Effects of iron surface pretreatment on kinetics of
aqueous nitrate reduction
Ya Hsuan Liou
a,∗, Shang-Lien Lo
a, Chin-Jung Lin
a, Wen Hui Kuan
b, Shih Chi Weng
a aResearch Center for Environmental Pollution Prevention and Control Technology, Graduate Institute of Environmental Engineering,National Taiwan University, Taipei 106, Taiwan, ROC
bDepartment of Environmental and Safety Engineering, Ming-Chi Institute of Technology,
Taishan, Taipei hsien 243, Taiwan, ROC Received 8 April 2005; accepted 27 June 2005
Available online 10 August 2005
Abstract
Using hydrogen gas at 400◦C to activate iron surface was proposed to remove nitrate (40 mg N L−1) in a HEPES buffer solution at pH value between 6.5 and 7.5. Compared with the nonpretreated iron, the first-order reaction rate constant (kobs) was increased 4.7 times by
pretreated iron, and the lag of the early period disappeared. Normalized to iron surface area concentration, the specific rate constant (kSA)
was increased approximately by a factor of 6 using hydrogen reduction (0.0020 min−1m−2L for nonpretreated iron and 0.0128 min−1m−2L for pretreated iron). The reactivity of aged iron covered by a complex mixture of iron oxides (soaking in nitrate solution for 60 days) were restored by hydrogen gas at 400◦C. Scanning electron microscopy (SEM) and temperature-programmed reduction (TPR) exhibited visibly cleaner without pitting and cracking and less oxygen fraction on pretreated iron surface relative to nonpretreated iron. Activation energies (Ea) of nitrate reduction over the temperature range of 10–45◦C were 46.0 kJ mol−1for nonpretreated iron, and 32.0 kJ mol−1for pretreated
iron, indicating chemical reaction control, rather than diffusion. The results indicated that this enhancement was attributed to the increase in active site concentration on iron surface by hydrogen reduction.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Nitrate; Iron; Pretreatment; Reduction
1. Introduction
Iron, the most commonly used material, is a highly reduc-tive metal for groundwater contaminated with organohalides [1–5], nitrate [6–10], heavy metals [9,11,12]and radioac-tive elements [13]. The disappearance of contaminants is attributed to a corrosion-like process, in which the iron donates electron to reduce target pollutants, accompanied by the dissociation of water. Generally, nonpretreated commer-cial iron is covered with a discontinuously passive layer of Fe2O3, formed during the high-temperature manufacturing process[14]. Additionally, a mixture of nonstoichiometric iron oxide and oxyhydroxide species may form in storage
∗Corresponding author. Tel.: +886 2 2362 5373; fax: +886 2 2392 8821.
E-mail address: d90541008@ntu.edu.tw (Y.H. Liou).
[15,16]. Prior to pretreatment, various valences of iron oxides predominantly controlled the reaction rate by restricting the diffusion of contaminants into the active sites on the iron sur-face. Thus, a pretreatment method is needed to remove the passive oxides layers to activate the iron surface.
The pretreatment methods, acid washing[2,4,17], chlo-ride ions treatment[18]and sonication[15], were proposed to remove the passive oxide layer prior to decontamination reaction, thereby increasing the available reactive sites and increasing the rate of the pollutant degradation at early time. At later time, the effect of both acid washing and chloride ions treatment provide little improvement. Evidences show that many fine iron particles were lost[16], and the oxidation rate of iron was highly accelerated due to increase concentra-tions of adsorbed H+and Cl−[4]. High energies inputs were employed for the pretreatment by sonication[15]. Thus, those 0304-3894/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
pretreatment methods may not be effective and convenient techniques for improving the drawbacks of iron.
In this study, the surface of iron was heated using reducing gas (20 vol.% H2/N2) at 400◦C for 3 h prior to the reduction of 40 mg N L−1 nitrate. The objective of this research was to investigate the effect of pretreatment of commercial iron on the nitrate reduction rates over a range of 10–45◦C. Spe-cially, the BET N2 adsorption analysis, scanning electron microscopy (SEM) and temperature programmed reduction (TPR) were used to compare the physical changes of the pre-treated and nonprepre-treated iron surface.
