Cadmium and sewage sludge
This essay concentrates on the heavy metal cadmium, and its occurrence in the environment, its pathways into and out of the human body and its movement into and out of Sewage sludge. Firstly, I will provide a clear definition of the Heavy Metal Cadmium.
What is Cadmium?
Cadmium is a naturally occurring element in the earth’s crust. Pure cadmium is a soft silver-white metal, but this form is not common in the environment. Rather, cadmium is most often encountered in combination with other elements such as oxygen (cadmium oxide), chlorine (cadmium chloride), or sulphur (cadmium sulphide). These compounds are all stable solids that do not evaporate, although cadmium oxide is often found as part of small particles present in air.
Most cadmium is obtained as a by-product from the smelting of zinc, lead or copper ores. Cadmium has a number of industrial applications, but it is used mostly in metal plating, pigments, batteries, and plastics.
Cadmium in the Environment
Small quantities of cadmium occur naturally in air, water, soil and food. For most people, food is the primary source of cadmium exposure, since food materials tend to take up and retain cadmium. For example, plants take up cadmium from soil; fish take up cadmium from water, and so on.
It is introduced into the environment from mining and smelting operations. Other cadmium emissions are from fossil fuel use, fertilizer application, sewage sludge disposal or galvanized pie corrosion. The largest source of these cadmium releases into the general environment is the burning of fossil fuels (such as coal or oil) or the incineration of waste materials. Cadmium may also escape into the air from zinc, lead or copper smelters. Working in or living close to a major source of airborne emissions such as these may result in higher than average exposure.
Pathways into the human body
Food and Drink
Cadmium can enter the blood by absorption from the stomach or intestines after ingestion of food or water. The amount of Cadmium ingested by an individual within a population can differ greatly due to dietary habits. For example, an individual who consumes a large amount of brown crabmeat or vegetables grown on land which is fertilised with sewage sludge are likely to have intakes of cadmium significantly above the average population level.
Only a small percentage of cadmium ingested with the food intake is absorbed, approximately 6%. However there are many factors, which can affect this figure, namely, physiological, such as age and the content of the diet.
Smoking is another important source of cadmium. Like most pants, tobacco contains cadmium, some of which is inhaled in cigarette smoke. Most people who smoke have about twice as much cadmium in their bodies as do non-smokers. Unlike other methods of cadmium uptake, much of the cadmium present in tobacco is absorbed.
Very little cadmium enters the body through the skin. Usually only about 1 to 5% of what is taken in by mouth is absorbed into the blood, while about 30-50% of that which is inhaled is taken up into the blood. Cadmium may escape into the air from the burning of fossil fuels or the incineration of waste. Inhalation of cadmium through airborne dusts can occur, but concentrations of cadmium are relatively low and therefore insignificant when compared to food ingestion. However, working or living close to a major source of airborne emissions may result in a higher than average exposure and build up. Once cadmium enters the body, it is very strongly retained; therefore even low doses may build up significant cadmium levels in the body if exposure continues for a long time.
Other Sources of Intake
Children are very vulnerable to cadmium poisoning due to their habitual chewing and licking of toys, pencils and other items. Toys made of vinyl, PVC or painted articles are the most dangerous for children to put in mouth. Many kitchen utensils are coated in metal glazes; these glazes through wear and tear are chipped off and in turn ingested through food consumption.
There is a very small and limited absorption of cadmium through the skin due to cosmetic or pharmaceutical application.
Table 1.0 below shows the absorption levels of cadmium through each of the methods above.
Table 1.0 Average intake and absorption of Cadmium by man in UK(ig)
SourceDaily intakeDaily absorption
* based on average diets
o based on long term airborne concentrations in urban areas.
** assuming 20 cigarettes/day
The Figures in Table 1.0 show clearly that those people who consume food containing a high percentage of cadmium, or those who smoke heavily, particularly unfiltered cigarettes are at a higher risk of suffering from Cadmium poisoning.
After dealing with the many intakes of Cadmium into the human body, I feel it is important to mention the pathways out of the body.
After ingestion of Cadmium into the human body, it can be excreted in a variety of ways. Most of the cadmium, which is ingested through food consumption, is passed directly through the alimentary canal and excreted within the faeces. Other forms of excretion involve the shedding of hair, skin and nails. But these are far more gradual forms of loss and would only be significant when over long periods of time.
The most significant pathway of excretion is by means of the kidneys. There is a very unfortunate inhibitor to this otherwise simple process. A cadmium protein complex exists within the human body called Metallothioneine; this substance is excreted and absorbed by the renal ducts. This and other circumstantial factors prevent the excretion and redistribution of cadmium within the kidney and therefore encourage considerable cadmium build up in the body.
The rates of renal excretion vary considerably between individuals, depending on metabolic rates, alcohol consumption and infections present.
Cadmium in Soils
As I mentioned within the section Cadmium in the environment’, cadmium both occurs naturally within soils and is present due to artificial means.
Naturally, cadmium is present in very low concentrations through rock and mineral derivation, organic substances and secondary material. Volcanic action transports cadmium up from the lower crustal regions of the earth to the land surface and to the atmosphere. This soil cadmium content, however, has a very limited availability and is not significant in comparison to other more artificial provisions of Cadmium.
The most common pathways that Cadmium finds into the soils are through processes such as atmospheric Deposition, irrigation water and fertiliser application. Atmospheric deposition occurs after the release of Cadmium into the atmosphere through burning of fossil fuels and tobacco smoke. Irrigation water is a significant source of cadmium contamination for soils, this is not through lack of treatment, but due to the materials which coat the pipework through which the water flows.
Fertiliser application provides a large percentage of the cadmium present in soils. It is sewage sludge which is one of the most prevalent types of fertiliser in this transfer of cadmium.
Many factors affect availability of soils Cadmium to plants, including pH, soil salinity, soil organic matter content, soil texture, soil micronutrients status, etc.
Transfer of Cadmium into Sewage Sludge
Approximately 20% of cadmium present in sewage sludge is derived from domestic sources which have been outlined in this chapter. Concentrated human waste can contain a high intensity of Cadmium. This level is however extremely variable depending on the location, but a mean level of 29 ppm dry matter appears to be a fairly accurate concentration.
The remaining 80% originates from industrial sources.