PYRIDINE PHOSPHONATES AND CHELATES THEREOF

CHEMICAL COMPOUNDS

FIELD OF THE INVENTION

This invention relates to new chemical compounds comprising pyridine

phosphonates. These compounds are especially useful in preparing coordination compounds, such as chelates. The resulting chelates are extremely useful as catalysts in oxidation-reduction processes, in particular the

conversion of H ₂ S to solid sulfur.

BACKGROUND OF THE INVENTION

Ligands, also known as complexing agents, can be any molecule, atom,

or ion that is attached to the central atom of a coordination compound. For

example, the ammonia molecule in [Co(NH ₃ ) ₆ ] ³⁺ and the chlorine atoms in P ₆ tCI are ligands. Commercially, ligands are used to modify the properties of metals or metal ions. Ligands interact differently with different metal ions

thus producing surprising and unexpected properties for the resulting

coordination compounds. Ligands with two or more donor atoms coordinated

to the same acceptor atom are known as chelating ligands.

The combination of chelating ligands with metals, such as, iron, copper,

cobalt, nickel or manganese results in the formation of metal chelates useful

as catalysts in the oxidation-reduction processes to convert H ₂ S to solid sulfur.

- 2 -

Hydrogen sulfide and alkyl mercaptans are a major source of pollution

of air streams since they are liberated as waste by-products in a number of

chemical processes, such as sulfate or kraft paper pulp manufacture, viscose

manufacture, sewage treatment, sulfurretting fatty oils, and production of

organic compounds, as well as in petroleum refining and in the production of

natural gas and of combustible gases from coal, such as in coking operations.

These sulfur compounds are also present in geothermal steam used in power

generating plants.

The use of an aqueous chelated metal catalyst solution for removing

hydrogen sulfide from a gas stream is well known in the art. However, those

processes relied on chelating ligands such as amino and/or

polyaminopolyacetic acids, e.g. nitrilotriacetic acid, ethylenediaminetetraacetic

acid, N-hydroxyethyl ethylenediamine triacetic acid, and diethylenet amine

pentaacetic acid and alkali metal salts thereof. In those prior art processes

a chelated metal catalyst solution is contacted with hydrogen sulfide-

containing gas, known as "sour gas", to effect oxidation of the hydrogen

sulfide to elemental sulfur and concomitant reduction of the metal chelate to

a lower oxidation state. The catalyst solution is then regenerated for reuse

by contacting it with an oxygen-containing gas to oxidize the metal chelate to

a higher oxidation state. The elemental sulfur is continuously removed from

the process as a solid product with high purity. Illustrative of these oxidation-

- 3 - reduction processes is the description contained in U.S. Pat. No. 4,622,212

(McManus et al.) and the references cited therein.

While the processes, and their associated catalyst composition, described in the art have achieved sufficient efficiency and stability for commercial utilization, a need still exists for alternate and significantly more

stable catalyst systems. This invention addresses this need and in particular

provides catalyst solutions having more stability against in-process oxidative

degradation of the active metal chelate. It also significantly reduces metal

sulfide precipitation, even when substoichiometric iron to sulfur mole ratios are

present in the absorber section of the process. This invention also increases

the stability of the metal chelating compound against hydrolysis in aqueous solutions having pH values up to and above 9.0. These and other

advantages will become evident from the following more detailed description

of the invention.

SUMMARY OF THE INVENTION

Accordingly, in one embodiment, the invention relates to new ligands

having the following formulae:

where, R is COOH or PO(OH) ₂ and R < R _j and R gis each selected independently from the group consisting of H, OH, alkyl containing 1 through

3 carbon atoms, NO _2ι SO ₃ H, Cl, Br, F, and CN.

Still another embodiment of the invention relates to a process for the

conversion of H ₂ S to sulfur in which the H ₂ S is contacted under conditions to convert H ₂ S with a solution containing an effective amount of a polyvalent metal chelate composition of the formula described above. Preferred catalyst

compositions for the removal of hydrogen sulfide from waste gas streams

have the following formulas: ML and ML ₂ where L has one of the following

formulas:

and where M is a polyvalent metal selected from the group consisting of iron,

copper, cobalt, nickel and manganese. A preferred polyvalent metal is iron and the series of reactions involved in catalytically oxidizing hydrogen sulfide

to elemental sulfur using an iron chelate catalyst can be represented by the

following reactions, where L represents either one of the pyridine based ligands described above:

(1 ) H ₂ S (gas) + H ₂ O(liq.)^ H ₂ S (aqueous) + H ₂ O (liq.)

