A risk analysis carried out by the UK Food Standards Agency has shown that all the important transition metal ions used as catalysts of oxo-biodegradation are widely distributed in agricultural soils and taken up by the foods that humans eat, and in drinking water. Some of them, typically iron, cobalt and manganese are essential trace elements, all obtained from foodstuffs and drinking water.
Transition Metals (Essential Trace Elements) in Food and Water (mg/kg)
(1) Iron: Abundant in meat, many vegetables and water
(2) Cobalt: Present in fish: 0.01, nuts: 0.09, cereals: 0.1
(3) Manganese: Present in green vegetables: 2.0, nuts: 15, bread: 8.0, other cereals: 6.8, tea: 2.7, water: up to 0.01
(4) Nickel: Present in oats:P 0.18, nuts: 1.8. Average daily intake: 0.016 mg/day (food and water)
UK Food Standards Agency Expert Group in Vitamins and Minerals (2003), Part 3 “Trace elements, Risk Assessment
For example, high concentrations of cobalt are found in fish (0.01 mg/kg), nuts (0.09 mg/kg), green leafy vegetables (0.009 mg/kg) and fresh cereals (0.01 mg/kg). Most of the cobalt ingested is inorganic. Fresh water concentrations of Co range from 0.001 to 0.01 mg/L. The mean population intake of Co is 0.012 mg/day. Cobalt is also included in some multi-constituent licensed medicines, at a maximum daily dose of 0.25 mg. Cobalt is an essential trace element, and Co deficiency has not been reported in humans (presumably because of its widespread availability from food and water).
Similarly, nickel is present in a number of enzymes in plants and micro-organisms, and in humans it influences iron absorption and metabolism. It is found in a variety of foods as ionic Ni, particularly in pulses and oats (0.18 mg/kg in miscellaneous cereals), and in nuts (1.77 mg/kg). Lower levels are found in water. Total intake of nickel by humans from all sources is up to 0.26 mg/day and no potential high intake groups have been identified. The average intake from food and drinking water is 0.16 mg/day.
The amount of transition metal ions available to plants from common soils is very much greater than could be produced from degradable plastics in the soil and is much higher than can be absorbed by plants8. Particular attention has been paid to cobalt and nickel. However agricultural soils contain very high concentrations of cobalt oxide (up to 100 ppm) and nickel oxide (up to 750 ppm). Sandstone and limestone contain 90 ppm and 10-20 ppm of nickel respectively and it can be calculated that in the ‘worst case scenario’, it would take 500 years to increase the nickel content of soil using typical nickel contents of degradable polyethylene mulching films by 1 ppm.