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In a system where protons and hydroxide ions are the only aqueous species, the point of zero net proton charge (pHPZNPC) of a mineral defines the pH at which the positively and negatively charged functional groups on its surface are equal (Drever, 1997). Ascertaining the pHPZNPC 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 pHPZNPC values of most clays have been reported, the variation of pHPZNPC 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 pHPZNPC 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 pHPZNPC 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 pHPZNPC value; (2) the pHPZNPC for kaolinite is around 5.6 to 6.6 while the pHPZNPC for illite and montmorillonite is in the range of 9 to10; (3) the pHPZNPC value decreases systematically with increasing IS for all three clay minerals studied; and (4) the change of pHPZNPC is linear with log(IS). A third method using surface complexation modeling (SCM) approach was applied to calculate the pHPZNPC 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.

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Given the high surface reactivity of clay minerals, it is assumed that flocculation will lead to metal accumulation in marginal marine settings. However, the degree of metal sorption to clays is impacted by solution pH and ionic strength, and it remains unknown whether riverine clays indeed serve as a metal sink once they encounter seawater where pH and ionic strength markedly increase. Here, we conducted cadmium (Cd) adsorption experiments to three types of common clay minerals – kaolinite, illite and montmorillonite. We found that 20–30% of Cd from illite and montmorillonite surfaces were desorbed when transitioning from freshwater to seawater pH and ionic strength conditions, while kaolinite showed no discernible differences. Synchrotron X-ray adsorption spectroscopy confirmed that Cd release corresponded to a change in bonding from outer- to inner-sphere complexes when clays encountered seawater pH and ionic strength conditions. If other trace nutrients (such as Cu, Zn, Co) adsorbed onto riverine clay minerals behave in a similar manner to Cd, we speculate that their desorption in marginal marine settings should exert a significant impact on the productivity of the biosphere.

Casts and molds of soft-bodied organisms in Ediacaran sandstones (“Ediacara-style” fossilization) have played an important role in reconstruction of the emergence and radiation of early complex macroscopic life. However, the preservational processes responsible for the Ediacara fossil record are still vigorously debated. Whereas classic studies proposed fossilization via rapid sulfide mineralization of carcass and matground surfaces, a more recent view posits silica as the key mineral involved in their preservation. We performed experiments in which a variety of soft-bodied organisms were exposed to silica-rich solutions at concentrations considered characteristic of Ediacaran seawater (2 mM). Our results document continuous precipitation of amorphous silica onto the surfaces of these organic tissues under constant and normal marine pH values (7.8). Mineral formation was accompanied by a progressive decrease in the dissolved silica (DSi) concentration of the experimental solution to levels well below amorphous silica saturation. Additionally, we find that the magnitude of silica precipitation is correlated to each organism’s functional-group chemistry, as measured by potentiometric acid-base titrations. We suggest that a wide range of soft-bodied organisms were prone to silicification in Ediacaran marine environments characterized by anactualistically high DSi concentrations. This provides further support for the model that the extraordinary moldic preservation of the Ediacara Biota was promoted by early silica cementation and that this mode of preservation can offer an accurate glimpse into the composition of those early animal ecosystems.

In marginal marine settings, understanding the role that clay minerals play in the transfer of metal cations from the water column to the seafloor is important if the composition of modern muds in the ocean is used as a proxy for the precursors of ancient shale deposits. In this study, we conducted potentiometric titrations of three naturally abundant clay minerals (kaolinite, montmorillonite and illite) in 0.56 M NaCl electrolyte solutions to ascertain the surface reactivity of each clay mineral at seawater ionic strength. Our results demonstrate that all three clay minerals were increasingly anionic as pH increases from 3 to 9, with montmorillonite having the highest negative-charge. The potentiometric titrations and adsorption data were modeled using both non-electrostatic (NEM) and electrostatic (constant-capacitance model, CCM) surface complexation models (SCMs). A two-site surface complexation model that included one basal siloxane surface site (≡X -) and one amphoteric edge site (≡SOH), provided an excellent fit for the protonation data of each mineral. Cd(II) adsorption experiments were conducted to quantify the magnitude of cation adsorption, which showed that 10 g montmorillonite, illite, and kaolinite powder could bind an equivalent of 4.2×10 -6 M, 3.8×10 -6 M, and 3.6×10 -6 M of Cd, respectively, under conditions relevant to seawater (initial Cd concentration of 8.9×10 -6 M, and pH=8). To place this value into the context of modern riverine clay inputs to the ocean, 13.5 ×10 9 tons of total suspended sediment are deposited annually (Milliman and Meade, 1983) of which 10 to 25% is clay (Manheim et al., 1970;Schroeder et al., 2015). The total amount of Cd adsorbed to suspended clay minerals entering the modern oceans could then amount to approximately 10 4 to 10 5 tons per year. The results of this study highlight the potential role of clay minerals in transporting metals from the ocean water column to the seafloor.

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