J.C.S. Faraday I, 1980,76,2542-2551New Zealand Allophanes: A Structural StudyBY RITA VANDICKELEN, GILBERT DE ROY AND ETIENNE F. VANSANT*Department of Chemistry, Universitaire Instelling Antwerpen,Universiteitsplein, 1, B-2610 Wilrijk, BelgiumReceived 18th October, 1979Three natural allophane samples from New Zealand, the Egmont, Waxy-Pan and iron-richRuapehu, were investigated by infrared spectroscopy, X-ray diffraction and Mossbauer spectroscopy.The specific surface area and the pore sizes of these samples increase as the pretreatment temperatureis raised, up to a temperature where a decrease of the surface area and a narrowing of the pores isobserved. This can be explained by two competing phenomena : the release of water from the poresand the collapse of other pores.In comparison with the Egmont and Waxy-Pan allophanes, theiron-rich Ruapehu sample contains considerably less pores of sufficient size to accommodate water.Its exceptional chemical composition and the infrared spectra, X-ray diffraction patterns andMossbauer spectra suggest the presence of a separate, amorphous iron (111) oxide phase in the poresof this allophane.Amorphous minerals are very important components of the fertile soils of NewZealand. They mainly consist of allophanes and oxides of silicon, aluminium andiron of volcanic origin. The presence of allophanes is essential to the soil fertilitybecause they are responsible for the high natural water content of the soil.The allophane minerals are composed of an aluminosilicate and a hydroxy-polyalumina phase.l The aluminosilicate phase is formed of randomly linked Si-and Al-tetrahedra and possesses a permanent negative charge. The octahedralpolyalumina phase contains a pH-dependent positive charge caused by broken bonds.A lot of plant nutrients including humic substances can be adsorbed on these chargedsites.The allophanes are present in the clay fraction (< 2 pm) of the soil in aflocculated form that is stable over a wide range of pH values (3-7) and allows a gooddrainage.andpoor infrared Nevertheless, it has been observed that these allophanesexhibit an equivalent of internal surface.6 In order to explain this, a structuralmodel was proposed by Kitagawa on the basis of electron microscopic data. Thismodel is still tentative and some differences in properties can even be explained byvariations of the chemical comp~sition.~This study is focused on the active surface of three New Zealand allophanes:the Waxy-Pan, the Egmont and the iron-rich Ruapehu.Their X-ray powderdiffraction patterns and infrared spectra will be examined and the coordination andvalency state of the iron atoms present will be investigated using Mossbauer spectro-scopic techniques.The randomness of the allophanes causes insignificant diffraction patternsEXPERIMENTALMATERIALSThe allophane samples originated from New Zealand (Waxy-Pan, Egmont and Ruapehu)and were kindly supplied by Dr. K. G. B. Theng, Soil Bureau, Lower Hutt, New Zealand.254R . VANDICKELEN, G . DE ROY A N D E.F . VANSANT 2543They were chemically analysed and their compositions are summarized in table 1. Thenitrogen gas used in the sorption experiments was from Matheson Gas Products (certifiedpurity > 99.99 %) and was not further purified before use.TABLE THE CHEMICAL COMPOSITIONS OF THE NEW ZEALAND ALLOPHANE SAMPLES (INWEIGHT %)iron-richWaxy-Pan Egmont RuapehuSi02A1203Fe203H20aNa20K2OCaOTi02p205total29.5239.494.3923.651.960.040.150.500.30100.0032.0932.887.0116.109.630.510.410.800.6011.341.1785.780.330.010.010.480.070.82100.03 100.01Q Obtained by calcination at 383 K.ADSORPTION METHODSThe adsorption experiments were carried out in a conventional constant-volume B.E.T.apparatus.The allophane samples (0.5-1 .O g) were fist outgassed Pa) at varioustemperatures (table 2). Because of their lack of thermal stability the heating was carriedout stepwise with increments of 50 K.Subsequently the adsorption-desorption isotherms of N2 were set up while cooling thesample to liquid nitrogen temperature (77 K) in order to determine the active surface areaof the allophanes.8 Furthermore, the pore size distribution was obtained from the hysteresisregion of this isotherm. For medium-sized and large pores the corrected modelless methodof Brunauer et a1.' was used. It is based on the capillary condensation