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Published online 15 February 2008
Published in Soil Sci Soc Am J 72:492-499 (2008)
DOI: 10.2136/sssaj2007.0086
© 2008 Soil Science Society of America
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SOIL CHEMISTRY

Boron Adsorption by Soils as affected by Dissolved Organic Matter from Treated Sewage Effluent

G. Communar and R. Keren*

Institute of Soil, Water and Environmental Sci., the Volcani Center, Agricultural Research Organization (ARO), P.O. Box 6, Bet Dagan 50250, Israel

* Corresponding author (rkeren{at}agri.gov.il).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 THEORY
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Although it is well known that treated sewage effluent enhances trace elements and nutrient solubility in soil solution through their complexation with dissolved organic matter (DOM), no information is available yet for B. The main purpose of this study was to evaluate the effect of DOM with B and native soil organic matter (OM) on B adsorption by soils. Batch equilibrium studies were conducted to measure the B adsorption by DOM (pH 7.7) that was selected from a municipal sewage plant. Effluent DOM was found to have a low affinity for the soils and its application resulted in a release of native soil OM into solution. The OM release was enhanced significantly by an increase in soil mass/solution volume ratio and effluent DOM concentration. The B adsorption capacity of DOM (294–333 mg kg–1) was less than that found for different humic acids (583–2235.6 mg kg–1). Nevertheless, the presence of DOM reduced the free-B concentration in solution due to formation of B–DOM complexes. As the total DOM concentration increased, the slope of the isotherms for B adsorption by soil decreased. All the B adsorption isotherms obtained for the different DOM concentrations merged into one isotherm, however, when free-B solution concentration was taken into consideration. The results suggest that the B–DOM complex did not interact with the soil.

Abbreviations: DOM, dissolved organic matter • OC, organic carbon • OM, organic matter • SAR, sodium adsorption ratio.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
THEORY
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Boron is an essential nutrient for normal plant growth but its positive influence is restricted by a narrow range of B concentrations in the soil solution; when solution B concentration exceeds a threshold value for the plant, it may be toxic. Since plants are susceptible mainly to soluble B species, their adsorption on soils is considered an important mechanism in maintaining nontoxic B levels (Keren and Bingham, 1985). The amount of B adsorbed by soils varies greatly, depending on soil constituents and solution pH values (Keren et al., 1981; Mezuman and Keren, 1981; Keren and Mezuman, 1981; Goldberg and Glaubig, 1985, 1986).

Boron interacts with a wide variety of ligands, creating inorganic and organic complexes. Therefore, when treated sewage effluents are used for irrigation purposes, the fact that B(OH)3 and B(OH)4– species can form complexes with DOM should be taken into account. Boric acid prefers to combine with carboxyl and hydroxyl groups. These ligand-exchange reactions have approximately the same complexation constants as those found for humic acid (Lemarchand et al., 2005) and prevail at low solution pH (<7.8). Increasing pH from 7.8 to 9.5 decreases B(OH)3 complexation, but increases B(OH)4– adsorption on hydroxyl-organic compounds. With further pH increase above 9.5, there is a sharp decrease in B adsorption by DOM as a result of the competition between B(OH)4– and OH– ions for available adsorption sites. In general, B interaction with DOM exhibits the same pH-dependence phenomenon as found for B adsorption on clay and oxide minerals (Mezuman and Keren, 1981; Keren and Bingham, 1985; Goldberg and Glaubig, 1985, 1986) but with higher adsorption capacity (Yermiyaho et al., 1988, 1995, 2001; Gu and Lowe, 1990; Meyer and Bloom, 1997; Lemarchand et al., 2005).

