Closed‐flow column experiments—Insights into solute transport provided by a damped oscillating breakthrough behavior

Abstract

Transport studies that employ column experiments in closed‐flow mode complement classical approaches by providing new characteristic features observed in the solute breakthrough and equilibrium between liquid and solid phase. Specific to the closed‐flow mode is the recirculation of the effluent to the inflow via a mixing vessel. Depending on the ratio of volumes of mixing vessel and water‐filled pore space, a damped oscillating solute concentration emerges in the effluent and mixing vessel. The oscillation characteristics, e.g., frequency, amplitude, and damping, allow for the investigation of solute transport in a similar fashion as known for classical open‐flow column experiments. However, the closed loop conserves substances released during transport within the system. In this way, solute and porous medium can equilibrate with respect to physicochemical conditions. With this paper, the features emerging in the breakthrough curves of saturated column experiments run in closed‐flow mode and methods of evaluation are illustrated under experimental boundary conditions forcing the appearance of oscillations. We demonstrate that the effective pore water volume and the pumping rate can be determined from a conservative tracer breakthrough curve uniquely. In this way, external preconditioning of the material, e.g., drying, can be avoided. A reactive breakthrough experiment revealed a significant increase in the pore water pH value as a consequence of the closed loop. These results highlight the specific impact of the closed mass balance. Furthermore, the basis for the modeling of closed‐flow experiments is given by the derivation of constitutive equations and numerical implementation, validated with the presented experiments.

Colloidal Stability and Mobility of Extracellular Polymeric Substance Amended Hematite Nanoparticles

“…Due to the closed loop in the experimental design, the concentration of a solute observed both in the effluent and in the mixing vessel reaches an equilibrium (Ritschel and Totsche, 2016a; Supplemental Material, Section S.10). Hence, the concentration of the HNP in the mixing vessel was observed during several pore volumes until equilibrium was established (Fig.…”

“…The mobility of the HNP was quantified by comparing the concentration of the colloids in the supply vessel with the volumetric dilution. The volumetric dilution represents the expected decrease in the concentration of the mixing vessel solution due to water‐filled pore space initially present in the columns (Ritschel and Totsche, 2016a) and was calculated from the water‐filled pore space and the volume of the mixing vessel as 0.62 for all columns. The concentration of pure HNP in the mixing vessel dropped drastically below the volumetric dilution and reached zero.…”

“…The mobility of HNP and EPS–HNP associates was investigated with closed‐flow column experiments (Totsche, 2001; Ritschel and Totsche, 2016a) under water‐saturated conditions. In this mode, the column effluent is redirected into the column inflow via a mixing vessel.…”

“…In this mode, the column effluent is redirected into the column inflow via a mixing vessel. We decided on this type of experiment because it allows full establishment of a flow equilibrium, while at the same time conserving the total mass of the system and minimizing experimental artifacts (Ritschel and Totsche, 2016a). Thus, the true mobile fraction of a solute or colloid can be exhaustively determined, especially in the case where surface interactions are strongly rate limited (Ritschel and Totsche, 2016b).…”

“…Attachment efficiencies were calculated from the aggregation kinetics and compared with the classical and extended Derjaguin–Landau–Verwey–Overbeek (DLVO) model reconstructions to investigate the mode of colloidal interaction and stabilization. The transport of HNP and EPS–HNP was studied using closed‐flow transport experiments (Totsche, 2001; Ritschel and Totsche, 2016a) to fully assess the mobility in a porous medium. Breakthrough data were analyzed using colloidal filtration theory according to Tufenkji and Elimelech (2004).…”