Comparison of equilibrium and non-equilibrium distribution coefficients for the human drug carbamazepine in soil

The distribution coefficient (K_D) for the human drug carbamazepine was measured using a non-equilibrium technique. Repacked soil columns were prepared using an Airport silt loam (Typic Natrustalf) with an average organic matter content of 2.45%. Carbamazepine solutions were then leached through the columns at 0.5, 1.0 and 1.5mLmin^-1 representing average linear velocities of 1.8, 3.5 and 5.3cmh^-1 respectively. Each flow rate was replicated three times and three carbamazepine pulses were applied to each column resulting in a total of 9 columns with 27 total carbamazepine pulses. Breakthrough curves were used to determine K_D using the parameter fitting software CXTFIT. Results indicate that as flow rate decreased from 5.3 to 1.8cmh^-1, K_D increased an average of 21%. Additionally, K_D determined by column leaching (14.7-22.7Lkg^-1) was greater than K_D determined by a 2h batch equilibrium adsorption (12.6Lkg^-1). Based on these K_D's carbamazepine would be generally characterized as non-mobile in the soil investigated. However, repeated carbamazepine applications resulted in an average 22% decrease in K_D between the first and third applications. Decreasing K_D is attributed to differences in sorption site kinetics and carbamazepine residence time in contact with the soil. This would indicate that the repeated use of reclaimed wastewater at high application rates for long-term irrigation or groundwater recharge has the potential to lead to greater transport of carbamazepine than K_D determined by batch equilibrium would predict.