunas et al. claimed that the target heavy metals' adsorption behavior was dominated by surface complexing, while the adsorption ability of the adsorbents was closely related to their surface molecular structure^[40].

  Nevertheless, common nanomaterials such as nano‐silica, nano‐ZVI, nano‐iron oxides, and nano‐titania are only suitable for the immobilization of one or several heavy metals but can hardly achieve the efficient remediation of multi‐heavy metal‐contaminated systems^[41-42]. We previously fabricated hydroxyapatite (HAP) nanostructures with different morphologies and found that the porous HAP nanoparticles (NPs) could rapidly and efficiently immobilize Pb²⁺, Cd²⁺, Cu²⁺, Co²⁺, Ni²⁺, Zn²⁺, and Hg²⁺ in solution, while the as‑ generated complexing compounds exhibited good stability^[21]. More importantly, HAP exhibits excellent environmental acceptance and degradability as well as high adsorption capacity^[43-44], making it promising in treating heavy metal‐contaminated soil and wastewater. A drawback of HAP lies in that it only exhibits fair adsorption ability to heavy metal ions, which corresponds to its less efficient removal capability for target heavy metals from the multi‐contaminated systems. Therefore, in this study we introduced —SH and —COOH dual‐function chelating groups onto the surface of spherical porous HAP as the carrier to reduce the steric hindrance by making use of alternate long‐short chain configuration, thereby achieving the rapid and highly efficient adsorption of Cd²⁺, Cu²⁺, Co²⁺, Ni²⁺, Zn²⁺, and Hg²⁺ from complex‐contaminated systems. This article reports the preparation and characterization of the target nano‐amendment as well as the evaluation of its adsorption performance towards coexistent heavy metal ions Cd²⁺, Cu²⁺, Co²⁺, Ni²⁺, Zn²⁺, and Hg²⁺ in the multi‐contaminated soil and water.

  1.   Experimental

  1.1   Reagents

  Phosphorus pentoxide (98%) and calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O, 99%) were purchased from Tianjin Kermel Chemical Reagent Company Limited. Absolute ethanol (99.7%) was bought from Anhui Ante Food Company Limited. Ammonia (25%‐28%) and toluene (99.5%) were provided by Luoyang Haohua Chemical Reagent Company Limited. Iminodiacetic acid (IDA, 98%), 3‐glycidyloxypropyltrimethoxy silane (KH‐560, 97%), and 3‐mercaptopropyl trimethoxysilane (MPS, 95%) were obtained from Aladdin Holdings Group Company Limited, and sodium hydroxide (96%) was supplied by Xilong Chemical Company Limited. The standard solution of Cd²⁺ (1 000 mg·L^-1) was formulated by the National Nonferrous Metals and Electronic Materials Analysis and Testing Center, and that of Co²⁺ (1 000 mg·L^-1) was provided by the National Steel Material Testing Center‐Central Iron & Steel Research Institute. The standard solutions of Hg²⁺ (1 000 mg·L^-1), Cu²⁺ (1 000 mg·L^-1), Zn²⁺ (1 000 mg·L^-1), and Ni²⁺ (1 000 mg·L^-1) were all commercially obtained from Beijing Haibin Hongmeng Standard Material Technology Company Limited. All the reagents are of analytical purity.

  1.2   Preparation of HAP NPs

  0.04 g of P₂O₅ was fully dissolved in 15 mL of absolute ethanol, while 0.24 g of Ca(NO₃)₂·4H₂O was fully mixed with 15 mL of distilled water. The P₂O₅ solution was added drop by drop into the Ca(NO₃)₂ solution with a constant pressure funnel. Upon completion of dropping, the pH of the reaction solution was adjusted to 10 with ammonia, followed by stirring at room temperature for 30 min. At the end of stirring, the reaction system was transferred to a 50 mL high‐pressure Teflon‐lined autoclave, heated to 165 ℃, and held for 12 h. After the reaction was accomplished, the autoclave was cooled to room temperature naturally, and the reaction solution was centrifuged (8 000 r·min^-1, 5 min). The resultant precipitate was washed with absolute ethanol and distilled water, each for three times, followed by drying at 60 ℃ to obtain HAP NPs.

