Water Air Soil Pollut (2014) 225:2213 DOI 10.1007/s11270-014-2213-5

Chemical Behavior of Lanthanum in the Presence of Soils Components: Adsorption and Humate Complexes M. Jiménez-Reyes & M. Solache-Ríos

Received: 12 August 2014 / Accepted: 4 November 2014 / Published online: 23 November 2014 # Springer International Publishing Switzerland 2014

Abstract The inorganic and organic components of soils from a forest and from a semiarid region were separated and characterized by different techniques. The kinetic adsorption behavior of lanthanum ions by the inorganic components of the soils indicated that the systems behave according to the kinetic model of pseudo-second order, so 3 h of contact was sufficient to achieve equilibrium. The sorption isotherm data, qe vs. Ce, were best adjusted to the Langmuir model for the soil of the forest; this means that a monolayer is formed and that the energy of adsorption is the same in all adsorption sites so the material is homogeneous. The data for the soil of the semiarid area were adjusted to the Freundlich model; this indicates that the material is heterogeneous and multilayers are formed. The competition between the lanthanum sorption by the inorganic components and the formation of a complex with humic acids was shown by a decrease of qe. The method of Schubert was used to determine the stoichiometric coefficient of the complex La:(HA)n, which was 1:1 for both soils, whereas the logβapp La;ðHAÞ was 5.1±0.2 for the n

soil of the forest and 7±1 for the semiarid soil. Keywords Adsorption . Humate complexes . Lanthanum . Soil M. Jiménez-Reyes : M. Solache-Ríos (*) Departamento de Química, Instituto Nacional de Investigaciones Nucleares, Carretera México-Toluca s/n, La Marquesa, Ocoyoacac, Edo. de México C.P. 52750, Mexico e-mail: [email protected]

1 Introduction The study of the geochemical behavior of rare-earth elements is interesting from the point of view of chemical industry wastes and the security of long-term deposits of radioactive waste (Marquadt 2000, Sun et al. 2014). Some rare-earth elements are present in nuclear waste, and these elements are chemically similar to the actinides, which are usually difficult to handle; lanthanum as a member of this group was chosen for the present work. Inorganic components of soil include sand, clay, and limestone. The organic components are fulvic acids (FA), soluble in solutions of pH=1; humic acids, which are the most abundant and soluble in solutions of pH= 10; and humin (insoluble in both pH 1 or 10 solutions), which may remain with the inorganic components when the separation is based on alternating the pH of solutions. Sorption and chemical interactions of lanthanides, mainly Eu(III), have been reported. The effect of humic acid (HA) and fulvic acid (FA) bound to hydrous alumina on the sorption mechanism of Eu(III) was determined; the results showed that inner-sphere surface complexation may contribute mainly to Eu(III) sorption and that a ternary surface complex is formed at the HA/ FA-hydrous alumina hybrid surfaces (Tan et al., 2008). The sorption of Eu(III) onto β -MnO2 depends on pH and it is independent of ionic strength, suggesting the formation of inner-sphere surface complexes (Sheng et al. 2014a). The adsorption of Eu (III) by β -MnO2 is promoted by the addition of humic or fulvic acids at low

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pH (Sheng et al. 2014b) The same kind of complexes was deduced for the system Eu(III)/γ -Al2O3, and in this case, Eu(III) sorption mechanism at pH 5.0, 6.5, and 8.0 was attributed to the formation of edge-shared and corner-shared surface complexes, and surface precipitation and/or surface polymerization tended to be the predominant mechanism at pH 10.0. The sorption percentage of Eu(III) increased with increasing ionic strength in the presence of humic acids (Yang et al. 2014; Sheng et al. 2014c). The effects of reaction temperature on Eu(III) interaction mechanism and micro-structure at the titanate nanotubes were studied and it was found that the adsorption process is endothermic and spontaneous (Sheng et al. 2013a). Complexation studies of Eu(III) with alumina-bound polymaleic acid showed that at high Eu loading, Eu is surrounded by one carboxylate group and one aluminol and at low metal ion concentration, a surface site corresponding to a 1:1 Eu/COO- stoichiometry was deduced (Montalvon et al. 2002). It is well known that inorganic and organic contaminants may simultaneously exist in many soil and water environment. Clay minerals have been extensively studied because of their strong sorption and complexation ability. Removal of lanthanum, and other elements of nuclear interest, from aqueous solutions has been studied with inorganic materials from the soil, such as clays (Chegrouche et al. 1997; Spencer et al. 2007, Sheng et al. 2012; Sheng et al. 2013b), and on the other hand, lanthanum-saturated materials have been used for the removal of anions (Wasay et al. 1996; Haghseresht et al. 2009). Kornilovich et al. (2000) observed that removal of lanthanides from aqueous solutions by clays, at various pH values, was influenced by the presence of organic matter in the system, which reduces the removal of lanthanides. Organic components such as humic acids are potential ligands for the formation of soluble complexes in aqueous media with metal cations, including rare-earth elements (Dierkcx et al. 1994; Grenthe and Puigdomench 1997; Marquadt 2000; Pandey et al. 2000; Tang and Johannesson 2010; Jiménez-Reyes and Solache-Ríos 2012). The purposes of the present work were (1) to isolate and characterize both inorganic and organic components of soils from an oyamel forest and from a semiarid zone, (2) to examine the retention mechanisms of one of the elements of the rare-earth group (lanthanum) by the inorganic components of soils by determining the kinetic and isotherms of the system, and (3) to determine the

