Inclusion of Montmorillonite Clays in Environmental Barrier Formulations to Reduce Skin Exposure to Water-Soluble Chemicals from Polluted Water

Abstract

Dermal exposures to environmental chemicals can significantly affect the morphology and integrity of skin structure, leading to enhanced and deeper penetration of toxic chemicals. This problem can be magnified during disasters where hazardous water-soluble chemicals are readily mobilized and redistributed in the environment, threatening the health of vulnerable populations at the impacted sites. To address this issue, barrier emulsion formulations (EVB) have been developed consisting of materials that are generally recognized as safe, with the inclusion of medical grade carbon or calcium and sodium montmorillonite clays (CM and SM). In this study, the adsorption efficacy of five highly toxic and commonly occurring contaminants of concern, including important hydrophilic pesticides (glyphosate, acrolein, and paraquat) and per- and polyfluoroalkyl substances were characterized. EVB showed properties such as high stability, spreadability, low rupture strength, and neutral pH that were suitable for topical application on the skin. The in vitro adsorption results indicated that EVB and EVB-SM were effective, economically feasible, and favorable barrier formulations for hazardous chemical adsorption, as supported by high binding percentage, low desorption rates for an extended period of time, and high binding affinity. A pseudo-second-order kinetic model was best fitted for the adsorption process and the Freundlich model fit the adsorption isotherms with negative enthalpy values indicating spontaneous reactions that involve physisorption. The study, with varying temperatures and pH, showed that the adsorption reaction was exothermic and persistent. The results indicated that EVB and EVB-SM can be used as effective barriers to block dermal contact from water-soluble toxic pollutants during disasters.

Graphical Abstract

1. INTRODUCTION

In the summer, a few hours of outdoor water activity at the beach, pool, or lake can result in dry, irritated, and wrinkled skin. Extended water exposure can cause stratum corneum swelling and a more porous and permeable skin barrier.^1,2 Cholesterol, squalene, and vitamin E are decreased in exposed areas, resulting in a lower ratio of squalene/lipids, poorer cohesion of the stratum corneum, and a higher erythematous index (redness).³ Exposure to environmental chemicals can damage and weaken the skin’s natural barrier resulting in an increase in amount and deeper penetration of chemicals which, in turn, can affect the morphology and integrity of skin structure. The damaged barrier also leads to higher transepidermal water loss, resulting in poor skin hydration affecting the rigidity and firmness of the skin.³ Because of these problems, there has been enhanced interest in developing anti-pollution skincare formulations that will reduce skin exposure to environmental chemicals.⁴ Many skincare companies have offered products to protect against the effects of air pollution, such as PM_2.5,⁵ but no products have been reported to block water-soluble, hazardous environmental pollutants.

The problem of skin exposure to hazardous chemicals can be further magnified and complicated during disasters such as hurricanes and flooding. During these events, the skin can be vulnerable to water-soluble pollutants that have been mobilized and redistributed from contaminated sites and sediment. Some of the most important chemical contaminants of water that are readily available for skin penetration include water-soluble pesticides such as glyphosate, acrolein, and paraquat, and per and polyfluoroalkyl substance (PFAS) such as perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA). These hazardous chemicals are highly toxic and associated with human disease, including non-Hodgkin’s lymphoma, renal cancer, testicular cancer, endocrine disorders, neurodegeneration (Parkinson’s and Alzheimer’s disease), infertility, low birth rate, and immunotoxicity.^6-9 Chemical exposures can be a serious problem for vulnerable populations of people and animals at the site of impact, as well as medical personnel and first responders who are in contact with these pollutants for an extended period of time. Therefore, viable strategies that are safe and effective to prevent or significantly reduce skin absorption of these hazardous environmental chemicals from polluted flood waters during disasters and emergencies are critical needs.

Sunscreen is an effective physical barrier that can prevent harmful impacts of UV rays and certain chemicals on the skin. Emulsions are the most common solvent vehicles for sunscreen lotions, and emulsified systems are composed of polar and nonpolar components (oil-in-water, O/W and water-in-oil, W/O).^10-13 W/O emulsions are generally perceived as heavier than O/W. However, this occlusive feature provides better protection,¹⁴ and thus, W/O emulsions are used as a vehicle in this study to develop effective barrier formulations.

