Removal of Pb(II) from aqueous solution by a zeolite–nanoscale zero-valent iron composite

TL;DR: In this article, a composite of zeolite and nanoscale zero-valent iron (Z-nZVI) overcomes these problems and shows good potential to remove Pb from water.

About: This article is published in Chemical Engineering Journal.The article was published on 2013-02-01 and is currently open access. It has received 272 citations till now. The article focuses on the topics: Zerovalent iron & Aqueous solution.

Summary

1. Introduction

• Heavy metals are problematic for ecosystems because of their toxicity and most heavy metals can be highly toxic even at very low concentrations.
• Pb is commonly used in several industries and in some locations large amounts of wastewaters containing high concentrations of Pb ions have been released.
• Tests with solution containing 1000 mg Pb(II)/L suggested that the capacity of the Z–nZVI is about 806 mg Pb(II)/g. Energy-dispersive X-ray spectroscopy showed the presence of Fe in the composite; X-ray diffraction confirmed formation and immobilization of Fe0 and subsequent sorption and reduction of some of the Pb(II) to Pb0.
• Alternatively, nanoscale zero-valent iron (nZVI) has shown good potential to remove metals and other aqueous pollutants.
• Zeolites have proven effective for environmental applications such as in PRBs for controlling the spread of cation-contaminated groundwater [18].

2.1. Materials and chemicals

• Naturally occurring zeolite was obtained from Alfa Aesar, A Johnson Matthey Co., Seoul, South Korea.
• After drying at 80 °C overnight, the zeolite was ground and sieved with a 100 mesh screen before use.
• Ethylenediaminetetraacetic acid (EDTA; DAE JUNG, Siheung, Korea) was >99% pure.
• Nanopure water (conductivity = 18 μΩ/m, TOC < 3 μg/L; Barnstead, Waltham, MA, USA) was used to prepare all reagents.
• A Pb stock solution was prepared by dissolving 1.60 g Pb(NO3)2 in 100 mL of degassed water and working concentrations were prepared by diluting the stock solution.

2.2. Preparation of the composite

• The Z–nZVI composite was prepared according to Wang et al. [21].
• The mixture was treated with ultrasound for 10 min, and then stirred vigorously at ambient temperature for 30 min.
• The reduction reaction is as follows: Fe2+ + 2BH4 – + 6H2O → Fe0 + 2B(OH)3 + 7H2 ↑ (1) After incubation, the black solids were separated from the solution using a vacuum filtration flask (0.45 μm membrane filter), washed several times with degassed water to remove residual sulfate, then vacuum-dried.

2.3. Characterization of the composite

• Field emission scanning electron microscopy (FE-SEM; Hitachi S-4700, Tokyo, Japan) was used to view the morphology and surface characteristics of the nZVI and zeolite.
• The characteristics of the Z–nZVI composite were obtained using biological transmission electron microscopy (Bio-TEM; Hitachi H-7650, Tokyo, Japan) and energy-dispersive X-ray spectra (EDS) were obtained using FE-SEM.
• Surface areas of the zeolite, nZVI, and Z–nZVI composite were measured by N2 adsorption using a Micromeritics ASAP (Accelerated Surface Area and Porosimetry) 2020 analyzer (BELSORP-MINI, BEL Japan, Inc., Osaka, Japan) [6].
• Infrared spectra of the zeolite, nZVI, and Z–nZVI composite powders were obtained in KBr pellets on a Perkin–Elmer Fourier transform infrared (FTIR) spectrophotometer (Irvine, CA, USA) in the diffuse reflectance mode at a resolution of 4 cm−1.

2.4. Pb(II) removal and release

• The initial pH of the solutions was adjusted using 0.1 M HCl or 0.1 M NaOH but was not controlled during the experiments.
• Samples were collected periodically up to 140 min and filtered using a 0.45 μm syringe filter.
• Pb(II) concentration in the filtrate was determined by ICP-AES (Inductively Coupled Plasma, Leeman Labs, Inc., Hudson, NH, USA).
• Zeolite was used as the control for this experiment.
• A sequential extraction procedure was applied to the Pb(II)-loaded Z–nZVI composite to determine Pb(II) availability, following the general procedures of Basta and Gradwohl [23] and Castaldi et al. [24].

2.5. X-ray diffraction

• To determine the nature of the Pb associated with the composite, X-ray diffractograms (XRD) of dried Z–nZVI were obtained after shaking with Pb solution.
• A Cu Kα incident beam (λ = 0.1546 nm) was used, monochromated by a nickel filtering wave at a tube voltage of 40 kV and current of 40 mA (Philips X’Pert Pro MPD, Eindhoven, Netherlands).

3.1. Characterization of the composite

• Typical SEM images of nZVI and zeolite and TEM images of the Z–nZVI composite are shown in Figure 1a–c.
• As previously reported, nZVI particles become aggregated (attributable to van der Waals and magnetic forces) [7, 26] and the aggregation decreases nZVI reactivity and mobility [27].
• Stabilizing supports such as zeolite have been used to prevent aggregation [28].
• EDS further confirmed the presence of Fe in the composite (Figure 1d).
• The FTIR spectrum of the Z–nZVI composite supports nZVI loading onto the zeolite.