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competition between that retention and the formation of the soluble complexes lanthanum–humic acids, including the determination of the stability constant of these complexes.

2 Materials and Methods 2.1 Separation and Characterization of Inorganic and Organic Components of Soils 2.1.1 Separation The sample of the soils from the oyamel forest (OF) and semiarid zone (SZ) were collected at a depth between 0 and 20 cm and a depth of 70 cm, respectively. These samples were left to dry at room temperature, percolated, and sieved. Later, they were dried at 60 °C and stored in polyethylene bags. The separation technique reported by Swift (1996) was modified to separate the components of the soils as follows: a sample of soil was suspended in water and the pH adjusted between 1 and 2 with 1 M HCl solution. The volume to mass ratio was 10:1 and the suspension was stirred for 1 h; fulvic acids (FA) remained in solution, and the solid was recovered by centrifugation at 6,000 rpm for 1 h. The pH of this solution was increased to alkaline in order to precipitate FA. A 0.1-M NaOH solution was added to the solid, maintaining the same proportion as above and after stirring for 4 h, the mixture was left overnight for sedimentation. Solids (containing inorganic materials) and supernatant (humic acids) were separated by centrifugation at 6,000 rpm for 1 h. The supernatant was again acidified to pH=1 and left for 16 h. The precipitate was dissolved in a 0.1-M KOH solution, and KCl was added to have a potassium concentration of 0.3 M. The process was repeated with the supernatant, alternating the pH. The precipitate of humic acids was suspended in a 0.3M HF/0.1 M HCl solution, in a plastic container. It was stirred overnight and then recovered by centrifugation at 6,000 rpm for 1 h. This process was performed twice. The precipitate was then placed in a dialysis tube and a water bath until it gave a negative result to chloride with silver nitrate. Finally, the HA were dried at 60 °C, and both the yield of the process and the percentage of ashes (2 h at 1,000 °C) were determined. The solids containing inorganic materials were washed with distilled water until the pH of the washing

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solution was almost neutral; they were then recovered by filtration and dried at 60 °C. The yield of the process was determined.

of these mixtures were carried out with a 1-M FeSO4 solution until a light green color appeared. A blank was titrated as well, and calculations were done as follows:

2.1.2 Characterization

%C org ¼

The materials obtained from the soils were characterized by (a) thermogravimetric analysis (SDT Q600 V20.9 Build 20 with DSC-TGA Standard module; TA Instruments Benelux, Belgium); (b) infrared spectrophotometry (Nicolet Magna IR-550, Thermo Scientific, USA); (c) electron microscopy (JEOL-6460LV to 20 kV; JEOL, USA, Inc.): the samples were placed on conductive copper tape, and the elemental composition was determined by dispersed energy spectroscopy (EDAX DX-4); and (d) X-ray diffraction (Siemens D-5000 using Kα radiation of copper and a mono-cromator of graphite; Siemens AG, Germany). The diffraction patterns were collected from 2.5 to 70° 2θ, with steps of 0.02° 2θ. Capacity of acid groups was determined by titration of HA in a 0.122-M NaOH solution with a 0.2-M HCl solution. Measurements of pH were done 3 min after each addition, and the agitation was constant. The reaction was followed with a combined electrode (Ag/AgCl) coupled to a pH-meter with a precision of 0.001 in pH units together with an automatic burette (Radiometer TIM900 Titrilab and ABU901; Radiometer Analytical, France); a double wall cell and a constant temperature circulator (Polyscience Circulator 12101-10; Polysciences, Inc., USA) were used. The amount of titrable acidic functional groups [A −] given in meq of NaOH per gram of humic acid was calculated as follows: ½A−  ¼