Treated sewage effluent is an important source of irrigation water in arid and semiarid regions where most of the cultivated soils contain a relatively small fraction of native OM (<1.0%) and mostly in the topsoil layers (5–20 cm). The contribution of this small OM content to B adsorption was found to be insignificant in comparison with B adsorption by clay and oxide minerals present in soils (Mezuman and Keren, 1981; Keren and Bingham, 1985); however, Yermiyaho et al. (1995) and Sharma et al. (2006) found an increase in B adsorption when OM was added to soil. On the contrary, Sarkar and Das (1990) and Marzadori et al. (1991) observed an increase in B adsorption by soil when the OM was completely removed. It is well documented, however, that (i) the interaction of DOM with soil is affected by the presence of OM and hydroxides in the clay fraction particles (McDowell and Wood, 1984; McDowell and Likens, 1988; Jardine et al., 1989; Donald et al., 1993) and (ii) the mobility of DOM depends on its nature since the affinity of the hydrophilic compounds to mineral surfaces is less than that of the hydrophobic compounds (Kaiser et al., 1996; Kaiser and Zech, 1997). Moreover, it was found (McDowell and Likens, 1988; Nodvin et al., 1986; Moore et al., 1992; Neff and Asner, 2001) that the interaction of DOM with soil may result in release of native OM and also trace elements into soil solution; decreased adsorption and enhanced transport of released trace elements due to their complexation with DOM are usually observed in such cases (Moore et al., 1992). Interaction of effluent DOM with native soil OM, B complexation with DOM, and adsorption of B and B–DOM complexes by soil may play an important role in assessing B solution concentration; however, no information on B adsorption and its mobility in soil in the presence of DOM is available yet. Understanding the mechanisms of these interactions will allow us to improve the model for B transport in soil (Communar et al., 2004; Communar and Keren, 2005, 2006). Therefore, the objectives of this study were (i) to estimate the adsorption affinity of DOM from effluent to B, (ii) to evaluate the effect of effluent DOM interaction with native soil OM, and (iii) to evaluate the effect of DOM on B adsorption by soil.


   THEORY
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 THEORY
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Sorption Model
The equilibrium B speciation in solution containing organic ligands (L) can formally be represented by the following reactions (Meyer and Bloom, 1997; Lemarchand et al., 2005):

Formula 1[1]

Formula 2[2]
where βh is the B hydrolysis constant, βL is the B(OH)3 complexation constant, and ai is the activity of species i such as B(OH)3, B(OH)4–, H+, HL–, and B(OH)3L–. Based on these equations, the concentration of inorganic B species [B(OH)3 and B(OH)4–] can be specified as CB = ABCBH, where CBH is the concentration of B(OH)3 and A = βhCH is a constant if the solution H– concentration CH is constant. For sewage effluent, we should consider HL– species as DOM and B(OH)2L– species as dissolved B–DOM complexes. With these definitions, the concentration of B–DOM complexes can be expressed as

Formula 3[3]
where EB-DOM = βL/(1 + A) is the reduced B–DOM complexation constant and CDOM is the solution DOM concentration. In Eq. [3], all the concentrations are in milligrams per liter, so the dimensionality of the complexation constant EB-DOM is liters per milligram. The formation of B–DOM complexes reduces the concentration of free B in solution. In this case, the total B and DOM concentrations in effluent are defined as

Formula 4[4]

Formula 5[5]
Thus, at least a three-component adsorption model can be used to describe B–DOM–soil interactions (Fig. 1 ). This model is similar to those proposed by Brunk et al. (1997) and Totsche and Kögel-Knabner (2004) for multicomponent solutions. We assumed that (i) the affinity of soil to B–DOM complexes is much lower than the free species, (ii) B and DOM species adsorb on different soil sites (without competition), and (iii) the native OM and surface-linked DOM are both available for B adsorption. Under these assumptions, the total mass concentration of B in the adsorbed phase (BBT) is

Formula 6[6]
where b (mg kg–1) is the concentration of B adsorbed by soil minerals and BOM (mg kg–1) is the concentration of B adsorbed by soil native OM and by surface-linked DOM. The concentration b is defined by the equation of Keren et al. (1981), which at a constant soil-solution pH can be written as (Communar et al., 2004)

Formula 7[7]
where bm (mg kg–1) is the soil adsorption capacity for B and kB (L mg–1) is the apparent B adsorption coefficient that depends on the affinity coefficients kHB, kB–, and kOH for the species B(OH)3, B(OH)4–, and OH–, respectively, to soil and pH. When the solution pH is constant, however, the apparent B adsorption coefficient can be considered to be constant. Interaction of B with soil OM (native + surface-linked DOM) can be described as a surface-complexation reaction and, therefore, the concentration BOM in Eq. [6] is defined as

