Aluminum and Iron(III) Chemistry - ACS Publications - American

26 Aluminum and Iron(III) Chemistry

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Some Implications for Organic Substance Removal J. Y. Bottero Equipe de Recherche sur la Coagulation-Floculation, UA 235 du Centre National de la Recherche Scientifique et GS Traitement Chimique des Eaux, BP40 54501 Vandoeuvre Cedex, France J. L . Bersillon Laboratoire de Recherche de la Lyonnaise des Eaux, 38 rue du Président Wilson, 78230 Le Pecq, France

Removal of organic substances by metallic salts is one of the most important processes in the water-treatment industry. Some of the subsequent treatment phases depend on the efficiency of the coagulation-flocculation phase. Development of new coagulant-flocculant agents is one way to improve the removal of organic compounds by coagulation. A new generation of aluminum (Al ) and iron hydroxides is being developed by prehydrolyzing aluminum or iron chloride salts to form active species. These polymers have greater reactivity than alum in removing organic material when the solution pH is lower or higher than the sweepflocculationzone. In acidic solutions (pH 6.5 by adsorption onto large particles. The polycations are stable over pH and time. Their structure is open and loose (fractal), ensuring a large available surface area for complexing or adsorbing molecules. The stability constants of organic compound-Al polycation complexes are higher than the stability constants of organic compound-Al or Fe monomer complexes. Prehydrolyzed coagulant-flocculant agents can be designed to adapt the treatment to specific raw-water characteristics. 13

0065-2393/89/0219-0425$06.00/0 © 1989 American Chemical Society

In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

426

AQUATIC HUMIC SUBSTANCES

NATURAL ORGANC I MATERA ILS ARE NOW THE PRM I ARY TARGET of wat treatment (I). These compounds (responsible for color, taste, and odor in water) have a potential impact on public health. Generally, these compounds originate in the soil and appear as humic or fulvic acids (2, 3). C N M R spectroscopy reveals that these complexes contain —50% aromatic - C O O H groups, —20% carbon bonded to phenolic O H , polysaccharides, proteins, and fatty acids. Their major building blocks are formed by structures like benzenecarboxylic acid (4). The chemistry of humic substances in water is somewhat different; carbohydrates and proteins are more prominent (5). Coagulation-flocculation, the most widely used treatment process for removal of organic substances in surface water, has received little attention as a potentially powerful process for removing soluble organic contaminants. Aluminum or iron flocculants are used in drinking-water treatment. From an engineering point of view, a better understanding of the interactions involved in organic-substance removal should lead to models that can be used in optimization of the process. Because coagulation is the first step in water treatment, its improvement will have large consequences on subse quent treatments (activated-carbon adsorption, ozonation, chlorination). The mechanisms involved (neutralization, flocculation, and adsorption on floes) and the efficiency of organic-compound removal by coagulation depend on the coagulant chemistry and structure (which vary with pH) and the oxidation pretreatments (ozonation). This chapter reviews the chemistry and structure of the mineral coagulants used (Al and Fe) and their effects on complexation, adsorption, and the relative kinetics of interaction with the organic functions in humic substances.


1 3

Mineral Coagtdant-Fhcctdant The aluminum coagulants in use are essentially alum ( A l ( S 0 ) · n H 0 ) . However, alternative coagulants have been developed in recent years. A l u minum chloride salts partially neutralized before use (polybasic aluminum chloride or PBAC) and hydrolyzed ferric salts are extensively studied in Europe. Some of them were successfully implemented in full-scale plants 10 years ago. In Japan, a partially hydrolyzed aluminum chloride ( [ O H ] ^ ^ / [Al] —1.9) containing sulfate ions (PACS) has been developed (Taki Fertilizer Company). P B A C can be optimized with respect to the quality of the raw water (temperature, organic content) by changing the hydrolysis ratio [ O H W [ A l ] (6). The ferric products are relatively more recent. Fe ions can be partially hydrolyzed before use, but the control of hydrolysis during preparation is a major problem (7). A new generation of well-controlled partially hydrolyzed iron coagulant is under study. 2

4

3


2

26.

