Remarkable phosphate removal from saline solution by using a novel trimetallic oxide nanocomposite

Phosphorus (P), as an indispensable element, holds a vital nourishing role in the growth of all living organisms (Shan et al. 2020). However, their excessive presence in the environment originating from industries, agricultural, and domestic wastewater sources could accelerate the overgrowth of undesirable algae and aquatic plants. This algae bloom leads to the eutrophication phenomenon, an important issue that damages the biodiversity and functionality of the aquatic ecosystem (Wu et al. 2017; Li et al. 2018; Ma et al. 2020). Therefore, controlling the amount of phosphorus is of great importance.

High amounts of salt are used in oil, textile, and leather industries. During their processes, a large amount of saline wastewater is produced, which finally enters surface waters. Moreover, high amounts of NaCl and Na₂SO₄ salts exist naturally in free waters as well (Stewart 2008). According to the results of various studies, the phosphorous removal efficiency decreases significantly with an increase in the amount of salinity in the solution (Lefebvre & Moletta 2006). Therefore, it is of high significance to find a method that functions well under this condition.

Thus far, conventional approaches such as chemical precipitation and biological treatment are incapable of removing low concentrations of phosphate; therefore, other methods are required so as to overcome the mentioned problems (Wu et al. 2017). Due to the simplicity in design, the effectiveness even at low concentrations of phosphorus, and the recycling capability, the adsorption method has been widely accepted as a preferable approach for phosphate removal (Wu et al. 2017; Yin et al. 2018; Dong et al. 2020). Since conventional adsorbents show low selectivity for phosphate adsorption in the presence of other competing anions (such as sulfate, chloride, and bicarbonate), the identification and development of adsorbents with high selectivity for this nutrient are crucial (Acelas et al. 2015). Recently, therefore, nano-based adsorbents have been developed due to their ease of use, low cost, and contamination removal capability from water and wastewater. So far, various nanocomposites have been used for phosphate adsorption. For example, Nie et al. (2019) achieved 44.14 mg P/g adsorption capacity by using Ti–NS nanocomposite at pH 6.5–7.5, nanocomposite dosage of 500 mg/L, initial phosphate concentration of 10 mg/L, and at 25 °C. In Zhou et al.’s (2018) study, the adsorption capacity of 37.86 mg P/g was reported by the use of nHFZO@I402 nanocomposite at pH 7, nanocomposite dosage of 750 mg/L, phosphate concentration of 5 mg/L, and at 25 °C. In another study, Wu et al. (2017) achieved the adsorption capacity of 83.5 mg P/g by using La(OH)₃/Fe₃O₄ nanocomposite at pH 4.3–6, nanocomposite dosage of 100 mg/L, initial phosphate concentration of 0.5–15 mg/L, and at 23 °C. In Nodeh et al.’s (2017) research, the adsorption capacity of 116.28 mg P/g was achieved by using MG@La nanocomposite at pH 6, nanocomposite dosage of 100 mg/L, initial phosphate concentration of 50 mg/L, and at 25 °C.

Moreover, the recyclability of an adsorbent is of great importance to evaluate its performance for practical applications (Hu et al. 2016). Nanoparticles are difficult to separate after usage in wastewater treatment, and commonly used adsorbent recycling systems suffer from several disadvantages. While separation methods such as centrifuge consume a lot of energy, and filtration is susceptible to clogging, magnetic separation is a faster and more effective method (Wu et al. 2017; Cai et al. 2019). Accordingly, the combination of magnetic nanoparticles (such as Fe₃O₄) with other nanomaterials makes the separation and recycling of the synthesized adsorbent more facile.

Earlier research conducted by Nezhadheydari et al. (2019) showed that Fe₃O₄/ZnO nanocomposite has a phosphate removal efficiency of 50% in 12 h, requiring modification. For this purpose, CuO nanoparticles were used because of their cost-efficiency, simple synthesis, and the fact that the combination of ZnO and CuO creates environmentally friendly nanocomposites. Also, through the formation of inner-sphere and/or outer-sphere complexes, CuO nanoparticles show a strong ligand sorption (of ⁠) (Mahdavi & Akhzari 2016).

