Adsorption and desorption of atrazine, deethylatrazine, and hydroxyatrazine by soil components

We studied the behavior of atrazine (AT), deethylatrazine (DEA), and hydroxyatrazine (HyA) in contact with humic acid, kaolinite, smectite, goethite, and ferrihydrite through adsorption kinetics and isotherms, and desorption isotherms, in laboratory batch systems. The initial triazine concentrations in solution ranged from 0.001 to 1 mg L-1. The adsorption of the three triazines by each component was significant (except for AT and DEA by the two iron oxides) and the adsorption equilibrium was reached after 72 h. The adsorption isotherms were described by Freundlich equations. Adsorption increased in the order iron oxides < clays < humic acid. On the humic acid and smectite, adsorption increased in the order DEA < AT much less than HyA, whereas on kaolinite it increased in the order AT < DEA much less than HyA. The desorption of the three triazines from each component was hysteretic. For each triazine, the amount of nonextractable residue (NER) followed the order kaolinite < smectite < humic acid. On humic acid, the amount of NER increased in the order DEA < AT < HyA, whereas on the clays it increased in the order AT < DEA < HyA. The results demonstrate: (i) the coexistence of heterogeneous adsorption sites on each soil component, (ii) the importance of the intrinsic characteristics of the triazine (e.g., its pKa) (iii) the likely time-dependent nature of hysteresis, and (iv) varying amounts of NER resulting from a combination of the number of adsorption sites occupied by the molecule and the energy level of the adsorption mechanisms involved.