4922
Ind. Eng. Chem. Res. 2004, 43, 4922-4934
Removal of Selenate by Fe and NiFe Nanosized Particles Kanchan Mondal,† Gautham Jegadeesan,† and Shashi B. Lalvani*,‡ Mechanical Engineering & Energy Processes, Southern Illinois University, Carbondale, Illinois 62901-6603, and Paper Science and Engineering, Miami University, Oxford, Ohio 45056
Irrigation drainage and industrial wastewaters often contain elevated levels of toxic anions such as selenate, arsenate, and chromate. Immobilizing the contaminants via reaction with zerovalent iron has been proven to be an effective remediation method. Ultrafine metallic and bimetallic particles were prepared by borohydride reduction of aqueous salt solutions and characterized. Nanosized zero-valent NiFe and iron particles rapidly reduced and immobilized selenate from aqueous solutions. Nearly 100% selenate removal was obtained in 5 h. The data show that, at identical solids concentrations, the use of NiFe particles accomplished 42 and 56% greater removals, respectively, than the use of Fe and Ni particles individually. At low selenate concentrations, the selenium removal rate followed the first-order rate kinetics that shifted to zeroth-order at higher concentrations. The maximum selenate removal (87% in 30 min at a solids concentration of 0.5 g/L) was obtained by bimetallic particles containing 70 at. % iron. High removal capacities were observed at pH lower than 8. A mechanism for the selenate removal has been developed. 1. Introduction
Table 1. Summary of Selenium Removal Technologies
Selenium, which is potentially toxic to all living organisms, creates serious environmental problems throughout the world. The environmental concern regarding selenium has been attributed to its potential to cause either toxicity or deficiency in humans, animals, and some plants within a very narrow concentration range. It has been observed that concentrations of selenate (SeO42-) as low as 10 parts per billion in water can cause death and birth deformities in waterfowls.1 As a result, the United States Environmental Protection Agency (U.S. EPA) designated 0.01 mg/L Se as the primary drinking-water standard.2 Selenium solubility and availability depend on the relative concentrations of various species present in soil solutions,3 which subsequently can govern biotoxicity and deficiency. Irrigation and drainage from seleniumrich soils leach selenium into the watersboth groundwater and surface water. Aqueous selenium exists predominantly as selenate (SeO42-) and selenite (SeO32-). Of the two species, selenate is the more stable in aqueous solutions and thus relatively more difficult to remove. The concentration and the chemical forms of selenium in soils or in drainage waters are governed by various physiochemical factors including oxidationreduction status, pH, and sorbing surfaces. Thermodynamic calculations3 indicate the likely existence of selenite and elemental selenium in reducing environments and of selenate in oxidizing environments. A brief summary of treatment technologies that are applicable for selenium removal from water is presented Table 1. As reported in Table 1, most of the previous investigations involved adsorption of selenium on ferrihydrite surfaces.4-23 In fact, the U.S. EPA24 selected the use of ferrihydrites as the best demonstrated available technology (BDAT) for selenium removal (in the form of * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: (513) 529 9278. † Southern Illinois University. ‡ Miami University.
technology precipitation adsorption
agent ferrihydrite
activated alumina
activated carbon ferric oxyhydroxide lanthanum oxide
Balistrieri and Chao (1987, 1990)4,5 Benjamin and Bloom (1981)6 Benjamin et al. (1982)7 Brown and Shrift (1980)8 EPRI (1980, 1985)9,10 Hayes et al. (1987)11 Hingston (1981)12 Hingston et al. (1968)13 Howard (1977)14 Peters et al. (1997)15 Manning and Burau (1995)16 Merrill et al (1986)17 Pengchu and Sparks (1990)18 Stiksma et al. (1996)19 Western States Petroleum Association (1995)20 Sparkman et al. (1990)21 Isaacson et al. (1994)22 Parida et al. (1997)23 Trussel et al. (1980, 1991)26,27 Yuan et al. (1983)28 Hornung et al. (1983)29 Batista and Young (1994, 1997)30,31 Jegadees et al. (2003)32 Jegadees et al. (2003)32 Jeffers et al. (1991)33 Corwin et al. (1994)34 Adutwum (1995)35 Maneval et al. (1985)36 Boegel and Clifford (1986)2 Virnig and Weerts (1993)37 Western States Petroleum Association (1995)20 Pontius (1995)38 Kapoor et al. (1995)39 Gleason et al. (1996)40
ion exchange
reverse osmosis emulsion liquid membranes nanofiltration reduction
ref(s)
ferrous hydroxide iron
Kharaka (1988, 1996)41,42 Afonso (1992)43 Fu et al. (1994)44 Murphy (1988,1989)45,46 Lien (1990)47 McGrew et al. (1996)48 Roberson (1999)49 Qiu et al.. (2000)50
