Cadmium: A Highly Toxic Metal with Extensive Commercial Use
A Highly Toxic Metal with Many Uses. The Agency for Toxic Substances and Disease Registry (ATSDR) publishes a priority list of substances determined to pose the most significant potential threat to human health due to their known or suspected toxicity and potential for human exposure at Environmental Protection Agency (EPA) National Priority List (NPL) hazardous waste sites in the United States.
Cadmium (Cd) ranked number 7 on this list in 2017  after benzene, polychlorinated biphenyls, vinyl chloride, mercury (Hg), lead (Pb), and number 1, arsenic (As). The NPL is a prioritization of substances based on a combination of their frequency, toxicity, and potential for human exposure at NPL sites. The high ranking of Cd is due, in part, to its presence at 1019 of 1669 NPL sites . Cadmium is one of the most toxic elements to which a person can be exposed at work or in the environment. Once absorbed, Cd is retained in the human body, accumulating throughout life, with a biological half-life of excretion of many years, depending on its body location. Breathing air with high levels of Cd can severely damage the lungs and may cause death. Exposure routes for the public are mainly mining or industrial sites where Cd is present as waste, food intake, and smoking cigarettes. Exposure to cadmium over a long-time period can result in (1) osteomalacia and osteoporosis with a propensity for fractures accompanied by severe bone pain and (2) renal tubular dysfunction . Cadmium toxicity is also suspected to be a causative factor in endometrial, breast, and lung cancers [3,4].
A toxicological profile on Cd has been prepared by ATSDR and EPA . This profile succinctly characterizes the toxicological and adverse health effects of Cd. Release of Cd into the environment was minimal prior to the Industrial Revolution that began in the early 19th century. Presently, Cd is emitted to soil, water, and air by non-ferrous (mainly zinc (Zn), Pb, and copper (Cu)) mining and refining (Cd is a by-product in ores containing these metals); manufacture and application of phosphate fertilizers (Cd is a common impurity in phosphate ores); fossil fuel combustion (Cd is present in coal); and waste incineration and disposal, particularly of spent technology products (the major use of Cd, approximately 80%, is in nickel-cadmium (Ni-Cd) batteries). Cadmium emitted by these processes can accumulate in aquatic organisms and agricultural crops (cadmium is selectively taken up by tobacco plant leaves and some leafy vegetables) . Two centuries ago, global mining and refining of ores was minimal and was localized to a few regions; no phosphate fertilizers were in use, minimal amounts of coal were being burned and none was used to produce electricity, a major global use in 2017; and no high technology products had yet been produced, Ni-Cd batteries were invented in 1899 but only introduced in large amounts in the early 1960s. Cadmium is also used in production of pigments (8%); coatings and platings (7%); stabilizers for plastics (1.2%); and non-ferrous alloys, photovoltaic devices, and others (0.8%) [2,6].
Cadmium and Zinc: Similar Chemically but with Widely Different Biochemical Functions. The toxicity of Cd has limited its uses . Discovered in 1817, this metal was named cadmium from the Latin word cadmia, Greek kadmeia. This is the ancient name for calamine or zinc carbonate. Cadmium received this name because of its close chemical similarity to Zn. Indeed, Cd is most often found in small quantities in Zn ores, such as sphalerite (ZnO) because of this similarity . Two closely related elements could not be more different in their effects on human health. Zinc biochemistry began in 1939 when the enzyme carbonic anhydrase was found to contain Zn, which was soon determined to be essential for its enzymatic activity . This enzyme, present in red blood cells, aids in the conversion of carbon dioxide to carbonic acid and bicarbonate ions. When red blood cells reach the lungs, the same enzyme helps to convert the bicarbonate ions back to carbon dioxide, which we breathe out. Without Zn, no respiration and no life.
