coagulant

Advances in Colloid and Interface Science100–102(2003) 475–502

0001-8686/03/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0001-8686Ž02.00067-2

Coagulation by hydrolysing metal salts

Jinming Duan , John Gregory *a b,

Ian Wark Research Institute, University of South Australia, Adelaide, SA 5095, Australiaa

Department of Civil and Environmental Engineering, University College London, Gower Street,b

London WC1E 6BT, UK

Received 20 May 2002; accepted 18 July 2002

Abstract

Aluminium and iron salts are widely used as coagulants in water and wastewater treatmentand in some other applications. They are effective in removing a broad range of impuritiesfrom water, including colloidal particles and dissolved organic substances. Their mode ofaction is generally explained in terms of two distinct mechanisms: charge neutralisation ofnegatively charged colloids by cationic hydrolysis products and incorporation of impuritiesin an amorphous hydroxide precipitate(‘sweep flocculation’). The relative importance ofthese mechanisms depends on factors such as pH and coagulant dosage. Alternativecoagulants, based on prehydrolysed forms of aluminium and iron, are more effective thanthe traditional additives in many cases, but their mode of action is not completely understood,especially with regard to the role of charge neutralisation and hydroxide precipitation. Somebasic features of metal hydrolysis and precipitate formation are briefly reviewed and theaction of hydrolysing coagulants is then discussed, with examples from the older literatureand from some recent studies on model systems. Dynamic monitoring of floc formation andbreakage can give useful insights into the underlying mechanisms. Although the results canbe reasonably well explained in terms of established ideas, a detailed understanding of the‘sweep flocculation’ mechanism is not yet available. There are also still some uncertaintiesregarding the action of pre-hydrolysed coagulants.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Aluminium; Coagulation; Flocculation; Hydrolysis; Iron; Water treatment

*Corresponding author. Tel.:q44-20-7679-7818; fax:q44-20-7380-0986.E-mail address: [email protected](J. Gregory).

476 J. Duan, J. Gregory / Advances in Colloid and Interface Science 100 –102 (2003) 475–502

1. Introduction

Hydrolysing metal salts, based on aluminium or iron, are very widely used ascoagulants in water treatment. ‘Alum’ or aluminium sulfate has been used for waterpurification since ancient times and was first mentioned by Pliny(approx. 77 AD).An interesting historical account has been given by Cohen and Hannahw1x.Hydrolysing coagulants have been applied routinely since early in the 20 centuryth

and play a vital role in the removal of many impurities from polluted waters. Theseimpurities include inorganic particles, such as clays, pathogenic microbes anddissolved natural organic matter. The most common additives are aluminium sulfate(generally known as ‘alum’), ferric chloride and ferric sulfate. Other products basedon pre-hydrolysed metals are also now widely used, including a range of materialsreferred to as polyaluminium chloride.Nearly all colloidal impurities in water are negatively charged and, hence, may

be stable as a result of electrical repulsion. Destabilisation could be achieved alongDLVO lines, either by adding relatively large amounts of salts or smaller quantitiesof cations that interact specifically with negative colloids and neutralise their charge.Highly charged cations such as Al and Fe should be effective in this respect.3q 3q

However, over the normal range of pH values in natural waters(say, 5–8), thesesimple cations are not found in significant concentrations, as a result of hydrolysis,which can give a range of products. Many hydrolysis products are cationic andthese can interact strongly with negative colloids, giving destabilisation and coagu-lation, under the correct conditions of dosage and pH. Excess dosage can givecharge reversal and restabilisation of colloids.At around neutral pH both Al(III ) and Fe(III ) have limited solubility, because of

the precipitation of an amorphous hydroxide, which can play a very important rolein practical coagulation and flocculation processes. Positively charged precipitateparticles may deposit on impurity particles(heterocoagulation), again giving thepossibility of charge neutralisation and destabilisation. A further possibility is thatsurface precipitation of hydroxide could occur, with similar consequences. Moreimportantly in practice, hydroxide precipitation leads to the possibility ofsweepflocculation, in which impurity particles become enmeshed in the growing precipitateand thus effectively removed.These additives can also remove dissolved natural organic matter(NOM), either

by charge neutralisation to give insoluble forms, or by adsorption on precipitatedmetal hydroxide.As well as simple hydrolysing salts, a range of commercial pre-hydrolysed

coagulants is available. These contain cationic hydrolysis products and are oftenmore effective than aluminium or iron salts.Although the broad principles of action of these coagulants are reasonably well

understood, there are still some uncertainties regarding the nature of the activespecies, the role of other salts, especially anions, in water, and the nature of theaggregates formed. The mode of action of pre-hydrolysed agents is not yet fullyunderstood. A review of the current state of knowledge will be given, with someexamples of recent experimental results on model systems.

477J. Duan, J. Gregory / Advances in Colloid and Interface Science 100 –102 (2003) 475–502

2. Hydrolysis of Al(III) and Fe(III)

2.1. Monomeric hydrolysis products

All metal cations are hydrated to some extent in water. It is reasonable to thinkin terms of aprimary hydration shell, where water molecules are in direct contactwith the central metal ion, and more loosely held water in a secondary hydrationshell. In the cases of Al and Fe , it is known that the primary hydration shell3q 3q

consists of six water molecules in octahedral co-ordinationw2x. Owing to the highcharge on the metal ion, water molecules in the primary hydration shell are polarisedand this can lead to a loss of one or more protons, depending on the solution pH.Effectively, this means that the water molecules in the hydration shell are progres-sively replaced by hydroxyl ions, giving a lower positive charge, according to thefollowing sequence(omitting co-ordinated water molecules for convenience):

3q 2q q yMe ™Me(OH) ™Me(OH) ™Me(OH) ™Me(OH)2 3 4

This is an oversimplified scheme, since it is known that dimeric, trimeric andpolynuclear hydrolysis products of Al and Fe can form. However, these can oftenbe ignored, especially in dilute solutions, and may not greatly affect the overallmetal speciation. Polynuclear hydrolysis products will be considered in Section 2.2.The hydrolysis scheme above will proceed from left to right as the pH is

increased, giving first the doubly- and singly-charged cationic species and then theuncharged metal hydroxide, Me(OH) . In the case of both aluminium and iron, the3

hydroxide is of very low solubility and an amorphous precipitate can form atintermediate pH values. This is of enormous practical significance in the action ofthese materials as coagulants. With further increase in pH, the soluble anionic formMe(OH) becomes dominant.y

4

Because of the formation of insoluble hydroxides(and also polynuclear species—see Section 2.2), the determination of hydrolysis constants can be difficult and thereare significant differences in some published values(see e.g. Wesolowski andPalmerw3x). Hydrolysis constants can be defined for successive deprotonations interms of the following equations:3q 2q qM qH OlM(OH) qH K2 1

2q q qM(OH) qH OlM(OH) qH K2 2 2

q qM(OH) qH OlM(OH) qH K2 2 3 3

y qM(OH) qH OlM(OH) qH K3 2 4 4

A solubility constant for the metal hydroxide is also needed:3q yM(OH) lM q3OH K3 S

Although the most stable solids are crystalline forms of metal hydroxides, suchas gibbsite and goethite in the case of Al and Fe, respectively, these are usuallyformed very slowly(typically weeks or months). In the context of coagulation


