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The importance of an initial aggregation step for the destabilization of an anatase colloidal suspension
Accepted Manuscript Title: The importance of an initial aggregation step for the ¨ destabilization of Please check the DOC ¨ headfor correctness.–>an anatase colloidal suspension Authors: L´ıvia Dias Campelo, Carlos Adolpho Magalh˜aes Baltar, Silvia Cristina Alves Franc¸a PII: DOI: Reference:
S0927-7757(17)30702-1 http://dx.doi.org/doi:10.1016/j.colsurfa.2017.07.053 COLSUA 21827
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
10-2-2017 15-7-2017 15-7-2017
Please cite this article as: L´ıvia Dias Campelo, Carlos Adolpho Magalh˜aes Baltar, Silvia Cristina Alves Franc¸a, The importance of an initial aggregation step for the destabilization of an anatase colloidal suspension, Colloids and Surfaces A: Physicochemical and Engineering Aspectshttp://dx.doi.org/10.1016/j.colsurfa.2017.07.053 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
THE IMPORTANCE OF AN INITIAL AGGREGATION STEP DESTABILIZATION OF AN ANATASE COLLOIDAL SUSPENSION
FOR
THE
Lívia Dias Campelo Federal University of Pernambuco (UFPE), Brazil Cidade Universitária, 50740-550 Recife - PE, Brazil
Carlos Adolpho Magalhães Baltar Federal University of Pernambuco (UFPE), Brazil Cidade Universitária, 50740-550 Recife - PE, Brazil
Silvia Cristina Alves França Centre for Mineral Technology (CETEM), Brazil Av. Pedro Calmon, 900 Cidade Universitária Rio de Janeiro-RJ Brazil 21941-908
Graphical Abstract
Highlights
Use of promoters of anatase particles pre-aggregation prior to flocculation with polyacrylamide.
Pre-aggregation was produced by using hexahydrate aluminum chloride (HAC) and dodecylamine (DDA).
Synergistic adsorption of polyacrylamide with HAC or DDA on particle surfaces proposed.
Flocculation was evaluated as a function of pH and surface charge.
Results showed a high performance on flocculation when particles were previously aggregated.
ABSTRACT The sedimentation rate is increased by aggregation of fine particles by flocculation. In colloidal suspensions, however, the floc formation is not sufficient for efficient sedimentation because the aggregates, even though containing large amounts of colloidal particles, still are very small and light, causing slow sedimentation rates. In colloidal systems, flocculation must be preceded by an initial aggregation step of the colloidal particles, using coagulation by an electrolyte or agglomeration by hydrophobic-association. This article compares the influence of initial aggregation through coagulation with aluminum chloride hexahydrated (IAHAC) and initial aggregation induced by dodecylamine (IADDA) on the flocculation of colloidal titanium dioxide particles (anatase) with a hydrolyzed polyacrylamide (HPAM). The results were evaluated in terms of turbidity and final compaction of the sediment. In both cases, the initial aggregation step provided a significant improvement in flocculation with HPAM. The IADDA generated relatively large flocs, the supernatant presented less turbidity, and the sediment was more compacted than with the flocs formed from IAHAC.
Key-words:
pre-aggregation;
compaction
of
sediments;
colloidal
system;
flocculation; floc size; polyacrylamide.
1. INTRODUCTION Particles with colloidal dimensions have an extremely slow sedimentation rate. According to Zajic (1971), particles with 10 nm diameter, for example, need one year to settle 1 mm. So, the removal of the dispersed ultrafine particles in the thickening step is very difficult, and the overflow for reuse accumulates colloidal particles, which
are harmful for flotation because they increase the consumption of reagents, cause slime coating, and contribute to rigidity of the froth (Sivamohan, 1990; Baltar, 2010a; Liu et al., 2013). Therefore, the removal of colloidal particles from the thickener is necessary to enable water recycling in mineral processing plants. A variety of reagents can be used to induce aggregation, therefore increasing the settling rate. The biggest and strongest aggregates are obtained with addition of water-soluble polymers (flocculants) with high molecular weight, usually between 106 and 107 g.mol-1 (Vreudge et al., 1975; Baltar, 2010b), whose chains are sufficiently long to bridge several particles at once. Flocculant polymers adsorbed onto particle surfaces can either stabilize or destabilize the colloidal particle dispersion. The outcome will depend on the molecular weight, charge density and type of polymer, besides the surface coverage. According to Gaudreault et al. (2009), a particle with even partially covered surface can attach to the bare surface of another particle by bridging flocculation, which depends directly of the degree of particle-polymer interaction, and hence surface coverage. However, the addition of the flocculant is not sufficient to destabilize a colloidal suspension. The small flocs formed continue having a slow settling rate due to the small mass of each particle (Baltar, 1997). Efficient solid-liquid separation requires previous aggregation, which can be obtained by particle coagulation or preaggregation. The addition of a multivalent cations (Ca2+, Al3+, Fe3+) can decrease the repulsive electrostatic forces between the colloidal particles, allowing the attractive van der Walls force to became dominant, in turn enabling the particles to aggregate (Wu et al., 2009; Lin et al., 2017). This aggregation is induced by interaction between colloidal particles covered with surfactant molecules (Baltar and Oliveira, 1998; Petzold et al., 2003; Petzold et al., 2008). A process consisting of pre-aggregation with surfactant followed by addition of an oppositely charged flocculant was used by Sadowski and Polowczyk (2004), producing larger flocs from a colloidal suspension of zinc and magnesium oxides. Farrokhpay and Filippov (2016) suggested the use of a dual aggregation and flotation process to concentrate a nickel laterite ore by floc flotation. The pre-adsorbed collector in the particle surface will play a similar role of multivalent cations, enabling the polymer adsorption and particle aggregation, previous to flotation. The characteristics of the flocs (size, structure, strength, etc.), and consequently their sedimentation performance, will be influenced by the conformation of the flocculant molecule on the surface of the initial aggregate (Baltar, 2010b), which in turn depends on the type of interactions between the macromolecules and the surface
of the aggregates (Baltar and Oliveira, 1998). HPAM adsorbs predominantly by hydrogen bonding in the initial aggregates formed by coagulation with the addition of an electrolyte (IAHAC), and hydrophobic interactions in the initial aggregates are induced by the action of a surfactant (IADDA). The objective of this study was to investigate the influence of the initial aggregation mechanism (coagulation of hydrophobic interactions) on the flocculation of a colloidal suspension with HPAM.
