CHEMICAL ENGINEERING TRANSACTIONS 

VOL. 81, 2020 

A publication of 

The Italian Association 
of Chemical Engineering 
Online at www.cetjournal.it 

Guest Editors: Petar S. Varbanov, Qiuwang Wang, Min Zeng, Panos Seferlis, Ting Ma, Jiří J. Klemeš 

Copyright © 2020, AIDIC Servizi S.r.l. 

ISBN 978-88-95608-79-2; ISSN 2283-9216 

Removal of Hexavalent Chromium Using Nanoscale Zero-

Valent Iron Stabilized by Poly (γ-Glutamic Acid) 

Chao Zhanga, Qing Liua, Renliang Huangb, Wei Qia, Zhimin Hea, Rongxin Sua,* 

a State Key Laboratory of Chemical Engineering, Tianjin Key Laboratory of Membrane Science and Desalination 

 Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China 
b School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, PR China 

 surx@tju.edu.cn 

Hexavalent chromium (Cr(VI)) is one of the most toxic components in heavy metal pollution. The reduction of 

highly toxic Cr(VI) into much less toxic trivalent chromium (Cr(III)) is a promising and common method of 

remediating Cr(VI) contamination. Nanoscale zero-valent iron (nZVI) is an important nanoparticle to facilitate 

environmental remediation. It was reported on the fabrication of a bionanocomposite (nZVI@PGA), i.e. nZVI 

stabilized and modified by poly (γ-glutamic acid) (PGA). The bionanocomposite was characterized by scanning 

electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier 

transform infrared spectroscopy (FTIR). The processes of Cr(VI) conversion by nZVI, PGA and nZVI@PGA 

were investigated and optimized. Compared to unmodified nZVI, nZVI@PGA displayed higher activity in acidic 

environment. The pH and PGA loading were optimized. The maximum unit removal of Cr(VI) by nZVI@PGA 

can reach 340 mg/g in acidic environment. The results demonstrated that nZVI@PGA can convert about 97.3 % 

Cr(VI) (initial concentration was 20 mg/L) in 30 min at pH=3. nZVI@PGA owns high stability and reactivity, and 

has potential to remove Cr(VI) of sewage in the future. 

1. Introduction

Hexavalent chromium pollution has become an increasingly common type of pollution, mainly due to paper, 

leather, and metallurgical industries (Dong et al., 2016). The main forms of chromium are hexavalent and 

trivalent, while hexavalent chromium (Cr(VI)) is easier to migrate and approximately 1,000 times more toxic than 

trivalent chromium (Cr(III)) (Chen et al., 2015). Cr(VI) is not only harmful to human health, but also to plants and 

microorganisms. It was now an urgent task to remove the Cr(VI) from wastewater. 

nZVI owns better adsorption and reduction effect in the field of hexavalent chromium removal from wastewater 

or contaminated soil (Mouhamadou et al., 2015). As a new type of nanomaterial, nZVI has a lot of advantages, 

such as large specific surface area, stronger adsorption and reduction capabilities (Dong et al., 2012). nZVI still 

has some deficiencies in the application of pollutant removal, including the agglomeration and facile oxidation 

(Guan et al., 2015). Due to the van der Waals and attractive magnetic forces, nZVI aggregates easily to form 

large particles, which caused the reactivity of nZVI to decrease severely (Xie et al., 2017). To solve this problem, 

many researchers have applied various carriers to stabilize nZVI and disperse nanoscale iron particles, such as 

carboxymethyl cellulose, bentonite, sodium alginate, resin, and biochar. As a macromolecular polymer, PGA 

was widely used in industrial fields due to its excellent adsorption performance (Elizabeth et al., 1999). PGA has 

been applied as an adsorbent to study the adsorption effect of divalent copper ions (Hu et al., 2017). To remove 

the divalent cobalt, the adsorption effect of PGA had been investigated (Zakariyah et al., 2017). The 

complexation of PGA with lead ions was carried out to remove the lead pollution (Magdolna et al., 2008). In the 

lab, the previous work had proved that nZVI modified by PGA had excellent performance of stability and 

dispersion, degrading p-chlorophenol more than 90 % in 30 min (Zhang et al., 2018). When the composite 

material includes PGA–dopamine and Fe-Pd nanoparticles, the degradation rate of p-chlorophenol almost 

reached 100 % in 60 min (Zhang et al., 2018). PGA has many carboxyl groups, which can provide static and 