Although found in the same family of elements as Zn, Cd has no known physiological function in the human body [9,10]. Zinc, on the other hand, is an essential element being involved in hundreds of enzyme and protein systems . The close chemical similarities of these metals suggest that Cd ingestion in humans could have an adverse effect on the function of numerous enzymes and proteins as Cd competes with Zn for binding sites on these vital biological molecules. Indeed, clinical manifestations of Cd poisoning indicate that many functions controlled by enzymes and proteins containing Zn have been compromised.
Exposure of the Population to Cadmium. Cadmium began to be mobilized in sizeable quantities into the ecosystem with the onset of the Industrial Revolution beginning in the late 19th century. Mining of Zn and Pb sulfide ores released Cd which was discarded at first because uses for it had not been found. The acute toxicity of airborne Cd, particularly as Cadmium oxide fumes, was first recognized in the 1920s among workers exposed in industrial settings such as electroplating, which was the first major use of Cd .
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In severe cases, initial symptoms rapidly progressed to severe pulmonary edema and chemical pneumonitis and death. Persistent respiratory effects lasting years were reported for workers surviving the initial effects. It was estimated that an 8-hour exposure to 1-5 milligram per cubic meter (mg m3) would be immediately dangerous. Extensive animal studies support the findings in humans that exposure to Cd results in lung damage. Elevated Cd levels occur in water sources near Cd-emitting industries, both current and historical.
Cadmium is found in tobacco plants where it is concentrated in the leaves. As a result, smokers have significantly elevated blood levels of Cd and are at risk of lung complications due to ingestion of this toxic metal [3,11]. The national geometric mean blood Cd level for adults in the U.S. is estimated to be 0.38 microgram per liter (µg L-1). It has been estimated that tobacco smokers are exposed to 1.7 µg L-1 per cigarette  and about 19% is inhaled when smoked. Blood Cd levels as high as 1.58 µg L-1 have been determined for heavy smokers in New York City. Measurement of Cd levels in body tissues confirms that smoking roughly doubles Cd body burden in comparison to not smoking. Thus, presence of Cd increases the risk of smoking in the global population. It is estimated that nearly 80% of the more than one billion smokers worldwide live in low- and middle-income countries, where the burden of tobacco-related illness and death is heaviest .
Cadmium is a natural constituent of calcium phosphate worldwide. Present extensive global use of phosphate fertilizers may increase the soil burden of Cd although a recent study concluded that metal levels found in fertilizers in North America do not pose risk to the public . Since Cd is taken up by plants, risk of Cd intake may increase over time in fertilized areas. Leafy vegetables such as lettuce and spinach, potatoes and grains, peanuts, soybeans, and sunflower seeds contain high levels of Cd, approximately 0.05-0.12 mg Cd per kg.
Use of Ni–Cd batteries has decreased in recent decades because of the high toxicity of Cd and the introduction of Ni metal hydride and lithium technologies which have superior energy density characteristics and performance properties. From 1997 to 2006, use of Ni-Cd batteries in the rechargeable battery market decreased from 56% to 18%. This decrease can be attributed, in part, to the marked increase in use of Li and Ni hydride batteries. However, a significant contributor to the decrease is recognition of the toxicity of Cd resulting in increasing numbers of government regulations that require replacement of Ni-Cd batteries with other battery technologies and the recycling of Cd from spent batteries rather than discard to waste dumps . Significant numbers of Ni-Cd batteries remain in use and the legacy of discarded batteries in landfills and other depositories remains worldwide with potential to contaminate the environment.
Weathering of NPL hazardous waste sites can present a risk to the surrounding environment and, eventually, to human health. Globally, electrical and electronic waste discharge totals approximately 50 million tonnes (mt) per year . A significant fraction of this waste is landfilled, often by unsafe means. Weathering of such deposits increases the risk of exposure to toxic metals, including Cd, that were used in product manufacture. The European Commission recently announced its decision to prohibit use of Cd in TVs and displays sold in Europe effective in October 2019 . This ban recognizes Cd as a hazardous heavy metal, like Hg or Pb with potential for eventual release into the environment. Dr. Michael Edelman, CEO of Nanoco, a world leader in the development of Cd-free quantum dots and other nanomaterials, said: “The European Commission is putting the health of consumers first and removing deadly Cd from these household products in an expedited timeframe.” This action is typical of many efforts worldwide to replace Cd in articles of commerce. A benefit of this action is that eventual disposal of these products in landfill or by incineration will not release Cd to the environment.
