Cadmium is Natural. Cadmium (elemental symbol Cd and CAS registry number 7440-43-9) is a member, along with zinc and mercury, of Group 12 (CAS IIB) of the Periodic Table of the Elements. It is generally characterized as a soft, ductile, silver-white or bluish-white metal, and is listed as 64th in relative abundance amongst the naturally occurring elements. Cadmium is found principally in association with zinc sulfide based ores and, to a lesser degree, as an impurity in lead and copper ores. It is also found in sedimentary rocks at higher levels than in igneous or metamorphic rocks, with the exception of course of the nonferrous metallic ores of zinc, lead and copper.
Cadmium is found throughout the environment from natural sources and processes such as the erosion and abrasion of rocks and soils, and from singular events such as forest fires and volcanic eruptions. Some typical levels of the natural occurrence of cadmium are as follows:
Atmosphere 0.1 to 5.0 nanograms* per cubic meter (ng/m3)
Earth’s Crust 0.1 to 0.5 micrograms** per gram (mg/g)
Marine Sediments 1 microgram** per gram (mg/g)
Sea Water 0.1 micrograms** per liter (mg/L)
*nanogram = 10E-9 grams (one billionith of a gram)
**microgram = 10E-6 grams (one millionith of a gram)
Cadmium is often described as a “heavy metal”, but the term has no meaning in fact since it possesses an average density, atomic number and atomic weight compared to other metals. The density of cadmium is 8.64 g/cm3 at room temperature compared to 1.85 g/cm3 for beryllium, the lightest of the metallic elements, to 19.3 g/cm3 for gold or tungsten, amongst the heaviest of the metallic elements. The atomic number for cadmium is 48, again right in the middle of the range of 4 for beryllium and 92 for uranium amongst the naturally occurring metallic elements. The atomic weight of cadmium is 112.1, which once again is very near the average atomic weight of the metallic elements. Beryllium is the lowest at 9 while other metallic elements with high atomic weights include lead at 207, bismuth at 209, and Thorium and Uranium at 232.
Cadmium is Useful. Most cadmium metal today is produced as a by-product of the extraction, smelting and refining of the nonferrous metals – zinc, lead and copper. Rather than disposing of it as waste into the environment, engineers have been able to utilize its unique properties for specific industrial applications. Cadmium is also produced from the recycling of spent nickel-cadmium batteries, its largest use, and secondary or recycled cadmium now accounts for about 23% of total cadmium supply.
Cadmium metal exhibits excellent sacrificial corrosion resistance, particularly in alkaline and seawater environments, possesses a low melting temperature and rapid ion electrical exchange activity, and maintains both high electrical and thermal conductivity, whether contained in an alloy or present in an oxide form. As pigments, cadmium compounds possess outstanding resistance to high temperatures and high stresses, good dispersion in polymers to produce strong coloring, high opacity and good tinting strength. Most cadmium pigments will remain colorfast for the life of the plastic, glass, ceramic or enamel into which they are incorporated.
The nickel-cadmium battery is based on the reversible electrochemical reactions of nickel and cadmium in a potassium hydroxide electrolyte. These cells are characterized by low cost, high energy and power densities, high cycle life, a wide operating temperature range, low self-discharge characteristics, and well-established mechanical and electrical durability. They are also easily recyclable, and have been utilized for many years in a wide range of applications, although they are now being replaced in the high-cost, high energy density applications by more advanced battery chemistries. Some cadmium electronic compounds exhibit semi-conducting properties and are utilized in solar cells and in small quantities in a variety of electronic applications such as detectors, relays, gates, and switches.
Because of their unique physical, mechanical, and electrochemical properties, cadmium metal and a few cadmium compounds (mainly cadmium sulfide and cadmium oxide/hydroxide) are used today in pigments, coatings, stabilizers, specialty alloys and electronic compounds, but mostly (about 85% in 2010) in rechargeable nickel-cadmium batteries. Almost all of these uses today are industrial uses as opposed to consumer uses. The recent trends in the consumption of cadmium in applications where cadmium is a deliberate addition and not an impurity are shown in Figure 1 below.
Figure 1. Trends in Cadmium Consumption Patterns from 2005 through 2010.
Cadmium Levels in the Environment. Cadmium levels in the environment increased rapidly in the period from 1800 to 1960, due predominantly to industrialization in the Western World and the huge increase in fossil fuel combustion during that period and since cadmium is a naturally occurring impurity found in all fossil fuels. For the 50 years since 1960, cadmium levels in the general environment have been decreasing due to improved emission control for fossil fuel combustion and improved technology for the production, use and disposal of cadmium and cadmium-containing products. These trends are shown in Figure 2 below.
Figure 2. Changes in cadmium concentrations of Greenland ice and snow from 1800 to 1995 (Boutron et al 1995). pg/g = picograms per gram. Picogram = 10E-12 g (one trillionith of a gram).