phenomena. Themicropore size distribution was obtained by the m.p. (micropore) method,1° an extensionof the so-called t-method of De Boer.'l Both methods use the desorption branch of theis0 t herm.X-RAY DIFFRACTIONThe diffraction patterns of th? allophane samples were recorded on a Scintag PAD-11automated diffractometer at a rate of 0.4"(28) min-l, using Cu-Ka radiation from a fine-focus X-ray tube.A Ni-filter was mounted between the flat powder specimen and theproportional counter in order to avoid Cu-Kp radiation as well as Fe-fluorescence.INFRARED SPECTROSCOPYInfrared spectra were obtained using a Beckmann 4240 spectrometer equiped with avariable reference beam attenuator. The allophane samples were pressed in a 2 % KBrdisc and recorded in air. The scanning rate was 600 cm-l min-l using the conventional slitprogram.MOSSBAUER SPECTROSCOPYThe Mossbauer spectra were recorded using a Canberra Quanta automated spectrometer.The source consisted of approximately 5 mCi of 57C0 in Pd as prepared by New Englan2544 NEW ZEALAND ALLOPHANES: A STRUCTURAL STUDYNuclear Corp., Boston, Mass.All data were obtained with the source and the absorberat room temperature. The spectrometer was calibrated using natural iron foil and sodiumnitroprusside. Isomeric shift data are expressed relative to the nitroprusside standard.The allophane samples were pressed in 150 mg 2 cm2 discs. Spectra were collected untilthe statistical count rate error became less than 3 :! of the adsorption peak height. Thespectra were subjected to a least-squares fit to a Lorentzian line shape and the Mossbauerhyperfine parameters were calculated from the fit assuming doublets.c I1 .o0.40.2 :'= 01.010 0.5 0 0.5 0 0.5 1r/nmFIG. 1.-Summary of the pore size distributions in New Zealand allophane samples pretreated atdifferent temperaturesR . VANDICKELEN, G . DE ROY AND E . F . VANSANT 2545TABLE 2.-sUMMARY OF PRETREATMENT TEMPERATURES AND SURFACE AREAS OF ALLOPHANESAMPLESpretreatmentsample temperature /K SBET/~' g-'Waxy-PanEgmontiron-rich Ruapehu29238848257829338047158229838347356347858160751727329 1350309473539449381I I I I I I I I I I20 10 8 6 4 2d/nmFIG. 2.-Powder diffraction patterns of (a) Waxy-Pan, (b) Egmont and (c) iron-rich Ruapehuallophanes. Traces of quartz (Q), feldspars (F) and mica-type minerals (M) are detected."1-8 2546 NEW ZEALAND ALLOPHANES: A STRUCTURAL STUDYRESULTSThe active surface area (SBET) of the Waxy-Pan, Egmont and iron-rich Ruapehuallophane samples, pretreated at different temperatures, was determined from thenitrogen adsorption-desorption isotherms (77 K) using the B-point method.* Thesedata are collected in table 2. From the desorption isotherms and the correspondingI I I I I4000 3000 2000 1500 1000 500klcm-'FIG. 3.4nfrared spectra of allophane samples: (a) Waxy-Pan, (6) Egmont and (c) iron-richRuapehu. The regions of physically as well as chemically adsorbed water vibrations (W) as well asthe " lattice " vibrations (L) are indicated.S,,, the pore size distributions were computed and correlated with the pretreatmenttemperatures (fig.1). These data reveal that both the surface area and the porewidths increase with increasing pretreatment temperatures, up to a certain temperaturewhere a collapse of pores is responsible for a lower S,,,R . VANDICKELEN, G . DE ROY AND E. F . VANSANT 2547In order to characterize the mineral composition and structure of the allophanesthe powder diffraction patterns and infrared spectra of the three New Zealandallophane samples were recorded and are shown in fig. 2 and 3, respectively.The iron phase, present in the allophane structure, was investigated from theMossbauer spectra (fig. 4). The fitted Lorentzian profiles are shown and the statisticalcount rate error is indicated by error flags. The hyperfine parameters are listed intable 3.These results indicate two iron centres in the Waxy-Pan and Egmontallophanes. However, in the iron-rich Ruapehu sample three different iron locationsin the mineral structure are observed.TABLE 3.-MOSSBAUER HYPERFINE PARAMETERS FOR THE NEW ZEALAND ALLOPHANE SAMPLEStype of I.S. Q.S. width occurrencesample site /mm s-1 /m s-l /mm s-l 1%Waxy-Pan I 0.672(2) 0.64(5) 0.342(6) 91 (2)I1 0.65(2) 1.09(4) 0.33(5)Egmont I 0.708(3) 0.616(9) 0.36(1) 9a3)I1 0.68( 1) 1.07(3) 0.29( 5) 8(3)iron-rich I 0.762(7) 0.71(1) 0.44(2) 59(3)Ruapehu I1 0.693(6) 1.36(2) 0.35(3) 23(3)111 0.55(1) 0.69(2) 0.34(3) 1N3)2.0 1.0 ox) -1.0(a)See caption overlea2548 NEW ZEALAND ALLOPHANES: A STRUCTURAL STUDYIII I i-, l ~ l l l l t f ~ ~ l ~ l l l l l l l l , l , l l ~ l t l l l , , ~ l l t l l l l l f l f ~ ~ , , ~ ~ l l I3.0 2 .o 1.0 0 -1.0 -2.0(c)FIG.4.