![]() ![]() Parts per million (ppm), parts per billion (ppb) and parts per trillion (ppt) – or mg/L, ug/L and ng/L – are the commonly used terms to describe very small amounts of substances. Some chemicals can cause toxicity at very low doses, so it is important to understand how low doses compare to one another. The toxicity of a chemical is all in the dose – scientists have different ways to measure toxicity. Taonga fish species such as tarore (sole), kahawai and pātiki (flounder) have shown higher sensitivity when compared to their similar species in Australia and North America. In the marine space, sea anemones have the highest sensitivity to zinc of those species tested, with reproduction rate impairment observed.In freshwater, water fleas are the most sensitive species to zinc among those tested, followed by green microalgae, fish (salmon and brown trout), crustaceans and aquatic snails.We also know a lot about how this toxicity varies, including how variable it is across organisms and different ecosystems and geographical regions. The toxicity of zinc to flora and fauna is well studied and understood. This means it has the potential to accumulate in receiving environments such as soils, sediments and organisms. When the organic compound of the pesticide degrades, the zinc – which does not degrade – gets released. It is also found in some multipurpose plant pesticides. This is because zinc is incorporated into agricultural products such as drenches and sheep dip to treat facial eczema. In rural areas, zinc-based pesticides and animal medicines are key sources of zinc entering soil and surface water. Further urban sources of zinc are treated effluents and sludges from wastewater treatment plants. Most of the zinc found in run-off is in a dissolved form (zinc ions), which is the form most available to organisms. When it rains enough to create run-off, traces of zinc from these sources get washed out and enter the environment through stormwater run-off. The main sources of zinc in urban areas are worn tyres, painted buildings and galvanised and coated roofs. So how does zinc move from where it’s meant to be into our environment? However, when the pH is below 5.5, zinc may become more available to plants and microorganisms tends. Soil particles with a pH of 5 or higher tend to bind with zinc, resulting in low mobility of zinc. In soils, zinc can be distributed in multiple ways – it can bind to soil particles or porewater (the water between soil particles).In acidic waters where the pH is low, zinc tends to dissolve and be in a free form, making it more available to aquatic organisms such as fish. In water, the pH among other factors will determine if zinc is in a freely dissolved form or is attached to sediment.Once released in the environment, zinc behaves in different ways depending on the characteristics of the environment: The form of zinc will determine its toxicity, with free zinc ions (Zn 2+) being the most available and toxic form to organisms. It can, however, take different forms (free zinc ions, zinc hydroxide or zinc sulphate) depending on environmental conditions. Zinc is a persistent element, this means it does not degrade or break down into smaller molecules called metabolites. This includes changes in plant growth and photosynthesis, and cellular damage and reproduction problems in animals. This can result in plants and animals obtaining an altered intake of other nutrients, which can have different impacts. At high concentrations, zinc can also outcompete other essential elements (such as calcium and copper) in biological processes. Environmental fate and exposure to non-target organismsĭespite its natural origin and beneficial properties, zinc can become toxic to organisms at certain doses. Zinc is also used in sunscreens, human and veterinary medications and health supplements, cosmetics, wood preservatives, paint pigments and anti-fouling paints, fertilisers, household products and as an agent in rubber vulcanisation, a process used by the tyre industry. In 2017, worldwide use of zinc was over 14 million metric tonnes! Its main use is in the steel industry where it is used (sometimes with other metals to form alloys) as a layer to prevent oxidation and rust. Humans use a lot of zinc – it is the fourth most widely used metal after iron, aluminium and copper. ![]()
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