Commercial sunscreen products commonly contain oxybenzone (benzophenone-3) due to its broad-spectrum UV coverage, and it has become one of the most widely used organic UVA filters in the US.^15,16 Despite established photoprotective effects, a number of important issues have arisen surrounding the use of oxybenzone in sunscreens, such as photoallergic potential and endocrine-disrupting potential.^17-19 Systemic absorption of oxybenzone has also been demonstrated, and the prevalence of exposure in the general U.S. population is estimated to be as high as 96.8%.²⁰ Importantly, recent studies have detected high levels of benzene, a Group 1 human carcinogen that is associated with leukemia in many sunscreens.^21,22 Due to this public health concern, there has been an increased demand for the use of natural ingredients in sunscreens.^12,15 Additionally, most of these products lack evidence for the binding and prevention of environmental chemicals or chemical mixtures and evidence supporting and validating binding efficacy and proof of concept. Therefore, to address these issues, a W/O emulsion formulation (Envirobloc, or EVB) with materials that are generally recognized as safe (GRAS) and also containing sun protection factor (SPF) has been developed. These materials have been researched extensively as active ingredients in sunscreen and should require no further safety evaluations.^23,24 In our study, carbon- and clay-based adsorbents have been incorporated in this formulation for their protection against UV radiation ranging from 250 to 400 nm,²⁵ and more importantly, to prevent the incursion of hazardous environmental pollutants from water. Montmorillonites, with high surface area, neutral color, tunable pore size, and biocompatibility, have been shown to adsorb environmental chemicals efficiently and have been included in EVBs for environmental decontamination.^26-36

In this study, an EVB formulation containing natural GRAS materials and adsorbents was developed to adsorb and prevent the incursion of highly toxic, water-soluble chemicals from polluted water. We hypothesize that the topical application of EVB formulation with montmorillonites embedded can effectively adsorb and facilitate the blockage of pesticides and PFAS when present in flooded water at contaminated sites.

2. MATERIALS AND METHODS

2.1. Preparation of Barrier Formulations.

The nonpolar component of the barrier formulation contained 64 g coconut oil, 32 g olive oil, 32 g shea butter, and 28 g beeswax and was heated in a water bath at 50 °C until all materials were liquified. They were stirred with 13 g zinc oxide, 132 g glycerin, 26 g urea, and ten drops of Vitamin E and essential oils. All components in this EVB emulsion are GRAS materials that are commonly included in other skincare products. After the EVB base formulation was cooled down to room temperature, hydrated adsorbents were added at 5–30% w/w. Medical grade activated carbon (AC) derived from the coconut shell (mesh size: 100–325; iodine number: 1100 mg/g; bulk density: 477–530 kg/m³) was obtained from the General Carbon Corporation (Paterson, NJ) and calcium and sodium montmorillonites (CM and SM) were obtained from Engelhard Corp. (Cleveland, OH).^29,34,37 The physicochemical properties of the carbon and clays have been previously characterized and reported.^28,29

2.2. Characterization.

To evaluate the physical stability of the barrier formulations, 5 g of each sample including EVB, EVB-AC, EVB-CM, and EVB-SM was subjected to three centrifugal cycles at 3000 rpm for 30 min in each cycle. This assay was performed at 27 ± 2 °C. Organoleptic characteristics (color, odor, and appearance), density, pH of point of zero charge (pH_pzc), and viscosity values were also assessed. Density was calculated based on the difference between pycnometer weight (without and with 2 mL of the sample) divided by the sample volume.³⁸ pH_pzc of EVB formulations was measured using the pH drift method.^28,39 Briefly, solutions of 0.1 M of NaCl were prepared to pH values between 2 and 11 using distilled and deionized water that was boiled for the removal of dissolved CO₂. EVB formulations at 100 mg were added to 20 mL of the NaCl solutions and were shaken for 48 h at 200 rpm and room temperature. The final pH of the solution was measured by the Pinnacle series M530P pH meter (Corning Electrochemistry, NY). The pH_pzc was determined as the pH of the NaCl solution that did not change after contact with the samples. Results were obtained from three measurements for each sample, and the assay was carried out in triplicate.