Toyama Bay, Japan: A Textbook Example of Cadmium Poisoning. The most serious case of Cd poisoning worldwide took place in Japan in the 20th century. The area involved was also the first to be recognized as being Cd polluted. This case is of special interest because it occurred in a localized area; involved interaction between a specific mining operation and a specific community downstream that was severely impacted by mining effluent over an extended time period; resulted in identification of a specific disease caused by Cd, but only after extensive scientific investigation, decades of denial by the industry involved, and intense suffering by the affected population; and became the basis for strong government sponsored environmental regulations but only after extensive scientific investigations. Aoshima  gives an account of the disaster in great and fascinating detail. Together with the Minamata Hg disaster , the Toyama Bay event had a strong influence on the development and enforcement of present strict Japanese laws and regulations of toxic substances in the workplace and environment. The Toyama Bay Cd incident is a textbook example of the effect of toxic metal poisoning on a community and the eventual successful cooperative effort by involved citizens, industry, and government to resolve the problem . It is hoped that the knowledge gained will be applied globally to shorten the time required for future incidents of Cd poisoning to be recognized and resolved.
The Jinzu River supplies water for about 20 km2 of fields for paddy rice culture in Japan in the Toyama Bay region, approximately 249 kilometers (155 miles) northwest of Tokyo . Approximately 30 km upstream of these fields along the Takahara River lies the large Kamioka lead and zinc mine and smelter. The Takahara River originates in the Japan Alps. In the late 19th century, mining activity rapidly developed at this mine. Fine particles of waste material or tailings were discharged into the Takahara River and washed down into the rice fields where they settled. Mining activity sharply increased during the World War I and World War II periods because of the need for these metals as Japan became industrialized. The inhabitants of the Jinzu River basin used the river water for drinking, cooking, washing, and bathing until around 1960. In 1946, local officials asked Kanazawa University scientists to investigate an endemic disease of unknown etiology that had been observed in the local population. Published results indicated a rheumatoid disease localized in the Jinzu River Basin with no cases in adjacent river basins. The disease was named itai-itai which means ouch-ouch in Japanese reflecting the intense pain associated with the condition. Skeletal deformation was extreme, and bone pain was common especially in the pelvic girdle and legs while walking. It required 20 years of careful scientific investigation to establish Cd as the causative agent in itai-itai disease. In 1968, the Japanese government recognized that itai-itai disease was caused by chronic Cd poisoning. Itai-itai disease was thus established as a disease caused by environmental Cd pollution, which begins with renal tubular dysfunction followed by secondary bone abnormalities that develop into osteomalacia. Based on cadmium determinations in the rice grown in the affected region, the authorities have replaced the upper layer of soil over 15 km2 of the paddy fields with non-polluted soil. This effort was started in 1980 and completed in 2012. A marked decline in cadmium levels of rice grains resulted. A graphic, descriptive presentation of the effects of Cd poisoning on affected individuals in Toyama Prefecture, Japan is available .
The number of persons affected by itai-itai disease in the 20th century in the Toyama region is not known, but must have been large. Cases of itai-itai disease and renal tubular dysfunction have also been found among residents in other cadmium polluted areas throughout Japan . Aoshima  observes that there is wide-spread cadmium contamination of agricultural soil in many areas of the world leading to the concern that itai-itai disease may be repeated elsewhere.
