Geochemical Transformations and Modeling of Two Deep‐Well Injected Hazardous Wastes

Abstract

AbstractTwo liquid hazardous wastes (an alkaline brine‐like solution and a dilute acidic waste) were mixed with finely ground rock samples of three injection‐related lithologies (sandstone, dolomite, and siltstone) for 155 to 230 days at 325°K‐10.8 MPa. The pH and inorganic chemical composition of the alkaline waste were not significantly altered by any of the rock samples after 230 days of mixing. The acidic waste was neutralized as a consequence of carbonate dissolution, ion exchange, or clay‐mineral dissolution, and hence was transformed into a nonhazardous waste.Mixing the alkaline waste with the solid phases yielded several reaction products: brucite, Mg(OH)2; calcite, CaCO3; and possibly a type of sodium metasilicate. Clay‐like minerals formed in the sandstone, and hydrotalcite, Mg6Al2‐CO3 (OH)16· 4H2O, may have formed in the siltstone at trace levels. Mixing the alkaline waste with a synthetic brine yielded brucite, calcite, and whewellite (CaC2O4· H2O). The thermodynamic model PHRQPITZ predicted that brucite and calcite would precipitate from solution in the dolomite and siltstone mixtures and in the alkaline waste‐brine system.The dilute acidic waste did not significantly alter the mineralogical composition of the three rock types after 155 days of contact. The model PHREEQE indicated that the calcite was thermodynamically stable in the dolomite and siltstone mixtures, and was detected in small quantities in aged‐solid samples. The sandstone‐waste system appeared to equilibrate with amorphous silica, whereas silica equilibria may not have been attained in the dolomite and siltstone systems.Computer models like PHRQPITZ and PHREEQE may be useful tools for estimating mineral equilibria in deep‐well scenarios, but there is a need to expand the database used in these kinds of calculations. Caution must be applied in interpreting the predicted equilibria. Fate modeling based on thermodynamic principles can predict simple geochemical interactions, but empirical, laboratory‐based investigations may be needed in addition to modeling for a reliable assessment of the fate of injected wastes.