Retention and Release of Commercial and Purified Phosphonate Scale Inhibitors on Carbonate Substrate

This paper describes a study of the interactions of phosphonate scale inhibitor with carbonate substrate. Much previous work has appeared on this topic, but here we present results which attempt to address some gaps identified in previous studies of this subject. The experimental programme focused on three main areas: (i) static adsorption/ compatibility analysis of phosphonate scale inhibitor at both 95°C and room temperature (RT). Static tests revealed that SI retention mechanisms are significantly more active at elevated temperatures compared to RT conditions, where only minimal adsorption was observed. At RT conditions with initial pHo = 4, while calcite dissolution occurs and Ca2+ may interact with SI, the formation of precipitate is minimal. Under these conditions, SI concentration primarily governs pH behaviour. These experimental results provided validation data for computational modelling work, which is presented in a separate study [1]. And: (ii) precipitation and re-dissolution tests of SI-Ca2+ complexes which were conducted across a temperature range of 20-95°C, with a subsequent larger-scale test at 95°C. For systems with high [Ca2+], the smaller-scale experiments yielded similar masses of precipitate (post-filtration and oven-drying) across all temperatures. Complex stoichiometry was determined using two methods: direct analysis of re-dissolved precipitates in distilled water/HCl, and indirect measurement of Δ[Ca] and Δ[SI] from supernatant solutions, both using Inductively Coupled Plasma - Optical Emission Spectroscopy (ICP-OES). The stoichiometric analyses revealed that excess [Ca2+] and initial pH of 8.5, rather than temperature, governed the reaction, resulting in near maximum possible complexation between Ca2+ and DETPMP in solution. The precipitates were characterized using ESEM-EDX and thermogravimetric analysis (TGA). ESEM-EDX surface imaging and compositional analysis demonstrated amorphous structures across all temperature conditions, while TGA results showed decreasing water content with increasing preparation temperature. Finally, (iii) Purified SIs obtained at 95°C were used to examine how the removal of phosphorus-containing impurities affects inhibition and adsorption performance. A series of inhibition efficiency (IE) and static adsorption experiments were conducted. The precipitated and redissolved DETPMP samples were evaluated against unmodified commercial DETPMP. Their effectiveness in preventing BaSO4 precipitation through Ba2+ interaction was assessed by measuring Δ[Ba2+] before and after SI addition to the brines using ICP analysis. Results demonstrated that purified materials exhibited similar barium sulphate inhibition efficiency to commercial products, indicating that impurities did not significantly influence the inhibition process. Comparative adsorption studies revealed higher apparent adsorption values for purified DETPMP, attributed to impurities in commercial products being measured as active DETPMP concentrations despite not participating in adsorption. It is shown how this can be easily corrected and accounted for in our results.