Decomposition of organolead compounds in aqueous systems

ENVIRONMENTAL

RESEARCH

25, 241-249 (1981)

Decomposition

of Organolead Compounds Aqueous Systems

A. W. P. JARVIE,*

R. N. MARKALL,~

in

AND H. R. POTTER+

Received February 26. 1980 Organolead compounds are generally more toxic khan inorganic lead compounds and if stable in water could be harmful to aquatic and higher species. The effect of a number of natural catalysts on the breakdown of lead alkyls in aqueous media was studied to evaluate the possible environmental impact of these materials. It was found that sunlight and various anions and cations promote the breakdown of organoleads in aqueous media.

INTRODUCTKIN

Organolead compounds enter the atmosphere by a number of routes, a major source being automobile exhaust fumes. Although the quantity of organolead in exhaust fumes (Hirschler and Gilbert, 1964) is only a very small percentage of the total tetraalkyllead (TAL) input of the engine, if organoleads were stable, high concentrations could build up in the atmosphere and be washed into waterways. Lead like mercury is generally much more toxic in its organic than inorganic forms (Shapiro and Frey, 1968) and organoleads in waterways could be harmful to aquatic life and if bioaccumulation took place, might even present a hazard to other species higher up the trophic scale. The purpose of the present study was to determine the influence of a number of possible natural catalysts on the decomposition of organolead compounds. The effect of sunlight, surface area, and various anions and cations on breakdown of organoleads in aqueous media was investigated. METHODS

Butyllead salts were prepared by the standard Grignard methods. All other alkyllead compounds were kindly provided by The Associated Octel Company, Ellesmere Port, South Wirral, England.

tins -l&rid rhromatogruphy 104 gas-liquid chromatograph @CD) operated in the pulsed injected from petroleum ether Ch-omosorb W (60-85 mesh)

klc). Analyses were carried out on a Pye model coupled to a Pye Unicam electron capture detector mode of 300°C with a ““Ni source. Samples were solution onto a 150-cm glass column packed with coated with 10% SE-30. Column temperature and

242

JARVIE,

MARKALL,

AND

POTTER

flow rate of nitrogen carrier gas were adjusted to give the optimum retention for the TAL present; samples suspected of containing tetraethyllead (TEL) were chromatographed at 100°C with a flow rate of 60 ml/min, those suspected of containing tetramethyllead (TML), at 64°C and 40 mlimin. Confirmation of the identity of the suspected TAL was by rechromatographing with a genuine sample and, for all except TML, by rechromatographing the sample on a 300-cm glass column packed with 10% 1,2,3-tris(2-cyanoethoxy)propane (TCEP) on Chromosorb W at 64°C and 40 mlimin flow rate. Peak areas were integrated electronically and quantified by comparison with those of standards of TAL in benzene. Analysis for Alkyllead

Salts

Samples (10 ml) of the aqueous extract suspected of containing alkyllead salts were analyzed by a calorimetric technique (Noden, 1973) using pyridylazoresorcinol (PAR), the absorption of the resulting solution being measured at 515 nm on a Pye Unicam SPSOO spectrophotometer. Reactions

in Aqueous Solutions

After shaking for the appropriate time 50 cm3 of solution (suspension in the case of TALs) was transferred to a separating funnel, benzene (50 cm3) was added and the mixture shaken for 5 min. The layers were separated, the benzene layer was analyzed for TAL by glc and the organolead salt in the aqueous layer was determined by the PAR technique, (Noden 1973). Decomposition

of Qrganolead

Compounds

on Silica

Silica gel (100-200 mesh) was used throughout. The silica was suspended in 50 cm3 of water before the addition of organolead compound. Organolead salts were added to the suspension by suitable additions of a 500 t&cm3 aqueous standard solution of the salt. Dilute solutions of TEL and TML were prepared by adding the TAL (50 cm3) to methanol (10 cm3) using a calibrated syringe; appropriate aliquots of this dilute solution were added to the silica. All suspensions were prepared in glass flasks (100 cm3) with glass stoppers: mixtures were stored in the dark prior to analysis. Analysis The suspension was transferred to a separating funnel and benzene (50 cm3) was added. The sample was shaken (10 min for TML, overnight for TEL) to achieve maximum extraction of TAL. The aqueous suspension of silica was run off into a glass column and the benzene layer was analyzed for TAL by glc. The organolead salts were eluted from the silica by slowly passing (15 cm3/min) aqueous acetic acid 2% (v/v) through the column. Successive fractions (50 cm3) were collected and neutralized by the addition of aqueous NaOH (5 M). The alkyllead content was determined by the PAR method. Normally 200 cm3 of aqueous acetic acid was sufficient to completely remove organolead salts from the column.

