Soil surface acidity plays a determining role in the atmospheric-terrestrial exchange of nitrous acid

Significance

Nitrous acid (HONO) emitted from soil plays an important role in regulating the oxidizing capacity of the atmosphere. Unexpectedly high emissions of HONO are observed from soils with close to neutral pH, where aqueous acid–base and Henry’s law equilibria would predict that nitrite is the dominant nitrogen species. We show that surface acidity of soil minerals rather than aqueous acid–base equilibria controls nitrite speciation in soil, implying that up to 70% of global soils are capable of emitting HONO due to their acidic or close to neutral pH; this suggests that soil nitrite, whether microbially derived or otherwise deposited, likely plays a more widespread role in terrestrial–atmospheric cycling of nitrogen affecting air pollution and climate than previously thought.

Abstract

Nitrous acid (HONO) is an important hydroxyl (OH) radical source that is formed on both ground and aerosol surfaces in the well-mixed boundary layer. Recent studies report the release of HONO from nonacidic soils, although it is unclear how soil that is more basic than the pK_a of HONO (∼3) is capable of protonating soil nitrite to serve as an atmospheric HONO source. Here, we used a coated-wall flow tube and chemical ionization mass spectrometry (CIMS) to study the pH dependence of HONO uptake onto agricultural soil and model substrates under atmospherically relevant conditions (1 atm and 30% relative humidity). Experiments measuring the evolution of HONO from pH-adjusted surfaces treated with nitrite and potentiometric titrations of the substrates show, to our knowledge for the first time, that surface acidity rather than bulk aqueous pH determines HONO uptake and desorption efficiency on soil, in a process controlled by amphoteric aluminum and iron (hydr)oxides present. The results have important implications for predicting when soil nitrite, whether microbially derived or atmospherically deposited, will act as a net source or sink of atmospheric HONO. This process represents an unrecognized mechanism of HONO release from soil that will contribute to HONO emissions throughout the day.

Nitrous acid (HONO) plays a significant role in regulating the oxidative capacity of the atmosphere as it is readily photolyzed to produce nitric oxide (NO) and hydroxyl radical (OH) (1). The latter product, OH, is an important atmospheric oxidant that leads to ozone and secondary aerosol formation (2, 3). Despite extensive research over three decades (4–12), HONO formation and loss processes are not completely understood, and current atmospheric models do not accurately predict nighttime and daytime levels (13–18). Models and field measurements of vertical HONO gradients indicate that reactions on ground surfaces may explain the inconsistencies (12, 13, 15, 19–21) and that the hydrolysis of NO₂ on these surfaces is the most important HONO source at night (22–25) that likely continues through the daytime (19). Recent investigations demonstrate that nitrite derived from microbial activity is a source of atmospheric HONO from nonacidic soil (i.e., soil pH >5) (26, 27). However, the mechanism responsible for HONO release from soil nitrite remains unclear because the pK_a of HONO in bulk water is ∼3 (28–30); this suggests that our understanding of bulk aqueous equilibria may not be applicable to understanding the speciation of soil-adsorbed N(III) species [N(III) = NO₂ˉ, HONO, H₂ONO⁺]. Here we report that the surface acidity of boundary layer minerals plays a critical role in the ability of a surface to release HONO or sequester it as nitrite (NO₂ˉ) or the nitroacidium ion (H₂ONO⁺) on the surface. We also demonstrate the disproportionate importance of aluminum and iron (hydr)oxides in controlling HONO uptake and release on soil surfaces.

In our previous study of HONO uptake onto soil surfaces at varied relative humidity (RH), we observed that as the RH increased above 30%, the mineral surfaces were covered by the equivalent of a monolayer of adsorbed H₂O and that mass transfer from the gas phase into the liquid phase and subsequent acid–base equilibria control the net uptake of HONO onto soil under atmospherically relevant conditions (31–33). It therefore seems reasonable that soil pH is an important variable in determining the uptake of HONO that deserves further attention. Soil is a complex mixture of amphoteric minerals, including aluminum and iron (hydr)oxides whose surface reactivity is pH dependent and can differ greatly from that measured in bulk water (34). Surface metal oxides are coordinatively unsaturated, and chemisorption of H₂O leads to the formation of neutral M–OH and charged M–Oˉ or M–OH₂⁺ groups (where M = Al³⁺ and Fe³⁺), depending on the pH of the environment (Fig. 1B) (35–37). Surface M–OH₂⁺ groups in particular are acidic due to the ability of the metal cation to withdraw electrons from the bound H₂O and weaken the O–H bond. By analogy, it is well known that aqua complexes of metal cations such as [Fe(OH₂)₆]³⁺ and [Al(OH₂)₆]³⁺ readily donate protons, an effect that is attenuated by outer-sphere hydration (hydrogen bonding) (38).

