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TECHNICAL REPORTS

Waste Management

Evaluation of Soils for Use as Liner Materials

A Soil Chemistry Approach

^a Department of Agronomy, Throckmorton Hall, Kansas State University, Manhattan, KS 66506
^b Now with the USDA-ARS National Soil Tilth Laboratory, Ames, IA 50011

^* Corresponding author (gmp{at}ksu.edu)

Received for publication July 30, 2004.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS THEORETICAL RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Movement of NH₄⁺ below animal waste lagoons is generally a function of the whole-lagoon seepage rate, soil mineralogy, cations in the lagoon liquor, and selectivity for NH₄⁺ on the soil-exchange sites. Binary exchange reactions (Ca²⁺–K⁺, Ca²⁺–NH₄⁺, and K⁺–NH₄⁺) were conducted on two soils from the Great Plains and with combinations of these soils with bentonite or zeolite added. Binary exchanges were used to predict ternary exchanges following the Rothmund–Kornfeld approach and Gaines–Thomas convention. Potassium and NH₄⁺ were preferred over Ca²⁺, and K⁺ was preferred over NH₄⁺ in all soils and soils with amendments. Generally, the addition of bentonite did not change cation selectivity over the native soils, whereas the addition of zeolite did. The Rothmund–Kornfeld approach worked well for predicting equivalent fractions of cations on the exchanger phase when only ternary-solution phase compositions were known. Actual swine- and cattle-lagoon solution compositions and the Rothmund–Kornfeld approach were used to project that native soils are predicted to retain 53 and 23%, respectively, of the downward-moving NH₄⁺ on their exchange sites. Additions of bentonite or zeolite to soils under swine lagoons may only slightly improve the equivalent fraction of NH₄⁺ on the exchange sites. Although additions of bentonite or zeolite may not help increase the NH₄⁺ selectivity of a liner material, increases in the overall cation exchange capacity (CEC) of a soil will ultimately decrease the amount of soil needed to adsorb downward-moving NH₄⁺.

Abbreviations: CEC, cation exchange capacity • EFA, equivalent fraction of ammonium • G, Gibb's free energy of exchange • RPD, relative percent difference • S1, Soil 1 • S1B, Soil 1 + bentonite • S1Z, Soil 1 + zeolite • S2, Soil 2 • S2B, Soil 2 + bentonite • S2Z, Soil 2 + zeolite

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS THEORETICAL RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
SOIL-LINED EARTHEN BASINS are often used to store and treat wastes from animal-feeding operations in the Great Plains. These lagoons range in size from 0.5 to 2.5 ha, with a depth of up to 6 m, and generally have compacted soil liners from 0.3 to 0.6 m thick (Ham and DeSutter, 2000). Liners may be constructed using soil present on-site or soil may be brought in from an external source. Bentonite may be added to the liner to reduce hydraulic conductivity. Historically, these soils have only been evaluated on their ability to restrict water movement and, although suggestions have been made that the CEC of the liner material should be included in the liner-evaluation procedure (Barrington and Madramootoo, 1989; Ham and DeSutter, 2000), chemical properties of the liner materials have gone largely unevaluated.

Recent reports have indicated that a substantial amount of NH₄–N can move into the soil underlying animal waste lagoons (DeSutter et al., unpublished data, 2005; Ham and DeSutter, 1999, 2000). Movement of NH₄⁺ into the soil is dependent on the whole-lagoon seepage rate and the concentration of NH₄⁺ in the liquor (Ham and DeSutter, 1999, 2000). Further movement of NH₄⁺ into the underlying soil also would be a function of the mineralogy and subsequent CEC of the soil (Barrington and Madramootoo, 1989; Ham and DeSutter, 2000) and the effect of competing cations for exchange sites (Lance, 1972).

Movement of NH₄⁺ into the soils underlying lagoons has been investigated by DeSutter et al. (unpublished data, 2005), Ham (2002), Ham and DeSutter (2000), Maule and Fonstad (1996), Miller et al. (1976), Parker et al. (1995), and Perschke and Wright (1998). In most instances, NH₄⁺ concentrations were greatest near the lagoon floor and decreased with depth beneath the lagoon. Although most NH₄⁺ concentrations decreased with depth, NH₄⁺ concentrations were still above expected background levels at the lowermost sampling interval indicating that the true depth of NH₄⁺ movement was not reached by sampling (DeSutter et al., unpublished data, 2005; Ham, 2002; Ham and DeSutter, 2000; Miller et al., 1976; Parker et al., 1995; Perschke and Wright, 1998).

