Inhibition of aluminum silicate scaling by phosphonate additives under geothermal stresses

•Phosphonate additives demonstrate variable stabilization effectiveness for aluminum silicate.•Phosphonates act as chelators/ligands for Al3+ ions, and thus, “turn off” their catalytic effect in silica polycondensation.•The silica precipitates that form in the presence of Al3+ ions are Al3+-absorbed amorphous silica, with variable Al3+ content.•Phosphonate entrapment into the precipitates occurs as a result of Al-induced formation of sparingly soluble Al−phosphonate “complexes”. The formation, precipitation, and deposition of so-called “aluminum silicate” in geothermal waters have been subjects of intense interest. Such scaling poses a severe threat to the smooth process of any water-supported industrial system that utilizes water that is high in both aluminum and silicate. The aim of our approach is the systematic study of the influence of phosphonate-based chemical additives on silica polycondensation chemistry in the presence of aluminum ions. The focus of this work is the prevention of “aluminum silicate” formation using a variety of well-known phosphonate additives. These are: 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC), hydroxyethylidene-1,1-diphosphonic acid (HEDP), amino-tris(methylenephosphonic acid) (AMP), hexamethylenediamine-tetrakis(methylene-phosphonic acid) (HDTMP), diethylenetriamine-pentakis(methylene phosphonic acid) (DTPMP) and bishexamethylenetriamine-pentakis(methylenephosphonic acid) (BHMTPAMP). Inhibition experiments were carried out in solutions of silicate (150 ppm, expressed as SiO2) and aluminum (a variety of concentrations, as Al) at pH 7.0. The phosphonate additives used were found to act as stabilizing agents, most likely by complexing the Al3+ cations and, thus, preventing “aluminum silicate” formation. On the basis of a plethora of experimental data, a number of useful functional insights have been generated, which add to building a more complete and comprehensive picture of the mechanism of “aluminum silicate” formation and stabilization. [Display omitted]