Effects of salinity on solute and ion transport in quasi-structural unstable sodic soil in a semi-arid environment in Botswana: simulations using mixed ions of sodium ([Na.sup.+]) and calcium ([Ca.sup.2+]).
Saline and sodic conditions are frequently observed in semi-arid regions like Botswana with the deterioration in permeability alleviated by maintaining the electrolyte concentration of percolating water above a critical threshold level (Levy et al. 2005). Problems related to the use of sodic/saline waters are global (Szabolcs 1992) especially in arid and semi-arid soils where the sodium (Na) and calcium (Ca) ions are the most common ions on the exchange complex (Miller and Donhue 1995). The behaviour of the ions on the exchange complex affects mechanism of ionic transport (Ersoz et al. 2001) with the cation exchange reactions being affected by exchanger species between solution and adsorbed phases (Gaston and Selim 1991; Evangelou and Lumbanraja 2002). Theoretically the cation exchange behaviour for the Na-Ca exchange system is described by; [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)
where CEC is the cation exchange capacity and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] is the activity of exchange phase of Na and Ca respectively. The exchange reactions of Na and Ca cation systems are in accordance to the following (Gapon 1933):
[Na.sub.x]/[Ca.sub.x] = [K.sub.G] * [Na]/[[Ca].sup.0.5] - or ESR = [K.sub.G] * (SAR) with SAR = [Na]/[square root of ([Ca])] (2)
where [Na.sub.x] and [Ca.sub.x] are fractions of the exchange complex occupied by those ions, [K.sub.G] is the Gapon constant and the square brackets indicate concentrations (in mmole/litre or mmol/L) of the ions in solution, ESR is the exchangeable sodium ratio and SAR is sodium adsorption ratio.
Excessive sodium ions on the exchange complex enhance swelling and dispersion which may have profound effects on soil structural stability of the pore matrix and decrease of permeability (Keren and Ben-Hur 2003) with decrease in electrolyte concentration particularly with increased exchangeable sodium percentage (ESP) or SAR. For example an increase in ESP can cause soil structural deterioration through clay swelling and dispersion and by slaking of silt-sized micro-aggregates (Abu-Sharar et al. 1987) thus blocking the water conducting pores. Further, using a monovalent NaCl electrolyte solution, Dikinya et al. (2006) attributed the decrease of permeability to mobilization and re-deposition of fine particles. Little, if any, research work dealing with the combined effects of salinity on the dynamics of permeability and ionic transport have been carried out. In particular to better simulate possible field conditions the dependence of permeability on salinity and rate of wetting should not be considered independently but simultaneously (Levy et al.2005).
The objectives of study are two-fold; (a) to examine the effects of the sodium adsorption ratio at decreasing electrolyte concentration on the dynamics of the permeability along soil columns and (b) to assess the Na-Ca exchange reactions and dynamics of the electrolyte breakthrough in saturated soil columns.
Materials and Methods
An agricultural soil sandy loam sample collected from top 20 cm of the soil for analysis and transport measurements. The soil often shows some build up of salts, especially sodium, in the subsoil, and has a potential for structural breakdown. The basic physico-chemical properties of the soil measured using standard methods (Day 1965; Klute 1986) are presented in Table 1. The soil is predominately kaolinitic with few traces of gybisite and has specific surface area of 6.8 [m.sup.2]/g.
Binary exchange reactions
Exchange reactions of Na-Ca were carried out employing a batch technique and using samples of the soils described in Tables 1. Ten grams (10g) of each sample in triplicate were saturated with the binary pair Na-Ca using 30 ml solutions of different cations ratios of Na/Ca as follows: r; 0.9, 0.7, 0.4, 0.2 and 0.1 and each added into separate 50 ml centrifuge tubes or vials. The 30 ml Na/Ca solution mixtures in the 50 ml vials were initially saturated with 0.5 M solution to ensure equilibration with exchanger sites for those ratios. The 0.5M saturated samples were placed on a reciprocating shaker for 24 hours (to disrupt soil aggregates), centrifuged and the supernatant discarded. This procedure was repeated twice. This was then followed by 0.005 M solutions by repeating the above procedure six (6) times to ensure that equilibrium is reached, with ammonia acetate (N[H.sub.4]OAc), added to the supernatant for cation extraction. The 30 ml of 1 M NH4OAc solution adjusted to a pH 6 (equivalent to pH of the soil) using acetic acid and ammonium hydroxide solution, was added to the supernatant, shaken for 24 hours and centrifuged with the supernatant retained. The supernatant samples were then weighed to determine the exact amount of cations for correction of the entrained supernatant. The supernatants were then analysed for cationic exchange capacities using Atomic Adsorption Spectrometer (AAS).