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Aluminum and Iron(IU) Chemistry

427

Aluminum Coagulant. The use of metallic coagulants presents a problem because little is known about the nature of the substance that precipitates when the solution is rapidly diluted at near-neutral p H . Many transient polymeric species have been suggested (8, 9) to explain the trans formations from A l ions to solid-phase compounds. Most of these data on transient polymeric species were obtained from highly indirect methods of investigation. Spectroscopic methods and radiation scattering have revolu tionized aluminum chemistry for systems that are far from equilibrium, as in the case of water treatment. Downloaded by UCSF LIB CKM RSCS MGMT on September 4, 2014 | http://pubs.acs.org Publication Date: December 15, 1988 | doi: 10.1021/ba-1988-0219.ch026

3 +

27

C h e m i s t r y of Coagulants and Floes. Solution-state A1 N M R pat terns of aluminum sulfate (or alum) dissolved in water exhibit two resonance peaks. The first, at 0 ppm, is relative to the monomers ( A l , A l , and A l ) in octahedral symmetry; the other corresponds to the complex Al(S0 ) (Figure la). The chemistry of PACS is somewhat different. The matrix so lution (—2.2 mol/L) contains more than 60% of the aluminum present as Al and 2-3% as dimers (Figure lb). The rest of the aluminum is layered octahedral medium-sized polymers. Dilution or a sharp increase in p H trig gers the formation of large particles or precipitates with the same local range order (i.e., aluminum atoms in octahedral coordination, as alum or the me dium polymers in the matrix solution, as shown by A l N M R in solid-state pattern in Figure 2). Alum makes a direct transition between monomers and precipitates. The sulfate ions form inner complexes with aluminum and inhibit the formation of transient polymeric species. 3 +

2 +

+

+

4

3 +

2 7

ALUM

(a) monomers

80

60

40

20

0

-20 ppm

27

Figure 1. Solution-state Al NMR spectra of alum (a) and PACS (b).


428


27


Figure 2. Solid-state Al NMR spectra of alum or PACS after precipitating at neutral pH. 100

50 0 -50-100 (ppm)

The products formed by hydrolyzing pure aluminum chloride solution are quite different. In these solutions the hydrolysis of A l with sodium hydroxide or sodium carbonate (10-12) yields the formation of soluble polycations. The most important parameters that govern the synthesis are fl = [ O H ] / [ A l ] , which varied from 0.5 to 3; [Al] from 10" to 0.2 m o l / L ; and Γ, the age of the solution. Solution-state A l N M R at 23.45 M H z (JO, 12) and 93.84 M H z (13) shows spectra that correspond to octahedral and tetrahedral symmetry of aluminum (Figure 3). From R = 0.5 to 2.3-2.4, a signal appears at 79.9 ppm from the reference (Al0 ~), corresponding to aluminum ions in octa hedral coordination. This resonance peak, which corresponds to monomers A l ( H 0 ) , A l ( O H ) ( H 0 ) , and A l ( O H ) ( H 0 ) (14), does not shift, but its width at half-maximum increases with R. The peak located at 17 ppm from the reference was attributed to the tetrahedral aluminum of the polymer Al (OH) Al 0 (H 0) _ . Its structure was described by Johansson in 1962 (15). The central A l atom is in tetrahedral coordination, whereas the 12 other aluminum atoms are octahedral (Figure 4). They are not observed because they do not have a symmetrical environment (12). The relative concentrations of each one of the species can be calculated from the areas under the peaks; the precision of the measurements is ±10%. For example, for A l = 10 m o l / L , Figure 5 shows that A l concentration corresponds approximately to 90% A l for fl = 2.0 or 2.2. Beyond these values, some aluminum is not detected by solution-state N M R because colloids are formed. The turbidity of the suspensions increases from 1 nephelometric turbidity unit (NTU) (R = 2.2) to - 5 0 N T U (fl = 2.5); they precipitate rapidly for fl > 2.6. A l solid-state magic angle spin ning (MAS) N M R spectra at 104 M H z relative to the particles formed from fl ^ 2.5 (Figure 6) look like the spectra obtained in solution state. The resonance band at —63 ppm is related to the tetrahedra of A l . The relative content of A l in the particles is obtained by comparing nor malized areas of the peak at 63 ppm with the peak resonance at —0 ppm (octahedral symmetry) (16). The data (Table I) show that the A l content in the small particles (fl = 2.5) and large particles (fl = 2.6-2.8) correspond nearly to 100%, at least for Τ < 1 h (13, 16). The A l content does not vary with time Τ for fl =2.6-2.8. At fl = 3 the precipitated solid is bayerite. 3 +

4

added

2 7

a d d e d

4

3 +

2

2 +

6

2

V I

1 2

2

I V

x

+

5

2

4

( 3 1 X ) +

4

2

12

x

1

1 3

added

2 7

1 3

1 3

1 3

1 3



26.