2. EXPERIMENTAL
2.1. Materials The pure anatase (TiO2) sample, with colloidal dimensions, was supplied by Sachtleben Chemie GmbH, in dry powder form (Hombitan Anatase). The particles had
an
average
size
(D50)
of
0.6
µm
(Figure
1),
determined
by
laser
diffractiontechnique, using Mastersizer 2000 (Malvern Instruments Ltd.), surface area of 1.18 x 104 m2.kg-1 and specific weight of 3900 kg.m-3. The purity of the sample was 99.0% TiO2.
Preliminary experiments showed that at pH 7, a suspension containing 24 grams of anatase per liter of distilled water was very stable. No settling was observed even after 30 days. The flocculant used was a partially hydrolyzed polyacrylamide (HPAM) manufactured by Kemira-Cytec, with high molecular weight (6-8 x 106 g.mol-1). The flocculant concentration ranged from 5 to 25 ppm (mg of dried HPAM/liter of solution). The surfactant was a commercial dodecylamine (DDA), supplied by Sigma-Aldrich, neutralized with hydrochloric acid before use. The electrolyte was prepared with aluminum chloride hexahydrate (HAC) supplied by Reagen. The pH was adjusted with diluted solutions of sodium hydroxide (NaOH) or hydrochloric acid (HCl), supplied by Merck. All experiments were performed using distilled water. Settling experiments were conducted at room temperature using 250 mL graduated cylinders. An IKA Eurostar digital mechanical agitator was used to conditioning the suspensions with reagents. The zeta potential values were determined by electroacoustic potential measurements with a Matec Instruments ESA-9000 (electroacoustic sonic amplitude) system. The suspension and supernatant turbidities
were determined by a Micronal B250 turbidimeter. The average diameter of the aggregates was determined by laser diffraction, as previously mentioned.
2.2. Methods The experiments were carried out in a beaker continuously stirred by a mechanical paddle stirrer. In each test, 6 grams of the anatase powder was added to 150 mL of DDA or HAC solutions. In this initial stage, the pH was adjusted with NaOH or HCl solutions and the suspension was stirred for 5 minutes, at 500 rpm, for homogenization and aggregate formation. Then the flocculant was added, under continuous stirring at 300 rpm during 3 minutes, the conditioning time required for floc formation and growth. Finally, the slurry was transferred to a graduated cylinder fitted with two sampling points for monitoring the variation of the residual turbidity. All experiments were performed in duplicate.
3. RESULTS AND DISCUSSION Figure 2 presents the variation of the residual turbidity with the HPAM concentration. The tests were performed at original pH (around 7) and at room temperature.
High values of residual turbidity remained even for HPAM concentrations as high as 25 ppm. This occurred because the flocs, although containing several particles, have low weight due to the colloidal particles’ small size (Baltar, 1997). The results confirmed that the destabilization of a colloidal suspension induced only by a flocculant is not effective. This suggests the need for initial aggregation of the colloidal particles prior to flocculation. The initial destabilization can be achieved by a multivalent cation (for electric double layer compression) or by a surfactant (Baltar and Oliveira, 1998; Petzold et al., 2008). The adsorption of polyacrylamide molecules on the hydrophilic surface can occur through hydrogen bonds while the adsorption of the polymer on the hydrophobized surface of the colloid particles can take place by means of adsorption bridging (Lin et al., 2017; Grabsch et al., 2013), as schematized in Figure 3. Li et al. (2016) reported the adsorption of HPAM on the oppositely charged Mg(OH) 2 particles as a function of its concentration, being explained by electrostatic attraction, hydrogen bonding, and bridging.
In this study, we compared the effects of aluminum chloride hexahydrate (HAC) and dodecylamine (DDA) as inducers of initial aggregation on flocculation with HPAM. Regarding the pre-aggregation phenomenon, the first set of experiments investigated the influence of pH in the particle aggregation, just by addition of HCl, from the original pH 7.1. The Table 1 shows the values of D50 for initial aggregates formed by the addition of HCl (IAHCl) as a function of pH.
It was noticed that the change in the solution pH induced an incipient particle aggregation, due to the presence of the potential determining ion H+; therefore with no significant increasing in the aggregate size. With the addition of HAC, the presence of Al 3+ ions and the Cl- counter ions compress the electrical double layer, reducing the zeta potential of the anatase surface. The fastest sedimentation (greatest aggregate) and lowest turbidity value were achieved when 0.5 x 10-3 M concentration of HAC was used (Table 2). This coagulant concentration is the one that most approximated the titanium dioxide to its isoelectric point (IEP), around pH 6, providing the most favorable conditions for coagulation and formation of larger IAHAC particles. For values above this concentration, the turbidity increases due to strong repulsion between positively charged particles, and consecutive redispersion of the particles (Wu et al., 2009).
The increase of IAHAC size with the increasing in HAC concentrations in the range of 0 to 0.5x10-3 M can be attributed to the decrease of zeta potential due to the hydrolysis reaction of aluminum, as follows: Al3+ + H2O = Al(OH)2+ + H+ Al(OH)2+ + H2O = Al(OH)2+ + H+
The hydrolysis reaction releases H+, a potential determining ion that promotes the titanium dioxide surface charge reversal at HAC concentrations up to 10 -3 M. As reported by Saukkoriipi (2010), the predominant form of the monomeric aluminum ion in the strongly acidic range (pH ≤ 4) is [Al(OH 2)6]3+(aq) – the aluminum hexahydrate cation; at pH of 5, [AlOH]2+ cation dominates, and at pH of 6 [Al(OH)2+]. So, as long as the HAC concentration was increasing, possibly the release of [Al(OH2)6]3+ cation promoted the pH decreasing. Figure 4 shows the decay of the residual turbidity after adding HPAM (10 ppm), as a function of time and different HAC concentrations. The turbidity was stabilized after
about 30 seconds for any HAC concentration. The increase of the HAC concentration reduced the residual turbidity up to 600 NTU.
The initial aggregation of colloidal particles can also be achieved by hydrophobic interactions of surfactant coated particles. A strong attractive force occurs, probably due to bridging flocculation, promoting a hydrophobic aggregation (Rattanakawin, 2003). Figure 5 shows the turbidity variation, after conditioning (5 minutes at 500 rpm) of the colloidal particles with DDA at pH 10.