 

                                                       DOI: 10.3303/CET2081215 
 

 
 

 
 
 
 

 
 

 
 
 

 
 

 
 
 

 
 

 

 
 

 
 
 

 
 
 

 
 
 

 
 
 

 
 
 

 
 

 
 

Paper Received: 12/06/2020; Revised: 28/06/2020; Accepted: 03/07/2020 
Please cite this article as: Zhang C., Liu Q., Huang R., Qi W., He Z., Su R., 2020, Removal of Hexavalent Chromium Using Nanoscale Zero-
Valent Iron Stabilized by Poly (γ-Glutamic Acid), Chemical Engineering Transactions, 81, 1285-1290  DOI:10.3303/CET2081215 

1285



spatial repulsion to disperse nano-scale iron particles (Elizabeth et al., 1999). nZVI modified by PGA is a stable 

system, which has potential to remove the pollution of Cr(VI) from water. 

To our knowledge, there is no report on nZVI@PGA application on the reduction of Cr(VI) to Cr(III). The major 

work of this paper includes preparation of PGA-modified nZVI materials (nZVI@PGA), characterization of 

nZVI@PGA, and application and optimization of nZVI@PGA for Cr(VI) removal. 

2. Materials and methods

Poly (γ-glutamic acid) (PGA, MW 100,000–50,000 kDa) was purchased from Yuanye Bio-Technology 

(Shanghai, China). Ferrous sulfate heptahydrate (FeSO4·7H2O, > 99.0 %), potassium borohydride (KBH4, 

97 %), potassium dichromate (K2Cr2O7, ≥ 99.8 %), diphenyl semicarbazide (C13H14N4O) and sodium hydroxide 

(NaOH, 96 %) were supplied by Aladdin Reagent Company (Shanghai, China). Hydrochloric acid (HCl, 36-

38 %), sulfuric acid (H2SO4, 95-98 %) were purchased from Jiangtian Chemical Technology (Tianjin, China). All 

chemicals were used without further purification. All water was purified by a water purification system (arium pro 

VF, Sartorius Germany). 

2.1 Preparation of nZVI@PGA 

The nZVI@PGA was prepared by the method of borohydride reduction. The total reaction volume was 50 mL. 

Then, 25 mg FeSO4·7H2O was added into PGA solution which included different amounts of PGA (0.1, 0.5, 1 

and 2 mg), and always under the protection of nitrogen to create an anaerobic environment. Next, 15 mg KBH4 

were dissolved in 3 mL deionized water to form a solution that dropped to the mixing system. When the Fe2+ 

reacted with BH4- to give Fe0, it was necessary to purge nitrogen for 20 min to eliminate the excess of hydrogen 

(Abel et al., 2017). pH was kept constant in the nZVI@PGA solution by addition of hydrochloric acid or sodium 

hydroxide. The preparation of nZVI@PGA was carried out in a water bath with constant temperature at 25 °C. 

The synthesized system was poured into a conical flask containing hexavalent chromium solution. 

2.2 Characterizations 

The morphology of nZVI@PGA was studied by scanning electron microscopy (S-4800, Hitachi, Japan). The 

composition and stability of nZVI@PGA were characterized by X-ray diffraction (D8-Focus, Bruker, Germany) 

and settlement experiment (Zhang et al., 2018). 

