Search

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. [L. 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. (C) 2009 Elsevier B.V. All rights reserved.

Lead contamination is an environmental concern due to its effect on human health. One of the ways of remediating lead contamination is the in situ stabilization of this heavy metal. This can be achieved by converting different types of lead species into Pb-phosphates (pyromorphites), which are among the least soluble Pb-bearing solids. Synthetic hydroxyapatite (Ca5(PO4)3OH) and natural apatite can be used for this purpose. Both hydroxyapatite and some natural apatites are effective in reducing the aqueous Pb concentration to below 15 μgT1 (EPA drinking water limit). The main mechanism in the case of hydroxyapatite is its dissolution and subsequent precipitation of pyromorphites. The mechanism and the effectiveness of natural apatites depend on the type of apatite, its composition, crystallinity and presence of impurities. Apatites are also effective in reducing the Pb concentration in leachets from contaminated soils in the pH range from 6 to 10. The percentage of Pb concentration reduction is from 60 to 99.9% depending on the type of contamination and type of apatite amendment applied. These data suggest that apatites have potential to provide cost-effective in situ Pb immobilization. Ljudske aktivnosti su dovele do promene distribucije i oblika u kojem se nalaze različiti elementi, uključujući olovo. Olovo je pretvoreno iz nereaktivnog, geohemijski stabilnog oblika u rastvorljiva jedinjenja te je tako postalo dostupno za biosferu. Jedan od mogućih pristupa saniranju zagađenja olovom je promena smera ovog procesa, odnosno transformacija reaktivnih i rastvornih jedinjenja ovog metala u stabilnije oblike. Ovim procesom se olovo ne ukljanja iz životne sredine, ali se sprečava njegova akumulacija u biosferi. Jedan od materijala koji se ispituju za korišćenje u ovu svrhu je sintetički hidroksiapatit (Ca5(PO4)3OH) i prirodni apatit. Ovi materijali omogućavaju formiranje Pb-fosfata (piromorfita) koji su veoma slabo rastvorljivi i u skladu sa tim, slabo dostupni biosferi. Korišćenje obe ove vrste apatita omogućava snižavanje koncentracije olova u vodenim rastvorima ispod 15 μgT1, što je gornja granica za pijacu vodu Američke agencije za zaštitu životne sredine (EPA). U slučaju hidroksiapatita osnovni mehanizam ovog procesa je njegovo rastvaranje i taloženje piromorfita. Efikasnost prirodnog apatita mnogo zavisi od sastava minerala, stepena kristaličnosti i količine prisutnih nečistoća. Mehanizam stabilizacije olova prirodnim apatitom nije u potpunosti utvrđen. Postoje dokazi koji potvrđuju mehanizam rastvaranje apatita i taloženja piromorfita, ali potrebna su dalja istraživanja u ovoj oblasti. Apatiti su takođe efikasni u smanjivanju koncentracije rastvorljivog olova u zagađenom zemljištu u opsegu pH od 6 do 10. Ovo smanjenje se kreće od 60 do 99.9 % u zavisnosti od vrste zagađenja i primenjenog apatita. Ovi podaci ukazuju da apatiti mogu biti korišćeni za in situ stabilizaciju olova.