  1.3   In situ surface modification of HAP NPs by IDA and MPS

  1.70 g of IDA, 20 mL of distilled water, and 0.50 g of NaOH were placed into a 50 mL beaker and ultrasonically dispersed, while NaOH solution (6 mol·L^-1) was used to adjust the pH to 11. The resultant mixed solution was transferred to a 100 mL three‐necked flask and stirred with an ice bath until the temperature reached 0 ℃. At this point, 1 mL of KH‐560 was dropped into the mixed solution and held for 4 h. Upon completion of the reaction, the reaction system was heated to 65 ℃, followed by condensation refluxing for 12 h to yield a KH‐560‐IDA solution. While the pH of 15 mL of the KH 560 IDA solution was adjusted to 2 with HCl solution (6 mol·L^-1), 0.50 g of HAP NPs was added. The as‐obtained mixed solution was evenly dispersed under ultrasonication, heated to 90 ℃, and continuously condensed and refluxed for 10 h. At the end of the reaction, the mixed solution was cooled in air and washed with absolute ethanol and distilled water, each for three times, and dried at 60 ℃ to produce HAP‐IDA NPs. 0.10 g of HAP‐IDA NPs and 30 mL of toluene were placed into a 50 mL three‐necked flask and ultrasonicated for 5 min. 0.20 mL of MPS was added to the resultant dispersion under magnetic stirring at room temperature for 2 h. Then, the dispersion was heated to 92‐95 ℃ and condensated‐refluxed for 12 h. Upon completion of the reaction, the mixed dispersion was cooled in air and sequentially washed with absolute ethanol and distilled water for a total of six times and dried at 60 ℃ to yield HAP‐IDA/MPS nano‐amendment.

  1.4   Evaluation of adsorption performance of HAP‐IDA/MPS

  0.10 g of HAP‐IDA/MPS nano‐amendment was added into a 50 mL centrifugal tube while 30 mL of the solution containing Cu²⁺, Cd²⁺, Co²⁺, Zn²⁺, Ni²⁺, and Hg²⁺ (Initial mass concentration for each kind of ions was 20 mg·L^-1; pH=7) was introduced. And a total of six of the same solution was formulated. The resulting mixed solutions were oscillated (180 r·min^-1) at 25 ℃ for 5, 10, 15, 20, 40, 60, 80, 100, and 120 min, respectively. At the end of the adsorption experiments, the mixed solutions were centrifuged (8 000 r·min^-1, 5 min), and the supernatants were analyzed by inductively coupled plasma atomic emission spectrometry (ICP‐AES, Optima 2100DV, USA) to determine the mass concentrations of the tested heavy metal ions. The mass of HAP was set at 0.050 and 0.10 g, and the shaking time was set at 1 h. The mass of HAP‐IDA/MPS and oscillation time were set as 0.10 g and 1 h to compare the adsorption capacity of HAP and HAP‐IDA/MPS. Similarly, the adsorption experiments for different mass concentrations (10, 20, 30, 40, 50, and 60 mg·L^-1) of Cu²⁺, Cd²⁺, Co²⁺, Zn²⁺, Ni²⁺, and Hg²⁺ at different pH values (3, 4, 5, 6, 7, 8, and 9) and different temperatures (20, 25, 30, 35, and 40 ℃) as well as the selective reactivity of HAP‐IDA/MPS nano‐amendment (0.10 g) towards heavy metals and its reusability were tested in the same modes.

  After adsorbing heavy metal ions, HAP‐IDA/MPS was filtered with a disposable filter and dried. The filtered and dried residue was placed into 30 mL of 1.0 mol·L^-1 HCl solution and shaken on a shaker at room temperature for 1 h to desorb the heavy metal ions from the surface of HAP‐IDA/MPS. Upon completion of shaking, the solution was centrifuged to collect HAP‐IDA/MPS for the next cycle of the adsorption experiment, which was conducted up to a total of six cycles.

  1.5   Instruments

  The as‐synthesized HAP‐IDA/MPS nano‐amendment was characterized with a Fourier transform infrared spectrometer (FTIR, VERTEX 70, Germany), a transmission electron microscope (TEM, JEOL‐2100, Japan; LaB6 filament, maximum acceleration voltage: 200 kV; point resolution: ≤0.23 nm, line resolution: ≤0.14 nm), a scanning electron microscope (SEM, JSM‐5600, Japan), and an X‐ray powder diffractometer (XRD, X′Pert Philips, Netherlands; 40 kV; 40 mA; Cu Kα target, λ=0.154 18 nm; 2θ=20°‐60°) in the presence of HAP and HAP‐IDA as the controls. Its thermal stability was evaluated with a thermogravimetric analyzer (TG, TGA/DSC 3⁺, Swiss). The mass concentrations of the tested heavy metal ions were determined by ICP‐AES. A high‐precision digital shaker (ATS‐03M2R, China) was used to conduct adsorption experiments.

