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ORGANIC CHEMISTRY OF LEAD IN NATURAL WATER SYSTEMS 

By R. L. WERSHAW 

I 

Natural soil-water and sediment-water systems are 

extremely complex, consisting of a myriad of interacting 

organic and inorganic components. In this section we 

shall be concerned widi tine interaction of one of the 

inorganic components, lead, with the organic 

components of these systems. 

The organic components of a soil-water system are an 

extremely diverse group of compounds (Saxby, 1969), 

including carbohydrates, amino acids, phenolic and 

quinonic compounds, organic acids, nucleic acids, 

enzymes, porphyrins and other heterocyclic compounds, 

lipids, terpinoids, and humic materials. (See Saxby, 1969; 

Spakhov and Spakhova, 1970; Greaves and Wilson, 1969; 

Kowalenko and McKercher, 1971; Ivarson and Sowden, 

1969; and Kononova, 1966, for more complete discussions 

of the organic compounds in soils.) 

In addition to the natural organic compounds present 

in soil, streams and lakes contain organic sediments and 

suspended solids that have been derived from municipal, 

agricultural, and industrial wastes. These wastes are made 

up of carbohydrates, proteins, nucleic acids, enzymes, 

lipids, and many of the other organic compounds of living 

systems. Oils, plasticizers, polymers, and an enormous 

number of omer organic compounds are discharged to 

natural waterways by manufacturing and chemical 

industries. 

The interaction of lead with the organic components of 

soil-water and sediment-water systems is still not well 

understood, but we shall review the work that has been 

done and attempt to draw what conclusions we can from 

the data available. 

A number of studies have shown that die organic matter 

of soils and stream sediments apparently binds metals and 

reduces fJieir mobility. However, as Mortensen (1963) has 

pointed out, although many workers have suggested that 

soil organic matter forms complexes with metals, and that 

these complexes are important in soil formation and plant 

nutrition, the evidence for existence of metal-soil organic 

matter complexes in nature is largely circumstantial. 

Although a relatively large number of publications are 

available on the interaction of metals wirii organic 

materials of soils and waters (see Mortensen, 1963; Saxby, 

1969), very few of the papers have dealt with interactions of 

lead and organic material, and of these, most have been 

concerned with lead in soil-water systems; there are very 

few on the interaction of lead with organic materials in 

streams and lakes. 

SURFACE WATER-SEDIMENT SYSTEMS 

Herrig (1969) and Hellman (1970a, b, 1971) have studied 

the heavy metals in die waters and sediments of the Rhine 

River and its tributaries. Herrig found that both inorganic 

and organic lead compounds were being introduced into 

rivers as industrial and municipal wastes; the most 

common of these were lead oxide, metallic lead, lead 

stearate, lead palmitate and tetraediyl lead. Much of this 

lead was concentrated in die suspended and bottom sediments of high organic content. The sediments were more 

effective in removing lead than in removing copper or zinc 

(Hellman, 1970a). 

The sediments of die lower Rhine contained about 10 

times the amount of heavy metals found in those of the 

upper Rhine and its tributaries. Lead concentrations as 

high as 0.2 percent were found in some of the sediments; 

high heavy-metal concentrations in the sediments were 

associated with high concentrations in the river water. 

The high concentrations are orders of magnitude higher 

than die natural background levels and can only be due to 

municipal and industrial pollution. The high 

concentration of lead in sewage sludge indicates that 

municipal waste can be a major contributor of lead 

pollution in some streams (Gross, 1970). Even diough die 

sediments remove large quantities of heavy metals from 

the water of rivers, diey are still not efficient enough to 

totally decontaminate die water, and therefore the river 

water often contains high heavy-metal concentrations. 

Hellman (1970a) measured lead concentrations in die 

water as high as 85 ns^h an ^ die average for die length of 

die Rhine river was 51 /tg/1. 

The lead in die organic components of a sediment enters 

die sediment from two sources: (1) The plant and animal 

remains that provide die organic compounds to die 

sediment, and (2) the water that comes into contact with 

die sediment. The chelation of lead by die chemical 

components of living organisms tends to cause lead to 

accumulate in living tissue (MacLean and odiers, 1969; 

13 
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14 LEAD IN THE ENVIRONMENT 

Danielson, 1970; Bryce-Smith, 1971). This accumulation 

of lead is no doubt a major source of lead in organic 

sediments. However, both the orgaraic and inorganic 

components of sediments will also remove lead from 

water. Therefore, as has been pointed out by Hellman 

(1970a), organic sediments will tend to decontaminate 

surface waters contaminated with lead and other heavy 

metals. However, one must also remember that sediments 

will tend to supply lead to waters widi low lead 

concentrations in order to maintain chemical 

equilibrium. Therefore, even if the lead pollution of a 

surface water were to cease immediately, the sediments 

would continue to supply lead to die water and to its fauna 

and flora. 