Formula 8[8]
where SOM (mg kg–1) is the concentration of OM in the soil. Assuming that equilibrium exists between effluent DOM and the adsorption sites of the soil's solid phase (native OM and clay minerals), the concentration SOM can be defined as (Huang and Lee, 2001)

Formula 9[9]
where kDOM (L kg–1) is the DOM–soil partitioning coefficient. Note that Eq. [9] is valid at low DOM concentration in soil solution. Alternatively, it is possible to describe B interaction with DOM as (Yermiyaho et al., 1988; Gu and Lowe, 1990; Meyer and Bloom, 1997; Lemarchand et al., 2005):

Formula 10[10]
in which bmDOM (mg kg–1) is the DOM adsorption capacity for B and kB-DOM (L mg–1) is the DOM affinity for B. Although the bmDOM value for B can be evaluated experimentally by using the dialysis bag technique (where the amount of DOM participating in the reaction is known), it is impossible to determine it for native OM present in the soil. Therefore, to describe B adsorption on clay minerals and soil OM sites, we used the following equation obtained from the combination of Eq. [6–9]:

Formula 11[11]


Figure 1
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  		Fig. 1. Schematic representation of the interactions between B, dissolved organic matter (DOM), and soil. The presence of DOM in soil solution leads to formation of nonadsorbed B–DOM complexes. Adsorption of free B and DOM occurs on different soil sites (i.e., with no competition) but a surface-linked DOM is considered as a potential sorbent for B. The DOM–soil interaction results in adsorption of DOM by soil or in release of native organic matter to soil solution.

 

   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 THEORY
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Solutions
The effluents of the treated sewage were sampled from two municipal wastewater treatment plants in Israel: the first one was from the Dan region and is based on an activated sludge technology (DOM-1) and the second one was from the city of Arad and is based on aeration technology (DOM-2). The effluents were filtered using a 0.45-µm filter (Polypure, Pall Corp., East Hills, NY) to exclude all of the suspended materials and were stored in a refrigerator not more than 2 to 3 d before use. The filtered effluents were analyzed for DOM, effluent composition, and pH. The total DOM concentration (mg organic C L–1) and pH were measured immediately after sampling and filtration using a combustion TOC analyzer (Fuji Electronics Co., Tokyo) and a glass electrode, respectively. Some characteristics of the studied sewage effluents are given in Table 1 . The organic C (OC) content in the filtered DOM-1 effluent was in the range of 35 to 45 mg L–1 (average 40 mg L–1) and in the filtered DOM-2 effluent ranged from 80 to 110 mg L–1 (average 95 mg L–1).


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  		Table 1. Composition of filtered effluents.

 
A portion of the filtered effluents was continuously circulated through a Prep/Scale ultrafiltration cartridge with a nominal molecular cutoff of 1000 Da until a sufficient DOM concentration had been reached. The electrical conductance of the concentrated DOM solutions was approximately 7 to 8.5 dS m–1. The dissolved electrolytes were removed by leaching with distilled water and using an ultrafiltration cartridge. This procedure was repeated until the electrical conductance decreased to 0.5 to 0.7 dS m–1. Thereafter, the concentrated DOM samples were freeze-dried and stored in a desiccator. A background solution of CaCl2 + NaCl at a total concentration of 20 mmolc L–1, sodium adsorption ratio (SAR) of 5 and pH of 7.7 was used for the preparation of the DOM solutions.

Soils
A loamy sand soil (Rhodoxeralf) from the coastal plain of Israel and a sandy loam soil (Haploxeralf) were used in this study. These soils are similar to those used in our previous studies (Communar et al., 2004; Communar and Keren, 2006, 2007). The soil samples collected from the surface (0.05–0.25 m) were oven dried (40°C), gently crushed to pass through a 2-mm sieve, thoroughly mixed, and analyzed for clay, silt, sand, free oxides, OM, and CaCO3. The mechanical composition was determined by means of a hydrometer (Klute, 1986), the free oxides by the method of Coffin (1963), the organic matter content (OC) by furnace combustion at 550°C, and the CaCO3 content by a volumetric method (Page et al., 1982). Some characteristics of the soils are listed in Table 2 . Soil pH was determined at a soil/water ratio of 1:2. The predominant clay mineral in the soils was montmorillonite and the weight-based OM content was about 0.55% in the loamy sand soil and 0.75% in the sandy loam soil. All soil analyses were conducted in triplicate.


