Mechanism of organic-driven homogeneous nucleation transition for Ca-organics co-removal in oil drilling wastewater reclamation

Fracturing wastewater mainly contains high concentrations of hardness-forming ions and organic compounds, which can hinder on-site treatment and reuse in drilling operations. In this study, a nucleation crystallization pelleting (NCP) process was developed. In a three-stage full-scale system, NCP reduced total hardness to <300 mg/L and removed >30% of organic compounds, achieving water reclamation and reuse. An electrospray Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) was used to examine the characteristics and properties of the removed organic compounds and their impact on the NCP system. The result showed that the preferentially removed species comprised highly unsaturated compounds, containing acidic, oxygenated, and sulfur-containing functional groups (e.g., -COOH, -OH, -S, and -S(=O)₂⁻). Using five betaines as model organic molecules, the combination of the experiments and CP2K calculations revealed two concurrent removal pathways at high pH: Ca²⁺ coprecipitation and adsorption on the CaCO₃ (1 0 4) surface, with adsorption dominated by van der Waals interactions. These Ca–organic species were adsorbed onto the CaCO₃ (1 0 4) surface and formed dense pellets under hydraulic shear. Continuous operation also showed the formation of loose flocs with progressive densification of the pellets. This phenomenon reflects the adsorption of organic compounds on active sites of the nucleation seeds, limiting Ca²⁺/CaCO₃ access to seed surfaces. Consequently, the growth of deposited organic compounds on active sites toward homogeneous nucleation by slowing heterogeneous crystallization. However, effluent hardness remained within on-site reuse standards across organic loadings, indicating that overall treatment. Therefore, this study elucidates the mechanisms underlying the simultaneous hardness and organic removal and how organic compounds influence NCP crystallization, providing a basis for optimizing and scaling NCP technology for fracturing wastewater reuse.