{Comments on the article Entitled adsorption of strontium on different sodium-enriched bentonites by Sonja R. Marinovic, Marija J. Ajdukovic, Natasa P. Jovic-Jovicic, Tihana M. Mudrinic, Bojana N. Nedic-Vasiljevic, Predrag T. Bankovic and Aleksandra D. Milutinovic-Nikolic, published in the Journal of the Serbian chemical society, volume 82, issue 4, 2017, pp. 449-463} J. Serb. Chem. Soc. 83 (3) 391–393 (2018) UDC 546.42’33+666.32:544.723: JSCS–5082 628.316.2(0.072) Letters to the Editor 391 LETTER TO THE EDITOR Comments on the article entitled Adsorption of strontium on different sodium-enriched bentonites by Sonja R. Marinović, Marija J. Ajduković, Nataša P. Jović-Jovičić, Tihana M. Mudrinić, Bojana N. Nedić-Vasiljević, Predrag T. Banković and Aleksandra D. Milutinović-Nikolić, published in the Journal of the Serbian Chemical Society, Volume 82, Issue 4, 2017, pp. 449–463 SLOBODAN K. MILONJIĆ*# Vinča Institute of Nuclear Sciences, University of Belgrade, Belgrade, Serbia (Received 16 December 2017, accepted 15 January 2018) Abstract. Recently, Marinović et al.1 have presented a study on the adsorption of strontium on different sodium-enriched bentonites. However, some experi- mental procedures and methods used, as well as presentation and interpretation of some data, are questionable, i.e., erroneous. Keywords: adsorption; bentonites; problematic interpretation. Recently, I have read the article of Marinović et al.1 on the adsorption of strontium on different sodium-enriched bentonites with great interest. But, it seems to me that a few scientific questions about the presentation and interpret- ation of some experimental data have to be raised. In EXPERIMENTAL, Adsorption experiments, the authors wrote that the amount of Sr2+ adsorbed, qt (mg g–1), after time t was calculated from the following mass balance relationship (Eq. (1)):1 0 ads t t c c q m − = (1) where c0 and ct are the concentration (mg dm–3) of Sr2+ in the initial solution and after adsorption time t, respectively. * E-mail: smiloni@vinca.rs # Serbian Chemical Society member. https://doi.org/10.2298/JSC171216017M ________________________________________________________________________________________________________________________Available on line at www.shd.org.rs/JSCS/ (CC) 2018 SCS. 392 MILONJIĆ The expression of Eq. (1), however, is incorrect. According to this equation, the obtained/resulting qt values have the unit mg g–1 dm–3. Equation (1) should be correctly expressed as Eq .(2): 0 ads ( )− = tt c c V q m (2) where V is the volume of solution (dm3) in authors’ case, 0.050 dm3. Therefore, the qt values calculated according to Eq. (1) are incorrect. The q values plotted on the y-axis of Figs. 3–7 and given in Tables II and III1 are also incorrect. There- fore, the values obtained from these data, as well as the discussions related to the adsorption of Sr2+ on different sodium-enriched bentonites are invalid, and con- sequently re-calculation of all parameters is strongly recommended. From Fig. 4,1 it is evident that the adsorbed amounts of Sr 2+ on Na-T, Na-W and Na-B bentonites in the initial pHi range ~4 to ~9 are constant, because the final pHf values are constant. Please note that the adsorbed amount depends on the equilibrium solution pH, i.e., pHf. On pages 457 and 459,1 the authors wrote: “This indicates that the ion exchange mechanism was the dominant adsorption mechanism” and “As pre- viously stated, the ion exchange mechanism could be regarded as the dominant adsorption mechanism”, respectively, giving however no experimental proof for such a statement. In the case of ion exchange, the amount of sorbed cation(s) from a solution (in this case Sr2+) should be equivalent to the amount of released cation(s) from the sorbent (in this case Na+, and other cations present in the investigated bentonites). In Fig. 5,1 the authors illustrate the effect of the contact time on the ads- orption of Sr2+ on three studied bentonites. It is evident that the adsorption equi- librium for Na-B and Na-T bentonites was reached within 20–25 min. However, in Fig. 6,1 they show linear plots for the pseudo-second-order kinetics. The use of data recorded after the reached equilibrium (in this case after 25 min), or very close