Plant-derived biochars exhibit large physicochemical heterogeneity due to variations in biomass chemistry and combustion conditions. However, the influence of biochar heterogeneity on biochar-metal interaction mechanisms has not been systematically described. We used flow adsorption microcalorimetry to study structure-sorption relationships between twelve plant-derived biochars and two metals (K(+) and Cd(2+)) of different Lewis acidity. Irrespective of the biochar structure, sorption of K(+) (a hard Lewis acid) occurred predominantly on deprotonated functional groups via ion exchange with molar heats of adsorption (ΔH(ads)) of -4 kJ mol(-1) to -8 kJ mol(-1). By comparison, although ion exchange could not be completely ruled out, our data pointed to Cd(2+) (a soft Lewis acid) sorption occurring predominantly via two distinct cation-π bonding mechanisms, each with ΔH(ads) of +17 kJ mol(-1). The first, evident in low charge-low carbonized biochars, suggested Cd(2+)-π bonding to soft ligands such as -C ═ O; while the second, evident in low charge-highly carbonized biochars, pointed to Cd(2+)-π bonding with electron-rich domains on aromatic structures. Quantitative contributions of these mechanisms to Cd(2+) sorption can exceed 3 times that expected for ion exchange and therefore could have significant implications for the biogeochemical cycling of metals in fire-impacted or biochar-amended systems.

Points of zero charge were determined on two highly weathered surface soils from Puerto Rico, an Oxisol and Ultisol, as well as mineral-standard kaolinite and synthetic goethite using three methods: (1) potentiometric titration measuring the adsorption of H + and OH � on amphoteric surfaces in solutions of varying ionic strength (I) (point of zero salt effect), (2) direct assessment of surface charges via non-specific ion adsorption as a function of pH and I (point of zero net charge), and (3) electroacoustic mobility of reversible particles as it varies with pH and I (isoelectric point). The first two methods yielded points of zero charge for kaolinite (2.7 -3.2) and synthetic goethite (7.4 -8.2) comparable to those reported previously, indicating the reliability of these analyses. The soil values ranged from 3.9 to 4.4 for the Oxisol and 2.3 to 3.7 for the Ultisol. Electroacoustic mobility, as measured by the AcoustoSizerk, is a parameter that has yet to be thoroughly tested for mineral or soil systems as a viable alternative to PZC assessment. The points of zero charge from electroacoustic mobility of kaolinite (3.8 -4.1) and synthetic goethite (8.1 -8.2) were similar to values obtained by electrophoretic mobility. Furthermore, the values found for the Oxisol (3.4 -3.5) and Ultisol (2.6 -2.7) were in the range expected for these soils.

The competitive adsorption of p-xylene and water vapors on soil materials was studied to elucidate the mechanisms responsible for vapor-phase sorption in the unsaturated zone. Isotherms obtained from the adsorption of p-xylene vapors on oven-dried silica gel, kaolinite, and Webster soil were nonlinear over the range of relative vapor pressures investigated. Increasing the relative humidity to 67 and 90% resulted in dramatic reductions in p-xylene sorption and a shift to isotherms that were linear at pxylene relative vapor pressure below 0.5. Estimates of p-xylene sorption based on partitioning into organic carbon did not account for the sorption of p-xylene on sorbents of low organic carbon content and at relative vapor pressures greater than 0.5. Dissolution of p-xylene into adsorbed water films was found to be insignificant at 67 and 90% relative humidity. In contrast, the adsorption of p-xylene at the gas-liquid interface, predicted by the Gibbs equation, contributed significantly to p-xylene sorption in the presence of water vapor. These findings indicate that the sorption of nonpolar organic vapors by hydrated soil materials should be described using a multimechanistic approach, which incorporates adsorption on mineral surfaces, adsorption at the gas-liquid interface, dissolution into adsorbed water, and partitioning into soil organic matter.or relative humidity (RH) increases, VOCs are displaced from adsorbent surfaces, resulting in the suppression of vapor-phase sorption (7,2,(6)(7)(8)(9). It is generally agreed that water molecules, because of their polar nature, effectively compete with nonpolar organic vapors for adsorption sites, such as exchangeable metal cations (6-9).f Approved for publication as Florida Agricultural Experiment Station Journal Series No. R-01738.

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A Winsor Type I surfactant/alcohol mixture was used as an in situ flushing agent to solubilize a multicomponent nonaqueous phase liquid (NAPL) as a single-phase microemulsion (SPME) in a hydraulically isolated test cell at Hill Air Force Base (AFB), Utah. The surfactant (polyoxyethylene(10) oleyl ether) and alcohol (1-pentanol) together comprised 5.5 wt % of the flushing solution. The NAPL was extremely complex, containing more than 200 constituents and a ‘pitch' fraction that was not solvent-extractable. The NAPL removal effectiveness of the SPME flood was evaluated using information from soil cores, partitioning tracer tests, and NAPL constituent breakthrough curves (BTCs) measured at three extraction wells. Soil core data indicated that approximately 90−95% of the most prevalent NAPL constituents were removed from the cell by the SPME flood. A comparison of pre- and postflushing partitioning tracer data indicated that about 72% of the measured NAPL volume was removed by the SPME flood. Integration of NAPL constituent BTCs indicated 55−75% removal of the target NAPL constituents when partitioning tracer data were used to estimate the initial amount of NAPL present and 60−175% removal of two target constituents when soil core data were used to estimate the amount of NAPL initially present. These results indicate that the SPME flood effectively removed the NAPL constituents of concern, but an insoluble anthropogenic residue was left behind.

