Influences of cation valence on water absorbency of crosslinked carboxymethyl cellulose

Anti-drought is a global challenge. The addition of superabsorbent polymers (SAPs) in soil can lower down the water percolation and evaporation. However, the salt-tolerance and repeating water absorbency (RWA) of prepared SAPs have not satisfied the requirements for implementation. This research investigated the influence of cation valence (Na⁺, Ca²⁺ and Al³⁺) on the structural variations of a crosslinked carboxymethyl cellulose (CCMC) using carboxymethyl cellulose (CMC) cross linked by epichlorohydrin (ECH). The results showed that higher addition of NaOH resulted in higher water absorbency (WA) due to the existence of more carboxyl group. The prepared CCMC sample with 5% CMC and 3% NaOH (CCMC₅₃) was a qualified SAP with WA of 969.0 g/g in deionized water. The salt resistance and the hydrophilicity of sample CCMC₅₃ decreased with the increase of cation valence in the solution. The introduction of Na⁺ resulted in the replacement of H⁺ from carboxyl group in sample CCMC₅₃-Na. The coordination of carboxyl group and Ca²⁺ was bidentate chelating and tridentate bridging for carboxyl group and Al³⁺. The introduction polyvalent cations benefited the stabilization of carboxyl group, however, retarded the swelling ability of sample CCMC₅₃ and hence resulted in lower WA_CS. The RWA of sample CCMC₅₃ in deionized water and salt solution dropped the most in the first absorption cycle and then reached constant after a few more cycles. It was necessary to control the first swelling degree of SAP in order to keep the RWA at a higher level.