Permeability and Breakthrough curves
Soil samples (< 2 mm) were initially mixed with 0.9-1 mm acid-washed sand (50% sand and 50% soil) to provide a rigid skeletal structure and to enhance water flow. The mixture was uniformly wet packed in a 400 mm long column (25 mm diameter), with holes bored at D1 (50mm), D2 (150 mm) and D3 (250 mm) for inserting pressure transducers for pressure head measurements (see Fig 1). These columns were packed at density of 1.69 g/[cm.sup.3] at saturated water content of 0.381[cm.sup.3]/[cm.sup.3]. The columns were then saturated with the desired solutions of different SARs: 5, 15 and 30 and with varying electrolyte concentrations; 1, 10 and 100 mmol/L with a complete saturation achieved by leaving the desired solution over night. A peristaltic pump was used to pump the feed solutions at constant flow rate of 1 [cm.sup.3]/min while an Agilent datalogger system (Agilent Technologies, 2003) was used for online pressure head measurements for permeability computations. The effluents, collected using fraction collectors, were measured for pH and electrical conductivity (EC), Na+ and [Ca.sup.2+] concentrations and particle release during leaching.
[FIGURE 1 OMITTED]
For the cationic Na/Ca breakthrough curves, the columns were initially equilibriated with 10 mmol/L Ca[Cl.sub.2] solution by passing 10 pore volumes of that solution. After this pre-conditioning, the input solution concentration was progressively changed through 10, and 2.5 mmol/L at SAR 15, and were leached at selected pulse times or pore volumes of about 18 for equilibrium with measurements of pressure changes. The effluents collected in fraction collectors were used for both Na/Ca concentrations analysis by Atomic Absorption Spectrometer (AAS) to obtain breakthrough curves.
Results and discussions
Binary exchange reactions
The experimental data on Ca-Na exchange on solution and exchange phases are presented in Table 2. Changes in selectivity over different Na/Ca ratios as marked by changes in exchange capacity ([SIGMA]Exi) presented in Table 2 demonstrated a non-exchanger preference for [Ca.sup.2+] than [Na.sup.+] ions. This minor variation in exchange capacity ([SIGMA]Exi) was consistent with smaller variation in exchanger phase composition suggesting nearly constant surface uniformity throughout the entire exchange isotherm. Similarly there were minor variations of solution pH of these Na-Ca binary systems across the entire isotherms (Table 2) with pH varying from 6.1 to 6.6. Ideally pH and ([SIGMA]Exi) or CEC should be constant across the entire isotherm to generate cation-exchange isotherm despite that the pH and CEC constancy in soil clay minerals is rarely obtained (Evangelou and Lumbanraja 2002).
Effects of sodium adsorption ratio on permeability
Permeability was computed using a The relative permeability (K/Ko) measured as a function of electrolyte concentration, following leaching of the soil columns with solutions of SAR 5, 15, and 30 are shown in Figure 2. Generally, the permeability decreased with time, with increasing SAR and with decreasing electrolyte concentrations.
[FIGURE 2 OMITTED]
It was clear that permeability was maintained for SAR 5 at a concentration of 100 mmol/L with a more rapid decrease at the lowest concentration (1 mmol/L) although this was consistent at all SARs. Low permeabilitity particularly at higher electrolyte concentrations is evidenced by kaolinitic nature of the soil material and similarly Anderson and Lu (2001) reported higher void ratios for Na/Ca-montmorilllonite than for kaolinite decreasing with increasing ionic strength. The dilution of high-sodicity soil irrigation water can cause induce swelling, aggregate slaking and particle clay dispersion (Bagarello et al., 2006) and the reductions in permeability here were likely to be caused by partial blocking of pores by dispersed clay particles, as evidenced by the appearance of suspended clay particles in the effluent during leaching.
The initial and the final pH remained constant during leaching whereas the electrical conductivity (EC) decreased with decreasing electrolyte concentration (Fig 2). This may imply that the pH did not influence any clay migration while the decrease of EC with electrolyte concentration may suggest an influence of EC on particle deposition which would subsequently clog pores or reduce permeability.
[FIGURE 3 OMITTED]
Dynamics of ionic transport: Breakthrough curves at critical concentrations
Figure 4 shows the breakthrough curves for leaching with 10 mmol/L CaCl2 followed by leaching at critical concentrations of SAR 15 at various electrolyte concentrations. The input together with equilibrium solutions are presented in Table 3.
The permeability initially decreased rapidly and stabilized at essentially constant but differing values at the three measuring points along the column with relatively little further change following change in the input concentration to 10mmol/L (Fig. 4). Figure 4 also show the outgoing effluent concentrations of sodium and calcium ions for the breakthroughs with different input concentrations (threshold concentration 10 mmol/L and turbidity concentration 2.5 mmol/L), while maintaining a constant input SAR of 15, together with the corresponding effluent SAR and relative permeability at positions D1 (50 mm), D2 (150mm) and D3 (250mm) along the columns for the input concentration of the sodium and calcium ions (Table 3).
