Effect of Heavy Metals On Sorption of Phenoxyalkanoic Herbicides by Calcareous Soils.

Sorption and desorption of phenoxyalkanoic acid herbicides on two calcareous soils, with varying content of organic carbon were studied. The higher content of organic carbon in soils resulted in higher sorption of the herbicides. This study also examined sorption of phenoxyalkanoic acid herbicides by the two soils when contaminated with Pb, Zn, and Cu, as a function of the solution concentration and temperature. Generally, the trend of sorption was in the order of: 2,4,5-trichlorophenoxy acetic acid (2,4,5-T) > 4-(2,4-dichlorophenoxy)-butyric acid (2,4-DB) > 2,4-dichlorophenoxy acetic acid (2,4-D) > 4-chloro-2- methylphenoxy-acetic acid (MCPA). Experimental results indicated that the equilibrium sorption and desorption data of the four phenoxyalkanoic acid herbicides fitted well the Freundlich isotherm. Phenoxy herbicides sorption was clearly lower after the addition of heavy metals. The distribution coefficient (K_d) and the organic carbon-normalized distribution coefficient (K_oc) for phenoxyalkanoic acid herbicides sorption by soils spiked with Pb, Zn or Cu at (0-250 mg kg^-1) were smaller than for their sorption by unspiked soils; the higher the content of heavy metals added to soils resulted in smaller K_d and K_oc values. Moreover, the influence of different metal ions (Pb²⁺, Zn²⁺, Cu²⁺) indicated that the presence of heavy metals suppressed the sorption of the investigated phenoxyalkaonic herbicides in the order: Pb²⁺ > Cu²⁺ > Zn²⁺. Temperature effects on the distribution coefficients (K_d) were investigated and thermodynamic parameters were calculated. An increase in temperature resulted in lower K_d values for the sorption of the phenoxy herbicides in soils and soils spiked with heavy metals. Thermodynamic studies suggested exothermic, spontaneous, physical sorption process. Adsorption kinetics results followed the pseudo-second-order kinetics model. The results are believed to provide useful insight for understanding the transport and fate of herbicides in natural environments.