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  		Table 2. Characteristics of soils used in the experiments.{dagger}

 
Batch Experiments
Boron–Dissolved Organic Matter Adsorption
The B adsorption on DOM (molecular mass >1000) was studied using dialysis bags (Spectrum Laboratories, Rancho Dominguez, CA) with a molecular weight cutoff of 1000 Da. Nine grams of freeze-dried sample of DOM-1 and DOM-2 were dissolved in 150 mL of the background solution to obtain a DOM concentration of 60 g L–1. The DOM solutions were adjusted to pH 7.7 (the average pH of the effluents) using 0.5 mol L–1 HCl or 0.5 mol L–1 OH and were allowed to stand at 24 ± 2°C for 24 h, which was enough time to achieve complete dissolution of the DOM (Lemarchand et al., 2005). Before the dialysis experiments, B and DOM solutions were tested separately to specify the time required for equilibration and to be sure that the DOM concentration in the bags remained constant during the experiments. The dialysis bags containing 5 mL of background solution at a B concentration of 10 mg L–1 and those containing 5 mL of DOM suspension (60 g L–1) were placed into 40-mL glass tubes with 15 mL of the background solution and the tubes were gently agitated on a platform shaker for 2 d. Data obtained from these test experiments indicated (i) that B concentrations inside and outside of the bags were equal and (ii) that release of DOM from the bags to the background solution was negligible. All of the subsequent dialysis-bag sorption experiments were performed at the same bag volume/solution volume ratio as used in the preliminary tests. The dialysis bags with aliquots (5 mL) of pH-adjusted DOM suspension (60 g L–1) were combined with 15 mL of the background solution at a B concentration ranging from 1 to 35 mg L–1. An equilibration time of 2 d was used throughout the experiments (set up in triplicate). At the end of experiments, the solution pH was measured and B concentration in the outside solution was determined by inductivity coupled plasma atomic emission spectroscopy (ICP–AES).

Dissolved Organic Matter–Soil Partitioning
Partitioning of the DOM–soil–solution system was determined by combining 35 mL of the background solution or filtered sewage effluents (DOM-1 at 40 mg OC L–1 and DOM-2 at 95 mg OC L–1) with varying amounts of dried soil (1, 2, 3, 6, 9, 15, and 20 g) in 50-mL polypropylene centrifuge tubes. The use of differing amounts of soil was related to the dependence of DOM–soil partitioning on the soil mass (ms) to solution volume (vs) ratio. Such dependence is a result of native OM presence in soils (Nodvin et al., 1986; Vance and David, 1992; Moore et al., 1992; Neff and Asner, 2001). Preliminary experiments conducted at a maximal ms/vs ratio of 0.57 kg L–1 indicated that a time of 24 h was sufficient to achieve equilibrium in all of the soil–solution systems; no changes in solution OC concentrations were observed from 6 to 24 h. Therefore, an equilibrium time of 24 h was chosen in the present study. The suspensions were gently agitated on a reciprocal shaker at 24 ± 2°C. Following equilibration, the suspensions were centrifuged for 30 min at 3000 rpm (1982 x g), filtered through membrane filters with a 0.45-µm pore size, and separate aliquots of the supernatant were analyzed for C-based DOM concentration by means of a combustion TOC analyzer. Since the pH of the solutions (7.6–7.8) differed only slightly from the soil pH (7.2–7.4), there were no substantial differences in final pH values (7.4–7.5) established after equilibration. All the DOM–soil partitioning experiments were done in triplicate for each treatment. The difference between analyses for OC in the three replicates was always <10%.

