Remediation of Nitrate-Contaminated Water by Fe0-Promoted Processes
L.L. Zawaideh, C.F. Chew, and T.C. Zhang
125B Engineering, Dept. of Civil Engineering, University of Nebraska-Lincoln at Omaha Campus, Omaha, NE 68182-0178, Phone: (402)554-3784; FAX: (402) 472-554-3288
The feasibility of using zero-valent iron powder to remediate nitrate-contaminated water was studied using bench-scale batch and fixed-bed column reactors. Operational parameters, such as Fe0 dosage (w/v), initial concentration of nitrate-nitrogen, pH, and the use of an organic buffer (HEPES), were studied to determine the effectiveness of nitrate removal using zero-valent iron powder. Nitrate-nitrogen was removed by 94% when 0.01M of HEPES was added to a non-shaking batch reactor containing 20 mg/l nitrate-nitrogen and 4% (w/v) of Fe0. Shaking was proved to be more efficient than no shaking. Using the response surface methodology it was found that nitrate removal was closely related to pH. At low pH (e.g., pH <2), the nitrate removal was fast and efficient (95 to 100% nitrate removal achieved); at high pH (e.g., pH > 11), the transformation of nitrate was fast and efficient only at low nitrate concentrations in the Fe0-H2O system; at normal pH range (pH = 6 to 8), nitrate removal was usually lower than 50% without buffer treatment. The addition of the organic buffer (HEPES) greatly enhanced the nitrate transformation in a wide pH range (e.g., pH = 2 to 11). Preliminary column experiments verified the batch experimental results on pH and buffer effects.
Keywords: zero-valent iron metal, nitrates, pH, buffer, remediation
Nitrate contamination in ground and surface water has become an increasingly serious environmental problem facing the United States. The overfertilization (artificial fertilizers and animal manure) and the increased use of groundwater have raised concerns about the nitrate contamination in groundwater. In Nebraska, nitrate concentration exceeded the maximum contaminant level (MCL) in more than 20 percent of the 5826 wells sampled between 1984 and 1988 (Spalding and Exner, 1991). In that area the average nitrate-N concentration increased 0.4 to 1.0 mg-NO3--N L-1 per year to average concentrations 1.5 to more than 2 times higher than MCL (Spalding and Exner, 1991).
Although numerous treatment processes have been developed to remove nitrate from water, in situ remediation of nitrate-contaminated water remains as an unsolved problem. In addition, many physical-chemical and biological processes developed are marginally cost-effective and/or have detrimental side effects on water quality. Therefore, it is imperative to explore new technologies for in situ and on-site remediation of nitrate-contaminated water. In this research, zero-valent iron-promoted processes were used to transfer nitrate in contaminated water.
Although environmental application of Fe0 metals was first reported in the patent literature (Sweeny and Fischer, 1972), the concept of using iron metal for environmental remediation was introduced by Gillham and his co-workers in research involving sorption of organics to well-casing materials (Reynolds, et al., 1990; Gillham and O'Hannesin, 1994; Gillham, 1995). Recently, great interest has been developed in use of zero-valent iron metal as a remediation tool. Iron-promoted processes have been evaluated for remediation of a wide variety of target compounds, including halogenated aliphatics (VOCs), polyhalogenated aromatics such PCBs, DDT, even TNT, atrazine, metals, and nitrate. These studies have demonstrated that the Fe0-promoted process is (1) relatively inexpensive and nontoxic, (2) faster and more energy effective than biotic remediation, and (3) free of clogging problems associated with nutrient inoculation.
