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Polymer adsorption: its correlation with flocculation and dewatering of kaolin suspension in the presence and absence of surfactants
Int. J. Miner. Process. 66 (2002) 183 – 202 www.elsevier.com/locate/ijminpro
Polymer adsorption: its correlation with flocculation and dewatering of kaolin suspension in the presence and absence of surfactants L. Besra a,*, D.K. Sengupta a, S.K. Roy b, P. Ay c a
Regional Research Laboratory, Council of Scientific and Industrial Research, Bhubaneswar 751 013, Orissa, India b Department of Metallurgical and Materials Engineering, IIT Kharagpur 721 302, WB, India c Lehrstuhl Aufbereitungstechnik, Brandenburgische Technische Universitaet, 03044 Cottbus, Germany Received 15 February 2002; received in revised form 8 May 2002; accepted 8 May 2002
Abstract The adsorption characteristics of polyacrylamide flocculants on kaolin surface have been studied at 25 jC as a function of concentration in the presence and absence of surfactants. The adsorption density of flocculants corresponding to maximum settling rate (Coptfloc) and minimum value of specific resistance of the cake to filtration (CminSRF) have been computed and compared with the adsorption density for monolayer coverage (Cl). It has been established in this study that the optimum flocculant concentration for the highest settling rate corresponds to about 50% coverage of the solid surface (i.e. Coptfloc c Cl/2) for untreated as well as surfactant-pretreated kaolin. Flocs suitable for filtration and dewatering are obtained by flocculation of either untreated or surfactantpretreated kaolin. But in each case, the requirement of polymer concentration for achieving the minimum specific resistance to filtration (SRF) is sufficiently lower than that required for optimum flocculation. The adsorption density of polymer corresponding to minimum SRF is less than about 25% of the plateau adsorption (i.e. CminSRF c 0.25 Cl). This has been found to be valid for flocculation with any of the anionic, cationic or nonionic polyacrylamide flocculants used in this investigation. D 2002 Published by Elsevier Science B.V. Keywords: flocculation; dewatering; polymer adsorption; surfactants
* Corresponding author. Tel.: +91-674-481-635; fax: +91-674-581-637. E-mail address: [email protected] (L. Besra). 0301-7516/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 3 0 1 - 7 5 1 6 ( 0 2 ) 0 0 0 6 4 - 9
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1. Introduction Flocculation of fine particles by high-molecular-weight polymers is a popular practice adopted for solid– liquid separation in many industries dealing with slurries of fine particles, waste water treatment, mineral processing and hydrometallurgical operations. A large volume of research and effort has gone into designing, synthesising and development of many new synthetic polymeric flocculants. The flocculation process primarily involves two basic steps: (i) transport of a particle to the closest distance of approach of another particle leading to collision, and (ii) adhesion of the particles resulting in aggregation. It has now been well established that the success of a flocculation by polymers depend on various factors related to the surface chemistry of the particles as well as on the ionic nature, molecular weight, charge density and bulk properties of the flocculants in the solution. Further, the most important influence affecting the extent and mechanism of flocculation is dependent primarily on the nature of adsorption of the polymer on the particle surface and also the conformation of the adsorbed polymer. Depending on the ionic nature and adsorption of polymers on the particle surface, several mechanisms have been put forward to be responsible for flocculation (Gregory et al., 1985): (i) charge neutralisation, (ii) bridging, (iii) charge patch mechanism, (iv) depletion flocculation. Of the above mechanisms, the charge neutralisation and bridging are very commonly encountered. It has also been established from theoretical calculations and experimentation that flocculation does not require high or complete coverage of the particle surface by the polymer. An ideal flocculant is one, which is able to neutralise a part of the surface charge responsible for repulsion, and adsorb with loops and tails extending into solution for bridging with other particles. Complete coverage of the surface with polymer adsorbed with flat configuration would lead to steric repulsion resulting in stable suspension. Smellie and LaMer (1958), Hogg (1984), Deason (1982) and many others proposed that not all collisions of particles are effective in producing flocculation and have related the collision efficiency factor (E) to the fractional surface coverage (h) by polymer. The original LaMer model and subsequently modified models can be generally described as: E ¼ f hð1 hÞ
ð1Þ
where f is the constant (1 < f < 2). The rate of flocculation is equal to the product of collision frequency and the collision efficiency factor E. It is clear from the above equation that E will have maximum value when h is 0.5, corresponding to 50% particle surface coverage with polymer (Somasundaran et al., 1997). The floc characteristics necessary for different methods of solid – liquid separation are, however, unique (Moudgil and Shah, 1986). As for example, sedimentation requires dense and large flocs with regularity in shape (preferably spherical), centrifugation requires strong, dense and large flocs and floc flotation requires low-density flocs with narrow size distribution (Sengupta et al., 1994). Therefore, the surface coverage and polymer adsorption required for different solid –liquid separation operations also will not be the same as that for flocculation. Filtration, which normally follows the flocculation step when dealing with very fine particles, requires porous, strong, permeable flocs. There has, however, been no report on quantification of surface coverage by polymer adsorption for optimum filtration and dewatering operations. So, we have made
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an attempt to correlate adsorption characteristics of three polymeric flocculants of different ionic nature in terms of their surface coverage with the flocculation as well as dewatering characteristics of kaolin suspension as a model system. Studies have also been made in the presence of three surfactants, which have been earlier reported to aid in dewatering properties of different suspensions.
2. Experimental 2.1. Materials and methods 2.1.1. Kaolin The kaolin clay used in this study is of high-quality paper coating grade obtained from Jashipur, Orissa, India. The particle size analysis using a Malvern Laser particle size analyser Model 3600E, from Malvern Instruments, UK, showed all the particles to be below 20 Am with only 11% particles are above 10 Am, and the d50 to be 4.8 Am. Its BET specific surface area is 8.62 m2/g and the point of zero charge is at pH value of 2.2. The other characteristic details of the sample are reported elsewhere (Besra et al., 2000). 2.1.2. Flocculants Three high-molecular-weight polyacrylamide flocculants of different ionic nature have been used in this investigation. They were kindly supplied by Allied Colloids, UK. The molecular weight as determined from the intrinsic viscosity method using the Mark – Houwink constants, K = 6.31 10 3 cm3 g 1 and a = 0.80 for PAM in distilled water (Brandup and Immergut, 1989) for all three polymers lie within 5 –7 106. The detail characteristics of the polymers are given in Table 1. 2.1.3. Surfactants Three representative surfactants of different ionic natures used in this investigation were supplied by Merck and Sigma, Germany. Some of their characteristics are given in Table 2. 2.2. Adsorption studies The adsorption of polyacrylamide flocculants and surfactants from solution onto kaolin surface was estimated by determining their depletion from the solution. The samples were Table 1 Characteristics of polymers used Polymers
Ionic type
Molecular weighta (g/mol)
Charge densityb (C/g)
Percent charged monomer
PAM-A PAM-C PAM-N
Anionic Cationic Nonionic
5.5 – 7 106 6.0 – 7 106 5.0106
260 + 150 1.07
30.0 19.0 –
a b
Determined by intrinsic viscosity method. Determined by polyelectrolyte titration using PCD.
