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CHEMICAL ENGINEERING TRANSACTIONS
VOL. 43, 2015
A publication of
The Italian Association
of Chemical Engineering
Online at www.aidic.it/cet
Chief Editors: Sauro Pierucci, Jiří J. Klemeš
Copyright © 2015, AIDIC Servizi S.r.l.,
ISBN 978-88-95608-34-1; ISSN 2283-9216
Chemical Reduction of Hexavalent Chromium (VI) in Soil
Slurry by Nano Zero Valent Iron
Mouhamadou Thierno Gueye, Elisabetta Petrucci, Luca Di Palma*
Dipartimento di Ingegneria Chimica Materiali Ambiente, Sapienza Università di Roma, via Eudossiana 18, 00184, Roma,
Italy
luca.dipalma@uniroma1.it
The increasing industrial development of recent decades has lead to the production of increasing quantities of
waste containing heavy metals, elements often harmful to the environment, which in the past were not
properly disposed of, thus inducing soil and groundwater pollution.
In particular, chromium (Cr) and its derivatives are largely used in industries such as textiles, electronics,
metallurgy, tanneries. Consequently, large quantities of this element were released into the environment due
to leakage or incorrect disposal.
Chromium is a transition element present in nature in three stable forms: metallic Cr, trivalent Cr(III) and
hexavalent Cr(VI). Metallic Chromium is rarely found in nature, mainly as natural chrome metallic inclusions in
diamonds, fragments of as meteorites and metal alloys in fluvial deposits. The trivalent form is characterized
by a relatively low toxicity, while the hexavalent chromium present in different compounds of industrial origin,
is considered highly toxic towards the respiratory system and carcinogenic.
In the present work, lab experiments of Cr(VI) contaminated soil clean-up by chemical reduction with
nanoparticles of zero valent iron (nZVI) are presented and discussed. The aim of the work was to optimize the
main operative parameters of the reduction process (pH, nZVI concentration, liquid/solid ratio). Cr(VI)
reduction using nZVI was found to obey a pseudo-first-order kinetic: the kinetic constant depended upon the
nZVI: Cr(VI) ratio. The use of nZVI in combination with sodium dithionite was also studied, by performing tests
in batch conditions at pH = 1.3, in order to assess the optimal ratio between nZVI and Cr(VI), and between
dithionite and Cr(VI). The results obtained showed an increase of Cr(VI) reduction rate with respect to the
tests carried only with nZVI: for long treatment times, up to 24 hours, an almost total removal of Cr(VI) was
achieved when a large excess of reagents was used.
1. Introduction
Heavy metals are common soil pollutants and constitute a threat for the ecosystems since they are not subject
to natural degradation (Alunno Rossetti et al., 2006). In particular, in the surrounding areas to galvanic
industries, textile and metallurgical high concentrations of hexavalent chromium are often found (Di Palma and
Verdone, 2012). Hexavalent chromium is highly toxic and on the basis of epidemiological and experimental
evidence has been classified by the IARC (International Agency for Research on Cancer) as a human
carcinogen, class I (Di Palma et al., 2005). It also has high water solubility and mobility in the environment
(Bartlett, 1991), making it a potential risk for contamination of groundwater and soil (Kozuh et al., 2000).
Technologies for Cr(VI) removal form soil include bioremediation (Chai et al., 2009), electrokinetic processes
(Gonzini et al., 2010), and oxidation/reduction processes (Singh et al., 2011), even by iron-based particles
(Cundy et al., 2008).
The present paper deals with the clean-up of a soil polluted by long term industrial activity. The aim of the
experimental work was to achieve the removal of hexavalent chromium through a decontamination treatment
based on the chemical reduction in the trivalent form, insoluble and less toxic (Di Palma et al., 2012).
DOI: 10.3303/CET1543110
Please cite this article as: Thierno Gueye M., Petrucci E., Di Palma L., 2015, Chemical reduction of hexavalent chromium (vi) in soil slurry by
nano zero valent iron, Chemical Engineering Transactions, 43, 655-660 DOI: 10.3303/CET1543110
655
Recently, the chemical reduction of Cr(VI) with zero valent iron nanoparticles (nZVI) has received
considerable attention in the field of environmental remediation and is becoming an increasingly used for the
treatment of hazardous and toxic waste and cleaning up groundwater and contaminated soils (Li et al., 2006).
In addition, a previous study also demonstrated that stabilized ZVI with carboxy-methyl-cellulose (CMC)
proved to be a very effective effective reducing agent against the Cr(VI) (Di Palma et al., 2015).