Currently, however, very little information on using Fe0 metal-promoted processes for remediation of nitrate-contaminated water is available. Siantar, et al. (1995) reported that nitrate at an initial concentration of 56.5 mg L-1 was reduced to nitrite by about 36 g-iron L-1 with a half-life of 3.3 minimum. The effect of SO42- and NO3- on the 1,2-dibromo-3-chloropropane (DBCP) transformation rate was also investigated. Singh et al. (1996) reported that treatment of a 60 mg NO3--N/L solution with 6% (w/v) Fe0 at pH 1.0 completely transferred all NO3- within 24 h. Nitrate loss from solution was inversely related to pH. Despite this previous work on nitrate removal using Fe0 powder, a clear understanding of the mechanisms involved under various conditions and protocols for remediation of nitrate-contaminated water by zero-valent promoted processes has not been achieved. Problems associated with controlling the pH of the process and the deactivation of the metal surface make the practical application of Fe0-promoted processes difficult.
In this study, the feasibility of using Fe0-promoted processes for remediation of nitrate-contaminated water was evaluated using batch and fixed bed-reactors. The focus of this study was concentrated on the effects of pH on nitrate removal and improvement of nitrate removal by adding an organic buffer. The objective of this paper is to present the preliminary results of the effects of operational parameters such as iron dosage (w/v), pH, and the use of an organic buffer (HEPES) on the performance of iron-promoted processes.
Materials and Methods
Batch reactors consisted of 150-mL Erlenmeyer flasks (VWR Scientific). Four upflow fixed-bed column reactors were constructed from 1.5-inch I.D. and 12-inch height acrylic tubing (obtained from American Plastic, Omaha, NE), with four sampling ports (Figure 1). The working volume (empty-bed reactor volume) was 260.5 mL. One column reactor contained 20% 100 mesh laboratory grade iron powder compacted with 80% silica sand. In this case, the sand and iron powder were not mixed with each other. The other column contained 100% 80 mesh industrial grade iron powder. The third column was used later in the research to compare mixing iron powder (20%) with sand (80%) instead of separating iron powder with sand. The fourth column was packed as the first column and was used to study the effect of the loading rate under variable pH values. Feed solution was delivered to the reactors by a Masterflex L/S peristaltic pump (Cole-Parmer Instrument Co., Niles, Illinois) with a standard constant flow pump head for each column.
The first test using batch reactors was to determine the effect of high and low pH and initial concentrations of nitrate on nitrate removal using zero-valent iron. In this test, the initial pH in batch reactors was set to 2 and 11 using either HCl or NaOH, respectively. The feed solution contained 5, 10, 20, and 60 mg L-1 NO3--N prepared from KNO3. Iron powder was then added (10% w/v) and the batch reactors were purged with N2 gas to replace the head volume. The reactors were sealed using parafilm to reduce evaporation. The flasks were placed on the shaker tray of a Reciprocal Shaking Water Bath (Precision Scientific INC., Chicago, IL) and shaken at 150 RPM for 24 hours at 25O ± 1 OC.
The second test using batch reactors was to study the effect of iron dosage and shaking on nitrate removal. In this design, the initial concentration of nitrate was 20 mg L-1; the initial pH was not adjusted (pH = 5.6); and 0.01M HEPES was added just before the test started. Eight batch reactors were prepared with iron dosages of 1%, 4%, 6%, and 10% (w/v), four of which were placed on the shaker tray and shaken at 150 RPM for 24 hours at 25O ± 1 OC. The other four were set on the bench at the same temperature for 24 hours.
Because the reaction of iron with water raises the pH, a buffer was used to
investigate the real effects of pH on the performance of the iron-promoted process. The
third batch test was to determine the effect of adding an organic buffer, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffer, in batch reactors. The
HEPES structure is shown below.
HEPES was chosen because it is not expected to form any precipitates or complexes with the ferrous iron as phosphate or carbonate buffers. In this design the initial nitrate concentration was 20 mg L-1; the iron dosage was 4%. The initial pH was unadjusted. The HEPES concentration was 0, 0.01, 0.05, 0.1, and 0.2 (M), respectively. To fully understand the HEPES-iron system, two additional batch tests were conducted. The first additional test was the addition of 0.05M Na2SO4 instead of the HEPES (initial pH was unadjusted) to see the effect of the sulphonic group; the second test was the addition of 0.01M phosphate buffer (a ratio of 1:1 H2PO4- and HPO42-) to see the effect of constant pH (pH of solution was around 7).
