By Alan Broughton
Arsenic levels in rice periodically hit the news. Arsenic is one of several toxic heavy metals found in foods – cadmium, lead, mercury and chromium are others. While these are naturally occurring elements in soil and rock, natural sources are not the main cause of contamination of agricultural produce. Principal sources include fertilisers, pesticides, mining, industrial waste and air pollution. This article focuses on the two most prevalent heavy metal contaminants – cadmium and arsenic.
Origins
There are two major sources of heavy metal contaminants in fertilisers. One is naturally occurring cadmium in rock phosphate, which is mined and processed to produce superphosphate and other soluble phosphatic fertilisers, or used directly. The other arises from the practice of adding industrial waste to fertilisers as a means of disposal, material containing mercury, arsenic, nickel, copper, zinc, uranium, lead, chromium and cadmium.
In the US any material that has some qualities as a fertiliser can be used on fields in the name of recycling, even low level radioactive waste from uranium processing. The California Public Interest Research Group identified 22 toxic elements in chemical fertilisers in 1996, all of which contained industrial waste from steel works, cement factories, paper making and electronic plants. The practice of adding industrial waste to fertilisers was revealed by Duff Wilson in Fateful Harvest (Wilson 2001). Until at least 2002 this practice was totally unregulated. Indeed, the US EPA stated (1997): “EPA’s longstanding policy encourages the beneficial reuse and recycling of industrial waste, including hazardous wastes, when such wastes can be used as safe and effective substitutes for virgin raw materials” (Asokakumar 2017). Few states in the US have any regulations for heavy metal content of fertilisers, only California, Washington and Oregon; nationally there are recommended maximum levels but they are not mandated (McLaughlin 2004).
In some other countries, including China, waste is added to fertilisers. Both the US and China are major fertiliser exporters. China is the biggest fertiliser producer in the world. China has set limits but these limits are not always well enforced. In 2002 zinc sulphate imported into Australia from China was found to contain 11% cadmium, over 100,000 times the limit. It originated from zinc smelters. It had been issued with an approval certificate by Chinese authorities. The fertiliser was being used by vegetable growers mainly in WA and Queensland. After examination the federal Department of the Environment classified it as hazardous waste (Ryle 2002).
In some other countries, including China, waste is added to fertilisers. Both the US and China are major fertiliser exporters. China is the biggest fertiliser producer in the world. China has set limits but these limits are not always well enforced. In 2002 zinc sulphate imported into Australia from China was found to contain 11% cadmium, over 100,000 times the limit. It originated from zinc smelters. It had been issued with an approval certificate by Chinese authorities. The fertiliser was being used by vegetable growers mainly in WA and Queensland. After examination the federal Department of the Environment classified it as hazardous waste (Ryle 2002).
Many countries have no regulations for fertiliser contaminants. There were still no regulations in India in 2014 (Asokokumar 2017). No official testing was done in Nigeria up to 2012; research has shown molybdenum levels in NPK fertilisers in Nigeria to be up to 1,000 times international standards (Chibueze et al 2012).
Blended fertilisers usually contain “filler”, which might be coal ash, sawdust, sand, soil or industrial waste.
Heavy metals have also been found in pesticides, particularly the glyphosate-based herbicides (Roundup for example). Arsenic, lead, chromium, cobalt and nickel are commonly found, while cadmium, mercury and aluminium levels are undetectable. Arsenic is the highest, above the permitted level in water supplies after the recommended dilution rate of the pesticide. The arsenic could come from petroleum derived additives such as POEA, or may be deliberately added to increase the effectiveness of the herbicide – arsenic is known for its herbicidal effects (Defarge et al 2018).
An Indian study (Sharma et al 2009) has shown a huge increase in cadmium and other heavy metal levels between the fields and the markets. The authors put this down to air pollution.
A Chinese study (Cao et al 2010) attributed the high levels of six heavy metals in rice and vegetables (chromium, copper, zinc, cadmium, mercury and lead) to a combination of mining, industrial processing, pesticides, chemical fertilisers and vehicle exhaust, and concluded that while all six were individually below hazardous levels, the cumulative effect was a risk to health.
Cadmium sources
Cadmium is particularly associated with kidney disease, but also contributes to cancers, osteoporosis, lung malfunction and immune system suppression. It is readily taken up by plants and transferred to people and animals.
Cadmium sources
Cadmium is particularly associated with kidney disease, but also contributes to cancers, osteoporosis, lung malfunction and immune system suppression. It is readily taken up by plants and transferred to people and animals.
The International Cadmium Association estimated in 2015 that 42% of the cadmium that accumulates in food comes from phosphatic fertilisers. Fossil fuel combustion contributes 22% and iron and steel production 17%. Irrigation water sometimes has a high cadmium content. Plants absorb cadmium more readily than they do other heavy metals.