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Table 2 Characteristics of surfactants used in this investigation Surfactant
Molecular formula
Molecular Ionic weight type (g/mol)
Cetyl trimethyl C16H33N(CH3)3 + Br 364.4 ammonium bromide (CTAB) Sodium dodecyl n-C12H25SO4 Na + 288.3 sulphate (SDS) Triton-X 100 p-t-(C8H17C6H4O(C2H4O)10H 628
Critical micelle HLB Aggregation concentration no. (cmc) (mM)
cationic
0.92
–
91
anionic
8.2
17.2
62
nonionic 0.28
13.5 135
treated with required amount of flocculant and surfactants while stirring at 25 jC, followed by equilibration for 24 h. The supernatants were then separated from the sediment by centrifugation for 90 min at 8500 rpm using a Remi Research Centrifuge from Remi Instruments, Bombay, India. Care was taken to see that no solid remained in suspension. Equilibrium concentrations of the flocculants and surfactants were determined spectrophotometrically with a Perkin-Elmer Lambda 2 model UV/VIS spectrophotometer using a 1-cm quartz cell. Calibration curves were used for quantitative estimation of polymers and surfactants. For the case when similar ionic nature of polymer and surfactant such as PAMA and SDS or PAM-C and CTAB coexist in the system, it was difficult to estimate their quantity accurately as both of them interact with the complex forming agent used for colour development during the spectrophotometric analysis. For such a system, we have employed two-stage method comprising of surface tension and polyelectrolyte titration for determination of surfactant and polymer. The amount of surfactant in the supernatant was first determined by reading the surface tension of the supernatant against the plot of surfactant concentration versus surface tension in the presence of polymer. The supernatant liquid was then titrated against the known concentration (0.001 N) of an anionic or cationic standard until neutralisation. The cationic poly-diallyl dodecyl ammonium chloride (Poly-DADMAC) and anionic sodium polyethylene sulphonate (PES-Na) have been used as the standard titrating agents. The volume of titrant actually consumed by the polymer in the supernatant is the difference between that consumed by both polymer plus surfactant and that required by only surfactant. The amount of flocculant present in the supernatant was then estimated by reading it directly from the plot of flocculant concentration versus volume of titrant required for neutralisation. All the measurements were conducted at the natural pH (6.4) of the slurry except in the study on the effect of pH as a variable, where the pH is adjusted with either 1 M HCl or NaOH solution. 2.3. Flocculation studies A known weight of the powder material was mixed and stirred in an appropriate volume of distilled water to obtain the desired slurry concentration and allowed to stand for 24 h. Then the suspension was stirred in a beaker provided with baffles using a 3-cm three-bladed stainless steel stirrer for 10 min at 500 rpm. The requisite amount of flocculant or its mixture with surfactants was then added dropwise followed by stirring
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for 1 min after addition. The stirring speed was then reduced to 300 rpm and continued for 1 min. In the case of surfactant pretreatment, the above step was preceded by addition of required amount of surfactant and equilibration for 24 h. The entire suspension was then transferred to a graduated cylinder and allowed to settle after inverting the cylinder five times. A clear liquid – slurry interface could be seen descending. The height of this interface was noted at a regular interval of time. Settling rates were estimated from the slope of the straight-line portion of the plot of interface height versus settling time. After the sedimentation test, the suspension was allowed to stand without disturbance for 24 h. The volume occupied by the sediment was then read directly from the graduated cylinder. Duplicate measurements made under similar experimental conditions suggest that the settling rates and sediment volumes are reproducible to within the error of 2.5%. 2.4. Filtration and dewatering studies All the filtration experiments were carried out with a vacuum filtration unit already described elsewhere (Besra et al., 2000). It consists of a filter holder, 10-cm diameter with 500-ml graduated reservoir. The base of the filter holder is a perforated plate with a provision to fix the filter paper on it. The Whatman 41 filter paper was used throughout the filtration studies. The slurry after desired treatment with polymers or surfactants is poured in to the filter holder for filtration to occur under a desired pressure difference (49 kPa) applied by vacuum. The volume of filtrate collected in the graduated cylinder was recorded at a regular time interval. These data were used to calculate the specific cake resistance using the integrated form of Darcy’s equation (Besra et al., 2000; Sengupta et al., 1997). A dewatering time of 10 min was allowed from the moment the liquid disappeared from top surface of the cake, after which the cake was carefully removed, weighed and dried at 105 jC until it attained a constant weight. Moisture content of the cake (in percent) was determined from the loss in weight on drying.
3. Results and discussion 3.1. Adsorption of surfactants on kaolin Fig. 1 shows the adsorption characteristics of surfactants on kaolin. In these plots, the amount adsorbed is plotted as a function of equilibrium concentration obtained after 24 h. In absence of any polymer, it shows a low adsorption density of anionic SDS that soon attains plateau leading to a typical Langmuir type curve. The low adsorption density of SDS may be a result of electrostatic repulsion between the negatively charged SDS and kaolin. The surface charge of kaolin was found to be negative in the whole range of pH values above 2.2, and is already reported elsewhere (Besra et al., in press). The effect of SDS, CTAB and TX 100 on the surface charge of kaolin/water interface has also been reported elsewhere (Besra et al., 2002a). The cationic surfactant CTAB exhibits maximum adsorption on kaolin due to attraction between the negative surface and positively charged CTAB. Its adsorption isotherm follows the typical H-type of curve as per the classification of Giles et al. (1960) indicating high affinity for the surface. Adsorption of nonionic
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Fig. 1. Adsorption characteristics of surfactants on kaolin.
surfactant TX 100 in the absence of electrostatic repulsion exhibits S-type curve of Giles et al. (1960) in the initial part indicating that as more solute is adsorbed, the easier it is for the additional amounts to become fixed. This implies a side-by-side association between the adsorbed molecules helping to hold them to the surface, i.e. cooperative adsorption. In order to quantify the adsorbed surfactants in terms of surface area coverage, calculations have been made considering that the area per molecule for SDS, CTAB ˚ 2 (Turner et al., 1999), 45 A ˚ 2 (Bandopadhyay et al., 1998), and 26.69 and TX 100 are 42 A 2 ˚ A (Anand et al., 1991), respectively. The estimation of surface area covered (A) by the surfactant has been calculated by multiplying the total number of surfactant molecules adsorbed on one gram of kaolin sample with the head area of the surfactant molecule and is given by: A¼
amount adsorbed ðg=gÞ Avogadro number molecular weight ðg=molÞ area per molecule ðm2 Þ
ð2Þ
The values of area covered as well as the percentage of BET surface area coverage when the initial surfactant dosage is 0.2 mg/g are presented in Table 3. It clearly shows that a negligibly small percentage of the BET surface area is covered by adsorption of surfactants. The percentage surface area coverage appears to be an under estimate because the BET surface area includes even pore spaces into which the surfactant molecule may ˚ 2) used not penetrate on account of their larger area compared to nitrogen molecule (17 A for BET surface area measurement. The evidence for presence of pores in kaolin has already been shown through morphological studies using SEM and have been reported elsewhere (Besra et al., 2000). The pores in kaolin and other clay minerals generally form
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Table 3 Surface coverage by adsorption of surfactants on kaolin Surfactant Dosage Amount No. of molecules Area covered Plateau adsorption (mg/g) adsorbed adsorbed per by surfactants density (mg/g) gram of kaolin (A) (m2/g) Cl Al (mg/g) (m2/g) SDS CTAB TX 100
0.2 0.2 0.2
0.11 0.20 0.132
2.98 1017 3.31 1017 1.27 1017
0.10 0.15 0.03
0.48 8.40 5.10
0.42 6.25 1.31
Percentage surface area coverage w.r.t. BET w.r.t. surface area monolayer adsorption 1.12 1.72 0.39
22.94 2.36 2.59
by the irregular stacking of the elementary clay sheets and the layered aggregates (Schoonheydt, 1995). 3.2. Relationship between flocculant adsorption and separation properties of kaolin The results of preliminary experiments on the kinetics of flocculant adsorption on kaolin are presented in Fig. 2. It shows that the adsorption of all three flocculants onto kaolin attains about 99% of their respective equilibrium adsorption density within first 1 – 2 min. The results of studies on the effect of pH on adsorption characteristics, settling behaviour and specific resistance of the cake to filtration (SRF) in the presence of all three flocculants on kaolin are depicted in Fig. 3. As the adsorption of PAM-A becomes negligibly small due to electrostatic repulsion, and the surface charge becomes more negative at higher pH, the settling and filtration rates are found to be very slow. Higher adsorption of PAM-C at high pH value is due to more electrostatic attraction of the negatively charged kaolin surface towards the cationic polymer. But the higher adsorption
Fig. 2. Kinetics of flocculant adsorption on kaolin.
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Fig. 3. Effect of pH on polymer adsorption, flocculation and filtration of kaolin suspension.
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of PAM-C has improved neither settling rate nor SRF. Instead, there is an increase in SRF at high pH value. This is the result of different conformation of PAM-C molecules at higher pH condition. It has been established by Sastry et al. (1999) through measurement of dielectric constants and reduced viscosities at different pH that the chain conformation of cationic polyacrylamides are more sensitive to hydrolysis of its amide moiety. It was shown that even smaller percentage of negative carboxylic groups in the alkaline conditions can develop more inter unit contacts as a result of attractive forces between the positive parts within the same chain. As a result the polymer chain may acquire a collapsed coil form of conformation under these conditions. As the collapsed coil conformation is not suitable for bridging, there is no more improvement in settling rate. For the case of PAM-N, even though same quantity of polymer adsorbs on kaolin at both acidic (pH = 2.3) and alkaline (pH = 10.95) conditions, better separation properties can be obtained at higher pH. It is due to a different conformation of PAM-N molecules at higher pH enabling it to interact simultaneously with more number of particles favourable for bridging (Somasundaran et al., 1997). Though the polyacrylamide flocculant used is known to be non ionic in nature, there is some degree of anionicity as found while estimating its charge density. It has also been reported that for the purpose of classifying flocculants, a polymer is considered non ionic if fewer than 1% of the monomer units are charged (Halverson and Panzer, 1980). Besides this, Sastry et al. (1999) found that non ionic polyacrylamide polymer had undergone about 10% hydrolysis in the alkaline region (pH c 10.5) leading to conversion of some amide groups to carboxylic moieties. Similar hydrolysis of the non ionic polyacrylamide flocculant (PAM-N) in the alkaline pH region could have resulted in an increase in negative charge density along the polymer chain and thus more inter unit repulsion. The polymer chain can acquire an extended form of conformation suitable for flocculation by bridging. The adsorption isotherms of the three flocculants at natural pH (6.4) are shown in Fig. 4. The adsorption characteristics on kaolin surface are dependent on the ionic nature of the
Fig. 4. Adsorption isotherms of the flocculants on kaolin (pH = 6.4).
