THE IDENTIFICATION AND CLASSIFICATION
OF WATER SOLUBLE METAL COMPLEXES
AND AN EXAMINATION OF THEIR
EFFECT ON THE METAL BINDING ABILITY
SUBMITTED IN PART FULFILMENT FOR THE
HONOURS DEGREE IN ENVIRONMENTAL
SLIGO INSTITUTE OF TECHNOLOGY
TABLE OF CONTENTS
Aims and objectives 10
Recommendations for further study
I would like to take this opportunity to extend my gratitude to all
the people who helped me during my project work and over the
last year in college , especially the following;
Dr. who gave his time and help in abundance.
The technical staff of Sligo I.T. science department.
The library staff of Sligo I.T. .
I would also like to thank the following ,my class for their
friendship and help, and for all the late nights out. To my housemates for all the
craic and more late nights. The wise man who shut Fureys down making us go
out even more to find a new pub. To the girls for having smooth legs even
though its a tough job. Of course anyone who had to put up with me .
Finally I would like to thank my mother for all her help
encouragement and nagging over the last few years, it got me this far.
“think where mans glory begins and ends
and say my glory was I had such friends”
Using solutions which contain various different metal complexes
,the ability of mosses to bind the metals was examined. The ability of the moss
to bind metal was then examined when chelating agents were present in the
solutions, the effect of varying the chelating agents concentration was also
All tests were carried out by monitoring the decrease in metal
concentration in solutions which were exposed to a quantity of moss. In the case
of metal complexes the samples were taken on varying dates after exposure, and
in the case of the chelating agents samples were taken hourly to examine effects
on the rate of metal uptake.
A test was also carried out to examine if the moss would uptake
the actual chelating agents themselves.
To understand why certain tests were carried out in this project
we must first understand the actual binding of metals to the moss surface. This
however is not a simple process in fact there is still much debate about the
process, some experts claim that the process involves cation exchange, however
there is evidence that this is not the case, this evidence does allow for the fact
that this process does play some part in the overall binding. Probably the most
likely process was put forward by Xue, Stumm and Sigg (1988) who conducted
tests on the metal binding ability of algae, they concluded that the process
involved surface complex formation between the algal surface and the metals.
The above mentioned work indicated that the surface of a
biological molecule contains various functional groups such as carboxylic,
amino, hydroxo, thio, and many others which interact coordinately with the
heavy metals . This however still does not explain all the characteristics of metal
binding in mosses, the process is understood to involve an equilibrium, between
metal bound to the moss and metal in solution. This is thought to be a part of an
adsorption like process which closely follows the Freundlich adsorption
Some of the experimental work conducted during this project
will attempt to clarify some of the theories involved in the metal binding of
mosses, one test will be aimed at seeing whether the moss can uptake the actual
complexing/chelating agents themselves.
The first set of tests will be aimed at the ability of the moss to
uptake metals that are complexed by simple inorganic ligands. If the metal
which is complexed does not have a full coordination sphere it should then be
possible for the ligands on the moss surface to bind to the metal coordinately,
thereby binding the metal. If however the metal has a full or crowded
coordination sphere it should be difficult for the moss ligands o approach and
bind to the metal. Of course here the lability of the ligands bound to the metal
becomes important, if the ligands are labile then it is reasonable to expect them
to be ejected from the metal in favour of the moss ligands. Therefore we could
expect that metals which contain a full coordination sphere of strong (non
labile) ligands ,will not be bound to the moss by coordinate processes. If then
metals in this form are bound to the moss there must also be physical adsorption
processes occurring at the moss surface.
A test will be carried out on one portion of moss to determine if
an equilibrium process is actually occurring. This will be achieved by
attempting to leach metals from the moss surface, if metal is leached from the
surface two conclusions can be drawn firstly that an equilibrium is present
between metal in solution and metal on the moss, proof of this is simple as in
clean water metal will leach from the moss surface to reestablish the
equilibrium. The second conclusion which could be drawn is that if an
equilibrium is present then metal must be physically adsorbed onto the moss
surface as well as metal bound to moss ligands, the equilibrium of course
would only effect physically adsorbed metal as the chemically adsorbed (bound)
metal would not be free to reenter the water as this would require bond breaking
which is highly unlikely to occur.
The other important test conducted here was the examination of
the effect of chelating agents on the uptake of metal by the moss. The two
chelating agents used were EDTA (ethelenediamminetetraacetic acid) and
EDTA is a common chelating agent used extensively in the analysis of
metals, the four oxygen and the two nitrogen atoms in the molecule bind to the
metal, the stability constants for metal EDTA complexes are high which
indicates that the EDTA chelate is non labile from most metals. This means that
the moss ligands should not be able to uptake the metal coordinately and once
more physical adsorption processes must be involved for uptake to occur.
A variation on this test will also be used in that the concentration of the
EDTA will be varied. It is expected that the presence of the bulky EDTA
molecule will slow the rate of binding but may also ultimately reduce the total
uptake. From the test hourly monitoring of metal concentration should indicate
this. If the rate of uptake is slower and assuming that the bonds between metal
and chelate are not broken then the rate of adsorption of metal EDTA complex
is slower than the uptake of metal alone. Of course if the total uptake is lower
then this would indicate a physical adsorption whose equilibrium is more biased
towards metal EDTA in solution. In other words the moss would have a
preference to free metal.