(2) H ₂ S (aqueous) ** H ⁺ + HS -

(3) HS ^" + 2(Fe L ₂ )→ S (solid) + 2(Fe ²⁺ L ₂ ) + H ^*

By combining equation (1) through (3) the resulting equation is:

(4) H ₂ S (gas) + 2(Fe ³⁺ L ₂ )→2H ⁺ + 2(Fe ²⁺ L ₂ ) + S (solid)

In order to have an economical workable process for removing hydrogen sulfide from a gaseous stream when a ferric iron chelate is used to effect catalytic oxidation of the hydrogen sulfide, it is essential that the ferrous iron

chelate formed in the above described manner be continuously regenerated

by oxidizing to ferric iron chelate on contacting the reaction solution with

dissolved oxygen, preferably in the form of ambient air, in the same or in a

separate contact zone. The series of reactions which take place when

regenerating the metal chelate catalyst can be represented by the following

equations:

(5) O ₂ (gas) + 2H ₂ O ** O ₂ (aqueous) + 2H ₂ O

(6) O ₂ (aqueous) + 2H ₂ O + 4 (Fe ²⁺ L ₂ ) ^→ 4 (OH ) + 4 (Fe ³⁺ L ₂ )

By combining equations (5) through (6), the resulting equation (7) is:

(7) V* O ₂ + H ₂ O + 2 (Fe ²⁺ L ₂ ) ^→ 2 (OH ) + 2 (Fe ³ * L ₂ )

And, when equations (4) and (7) are combined, the overall process can be

represented by the following equation:

(8) H ₂ S (gas) + ¹ / ₂ O ₂ (gas) ^→ S (solid) + H ₂ O (liq.)

As gaseous hydrogen sulfide has a low solubility in an acidic aqueous

solution, the catalytic oxidation of the hydrogen sulfide is preferably carried

out in an aqueous alkaline solution because hydrogen sulfide gas is absorbed

more rapidly and hydrosulfide ions are produced at significantly increased

rates when the reaction solution has a higher pH value. When the continuous

catalytic oxidation-reduction reaction solution is maintained at the higher pH

values and a conventional chelating agent used, an insoluble precipitate of

ferric hydroxide is formed which removes iron from the reaction solution and

reduces the concentration of catalytic reagent. The precipitation of ferrous

sulfide is likewise highly detrimental, as this also reduces the concentration

of the catalytic reagent in the reaction solution and fouls both the sulfur

product and process equipment. It is therefore desirable to eliminate or

minimize the loss of iron from the reaction solution, particularly at the higher

pH values, in order to improve the process for removing hydrogen sulfide from

a stream of industrial process gas. It has been found that not all iron chelating agents capable of forming a complex in aqueous solutions with iron in the ferric valence state (Fe ^3* ) or

in the ferrous valence state (Fe ²⁺ ) are suitable for use over the broad range of operating conditions employed for this oxidation-reduction system for the removal of hydrogen sulfide.

Among the iron chelate reagents which have been used in prior art

processes for removing hydrogen sulfide are the polyaminopolyacetic acid-

type chelating agents, such as ethylenediamine tetraacetic acid and the alkali

metal salts thereof.

Another chelate compound that has been tried without success is

described in U.S. Patent No. 5,273,734 (Sawyer et al.) and contains a ligand

having the following formula:

Wherein R is selected from H and alkyls containing 1 through 3 carbon

atoms. Polyvalent metal chelating catalysts made using one specific type of

the Sawyer et al. ligand is the iron chelate of 2,6-pyridinedicarboxylic acid, a

chelating agent which exhibits good resistance to catalyst degradation but

unfortunately experiences unacceptable precipitation of ferric hydroxide from

its ferric chelate when operating at the normal pH of the process. Using the

ligands of the present invention to prepare polyvalent metal chelate catalyst

yields catalyst solutions that resist degradation and do not form appreciable

precipitates of ferric hydroxide when operated at basic conditions of pH 9.0

or higher. In addition, conversion of the ferrous complex to iron sulfide is

prevented.

Accordingly, a primary object of the present invention is to provide a

novel ligand composition useful for preparing polyvalent metal chelate

compounds.

It is also an object of this invention to provide a novel and improved

catalyst composition for use in oxidation-reduction systems for removing

hydrogen sulfide from fluid streams which avoids difficulties encountered in

the prior art systems of this type.

