An Introduction to Environmental Chemistry

only in the Amazon, but in Latin America as a whole, the Pacific Rim countries,

and Africa, which together produce thousands of tonnes of gold each year.

During the amalgamation process Hg^0 is easily introduced into the local soils,

rivers and atmosphere. Although the effects of leaching of Hg^0 from waste tips

and direct spillages into water courses are reduced by the low solubility of Hg^0

in water (solubility of 0.12¥ 10 -^6 mol l-^1 ), the mercury is potentially subject to

a number of chemical transformations that make it harmful to organisms.

During the final stages of gold processing, burning of the Au-Hg amalgam in

open pans allows evaporation of Hg^0 as vapour to the local atmosphere. The

annual flux to the atmosphere by this process is around 80 tonnes of Hg^0 , approx-

imately 67% of Brazil’s total mercury emission. In the atmosphere Hg^0 can be

dispersed globally (atmospheric residence time around 1 year) but much is readily

oxidized to Hg^2 +. The reaction occurs in water droplets, the oxidant is ozone (see

also Section 3.9) and experiments suggest that mercuric oxide is produced, for

example:

eqn. 5.22

The oxide might then be ionized, for example by:

eqn. 5.23

This ionic mercury (HgII) adheres to aerosols and thus has a short (days to weeks)

residence time in the atmosphere; rainfall delivers it to the local soils and rivers.

Ionic mercury is readily methylated (eqn. 5.24) by both abiotic and biotic path-

ways. However, most scientists now agree that methylation by anaerobic sulphate

reducing bacteria (SRB) is most important.

eqn. 5.24

In equation 5.24 the Hg^2 +ion forms a covalent compound dimethylmercury

(Hg(CH 3 ) 2 ) by bonding with the methyl anion (CH 3 - ). The methyl anion is a

derivative of vitamin B 12 called methylcobalamin, a common constituent of the

bacteria themselves. At low to neutral pH, methyl mercury (CH 3 Hg+) forms in a

similar way to dimethylmercury (Fig. 5.15). Methyl mercury is a potent toxin

because it is soluble in the fatty tissues (lipophylic, see Box 4.14) of animals, ulti-

mately attacking the central nervous system. It enters the food chain either

directly absorbed from the water (e.g. into fish) or when plankton, a food source

for fish, feed on the methyl mercury-rich bacteria. Each step in the food chain

increases the concentration of mercury in the organic tissues (biomagnification)

because organisms cannot eliminate mercury as fast as they ingest it (Fig. 5.15).

As a result, mercury-contaminated fish are the main cause of mercury poisoning

in humans. This is a particularly alarming situation for the Amazon region where

fish are the staple food of the poor. Unfortunately, this pattern of mercury

poisoning is very well known. Between 1953 and 1975 in the now infamous

Japanese fishing village of Minamata, over 600 people were poisoned (115

Hg CH Hg CH

SRB

2

(^2332)

+-+ ()

(methylcobalamin)

2 H+++Æ+HgO Hg^2 H O 2

Hg^0 +ÆO 32 HgO O+

172 Chapter Five