It is possible to observe from results displayed in the Figure 5 that the turbidity values, for all DDA concentrations, stabilized after 30 seconds. Moreover, the turbidity value decreased when the flocs were produced through IADDA, and depend on the DDA concentration. There was a clear correlation of DDA concentration, intensity of the hydrophobic interactions, initial aggregate size and floc sedimentation rate. The surfactant concentration (10-3 M at the initial aggregation stage) that provided the highest sedimentation rate in the flocculation stage also produced greater particle size via IADDA (102.1 µm), as presented in Table 3. The increase of DDA concentration to 5x10-3 M promoted a decrease of the pre-aggregate size (Table 3), probably due to the formation of micelles, which promoted surface charge reversion and decreased the hydrophobic forces (Baltar and Oliveira, 1998). These results support the hypothesis that the efficiency of flocculation increases with the size of the initial aggregate.
The increase of pH with rising DDA concentration can be related to the protonation reactions of secondary amine in water, as follows: RNH2 + H2O = RNH3+ + OHConsidering this dissociation equation, as long as the DDA concentration increases, more OH- will be released in the solution, promoting pH increasing. The equilibrium [RNH2]=[ RNH3+] will be observed at pH 10.6 (Wang, 2016).
The variation of turbidity as a function of the HPAM concentration is shown in the Figure 6, in both direct flocculation (flocculation without initial aggregation stage) and flocculation via IAHAC (0.5 x 10-3M HAC) or IADDA (1.0 x 10-3M DDA). Table 4 shows the average floc size in each flocculation system. The individual colloidal particles presented D50 of 0.6 µm (Table 4). The direct flocculation with HPAM generated flocs with D50 of 0.9 µm and residual turbidity around 100,000 NTU (HPAM concentrations from 10 ppm), as presented in Figure 5. The simple coagulation with HAC addition (0.5 x 10-3 M) produced an IAHAC with average size of 17.9 µm and a supernatant turbidity around 50,000 NTU. The IAHAC flocculation produced flocs with D50 of 97.9 µm and a residual turbidity around ten times lower than the original values. The largest initial aggregates were mainly driven by hydrophobic interactions among the DDA molecules adsorbed on the colloidal anatase surface. From these larger hydrophobic particles obtained by IADDA, the HPAM addition generated flocs with D50 of 414 µm, reaching higher settling rates and more efficient turbidity removal (above 100 NTU), adequate for water reuse (Figure 6). The hydrophobicity of the initial aggregate also influenced the compaction height of sediment. For all HPAM concentrations, the lowest compaction height was obtained when the initial aggregate was produced with a DDA concentration of 10 -3 M (Figure 7). The aggregation and release of water in the floc structure are illustrated schematically in Figure 8. The drawings are not to scale, but depict the phenomena involved in the aggregation and dewatering process. The pressure exerted by the upper layers causes the compression of the sediment due to the release of the entrapped water. In the case of hydrophilic particles, a portion of the water remains on their surface due to hydrogen bonds (Figure 8A) produce aggregates that settle easily forming a voluminous sediment, proper to the structures obtained with anionic polymers (Grabsch et al., 2013). In turn, when hydrophobic particles/aggregates form the sediment, the mandatory interaction is water-particle surface (Figure 8B), so the water release provides greater compaction of sediment. Regarding the range of DDA concentrations tested, this is the one that reduces the most the amplitude of the zeta potential of anatase (Figure 9); in other words, that the concentration corresponding to the highest occupation of negative surface sites by the surfactant, promoting the best hydrophobic surface condition.
The results shown in Figure 9 reinforce the hypothesis that the hydrophobic forces influence the sediment height. The most compacted sediments were obtained when the colloidal particles presented lower negative surface charge (-12 mV) and with DDA concentration up to 0.5 x 10-3 M, corresponding to the best condition of surface coverage. 4. CONCLUSIONS The importance of an initial aggregation step on the flocculation of a colloidal anatase suspension was demonstrated. No sedimentation was observed when HPAM was directly applied in the colloidal suspension. The turbidity remained too high (100,000 NTU) even for HPAM concentrations up to 25 ppm. The destabilization of the colloidal suspension only possible after an initial aggregation step, with a salt (HAC) or a surfactant (DDA). There was an unquestionable relationship between the size of the aggregate formed in the initial stage (IAHAC or IADDA) and the posterior flocculation efficiency with HPAM. The best flocculation results were observed at concentrations of the destabilizing agents corresponding to the one that reduced the most the amplitude of the zeta potential of anatase, and therefore produced the greatest aggregate size (97.9 µm and 414.7 µm, for IAHAC and IADDA, respectively). In both cases, the reduction of the aggregate size was related to the increased amplitude of the zeta potential, caused by the increase of reagents concentration. In the case of HAC the reversion occurred due to reduction of the pH value, while in the case of DDA, this happened possibly because of formation of hemi-micelles. Furthermore, the characteristics of the flocs were strongly influenced by the nature of the attractive interactions that prevailed in the initial aggregation. DDA produced hydrophobic aggregates (IADDA), generating flocs with larger size and more compacted sediments compared with flocs obtained through IAHAC. It is interesting to point the relation between the hydrophobicity of the initial aggregate (IADDA) and the compaction height of the sediment obtained by flocculation. The greatest compaction of sediments were observed to the most reduced amplitude of the zeta potential of anatase, which can be attributed to the easier release of the trapped water in the compaction zone, due to the hydrophobic environment inside the flocs. Considering cost as a limiting operational parameter, the type, combination and dosage of reagents should be carefully evaluated to reach the appropriate aggregate characteristics for efficient dewatering, depending on the industrial application.
5. ACKNOWLEDGMENTS The authors are grateful to Leila Baltar for the zeta potential measurements; Marcelo Gomes for the particle size analysis and Ana Paula Terto for supplying the titanium dioxide sample.
REFERENCES Baltar, C.A.M., 1997. Influência da Interação Polímero-Surfatante na Floculação de uma Sílica Coloidal com Poliacrilamida. Tese de Doutorado no Programa de Engenharia Metalúrgica e de Materiais, COPPE, Universidade Federal do Rio de Janeiro, 197 p. Baltar, C.A.M., 2010. Flotação no Tratamento de Minérios, 2ª edição. Editora da Universidade Federal de Pernambuco, 238 p. Baltar, C.A.M., 2010. Processos de Agregação. In.: Tratamento de Minérios, 5ª edição. Luz, A.B.; Sampaio, J.A.; França, S.C.A. (Eds), capítulo 13. CETEM/MCT, p. 557-594. Baltar, C.A.M.; Oliveira, J.F., 1998. Flocculation of colloidal silica with polyacrylamide and the effect of dodecylamine and aluminium chloride pre-conditioning. Minerals Engineering, 2 (5), p. 463-467. Fan, A.; Turro, N.J.; Somasundaran, P., 2000. A study of dual polymer flocculation. Colloids and Surfaces. A: Physicochemical and Engineering Aspects, 162, p. 141-148 (doi:10.1016/S0927-7757(99)00252-6). Farrokhpay, S.; Filipov, L., 2016. Challenges in processing nickel laterite ores by flotation.
