Flocculation Mechanism of Suspended Particles Using the Hydrophilic / Hydrophobic Transition of a Thermosensitive Polymer

We examined the f locculation of suspended particles using a thermosensitive polymer which undergoes a reversible hydrophilic/hydrophobic transition when heating or cooling its aqueous solution. As a thermosensitive polymer poly(N-isopropylacrylamide) (polyNIPAM), whose transition temperature is about 32°C, was used. Flocculation experiments were performed by the jar test using kaolin suspension. In the case of operating temperature lower than the transition temperature of polyNIPAM, there was an optimum polymer dosage, and by dosing excessively kaolin particles were dispersed stably in the same manner as conventional polymeric f locculants. However, f loc formation was observed by heating the suspension above the transition temperature under the excess polymer dosage. Furthermore, by cooling the f loc-containing suspension down to below the transition temperature again, the f locs were disorganized to the particles. These phenomena show that the f loc formation caused by heating to above the transition temperature is due to the hydrophobic interaction of polyNIPAM molecules adsorbed on the particles.


Introduction
Polymeric f locculants, which are widely used in the f locculation and dehydration of suspensions, are generally hydrophilic polymers and therefore have the drawback that the f locs contain large amounts of water which cannot be easily removed.In our previous paper (Sakohara and Nishikawa, 2000) we proposed the use of thermosensitive polymers as a way to alleviate this problem.
Thermosensitive polymers in aqueous solutions become soluble (hydrophilic) at low temperatures, but are insoluble (hydrophobic) at high temperatures.This hydrophilic/hydrophobic transition is reversible and occurs at the transition temperature, which is determined by a primar y structure of polymer (Ito, 1989).In our previous paper (Sakohara and Nishikawa, 2000) by using the plunger test the compaction of a highly concentrated kaolin suspension with poly(N-isopropylacrylamide) (polyNIPAM), a representative thermosensitive polymer, was examined.The suspended particles were fully mixed with polyNIPAM, and the mixture was heated to above the transition temperature.Then by applying an appropriate external force, the compacted flocs was readily obtained.
We proposed the mechanism illustrated in Fig. 1 for the formation of such compacted flocs employing thermosensitive polymers.First, the polymer molecules are fully adsorbed onto the suspended particles below the transition temperature (process 1, left).If the conventional hydrophilic polymer is used, the particles disperse stably under these conditions.But in case of thermosensitive polymer, by heating the suspension to above the transition temperature (process 2, center), the adsorbed polymer molecules change to hydrophobicity, in turn making the surfaces of the suspended particles hydrophobic, and resulting in the f locculation (f loc formation) of the suspended particles due to the hydrophobic interaction.Applying an appropriate external force at this point will cause the rearrangement of the particles, thereby forcing out the water and readily allowing compaction.On the other hand, since the hydrophilic/hydrophobic transition of thermosensitive polymers is reversible, by cooling the mixture down below the transition temperature (process 3, right) the flocs disorganize and return to stable suspended particles.
However, the plunger test in our previous paper was meant to obtain the compacted flocs and did not allow us to adequately investigate this flocculation mechanism.In this research, therefore, we performed the f locculation experiments and examined the validity of this model by using an agitator tank.

Materials and Methods
As a thermosensitive polymer we used poly(N-isopropylacrylamide) (polyNIPAM), which were synthesized by the same methods as in our previous paper (Sakohara and Nishikawa, 2000).The molecular weight is about 3.95҂10 6 , and its transition temperature in water is about 32°C.As a sample suspension, kaolin (Katayama Chemicals, Inc.) suspension with a concentration of 20 kg/m 3 , containing 0.5 mM sodium hydroxide as a dispersant, was used.The transition temperature of polyNIPAM in the aqueous solution of 0.5 mM sodium hydroxide is about 34°C, slightly higher than that in water.
We performed the f locculation experiment with an acrylic resin agitation tank which had an inside diameter of 109 mm and a height of 135 mm, and which was fitted with six baff le plates.Into this tank we put 1 dm 3 of the test suspension, and submerged it in a thermostatic bath at the prescribed temperature.The suspension was stirred at 300 rpm using a turbine blade (six f lat blades, 50 mm in diameter) installed at half the liquid depth.After the temperature of suspen-sion reached the prescribed temperature, the desired amount of polymer was added, and the mixture was stirred rapidly (300 rpm) and then slowly (150 rpm) under each of three experimental conditions described below.Although the slow stirring speed was faster than that generally used in the jar test, this speed was chosen to ensure good contact between the suspended particles and the polymer molecules, or among suspended particles.The f locculation performance was evaluated by settling for 30 minutes after stirring finished.The sample supernatant was taken from the depth of 50 mm below the surface, and the measurement of transmittance was performed at a wavelength of 600 nm using a spectrophotometer.
The f locculation experiment was conducted under each of the following three conditions to confirm the viability of the mechanism proposed in Fig. 1.
(1) Flocculation experiment below and above the transition temperature The mechanism in Fig. 1 assumes that the hydrophilic polyNIPAM molecules adsorb onto suspended particles shown as in process 1 in the figure .To verify this we examined the flocculation performance at 30°C, below the transition temperature of polyNIPAM, and at 40°C, above the transition temperature.The desired amount of polyNIPAM was added to the suspension after the suspension had attained the prescribed temperature, and the stirring was performed at high speed for 30 min, and then at low speed for 20 min. (

2) Flocculation experiment by heating
We performed the following experiment to verify process 2 in Fig. 1, that is, the formation of f locs by heating the suspension.PolyNIPAM was added at 30°C, below the transition temperature, and the mixture was stirred at high speed for 10 min.The ther- (3) Flocculation experiment by cooling suspension back below the transition temperature after heating We performed the following experiment to verify process 3 in Fig. 1, in which flocs formed by heating will again disperse when the suspension is cooled below the transition temperature (reversibility).The suspension was stirred at low speed for 20 min under the heating conditions described above, and then the thermostatic bath water was quickly replaced with water of 25°C, and slow stirring was continued for another 50 min.The closed circles in Fig. 2 show the suspension's temperature change.