Yet another object of the present invention is to provide an improved

continuous process for the removal of hydrogen sulfide from a gaseous

stream without causing loss of catalyst while operating the process at the

most efficient range of pH values.

Finally, still another object is to provide a stable polyvalent metal

chelate catalyst that increases the capacity of conventional oxidation-reduction

processes which are presently limited by the extent of degradation of the conventionally used chelating agents and relative economics of alternate prior

art processes.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts commercially available compounds that can be used to initiate the synthesis of the ligands of this invention.

Figure 2 schematically illustrates a synthesis scheme used to create a

preferred embodiment of this invention.

Figure 3 schematically illustrates another synthesis scheme used to create a preferred embodiment of this invention.

Figure 4 graphically compares the degradation of two iron chelate

catalysts; Fe(lll)-2P6C and Fe(lll)-NTA in the absence of stabilizer, Figure 5 graphically illustrates the degradation of Fe (lll)-NTA with and

without Na ₂ S ₂ O ₃ ^• 5H ₂ O stabilizer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The novel chemical compounds of this invention are characterized in

that each contains a pyridine group and a phosphonic acid group. Since

these ligands are heretofore unknown, and therefore not commercially

available, each must be synthesized using a series of chemical reaction steps.

Commercially available chemical compounds that can be used as starting

materials in the chemical synthesis of these ligands are shown in FIG. 1. Any

synthesis procedure known to the art may be used to make a specific ligand

from the many different species of ligand that are defined in this invention.

Two species in particular have worked extremely well as chelating agents in

preparing catalyst for the oxidation of H ₂ S to solid sulfur.

The first of these ligands is pyridine-2-phosphonic-6-carboxylic acid

(hereinafter abbreviated as "2P6C"). One method by which 2P6C may be

synthesized is illustrated in FIG. 2 and begins with treating picolinic acid

(20.0g) with 125 ml glacial acetic acid and 22 ml 30% H ₂ O ₂ . The reaction

mixture is then heated to 70-80 °C for 3 hrs. Another 11 ml of 30% H ₂ O ₂

solution is then added and the temperature is maintained at 70°C for 18 hr.

Most of the solvent is removed by evaporation, and the residue is cooled to

room temperature and allowed to stand in a refrigerator (~5°C) for 2 hrs. The

white solid that precipitates is filtered off and dried at 60 °C under vacuum for

2 hrs. This procedure yields compound 1 of FIG. 2. To 20.3 g of compound

1, 130 ml of EtOH is added and the mixture is treated with HCl gas for 30

minutes. Then the solution is heated to reflux for 30 minutes. The solvent

is removed by evaporation under reduced pressure and to the resulting

residue, 130 ml of EtOH is added and the above procedure is repeated.

Compound 2 is obtained as a white solid and is used directly in the next step.

Dimethylsulfate (15.2g) is slowly added to compound 2 over a 30 minute

period. It was then heated at 80-90°C for 3 hours to ensure complete

reaction. The reaction mixture, a yellow oil, is used directly in the next step.

n-Butyllithium (15.0 ml of 23% solution in hexane) is added dropwise to 25 ml

of diethylphosphite at -20.0 to 0 °C over a 2 hr. period. To the resulting

solution of lithium diethylphosphite, compound 3 in diethyl phosphite (40 ml)

is added over 1 hr. at -15°C. The reaction mixture is stirred at room

temperature for 16 hrs. and heated at 70° C for 2 hrs. After the mixture has

cooled to room temperature, 140 ml of water is added. The mixture is then

extracted three times with 75 ml CH ₂ CI ₂ . The organic extracts are combined

and are extracted with 100 ml of 4 M HCl. The organic solvent is removed

by evaporation under vacuum. Compound 4 is obtained as a yellow oil.

Compound 4 (24.7 g) is heated under reflux with 150 ml concentrated HCl for

16 hrs. After the mixture is allowed to cool to room temperature, the solution

is extracted four times with 50 ml CH ₂ CI ₂ and the organic extracts are

discarded. The aqueous acid solution is evaporated, treated with a small

amount of water and the white solid is filtered off. The residue is washed with

- 13 - a small amount of cold water and compound 5 is produced after drying under

a vacuum over P ₂ O ₅ for 16 hrs.