Fig. 1.

An illustration of the speciation of HONO in natural systems. (A) Partitioning between bulk aqueous and gas phases is determined by Henry’s law. Once in the aqueous phase, HONO dissociates into NO₂ˉ or may be protonated to form the nitroacidium cation (H₂NO₂⁺), depending on the pH. (B) Speciation of HONO on soil surfaces depends on the surface acidity, which is controlled in part by amphoteric metal oxides present.

The existence of chemisorbed H₂O in the soil system means that HONO and NO₂ˉ not only interact with species such as H₃O⁺ in bulk water as shown in Fig. 1A, but they may also undergo proton transfer reactions with mineral substrates (Fig. 1B); if this is true, then it is possible that the apparent pK_a of the NO₂ˉ/HONO system could be influenced by the availability of surface protons, whose surface density may not necessarily be reflected by the bulk pH. In one example of how surface acidity is different from bulk acid–base equilibria, the pK_a of ammonia was shown to be lowered from 9.3 in bulk water to 8.4 on a hydrated hematite interface (39).

We propose that the degree of protonation of amphoteric soil mineral surfaces yielding M–Oˉ, M–OH, and M–OH₂⁺ determines the pH dependence of HONO uptake onto soil mineral surfaces rather than its pK_a in bulk water. To test this hypothesis, we measured the uptake coefficients for HONO adsorption as a function of soil pH and monitored the evolution of HONO from nitrite additions to pH-adjusted soil and other model substrates under atmospherically relevant conditions of pressure (1 atm), HONO mixing ratios (10 ppb), and relative humidity (30% RH). In addition, we used potentiometric titrations to determine the surface charge of each substrate to better understand the relationship between surface acidity and the speciation of N(III) in the soil system. This study provides a fundamental understanding of how atmospherically significant emissions of HONO arise from soils with close to neutral pH where traditional aqueous phase equilibria would predict that nitrite is the dominant nitrogen species. To our knowledge, the surface acidity of soil minerals has never before been linked to the pH-dependent exchange of HONO between the terrestrial and atmospheric environments. The results have important implications for predicting when soil NO₂ˉ, whether microbially derived or otherwise deposited, will act as a net source or sink of atmospheric HONO. Including this mechanism within parameterizations of HONO sources will improve the accuracy of atmospheric models used for air pollution prediction and control.

Results and Discussion

Dependence of HONO Uptake on pH.

A coated-wall flow reactor coupled to a chemical ionization mass spectrometer (CIMS) was used to study the adsorption of HONO on pH-adjusted agricultural soil and other soil components. Uptake of HONO onto the substrate-coated surface was initiated by retracting the moveable injector to a position upstream of the coated tube, thereby exposing the substrate layer to a well-mixed and laminar flow of HONO and humidified air. The uptake of HONO onto a surface is described by the uptake coefficient, γ_net, a unitless measure of the fraction of collisions with the wall surface that results in the removal of the species from the gas phase (40–42). This ratio reflects the balance of HONO fluxes due to adsorption (J_ads) and desorption (J_des) divided by the total flux of HONO molecules hitting the surface (J_coll), as illustrated by Eq. 1.

$${\gamma }_{net}=\frac{J_{ads}-J_{des}}{J_{coll}}$$ [1]

When considering the pH dependence of γ_net for HONO, we expect a parabolic trend reflective of the pH-dependent speciation of HONO: Adsorption is most likely at high pH where HONO can remain on the surface as nitrite (NO₂ˉ), or at very low pH where nitroacidium ion (H₂ONO⁺) is the predominant form of N(III) (Fig. 1). At high pH, HONO is expected to react with basic M–Oˉ groups, leaving behind solvated nitrite and covalently bonded nitrate, nitrito (–ONOˉ), and nitro (–NO₂ˉ) groups, the existence of which is supported by infrared spectroscopy studies on related metal oxide systems (43–46). At the acidic and basic extremes, the γ_net is high because adsorption outcompetes desorption. Considerations of bulk aqueous equilibria for HONO (Fig. 1A) suggest that the lowest γ_net values would be expected in the range of pH 2–3, because this is where desorption of volatile HONO becomes possible. Wetted-wall flow tube experiments show that values of γ for the uptake of HONO into bulk water are indeed lowest at between pH 2 and 3 (47). Experiments on soil surfaces, however, show the occurrence of the minimum at pH 6.2 (Fig. 2A), nearly 3 pH units above the pK_a of HONO, which suggests that the interaction of HONO with solid surfaces is different from its interaction with water and that the surface charge of the amphoteric minerals in soil plays an important role.