2. Experimental 2.1. Chemicals
Potassium nitrate was purchased from Aldrich (99+%, Milwaukee, WI). The chemicals used was N-[2-hydroxy-ethyl]piperazine-N-[2-ethanesulfonic acid] acid (HEPES, Sigma) for pH control. The zero valent iron used was iron powder (99.6%, electrolytic and finer than 100 mesh) obtained from J.T. Baker. All aqueous solutions were made in water purified with a Milli-QTMsystem (18.2 M cm−1). The desired concentrations of nitrate, 40 mg N L−1, in Ar-purged water were prepared by dilution of a 1000 mg N L−1 stock solution. Adding the buffer, 40 mM HEPES, to control the pH of the solution at the range 6.5–7.5.
2.2. Surface treatment
The iron was heated in a flow of H2/N2 (20 vol.%, 50 mL min−1) from ambient to 400◦C. Keeping at 400◦C for 3 h to completely reduce the aged oxide layer on the iron surface into zero valences. After cooling down to room tem-perature, the flow of H2/N2was then replaced by Helium gas (50 mL min−1) to purge the reduced sample for 10 min. The H2-reduced iron must be stored in a drying box. When a loss of reactivity of aged iron occurs due to a build up of iron oxide layers, the same process was used to recover its activity after drying iron particles.
2.3. Characteristics of iron surface
Surface areas were determined by BET N2 adsorption analysis on a Coulter SA3100 surface area analyzer (Coul-ter Co., Hialeach, FL). The morphology of the surface of the iron was viewed with scanning electron microscopy (SEM). Temperature programmed reduction (TPR) studies were per-formed to determine the quantity of iron oxide with the appa-ratus similar to that described previously[19]. In that, a flow of H2/Ar (20 vol.%, 100 mL min−1) was used as reducing gas. The oven temperature was programmed from ambient to 450◦C at rate of 10◦C min−1and keeping it at 450◦C for 1 h. The peak of H2consumption was assigned to Fe(III)→ Fe0,
represented as Eq.(1).
Fe2O3+ 3H2→ 2Fe0+ 3H2O (1) The quantity of H2consumption was obtained by com-paring the area of this peak to that of 1 mL H2(40.9mol at 1 atm, 25◦C) passing through the reactor of TPR. Simulta-neously, the total number of Fe2O3atoms was calculated by multiplying by a factor (1/2), consistent with the stoichiom-etry of Eq.(1).
2.4. Reactor system
All experiments as function of time were performed with 65 mL serum bottles. In each bottle, 0.5 g iron particles and 65 mL of Ar-purged buffered 40 mg N L−1 solution were added, leaving no headspace. Immediately, the vials were capped with Teflon silicone septa and aluminum seals, and then mixed at 200 rpm using a reciprocal shaker water bath (Yihder, BT-350R) at 10, 25, 35 and 45◦C.
2.5. Sample analysis
One milliliter of aqueous solution was collected from the serum bottle by a syringe through the septa, and simultane-ously another needle was used to inject argon gas to replace the liquid removal. Nitrate was measured using an ion chro-matograph (Dionex DX-100TM).
3. Results and discussion 3.1. Effect of surface pretreatment
The reaction rate was evaluated with nitrate solution (40 mg N L−1) containing 0.5 g of pretreated iron, and as comparison with nonpretreated iron (Fig. 1). The reduction of nitrate followed pseudo-first-order kinetics with respect to
Fig. 1. Kinetics of nitrate removal as function of reaction time in the presence of nonpretreated iron, pretreated iron and regenerated iron at 25◦C.
the concentration of nitrate: r = −d[NO3−]
dt = kobs[NO3
−] (2)
where kobs is the observed pseudo-first-order reaction rate constant (min−1). The reduction of nitrate using the non-pretreated iron exhibited a stagnation phenomenon at the first 20 min of the reaction, and then attenuated at rate of 0.0081 min−1. A build up of iron oxide layer, resulting from contacting with the ambient oxygen during manufac-turing and transportation process, exhibited a stagnation phe-nomenon during the reduction of nitrate. The iron was heated in a flow of H2/N2gas at 400◦C to reduce the passive iron oxides into zero valence and then added into 65 mL of Ar-purged buffered 40 mg N L−1solution. The results indicated that not only the kobs was promoted by a factor of about 4.7–0.0384 min−1but also the lag of the early period disap-peared. Generally, the nitrate reduction rate is proportional to the amounts of exposed iron surface. Therefore, regarding to the iron activity per unit surface area, the kobsis necessar-ily to normalize according to the surface area and the mass concentration of iron particles. The surface area normalized rate constant (kSA) can be calculated by Eq.(3).