2.3. Adsorption Screening.

Individual chemical stock solutions containing 2 ppm (mg L⁻¹) glyphosate, 1.3 ppm acrolein, 0.3 ppm paraquat, and a mixture of 0.1 ppm PFOA and PFOS in pH7 water were prepared from pure crystals of chemicals (Thermo Fisher, Waltham, WA). These concentrations of chemicals were determined based on the maximum levels detected in the aquatic system in the US.^40-46 EVB formulations were applied evenly on 15.8 mm filter papers (Millipore Co., Bedford, MA) at 0.1 g and placed at the bottom of a 24-well plate. Chemical stock solutions at volumes of 2 mL were added to each well and mixed with EVB formulations on a rocking platform (VMR, Hamburg, Germany) at 600 rpm and 37 °C for 2 h, which is the suggested duration for reapplication of sunscreen products. Controls included 2 mL of blank solution (pH 7 water), chemical solution, and EVB formulations. All samples were then centrifuged at 2000g for 20 min. The aliquot of the supernatant was collected and filtered through a Strata C18-E column (55 μm, 70 Å) (Phenomenex, Torrance, CA) following previous methods.^47-50 Briefly, columns were preconditioned with 2 mL of methanol and 2 mL of water. For hydrophilic pesticides, columns were loaded with samples and eluted directly with 1 mL of water into injection vials. For PFAS with 8-carbon chains, columns with samples loaded were washed twice with water and dried under high vacuum (10–15 mm Hg) for 5 min and then eluted with 1 mL of methanol. The filtrate containing free chemicals was individually detected using a Waters HPLC or Waters Acquity LC–MS/MS (Milford, MA) following previously described methods.^30,31,51 The reduction rates (%) of each chemical after treatment were calculated as the difference between chemical controls and treatment with EVB, EVB-AC, EVB-CM, and EVB-SM at different doses.

2.4. Adsorption Kinetics.

Adsorption kinetics is important in that it controls the efficiency of the process, and the models can correlate the adsorbate uptake rate with its concentration. Also, it is important to study the effectiveness of the EVB formulations in adsorbing chemicals following realistic exposure durations (within a few hours).¹

For estimating the optimal amount of adsorbent per unit mass of the adsorbate, 0.05 g of EVB and EVB-SM were placed in contact with chemical solutions at various concentrations, that is, glyphosate at 4, 2, and 0.5 ppm; acrolein at 0.6 and 0.15 ppm; paraquat at 0.45, 0.3, and 0.15 ppm; PFAS at 0.3, 0.2, and 0.1 ppm. These chemical/formulation mixtures were agitated at 37 °C in pH 7 water for a total of 4 or 6 h until equilibrium was achieved. The kinetics of adsorption were determined by analyzing the adsorption of chemicals from the aqueous solution at different time intervals (10 min, 30 min, 1 h, 2 h, 4 h, and 6 h). To analyze the adsorption rate, pseudo-first-order, pseudo-second-order, and Elovich models were used to investigate the adsorption kinetics of chemicals onto the EVB formulations.^52,53 The nonlinear pseudo-first-order rate equation is expressed as follows

$$q_{{}_t}=q_{{}_{\mathrm{e}}}\times [1-\text{exp}(-K_1\times t)]$$ (1)

where q_e (mg/kg) and q_t (mg/kg) are the amounts of chemical adsorbed at equilibrium and at time t. K₁ (min⁻¹) is the rate constant of the first-order biosorption process.

The pseudo-second-order kinetic model is expressed as

$$q_{{}_t}=\frac{K_2\times q{e}^2\times t}{1+qe\times K_2\times t}$$ (2)

where q_e and q_t are the amounts of the chemical removed per unit mass of EVB formulations (mg/kg) at equilibrium and at time t (min), and K₂ (mg/kg min) is the pseudo-second-order rate constant.

The Elovich equation is one of the most useful models for describing activated adsorption reactions.⁵⁴ The Elovich equation is expressed as given below

$$q_{{}_t}=\frac{1}{b}\text{ln}ab+\frac{1}{b}\text{ln}t$$ (3)

where a is the initial adsorption rate (mg/kg min) and b is related to the extent of surface coverage and the activation energy involved in chemisorption (mg/kg). A trial and error procedure, which was applicable to computer operation for nonlinear models, was developed and used to determine the kinetic parameters by minimizing the squared deviation and coefficient of determination between experimental data and predicted values using the “solver” add-in with Microsoft’s spreadsheet.⁵⁵

Additionally, to study the effect of solution pH on the kinetics of adsorption, the chemical/formulation mixtures were dissolved at pH 4, 7, and 8.4 for 2 h to simulate the pH of realistic acid rain, neutral water, and marine/brackish water, respectively. Furthermore, the effect of the contact temperature on the kinetics was also investigated by agitating the chemical/formulation mixtures at three different temperatures (4, 24, and 37 °C) for 2 h. In these studies, chemical solutions were included at the same levels as in the screening study, (2 ppm glyphosate, 1.3 ppm acrolein, 0.3 ppm paraquat, and 0.1 ppm PFAS). EVB and EVB-SM formulations were included at the same levels as in the kinetic study (0.05 g).