Ham and DeSutter (1999) developed an export model to predict the depth of soil (Z_eq), in m, needed to adsorb 100% of the downward moving NH₄⁺ from animal waste lagoons:

[1]

where [NH₄⁺] is the ammonium charge concentration (cmol_c m^–3) in the lagoon liquor; S_t is the total depth of seepage over the time period of interest (m); _b is the bulk density of the soil (kg m^–3); CEC is the cation exchange capacity of the soil (cmol_c kg^–1); and EFA is the equivalent fraction of NH₄⁺ on the soil's exchange sites. Although Ham and DeSutter (2000) used an EFA of 0.87 based on the work of Lance (1972), research suggests that the actual EFA in the upper 0.5 m of cattle and swine lagoons may actually be only as much as 0.22 and 0.44, respectively (DeSutter et al., unpublished data, 2005). The EFA of a soil is a function of the selectivity of the soil for NH₄⁺ relative to that of other cations (i.e., Ca²⁺, Mg²⁺, K⁺, and Na⁺) in the waste for exchange sites (Ham, 2002).

The model was used to predict Z_eq, in cm, and compare differing scenarios of CEC and EFA when using a constant concentration of NH₄⁺ in the waste of 910 mg NH₄–N L^–1 (6500 cmol_c m^–3), a bulk density of 1400 kg m^–3, a seepage rate of 1.1 mm d^–1, and a time of one year (Fig. 1) . If a soil has an EFA of 0.1 and CEC of 10 cmol_c kg^–1, about 180 cm of soil is needed to adsorb 100% of the downward-moving NH₄⁺ per year, compared with 40 cm of soil if the EFA is 0.5. Also, a 75-cm reduction in Z_eq is achieved when the EFA of a 20 cmol_c kg^–3 soil is increased from 0.1 to 0.5. The model and Fig. 1 clearly show that increasing the EFA, CEC, or both can greatly reduce the amount of soil needed to adsorb downward-moving NH₄⁺. Therefore, when evaluating soils for use as liners, choosing soils or soil amendments that improve protection of ground water quality by way of soil chemical properties should be considered. Thus, the objectives of this research were to (i) develop the methodology to evaluate two soils on their ability to selectively adsorb NH₄⁺ by using binary (Ca²⁺–K⁺, Ca²⁺–NH₄⁺, and K⁺–NH₄⁺) and ternary exchange (Ca²⁺–K⁺–NH₄⁺) reactions; (ii) evaluate these soils with amendments of bentonite or zeolite; and (iii) use the ternary model and information from actual swine- and cattle-waste lagoons to predict the depth of soil needed to adsorb 100% of the downward-moving NH₄⁺.

View larger version (22K): [in this window] [in a new window]   Fig. 1. Simulation of the depth of soil (Zeq; Eq. [1]) needed to adsorb 100% of the NH4+ moving into the underlying soil of a swine-waste lagoon under various soil conditions after 1 yr. Bd, bulk density of the soil; St, whole-lagoon seepage rate; CEC, cation exchange capacity.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS THEORETICAL RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

The <2.0-mm fractions of S1 and S2 were analyzed for sand, silt, and clay following the pipet method outlined in Gee and Bauder (1986). The soils were pretreated in an 80°C water bath with 30% H₂O₂ and 1 M NaOAc to remove organic matter and carbonates before the determination of the clay concentration. The difference between 1000 g kg^–1 and the sum of the sand (>53 µm) and silt (2–53 µm) concentrations was determined to be the concentration of clay (<2 µm). Concentrations of sand, silt, and clay in the Soil 1 + bentonite (S1B), Soil 1 + zeolite (S1Z), Soil 2 + bentonite (S2B), and Soil 2 + zeolite (S2Z) soils were determined by using a modified hydrometer method of Gee and Bauder (1986). The clay fraction was determined by taking a hydrometer reading at a time determined by Stokes' law (about 8 h) after mixing. After the clay fraction was determined, the sand fraction was separated with a 53-µm sieve, and the mass of sand was determined by gravimetric methods after drying at 105°C for 24 h. Silt was determined to be the difference between 1000 g kg^–1 and the sum of the clay and sand fractions.