Fig. 2.

A comparison of HONO speciation and surface acidity. This matrix shows the agreement between measurements of (A) HONO uptake; (B) HONO production from nitrite; and (C) surface charge for the three substrates: agricultural soil, kaolinite, and 2% wt/wt AlCl₃ in kaolinite. Lines on the HONO uptake and the HONO production plots are connections of the data to guide the eye, and the potentiometric titration has a polynomial fit used to derive the point of intersection, the PZSE. The error bars represent the 95% confidence interval of the mean of replicate measurements. The pH values highlighted by yellow bars represent the intersection of minimal HONO uptake, the beginning of HONO production from soil nitrite, and the PZSE, all of which are indicative of neutral surface charge.

To further explore the interaction of HONO with mineral surfaces, we chose kaolinite as a model system because it is both present in our soil sample (SI Appendix) and its surface properties have been well characterized in previous studies (48–52). Kaolinite is a phyllosilicate mineral of alternating tetrahedral silicon and octahedral aluminum sheets that has been shown to have a neutral surface charge at pH ∼3 (48, 53); i.e., at a pH of 3, the surface density of M–OH₂⁺ and M–Oˉ groups is equal. We also modified the kaolinite surface by adding 2% wt/wt AlCl₃, because this has been shown to coat the kaolinite with amorphous Al³⁺-based hydrolysis products, e.g., Al(OH)₃. The coating shifts its neutral surface charge to several pH units above that of pure kaolinite while maintaining the integrity of the clay (49). These two substrates represent surfaces where Al–OH₂⁺ groups are favored under acid (for kaolinite) or close to neutral pH values (for kaolinite + AlCl₃), a difference that we would expect to dramatically impact the pH of the minimum in HONO uptake. As shown in Fig. 2A, the trends in HONO uptake on both surfaces show minima at pH 3.0 and 6.9 for the kaolinite and the AlCl₃ + kaolinite mixture, respectively, that track the surface charge of the substrate rather than the pK_a of HONO in bulk water.

Nitrite Protonation as a Function of Soil pH.

A series of experiments was conducted to measure the amount of HONO evolved as, in each case, 1 nmol of aqueous NO₂ˉ was added onto pH-adjusted substrates to directly probe the influence of substrate pH on the J_des term in Eq. 1. Fig. 2B shows titration curves for NO₂ˉ additions to soil, kaolinite, and kaolinite + AlCl₃ mixtures. Most notably, the inflection points in the titration curves in Fig. 2B correspond to the minima in HONO uptake for the respective substrates shown in Fig. 2A. At pH values significantly above the HONO uptake minima, where basic M–O⁻ species dominate the surface of minerals, HONO was not produced due to the low density of exchangeable surface protons. Because the surface is protonated at lower pH, the proportion of M–OH₂⁺ groups increases and the surface is capable of protonating NO₂ˉ to form HONO. Consistent with these results is the observation that the presence of surface-adsorbed H₂O is required to observe any dependency of γ_net on pH (SI Appendix, Fig. S1). Water is clearly required to hydrate the oxide surfaces and promote proton transfer from acidic M–OH₂⁺ groups to NO₂ˉ (54). Fig. 2B shows that soil and the 2% AlCl₃ in kaolinite both demonstrated the ability to protonate nitrite at pH values that are much greater than the pK_a of HONO in the bulk aqueous phase. Once again, these experiments confirm the importance of the substrate surface chemistry—namely, the degree of protonation of the metal oxides, rather than typical bulk aqueous behavior.

Effect of Soil Surface Charge on HONO Uptake and Release.

