Organo-mineral interactions mask the true sorption potential of biochars in soils

Abstract

The sorption of carbaryl (1-naphthyl methyl carbamate) and ethion [O,O,O′,O′-tetraethyl S,S′-methylene bis(phosphorodithioate)] was studied in whole soils as well as after treatment of soil with 2% hydrofluoric acid (HF) to remove paramagnetic materials and to oxidize most forms of labile carbon by photo-oxidation with high energy (UV) on < 53 μ m fractions. The sorption coefficient (K_d) values for carbaryl and ethion in soils did not follow the order of their organic carbon (OC) content, and specially their char content However, the K_oc values in < 53 μ m fractions after hydrofluoric acid/photo-oxidation with high energy (hydrofluoric acid/ultraviolet; HF/UV) treatment were found to be much higher than those in bulk untreated soils. The effect of organic matter chemistry was determined by correlating K_oc values of contaminants in bulk soils or 53 μ m fractions against sample aromaticity. A poor correlation of K_oc in bulk soil and aromatic C values of both carbaryl and ethion was observed. However, the correlation between the K_oc and the aromatic fraction of C after the HF/UV treatment improved significantly, reflecting the contribution of char fraction of carbon in soils towards sorption of pesticides. The increase in sorption after HF/UV treatment suggested that the sorption potential of biochars, which are expected to contribute significantly to contaminant sorption due to their high surface area, can remain masked by the organo-mineral interactions of char in whole soils. This has implications for the modification of surfaces of the freshly applied biochars in soils due to organo-mineral interactions.

Keywords:

• Carbaryl
• ethion
• sorption
• soils
• chars

Acknowledgments

We acknowledge J. O. Skjemstad and colleagues for providing soil samples and their nuclear magnetic resonance (NMR) data.

This paper was presented at the IUPAC sponsored International Conference on Agrochemicals Protecting Crop, Health and Natural Environment held in New Delhi, India, January 8–11, 2008.