Summarized results

5.2 Vapor retardation 91

(v200 cm h⁻¹). Hence, the focus laid on the velocities used and higher values were beyond the scope.

Mass balance analyses were performed to obtain mass recovery (r) from each breakthrough curve. Mass recovery was calculated from concentration and flow measurements and were normalized with respect to the injected mass. In general, mass recoveries of argon and CS2

showed good results. The mean recovery of argon calculated from all vapor-retardation ex-periments conducted yielded r_Ar= 0.995±0.007 and confirmed complete mass recovery. The mean recovery of CS2was r_CS2= 0.981±0.084 (without the experiments of Series 3), thus only suggesting slight mass losses. Mass recoveries of all experiments are given in Table A.2 in the appendix. The mass balance and complete recovery proved the reliability and quality of the results gained from these column experiments.

The results discussed above excluded Series 3 conducted in Geba fine sand. Series 3 was the second saturation and drainage cycle which was carried out to establish a different static water saturation than in Series 2. However, significant CS₂ mass losses became more pronounced with each experiment in this series, eventually leading to its exclusion from the results.

Recoveries of CS₂ decreased from r_CS2= 0.854 in the first experiment of Series 3 down to r_CS2= 0.010. This mass loss of CS2was caused by biodegradation which was confirmed by the smell of hydrogen sulfide in the column outflow.Cox et al.(2013) found carbonyl sulfide (COS) and hydrogen sulfide (H₂S) as by-products during CS₂ biodegradation in their experiments.

The mass balance analysis of the experiments enabled for determining mean degradation rates of CS₂which were calculated from the CS₂mass rate and the recovery. The mean degradation rates ranged from 0.12 to 1.28 mg h⁻¹ depending on the respective seepage velocity applied in the experiments. The experiments showed that biodegradation may have a considerable poten-tial for mitigating the contaminant mass transfer by vapor migration to the underlying aquifer.

However, the quantification of biodegradation of CS₂was beyond the scope of this work but should be addressed in future research.

all experiments conducted within a series. The retardation of CS2 was expected to be affected by the mean water saturation or the non-uniform water-saturation profiles in the case of Geba fine sand.

Different seepage velocities ranging from 25 to 200 cm h⁻¹were applied to characterize the va-por transva-port and to evaluate their impact on retardation. The range was chosen based on the velocities observed during the experimental investigation into density-driven vapor migration (Sec. 5.1). The experiments showed that the velocities affected diffusion/dispersion of the gases due to the corresponding residence time in the porous medium and due to mechanical mixing.

This effect was illustrated by the skewness of the breakthrough curves shown in Figure 5.15 and 5.16. The skewness of the BTCs was negatively correlated to the seepage velocity. Disper-sion coefficients as a function of seepage velocity were obtained from the temporal-moment analysis for experiments in moist conditions. These ranged from D_Ar= 0.089 to 0.142 cm²s⁻¹ and D_CS2= 0.033 to 0.074 cm²s⁻¹ as a function of the seepage velocity and the porous medium.

Based on the graph of dispersion coefficients as a function of seepage velocity (Fig. 5.17), linear regressions were used to determine the effective binary diffusion coefficient of argon and CS₂as well as the dispersivity of the two porous media. In fine glass beads, effective binary diffusion coefficients of argon wereD_Ar^∗ = 0.0909 cm²s⁻¹and of CS₂wereD^∗_CS

2= 0.0263 cm²s⁻¹. In Geba fine sand, coefficients of argon wereD_Ar^∗ = 0.0966 cm²s⁻¹and of CS₂wereD_CS^∗ ₂= 0.0332 cm²s⁻¹. The effective binary diffusion coefficients of the experiments were higher than those calculated from the theoretical approach (Millington and Quirk, 1961) given in Table 5.8. The higher experi-mental values could be a result of the porous media used in this work which were characterized by a uniform and narrow grain-size distribution. Furthermore, the theoretical approach takes into account the porosity only and neglects material characteristics such as grain-size or pore-size distribution which affect diffusion/dispersion. This confirms that theoretical approaches do not apply satisfactorily to a wide variety of materials. Dispersivity was obtained from the slope of the linear regression and differed slightly for argon and CS₂. A mean dispersivity of α_GBfine= 0.958 cm in fine glass beads and α_Geba= 0.432 cm in Geba fine sand was determined in moist conditions. Since CS₂ was affected by retardation, the dispersivity gained from the breakthrough curve of argon was considered more reliable.

