War. Res. Vol. 24, No. 12, pp. 1519-1526, 1990 0043-1354/90 $3.00 + 0.00 Printed in Great Britain. All rights reserved Copyright ~ 1990 Pergamon Press pk

CHEMICAL ASPECTS OF COAGULATION USING ALUMINUM SALTS I. HYDROLYTIC REACTIONS

OF ALUM AND POLYALUMINUM CHLORIDE

Jow~ E. VAN BENSCHOTEN I ~ and JAMES K. EDZWALD 20 'Department of Civil Engineering, State University of New York at Buffalo, Buffalo, NY 14260 and

2Department of Civil Engineering, University of Massachusetts, Amherst, MA 01003, U.S.A.

(First receired June 1989; accepted in revised form May 1990)

Abstract--This is the first of a two-part series of papers investigating the chemistry of AI coagulants. This paper examines hydrolysis reactions of alum and polyaluminum chloride (PACI). Part II of the series addresses the coagulation of fulvic acid by these coagulants.

Monomeric, polymeric and precipitated AI were identified based on a timed spectrophotometric analysis. At typical AI doses used in water treatment, alum showed no evidence of polymer formation. PACi consists of preformed polymers which are stable upon dilution below pH 6 and over the time frames encountered in water treatment. Solubility studies showed that alum and PACI precipitate to form different solid phases. Alum precipitates are adequately described by amorphous AI(OH)3(s) solubility. The polymeric structure of PAC! is retained upon precipitation yielding a solid phase with different light scattering characteristics, electrophoretic mobility and solubility than alum floc. AI hydrolysis is interpreted as a coordination reaction between AI and OH-. Effects of low temperature on alum are shown to be a result of changes in OH- caused, in part, by the temperature dependence of the ion product of water. Hydrolysis products in PACI are preformed and therefore less sensitive to/n situ hydrolysis than alum. Results suggest that when using alum, some of the adverse effects of low temperatures may be mitigated by an increase in pH, thereby maintaining a constant concentration of the complexing ligand, OH-.

Key words--coagulation, hydrolysis, alum, polyaluminum chloride

INTRODUCTION

Aluminum salts are extremely versatile coagulants in the treatment of potable water. Using AI based coagulants, waters of widely differing chemical characteristics and biological quality can be success- fully treated. Alum [AI2(SO4)3.'nH20] is the most widely used coagulant in the water treatment indus= try. Alum is also extensively used in both industrial and municipal wastewater treatment. In recent years, polymerized forms of AI have been used increasingly in water treatment. Polyaluminum chloride (PACI) is most common in this regard, although polyaluminum sulfates have also been investigated.

The aqueous chemistry of A1 is complex and upon addition of an AI coagulant in water treat= ment, multiple reaction pathways are possible. For example, coordination between AI and inorganic ligands (e.g. PO~-, F - , O H - , SO~-) and organic ligands (e.g. humic materials) can occur as parallel competitive reactions. In addition, serial formation of A! monomers, A1 polymers and AI(OH)3(s) may take place, depending on pH. Coagulation occurs by interaction of A1 hydrolysis products with contami- nants such as dissolved natural organic matter and colloidal inorganic or organic particles. The mech- anism by which AI functions depends on which AI species react to remove dissolved or colloidal con- taminants. Destabilization involving AI monomers

is referred to as charge neutralization or, in the case of dissolved organic substances, charge neutral- ization/precipitation (Edzwald, 1986; Hundt and O'Melia, 1988). Coagulation of colloidal particles in the presence of Al(OH)s(s ) is termed enmeshment or sweep floe. Dissolved organics can be removed by adsorption on Al precipitates.

