The Role of Fe-Bearing Phyllosilicates in DTPMP Degradation under High-Temperature and High-Pressure Conditions

To ensure safer and more efficient unconventional oil/gas recovery and other energy-related subsurface operations, it is important to understand the effects of abundant Fe-bearing phyllosilicates on the degradation of phosphonates, which are applied to inhibit scale formation. In this study, under subsurface relevant conditions (i.e., slightly oxic owing to oxygen-containing injection, 50-95 °C, and 102 atm CO₂), we reacted 0.5 mM DTPMP (diethylenetriaminepenta(methylene)phosphonate, a model phosphonate) with three phyllosilicates: an Fe-poor muscovite, an Fe(II)-rich biotite, and an Fe(III)-rich nontronite. The three phyllosilicates induced different effects on DTPMP degradation, with no distinguishable effect by muscovite, slight promotion by nontronite, and remarkable promotion by biotite. We found that Fe associated with phyllosilicates is key to the redox degradation of DTPMP: reactive oxygen species (ROS) were generated through the reduction of molecular oxygen by Fe(II) adsorbed on the mineral surface or in the mineral structure, and the hydroxyl radicals further degraded DTPMP to form phosphate, formate, and DTPMP residuals. In addition, DTPMP degradation was favored at higher temperatures, probably resulting from more exposed reactive Fe(II) sites created by enhanced biotite dissolution and also from faster electron transfers. Dissolved Fe and Al precipitated with phosphate or degraded DTPMP and formed secondary minerals. This study provides new information about how DTPMP degradation is affected by the presence of Fe-bearing phyllosilicates under high-temperature and high-pressure conditions and has implications for engineered subsurface operations.