International
Journal
of
Mineral
Processing,
151,
59-67
(doi.org/10.1016/j.minpro.2016.04.007). Gaudreault, R.; Cesare, N.; Weitz, D.; van de Ven, T.G.M., 2009. Flocculation kinetics of precipitated calcium carbonate. Colloids and Surface A: Physicochemical and Engineering Aspects, 340, p.56-65 (doi:10.1016/j.colsurfa.2009.03.008). Grabsch, A.F., Fawell, P.D., Adkins, S.J., Beveridge, A., 2013. The impact of achieving a higher aggregate density on polymer-bridging flocculation. International Journal of Mineral Processing, 124, p. 83-94 (doi:10.1016/j.minpro.2013.04.011). Li, F., Ye, L., Li, Y., Wu, T., 2016. Investigation into the adsorption of partially hydrolyzed polyacrylamide onto in situ formed magnesium hydroxide particles. RSC Advances, 6, p. 31092-31100 (doi:10.1039/C6RA02308H). Liu, W.; Moran, C.J.; Vink, S., 2013. A review of the effect of water quality on flotation. Minerals Engineering, 53, p. 91-100 (doi:10.1016/j.mineng.2013.07.011). Lin, Z.; Li, P.; Hou, D.; Kuang, Y.; Wang, G., 2017. Aggregation mechanism of particle: effect of Ca2+ and polyacrylamide on coagulation and flocculation of cola slime water containing illite. Minerals, 7, 30, 10 p (doi:10.3390/min7020030).
Petzold, G.; Mende, M.; Lunkwitz, K.; Schwarz, S.; Buchhammer, H.M., 2003. Higher efficiency in the flocculation of clay suspensions by using combinations of oppositely charged polyelectrolytes. Colloids and Surfaces. A: Physicochemical and Engineering Aspects, 218, p. 47-57 (doi:10.1016/S0927-7757(02)00584-8). Petzold, G.; Dutschk, V.; Mende, M.; Miller, R., 2008. Interaction of cationic surfactant and anionic polyelectrolytes in mixed aqueous solutions. Colloids and Surfaces.
A:
Physicochemical
and
Engineering
Aspects,
319,
p.
43-50
(doi:10.1016/j.colsurfa.2007.06.011). Rattanakawin, C., 2003. Aggregate size distributions in hydrophobic flocculation. Songklanakarin Journal of Science and Technology, 25, p. 477-483. Saukkoriipi. J., 2010. Theoretical study of the hydrolysis of aluminum complexes. Acta Universitatis Ouluensis, A 554, Finland. Sadowski , Z.; Polowczyk, I., 2004. Agglomerate flotation of fine oxide particles. International Journal of Mineral Processing, 74, 85-90. Sivamohan, R., 1990. The problem of recovering very fine particles in mineral processing – A review. International Journal of Mineral Processing, 28, p. 247-288. Vreudge, M.J.A.; Poling, G.W., 1975. The effect of flocculation on reclaim water quality for flotation. Canadian Mining and Metallurgical Bulletin, p. 1-6. Wang. D., 2016. Flotation Reagents: Applied Surface Chemistry on Minerals Flotation and Energy Resources Beneficiation. Volume 1: Functional Principle. Springer Ed., XIV, 382p. Wu, X.; Ge, X.; Wang, D.; Tang, H., 2009. Distinct mechanisms of particle aggregation induced by alum and PACl: floc structure and DLVO evaluation. Colloids and
Surface
A:
Physicochemical
and
Engineering
Aspects,
347,
p.56-63
(doi:10.1016/j.colsurfa.2008.12.005). Zajic, J.E., 1971. Water Pollution – Disposal and Reuse. 2nd ed. New York, Marcel Dekker, Inc. Zhu, M., Wang, H., Keller, A.A., Wang, T., Li, F., 2014. The effect of humic acid on the aggregation of titanium dioxide nanoparticles under different pH and ionic strengths.
Science
of
the
Total
(doi:10.1016/j.scitotenv.2014.04.036).
Environment,
n.
487,
p.
375–380
Figure 1 - Particle size distribution of the anatase sample
Figure 2 - Influence of the concentration of HPAM on the turbidity of the anatase suspension, after 30 seconds of sedimentation
Figure 3 – Schematization of HPAM interactions
Figure 4 – Variation of the turbidity after flocculation with HPAM (10 ppm) as a function of time and HAC concentrations
Figure 5 - Influence of the DDA concentration, in the initial aggregation stage, on the turbidity of the suspension flocculated with HPAM at pH 10
Figure 6 - Turbidity as a function of HPAM concentration for direct flocculation and flocculation from IAHAC and IADDA
Figure 7 - Influence of the HPAM concentration on the sediment height after 72 hours
(A)
(B)
Figure 8 – Illustration of the hydrophobic environment inside of the floc formed through IADDA (drawings illustrative cartoons, not to scale)
Figure 9 - Influence of surfactant concentration on the zeta potential and sediment compaction after 72 hours. Tests carried out with HPAM (10 ppm) at pH 10
Table 1 - D50 for IAHCl as a function of pH pH
IAHCl D50 (µm)
7.1
0.6
6.4
1.0
4.4
1.7
3.6
3.7
3.3
4.0
Table 2 – pH values and D50 for IAHAC, as a function of the concentration of aluminum chloride (no HPAM). HAC (X 10-3M)
pH
IAHAC D50 (µm)
0
7.1
0.6
0.1
6.8
2.4
0.5
3.3
17.9
1.0
3.0
14.8
5.0
2.9
7.1
Table 3 - pH and IADDA D50 as a function of DDA concentration (no HPAM). DDA (X 10-3M)
pH
IADDA D50 (µm)
0
7.0
0.6
0.1
7.2
8.1
0.5
7.2
37.8
1.0
7.5
102.1
5.0
7.7
28.1
Table 4 – Average size of the anatase aggregates for each tested system. HPAM NO
YES
HAC (X 10-3M)
DDA (X 10-3M)
D50 (µm)
-
-
0.6
0.5
17.9 1.0
102.1
-
-
0.90
0.5
-
97.9
-
1.0
414.7
S0927-7757(17)30702-1 http://dx.doi.org/doi:10.1016/j.colsurfa.2017.07.053 COLSUA 21827
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
10-2-2017 15-7-2017 15-7-2017
Please cite this article as: L´ıvia Dias Campelo, Carlos Adolpho Magalh˜aes Baltar, Silvia Cristina Alves Franc¸a, The importance of an initial aggregation step for the destabilization of an anatase colloidal suspension, Colloids and Surfaces A: Physicochemical and Engineering Aspectshttp://dx.doi.org/10.1016/j.colsurfa.2017.07.053 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
THE IMPORTANCE OF AN INITIAL AGGREGATION STEP DESTABILIZATION OF AN ANATASE COLLOIDAL SUSPENSION
FOR
THE
Lívia Dias Campelo Federal University of Pernambuco (UFPE), Brazil Cidade Universitária, 50740-550 Recife - PE, Brazil
Carlos Adolpho Magalhães Baltar Federal University of Pernambuco (UFPE), Brazil Cidade Universitária, 50740-550 Recife - PE, Brazil
Silvia Cristina Alves França Centre for Mineral Technology (CETEM), Brazil Av. Pedro Calmon, 900 Cidade Universitária Rio de Janeiro-RJ Brazil 21941-908
Graphical Abstract
Highlights
Use of promoters of anatase particles pre-aggregation prior to flocculation with polyacrylamide.