C OH þ 10−pH −10pH−pKw mHA

ð1Þ

where COH = initial meq of NaOH minus added meq of HCl, pH is the experimental value for each addition, pKw is the ionic product of water, and mHA is the mass of the sample in grams. Organic carbon was determined by both EDAX and the Walkey and Black methods (Norma Oficial Mexicana 2002). Samples of HA were treated with a 0.166-M K2CrO7 solution and concentrated sulfuric acid for 30 min; concentrated phosphoric acid and a solution of diphenylamine were then added. Titrations

0:39  ðB−S Þ  M mHA

ð2Þ

where B and S are the volumes of ferrous sulfate solution, used for blank and sample, respectively; M is the concentration in molarity; and mHA is the mass of the sample in grams. Molecular weight and the indicator of humification were determined (Chin et al., 1994) with a UV–Vis Perkin Elmer, Lambda 10 spectrophotometer. Absorbance of HA solutions between 2×10−5 and 8×10−5 g/cm3 were measured at 280, 465, and 665 nm. Molecular weight could be calculated by the following equation: M w ¼ 3:99ε þ 490

ð3Þ

where ε is the slope value of the graph of absorbance at 280 nm versus organic carbon concentration (in M). The indicator of humification (E4/E6) was calculated as the ratio of absorbance at 465 and 665 nm, respectively. 2.2 Sorption: Kinetic and Isotherms Solutions were labeled with the isotope 140La for the determinations of the sorption kinetic behaviors and isotherms of lanthanum ions by inorganic materials of the OF and the SZ. This isotope was obtained by irradiating a 7.2×10−4 M lanthanum nitrate (Merck KGaA, Germany) solution, pH = 4, in the nuclear reactor TRIGA MARK III of the Nuclear Center of Mexico, with a neutron flux of 9.9×1012 cm−2 s−1 for 12 min. The 140La was identified by its gamma radiation spectrum (detector GeH coupled to a multichannel analyzer) and its half life (40 h). Inorganic materials (0.05 g) were shaken with solutions of lanthanum (5 mL) labeled with the radioactive isotope ([La3+]initial = 1.4× 10−5 M, for SZ and 3×10−5 M or 1.3×10−3 M, for OF) from 3 min to 120 h. After centrifuging at 6,000 rpm for 10 min, aliquots were taken for the measurements of radioactivity, which were performed with a gamma radiation detector of NaI (Tl) coupled to a monochannel analyzer. The reference was a

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solution containing the same amount of 140La used in each experiment, whose radioactivity was measured in the same experimental conditions. The material (0.05 g) and 5 mL of solution were shaken for 3 h and the [La3+]initial ranges were between 8.8×10−4 M and 1.7×10−3 M for OF and 4.2×10−5 M and 2.8×10−4 M for SZ for the sorption isotherms. Initial pH of the mixture sorbent–lanthanum solution (4.81 ± 0.07) did not change between 1 and 24 h of contact. The pH of the humic acid solution was 7; therefore, the pH depended on the quantity added of this solution; neither in these cases the initial pH changed along the contact time. All experiments were done by duplicate or triplicate. The sorption capacities of the inorganic materials for lanthanum ions for a specific contact time (qt) or at the equilibrium (qe) were calculated as follows: qt

or

qe ¼

3þ

3þ

ð4Þ where m(La3 +)I and m(La3+)F are the initial and final mass (milligrams) of La3 + in solution, respectively; m(La3+)IM is the mass (milligrams) of La3 + adsorbed by the inorganic materials of soils (IM); and mIM the mass of the IM in grams (0.05 g). Adsorbed mass was calculated considering the known initial mass and the radioactivity measured for each experiment and the reference; loss of radioactivity on the polyethylene tubes was not observed. The percentage of La3+ adsorbed by IM was calculated as follows: % La3þ sorbed ¼

mðLa3þ ÞIM  100 mðLa3þ ÞI

ð5Þ

2.3 Determination of the Stability Constant of the Humates of Lanthanum Commercial humic acid (Aldrich, Germany; here named AHA) was purified for these experiments. IM (0.05 g) was left in contact with solutions of lanthanum (5 mL) labeled with the radioactive isotope 140La. The reactions for the formation of the complexes are as follows:
3− j La3þ þ j ðHAÞn ⇌La ðHAÞn j

