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Abstract
This study was conducted to investigate chemical reduction efficiency of nitrate by synthesis nanoscale
zero-valent iron (NZVI) in aqueous solution, under aerobic condition. TEM image shows
synthesis nano zero-valent iron has a size in the range of 40–150nm. Experimental results suggest
that the reduction efficiency of nitrate decreased quickly with increasing initial pH value from 4
to 10 increased considerably with the increasing dosage of nanoscale zero-valent iron from 0.25
to 1gl-1 and did not vary much with initial nitrate concentration changing from 30 to 50 mg l-1 (NO
3
-N). With reductive denitrification of nitrate by nano zero-valent iron, the removal rate of nitrate
reached 80% in 60 min with nano zero-valent dosage of 1.0gl−1
and pHin4, in room temperature.
Introduction
Nowadays, regarding to increasing demand on safe drinking water, removal of widespread
pollutants such as nitrate is creating a significant challenge in water treatment industry. Anthropogenic
sources such as nitrogen fertilizer, nitrogen pesticides and industrial waste effluent discharge
account for most nitrate contamination of ground and surface waters [1]. Elevated nitrate
concentrations in drinking water supplies present a potential risk to public health. In infants NO3
is reduced to NO2
, which combine with hemoglobin in the blood to form methemoglobin leading
to cyanosis in babies under six month old [2]. A research conducted by Mayo Clinic Center
in Minnesota also showed that drinking tap water with a high concentration of nitrate would
have a higher risk of causing bladder cancer and ovary cancer [3]. Therefore many countries have
regulated the concentration of nitrate in drinking water. In the US, EPA established a maximum
contaminant level (MCL) of 10mg/L NO3
-N for drinking water [4]. In Iran the regulatory thresholds
for NO3
-N in drinking water sources are set as 10mg/L which is equivalent to 44.82 mg/L NO3
.
Current technology to remove nitrate from water include ion exchange, reverse osmosis, biological
denitrification and chemical reduction [3]. Among different water treatment methods, using
nano materials such as nano zero-valent iron as a new method has a good potential for removal of
nitrate. Researchers have studied the use of zero-valent iron in halogenated organics, azoaromatic
nitroaromatics and the treatment of different kinds of compounds such as inorganic compounds
like heavy metals [5]. In recent years, there has been a growing interest in the use of zero-valent iron in the treatment of water containing nitrate. Previous studies have demonstrated that nitrate
could be completely reduced by metallic iron under anoxic and aerobic conditions; furthermore,
the major reduction product was ammonia [6,7]. Nano zero-valent iron in contrast with iron powder
have some advantages of high specific surface area, high active surface, easily being sca- tered
and so on, which lead to the increasing denitrification rate of nitrate [5]. In this paper, we studied
the parameters which affect on the effectiveness of nitrate removal by synthesis iron nano
particles. These parameters include pH, dosage of nanoscale Fe˚ and nitrate concentration. The
Purpose of this study is to improve our understanding of denitrifcation condition by nanoscale Fe.
2. Materials and Methods
2.1. Chemicals and materials
The following chemicals were purchased from Merck: NaBH4
(for synthesis), FeSO4
.7H2
O (98%),
Methanol (99%), NaOH (99%), H2
SO4 (98%), KNO3
(98%). Nitrate reagent (Nitraver 5) was obtained
from Hack company.
2.2. Method for nanoscale Fe0
synthesis
In a typical synthesis of Fe˚ Nanoparticles by borohydride reduction, 4.0g of FeSO4
·7H2O was dissolved
in 200mL of 30% methanol and 70% de-ionized water (v/v). The pH was adjusted to about
6.8 by 3.8M NaOH. Then 1.5 g of NaBH4
powder was dissolved in 10mL de-ionized water and the
solution was added incrementally to the mixture in ultrasonic shaker at 25ºC temperature for 45
min After addition of all of the NaBH4
solution the mixture was stirred in jar test for another 45
min and then centrifuged for 15 min at 5000 rpm. The solid particles were washed at least three
times with methanol and then dried for 4hr under vacuum condition, and then broken up with a
spatula to form a fine black powder and immediately added to the aqueous solution to react with
nitrate. The ferrous iron was reduced to zero-valent iron according to the following reaction Eq(1):
Fe (H2
O)6
+ 3BH4
+ 3H2
O → Fe˚ + 3B(OH)3 + 10.5 H2 (1)
2.3. Preparation of aqueous nitrate solution
Different concentrations of nitrate in aqueous solution were prepared by dissolving desired
quantities of KNO3 in de-ionized water. An initial concentration of 30mg/l NO3
-N (133 mg/l NO3
)
was used for studying the effects of pH and dosage of iron nanoparticles, whereas 50mg/l NO
3
-N (222 mg/l NO3
) also was used for studying the effect of nitrate concentration on removal
effectiveness.