There have been major clusters of fertiliser-caused kidney disease in various parts of the world (Asokakumar 2017). In Sri Lanka 20,000 people have died and 450,000 more are affected. A group of Sri Lankan doctors associated with a WHO report into the problem blamed arsenic and cadmium in food traced back to chemical fertilisers and pesticides, as there was no other possible source. Triple phosphate fertiliser had the highest arsenic content of all fertilisers, 31 mg/kg, and high levels of cadmium. Other kidney disease clusters occur in Nicaragua, El Salvador, Haiti and Srikalulam District in the Indian state of Andhra Pradesh. More recent research (Jayasumana et al 2014) has found a strong link between a combination of heavy metals, hard water (high in calcium and magnesium) and glyphosate herbicide with kidney disease in these areas of the world.
Highest levels of cadmium in fertilisers sold in Australia have been found in phosphorus and trace element fertilisers; levels in nitrate and potash fertilisers are low. Maximum allowed in phosphatic fertilisers in Australia is 0.3 mg of cadmium per kilo of phosphorus; pasture grades of superphosphate are generally lower than 0.25 mg while premium grades are less than 0.1. Monoammonium phosphate (MAP) and diammonium phosphate (DAP) have lower levels. Poultry manure is another source of cadmium, because of the alleged practice of adding phosphate to poultry rations (Jinadasa 1999). Natural levels of cadmium in Australian soils range from 0.1 up to 0.7 kg/ha.
Currently used phosphate sources contain much less cadmium that that from the Pacific Islands, though the build-up from the previous applications is still there in soils. Nauru phosphate contained 0.641 mg/kg P, by far the highest in the world. The Duchess mine deposits in Australia contain 0.5 mg/kg, while Morocco is 0.24 and Tunisia 0.108. Much of the current imports are from North Africa. Some cadmium comes from the sulphuric acid used in the solubilisation of rock phosphate to manufacture superphosphate and other phosphatic fertilisers (McLaughlin 2004).
The Fertiliser Industry Federation of Australia has a code of practice for cadmium levels in fertilisers, but regulations in Australia are state-based. The Australian National Cadmium Minimisation Committee was set up in 2000 to minimise cadmium residues in soils and crops. Victorian regulations (DEDJTR 2016) make it illegal to manufacture or sell fertiliser with greater than 0.3 mg/kg P of cadmium, and warning labels are required if the level exceeds 0.001 mg/kg. Lead and mercury levels are also limited and warnings required.
Uptake of cadmium by plants increases as the soil pH decreases. Uptake is also correlated with low organic matter levels, low clay content in soils, and deficient zinc. High chloride levels, from salinity or potassium chloride fertiliser, also increase uptake. Concentrations are highest in leafy vegetables, followed by tubers, then seeds and grains, then fleshy fruits. Capeweed and the brassica family of vegetables are accumulators of cadmium.
Cadmium in food
The average American ingests 10-51 micrograms of cadmium daily; in Britain it is 10-20, Sweden 11-18, and Japan 20-70. In the European Union, studies show that adults on average consumed close to or slightly more than the tolerable weekly intake of cadmium; vegetarians, children and smokers took in twice the tolerable level. Cigarette smokers absorb 1-3 micrograms of cadmium per day from cigarettes. Maximum daily intake should not exceed 70 micrograms per day according to the WHO, though there is evidence that significantly less than this can be harmful, particularly for children.
Between 2% and 6% of the cadmium ingested remains in the body, accumulating in the kidneys, and up to 10% for people deficient in iron, calcium or protein. The majority is expelled in urine.
The World Health Organisation recommends maximum concentrations of cadmium in food as follows: 40 micrograms/kg for polished rice, 20 for wheat, tubers and leafy vegetables and 5 for other foods. Where people eat rice three times a day, as in many Asian countries, the intake is high, corresponding to the high incidence of kidney disease. Almost all rice samples tested in the Indian state of Kerala contained cadmium levels greater than the food safety standard. Kerala has a large proportion of kidney disease sufferers (Asokakumar 2017). In the city of Isfahan in Iran 74% of vegetable samples contained above the WHO and FAO tolerance levels of cadmium; 51% contained above tolerance levels of lead (Rahimi et al 2015).
Food Standards Australia and New Zealand has set Maximum Levels for cadmium, above which the product cannot be sold as human food; these are 100 micrograms/kg for leafy and root vegetables, rice and wheat, and meat 50 (McLaughlin 2004). The maximum level in wheat was raised from 50 micrograms/kg to 100 in 1997; this level is sometimes exceeded. Horticulture Australia says that the Maximum Level for vegetables has also sometimes been exceeded, but in general levels in Australian produce are low in comparison with other countries (Horticulture Australia 2003).
