测量模拟土体zeta电位的简易流动电位装置及其使用方法

An Improved Apparatus Driven by Gravity for Streaming Potential Measurement of Simulated Soil

本文展示了一款可应用于土壤学领域研究的简易流动电位测量装置。为验证该装置的可行性,采用石英砂和包铁石英砂模拟土体所具有的多孔结构和表面电荷特征,基于流动电位法测量了石英砂在不同浓度(0.01、0.05、0.1、0.5、1.0、2.0和5.0 mmol·L^(-1))NaCl溶液中的zeta电位和石英砂与包铁石英砂在不同pH(4.4、5.2、6.1、6.6、6.9、7.7和8.0)电解质溶液中的zeta电位。同时采用电泳法测量了石英砂胶体在不同pH电解质溶液的zeta电位以作比较。结果表明:在考虑表面电导时,流动电位法测得石英砂在不同浓度NaCl溶液中的zeta电位随溶液浓度的升高逐渐往正值方向移动。这是由于随着离子强度增加,石英砂的双电层厚度受到压缩,zeta电位的绝对值逐渐降低;相同离子强度下,随溶液pH升高,石英砂和包铁石英砂表面官能团发生去质子化作用,流动电位法测得石英砂和包铁石英砂的zeta电位均随pH升高而逐渐降低;由于电荷屏蔽作用,在带负电荷的石英砂表面包被带正电荷的氢氧化铁后,流动电位法测得包铁石英砂的等电点pH(IEP)介于石英砂和氢氧化铁的IEP。此外,流动电位法测得的石英砂颗粒在不同pH电解质溶液中的zeta电位与电泳法测得的石英砂胶体在不同pH电解质溶液中的zeta电位具有较好的一致性。可见采用自制流动电位装置所获结果均与理论预测及常规电泳法测定结果相符,准确度较高。自制流动电位装置结构简单、操作方便、加工制造容易。该装置可被土壤学领域的研究者借鉴参考,以望为土壤电化学研究工作做出贡献。

【Objective】Theoretically,surface electrochemical properties of bulk soil can be well characterized by streaming potential measurement.Since the streaming potential measurement is usually adopted to measure the zeta potential of charged solid with large size(e.g.,bulk soil).Nevertheless,commercial streaming potential apparatus is cumbersome to operate and expensive.This limits the application and development of streaming potential measurement in soil research.【Method】In this study,a laboratory-made streaming potential apparatus was developed to measure the zeta potential of simulated bulk soil.Packed quartz grains and Fe oxide-coated quartz grains column were used to mimic the porous structure and surface electrochemical characteristics of bulk soil.The zeta potentials of quartz grains in NaCl electrolytes with different ionic strength(0.01,0.05,0.1,0.5,1.0,2.0,and 5.0 mmol·L^(-1))were measured by the streaming potential measurement.Also,using this self-made streaming potential apparatus,we measured the zeta potentials of quartz grains and Fe oxide-coated quartz grains in electrolytes with same ionic strength(20μS·cm^(-1))but different pHs(4.4,5.2,6.1,6.6,6.9,7.7,and 8.0).In addition,the zeta potentials of colloidal quartz in different pH’s electrolytes measured by traditional electrophoresis measurement was used to verify the accuracy of the streaming potential measurement.【Result】The results show that,with increasing ionic strength,the zeta potential of quartz grains measured by steaming potential measurement became less negative due to compression of the diffusion layer of the electric double layers on charged solid surfaces at higher electrolyte concentrations.Similarly,with increasing electrolyte pH,the zeta potentials of both quartz and Fe oxide-coated quartz grains measured by steaming potential measurement decreased because of the greater deprotonation effect of functional groups on the surface of quartz and Fe oxide-coated quartz grains.When Fe oxide was adsorbed on the quartz surface,the zeta potential of Fe oxide-coated quartz grains was higher than that of uncoated quartz grains.This was because of charge neutralization between positively charged Fe oxide and negatively charged quartz grains.The measured IEP(isoelectric point)of the Fe oxide-coated quartz grains was between the IEPs of quartz grains and Fe hydroxide.There was a good consistency between the zeta potentials of quartz grains measured by streaming potential and the zeta potentials of quartz colloids measured by electrophoresis measurement.【Conclusion】The results obtained by using the laboratory-made streaming potential measurement apparatus are consistent with the theoretical prediction and electrophoresis measurement.Also,the structure of the laboratory-made streaming potential apparatus is simple,its operation is convenient,and the cost to build this apparatus is low compared to commercial apparatus.Thus,the streaming potential measuring apparatus used in this study can be used as a powerful tool to conduct studies in the field of soil electrochemistry.