The chemical EDTA is used in a range of products, including personal care products. Because EDTA is been used so widely and is not readily biodegradable, it is now a major pollutant in many urban waterways. This is a concern because EDTA binds strongly to many metals.
Heavy metals, such as lead and mercury, are present in many urban environments. A lot of the time, the metals are bound up in complexes that cannot be dissolved in water. Luckily, this helps to minimise exposure. However, EDTA can help heavy metals to dissolve in water. When heavy metals are dissolved, it is easier to absorb them and drink them. Plants and animals can be exposed to the heavy metals and their toxic effects when this happens.
Even if the amount of heavy metals dissolved is low, the extra exposure can be a problem through bioaccumulation. An animal at the top of the food chain will not only be exposed to heavy metals in their drinking water (and the water they live in, if it is an aquatic animal), but also any heavy metals that the animals and plants that they eat have absorbed.
Other metals are important nutrients in the environment. For example, iron is needed for algae growth. As for heavy metals, EDTA can help more iron dissolve in the water. While this is great for the algae, it can create an algae bloom and put the broader ecosystem out of balance. In the worst cases, the extra algae uses up more oxygen from the water which can suffocate aquatic animals.
The toxicity of EDTA itself can also differ depending on what sort of metal it is bound to. Some studies have found that EDTA is more toxic to plants when it is not bound to a metal, compared to when it is bound to iron. Other studies have found that the toxicity of the metal can change when it is bound to EDTA. For example, copper can be more toxic to aquatic invertebrates when it is bound to EDTA (and not just because EDTA helps to dissolve it). These factors can make it difficult to predict exactly what the impact of EDTA in the environmental will be.