Ferrates Synthesis, Properties, and Applications in Water and

Chapter 16

Electrochemical Fe(VI) Water Purification and Remediation Downloaded by UNIV OF MISSOURI COLUMBIA on March 10, 2013 | http://pubs.acs.org Publication Date: July 25, 2008 | doi: 10.1021/bk-2008-0985.ch016

Stuart Licht, Xingwen Yu, and Deyang Qu Department of Chemistry, University of Massachusetts at Boston, Boston, MA 02125

A novel on-line electrochemical Fe(VI) water purification methodology is introduced, which can quickly oxidize and remove a wide range of both inorganic and organic water contaminants. Fe(VI) is an unusual and strongly oxidizing form of iron, which provides a potentially less hazardous water purifying agent than chlorine. However, a means of Fe(VI) addition to the effluent had been a barrier to its effective use in water remediation, as solid Fe(VI) salts require complex (costly) syntheses steps, and solutions of Fe(VI) decompose. On-line electrochemical Fe(VI) water purification avoids these limitations, in which Fe(VI) is directly prepared in solution from an iron anode as the FeO " ion, and is added to the contaminant stream. Added FeO decomposes, by oxidizing water contaminants. Demonstration of this methodology is performed with inorganic contaminants sulfides, cyanides and arsenic; and water soluble organic contaminants including phenol, aniline and hydrazine. In addition, removal of the oxidation products by following an activated carbon filter at the downstream of the on-line configuration is also presented. 2

4

2-

4

Chlorination has been a mainstay of tertiary water treatment and drinking water science. As a stepping stone to the future, water utilities have sought alternative, less hazardous, broadly applicable and more cost effective oxidative

268

© 2008 American Chemical Society

In Ferrates; Sharma, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

269 methodologies to chlorination. Fe(VI) is an unusual and strongly oxidizing form of iron. As a strong oxidant, Fe(VI) has been investigated as a less hazardous water purifying agent for several decades as a safer alternative to the chlorination purification of water (1-3). The Fe(VI) oxidant for water treatment is present as the soluble aqueous FeO species, which is then reduced to environmentally benign ferric oxide products. Our generalized oxidation mechanism for Fe(VI) treatment of a contaminant is presented in eq 1, with the Fe(III) iron product ferric oxide in various states of hydration (l/2Fe O FeOOH, etc.): 2-

4

Downloaded by UNIV OF MISSOURI COLUMBIA on March 10, 2013 | http://pubs.acs.org Publication Date: July 25, 2008 | doi: 10.1021/bk-2008-0985.ch016

2

3,

aFe(VI) + bContaminant → cFe(III) + dDe-toxified(reduced)Contaminant (1) The field of Fe(VI) compounds for charge storage was introduced in 1999, and at that time the term super-iron was coined to refer to the class of materials which contain "super-oxidized" iron in the unusual hexavalent state (4). The synthesis of alkali and alkali earth super-irons has been presented and the charge transfer chemistry of the super-irons in both aqueous and non-aqueous media has been investigated (4-32). In previous water purification studies, the Fe(VI) salt, K FeO , has been used to oxidize and remove arsenic (3, 33), and for the removal from water of ammonia, cyanide, thiocyanate and sulfide (34), and been used for the effective treatment of biosolids (35). Viral inactivation by K FeO has been demonstrated (36), as well as algae removal (37). Fe(VI) effectively oxidizes a wide variety of organics including phenol (38), alcohols (39-43), toluene and cycloalkanes (39), ketones and hydroquinones (42), carbohydrates (44), and aminobenzene (45). Super-iron, FeO , provides an environmentally friendly, potentially cost effective oxidant, in lieu of chlorine, but delivery challenges of this oxidant is a principal limitation to its implementation by water utilities (46). In the new on line electrochemical water purification methodology, Fe(VI) is now directly prepared in an alkaline solution from the Fe metal anode as the Fe0 ion, and is added to the contaminant stream for water treatment on an "as need" basis (3). Prior barriers for Fe(VI) addition to an effluent for use in water remediation, were due to synthesis and solution phase stability challenges. The chemical synthesis of solid super-iron salts, principally by hypochlorite reaction with ferric salts, can be complex and costly, and only highly purified K FeO maintains long-term retention of the hexavalent state. Solutions of dissolved super-iron salts will oxidize water, and hence cannot be effectively stored or shipped as an oxidant for water treatment. On-line delivery system for superiron water purification will avoids these limitations (3). In addition to chemical syntheses, we have recently explored solid super-iron salt syntheses utilizing Fe(VI) electrochemically generated from a simple iron metal (3-29). On-line electrochemical super-iron, rapid, effective, treatment of a range of contaminants will be presented and summarized in this paper. 2