To correlate the pH dependence of γ_net to the particle surface charge, potentiometric titration was used to determine the point of zero salt effect (PZSE), the common intersection point where the H⁺ and OHˉ activities are unaffected by changes in electrolyte concentrations, indicating a neutral surface charge (37, 55, 56). Potentiometric titration is a common method for finding the suspension pH at which the net surface charge is zero, i.e., the point of zero charge (PZC) based on the amount of H⁺ adsorbed to exchangeable surface sites (37, 55, 57). At the PZSE of a substrate, the surface is partially hydroxylated and the surface density of M–OH₂⁺ and M–Oˉ groups is equal. As shown in Fig. 2C, the PZSE values for soil, kaolinite, and the AlCl₃ mixture occur at pH values of 6.1, 3.4, and 7.2, respectively. The PZSE values correspond to the minimum in HONO uptake efficiency (Fig. 2A) and the beginning of HONO desorption observed for the nitrite titrations (Fig. 2B), because the surface is neither sufficiently basic to retain the HONO as nitrite nor acidic enough to retain the HONO as the nitroacidium cation. Thus, the J_des term in Eq. 1 is greatest at the PZSE. The excellent agreement among the occurrence of the substrate PZSE values, the pH of minimum HONO uptake, and the onset of HONO desorption strongly suggests that it is the presence of amphoteric metal oxides that determines whether HONO is released or remains adsorbed to soil.

Soil Components Affecting HONO Adsorption and Desorption.

To further investigate the relative importance of the different components of the soil matrix on HONO uptake, we performed a series of experiments on treated soil. The substrates included soil oxidized with hydrogen peroxide and a reconstituted fraction composed of the oxidized soil and 3% wt/wt of an isolated soil humic acid to examine the effect of soil organic matter on the pH dependence of HONO uptake to soil surfaces. Fig. 3 shows that the pH of minimum HONO uptake was lower by ∼2 pH units when soil is oxidized, relative to unmodified soil. To further investigate the role of organic matter, the oxidized soil was reconstituted with 3% wt/wt Elliott soil humic acid to simulate the organic matter content of the native soil. Elliott soil humic acid is isolated from soil collected in Joliet, IL, and is typical of the soil organic matter common to the US states of Indiana, Illinois, and Iowa (58). As shown in Fig. 3, the minimum HONO uptake at pH 3.9 for the reconstituted soil was similar to that of the oxidized soil, suggesting that soil organic matter has little to no effect on the pH dependence of HONO uptake onto soil.

Fig. 3.

The dependence of HONO uptake on mineral content and organic matter. (A) The pH dependence of HONO uptake on treated soil samples exposed to 10 ppb of HONO in air at 30% relative humidity: reconstituted soil, oxidized soil, and whole soil. The dashed lines are guides for the eye. The error bars represent the 95% confidence interval of the mean of replicate measurements. (B) Absolute change in wt% of elements in soil upon oxidation with H₂O₂, as determined by EDX.

Although hydrogen peroxide removes soil organic matter, it can also induce severe structural and chemical changes in the soil that result in the mobilization of iron and aluminum ions from mineral surfaces and loss of structural OH groups (59–62). The loss of iron and aluminum is confirmed by energy-dispersive X-ray spectroscopy (EDX) measurements, which show that the iron and aluminum content in our soil decreases by 1–2% upon oxidation (Fig. 3). Hints into the nature of the iron- and aluminum-containing minerals affected by soil oxidation are obtained by comparing EDX results with those obtained using X-ray powder diffraction (XRD; SI Appendix), which reveals that the main crystalline phases present in the soil are quartz, feldspar, amphibole, mica, smectite, and kaolinite. None of these minerals are expected to possess a PZC above 4, and the notable absence of crystalline Fe³⁺ and Al³⁺ (hydr)oxides in the XRD pattern of the unmodified soil suggests that small amounts of amorphous iron oxides and aluminum hydroxides, not detectable by XRD, are likely present (34). It is important to note that the determined soil PZSE represents an ensemble of many solid phases whose individual points of zero charge range from 1 to 8 (Fig. 4). It would appear that the PZSE of our soil sample is heavily influenced by amorphous Fe³⁺ and Al³⁺ (hydr)oxides present in the soil, both of which have PZC values in the range of 7–9 (Fig. 4) and are known to cover mineral grains with a large surface area despite their low relative abundance (53, 63). Similar to our kaolinite + AlCl₃ experiments, Sakurai et al. (50) demonstrated that even small percentages of iron or aluminum hydroxides (2–10%) added to soils can significantly shift the PZC to higher values, in some cases by as much as 4 pH units. Elliott and Sparks (63) also demonstrated that when soil was treated to remove amorphous iron oxide coatings, the soils shifted from having a PZC of ∼7.5 to behavior more like silica with a PZC of ∼2. In conclusion, the data suggest that it is the loss of amorphous iron and aluminum (hydr)oxide in the soil that best explains the significant decrease in the pH of minimum uptake from the 6.2 of native soil to the 4.0 observed for oxidized soil.

