Sorption and desorption of herbicides by soil humic acid fractions

Results obtained with a dialysis membrane technique for the study of the sorption and desorption of herbicides by a fraction of the organic matter extracted from soil are reported. The humic acid (HA) fraction of a Typic Haplumbrept soil, extracted with 0.1 M NaOH plus 0.1 M Na4P2O7, was cut off at 5,000 Da. The water solution of HA>(5,000) was transferred to a 1,000-Da dialysis tube and inserted in a shaded flask containing a water solution of the herbicides (paraquat or linuron). The sorption of herbicide by the humic acid was determined by measuring at different time intervals (0, 0.5, 1, 2, 4, and 24 h) the decreasing concentration of the herbicide in the solution external to the dialysis tube. Measurement of the herbicide concentration was carried out spectrophotometrically in the lambda = 200-300 nm UV region. The results obtained indicate that the procedure adopted can be useful to study the interactions between herbicide and fractions of soil organic matter with relatively large molecular sizes (>1,000 Da).