2

4

4

2-

4

2-

4

2


4

270

Electrochemical Fe(VI) Water Remediation Configurations


On-line electrochemical Fe(VI) water remediation configuration. The on-line electrochemical Fe(VI) water remediation configuration is shown as Scheme 1 (5). This configuration generally includes: 1) a Fe(VI) generation section; 2) an effluent oxidation section. The Fe(VI) generation section is a 2-electrode electrochemical flow liquid cell. An alkaline solution is used as the electrolyte for Fe(VI) generation. The anode may consist of an iron sheet, but a higher surface area iron anode (75,27,52), increases the rate per volume of Fe(VI) formed in solution. The cathode in this on-line Fe(VI) remediation is limited to materials which are stable when immersed in an alkaline, reductive environment. Cathodes used have included nickel and nickel oxide, platinum, gold, graphite, carbon black, iridium oxide or ruthenium oxide. The anode and the cathode electrode are controlled by an external power supply. As seen in Scheme 1, the Fe(VI) formation compartment may have an optional separator between the cathode and anode. When the anode is downstream of the cathode, the separator is generally not required. If the anode were in proximity of the cathode, then a portion of the anodically formed Fe0 " could be lost at the cathode. When used, a separator between the iron anode and the cathode can be either a non conductive separator configured with open channels, grids or pores, such as a ceramic frit, or a membrane to impede Fe0 ' transfer, such as a cation selective membrane (eg. Nation 350, Dupont®). The water to be purified, and the solution containing electrochemically synthesized Fe0 " are brought together by means of a pump to the oxidation section. In our study, all Fe(VI) formation electrolytes are prepared with NaOH (analytical grade), and triply deionized water. The concentration of NaOH can 2

4

2

4

2

4

Scheme 1. On-line electrochemical Fe(VI) water purification.


271 range from 1 M to saturation. A concentrated alkaline solution provides higher current efficiency, and higher rates of Fe(VI) formation at lower overpotential (27). However, the solubility of Fe(VI) will decrease with increasing NaOH concentration (4,5), and therefore 10 M NaOH is observed as an optimal concentration. Small diameter, high surface area, coiled iron wire is used as the anode, preparedfromFluka (0.1% Cu, 0.1% Ni and 0.7% Mn) iron wire, d=0.2 mm diameter, L = 128 m coiled length, and has a surface area, A = Lrcd = 800 cm . Anode pretreatment consists of 3 minutes sonication in 1:3 HC1 (concentrated HCkwater), followed by washing with triply deionized water (Nanopure water system of 18.2 megaohm resistivity) to pH=7. 80 cm planar nickel sheet is used as the cathode. A 2 mA/cm current density is observed to provide the highest current efficiency for Fe(VI) yield, therefore, constant current of 2 x 800 = 1600 mA (1.6 A) is applied for Fe(VI) formation. The flow cell is operated at room temperature (22 °C). In the oxidation section, the contaminated effluent steam to be purified is brought together with the Fe(VI) stream [solution phase Fe(VI), as Fe0 '] generated in the Fe(VI) generation section. The flow rate of the effluent to be treated, as well as the flow rate of the generated Fe(VI) solution, are separately rhanaged with two Control Company variable flow mini-pumps, Model number 3385, using 0.5-4.8mm (I.D.) tubing. Upon mix of the two streams, Fe(VI) contacts the contaminants and the contaminants are oxidatively decomposed by Fe(VI), which reduces to Fe(III). The length of the oxidation section can be modified, to control the time for the reaction between Fe(VI) and contaminants. For a relatively slower reaction rate, a longer oxidation section is required to provide sufficient time for the mixed streams to interact. Effectiveness of the contaminants removal is probed downstream of the configuration, by measuring the contaminants' concentration. 2


2

2

2

4

In-line electrochemical Fe(VI) water purification configuration An in-line electrochemical Fe(VI) water purification system, as represented in Scheme 2, is an alternate configuration (3) for effluent treatment. This configuration, utilizes the water to be treated as an electrolyte for the electrochemical formation of Fe(VI) species, and the water to be purified is in contact with, and flows over, the Fe0 ' generating anode. However with this in-line configuration, the iron electrode is exposed to the untreated water and vulnerable to fouling. 2

