While talking around the coffee table about the history of metal poisoning I recalled an event which occurred in the west of England back in the late 1980s. My own view is that heavy metal poisoning is an important subject as it may have resulted or at least contributed to the fall of Rome. The hypothesis is that due to their great use of lead (for pipes, containers and even as a wine additive) the Roman leaders were ingesting large amounts of lead which caused dire brain damage which in turn caused some mental health trouble which can explain some of the vile outrages of emperors such as Caligula. I reason that a society run by someone like Caligula is doomed to fail as he was not competent to run a large and complex empire.
But on the other hand it is important to understand that some of the people who recorded the events of the reign of Caligula were very keen to drag his name into the mud, they may have wanted to smear him by suggesting that he was a sexual pervert and insane that he was a poor leader. In those days insanity and sexual excesses were associated with poor government. But lets assume that Caligula was as bad as he is commonly thought to be.
I hold the view that if this wicked Caligula was alive today in the UK it is likely that he would either be in prison or in a high security psychiatric hospital such as Broadmoor or Rampton. For those of you who do not know, Broadmoor and Rampton are hospitals in the UK were some very dangerous people are looked after and kept away from the general public. Many of the people in these hospitals have done some dire acts, mostly they are people who were either judged to be too mentally ill to be responsible for their crimes or people who while serving prison sentences for violent crimes have suffered a dire decline in their mental health while in Jail (Such as the Yorkshire ripper).
But let’s get back to the chemistry, back in the late 1980s an industrial accident with serious off site consequences occurred. I think that it is equivalent to either a level four or maybe a level five nuclear accident on the nuclear accident scale. I am basing this judgement on the off site effect which was exerted via the public drinking water supply.
The event occurred at a place called Camelford, what happened was that a delivery driver arrived at an unattended water treatment site. He was there to deliver a chemical which is used for treating water (aluminium sulphate); this chemical should be added to water in very small amounts with careful control of the pH to help to remove solid particles and some metal ions from the water. Instead he poured twenty tons of aluminium sulphate solution into the wrong tank; it was the storage tank for the final product water where it was held just before it was sent down pipes to the general public.
A range of things could have prevented or maybe mitigated the accident, for example a well enforced rule that delivery drivers have to be meet by a competent site worker who checks the identity of the substance to be delivered and then supervises the unloading into the correct part of the waterworks would have stopped the accident from occurring. A better accident response would have reduced the exposure of the general public to the contaminated water, but sadly people were misinformed and were wrongly told that the water was safe to drink.
Sadly Dr Foreman does not have a time machine, so sadly I can not change the past. What happened was that the aluminium sulphate underwent a hydrolysis reaction. Hydrolysis is either a splitting up by water or I think it is better thought of as water being split up. What happens in a hydrolysis reaction of a metal is that the metal cation will bind to a water molecule.
The more intense the positive electric charge on the metal atom is the more the metal is able to pull electron density out of the waters and thus make the water hydrogens more positive than they would be if the water was not bonded to a metal.
If the metal is able to exert a strong pull on the electrons in the water then the metal can make the water act as an acid. The electrons which are used to bond a hydrogen to the oxygen in the water are transferred to the oxygen forming a hydroxyl anion and a proton.
Because the proton is so intensely charged it quickly bonds onto a water molecule to form the solvated proton, so do not worry it is not the “same” proton as a fast moving proton which has come flying out of a cyclotron. The proton in aqueous chemistry to give its full name is a solvated or hydrated proton (H3O+)
Normally many people think of a hydrolysis reaction as forming hydroxyl ligands (HO-) but I think that if taken to an extreme it can form oxide ligands (O2-). Strictly speaking this reaction should be called hydroxylsis, but it has the same mechanism so I think it should be lumped together with normal hydrolysis.
An easy example is iron(III) in water, in very acidic water iron(III) forms a pale purple coloured positively charged molecule with a charge of +3, one central iron and six waters arranged around it in an octahedral manner.
When the water is not very acidic one of the water molecules bonded to the iron splits up to give a hydroxyl anion bonded to the iron and a proton which goes off into the water to play, acidify and do those things which protons do.
The equation is
[Fe(H2O)6]3+ → [Fe(OH)(H2O)5]2+ + H+
What has happened is that the metal complex has given up a proton into the water and it has lowered its positive charge. These two features are common to all hydroyslsis events.
Like iron the aluminium has a charge of +3 as it has lost three electrons when it forms a typical aluminium compound. Unlike iron the aluminium trication (three plus ion) is much smaller. As the smaller a charged sphere is the more intense the electric field on the surface is then the aluminium cation is more able to split up water with its electric field. As a result the aluminium is more able to split up the water and form acid than the iron.
