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  2435 Environmental Toxicology and Chemistry, Vol. 24, No. 9, pp. 2435–2444, 2005   2005 SETACPrinted in the USA0730-7268/05 $12.00    .00 ELECTRON PARAMAGNETIC RESONANCE ANALYSIS OF THE DISTRIBUTION OF AHYDROPHOBIC SPIN PROBE IN SUSPENSIONS OF HUMIC ACIDS, HECTORITE,AND ALUMINUM HYDROXIDE–HUMATE–HECTORITE COMPLEXES M ATTEO  S PAGNUOLO ,†‡ A STRID  R. J ACOBSON ,† and P HILIPPE  B AVEYE *† †Department of Crop and Soil Sciences, Cornell University, 1006 Bradfield Hall, Ithaca, New York 14853, USA‡Dipartimento di Biologia e Chimica Agro-forestale ed Ambientale, Universita` degli Studi di Bari, via Amendola 165/a, 70126 Bari, Italy(  Received   15  November   2004;  Accepted   21  March  2005) Abstract —Until recently, there were no techniques capable of direct observation of the microscale locations where nonpolarorganiccompounds accumulate when associated with natural geosorbents. The ability of electronparamagneticresonance(EPR)spectroscopyto monitor and elucidate directly the different molecular-scale environments of paramagnetic spin probes has been demonstratedlately in model soils, yet it remains untested in complex systems. In this general context, the present investigation was aimed atassessing the extent to which EPR could be used to monitor the sorption of 4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy benzoate(TEMPO benzoate), a hydrophobic spin probe, on a smectite (hectorite), two humic acids, and their complexes in the presence orabsence of aluminum hydroxide. Results demonstrate that EPR is able to monitor easily adsorption on these sorbents in batch-styleexperiments. Distribution coefficient ( K  d ) values of 455.4 and 483.1 ml/g were found for the adsorption of TEMPO benzoate onhectorite–humic acids complexes, compared to respective  K  d  values of 46 and 147 ml/g predicted solely on the basis of the massof humic acids present in the complexes. These observations confirm the significant role of hectorite for the sorption of hydrophobiccompounds, together with humic acids, contrary to common belief that emphasizes the almost exclusive sorptive role of organicmatter. In addition, for the first time, EPR is able to provide evidence that hydrophobic molecules in the presence of geosorbentscan segregate in multimolecular clusters that are in equilibrium with aqueous probe concentrations below the probe’s solubilitythreshold. Possible consequences of this clustering process in terms of the fate and transport of hydrophobic compounds in subsurfaceenvironments are discussed. Keywords —Electron paramagnetic resonance Cluster formation Spin probe Xenobiotic sorption Hectorite INTRODUCTION Sorption is a key factor determining the transport and re-activity of organic chemicals in soils and sediments, and italso significantly influences the bioavailability of organic xe-nobiotics in the environment [1]. Until very recently, exper-imental methods available to study sorption processes reliedexclusively on macroscopic observations [2]. Analyses of sorption isotherms and of the kinetics of sorption/desorptionof organic chemicals have unveiled a number of aspects of sorption that have significant practical import. Various studies,for example, have shown sorption processes to be fast initially,and then to slow down considerably, exhibiting characteristictwo-step kinetics [2–4]. Sorption isotherms often arenonlinear, and the degree of their nonlinearity increases with increasedsorption time. In most cases, desorption appears to be signif-icantly slower than sorption.In spite of the practical relevance of the information theyprovide, macroscopic methods cannot afford any direct insightinto the molecular-scale mechanisms of sorption, because theyaverage out the contribution of all of the specific interactionsbetween chemical contaminants and the various sorbents pre-sent in the geosorbent matrix. At best, via fitting of phenom-enological or mechanistic models, they can serve as a basisto suggest sorption scenarios. Unfortunately, in systems asstructurally complex as soils and sediments, modeling effortsare fraught with uncertainties, especially giventhewidevarietyof chemicals to be considered. For example, most models cur-rently available to describe the fate and distribution of hydro- * To whom correspondence maybeaddressed(pcb2@cornell.edu). phobic organic chemicals in soils and sediments are based onthe premise that these chemicals partition into