The Influence of Solution Chemistry on Air-Water Interfacial Adsorption and Transport of PFOA in Unsaturated Porous Media

Abstract

There is great interest in the transport behavior of PFAS in the vadose zone, and the impact of leaching on groundwater contamination. Air-water interfacial adsorption is an important process for PFAS retention in unsaturated porous media, and it is influenced by many factors including solution conditions such as ionic strength. The present study employed miscible-displacement column experiments to investigate the impact of ionic strength and pH on perfluorooctanoic acid (PFOA) retardation and transport under dynamic water-flow conditions. The results showed that retardation under unsaturated conditions was affected significantly by changes in ionic strength, whereas there was minimal impact for saturated conditions. This indicates that air-water interfacial adsorption, which was a major source of retardation, was influenced significantly by changes in ionic strength while they had a minor impact on solid-phase adsorption. The impact of changes in ionic strength on the magnitude of air-water interfacial adsorption observed for the column experiments was consistent with measured surface-tension data. The impact of changes in pH were less significant compared to that of ionic strength for transport under unsaturated conditions. These results illustrate the influence of solution chemistry on PFAS adsorption and transport under unsaturated conditions. This solution-dependent behavior should be considered when characterizing PFAS transport in soils and the vadose zone.

1. Introduction

Per- and poly-fluoroalkyl substances (PFAS) have become environmental contaminants of great concern. Concomitantly, interest has expanded in characterizing the occurrence and transport behavior of PFAS in soil and the potential for leaching through the vadose zone to groundwater (e.g., Xiao et al., 2015; Brusseau, 2018; Hatton et al., 2018; Anderson et al., 2019; Brusseau et al., 2019; Dauchy et al., 2019). Transport of PFAS in the vadose zone is complicated by the additional retention process of adsorption at air-water interfaces for unsaturated systems (Brusseau, 2018; Lyu et al., 2018; Brusseau et al., 2019). As these authors discuss, air-water interfacial adsorption of PFAS is influenced by many factors. One important factor is solution conditions, such as ionic strength and pH.

It is well established that the surface activity of surfactants is sensitive to ionic strength, with surface activity typically increasing with greater ionic strength. Such impact of ionic strength on surface activity has long been demonstrated for PFAS, specifically for example for PFOA (Shinoda and Nakayama, 1963; Downes et al., 1995; An et al., 1996). More recent environmentally focused work has shown the same impact of ionic strength on surface activity of PFOA, perfluorooctanesulfonate (PFOS) and other PFAS (Lyu et al., 2018; Brusseau et al., 2019; Brusseau and Van Glubt, 2019; Costanza et al., 2019; Silva et al., 2019).

The results of this prior research indicate that the magnitude of air-water interfacial adsorption of PFAS will be influenced by ionic strength, which is an important factor to understand for accurate characterization of PFAS transport. However, all of the prior research regarding the impact of ionic strength on PFAS surface activity is based on surface-tension measurements, which are conducted using batch-type systems. No direct investigation of this phenomenon has been conducted for actual water-flow and solute-transport conditions. Neither has the impact of pH. The objective of the present study is to investigate the impact of solution conditions, specifically ionic strength and pH, on PFOA transport under unsaturated-flow conditions.

2. Materials and Methods

2.1. Materials

PFOA (CAS#335-67-1) of 98% purity was purchased from AIKE Reagent (China). Sodium chloride was used as the background electrolyte for all experiments. Solutions were prepared using distilled, deionized water.

A natural quartz sand whose mean diameter is 0.35 mm was used for the miscible-displacement experiments. This medium exhibits a low magnitude of solid-phase adsorption for PFOA (Lyu et al., 2018). It was selected to specifically focus on the impact of changes in solution chemistry on air-water interfacial adsorption.

The columns used in this study were constructed of acrylic to minimize interaction with PFOA, and were 15 cm long with inner diameter of 2.0 cm. Flow distributors were placed in contact with the porous media on the top and at the bottom of the column to help promote uniform fluid distribution and to support the media. Peristaltic pumps (BT100–02, Baoding Qili Precision Pump Co., Ltd, China) were used to provide fluid flow. As noted below, analysis of background samples collected from the column effluent revealed the absence of any interferences associated with the column or apparatus for PFOA determination.

2.2. Miscible-Displacement Experiments

The miscible-displacement experiments were conducted using methods we have used in prior studies (Brusseau et al., 2007, 2015, 2019; Lyu et al., 2018). Details are available in the cited references. A PFOA input concentration (C₀) of 1 mg/L was used for the miscible-displacement experiments. This concentration is well within the range of soil concentrations reported for contaminated sites, which range upward of several hundred mg/kg (e.g., Anderson et al., 2019; Dauchy et al., 2019).

