Settling Rate Of Particles Research Articles

Investigating the parametric dependence of the impact of two-way coupling on inertial particle settling in turbulence

Tom et al. (J. Fluid Mech., vol. 947, 2022, p. A7) investigated the impact of two-way coupling (2WC) on particle settling velocities in turbulence. For the limited parameter choices explored, it was found that (i) 2WC substantially enhances particle settling compared with the one-way coupled case, even at low mass loading $\varPhi _m$ and (ii) preferential sweeping remains the mechanism responsible for the particles settling faster than the Stokes settling velocity in 2WC flows. However, significant alterations to the flow structure that can occur at higher mass loadings mean that the conclusions from Tom et al. (J. Fluid Mech., vol. 947, 2022, p. A7) may not generalise. Indeed, even under very low mass loadings, the influence of 2WC on particle settling might persist, challenging the conventional assumption. We therefore explore a much broader portion of the parameter space, with simulations covering cases where the impact of 2WC on the global fluid statistics ranges from negligible to strong. We find that, even for $\varPhi _m=7.5\times 10^{-3}$ , 2WC can noticeably increase the settling for some choices of the Stokes and Froude numbers. When $\varPhi _m$ is large enough for the global fluid statistics to be strongly affected, we show that preferential sweeping continues to be the mechanism that enhances particle settling rates. Finally, we compare our results with previous numerical and experimental studies. While in some cases there is reasonable agreement, discrepancies exist even between different numerical studies and between different experiments. Future studies must seek to understand this before the discrepancies between numerical and experimental results can be adequately addressed.

Assessing the effective settling of mineral particles in the ocean with application to ocean-based carbon-dioxide removal

Ocean alkalinity enhancement (OAE), a potential approach for atmospheric carbon dioxide removal (CDR), can involve introducing milled mineral particles into the ocean to promote carbon dioxide uptake. The effectiveness of this method relies on particles remaining in the ocean mixed layer while dissolution takes place, which depends on particle settling rates. Conventionally, particle settling rates are assessed using the Stokes settling velocity in stagnant conditions. However, recent numerical modeling reveals that in dynamic, stratified ocean environments, sediment vertical transport can be up to an order of magnitude faster than Stokes settling because of two types of fluid instabilities that can take place at the mixed layer base. Here, we estimate effective settling velocities in the presence of these instabilities and assess the implications for the efficacy of this particular OAE approach for CDR. The new effective settling rate estimates are sufficiently rapid that there is negligible particle dissolution before particles settle out of the mixed layer. This result is independent of initial particle size for the range of sizes considered here. Findings underscore the importance of considering ocean dynamics and stratification in assessing particle settling rates and provide valuable insights for optimizing OAE applications in diverse marine settings.

Influence of different inlet modes on hot carrier gas flow in magnesium nitrate pyrolysis furnace

Spray pyrolysis of magnesium nitrate is a way to prepare magnesium oxide solid particles. The gas–solid flow process after the generation of solid particles inside the pyrolysis furnace was studied in this article. Six different inlet modes of hot-carrier gas were designed, and the hot-carrier gas enters the pyrolysis furnace at the same flow rate through the six different inlet modes. The influence of varying inlet methods on the actual running distance of gas flow and the particle sedimentation rate was deeply studied in this article. The longer the distance the hot-carrier gas runs in the pyrolysis furnace, the more conducive it is to full contact with the material. The higher the particle settling rate, the more magnesium oxide particles are collected in the pyrolysis furnace. ANSYS Fluent was used for numerical simulation in this study. The results indicate that when using a tangential inlet, the actual running distance of the hot-carrier gas in the pyrolysis furnace is longer than that of the normal inlet. The actual running distance of the thermal carrier gas at the three tangential inlets is 46.5 m, and that at a single normal inlet is 24.7 m. The former is 88% more than the latter. The sedimentation rate of solid particles at three tangential inlets is 32.24%, and that at a single normal inlet is 12.06%. The former is 167% higher than the latter. The three tangential inlet mode is the best way to increase the actual running distance of the hot-carrier gas and improve the solid particle settling rate in the pyrolysis furnace. The pyrolysis furnace with multiple tangential inlets has two advantages. On the one hand, increasing the running distance of the hot-carrier gas in the furnace is conducive, thereby making the contact between the hot-carrier gas and the material more sufficient. On the other hand, it can increase the capture rate of product particles, improve the product yield, and reduce the dust content of pyrolysis tail gas.