the organicmatter [4–6] or adsorb hydrophobically to it [7,8]. Experi- mental evidence strongly suggests, however, that expandablephyllosilicates also have a high capacity to sorb hydrophobicmolecules [9–11]. Very recently, the important roleofdifferent smectites on phenanthrene sorption has been reported [12].However, any attempt to find correlations with chemical prop-erties of smectites failed. The postulated mechanism of inter-action was capillary condensation into micro- and nanoporesinside the mineral superstructure. In soils or sediments,organicmatter and clay particles usually are associated very tightly,so that their individual contributions to the overall reactivityof soils generally cannot be deduced from overall sorptionisotherms or by comparison with sorption isotherms obtainedwith pure constituents.Until recently, there were no techniques available for directidentification of microscale sites where hydrophobic organicxenobiotics associate with soil or sediment constituents. How-ever, electron paramagnetic resonance (EPR) spectroscopy hasnow emerged as a credible candidate [13–16]. With some con- straints, EPR ideally is suited to study the interactions of spe-cific organic molecules (either spin probes or spin-labeledcompounds) in model soil systems designed to be free of in-terfering paramagnetic groups. Nitroxide spin probes, whichare low molecular weight, stable organic radicals, are availablewith different chemical properties such as polarity and charge,and thus may cover the range of physico-chemical propertiesof common organic contaminants. The shape of the EPR spec-tra of spin probe molecules is very sensitive to the microscale  2436  Environ. Toxicol. Chem.  24, 2005 M. Spagnuolo et al.Fig. 1. Chemical structure of the spin probe 4-hydroxy-2,2,6,6-tetra-methyl-piperidinyloxy benzoate (TEMPO benzoate). The dot adjacentto the N-O group represents an unpaired electron. environment in which they are located. Spin probes and spin-labeled compounds have been used extensively in biomedicalresearch to study the structure and functions of enzymes, andof oriented lipid systems simulating biological membranes[17]. Spin probes have been used in environmental systemsinvolving, for example, clay minerals [18–20] and aluminum oxides [21]. Other studies have been conducted to ascertainthe location of hydrophobic sites in humic acids [13] and/ortheir role in enhancing the solubility of hydrophobic spinprobes such as 5-SALS (a stearic acid spin-label with a ni-troxide free radical in position 5 of the hydrocarbon chain)[15]. Recently, EPR spectroscopy has been used to monitorthe distribution of a charged and a polar spin probe (Tempa-mine  and Tempol, respectively) in dispersed and flocculatedhectorite suspensions [14] and hectorite–humic acid organ-omineral complexes [16], as well as the bioavailability of Tem-pol in hectorite systems (A. Dumestre et al., Cornell Univer-sity, unpublished data). These studies largely have confirmedthe benefits expected from the use of EPR spectroscopy underthe experimental conditions considered. However, similar re-search remains to be carried out with hydrophobic spin probesin more environmentally relevant model soil systems, con-taining not just clay minerals but also organic matter and metalhydroxides.In this general context, the objective of the present studywas to carry out a detailed analysis of the extent to which EPRspectroscopy could be used to monitor the sorption and dis-tribution of a nonpolar spin probe, 4-hydroxy-2,2,6,6-tetra-methyl-piperidinyloxy benzoate, (4-hydroxy TEMPO benzo-ate) in a number of model soil systems involving hectorite andhumic acids, alone or complexed, in the presence or absenceof aluminum hydroxide, a common metal hydroxide in soilsthat has been shown to enhance bonding between humic acidsand clay minerals [22]. MATERIALS AND METHODS Spin probe The selected spin probe, 4-hydroxy-2,2,6,6-tetramethyl-pi-peridinyloxy benzoate (4-hydroxy TEMPO benzoate), was ob-tained from Aldrich Chemical (Milwaukee, WI, USA). It con-tains a stable paramagnetic nitroxyl radical (Fig. 1). Its func-tional group (benzoate) in position 4, and to some extent alsoits tetramethyl piperidinyloxy structure, confer hydrophobicproperties to the molecule. The octanol-water partition coef-ficient ( K  ow ) was measured by equilibrating a 10:1 water:oc-tanol mixture for 24 h and then determining the concentrationof TEMPO-benzoate in the water fraction by high-performanceliquid chromatography (HPLC). The value obtained (4.3   10 2 ) is comparable to those reported in the literature for chlo-robenzenes [23]. The spin probe was dissolved in methanol toprepare a stock methanol solution with a spin probe concen-tration equal to 10 mM.  