Ability of natural, acid-activated, and surfactant-modified Terra Rossa soils to sorb triazine herbicides and their degradation products

Two Terra Rossa soils that were collected in the Mediterranean region of Croatia and Turkey were tested in their natural, acid-activated, and surfactant-modified forms as sorbents for hydrophobic triazine groundwater pollutants in terms of their potential application to the multi-mineral landfill barriers. The soils were predominantly composed of kaolinite, followed by quartz, calcite, iron-bearing oxides/oxyhydroxides, and several other types of clay. The natural soils were poor in organic carbon (OC ≤ 2%), were similar in chemical composition, and were significantly different in their cation-exchange capacity (CEC). Sorption experiments were conducted with hydrophobic, weakly basic and ionisable triazine herbicides (atratone, ametryn, and atrazine) and three atrazine dealkylated degradation products. The Freundlich sorption parameters were evaluated and compared with respect to the physicochemical properties of the sorbates and sorbents. The low affinities of the natural soils for the sorption of all of the tested triazines were significantly enhanced by soil acid activation. The highest fractions of sorbed triazines were observed in the acid-activated soil with the higher CEC: 90% for the most basic and partially protonated atratone and ametryn and 78% and 61% for the more polar degradation products deisopropylatrazine and didealkylated atrazine, respectively. The enrichment of natural soils with OC by the sorption of the cationic surfactant hexadecyltrimethylammonium bromide (HDTMA) was more efficient in soil with higher CEC. The sorption intensities of all of the triazines except the most polar didealkylated atrazine increased significantly with increasing soil OC and correlated positively with the hydrophobicity of the compound. The sorption capacities of the HDTMA-modified soils were greater for most compounds than the capacities of the acid-activated soils. Based on the calculated sorbate fractions that were resistant to desorption, atratone and ametryn would be expected to be retained longest in the acid-activated soils, while the soil modification with HDTMA primarily prolonged the retention of ametryn and atrazine. The results indicated that, unlike natural Terra Rossa soils, their acid-activated or surfactant-modified forms could be applicable as components of an organophilic layer in multi-mineral barriers for retention of triazines and similar chemicals.