CHEMISTRY OF THE INTERACTIONS 

Saxby (1969) has reviewed die literature of metalorganic chemistry in the geochemical cycle. He found that 

lead has been detected in the carbonaceous fractions of 

soils, coals, petroleum, bituminous rocks and asphalts, 

shales, sedimentary sulfides, and phosphate rocks. In 

general, however, very little is known about the molecular 

associations of metals in geological materials. Therefore, 

in our discussion we shall have to draw heavily upon 

studies that have been made on synthetic organic systems 

and medically important biological systems; however, the 

general chemical principles elucidated in these studies 

should also hold true for natural soil-water systems. 

Two different types of organic lead compounds may be 

found in nature: (1) the so-called organ©-lead compounds, 

in which die lead is covalently bonded to carbon atoms 

and the metal complexes, and (2) chelates, in which the 

lead is ionically bonded to organic ligands. 

Lead normally exhibits bodi divalent and tetravalent 

states in organic compounds. When lead is covalently 

bonded to carbon it is generally more stable in the 

tetravalent state; relatively stable divalent lead derivatives 

are known, however. (See Shapiro and Frey, 1968, for a 

more complete discussion of lead bending in organic 

compounds.) 

Covalently bonded organo-lead compounds in natural 

systems undergo slow oxidation by air and photolysis 

when exposed to light. Most of diese compounds except 

those in which die lead-carbon bonds are highly polar, 

such as the perfluoroalkyl lead compounds and RjPbX 

and RjPbX compounds in which X is an anion, are not 

hydrolyzed by water. In coordination compounds, such as 

metal complexes and chelates, ionic lead is normally 

divalent; however, organo-lead cations in which the lead is 

tetravalent may also be complexed by organic ligands. 

Most of die detailed studies on die eomplexing of lead 

and other metals by organic ligands have been conducted 

by biochemists on the components of living systems; the 

same ligands however are present in die organic material 

of soils, natural waters, and sewage effluents. 

Lead is generally most strongly bound to sulfur 

containing ligands and to phosphoryl groups (Passow, 

1970), but it also forms coordination bonds with other 

ligands. 

Vallee and Wacker (1970) have reviewed the chemistry 

of metalloproteins and have pointed out that lead interacts 

with proteins and enzymes by binding to carboxyl groups 

or sulfhydryl groups. The binding of lead and other heavy 

metals to a wide variety of enzymes inhibits the activity of 

the enzymes (Morrow and others, 1969; Hernberg and 

.Nikkanen, 1970; Vallee and Wacker, 1970). Li and 

Manning (1955) have demonstrated diat, in general, lead 

forms more stable complexes with proteins than do 

cadmium, zinc, or copper. 

The binding of lead to enzymes and the consequent 

inhibition of enzymatic activity will undoubtably affect 

die biological reactions that take place in streams, soils, 

and sewage treatment facilities, but unfortunately there 

are very few data on die interactions of lead with soil and 

water biochemical systems. However, the poisoning of 

enzyme systems in natural waters by heavy metals can be 

expected to drastically alter the rates of decomposition of 

organic wastes in these systems. 

Heavy-metal cations may not always act as enzyme 

poisons in soil systems. Ladd and Butler (1970) have 

shown that divalent cations are effective in reducing the 

inhibition of the enzyme protease by humic acid. It 

appears that die humic acid binds to the enzyme, thereby 

preventing it from combining with substrate. If metal 

cations are present in the system, active sites on the humic 

acid will apparently preferentially bind to the cations and 

be unavailable for binding the enzyme. Lead also interacts 

widi polynucleotides and viruses. Lead ions will cause the 

depolymerization of some polynucleotides such as 

ribonucleic acid (RNA) (Farkas, 1968) and 

deoxyribonucleic acid (Eichhorn, 1962; Izatt and others, 

1971). In the depolymerization of RNA it appears that the 

lead binds die phosphate groups. This neutralizes the 

charge on the phosphate groups and renders the phosphodiester bonds more susceptible to hydrolysis by 

hydroxyl groups (Farkas, 1968). In die binding of lead to 

tobacco mosaic virus molecules the lead may displace 

protons of the hydroxyl groups (Fraenkel-Conrat, 1965). 

In a amino acids, in more complex molecules containing a amino acids, and in molecules similar to 8-

hydroxyquinoline and quinoline acid, lead and many 

odier metals form five-membered chelate rings with 

reactive ligands. Charles and Freiser (1952), in their work 

on five-membered chelate rings, have measured the 

stability constants of lead chelates of o-aminophenol and 

o-aminobenzenediiol. Their data indicate that theorderof 

decreasing stability for metal o-aminophenol is as follows: 

Cu, Ni, Zn, Co, Pb; for o-aminobenzenethiol, however, the 

order is Cu, Ni, Pb, Zn, Co. Lead forms more stable 

complexes with o-aminobenzenethiol dian with o-
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ORGANIC CHEMISTRY OF LEAD IN WATER 15 

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h 

aminophenol, apparently because the lead-sulfur bond is 

stronger dian the lead-oxygen bond. 

