George F. Vance

Selenium adsorption on Mg-Al and Zn-Al layered double hydroxides

Layered double hydroxides (LDHs) have high anion exchange capacities that enhances their potential to remove anionic contaminants from aqueous systems. In this study, different Mg–Al and Zn–Al LDHs were synthesized by a coprecipitation method, with the products evaluated for their ability to adsorb selenite (SeO32−) and selenate (SeO42−). Results indicated the adsorption isotherm for SeO32− retention by Mg–Al and Zn–Al LDHs could be fitted to a simple Langmuir equation with the affinity of SeO32− on Zn–Al LDH higher than that on Mg–Al LDH. The adsorption trends for both SeO32− and SeO42− on LDHs were similar under the experimental conditions. The SeO32− adsorption was rapid and was affected by the initial SeO32− concentration. The quasi-equilibrium for 0.063 and 0.63 cmol/l SeO32− solutions was obtained within the first 30 and 60 min of adsorption, respectively. The maximum adsorption of SeO32− on Mg–Al LDH was higher than that of Zn–Al LDH and decreased with an increase in the LDH mole ratio of Mg/Al. The high pH buffering capacities and the SeO32− adsorption for Mg–Al and Zn–Al LDHs was a function of pH. Competing anions strongly affected the adsorption behavior of SeO32− with SeO32− adsorption increasing in the order: HPO42−<SO42−<CO32−<NO3−. The release of adsorbed SeO32− depended upon the type of competing anion in the aqueous solution. For example, with CO32− the adsorbed SeO32− could be desorbed completely from Mg–Al LDH. X-ray diffraction patterns indicated that d-spacing increased when SeO42− was adsorbed, but not with SeO32− adsorption.

Carbon mobilization from the forest floor under red spruce in the northeastern U.S.A.

Global climate change may alter soil temperature and moisture conditions, increasing the need to understand how these basic factors affect C dynamics. This is particularly important in boreal forests, which often have large C pools in the forest floor and mineral horizons. We examined the effects of temperature and precipitation frequency on C dynamics in forest floor horizons from eight red spruce sites in the northeastern U.S. using column leaching experiments. Intact and sieved forest floor samples were incubated at 3, 10 or 20°C and leached either daily, once per week, or twice per week during 14 to 39 days using simulated throughfall solutions (pH 2.7 or 4.0). Leachate DOC and CO2 production were measured along with soil C and N concentrations. For intact samples, losses of C as DOC and as CO2 increased with increasing temperature, and the increase (Q10) was usually greater between 3 and 10°C than between 10 and 20°C. There was a greater response of CO2 to temperature than of DOC (e.g. Howland sieved soil Q10s of 1.9 and 7.2 for CO2 and 1.5 and 2.0 for DOC at 3–10 and 10–20°C ranges, respectively). More frequent leaching increased steady state DOC mobilization (e.g. 145 and 58 μg g−1 forest floor d−1 for daily and weekly leachings at 10°C, respectively), but not CO2 evolution (e.g. 79 and 74 μg CO2C g−1 forest floor d−1 for daily and weekly leachings at 10°C, respectively). Across the eight sites DOC loss and CO2 evolution varied by factors of 3.6 and 4.0, respectively. Both CO2 evolution and DOC in leachates calculated as fluxes were correlated (r = 0.73 and 0.87 respectively, n = 8) with the C-to-N ratios of the samples (C-to-N ratios ranged from 27 to 58), which could be explained by N limitations that triggered selective lignin degradation, differences in degree of humification of the material, or position on a west-to-east pollution gradient. Although higher temperatures and more frequent leaching increased DOC mobilization, and higher temperatures increased CO2 evolution, both treatments and site to site variation illustrate the complexity of the response of forest-floor C pools to manipulations.

Surfactant-enhanced adsorption of organic compounds by layered double hydroxides

Abstract Clay-based adsorbents were synthesized by incorporating anionic surfactants, sodium octylsulfate (SOS), sodium dodecylsulfate (SDS), sodium 4-octylbenzenesulfonate (SOBS), and sodium dodecylbenzenesulfonate (SDBS), into magnesium aluminum layered double hydroxide (Mg–Al LDH) via ion exchange. Adsorption isotherms indicated the amount of surfactants intercalated decreased in the order: SDS>SOBS>SDBS>SOS. X-ray diffraction analysis of the organo-LDHs revealed that surfactant molecules could adopt various configurations within the Mg–Al LDH interlayer space, with SOS forming bilayers whereas others exhibited monolayer arrangements. Intercalation of surfactants into Mg–Al LDH resulted in a decrease in surface area as determined by BET analysis. The products were also examined for their ability to adsorb organic pollutants. The adsorption results indicated that both 1,2,4-trichlorobenzene and 1,1,1-trichloroethane could be adsorbed by all of the organo-LDHs studied, presumably due to partitioning, with adsorption affinity dependent upon the type of surfactant used. Adsorption results indicated anionic surfactants intercalated into Mg–Al LDH form a more effective partition phase than octanol.

