The adsorption and configuration of octyl hydroxamic acid on pyrochlore and calcite

Colloids and Surfaces A:Physicochem.Eng.Aspects 411 (2012) 80–86Contents lists available at SciVerse ScienceDirectColloids and Surfaces A:Physicochemical andEngineeringAspectsj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /c o l s u r faThe adsorption and configuration of octyl hydroxamic acid on pyrochlore and calciteXiao Ni,Qi Liu ∗Department of Chemical and Materials Engineering,University of Alberta,Edmonton,AB T6G 2V4,Canadah i g h l i g h t sHydroxamic acid chemisorbed on pyrochlore,but physically adsorbed on calcite.At low concentration,hydroxamic acid formed a vertically oriented monolayer on pyrochlore.At low concentration,hydroxamic acid formed a horizontally oriented monolayer on calcite.g r a p h i c a la b s t r a c tAt low concentrations,octyl hydroxamic acid formed a vertically oriented monolayer on pyrochlore surface,but a horizontally oriented monolayer on calcitesurface.a r t i c l ei n f oArticle history:Received 16March 2012Received in revised form 25June 2012Accepted 2July 2012Available online 16 July 2012Keywords:Hydroxamic acid Adsorption density Pyrochlore Calcitea b s t r a c tXPS,adsorption density and ToF-SIMS measurements were performed on pyrochlore and calcite treated with octyl hydroxamic acid (OHA).XPS results indicated a slight shift of the binding energies of the Ca 2p electrons (0.19eV)on calcite after OHA treatment but the N 1s binding energy spectrum of OHA did not change,showing a physical adsorption mechanism.On OHA-treated pyrochlore,the binding energies of both Nb 3d and Ca 2p electrons shifted (0.24and 0.35eV,respectively),and the N 1s electron binding energy peak of OHA split from 400.3eV to 400.7and 398.5eV,representing the neutral R NH OH and the ionized R NH O −groups.The ratio of the integrated areas of the 400.7eV peak to the 398.5eV peak was 3.8to 1.Therefore OHA possibly adsorbed on pyrochlore by both physisorption and chemisorption,with the number of OHA molecules involved in the adsorption at 3.8to 1.The adsorption isotherm of OHA on pyrochlore showed two plateaus,with the first plateau corresponding to a vertically oriented monolayer of OHA.The second plateau was 4times higher than the first plateau.Therefore,the adsorption isotherm was consistent with XPS results,in that the vertically oriented monolayer (first plateau)formed the chemisorbed layer,and the second plateau contained mostly physisorbed OHA.The adsorption isotherm of OHA on calcite showed a lower plateau which corresponded to horizontally oriented OHA.ToF-SIMS positive ion images on OHA treated mixtures of pyrochlore and calcite confirmed that OHA preferentially adsorbed on pyrochlore.© 2012 Elsevier B.V. All rights reserved.∗Corresponding author.Tel.:+17804928628;fax:+17804922881.E-mail address:qi.liu@ualberta.ca (Q.Liu).1.IntroductionAlkyl hydroxamic acid is an effective collector for many oxide minerals in froth flotation including copper [1],iron [2],tin [3],tungsten [4],and rare earth metal minerals [5,6].It is also a selec-tive collector for niobium oxide minerals from carbonatite ores.It has been shown previously that a commercial reagent Aero 6493,0927-7757/$–see front matter © 2012 Elsevier B.V. All rights reserved./10.1016/j.colsurfa.2012.07.005X.Ni,Q.Liu/Colloids and Surfaces A:Physicochem.Eng.Aspects411 (2012) 80–8681Fig.1.Structure of octyl hydroxamic acid and its metal chelates in bulk solution. whose major ingredient is hydroxamic acid,could recover over85% of niobium minerals(mostly pyrochlore)from a mill feed sam-ple taken from an operating mine in eastern Canada at less than 30%mass pull(with carbonate mineral recovery at under30%)[7]. Its natural selectivity between niobium-bearing minerals and car-bonate minerals inflotation is worth studying.Octyl hydroxamic acid(OHA)was subsequently synthesized in our labs and tested in batchflotation of the same ore sample,and similar results as the commercial collector Aero6493were obtained.The hydroxamate group can form chelates with many transi-tion metal cations