Position Papers
Transition Metal Salts
Questions are sometimes asked about the effect of transition metal compounds (often inappropriately called "heavy metals") present in oxo-biodegradable plastics.
The heavy metals known to science are lead, mercury, cadmium and chromium. None of these are used in pro-oxidant additive for oxo-biodegradable plastics.
The catalyst in the additive is a metal salt, and the actual content of metal in a salt is typically less than 10%. Further, a salt of a material is significantly different to the material itself. Take sodium and chlorine for example. Separately they are very dangerous; but as sodium chloride they are regularly consumed as table salt.
Further, it is appropriate to consider how little material is being used. The additive (masterbatch) is needed at only 1% of the finished film. Within the masterbatch itself the active ingredients amount to less than 10%, and the metal content is significantly less than 10% of that figure!
Even so, extensive research has been carried out on potential eco-toxic effects of particulate and extensively degraded oxo-biodegradable plastics when mixed with soil. Tests included seed germination and plant growth rates, compared with the same soil without degraded plastics [3,4], the effects on macro-organisms (worms, daphnia, etc.) in the soil [4] and on the accumulation of transition metal ions in the stems, leaves and fruit of plants during the growing season [2,5].
With the present range of degradable plastics which incorporate fractions of a percent of transition metal ions, as indicated above, there are no negative effects shown by any of the above tests. It is also recognised in horticulture that fragmented oxo-biodegradable plastic films play a positive role as soil conditioners [6].
The commonly used transition metal compounds in commercial oxo-biodegradable plastics are manganese, iron, cobalt and nickel. As indicated above, none of these are "heavy metals" and none have been shown to be eco-toxic.
Indeed, all the above transition metal ions are required in human and plant nutrition, and are absorbed from foodstuffs and water. Far from being dangerous they are considered to be "essential" minerals required in oxygen transport systems. The non-toxicity of iron, which is present in blood haemoglobin, catalase and peroxidases and of manganese, required for manganese peroxidase, have never been questioned [7].
The UK Food Standards Agency has carried out a risk assessment [8] on trace elements, and the following is a summary of their findings.
High concentrations of cobalt are normally 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. Although cobalt is an essential trace element, Co deficiency has not been reported in humans (presumably because of its widespread availability from food and water).
Gastrointestinal absorption of cobalt depends on the dose. Very low doses are almost completely absorbed, whereas larger doses are less well absorbed. Most excess cobalt is excreted in urine. The only toxicity data for cobalt reported in the literature was in 1960, when heavy beer drinkers suffered cardiomyopathy as a result of the use by the brewing industry of cobalt chloride as a "foam stabiliser" at (1.0-1.5) mg/kg. Ethanol and cobalt have a synergistic effect in reducing blood flow causing damage to the heart. Massive doses of cobalt salts (30 mg/day), evaluated as a treatment for anaemia's led to skin rashes and hot flushes. Prolonged use of cobalt "therapy" led to depression in iodine uptake.
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. Nickel is excreted in urine and in sweat.
Acute nickel exposure is associated with nausea, vomiting abdominal discomfort and diarrhoea. The lowest reported oral dose associated with acute effects of nickel in humans was 1.2 mg in a 60 kg adult. Chronic inhalation of nickel and its compounds is associated with lung cancer in humans and in animals but orally administered nickel was found not to be carcinogenic. It was the exposure of humans to nickel during mining that led to the believe that nickel is carcinogenic however it is imbibed but administration of nickel compounds orally has shown that the main effects in humans is in skin sensitisation but only over 5.6 mg.
From the above, it can be understood how and why the common transition metals are obtained by humans as essential nutrients. It will also be useful when discussing "dangerous substances" in the environment to see how they are absorbed into the food chain from the soil. In fact, the amounts of transition metal ions available to plants from common soils is much higher than can be absorbed by the plants [2] and is very much greater than would be produced from degradable plastics in the soil.
Particular attention has been paid to cobalt and nickel for the reasons discussed above. Volcanic 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 [2]. However, the amount of nickel taken up by the plant appears to have little to do with its concentration in the soil.