In the fourth set of batch reactors, response surface methodology was used to study the effect of pH and iron dosage together on nitrate removal. In this design, 12 batch reactors were prepared following the same procedure aforementioned (Table 1). The initial nitrate-nitrogen concentration was 60 mg L-1. pH and iron dosage were two independent variables, while nitrate concentration remaining in the solution after the test was the dependent variable.
The fifth batch test was designed, using response surface technology, to determine the effect of HEPES, pH, and iron dosage on nitrate removal. In this design, 20 batch reactors were prepared using the same procedure described above. The initial concentration of nitrate was 60 mg L-1. After the buffer (HEPES) was added in the batch reactors, the initial pH was regulated using either HCl or NaOH based on the experimental design shown in Table 2. After the test, the pH in the batch reactors was measured again.
Response surface methodology is an approach to experimental design which allows the investigator to optimize responses obtained from the experiment. In this research, a rotatable central composition design was used to design two sets of batch tests. A central composite design consists of a 2k factorial (coded to the usual ±1 notation) augmented by a 2k axial points (±, 20 0, 0, ..., 0), (0, ±, 0, ..., 0), (0, 0, ±, ..., 0), ... (0, 0, 0,..., ±) and nc center points (0, 0, ..., 0) (Montgomery, 1991). A central composite design is made rotatable by the choice of . The value of for rotatability depends on the number of points of the design (nf), where = (nf)1/4. The response surfaces were approximated using second-order polynomial functions
After completion of the fourth and fifth batch tests, STATGRAPHICS (STSC, Inc., 1986) was used to (1) conduct multiple regression, (2) produce multiple X-Y-Z plots, and (3) produce response surface plots. Multiple regression allows us to examine the relationship between the dependent variable with one or more independent variables.
Chemicals, Materials, and Analytical Methods
100-mesh iron powder (Certified Grade, 95%) with a nominal S content < 0.02% was obtained from Fisher Scientific (Fair Lawn, NJ). The organic buffer 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) was also obtained from Fisher Scientific. Nitrate standard solutions were prepared from potassium nitrate (KNO3) obtained from Fisher Scientific. Distilled waster was used to prepare standard solutions.
Parallel samples from batch reactors were filtered using Nitrocellulose filter circles, 47 mm in diameter with a pore size of 0.45 µm (Gelman Sciences, Ann Arbor, MI). The pH of the samples was then measured and recorded using Standardized Fisher Scientific Accu-pHast polymer combination electrode with Accumet 925 pH meter. A 100 mL sample was used for the nitrate analysis using the nitrate electrode method (Orion Nitrate Electrode Method). The electrode potential was measured and recorded in mV using Orion Nitrate Electrode Model 93-07 with Orion Model 90-02 Double Junction Reference Electrode. The concentration of the samples was then determined from the standard curves.
Figure 2 shows the effect of initial pH and nitrate concentrations on nitrate removal. As the initial nitrate concentration increased, the nitrate removal efficiency increased at pH of 2 and decreased at pH of 11. When the initial nitrate concentration was lower than 10 to 15 mg L-1, the nitrate removal at pH of 11 was higher than that at pH of 2; when the initial nitrate concentration was higher than 15 mg L-1, the nitrate removal was usually higher than 85% at pH of 2 and was approximately 40% at pH of 11. At pH of 2, the final nitrate-nitrogen concentration remaining in the batch reactor is usually around a low concentration (i.g., 2 mg L-1). Therefore, a higher nitrate removal efficiency always corresponds to a higher initial nitrate concentration. At pH of 11, however, the nitrate removal was limited, with a lower nitrate removal efficiency corresponding to a higher initial nitrate concentration. Therefore, we speculated that the mechanism involved for nitrate removal by Fe0 metal in these aqueous systems at pH of 2 was different from that at pH of 11 (see discussion below).
