Gengxin Xie

Biosorption of zinc(II) from aqueous solution by dried activated sludge

The biosorption potential of dried activated sludge as a biosorbent for zinc(II) removal from aqueous solution was investigated. The effects of initial pH, contact time, initial zinc ion concentration, and adsorbent dosage on the biosorption processes were determined, and the equilibrium data were modeled by the Langmuir and Freundlich isotherms. The Langmuir isotherm model (R2 = 0.999) was proved to fit the equilibrium data much better than the Freundlich isotherm model (R2 = 0.918). The monolayer adsorption capacity of dried activated sludge for zinc(II) was found to be 17.86 mg/g at pH of 5 and 25 degrees C. The kinetic data were tested using pseudo first- and second-order models. The results suggested that the pseudo second-order model (R2 > 0.999) was better for the description of the adsorption behavior of zinc(II) onto the dried activated sludge. Fourier transform infrared spectral analysis showed that the dominant mechanism of zinc(II) biosorption onto the dried activated sludge was the binding between amide groups and zinc ions.

Effects of surfactants and salt on Henry's constant of n-hexane

n-Hexane biological removal is intrinsically limited by its hydrophobic nature and low bioavailability. The addition of surfactants could enhance the transport of volatile organic compounds (VOCs) and change the gas-liquid equilibrium of VOCs. In this paper, the effects of four surfactants, sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), tert-octylphenoxypoly-ethoxyethanol (Triton X-100), polyoxyethylene (20) sorbitan monooleate (Tween 80), and sodium nitrate on apparent Henrys constant of n-hexane in surfactant solutions were investigated. The apparent Henrys constants were significantly reduced when surfactants concentrations exceeded their critical micelle concentrations (cmcs). On a cmc basis, the anionic surfactant SDS was found to have the greatest effect on the apparent Henrys constant with CTAB succeeding, then followed by Triton X-100 and Tween 80. However, the apparent Henrys constant of n-hexane decreased even more rapidly when Triton X-100, a nonionic surfactant, was added than when the ionic surfactant of SDS or CTAB was applied under identical mass concentration and other conditions. These results suggest that Triton X-100 have the biggest solubilization of n-hexane among the four surfactants. Sodium nitrate slightly decreased the apparent Henrys constant of n-hexane in surfactant solutions, and could be considered as a cosolvent in the surfactant-(n-hexane) solution. In addition, the relationship between apparent Henrys constant and surfactant concentration was further developed.

Spectroscopic studies of dye-surfactant interactions with the co-existence of heavy metal ions for foam fractionation

The interaction between a cationic dye Methylene Blue (MB) and an anionic surfactant sodium dodecyl sulfate (SDS) with the presence of Cd2+ was investigated spectrophotometrically in a certain concentration range. The spectrophotometric measurements of dye-metal ion-surfactant system were carried out. The results indicated that the SDS concentration had a significant influence on the dye spectrum, while the addition of Cd2+ hardly caused change of the maximum value of absorbance. According to this observation, we concluded that electrostatic and hydrophobic interaction between dye and surfactant occurred up to a certain level, and the homo-ions Cd2+ almost exerted no effect on the dye-surfactant complexation, establishing a theoretical foundation for simultaneous removal of organic dye and heavy metal using foam fractionation. Meanwhile, the effects of their interaction on foam performance were investigated. The results showed that the addition of Cd2+ favored the tendency to ameliorate foam properties just contrary to MB. The feasibility of foam separation for dye and heavy metal removal from simulated wastewater was also confirmed using a continuous foam fractionator. In the simultaneous removal process, with the initial SDS concentration ranging from 0.5 to 5.0 mmol/L, the maximum removal efficiencies of MB and Cd2+ were obtained as 99.69% and 99.61%, respectively. The enrichment ratios were reduced from 24.34 to 7.65 for MB and from 22.01 to 3.35 for Cd2+.

Enzyme–Substrate Binding Landscapes in the Process of Nitrile Biodegradation Mediated by Nitrile Hydratase and Amidase

The continuing discharge of nitriles in various industrial processes has caused serious environmental consequences of nitrile pollution. Microorganisms possess several nitrile-degrading pathways by direct interactions of nitriles with nitrile-degrading enzymes. However, these interactions are largely unknown and difficult to experimentally determine but important for interpretation of nitrile metabolisms and design of nitrile-degrading enzymes with better nitrile-converting activity. Here, we undertook a molecular modeling study of enzyme–substrate binding modes in the bi-enzyme pathway for degradation of nitrile to acid. Docking results showed that the top substrates having favorable interactions with nitrile hydratase from Rhodococcus erythropolis AJ270 (ReNHase), nitrile hydratase from Pseudonocardia thermophila JCM 3095 (PtNHase), and amidase from Rhodococcus sp. N-771 (RhAmidase) were benzonitrile, 3-cyanopyridine, and l-methioninamide, respectively. We further analyzed the interactional profiles of these top poses with corresponding enzymes, showing that specific residues within the enzyme’s binding pockets formed diverse contacts with substrates. This information on binding landscapes and interactional profiles is of great importance for the design of nitrile-degrading enzyme mutants with better oxidation activity toward nitriles or amides in the process of pollutant treatments.