Insights to the chemical aspects of coagulation can be gained by an examination of the hydrolysis of Al. Studies of the hydrolytic reactions of AI have been used extensively to explain coagulation mechanisms of humic substances (Hundt and O'Melia, 1988; Dempsey et al., 1984; Edwards and Amirtharajah, 1985; Mangravite et al., 1975). The AI species that are observed at a given pH are assumed to be present in the actual coagulation process under similar con- ditions. Although such studies may lend insight to coagulation mechanisms, the interpretation of hydrolysis study results for water treatment plant conditions is not straightforward. It is well known, for example, that AI speciation is affected by the AI concentration, mixing, type and concentration of anion, and aging (Stol et aL, 1976; De Hek et al., 1978; Bersillion et al., 1983). Not surprisingly, many different AI species, especially polymers, have been proposed. Even when AI hydrolysis is studied under conditions similar to those in water treatment, there is no guarantee that similar species exist in the

1519

1520 Joh~ E. V^N B~NSCHOaZN and JAMES K. EDZWALD

presence of humic substances since hydrolysis and interactions between A1 and humic materials are competitive reactions (Randtke, 1988). Furthermore, differences in AI speciation would be expected between preformed polymers and hydrolysis products formed in situ.

In this paper, the hydrolysis reactions of alum and PACI are examined. The objectives of the paper are: (1) to analyze and compare the hydrolytic reactions of alum and PACI; and (2) to evaluate how the chemistry of these coagulants may impact their per- formance in water treatment. The coagulation of humic substances by alum and PACI is addressed in Part II of this series (Van Benschoten and Edzwald, 1990a).

MATERIALS AND METHODS

Materials

PACI was prepared following the procedures of Parthasarathy and Buttle (1985). A 2 I. Wagner type floccu. iation jar was used as the titration vessel. A plexiglass top with ports for stirrer, nitrogen tube, titrant tube, and pH probe was fitted to the jar. An impeller mixer was used during the titration to provide vigorous mixing (1550 rpm). Temperature was controlled at 25 4- 0.2°C by immersion of the 2 i. fiocculation jar in a thermostatically controlled water bath. Titrant was added at a flow rate of about 0.2 ml/min. Nitrogen was continually bubbled through the solution to remove inorganic carbon.

One liter of 0.1 M AICI3 solution was titrated with 250 ml of 1.0 N NaOH. Reagents used in the titration were pre- pared using deionized water made carbonate free by boiling. The titration required approx. 22h to complete. The final PACI solution was 0.08 M A1 with a ligand number (OH,,k~/AIT) of 2.5. The initial and final pH values were 3.3 and 5.3, respectively.

The PACI solution was slightly cloudy at the end of the titration, but cleared after a day or two. Similar results were observed by Bersillon et aL (1980).

Experimental methods

The hydrolysis of alum [AI,(SO4)f 18H20] and PACI was studied in jar test experiments. The standard procedure was to add coagulant to a beaker (250 ml) containing deionized water which had been previously adjusted to an acidic level (pH~e4) using 0.IN HCI. An AI dose of 10-3'3M (13.5 mR/l) was used for many experiments and is typical of the required dose of AI needed to coagulate a water high in DOC. Following coagulant addition, the pH was then adjusted with 0.1 N NaHCO3 and/or NaOH to the desired value. The pH adjustment was made under vigorous stirring with a magnetic stir bar. The beaker was then transferred to a six paddle stirrer (Phipps & Bird) and stirred at 100 rpm (Z = 70s- ' ) while coagulant addition and pH adjustment were carried out for the next sample. When the coagulant addition steps were completed, all test solutions were mixed at 20rpm ( g = 1 0 s -~) for 30min, followed by l h of settling. At the end of the sedimentation period, samples were analyzed for pH, temperature, dissolved aluminum (0.22/~m membrane filters, Millipore type GS), turbidity and eloctrophoretic mobility.

Similar experiments were also conducted at 4°C making use of a thermostatically controlled water bath.

Analytical methods

AI was measured using a colorimetric procedure based on the reaction of AI with 8-hydroxyquinoline (James et al., 1983). The Al-quinolate complex was extracted in butyl

acetate. Details of the analytical procedure are given else- W ~ (Van Bensehotetl and Edzwald, 1990b).

The method used to characterize PACI and alum sol- utions was a timed spectrophotometric procedure involving reactions of AI with ferron (Smith, 1971). Three fractions designated AI" (monomers), AI b (polymers) and AI" (pre- cipitated AI) were determined by the procedure. The PACI solution was characterized periodically over a 1 year period using this procedure.

