Zinc and cadmium adsorption from wastewater using hydroxyapatite synthesized from flue gas desulfurization waste

Heavy metals such as cadmium, cobalt, zinc, lead, copper, chromium, iron, mercury and arsenic can cause serious health problems and damage the environment, when their concentrations pass the permissible limits (Naushad et al. 2015; Jamshaid et al. 2017; Saleh et al. 2019; Hasanpour & Hatami 2020; Palash et al. 2020). With rapid development of industrialization, increasing levels of heavy metals such as zinc and cadmium are discharged to the environment constituting significant danger to the living (Liu et al. 2018). Zinc ions are discharged from the fabric, wood, metal coating, mining, ceramic, battery production, drug, sun blocks and deodorant industries and cadmium ions are discharged by the metallurgy, machinery, mining and electroplating industries to wastewater (Ngabura et al. 2018; Vardhan et al. 2019; Kumar & Pakshirajan 2021). Respiratory, renal, skeletal and cardiovascular system damage and lung, kidney, prostate and stomach cancers appear in children, because of the cadmium toxicity. Zinc is necessary for physiological and metabolic activities of many organisms, but high amounts of zinc can be toxic to them (Kinuthia et al. 2020; Palash et al. 2020). The US EPA's regulatory cadmium limit is 0.005 mg/L and zinc limit is 5 mg/L in drinking water. The World Health Organization recommended safe limit for cadmium is 0.003 mg/L and for zinc is 3 mg/L in wastewater (Javed & Usmani 2013; Ngabura et al. 2018; W.H.O. 2018; Kinuthia et al. 2020). Consequently, these heavy metals must be removed from wastewater to preserve human health and the environment. A large number of chemical or physical technologies have been employed, like coagulation, ion exchange, osmosis, photocatalysis, phytoremediation, membrane separation, reverse osmosis, electro floatation, adsorption etc., for waste water remediation (Ihsanullah et al. 2015; Jamshaid et al. 2017; Sharma & Naushad 2020). Adsorption has been acknowledged as the most economical removal method of zinc and cadmium from wastewater in literature (Yan et al. 2014; Chen et al. 2020). Hydroxyapatite (HAP) and its composites are utilized as a significant adsorbent for adsorption of many pollutants, like heavy metals, from wastewater (Pai et al. 2020). HAP, Ca₁₀(PO₄)₆(OH)₂, is one of the most biocompatible inorganic materials used in the human body, and it has shown good binding capacity with metallic ions from wastewater (Ibrahim et al. 2020; Jiang et al. 2020). HAP can be synthesized using chemical precursors like calcium and phosphorus, using various techniques including, dry, wet and high-temperature methods. Scientists tried to evolve cost-effective calcium sources, instead of using expensive reagents, in order to reduce HAP costs (Liu et al. 2018; Mohd Pu'ad et al. 2020). Flue gas desulfurization (FGD) gypsum is an industrial by-product produced during the FGD process in coal-fired power plants. Its major composition is CaSO₄.2H₂O, so it is an ideal calcium source. A significant amount of FGD waste is discharged directly, which occupies an extensive quantity of land resources and causes high levels of environmental pollution. Therefore, finding a suitable process to change waste FGD to valuable products can be an economical solution to this problem. The precipitation and adsorption yield of FGD waste is high, and it could be used for environmental purposes. Nowadays, phosphate has been used for adsorption and immobilization of heavy metal from water and soil as a cost-effective and environmentally friendly technology. The FGD-HAP exhibited a high efficiency in the removal of aqueous heavy metals. Yan et al. (2014) studied Pb⁺² and Cd⁺² removal from wastewater using HAP synthesized from FGD waste, and Liu et al. (2018) used FGD-HAP to immobilize Pb⁺² and Cu⁺² in aqueous solution and soil (Yan et al. 2014, 2020; Liu et al. 2018; Koralegedara et al. 2019; Li et al. 2019).