In this paper we have tested the use of the reducing agent in the treatment of polluted industrial soil. To this
purpose, as reducing agents, zero valent iron nanoparticles (nZVI) stabilized with (CMC), and a combination
of nZVI stabilized and sodium dithionite were studied. The tests were carried out in batch mode, at different
ratios between nZVI and dithionite and chromium, as well as varying the pH of the reducing solution.
The objective of this work was to optimize the main parameters (pH, concentration of Fe(0), solid-liquid ratio)
to determine the optimum conditions for the reduction of Cr(VI). The experiments were conducted on samples
of contaminated soil collected in an area affected by activities of coating and surface finishing of steels,
characterized by high concentrations of Cr(VI) and other heavy metals. It was therefore developed a kinetic
model with which to correlate the experimental data and obtain an estimate of the kinetic constants of the
reaction.
2. Materials and Methods
2.1 Soil characterization
Soil characterization was carried out through commonly used protocols (Liu and Evett, 2002). The main
characteristics of the soil are reported in Table 1.
Table 1: Selected characteristics of the soil
Parameter Value
pH 7.54
C.E.C. (meq/100 g) 9.6
Organic carbon (g/kg) 14.45
Organic matter (g/kg) 24.91
Metals concentration was determined after acid digestion technique according to the EPA 3050B method (Liu
and Evett, 2002) followed by atomic absorption spectrophotometry (FAAS) analysis, using a Agilent AA DUO
240 Fs instrument. The concentration of Cr(VI) was determined by the colorimetric method of
diphenylcarbazide (Bartlett, 1991) after alkaline digestion according to the EPA 3060A method (US EPA,
1996) using a UV-visible spectrophotometry (T80+, PG Instruments, Ltd.). Table 2 reports the mean
concentration of heavy metals in the soil, and the limits for a civil reuse (mg/kg), or industrial reuse (mg/kg)
according to Italian regulation (Italian Environmental Regulation, 2006).
Table 2: Metals in soil
Metal Concentration
(mg/L)
Limit for industrial reuse
(mg/kg)
Limit for civil reuse
(mg/kg)
Cr 155.2 150 800
Cr(VI) 54.4 2 15
Ni 156.2 120 500
Mn 817.6 - -
Fe 36267 - -
Pb 18.0 100 1000
2.2 Experimental procedure
Nano zero valent iron particles were prepared from a 1 g/L Fe2+ aqueous solution, by reacting with sodium
borohydride (NaBH4) at room temperature and in a free oxygen atmosphere. As dispersing agent sodium
CarboxyMethyl Cellulose (CMC) was used, at a CMC/Fe2+=0.005 molar (He et al., 2007; He and Zhao, 2007).
The reaction of formation of zero valent iron is as follows:
Fe(H2O)62+ + 2BH4- → Fe0↓ + 2B(OH)3 + 7H2↑ (1)
The size of nZVI and soil particles were measured using a Malvern Instruments Zetasizer 3600 Zen.
656
The reduction tests with nZVI were performed in batch mode, by mixing 5 g of soil in an orbital shaker at 120
rpm with 50 mL of the reducing solution. The stoichiometric amount of nZVI was calculated according to the
following equation:
3Fe0 + Cr2O72- + 7H2O → 3Fe2+ + 2Cr(OH)3 + 8OH- (2)
The experiments were performed at room temperature (20 ± 1 °C), and at the end of each test, the soil sample
was filtered through a 0.45 µm Whatman membrane filter, and the reaction was stopped by washing the soil
with distilled water. The pH in the liquid phase was measured using a GLP42+ Hanna Instrument. The residue
amount of metals and Cr(VI) in soil after treatment was determined according the above mentioned procedure.
In a first series of tests devoted to the reduction process optimization, selected concentration of Fe0 were
used. With respect to the stoichiometric ratio, excesses of iron equal to 15, 20 and 25 times, corresponding to
concentrations of Fe0 equal to 0.131, 0.175 and 0.219 g / L, were adopted.
In a second series of tests, carried out at room temperature and pH = 1.3, the combination of Fe0 and
dithionite was tested. The tests were conducted at molar ratios combined Fe0 / Cr (VI) and Na2S2O4 / Cr (VI)
equal to 15-15; 20-20, 25-25 (hereafter named as Fe15-D15, Fe20-D20 and Fe25-D25, respectively). All tests
were conducted in triplicate.
