Particle formation and growth in dilute aluminum(III) solutions

Water Res. Vol. 18, No. 4. pp. 479-488. 1984 Printed in Great Britain. All rights reser',ed

0043-1354,84 $3.00+0.00 Copyright C 1984 Pergamon Press Ltd

PARTICLE FORMATION A N D GROWTH IN DILUTE

ALUMINUM(III) SOLUTIONS CHARACTERIZATION OF PARTICLE SIZE DISTRIBUTIONS AT pH 5.5 WILLIAM J. SNODGRASS, MARK M . CLARK a n d CHARLES R. O'MELIA

Department of Geography and Environmental Engineering, The Johns Hopkins University, Baltimore. MD 21218, U.S.A. ( Recei~.ed August 1983)

Abstract--Particle formation and growth over the 1-40#m size range in dilute aluminum solutions (approx. 2 x 10-* M) have been studied using an electronic particle counter. Sulfate, fulvate and hydroxide ion accelerate the rate of particle formation and changes of the particle size distribution over time. Increasing ionic strength (inert electrolyte) produces similar but less dramatic effects. Combinations of sulfate and fulvic acid or sulfate and inert electrolyte further accelerate the rate of particle formation. Aluminum chloride solutions at moderate ionic strength are devoid of supramicron particles after several days. A conceptual pathway model is developed which suggests that two different solids are formed when aluminum is added to fulvic acid solutions: an aluminum-fulvate precipitate and AI(OH)3(s). The first solid dominates in fulvic acid solutions at pH ~5.5.

INTRODUCTION

This paper is written with three objectives: (i) to examine the rate of particle formation ( > l / a ) in dilute solutions of aluminum salts and dissolved natural organics at pH ~ 5.5; (ii) to examine the characteristics of the resulting particle size distributions over time and (iii) to suggest a mechanistic interpretation for the results. To accomplish these objectives, the paper is divided into three areas. Firstly, the characteristics of the removal of organics from solution by two aluminum salts in coagulation processes of water treatment plants are reviewed. Secondly, results are presented for experiments which are designed to explore particle formation at pH ~ 5.5. Finally a mechanistic interpretation of the experimental results is suggested in terms of the chemistry of aluminum and reaction pathways for particle formation.

THE REMOVAL OF NATURAL ORGANIC

SUBSTANCES WITH ALUMINUM SALTS Aluminum salts are widely used as coagulants in water treatment to remove turbidity and color. Turbidity is the scattering of light by suspended particles while color results from the absorption of light by dissolved substances. Suspended particles are usually solids while molecules causing color are often naturally occurring organic matter called humic substances. The removal of turbidity and color from water supplies is important for aesthetic and operational considerations and their possible association with health hazards. The removal of humic sub-

stances is currently receiving much attention because they have been identified as precursers for potential carcinogens formed by reactions with chlorine. Humic substances are acidic, randomly polymerized, hydrophilic, chemically complex molecules having a wide range of molecular weights. They are ubiquitous in surface waters. They comprise the majority of the dissolved organic fraction in most waters and are generally resistant to degradation by bacteria. Sources of humic substances for research include extracts from soils, lake sediments and, more rarely, the water column of lakes. The humic materials used in this research are fulvic acids, the predominate form of humics in surface waters. An extract from a lake water was selected as a useful model compound for examining reactions typical of those occurring in water treatment plants. The pathways and mechanisms of aluminum reacting with turbidity are perhaps better understood than those involving its reactions with organics. Turbidity removal can be achieved in two ways: charge neutralization and AI(OH)~ precipitation (Stumm and O'Melia, 1968). Charge neutralization involves the adsorption of positively charged hydroxo polymers of aluminum onto negatively charged particles. This neutralizes the charge on the particle and permits aggregation to occur during flocculation processes. Precipitation involves the formation of an aluminum hydroxide solid with which turbidity can collide and aggregate. Color removal involves at least two mechanisms (Dempsey et al., 1982). First, humic substances (negatively charged polyelectrolytes) can be precipitated by cationic aluminum species. Second, they can absorb onto AI(OH)3 precipitate.

479

WILLIAMJ. SNOOGRASSet a[.

