Kalliat T. Valsaraj

Adsorption and Reaction of Trace Gas-Phase Organic Compounds on Atmospheric Water Film Surfaces: A Critical Review

The air-water interface in atmospheric water films of aerosols and hydrometeors (fog, mist, ice, rain, and snow) presents an important surface for the adsorption and reaction of many organic trace gases and gaseous reactive oxidants (hydroxyl radical (OH(.)), ozone (O(3)), singlet oxygen (O(2)((1)Delta(g))), nitrate radicals (NO(3)(.)), and peroxy radicals (RO(2)(.)). Knowledge of the air-water interface partition constant of hydrophobic organic species is necessary for elucidating the significance of the interface in atmospheric fate and transport. Various methods of assessing both experimental and theoretical values of the thermodynamic partition constant and adsorption isotherm are described in this review. Further, the reactivity of trace gases with gas-phase oxidants (ozone and singlet oxygen) at the interface is summarized. Oxidation products are likely to be more water-soluble and precursors for secondary organic aerosols in hydrometeors. Estimation of characteristic times shows that heterogeneous photooxidation in water films can compete effectively with homogeneous gas-phase reactions for molecules in the atmosphere. This provides further support to the existing thesis that reactions of organic compounds at the air-water interface should be considered in gas-phase tropospheric chemistry.

Contaminant fluxes from sediment due to tubificid oligochaete bioturbation

The release of the hydrophobic organic compounds pyrene, dibenzofuran and phenanthrene from bioturbated freshwater sediments was studied in laboratory microcosms. Initial Tubificid oligochaete densities of 0, 6700 and 2.67 × 104 individuals · m−2 were employed. Under oxygen saturated conditions, the difference between the contaminant fluxes from the high-density bioturbated microcosms and controls remained essentially constant at 37 and 70 ng · cm2−2 · d−1 for pyrene and phenanthrene, respectively, corresponding to effective mass transfer coefficients of 0.16 and 0.37 cm/y. Under hypoxic conditions, worm defecation on the sediment surface increased and led to significantly increased fluxes to a maximum of 380, 490 and 940 ng · cm−2 · d−1, for pyrene, phenanthrene and dibenzofuran, respectively. Average bioturbation fluxes in the high-density microcosms of 246, 258 and 310 ng · cm−2 · d−1 for the respective compounds corresponded to effective mass transfer coefficients of 1.7, 3.2, and 7.5 cm/yr. Initial release rates from medium-density microcosms (25% of high density) were typically half the release rate of the high-density microcosms, indicating greater organism activity per individual at the lower density. The increased flux with the more soluble compounds likely reflects more rapid release at the sediment surface and the increased importance of porewater pumping over sediment particle reworking for migration of these compounds.

Separation of Organic Dyes from Wastewater by Using Colloidal Gas Aphrons

Abstract Auxiliary chemicals used in the preparation of water-based dyes have been reported to pose problems in conventional wastewater treatment processes. The dye manufacturing industries have to find alternate wastewater treatment processes to remove dyes from their waste effluents. This paper investigates a novel method of separation of dyes by flotation using colloidal gas aphrons (CGAs). The results indicate that this is an effective method for the separation of synthetic dyes from wastewater. The mechanism of removal has been shown to be ion coupling of the oppositely charged species of the surfactant forming the CGA and the dye and flotation of the ion-dye complex on the surface of CGA microbubbles.

Immobilization of aqueous Hg(II) by mackinawite (FeS).

As one of the major constituents of acid volatile sulfide (AVS) in anoxic sediments, mackinawite (FeS) is known for its ability to scavenge trace metals. The interaction between aqueous Hg(II) (added as HgCl(2)) and synthetic FeS was studied via batch sorption experiments conducted under anaerobic conditions. Due to the release of H(+) during formation of hydrolyzed Hg(II) species which is more reactive than Hg(2+) in surface adsorption, the equilibrium pH decreased with the increase in Hg(II)/FeS molar ratio. Counteracting the loss of FeS solids at lower pH, the maximum capacity for FeS to remove aqueous Hg(II) was approximately 0.75 mol Hg(II) (mol FeS)(-1). The comparison of X-ray power diffraction (XRPD) patterns of synthetic FeS sorbent before and after sorption showed that the major products formed from the interaction between FeS and the aqueous Hg(II) were metacinnabar, cinnabar, and mercury iron sulfides. With the addition of FeS at 0.4 g L(-1) to a 1 mM Hg(II) solution with an initial pH of 5.6, Fe(2+) release was approximately 0.77 mol Fe(2+) per mol Hg(II) removed, suggesting that 77% of Hg(II) was removed via precipitation reaction under these conditions, with 23% of Hg(II) removed by adsorption. Aeration does not cause significant release of Hg(II) into the water phase.

Adsorption of Trace Hydrophobic Compounds from Water on Surfactant-Coated Alumina

Abstract The adsorption of several hydrophobic organic compounds (HOCs) from water on alumina coated by sodium dodecylsulfate surfactant “hemimicelles” is discussed. This provides a convenient way of concentrating them on a metal oxide solid support such as alumina. Alumina can be subsequently regenerated with a minimum effort such as a change of pH of the aqueous solution. Adsorption studies showed that the partition constants of HOCs with hemimicelles of sodium dodecylsulfate on alumina is linearly related to their octanol-water partition constants. Surfactantcoated alumina was a better adsorption surface for HOCs than natural soils.

