Properties of NP suspensions and flotation assisting reagents

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NPs from domestic and industrial wastewater

Carbon, silver, silica, titania, zinc and cerium dioxide are most commonly used among different types of NPs (see in Fig. 1.3). It is inevitable that those nanoscale products and their by-products find one release route among others into the aquatic environment via domestic and industrial wastewater discharges. Typically, there is a growing global demand on fuel additives, lubricants, and catalysts because of their enhanced performance achieved through the infusion of nanoscale cerium oxide particles. Generally, these nanotechnologically fuel additives, lubricants and catalysts are likely to be emitted through various waste streams into the air, water or soil systems. Ultimately, they will end up into the aquatic and terrestrial environments through surface-runoffs, spillages during use, and leakages from vehicles, or via sewage drainage systems. This raises serious concerns of dealing with nanowastes from both point and non-point sources.

Sources of nanoparticles (NPs) in the aqueous environment

Another instance is the cosmetics and personal care sector which constitute the largest number of nanoproducts (> 50 %) currently available in the global market (Musee, 2011). The usage of nanomaterials is favoured for the following reasons: ability to absorb and reflect UV light while remaining transparent (e.g. TiO2), a better antioxidant than vitamin E (e.g. fullerene), antibacterial properties (e.g. nano-Ag), and anti-aging skin properties (e.g. nanosomes and gold particles) (Yang et al., 2007; Benn and Westerhoff, 2008; Labille et al., 2010). Consequently, the waste streams containing nanomaterials will be introduced directly to the aquatic environments at application phase — through the sewage systems as a result of showering and washing process.

NPs from wastewater treatment plants (WWTPs) as second pollutants

Some metallic NPs may dissolve (e.g., silver-, zinc-, iron- or copper-based) or biodegrade (e.g., fullerenes) in wastewater, subsequently adsorb to settable biomass, and finally precipitate as inorganic solids or form stable aqueous complexes. Because of the dense biological communities in WWTP unit processes, under typical conditions, more than 90 % of the nanomaterials may attach to biomass and be removed in wastewater treatment plants (WWTPs). Inclusion of membrane filtration to strengthen gravity settling has the potential to increase NP removal (Springer et al., 2013; Westerhoff et al., 2013). However, after the biosolids are disposed to the land surface and spread to fields, landfills, or incineration, their fate needs to be further considered. From this standpoint, municipal WWTPs are particularly important sources of contaminant release into the environment, as they provide potential pathways of nanopollutants into surface waters, soils, and air through treated effluent, biosolids, and plant-generated aerosols (Kiser et al., 2010).

Pathways of NP entry into living bodies via the aqueous environment

As reported by Moore (2006), apart from the NP uptake by inhalation or ingestion which is the major route in terrestrial organisms, in the aqueous environment there may be more routes of entry such as direct passage across gill and other external surface epithelia. Ultimately, those NPs possibly accumulate in human bodies through the food chain. The natural propensity for many NPs to bind transition metals and organic chemical pollutants is believed to enhance the toxicity of some NPs; and the ability of those particles to penetrate the body and cell (e.g. through fluid-phase endocytosis and cavelae in Fig. 1.4) provides potential routes for the delivery of NP-associated toxic pollutants to sites where they would not normally go.

Water treatment application and case study of the NP removal from aqueous body

A growing number of study and trials on laboratory and WWTP scales have been addressed to the NP release into aquatic environment via controlling the treated effluent discharge; meanwhile great efforts have been dedicated to govern the terrestrial environment via sludge disposal or application to land (Hwang et al., 2011). The efficient NP removal is particularly important in view of preventing the considerable NP ecotoxicity for a long-term persistence and evidence. Herein, several typical attempts and application on the NP wastewater treatment have been summarized in Table. 1.1. Traditional and improved techniques, such as flotation, coagulation, filtration, adsorption, were used to deal with different kinds of NP suspensions or wastewater including chemical mechanical polishing effluent, municipal sewage, etc, and high treating efficiencies and helpful indications for further technical development were obtained.

Flotation in the NP separation from water

Flotation, the water purification process based on the adsorption or attachment of materials on the surface of gas bubble passing through a solution or suspension, has always been considered among the most effective techniques in particle separation (Ramirez et al., 1999). Its great potential on the NP and submicron particle removal from wastewaters has been proved via the successful application of carrier flotation, agglomerate flotation, emulsion flotation and oil-in-water flotation in increasing flotation rates of ultrafines (George et al., 2004). In our research group, work has been dedicated to the NP separation by flotation and aggregation. Liu et al (2010, 2012, 2013) developed a laboratory scale flotation apparatus for NP separation from liquid media, and also studied the interaction between silica NPs and additives (AlCl3 and CTAB, respectively) by means of kinetics and thermodynamics aggregation researches. Flotation tests in the absence and presence of additives highlighted the importance of flotation assisting reagents on achieving high NP removal efficiencies. It was found that the charge neutralization dominated mechanism for the agglomeration of NP suspension in the presence of either AlCl3 or CTAB. Besides, for the AlCl3 involved NP aggregation, a possible “hair layer” for NP at the high ionic strength and sweep flocculation based on the precipitation of Al(OH)3 could play leading roles at the thermodynamic equilibrium. For the NP-surfactant (CTAB) aggregation, “depletion flocculation” or “volume-restriction” was proposed to explain the aggregatio n of nanosilica at high zeta potential conditions apart from the electrostatic interaction. Critical coagulation concentrations of additive reagents were determined and high NP removals (> 99 %) were obtained.