480

Coagulation experiments using dilute fulvic acid solutions have been carried out using alum [AI:(SO~)3' 18H_,O] and aluminum chloride (O'Melia and Dempsey, 1982). A typical set of jar test experiments for alum coagulation of fulvic acid is shown in Fig. 1. The aluminum dose was 10-4M (aluminum c o n c e n t r a t i o n = 2 . 7 m g l - ' ) . The fraction of fulvic acid remaining in solution is plotted as a function of pH at the end of the testing period; the upper curve presents results for removal after coagulation and sedimentation while the lower curve includes removal by membrane filtration of the supernatant solution. These results are interpreted as follows (O'Melia and Dempsey, 1982). At neutral pH, an aluminum precipitate forms to which fulvic acid absorbs; the solid is sufficiently large that it settles after flocculation. Below pH 6, particles form, as indicated by removal with a 0.45 # m filter. However, they are too small to allow effective removal by flocculation and settling. These small particles are hypothesized herein to be an "aluminum-fulvate'" precipitate formed by the reaction of a negative polyelectrolyte (fulvic acid) with a positive one (polynuclear hydroxo complexes of aluminum). Similar results were found for aluminum chloride except that the pH at which settleable solids appear (upper curve) is higher (~0.5-1.0 pH units). Some of these results can be examined by considering the kinetic effects caused by inorganic anions (Vermeulen et al., 1975; Stol et al., 1976; De Hek et al., 1978) and the ligand number of the solution, r. The ligand number is the average number of moles of hydroxide ions bound per mole of aluminum in the system. In solutions of aluminum chloride, Stol et al. (1976) demonstrate that a precipitate of Al(OH)3 does not occur until r exceeds a value of approx. 2.4. In solutions of aluminum sulfate, a precipitate of AI(OH) 3 occurs when R exceeds approx. 0.5. They propose that in aluminum chloride solutions, stable polynuclear complexes are formed whose high posi12 /o~

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tive charge precludes nucleation and subsequent crystal growth. They propose that sulfate, which forms outer sphere complexes with aluminum ions, screens the positive charges on polynuclear aluminum species and allows nucleation and particle growth. Sulfate. thus, has effects upon formation of particles in both aluminum salt systems and in aluminum-fulvate systems. In the former, it catalyzes formation of AI(OH)~(s). In the latter, it may (i) catalyze formation of AI(OH)j(s) onto which fulvic acid can adsorb. (ii) catalyze formation of aluminum polymers which react with negatively charged fulvic acid molecules and/or (iii) destabilize positively charged complexes and colloids of aluminum-fulvic acid which then grow to large molecules. Higher r values induce faster formation of Al(OH)3(s) and change the number of protons associated with the fulvic acid molecule. The experiments reported in this paper are designed to explore particle growth phenomena in the pH region (~5.5) where filtration but not settling achieve particle removal in jar tests. The results further elucidate the removal mechanisms hypothesized previously as important in this region. EXPERIMENTAL METHODS A summary of experiments reported in this paper is given in Table I. The total aluminum concentration (AIT) is 1.8x 10-4M, the fulvic acid (FA) concentration is 3.5 mg l-' as TOC: the pH is approx. 5.5 for all experiments except for numbers 6 and 7. The majority of experiments were conducted in a sodium chloride solution (10g 1-I) to maintain constant ionic strength (0,17 M): this NaCI solution is hereafter called electrolyte. The first five experiments explore the effects of sulfate upon particle formation in aluminum solutions while the next two (Experiments 6 and 7) examine the effect of pH on these solutions. Experiments 8 and 9 provide an examination of the interaction of fulvic acid with alum and aluminum chloride solutions. The last three experiments (Experiments 10-12) examine AI-FA reactions in solutions of lower ionic strength. Stock solutions ofAICls, Na_,SO4, NaCI and NaOH were prepared from reagent grade chemicals dissolved in distilled water. Stock solutions containing desired AI:SO4 ratios were made by combining appropriate volumes of the AICI3 and Na,SO4 stock solutions. The concentrated fluvic acid solution was obtained in a previous study (FA No. t; Dempsey, 1981; Dempsey et al., 1981). It was isolated from the waters of Lake Drummond. Virginia by absorption on XAD-2 resin at pH 2, recovered by elution at pH 3.5, and stored in the dark at 4'C. All stock solutions were filtered through 0.45 pm membrane filters. The electrolyte solution was prepared in a continuous filtration unit utilizing a 0.2 gm cartridge filter, resulting in a negligible background particle count. The standard experiment involves dilution of 30ml of stock aluminum solution (3.06 × 10-3M) into 460ml of electrolyte in a 600 ml beaker placed on a magnetic stirrer. Under vigorous stirring, I0 ml of base (0.0215 M NaOH) was added from a volumetric pipette with the pipette tip below the solution surface, and as close as possible to the magnetic stirrer. The addition procedure took approx. 5 s. Time zero for the experiment was taken as the time of initial base addition. For the fulvic acid-aluminum studies, 2.15 ml of stock fulvic acid (0.811 mg ml-') was mixed with 11.3 ml of base and added to the aluminum-electrolyte solution to give a fulvicacid concentration of 3.5 mg I-t as total organic

Particle formation and growth in dilute aluminum(Ill) solutions

481

Table 1. Description of experiments and time required for particle formation* Time te form Experiment Description particles (r) l

A l u m ( a l u m i n u m sulfate) in electrolyte (AI:SOa = 1:1.5)