  2.   Results and discussion

  2.1   Structure characterization

  Fig. 1 shows the SEM image and particle size distribution of HAP, as well as the SEM and TEM images of HAP‑IDA/MPS. The as‐prepared HAP appeared as irregularly shaped particles with relatively rough surfaces and good dispersibility (Fig. 1a, 1b), and its particle size was about 75 nm (Fig. 1f). After dual surface‐modification, the morphology of HAP‐IDA/MPS remained nearly unchanged (Fig. 1d), i.e., it still emerged as irregularly shaped particles. Corresponding TEM image demonstrated that HAP‐IDA/MPS had a porous bulk structure (Fig. 1e). Besides, the EDS (energy dispersive X‐ray spectrum) mappings showed that the N content (mass fraction) in HAP‐IDA was 0.90% (Fig.S1c and S1d, Supporting information), higher than the N content of HAP (0.22%, Fig.S1a and S1b); and N and S elements were of even distribution on the surface of HAP‐IDA/MPS (Fig.S1e and 1f), which proves the successful surface‐capping of HAP by IDA and MPS.

  Figure 1

  

  Figure 1.  SEM images of HAP (a, b), HAP‐IDA (c), and HAP‐IDA/MPS (d); TEM image of HAP‐IDA/MPS (e); Particle size distribution of HAP (f)

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  The FTIR spectra of HAP, HAP‐IDA, and HAP‐IDA/MPS are presented in Fig. 2. Both HAP and HAP‐IDA/MPS exhibited the strong characteristic absorption bands of HAP at 3 568, 1 740, 1 094, and 570 cm^{-1 [8-9]}, which proves that the dual surface‐modification by IDA and MPS had nearly no influence on the chemical feature of HAP, and HAP‐IDA/MPS still contained HAP as the major component. Besides, the absorption bands at 2 926 and 2 864 cm^-1 are assigned to the stretching vibrations of the fragments —CH₃ and —CH₂ in IDA^[43]; and those absorption bands in the wavenumber range of 3 000‐3 500 cm^-1 are ascribed to the stretching vibrations of O—H^[45]. Compared with HAP, HAP‐IDA exhibited stronger absorption signals at 3 000‐3 500 cm^-1 and 1 740 cm^-1, which indicates that IDA was successfully grafted onto the surface of HAP (The absorption band of —NH₂ in IDA was in the range of 3 000‐3 500 cm^-1, 1 740 cm^-1 belongs to the stretching vibration peak of —COOH). Furthermore, HAP‐IDA/MPS showed the stretching vibration of the group —SH at 2 550 cm^{-1 [16]}. These FTIR data give evidence of the successful synthesis of HAP‐IDA/MPS nanocomposite.

  Figure 2

  

  Figure 2.  FTIR spectra of HAP, HAP‐IDA, and HAP‐IDA/MPS

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  In the presence of HAP‐IDA/MPS as an example, XRD analysis was conducted to investigate whether dual surface modification affected the crystal structure of HAP. As shown in Fig. 3, HAP‐IDA/MPS nanostructure exhibited characteristic diffraction peaks at 25.8°, 31.7°, 32.2°, 39.7°, 46.7°, and 49.3° which correspond to the (002), (211), (112), (310), (222), and (213) facets of HAP (PDF No.46‐0905). This proves that the dual surface modification has nearly no influence on the crystal structure of HAP. Meanwhile, the sharp XRD peaks demonstrated that HAP‐IDA/MPS nanostructure had high crystallinity, and the absence of other characteristic diffraction signals showed that the synthesized nanostructure was highly pure.

  Figure 3

  

  Figure 3.  XRD pattern of HAP‐IDA/MPS

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  2.2   Thermal stability and adsorption ability

  TG analysis of HAP, HAP‐IDA, and HAP‐IDA/MPS was conducted under N₂ atmosphere, with the results shown in Fig. 4. HAP exhibited a 5% of weight loss after 300 ℃. For HAP‐IDA and HAP‐IDA/MPS, a sharp weight decrease was observed between room temperature and 300 ℃, attributed to moisture evaporation, and above 300 ℃, the decomposition of introduced organic components (IDA and MPS) commenced. Within the temperature range of 300‐1 000 ℃, HAP‐IDA showed a weight loss of 15%, while HAP‐IDA/MPS demonstrated a higher weight loss of 18%. The greater weight loss observed in HAP‐IDA/MPS compared to HAP‐IDA further confirms the successful grafting of MPS onto the HAP‐IDA surface.

