Commercial and Purified Phosphonate Scale Inhibitors in Carbonate Systems: Retention, Release, and Supernatant Reactivity

This paper describes a study of the interactions of diethylenetriamine penta(methylene phosphonic acid) (DETPMP), one of the most widely used phosphonate scale inhibitors (SIs), with carbonate substrates. Much previous work has appeared on this topic, but here, we present results addressing key gaps identified in earlier studies. The experimental program focused on four main areas: (i) static adsorption/compatibility analysis of DETPMP at both 95 and 21 °C. Static tests revealed that SI retention mechanisms are significantly more active at elevated temperatures compared to 21 °C, where only minimal adsorption was observed. At 21 °C with initial pH₀ = 4, calcite dissolution occurs, and Ca²⁺ may interact with DETPMP, but precipitate formation is minimal. The SI’s concentration primarily governs pH behavior under these conditions. The experimental results presented here provide data to validate computational modeling work reported in a separate study (Kalantari Meybodi et al., Colloids Surf. A Physicochem. Eng. Asp. 2024, 133535). (ii) Precipitation and re-dissolution tests of DETPMP_Ca complexes, which were conducted across a temperature range of 21–95 °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. The stoichiometries of the DETPMP_Ca complexes formed were determined using two methods: direct analysis of redissolved precipitates in distilled water/HCl and indirect measurement of Δ[Ca] and Δ[DETPMP] from supernatant solutions, both using Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). The stoichiometric analysis revealed that the excess [Ca²⁺] and initial pH of 8.5, rather than temperature, governed the reaction, almost achieving maximum complexation (5 ligands) 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 that the complexes had amorphous structures under all temperature conditions, while TGA results showed that the complexes had decreasing water content with increasing preparation temperature. (iii) The purified SIs obtained at 95 °C were used to examine how removing phosphorus-containing impurities affects inhibition and adsorption performance. A series of inhibition efficiency (IE) and static adsorption experiments compared purified and commercial DETPMP. Their effectiveness in preventing BaSO4 precipitation was assessed by measuring Δ[Ba²⁺] before and after the addition of DETPMP to the brines using ICP analysis. Results demonstrated that purified materials exhibited very similar barium sulfate inhibition efficiency to commercial products, indicating that the impurities did not significantly influence the inhibition process. Comparative adsorption studies revealed higher apparent adsorption values for purified DETPMP, attributed to the impurities in the commercial products being measured as active DETPMP concentrations despite not participating in the adsorption process. It is shown how this can easily be corrected and accounted for in our results. (iv) The supernatant solution from the DETPMP_Ca complexation process was analyzed following initial precipitation. The effectiveness of this supernatant solution was then evaluated for BaSO₄ scale inhibition. The results indicated that these residuals showed almost no efficiency in inhibiting BaSO4 formation. Subsequently, precipitation experiments were conducted to further investigate the complexation and precipitation behavior of the supernatant materials, revealing that only small amounts of DETPMP_Ca complexes were actually present; i.e., the relatively high apparent “DETPMP” concentration was mainly (P-containing) impurity and not DETPMP.