Table 1Hydrolysis and solubility constants for Al and Fe for zero ionic strength and 258C. (Values taken3q 3q

from referencesw3x and w4x)

pK1 pK2 pK3 pK4 pKSam

Al 3q 4.95 5.6 6.7 5.6 31.5Fe3q 2.2 3.5 6 10 38

mechanisms, it is more relevant to consider the solubility of the amorphousprecipitates that form initially. However, solubility constants for the amorphousforms, K are not known precisely and only estimated values can be quoted. TableSam

1 gives values for hydrolysis and solubility constants(in pK form), taken fromWesolowski and Palmerw3x for Al and from Flynn w4x for Fe. The values are forconditions of zero ionic strength and 258C. Referencew3x gives extensive data forAl at other temperatures and ionic strengths.Using the values in Table 1, it is possible to plot, as a function of pH, the

concentrations of the various species in equilibrium with the amorphous hydroxideprecipitate. Such diagrams are shown in Fig. 1 for Al and Fe. The total amount ofsoluble species in equilibrium with the amorphous solid is effectively the solubilityof the metal and it can be seen that in each case there is a minimum solubility at acertain pH value. For Al this is approximately pH 6, at which the solubility is ofthe order of 1mM. Measurements of Al solubility as a function of pH givereasonable agreement with such calculationsw5x, even though only monomericspecies are included. For Fe, the minimum solubility is much lower—less than 0.01mM, and the corresponding region is broader than for Al.Martin w6x has pointed out a significant difference in the hydrolysis behaviour of

Al and Fe, which is apparent from the values in Table 1 and the computed resultsin Fig. 1. The hydrolysis constants for Al cover a much narrower range than thosefor Fe. The latter are spaced over approximately 8 pH units, whereas all of the Aldeprotonations are ‘squeezed’ into an interval of less than 1 unit. Martin explainedthis feature by the transition from the octahedral hexahydrate Al .6H O to the3q

2

tetrahedral Al(OH) . This makes the successive hydrolysis steps co-operative iny4

nature. All of the hydrolysed species for Fe retain the octahedral co-ordination3q

and the stepwise deprotonations show the expected spread of pK values. Thisdifference is clearly seen in the plots of species distributions in Fig. 2, which showthe mole fraction of the various soluble hydrolysis products in equilibrium with theamorphous precipitate. The ferric species each attain significant relative concentra-tions in solution at appropriate pH values, whereas for Al, apart from a narrow pHregion approximately 5–6, the dominant soluble species are Al and Al(OH) at3q y

4

low and high pH, respectively.

2.2. Polynuclear species

As well as the simple monomeric hydrolysis products discussed above, there aremany possible polynuclear forms that could be consideredw2x. For Al, these include


Fig. 1. Concentrations of monomeric hydrolysis products of Fe(III ) and Al(III ) in equilibrium with theamorphous hydroxides, at zero ionic strength and 258C.

Al (OH) and Al (OH) and there are equivalent species for Fe. Formation4q 5q2 2 3 4

constants for the dimers and trimers are known, but, for practical purposes, they donot significantly affect the speciation shown in Figs. 1 and 2. Martinw6x showedthat the Fe dimer, Fe(OH) , could become significant in acid solutions(pH-3),4q

2 2

but the corresponding Al dimer does not occur to any significant extent in saturatedsolutions of Al(OH) . From the standpoint of coagulation with simple Al and Fe3

salts, only monomeric hydrolysis products and the amorphous hydroxide precipitateneed be considered.Polynuclear hydrolysis products can be prepared in significant amounts under

certain conditions. The best known of these is Al O(OH) or ‘Al ’, which can7q13 4 24 13

be formed by controlled neutralisation of aluminium salt solutions or by severalother methods. This tridecamer has the so-called ‘keggin’ structure, consisting of acentral tetrahedral AlO unit surrounded by 12 Al octahedra with shared edges.5y

4

The tetrahedral and octahedral Al sites can be easily distinguished in the Al NMR27

spectrumw7x. The structure has also been confirmed by small angle X-ray methods


Fig. 2. Proportions(mole fractions) of dissolved hydrolysis products in equilibrium with amorphoushydroxides.

w8x and by potentiometric titrationw9x. Under appropriate conditions, Al forms13

fairly rapidly and essentially irreversibly, remaining stable in aqueous solutions forlong periods. The tridecamer has been detected in the natural aquatic environment,an acid forest soil waterw10x. The Al unit has an ionic radius in solution of13

approximately 1.3 nmw8x.Other polynuclear species, such as the octamer, Al(OH) , have been proposed,4q

8 20

based on coagulation dataw11x. However, there is no direct evidence for the octamerand it is unlikely to be significant in practice.The titration of an aluminium salt solution with base typically gives a curve like

that in Fig. 3, in which four regions can be distinguished, based on the amount ofadded baseB (sOHyAl). In Region 1 the base neutralises free acid produced byspontaneous hydrolysis, giving a rapid increase of pH. Above approximately pH 4,hydrolysed species are formed and there is an extended phase(Region 2) where pHincreases only slowly, since added base is consumed by hydrolysis. In this region,


Fig. 3. Titration curves for neutralisation of aluminium salt solutions, showing variation of pH withadded baseB (sOHyAl)

large quantities of polymeric species can be formed and become the predominantsoluble species at highB values. During titration, depending on mixing conditions,added base can give local supersaturation and precipitation of the amorphoushydroxide, or excess formation of the Al(OH) ion. These conditions may favoury

4

the formation of the Al polymer, although the details are not clear.13

Further increase ofB, to the range 2.4–2.8, gives Region 3, in which a smallshoulder appears in the curve, following a sharp rise in pH. In this region,supersaturation of the solution with respect to amorphous Al(OH) and rapid3

precipitation occurs. This shoulder disappears in the presence of highly chargedanions, such as sulfate, which can promote hydroxide precipitationw12x. In Region4, the added base reduces the positive surface charge of the colloidal hydroxideparticles and visible precipitates are formed. Further addition of base gives a rapidincrease in pH.The speciation of Al solutions can be conveniently studied by a timed colorimetric

reaction with ferron reagent(8-hydroxy-7-iodo-5-quinoline sulfonic acid). Thismethod was introduced by Smithw13x and is based on the observation that differentforms of Al react at varying rates with ferron. Mononuclear Al species(Al ) reacta

almost instantaneously and polynuclear species(Al ) much more slowly. Colloidalb

or precipitated Al(Al ) shows practically no reaction with ferron. The proportionc

of polynuclear species(Al ) determined by the ferron technique corresponds quiteb

well with results from membrane filtration testsw14x and with the proportion ofAl from NMR studies w15x. Typically, the proportion of the various species is13

determined as a function of the degree of neutralisation,B. For 0.1 M AlCl3solution, titrated with 0.5 M NaOHw13x, the proportion of Al increased fromb

approximately 33% to 83% asB was raised from 1.0 to 2.5. For values ofB above2.5, the amount of precipitate(Al ) increased significantly. In the same study, itc

was shown that the Al fraction was quite stable to dilution and to changes in pH,b


and only slowly converted to other forms. This is in marked contrast to monomerichydrolysis products, which respond very rapidly to changes in chemical conditions.It is doubtful whether Al forms under conditions where aluminium salts are13

added to water at around neutral pH, to give low Al concentrations(typical ofwater treatment conditions). In this case, it is thought that monomeric hydrolysedspecies predominate in solution and that amorphous precipitates form without theinvolvement of Al species(see below).13

Polymeric hydrolysis products of ferric salts can also be prepared and characterisedby ferron analysisw14,16x. In partially neutralised ferric chloride solutions, it hasbeen suggested that, in the range ofB (OHyFe) up to 1.0, monomeric and di- andtrimeric species are predominant in solutionw17x. The iron trimer, Fe(OH) , was5q

3 4

postulated as the nucleus for phase transition in supersaturated ferric chloridesolutionsw18x, which may build up larger polymers such as Fe and Fe with size6 9

approximately 1.66 nmw19x. Bottero et al.w20x reported the formation of Fe24polycation in hydrolysed ferric chloride solution and they observed that fractalaggregates form with Fe as subunits, by using a small angle X-ray scattering24

(SAXS) technique. In such solutions, polycations are first produced, and furtherneutralisation causes aggregation of these polycations leading to formation of fractalpolymers.

