Stable isotope fractionations during reactive transport of phosphate in packed-bed sediment columns

Characterizing reactivity and fate of contaminants in subsurface environments that are isolated from direct visualization is a major challenge. Stable isotopes coupled with concentration could be used as a potential tool to quantitatively analyze the chemical variability of the contaminant during reactive transport processes in the subsurface environment. This study was aimed at determining whether abiotic reactions of phosphate during its transport involve fractionation of oxygen isotopes in phosphate (δ18Op). It included the effects of solution chemistry and hydrodynamics on δ18Op values during phosphate transport through a packed-bed column prepared by using natural sediment collected from the Cape Cod aquifer in Massachusetts. Results show that the isotopic fractionation between effluent and influent phosphate at early stage of transport could be ~ 1.3‰ at higher flow rates with isotopically-light phosphate (P16O4) preferentially retained in the sediment column. This fractionation, however, decreased and became insignificant as more phosphate passed through the column. Mobilization of phosphate initially sorbed onto sediments caused a large kinetic isotopic fractionation with isotopically-light phosphate preferentially remobilized from the sediment column, but over longer time periods, this fractionation decreased and became insignificant as well. These results collectively suggest that abiotic reactive transport processes exert minimal influence on the δ18Op composition of subsurface systems. Alternatively, fluctuation in flow rate and subsequent remobilization of phosphate could be detectable through transient changes in δ18Op values. These findings extend the burgeoning application of δ18Op to identify the different sources and geochemical processes of phosphate in the subsurface environments.