Figure 3 shows the effects of iron dosage and shaking conditions on nitrate removal. In this test, the initial pH of the batch reactors was usually around 5.6 without any adjustment. To eliminate the pH effect on nitrate removed, which might result from a different iron dosage, an organic buffer (HEPES) was used (0.01M was the optimal buffer concentration found in our experiments, see below). After the addition of HEPES, the pH of the batch reactors was 6.95, and the final pH after the test was 7.10. Shaking the batch reactors nitrate removal (see Fig. 3). Without shaking, increasing iron dosage from 1% to 6% enhanced the nitrate removal due to an increases of contact surface areas of iron powder. Further increasing the iron dosage more than 6% did not increase contact surface areas of iron powder, resulting in a decrease of the nitrate removal. On the contrary, shaking the batch reactors increased the efficiency of mass transfer by constantly introducing new contact surface areas of iron powder to nitrate. When the iron dosage was higher than 6% Fe0 (w/v), the removal remained constant, indicating that for initial nitrate-nitrogen concentration of 20 mg L-1, 6% of Fe0 was the minimal dosage to complete nitrate transformation.
Figure 4 shows the effect of using an organic buffer (HEPES) on nitrate removal. To concentrate on the buffer effect, we chose to: (1) use no shaking conditions, which would eliminate the enhancement of mass transfer; (2) use an initial nitrate concentration of 20 mg L-1, which was the concentration that demonstrated a very obvious nitrate removal difference between low pH and high pH (see Figure 2); and (3) use 4% Fe0 dosage, which was less than minimal iron dosage, insuring that any enhancement of the nitrate removal would not result from excess iron powder. As shown in Figure 4, an optimal nitrate removal efficiency (97%) was obtained at a buffer concentration of 0.01M; for buffer concentrations higher than 0.05M, the nitrate removal remained approximately constant at about 80%. Therefore, the buffer concentration of 0.01M was chosen in later experiments unless otherwise addressed.
Figure 5 shows the comparison between the effect of HEPES, Na2SO4, and HPO4--H2PO4-2 on nitrate removal. When the phosphate buffer was used, about 30% of the nitrate was removed. When the sodium sulfate was used, about 40% of the nitrate was removed; while around 98% of the nitrate was removed when the HEPES buffer was used. These tests demonstrates that the sulfonic acid group is a stronger reductant than the sulfanic acid group or phosphate group. More studies are needed to fully understand the HEPES-iron powder systems.
Response Surface Technology
Figure 6 shows the effects of pH and iron dosage on nitrate removal obtained from the batch tests based on the response surface technology. Based on the experimental results shown on Figure 6a, a polynomial equation to represent the response surface of nitrate concentration remaining after batch tests (y) vs. Fe0 dosage and pH was obtained with the coefficient of determination, R2 being 0.95. The statistical model fitting results showed that the multiple regressions were very successful (data not shown). Figure 6b shows the response surface, indicating that (1) nitrate removal is inversely related to pH when pH is below 8.5 (Table 1 and Figure 6); (2) when pH is less than 2, nitrate removal is higher than 90% for iron dosages between 3.4 and 13.4 mg/L; (3) when pH is between 6 and 8, nitrate removal is less than 50% ; and (4) when pH is higher than 8.5, nitrate removal increases.
Since the pH within the reactor increased during the batch experiments, it was very difficult to evaluate the effect of iron dosage alone on nitrate removal. To eliminate confusion between pH and dosage, we included the organic buffer (HEPES) as another parameter in batch tests based on the response surface technology. Table 2 shows the rotatable central composite design and results of the batch tests. Based on Statgraphics analysis, no good regression equation could be found; the best coefficient of determination R2 found was 0.283. Further analysis of the results shown in Table 2 revealed that the effect of the organic buffer HEPES on nitrate removal was so strong that it hid the effects of pH and iron dosage on the performance of the Fe0-water system with nitrate. Nitrate removal was minimal when no buffer was used (run 7), no iron dosage was added (run 1), or when the initial pH was extremely low (run 8). Otherwise, nitrate removal was greater than 85% and in most reactors more than 90%. HEPES, thus, greatly increased nitrate removal.
