Removal of Cu (II) and Pb (II) from Aqueous Solution using engineered Iron Oxide Nanoparticles

Abstract

Nano-sized Fe₃O₄ and Fe₂O₃ were synthesized using a precipitation method. The nanomaterials were tested as adsorbents for the removal of both Cu²⁺ and Pb²⁺ ions. The nanomaterials were characterized using X-ray powder diffraction to determine both the phase and the average grain size of the synthesized nanomaterials. Batch pH studies were performed to determine the optimum binding pH for both the Cu²⁺ and Pb²⁺ to the synthesized nanomaterials. The optimum binding was observed to occur at pH 4 and above. Time dependency studies for Cu²⁺ and Pb²⁺ showed the binding occurred within the first five minutes of contact and remained constant up to 2 hours of contact. Isotherm studies were utilized to determine the binding capacity of each of the nanomaterials for Cu²⁺ and Pb²⁺. The binding capacity of Fe₃O₄ with Cu²⁺ and Pb²⁺ were 37.04 mg/g and 166.67 mg/g, respectively. The binding capacities of the Fe₂O₃ nanomaterials with Cu²⁺ and Pb²⁺ were determined to be 19.61 mg/g and 47.62 mg/g, respectively. In addition, interference studies showed no significant reduction in the binding of either Cu²⁺ or Pb²⁺ to the Fe₃O₄ or Fe₂O₃ nanomaterials in the presence of solutions containing the individual ions Na⁺, K⁺, Mg²⁺ and Ca²⁺ or a solution consisting of a combination of all the aforementioned cations in one solution.

1. Introduction

Contamination of potable water is a major global environmental and human heath concern as more and more countries become industrialized. Both copper and lead are of interest due to numerous health concerns associated with exposure to these heavy metals. At low concentrations, lead has been shown to affect the nervous and renal systems; lead has been shown to negatively affect the synthesis of heme (1,2). Exposure to high levels of copper has been associated with liver damage nausea, abdominal pain, diarrhea, and vomiting (3). Many common industrial processes such as battery manufacturing, battery recycling, paint manufacturing, metal smelting, oil refining, mining, electroplating, and the manufacturing of alloys; produce large amounts of copper and lead contaminated waste. Disposal of copper and lead contaminated wastes have been shown to contaminate natural clean water (4, 5). Heavy metal contamination is of a concern due to the difficulty of the remediation of heavy metals from soils (1).

Various methods have been studied for the removal of heavy metal ions, which include: ion exchange, biosorption, activated carbon sorption, solvent extraction, precipitation, coagulation, and adsorption (6, 7). Although these methods have been shown to be effective for the remediation of heavy metals, many of the aforementioned methods are either costly or complicated to implement. Adsorption, on the other hand, is relatively inexpensive and easy to implement. The use of nanometer-sized adsorbents has the advantage of higher surface area for increased reactivity (8) Recent studies have shown that nano-sized Fe₃O₄ and Al₂O₃ were effective for the removal of lead, with Fe₃O₄ having a higher binding capacity than the Al₂O₃ (9). A second study showed that Fe₃O₄ had a binding capacity 10 times higher for copper and lead than a CuO nanomaterial (10). The use of Fe₃O₄ nanomaterials have the added benefit of superparamagnetism and can be easily removed using a magnet, thus the removal of contaminates can be achieved using magnets/magnetic fields.

In this study nano-sized Fe₃O₄ and Fe₂O₃ were synthesized using a titration/precipitation method. The method involved the titration of solutions of iron(III) chloride or iron (II) chloride with sodium hydroxide. The resulting nanoparticles were characterized using X-ray diffraction to determine their crystal structure and average grain size. The nanomaterials were then investigated for their potential as adsorbents for Cu²⁺ and Pb²⁺ in solution. Batch studies were performed to determine the optimum binding pH, equilibration time, and binding capacity. The binding capacity studies were performed using isotherm studies and found to follow the Langmuir isotherm. In addition, the effects of K⁺, Na⁺, Ca²⁺, and Mg²⁺ ions on the binding of both copper and lead were also investigated at the optimal binding pH.