BOTTERO & BERSILLON

429

Aluminum and lron(IH) Chemistry

FU2.3 R =

2.4

AI(H 0)|

FU2.5

2

+

AI 0 (OH)J; 1 3

4

80ppm

J

Figure 3. Solution-state Al NMR spectra typical of PACl as a function of R = [OH]/ [Al]. The peak at 80 ppm from Al(H 0) is an integration reference needed to calculate the relative concentration of each one of the species. (Reproduced from ref. 12. Copyright 1980 American Chemical Society.) 27

3+

2

6

IV

VI

Figure 4. Structure of the Al OMi (OH)24 (H 0) polycation. (Reproduced with permission from ref. 16. Copyright 1987 Academic.) 2

7+

2

l2


430


100

"AI13" ι Polymers


0.1

M 50

_

40

t

30

20 H10

•a

5

0

R [OH] / [Alt ] Figure 5. Distribution of the different hydroxy aluminum species (monomers and Aln) in PACl solutions from NMR data, versus R = [OH]/[Al] ( ) and subsequent turbidity variation ( ).

AI

VI

(ppm) I

I

80

I

I

40

I

I

0

I

I

-40

I

1

-80

27

Figure 6. Al MAS NMR spectra of colloids in R = 2.5 and 2.6 solutions versus time. The peak at 63 ppm is rehtive to the tetrahedral coordinated Al ofAl . The large peak at ~0 ppm is the octahedral coordinated Al of Al and other species when the age of the solutions increases. l3

13


26.

BOTTERO & BERSILLON

Aluminum and lron(Hl) Chemistry

Table I. Concentration of A l i in Colloids Spectrum Sample Al

431

3

a


13

1 2 3 4 5 6 7 8 9 10

R R R R R R R R R R

= = = = = = = = =

2.5; 2.5; 2.5; 2.5; 2.6; 2.6; 2.8; 2.8; 3.0; 3.0;

Τ Τ Τ Τ Τ Τ Τ Γ Τ Τ

= = = = = = = = = =

8.8 4.7 4.2 3.9 8.5 6.7 6.7 6.6 1-2 (bayerite) 0 (bayerite)

lh 2h 1 day 1 week lh 1 week lh 1 week lh 1 week

27

NOTE: Colloids obtained from Al MAS NMR patterns and cal culated from A F / ( A F + A H ratio. The theoretical ratio is 1/13 = 7.7%. FromR = 2.8, theflocculationoccurs in the solution. V1

°Αΐ /Σ(ΑΡ + A1 ). ιν

The A l content does not change very much (—80-100%) from R = 2.2 to 2.8. This behavior implies that the charge of A l polymers decreases as p H or R increases. The modifications attributable to dilution and p H shock that characterize the use of aluminum salts in water treatment have been evaluated. The dilution, in demineralized water, of R = 2 (—90% of isolated A l ) , R = 2.5 (-40% of isolated A l and 60% of aggregated A l ) from A l = 10" to 10" m o l / L at p H 6.5, 7.5, and 8.5 led to formation of floes. Solid-state M A S A l N M R spectra of these floes precipitated at p H 7.5, [Al] = 10 m o l / L , and from R = 2.0 and 2.5 (Figure 7) are similar to those of floes precipitated in acidic solutions (Figure 6). Table II summarizes the A l content of the floes at various precipitation p H and aging times. The A l content decreases as precipitation p H and aging time increase. The polyaluminum chloride (PAC1) coagulants are different from sulfate or polyaluminum chlorosulfate (PACS) coagulants because A l exists only in the PAC1 solutions and because the dilution and p H increase do not modify the local range order (i.e., the A l ions are not immediately destroyed). 1 3

