The formation of a structural framework in gelled Wyoming bentonite: direct observation in aqueous solutions

Rethinking the relationships between gel like structure and sludge dewaterability based on a binary gel like structure model: Implications for the online sensing of dewaterability

The digital transformation of sludge treatment processes requires online sensing of dewaterability. This topic has been attempted for many years based on macroscopic shear rheology. However, the relationship between rheological behavior and dewaterability remains noncommittal, and the reason is unclear. Herein, a binary gel-like structure model was proposed including the interactions network at the supra-flocs level and the gel-like structure at the flocs level. Multiple advanced techniques including optical tweezers were employed to precisely understand the binary gel-like structure and to classify the correlation mechanism between this gel-like structure, rheological behavior, and dewaterability. The analysis of sludge from eight wastewater treatment plants showed the binary gel-like structures at both supra-flocs and flocs levels have significant relationships with sludge dewaterability (p < 0.05). Further deconstruction of the sludge viscoelastic behavior illustrated that the gel-like structure at the supra-flocs level dominates the rheological behavior of sludge. Moreover, the direct description of the binary gel-like structure in four typical sludge treatment processes highlighted the importance of the flocs level's structure in determining the dewaterability. Overall, this study revealed that shear rheology may prefer to stress the interactions network at the supra-flocs level but mask the flocs level's structure, although the latter is important. This observation may provide a general guideline for the design of robust sensors for dewaterability.

Dewatering behavior and regulation mechanism of montmorillonite nanosheet in aqueous solution

Two-dimensional montmorillonite nanosheet (MMTNS) is desirable building block for fabricating multifunctional materials as due to its extraordinary properties. In practical applications, however, the concentration of MMTNS prepared by exfoliation is normally too low to be used for material assembling. The general thermal-concentration method is effective, however, it can be time-consuming and require a lot of energy. In this case, the remarkable dispersion stability of MMTNS is worth noting. Herein, the extraordinary dispersion stability of MMTNS derived from electrostatic and hydration repulsion was firstly revealed by molecular dynamics (MD) simulation, which caused the poor dewatering of MMTNS. Further, based on the surface and structural chemistry of MMTNS, a series of strategies, involving charge and cross-linked structure regulation on the edge surface, as well as electrical double-layer modulation and calcification modification based on the electrolytes, were proposed to inhibit the dispersion and enhance the aggregation of MMTNS. Intriguingly, a novel chemical, Tetraethylenepentamine (TEPA) was applied in the dewatering of MMTNS. The TEPA not only act as a cross-linker to bond with MMTNS into an easy-to-dewatering 3D network structure, but also act as a switch for effortless viscosity tuning. Meanwhile, the dual function of electrolytes for electrical double layer compression and calcification modification of MMTNS was investigated by DLVO theory and structural analyses. This work offers explicit directions for improving the dewatering performance of MMTNS to meet the requirements of practical implementation.

Ionic strength and polyelectrolyte molecular weight effects on floc formation and growth in Taylor-Couette flows

Polyelectrolyte-driven flocculation of suspended particulate in solution is an important process in a variety of industrial processes such as drinking water treatment and composite material synthesis. Flocculation depends on a wide variety of physicochemical and hydrodynamic properties, which affect floc size, growth rate, and floc morphology. Floc formation and growth behavior is explored here using two different molecular weights of a cationic polyacrylamide flocculant and anisotropic Na-bentonite clay particles under a variety of solution ionic strengths. A Taylor-Couette cell with radial injection capabilities was used to study the effects of solution ionic strength and polyelectrolyte molecular weight on floc size, growth rate, and floc morphology during the flocculation process with a constant global velocity gradient. The floc size generally decreased with increasing ionic strength whereas the floc growth rate initially increased then decreased. This likely occurred due to charge screening effects, where increased bentonite aggregate size and a less expanded polyelectrolyte conformation at higher ionic strengths results in a decreased ability for the polyelectrolyte to bridge multiple bentonite aggregates. The densification of bentonite aggregates at higher ionic strengths resulted in floc morphologies that were more resistant to shear-induced breakage. With the exceptions of optimal dose concentration and dispersion coefficients, there were no clear differences in the floc growth rate behaviors for the two molecular weights studied. This work contributes to an improved understanding of the physicochemical complexities of polyelectrolyte-driven flocculation that can inform dosing requirements for more efficient industrial operations.

In situ polymer flocculation and growth in Taylor-Couette flows

Flocculation of small particulates suspended in solution is a key process in many industries, including drinking water treatment. The particles are aggregated during mixing to form larger aggregates, known as flocs, through use of a polyelectrolyte flocculant. The flocculation of these particulates in water treatment, however, are subject to a wide spatial variation of hydrodynamic flow states, which has consequences for floc size, growth rate, and microstructure. Floc assembly dynamics are explored here using a commercially available cationic polyacrylamide, commonly used in water treatment, and anisotropic Na-bentonite clay particles under a variety of hydrodynamic mixing conditions. A Taylor-Couette cell with the unique ability to radially inject fluid into the rotating annulus was used to study how specific hydrodynamic flow fields affect assembly and structure of these materials during the flocculation process. Faster floc growth rates and decreased floc fractal dimensions were observed for higher order flow states, indicating improved mass transfer of the polymer flocculant and breakage at the edges of the flocs (shear rounding), respectively. This work sheds more light on the complexities of polymer-induced flocculation, towards improving dosing and efficiency of large-scale operations.