Adsorption and desorption processes of boron in calcareous soils

Chemosphere 80 (2010) 733–739

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Adsorption and desorption processes of boron in calcareous soils Aziz Majidi, Rasoul Rahnemaie *, Akbar Hassani, Mohammad Jafar Malakouti Department of Soil Science, Tarbiat Modares University, P.O. Box 14115-336, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 22 February 2010 Received in revised form 16 May 2010 Accepted 19 May 2010 Available online 12 June 2010 Keywords: Adsorption Boron Calcareous soil Desorption Hysteresis

a b s t r a c t Boron (B) availability is regulated by its equilibrium concentration that in turn is buffered by adsorption and desorption reactions. Ionic strength, pH, OM content, and the type and amount of minerals are the major factors affecting B sorption reactions. To evaluate the influence of calcium carbonate equivalent (CCE) and ionic strength on B chemical behavior, its adsorption and desorption isotherms were measured in eight calcareous soils differed in CCE (0–85%). Adsorption and desorption data were described by the Langmuir and the Linear adsorption equations, respectively. No statistically significant relation was found between model parameters and soil properties. However, in comparison, soils with higher reactive particles (clay and OM) and higher pH adsorbed more boron. Removing CCE from a soil sample (CCE = 18%) lowered B adsorption maximum by 35%. In contrast, increasing electrolyte concentration (0.01 M NaCl) to 0.1 and 0.5 M caused to increase B adsorption maximum by 30% and 75%, respectively. At the equi-molar concentration, CaCl2 increased B adsorption stronger than NaCl. The positive effect of ionic strength was attributed to a better screening of surface charges and compaction of double layer thickness. Desorption data were deviated from adsorption isotherms only at equilibrium concentrations smaller than 2 mM. Analysis of boron solution speciation and adsorption–desorption data revealed that B is mainly adsorbed as spectroscopically proved outer-sphere complex in the studied soil samples. The experimental data and model prediction could be used to manage B bio-availability and to optimize remediation processes in calcareous soils. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Boron (B) is an essential microelement for plants that its uptake is regulated by B concentration in soil solution. Boron concentration in soil solution is buffered by the adsorbed B on mineral surfaces through adsorption and desorption reactions. Since the range between deficiency and toxicity limits of B for plants is very narrow, any change in B equilibrium concentration may turn to considerable influence on plant growth (Keren and Bingham, 1985). Thus, adsorption and desorption reactions are very important in the management of B bio-availability. In addition, accumulation of B in surface soils is a common problem in many calcareous soils irrigated with water containing high B. Application of any remediation plan in these soils needs information on the B adsorption and desorption reactions. During last few decades, adsorption and desorption behavior of boron has intensively been studied in soil and on minerals. Ionic strength, pH, organic matter, and the type and amount of minerals have been identified as the most important factors influencing B reactions in soils and in other natural systems (Keren and O’Connor, 1982; Goldberg and Glaubig, 1988; Singh and Mattigod, * Corresponding author. Tel.: +98 21 4829 2278; fax: +98 21 4829 2200. E-mail address: [email protected] (R. Rahnemaie). 0045-6535/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2010.05.025

1992; Goldberg et al., 1993; Debussetti et al., 1995; Hobbs and Reardon, 1999; Sharma et al., 2006; Chen et al., 2009; Keren and Communar, 2009). Adsorption isotherms of B exhibit a trend similar to most oxyanions, i.e. a Langmuir type of adsorption; however, its adsorption edges are rather specific, like silicon (Si). At low pH, adsorption is weak. It gradually increases and reaches to a maximum at pH range 7–9, depending on the type of soil and minerals. At higher pH range, its adsorption gradually decreases to a minimum value (Keren and Sparks, 1994; Debussetti et al., 1995). The first two parts of this trend originate from the solution speciation of B. However, decrease in adsorption at high pH is due to a drop in surface potential on minerals with pH-dependent charge. In addition to pH, ionic strength has a noticeable effect on B adsorption. This effect has been measured small for oxides but large for clay minerals and soils (Keren and O’Connor, 1982; Mattigod et al., 1985; Goldberg et al., 1993; Keren and Sparks, 1994). Keren and O’Connor (1982) and Keren and Sparks (1994) argued that negative electrical field associated with the planar surface of clay minerals affects on the B adsorption on edge surfaces. Therefore, an increase in ionic strength suppresses the negative electrical field of planner surface and allow for higher adsorption of B. Furthermore, different types of minerals in soil exhibit different charge and potential behavior that affects on the B adsorption (Keren and O’Connor, 1982; Keren