-Mossbauer spectra of allophane samples : (a) Waxy-Pan, (b) Egmont and (c) iron-richRuapehu. The sites are named as follows : site I, Fe3+ in a symmetric octahedral surrounding ;site 11, Fe3+ in a cis-dihydroxyoctahedron ; site 111, a separate Fe203 phase. For more information,see textR . VANDICKELEN, G . DE ROY AND E . F. VANSANT 2549DISCUSSIONThe chemical analysis (table I) shows a marked difference in the composition of,on the one hand, the Egmont and Waxy-Pan allophanes and, on the other hand,the iron-rich Ruapehu sample concerning the SO2, Al,03, Fe,03 and H,O content.Although the allophanes mainly consist of Si02 and A1203 they can exhibit alarger surface area as compared with the composition-weight median of amorphoussilica and alumina (385 m2 g-l and 140 m2 g-l after 383 K pretreatment, respectively).A more symmetric arrangement of the Si- and Al-units must be responsible for thisbehaviour, so that more and wider pores can be formed.In order to investigate the allophane " framework ", the structural model asproposed by Kitagawa was tested with the experimental adsorption, pore sizedistribution, infrared, X-ray and Mossbauer data.According to the proposedmodel, the allophanes are composed of 5.5 nm spherical particles, surrounded by amonolayer of water. The allophane microaggregates consist of these particles in aclose-packed arrangement. In an aqueous environment all space between theseparticles is filled with water, but in the air-dry allophane two types of cavities appearbecause there is not enough water left to fill all the free space.Upon heating to383 K, loss of the adsorbed water results in the formation of microaggregates due tostrong physical interactions between the unit particles.The observed variations of the specific surface area (SBET) of the allophane samplesinvestigated as a function of the pretreatment temperature and the correspondingpore size distributions (table 2, fig. 1) fitted well in the proposed allophane model.The increase of SBET on pretreatments from 298 to 383 K indicates the removal ofphysically adsorbed water from the pores between the unit particles. The calculatedpore widths indeed become larger.Pretreatments from 383 to 473 K cause a furtherincrease in the surface area of the Waxy-Pan and the Egmont samples due to therelease of physically as well as chemically adsorbed water. The increasing pore sizesreflect this trend. Eventually, as the samples are pretreated at higher temperatures,SBET declines and the pore sizes are reduced by the removal of structural water and acollapse of the internal pores.The iron-rich Ruapehu sample, however, displays different behaviour in thetemperature range 383-473 K. Its surface area declines while the pore sizes are stillincreasing. This can be explained by the collapse of an important amount of smallpores. This behaviour is probably related to the exceptional chemical compositionof this sample and its extremely low water content.The absence of pores of sufficientsize can account for this phenomenon, but the elevated iron content suggests thepresence of a separate iron oxide phase in this allophane.The X-ray diffraction patterns of all three samples show only very weak intensities(fig. 2). No evidence is found for any crystalline phase, except for the Egmontallophane sample, where small amounts of quartz (Q), feldspar (F) and mica-typeminerals fM) were detected.13 The patterns show a striking similarity to thediffractograms of Japanese allophanes,14 with broad bands at d = 0.8-1.0 nm andd = 0.30-0.35 nm. The iron oxide in the iron-rich Kuapehu sample should be eitherin a highly dispersed crystalline form or in a completely amorphous form, present as aseparate phase or randomly distributed in the allophane phase.The infrared spectra of these samples are also comparable to those of Japaneseallophanes 4p l4 showing as main features intense bands at 3400-3500 and 1650-1630 cm-1 due to water vibrations and two bands near 1000 and 600 cm-1 charac-teristic of a l l o p h a n e ~ .