selenite). However, the study concluded that insignificant selenate removal ( bicarbonate/carbonate ≈ Se(IV) > oxalate > fluoride ) Se(VI) > sulfate. Hayes11 postulated that selenate adsorbs as an outer-sphere hydrated complex and thus it can be easily replaced by other solution anions such as sulfate. This was confirmed by other researchers.5,51 A number of studies26-31 concluded the following decreasing order of the preference for adsorption on alumina: hydroxide > phosphate > fluoride > As(V) > Se(IV) . sulfate > Se(VI) > bicarbonate > chloride > nitrate > As(III). Trussel et al.26 also observed that sulfate and bicarbonate had no effect on the adsorption of Se(IV) but greatly affected that of Se(VI). Yuan,28 however, demonstrated successful Se(VI) removal by reducing the pH of the solution to 4 or less. Adsorption on novel polymeric materials (chitin and chitosan)54 with a high concentration of amine groups has also been investigated for the ability of these materials to remove selenium and arsenic oxyanions52,53 from aqueous solutions. Zero-valent iron has been reported to degrade many chlorinated hydrocarbon solvents effectively via reductive dehalogenation55,56 and to immobilize redox sensitive inorganic contaminants such as selenate,47-57 arsenate and arsenite,58-60 chromate,61-63 molybdate,64 etc. The removal mechanism appears to be reductive precipitation followed by sorption on the corroded iron surface. Zero-valent iron is an attractive alternative for removing selenium from water via reduction of selenium oxyanions to elemental selenium (Se0). In an X-ray absorption near-edge structure (XANES) study of the reaction of selenate with iron filings, the product profile showed the following fate of selenium: 74% elemental, 17% Se(IV), and 9% Se(VI).50 In the redox reaction, selenate is the electron acceptor, while the Fe metal acts as an electron donor. Equations 1 and 250 illustrate possible reactions for the reduction and deposition of selenium on iron surfaces.
Fe0 + SeO42- + H2O f Fe2+ + SeO32- + 2OH-
∆G ) -84.7kJ/mol (1)
3Fe0 + SeO42- + 4H2O f 3Fe2+ + Se0(s) + 8OH-
∆G ) -105.5 kJ/mol (2)
Various other reductants, such as ferrous hydroxide,43-45 zinc, and aluminum, have been used to produce elemental selenium or metal selenides. Preliminary investigations reported in this study have also shown that, as compared to Ni and Fe, NiFe bimetallic particles removed selenate at a faster rate. In this paper, the removal of selenate from aqueous solutions using nanosized particles of bimetallic NiFe was investigated. The bimetallic particles produced by the borohydride-assisted reduction of metal salt solutions were thoroughly characterized. The effects of the
initial selenate concentration, particle solids concentration, pH, temperature, competing anions, and Ni content of the particles on the removal of selenate from synthetic solutions were studied. A possible mechanism for the removal process is presented. The kinetics of selenate reduction at the bimetallic particle surface was evaluated. As a comparison, a number of adsorbents including activated alumina, activated carbon, fullerenes, cellulose, chitin, and chitosan were employed to study selenate removal as well. 2. Experimental Section 2.1. Materials. 2.1.1. Metal and Bimetallic Powders. The metallic powders (M) are formed by reaction with sodium borohydride according to the following reaction
2M2+ + 2H2O + BH4- f 2M0(s) + BO2- + 4H+ + 2H2(g) The precipitation of bimetallic powders (M1 and M2) occurs according to the following reaction, which involves simultaneous reduction of the metal ions in aqueous solution by sodium borohydride
M12+M22+ + 2H2O + BH4- f M10(s)M20(s) + BO2- + 4H+ + 2H2(g) Transition metal powders were produced by the reduction of 1 M salt solutions with 0.01 g L-1 sodium borohydride. The bimetallic powders were produced via the same method using solutions with metal molar ratios of 1:1. A salt solution (100 mL) was placed in a vial to which 50 mL of borohydride solution was added dropwise. The precipitated solids were centrifuged to remove the water and then dried at 85 °C in nitrogen atmosphere for 24 h. The solids were stored in airtight vessels under nitrogen. To identify the best method for producing similarly sized ultrafine particles, five preparatory routes were used (Figure 1). The general procedure for producing these powders is provided in the previous paragraph. The following descriptions explain the differences in preparation methodologies of the five routes. Preparation of the bimetallic NiFe powder via pathways A-C did not involve the use of any mechanical agitation, whereas pathways D and E utilized ultrasonic agitation. Sample A was produced via borohydride addition to the as-prepared solution containing nickel chloride and ferric chloride. For the preparation of sample B, 0.5 g/L of sodium dodecyl sulfate (surfactant) was added to an identical solution as used for sample A prior to borohydride addition. The pH of the solution (identical to that used in pathway A) was adjusted to 6.5 and then borohydride was added to produce sample C. Samples D and E were produced under continuous ultrasonic agitation (260 W, 46 kHz) during sodium borohydride addition to solutions identical to those used for samples A and B, respectively. 2.1.2. Adsorbents. R-Alumina and γ-alumina were obtained from Fisher Scientific (Chicago, IL). Darco S51 was provided by Norit Americas Inc (Atlanta, GA). The three commercial fullerenes, as-produced fullerenes (AsF), toluene-extracted fullerenes (TEF), and tolueneextracted heat-treated fullerenes (TEHTF) were obtained from MER Corporation (Tucson, AZ). Chitin was
4924 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004
Figure 1. Flowsheet for ultrafine NiFe particle preparation.