To assess the influence of soil native OM on DOM–soil partitioning, additional experiments were conducted using the soils preliminarily leached with the background solution. The samples of the loamy sand and sandy loam soils (1, 2, 3, 6, 9, 15, and 20 g) weighed into 50-mL polypropylene centrifuge tubes were leached with the background solution to extract native OM; soil sample leaching continued until the OC concentration in the leachate was <2 to 5 mg OC L–1. Thereafter, 35 mL of the DOM-1 and DOM-2 effluents were added to the leached soil samples and adsorption studies were performed in the same manner as described previously.

Boron Adsorption by Soils
Boron adsorption by soils was studied by shaking 15 g of whole loamy sand and sandy loam soils in 50-mL polypropylene centrifuge tubes containing 35 mL of the background solution and filtered sewage effluents (DOM-1 at 40 mg OC L–1 and DOM-2 at 95 mg OC L–1) with a B concentration ranging from 1 to 12 mg L–1. The apparent B adsorption equilibrium was achieved within the first several hours as determined by kinetic experiments; however, a typical equilibration time of 1 d was used throughout all batch experiments. The batch experiments were performed in triplicate at 24 ± 2°C. After equilibration, the solution pH was measured and the supernatant obtained after centrifugation and filtration was analyzed for B and DOM. Boron was determined by means of ICP–AES and OC concentration was determined by using a combustion TOC analyzer.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 THEORY
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Boron Adsorption by Dissolved Organic Matter
The B adsorption isotherms for DOM (pH 7.7) obtained from the dialysis experiments are shown in Fig. 2 . At any given B solution concentration, the amount of adsorbed B was somewhat higher for DOM-1 than for DOM-2. The Langmuir equation (Eq. [11]) fitted well the experimental results for both DOM suspensions (with correlation coefficients r2 ~ 0.975–0.985 at P < 0.01). The coefficients bmDOM and kB-DOM estimated from the linear plot of the Langmuir equation (CB/BDOM vs. CB) are given in Table 3 . The maximum B adsorption and the affinity of DOM-1 for B (bmDOM = 333mg kg–1, kB-DOM = 0.012 L mg–1) were slightly different from those obtained for DOM-2 (bmDOM = 294 mg kg–1, kB-DOM = 0.009 L mg–1). The adsorption capacities of DOM for B obtained in our study were lower than the capacity of 583 mg kg–1 found for OM containing humic and fulvic acid fractions at ratios of 74 and 26, respectively (Yermiyaho et al., 1988) as well as the values (712–2235.6 mg kg–1) reported by Gu and Lowe (1990), Meyer and Bloom (1997), and Lemarchand et al. (2005) for different humic acid samples. The smaller adsorption capacity of DOM for B is probably due to the relatively small content of humic acid in the effluents. The binding of Ca+2 (from the background solution with SAR of 5) on carboxylic groups could also decrease the number of protons in DOM compounds, as emphasized by Lemarchand et al. (2005). On the other hand, the data shown in Fig. 2 clearly indicated that the presence of DOM could reduce the free-B concentration in solution due to formation of B–DOM complexes.


Figure 2
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  		Fig. 2. Boron concentration adsorbed on the flocculated dissolved organic matter (DOM) from sewage as a function of aqueous B concentration (Langmuir adsorption isotherm) at pH = 7.7 and at 24 ± 2°C. The symbols represent the experimental data. Errors bars take into account an uncertainty in B chemical analysis. The dashed lines were calculated using the Langmuir equation (Eq. [10]) and the DOM adsorption capacity for B, bmDOM, and the DOM affinity for B, kB-DOM, of Table 3.

 

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  		Table 3. Parameters of the Langmuir equation for B adsorption on dissolved organic matter (DOM) suspensions. Standard deviations are given in parentheses.

