Chemical Control of Magnesium Silicate Precipitation in Magnesium- and Silica-Loaded Process Waters by Phosphonate-Functionalized and Systematically Elongated Linear Amino Acids

The application of five systematically structured homologous phosphonated linear amino acids (PLAAs) as inhibitors of magnesium silicate scale formation is reported. The general formula of PLAAs is HOOC-(CH2)n-N-(PO3H2)2 (PLAA-C2, n = 1; PLAA-C3, n = 2; PLAA-C4, n = 3; PLAA-C5, n = 4; PLAA-C6, n = 5). These antiscalants are considered environmentally acceptable and are derived from their respective inherently biodegradable amino acids. The ethylamino-N-di(methylenephosphonate) analogue (C2D), which contains an ethyl instead of a –COOH group, was also tested for comparison reasons. The PLAAs and C2D were found to act as stabilizing agents, most likely by complexing the Mg2+ cations, thus preventing magnesium silicate formation. All additives exhibit similar dosage-dependent performance, and neither the presence of the alkyl carboxylate moiety nor its length influences their inhibitory activity. The formed precipitates are Mg2+-absorbed amorphous silica that incorporates an entrapped inhibitor. The extent of Mg2+ incorporation into the precipitate is not affected by either the antiscalant nature or dosage. To further shed light on the mode of antiscalant binding/chelation to Mg2+ ions, the “complexes” Mg-PLAA-C2, Mg-PLAA-C6, and Mg–C2D were synthesized and structurally characterized, revealing that both phosphonate groups bind to Mg2+ but the carboxylate moiety does not interact with the Mg2+ center.