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and Sparks, 1994). The interaction of B with OM and its components, i.e. HA and FA, is more complicated than its interaction with clays and oxides (Keren and Communar, 2009). OM binds B stronger and larger than clay minerals (Yermiyahu et al., 1995) and at the same time, due to similarity in charge, they may compete with B for adsorption on minerals. This interaction lowers binding of B by minerals (Marzadori et al., 1991; Xu and Peak, 2007). Although the importance of metal (hydr)oxides, OM and clay minerals on boron adsorption has been quantified, however the effect of calcium carbonates still needs to be clarified. Elrashidi and O’Connor (1982) reported that the presence of calcite is not statistically significant in B adsorption. In contrast, a negative correlation between B adsorption and carbonate minerals has been observed by Hingston (1964). On the other hand, Goldberg and Forster (1991) measured relatively high adsorption of B on calcite. Adsorption data provide valuable information for interpretation of ion adsorption in soil and on soil minerals. However, from the perspective of plant nutrition and for evaluation of environmental remediation processes, desorption isotherms are more appropriate. This advantage arises from the fact that adsorption and desorption isotherms of ions usually do not coincide. The observed hysteresis is partly due to the heterogeneity in adsorption affinities and partly due to the pathways through which adsorption and desorption occur (Yin et al., 1997). To increase our understanding of boron adsorption and desorption behavior in calcareous soils, in this study we measured adsorption and desorption of boron in eight soils differed in calcium carbonate content. The effects of removing calcium carbonate, change in ionic strength, and the type of electrolyte solution on boron adsorption were also evaluated.

2. Materials and methods The experimental soil samples were collected from different soil series (0–20 cm) representing eight soils with different amount of calcium carbonate equivalent (CCE). The collected samples were then air dried and passed through a 2-mm sieve, homogenized, and stored in plastic containers. The CCE content was determined by titration method. The reactive fraction of CCE (CCEr) was measured using the ammonium oxalate method. Organic carbon (OC) and particle size distribution were determined by the wet oxidation method of Walkley and Black and hydrometer method, respectively (Page et al., 1982; Klute, 1986). The amount of amorphous and crystalline iron and aluminum in experimental soil samples were determined by ammonium oxalate and dithionite–citrate– bicarbonate (DCB) methods (Coffin, 1963; Schwertmann, 1973). The adsorption experiments were carried out in batch systems. Soil samples (8.12 g) were added to 50 mL polypropylene centrifuge tubes and equilibrated with 25 mL of 0.01 M CaCl2 solution, containing varying concentration of H3BO3. Initial concentration of B ranged from 0 to 10 mM. The centrifuge tubes were equilibrated for 20 h on an end-over-end shaker (180 rpm) in a constant temperature room (25 ± 1 °C). The suspensions were then centrifuged (4000g for 15 min, 25 °C) and the aliquot of clear supernatants were taken and filtered through Whatman No. 42 filter paper. The supernatants were analyzed for B concentration using the colorimetric azomethine-H method (Bingham, 1982), in which B forms a stable complex with azomethine-H at pH 5.1. The solution pH was buffered with glacial acetic acid and ammonium acetate. After stabilization of developed color (15–30 min), absorbance was measured at 420 nm. The equilibrium pH was measured in the remixed remained of the suspensions. No change in pH was observed compares to the control sample. Boron desorption isotherms were measured in soils 2, 3, and 4 for five initial B concentration. The centrifuged residues of adsorption experiments were weighted to determine the amount of oc-