Electrophoretic mobility (EPM) was measured using a microelectrophoresis apparatus (Rank Brothers). For the cold water studies, samples were continuously chilled while EPM was measured.

Turbidity was measured using a Hach Model 18900 Ratio Turbidimeter.

RESULTS AND DISCUSSION

PAC! and alum

PACI was characterized periodically over a 1 year period using the timed spectrophotometric method. The time dependence of the reaction between AI and ferron has been described as (Smith, 1971):

Al(t) = AI* + Alob(l -- e -~') (I)

where

Al(t) = AI that has reacted with ferron at time t AI* = monomeric aluminum AI b = AI b in solution at time 0

k - - f i r s t order rate constant for type b AI (min-I) .

Using a least squares procedure, model parameters were estimated for the results of the timed spectro- photometric analysis. Parameter values for the characterization results (5 occasions) are shown in Table 1. The variables in the model (AI0 b and k) are significant at the 0.0001 level and multiple r 2 values are generally 0.99 or higher. Although some varia- bility between the analyses is evident, the periodic determinations are consistent. Polymerized AI (AI b) accounts for about 90% of the total aluminum. Monomeric species (AI') and precipitated AI (AI¢) are small fractions of the total. The rate constants in Table 1 reflect the reactivity of AI b with ferron. No obvious time dependence in the rate constant is evident, indicating the nature of the polymers had not changed during the course of the study. The k values observed in this study are similar to determinations made by several investigators for solutions with similar ligand values (Smith, 1971; Panhasarthy and

Table I. Summary of PACI characterization on five occasions

k ( x 100) Days* %AI' %AP %AF (rain -l)

8 2.1 90.1 7.7 2.8 52 8.2 92.0 - - 4,1

193 3.1 82.6 14.2 5,4 204 0.4 91.1 8.5 5.3 326 2.8 93.8 3.4 4.2

Mean 3,3 89.9 8.4 4.4 SD 2.9 4.3 4.4 I. I

*Days following PACI preparation.

Aluminum hydrolysis reactions 1521

0.30

"~ 0.25

0.20

i 0.15

0.10 ,,,t < 0 . 0 5

0.00

PACI. ALe

~ ~ - --AL b -

1.05

0.86 0.20

i o .16

C ~ o .~ '~ ~ o.12

i - := o

0.14 <~ e, 0.08 t .o

< I I I TAL° o.oo 50 100 150 200 250

T i m e i s )

Fig. 1. Absorbanc¢ vs time for alum and PACI reacting with ferron. Alum and PACI at 10 -4.7 and 10 -45 M AI, respect- ively. AI fractions determined by timed reaction with ferron

illustrated for PACI sample.

Buffle, 1985). The time invariance of the reaction rate constant has also been observed (Parthasarthy and Buffle, 1985).

A comparison of PACI with alum using the timed spectrophotometric procedure is shown in Fig. 1. The concentrations of PACI and alum in these samples were 10 -4's M (0.85 mg AI/i) and 10 -4.7 M (0.54regAl/l), respectively. The large increase in absorbance for PACI over time indicates the presence of AI polymers. There is little change in absorbance with time for alum, confirming that the dissolved species are primarily monomers.

To investigate the effect of pH on the polymeric A! species in PACI, filtered samples from PACI solutions at three pH levels were evaluated using the timed spectrophotometric procedure. The results are shown in Fig. 2. At pH 4.2 and 5.9, most of the dissolved AI is in a polymeric form, although at the latter pH some particulate AI may have formed judging from the lower absorbance on the plateau of the curve com- pared to the pH 4.2 sample. At pH 7.4, most of the AI had precipitated (see Solid phase and solubility), and following filtration the remaining dissolved species appear to be monomers. These results indicate that below pH 6, the polymeric species in PACI should be stable under water treatment conditions and available for coagulation.