2. Experimental

2.1. Synthesis of iron nanomaterials

Fe₃O₄ and Fe₂O₃ was synthesized via a slow titration/precipitation method. Fe₃O₄ was synthesized using a 1.0 L 30 mM solution of iron (II) chloride, which was titrated drop wise with 100 mL of 1 M sodium hydroxide. The 30 mM iron (II) chloride was prepared by dissolving 30 mmol (5.964 g) of iron (II) chloride in 1.0 L of 18 MΩ Millipore water. The Fe₂O₃ nanomaterial was synthesized using the same method a 1.0 L 30 mmol of iron (III) chloride was titrated using 100 mL of 1 M sodium hydroxide. The 30 mM FeCl₃ solution was prepared by dissolving (8.109 g) of iron (III) chloride in 1 L of 18 MΩ Millipore water. The 1 M sodium hydroxide was prepared by dissolving 40.0 g of sodium hydroxide in 18 MΩ Millipore water. The titration of the iron(II) and iron(III) solutions was performed at rate of 1 mL min⁻¹ under vigorous stirring. Subsequent to the addition of the NaOH to the iron solutions the samples were boiled for 1 hr under vigorous stirring. The solutions were then cooled to room temperature, centrifuged at 3500 rpm for 5 minutes, decanted, suspended again in 18.0 MΩ deionized water centrifuged. The nanomaterials were washed three times using 18.0 MΩ deionized water followed by centrifugation to remove any by-products and unreacted starting material. The resulting nanoparticles were oven dried overnight at 70°C.

2.2. Characterization of iron nanomaterials

The synthesized Fe₃O₄ and Fe₂O₃ nanomaterials were characterized using a Rigaku Miniflex XRD equipped with a scintillation detector. The samples scans were collected from 20° to 60° (2θ) with a count time of 1 s and a step of 0.01° in 2θ. The resulting diffraction patterns were then fitted using the Le Bail fitting procedure and crystal structure data from the literature (11, 12). The Average grain size of the nanoparticles was determined using Scherrer’s equation and the full width at half maximum of three independent diffraction peaks.

2.3. pH study

Stock solutions of either Cu²⁺ or Pb²⁺ were prepared at concentrations of 300 ppb. The pH of the stock solutions were adjusted to 2,3,4,5, 6 using either dilute nitric acid or dilute sodium hydroxide. From the pH adjusted stock solutions 4 mL aliquots of the solution was then placed into 5 mL tubes, which were preloaded with 10 mg of either Fe₃O₄ or Fe₂O₃ nanomaterials. In addition, control reactions were also maintained for all reactions, which consisted of 4 mL aliquots of the Cu²⁺ or Pb²⁺ solutions without the nanomaterials. All samples and control reactions were performed in triplicate for statistical purposes. The tubes were capped and placed on a speci-mix rocker and equilibrated for 1 hr. After equilibration the tubes were centrifuged for 5 min at 3000 rpm, decanted, and saved for analysis. Metal analysis was performed using a PerkinElmer AAnalyst 800 atomic absorption spectrometer operated in graphite furnace mode. All calibration curves obtained for analysis had correlation coefficients (R²) of 0.99 or better.

2.4. Time Dependency Studies

Samples consisting of 10mg of either Fe₃O₄ or Fe₂O₃ were weighted out and placed in 5 mL tubes and 4 mL aliquots of pH adjusted (pH 4) 300 ppb solution of either Cu²⁺ or 300 ppb Pb²⁺ were added to the tube. The tubes were then capped and equilibrated for the following times: 5,10, 15, 20, 30, 60, 120, and 240 min. Control samples consisting of either the Cu²⁺ or Pb²⁺ ions, without the iron oxide nanoparticles were maintained and treated the same as the reaction samples. All samples and controls were performed in triplicate for statistical purposes. After equilibration the test tubes were centrifuged for 5 min at 3000 rpm, decanted, and the supernatants were saved for analysis. A PerkinElmer AAnalyst 800 atomic spectrometer was used to analyze the samples. Cu²⁺ and Pb²⁺ concentrations were determined using calibration curves with correlation coefficients (R²) of 0.99 or better.