A study of biochar physiochemistry and particle size distribution influencing the properties of water-based slurry fuels

Biochar-based slurry is a fossil-free-liquid fuel derived from a renewable source, biomass. This study aims to examine the properties of this fuel as an alternative to coal-water slurries. The slurries were produced by suspending 40 wt% biochar in a solution made of water and a surfactant. Two biochar types from chemically treated and untreated rice straw (RS) were utilized to assess the impact of particle-particle interactions and biochar physicochemical composition on slurry properties, including stability, rheology, and heating value. Additionally, three particle size distributions (PSD), two unimodal and a bimodal, were used to analyze the effect of PSD on the abovementioned properties.All slurries had an average energy content of 7.32 ± 0.27 MJ/kg. The stability of the slurry was higher for fine particles from treated RS with unimodal PSD (Dv50 8.8 ± 0.68 μm). However, slurries containing fine and coarse particles with bimodal PSD (Dv50 15.8 ± 0.64 μm) had relatively lower apparent viscosities of 342.1 and 336.55 mPa.s at a shear rate of 100 s−1 for slurries made of biochar from treated and untreated RS, respectively. Slurries containing coarse particles from treated and untreated RS with unimodal PSD (Dv50 18.6 ± 0.32 μm) led to higher viscosities and particle settling rates. Biochar morphology and chemical surface constitution significantly influenced slurry stability, while PSD greatly impacted rheological results.

Persistence of E. coli in Streambed Sediment Contaminated with Faeces from Dairy Cows, Geese, and Deer: Legacy Risks to Environment and Health.

Legacy stores of faecal pollution in streambed sediments can result in delayed impacts on environmental quality and human health if resuspended into the overlying water column. Different catchment sources of faecal pollution can contribute to a legacy store of microbial pollutants, with size of stores influenced by microbial die-off and faecal accrual rates in the streambed. The aim of this study was to use a mesocosm experiment to characterise the persistence of E. coli derived from faeces of dairy cows, deer, and geese once introduced to streambed sediment under different temperature regimes. The settling rate of solid constituents of faecal material into streambed sediment once delivered into an aquatic environment was also quantified. The persistence patterns of E. coli in streambed sediment were found to vary as a function of faecal source and temperature; die-off of E. coli in sediment contaminated with goose faeces was more rapid than in sediments contaminated with dairy cow or deer faeces. Goose faeces also recorded a more rapid settling rate of faecal particles through the water column relative to dairy cow and deer faeces, suggesting a more efficient delivery of E. coli to streambed sediments associated with this faecal source. Our findings provide new evidence to improve understanding of the potential longer-term risks to both the environment and public health posed by sediments when contaminated with livestock, wildlife, and wildfowl faeces.

Transient nuclear criticality excursion analysis of highly dispersed particulate three-phase fluidised systems

