Modeling Adsorption Of Inorganic Pollutants Onto Polymer-based Hydrous Ferric Oxide Nanocomposites

Immobilization of hydrous ferric oxide particles(HFO)inside porous solid hosts is an effective approach to improve their applicability in water treatment.The resultant composites are expected to overcome the bottleneck of HFO nanoparticles in application as well as to incorporate advantages of both nanoparticles and hosts.Thus,they have received increasing attention in both laboratory study and industry.The effect of porous host on the intrinsic physicochemical properties and reactivity of the encapsulated hydrous ferric oxides(HFOs)is crucial to better understand how HFOs interact with ionic pollutants inside the pore region.In this study,we prepared a series of hybrid adsorbents by dispersing nanosized HFOs inside porous polymeric supports of varying pore structure and surface chemistry.Based on the macroscopic experimental data sets,surface complexation model(SCM)was used to describe the adsorption behavior,and to quantitatively evaluate the surface chemistry and adsorption performance of the HFO inside the nanopores of the hosts.Firstly,a commercial chloromethylated polystyrene(CMPS)resin was employed as a host to prepare a HFO-loaded hybrid material(HFO-CMPS).The acid-base titration results showed that CMPS is an inert macroreticular adsorbent without any active functional groups.The one-site constant capacitance model(CCM)can successfully describe the acid-base behavior of the encapsulated HFOs.The adsorption edge of the inorganic ions,i.e.,Cu(?),Ni(?),Pb(?),As(?)and P(?),on HFO-CMPS indicated that inner-sphere complexes are formed during adsorption,which is similar to the bare HFO reported elsewhere.The adsorption edge was fitted by CCM to determine the adsorption constants.Results indicated that CCM could well predict the adsorption of the five ions onto HFO-CMPS under different solution conditions.Surface complexation model was employed to quantitatively evaluate the changes of surface acid-base chemistry as well as to investigate the specific solid-liquid interfacial behaviors of HFOs upon loading.As observed,the HFOs immobilized inside CMPS exhibited different performance from the bare HFOs.The potentiometric titration curves of HFO-CMPS at three ionic strengths showed markedly weaker ionic strength dependence as compared to the bare HFO particles.It is possibly due to the buffering effect of CMPS host for the compression of electrical double layer(EDL).The intrinsic equilibrium constants for the acid-base reactions of the surface sites of HFOs distinctly changed upon loading,that is,the log K(+)values decreased,while its log K(-)values increased,resulting in its pHpzc value shifting from?8.2 to?6.3.The modelling results 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 different 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 into acidic solution was observed,and may be due to its weaker pH buffering capacity,which possibly results from the surface charges dependent upon particle size.The available results indicated that the porous hosts play a significant role in the properties of the encapsulated metal oxides for their application in water treatment.One of the main objectives of this work was to elucidate the effect of the host pore size on the intrinsic surface properties of HFO encapsulated inside polymeric hosts.Here,we fabricated three polystyrene-based nano HFOs and explored the effect of host pore structure on surface chemistry of the immobilized HFOs.The potentiometric titration results show that the site density increased with the decreasing size of loaded HFO.Moreover,the affinity for H+was weaker and that for OH-was stronger for the smaller nanoparticles.Based on the optimized acidity constants,the distribution of surface species were determined,indicating that,the relative content of negatively charged species on HFO particles increased with the increase of the specific host surface area and resulting in a lower pHpzc shift.Additionally,adsorption of As(III)and As(V)suggested that the adsorption capacity was improved as the surface area of the host increased.Generally,the intrinsic chemical properties of the encapsulated HFO were significantly affected by the pore structure of host,which is believed to favor the structure optimization of the hybrids.In summary,we believe that this study is crucial to reveal the underlying mechanism of preferable pollutants removal by the immobilized HFOs.Moreover,it could help to guide preparation of iron oxide-based nanocomposites for environmental remediation.