1-s2.0-S030147970500232X-main

Journal of Environmental Management 79 (2006) 215–220
www.elsevier.com/locate/jenvman
The application of potassium ferrate for sewage treatment
Jia-Qian Jianga,*, Alex Panagoulopoulosa, Mike Bauerb, Pete Pearceb
b
a
School of Engineering (C5), University of Surrey, Guildford, Surrey GU2 7XH, UK
Thames Water Limited, Spencer House, Manor Farm Road, Reading, Berkshire RG2 0JN, UK
Received 6 July 2004; revised 7 May 2005; accepted 20 June 2005
Available online 22 September 2005
Abstract
The comparative performance of potassium ferrate(VI), ferric sulphate and aluminium sulphate for the removal of turbidity, chemical
oxygen demand (COD), colour (as Vis400-abs) and bacteria in sewage treatment was evaluated. For coagulation and disinfection of sewage,
potassium ferrate(VI) can remove more organic contaminants, COD and bacteria in comparison with the other two coagulants for the same
doses used. Also, potassium ferrate(VI) produces less sludge volume and removes more contaminants, which should make subsequent sludge
treatment easier.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: Coagulation; Disinfection; Potassium ferrate(VI); Sewage treatment; Sludge production
1. Introduction
Environmental regulation and public health concerns
require that sewage collected from communities must be
treated to the given standards before it is returned to surface
waters or reused. Sewage contains a range of contaminants,
which are either suspended or dissolved. Based on their
characteristics, the contaminants in the sewage can be
divided into four major groups, namely, total solids (e.g.
suspended and colloidal solids), inorganic constituents
(phosphorus, nitrates and heavy metals), organic constituents (e.g. proteins, carbohydrates and oils and fats) and
microorganisms (e.g. bacteria, fungi, protozoa and viruses),
of which about two thirds are organic constituents.
Most sewage treatment works consist of screens, grid and
fat removal tanks, primary settlers, activated sludge
processes or biofiltrators and secondary clarifiers. Primary
treatment aims to enhance the removal of colloidal particles,
organic constituents and harmful microorganisms and leads
to less remaining particles and organic contaminants; this is
favourable to the subsequent biological and physicochemical treatment processes.
* Corresponding author. Tel.: C44 1483 686609; fax: C44 1483 450984.
E-mail address: [email protected] (J.-Q. Jiang).
0301-4797/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jenvman.2005.06.009
Coagulation and oxidation/disinfection are two important unit processes for water and wastewater treatment.
Coagulation destabilizes colloidal contaminants and
transfers small particles into large aggregates and adsorbs
dissolved organic materials onto the aggregates, which can
then be removed by sedimentation and filtration. Disinfection is designed to kill the harmful organisms (e.g. bacteria
and viruses) and oxidation is used to degrade various
organic contaminants.
A wide range of coagulants and oxidants/disinfectants
can be used for water and wastewater treatment. The most
common coagulants used include ferric sulphate, aluminium
sulphate, and ferric chloride, and the oxidants/disinfectants
used are chlorine, sodium hypochlorite, chlorine dioxide,
and ozone (Jiang and Lloyd, 2002). An efficient water
treatment chemical reagent should ideally be able to
disinfect microorganisms, partially degrade and oxidise
the organic and inorganic contaminants, and remove
colloidal/suspended particulate materials and heavy metals.
A potential chemical reagent that meets these criteria is a
ferrate(VI) salt (e.g. potassium ferrate), which is a very
strong oxidant; under acidic conditions, the redox potential
of ferrate(VI) ions is strongest among all oxidants/
disinfectants used for water and wastewater treatment. It
is also a coagulant; during the oxidation/disinfection
process, ferrate(VI) ions are reduced to Fe(III) ions or ferric
hydroxide, and this simultaneously generates a coagulant in
a single dosing and mixing unit process (Jiang and Lloyd,
216
J.-Q. Jiang et al. / Journal of Environmental Management 79 (2006) 215–220
Table 1
Characteristics of sewage (January–March 2003)
Due to the variation in sampling date and time and flow rate,
the characteristics of the sewage varied and this can be seen
in Table 1.