obtained from Fisher Scientific (Chicago, IL). Ferric hydroxide and chitosan (deacetylated chitin) were also obtained from Fisher Scientific (Chicago, IL). 2.2. Material Characterization. The particle size distribution was obtained using a Laser Microtrac Particle Size Analyzer. Energy-dispersive X-ray spectroscopy was utilized to quantify the relative content of each metal in the bimetallic particles. The quantification of the Ni and Fe concentrations in the leachates was conducted using a Buck Instruments VG210 atomic absorption spectrophotometer. A Quantachrome Nova 2000 BET analyzer was used to estimate the surface areas of the powders. Photomicrographs were obtained with a Hitachi transmission electron microscope at a magnification of 100 000. 2.3. Selenate Removal Studies. The stock solutions of selenate were prepared using sodium selenate. No anionic species were added unless otherwise noted. Batch experiments were conducted using 25 mL of stock solution whose pH was adjusted to a desired value by adding 0.1 N HCl or NaOH at the start of the experiment. The pH was noted at the beginning and the end of each experiment. The experiments were conducted under constant agitation, using a magnetic stirrer, at a controlled temperature of 298 K for 100 h (except where
noted). The samples were filtered using a fine filter paper and analyzed for selenium concentrations. 2.4. Analysis. The selenium concentration was analyzed by the method EPA 200.8 using the inductively coupled plasma mass spectrometer (ICP-MS) available at the DWR Bryte Laboratory, West Sacramento, CA. In some studies, especially involving synthetic selenatecontaining solutions with no sulfate anion, analyses were performed using ion chromatography. The analytical data obtained from the ion chromatograph (Dionex DX 500) and ICP-MS are in excellent agreement (r2 ) 0.97). 3. Results and Discussion 3.1. Screening Test: Comparison of Selenate Removal Using Transition Metals and Bimetallic Powders. In a set of preliminary tests, transition metal powders of Ni, Fe, Co, and Cu and bimetallic powders (M1/M2 ) 1:1) of NiFe, NiCo, NiCu, FeCo, FeCu, and CoCu were employed for the selenate removal from a synthetic solution containing 51 mg/L of selenium at 25 °C for 30 min. The solids concentration used in each experiment was 0.5 g/L. For the sake of comparison, experiments employing two commercially available adsorbents, γ-alumina and activated carbon (Darco S51)
Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4925
Figure 2. Final concentrations obtained in removal studies of metallic and bimetallic powders of transition metals. Experiments were conducted at 25 °C for 30 min at a solids concentration of 0.5 g/L. Synthetic solutions containing 51 mg/L were used.
were also conducted for 72 h using a solids concentration of 4 g/L. The data in Figure 2 show that the use of iron, nickel and cobalt powders results in significant selenate removal (ca. 60.0, 54.0, and 65.2%, respectively) as compared to activated carbon and alumina (ca. 10.1 and 22.6%, respectively). The removal of selenate by the metallic powders probably occurs by chemical reduction of selenate to selenite followed by further reduction to selenium.49,50 The metal is oxidized during the reaction. The thermodynamic software HSC67 was employed to postulate possible reactions at room temperature (25 °C). There are two probable mechanisms for selenate reduction depending on the pH of the solution. According to theoretical thermodynamic evaluation and the overall reaction suggested by Qiu et al.,50 the following set of reactions are proposed to describe a possible mechanism through which selenate is reduced to selenite under neutral and basic pH conditions
powder must be able to lose its electrons in aqueous solutions. The formal cell potentials for the metal oxidation are 408, 236 and 282 mV vs NHE for iron, nickel, and cobalt, respectively. However, the formal potential for copper is -339 mV vs NHE, indicating that spontaneous copper oxidation is not thermodynamically favored in neutral aqueous solutions, and as a result, negligible selenium removal was observed. Similar reaction pathways can be evoked for the reduction reactions of selenite to elemental selenium49 and selenide (based on thermodynamic evaluation). The selenide can exist either as hydrogen selenide or metal selenide. The overall reactions postulated are as follows
Anodic
Another pathway for selenate removal could involve direct reduction to selenium
M0 f M2+ + 2e2H2O + 2e- f 2H* + 2OHSeO4
3M0 + SeO32- + 3H2O f 3M2+ + Se2- + 6OH-
3M0 + SeO42- + 4H2O f 3M2+ + Se0(s) + 8OH-
Cathodic
2-
2M0 + SeO32- + 3H2O f 2M2+ + Se + 6OH-
2-
+ 2H* f SeO3
+ H2O
M0 + SeO42- + H2O f M2+ + SeO32- + 2OH0 E cathodic ) 0.030 V vs NHE
The electrons made available by the dissolution of the metal results in water deprotonation. The hydrogen radical (H*) thus formed reduces selenate to lower oxidation states and to elemental selenium. At low pH values, the following selenate reduction pathway is postulated
SeO42- + 2H+ + 2e- f SeO32- + H2O On the basis of the above mechanisms, the metallic