An initial increase in the effluent Na concentration following commencement of the input of the 10mmol/L SAR 15 solution occur rapidly, and is followed by subsequent rapid decreases with decreasing electrolyte concentration of the input solution to 2.5mmol/L. The calcium concentrations in the effluent decreased progressively with decreasing input concentrations as would be expected. However more than 15 pore volumes are required for the effluent SAR to reach that of the input solution. This rapid decrease in permeability presumably reflects a more open pore structure associated with the presence of micro-aggregates. As is to be expected the electrical conductivity closely follows the decrease with normality of the solutions.
[FIGURE 4 OMITTED]
A most surprising feature of the results is the relative uniformity of the response at the different measuring points along the column despite the clear indication of a gradual breakthrough of the SAR front moving through the columns during the change from the 10mmol/L Ca[Cl.sub.2] solution to 10mmol/L at SAR 15. This change is initially accompanied by rather erratic permeability behaviour before near uniformity occurs. At the threshold concentration one would expect slaking to occur with perhaps some dispersion. The subsequent reduction in input concentration to 2.5mmol/L (the turbidity concentration) occurs rapidly in the effluent producing uniformly lower permeability along the columns. However there seems to be a relationship permeability and particle release (Fig. 5). For example at pore volume of 7, a significant particle release is accompanied by a dramatic drop in permeability. This time lag of K/Ko with particle release is consistent as also reflected at pore volume of 23 when a dilution of 10 to 2.5 mmol/L was marked by an increased particle release and a decrease in K/Ko. This suggests that as larger particles concentration are released, they tend to partial clog or clog pores to substantial decrease K/Ko.
[FIGURE 5 OMITTED]
Summary and conclusions
The effects of composition of mixed ions on exchange reactions demonstrated that strong preference for [Ca.sup.2+] ions with a stronger preference for Na on the exchange complex of the agricultural soil. Generally, the saturated hydraulic conductivity of both soils has been shown to decrease with time, with increasing SAR and with decreasing electrolyte concentration. The permeability was maintained for SAR 5 at a concentration of 100 mmol/L. A more rapid decrease in permeability particularly at the lowest concentration (1 mmol/L) although this was consistent at all SARs. Significant differences in permeability were observed in the passage of fronts of decreasing electrolyte concentrations for Ca[Cl.sub.2] and SAR 15 solutions through the soil columns reflecting structural alterations (slaking) of the media, the nature of the particles mobilised and the extent of straining (self-filtration) within the porous structure at any given point in the column.
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(1) Otlogetswe Totolo and (2) Oagile Dikinya
(1,2) Department of Environmental Science, University of Botswana, Private Bag 0022, Gaborone, Botswana
Emails: (1) email@example.com and firstname.lastname@example.org
(1) Corresponding author
Table 1: Selected physico-chemical characteristics of soil measured. Property Sand(%) 83.3 Silt (%) 6.6 Clay (%) 3 10.1 Bulk Density (g/[cm.sup.]3) 1.61 Electrical conductivity ([micro]S/cm) 42 pH (water) 6.1 CEC (cmol/kg) 3.4 ESP (%) 2.5 Organic Carbon (%) 0.3 CEC--cation exchange capacity ESP--exchangeable sodium percentage Table 2. Sodium-Calcium exchange on soils for different concentrations ratios r. r (Na/Ca) pH ExNa ExCa [SIGMA]Exi mmol/kg) 0.9 6.1 1.10 11.50 12.60 0.7 6.6 0.64 11.10 11.74 0.4 6.3 0.46 11.14 11.86 0.2 6.6 0.20 12.50 12.70 0.1 6.6 0.18 12.20 12.38 Exi--denotes concentration of absorbed cations and [SIGMA]Exi--total adsorbed cations Table 3: Input and equilibrium conditions during measurements. Solution Input conditions (or Equilibrium conditions concentrations solution) Na Ca SAR EC Na Ca SAR --mmol/L-- (%) dS/ --mmol/L-- (%) m 10 mmol/L n.a 9.9 n.a 2.33 n.a 10.1 n.a Ca[Cl.sub.2] SAR15 9.52 0.39 15.24 1.28 10.10 0.44 15.2 10mml/L SAR15_2.5 2.5 0.03 14.6 0.35 2.61 0.03 14.4 mmol/L 3 Solution Equilibrium concentrations conditions EC PVeq dS/m (-) 10 mmol/L 2.29 10.4 Ca[Cl.sub.2] SAR15 1.30 18.0 10mm ol/L SAR15_2.5 0.37 18.0 mmol/L PVeq = pore volume to reach equilibrium and EC= Electrical conductivity (dS/m), n.a= not applicable.