Parameter
Range
pH
Turbidity (NTU)
Total suspended solids (mg LK1)
UV254-abs (cmK1)
Colour as Vis-abs at 400 nm (cmK1)
Total COD (mg LK1)
Soluble CODa (mg LK1)
Total Coliform (per 100 mL)
Faecal Coliform (per 100 mL)
7.25–7.88
29.4–73.3
97–303
0.168–0.316
0.011–0.041
353–527
154–194
4!108–2.2!109
3.3!108–2!109
a
Samples were filtered with 0.45 mm filter before the COD measurement.
2002). More importantly, ferrate(VI) is also an environmental friendly treatment chemical, which will not produce
any harmful by-products in the treatment process. Previous
studies have shown that potassium ferrate (VI) can perform
superiorly in degrading various synthetic and natural
organic pollutants (Bartzatt and Carr, 1986; Sharma and
Bielski, 1991; Bielski et al., 1994; Norcross et al., 1997;
Gulyas, 1997; White and Franklin, 1998; Sharma, 2002;
Jiang and Wang, 2003a), inactivating harmful microorganisms (Murmann and Robinson, 1974; Gilbert et al., 1976;
Schink and Waite, 1980; Kazama, 1995; Jiang and Wang,
2003b), and coagulating colloidal particles and heavy
metals (Bartzatt et al., 1992; Jiang et al., 2001; Jiang, 2003).
This paper explores the potential of using potassium
ferrate(VI) for sewage treatment. The performance of
potassium ferrate(VI) was compared with conventional
coagulants in terms of reducing the turbidity and total and
dissolved COD, the inactivation of bacteria, and the
comparative sludge production.
2. Sewage
Sewage samples were collected from a Sewage Treatment Works of Thames Water, and obtained from the plant
influent prior to the primary grit chamber. Samples were
collected in high-density polyethylene (HDPE) jerricans,
which were washed and rinsed with tap water before use.
3. Experimental procedures for coagulation
and disinfection
Comparative performance in treating sewage with
potassium ferrate (FR) and two traditional coagulants, ferric
sulphate (FS) and aluminium sulphate (AS) was evaluated
by a standard jar test procedure. The FR was prepared in the
laboratory following a modified chlorination procedure
(Thompson et al., 1951), in which a strong alkaline
hypochlorite solution was made by the reaction of a given
amount of KMnO4, HCl and KOH, and then reacted with
ferric nitrate to form the FR. AS and FS were provided by
Merck Ltd, UK. The coagulation pH was adjusted to either 5
or 7 in the use of FR but was not adjusted when using AS
and FS. The final pH varied from 6.75 to 7.48 depending on
the coagulant doses. The selected coagulant doses ranged
from 0.07 to 0.56 mmol LK1 as either Al or Fe and this is
shown in Table 2.
Blank samples were used for all jar tests conducted,
where no coagulant was added, but the sample pH was
adjusted if it was necessary. Upon the coagulant being
dosed, the sewage sample was rapidly mixed at a speed of
400 rpm for a period of one minute and then allowed to
flocculate at a speed of 35 rpm for a period of 20 min. After
slow mixing finished, the samples were allowed to settle for
a period of 60 min.
At the end of sedimentation, 100 mL of supernatant was
withdrawn for the measurement of various quality parameters, including turbidity, colour (as Vis400-abs), total
and dissolved COD and total and faecal coliforms. The
removal percentages were calculated based on the parameters of the blank samples (no coagulant addition, only
pH adjustment). The pH was adjusted with the addition of
either sulphuric acid or sodium hydroxide. The procedures
for measurement of water quality parameters followed the
Standard Methods (AWWA et al., 1992).
Table 2
Chemical doses for sewage coagulation
AS
FS
FR
(mg Al LK1)
(mmol Al LK1)
(mg Fe LK1)
(mmol Fe LK1)
(mg Fe LK1)
(mmol Fe LK1)
4
8
12
15
0.15
0.30
0.44
0.56
6
10
16
22
28
0.11
0.18
0.29
0.39
0.50
4
7
10
15
18
22
26
30
0.07
0.13
0.18
0.27
0.32
0.39
0.46
0.54
J.-Q. Jiang et al. / Journal of Environmental Management 79 (2006) 215–220
FR (33.8 NTU)
AS (31.1NTU)
FR (416ppm)
FS (33.1 NTU)
% Removal of total COD
% Removal of turbidity
AS (413ppm)
FS (410ppm)
40
100
80
60
40
20
0
35
30
25
20
15
10
5
0
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.00
Coagulant dose as Al or Fe (mmol L-1)
Results of coagulation and disinfection of sewage are
presented and discussed as follows. For all figures
presented, the numbers in brackets represent the values of
the sewage quality parameters of the blank samples, which
underwent the jar test including the pH adjustment but
without the coagulant addition.