~ = 15 min

2 3

AICIj in electrolyte r > 5 days AI and SO~ in electrolyte r = 51 min (AhSO= = I:l) 4 AI and SOj in electrolyte I ~
carbon. The additional base was required to neutralize the acidified fulvic acid stock solution. Solutions were continuously stirred and the pH monitored; occasional measurements of monomeric aluminum were made (Bersillon et al., 1980). Samples were periodically withdrawn by pouring solution from the beaker into a sample vial for particle counting. For experiments extending to more than 2 h, the beaker was covered to minimize the influences of dust upon particle reactions. The particle size distributions were measured using an "Elzone" particle counter* interfaced with an EZ-211 minicomputer. The system provides raw data in a 128 (or 256) channel format and permits rapid instrument calibration, a calculation of various characteristics o f the particle distribution, and the writing o f one's own software. A 5-point quadratic smoothing algorithm was used for particle number distributions reported herein. Particle surface area was calculated assuming a spherical particle whose diameter is equivalent to that of the mid-point o f each channel. A 30/tin diameter orifice was used in all experiments; calibration using two primary standards, 2.02 and 4.95/a m latex spheres yielded a detectable size range of 0.80-11#m. However, because it has been determined that counts in the first 10-15 channels are attenuated for the machine parameters used in this study, a detection range for the 3 0 # m orifice o f 1-11 # m is used in this work. Plots o f instrument response versus standard particle concentration are linear; this supports the manufacturer's claim of less than 1% coincidence error for typical concentrations reported herein. A sample size o f 12 #1 was used for particle counting with the 30 # m orifice. This results in a sampling time o f 10 s. The flow rate is substantially reduced from that used for the 95/~m orifice. Comparison of results from the 30 ,urn orifice and the 95 # m orifice suggests that some breakage o f large particles occurs when the smaller orifice is used. The particle number distribution measured with the 3 0 # m orifice has a slightly smaller upper range as well as an increase of numbers in the lower size ranges. pH was measured with a Fisher Accumet Model 620 pH meter. Solution temperature was measured frequently, and was approximately the room temperature o f 26°C.

RESULTS

T h e total n u m b e r o f particles, particle surface area, a n d particle v o l u m e p e r unit liquid v o l u m e are s h o w n in Fig. 2 for E x p e r i m e n t I ( a l u m s o l u t i o n ) a n d the s o l u t i o n p H is s h o w n in Fig. 3. T h e first two o b s e r v a t i o n s in Fig. 2 (3 a n d 10 min) r e p r e s e n t b a c k g r o u n d noise ( < 9 0 particles p e r 12#1). T h e r e is an initial p e a k o f total n u m b e r o f particles (17 min), followed by a d e c r e a s e in n u m b e r s to a p p r o x . 25 rain a n d then a n increase to a s u b s e q u e n t p l a t e a u at 4 0 - 5 4 min. T h e

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Fig. 2. Total number (No. m l - I ) , volume (#m~ml -~) and surface are ~ m 2 ml-~), o f particles over time for alum in electrolyte (Experiment 1).

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Fig. 3. pH of alum and aluminum chloride solutions over time (Experiments I and 2). total number at the plateau is less than at the initial peak. The curves for total surface area and total volume suggest that these increase over time, with perhaps a slight peak at 17 rain. The plateau values at 40-54 rain are substantially larger than the initial peaks. The pH attains a nearly constant value after 2 rain (Fig. 3, pH = 5.45). The particle number distribution (AN/A log alp) is defined as the number of particles per ml per log diameter increment in microns plotted against the equivalent diameter of the midpoint of the log diameter increment in microns. For Experiment I, the particle number distribution (Fig. 4) shows three noteworthy characteristics over time. Initially there is a quasi-unimodal distribution in the small size range

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Fig. 4. Particle number distribution for the alum system in electrolyte over time [Experiment 1. Units for ordinate: number of particles per ml per log diameter ~ m ) increment.]

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(a mode at approx. 1.3 ~zm): this distribution develops into a bimodal distribution at 21 rain (modes at approx. 1.3 and 2.5 #m) and then into a unimodal distribution whose size is substantially larger than the first distribution (mode at approx. 2.gpm). These results are interpreted as indicating that particles smaller than 1 #m move through the 1,urn detection limit, 10-15 min after base addition. The portion of the particle number distribution which is submicron and which contributes to the particle distribution in the diameter range of 1.0-1.3 [~m decreases over time. The characteristics of the number distribution of Fig. 4 provide some explanation for the shape of curves in Fig. 2. The peak at 17 rain in the total number curve results from a large number of small particles moving into the detectable size range (I-2.5 # m). They reach a maximum at 17 min. Aggregation after 17 rain is sufficiently fast that the total number, the total surface area and the number of small particles decrease, while the number of larger particles (2.5-4pm) increases. Aggregation of these supramicron, small particles is faster than the rate of movement of submicron particles through the l # m detection limit, causing the decrease in total number of particles and the decrease in number of particles in the 1-2.5 p m size range. The general increase in the total volume curve after 17 rain and the increase in surface area after 23 rain suggests that aggregation of supramicron with submicron particles is significant. If only aggregation of particles in the detectable range is significant, the total surface area would decrease and the total volume would remain constant. After 40 min, any changes due to aggregation are slow as all curves are essentially constant. The number distribution curve for t = 40 min is not shown but is essentially the same as that for 58 rain. A small decrease in total volume is suggested by Fig. 2 between 17 and 19 rain. Such phenomena can be explained by particle counting errors, particle breakup or ripening. Ripening (the dissolution of thermodynamically unstable particles formed previously under more supersaturated conditions, e.g. Lieser, 1967) is unlikely, given the slow dissolution kinetics of aluminum polymers and solids observed by Stol et al. (1978). More measurements are required in the vicinity of 17-19rain before the suggested decrease can be accepted as factual. The two mechanisms which could determine the observed changes in particle number distribution in the alum system are aggregation and crystal growth. The exact degree of supersaturation cannot be calculated for our system because of uncertainty about the solubility product of Al(OHh(s). Estimates are given in Table 2 and compared to Groeneweg's (1980) observations for crystal growth from caustic solutions. Measurements show that little aluminum ( < 5 x 10-6M) is left in a monomeric form within 1 rain of base addition, suggesting that little aluminum is available for crystal growth. The dramatic