  Figure 4

  

  Figure 4.  TG curves of HAP, HAP‐IDA, and HAP‐IDA/MPS

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  Fig. 5 shows the immobilization rates of Cu²⁺, Zn²⁺, Ni²⁺, Co²⁺, Cd²⁺, and Hg²⁺ ions versus adsorption time. It is seen that all the tested heavy metal ions were quickly adsorbed by HAP‐IDA/MPS. The adsorption equilibrium was achieved in 20 min, and the immobilization rates for Hg²⁺, Cu²⁺, Zn²⁺, Ni²⁺, Co²⁺, and Cd²⁺ ions were recorded as 98.1%, 59.7%, 25.1%, 13.9%, 12.7%, and 7.7%, respectively. Therefore, it can be concluded that HAP‐IDA/MPS preferentially adsorb Hg²⁺, followed by Cu²⁺, Zn²⁺, Ni²⁺, Co²⁺, and Cd²⁺. To ensure the complete adsorption of the adsorbent, we carried out the adsorption experiments with 0.1 g of the synthesized adsorbent at a temperature of 25 ℃, a solution pH of 7, and an adsorption time of 1 h.

  Figure 5

  

  Figure 5.  Relationship between the immobilization rates of heavy metal ions by HAP‐IDA/MPS and adsorption 16:54:02

  Conditions: 0.1 g of the synthesized adsorbent, 20 mg·L^-1 of heavy metals, 25 ℃, pH=7.

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  To investigate the effect of the initial mass concentration of Cu²⁺, Zn²⁺, Ni²⁺, Co²⁺, Cd²⁺, and Hg²⁺ on their immobilization rate by HAP‐IDA/MPS, we conducted a series of adsorption experiments. As shown in Fig. 6, their adsorption within the initial mass concentration range of 10‐60 mg·L^-1 followed the order of Hg²⁺, Cu²⁺, Zn²⁺, Ni²⁺, Co²⁺, and Cd²⁺, which is basically in agreement with that shown in Fig. 5. At the same time, the immobilization rate for Hg²⁺ remained around 100% when its initial mass concentration was in the range of 10‐30 mg·L^-1, which further indicates that HAP‐IDA/MPS preferentially adsorbs Hg²⁺ in the presence of the coexistent heavy metal ions. The reason lies in the fact that different heavy metal ions react with —SH (from MPS) or —COOH (from IDA) to form compounds with different solubility products. Namely, the solubility product constant for HgS, CuS, CdS, ZnS, NiS, and CoS is in the order of 4.0×10^-53, 6.3×10^-36, 7.9×10^-27, 2.9×10^-25, 1.4×10^-24, and 4.0×10^-21, respectively, which sequentially corresponds to their immobilization rate dependent on the competitive adsorption of relevant sulfides (except for CdS). Moreover, the solubility product constant for the compounds of the heavy metal with —COOH is unavailable; it would be infeasible to discuss the difference in the immobilization rates of various heavy metals from this perspective. Whatever, the exception for CdS could be related to the joint combinations of —SH and —COOH with the heavy metal ions. In other words, the —COOH group of HAP‐IDA/MPS can chemically react with Cd²⁺ to produce a compound with a relatively high solubility product constant.

  Figure 6

  

  Figure 6.  Relationship between immobilization rates of heavy metal ions and their initial mass concentrations

  Conditions: 0.1 g of the synthesized adsorbent, 20 mg·L^-1 of heavy metals, 25 ℃, pH=7.