Fig. 1 shows the comparative performance of turbidity
removal with three coagulants. In general, AS and FS can
remove about 80–85% of turbidity and their performance
was similar. However, over the dose range studied, FR can
achieve 85–95% turbidity removal; the advantage of using
the FR thus appears. Moreover, the superior performance of
FR has been demonstrated for colour removal as shown in
Fig. 2. FR can remove approximately 50% more colour than
AS and FS for the same doses when the chemical doses are
greater than 0.18 mmol LK1. This is also the case when
comparing the total and dissolved COD removal (Figs. 3
and 4). Percentage removal of total COD with FR was 5–
20% more than that with FS, and 5–28% more than that with
AS for the same doses. The overall removal performance for
dissolved COD was not as good as that of total COD, but FR
still can remove 5–10% more dissolved COD than AS and
FS (Fig. 4).
The outstanding performance in reduction of COD with
ferrate(VI) is consistent with previous studies, where a range
AS (0.010)
FS (0.010)
100
80
60
40
20
0
0.00
0.20
0.30
0.40
0.50
0.30
0.40
0.50
0.60
of organic contaminants have been shown to be readily
oxidised by ferrate (VI). The organic compounds investigated were alcohol (Norcross et al., 1997), carboxylic
compounds (Bielski et al., 1994), amino-acids (Sharma and
Bielski, 1991), phenol (Rush et al., 1995), organic nitrogen
compounds (Carr and Erickson, 1988), aliphatic sulphur
(Bartzatt and Carr, 1986), nitrosamines compounds (Read et
al., 1998), recalcitrant organics (Gulyas, 1997), thiourea
(Sharma et al., 1999), chlorine oxyanions (Carr and
Mclaughlin, 1988) and hydrazine compounds (Johnson and
Hornstein, 1994). All these organic contaminants could be
present in the wastewater, which is a mixture of municipal
sewage and industrial effluents. The high oxidation efficiency
of ferrate(VI) in reducing COD can be attributed to the high
redox potential; under acidic conditions, the redox potential
of ferrate (VI) ions is higher than that of molecular ozone, and
ferrate(VI) possesses the strongest oxidation capacity among
all oxidants/disinfectants realistically applicable to water and
wastewater treatment (Jiang and Lloyd, 2002).
Due to its high re-dox potential, FR not only oxidises
organic matter present in sewage, but also kills bacteria that
traditional coagulants lack the power to do. This study
demonstrates that at both pH 5 and 7, FR can achieve more
than 4Klog10 bacteria inactivation, whilst AS and FS can
FR (165ppm)
0.60
AS (164ppm)
FS (164ppm)
0.30
0.50
40
35
30
25
20
15
10
5
0
0.00
0.10
0.20
Fig. 3. Comparative total COD removal performance with three coagulants.
% Removal of soluble COD
4. Results of coagulation and disinfection
FR (0.012)
0.10
Coagulant dose as Al or Fe (mmol L-1)
Fig. 1. Comparative SS removal performance with three coagulants.
% Removal of colour
(as Vis-abs, 400 nm)
217
0.10
0.20
0.40
0.60
Coagulant dose as Al or Fe (mmol L-1)
Coagulant dose as Al or Fe (mmol L-1)
Fig. 2. Comparative colour removal performance with three coagulants.
Fig. 4. Comparative dissolved COD removal performance with three
coagulant.
218
J.-Q. Jiang et al. / Journal of Environmental Management 79 (2006) 215–220
Table 3
Comparative performance of bacteria inactivation (in log10 terms)
a
Coagulation pH
Total Coliform
Faecal Coliform
a
Simhoff : unsettled
suspended solids in
the supernatant of
Imhoff cone after
sedimentation
b
AS
FS
FR
6.75–7.48
0.89–1.05
0.96–1
6.75–7.48
0.89–1
1–1.05
5
O4
O4
7
O4
O4
a
AS and FS dose required was O0.50 mmol LK1 as either Al or Fe(III).
