保水剂在尿素和阳离子溶液中的吸水性能及养分吸附特征

【目的】电解质种类和浓度影响保水剂的吸水性能。探明保水剂与尿素、不同价态阳离子间的相互作用，对于正确使用和开发水肥调控性能优越的缓释材料具有重要意义。【方法】试验选用聚丙烯酰胺–丙烯酸盐型保水剂P(AA-AM)]和聚丙烯酸盐型保水剂 (PAA)。以分析纯盐酸盐和尿素分别配置了浓度1～256 mmol/L的尿素、NH₄+、Na+、K+、Mg2+、Ca2+、Fe3+、Al3+溶液。称取保水剂0.3000 g于75 μm尼龙袋中，分别置于以上各系列溶液和去离子水300 mL中，静置12 h。取出保水剂袋，沥水30 min，以差减法测定保水剂吸水倍率和对溶质的吸附量。【结果】在溶质浓度从1 mmol/L增加到256 mmol/L时，尿素溶液中保水剂的吸水倍率没有显著下降，而各阳离子溶液中保水剂的吸水倍率显著下降，保水剂的相对吸水倍率与尿素、各种离子浓度之间呈显著幂函数的减函数关系，最终下降幅度由小到大依次为尿素、一价阳离子、二价阳离子、三价阳离子。保水剂对溶液中尿素和各种阳离子的吸附量随溶液浓度的增加而增加，P(AA-AM) 的最大吸附量:尿素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、Al3+ 12.74 mmol/g。保水剂对尿素和一、二价阳离子的吸附特征可用Freundlich等温吸附模型表征，对三价阳离子的吸附特征可用Langmuir等温吸附模型表征。尿素和二、三价阳离子对P (AA-AM) 吸水倍率的影响均小于其对PAA的影响（P < 0.05），一价阳离子对P (AA-AM) 和PAA影响差异不明显（P > 0.05）。P (AA-AM) 吸附尿素和阳离子的能力要大于 PAA（P＜0.05）。【结论】保水剂对尿素的吸附量远高于阳离子，而尿素浓度基本不影响保水剂的保水性能。三价、二价和一价阳离子可显著降低保水剂的吸水倍率，阳离子浓度与保水剂的吸水倍率之间呈幂函数关系。保水剂对一价阳离子的最大吸附量大于对二价阳离子的吸附量，也远大于对三价阳离子的吸附量。聚丙烯酰胺–丙烯酸盐型保水剂P(AA-AM)]对溶质的吸附性能大于聚丙烯酸盐型保水剂 (PAA)。因此，把保水剂作为缓释包膜材料包裹尿素比较适宜，但不适宜包裹盐类肥料。

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.