The aim of this study was to perform sensitivity analysis, investigating how different fluidisation and sedimentation characteristics of three-phase wetted UO2 powder beds, may affect a transient nuclear criticality excursion initiated through the addition of water into a fissile powder bed. This type of postulated nuclear criticality accident scenario may occur in nuclear fuel fabrication facilities when a fire is fought through the use of water, supplied via an automatic or manual fire-suppression system. A similar scenario may also develop as a result of water leaks or flooding of the process area housing UO2 powder. The article introduces a model for gas-bubble induced fluidisation of a UO2 powder bed and examines how this phenomenon may affect the neutron kinetic response of a three-phase fluidised fissile powder system. Empirical analysis has shown that fissile suspensions form agglomerated structures when suspended in water, at agglomerate sizes that range from 18 μm to 40 μm. Simulation results indicate that both the critical gas velocity and rate of fluidisation may significantly affect transient nuclear criticality excursion dynamics. The re-distribution of fissile mass into a highly dispersed suspension generally reduces the reactivity of the system, however, depending on the H/U ratio, a positive reactivity may be added to the system. Low Péclet numbers in the suspension suggest that gas-bubble induced motion of the suspension causes a highly dispersive flow field. An oscillatory power response is predicted for low critical gas velocities where the reactivity of the system is predominantly governed by the re-distribution of fissile mass within the system. The frequency of these oscillations is greater for a higher hindered settling rate of powder particles. At a higher critical gas velocity, the transient nuclear criticality excursion is governed by the voidage reactivity feedback, making the response quite independent of fluidisation. In all cases, large volumes of UO2 powder may leave the domain due to overflowing of the suspension. Transient nuclear criticality excursions in UO2 powder beds with a low critical gas velocity are terminated once the bed becomes fully saturated.

Investigation of the dynamics of particle settling during separation condensing gases

The experience of operating gas separation process units shows that it is not always possible to achieve effective separation of liquid particles. Traditional calculation formulas for determining the settling rate of particles in separators are based on the thermodynamic equilibrium of the main parameters of the incoming gas flow (pressure, density). At the same time, well production, in particular condensing gases, is a nonequilibrium system, which is characterized by a certain delay (relaxation times) in changing parameters. As a result of this delay, the settling rate of particles in the separator does not have time to reach the steady state Stokes regime corresponding to effective separation. The paper proposes a nonequilibrium model for the sedimentation rate of particles and developed an algorithm for its numerical implementation. Using this algorithm, multivariate computational experiments were carried out to study the dynamics of particle settling. It was found that for the effective separation of liquid particles in a gravity separator, the relaxation time for the settling velocity of the particles should not exceed 10 sec. Keywords: gas separation; gravity separator; particle settling rate; relaxation time.

MOF/Polymer Mixed-Matrix Membranes Preparation: Effect of Main Synthesis Parameters on CO2/CH4 Separation Performance.

Design and preparation of mixed-matrix membranes (MMMs) with minimum defects and high performance for desired gas separations is still challenging as it depends on a variety of MMM synthesis parameters. In this study, 6FDA-DAM:DABA based MMMs using MOF-808 as filler were prepared to examine the impact of multiple variables on the preparation process of MMMs, including variation in polymer concentration, filler loading, volume of solution cast per membrane area, solvent type used and solvent evaporation rate, and to identify their impact on the CO2/CH4 separation performance of these membranes. Solvent evaporation rate proved to be the most critical synthesis parameter, directly influencing the performance and visual appearance of the membranes. Although less dominantly influencing the MMM performance, polymer concentration and solution volume also had an important role via control over the casting solution viscosity, particle agglomeration, and particle settling rate. Among all solvents studied, MMMs prepared with chloroform led to the best performance for this polymer-filler system. Chloroform-based MMMs containing 10 and 30 wt.% MOF-808 showed 73% and 62% increase in CO2 permeability, respectively, without a decrease in separation factor compared to unfilled membranes. The results indicate that enhanced gas separation performance of MMMs strongly depends on the cumulative effect of various synthesis parameters rather than individual impact, thus requiring a system-specific design and optimization.

Accounting for the effect of particle density and size on stratification in mineral jigs: A preliminary model based on particle settling rates

A model is developed that accounts for the effect of particle density and size on stratification in mineral processing contexts. It is based on the premise that stratification is driven by differences in the settling rates of particles in a stratifying particle bed. A number of arbitrary assumptions are made to avoid the complications associated with modelling settling rates in that environment. Despite the arbitrariness and simplicity of the assumptions made, the model fits stratification data remarkably well. It reduces to the well-validated King model in the context of density stratification, and also fits size stratification data very well. In more general contexts where both particle density and size vary, the fits to experimental data are also good although some anomalies occur. Better fits are anticipated from a more rigorously derived version of the model that is presented in a follow up paper.