Mineralogies of the clay fraction of S1 and S2 were determined by using X-ray diffraction (XRD) techniques outlined in Ransom et al. (1988). The clay fractions from both S1 and S2 were composed of mica–smectite–kaolinite assemblages. Specific surface areas of the Mg-saturated clay fractions of S1 and S2 were determined according to a modified ethylene glycol monoethyl ether (EGME) method outlined in Carter et al. (1986) in which clays remained Mg-saturated instead of being Ca-saturated. Total C was determined by combustion (CN2000; LECO, St. Joseph, MI). Saturated hydraulic conductivities were determined once for each soil and soil plus amendment following ASTM Designation D 5084-90 (American Society for Testing and Materials, 1991).

Binary-exchange reactions were conducted on Ca- or K-saturated exchangers. One gram of the <1.0-mm size fraction of each exchanger was equilibrated with 4 L of a solution containing varying equivalences of Ca²⁺ and K⁺, Ca²⁺ and NH₄⁺, and K⁺ and NH₄⁺ (Tables 1 and 2) using a flow-through approach. Solutions were prepared using gravimetric methods from 2 M stock solutions of chloride salts and deionized water while maintaining a constant ionic strength of 0.05 mol L^–1. The flow-through system consisted of a 47-mm Teflon filter apparatus equipped with a 45-µm filter, peristaltic pump, and Mariotte bottle. For S1 and S2, eleven solutions were used for each binary-exchange reaction. Based on the information provided from the S1 and S2 reactions, S1B, S1Z, S2B, and S2Z were only equilibrated with five solutions. All equilibrations with exchangers were repeated twice and were conducted at 25°C. A 50-mL aliquot of each solution was removed before exposure with the exchanger and was saved for analysis. After equilibrations were achieved, each sample was washed with 100 mL of 95% ethanol. Cations were extracted from a 0.5-g subsample of each exchanger for 30 min using 10 mL of 0.5 M Na-acetate. Calcium and K⁺ concentrations in the solutions and the 0.5 M Na-acetate extract were determined with inductively coupled plasma–atomic emission spectroscopy (Accuris 141; Fisons Instruments, Beverly, MA). Ammonium N concentrations from solutions and exchanger extracts were determined by the salicylate–hypochlorite method (Crooke and Simpson, 1971) and were analyzed with an autoanalyzer (Alpkem, Clackamas, OR). The CEC value of each exchanger (cmol_c kg^–1) after equilibration with a solution was determined to be the sum of the charges of the two cations from the binary reactions. Individual CEC values of all the binary reactions from respective exchangers were summed and divided by the total number of binary reactions to derive the average CEC for each exchanger.

View this table: [in this window] [in a new window]   Table 1. Binary-exchange (Ca–K, Ca–NH4, and K–NH4) solution and exchanger compositions for Soil 1 (S1) and Soil 2 (S2).

View this table: [in this window] [in a new window]   Table 2. Binary-exchange (Ca–K, Ca–NH4, and K–NH4) solution and exchanger compositions for Soil 1 + bentonite (S1B), Soil 1 + zeolite (S1Z), Soil 2 + bentonite (S2B), and Soil 2 + zeolite (S2Z¶).

	THEORETICAL

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS THEORETICAL RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

[2]

where z_i is the valence of cation I_i and (exch) and (aq) are the exchanger and solution phases, respectively, the Vanselow selectivity coefficients (K_V) and equivalent fractions of cations on the exchanger (N_i) were computed for each binary exchange reaction as defined by Sparks (2003). Solution concentrations were used to calculate solution-phase ionic strengths and the Davies equation was used to calculate the single-ion activity coefficients used to estimate activities from concentrations (Sparks, 2003). Binary-exchange isotherms were constructed according to the approach outlined in Sparks (2003), in which the equivalent fraction on the exchanger phase was plotted as a function of the equivalent fraction in the solution phase. Each exchange isotherm is the average of the two replications. Nonpreference exchange isotherms for the divalent–univalent mixtures were calculated according to the approach outlined in Sposito (2000). Nonpreference isotherms for the univalent–univalent exchanges were 1:1 lines. Gibb's free energies (G_ex⁰) were calculated for each binary-exchange reaction from:

[3]

and:

[4]

where K_ex is the equilibrium exchange constant; R is equal to 8.314 J mol^–1 K^–1; and T (K) is the reaction temperature (Sparks, 2003). Each reported Gibb's free-energy value is the average of two replications.