4

On-line electrochemical Fe(VI) - activated carbon water purification configuration. Based on the on-line electrochemical Fe(VI) water purification configuration introduced above, an on-line electrochemical Fe(VI) - activated


272 power supply

optional separator

untreated water

Iron containing anode

Fe(VI) oxidation

purified water


Scheme 2. In-line electrochemical Fe(VI) water purification.

carbon water purification configuration is further developed by extending downstream 'an activated carbon filter, and probed for the rapid removal of organic toxins. This expanded configuration is shown as Scheme 3. In addition to the 1) Fe(VI) generation section, and 2) effluent oxidation section, this exapnded configuration also includes 3) a products removal section.

Scheme 3. on-line electrochemical Fe(VI) - activated carbon water purification.

The Fe(VI) generation section and the effluent oxidation section in the Scheme 2 on-line electrochemical Fe(VI) - activated carbon water purification configuration, are the same as Scheme 1. The final products removal section in this configuration is prepared as a 5 cm long activated carbon filter. It is used to absorb the insoluble oxidation products of the treated water, such as suspensions of minute particles, or those present as emulsions of water insoluble liquids. If a large amount of insoluble oxidation products are produced from the effluent that is treated, it would be necessary to separate the bulk insoluble oxidation


273


products prior to the stream flowing through the activated carbon filter; however, this was not observed as necessary, to treat the relatively small volumes of contaminants used in this study. Hence, the stream is directly introduced into the product removal section after the oxidation section. In this paper, the on-line electrochemical Fe(VI) water remediation configuration shown as Scheme 1 will be used for the demonstration of inorganic contaminated water purification. While, organic contaminated water remediation will be demonstrated with on-line electrochemical Fe(VI) activated carbon water purification shown as Scheme 3.

Electrochemical Fe(VI) Generation and Analysis Fe(VI) Formation Fundamentals Fe(VI) can be preparedfromalkaline hypochlorite oxidation of Fe(III) salts (4, 10, 12). The electrochemical Fe(VI) water purification methodology is based on the novel on-line electrochemical formation and addition of Fe(VI) to oxidize and treat contaminants. Fe(VI) is added in the form of Fe0 " ion. Electrochemical formation of the Fe0 " is accomplished at a positively biased iron anode in contact with an alkaline aqueous solution. High pH acts to enhance the rate of formation of the Fe(VI) species, favoring utilization of concentrated hydroxide electrolytes, similarly high surface area anodes, such as wound iron wire, favor rapid Fe0 ' formation (21). At sufficiently anodic potentials in alkaline media, an iron anode is directly oxidized to the Fe(VI) species, Fe0 ", in accord with the oxidation reaction: 2

4

2

4

2

4

2

4

2

Fe + 80H' -> Fe0 " + 4H 0 + 6e" 4

(2)

2

The process occurs at potentials > 0.6 V vs the standard hydrogen electrode, SHE, consistent with the alkaline rest potentials related to the formation of Fe(II), Fe(III) and Fe(VI)fromFe(0): Fe + 20H- -> Fe(OH) + 2e; E°= -0.8 V vs SHE

(3)

Fe(OH) + OH' — FeOOH + H 0 + e; E°= -0.7 V vs SHE

(4)

2

2

2

2

FeOOH + 50H- — Fe0 " + 3H 0 + 3e"; E°= +0.6 V vs SHE 4

2

(5)

Isolation of Fe(VI) away from the cathode minimizes losses due to the cathodic back-reduction of the synthesized Fe(VI), eq 6, and instead favoring the cathodic evolution of hydrogen, eq 7: 2

Fe0 * + 3H 0 + 3e' -> FeOOH + 50H 4

-

2


(6)

274 6H 0 + 6e" — 60H" + 3H 2

(7)

2

With 6 Faraday of charge transfer through the anode and cathode per equivalent of Fe, eq 2 and 7 combined yield the net Fe(VI) solution phase alkaline synthesis reaction: 2

Fe + 20H' + 2H 0 — Fe0 " + 3H


2

4

(8)

