On the Kaolinite Floc Size at the Steady State of Flocculation in a Turbulent Flow

Aggregation behavior of polyethylene microplastics in the nearshore environment: The role of particle size, environmental condition and turbulent flow

Estuary and coastal waters are hotspot areas for microplastics (MPs) pollution. MPs of varying sizes converge in this complex nearshore environment. Aggregation is an important process that affects the transport and fate of MPs in the aqueous environment. Nevertheless, the influence of different factors on the aggregation behavior and the aggregates structure of MPs is unclear. In this study, the aggregation behavior and the aggregates structure of polyethylene microplastics (PEs) of different sizes under the impact of nearshore environmental conditions (i.e., salinity gradient, dissolved organic matter-DOM, turbulent flow) were investigated. The results show that particle size was the dominant factor affecting the stability of PEs in the aqueous environment, and the critical coagulation concentration (CCC) of PEs shifts to the right with increasing size. It was also found that the size of PEs stable aggregates is negatively correlated with the turbulent kinetic energy dissipation rate. The particle size of PEs can significantly affect the fractal dimension (FD) of stable aggregates, and the smaller the particle size, the more compact the aggregates formed. Moreover, salinity and DOM control the size and FD of PEs stable aggregates through different mechanisms. The findings of this study will be helpful for the prediction of the transport and fate of MPs in the aqueous environment.

The role of suspended extracellular polymeric substance (EPS) on equilibrium flocculation of clay minerals in high salinity water

Biophysical cohesive mud, consisting of clay minerals and extracellular polymeric substance (EPS), plays significant role in determining sediments, nutrients and pollutants transport in estuarine and coastal systems. Series of laboratory jar experiments have been conducted aiming at filling the gap of knowledge regarding how biological cohesive EPS affects equilibrium flocculation of EPS-mineral mixtures. Four types of common clay (chlorite, kaolinite, illite and montmorillonite) were chosen due to their abundance in estuarine mud and distinct crystal chemistry and structures. Turbulent shear throughout all the experimental runs were constantly provided at a mean shear parameter of G ≈ 15 s-1 being equivalent to high tidal influenced estuarine turbulent environment. The results reveal that adding EPS increases the equilibrium floc size evidently. The pure mineral flocs show unimodal equilibrium floc size distribution (eFSD) with single peak located at microfloc range (<200 μm) while the EPS-mineral flocs show bimodal eFSD with a secondary peak located in macroflocs range (>200 μm) mostly. Moreover, EPS largely reduces the effective density in EPS-mineral flocs by 1∼2 magnitude. Most importantly, the terminal settling velocity of flocs shows size-dominated in uniform mineral floc cases but density-dominated in EPS-mineral mixture floc cases especially in macroflocs. To model a full floc size or settling velocity distributions in natural environments, furtherly quantification of EPS functions within the large-sized non-fractal mixture floc individually becomes a necessity.

Dynamics of fluvial hydro-sedimentological, nutrient, particulate organic matter and effective particle size responses during the U.K. extreme wet winter of 2019–2020

The floc size distribution of suspended sediment is a critical driver for in-channel sedimentation and sediment-associated contaminant and nutrient transfer and fate in river catchments. Real-time, in situ, floc size characterisation is possible using available technology but, to date, limited high resolution floc data have been published for fluvial systems draining upland extensive grassland catchments. To that end, suspended sediment floc size distribution and turbidity were characterised at 15-minute intervals using Laser In-Situ Scattering and Transmissometry (LISST) diffraction and a YSI turbidity sonde for six storm events in the upper River Taw (15 km²) catchment in SW England. Maximum event discharges (Q) ranged between 4.3 and 20.0 m³ s⁻¹, with clockwise hysteretic responses (HI = 0.18–0.48) of total suspended solid concentrations (TSS) and Q. The sediment flushing index was highest in the early autumn (0.93) and storm event TSS fluxes varied from 0.04 to 2.9 t km⁻². This suggests a change in sources or composition of sediment during higher Q and highly variable patterns of sediment flux from event-to-event. The proportion of particulate organic matter (POM) to TSS was highly variable (5–89%) and did not increase with Q, indicating POM source limitation. The fine-grained tail (D₁₀ and D₁₆) of the floc size distributions decreased during hydrograph rising limbs, with the finest floc sizes associated with the highest TN and TP concentrations at peak Q. The results suggest that dynamic interactions between wet ground and extreme rainfall events can flush significant amounts of sediment from the relatively undisturbed extensive grassland upland catchment. We strongly encourage a sensors-based approach to reveal the spatio-temporal complexity of floc size and associated pollutant export during high Q generated by extreme rainfall since this can help to elucidate processes and mechanisms and generate high-resolution data for water quality modelling without significant user intervention.

•

In situ flocs size and turbidity were studied in sequential high discharge events.

