5.2 Acidity and heavy metals
Acidity can be detrimental to life forms. Table 2 shows the effects of low pH on fish.
|6.0–6.4||Unlikely to be harmful except when carbon dioxide levels are very high (1000 mg l−1)|
|5.0–5.9||Not especially harmful except when carbon dioxide levels are high (20 mg l−1) or ferric ions are present|
|4.5–4.9||Harmful to the eggs of salmon and trout species (salmonids) and to adult fish when levels of calcium, sodium and chloride are low|
|4.0–4.4||Harmful to adult fish of many types that have not been progressively acclimated to low pH|
|3.5–3.9||Lethal to salmonids, although acclimated roach can survive for longer|
|3.0–3.4||Most fish are killed within hours at these levels|
In addition, highly acidic waters can dissolve heavy metals, especially if the pH is below 3. This occurs in the case of mine wastewaters. The metals present tend to be brought into solution, especially iron, zinc, lead and molybdenum.
The information that is available on metal pollutants tends to refer to the total concentration of the metals; unfortunately this provides little information on a metal’s bioavailability (i.e. the ability of an organism to take up the metal) or how long the metal will stay in solution before it is removed into sediment. To be able to predict the adverse effects of metal pollutants in water it is necessary to determine the physical form or chemical speciation of the metal present, since metals can exist in a wide range of forms. The subject is too complicated to be covered fully here, but the following two theoretical examples are extreme cases of metal pollution to show the range of events that can occur.
- Example 1: After treatment to remove most of the lead, the effluent from a lead–acid battery factory is discharged into a river. The lead concentration in the effluent is still about 4 g m−3, with a trace of particulate lead in the form of lead sulfate. The lead sulfate settles out quickly. The soluble lead becomes attached to particles greater than 12 µm in size and settles out in the river. The discharge therefore causes a relatively small increase in the lead content of the river.
- Example 2: A sewage effluent is contaminated with cadmium from industrial sources. The cadmium is in the form of organic complexes formed from the organic-rich sewage in the sewage treatment process. In the river this form of cadmium does not settle out and is carried downstream over a long distance, remaining available to biological life throughout.
Metals dissolved in water can enter the food chain by a variety of mechanisms. For example:
- Phytoplankton absorb metals by diffusion across the external membrane.
Fish can take in metals by
- diffusion across the membrane of their gills
- ingesting metals, though metals taken in this way are not as readily bioavailable.
- Filter feeders such as oysters and cockles inhabit the surface of sediments and consume considerable amounts of particulate matter from the water that passes through them; they accumulate metals by ingestion.