Pre-aggregation was produced by using hexahydrate aluminum chloride (HAC) and dodecylamine (DDA).
Synergistic adsorption of polyacrylamide with HAC or DDA on particle surfaces proposed.
Flocculation was evaluated as a function of pH and surface charge.
Results showed a high performance on flocculation when particles were previously aggregated.
ABSTRACT The sedimentation rate is increased by aggregation of fine particles by flocculation. In colloidal suspensions, however, the floc formation is not sufficient for efficient sedimentation because the aggregates, even though containing large amounts of colloidal particles, still are very small and light, causing slow sedimentation rates. In colloidal systems, flocculation must be preceded by an initial aggregation step of the colloidal particles, using coagulation by an electrolyte or agglomeration by hydrophobic-association. This article compares the influence of initial aggregation through coagulation with aluminum chloride hexahydrated (IAHAC) and initial aggregation induced by dodecylamine (IADDA) on the flocculation of colloidal titanium dioxide particles (anatase) with a hydrolyzed polyacrylamide (HPAM). The results were evaluated in terms of turbidity and final compaction of the sediment. In both cases, the initial aggregation step provided a significant improvement in flocculation with HPAM. The IADDA generated relatively large flocs, the supernatant presented less turbidity, and the sediment was more compacted than with the flocs formed from IAHAC.
Key-words:
pre-aggregation;
compaction
of
sediments;
colloidal
system;
flocculation; floc size; polyacrylamide.
1. INTRODUCTION Particles with colloidal dimensions have an extremely slow sedimentation rate. According to Zajic (1971), particles with 10 nm diameter, for example, need one year to settle 1 mm. So, the removal of the dispersed ultrafine particles in the thickening step is very difficult, and the overflow for reuse accumulates colloidal particles, which
are harmful for flotation because they increase the consumption of reagents, cause slime coating, and contribute to rigidity of the froth (Sivamohan, 1990; Baltar, 2010a; Liu et al., 2013). Therefore, the removal of colloidal particles from the thickener is necessary to enable water recycling in mineral processing plants. A variety of reagents can be used to induce aggregation, therefore increasing the settling rate. The biggest and strongest aggregates are obtained with addition of water-soluble polymers (flocculants) with high molecular weight, usually between 106 and 107 g.mol-1 (Vreudge et al., 1975; Baltar, 2010b), whose chains are sufficiently long to bridge several particles at once. Flocculant polymers adsorbed onto particle surfaces can either stabilize or destabilize the colloidal particle dispersion. The outcome will depend on the molecular weight, charge density and type of polymer, besides the surface coverage. According to Gaudreault et al. (2009), a particle with even partially covered surface can attach to the bare surface of another particle by bridging flocculation, which depends directly of the degree of particle-polymer interaction, and hence surface coverage. However, the addition of the flocculant is not sufficient to destabilize a colloidal suspension. The small flocs formed continue having a slow settling rate due to the small mass of each particle (Baltar, 1997). Efficient solid-liquid separation requires previous aggregation, which can be obtained by particle coagulation or preaggregation. The addition of a multivalent cations (Ca2+, Al3+, Fe3+) can decrease the repulsive electrostatic forces between the colloidal particles, allowing the attractive van der Walls force to became dominant, in turn enabling the particles to aggregate (Wu et al., 2009; Lin et al., 2017). This aggregation is induced by interaction between colloidal particles covered with surfactant molecules (Baltar and Oliveira, 1998; Petzold et al., 2003; Petzold et al., 2008). A process consisting of pre-aggregation with surfactant followed by addition of an oppositely charged flocculant was used by Sadowski and Polowczyk (2004), producing larger flocs from a colloidal suspension of zinc and magnesium oxides. Farrokhpay and Filippov (2016) suggested the use of a dual aggregation and flotation process to concentrate a nickel laterite ore by floc flotation. The pre-adsorbed collector in the particle surface will play a similar role of multivalent cations, enabling the polymer adsorption and particle aggregation, previous to flotation. The characteristics of the flocs (size, structure, strength, etc.), and consequently their sedimentation performance, will be influenced by the conformation of the flocculant molecule on the surface of the initial aggregate (Baltar, 2010b), which in turn depends on the type of interactions between the macromolecules and the surface
of the aggregates (Baltar and Oliveira, 1998). HPAM adsorbs predominantly by hydrogen bonding in the initial aggregates formed by coagulation with the addition of an electrolyte (IAHAC), and hydrophobic interactions in the initial aggregates are induced by the action of a surfactant (IADDA). The objective of this study was to investigate the influence of the initial aggregation mechanism (coagulation of hydrophobic interactions) on the flocculation of a colloidal suspension with HPAM.