Sulfur mass balance and speciation in the water cap during early-stage development in the first pilot pit lake in the Alberta Oil Sands

Environmental context Base Mine Lake (BML) is a demonstration pilot pit lake for long term oil sands tailings reclamation in Alberta, Canada. This study quantified BML water cap sulfur mass balance and speciation to help understand potential risks to oxygen levels during its early-stage development. Results provide important insights for the adaptive management of water-capped oil sands tailings reclamation. Rationale Sulfur cycling is crucial to the persistence of oxygen in the water cap of Base Mine Lake (BML), the first demonstration oil sands pit lake for water capped tailings technology (WCTT) in Alberta, Canada. Methodology Here, we report on the first investigation of sulfur mass balance and sulfur speciation (SO4 2−, SO3 2−, S2O3 2−, S0 and ƩH2S) over seasonal, annual and spatial scales in BML. Results and discussion High aqueous total sulfur concentration (1.7–2 mM), dominated by sulfate (>75%), decreased over the study period (2015–2021), due to the consolidation of fluid fine tailings (FFT) and operational pump-in and pump-out activities. Expanded BML water cap S biogeochemical cycling occurred after a 2016 alum amendment. Late summer hypolimnetic anoxia emerged post-alum (2017–2021), coincident with detectable total sulfide (ƩH2S) and elemental sulfur (S0) concentrations and expanded sulfur-reducing bacteria activity in anoxic bottom waters. Post spring turnover resuspended FFT and particle settling rates also likely increased post-alum, supported by the observed migration of epilimnetic highest sulfite concentration (pre-alum) to metalimnetic waters (post-alum). These sulfide containing particles are likely the primary reduced S substrate for spring–summer sulfur-oxidising bacteria activity, as winter aqueous reactive S species (ƩH2S, S0, S2O3 2 − and SO3 2−) were non-detectable across years. Concentrations of reactive S species reached up to 200 µM, posing risks to BML O2 levels (maximum 300–350 µM). Results of this study establish the interactive effects of physical and biogeochemical processes, as well as operational activities in emergent S risks to water cap oxygen levels, a key criterion for success of this reclamation tailings technology.

Synthesis and characterization of novel flake-shaped carbonyl iron and water-based magnetorheological fluids using laponite and oleic acid with enhanced sedimentation stability

In this study, micron-sized flake shaped carbonyl iron (CI) water-based MR fluids were prepared with adding laponite and oleic acid as an additive and surfactant, respectively. The MR suspensions are comprised of the fixed CI particles and water weight %, while weight % of laponite and oleic acid changes from 1 to 3 wt% and 0.5 to 1.5 wt%, respectively. The remarkable enhancement in magnetorheological properties was obtained with improved sedimentation stability for CI/water MR suspensions with the addition of laponite and oleic acid. It was found that at the lowest magnetic field strength, the higher laponite concentration is effective, while at the highest magnetic field strength, the smaller concentration was effective. It was because of the combined effect of the field-induced CI chains and the laponite clay gel network. Its storage moduli showed a stable plateau area for whole angular frequencies, suggesting distinguished solid-like behavior of the MR fluid. Finally, a novel correlation was obtained between the initial settling rate of the CI particles and magnetorheological behavior of CI/laponite/OA MR suspensions with 1 wt% laponite and 0.5 wt% oleic acid, which has less zero-field, high on-state shear stress with enhanced sedimentation stability. The prepared MR fluids are a reliable industrial application vibration-isolation, clutch, and brake.

Modelling and Experimental Investigation on the Settling Rate of Kaolinite Particles in Non-Ideal Sedimentation Stage under Constant Gravity.

We compared the catalytic effects of two polymers (soluble starch and apple pectin) on the flocculation of kaolinite suspension. Moreover, the relationship between the zeta potential value and the time when kaolin particle sedimentation occurred was verified, and the mechanism of flocculation was analyzed. Additionally, a constitutive model was proposed to simulate the non-ideal sedimentation of clay particles in an aqueous system under constant gravity. This model not only considers the inhomogeneity of the solute but also simulates the change in clay concentration with time during the deposition process. This model proposes a decay constant (α) and sedimentation coefficient (s). The model can also be used to calculate the instantaneous sedimentation rate of the clay suspensions at any time and any depth for the settling cylinder. These sedimentary characteristics were simulated by adopting the established deposition model. The results show that the model is capable of predicting the time required for the complete sedimentation of particles in the aqueous system, suggesting the feasibility of engineering wastewater treatment, site dredging, etc.

