As a final sidenote on the above, to say that halloysite is out...

As a final sidenote on the above, to say that halloysite is out of the picture for phosphate removal because haematite, for example, adsorbs multiples more phosphate than halloysite would also be a bit off.
As seems to be coming clear, it is halloysite plus something that transforms halloysite towards the supermaterial bracket. Halloysite is a team player!! For example, with relevance to phosphate removal, it is halloysite combined with lanthanum oxycarbonate that wipes the floor with other products for phosphate adsorption ( Weir et al, 2019). I think I am correct in stating that whilst haematite is approx ten times more effective than halloysite, halloysite plus lanthanum oxycarbonate is then somewhat over five times as effective as haematite in phosphate adsorption.