2.5. Adsorption isotherm

Isotherm studies were performed under similar conditions as the time dependency studies, with the exception of a fixed 1 hr equilibration time. Solutions of Cu²⁺ and Pb²⁺ at the following concentrations: 5, 10, 50, 100, and 1000 ppm were adjusted to a pH 4. A 4.0 mL aliquot of the pH adjusted Cu²⁺ or Pb²⁺ solution was added to 5 mL test tubes, containing either 10 mg of Fe₃O₄ or 10 mg of Fe₂O₃. The tubes were capped, equilibrated for 1 hr, and centrifuged for 5 min at 3000 rpm. In addition, control samples consisting of either the Cu²⁺ or Pb²⁺ solutions were maintained through the equilibration time and treated the same as the samples. The reaction sample and control samples were prepared in triplicate for statistical purposes. Subsequent to centrifuging the supernatants were saved for analysis with a PerkinElmer ICP-OES Optima 8300. Calibration curves with correlations coefficients (R²) values of 0.99 or better were obtained for all analyses.

2.6. Binding interference

The effects of potential interference of secondary cations Na⁺, K⁺, Mg²⁺, and Ca²⁺ to Cu²⁺ and Pb²⁺ binding onto Fe₃O₄ and Fe₂O₃ was determined by preparing solutions of 0.3, 3, 30, 300, and 3000 ppm of Na⁺, K⁺, Mg²⁺, and Ca²⁺ spiked with 300 ppb of either Cu²⁺ or Pb²⁺. In addition, a mixture of all four cations was prepared with each cation at the aforementioned concentrations in the presence of 300 ppb of either Cu²⁺ or Pb²⁺. All solutions were pH adjusted to 4.0 and 4.0 mL aliquots were added to tubes containing 10 mg of either Fe₃O₄ or Fe₂O₃. Control solutions were also prepared consisting of the mixture of cations without the nanomaterials present. The test tubes were capped, equilibrated for 1 hr, centrifuged for 5 min at 3000 rpm, decanted, and the supernatants were saved for analysis. Analysis was performed using a PerkinElmer ICP-OES Optima 8300 using calibration curves with correlations coefficients (R²) values of 0.99 or better.

3. Results

3.1. X-ray Diffraction of synthesized iron oxides

The XRD patterns for the Fe₃O₄ and Fe₂O₃ synthesized are presented in Fig 1. Fig. 1A shows the XRD pattern for Fe₃O₄ contains the 220, 311, 400, 422, and 511 Miller indices, indicating that the magnetite phase was obtained (13). The observed Miller indices correspond to the diffraction peaks observed at the following angles 30.11, 35.44, 37.11, 43.11, 47.11, 53.44, and 57.00 in 2θ. Fig. 1B shows the XRD pattern obtained for Fe₂O₃ synthesis which, contains the 111, 040, 113, 201, 202, 212, 214, and 006 Miller indices corresponding to ε-Fe₂O₃ (14). The diffractions peaks were observed at the following angles 21.46, 33.58, 35.07, 36.91, 40.38, 41.59, 53.49, and 59.44 in 2θ. The fittings of the diffraction patterns were performed using the Le Bail fitting procedure in the FullProf suite and crystallographic data from the literature (10, 11).

Fig. 1.

A. Powder X-ray diffraction pattern of synthesized Fe₃O₄. B. Powder X-ray diffraction pattern of synthesized Fe₂O₃.

The average grain size of each of the nanomaterials was calculated using Scherrer’s equation and the full width half maximum (FWHM) of three independent diffraction peaks. Scherrer’s equation is provided below:

$$d=\frac{0.9\lambda }{\beta cos\frac{2\theta }{2}}$$

Where d is the nanomaterials diameter, λ is the wavelength of the copper x-ray source (1.54 Å), β is FWHM of the peak, and 2θ/2 is the diffraction angle. Using the Scherrer’s method the average grain size of the synthesized Fe₃O₄ nanomaterial was 19.9 nm ± 0.2 nm, whereas the synthesized Fe₂O₃ material had an average grain size of 12.9 nm ± 1.4 nm. Luther et al., as well as Parsons et al., have observed similar results for iron oxide nanomaterials synthesized using this precipitation technique (15–17). These authors have shown nanomaterials synthesized through the precipitation technique have average grain sizes of 17.0 nm (Fe₃O₄), 12 nm (Fe₂O₃), 28 nm (Fe₃O₄),, and 19 nm (Fe₃O₄).

