Differential roles of humic acid and particulate organic matter in the equilibrium sorption of atrazine by soils

Recent studies have indicated that soil organic matter (SOM) may consist of physically and chemically different fractions, including particulate organic matter (POM), such as black carbon and unburned coal materials. The present study examined the differential roles of three different SOM fractions isolated from a peat and a topsoil in the equilibrium sorption of the herbicide atrazine (ATZ). The SOM fractions isolated from the two samples included humic acids (HAs), base-extracted humin (HM), and POM after demineralization of HM. A batch technique was employed to measure both the nonequilibrium ATZ sorption on the original and HA samples and the equilibrium ATZ sorption and desorption. The results showed that the phase-distribution relationships measured under nonequilibrium conditions were more linear and had lower sorption-capacity parameters compared with their respective isotherms measured under equilibrium conditions. The sorption isotherms were variously nonlinear, with POM exhibiting the greatest organic carbon-normalized sorption capacity. There existed apparent sorption-desorption hysteresis for each sorbent-sorbate system. It appeared that the extracted HAs could facilitate hydrolysis of ATZ when the reaction time extended to 4 d or longer. The equilibrium sorptive behavior of the HAs therefore was not examined. The present study indicated that both original samples showed lower organic carbon-normalized sorption distribution coefficients compared with their respective SOM fractions, suggesting that a fraction of sorption sites in soil aggregates were not accessed by ATZ.