2

Fe(VI) Analyses 2

Fe(VI), dissolved as Fe0 ", is evident as a purple color in aqueous solution, and has a distinctive UV/Vis spectrum. The visible absorption of Fe(VI) in highly alkaline solution is presented in the inset of Figure 1 (75). This shows a maximum absorption at 505 nm, an absorption shoulder at 570 nm. The molar absorptivity measured at 505 nm is 1070±30 M" cm' . The molar absorptivity remained constant up to 200 mM. Similarly, at a fixed ferrate concentration, the measured absorbance was highly independent of the alkali hydroxide cation or hydroxide concentration in solution (4, 75). Figure 1 shows the linear relationship between the 505 nm absorbance and Fe0 " concentrations, and with the slope about 1070 M" . In our study, Fe0 ' concentration is determined with a Hewlett Packard 8453 UV/Vis photodiode array spectrophotometer by the 1070 M" cm" molar absorptivity at A=505 nm with 385 nm baseline correction, and with dilution as required to reach the analyzable Fe0 " concentration range (75). 4

,

1

2

4

1

2

4

1

1

2

4

Figure 1. The 505 nm absorbance offerrate (VI) solutions of various concentrations at 505 nm. Inset: Vis. absorption spectra ofFeO/' in alkaline aqueous solution.


275 Fe(VI) generation in the on-line electrochemical flow cell Study of the parameters which influence Fe(VI) generation, such as electrolyte (NaOH) concentration, iron anode surface area and current density, and temperature, have been studied in details elsewhere in order to optimize yield for the electrochemical synthesis of Fe(VI) salts (15, 21). In this water purification study, the following conditions provide effective dissemination of the Fe(VI) for remediation: surface area of Fe wire anode: 800 cm in a 40ml anode chamber of the flow cell; NaOH electrolyte concentration: 10 M; current density applied to the Fe anode: 2mA/cm " (total current applied: 800 cm x 2 mA/cm = 1.6 A), operated at room temperature (22°C). Under these conditions, the cell reached a steady state Fe(VI) concentration in approximately half an hour as shown in Figure 2. Specifically, anodic oxidation of 10, 20, 30 or 60 minutes respectively, generated concentrations of 21, 33, 38 and 39 mM Fe0 ", respectively (measured by 505 nm absorption spectroscopy as discussed above). 2


2

2

2

4

i

0

1

r

10

20

30 40 Time, min

50

60

Figure 2. FeO/~ concentration increase with the time of application of a constant anodic current in the flow cell

Inorganic Contaminated Water Purification Inorganic contaminants remediation for water purification will be demonstrated with the on-line electrochemical Fe(VI) water purification shown as Scheme 1. Contaminated water samples are prepared with sulfide, cyanide and arsenite.


276 Analyses of inorganic contaminants sulfide, cyanide and arsenite Inorganic contaminants water samples are prepared with sulfide, cyanide and arsenite. Sulfide concentration is analyzed with a model 27504-28 ColeParmer® Silver/Sulfide electrode and cyanide with a model number 27504-12 Cole-Parmer® Cyanide electrode, both using an Orion Research expandable ionAnalyzer EA 920 measured as a function of time using LabView data acquisition. Unlike sulfide, and cyanide, ion selective analysis was not available for arsenic determination. Instead arsenic concentration is determined by 3 electrode cyclic voltammetry on an 0.5 mm platinum electrode scanned at 50 mV/s, with a Pt counter electrode and silver/silver chloride reference electrode; accomplished by monitoring the relative decrease in the observed As(III) peak current on Pt at +0.13V, which generates a linear response of peak current over the 0.5 to 10 mM concentration NaAs02 ' M NaOH, as shown in Figure 3.


2

m

£, V Figure 3. cyclic voltammetry of the inert Pt electrode in different concentration arsenite solution. Inset: linear relationship between peak current and arsenite concentrations.

Sulfide, cyanide and arsenite contaminated water treatment with on-line electrochemical Fe(VI) water purification configuration 2

Fe(VI) was readily formed on-line as the Fe0 " species in the flowing 10 M NaOH solution, and was added to an effluent to be treated in accord with the Scheme 1. The effect of Fe(VI) on a contaminant was probed by cycling at 4


277


various controlled time intervals, a constant Fe(VI) formation current. Fe(VI) was added in this stepped manner to a purposely prepared, sample contaminated effluent, by switching on and off, the anodic (Fe metal electrode) oxidation current, as represented in Figure 4. The contaminant's concentration were measured down-line of the Fe(VI) addition, after the combined effluent and Fe(VI) streams had intermingled for 3 minutes.

15 30 45 60 75 90 105 120 Time, min Figure 4. Sketch of stepped addition of Fe(VI) to effluent and time control arrangement. On: switch on the constant anodic current (lower horizontal line); off: switch off the anodic current (upper horizontal line).

