Conjunctive effect of CMC–zero-valent iron nanoparticles and FYM in the remediation of chromium-contaminated soils
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Chromium is an important industrial metal used in various products and processes but at the same time causing lethal environmental hazards. Remediation of Cr-contaminated soils poses both technological and economic challenges, as conventional methods are often too expensive and difficult to operate. Zero-valent iron particles at nanoscale are proposed to be one of the important reductants of Cr(VI), transforming the same into nontoxic Cr(III). In the present investigation, soils contaminated with Cr(VI) are allowed to react with the various loadings of zero-valent iron nanoparticles (Fe0) for a reaction period of 24 h. Fe0 nanoparticles were synthesized by the reduction of ferrous sulfate in the presence of sodium borohydride and stabilized with carboxy methyl cellulose and were characterized by scanning electron microscopy, energy dispersion spectroscopy, X-ray diffraction, UV–vis spectrophotometer, Fourier transform-infra red spectrophotometer, Raman spectroscopy, dynamic light scattering technique and zeta potential. Further, this work demonstrates the potential utilization of farm yard manure (FYM) and Fe0 nanoparticles in combination and individually for the effective remediation of Cr(VI)-contaminated soils. An increase in the reduction of Cr(VI) from 60 to 80 % was recorded with the increase in the loading of Fe0 nanoparticles from 0.1 to 0.3 mg/100 g individually and in combination with FYM ranging from 50 to 100 mg/100 g soil.
KeywordsChromium Farm yard manure Nanoparticles Remediation Zero-valent iron Zeta potential
Chromium which falls in the heavy metal category has been known for its toxicity since a century or more (Barceloux and Barceloux 1999). According to its toxicity, Cr was classified as a primary pollutant and ranked as second among many toxic metals in the environment for frequency of occurrence at Department of Energy (DOE) sites (Sparks 1995). WHO-allowed maximum concentration of Cr(VI) in drinking water is 0.05 mg/l. Chromium compounds are used in various industries (e.g. textile dying, tanneries, metallurgy, metal electroplating, electronic, and wood preserving); hence, large quantities of Cr have been discharged into the environment due to improper disposal and leakage (Kimbrough et al. 1999). Cr exists primarily in two-valence states, i.e., trivalent Cr(III) and hexavalent Cr(VI). Cr(VI) is highly toxic, soluble, and mobile in the aquatic systems (Cheryl and Susan 2008). Moreover, Cr(VI) has been classified as a potential carcinogen, mutagen, and teratogen and has acute toxicity for different biological systems. Cr(III), on the other hand, is less toxic, immobile, and readily precipitates as Cr(OH)3 under alkaline or even slightly acidic conditions (Puls et al. 1999). Cr(III) is also an important micronutrient in the biological activity of insulin, is relatively stable and has low solubility in aqueous solution. The compounds of Cr(III) are reported to be 10–100 times less toxic than those of Cr(VI) (Wei et al. 1993).
Several techniques for Cr(VI) removal such as ion exchange, filtration, electrochemical precipitation, activated carbon adsorption, bioremediation, etc., have been reported in literature. However, these conventional methods are relatively expensive and complicated. Ionic state of the reductant plays an important role in remediation of the heavy metals. In this context, zero-valent iron (Fe0) nanoparticle technology offers a potential advantage over conventional methods because of its unique physico-chemical properties, enhanced surface energy, non-toxicity and economical viability. Fe0 nanoparticles have long been used in the electronic and chemical industries due to their magnetic and catalytic properties. Nowadays, use of Fe0 nanoparticles is becoming an increasingly popular method for treatment of hazardous and toxic wastes and for remediation of contaminated soil and ground water (Li and Zhang 2006; Lien et al. 2006). Fe0 nanoparticles provide a high surface-to-volume ratio, which promotes mass transfer to and from the solid surface resulting in high potential for contaminant removal and degradation (Martin et al. 2008). Mixing Fe0 and sand (yielding Fe0/sand filters) at suitable proportions has been proposed as a promising option for safe drinking water production (Chicgoua Noubactep et al. 2010). Diminutive size and large specific surface area of Fe0 nanoparticles translate to enhanced reactivity for contaminant remediation. Generally, because of size effects and much surface free-energy, nanoparticles in aqueous media tend to aggregate rapidly (Yang et al. 2007) or react quickly with the surrounding media such as dissolved oxygen or water, resulting in much larger chains and loss of reactivity and transfer in contaminated sites. Therefore, it is important to increase the stability and transportability of nanoparticles into the subsoil and groundwater. In this case, various particle-stabilizing agents have been employed, including resin and starch (He and Zhao 2005), chitosan and carboxymethyl cellulose (Geng et al. 2009).