2.5. Adsorption/Desorption Isotherms and Thermodynamics.

Chemical solutions at the same concentration as in previous studies including 2 ppm glyphosate, 1.3 ppm acrolein, 0.3 ppm paraquat, and a mixture of 0.1 ppm PFOA and PFOS were prepared in pH 7 water to simulate realistic contamination levels in US water. Then, 4 mg EVB and EVB-SM formulations were applied evenly on 15.8 mm filter papers and placed at the bottom of 24-well plates. Controls included 2 mL of blank water solution, chemical solution, and EVB formulations in water. All samples were vibrated at 37 °C and 600 rpm using a rocking platform for 2 h, as 2 h is the suggested reapplication duration for most sunscreen products. The adsorption of glyphosate was also conducted at 4 and 24 °C to determine the interaction enthalpy. The chemical/formulation complex was then separated from the solution by centrifugation at 2000g for 20 min, and the supernatants were filtered through the Strata C-18E column (55 μm, 70 Å). The filtrates containing free chemicals were detected using a Waters HPLC or Waters Acquity LC–MS/MS as mentioned above.

The desorption of chemicals from the surface of formulations was tested at the end of adsorption experiments. Chemical-loaded formulations on the filter paper were separated from the chemical solutions and then filled with 2 mL of pH 7 water and agitated at 500 rpm and 37 °C for 24 h. Then, the solution was centrifuged at 2000g for 20 min and passed through the Strata C-18E column (55 μm, 70 Å) for analysis. The amount of chemical that remained bound after desorption was determined as the difference between the initial adsorbed and the desorbed amount. For both adsorption and desorption, the dry weight of formulations on filter papers, before and after the experiments, was determined for selected concentrations, and no significant change was observed.

2.6. Data Calculations and Curve Fitting.

The quantity of bound chemicals was calculated by the concentration difference between control and test groups and expressed as mg/kg of EVB formulations on the isotherm plots. Table-Curve 2D (Systat Software, Inc, Palo Alto, CA) with R programming was used to plot adsorption data and derive values for the variable parameters. The R code was used to calculate adsorption values and fitness to standard models based on maximum likelihood estimation, and standard deviations and confidence bands were calculated using the information matrix method.^56,57 The adsorption isotherms were plotted by the Freundlich model using mean values of observed data points from independent triplicate analyses. The Freundlich isotherm was used to describe the adsorption characteristics for a heterogeneous surface, as represented by the following equation

$$q_{{}_{\mathrm{e}}}=K_{\mathrm{f}}{C}_{\mathrm{e}}^{1∕n}$$ (4)

where K_f = Freundlich distribution constant; 1/n = degree of heterogenicity.

Adsorption parameters coupled with the van’t Hoff equation were used to calculate enthalpy (ΔH)

$$\Delta H=\frac{-R\text{ln}\left(\frac{K_{\mathrm{d}2}}{K_{\mathrm{d}1}}\right)}{\left(\frac{1}{T_2}\right)-\frac{1}{T_1}}$$ (5)

where R (gas constant) = 8.314 J/mol/K and T (absolute temperature) = 273 + t (°C).

The percentage of desorption was calculated by the following equation

$$\%\text{desorption}=100\times \left(1-\frac{Q_{\mathrm{e}}}{q_{\mathrm{e}}}\right)$$ (6)

where q_e = amount bound at equilibrium in the adsorption study (g/kg) and Q_e = amount that remained bound in the desorption study (g/kg).

2.7. Statistical Analysis.

All experiments included blanks and negative controls and were independently triplicated. A one-way ANOVA followed by a post-hoc Tukey test was used to determine statistical significance. The reduction % in the screening study and mass bound in the kinetic studies were calculated for standard deviation and p-value. Bonferroni correction was used for multiple test corrections.⁵⁸ Results were considered significant at p ≤ 0.05.