Hectorite The reference smectite (phyllosilicate or clay mineral) usedin this study is hectorite SHCa-1 from San Bernardino County(CA, USA), obtained from the Source Clay Minerals Repos-itory (West Lafayette, IN, USA) of the Clay Mineral Society.It contains 34.7% SiO 2 , 15.3% MgO, 23.4% CaO, 2.18% Li 2 O,0.69% Al 2 O 3 , 0.25% FeO, and 0.02% Fe 2 O 3  [23]. HectoriteSHCa-1 is known to contain carbonate impurities (24.8%), andFourier Transform Infra-Red spectroscopy did indeed revealabsorbance bands at 1,439/cm and 885/cm. However, thesebands were not evident in the   2-  m fraction obtained aftersedimentation. Even the fraction  2  m still showed the pres-ence of impurities, which prevented the calculation of thestructural formulae [24]. A tentative structural formula of (Mg 0.56  Na 0.42  K 0.05 )(Mg 4.6  Li 1.39  Ti 0.01 )(Si 7.75  Al 1.17  Fe[III] 0.05 ) hasbeen suggested for hectorite SHCa-1, along with a specificsurface area of 63.19 m 2  /g and a cation exchange capacity of 43.9 meq/100 g [25]. Hectorite was chosen because of its lowiron content, which was necessary to prevent interferenceswith the EPR spectra of the spin probe. Hectorite SHCa-1 wassaturated with Ca 2  by repeated exchange with 0.5 M CaCl 2 .After the last saturation, excess salts were removed by washingtwice with deionized water. The suspension then was freeze-dried and stored at room temperature.  Humic acids Two different humic acids were used. A dark brown(10YR2/3), calcareous soil from Foresta Umbra(FU),Provinceof Foggia (Puglia), Italy, covered with  Quercus cerris , wasextracted with 0.5 M NaOH–0.1 M Na 4 P 2 O 7  (100 g of soil perL of extracting solution) under a N 2  atmosphere for 24 h withvigorous shaking to produce the first humic acid, labeled FU-HA. The material sold by Aldrich Chemical as a humic acidand labeled hereafter as Aldrich-HA was dissolved in 0.1 MNaOH under N 2 .Suspensions of the two HAs were centrifuged at 6,000  g for 20 min. The HAs then were precipitated overnight by acid-ifying the supernatants with 6 M HCl to pH 2 [26,27]. Aftercentrifugation at 8,000  g  for 20 min, the pellets were resus-pended in 0.1 M KOH under nitrogen, reprecipitated with HCl,and centrifuged as described above. Finally, they were treatedwith 0.1 M HCl and 0.1 M HF to reduce the ash content [28],dialyzed against deionized water with hourly changes, andfreeze dried. The elemental composition was determined withan Elemental Analyzer 1108 Carbon, Hydrogen, Nitrogen,Sul-fur Analyzer (Fisons Instruments, Lucino di Rodano, Italy).The determination of the E 4  /E 6  ratio (ratio of absorbances at464 and 665 nm), which commonly is used as an index of thedegree of condensation or humification of aromatic constitu-ents, together with fluorescence and Fourier Transform Infra-Red spectroscopic analyses, helped to characterize the humic  Hydrophobic spin probe distribution in hectorite suspensions  Environ. Toxicol. Chem.  24, 2005 2437Table 1. Selected characteristics of the Foresta Umbra (FU) and Aldrich humic acids (Aldrich HA) usedin the experimentsSource of humic acidElemental analysis% C % H % N % S % OWatercontent(%)Ashcontent(%) E 4  /E 6 FU-HA a Aldrich-HA b 50.354. a Province of Foggia (Puglia), Italy. b Aldrich Chemical (Milwaukee, WI, USA). acid samples under investigation [29]. Results of the elementalanalyses, E 4  /E 6 , and ash contents are shown in Table 1. Organo-mineral complexes Clay–humic acid complexes were synthesized using a pro-cedure based on those developed by Singer and Huang [22]and Violante et al. [30] for the formation of OH-Al-humate-smectite complexes. Five hundred milliliters of 6 mMAl(NO 3 ) 3  were titrated potentiometrically to pH 5 by addingCO 2 -free, standardized, 0.25 M NaOH at a rate of 0.5 ml/min.Thereafter, 10 ml of 50 mg/ml FU-HA or Aldrich-HA, pre-viously suspended in water at pH 7 for 24 h, were added andthe suspensions were stirred for 2 h. After the addition of 100ml of 100 mg/ml Na-hectorite dispersed in deionized water,the pH was adjusted to 7 with 0.25 M NaOH (0.5 ml/min).Volumes then were brought to 1 L with deionized water andthe suspensions were shaken continuously for 24 h. The twocomplexes obtained with this procedure