An EXAFS investigation of the mechanism of competitive sorption between Co(Ⅱ) and Ni(Ⅱ) at γ-alumina/solution interface

Co(Ⅱ)and Ni(Ⅱ)are two common toxic heavy metals,and may simultaneously exist in contaminated water,soil,and sediment systems in Earth’s surface environment.Under this circumstance,competitive adsorption between the two metals may influence their migration,toxicity,and bioavailability.In this research,the competitive sorption of Co(Ⅱ)and Ni(Ⅱ)on γ-Al_2O_3was studied using both macroscopic sorption experiments and extended X-ray absorption fine structure(EXAFS)spectroscopy.Results suggest that Ni(Ⅱ)reduced the amount of Co(Ⅱ)sorption in a binary-solute system at pH 6.0.This is because both Co(Ⅱ)and Ni(Ⅱ)form inner-sphere surface complexes during sorption on γ-Al_2O_3and compete for the surface reactive sites.However,Co(Ⅱ)exhibited a negligible influence on sorption amount of Ni(Ⅱ)under the same conditions,which suggests Ni(Ⅱ)has a stronger affinity to alumina surface.At pH 7.5,Co(Ⅱ)and Ni(Ⅱ)sorption density were much higher than that at pH 6.0,but there no mutual competitive effect was observed.EXAFS analysis further revealed that formation of layered double-hydrated precipitates was the dominant sorption mechanism for both Co(Ⅱ)and Ni(Ⅱ)at pH 7.5.Because this type of sorption does not rely on surface reactive sites,there was no competition between Co(Ⅱ)and Ni(Ⅱ).This finding sheds light on risk assessment and remediation of Ni/Co pollution.

Competitive sorption between 17α-ethinyl estradiol and bisphenol A/4-n-nonylphenol by soils

The sorption of 17α-ethinyl estradiol （EE2）, bisphenol A （BPA）, and 4-n-nonylphenol （NP） in single systems and the sorption of EE2 with different initial aqueous concentrations of BPA or NP were examined using three soils. Results showed that all sorption isotherms were nonlinear and fit the Freundlich model. The degree of nonlinearity was in the order BPA （0.537-0.686） 〉 EE2 （0.705-0.858） 〉 NP （0.875-0.0.951） in single systems. The isotherm linearity index of EE2 sorption calculated by the Freundlich model for Loam, Silt Loam and Silt increased from 0.758, 0.705 and 0.858, to 0.889, 0.910 and 0.969, respectively, when BPA concentration increased from 0 to 1000 μg/L, but the effect of NP was comparably minimal. Additionally, EE2 significantly suppressed the sorption of BPA, but insignificantly suppressed that of NE These findings can be attributed to the difference of sorption affinity of EE2, NP and BPA on the hard carbon （e.g., black carbon） of soil organic matter that dominated the sorption in the low equilibrium aqueous concentration range of endocrine-disrupting chemicals （EDCs）. Competitive sorption among EDCs presents new challenges for predicting the transport and fate of EDCs under the influence of co-solutes.

Sorption mechanism of naphthalene by diesel soot: Insight from displacement with phenanthrene/p-nitrophenol

The nonlinear sorption of hydrophobic organic contaminants(HOCs)could be changed to linear sorption by the suppression of coexisting solutes in natural system,resulting in the enhancement of mobility,bioavailability and risks of HOCs in the environment.In previous study,inspired from the competitive adsorption on activated carbon(AC),the displaceable fraction of HOCs sorption to soot by competitor was attributed to the adsorption on elemental carbon fraction of soot(EC-Soot),while the linear and nondisplaceable fraction was attributed to the partition in authigenic organic matter of soot(OM-Soot).In this study,however,we observed that the linear and nondisplaceable fraction of HOC(naphthalene)to a diesel soot(D-Soot)by competitor(phenanthrene or p-nitrophenol)should be attributed to not only the linear partition in OM-Soot,but also the residual linear adsorption on EC-Soot.We also observed that the competition on the surface of soot dominated by external surface was different from that of AC dominated by micropore surface,i.e.,complete displacement of HOCs by p-nitrophenol could occur for the micropore surface of AC,but not for the external surface of soot.These observations were obtained through the separation of EC-Soot and OM-Soot from D-Soot with organic-solvent extraction and the sorption comparisons of D-Soot with an AC(ACF300)and a multiwalled carbon nanotube(MWCNT30).The obtained results would give new insights to the sorption mechanisms of HOCs by soot and help to assess their environmental risks.