2.3. Precipitate formation

Precipitated metal hydroxides can be formed in various ways, such as byneutralisation of the metal salt toB (OHyM) ratios of approximately 3, as mentionedabove. However, the precise mechanisms and structure of the precipitate have beenthe subject of much debate and there is a rather extensive and confusing literatureon the topic. We shall restrict attention here mainly to aluminium. In the case ofiron, there are redox as well as hydrolytic reactions to considerw21x and the subjectbecomes quite complex.An early hydrolysis–precipitation model for aluminium involved the two-dimen-

sional growth of hexameric ring unitsw22x, although there is no direct experimentalevidence for this so-called ‘coreqlinks’ model. Several studiesw23,24x havesuggested that the initial stage in the formation of gels by neutralisation of aluminiumsolutions consists of the aggregation of tridecamer(Al ) units, although detailed13

mechanisms are still not clear. Bottero et al.w24x concluded, from NMR, infra-redand small angle X-ray studies, that the nature of the clusters formed depends verymuch on the OHyAl ratio. For Bs2.5 at short times, chain-like clusters of lowfractal dimension and size of approximately 40 nm are formed. With increasingBthe clusters become larger and more compact and there is a progressive loss of thetetrahedral Al.The initially precipitated form undergoes rearrangement on ageing or heating and

eventually attains long-range crystalline order. However, in the context of coagulationby hydrolysing metal salts, it is the rapidly-formed amorphous precipitate, which isof most interest.


Details of the neutralisation procedure, such as temperature, stirring conditionsand base injection rate can have a very important influence. Ohman and Wagbergw25x showed that different routes to a neutralised Al solution could give significantlydifferent precipitate properties. They used a stopped-flow ‘flash’ neutralisationtechnique with AlCl solution, a partially prehydrolysed AlCl solution and a3 3

solution of sodium aluminate. These showed broadly similar titration curves, butsignificant differences in the particle size atB values of 3 or more. In all cases theparticles showed a dramatic increase in size at pH values of approximately 7 orgreater.The effect of mixing conditions on aluminium precipitation has been considered

in some detail by Clark et al.w26x. They studied the neutralisation of AlCl solutions3

in a stirred tank reactor and considered the kinetics of hydrolysis reactions inrelation to characteristic mixing time scales. This work showed that there is acompetition between the formation of polynuclear hydrolysis products and precipi-tated solid. With more intense mixing, the results indicated that precipitation wouldbe favoured.Neutralisation of fairly concentrated metal solutions by added base is not directly

relevant to the use of hydrolysing metal coagulants in practice, where the metal saltis added to water, usually containing excess alkalinity, to give a final concentrationof approximately 0.1 mM or less. In this case, neutralisation would occur rapidlyand it is very likely that precipitation would occur without the formation ofsignificant polynuclear species such as Alw5x. It has been shown that precipitates13

formed by addition of aluminium sulfate and polyaluminium chloride(a prehydro-lysed solution containing Al) give different solid phases. In the latter case, the13

polymeric structure is maintained in the precipitate and after re-dissolution in acidw5x. Similar behaviour has been found for ferric sulfate and a pre-hydrolysed form(polyferric sulfate) w27x.The surface charge characteristics of precipitated metal hydroxides are of great

importance in coagulation. In common with oxides and other minerals they showan isoelectric point(i.e.p.) at which the apparent(electrokinetic) surface charge iszero. At pH values below the i.e.p. the precipitate is positively charged and athigher pH values it has a negative charge. The value of the i.e.p. depends on thepreparation details and on the solution composition, so that there are significantdifferences in values reported in the literature.Precipitation from aluminium chloride solutions gives a solid with an i.e.p. in the

region of 9 w25x, whereas from aluminium sulfate the value is closer to 8w5x. Foramorphous ferric hydroxide the i.e.p. is somewhat lower. For both aluminium andferric salts, pre-hydrolysed forms give precipitates with i.e.p. values shifted upwardsby one or more pH units.The varying charge with pH can greatly affect the precipitation process. At around

neutral pH for aluminium, the initially formed colloidal precipitate is positivelycharged and, hence, is colloidally stable. As the pH is increased towards the i.e.p.,the stability decreases and the particles can aggregate into large, settleable flocs.Hayden and Rubinw28x carried out a light scattering study on precipitation from an


aluminium nitrate solution(1 mM) as a function of pH. At low pH, the solutionappeared clear, but showed a Tyndall beam, indicating the presence of very small,colloidal particles. As the pH increased, the particle size increased, giving higherturbidity. Above approximately pH 7 much larger particles were formed, whichsettled rapidly to give a reduced turbidity. The i.e.p. in this case was approximately8, in the middle of the pH range where settleable flocs were produced. The resultsof Ohman and Wagbergw25x are consistent with these findings, although theirprecipitates were from AlCl solutions and showed a rather higher i.e.p. value. It is3

worth noting that the i.e.p. for Al(OH) occurs at a pH value well above that of3

minimum solubility (Fig. 1), so that the largest flocs do not correspond with themaximum amount of precipitate.The presence of highly charged anions, such as sulfate, can have a large effect

on hydroxide precipitation. Sulfate can reduce the positive charge of the precipitatein the acid region, so that large flocs are formed over a wider pH range. This wasclearly shown by Hayden and Rubinw28x and others. The sulfate effect has beenknown since the 1930sw1x and is very important in practice since aluminium andferric sulfates are commonly used as coagulants and natural waters can containsignificant amounts of sulfate.

3. Mechanisms of coagulation

3.1. General

Natural waters contain a very wide variety of particulate impurities. These includeinorganic substances such as clays and metal oxides, various organic colloids andmicrobes such as viruses, bacteria, protozoa and algae. Aquatic particles cover abroad range of particle size, from nm to mm dimensions and present a significantchallenge in water treatment technology. For smaller particles, separation efficiencycan be greatly enhanced by aggregation to give an increased size(coagulationyflocculation).Over the usual range of natural water pH(say, 5–9) particles nearly always carry

a negative surface charge. This may be because the water pH is above the isoelectricpoint, which is usually the case for particles of biological origin. Even for mineralparticles with a fairly high i.e.p., adsorption of natural organic matter usually givesa negative surface chargew29x. Because of their surface charge, aquatic particles areoften colloidally stable and resistant to aggregation. For this reason, coagulants areneeded to destabilise the particles.According to the classical ideas of colloid stability, destabilisation can be brought

about by either:

● an increase in ionic strength, giving some reduction in the zeta potential and adecreased thickness of the diffuse part of the electrical double layer, or

● specific adsorption of counterions to neutralise the particle charge.