3. RESULTS AND DISCUSSION

3.1. Characterization.

Topical formulations should exhibit acceptable mechanical characteristics, such as easy application and suitable spreadability, enabling skin adhesion.³⁸ In this study, all tested formulations possessed desirable parameters for the application of a cream product onto the skin surface. For example, the formulations were easily applied and spread evenly onto human skin, and no visible difference in color or texture was observed after washing under running water for 10 min, suggesting high adhesiveness. EVB formulations were white, creamy, and shiny in appearance. They had a characteristic odor, a pH_pzc around 7.2 that was compatible with the skin, and density around 1.12 ± 0.1 g/mL. According to the scientific literature, these are all desirable characteristics for a cosmetic product that will be applied onto the skin. Depending on the concentration of the selected components, formulations with 5% inclusion of adsorbents exhibited the optimal skin sensation, viscosity, and uniform texture; therefore, EVB test formulas were amended with 5% AC, SM, and CM in the following adsorption experiments.

3.2. Adsorption Screening.

The binding results for the 5 chemicals onto EVB formulations, with and without adsorbents, are shown in Figure 1. The base EVB formulation effectively adsorbed and removed acrolein by 42 ± 5.8%, paraquat by 53 ± 6.0%, and PFOA by 41 ± 5.2%, with limited effects on glyphosate and PFOS. Importantly, the inclusion of 5% CM and especially SM significantly enhanced the removal rate for glyphosate (55 ± 5.2%), acrolein (59 ± 9.3%), and PFOS (21 ± 3.2%), possibly because of the high expansibility of SM clays in water that facilitated the attraction of chemicals into active binding sites. This is in alignment with our previous results suggesting that these toxins were adsorbed onto the surfaces of montmorillonites.^30,31,59 In the following experiments, SM was included in the composition for the comparison with base EVB formulations, and not carbon, to avoid the black color of the EVB-AC composition.

Figure 1.

Percent reduction in the adsorption of (A) glyphosate, (B) acrolein, (C) paraquat, (D) PFOS, and (E) PFOA by various EVB formulations.

3.3. Effect of Initial Chemical Concentration and Contact Time on Adsorption.

Contact time is one of the important parameters for a successful adsorption application. To examine the effect of initial chemical concentration, a time course study was carried out with EVB and EVB-SM at 37 °C and pH 7. The experiment was conducted for 4–6 h to ensure that reaction equilibrium was reached. The effect of contact time on the adsorption of the five chemicals by EVB and EVB-SM is shown in Figure 2. The amount of chemicals bound to the formulations (mg/kg) increased with increasing initial concentrations for all five chemicals. This was attributed to the fact that the driving force, which depended on the concentration gradient, increased with the increasing initial concentrations.⁶⁰ These results suggest that the available sites on the EVB formulations are sufficient for binding and have not reached saturation. It was observed that the adsorption process of glyphosate reached equilibrium within 60 min for all concentrations, the adsorption of paraquat, PFOS, and PFOA reached equilibrium within 120 min, and the adsorption of acrolein required 240 min to reach equilibrium. Importantly, these chemicals remained bound and the adsorption capacity achieved a constant value after equilibrium had been reached, suggesting that the adsorption interactions were stable with limited dissociation. In most cases, 90% of the chemicals were adsorbed within the first 10 min. This fast rate was possibly due to excellent surface adsorption and high binding affinity for these environmental chemicals.

Figure 2.

Effect of various initial chemical concentrations on the adsorption of (A) glyphosate, (B) acrolein, (C) paraquat, (D) PFOS, and (E) PFOA on 0.05 g EVB and EVB-SM at 37 °C and pH 7.

3.4. Kinetic Models.

Kinetics and equilibrium of the adsorption process are the two major parameters to evaluate adsorption dynamics. The above time course results were analyzed by three common nonlinear kinetic models. The adjusted correlation coefficient (R²) values for the pseudo-second-order kinetic model for all chemicals at all concentrations were higher than the values for the pseudo-first-order and Elovich models. Specifically, the value of R² for the pseudo-second-order model was ≥0.98 for EVB-SM and ≥0.74 for EVB for all chemicals at all concentrations. Additionally, the calculated adsorption capacities (q_e, cal) by the pseudo-second-order kinetic model were close to the experimentally observed q_e (exp) values for all chemicals. Therefore, it was suggested that the pseudo-second-order model was more suitable for describing the adsorption of glyphosate, acrolein, paraquat, PFOS, and PFOA onto EVB and EVB-SM formulations, and this data is shown in Table 1. To delineate binding mechanisms and interaction energy, adsorption/desorption isotherms and thermodynamic studies were also conducted.

Table 1.