 
Dissolved Organic Matter–Soil Interaction
The amount of OM released from dried loamy sand and sandy loam soils into the background solution is illustrated in Fig. 3 . Each point of the plot corresponds to a given soil mass/solution volume ratio (ms/vs). The released DOM concentration increased linearly with the increase of ms/vs ratio. At ms/vs value of 0.57 kg L–1, the released DOM concentration was 30 mg L–1 for the loamy sand soil and 61 mg L–1 for the sandy loam soil. A good linear correlation (r2 ~ 0.98–0.99 with P < 0.01) between the amount of released OM (vsCDOM/ms) and solution DOM concentration (cDOM) was found for the two soils exposed to the background solution. From this plot, it is possible to obtain the potential amount of OM (SOMin) that can be released from the soil into the background solution and the DOM partitioning coefficient (kDOM) can be estimated by fitting the following equation to the data:

Formula 12[12]
The values for SOMin and kDOM are given in Table 4 . Although the average OM content of the two soils was approximately the same (~0.6%), the SDOMin value (144 mg kg–1) for the sandy loam soil with 17.8% clay content was approximately twice the value (68 mg kg–1) of the loamy sand soil with 6% clay content. On the contrary, the values for DOM–soil partitioning coefficients kDOM were close for both soils. Therefore, a linear isotherm (Eq. [9]) using an average value of kDOM (0.575 L kg–1) described successfully all of the experimental points plotted in Fig. 3, where the concentration of adsorbed DOM (SOM) is specified as SOM = (SOMin – vsCDOM/ms).


Figure 3
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  		Fig. 3. Concentration of dissolved organic matter (DOM) as organic C (OC) released from whole loamy sand and sandy loam soils by background solution (Na adsorption ratio = 5.0, pH = 7.7). The symbols represent the experimental data. The dashed lines represent fitting Eq. [12] to the data to obtain the DOM–soil distribution coefficient kDOM and initial reactive soil pool SDOM values given in Table 4. The solid line was calculated using Eq. [9] and kDOM value of Table 4.

 

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  		Table 4. Dissolved organic matter (DOM)–soil distribution coefficients. Standard deviations are given in parentheses.

 
In general, the application of effluents enhanced the release of native OM to solution for both soils (Fig. 4 ), but a greater impact was observed for the sandy loam soil. For both soils, the concentration of OM released by effluent DOM-1 (Fig. 4a) was smaller than that obtained for effluent DOM-2 (Fig. 4b). The plots vs(CDOM0 – CDOM)/ms vs. CDOM (Fig. 4a and 4b) were strongly nonlinear, indicating that the presence of native OM influences the DOM–soil partitioning. For those systems in which DOM–soil interactions occurred in the presence of native OM, it was necessary to apply the initial mass (IM) isotherm approach (Nodvin et al., 1986) to specify the amount of OC that could readily be exchanged with the added DOM. The usefulness of IM isotherms in DOM adsorption or desorption studies was demonstrated by several investigators (Vance and David, 1992; Moore et al., 1992; Neff and Asner, 2001). In our case, when DOM–soil partitioning batch experiments were conducted at varying ms/vs ratios and a constant initial DOM concentration (CDOM0) in the effluent, the IM isotherm could be written as

Formula 13[13]
where m and {Delta} = (1 – m)(SDOMin/CDOM0) are the intercept and the slope, respectively, of a linear regression (Eq. [13]). The intercept (m) and slope ({Delta}) values were used to calculate the reactive soil pool for DOM

Formula 14[14]
The intercept of the regression line on the ms/vs axis (at null-point concentration) was used to specify the ms/vs ratio at which there was no DOM adsorption or OM release from the soil.


Figure 4
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  		Fig. 4. Amount of dissolved organic matter (DOM) released from whole loamy sand and sandy loam soils as a function of DOM concentration as organic C (OC) for (a) DOM-1 and (b) DOM-2 effluents, and and linear regression (Eq. [13]) of relative DOM concentration (CDOM/CDOM0) vs. soil mass/solution volume ratio (ms/vs) for (c) DOM-1 and (d) DOM-2 effluents. The slope ({Delta}) and the intercept (m) values of linear regressions are given in Table 5. Errors bars indicate standard deviations in DOM analysis.

 

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  		Table 5. Parameters of initial mass isotherm for dissolved organic matter (DOM)–soil interaction. Standard deviations are given in parentheses.

