This class of polymer is also referred to as superabsorbent polymer(SAP).
Disposable diapers/nappies are likely to be the sodium form. Sodium forms are typically cheaper. Further, the potassium form of the salt is not as soft and would be less suitable for cloth-like wearables.
Polyacrylate SAP does not biodegrade in soil very fast.
For the main chain polymer:
...if at all, 0.12-0.24 % per 6 months.
1: A manufacturer of agricultural/hygiene polyacrylate SAP
2: Wilske, B.; Bai, M.; Lindenstruth, B.; Bach, M.; Rezaie, Z.; Frede, H.-G.; Breuer, L. Biodegradability of a polyacrylate superabsorbent in agricultural soil. Environmental Science and Pollution Research 2013, 21 (16), 9453–9460.Springer
Side note: All diapers with an absorbent layer are SAP, either polyacrylate or polysaccharide (rarely). Polysaccharide is the more expensive / less effective 'green' alternative, and if your product contains this, it will tout it on the label.
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