Adsorption Constants Calculated by the Nonlinear Pseudo-Second-Order Model and the Experimental Binding Capacities for Chemicals at Different Initial Concentrations

		EVB				EVB-SM
chemicals	conc (Ci, mg L−1)	qe (exp, mg kg−1)	qe (cal, mg kg−1)	K 2	R adj 2	qe (exp, mg kg−1)	qe (cal, mg kg−1)	K 2	R adj 2
glyphosate	4	60 ± 2	59 ± 3	2.6 × 10−3	0.99	67 ± 0.2	65 ± 0.3	1.1 × 10−2	1
	2	9.5 ± 0.9	8.6 ± 1	8.9 × 10−3	0.74	28 ± 0.3	28 ± 0.5	1.4 × 10−2	1
	0.5	2.7 ± 0.5	1.8 ± 0.8	6.6 × 103	0.87	6.7 ± 0.03	6.1 ± 0.04	5.4 × 107	1
acrolein	0.6	43 ± 1	50 ± 2	9.9 × 10−5	0.97	63 ± 1.1	70 ± 2	1.0 × 10−4	0.93
	0.15	10 ± 0.2	10 ± 0.2	3.4 × 10−3	0.99	14 ± 0.7	11 ± 0.6	6.9 × 10−2	0.99
paraquat	0.45	17 ± 0.7	15 ± 0.5	4.8 × 101	1	18 ± 0.6	16 ± 1.0	4.9 × 103	1
	0.3	7.2 ± 0.3	5.0 ± 0.5	2.2 × 103	1	9.1 ± 0.4	7.4 ± 0.9	7.9 × 102	1
	0.15	5.1 ± 0.08	4.3 ± 0.1	8.6 × 104	0.99	5.2 ± 0.08	4.4 ± 0.01	9.7 × 104	0.99
PFOS	0.3	2.3 ± 0.05	1.4 ± 0.07	9.4 × 10−1	0.99	2.6 ± 0.02	1.6 ± 0.02	2.1 × 10−1	1
	0.2	2.1 ± 0.01	1.3 ± 0.02	5.9 × 10−1	1	0.98 ± 0.02	0.99 ± 0.02	3.5 × 10−1	1
	0.1	0.88 ± 0.06	0.62 ± 0.09	2.1 × 10−1	0.95	0.75 ± 0.05	0.81 ± 0.06	4.3 × 10−2	0.99
PFOA	0.3	8.3 ± 0.3	7.9 ± 0.4	2.8 × 10−3	0.80	6.2 ± 0.07	7.3 ± 0.1	3.4 × 10−3	1
	0.2	7.1 ± 0.3	6.2 ± 0.3	7.2 × 10−3	0.98	6.3 ± 0.1	5.8 ± 0.2	6.4 × 10−3	0.98
	0.1	3.0 ± 0.05	2.2 ± 0.08	5.3 × 10−2	0.99	2.6 ± 0.2	3.6 ± 0.2	3.7 × 10−3	1

3.5. Effect of Contact Temperature and Solution pH.

To study the effect of temperature on the kinetics of adsorption, the chemical/formulation complexes were reacted in pH 7 water for 2 h at three different temperatures (4, 24, and 37 °C). As shown in Figure 3, the adsorption amount of all five chemicals onto EVB and EVB-SM decreased with increased temperature. This result indicated that the adsorption of EVB and EVB-SM was exothermic, with release of heat. Another possible reason contributing to the lower binding at higher temperatures was the fact that chemical solubility in water can be increased with temperature, reducing the interaction with EVB formulations.

Figure 3.

Effect of contact temperature on the adsorption of (A) glyphosate, (B) acrolein, (C) paraquat, (D) PFOS, and (E) PFOA on 0.05 g EVB and EVB-SM in pH 7 water.

The adsorption results at different pH values (4, 7, and 8.4) are shown in Figure 4. The chemical adsorption onto EVB and EVB-SM showed a minimal effect by pH, possibly because the charge on the chemicals within the pH range from 4 to 8.4 remained the same, that is, glyphosate and PFAS were negatively charged, paraquat was permanently positively charged, and acrolein was neutral. This indicates that the adsorption of all five chemicals was consistent and reproducible onto EVB and EVB-SM formulations.

Figure 4.

Effect of solution pH on the adsorption of (A) glyphosate, (B) acrolein, (C) paraquat, (D) PFOS, and (E) PFOA on 0.05 g EVB and EVB-SM at 37 °C.