3.2. pH binding studies

The effects of pH on the binding of copper and lead to Fe₃O₄ and Fe₂O₃ are shown in Figure 2. Figure 2A shows the binding of copper and lead to Fe₃O₄ nanomaterial. As can be seen in Figure 2A at pH values below 3, both Cu²⁺ and Pb²⁺ binding to the Fe₃O₄ nanomaterial is very low, less than 15% and 5%, respectively. However, at pH 3 and above the binding increased to greater than 90% for Pb²⁺. Cu²⁺ binding to the nanomaterials was observed to increase with increasing pH. However, the maximum binding for the Cu²⁺ was achieved at pH 4 and above. Figure 2B shows the binding of Cu²⁺ and Pb²⁺ to Fe₂O₃ from pH 2 through pH 6. Cu²⁺ showed little to no binding at pH 2 and an increase to approximately 80% binding at pH 3 and above. Similarly, observed Pb²⁺ binding to Fe₂O₃ was low at pH 2, below 20%, which increased to 80% at pH 3 and remained relatively constant through pH 5. However, the Pb²⁺ binding was observed to decreased slightly to approximately 65% at pH 6. The reduction in binding may have been due to Pb(OH)_X species formation, which have been observed to start forming at pH 5 and above (18). The formation of the hydroxide species observed for Pb²⁺ in solution may inhibit to adsorption of the Pb²⁺ ions to the surface of the nanomaterials. Similar studies have observed comparable results for the removal of lead (10, 19). However, Mahdavi et al, studying the removal of heavy metal ions using Fe₃O₄, ZnO and CuO obtained less than 20% removal of copper up to pH 6, with a drastic increase to nearly 100% at pH 7. Katsumata et al. found effective removal of both copper and lead (over 90%) with Fe₃O₄ (20). Yu et al. observed optimal binding of copper and lead at pH 4 using sawdust as the adsorbent (21). Machida et al showed the adsorption of Pb²⁺ from aqueous solution using carbon based materials occurred optimally at pH 5.5 (18). The selection of pH 5.5 as the optimal binding pH was due to the speciation of the Pb²⁺ ions in solution. At pH 5.5 and below the majority of lead ions in solution are present as the single hydrated lead ions (Pb²⁺) (18). Above pH 5.5 hydrated Pb species have been found to exist in solution, which may change the binding properties of the ions (18).

Fig. 2.

A. pH profile for the binding of copper and lead to Fe₃O₄, pH 2 to pH 6. B. pH profile for the binding of copper and lead to Fe₂O₃, pH 2 to pH 6.

3.3. Time Dependency Studies

The results from time dependency studies are shown in Fig. 3. Fig. 3A shows the binding of Cu²⁺ and Pb²⁺ to the Fe₃O₄ nanomaterials, as can be seen in Fig. 3A the binding was independent of time. The Cu²⁺ solution concentration was reduced by approximately 90% within the first 5 minutes of contact and remained constant up to 120 minutes of contact time. However, a slight increase (approximately 10%) in the Cu²⁺ binding was observed at the 240 minutes time interval. Similarly the Pb²⁺ binding remained relatively constant at approximately 85% from the initial 5 minute contact time to 240 minute contact time. Fig. 3B shows the binding of both Cu²⁺ and Pb²⁺ to the Fe₂O₃ nanomaterials. As can be seen in Figure 3B the Cu²⁺ binding remained below 70% for the first 15 min of contact time with Fe₂O₃ and maximum binding was observed at 20 min, with little to no change up to the 240 minute contact time. Similarly, Pb²⁺ ions showed a small increase in binding percentage binding at the 20 minute contact time. Initially the Pb²⁺ removal was observed to be approximately 70% at 5 min and was observed to increase to approximately 90% at 20 min and remaining constant through to the 240 min of contact time. The results obtained in this study are significantly higher than previously conducted studies using Fe₃O₄. Mahdavi et al., found between 40%–50% lead removal through 100 min of contact time, this value increase to 60% lead removal at 120 min (10). Similarly, copper removal remained approximately 40% through 120 min of contact time. A second study, achieved 35% after 2 h of contact with Fe₃O₄, with only a slight increase to 40% after 5 h of contact (8). The observed differences in the percentage binding may be due to many different factors, primarily the concentrations at which the reactions were performed. In the present study 0.3 ppm of either Cu²⁺ or Pb²⁺ was used for primary investigations. However, many of the comparable studies with respect to pH are in the ppm concentration ranges (18, 22–24). The observed increase in the binding of metal ions from solution with increasing time has been observed in the literature. Ions such as gold(III) show this phenomenon and it is generally related to the reduction of the gold(III) ion to gold(0). The reduction of gold ions in solution using many different biomasses has been highlighted in the literature (25,26). The reduction of silver(I) ions in solution binding to biomass has also been shown using Basil and Cinnamon zeylanicum bark(27,28). In addition, it has also been shown in the literature that magnetite (Fe₃O₄) has the capability to reduce copper(II) ions as well as Vanadium(V), and chromate ions (29). The observed increases in the adsorption of both the Cu²⁺ and Pb²⁺ ions at the 20 minute contact time may be due to a change at the surface after complexation of the ions. Alternatively, on the surface of the Fe₂O₃ residual protons may be blocking the binding of the metal ions, once the protons are removed then the binding of the Cu²⁺ and Pb²⁺ may occur.

