Environ. Sci. Technol. 1993, 27, 2606-261 1
Adsorption Mechanisms of Monobutyltin in Clay Minerals M. Carmen Hermos/n,* Pledad Martln, and Juan CorneJo
Instituto de Recursos Naturales y Agrobiologia d e Sevilla, CSIC, Apartado 1052, Sevilla 41080, Spain Two phyllosilicates of high permanent charge and expandable (swelling) structure, montmorillonites, and two silicates of low permanent charge and nonexpandable structure, kaolinite (phyllosilicate) and sepiolite (fibrous silicate), were used as sorbents to elucidate the capacity and mechanism of adsorption of monobutyltin (MBT) species, as related to the presence and dynamics of MBT in sediments and particulate matter. Adsorption kinetics and isotherms, desorption measurements, and studies of organotin-clay complexes by X-ray diffraction and FTIR spectroscopy indicated that the MBT adsorption process is a cationic exchange on these clay minerals, besides an additional adsorption of neutral (MBT)Cl3 attracted by the lipophilic moiety of the first MBTn+adsorbed. The expandable (swelling) minerals showed much higher adsorption capacity because MBT adsorbed in their interlayer spaces where MBTn+species, besides (MBT)CIS,make "aggregates" or "clusters" that propelled some montmorillonite layers from 14 to 22 A. The results show the important role of clay minerals in the presence and dynamics of MBT in sediments as bound residues. The use of montmorillonite as a filter for MBT-contaminated waters is also suggested. Introduction
The sorption capacity of sediments and particulate matter is intimately related to their colloidal or clay fraction. A large proportion of pollutants are found to be associated with the clay fraction of particulate matter and sediments (1, 2), indicating that adsorption by the components of this fraction is a major control mechanism determining the distribution and fate of contaminants in aquatic systems. The clay minerals are, besides iron oxides and oxyhydroxides, the major inorganic components of particulate matter and sediments (3, 4). Clay minerals are aluminum or magnesium silicates of layered structure having permanent negative charge which is compensated by hydrated inorganic cations which are exchangeable (5). Clay minerals due to their high surface area and reactivity are very effective sorbents for organic contaminants of cationic or polar character (5, 6), and hence can act as organic contaminant carriers (2). Organotin species have attracted the attention of the environmental protection agencies of diverse countries because their increasing usage have raised the possibility of environmental pollution (7). The presence of the organotin compound has been reported in water and sediments from coastal environments of the and their toxicity USA and Canada (8)and of Europe (9), effect on nontarget organisms has been shown (7). The inputs of butyltin compounds in the environment come from their use as biocides, as stabilizers for poly(viny1 chloride) (PVC) polymers, as catalysts for industrial processing, and in antifouling paints (10). The highest toxicity corresponds to tributyltin (TBT) species, but their less toxic degradation products dibutyltin (DBT) and monobutyltin (MBT) species are also of environmental concern (10, 11). MBT and DBT occurrence has been 2606
Envlron. Scl. Technoi., Vol. 27, No. 12. 1993
related to degradation pathways of TBT by microbial activity or biological and photochemical reactions (11,12); however, there is increasing evidence of direct leaching from PVC pipes (13). Quevauiller (14), studying the stability of butyltin species in turbid water, stressed the need to investigate the adsorption/desorption process of butyltin in particulate matter. Recently (15) a study on adsorption/desorption of TBT in sediments of the San Diego and Pearl Harbor bays related the sorption capacity of sediments with the clay fraction content, especially with expandable and high cation exchangecapacity (CEC)minerals. The complexity of natural sediments and particulate matter makes difficult