Figure 7 shows the effect of pH on nitrate removal in column reactors at an HRT of 43.3 h and the initial nitrate-nitrogen concentration of 60 mg L-1. Nitrate removal was 88% in the column containing 20% Fe0 (100 mesh, lab grade) and 94.6% in the column containing 100% industrial grade 80 mesh iron powder at a feed pH of 2; nitrate removal was 51% for the 20% Fe0 column; and 35% for the 100% Fe0 column at a feed pH of 11. There was less removal when the pH was set to 4 (ca. 3.3%), 5.8(ca. 6.3%), and 8 (ca. 2.5% for 20% column, and 21% for 100% column). These results verified the results obtained in batch tests.
Figure 8 shows the effect of HRT on nitrate removal and the difference between placing iron powder as a wall or mixing it with sand. For the column with a 20% or 100% iron treatment wall, as the HRT increased, the nitrate removal efficiency increased. On the contrary, the column with 33% Fe0 powder mixed with sand showed that, as the HRT increased, the removal efficiency decreased. During the collection of samples, it was noticed that for the 20% and 100% Fe0 columns, the water samples were cloudy and orange while the samples collected from the column with 33% Fe0 mixed with sand were clear. These tests show that mixing iron powder with sand is more efficient than placing it as a wall.
Major conclusions about the pH effect on nitrate transformation by Fe0, based on this study, are (1) at low pH (e.g., pH < 2), the nitrate transformation is fast and efficient; (2) at high pH (e.g., pH > 11), the nitrate transformation is fast and efficient only for a low nitrate concentration in the Fe0-H2O system; and (3) at normal pH range (pH = 6 to 8), nitrate removal is usually lower than 50% without buffer treatment. The addition of the organic buffer (HEPES), however, greatly enhances the nitrate transformation in a wide pH range (e.g., pH = 2 to 11). It is our speculation that three different mechanisms were involved in the pH effects.
The reaction of iron with nitrate-contained water is an electrochemical process in which the iron is oxidized. The following are half-reactions and their standard potentials (Bard, et al., 1985):
Fe2+ + 2e- Fe0 E0 = -0.440 V (1)
NO3- + 3H+ + 2e- HNO2- + H2O E0 = 0.940 V (2)
NO3- + 3H+ + 2e- NO2- + H2O E0 = 0.935 V (3)
NO3- + 4H+ + 3e- NO(g) + H2O E0 = 0.957 V (4)
2NO3- + 12H+ + 10e- N2(g) + 6H2O E0 = 1.246 V (5)
NO2- + 2H+ + e- NO(g) + H2O E0 = 1.202 V (6)
2NO2- + 8H+ + 6e- N2(g) + 4H2O E0 = 1.520 V (7)
HNO2- + H+ + e- NO(g) + H2O E0 = 0.996 V (8)
2NO(g) + 2H+ + 2e- N2O(g) + H2O E0 = 1.590 V (9)
N2O(g) + 2H+ + 2e- N2(g) + H2O E0 = 1.770 V (10)
NO3- + 10H+ + 8e- NH4+ + 3H2O E0 = 0.880 V (11)
NO2- + 8H+ + 6e- NH4+ + 2H2O E0 = 0.890 V (12)
Therefore, the minimum net potential difference between electrode reactions of eq. (1) and eqs. (2) to (12) is 1.32 V, which is thermodynamically very favorable under most conditions. The final products may be either NH4+ or gases such as N2. Based on eqs. 2 to 12, all the half-reactions involved in nitrate transformation are favorable at low pH conditions. This is coincident with the accelerated Fe2+ formation (corrosion) at low pH.
