Synergistic inhibition effect and mechanism of polycation and polyanion on colloidal silica

Inhibition of silica scaling with functional polymers: Role of ionic strength, divalent ions, and temperature

High concentrations of dissolved silica in saline industrial wastewaters and brines cause silica scale formation, significantly hampering the efficacy of diverse engineered systems. Applying functional polymers as scale inhibitors in process feedwater is a common strategy to mitigate silica scaling. However, feedwater characteristics often vary widely, depending on the specific processes, making the inhibition of silica scaling challenging and complex. In this study, we systematically investigate the role of ionic composition, specifically ionic strength and divalent ions, and solution temperature, in inhibiting silica scaling using molecularly designed amine/amide polymers. The inhibitor demonstrates effective stabilization of silicic acid, with inhibition efficiency of 74 and 55 % in the absence and presence of 20,000 ppm NaCl, respectively. However, further increasing the ionic strength of oversaturated silicic acid solutions significantly diminishes inhibition performance, rendering it ineffective at 180,000 ppm NaCl. Divalent inorganic cations exhibit a stronger impact on reducing inhibition efficiency compared to sodium ions. Molecular dynamics simulations reveal a competition mechanism between anionic silicic acid reactants (i.e., H3SiO4-) and chlorides for binding to ammonium groups within the polymeric inhibitor. Additionally, cations form clusters with H3SiO4- ions, hindering their stabilization with polymeric inhibitor. Notably, at elevated temperatures, the inhibitor achieves near-perfect inhibition for 500 ppm silicic acid solutions. This comprehensive assessment provides important insights into the effectiveness of silica scaling inhibitors under solution conditions relevant to real-world applications, addressing the challenges posed by varying solution parameters in diverse industrial processes.

Molecular Design of Functional Polymers for Silica Scale Inhibition

Silica polymerization, which involves the condensation reaction of silicic acid, is a fundamental process with wide-ranging implications in biological systems, material synthesis, and scale formation. The formation of a silica-based scale poses significant technological challenges to energy-efficient operations in various industrial processes, including heat exchangers and water treatment membranes. Despite the common strategy of applying functional polymers for inhibiting silica polymerization, the underlying mechanisms of inhibition remain elusive. In this study, we synthesized a series of nitrogen-containing polymers as silica inhibitors and elucidated the role of their molecular structures in stabilizing silicic acids. Polymers with both charged amine and uncharged amide groups in their backbones exhibit superior inhibition performance, retaining up to 430 ppm of reactive silica intact for 8 h under neutral pH conditions. In contrast, monomers of these amine/amide-containing polymers as well as polymers containing only amine or amide functionalities present insignificant inhibition. Molecular dynamics simulations reveal strong binding between the deprotonated silicic acid and a polymer when the amine groups in the polymer are protonated. Notably, an extended chain conformation of the polymer is crucial to prevent proximity between the interacting monomeric silica species, thereby facilitating effective silica inhibition. Furthermore, the hydrophobic nature of alkyl segments in polymer chains disrupts the hydration shell around the polymer, resulting in enhanced binding with ionized silicic acid precursors compared to monomers. Our findings provide novel mechanistic insights into the stabilization of silicic acids with functional polymers, highlighting the molecular design principles of effective inhibitors for silica polymerization.

Probing interaction forces associated with calcite scaling in aqueous solutions by atomic force microscopy

The prevention of calcite aggregation and scaling remains a challenging problem in aqueous based systems and environmental science. Decades of research studies have proposed microscopic mechanisms of aggregation control, but experiments at the nanoscale and molecular level are rarely conducted. Here we show that the nanoscale topographic features of calcite during its aggregation depend significantly on the intermolecular and surface forces involved in this process. By measuring the forces between a calcite or silica particle and a calcite surface in aqueous solutions using atomic force microscopy, we found that higher solution pH and inhibitor concentration and lower salinity resulted in a system of stronger repulsion and weaker adhesion, which is favorable for reducing the possibility of calcite aggregation and surface deposition. Conflicting roles of Mg²⁺ in calcite aggregation prevention, being positive in acidic pH and negative in alkaline pH, were also observed. The nanoscale structural changes of calcite, visualized by atomic force microscopy or scanning electron microscopy, indicated a size dependence of aggregated and deposited calcite crystals on the calcite-calcite and calcite-silica interactions, respectively. The generalized framework of the calcite aggregation mechanism achieved in this work can be extended to other types of systems and provides a basis for investigating the anti-aggregation strategy of calcite from industrial and environmental perspectives.