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  Table 1 shows the adsorption abilities of HAP and HAP‐IDA/MPS for coexistent Cu²⁺, Cd²⁺, Co²⁺, Zn²⁺, Ni²⁺, and Hg²⁺ with the initial mass concentration of 20 mg·L^-1. It can be seen that HAP‐IDA/MPS exhibited higher adsorption capacity for heavy metal ions under investigation than HAP. Notably, HAP‐IDA/MPS displayed excellent adsorption ability for Hg²⁺: it provides an immobilization rate of nearly 100% at a dosage of 0.10 g, being much superior to HAP (about 0% for the same cation). Additionally, 0.10 g of HAP‐IDA/MPS had a total adsorption capacity of 13.7 mg·g^-1 for the six kinds of heavy metal ions, about 4.3 times as much as that of HAP (3.2 mg·g^-1). This is because, on the one hand, HAP only contains active anion PO₄^3-, which exhibits limited adsorption ability for the tested heavy metal ions. On the other hand, dual surface‐capped HAP‐IDA/MPS possesses —COOH and —SH chelating groups in association with PO₄^3- that can synergistically react with the tested heavy metal ions, thereby significantly augmenting the adsorption efficiency toward Cu²⁺, Cd²⁺, Co²⁺, Zn²⁺, Ni²⁺, and Hg²⁺.

  Table 1

  

  Table 1.  Comparison of adsorption efficiency of HAP and HAP‐IDA/MPS for mixed heavy metal ions

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  Adsorbent	pH	Temperature / ℃	ρ / (mg·L-1)	Time / h	Adsorbent quality / g	Immobilization rate / %
  						Cu2+	Cd2+	Co2+	Zn2+	Ni2+	Hg2+
  HAP	7	25	20	1	0.050	8.40	7.50	9.70	9.80	10.20	0
  HAP‐IDA/MPS					0.050	35.00	7.75	7.50	0	9.30	94.06
  HAP					0.10	10.40	8.05	12.30	10.10	12.50	0
  HAP‐IDA/MPS					0.10	72.43	9.35	15.40	18.70	12.70	100.00

  In terms of the adsorption mechanism of HAP‐IDA/MPS for the tested heavy metal ions, three aspects need to be considered. Firstly, the chelating groups —SH (from MPS) and —COOH (from IDA) are of significance for the adsorption of the tested heavy metal ions, because the —SH group can form strong complexes with heavy metal ions particularly Hg²⁺, while the —COOH group can interact with other metal ions such as Cu²⁺, Cd²⁺, Zn²⁺, Ni²⁺, and Co²⁺. The formation of the adsorbed complexes is driven by the differences in the solubility product constants of the corresponding metal sulfides and carboxylate complexes. Secondly, the adsorption mechanism of HAP‐IDA/MPS for the tested heavy metal ions is highly dependent on the competitive adsorption of different metal ions at its available active sites. The preferential adsorption of certain ions like Hg²⁺ can be attributed to the lower solubility product constant of HgS compared to other metal sulfides. This means that the adsorption process is not only dependent on the presence of the functional groups but also on the inherent chemical properties of the metal ions. Thirdly, HAP‐IDA/MPS allows the tested heavy metal ions to reach adsorption equilibrium within a short time (20 min), demonstrating its rapid adsorption ability. In one word, the adsorption mechanism of HAP‐IDA/MPS for the tested heavy metal ions is a complex interplay of functional group interactions, competitive adsorption dynamics, and adsorption kinetics related to the high surface area and large porosity of the HAP‐IDA/MPS nano‐amendment.

  Fig. 7a shows the adsorption capacities of HAP‐IDA/MPS for Cu²⁺, Cd²⁺, Co²⁺, Zn²⁺, Ni²⁺, and Hg²⁺ ions at different pH values (3‐9). The adsorption process is dominated by the electrostatic interaction between the adsorbent active sites and metal cations. Under acidic pH conditions, there is competitive adsorption between H⁺ and metal ions on the surface of HAP‐IDA/MPS. As the pH increased, the functional groups —SH and —COOH of HAP‐IDA/MPS were deprotonated to carry negative charges and easily bind to the electron‐deficient metal ions, thereby yielding metal hydroxide precipitates under alkaline conditions (pH > 7). Precipitation may coat the adsorbent surface, hindering further adsorption reactions or altering the adsorption mechanism and efficiency. A pH of 7 represents a neutral condition that not only avoids the competitive adsorption of H⁺ ions under acidic conditions but also prevents the excessive formation of metal hydroxide precipitates in alkaline environments. At pH=7, the electrostatic interactions between the adsorbent and metal ions can be effectively balanced, allowing the active sites of HAP IDA/MPS to efficiently bind with metal cations. This ultimately achieved an optimal adsorption performance. Therefore, we selected pH 7 as the experimental condition for the adsorption experiments. Besides, in Fig. 7b, an increase in temperature (20‐40 ℃) could slightly promote the adsorption of Cu²⁺, Cd²⁺, Co²⁺, Zn²⁺, Ni²⁺, and Hg²⁺ by the HAP‐IDA/MPS remediation agent, possibly due to enhanced intermolecular interaction at elevated temperatures. In practical environmental remediation or industrial applications, 25 ℃ is a commonly adopted and easily controllable temperature condition. Compared to higher or lower temperatures, 25 ℃ aligns more closely with natural ambient temperatures or room‐temperature industrial processes, facilitating practical implementation without requiring complex temperature‐control equipment or excessive energy consumption. Therefore, we set the optimal adsorption temperature as 25 ℃.

