Sorption and leaching behaviour of polar aromatic acids in agricultural soils by batch and column leaching tests

Abstract

SummaryAromatic acids can reach the soil from direct anthropogenic activities or, indirectly, from the degradation of many aromatic compounds, such as pesticides or polycyclic aromatic hydrocarbons. Because of the anionic character of aromatic acids at the pH of most soil and sediment environments, they are expected to move rapidly through the soil profile and to pose a great risk of ground water contamination. We designed batch and column leaching tests to characterize the behaviour of three aromatic acids differing in their chemical structures, picloram (4‐amino‐3,5,6‐trichloropicolinic acid), phthalic acid (2,2‐benzenedicarboxylic acid), and salicylic acid (2‐hydroxybenzoic acid), in four European soils with different physicochemical characteristics. Batch experiments revealed that the persistence of the three acids in soil:water suspensions decreased in the order: picloram ≫ phthalic acid > salicylic acid, and their dissipation curves were relatively independent of soil type. Sorption by the soils, their clay‐size fractions and model sorbents indicated much greater affinity of soil constituents for salicylic acid than for picloram or phthalic acid, most likely due to the ability of salicylic acid to form bidentate complexes with positively charged soil components. The extent of leaching of the aromatic acids in hand‐packed soil columns decreased in the order: picloram (90–96%) > phthalic acid (25–90%) > salicylic acid (0–37%), which was consistent with the sorption and persistence results of the batch tests. The organic C content, the amount of small‐size pores, and the initial concentration of aromatic acid in soil appeared to be important factors influencing the leaching patterns of phthalic acid and salicylic acid in the soils studied, but did not greatly influence the leaching pattern of picloram. Sorption and leaching of polar aromatic acids in soil can therefore vary considerably depending on the structural characteristics of the aromatic acid or soil type.