the interpretation of adsorption behavior. The use of a model may allow the fundamentals of adsorption mechanisms in determining the dynamics of contaminants in natural systems at a molecular level to be understood, as it has been recently suggested (16). Recent studies (17, 18)of the adsorption of B T species in model systems (iron oxides and fulvic acids) showed that maximum adsorption occurred for MBT species, through a mixed process including polar and hydrophobic forces. The objective of this work was to assess the capacity and mechanism of adsorption of MBT on clay minerals, as related to their structural characteristics and surface properties. The use of pure mineral components to ascertain the behavior of organic pollutants has been showed to be very useful in soil environments (19-22). Materials and Methods
Mineral Sorbents. The model sorbents used were three clay mineral types: (a) two high permanent layer charge (CEC, cation exchange capacity) phyllosilicates, smectites or montmorillonites, samples SAz and SWy from the Clay Mineral Repository from CMS (23), (b) a very low permanent charge phyllosilicate, kaolinite, sample KGa from the same source, and (b) a fibrous silicate, sepiolite (Sp),with low permanent charge, from a Vallecas (Spain) deposit (24). The surface properties of these sorbents are summarized in Table I, and their schematic structures are shown in Figure 1. The montmorillonite has a swelling or expandable structure that makes the interlamellar surface accessible to interchange the cations and to polar molecules (51, and from this fact, Saz and Swy have very high glycerol surface values (internal surface). Kaolinite and sepiolite have rigid or nonexpandable structures; however, sepiolite has a high SBET value because this sample has a high microporosity (25). MBT Solutions and Analytical Technique. The compound used was monobutyltin trichloride from Aldrich with a 95 % purity. Since this product is highly insoluble in water, the solutions were prepared in 5% methanol/ water, which also helped to improve the sensivity of the analytical method (26). The pHs of these solutions were always between 2.2 and 2.5, which was enough for the stability of MBT species without further acidification (26). The solutions were prepared and stored in polypropylene bottles. 0013-936X/93/0927-2606$04.00/0
0 1993 American Chernlcal Soclety
Table I.
clay
Surface Properties of Clay Minerals S B d
SG$
(m2g-l)
(m2g-l)
layer chargec (chargeunitlunit cell)
CEC (requivlg)
SAz SWy Sep KGa
1200 1.14 97 820 764 0.69 31 662 120 0 283 230 40 0 10 16 a SBmmeasuresthe externalsurfaces. b SG,. measuresthe external and internal surfaces. Jaynes and Boyd (3). a)s m t i t r
silicate layer
layer
000 0
I
.
,5
.
"
,, $
'.
I.
.
.
'
0 ,
I
T
0
Silicate
Exchangeable inorganic cation,
m
0
b)Kaolinite
7/h;Y/flh-l/4
I-
Hydrogen bonds
(
.
.
10
20
30
40
SO
Times (hours)
Figure 2. Evolution of MBT in solution in the presence of clay minerals with time.
.
si1ic.lte layer
silicate layer
adsorption isotherms were obtained by plotting C,(pmol/ Ce (pmol/mL). Desorption Measurements. The desorption of MBT from the sorbents was done from the adsorption bottles by substitution of half of the equilibrium concentration by an equal volume of the solvent. Three successive dilutions were run, and after 24 h the corresponding equilibrium MBT concentration was measured. X-ray Diffraction and FTIR Spectroscopy of Organotin-Clay Mineral Complexes. The sorbent corresponding to the maximum adsorption value at the isotherm was washed with methanol/water (5%) and air dried. The organotin-clay complexes thus obtained were studied by X-ray diffraction as oriented specimens on glass slides and by the FTIR spectroscopy technique as KBr disks. X-ray diffraction was done on a Siemens D-500 Kristalloflex and FTIR spectra were recorded in a FTIR Nicolet 5 PC. g) versus equilibrium concentration,
b)Sopiolita -Silicate
fibr
Internal channel External channel
Flgure 1. Schematic structures of clay minerals used In this study.