Humic acid was also used in a similar test . Studies conducted by
Aualitia and Pickering show that Lead and Copper sorbed onto humic acid are
non labile to anodic stripping voltametry however, there is an equilibrium
between metal in solution and metal sorbed to humic acid. In the context of this
test, it is known that the moss does not uptake the humic acid, therefore
depending on this equilibrium the rate of metal uptake will be affected. The
reason for this is that there is only a specific proportion of metal free in the
solution the amount being dependant on the bias of the equilibrium, as this is
taken up by the moss the concentration of free metal in solution is reduced and
the metal humic acid equilibrium is disturbed and metal is released from the
humic acid to reestablish the equilibrium this process continues until both
equilibria are in synch that is that the equilibria between the metal and the moss
and metal and humic acid are themselves in equilibrium, this should leave more
metal not bound to the moss, but also should slow the rate of uptake as the
equilibria balance each other.
As the humic acid has different affinities for metals we should see
varying changes in the uptake across the metals, the metals to be used in this test
are Lead, Copper and zinc as these form water soluble complexes with humic
acid. These are uptaken by the moss to a considerable level these will also be
used in the tests involving EDTA so that a comparison of the two chelating
agents can be conducted.
A test will be carried out using EDTA and metal leached moss to see
whether the moss is capable of uptaking the actual chelating agent itself and the
extent of the uptake of the EDTA. This test is to be used to examine if the
uptake of metals bound to EDTA is equivalent to the uptake of EDTA, this
could be used to determine if the metals made any difference to the uptake of
EDTA if of course EDTA itself is actually adsorbed. The term adsorbed is used
here as the EDTA molecule will be in anionic form, as the moss ligands
themselves are anions it should not be possible to bind coordinately with the
EDTA. This test should show if adsorption processes are occurring at the moss
surface. If this is the case then it would be reasonable to expect other anions to
be adsorbed on the moss surface. This can be reasonably expected as moss can
be best described as a “scavenger” taking up everything that is placed in solution
with it in order to survive. If this is occurring then the uptake of complexes may
not be governed by the uptake of metals but more by the uptake of the
The pH of a number of solutions shall be measured the information
gained from this should be able to clarify the role of cation exchange in the
metal uptake process. As all the ligands mentioned contained labile hydrogen
atoms it is reasonable to assume that some of these protons will exchange for
metal atoms during the binding process thereby allowing the metals to bind
coordinately with the ligands. It is this that will be monitored in this test. If a
significant lowering in pH is monitored then it is reasonable to assume that this
cation exchange process is significant, however a slight lowering of solution pH
would indicate that cation exchange plays only a slight part in the metal binding
processes occurring at the surface of the moss.
The above tests have been specifically chosen for two reasons firstly
they will show the effect that using complexed forms of metal will have on the
ability of moss to bind the metal. The tests will also aid in the understanding of
the processes involved in the metal binding of moss, the tests should indicate
some of the processes involved and prove that they are actually occurring.
AIMS AND OBJECTIVES
The aim of this work is to examine the effect that ligands bound to
metals will have on the ability of moss to bind that metal, the study will also
encompass chelating agents and their effects.
The object of this is to gain a further understanding of the processes
occurring at the moss surface, the work will elucidate which processes are
actually occurring and should be capable of indicating the extent and therefore
the importance of each of the theorised mechanisms.
Much of the experimental portion of this project is fairly similar and to
avoid repetition a general outline of the tests will be given firstly.
For the majority of tests a portion of moss is placed in a solution
containing either metal or metal complex. 10ml samples are then extracted from
the solution at predetermined or convenient times. The samples are then filtered
to remove any residual moss (to ensure that the instrument sample intake
capillary is not blocked). Once filtered the samples are analysed for the
appropriate metal using flame atomic absorption using a Perkin Elmer flame
atomic absorption spectrophotometer.
METAL COMPLEX UPTAKE TESTING
Solutions containing 10 parts per million of metal are prepared,
500mls of these solutions are placed in a beaker which contained 600mls of
loose packed washed moss. Samples were then taken at days convenient to lab
usage. The metal complexes used were Zinc nitrate ( ), Copper chloride ( ),
Manganese chloride ( ) and
METAL CHELATE UPTAKE TESTING
For the purpose of these tests three metals were chosen those
were Lead, Copper and Zinc the chelating agents chosen were EDTA and
Humic acid. Solutions were then prepared from these, the working solutions
contained the following: each solution contained 10ppm of metal, 2,5 and
10ppm of chelating agent were then added to separate solutions. Roughly 30g of
washed moss were then placed into 600mls of each solution. Samples were then
extracted from each solution on an hourly basis for twelve hours, samples were
also taken at 24 and 30 hours after introduction.
MOSS LEACHING TEST
A portion of moss was placed into roughly 600mls of distilled
water at intervals samples were extracted. After each sample the moss was
washed and placed into fresh water and the process was repeated. The samples
were then analysed for a number of relevant metals.
EDTA UPTAKE TESTING
A portion of the moss from the above test was washed and placed
into contact with 500mls of a 0.1 molar EDTA solution after 24 hours four 20ml
samples were extracted and were analysed for EDTA content.
The EDTA analysis involved the samples being titrated using a
0.1 molar Copper solution and an Erichrome Black T (EBT) indicator.