Abstract:

In the process when inlet bulk solution with low or medium concentration was involved, the scaling was apt to appeared on the nanofiltration (NF) membrane surface in the retentate side. Therefore, the study of surface electrokinetic phenomena of NF membrane termed as DK membrane under threshold concentration condition is very important. In this paper, the surface charge properties of DK nanofiltration membrane were studied using streaming potential method, and a series of two-component ternary electrolyte solutions (K₂SO₄+CaCl₂) with the molar ratio of 1:1 were involved as the study objects. Using the home-made tangential streaming potential test device, DK membrane was characterized with streaming potential (E_s), Zeta (ζ) potential and other measurements encountering with a wider scale-prone ionic concentration (0.5-18 mol·m^-3), operating pressure (25-200 kPa) and feed pH (3.0-10.0). The results showed that at operating pressure of 180 kPa, with the scale-prone ionic concentration of feed solution ranged from 0.5 to 18 mol·m^-3, it turned out E_s and ζ -potential values increased from -87.37 and -59.13 mV to -3.41 and -36.34 mV, respectively. It becomes less negative and tends to be neutral due to electrostatic charge shielding and compression of the diffuse layer. But the calculated surface charge density (σ) value always increased from 13.94 to 34.86 mC·m^-2 under the same operating conditions. It could be inferred that under the condition of non-alkaline sparingly soluble salt CaSO₄ occurring in brine side, there are still some surface electrokinetic phenomena with low or medium level on NF membrane surface.

Key words: DK nanofiltration membrane, boundary layer, threshold concentration, electrokinetic phenomenon, Zeta potential

摘要：

荷电纳滤（NF）膜在相似浓度范围的进水端主体溶液软化过程中，浓水端的膜面处易出现结垢污染，因此开展临阈浓度条件下膜表面动电现象研究是十分必要的。本文选定DK型NF膜和c_K2SO4：c_CaCl2=1：1的双组分溶液为载体，以CaSO₄·0.5H₂O为典型代表的非碱性难溶垢为研究对象，通过自制的切向流动电位测试系统，分别考察临阈浓度条件下，进水端主体溶液中不同构晶离子浓度（0.5～18 mol·m^-3）、操作压力（25～200 kPa）和不同pH（3.0～10.0）对该NF膜流动电位、Zeta电位和膜表面电荷密度的影响效应。当操作压力恒定为180 kPa且逐渐增加构晶离子浓度（0.5～18 mol·m^-3）时，DK膜面流动电位和Zeta电位绝对值分别从87.37和59.13 mV下降至3.41和36.34 mV，均呈先急剧后缓慢下降的变化趋势。这可能是由于静电屏蔽和扩散层压缩共同作用的结果。而同时膜面电荷密度从13.94 mC·m^-2逐渐增加至34.86 mC·m^-2。研究表明，当NF浓水端膜面处开始出现结垢时，膜表面仍有中低强度的动电现象。

关键词: DK纳滤膜, 边界层, 临阈浓度, 动电现象, Zeta电位