The analytical method to measure MBT concentrations in solutions was atomic absorption spectrophotometry (AAS)in a Perkin-Elmer 2380 using a dinitrogedacetylene flame with a 5-cm burner and an activation hollow-cathode lamp operated at 13 mA. The absorbance readings were done at 286.3 nm and a slit of 0.7 mm. The standard concentrations used were from 40 to 400 ppm Sn. Adsorption Kinetics and Isotherms. The sorbent/ solution ratio used for both experiments was 0.1 g/lOO mL for montmorillonites and 0.2 g/20 mL for sepiolite in order to obtain a sorption percentage measurable by the analytical technique used to determine organotin species (AAS). The adsorption experiments were carried out at the pH of the MBT solutions used (pH N 2.5) to ensure MBT stability (hydrolysis and precipitation) at the concentration used in the adsorption experiments. The final pH measured in the supernatant was practically the same as that of the initial solutions, except for that of sepiolite, which increased approximately one unit. The minerals were mixed with the solutions in polypropylene centrifuge tubes with screw caps. For kinetic studies the suspensions were shaken at room temperature (22 f 2 "C) for times ranging from 15 min to 48 h. After each time, the suspensions were centrifuged at 18 000 rpm for 10min and the supernatants analyzed after filtering (Dynagard, 0.2 pm). All points were duplicated, and two blanks ((1) the initial solution without clay and (2) clay with solvent (5% methanol/water at pH 2.5)) were run to correct possible methodology errors. The kinetics of adsorption was studied with 3 mM MBT solution, and adsorption isotherms were carried out using MBT concentrations ranging from 0.87 to 5.21 pmol/mL and using a shaking time of 24 h, at which it could be considered that pseudoequilibrium was reached (kinetic study, aee below). For both adsorption kinetics and isotherms the amount of MBT adsorbed, C,, was calculated from the difference between the initial, Ci,and equilibrium, C,, MBT concentrations in the respective solutions. For adsorption kinetics the MBT solution concentration (Ce)and amount adsorbed, C, (pmol/g) were plotted versus time, t (h). The
Results and Discussion The nature of the MBT species in the experiments was unknown but from the pH of the adsorption solutions and according to previous researchers (17,18) the aqueous species would be predominantly cationic, such as MBTS+, (MBT)C12+,and (MBT)ClZ+,with some neutral (MBT). C13. These ionic species can interchangewith the inorganic cations, compensating for structural charge of the clays (5, 6 ) . The adsorption of MBT on the clays was monitored as a function of time, and the evolutionof MBT concentration in solution versus time is plotted in Figure 2 for all clays studied. The kinetic profiles were similar for all clays, but the decrease of MBT was higher for montmorillonites than that for sepiolite and kaolinite, even when the solid/ solution ratio was much higher for sepiolite and kaolinite (0.2 g/20 mL) than for montmorillonites (0.1 g/100 mL). The kinetic profiles of all clays suggest that MBT adsorbs through two steps: an initial fast adsorption until the first (montmorillonites),second (kaolinite), or fifth (sepiolite) hour, and after that a slower adsorption for montmorillonites and sepiolite until 48 h (no further adsorption was detected in kaolinite). The kinetic data were assayed to fit a first-order kinetic model, which is usual in clay mineral adsorption reactions, involving exchangeable cation sites (27):
C, = Coe-kt
(1) where COis the initial solute (MBT) concentration, Ce is Environ. Sci. Technol., Voi. 27, No. 12, 1993 2607
Table 11. Correlation Coefficients and First-Order Kinetic Constants for Adsorption of MBT on Clay Minerals
first step mineral
k
r
SAz
0.93c
second step
co
X
10-2 (pmol/mL)
37.5
SWy 0.95b 59.3 Sep 0.9W 2.0 KGa 0.95b 4.0 a P < 0.001. P < 0.05. P
k
r
X
2.8 0.93b 2.7 0.93b 2.8 0.9gb 2.9 < 0.1.
CO 1t2 (pmol/mL)
0.2 0.6 0.2
1.8 1.2 2.2
c
C 3
k-
m I
RGa 0 2g/20rnl -a-a-A-A-A-
0
0.5
A0
50
Flgure 4. Time dependence of MBT adsorbed on clay minerals.
0.3
0.1
->
0.0
v
0.2 0
m
30
20
10
Times (hours)
0.4
0
1 -
1
2500
i
_I
2500
2000
1500 1000
-0.1
-0.2
0
10
20
30
50
40
Time (hours)
Figure 3. First-order klnetic plots for MBT adsorption on clay minerals.
I 0
Table 111. MBT Adsorption Isotherm Parameters from Langmuir and Freundlich Equations from Linear (1) and Nonlinear (nl) Estimations
Langmuir clay SAz
SWy Sep KGa P
r
L C, (~mol/g) 1 nl 1 nl r
0.99a 14 15 1911 1879 0.96a 0.99a 60 85 2439 2369 0.9ga 0.990 18 15 171 176 0.82O 53 42 0.90b 0.98a 4 7 < 0.001. P < 0.01. P < 0.05.
