1464
Ind. Eng. Chem. Res. 1987, 26, 1464-1468
Flocculation of Suspension by Binary (Polycation-Polyanion) Flocculant Akira Suzuki* and Isamu Kashiki Department of Chemistry, Faculty of Fisheries, Hokkaido University, Hakodate 041, Japan
T h e concept of an associated colloidal flocculant (ACF), previously proposed, was applied to the development of a practical and powerful binary flocculant composed of two kinds of easily available polyelectrolytes, polyethylenimine hydrochloride (PEI) and potassium polyvinyl sulfate (PVSK). Such binary ACF flocculants were prepared by mixing the two electrolytes in aqueous solutions. The flocculating power of ACFs was tested through clarification experiments on quartz sand suspensions. It was found that use of both polyelectrolytes simultaneously not only greatly improves their power but also widely extends the concentration region where the polyelectrolytes work effectively, as compared with their use individually. To reveal the working mechanism, a light-scattering technique was applied to evaluate the degree of colloid formation between the two polyelectrolytes, and the adsorption of the electrolytes onto the quartz sand was studied. The results showed that the flocculating power of this flocculant comes from colloid formation between the two electrolytes and that a slight excess of the polycations (PEI),remaining soluble in the medium, is necessary for the effectiveness of the ACF. It is well-known that the flocculating power of polymer flocculants increases in general with increasing molecular weights (Linke and Booth, 1960; Healy and La Mer, 1965; Sakaguchi and Nagase, 1966). However, such flocculants should not be practical because they can be no longer dispersed in medium. As a solution to this problem, the authors have previously presented a new concept of an associated colloidal flocculant (ACF) (Kashiki et al., 1982; Kashiki and Suzuki, 1986). The ACF can be defined as a flocculant that is hardly soluble in water, so that it is dispersed in the state of colloidal particles through the association of molecules or ions. Thus, formed colloidal particles are to have a greatly high “molecular weight”. The purposes of this work are roughly summarized as the following: (1)to develop a powerful and practical ACF flocculant composed of two kinds of easily available polyelectrolytes with opposite electric charges, (2) to elucidate its working mechanism, and (3) to give fundamental information about its conditions of proper use. Experimental Section Clarification Experiments. 1. Flocculant. The flocculant was prepared by mixing two kinds of aqueous solutions. One contained polyethylenimine hydrochloride (PEI) as a cationic polyelectrolyte. HC1 was added to neutralize PEI, which has weak alkaline properties in water. The other contained potassium polyvinyl sulfate (PVSK) as an anion. The chemical structures of PEI and PVSK are shown in Figure 1. The concentrations of both solutions were adjusted to the same value, 1mmol/L, with respect to the ionic segments (1mequiv/L). Mixtures of two solutions were prepared in various volume ratios and were used as flocculants for clarification experiments. 2. Suspension. Quartz sand suspensions were used as suspended solids to be clarified. Quartz sand was thoroughly washed with distilled water and then was pulverized by a ball mill to reach a specific surface of about 18000 cm2/g as determined by an air permeability method. One gram of the powder was boiled in water, and an appropriate amount of water was added to make a 1 wt % suspension. 3. Method. In order to evaluate the power of the flocculant, clarification experiments were carried out on
* Author
to whom correspondence should be addressed.