1 3

13

1

1 3

3

13

2 7

3

1 3

1 3

1 3

1 3

Structure of the Coagulants and Floes. The structure of active species in matrix solutions and the relative precipitated floes at neutral p H have a great consequence on removal of organic matter and solids. Such structures can be investigated by radiation scattering (small-angle X-ray or neutron scattering) techniques. These methods permit definition of the size, shape, and structure of homogeneous particles or agregates. If fractallike particles are present, the slope of the logarithm of the scattering intensity versus the logarithm of the wave vector can be expressed as

(1)


432



100 50 0 -50-100 (ppm)

27

Figure 7. Al MAS NMR spectra ofPACl floes, R = 2 and 2.5, precipitated at pH 7.5 after diluting at 1Q- mol/L. 3

Table II. pH6.5 Floccufont 10 min 1 h 80 80 R = 2.0 1 100% 80 75 R = 2.5 1 100%

100 50 0 -50*-100 (ppm)

Al» Content in Precipitated Floes pH 8.5 pH 7.5 10 min 1 h 24 h 24 h 10 min lh

24 h

80 Ν/Α

I

80

80

60

45

35

80

60

60

60

35

25

100% Ν/Α

l

3

NOTE: Floes precipitated from R = 2 and 2.5 after diluting Al solutions from 10 to ΙΟ" M at pH 6.5, 7.5, and 8.5, as calculated by ^Al MAS NMR experiments.

where 2 Θ and λ are the scattering angle and the wavelength, respectively. This relationship yields the fractal dimension, Df. Dp which varies between 1 and 3, characterizes the structure of the aggregates (i.e., their density). Low, medium, and high Devalues correspond to linear, branched, and dense aggregates, respectively. P B A C coagulants have been extensively studied. In R = 2 solution, the A l polymers have a radius of 12.6 À, which cor responds to the radius of the polymers plus the thickness of the hydration shell (17). In R = 2.5 solutions, A l aggregates have a radius of gyration (r) of —90 Â and a fractal geometry with a fractal dimension D —1.43 (16, 18). This coagulant solution is made of isolated and aggregated A l (Figure 8). In R = 2.6 solutions, the A l aggregates are bigger (r > 300 A) and have a fractal geometry with a fractal dimension D —1.7-1.8. After floc culating at —pH 7, these aggregates form large floes of A l with a fractal dimension D —1.8 (29). A very loose structure that provides a large surface area for adsorption of organic molecules is certainly more useful than a dense 1 3

1 3

f

1 3

1 3

f

1 3

f


26.

BOTTERO & BERSILLON

Aluminum and lron(III) Chemistry

433

microporous aggregate on which a large part of the surface area is not ac cessible.

Specific Surface Area of Floes and Surface Charge.

Alum and

PAC1 floe particles obtained at p H >6 present large specific surface areas that have been measured by using adsorption of anionic surfactants (20 and Rakotonarivo, C N R S 1987, unpublished data). The specific surface area ob tained with PAC1 depends on p H of precipitation and on initial H = [OH]/[Al], The specific surface areas decrease as p H and R increase (Table III). The specific surface area of alum floes is much lower. The surface charge distribution can be evaluated by electrophoresis and potentiometric titration. From electrokinetic potential (ζ) and potentiometric titration data in N a C l


a d d e d

Figure 8. Schematic representation of PACl R = 2.5 solution.

pH 6.5 7.5

Table III. Specific Surface Area Values R =2 R = 2.5 587 774 601 540

R =0 1100 800

NOTE: All values are in square meters per gram; calculated from the adsorption isotherms of long-alky 1-chain soaps (21) or long-alkylchain sulfonates (20).


434


electrolyte, the difiuse-layer charge σ and the surface charge σ can be calculated. Then the active site number N and the surface reaction constant K, can be evaluated. Four equilibria between surface sites and liquid could be postulated for the purpose: ά

0

s

+

A l ( O H ) t = b Al(OH) + H

+

(2)

2

Al(OH) A

ART + H

A l ( O H ) * C l - ±=b A l ( O H ) Downloaded by UCSF LIB CKM RSCS MGMT on September 4, 2014 | http://pubs.acs.org Publication Date: December 15, 1988 | doi: 10.1021/ba-1988-0219.ch026

2

AlONa

2

+

+

+

(3)

+ CI"