Freundlich Kr (rmol/g)
nt
1
0.12 0.04 0.10 0.16
nl
1
nl
0.15 1681 1670 0.04 2332 2329 0.12 157 161 40 0.15 38
the equilibrium solute concentration at time t , and k is the rate constant of the process. The best fit was obtained considering the adsorption process occurring in two successive first-order steps, as it was suggested from the kinetic profiles (Figure 2) and is usual in expandable clays (28). However, for the KGa sample the second step did not occur; the overall adsorption occurred in the first 2 h. Table I1 shows the linear fitting parameters obtained for MBT adsorption kinetics in all sorbents, and Figure 3 shows the linear plot of these first-order reactions for all clays. These two steps could correspond to an initial fast adsorption of MBTn+species by a cation exchangereaction attracted by Coulombic forces, followed by a slow adsorption of MBTn+ and MBT neutral species attracted by lipophilic bonds to the alkyl moiety of cationic MBT previously adsorbed. This has been shown to occur for other organic cations on clays (29-31). The rate constant values in Table I11 show that the MBT adsorption process is much faster for the high CEC and expandable clays, SAz and SWy, which also have high internal surface areas (Table I, Sar),indicating interlayer adsorption in these minerals. This is also suggested by the lower rate found for the SAz sample having the higher layer charge (Table I) that makes the opening of the silicate 2608
Envlron. Scl. Technol., Vol. 27, No. 12, 1993
1
2
3
MET Concentration C e (pmol/ml)
Flgure 5. Adsorption isotherms of MBT on clay mlnerals.
layers more difficult, as compared with SWy with a lower layer charge. Sepiolite gave the lower rate constant for the first step (Table 11),suggesting low accessibility of the adsorption sites which could be due to the high porosity of this sample (Table I, &ET). Hermosin et al. (25)have reported that almost half of the surface area (&ET) of this sepiolite is due to microporosity. The rate constant for the second step was very low, except that for the SWy sample, which had a slightly higher value. Figure 4 shows the evolution of the amount of MBT adsorbed with the time for all clays. Two features are evident from these uptake curves, which confirm the above considerations. First, a fast initial adsorption occurred in phyllosilicates in the first hour with 80% for SWy, 90% for SAz, and 100% for KGa (first 2 h) of the total loading at 48 h, whereas for sepiolite this initial adsorption was slower with only 55 7% of the total loading in the first 5 h. Second,pseudoequilibriumcould be assumed to be reached at 24 h, when more than 95% of the 48-h loading had adsorbed in all clays. The adsorption isotherms of MBT on the three clay minerals are shown in Figure 5. These isotherms were of the H type, according to the classification of Giles et al. (32) for all samples, indicating a specific and strong interaction or high affinity between the solute and sorbent. These types of isotherms have been reported for adsorption of diverse organic cations on clay minerals (24-31), adsorbing by the cation exchange mechanism. However, the MBT adsorption level was much higher for expandable clay montmorillonites (SA2 and SWy) than for nonexpandable clays sepiolite and kaolinite. This lower adsorption on sepiolite and kaolinite suggestsweaker affinity as corresponds to the lower CEC of these minerals (Table I). These adsorption data were fitted to the adsorption equations for the solid/liquid interphase process (33):
(a) Langmuir equation CJC, = CJC,
+ l/C,L
(b) Freundlich equation C, = KfC,"'
15.2
(2)
A
(3)
where C, is the solute (adsorbate) concentration in the solution at equilibrium (pmol/mL),C, the amount of solute adsorbed in the solid at equilibrium (pmol/g), C m the adsorption maximum for the sorbent, L a constant related to the adsorption energy, Kf the Freundlich adsorption capacity parameter, and nf the Freundlich intensity parameter. C, and Kf can be used as measures of the relative adsorption capacity (34). The estimation of the above parameters was done by linear correlation and by nonlinear application of eqs 2 and 3. Table I11 shows the parameters corresponding to both equations and both estimation methods, for MBT adsorption data on the three clay minerals. The parameter values from both estimation methods were very similar. The correlation coefficient values indicated a good fit for both equations, being slightly better for the Langmuir equation as usual for the H- and L-type isotherms (32). Taking into account that the Langmuir model is based on the assumption that adsorption occurs in specific surface sites with homogeneous adsorption energies (limited adsorption value, C,) and no interaction between adsorbed species and that Freundlich assumed different adsorption sites, with diverse adsorption energies and interactions among adsorbedspecies (34),it seems reasonable to suggest that MBT adsorption on clay minerals occurs through a mixed process of both assumptions. C, and Kfvalues show that MBT the adsorption capacity of these clay minerals increased: KGa < Sp