The formal cell potentials of each of the above reactions are positive when M is Fe, Ni, or Co. Thus, the reduction of the selenium species to lower oxidation states by these metals is thermodynamically favored. However, for these reactions to occur, selenate must first adsorb onto the surface and then be reduced by electrons that are transported from M0, which is located beneath the native oxide overlayer. As the metal oxide film grows, the transport of the electrons through the layer is hindered and results in the inhibition of the reduction of selenate. For example, zero-valent iron corrodes in aqueous solutions by forming corrosion products such as magnetite and maghemite on the surface,64 reducing the effectiveness of the immobilization capacity of the iron particles. It has been observed in the past that the addition of nobler metals such as Pd significantly enhances the rate of reduction and reduces the effect of corrosion products
4926 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004
Figure 3. TEM images of NiFe bimetallic particles obtained using five different methods of preparation.
formed on the surface on the reduction of halogenated solvents. The substitution of Ni for Pd was also seen to have a similar effect on the remediation of these solvents. It has been hypothesized that the actual reduction takes place at the noble metal sites that are protected galvanically by the iron. From the data obtained for the individual metal reductants, hypothetical selenate removal by the bimetallic powder was estimated assuming that no catalytic
or synergistic effect exists. The hypothetical removals were approximately 20% greater than the actual selenate removals obtained for powders that did not contain nickel. When nickel was present, up to 224% enhancement (e.g., for NiFe) over the hypothetical removal was observed. Thus, the data show that the presence of nickel in bimetallic reductants imparts catalytic reactivity. In 30 min, 86.9% removal of selenium was obtained, which corresponds to 88.6 mg/g of
Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4927 Table 2. Particle Characteristics sample NiFe-A NiFe-B NiFe-C NiFe-D NiFe-E Fe-A
mean 9.9 18.8 15.3 6.6 9 17.7
size (µm) median 90% passing 6 14 12 5 10 11
31.5 41.7 41.7 15.0 17.7 24.3
Table 3. Selenate Removal Studies Using NiFe and Fe Particles (5 h)a specific surface area (m2/g) 41.44 45.11 191.42 39.88 88.42 41.22
specific removal of selenate by NiFe. NiCu, however, showed a slight (8%) decrease from the hypothetical removal. Further experiments were conducted using NiFe bimetallic powders. Because the zero-valent iron (especially in fine powder form) is known to be an effective agent for selenate removal,49,50 it was used for the purpose of comparison in this study. 3.2. Nanosized Particle Characteristics. 3.2.1. TEM. Owing to the different methods of preparation, distinct differences in the morphology and particle size among these samples were observed in the TEM photomicrograph (Figure 3). The data show that the sample prepared by pathway A consists of spherical particles 10-100 nm in size, whereas that prepared by pathway B is larger (50-200 nm). Needlelike particles were observed when the pH was maintained at near-neutral values (pathway C).Ultrasonic agitation resulted in the precipitation of larger particles from the as-prepared salt solution (pathway D). However, application of ultrasonic agitation during particle formation from the solution containing a surfactant (pathway E) resulted in finer-sized particles (10-50 nm) as compared to the case when no agitation was provided (pathway B). It is evident from the figure that the particles are of nanosize irrespective of the pathway. However, because of agglomeration in the solution, the mean particles sizes range from 5 to 15 µm (Table 2). 3.2.2. Particle Size Analysis: The data shown in Figure 4 clearly indicate that the particles formed via pathways D and E have the narrowest size distributions. Thus, ultrasonic agitation is required to maintain a narrow size distribution. The median and average sizes of the particles produced by the five different pathways (Table 2) are within the range of 5-14 and 6.6-18.8 µm, respectively. In addition, greater than 90% of the agglomerated particles formed via borohydride reduction (regardless of the pathway) are finer than 50 µm. Thus, it can be concluded that ultrafine particles are formed via this process. 3.2.3. BET Surface Area: The data on the surface areas of the particles are also provided in Table 2. The surface area of the powder prepared via pathway A was found to be 41.44 m2/g, and it increased to 45.11 m2/g when a surfactant was added to the solution (pathway B). The highest surface area (191 m2/g) was obtained by adjusting the pH of the precursor solution (pathway C). The data clearly show that providing enhanced mechanical agitation during powder preparation in the presence of a surfactant resulted in enhanced surface areas. The lowest surface area (39.88 m2/g) was obtained via pathway D, probably as a result of low porosity due to the increased surface tension in the absence of surfactant leading to a greater degree of agglomeration. The data obtained from the BET and TEM analyses are in close agreement.