Other groups of compounds such as carbohydrates, 

carboxylic acids, lipids, phenols, and oxygenated 

isoprenoids that have electron donor ligands similar to 

those of the proteins mentioned above would also be 

expected to complex lead. In this regard Hoogeveen (1970) 

has demonstrated that lead is bound by phospholipids. 

Martin, Ervin, and Shepherd (1966) and Martin and 

Richards (1969) have shown that copper, zinc, iron, and 

aluminum form complexes with soil polysaccharides. 

These metal-polysaccharide complexes are generally more 

resistant to decomposition in soils than are uncomplexed 

polysaccharides. Although there are no experimental data 

on lead-polysaccharide complexes in soil, one would 

expect them to have properties similar to those of other 

heavy-metal polysaccharide complexes. 

It is well established that soil humic materials such as 

humic acid and fulvic acid form complexes with metals 

(Kononova, 1966). Aleksandrova (1967) has classified the 

interactions of metals with humic materials into three 

categories: (1) Formation of heteropolar salts (an ion 

exchange phenomenon between strong bases and humic 

materials), (2) formation of coordination complexes, (3) 

formation of adsorption complexes with nonsilicate 

sesquioxides (adsorption of humus on sesquioxide gels). 

Most of die interactions of lead with humic materials that 

have been studied fall under the second category. 

Humic acids and fulvic acids form stable complexes 

with lead (II) ions and other metal ions; these complexes 

may be separated from a mixture of humic acid or fulvic 

acid metal complexes by gel permeation chromatography 

(Mucke and Kleist, 1965; Klbcking and Miicke, 1969). 

Therefore, it should be possible to isolate intact, humicmetal complexes from soils and sediments. 

Bondarenko (1968) found diat the presence of fulvic acid 

in water increased die rate of solution of lead sulfide 10-60 

times over a water solution at the same pH that did not 

contain fulvic acid. At pH values near 7, soluble leadfulvic acid complexes were present in solution; at initial 

pH values between 7.4 and about 9, die lead-fulvic 

complexes partially decomposed, and lead hydroxide and 

carbonate were precipitated. At initial pH values of about 

10, the amount of lead-fulvic acid complexes again 

increased. Bondarenko (1968) attributed diis increase to 

dissociation of phenolic groups at high pH values, a 

phenomenon which increases die complexing capacity of 

die fulvic acid. 

In addition to interactions in which a lead ion is bound 

by a single organic molecule, a single metal ion can be 

bound by two different organic molecules. It has been 

demonstrated, for example, that polyvalent cations act as 

bridges between clay minerals and humic and fulvic acids 

(Greenland, 1971). 

SUMMARY AND CONCULSIONS 

The complexing of lead by most of the common sulfur-, 

phosphorus-, oxygen- and nitrogen-containing ligands 

means that lead will accumulate in both the living and 

nonliving organic components of soil-water and 

sediment-water systems. The living and non-living 

organic components are not independent of each other, 

but are constantly interacting, as the living components 

metabolize die nonliving components of the system and 

then die, contributing their remains to the pool of nonliving compounds in the system. Some of the lower forms 

of life are consumed in the process by the higher forms of 

life and in diis way chemical elements from the sediments 

are introduced into the food chain of the system; this food 

chain often ends with man. The fate of heavy metals in this 

process has not been elucidated; however, the sediment in 

a contaminated surface-water body will serve as a large 

reservoir which can provide lead and other metals to the 

biota of die system even after heavy metal pollutants have 

ceased to be introduced into the system. Unfortunately, in 

the case of heavy metals, much more attention has been 

paid to heavy metals dissolved in the water phase of surface 

water than to die complexed metals in the sediment phase 

which, in general, probably contain a considerably higher 

amount of metals than the water. The sediments will also 

act as a reservoir for providing dissolved heavy metals to 

the water when contamination levels are low. 

High concentrations of lead in soils and sediments will 

have a marked effect upon enzyme systems, acting in some 

instances to reduce inhibition by other components of the 

systems. The alteration of the activity of some of the most 

basic components of living systems is certain to have a 

profound effect upon the biological and chemical processes taking place in natural systems. 

ACKNOWLEDGMENT 

This work could not have been completed without the 

diligent efforts of Miss Sarah Booker who did most of the 

literature search. 

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16 LEAD IN THE ENVIRONMENT 

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