 
All the IM isotherms (Fig. 4c and 4d) showed a linear relationship between changes in DOM concentration vs. soil mass/solution volume ratio; OM concentration released by effluents increased linearly with increasing ms/vs ratio from 0.0286 to 0.57 kg L–1. For both soils exposed to effluents, null-point concentrations matched the smallest ms/vs ratio of 0.0286 kg L–1, indicating that the release of native OM prevailed over DOM adsorption in these soil–solution systems. Similar to the background solution, the SDOMin values (Table 5 , calculated from Eq. [14]) exhibited the same dependency on the clay content; the SDOMin values (76–178 mg kg–1) for the sandy loam soil were smaller than those obtained for the loamy sand soil (120–232.29 mg kg–1) and for both soils the SDOMin was higher for DOM-2 than for DOM-1. It indicated that native OM present in these soils behaved as DOM and that under competitive interaction, the release of OM prevailed over the adsorption of DOM. All other conditions being equal, such a prevalence of OM release considerably depended on the ms/vs ratio and, therefore, one could expect that DOM concentrations forming as the effluents moved through dried soils could be higher than those measured under batch conditions (at a relatively low ms/vs ratio). The intercept of the IM isotherm m was considered to be a measure of DOM affinity to soil (Nodvin et al., 1986) and is commonly used in calculation of the DOM–soil distribution coefficient kDOM from

Formula 15[15]
Because such coefficients appear to depend on the vs/ms ratio, however, some correction of kDOM obtained from Eq. [15] is required before its use in DOM transport simulations.

Leaching the soils with background solution before the effluent application resulted in increased adsorption of DOM (Fig. 5 ). Since all of the plots of CDOM0/CDOM – 1 vs. ms/vs were linear at various combinations of sewage effluents and soils, the values of kDOM for DOM-1 and DOM-2 adsorption were estimated with the following equation assuming that the effect of residual OM on DOM–soil partitioning was negligible:

Formula 16[16]
The calculated adsorption coefficient, kDOM, values are presented in Table 4. One can see that the values are close to those obtained for the OM-releasing process. The low values of kDOM suggest that DOM adsorption by soils proceeded mainly through hydrophobic and Van der Waals interactions of carboxylic and phenolic groups present in the DOM molecules and in native soil OM (Jardine et al., 1989). It is also possible that residual OM not removed from the soils by the background solution could influence the DOM–soil partitioning (McDowell and Wood, 1984; Moore et al., 1992). The fact that removal of OM from a subsurface soil with H2O2 increased the DOM adsorption significantly (Jardine et al., 1989) may support this hypothesis.


Figure 5
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  		Fig. 5. Representative isotherms for dissolved organic matter (DOM) adsorption by the loamy sand and sandy loam soils leached with the background solution (Na adsorption ratio = 5.0, pH = 7.7). The symbols represent the experimental data. The solid line was calculated using Eq. [16] and the DOM–soil distribution coefficient values, kDOM, given in Table 4 for DOM adsorption. Error bars indicate standard deviations in DOM analysis.

 
Boron Adsorption by Soils
The isotherms of B adsorption on dried, sieved loamy sand and sandy loam soils exposed to the background solution and DOM-1 (40 mg OC L–1) and DOM-2 (95 mg OC L–1) effluents are shown in Fig. 6a and Fig. 7a , respectively. The sandy loam soil, containing more clay minerals, adsorbed more B than the loamy sand soil but for each soil, B adsorption decreased as the DOM concentration increased.


Figure 6
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  		Fig. 6. Boron adsorption isotherm for the loamy sand soil at different dissolved organic matter (DOM) concentrations: (a) adsorbed B vs. total B solution concentration and (b) adsorbed B vs. free-B concentration in solution. The symbols represent the experimental data. Errors bars indicate standard deviations. The lines were calculated using Eq. [17] and [18] and the adsorption coefficients bm, kB, and EB-DOM of Table 6.

 

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  		Table 6. Parameters of the Langmuir equation{dagger} for B adsorption on soil at various dissolved organic matter (DOM) concentrations.

 

Figure 7
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  		Fig. 7. Boron adsorption isotherm for the sandy loam soil at different dissolved organic matter (DOM) concentrations: (a) adsorbed B vs. total B solution concentration and (b) adsorbed B vs. free-B concentration in solution. The symbols represent the experimental data. Errors bars indicate standard deviations. The lines were calculated using Eq. [17] and [18] and the soil adsorption capacity for B, bm, the B affinity coefficient, kB, and the B–DOM complexation coefficient, EB-DOM, of Table 6.

