Zhong-Yi Li1,2 Ren-Kou Xu1 Jiu-Yu Li1 Zhi-Neng Hong1

ELECTRONIC SUPPLEMENTARY MATERIAL

soils, sec 2 • global change, environ risk assess, sustainable land use • research article

Effect of clay colloids on the zeta potential of Fe/Al oxide-coated quartz: a streaming potential study

Zhong-yi Li1,2 Ren-kou Xu1Jiu-yu Li1Zhi-neng Hong1

Received: 17 March 2016 / Accepted: 19 May 2016

Responsible editor: Huijun Zhao

1State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, P.O. Box 821, Nanjing, China

2University of Chinese Academy of Sciences, Beijing 100049, China

Ren-kou Xu

Theoretical basis of streaming potential method

Streaming potential producedin a pile of Fe/Al oxide-coated quartz in NaCl solution is illustrated in Fig.S2. When electrolyte solution flows through the pore space of grains at a velocity v under the driving force ΔP, the movement of Na+ and Cl– in the pores constitutes a streaming current, Is. Anions accumulate along the flow direction because of the electrostatic attractive force of the coated quartz. Accordingly, there is a charge difference between the ends of grains and an electric potential, termed the streaming potential (ΔE), is produced. The streaming potential causes the conduction current (Ic) which flows in the opposite direction toIs. When a steady state is achieved, Ic = Is, which can combine the Poiseuille’s equation, Poisson’s equation and Ohm’s law together. Then, the zeta potential (ζ) of Fe/Al oxide-coated quartz can be calculated by streaming potential measurement (Hunter 1981):

(1)

Whereε is the dielectric constant of the medium;η is the viscosityof the medium;RL is the total electric resistance of the pore space filled with the electrolyte used for sample measurement, KHL is the electric conductivity of highly concentrated electrolyte and RHLis the total electric resistance of the pore space with highly concentrated electrolyte. It assumes that the surface conductance of quartz is suppressed at sufficiently high electrolyte concentration. So, KHLRHL is used to adjust the formation factor of the measuring cell after grain loading. The high concentration of electrolyte used in the present study was 0.1 M potassium chloride (KCl) (Elimelech et al. 1994).The electrolyte conductivity of 0.1 KCl was measured by EC 215 conductivity meter (HANNA Instruments, Italy).

When NaCl solution contains colloidal clay particles, such as kaolinite or montmorillonite (Fig.S2), there is an interaction between clay colloids and Fe/Al oxide-coated quartz. A proportion of the colloids will deposit on the surface of grains because of the electrostatic attraction force or physical straining and the surface properties of the coated quartz are directlychanged. The remainder of the colloids will not deposit on the surface of grains but continue to flow through it under the action of the ΔP.The electrostatic attraction force of oppositely charged particles can induce the overlapping of the diffuse layers of EDLs between the clay particles and the coated quartz. The overlapping of these diffuse layers can affect the distribution of ions in the EDL and change the electrochemical properties of the coated quartz. Regardless of whether there is deposition of clay particles, the clay particles affect the electrochemical properties of the coated quartz. Thus, the streaming potential is changed, since the variation of electrochemical properties on the surface of Fe/Al oxide-coated quartz changes the ions’ distribution in pore space. After reaching dynamic equilibrium between clay particles and coated quartz, the streaming current, conduction current and streaming potential are Is', Ic' and ΔE', respectively (Fig.S2). The apparent zeta potential (ζ') of Fe/Al oxide-coated quartz in contact with clay suspensions can also be calculated using Eq (1). Then, the change of zeta potential (Δζ=ζ –ζ') at the surface of grain in electrolyte and in clay suspensions is obtained. Therefore, streaming potential measurement is a theoretically useful tool to characterize the effect of micro-particle on the electrochemical properties of large-sizematerial in situ.

Figures

Fig. S1 Schematic of transport of micro-particles in the soil pore space with the movement of soil solution

Fig.S2 A sketch of the streaming potential formed by a pile of Fe/Al oxide-coated quartz in NaCl solution without (a) or with (b) colloidal clay particles

Fig. S3 Schematic representation of streaming potential apparatus

Fig.S4 The X-ray diffraction spectra of Fe oxide-coated and Al oxide-coated quartz

Fig.S5 The X-ray diffraction spectra of kaolinite and montmorillonite

References

Elimelech M, Chen WH, Waypa JJ (1994) Measuring the zeta (electrokinetic) potential of reverse osmosis membranes by a streaming potential analyzer. Desalination 95:269286

Hunter RJ (1981) Zeta Potential in Colloid Science: Principles and Applications. Academic press, London