In an Fe0-H2O system, the major three reductants formed are iron metal, ferrous iron, and hydrogen resulting from corrosion. These reductants suggest three pathways for abiotic degradation in a Fe0-H2O-oxidant (e.g., alkylhalides, RX) system (Matheson and Tratnyek, 1994): (1) direct electron transfer from iron metal at the metal surface to the adsorbed oxidant (Fe0 + RX + H+ Fe2+ + RH + X-); (2) reduction by Fe2+ (2Fe2+ + RX + H+ 2Fe3+ + RH + X-), where Fe2+ is from either 2Fe0 + O2 + 2H2O 2Fe2+ + 4OH- when dissolved oxygen is present or Fe0 + 2H2O Fe2+ +H2 + 2OH- under anaerobic conditions (both cases of corrosion result in an increase of pH in weakly buffered system); and (3) catalyzed hydrogenolysis by the H2 that is formed by reduction of H2O during anaerobic corrosion (H2 + RX RH + H+ + X-), where H2 is from Fe0 + 2H+ Fe2+ + H2(g).
At low pH, Fe2+ is formed as a result of the increased aqueous corrosion. Forms of ferrous iron affect its strength as a reductant. In the batch reactors where nitrate was removed, iron corrosion was observed. Corrosion was characterized by having a rusty color with finer iron particles. Inner-sphere complexations of Fe2+ to metal oxides can create more reducing species (Stumm, 1992), which in turn reduces the nitrate. In the first batch test (Figure 2), response surface tests (Table 1), and the column test (Figure 7), nitrate removal efficiencies were higher at low pH (e.g., pH of 2) than those at high pH (e.g., pH of 11). We surmised that this was due to the formation of Fe2+ at low pH and the formation of some solid compounds such as Fe(OH)2, Fe(OH)3, or FeCO3 at high pH, depending on the redox potential in the system. These solid compounds would eventually form a surface layer on the Fe0 surface that would inhibit further decomposition of Fe0 (Stumm, et al., 1992). Even if nitrate concentration in the batch test at pH of 11 increases, the nitrate being removed will be limited by the available iron surface before the surface is covered by the passive film. Therefore the nitrate removal will not increase with initial nitrate concentration at pH of 11.
Effect of HEPES Buffer
The corrosion process involves a number of different reactions that lead to an increase in pH. The use of the buffer in the third batch reactor test (Figures 4 and 5) and the second response surface tests (Table 2) was to determine the effect of maintaining the pH during the test. Adding the HEPES buffer to the Fe0-H2O with nitrate resulted in immediate corrosion of the iron. In the response surface batch test, some samples turned orange (rusty) as soon as the shaking started. Some samples required filtration more than one time, and some maintained the light orange color. Compared with sodium sulfate and phosphate buffer, HEPES buffer was the best in enhancing the nitrate removal efficiency. Harms, et al. (1995) reported that addition of sulfur compounds, such as sulfate, HEPES, sulfides (NaS2 and FeS) and pyrite, accelerated the iron-induced degradation of carbon tetrachloride under aerobic conditions. Schreier and Reinhard (1995) reported that HEPES greatly enhanced the transformation of PCE. Our results are inconsistent with these researchers.
Schreier and Reinhard (1995) explained that the role of the HEPES buffer in iron-water with nitrate systems appears to be pH control rather than the presence of the reducible sulfonic acid group of the buffer. We found decreasing the HEPES buffer concentration from 0.2M to 0.01M resulted in minor decrease in the nitrate removal, which is consistent with Schreier and Reinhard (1995). However, the pH control argument cannot explain why the phosphate buffer only resulted in 30% nitrate reduction while the sodium sulfate buffer resulted in 40% nitrate reduction in the same system (Figure 5). In fact, phosphates, silicates, polyphosphates, or polymeric ligands may have dissolution-enhancing effects of the passive film on iron metal surface at low pH and inhibition effects at neutral or alkaline pH values (Stumm and Morgan, 1996). Therefore, we must consider the dissolution-enhancing effects of the passive film on iron metal surfaces.