However, to the best of our knowledge, there are no reports available on the application of Fe₃O₄/ZnO/CuO for phosphate removal. In addition, the functionality of this trimetallic oxide nanocomposite for phosphate adsorption remains unclear and requires more investigations.

Motivated by these concepts, the main objectives of this study are to (1) fabricate and characterize a novel recyclable trimetallic oxide nanocomposite for phosphate removal, (2) improve the phosphate sorption efficiency and the capacity of the trimetallic oxide adsorbent, (3) accelerate phosphate removal by modifying Fe₃O₄/ZnO, and (4) investigate the kinetic studies of phosphate removal in the presence and absence of salinity by Fe₃O₄/ZnO/CuO. This study proposed a novel adsorbent as a promising candidate for effective phosphate removal from saline solutions.

In this study, all chemicals were of analytical grade and used without further purification. Iron(III) chloride hexahydrate (FeCl₃·6H₂O), zinc acetate dihydrate (Zn(CH₃COO)₂2H₂O), ethylene glycol, polyethylene glycol, sodium acetate (C₂H₃NaO₂), copper(II) acetate dihydrate (Cu(CO₂CH₃)₂ · 2H₂O), sodium hydroxide (NaOH), ethanol and distilled water were used for synthesizing the nanocomposites. KH₂PO₄ was used as a pollutant, and NaCl and Na₂SO₄ were used for investigating the effect of salinity on phosphate adsorption. Molybdovanadate reagent was also used for analyzing the concentration of phosphate in the solution, all of which were purchased from Merck.

The equipment used in this research was Hach DR4000U spectrophotometer, Metrohm 691 pH meter, Korea Tech DSA-series ultrasonic bath, Shafaq magnetic stirrer (made in Iran), Fara Azma vacuum oven (made in Iran), 101 Sigma centrifuge, Stainless Teflon autoclave (Made in Iran), Tescan MIRA3 (for FE-SEM and EDX analysis), XRD Philips X'pert XPD, and Frontier FT-IR (made in the USA).

To synthesize the nanoadsorbents, the following steps were taken for each of the nanocomposites.

Fe₃O₄ and ZnO nanoparticles were synthesized by co-precipitation and sol-gel methods, respectively (Fu & Zhu 2016; Hasnidawani et al. 2016). To prepare the nanoparticles, 1 g of FeCl₃·6H₂O was poured into 40 ml of ethylene glycol and stirred to obtain a clear solution. Then, 4.3 g of sodium acetate and 1 g of polyethylene glycol were added to the solution, stirred at room temperature for 30 min, and put in an autoclave for 8 h at 200 °C. Then, it was washed several times with ethanol and dried at 60 °C in the vacuum oven. Then, 0.5 g of the dried compound was dispersed in 100 ml of ethanol. Two solutions were prepared: one containing 2 g of Zn(CH₃COO)₂·2H₂O in 15 ml of distilled water and the other containing 8 g of sodium hydroxide in 10 ml of distilled water; both of which were stirred at room temperature for 5 min. The resulting solution was added drop-wise to a mixture of Fe₃O₄ and ethanol. The final solution was kept at room temperature for 1 h to allow all sediments to settle. Then, it was centrifuged several times with ethanol and was dried at room temperature.

CuO nanoparticles were synthesized using the hydrolysis method (Tran & Nguyen 2014). To prepare these nanoparticles, 0.6 g of copper acetate (II) was dissolved in 50 ml of distilled water on a stirrer at 100 °C, and 0.01 g of NaOH was added. Then, 0.1 g of the synthesized Fe₃O₄/ZnO nanocomposite was dispersed in 25 ml of distilled water, added to the copper solution, and stirred for 1 h at 80 °C. After hydrolysis, the precipitate in the solution was washed several times with distilled water and ethanol, and then the resulting substance was dried at 200 °C for 2 h.

