Aqueous Sorption Of Typical Aromatic Pollutants On Nanomaterials Modified With Cetyl Pyridinium Chloride

Investigation of organic pollutants sorption by nanomaterials is not only important for environmental risk assessment of nanomaterials, but also for development of nano-sorbents. Surfactants could coexist with nanomaterials in the environment, and could affect the sorption of organic pollutants on nanomaterials, then had been employed to synthesize composite organic sorbents (e.g. organobenonite). Therefore, study of influence of surfactants on organic pollutants sorption by nanomaterials would be helpful for development of high efficient nano-sorbents and environmental risk assessment of nanomaterials and organic pollutants. In this dissertation, sorption of organic pollutants on nanomaterials and its influence factor was reviewed. A single-walled carbon nanotube (SWCNT) and a Nano-SiO2 were chosen here as hydrophobic and hydrophilic nanomaterials, respectively, to investigate the effect of cetylpyridinium chloride (CPC, a cationic surfactant) on aqueous sorption of typical aromatic pollutants by these two nanomaterials, and mechanism of sorption, cosorption and desorption of aromatic pollutants on/from surfactant-modified nanomaterials was explained. The main conclusions of this dissertation were as follows:1 Substantially decreased sorption of naphthalene on SWCNT by CPC in aqueous phase was observed, suggesting surfactants would significantly alter the interaction between carbon nanotubes and organic pollutants and their biological effects. Two mechanisms were responsible for this result:(1) competition sorption between naphthalene and CPC; (2) partition of naphthalene into CPC hemimicelles on SWCNT, and partition ability of the hemimicelles decreased with increased CPC adsorption on SWCNT as hemimicelles became much tighter. Effect of competition by CPC was more significant than enhancement of CPC hemimicelles, so naphthalene sorption on SWCNT decreased quickly with increased adsorption of CPC. A partition-adsorption model was introduced to describe the partition fraction of naphthalene into adsorbed CPC hemimicelles as well as the adsorption fraction of naphthalene on SWCNT.2 Significantly enhanced sorption of organic pollutants on Nano-SiO2 by CPC in aqueous phase was observed, with a sorption coefficient 3.5～9 time higher than natural organic matter, suggesting surfactant modification probably be a good method to develop high efficient nano-sorbents. Sorption mechanism of organic pollutants on CPC-modified Nano-SiO2 was partition, specifically linear partition for nonpolar organic pollutants (such as naphthalene), and nonlinear partition for polar and hydroxyl organic pollutants (such as 4-nitrophenol). The linear partition was contributed by hydrophobic effect only, while the nonlinear partition was contributed by both hydrophobic effect and hydrogen bonding. The sorption enhancement by CPC was to increase sorption capacity, which was positively correlated with CPC content in modified Nano-SiO2, while sorption affinity was relatively constant.3 Sorption mechanisms of 23 kinds of typical aromatic pollutants on CPC-modified Nano-SiO2 were figured out. A sorption predictive model of aromatic pollutants on modified Nano-SiO2, based on solvation parameters (V1,π*,βm,αm), octanol solubility (So) and organic carbon content of modified Nano-SiO2, was established, which would be helpful for development of modified nano-sorbents and prediction of surfactant effect on organic pollutant sorption by modified nanomaterials. It was observed:(1) sorption enthalpy (△H) of aromatic pollutants by modified Nano-SiO2 was -3～-28 kJ/mol. Moreover, AH of polycyclic aromatic hydrocarbons (PAHs) and nitrobenzenes (αm=0, hydrophobic effect only) remained constant, while AH of phenols and anilines (αm>0, with hydrogen bonding) decreased obviously with increased equilibrium concentration of aromatic pollutants because of limitation by the number of active adsorption site of hydrogen bonding on modified Nano-SiO2. (2) The sorption affinity of PAHs and nitrobenzenes (E) was 5.71 kJ/mol, while E values for phenols and anilines was positively correlated with their am, which further proved that hydrogen bonding took an important role in phenols and anilines sorption. (3) Sorption capacity of aromatic pollutants was positively correlated with their So.4 Cosorption mechanisms of typical aromatic pollutants on CPC-modified Nano-SiO2 were figured out:(1) aromatic pollutants with linear partition (such as naphthalene, phenanthrene and pyrene) would not affect each other’s sorption, and also would not affect the sorption of aromatic pollutants with nonlinear partition (such as 2,4-dichlorophenol and 4-chloroaniline). (2) Aromatic pollutants with nonlinear partition (such as 2,4-dichlorophenol and 4-chloroaniline) would not affect the sorption of aromatic pollutants with linear partition (such phenanthrene)when the equilibrium adsorbed concentration (qe) is below 125 mg/g, while 2,4-dichlorophenol would enhance the sorption of phenanthrene when qe>125 mg/g. It was because that the partition ability of coated CPC was enhanced by the sorption of a large quantity of 2,4-dichlorophenol, similar with synergistic extraction effect. (3) Aromatic pollutants with nonlinear partition (2,4-dichlorophenol and 4-nitrophenol, 4-nitrophenol and 2-nitroaniline) would compete with each other on sorption. The sorption isotherms of aromatic pollutants became more linear under competition, suggesting active adsorption site of hydrogen bonding could be competed, and sorption isotherms would be linear when various aromatic pollutants were coexisting. Besides, aromatic pollutants had no desorption hysteresis from modified Nano-SiO2, including linear partition and nonlinear partition, suggesting the organic pollutants would be totally released when modified Nano-SiO2 was taken in by organisms, then result in high environmental risk.