cluded solution, and then re-suspended in 25 mL of boron-free 0.01 M CaCl2. The tubes were shaken for 20 h, centrifuged, and then clear supernatants were removed. This sequence of operations was repeated for four times. Boron concentration was measured in supernatants separately for each desorption step. The desorbed B was calculated for each step taking into account the amount of B remained in the occluded solution. To examine the effect of ionic strength, boron adsorption isotherms were measured in soil 6 in 0.01, 0.10, and 0.50 M NaCl solutions. Furthermore, for evaluation of the importance of carbonate minerals on B adsorption, carbonates were removed from soil 6 using a modification of the procedure described by Kunze and Dixon (1986). The soil sample was washed three times with 0.5 M Naacetate solution, adjusted at pH = 5 by the addition of glacial acetic acid, then washed twice with deionized water, air dried, and passed through a 2-mm sieve. B adsorption in the treated soil was determined similar to the procedure described above. The native adsorbed B in soil samples was estimated from the equilibrium concentration of B in blank samples using the parameterized adsorption isotherm equations. In addition, adsorption isotherm of phosphate was measured in soil 6 assuming a similar adsorption mechanism for boron and phosphorus. In this experiment, 3.12 g soil sample was added to 50 mL polypropylene centrifuge tubes and equilibrated with 25 mL of 0.01 M CaCl2 solution, differing in P concentration (0–0.65 mM). After equilibration and centrifuging, both B and P concentrations were measured in the supernatants. 2.1. Computations and modeling The amount of adsorbed ions was calculated by the difference between the initial and final equilibrium concentration. In the desorption experiments, the B concentration retained by the soil in the suspension after each desorption step was calculated according to (Yin et al., 1997):

Ci ¼ Ci1  ðci  ci1 =nÞ=w

ð1Þ

where Ci and Ci1 are the concentrations of B remaining in the soil at the end of the ith and (i  1)th desorption steps (mmol kg1), ci and ci1 are B concentrations in the solution at the end of the ith and (i  1)th desorption steps (mM), n is dilution factor, and w is the solid concentration in the suspension (kg L1). Experimental adsorption and desorption data were respectively described with the Langmuir (Eq. (2)) and the Linear (Eq. (3)) adsorption isotherm equations.

C ¼ Cmax

KLc 1 þ KLc

C ¼ Kdc þ d

ð2Þ ð3Þ

where C represents the amount of boron adsorbed (mmol kg1), c equilibrium concentration of the B in solution (mM), Cmax maximum adsorption capacity of soil for B (mmol kg1), and KL (L mmol1) and Kd (L kg1) approximations of adsorption energy, and d is a measure of irreversible adsorbed B (mmol kg1). 3. Results and discussion 3.1. Chemical and physical properties of soils Some properties of the experimental soil samples including soil particle size distribution, total calcium carbonate equivalent (CCE), reactive calcium carbonates (CCEr), OC, and amorphous and crystalline iron and aluminum (hydr)oxides are presented in Table 1. The CCE content in soil samples was varied from 3% to 85%, indicat-

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A. Majidi et al. / Chemosphere 80 (2010) 733–739 Table 1 Some chemical and physical properties of soil samples; except pH, data are given in percent.

a b

Soil no.

Soil series

CCE

CCEra

Clay

Silt

OC

pHb

1 2 3 4 5 6 7 8

Fine loamy, mixed, thermic-lithic rendoll Fine loamy, mixed, mesic-vertic calcixerolls Fine loamy, mixed, mesic-fluentic Haploxerolls Coarse loamy, carbonatic, mesic- typic calcixerepts Coarse loamy, mixed thermic-typic torroflents Coarse loamy, mixed thermic-typic torroflents Fine loamy, mixed, thermic-typic haplogypsids Fine loamy, calcareous, mesic-typic xerorthents

0 3.0 3.4 7.1 12 18 30 85

0 0.64 0.48 1.44 2.8 9.2 28.4 9.2

41.7 34 24 32 14.3 28.5 38.8 24.5

53.0 55 47 45 19.4 45.0 45.9 42.8

1.36 1.07 1.07 1.13 0.23 0.23 0.72 0.20

4.8 7.9 7.8 7.8 7.7 7.4 8.1 7.6

Amorphous

Crystalline

Al

Fe

Al

Fe

0.20 0.09 0.05 0.05 0.04 0.03 0.06 <0.01

0.37 0.14 0.15 0.14 0.04 0.06 0.08 0.04

1.20 0.08 0.05 0.02 0.43 0.36 0.49 0.01

2.30 0.63 0.59 0.69 0.43 0.63 0.67 0.10

CCEr, reactive calcium carbonate equivalent. Soil pH was measured in 1–3 solid-solution ratio similar to the adsorption isotherm experiments.