Sulfide was the first contaminant studied. Aqueous sulfide and sulfur (polysulfide) chemistry is complex. In all but the most alkaline solutions, sulfide solutions predominantly contain the (hydrosulfide) HS", rather than the S", species, and partial oxidation of the sulfide solution generates a mix of polysulfides, S ", and thiosulfate, S 0 ', whose speciation varies with sulfide concentration and pH (47-52). H S is fully oxidized by excess ferrate to sulfate, S0 -, whereas, at high pH sulfite and thiosulfate products can also occur (2), and the latter can predominate (47). Theoretically, oxidative removal of sulfide to thiosulfate, S 0 ", requires a 4/3 molar ratio consistent with eq. 1, when treated by Fe(VI) as: 2

2

2

x

2

3

2

2

4

2

4Fe0

4

2

3

2

+ 3S ' + 23/2H 0 — 4Fe(OH) + 3/2S 0 ' + 1 10H'

2

2

2

3

2

(9)

3

We observe that the oxidation product is thiosulfate (i) via thiosulfate's characteristic FTIR absorption peak at 996 cm' , and (ii) the stoichiometric ratio of Fe(VI) required to fully remove sulfide, as measured by the sulfide ion selective electrode. By this latter technique, a molar ratio > 1.4: 1 of Fe0 " to Na S, is observed to remove all sulfide from 0.01 M Na S in 1 M NaOH (while a 1.3:1 molar left trace sulfide). In accord with the stoichiometric ratio of eq. 9, 1

2

4

2

2


278 2

the current controlled concentration of added Fe0 ' was 4/3 that of the sulfide concentration. The variation of sulfide concentration in time of on-line electrochemical Fe(VI) treated 0.01 M Na S in 1 M NaOH was measured using sulfide selective electrode analysis. This was calibrated using the same experimental configuration, but periodically interrupting the sulfide solution flow, and instead of the Fe(VI) solution, flowing a sulfide free (and Fe(VI) free) solution. The sulfide calibration is presented in the bottom section of Figure 5. Figure 5 also presents the on-line electrochemical treatment of sulfide by Fe(VI), with, or without anode controlled Fe0 " addition. As seen in the top section of the figure, the sulfide is completely removed by the Fe(VI). 4

2

2


4

0

20

40

60 80 Time, min

100

120

Figure 5. Top (a): Remediation of sulfide by on-line Fe(VI) treatment. The variation of sulfide concentration upon FeO/~ addition to sulfide effluent flow. The iron anode is alternately cycledfrom 0 current to constant oxidation current mode, as described in Figure 4. Bottom (b): Sulfide concentration measurement and calibration, exposing the downstream sulfide ion selective electrode to the 1 M NaOH flowing solution alternately with, and without, 0.01 MNa S. 2

Generally in this study, the effluent studied was 1 M NaOH, at approximately pH 14 (55), containing different contaminants. However, the Fe0 " is also effective for oxidation remediation of water at lower pH. Figure 6, summarizes sulfide removal at lower pH by Fe(VI), in both pH 12 and 13 solutions, and again the complete removal of sulfide is observed. Water to be 2

4


279 0.012 0.0106-r3=o-°--Q=Q^g=8=5^a^? - — add FeO. £ 0.008 ©

Remediation of sulfide by Fe(VI) treatment at low p H

0.006 — • — pH=12 §

0.004

Beyond 5 min, the pH 12 points are

— o — H=13


P

overshadowed by the pH 13 symbols

/

© 0.002 u 0.000

•-•-•-•-•-•-•-o-o-a-a-D-o-D-D-a-o-o-D

6

10

8

12

-

14

Time, min Figure 6. Remediation of sulfide by Fe(VI) treatment at lower pH The variation of sulfide concentration upon FeO/~ addition. pHvalues of 12 and 13 controlled by NaOH addition to 0.01 MNa S with FeOf added as K Fe0 . 2

2

4

treated may also be at neutral pH, although the treated stream will be raised to higher pH by addition of the alkali media used to generate the Fe(VI). Aqueous alkaline oxidation of cyanide generates cyanate (54, 55), which will form upon Fe(VI) addition, in accord with: 2

2Fe0 "+ 3CN'+ 5H 0 — 2Fe(OH) + 3CNCX + 40H' 4

2

3

(10)

