Introduction

Large quantities of limestone sludge are produced in the quarrying, cutting, and crushing of limestone. Limestone sludge, comprising 85% water and 15% limestone powder, poses a serious threat to the environment [1]. The United Kingdom produces roughly 21.2 million tons of limestone sludge per year, Greece produces 18 million tons, and Turkey produces 30 million tons [2]. Taiwan produces about 12,000 tons of limestone sludge every year, most of which is piled in areas adjacent to production facilities. When disturbed by heavy rains and wind, these materials pose a health hazard (e.g., asthma) [3]. Sludge is currently disposed of in sanitary landfills in Taiwan. However, rapid urbanization has made locating suitable landfill sites increasingly difficult. The increase in the demand for natural resources and a scarcity of environmentally acceptable solid waste disposal sites have motivated several municipalities in Taiwan to consider resource recovery as an alternative. The CaO that makes up roughly 60% of limestone sludge [4] can be reused as filling material, natural aggregate, asphalt mixture, cement replacement material, and light brick material [5].

Water pollution by heavy metals poses a significant threat to the environment worldwide. Unlike most organic pollutants, heavy metals can have direct or indirect adverse effects on plants and humans, even at very low concentrations. The nonbiodegradability and long-term persistence of heavy metals leads to their accumulation in organisms throughout the food chain with a range of corresponding health disorders [6, 7]. The use of copper (Cu) in the manufacture of pesticides, paper, electronics, automotive parts, and fertilizers has led to the continuous release of large quantities of copper(II) from industrial wastewater into the environment. Cu is an essential trace element for organisms in natural ecosystems; however, excessive Cu2+ is a serious problem [8, 9]. Numerous methods have been developed for the removal of heavy metals from water, including chemical precipitation, ion exchange, adsorption, membrane filtration, and electrochemical processing [10,11,12,13,14]. Adsorption methods tend to be simple, flexible, cost-effective, and easy to implement [15, 16]. Researchers have developed numerous adsorbents, including zeolites [17], montmorillonite [18], diatomaceous earth [19, 20], and hydroxyapatite [21]. Note that these adsorbents vary considerably in terms of removal efficiency, preparation complexity, and cost.

Hydroxyapatite (HAP; Ca10(PO4)6(OH)2) is a member of the apatite mineral family and the main component of bones and teeth in vertebrates. HAP is widely used in wastewater purification [22] and soil remediation [23], due to its ability to fix heavy metal ions, such as Zn2+ and Cu2+ [24]. The immobilization of metal ions on HAP surfaces involves a number of processes, including ion exchange, surface complexation, HAP dissolution to form new metal phosphates, and the replacement of Ca2+ in HAP with other metal ions during recrystallization [25,26,27]. Ayodele et al. [28] used hydroxyapatite from eggshells to remove copper and nickel ions from simulated industrial wastewater. Wang et al. [29] used hydroxyapatite-biochar nanocomposite for the adsorption of Pb(II), Cu(II) and Zn(II). Wang et al. [30] used microspheres of synthetic abalone shell to remove Cu2+ from an aqueous solution. Chen et al. [31] prepared hydroxyapatite from sewage sludge as an absorbent to remove Cu 2+ and Cd 2+ from aqueous solutions. This study examined the efficacy of using limestone sludge as a source of calcium in the hydrothermal synthesis of ecological HAP (eco-HAP) for the removal of Cu2+ from water. The resulting materials underwent characterization via X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) and thermal field emission scanning electron microscope (FE-SEM). This study also sought to identify key factors affecting the total Cu2+ adsorption, adsorption kinetics, and adsorption isotherms.