FR achieved O 4 log10 bacteria inactivation at doses !0.27 mmol LK1
as Fe(III).
Ssludge deposit : sludge in
the bottom of Imhoff
cone after coagulation
and sedimentation,
including sludge from
contaminants removed
and the coagulant used.
b
only achieve approximately 1Klog10 bacteria inactivation
(Table 3).
Chlorine is commonly used as a disinfectant for drinking
water and wastewater treatment. Since the discovery of
chlorinated by-products (e.g. trihalomethanes), which cause
potentially negative health effects in human beings (Rook,
1974), great efforts have been made to either minimise the
concentration of natural/synthetic organic compounds prior
to disinfection, or remove the chlorinated by-products after
disinfection. However, this will greatly increase the overall
cost of water treatment.
Alternative disinfectants (e.g. bromine, iodine, chlorine
dioxide, and ozone) have thus been considered to replace the
chlorine. However, they form a range of other by-products,
which are also considered to be toxic to some extent to the
human population and to aquatic life.
For these reasons potassium ferrate(VI) has been
investigated as an alternative to chlorine for the disinfection
of water and wastewater. The disinfecting properties of
ferrate(VI) were first observed by Murmann and Robinson
(1974) when they investigated the effectiveness of
ferrate(VI) as a disinfectant to kill two pure laboratory
cultures of bacteria (Non-recombinant Pseudomonas and
Recombinant Pseudomonas). Later, the study conducted by
Gilbert et al. (1976) showed that at pH 8.2 and a dose of
6 mg LK1 as Fe, ferrate(VI) could kill 99.9% of Escherichia
coli (E. coli) when the contact time was 7 min. Another
study using ferrate(VI) to disinfect secondary effluent
(Waite, 1979) revealed that 99.9% of total coliforms and
97% of the total viable bacteria could be removed by
ferrate(VI) at a dose of 8 mg LK1 as FeO2K
4 . Our previous
studies of using ferrate(VI) for drinking water treatment
(Jiang et al., 2001; Jiang, 2003; Jiang and Wang, 2003b)
have also confirmed that ferrate (VI) has sufficient
disinfection capability to kill E. coli. At pH 8.0, at a dose
of 6 mg LK1 as Fe(III) and a contact time of 30 min,
ferrate(VI) can achieve more than 5–6 log10 inactivation of
E. coli. To achieve the same disinfection efficiency for the
same conditions, combined chemical doses (chlorine and
ferric sulphate) were required; which were 10 mg LK1
chlorine (as Cl) plus 4 mg LK1 ferric sulphate (as Fe). The
studies also demonstrated that the disinfecting ability of
ferrate(VI) increased markedly if the water pH was
below 8.0.
Fig. 5. Schematic representation of mass balance after chemical
coagulation.
5. Study of sludge production
The sludge generated from primary sedimentation and
coagulation processes has a large volume and contains a
high percentage of water, which requires an extra treatment
process to handle. Water industries wish to use a coagulant
with good treatment performance and less sludge production and thus ease the sludge treatment process.
Comparison of the sludge production is thus useful for the
selection of coagulants.
Sludge production under the optimum operating conditions was studied by the use of a cone-shaped vessel called
an Imhoff cone (Fig. 5). The optimum treatment conditions
resulting from the previous studies were selected for the
sludge production experiments and these can be seen in
Table 4.