Aqueous cyanide may be treated by Fe(VI) through oxidative decomposition as described by eq 10. Calibration of the cyanide concentration, as measured by ion selective electrode analysis, for a 1 M NaOH solution with, or without, 0.01 M cyanide is presented in the bottom section of Figure 7. Removal of cyanide is in accord with the stoichiometric ratio of 2 Fe(VI) per 3 cyanide in eq 10. However, the rate of oxidation is slower than that observed for sulfide. As seen in Figure 8, whereas the Fe(VI) rate of sulfide oxidation occurs in less than 1 minute, the rate of cyanide oxidation is longer. It is seen in the figure that only 90% of the initial cyanide concentration is removed in thefirst3 minutes. This also occurs for on-line Fe(VI) remediation of cyanide, when the effluent and Fe(VI) streams are permitted to intemix for only 3 minutes prior to downstream analysis. As seen in the top section of Figure 7, approximately 90% of the cyanide, to ImM, is removed by the Fe(VI) addition during this time. In accord with Figure 8, the fraction of cyanide removed increases with increasing mixing time of the cyanide effluent and Fe(VI) streams.


280 .(a)

12

lOmMNaCN

, Fe(VI) off

8


2 C 12 °

\

Fe(VI) on

4

0

N

l

0 mM

J

'

"

Without NaCl

20

J

^lOmMNaCN^

/ WithNaCNv.

4

1 mM

40

60 80 Time, min

100

120

Figure 7. Top (a): Remediation of cyanide by on-line Fe(VI) treatment. The variation of cyanide concentration upon Fe0 ' addition to the 0.01 Mcyanide effluent flow. The iron anode is alternately cycledfrom 0 current to constant oxidation current mode, as described in Figure 4. Bottom (b): Cyanide concentration measurement and calibration, exposing the downstream cyanide ion selective electrode to the 1 M NaOH flowing solution alternately with, and without, 0.01 MNaCN. 2

4

0.012

J

2

Oxidization rate of S and CN Fe(VI)

0.008 Measure here, 3 min

-a— Sulfide

0.004

- o — Cyanide

s o U 0.000

10

20

30

40

50

60

Time, min 2

Figure 8. Comparison of the rate of 0.01 MS ' or 0.01 M CN oxidization by FeO '. Respective cyanide and sulfide concentrations are measured by ion selective electrode. 2


281 For the arsenic contaminant, arsenite, As(III) (for example as added NaAs0 ), the oxidation product upon Fe(VI) addition is arsenate, As(V), with the overall reaction given by: 2

2

-

3

Fe0 " + 3/2As0 - + OH +H 0 - » Fe(OH) + 3/2As0 " 4

2

2

3

(11)

4

2

3

Hence, the Fe0 " required to oxidize 1 mole of As0 * is 2/3 mole. Figure 9 presents the result of on-line electrochemical treatment of arsenic by Fe(VI). As seen in figure, in a manner analogous to the sulfide treatment, the Fe(VI) removal of arsenic, as As(III) in the 0.01 NaAsC>2 M effluent, is complete. Downloaded by UNIV OF MISSOURI COLUMBIA on March 10, 2013 | http://pubs.acs.org Publication Date: July 25, 2008 | doi: 10.1021/bk-2008-0985.ch016

4

3

1

0.012

1

J

1

^Fe(VI) o f f

T

•

|

10 m M NaAsO,

N

0.008

-

0.004 Fe(VI) on 0.000 . ...i

0

0 mM /

» .

i

20

40

.

i

i

60

i

80

.

i

100

120

Time, min Figure 9. Remediation of arsenic by on-line Fe(VI) treatment. The variation of arsenic concentration upon Fe0 ' addition to the 0.01 M NaAs0 in 1 MNaOH effluent flow. The iron anode is alternately cycledfrom 0 current to constant oxidation current mode, as described in Figure 4. 2

4

2

Simultaneous treatment of multiple contaminants in an effluent flow by Fe(VI) on-line remediation was also found to be effective. Figure 10, presents the treatment, by on-line electrochemically generated Fe(VI), of a solution containing both 0.01 M sulfide and cyanide. The Fe(VI) concentration is anodically controlled in accord with the sum of eqs 9 and 10. The concentration of sulfide and cyanide in the treated effluent, is observed to be consistent with the treatment of the individual contaminant components. Specifically, as observed in Figure 10 the sulfide removal is complete, and 90% of the initially cyanide is oxidized as limited by the (3 minute) intermix time, prior to detection, of the effluent Fe(VI) and contaminant streams.


