So far as I know, you want to use electrolysis on fairly saturated clean solutions to obtain good electrical efficiency of the process. I only know that electrolysis of PGMs solutions is fairly unusable for separate one from another in general. But with 0,5g/L PGMs, the current efficiency will be very poor, producing lots of oxidizing gases arising from electrolysis to obtain some sane current densities (my opinion).
But for scavenging them... Maybe it will be simpler to just cement them out on something.
As you named Na, Ca and Al already in solution... Why not to use iron/aluminium for cementation ?
I maybe see the point of not creating pools of liquid waste, but if there is no other contaminant such as copper or other reducible metals like nickel or tin, why not go with bulk iron sheet or bulk aluminium ingot to precipitate them out ? Even if some copper/tin/nickel etc is present, you will remove it from the huge ammount of liquid, which ease the disposal process issues. And concentrate the values into something very compact.
Ready to melt and purify by electrolysis. Or dissolved very easily (very fine powder created) in AR and processed with hydrometallurgy.
For that small ammount of PGMs, there will be no big consumption of metal anyway. With aluminium, there will be beter charge-mass ratio for cementation = meaning much less weight of aluminium will be dissolved, than eg iron for the same purpose. And with pH around 1, HCl isn´t particularly strong (I doubt that it is stronger than 1%, but it could be measured by titration), so reaction of HCl with eg iron won´t be fast. Good agitation of the liquid with pump or mixer will speed the process quite a bit.
Minimal unnecessary contamination with iron will happen, and acidity will go way down (closer to fulfill the enviromental/official measures of what you can safely pour down the drain/river) after treatment. Maybe even beneficial to do it like this