Fig. 4.

The range of measured PZC for soil, common minerals, and soil constituents (53, 63, 67, 69). Note that bentonite consists largely of the clay mineral smectite.

Environmental Implications.

This study presents direct evidence that reactive uptake and release of HONO on soil surfaces are pH dependent, consistent with the ability of amphoteric mineral surfaces to influence the speciation of N(III) in the soil environment, independent of the acid–base equilibria expected in bulk water. The data support a mechanism whereby efficient uptake of gas-phase HONO at high soil pH is driven by the reaction of HONO with basic M–Oˉ groups to generate aqueous nitrite and covalently bonded nitrate, nitrito, and nitro groups, whereas at low pH values, higher uptake efficiency is driven by the formation of surface-adsorbed H₂ONO⁺ cations. At intermediate pH values, any nitrite contributed by HONO to the surface will be protonated by M–OH₂⁺ groups and a reversible equilibrium will be established. These findings suggest that future parameterization of HONO uptake onto soil surfaces cannot only consider Henry’s law partitioning to and from an adsorbed or pore water phase as previously assumed (31) but must also take into account the pH-dependent reactivity of mineral surfaces that controls N(III) speciation in this system.

This work has important implications for understanding how soil pH determines whether soil NO₂ˉ will be a net sink or source of HONO to the atmosphere and provides the theoretical framework to support recent reports of atmospherically significant HONO emissions from soils with pH values of up to 8.8, far above the pK_a of HONO (26, 27). Whether soil nitrite is accumulated via atmospheric deposition of HONO or by microbial nitrification, it can act as a source of HONO to the atmosphere in the presence of ubiquitous aluminum and iron (hydr)oxides, in addition to being influenced by deposition of photochemically derived acids as suggested by VandenBoer et al. (19). It is apparent that outgassing of nocturnally accumulated soil nitrite over the course of the next day could represent an important source of daytime HONO and OH radical. Previously, it was shown that atmospheric–terrestrial exchange of HONO could be approximated based on the aqueous pK_a of HONO, aqueous N(III) concentrations, Henry’s law, and pore-water pH (27). However, our results show that not accounting for the surface acidity of soil minerals may lead to an underestimation of the amount of HONO outgassed from soil.

The importance of soil as a widespread source of atmospheric HONO is apparent from land-cover analyses that classify the Earth’s surface as 15% bare soil, with an additional 13% considered cropland that often remains as bare soil surfaces through fallow seasons (64). The knowledge that surface acidity is the dominant factor in nitrite protonation and release as HONO, rather than bulk pH, changes the proportion of soil types possibly involved in HONO production from the small subset with pH <3 to the vast majority of acidic and nonacidic soils—a staggering 70% of global soils (65). As Fig. 4 demonstrates, soils are extremely heterogeneous and their reactivity in terms of being able to protonate soil nitrite will be heavily influenced by their constituents, which include both the mineral grains and coatings of amorphous iron and aluminum (hydr)oxides (50). HONO release and uptake are clearly coupled to soil mineralogy, and an understanding of the soil characteristics is needed to fully understand the atmospheric–terrestrial exchange of N(III) in the global nitrogen cycle. This additional source of HONO acts as a precursor to OH and thus plays a role in determining ozone and aerosol formation and in limiting the lifetime of greenhouse gases in the atmosphere.

Methods

Soil Characterization.

Soil samples were collected from an agricultural field located in Bartholomew County, IN (39.17, −85.89) at a depth of 0–5 cm in May 2013. The soil was autoclaved for 1 h at 394 K for three successive days and sieved with a 120-mesh sieve. The soil is classified as Genesee, a fine loamy alluvium comprised of quartz, aluminosilicates, and 1% organic matter (66). The unadjusted soil pH was measured as recommended by Hendershot et al. (67) [1:2 soil/water (vol/vol); Thermo Scientific, OrionStar A211 pH meter] and was determined to be 6.48. SEM (FEI Quanta 600 FEG instrument) was used to characterize the soil–glass interface and soil surface; the coating covered the entire inner surface of the tube and was uniform to the eye.

