Thursday, 13 July 2006

Impact of Dissolved Organic Matter (DOM) on Atrazine Sorption by Montmorillonite.

Jianming Xu1, Ling Wanting1, and Yanzheng Gao2. (1) Institute of Soil and Water Resources and Environmental Science, Zhejiang Univ, 268 Kaixuan Road, Hangzhou, China, (2) College of Resource and Environmental Sciences, Nanjing Agricultural Univ, 1 Weigang Road, Nanjing, 210095, China

Because of the contrasting composition of DOM, and the different characteristics of both soil and nonionic organic chemicals (NOC), reports on the effect of dissolved organic matter (DOM) on sorption of NOC by soil are still contradictory till now. Insight into the influence of DOM on atrazine sorption by homoionic montmorillonite (Mont) was gained using batch sorption techniques. Two DOMs with different hydrophobic/hydrophilic ratio from agricultural waste were used. One was in composition of 39% hydrophobic and 61% hydrophilic fraction and designed as A1. The other designed as A2 had a 60% hydrophobic and 40% hydrophilic fraction. And the high-molecular-weight fractions (>10000 dalton) of A2 were much larger than those of A1. Homoionic K-, Al-, and Fe-Mont were obtained by dispersing Mont repeatedly with saturated KCl, AlCl3, and FeCl3 solution, respectively. Sorption of DOMs, atrazine, and atrazine in the present of DOMs with different adding levels, on each homoionic Mont were measured, and the results showed: (1) The sorption of A2 on all Monts tested was generally greater than that of A1, and both DOMs showed a higher sorption capacity on Al- and Fe-Mont than K-Mont. It was found that more hydrophobic fraction of DOM was adsorbed to Mont than hydrophilic fraction. 2) The sorption of atrazine on each Mont could be described well using the linear isotherm. And the experimented linear distribution constants (Kd) for non-homoionic, K-, Al-, and Fe-Mont were 2.87, 2.66, 3.31, and 3.18, respectively. (3) The sorption of atrazine was influenced obviously by A2, which increased with the increasing adding level of A2 irrespective of different homoionic Monts. And the enhancement (DKd) of Kd* in the presence of A2 (the apparent distribution constant in the presence of DOM) versus Kd was more significant for Al- and Fe-Mont. Also, the distribution coefficients (Kat/soc) of atrazine between water and the sorbed DOM on each Mont were calculated. Kat/soc were much larger than Kd and Kat/doc (the association coefficient of atrazine with DOM in water), indicating that the sorption sites provided by A2 might have a high capacity for atrazine sorption. This might be the dominant mechanism ascribing for the enhanced sorption of atrazine by Monts in the presence of A2. (4) The presence of A1, however, decreased atrazine sorption on Monts. In general, this depression effect tended to enhance with the increasing level of A1 from 0 to 80 mg/L. The Kd* values for atrazine sorption by non-homoionic, K-, Al-, and Fe-Mont in the presence of A1 at 80 mg/L adding level were 20.2%~39.0% smaller than their corresponding Kd values. The higher percentage of hydrophilic fraction of A1 compared to A2 enhanced the hydrophility of mineral surfaces as a result of A1 adsorption by Monts. The competition between A1 and atrazine for sorption sites on minerals might account for the inhibited sorption of atrazine in the presence of A1.

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