init conc (mg/L)
1 g/L
NiFe 2 g/L
1 g/L
Fe 2 g/L
5 g/L
(A) specific removal (mg/g)b,c 2.49 1.00 3.78 4.88 2.00 39.94 25.73 10.60 65.88 49.25 20.60 55.80 24.60 96.40 36.80 98.00 -
2.49 23.25 45.69 -
1.00 9.69 18.71 -
ND 4.496 8.62 -
ND 2.540 6.46 -
5 g/L
1.06 5 10 51 53 100 103 123 565 1130
1.04 4.73 9.64 50.10 83.40 97.60 -
1.06 5 10 51 53 100 103 123 565 1130
(B) final concentration (mg/L) NDd 0.270 ND ND 1.217 0.360 0.238 ND 11.062 2.900 1.550 ND 34.122 19.600 4.500 ND 25.4 11.400 ND 372.200 381 934.2 -
a Synthetic solutions with an initial pH of 7.7 were used. Specific removal ) (initial concentration - final concentration)/ (solids concentration). c Units of milligrams of selenate removed per gram of reductant. d ND ) not detected (detection limit 0.016 mg/L) b
3.2.4. Composition. The composition, as determined by energy-dispersive X-ray spectroscopy (EDS), of the particles obtained from precursor solutions containing Ni/Fe in a molar ratio of 1:1 was found to be 50 at. % Ni and 50 at. % Fe irrespective of the method chosen for synthesis. The Ni concentration in solution was varied from 0 to 100%, and the resulting Ni content of the powder formed was observed to be identical to the solution concentration. 3.3. Effect of Concentration and Removal Agent Concentration. Table 3 contains the data on specific removals (milligrams of selenate per gram of reductant) of selenium and the final vs initial concentration of selenium (5-1000 mg/L) for solids concentrations of 1, 2, and 5 g/L of NiFe and Fe powders. The residence time in these experiments was 5 h. The data show that, for a given initial Se(VI) concentration, the percent removal increases with the solids concentration of the powder. The amount of selenate removed (per unit mass of powder) increases with the amount of initial impurity [Se(VI)] in the solution. As compared to traditional adsorbents, very high specific removal capacities (50 mg/g) are observed from a solution containing approximately 50 mg/L of Se(VI). This is a 25% increase over the removal capacity of iron powder (40 mg/g) under similar conditions. It is interesting to note that complete removal of the contaminant was observed when the initial Se content used was 1 mg/L even at a relatively low solids concentration of the NiFe powder (1 g/L). As discussed earlier, the reduction of selenate to selenium can result from direct reduction or through an intermediate selenite formation. The hypothetical reactions are k1
SeO42- 98 Se k2
k3
SeO42- 98 SeO32- 98 Se
4928 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004
Figure 4. Comparison of size analysis.
The rate equations for the above reactions are
CSeO42- ) CSeO42-,0e-k1t
(A)
and
CSeO42- )
k3 k2 C e-k2t C e-k3t k3 - k2 SeO42-,0 k3 - k2 SeO42-,0 for k3 * k2 (B)
CSeO42- ) (1 + k3t)CSeO42-,0e-k3t
for k3 ≈ k2 (C)
The ln(CSeO42-) vs ln(CSeO42-,0) plots (not shown) based on the data in Table 3 provided a good fit (r2 ) 0.91) at low selenate concentrations. It can thus be concluded that the reaction isa first-order with respect to the selenate concentration and can be represented by the above-described reaction pathways. However, the reaction kinetics was observed to shift toward zeroth-order with respect to the selenate concentration when the initial concentration was greater than 50 mg/L. This indicates that the concentration of selenate ions in the aqueous solutions was significantly greater than that of the sites available for the reactions. Such shift in the reaction order with concentration has been reported in the literature with respect to solid-liquid reactions.56,65,67 The data do not confirm or reject the hypothesis of direct reduction of selenate to elemental selenium. The firstorder rate constant was estimated to be 0.01 min-1, and the zeroth-order rate constant was 36.10 mg L-1 min-1. Figure 5 is a plot of the specific selenate removal vs the NiFe and Fe suspension density using synthetic solutions containing initial Se(VI) concentrations of approximately 50 and 123 mg/L. It is seen that, at a
Figure 5. Effect of solids concentration on the selenate specific removal. Experiments were conducted at 25 °C for 5 h. Synthetic selenate-containing solutions (initial pH ) 7.7) were used.
solids concentration of 0.1 g/L, the specific removal by Fe powder from the 50 mg/L selenate solution (Figure 5) was 155 mg/g whereas that by NiFe powder was 225 mg/g. Upon increasing the initial Se(VI) concentration to 123 mg/L, the specific removal by NiFe increased to 303 mg/g. This indicates that the specific removal is proportional to the selenate concentration. The data also show that the specific removal drops sharply with solids concentration. However, when increasing the solids concentration was increased beyond 1 g/L (specific removal of 45-55 mg/g for NiFe), the specific removal did not change significantly. The final concentration vs powder solids concentration in the solution after 5 h was observed to show an exponential decay with the NiFe solids concentration.
Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4929
A more complex equation involving a negative power and an exponential decay function was found to closely emulate the experimental data that involved the use of Fe powders. The following equations, representing the change of concentration with respect to the change in solids concentration, are obtained by differentiating the fitted functions (to the experimental data) with respect to initial solids concentration
dCSeO42-,5 dCFe,0
1 ) - (3k5CFe,0k6 + k7CSeO42-,5) 4
dCSeO42-,5 dCNiFe,0
) -k4CSeO42-,5
(D)
(E)
The values of k4, k5, k6, and k7 were estimated to be 1.5 L/g, 0.12 (mg/L)(g/L)0.7, -1.70, and 0.20 L/g, respectively. The factor of 3 in eq D reflects the number of atoms of iron needed for the reduction of 1 atom of selenate. As expected, the rate of change of the concentration with respect to a change in the initial solids concentration is a function of the iron solids concentration and the selenate concentration. It is concluded that both the iron and selenium oxyanions participate in the reduction reaction. It should be noted that the oxidized iron (in the form of hydroxides and oxyhydroxides) can further adsorb selenate and the intermediate selenite. However, it is observed that the rate of change of the selenate concentration with respect to a change in the initial solids concentration (NiFe) is a function of the selenate concentration only (eq E). 3.4. Proposed Mechanism. It has been hypothesized that the electrons64 made available at the nickel surface by the corrosion of iron68 are responsible for selenate reduction. However, the kinetics of iron corrosion is probably much faster than the surface reduction reaction and, thus, that the chemical reaction is ratelimiting. Thus, in the presence of adequate amount of iron moieties, it is possible that the nickel surface acts as a catalyst for the reduction reaction and, as a result, the overall rate of change of the selenate concentration with respect to the initial solids concentration is a function of selenate concentration only. The exponential decay of the concentration with respect to the NiFe solids concentration, which is indicative of the catalytic action occurring during the initial 5 h, can be explained via the following chemistry
Case A Fe0 f Fe2+ + 2eNi
2H2O + 2e- 98 2H* + 2OHNi + H* f Ni-H 2Ni-H + SeO42- f 2Ni + SeO32- + H2O 4Ni-H + SeO42- f 4Ni + Se + 2OH- + 2H2O In the above-described mechanism, iron corrodes galvanically, resulting in the formation of nickel hydride. The nickel hydride then reduces the selenate, and free nickel surface is formed to adsorb newly generated H* (active atomic hydrogen or hydrogen radical).
Table 4. Final Selenate Concentrations after 0.5 h of Treatment with NiFe Powders with Different Ni Concentrations from a 50 mg/L Selenate Solution at 25 °C and a pH of 7.7. (0.5 g/L NiFe) at. % Ni.
final conc (mg/L)
specific removal (mg/g)
removal (%)
0 10 30 50 70 90 100
23.43 18.5 6.49 6.68 17.15 18.50 20.40
55.14 65.00 89.02 88.64 67.70 65.00 61.20
54.06 63.73 87.27 86.90 66.37 63.73 60.00
Because nickel is more noble than iron, iron corrodes to protect the more noble metal as in a bimetallic couple.68 This leads to the second pathway. Metallic nickel can lose electrons to reduce selenate by the mechanism described for the single-metal powder (eqs 5-7). The nickel ion thus formed is reduced to metallic nickel by electron released by Fe/Fe2+ and Fe2+/Fe3+ redox reactions and movement of the electrons released through the particle body to the nickel surface. Thus, as long as there is an electronic bridge between the two metals and the presence of metallic iron, selenate reduction at the nickel surface will continue despite the formation of an iron oxide layer that prevents the generation of H*. This pathway also explains the catalytic behavior of the NiFe powder. The following equations illustrate the above mechanism
Case B Ni0 + SeO42- + H2O f Ni2+ + SeO32- + 2OH3Ni0 + SeO42- + 4H2O f 3Ni2+ + Se0(s) + 9OHFe0 f Fe2+ + 2eNi2+ + 2e- f Ni0(s) Either of the two reaction pathways outlined above describes the catalytic phenomena observed when NiFe was employed. It is possible that selenate reduction occurs via both pathways simultaneously. That described by pathway A is predominant in the early stages, and that described by pathway B is predominant in the latter stages (when a thick layer of iron oxide is formed on the surface, resulting in the cessation of the formation of H*). When the reaction is carried out for a longer time (complete loss of Fe0), nickel is expected to provide additional reduction of selenate by undergoing oxidation to nickel ion. In addition, the ferrous ions formed will form iron hydroxide and iron oxyhydroxide at the particle surface and result in further reduction of selenate via adsorption. 3.5. Effect of Ni Content. According to the mechanism postulated above, it can be concluded that the relative nickel content in the NiFe particles should be lower than that of the iron to effectively reduce the selenate to elemental selenium. Experiments were conducted to study the effect of the amount of Ni present in the bimetallic NiFe powders on the removal of selenate. The selenate removal experiments were carried out using 0.5 g/L solids concentration in a synthetic solution of 50.04 mg/L (initial) concentration of Se(VI) for a period of 0.5 h. The data in Table 4 show that the maximum selenate reduction occurs when the Ni con-
4930 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004
Figure 6. Effect of pH on the selenate specific removal. Experiments were conducted at 25 °C for 5 h at a solids concentration of 1 g/L iron and NiFe powders. Synthetic solutions containing 5.4 mg/L were used.