approximately higher than 2 mM, apparent adsorption affinity gradually decreases moving towards an adsorption maximum. At high equilibrium concentrations, soil samples show relatively large differences in the capacity of boron binding. Of the soil samples, the highest and lowest amounts of B adsorption were measured in soil 2 and soil 5, respectively. Soil 2 contained higher amount of reactive particles (clay and OC) than soil 5. Thus, the difference in total reactive surfaces may explain most of the difference in adsorption maximum in these two soils. In addition, soil 2 had the highest pH value (7.9) among soil samples except soil 7. A higher pH value is closer to the pH range in which adsorption maximum has been observed in soil or on minerals (Goldberg, 2004; Lemarchand et al., 2005; Xu and Peak, 2007). In contrast, soil 1 has the lowest pH value (4.8) that may denote on the low B adsorption whereas adsorption maximum in this soil is intermediate among soil samples. This may be related to the relatively high amount of iron and aluminum (hydr)oxides in this soil (Table 1) (Goldberg et al., 1993; Debussetti et al., 1995; Kim and Kirkpatrick, 2006). For description of adsorption data, Freundlich, Langmuir, Toth, and Langmuir–Freundlich (LF) adsorption isotherm equations were used. The first two equations have the advantage of less adjustable parameters, i.e. two parameters in comparison with three parameters in Toth and LF equations. Analysis revealed, from a statistical point of view, the Langmuir equation describes adsorption data better or similar to the other equations. Thus, due to having a thermodynamic basis and fewer adjustable parameters, the Langmuir

ing all soil samples, except soil 1 with CCE  0%, are calcareous. The amount of CCEr, measured by ammonium oxalate method, is an approximate of the reactive surface of carbonate minerals in soil. Due to the presence of Ca2+ and Mg2+ ions in the solution and in exchangeable phase that react with oxalate ions, this method somewhat overestimate CCEr (Loeppert and Suarez, 1996). This source of error was approximated and corrected for soil samples by assuming soil 1 (CCE  0%) as the blank sample. No reasonable relationship was obtained between CCE and CCEr. As an example, the difference in CCE content was determined 67% between soil 6 and soil 8, whereas CCEr was measured equal in these two soil samples. This may suggest that CCEr is related to the particle size distribution of carbonate minerals rather than to the CCE content. The amount of Fe and Al (hydr)oxides in soil samples were found relatively low, ranging from 0.11% in soil 8 to 3.49% in soil 1. Soil 1 has the lowest pH value (4.8) compares to other soil samples that range from 7.4 to 8.1. 3.2. Boron adsorption isotherms Boron adsorption was measured as a function of equilibrium concentration in soils 1–8. The obtained results are presented in Fig. 1. The isotherms virtually comprises of two parts: a high affinity part at low equilibrium concentration, where adsorption increases almost linearly with increasing equilibrium concentration and a plateau where rate of adsorption reduces with an increase in solution concentration (Fig. 1). At equilibrium concentrations

7

adsorbed B, mmol kg-1

Soil 2 6

Soil 4

5

Soil 3 Soil 7

4

Soil 8 3

soil 1

2

Soil 6

1

Soil 5 calculated

0 0

2

4

6

equilibrium B conc, mmol

8

10

L-1

Fig. 1. Experimental boron adsorption data (symbols) with corresponding model description (solid lines) for soils 1–8 using the Langmuir equation.

A. Majidi et al. / Chemosphere 80 (2010) 733–739

Table 2 Adsorption maximum (Cmax)a and ‘‘affinity constant” (KL) of interaction of boron with different soil samples as derived from modeling of adsorption data with the Langmuir equation.

a

Soil no.