0888-5885/87/2626-1464$01.50/0
the suspensions. A 100-mL suspension containing 1 g of solids was finally formed by adding a certain amount of flocculant and then was stirred with a magnetic stirrer for 5 min in a flask at room temperature. The suspension was poured into a 100-mL glass cylinder and was left to stand for a certain time (usually 30 s). A 1-mL sample was taken from the supernatant layer at the position 3 cm under the surface. The absorbance of the sample was measured by a Hitachi 100-20 spectrophotometerwith 10-mm cells. The wavelength used was 600 nm. At this wavelength, the absorbance of the sample, A, was almost proportional to the concentration of solids, T. For the judgment of flocculating power, the relative turbidity, T / T o ,or A/Ao was employed,where Toand A. denote T and A in the absence of the flocculant, respectively. Detection of Colloid by a Light-Scattering Method. In order to investigate the dispersing state of the flocculant in various mixing ratios, a simple light-scattering method was applied. The method is often utilized for the determination of size or concentration of colloidal particles because it is very sensitive to the physical condition of dispersed material in liquid. The device used was the spectrofluorometer (FP-550A, Japan Spectroscopic Co., Ltd.). It enabled the light scattered by colloidal particles in a 10-mL rectangular cell to be measured at an angle perpendicular to the incident beam. The wavelength used was 500 nm. From the scattered-light intensity data obtained by this method, it was estimated that some colloidal substance was present in the flocculant. Adsorption of PEI on Quartz Sand Surface. By use of the usual adsorption technique, the measurement of adsorbed amounts of PEI on quartz sand surfaces was carried out. One-hundred milliliter suspensions containing 0.5 g of powder were contacted with various amounts of PEI solutions for 5 min at 20 “C. The suspensions were centrifuged and then filtered with a membrane filter having O.l-gm pore sizes. The concentration of PEI remaining in the filtrate was determined by a colloid titration method (Senju, 1969), in which PEI was titrated with a standard PVSK solution in a recording autotitrator with a photometric attachment (RAT-11 type, Hiranuma Sangyo Co., Ltd.). Results and Discussion Flocculation by PEI. As a preliminary step, clarification experiments for quartz sand by PEI were conducted 0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 7, 1987 1465 -CH2-CH
2
-NH-
-CH
1.0
-CH-
+
-
PEI-PVSK (PEI 10 m L )
2 1 OSOgK
by
(b)
(a)
Figure 1. Chemical structures of (a) PEI and (b) PVSK.
I
1.0
t
\
by P E I
1
I
0.5 c c
\
FLOCCULATION REGION
--
.
0.25
\
0.5
0.0
0.25
0
Figure 2. Flocculation of quartz sand suspensions by PEI alone (solid circles) and by PEI-PVSK mixture (open circles). In the preparation of the PEI-PVSK mixture, various volumes of PEI were added to a fixed volume (10 mL) of PVSK. The horizontal line refers to the maximum flocculating power obtained from the use of PEI alone.
to obtain a reference standard for the flocculating power of the PEI-PVSK mixture. Since PEI is a polycation and quartz sand possesses a lot of negatively charged adsorption sites on its surface, the former can easily be adsorbed onto the latter. As is well-known, appropriately covered solids with oppositely charged (po1y)ions generally result in flocculation. The effect of PEI dosage on the flocculation of suspension is represented by solid-circle data in Figure 2. Optimum flocculation took place at the PEI dosage of 4 mL, and the corresponding TITo value was 0.25. Flocculation by Binary Mixtures of PEI and PVSK. The open-circle data in Figure 2 and 3 show the results of other clarification experiments in which PEIPVSK mixtures were used as flocculants. In the preparation of the flocculants in these experiments, the volume of one solution (PVSK in Figure 2; PEI in Figure 3) was kept constant at 10 mL; the other was widely varied. The horizontal lines in these figures refer to the maximum possible flocculation ( T / T o= 0.25) observed by separate use of the single flocculant PEI, obtained from the foregoing experiment. Provided that the mixture was in an appropriate condition (11.5-26 mL of PEI to 10 mL of PVSK in Figure 2, and 2.5-9 mL of PVSK to 10 mL of PEI in Figure 3), the simultaneous use of PEI and PVSK clearly improved the flocculation effect. This directly leads to the following conclusion: The flocculating power of PEI-PVSK mixtures must be caused by some kind of interaction between PEI and PVSK because the separate use of each polyelectrolyte never indicates that TITo < 0.25. Experiments similar to Figure 2 were subsequently carried out for various PVSK dosages. From each experiment, critical PEI dosages corresponding to both sides of a flocculation region in which T I T , < 0.25 were obtained. The open- and solid-circle data in Figure 4 show the lower and upper critical dosages, respectively. Discussion of the working mechanism of flocculation in this region will be developed later. Dispersed State of PEI-PVSK Mixture in Water. From the clarification experiments presented above, it was suggested that PEI and PVSK cooperates with each other
5
10
'PVSK ImL1 Figure 3. Flocculation of quartz sand suspensions by PEI-PVSK mixtures. Various volumes of PVSK were added to a fixed volume (10 mL) of PEL 0.02
0 El
J
I
-
0.01
I
, ,,
W
a 2
"."0.0
0.005 NpVSK
0.01
[mesl
Figure 4. Flocculation region by use of PEI-PVSK mixtures prepared by the addition of PEI to PVSK. The detail meaning of "flocculation region" is given in the text.