1 L of sewage was placed in the Imhoff cone, then the
coagulant was added at the given optimum dose as shown in
Table 4
The optimum conditions for sludge production experiments
Coagulant
Optimum dose as Al(III) or Fe(III)
(mg L
Blank
AS
FS
FRlow
FRhigh
)
K1
0
10
20
15
20
(mmol L
K1
0
0.37
0.36
0.27
0.36
pH
)
7
7
7
7
7
Table 5
Wastewater sludge volumes
Coagulant
Dose as Al(III) or
Fe(III) (mmol LK1)
Sludge production
(in mL)
Blank
AS
FS
FRlow
FRhigh
0
0.37
0.36
0.27
0.36
20
40
40
30
35
J.-Q. Jiang et al. / Journal of Environmental Management 79 (2006) 215–220
219
Table 6
Mass balance of sewage sludge
Coagulant and
dosagea
AS (0.37)
FS (0.36)
FRlow (0.27)
FRhigh (0.37)
a
SS in raw sewage
SS in supernatant (Imhoff)
Sludge deposit (Imhoff)
Coagulant produced sludge
Contaminants
removed
(g)
(g)
(mL)
(g)
(mL)
(g)
(g LK1 sewage)
0.382
0.382
0.382
0.382
0.0364
0.0204
0.0335
0.0285
960
960
970
965
0.47
0.49
0.51
0.54
40
40
30
35
0.035
0.042
0.032
0.042
0.017
0.046
0.063
0.088
Dosages were presented in brackets, as Al or Fe, mmol LK1.
Table 4, and the mixture was rapidly mixed at a speed of
400 rpm for one min and then allowed to flocculate at a
speed of 35 rpm for a period of 20 min. After fast and slow
mixing, the solution was allowed to settle for a period of
60 min. The sludge volume produced was measured and is
shown in Table 5. It can be seen that a slightly lower volume
of sludge was produced with FR at a similar dose compared
with AS and FS.
The total suspended solids (SS) in both sludge deposit
and supernatant in the Imhoff cone were measured
following the standard methods (AWWA et al., 1992),
and the results were used to calculate the mass balance of
sludge production.
The components considered to contribute to the sludge
production are the contaminants in the raw sewage and the
solids produced from the coagulant. Based on the mass
balance, the contaminants removed for a given 1 L sewage
sample can be calculated using Eq. (1):
½Removed contaminants
Z SSludge deposit KSRaw KSImhoff KScoagulant
(1)
where:
SSludgedeposit is the mass of the sludge in the bottom of
Imhoff cone after coagulation and sedimentation,
SRaw is the mass of suspended solids in raw sewage,
SImhoff is the mass of suspended solids remained in the
supernatant after sedimentation, and
SCoagulant is the mass of the sludge produced by adding
the coagulant.
Table 7
Comparative performance of coagulants at optimum dose
pH
Optimum dose as Al(III) or Fe(III)
(mmol LK1)
Turbidity removal (%)
Colour (Vis400-abs) removal (%)
Total COD removal (%)
Dissolved COD removal (%)
Bacteria inactivation
(in log10 terms)a
a
AS
FS
6.75–7.48
0.37
6.75–7.48
0.36
80
50
6
4
1
86
50
16
7
1.05
FR
The right part of Eq. (1) can be determined by the
measurement of suspended solids in various samples
together with a theoretical calculation to determine the
sludge produced from the coagulant used. In terms of the
previous studies (Montgomery, 1985), 1 g of Al produces
3.46 g of dry aluminium hydroxide and 1 g of Fe(III)
produces 2.1 g of ferric hydroxide. Using the established
ratio and the coagulant dosages, the coagulant sludge can be
estimated. Thus, the left part of Eq. (1), the net contaminants
removed from 1 L of sewage, can be determined. The study
results can be seen in Table 6. It is evident that FR can
remove more contaminants (e.g. organic materials as COD
or colour) and produce less wet sludge than FS and AS at an
equivalent dose; this should then make the handling of the
resulting sludge easier.
6. Conclusions
In sewage treatment, potassium ferrate(VI) can remove
50% more colour (Vis400-abs) and 30% more COD, and
inactivate 3-log10 more bacteria in comparison with AS and
FS at the same or even smaller doses (Table 7). In addition,
using ferrate(VI) produced less sludge volume, which
should then make sludge treatment easier.
Acknowledgements
The authors thank the UK Engineering and Physical
Science Research Council (EPSRC) and Thames Water that
provided a CASE studentship for A. Panagoulopoulos. The
views expressed in this paper are not necessarily representing that of Thames Water.
7
0.36
94
92
32
14
O4
AS and FS achieved 1Klog10 bacteria inactivation at doses O0.
50 mmol LK1 as either Al or Fe, whilst FR achieved O 4Klog10 bacteria
inactivation at doses !0.27 mmol LK1 as Fe.
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