Figure 7. Effect of temperature on the selenate specific removal. Experiments were conducted at 25, 45, and 65 °C using synthetic solutions of initial pH of 7.7 for 5 h at a solids concentration of 1 g/L NiFe and Fe powder.
tent in the bimetallic powder is between 30 and 50 at. %. For particles with nickel contents outside this range, the selenate removal is observed to decrease. As explained by the reaction pathways described earlier, more than 50 at. % Fe is required for nickel surface protection from corrosion. When the iron content is more than 70%, the number of active sites on the surface is not high enough to effectively reduce selenate, thus giving the observed lowered removals. 3.6. Effect of pH. Figure 6 shows the influence of solution pH on the specific removal of Se(VI) by 1 g/L NiFe and Fe powders from a 5.4 mg/L selenate-containing synthetic solution. The experiments were run for 5 h. It is observed that the minimum final concentration of selenate (0.297 mg/L), corresponding to 94.5% removal, is obtained at a pH of 7.7. Almost no removal is observed at a pH of 11, whereas 88% removal is obtained at a pH of 3.5 (final concentration ) 0.63 mg/L). The decrease in selenate removal at a pH of 3.5 (as compared to 7.7) might be a result of competition between selenate reduction and hydrogen evolution sincsinbe the proton concentration is several orders higher. The negligible removal observed at a solution pH of 11 is due to the formation of Ni(OH)2 on the surface, resulting in the cessation of selenate reduction. When Fe powder was used in a solution of pH of 11, almost no selenate removal is observed. The negligible removal at high solution pH is a result of the precipitation of hydroxides that cover the active metal surface. A total of 77.4 and 90% removal was observed in experiments conducted for 5 h at pH values of 7.7 and 3.5. 3.7. Effect of Temperature. The effect of temperature on the selenate removal by 1 g/L NiFe and Fe powders from 5.4 and 120 mg/L selenate solutions is shown in Figure 7. The initial pH of the solution was 7.7, and the experiments were conducted for 5 h. The data show that higher removals were achieved at elevated temperatures. In fact, no selenate in the solutions (initial content ) 5.4 mg/L) obtained from experiments conducted at temperatures greater than 45 °C was detected using NiFe powder. When the temperature was increased from 25 to 65 °C, the selenate removal (from a 120 mg/L selenate solution) was found to increase from 45.8 to 91.5%, corresponding to an increase of nearly 100%. The increase in removal rate can be explained in part by the Arrhennius equation.
