Kinetics and Thermodynamics of Phosphorus Sorption By Goethites: Effects of Biochar Application.

Plant availability of phosphorus (P) in soils is controlled by, inter alia, the presence of oxides of iron and aluminum, which are widely distributed in soils. These oxides readily bind P, thus reducing the plant availability of this critical nutrient. Many studies have examined ways of improving soil fertility by minimizing P sorption to oxides, with one of the promising approaches being the use of biochar. Results from a handful of studies evaluating the effects of biochar on P sorption by goethite suggest that use of biochar can decrease P sorption by goethite, thus enhancing P bioavailability for plant uptake.  However, to date, the kinetics and temperature dependence of biochar influence on P sorption have not been investigated. This laboratory batch experiment examined the effects of biochar application and temperature (15, 25, and 35°C) on the kinetics and thermodynamics of P sorption by laboratory-synthesized goethite and aluminum-substituted goethite. Results showed that biochar application enhanced P sorption on both oxides. Phosphorus sorption by the goethites was adequately described by a pseudo-first order kinetic model at all temperatures tested. Sorption rate constants were statistically similar at 15 and 25°C but lower at 35°C. Phosphorus sorption by biochar-amended oxides was associated with low activation energy, indicating that the sorbed P can still become available to plants.