2. EXPERIMENTAL
2.1. Materials The pure anatase (TiO2) sample, with colloidal dimensions, was supplied by Sachtleben Chemie GmbH, in dry powder form (Hombitan Anatase). The particles had
an
average
size
(D50)
of
0.6
µm
(Figure
1),
determined
by
laser
diffractiontechnique, using Mastersizer 2000 (Malvern Instruments Ltd.), surface area of 1.18 x 104 m2.kg-1 and specific weight of 3900 kg.m-3. The purity of the sample was 99.0% TiO2.
Preliminary experiments showed that at pH 7, a suspension containing 24 grams of anatase per liter of distilled water was very stable. No settling was observed even after 30 days. The flocculant used was a partially hydrolyzed polyacrylamide (HPAM) manufactured by Kemira-Cytec, with high molecular weight (6-8 x 106 g.mol-1). The flocculant concentration ranged from 5 to 25 ppm (mg of dried HPAM/liter of solution). The surfactant was a commercial dodecylamine (DDA), supplied by Sigma-Aldrich, neutralized with hydrochloric acid before use. The electrolyte was prepared with aluminum chloride hexahydrate (HAC) supplied by Reagen. The pH was adjusted with diluted solutions of sodium hydroxide (NaOH) or hydrochloric acid (HCl), supplied by Merck. All experiments were performed using distilled water. Settling experiments were conducted at room temperature using 250 mL graduated cylinders. An IKA Eurostar digital mechanical agitator was used to conditioning the suspensions with reagents. The zeta potential values were determined by electroacoustic potential measurements with a Matec Instruments ESA-9000 (electroacoustic sonic amplitude) system. The suspension and supernatant turbidities
were determined by a Micronal B250 turbidimeter. The average diameter of the aggregates was determined by laser diffraction, as previously mentioned.
2.2. Methods The experiments were carried out in a beaker continuously stirred by a mechanical paddle stirrer. In each test, 6 grams of the anatase powder was added to 150 mL of DDA or HAC solutions. In this initial stage, the pH was adjusted with NaOH or HCl solutions and the suspension was stirred for 5 minutes, at 500 rpm, for homogenization and aggregate formation. Then the flocculant was added, under continuous stirring at 300 rpm during 3 minutes, the conditioning time required for floc formation and growth. Finally, the slurry was transferred to a graduated cylinder fitted with two sampling points for monitoring the variation of the residual turbidity. All experiments were performed in duplicate.
3. RESULTS AND DISCUSSION Figure 2 presents the variation of the residual turbidity with the HPAM concentration. The tests were performed at original pH (around 7) and at room temperature.
High values of residual turbidity remained even for HPAM concentrations as high as 25 ppm. This occurred because the flocs, although containing several particles, have low weight due to the colloidal particles’ small size (Baltar, 1997). The results confirmed that the destabilization of a colloidal suspension induced only by a flocculant is not effective. This suggests the need for initial aggregation of the colloidal particles prior to flocculation. The initial destabilization can be achieved by a multivalent cation (for electric double layer compression) or by a surfactant (Baltar and Oliveira, 1998; Petzold et al., 2008). The adsorption of polyacrylamide molecules on the hydrophilic surface can occur through hydrogen bonds while the adsorption of the polymer on the hydrophobized surface of the colloid particles can take place by means of adsorption bridging (Lin et al., 2017; Grabsch et al., 2013), as schematized in Figure 3. Li et al. (2016) reported the adsorption of HPAM on the oppositely charged Mg(OH) 2 particles as a function of its concentration, being explained by electrostatic attraction, hydrogen bonding, and bridging.
In this study, we compared the effects of aluminum chloride hexahydrate (HAC) and dodecylamine (DDA) as inducers of initial aggregation on flocculation with HPAM. Regarding the pre-aggregation phenomenon, the first set of experiments investigated the influence of pH in the particle aggregation, just by addition of HCl, from the original pH 7.1. The Table 1 shows the values of D50 for initial aggregates formed by the addition of HCl (IAHCl) as a function of pH.
It was noticed that the change in the solution pH induced an incipient particle aggregation, due to the presence of the potential determining ion H+; therefore with no significant increasing in the aggregate size. With the addition of HAC, the presence of Al 3+ ions and the Cl- counter ions compress the electrical double layer, reducing the zeta potential of the anatase surface. The fastest sedimentation (greatest aggregate) and lowest turbidity value were achieved when 0.5 x 10-3 M concentration of HAC was used (Table 2). This coagulant concentration is the one that most approximated the titanium dioxide to its isoelectric point (IEP), around pH 6, providing the most favorable conditions for coagulation and formation of larger IAHAC particles. For values above this concentration, the turbidity increases due to strong repulsion between positively charged particles, and consecutive redispersion of the particles (Wu et al., 2009).
The increase of IAHAC size with the increasing in HAC concentrations in the range of 0 to 0.5x10-3 M can be attributed to the decrease of zeta potential due to the hydrolysis reaction of aluminum, as follows: Al3+ + H2O = Al(OH)2+ + H+ Al(OH)2+ + H2O = Al(OH)2+ + H+
The hydrolysis reaction releases H+, a potential determining ion that promotes the titanium dioxide surface charge reversal at HAC concentrations up to 10 -3 M. As reported by Saukkoriipi (2010), the predominant form of the monomeric aluminum ion in the strongly acidic range (pH ≤ 4) is [Al(OH 2)6]3+(aq) – the aluminum hexahydrate cation; at pH of 5, [AlOH]2+ cation dominates, and at pH of 6 [Al(OH)2+]. So, as long as the HAC concentration was increasing, possibly the release of [Al(OH2)6]3+ cation promoted the pH decreasing. Figure 4 shows the decay of the residual turbidity after adding HPAM (10 ppm), as a function of time and different HAC concentrations. The turbidity was stabilized after
about 30 seconds for any HAC concentration. The increase of the HAC concentration reduced the residual turbidity up to 600 NTU.
The initial aggregation of colloidal particles can also be achieved by hydrophobic interactions of surfactant coated particles. A strong attractive force occurs, probably due to bridging flocculation, promoting a hydrophobic aggregation (Rattanakawin, 2003). Figure 5 shows the turbidity variation, after conditioning (5 minutes at 500 rpm) of the colloidal particles with DDA at pH 10.