Phosphorus retention in sandy coastal plain soils can be low enough that surface water quality is jeopardized by agricultural P loadings. Objectives of this study were to: (i) determine if discrete morphological characteristics could effectively differentiate sandy soils with respect to P retention and (ii) test the efficacy of a rapid chemical assessment of relative P adsorption (single‐point isotherm) for sandy materials. Soil samples from 96 surface and subsurface horizons of randomly selected Alaquod, Quartzipsamment, and Paleudult pedons were used. These pedons had previously been described, sampled, and characterized as part of the Florida Cooperative Soil Survey Program. Two groups of uncoated Quartzipsamments (<5% silt plus clay) were distinguished: those having “clean” (coating‐free) and “slightly coated” grains. Eluvial horizons from Alaquods were also designated as clean because of a dominance of coating‐free sand grains. Single‐point P adsorption isotherms were obtained for all samples, and multipoint adsorption and desorption isotherms for 21 of the 96 samples. Single‐point isotherms effectively arrayed sandy material with respect to relative P adsorption. Sand‐grain coatings significantly enhanced P adsorption and resistance to desorption. All clean samples readily desorbed P regardless of origin or amount adsorbed. Thus clean sands pose a greater hazard for P leaching than sands with grain coatings. Clay content was closely related to P adsorption, but silt content was not. The P‐retention distinction between clean and other Quartzipsamments is more marked than the “uncoated vs. coated” USDA family criterion, the latter being confounded by the inclusion of silt content.

Flow calorimetry, which is ideally suited for measuring reactions oc curring at the liquid/solid interface, has been used to study the surface chemistry of many types of solids, but little use of it has been made in the study of surface reactions of soils. The purpose of this study was to demonstrate the application of flow calorimetry to the study of two fun damental soil chemical processes, namely cation exchange and phosphate sorption. Surface horizon samples of a Typic Acrorthox and a Typic Tropohumult from Puerto Rico, a strong acid cation exchange resin (Dowex 50W-8), and an amorphous Al(OH) 3 were used. Heats for K/Ca exchange on the Dowex resin and the Oxisol, and K/Na exchange on the Ultisol, were consistent with literature values that were obtained using conventional batch calorimetry or derived from the temperature depen dence of the exchange constant. Although peak areas associated with a given pair of exchange reactions were equal, peak shapes were generally not equivalent, indicating differences in the rate at which the two reac tions occurred. For example, Ca displacing exchangeable K occurred more rapidly than the reverse reaction on the Dowex resin. The reaction of phosphate with the Ultisol and amorphous Al(OH) 3 was exothermic. Exposure of the soil to several cycles of phosphate was sufficient to sat urate the sorption sites, as evidenced by the loss of a detectable heat sig nal. However, phosphate reactive sites were regenerated by flushing the column with a salt solution at pH 10. Precipitation of Al-phosphate was shown to be endothermic, confirming that precipitation was not the pri mary mechanism for phosphate sorption in this study.

Surfactants can enhance the removal of nonaqueousphase-liquids (NAPLs) from porous media by two very different mechanisms: (1) increased solubilization that occurs in the presence of surfactant micelles and (2) mobilization of NAPL ganglia held by capillary forces. Solubilization technologies pose less risk of uncontrolled NAPL migration and are less complex to design. Since dense-NAPLs, like PCE, pose the greatest risk of migration, there is a need for more information on surfactants that are capable of solubilizing DNAPLs and removing them as water continuous, low viscosity, microemulsions without mobilization. Forty-two commercial, water-soluble surfactants were screened for PCE solubilization in batch experiments and one nonionic (Brij 97) and one anionic (SDS) surfactant were further evaluated in column experiments. Of the 42 surfactants that were screened, 12 nonionic surfactants with HLB values between 10.8 and 13.2 solubilized the most PCE. However, as PCE solubilization exceeded ∼40 000 mg/L, macroemulsion stability became a problem. Addition of IPA did not affect the amount of PCE solubilized but decreased macroemulsion stability, resulting in more rapid formation of Winsor Type I microemulsions. The most efficient surfactants were Brij 97 and Ritoleth 10, both being ethoxylated oleyl alcohol ethers. At 3 wt % IPA and surfactant, these surfactants solubilized >70 000 mg PCE/ L. In column experiments, Brij 97/IPA removed >92% of the residual PCE in 11 pore volumes without mobilizing the PCE. An SDS/pentanol/IPA system removed 98% in ∼9.5 pore volumes, but about 30% of the PCE was mobilized. Removal of PCE was rate-limited with Brij 97/IPA, suggesting that flow interruption may be an effective remediation strategy.