kSA= kobs
ρa (3)
ρa is the surface area concentration of iron in m2L−1, and here, the kSA is a parameter of assessment of the overall surface reactivity. The BET surface areas are 0.516 m2g−1 for nonpretreated iron and 0.391 m2g−1for pretreated iron (Table 1). The value ofρawas 3.97 m2L−1for nonpretreated iron and 3.01 m2L−1for pretreated iron in the batch exper-iments. Thus the kSA for nonpretreated and pretreated iron were 0.0020 and 0.0128 min−1m−2L as shown inTable 1. The reactivity of pretreated iron was higher relative to non-pretreated iron as indicated by a larger kSA for pretreated iron. Hence the active site concentration on pretreated iron was increased due to the transformation of iron oxides into zero valences by H2reduction.
3.2. Characteristics of iron surface
The characteristics of exposed iron surfaces, nonpre-treated and reduced by H2, were compared. Clearly, using the H2-reducing pretreatment method, the specific surface area of pretreated iron decreased as compared with nonpretreated iron (Table 1). The morphology of these two iron surfaces were analyzed using SEM. The pretreated iron inFig. 2(a)
Fig. 2. SEM images of (a) pretreated iron surface and (b) nonpretreated iron surface with a magnification of 5000×.
exhibited visibly cleaner without pitting and cracking rela-tive to the nonpretreated iron inFig. 2(b). This change was due to the reduction of fluffy iron oxides on iron surface into solid zero-valence and therefore the specific surface area of pretreated iron was decreased.
The amount of H2consumed by the samples of iron (non-pretreated and (non-pretreated iron) was obtained by TPR (Fig. 3); the total number of Fe2O3 atoms was calculated by multi-plying by a factor consistent with the stoichiometry of Eq. (1). Then, the total mass of Fe2O3 on the surface of the iron was normalized to the specific surface area in the units of mg m−2.Table 2shows the relevant values. The Fe2O3 Table 1
The values of pseudo-first-order rate constants (kobs) and surface area normalized rate constant (kSA) of nitrate reduction under each reductant
Reductants BET specific surface area (m2g−1) k
obs(min−1) kSA(min−1m−2L) Observed first-order rate constant of each
reductants with coefficient of determination (r2)
Nonpretreated iron 0.516 0.0081 0.0020 0.95
Pretreated iron 0.391 0.0384 0.0128 0.99
Table 2
Fe2O3mass per unit exposed iron surface area for nonpretreated, pretreated and regenerated iron
H2consumption of 1 g iron
sample (mol) (1)
H2consumption per unit exposed iron surface
area (mol m−2) (2)=(BET surface area)(1) ×1 g
Fe2O3mass per unit exposed iron surface area
(mg m−2) (3)=(2)×10−6×55.8×103 3
Nonpretreated iron 117.4 227.5 8.46
Pretreated iron 15.6 39.9 1.48
Regenerated iron 18.9 42.8 1.59
mass per unit exposed iron surface area of nonpretreated iron reached 8.46 mg m−2. This sample had been unsealed for 9 months and then stored in a drying box. After pretreatment by reducing with H2, the iron oxides quickly converted into zero-valent iron, and only few O atoms remained on the H2 -reduced iron surface.
3.3. Temperature effect
Temperature is an important factor in control the reac-tion rate of chemical or physical processes. Su and Puls[17] demonstrated that the calculated activated energy (Ea) by evaluating the rate constants over a temperature range can be viewed as the quality of energy of the slowest reaction step. Thus, the rate-limiting step in the reaction of a metallic iron–nitrate–water system must be either a chemical reaction or a diffusion process, as determined by Eavalue. Generally, a physical process, diffusion, requires less energy than a chem-ical process, such as reduction. Su and Puls[17]stated that an Eavalue of around 15 kJ mol−1was the most often cited value for diffusion-controlled processes. Firstly, the reduc-tion rate constants (kobs) in Eq.(2)were evaluated in a batch system at 10, 25, 35, and 45◦C individually in contact with nonpretreated and pretreated iron, respectively (Fig. 4). The kobsmeasured in batch experiments exhibited a temperature dependency consistent with the Arrhenius equation:
kobs= A exp−EaRT (4)
where Ea is the activation energy (kJ mol−1), A the pre-exponential factor (min−1m−2L), R the molar gas constant
Fig. 3. Consumption of H2 during temperature programmed reduction of
nonpretreated, pretreated and regenerated iron.
