The removal of contaminants from wastewaters is a major challenge in the field of water pollution. Among numerous techniques available for contaminant removal, adsorption using solid materials, named adsorbents, is a simple, useful and effective process. The adsorbent matter can be mineral, organic or biological. Activated carbon is the preferred, conventional material at the industrial scale. Activated carbon is extensively used not only for removing pollutants from wastewater streams, but also for adsorbing contaminants from drinking water sources, e.g., groundwater, rivers, lakes and reservoirs. However, the widespread use of activated carbon is restricted due to a high cost. In the last three decades, numerous approaches using non-conventional adsorbents have been studied for the development of cheaper and more effective adsorbents to eliminate pollutants at trace levels. This review gives an overview of liquid-solid adsorption processes using conventional and nonconventional adsorbents for pollutant removal. The manuscript outlines the principles of adsorption and proposes a classification for adsorbent materials. Finally, the various mechanisms involved in the adsorption phenomena are discussed.

The inhibition activities of two antifreeze proteins (AFPs) on the formation of tetrahydrofuran (THF) clathrate hydrate have been tested. AFPs from fish (wfAFP) and insect (CfAFP) changed the morphology of growing THF hydrate crystals. Also, both AFPs showed higher activities in inhibiting the formation THF hydrate than a commercial kinetic inhibitor, poly(vinylpyrrolidone) (PVP). Strikingly, both AFPs also showed the ability to eliminate the "memory effect" in which the crystallization of hydrate occurs more quickly after the initial formation. This is the first report of molecules that can inhibit the memory effect. Since the homogeneous nucleation temperature for THF hydrate was measured to be 237 K, close to that observed for ice itself, the action of kinetic inhibitors must involve heterogeneous nucleation. On the basis of our results, we postulate a mechanism for heterogeneous nucleation, the memory effect and its elimination by antifreeze proteins.

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The apparent molar volumes (V φ ,S) of a homologous series of hydrocarbon (hc) (C x H2 x +1CO2Na, x = 2, 5−9, 11, 13) and perfluorocarbon (fc) (C x F2 x +1CO2Na, x = 1, 3, 4, 6−9) surfactants (S) have been determined in water and in binary solvent (H2O + β-cyclodextrin (β-CD)) systems at 25 °C. The apparent molar volumes of β-CD (V φ ,CD) in water and in binary (H2O + S) systems containing hc and fc surfactants have also been obtained. The results show that the magnitudes of V φ ,S and V φ ,CD are greater in ternary solutions than in the binary aqueous systems. The apparent molar volumes at infinite dilution ( ) of β-CD and of the surfactants in ternary solutions are observed to depend on the following factors:  (i) the magnitude of the binding constant (K i ), (ii) the alkyl chain length of the surfactant, (iii) the mole ratio of the host to guest species, (iv) the host/guest stoichiometry, and (v) the physicochemical properties of the surfactant. The volumetric properties of the ternary systems have been analyzed in terms of the complexed and uncomplexed species by application of Young's rule. The formation of β-CD/surfactant complexes having 1:1 and 1:1 plus 2:1 stoichiometries were successfully modeled using two-site and three-site models, respectively.

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A 19F NMR chemical shift study of a homologous series of perfluorocarbon (fc) [C x F2 x +1CO2Na, x = 3, 4, 6−9] surfactants (S) has been carried out in D2O and in binary solvent (D2O + cyclodextrin (CD)) systems at 22 °C. Both β-CD and modified cyclodextrins were used. Complementary 1H NMR chemical shift data for the cyclodextrins in binary solvent (D2O + S) systems were also obtained. Values of the complex-induced chemical shifts (CIS) for selected host or guest nuclei are observed to increase with increasing alkyl chain (C x ) length of the surfactant when the CD/S complex has a 1:1 stoichiometry. However, for complexes having stoichiometries other than 1:1 CD/S, somewhat different trends in the CIS values were observed. Binding constants (K i) have been obtained from the analysis of 19F and 1H CIS values of the CD/S systems using equilibrium models in which 1:1, 1:1 plus 2:1, 1:1 plus 1:2 complexes, and uncomplexed species are present. In general, K i increases as C x increases; however, differences in the binding affinity, stoichiometry, and inclusion geometry of the CD/S complexes were observed to depend on the type of cyclodextrin. The latter can be understood in terms of the steric effects created by the introduction of alkyl groups in the annulus region of the cyclodextrin.

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