±â* A l O - + N a

(4)

+

(5)

Reactions 4 and 5 correspond to the complexation of positive and neg ative sites by C l " and N a ions. The variables N and K are optimized to fit ζ - ρ Η and σ - ρ Η curves (21). The active-site-number density and surfacereaction p K of floes precipitated from PAC1 solutions Η = 2.5 or 2 in N a C l electrolyte have been calculated by computer (21). The results (Figure 9) show that at neutral p H the sites are predominantly neutral: 60% is Al(OH); 20-40% of positive sites are A l ( O H ) complexed by C l " ions. The physical and chemical relations between PAC1 solutions and PAC1 floes are summarized in Figure 10. Alum and unhydrolyzed aluminum chlo+

s

t

0

+

2

loot

Figure 9. Distribution of the active surface sites of PAClfloesversus pH in deionized water.



26.

Aluminum and Iron(III) Chemistry

BOTTERO & BERSILLON

435

Figure 10. Representation of the physical and chemical evolution of PACl solutions versus R = [OH]/[Al], dilution, and pH increase. ride coagulant solutions are extensively modified by dilution and p H increase during treatment; however, this does not happen with P A C l and only partly with PACS. Iron Coagulant. The chemistry of iron ions is not as developed as that of aluminum because no local probe exists except X-ray absorption fine structure (EXAFS), which can play the same role as N M R . Nevertheless, different-sized colloids ( F e ( H 0 ) , F e ( H 0 ) , F e ( H 0 ) ) exist in equi librium with monomers (22-25). The size of these colloids (5-40 nm) depends on the time of aging. In supersaturated iron(III) nitrate solutions, studied by methods such as base titration, ultracentrifugation, and IR spectroscopy (26-28), the precursor for the formation of the organized phase is amorphous. The critical nucleus is postulated to contain 16-32 atoms. Small polycations 3 +

2

6

2 +

2

5

+

2

4


436

AQUATIC H U M I C SUBSTANCES

5+

3


6 +

such as [ F e ( O H ) ] and [ F e 0 ( O H ) ] , in which octahedra share edges and corners, control both nucleation and growth steps by associating in larger polymers (29). Direct methods were used to follow the size and the growth of iron(III) clusters during the first steps of aging, just after the addition of base. Data from small-angle X-ray scattering were recorded every 50 s. Small polymers were detected (from 0.5- to 0.9-nm diameter as time increased). They formed linear clusters by aggregating the small polymers (30). The local range order as revealed by the E X A F S technique is similar to that of goethite, as R = [OH]/[Fe] > 1.5 (31). The radial distribution function (Figure 11) reveals the presence of a first distance at —0.2 nm, characteristic of F e - O or F e - C l distances. As hydrolysis increases, a second and a third distance appear to indicate F e - F e distances. The F e octahedra are shared either by edges (0.295 nm) or by vertices (0.330 nm). From R = 1.5 the spectra exhibit the same shape as that of goethite. The precipitates formed at near-neutral p H have the same structure as goethite, a short range order similar to that of polymers in acidic solutions (Combe, J. M . , C N R S , 1988, personal com munication). 4

4

4

Mechanisms of Organic Material Removal The removal of humic material can be achieved through complexation and adsorption. These mechanisms depend on p H and the physical chemistry

Figure 11. Radial distribution function of iron(III) hydroxide solutions versus R = [OH]/[Fe] through EXAFS technique.



26.

