Post-Recycling Sodium Polyacrylate with Nanostructural Halloysite Additive as the Basic Components of the New Hybrid Soil Modifier—The Circular Economy Rules for SAP in Agriculture

Post-recycling sodium polyacrylate (SPA) derived from the selective recycling of personal hygiene products was tested as a potential key component of soil modifiers. Recycled SPA in combination with layered aluminosilicate (halloysite) with the sorption capacity of fertilizers allows for the creation of a fully functional, original system responsible for both water retention in soil and controlled dosing of fertilizers. The use of post-recycling SPA (possibly partially supplemented with virgin SPA) is advantageous for the natural environment. The results of different water types retention in post-recycling SPA (deionized water, rainwater, tap water) are discussed. The most beneficial effects of R-SPA swelling and water accumulation are achieved when it comes into contact with rainwater. The clear impact of Mg²⁺ ions on the sorption capacity of R-SPA introduces the need for a technological compromise between the solution composition (N, P, K or N, P, K, Mg) affecting its fertilization value and the possibility of obtaining maximum R-SPA swelling. The sorption capacity of R-SPA can be slightly increased by using R-SPA/V-SPA mixtures, but R-SPA still plays a decisive role in this system. The possibility of technological adjustment of the leaching rate of fertilizer components under dynamic conditions using different combinations of (R-SPA/V-SPA)/HAL was demonstrated by column tests.