Suitable conditions for Cr(VI) reduction occur where organic matter is present and Cr(VI) reduction is enhanced in acidic rather than alkaline soils (Bartlett and Kimble 1976). Various organic materials, such as powdered leaves (Suseela et al. 1987) and Scotch pine (Pinus sylvestris L.) bark (Alves et al. 1993) have been used to remove Cr(VI) from industrial effluents. The natural organic matter (NOM) such as humic acid with Fe0 nanoparticles plays an important role in Cr(VI) reduction by adsorption phenomena (Singh et al. 2011). Hence, in the present study it is proposed to use Fe0 nanoparticles synthesized by using borohydride as reducing agents of ferrous ions and CMC as stabilizing agent and were tested for their potential in the remediation of Cr(VI)-contaminated soils individually and in combination with the farm yard manure (FYM).
Materials and methods
Chemicals and solutions
Ferrous sulfate heptahydrate (FeSO4·7H2O) and sodium borohydride (NaBH4) were purchased from Merck, India. 1,5-Diphenylcarbazide (C13H14N4O) was procured from SD Fine Chemicals Ltd., India, carboxy methyl cellulose (CMC) and ethanol (C2H5OH) obtained from Merck, India, were used without any dilution.
Collection of soil samples
Soil sample was collected from the CL leather industry, Vellore, Tamilnadu following US EPA standard operating procedures for soil sampling (US EPA 2000) in the month of September 2012. After collection, all the samples were air dried, passed through a 2 mm sieve, packed in plastic bags and then stored in dark at 4 °C. Estimation of the heavy metals concentration in soil samples in triplicate was carried out using ICP-OES technique.
Synthesis of Fe0 nanoparticles
The CMC-stabilized Fe0 nanoparticles were freshly synthesized by following the methods reported by Wei et al. (2004) and He and Zhao (2005). In brief, the iron nanoparticles were synthesized by adding stoichiometric amounts of NaBH4 in 30 % ethanol solution at a BH4−/Fe2+ molar ratio of 2.0 dropwise to a 1,000 mL three-necked flask containing FeSO4·7H2O aqueous solution and CMC with electrical stirring at ambient temperature. Before use, deionized water and the CMC solution were purged with purified N2 to remove dissolved oxygen.
Characterization of Fe0 nanoparticles
Fe0 nanoparticles as prepared were characterized using multiple techniques like SEM, EDS, UV–vis, FT-IR, Raman spectroscopy, dynamic light scattering (DLS) for particle size determination and zeta potential.
SEM and EDS
Morphological studies of as prepared CMC-stabilized Fe0 nanoparticles were carried out by using scanning electron microscope (SEM) fitted with an energy dispersive spectrophotometer (EDS) system (model CARL-ZEISS EVO MA 15). The sample was observed at 10,000× magnification with an accelerating voltage of 20 kV. The energy dispersive spectrograph reveals strong signal in the iron region and confirms the formation of iron nanoparticles.
The localized surface plasmon resonance (LSPR) of Fe0 nanoparticles stabilized with the CMC was recorded using Shimadzu UV 2450 UV–vis spectrophotometer and their absorption maxima at 300 nm which is a characteristic of monodispersed iron nanoparticles observed.
