Acid-base properties of kaolinite, montmorillonite and illite at marine ionic strength

Abstract

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.

Rational Design of Porous Nanofiber Adsorbent by Blow‐Spinning with Ultrahigh Uranium Recovery Capacity from Seawater

“…Natural montmorillonite usually exhibits a negative surface charge at seawater pH, which inhibits the adsorption of uranyl because of charge repulsion . To address this disadvantage, in this study, a montmorillonite substance that exhibits neutral charge at weakly alkalescent pH was selected to develop the target fiber material.…”

Change in the site density and surface acidity of clay minerals by acid or alkali spills and its effect on pH buffering capacity

“…Potentiometric titration has been used to quantify the density of active reaction sites of clay minerals and determine the surface acidity constants (Ka) of those sites 25,26,43,44,46,54–61 . The protonation behaviors of three different kaolinite or montmorillonite (untreated, acid-spilled, and alkali-spilled) were investigated using an automatic potentiometric titrator (G10S, Mettler-Toledo, Switzerland).…”

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

“…Although this work is currently being conducted, there are existing data available that consider clay mineral reactivity in such settings. For instance, experimental results of Cd adsorption onto montmorillonite showed that almost 90% of Cd adsorbed onto montmorillonite at pH=6 and at 0.001 M ionic strength (Gu et al, 2010), while this number decreased to only 10% if ionic strength was increased to mimic seawater (0.56 M) (Liu et al, 2018). The increase of pH has inverse effect with IS, where the increase of pH from 6 to 8 results in an additional 40% of Cd adsorption (Liu et al, 2018).…”

“…Over the past decade, our understanding of the mechanisms underpinning the adsorption of ions to geologic media has considerably improved through the use of sorption isotherms and experimentally-derived surface complexation modeling (e.g., Borrok and Fein, 2005;Alessi and Fein, 2010;Wang et al, 2010;Komárek et al, 2015;Liu et al, 2015;Alam et al, 2018;Liu et al, 2018;). Adsorption is generally governed by two parameters: the surface charge properties of the adsorbent and the chemical complexation of aqueous adsorbates onto specific surface functional groups (Dzombak and Morel, 1990).…”