Mud in rivers transported as flocculated and suspended bed material

Riverine transport of silt and clay particles—or mud—builds continental landscapes and dominates the fluxes of sediment and organic carbon across Earth’s surface. Compared with fluxes of sand-sized grains, mud fluxes are difficult to predict. Yet, understanding the fate of muddy river sediment is fundamental to the global carbon cycle, coastal landscape resilience to sea-level rise, river restoration and river–floodplain morphodynamics on Earth and Mars. Mechanistic theories exist for suspended sand transport, but mud in rivers is often thought to constitute washload—sediment with settling velocities so slow that it does not interact with the bed, such that it depends only on upstream supply and is impossible to predict from local hydraulics. To test this hypothesis, we compiled sediment concentration profiles from the literature from eight rivers and used an inversion technique to determine settling rates of suspended mud. We found that mud in rivers is largely flocculated in aggregates that have near-constant settling velocities, independent of grain size, of approximately 0.34 mm s−1, which is 100-fold faster than rates for individual particles. Our findings indicate that flocculated mud is part of suspended bed-material load, not washload, and thus can be physically described by bed-material entrainment theory. Fast settling rates of suspended silt and clay particles suggest that mud in rivers is largely flocculated and part of suspended bed-material load.

Insights into the stability of polytetrafluoroethylene aqueous dispersion: Role of surfactant

Polytetrafluoroethylene (PTFE) has attracted significant interest from both industry and academia owing to its remarkable properties such as wear and corrosion resistance, low surface energy and low dielectric constant. High stability and high solid content (~60%) are primary criteria for PTFE aqueous dispersions toward preparing various end-use articles; however, they are difficult to be achieved simultaneously. Additionally, the stabilization mechanism has not been clearly unraveled and understood till date. Herein, four types of surfactants including cationic dodecyltrimethylammonium bromide (DTAB), anionic sodium dodecylsulfate, zwitterionic lauryl betaine (LB), and nonionic lauryl alcohol polyoxyethylene Brij 30, are used to stabilize 60% PTFE dispersions from the as-polymerized dilute mixture. The stabilities of these dispersions are quantitatively and objectively characterized for the first time using a Turbiscan analyzer based on multiple light scattering technology. While DTAB causes flocculation, the other three surfactants can formulate concentrated dispersions. The dispersion with LB loses the stability in a short time, whereas Brij 30-stabilized dispersion exhibits high stability due to the formation of a network of wormlike micelles. It is determined that the instability of surfactant-stabilized dispersion stems from particle sedimentation rather than coalescence or aggregation. The size of dispersed particle and dispersion viscosity play dominant roles in controlling the particle settling rate, thereby affecting the stability. This study affords useful insights into the stability of concentrated PTFE dispersion, which can guide the selection of suitable surfactants to prepare PTFE dispersions with desired stabilities.

A low‐cost underwater particle tracking velocimetry system for measuring in situ particle flux and sedimentation rate in low‐turbulence environments

AbstractWe describe a low‐cost three‐dimensional underwater particle tracking velocimetry system to directly measure particle settling rate and flux in low‐turbulence aquatic environments. The system consists of two waterproof cameras that acquire stereoscopic videos of sinking particles at 48 frames s−1 over a tunable sampling volume of about 45 × 25 × 24 cm. A dedicated software package has been developed to allow evaluation of particle velocities, concentration and flux, but also of morphometric parameters such as particle area, sinking angle, shape irregularity, and density. Our method offers several advantages over traditional approaches, like sediment trap or expensive in situ camera systems: (1) it does not require beforehand particle collection and handling; (2) it is not subjected to sediment trap biases from turbulence, horizontal advection, or presence of swimmers, that may alter particulate load and flux; (3) the camera system enables faster data processing and flux computation at higher spatial resolution; (4) apart from the particle settling rates, the particle size distribution, and morphology is determined. We tested the camera system in Lake Stechlin (Germany) in low turbulence and mean flow, and analyzed the morphological properties and settling rates of particles to determine their sinking behavior. The particle flux assessed from conventional sediment trap measurements agreed well with that determined by our system. By this, the low‐cost approach demonstrated its reliability in low turbulence environments and a strong potential to provide new insights into particulate carbon transport in aquatic systems. Extension of the method to more turbulent and advective conditions is also discussed.