3. Results and Discussion
3.1 Effect of nZVI concentration and pH
The study reveals that treating polluted soil with a nZVI aqueous solution, the chromium removal was time
dependant and increasing with Fe0 concentration, as shown in Figure 1. In the test performed at the
concentration of 15 times excess, Cr(VI) removal of about 70% was achieved: the residue amount in the soil
was 16.1 mg/kg then higher than the limit for both residential and industrial reuse according to Italian
Regulation (Italian Environmental Regulation, 2006, see Table 2). Residue level below this threshold was not
achieved either increasing the Fe0/Cu(VI) ratio up to 25, or prolonging the reaction up to 24 hours of treatment
(data not shown). This behaviour can be explained considering that that the Cr(VI) reductive reaction occurred
on the Fe0 nanoparticles surfaces. As the Fe0 nanoparticles mass concentration increased, the reactive sites
proportionally increased, thus determining a corresponding increase of Cr(VI) removal efficiency.
0
10
20
30
40
50
60
70
0 20 40 60 80 100 120 140
C
r(
V
I)
(
m
g
/k
g
)
Time (min)
x 15
x 20
x 25
Figure 1: Cr(VI) residue in the soil treated with nZVI
In addition, during treatment alkaline conditions were established, depending on the concentration of Fe0, due
to the reaction (2) and the side reaction of Fe0 with water producing hydroxyl ion according to the (Liu et al.,
2005):
Fe0 + 2H2O → Fe2+ + H2 + OH- (3)
However, the final pH of the slurries were 7.64, 7.43 and 7.17 for initial pH 7.76, 7.73 and 7.63 for the tests
conducted with x15, x20, and x25 excess, respectively. Such a slight decrease in pH after 120 min of reaction
was due to the consumption of hydroxides by trivalent chromium and iron, to form oxy-hydroxides of Cr and
Fe on the surface of the nanoparticles (Powell et al., 1995), despite their release according to the above
mentioned reactions 2 and 3 (Orth and Gillham, 1996). The formation of oxy-hydroxides of Cr and Fe on the
surface of the nanoparticles during the reaction also determined the passivation of reactive surface (Lee et al.,
657
2003), thus explaining the reduction of reagent efficiency over time, as already observed in other studies
(Rivero-Huguet and Marshall, 2009).
3.2 Kinetics
Figure 2 shows representative kinetic profiles accounting for Cr(VI) reduction using nZVI in the slurry reactor
under agitation (120 rpm). Good correlation coefficients were obtained from the linear regression analysis,
thus revealing that Cr(VI) reduction using nZVI obeys a pseudo-first-order kinetic model, as found in other
studies in literature (Franco et al., 2009).
y = -0,0714x
R² = 0,98442
y = -0,0335x
R² = 0,97276
-3
-2,5
-2
-1,5
-1
-0,5
0
0,5
1
0 5 10 15 20 25 30 35
ln
(C
/C
0)
Time (min)
x25
x15
Figure 2: Kinetics of pseudo-first order of the Cr(VI) reduction
Considering that the redox process obeys a pseudo-first-order kinetic model (Wang et al., 2010), the reduction
of Cr(VI) taking place in the slurry reactor can be described by the following relation (Xu and Zhao, 2007):
ln Cr (VI ) = ln Cr (VI )0 ⋅ kobs
* ⋅ t (4)
where [Cr(VI)] is the instantaneous concentration of Cr(VI) (mg/L), t is the treatment time (min) and k*obs is the
overall kinetic rate constant for the heterogeneous redox process (min-1).
Taking into account the heterogeneous nature of the redox process carried out under constant and intense
agitation and the collisions among the suspended soil microparticles (SSM), as well the collisions of these
particles with the nZVI, at k*obs can represented by the (Franco et al., 2009):
k
obs
* = k
h
⋅ Cr (VI )
0
⋅ Fe
0
0{ }
y
= k
sa
⋅ a
s
⋅ Z
h
⋅θ ⋅ A
SSM( )( ) ⋅ Cr (VI )0 ⋅ Fe0 0{ }
y
(5)
where [Cr(VI)]0 and [Fe0]0 are the initial concentrations of Cr(VI) and nZVI, respectively, and kh and y are
empirical constants for the specific redox process.
The parameter kh is the heterogeneous kinetic rate constant, representing the redox process taking place at
the surface of the nZVI under perfect mixing conditions, in contact with SSM, and:
- Zh is the heterogeneous collision frequency factor;
- ksa is the specific kinetic rate constant related to the active surface area of iron nanoparticles (l.min-1 m-2) (He
et al., 2007);
- as is their average specific surface area (m2 g-1)
- θ.A(SSM), with 0 ≤ θ ≤ 1, is the average active surface area of the SSM where θ is the fraction of the SSM
surface containing Cr(VI).
Thus, once obtained k*obs from the kinetics data (Figure 2), and measured the size of nZVI and SSM, the
values of the kinetic parameters were calculated, as shown in Table 3.