DECOMPOSITION

Rerno\~al of TEL .from

OF

ORGANOLEAD

COMPOUNDS

1-43

Glass

Benzene (100 cm3) was added to the flask and shaken for 1 hr; the TEL content of the benzene was determined by glc. Subsequent portions of benzene were added until TEL was no longer detected in the extract. RESULTS Trtraalkyle~~ds TAL compounds are very insoluble (Feldhake and Stevens, 1963) in water (e.g., TEL, 6-9 x lo-’ mole/liter). Their true solubihties were too low for our purposes and as a compromise, higher concentrations of TAL ( 12- 1.5 x IO-” mole/liter) in water were used and the suspension shaken regularly. To sample, a portion was removed immediately after shaking and analyzed in the normal way. A “solution” of TEL in distilled water in the dark was relatively stable with only 2% decomposition to Et,Pb+ over 77 days. Originally it was thought that decomposition was fast since the TEL disappeared rapidly from solutions (Fig. 1): however, the apparent reaction of TEL was not accompanied by the formation of reasonable quantities of any other organolead product. This appeared anomalous and on further investigation, it was found that the TEL was being adsorbed onto the glass walls of the reaction vessel; the TEL was recoverable unchanged from the walls by extraction with benzene. The dark decomposition of TEL to Et,Pb+ is catalyzed by CU” and Fe’+ ions (see Fig. I), CuZ’ being more effective than Fe”

244

JARVIE,

MARKALL,

AND POTTER

in promoting the breakdown. In these reactions, Et,Pb+ was the main organolead product; only traces of Et,Pb*+ were ever detected. In the catalysis by Fe*+ and Ct.?+ (1 ppm) the large loss of TEL from solution in the early stages of the reaction is probably like the dark reaction due to adsorption onto the glass walls of the flask. Evidence for this is the lack of formation of Et,Pb+. Ni*+ and a number of common anions including Cl- and SO< had no effect on the decomposition reaction. The reactions of CU*+ with TALs in alcoholic solution (Bawn and Johnson, 1960) have been studied in detail, and a mechanism has been proposed for the reaction: Cu’+ + R,Pb + Cu+ + R,Pb+ + R., Cu’ + R,Pb + RCu + R,Pb+. It seems probable that the aqueous reaction will follow the same pathway. The reaction of Fe3+ with TEL has also been studied previously (Gilman and Apperson, 1939) and probably follows a similar pathway to the Cu*+ reaction, Fe3+ being reduced to Fe*+. The latter process has been used to remove TEL from petrol (Choudhuri et al., 1961). In this work iron solutions were chosen for study because iron occurs so commonly in natural water systems and in such abundance. When exposed to sunlight aqueous solutions of TEL rapidly decomposed to give Et,Pb+. The maximum concentrations of Et,Pb+ were attained after 4 days; after 15 days only 1% TEL remained. Aqueous solutions of TML reacted fairly rapidly in both light and dark. After 22 days 59 and 16%, respectively, had reacted to give Me,Pb+ (see Fig. 2). There was evidence that in the dark TML like TEL was removed from solution by adsorption into the glass walls.

TlHE

FIG. 2. Decomposition

tight Dark

n .

TML

tight Dark

0 .

Me$‘b+

(ws)

of TML and the formation of Me,Pb+ in aqueous solution.

DECOMPOSITION

OF

ORGANOLEAD

245

COMPOUNDS

Both TML and TEL were totally adsorbed from aqueous solution Onto silica. The adsorbed species underwent relatively rapid reactions to give R&‘b+ as the only detectable organolead product (Table 1). After about a month, 97% of the TEL and 55% of the TML had reacted. It has been reported previously that the reaction of R4Pb with carboxylic acids is promoted by adsorption onto silica gel and catalysis with silica gel has been used for the synthesis of R,PbX derivatives of the weaker carboxylic acid (Browne and Reid, 1972). In contrast other workers have noted that R,Pb compounds are not adsorbed to any extent by sediments (Schmidt, 1977). It is possible to rationalize these apparently conflicting observations. We have observed both in this study and in previous investigations (Jarvie er al., 1977) that solutions of inorganic lead and mercury salts release alkylleads from silica and sediment surfaces. It may be that the sediment investigated by Schmidt already contained inorganic lead and other heavy metal salts. Such sediments would have a much lower affinity for alkylleads. In summary, the order of reactivity of the TALs under the various conditions is: TML > TEL TEL > TML TEL > TML

Dark Light Silica

The reversal in the order of the light and dark reactions indicates that these reactions follow different mechanisms. No mechanistic studies were attempted in this investigation. Previous studies of the photolytic decompositions of TALs have shown that photolysis produces much the same results as pyrolysis. A radical mechanism was proposed by Pratt and Purnell(1964) to account for the products formed in the pyrolysis reaction, the first step being R,Pb + R,Pb.

