versión On-line ISSN 0718-2791
R.C. Suelo Nutr. Veg. v.9 n.1 Temuco 2009
RC.Suelo Nutr. Veg. 9 (1) 2009 (55-68) J. Soil Sc. Plant Nutr. 9 (1) 2009 (55-68)
KINETICS OF MOLYBDATE AND PHOSPHATE SORPTION BY SOME CHILEAN ANDISOLS
Cinética de adsorción de molibdato y fosfato en algunos Andisoles chilenos
Erika Marina Vistoso G.1, Nanthi S. Bolan2, Benny K. G. Theng3, María de la Luz Mora4
1National Institute for Agricultural Research (INIA), Remehue Research Centre, P.O. Box 24-0, Osorno, Chile.
2Centre for Environmental Risk Assessment, University of South Australia, SA 5095, Australia.
3Landcare Research, Private Bag 11052, Manawatu Mail Centre, Palmerston North 4442, New Zealand.
4Departamento de Ciencias Químicas, Facultad de Ingeniería, Ciencias y Administración, Universidad de la Frontera, P.O. Box 54-D, Temuco, Chile. Corresponding author:firstname.lastname@example.org
The kinetics for the sorption of molybdate and phosphate by four Chilean Andisols have been determined. About 55%o of the molybdate and 61% of the phosphate was sorbed in the first 0.5 h, after which sorption slowly increased, reaching 90%o for molybdate and 97% for phosphate after 72 h. At the same time, OH¯ ions were released into the external solution, raising its pH by 0.85 units for molybdate and by 0.65 units in the case of phosphate. These observations indicated that both anions were sorpbed by a ligand exchange mechanims. Among the five kinetic models examined (),Table, 2 the Elovich equation gave the best fit of the experimental data (R2 = 0.93 to 0.97, standard error = 0.35 to 0.94). The sorption rate constant (α) for both anions was related to the organic matter (OM) content of the soils, especially the content of Al- and Fe-humus complexes. The α values for molybdate were 2.24x1015 mmol kg-1 h-1 for the Vilcun soil (15% OM), 2.49 x1012 mmol kg-1 h-1 for the Pemehue soil (16% OM), 8.76x1010 mmol kg- h-1 for the Osorno soil (20% OM), and 3.11x107 mmol kg-1 h-1 for the Piedras Negras soil (24% OM). The corresponding values for phosphate were 3.89x107, 5.21x101 , 3.11 x1012 and 1.08x1016 mmol kg-1 h-1. The desorption rate constant (β) for the four soils (in the above order) ranged from 0.47 to 0.28 for molybdate, and 0.22 to 0.39 mmol kg-1 h-1 for phosphate. The results suggest that the mineralogical composition and organic matter content of the Andisols control the kinetics for the sorption of both molybdate and phosphate. Molybdate appeared to have a high affinity for Fe- and Al-oxides, while phosphate was largely sorbed to Fe-and Al-humus complexes.
Keywords: Al- and Fe-oxide, Chilean Andisols, kinetic model, molybdate and phosphate.
Se determinaron las cinéticas de adsorción de molibdato y fosfato en cuatro Andisoles Chilenos. Cerca del 55% del molibdato y 61% del fosfato fueron adsorbidos en las primeras 0,5 h; después la adsorción aumentó lentamente, donde se adsorbió aproximadamente el 90% del molibdato y el 97% del fosfato después de 72 h. Al mismo tiempo, aumentó la concentración de iones OH¯ en la solución de suelo, aumentando el pH en 0,85 unidades para molibdato y en 0,65 unidades en el caso de fosfato. Estas observaciones indican que ambos aniones son adsorbidos por el mecanismo de intercambio de ligandos. Entre los cinco modelos cinéticos examinados (Tabla 2), la ecuación de Elovich mostró los mejores ajustes de los datos experimentales (R2 = 0,93 a 0,97; SE = 0,35 a 0,94). La velocidad de adsorción (α), para ambos aniones, se relacionó con el contenido de materia orgánica (MO) de los suelos, especialmente con los contenidos de los complejos humus-Al y humus-Fe. Los valores de α para molibdato fueron 2,24x1015 mmol kg- h-1 en el suelo Vilcún (15% MO); 2,49 x1012 mmol kg-1 h-1 en el suelo Pemehue Í16% MO); 8,76x1010 mmol kg-1 h-1 en el suelo Osorno (20% MO), y 3,11x107 mmol kg- h-1 en el suelo Piedras Negras (24% MO). Los valores correspondientes para fosfato fueron 3,89x107; 5,21x1010; 3,11x1012; y l,08x1016 mmol kg-1 h-1. La velocidad de desorción (β) en los cuatro suelos (en el orden anterior) fluctuó en el rango de 0,47 a 0,28 para molibdato, y 0,22 a 0,39 mmol kg-1 h-1 para fosfato. Los resultados sugieren que la composición mineralógica y el contenido de materia orgánica de estos Andisoles controlan la cinética de adsorción de molibdato y fosfato. Molibdato parece tener mayor afinidad por los óxidos-Fe y óxidos-Al, mientras que fosfato es mayormente adsorbido por los complejos humus-Fe y humus-Al.