Particle formation and growth in dilute aluminum(HI) solutions Table 2. Estimation of degree of supersaturation in aluminum systems l, Definitionof terms a = relative degree of supersaturation = solution concentration minus the equilibrium concentration divided by equilibrium concentration K,,, = solubility product for AI(OH)3(s) 2. Literaturevalues for K,o range from l 0 -3t ~i tO 10 T M 3. Corresponding values for a are 50--420 3. Rate of change of mode for alum in electrolyte system (from 1.3 to 2.9 ,am over 30 rain) represents a linear, sphericalequivalent growth rate of I ,am h -~ 4. For 1 ~<~ g 100, Groeneweg (1980) observed crystal growth rates of 10-1000/~mh-~ at 85~C for AI(OH)j(s) from caustic solutions. For an Arrhenius activation energy of 6000Jmol -I this growth rate is 0.1-10,amh -~ at room temperature

change in particle number distribution over time is best attributed to an aggregation mechanism. Hence for conceptual and preliminary purposes, it can be assumed that aggregation is the only significant mechanism determining particle growth in the size range observed. The pH of an aluminum chloride solution over time (Experiment 2) is shown in Fig. 3. The pH approximates that of the aluminum sulfate solution. No particles greater than I y m were detected during several days o f mixing. This observation supports the role of sulfate as a catalyst for the precipitation of aluminum hydroxide. Significantly different behavior is observed for particle formation in fulvic acid solutions containing aluminum. The total particle number, surface area and particle volume over time and the particle size distribution over time are given respectively in Figs 5

483

and 6 for the alum-fulvic acid system (Experiment 8). The maximum number of particles are observed within l min of base addition, i.e. almost instantaneously given time requirements for base addition, sampling and particle counting. The total number decreases over the first 5 min and maintains a constant value thereafter. The total surface area is approximately constant during the first 5 min, while particle volume increases. After the first 5 rain, both particle surface area and particle volume increase slowly. The particle number distribution initially has a quasi-unimodal form (mode at approx. 1.2/~m); this rapidly changes into a more uniform distribution (at 3 min) and then into a unimodal form at a larger size (mode at approx. 3.7 gm). Most of the transport of particles from the low size range occurs in the first 5 min, coincident with the changes in total numbers. For a fulvic acid-aluminum chloride system in electrolyte (Experiment 9, Fig. 7), similar behavior to that of the a l u m - F A solution is observed. The curve for total number of particles has the same rapid decrease followed by plateau development. The total surface area and particle volume show the same trends. Their numerical values differ from those of the alum-fulvic acid solution by up to 50~'o but the significance of this difference requires further assessment. The changes in particle number distribution for aluminum-fulvic acid solutions (e.g. a l u m - F A ; AICI3-FA) are interpreted similarly to that for alum solutions. A submicron particle size distribution moves through the I ,urn detection limit and is then observed by the particle counter. Above I ym, par-

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Fig. 6. Panicle number distribution for alum-fulvic acid system over time [Experiment 8. Units for ordinate: number of particles per ml per log diameter (#m) increment.]

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ticle aggregation dominates the shaping of the distribution, moving small size particles into the larger size range. The main differences between the A I - F A system and the alum system are hypothesized to be 2-fold. Firstly the solid formed is hypothesized to be an "aluminum fulvate", a solid different from that formed in the alum solutions. Secondly the rate of formation of particles of the size of a nucleus and the rate of aggregation are faster in the aluminum-fulvic acid solution. There are two other differences of potential significance. Firstly, the total volume and surface area curves of the aluminum-fulvic acid solutions are relatively more constant throughout time. Secondly, the total volume, surface area and number of particles are 2-5 times larger in the fulvic acid solutions; the maximum particle diameter and the mode of the particle size distribution in the aluminum-fulvic acid system are larger than that of the alum system. These observations suggest that the mass of solids formed in the A I - F A system is larger than in the alum system and that most of the mass of solids in the measurable and non-measurable size range is observed much quicker in the A I - F A system than in the alum system. But the extra volume of solid in the fulvic acid system does not a priori infer that there is more aluminum in the supramicron fraction because of the larger size of the fulvic acid molecule compared to that of the hydroxyl ion (the fulvic acid is estimated to have a weight averaged molecular weight range of 5000-10,000 from exclusion studies with Sephadex 6-25 and Sephadex 6-50; the smallest particles are 1500-2500MW from small angle X-ray scattering studies. Dempsey and O'Melia, 1981).