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flocculants. The ionic flocculants (PAM-A and PAM-C) shows a Langmuir type curve signifying monolayer adsorption on kaolin surface, which is typical of most polyelectrolyte adsorptions. The isotherm for PAM-N exhibits multilayer nature of adsorption on kaolin surface similar to the H-3 type of curve according to the classification of Giles et al. (1960), and thus signifying high affinity of PAM-N towards the surface. The effectiveness of flocculation in this study has been evaluated by measuring the settling rate. The dynamic flocculation conditions are very close to equilibrium state as more than 99% of equilibrium adsorption of the flocculants on kaolin takes place within 1– 2 min. The untreated kaolin suspension exhibits a very slow settling rate of only about 0.03 cm/s. A substantial increase in settling rates has been observed on flocculation by all three flocculants. The minimum dosage of flocculants required to obtain the maximum or plateau value of settling rates have been considered as the optimum concentration. To precisely determine the optimum dosage of flocculants, the settling rates have been plotted against the ratio of corresponding adsorption density (C) to the adsorption density at first plateau or monolayer coverage (Cl) and shown in Fig. 5. It clearly shows that maximum or the plateau value of settling rate for all three flocculants starts at C/Cl value of about 0.5. This means that the optimum condition for flocculation of kaolin by flocculants occurs under conditions (Copt.floc c Cl/2), which is in agreement with LaMer and Healy (1963). Table 4 gives a summary of optimum flocculation concentrations, adsorption density and its relationship with the monolayer/first plateau adsorption density for all three flocculants. It also presents the ratio of area coverage calculated on the basis of surface area covered by monolayer adsorption. The surface area coverage is calculated assuming the average area ˚ 2 (Nabzar et al., 1985) and that each monomer per monomer of polyacrylamide to be 25 A occupies some part of the surface. The molecular weights of nonionic, anionic and cationic acrylamide monomers constituting the flocculants are 71, 94 and 150, respectively. The
Fig. 5. Correlation between fractional surface coverage with settling rate of 5% (w/v) kaolin suspension by flocculants.
Table 4 Surface coverage by adsorption of flocculant on bare and surfactant-pretreated kaolin for best flocculation conditions Flocculant
PAM-C
PAM-N
kaolin 0.2 mg/g SDS-pretreated kaolin 0.2 mg/g CTAB-pretreated kaolin 0.2 mg/g TX 100-pretreated kaolin kaolin 0.2 mg/g SDS-pretreated kaolin 0.2 mg/g CTAB-pretreated kaolin 0.2 mg/g TX 100-pretreated kaolin kaolin 0.2 mg/g SDS-pretreated kaolin 0.2 mg/g CTAB-pretreated kaolin 0.2 mg/g TX 100-pretreated kaolin
Adsorption density of flocculants corresponding to monolayer coverage
Initial concentration of flocculant corresponding to maximum settling rate (mg/g)
Corresponding adsorption density of flocculant (Coptfloc) (mg/g)
No. of monomers adsorbed per gram of kaolin (calculated)
Surface area covered by adsorbed flocculant (Aoptfloc) (m2/g)
Coptfloc/Cl or Aoptfloc/Al
Cl (mg/g)
Al (m2/g)
0.079 0.100
0.155 0.196
0.125 0.250
0.040 0.049
3.14 1017 3.86 1017
0.085 0.104
0.506 0.491
0.328
0.644
0.522
0.158
1.24 1018
0.335
0.482
0.162
0.318
0.379
0.080
6.29 1017
0.169
0.493
1.500 1.156
2.863 2.206
0.750 0.750
0.730 0.600
5.57 1018 4.58 1018
1.548 1.273
0.487 0.519
1.082
2.065
0.500
0.490
3.74 1018
1.039
0.452
1.350
2.576
0.750
0.670
5.11 1018
1.421
0.496
0.892 0.75
1.891 1.590
0.506 0.506
0.470 0.384
3.99 1018 3.26 1018
0.998 0.815
0.527 0.510
0.692
1.467
0.506
0.351
2.98 1018
0.744
0.507
0.672
1.425
0.379
0.326
2.97 1018
0.692
0.485
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PAM-A
Substrate
193
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estimation of approximate surface area covered (A) by flocculants has been made by multiplying the number of monomers adsorbed with area per molecule and is given by: A¼
amount adsorbed ðg=gÞ Avogadro number monomer molecular weight ðg=molÞ area per monomer ðm2 Þ
ð3Þ
The plateau adsorption density (Cl) of PAM-A on kaolin is about 0.079 mg/g solid. It can be clearly seen from both Fig. 5 and Table 4 that the adsorption densities corresponding to maximum settling rate (0.19 cm/s) are about 50% of its plateau adsorption. The corresponding values of adsorption densities for maximum settling rate due to PAM-C and PAM-N are also about 50% of their respective adsorption density at first plateau/monolayer coverage (Cl). So in general, a surface coverage of about 0.5 fraction of plateau adsorption (Cl) by the flocculants is required for highest settling rate irrespective of their nature of charges. The filtration characteristics of kaolin suspensions have been assessed through measurement of the specific resistance of the cake to filtration (SRF). Details of the method for SRF estimation is given elsewhere (Besra et al., 2000). Lower SRF yields better filtration characteristics. The SRF for all three flocculants reduces with increasing concentration up to a certain optimum value where the SRF is minimum. Fig. 6 shows the plots between SRF versus the ratio of corresponding adsorption density (C) to the adsorption density at first plateau or monolayer coverage (Cl). Table 5 also presents the summary of optimum concentrations for minimum SRF, adsorption density and its relation with monolayer adsorption density (Cl) and surface coverage by all three flocculants. It clearly indicates that the optimum concentration of flocculants corresponding to minimum SRF (CminSRF) for all three flocculants are much lower than that for
Fig. 6. Correlation between fractional surface coverage with SRF of 5% (w/v) kaolin suspensions by flocculants.
Table 5 Surface coverage by adsorption of flocculant on bare and surfactant-pretreated kaolin for conditions corresponding to minimum SRF Flocculant
PAM-C
PAM-N
kaolin 0.2 mg/g SDS-pretreated kaolin 0.2 mg/g CTAB-pretreated kaolin 0.2 mg/g TX 100-pretreated kaolin kaolin 0.2 mg/g SDS-pretreated kaolin 0.2 mg/g CTAB-pretreated kaolin 0.2 mg/g TX 100-pretreated kaolin kaolin 0.2 mg/g SDS-pretreated kaolin 0.2 mg/g CTAB-pretreated kaolin 0.2 mg/g TX 100-pretreated kaolin
Adsorption density of flocculants corresponding to monolayer coverage
Initial concentration of flocculant corresponding to minimum SRF (mg/g)
Corresponding adsorption density of flocculant (CminSRF) (mg/g)
No. of monomers adsorbed per gram of kaolin (calculated)
Surface area covered by adsorbed flocculant (AminSRF) (m2/g)
CminSRF/Cl or AminSRF/Al
Cl (mg/g)
Al (m2/g)
0.079 0.100
0.155 0.197
0.063 0.063
0.02 0.024
1.57 1017 1.89 1017
0.039 0.047
0.25 0.24
0.328
0.645
0.063
0.065
5.11 1017
0.128
0.20
0.162
0.318
0.063
0.040
3.14 1017
0.079
0.25
1.500 1.156
2.863 2.207
0.25 0.25
0.25 0.23
1.91 1018 1.76 1018
0.477 0.439
0.17 0.20
1.082
2.065
0.25
0.25
1.91 1018
0.477
0.23
1.350
2.577
0.25
0.25
1.91 1018
0.477
0.185
0.892 0.750
1.892 1.591
0.253 0.253
0.23 0.20
1.95 1018 1.69 1018
0.488 0.424
0.257 0.26
0.692
1.468
0.126
0.125
1.06 1018
0.265
0.18
0.672
1.425
0.126
0.120
1.02 1018
0.254
0.178
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PAM-A
Substrate
195
196
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flocculation (Coptfloc). The minimum value of SRF occurs at the C/Cl value of about 0.25 for PAM-A, 0.17 for PAM-C and about 0.26 for PAM-N, respectively. This means that the optimum condition for filtration in the presence of flocculants occurs under conditions when the adsorption density is less than about 25% of plateau adsorption (Coptfloc c 0.25 Cl). 3.3. Relationship between flocculant adsorption and separation properties of surfactantpretreated kaolin The results on pretreatment of kaolin with anionic, cationic and nonionic surfactants followed by flocculation with anionic, cationic or nonionic flocculant are presented in
Fig. 7. Correlation between fractional surface coverage by flocculants and (a) settling rate and (b) SRF of 5% (w/ v) SDS-pretreated kaolin suspension.
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Table 4. It indicates that the adsorption of PAM-A is enhanced due to preadsorption of all the three surfactants. But in the cases of PAM-C and PAM-N the adsorption decreases. The decrease in adsorption has been explained to be due to blockage of the surface sites by surfactant molecules (Besra et al., 2002a,b). The increase in the amount of PAM-A adsorption on kaolin due to surfactant preadsorption perhaps enables more number of the flocculant molecule available to take part in bridging the particles resulting in higher setling rate. But it does not necessarily mean that high adsorption of flocculant will lead to better flocculation. As, for example, the blockage of some surface sites by surfactants results in reduction in adsorption of PAM-C and PAM-N. In these cases, the improvement
Fig. 8. Correlation between fractional surface coverage by flocculants and (a) settling rate and (b) SRF of 5% (w/ v) CTAB-pretreated kaolin suspension.