to form a new type of flocculant, provided the mixture satisifies a certain condition. Accordingly, it is important to know which states both electrolytes take, that is, soluble (ionic) or insoluble (colloidal). The light-scattering technique has been utilized often to detect changes in the average size of particles suspended in a liquid. According to Rayleigh's rule (Jirgensons and Straumanis, 1962b), the light intensity scattered by a colloid's surface increases with the size of the colloidal particles, provided the concentration of particles is kept constant. Therefore, the measurement of scattered light intensity should give some useful information about the dispersed state of PEI and PVSK. The experiment was conducted according to the following procedure: A 90-mL solution containing 3 mL of PVSK (or PEI) was prepared and titrated with a PEI (or PVSK) solution. The successive change in the scattered light intensity, H , with the equivalent mole ratio of PEI to PVSK in the solution was measured. The results are shown in Figure 5, where solid and broken lines distinguish the cases in which the titrants were PEI and PVSK, respectively. The figure shows that the scattered light intensity undergoes a sudden change when NPEI/NPVSK is slightly greater than 1. In other words, some colloidal
1466 Ind. Eng. Chem. Res., Vol. 26, No. 7, 1987 10 0
-'
titrated w i t h PEI
t i t r a t e d w i t h PVSK
0
10 - I
i.n-)
1
1
I
I 1 I I l l 10 -1
10-2
CpEI
I
I
I
I I I l l 1 1
[meq/LI
Figure 7. Freundlich plot for the adsorption of PEI on the quartz sand.
Figure 5. Variation of the intensity of scattered light, H , with the addition of one polyelectrolyte to the other. The open circles refer to the case where the polycations (PEI) were added to a fixed volume of the polyanions (PVSK), the closed circles to the reversed case. The arrows on the curves indicate the directions of the paths along with the titrations proceeded. 0.04
. i
0
oi
L
0.02
M
Y
a
u
2.0 xi 0 - 3 0.0
0.0
0.02
0.04
cpySK [meq/Ll Figure 6. Relation between the concentrations of PEI, CPEI. and PVSK, CPVsK,a t the point where the colloid was formed suddenly. The keys for the two kinds of circles are the same as shown in Figure 5.