In both cases, additives effective for negative particles should be salts with highlycharged cations. It is unlikely that a sufficient increase in ionic strength would be a


practical destabilisation method, but counterion adsorption is much more promising,since quite small amounts of additive would usually be sufficient. Aluminium andiron salts give cationic hydrolysis products that are strongly adsorbed on negativeparticles and can give effective destabilisation.Polymeric additives can also be used to cause aggregation of particles and they

may act either bypolymer bridging or charge neutralisation(including ‘electrostaticpatch’ effects) w30x. The action of hydrolysing metal coagulants can involve similarmechanisms.We shall initially consider the action of coagulants with respect to particulate

impurities in water with only ‘simple’ salts of Al and Fe. Later sections will dealwith pre-hydrolysed coagulants and dissolved organic matter.

3.2. Charge neutralisation

At very low concentrations of metal, only soluble species are present—thehydrated metal ion and various hydrolysed species, which, assuming only monomericforms will depend on solution pH, as shown in Fig. 1. It is generally thought thathydrolysed cationic species such as Al(OH) are more strongly adsorbed on2q

negative surfaces than the free, hydrated metal ionw31x. Adsorbed metal ions maybe in the form of outer sphere or inner sphere complexesw32x. In the former casethere is at least one water molecule separating the cation from the surface, i.e. thecation retains its hydration shell. Inner sphere complexes involve the direct co-ordination of the metal ion to surface groups, with no intervening water.Models for surface complex formation(e.g. Stummw32x), are mainly for metal

oxide surfaces, and involve specific parameters, such as binding constants. Theprocess is analogous to complex formation in solution and only monolayer coveragecan occur. However, it is known that hydrolysing coagulants can neutralise thenegative surface charge of many types of particle, including bacteria and clays, andit is unlikely that a specific complexation reaction will provide an explanation ofall the observed effects.Generally, charge neutralisation with aluminium salts occurs at quite low metal

concentrations—typically a fewmM at around neutral pH. Letterman et al.w33xfound that, for several inorganic suspensions at pH 6, the amount of aluminiumneeded to bring the electrophoretic mobility to zero was in the region of 5mM Alper m of particle surface. Inspection of Fig. 1 shows that, even at very low2

concentrations, the solubility limit of the hydroxide may be exceeded. Also, atneutral pH, cationic hydrolysis products should represent only a tiny fraction of thetotal soluble Al(Fig. 2), the dominant form being the aluminate ion. This suggeststhat the effective charge-neutralising species may be colloidal hydroxide particles,which should be positively charged up to approximately pH 8. Even when the bulkhydroxide solubility is not exceeded, a form ofsurface precipitation may take place.James and Healyw34x among others have suggested that adsorption of solublehydroxide can lead to a layer of amorphous hydroxide precipitate, by surfacenucleation and precipitation.


A surface precipitation model was proposed by Farley et al.w35x to explain cationadsorption on oxide surfaces. In this model, when cations adsorb to the surface ofa mineral a precipitate of the cation with the constituent ions of the mineral surfacemay form at high surface coverage. This allows sorption to occur through acontinuum between surface complex formation and bulk solution precipitation ofthe sorbing ion. As the cation is complexed at the surface, a new hydroxide surfaceis formed. In the model, cations at the solid(oxide) water interface are treated assurface species, while those not in contact with the solution phase are treated assolid species forming asolid solution. In this case, at low adsorbing cationconcentration, surface complexation is the dominant mechanism. However, as thesorbate concentration increases, both the surface complex concentration and theamount of the surface precipitate increase until the surface sites become saturated.Surface precipitation then becomes the dominant sorption mechanism. Furthermore,as bulk solution precipitation is approached, the amount of the surface precipitatebecomes large. It follows that, as metal is adsorbed on a surface, a new hydroxidesurface will be formed, allowing further mass transfer of metal to the solid phase.This can give multilayer sorption, in contrast to surface complex formation.In practice it is often quite difficult to distinguish between surface precipitation

and the deposition of colloidal hydroxide particles which have been precipitated inbulk solution. A combination of these effects is included in thePrecipitation ChargeNeutralisation (PCN) model, which was introduced by Dentelw36x to explaincoagulation by hydrolysing metal salts in water treatment. A schematic illustrationof the processes involved is given in Fig. 4. The PCN model has been presented ina quantitative formw37x although this aspect will not be covered here.According to the PCN model, coagulation with aluminium or iron salts involves

three steps:

1. Destabilisation begins after addition of a dose of coagulant that exceeds theoperational solubility limit of aluminium(or iron) hydroxide.

2. Aluminium or iron hydroxide species are then deposited onto colloidal surfaces,as shown in Fig. 4. This figure shows that metal hydroxide could end up onparticle surfaces by several possible pathways.

3. Under typical conditions, metal hydroxide is positively charged, while the originalcolloidal particles are negatively charged. So the deposition process can result incharge neutralisation or charge reversal of the colloidal particles at certain doses,as shown in a simplified manner in Fig. 5.

If the positively charged adsorbed species are in the form of isolated regions,then a form of ‘electrostatic patch’ attraction may be important, as in the case ofpolyelectrolytesw30x, although this does not seem to have been examined systemat-ically for hydrolysing coagulants.It is important to note that the PCN model does not consider bulk hydroxide

precipitation and ‘sweep flocculation’, which will be discussed in the next section.There is no doubt that, at the correct dosage, charge neutralisation by adsorbed

hydrolysis products andyor hydroxide precipitate can cause negatively chargedparticles to become destabilised and hence to coagulate. When electrophoretic


Fig. 4. Schematic illustration of the concept of the Precipitation Charge Neutralisation(PCN) model.(After Dentel w36x.)

Fig. 5. Deposition of metal hydroxide species on oppositely-charged particles, showing charge neutral-isation and charge reversal.(After Dentel w36x.)

mobility (EM) measurements are carried out, it is evident that the optimumcoagulation dosage corresponds with the condition where the zeta potential of theparticles is close to zero. Some results for kaolin suspensions(50 mgyl) coagulatedwith aluminium sulfate(‘alum’) at pH 6, are shown in Fig. 6w38x. This shows theresidual turbidity of the suspensions after standard stirring and settling conditions(jar test) as well as the electrophoretic mobility of the particles, soon after coagulantaddition. Since the minimum residual turbidity occurs at approximately 8mM Al,this is the ‘optimum dosage’ for particle separation. The EM value is very close tozero at this point, indicating that charge neutralisation is responsible for thedestabilisation of the clay particles. At slightly higher alum dosages, the EMbecomes positive and the residual turbidity increases, indicating that charge reversalcauses restabilisation of the particles. Very similar results were obtained for kaolinby Letterman and Vanderbrookw39x. According to Fig. 1, 8mM Al at pH 6 isslightly above the solubility limit of amorphous Al(OH) .3At higher pH values the optimum alum dosage increases because of the decreased

positive charge of the adsorbed species. In such systems it appears that theelectrokinetic properties of the particles are very like those of the amorphoushydroxide precipitatew33x, with an isoelectric point in the pH region 8–9, depending


Fig. 6. Electrophoretic mobility(EM) and residual turbidity for kaolin suspensions(50 mgyl) with lowdosages of aluminium sulfate(‘alum’) at pH 6.(Replotted from data of Duanw38x.)

on the anions present in solution. At around the i.e.p. the particles do not becomepositively charged, even at high Al dosages and so no restabilisation is observed.If charge neutralisation is the predominant destabilisation mechanism, then there

should be a stoichiometric relationship between the particle concentration and theoptimum coagulant dosagew40x. At low particle concentrations, low coagulantdosages should be required. Under these conditions coagulation rates can be verylow, which causes problems in water treatment. Another practical difficulty is thatthe optimum coagulant dosage range can be quite narrow, which means that ratherprecise dosing control is needed.Both of these difficulties can be overcome by using higher coagulant dosages,

where extensive hydroxide precipitation occurs, givingsweep flocculation.