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in flocculation has been explained, as more number of flocculant segments is available to interact with neighbouring particles (Besra et al., 2002a,b). It can be seen from Tables 4 and 5 that flocculant around 50% of the plateau adsorption is required for the maximum settling rate and around 25% or slightly less is required for the minimum SRF for SDS, CTAB and TX 100-pretreated kaolin, respectively. It is further seen from Figs. 7 – 9 that when the adsorption is either enhanced or reduced by pretreatment with surfactants, the requirement for the highest settling rate is about half of the plateau adsorption of the flocculant on the surfactant-pretreated kaolin. The requirement for minimum SRF is about one-fourth of the plateau adsorption. The reason for achieving best flocculation conditions at 50% of the surface coverage can be explained on the basis of the LaMer and Healy (1963) model in which the collision efficiency factor
Fig. 9. Correlation between fractional surface coverage by flocculants and (a) settling rate and (b) SRF of 5% (w/ v) TX 100-pretreated kaolin suspension.
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E is also maximised when the fractional surface coverage h is 0.5. The explanation for lower flocculant dosage of about 25% of plateau adsorption for best filtration condition can be made on the basis of the work reported by Sengupta et al. (1997). It has been reported that larger flocs do not necessarily improve separation properties. They have shown that while adding flocculant in small quantities in a stirred suspension of kaolinite, the average floc size gradually increases in a first few dosages. In the present case, the small addition of flocculant forms tiny flocs, which are strong enough, but further addition of flocculant can increase the average floc size by aggregating these tiny flocs resulting in higher rate of settling. But these aggregates may not be strong enough to withstand the pressure of filtration or they may be more compressible than the tiny flocs. This may be the reason for which only about 25% or even lower surface coverage is required for the best performance in filtration process. Flocculant dosages higher than 25% of that required for attaining plateau in the adsorption isotherms leads to weaker and compressible flocs resulting in poorer performance in filtration. Table 6 summarises the results on separation properties of kaolin without and with surfactant pretreatment. Pretreatment with surfactants in most of the cases improves settling behaviour and filtration parameter (SRF). The results have been quantified by Table 6 Summary of values of best settling rates and and minimum SRF Flocculant
No flocculant PAM-A
PAM-C
PAM-N
a b
Substrate
kaolin kaolin 0.2 mg/g SDS-pretreated kaolin 0.2 mg/g CTAB-pretreated kaolin 0.2 mg/g TX 100-pretreated kaolin kaolin 0.2 mg/g SDS-pretreated kaolin 0.2 mg/g CTAB-pretreated kaolin 0.2 mg/g TX 100-pretreated kaolin kaolin 0.2 mg/g SDS-pretreated kaolin 0.2 mg/g CTAB-pretreated kaolin 0.2 mg/g TX 100-pretreated kaolin
Best flocculation condition
Best filtration condition
Best settling rates (cm/s)
Flocculation improvement factora
Minimum SRF (m/kg)
0.028 0.198 0.611
1.00 7.00 21.59
7.81 1011 4.45 1011 2.23 1011
1.00 1.76 3.50
0.522
18.45
2.22 1011
3.55
0.473
16.72
2.23 1011
3.50
0.361 0.318
12.74 11.25
9.96 1010 5.53 1010
7.84 14.12
0.351
12.41
5.52 1010
14.15
0.382
13.51
1.00 1011
7.81
0.575 1.207
20.34 42.64
1.11 1011 1.11 1011
7.04 7.04
1.210
42.76
1.11 1011
7.04
1.220
43.11
2.23 1011
3.50
Filtration improvement factorb
Flocculation improvement factor=(settling rate after flocculation)/(settling rate without flocculation). Filtration improvement factor=(SRF without flocculation)/(SRF after flocculation).
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defining a flocculation and filtration improvement factor and are included in Table 6. A comparison of the improvement factors reveal that nonionic flocculant PAM-N is most effective for improving flocculation behaviour, whereas cationic flocculant PAM-C is most effective for improving filtration behaviour of kaolin suspensions. Pretreatment of kaolin with surfactants substantially increases the settling rates on flocculation by all three flocculants as indicated by an increase in flocculation improvement factor. Most improvement in settling rate has been observed for anionic flocculant PAM-N. Surfactant pretreatment also improves the filtration characteristics in most of the cases. The maximum improvement in filtration characteristics has been obtained for flocculation by PAM-C of kaolin pretreated with surfactant CTAB and SDS. The cationic flocculant PAM-C adsorbs comparatively more on SDS-pretreated kaolin than on CTAB-pretreated one (Table 4). The better performance in both the cases compared to the other conditions can be explained to be the result of combined effect of charge neutralisation and bridging (Besra et al., 2002b).
4. Conclusions Based on the data presented, the following conclusions can be made: (i)
The choice of flocculant for separation by different operations can be made by comparing the best settling rates and minimum SRF obtained through flocculation. (ii) The non ionic polymer PAM-N is most effective for improving settling rates of kaolin suspension in comparison to anionic (PAM-A) and cationic (PAM-C) flocculants. The flocculation by non ionic flocculant occurs by bridging. (iii) The cationic polymer PAM-C is more effective for improving filtration characteristics of kaolin suspension in comparison to anionic (PAM-A) and non ionic (PAM-N) flocculants.
Fig. 10. Schematic representation of the relationship between fractional surface coverage and optimum conditions for flocculation and filtration.
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(iv) In most of the cases, preadsorbed surfactants on kaolin further improves both settling rates and filtration characteristics on flocculation. (v) The floc properties required for separation by different methods are different, and the dosing conditions of flocculant for best flocculation and filtration are not same. (vi) About 50% coverage of the particle surface by polymer adsorption is necessary for optimum flocculation where maximum settling rate occurs. This condition is valid for the surfactant-pretreated kaolin also. (vii) The best conditions for separation by filtration occurs at sufficiently lower flocculant dosages and is close to or less than 25% of the adsorption density for monolayer coverage (i.e. CminSRF c 0.25 Cl). (viii) The adsorption density corresponding to best flocculation and filtration conditions can be correlated with the monolayer adsorption density of the polymers by the schematic illustration given in Fig. 10.
Acknowledgements One of the authors (LB) is thankful to the German Academic Exchange Service (DAAD) for the financial grant to carry out a part of this work. We are thankful to Ms. Valeria Thalheim for her assistance in quantitative analysis of flocculants and surfactants using UV/VIS spectrophotometer. Permission of Dr. V.N. Mishra, Director, Regional Research Laboratory, Bhubaneswar, for publishing this paper is duly acknowledged.
References Anand, K., Yadav, O.P., Singh, P.P., 1991. Studies on the surface and thermodynamic properties of some surfactants in aqueous and water + 1,4-dioxane solutions. Colloids Surf. 55, 359 – 364. Bandopadhyay, S., Shelly, J.C., Tarek, M., Moore, P.B., Klein, M.L., 1998. Surfactant aggregation at a hydrophobic surface. J. Phys. Chem., B 102, 6318 – 6322. Besra, L., Sengupta, D.K., Roy, S.K., 2000. Particle characteristics and their influence on dewatering of kaolin, calcite and quartz suspensions. Int. J. Miner. Process. 58 (2), 89 – 112. Besra, L., Sengupta, D.K., Roy, S.K., Ay, P., 2002a. Studies on flocculation and dewatering of kaolin suspensions by anionic polyacrylamide flocculant in the presence of some surfactants. Int. J. Miner. Process. (in press). Besra, L., Sengupta, D.K., Roy, S.K., Ay, P., 2002b. Flocculation and dewatering of kaolin suspensions in the presence of polyacrylamide (PAM) and surfactants. Int. J. Miner. Process. (in press). Besra, L., Sengupta, D.K., Roy, S.K., Ay, P., 2002c. Flocculation and dewatering of kaolin suspensions by cationic polyacrylamide (PAM-C) flocculant in presence of some surfactants. Sep. Purif. Technol. (Communicated). Brandup, J., Immergut, E.H., 1989. Polymer Handbook, 3rd edn. Wiley, New York. Deason, D.M., 1982. In: Attia, Y.A. (Ed.), Flocculation in Biotechnology and Separation Systems. Elsevier, Amsterdam, pp. 21 – 30. Giles, C.H., MacEwan, T.H., Nkhaw, S.N., Smith, D., 1960. Studies in adsorption: Part XI. A system of classification of solution adsorption isotherms and its use in diagnosis of adsorption mechanisms and in measurement of specific surface. J. Chem. Soc., 3973 – 3993. Gregory, J., 1985. The use of polymeric flocculants. Proc. of the Engineering Foundation Conference on Flocculation Sedimentation and Consolidation, The Cloister, Sea Island, Georgia, USA, Jan. 27 – Feb. 1. American Institute of Chemical Engineers, New York, pp. 125 – 137. Halverson, F., Panzer, H.P., 1980. Flocculating agents. Encyclopedia of Chemical Technology, 3rd edn., vol. 10. Wiley, New York, pp. 489 – 523.