Phosphate adsorption from the synthesized wastewater using Fe₃O₄/ZnO and Fe₃O₄/ZnO/CuO nanocomposites was investigated by the one-factor-at-a-time (OFAT) method. The values of the examined parameters are presented in Table 1.

After obtaining the optimum conditions for phosphate removal, phosphate adsorption tests were conducted in the presence of NaCl and Na₂SO₄ salts, which are the predominant salts in both industrial wastewater and free waters (Stewart 2008). Then, the amount of adsorption efficiency was determined.

To determine pH at the point of zero charge (pH_pzc) for each nanocomposite, the following approach was used. First, 0.01 M NaCl solution was added to sealed vials, and the initial pH of each vial was adjusted to the values between 2 and 10 using pH regulator solution. 30 mg of each nanocomposite was added to vials with different initial pH and shaken for 48 h at room temperature. Using a pH meter, the final pH of each solution was determined. The initial and final graphs were plotted, and pH_pzc is the interception point of the pH_final vs. pH_initial curve and the pH_initial = pH_final line (Órfão et al. 2006).

To determine the characteristics of the synthesized nanocomposites, FE-SEM, EDX, FT-IR, and XRD analyses were performed, the results of which are presented in Figures 1–4.

To investigate the morphology and dimensions of Fe₃O₄/ZnO and Fe₃O₄/ZnO/CuO, FE-SEM images of both fabricated nanocomposites are presented in Figure 1. Figure 1(a) shows the morphology of Fe₃O₄/ZnO, which has a rod-shaped structure, and the dimension of this nanocomposite varies from 20 to 30 nm. In Hong et al.’s (2008) and Xia et al.’s (2011) studies, the average dimension of Fe₃O₄/ZnO nanocomposite was in the ranges of 25–30 and 20–30 nm, respectively. As can be seen in Figure 1(b), Fe₃O₄/ZnO/CuO has a spherical structure, and the dimension of this nanocomposite varies in the range of 25–36 nm.

X-ray diffraction analysis was performed to identify the chemical composition of the synthesized samples. X-ray diffraction spectra for Fe₃O₄/ZnO and Fe₃O₄/ZnO/CuO are shown in Figure 2(a) and 2(b), respectively. The constituent materials of the two synthesized nanocomposites are presented in Table 2. Based on results, Fe₃O₄/ZnO has O, Fe, and Zn elements, and the trimetallic oxide adsorbent contains O, Fe, Zn, and Cu. According to Figure 2(a) and 2(b), broad peaks of Zn, O, Fe, and Cu elements indicate a strong bond between Fe₃O₄ and ZnO nanoparticles in Fe₃O₄/ZnO and between Fe₃O₄, ZnO, and CuO nanoparticles in the Fe₃O₄/ZnO/CuO nanocomposite (Farrokhi et al. 2014; Tju et al. 2017).

FT-IR was used to qualify the chemical bonds between the functional groups of Fe₃O₄/ZnO and Fe₃O₄/ZnO/CuO nanocomposites. The synthesized samples were subjected to wavenumbers in the range of 400–4,000 cm⁻¹, the results of which are presented in Figure 3. According to Figure 3(a), the adsorption in 400 and 590 cm⁻¹ are associated with stretching bands of Zn–O and Fe–O, respectively. Moreover, the broad peak around 1,700–3,400 cm⁻¹ is related to the stretching vibrations of O–H bond in water molecules adsorbed on the surface of Fe₃O₄/ZnO nanocomposite. As shown in Figure 3(b), the adsorption in 400, 420, and 590 cm⁻¹ are related to stretching bands of Zn–O, Cu–O, and Fe–O, respectively. Besides, the wide peak in 1,700–3,400 cm⁻¹ is due to the stretching vibrations of O–H bond in water molecules adsorbed on the surface of Fe₃O₄/ZnO/CuO (Xie et al. 2015; Kulkarni et al. 2017; Tju et al. 2017). The agreement of XRD and EDX analyses indicates the successful formation of fabricated nanocomposites (Taufik & Saleh 2017).

