Clean fresh water is a scarce resource. Areas of the world suffering from drought have to filter the salt out of seawater to make it drinkable. In other areas, the water may instead have a high content of toxic compounds, such as arsenic.
If you think about a water filter as a kind of strainer with tiny holes through it, you would assume that since it does a pretty good job of filtering out the small ions of normal table salt, sodium, and chloride, from seawater it would work even better for the larger arsenic compounds. This is however not the case when it comes to desalination – the technology for producing fresh water from seawater; quite the opposite actually. While sodium and chloride are removed effectively, other, much larger contaminants pass through the filtration materials that are typically used. That indicates there must be another mechanism at work here.