Solid phase and solubility

The timed spectrophotometric procedure ~as also used to investigate the solid phases formed by precipi- tation of alum and PACI. Alum and PACI solutions were first titrated to pH 7.5 with NaOH to induce precipitation. The precipitate in each case was then dissolved by acidification to pH 4 and immediately measured by the timed spectrophotometric pro- cedure. The hypothesis in this experiment was that if identical solid phases were formed by each coagu- lant, then upon dissolution the dissolved species should show identical reactivity with ferron. Further- more, if the solid phase was a freshly precipitated amorphous AI hydroxide, monomers would be expected after acidification to pH 4. The results of this experiment for alum (Fig. 3) indicate that this is

004

0.00

I)H 4.21 ...o.....o

pH 7.41

20 40 6o 80 100 12o t40 16o 16O 20o

T i m e ( r a i n )

Fig. 2. Absorbance vs time for PACI (10 -33 M AI) at three pH levels.

the case. For PACI, however, the polymeric structure still exists upon dissolution of the PACI precipitate. The redissolved precipitate was about 90% AI poly- mers, the same as the PACI stock solution. Evidently, the PACI precipitate is a coalition of polymers where the individual polymeric subunits remain intact, at least over a short time interval following precipitation.

That PACI and alum form different solid phases was supported by the results of microscopic inspec- tion of the precipitates. The PACI precipitate often appeared as small (<25/~m) spheres, clusters of small spheres and even chain-like structures. Only at high pH (>9) were particles observed that were similar to alum flocs. Alum flocs usually appeared as fluffy, porous structures, ranging in size from 25 to 100/~m. These results are in basic agreement with recent studies which have shown that for short aging times, PACI precipitates are composed of Al~ sub- units with some polymerization of the octahedra (Bottero et al., 1987; Bertsch, 1987). The shape and size of the particles were dependent on ligand number and age. For ~ -- 2.6, Bottero et al. (1987) observed particles in chain-like clusters (diameter~0.1/zm); at ~ = 3.0 and short aging times, particles consisted

14 -

12

10

E 6

4

PACL

ALum

I I I I I I f I I 0 2 0 4 0 6 0 e o 100 120 140 160 180

T ime { rain )

Fig. 3. Reactivity of redissolved precipitates of alum and PACI with ferron.

WR 24~ 12--<3

1522 JOH~ E. VAN BENSCHOTEN and JA.~.S K. EDZWALD

of a mosaic of platelets (length ~ 0.04~/~m), smaller size of these particles compared to the PACI precipitates observed in this study is understandable considering that the study methods reported here involved pH adjustment (pH range from 5 to 10) and flocculation to promote particle aggregation.

Solubility studies of alum and PACI were con- ducted to gain insights to the dissolved Al speciation and solid phases formed by precipitation of these coagulants. Solubility diagrams are widely used as an aid in interpreting coagulation results, although the particular choice of dissolved species and solid phase solubility product is somewhat arbitrary. The AI(OH)3 solid phase has sometimes been assumed to be amorphous (Edwards and Amirtharajah, 1985; Dempsey et al., 1984), while another investigation revealed a more crystalline solid phase (Dempsey, 1989). The existence of polymeric species is often uncertain and in some cases polymers are assumed to be present and in others only monomers are considered. A review of AI hydrolysis studies reveals a seemingly bewildering array of species and con- stants to choose from, but as noted in a recent review (Bertsch, 1989) many arguments for monomeric or polymeric hydrolysis schemes are irrelevent since many experimental results are system specific. The specific conditions for AI hydrolysis following alum addition in water treatment systems include the following: relatively dilute AI concentrations, fast addition of base if pH is adjusted during coagulation, presence of sulfate, OH/Ai ratios > 2 and reactions involving freshly precipitated solids. The first three conditions are those for which monomeric forms of Al are expected, although some uncertainty exists due to the high ligand numbers that are characteristic of water treatment conditions. It would be expected that fresh precipitates formed in water treatment would be a metastable, amorphous AI(OH)3(s). Freshly precipitated AI hydroxide has often been character-

ized as amorphous (De Hek et al., 1978; Smith and Hem, 1972; Hem, 1972; Hem and Roberson, 1967). Although there is considerable evidence suggesting that in the acidic pH range Al solubility may be controlled by an Al-SO4 precipitate (Hayden and Rubin, 1974; Singh, 1969), other investigators (De Hek et al., 1978) dispute such findings.

