Introduction

As discussed in Chapter 8 of the companion book, the emission of nitric oxide (NO) is problematic due to

• its contribution to ozone depletion in the ionosphere (i.e., NO + O3 —► N02 + 02)

• the production of smog in the troposphere

• its proclivity to attach to hemoglobin (preventing dioxygen transport)

• its role as a precursor of acid rain

• its high reactivity (it is a radical)

NO is thermodynamically unstable at room temperature and pressure. As soon as it comes in contact with dioxygen, it is oxidized to nitrogen dioxide (a toxic, brown acidic gas).

Many processes have been proposed and used for NO removal from combustion gases. Dry processes include its reduction with NH3 and N2H4. Wet processes are often aimed at the simultaneous removal of SO2 and NO. However, owing to the low solubility of NO in water (1.25 x 10~3 Matm-1 at 50°C), chemical reactions in the gas phase or in aqueous media are needed to solubilize it. Practically all of the NO wet scrubbing methods are based on its oxidation (e.g., with H202), reduction (e.g., with S032"), or complexation (e.g., with aminopolycarboxylates). In the present experiment we will focus on this last approach.

Complex formation removes gaseous pollutants when these can be used as ligands for selected metal ions, typically in aqueous solution. The complexes thus formed can be chemically or electrochemically oxidized or reduced, whereby the pollutant (acting as a ligand) is destroyed to produce less harmful species, and the metal ion becomes ready to reinitiate the cycle.

Removal of NO by complexation can be as high as 70-80%, with typical enhancement factors of 100500 as compared to its absorption in pure water. Several iron thiochelates (e.g., with 2,3-dimercapto-1-propanesulfonate) are particularly effective for its removal. Iron aminopolycarboxylate chelates (e.g., Fe-EDTA) are also effective to a satisfactory degree.

An Fe-EDTA chelate will be used here due to the wider availability and lower cost of aminopolycarboxylates as compared to thiochelates. In addition, EDTA is a strong complexing agent for many metal ions, although unfortunately this same property makes it a persistent pollutant.

The complexing ability of metal chelates can be selective for certain oxidation states. For example, [Fe(II)EDTA] forms with NO the corresponding (mono)nitrosyl complex [Fe(II)NO(EDTA)], whereas the Fe(III) chelate cannot complex it. For this reason, a practical NO scrubbing system must

—o—H4EDTA -4- (H3EDTA)—»- (H2EDTA)-2 -*- (HEDTA)-3 —(EDTA)-4

—o—H4EDTA -4- (H3EDTA)—»- (H2EDTA)-2 -*- (HEDTA)-3 —(EDTA)-4

Figure 1. EDTA speciation with pH.

Figure 1. EDTA speciation with pH.

include the Fe(III) to Fe(II) reduction step. Reducing agents for this step include S032- and HSO^ ions formed by the dissolution of SO2 (which frequently accompanies NO as an emission gas). Other reducing agents suitable for this purpose are S2042-, S2~, ascorbic acid, iron metal, glyoxal, and electrochemical reduction. The NO-reduction products depend on the reduction method used and include NH4+, NH3, N2,N20, NH2(S03H), and H0N(S03)22".

Solution pH can be of paramount importance to have an adequate ligand speciation (see Section 2.1). For example, EDTA can typically be present as five different chemical species depending on the pH of the medium: H4EDTA, H3EDTA~, H2EDTA2", HEDTA3-, and EDTA4" (see Figure 1). Its complexes can then be anionic, cationic, or neutral, and this can have dramatic effects on their properties and behavior. A case in point is the EDTA complexation of Fe3+, in which the anionic complex with EDTA4-, [FeEDTA]-, is some 20 orders of magnitude more stable than the corresponding neutral complex with HEDTA3", [FeHEDTA], (For ligand speciation calculations one can use the chemical-species distribution algorithm described in Section 2.1 and developed by Rojas, 1995).

In the experiment presented below, the [Fe(II)EDTA] complex will be prepared and used to demonstrate the reactive dissolution of an insoluble polluting gas (NO) by complex formation. The NO in the resulting complex is then reduced and the [Fe(II)EDTA] complex is regenerated.

Experimental Procedure

Estimated time to complete the experiment: 2 h

Materials

Reagents

2 5-mL vials

Fe(ii)EDTA

2 20-mL filter flasks

KNO2

2 10-mL syringes

1 M H2SO4

1 1-mL graduated pipet

1 MKOH

1 2-mL graduated pipet

FeS04 ■ 7H20

2 10-mL beakers

Na2H2EDTA

3 25-mL Erlenmeyer flasks

Na2S204

2 plastic septa

nitrogen gas

1 well plate

D. i. water

1 plastic cap

1 spectrophotometer cell

1 spectrophotometer

20 cm of rubber tubing

1 magnetic stirrer

This experiment consists of five steps: a) Preparation of NO, b) preparation of a basic trap, c) preparation of [Fe(II)EDTA], d) removal of NO by [Fe(II)EDTA], and e) reduction of the corresponding iron nitrosyl complex to regenerate the chelate.