•

Hydrographs characterised by clockwise hysteresis with a secondary sediment peak.

•

Proportion of POM to TSS in the physical samples was highly variable (5–89%).

•

Fine (D₁₀ and D₁₆) floc size diameter was lowest during peak discharges.

•

Floc size exerted less influence on TP and TN concentrations.

Chemometric Optimisation of a Copper Sulphide Tailings Flocculation Process in the Presence of Clays

The presence of fine and ultra-fine gangue minerals in flotation plants can contribute to sub-optimal valuable ore recovery and incomplete water recycling from thickeners, with the performance of the latter equipment relying on adequate flocculation. In order to study the dependence of the flocculation process on the suspension-flocculant mixing conditions, a series of experiments—chosen using chemometric analysis—were carried out by varying mixing conditions, solid concentration, water salinity and flocculant dosage. To this purpose, two different tailings (both featuring coarse and fine content) were considered and a response surface methodology based on a Doehlert experimental design was used. The results suggest that the operational conditions to optimise the flocculated tailings settling rate and the suspended solids that report to a thickener overflow are not necessarily the same. This is a reasonable outcome, given that the settling rate depends on the coarse aggregates generated in the slurry, while the overflow solids content is governed both by either fine particle content (and its characteristics) or small aggregates. It is inferred that to maximise dewatering performance two stages should be involved—a separate treatment of the thickener overflow to remove fine content and thickening at optimal flocculant dosage to enhance this process.

Effects of turbulent aggregation on clay floc breakup and implications for the oceanic environment

Understanding how turbulence impacts marine floc formation and breakup is key to predicting particulate carbon transport in the ocean. While floc formation and sinking rate has been studied in the laboratory and in-situ, the breakup response to turbulence has attracted less attention. To address this problem, the breakup response of bentonite clay particles flocculated in salt water was studied experimentally. Flocs were grown in a large aggregation tank under unmixed and mixed aggregation conditions and then subjected to turbulent pipe flow. Particle size was quantified using microscope imaging and in-situ measurements obtained from standard optical oceanographic instruments; a Sequoia Scientific LISST-100X and two WET Labs ac-9 spectrophotometers. The LISST instrument was found to capture the breakup response of flocs to turbulent energy, though the resulting particle size spectra appear to have underestimated the largest floc lengthscales in the flow while overestimating the abundance of primary particles. Floc breakup and the resulting shift towards smaller particles caused an increase in spectral slope of attenuation as measured by the ac-9 instruments. The Kolmogorov lengthscale was not found to have a limiting effect on floc size in these experiments. While the flocs were found to decrease in overall strength over the course of the two-month experimental time period, repeatable breakup responses to turbulence exposure were observed. Hydrodynamic conditions during floc formation were found to have a large influence on floc strength and breakup response. A non-constant strength exponent was observed for flocs formed with more energetic mixing. Increased turbulence from mixing during aggregation was found to increase floc fractal dimension and apparent density, resulting in a shift in the breakup relationships to higher turbulence dissipation rates. The results suggest that marine particle aggregation and vertical carbon transport concepts should include the turbulence energy responsible for aggregate formation and the resulting impact on floc strength, density, and the disruption potential.

An Effective Flocculation Method to the Kaolin Wastewater Treatment by a Cationic Polyacrylamide (CPAM): Preparation, Characterization, and Flocculation Performance

P(AM-DMC) (PAD) was synthesized by ultraviolet- (UV-) initiated copolymerization with methacryloxyethyl trimethyl ammonium chloride (DMC) and acrylamide (AM) as the monomers and initiator 2,2-azobis [2-(2-imidazolin-2-yl) propane] dihydrochloride (VA-044) as the photoinitiator. Parameters that affect the molecular weight were reviewed by using the single-factor approach. The results showed that the molecular weight (MW) of PAD could come to 7.88 × 106 Da with the optimum polymerization conditions as follows: monomer concentration of 30%, monomer mass ratio m(AM) : m(DMC) of 3 : 1, initiator concentration of 0.6‰, illumination time of 80 min, solution pH value of 4.5, and incident light intensity of 1000 μW cm−2. The PAD was represented by several instruments. The results of FTIR and 1H NMR showed that PAD was indeed polymerized by AM and DMC. The results of TGA showed that PAD was very stable at room temperature while the result of SEM revealed that PAD had a porous structure and rough surface. For PAD used as flocculant in kaolin wastewater treatment, the results confirmed that, at optimal conditions, the turbidity and the floc size d50 could reach to 5.9 NTU and 565.936 μm, respectively, at the optimal conditions (pH = 7.0 and dosage = 2 mg l−1). Kaolin wastewater flocculation test outcome reveals that the PAD with high cationic degree and intrinsic viscosity could boost the charge neutralization and bridging capability. Consequently, the result is an excellent flocculation performance of treating kaolin wastewater.