Chemical characteristics and acidity of soluble organic substances from a northern hardwood forest floor, central Maine, USA

Our understanding of the chemistry, structure, and reactions of organic substances in forest floor leachates is limited and incomplete. Therefore, we examined the organic and inorganic chemistry of forest floor leachates collected from a hardwood forest in central Maine over a two-year period (1987–1989), including detailed study of dissolved organic carbon (DOC). Seasonal variations in NH4+, NO3−, K+, and total Al were believed due to organic matter decomposition and release. Leaching of other base cations closely followed that of NO3−. Snowmelt resulted in NO3− levels that decreased in time due to flushing of mineralization/nitrification by-products that had accumulated during the winter months. Total DOC ranged from 2228 to 7193 μmol L−1 with an average of 4835 μmol L−1. Monosaccharides and polyphenols constituted 3.9% (range of 3.4 to 4.4%) and 3.0% (2.2 to 3.7%) of the DOC, respectively, which suggests DOC may contain partially oxidized products that are possibly of a lignocellulose nature. Fractionation of the forest floor DOC indicated high organic acid contents (hydrophobic and hydrophilic acids) that averaged 92% of the total DOC. Organic acids were isolated and analyzed for elemental content (C, H, N, and S), and determination of UV absorptivity (E4E6) ratios, CuO oxidation products, FT-IR and 13C-NMR spectra, and acidity by potentiometric titration. Results from these analyses indicate the organic acids in the forest floor leachates are similar to fulvic acids. Hydrophobic and hydrophilic acids had average exchange acidities of 0.126 and 0.148 μeq μmol−1 C, respectively, and pKa, of 4.23 and 4.33. Their FT-IR and 13C-NMR spectra suggest they are primarily carboxylic acids, with aliphatic and aromatic structure. An organic charge contribution model was developed using titration data, DOC fractionation percentages, and the total DOC in the forest floor leachates. Application of the model to all solutions accounted for 97% of the charge balance deficits. Adjusted values for the flux of C and organic acidity due to organic solutes in forest floor leachates indicated translocation of 112 to 260 kg C ha−1 yr−1 and 460 to 1330 eq ha−1 yr−1, respectively, to the underlying mineral subsurface horizons.

Adsorption of dicamba (3,6-dichloro-2-methoxy benzoic acid) in aqueous solution by calcined-layered double hydroxide

Layered double hydroxides (LDHs) are anionic clays with high anion exchange capacities. Calcination of LDHs increases their anion-exchange capacities significantly, resulting in calcined-LDHs that are better adsorbents for removal of anion pollutants than uncalcined-LDHs. In this study, layered double hydroxides, LDH–CO32− and calcined-LDH, were prepared and characterized by FT-IR and X-ray diffraction analysis, with the products evaluated for their ability to adsorb dicamba (3,6-dichloro-2-methoxy benzoic acid), an ionizable organic pesticide. Results indicated that dicamba could be adsorbed on calcined-LDH, but no adsorption occurred with LDH–CO32−. The adsorption isotherms of dicamba on calcined-LDH are typical S-type curves, suggesting a hydrophobic adsorption mechanism was involved. Dicamba adsorption on calcined-LDH was a rapid process that reached a quasi-equilibrium within 30 min. Competing anions strongly affected the adsorption process, with dicamba adsorption, in the presence of different anionic species, increasing in the order: SO42−<HPO42−< CO32−<NO3−≈F−≈Cl−≈Br−≈I−. Adsorbed dicamba on calcined-LDH could be desorbed completely, with the release rate dependent upon the type of competing anion in the aqueous solution. FT-IR and X-ray diffraction analysis verified that adsorbed dicamba formed a monolayer that was oriented perpendicularly from the interlayer mineral surfaces.

Hybrid organic–inorganic derivatives of layered double hydroxides and dodecylbenzenesulfonate: Preparation and adsorption characteristics

Layered double hydroxides (LDHs) are anionic clays that can be synthesized under laboratory conditions. In this study, different LDHs were synthesized by a coprecipitation method, with the parent products and calcined derivatives evaluated for their ability to adsorb the anionic surfactant, dodecylbenzenesulfonate (DBS). Adsorption isotherms for DBS retention on LDHs were typical L-type curves, with adsorption data conforming to a simple Langmuir equation. Langmuir maximum adsorption of DBS on calcined-LDH was significantly higher than that on uncalcined-LDHs. Organo-LDHs were also synthesized by incorporating DBS into LDHs via ion-exchange, reconstruction of calcined-LDH and in-situ synthesis methods. X-ray diffraction analysis of organo-LDHs revealed that DBS was intercalated into LDHs with the mono-layer DBS molecules oriented perpendicularly to LDH surfaces. Intercalation of DBS into LDHs decreased surface area according to BET analysis. The adsorption capacities of organo-LDHs for trichloroethylene (TCE) and tetrachloroethylene (PCE) were substantially greater than the original LDH materials. Adsorption of organic compounds by organo-LDHs was due to a partitioning mechanism.

