Enhanced scale inhibition against Ca3(PO4)2 and Fe2O3 in water using multi-functional fluorescently-tagged antibacterial scale inhibitors

Abstract

Formation and deposition of scales, especially the refractory Ca₃(PO₄)₂ and Fe₂O₃, is an obstacle for water treatment and reuse. In situ concentration detection of P-free scale inhibitors also remains challenging. In this work, well-designed P-free copolymers, FM–AA–APEO with ternary units (acrylic acid (AA), allyl-polyethylene oxide (APEO), and fluorescent monomer (FM)), were employed as scale inhibitors against Ca₃(PO₄)₂ and Fe₂O₃. The effects of ratios among different units were studied, and the optimized molar percentages of FM, AA, and APEO were 1.56%, 87.50%, and 10.94%, respectively. The optimized FM–AA–APEO exhibited much better scale inhibition (higher efficiency and lower dosage required) and antibacterial performance than commercial products. Fluorescence intensity and inhibitor concentration had good correlation (R² > 0.99) regardless of the absence or presence of metal ions, providing a basis for in situ concentration detection. Scale inhibition mechanism investigations from micro- to macro-viewpoints demonstrated that the enhanced performance of FM–AA–APEO resulted from the synergistic effects of ternary compositions: AA units tightly bound metal atoms on small crystal particles via coordination and provided negative surface charges for electrostatic repulsion; APEO units with stretched hydrophilic sidechains contributed to steric repulsion among the particles; FM units not only exhibited designed in situ concentration determination ability (fluorescent groups in FM) and antibacterial effects (quaternary ammonium groups in FM), but also had unexpected strong coordination with Fe(III) atoms for strengthening Fe₂O₃ scale inhibition. Understanding of the above mechanism provided more specific design strategies for the development and application of scale inhibitors in water.