Materials and methods

Materials

The limestone sludge samples were collected from a stone processing plant rom Hualien County in Taiwan. Limestone sludge was washed with distilled water to remove impurities and dried in an oven at 105 °C overnight, whereupon the resulting solids were sieved to a particle size of 74 μm. The limestone sludge was alkaline (pH = 11.70), with a specific gravity of 2.91 and a density of 2.21 g/cm3. The limestone sludge comprised the following: ash (86.31%), combustible matter (13.45%), and moisture (0.24%). The limestone sludge comprised the following: CaO (94.30%), MgO (1.97%), and SiO2 (1.73%). Using X-ray powder diffraction (XRD), it determined that the main crystal phase limestone sludge was calcium carbonate (JCPDS No. 085-1108). PO43− from ((NH4)2HPO4) (diamine hydrogen phosphate, 98.5%) with HNO3 (nitric acid solution, 65%) was used to extract the Ca source. NaOH (sodium hydroxide solution, 65%) was used to adjust the alkalinity of the solution.

Hydrothermal synthesis of hydroxyapatite

The limestone sludge solids were dried and crushed to form a powder. This study dissolved 25 g of the powder in 100 mL of 3 M HNO3 and then filtered the mixture. (NH4)2HPO4 solution (100 mL) was gradually added to the resulting solution until Ca/P ratios of 1, 1.67, 2.33, and 3 were reached. NaOH solution was added to maintain the pH of the solution at 10. Mixture details are listed in Table 1. The resulting mixture was then sealed in a Teflon-coated stainless steel container to perform autoclave and hydrothermal conversion at 120–180 °C over a period of 8 h. The autoclave was allowed to cool naturally to room temperature, whereupon the resulting product was washed using deionized water, collected as a powder, and dried at 90 °C for 24 h prior to analysis.

Table 1 Sample codes and corresponding experimental conditions
Full size table

Material characterization

The chemical composition of the raw materials was analyzed using an X-ray fluorescence analyzer (XRF, Rigaku NEX CG). The pore structure and crystallinity of the eco-HAP were determined by XRD (Rigaku Miniflex, Japan) using diffraction data collected between 2θ = 5–80°. In accordance with the methods outlined by Iqbal et al. [32], this study calculated % phase A as the sum of the area under the 10 main peaks centered at 2θ = 10.28°, 22.88°, 25.84°, 28.98°, 31.72°, 32.62°, 40.05°, 45.27°, 45.27° and 45.24°, as follows Eq. (1). The crystal size of eco-HAP was calculated using the Scherrer equation, as follow Eq. (2).

$$\% Phase A = {\raise0.7ex\hbox{${Sum\,\,of\,\,area\,\,under\,\,XRD\,\,peak\,\,of\,\,phase\,\,A}$} \!\mathord{\left/ {\vphantom {{Sum\,\,of\,\,area\,\,under\,\,XRD\,\,peak\,\,of\,\,phase\,\,A} {Total\,\,area}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${Total\,\,area}$}}\,\, \times \,\,100$$
(1)
$$\Delta \left( {2\theta } \right) = {\raise0.7ex\hbox{${0.9\lambda }$} \!\mathord{\left/ {\vphantom {{0.9\lambda } {Dcos\theta }}}\right.\kern-0pt} \!\lower0.7ex\hbox{${Dcos\theta }$}}$$
(2)

where Δ(2θ) indicates the peak width at half maximum (002) reflection intensity; λ is the wavelength of CuKα (0.154056 nm); D (nm)) is the crystal size; and θ is the Bragg angle [33].

FE-SEM (Nova NanoSEM 230) was used to characterize the surface and structural morphologies. Nitrogen adsorption and desorption isotherms were obtained using a Tristar 3000 (Micromeritics) and the BET method was used to determine the specific surface area. The BJH method was used to calculate the total pore volume and pore diameter.

