Introduction

Widespread use of lead in various industries such as battery manufacturing, radiation shielding, acid metal plating, ammunition, tetraethyl lead manufacturing, ceramic and glass industries printing, painting, water piping and other industries can release lead into environment (Mager et al. 2011). Lead can cause negative effects on the environment when it accumulates in living systems. Lead poisoning causes damage to the human biological systems such as nervous system, liver, kidney, brain and reproductive system (Fergusson 1990).

Several different processes have been developed for removal of heavy metals from aqueous media such as electrodialysis, chemical precipitation, adsorption, solvent extraction, reverse osmosis, ultrafiltration or ion exchange (Gritlin 1988; Davis and Comwell 1991). Adsorption process has been shown good effectiveness, efficiency, economically feasibility to remove heavy metal such as application of waste biomass (Cheng et al. 2016), modified ligno-cellulosic material (Huang et al. 2015) and wheat straw (Wu et al. 2019). Many different adsorbents such as natural zeolite (Faghihian et al. 1999; Pansini and Collella 1990), aquatic plant (Srivastav et al. 1993), fly ash sub grades (Laumakis et al. 1995), ion exchange resin (Hewitt et al. 1991), activated carbon (Reed et al. 1994; Gupta et al. 1997) have been used for lead removal from aquatic samples.

Synthesis and application of nanocompounds for various purposes have been considered such as the application of mesoporous compounds as catalyst (Malgras et al. 2016, 2018), synthesis of nanocrystals with special optical and electronic properties (Li et al. 2014; Tan et al. 2017), the use of nanoparticles as adsorbent for water and wastewater treatment (Hamidzadeh et al. 2015). Various nanomaterials were developed to removal heavy metals from aqua samples. The physical and chemical properties of nanomaterials have led to enhance the removal efficiency (Lu and Astruc 2018).

Hydroxyapatite (HA) is a class of calcium phosphate-based bioceramics with chemical composition of Ca10(PO4)6(OH)2. HA is used for different applications such as drug delivery (Pon-On et al. 2011; Zhang et al. 2010), coating implants (Kold et al. 2006) and other biomedical areas (Chen et al. 2011a, b). It is also an ideal material for removing heavy metals. The morphology and structure of HA are strongly affected by the method of preparation. Various different methods have been developed for synthesis of hollow spheres HA such as microwave-assisted hydrothermal routes (Wang et al. 2011), DNA-template hydrothermal methods (Qi et al. 2012), ion-assisted mineralization methods (Jiang et al. 2012), and others.

The egg shells contain natural organic and inorganic compounds, while a large amount of them is discarded as biowaste every day. To recycle them, this biowaste can be used to make effective adsorbents to remove toxins from the water. In the present work, the flower liked HA, which synthesized from egg shells by microwave oven, was used for lead ion removal from water samples.

Methods and materials

Ethylene diamine tetra acetic acid (EDTA), sodium hydroxide, sodium dihydrogen phosphate and sodium hypochlorite were prepared from Merck, Germany. Lead stock solution (1000 mg L−1) was purchased from Merck, Germany. Double distilled water was used for all experiments.

For preparation of nanoadsorbent, the egg shells were collected from household consumption for a week. The egg shells were placed in boiling water for 30 min and then washed to remove inner membranes with cold water. They were placed in a sodium hypochlorite solution (1 M) for 2 days. They were washed and dried at 110 °C for 5 h and then powdered.

500 ml EDTA 0.1 M was added to 5 g egg shells powder and stirred for 2 h until all carbon dioxide was removed from solution. Then, 2.50 ml Na2HPO4 0.6 M was slowly added to breaker for 30 min and then pH of solution was adjusted to 13 by sodium hydroxide solution. The solution was put in microwave with 680 W powers for 30 min. The resulted precipitate was filtered and washed with water tree times and dried at 110 °C for 5 h (Kumar and Girija 2013).

The scanning electron microscopy (SEM) image of synthesized nanostructure (Fig. 1) was obtained by the SEM (TSCAN, Check). As shown in Fig. 1, the flowerlike HA nanostructure formed leafs with width less than 100 nm and length about 500 nm which are extending radially from center.

Fig. 1
figure 1

SEM image of synthesized HA with microwave

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FT-IR spectrum of nanostructure (Fig. 2) showed in 450–4000 cm−1 resolution was obtained by PerkinElmer (USA) spectrum 2 with KBr pellet technique at temperature room. FT-IR spectrum was recorded in 3 regions (ν1) 873.5 cm−1 (ν2) 1416.4 cm−1, and (ν3) 1458.3 cm−1 which represents formation of hydroxyl apatite.

Fig. 2
figure 2

FT-IR spectrum of synthesized HA

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In discontinuous method, lead solution 100 mg L−1 was prepared from stock solution (1 g L−1). 10 ml of 100 mg L−1 solution was transferred into 50-ml breaker, and then 0.01 g of nanoadsorbent was added, stirred for 1 h and centrifuged. Lead concentration was determined with atomic absorption technique (Varian 200).

Elimination efficiency of lead ion was improved by investigation of effective parameters such as contact time, initial concentration, pH solution and dose of nanoadsorbent.