3.3 Combination of nZVI and dithionite for Cr(VI) reduction
The method uses the nZVI in combination with dithionite (Na2S2O4 = D) in equimolar ratio, to inhibit oxidation,
aggregation and precipitation of ferrous iron, which mitigates the surface passivation of nZVI. The mechanism
658
of Cr(VI) treatment in Na2S2O4 applications involves the conversion of Fe(III) in soils to Fe(II) by Na2S2O4 and
the subsequent reduction of Cr(VI) to Cr(III) by Fe(II) to form the CrxFe1-x(OH)3 solid (Paul et al., 2002).
Sodium dithionite in water undergoes dissociation and disproportionation reactions to form primarily sulfoxyl
radicals (SO2•-), sulfites (SO32-), and thiosulfates (S2O32- ) via following equations:
S2O42- → 2SO2•- (6)
4SO2•- + H2O → 2SO32- + S2O32- + H+ (7)
Table 3: Values of kinetic parameters
Fe0/Cr(VI)
(mol/mol)
[Fe0]
(g/l)
K*obs
(min-1)
ksa
(l.m-2.min-1)
as
(m2.g-1)
Θ A(SSM)
(m2)
Zh
(l.g-1.m-2)
Y
15/1 0.131 0.0335 4.27.10-8 5.98.106 0.31 2.07 287.25 1
25/1 0.175 0.0714 5.45.10-8 5.98.106 0.31 2.07 287.25 1
During dissociation reactions, dithionite can reduce structural iron in clays and dissolve and reduce
amorphous and some crystalline Fe(III) oxides to produce one or more Fe(II) species (Szecsody et al., 2004):
SO2•- + Fe3+ + H2O → Fe.2+ + SO32- + 2H+ (8)
The oxidation of 1 mol of dithionite by Fe(III) to 2 mol of sulfite ultimately resulted in the production of 4 mol of
acid. To limit the precipitation of Fe(OH)2 which can inhibit the dissemination of Fe(II), the test were perfomed
in a pH range between 2 and 5 by adding H2SO4. In such conditions, the reduction of Cr(VI) by the Fe (II) can
be written as:
HCrO4- + 3Fe2+ + 7H+ ↔ Cr3+ + 3Fe3+ + 4H2O (9)
The results of the laboratory tests carried out using combined solutions of nano zero valent iron and dithionite,
indicate that treatment is more effective in long term compared to that made with only Fe0 (Figure 3). Thus,
the combination resulted in both an almost complete elimination of Cr(VI) using the solution Fe25-D25:
lowering of the concentration of couple Fe-D, the residual Cr(VI) was below 2 mg/kg after treatment with the
Fe20-D20 solution, thus allowing a civil reuse of the soil (Italian Environmental Regulation, 2006), and below
the limit for industrial reuse when the Fe15-D15 solution was used.
0
10
20
30
40
50
60
70
0 5 10 15 20 25 30
Cr
(V
I)
(m
g/
kg
)
Time (h)
Fe15-D15
Fe20-D20
Fe25-D25
Figure 3: Cr(VI) residue as a function of time by combining solution of nZVI and dithionite
4. Conclusions
In the present work the reduction of hexavalent chromium in contaminated soil, through a decontamination
treatment based on chemical reduction was investigated. As reducing agents solutions of stabilized zero
valent iron nanoparticles (nZVI) and a combination of nZVI and sodium dithionite were used.
The results showed that increasing the ratio Fe0/Cr(VI)0 the percentage of removal of chromium also
increased. After 120 min of treatment, using Fe0 excess of 15, 20 and 25 times, respectively, Cr(VI) content in
the soil was reduced of to 70.3 %, 86.6 %, 91.1 %. Tests conducted prolonging contact time up to 24 h did not
show significant increasing of performances, due to progressive Fe0 inactivation. Only when the higher
reagent excess was used it was possible to obtain levels of Cr(VI) residues in the soil such as to permit its
reuse for commercial or residential purpose in accordance with current legislation in Italy. Cr(VI) reduction
659
obeyed a pseudo-first-order kinetic, and was correlated, in accordance to literature model, to the collision
frequency between SSM and nZVI and the average specific surface area available for the redox process.
To investigate the possibility of further reducing Cr(VI) level, in a second series of tests, the reductant solution
consisted in a mixture of nZVI and sodium dithionite. Tests were carried out in batch conditions at pH = 1.3, to
assess the optimal ratio between Fe0/Cr(VI)0 between and dithionite/Cr(VI)0, showed a similar increase of the
reduction rate of Cr(VI) with respect to the tests carried only with nZVI. Furthermore, for long treatment times
(up to 24 h) it was possible to obtain in this case an almost total removal of Cr(VI), though using a large
excess of reagents (25 times with respect the stoichiometric amount).
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