+ R..

It would be expected for reactions following this pathway that reactivity would increase with the stability of the R. radical, and the reactivity for the light reaction. TEL > TML, is consistent with this mechanism. The observed reactivity of the dark reaction, TML > TEL, suggests a simple hydrolysis, the rate of nucleophilic attack by water at the Pb atom decreasing as the size of the groups around the lead increases.

DECOMPOSJTION Percentage

recovery

of TEL

added

Time (days)

TEL

0 3

78 63

23

7 13 17 20 39

54 19 22 I9 3

33 63 55 56 70

Et:,Pb-

TABLE I OF TALs

ON SII.ICA Percentage

recovery

of TML

added

Time (days)

TML

0 7

66 35

-

14 28 49

29 I5 8

IX 25 29

Me.,Pb’ 5

246 Trialkyllead

JARVIE,

MARKALL,

AND POTTER

Compounds

In the absence of light aqueous solutions of triethyl- and tributyllead chloride showed no signs of decomposition after 12 months. Trimethyllead chloride reacted very slowly and there was some 1% reaction after 220 days. Cations such as Cu*+ and Fe3+ which promoted the decomposition of the tetraalkyl compounds had no effect on the reactions of the trialkyl compounds. It has been reported elsewhere (Huber and Schmidt, 1976) that various anions catalyzed the disproportionation of the tri- and dialkyllead salts and we found that sulfide ions were particularly effective in promoting disproportionation; the sulfide reaction was studied in depth and details will be reported later. Sunlight accelerated the breakdown of the trialkyl compounds. Over a U-day period, there was some 4% loss of Me,Pb+, 25% loss of Bu,Pb+, and 9% loss of Et,Pb+ (Fig. 3). The detectable product in all cases was inorganic lead; however, with Et,Pb+ a trace of Et2Pb2+ was also observed (see Table 2). The light reactions of Me,Pb+ and Bu,Pb+ were studied under identical conditions, whereas Et,Pb+ solutions were studied under different conditions (i.e., different times of year, therefore, different amounts of heat, light, etc.), so the relative reactivity of the three systems cannot be compared. Both Et,Pb+ and Me,Pb+ were totally adsorbed from aqueous solutions by silica, which promoted the decomposition of the trialkyl compounds to a slight extent, but the breakdown pattern was rather peculiar. Some reaction occurred in the first 14-21 days and subsequent reaction was exceedingly slow (Table 2). The results for the R,Pb+ systems indicate a similar reactivity for Me,Pb+ and Et,Pb+ on silica; however, analysis of the products from the reaction of TALs on silica would suggest that Et,Pb+ is rather more stable than Me,Pb+ on silica surfaces. The order of reactivity under the various conditions is: Dark Light Silica

Me,Pb+ > Et,Pb+ > Bu,Pb+ Bu,Pb+ > Me,Pb+ Et,Pb+ > Me,Pb+ MejPb+ Suj Pb+ Et3 Pb’

FIG. 3. Decomposition

0 ca .

of aqueous solutions of R,PbCl in sunlight.

DECOMPOSITION

OF

ORGANOLEAD TABLE

DECOMPOSITION

Percentage

recovery

of Et,Pb+

OF

2 R:,Pb’

247

COMPOUNDS

ON

SILICA

added

Percentage Time (days)

Me,,Pb-

0 14 11

95 8.5 x3

Time (days)

Et:,Pb’

Et,Pb”’

0 1-I 42 84

94 76 78 69

0 7 5 6

recovery

of Me,,Pb+

added

Me,Pb2‘ 0 0 0

Aqueous solutions of R2Pbz+ compounds undergo a slow disproportionation in the dark. After about a month 10% of a MezPb2+ solution had decomposed and over the same period, some 6 and 4% of the Et,Pb2’ and Bu,Pb’+ respectively had reacted (see Fig. 4). Formation of R,Pb+ products indicates a disproportionation reaction. In the light after 40 days the Bu,Pb *+, Et,Pb+ , and Me,Pb2- compounds had reacted some 70, 25, and 5%, respectively, As normal the light reaction shows increased reactivity and the order is reversed. The light reaction like the dark reaction is a disproportionation. The order of reactivity under the various conditions is: Dark Light