Effect of Temperature on Floc Formation
Fig. 3 shows the f locculation performance at 30°C and 40°C.Since polyNIPAM is hydrophilic at 30°C, it acts like the conventional hydrophilic polymeric f locculant: The transmittance of the supernatant increased with increasing the polymer dosage, and then decreased after peaking.This means there is an optimum polymer dosage.At and near that dosage, the bridging adsorption of polyNIPAM on the suspended kaolin particles encourages floc formation, and the transmittance of supernatant increases.But when too much polyNIPAM is added, the surfaces of the suspended kaolin particles are covered with polyNIPAM, and thereby disperse stably, which reduces transmittance (Sakohara et al., 1980).At 40°C, however, f locs were hardly formed no matter what the additive amount of polymer, and the supernatant transmittance remained low.It is considered due to the fact that the added polyNIPAM molecules immediately became hydrophobic and that they could not adsorbed onto the kaolin.As a result, f locs did not form.Therefore, to make kaolin adsorb the polyNIPAM as in the Fig. 1 model, the operation must be performed below the transition temperature.
We have seen no instances as in our experiment in which polyNIPAM was used as the flocculant, but there are some reports of the f locculation of inorganic particle suspensions by copolymers of NIPAM and cationic monomers (Guillet et al., 1985;Deng et al., 1996).When this kind of cationic polymer is used, f loc formation has been observed even if the polymer is added to a suspension at the temperature higher than the transition temperature.In this kind of polymer, the portion of polyNIPAM that has become hydrophobic agglomerates through hydrophobic interaction, and forms polymer fine particles with cationic components on their surfaces.It appears that these surface with cationic components function as f locculants.As a result, they cause flocculation even above the transition temperature.

Effect of Heating Suspension on Floc
Formation Fig. 4 compares the f locculation performance obtained by heating suspension to 60°C after poly-NIPAM adsorption at 30°C, and that obtained at 30°C.By heating from 30°C to 60°C, the transmittance increased and then decreased with increasing poly-NIPAM dosage.By adding more polyNIPAM the transmittance increased again, and then remained fairly steady (range A in the figure).In this range the supernatant had very low transmittance at 30°C, meaning that hardly any f locs were formed.This shows that f locs formed by heating the suspension after the surfaces of the suspended particles were covered enough with polyNIPAM.In fact, before the temperature of agitator tank was raised (when it was still 30°C), we did not discern f loc formation, but when the tank was heated we clearly discerned f loc formation with the unaided eye at the point the temperature of suspension exceeded the transition temperature.This seems to bear out the mechanism illustrated in Fig. 1: By heating a suspension of kaolin particles whose surfaces are covered enough with polyNIPAM, the polyNIPAM becomes hydrophobic, which in turn makes the particle surfaces hydrophobic, thereby making particles adhere to each other and form f locs due to the hydrophobic interaction.
Even when the suspension was heated, the transmittance increased with the same polymer dosage as the optimum dosage for 30°C (range B in the figure).The reason appears to be that f loc formation during the 10 min of rapid stirring after adding polyNIPAM to an almost optimum amount increased supernatant transmittance.This fact suggests that even though polyNIPAM adsorbed onto the surfaces of suspended particles might become hydrophobic, they do not readily desorb from the suspended particle surfaces.As stated in our previous paper (Sakohara and Nishikawa, 2000), this is likely because polymer adsorption onto the suspended particles is multipoint adsorption by functional groups in the polymer molecules.

Reversibility of Floc Formation by
Hydrophobic Interaction The closed circle in Fig. 4 shows one example of the f locculation performance by adding polyNIPAM, by raising the suspension temperature from 30°C to 60°C, then by cooling it down to below the transition temperature.The polymer dosage was 0.02 kg/kgkaolin.As the figure shows, the suspension returned to a nearly completely dispersed state.We also observed clearly with the unaided eye that the floc quickly disappeared when the suspension temperature fell below the transition temperature.This shows that f locculation (f loc formation) caused by the hydrophobic interaction occurs reversibly in response to the change in temperature.

Conclusion
We investigated the flocculation mechanism utilizing the hydrophilic/hydrophobic transition of poly-NIPAM, a representative thermosensitive polymer, by using an agitation tank to perform a f locculation experiment on a kaolin suspension.Since flocculation hardly occurred above the transition temperature (about 32°C), the adsorption operation of polyNIPAM onto suspended kaolin particles needs to be performed in a hydrophilic state below the transition temperature.When the kaolin particle surfaces are coated enough with polyNIPAM by increasing the polymer dosage, kaolin particles disperse stably below the transition temperature, but f locs form by heating the suspension.When the flocculated suspension is cooled back to below the transition temperature, particles disperse again.From these results, it can be concluded that the flocculation mechanism due to the hydrophobic interaction proposed in our previous paper (Sakohara and Nishikawa, 2000) is valid.

Fig. 1
Fig. 1 Mechanism causing flocculation of suspended particles by using a thermosensitive polymer

Fig. 3
Fig. 2 Temperature change in the f locculation tank

Fig. 4
Fig. 4 Effects on flocculation performance of heating and cooling the suspension