3.6. Adsorption/Desorption Isotherms.

Isothermal data reflecting the adsorption of glyphosate, acrolein, paraquat, and PFAS onto EVB (Figure 5) and EVB-SM (Figure 6) surfaces were plotted in a Freundlich model, indicating heterogeneous binding sites. Based on the International Union of Pure and Applied Chemistry (IUPAC) classification of isotherms for the purpose of structural characterization, the adsorption isotherms of glyphosate and PFAS fit type III, indicating surface binding interactions mainly at macroporous sites, whereas the isotherms of acrolein and paraquat fit type I, indicating the major surface binding interactions at microporous sites on the surfaces of EVB and EVB-SM.^61,62 The values of Freundlich constants K_f and n were obtained from the plots of C_e versus q_e and are shown in Table 2 for EVB and Table 3 for EVB-SM. For adsorption isotherms of acrolein and paraquat onto EVB-SM, 1/n values were less than 0.7. It described a highly curved shape on the isotherm plot indicating that the adsorption on EVB-SM was favored with high heterogeneity in binding sites.⁶³ These curved plots can also fit a Langmuir model indicating saturable binding sites available to the chemicals. On the other hand, the adsorption isotherms of glyphosate and PFAS onto EVB-SM showed 1/n values > 1, indicating S-type isotherms and relatively high adsorption ability. These are relatively uncommon but are often observed at low concentration ranges for compounds containing a polar functional group. It has been hypothesized that, at low concentrations, such chemicals were in competition with water for adsorption sites. The high reproducibility of the adsorption data and the high fitness to the Freundlich model were predicted by the low mean squared error (MSE) values and 95% confidence interval bands from independent triplication.

Figure 5.

Adsorption isotherms of (A) glyphosate, (B) acrolein, (C) paraquat, (D) PFOS, and (E) PFOA on surfaces of 4 mg EVB, plotted by the Freundlich model. Data represent the mean adsorption (mg/kg) at each concentration, run in triplicate. Bands indicate 95% confidence intervals on the mean response.

Figure 6.

Adsorption isotherms of (A) glyphosate, (B) acrolein, (C) paraquat, (D) PFOS, and (E) PFOA on surfaces of 4 mg EVB-SM surfaces, plotted by the Freundlich model. Data represent the mean adsorption (mg/kg) at each concentration, run in triplicate. Bands indicate 95% confidence intervals on the mean response.

Table 2.

Parameters of Freundlich Adsorption/Desorption Isotherms on EVB

	chemicals adsorbed			chemicals remaining after desorption
chemicals	K f	n	MSE	K f	n	MSE	desorption %
glyphosate	65 ± 6	0.81 ± 0.1	390	51 ± 5	0.74 ± 0.2	383	15.9
acrolein	127 ± 9	2.3 ± 0.5	237	81 ± 5	2.2 ± 0.4	92	39.5
paraquat	125 ± 3	2.6 ± 0.5	94	117 ± 22	2.3 ± 0.4	59	10.7
PFOS	8260 ± 13	0.39 ± 0.09	2.3	8810 ± 78	0.33 ± 0.04	0.37	24.4
PFOA	37 ± 1	2.0 ± 0.3	2.0	30 ± 3	1.8 ± 0.2	1.3	34.1

Table 3.

Parameters of Freundlich Adsorption/Desorption Isotherms on EVB-SM

	chemicals adsorbed			chemicals remaining after desorption
chemicals	K f	n	MSE	K f	n	MSE	desorption %
glyphosate	69 ± 6	0.77 ± 0.1	209	49 ± 5	0.67 ± 0.1	241	19.5
acrolein	258 ± 20	3.9 ± 0.7	788	174 ± 12	3.2 ± 0.6	362	31.2
paraquat	134 ± 28	2.8 ± 0.3	124	161 ± 10	2.8 ± 0.4	63	6.42
PFOS	3410 ± 113	0.37 ± 0.07	0.46	2320 ± 27	0.37 ± 0.06	0.19	25.4
PFOA	881 ± 43	0.65 ± 0.07	1.2	2270 ± 49	0.50 ± 0.09	0.13	30.3

The desorption study showed that more than half of the chemicals remained bound on formulations after washing with pH 7 water for 24 h. Specifically, the desorption rate from EVB-SM surfaces (ranging from 6.42 to 31.2%) was less than that from EVB surfaces (ranging from 10.7 to 39.5%), suggesting enhanced binding sites and stability with the inclusion of montmorillonite clays.