The Rothmund–Kornfeld approach outlined in Bond (1995) and Bond and Verburg (1997) was followed, and ternary exchange between Ca²⁺–K⁺–NH₄⁺ was predicted from Gaines–Thomas binary-exchange reactions (Ca²⁺–K⁺, Ca²⁺–NH₄⁺, and K⁺–NH₄⁺). The Gaines–Thomas working form of the Rothmund–Kornfeld equation is:

[5]

where _i is the activity coefficient of cation I_i in the solution phase; C is the concentration of cation I_i in the solution phase in mol L^–1; and k_ij and n_ij are the Rothmund–Kornfeld empirical exchange constants. Using the linear model:

[6]

the Rothmund–Kornfeld empirical exchange constants were determined by fitting the binary data to a plot of:

[7]

where log k_ij and n_ij are the intercept and slope, respectively. The empirical exchange constants are then used to solve for the predicted ratios of cations on the exchanger phase N_K/N_NH4, N_Ca/N²_K, and N_Ca/N²_NH4 by:

[8a]

[8b]

[8c]

where A is the solution phase activity (_iC_i) in a ternary solution. Predicted equivalent fractions of each cation on the exchanger phase are then determined by describing the mass balance of the exchanger phase as:

[9]

and solving the following quadratic equations:

[10a]

[10b]

[10c]

where = N_K/N_NH4, ß = N_Ca/N²_K, and = N_Ca/N²_NH4. Through the use of Eq. [9] and [10], there are n + 1 equations for n unknowns, which makes it possible to calculate the three equivalent fractions of cations on the exchanger using only Eq. [9] and two of Eq. [10] (Bond and Verburg, 1997). Validity of the model was evaluated by equilibrating the exchanger phases with a ternary solution containing equal equivalences of Ca²⁺, K⁺, and NH₄⁺. Also, each exchanger was equilibrated with solutions containing high equivalences (0.69) of each cation, Ca²⁺, K⁺, or NH₄⁺, while keeping equivalent fractions of the remaining cations constant. All ternary solutions were prepared with a target ionic strength of 0.05 mol L^–1 and repeated twice. In addition, each exchanger was equilibrated with the previously described four solutions containing a background Na⁺ concentration of 0.013 mol L^–1. Additions of Na⁺ to binary-exchange reactions have been shown to not influence the exchangers' cationic preference (Sposito et al., 1983). This concentration of Na⁺ is similar to what one may expect in swine and cattle waste lagoons in Kansas (Ham, 2002). All equilibrium reactions were conducted twice, and the reaction temperature was held constant at 25°C. Finally, lagoon-liquor data from existing swine and cattle waste lagoons in Kansas were used to predict the equivalent fractions of Ca²⁺, K⁺, and NH₄⁺ on the exchanger phase. These results were used in conjunction with Eq. [1] to predict the depth of soil needed (cm) to adsorb 100% of the downward-moving NH₄⁺ after 20 yr of operation.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS THEORETICAL RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Characterization of Exchangers
The clay contents of S1 and S2 were 354 and 239 g kg^–1, respectively (Table 3). The addition of bentonite increased the concentration of clay in the S1 and S2 soils whereas the addition of zeolite decreased the concentration of clay. Addition of the amendments slightly decreased the total C in the S1, S1B, and S1Z exchangers, from 7.8 g kg^–1 to 7.2 and 7.0 g kg^–1, respectively (Table 3). The total C remained fairly constant at about 4.0 g kg^–1 in the S2, S2B, and S2Z exchangers.