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Hogg, R., 1984. Collision efficiency factor for polymer flocculation. J. Colloid Interface Sci. 102 (1), 232 – 236. LaMer, V.K., Healy, T.W., 1963. Adsorption – flocculation relations of macromolecules at the solid – liquid interface. Rev. Pure Appl. Chem. 13, 112 – 132. Moudgil, B.M., Shah, B.D., 1986. Selection of flocculants for solid – liquid separation processes. In: Muralidhara, H.S. (Ed.), Advances in Solid – Liquid Separation. Battelle Press, Columbus, pp. 191 – 294. Nabzar, L., Pefferkorn, E., 1985. An experimental study of kaolinite crystal edge – polyacrylamide interactions in dilute suspensions. J. Colloid Interface Sci. 108 (1), 243 – 248. Sastry, N.V., Dave, P.N., Valand, M.K., 1999. Dilute solution behaviour of polyacrylamides in aqueous media. Eur. Polym. J. 35, 517 – 525. Schoonheydt, R.A., 1995. Clay mineral surfaces (Chapter 9). In: Vaughan, D.J., Pattrick, R.A.D. (Eds.), Mineral Surfaces. Chapman & Hall, London, pp. 303 – 332. Sengupta, D.K., Attia, Y.A., Hamza, H.A., 1994. Enhanced filtration dewatering of industrial sludge by flocculation. Fluid/Part. Sep. J. 7 (4), 165 – 171. Sengupta, D.K., Kan, J., Al Taweel, A.M., Hamza, H.A., 1997. Dependence of separation properties of flocculation dynamics of kaolinite suspension. Int. J. Miner. Process. 44, 73 – 85. Smellie, R.H., LaMer, V.K., 1958. Flocculation, subsidence and filtration of phosphate slimes: VI. A quantitative theory of filtration of flocculated suspensions. J. Colloid Interface Sci. 23, 589 – 599. Somasundaran, P., Markovic, B., Krishnakumar, S., Yu, X., 1997. Colloid system and interface—stability of dispersions through polymer and surfactant adsorption. In: Birdi, K.S. (Ed.), Handbook of Surfaces and Colloid Chemistry. CRC Press, Boca Raton, FL, pp. 559 – 601. Turner, S.F., Clarke, S.M., Rennie, A.R., Thirtle, P.N., Cooke, D.J., Li, Z.X., Thomas, R.K., 1999. Adsorption of sodium dodecyl sulfate to a polystyrene/water interface studied by neutron reflection attenuated total reflection infra-red spectroscopy. Langmuir 15, 1017 – 1023.
Polymer adsorption: its correlation with flocculation and dewatering of kaolin suspension in the presence and absence of surfactants L. Besra a,*, D.K. Sengupta a, S.K. Roy b, P. Ay c a
Regional Research Laboratory, Council of Scientific and Industrial Research, Bhubaneswar 751 013, Orissa, India b Department of Metallurgical and Materials Engineering, IIT Kharagpur 721 302, WB, India c Lehrstuhl Aufbereitungstechnik, Brandenburgische Technische Universitaet, 03044 Cottbus, Germany Received 15 February 2002; received in revised form 8 May 2002; accepted 8 May 2002
Abstract The adsorption characteristics of polyacrylamide flocculants on kaolin surface have been studied at 25 jC as a function of concentration in the presence and absence of surfactants. The adsorption density of flocculants corresponding to maximum settling rate (Coptfloc) and minimum value of specific resistance of the cake to filtration (CminSRF) have been computed and compared with the adsorption density for monolayer coverage (Cl). It has been established in this study that the optimum flocculant concentration for the highest settling rate corresponds to about 50% coverage of the solid surface (i.e. Coptfloc c Cl/2) for untreated as well as surfactant-pretreated kaolin. Flocs suitable for filtration and dewatering are obtained by flocculation of either untreated or surfactantpretreated kaolin. But in each case, the requirement of polymer concentration for achieving the minimum specific resistance to filtration (SRF) is sufficiently lower than that required for optimum flocculation. The adsorption density of polymer corresponding to minimum SRF is less than about 25% of the plateau adsorption (i.e. CminSRF c 0.25 Cl). This has been found to be valid for flocculation with any of the anionic, cationic or nonionic polyacrylamide flocculants used in this investigation. D 2002 Published by Elsevier Science B.V. Keywords: flocculation; dewatering; polymer adsorption; surfactants
* Corresponding author. Tel.: +91-674-481-635; fax: +91-674-581-637. E-mail address: [email protected] (L. Besra). 0301-7516/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 3 0 1 - 7 5 1 6 ( 0 2 ) 0 0 0 6 4 - 9
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1. Introduction Flocculation of fine particles by high-molecular-weight polymers is a popular practice adopted for solid– liquid separation in many industries dealing with slurries of fine particles, waste water treatment, mineral processing and hydrometallurgical operations. A large volume of research and effort has gone into designing, synthesising and development of many new synthetic polymeric flocculants. The flocculation process primarily involves two basic steps: (i) transport of a particle to the closest distance of approach of another particle leading to collision, and (ii) adhesion of the particles resulting in aggregation. It has now been well established that the success of a flocculation by polymers depend on various factors related to the surface chemistry of the particles as well as on the ionic nature, molecular weight, charge density and bulk properties of the flocculants in the solution. Further, the most important influence affecting the extent and mechanism of flocculation is dependent primarily on the nature of adsorption of the polymer on the particle surface and also the conformation of the adsorbed polymer. Depending on the ionic nature and adsorption of polymers on the particle surface, several mechanisms have been put forward to be responsible for flocculation (Gregory et al., 1985): (i) charge neutralisation, (ii) bridging, (iii) charge patch mechanism, (iv) depletion flocculation. Of the above mechanisms, the charge neutralisation and bridging are very commonly encountered. It has also been established from theoretical calculations and experimentation that flocculation does not require high or complete coverage of the particle surface by the polymer. An ideal flocculant is one, which is able to neutralise a part of the surface charge responsible for repulsion, and adsorb with loops and tails extending into solution for bridging with other particles. Complete coverage of the surface with polymer adsorbed with flat configuration would lead to steric repulsion resulting in stable suspension. Smellie and LaMer (1958), Hogg (1984), Deason (1982) and many others proposed that not all collisions of particles are effective in producing flocculation and have related the collision efficiency factor (E) to the fractional surface coverage (h) by polymer. The original LaMer model and subsequently modified models can be generally described as: E ¼ f hð1 hÞ
ð1Þ
where f is the constant (1 < f < 2). The rate of flocculation is equal to the product of collision frequency and the collision efficiency factor E. It is clear from the above equation that E will have maximum value when h is 0.5, corresponding to 50% particle surface coverage with polymer (Somasundaran et al., 1997). The floc characteristics necessary for different methods of solid – liquid separation are, however, unique (Moudgil and Shah, 1986). As for example, sedimentation requires dense and large flocs with regularity in shape (preferably spherical), centrifugation requires strong, dense and large flocs and floc flotation requires low-density flocs with narrow size distribution (Sengupta et al., 1994). Therefore, the surface coverage and polymer adsorption required for different solid –liquid separation operations also will not be the same as that for flocculation. Filtration, which normally follows the flocculation step when dealing with very fine particles, requires porous, strong, permeable flocs. There has, however, been no report on quantification of surface coverage by polymer adsorption for optimum filtration and dewatering operations. So, we have made
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an attempt to correlate adsorption characteristics of three polymeric flocculants of different ionic nature in terms of their surface coverage with the flocculation as well as dewatering characteristics of kaolin suspension as a model system. Studies have also been made in the presence of three surfactants, which have been earlier reported to aid in dewatering properties of different suspensions.
2. Experimental 2.1. Materials and methods 2.1.1. Kaolin The kaolin clay used in this study is of high-quality paper coating grade obtained from Jashipur, Orissa, India. The particle size analysis using a Malvern Laser particle size analyser Model 3600E, from Malvern Instruments, UK, showed all the particles to be below 20 Am with only 11% particles are above 10 Am, and the d50 to be 4.8 Am. Its BET specific surface area is 8.62 m2/g and the point of zero charge is at pH value of 2.2. The other characteristic details of the sample are reported elsewhere (Besra et al., 2000). 2.1.2. Flocculants Three high-molecular-weight polyacrylamide flocculants of different ionic nature have been used in this investigation. They were kindly supplied by Allied Colloids, UK. The molecular weight as determined from the intrinsic viscosity method using the Mark – Houwink constants, K = 6.31 10 3 cm3 g 1 and a = 0.80 for PAM in distilled water (Brandup and Immergut, 1989) for all three polymers lie within 5 –7 106. The detail characteristics of the polymers are given in Table 1. 2.1.3. Surfactants Three representative surfactants of different ionic natures used in this investigation were supplied by Merck and Sigma, Germany. Some of their characteristics are given in Table 2. 2.2. Adsorption studies The adsorption of polyacrylamide flocculants and surfactants from solution onto kaolin surface was estimated by determining their depletion from the solution. The samples were Table 1 Characteristics of polymers used Polymers
Ionic type
Molecular weighta (g/mol)
Charge densityb (C/g)
Percent charged monomer
PAM-A PAM-C PAM-N
Anionic Cationic Nonionic
5.5 – 7 106 6.0 – 7 106 5.0106
260 + 150 1.07
30.0 19.0 –
a b
Determined by intrinsic viscosity method. Determined by polyelectrolyte titration using PCD.