According to the Schubert method [Schubert 1948], the constant can be obtained using the following equation:   λ0 log −1 ¼ logβapp ð7Þ La; jðHAÞn þ j*log½HA λ where λ0 and λ are the distribution constants of metal between the IM and the solution, in the absence and in the presence of the ligand, respectively, and j is the ligand to metal ratio of the complex. These distribution constants are calculated as follows: λ0 orλ ¼

mðLa ÞI −mðLa Þ F mðLa ÞIM ¼ mIM mIM 3þ

where 1≤j≤3. The apparent formation complex constant is defined as follows: h
3− j i La ðHAÞn j ð6Þ βapp  j La; jHA ¼ ½La3þ  ðHAÞn

ðaÞ

¼

mðLa3þ Þinitial −mðLa3þ Þsolution mðLa3þ Þsolution mðLa3þ ÞIM mðLa3þ Þsolution

ð8Þ

3 Results and Discussion 3.1 Separation and Characterization The materials obtained from the OF soil were black powders; the IM was of granular appearance, and both HA and FA were crystalline. The yields were 82, 2.6, and 0.4 % for IM, HA, and AF, respectively. The IM of the SZ was beige and granular (99 %), and HA was black and crystalline (0.03 %) with a percentage of ashes at 1,000 °C of 3.5 %; FA was imponderable. The main components of the inorganic materials identified by X-ray diffraction were as follows: the feldspars disordered albite (NaAlSi3O8) and calcium ordered albite (Na,Ca) (Si, Al)4 O8), quartz (SiO2), and montmorillonite ((Na,Ca)0.3(Al,Mg)2 Si4O10(OH)2 ·n(H2O)). The micrographs (×500) of the IM of both soils show rough particles while those of HA and FA (×1,000) show smooth or slightly rough surfaces. The main elements found by EDS are shown in Table 1; other elements were found with concentrations <0.1 % (Na, Mg, S, Cl, K, Ca, Ti, Mn, and Ba). The presence of silicon, aluminum, calcium, and iron confirm once again that

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Table 1 Composition of the materials separated from the soils. Data obtained by EDS and (*) the method of Walkey and Black Element

FA (OF)

HA (OF)

IM (OF)

IM (SZ)

C

45±3 %

53±1 (54.6) %*

16±0.2 %

6±6 %

O

36±5 %

42±3 %

51±1 %

56±1 %

Al

0.9±0.9 %

1.4±0.1 %

5±2 %

8±2 %

Si

0.3±0.1 %

–

14±2 %

22±1 %

Ca

<0.2 %

–

4±2 %

2±1 %

Fe

–

<0.3 %

6±4 %

3±2 %

both IMs contain materials like sand, feldspars, and clays. The presence of carbon, mainly in the IM of the OF soil, may indicate the presence of insoluble organic substances like humines. The presence of fluorine (ca. 15 %) and silicon in the IM of AF shows that the material was not completely pure. Regardless of origin, country, or continent, humic acids contain a percentage of carbon ranging from 40 to 60 % (Licona Sánchez et al. 2008), so humic acids were suitably purified. HA is mainly composed of carbon and oxygen (95 %) with an atomic ratio O/C of 0.8; the same value was found for AF. According to Rupiasih and Vidyasagar (2008), this value is high and indicates a high content of carbohydrates. Figure 1 shows the thermogravimetric diagram of the three materials separated from the OF soil. The main processes are (a) dehydration observed at ≤130 °C, (b) decarboxylation of cellulose (ca. 310 °C), and (c) decomposition of lignin and char formation by the

decomposition of cellulose (ca. 430 °C). These results agree with those reported in the literature (Lopez-Capel et al. 2005; Jiménez-Reyes and Solache-Ríos 2012). The remaining percentages of mass at 800 °C were 48.5 % for HA, 36.6 % for FA, and 83 % for IM. The bands of infrared spectra (Fig. 2a) of humic and fulvic acids of OF were assigned as follows (Stevenson and Goh 1971; White 1971; Choppin 1988; McDonnell et al. 2001): (a) the strong band at 3,400 cm−1 reflect the OH groups, including those of alcohol, phenols, and carboxylic acids; in some cases, N-H groups may contribute to absorption in this region; (b) the bands at 2,924 and 2,850 cm−1 correspond to the C-H stretching of methyl and methylene groups of aliphatic chains; (c) the 1,730-cm−1 band is due largely to stretching frequencies of C=O of carbonyl and carboxyl groups; (d) the 1,640 cm−1 band is assigned to the COO- group; (e) a broad band at the region between 1,350 and 1,500 cm−1 is probably due to OH deformation and the C-O stretching of OH phenolic groups or C-H deformation of CH2 and CH3; (f) the shoulder at 1,270 cm−1 can be due to C-O stretching and OH deformation of COOH groups; and (g) 1,030 cm−1: C-O stretching of polysaccharides. Both spectra are similar; however, the bands at 2,924 and 2,850 cm−1 and the region 1,350–1,500 cm−1 corresponding to the aliphatic C-H are quite strong and clearly defined for HA and not for AF. The HA spectrum is qualitatively similar to that previously reported for the commercial AHA (Jiménez-Reyes and SolacheRíos 2012).