Electrolyteb

KL

Cmax

RMSE

R2

1 2 3 4 5 6 7 8

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

0.421 ± 0.064 0.116 ± 0.021 0.308 ± 0.048 0.260 ± 0.03 0.892 ± 0.113 0.533 ± 0.067 0.372 ± 0.086 0.430 ± 0.109

2.292 ± 0.139 11.707 ± 1.352 3.808 ± 0.276 5.753 ± 0.317 1.018 ± 0.040 1.753 ± 0.079 2.902 ± 0.283 2.312 ± 0.232

0.129 0.276 0.148 0.180 0.061 0.088 0.233 0.217

0.97 0.98 0.97 0.98 0.96 0.97 0.92 0.90

After removing CCE 6 0.01

0.952 ± 0.199

1.135 ± 0.071

0.114

0.88

Salt effect 6 0.01 6 0.10 6 0.50

0.358 ± 0.06 0.344 ± 0.067 0.268 ± 0.045

1.658 ± 0.113 2.134 ± 0.173 2.915 ± 0.223

0.065 0.095 0.094

0.98 0.98 0.99

Cmax and KL are respectively based on mmol kg1 and L mmol1.

b

CaCl2 was used to adjust salt concentration; in salt effect experiments NaCl was used instead.

equation was chosen for further description of experimental data. Parameter values derived from modeling of adsorption data by the Langmuir equation for individual soil samples are given in Table 2 and the model descriptions are demonstrated in Fig. 1. Calculating mole fraction for adsorbed B, i.e. h = C/Cmax, indicated that 0.48 (soil 2) to 0.89 (soil 5) of predicted available adsorption sites were occupied by B at maximum loading. This is in line with the above analysis where maximum of adsorption per gram of soil samples (Cmax) was found in soil 2. That is, soil 2 with relatively high amount of reactive particles per gram of soil has the maximum capacity for B adsorption, however at an identical equilibrium concentration; it has the lowest value of h. Boron equilibrium concentration in blank samples was measured to be smaller than 0.05 mM with an exception for soil 7 where it was 0.26 mM. For modeling adsorption data, the B concentration in blank samples assumed to be zero and the data were adjusted accordingly. The amount of boron adsorbed natively in soil samples, Cnat, was calculated back using the parameterized Langmuir model. The performed calculations revealed small values for Cnat that numerically were close to the B equilibrium concentration, i.e. a 1:1 partitioning at very low B concentration. The Cnat was also approximated using a phosphate adsorption isotherm (data are not shown) measured in soil 6 where equilibrium concentrations of both B and P were determined. In this experiment, no change in B equilibrium concentration was observed with change in equilibrium concentration of phosphate. In a similar way, phosphate equilibrium concentration was measured in a few adsorption isotherms of B. Here also no noticeable change in phosphate equilibrium concentration was observed. These facts imply that at the studied conditions, there is no significant adsorption interaction between B and P. Thus, addition of B or P does not influence on the equilibrium concentration of the second element. 3.3. Boron solution speciation in relation to adsorption

is attracted much stronger by the positively charged surface sites. Therefore, boron adsorption is gradually increases with an increase in solution pH. This process is then counteracted by deprotonation of surface groups that lowers the surface potential for the adsorption of B. This type of adsorption is similar to the adsorption of H4SiO4 (Hiemstra et al., 2007), but it is different with other oxyanions such as phosphate (Rahnemaie et al., 2007) and arsenate (Manning and Goldberg, 1996). Thus, one may conclude that in soil 1 (pH = 4.8), boron is mainly adsorbed as outer-sphere surface complex, however in other soil samples (pH > 7.4), it is adsorbed as both outer-sphere and inner-sphere surface complexes depending on the pH value and surface potential. With respect to this analysis, the absence of adsorption interaction between B and P mainly arises from their solution speciation. At the pH range of studied soil samples (<8), B mainly exists as molecule H3 BOo3 and 2 o P as H2 PO 4 and HPO4 . The uncharged H3 BO3 is adsorbed mainly as outer-sphere surface complex while phosphate ions are adsorbed mush stronger by formation of inner-sphere surface complexes. Therefore, it is quite impossible to observe measurable adsorption interaction between these two elements at the studied conditions. On the other hand, one may argue that the weak competition between boron and phosphorus might be related to the low loading that prevent ion competition to be observed experimentally. 3.4. The effect of CCE on boron adsorption An important part of the calcareous soil samples is the minerals of calcium and magnesium carbonates that may have an important effect on B adsorption. To evaluate this factor, carbonates were removed from a sub-sample of soil 6 (CCE = 18%). Boron adsorption isotherm was measured in the treated soil and compared with its adsorption before carbonate removal. The experimental data and model predictions are illustrated in Fig. 2 for these two scenarios. The derived Langmuir model parameters are also given in Table 2. Based on the Langmuir model calculation (Table 2), after removing carbonate minerals B adsorption maximum was reduced to 1.135 mmol kg1; i.e. about 35% reduction in adsorption. Such a decrease in boron adsorption has been observed by Goldberg and Forster (1991). They reported 35% and 15% reduction in B adsorption in soils with respectively 13% and 15% calcium carbonate. Reduction in Cmax is accompanied with almost a double increase in adsorption affinity (KL) that is not actually seen in the experi-