  Figure 7

  

  Figure 7.  Effect of solution pH at 25 ℃ (a) and temperature at pH=7 (b) on the adsorption of heavy metal ions by HAP-IDA/MPS

  Conditions: 0.1 g of the synthesized adsorbent, 20 mg·L^-1 of heavy metals.

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  To further verify the selective reactivity of HAP‐IDA/MPS nano‐amendment toward the tested heavy metal ions, we fixed the mass of HAP‐IDA/MPS nano‐amendment at 0.10 g and tested its adsorption performance toward coexisting Cu²⁺, Cd²⁺, Co²⁺, Zn²⁺, Ni²⁺, Hg²⁺, Ca²⁺, and Mg²⁺. As can be seen in Fig. 8a, the immobilization rates of Cu²⁺, Cd²⁺, Co²⁺, Zn²⁺, Ni²⁺, and Hg²⁺ were 76.12%, 10.12%, 14.85%, 20.83%, 13.57%, and 98.1%, respectively. This indicates that among the six kinds of heavy metal ions, the HAP‐IDA/MPS nano‐amendement preferentially passivates Hg²⁺ and Cu²⁺. The presence of Ca²⁺ and Mg²⁺ has little impact on the adsorption performance, which demonstrates that HAP‐IDA/MPS nano‐amendment had high selective reactivity towards heavy metal ions Cu²⁺, Cd²⁺, Co²⁺, Zn²⁺, Ni²⁺, and Hg²⁺.

  Figure 8

  

  Figure 8.  Selective reactivity of HAP‐IDA/MPS nano‐amendment towards the tested heavy metal ions (a) and immobilization rates of the cations during six recycle of adsorption experiments (b)

  Conditions: 0.1 g of the synthesized adsorbent, 20 mg·L^-1 of heavy metals, 25 ℃, pH=7.

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  Moreover, reusability is one of the most important and valuable properties of an adsorbent. The immobilization rates of Cu²⁺, Cd²⁺, Co²⁺, Zn²⁺, Ni²⁺, and Hg²⁺ by HAP‐IDA/MPS nano‐amendment after being recycled for up to six times are presented in Fig. 8b. After six consecutive cycles, the immobilization rates of Hg²⁺ and Cu²⁺ by HAP‐IDA/MPS were consistently maintained above 60%, which indicates that the HAP‐IDA/MPS nano‐adsorbent has good reusability for the removal of the toxic heavy metal ions in aqueous solutions.

  Scheme 1 shows the schematic diagram for the preparation of the double‐chelated HAP‐IDA/MPS nano‐adsorbent with alternate long‐short chain configurations allowing rapid and efficient adsorption of coexisting multi‐heavy metal ions. Namely, HAP NPs were prepared by a one‐step hydrothermal method. Then, IDA and MPS are grafted onto the surface of HAP to afford dual surface‐capped nano‐amendment HAP‐IDA/MPS, during which MPS and HAP are mediated by IDA. Specifically, HAP is first surface modified by IDA to form HAP‐IDA complex containing —COOH functional group. Subsequently, MPS is introduced onto the surface of HAP‐IDA to accomplish the dual‐surface modification, during which the —SH functional group in MPS reacts with the —COOH on the surface of HAP‐IDA to form the HAP‐IDA/MPS complex. In this way, MPS reacts with the surface —COOH of the modified HAP‐IDA rather than with HAP directly. The resultant bifunctional surface modification (IDA and MPS) enables HAP‐IDA/MPS to more effectively adsorb a variety of heavy metal ions, especially in complex pollution systems, significantly enhancing its heavy metal removal ability. Compared with HAP, the bichelated —COOH and —SH groups endow HAP‐IDA/MPS with superior and synergistic immobilizing ability towards Cu²⁺, Cd²⁺, Co²⁺, Zn²⁺, Ni²⁺, and Hg²⁺. Besides, the alternate long‐short chain configuration contributes to effectively reducing the steric hindrance and achieving rapid, efficient, and synergistic adsorption of the coexistent heavy metal ions.