 
Note that there is not a common opinion on the influence of native OM and DOM on B adsorption, and data obtained by various investigators are often ambiguous. For example, Mezuman and Keren (1981) found no changes in B adsorption after removal of about 50% of the native OM from soil, whereas Sarkar and Das (1990) and Marzadori et al. (1991) observed an increase of B adsorption when OM was completely removed from soils. Alternatively, Yermiyaho et al. (1995) and Sharma et al. (2006) observed an increase in B adsorption when OM was added to soils. In our case, DOM rather than a solid OM phase was added to the soils and release of OM rather than DOM adsorption took place on the interaction of effluents with the whole soils. The DOM concentrations measured during B adsorption experiments (Table 6 ) were the same as found for DOM–soil partitioning experiments at a ms/vs ratio of 0.428 (see Fig. 3 and 4).

The reduction in B adsorption with an increase in DOM concentration (Fig. 6a and 7a) may support the assumption that B–DOM complexes do not interact with the soils. Combining Eq. [3] and [4], a free-B solution concentration can be defined as CB = CBT(1 + EB-DOMCDOM)–1 and substituting this expression for CB in the Langmuir equation (Eq. [7]) yields

Formula 17[17]
where kB* is the apparent B adsorption coefficient, being a function of DOM concentration:

Formula 18[18]
Thus, to assess the effect of DOM on B adsorption by soil, one should know the B–DOM complexation constant.

A two-step procedure was used to determine the values bm, kB, and EB-DOM from the experimental data shown in Fig. 6a and 7a. First, the adsorption coefficients bm and kB* were estimated from the linear plot of CBT/b vs. CBT (obtained from Eq. [17]). The solid lines in Fig. 6a and 7a were calculated using Eq. [17] and the fitted values bm and kB* given in Table 6. As expected, the bm value for the loamy sand soil (19.6 mg kg–1) was lower than that obtained for the sandy loam soil (31.2 mg kg–1), whereas the B adsorption capacity of the two soils was not affected by DOM. The values of B adsorption capacity obtained in this study are similar to those obtained by Communar and Keren (2006) for the same soils leached with the background solution. This supports the finding that soluble OM does not affect the adsorption capacity of the tested soils for B. On the contrary, the fitted kB* values decreased as DOM concentration increased (Table 6). Plotting 1/kB* vs. CDOM (Eq. [18]), one can obtain the B affinity coefficients (kB = 0.0620 L mg–1 for the loamy sand soil and kB = 0.0552 L mg–1 for the sandy loam soil) for zero DOM concentration and the values for the B–DOM complexation coefficient (EB-DOM = 0.005 L mg–1). Using the fitted values for bm, kB, and EB-DOM (Table 6), Eq. [17] and [18] allow reproduction of the B isotherms in terms of free-B solution concentration. In this case, all of the B isotherms obtained at different DOM concentration merged into one isotherm, as shown in Fig. 6b and 7b. These results also support the above-mentioned assumption that B–DOM complexes do not interact with the soils. Thus, irrigation with treated municipal wastewater effluent reduced B adsorption by soil due to the formation of nonadsorbing B complexes with DOM from effluent and that released from native soil OM.

Note that EB-DOM = 0.005 L mg–1 obtained for the mixture of effluent DOM with OM compounds released from the soils was approximately half of the average value for kB-DOM (0.010 L mg–1) obtained from the dialysis-bag experiments. Such a difference between EB-DOM and kB-DOM coefficients is probably caused by differences in B affinity for DOM released from native soil OM and DOM from effluents, i.e., the affinity for DOM released from soils was somewhat smaller than for effluent DOM.


   ACKNOWLEDGMENTS
 
This research was supported in part by GIF, German–Israeli Foundation, and a grant from the Chief Scientist, Ministry of Agriculture and Rural Development. We would like to thank Ms. Ludmila Tsechansky for technical assistance with the analysis throughout this project.


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Received for publication February 27, 2007.


   REFERENCES
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