The retardation of CS₂ was quantified based on the experiments in dry and moist porous me-dia. The injection of a slug and subsequent chase allowed for the separate evaluation employ-ing the temporal-moment analysis which has been adapted for step input. The breakthrough of CS₂was first related to that of argon and second compared with breakthroughs in dry porous media. This ensured reproducible results and eliminated systematic errors or set-up related errors (Fig. 5.18 and 5.19). The experiments conducted showed a clear retardation of CS2 in moist porous media as a function of water saturation (Fig. 5.20). In fine glass beads, the re-tardation coefficient increased from R_GBfine= 1.09 to 1.16 with water saturation increasing from Sw= 0.075 to 0.155. Retardation in fine glass beads compared very well with the theoretical retardation coefficient (Eq. 3.11) taking into account the partitioning into the aqueous phase.

The results benefited from the uniform water-saturation profile in the column observed with fine glass beads. In all experiments, a slightly higher retardation of the chase by a factor of 1.05 compared to that of the slug was observed. Retardation in Geba fine sand was stronger than predicted by the theoretical coefficient which was believed to be influenced by the particular

5.2 Vapor retardation 93

water-saturation profile. A mean retardation coefficient of R_Geba= 1.32 was determined at a water saturation of S_w= 0.162. The pronounced increase in water saturation toward the bottom probably contributed to the retardation due to the higher reservoir available for dissolved CS₂ despite the smaller interfacial area. Retardation coefficients as a function of (seepage) velocity revealed only a minor dependency and suggested a slight tendency toward a reduced retarda-tion at higher velocities. Further repetiretarda-tions and experiments with higher velocities would be required to investigate this behavior.

Mass balance of argon and CS2confirmed complete mass removal. A mean recovery of argon yielded r_Ar= 0.995±0.007 and of CS₂r_CS2= 0.981±0.084. These high recovery values support the experimental results presented in this work. In the last series of vapor-migration experi-ments conducted in Geba fine sand, the mass balance revealed significant losses of CS2 increas-ing with every experiment conducted. This series of experiments revealed biodegradation of CS₂ due to microbial growth after the last saturation-and-drainage cycle. This was confirmed by the smell of hydrogen sulfide in the column outflow which Cox et al.(2013) has reported to be a by-product of degradation in soils. Mean degradation rates up to 1.28 mg h⁻¹were de-termined from mass balance analyses. These findings demonstrate the potential of biodegra-dation to reduce the total CS2mass in case of a contamination in the unsaturated zone and of migrating vapor plumes eventually threating the underlying aquifer.

The vapor-retardation experiments conducted in this work successfully allowed for quantify-ing the retardation of CS₂vapor as a function of water saturation and seepage velocity in two different porous media. It was shown that the migrating CS₂vapor was retarded due to parti-tioning into the aqueous phase. Retardation coefficients up to 1.20 and 1.36 were measured in fine glass beads (S_w= 0.162) and in Geba fine sand (S_w= 0.162), respectively. The breakthrough of the slug and of the chase was observed and evaluated, the latter demonstrating a complete removal of the gaseous CS2confirmed by mass balance analyses. This observation clearly pro-motes the remediation of a liquid CS₂spill using soil-vapor extraction. SVE was investigated in detail in 2-D experiments and their results are discussed in the following section.

Key findings

• The retardation of CS₂ was quantified in column experiments for two porous media at different water saturations by comparison with the conservative tracer argon.

• The temporal-moment analysis (TMA) was successfully applied to quantify diffu-sion/dispersion of CS2and argon as well as retardation of CS2from concentration break-through curves.

• The effective binary diffusion coefficient at the given experimental conditions was found to be slightly higher than theoretical values based on the approach byMillington and Quirk (1961).

• The impact of different seepage velocities on the breakthrough curves and thus on the dispersion coefficient was observed.

• The retardation coefficient of CS2 increased with increasing water saturation and com-pared very well with the theoretical approach for fine glass beads. A pronounced higher retardation was observed in Geba fine sand due to the different grain-size distribution and the particular water-saturation profile.

• Clear evidence of the biodegradation of CS2was found in the last series of experiments in Geba fine sand confirmed by the mass balance analysis.

• The experiments conducted clearly proved that a migrating CS2-vapor plume in the un-saturated zone is retarded and that dissolved CS₂is amenable to biodegradation.