To investigate the binding mechanisms and energy, thermodynamic studies on glyphosate were conducted at three different temperatures (4, 24, and 37 °C) to calculate the enthalpy (ΔH). The adsorption of glyphosate on the surfaces of EVB and EVB-SM at 4 and 24 °C is shown in Figure 7. The average enthalpy values derived from the van’t Hoff equation were equal to −7.5 ± 0.5 kJ/mol for EVB and −16 ± 4 kJ/mol for EVB-SM (Table 4). The negative signal indicated that the adsorption reaction was exothermic and spontaneous, which is consistent with adsorption results at varying temperatures, as shown in Figure 3. The absolute values of enthalpy (∣ΔH∣ < 20 kJ/mol) indicated that physisorption mechanisms (such as van der Waals forces and hydrogen bonds) were involved in the reaction. This aligned with the finding that the Freundlich model was the best fit for adsorption isothermal plots.

Figure 7.

Adsorption thermodynamics of glyphosate at 4 and 24 °C on surfaces of (A) 4 mg EVB and (B) 4 mg EVB-SM, plotted by the Freundlich model. Data represent the mean adsorption (mg/kg) at each concentration, run in triplicate. Bands indicate 95% confidence intervals on the mean response.

Table 4.

Parameters of Freundlich Thermodynamics of Glyphosate on EVB and EVB-SM

	EVB				EVB-SM
	K f	N	MSE	ΔHave (kJ/mol)	K f	N	MSE	ΔHave (kJ/mol)
24 °C	73 ± 4	0.85 ± 0.08	76	−7.5 ± 0.5	82 ± 3	3.4 ± 1	40	−16 ± 4
4 °C	93 ± 2	0.83 ± 0.05	48		147 ± 9	0.64 ± 0.07	424

4. CONCLUSIONS

Recreational sunscreens are made for outdoor activities which frequently involve extended periods of water exposure. Importantly, exposure to water can cause stratum corneum to swell and enhance the skin permeability to chemical contaminants in water. Furthermore, this problem can be magnified during disasters such as floods and hurricanes, when water-soluble pollutants in the environment can be mobilized from hazardous waste sites and redistributed in contaminated sediments and flood waters, thus threatening the health of people and animals during these events. Therefore, the development of safe formulations that possess water resistance and stability, with the potential to resist hygroscopic pressure, maintain the integrity of the barrier, and reduce dermal exposures to highly toxic environmental chemicals is warranted. In this study, a W/O emulsion formulation (EVB) was developed using all GRAS materials including active adsorbents to mitigate dermal contact to water-soluble pollutants in water.

Novel EVB barrier formulations, with and without montmorillonite clay inclusion, exhibited stability for 3 months and a neutral color. Properties, such as high spreadability and adhesiveness, neutral pH comparable to skin pH, and viscoelastic behaviors, were suitable for topical application. Our base EVB is comparable to commercial sunscreens with a total SPF of 30. Moreover, it only contains all GRAS materials. The present research indicates that EVB and EVB-SM can be used as field-practical, cost-effective, and efficacious barrier formulations to reduce skin absorption of carcinogenic and highly toxic environmental chemicals such as glyphosate, acrolein, paraquat, PFOS, and PFOA from polluted water. This conclusion is supported by the following in vitro adsorption results: (1) the EVB binding results of all five chemicals at the maximum level detected in US water systems showed adsorption with high capacity, (2) the inclusion of montmorillonites at 5% w/w in the EVB formulation enhanced the binding percentages of glyphosate, acrolein, and PFOS, compared to EVB alone, (3) the adsorption of all five chemicals reached equilibrium within 240 min, with stable binding onto EVB and EVB-SM surfaces, and no observable dissociation, (4) the adsorption has high affinity and 90% binding occurred in the first 10 min of contact, (5) the adsorption of chemicals onto EVB and EVB-SM surfaces followed pseudo-second-order, (6) the kinetic studies with varying temperatures and pH showed that binding onto EVB and EVB-SM surfaces was exothermic for all chemicals, and pH had a limited effect on the binding interaction, and (7) the adsorption/desorption isotherms and thermodynamics demonstrated that adsorption onto EVB and EVB-SM fit the Freundlich model with lower than 40% desorption rate and negative enthalpy values, indicating physisorption and spontaneity of the reaction. Due to their high adsorption efficacy, these novel EVB barrier formulations show promise for use by first responders and vulnerable populations to reduce common chemical exposures from flood waters following disasters. They may also be used as enhanced sunscreens, with value-added protection from environmental chemicals in polluted water. Further studies are ongoing to investigate other natural products and active sorbents for inclusion in the base EVB formulation for the adsorption of environmental chemicals and microbes from polluted water.