Eco-friendly removal of strontium from produced water of an oilfield in western Iran using biosurfactant derived from Malva sylvestris leaves.

Oilfield-produced water is a major environmental concern due to elevated concentrations of dissolved metals, notably Sr, which can form insoluble scales as SrSO₄ and facilitate the co-precipitation of naturally occurring radioactive materials. In this study, for the first time, a biosurfactant extracted from Malva sylvestris leaves, rich in saponins, was explored as a green and biodegradable agent for Sr removal from highly saline produced water collected in western Iran. Batch experiments investigated the effect of temperature (25 °C and 60 °C) and biosurfactant concentration (0.006, 0.015, and 0.024 g/mL) on removal efficiency. The removal mechanism is primarily attributed to coordination between Sr ions and the deprotonated carboxylate groups in saponin molecules, forming stable Sr-saponin complexes. Elevated temperature enhances this process through increased molecular mobility, partial dehydration of Sr2⁺ hydration shells, and favorable thermodynamic shifts, as indicated by Van't Hoff analysis. Increasing biosurfactant concentration supplies more active binding sites and improves selectivity over competing cations such as Ca2⁺ and Mg2⁺. Time-dependent experiments further revealed that complexation starts after 2 days, and complete sedimentation was observed at the end of the test (day 5). The absence of further Sr reduction between days 5 and 10 indicates the long-term stability of the complexes and their resistance to re-dissolution. A peak Sr removal efficiency of 63.6% was achieved at 60 °C and 0.024 g/mL with no significant change in the pH of the aqueous phase, demonstrating the potential of M. sylvestris for stable and efficient Sr removal from real oilfield produced water. This approach provides a novel, sustainable, and cost-effective alternative to conventional synthetic chemicals for produced water treatment.