According to Stumm (1992), the composition of the passive film may consist of an oxide of Fe3-xO4 with a spinal structure varying in composition from Fe3O4 (magnetite), in oxygen-free solutions, to Fe2.67O4 in presence of oxygen. It may also consist of a duplex layer consisting of an inner layer of Fe3O4 and an outer layer of -Fe2O3. The dissolution rate of iron metal is then apparently related to the dissolution rate of the passive film. It is our postulation that the addition of the HEPES buffer seems to have two functions to reduce passivity of iron metal, that is, (1) function as a reductant and (2) formation of surface complex. HEPES tends to lower the electrode potential and favor the reductive dissolution of passive iron oxide films. HEPES may also be able to form surface complexes that can facilitate the electron transfer to the oxide and tend to dissolve the passive film. Harms, et al. (1995) explained that sulfate (SO42-) is known to remove iron oxides and hydroxide on the iron surface, which is the destruction of the protective layer, and form FeSO4 which is a more soluble corrosion product. When corrosion occurs, Fe2+ is formed which is a good nitrate reductant. Obviously, more research is needed in this area to obtain direct experimental evidence.
Zero-valent iron-promoted processes can be used to treat nitrate-contaminated water. Based on this study, the major conclusions are as follows:
1. At low pH (e.g., pH < 2), the nitrate transformation is fast and efficient.
2. At high pH (e.g., pH > 11), the nitrate transformation is fast and efficient only for a low nitrate concentration in the Fe0-H2O system.
3. The organic buffer (HEPES) greatly enhances the nitrate transformation in a wide pH range (e.g., pH = 2 to 11).
4. It is our speculation that, in our batch and column reactors, the corrosion of Fe0 to Fe2+ was achieved under low pH and when the HEPES buffer was used; at high pH range (pH > 11), however, another mechanism may be involved.
5. Mass transport of substrate to the iron surface proved to be important in achieving high removal efficiency, showing that it is important to introduce mixing as an experimental variable.
6. In the column reactors, mixing iron powder with sand proved to be more efficient than placing it as a wall. At an HRT of 28 h., nitrate removal was around 99% in the column with 33% Fe0 mixed with sand, compared to 43% for the column of 100% Fe0 and 33% for the column of 20% Fe0 packed as an iron treatment wall.
Although this article has been funded in part by the U.S. Environmental Protection Agency under assistance agreement R-819653 through the Great Plains/Rocky Mountain Hazardous Substance Research Center headquartered at Kansas State University, it has not been subjected to the agency's peer and administrative review and therefore may not necessarily reflect the views of the agency and no official endorsement should be inferred. The Water Center/Environmental Programs at the University of Nebraska-Lincoln and the Center for Infrastructure Research at UNL have provided 1:1 matching to this study. During the whole study, Drs. P. Shea and S. Comfort in the Agronomy Department at UNL provided technical advice, which is greatly appreciated.
Gillham, R.W., and O'Hannesin, S.F. (1994) Enhanced degradation of halogenated aliphatics by zero-valent iron. Groundwater, 32:958-967.
Harms, S., Lipczynska-Kochany, E., Mibum, R., Sprah, G., and Nadarajah, N. (1995) Degradation of carbontetrachloride in the presence of iron and sulphur-containing compounds. In Preprints of Papers Presented at the 209th ACS National Meeting, Anaheim, CA, April 2-7, 1995 35:825-828.
Matheson, L.J., and Tratnyek, P.G. (1994) Reductive dehalogenation of chlorinated methanes by iron metal. Environ. Sci. Technol., 28:2045-2053.
Montgomery, D. (1991) Design and Analysis of Experiments, Wiley; New York, 3rd ed., p. 641.