2.4. Experiments for chemical reduction of nitrate by iron
Five hundred milliliter of aqueous nitrate solution of a selected concentration was first put in
each of glass beakers for each set of experiments. Freshly prepared nanosized ZVI at arbitrary
concentration (1, 0.5 or 0.2g/L) was added to each glass beaker. Chemical reduction of nitrate by
nanosized ZVI at ambient temperature, and desired pH(4,7,10) and/or desired nitrate concentrations
(30, 50 mg/l) were simultaneously conducted in various glass beakers using a jar test apparatus at a mixing rate of 300 rpm. In each set of experiments the reaction vessels were removed
one by one from the jar test apparatus at intervals of 10 min during the 60-min reaction. Periodically,
20ml of the aqueous solution passed through a 0.45 µm membrane filter to separate iron
nanoparticles. The concentrations of unreacted nitrate (NO3) were determined by VIS spectrophotometer
(VIS DR2800, HACH). The pH value was measured using pH meter (satorius-150).
2.5. Mechanism of denitrification by nanoscale Fe0
In aqueous systems, zero-valent iron (Fe˚) is readily oxidized to ferrous ion (Fe+2) by many substances.
under anaerobic condition, H+
is the only electron acceptor that will be involved in the
reaction. Therefore, the overall process of corrosion in anaerobic Fe˚–H2
O system is described by
the following reaction [6], Eq(2). But Under aerobic conditions dissolved oxygen would play a
role of the electron acceptor in the cathodic half-reaction. In this case, the primary reaction yields
only OH
and not H2
[8], Eq(3):
Fe˚ + 2H2
O→ Fe+2 + H2 +2OH- (2)
2Fe˚ +O2
+ 2H2
O→ 2Fe+2 +2OH- (3)
The reaction and its mechanisms between nitrate and ZVI is a true redox reaction (Yang and Lee,
2005). Several studies have indicated the final products of chemical reduction of nitrate by ZVI
could be N2
or NH3
depending on the experimental conditions [6,7,8]. But certainly, the main
product of this reaction is ammonium [7], (Eq(4)):
4Fe˚ + NO3
+ 7H2
O→ 4Fe +2 + NH4
+ + 10OH- (4)
3. Results and Discussion
3.1. Characterization of nanoscale Fe0
The particle size are determined by PHILIPS (EM208 S, the Netherlands), transmission electron
microscopy (TEM) at 100 kV of acceleration voltage. Figure 1, shows TEM image of the synthesized
nanoscale Feº. The particles are spherical with the size ranging from 60 to 120 nm in
diameter.
Effect of Fe0
dosage on nitrate reduction by nanoscale Fe0
Fe˚ dosage is a significant variable parameter in nitrate reduction by nanoscale Feº. Since the
denitrifcation of nitrate by Feº involves reaction at the metal surface, it was anticipated that the
quantity of metal surface area should strongly influence the efficiency of nitrate reduction. In
this study we used three different dosage of nanoscale Feº (0.2, 0.5, 1 g/L). As shown in figure
2, with the dosage increasing, the removal efficiency of nitrate become higher and higher, In
0.2g/L Feº dosage, after 60 min nitrate removal efficiency reached 57%, then increasing Fe dosage
of 0.2 to 0.5, cause increasing efficiency to 70% ,and finally in 1g/L dosage after 60min, nitrate
removal reached near 80% . Therefore with increasing Fe˚ concentration, metal surface area also
increased, and efficiency of nitrate reduction will be increased [2,7,9].