Well, this is an opinion from the Internet, so it's worth what you paid for it, or perhaps considerably less, but here goes ...
My DW uses a water absorbing/releasing gel in flower pots. I'm guessing it's probably sodium polyacrylate based. We keep the soil separate and it's our preference to not use it in our tomato growing barrels.
But really, I'm not finding anything about sodium polyacrylate that would personally stop me from using those totes for garden water. It has such an immense affinity for water that filling it and letting it sit for a while should remove most traces, and I can't see how it would seep into the structure of the poly the way some chemicals do.
If I were being extra careful, I would perhaps wash them out with a soap or TSP solution, and discard the first filling after it has sat for a couple of weeks.
That's my 2 cents' worth.
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