Comparison of pyrophosphate and orthophosphate removal by boehmite and kaolinite

Considerable quantities of pyrophosphate and orthophosphate are introduced to soils as fertilizers and fire retardants. While the fate of orthophosphates added to soils has been extensively studied, less attention has been paid to the behaviour of pyrophosphates in soils. In this work sorption edge and surface charge measurements of orthophosphate and pyrophosphate by boehmite and kaolinite, were conducted at 10, 25 and 50 °C between pH 3.5–11.5. Pyrophosphate and orthophosphate sorption edges differ significantly, showing greater quantities of phosphorous as pyrophosphate sorbed by the minerals in lower pH. Boehmite consistently had higher pyrophosphate uptake across the three temperatures compared to kaolinite. For kaolinite, temperature changes had a more pronounced influence on pyrophosphate sorption. Temperature increases also shifted boehmite point zero charge to lower pH. Analysis of pyrophosphate isotherm data suggests outer-sphere electrostatic attraction is the principal interaction between absorbent and absorbate material at pH 8. The difference observed in sorption results indicate that the type and structure of various phosphates will affect sorption to different mineral types and stresses the importance of studying pyrophosphate sorption processes under a range of environmental conditions.