The second species of ligand of this invention that shows favorable

performance as a part of a catalyst composition for the conversion of H ₂ S to

solid sulfur in waste gas streams is pyridine-2,6-diphosphonic acid (hereinafter

abreviated as "2,6-PDPA"). To synthesize 2,6-PDPA, the scheme illustrated

in FIG. 3 can be used, where 31.5 ml (0.075 mole) n-butyllithium (23% in

hexane) is added dropwise to 12.5 g (0.090 mole) of diethylphosphite at -20.0

to -30.0 °C over a 2 hr. period. To the resulting solution of

lithiumdiethylphosphite, compound 1 of FIG. 3, a solution of N-metho-

xypyridiniummethylsulfate, compound 3, (from 7.15 g (0.075 mole) pyridine N-

oxide, compound 2, and 9.5 g (0.075 mole) of dimethylsulfate in

diethylphosphite (20 ml) is added at -15.0 °C in 1 hr. The reaction mixture

is stirred at room temperature for 16 hrs. then 50 ml of water is added. The

mixture is then extracted three times with 38 ml of chloroform, and the organic

extracts are combined and extracted with 4N HCl, neutralized with base, and

reextracted with chloroform. From this chloroform solution, after distillation

10.8 g of compound 4 is obtained. Compound 4, 10.8 g, 36 ml of glacial

acetic acid and 6 ml of hydrogen peroxide are mixed and the reaction mixture

is heated to 70.0-80.0 °C for 3 hrs. Another 3 ml of hydrogen peroxide is

added and the temperature is maintained at 70.0 °C for 16 hrs. The solvent

is removed by rotovaporation and to the residue, 100 ml of chloroform is

added, the organic phase is washed with concentrated sodium carbonate

solution, and then dried with magnesium sulfate for 16 hrs. After the solvent

is removed compound 5 is obtained. Dimethylsulfate (4.3 g) is then slowly

added to 7.9 g of compound 5 over 30 min. This reaction mixture is heated

with a steam-bath for 2 hr. to ensure complete reaction. The compound is

purified by silica gel, with a mixed solvent of methylene chloride and methanol

used as eluant. After the solvents are removed, compound 6 is obtained.

To prepare compound 7 the same procedure as used in the preparation

of compound 4 is used. Compound 7 (3.0 g) is next heated under reflux with

30 ml of 6 M HCl for 12 hr. The solvent is removed by evaporation under

reduced pressure, and a yellow oil is obtained. Addition of methanol yields

a yellow solid which is then recrystallized three times from methanol. The

sample of pure product 2,6-pyridinediphosphonic acid, compound 8, is

obtained. Although the above-described syntheses are presented in great detail,

there exists many alternative routes that allow these novel ligands to be

prepared and subsequently used in catalyst compositions in combination with

polyvalent metals. Further, the invention thus far has been described with

particular emphasis on the use of iron as the polyvalent metal of choice,

however, other polyvalent metals that form chelates with the novel ligands

described above can also be used. Such additional polyvalent metals include

copper, cobalt, vanadium, manganese, platinum, tungsten, nickel, mercury, tin

and lead.

The chelated metal catalyst solution of the present invention is

preferably prepared by dissolving a suitable polyvalent metal salt in water,

separately dissolving the chelating agent in water, and mixing the two

solutions to provide a concentrate. The pH of the concentrate is adjusted by

adding the required amount of an alkaline material, such as sodium hydroxide

or sodium carbonate, to provide a concentrate of desired neutral or alkaline

pH. An appropriate amount of the concentrate can be diluted with water as

required to obtain the desired amount of operating solution having the desired

polyvalent metal content. The polyvalent metal content of the operating

solution can vary over a wide range, dependent upon the gas being treated

and other factors. Typically, when an iron chelate catalyst is used, the iron

content of the operating solution may be from about 5 ppm to about 5000

ppm, with 200 to 2000 ppm being preferred, although in some applications the

iron content can be >5000 ppm. The amount of chelating agent should be at

least sufficient to chelate all of the iron in the solution and preferably

somewhat in excess of that amount.

In a preferred embodiment the chelating agents of this invention are

used in sufficient amount so that the polyvalent metal is chelated

predominantly with two moles of the chelating agents per mole of polyvalent

metal. The mole ratio of chelating agent to iron should be at least about 2:1

to ensure that substantially all of the polyvalent metal is present as the dimer

form of the chelating agent-metal complex, although acceptable results are

obtained at less than the 2:1 ratio.

Although the art is replete with different processing flow schemes and

conditions to effect the conversion of H ₂ S to solid sulfur, none of the known

processes have utilized the catalysts of this invention. Any of the various

methods well known in the art can be used to effect the required intimate

contact between the hydrogen sulfide-containing gas and the aqueous catalyst

solution, including an aerobic system in which the oxidation of hydrogen

sulfide and the regeneration of the catalyst solution are carried out

simultaneously in the same reaction vessel. Alternately, an anaerobic system

can be used where oxidation of hydrogen sulfide and regeneration of the

catalyst solution are effected in separate vessels or reaction zones.