Lignine decomposition +char from cellulose

Cellulose decarboxilation

HA

Dehydration

IM-OF Oxides

FA

Fig. 1 Thermogravimetric diagram of the fractions of the soil of oyamel forest

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Water Air Soil Pollut (2014) 225:2213 140

100 80 60 40

transmitancenc

120

20 0 3900

3400

2900

2400

1900

1400

900

400

wavenumber cm-1 Fig. 2 IR spectra of the organic fractions of the soil of oyamel forest soil (continuous line). The inorganic materials of the oyamel forest soil IM(OF) and the semiarid zone soil IM(SZ) (discontinuous line)

The bands of infrared spectra (Fig. 2b) of the inorganic materials of both SZ and OF soils were assigned as follows (Hlavay et al. 1978; Eren and Afsin 2008; Darehkordi et al. 2012): (a) the absorption band at 3,629 cm−1 is due to stretching vibrations of structural OH groups of montmorillonite, (b) the broad band at 3,415 cm−1 corresponds to the H2O-stretching vibrations, with a shoulder near 3,330 cm−1, due to an overtone of the bending vibration of absorbed water observed between the foils of the montmorillonite at 1,651 cm−1, (c) the band at 1,047 cm−1 is related to the stretching vibrations of Si-O groups, (d) the band corresponding to AlAlOH, bending vibrations was observed at 914 cm−1, (e) bands at 802, 695, and 470 cm−1 are typical of quartz:

Fig. 3 Titration curve of HA from the oyamel forest soil

Si-O symmetrical stretching, symmetrical bending, and asymmetrical bending vibrations, respectively, (f) bands at 775 and 470 cm−1 are usually observed for feldspar. In general, the spectrum of SZ is the best defined. Figure 3 shows the titration curve of HA from OF soil, and Table 2 includes the pH values of inflexions, the functional groups whose pKs correspond to those values and concentrations of each one (in meq NaOH/g HA). Acetate, kojate, and phenolate groups form part of the cellulose and lignin molecules while the 1-naftolate may come from the degradation of insecticides applied to the ground. Kojate and phenolate were previously identified in AHA; however, the concentration (in NaOH meq/g HA) is lower for the kojato and higher

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Table 2 Capacities of the acid groups in HA of the OF soil pH

Functional group

4.5±0.2

Acetate, pKa=4.56

1.8±0.1

7.3±0.3

Kojate, pKa=7.6a

3.8±0.4

9.2±0.1

1-Naftolate, pKa=9.14b

6.6±0.1

10±0.1

Fenolate, pKa=9.9

9.6±0.5

a

Kojic acid:

b

1-naphtol:

[A], meq NaOH / g HA

for the phenolate in HA than in AHA (Jiménez-Reyes and Solache-Ríos 2012). Table 1 shows the concentration of total organic carbon for the HA using Walkey and Black’s (Norma Oficial Mexicana 2002) method, which is statistically identical to that obtained by EDS. The total organic carbon of HA was taken into account to calculate the concentration of solutions measured by spectrophotometry at 280 nm. This wavelength was chosen because phenolic compounds, benzoic acids, and polyenes, among other compounds, absorb in this region (Rupiasih and Vidyasagar 2008). The equation obtained was as follows:   Absorbanceð280 nmÞ ¼ 458 C org ; r2 ¼ 0:9999