Preliminary tests were conducted with a nonreactive tracer solution (pentafluorobenzoic acid) to ensure that the columns were packed well and to characterize hydrodynamic conditions. Additional preliminary experiments were conducted with PFOA under saturated conditions to determine the impact of solid-phase adsorption on retardation and transport. Experiments were then conducted under unsaturated conditions to determine the additional impact of air-water interfacial adsorption.

Experiments were conducted to examine the impact of solution conditions on retardation. Conditions for all experiments are presented in Table 1. Electrolyte composition was varied for one set of experiments, with the concentration of NaCl set at 0.1, 0.01, 0.005, and 0.001 M. The solution pH was 7 for these experiments. The pH of the solution was varied for the second set, with values of 5, 6, 7, and 8. The pH was adjusted using HCl and NaOH. The NaCl concentration was set to 0.01 M for this set of experiments.

Table 1.

Experiment conditions and measured retardation factors for PFOA from miscible-displacement experiments.

Expt	Water Saturation	Ionic Strength (M)	pH	Measured Retardation Factor	AWIAa
1	0.68	0.01	7	2.0 (1.96–2.07)b	0.56
2	0.68	0.1	7	2.4	0.84
3	0.67	0.005	7	1.7	0.23
4	0.67	0.001	7	1.2	0.08
5	0.76	0.01	7	1.8	0.48
6	0.74	0.01	5	2.0	0.63
7	0.78	0.01	6	1.8	0.47
8	0.77	0.01	8	1.7	0.42

^aAWIA = K_iA_i/θ_w

^b95% confidence interval for 3 experiments; data originally reported in Lyu et al., 2018

2.3. Chemical and Data Analysis

Samples of column effluent were collected in polypropylene tubes and analyzed immediately after collection. PFOA was analyzed by high-performance liquid chromatography (Agilent Model 1100, USA) and tandem mass spectrometry (TSQ quantum, Thermo Scientific, USA), i.e., LC-MS/MS. The column was an Agilent C18 maintained at 40 °C. The dual mobile phase comprised 5 mM ammonium acetate and acetonitrile applied in a 60:40 gradient at a flow rate of 0.2 mL/min. The column temperature was set at 40 °C. The aqueous samples were injected directly, with injection volumes of 2 μL. Retention time was consistently ~4.3 min. MS/MS conditions were as follows: ionization mode: ESI-; capillary temperature: 300 °C; nebulizer temperature: 300 °C. Standard QA/QC protocols were employed. Blanks, background samples, and check standards were analyzed periodically for each sample set. The results for the first two were lower than the quantifiable detection limit. The calibration curve attained a coefficient of determination (r²) larger than 0.999. The quantifiable detection limit was ~0.3 ug/L. Background aqueous samples collected from the column effluent before injection of PFOA revealed no measurable PFOA concentrations or other interferences for all experiments.

Measured retardation factors were determined for each miscible-displacement experiment by the standard method of calculating the area above the breakthrough curve. The retardation factor is defined as (e.g., Lyu et al., 2018):

$$\text{R }=1+{\text{ K}}_{\text{d}}{\rho }_{\text{b}}/{\theta }_{\text{w}}+{\text{ K}}_{\text{i}}{\mathrm{A}}_{\text{i}}/{\theta }_{\text{w}}$$ (1)

where K_d is the solid-phase adsorption coefficient (cm³/g), K_i is the air-water interfacial adsorption coefficient (cm³/cm²), A_i is the specific air-water interfacial area (cm²/cm³), ρ_b is porous-medium bulk density (g/cm³), and θ_w is volumetric water content (−). The K_i can be determined from measured surface tension data by:

$$K_i=\frac{\Gamma }{C}=\frac{-1}{\text{ RTC }}\frac{\partial \gamma }{\partial lnC}$$ (2)

where γ is the surface tension (mN/m), Γ is the adsorbed concentration (mol/cm²), C represents the aqueous phase concentration (mol/cm³), T is temperature (K), and R is the gas constant (erg/mol °K). Additional details are reported in Lyu et al. (2018) and Brusseau et al. (2019).

3. Results and Discussion

The breakthrough curves for transport of the nonreactive tracer in the sand-packed columns exhibit ideal behavior for both saturated and unsaturated conditions, as discussed by Lyu et al. (2018). The breakthrough curves for PFOA transport under saturated-flow conditions with different ionic-strength solutions are presented in Figure 1. There is a small impact of ionic strength on transport, with the retardation factor increasing by less than 20% for the change in ionic strength from 0.001 to 0.1 M. The relatively minor impact of changes in ionic strength on the magnitude of solid-phase adsorption is expected for this sand, given its absence of clay minerals and minimal metal-oxide content, and overall small degree of adsorption.

Figure 1.