2.3 Batch experiments of Cr(VI) removal 

1 g/L stock solution of Cr(VI) was prepared by dissolving potassium dichromate in water at 25 °C. Then, the 

prepared nZVI@PGA system was poured into four 150 mL conical flasks containing the stock solution of Cr(VI) 

(1, 2, 2.5, 4 mL), so that the initial Cr(VI) concentration was 20, 40, 50, 80 mg/L. In these mixed systems, 

different mass ratio Fe:Cr(VI) (5:1, 2.5:1, 2:1, 1.25:1) were obtained to explore the effect of initial hexavalent 

chromium concentration on Cr(VI) removal. At the same time, it is also necessary to consider the effect on the 

removal of Cr(VI) at different pH (3, 6, 9) and temperature (15, 25, 35, 45 °C) values. According to the time node 

of the experimental design, 1 mL of sample solution was took out from the mixed systems each time. The 

absorbance of Cr(VI) was measured with a spectrophotometer at a wavelength of 540 nm. The Cr(VI) 

concentration was calculated according to the calibration curve, designed in the 0.02 mg/L-5 mg/L concentration 

range, and having a coefficient of determination of 0.9996. All experimental measurements were repeated twice. 

3. Results and discussion

3.1 Characterization of nZVI@PGA 

Figure 1: SEM micrographs of the (a) nZVI  and (b) nZVI@PGA 

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The morphologies and size of nZVI and nZVI@PGA were characterized by SEM. As shown in Figure 1a, in the 

absence of PGA in synthesis process, the resulted nZVI formed chain aggregations. Figure 1b shows that the 

majority of nZVI@PGA had relatively spherical shapes and smooth surfaces in the size range of 50-150 nm. In 

general, the results proved that nZVI was easily oxidized and PGA could inhibit the aggregation and restrain 

oxidation. 

3.2 Cr(VI) removal by nZVI@PGA 

Under acidic conditions (pH=3), the removal of Cr(VI) with nZVI@PGA was studied through batch tests. Figure 

3a shows the amount of Cr(VI) (20 mg/L) removed followed the order PGA<nZVI<nZVI@PGA. Compared with 

nZVI and PGA, nZVI@PGA was the most effective for Cr(VI) removal, and the amount removed in 30 min was 

twice compared to that obtained with nZVI. Bare PGA had little effect on the removal of Cr(VI) with approximately 

3 % removal in 150 min. According to the theory of surface-mediated reactions, the reactivity was directly 

proportional to the effective surface area (Choe et al., 2001). nZVI@PGA presents better dispersion and a large 

specific surface area, indicating that the removal of Cr(VI) was more efficient and the removal amount can reach 

97 % in 30 min. As shown in Figure 3a, nZVI@PGA was much more efficient and faster on the removal of Cr(VI). 

In this study, when the mass ratio Fe:Cr(VI) was 5:1 and pH=3, nZVI@PGA exhibited high activity on the removal 

of Cr(VI), which shows that nZVI@PGA is a promising nanomaterial. The sketch of Cr(VI) removal was shown 

in Figure 2. 

Figure 2: Schematic diagram for the synthesis of nZVI@PGA and the removal of Cr(VI) 

Figure 3: (a) Time curve of Cr(VI) (20 mg/L) removal using PGA, nZVI and nZVI@PGA (pH=3.0, T=25 °C); (b) 

Effect of Cr(VI) (40 mg/L) removal under different PGA loadings (pH=3.0, T=25 °C) 

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3.3 Effect of nZVI to PGA mass ratio on the removal of Cr(VI) by nZVI@PGA 

As shown in Figure 3b, the effects of Cr(VI) removal was studied by changing the nZVI:PGA mass ratio (50:1, 

10:1, 5:1 and 2.5:1). The result shows that the maximum removal amount was 79.15 % when the mass ratio 

nZVI:PGA was 5:1. Known from the characterized SEM image, nZVI particles were easier to gather when the 

proportion of PGA was small. When the PGA mass ratio increased, the nZVI particle was present better 

dispersion, which provided more active sites for Cr(VI) removal. Excessive PGA loading prevented nZVI from 

reacting with Cr(VI), indicating that active sites and specific surface area of nZVI were reduced. Many studies 

had proven this phenomenon, for example, nZVI supported by biochar to remove the methyl orange (Han et al., 

2015). The best mass ratio nZVI:PGA was 5:1, which showed efficient removal effect for the Cr(VI). 