particles are generated in the flocculant at the condition of a slight excess of PEI over PVSK. The figure also informs that there is little difference in size between the colloids formed by the two methods, because they gave the same scattered light intensities (H= 118). To obtain a more general critical condition a t which a binary flocculant (PEI-PVSK) is suddenly colloidized, similar titration experiments to Figure 5 were carried out varying PVSK (or PEI) concentration in tested solutions. Figure 6 shows the relationship between the concentrations of PEI and PVSK at the critical condition. The experimental data are in good accordance with a straight line mequiv/L. having a slope of 1and the intercept of 2 X
The slope value means that the colloid is composed of equimolar PEI and PVSK. On the other hand, the meaning of the intercept is rather important. Its value represents an excess concentration at which the binary flocculant undergoes a kind of phase transition. The primary binary flocculant must be converted to a secondary state of ACF, when the concentration of free PEI ions in a flocculation system reaches 2 X mequiv/L. Adsorption of PEI on Quartz Sand. Polycations such as PEI can in general be adsorbed on a negative solid surface. In an actual flocculation system, most PEI molecules added as a component of the flocculant react with PVSK, while some part of PEI is adsorbed on a quartz sand surface and the rest remains in the medium. Accordingly, for the more precise discussion of the flocculation mechanism, it is of much importance to know the quantities of the PEIs in these three states, separately, that is, to obtain the adsorption isotherm of PEL Preliminary adsorption experiments were carried out to get the time required for reaching equilibrium. The results showed that the adsorption process proceeded fairly quickly and 5 min was enough to reach the equilibrium. Subsequently, the experiments for obtaining the adsorption isotherm a t 20 "C were carried out at a constant contact time of 5 min. The results were well fitted to Freundlich's equation, as in Figure 7. The equation of the adsorption isotherm was obtained as, X = 1.59 X 10-2C'.a, where X is the adsorbed amount of PEI per unit mass of quartz sand [mequiv/g of solid], and C is the equilibrium concentration of dissolved PEI [mequiv/L]. This relationship will be utilized in the next section to discuss the working mechanism of the PEI-PVSK mixture. Dispersed S t a t e of a PEI-PVSK Mixture i n a Flocculation System. PEI or PVSK in the present system must take one of the following three forms: (1) adsorbed on the solids' surface, (2) free ions in medium, or (3) equimolar complex of PEI and PVSK. The adsorbed form of PVSK should be omitted, since it cannot be adsorbed on the solids dealt with in this work. If we confine the discussion to the flocculation region defined by T I T , < 0.25, all the PVSK molecules must be converted to the equimolar complexes of PEI-PVSK, because the dosage of PEI is always more than that of PVSK, and the reaction between PEI and PVSK can be regarded as almost irre-
Ind. Eng. Chem. Res., Vol. 26, No. 7 , 1987 1467 1.0
c*
-
10 [ d L I
-
0.010
I
I
. c ' c
-
0.5
0
u
Y
REGION
FLOCCULATION
0.005 z a
0.0 0.0
Figure 9. Test of the flocculating power of the ACF, PEI-PVSK, on the quartz sand suspensions at the optimal mixing condition ( A N = 3.3 X mequiv). N f denotes all the added equivalent moles of the two electrolytes. 0.0 0.0
0.005 NpySK
0.010
[mwl
Figure 8. Comparison of the lower critical dosage predicted theoretically (shown by the full line) with the experimental data (open circles). The experimental data on the upper critical dosage are also presented by the closed circles.
versible. This gives the equatons of material balance with respect to PET and PVSK as (1) NPEI= CPEIV+ mX + NPEI-PVSK NPVSK= NPEI-PVSK (2) where NpEI, N P v s ~and , NpEI-pVSK [mequiv] represent the amounts of PEI, PVSK, and PEI-PVSK compounds, respectively, and CpEI [mequiv/L], V [L], m [g], and X [mequiv/g of solid] represent the PEI concentration remaining in the medium, the volume of the suspension, the mass of solids, and the adsorbed amount of PEI per unit mass of solids, respectively. The elimination of the term containing NpEI-pVSK from eq 1 and 2 gives (3) NPEI= NPVSK + CPEIV+ mX when we define
AN = NPEI- NPVSK eq 3 is alternately expressed as AN = CpEIV + mX
(4)
(5)
When the adsorption isotherm obtained from the previous adsorption experiments is applied, eq 5 is rewritten as