are referred to in thistext as FU-Al-hectorite and Aldrich-Al-hectorite. In addition,100 ml of a 100-mg/ml Na-hectorite-water suspension and 10ml of 50 mg/ml FU-HA were adjusted to 1 L with deionizedwater and mixed without the addition of any Al(NO 3 ) 3  in orderto evaluate the role of aluminum hydroxide on the behaviorof the organo-mineral complexes. The resulting complex islabeled as FU-hectorite. Finally, as with the srcinal hectoritesample, all the clay complexes were exchanged with Ca 2  ,washed twice with deionized water, and freeze dried. In theirfinal states, the complexes contain approximately 5% byweight of organic matter. Sorption isotherms Sorption was carried out in aqueous suspensions containing0.005 M CaCl 2  (to prevent hydrolysis of hectorite), and either5 mg/ml HAs (FU-HA or Aldrich-HA) or 25 mg/ml hectoriteor hectorite-HA complexes (FU-Al-hectorite, Aldrich-Al-hec-torite, and FU-hectorite). Abiotic controls received an addi-tional 2 mg/ml of sodium azide (microbial inhibitor). The sus-pensions were titrated to pH 7 by adding Ca(OH) 2  or HCl andthen agitated for 24 h in order to dissolve the HAs and dispersethe hectorite and organo-mineral complexes. For each sorbent,sorption isotherms of the 4-hydroxy TEMPO benzoate spinprobe were obtained by varying its concentration in solutionfrom 0.01 to 0.10 mM. To avoid artifacts, the methanol con-centration in all the samples was kept at a constant concen-tration of 1% by volume. It was not possible to extend therange of the spin probe concentrations because of solubilityand instrumental detection limits. Sorption in systems con-taining HAs was evaluated after 24 h. In order to monitor theevolution of the sorption process over time in the suspensionscontaining hectorite, HA-hectorite, and HA-AlOH-hectoritecomplexes, samples were subjected to end-over-end shakingfor 24 h and for 15 d. All the samples were analyzed by EPRspectroscopy and HPLC as described in the  EPR Analysis  and  HPLC Analysis sections.  The experiments were run in trip-licate.  EPR analysis Samples for EPR analysis were obtained by filling the tipof a capillary tube with the entire suspension when humicacids were studied as sorbents, or with the suspension, pellet,and supernatant (obtained after centrifugation) in experimentsinvolving hectorite or HA-hectorite complexes. Each capillarytube was sealed promptly with plasticine and placed in a quartzEPR tube. Room temperature (20  C) EPR spectra were run at9.76 GHz (X-band) using a Bruker ESR 200 SRC spectrometer(Bruker Biospin AG, Fa¨llanden, Switzerland), with the mod-ulation and power set to 2.0 Gauss and 10 mW, respectively,to minimize signal distortion and power saturation effects.Higher power settings (20–50 mW) were necessary to improvethe detection limit at low spin probe concentrations(1–10  M).The correlation time   R  was measured to assess the rota-tional diffusion of the probe. It was calculated from the three-line spin probe spectra according to the equation [31]    0.65 W   (  R    2)  R  o    (1)where 1/2 1/2  R    ( h  /  h  )    ( h  /  h  )   o   1 o   1  (2)and  h  1 ,  h o , and  h  1  are the measured heights of the low,middle, and high field lines, respectively, and  W  o  is the widthof the middle line (Gauss units). Rotational correlation timeis interpreted as the length of time over which molecules main-tain a particular orientation before random thermal motionreorients them. This analysis can only be applied to spectrathat are generated from a population of spin probe undergoingrapid self-exchange (rotational correlation times lower than 10nanoseconds [ns]).The total spin probe concentration was evaluated by mea-suring the central peak height ( h 0 ) of the sample relative topeak heights of standard probe solutions. The quantificationobtained by EPR analysis was validated by the HPLC analysesreported in the following section.  