3.3. Sweep flocculation

It has long been recognisedw41x that, in many cases, optimal removal of particlesfrom water is achieved under conditions of rapid and extensive hydroxide precipi-tation. In the case of aluminium coagulants, optimum pH values are approximately7, close to the minimum solubility(Fig. 1) but close enough to the i.e.p. to givefairly rapid aggregation of the colloidal precipitate particles. Although details arenot fully understood, it seems clear that impurity particles are enmeshed in agrowing hydroxide precipitate and are effectively removed from suspension. Thisprocess has become known as ‘sweep flocculation’ since particles are ‘swept out’of water by an amorphous hydroxide precipitate.Sweep flocculation generally gives considerably improved particle removal than

when particles are destabilised just by charge neutralisation. At least part of thereason is the greatly improved rate of aggregation, because of the increased solidsconcentration. Hydroxide precipitates tend to have a rather open structure, so thateven a small mass can give a large effective volume concentration and, hence, a


high probability of capturing other particles. It is also possible that binding(‘bridging’) of particles by precipitated hydroxide may give stronger aggregates.Increasing the coagulant dosage in the sweep region gives progressively largervolumes of sedimentw42x but, beyond the operational optimum dosage, there islittle further improvement in particle removal.The different mechanisms outlined above have led to the definition of fourzones

of coagulant dosage, with the following consequences for negatively chargedparticles:

Zone 1: Very low coagulant dosage; particles still negative and hence stable.Zone 2: Dosage sufficient to give charge neutralisation and hence coagulation.Zone 3: Higher dosage giving charge neutralisation and restabilisation.Zone 4: Still higher dosage giving hydroxide precipitate and sweep flocculation.

The example in Fig. 7 shows the results of jar test and EM measurements forkaolin suspensions with alum at pH 7w38x. Below approximately 8mM Al there isessentially no reduction in turbidity, since the particles are negatively charged andcolloidally stable(Zone 1). There is a narrow range of lowered turbidity in theregion of 15mM Al, which is close to the dosage where the EM is reduced to zero(Zone 2). By 20 mM Al, the particles are positively charged and completelyrestabilised, since the residual turbidity is no lower than that for the original claysuspension(Zone 3). Beyond approximately 60mM Al the turbidity falls again asa result of sweep flocculation(Zone 4). It is very significant that a substantialchange in residual turbidity occurs in a region of alum dosage where the EM of theparticles is still positive and shows no appreciable reduction. Although there is agradual reduction in EM as the alum dosage is increased, this is not obviouslyrelated to the degree of turbidity removal. It is also worth noting that the residualturbidity in Zone 4 is significantly lower than in Zone 2, indicating a much greaterdegree of clarification by sweep flocculation.These experiments were supplemented by dynamic measurements of floc growth,

using a simple optical monitoring techniquew43x, based on the principle of ‘turbidityfluctuations’. This gives a semi-empiricalFlocculation Index (FI), which is stronglycorrelated with floc size. Fig. 8 shows the change in FI with time for two differentalum dosages at pH 7: 5 and 40mM Al (SO ) (10 and 80mM Al ). The first of2 4 3

these is well within Zone 2, where charge neutralisation is the operative mechanismand the higher dosage is that where the onset of optimal sweep flocculation occurs.There are very significant differences between the two curves in Fig. 8. For 5

mM alum, the FI value begins to increase very soon after dosing(the first minutecorresponds to the ‘rapid mixing’ phase of the jar test, where little floc growthoccurs). Flocs then grow quite slowly and the FI reaches a plateau, correspondingto a limiting floc size, which depends on the stirring rate. This is consistent withthe fairly rapid adsorption of charge-neutralising species on the kaolin particles,followed by quite slow coagulation. The aggregates(flocs) formed are quite weakand grow only to a rather small size. The rapid charge neutralisation is also indicatedby the fact that all of the EM reduction occurs during the initial rapid mix phase.


Fig. 7. As Fig. 6, but over a wider range of alum dosages and at pH 7.(Replotted from data of Duanw38x.)

Fig. 8. Dynamic monitoring of kaolin suspensions at two dosages of alum, under the same conditionsas for Fig. 7.(Replotted from data of Duanw38x.)

For the dosage of 40mM Alum, the onset of floc formation is considerablydelayed—significant rise in the FI value does not begin until approximately 5 minafter dosing. However, a very rapid rise then occurs and the FI reaches a valuenearly four times that in the other case, indicating much larger flocs. The lag timeobserved is related to the time required to form relatively large amorphous hydroxideprecipitate particles. In the same studyw38x it was shown that particles of aluminiumhydroxide in the same solution at pH 7, but without kaolin, took several minutes togrow to a detectable size and the delay was of the same order as that observed forthe onset of flocculation in Fig. 8. The delay can be considerably reduced byincreasing the alum dosage or by increasing the pH to approximately 8, which isthe i.e.p. of the hydroxide precipitate in this system. However, even with a more


rapid onset of flocculation, the rate of growth is not significantly greater and thefinal FI value is about the same.These results confirm that there are very important differences between destabi-

lisation by charge neutralisation and sweep flocculation. In particular, flocs formmore rapidly(perhaps after an initial delay) and can become much larger in thecase of sweep flocculation, so that a greater degree of separation can be achieved.It seems that these effects are closely connected with the formation of a bulkhydroxide precipitate, initially in the form of very small colloidal particles(a fewnm in size), which are positively charged at around neutral pH. It is likely thatsome of these particles form a coating on the impurity particles, reversing theircharge. Subsequently, aggregation of the colloidal hydroxide particles occurs, eitheron the particle surfaces(a form of heterocoagulation) or in bulk solution. Detailsof this process are still not clear, but microscopic observation of flocs producedunder ‘sweep’ conditions show the original impurity particles embedded in anamorphous precipitate. A schematic diagram showing a possible sequence of eventsin sweep flocculation with aluminium salts is given in Fig. 9.The Smoluchowski theory for particle aggregation in shear fields(orthokinetic

flocculation) leads to the conclusion that flocculation rate is directly proportional tothe effective particle volumew44x. Growing hydroxide precipitate consists of verysmall primary particles in a rather open, fractal structure and it is easy to show thatthe effective floc volume can become quite large, even for low coagulant doses. Itis found that the settled floc volume increases in proportion to the dosage ofhydrolysing coagulants under sweep conditionsw42x. This probably accounts for theenhanced aggregation rate in sweep flocculation. Simply neutralising the particlecharge with a rather thin layer of adsorbed species would not give a significantlyincreased collision radius.Although the broad principles are reasonably well understood, there are several

complications with hydrolysing coagulants, which can be important in practice.These will be discussed in the next sections.

