Nanohydroxyapatite and its polycaprolactone nanocomposite for lead sorbent from aqueous solution

INTRODUCTION

Water is one of the most important natural sources in the world for the survival of all living beings and the development of humans. The water pollution with heavy metal is becoming a more serious problem of the economic development and rapid industrialization from recent years [1, 2]. Lead is the most important heavy toxic metal in the environment. The lead nature is non-biodegradable pollutant with detrimental effects on human health [3]. This element tends to accumulate in living organisms and leads to serious illnesses [4]. Lead absorption has become a global challenge due to the widespread use of lead metal in various industrials [5, 6].

Nanotechnology has attracted a great attention because of unique properties of nanomaterials and nanostructures [7] in various fields such as drug delivery [8-10], antibacterial activity [11-13], food packaging [14, 15], energy [16], sensor [3], agriculture [17], dental [18], absorption of hazardous material [19, 20], and etc. Various methods have been developed to remove heavy metals from water such as chemical precipitation [21], ion exchange [22], reverse osmosis [23], and adsorbent [24]. Recently, adsorbent nanomaterial was applied for the removal of lead from aqueous solutions [25, 26]. Easy separation of sorbent from the water is expanding as beneficial and functional properties for absorption of heavy metals. Therefore, it is important to develop the use of adsorbents based on magnetic [27-30] and polymer [27, 31] materials. In the recent years, hydroxyapatite polymer composite was used to lead(II) uptake from aqueous solutions [32]. Hydroxyapatite is a mineral porous material and it can be acted as sorbent of lead ions [33]. Polycaprolactone is one of the most commonly-used polymers for the removal of hazardous, carcinogenic, and toxic pollutants of heavy metals from aqueous solutions. According to previous report, polycaprolactone nanofibrous materials were modified by clay and zeolite nanoparticles for lead adsorption [34]. Also in another report, cyclodextrin-polycaprolactone titanium dioxide nanocomposites were used as a sorbent for the removal of lead in aqueous waste [35]. In present study, nanohydroxyapatite/polycaprolactone nanocomposites were prepared by a simple method and lead absorption was investigated by nanohydroxyapatite and n-HA/PCL nanocomposites from aqueous solution.

MATERIALS AND METHODS

Materials

All chemicals were analytical grade. Ultra-pure water was used for the preparation of all reagents solutions. Lead acetate salt and poly (ε-caprolactone) (with 1400 g/mol molecular weight) were purchased from Merck company (Germany), and nanohydroxyapatite bought from Pardis Pajoohesh Fanavaran-e Yazd company (Iran).

Methods

PCL nanocomposites were prepared by solution casting method with different percentages of nanohydroxyapatite. PCL and n-HA were dissolved in chloroform solvent and kept stirring for 30 min at 40 ˚C. The n-HA powder was placed to ultrasonic bath for better dispersion. Then nanohydroxyapatite solution was added to PCL solution under magnetic stirring system. The final solution was transferred to the plate and allowed to dry. PCL nanocomposites with 5 and 10 percentage of n-HA were shown better results for lead absorption.

Characterization

The crystalline structure of sample was investigated by X-ray diffraction utilizing Cu Kα X-ray radiation with a voltage of 40 kV and a current of 30 mA by X’pert pro diffractometer (X’ Pert Pro model, Panalytical, Peru). Field emission scanning electron microscope was employed to observe morphology and size (Sigma VP model, ZEISS, Germany). The surface area was determined using nitrogen gas sorption by n-HA samples at 298 K and 0.88 atmosphere pressure (BElSORP Mini model, Microtrac Bel Corp, Japan). Lead absorption was evaluated by UV–Vis spectroscopy (GENESYS 30 model, Thermo Scientific, America).

RESULTS AND DISCUSSION

XRD

The XRD pattern of samples was measured in 2θ range 5-80° that used to identify the crystalline structure (Fig. 1). X-ray diffraction patterns were showed characteristic peaks between 2θ range from 30 to 40° according to the previous reports [36, 37]. The crystal structural of n-HA has been preserved after lead absorption. The XRD of PCL approved two characteristic peaks of the crystalline structure according to the previous report [38]. Due to the high percentage of polymer in the nanocomposite, characteristic peaks of nanohydroxyapatite were not observed.

FE-SEM

The FE-SEM images were shown for n-HA before and after lead adsorption and n-HA/PCL nanocomposite (Fig. 2). According to the results, nanohydroxyapatite had needle-like morphology and the particle size was estimated to be 50 (before lead adsorption) and 80 nm (after lead adsorption). Lead absorption was caused the increase of particle size due to phenomenon of swelling. The FE-SEM of n-HA/PCL nanocomposite was shown in the form of image from the cross section. This result presented for the first time.

BET

The Brunauer–Emmett–Teller (BET) analysis was used for determination of surface area of nanohydroxyapatite by N₂ adsorption before and after lead absorption (Fig 3). Based on the results, the surface area of n-HA decreased with lead adsorption from 89.966 m²/gr to near-zero. In fact, lead approximately filled up the pores and there is not any residual porosity.

UV–Vis spectroscopy

The lead absorption was investigated by UV–Vis spectroscopy. The calibration curve of lead was examined at λ_max = 208 nm with concentration of 5, 10, 20, 30, and 40 ppm (Fig 4. a). The adsorption diagram was investigated at different n-HA amounts including 0.1, 0.25, and o.5 g with a constant concentration of 250 ppm of lead at different times (Fig 4. B). Based on the results, increase of sorbent amount was resulted to increase of adsorption due to increase of surface area.

The absorption was evaluated in different lead concentrations including 50, 80, 100, 150, and 250 ppm by n-HA and n-HA/PCL nanocomposite at different times (Fig. 5). In general, the increase of concentration was resulted to increase of lead adsorption. The nanocomposites were examined with two concentrations of lead including 100 and 250 ppm. Based on nanocomposite results, the increase of percentage had higher effect on the increase of lead adsorption than the increase of concentration. Also, high concentrations were resulted to increase of inhibition and decrease of absorption.

Lead adsorption was studied in various pH of solution including acidic (pH=2), neutral, and basic (pH=10) for 0.25 g n-HA with 250 ppm of lead concentration and 10% nanocomposite with 100 ppm of lead concentration (Fig. 6). The higher pH was caused more surface active sites, decrease of competition between positive charges, and increase lead adsorption through the electrostatic force of attraction. The result is according to the previous reports [5, 37].

Lead adsorption was studied in different temperature including 25 (ambient), 40, 60, and 80 °C for 0.25 g n-HA with 250 ppm of lead concentration and 10% nanocomposite with 100 ppm of lead concentration (Fig. 6). Based on the result, the increase of temperature was resulted to increase of lead adsorption because of kinetic energy and Brownian motion. According to the previous report, temperature is directly related to the potential of lead adsorption by sorbent [37].

CONCLUSIONS

In this research, n-HA and n-HA/PCL nanocomposite were used for lead adsorption. The effect of different parameter including pH and temperature of solution, amount and concentration of sorbent was shown on lead adsorption by n-HA and its nanocomposite. The results presented that n-HA and n-HA/PCL nanocomposite can represent an economical source of lead sorbent from aqueous solution to develop environmental applications. The future prospects can be developed great application of this nanocomposites to pharmaceutical and medical.

CONFLICT OF INTERST

The authors report no conflict of interest.