Despite these assurances and the FSANZ policy, a report in 1999 (Jinadasa et al 1999) published by the Rural Industries Research and Development Corporation found more than 60% of the leafy vegetables grown in the Sydney region equalled or exceeded the National Health and Medical Research Council maximum permitted concentration. About 40% of the cadmium in a normal Australian diet comes from leafy vegetables.
A sampling of rice on sale in Australia (Rahman et al 2014a) found levels of cadmium were well below Maximum Levels; Australian grown rice was lower than imported Chinese, Japanese, Thai and Indian rice. However, Australian rice had on average double the Maximum Level for lead, with brown rice showing five times the Maximum Level; Thai rice was also very high in lead. For vegetables, the study found about half the Australian vegetables contained above ML for cadmium and most were slightly higher in lead than ML.
The RIRDC report (Jinadasa et al 1999) warned that addition of cadmium in the vegetable cropping areas of greater Sydney were at least 10 times higher than loss of cadmium from soil by leaching and harvest. The equilibrium state of cadmium was estimated to be 1-5 grams/ha/year of addition – a 1994 study found 40 grams/ha/year were applied in fertiliser. Farming advice from organisations such as the CSIRO says to seek out low-cadmium phosphatic fertilisers and do not use more than required, a rather pathetic response. The CSIRO warns: “Research shows applications of phosphorus fertiliser are increasing soil cadmium levels as more cadmium is being added than is removed by leaching or harvest” (Jackson 2003). So, despite the reduction strategy, levels continue to rise. The European Union on the other hand has a zero net cadmium accumulation policy for soil (McLaughlin 2004).
Arsenic
The main source of arsenic in Bengal (both West Bengal state in India and Bangla Desh) where millions of people are affected by arsenocosis and thousands are dying, is ground water, taken from aquifers naturally high in arsenic and used for irrigation and drinking water (McLaughlin 2004).
Much of the arsenic in the Australian environment though is not naturally occurring. It is caused by several different human activities (Smith et al 2003). The most important is mining, particularly gold mining, dating back to the 1850s and still contaminating both ground and surface waters. Arsenical herbicides were used along railway lines and around electricity poles; arsenical insecticides were common from the late 1800s till 1960 in orchards and vineyards; and sheep and cattle dips commonly used arsenic based lice, blowfly and tick treatments. Other sources include power stations, timber treatment works (and the illegal burning of treated timber), dust storms, volcanic eruptions and forest fires.
Arsenic is not taken up as readily by plants as cadmium and is mainly concentrated in the roots. However, the edible parts of some plants are affected, particularly silver beet and rice.
Researchers (Rahman et al 2014b) found very high arsenic levels in Australian rice. Highest was organic brown rice at 438 micrograms/kg, well above the WHO/FAO maximum levels of 300 (but below the Food Standards ANZ maximum levels of 1,000). Imported rices had much lower levels, except Italian Arborio rice at 547. Thai varieties averaged 172. The levels in Australian organic brown rice are a health hazard especially for those who eat rice more than once a day. It is especially hazardous for babies being fed on rice-based baby foods. Even 200 micrograms/day has been found to cause human genetic damage, increasing the risk of cancer (Bannerjee 2013). FSANZ says there is no need for concern (Sams 2016).
Another study (Francisca et al 2015) found levels of between 90 and 330 micrograms/kg in Australian rice; highest levels were in Arborio and sushi varieties.
What should producers and consumers do?
Organic farming systems generally reduce cadmium uptake in plants, but not arsenic. Australian organic brown rice, for example, was found to contain higher heavy metal levels, including arsenic, than non-organic brown rice (Rahman et al 2014a, 2014b).
Rice produced without flooding accumulates considerably lower arsenic levels than flooded, however this may not be feasible for producers, as one of the purposes of flooding in southern Australian rice growing areas is to prevent chilling during grain filling.
Organic farmers must be aware that rock phosphates, allowed under organic standards, including in blended fertilisers, come from the same source as that used for manufacturing superphosphate. Their cadmium content may be even higher because some cadmium is taken out in the processing. Some poultry manures might also be a source for cadmium. Most pelletised complete organic fertilisers are based on composted poultry litter. Composting does not reduce heavy metal levels though it may reduce availability to plants. High soil organic matter levels reduce cadmium uptake by crops. The soil pH must be above 5.5, preferably 6.2 to 6.5, to minimise uptake.
All farmers should be very wary about trace element fertilisers, particularly imported zinc sulphate.
For consumers there is not much that can be done about rice. Brown rice is more nutritious than white rice because not only arsenic but other minerals too are concentrated in the bran which is removed in milling. Using large amounts of cooking water leaches some arsenic out of the rice but also leaches other minerals needed for health. Best option is to restrict rice consumption to once or twice a week to reduce intake.
There is a strong case for developing rice growing methods that minimise arsenic uptake, and a stronger one for tighter regulation and monitoring of heavy metals in fertilisers. The practice of recycling industrial waste in fertiliser must end.
Alan Broughton, February 2018.
References
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