3.4. Floc strength and breakage

Practical applications of hydrolysing coagulants are nearly always under conditionsof turbulent fluid motion. Mixing of coagulant involves quite intense agitation(‘rapid mix’) for a short time and this is usually followed by a longer period ofgentler mixing, either in a stirred tank or some form of hydraulic flocculator. Thepurpose of the second phase is to promote orthokinetic collisions of particles andhence floc growth. Flocs grow initially at a rate that depends on the energydissipation(or applied shear), as well as on the particle concentration and collisionefficiency. As flocs become larger, further growth is restricted by the applied shearfor essentially two reasons. Existing flocs may be broken as a result of disruptiveforces w45x and the collision efficiency of particles in a shear field becomes loweras particle size increasesw46x. A dynamic balance between floc growth and breakageoften leads to a steady-state floc size distribution, where the limiting size dependson the applied shear ratew47x.


Fig. 9. Schematic diagram showing the interaction of aluminium species with initially negatively chargedparticles in water. The particles on the right hand side are initially stable and then become destabilisedby charge neutralisation. At higher coagulant dosages they can become restabilised by charge reversaland incorporated in a flocculent hydroxide precipitate(‘sweep flocculation’).

If the effective shear rate is increased, pre-formed flocs can be broken, in amanner, which depends on the floc size relative to the turbulence microscalew47x.Flocs formed by hydrolysing coagulants tend to be rather weak, so that breakageoccurs readily. In the case of sweep flocculation, this breakage is not fully reversible,so that flocs do not completely re-form when the original shear conditions arerestored. This effect is well known in practice, but has received rather littlesystematic attentionw48x. Recent workw49x has given more detailed information onthe subject, although the underlying mechanisms are still not well understood.Using the same dynamic monitoring method mentioned earlier, Yukselen and

Gregory w49x showed that flocs formed with kaolin suspensions and aluminiumsulfate were broken irreversibly at high stirring speeds and that the degree ofirreversibility depended on the time of breakage. An example of their results isshown in Fig. 10. In this case a kaolin suspension(50 mgyl) was flocculated byalum at a dosage of 130mM Al at around neutral pH(i.e. well within the sweepfloc regime). Flocs were formed by stirring for 10 min at 50 rpm and then thestirring rate was increased to 400 rpm. These stirring rates correspond to meanshear rates(G values) of approximately 25 and 520 s , respectively. The highy1

stirring speed was maintained for between 10 and 300 s and was then reduced tothe original 50 rpm. From the change in Flocculation Index with time, it is clearthat, under slow stirring conditions, flocs grew fairly rapidly to a limiting size.When the stirring speed was increased, there was an immediate and rapid drop inthe FI value, by a factor of nearly 10, showing very substantial breakage of flocs.Most of the breakage was achieved after approximately 10 s. When the stirring ratewas returned to 50 rpm, some re-growth of flocs occurred, but not back to the


Fig. 10. Dynamic monitoring of kaolin suspensions, showing formation, breakage and partial re-forma-tion of flocs. In all cases suspensions were dosed with alum(130mM Al ) and stirred at 50 rpm for 10min. A higher stirring speed(400 rpm) was then applied for times of 10–300 s(as indicated on curves),followed by further stirring at 50 rpm.(From w49x.)

previous FI value. Furthermore, the degree of recovery decreased for longer breakagetimes. This is especially apparent for the 300 s breakage case.As yet, there is no adequate model to explain these findings. The effect seems to

be specific to the sweep flocculation case, since flocs formed using cationicpolyelectrolytes, which destabilise the clay particles by charge neutralisation andelectrostatic patch effects, showed almost complete re-formation after breakage. Itis likely that breakage of metal hydroxide flocs involves rupture of chemical bonds,which are unable to re-form.

3.5. Effect of anions

Solution chemistry has considerable influence on coagulation by hydrolysingmetal ions. This influence will depend on how strongly anions can co-ordinate withaluminium in terms of replacement of hydroxyl ionw50x, or how they affect thekinetics of precipitationw51x. Matijevic w31x suggested that it is the solutionchemistry, particularly the pH and destabilising anion concentrations that determinewhether or not the precipitate coated particles will be flocculated. Letterman andVanderbrookw39x suggested that Zone 4 coagulation would be controlled by thesolubility of the adsorbed aluminium hydroxide precipitate and surface ionisationand sulfate complex formation reactions.Among the anions, nitrate has very little tendency to co-ordinate with metal ions,

and does not have a significant influence on destabilisation with metal coagulants.However, anions such as bicarbonate, chloride, sulfate, etc. have considerable effectson coagulation by aluminium salts.


The effects of bicarbonate ion on coagulation of kaolin particles by aluminiumsalts have been studiedw51,52x. The traditional view of the significance of thebicarbonate ion in the coagulation or flocculation of suspensions using hydrolysingcoagulants has been associated with its contribution to the alkalinity(buffer capacity)of the water. The importance of maintaining a buffered solution is related to thechemical and physical characteristics of hydroxide precipitatesw51x. If a suspensiondoes not have sufficient alkalinity, addition of the hydrolysing metal salt maysignificantly depress solution pH, so that electrical charge andyor colloidal propertiesof the precipitate would be greatly affected. Precipitation might even be preventedif the solution pH is reduced beyond the solubility range of the hydroxide. Becauseferric hydroxide is much less soluble than aluminium hydroxide(Fig. 1) the formeris precipitated over a much broader pH range. For this reason, sweep flocculationby ferric salts is less sensitive to pHw41x.Generally, bicarbonate, sulfate and chloride, have little or no effect on the pH of

aluminium precipitation. However, they may exert great influence on the range ofpH values where the initial precipitate can aggregate to settleable flocsw28x.Sulfate is a moderately strong co-ordinator with aluminium, and the presence of

sulfate ion extends the pH range of coagulation towards the acid side under normalcoagulation conditionw39,52x. Near to the isoelectric pH of freshly precipitatedaluminium hydroxide, coagulation and destabilisation of particles is due to thecoating of the inherently unstable aluminium hydroxide possibly resulting fromionisation of the precipitate surface or adsorption of sulfate anions. The presence ofsulfate in solution can reduce significantly the positive charge of aluminiumhydrolysis products.It has been shown that dissolved silica(silicic acid) can exert significant effect

on coagulation with alum and ferric chloridew38x. The presence of dissolved silicacan promote or inhibit coagulation of kaolin clay suspensions depending onconcentration of silica and solution pH at a normal alum dosages. At pH 6, dissolvedsilica can promote alum coagulation greatly at some low concentrations, while atneutral or alkaline pH, a strong detrimental effect on coagulation was observed.This was found to be related to the effect of dissolved silica on aluminiumprecipitation w53x. If the presence of dissolved silica can promote aggregation ofinitial crystallites of aluminium hydroxide, it promotes coagulation of kaolinsuspension by alum, and vice versa.

