Adsorption of Cu(II) and Cd(II) by Na-montmorillonite at low surface coverage in the presence and absence of water soluble complexing organic ligands Public Deposited

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  • The formation of solution and solid complexes with heavy metals will partially determine their mobility and plant availability in soil systems. Experiments were initiated to determine the effect of pH, adsorbent and initial metal concentrations, and the presence of water soluble ligands on the adsorption of Cd and Cu by Na-montmorillonite. Batch adsorption experiments over a pH range of 4 to 8.5 were performed under humidified N₂ gas in a constant ionic strength medium of 0.01 M NaC10₄ at 25°C for 2 hrs. This equilibrium period was chosen in accordance with results from kinetic studies at 50 μM CdT and CuT on Na-montmorillonite (2 g liter ⁻¹) which revealed that equilibrium occurred within 30 min. The amount of metal adsorbed was determind as the difference between the total metal added (MeT) and the metal in solution at equilibrium (MeTS). Total soluble metal (MeTS) values were undersaturated with respect to metal hydroxide solids and since CO₂ was excluded from the reaction vessels metal carbonate precipitation was eliminated. The pH reversible adsorption of 50 - 0.1 μM CdT and 50 - 0.5 μM CuT by Na-montmorillonite (2 g liter⁻¹) increased from 70 to 90% and 80 to 98%, respectively, over a pH range of 4 to 8.5. Linear adsorption isotherms at constant pH values of 4, 6, and 8 were developed utilizing data from the adsorption experiments at six initial CdT and five initial CuT concentrations. Adsorption of 2 μM CdT from solution decreased as the solid concentration was decreased from 2 g liter⁻¹ to 0.2 and 0.02 g liter⁻¹. However, adsorption in terms of mol Cd g clay⁻¹ increased with decreasing adsorbent concentration (ST) indicating that fractional coverage on clay sites increased as ST decreased. The adsorption pH dependence also increased as ST decreased and it appears that as the amount of Cd adsorbed approaches saturation with respect to the calculated amount of constant potential edge sites, the adsorption pH dependence approaches a value near the expected pH dependence inferred from CEC measurements. This provides partial support that adsorption of Cd and Cu at low surface coverage was controlled by constant potential sites rather than constant charge sites. In the presence of a forest litter layer extract (1.8 mM CT), the adsorption of 50 - 0.1 μM CdT and 50 - 0.5 μM CuT by Na-montmorillonite (2 g liter⁻¹) decreased from 75 to 65% and 60 to 25%, respectively, over a pH range of 4 to 8.5. Activity measurements for Cd²⁺ and Cu²⁺ indicated that more Cu was complexed by organic ligands than Cd explaining the greater decrease observed for Cu adsorption in the presence of the forest litter layer extract. Adsorption of Cd and Cu at low metal concentrations by Na-montmorillonite in the absence of organic ligands was probably controlled by constant potential edge sites. Organic ligands present in the forest litter layer extract formed more non-adsorbing complexes with Cu than Cd which resulted in a more dramatic decrease in Cu adsorption as pH increased. Organic ligands in soil solutions will therefore have a greater effect on mobilizing. Cu and/or determining solution speciation than they will on Cd.
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