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Table 2 Characteristics of surfactants used in this investigation Surfactant
Molecular formula
Molecular Ionic weight type (g/mol)
Cetyl trimethyl C16H33N(CH3)3 + Br 364.4 ammonium bromide (CTAB) Sodium dodecyl n-C12H25SO4 Na + 288.3 sulphate (SDS) Triton-X 100 p-t-(C8H17C6H4O(C2H4O)10H 628
Critical micelle HLB Aggregation concentration no. (cmc) (mM)
cationic
0.92
–
91
anionic
8.2
17.2
62
nonionic 0.28
13.5 135
treated with required amount of flocculant and surfactants while stirring at 25 jC, followed by equilibration for 24 h. The supernatants were then separated from the sediment by centrifugation for 90 min at 8500 rpm using a Remi Research Centrifuge from Remi Instruments, Bombay, India. Care was taken to see that no solid remained in suspension. Equilibrium concentrations of the flocculants and surfactants were determined spectrophotometrically with a Perkin-Elmer Lambda 2 model UV/VIS spectrophotometer using a 1-cm quartz cell. Calibration curves were used for quantitative estimation of polymers and surfactants. For the case when similar ionic nature of polymer and surfactant such as PAMA and SDS or PAM-C and CTAB coexist in the system, it was difficult to estimate their quantity accurately as both of them interact with the complex forming agent used for colour development during the spectrophotometric analysis. For such a system, we have employed two-stage method comprising of surface tension and polyelectrolyte titration for determination of surfactant and polymer. The amount of surfactant in the supernatant was first determined by reading the surface tension of the supernatant against the plot of surfactant concentration versus surface tension in the presence of polymer. The supernatant liquid was then titrated against the known concentration (0.001 N) of an anionic or cationic standard until neutralisation. The cationic poly-diallyl dodecyl ammonium chloride (Poly-DADMAC) and anionic sodium polyethylene sulphonate (PES-Na) have been used as the standard titrating agents. The volume of titrant actually consumed by the polymer in the supernatant is the difference between that consumed by both polymer plus surfactant and that required by only surfactant. The amount of flocculant present in the supernatant was then estimated by reading it directly from the plot of flocculant concentration versus volume of titrant required for neutralisation. All the measurements were conducted at the natural pH (6.4) of the slurry except in the study on the effect of pH as a variable, where the pH is adjusted with either 1 M HCl or NaOH solution. 2.3. Flocculation studies A known weight of the powder material was mixed and stirred in an appropriate volume of distilled water to obtain the desired slurry concentration and allowed to stand for 24 h. Then the suspension was stirred in a beaker provided with baffles using a 3-cm three-bladed stainless steel stirrer for 10 min at 500 rpm. The requisite amount of flocculant or its mixture with surfactants was then added dropwise followed by stirring
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for 1 min after addition. The stirring speed was then reduced to 300 rpm and continued for 1 min. In the case of surfactant pretreatment, the above step was preceded by addition of required amount of surfactant and equilibration for 24 h. The entire suspension was then transferred to a graduated cylinder and allowed to settle after inverting the cylinder five times. A clear liquid – slurry interface could be seen descending. The height of this interface was noted at a regular interval of time. Settling rates were estimated from the slope of the straight-line portion of the plot of interface height versus settling time. After the sedimentation test, the suspension was allowed to stand without disturbance for 24 h. The volume occupied by the sediment was then read directly from the graduated cylinder. Duplicate measurements made under similar experimental conditions suggest that the settling rates and sediment volumes are reproducible to within the error of 2.5%. 2.4. Filtration and dewatering studies All the filtration experiments were carried out with a vacuum filtration unit already described elsewhere (Besra et al., 2000). It consists of a filter holder, 10-cm diameter with 500-ml graduated reservoir. The base of the filter holder is a perforated plate with a provision to fix the filter paper on it. The Whatman 41 filter paper was used throughout the filtration studies. The slurry after desired treatment with polymers or surfactants is poured in to the filter holder for filtration to occur under a desired pressure difference (49 kPa) applied by vacuum. The volume of filtrate collected in the graduated cylinder was recorded at a regular time interval. These data were used to calculate the specific cake resistance using the integrated form of Darcy’s equation (Besra et al., 2000; Sengupta et al., 1997). A dewatering time of 10 min was allowed from the moment the liquid disappeared from top surface of the cake, after which the cake was carefully removed, weighed and dried at 105 jC until it attained a constant weight. Moisture content of the cake (in percent) was determined from the loss in weight on drying.
3. Results and discussion 3.1. Adsorption of surfactants on kaolin Fig. 1 shows the adsorption characteristics of surfactants on kaolin. In these plots, the amount adsorbed is plotted as a function of equilibrium concentration obtained after 24 h. In absence of any polymer, it shows a low adsorption density of anionic SDS that soon attains plateau leading to a typical Langmuir type curve. The low adsorption density of SDS may be a result of electrostatic repulsion between the negatively charged SDS and kaolin. The surface charge of kaolin was found to be negative in the whole range of pH values above 2.2, and is already reported elsewhere (Besra et al., in press). The effect of SDS, CTAB and TX 100 on the surface charge of kaolin/water interface has also been reported elsewhere (Besra et al., 2002a). The cationic surfactant CTAB exhibits maximum adsorption on kaolin due to attraction between the negative surface and positively charged CTAB. Its adsorption isotherm follows the typical H-type of curve as per the classification of Giles et al. (1960) indicating high affinity for the surface. Adsorption of nonionic
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Fig. 1. Adsorption characteristics of surfactants on kaolin.
surfactant TX 100 in the absence of electrostatic repulsion exhibits S-type curve of Giles et al. (1960) in the initial part indicating that as more solute is adsorbed, the easier it is for the additional amounts to become fixed. This implies a side-by-side association between the adsorbed molecules helping to hold them to the surface, i.e. cooperative adsorption. In order to quantify the adsorbed surfactants in terms of surface area coverage, calculations have been made considering that the area per molecule for SDS, CTAB ˚ 2 (Turner et al., 1999), 45 A ˚ 2 (Bandopadhyay et al., 1998), and 26.69 and TX 100 are 42 A 2 ˚ A (Anand et al., 1991), respectively. The estimation of surface area covered (A) by the surfactant has been calculated by multiplying the total number of surfactant molecules adsorbed on one gram of kaolin sample with the head area of the surfactant molecule and is given by: A¼
amount adsorbed ðg=gÞ Avogadro number molecular weight ðg=molÞ area per molecule ðm2 Þ
ð2Þ
The values of area covered as well as the percentage of BET surface area coverage when the initial surfactant dosage is 0.2 mg/g are presented in Table 3. It clearly shows that a negligibly small percentage of the BET surface area is covered by adsorption of surfactants. The percentage surface area coverage appears to be an under estimate because the BET surface area includes even pore spaces into which the surfactant molecule may ˚ 2) used not penetrate on account of their larger area compared to nitrogen molecule (17 A for BET surface area measurement. The evidence for presence of pores in kaolin has already been shown through morphological studies using SEM and have been reported elsewhere (Besra et al., 2000). The pores in kaolin and other clay minerals generally form
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Table 3 Surface coverage by adsorption of surfactants on kaolin Surfactant Dosage Amount No. of molecules Area covered Plateau adsorption (mg/g) adsorbed adsorbed per by surfactants density (mg/g) gram of kaolin (A) (m2/g) Cl Al (mg/g) (m2/g) SDS CTAB TX 100
0.2 0.2 0.2
0.11 0.20 0.132
2.98 1017 3.31 1017 1.27 1017
0.10 0.15 0.03
0.48 8.40 5.10
0.42 6.25 1.31
Percentage surface area coverage w.r.t. BET w.r.t. surface area monolayer adsorption 1.12 1.72 0.39
22.94 2.36 2.59
by the irregular stacking of the elementary clay sheets and the layered aggregates (Schoonheydt, 1995). 3.2. Relationship between flocculant adsorption and separation properties of kaolin The results of preliminary experiments on the kinetics of flocculant adsorption on kaolin are presented in Fig. 2. It shows that the adsorption of all three flocculants onto kaolin attains about 99% of their respective equilibrium adsorption density within first 1 – 2 min. The results of studies on the effect of pH on adsorption characteristics, settling behaviour and specific resistance of the cake to filtration (SRF) in the presence of all three flocculants on kaolin are depicted in Fig. 3. As the adsorption of PAM-A becomes negligibly small due to electrostatic repulsion, and the surface charge becomes more negative at higher pH, the settling and filtration rates are found to be very slow. Higher adsorption of PAM-C at high pH value is due to more electrostatic attraction of the negatively charged kaolin surface towards the cationic polymer. But the higher adsorption
Fig. 2. Kinetics of flocculant adsorption on kaolin.