Reference is made to the Thompson U.S. Pat. No. 4,189,462 patent for a

detailed explanation of the two types of processing systems. In addition, the

oxidation-reduction processes disclosed in Hardison U.S. Pat. Nos. 5,139,753

and 5, 160,714 describe alternative processes that can use the novel

compositions of this invention. Also suitable for practicing this invention is the

autocirculation process described in the Hardison U. S. Pat. No. 4,238,462

and the Mancini et al. U.S. Pat. No. 4,01 1 ,304 which describes a control

system for use in such a process. The Thompson, Hardison and the Mancini

et al. patents are all incorporated herein by reference.

The contacting of the hydrogen sulfide-containing gas with the

operating solution in the hydrogen sulfide oxidation step is often carried out

at ambient conditions of temperature and pressure, but temperatures of from

about 5° to about 65° C and pressures ranging from subatmospheric to 100

atmospheres or greater can be used. A pH ranging from about 5.5 to about

10.5 is usually maintained, although higher pH can be used. In an anaerobic

system the regeneration of the reduced catalyst solution is effected by

contacting the catalyst solution with air or other oxygen-containing gas at

ambient conditions, although higher pressures and lower temperatures can be

employed in some circumstances.

Although remarkably stable operations are obtained using the chelated-

polyvalent metal catalyst of this invention, an even more stable operation is

possible through the use of well known chemical stabilizers, for example,

ammonium thiosulfate, alkali metal thiosulfates, alkaline earth metal

thiosulfates, ammonium thiosulfate and thiosulfate ion precursors. In addition

to the alkaline thiosulfates, certain lower molecular weight aliphatic alcohols

may also be used as stabilizing additives to retard or prevent chelate

degradation in accordance with the present invention. Preferred materials in

this category are the monohydroxy alcohols having 3 to 5 carbon atoms,

particularly the alcohols such as t-butanol and isopropanol. The dihydroxy

alcohols such as ethylene glycol and propylene glycols may also be used.

The concentration of the alcohol additives in the operating solution may be

from about 20 to about 100 g/L. The McManus et al. U.S. Pat. No. 4,622,212 describes in detail the nature and chemical effects of these stabilizers and is

incorporated herein by reference. To more fully describe the invention the following specific example is presented but is not to be construed as limiting

the scope of the invention. As will be understood by those skilled in the art,

the solutions or mixtures employed to practice this invention may contain other

materials or additives for select given purposes. For example, the use of buffering agents, microbiological growth control agents, antifoaming additives

and wetting agents may be employed, as well as other specific additives for simultaneous treatment of organic sulfur species, such as, COS and CS ₂ .

EXAMPLE

The degradation resistances of pyridine-2-phosphonic-6-carboxylic acid

(2P6C) and pyridine-2,6-diphosphonic acid (2,6-PDPA), prepared by the

synthesis schemes presented in FIGS. 2 and 3, respectively, were evaluated

as described below. Both ligands were evaluated in the iron chelate form.

A. DEGRADATION WITHOUT STABILIZATION

1. Degradation of Fe(lll,-2P6C at pH 8.5 and 25° C (FedlHO .018M. 2P6C

0.036 M: H,S flow rate = 2.0 mL/min.)

The redox reaction was carried out in a continuous glass reaction

apparatus as described in D. Chen, R. J. Motekaitis, A.E. Martell and

D. McManus, Can. J. Chem., 71 , 1524 (1993) which is incorporated

herein by reference.

Preparation of iron-free sample. To 4.0 mL of solution taken from the

continuous reactor, 100 mg of NaOH in 20 mL water were added

dropwise with stirring. The mixture was heated to 70-80° C for 30 min.

The Fe(OH) ₃ that precipitated was filtered off carefully and was washed

several times with water. The filtrate was combined with the washings

and was transferred to a 50 mL volumetric flask. The pH of the

solution was adjusted with acid to 4.2, and then diluted with distilled

water to 50 mL.