  Scheme 1

  

  Scheme 1.  Schematic illustration of the synthesis of dual‐surface‐capped HAP‐IDA/MPS

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  3.   Conclusions

  A one‐step hydrothermal method was utilized to prepare porous HAP nanoparticles dual‑surface capped by IDA and MPS. The as‐synthesized HAP‐IDA/MPS nano‐amendment exhibited alternate long‐short chains of —COOH and —SH bichelated species. It is superior to HAP in immobilizing Cu²⁺, Cd²⁺, Co²⁺, Zn²⁺, Ni²⁺, and Hg²⁺, which is because its —COOH and —SH chelating groups in association with PO₄^3- can synergistically react with the tested heavy metal ions, thereby significantly augmenting the adsorption efficiency towards Cu²⁺, Cd²⁺, Co²⁺, Zn²⁺, Ni²⁺, and Hg²⁺. Particularly, the immobilization rate for Hg²⁺ was the highest, which could be because it chemically reacts with —SH to form sulfide with the lowest solubility product constant. HAP‐IDA/MPS could rapidly and synergistically immobilize the tested heavy metals with high efficiency, being a novel nano‐amendment with a promising prospect.

  
  Acknowledgements: The authors acknowledge the financial support provided by Natural Science Foundation of Henan Province of China (Grant No.232300420166), Henan Provincial Key Research, Development and Promotion Special Project (Science and Technology Breakthrough, Grant No.252102231052), the Fellowship of China Postdoctoral Science Foundation (Grant No.2021M690913), Project of International Cooperation and Exchanges of the National Natural Science Foundation of China (Grant No.82020108017), the Open Foundation of State Environmental Protection Key Laboratory of Soil Environmental Management and Pollution Control (Grant No.MEESEPC202310), and Science and Technology Project of Kaifeng City (Grant No.2202002). Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  SEM images of HAP (a, b), HAP‐IDA (c), and HAP‐IDA/MPS (d); TEM image of HAP‐IDA/MPS (e); Particle size distribution of HAP (f)

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  Figure 2  FTIR spectra of HAP, HAP‐IDA, and HAP‐IDA/MPS

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  Figure 3  XRD pattern of HAP‐IDA/MPS

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  Figure 4  TG curves of HAP, HAP‐IDA, and HAP‐IDA/MPS

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  Figure 5  Relationship between the immobilization rates of heavy metal ions by HAP‐IDA/MPS and adsorption 16:54:02

  Conditions: 0.1 g of the synthesized adsorbent, 20 mg·L^-1 of heavy metals, 25 ℃, pH=7.

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  Figure 6  Relationship between immobilization rates of heavy metal ions and their initial mass concentrations

  Conditions: 0.1 g of the synthesized adsorbent, 20 mg·L^-1 of heavy metals, 25 ℃, pH=7.

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  Figure 7  Effect of solution pH at 25 ℃ (a) and temperature at pH=7 (b) on the adsorption of heavy metal ions by HAP-IDA/MPS

  Conditions: 0.1 g of the synthesized adsorbent, 20 mg·L^-1 of heavy metals.

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  Figure 8  Selective reactivity of HAP‐IDA/MPS nano‐amendment towards the tested heavy metal ions (a) and immobilization rates of the cations during six recycle of adsorption experiments (b)

  Conditions: 0.1 g of the synthesized adsorbent, 20 mg·L^-1 of heavy metals, 25 ℃, pH=7.

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  Scheme 1  Schematic illustration of the synthesis of dual‐surface‐capped HAP‐IDA/MPS

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  Table 1.  Comparison of adsorption efficiency of HAP and HAP‐IDA/MPS for mixed heavy metal ions

  Adsorbent	pH	Temperature / ℃	ρ / (mg·L-1)	Time / h	Adsorbent quality / g	Immobilization rate / %
  						Cu2+	Cd2+	Co2+	Zn2+	Ni2+	Hg2+
  HAP	7	25	20	1	0.050	8.40	7.50	9.70	9.80	10.20	0
  HAP‐IDA/MPS					0.050	35.00	7.75	7.50	0	9.30	94.06
  HAP					0.10	10.40	8.05	12.30	10.10	12.50	0
  HAP‐IDA/MPS					0.10	72.43	9.35	15.40	18.70	12.70	100.00

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