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Fig. 3. Effect of pH on polymer adsorption, flocculation and filtration of kaolin suspension.
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of PAM-C has improved neither settling rate nor SRF. Instead, there is an increase in SRF at high pH value. This is the result of different conformation of PAM-C molecules at higher pH condition. It has been established by Sastry et al. (1999) through measurement of dielectric constants and reduced viscosities at different pH that the chain conformation of cationic polyacrylamides are more sensitive to hydrolysis of its amide moiety. It was shown that even smaller percentage of negative carboxylic groups in the alkaline conditions can develop more inter unit contacts as a result of attractive forces between the positive parts within the same chain. As a result the polymer chain may acquire a collapsed coil form of conformation under these conditions. As the collapsed coil conformation is not suitable for bridging, there is no more improvement in settling rate. For the case of PAM-N, even though same quantity of polymer adsorbs on kaolin at both acidic (pH = 2.3) and alkaline (pH = 10.95) conditions, better separation properties can be obtained at higher pH. It is due to a different conformation of PAM-N molecules at higher pH enabling it to interact simultaneously with more number of particles favourable for bridging (Somasundaran et al., 1997). Though the polyacrylamide flocculant used is known to be non ionic in nature, there is some degree of anionicity as found while estimating its charge density. It has also been reported that for the purpose of classifying flocculants, a polymer is considered non ionic if fewer than 1% of the monomer units are charged (Halverson and Panzer, 1980). Besides this, Sastry et al. (1999) found that non ionic polyacrylamide polymer had undergone about 10% hydrolysis in the alkaline region (pH c 10.5) leading to conversion of some amide groups to carboxylic moieties. Similar hydrolysis of the non ionic polyacrylamide flocculant (PAM-N) in the alkaline pH region could have resulted in an increase in negative charge density along the polymer chain and thus more inter unit repulsion. The polymer chain can acquire an extended form of conformation suitable for flocculation by bridging. The adsorption isotherms of the three flocculants at natural pH (6.4) are shown in Fig. 4. The adsorption characteristics on kaolin surface are dependent on the ionic nature of the
Fig. 4. Adsorption isotherms of the flocculants on kaolin (pH = 6.4).
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flocculants. The ionic flocculants (PAM-A and PAM-C) shows a Langmuir type curve signifying monolayer adsorption on kaolin surface, which is typical of most polyelectrolyte adsorptions. The isotherm for PAM-N exhibits multilayer nature of adsorption on kaolin surface similar to the H-3 type of curve according to the classification of Giles et al. (1960), and thus signifying high affinity of PAM-N towards the surface. The effectiveness of flocculation in this study has been evaluated by measuring the settling rate. The dynamic flocculation conditions are very close to equilibrium state as more than 99% of equilibrium adsorption of the flocculants on kaolin takes place within 1– 2 min. The untreated kaolin suspension exhibits a very slow settling rate of only about 0.03 cm/s. A substantial increase in settling rates has been observed on flocculation by all three flocculants. The minimum dosage of flocculants required to obtain the maximum or plateau value of settling rates have been considered as the optimum concentration. To precisely determine the optimum dosage of flocculants, the settling rates have been plotted against the ratio of corresponding adsorption density (C) to the adsorption density at first plateau or monolayer coverage (Cl) and shown in Fig. 5. It clearly shows that maximum or the plateau value of settling rate for all three flocculants starts at C/Cl value of about 0.5. This means that the optimum condition for flocculation of kaolin by flocculants occurs under conditions (Copt.floc c Cl/2), which is in agreement with LaMer and Healy (1963). Table 4 gives a summary of optimum flocculation concentrations, adsorption density and its relationship with the monolayer/first plateau adsorption density for all three flocculants. It also presents the ratio of area coverage calculated on the basis of surface area covered by monolayer adsorption. The surface area coverage is calculated assuming the average area ˚ 2 (Nabzar et al., 1985) and that each monomer per monomer of polyacrylamide to be 25 A occupies some part of the surface. The molecular weights of nonionic, anionic and cationic acrylamide monomers constituting the flocculants are 71, 94 and 150, respectively. The
Fig. 5. Correlation between fractional surface coverage with settling rate of 5% (w/v) kaolin suspension by flocculants.
Table 4 Surface coverage by adsorption of flocculant on bare and surfactant-pretreated kaolin for best flocculation conditions Flocculant
PAM-C
PAM-N
kaolin 0.2 mg/g SDS-pretreated kaolin 0.2 mg/g CTAB-pretreated kaolin 0.2 mg/g TX 100-pretreated kaolin kaolin 0.2 mg/g SDS-pretreated kaolin 0.2 mg/g CTAB-pretreated kaolin 0.2 mg/g TX 100-pretreated kaolin kaolin 0.2 mg/g SDS-pretreated kaolin 0.2 mg/g CTAB-pretreated kaolin 0.2 mg/g TX 100-pretreated kaolin
Adsorption density of flocculants corresponding to monolayer coverage
Initial concentration of flocculant corresponding to maximum settling rate (mg/g)
Corresponding adsorption density of flocculant (Coptfloc) (mg/g)
No. of monomers adsorbed per gram of kaolin (calculated)
Surface area covered by adsorbed flocculant (Aoptfloc) (m2/g)
Coptfloc/Cl or Aoptfloc/Al
Cl (mg/g)
Al (m2/g)
0.079 0.100
0.155 0.196
0.125 0.250
0.040 0.049
3.14 1017 3.86 1017
0.085 0.104
0.506 0.491
0.328
0.644
0.522
0.158
1.24 1018
0.335
0.482
0.162
0.318
0.379
0.080
6.29 1017
0.169
0.493
1.500 1.156
2.863 2.206
0.750 0.750
0.730 0.600
5.57 1018 4.58 1018
1.548 1.273
0.487 0.519
1.082
2.065
0.500
0.490
3.74 1018
1.039
0.452
1.350
2.576
0.750
0.670
5.11 1018
1.421
0.496
0.892 0.75
1.891 1.590
0.506 0.506
0.470 0.384
3.99 1018 3.26 1018
0.998 0.815
0.527 0.510
0.692
1.467
0.506
0.351
2.98 1018
0.744
0.507
0.672
1.425
0.379
0.326
2.97 1018
0.692
0.485
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PAM-A
Substrate
193
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estimation of approximate surface area covered (A) by flocculants has been made by multiplying the number of monomers adsorbed with area per molecule and is given by: A¼
amount adsorbed ðg=gÞ Avogadro number monomer molecular weight ðg=molÞ area per monomer ðm2 Þ
ð3Þ
The plateau adsorption density (Cl) of PAM-A on kaolin is about 0.079 mg/g solid. It can be clearly seen from both Fig. 5 and Table 4 that the adsorption densities corresponding to maximum settling rate (0.19 cm/s) are about 50% of its plateau adsorption. The corresponding values of adsorption densities for maximum settling rate due to PAM-C and PAM-N are also about 50% of their respective adsorption density at first plateau/monolayer coverage (Cl). So in general, a surface coverage of about 0.5 fraction of plateau adsorption (Cl) by the flocculants is required for highest settling rate irrespective of their nature of charges. The filtration characteristics of kaolin suspensions have been assessed through measurement of the specific resistance of the cake to filtration (SRF). Details of the method for SRF estimation is given elsewhere (Besra et al., 2000). Lower SRF yields better filtration characteristics. The SRF for all three flocculants reduces with increasing concentration up to a certain optimum value where the SRF is minimum. Fig. 6 shows the plots between SRF versus the ratio of corresponding adsorption density (C) to the adsorption density at first plateau or monolayer coverage (Cl). Table 5 also presents the summary of optimum concentrations for minimum SRF, adsorption density and its relation with monolayer adsorption density (Cl) and surface coverage by all three flocculants. It clearly indicates that the optimum concentration of flocculants corresponding to minimum SRF (CminSRF) for all three flocculants are much lower than that for
Fig. 6. Correlation between fractional surface coverage with SRF of 5% (w/v) kaolin suspensions by flocculants.