2

adsorbed B, mmol kg-1

736

1 0.01 M NaCl 0.1 0.5 0.01 M CaCl2 CCE removed

0 0

Soil solution pH significantly influences boron adsorption by changing B solution speciation and surface charge. Calculating the solution speciation in a system similar to the solution of soil samples indicated that at pH value less than 7, almost 100% of dissolved B is in the form of H3 BOo3 : This species is dominant up to pH  9.2. Electrostatically, the uncharged molecule H3 BOo3 is not favorable for adsorption by the charged surface sites. Above pH 7, H3 BOo3 is gradually deprotonated and borate ion is formed that

2

4

6

equilibrium B conc, mmol

8

10

L-1

Fig. 2. Boron adsorption isotherms in soil 6 at three levels of NaCl solution in comparison with 0.01 M CaCl2 and with B adsorption in soil after removing carbonate minerals (CCE). Solid lines, dashed line, and dotted line respectively represent description of experimental data as function of NaCl concentration, in the presence of CaCl2 solution, and after removing CCE (in 0.01 M CaCl2) using the Langmuir adsorption equation. Corresponding model parameters are given in Table 2.

A. Majidi et al. / Chemosphere 80 (2010) 733–739

mental data. The discrepancy between the trend of experimental data and the change in the calculated affinity constant can be well explained with a high negative correlation between model parameters (r > 0.8). The affinity constant (KL) in the Langmuir equation is strongly related to the slope of the adsorption isotherm at very low equilibrium concentration. However, the slope gives the product K. Cmax and not just K (Kinniburgh, 1986). The K value can be approximated unequivocally where adsorption maximum is known. As a modeling exercise, one may choose the K value derived for the treated soil sample, i.e. K = 0.95, and apply it for the non-treated experimental data and then optimize adsorption maximum on the experimental data. This calculation leads to Cmax = 1.482 mmol kg1 with a slight decrease in the goodness of fit (R2 = 0.94). Based on this calculation, the change in the adsorption maximum after carbonate removal is about 23%. It is more convenient to have an identical affinity constant for any ion adsorbed in soils regardless of their adsorption maximum (soil type). However, as pointed above, an independent K cannot be derived using the empirical adsorption isotherm equations including the Langmuir equation. Furthermore, soils are very different in physical and chemical properties and even within a particular soil; the solid part is heterogeneous implying that the derived K is an average value for the interaction of ion with different constituents of the soil. Due to the heterogeneity among studied soil samples (Table 1) and high correlation found between the model parameters (r > 0.8), no statistically significant relation was found between the soil properties and the Langmuir model parameters. Added to the complexity among soil samples, as pointed it out above even within a particular soil, there are different reactive surfaces that have their own affinity and capacity for binding of B. Thus, derived values for K and Cmax are averages for interaction with different reactive surfaces. Nevertheless, to have an overview of the effect of soil characteristics on model parameters, it was assumed that B is adsorbed with the same adsorption affinity in all soils, and thus the difference among soil samples was attributed to the Cmax. In this approach, experimental data were successfully described by an extended Langmuir equation (the model prediction is not shown) and the affinity constant, K, and adsorption maximum, Cmax, were optimized on all experimental data of soils 1–8 simultaneously. The value of K was obtained 0.23 and Cmax was found 2.90, 7.91, 4.34, 6.03, 1.57, 2.38, 3.53, and 2.96 for respectively soils 1–8. The derived Cmax with this method showed a positive relation with the amount of reactive particles (clay and OM content). 3.5. Effect of ionic strength on boron adsorption To quantify the effect of ionic strength, boron adsorption isotherms were measured at three NaCl levels in soil 6 as presented in Fig. 2. The derived model parameters are given in Table 2. Experimental data and model prediction using the Langmuir equation denote on the large effect of salt concentration on the boron adsorption, i.e. about 30% and 75% increase in Cmax due to increase of salt concentration from 0.01 to 0.1 and 0.5 M. Salt effect is negligible at very low equilibrium concentration; however, it gradually increases with increasing the equilibrium concentration of boron. The positive effect of salt concentration on boron adsorption can be attributed to a better screening of surface charge on minerals with pH-dependent charge (Rahnemaie et al., 2006b). At pH range lower than the point of zero charge (PZC), salt effect increases the net positive charge on the surface, and above PZC it causes an increase in the negative charge. Therefore, increase in pH gradually decreases adsorption of anions due to reduction in the surface potential (Rahnemaie et al., 2007). An adverse effect is observed for the adsorption of cations. On the other hand,

