The Effects of Phosphonate-Based Scale Inhibitor on Brine-Biotite Interactions under Subsurface Conditions

To explore the effects of scale inhibitors on subsurface water-mineral interactions, here batch experiments on biotite dissolution (0-96 h) were conducted in solutions containing 0-1.0 mM diethylenetriaminepenta(methylene)phosphonate (DTPMP, a model scale inhibitor), at conditions simulating subsurface environments (95 °C and 102 atm CO₂). The phosphonate groups in DTPMP enhanced biotite dissolution through both aqueous and surface complexations with Fe, with more significant effects at a higher DTPMP concentration. Surface complexation made cracked biotite layers bend, and these layers detached at a later stage (≥44 h). The presence of DTPMP also promoted secondary precipitation of Fe- and Al-bearing minerals both in the solution and on the reacted biotite surfaces. With 1.0 mM DTPMP after 44 h, significant coverage of biotite surfaces by precipitates and less detachment of cracked layers blocked reactive sites and inhibited further biotite dissolution. Furthermore, adsorption of DTPMP made the reacted biotite basal surfaces more hydrophilic, which may affect the transport of reactive fluids. This study provides new information on the impacts of phosphonates in brine-mineral interactions, benefiting safer and more environmentally sustainable design and operation of engineered subsurface processes.