Experimental assessment of CO2-mineral-toxic ion interactions in a simplified freshwater aquifer: implications for CO2 leakage from deep geological storage.

The possible intrusion of CO2 into a given freshwater aquifer due to leakage from deep geological storage involves a decrease in pH, which has been directly associated with the remobilization of hazardous trace elements via mineral dissolution and/or via desorption processes. In an effort to evaluate the potential risks to potable water quality, the present study is devoted to experimental investigation of the effects of CO2 intrusion on the mobility of toxic ions in simplified equilibrated aquifers. We demonstrate that remobilization of trace elements by CO2 intrusion is not a universal physicochemical effect. In fact goethite and calcite, two minerals frequently found in aquifers, could successfully prevent the remobilization of adsorbed Cu(II), Cd(II), Se(IV), and As(V) if CO2 is intruded into a drinking water aquifer. Furthermore, a decrease in pH resulting from CO2 intrusion could reactivate the adsorption of Se(IV) and As(V) if goethite and calcite are sufficiently available in underground layers. Our results also suggest that adsorption of cadmium and copper could be promoted by calcite dissolution. These adsorbed ions on calcite are not remobilized when CO2 is intruded into the system, but it intensifies calcite dissolution. On the other hand, arsenite As(III) is significantly adsorbed on goethite, but is partially remobilized by CO2 intrusion.

Nucleation and growth of chrysotile nanotubes in H2SiO3/MgCl2/NaOH medium at 90 to 300 °C.

Herein, we report new insights into the nucleation and growth processes of chrysotile nanotubes by using batch and semi-continuous experiments. For the synthesis of this highly carcinogenic material, the influences of temperature (90, 200, and 300 °C), Si/Mg molar ratio, and reaction time were investigated. From the semi-continuous experiments (i.e., sampling of the reacting suspension over time) and solid-state characterization of the collected samples by XRPD, TGA, FTIR spectroscopy, and FESEM, three main reaction steps were identified for chrysotile nucleation and growth at 300 °C: 1) formation of the proto-serpentine precursor within the first 2 h of the reaction, accompanied by the formation of brucite and residual silica gel; 2) spontaneous nucleation and growth of chrysotile between about 3 and 8 h reaction time, through a progressive dissolution of the proto-serpentine, brucite, and residual silica gel; and 3) Ostwald ripening growth of chrysotile from 8 to 30 h reaction time, as attested to by BET and FESEM measurements. Complementary results from batch experiments confirmed a significant influence of the reaction temperature on the kinetics of chrysotile formation. However, FESEM observations revealed some formation of chrysotile nanotubes at low temperatures (90 °C) after 14 days of reaction. Finally, doubling the Si/Mg molar ratio promoted the precipitation of pure smectite (stevensite-type) under the same P (8.2 MPa)/T (300 °C)/pH (13.5) conditions.