A Novel Procedure for Performance Assessment of Paste Thickeners

ABSTRACT High stability and low settling rate of phosphate fines in water suspensions encountered the dry area-located mines with serious environmental and technical challenges. The best suggestion for these mines is paste thickener installation for dewatering. In this research, the novel procedure has been proposed for evaluating the potential of paste thickeners in dewatering mining tailings. The case study is Esfordi phosphate complex (Iran). Initially, the selection criteria of a proper flocculant (environmental effects, price, particle settling rate, turbidity of overflow water, and underflow water content) were evaluated for current consuming flocculant (A-27) and 12 other ionic and nonionic flocculants. Then, flocculation performance was ranked based on the type and concentration, applying multiple criteria decision-making (MCDM) techniques of ORESTE, MAPPAC, and ELECTRE. Subsequently, the pilot-scale tests were performed for the selected flocculants and the operational conditions were evaluated for the maximum underflow solid content of 70 wt.%. The results of the present research showed that installation and application of this type of thickener could be effective for the case study.

Phytoplankton calcifiers control nitrate cycling and the pace of transition in warming icehouse and cooling greenhouse climates

Abstract. Phytoplankton calcifiers contribute to global carbon cycling through their dual formation of calcium carbonate and particulate organic carbon (POC). The carbonate might provide an efficient export pathway for the associated POC to the deep ocean, reducing the particles' exposure to biological degradation in the upper ocean and increasing the particle settling rate. Previous work has suggested ballasting of POC by carbonate might increase in a warming climate, in spite of increasing carbonate dissolution rates, because calcifiers benefit from the widespread nutrient limitation arising from stratification. We compare the biogeochemical responses of three models containing (1) a single mixed phytoplankton class, (2) additional explicit small phytoplankton and calcifiers, and (3) additional explicit small phytoplankton and calcifiers with a prognostic carbonate ballast model, to two rapid changes in atmospheric CO2. The first CO2 scenario represents a rapid (151-year) transition from a stable icehouse climate (283.9 ppm) into a greenhouse climate (1263 ppm); the second represents a symmetric rapid transition from a stable greenhouse climate into an icehouse climate. We identify a slope change in the global net primary production response with a transition point at about 3.5 ∘C global mean sea surface temperature change in all models, driven by a combination of physical and biological changes. We also find that in both warming and cooling scenarios, the application of a prognostic carbonate ballast model moderates changes in carbon export production, suboxic volume, and nitrate sources and sinks, reducing the long-term model response to about one-third that of the calcifier model without ballast. Explicit small phytoplankton and calcifiers, and carbonate ballasting, increase the physical separation of nitrate sources and sinks through a combination of phytoplankton competition and lengthened remineralization profile, resulting in a significantly higher global nitrate inventory in this model compared to the single phytoplankton type model (15 % and 32 % higher for icehouse and greenhouse climates). Higher nitrate inventory alleviates nitrate limitation, increasing phytoplankton sensitivity to changes in physical limitation factors (light and temperature). This larger sensitivity to physical forcing produces stronger shifts in ocean phosphate storage between icehouse and greenhouse climates. The greenhouse climate is found to hold phosphate and nitrate deeper in the ocean, despite a shorter remineralization length scale than the icehouse climate, because of the longer residence times of the deep water masses. We conclude the global biogeochemical impact of calcifiers extends beyond their role in global carbon cycling, and that the ecological composition of the global ocean can affect how ocean biogeochemistry responds to climate forcing.