3.2. Removal of Pb(II) from water

• Adjusting the initial pH to 4 dissolved the passivating Fe (oxy) hydroxide layer on nZVI surfaces [38]; the solution pH increased to 7.7 during equilibration due to reaction of nZVI with water [39].
• The enhanced effectiveness of the Z–nZVI composite for Pb(II) removal is likely due to its much larger specific surface area than that of zeolite alone.
• The zeolite supporting material prevented aggregation of nZVI, thereby providing more surface area for Pb(II) sorption [31].
• Results are consistent with previous studies reporting adsorption of Pb(II) by kaolinite-supported nZVI, and Cr(VI) and Pb(II) adsorption by resin-supported nZVI [31, 40].

3.2.1. Effect of Pb(II) concentration

• Removal efficiency varied with initial con- centration (Figure 4).
• The decrease in removal efficiency to 80.6% at the higher concentration (1000 mg/L) suggests that the capacity of the Z–nZVI is about 806 mg Pb(II)/g, which was exceeded under the conditions of the experiment, as observed for removal of Pb(II), Cu(II), and Zn(II) by natural zeolite [41] and Cr(VI) ions by a bentonite–nZVI composite [42].

3.2.2. Effect of initial pH

• Solution pH can have a significant influence on the adsorption of heavy metals, due to metal speciation, surface charge, and functional group chemistry of the adsorbent [43].
• The difference in pH would have a minimal effect on the surface charge of zeolite [44].
• Error bars indicate standard deviations of the means; where absent, bars fall within symbols.
• The authors results suggest that rapid diffusion of Pb2+ into the Z–nZVI matrix and adsorption were optimized by adjusting the initial pH to 4, and were followed by reduction to Pb0 by Fe0.
• The more acidic solution pH facilitates these processes through dissolution of the passivating Fe (oxy)hydroxide layer on nZVI surfaces [38].

3.2.3. Effect of temperature

• Temperature is an important factor affecting adsorption and would be generally expected to increase with decreasing temperature due to the exothermicity of cations for an adsorbent surface.
• More efficient removal at higher temperatures is likely due to desolvation of Pb cations [17] and more rapid diffusion into the internal pores of the composite particles.
• Results are consistent with the greater adsorption of Pb(II) on NKF-6 zeolite [17] and Cr(VI) on a bentonite-nZVI composite [42] with increasing temperature.

3.2.4. X-ray diffraction

• XRD patterns of the Z–nZVI composite were recorded before and after shaking with the aqueous solution alone (Figures 7a and b, respectively) or with the Pb solution (Figure 7c).
• Fe(II) adsorbed to the zeolite was likely reduced to Fe0 and immobilized on the surface, as described by Lee et al. [19].
• Peaks 5–10, appearing in Z–nZVI after shaking with aqueous solution, can be attributed to the formation of iron oxides, primarily magnetite (Fe3O4), maghemite, and lepidocrocite from Fe0 oxidation [31].
• The XRD analyses support formation and immobilization of Fe0, as well as sorption and reduction of Pb(II) to Pb0, on the composite.

3.3. Availability of Pb removed by the composite

• The Pb(II)-loaded composite was sequentially shaken with extractant solutions of increasing removal capacity to determine the availability of Pb associated with the composite.
• In contrast, the fraction extracted with EDTA, consid- Figure 6.
• The low quantity of Pb(II) recovered in the water-soluble and Ca(NO3)2-extractable fractions (Table 1) indicates low bioavailability of the Pb removed by the Z–nZVI composite.
• These fractions likely consist of Pb2+ electrostatically adsorbed to external surfaces of the composite [24].
• The large fraction of Pb(II) extracted with EDTA likely consists of more strongly bound Pb(II) and precipitated lead hydroxide complexes on active sites within the zeolite-based matrix of the composite [41, 44].

4. Conclusions

• Zeolite was an effective dispersant and stabilizer of nZVI in a composite support system, reducing aggregation and increasing specific surface area.
• Batch experiments indicated that the Z–nZVI composite was superior to zeolite in removing Pb(II) from aqueous solution.
• XRD confirmed that the composite adsorbed the Pb(II) ions and subsequently reduced some of them to Pb0.
• Further studies are needed to assess the potential of the material to remove other metals and organic pollutants.
• This paper was supported by research funds of Chonbuk National University for 2010 Campus Faculty Exchange Program.

Figures

Figure 3. Removal of Pb(II) by 0.1 g of Z–nZVI compared to zeolite alone after shaking with 100 mL of aqueous solution containing 100 mg Pb(II)/L for 140 min at pH 4 and 35 °C. Error bars indicate standard deviations of the means; where absent, bars fall within symbols.

Figure 2. FT-IR spectra of (a) nZVI, (b) Z–nZVI, and (c) zeolite.