in bulk solution[8–11].More specifically,the hydroxyl groups and the carbonyl groups in the hydroxamate form a stablefive-membered ring chelate with metal cations[8–11].The carbonyl group participates in the chelation by donating its lone pair electrons[8–11](Fig.1).Research has also shown that they can also form chelates with metal atoms on mineral surface[3,6,12–14]. Thus hydroxamic acid are capable of bonding with mineral surface by“inner sphere adsorption”like carboxylic acid[15,16];but the formed chelates are stronger than the coordination bonded com-plex of corresponding carboxylic acids[8,17,18].Amphiphilic molecules which form chemical bonds(surface anchoring)with atoms or moieties on solid substrate can result in a self-assembled closed packing monolayer significantly increasing the substrate’s hydrophobicity.Chelating reaction formed between polydentate ligand and metal atoms on substrate surface can lead to a closed packed monolayer[19–21].Saturated alkyl chain in surfac-tant is moreflexible than chains with unsaturated bonds,and they tend to form linear conformation due to the hydrophobic inter-action between hydrophobic chains nearby if in a close packed self-assembled monolayer[22].With increasing surfactant concen-tration in the solution,the adsorbed molecules’orientations would gradually change to parallel to each other until the monolayer is formed[23].It is suggested that hydroxamic acid can also form such a monolayer on the metal oxide surface through aforementioned chelating reaction[24–26].However,the reported work concerned the adsorption of alkyl hydroxamic acid on calcium-bearing min-erals such asfluorite or phosphate[26,27].It is interesting to note that hydroxamic acid could adsorb on different mineral surfaces but the resultant imparted hydrophobicity andfloatability could be very different,such as on pyrochlore and calcite[7].This work reports results of a study on the adsorption mechanism of octyl hydroxamic acid(OHA)on the surfaces of pyrochlore and calcite, more importantly,the configuration of the adsorbed molecules on the surfaces and the resultant hydrophobicity of the mineral surface after adsorption.2.Materials and methods2.1.Materialsn-Octyl hydroxamic acid(OHA)was used in this work,and it was synthesized in our lab from the corresponding ester follow-ing the procedures of Devlin et al.and Pirrung and Chau[28,29]. The synthesized octyl hydroxamic acid was precipitated in acidic deionized water(pH=2),washed with acetone/water mixture and dried in a vacuum desiccator.The dried hydroxamic acid was a white and feather like powder,which was kept in a refrigerator. Sodium hexametaphosphate,sodium hydroxide and hydrochloric acid were all purchased from Fisher Scientific.Distilled water was used in all tests.Two high purity mineral samples,pyrochlore and calcite,were employed in this study to represent the niobium containing value mineral and the carbonatite gangue.High purity mineral samples of pyrochlore,in a powder form,was obtained from COREM,Que-bec,Canada,which assayed59.8%Nb2O5,10.5%CaO and2.73% SiO2.Since the theoretical Nb2O5content in pure pyrochlore is 66.55%,this sample contained approximately91%pyrochlore.Cal-cite(58.34%CaO)sample were purchased from Ward’s Scientific Establishment,Ontario,Canada.All the mineral samples were crushed,ground in a mechan-ical agate mortar/pestle grinder and screened to collect the −25␮m fraction for adsorption isotherm measurements.The spe-cific surface areas of the−25␮m pyrochlore and calcite samples were determined on Quantachrome Autosorb1MP(Quantachrome Instruments,USA)using nitrogen as adsorbate based on the BET theory,and the results are0.88and1.27m2/g,respectively.For XPS measurements the−25␮m fraction was further ground to approx-imately2␮m.2.2.X-ray photoelectron spectroscopyThe X-ray photoelectron spectroscopy(XPS)characterization of samples was carried out on a Kratos Axis165spectrometer with monochromatized Al K␣1,2radiation as an excitation source (h =1486.6eV).The analysis area was about400␮m×700␮m. Survey scans were conducted using an electron