Interactions of atrazine with humic substances of different origins and their hydrolyzed products

Three humic acids (HA) extracted, respectively, from a volcanic soil, a North Dakota Leonardite, and an oxidised coal and their hydrolysed products obtained after acidic hydrolysis, were studied for their interaction with atrazine. Adsorption kinetics showed that HA from oxidised coal had the highest capacity of adsorbing atrazine (84%) followed in order by HAs from Leonardite (45.3%) and volcanic soil (31.7%). A reverse order is shown by the hydrolysed products, HA from volcanic soil being the highest adsorber (69.2%) followed by Leonardite HA (45.6%) and by oxidised coal HA (27%). Desorption kinetics, mass balance extractions and adsorption isotherms confirmed these results. Characterization by 13C NMR indicated that humic acid aromaticity decreased and aliphaticity increased in the order: oxidised coal, Leonardite, and volcanic soil. Chemical characterization showed that total, carboxylic, and phenolic acidities increased after hydrolysis for all HAs and failed to explain the observed differences in atrazine adsorption. Molecular weight distribution by gel permeation chromatography showed that the structural modifications induced by hydrolysis are able to explain the different adsorbing capacities of HA samples. Molecular dimension and concentration of chromophore groups seemed to govern atrazine adsorption. These results indicate that (1) atrazine is mainly adsorbed through a charge-transfer mechanism between electron-poor groups of HAs and electron-rich atoms in atrazine and (2) more atrazine is adsorbed the higher the aromaticity, the polycondensation, and the molecular size of HAs.