Large Eddy simulations of particulate flows inside a square differentially heated cavity at Rayleigh numbers up to 1011

The Differentially Heated Cavity (DHC) has often served as a suitable geometric model for naturally driven particulate flows inside closed volumes. While providing deep insights of the flow dynamics, 3D DNS or 3D LES simulations of DHC flows are very time-consuming and their potential extension to higher Rayleigh (Ra) numbers typical of room dimensions is problematic. In this study, therefore, a 2D-LES Euler-Lagrange approach is used to predict turbulent particle transport in a DHC.We first validate our predictions against a 2D DNS database at Ra 109 and 1010 with particles having diameters greater than 15 μm. We show that our LES results are in very agreement with the DNS data. In a second step, we extend our simulations to a fully turbulent Ra number 1011 and particle diameters between 0.5 and 10 m. Details are provided on the particle deposition locations and airborne concentration decay rate with time. We show in particular that as the flow becomes more turbulent, the particle settling rates approach those predicted by the simple stirred settling model.

Characterization of a hybrid polyacrylamide and its flocculation properties in cyanide tailing suspensions.

An inorganic-organic hybrid flocculant Al(OH)3-polyacrylamide (Al-PAM) with narrow molecular weight distribution was synthesized using inverse microemulsion polymerization. The hybrid polymer Al-PAM was characterized by Infrared spectroscopy, thermogravimetric analysis, transmission electron microscopy and scanning electron microscopy, and it was found that it had a 'star-like' structure in which Al(OH)3 colloidal particles acted as cores linking PAM chains. The properties of Al-PAM were investigated in flocculating 10 wt% cyanide tailing suspensions. It was found that as the amount of Al-PAMM1 with high molecular weight and aluminum content increased, the initial settling rate of particles accelerated, achieving the maximum 6.6 m/h, 17.3 times the rate of the control without flocculants. The turbidity of the supernatant decreased to 35 ± 2 NTU accordingly, compared to 353 ± 2 NTU of that in the control, which meant that 90.0% of turbidity was removed from the cyanide tailing suspensions. The flocculation mechanism was further explored by floccule size and ζ potential measurements. The superior performance of cationic Al-PAM in flocculating negatively charged particles compared to commercial non-ionic GG indicated that electrostatic repulsion between tailing particles was a crucial factor in deciding the flocculation performance of the polymer. The study demonstrated that both charge neutralization and bridge adsorption were conductive to the particle flocculation.

Assessing nanomaterial exposures in aquatic ecotoxicological testing: Framework and case studies based on dispersion and dissolution

The unique behavior of engineered nanomaterials (ENM) in aqueous media and dynamic changes in particle settling, agglomeration and dissolution rates is a challenge to the consistency, reliability and interpretation of standard aquatic hazard bioassay results. While the toxicological endpoints (e.g., survival, growth, reproduction, etc.) in ecotoxicity bioassays are largely applicable to ENMs, the standard methods as written for dissolved substances are confounded by the dynamic settling, agglomeration and dissolution of particulate ENMs during the bioassay. A testing framework was designed to serve as a starting point to identify approaches for the consistent conduct of aquatic hazard tests that account for the behavior of ENMs in test media and suitable data collection to support representative exposure metrology. The framework was demonstrated by conducting three case studies testing ENMs with functionally distinct characteristics and behaviors. Pretests with a temporal sampling of particle concentration, agglomeration and dissolution were conducted on each ENM in test media. Results indicated that a silver nanoparticle (AgNP) powder was not dispersible, a nano-TiO2 powder was dispersible but unstable, and a polyvinylpyrrolidinone-coated AgNP was relatively stable in test media. Based on these functional results, Ceriodaphnia dubia bioassays were conducted to compare different exposure summary methods (nominal, arithmetic average, geometric average, time-weighted average) for calculating and expressing toxicity endpoints. Results indicated that while arithmetic means were effective for expressing the toxicity of more stable materials, time-weighted averaged concentrations were appropriate for the unstable nano-TiO2.