analyzer with a pass energy of160eV and a step size of0.35eV,and then high resolution scans were carried out within specific binding energy ranges at a constant pass energy of20eV and a step size of0.1eV.All measure-ments were carried out at pressures below10−8Pa.All the spectra obtained were calibrated using C1s at284.8eV.Sample powders were loaded into the sample holder by creating a surface as smooth as possible by unaided eyes.The integral areas of selective peaks on the full survey scan of minerals were divided by relative sen-sitivity factors(RSFs)and normalized,from which relative atomic concentrations was obtained.The OHA treated mineral samples were prepared as follows:first,50mL0.5wt%(31mmol/L)OHA was prepared in a100mL beaker at pH8.5,which was the natural pH in batchflotation.Then 1.5g of the mineral sample was added to the beaker and the mixture was stirred with a magnetic stir bar for1h under room temperature. The conditioned pulp was subsequently centrifuged and decanted. The mineral particle sediments were washedfive times byfive aliquots of50mL deionized water at pH8.5(adjusted by sodium hydroxide)and dried in a vacuum desiccator until analysis.2.3.Adsorption isotherm determinationAdsorption densities of the OHA on the pyrochlore and cal-cite minerals were measured at pH8.5and room temperature (20◦C).One gram of the−25␮m mineral(pyrochlore or calcite)was added to a conicalflask,then150mL solution with different OHA concentrations was added.Control blank was run using150mL 5mmol/L OHA but no mineral was added.Natarajan et al.[13]have shown that usually30min is sufficient for adsorption/desorption of hydroxamic acid to attain equilibrium on mineral surfaces.We used a4h equilibration time to ensure that the adsorption has reached equilibrium.The control blank test indicated that the OHA solution concentration did not detectably change after4h shaking at pH8.5,82X.Ni,Q.Liu/Colloids and Surfaces A:Physicochem.Eng.Aspects411 (2012) 80–86verifying that the concentration difference after adsorption is not caused by decomposition.Theflask was stoppered and agitated in a Jeio Tech SI600shak-ing incubator for4h at180rpm and room temperature.Theflask was then removed and allowed to stand still for2h,and the upper portion of the solution wasfiltered using a0.2␮m pore size syringe filter for colorimetric measurement of the OHA concentration.The colorimetric measurements were carried out on a Jenway6405 UV/vis spectrophotometer.The concentration of OHA was deter-mined by the absorbance of its Fe3+chelates at520nm.Although hydroxamic acid absorbs UV light,direct UV light scanning was found inaccurate.On the other hand,the OHA–ferric chelate shows strong absorbance at visible light region(520nm)andfits well with Beer–Lambert law[30,31].The adsorbed octyl hydroxamic acid was determined by the difference of the initial and equilib-rium OHA concentration.It was verified that calcium ions were not able to form colored chelates or precipitates with hydroxamic acid even at a very high calcium concentration(20mmol/L),so that this method is still valid for semi-soluble carbonate miner-als such as calcite.A standard calibration curve was obtained with R2>0.99within the OHA concentration range of0.3–2.4mmol/L. Dilution was used before absorbance measurement if the solution’s concentration was higher than the aforementioned calibration con-centration range.2.4.ToF-SIMS imagingSecondary ion mass images were obtained from a ToF-SIMS IV instrument(ION-TOF GmbH).Bi+ions were used as an analytical source operated at25kV and a pulsed target current of approxi-mately1.0pA.Low-energy electrons were used to compensate for surface charging caused by primary ion beam on the insulating sur-faces.The mass spectra were calibrated by positive ion peaks of H+, CH3+and Na+.136.2␮m×136.2␮m size images obtained by map-ping specific ion intensity were collected in a burst alignment mode with pixels of128×128per image.Total primary ion beam dose for analyzed area was kept below1×1012ions per cm2,ensuring static conditions.Data analysis was carried out