Palabras claves: Oxido de Al y Fe, andisoles chilenos, modelo cinético, molibdato y fosfato.
Chilean Andisols are acidic, have variable charge characteristics and a high anion sorption capacity (Mora et al., 2005). Andisols also have a large propensity for retaining P, Mo, and Se (Parfitt, 1978; Mora and Canales, 1995a,b; Barrow, 1999; Barrow, 2005; Vistoso et al., 2005; Cartes et al., 2005; López et al., 2007). Thus, farmers need to apply high rates of P and Mo fertilizers for legume production in volcanic soils of Sourthen Chile. In practice, this is done by placing the fertilizers as bands near the seed or coat over the seeds. Sorption of molybdate and phosphate by soil constituents controls the mobility and bioavailability of Mo and P in soil-solution-plant systems (Barrow, 1999). Both molybdate and phosphate are strongly sorbed to the variable charge mineral constituents (Al-oxides, Fe-oxides, allophane, imogolite) of volcanic soils (Karimian and Cox, 1978; Goldberg and Forster, 1998; Brown and Parks, 2001). The principal sorption mechanism is ligand exchange involving the formation of inner-sphere surface complexes (Zhang and Sparks, 1989; Goldberg et al., 2002; Goldberg et al., 2008). Thus, P and Mo deficiencies often occur in highly weathered acid soils.
Chemical reactions in soil, including anion sorption from aqueous solutions, occur on a millisecond time scale (Sparks, 1999). Since sorption kinetics have a controlling influence on anion mobility, transport, and bioavailability, their assessment is a prerequisite for the efficient application of Mo and P fertilizers. Various models have been used to describe the kinetics for the sorption of phosphate by soils and soil constituents (Chien and Clayton, 1980; Aharoni etal., 1991; Freese etal., 1995). Little information, however, is available of the kinetics of molybdate sorptionby soils, let alone by volcanic soils.
The aim of this research was to compare the effect of reaction time and the effect of soil properties on molybdate and phosphate adsorption in a range of Andisols from southern Chile.
Many models such as the homogeneous and heterogeneous surface site models have been used to describe the sorption of Mo and P by soils (Zhang and Sparks, 1989; Sparks, 1999; McGechan and Lewis, 2002). Because of the mathematical complexity involved, sorption rates have commonly been described in terms of zero-order kinetic (1), first-order kinetic (2), and second-order kinetic (3) kinetics all of which assume that the rate of change in solute concentration is proportional to the concentration in solution and the number of empty sites on the sorbent:
q, = q0-K0t (1)
In q, = lnq0 — Kit (2)
\lq,= \lq0 + K2t (3)
Here qt, and q0denote the quantity at time (t) and time zero, respectively; K0, K¡, and K2are the corresponding rate constants. Another kinetic model is the parabolic diffusion equation (4) which assumes that the rate-limiting step is the diffusion of anions either from the solution into the surface, or from the surface into intraparticle pores:
q, = a+Kd1/2 (4)
where, qt is the sorbed amount at time t; and Kd are constants.
A widely used model is the Elovich equation:
dq/dt = a exp (β-q) (5)
where, q is the sorbed amount at time (t); α and β are constants. Equation (5), originally developed to describe the kinetics for the chemisorption of gases to solid surfaces, was modified by Chien and Clayton (1980) who assumed that the activation energy of sorption increases linearly with surface coverage. The α parameter can be considered as the initial rate constant since dq/dt -> α when q This means that the rapid initial sorption is not governed by the exponential law. Assuming that q = 0att=0, and integrating equation (5) yields:
q = (1/β) In (1 +α βt) (6)
By assuming that αβt »1, equation (6) may be further simplified to:
qt = (1/β) In (αβ) +(1/β) In t (7)
where, q is the amount of anión sorbed at time (t),α and β are parameters and Kegis the equilibrium constant (calculated by α/ β ). Chien and Clayton (1980) determined α and β (7), and used these parameters to compare the kinetics of phosphate release and sorption in soils.
