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Chloriteremovalwithferrousions
ArticleinDesalination·June2005
ImpactFactor:3.76·DOI:10.1016/j.desal.2004.11.013
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CarloCollivignarelli
UniversitàdegliStudidiBrescia
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Desalination 176 (2005) 267–271
Chlorite removal with ferrous ions
Sabrina Sorlini*, Carlo Collivignarelli
Department of Civil Engineering, University of Brescia, via Branze 38, 25123 Brescia, Italy
Tel. +39 (030) 371-5826; Fax. +39 (030) 371-5503; email: [email protected]
Received 28 October 2004; accepted 5 November 2004
Abstract
Effective use of chlorine dioxide as an alternative disinfectant in water treatment may require removal of the byproduct chlorite ion (ClO2!). The goal of this research was to investigate the use of ferrous iron (Fe2+) for the chemical
reduction of ClO2! from drinking water in order to define the operating conditions, process efficiency with different
pH conditions and organic carbon concentration and the potential formation of chlorate during this process. The main
results show that the reaction between the ferrous ion and chlorite is very fast (5–15 s) over a range of pH 6.5–8.0; in
this condition a ferrous ion dose of 3.31 mg Fe/mg ClO2! completely reduced chlorite to chloride, producing minimal
residual soluble iron. For pH higher than 8.0–8.5, chlorite removal is lower due to the natural transformation of the
ferrou ions to ferric hydroxide. Within these pH values, chlorite can be removed completely with ferrous ion
concentrations higher than the stoichiometric value. Moreover, the application of ferrous salts for chlorite removal
during the coagulation process enhances the performance of the coagulation and flocculation treatment.
Keywords: Disinfection by-products; Chlorite; Ferrous ions
1. Introduction
Chlorine dioxide is a strong oxidant and disinfectant that does not form trihalomethanes
(THMs). However, chlorine dioxide application
can generate, through secondary reactions, both
organic and inorganic [chlorite (ClO2!) and
*Corresponding author.
chlorate (ClO3!)] disinfection by-products. Chlorite was found toxic at more than 0.2 mg/L on
Daphnia magna [1–3].
For potential toxic effects on human health,
the current Italian regulation on drinking water
(legislative decree No. 31 2/2/2001 [4], accomplishment of the European directive 98/83/UE)
introduced a maximum concentration of 200 µg/L
for chlorite (with a transitory limit of 800 µg/L
Presented at the Seminar in Environmental Science and Technology: Evaluation of Alternative Water Treatment
Systems for Obtaining Safe Water. Organized by the University of Salerno with support of NATO Science Programme.
September 27, 2004, Fisciano (SA), Italy.
0011-9164/05/$– See front matter © 2005 Elsevier B.V. All rights reserved
268
S. Sorlini, C. Collivignarelli / Desalination 176 (2005) 267–271
until December 2006), which is lower than US
EPA limitations (maximum concentration level =
1000 µg/L; maximum concentration level goal =
800 µg/L) [5]. This difference is due to the fact
that the EPA’s limitations were defined on the
basis of some new studies on chlorite toxicology
[6] not considered in the WHO Guidelines in
1993 [7,8].
Chlorite removal could greatly enhance the
potential for chlorine dioxide use in drinking
water treatment. Recent studies have investigated
different strategies for chlorite removal:
C adding reduced-sulfur compounds such as
sulfur dioxide and sodium sulfite [9]
C adding some salts, such as ferrous chloride
and ferrous sulfate [10]
C applying powdered (PAC) or granular (GAC)
activated carbon [9,11].
organic carbon. The influence of water characteristics on the efficiency of this process was
studied on three different surface waters characterized by different organic matter concentrations
(TOC = 1.7, 2.8 and 3.6 mg/L).
During chlorite removal with ferrous ions,
chlorite reduction to chloride is a very fast reaction that is concluded in 3–5 s at a pH range of
5–7, with a stoichiometric dosage of ferrous ions,
3–3.1 mg Fe/mg ClO2! [10], following the reaction:
2.2. Experimental
4 Fe2+ + ClO2! + 10 H2O ÷ 4 Fe(OH)3(s)
+ Cl! + 8 H+
(1)
During this process, ferrous ions (Fe2+) are oxidized to Fe3+ in the form of insoluble Fe(OH)3,
which can easily be removed by means of sedimentation and/or filtration. During a coagulation/
flocculation process a lower reagent dose can be
applied due to the coagulation/flocculation effect
of ferrous ions.