737

adsorption of omnipresent anions like phosphate, sulfate, and HS lowers the isoelectric point of minerals. Hence, in fertile soils, even in the pH range of 7–8, one may expect a net negative charge on mineral surfaces with relatively high PZC. Therefore, the salt effect may also be due to compaction of DDL in response to increase in salt concentration. A decrease in DDL thickness lowers the electrostatic repulsive forces that in turn enhance B adsorption. For clay minerals, Secor and Radke (1985) using the Poisson–Boltzmann equation and Keren and Sparks (1994) by analyzing B adsorption isotherms have concluded that spillover of negative electrical field associated with the planar surface into the edge surface reduces B adsorption. Increasing ionic strength suppresses the DDL on planar surface and therefore more negative borate ions are able to move close enough to interact with the adsorption sites located on the edge surfaces. Corresponding mole fraction of adsorption data demonstrated in Fig. 2 was calculated. Increase of salt concentration from 0.01 to 0.1 M had no noticeable effect on mole fraction of adsorbed boron, i.e. all the new sites formed due to the increase in salt concentration were occupied by B. However, in the presence of 0.5 M NaCl a systematic reduction (5%) in mole fraction of adsorbed B was observed. Decrease in the ratio of boron to the available adsorption sites and/or adsorption interaction of chloride ions with boron could be the possible descriptive reasons. The former is negligible, since the larger fraction of initial B exists in the solution. However, at high salt concentration chloride ions may compete with boron for surface adsorption sites. In addition, the high accumulation of Cl ions near the surface increases electrostatic repulsion energy. In addition to the salt concentration, a significant difference was found when the type of electrolyte ions was compared at the same molar concentration. Experimental data and model description for boron adsorption in the presence of either 0.01 M NaCl or 0.01 M CaCl2 are demonstrated in Fig. 2. Boron adsorption is much stronger in the presence of CaCl2 solution even at very low equilibrium concentration. Similar to the general effect of ionic strength, this effect may also be partly related to better screening of the surface charge in the presence of calcium than sodium ions as for instance has been observed for goethite (Rahnemaie et al., 2006a). It might also be related to a stronger lowering repulsion forces by reducing double layer thickness (Keren and O’Connor, 1982). 3.6. Boron desorption isotherms Boron desorption isotherms were measured in soil 2, 3, and 4 for five initial B concentrations of 0.93, 1.85, 3.7, 5.9, and 9.3 mM. Desorption data are given in Fig. 3 along with adsorption data. In the first desorption step, a large part of the adsorbed boron was desorbed from the soil samples. However, in steps 2, 3, and 4 relatively a smaller part of the adsorbed B was released. For instance, for an initial loading 0.93 mM B, in the first step 42%, 41%, and 58% of the adsorbed B (respectively 0.96, 0.73, and 0.68 mmol kg1) was removed from soil 2, 3, and 4 (Fig. 3). However, in all steps 2, 3, and 4 respectively 13%, 9% and 12% of the initially adsorbed B was released. Datta and Bhadoria (1999) reported 27–86% desorption for B in some acidic soils. Graphically, the trend of desorption data shows no hysteresis at the higher range of studied equilibrium concentration (Fig. 3). At the lower range (<2 mM), however, desorption isotherms deviate from the adsorption isotherms implying a relatively strong hysteresis effect. The pronounced hysteresis may denote on different types of adsorption mechanism at low and high equilibrium concentration or it may indicates that a small part of adsorbed B is on the sites that are diffused to bulk solution very slowly (Griffin and Burau, 1974) . Using spectroscopic techniques, two types of adsorption mechanisms have been identified for boron adsorption on different min-