to the equilibrium, for a significant interval of time is likely to lead to erroneous conclusions in the adsorption kinetics analysis. The measurements of the adsorption of Sr2+ on the studied bentonites should be started at an initial time shorter than 10 min. The dimension of the Freundlich adsorption constant given in Table II, as KF in dm3 mg–1, is incorrect. The correct dimension is (mg g–1)/(mg dm–3)n. The values for the energy of adsorption (E) according to the Dubinin–Raduskevich isotherm model are given in Table III. On the basis of the calculated E values the authors wrote: “The calculated E values were similar for all investigated ads- orbents and slightly higher than 8 kJ mol–1, indicating that the type of adsorption was chemisorption.” In the same table, the authors gave the Freundlich ads- orption constant (n) data, from 5.78 to 9.558, for all investigated bentonites. ________________________________________________________________________________________________________________________Available on line at www.shd.org.rs/JSCS/ (CC) 2018 SCS. COMMENTS ON AN ARTICLE PUBLISHED IN JSCS 393 Based on this (n > 1), it means that the adsorption process is physical. It is evident that the given data, and the conclusion drawn from them, are in dis- agreement. Finally, all the results and discussions related to the adsorption of Sr2+ on studied bentonites, as well as drawn conclusions are highly questionable. I hope that these comments, intended to prevent future mistakes, will be of assistance not only to the authors of commented paper1 but also to others working in this field. To conclude, I suggest the authors of the paper,1 as well as other researchers and reviewers working in the field of adsorption/sorption from solutions, to read the paper (A critical review) of Tran et al.2 devoted to mistakes and incon- sistencies regarding the adsorption of contaminants from aqueous solutions. И З В О Д КОМЕНТАРИ НА РАД ПОД НАСЛОВОМ АДСОРПЦИЈА СТРОНЦИЈУМА НА РАЗЛИЧИТИМ НАТРИЈУМОМ ИЗМЕЊЕНИМ БЕНТОНИТИМА, АУТОРИ: САЊА МАРИНОВИЋ, МАРИЈА АЈДУКОВИЋ, НАТАША ЈОВИЋ-ЈОВИЧИЋ, ПРЕДРАГ БРАНКОВИЋ, ТИХАНА МУДРИНИЋ, БОЈАНА НЕДИЋ-ВАСИЉЕВИЋ И АЛЕКСАНДРА МИЛУТИНОВИЋ-НИКОЛИЋ, ОБЈАВЉЕНОГ У JOURNAL OF THE SERBIAN CHEMICAL SOCIETY, VOLUME 82, ISSUE 4, 2017, СТР. 449–463 СЛОБОДАН К. МИЛОЊИЋ Институт за нуклеарне науке “Винча”, Универзитет у Београду, Београд Недавно је у Journal of the Serbian Chemical Society објављен рад (JSCS 82(2017)449– –463) под насловом “Адсорпција стронцијума на различитим натријумом измењеним бентонитима”, чији су аутори Сања Мариновић, Марија Ајдуковић, Наташа Јовић-Јо- вичић, Предраг Банковић, Тихана Мудринић, Бојана Недић Васиљевић и Александра Милутиновић-Николић. Циљ ових добронамерних коментара је био да се укаже на: погрешно израчунавање појединих величина, грешке у интерпретацији добијених/изра- чунатих резултата као и грешке изнете у закључцима рада. Аутор ових коментара верује да ће изнети коментари бити од користи не само ауторима предметног рада већ и оста- лим ауторима и рецензентима који раде у области адсорпције/сорпције из раствора. (Примљено 16. децембра 2017, прихваћено 15. јануара 2018) REFERENCES 1. S. R. Marinović, M. J. Ajduković, N. P. Jović-Jovičić, T. M. Mudrinić, B. M. Nedić- -Vasiljević, P. T. Branković, A. D. Milutinović-Nikolić, J. Serb. Chem Soc. 82 (2017) 449 2. H. N. Tran, S.-J. You, A. Hosseini-Bandegharaei, H.-P. Chao, Water Res. 120 (2017) 88. ________________________________________________________________________________________________________________________Available on line at www.shd.org.rs/JSCS/ (CC) 2018 SCS. << /ASCII85EncodePages false /AllowTransparency false /AutoPositionEPSFiles true /AutoRotatePages /None /Binding /Left /CalGrayProfile (Dot Gain 20%) /CalRGBProfile (sRGB IEC61966-2.1) /CalCMYKProfile (U.S. Web Coated \050SWOP\051 v2) /sRGBProfile (sRGB IEC61966-2.1) /CannotEmbedFontPolicy /Error /CompatibilityLevel 1.4 /CompressObjects /Tags /CompressPages true /ConvertImagesToIndexed true /PassThroughJPEGImages true /CreateJobTicket false /DefaultRenderingIntent /Default /DetectBlends true /DetectCurves 0.0000 /ColorConversionStrategy /CMYK /DoThumbnails false /EmbedAllFonts true /EmbedOpenType false /ParseICCProfilesInComments true /EmbedJobOptions true /DSCReportingLevel 0 /EmitDSCWarnings false /EndPage -1 /ImageMemory 1048576 /LockDistillerParams false /MaxSubsetPct 100 /Optimize true /OPM 1 /ParseDSCComments true /ParseDSCCommentsForDocInfo true /PreserveCopyPage true /PreserveDICMYKValues true /PreserveEPSInfo true /PreserveFlatness true /PreserveHalftoneInfo false /PreserveOPIComments true /PreserveOverprintSettings true /StartPage 1 /SubsetFonts true /TransferFunctionInfo /Apply /UCRandBGInfo /Preserve /UsePrologue false /ColorSettingsFile () /AlwaysEmbed [ true ] /NeverEmbed [ true ] /AntiAliasColorImages false /CropColorImages true /ColorImageMinResolution 300 /ColorImageMinResolutionPolicy /OK /DownsampleColorImages true /ColorImageDownsampleType /Bicubic /ColorImageResolution 300 /ColorImageDepth -1 /ColorImageMinDownsampleDepth 1 /ColorImageDownsampleThreshold 1.50000 /EncodeColorImages true /ColorImageFilter /DCTEncode /AutoFilterColorImages true /ColorImageAutoFilterStrategy /JPEG /ColorACSImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /ColorImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /JPEG2000ColorACSImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /JPEG2000ColorImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /AntiAliasGrayImages false /CropGrayImages true /GrayImageMinResolution 300 /GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 300 /GrayImageDepth -1 /GrayImageMinDownsampleDepth 2 /GrayImageDownsampleThreshold 1.50000 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages true /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /GrayImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /JPEG2000GrayACSImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /JPEG2000GrayImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /AntiAliasMonoImages false /CropMonoImages true /MonoImageMinResolution 1200 /MonoImageMinResolutionPolicy /OK /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 1200 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.50000 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict << /K -1 >> /AllowPSXObjects false /CheckCompliance [ /None ] /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile () /PDFXOutputConditionIdentifier () /PDFXOutputCondition () /PDFXRegistryName () /PDFXTrapped /False /CreateJDFFile false /Description << /ARA /BGR /CHS /CHT /CZE /DAN /DEU /ESP /ETI /FRA /GRE /HEB /HRV (Za stvaranje Adobe PDF dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. Stvoreni PDF dokumenti mogu se otvoriti Acrobat i Adobe Reader 5.0 i kasnijim verzijama.) /HUN /ITA /JPN /KOR /LTH /LVI /NLD (Gebruik deze instellingen om Adobe PDF-documenten te maken die zijn geoptimaliseerd voor prepress-afdrukken van hoge kwaliteit. De gemaakte PDF-documenten kunnen worden geopend met Acrobat en Adobe Reader 5.0 en hoger.) /NOR /POL /PTB /RUM /RUS /SKY /SLV /SUO /SVE /TUR /UKR /ENU (Use these settings to create Adobe PDF documents best suited for high-quality prepress printing. Created PDF documents can be opened with Acrobat and Adobe Reader 5.0 and later.) >> /Namespace [ (Adobe) (Common) (1.0) ] /OtherNamespaces [ << /AsReaderSpreads false /CropImagesToFrames true /ErrorControl /WarnAndContinue /FlattenerIgnoreSpreadOverrides false /IncludeGuidesGrids false /IncludeNonPrinting false /IncludeSlug false /Namespace [ (Adobe) (InDesign) (4.0) ] /OmitPlacedBitmaps false /OmitPlacedEPS false /OmitPlacedPDF false /SimulateOverprint /Legacy >> << /AddBleedMarks false /AddColorBars false /AddCropMarks false /AddPageInfo false /AddRegMarks false /ConvertColors /ConvertToCMYK /DestinationProfileName () /DestinationProfileSelector /DocumentCMYK /Downsample16BitImages true /FlattenerPreset << /PresetSelector /MediumResolution >> /FormElements false /GenerateStructure false /IncludeBookmarks false /IncludeHyperlinks false /IncludeInteractive false /IncludeLayers false /IncludeProfiles false /MultimediaHandling /UseObjectSettings /Namespace [ (Adobe) (CreativeSuite) (2.0) ] /PDFXOutputIntentProfileSelector /DocumentCMYK /PreserveEditing true /UntaggedCMYKHandling /LeaveUntagged /UntaggedRGBHandling /UseDocumentProfile /UseDocumentBleed false >> ] >> setdistillerparams << /HWResolution [2400 2400] /PageSize [612.000 792.000] >> setpagedevice