Impact of Selected Biomolecules on CaCO3 Scale Formation at pH 8

Summary Mineral scale formation is a challenging issue in oil and gas production, causing economic losses and safety problems. Scale inhibitor is one of the most economical and effective methods to mitigate scale damage, and green scale inhibitors with better biodegradability, less bioaccumulation, and less toxicity are needed. Biomolecules, such as amino acids, polysaccharides, and phospholipids, exhibit potential as green scale inhibitors because of their prevalence in the biosphere. However, to the best of our knowledge, systematic study of the quantitative effects of biomolecules on mineral crystallization kinetics remains scarce, preventing their industrial applications. In this study, the induction time (tind) of calcium carbonate (CaCO3) with or without nine biomolecules, including four amino acids [L-aspartic acid (L-Asp), L-glutamic acid (L-Glu), L-histidine (L-His), and L-arginine (L-Arg)], three polysaccharides [pullulan (PL), xanthan gum (XG), and carboxymethyl cellulose (CMC)], and two phospholipids [phosphatidyl ethanolamine (PE) and phosphatidyl serine (PS)], was measured using a laser apparatus at conditions of a calcite saturation index (SIcalcite) from 1.00 to 1.90, pH from 7.78 to 8.05, temperature (T) from 25°C to 90°C, and biomolecule concentrations from 0 to 500 mg/kg water (H2O). The results showed that negatively charged amino acids (L-Asp, L-Glu) and polysaccharides (CMC) show inhibitive effects, while the positively charged amino acid (L-Arg) shows promotion effects, and the neutral biomolecules (L-His, PL, and XG) show negligible effects. However, the positively charged PE also shows inhibitive effects, while the negatively charged PS shows negligible effects. In addition, under conditions with elevated calcium ion (Ca2+) concentration and lower carbonate ion (CO32−) concentration at 25°C, the inhibition efficiencies of the negatively charged L-Asp and CMC are enhanced, while that of the positively charged PE remains unchanged. As a result, it is proposed that the interactions between aqueous ions, crystals (or nuclei), and the biomolecules are based not only on electrostatic interaction but also on competition between aqueous species and surface adsorption, governing CaCO3 crystallization kinetics. The morphology changes measured by scanning electron microscope showed that the biomolecules that inhibit CaCO3 crystallization (L-Asp, L-Glu, CMC, and PE) tend to precipitate near-spherical CaCO3 solids with smooth surfaces and edges, while the biomolecules promoting CaCO3 crystallization (L-Arg) are inclined to precipitate CaCO3 with better structured rhombohedral crystals, and biomolecules showing no impact (L-His, PL, and XG) on the crystallization have no effects on morphology. This study provides experimental data and theoretical support for the development of new green scale inhibitors from naturally present biomolecules.