3.4 Effect of pH on the removal of Cr(VI) by nZVI@PGA 

Figure 4 shows that the initial pH of nZVI@PGA had a significant effect on the removal of Cr(VI). When the 

initial pH increased, the amount of Cr(VI) removed was severely reduced, which can only reach 18.6 % at pH=9 

and 28.6 % at pH=6 in 150 min. The reduction rate of nZVI@PGA decreased significantly with the increased 

initial pH, indicating that lower pH was beneficial for the removal of Cr(VI). According to previous literature 

reports, HCrO4− was the main form of Cr(VI) in the acidic solution and CrO42- in the alkaline solution (Dinesh et 

al., 2006). Under acidic conditions, the main reaction was shown in Eq 1. 

2HCrO4
-
 + 3Fe

0
 + 14H

+
 → 2Cr

3+
 + 3Fe

2+
 + 8H2O (1) 

In acidic environment, H+ ions speed up the reaction with HCrO4−, and the electrostatic interactions between 

nZVI@PGA and Cr(VI) increased the adsorption of Cr(VI) onto the surface for removal (Shi et al., 2011). The 

electrostatic repulsion between nZVI@PGA and CrO42- might inhibit the Cr(VI) absorption onto the composite 

material in alkaline environment. Cr(III) and Fe(III) produced during the reactions were easily accumulated on 

the surface of nZVI@PGA, which prevented further reduction (Fang et al., 2011). 

Figure 4: Cr(VI) (40 mg/L) removal by nZVI@PGA at different initial pH (T=25 °C) 

3.5 Effect of initial Cr(VI) concentration on the removal of Cr(VI) by nZVI@PGA 

Figure 5a shows the effect of initial Cr(VI) concentration for the removal of Cr(VI) at pH=3. When the initial Cr(VI) 

concentration was 40, 50, and 80 mg/L, the amount of Cr(VI) removed was about 79.2 %, 68.8 %, 51.1 % in 

150 min. Besides, nZVI@PGA removed Cr(VI) faster, which can achieve 97.3 % in 30 min when initial Cr(VI) 

concentration was 20 mg/L. When the initial Cr(VI) concentration was raised from 20 to 80 mg/L in acidic 

environment, the unit removal capacity of nZVI@PGA increased from 160 to 340 mg/g. Compared with other 

composite materials, such as activated carbon supported nZVI (AC/nZVI) (Soroosh et al., 2018), biochar 

supported nZVI (nZVI-BC) (Fan et al., 2019), and silicon rich biochar supported nZVI (nZVI-RS700) (Qian et al., 

2019), nZVI@PGA was more efficient for the removal of Cr(VI) as shown in Table 1. The unit remova (Ur, mg/g) 

of nZVI@PGA was calculated by Eq 2,.where MCr was the mass of Cr(VI) removed, and MFe was the total mass 

of nZVI@PGA in the reaction. The unit removal capacity could decrease when nZVI was overloaded (Zhang et 

al., 2018). When the initial concentration of Cr(VI) was excessive, nZVI will be oxidized quickly so that the 

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Cr(III)/Fe(III) oxides/hydroxides stacked on the surface of nZVI@PGA, making it unavailable for further reduction 

(Liu et al., 2010). 

Ur = MCr / MFe (2) 

Table 1: The unit removal of nZVI@PGA compared with other composite material (pH=3.0) 

Composite material AC/nZVI nZVI-BC nZVI-RS700 CMC-nZVI nZVI@PGA 

Unit removal (mg/g) 3.3 14.5 111.9 136.0 162.2 

Time (h) 48 5 24 0.5 0.5 

3.6 Effect of temperature on the removal of Cr(VI) by nZVI@PGA 

As shown in Figure 5b, when the initial temperature increased, nZVI@PGA removed Cr(VI) more efficiently. 