BOTTERO & BERSILLON

Aluminum and lron(HI) Chemistry

437

of the mineral coagulant. They are detected by indirect evidence. Extensive projects have measured color and total organic carbon removal after treat ment with alum, P A C l , or P A C S . Jar-test experiments with organic raw water have led to a design and operation diagram with alum as coagulant (32, 33). The diagram shows restabilization zones and two regions of removal that are located approximately at p H 5.5 and -4.5 7 adsorption occurs on large aluminum floes; the stoichiometry no longer exists, and the floe surface reacts the same as any oxide surface (21). The choice of coagulant for the removal of fulvic acids depends on the F A type and concentration, p H , mineral complexing concentration ( S 0 ~ or P04 ") (32). The same kind of result has been obtained with P A C S ; this shows a clear relationship between the size and charge of the floes formed (35). Organic-substance removal by coagulation in the presence of montmorillonite and kaolinite clay particles has been investigated by Dempsey et al (36). The benefits of treatment with P A C l instead of alum have been investigated as a function of p H , raw water composition, and mixing con ditions. At low contaminant concentration, P A C l is a better coagulant than alum at p H values 7. The relative benefit of P A C l over alum de creases at higher contaminant concentration. Turbidity slightly influences color removal. The presence of fulvic acids increases the dose of coagulant required to remove turbidity. Ozone treatment has been investigated as a way to modify the nature of raw organic material and increase its affinity for coagulants. Low concen trations of ozone (0.2-0.6 mg/mg of dissolved organic carbon) are sufficient to oxidize the adsorbable high-molecular-weight fraction (37). A n excess of ozone dosage decreases the affinity of humic substances for floes, presumably because of small units humic formation. In the presence of aluminum the metal-humate complexes are more soluble after ozonation, and removal efficiency can be lower (38). The 1:1 compounds formed through ozonation treatment excluded the formation of dicarboxylic acids (39). The removal of dibutyl phthalate and diethyl phthalate (one of the principal groups of ozon ation products of humic materials) by A l or Fe coagulation (Figure 12) is 2

4

3



438


Concentration at equilibrium Figure 12. Adsorption isotherms of DBF (dibutyl phthalate) and DEP (diethyl phthalate) onto Fe (R = 2.5) and PACl hydroxide (R = 2.5)floesat pH 8 and Τ = 20 °C. (Reproduced with permission from ref. 40. Copyright 1981 Pergamon.) organic-size dependent. The amount adsorbed increased with the molecular weight because of the increasing strength of the lateral bonds between mol ecules in the adsorbed phase (cooperative adsorption) (40). In a laboratory study that used representative low-molecular-weight species such as salicylic acid, glycyltyrosine, cellobiose, and N-acetylglucosamine, low-dosage ozone yielded polymerization into macromolecules. Jar tests performed with alum showed improved organic removal (41). The large discrepancies between research reports show that more basic inves tigations are needed. The mechanisms of the ozonation-coagulation process are not well understood. To approach the physical origin of such differences, it is necessary to evaluate the origin of the relationship between organic functions and alu minum cations or polycations (complexation), as well as the parameters gov erning the adsorption. In the literature that deals with complexation and adsorption of organic substances by aluminum, the major data concern alu minum monomeric species. Complexation. The origin of the humic-substance affinity for alu minum floes is certainly substituted hydroxybenzoic acids (42). Many studies have been done of the environmental implications of the aluminum- or iron-organic substance complexation. The aluminum-complexation reaction by salicylate (43), sulfosalicylate (44), acetate (45), and fulvic acids (46) is of metal-ligand complex origin, as proposed by the Eigen theory (47). Kinetically, the first step is an outer-sphere complex formation. The second step, much slower, is an inner-sphere complex formation accomplished by dis placing a water molecule from the hydration shell. The complexation of A l In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

26.

Aluminum and lron(Hl) Chemistry

BOTTERO & BERSILLON

439

monomers (pH monomer*A + H

+

(6)

+

where the acid A H is dissociated to form A " and H and K = k lk_ . The rate constants k that characterize the complexation with salicylate ions are of the order of 1, 1000, and 3000 m o l s with A l , A l ( O H ) , and A l ( O H ) , respectively. The stoichiometry is always 1:1. The stability con stant of the complex is Κ ~ 10 . The results are quite different with a mixture of monomers and A l and with a pure A l solution (48). The formation kinetic constant between A l and the salicylate ions is k ~ 4000 mol V , k_ ~ 10 s" , and the stability constant Κ = 10 . These data show that the complexation kinetic with A l is larger than with monomers and that the complexes A l -organic acids are more stable. The stoichiometry of such complexes is six salicylate ions per A l ; the charge of the complex is ~ 0 . In the presence of minerals, more coagulant is needed to ensure a good removal of organic materials (36). Two kinds of reaction take place. The coagulant is adsorbed onto solids and organic compounds are adsorbed onto coagulants. The kinetic of coagulation-flocculation of minerals by aluminum is fast, because of the high affinity of A l for solid surfaces present in water (clays, silica). The half-time of silica aggregation by A l is less than 3 X 10~ s (49). From studies of complexation (43-45), the half-time reaction between small organic acids and A l is higher than 0.07 s. This could mean that a large part of the coagulant, because it is first adsorbed onto solids, is not available for the adsorption or complexation of organic substances. (