Role of Soil Organic Acids in Mineral Weathering Processes

The soluble organic acids in soils consist largely of complex mixtures of polymeric compounds referred to collectively as fulvic and humic acids. These compounds are relatively refractory, and are broken down only slowly by microorganisms. Low-molecular-mass acids (e.g., acetic, oxalic, formic) exist in dynamic balance: they are rapidly produced and consumed by microorganisms. Their concentrations may be quite high where rapid decomposition of plant material is taking place, and in microenvironments adjacent to roots and fungal hyphae, but are generally low (typically less than 1 mM) in bulk soil solution. Concentrations of organic acids are generally highest in the organic layer at the top of the soil profile and decrease with depth. Particularly high concentrations of dissolved organic acids occur in peatlands and waterlogged soils.

Dissolved Organic Carbon and Sulfate Sorption By Spodosol Mineral Horizons

Dissolved organic carbon (DOC) can influence the mobility and sorption of inorganic solutes in soils. Soil exchange sites can be occupied by DOC thus preventing the retention of SO42−, which enters the soil from acidic deposition. To determine the importance of DOC and SO42− interactions with soil, we examined the sorption of DOC and SO42− by several mineral horizons of three Spodosols. In general, sorption of DOC was found to be comparable between spodic horizons within each Spodosol, although sorption characteristics indicated some differences among soils. Due to the release of large amounts of DOC, analysis of sorption isotherms was characterized by the initial mass isotherm method instead of Langmuir or Freundlich techniques. Using the initial mass isotherm, we found that the carbon reactive soil pools (RSP) corresponded closely to total soil carbon (Ct) levels (e.g., RSP = 5.67 Ct − 8.44, r2 = 0.856, n = 10). Temperature (276 and 294 K) and pH (equilibrium solution pH ranged from 4.2 to 5.2), however, had little influence on DOC sorption. Sulfate sorption, evaluated on the horizon of each soil with the highest extractable SO42−, was found to be distinctly different for each of the horizons. In the presence of DOC (5.6 mmol C L−1), SO42− sorption decreased (e.g., 5–22% reduction at 10 mmol(−) kg−1 SO42− addition). Although DOC reduced the amount of SO42− adsorbed by each soil, the great affinity that mineral horizons have for DOC can rapidly reduce DOC concentration in soil leachates percolating through the soil profile. This would decrease the competitive DOC effect on SO42− sorption, and SO42− would be retained in lower mineral horizons. Therefore, soil contact is proposed as the most important factor in controlling terrestrial inputs of both DOC and SO42− to surface waters.

Sorption of trichloroethylene by organo-clays in the presence of humic substances

Abstract We examined trichloroethylene sorption by organo-clays [hexadecyltrimethylammonium (HDTMA) and didodecyldimethylammonium (DDDMA) exchanged smectites (SWy-1 and SAz-1)] in aqueous solutions both in the presence and absence of humic substances (Aldrich humic acid and peat fulvic acid). The results indicated the uptake of humic substances by organo-clays and the effects of humic substances on trichloroethylene sorption by organo-clays were dependent on the type of organo-clay and humic substance. Selection of an appropriate combination of clay and organic cation can optimize the removal of trichloroethylene as well as humic substances from aqueous solutions by organo-clays. Organo-clays may be an alternative to activated carbon sorbents in some specific water treatment processes.

Sorption of benzene, toluene, ethylbenzene, and xylene (BTEX) compounds by hectorite clays exchanged with aromatic organic cations

Adsorptive-type organoclays, where hydrocarbons adsorb directly to the siloxane surfaces, were studied to find new organic cations and to determine the parameters that produce effective sorbents. Organoclays were prepared from hectorite by cation exchange with small, aromatic organic cation salt solutions. Trimefhylphenylammonium (TMPA) chloride was obtained and iodide salts of commercially-unavailable aromatic cations were synthesized and used to prepare organoclays. An aqueous mixture of benzene, toluene, ethylbenzene, and xylenes (BTEX) consistent with the composition of unleaded gasoline was used in sorption isotherms to compare the sorptive properties of the organoclays. Only the TMPA, methylphenylpyridinium (MPPyr), and trimethylammonium indan (Indan) organoclays were effective BTEX sorbents. Organoclays prepared from methylpyridinium (MPyr), trimethylammonium biphenyl (Biphenyl), and trimethylammonium fluorene (Fluorene) were poor sorbents. The MPPyr and TMPA organoclays preferentially sorbed ethylbenzene, whereas the Indan organoclay preferentially sorbed benzene and toluene. Langmuir-type sorption isotherms for the TMPA, MPPyr, and Indan organoclays implied surface adsorption, whereas linear isotherms suggested that partitioning was the sorptive mechanism for the MPyr, Biphenyl, and Fluorene organoclays. Water hydrating the small MPyr cation and the larger bulk of the Biphenyl and Fluorene cations may have blocked BTEX access to the interlayer siloxane surfaces. Although the rather bulky MPPyr and Indan cations produced effective organoclays, compact size and low hydration are organic cation properties that typically yield effective adsorptive-type organoclays.