[Investigating Adsorption of Naphthalene to Organo-Modified Montmorillonites and Influencing Factors]

Abstract

There are many sites contaminated by polycyclic aromatic hydrocarbons (PAHs) or combined PAHs-heavy metal in China, which pose serious health-risks to local people and environments. Dissipative quartz crystal microbalance (QCM-D) was applied to investigate the adsorption of naphthalene to two organic-modified smectite clays (cetyltrimethyl ammonium bromide modified montmorillonite, CTAB-SMF, and 3-mercapto propyl trimethoxy silane modified montmorillonite, TMSP-SMF) and original SMF, together with batch adsorption experiments. The results, based on in-situ online QCM-D experiments, showed that the adsorption sites of CTAB-SMF on naphthalene were mainly CTAB grouped between the interlayers of clay particles, while TMSP-SMF's were TMSP grouped on the surfaces of clay particles. The isotherms of naphthalene adsorption to CTAB-SMF fitted well (R²>0.92) with the Freundlich model, while the adsorption isotherms to TMSP-SMF and SMF fitted well with the Langmuir model (R²>0.96). The parameters of CTAB-SMF (n>1) indicated that the adsorption sites of naphthalene to CTAB-SMF were heterogeneous and the adsorption increases at higher equilibrium concentration. The adsorption capacity q_max of TMSP-SMF is significantly greater than that of SMF (P<0.05). Based on the calculation, the value of ΔH_obs for these three clays (CTAB-SMF, TMSP-SMF, and SMF) were within the range of -30 to -10 kJ·mol^-1, which indicated that it was a spontaneous exothermic physical process. The solution ionic strength could decrease the adsorption of naphthalene to SMF, but had limited effects on naphthalene adsorption to the two organo-modified clay particles. The existence of Cu²⁺ in the solution could enhance naphthalene adsorption to the three particles, and naphthalene could improve Cu²⁺ adsorption to these two modified clays. The results of this study show that QCM-D is a useful technique to indicate the changes of clay film during the process of adsorption, and the two organo-modified clay particles have the potential to be remediation materials to remediate PAHs or PAHs-heavy metal-contaminated sites.

Keywords: adsorption; compound contamination of PAHs & heavy metals; dissipative quartz crystal microbalance (QCM-D); naphthalene; organo-clay particles; polycyclic aromatic hydrocarbons (PAHs).