Surface Chemistry of Nanosized Hydrated Ferric Oxide Encapsulated Inside Porous Polymer: Modeling and Experimental Studies

Elucidating the effect of porous host on the intrinsic physicochemical properties and reactivity of the encapsulated nanosized hydrous ferric oxides (HFOs) is believed important to better understand how HFOs interact with ionic pollutants inside the pore region. Here we prepared a hybrid adsorbent (HFO–CMPS) by dispersing nanosized HFOs onto an inactive porous polymeric support (i.e., chloromethylated polystyrene, CMPS). A surface complexation model (SCM) was employed to quantitatively evaluate the acid–base reactions and adsorption behaviors of HFO–CMPS as compared to the bare HFOs. Results demonstrated that the intrinsic equilibrium constants for acid–base reactions of surface sites of HFOs distinctly changed upon loading, that is, the log K (+) values decreased, while its log K (−) values increased, resulting in its pH pzc value shifting from ∼8.2 to ∼6.3. Meanwhile, the titration curves of HFO–CMPS showed a markedly weaker dependence upon ionic strength. The results of model fittings of Cu(II) and As(V) adsorption indicate that the change of Coulombic terms, reflecting the effect of the electrical potential on the adsorption activities, played an important role in the difference in pH-dependent adsorption of Cu(II) and As(V) between HFO–CMPS and bare HFOs. Additionally, the greater tendency of the encapsulated HFOs to dissolve in acidic solution was observed and may be due to its weaker pH buffering capacity, which possibly results from size-dependency of surface charges. All the results indicated that porous hosts play a significant role in the properties of the attached metal oxides for their application in water treatment.