When the initial temperature increased from 15 to 45 °C, the removal amount was from 71.6 % to 89.7 % in 

150 min. The ion diffusion became more intense as temperature was raised, giving nZVI@PGA more chance 

to react with Cr(VI) (Shi et al., 2011). Consequently, when the temperature increased, the reaction rate was also 

raised, which significantly reduced the time for the removal of Cr(VI) (Manning et al., 2007). 

Figure 5: Degradation curves of Cr(VI) removal at  (a) different initial concentrations (20, 40, 50, 80 mg/L) and 

(b) different temperatures (pH=3.0) 

4. Conclusions

The main work was to investigate the feasibility of the Cr(VI) removal with nZVI on support of PGA. The results 

show that nZVI@PGA was better than nZVI, having excellent reduction effect for Cr(VI). When the mass ratio 

nZVI:PGA was 5:1, nZVI@PGA showed a better stability and dispersion. Cr(VI) removal by nZVI@PGA can 

reach 97.3 % in acidic solution, indicating that low pH can promote the reaction. The maximum unit removal of 

nZVI@PGA was 340 mg/g, which was more efficient than AC/nZVI, nZVI-BC and nZVI-RS700. The higher 

temperature facilitates the diffusion of ions, which is beneficial for the removal of Cr(VI). Besides, the 

Cr(III)/Fe(III) hydroxides produced precipitation, eliminating potential secondary pollution issues to the 

environment. The results indicate that nZVI@PGA is an efficient composite material for the removal of Cr(VI), 

especially in acidic environment. 

Acknowledgments 

This study was supported by the Natural Science Foundation of China (No. 51473115) and Wuqing S&T 

Commission (WQKJ201726 and WQKJ201806).  

References 

Abel M.M., Gunel R.A., Ulviyya A.H., Mahammadali A.R., Luca D.P., 2017, Synthesis and application of zeolite 

and glass fiber supported zero valent iron nanoparticles as membrane component for removal nitrate and 

Cr (+6) ions, Chemical Engineering Transactions, 60, 163–168. 

Chen T., Zhou Z.Y., Xu S., Wang H.Y., Lu W.J.,2015, Adsorption behavior comparison of trivalent and 

hexavalent chromium on biochar derived from municipal sludge, Bioresource Technology, 190, 388–394. 

Choe S., Lee S.H., Chang Y.Y., Hwang K.Y., Khim J., 2001, Rapid reductive destruction of hazardous organic 

compoundsby nanoscale Fe0, Chemosphere, 42(4), 367-372. 

T = 15 °C 

T = 25 °C 

T = 35 °C 

T = 45 °C 

1289



Dinesh M., Charles U.P., 2006, Activated carbons and low cost adsorbents for remediation of tri- and hexavalent 

chromium from water, Journal of Hazardous Materials, 137, 762-811. 

Dong H.R., Guan X.H., Irene M.C.L., 2012, Fate of As(V)-treated nano zero-valent iron: determination of arsenic 

desorption potential under varying environmental conditions by phosphate extraction, Water Research, 

46(13), 4071–4080. 

Dong H.R., He Q., Zeng G.M., Tang L., Zhang C., Xie Y.K., Zeng Y.L., Zhao L., Wu Y.N., 2016, Chromate 

removal by surface-modified nanoscale zero-valent iron: Effect of different surface coatings and water 

chemistry, Journal of Colloid and Interface Science, 471, 7–13. 

Elizabeth G., Thomasin C.M., Jose R.G., James A.H., 1999, Characterization of immobilized poly-L-aspartate 

as a metal chelator, Environmental Science Technology, 33, 1664–1670. 

Fan Z.X., Zhang Q., Gao B., Li M., Liu C.Y., Qiu Y., 2019, Removal of hexavalent chromium by biochar 

supported NZVI composite: Batch and fixed-bed column evaluations, mechanisms, and secondary 

contamination prevention, Chemosphere, 217, 85-94. 

Fang Z.Q., Qiu X.Q., Huang R.X., Qiu X.H., Li M.Y., 2011, Removal of chromium in electroplating wastewater 

by nanoscale zero-valent metal with synergistic effect of reduction and immobilization, Desalination, 280(1-

3), 224-231. 