The times to form particles in other solutions considered in this work are given in Table 1. The time for particle formation is defined as the time after base addition at which a detectable number of particles greater than 1 ~m is observed. A few values are given as ranges because measurements were not made sufficiently close to the time of particle formation. The first five experiments, involving a graduation in sulfate concentrations from a AI: SO~ ratio of 1 : 1.5 to 1:0, show the significant effects of sulfate upon the time for particle formation. Especially significant is the variation from an AI:SO4 ratio of 1:1.5 to one of l:0.5. The 3-fold decrease in sulfate increases the time to form detectable particles by up to 12 times. No particles are detected in the aluminum chloride solution over several days. A higher initial pH significantly increases the rate of particle formation while a lower pH decreases the rate. Redefining r in a slightly different fashion as the ratio of base added (OH; M) to the amount of aluminum present in solution (Air, M), the three respective pHs of 5.3, 5.45 and 5.9 (Experiments 6, 1 and 7) are induced by r values of 2.2, 2.3 and 2.5. De Hek et al. (1978) observed the formation of a solid in aluminum sulfate solutions at r values of 0.5 and greater for All- concentrations ranging from 5 x 10 -~ to 5 x 10-2M and at r = 1.0 for concentrations of 5 x 10 -4 M. No precipitate formed until an r value of 2.7 was reached in relatively concentrated aluminum solutions (5 x 10 -~ M) free from sulfate. No data are given for lower concentrations of AI free from sulfate, but it is reasonable to assume that a similar value of r is required. Accordingly our results are consistent with those of De Hek et al., although with respect to precise r values at which solid formation is observed, some deviation is expected because of differences in base addition techniques. Ionic strength has significant effects upon the kinetics of particle formation time. The time to form particles in the alum-fulvic acid system is the same in distilled water (Experiment 11) as in elecrolyte (Experiment 8) but it is slower for the alum system (compare Experiment I0 to Experiment 1) and substantially slower for the AICI3-FA system (compare Experiment 12 to Experiment 9). Because the particle formation time for the AICI3-FA system is substantially longer in distilled water than in electrolyte, it is plausible that the time for particle formation for a l u m - F A in distilled water compared to a l u m - F A in electrolyte is also longer but that the differences cannot be detected by the particle analyzer due to the short times involved ( < 1 rain). The data for distilled water systems should be viewed with some circumspection because of potential errors resulting from particle breakup during sample preparation. These potential errors are observed in the following measurements. The electrolyte systems are counted directly while the distilled water systems are diluted with NaCI solution 1:4 to provide sufficient conductivity for the particle analyzer. Addi-

Panicle formation and growth in dilute aluminum(IIl) solutions tional experiments and measurements were made to examine the effect of dilution upon the particle number distribution. The majority of these measurements were made upon the electrolyte systems because an accurate measurement of their particle number distribution is available. In one experiment an alum-electrolyte system was diluted I:1 with NaC1 solution 1 h after base addition. This caused a tripling in the number of particles and a lower mode than that of the undiluted sample; however the total particle volume of the diluted sample was approximately the same as the undiluted sample. Similar behavior (i.e. an increase in number of particles, a lower mode, a loss of particles from upper size ranges and constant particle volume) was observed when 2-3-day-old solutions of the alum-electrolyte system, the alum-fulvic acid--electrolyte system and the alum--distilled water system were diluted. The distilled water sample had one difference: there was a loss of particles from the largest size but the mode did not significantly shift. Whether this means that less particle break-up occurs upon dilution of distilled water samples is unclear.

Fig. 8. It is based upon the model of De Hek et al. (1978), extended to account for the data and systems of this work. The width of the arrows leading to formation of small polynuclear complexes and their reactions to form larger particles are designed to suggest the possible relative rates of reactions for solution conditions. The relative rates may change for higher or lower concentrations of aluminum, hydroxyl ion and fulvic acid. Dashed lines are sketched to describe probable reactions which directly or through subsequent reactions produce particles detectable by the particle analyzer. While inert electrolyte influences the rate of aggregation, and ionic strength and sulfate may influence crystal growth rates, no attempt is made here to include these effects in the width of "dashed arrows". Accordingly some of the differences in width of arrows in Fig. 8 attributed to the rate of nucleus formation may in fact result from differences in later pathways because the rate information of this study is based upon the rate of formation of supramicron particles. Dotted lines are sketched to describe possible reactions which may occur in other systems with different kinetics. The rate of conversion of AI3* to AIOH -'÷ is immediate, involving the liberation of H ÷ from one of the six waters octahedrally coordinated with the aluminum ion; estimates place the half time of this reaction in the order of 10-4S or smaller (Holmes e t