The solubility of Al for alum and PACi additions to deionized water at 20-25°C is shown in Fig. 4. For alum, two Al doses were used as indicated by arrows in Fig. 4. The soluble species used in constructing the curve in Fig. 4(a) were Ai 3+, AI(OH) 2+ and AI(OH);. The dotted line is the theoretical solubility of Al if a fourth monomer, AI(OH)~, is also included in the calculations. Amorphous AI(OH)3 is assumed to be the solid phase controlling solubility. AI hydrolysis constants are shown in Table 2. A temperature of 25:C has been used in the calculations.

Although investigations of AI speciation in water treatment have often included the AI(OH)~ monomer, the data shown in Fig. 4(a) clearly do not correspond to solubility as defined by all four monomers (another AI monomer, Al(OH) °, has been postulated, although equilibrium constants are con- sidered unreliable and the existence of the species is often questioned). Whereas the hydrolysis constants for AI(OH) 2+ and AI(OH)~" are well established, the constant for AI(OH)~" is considered to be too high (Baes and Mesmer, 1976). Also, K,o values for Al(OH)3(s) vary widely and depend on the type of solid material present. Hayden and Rubin (1974) report K~o = l0 -3L6 for amorphous AI(OH)3 which is in good agreement with the AI solubility observed in these studies for alkaline conditions. On the acidic portion of the solubility diagram in Fig. 4(a), the experimental data appear slightly more insoluble than the theoretical curve. The data points near pH 4 are undersaturated with respect to the solid phase, but the remaining points between pH 4.5 and 6.5

o l= -S

-S c:n o .J

- 2

- 3

- 4

- 7

(o)

- 8

/ ,i • ". ( * "

ALum

I I I I I I I 4 5 6 7 8 9 10

pH

- 2 - -

E

o

(b)

- 8 3

\ °

, 4 S 6 7 8 9 ~0

pH

Fig. 4. Experimental solubility data for dissolved aluminum in solutions of (a) alum and (b) PAC1 at 20-25:C. (a) Alum added at log Al z -3.3 (Q) and log Al == - 4 (O) as shown by arrows. Solid line is theoretical solubility of aluminum in equilibrium with Al(OH)3(am) (pK, = 31.5). Species include AP +, Al(OH) z+ and AI(OH)~'. Dotted line is aluminum solubility if AI(OH)~" is included. (b) PACI added at log AI = -3.3. Solid lines are theoretical solubility for specified A! species. Dashed line shows trend of

experimental data at slope of -7, the same as for the Al, polymer.

Aluminum hydrolysis reactions

Table 2. Aluminum hydrolys/s constants

Reaction pK Reference

AI j÷ + H20 = AI(OH) 2÷ + H ÷ pKI, " 4 . 9 9 Ball et aL (1980) AI ~÷ + 2H20 = AI(OH)~" + 2H ÷ pK,2 = I0.I Ball e ta / . (1980) AI 3+ + 4HzO = AI(OH)~" + 4H + PK,4 = 23.0 Ball et al. (1950)

AI(OH)3(s ) = AI ~÷ + 3OH- pK., . . 31.6 Hayden and Rubin (1974)

1523

indicate a more insoluble solid phase than theoretical predictions.