This sequence is conveniently performed, for example, in two 5-mL vials and a 20-mL filter flask (see Ibanez, 2000). The vials need to be equipped with septa and connected to the filter flask by means of short pieces of plastic or rubber tubing (e.g., 35 mm in diameter). A well plate (with a well capacity of at least 3 mL) equipped with plastic caps and tubes for every well to convey the produced gases from one well to the next, can also be used (at least four wells are required).

a) Preparation of NO

NO can be conveniently prepared by the reaction represented in equation 1:

Place 1 mL of deionized or distilled water and about 20 mg of KN02 or NaN02 in one of the 5-mL vials (also called the preparation vial). Cap it with a two-hole rubber septum equipped with two pieces of tubing inserted through it, one of which is connected to a nitrogen gas tank, and the other to the filter flask, as shown in Figure 2. Be ready to add approximately 0.2 mL of 1 M H2S04 with a syringe through the rubber septum (secure a tight fit to prevent the escape of gas), but do not add the acid until the entire setup is ready (i.e., after step c). Caution: NO gas may be irritating to the eyes, throat, and nose, and it is known to have other biological effects including increased vasodilatation. N02 is toxic.

Open to atmosphere Plastic syringe or t0 an additional trap

Open to atmosphere Plastic syringe or t0 an additional trap

Figure 2. Experimental set-up for the formation of the Fe-EDTA nitrosyl complex using vials and flasks. (Adapted from Ibanez, 2000).

For these reasons, it is strongly recommended that the experiment be performed under a fume hood in a well-ventilated area.

b) Preparation of a basic trap

A KOH trap is prepared by placing 10 mL of 1 M KOH in the filter flask (also called the basic trap). Connect the exit tube from the first vial to this flask and dip the tube in the basic solution. Connect the exit tube from this flask to the second 5-mL vial in the same manner so as to transport the NO into the chelate solution that will be prepared in this vial, see Figure 2.

c) Preparation of the [Fe(II)EDTA] chelate

Place 5 mL of distilled or deionized water in a

10-mL beaker. While stirring, add the necessary amount of an Fe(II) salt (for example, FeS04 • 7H20) to form a 0.3 M Fe2+ solution. Transfer enough of this solution to a spectrophotometer cell and take its absorption spectrum. Return the solution into the beaker.

Then, add enough EDTA (for example, its disodium salt) to form the 1:1 iron chelate and stir. Excess EDTA can be used to ensure complete chelate formation. Avoid low pH values because the EDTA may form its tetraprotonated, insoluble form or else the positively charged complex, [Fe(H3EDTA)]+, as deduced from Figure 1. Both forms are unsuitable for the present experiment.

Transfer 1-2 mL of this freshly formed chelate solution into a spectrophotometric cell and obtain a scan in the visible region with a UV-VIS spectrophotometer. Transfer another 2 mL of this chelate solution to the second 5-mL vial (also called the scrubbing vessel). Make sure that all the tubes and the syringe are now connected. Nitrogen gas can be used as the carrier gas. Initiate gentle nitrogen bubbling so as to carry the NO from the preparation well into the basic trap and into the scrubbing vessel.

[A simpler method consists of using a large syringe to inject air continuously to the preparation vial for this same purpose; a certain disadvantage in using this alternative approach is that some NO will be oxidized to N02, and some Fe(II) will be oxidized to Fe(III), but this may be offset by its simplicity because the use of a compressed gas is avoided].

Caution: Do not start producing NO gas until the entire set-up is ready, the iron chelate has been produced (see below), and the carrier gas is either flowing or ready to be pumped into the system.

Now, inject the acid into the preparation vial to initiate NO formation as described in a).

d) Removal of NO by [Fe(II)EDTA], producing [Fe(II)NO(EDTA)]

After a few minutes of bubbling the carrier gas containing NO through the scrubbing solution, a color change will be observed (from pale yellow to olive green) due to the formation of [Fe(II)NO(EDTA)]. At this point, stop the production of NO by opening the preparation well and flushing it with running water. The small amount of residual nitrite can be flushed down the drain. Transfer 1-2 mL of the green solution from the scrubbing well (containing the nitrosyl complex) to a spec-trophotometric cell, and take its visible absorption spectrum.

e) Reduction of the [Fe(II)NO(EDTA)] complex to regenerate the [Fe(II)EDTA] chelate

In order to reduce the NO in the nitrosyl complex and regenerate the iron chelate, return the solution from the spectrophotometer cuvette into the scrubbing vessel, and add some 20 mg of dithionite powder (sodium hydrosulfite, Na2S204) to it. Stir. (Note: if not enough dithionite were added, the mixture becomes red with time!) After some time, a color change back to the original pale-yellow is observed, signaling the destruction of the NO complex and the regeneration of the [Fe(II)EDTA] chelate. It is a good practice to set aside a small amount of the untreated chelate for color comparison. (Sodium sulfite can also be used for this reduction step, although the color change will be somewhat less spectacular). Use some of this reduced solution to obtain its visible spectrum.

The three spectra (i.e., before chelation, after chelation, and after reduction) can be now compared. An intense absorption band is observed in the second spectrum, which is absent in the other two. This demonstrates the formation and destruction of the NO complex.

158 Name

Section

15. Removal of Nitric Oxide by Complex Formation _Date_

Instructor_Partner_

0 0

Post a comment