C O N C E P T U A L PATHWAY M O D E L FOR PARTICLE F O R M A T I O N

A conceptual pathway model for particle formation in dilute aluminum solutions is presented in

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Fig. 8. Conceptual pathway model for formation of aluminum particles in dilute AI solutions. Symbols: B = reaction induced by base addition; FA = fulvic acid reaction: (1)= high ionic strength and sulfate present; (2) = high ionic strength or sulfate present; (3) = appropriate solution conditions: CG = crystal growth; Agg = aggregation; Rip = ripening; Aging = aging. WR, 1 8 ~

486

WILLIAM J. SNODGRASSet al.

al., 1968). The rate of formation of small polynuclear

complexes such as hydroxo dimers and trimers of aluminum from monomers is slower, but relatively faster than reactions involving the growth of complexes to larger species. The fastest of the next series of reactions are those involving aluminum polymers with fulvic acid. The experimental results suggest that reactions of alum or AICI~ with FA in electrolyte are equally fast ( < 1 min); but the alum-FA reactions in distilled water ( < 1 rain) are substantially faster than those between A1CI3 and FA in distilled water (2 h < r < 1.5 day). It is probable that the alum-FA reactions are faster than AICI3-FA reactions in electrolyte solutions but that the differences cannot be detected in our experiments. The slow particle formation kinetics for the AICI3-FA distilled water system (arrow 5) can be explained by a restabilization mechanism and/or the formation of large polynuclear complexes. A restabilization mechanism involves the formation of aluminum fulvate particles which are coated with aluminum species whose net positive charge prevents or minimizes the rate of aggregation of small particles into large particles. On the other hand, the formation of larger polynuclear complexes of aluminum in solutions of high ionic strength or sulfate concentration and the reaction of these complexes with negatively charged FA could explain the slower kinetics of the AIC13-FA-distilled water system. In similar systems, the jar test results of Dempsey et al. (1982) carried out in distilled water are interpreted as indicating that restabilization is occurring. At the solution concentrations of our experiments, their alum-FA system showed the formation of nonsettleable but filterable (0.45/~m) particles whereas their AICI3-FA system showed that no filterable particles formed after 15 min of mixing and 3 h of settling. The jar test work of Edzwald (personal communication) suggests that there is a stoichiometry for the charge neutralization of FA polymers by AI polymers at a mass ratio for AIr to FA as TOC of 0.5 (mgmg -~) in alum-FA distilled water systems. This would suggest that our FA system (AIT:FA = 1.4) is somewhat overdosed with aluminum. Restabilization would explain the differences between the kinetics of AICIj-FA in electrolyte (high I) and in distilled water (low I). It is plausible that restabilization is not apparent in the alum-FA--distilled water system because sulfate reduces the formation of larger polymers which react with FA, minimizes the quantity of polymers available for restabilization of the particle and/or destabilizes positive colloids. The formation of supramicron particles of AI(OH)3(s) (arrows 7-10) in aluminum salt solutions is fastest when sulfate and electrolyte are present. The effect of sulfate is interpreted in terms of its role as an outersphere complex, shielding highly charged aluminum polymers from each other to permit AI(OH)3(s) particle formation and growth. The effect of ionic strength is to decrease the thickness of diffuse

layer about these particles, permitting more efficient aggregation and perhaps influencing the kinetics of nucleation. In comparing the relative effects of sulfate (0-2.7 × 10 -~M) and ionic strength (approx. 10-3-0.17 M) upon the kinetics of particle formation, the role of sulfate is primary. However. particles may form in aluminum chloride systems if the ionic strength is sufficiently high. Stol et al. (1978) observed particle formation at lower OH/AL ratios in aluminum nitrate solutions of high ionic strength ( ~ 2.8 M; their work suggests that aluminum nitrate and aluminum chloride systems have similar properties). As the width of the arrows of Fig. 8 suggests, the kinetics of AI(OH)3(s) precipitation (arrows 7 and 8) are faster in solutions of high ionic strength and/or in those having AI:SOa ratios <1:0.5 than the kinetics of solid formation in AIC13-FA-distilled water systems (arrow 5). The kinetics of AI(OH)3(s) formation are dependent upon the solution pH (and r value). At a pH of 5.8-5.9, they are of the same order as the aluminum-FA reactions but much slower at a pH of 5.3. At lower r values and in the absence of sulfate, others have indicated that large polynuclear complexes are formed which are stable for months because of a high activation energy for nucleation. This stability is partially dependent upon use of a homogeneous base addition technique. These polynuclear complexes do not appear to form in solutions containing sulfate or in sulfate free solutions with r > 2.7; rather amorphorus or microcrystalline particles form. Whether these polynuclear complexes can aggregate or grow into particles is unclear. One school of thought which has some credibility suggests that they are not sites for growth. Growth would not occur in a significant fashion if their structure is different from that of a particle on which growth will occur, or, if the solution is no longer supersaturated with respect to the polynuclear complex (e.g. see Lieser, 1969). If they participate in crystal growth, they do so by dissolving, furnishing molecules for the growth of other particles. Since the time scale of dissolution is much longer than an hour, dissolution of polynuclear complexes cannot be significant in Our experiments. Two pathways are shown in Fig. 8 to reflect reactions dependent only upon r---one leads to particle formation (arrow 9) while the second leads to formation of stable polynuclear complexes (arrow 10). Whether polynuclear complexes, which are stable for months, would form in our aluminum chloride solution (arrow I0) is unclear because of the effects of localized concentration gradients surrounding NaOH droplets during base addition; given the results of de Bruyn and co-workers, it is improbable. The dashed and dotted arrows show that for particle growth from a nucleus stage to a supramicron particle, aggregation, crystal growth, ripening and aging are probable mechanisms. For the experiments reported herein, aging is not relevant (it is a