Me,Pb?+ > Et,Pb” Bu,PW+ > Et,Pb”+

> Bu,Pb”’ > Me,Pb”

DISCUSSION

These and other observations indicate that TALs emitted from automobile exhausts and washed into waterways can undergo decomposition by various pathways:

Pb2+

+

R3Pb*

Pb’+

+

R$‘b*

2 R,Pb”

TIME

FIG. 4. Decomposition

Pb++

Bu2 o n

B

Me2Pb++ Et2Pb++ Bu2Pb++

of aqueous solutions of R,PbC&.

DAYS

Pb++

Et2 0

A 0

Me2Pb++

Light

Dark

DECOMPOSlTlON

OF ORGANOLEAD

COMPOUNDS

‘49

Breakdown of TALs and the various intermediate organolead compounds to inorganic lead is promoted by sunlight and in any aqueous system exposed to sunlight decomposition would be rapid. Dark reactions in a pure water system would be slow. Fortunately natural water systems contain a variety of anions and cations and sediments which could speed up the breakdown of organic lead to inorganic lead. There are a number of reports (Wong ef ctl., 1975; Huber and Schmidt, 1976) of the biological methylation of inorganic compounds and organic lead compounds could accumulate in waterways as a result of microbial action. However, the sporadic nature and lack of reproducibility of the observations on natural methylation make it unlikely Lhat significant quantities of organolead compounds will be produced by this route. In any case, whatever their origin any organolead compounds formed in aqueous systems will be subject to the decomposition processes described above. REFERENCES Bawn, C. E. H., and Johnson, R. (1960). Alkyl derivatives of group 1 metals. III. Kinetics of decomposition of ethylcopper (I) in ethanol. J. CIze~rf. Sot,., 4162-4165. Browne, 0. H.. and Reid. E. E. (1972). Some reactions of lead tetra-ethyl. .I. A!?lrr. C/I~)~I. Sot,. 49. 830-838. Choudhuri, B. K.. Viswanatham C. R.. Vats. S. S.. and Aijar. A. R. (1961). Deleading of gasoline. Dqf. SC,;..I. 2. 34-36. Feldhake. C. J., and Stevens. C. D. (1963). The solubility of tetraethyllead in water. J. (‘iwt~i. E/I:>. Dtrtci 8, 196, 197. Gilman, H., and Apperson. L. D. (1939). Reactions between organolead compounds and some metallic halides. J. 01:~. C/IC~I?I.4, 162-168. Hirschler, D. A.. and Gilbert. L. F. (1964). Nature of lead in automobile exhaust gas. AK/I. Et~~,irc~t~. NfJtrltl7 8, 297-313. Huber, F.. and Schmidt. U. (1976). Methylation of organolead and lead (II) compounds to (CH,,), Ph hy microorganisms. Nltrlrvr (Londor~) 259, 157, 1.58. Jarvie, A. W. P.. Markall, R. N., and Potter, H. (1977). Detection and determination of alkylead compounds in natural water. Wafer Po/lu~. C’or~wr~/ 76, 123- 128. Noden, F. G., (‘I trl. (1973). Determination of tetraalkyi, trialkyl dialkyl. and inorganic lead compounds from various sources, private communication, Analytical Services Group. Associated Octel Co. Pratt. G. L.. and Purnell. J. H. (1964). Gas phase reactions of ethyl radicals with nitric oxide. 7‘~(1rr.\. Ftrrtrcltrr SW. 60, 37 I-377. Pratt. G. L.. and Purnell. J. M. (1964). Pyrolysis of tetraethyllead. Tvtiir.\. Flrf&rix SC,<..60, 519-526. Schmidt. U. (1977). “Biomethylation of Pb”- and Lead Alkyl Compounds and Research into inhibition of Growth of Bacteria by Lead Alkyl Compounds,” p. 25. Ph.D thesis. Dortmund University. Shapiro. H.. and Frey, F. W. (1968). “The Organic Compounds of Lead.” Interscience. New York. Wong. P. T. S.. Chau. Y. K.. and Luxon. P. L. (1975). Methylation of lead in the environment. ,Yo/ffr(~ U.~mlr~~~~253. 263. 264.