Fig. 3.

A. Effects of contact time on the binding of copper and lead to Fe₃O₄. B. Effects of contact time on the binding of copper and lead to Fe₂O₃.

3.4. Isotherm binding studies

The binding capacities for both the Fe₃O₄ and Fe₂O₃ with Cu²⁺ and Pb²⁺ are shown in Table 1. The capacities in Table 1, for both the Cu²⁺ and Pb²⁺ ions are for 1 h of contact with each of the respective nanomaterials at the optimum binding pH. The binding of both Cu²⁺ and Pb²⁺ to the Fe₃O₄ and Fe₂O₃ nanomaterials was found to follow the Langmuir Isotherm model. The isotherm data were plotted were fitted using the Langmuir Isotherm equation in linear form, which resulted in linear plots with R² values of 0.99 or better. The linear form of the Langmuir equation is shown below:

$$\frac{1}{q_e}=\left(\frac{1}{Q_{0}b}\ast \frac{1}{C_e}\right)+\frac{1}{Q_0}$$

Table 1.

Binding capacities for copper and lead to Fe₃O₄ and Fe₂O₃ after 1 h of contact time at room temperature.

	Fe3O4		Fe2O3
Copper	37.04 mg/g	0.59 mmol/g	19.61 mg/g	0.31 mmol/g
Lead	166.67 mg/g	0.80 mmol/g	47.62 mg/g	0.23 mmol/g

Where q_e is the equilibrium concentration of the metal (copper or lead), Q_o is the binding capacity of the nanomaterial (Fe₃O₄ or Fe₂O₃), b is a constant, and C_e is the initial metal concentration. By plotting 1/q_e vs 1/C_e and employing the linearized Langmuir equation the intercept of the line can be used to calculate the binding capacity of the adsorbent. The Langmuir adsorption model for lead removal using Fe₃O₄ was also observed in a study conducted by (5). Studies conducted using amino functionalized Fe₃O₄ (30), biomass coated Fe₃O₄ (31) and γ-AlOOH @ SiO₂/Fe₃O₄ (32) similarly, observed Langmuir adsorption for both copper and lead. In the present study the binding of copper and lead to both the Fe₃O₄ and Fe₂O₃ was found to be fitted best using the Langmuir adsorption model. The Langmuir adsorption model suggests single layer of metal ions adsorbing to the surface of the nanomaterial.