282

0.012

(a): Cyanide

0.008

Fe(VI) off

1 mM

Fe(VI) on

/

. 0.004 e


•2 o.ooo

1

0.012

(b): Sulfide

, Fe(VI) off,

§ 0.008 U

I

0 mM

0.004

Fe(VI) on I

0.000 20

40

60 80 Time, min

) 100

120

Figure 10. Simultaneous remediation of cyanide and sulfide by on-line Fe(VI) treatment. The effluent contains 0.01 Msulfide and cyanide. The time variation of the concentration of each [Top (a): cyanide and Bottom (b): sulfide], upon FeOf~ addition to the effluent flow, is shown. The iron anode is alternately cycledfrom 0 current to constant oxidation current mode, as described in Figure 4.

Organic Contaminated Water Purification Organic contaminants water purification was demonstrated with the on-line electrochemical Fe(VI) - activated carbon water purification shown as Scheme 3. Organic contaminated water samples were prepared with aniline, phenol and hydrazine.

Measurement of aniline, phenol and hydrazine. Both aniline and phenol are quantitatively analyzed with UV/Vis spectroscopy using a series of standard concentrations of each. As seen in Figure 1 la, two characteristic peaks of aniline at 230 nm and 280 nm are candidates for


283 the quantitative analysis of aniline in water. However in the UV region, the absorbance by hydroxide overlaps and interferes with analine's 230 nm absorbance. Hence, although aniline's absorbance is larger at 230 nm than at 280 nm, in a medium in which the hydroxide concentration can vary, the 280 nm absorption provides a more quantitative measure for aniline analysis. In a similar manner, it is evident in Figure lib, that phenol absorption at 270 nm is more useful for quantitative phenol analysis, due to the close proximity of the UV hydroxide absorbance to the alternate 211 nm phenol absorption. The linear variation of absorbance, A, with either mM concentration of aniline, C , or phenol, C , is presented in the insets of Figure 1 la and Figure 1 lb respectively, and yields the linear relationships for respective aniline and phenol UV/Vis analysis: For aniline: A (280 nm) = 1.288 C (12) a


p

a

For phenol: A (270 nm) = 1.457 C p

a

p

(13)

The hydrazine concentration is analyzed with PDMAB method (ASTM D 1385-01), using a 0-500 ppm VACUettes Kit®.

Con ceil ti ation of aniline. m M Concentration of phenol. m M Figure 11. Quantitative analyses of aniline and phenol with UV/Vis spectroscopy, (a): The 230 nm and 280 nm absorbance of aniline solutions at various concentrations; (b): The 211 nm and 270 nm absorbance ofphenol solutions at various concentrations. Inset: Representative UV/Vis spectra obtained at the indicated concentration of aniline and phenol solutions.


284


Oxidative treatment and removal of aniline, phenol and hydrazine. Hydrophilic groups (amido and hydroxyl) in aniline, phenol and hydrazine facilitate a high degree of water solubility, and each is known as a hazard contaminant in industrial wastewater (56-58). Each is in a relatively reduced state, and in principal should be susceptible to oxidation in aqueous medium. Furthermore, the oxidation products of these organics are either insoluble or gas products which under the appropriate conditions can facilitate product removal. The oxidation product of aniline by Fe(VI) is azobenzene (39). The oxidation of phenol by Fe(VI) is more complex, with the principal oxidation product determined as p-benzoquinone (38). The respective oxidative reaction of aniline or phenol by Fe0 " may be generalized as: 2

4

2

3C H N + 2Fe0 ' + 2H 0 — 3/2C H N=NH C + 2Fe(OH) + 40H" (14) 6

7

4

2

6

5

5

6

3

2

3C H OH + 4Fe0 ' + 7H 0 -> 3 C H 0 + 4Fe(OH) + 80H" 6

5

4

2

6

4

2

3

(15)