Batch adsorption

In a volumetric flask, Cu(NO3)2 was dissolved in deionized water to prepare a standard Cu2+ solution (100 mg/L). The solution was then diluted to the desired concentration using deionized water. The volume of the tested metals bearing aqueous solutions was fixed at 100 mL. All adsorption experiments were performed under a controlled temperature with the contents of the flask stirred at 100 rpm. This study examined the adsorption of Cu2+ by eco-HAP as a function of the following adsorption conditions: adsorbent quantity (0.1–2 g/L), reaction time (0–180 min), and initial Cu2+ concentration (10–100 mg/L) The pH of the solution was maintained at 5 through the addition of 0.1 M of NaOH solution. All experiments were performed in triplicate to confirm their reproducibility, the results of which were averaged. Note that the relative standard deviation was less than 3% of the average. Immediately after adsorption, the samples underwent solid–liquid separation using a centrifuge, whereupon flame atomic absorption spectrometry (FLAA) was used to analyze the remaining Cu2+ concentration in the supernatant. The removal percentage (%, R) and adsorption capacity (qe, mg/g) of the adsorbent were, respectively, calculated using the following formulas:

$${\text{\% }}R = {\raise0.7ex\hbox{${C_{{\mathfrak{i}}} - C_{{\text{e}}} }$} \!\mathord{\left/ {\vphantom {{C_{{\mathfrak{i}}} - C_{{\text{e}}} } {C_{0} }}}\right.\kern-0pt} \!\lower0.7ex\hbox{${C_{0} }$}} \times 100\%$$
(3)
$$q_{e} = {\raise0.7ex\hbox{$V$} \!\mathord{\left/ {\vphantom {V M}}\right.\kern-0pt} \!\lower0.7ex\hbox{$M$}} \times \left( {C_{{\mathfrak{i}}} - C_{e} } \right)$$
(4)

where Ci and Ce (mg/L) are the initial concentration and final concentration of the Cu2+ solution, respectively; V(L) is the volume of the Cu2+ solution; and M (g) is the quality of the penetrant.

Results and discussion

XRD crystal phase analysis

This study’s objective was to convert calcium carbonate in the limestone sludge precursor material into eco-HAP. Figure 1a presents the XRD patterns of the eco-HAP synthesized at a hydrothermal temperature of 120 °C with various Ca/P molar ratios. The eco-HAP synthesized using a Ca/P molar ratio of 1 (LA3) was comparable to the standard chart JCPDS (09-0432) with the following diffraction peaks at 2θ: 25.90°, 31.74°, 32.54°, 40.19°, 45.30°, 49.46°, and 53.12°. This study posits that under high-temperature and high-pressure hydrothermal synthesis conditions, the Ca2+ first reacted with PO43− to form β-TCP (Ca3(PO4)2), which reacted with Ca2+ and OH− to form eco-HAP crystals. Sample LA3 also presented a diffraction peak of beta tri-calcium phosphate (β-TCP) at 34.37°, a byproduct of eco-HAP synthesis. This study posits that this was due to a small quantity of Ca2+ in the mixed solution forming a precipitate of Ca(OH)2 under high alkaline conditions (pH = 10), such that there was insufficient Ca2+ to react with all of the β-TCP [34].

Fig. 1
figure 1

XRD patterns of eco-HAP

Full size image

Using the same hydrothermal temperature (120 °C) with a Ca/P molar ratio of 1.67, 2.33, or 3 (samples LA5, LA7, and LA9), this study observed 2θ diffraction peaks at 25.90° and 31.74°. This study posit that the high diffraction peak intensity of sample LA7 (Ca/P = 2.33) can be attributed to the availability of Ca2+ to react with PO43− and OH− to form eco-HAP [35]. Increasing the Ca/P molar ratio to 3.0 decreased the relative content of PO43−, such that a portion of Ca2+ reacted with OH− to form Ca(OH)2, which was unsuitable for the formation of eco-HAP, resulting in a weaker diffraction peak.

Figure 1b presents the XRD patterns of eco-HAP at a hydrothermal temperature of 150 °C with various Ca/P molar ratios. Samples with a Ca/P molar ratio of 1 (LB3) presented 2θ diffraction peaks at 25.84°, 31.72°, 32.62°, 40.05°, 45.27°, 49.42°, and 53.21°. It appears that the first reaction involved the conversion of the aragonite phase (hexagonal) into calcite (hexagonal), which reacted with phosphate to form β-TCP (rhombohedral), eventually leading to the formation of a thermodynamically stable form of calcium phosphate (hexagonal) [36]. Sample LB3 also presented a diffraction peak at 34.37°, indicative of β-TCP. As with the eco-HAP synthesized at a hydrothermal temperature of 120 °C, increasing the molar ratio to 1.67, 2.33 or 3.0 produced diffraction peaks at 25.90° and 31.74°; however, the intensity of these peaks was higher, indicating higher crystallinity.