Table 1 shows that both copper and lead have high binding capacity for both of the adsorbents tested. Lead was determined to have a binding capacity of 166.67 mg/g for Fe₃O₄ and 45.45 mg/g for Fe₂O₃. These results are significantly higher than Fe₃O₄ binding capacities reported by others (8,19). Additional studies obtained Fe₃O₄ capacities only slightly lower, 101.4 mg/g and 111.1 mg/g, than the capacities obtained in the present study (10, 33). The results obtained for lead in this study were also compared binding capacities for lead binding to similar other metal oxides. Recillas et al., found high uptake of lead with TiO₂ was 159 mg/g and CeO₂ bound 189 mg/g, Mahdavi et al., achieved 119.1 mg/g lead binding with ZnO nanoparticles, but only 14.2 mg/g binding with CuO nanoparticles (8,10). Further comparing the Fe₃O₄ nanoparticles synthesized in the present study showed higher binding capacities than those observed in biosorbents such as sawdust, neem bark, and macrophyte biosorbent materials (33, 35). Studies that utilized Fe₃O₄ functionalized with various materials showed mixed results with binding capacities ranging from 64.50 mg/g to 598.8 mg/g (5, 33, 36). Using an Iron based material, Castaldi et al. showed that iron based water treatment residuals had adsorption capacities of 0.194 and 0.106 mmol/g or 40.20 and 6.73 mg/g for Pb²⁺ and Cu²⁺ ions at pH 4.5 (37). In the same manuscript Castaldi et al. showed that an Al based water treatment residual has capacities of 0.073 mmol/g and 0.065 mmol/g, or 15.2 and 4.13 mg/g, for Pb²⁺ and Cu²⁺, respectively (37).

Binding capacities of 37.04 mg/g and 19.31 mg/g were observed for copper to Fe₃O₄ and Fe₂O₃, respectively. Recent studies in the literature have shown variable results, ranging from very low capacities of 6 mg/g to high capacities of 52 mg/g(10, 38–40). Higher copper removal has been reported with the use of ZnO and CuO as well as Fe₃O₄ functionalized with polymers and polymeric beads (10,31, 38–40). However, these binding capacities are not specifically for the nanomaterial under investigation, but a combination of the nanomaterial and the ligand used to functionalize the nanomaterial. The use of macrophyte biomaterial obtained varying removal capacities ranging from 19.7 mg/g to 40.8 mg/g, with the removal dependent on the species of macrophyte used in the study. The variation of binding capacities observed for copper and lead to various materials suggest that binding capacity is highly dependent on particle size and experimental conditions.

3.5. Effects of coexisting cations

The effect of naturally occurring interfering cations ions Na⁺, K⁺, Ca²⁺, or Mg²⁺ on the binding of copper and lead to Fe₃O₄ are shown in Fig. 5. As can be seen in Fig 5A–D the presence of these metal ions had no significant effect on the binding of either copper or lead to the Fe₃O₄ nanomaterial. However, a small decrease in copper binding was observed in the presence of Mg²⁺ at 3000 ppm. A study conducted by Dong et al., found that no interference occurred with the binding of lead to hydroxyapatite coated Fe₃O₄ in the presence of the same ions (33). A second study showed that Ca²⁺ ions had no observable effect on the binding of lead to Fe₃O₄ at a Ca²⁺ concentration of 100 ppm (19). It was also observed in the study by Nassar et al. that the addition of Co²⁺, Ni⁺, and Cd²⁺ ions at 100 ppm had no significant effect on the binding of lead to Fe₃O₄. Fig. 6 shows the effects of the same metal ions on the binding of copper and lead to synthesized Fe₂O₃ nanomaterial. Fig. 6A and 6B also show that both Na⁺ and K⁺ ions had no significant effect on the binding of copper to the Fe₂O₃ nanomateiral. However, the Pb²⁺ binding to Fe₂O₃ was observed to decrease to approximately 70% in the presence of Na⁺ at 0.3 ppm. The binding of the Pb²⁺ ions increased continually after the initial decrease with increasing Na⁺ concentration, reaching 95% removed at a Na⁺ concentration of 3000 ppm. This initial binding decrease in the presence of low concentrations of interfering ions has been observed by other authors. The binding of Cu²⁺ (Fig. 6C) was found to decrease at high concentrations of Mg²⁺ ions, whereas lead binding is unaffected up to 3000 ppm. Copper binding decreased to 85% removal at 0.3 ppm Ca²⁺ and 88.5% removal at 3 ppm Ca²⁺. After this initial decrease, binding increased to greater than 95% removal at concentrations 30 ppm and above. Lead binding decreased to 90% at 0.3 ppm Ca²⁺, then increased to complete removal at concentrations of 3 ppm and above.

Fig. 5.