Binary Exchange
The repeatability of the cation concentrations on the exchangers was generally less than a 10% relative percent difference (RPD) from the means indicating that the flow-through method worked very well for equilibrating an exchanger phase with a solution phase. Thus, the average of the two replications was reported (Tables 1 and 2). For S1 and S2, complete exchange of the initial saturating cation was observed for all reactions when the saturating cation was not present in the equilibrating solutions (Table 1). Because of this behavior, equivalent fractions of 1.0 and 0.0 were added to the S1B, S1Z, S2B, and S2Z datasets to allow the G_ex⁰ to be calculated for all six exchangers to be comparable. The RPD from the means of the G values, reported in Fig. 2 through 4 , indicate that the smallest RPD was from Ca²⁺–K⁺ exchange reaction on the S2B exchanger (0.3%) and the largest RPD was from the K⁺–NH₄⁺ exchange reaction on the S1B exchanger (44.7%). In general, most RPDs for the G_ex⁰ values were less than 10%.

View larger version (28K): [in this window] [in a new window]   Fig. 2. Calcium–K+ exchange isotherms and values for the Gibb's free energy of exchange (G) for two Great Plains soils: S1 (a) and S2 (b) alone, and with the addition of bentonite and zeolite at a concentration of 60 g kg–1. The relative percent difference (RPD) from the mean of the G values is presented.

View larger version (26K): [in this window] [in a new window]   Fig. 4. Potassium–NH4+ exchange isotherms and values for the Gibb's free energy of exchange (G) for two Great Plains soils: S1 (a) and S2 (b) alone, and with the addition of bentonite and zeolite at a concentration of 60 g kg–1. The relative percent difference (RPD) from the mean of the G values is presented.

View larger version (28K): [in this window] [in a new window]   Fig. 3. Calcium–NH4+ exchange isotherms and values for the Gibb's free energy of exchange (G) for two Great Plains soils: S1 (a) and S2 (b) alone, and with the addition of bentonite and zeolite at a concentration of 60 g kg–1. The relative percent difference (RPD) from the mean of the G values is presented.

Isotherms and G_ex⁰ values both indicate a preference for NH₄⁺ over Ca²⁺ for the S1, S1B, and S1Z exchangers (Fig. 3a). The addition of zeolite to the S1 exchanger increased NH₄⁺ preference over both the S1 and S1B exchangers (Fig. 3a). Exchanger S2Z had the greatest preference for NH₄⁺ over Ca²⁺, as indicated by the exchange isotherm and a G_ex⁰ value of –8.50 kJ mol^–1 (Fig. 3b). The addition of bentonite to both the S1 and S2 exchangers had little affect on the preference of S1B and S2B exchangers for NH₄⁺ over Ca²⁺ (Fig. 3a and 3b).

Exchange isotherms and G_ex⁰ values indicated a slightly greater preference for K⁺ over NH₄⁺ for all six exchangers (Fig. 4). Addition of bentonite and zeolite to S1 numerically increased the G_ex⁰ values for K⁺ over NH₄⁺, but statistical differences were not determined (Fig. 4a). Gibb's free energy values for the S2, S2B, and S2Z exchangers were very similar (Fig. 4b).

Ternary Exchange
Binary-exchange data were used to calculate the intercept and slope, log k_ij and n_ij, respectively, of the plot of Eq. [7] (Table 4). The r² values of these plots indicate a good fit of the log-transformed data. Values of r² ranged from 0.985 to 0.999 from all the plots. Thus, the determined values of k_ij and n_ij can be considered good estimates of the empirical exchange constants. Through the use of Eq. [8], [9], and [10], k_ij and n_ij were used to estimate N_i for each cation in a ternary system. These predicted N_i values were compared with the measured N_i for each cation in the ternary system. The relationship between the predicted and measured N_i on the S1 exchanger was very good, with an r² of 0.966 and a root-mean square (RMS) of 0.038 (Fig. 5a) . The addition of bentonite did not hinder the predictability of N_i, but addition of zeolite decreased the r² value to 0.881 and increased the RMS to 0.055. Adding a Na⁺ background to the equilibrium solutions did not decrease the model's ability to predict N_i in a ternary system (Fig. 5b). Both the r² and RMS values of S1 and S1B slightly improved when Na was present in solution, although the r² and RMS values from the S1Z exchanger slightly deteriorated with the presence of Na⁺ in the solution. No significant differences were observed between the exchangers with, and without, Na⁺ present in the solution phase (data not shown).