Figure 1c presents the XRD patterns of eco-HAP at a hydrothermal temperature of 180 °C with various Ca/P molar ratios. Samples with a Ca/P molar ratio of 1 (LC3) presented 2θ diffraction peaks at 2θ = 25.81°, 31.73°, 32.58°, 40.10°, 45.24°, 49.40°, and 53.18°. Strong peaks at 2θ = 25.81°, 31.7°, and 32.58°, respectively, corresponded to the (002), (211), and (300) planes of eco-HAP. The intensity ratio of the (002) to the (211) plane in their samples was far higher than that of the standard, indicating that the crystallization of eco-HAP occurred along the c-axis (Feng et al. 2016) [37]. Increasing the Ca/P molar ratio to 1.67, 2.33, and 3 (LC5, LC7 and LC9) led to an increase in the strength of the 2θ diffraction peaks at 25.90° and 31.74°. Increasing the hydrothermal temperature from 120 to 180 °C increased the intensity of the diffraction peaks at 25.81°, 31.73°, 32.58°, 40.10°, 45.24°, 49.40°, and 53.18°, which is indicative of higher crystallinity.

Table 2 lists the crystallinity of eco-HAP samples synthesized at various temperatures with various Ca/P molar ratios. In samples synthesized at 120 °C, the crystallinity varied as a function of Ca/P ratio, as follows: 1 (73.78%), 1.67 (78.43%), 2.33 (83.10%), and 3.0 (70.07%). The obvious drop in crystallinity under a Ca/P ratio of 3.0 can be attributed to a decrease in the quantity of PO43−, such that a portion of Ca2+ reacted with OH− to form Ca(OH)2, which is ill-suited to the formation of eco-HAP crystals (Liu et al., 2018). Note that a high Ca/P ratio had the same effect, regardless of synthesis temperature. Increasing the hydrothermal synthesis temperature to 180 °C was shown to increase crystallinity, regardless of Ca/P ratio.

Table 2 The crystallinity of eco-HAP
Full size table

FE-SEM analysis

Figure 2a presents FE-SEM images of eco-HAP synthesized at a hydrothermal temperature of 120 °C with various Ca/P molar ratios. A Ca/P molar ratio of 1 resulted in eco-HAP particles with an elliptical shape. Increasing the Ca/P molar ratio to 1.67 resulted in slightly elongated particles. A Ca/P molar ratio of 2.33 or 3.0 resulted in particles with a pronounced rod shape, a length of 144–201 nm, and a width of 25–30 nm. Liu et al. (2018) [35] reported that under high-temperature and high-pressure conditions, the reaction of Ca(OH)2 with PO43− resulted in eco-HAP with high-intensity diffraction peaks. This study posits that increasing the Ca/P molar ratio increased the amount of Ca(OH)2 available in the solution to react with PO43−, resulting in the formation of rod-shaped eco-HAP. Note that these results are consistent with the XRD patterns.

Fig. 2
figure 2

FE-SEM image of eco-HAP

Full size image

Figure 2b presents FE-SEM images of eco-HAP synthesized at a hydrothermal temperature of 150 °C with various Ca/P molar ratios. A Ca/P molar ratio of 1 resulted in eco-HAP particles in the shape of short rods. Note that the length of the rods increased with the Ca/P ratio. Note also that the form of the eco-HAP crystals depended on the Ca/P molar ratio in the precursor solution as well as the synthesis temperature and synthesis time. The eco-HAP crystals nucleated and grew from single sites to form rod-shaped particles. The agglomeration of adjacent crystals led to the formation of bundle-like structures, which formed in a stacked configuration.