Silicic Acid Removal by Metal-Organic Frameworks for Silica-Scale Mitigation in Reverse Osmosis

Reverse osmosis (RO) membranes are susceptible to silica scaling, resulting in irreversible degradation of membrane performance. This work covered the fabrication of MIL-101(Fe) for silicic acid adsorption to alleviate the silica scaling of RO membranes. The effect of pH, mixing time and initial concentration on silicic acid adsorption of MIL-101(Fe) was appraised in detail. The adsorption experiments demonstrated that MIL-101(Fe) possessed an excellent adsorption ability for silicic acid with the maximum adsorption capacity reaching 220.1 mgSiO₂·g^-1. Data fitting confirmed the pseudo-second-order equation and Freundlich equation were consistent with silicic acid adsorption on MIL-101(Fe). Finally, a simulated anti-scaling experiment was carried out using a feed solution pretreated by MIL-101(Fe) adsorption, and the permeance exhibited a much lower decline after 24 h filtration, confirming that MIL-101(Fe) exhibits an excellent application potential for silica-scale mitigation in RO systems.

Study on the Corrosion and Scale Inhibition Mechanism of the Thiourea-Modified Polyepoxysuccinic Acid (CNS-PESA)

In this paper, thiourea-modified polyepoxysuccinic acid (CNS-PESA) was synthesized and used as a corrosion and scale inhibitor for the circulating cooling water system of the factory. Polyepoxysuccinic acid (PESA) has been widely used in industrial water treatment due to its good corrosion inhibition and scale inhibition performance. It also acts as a reliable material for dispersion of iron oxide, good biodegradation efficiency, and phosphorus-free content. The scale inhibition performance for calcium carbonate and calcium phosphate scale with corrosion inhibition performance using different inhibitor concentrations, temperatures, pH, and calcium ion concentration on scale inhibition efficiency were studied. The scanning electron microscope (SEM) analysis on the scale and corrosion of carbon steel with two methods as GB/T 18175-2000 “Measurement of inhibitors in water treatment: rotation coupon method” and electrochemical method for CNS-PESA and PESA were utilized. Compared to using CNS-PESA and PESA for calcium carbonate and calcium phosphate, the scale inhibition rates were increased by 6% and 18.7%, and the corrosion inhibition rate increased by 22.4%. The scale inhibition efficiency of CNS-PESA is best when the calcium ion concentration is less than or equal to 350 mg/L. Moreover, the thiourea-modified polyepoxysuccinic acid (CNS-PESA) was phosphorus-free corrosion and scale inhibitor and minimizes environmental pollution problems.

Rhamnolipids produced under aerobic/anaerobic conditions: Comparative analysis and their promising applications

This research comprises a comparative study of the properties, rhl genes expression, and structural difference in rhamnolipids produced under different oxygen conditions via Pseudomonas sp. CH1. The critical micelle concentration (CMC) of rhamnolipids produced under aerobic conditions (RAO) was 100 mg/L. In contrast, rhamnolipids produced under anaerobic conditions (RNO) had a low CMC of 40 mg/L. RNO comprised six rhamnolipids homologs, and the proportion of mono-rhamnolipids was up to 87.83%; meanwhile, the percent ratio of di-rhamnolipids and mono-rhamnolipids in RAO was 63.1:36.9. Additionally, diversified applications for solubilization of hydrophobic pollutants and reduction in heavy oil viscosity were investigated. The addition of RNO greatly enhanced the solubility of phenanthrene in water, from 1.29 mg/L to 193.14 mg/L, a 148.7-fold increase. Moreover, the viscosity of heavy oil decreased by over 90% for both kinds of rhamnolipids, whereas RAO effectively reduced the viscosity even at a low temperature (10 °C). The findings of this study provide insights into the versatile potential applications of rhamnolipids produced under different oxygen conditions.