ð9Þ

The value of ε=458 was applied to Eq. 3 to calculate the molecular weight of AH, which was 2,316 g mol- 1. This value is of the same order of magnitude, although smaller than those reported previously for AHA, 2,850 (Jiménez-Reyes and Solache-Ríos 2012) and 3,070 g mol−1 (Chin et al. 1994). The indicator of humification (proportion of organic humic substances with respect to other chemical species in the sample) was calculated as the ratio of the absorbance at 465 and 665 nm and was E4/E6 =5.6±0.1. Previous reported values for AHA are 5.7 (Jiménez-Reyes and SolacheRíos 2012) and 7.6 (Chin et al. 1994). The humic acid obtained from the OF was a mixture of organic compounds with a high content of aliphatic chains and high molecular weight; the pH of the aqueous solutions (7×10−3 M) was >8. In such conditions, the rare-earth elements form insoluble complexes in aqueous media. The pH of commercial humic acids solutions is 6; thus, AHA was used for the determination of the stability constant of the humates of lanthanum.

3.2 Sorption Figure 4 shows the results obtained on the kinetic behavior of the systems [La3 +]initial =3×10−5 M–IM(OF) and [La3 +]initial =1.4×10−3 M–IM(SZ); that corresponding to [La3 +]initial =1.3×10−3 M–IM(OF) is not included in the figure. Error bars were in all cases between 1 and

Fig. 4 Adsorption of lanthanum by the inorganic materials as a function of time and adjustment of data to the pseudo-second order kinetic model. Rhombs OF (pHeq =6.7, [La3+]initial =3×10−5 M); circles SZ (pHeq =4.9, [La3+]initial =1.4×10−5 M)

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6 % of average values. The equilibrium for [La3 +]initial = 3×10−5 M–IM(OF) was reached very quickly (0.25 h) 63±4 % of lanthanum was retained in the solid; whereas for [La3+]initial =1.3×10−3 M–IM(OF), between 3 and 22 h, 93±6 % of lanthanum was retained. For [La3 + ]initial =1.3×10−3 M–IM(SZ), the time to reach equilibrium was about 2 h of contact the solid retained 89±3 % of lanthanum. The experimental data were treated with the Lagergren, Elovich, modified Freundlich, and pseudosecond order kinetic models; the best fit was found with the last model (Fig. 4). This model is described by means of the following equation: t 1 1 ¼ þ t qt K 2 q2max qmax

ð10Þ

Table 3 shows the parameters obtained with this model based on the assumption that the rate-limiting step may be chemisorption, which involves valence forces through the sharing or exchange of electrons between adsorbent and adsorbate (Ho and McKay 1999). Independently of the sorbent, the values of qmax are related to the initial concentrations of lanthanum (see Table 3) by means of the following equation: qmax = 14,093 [La3+]initial (in molarity)—0.04 (R2 =1). Thus, adsorption depends on the initial concentrations of lanthanum. The sorption isotherm data obtained from Ce and qe were treated with the models of Langmuir, Freundlich, and Langmuir–Freundlich. For IM(OF), the best fit was achieved with the Langmuir model (Fig. 5a), and the equation obtained was as follows: Ce 1 1 ¼ C e þ K L ¼ 0:0605C e þ 0:0676; R2 q0 q0 qe ¼ 0:99

ð11Þ

Table 3 Parameters obtained from the experimental data and the model of pseudo-second order for the adsorption of La3+ by IM(OF) and IM(SZ) (Eq. 10) Sorbent [La3+], M Equations

Parameters

IM(OF) 3×10−5

K2 =9.4 g mg−1 h−1 qmax =0.36 mg g−1

t/qt =2.81 t+0.84 R2 =0.9997

IM(OF) 1.3×10−3 t/qt =0.055 t +0.007 K2 =0.41 g mg−1 h−1 qmax =18.3 mg g−1 R2 =0.9997 IM(SZ) 1.4×10−5 t/qt =5.41 t +0.40 R2 =0.9994

K2 =73.4 g mg−1 h−1 qmax =0.18 mg g−1

Where qe and Ce refer to the concentration of solid phase (milligrams per gram) and liquid phase (milligrams per liter) after sorption equilibrium, respectively, K L (0.89 L mg−1) is the constant of the Langmuir isotherm and q0 (16 mg/g) is the maximum adsorption capacity. This model considers that sorption takes place at defined sites on the surface of the adsorbent, forming a monolayer, and that the energy of adsorption is the same in all adsorption sites, so the material is considered homogeneous. The viability of the process, characteristic of the Langmuir isotherm, can be expressed by a nondimensional term RL: RL ¼

