Retention of dissolved organic carbon from vinasse by a tropical soil, kaolinite, and Fe-oxides

Abstract

Vinasse is a sugarcane distillery waste water commonly applied to soil in Brazil. The content of dissolved organic matter (DOM) in vinasse is high, varying from 10,973 to 14,801 mg l −1 of dissolved organic carbon (DOC). This study examines the capacity of an Ultisol (from a sugarcane-growing region), kaolinite and synthetic Fe-oxides to retain DOM extracted from vinasse. Adsorption isotherms of the A, BA, Bt1, Bt3 and Bt4 horizons of the Ultisol, as well as of those of the kaolinite and Fe-oxides, were obtained through batch equilibrium experiments using DOC concentrations, simulating vinasse additions of 0, 43, 86, 344, 688 and 1376 m 3 ha −1. Adsorption isotherms of DOM extracted from vinasse on different horizons of the Ultisol indicated that DOM adsorption increased with depth. Distinct adsorption capacity of these horizons was related to differences in dithionite-citrate-bicarbonate (DCB) extracted Fe, native organic matter and clay content. At an initial DOC concentration of 160 mg l −1 (equivalent to the application of 688 m 3 vinasse ha −1), the A horizon retained 7.1% and the Bt4 horizon retained 27.4% of the total DOC in solution, suggesting that this soil has a relatively low adsorption capacity for DOM from vinasse. Adsorption isotherms of DOC on pure kaolinite, synthetic goethite and hematite showed that the maximum adsorption capacity of the Fe oxides was as much as about five times the maximum adsorption capacity of kaolinite. Desorption studies indicated that while all DOC adsorbed by kaolinite were completely desorbed, only 28 to 35% of the adsorbed DOC were desorbed by Fe-oxides. These findings highlight the importance of the goethite and hematite on the adsorption of DOM in tropical soils.