[defrost]

Dull speculation is more on the nail.

Do you know how many billions of dollars worth of platinum group elements (platinum; Pt, palladium; Pd and rhodium; Rh) are literally lying on the roads and roadsides of G20 countries?

As dust from in use catalytic converters it's substantial. And unrecovered.

There's a problem with economic feasibilty - extraction costs (digging up and processing every road surface and road side) are prohibitive - greater than the value of the material.

From your [2]

    The advance is still not likely cheap enough to compete with mining lithium on land, 

In a nutshell this is still very much the issue with mining seawater for Lithium, Uranium, etc.

The 0.2 parts per million (PPM) is a problem, a literal ceiling on diminishing returns, the more you extract the less remains and it's tricky to keep processed seawater from mixing back with unprocessed.