Particle formation and ~owth in dilute aluminum(Ill) solutions term which appears to be used for longer than 1 h, describing processes after initial kinetics have given way to longer term processes) and crystaUine particle formation not probable. Aluminum fulvate is hypothesized to form only by aggregation of their nuclei, assuming that the aluminum fulvate is formed only by the reaction of aluminum species with the fulvic acid. Supramicron particles of AI(OH)3(s) can form both by crystal growth and aggregation; it is hypothesized that aggregation dominates in the alum system. Ripening involves dissolution of thermodynamically unstable small particles; dissolution furnishes material for growth of other particles. Ripening is a possible mechanism in some systems, but is hypothesized to be insignificant in our experimental systems. The rapid appearance of particles through the "'1/am detection limit" in fulvic acid systems suggests that the aggregation rate may be faster than in the aluminum salt systems. For particles less than 1/am in size, perikinetic flocculation (Brownian motion) should dominate over orthokinetic flocculation (fluid shear) as the rate-controlling transport mechanism. The rate of perikinetic flocculation is primarily a function of the number of particles, the efficiency of particle collisions and is independent of fluid shear. Assuming the same efficiency of particle collisions, a larger number of particles is the most reasonable explanation for the faster rate of detectable particle formation in FA systems. Fulvic acids of the order of 5000 MW have a probable size of 25/~. The nucleus for aluminum hydroxide may have a size of approx. 40 A, assuming it to be the same size as that of ferric hydroxide (Dousma and de Bruyn, 1978). Base addition causes aluminum polymers to form. Subsequent reaction with a fulvic acid molecule results in the rapid formation of particles of nucleus size or larger, while such reactions over the same time frame in an aluminum salt solution produce particles of subnucleus size. It is probable that the role of fulvic acid is to induce the formation of particles whose structure is different than the basic structure ofAl(OH)3(s ) and whose initial size and number are larger than that of the AI(OH)3(s) nucleus. Another plausible explanation for these observations is that the solutions have similar nucleation kinetics and that sulfate ion, ionic strength and fulvic acid merely affect aggregation. From this perspective, both sulfate and ionic strength would affect colloid stability (sulfate through surface complexation reactions with surface groups; ionic strength through compression of the diffuse electrical layer about these particles). Fulvic acid could also affect colloid stability by neutralizing the positive charge of AI(OH)~(s) particles or acting as polymer to provide a physical bridge between two AI(OH)3(s) particles. The probable association of a sulfate ion as an outer orbital complex to an aluminum atom and the significant differences of detectable particle formation time in systems of different AI:SO4 ratios argues for

487

the primary effect of sulfate as being upon nucleation kinetics. The possible effect of sulfate ion upon the efficiency of particle collisions (rate of aggregation) is probably small. Ionic strength must also affect nucleation kinetics--particles formed at high ionic strength in the aluminum nitrate systems of Stol et al. (1978); the primary effect is probably upon colloid stability. With respect to fulvic acid, it is difficult to accept that small aluminum polymers would not react with fulvic acid molecules. The larger volume in the AI-FA systems than in the alum system suggests incorporation of FA into a particle. Viewing the reactions of aluminum as being along the two parallel pathways of aluminum polymers reacting with fulvic acid and with themselves, a conclusion that the aluminum polymer-FA pathway dominates in the systems at pH5.5 is consistent with the data presented herein. It is probable that the second pathway (aluminum polymers reacting with themselves) dominates at a high pH (e.g. pH 7.5). IMPLICATIONS FOR WATER TREATMENT

Two implications for the design and operation of the flocculation basin and settling basin result from the experiments on the distilled water system. (1) In drinking water supplies which are relatively acidic (pH 5.5) and free from particles, which contain low concentrations of TOC and to which approx. 10 -4 M alum is added, a 2 min rapid mix is sufficient to produce particles whose aggregation will be influenced by fluid shear in the flocculation basin when the AI salt is alum. However the first portion of the flocculation tank may be ineffective in promoting flocculation if the AI salt is AICI3 due to the formation of submicron particles whose rate of aggregation is slow due to chemical stability and whose size dictate that Brownian motion rather than fluid shear control the rate of particle collisions. (2) Particles formed in alum-fulvic acid systems of pH approx. 5.5 have a size range of the order of I-5/am after 1 h of mixing. The equilivalent Stokes settling velocity is of the order of 10-7-10-Scms -~ assuming, as an upper limit, the density of 2.4 g cm-3 for aluminum hydroxide (Groeneweg, 1980) as an approximation for that of aluminum futvate. Removal of such particles by 4 h of settling is inconsequential, but they can be removed by filtration with a 0.45/am membrane filter. This is consistent with the jar test results observed by Dempsey et al. (1982) at pH approx. 5.5. Whether these particles are removed by conventional sand filtration is primarily dependent upon their colloidal stability. SUMMARY

