PARENT SESSION
3C Soil Contamination
9:00 AM to 7:00 PM, Tuesday, 08 May 2001

(T/EH129) Desorption of herbicide residues from a laboratory-contaminated soil as influenced by aging time.

Rodríguez-Cruz, Mª Sonia¹, Sánchez-Martín, Mª Jesus¹, Sánchez-Camazano, María¹, ¹

ABSTRACT- The presence of hydrophobic herbicides in groundwater, arising from diffuse or point located pollution of soils, is every time more frecuent. In recent years, increasing numbers of studies have addressed the behaviour and transport of herbicides in the soil, mainly studying the mechanisms of adsorption-desorption as the most significant among those controlling the movement of these compounds in this medium. However, one of the limitations to laboratory experiments on desorption, which are usually carried out to determine the reversibility of adsorption and which attempt to simulate field conditions, is that the effect of the residence time, or aging time, of the herbicide in the soil is not usually taken into account. This is of special interest when dealing with hydrophobic herbicides and soils with high organic matter contents. In the light of the above, in the present work we were prompted to study the effect of the aging time (0, 3 and 9 months) on the desorption (desorption isotherms) and on the desorption rate (desorption kinetics) of linuron and atrazine from a soil that had been artificially contaminated at the laboratory. Linuron and atrazine are two hydrophobic herbicides with Kow values of 2.5 and 3, respectively; the organic matter content of the soil used was 7.3%. The values of the Freundlich desorption constants, Kfd, increased with the aging time of the residues from 18.7 for the non-aged soil to 27.8 for the soil aged for 9 months in the case of atrazine, and from 43.4 to 44.4 for linuron. The hysteresis coefficients, H, (nf isotherm of adsorption/nf isotherm of desorption) increased with the aging time from 8.80 to 40.5 and from 2.89 to 16.1 for linuron and atrazine, respectively. The percentages desorbed diminished with the aging time from 35.6 (time 0) to 12.5 (9 months) for atrazine and from 8.0 to 3.94 for linuron. The desorption patterns were biphasic, a slow desorption following an initial fast phase. The implications of the intensification of the slow and incomplete desorption of the two hydrophobic herbicides with the increase in aging of the residues are very important. Both modifications increase the limitations to application of remedial technologies to soils and groundwaters.

Key words: Herbicide residues, Soil, Desorption, Aging time