Figure 2c presents FE-SEM images of eco-HAP synthesized at a hydrothermal temperature of 180 °C with various Ca/P molar ratios. A Ca/P molar ratio of 1 resulted in eco-HAP particles with a distinct rod shape, measuring 152.7 nm in length and 34.6 nm in width. Increasing the molar ratio to 3.0 increased the length of the particles to 241.9 nm and the width to 60.6 nm. Zhang et al. [38] reported that the Ca/P molar ratio had little effect on the length of crystals. Arfa et al. [39] reported that the Ca/P molar ratio was affected by the pH value but not the synthesis temperature. Taken together, these results indicate that synthesis temperature and Ca/P molar ratio affected the crystal size, crystallinity, and peak intensity of the eco-HAP.

Nitrogen adsorption and desorption curves

Figure 3a presents the N2 adsorption/desorption isotherms of eco-HAP. The eco-HAP samples synthesized at 120 °C with a Ca/P molar ratio of 1 presented a type-IV adsorption isotherm curve (IUPAC classification), indicating a mesoporous structure and small nanoplate aggregates [40] and H3-type hysteresis loop. During adsorption, the number of N2 molecules in the pores increased with the pressure, and the capillary condensation of N2 molecules led to isotherm retention [41]. The maximum adsorption capacity LA3 was 299.42 cm3/g STP with a specific surface area of 115.04 m2/g. Eco-HAP samples synthesized at 120 °C with a Ca/P molar ratio of 2.33 presented a maximum adsorption capacity 199.02 cm3/g STP and a specific surface area of 49.44 m2/g. Wei et al. [42] reported that the crystallinity of eco-HAP is inversely proportional to the specific surface area of eco-HAP and the corresponding number of active sites.

Fig. 3
figure 3

Nitrogen adsorption isotherm and desorption curve of eco-HAP

Full size image

Samples synthesized at 150 °C presented the same trend as those synthesized at 120 °C. The eco-HAP samples synthesized at 180 °C with a Ca/P molar ratio of 1 presented a specific surface area of 74.69 m2/g and a maximum adsorption capacity of 277.21 cm3/g STP, both of which are below those of samples synthesized at 120 °C.

Figure 4 presents the pore size distribution of eco-HAP. The eco-HAP samples synthesized at 120 °C with a Ca/P molar ratio of 1 presented a bimodal mesopore size distribution (3.21 nm and 14.44 nm). Increasing the Ca/P molar ratio to 2.33 resulted in a unimodal pore size distribution with significantly smaller pores (2.91 nm) [42]. The eco-HAP samples synthesized at 150 °C with a Ca/P molar ratio of 1 presented a bimodal mesopore size distribution (3.19 and 16.21 nm). Increasing the Ca/P molar ratio to 2.33 increased the pore size to 3.05 nm. The eco-HAP samples synthesized at 120 °C with a Ca/P molar ratio of 1 presented a bimodal mesopore size distribution (4.21 and 24.44 nm). Increasing the Ca/P molar ratio to 2.33 increased the pore size to 2.14 nm.

Fig. 4
figure 4

Pore size distribution of eco-HAP

Full size image

Adsorption kinetics of Cu2+ in eco-HAP

Figure 5a presents a graph showing the adsorption kinetics of Cu2+ in eco-HAP. The eco-HAP samples synthesized at 120 °C completed the reaction process within 20 min, wherein a rapid initial interaction between Cu2+ and eco-HAP gradually tapered off [8]. The adsorption capacity varied with the Ca/P molar ratio as follows: Ca/P = 1 (210 mg/g) and Ca/P = 2.33 (121.8 mg/g). Note that the adsorption capacity was inversely proportional to crystallinity.

Fig. 5
figure 5

Adsorption kinetics of Cu2+ in eco-HAP

Full size image

The eco-HAP samples synthesized at 150 °C completed the reaction process within 20 min. The adsorption capacity varied with the Ca/P molar ratio as follows: Ca/P = 1 (172.65 mg/g) and Ca/P = 2.33 (100 mg/g).

