Adsorption Properties And Mechanisms Of Typical Oxyanion Pollutants Onto Layered Double Hydroxides

Layered double hydroxides (LDHs) are anionic clays that composed of positively charged brucite-like sheets and the intercalated anions with molecular formula of [M1-x2+Mx3+·(OH)2]x+(An-)x/h·mH2O. It is a novel functional material efficient in removing anions from water due to its large surface area and high ion-exchange capacity. In addition, LDHs can be easily regenerated by calcinating as the structure of calcinated LDHs (CLDHs) could be recovered after incorporating water and anions ("memory effect"). Therefore, LDHs would be a promising material for treating water anionic pollutions. In this dissertation, the synthesis methods, structure characteristics and oxy-anion adsorption properties of LDHs were reviewed, and then it is noted that, despite the excellent capability of LDHs to remove pollutants, the corresponding relationship between the LDH structure and the LDH adsorption properties wants further studies. In this work, adsorption of oxy-anions to a series of LDHs with different composition (including graphene intercalating) was investigated. The composition structure, surface functional groups and morphology of the synthesized LDHs were characterized by ICP-MS, XRD, FTIR, Raman, TG-DTG, SEM and TEM. Adsorption properties and mechanisms of an inorganic anion (CIO4-) and an organic anion (phenol anion) on LDHs were illustrated. Specifically, the uptake and molecular interaction mechanisms between CIO4-and the LDHs (CLDHs), the adsorption properties of phenol onto the CLDHs under different adsorption conditions, the thermal recycle performance of CLDHs and the strengthening mechanism of graphene in the adsorption process were demonstrated. The main original conclusions of this work are drawn as follows.(1) Elucidated the adsorption properties, mechanisms and the structure-function relationship of perchlorate onto LDHs and CLDHs. The adsorption of CIO4-onto the parent LDHs was very weak through surface adsorption or ion-exchange, and adsorp-tion amount was about10mg/g. The CLDHs exhibited a high adsorption capacity of CIO4-(adsorption amount of280mg/g), driven by the structural memory effect of the CLDHs with perchlorate as an interlayer anion and the hydrogen bond that formed between the CIO4-and the surrounding hydroxyl group in the recovered LDHs. The CIO4-uptake by the CLDHs was enhanced as the M2+/M3+value increased, while the positive layer charge density was decreased, which illustrated the adsorption mechanism cannot be explained by anion exchange alone. Inflect points were found along with the ClO4-concentration increased in the adsorption isotherm, which stated multi-interactions were existed among the ClO4-and hydroxide layer. The change of M3+cation (from Al3+to Fe3+) mainly resulted in the layer charge density decreased caused for the larger ion radius of Fe3+than Al3+, then the electrostatic interaction between the ClO4-and the hydroxide layer was weakened, and the adsorption amount decreased to121mg/g. In a comparison, the change of M2+cation (from Mg2+to Zn2+) led to a good match of the interlayer space symmetry (D3h) with the carbonate, so the carbonate possessed highly stability in the interlayer and formed strongly hydrogen bond with the surrounding hydroxyl group, which finally result in a very low adsorption capacity (11mg/g) driven by electrostatic interaction.(2) Verified the adsorption properties, adsorption mechanisms and thermal-recycle properties of phenol onto the CLDHs under different adsorption conditions. The CLDHs possess highly adsorption capacity about the phenol with a highest adsorption amount of216mg/g, in which the phenol was adsorbed as an interlayer anion to formed LDHs-phenol. CLDHs was also a recyclable adsorbent, and the adsorbed phenol in the LDHs-phenol can be thoroughly decomposed through thermal treatment with the adsor-bent regeneration simultaneously. As for the rich water content in the LDHs interlayer, the existing phenol in the interlayer tend to combine with the hydroxyl group or water molecular through weak hydrogen bond. And the weak interaction between phenol and the LDHs, greatly enlarged the interlayer space, in turn promoted the phenol adsorption.(3) Clarify the contribution of graphene in the adsorption of oxyanion onto the complex of LDHs and graphene. In detail, the incorporation of graphene can improve the dispersity of LDHs, and can increase the microenvironment hydrophobic property, which result in the improvement of adsorption amount (410mg/g). Hydration was weakened for the decrease of water content in the interlayer space, thus strengthen the electrostatic attraction between the ClO4-with the LDHs layer. On the other hand, the incorporation of graphene significantly promote the affinity of LDHs to the phenol, effectively diminished the interlayer space and facilitated the interaction between the phenol and the LDHs layers (like electrostatic interaction, hydrogen bonding and π-π interaction).