XRD patterns were generated using Cu Kα radiation (40 kV, 40 mA) to identify the crystalline components of both adsorbents. All XRD patterns were obtained over a 2ϴ range of 5–80°. Figure 4(a) and 4(b) illustrates XRD patterns of Fe₃O₄/ZnO and Fe₃O₄/ZnO/CuO nanoadsorbents, respectively. Fe₃O₄ nanoparticles have diffraction angles at about 2ϴ = 30, 35, 43, 53, 57, 62, 66, 71, 74, 75, and 79, ZnO nanoparticles at 2ϴ = 32, 34, 36, 47, 56, 63, 66, 68, and 69 (Farrokhi et al. 2014; Vakili Tajareh et al. 2019), and CuO particles have diffraction angles at 2ϴ = 32, 35, 38, 48, 53, 58, 61, 65, 66, and 67 (Taufik et al. 2016).

To determine the optimum values of parameters (the initial pH of the solution, the concentration of nanocomposite, and the initial concentration of phosphate), required examinations were conducted. Then, the optimum conditions for each of the desired parameters were determined. The effect of different dosages of salinity and adsorption kinetics under optimum conditions were also investigated.

Since the optimum pH value of the trimetallic oxide nanocomposite (pH = 4) is higher than that of Fe₃O₄/ZnO (pH = 3), the amount of material needed to reduce pH will decrease. Moreover, Fe₃O₄/ZnO/CuO has an efficiency of 96.13% at pH = 4, with an increase of nearly 16% compared to Fe₃O₄/ZnO with a removal efficiency of 80.46% at pH = 3. Furthermore, at pH in the wide range of 3–6, the trimetallic oxide adsorbent is highly efficient. These results show a significant improvement in the quality of Fe₃O₄/ZnO/CuO and its more economical use.

To investigate the effect of the adsorbents dosage, different amounts of them were tested, the results of which are presented in Figure 6. As can be observed in Figure 6(a), at first, the adsorption efficiency rises with an increase in the concentration of both nanocomposites and decreases slightly after reaching the maximum value. This trend can be ascribed to an increase in adsorbent dosage, which leads to the lack of access to the unsaturated active sites in the adsorbents (Farrokhi et al. 2014; Salehi & Hosseinifard 2020). Also, when the adsorbent dosage exceeds 100 mg/L, the particles become agglomerated, which culminates in a decrease in surface area, and thus a reduction in the adsorption efficiency (Keramati & Ayati 2019). Another important point is that the value of adsorption capacity increased while using a solution that contains 100 mg/L of the trimetallic oxide nanocomposite (156.35 mg/g) in comparison with the same amount of Fe₃O₄/ZnO (130.76 mg P/g). This increment can be attributed to the CuO nanoparticles. Since these particles exhibit a strong ligand sorption (of ⁠) through the formation of inner-sphere and/or outer-sphere complexes, the phosphate adsorption efficiency and capacity increase significantly (Mahdavi & Akhzari 2016).

To determine the optimum phosphate concentration, different amounts of this contaminant were examined, the results of which are presented in Figure 7. Based on the results, by increasing the initial concentration of phosphate, the adsorption rate raised at first and then decreased. This trend could be related to the presence of sufficient active sites in the adsorbent compared to the initial concentration of the contaminant. By increasing the concentration to 50 mg/L, the adsorption rate for Fe₃O₄/ZnO and Fe₃O₄/ZnO/CuO nanocomposites reached 80.46% and 96.31%, respectively. Then the adsorption efficiency, in concentrations of more than 50 mg/L, decreased due to the reduction in the number of active sites in comparison to the dosage of contaminant (Hallaji et al. 2015; Nakarmi et al. 2020). In several studies, increasing the initial phosphate concentration has been associated with a decrease in the phosphate removal efficiency (Bozorgpour et al. 2016; Nodeh et al. 2017). Besides, by comparing the graphs in Figure 7(a), it can be concluded that the trimetallic oxide nanocomposite can maintain its high adsorption efficiency even at a low concentration of phosphate (10 mg/L) in comparison to Fe₃O₄/ZnO.