The eco-HAP samples synthesized at 180 °C completed the reaction process within 20 min. The adsorption capacity varied with the Ca/P molar ratio as follows: Ca/P = 1 (110 mg/g) and Ca/P = 2.33 (55 mg/g). Again, adsorption capacity was inversely proportional to crystallinity.

The diffusion mechanism involved in the adsorption process was characterized using pseudo-first-order and pseudo-second-order kinetic models, based on the following formulas:

$${\text{l}}n\left( {q_{e} - q_{t} } \right) = lnq_{e} - k_{1} t$$
(5)
$${\raise0.7ex\hbox{$t$} \!\mathord{\left/ {\vphantom {t {qt}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${qt}$}} = {\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 {k_{2} q_{e}^{2} }}}\right.\kern-0pt} \!\lower0.7ex\hbox{${k_{2} q_{e}^{2} }$}} + {\raise0.7ex\hbox{$t$} \!\mathord{\left/ {\vphantom {t {q_{e} }}}\right.\kern-0pt} \!\lower0.7ex\hbox{${q_{e} }$}}$$
(6)

where qe and qt (mg/g), respectively, indicate the amount of Cu2+ adsorbed on the eco-HAP (mg/g) when the adsorption reached equilibrium and time t (min); and k1 and k2 indicate the pseudo-first-order and pseudo-second-order rate constants (g/mg/min).

Table 3 lists the kinetic parameters of eco-HAP adsorbed lead ions based on the pseudo-first-order and pseudo-second-order kinetic models. Overall, the fitting effect of the pseudo-second-order kinetics model was better than that of the pseudo-first-order model, with an R2 value of 0.9990–1.000. These results show that the eco-HAP controlled the exchange of ions and other chemical interactions between the adsorbent and adsorbate.

Table 3 Adsorption kinetic parameters of Cu2+ on eco-HAP
Full size table

Removal efficiency of Cu2+ from aqueous solutions

Figure 6 presents a graph illustrating the influence of adsorbent quantity on the performance of the eco-HAP (LA3) in terms of Cu2+ removal (synthesis temperature = 120 °C and Ca/P molar ratio = 1). Increasing the quantity of adsorbent from 0.1 to 2 g/L increased the proportion of removed copper ions from 27 to 99.40%, due presumably to an increase in the number of adsorption sites. Overall, the removal performance of the eco-HAP increased with adsorbent quantity, regardless of the Ca/P molar ratio. Nonetheless, samples synthesized with a Ca/P molar ratio of 3.0 (2 g/L) removed only 79.75% of the dissolved copper ions, due presumably to higher crystallinity, which reduced the number of available adsorption sites as well as the dissolution of phosphate and hydroxyl radicals. The removal efficiency of samples synthesized at 120 °C (99.99%) exceeded that of samples synthesized at 180 °C (80.68%), presumably due to enhanced crystallinity.

Fig. 6
figure 6

The removal efficiency of eco-HAP removing Cu2+ from aqueous solutions

Full size image

Cu2+ adsorption isotherms of eco-HAP

Adsorption isotherms can be used to characterize the surface of adsorbents at the micro level as well as the adsorption capacity, adsorption strength, and absorption state. In this study employed the Langmuir and Freundlich equation as follows:

$$Q_{e} = H_{h} C_{e}$$
(7)
$${\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 {q_{e} }}}\right.\kern-0pt} \!\lower0.7ex\hbox{${q_{e} }$}} = {\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 {q_{m} }}}\right.\kern-0pt} \!\lower0.7ex\hbox{${q_{m} }$}}KC_{e} + {\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 {q_{e} }}}\right.\kern-0pt} \!\lower0.7ex\hbox{${q_{e} }$}}$$
(8)
$$Lnq_{e} = lnK_{f} + nlnC_{e}$$
(9)

Figure 7 presents the Langmuir models of eco-HAP synthesized at temperatures of 120, 150, and 180 °C as a function of Ca/P molar ratio. The R2 values of eco-HAP in the adsorption of Cu2+ were as follows: 120 °C (0.96–0.98), 150 °C (0.97–0.99), and 180 °C (0.96–0.99). Overall, the Ca/P molar ratio was proportional to R2 = 1.