Breakthrough curves for transport of PFOA under saturated conditions with various ionic strength solutions. Relative concentration is the effluent concentration divided by the input concentration; pore volumes represent the solution discharge divided by the fluid volume retained in the column.

The breakthrough curves for PFOA transport under unsaturated-flow conditions with different ionic-strength solutions are presented in Figure 2. It is clearly observed that the curves for the higher ionic-strength solutions are shifted rightward, exhibiting greater retardation. The measured retardation factors for the reduced ionic-strength (NaCl = 0.001 and 0.005 M) experiments are smaller (closer to 1) than the values for the corresponding experiments with higher ionic strength (compare experiments 1–2 versus 3–4 in Table 1). In addition, the retardation factor for the NaCl = 0.1 M solution is greater than the value for the 0.01 M solution.

Figure 2.

Breakthrough curves for transport of PFOA under unsaturated conditions (S_w = 0.68) with various ionic strength solutions. Relative concentration is the effluent concentration divided by the input concentration; pore volumes represent the solution discharge divided by the fluid volume retained in the column.

Figures 1 and 2 can be compared to evaluate the relative impact of changes in ionic strength on retardation. The comparison reveals that the impact is much greater for the unsaturated-flow experiments compared to those conducted under saturated conditions. For example, the retardation factor doubles from the lowest to highest ionic strength for unsaturated conditions, versus the <20% change for saturated conditions. This indicates that the impact of ionic strength is much greater for air-water interfacial adsorption compared to solid-phase adsorption for this medium.

The impact of ionic strength on the magnitude of PFOA adsorption at the air-water interface can be evaluated by comparing the AWIA values reported in Table 1. Because the ionic-strength experiments were conducted with nearly identical water saturations, both the A_i and θ_w terms in the AWIA parameter are essentially the same for all experiments. Therefore, the range of values reported for AWIA represents in effect the range in K_i values. It is observed that AWIA, and hence the column K_i, ranges by a factor of approximately 10 from the lowest to highest ionic strength.

The results of the column experiments can be compared to the surface-tension data, which are presented in Figure 3. The surface activity of PFOA is observed to increase with increasing ionic strength, as observed for prior surface-tension measurements (Lyu et al., 2018; Brusseau and Van Glubt, 2019; Costanza et al., 2019; Silva et al., 2019). The K_i values determined from the surface-tension data reported in Figure 3 range by a factor of approximately 8, from 0.001 cm to 0.0082 cm, for deionized water and 0.1 M NaCl, respectively. This range is similar to the range of AWIA observed for the column data. Thus, there is good consistency between the two sets of data.

Figure 3.

Surface tension measurements for PFOA in aqueous solutions as a function of ionic strength.

As discussed by Brusseau and Van Glubt (2019), increasing the electrolyte concentration reduces electrostatic repulsion among the ionic headgroups at the air-water interface. It also increases the activity of the hydrophobic tail in solution. Both of these phenomena result in greater surface activity and, therefore, increase interfacial adsorption.

The breakthrough curves for PFOA transport under unsaturated-flow conditions with different pH solutions are presented in Figure 4. A much smaller difference is observed among the breakthrough curves compared to the influence of ionic strength (Figure 2). The retardation factor is slightly lower for pH = 8 and slightly higher for pH = 5, compared to pH = 7 (Table 1). The AWIA decreases by a factor of 1.5 between the lowest and highest pH. The increase in air-water interfacial adsorption at lower pH is consistent with the observed impact of acid addition upon surface tensions of solutions containing PFOA or PFOS (Talbot, 1959). The impact of pH was attributed to the influence of H⁺ on the dissociation status of the PFAS anion.

Figure 4.

Breakthrough curves for transport of PFOA under unsaturated conditions (S_w = 0.76) with various pH solutions. Relative concentration is the effluent concentration divided by the input concentration; pore volumes represent the solution discharge divided by the fluid volume retained in the column.

4. Conclusion

Prior research has established that the adsorption of PFOA and other PFAS at the air-water interface is influenced by solution ionic strength and pH. However, this research was based on surface-tension measurements in batch-type systems. The present study employed column experiments to investigate the impact of ionic strength and pH on PFOA retardation and transport under dynamic water-flow conditions. The results showed that retardation under unsaturated conditions was affected significantly by changes in ionic strength, whereas there was minimal impact for saturated conditions. This indicates that air-water interfacial adsorption, which was a major source of retardation, was influenced significantly by changes in ionic strength while they had a minor impact on solid-phase adsorption. The impact of changes in ionic strength on the magnitude of air-water interfacial adsorption observed for the column experiments was consistent with measured surface-tension data. The impact of changes in pH were less significant compared to that of ionic strength for transport under unsaturated conditions. These results illustrate the influence of solution chemistry on PFAS adsorption and transport under unsaturated conditions. This solution-dependent behavior should be considered when characterizing PFAS transport in the vadose zone.