Materials and methods
Four Andisols (Vilcun, Pemehue, Osorno and Piedras Negras) with contrasting chemical properties were sampled from locations in Southern Chile (37° 35' -44° 04' LS and 70° 50'-71° 35' LW). The samples were taken from 0-20 cm, air-dried and sieved 2 mm . The chemical characteristics of the soils were determined according to the methods described by Sadzawka et al. (2006). Soil pH (water) was measured potentiometrically using a soihsolution ratio of 1:2.5. Organic matter was estimated by a modified Walkley-Black wet digestion method adapted for Chilean soils by Sadzawka et al. (2006). The different forms of Al and Fe oxides in the soils were determined by selective extraction (dissolution) with acid ammonium oxalate (Al0, Fe0), dithionite-citrate-bicarbonate (Ald, Fed), and Na-pyrophosphate (Alp, Fep) as described by Blakemore et al. (1987). The concentrations of Al and Fe in the various extracts were determined by atomic absorption spectrophotometry (AAS). Allophane and ferrihydrite contents were calculated using the methods proposed by Parfitt and Wilson (1985) and Parfitt and Childs (1988). The chemical characteristics of the soils together with the concentrations of different forms of Al, Fe, and Si, and the contents of allophane and ferrihydrite, are listed in Table 1.
The kinetics of molybdate and phosphate sorption by the four Andisols were determined at 298 K using the batch method. A background electrolyte solution (0.1 M KC1) containing 5.0 mmol L-1 of molybdate (as Na2Mo04x2H20) or phosphate (as K2HP04) was added in duplicate to polypropylene centrifuge tubes at soihsolution ratio of 1:20. The pH was adjusted to 5.0 by addition of dilute HCl or KOH solution in a N2 atmosphere so as to eliminate C02. The tubes were shaken for 72 h and the end of the reaction time, the suspensions were centrifuged at 10,000 g for 10 minutes and aliquots of the supernatants were analyzed. The concentration of Mo in the equilibrium solution was determined by AAS and that of P by colorimetry (Murphy and Riley, 1962).
The following models were used to describe the sorption of Mo and P: zero-order kinetic (1), first-order kinetic (2), second-order kinetic (3), parabolic diffusion (4), and Elovich equation (7).
The experimental data were statistically analyzed using SPSS version 13. The fit of each kinetic model was evaluated by the determination coefficient (R2) and standard error (SE) of Chien and Clayton (1980). The values of R2 and SE were used to compare the applicability of the different models at a confidence level of 95%. Furthermore, the parameters of the Elovich equation model were correlated with the chemical properties of the Andisols.
RESULTS AND DISCUSSION
The initial step of molybdate sorption by the Vilcún and Pemehue soils was faster than that by the Osorno and Piedras Negras soils samples (Figure 1). In accord with Barrow's (1999) studies with the same anions but with other soils, the sorption of phosphate by the Andisols was twice as rapid as that of molybdate, except for the Vilcún soil. Sorption increased with time, reaching an apparent equilibrium after 72 h. The sorption of both molybdate and phosphate showed two clearly defined steps: A fast initial reaction (up to 30 minutes), followed by a slow adsorption (up to 72 h). About 55.4% of the total amount of molybdate and 61.7% of the total amount of phosphate, were sorbed after 30 minutes. At 72 h, 89.7% of molybdate and 96.8% of phosphate were sorbed. Similar results were obtained by Zeng et al. (2003) for the sorption of phosphate by two Andisols. The same pattern was observed for the sorption of molybdate and phosphate by Al- and Fe-hydroxides, goethite, and acid soils (Bolan etal., 1985; Zhang and Sparks, 1989; Freese et al., 1995; Barrow, 1999; Brinton and O'Connor, 2003; do Carmo Horta and Torrent, 2007). Barrow (1999) has suggested that the "slow" reaction is due to diffusion of sorbed anions from surface sites into micropores within the structure of the soil constituents.
There was a steady increase in the pH of the supernatant solution as sorption progressed. After 72 h the pH of the equilibrium solution increased by 0.85 units for molybdate and 0.65 units for phosphate (Figure 2). These observations is supported by the literature because sorption occurs by ligand exchange mechanism; that is, the replacement of hydroxyl groups, exposed on surface sites, by either molybdate or phosphate in solution (Zhang and Sparks, 1989; Barrow, 1999; Goldberg et al., 2008).
We tested five different kinetics models using the least square regression analysis to describe the sorption of molybdate and phosphate in the Andisols (Table 2). The fitting by the linear models (equations 1, 2, 3 and 4) were unsuccessful in that the R2 were low (0.32-0.68) with, high SE (1.12-2.67). Similarly, Bolan et al. (1985) and Zeng et al. (2003) found that the sorption of phosphate could not be adequately described by a zero-, first-, or second- order kinetic; nor by the parabolic diffusion model. No information, however, is available on modelling the kinetics for the sorption of molybdate by Andisols.