using IonImage(version 4.1)software.Samples tested in ToF-SIMS were prepared as follows:3g of mineral mixture(pyrochlore:calcite at1:1)was agitated in50mL of0.5mmol/L OHA solution for5min at pH8.5,then it wasfiltered and rinsed byfive aliquots of50mL deionized water at pH8.5and dried in a vacuum desiccator.A pellet for analysis was made using 1g of dried mineral mixtures in a mechanical press.3.Results and discussion3.1.X ray photoelectron spectroscopy analysisA stable self-assembled monolayer at low adsorbate concen-tration on a substrate may be facilitated by chemical adsorption of reagents on the surface,i.e.chemical bonds formed between reagent and surface.The chemical shift of the electron binding energy in XPS spectra shows a change in the chemical state of the atoms.Therefore,the obvious shift of the binding energy should be observed after chemical adsorption has occurred.Fig.2shows the binding energies of the Nb3d electrons of pyrochlore before and after treatment by OHA,and the Ca2p electrons of calcite before and after treatment by OHA.As can be seen from Fig.2a,there are two pairs of Nb3d doublets.This may be due to the existence of columbite(which also contains niobium)in the pyrochlore sam-ple.Since pyrochlore is the dominant mineral component of this high purity sample,the larger doublet likely represents pyrochlore. The binding energy of Nb3d5/2on OHA treatedpyrochlore Fig.2.XPS spectra of(a)Nb3d of untreated and OHA treated pyrochlore and(b)Ca 2p of untreated and OHA treated calcite.surface was decreased by0.24eV while that of Ca2p3/2was decreased by0.35eV after OHA adsorption.However,for the cal-cium atoms on the OHA treated calcite surface,the binding energy of the Ca2p3/2electrons dropped by only0.19eV(Fig.2).Therefore, both the calcium and niobium atoms on pyrochlore mineral surface could be active sites for hydroxamic acid to form chemical bonds, while the adsorption of OHA on calcite was of a weaker interaction. The decrease of the electronic binding energies of the respective cations on the mineral surface indicated that these cations accepted electron clouds after OHA adsorption,i.e.that they were reduced. This was likely caused by the donation of the lone pair electrons on the carbonyl oxygen as discussed earlier.The relative atomic ratios on the treated and untreated pyrochlore and calcite surfaces are listed in Table1.The increase of C/Nb and N/Nb ratios on both mineral surfaces after OHA treatment shows that the OHA was adsorbed on the mineral surfaces.X.Ni,Q.Liu/Colloids and Surfaces A:Physicochem.Eng.Aspects411 (2012) 80–8683 Table1Relative atomic ratio on the mineral surfaces.Sample C/Nb N/Nb Sample C/Ca N/CaPyrochlore 6.650.00Calcite 3.200.00OHA treated pyrochlore10.260.66OHA treated calcite8.220.68The presence of adsorbed hydroxamic acid on the mineral sur-face can be best shown by comparing the N1s signals from the XPSspectra.N1s signal is also the best indicator of chemical state ofatoms in OHA molecules since the other two elements,oxygen andcarbon,in OHA is easily overlapped by signals from substrate and/orcontaminants.High resolution spectra of N1s of the OHA as well ason the treated and untreated minerals are shown in Fig.3.As canbe seen,on both untreated pyrochlore and calcite,no obvious N1speaks were detected.However,after treatment by OHA,the N1speaks were observed on both minerals.After the N1s spectra werede-convoluted into components,different adsorption mechanismson the two minerals were revealed.On OHA treated pyrochlore surface,the N1s spectrum was madeof two components:one at398.5eV and the other at400.7eV.The one at400.7eV was attributed to nitrogen atoms in the neu-tral molecules,i.e.the protonated form of the hydroxamic acid,R CO NH OH,and the peak at398.5eV was due to the nitro-gen in the deprotonated hydroxamate groups,R CO NH O−[24].Since deprotonation is thefirst step of chemical adsorption ofthe hydroxamic acid on the mineral surface,the peak at398.5eVwas attributed to chemically adsorbed octyl hydroxamic acidmolecules.The integrated area ratio between these two peaks