Table 5 Surface coverage by adsorption of flocculant on bare and surfactant-pretreated kaolin for conditions corresponding to minimum SRF Flocculant
PAM-C
PAM-N
kaolin 0.2 mg/g SDS-pretreated kaolin 0.2 mg/g CTAB-pretreated kaolin 0.2 mg/g TX 100-pretreated kaolin kaolin 0.2 mg/g SDS-pretreated kaolin 0.2 mg/g CTAB-pretreated kaolin 0.2 mg/g TX 100-pretreated kaolin kaolin 0.2 mg/g SDS-pretreated kaolin 0.2 mg/g CTAB-pretreated kaolin 0.2 mg/g TX 100-pretreated kaolin
Adsorption density of flocculants corresponding to monolayer coverage
Initial concentration of flocculant corresponding to minimum SRF (mg/g)
Corresponding adsorption density of flocculant (CminSRF) (mg/g)
No. of monomers adsorbed per gram of kaolin (calculated)
Surface area covered by adsorbed flocculant (AminSRF) (m2/g)
CminSRF/Cl or AminSRF/Al
Cl (mg/g)
Al (m2/g)
0.079 0.100
0.155 0.197
0.063 0.063
0.02 0.024
1.57 1017 1.89 1017
0.039 0.047
0.25 0.24
0.328
0.645
0.063
0.065
5.11 1017
0.128
0.20
0.162
0.318
0.063
0.040
3.14 1017
0.079
0.25
1.500 1.156
2.863 2.207
0.25 0.25
0.25 0.23
1.91 1018 1.76 1018
0.477 0.439
0.17 0.20
1.082
2.065
0.25
0.25
1.91 1018
0.477
0.23
1.350
2.577
0.25
0.25
1.91 1018
0.477
0.185
0.892 0.750
1.892 1.591
0.253 0.253
0.23 0.20
1.95 1018 1.69 1018
0.488 0.424
0.257 0.26
0.692
1.468
0.126
0.125
1.06 1018
0.265
0.18
0.672
1.425
0.126
0.120
1.02 1018
0.254
0.178
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PAM-A
Substrate
195
196
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flocculation (Coptfloc). The minimum value of SRF occurs at the C/Cl value of about 0.25 for PAM-A, 0.17 for PAM-C and about 0.26 for PAM-N, respectively. This means that the optimum condition for filtration in the presence of flocculants occurs under conditions when the adsorption density is less than about 25% of plateau adsorption (Coptfloc c 0.25 Cl). 3.3. Relationship between flocculant adsorption and separation properties of surfactantpretreated kaolin The results on pretreatment of kaolin with anionic, cationic and nonionic surfactants followed by flocculation with anionic, cationic or nonionic flocculant are presented in
Fig. 7. Correlation between fractional surface coverage by flocculants and (a) settling rate and (b) SRF of 5% (w/ v) SDS-pretreated kaolin suspension.
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197
Table 4. It indicates that the adsorption of PAM-A is enhanced due to preadsorption of all the three surfactants. But in the cases of PAM-C and PAM-N the adsorption decreases. The decrease in adsorption has been explained to be due to blockage of the surface sites by surfactant molecules (Besra et al., 2002a,b). The increase in the amount of PAM-A adsorption on kaolin due to surfactant preadsorption perhaps enables more number of the flocculant molecule available to take part in bridging the particles resulting in higher setling rate. But it does not necessarily mean that high adsorption of flocculant will lead to better flocculation. As, for example, the blockage of some surface sites by surfactants results in reduction in adsorption of PAM-C and PAM-N. In these cases, the improvement
Fig. 8. Correlation between fractional surface coverage by flocculants and (a) settling rate and (b) SRF of 5% (w/ v) CTAB-pretreated kaolin suspension.
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in flocculation has been explained, as more number of flocculant segments is available to interact with neighbouring particles (Besra et al., 2002a,b). It can be seen from Tables 4 and 5 that flocculant around 50% of the plateau adsorption is required for the maximum settling rate and around 25% or slightly less is required for the minimum SRF for SDS, CTAB and TX 100-pretreated kaolin, respectively. It is further seen from Figs. 7 – 9 that when the adsorption is either enhanced or reduced by pretreatment with surfactants, the requirement for the highest settling rate is about half of the plateau adsorption of the flocculant on the surfactant-pretreated kaolin. The requirement for minimum SRF is about one-fourth of the plateau adsorption. The reason for achieving best flocculation conditions at 50% of the surface coverage can be explained on the basis of the LaMer and Healy (1963) model in which the collision efficiency factor
Fig. 9. Correlation between fractional surface coverage by flocculants and (a) settling rate and (b) SRF of 5% (w/ v) TX 100-pretreated kaolin suspension.
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199
E is also maximised when the fractional surface coverage h is 0.5. The explanation for lower flocculant dosage of about 25% of plateau adsorption for best filtration condition can be made on the basis of the work reported by Sengupta et al. (1997). It has been reported that larger flocs do not necessarily improve separation properties. They have shown that while adding flocculant in small quantities in a stirred suspension of kaolinite, the average floc size gradually increases in a first few dosages. In the present case, the small addition of flocculant forms tiny flocs, which are strong enough, but further addition of flocculant can increase the average floc size by aggregating these tiny flocs resulting in higher rate of settling. But these aggregates may not be strong enough to withstand the pressure of filtration or they may be more compressible than the tiny flocs. This may be the reason for which only about 25% or even lower surface coverage is required for the best performance in filtration process. Flocculant dosages higher than 25% of that required for attaining plateau in the adsorption isotherms leads to weaker and compressible flocs resulting in poorer performance in filtration. Table 6 summarises the results on separation properties of kaolin without and with surfactant pretreatment. Pretreatment with surfactants in most of the cases improves settling behaviour and filtration parameter (SRF). The results have been quantified by Table 6 Summary of values of best settling rates and and minimum SRF Flocculant
No flocculant PAM-A
PAM-C
PAM-N
a b
Substrate
kaolin kaolin 0.2 mg/g SDS-pretreated kaolin 0.2 mg/g CTAB-pretreated kaolin 0.2 mg/g TX 100-pretreated kaolin kaolin 0.2 mg/g SDS-pretreated kaolin 0.2 mg/g CTAB-pretreated kaolin 0.2 mg/g TX 100-pretreated kaolin kaolin 0.2 mg/g SDS-pretreated kaolin 0.2 mg/g CTAB-pretreated kaolin 0.2 mg/g TX 100-pretreated kaolin
Best flocculation condition
Best filtration condition
Best settling rates (cm/s)
Flocculation improvement factora
Minimum SRF (m/kg)
0.028 0.198 0.611
1.00 7.00 21.59
7.81 1011 4.45 1011 2.23 1011
1.00 1.76 3.50
0.522
18.45
2.22 1011
3.55
0.473
16.72
2.23 1011
3.50
0.361 0.318
12.74 11.25
9.96 1010 5.53 1010
7.84 14.12
0.351
12.41
5.52 1010
14.15
0.382
13.51
1.00 1011
7.81
0.575 1.207
20.34 42.64
1.11 1011 1.11 1011
7.04 7.04
1.210
42.76
1.11 1011
7.04
1.220
43.11
2.23 1011
3.50
Filtration improvement factorb
Flocculation improvement factor=(settling rate after flocculation)/(settling rate without flocculation). Filtration improvement factor=(SRF without flocculation)/(SRF after flocculation).
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defining a flocculation and filtration improvement factor and are included in Table 6. A comparison of the improvement factors reveal that nonionic flocculant PAM-N is most effective for improving flocculation behaviour, whereas cationic flocculant PAM-C is most effective for improving filtration behaviour of kaolin suspensions. Pretreatment of kaolin with surfactants substantially increases the settling rates on flocculation by all three flocculants as indicated by an increase in flocculation improvement factor. Most improvement in settling rate has been observed for anionic flocculant PAM-N. Surfactant pretreatment also improves the filtration characteristics in most of the cases. The maximum improvement in filtration characteristics has been obtained for flocculation by PAM-C of kaolin pretreated with surfactant CTAB and SDS. The cationic flocculant PAM-C adsorbs comparatively more on SDS-pretreated kaolin than on CTAB-pretreated one (Table 4). The better performance in both the cases compared to the other conditions can be explained to be the result of combined effect of charge neutralisation and bridging (Besra et al., 2002b).
4. Conclusions Based on the data presented, the following conclusions can be made: (i)
The choice of flocculant for separation by different operations can be made by comparing the best settling rates and minimum SRF obtained through flocculation. (ii) The non ionic polymer PAM-N is most effective for improving settling rates of kaolin suspension in comparison to anionic (PAM-A) and cationic (PAM-C) flocculants. The flocculation by non ionic flocculant occurs by bridging. (iii) The cationic polymer PAM-C is more effective for improving filtration characteristics of kaolin suspension in comparison to anionic (PAM-A) and non ionic (PAM-N) flocculants.
Fig. 10. Schematic representation of the relationship between fractional surface coverage and optimum conditions for flocculation and filtration.
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201
(iv) In most of the cases, preadsorbed surfactants on kaolin further improves both settling rates and filtration characteristics on flocculation. (v) The floc properties required for separation by different methods are different, and the dosing conditions of flocculant for best flocculation and filtration are not same. (vi) About 50% coverage of the particle surface by polymer adsorption is necessary for optimum flocculation where maximum settling rate occurs. This condition is valid for the surfactant-pretreated kaolin also. (vii) The best conditions for separation by filtration occurs at sufficiently lower flocculant dosages and is close to or less than 25% of the adsorption density for monolayer coverage (i.e. CminSRF c 0.25 Cl). (viii) The adsorption density corresponding to best flocculation and filtration conditions can be correlated with the monolayer adsorption density of the polymers by the schematic illustration given in Fig. 10.
Acknowledgements One of the authors (LB) is thankful to the German Academic Exchange Service (DAAD) for the financial grant to carry out a part of this work. We are thankful to Ms. Valeria Thalheim for her assistance in quantitative analysis of flocculants and surfactants using UV/VIS spectrophotometer. Permission of Dr. V.N. Mishra, Director, Regional Research Laboratory, Bhubaneswar, for publishing this paper is duly acknowledged.
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