Operating conditions. Analytical column: l-SIL 5 C8, reverse phase 4.5

x 150 mm; Column temperature: ambient; mobile phase flow rate: 1.5

mL/min; Detector: ^' UV at 275 nm, 0.20 AUFS; Sample volume: 20

microliter; Recorder: 10 mV full-scale; Chart speed: 0.5 cm/min. The

mobile phase contained: 0.0010 M Cu (II), 60:40 H ₂ O.MeOH, 2%

HOAc, 0.2% CH ₃ (CH ₂ ) ₁₅ (CH ₃ ) ₃ NBr, adjusted to pH 4.2 with NaOH solution.

The degradation of Fe(lll)-2P6C is shown in Table 1 and Figure 4. The results show that 2P6C degrades very slowly during the redox

reaction. After the redox reaction was carried out for 100 hr about 20%

of the 2P6C was lost, presumably by hydroxylation. Hydroxylation of the pyridine ring does not significantly, adversely affect the chelating ability of the 2P6C, hence no deleterious effect on the process is

observed.

Table. 1. Degradation of Fe (lll)-2P6C at pH 8.5 and 25°C (Fe (III) 0.018 M, 2P6C 0.036M)

Time, hr 2P6C M 2P6C % Remaining

0 0.036 100.0

25 0.034 94.4

50 0.031 86.1

84 0.030 83.3

100 0.029 80.6

2. Degradation of Fe TIII, -2.6 PDPA at pH 8.5 and 25°C: Fe (III) 0.0090

M: 2.6 PDPA 0.018 M and 25°C. H-.S flow rate = 2.0 mL/min.)

The concentration of 2,6 PDPA was analyzed by the following uv-vis

spectrophotometric measurements: to 1.0 mL of solution taken from the

continuous reactor, 150 mg of NaOH in 15 mL water was added with stirring.

The mixture was heated with a steam-bath for 2 hr, then allowed to stand at

room temperature for 2 hr. The Fe(OH) ₃ that precipitated was filtered off

carefully and washed several times with water. The filtrate combined with the

washings, was transferred to a 50 mL volumetric flask, and diluted to 10 mL

with 6M HCl. This solution was ready for uv-vis spectrophotometric analysis.

The results show that after the redox reaction was carried out for 100

hr, no significant degradation was observed. On the basis of the results

obtained the degradation of 2,6 PDPA is estimated to be less than 5% over

a 100 hr period of operation.

3. Degradation of NTA

Using the same test conditions described above, the degradation of

nitrilotriacetic acid (NTA) was evaluated. Table 2 and Figure 4 compares the

measured rate of degradation of 2P6C and NTA iron chelate systems starting

with the same concentration of Fe (III) in each case. It is obvious from Figure

4 that 2P6C degrades much more slowly than NTA.

Table 2. Degradation of Fe (lll)-NTA at pH 8.5.

Time, hr NTA (M) NTA % Remaining

0 0.0362 100.0

12.0 0.0274 75.7

20.0 0.0238 65.7

30.0 0.0190 52.4

40.0 0.0179 49.4

64.0 0.0119 32.9

77.0 0.0095 26.2

B. DEGRADATION IN THE PRESENCE OF STABILIZER

1. Nitrilotriacetic Acid (NTA)

Degradation of NTA is much slower when thiosulfate is added to

scavenge the radical oxidant, probably the hydroxyl radical, which is considered responsible for the oxidative degradation of NTA. Table 3 and

Figure 5 contain the data obtained from a run with the continuous glass

reaction apparatus described above.

Table 3. Degradation of Fe (lll)-NTA at pH 8.5 with 32g Na ₂ S ₂ O ₃ (per liter)

Time, hr NTA (M) NTA % Remaining

0 0.0362 100.0

14.0 0.0344 95.0

26.0 ^" 0.0333 92.0

39.0 0.0320 88.4

50.0 0.0317 87.6

66.0 0.0306 84.5

80.0 0.0297 82.0

99.0 0.0293 80.9

The Phosphonate Ligands

When similar degradation runs were carried out with 2P6C and 2,6

PDPA in the presence of thiosulfate, no detectable degradation was observed after 100 hr of operation. Experimental runs were also conducted with 2P6C,

with and without K ₂ SO ₃ added as a stabilizer. Tables 4, and Figure 6,

illustrate the improvement in stability.

Table 4 2P6C, 5% K ₂ SO ₃

Time, hr 2P6C M % Remaining

0 0.036 100.0

2 0.036 100.0

20 0.035 97.7

44 0.035 97.6

68 0.035 97.4

92 0.036 98.9

Therefore it is seen that thiosulfate lowers the rate of degradation of all

three chelating agents, however, the rate of degradation of NTA is still

measurable. The rates of degradation of the phosphonates are virtually unmeasurable.