The lead adsorption into the nano-HA was calculated by following equation

$$q_{e} = \left( {c_{0} } \right. - \left. {c_{e} } \right)\frac{v}{w}$$

Where qe is the dose of adsorbed metal per grams of Nano HA (mgg−1), v is volume of solution (L), w is the weight of adsorbent (g), and the C0 and Ce are the initial and equilibrium concentration of metal in solution (mgL−1), respectively.

Results and discussion

In the present study, adsorption process was used as refining technology to remove lead ions from water and wastewater samples by nanostructure adsorbent. Elimination efficiency of lead ion was improved by investigation of effective parameters such as temperature, contact time, initial concentration and pH solution.

To investigate the effect of pH on removal process, 15 ml of 20 mg L−1 of lead solution was exposed to 0.01 g nano HA at 24 ± 1 °C with 500 rpm stirred rate for an hour. The pH value was varied from 3 to 9 by addition of 1 molL−1 hydrochloric acid or sodium hydroxide solutions. pH is the effective parameters in removing process of heavy metal ions from wastewater samples. As shown in Fig. 3a, the percentages of lead ion removal at different pH were shown that maximum removal was occurred in pH = 7, whereas this pH is optimal pH and would be used for other experiments.

Fig. 3
figure 3

The effects of a pH, b contact time and c dose of adsorbent on removal of lead ion by HA nanoparticles

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The contact time is one of the most effective factors in the adsorption process. To investigate the effect of contact time on adsorption, the experiments were done in different contact times from 5 to 60 min with 20 mg L−1 initial concentration of lead ions and 0.01 g nanoparticles weight at pH = 7 and 24 ± 1 °C temperature. Figure 3b shows that the rate of adsorption in the early stages is rapid and gradually decreases over time and after reaching the equilibrium state, it remains almost constant. The adsorption of lead ions was reached to equilibrium state after 15 min with 99.8% removal.

The adsorption of lead ions raised by increasing dose of adsorbent due to increase adsorption sites. The amounts of adsorption were increased by increasing the dose of nanoparticle from 0.001 to 0.05 g (Fig. 3c).

Adsorption isotherms are intended to describe adsorption capacity in order to facilitate the evaluation of this process. Among the various types of available isotherms, Langmuir and Freundlich isotherms, which are commonly used for adsorption modeling, are studied. In this study, 100 ml of lead solution at concentrations of 15–1000 mg L−1 was contacted for 15 min with 10 mg of nanoparticles at pH 7, 500 rpm and 24 °C, and then it was filtered with a filter paper and vacuum pump. Finally, the concentration of the remaining ions was read by the atomic adsorption device, the amount of adsorbed ions was measured, and the lead adsorption isotherms were plotted. The fitting of Langmuir and Freundlich adsorption models was studied, and its results were investigated for isotherms adsorption and modeling.

Used Langmuir equation for equilibrium adsorption is:

$$\frac{{c_{e} }}{{q_{e} }} = \frac{1}{{Q_{0} \times b}} + \frac{{c_{e} }}{{Q_{0} }}$$

Where Ce is equilibrium concentration of metal in solution (mgL−1), qe is the dose of adsorbed metal per unit weight adsorption (mgg−1), Q0 and b are Langmuir constants that related to the adsorbent capacity and adsorbent energy. Ce/qe versus Ce is plotted in Fig. 4a, and the Langmuir constant is tabulated in Table 1.

Fig. 4
figure 4

a Langmuir and b Freundlich isotherms

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Table 1 Langmuir and Freundlich constants of absorption isotherms curves
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Adsorption process was modeled with Freundlich isotherm with following equation

$$q_{\text{e}} = k_{\text{f}} c_{\text{e}}^{{\frac{1}{n}}}$$

where Ce is the equilibrium concentration of metal in solution (mg L−1), qe is the dose of adsorbed metal (mg g−1), kf and n are Freundlich adsorption constants. The Logarithmic equation of Freundlich model can be writhen as:

$$\log q_{\text{e}} = \log k_{\text{f}} + \frac{1}{n}\log c_{\text{e}}$$

By plotted Log qe versus log Ce, the straight line was resulted that the slop and intercept of line represented n−1 and log kf, respectively. Figure 4b shows the Logarithmic plot of Freundlich model, and the Freundlich adsorption constants is tabulated in Table 1. The obtained result in Table 1 showed that adsorption process had compatibility with Langmuir isotherm.

In this research, the nanostructure hydroxyapatite adsorbent was synthesized from eggshell and used to eliminate lead ion from water samples. The results showed that the removal procedure was done with low adsorption energy (8.3 × 10−2 L mg−1) and high (322.6 mg g−1). The adsorption capacity was measured after 2 months, and no significant difference was observed. Table 2 shows the maximum adsorption capacity of some adsorbent to remove lead ion which published in 2018 papers. Although, some of the maximum adsorption capacities, reported in Table 2, are greater than those obtained in this study, the effective nanostructure hydroxyapatite adsorbent has been synthesized from recycled biowaste by a simple and inexpensive method.

Table 2 Comparative values of maximum adsorption capacity (Qm) of some adsorbent to remove lead ion from water which reported in 2018 publications
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