is3.8:1,indicating that the ratio of physically adsorbed OHA to chemi-cally adsorbed OHA was about3.8to1.XPS analysis[25]ofadsorbedFig.3.N1s spectra of OHA,untreated and treated pyrochlore andcalcite.Fig.4.Adsorption isotherm of OHA on calcite at pH8.5(dashed lines:calculatedhorizontal orientation and vertical orientation saturated adsorption density).hydroxamic acid monolayer on metal oxides indicated that on tita-nium oxide and aluminum oxide,the deprotonated molecules werealso observed,and the ratios of protonated versus deprotonatedOHA were2.5:1and2.3:1,respectively.On the other hand,on thetreated calcite surface,only a single peak was detected at400.3eV,which matches exactly the single N1s peak in the protonated OHA.The peak at398.5eV was absent.Therefore,the XPS spectra of OHA treated pyrochlore indicated chemical shift of binding energies of electrons in the atoms in bothadsorbate(nitrogen in OHA)and adsorbent(niobium and calciumin pyrochlore)after adsorption.In contrast,the chemical shift ofthe N1s in OHA could not be found on treated calcite surface,and the chemical shift of Ca1s on treated calcite was also subtle.This indicated that OHA was likely adsorbed on calcite by physicaladsorption whereas on pyrochlore by chemisorption.3.2.Adsorption isothermThe adsorption isotherm of OHA on calcite is shown in Fig.4.The stepwise increase in the adsorption density was observed for OHAadsorption on calcite at low OHA concentration.It was proposedthat OHA adsorption on calcite could have different orientations[32]:at low concentration,its molecules tend to lieflat on the sur-face whereas at high concentration,they tend to form close-packuntil reaching a dense packing corresponding to an edge-on ver-tical monolayer surface coverage.Based on this hypothesis,theadsorption densities corresponding to a horizontal or a verticalmonolayer orientation were calculated,using the lengthwise andcross-sectional area size of OHA of55and20.5˚A2,respectively.Thecalculated values of horizontal and vertical saturation adsorptiondensities are3and8␮mol/m2,respectively.[Note:as the numberof adsorbed monolayers=molecular size(m2)×adsorption den-sity(mol/m2)×Avogadro number(6.02×1023),it follows that theadsorption density(mol/m2)at one monolayer coverage is theinverse of molecular size(m2)times the Avogadro number,wherethe molecular size is either55or20.5˚A2,for lengthwise and cross-sectional areas of the OHA.]By examining the data in Fig.4it can beseen that the horizontal oriented monolayer of OHA was observedbelow an equilibrium OHA concentration of1mmol/L.However,itis less certain if the vertical oriented monolayer was established oncalcite surface.It is suggested that physically adsorbed surfactantsmay not organize into close-packed vertically oriented monolayerat low concentrations[33–35].At higher OHA concentrations,theadsorption density on calcite increased drastically,indicating ran-dom stacking of OHA molecules on top of the adsorbed layers.The adsorption isotherm of octyl hydroxamic acid on pyrochlore surface is shown in Fig.5.Unlike on calcite surface,there was nohorizontal oriented OHA monolayer on the pyrochlore.Even at84X.Ni,Q.Liu /Colloids and Surfaces A:Physicochem.Eng.Aspects 411 (2012) 80–86Fig.5.Adsorption isotherm of OHA on pyrochlore at pH 8.5(solid lines:averaged adsorption densities at these two plateaus;dashed line:calculated vertical orienta-tion saturation adsorption density for chemically adsorbed monolayer).very low equilibrium OHA concentrations,vertical oriented OHA monolayers formed on the pyrochlore surface.This may be due to the formation of chemical chelates on the pyrochlore,which have greater stability.These stable complexes could be formed even at a low concentration until the active sites on the surface are com-pletely occupied.As OHA concentration increases,a second plateau was observed on the adsorption isotherm.This should be the mul-tilayer formed by the physically adsorbed OHA molecules on top of the first chemically adsorbed monolayer.It is interesting to note that based on the adsorption density data in Fig.5,the