In this study, FGD waste fabricated from two different coal power plants was converted to HAP by hydrothermal method. The aim of this study is to use synthesized FGD-HAP as an alternative low-cost adsorbent to remove zinc and cadmium from wastewater. FGD-HAP synthesis conditions (waste type and H₃PO₄/CaSO₄ molar ratio) were determined and the effects of experimental conditions (adsorbent dose, zinc and cadmium concentration, and contact time) on the adsorption performance were examined. FGD waste and synthesized FGD-HAP were characterized by X-Ray Diffraction (XRD), Fourier Transformed Infrared Spectroscopy (FT-IR), Scanning Electron Microscope (SEM) and Brunauer-Emmett-Teller (BET) instruments. The zinc and cadmium concentration was measured by Inductively coupled plasma atomic emission spectroscopy (ICP-AES). Adsorption yields were calculated between 98.53% and 100%. The adsorption data were well explained by a second-order kinetic model and the Freundlich isotherm model fits the equilibrium data. The adsorption results demonstrated that FGD waste is an effective source to synthesize HAP, which is used as an adsorbent for zinc and cadmium removal from wastewater due to high adsorption capacity. The major distinction of this study from previous studies is the use of FGD-HAP synthesized using FGD waste generated by two different coal power plants as an alternative cost-effective adsorbent to remove zinc and cadmium from wastewater, also contributing to environmental preservation and the economy. FGD waste pollution can be eliminated and FGD waste can be changed into valuable product.

FGD waste used in this study was supplied from Orhaneli (Bursa) and Cayırhan (Ankara) coal-fired power plants in Turkey. The chemical reagents, phosphoric acid (85%) and ammonia solution (25%), used for HAP synthesis, and Zn(NO₃)₂.6H₂O (98%) and Cd(NO₃)₂.4H₂O (99%), used for synthetic wastewater preparation, were purchased from Merck (Darmstadt, Germany).

After determining the optimum mole ratio, experiments were performed at various temperatures (20 °C, 30 °C and 40 °C) to identify the temperature effect on the FDG-HAP synthesis. The appropriate time for complete reaction was decided by repeating the experiment at various time intervals (1, 2 and 4 h). The produced FDG-HAP was filtered and was dried at 80 °C for 12 h. The dried product was milled and the product powder was stored in low-density polyethylene bags at room temperature (Mousa & Hanna 2013; Yan et al. 2014; Koralegedara et al. 2019).

The crystalline phases of the FGD waste and synthesized FGD-HAP were analyzed by PANalytical Xpert Pro XRD (PANalytical B.V., Almelo, The Netherlands) at 45 kV and 40 mA, using X-rays produced with Cu-Kα tube. The FT-IR spectra was investigated using a PerkinElmer Spectrum One FT-IR spectrometer (Waltham, MA, USA), equipped with a universal attenuation total reflectance sampling accessory, having spectral range between 4,000 and 650 cm⁻¹. The surface properties and morphology of the synthesized FGD-HAP were examined with an Apollo 300 field-emission SEM (CamScan, Oxford, UK) equipped with a back-scattering electron detector at 15 kV, and 1000× and 5000× magnification was set. The BET surface areas of FGD-HAP adsorbents were measured on a Micromeritics ASAP 2020 instrument using N₂ adsorption after degassing the adsorbent at 300 °C for 3 h. The concentration of zinc and cadmium ions in synthetic wastewater was measured by PerkinElmer Optima 2100 DV ICP-OES equipped with an AS-93 autosampler (PerkinElmer, CT, USA).

Here, q_e is the amount of zinc and cadmium adsorbed per gram of adsorbent (mg g⁻¹), C_o is the initial zinc and cadmium concentration (mg/L), C_e is the concentration of zinc and cadmium that remained unadsorbed in the solution (mg/L), V is the volume of zinc and cadmium solution (L), and M is the amount of adsorbent (g).

XRD patterns of Cayırhan and Orhaneli FGD wastes are shown in Figure 1. According to the XRD results, Cayırhan FGD waste was identified as a mixture of bassanite (pdf. no: 00-033-0310; CaSO₄.1/2H₂O) and calcite (pdf. no: 00-001-0837; CaCO₃), and Orhaneli FGD waste was identified as a mixture of bassanite (pdf. no: 00-033-0310; CaSO₄.1/2H₂O) and gypsum (pdf. no: 01-074-1433; CaSO₄.2H₂O). The high ratio of calcium in both compounds found in the structure indicates that they can be used in the synthesis of HAP. The peaks observed in the XRD pattern at 15°, 30°, 40° and 50° are characteristic peaks of HAP (Hokkanen et al. 2018).

