Mesoporous Silica and Organosilica Nanomaterials as UV-Blocking Agents.

Mesoporous silica nanoparticles (MSN) and periodic mesoporous organosilica nanoparticles containing bridging benzene (PMOBTB) and ethane (PMOBTE) moieties are synthesized, characterized, and evaluated for application in skin protection from UVA/UVB sun irradiation. Furthermore, the influence of surface functionalization with chelating 3-(2-aminoethylamino)propylsilane and Zn2+ ions on the UV-blocking ability of MSN is evaluated, along with the photostability and capability of the synthesized nanomaterials to carry avobenzone, a known UV-absorbing agent. The obtained results reveal promising characteristics of MSN and PMO materials with regard to their potential for sunscreen applications, which could be beneficial in terms of alleviating concerns about health and environmental hazards of sunscreen ingredients.

Authors' response to comments on "Factors influencing the removal of divalent cations by hydroxyapatite".

Recently, it has come to our attention, that a polemical note by Milonjic [S.K. Milonjic, Comments on "factors influencing the removal of divalent cations by hydroxyapatite", J. Hazard. Mater. 162 (2009) 1588-1589], has been published raising several critical comments on some experimental procedures, as well as the presentation and interpretation of some data found in the cited paper by Smiciklas et al. [I. Smiciklas, A. Onjia, S. Raicevic, Dj. Janackovic, M. Mitric, Factors influencing the removal of divalent cations by hydroxyapatite, J. Hazard. Mater. 152 (2008) 876-884]. Since we have not received any prior copy of this discussion through the process involved in the acceptance of these comments, we now take the opportunity of responding. We have reviewed some scientific papers dealing with cation sorption, in order to demonstrate variety of experimental approaches used for batch sorption tests, regarding solution pH. Finally, we justified experimental methods and data interpretation methods we chose to perform in the mentioned manuscript.

Factors influencing the removal of divalent cations by hydroxyapatite.

The effect of pH, contact time, initial metal concentration and presence of common competing cations, on hydroxyapatite (HAP) sorption properties towards Pb(2+), Cd(2+), Zn(2+), and Sr(2+) ions was studied and compared using a batch technique. The results strongly indicated the difference between the sorption mechanism of Pb(2+) and other investigated cations: the removal of Pb(2+) was pH-independent and almost complete in the entire pH range (3-12), while the sorption of Cd(2+), Zn(2+) and Sr(2+) generally increased with an increase of pH; the contact time required for attaining equilibrium was 30 min for Pb(2+) versus 24h needed for other cations; maximum sorption capacity of HAP sample was found to be an order of magnitude higher for Pb(2+) (3.263 mmol/g), than for Cd(2+) (0.601 mmol/g), Zn(2+) (0.574 mmol/g) and Sr(2+) (0.257 mmol/g); the selectivity of HAP was found to decrease in the order Pb(2+)>Cd(2+)>Zn(2+)>Sr(2+) while a decrease of pH(PZC), in respect to the value obtained in inert electrolyte, followed the order Cd(2+)>Zn(2+)>Pb(2+)>Sr(2+); neither of investigated competing cations (Ca(2+), Mg(2+), Na(+) and K(+)) influenced Pb(2+) immobilization whereas the sorption of other cations was reduced in the presence of Ca(2+), in the order Sr(2+)>Cd(2+)>or=Zn(2+). The pseudo-second order kinetic model and Langmuir isotherm have been proposed for modeling kinetic and equilibrium data, respectively. The sorption of all examined metals was followed by Ca(2+) release from the HAP crystal lattice and pH decrease. The ion exchange and specific cation sorption mechanisms were anticipated for Cd(2+), Zn(2+) and Sr(2+), while dissolution of HAP followed by precipitation of hydroxypyromorphite (Pb(10)(PO(4))(6)(OH)(2)) was found to be the main operating mechanism for Pb(2+) immobilization by HAP, with the contribution of specific cation sorption.