Differences in test data at acidic and alkaline pH were examined by calculating and plotting log AI 3+ vs pH. The concentration of AP + was calculated using the chemical equilibrium model MINEQL, assuming that the three monomeric species discussed previously were present. Theoretically, the slope of an Al 3+ vs pH plot should be - 3 with an intercept equal to log (K~/K~). Dempsey (1989) has shown that between pH 4.75 and 6.5 a log Al 3+ vs pH plot exhibited a slope of -2.96 and a K,o of 32.8 corre- sponding to microcrystalline gibbsite. Assumed AI species included Al 3+, AI(OH) 2+, AI(OHH)~', AI(OH) ° and AI(OH)~-. In this study, a regression line for pH > 6 yielded a slope of -2.99 and an intercept corre- sponding to a log F~ = -31.5. Regression results in the pH 5-6 range gave a slope = -2 .4 and a log K~o = -35.2. While the results above pH 6 suggest that the precipitated material is amorphous AI(OH)3, the lack of a slope = - 3 for pH < 6 is indicative of either an incorrect choice of AI species or equilibrium constants, or that soluble AI species are controlled by a solid phase other than Al(OH)3(am). Interest- ingly, theoretical solubility calculations for amor- phous forms of both AI(OH)s and basaluminite [AI4(OH)IoSO4.5H20, log K=24.0] (Lindsay and Walthall, 1989) show that in the acidic pH range, AI solubility is controlled by basaluminite. Assuming SO~- is constant, a log AP + vs pH plot for basaluminite solubility would exhibit a slope of -2.5, which is close to the value observed in these studies. However, because of a lack of direct exper- imental evidence, the existence of a mixed AI-SO4 precipitate is uncertain.

The solubility data for PACI [Fig. 4(b)] show that at a dose of 10-s3 M (13.5mg AI/I) the AI species in this coagulant did not precipitate below pH 7. The data in Fig. 4(b) include not only the solubility data described previously, but also data points from exper- iments with slow mixing periods of 4, 6 and 24 h. These longer term tests, undertaken to determine if kinetic factors were important in the precipitation reactions, revealed no differences in precipitation over time. Although PACI precipitates are known to undergo structural rearrangement over time scales as short as 24 h (Bottero et aL, 1987), such changes, if they occurred, did not appear to significantly alter solubility relationships.

Also included in Fig. 4(b) are theoretical lines for All304(OHl)~ [pK = 98.73, Baes and Mesmer (1976)] and AI(OH)~- in equilibrium and Al(OH)3(am). Although the experimental data tend to follow

the slopes of these lines, they are shifted toward higher pH. The slopes are significant because on a log Al vs pH plot, the slope of a line for a given species is equal in magnitude to the the charge of that species (note that the sign is reversed, how- ever). The dashed line in Fig. 4(b) has a slope of - 7 , the same as the line for the AI,3 polymer. The data tend to follow this line, possibly indicating that a highly charged polymeric species, or com- bination of species, is controlling solubility in this pH range. The data at alkaline pH are less soluble than the theoretical curve for AI(OH)~'. This is interpreted to mean that the solid phase in these experiments is probably not an amorphous A1 hydroxide precipitate, an observation supported by results of the timed spectrophotometric procedure (Fig. 3).

Temperature effects

In Fig. 5, solubility data at 4°C (open circles) are included in the solubility diagrams. For alum, the dashed line is the theoretical solubility of Al at 4°C. Compared to the data at 20-25°C, there has clearly been a shift in solubility. The standard enthalpy of AI(OH)3(s) is reported as 22.8kcal/mol (Smith and Martell, 1974) which results in decreased Al solubility with temperature. As discussed subsequently, decreasing temperatures also result in changes in the ion product of water and cause a shift in the solubility curve toward higher pH.

The data at 4°C for PACI show a similar trend above pH 7. The lines in Fig. 5(b) have been drawn through the points to show data trends; they do not represent theoretical solubility curves.

The results of tests using alum [10-3"3M (13.5mgAl/l)] in deionized water are shown in Fig. 6. At 25°C, stable particles were initially formed at about pH 4.5 as shown by the peak in turbidity and the decrease in the concentration of dissolved aluminum. As the pH was increased, the precipitate was destabilized as indicated by decreases in both turbidity and EPM values. The isoelectric point (pHli~) of the A! precipitate occurs at about pH 7. Below pH 4.5 and above pH 8, most of the A! was present as soluble species.

The effect of low temperature is two-fold. First, the pH of precipitation was increased to about 5.5. Both turbidity and AI data illustrate this shift in solubility. The second effect is that at 4°C, particles appeared to maintain a positive charge at higher pH than at 20--25°C. The pI-l~ is increased to about pH 9 at 4:C.