In general the more highly charged a metal cation is, and the smaller it is then the greater the metal ion’s tendency to split up water in a hydrolysis reaction will be.
In this process the water molecules bonded to the aluminium are converted into hydroxide anions. For example if a aluminium salt is treated with 1 equiv of a base then the following dinuclear complex can form. The term dinuclear means that the complex has two metal atoms (nuclei) in it. Here the hydroxyl ligand is acting as a bridging ligand between two aluminium atoms thus forming the dimer.[i]
We can use the equations E = Q / 4∏eor2 and V = Q/ 4∏eor to understand what is going on. If we take a octahedral hexaaqua complex of aluminium ion (six water molecules arranged in an octahedral manner around an three plus aluminium) then according to X-ray crystallography[ii] the water oxygens are 1.87 Å (187 pm, 187 x 10-12 m or 0.000000000187 meters) from the aluminium atom.
We can now calculate the electric field which these oxygens experience
Using the constants
Q = 3 x 1.602 x 10-19 C
eo = 8.85 x 10-12 F / m
r = 1.87 x 10-10 m
We can make an estimate of the electric field strength in terms of volts per meter at the water oxygens. I work it out as being 124 GV m-1 which is a very high electric field strength. Note that typically dry air will fail as a dielectric at about 1 MV m-1. If we choose a slightly larger metal ion such a chromium(III) then on average the water oxygens are 1.961 Å from the metal centre. If we do the same calculation then we now get a value of 112 GV m-1 which is a bit lower than the value for the aluminium complex.[iii]
If we go to a still larger metal ion (ruthenium (III)) in a hexaaqua complex then the water oxygens on average are 2.122 Å from the metal atom,[iv] now the electric field at the water oxygens will be only 96 GV m-1.
If we do these calculations using the crystallographic data for the dinuclear aluminium complex then we should get another answer.
The equatorial waters at the end of the complex are on average 1.902 Å from the aluminium atom while the axial oxygens are on average 1.963 Å from the nearest aluminium atom. This causes a tiny reduction in the electric field for these oxygens so that based on the distance alone from the nearest aluminium they would experience fields of 120 GV m-1 and 112 GV m-1.
But we also have the field (of the opposite sign) from the hydroxyl anions. These charges and the charge of the other aluminium need to be taken into account. I got a final electric field strength of 113 GV m-1 for the equatorial waters. When the calculations were repeated for the axial waters I got a value of 109 GV m-1. While the electric fields might seem large it is important to understand that they are now only about 90 % of what they were before the hydrolysis reaction. So while the product of the hydrolysis can undergo a further reaction the hydrolysis reaction forms a product which is less able to undergo the reaction than the parent compound.
If the process continues then more hydroxide and even oxide ligands will form. This formation of these liagnds will lead to an infinite array of aluminium atoms linked by oxygens being formed.
The intended use of the aluminium sulphate at the water works was to form particles of such a solid, these particles help to clear the water by sweeping the solid bits in the water to the bottom. When the process is done correctly the pH is controlled by the addition of a base to stop the water becoming acidic, but during the accident no attempts were being made to control the pH of the mixture.
During the accident the acid formed by the hydrolysis of the aluminium sulphate caused corrosion of copper pipes and other surfaces which then made the water even more toxic. A study of the hair of some of the people from Camelford strongly suggests that at the time of the water works accident that they were exposed to lead. It is reasoned that the lead was mobilised by the acidic water.[v]
[i] E.A.Mainicheva, O.A.Geras’ko, L.A.Sheludyakova, D.Yu.Naumov, M.I.Naumova, V.P.Fedin, Izv.Akad.Nauk SSSR,Ser.Khim.(Russ.)(Russ.Chem.Bull.), 2006, 261
[ii] D.G.Samsonenko, M.N.Sokolov, A.V.Virovets, N.V.Pervukhina, V.P.Fedin, Eur.J.Inorg.Chem., 2001, 167.
[iii] D.G.Samsonenko, O.A.Geras’ko, T.V.Mit’kina, J.Lipkowski, A.V.Virovets, D.Fenske, V.P.Fedin, Koord.Khim.(Russ.)(Coord.Chem.), 2003, 29, 178.
[iv] P.Bernhard, H.-B.Burgi, J.Hauser, H.Lehmann, A.Ludi, Inorganic Chemistry, 1982, 21, 3936.
[v] J.P. Powell, S.M. Greenfield, R.P.H. Thompson, J.A. Cargnello, M.D. Kendall, J.P. Landsberg, F. Watt, H.T. Delves and I. House, Analyst, 1995, 120, 793.