Coated-Wall Flow Tube Studies.

Five substrates were used in this study: Genesee soil, kaolinite (Fluka Analytical), a 2% wt/wt AlCl₃ (Sigma Aldrich) in kaolinite mixture, oxidized soil, and reconstituted soil. The substrate slurry pH was adjusted using H₂SO₄ (Fluka Analytical) and NaOH (EMD Chemicals) solutions, and was measured using the Thermo Scientific, OrionStar A211 pH meter; due to the heterogeneity of the substrates and the variations in slurry composition, an error of ±0.2 is assumed. The slurry composition (2:1 soil/water and 1:1 kaolinite/water, wt/wt) was chosen based on the suitability for coating. The slurry of substrate and deionized water (18.2 MΩ·cm; Milli-Q Integral) was dripped onto the inner walls of the cylindrical glass tube inserts and dried overnight in an oven at 393 K, yielding dry weights that were determined to be well within the mass independent region to best represent an infinitely thick layer of substrate (soil: 1.5 ± 0.2 g; kaolinite: 0.7 ± 0.1 g) (31).

Uptake of HONO onto the substrate-coated surface was initiated by retracting the moveable injector to a position upstream of the coated section, exposing the substrate layer to a well-mixed and laminar flow of HONO and humidified air (SI Appendix). Uptake coefficients reported here are derived from the steady-state portion of the signal vs. time and are based on the geometric surface area; this best represents real boundary layer soil surfaces and provide results that are directly comparable to field measurements and can be implemented into models.

Soil Oxidation and Reconstitution.

Based on a modification of the method of Favilli et al. (68), a slurry of soil (120 g), water (150 mL), and hydrogen peroxide (Macron Fine Chemicals; 30% wt/wt, 500 mL) was stirred in a vented flask for 30 d. The soil was filtered and rinsed with water (1 L). More H₂O₂ (400 mL) was added portion-wise over 48 h to ensure complete reaction (i.e., to the point where no more gas was evolved), and the soil was filtered and rinsed again thoroughly. The reconstituted soil was comprised of a mixture of the oxidized soil and 3% wt/wt of Elliott soil humic acid (International Humic Substance Society).

Nitrite Titration Studies.

Three substrates (soil, kaolinite, and a 2% wt/wt AlCl₃ in kaolinite mixture) were used in this study, and the pH-adjusted slurries were prepared and adjusted using the method described above. However, in this case the substrate was then dried and the powder (0.1 ± 0.02 g) was packed into a small Teflon trough (13 mm inner diameter, 2 mm depth). This trough was placed beneath a port inside the flow tube (flow: 2,180 cm³⋅min^–1) where a rubber stopper supported a 10-μL Hamilton syringe. After the trough was inserted and had been allowed to equilibrate with air at 30% RH, a 5-μL drop of 200 μM NaNO₂ was added and the resulting outgassed HONO was monitored and averaged over a 10-min interval. This procedure was carried out with two fresh surfaces for each substrate and pH.

Potentiometric Titrations.

Suspensions of soil (100 g⋅L⁻¹) in 0.01 and 0.1 M NaCl were adjusted to range from pH 2.3 to 10.2 using HCl and NaOH. The suspensions were kept at room temperature in sealed 20-mL vials and shaken twice a day for 3 d. The pH of each was then measured again on the third day. The initial amount of acid or base minus the amount necessary to adjust a blank NaCl solution to the final equilibrium pH was used to estimate the amount of H⁺ and OHˉ adsorbed by the soil or clay samples (56). Aqueous substrate dispersions were titrated at two different ionic strengths to allow for identification of the inflection point of the titration curve (the common intersection point that occurs at the PZSE) that is not sensitive to changes in ionic strength. The amount of H⁺ adsorbed and released depends on ionic strength above and below this point, which affects the thickness of the diffuse double layer and hence the degree to which the surface charge is shielded (37).

Energy-Dispersive X-ray Spectroscopy.

EDX with an Oxford Inca detector was used to determine the elemental composition of the soil. The results for the native soil (in weight %) are as follows: O, 63; Si, 19; C, 10; Al, 5; Fe, 2; Ca, 0.4; and K, 1. The results for the oxidized soil (in weight %) are as follows: O, 56; Si, 33; C, 3; Al, 4; Fe, 1; Ca, 0.7; and K, 2 (SI Appendix).