In this work, particle formation and growth in dilute aluminum solutions (1.8 x 10-aM) at pH 5.5 have been characterized over the 1--40/am size range using an electronic particle counter. Particles of size

488

WILLIAMJ. SNODGRASSet al.

greater than 1 tam rapidly form in all solutions except one involving AICI3 in inert electrolyte (ionic strength 0.17 M). The rate of detectable particle formation is increased by higher pH (more hydroxyl ions), the presence of sulfate ions, and the addition of fulvic acid. Higher ionic strength further increases the particle formation rate in systems containing sulfate ion and/or fulvic acid. A conceptual pathway model for particle formation in different aluminum systems is presented (Fig. 8). The formation of two different solids, aluminum fulvate and aluminum hydroxide, is allowed by two different reaction pathways which compete for aluminum. The model stresses the role of sulfate in aluminum polymer formation and in the formation of both solids. It views the role of pH as being an important controller of particle formation; the role of ionic strength is viewed as secondary in importance. The relative importance of sulfate and pH on particle formation is not examined because of the narrow range of pH values explored.

Acknowledgements--Primary funding for this research project was provided by the Office of Research and Development, the U.S. Environmental Protection Agency under grant number R-808104. The Environmental Protection Agency does not necessarily endorse the products used in this research. The conclusions are those of the authors and do not necessarily represent the opinion, policies or recommendation of the Environmental Protection Agency. The remaining funding was provided by the Johns Hopkins University. The authors were stimulated to explore some of the systems considered herein using the particle analyzer by the early jar test results and inquisitiveness of a former colleague, Dr Brian Dempsey, ~ow at the University of Missouri-Rolla. Discussions with Dr Wen, Department of Chemical Engineering, Johns Hopkins University, on certain points about nucleation phenomena are gratefully acknowledged.

REFERENCES Bersitlon J. L.. Hsu P. H. and Fiessinger F. (1980) Characterization of hydroxy-aluminum solutions. Soil Sci. Soc. Am. J. 44, 630-634. De Hek H., Stol R. J. and de Bruyn P. L. (1978) Hydrolysisprecipitation studies of Aluminum (lid solutions. 3. The role of the sulfate ion. J. Colloid Interface Sci. 64, 72-89. Dempsey B. A. (1981) The protonation, calcium complexation and adsorption of a fractionated futvic acid. Ph.D. Dissertation. University of North Carolina at Chapel Hill. Dempsey B. A. and O'Melia C. R. (1982) The proton and calcium complexation of four fulvic acid fractions. In Aquatic and Terrestrial Humic Materials (Edited by Christman R. F. and Gjessing E. T.), Chap. 12. Ann Arbor Science. Ann Arbor, MI. Dempsey B. A., Ganho R. H. and O'Melia C. R. (1982)The coagulation of humic substances using aluminum salts. Paper presented at the Annual American Water Works Association Conference, Miami Beach, FL. Dousma J. and de Bruyn P. L. (1978) Hydrolysisprecipitation studies of iron solutions. II. Aging studies and the model for precipitation from Fe (III) nitrate solutions. J. Colloid Interface Sci. 64, 154-170. Groeneweg P. G. (1980) Development of an AI(OH)3crystallization model based on population balance. Ph.D. Dissertation, McMaster University, Hamilton, Ontario, Canada. Homes L. P., Cole D. L. and Eyring E. M. (1968) Kinetics of aluminum for hydrolysis in dilute solutions. J. phys. Chem. 72, 401. Lieser K. H. (1969) Steps in precipitation reactions. Angewandte Chem. 8, 188-201. O'Melia C. R. and Dempsey B. A. (1982) Coagulation using polyaluminum chloride. Proceedings of the 24th Annual Public Water Supply Engineers" Conference, pp. 5-15. Champaign, IL. Stol R. J., Van Helden A. K. and de Bruyn P. L. (1976) Hydrolysis precipitation studies of aluminum (lid solutions 2. A kinetic study and model. J. Colloid Interface Sci, 57, 115-131. Stumm W. and O'Melia C. R. (1968) Stoichiometry of coagulation. J. Am. War. Wks Ass. 60, 514-539. Vermeulen A. C., Geus J. W., Stol R. J. and de Bruyn P. L. (1975) Hydrolysis precipitation studies of aluminum (III) solutions 1. Titration of acidified aluminum nitrate solutions. J. Colloid Interface Sci. 51, 449-458.