Understanding mineral aggregation is crucial to many geochemical systems, including sedimentation and nuclear waste disposal. In sedimentary systems porosity originates from the initial grain deposition, and is modified by compaction, precipitation, and recrystallization. In this study, we used ultra-/small-angle neutron scattering ((U)SANS) to investigate the effect of salt concentration on compaction by characterizing the changes of porosity, including size distribution, volume fraction, and specific surface area (SSA) in samples exposed to various times and rates of centrifugation. Tumbler cell experiments were also used to evaluate aggregation in various salt solutions. In the tumbler experiments three minerals (barite, kaolinite, and calcite) were exposed to D2O and different concentrations of NaCl (0.1 and 1 molal), Na2SO4 (0.01 and 0.1 molal), and Na4P2O7 (0.01 and 0.001 molal). Slightly higher volume fraction and SSA were found at 0.1 molal of Na2SO4 for all the minerals measured, while there were significant increases in volume fraction and SSA at 0.01 molal Na4P2O7 for barite and calcite. (U)SANS measurements of centrifuged double-plate cells were used to quantify the effect of external force on minerals settling as a proxy for burial. Pore volume fraction (but not SSA) of barite in D2O has an apparent increase with increasing time and speed of the centrifuging process, while pore volume fraction and SSA of kaolinite tend to decrease in D2O and all NaCl conditions. Pore volume fraction and SSA of barite in NaCl conditions and calcite in D2O and NaCl conditions tend to first reduce and then increase. The non-intuitive parts of these results may reflect the increase in the number of pores in the (U)SANS size range (&lt;~20 µm) through compaction of larger pores. In addition, molecular dynamics simulation was conducted to provide the mechanisms of mineral aggregation under different salt solutions and concentrations and external forces in atomic scale. This research highlights the importance of interfacial properties and external force on mineral aggregation in solution environments.

Investigating effects of salt concentration and external force on compaction of mineral nanoparticles

ACS Spring 2022

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