Some authors [12] found that chlorite removal
can be inhibited with high oxygen concentration
due to the consumption of oxygen for ferrous ion
oxidation. The goal of this study is to define the
operating conditions of this process, particularly
as concerns the stoichiometry between ferrous
ions and chlorite and the efficiency of chlorite
removal at different pH conditions and dissolved
2. Materials and methods
2.1. Water characteristics
The experimental tests were performed on
three different water samples (Table 1) characterized by different organic matter and inorganic
ion concentrations. Chlorite was artificially
spiked in water with a concentration of 1 or
2 mg/L by adding a sodium chlorite solution
(with 25% concentration).
Batch experimental tests were performed with
a jar test apparatus with the following procedures:
C determination of the minimum time required
to complete chlorite/ferrous ion reaction: one
beaker was filled with 250 mL of raw water;
a stoichiometric dose of ferrous ion was added
Table 1
Water characteristics
Parameter
Treated
Raw river
groundwater water
Raw lake
water
pH
Turbidity, NTU
TOC, mg/L
UV254 nm, 1/cm
DUV, 1/cm
ClO2!, mg/L
Cl!, mg/L
NO3!, mg/L
SO4!!, mg/l
7.20
0.40
1.7
0.005
0.006
0.098
14.0
34.0
34.0
7.74
9.30
3.6
0.093
0.062
<DL
198.0
24.0
105.0
7.86
8.36
2.8
0.078
0.054
<DL
13.0
19.0
52.0
DL, detection limit; DUV, absorbance UV 254 nm after
0.45 µm membrane filtration.
S. Sorlini, C. Collivignarelli / Desalination 176 (2005) 267–271
and mixed at 120 rpm for 1 min. Chlorite
residual concentration in water was detected at
different times after the addition of ferrous
ions (30, 60, 180, 600 s);
C determination of the optimum ferrous ion
dose: three beakers were filled with 250 mL of
water and different ferrous ion concentrations
were added (75%, 100%, 125% of the stoichiometric value); they were mixed at
120 rpm for 1 min, and after 30 s contact time
chlorite residual concentration was detected;
C determination of optimum pH: three beakers
were filled with 250 mL of water at different
pH conditions (7.0, 7.86, 8.5). The optimum
ferrous ion concentration, determined in the
previous step, was added into each beaker and
they were mixed at 120 rpm for 1 min; after
30 min contact time chlorite residual concentration was detected.
A stock ferrous ion solution (FeCl2) was of
analytical grade with 14% ferrous ion concentration. The stock chlorite solution was prepared
daily by adding sodium chlorite (NaClO2) to
distilled water. All the tests were performed at
15°C temperature.
2.3. Analytical methods
The following parameters were analysed: pH
(pH meter 713, Metrohm); ion analysis, particularly chlorite, was performed by direct injection into an ion chromatograph (Dionex, series
4500 series, column AS9, pre-column AS9SC,
with an eluent solution of Na2CO3 0.002 M,
NaHCO3 0.00075 M and with a conductivity
detector); TOC (total carbon monitor 480, Carlo
Erba); absorbance UV 254 nm and DUV (after
0.45 µm membrane filtration) (Beckman DU.70
with quartz cell of 1 cm); NH4+, Ca2+, Mg2+, Fe2+,
total Fe, soluble Fe, Mn, and alkalinity were
analysed according to Water Analysis Methods
IRSA/CNR [13].
269
3. Results and discussion
The results have been analysed only for raw
river water, as both treated groundwater and raw
lake water showed a similar behavior. Ferrous ion
reaction with chlorite at a stoichiometric ratio of
3.31 Fe2+/ClO2! at natural pH (pH 7.86) is very
rapid, as it is completed in 30–60 s (Fig. 1).