The Toyama Bay disaster was important in several ways. First, it was the first large-scale cadmium pollution event. Cadmium was established as the culprit and the study of the individuals affected provided valuable information concerning Cd poisoning. Second, governments worldwide became aware of the severity of Cd poisoning and standards began to be established for Cd exposure in the workplace, for children, and in the environment. Third, this Cd pollution disaster together with the Minamata Bay Hg pollution disaster , had a strong influence on developing environmental awareness by the Japanese Government. As a result, Japan has become a global leader in promoting toxic metal pollution control and recycling of metals.
Production and Recycling of Cadmium. There has been a significant reduction in the use of Cd in commercial products during the past few decades. In 2015, only two companies in the United States produced refined cadmium . One company recovered cadmium as a by-product from zinc leaching of roasted sulfide concentrates. The other company recovered secondary cadmium metal from spent nickel-cadmium batteries and other cadmium-bearing scrap. Production amounts were proprietary and not released. Most of global primary cadmium production in 2015 took place in China, Republic of Korea, and Japan. Most consumption of cadmium occurred in Belgium, China, India, and Japan. Ni-Cd battery production accounted for more than 80% of global cadmium consumption. Recent regulations, particularly in Europe and China, limiting use of Ni-Cd batteries should reduce significantly the quantity manufactured in the future . Future use of Ni-Cd batteries may be found only in specialty areas. Remaining consumption was in pigments, coatings and plating, stabilizers for plastics, nonferrous alloys, and other specialized uses. Cadmium telluride photovoltaic modules have promise for use in solar cell technology. However, in these applications, the toxicity of cadmium is of concern.
Recycling of cadmium is estimated to be in the 10-25% range . Secondary cadmium is mainly recovered from spent consumer and industrial Ni-Cd batteries. Dumping or incineration of spent Ni-Cd batteries can cause serious environmental and human health problems. However, the high content of Ni and Cd in these spent batteries make them an excellent secondary source for these metals. Methods used for Cd separation and recovery including pyrometallurgical, solvent extraction, and bioleaching are expensive and inefficient . The need for economic and environmentally friendly processes to separate and recovery Cd and Ni from these spent batteries has been observed . Such efficient technologies should be effective in minimizing capital outlay and recovery of these metals for reuse or proper disposal.
Cadmium Separation and Recovery from Primary and Secondary Sources Using Molecular Recognition Technology (MRT). MRT is a mature technology that employs green engineering and green chemistry principles to make selective separations of metals from primary and secondary sources [23,24]. Recovery rates of individual metals are high, ranging from 99% to 99.99%, depending on the demand. The MRT process is simple in design and operation, since high metal selectivity is present eliminating the need for large numbers of stages and chemicals. Separated metals are recovered for reuse or safe disposal. No solvents are used. Two examples are given of the use of MRT processes to separate and recover Cd.
Removal of Cadmium as an impurity in Production of High Purity Cobalt [23,24]. High purity cobalt is required in many electronic and other high-technology products. Cadmium is an important impurity in cobalt ore and its concentration must be controlled to meet Co purity specifications. The need for high purity Co has been accelerated by its use in Li batteries, which are required for rapidly growing energy technologies involving electric vehicles. Three technologies were evaluated for the selective removal of trace amounts (~6 mg L-1) of Cd from a Co-electrolyte solution containing 60 mg L-1 Co and other metal impurities including magnesium and manganese : (1) MRT, (2) solvent extraction using di-2-ethylhexyl phosphoric acid (D2EHPA), and (3) adsorption using amino-methyl phosphoric acid resin (Purolite S950).
The plot in Figure 1 of the log of the selectivity factor (SF) for these three technologies demonstrates that MRT is much more effective at selectively removing Cd than either solvent extraction with D2EHPA or adsorption with Purolite S950. The higher selectivity of the MRT process allows Cd to be recovered as a pure product that can be sold to help defray separation costs, rather than disposed of in a landfill or other waste repository. MRT has significant economic advantages over the other two technologies. Higher SF values allow separations to be made in single passes with MRT eliminating many process steps, higher chemical costs, higher labor costs, and larger space and equipment requirements. No organic solvents are used in MRT processes eliminating significant costs associated with use and disposal of these chemicals. Minimal waste is generated using MRT processes.