{

{


{

1

1

3 +

2 +

+

2

5

1 3

1 3

1

1 3

p

1

30

p

1 3

13

13

3

A d s o r p t i o n . In many water-treatment plants the ρ H is rarely lower than 6.5, so the first step in the organic-substance removal process is ad sorption onto large particles of coagulant. Three factors influence the uptake of organic material: the specific surface area, the surface charge, and the extent of the most active (energetic) sites (50). As shown in Table III, the specific surface area of P A C l floes depends on the neutralization ratio R and the p H as measured from the adsorption of surfactants (20, 51). For alum floes in water, the specific surface area is lower: 200-400 m / g of floe (Bottero, C N R S , unpublished data). Few data exist concerning iron coagulants. It seems that 600-m /g floe particles can be obtained (52), but generally the specific surface area of hydrous iron oxyhydroxides is 100-200 m / g . The specific surface area in water has to be the highest. In P A C l floes the presence of an A l subunit (—1000 m /g) leads to a very large available surface area for the uptake of organic com pounds. If the surface area of floes obtained from R = 0 or alum salts can 2

2

2

2

1 3



440


be important, the affinity is lower for the reactions of monomers with small organic acids (43-46). The surface charge of hydroxylated mineral floes is generally positive at the p H of water (pH 6-8). In this case the adsorption is partly of elec trostatic origin. The value of the isoelectric point (IEP) or zero point of charge (ZPC) is important, and the extent of the removal can be determined at constant surface area. Differences exist between the coagulants. The sur face charge of P A C l floes is positive until p H —8.5 (21). The point of zero charge of alum floes or hydrous iron oxyhydroxides is —7 and 6.5 to —8.5, respectively (21, 52). In the case of alum, the affinity of S 0 " (nonindifferent electrolyte) for aluminum can partly mask the adsorption sites; because of this affinity, the sulfate ions would not be totally exchanged by organic substances. O n P A C l floes the weak affinity of C l " ions for the surface in creases the available site number compared with alum. Some researchers report that the interactions of humic acids and alu minum salts at neutral p H are primarily a surface complexation on floes, the anionic humate molecules are exchanged against O H , and the stoichiometry is 1:1 (one acidic group complexed by one Al) (38). This result is not obtained from the adsorption of sodium decanoate and sodium dodecanoate onto P A C l floes at 6.5 < p H < 7.5. For this kind of molecules the adsorption mechanism is mainly of electrostatic origin without O H exchange (51). If cooperative effects exist between molecules in the adsorbed phase (van der Waals at2

4

8

6

•

R . O

*

Rr2.0

ο

R=2.5

j ^

o X CM

ε

3 X9

0

2

4

6

Ce(mg/I)

Figure 13. Normalized adsorption isotherms of fulvic acids onto PACl floes versus R = [OH]/[Al] at pH 7.2 ± 0.15. The specific surface area values were obtained from refs. 20 and 21. In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.


26.

BOTTERO & BERSILLON

Aluminum and Iron(III) Chemistry

441

traction between alky] chains), they do not generate the adsorption when the adsorbate-adsorbent interaction energy is large (50). The surface of floes is energetically heterogeneous, and the distribution or extent of this heterogeneity depends on the p H and the nature of the coagulant (20, 51). A study of the removal of a river humic acid by P A C l floes at p H = 7.2 ± 0.15 (53) showed that the surface of floes precipitated from R = 2.0 and 2.5 is more efficient than the one from R = 0. For a same adsorbed amount, the equilibrium concentration decreases with R = [OH] / [ A l ] (Figure 13). It means that the more energetic sites are more extended on R = 2.0 and R = 2.5 floes than on R = 0 floes. Few data exist on in situ adsorption of humic substances onto iron floes. At p H —7, polymerized iron chloride improves humic-substance removal as compared with iron chloride, especially at low temperature (54). added

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for review January 26, 1988.

ACCEPTED

for publication May 4, 1988.


Aluminum and Iron(III) Chemistry - ACS Publications - American

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