Change of the point of zero net proton charge (pHPZNPC) of clay minerals with ionic strength

In a system where protons and hydroxide ions are the only aqueous species, the point of zero net proton charge (pH(PZNPC)) of a mineral defines the pH at which the positively and negatively charged functional groups on its surface are equal (Drever, 1997). Ascertaining the pH(PZNPC) of clay minerals, a ubiquitous component in soils, sediment and rivers, is useful in predicting its electrostatic interactions with charged aqueous species, colloids, and bacteria. While the pH(PZNPC) values of most clays have been reported, the variation of pH(PZNPC) with changing solution ionic strength (IS) and the mathematic relationship between them are not well-understood but critical in assessing the surface reactivity of clays in aqueous solutions ranging from freshwater to brines. To address this gap, we studied the relationship of the pH(PZNPC) of three clay minerals (kaolinite, illite, and montmorillonite) at seven ionic strengths (from 0.001 to 0.1 M). Titration data for each clay were used to calculate the pH(PZNPC) by two methods previously documented in literature: (1) using the difference between the blank titration and clay titrations, and (2) by the difference between the number of protons added during titration and the number of protons remaining in solution. The results show that: (1) unlike simple metal oxides (e.g., hematite, gibbsite, quartz), titration curves of clay minerals at different IS do not intersect at a common pH(PZNPC) value; (2) the pH(PZNPC) for kaolinite is around 5.6 to 6.6 while the pH(PZNPC) for illite and montmorillonite is in the range of 9 to10; (3) the pH(PZNPC) value decreases systematically with increasing IS for all three clay minerals studied; and (4) the change of pH(PZNPC) is linear with log(IS). A third method using surface complexation modeling (SCM) approach was applied to calculate the pH(PZNPC) of the clay minerals, and the results match well with Method 2. Our results allow for a more accurate estimation of clay surface charge property in aqueous environment, which, in turn, will improve model predictions of the adsorption of charged species in systems in which ionic strength changes.