Effect of admicellar properties on adsolubilization: column studies and solute transport

Mixtures of anionic and cationic surfactants exhibit synergistic behavior as evidenced by low critical micelle concentrations (CMC) of the mixed system, increased surface activity, and improved detergency performance. The adsorption of a single-head anionic surfactant, sodium dodecyl sulfate (SDS), in mixture with a twin-head cationic surfactant, pentamethyl-octadecyl-1,3-propane diammonium dichloride (PODD), showed synergism of adsorption onto silica when present at a mixing ratio of 1:3 (cationic-rich), and also demonstrated lower surfactant desorption with water flushing of columns packed with the surfactant-modified media. In addition, the proportion of the mixed surfactants in the admicelles moved from the initial ratio of 1:3 towards equimolar after rinsing the surfactant-modified silica absorbent. The retardation of organic solutes passing through columns packed with modified-silica adsorbent increased nominally three fold for silica modified with mixed surfactants versus single surfactants (retardation factors increase from 4.0 to 12.8 for styrene and from 32.1 to 90.2 for ethylcyclohexane for single and mixed surfactants, respectively). Thus, this study demonstrates that mixed surfactant systems more effectively modified the silica surface than single surfactant systems both in terms of enhanced retardation of organic solutes and in terms of reduced surfactant desorption.     

Mixtures of anionic and cationic surfactants with single and twin head groups: Adsorption and precipitation studies

This research reports on the adsorption and precipitation of mixtures of anionic and cationic surfactants having single and twin head groups. The surfactant mixtures investigated were: (i) a single-head anionic surfactant, sodium dodecyl sulfate (SDS), in a mixture with the twin-head cationic surfactant pentamethyl-octadecyl-1,3-propane diammonium dichloride (PODD)—adsorption was studied on negatively charged silica; and (ii) a twin-head anionic surfactant, sodium hexadecyl-diphenyloxide disulfonate (SHDPDS), and the single-head cationic surfactant dodecylpyridinium chloride (DPCI)—adsorption was studied on positively charged alumina. Whereas the mixed surfactant system of SHDPDS/DPCI showed adsorption on alumina that was comparable to the of SHDPDS alone, the mixed surfactant system of SDS/PODD showed increased adsorption on silica as compared with PODD alone. The adsorption of the SDS/PODD mixture increased as the anionic and cationic system approached an equimolar ratio. Precipitation diagrams for mixtures of single- and twin-head surfactant systems showed smaller precipitation areas than for single-head-only surfactant mixtures. Thus, the combination of single- and double-head surfactants helps reduce the precipitation region and can increase the adsorption levels, although the magnitude of the effect is a function of the specific surfactants used.     

Mixtures of anionic and cationic surfactants with single and twin head groups: Solubilization and adsolubilization of styrene and ethylcyclohexane

Mixtures of anionic and cationic surfactants with single and twin head groups were used to solubilized styrene and ethylcyclohexane into mixed micelles and adsolubilize them into mixed admicelles on silica and alumina surfaces. Two combinations of anionic and cationic surfactants were studied: (i) a single-head anionic surfactant, sodium dodecyl sulfate (SDS), with a twin-head cationic surfactant, pentamethyl-octadecyl-1,3-propane diammonium dichloride (PODD), and (ii) a twin-head anionic surfactant, sodium hexadecyl-diphenyloxide disulfonate (SHDPDS), with a single-head cationic surfactant, dodecylpyridinium chloride (DPCl). Mixtures of SDS/PODD showed solubilization synergism (increased oil solubilization capacity) when mixed at a molar ratio of 1∶3; however, the SHD-PDS/DPCl mixture at a ratio of 3∶1 did not show solubilization enhancement over SHDPDS alone. Adsolubilization studies of SDS/PODD (enriched in PODD) adsorbed on negatively charged silica and SHDPDS/DPCl adsorbed on positively charged alumina showed that while mixtures of anionic and cationic surfactants had little effect on the adsolubilization of styrene, the adsolubilization of ethylcyclohexane was greater in mixed SHPDS/DPCl systems than for SHDPDS alone. Finally, it was concluded that whereas mixing anionic and cationic surfactants with single and double head groups can improve the solubilization capacity of micelles or admicelles, the magnitude of the solubilization enhancement depends on the molecular structure of the surfactant and the ratio of anionic surfactant to cationic surfactant in the micelle or admicelle.     

Sorption and transport of acetaminophen, 17alpha-ethynyl estradiol, nalidixic acid with low organic content aquifer sand

The sorption and transport of three pharmaceutical compounds (acetaminophen, an analgesic; nalidixic acid, an antibiotic; and 17α-ethynyl estradiol, a synthetic hormone) were examined by batch sorption experiments and solute displacement in columns of silica, alumina, and low organic carbon aquifer sand at neutral pH. Silica and alumina were used to represent negatively-charged and positively-charged fractions of subsurface media. Column transport experiments were also conducted at pH values of 4.3, 6.2, and 8.2 for the ionizable nalidixic acid. The computer program UFBTC was used to fit the breakthrough data under equilibrium and nonequilibrium conditions with linear/nonlinear sorption. Good agreement was observed between the retardation factors derived from column model studies and estimated from equilibrium batch sorption studies. The sorption and transport of nalidixic acid was observed to be highly pH dependent, especially when the pH was near the pK_a of nalidixic acid (5.95). Thus, near a compound's pK_a it is especially important that the batch studies be performed at the same pH as the column experiment. While for ionic pharmaceuticals, ion exchange to oppositely-charged surfaces, appears to be the dominant adsorption mechanism, for neutral pharmaceuticals (i.e., acetaminophen, 17α-ethynyl estradiol) the sorption correlated well with the K_ow of the pharmaceuticals, suggesting hydrophobically motivated sorption as the dominant mechanism.     