View larger version (30K): [in this window] [in a new window]   Fig. 5. (a) Equivalent fraction of cations on the Soil 1 (S1), Soil 1 + bentonite (S1B), and Soil 1 + zeolite (S1Z) exchanger phases predicted from the Rothmund–Kornfeld equation and Gaines–Thomas convention vs. laboratory-measured equivalent fractions on the S1, S1B, and S1Z exchangers. (b) Also with Na added to the ternary solutions at 0.013 mol L–1. Root-mean square (RMS) = [( predicted N – measured N)2/n]0.5.

View larger version (30K): [in this window] [in a new window]   Fig. 6. (a) Equivalent fraction of cations on the Soil 2 (S2), Soil 2 + bentonite (S2B), and Soil 2 + zeolite (S2Z) exchanger phases predicted from the Rothmund–Kornfeld equation and Gaines–Thomas convention vs. laboratory-measured equivalent fractions on the S2, S2B, and S2Z exchangers. (b) Also with Na added to the ternary solutions at 0.013 mol L–1. Root-mean square (RMS) = [( predicted N – measured N)2/n]0.5.

Although the benefit of adding bentonite and zeolite to soils with respect to the predicted N_NH4 values may be small, these slight increases (Table 6), coupled with increases in CEC (Table 3), will ultimately decrease the depth of soil needed to adsorb downward-moving NH₄⁺. The value of Z_eq required after 20 yr for the six exchangers ranged between 223 and 318 cm for Swine 1 and between 425 and 545 cm for Swine 2 (Table 6). For both Swine 1 and Swine 2, the benefit of adding bentonite to the native soils was very subtle, but the addition of zeolite to S1 and S2 decreased Z by 55 and 73 cm, respectively, in the Swine 1 lagoon. Although Z_eq was as much as two times greater in Swine 2 than in Swine 1, the trend of results was very similar. Bentonite additions to S1 and S2 did very little in decreasing Z_eq for Swine 2, but additions of zeolite reduced the required depth by up to 86 cm. Bentonite additions only decreased Z_eq in the S2B exchangers over the S2 in both cattle lagoons by 10 cm or less (Table 6). Conversely, the S1B exchangers for both cattle lagoons slightly increased the predicted amount of soil, compared with S1. Again, zeolite additions to S1 and S2 greatly decreased Z_eq for both lagoons by as much as 133 cm, compared with the Z_eq of the native soils.

Overall, additions of bentonite to the native soils were not very effective at reducing Z_eq in any of the lagoons. Considering all S1B and S2B combinations with lagoons, the average reduction in Z_eq was only about 6 cm, although additions of zeolite to both soils decreased the Z_eq from all lagoons by an average of about 78 cm when compared with that of the native soils. Thus, from a soil chemistry perspective, the benefits of adding bentonite to the soil to reduce Z_eq may not be as important as the beneficial decrease of a soil's saturated hydraulic conductivity (Table 3). Conversely, zeolite additions to native soil greatly decreased the required Z_eq under lagoons, which can mostly be attributed to increases in the CEC of the mixtures. Additions of zeolite, however, did tend to increase the saturated hydraulic conductivities of the soil and may not be appropriate for use with some soils.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS THEORETICAL RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The CEC of native soils may be increased through the use of amendments such as bentonite and zeolite. Additions of zeolite at a rate of 60 g kg^–1 to two soils that were previously approved for use as compacted soil liners increased the CEC of the mixtures by as much as 4.0 cmol_c kg^–1. Binary-exchange reactions between Ca²⁺–K⁺, Ca²⁺–NH₄⁺, and K⁺–NH₄⁺ that were conducted on these soils, and soils plus amendments, indicated that the univalent cations, K⁺ and NH₄⁺, were preferred over Ca²⁺ on the exchangers. Also, K⁺ was preferred over NH₄⁺ in all instances. Mixtures of soil plus zeolite consistently had the greatest preference for K⁺ or NH₄⁺ over Ca²⁺ and for K⁺ over NH₄⁺, compared with preferences of the native soils and soils with bentonite added. But, preferences for K⁺ and NH₄⁺ over Ca²⁺ and K⁺ over NH₄⁺ in the native soils and soil–bentonite mixtures were nearly identical, which suggests the presence of similar smectite clays in the soils and bentonite amendments.