Cadmium emissions in the metals industries in Western or OECD nations are now tightly controlled due to significant improvements in both processing techniques and pollution control technology and to stricter regulation and legislation. On a global scale, however, emission problems remain with respect to non-OECD nations and with regard to emissions from sources produced on very large scales where cadmium is present as a minor impurity.
Emissions of cadmium from the incineration of municipal solid waste, medical waste or other types of waste have generally been successfully addressed by the application of best available technology that is capable today of capturing more than 99% of the incinerator fume cadmium emissions. It should also be noted that most municipal solid waste today contains both materials that have deliberate cadmium additions and those where cadmium is present as an incidental impurity. Both would contribute to cadmium emissions from municipal solid waste and thus both should be addressed.
Another factor to consider is the bioavailability of cadmium from the products in which it is present. In many of its deliberate applications, cadmium or cadmium compounds are imbedded in the product’s matrix, do not readily leach from the product, and are therefore not readily bioavailable. A notable example is products colored by cadmium sulfide pigments that are encased in plastics, glasses, ceramics or enamels, and which are therefore completely insoluble.
Other cadmium-containing products such as batteries, coatings and alloys are recyclable, although at present only NiCd batteries are available in sufficient quantities with sufficient amounts of nickel and cadmium present to economically justify their recycling. Adequate technology and capacity for recycling all of the world’s NiCd batteries exists today. The principal deterrent to higher recycling rates for NiCd batteries is the lack of well-organized battery collection systems in some areas of the world to collect the well-dispersed small, portable consumer batteries. These systems do exist in the United States, Canada, Europe and Japan, but are lacking elsewhere. Industrial NiCd batteries are very efficiently collected and recycled, and enjoy recycling rates over 90%.
Cadmium and Human Health. The effect of cadmium exposure on human health has been studied extensively since the 1950s following the occurrence of Itai Itai disease in Japan as the result of post-menopausal women with low iron and zinc levels ingesting cadmium-contaminated rice. In the human body, cadmium accumulates mainly in the kidneys and, at high enough levels, can reach a critical threshold and lead to kidney failure. Studies (Bernard & Buchet 1998) have shown that kidney effects may be reversible at low exposure levels once the cadmium exposure is reduced or removed. At sufficiently high enough exposures, bone effects occur as well leading to Itai Itai disease.
The major route (98%) of cadmium intake for the non-smoking and non-occupationally-exposed population is through ingestion of food and water. This cadmium input results largely from trace amounts of cadmium in foodstuffs that are taken up from soils and waters. The cadmium in soil is a result of its natural presence, the application of phosphate fertilizers or sewage sludge to agricultural land, atmospheric deposition, or deposition from cadmium-containing waters
Investigations around the world have shown that, for the general population, the average cadmium intake is low compared to the World Health Organization’s (WHO) standard for tolerable cadmium intake and that cadmium intake levels have in fact been decreasing over the past 20 years. The current cadmium tolerable intake standard established by the Joint Expert Committee on Food Additives (JECFA) of WHO is 25 micrograms per kilogram of body weight per month (mg/kg bw-mo). Average cadmium intakes in 1960 were about 15 mg/kg bw-mo but by 2000 had decreased to about 5 mg/kg bw-mo, well below the current WHO JECFA standard established in 2010. These results are shown graphically below in Figure 3.
Figure 3. Average Monthly Cadmium Intake for the General Population
Most cadmium emissions and human cadmium exposure for the general population results from cadmium contained as a minor impurity in high-volume products such as fossil fuels, fertilizers, iron and steel production and cement production. Na
tural sources is actually also a significant contributor to total cadmium emissions and human cadmium exposure. The production of cadmium, cadmium compounds and cadmium products, their use, and final disposal represent only a small fraction of total cadmium emissions and human cadmium exposure. These results are shown in Figure 4 below and show that the production, use and disposal of products to which cadmium has deliberately been added account for less than 2% of the total sources of cadmium emissions and human cadmium exposure.
Workers may be occupationally exposed to cadmium in industrial environments. However, occupational exposure standards for cadmium throughout the world have now been reduced to completely safe levels, workers’ exposure to cadmium is strictly controlled by rigorous industrial hygiene practices, and workers are continuously tested by biological monitoring to prevent any unacceptable health risks to those who might be occupationally exposed. While the renal (kidney) effects of cadmium are well-known and accurately established, the carcinogenicity of cadmium remains an area of controversy. Some jurisdictions have classified cadmium as a known human carcinogen while others have indicated that it is a possible or probable human carcinogen. Many of the earlier studies on which these classifications were based have now been contested by newer studies such as those of Sorahan which have indicated that confounding exposures to other carcinogens may have been responsible for the observed effects.
Figure 4. Relative Contributions of Different Sources to Human Cadmium Exposure (Regoli, Meeting of UNECE Task Force on Heavy Metals, 16 March 2005, Berlin)