3.6. Temperature effects

It is commonly found that hydrolysing metal coagulants perform less well at lowtemperatures. Temperature effects may be due to physical or chemical factors.Physically, water temperature may affect particle transport processes or particle

collision rates, primarily through the effect on viscosity, and thus on the mixingenergy dissipated in water. It is known that the orthokinetic collision rate greatlyexceeds the perikinetic rate due to Brownian diffusion for particles whose size isgreater than approximately 1mm. In orthokinetic coagulation, where particlecollisions are provided by fluid shear, it has been suggested that, as the viscosity


increases with decreasing temperature, the poor rapid-mixing conditions caused bylow water temperature might lead to inhomogeneous distribution of coagulantspecies in the water, which results in a poor coagulationw54x. However, Hansonand Cleasbyw55x concluded that the effects of temperature on coagulation couldnot be explained by the effect on parameters such as energy dissipation andturbulence microscale. Furthermore, much of the difference observed between alumcoagulation at 20 and 58C is related to floc strength and not to turbulent flow fieldcharacteristics. It was found that the system chemistry was more important than thechoice of energy input parameters at different water temperatures.Chemical influence on coagulation by hydrolysing metal salts due to variation of

water temperature may be related to the effect on hydrolysis reactions, precipitationand solubility of the metal hydroxide.Kang and Cleasbyw56x reported that, decreasing water temperature from 25 to 5

8C results in lowering the minimum solubility of Fe(OH) by 0.2 log unit and3

shifting it approximately 0.4 pH unit to the alkaline side. Dempseyw57x showedthat, for a temperature change from 25 to 18C, the theoretical solubility minimumof Al(OH) shifts 0.6–0.8 pH units to the alkaline side and is lowered by3 s( )

approximately 0.7 log unit. Similar observations were reported by Driscoll andLettermanw58x and Hem and Robersonw59x.Water temperature may also affect rate of the metal ion hydrolysis reactions and

establishment of equilibrium of solid phase with dissolved species in solution. Withincreasing temperature and pH, the rate of hydrolysis of Fe(III ) salts is acceleratedand the formation time of soluble polymeric iron species is reported to decreaserapidly w4,60x. In addition, the rate of approach to the equilibration concentration ofaluminium hydroxide is significantly enhanced with increasing temperaturew59x.However, Morris and Knockew61x reported that rate of aluminium or iron(III )precipitation was not significantly affected over a temperature range of 1–238C.It has been observed that, at low temperature, optimal coagulation pH shifts to a

higher value when using Fe(III ) or Al(III ) coagulantsw5,55,62x. If pOH was keptconstant, coagulation kinetics with ferric sulfate at 20 and 58C were nearly identicalw55x. Van Benschoten and Edzwaldw5x found that the pH at which Al precipitationoccurs increased from 4.6 at 258C to 5.5 at 48C; and that the isoelectric point ofAl precipitates shifted from pH 7 at 258C to pH 9 at 48C. It appeared that Alprecipitates at 48C maintained a positive charge at higher pH than at 258C.It is possible that the temperature dependence of hydrolysis equilibria can also

influence the species adsorbed on particles, and thus the coated particle surfaceproperties. Gray et al.w14x suggested that slowing the rate of hydrolysis andprecipitation reactions of metal coagulants in lower-temperature water is beneficialto some conditions, probably due to permitting hydrolysed species to react moreextensively with particles. In contrast, Morris and Knockew61x concluded that theeffect of low temperature on coagulation efficiency in terms of turbidity removalwas not related to reduced metal hydroxide precipitation rates. Instead, the poorturbidity removal at low temperature may be attributed to the floc characteristics.In cold water conditions, flocs are formed rather slowly and are smaller than those


formed at normal water temperaturew61,63x. Hanson and Cleasbyw55x found thatboth iron and alum flocs formed at 58C with kaolin clay, even at constant pOH,were much weaker that those at 208C.It was found that, under low-temperature conditions, Fe(III ) coagulants can

produce much better turbidity and colour removal than alumw61,64x. The betterperformance of Fe(III ) over alum is believed to be due to the faster rate ofprecipitation with Fe(III ) and the formation of larger flocs under low-temperatureconditions. Haarhoff and Cleasbyw65x found that FeCl is a better coagulant than3

alum for turbidity removal at low temperature in direct filtration at the same molardosage of Fe and Al . Hanson and Cleasbyw55x also observed that, at 58C,3q 3q

ferric sulfate coagulation of kaolin suspensions yielded better efficiency than alum,the iron floc being significantly stronger than the alum floc at temperatures 5 and20 8C. Pre-hydrolysed products are also more effective than conventional coagulantsat low temperatures(see below).

4. Pre-hydrolysed coagulants

As well as traditional coagulants, based on Al and Fe salts, there are now manycommercial products that contain pre-hydrolysed forms of the metals, mostly in theform of polynuclear species(see Section 2.2). In the case of Al, most materials areformed by the controlled neutralisation of aluminium chloride solutions and aregenerally known as polyaluminium chloride(PACl). It is believed that many ofthese products contain substantial proportions of the tridecamer Al . Some infor-13

mation on the preparation of such materials and those based on iron is availablew15x, but some important details are commercially sensitive and not easily found.In the case of aluminium sulfate, it is difficult to prepare pre-hydrolysed forms withhigh B values, because sulfate encourages hydroxide precipitation. The presence ofsmall amounts of dissolved silica can substantially improve the stability up toBvalues of approximately 1.5w66x. The resulting product is known as polyaluminos-ilicate sulfate(PASS).Pre-hydrolysed materials are often found to be considerably more effective than

the traditional coagulantsw67x. PACl products seem to give better coagulation than‘alum’ at low temperatures and are also claimed to produce lower volumes ofresidual solids(sludge). Because they are already partially neutralised, they have asmaller effect on the pH of water and so reduce the need for pH correction.However, the mechanisms of action of PACl and similar products are still not wellunderstood.Most explanations are in terms of the high charge associated with species such

as Al and the consequent effectiveness in neutralising the negative charge of13

colloids in water. The relatively high stability of Al means that it should be more13

readily available for adsorption and charge neutralisation at around neutral pH,whereas conventional ‘alum’ undergoes rapid hydrolysis and precipitation. However,charge neutralisation cannot be the only mechanism of destabilisation, otherwiseonly ‘Zone 2’ coagulation would occur, which is less effective than sweepflocculation. It is still not clear what role hydroxide precipitation plays in the action


of pre-hydrolysed coagulants. Sulfate also plays an important role in precipitationwith PACl w68x.It has been shown recentlyw42x that the volume of sediment produced in

coagulation of clay suspensions by commercial PACl products is proportional to thecoagulant dosage. This implies that some form of sweep flocculation is operating,since the volume of hydroxide precipitate would be expected to depend on theamount of coagulant added.From dynamic studies with a range of hydrolysing coagulantsw42x it has been

shown that PACl products give more rapid flocculation and stronger flocs than for‘alum’ at equivalent dosages. However, in all cases, floc breakage was irreversibleto some extent, as found previously for alum(Fig. 10). This is further evidencethat the pre-hydrolysed products do not act simply by charge neutralisation and thatsome form of sweep flocculation is involved. It is very likely that the nature of theprecipitate differs in the case of PACl materials, as has been shown for precipitationfrom solutions of Al (see Section 2.3), but a detailed understanding of flocculation13

mechanisms with these coagulants is still lacking.