(0.008314 kJ mol−1K−1), and T is the absolute temperature (K). The activation energy for the reaction was obtained from the slope of a plot of ln(kobs) versus 1/T using lin-ear least-square analysis.Table 3presents the relevant val-ues. The Ea calculated using Arrhenius law were 46.0 and 32.0 kJ mol−1for nonpretreated and pretreated iron, respec-tively. This result indicates temperature effect was more significant for nonpretreated iron relative to pretreated iron. These data are large enough to indicate that the chemical reactions, rather than diffusion, dominate the rate of nitrate loss in the iron-water systems. The difference of Eabetween nonpretreated and pretreated iron could be a result of dif-ferent active sites on iron surface controlling the nitrate reduction.
Fig. 4. Kinetics of nitrate removal as function of reaction time in the presence of (a) nonpretreated iron and (b) pretreated iron over the temperature range of 10–45◦C.
Table 3
Observed first-order rate constants at 10, 25, 35 and 45◦C and activation energies for nonpretreated and pretreated iron
Reductant Observed first-order rate constant (kobs, min−1) Activation energy (kJ mol−1)
T = 10◦C T = 25◦C T = 35◦C T = 45◦C
Nonpretreated iron 0.0023 0.0081 0.0121 0.0202 46.0
Pretreated iron 0.018 0.0384 0.0602 0.0779 32.0
3.4. Regeneration
The removal of passive oxide layer using acid washing and sonication to restore the reactivity of metallic iron has been reported. Gui et al.[20] pointed out that the acid solution (H2SO4) for Ni/Fe regeneration removed some of the cov-ered corrosion products, thereby making iron and nickel more accessible to the NDMA molecules in the solution. How-ever, a loss for recoverable active sites on Ni was caused through the regeneration process with acid solution. The use of sonication to regenerate the surface reactivity remov-ing superficial deposits did not successfully remove oxide layers and thereby was no different in reaction rate from the nonsonicated samples [21]. This study used a flow of H2/N2 (20 vol.%, 50 mL min−1) to flush the aged iron sur-face at 400◦C in a closed oven, similar to the pretreatment method described above. The regeneration experiment was performed to aged iron with continual soaking in nitrate solu-tion for 60 days. Before regenerating, the reactivity of these aged iron decreased 50–60% (about 0.0217 min−1) as com-pared to the fresh pretreated iron. As shown inTable 1, the kobs was 0.0410 min−1and the kSAwas 0.0121 min−1m−2L. The value of kSAfor regenerated iron was similar to that for fresh pretreated iron whereas the value of kobswas rose by a factor of about 1.1 due to the increase in the BET specific surface area. The reactivity of the aged iron was completely restored. Unlike acid washing process, no acid wastewater and sludge were produced using H2reduction process. Hence H2 reduc-tion process is an effective and promising method to activate iron surface.
4. Conclusions
Iron surface was activated using hydrogen gas at 400◦C to degrade nitrate (40 mg N L−1) in a HEPES buffer solution at pH value between 6.5 and 7.5. The results obtained in this study have demonstrated the following:
(1) Compared with the nonpretreated iron, the both values of kobsand kSAwere increased 4.7 and 6 times by pretreated iron, and the lag of the early period disappeared. (2) The physical changes on iron surfaces were investigated
by BET analysis, SEM and TPR. The results indicated the increase in active site concentration on the pretreated iron resulted from the transformation of iron oxides into zero valence.
(3) The values of Eaof nitrate reduction over the temperature range of 10–45◦C were 46.0 kJ mol−1for nonpretreated iron, and 32.0 kJ mol−1 for pretreated iron, indicating chemical reaction control, rather than diffusion. (4) The reactivity of aged iron with continual soaking in
nitrate solution for 60 days was completely restored by hydrogen gas at 400◦C.
Acknowledgement
The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under contract no. NSC 93-2211-E-002-035.
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