Antimikrobni materijali na bazi hidroksiapatita su potencijalno atraktivni za širok spektar primena u medicini. Monofazni srebro-, bakar- i cink-dopirani hidroksiapati (MxCa10-x(PO4)6(OH)2, M = Ag, Cu ili Zn, 0,002 ≤ x ≤ 0,04) su sintetizovani korišćenjem metode neutralizacije. Ova metoda se sastoji od rastvaranja oksida metala (Ag2O, CuO ili ZnO) u rastvoru H3PO4, i sporog dodavanja rastvora u suspenziju Ca(OH)2 radi homogene raspodele dopantnog jona. Kvantitativna elementala analiza je pokazala da su joni srebra, bakra i cinka u potpunosti ugradili u hidroksiapatit. Rendgenostrukturna analiza je pokazala da su svi dopirani uzorci veoma slične strukture sa čistim hidroksiapatitom i u skladu su sa ASTM podacima (kartica 9-432). Svi difrakcini pikovi su oštri i dobro razdvojeni, što ukazuje da je dobijeni monofazni i dobro iskristalisani hidroksiapatiti. Posmatranja transmisionim elektronskim mikroskopoma su pokazala da su morfologije čestice sintetisanih hidroksiapatita veoma slične, nepravilnog oblika i nano veličine. Prosečna dužina čestica u svim uzorcima je oko 70 nm i oko 15-25 nm u prečniku. Antimikrobna aktivnost uzorka su testirani na mikroorganizme koji mogu da izazovu infekcije vezane za implantate: Staphilococcus aureus, Escherichia coli i Candida albicans. Rezultati antimikrobnih testova su pokazali da svi dopirani uzorci hidroksiapatita imaju antimikrobnu aktivnost. Antimikrobna efikasnost materijala je u funkcija vrste i količine dopantnog jona. Aktivnost je najviše izraženo sa uzorcima hidroksiapatita dopiranih jonima srebra. Antimikrobno dejstvo uzoraka dopiranog sa najvećom količinom srebra na ćelije mikroorganizama je posmatrana pomoću mikroskopa atomskih sila. Studije su pokazale da dopirani uzorak hidroksiapatita stvara značajne morfološke promene na ćelijama mikroorganizama koje mogu biti uzrok njihove smrti. Test hemolize je izveden na srebro- i cink-dopirane uzorke kako bi se ispitala njihova hemokompatibilnost. Stepen hemolize uzoraka je bio manji od 3% i zbog toga, se može zaključiti da su nehemolitički. Tako, pripremljeni metal dopirani hidroksiapati su perspektivni za inženjering koštanog tkiva i mogu se primeniti kao materijali za mikrobiološki tretman zagađene vode, kao aditivi polimera i za opštu upotrebu. Antimicrobial materials based on hydroxyapatite are potentially attractive in a wide variety of medical applications. Monophase silver-, copper- or zinc-doped hydroxyapatite (MxCa10−x(PO4)6(OH)2; M = Ag, Cu or Zn; 0.002 ≤ x ≤ 0.04) were synthesized using a neutralization method. This method consists of dissolving metal oxide (Ag2O, CuO or ZnO) in a solution of H3PO4, and the slow addition of a solution to the suspension of Ca(OH)2 was applied for the purpose of homogenous distribution of dopant ions. The quantitative elemental analysis showed that the silver, copper and zinc ions fully incorporated into the hydroxyapatite. X-ray diffraction analysis showed that all doped samples are very similar in structure to pure hydroxyapatite and in accordance with ASTM data (Card 9-432). All of the diffraction peaks were sharp and well resolved, indicating the obtained monophase and well crystallized hydroxyapatite. Observation of transmission electron microscopy showed that the morphology of the synthesized hydroxyapatite particles are very similar, irregular shaped and nano sized. The average length of particles in all samples is about 70 nm and about 15–25 nm in diameter. Antimicrobial activities of the samples were tested for microorganisms that can cause implant-related infections including: Staphylococcus aureus, Escherichia coli and Candida albicans. The results of antimicrobial tests demonstrate that all doped hydroxyapatite samples showed antimicrobial activity. Antimicrobial efficacy of these materials is a function of types and quantities of dopant ions. The activity is most pronounced with the silver-doped hydroxyapatite samples. Antimicrobial effects of silver-doped samples with the most silver on microorganism’s cells are observed by the atomic force microscope. The studies show that a silver-doped hydroxyapatite sample causes considerable morphological changes of microorganism cells which might be the cause of cells’ death. A hemolysis test was performed on silver and zinc-doped samples to determine their hemocompatibility. The hemolysis ratios of the samples are lower than 3% and therefore, it can be concluded that they are nonhemolytic. Thus, prepared metal-doped hydroxyapatite nanopowders promising for bone tissue engineering and can be applied as materials for the treatment of microbiologically polluted water, as a polymer additive and for general use.