A new adsorption model to quantify the net contribution of minerals to butachlor sorption in natural soils with various degrees of organo-mineral aggregation

The effect of interactions between soil minerals and organic matter as a function of aggregate size on butachlor sorption was quantified in natural soils with various degrees of organo-mineral aggregation. The smallest size clay microaggregates sorbed most butachlor (58% to 71%) and the fine sand fraction sorbed the least (less than 4.3%). When normalized to organic carbon, butachlor sorption to the clay microaggregates was even smaller than to the silt and sand fractions under specific soil conditions. The sum of sorption to the different fractions was, on average, above 78% greater than sorption to the bulk soils, with the greatest differences in the soils with relatively higher ratios of clay to soil organic carbon (RCO). This suggests that minerals can physically protect favorable sorption sites within soil organic matter (SUM), and inhibit butachlor sorption by influencing SUM physical conformation. Comparisons of changes in butachlor sorption coefficients (both K-d and K-oc) in two different series of soils, with the same mineral components but gradients of total organic carbon (TOC) and RCO values also showed that minerals can directly contribute to soil butachlor sorption processes, which may be even more pronounced in soils with relative higher RCOs. A new adsorption model was proposed and verified to quantify the net contribution of minerals to butachlor sorption, based upon 38 different soils. This study has increased our ability to quantify the positive direct contribution of soil minerals and their negative indirect contribution through associated effects on SOM physical conformation during butachlor sorption in natural soils. (C) 2014 Elsevier B.V. All rights reserved.