Reynolds, G.W., Hoff, J.T., and Gillhan, R.W. (1990) Sampling bias caused by materials used to monitor halocarbons in groundwater. Environ. Sci. Tech., 24:132-142.
Schreier, C.G., and Reinhard, M. (1995) Transformation of chlorinated ethylenes by iron powder in 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffer. In Preprints of Papers Presented at the 209th ACS National Meeting, Anaheim, CA, April 2-7, 1995 35:833-835.
Siantar, D.P., Schrefer, C.G., and Reinhard, M. (1995) Transformation of the pesticide 1,2-dibromo-3-chloropropane (DBCP) and nitrate by iron powder and by H2/Pd/AL2O3. In Preprints of Papers Presented at the 209th ACS National Meeting, Anaheim, CA, April 2-7, 1995 35:745-748.
Singh, J., Zhang, T.C., Shea, P.J., Comfort, S.D., Hundal, L.S., and Hage, D.S. (1996) Transformation of atrazine and nitrate in contaminated water by iron-promoted processes. In WEFTEC'96 Remediation of Soil and Groundwater & Industrial Wastes (Vol 3) The Proceeding of the WEF 69th Annual Conference & Exposition, pp. 143-150. Dallas, TX, October 5-9, 1996.
Spalding, R.F., and Exner, M.E. (1991) Nitrate contamination in the contiguous united states, NATO ASI series, G 30, Nitrate Contamination, Ed. By Bogardi, I., and Kuzelka, R.D., p. 13-48, Springer-Verlag, Berlin.
STSC, Inc. (1986) STATGRAPHICS.
Stumm, W., and Morgan, J. (1996) Aquatic Chemistry; Chemical Equilibrium and Rates in Natural Waters. Wiley, 3rd ed., New York, p. 1022.
Sweeny, K.H., and Fischer, J.R. (1972) Reductive degradation of halogenated pesticides.
U.S. Patent No. 3640821. Feb. 8, 1972.
Fe0 Sampling ports
20% Fe0 + Silica Sand 100% Industrial Iron
Figure 1. Schematic of upflow fixed-bed column reactors.
Figure 2. Effect of initial pH and nitrate concentration on nitrate removal.
Figure 3. Effect of iron dosage and shaking on nitrate removal.
Figure 4. Effect of buffer (HEPES) concentrations on nitrate removal.
Figure 5. Effect of HEPES, Na2SO4 and HPO4--H2PO42- on nitrate removal.
Figure 6. (Top) Experimental Results after 24-h Fe0 treatment. (Bottom) Response surface for evaluation of the inter-relationship among pH, Fe0 dosage, and nitrate concentration remaining in the solution.
Figure 7. Effect of pH on nitrate removal in column reactors.
Figure 8. Effect of HRT on nitrate removal in column reactors, HEPES = 0.01M, nitrate
initial concentration was 90 mg/L.
Table 1. The rotatable central composite design and experimental results of batch tests for evaluation of the effects of pH and iron dosage on nitrate removal*.
Factorial as Star Points
|Center Points||Semi Range||-1.414||-1||0||1||1.414|
|Run Number||Random Number||pH
* Initial concentration of nitrate is 60 mg/L.
Table 2. The rotatable central composite design and experimental results of batch tests for evaluation of the effects of pH, HEPES, and iron dosage on nitrate removal*.
Factorial as Star Points
|Center Points||Semi Range||-1.682||-1||0||1||1.682|
|Run Number||Random Number||pH
* Initial nitrate concentration was 60 mg/L. In this experimental design, 0 means central point; "-1" can
be obtained using semi-range - central point (5 - 3 = 2); "+1" can be obtained using central point + semi
range (5 + 3 = 8); "-1.682" can be obtained using (semi-range * -1.682 ) - central point (3* -1.682 - 5 = -0.05); and "1.682" can be obtained using (semi-range * 1.682 ) - central point (3* 1.682 + 5 = 10.05).
Run number is the design number, while the random number is the actual experimental number.