~ ~ - ~ The water vibrations are due to both physically adsorbe2550 NEW ZEALAND ALLOPHANES: A STRUCTURAL STUDYwater and to structural hydroxyl groups. The large width of the 3400-3500cm-lband suggests complex hydrogen bonding.Vibrations in the SO4- and Al0,-tetrahedra occur in the region of 1250-850 cm-l l 7 and all allophane samples possess a broad and intense band in thisregion.A second band can be distinguished in the 750-400cm-l region, due tovibrations between adjacent Si- and/or Al-tetrahedra.l The 473,550 and 600 cm-labsorptions suggest the presence of iron in an octahedral oxygen surrounding.A weak but significant peak at 780 cm-I is observed in the Egmont sample. Itcan be attributed to quartz. This is in perfect agreement with the observed X-raydiffraction pattern of this sample.The infrared technique gave no answer on the state of the iron oxide in the iron-rich Ruapehu, Waxy-Pan and Egmont allophanes. hlossbauer spectra were thereforerecorded and analysed. Three different iron sites are detected in the allophanes.The relative abundances of iron atoms in these sites are plotted as a function of theiron content of the samples (fig.5). The iron atoms in site I are surrounded by oxygenatoms in a regular octahedral way. They are probably isomorphically substitutedfor aluminium in the octahedral polyalumina phase of the a1lophane.l The ironatoms in site I1 are octahedrally surrounded by oxygen ligands, two of them being0 0.2 0.4 0.6 0.8Fe203/(Si02 + AI2O3 + Fe203)FIG. 5.-The relative abundances of allophane iron atoms in the different sites as a function of thehydroxyl groups (in cis position) caused by bond cleavage. The resulting decreaseof symmetry causes the drastic rise of the quadrupole splitting. As more iron ispresent in the allophane, the number of broken bonds will increase, so more sites Iwill be converted into sites 11. In allophanes with a high iron content a third site(111) appears. Its parameters can be explained in two different ways: either as afree Fe,03 phase or as Fe3+ in a tetrahedral environment.lg~ 2o If the latter explana-tion is assumed, the iron would substitute for alumina in the aluminosilicate phase,causing the iron-rich Ruapehu to have nearly the same pore size distribution as theother samples.This is in conflict with our observations (fig. 1). A separate,total iron content. The sites are named as in fig. 4R. VANDICKELEN, G. DE ROY AND E. F. VANSANT 255 1amorphous Fe,O, phase thus has to be implemented. The absence of pores of0.6-0.7 nm, as compared with the Egmont and Waxy-Pan samples, suggests that thisphase acts as a binding agent, filling the pores between the allophane unit particles,thereby excluding the water molecules.In general, we can conclude that the New Zealand Waxy-Pan and Egmontallophanes behave in a very similar way to their Japanese analogues. The iron-richRuapehu sample, however, has a different chemical composition, which is reflectedin its structure as an amorphous, pore-filling iron oxide phase.E.F. V. thanks the National Science Foundation (Belgium) and the BelgianGovernment for their financial support and the Soil Bureau, D.S.I.R., Lower Hutt(New Zealand) for the research facilities. G. De R. thanks I.W.O.N.L. Belgiumfor a research grant. The powder diffraction patterns were recorded on the PAD-I1diffractometer of the " Laboratorium voor Oppervlaktescheikunde ", CatholicUniversity of Louvain (K.U.L.), Leuven (Belgium).The authors thank Prof. J. B.Uytterhoeven and W. J. Mortier for their kind collaboration.L. P. Van Reeuwijk and J. M. De Villiers, Agrochemophysica, 1970, 2, 77.K. S. Birrell and M. Fieldes, J. Soil Sci., 1952, 3, 156.M. Fieldes, New Zealand J. Sci., 1966, 9, 591.M. Fieldes and R. J. Furkert, New Zealand J. Sci., 1966, 9, 608.H. H. Adler, Prelim. Rep. No. 6, Amer. 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Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds (Wiley Interscience,London, 1970).l9 G. M. Bancroft, Mossbauer Spectroscopy: an Introduction for Inorganic Chemists and Geo-chemists (McGraw-Hill, London, 1973).2o Chemical Applications of Mossbauer Spectroscopy, ed. V. I. Goldanskii and R. H. Herber(Academic Press, London, 1968).America, Special Paper 126, 1970).M. Fieldes, New Zealand J. Sci. Technol., 1955, B37, 336.1975), pp. 292-295.(PAPER 9/1657