1 1 þ K LC0

ð12Þ

where KL =0.89 L mg−1 and C0 is the initial lanthanum concentration. The calculated values for the C0 interval, between 122 and 244 mg L−1, were less than 0.01, indicating favorable conditions for the sorption. The sorption isotherm data for IM(SZ) were treated with the same models mentioned before. The best fit was achieved with the Freundlich model (Fig. 5b), and the equation obtained was as follows: logqe ¼

1 logC e þ logK F ¼ 1:56C e þ 0:74; R2 n

¼ 0:97

ð13Þ

The parameters calculated were Kf = 5.54 L(1/n) g−1 mg(1/n)−1 and 1/n=1.56. This model implies a nonlineal adsorption due to a heterogeneous surface on which the adsorption sites and their energies are exponentially distributed, giving rise to a multilayer formation. It is important to note that carbonate ions did appear during experimentation because the purpose of this work was to determine the chemical behavior under environmental conditions. The distribution diagrams considering [CO23 +]=50 [La3+]initial (Jiménez-Reyes et al. 1999; Puigdomenech, 2010) reveal the following features: (a) for the IM(OF) experimental conditions (pHeq =6.8), the predominant species are the insoluble La2(CO3)3 ·8H2O (ca. 90 %), La3+ (ca. 8 %), and LaCO+3 (ca. 2 %); and (b) for IM(SZ) experimental conditions (pHeq =5.8), ca. 50 % of each La2(CO3)3 ·8H2O and La3+ are present. It is likely that at these pH values, lanthanum may be found partially adsorbed and the rest precipitated as carbonate on both IMs. Both IMs contain

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Fig. 5 Sorption isotherm of lanthanum by a IM(OF): adjustment of data to the Langmuir model and b IM(SZ): adjustment of data to the Freundlich model

montmorillonite and iron, and particularly IM(OF) also contains organic material; therefore, the adsorption of lanthanum could be explained by means of the following contributions: (a) competition between cation exchange and bounding to the clay surface by sharing one or several ligands, generally oxygen (Bhattacharyya and Gupta, 2008); (b) the possible presence of iron oxohydroxides may form the FeOHLa3+ (Pepper et al., 2006); and (c) the remaining organic material of IM(OF) may form lanthanum complexes. The effect of HA and FA bound to the sorbent, let the formation of inner-

sphere surface complexation of europium and other ions; the presence of humic acids enhances the sorption (Tan et al. 2008; Sheng et al. 2013b, 2014d; Yang et al. 2014). Moreover, surface chemical nature such as acidic sites and basic sites of humic acids is very useful to explain the experimental results and possible adsorption mechanisms (Tan et al. 2008; Sheng et al. 2013b, 2014d). On the contrary, for the present experiments, the presence of humic acids removes the lanthanum from the sorbent and then the humic lanthanum complexes remain in solution.

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Table 4 Experimental data concerning the determination of the stability constant of lanthanum humates Sorbent [La3+]i, M (pHeq)

[AHA]/[La3+]

La3+ in sorbent (%)

qe

Schubert method equation

IM(OF) 1×10−4 (6.8)

0 2.8–4.9

98.5±0.01 48±5

1.37±0.001 0.68±0.08

y=5.0+0.95 x (R2 =0.69)

IM(OF) 1×10−4 (6.8)

0 5.6–7.0

98.5±0.01 16±3

1.37±0.001 0.22±0.04

y=5.2+0.94x(R2 =0.98)

IM(OF) 1.4×10−5 (6.8)

0 7.8–11.7

98±0.1 75±5

0.2±0.01 0.16±0.01

y=4.9+0.97x(R2 =0.98)

15.6–19.5

65±1

0.13±0.001

IM(SZ) 5.6×10−5 (5.9)

0 0.8–4.9

95±0.8 2±2

0.77 0.01±0.01

y=6.7+1.0x(R2 =0.98)

Equation for the Schubert method (Eq. 7)

The presence of humic acids in solution gives rise to the formation of the soluble complex in an aqueous medium. The competition between the sorption of lanthanum by both IM and the formation of the complex with humic acids was demonstrated by gradual decreases of qe and of the remaining percentage of lanthanum in the solid (see Table 4). In the absence of humic acids ([AHA]/[La3+]=0), virtually all lanthanum is captured by both solids. Solubilization of lanthanum from IM(OF) depends on the initial concentration of the metal: 16 % remains on the solid for 1×10−4 M and [AHA]/[La3+]=7 whereas 65 % remains on the solid for

