Sorption Of Nonionic Organics To Mineral Micropores:Effect Of Metal Cation Hydration

Molecular-scale physic-chemical processes at mineral-water interfaces?MWI?play important roles in the fate and transport of contaminants in soils and sediments.For soils and sediments with low organic carbon?<0.1%?,sorption of contaminants into the mineral phase is commonly assumed to be the major mechanism for contaminant retention.The influence of K⁺and Ca²⁺on the sorption of nonionic aromatic contaminants?1,4-dinitrobenzene and p-xylene?on a series of microporous zeolite minerals?HZSM-5?with various surface charge densities were investigated.For zeolites with high or low charge density?>1.78 sites/nm² or<0.16 sites/nm²?,K⁺and Ca²⁺had negligible influence on the sorption of organics,which mainly occurred at the hydrophobic nanosites.For zeolites with charge density in the moderate range?0.16¹.78 sites/nm²?,the sorption of organics was strongly dependent on cation hydration effect.K⁺with lower hydration free energy?-78.6 kcal/mol?greatly favored sorption of organics to the micropores,while Ca²⁺with higher hydration free energy?-369.9 kcal/mol?has no effect on the sorption of organics.The result of Differential scanning calorimetry?DSC?showed that the dehydration behaviors of zeolites with charge density of 0.27 sites/nm²?K/ZSM-5?46?,and Ca/ZSM-5?46??were quite different,and more positively correlated to the hydration free energies of the surface cations.It was found that K/ZSM-5?46?displayed a single endotherm peak at 42 ^oC,while for Ca/ZSM-5?46?,two overlapping endotherm peaks at 45 ^oC and 78 ^oC were observed.The DSC results clearly indicate that water binding in K/ZSM-5?46?was relatively weaker and can be more easily removed compared to Ca/ZSM-5?46?,which held water molecules more tightly and retained to higher temperatures.X-ray photoelectron spectroscopy?XPS?results showed zeolites with charge density of 0.16sites/nm² and 1.78 sites/nm²?HZSM-5?5.4??and HZSM-5?470?),the ratio of the N1s?400 eV?and N1s?402 eV?peak areas N1s?400 eV?:N1s?402 eV?,was comparable in the absence and presence of K⁺and Ca²⁺?i.e.,0.2-0.3 for HZSM-5?5.4?and0.4-0.6 for HZSM-5?470?,respectively?.But for HZSM-5?46?,N1s?400 eV?:N1s?402 eV?ratio was much higher?1.0?,and significantly increased to 2.4 in the presence of K⁺,while Ca²⁺had little effect.DSC and XPS results indicated that K⁺can reduce the water affinity and promote specific sorption of organics in the zeolites with moderate charge density.The above mechanisms were successfully applied to explain the retention of 1,4-dinitrobenzene and p-xylene on four natural minerals?smectite,illite,kaolinite and mordenite?.This study shed new insights on how cation hydration influences sorption interactions of nonionic organic contaminants at mineral-water interfaces as a function of mineral charge density.It is of great significance to further explain the phenomenon that organic pollutants are permanently retained in the soil,reveal the migration and transformation of pollutants and ecological risks,and provide theoretical guidance for soil organic pollutant management and remediation technology.