We have fitted the sorption data to the Elovich equation (Figure 3) where the R2 values were highest ( 0.92) and SE lowest ( 0.94) (Table 2). Other have obtained similar results (Freese et al. (1995), Zeng et al. (2003) and Li et al. (2005) obtained similar results for different types of soils.
Kinetic constants from the Elovich equation
The Elovich parameters for the sorption of molybdate and phosphate are listed in Table 3. Except for the Vilcún and Pemehue soils, the values of sorption rate constant (α) for phosphate were much higher than those for molybdate. This agrees with Barrow (1999) who indicates that the reactivity of phosphate in soil is much higher than that of molybdate. Piedras Negras soil gave a higher α value for phosphate than the other soils. According to the parameters estimated by the Elovich equation (Table 3) it can be inferred that the α values for phosphate and molybdate depend on the soil type. The β values did not vary much between the four soils, while the sorption rate constants (α) were higher than the desorption constants (β). Since β values for molybdate were higher than those for phosphate, the availability of molybdate in these Andisols would be higher than that of phosphate. The equilibrium constants (Keq) for molybdate increased as the organic matter content of the soils decreased, while the Keq values for phosphate showed the opposite tendency (Table 3). The Keqvalues indicate that the molybdate sorption capacity of these Andisols is related to the content of free Al- and Fe-oxides (Ald, Fed). In contrast, the Keq values for phosphate are related to the content of Al- and Fe-humus complexes (Alp, Fep). These results suggest that Al- and Fe-humus complexes control the kinetic behaviour of these anions. Many workers (Barrow, 1970; Karimian and Cox, 1978; Garcia-Rodeja and Gil-Sotres, 1997; Goldberg and Forster, 1998; Brown and Parks, 2001; Hirade and Uchida, 2004; Goldberg et al, 2008) have shown that molybdate and phosphate in soil are strongly sorbed to, and associated with, Aland Fe-oxides. Being rich in free Al- and Fe-oxides (and allophane), volcanic soils generally have a high capacity for sorbing molybdate and phosphate. For this reason, the phytoavailability of Mo and P in Chilean Andisols would be low. The present results are consistent with this expectation.
Figure 3: Kinetic data for the sorption of molybdate (a) and phosphate (b) by Vilcun, Pemehue, Osorno and Piedras Negras soils at 298 K in the presence of 0.1 M KCl, showing the closeness of fit to the Elovich equation (solid lines).
Figura 3: Datos cinéticos de la adsorción de molibdato (a) y fosfato (b) en los suelos Vilcún, Pemehue, Osorno and Piedras Negras, a 25 + 0,1 °C en presencia de 0,1 M KCl como electrolito de soporte, mostrando los datos modelados con la ecuación de Elovich (líneas sólidas).
Relationship between kinetic constants and various soil properties
As already mentioned, Al- and Fe-oxides in soils are very reactive toward molybdate and phosphate. Table 4 gives the coefficients for the correlations between soil properties and the Elovich parameters for the sorption of molybdate and phosphate. The results lend further support to the hypothesis that the rate of molybdate sorption (α) is positively correlated with free Al- and Fe-oxides (Ald, Fed) and negatively correlated with Al0 and ferrihydrite contents (p<0.05). In contrast, the α values for phosphate were positively correlated with the content of poorly crystalline Al-oxides (Al0), allophane, and Al- and Fe-humus complexes (Alp, Fep). On the other hand, the rate of desorption (β) for molybdate was positively correlated with Ald and Fed and negatively correlated with organic matter, Fe0, Alp, allophane and ferrihydrite contents. The correlation coefficients relating phosphate β values to soil properties were similar in magnitude to those of molybdate, but with an opposite sign. The results for phosphate are in line with previous measurements of the kinetics of phosphate sorption and release (Garcia-Rodeja and Gil-Sotres, 1997; Zeng et al., 2003; do Carmo Horta and Torrent, 2007).
The content of free Fe and Al oxides appears to be the principal factor affecting the rate and capacity of molybdate sorption by Chilean Andisols. In the case of phosphate, on the other hand, it is the content of allophane, Al-humus and Fe-humus complexes that has a controlling influence on sorption and release. From the correlation between the Elovich parameters and soil properties we infer that availability of Mo in Andisols is higher than that of P. Nevertheless, Mo availability in volcanic soils may be inadequate for crop production. The high, negative correlation between the desorption constant for molybdate and the content of allophane or ferrihydrite further suggests that plant roots are not very efficient in taking up Mo. Thus, volcanic soils may not be able to supply sufficient Mo to meet the requirement of legumes for nitrogen fixation.
This research was supported by the National Research Council of Chile (FONDECYT project 1020934), the International Cooperation Grant (project 7020934), and the Education Ministry of Chile (MECESUP FRO 0309).
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