In addition, it is known that the solubility of oxygen in water decreases with temperature, resulting in an anaerobic environment, which favors the formation of reduced forms of selenium (such as selenite and selenide) that are relatively more amenable to further reduction to elemental selenium. The absence of oxygen also reduces the degree of formation of OH- ions due to the reduction of water (2H2O + O2 + 4e- f 4OH-), a reaction that competes with selenate reduction. As a result, the degree of the loss of active surface via reaction with dissolved oxygen decreases with temperature. As a result, a 40 °C increase in temperature results in a 100% enhancement in the percent removal of selenium. When Fe powder was used, the data showed that the selenium removal (initial concentration ) 5.4 mg/L) increased from 77.4% at 25 °C to 94.6% at 65 °C, an increase of 22%. This corresponds to a change in the final concentration of selenate in the solution from 1.21 mg/L (at 25 °C) to 0.29 mg/L (at 65 °C). Assuming a first-order homogeneous reaction (5.4 mg/L solution), an activation energy and frequency factor of 29.5 kJ/ mol and 335 min-1, respectively, were obtained for NiFe powder. In contrast, the activation energy and the frequency factor calculated for the reaction using Fe0 as the reductant were 14.3 kJ/mol and 1.44 min-1, respectively. The high activation energy for selenate reduction when NiFe is used can be related to the less positive formal potential for Ni as compared to Fe oxidation.68 The frequency factor is nearly 200 times higher for NiFe than for Fe0. Thus, the addition of Ni to Fe significantly accelerates the selenate reduction. 3.8. Kinetic Studies. Figure 8 contains the kinetic data on selenate removals from 100 mg/L (initial concentration) selenate-containing solution using two different NiFe solids concentrations (5 and 0.67 g/L) over a period of 2 h. Removal kinetics using iron powders (5 g/L) are also included. In all these cases, the selenium concentration is observed to drop rapidly with time for the first 2 min of operation. This decrease is rather precipitous when NiFe (5 g/L) powder is employed. The rate of selenate removal by iron powders, however, decreases and reaches a value of nearly 8 mg/L at the end of 30 min. No significant reduction in the selenium is obtained at longer residence times. It has been reported in the past69 that the reduced form of selenium on iron is often reoxidized if the reductant remains in the synthetic solution over long
Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4931 Table 6. Leach Testa Results agent
final pH
Ni (µg leached/g)
Fe (µg leached/g)
Ni0.1Fe0.9 Ni0.3Fe0.7 Ni0.7Fe0.3 Ni0.9Fe0.1
7.7 7.3 7.6 7.9
3 3 2.5 0.625
0.250 chitin (ca. 29.90% removal) > chitosan (ca. 27.30% removal) >ApF (ca. 25.50% removal) > TETHF (ca. 22.30% removal) > Darco HDB (ca. 18.14% removal) > cellulose (ca. 15.77% removal). In comparison, Fe powder reduced the selenate concentration from 5.04 to 0.49 mg/L (∼90%
4932 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 Table 7. Comparison of Removal Percentages for Synthetic Solutions with/without Competing Anions after Removala removal (%) removal agent
no impurities
10 g/L chloride
2.5 g/L nitrate
γ-alumina R-alumina chitosan chitin Darco S51 Darco HDB AsF TEHTF cellulose Fe NiFe
57.4 22.8 31.8 33.8 42.6 19.0 31.6 30.0 28.6 90.2 99.7
12.2 8.2 1.8 3.6 14.8 9.4 4.2 5.8 3.2 74.0 93.4
39.0 20.0 20.4 21.0 21.4 10.6 22.4 16.0 35.4 84.4 98.8
a Experiments were conducted at 25 °C for 300 min at a solids concentration of 1 g/L. The initial concentration of selenium was 5 mg/L.
removal). The removal capacity upon use of NiFe bimetallic powder was observed to be nearly 100%, with a final selenium concentration of 0.015 mg/L. 3.11. Effect of Competing Anions. Figure 10 and Table 7 show the effects of 10 and 2.5 g/L chloride and nitrate impurities on the removal effectiveness of various adsorbents and Fe and NiFe produced in the laboratory as characterized by the amount of selenium present in the solution after the treatment. The initial concentration of selenate in the solution was ca. 5.04 mg/L. It is seen in the Table 7 that the presence of 10 g/L chloride resulted in a much lower selenate removal than when the solution that contained no anions. The reduction of the selenium removal due to the presence of chloride was more pronounced in the case of adsorbents that showed somewhat significant selenate removal, namely, γ-alumina and activated carbon (Darco S51). The selenium removal by γ-alumina in the absence of anions was found to be 52.0%, which reduced to 12.9% upon the addition of chloride ions. A similar decrease in the selenate removal by activated carbon (Darco S51) was observed (57.9 vs 15.5%). The presence of chloride adversely affected the removal capacity of NiFe and Fe particles, with the presence of chlorides reducing the Se(VI) removals from 99 and 91.1% to 74 and 70%, respectively. However, as compared to the other adsorbents employed in this study, the capacity of the Fe and NiFe powders to remove selenate was less adversely affected. For example, as compared to a 75.19% decrease in the selenate removal capacity of Darco S51 activated carbon due to the presence of chlorides, the decrease in the selenate removal capacity of NiFe was calculated to be only 25.25%. The primary mechanism by which chloride ions could inhibit the reduction of selenate by the metallic and bimetallic particles is via pitting-type reactions. As a result, the electrons produced by iron oxidation are not available for selenate reduction. When the impurity used was a nitrate anion (2.5 g/L), the selenate removal by the adsorbents and the reductants was also adversely affected. However, the decrease in selenium removal due to the presence of nitrate anions was less significant than what was observed for solutions to which chloride was added. The influence of the presence of sulfate on selenate removal was also investigated. It was observed that the presence of 2.5 g/L sulfate impurities greatly reduced the selenate removal by NiFe powder from nearly 100 to 71.5% (final Se concentration ) 1.44 mg/L). As compared to the oxyanions of selenium, the chlorides, nitrates, and sulfates
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Received for review September 19, 2003 Revised manuscript received March 24, 2004 Accepted March 26, 2004 IE030715L
