1. Green synthesis of Fe nanoparticles for chromate removal
C.Mystrioti1*, A Xenidis1, N. Papassiopi1
1Sch. of Mining and Metallurgical Eng., National Technical University of Athens, 15780, Greece
Life10 env/gr/000601
The maximum extraction of polyphenols per g of herbs was achieved at a ratio of herb/water (S/L) equal to g/L. The reduction of ferric iron with the herb extracts was partial at all conditions investigated and its degree has not exceeded 50%. It is estimated that the higher reduction of Fe(III) to Fe(0) was achieved by the mixing ratio VFe/Vext 1:2 when higher concentration of phenols are available (Fig.1).
It seems that the iron nanoparticles prepared using extracts were mostly spherical with particle size distribution from 5-60 nm as shown in Fig. 2. Using clove extract the NPs are not easily seen by TEM because are covered by polyphenols. As seen in all the images in Fig. 2, there are not any aggregates formed. A representative SAED spectrum is shown in Fig. 2. The arrangement of bright spots is typical of the face-centered cubic crystal system of elemental iron.
Introduction
Methods
The contamination of groundwater with hexavalent chromium is a major environmental issue in Greece. Higher concentrations than 5000μg/L of Cr(VI) have been detected recently in the river basin of Asopos river (Central Greece). For this reason an intense research activity is going on, aiming at the development of effective Cr(VI) remediation technologies.
The last years nanoscale zero valent iron (nZVI) tend to be one of the most promising reductants combining effectiveness and low cost (Noubactep, 2008). Research conducted using iron nanoparticles indicated that the reduction of pollutants occurs at a rate several orders of magnitude higher compared to the rates observed with granular ZVI particles (Crane and Scott, 2012), due to:
smaller particle size,
larger specific surface area
higher surface reactivity
A green synthesis procedure was used for the one step production of nZVI using herbs with high polyphenol content. Plant extracts contain active polyphenol compounds, which reduce the trivalent iron to iron nano-particles by combining extracts with ferric iron salts. This method for producing nZVI is considered the most environmental friendly (Hoag, 2009).
The aim of this research was to examine the alternative raw materials for the synthesis of stable nZVI dispersions :
Camellia sinensis (green tea, GT),
Syzygium aromaticum (clove, CL),
Mentha spicata (spearmint, SM),
Punica granatum juice (pomegranate, PG)
Red Wine (RW)
For the production of nZVI, the polyphenols extract was introduced to the ferric ion solution at volume ratios 1:2, 1:1, 2:1. The extracts were prepared by immersion of the herb to water at 80 oC for 5 minutes. The effect of herbs weight per litter of water (5, 10, 20, 50 and100 g/L) on polyphenol content in the final extract was investigated. Although wine and pomegranate juice were used as received.
To evaluate the degree of Fe(III) reduction to elemental Fe(0), a portion of 20 ml for each nZVI suspension was subjected to centrifugal ultrafiltration. The filtrate was analyzed for total Fe and Total polyphenol content (TPC). Morphological analysis of the nZVI suspensions was performed by means of TEM.
The reduction of hexavalent chromium was evaluated using an aqueous solution Cr(VI) with initial concentration (0.96 mM) and nZVI suspensions. The tests were carried out with molar ratio of Fetot/Cr(VI) equal to 3 mole/mole. The flasks were left for 2 days under continuous agitation (200 rpm) at ambient temperature (~22°C). The samples were analysed for pH, ORP, Cr(VI), total Cr and Fe by (AAS-FE). Cr(VI) was analysed by using the USEPA 7196a method.
Figure 3
It is noted that the preponderant specie of Cr(VI) in the acidic and slightly acidic solutions up to pH=6.5 is HCrO4-. It should be mentioned however that Fe(0) may be consumed by many other parallele reactions, including oxidation by atmospheric O2, or corrosion by the acidic pH of the solutions . The highest reduction, corresponding to %, was obtained using the nZVI suspension produced with PG juice and RW respectively. The other three nZVI suspensions, i.e. CL-nZVI, GT-nZVI, and SM-nZVI, had a comparable effectiveness corresponding to 70% reduction of chromate.
Conclusions
Polyphenol coated nano Fe(0) particles with different plant extracts (GT, CL, PG, SP, RW) have been synthesized and used to reduce hexavalent chromium in aquatic solutions. Results of synthesis polyphenol coated nZVI indicated that the higher reduction of Fe(III) to Fe(0), was achieved with the pomegranate juice (PG), 53% and 40.2% with clove extract (CL), while the lowest reduction was achieved with the SM extract (15.6%). It was also found that RW -nZVI is the most effective medium to reduce chromium(VI), as it exhibits the highest reduction rate, i.e. 98%, within 2 days.
Results
Figure 2
The results of chromium (VI) reduction, using the nZVI produced with the five plant extracts, are presented in Fig 3. The tests were carried out by adding a constant volume of nZVI suspensions in the 0.96 mM Cr(VI) solution. With the mixing ratio Fetot/Cr(VI) equal to 3 mole/mole, the total iron added in the samples corresponds to 2.88 mM, but the elemental iron, Fe(0) varies from 0.45 mole for SM to 1.015mole for RW.
The required stoichiometric ratio is 1 mol of Fe(0) per mol of Cr(VI) for Cr(VI) reduction by Fe(0) (Eq. 1):
Fe0 + HCrO4- + 7H+ → Fe+3 + Cr H2O (1)
Acknowledgements
This research is financed by (CHARM) LIFE 10 ENV/GR/ a project titled “Chromium in Asopos Groundwater System: Remediation Technologies and Measures (CHARM)” aims to contribute significantly to the solution of this complex problem that seriously threats sustainable development of groundwater bodies.
Noubactep C., (2008), A critical review on the mechanism of contaminant removal in Fe0–H2O systems, Environ. Technol. 29, 909–920.
Crane R.A., Scott T.B., (2012), Nanoscale zero-valent iron: future prospects for an emerging water treatment technology. Journal of Hazardous Materials, 211– 212.
Hoag G. E., (2009), Degradation of bromothymol blue by ‘greener’ nano-scale zero-valent iron synthesized using tea polyphenols, Journal of Materials Chemistry. 19(45):
References
Figure 1