Table 1 shows the effect on plant uptake of nickel sulphate applied to the soil to simulate the deposition of nickel from degradable polyethylene mulching films by up to 180 years of application to the same soil [2]. It is clear that the accumulation of nickel in various parts of the plant remains constant within experimental limits, whatever the concentration of nickel in the soil.
Furthermore, 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 [1].
Table 1 - the accumulation of nickel in melons (ppm, measured by atomic absorption) grown in soils containing increasing amounts of nickel sulphate1 [25] Control 60 years 120 years 180 years leaves 17.3 15.2 13.5 13.7 stems 5.0 4.5 5.2 5.0 flesh 2.7 2.0 3.0 3.2 skin 3.0 3.5 3.2 3.0
1The soil was sprayed with NiSO4 to give nickel concentrations in the topsoil equivalent to the accumulation from S-G mulching films used for the number of years indicated.
ConclusionSynthetic hydrocarbon polymers (e.g. polyolefins, polystyrene and synthetic rubbers) biodegrade in the environment by the same abiotic and biotic processes as naturally occurring polymers (e.g. natural rubber, resins and lignin).
In the case of conventional commercial plastics the rates of formation of oxidation products depend on the presence of pro-oxidant transition metal ions (Mn, Fe, Co, Ni) and commercial antioxidants. The lifetimes and hence biodegradation times of commercial biodegradable polyolefins may vary by orders of magnitude depending on the application. The time-scale from the end of the user life to final conversion in the environment to carbon dioxide, water and biomass lies within the range of many natural product wastes such as straw and related lingo-cellulosic materials.
The oxidation products of both natural and synthetic hydrocarbon polymers biodegrade rapidly and are absorbed by microbial cells. Consequently, abiotic or biotic oxidation is normally rate controlling and there is no accumulation of low molar mass products in the environment.
The rates of abiotic peroxidation of carbon-chain polymers in the environment can be predicted from laboratory tests and it has been shown that the biodegradation rates of the oxidation products correlate with mass loss of the polymer. The purpose of eco-toxicological tests is to ensure that neither the fragmented polymers nor their oxidative breakdown products have an adverse effect on plants or to humans and animals that may consume the crops.
It is clear from the published work that the four usual pro-oxidant transition metal ions (Mn, Fe, Co and Ni) are no more toxic in the environment than the abundant naturally occurring minerals and that on the contrary both are the source of essential elements for human nutrition.
References;-
[1] D.Gilead in Degradable Polymers: Principles and Applications, 1st Edition, Editors, G.Scott and D.Gilead, Chapman & Hall (Kluwer), 1995, Chapter 10.
[2] A.Fabbri in Degradable Polymers: Principles and Applications, 1st Edition, Editors, G.Scott and D.Gilead, Chapman & Hall (Kluwer), 1995, Chapter 11.
[3] S-R.Yang and C-h.Wu in Degradability, Renewability and Recycling, 5th International Scientific Workshop on biodegradable Plastics and Polymers, Macromolecular Symposia 144, Eds., A-C. Albertsson, J. Feijen, G. Scott and M. Vert., Wiley-VCH, 1999, 101-112.
[4] D.M. Wiles, J-F. Tung, B.E. Cermak, C. W. J. Hare and, J.G. Gho, Proceedings of the Biodegradable Plastics 2000 Conference, Frankfurt, June 6 & 7 (1990).
[5] G.Cassalicchio, A.Bretoluza and A.Fabbri, Plasticulture, 86, 21-28 (1990).
[6] H.O.W.Eggins, J.Mills, A.Holt and G.Scott in Microbial Aspects of Pollution, Editors, G.Sykes and F.A. Skinner, Academic Press, 1971, pp 267-277.
[7] G. Scott, Antioxidants in science, technology, medicine and nutrition, Albion Publishing, 1997
[8] Food Standards Agency Expert Group on Vitamins and Minerals (2003), Risk Assessment.