5. Interaction with dissolved organic matter

So far, we have only considered the removal ofparticles from water byhydrolysing coagulants. However, many natural waters contain dissolved organicsubstances, which also need to be removed. Natural organic matter(NOM) in watermay impart undesirable colour to water and some constituents can form carcinogenicsubstances when water is chlorinated. NOM consists of a huge variety of organiccompounds including simple sugars, amino acids, organic acids, proteins and manyothers. In most cases, so-called ‘humic substances’ are major components of aquaticNOM. This term covers a range of complex organic materials that exist in all soiland water environments and are thought to originate from decomposition of plantand animal remains. They are classified according to their aqueous solubility, withfulvic acids being more soluble than humic acids. Fulvic acids predominate in mostwaters and have lower molecular weight, typically in the region of 500–2000.Humic acids are reported to have a wide range of molecular weights, from a fewthousand up to as high as 100 000, although the high values may be a result ofaggregation.Essentially, humic substances can be regarded as natural anionic polyelectrolytes,

of rather indeterminate structure. They have various functional groups, includingcarboxylic and phenolic, and a framework of randomly condensed aromatic rings.A hypothetical structure is shown in Fig. 11. Because of the ionisation of carboxylgroups, humic substances will have anionic charge at pH values higher thanapproximately 4 and are generally soluble under these conditions.It has long been known that humic substances can be effectively removed from

water by hydrolysing coagulants and there have been many studies on this subjectw69,70x. Humic substances adsorbed on aquatic particles can give enhanced stabilityand increased coagulant dosages may be neededw71x. In fact, for waters with highconcentrations of organic matter, there is often a stoichiometric relation between the


Fig. 11. Hypothetical molecular structure of humic acid, showing important functional groups.

organic content(usually measured asdissolved organic carbon, DOC) and therequired coagulant dosagew72x. Optimum pH for removal of dissolved organics isusually rather less(typically pH 5–6) than that for removal of suspended particles.When the coagulation process in water treatment is specially modified to ensuregood removal of organic matter it is often known asenhanced coagulation.There are two likely mechanisms for the removal of humic substances by

hydrolysing metal coagulants:

● Binding of metal species to anionic sites, thus neutralising their charge andgiving a reduced solubility. For fairly large molecules, this can lead to precipitationof the metal–humic complex, to form particles that can be removed bysedimentation or filtration.

● Adsorption of humic substances on amorphous metal hydroxide precipitate. AtpH values approximately 5–6, the humic substances are negatively charged andAl and Fe hydroxides are positively charged, which would give strong adsorptionand some charge neutralisation. Pre-formed ferric floc has been shown to be agood adsorbent for humic substancesw73x.

In many practical cases it is not easy to distinguish between the precipitation andadsorption mechanisms. In a recent studyw74x both were shown to operate,depending on pH and coagulant dosage. Humic substances isolated from lakesediments in Xi’an, China were coagulated with aluminium sulfate at different pHvalues and over a range of dosages. The removal was monitored by the reductionin UV absorbance(at 254 nm) after a standard coagulation and sedimentationprocedure. The electrophoretic mobility of the destabilised particles was determinedimmediately after rapid mixing of the coagulant. In this case the EM values wereconverted to zeta potentials.The results in Fig. 12 show the residual UV absorbance and zeta potentials as

functions of alum dosage at pH 5.0. The alum dosage is expressed on the basis of


Fig. 12. Removal of aquatic natural organic matter by coagulation with alum at pH 5, showing residualUV absorbance at 254 nm and zeta potentials of coagulated material. Alum dosages are shown relativeto organic carbon content of the water sample(mg Alymg TOC).

humic substance concentration as mg Alymg of total organic carbon(TOC). Thereis a region of significant reduction in UV absorbance between approximately 0.1and 0.2 mg Alymg TOC, which corresponds very well with the point at which thezeta potential reverses sign. This is strong evidence for a simple charge neutralisationmechanism under these conditions. At higher alum dosages there is another regionwhere the absorbance is reduced, but this is not correlated with any significantchange in zeta potential. The implication is that adsorption on a hydroxide precipitateis responsible for the removal.At pH 7, the results are quite different. Fig. 13 shows that there is an appreciable

reduction in UV absorbance at approximately 0.2 mg Alymg TOC, which graduallyimproves at higher dosages. The zeta potential remains negative over the wholedosage range, but approaches zero at the highest dosages. It is very likely that theremoval of humic substances under these conditions is entirely by adsorption onprecipitated aluminium hydroxide. The reduction in absorbance is slightly greater atthe higher pH, especially at the higher alum dosages.Pre-hydrolysed coagulants are also effective in removing dissolved organic matter

from water, but in this case they often show no significant improvement overtraditional coagulantsw14x. It is known that polynuclear Al species, such as Al ,13

can be depolymerised in the presence of organic matter in natural watersw75x.

6. Conclusions

Although the broad principles governing the action of hydrolysing coagulants arereasonably well understood, there are several important gaps in knowledge, whichare of both fundamental and practical interest. For simple aluminium and ferric saltsat low dosages, it is well established that charge neutralisation can be an effectivemeans of destabilising colloidal particles. The precise nature of the cationic speciesis not known in detail, but it is likely that some form of surface precipitation is


Fig. 13. As for Fig. 12, but for coagulation at pH 7.

involved. Charge neutralisation can only be achieved at pH values below theeffective i.e.p. of the metal hydroxide(approx. 8–9 in the case of Al). As the i.e.p.is approached, larger coagulant dosages are needed to achieve charge neutralisation.At excess coagulant dosage particle charge is reversed and restabilisation occursand in many cases the range of effective coagulant dosages is quite narrow. Whencharge neutralisation is the only destabilisation mechanism, and for rather dilutesuspensions, then the rate of coagulation can be low and relatively small aggregatesare formed, giving poor removal efficiency.At higher coagulant dosages, bulk precipitation of metal hydroxide occurs, which

can give large flocs of rather open structure. Impurity particles originally present inwater become incorporated in these flocs and can be very effectively removed. This‘sweep flocculation’ mechanism is generally more rapid than coagulation by chargeneutralisation and gives larger flocs. There is often no clear correlation betweenparticle charge and the onset of sweep flocculation. In some cases, there is asignificant lag time between dosing the coagulant and the onset of appreciable flocgrowth, which is thought to be related to the time required for aggregation of theprimary precipitate particles. This process is greatly dependent on the pH of thesolution and the presence of certain anions. Flocs formed by hydroxide precipitatesare rather weak and breakage is not fully reversible.Pre-hydrolysed coagulants are often more effective than simple Al and Fe salts.

Part of the reason has to do with highly charged cationic species, such as Al ,13

which are rather stable and have a better opportunity to adsorb on negative colloidsand neutralise their charge. However, at practical dosages of these coagulants, thesolubility of the metal hydroxides is greatly exceeded and it is highly likely thatprecipitation plays an important role. The improved performance of these materialsis probably due to the different nature of the precipitate formed, although moredetailed studies are needed.As well as effectively removing colloidal particles, hydrolysing coagulants can

also be used to remove dissolved natural organic matter from water. In this case, a


precipitation–coagulation mechanism can be important, in which anionic sites onthe organic molecules are neutralised by hydrolysed metal species, so that thesolubility is greatly reduced. Adsorption of organic matter on amorphous hydroxideprecipitate can also be effective, although this generally requires higher coagulantdosages. The relative importance of these mechanisms depends primarily on pH.

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