1.4×10−5 M, even if [AHA]/[La3+] is ca. 20. On the contrary, lanthanum solubilization does not require a large quantity of humic acids when it is adsorbed by IM(SZ) ([AHA]/[La3 +]<1). This behavior allowed the determination of the stability constant of the complex formed in solution. According to the equations obtained from the application of the Schubert method (see Fig. 6 and Table 4), the
 slopes of the lines of log λλ0 −1 vs. [AHA] had values of 1±0.02, indicating the stoichiometry of 1:1 for the lanthanum humate; that means that a molecule or a molecular site is participating in the formation of the soluble complex in the aqueous medium. Considering the axis intersection of ordinates of the equations (Table 4), the value of logβapp obtained in the presence of La;ðHAÞ

Fig. 6 Schubert method for the determination of the stability constant of the complex of humate with lanthanum. Rhombes, IM(OF); [La3+] = 1 × 10−4 M, pHeq = (6.8); squares IM(OF);

[La3+]=1×10−4 M, pHeq =(6.8); triangles IM(OF); [La3+]=1.4× 10 −5 M, pH eq = (6.8); circles IM(SZ); [La 3+ ] = 5.6 × 10 −5 , pHeq =(7.4)

3.3 The Stability Constant of the Humates of Lanthanum

n

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IM(OF) was 5.1±0.2, independent of the lanthanum’s initial concentration. In the presence of IM(SZ), the logβ app obtained was slightly higher, 6.7±1. La;ðHAÞ n

In a previous report, using the ion exchange resin Dowex 50W X8 as a sorbent (Jiménez-Reyes and Solache-Ríos 2012) and initial relationships of [AHA]/ [La3+] between 0 and 11, the stoichiometry of the lanthanum humates were 1:1, 1:2, and 1:3. Particularly for the stoichiometry 1:1, the values of logβapp La;ðHAÞ at pH n

4.9 and 5.9 were 6.29 and 7.61, respectively. In a previous work (Marquadt 2000) using the resin BioRex70 and an Aldrich humic acid with a pH of 5, the stoichiometric coefficient was 0.62 and logβ app La;ðHAÞ =2.7. The n

value of logβapp La;ðHAÞ of this research is different from n

that mentioned above; this behavior shows the importance of the material in which lanthanum is adsorbed. The diversity of values is explained by the parameters already mentioned (Grenthe and Puigdomench 1997): pH, concentration of the metal, method of purification of humic acids, and experimental conditions of the sorption system. Although there was no competition between the carbonate ions and humic acids in the aquifer sediments [Tang and Johannesson 2010], a complex similar to that reported for europium (Dierkcx et al. 1994) is proposed for lanthanum, which could explain its solubility in the presence of humic acids: La (CO3) (HA)n.

The kinetic behavior of the systems [La3+]— inorganic materials indicated that the rate-limiting step may be chemisorption. Both inorganic materials adsorb lanthanum efficiently. The sorption isotherm data were best adjusted to the Langmuir model for the soil of the forest; whereas, the data for the soil of the semiarid area were best adjusted to the Freundlich model. This indicates differences between the surfaces of these materials. Humic acids compete with the sorption, forming a lanthanum humate with a stoichiometry of 1:1 that is soluble in an aqueous medium. The logβapp La;ðHAÞ value n

and the stoichiometry of the complexes depend upon the experimental conditions, including the physicochemical properties of sorbent. The results of this research are important to understand the interaction mechanisms of lanthanides in soils, which are interesting from the point of view of chemical industry wastes and the safe disposal of radioactive wastes. Ackowledgments We thank F. Monroy Guzmán and E. Fernández Ramírez for the donation of the samples of the semiarid zone soil. The technical support of E. Morales Moreno, I. Z. López Malpica, J. Muñoz Lujano, and M. Villa Tomasa was much appreciated as well as the participation of the students: M. I. Valencia Flores, A. Hernández Jiménez, and B. Portillo Rodríguez.

References 4 Conclusions The inorganic and organic components of the soil of an oyamel forest and a semiarid zone were separated and characterized properly. As expected, organic materials are quite scarce in the semiarid soil and abundant in the forest soil; presumably a good quantity of humines remains together with the inorganic material of this last soil. Both inorganic materials are mainly composed by: feldspars, quartz, and montmorillonite. Organic materials decompose according to cellulose and lignin; the functional groups of these molecules were identified, as well as the 1naftolate a degradation product of insecticides applied to the forest ground. Particularly, the humic acid obtained from the OF was a mixture of organic compounds with a high content of aliphatic chains and high molecular weight.

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