A. Interference study of copper and lead binding to Fe₃O₄ in the presence of increasing concentrations of Na⁺ ions. B. Interference study of copper and lead binding to Fe₃O₄ in the presence of increasing concentrations of K⁺ ions. C. Interference study of copper and lead binding to Fe₃O₄ in the presence of increasing concentrations of Mg²⁺ ions. D. Interference study of copper and lead binding to Fe₃O₄ in the presence of increasing concentrations of Ca²⁺ ions.

Fig. 6.

A. Interference study of copper and lead binding to Fe₂O₃ in the presence of increasing concentrations of Na⁺ ions. B. Interference study of copper and lead binding to Fe₂O₃ in the presence of increasing concentrations of K⁺ ions. C. Interference study of copper and lead binding to Fe₂O₃ in the presence of increasing concentrations of Mg²⁺ ions. D. Interference study of copper and lead binding to Fe₂O₃ in the presence of increasing concentrations of Ca²⁺ ions.

The effects of multiple coexisting alkali and alkali earth (Na⁺, K⁺, Ca²⁺, and Mg²⁺) are presented in Fig. 7. Fig. 7A shows that the presence of all of the coexisting ions has little to no effect on the binding of copper or lead to Fe₃O₄. Fig. 7B shows similarly that the presence of these ions do not affect the binding of copper or lead to Fe₂O₃. The effects of Na⁺, K⁺, Ca²⁺, or Mg²⁺ ions on the removal of lead were similarly observed using biosorbents (41). Deng et al., found that the presences of these ion individually had little effect on the removal of lead using Cladophora fascicularis green algae (41). Similar results were obtained for binding copper using the using Cladophora fascicularis algae biosorbent (42). There appears to be an antagonistic effect between some of the cations in solution and Pb²⁺ or Cu²⁺ especially at the low concentrations of both ions. However, the data indicates that the Cu²⁺ and Pb²⁺ ions are preferentially bound from aqueous solution, due to the high binding of Cu²⁺ and Pb²⁺ from solution in the presence of extremely high concentrations of the interfering cations. The combined interference study the molar ratio of the ions to the Cu²⁺ was 86147:1 and the Pb²⁺ was 282184:1. Even in these extremely high amounts of cations present in solution the Cu²⁺ and Pb²⁺ binding was still above 90%, which indicates a strong affinity of both the Cu²⁺ and Pb²⁺ ions to the nanomaterials.

Fig. 7.

A. Interference study of copper and lead binding to Fe₃O₄ in the presence of increasing concentrations of Na⁺, K⁺, Mg²⁺, and Ca²⁺. B. Interference study of copper and lead binding to Fe₂O₃ in the presence of increasing concentrations of Na⁺, K⁺, Mg²⁺, and Ca²⁺.

4. Conclusion

Optimal binding conditions for copper and lead to two synthesized nanomaterials, Fe₃O₄ and Fe₂O₃, were explored. Binding was found to be pH dependent, with pH 4 chosen as the optimal binding pH. Copper and lead binding was also found to be time independent, with high removal occurring within the first 5 min of contact with the Fe₃O₄ nanomaterial and within the first 20 min of contact with Fe₂O₃ nanomaterial. Nanomaterial binding was tested in the presence of competing alkali metals and alkaline earth metals for any possible interference. The presence of Na⁺, K⁺, Mg²⁺, and Ca²⁺ ions showed no significant effect on the binding of either Cu²⁺ or Pb²⁺ to either Fe₃O₄ or Fe₂O₃. The data indicates that nano-sized iron oxides can provide high removal of copper and lead without the need of additional modifications to the surface with organic ligands or chelating agents.

Fig. 4.

A. Binding affinity of copper or lead to Fe₃O₄ in the presence of increasing concentrations of copper and lead. B. Binding affinity of copper or lead to Fe₂O₃ in the presence of increasing concentrations of copper and lead.

Highlights.

• Non-stabilized iron oxide nanoparticles were synthesized from 12–20 nm in size

• Effects of pH, time, and interfering ions were investigated for low level Cu²⁺ and Pb²⁺ removal

• Pb²⁺ had higher binding capacity to the Fe₃O₄ nanomaterial

• Cu²⁺ had higher binding capacity and Fe₂O₃ nanomaterial

• Both Cu2+ and Pb2+ binding followed the Langmuir isotherm model.