Water absorbing capacity and nutrients adsorption characterization of super absorbent polymer in the presence of urea or cations

Objectives Water absorbing capacity of super absorbent polymer (SAP) is impacted by the type and concentration of co-existing electrolyte ions. Study of the interaction between SAP and urea or cations will help the correct choice of SAPs when use them in the coating materials of fertilizers.

Methods Poly acrylate-co-acrylamide P(AA-AM) and polyacrylate (PAA) were used as the tested SAPs in this study. 1–256 mmol/L serious cation (urea) solutions were prepared using analytical purity grade urea, NH₄Cl, NaCl, KCl, MgCl₂, CaCl₂, FeCl₃ and AlCl₃ individually. 0.3000 g of the SPAs into nylon bags (75 μm) were put into 300 mL of above solutions and deionized water and stood for 12 h. Then take the bags out of the solution and dried at air for 30 min. Difference subtraction methods were used to calculate the relative absorbance capacity to water and all the tested solutes.

Results When the concentration of solute was increased from 1 to 256 mmol/L, the water absorbency of SAPs in urea solution was not decreased obviously, while in all the cation solutions decreased sharply. The relationship between the relative water absorbency of SAPs and the concentrations of cations could be fitted by power function. The decrease effects on water absorption capacity was in order of urea,monovalent catioans(NH₄⁺, Na⁺, K⁺) bivalent cations(Mg²⁺, Ca²⁺) and trivalent cations(Fe³⁺, Al³⁺). With the increase of solute concentration, the absorbed amount of solutes increased. The largest absorption of P(AA-AM) was urea 64.66 mmol/g, Na⁺ 45.24 mmol/g, K⁺ 34.26 mmol/g, NH₄⁺ 32.63 mmol/g, Mg²⁺ 30.09 mmol/g, Ca²⁺ 23.96 mmol/g, Fe³⁺ 8.07 mmol/g and Al³⁺ 12.74 mmol/g. The adsorption of SAPs to urea, monovalent and bivalent cations could be well described with Freundlich isotherm model, and that to trivalent cations with Langmuir isotherm model. The impaction of monovalent cations on the water absorbency of P (AA-AM) and PAA was not significantly different, while the impaction of divalent and trivalent cations on P(AA-AM) was less than that on PAA (P < 0.05). The adsorption capacity of P (AA-AM) for urea and cations was higher than that of PAA (P < 0.05).

Conclusions The adsorption capacity of SAPs to urea is much higher than that to cation, and the concentration of urea does not affect the water retention property of SAPs. The maximum adorption capacity of SAPs to monovalent cation is larger than that of divalent cation and also much larger than that of trivalent cation. Cations can significantly reduce the water absobency of SAPs, and the relationship between the concentration of cation and the water absobency of SAPs is a power function. Both the tolerance and adsorption of P(AA-AM) to cations are slightly higher than those of PAA. Generally, SPAs are suitable to be added into the coating material for urea, and not for salt-based fertilizers.