Stability and Migration Characteristics of SDS and SiO2 Colloidal Gas Aphron and Its Removal Efficiency for Nitrobenzene-Contaminated Aquifers

Abstract

The instability of colloidal gas aphrons (CGAs) in aquifers is one of the major obstacles influencing the remediation efficiency of CGAs-flushing. In this study, hydrophobic nano-SiO2 (HN-SiO2) was utilized to enhance the stability of CGAs. The batch experiments were carried out to investigate the effect of hydrogeochemical conditions on the stability of HN-SiO2 stabilized CGAs in aquifers, such as ions, pH, and nitrobenzene (NB); 2-D tank experiments designed with various injection velocities and medium particle sizes were conducted to determine the factors influencing the migration of CGAs in the aquifer; and 1-D column experiments were established to evaluate the remediation efficiency of HN-SiO2 stabilized CGAs flushing for NB removal. The results indicated that HN-SiO2 had a favorable effect on the stability of CGAs, which enhanced the half-life of CGAs from 328 to 1,050 s. HN-SiO2 particles formed a shell film on the gas-liquid interface of CGAs to slow down the gas escape and drainage velocity. The stability of CGAs decreased while pH<6 or pH>8, whereas this was enhanced with the addition of 0.1 wt.% NaCl. The 2-D tank experiments demonstrated that the HN-SiO2 stabilized CGAs had better stability when they flowed in aquifers compared to sodium dodecyl sulfate (SDS) CGAs. The fine medium and high injection velocity facilitated CGAs forming the larger radius remediation area. The injection pressure was inversely proportional to the medium grain sizes, whereas directly proportional to the injection velocities. In addition, column experiments demonstrated the remediation efficiency of HN-SiO2 stabilized CGAs (81.7%) flushing for NB removal was higher than that of SDS CGAs (61.4%) flushing. Therefore, in comparison with SDS CGAs, HN-SiO2 stabilized CGAs were more efficient reagents for NB removal. CGAs-flushing technique using HN-SiO2 stabilized CGAs is feasible to remedy the contaminated aquifers. The contamination caused by dense nonaqueous phase liquids in aquifers has a great threat to ecosystems and human health. Colloidal gas aphron (CGA) flushing has been proposed as an effective remediation technology for dense nonaqueous liquids contaminated aquifers. However, the instability of colloidal gas aphrons in aquifers is one of the major obstacles influencing remediation efficiency. In this work, the hydrophobic nano-SiO2 was used to improve its stability. Besides, the migration of colloidal gas aphron generated with sodium dodecyl sulfate (SDS) and SDS/SiO2 was investigated, and the remediation efficiency for nitrobenzene removal of SDS/SiO2 colloidal gas aphrons flushing was evaluated. SiO2 had a favorable effect on the stability of colloidal gas aphrons, enhancing the half-life of colloidal gas aphrons from 328 to 1,050 s, and the enhancement effect was more evident with the increasing SiO2 concentration. The tank experiment results indicated that SDS/SiO2 colloidal gas aphron had fine migration performance and wide applicability in aquifer environments. In addition, column experiments demonstrated the remediation efficiency of hydrophobic nano-SiO2 stabilized colloidal gas aphrons flushing for nitrobenzene removal was higher than that of SDS colloidal gas aphrons flushing. Therefore, the colloidal gas aphron flushing technique using SDS/SiO2 colloidal gas aphrons is feasible to remedy the contaminated aquifers.