Concentration of the separated Co in the MRT process by a factor of 100 or more in the column elution step allows for its easy recovery rather than disposal as waste as is the case with the other technologies. Recovery is important, especially in the case of toxic Cd, due to the danger of dispersion of the discarded metal into the commons where it may enter the ecosystem through air, soil, or water pollution. Control of the fate of the metal as is done in MRT processes is important because once the metal enters the commons it becomes unrecoverable.
Removal, Separation and Recovery of Heavy Metals from Industrial Waste Streams using MRT . Navy, Army, and Air Force industrial wastewater treatment plants receive 90% by volume of metal laden wastewaters from electroplating, parts cleaning and paint stripping operations. Treatment of these industrial wastewaters using conventional hydroxide precipitation methods generates hazardous metal sludge, which is sent to landfill as Resource Conservation and Recovery Act F006 hazardous waste. MRT was tested as part of a Department of Defense program to evaluate advanced techniques to effectively recycle and reclaim metals from industrial waste waters .
Results of a demonstration, conducted at Puget Sound Naval Shipyard (PSNS) from 1999 to 2001, showed that all heavy metals regulated under the Clean Water Act pre-treatment standards were recovered successfully using the MRT process. Following treatment by the MRT process, metal ion concentrations in the effluent stream were two orders of magnitude below PSNS monthly regulatory discharge limits and much lower than those achievable with conventional precipitation technology. Analytical results showed that benign alkaline earth and alkali metals passed through the MRT column without retention, as expected. Mass balance analysis confirmed that the MRT column capacity was five orders of magnitude greater than that of typical IX columns.
Columns loaded with SuperLig® 327 were effective for removal of bivalent Cd, Pb, Zn, Cu, and Ni as a group from an acid/alkali waste stream feed solution. Separation of individual metals can subsequently be made using other SuperLig® products. Cr(III) is removed using SuperLig® 310 mixed with SuperLig® 327 in the column. Any Cr(VI) present can be extracted using SuperLig® 307. Mg, Ca, Na, and K present in the feed solution had no affinity for these SuperLig® products and passed through the column to raffinate. Results in the Table show the effectiveness of this separation procedure in removing heavy metals to concentrations significantly below discharge limits and at 98.9 % or greater extraction efficiencies.
Table. MRT Acid/Alkali Waste Stream Performance Data for Removal of Hazardous Metals as a Group .
Conclusions. Release of Cd into the environment has increased markedly since the onset of the industrial revolution in the mid-1800s. The toxicity of Cd has been recognized and efforts have been made to reduce its use in consumer products. Nevertheless, there is still ample opportunity for human exposure to this toxic metal. Cadmium is used in many different industries such as electroplating, pigments, synthetic chemicals, ceramics, metallurgical and photographic products, and electronics . Waste from these industries provides ample opportunity for Cd dispersal into the environment. The presence of Cd as a natural constituent of tobacco leaves poses a significant risk to smokers worldwide. As an impurity in sulfide ores, Cd may be discarded into tailings or landfill as waste. Separation and recovery of Cd from secondary sources by conventional technologies is usually not economic resulting in discard into landfill or incineration. There is a need for green engineering and green chemistry technologies capable of selectively separating Cd from primary and secondary sources and recovering it for reuse or safe disposal economically. MRT has this capability using highly Cd-selective SuperLig® products in a simple separation process with favorable economics. Going forward, increased awareness by industrial companies and government agencies of the need to recover Cd and other toxic metals to prevent their release to the environment should lead to greater use of green technologies such as MRT for this purpose.
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Reed M. Izatt received a BS degree in Chemistry from Utah State University (1951) and a PhD degree in Chemistry with an Earth Sciences minor ... <Read more about Reed M. Izatt, PhD>