Copper Sorption and Transport in an Acidic Brown Soil

The sorption and transport of copper (Cu) in an acidic brown soil were studied using batch and column experiments. The results showed that Cu adsorption fitted better Langmuir isotherm at low pH (3.13) whereas Freundlich equation fitted better at high pH (5.87), and affinity (K and K-F) increased significantly from 0.00676 to 0.0121 L mg(-1), and from 33.05 to 135.98, respectively, with pH increase, resulting in a very great increase in adsorption capacity (Q(max)) from 970 to 2272 mg kg(-1). Its kinetics was found to be better described by a pseudo-second order model (R-2 > 0.997), where sorption rate (k(2) and h) was as low as 0.0237 and 0.0013 kg mg(-1) d(-1), and 23.89 and 106.12 mg kg(-1) d(-1), respectively, at pH 3.19, much lower (10-20 times) than those at pH 6.92. At pH 5.87, the breakthrough curve of Cu showed substantial retardation and low peak concentration (C/C-0 = 0.64); whereas at pH 3.17, full breakthrough (C/C-0 = 1) was observed, meaning great increase in mobility of Cu. Generally, two different mechanisms governed Cu sorption and transport: CuOH+ was precipitated on clay mineral surface and weaker complexation with DOM at higher pH (>5); whereas Cu2+ adsorbed to SOM surface and stronger complexation with DOM at lower pH (