The FTIR spectra of the CMC-stabilized Fe0 nanoparticles were recorded in the transmission mode at room temperature using KBr pellet technique (1:20). The KBr was dried in a dryer at 200 °C for 24 h, then 560 mg KBr was homogenized with sample and afterwards ground to fine powder with a mortar and pestle. FTIR-Tensor 27(Bruker) infrared spectrophotometer was used to determine the spectra of the sample which was mixed with spectrally pure KBr and pressed to form thin plates (radius 1 cm, thickness 0.1 cm), then subjected to IR spectroscopic analysis in the spectral range 500 and 4,000 cm−1.
The method of X-ray diffraction (XRD) was used to investigate the material structure of iron nanoparticles. The XRD analysis was conducted with a Philips XRD 3100 diffractometer (Philips electronics Co.,) at 45 kV and 30 mA. It used copper Kα radiation and a graphite monochromator to produce X-rays with a wavelength of 1.54 Å. Iron nanoparticles were placed in a glass holder and scanned from 20° to 90°. This scan range covered all major species of iron and iron oxides. The scanning rate was set at 2.0°/min.
Zeta potential and particle size determination
The particle size and particle distribution width by DLS were obtained by using SZ-100 nanoparticle, a flexible analytical tool for characterizing the physical properties of small particles. The configuration and application of the system can be used as a particle size analyzer and also to measure zeta potential. Zeta potential was taken under the temperature of 25 °C; the dispersion medium has the viscosity 0.893 mPa and the electrode voltage at 3.4 V.
Raman spectroscopy allows characterization of many types of samples without any specific preparation. The Raman spectra were obtained using a Lab RAM HR visible single spectrometer equipped with a microscope and a Peltier-cooled CCD detector. The 633 nm He–Ne laser line was used for excitation. Raman measurements were performed at room temperature and atmospheric pressure. The characteristic peak positions of iron-based particles were determined in the Raman region of interest in this investigation: 100–1,200 cm−1. The most intense peak positions for Raman spectra present in our sample for Fe0 are at 223.6, 291.2 cm−1.
Colorimetric method for analysis of Cr(VI)
Heavy metals present in the soil samples determined by ICP-OES
Metal conc. in (ppm)
0.524 ± 0.05
0.362 ± 0.08
36.080 ± 2.5
0.111 ± 0.01
8.514 ± 1.6
0.731 ± 0.09
117.119 ± 5.3
0.352 ± 0.09
9.206 ± 1.2
20.022 ± 1.8
0.932 ± 0.1
0.421 ± 0.07
12.532 ± 1.20
0.813 ± 0.14
0.940 ± 0.20
0.551 ± 0.07
0.505 ± 0.04
1.493 ± 0.12
Effect of Fe0 concentration
Experiments were carried out to determine the concentration at which maximum reduction of Cr(VI) occurs. Various concentrations ranging from 0.1 to 0.3 mg of CMC–Fe0 nanoparticles were added to 100 g of Cr(VI)-contaminated soil. After that the reaction mixture was extracted and analyzed for Cr(VI).
Effect of organic amendment (farm yard manure) on removal efficiency of Cr(VI) by Fe0 nanoparticles
To investigate the effect of organic amendment (FYM) on removal efficiency of Cr(VI) by Fe0 nanoparticles, two different concentrations (50, 100 mg) of FYM was spiked in 100 g of Cr(VI)-contaminated soil samples. These samples were treated with 0.1, 0.2, 0.3 mg of Fe0 nanoparticles. After that the reaction mixture was extracted and analyzed for Cr(VI).
Results and discussion
It is desirable that the reduction of Cr(VI) present in soil is accompanied by immobilization in soil of the corresponding insoluble reduced chromium species. The efficiency presented by ZVI for the remediation of soil containing Cr(VI) can be evaluated comparing the different findings. As expected the analysis of findings obtained for ZVI with increased FYM revealed that S1N3F2 presented a higher efficiency for reduction of Cr(VI) (81 %) when compared to the other treatments.
