Dear Dave, 

As I can see you're from South Africa. The water hyacinth ( https://en.wikipedia.org/wiki/Eichhornia_crassipes ) is famous about its Arsenic and heavy metal removal capacity- but this is a plant originated from South-America, and considered to be invasive in Africa, so it is not really applicable in your case. 

I would either try to pilot a floating bed wetland of water hyacinth with the plants contained as much as possible (using barriers and filtration methods to prevent invasion) , or try to piloting algae as this study suggests ( https://link.springer.com/article/10.1007/s13201-015-0300-4 ). 

My other way of approach would be setting up a flow-through LECA hydroculture of land originated Arsenic-accumulating species like  Isatis cappadocica, or more likely Pteris vittata , which is native in South-Africa. 

We have an excellent partner to work with in SA, a horticultural expert, helping us procuring very obscure local plants. If you would like to contact him, please send me a private message. 

Sincerely, 

László