The Rothmund–Kornfeld approach, according to Gaines–Thomas convention, yielded good predictions of the exchanger phase composition in a ternary system by use of simple binary-exchange reactions. Although these experiments only addressed Ca²⁺, K⁺, and NH₄⁺, the addition of Na⁺ to the solution phase, essentially producing a quaternary system, did not hinder the ability of the model to effectively predict equivalent fractions of Ca²⁺, K⁺, or NH₄⁺ on the exchanger phase. Research suggests that combining Mg²⁺ with Ca²⁺ (Feigenbaum et al., 1991) may also be a viable option for increasing the practicality of the model when investigating environmental issues.

The Rothmund–Kornfeld approach, in conjunction with the knowledge of lagoon-liquor chemistries, may provide a useful tool to aid in the construction of soil liners for animal and municipal waste lagoons (Ham and DeSutter, 2000). This approach allows one to use soil from a proposed construction site and evaluate a soil's ability to selectively adsorb NH₄⁺ on its exchange sites. Results from the ternary-prediction experiments indicate that soils from the Great Plains that are used for liner materials could retain about 53 and 23% of the NH₄⁺ from swine and cattle lagoons on their exchange sites. Additions of bentonite or zeolite to these soils may not improve the equivalent fraction of NH₄⁺ on the exchange sites, when used in conjunction with a swine-waste lagoon. When NH₄⁺ concentrations are similar to those found in cattle lagoons, however, the addition of zeolite to soils could help increase the equivalent fraction of NH₄⁺ on the exchange sites.

Another useful application of the Rothmund–Kornfeld approach, when used in conjunction with Eq. [1], allows for the prediction of the equivalent depth of soil needed to adsorb 100% of the downward-moving NH₄⁺ from waste lagoons. In general, swine lagoons needed a greater equivalent depth of soil, compared with cattle waste lagoons, to adsorb all the NH₄⁺. The equivalent depth of soil needed to adsorb 100% of the NH₄⁺ from a swine lagoon after 20 yr of operation was as much as 5 m, whereas the soil needed for a cattle lagoon was only as much as about 3 m.

Additional research that should be explored is how effectively the predicted Z_eq values compare with actual depths of NH₄⁺ movement below swine and cattle lagoons and how downward-moving organic C from animal waste lagoons affects an exchanger's cation selectivity preference and cation exchange capacity. Model validation through the investigation of decommissioned swine and cattle lagoons would allow researchers and regulators to better assess the potential costs for N cleanup and also better identify lagoon construction sites. Increases in CEC are expected with the addition of organic carbon (Parfitt et al., 1995), but some uncertainty in NH₄⁺ selectivity still exists when organic C concentrations are increased (Chung and Zasoski, 1994). Another area of exploration is the evaluation of calcareous soils for use as soil liners. Dissolution of carbonates may increase the Ca²⁺ and/or Mg²⁺ content in calcareous soil and, thus, compete with the cations in the lagoon liquor for soil exchange sites and subsequently increase the amount of soil needed to adsorb downward-moving NH₄⁺. Prediction of actual concentration profiles of NH₄⁺ beneath the lagoon may be possible by incorporating exchange theory with transport theory (Mansell et al., 1988; Mansell et al., 1993; Bond, 1997). Investigating other amendments, such as vermiculite, that may have a greater ability to selectively adsorb NH₄⁺ (Evangelou and Lumbanraja, 2002) may help reduce the downward movement of NH₄⁺.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS THEORETICAL RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Contribution no. 04-385-J from the Kansas Agricultural Experiment Station, Manhattan, KS.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS THEORETICAL RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

• American Society for Testing and Materials. 1991. ASTM Designation D 5084-90. Standard test method for measurement of hydraulic conductivity of saturated porous materials using a flexible wall permeameter. Annu. Book ASTM Stand. 4.08:1070–1077.
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