1524 JOHN E. V^N BENSCHOT-r~ and JAM~ K. EDZWALD

-2

-3

-4

E - 5

O~ - 6 0 ..J

- 7

- 8 4

\\ \ 25" ~e

\ , / 0 o

"~ ' o ; / # - 00% # ~ / /

AI.um

(a) I I I I I 5 6 7 8 g

pH

-2

-:3

- 5

~, - 6 o - /

- 7

i - 8 10

\

• o * * * * ~ q ~ e ~ *

b / -- PACL (b)

I I I I I I 4 5 6 ? 8 g 10

pH

Fig. 5. Solubility plots of dissolved aluminum for alum and PACI in deionized water at 25 and 4°C vs experimental data. (a) Solid and dashed fines are theoretical solubility of aluminum in equilibrium with Al(OH)~(am) (pK. = 31.5). Co) Solid and dashed lines drawn to show data trends and do not represent

theoretical solubility curves.

Results using PACI [5.04 x 10-' M (13.6 mgAl/l)] are shown in Fig. 7. Differcnccs between PAC! and alum arc evident. PACI does not appear to prccipitate until about pH 7 as shown by the dissolved AI data, although turbidity and EPM data indicate particle formation. Also, turbidity curves for PACI arc shaped differently than those for alum. PACI exhibits a gradual incrcase in turbidity, reaching a maximum value at about one pH unit above the pH of prccipi- ration. These data suggest that below pH 7 PAC! precipitated, but the resulting solid phase was not filterable using 0.22 pm porc size membrane filters. For alum, the maximum in turbidity and the re- duction in dissolved AI occurred at almost the same pH. Furthermore, although the AI dose was nearly

the same for each coagulant, the maximum turbidity for PAC! was about 1.4 NTU compared to nearly 8 NTU for alum. A trcnd of higher turbidity with alum has also bccn observed in tests involving coagu- lation of humic materials (see Part II, Van Bcnscho- ten and Edzwaid, 1990). These differcnccs support the conclusion that different solid phases have formed.

EPM data show higher positively charged particles for PACI than for alum, and the particles formed from PACi maintained a positive charge to pH 9 while those for alum did not. The availability of positively charged particles over a wide range is an important consideration for coagulation of negatively charged colloids. The effect of temperature on PACI was less pronounced than for alum. Particle mobility

~ o - I I 3 4 5 6 7 8 9 10

pH

~, 1 -o- -.., CP

~[ - I " - o , e ~

(i. "" t u - 2 I l l I I

3 4 5 6 ? 8 9 pH

f , . 3 4 5 G 7 8 9 10

pH

I 1o

I 11

Fig. 6. Experimental results for alum (5 x 10 -4 M Ai) in deionizcd water at two temperatures.

i • T " 4 ~ PACL

1.0 0 T=2~C

o0 .:?: \ 'oo , I vr. :r

3 4 5 6 7 8 9 10 pH

0 I I I I I I n I 3 4 5 6 7 e 9 10

pH

12

9

6

3

0 3

E

Q

I I 4 5 8 7 8 9 10

pH

Fig. 7. Experimental results for PACI (5.04 × 10 -4 M AI) in deionized water at two temperatures.

Aluminum hydrolysis re~'tions 1525

data for PACI appeared to be affected by temperature change to some degree, but effects of temperature on turbidity and dissolved AI concentrations were slight.

Tests involving alum and PACI in deionized water reflect AI hydrolytic reactions since OH- was the only complexing ligand that systematically varied in the experiments. To examine the effect of ligand concentration on jar test results, the data for alum in Fig. 6 were replotted as Fig. 8 with pOH as the independent variable rather than pH. The results show that the effect of temperature appears to be largely accounted for by changes in the OH- concen- tration. The standard enthalapy of reaction for the dissociation of water is 13.36kcal (Garreis and Christ, 1965) resulting in a change in the ion product of water, K~, from 10 -I( at 25°C to 10 -j4s at 4°C. Consequently, at constant pH, the hydroxyl ion concentration decreases with decreasing temperature. In addition to the ion product of water, temperature would also affect the solubility product of AI(OH)3(s) as well as intrinsic surface ionization constants. Although possible effects on these constants is recog- nized, from a practical standpoint the effect of OH- is of most interest since pH can be controlled.