When the ferrous ion dosage is about 75% of the
stoichiometric dose, the maximum chlorite removal, about 83%, is reached after 30 s. When
the ferrous ion dosage is 100% of the stoichiometric value, chlorite is completely removed
(98–100%) after 60 s. Furthermore, when ferrous
ion is 125% of the stoichiometric value, 100%
removal is obtained after 30 s. The final pH value
was 7.65.
The influence of pH on the process efficiency
was evaluated by means of batch tests with a stoichiometric dose of ferrous ion (3.31 Fe2+/ClO2!)
and different pH conditions (7.0, 7.86 and 8.5);
chlorite removal was evaluated 30 s after ferrous
ion addition. The results, represented in Fig. 2,
show that the highest chlorite removal (99.6%) is
obtained at pH 7.0, which is the optimal pH condition. However, for higher pH, chlorite removal
efficiency is lower (98% with pH 7.86 and 80%
with pH 8.5). In fact, for pH higher than 8.0, the
prevalent iron form in water is the ferric ion that
generates insoluble ferric hydroxide. The result is
Fig. 1. Chlorite removal with time after adding ferrous
ions (75%, 100% and 125 % of stoichiometric dose).
270
S. Sorlini, C. Collivignarelli / Desalination 176 (2005) 267–271
Fig. 2. Chlorite residual concentration at different pH conditions
with a dose of stoichiometric ferrous ions.
Fig. 3. Chlorite removal with different ferrous ion doses
and pH conditions.
Fig. 4. Residual soluble iron with different ferrous ion
doses and pH conditions.
that lower ferrous ion is available for reaction
with chlorite and, consequently, a lower chlorite
removal can be obtained.
For different pH conditions different ferrous
ions doses were applied in order to define the
optimal dose with respect to pH. The results of
Fig. 3 show that for pH 7.0 and 7.86 the maximum chlorite removal can be obtained with the
stoichiometric value, while with a pH of 8.5 an
additional 25% amount of Fe2+ with respect to the
stoichiometric one is required. The final pH
values were 6.96, 7.67 and 8.3 with initial pH
values, respectively, of 7.0, 7.86 and 8.5.
No soluble iron after treatment (Fig. 4) is
detected for a ferrous ion stoichiometric dosage at
any pH conditions due to the complete reaction
between ferrous ions and chlorite. When the ferrous ion concentration is 125% of the stoichiometric value, a residual soluble iron is detected
only for pH lower than 8.0 [0.4 mg/L for pH of
7.0 and 0.55 mg/L for pH of 7.86 (natural pH)],
while for pH 8.5 the final iron concentration is
very low (0.01 mg/L) due to insoluble ferric
hydroxide formation.
The consequence is that the iron maximum
allowable concentration of 200 µg/L, indicated by
S. Sorlini, C. Collivignarelli / Desalination 176 (2005) 267–271
Legislative Decree 31/01, can be observed for
any pH condition with 100% of the stoichiometric ferrous ion dose or lower, while for a
higher dose (125% Fe2+ of the stoichiometric
value), this maximum concentration can be observed only for a pH higher than 8.0.
Chlorate concentration detected in all the
experimental tests was lower than the detection
limit of 3 µg/L.
4. Conclusions
Complete removal of chlorite can be obtained
with a ferrous ion stoichiometric dose at neutral
pH. At higher pH conditions (pH >8) an additional 25% Fe2+ dose is required to reach complete chlorite removal. The influence of organic
matter concentration on the process efficiency is
negligible.
Chlorite removal with ferrous ions can be successfully applied on new potabilisation plants or
as an up-grading stage of existing plants (by
adding ferrous ions in existing coagulation/
flocculation tanks or before sand filtration). This
process is very interesting for its simplicity and
economy.
Acknowledgements
The authors are grateful to Dr. Mario Belluati
(Caffaro S.p.a., Brescia), for the useful suggestions offered during the development of the
experiment; to engineers Ferrari Silvia and Tosini
Sara and the CTA personnel of Caffaro for their
assistance during the performance of the tests.
The valuable contribution by Luigi Rizzo and
Ceyda Uyguner is appreciated.
271
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