Kinetic modeling of molybdenum sorption and transport in soils

In this investigation, batch and column experiments were conducted to investigate the molybdenum (Mo) sorption and transport processes on a neutral-pH soil (Webster loam) and an acidic soil (Mahan sand) in Ca²⁺ and K⁺ background solutions. Batch results showed that the adsorption of Mo was strongly non-linear in both soils and amount of Mo sorbed in the acidic soil was larger than the neutral soil. The Freundlich distribution coefficients (K_f) and Langmuir sorption maxima (S_max) in Ca²⁺ background solution are larger than that in K⁺ solution, indicating greater Mo sorption in Ca²⁺ than in K⁺. Experimental breakthrough curves (BTCs) demonstrated that mobility of Mo was higher at neutral condition than that at acidic condition. A multi-reaction transport model (MRTM) formulation with two kinetic retention reactions (reversible and irreversible) well described Mo transport for Webster soil. However, MRTM model which accounts for equilibrium and kinetic sites is recommended for Mo transport in Mahan soil, reflecting different soil properties. Based on inverse modeling, the sorption forward rate coefficients (k₁) obtained from Ca²⁺ in both soils are larger than that from K⁺, which consistent with batch experiment. Overall, MRTM model was capable of describing the Mo transport behavior under different geochemical conditions.