On the basis of solution chemistry and X-ray photoelectron spectroscopy data, Li et al. (2008) determined the average value of x, and it was found to be approximately 0.667. This Cr–Fe oxyhydroxide shell is stable and serves as a sink for Cr(VI). Fe0 nanoparticles exhibit characteristics of both iron oxyhydroxides (i.e., as a sorbent) and metallic iron (i.e., as reductant). For Cr(VI) removal, Fe0 nanoparticles mainly act as a reductant. The chemical reduction of Cr(VI) to Cr(III) is accompanied by chromium immobilization in soil via formation of an insoluble mixed oxide compound (Débora Franco et al. 2009). The reduced Cr(III) can be incorporated into the iron oxyhydroxide shell forming (Cr x Fe1−x) (OH)3 or Cr x Fe(1−x) OOH at the surface. At high initial Cr concentration, this structure may serve as a passive layer at the surface and hinder further reduction of Cr(VI). After reduction, Cr(III) may be removed through the precipitation or co-precipitation in terms of (Cr x Fe1−x)(OH)3 or Cr x Fe(1−x)OOH as its E0 is (−0.74 V) which is slightly more than E0 of Fe0 nanoparticle.
The statistical results for the above experiment revealed that there is a consistency in results.
Our speculation is that the FYM with Fe0 nanoparticle-assisted reduction of Cr(VI) in this study may be explained as per the earlier citations by the researchers. The organic amendment which is rich in ammoniacal N oxidizes to nitrate NO3− resulted in the release of protons (Bolan et al. 2003). This may be one of the reasons for the decrease of soil pH that can inhibit the iron corrosion thereby prolonging the life time of nano Fe0. On the other hand the FYM can also transfer electrons from Fe0 to reduce Cr(VI) in the soil. Hence, the reduction of Cr(VI) increased in the presence of FYM-assisted Fe0 nanoparticles than unassisted nano Fe0 particles. This is supported by earlier studies which have shown that the reduction of Cr(VI) to Cr(III) with Fe0 nanoparticles was found to increase with the decrease in soil pH. (Debora Franco et al. 2009).
Further, Paul and Beauchamp(1989) reported that the easily oxidisable organic carbon and DOC fractions provide the energy source for microorganisms involved in the reduction of metals such as, Cr[i.e., Cr(VI) to Cr(III)]. Hence the DOC, ammoniacal nitrogen and microorganisms in organic amendments influence the performance of Fe0 nanoparticles. Several authors reported the chromium detoxification by microorganisms by various processes such as biotransformation, bioremediation and biosorption. The carbonyl, hydroxyl, sulphydryl, amino and phosphate groups on the microorganisms play a vital role in reduction of Cr(VI) (Chand et al. 1994). On the basis of above studies, it may be concluded that the FYM used in the present study may have high microbial activity and also assists Fe0 nanoparticles in Cr(VI) reduction. Further studies in this direction will help to elucidate the detailed mechanism of the role of microbial activity in Fe0 nanoparticle-mediated reactions.
CMC-stabilized Fe0 nanoparticles were synthesized and were used to reduce Cr(VI). The CMC-stabilized Fe0 nanoparticles exhibited higher removal efficiency than those prepared without a stabilizer because the CMC acted as a good dispersant to prevent Fe0 nanoparticles from agglomerating. Addition of FYM to Fe0 nanoparticles facilitates the effective reduction of Cr(VI) concentration. The study further suggests that the stabilized Fe0 nanoparticles may be used for in situ reductive immobilization of Cr(VI)-contaminated soils or other Cr(VI)-laden solid wastes, which may lead to an innovative remediation technology that is likely more cost-effective and less environmentally disruptive.
The authors are thankful to Department of Physics, SV University for providing the data on Raman spectroscopy.
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