Phosphate adsorption for both nanocomposites was tested in 1 h. Considering the graphs illustrated in Figures 5(b), 6(b), and 7(b), the equilibrium time for Fe₃O₄/ZnO and Fe₃O₄/ZnO/CuO nanocomposites was attained in 30 and 15 min, respectively. This noticeable decrease in the equilibrium time indicates a significant improvement of the trimetallic oxide adsorbent in comparison to Fe₃O₄/ZnO due to the reduction in the consumed energy during the process.

To investigate the ability of the synthesized nanocomposites in phosphate removal in the presence of salinity, the influence of different anions was investigated. The effect of chloride and sulfate anions, as a predominant ion that generally coexists with phosphate in both industrial wastewater and free waters, was examined. These ions were tested at concentrations of 10, 20, 30, 40, 50, and 60 mg/L, and their results are presented in Figure 8(a) and 8(b) for Fe₃O₄/ZnO and Fe₃O₄/ZnO/CuO nanocomposites, respectively. As can be seen, the effect of both salts on the phosphate removal efficiency was almost constant. The presence of both ions had an interfering effect on the phosphate adsorption and led to a decrease in the adsorption efficiency. The investigated anions' chemical attraction to the adsorption sites can be explained by the Hofmeister order, i.e., (Luo et al. 2020). However, salinity had less effect on the trimetallic oxide adsorbent in comparison to Fe₃O₄/ZnO. Also, Fe₃O₄/ZnO/CuO had a higher adsorption capability even at high salinity concentrations (60 mg/L) (less than 16% decrease in efficiency). On the other hand, Fe₃O₄/ZnO had a significant efficiency reduction in the presence of salinity (more than 36%). Moreover, Fe₃O₄/ZnO/CuO achieved higher efficiency in less time, indicating its higher ability for phosphate removal. According to the results, the phosphate removal efficiency in the presence of the mentioned concentrations of NaCl salinity using Fe₃O₄/ZnO was 70.95, 66.12, 56.31, 57.4, 48.6, and 39.81%, respectively, while this efficiency for trimetallic oxide nanocomposite was 90.74, 89.32, 88.02, 85.62, 83.45, and 80.37%, respectively. The phosphate removal efficiency in the presence of the mentioned concentrations of Na₂SO₄ salinity while using Fe₃O₄/ZnO was 76.4, 75.3, 55.3, 53.9, 49.5, and 43.72%, respectively, and this efficiency using Fe₃O₄/ZnO/CuO was 90.13, 88.48, 87.23, 86.33, 83.69, and 81.05%, respectively. The decrease in the contaminant removal efficiency in the presence of salinity can be due to the competition that occurs between ions. Similar results have been reported in several studies. In Hong et al.’s (2017) study, Fe₃O₄@SiO₂ nanocomposite was used in the presence of anion, which reduced the phosphate removal efficiency by 29%. In a study by Hu et al. (2020), Zr@MCS nanoadsorbent was used for phosphate removal in the presence of anion, which reduced the adsorption efficiency by more than 15%.