It is possible to observe from results displayed in the Figure 5 that the turbidity values, for all DDA concentrations, stabilized after 30 seconds. Moreover, the turbidity value decreased when the flocs were produced through IADDA, and depend on the DDA concentration. There was a clear correlation of DDA concentration, intensity of the hydrophobic interactions, initial aggregate size and floc sedimentation rate. The surfactant concentration (10-3 M at the initial aggregation stage) that provided the highest sedimentation rate in the flocculation stage also produced greater particle size via IADDA (102.1 µm), as presented in Table 3. The increase of DDA concentration to 5x10-3 M promoted a decrease of the pre-aggregate size (Table 3), probably due to the formation of micelles, which promoted surface charge reversion and decreased the hydrophobic forces (Baltar and Oliveira, 1998). These results support the hypothesis that the efficiency of flocculation increases with the size of the initial aggregate.
The increase of pH with rising DDA concentration can be related to the protonation reactions of secondary amine in water, as follows: RNH2 + H2O = RNH3+ + OHConsidering this dissociation equation, as long as the DDA concentration increases, more OH- will be released in the solution, promoting pH increasing. The equilibrium [RNH2]=[ RNH3+] will be observed at pH 10.6 (Wang, 2016).
The variation of turbidity as a function of the HPAM concentration is shown in the Figure 6, in both direct flocculation (flocculation without initial aggregation stage) and flocculation via IAHAC (0.5 x 10-3M HAC) or IADDA (1.0 x 10-3M DDA). Table 4 shows the average floc size in each flocculation system. The individual colloidal particles presented D50 of 0.6 µm (Table 4). The direct flocculation with HPAM generated flocs with D50 of 0.9 µm and residual turbidity around 100,000 NTU (HPAM concentrations from 10 ppm), as presented in Figure 5. The simple coagulation with HAC addition (0.5 x 10-3 M) produced an IAHAC with average size of 17.9 µm and a supernatant turbidity around 50,000 NTU. The IAHAC flocculation produced flocs with D50 of 97.9 µm and a residual turbidity around ten times lower than the original values. The largest initial aggregates were mainly driven by hydrophobic interactions among the DDA molecules adsorbed on the colloidal anatase surface. From these larger hydrophobic particles obtained by IADDA, the HPAM addition generated flocs with D50 of 414 µm, reaching higher settling rates and more efficient turbidity removal (above 100 NTU), adequate for water reuse (Figure 6). The hydrophobicity of the initial aggregate also influenced the compaction height of sediment. For all HPAM concentrations, the lowest compaction height was obtained when the initial aggregate was produced with a DDA concentration of 10 -3 M (Figure 7). The aggregation and release of water in the floc structure are illustrated schematically in Figure 8. The drawings are not to scale, but depict the phenomena involved in the aggregation and dewatering process. The pressure exerted by the upper layers causes the compression of the sediment due to the release of the entrapped water. In the case of hydrophilic particles, a portion of the water remains on their surface due to hydrogen bonds (Figure 8A) produce aggregates that settle easily forming a voluminous sediment, proper to the structures obtained with anionic polymers (Grabsch et al., 2013). In turn, when hydrophobic particles/aggregates form the sediment, the mandatory interaction is water-particle surface (Figure 8B), so the water release provides greater compaction of sediment. Regarding the range of DDA concentrations tested, this is the one that reduces the most the amplitude of the zeta potential of anatase (Figure 9); in other words, that the concentration corresponding to the highest occupation of negative surface sites by the surfactant, promoting the best hydrophobic surface condition.
The results shown in Figure 9 reinforce the hypothesis that the hydrophobic forces influence the sediment height. The most compacted sediments were obtained when the colloidal particles presented lower negative surface charge (-12 mV) and with DDA concentration up to 0.5 x 10-3 M, corresponding to the best condition of surface coverage. 4. CONCLUSIONS The importance of an initial aggregation step on the flocculation of a colloidal anatase suspension was demonstrated. No sedimentation was observed when HPAM was directly applied in the colloidal suspension. The turbidity remained too high (100,000 NTU) even for HPAM concentrations up to 25 ppm. The destabilization of the colloidal suspension only possible after an initial aggregation step, with a salt (HAC) or a surfactant (DDA). There was an unquestionable relationship between the size of the aggregate formed in the initial stage (IAHAC or IADDA) and the posterior flocculation efficiency with HPAM. The best flocculation results were observed at concentrations of the destabilizing agents corresponding to the one that reduced the most the amplitude of the zeta potential of anatase, and therefore produced the greatest aggregate size (97.9 µm and 414.7 µm, for IAHAC and IADDA, respectively). In both cases, the reduction of the aggregate size was related to the increased amplitude of the zeta potential, caused by the increase of reagents concentration. In the case of HAC the reversion occurred due to reduction of the pH value, while in the case of DDA, this happened possibly because of formation of hemi-micelles. Furthermore, the characteristics of the flocs were strongly influenced by the nature of the attractive interactions that prevailed in the initial aggregation. DDA produced hydrophobic aggregates (IADDA), generating flocs with larger size and more compacted sediments compared with flocs obtained through IAHAC. It is interesting to point the relation between the hydrophobicity of the initial aggregate (IADDA) and the compaction height of the sediment obtained by flocculation. The greatest compaction of sediments were observed to the most reduced amplitude of the zeta potential of anatase, which can be attributed to the easier release of the trapped water in the compaction zone, due to the hydrophobic environment inside the flocs. Considering cost as a limiting operational parameter, the type, combination and dosage of reagents should be carefully evaluated to reach the appropriate aggregate characteristics for efficient dewatering, depending on the industrial application.
5. ACKNOWLEDGMENTS The authors are grateful to Leila Baltar for the zeta potential measurements; Marcelo Gomes for the particle size analysis and Ana Paula Terto for supplying the titanium dioxide sample.
REFERENCES Baltar, C.A.M., 1997. Influência da Interação Polímero-Surfatante na Floculação de uma Sílica Coloidal com Poliacrilamida. Tese de Doutorado no Programa de Engenharia Metalúrgica e de Materiais, COPPE, Universidade Federal do Rio de Janeiro, 197 p. Baltar, C.A.M., 2010. Flotação no Tratamento de Minérios, 2ª edição. Editora da Universidade Federal de Pernambuco, 238 p. Baltar, C.A.M., 2010. Processos de Agregação. In.: Tratamento de Minérios, 5ª edição. Luz, A.B.; Sampaio, J.A.; França, S.C.A. (Eds), capítulo 13. CETEM/MCT, p. 557-594. Baltar, C.A.M.; Oliveira, J.F., 1998. Flocculation of colloidal silica with polyacrylamide and the effect of dodecylamine and aluminium chloride pre-conditioning. Minerals Engineering, 2 (5), p. 463-467. Fan, A.; Turro, N.J.; Somasundaran, P., 2000. A study of dual polymer flocculation. Colloids and Surfaces. A: Physicochemical and Engineering Aspects, 162, p. 141-148 (doi:10.1016/S0927-7757(99)00252-6). Farrokhpay, S.; Filipov, L., 2016. Challenges in processing nickel laterite ores by flotation.