Competitive sorption and transport of Pb2+, Ni2+, Mn2+, and Zn2+ in lateritic soil columns

Knowledge of sorption and transport of heavy metals in soils in the presence of other metals is crucial for assessing the environmental risk of these metals. Competitive sorption and transport of four metals, Pb(2+), Ni(2+), Zn(2+), and Mn(2+), were investigated using multi-metal column experiments with lateritic soils obtained from a gold mine impacted by acid mine drainage. Based on Pb(2+) breakthrough time for single-metal system at a pH of approximately 5, the sorption capacity of Pb(2+) was estimated to be higher in lateritic soil than the other metals. For multi-metal systems, the estimated retardation factors for the metals from highest to lowest were: Pb(2+)>Zn(2+)～ Ni(2+)>Mn(2+), suggesting the mobility of metals through lateritic soil for a multi-metal system would be in the order of Mn(2+)>Ni(2+)～ Zn(2+)>Pb(2+). For binary and multi-metal systems, the estimated sorption capacities of individual metals were found to be lower than the sorption capacities in single metal system - indicating possible competition for sorption sites. Mass recoveries estimates showed that the sorption of metals was more reversible under competitive multi-metal systems than in single metal systems.     

Sorption of acetaminophen, 17alpha-ethynyl estradiol, nalidixic acid, and norfloxacin to silica, alumina. and a hydrophobic medium

Two pure minerals and a hydrophobic medium were selected to study sorption of pharmaceuticals. The sorption of four pharmaceuticals, acetaminophen (analgesic), 17alpha-ethynyl estradiol (synthetic hormone), nalidixic acid (antibiotic), and norfloxacin (antibiotic), was evaluated with silica, alumina, and Porapak P (a hydrophobic medium). Alumina and silica were selected to represent positively charged and negatively charged aquifer mineral surfaces at neutral pH, respectively, while Porapak P was selected to represent the hydrophobic organic content of an aquifer medium. At neutral pH, acetaminophen, the least hydrophobic pharmaceutical, showed no significant sorption to any of the media, while 17alpha-ethynyl estradiol, the most hydrophobic pharmaceutical, showed significant sorption to Porapak P. Nalidixic acid, which has a carboxyl functional group that is anionic at neutral pH, showed significant adsorption to the positively charged alumina. Norfloxacin, with both a carboxyl (anionic) and a piperazynyl (cationic) group, can exist in four forms (neutral, cationic, anionic, and zwitterionic) depending on the aqueous pH. Norfloxacin also showed significant adsorption than nalidixic acid. Both nalidixic acid and norfloxacin adsorbed to silica and Porapak P to a much lower extent. The pH dependence of nalidixic acid and norfloxacin adsorption to silica and alumina was also studied by varying the pH between 4 and 11. The maximum adsorption of nalidixic acid to alumina occurred near its pKa (pH approximately 6), where the combination of cationic alumina and anionic nalidixic produced maximum adsorption. The maximum adsorption of norfloxacin to alumina was observed at pH approximately 7, which was the region where the zwitterionic form dominated. This research demonstrates that the adsorption of ionizable pharmaceuticals is strongly dependent on the system pH, the pharmaceutical properties (pKa and hydrophobicity), and the nature of the surface charge (point of zero charge). For pharmaceuticals that are uncharged at environmentally relevant pH values, the main sorption factor is their solubility or hydrophobicity; for charged forms, ion exchange is also an important adsorption mechanism.     

Photocatalytic oxidation of cyanide in aqueous titanium dioxide suspensions: Effect of ethylenediaminetetraacetate

The kinetics of the photocatalytic oxidation of cyanide in aqueous TiO₂ suspensions was investigated as a function of catalyst loading (0.1–5.0 g l^?1), air-flow rate (0.2–1.1 l min^?1), and the concentration of ethylenediaminetetraacetate, EDTA (0.4–40 mM) at pH 13.0. The cyanide oxidation rate did not vary with the TiO₂ loading while a slight increase in the degradation rate with an increase in the air-flow rate was found. Cyanate (NCO^?) was the only product of the cyanide decomposition. The effect of EDTA on the photocatalytic oxidation of cyanide was examined at different molar ratios of EDTA to cyanide (0.1–10.5) by keeping the initial cyanide concentration at 3.85 mM. EDTA retarded the photocatalytic oxidation of cyanide and the decrease in the oxidation rate was not proportional to the molar ratio of EDTA to cyanide. The first-order rate constant, k (min^?1) for the oxidation of cyanide in the presence of EDTA may be expressed as k = 3.38 × 10^?3 × ([EDTA]/[CN^?])^?0.23. A mechanism of the oxidation of cyanide by a photocatalytic process in absence and presence of EDTA is presented.