Aqueous solubility enhancement and desorption of hexachlorobenzene from soil using a plant-based surfactant

A plant-based surfactant extracted from fruit pericarps of Sapindus mukorossi (Ritha) is proposed for remediation of contaminated soil from a local hazardous waste site. Natural surfactants can be prepared using a very simple water extraction of fruit pericarp powder. Natural surfactant solutions are employed to enhance the aqueous solubility of a hydrophobic organic compound, hexachlorobenzene (HCB), and to desorb HCB from soils in batch and one-dimensional flow-through soil column experiments. The solubility of HCB in natural surfactant solutions increased linearly with surfactant concentration beyond the critical micelle concentration. The mass of dry Ritha powder required to solubilize 1 mg of HCB in 1 liter of water was comparable to sodium dodecylsulfate solution and other commercial surfactants. HCB concentration in the aqueous solutions approached 90% of the HCB solubility in the respective natural surfactant solutions when soils contaminated to high levels were used for desorption. HCB recovery was up to 90% of the total HCB for soils contaminated with lower levels. Desorption behavior observed for natural surfactant solutions was similar to SDS solutions. Natural surfactant solutions performed more efficiently than a simple water flood in recovering HCB from one-dimensional soil columns. The HCB concentration in the effluent was found to be as high as 80% of the surfactant-enhanced HCB solubility in respective solutions. The results of this study provide a strong case for pursuing natural surfactant solutions in further research.

Soil flushing of residual transmission fluid: Application of Colloidal Gas Aphron suspensions and conventional surfactant solutions

Abstract An innovative technology using Colloidal Gas Aphrons (CGAs) is applied in this study to flush residual levels of a light non-aqueous phase liquid (LNAPL) such as automatic transmission fluid (ATF) from a Superfund site soil. Performance of CGA suspensions is compared with that of conventional aqueous anionic surfactant solutions under both gravity stable (downflow) and gravity unstable (upflow) conditions. CGA suspensions were found to be more effective in washing ATF under both downflow and upflow modes. The displacement of ATF from the soil pores seems to be the mechanism of removal in the case of water floods. Increasing the surfactant concentration did not increase the removal rate correspondingly. The pressure required to pump the CGA suspension was much lower than that required for conventional surfactant solutions or water flood. Results show enough promise that more research should be directed at this potential technology for in situ remediation of contaminated aquifers.

Application of surfactant solutions and colloidal gas aphron suspensions in flushing naphthalene from a contaminated soil matrix

Abstract The applicability of conventional surfactant solutions and colloidal gas aphron (CGA) suspensions to remove naphthalene from a contaminated soil matrix was investigated. Laboratory studies were conducted to evaluate the relative suitability of nonionic, cationic and anionic surfactants in removing naphthalene from the soil. Nonionic surfactant was found to be more efficient than the ionic surfactants in removing napthalene from contaminated soil matrix in batch experiments and was selected for further flushing studies. The presence of surfactant reduced the volatilization of naphthalene. Micellar solubilization appears to be the primary mechanism of removal using both surfactant solutions and CGA suspensions. Increasing the concentration of the surfactant solution enhanced the percent removal of naphthalene. Using a CGA suspension as a flushing medium may result in channeling and pore clogging in the soil matrix, thereby affecting the overall efficiency of the process.

Soil flushing using colloidal gas aphron suspensions generated from a plant-based surfactant

Abstract Natural surfactant solutions obtained from the fruit pericarps of Sapindus mukorossi , commonly known as Ritha or soapnut, are tested for their ability to remove hydrophobic organic compounds (HOCs) from soil. Colloidal gas aphron (CGA) suspensions generated using the surfactant are used in this study to flush an HOC from a representative soil. Soil is spiked with a chlorinated hydrocarbon, hexachlorobenzene (HCB), serving as a model HOC representative of contamination at a Superfund site north of Baton Rouge, LA. The recovery of HCB from soil columns using CGA suspensions was considerably larger than that for a waterflood. HCB recoveries in the effluent reached a maximum by the fifth pore volume and remained fairly constant for soils contaminated with high levels of HCB. This maximum HCB concentration in the column effluent was proportional to HCB solubility in the corresponding surfactant solutions. Natural surfactant performed marginally better in the form of conventional solutions than CGA suspensions at similar concentration in recovering HCB. HCB removal increased with increasing surfactant concentration due to increased aqueous solubility. The pressure buildup across the soil column remained fairly low when natural surfactant was used at concentrations up to 1%. Alternating the flushing media between CGA and water neither enhanced the recovery of HCB nor changed the pressure buildup across the soil column.

A Study of the Size Distribution and Stability of Colloidal Gas Aphrons Using a Particle Size Analyzer

Abstract Application of colloidal gas aphrons (CGA) in decontaminating soils and aqueous solutions is one of the emerging innovative technologies. This paper addresses the size distribution and stability of CGAs as studied by using a particle size analyzer. Cationic, anionic, and nonionic surfactants were used to generate the CGAs. Size distribution spectrum and volume fraction of microbubbles in sample solutions were studied as functions of time. The effects of surfactant concentrations used to produce CGAs and the presence of an electrolyte, such as sodium chloride, on the characteristics of the suspension were also studied.