Preparation, swelling, and slow-release characteristics of superabsorbent composite containing sodium humate

A series of multifunctional poly(acrylic acid-co-acrylamide)/Ca-montmorillonite/sodium humate superabsorbent composites with slow-release sodium humate were prepared by graft copolymerization reaction with partially neutralized acrylic acid (AA), acrylamide (AM), Ca-montmorillonite (Ca-MMT), and sodium humate (SH) in aqueous solution. The effects of SH and Ca-MMT contents on water absorbency were studied, and the highest water absorbency was obtained when the SH-to-MMT ratio was 3:2 (total amount of SH and Ca-MMT in the feed was fixed at 50 wt %). The swelling kinetics in distilled water and swelling behaviors in various pH solutions for the developed composites were systematically investigated. A two-stage release characteristic of SH in distilled water was observed, and the differences before and after SH release were characterized by FTIR, XRD, SEM, and TGA. With increasing SH and Ca-MMT contents, water absorbency differed, suggesting that SH was mainly chemically bonded with the polymer matrix, whereas Ca-MMT particles were mainly physically filled in the polymeric network.