Fig. 7
figure 7

Langmuir diagram of Cu2+ adsorption by eco-HAP

Full size image

Figure 8 presents the Freundlich models of eco-HAP. The R2 values of eco-HAP in the adsorption of Cu2+ were as follows: 120 °C (0.94–0.98), 150 °C (0.93–0.96), and 180 °C (0.90–0.97). Overall, the Ca/P molar ratio was proportional to R2, and the adsorption isotherm curves were more closely described using the Langmuir model.

Fig. 8
figure 8

Freundlich diagram of Cu2+ adsorption by eco-HAP

Full size image

Adsorption mechanism

Varied parameters, initial adsorption concentration and adsorption time, are very vitalin evaluating the adsorption capacity of heavy metal ions. As for the adsorption equilibrium isotherms and the adsorption kinetics, they are important measurements in diagnosing the adsorption mechanism and evaluating the adsorption efficiency. In this experiment, a series of batch equilibrium experiments were carried out to identify the effects of adsorption time (Fig. 5) and initial adsorption concentration (Figs. 7 and 8) on the adsorption of Cu2+. The mechanisms of eco-HAP removal Cu2+ in an aqueous solution are shown in Fig. 9. For the adsorption mechanism of Cu2+, they were adsorbed on eco-HAP, which mainly relied on the ion exchange through substituting Ca2+ from the apatite lattice by a diffusion process according to Eq. (10):

$$\equiv Ca_{10} (PO_{4} )_{6} \left( {OH} \right)_{2} + xCu^{2 + } \to \equiv Ca_{10 - x} Cu_{x} PO_{4} )_{6} \left( {OH} \right)_{2} + xCu^{2 + }$$
(10)
Fig. 9
figure 9

The mechanism of Cu2+ removal on eco-HAP

Full size image

Conclusion

The limestone sludge is a promising alternative to expensive hydroxyapatite due to its low costs, satisfactory separation performance and sustainable feature. First, as a waste-to-resource strategy they are promising solutions to the waste disposal problems. Second, using wastes for limestone sludge reduces the raw material costs and the need for expensive and unrenewable raw materials (e.g. CaO). Third, synthetic HAp available on the market are very expensive due to the use of high-purity chemical reagents in the production of HAp. Finally, waste-derived hydroxyapatite could offer excellent pollutant removal performance due to its synergetic effect of adsorption properties. Therefore, more research efforts should be devoted to this exciting area to achieve long-term sustainability and move towards a circular economy.

This study developed a novel approach to the hydrothermal synthesis of eco-hydroxyapatite (eco-HPA) from recycled limestone sludge for the removal of Cu2+ from aqueous solutions. In the hydrothermal synthesis of eco-HAP under high-temperature and high-pressure conditions, Ca2+ first reacted with PO43− to form β-TCP, which then reacted with Ca2+ and OH- to form eco-HAP crystals. Note that the sample synthesized at 120 °C with a Ca/P molar ratio of 1 presented a 2θ diffraction peak of β-TCP at 34.37°, a byproduct of eco-HAP synthesis. The as-prepared eco-HAP presented good adsorption capacity in the removal of Cu2+ from aqueous solutions. Under a hydrothermal temperature of 120 °C and Ca/P molar ratio of 1, it obtained hydroxyapatite (LA3) with an adsorption capacity of 210 mg/g. The adsorption kinetics closely fit the pseudo-second-order kinetic model (R2 = 0.9990–1.000). The isotherms presented the strongest correlation with the Langmuir model (R2 = 0.97–0.99). These results show that the eco-HAP controlled the exchange of ions and other chemical interactions between the adsorbent and adsorbate. The stability of the proposed material after adsorbing Cu2+ would also prevent secondary contamination.