Effect of pH, temperature and co-existing anions on the Removal of Cr(VI) in groundwater by green synthesized nZVI/Ni
Graphical abstract
Introduction
Hexavalent chromium, one of common heavy metal pollutants, exists in soil and groundwater excessively because of its widely use in industries, such as chrome plating, dyes and pigments, leather and wood preservation (Albadarin et al., 2012). Cr(VI) is more soluble and toxic than Cr(Ⅲ) and can cause dermatitis, rhinitis and cancer (Vinod et al., 2010). Cr(VI) exists in the form of anion in underground environment, however, there are lots of negative charges in aquifer media. So that, Cr(VI) is hard to be absorbed and it could migrate in groundwater easily, causing serious environmental problems.
In recent years, nanoscale zero-valent iron (nZVI) has attracted attention because its efficiency of reducing Cr(VI) with low toxicity from aqueous solution (Gheju, 2011, Montesinos et al., 2014). However, when exposed to the air, nZVI would be oxidized rapidly and the reduction rate would greatly decrease because of an oxidation layer formed on the surface of nZVI (Phenrat et al., 2007). Loading second metal, such as Pd, Co, Cu and Ni, on the surface of nZVI could prevent oxidization of nZVI and accelerate the degradation of contaminants because the second metal could act as catalyst to decrease the activation energy and accelerate the reduction process (Huang et al., 2013, Qin et al., 2016, Wang et al., 2015, Barnes et al., 2010). Among them, nickel is cheap, abundant and has higher economic benefit. Research from Kadu et al. (2011) showed that a completely reduction of Cr(VI) was achieved within 10 min in 25 mg L−1 Cr(VI) solution with 2 g L−1 Fe-Ni nanoparticles and the generation of reactive H·could enhance the reduction capacity when pH was lower than 4. Zhu et al. (2017) used nano zero valent iron/nickle (nZVI/Ni) to remediate Cr(VI) contaminated soil leachate, the reduction efficiency reached 99.84% when the dosage of nZVI/Ni was 0.08 mg mL−1, pH value was 5 and temperature was 303 K. In addition, co-existence ion, such as SO42-, HCO3- and CO32-, could inhibit the reduction rate.
Liquid phase reduction method using sodium borohydride as the reducing agent is the main method to prepare nZVI. However, sodium borohydride was toxic, corrosive and flammable. Flammable hydrogen generated through this method is dangerous and limits large-scale production of nZVI (Li et al., 2006). Nowadays, green synthesized nZVI using plant extracts has been proposed. The components in the plant extracts, such as polyphenols, protein, vitamins and flavone can be an alternative to borohydride to act as reducing agent (Fazlzadeh et al., 2016). Plant extracts have high solubility which can be extracted using deionized water, reacting with ferrous ion in solution and forming nZVI (Machado et al., 2013, Mohan et al., 2013). Additionally, these components also contribute to protect nZVI from oxidation and agglomeration, thus improving the stability of nanoparticles (Gao et al., 2016). Tea polyphenols extracted from green tea are cheap, safe and east to get, has been widely used as reductant, dispersant and capping agent to synthesize nZVI (Wang et al., 2017). Previous researches showed that nZVI prepared using green tea extracts could remediate Cr(VI)-contaminated water efficiently. For instance, Soliemanzadeh and Fekri (2017) prepared bentonite-supported nZVI using green tea extracts as reductant to remove Cr(Ⅵ) in solution. Results showed that the adsorption capacity reached 66 mg g−1 at pH 5, 0.05 M ionic strength. Mystrioti et al. (2015) used green tea extracts to synthesize nZVI suspension, which could remove 69.4% of Cr(VI) in aqueous solution within 2 days when initial Cr(VI) concentration was 50 mg L−1, Fe(tot) was 160 mg L−1, pH value and temperature was 4.58 and 295 K, respectively. Chrysochoou and Reeves (2017) prepared nZVI suspensions using green tea extracts (GT-nZVI), the maximum amount of reduced at pH 2.5 was 10.7 μmol Cr(VI) per mL of GT-nZVI. Green synthesized nZVI showed high performance on the removal of contaminants, however, only little reports using green synthesized method to synthesize iron-system bimetallic materials (Kumar et al., 2012, Smuleac and Varma, 2011). Moreover, the characteristic and adsorption capacity of green synthesized iron-nickel bimetallic nanoparticles has not been reported yet. Nanoscale zero-valent (nZVI) is one of the main reductant for in-situ remediation for groundwater (Mueller et al., 2012).
In this study, novel nanoscale zero-valent iron/nickel bimetal (GT-nZVI/Ni) was prepared using green tea extracts to evaluate the performance on removing Cr(VI) in solution. The bimetal was characterized using SEM, XRD, XPS and BET analyses. Batch experiments were carried out to investigate the effect of different factors on the removal of Cr(VI), including initial Cr(VI) concentration, pH, co-existing anions and temperature. The Langmuir-Hinshelwood first order kinetic model was used to analyze the reduction process. Finally, the mechanism of the removal of Cr(VI) using GT-nZVI/Ni was studied.
Section snippets
Materials
Ferrous sulfate (FeSO4·7H2O), nickel sulfate (NiSO4·6H2O), potassium dichromate (K2Cr2O7) and anhydrous alcohol (C2H5OH) were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Diphenylcarbazide (C13H14N4O), sulfuric acid (H2SO4), and phosphoric acid (H3PO4) were supplied by Tianjin Kemiou Chemical Reagent Co., Ltd., Tianjin, China. Green tea used in this study was Green Tea Suzhou Biluochun Tea Co., Ltd., Suzhou, China. Double distilled water was used in all experiments.
Characterization of GT-nZVI/Ni
Fig. 1(a) shows the SEM image of prepared GT-nZVI/Ni. Fig. 1(a) showed that the aggregates were formed by nanoparticles which were in spherical shape and had the size in the range of 80–150 nm. The specific surface area (SSA) of GT-nZVI/Ni was 22.2 m2 g−1, proving that GT-nZVI/Ni had large specific surface area due to this aggregate structure which was made up by small particles.
The XRD patterns of GT-nZVI/Ni are shown in Fig. 1(b). In Fig. 1(b), the broad humps about 2θ = 24.8° was attributed
Mechanism of removing Cr(VI) using GT-nZVI/Ni
The mechanism of removing Cr(VI) by GT-nZVI/Ni was analyzed. Cr(VI) was easily adsorbed on the surface of GT-nZVI/Ni because its specific surface area and larger pore volume, leading to the enrichment of Cr(VI) on GT-nZVI/Ni. The standard electrode potentials of Fe°/Fe2+ and Cr2O72-/Cr3+ are − 0.440 V and 1.33 V, respectively. Therefore, zero valent iron was a highly effective reductant for Cr(VI).
XPS analysis was used for elucidating the surface state of GT-nZVI/Ni after reaction with Cr(VI).
Conclusions
Nanoscale zero-valent iron/nickel bimetal (GT-nZVI/Ni) was synthesized using extracts from green tea. Results showed that acidic conditions and high temperature could promote removal efficiency of Cr(VI), however co-existing anions involving CO32-, HCO3−, SO42- and NO3− inhibited the process. The removal of Cr(VI) by GT-nZVI/Ni was satisfactorily represented by the Langmuir-Hinshelwood's first order reaction kinetic model and the process was a chemically-dominated adsorption, which was
Acknowledgments
This work was sponsored by the National Natural Science Foundation of China (Project No.21276174) and Natural Science Foundation of Shanxi province (Project No.2013011040-1).
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