International
Journal
of
Mineral
Processing,
151,
59-67
(doi.org/10.1016/j.minpro.2016.04.007). Gaudreault, R.; Cesare, N.; Weitz, D.; van de Ven, T.G.M., 2009. Flocculation kinetics of precipitated calcium carbonate. Colloids and Surface A: Physicochemical and Engineering Aspects, 340, p.56-65 (doi:10.1016/j.colsurfa.2009.03.008). Grabsch, A.F., Fawell, P.D., Adkins, S.J., Beveridge, A., 2013. The impact of achieving a higher aggregate density on polymer-bridging flocculation. International Journal of Mineral Processing, 124, p. 83-94 (doi:10.1016/j.minpro.2013.04.011). Li, F., Ye, L., Li, Y., Wu, T., 2016. Investigation into the adsorption of partially hydrolyzed polyacrylamide onto in situ formed magnesium hydroxide particles. RSC Advances, 6, p. 31092-31100 (doi:10.1039/C6RA02308H). Liu, W.; Moran, C.J.; Vink, S., 2013. A review of the effect of water quality on flotation. Minerals Engineering, 53, p. 91-100 (doi:10.1016/j.mineng.2013.07.011). Lin, Z.; Li, P.; Hou, D.; Kuang, Y.; Wang, G., 2017. Aggregation mechanism of particle: effect of Ca2+ and polyacrylamide on coagulation and flocculation of cola slime water containing illite. Minerals, 7, 30, 10 p (doi:10.3390/min7020030).
Petzold, G.; Mende, M.; Lunkwitz, K.; Schwarz, S.; Buchhammer, H.M., 2003. Higher efficiency in the flocculation of clay suspensions by using combinations of oppositely charged polyelectrolytes. Colloids and Surfaces. A: Physicochemical and Engineering Aspects, 218, p. 47-57 (doi:10.1016/S0927-7757(02)00584-8). Petzold, G.; Dutschk, V.; Mende, M.; Miller, R., 2008. Interaction of cationic surfactant and anionic polyelectrolytes in mixed aqueous solutions. Colloids and Surfaces.
A:
Physicochemical
and
Engineering
Aspects,
319,
p.
43-50
(doi:10.1016/j.colsurfa.2007.06.011). Rattanakawin, C., 2003. Aggregate size distributions in hydrophobic flocculation. Songklanakarin Journal of Science and Technology, 25, p. 477-483. Saukkoriipi. J., 2010. Theoretical study of the hydrolysis of aluminum complexes. Acta Universitatis Ouluensis, A 554, Finland. Sadowski , Z.; Polowczyk, I., 2004. Agglomerate flotation of fine oxide particles. International Journal of Mineral Processing, 74, 85-90. Sivamohan, R., 1990. The problem of recovering very fine particles in mineral processing – A review. International Journal of Mineral Processing, 28, p. 247-288. Vreudge, M.J.A.; Poling, G.W., 1975. The effect of flocculation on reclaim water quality for flotation. Canadian Mining and Metallurgical Bulletin, p. 1-6. Wang. D., 2016. Flotation Reagents: Applied Surface Chemistry on Minerals Flotation and Energy Resources Beneficiation. Volume 1: Functional Principle. Springer Ed., XIV, 382p. Wu, X.; Ge, X.; Wang, D.; Tang, H., 2009. Distinct mechanisms of particle aggregation induced by alum and PACl: floc structure and DLVO evaluation. Colloids and
Surface
A:
Physicochemical
and
Engineering
Aspects,
347,
p.56-63
(doi:10.1016/j.colsurfa.2008.12.005). Zajic, J.E., 1971. Water Pollution – Disposal and Reuse. 2nd ed. New York, Marcel Dekker, Inc. Zhu, M., Wang, H., Keller, A.A., Wang, T., Li, F., 2014. The effect of humic acid on the aggregation of titanium dioxide nanoparticles under different pH and ionic strengths.
Science
of
the
Total
(doi:10.1016/j.scitotenv.2014.04.036).
Environment,
n.
487,
p.
375–380
Figure 1 - Particle size distribution of the anatase sample
Figure 2 - Influence of the concentration of HPAM on the turbidity of the anatase suspension, after 30 seconds of sedimentation
Figure 3 – Schematization of HPAM interactions
Figure 4 – Variation of the turbidity after flocculation with HPAM (10 ppm) as a function of time and HAC concentrations
Figure 5 - Influence of the DDA concentration, in the initial aggregation stage, on the turbidity of the suspension flocculated with HPAM at pH 10
Figure 6 - Turbidity as a function of HPAM concentration for direct flocculation and flocculation from IAHAC and IADDA
Figure 7 - Influence of the HPAM concentration on the sediment height after 72 hours
(A)
(B)
Figure 8 – Illustration of the hydrophobic environment inside of the floc formed through IADDA (drawings illustrative cartoons, not to scale)
Figure 9 - Influence of surfactant concentration on the zeta potential and sediment compaction after 72 hours. Tests carried out with HPAM (10 ppm) at pH 10
Table 1 - D50 for IAHCl as a function of pH pH
IAHCl D50 (µm)
7.1
0.6
6.4
1.0
4.4
1.7
3.6
3.7
3.3
4.0
Table 2 – pH values and D50 for IAHAC, as a function of the concentration of aluminum chloride (no HPAM). HAC (X 10-3M)
pH
IAHAC D50 (µm)
0
7.1
0.6
0.1
6.8
2.4
0.5
3.3
17.9
1.0
3.0
14.8
5.0
2.9
7.1
Table 3 - pH and IADDA D50 as a function of DDA concentration (no HPAM). DDA (X 10-3M)
pH
IADDA D50 (µm)
0
7.0
0.6
0.1
7.2
8.1
0.5
7.2
37.8
1.0
7.5
102.1
5.0
7.7
28.1
Table 4 – Average size of the anatase aggregates for each tested system. HPAM NO
YES
HAC (X 10-3M)
DDA (X 10-3M)
D50 (µm)
-
-
0.6
0.5
17.9 1.0
102.1
-
-
0.90
0.5
-
97.9
-
1.0
414.7