ratio of chemically adsorbed OHA to physically adsorbed OHA was 1:4.ThisFig.6.Adsorption isotherm at low OHA dosage on pyrochlore and calcite at pH 8.5(dashed lines:calculated horizontal orientation and vertical orientation saturated adsorption density).number matches with XPS results,which showed that the ratio was 1:3.8as discussed earlier.Fig.6shows a comparison of the adsorption densities of OHA on pyrochlore and calcite at low equilibrium concentration (which was used in the batch flotation tests).A vertical line indicates the exact OHA concentration in the batch flotation tests),and the adsorption density of OHA on pyrochlore was higher than that on calcite at this concentration.The adsorption density difference at low OHA concentration was due to the higher affinity of OHA to pyrochlore which caused a vertical orientation of the OHA on pyrochlore.But they were in a horizontal orientation on the calcite surface,this horizontal ori-entation on calcite surface prevented more OHA molecules from being adsorbed.The vertical orientation on the pyrochloresurfaceFig.7.Proposed self-assembled monolayer of OHA on pyrochlore and horizontal oriented monolayer on calcite at low OHAconcentration.Fig.8.Positive ion intensity images of 136.2␮m ×136.2␮m size on OHA treated pyrochlore and calcite mixture (weight ratio 1:1):(a)Nb +;(b)Ca +;(c)C 8H 17NO 2+.X.Ni,Q.Liu/Colloids and Surfaces A:Physicochem.Eng.Aspects411 (2012) 80–8685not only led to higher adsorption density,but also to higher sur-face hydrophobicity as the hydrocarbon tail of the OHA is oriented outwards to the bulk aqueous solution(Fig.7).At high concentra-tions,the adsorption density of OHA on the calcite surface increased faster than that on pyrochlore surface.This explained why simply increasing OHA dosage would have a negative effect on niobium flotation selectivity previously observed in batchflotation.3.3.ToF-SIMS imaging analysisThe adsorption density measurements were carried out on high purity single minerals and while they showed the relative affinity of OHA to the minerals,they did not show the competitive adsorp-tion when both minerals were present in theflotation pulp where OHA was petitive adsorption of OHA onto the mineral mixtures was studied using time offlight secondary ion mass spec-troscopy(ToF-SIMS).Image acquisitions using spectra of positive ions Ca+,Nb+,C8H17NO2+were carried out(Fig.8).Positive ion Nb+(Fig.8a)was selected to identify pyrochlore distribution in the mineral mixtures since it was the only source of niobium in the mix-ture.Signals of Ca+ion(Fig.8b)can be detected on both pyrochlore and calcite surface since they both contain calcium.However,its intensity was higher on calcite as calcite(58.34%CaO)had higher content of calcium than that of pyrochlore(10.5%CaO).Positive ion C8H17NO2+(Fig.8c)was used to determine the distribution of OHA on the mineral mixtures.It can be seen that the pattern of Fig.8c is very similar to Fig.8a,indicating that higher OHA fragment ion intensity corresponded to higher Nb intensity,which shows that OHA was adsorbed mostly on the pyrochlore.4.ConclusionsXPS measurements confirmed that the adsorption of octyl hydroxamic acid(OHA)onto pyrochlore surface was through chemisorption.On the other hand,the OHA only adsorbed on calcite surface through weaker physical adsorption.The strong chemisorption of the OHA on pyrochlore surface led to a vertical head-on orientation of the molecules so that the pyrochlore was strongly hydrophobized even at low OHA concentrations,whereas the weak adsorption of OHA on calcite led to a horizontal orienta-tion of the adsorbed OHA molecules with low adsorption density and low imparted surface hydrophobicity.This was confirmed by both adsorption isotherm measurements on the high purity single minerals of pyrochlore and calcite and ToF-SIMS imaging data on the mixtures of the two minerals.The results explain the observed selectivity of hydroxamic acid in theflotation of niobium oxide minerals(mostly pyrochlore)from carbonatite Nb ores. 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