THE ACID-BASE PROPERTIES OF CLAY MINERALS AS A POTENTIAL BUFFER FOR SEDIMENT PORE WATER PH AND CARBONATE SATURATION DURING MICROBIAL IRON REDUCTION

Clay minerals contain pH dependent charge sites that can accept or donate protons from aqueous solutions. Previous research has overlooked how these acid-base properties may affect diagenetic reactions in marine sediments. Given that clay minerals are often deposited with elevated concentrations of organic carbon, their influence on pore fluid pH and carbonate saturation during contemporaneous biogeochemical reactions involving organic carbon must be investigated.

Here, we present a chemical model for microbial Fe reduction that accounts for the buffering capacity of clay mineral-rich sediments. The model is based on equations describing speciation, mass and charge balances, and predicts the evolution of pH and carbonate saturation as a function of moles Fe³⁺ reduced. Clay mineral-rich sediments were obtained from International Ocean Discovery Program (IODP) cores including the Nankai mudstone from IODP Expedition 322 and the Ursa mudstone from IODP Expedition 308. These clay mineral-rich sediments were titrated in nanopure water to obtain their acid dissociation constant (pKa) and total acidity, both of which are key model inputs. The Nankai mudstone has a pKa value of ~4.8 and a total acidity of ~0.14 mmol g^-1, while the Ursa mudstone has a pKa of ~3.9 and a total acidity of ~0.05 mmol g^-1.

Our model results for the Nankai mudstone show that with an initial pH of 6.5 and the consumption of 0.5 mM Fe³⁺ by microorganisms the pH is 0.57 log units lower than for microbial Fe reduction alone (without consideration of clay minerals). Alternatively, if this reaction begins at a pH of 8 there is no change in pH after the consumption of 0.5 mM Fe³⁺ by microorganisms. Model results for the Ursa mudstone show that there is no change in pH after the consumption of 0.5 mM Fe³⁺ by microorganisms no matter whether the reaction begins at a pH of 6.5 or 8. These results illustrate that the sediment pKa value must be within ~2 log units of the initial pH value when Fe reduction begins in order for clay minerals to be effective pH and carbonate saturation buffers. This heightened understanding of the acid-base properties of clay mineral-rich sediments has the profound implication that authigenic carbonate cementation could be limited or prevented during microbial Fe reduction when the initial pore water pH is within a few log units of the sediment pKa.