AN = ~ [ C P E+I (m/V)(1.59x 10-2)cp~t'65] (6)
As the experimental condition in the present work was fixed as V = 0.1 L and m = 1 g, eq 6 leads to Aiv = 0.1[CpEI
+ lO(1.59 x
10-z)cpE~65] (7)
On the other hand, the colloidal particles of PEI-PVSK complexes began to be formed when the concentration of mequiv/L. Assuming that this free PEI reached 2 x colloid did work as a flocculant, the CpEIof 2 X lo9 should be a critical value above which flocculation takes place. Substitution of this value into eq 7 yields
iw = 4.8 x 10-4
(8)
To confirm the model, eq 8 was compared with the experimental results as shown in Figure 8, in which the data
of Figure 4 were replotted. It can be seen that eq 8, which is represented by a horizontal solid line in this figure, is in good accordance with a lower limit of the flocculation region. This strongly supports the idea that the positively charged PEI-PVSK colloid has have a flocculating power, since colloidal particles with an excess of cations must be positive according to the Paneth-Fajans rule (Jirgensons and Straumanis, 1962a). In this figure it is also seen that the flocculation region extends to the excess side of PEI, as PVSK dosage is increased. But the present study cannot refer to the upper limit of the PEI dosage. Flocculation by a Binary Colloidal Flocculant. Figure 9 shows the clarification test with the PEI-PVSK colloidal flocculant in the optimum mixture ratio (AN = 3.3 X on 1wt 9i quartz sand suspensions. Nf refers to the total dosage of PEI-PVSK flocculant. This figure suggests that the flocculating power of this flocculant is good for two reasons: a small dosage such as 0.02 mequiv i s enough to flocculate the suspension. This value is equivalent to a 2.2-mg dosage to a 100-g suspension (that is, 22 ppm). The restabilization phenomenon by an overdose of the flocculant is not observed.
Conclusions The earlier proposed concept of an associated colloidal flocculant (ACF) has been applied to the preparation of a powerful and practical flocculant from two kinds of easily available polyelectrolytes (PEI and PVSK). The two polyelectrolytes in aqueous solutions were mixed in various mixture ratios, and their flocculating power was tested through standard clarification experiments on quartz sand suspensions. It was found that the simultaneous use of the two polyelectrolytes not only greatly improved the power but also widely extended the flocculation region within which the polyelectrolytes work effectively. To reveal the working mechanism, a light-scattering technique was applied to determine the degree of colloid formation between the two polyelectrolytes, and the adsorption of the electrolytes onto the quartz sand was derived. The results showed that the flocculating power of the flocculant comes from the colloid formed between the two electrolytes and that a slight excess of the polycations (PEI) remaining soluble in the water phase is necessary for the effectiveness of the ACF. The clarification test was carried out, using the PEIPVSK colloidal flocculant in its optimum mixture ratio, on 1 wt % quartz sand suspensions. The result showed
Ind. Eng. Chem. Res. 1987,26, 1468-1472
1468
that the power of this flocculant was good for two reasons: (1) a small dosage of only about 22 ppm was enough to flocculate the suspension; (2) the restabilization phenomenon caused by an overdose of the flocculant was not observed. Nomenclature A = absorbance A,, = standard value of A C = concentration of polycation m = mass of suspended solids N = equivalent moles of polyelectrolyte or PEI-PVSK compound T = residual turbidity To = standard value of T X = adsorbed amount of polyelectrolyte V = volume of flocculation system
PVSK = potassium polyvinyl sulfate PEI-PVSK = complex of PEI and PVSK Registry No. PEI.HC1, 26338-45-4;PVSK, 26837-42-3. Literature Cited Healy, T. W.; La Mer, V. K. J. Colloid Sci. 1965, 9, 545. Jirgensons, B.; Straumanis, M. E. In A Short Textbook of Colloid Chemistry; 2nd revised ed.; Pergamon: London, 1962a; Chapter 5. Jirgensons, B.; Straumanis, M. E. In A Short Textbook of Colloid Chemistry; 2nd revised ed.; Pergamon: London, 1962b; Chapter 6. Kashiki, I.; Suzuki, A. Znd. Eng. Chem. Fundam. 1986, 25, 120. Kashiki, I.; Suzuki, A.; Gotoh, K. Kagaku Kogaku Ronbunshu 1982, 8, 73. Linke, F.; Booth, R. B. Trans. Am. SOC.Mech. Eng. 1960,217,364. Sakaguchi, K.; Nagase, K. Bull. Chem. SOC.Jpn. 1966, 39, 88. Senju, R. Koroido Tekitei-hou; Nankodo: Tokyo, 1969.