HPLC analysis The concentrations of 4-hydroxy TEMPO benzoate weredetermined with a Perkin-Elmer LC Model 410 pump HPLC,(Perkin-Elmer, Wellesley, MA, USA) and a Waters SymmetryC-18 column (Milford, MA, USA; 15 cm  3.9 i.d.  5  m),and a diode-array ultraviolet-vis detector (Perkin-Elmer,Series200) set at 232 nm. The mobile phase was a 30:70% water:methanol (volume/volume [v/v]) isocratic solution eluted at 1/ min.  2438  Environ. Toxicol. Chem.  24, 2005 M. Spagnuolo et al.Fig. 2. ( a ) Typical electron paramagnetic resonance (EPR) spectrumof 4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy benzoate (TEMPObenzoate) in a 0.1-mM aqueous solution, and ( b ) corresponding spec-trum in hectorite suspension at identical probe concentration and gain. RESULTS AND DISCUSSION  EPR spectra in aqueous solutions The characteristics of the EPR spectra of 4-hydroxy TEM-PO benzoate are similar to those of Tempol and Tempamine  used in an earlier study [14] and need not be recounted indetail here. In a nutshell, because the unpaired electron of theTEMPO benzoate molecule is situated on the N-O   -orbital,the nuclear spin of   14 N (  I     1) splits the EPR signal into threehyperfine lines corresponding to the  M   I     1, 0,   1 quantumnumbers (see Fig. 2a).Calculation of the rotational correlation time    R  of 4-hy-droxy-TEMPO benzoate in aqueous solutions on the basis of the relative heights of the three peaks in the EPR spectrumgives a value of 0.09 ns, slightly longer than that of 0.031 nsfor Tempol and similar to 0.088 ns for Tempamine  [14]. Ro-tational mobilities of the order of 0.01 to 0.1 ns are expectedfrom the Debye diffusion model applied to small moleculesin water [19].In aqueous solutions with spin probe concentrations up to0.13 mM, the height ( h o ) of the central peak is directly pro-portional to the spin probe concentration. At concentrationsabove 0.2 mM, however,  h o  levels off. Therefore, a calibrationcurve was set up from 0.001 to 0.13 mM. The HPLC analysesof TEMPO benzoate acqueous solutions fully corroborated theresults obtained by measuring the central peak height of theEPR signal. Sorption isotherms On the basis of the linear correlation in the 0.001- to 0.13-mM concentration range, the central peak height,  h o , of theprobe’s EPR spectra can be converted directly to a spin probeconcentration. The EPR spectra were measured directly in theHA suspensions, which do not sediment readily, but were mea-sured in supernatant solutions after centrifugation of the sus-pensions of hectorite and organo-mineral complexes. TheHPLC analyses of supernatants, obtained after centrifuging thehectorite and organo-mineral suspensions, resulted in spinprobe concentrations similar to those deduced from EPR mea-surements. These considerations made it possible to calculate,by difference, the amount of probe that was sorbed to, or atleast closely associated with, the solid constituents and humicacids. These two spin probe concentrations, in the supernatantand in the solid phase, can be correlated to yield sorptionisotherms of the probe in the different systems. When theseisotherms are reasonably linear, their slope, denoted by  K  d ,may be computed and may provide information on the sorptiveaffinity of the probe molecules for the solid phases present inthe systems.Analysis of the EPR spectra for 4-hydroxy TEMPO ben-zoate in the HA suspensions resulted in signal attenuations of 83 and 94% for Aldrich-HA and FU-HA, respectively. For-tunately, however, the mean    R  values of 0.094 and 0.097 nsin the FU-HA and Aldrich-HA suspensions, respectively, bothwere very close to the    R  value of the spin probe in aqueoussolution (0.09 ns). This observation could be accounted for ina number of different ways. In the first scenario, a solution-like EPR signal could have dominated almost entirely other,comparatively weaker, EPR signals produced by sorbed spinprobe molecules. In the second scenario, humic acid–spinprobe complexes could have tumbled at about the same rateas free spin probe molecules. Given the sheer mass of humicacid molecules, the second situation is not very likely. In thethird scenario, TEMPO benzoate could sorb to the humic acidmolecule via the phenyl group on the TEMPO benzoate mol-ecule. In this case, the tetramethyl-piperidinyloxy part of themolecule would be free to rotate around the ester bond, andthis segmental motion could account for the rotational cor-relation time observed. Nevertheless, to ascertain that theprobe molecules producing the EPR signal were not sorbedonto the humic acids, suspensions of the humic acids con-taining 4-hydroxy TEMPO benzoate were placed in one com-partment of a dialysis cell, separated by a dialysis membrane(3 kDa cut-off) from the other compartment, which was filledwith background electrolyte. The experimental results clearlysuggest that the probe was free to diffuse through the dialysismembrane, because after 3 h the EPR signals were identicalin both compartments of the dialysis cell, and the EPR signalintensity in both compartments was