These results illustrate that aspects of particle formation are temperature dependent. It has often been reported that treatment plant performance is adversely impacted by cold temperatures. For alum, the effects of cold temperatures should be diminished by an increase in pH. At 4°C, for example, an increase of approx. 0.8 pH units (e.g. practicing coagulation at

I-- ALUM z

D

.O k.

I / I . _ " ~ _ J - , a Ae.L ~ I - 0~ ,¢ - - ' ~ ~ . I I lo 9 8 7 6 S 4 3

pOH

-, I- ° \

LU 10 9 8 7 6 5 4 3

pOH

o I I 11 t0 9 e 7 6 5 4 3

pOH

e Tm,~C

0 T = 2~BC

Fig. 8. Experimental data for alum (5 x 10 -4 M AI) in deionized water at two temperatures. Abscissa values calcu. lated as pOH = p/~ - pH, where p/~ is 14.0 and 14.8 at 25

and 4°C, respectively.

pH 6.8 rather than pH 6) should result in similar trends in AI solubility and particle stability as occur for alum coagulation at 25°C at pH 6 (Figs 5 and 6). Relationships between temperature, pH and treat- ment performance have been observed by Camp et al. (1940) in a study involving ferric salts. It was found that adverse effects of cold temperatures could be mitigated by an increase in the coagulation pH of about 0.5 units. Similar findings were reported by Mohtadi and Rao (1973).

For PACI, the results of replotting data with pOH as the independent variable showed a divergence in the curves. In contrast to alum which hydrolyzes/n situ, the Al species in PAC! are preformed and the precipitation of PACI results in a different solid phase than the amorphous AI(OH) 3 precipitate formed from alum.

C O N C L U S I O N S

Based on the results of this research, the following conclusions are made.

(l) The hydrolysis o ra l following addition of alum to deionized water at concentrations typically used in water treatment shows little evidence of polymer formation. Al solubility can be adequately described by the presence of three monomeric species: AP +, Al(OH) z+ and AI(OH)~'. Application of a timed spectrophotometric method to alum in deionized water showed that Al was present as monomers almost entirely.

(2) PACI was characterized by a timed spectro- photometric method which showed that approx. 90% of the A1 was in a polymerized form. When used in jar test type studies at pH < 6, the Al polymers were stable in dilute solutions and for the time scales typical in water treatment.

(3) Experimental results including analysis of redis- solved precipitates, solubility tests, turbidity data and EPM measurements indicated that alum and PACI precipitate to form different solid phases. The poly- meric structure remains intact within the PAC! pre- cipitate and particles are more positively charged and produce lower turbidity than alum floe.

(4) Temperature exerts a strong effect on solutions containing alum. Turbidity, electrophoretie mobility and Al solubility are temperature dependent. The underlying effect has been shown to be chemical in nature. Al hydrolysis can be viewed as a complex- ation reaction involving hydroxyl ions. At constant pH, the hydroxyl ion concentration changes with temperature due to the temperature dependence of the ion product of water. The data suggest that when using alum, some of the adverse effects due to cold temperatures may be offset by an increase in the coagulation pH.

(5) PACI is much less affected by temperature than alum. At constant pH, low temperature produces only minor changes in turbidity, electrophoretic mobility and Al solubility. Although the precipitation

1526 JOXN E. V^~ B~SSCMOT~ and JAMES K. EDZWALD

mechanisms for PACI appear to be different than for alum, the fundamental mechanisms are not clearly understood.

Acknowledgements--The research described in this paper has been funded, in part, by the Risk Reduction Engineering Laboratory, U.S. Environmental Protection Agency (U.S. EPA), under Cooperative Agreement CR-812639. The assistance of Kim R. Fox, U.S. EPA Project Officer, is greatly appreciated. The content and conclusions are the views of the authors and do not necessarily reflect the views and policies of the U.S. EPA. Mention of trade names or commercial names does not constitute endorsement or recommendation for use.

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