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General mechenisms of particle involved flotation process

It is extensively recognized that the key of flotation process is the bubble-particle capture which is the consequence of i) bubble particle collision, ii) particle attachment to the bubble and iii) the stabilization of particle-bubble aggregates: Ecapt Ecoll Eatt Estab , where Ecoll is the collision efficiency, Eatt is the attachment efficiency and Estab the stability efficiency of the particle-bubble aggregates (Huang et al., 2011). The transport of particles towards the bubble surface is shown schematically in Fig. 1.5. Without loss of generality, three predominant transport mechanisms are considered, including interception, gravity (sedimentation) and Brownian diffusion. The transport of particles by interception is due to the liquid flow, which carries the particles along the liquid streamlines. The particles come into contact with the bubbles because of their finite size. Transport by gravity is significant for solid particles which have a density greater than that of water. The influence of inertial forces and gravity on the particle transport is insignificant for NPs; and transport by Brownian diffusion is then the most important mechanism. The combination of these three transport mechanisms allows the determination of the transition between the Brownian diffusion and hydrodynamic-gravity collection mechanisms (A. V. Nguyen et al., 2006).

Table of contents :

1 General review of nanoparticle in wastewater and relative separation techniques
1.1 Sources of nanoparticles (NPs) in the aqueous environment
1.1.1 Classification of nanomaterials
1.1.2 NPs from domestic and industrial wastewater
1.1.3 NPs from wastewater treatment plants (WWTPs) as second pollutants
1.2 Threat of NPs to human beings and the aqueous environment
1.2.1 Pathways of NP entry into living bodies via the aqueous environment
1.2.2 Hazard and toxicity of NPs
1.3 Techniques already applied in the NP wastewater treatment
1.3.1 Challenges in the NP wastewater treatment
1.3.2 Water treatment application and case study of the NP removal from aqueous body
1.4 Flotation in the NP separation from water
1.4.1 Flotation mechanisms of the NP separation by flotation
1.4.1.1 General mechenisms of particle involved flotation process
1.4.1.2 Flotation of NPs
1.4.2 Approaches of improving the NP removal efficiency in flotation
1.4.2.1 Decreasing the bubble size
1.4.2.2 Enhancing the particle-bubble interaction
1.5 Conclusions
2 Properties of NP suspensions and flotation assisting reagents
2.1 An overview of characterization techniques
2.2 TNP suspension and HA solution
2.2.1 TNPs and TNP suspension
2.2.1.1 Characterization of TNPs in the initial suspension
2.2.1.2 Colloid behaviors of TNP suspensions
2.2.2 Properties of flotation assisting reagent — HA
2.2.2.1 Preparation of HA solution
2.2.2.2 Characterization of HA and HA solution
2.3 SNP suspensions and surfactant solutions
2.3.1 SNPs and SNP suspensions
2.3.1.1 Characterization of SNPs in the inital suspension
2.3.1.2 Colloid behaviors of SNP suspensions
2.3.2 Properties of flotation assisting reagent — surfactant
2.4 Conclusion
3 DAF of TNPs with HA assistance
3.1 TNP surface modification with alkaline HA solution
3.1.1 Experimental method
3.1.2 Surface charge of TNP-HA aggregates
3.1.3 PSD pattern of TNP-HA suspension
3.1.4 FT-IR analysis
3.1.5 Residual DOC and turbidity in the TNP-HA suspension
3.1.6 Discussion of the TNP surface modification with alkaline HA solution
3.2 TNP-HA adsorption-aggregation experiments with controlling pH
3.2.1 Experimental procedure
3.2.2 Adsorption isotherms
3.2.3 Properties of HA-TNP aggregates obtained from adsorption-aggregation
3.2.3.1 Zeta potential and pH value
3.2.3.2 Aggregate size
3.2.3.3 Fractal structure
3.3 Laboratory scale continuous DAF experiments
3.3.1 Experimental device, methods and procedure
3.3.1.1 Description of laboratory scale continuous DAF apparatus
3.3.1.2 Bubble production
3.3.1.3 Capacity of the DAF system
3.3.1.4 Determination of hydraulic retention time (HRT)
3.3.1.5 Evaluation of DAF performance
3.3.2 Tentative theoretical determination of operating conditions
3.3.3 Preliminary DAF tests (using Lot #1)
3.3.4 Expanded DAF tests (using Lot #2)
3.3.4.1 DAF of TNPs with pH adjustment
3.3.4.2 DAF of TNPs with HA assistance
3.3.4.3 Discussion of flotation mechanisms
3.4 Conclusions
4 CGA-flotation of SNPs
4.1 CGA generation and characterization
4.1.1 Design of laboratory scaled CGA generator and CGA generation
4.1.2 CGA characterization
4.1.2.1 Visual observation
4.1.2.2 Stability of CGAs — half life τS
4.1.2.3 Size measurement
4.1.2.4 Zeta potential
4.2 Exploration of SNP-surfactant interaction mechanisms by adsorption-aggregation
4.2.1 Experimental method
4.2.2 Adsorption isotherms
4.2.3 Properties of SNP-surfactant aggregates
4.2.3.1 Zeta potential and pH value
4.2.3.2 Aggregate size analysis
4.2.3.3 Fractal structure
4.3 Continuous CGA-flotation separation of SNPs
4.3.1 Experimental procedure
4.3.2 Effect of surfactant concentration in the flotation cell (Csurf’)
4.3.3 Effect of SNP size
4.3.4 Effect of initial volume concentration of SNPs
4.3.5 Effect of initial pH of SNP suspension
4.4 Comparison between CGA-flotation and DAF
4.4.1 Flotation performance
4.4.2 Reduction of surfactant addition
4.4.3 Mechanism comparison between CGA-flotation and DAF
4.5 Conclusions
5 Conclusions and prospects
5.1 Summary of the present dissertation
5.2 Future work and prospects
References.

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