Based on eq 14 and eq 15, the oxidative removal of aniline or phenol to azobenzene and benzoquinone respectively requires either a 2/3, or a 4/3, molar ratio of F e 0 \ 8 mM aniline and 5 mM phenol solution are prepared as sample contaminated water effluents, and treated with various flow rates of the Fe(VI) stream. In accord with Figure 2, a constant concentration of 39±1 mM Fe(VI) is obtained in half an hour after initiation of oxidative current in the flow cell. The contaminant water effluent (including aniline or phenol) and Fe(VI) stream are then brought together into the oxidation section of Scheme 3 with the flow ratio of Fe(VI) : contaminated water, varying over a twenty fold range (from= 1:1 to 1:20). This provides Fe(VI) to contaminant molar ratios varyingfrom-5:1 up to 1:4 for Fe(VI) to aniline; and from -8:1 to 1:2.5 for phenol. Upon 30 minutes treatment, the residual aniline or phenol in the treated water is analyzed before the mixed streams flow through the activated carbon. Figure 12a and Figure 12b show that a flow rate ratio of 1:4 for Fe(VI) stream to contaminated water stream is necessary to sufficiently oxidize aniline or phenol contaminants. Molar ratios of Fe(VI) required for completely oxidizing aniline or phenol derived from Figure 12a and Figure 12b are 1.2:1 for Fe(VI) to aniline, and 2:1 for Fe(VI) to phenol. Both are higher than theoretical calculation based on the stoichiometric ratios in eq 14 and eq 15 (molar ratio > 2/3 for Fe(VI) to aniline; and > 4/3 for Fe(VI) to phenol). 2

4

Contaminated water sample prepared with 8 mM aniline and 5 mM phenol are then fully treated with the on-line electrochemical Fe(VI) - activated carbon water treatment configuration shown as Scheme 3, with a flow rate ratio of Fe(VI): effluent contaminant equal to 1 : 4. After the two streams of Fe(VI) and contaminated water (containing aniline or phenol) are brought together, the mixed streams flow through the oxidation section for either 5 min, 15 min or 30


285 10

—i 1 1 1 r ,

(b)

(a) Aniline solution

8

8

mM

5 1 :20

• 1 : 20

6


U

0

s

JS

2§

u

1: 5m 0.4

1:4

1: 1 1:

•—

0.0 0.2

4

mM

E "© a

2 B O

1—

• 1:10

1 : 10

4

1

Phenol solution

0.6

0.8

1.0

0.0

0.2

0.4

1: 1

1:2 •— 0.6 0.8

4>

1.0

Flow ratio of Fe(VI) to aniline or phenol

Figure 12. The oxidation of aniline and phenol by Fe(VI) at different flow ratio ofFe0 ' (using anodically generated 39±1 mM FeO/~) to (a): 8.0 mM aniline and (b): 5.0 mM phenol solution. 2

4

min, and then flow through the activated carbon filter. After this treatment time, the samples are diluted tenfold (to the within the concentration range of the analysis) and analyzed with UV/Vis spectroscopy. As in the solid-circle curves in Figure 13a and Figure 13b, the characteristic peaks for aniline at 280 nm and phenol at 270 nm fully disappear after 5 minutes of Fe(VI)/effluent mix. Spectra measured after 15 or 30 minutes (not shown) are virtually identical to those measured. Hence, 5 minutes provides an upper limit to the requisite reaction time for complete effluent remediation. Also notable in the solid-circle curves is the efficacy of the activated carbon, evident in the absence of any product peak in the 300-320 nm range. Whereas, these product peaks are evident in the figure, when either effluent is treated and removed by Fe(VI) in the absence of an activated carbon filter. For comparison, the pure azobenzene (the oxidation product of aniline, evident at 320 nm) and p-benzoquinone (the oxidation product of phenol, evident at 300 nm) are also included in Figure 13. In principal, oxidative removal of hydrazine to nitrogen requires a 4/3 molar ratio of Fe0 "to N H in accordance with (55): 2

4

2

4

2

3N H + 4Fe0 ' + 4H 0 - * 3N + 4Fe(OH) + 80H" 2

4

4

2

2

3

(16)

Hydrazine oxidation is performed in a simplified Scheme 3 configuration, without inclusion of the activated carbon filter to remove the oxidation product, as the oxidation product of hydrazine is nitrogen (N ) (59). The water contaminant effluent sample is prepared as 5.6 mmol/L hydrazine. Rapid reaction is readily observed between Fe(VI) and hydrazine; seen as a 2


In Ferrates; Sharma, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008. 30 min •

250

300

350

I

250

Wavelength, nm

400

— Fe(VI)/activated carbon treated (a): aniline; (b): phenol ellfuent

300

- ° — Fe(VI) treated (a): aniline; (b):phenol ellfuent, without activated carbon

350

400

— (a): Pure azobenzene (saturated); (b) Purep-benzoquinone (saturated), without activated carbon.

— Untreated effluent (a): 8.0 m M aniline, diluted 10 times; (b): 5.0 m M phenol, diluted 10 times

solution, 5 m M

' Untreated phenol

c

Ferrates Synthesis, Properties, and Applications in Water and

Recommend Documents