Subscripts PEI = polyethylenimine hydrochloride
Received for review October 7, 1985 Accepted April 15, 1987
Oxidation Kinetics of FeII-edta and FeII-nta Chelates by Dissolved Oxygen Eizo Sada,* Hidehiro Kumazawa, and Hiroshi Machida Department of Chemical Engineering, Kyoto University, Kyoto 606, Japan
It has been found in our previous work that the degree of removal of NO by aqueous solutions of Na2S03with added Fe'I-edta mainly depends on the concentration of Fe2+,and the concentration of Fe2+is determined from a balance of the rates of oxidation and reduction of iron. Thus, the reaction kinetics for oxidation of Fe"-edta or Fe'I-nta, which is another promising chelating agent to Fe2+ for removal of NO by dissolved oxygen, were investigated by using a bubble column reactor. The oxidation reaction of Fen-edta was found to be first-order with respect to dissolved oxygen and about half-order with respect to Fe"-edta. The oxidation was suppressed by about 30% by adding 20% EDTA in excess. The oxidation rate of Fe"-nta was shown to be 1st-order in dissolved oxygen concentration and approximately 0.7-order in FeLnta concentration. The oxidation, however, could not be suppressed at all by adding NTA in excess. Wet scrubbing processes have a potential of effectively removing both nitrogen and sulfur oxides (NO, and SO,) from stationary combustion facilities simultaneously. Aqueous solutions of Na2S03with added FeEedta chelate seem to be promising absorbents to fulfil such a possibility. To establish the procedure for inevitable regeneration or treatment of used absorbent, it is necessary to clarify the whole scheme of the complex liquid-phase reactions. In our previous work (Sada et al., 1984,1986),the pathways of the liquid-phase reactions were completed and presented in the form of a map. It has been shown that the degree of removal of NO depends on the concentration of Fe2+, which is determined from a balance of oxidation and reduction of iron. Furthermore, it was found that SO2 coexisting with Fez+in the solution reflected the decrease in pH of the absorbent through the reaction of SO2 with S032-and that the removal of NO during the simultaneous absorption of NO and SO2 can be predicted from corresponding experimental results for the absorption of only NO into an absorbent of the same pH value. Regarding reaction kinetics, both the reduction of Fe3+ to Fez+by HS03- with coexisting EDTA and the oxidation of Fez+ to Fe3+by NO in the presence of Na#03 were investigated. It should be emphasized that Fez+ is oxidized by NO in the presence of Na2S03,though it is not oxidized at all in the absence of Na2S03. The rate of the reduction can be 0888-S885/ 87 12626-1468$01.50/0
expressed as first-order with respect to both Fe'Qdta and HSO - concentrations and minus first-order with respect to FJI-edta concentration. The rate of the apparent oxidation can be expressed as first-order with respect to both Fe(NO)(edtaY+ and Na2S03 concentrations and minus first-order with respect to Fe"'(edta)- Concentration. Flue gases emitted from stationary combustion sources normally contain several percent oxygen. In view of the above-stated information that the degree of femoval of NO mainly depends on the Fez+concentration, the oxidation kinetics of Fe"-edta by dissolved oxygen should be established more completely. For the sake of comparison, the oxidation kinetics of Fe"-nta were also investigated. nta stands for nitrilotriacetic acid, which is another promising chelating agent to Fe2+for NO removal (Lin et al., 1982). Experimental Section The experiments on oxidation of aqueous Fe"-edta solutions by dissolved oxygen were carried out in a bubble column reactor which is similar to one used in previous work (Sada et al., 1986). The bubble column was operated continuously with respect to the gas phase and batchwise with respect to the liquid phase. The gas sparger was a ball filter (G3,lO mm in diameter). The liquid volume free of gas bubbles was 1000 cm3. The total gas flow rate was 0 1987 American Chemical Society