Figure 1. SEM images of (a) nZVI particles and (b) zeolite; TEM image (c) and EDS (d) of the Z–nZVI composite.

Table 1. Release of lead by sequential extraction of 1 g of Pb-containing Z–nZVI composite with 25 mL of H2O, 0.1 M Ca(NO3)2, and 0.1 M EDTA.

Figure 7. XRD patterns of (a) the Z–nZVI; (b) Z–nZVI after shaking with aqueous solution alone; and (c) Z–nZVI after shaking with aqueous solution containing 250 mg Pb(II)/L. 1,4 = SiO2; 2,12 = Fe0; 3,8 = γ-Fe2O3; 5–10, 13 = iron oxides; 11,14 = Pb0.

Frequently asked questions

Q1. What have the authors contributed in "Removal of pb(ii) from aqueous solution by a zeolite–nanoscale zero-valent iron composite" ?

Kim et al. this paper proposed a composite of zeolite and nanoscale zero-valent iron ( Z-nZVI ) to remove heavy metals from water. 

Q2. What have the authors stated for future works in "Removal of pb(ii) from aqueous solution by a zeolite–nanoscale zero-valent iron composite" ?

Further studies are needed to assess the potential of the material to remove other metals and organic pollutants. 

Q3. What is the effect of zeolite on the surface area of a composite?

Zeolite was an effective dispersant and stabilizer of nZVI in a composite support system, reducing aggregation and increasing specific surface area. 

Q4. What was used to view the morphology and surface characteristics of the nZVI and?

Field emission scanning electron microscopy (FE-SEM; Hitachi S-4700, Tokyo, Japan) was used to view the morphology and surface characteristics of the nZVI and zeolite. 

Q5. What is the capacity of zeolite as a reductant?

The capacity of Fe0 as a reductant [20], combined with the properties of zeolite, should promote efficient removal and reduction of Pb(II) to Pb0. 

Q6. What was the fraction extracted with Ca(NO3)2?

The fraction extractable with Ca(NO3)2 comprised exchangeable Pb, which was about 2.3% of the Pb initially removed by the Z–nZVI composite. 

Q7. What is the effect of pH on the removal of heavy metals?

Solution pH can have a significant influence on the adsorption of heavy metals, due to metal speciation, surface charge, and functional group chemistry of the adsorbent [43]. 

Q8. What is the common use of zeolites?

Zeolites have proven effective for environmental applications such as in PRBs for controlling the spread of cation-contaminated groundwater [18]. 

Q9. What is the effect of the XRD on the Pb(II)-loaded?

The Pb(II)-loaded composite was sequentially shaken with extractant solutions of increasing removal capacity to determine the availability of Pb associated with the composite. 

Q10. What is the abundant fraction of Pb(II) extracted with EDTA?

The large fraction of Pb(II) extracted with EDTA likely consists of more strongly bound Pb(II) and precipitated lead hydroxide complexes on active sites within the zeolite-based matrix of the composite [41, 44]. 

Q11. What was the process of extracting the Pb composite?

After the extractions, the composite was dried overnight at 105 °C and digested with 0.1 M HNO3 and 0.1 M HCl to recover Pb0 and other non-exchangeable Pb (likely present as Pb oxides or mixed Pb–Fe oxides). 

Q12. How much of the Pb was removed from the zeolite?

Results indicate that the composite effectively removed 96.2% of the Pb from aqueous solution (96.2 mg/g) within 140 min, while the zeolite alone only removed 39.1% (39.1 mg/g). 

Q13. What is the effect of the initial pH on the adsorption of Pb2+?

Their results suggest that rapid diffusion of Pb2+ into the Z–nZVI matrix and adsorption were optimized by adjusting the initial pH to 4, and were followed by reduction to Pb0 by Fe0. 

Q14. What is the effect of the Z–nZVI composite on Pb(II)?

The enhanced effectiveness of the Z–nZVI composite for Pb(II) removal is likely due to its much larger specific surface area than that of zeolite alone. 

Q15. What is the effect of the Z–nZVI on the Pb(II)?

Their results suggest that a large fraction of the Pb(II) removed by the Z–nZVI was incorporated into the internal matrix of the composite. 

Q16. What was the procedure used to determine the Pb(II) availability of the Z–?

A sequential extraction procedure was applied to the Pb(II)-loaded Z–nZVI composite to determine Pb(II) availability, following the general procedures of Basta and Gradwohl [23] and Castaldi et al. [24]. 

Q17. What is the main reason why heavy metals are toxic?

Heavy metals are problematic for ecosystems because of their toxicity and most heavy metals can be highly toxic even at very low concentrations. 

Q18. What is the way to remove Pb from water?

A composite of zeolite and nanoscale zero-valent iron (Z–nZVI) overcomes these problems and shows good potential to remove Pb from water. 

Q19. What is the average surface area of the composite?

The mean surface area of the composite was 80.37 m2/g, much greater than zeolite (1.03 m2/g) or nZVI (12.25 m2/g) alone, as determined by BET-N2 measurement.