To evaluate the rate of phosphate adsorption on the surface of the nanocomposites, pseudo-first-order and pseudo-second-order kinetic models were applied to the adsorption data. Their results under two situations – the presence and absence of salinity – are presented in Table 3. The pseudo-second-order kinetics has a higher correlation coefficient (R²) than the pseudo-first-order (closer to 1) in the absence of salinity in the system. So, it can be concluded that phosphate adsorption kinetics follows the pseudo-second-order model. Different types of adsorbents had similar results (Bozorgpour et al. 2016; Nodeh et al. 2017; Wu et al. 2017; Zhou et al. 2018; Hao et al. 2019; He et al. 2020). According to the results, the rate constant in the kinetic equation for Fe₃O₄/ZnO and Fe₃O₄/ZnO/CuO is 0.001 and 0.0015, respectively. The 50% increase in the adsorption rate while using Fe₃O₄/ZnO/CuO indicates its higher capability in phosphate adsorption. Moreover, the Fe₃O₄/ZnO/CuO maximum adsorption capacity (156.35 mg P/g) is approximately 16.37% higher than that of Fe₃O₄/ZnO (130.76 mg P/g).

Adsorption kinetics was also applied on phosphate removal in the presence of salinity (NaCl and Na₂SO₄ salts with a concentration of 60 mg/L) in the system. The concentration of 60 mg/L was chosen due to its substantial effect on the reduction of phosphate removal efficiency, the results of which are presented in Table 3. By comparing the values of the correlation coefficient (R²), it can be seen that the phosphate adsorption in the presence of salinity follows the pseudo-second-order model.

According to Table 3, the adsorption rate by using Fe₃O₄/ZnO and Fe₃O₄/ZnO/CuO nanocomposites in the presence of NaCl in the system (K₂ = 0.0007 and K₂ = 0.0013, respectively) is approximately 28 and 30%, respectively. This rate is higher than the condition that Na₂SO₄ exists in the system (K₂ = 0.0005 and K₂ = 0.0009, respectively), which shows that the interfering effect of NaCl on the adsorption efficiency is slightly less than Na₂SO₄. According to the results, Fe₃O₄/ZnO/CuO had an adsorption rate of about 45% higher than Fe₃O₄/ZnO. Also, using Fe₃O₄/ZnO and Fe₃O₄/ZnO/CuO adsorbents in the presence of NaCl in the system reduced the reaction rate by 30 and 13%, respectively, and by 50 and 40% in the presence of Na₂SO₄, respectively, compared to the absence of salinity in the system. Generally, the results indicate the better quality and performance of the modified trimetallic oxide nanocomposite.

To investigate the reusability of both nanocomposites, the sorption–desorption experiments were repeated for 10 cycles. The bimetallic and trimetallic oxide nanocomposites were dried at room temperature and then regenerated by 1 mol/L NaOH solution with a contact time of 20 min. This desorption reagent could efficiently regenerate both of the adsorbents due to the tendency to replace OH⁻ particles with particles. The NaOH-regenerated nanocomposites were reused for phosphate removal under optimum conditions. As illustrated in Figure 9, the results showed that with the use of Fe₃O₄/ZnO, the phosphate removal efficiency had a 2–3% decrease in each cycle and after the 10th cycle, the efficiency declined up to 20%. On the other hand, by using the modified trimetallic oxide nanocomposite, the phosphate adsorption efficiency had less than 1% decrease after each cycle, while, in the 10th cycle, it declined to less than 10%.

In this study, two magnetically separable Fe₃O₄/ZnO and modified Fe₃O₄/ZnO/CuO nanocomposites were synthesized and characterized. Then, their ability to adsorb phosphate and the effect of salinity on their performance were investigated and compared. According to the results, the trimetallic oxide nanocomposite had a high capacity to adsorb phosphate in the absence and particularly in the presence of salinity, while Fe₃O₄/ZnO showed less resistance to salinity and had a lower phosphate removal efficiency. Results demonstrated about 50% increase in phosphate adsorption rate by using Fe₃O₄/ZnO/CuO and its significant improvement in comparison to Fe₃O₄/ZnO, due to the decrease in reaction time and energy, and the increase in the phosphate removal efficiency. Regeneration and reusability studies showed that the trimetallic oxide nanocomposite had a notable improvement in comparison to Fe₃O₄/ZnO due to its negligible adsorption efficiency reduction after each cycle. Moreover, the pseudo-second-order kinetic model perfectly described phosphate removal for both nanocomposites in the presence and absence of salinity.

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