Guan X.H., Sun Y.K., Qin H.J., Li J.X., Irene M.C L., He D., Dong H.R., 2015, The limitations of applying zero-

valent iron technology in contaminants sequestration and the corresponding countermeasures: the 

development in zero-valent iron technology in the last two decades (1994–2014), Water Research, 75, 224–

248. 

Han L., Xue S., Zhao S.C., Yan J.C., Qian L.B., Chen M.F., 2015, Biochar supported nanoscale iron particles 

for the efficient removal of methyl orange dye in aqueous solutions, Plos One, 10(7), e0132067. 

Hu P.G., Zhang F.S., Shen F., Yu X., Li M.S., Ni H., Li L., 2017, Poly-g-glutamic acid coupled pseudomonas 

putida cells surface-displaying metallothioneins: composited copper(II) biosorption and inducible flocculation 

in aqueous solution, RSC Advances, 7, 18578–18587. 

Liu T.Y., Zhao L., Sun D.S., Tan X., 2010, Entrapment of nanoscale zero-valent iron in chitosan beads for 

hexavalent chromium removal from wastewater, Journal of Hazardous Materials, 184(1-3), 724-730. 

Magdolna B., Anna-Lena K., Reka M.M., John F.H., Lajos D., Bo N., Janos B., 2008, Nanoparticles formed by 

complexation of poly-gamma-glutamic acid with lead ions, Journal of Hazardous Materials, 153, 1185-1192. 

Manning B.A., Kiser J.R., Kwon H., Kanels S.R., 2007, Spectroscopic investigation of Cr (III)-and Cr (VI)-treated 

nanoscale zerovalent iron, Environmental Science Technology, 41, 86-592. 

Mouhamadou T.G., Elisabetta P., Luca D.P., 2015, Chemical reduction of hexavalent chromium (VI) in soil slurry 

by nano zero valent iron, Chemical Engineering Transactions, 43, 655–660. 

Qian L.B., Shang X., Zhang B., Zhang W.Y., Su A.Q., Chen Y., Da O., Lu H., 2019, Enhanced removal of Cr(VI) 

by silicon rich biochar-supported nanoscale zero-valent iron, Chemosphere, 215, 739-745. 

Shi L.N., Zhang X., Chen Z.L., 2011, Removal of chromium (VI) from wastewater using bentonite-supported 

nanoscale zero-valent iron, Water Research, 45(2), 886-892. 

Soroosh M., Hyeunhwan A., Dongwon C., Jaeyun M., 2018, Activated carbon impregnated by zero-valent iron 

nanoparticles (AC/NZVI) optimized for simultaneous adsorption and reduction of aqueous hexavalent 

chromium: Material characterizations and kinetic studies, Chemical Engineering Journal, 353, 781-795. 

Xie Y.K., Dong H.R., Zeng G.M., Tang L., Jiang Z., Zhang C., Deng J.M., Zhang L.H., Zhang Y., 2017, The 

interactions between nanoscale zero-valent iron and microbes in the subsurface environment: A review, 

Journal of Hazardous Materials, 321, 390–407. 

Zakariyah A.J., Tawfik A.S., Shaikh A.A., 2017, Biogenic glutamic acid-based resin: its synthesis and application 

in the removal of cobalt(II), Journal of Hazardous Materials, 327, 44–54. 

Zhang S.Y., Zhang C., Liu M.Y., Huang R.L., Su R.X., Qi W., He Z.M., 2018, Poly (γ-Glutamic Acid) Promotes 

Enhanced Dechlorination of p-Chlorophenol by Fe-Pd Nanoparticles, Nanoscale Research Letters, 13(1), 

219. 

Zhang S.Y., Zhang C., Liu M.Y., Huang R.L., Su R.X., Qi W., He Z.M., 2018, Dechlorination of p-chlorophenol 

by Fe-Pd nanoparticles promoted by poly (γ-glutamic acid)–dopamine composite, Chemical Engineering 

Transactions, 70, 2095–2100. 

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