XRD patterns of synthesized FGD-HAP were measured at different H₃PO₄/FGD mole ratios, reaction temperatures and reaction times. The XRD score of a compound can be defined by the similarity of the peak intensities (%) and locations of the phase to the pdf card pattern of the reference mineral. A continuous increase was observed until the mole ratio was equal to 4; when the mole ratio was equal to 4.79 the XRD scores decreased. This situation indicated that increasing H₃PO₄/FGD mole ratio in reaction medium contributed to the FGD-HAP formation until the mole ratio was equal to 4, and with the decreasing H₃PO₄/FGD mole ratio, XRD scores of samples were increased dramatically. Due to the highest XRD scores for both wastes, the H₃PO₄/FGD mole ratio of 4:1 was selected as optimum (Figure 2). Table 1 shows XRD scores of synthesized FGD-HAP, where only pure HAP was observed. This confirms that pure HAP was synthesized successfully. As a result of the preliminary experiments, it has been observed that temperature and time did not affect the FDG-HAP synthesis efficiency (Mousa & Hanna 2013; Zhang et al. 2016; Sari et al. 2017).

The FT-IR spectra of Cayırhan and Orhaneli FGD wastes are shown in Figure 3(a). The peaks seen at 3,608–3,554 cm⁻¹ (Cayırhan) and 3,607–3,553 cm⁻¹ (Orhaneli) were related to the O-H stretching and H₂O bending vibration of basanite. The band at 1,617 cm⁻¹ could be ascribed to the H-OH bonding in water. The peaks observed between 1,007 and 1,141 cm⁻¹ belong to SO₄²⁻ vibration bands. The band observed at 874 cm⁻¹ was assigned to the CO₃ group. The peak seen at 658 cm⁻¹ is the characteristic peak of CaSO₄ (Kang et al. 2019). Figure 3(b) represented the FT-IR spectrum of synthesized FGD-HAP. The peak at 3,429 cm⁻¹ (Orhaneli FGD-HAP) was assigned to hydroxyl groups. The peaks at 3,152, 1,648 (Cayırhan FGD-HAP) and 1,638 cm⁻¹ (Orhaneli FGD-HAP) can be attributed to the adsorbed water. The peaks at 563, 602, 1,035 (Cayırhan FGD-HAP), 1,030 (Orhaneli FGD-HAP) and 1,100 cm⁻¹ corresponded to the stretching vibration of the phosphate groups. The peaks observed at 1,401 and 867 cm⁻¹ correspond to the carbonate groups, demonstrating carbonate partially substituted for the phosphate while FGD-HAP is subjected to atmosphere. The 1,401 cm⁻¹ peak may also be due to atmospheric carbon dioxide from the air environment in the preparation phase. The signals of synthesized FGD-HAP coincided to a great extent with the major absorbance signals of HAP (El Asri et al. 2010; Salah et al. 2014; Yan et al. 2014; El-Zahhar & Awwad 2016; Zou et al. 2019; Jiang et al. 2020).

The surface morphology of FGD-HAP particles synthesized from Cayırhan and Orhaneli FGD wastes was examined by SEM at 1000× and 5000× magnification. The SEM images obtained are given in Figure 4. The synthesized FGD-HAP surface, has a medium grain size porous surface structure which provides a suitable area for zinc and cadmium adsorption from wastewater. The reason for this morphology is the presence of the CO₃ group and the high calcium content, which limit the growth of FGD-HAP crystals. The small particle size increases the specific surface area and the porous structure allows adsorption not only on the surface but also between the pores. As a result, synthesized FGD-HAP has high adsorption properties (El-Zahhar & Awwad 2016; Deb et al. 2019; Liu et al. 2021). The BET surface area of Cayırhan FGD-HAP was 85.224 m²/g and Orhaneli FGD-HAP was 82.652 m²/g. The high surface area of FGD-HAP has proved that it can be used in zinc and cadmium adsorption process.