exactly half the initialintensity in the humic acid–4-hydroxy TEMPO benzoate com-partment. Therefore, it is safe to assume that the observed EPRsignals in the HA suspensions emanated only from free probemolecules in solution, and that it is possible to calculate bydifference the amount of TEMPO-benzoate sorbed to HAs.Figure 3 shows the isotherm of TEMPO-benzoate with thetwo humic acids studied. The  K  d  for FU-HA (2,952 ml/g) ismore than three times higher than that of Aldrich-HA (925ml/g). These  K  d  values are consistent with the properties of the two humic acids. Fluorescence and Fourier Transform In-fra-Red spectra (data not shown) together with E 4  /E 6  values(Table 1) indicate a more pronounced complexity, aromaticity,and condensation of FU-HA with respect to the commercialAldrich-HA. These features tend to increase the hydrophobic-ity of FU-HA molecules relative to Aldrich-HA molecules,and could account for the higher affinity of FU-HA for 4-hydroxy TEMPO benzoate.The scientific literature is replete with statements that ‘‘soilorganic matter is the principal sorbing component of hydro-phobic contaminants in soils and sediments’’ [32], or that be-yond a threshold value of 0.1% organic carbon, sorption iscontrolled solely by the organic fraction in soils [33]. Fromthat standpoint, one would have predicted that the sorptivecapacity of the organo-mineral complexes for TEMPO ben-  Hydrophobic spin probe distribution in hectorite suspensions  Environ. Toxicol. Chem.  24, 2005 2439Fig. 3. Sorption isotherms of 4-hydroxy-2,2,6,6-tetramethyl-piperi-dinyloxy benzoate (TEMPO benzoate) by Aldrich Chemical humicacid (Aldrich-HA; Milwaukee, WI, USA) and Foresta Umbra humicacid (FU-HA; Puglia, Italy) after 24 h of interaction. Coefficients of variation: 2.6 to 7.3%.Fig. 4. Sorption isotherms of 4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy benzoate (TEMPO benzoate) by Ca-hectorite and humic acid–hectoritecomplexes after interaction for 24 h and 15 d. Coefficients of variation: 1.8 to 6.1%. HA    humic acid. zoate should be sharply lower than that for the HAs. Indeed,humic acids amount, at most, to 5% by mass in the complexes,so that the  K  d  values of the spin probes in these systems, inprinciple, should be reduced by a factor of at least 20 comparedwith the  K  d  values found with the pure HAs. The resultingestimated  K  d  values then would be of the order of 147 ml/gfor complexes containing FU-HA and 46 ml/g for those con-taining Aldrich-HA. In principle,  K  d  values lower by morethan a factor of 20 could occur if smectite particles somehowwere blocking a portion of the adsorption sites on the HAs.In spite of these predictions, the  K  d  values observed afteronly 24 h for the sorption of the spin probe on the organo-mineral complexes far exceeded the  K  d  values of the spin probecalculated on the basis of a 5% HA content in the complexes(Fig. 4). After 15 d, the  K  d  value for the complexes containingFU-HA was more than three times higher than the estimateabove, whereas that for the Aldrich-AlOH-hectoritecomplexeswas about 10 times higher. Furthermore, there was no apparentcorrelation between the  K  d  values and the nature of the humicacid involved in the complexes. The  K  d  value of the spin probesorbed to the FU-AlOH-hectorite complexes (483.1 ml/g) wasonly marginally higher than that for spin probe sorbed to theAldrich-AlOH-hectorite complexes (455.4 ml/g), in spite of the sizeable difference between the  K  d  values of the spin probesorbed to the FU-HA and Aldrich-HA (Fig. 3). Similarly, thepresence of aluminum hydroxide did not seem to affect sig-nificantly the observed  K  d  value of the spin probe, other thanto speed up sorption in the early stages. In the FU-hectoritecomplexes, the  K  d  was relatively low (239.3 ml/g) after 24 h,compared to the value (345.3 ml/g) in the FU-AlOH-hectoritecomplexes. This difference disappeared over the following 14d. The absence of aluminum hydroxide may have decreasedor alleviated diffusional processes hindering the movement of spin probe molecules in the FU-hectorite complexes. Suchdiffusion limitations would be expected, particularly if humicacid molecules coat the surfaces of hectorite particles [30].The unexpectedly high  K  d  values found in the organo-min-eral complexes appear related to the fact that the clay, hectoriteSHCa-1 used in the experiments has a nonnegligible affinity
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