SODIUM ADSORPTION AND SELECTIVITY COEFFICIENTS FOR CA-NA EXCHANGE ON PEAT: IMPLICATIONS FOR ROAD SALT CONTAMINATION OF PEATLAND SOILS

Road salt (NaCl) runoff elevates sodium concentrations in peatlands, altering water and soil chemistry. Cation exchange reactions affect the storage or release of Na from peat soil, and the degree of Na exchange may be tempered by Ca. This study investigates how the sodium adsorption ratio (SAR = [Na]/([Ca]+[Mg])^0.5) and ionic strength (I) of different NaCl-CaCl₂ solutions affect the exchangeable sodium percentage (ESP) and selectivity coefficients for Ca-Na exchange on peat. Reagents of NaCl-CaCl₂ having SAR values of 1, 2.5, 5, 7, 10, 20, 40, and 60 mM^0.5 and I values of 1, 5, 10, 20, and 50 mM were reacted with peat (catotlem) collected from Hawley Bog, MA. Following equilibration, the entrained solution was analyzed for Ca, Na, Mg, K and pH. Exchangeable cations were determined following reaction with 1M NH₄-Acetate (base cations) and 1M KCl (for H⁺) after correcting for entrained solution chemistry.

At all ionic strengths, Freundlich adsorption isotherms showed more sorbed Ca and Na with higher Ca and Na in the entrained solution. However, the rate of increase differed for Ca and Na. Sorbed Ca leveled off at higher Ca concentrations, and the amount of sorbed Na was minimal at low Na concentrations but increased exponentially at higher concentrations. Similarly, exchangeable sodium percentage (ESP) increased with greater SAR. Sodic ESP levels (>6%) occurred at all I values. Strongly sodic ESP (>15%) occurred when SAR was greater than 20. Therefore, Na readily exchanges with Ca and is preferentially adsorbed by peat at low Ca concentrations.

Na-Ca selectivity coefficients determined by Langmuir power functions for all SAR reagents at the same I show a preference for Na over Ca on exchange sites (K_ex > 1), with Na preference decreasing at higher I. Na-Ca selectivity coefficients calculated using the conventional Vaneslow equation for individual SAR solutions show similar results where K_v ≥ 1, and is lower at higher I. However, Ca is preferred over Na (K_v < 1) at I = 50 and at low SAR when I = 20. Independent of method, both selectivity coefficient models demonstrate that Na is more readily sorbed by peat when the proportion of Na to Ca is high, but Na preference is weakened at higher ionic strength. This suggests that the potential for Na exchange onto peat may be lessened at higher levels of road salt contamination, particularly in the presence of Ca.