Heavy metals like cadmium, zinc, lead, nickel, copper, mercury and chromium are primary pollutants in industrial wastewaters, causing a serious risk to public health and the environment when they exceed the permissible limits (Saleh et al. 2019; Turkmen Koc et al. 2020). The use of FGD-HAP as an adsorbent for zinc and cadmium was realized by conversion of waste FGD to HAP with hydrothermal method. Then, 1 or 2 g/L of synthesized FGD-HAP adsorbent was mixed with a stock solution. The stock solutions were prepared from standard zinc and cadmium solutions with concentrations of 30, 40 and 50 mg/L. For the adsorption experiments, 50 mL of treatment solution was used with 500 rpm stirring speed, at room temperature (22 ± 0.5 °C), for 1, 2, 3 or 4 h. The solution was separated from the adsorbent using filter paper at the end of the adsorption experiments. Experiments were repeated three times. The concentration of zinc and cadmium was measured by ICP-OES. The results of analyses are given in Figure 5. The maximum zinc and cadmium removal efficiency of the FGD-HAPs was calculated between 98.53% and 100%. The zinc adsorption capacity of Cayırhan FGD-HAP was higher than Orhaneli FGD-HAP, but on the contrary, the cadmium adsorption capacity of Cayırhan FGD-HAP was lower than Orhaneli FGD-HAP. The adsorption efficiency increases when the amount of adsorbent increases. This increase could be explained by an increased number of active sites for zinc and cadmium adsorption on the HAP surface (Ivanets et al. 2019; Long et al. 2019; Jiang et al. 2020).

A comparison of maximum monolayer adsorption capacity of zinc and cadmium of various adsorbents is shown in Table 2. The maximum monolayer adsorption capacity of FGD-HAP is higher than the other adsorbents shown in Table 2.

The results in Table 3 indicate that the Freundlich isotherms fits better with the experimental data than Langmuir, proposing a better R² value. This result proves the heterogeneous and porous nature of the FGD-HAP adsorbents. The adsorption in this study is a series process of multilayer adsorption. The value of n greater than 1 (n > 1) in the Freundlich model indicates that the conditions were favorable and the FGD-HAP is an encouraging adsorbent to remove zinc and cadmium from wastewater. These results confirmed that Cayırhan FGD-HAP is more effective for zinc and cadmium adsorption.

In this study, low-cost adsorbent synthesized using FGD waste generated by Cayırhan and Orhaneli coal power plants was used for zinc and cadmium adsorption from wastewater, FGD waste pollution was eliminated and FGD waste was changed into valuable product. FGD waste and synthesized FGD-HAP were characterized by XRD, FT-IR, SEM and BET devices. The results are summarized as follows:

• 1.

  Optimum H₃PO₄/FGD mole ratio was selected 4:1 because of the high XRD scores.

• 2.

  It was seen from FT-IR spectra of FGD-HAP that HAP was synthesized succesfully from FGD waste.

• 3.

  It was seen from SEM images of the synthesized FGD-HAP that the adsorbent surface has a medium grain size porous surface structure providing a suitable area for zinc and cadmium adsorption from wastewater. The BET analysis results also support this outcome.

• 4.

  Either 1 or 2 g/L of synthesized FGD-HAP adsorbent was mixed with a stock solution prepared by dissolving 30, 40 or 50 mg/L zinc or cadmium solutions for 1, 2, 3 or 4 h. The zinc and cadmium concentrations were determined by ICP-AES. The maximum removal efficiency of the FGD-HAPs was calculated between 98.53% and 100%. The zinc adsorption capacity of Cayırhan FGD-HAP was higher than Orhaneli FGD-HAP, but on the contrary, the cadmium adsorption capacity of Cayırhan FGD-HAP was lower than Orhaneli FGD-HAP. The adsorption efficiency increases when the amount of adsorbent increases.

• 5.

  Kinetic studies showed that the second-order kinetic model explains the adsorption process with a high correlation coefficient. The Freundlich isotherm model gives the best result to the equilibrium experimental data.

This result proves the heterogeneous and porous nature of the FGD-HAP confirming that FGD-HAP, is a successful adsorbent for removal of zinc and cadmium from wastewater and Cayırhan FGD-HAP is more effective for zinc and cadmium adsorption. Based on the results of the present work, FGDs waste can be used as an effective source to produce FGD-HAP, which is used as an economical adsorbent for the treatment of wastewaters containing zinc and cadmium metal ions because of its excellent adsorption performance. Using the produced adsorbent in industrial wastewater for adsorption of different heavy metals can be recommended for future research. Besides, in this study, the adsorption capacity of FGD-HAP is studied from a single-component solution for zinc and cadmium. FGD-HAP adsorption capacity studies for multi-metal solutions must be done in future studies.

This study was supported by project 2015-07-01-YL04 of Yıldız Technical University Scientific Research Projects Coordinator.

All relevant data are included in the paper or its Supplementary Information.