Adsorption of Picloram (4‐Amino‐3,5,6‐Trichloropicolinic Acid) on Panoche, Ephrata, and Palouse Soils: A Thermodynamic Approach to the Adsorption Mechanism

Abstract The adsorption of picloram (4‐amino‐3,5,6‐trichloropicolinic acid) was studied on Panoche clay loam (at pH 4.2 and 1.2), Ephrata sandy loam, and Palouse silty loam. The adsorption processes could be described by the Freundlich equation with Freundlich constants, K , 1/ n , and percentage of adsorption at 25C: respectively, 0.210, 0.92, and 3–5.6% for Panoche at pH 4.2; 2.5, 0.93, and 30% for Panoche at pH 1.2; 0.172, 0.99, and 1.9–3.6% for Ephrata; and 1.74, 0.92, and 26–33% for Palouse. Also evaluated were the standard free energy, entropy, and enthalpy associated with the adsorption processes. The values of Δ G ° (kcal/mol), Δ S ° (cal/mol per degree), and Δ H ° (kcal/mol) were: respectively, −1.2±0.1, −14±2, and −5.3±0.6 for Panoche at pH 4.2; −2.98±0.05, −59±2, and −20±1 for Panoche at pH 1.2; −1.80±0.06, −24±3, and −9±1 for Ephrata; and −3.11±0.05, −3.3±1.5, and −4.1±0.4 for Palouse. Thermodynamic parameters were useful in assigning adsorption mechanisms to the four picloram‐adsorbent‐water systems.