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A. Majidi et al. / Chemosphere 80 (2010) 733–739

for each soil in spite of change in the initial adsorbed B. Similarity in the Kd values denotes on a common adsorption behavior, as pointed out above, at the studied range of equilibrium concentration. However, d values were slowly increased with an increase in the initial B concentration. The corresponding mean slope and intercept were calculated for each soil sample, representing in parameterized desorption equations, as given in Eqs. ()()()(4)–(6) for soil samples 2, 3, and 4 respectively.

adsorbed B, mmol kg-1

7 6 5 Soil 2 0.93 mM 1.85 3.70 5.92 9.25 modeled

4 3 2 1 0 0

2

4

6

8

10

adsorbed B, mmol kg -1

3

2

2

4

6

8

10

adsorbed B, mmol kg-1

5 4 3

Soil 4 0.93 mM 1.85 3.70 5.92 9.25 modeled

2 1

2

4

6

8

ð5Þ

C ¼ 0:58c þ 0:56

ð6Þ

There is a large capacity for boron adsorption in calcareous soils. This capacity provides a reliable source of boron for plant nutrition by buffering equilibrium concentration. Ionic strength has a positive effect on boron adsorption; this effect is stronger in the presence of a divalent ion such as calcium than monovalent sodium ions. A large part of the adsorbed B releases relatively easy by diluting equilibrium solution, implying that outer-sphere complexation is the major binding mechanism for B adsorption in calcareous soils. Desorption isotherms denote that accumulated boron in soil can be easily leached out by applying a desirable leaching fraction. References

0 0

C ¼ 0:53c þ 0:24

4. Conclusion

0 0

ð4Þ

The obtained d values are much larger than the calculated natively adsorbed B, implying that soil samples have much larger capacity for adsorption of boron by inner-sphere complexation. It may also suggest that four times desorption with 20 h equilibration time has not been long enough to desorb and bring boron into the solution phase.

Soil 3 0.93 mM 1.85 3.70 5.92 9.25 modeled

1

C ¼ 0:70c þ 0:77

10

equilibrium B conc, mmol L-1 Fig. 3. Boron adsorption (symbol with soil no) and desorption (symbols with initial B loading) isotherms measured in soils 2–4. The solid lines represent the Langmuir model prediction for the adsorption data.

erals; a high affinity inner-sphere complex and a low affinity outersphere complex (Su and Suarez, 1995; Xu et al., 2001; Peak et al., 2003). Taking boron solution speciation (Section 3.3) and spectroscopic proved surface species into account, one may conclude that in studied soils with pH varying from 7 to 8, boron is mainly adsorbed as outer-sphere complex. Thus, in the first desorption step most B is removed from the soil particle surfaces. The rest of B that is bounded stronger desorbed more slowly. In addition, formation of surface precipitates has been claimed for observed hysteresis (Elrashidi and O’Connor, 1982). As can be seen in Fig. 3, the experimental desorption data demonstrate a linear trend and thus cannot be described correctly by the non-linear Langmuir equation similar to the adsorption isotherm data. All desorption isotherms were perfectly (R2  0.99) described using the Linear equation (Eq. (2)). In this equation, the slope Kd represent the desorption affinity and d is a measure of irreversible portion of the adsorbed boron after four successive desorption steps. The calculated Kd values were rather the same

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