[Advances in Chemistry] Aquatic Humic Substances Volume 219 (Influence on Fate and Treatment of Pollutants) || Aluminum and Iron(III) Chemistry

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  • 26 Aluminum and Iron(III) Chemistry 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 Prsident 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 coagu-lation-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 (Al13) 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 sweep flocculation zone. In acidic solutions (pH 6.5 by adsorption onto large particles. The poly-cations 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 com-pound-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.

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

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

  • 426 AQUATIC HUMIC SUBSTANCES

    NATURAL ORGANIC MATERIALS ARE NOW THE PRIMARY TARGET of water 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). 1 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 wil l have large consequences on subsequent 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.

    Mineral Coagtdant-Fhcctdant The aluminum coagulants in use are essentially alum (Al 2 (S0 4 ) 3 n H 2 0 ) . However, alternative coagulants have been developed in recent years. A lu 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.

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

  • 26. BOTTERO & BERSILLON 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 transformations from A l 3 + 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 revolutionized aluminum chemistry for systems that are far from equilibrium, as in the case of water treatment.

    Chemis t ry of Coagulants and Floes. Solution-state 2 7A1 N M R patterns of aluminum sulfate (or alum) dissolved in water exhibit two resonance peaks. The first, at 0 ppm, is relative to the monomers ( A l 3 + , A l 2 + , and A l + ) in octahedral symmetry; the other corresponds to the complex Al(S0 4 ) +

    (Figure la). The chemistry of PACS is somewhat different. The matrix solution (2.2 mol/L) contains more than 60% of the aluminum present as A l 3 + 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 triggers the formation of large particles or precipitates with the same local range order (i.e., aluminum atoms in octahedral coordination, as alum or the medium polymers in the matrix solution, as shown by 2 7 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.

    ALUM (a) monomers

    80 60 40 20 0 -20 ppm

    Figure 1. Solution-state 27Al NMR spectra of alum (a) and PACS (b).

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

  • 428 AQUATIC HUMIC SUBSTANCES

    Figure 2. Solid-state 27Al 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 3 + 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 d d e d / [ A l ] , which varied from 0.5 to 3; [Al] from 10"4 to 0.2 mol /L; and , the age of the solution.

    Solution-state 2 7 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 a d d e d = 0.5 to 2.3-2.4, a signal appears at 79.9 ppm from the reference (Al0 4 ~), corresponding to aluminum ions in octahedral coordination. This resonance peak, which corresponds to monomers A l ( H 2 0 ) 6 3 + , A l ( O H ) ( H 2 0 ) 5 2 + , and A l ( O H ) 2 ( H 2 0 ) 4 + (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 A l 1 2 V I ( O H ) x A l I V 0 4 (H 2 0) 1 2 _ x ( 3 1 X ) + . 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 1 mol /L , Figure 5 shows that A l 1 3 concentration corresponds approximately to 90% A l for fladded = 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. 2 7 A l solid-state magic angle spinning (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 1 3 . The relative content of A l 1 3 in the particles is obtained by comparing normalized 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 1 3 content in the small particles (fl = 2.5) and large particles (fl = 2.6-2.8) correspond nearly to 100%, at least for < 1 h (13, 16). The A l 1 3 content does not vary with time for fl =2.6-2.8. At fl = 3 the precipitated solid is bayerite.

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

  • 26. BOTTERO & BERSILLON Aluminum and lron(IH) Chemistry 429

    A I ( H 2 0 ) | +

    AI 1 3 0 4 (OH ) J ;

    80ppm J

    Figure 3. Solution-state 27Al NMR spectra typical of PACl as a function of R = [OH]/ [Al]. The peak at 80 ppm from Al(H20)63+ is an integration reference needed to calculate the relative concentration of each one of the species. (Re-

    produced from ref. 12. Copyright 1980 American Chemical Society.)

    Figure 4. Structure of the AlIVOMi2VI (OH)24 (H20)l27+ polycation. (Repro-duced with permission from ref. 16.

    Copyright 1987 Academic.)

    F U 2 . 3

    R = 2 . 4

    F U 2 . 5

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

  • 430 AQUATIC HUMIC SUBSTANCES

    100

    "AI13" Polymers

    0.1 M 5 0 _

    4 0 t

    3 0

    20 H10

    0

    a 5

    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 ( ).

    AIV I

    (ppm) I I I I I I I I I 1

    8 0 4 0 0 - 4 0 - 8 0

    Figure 6. 27Al 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 ofAll3. The large peak at ~0 ppm is the octahedral coordinated Al of Al13 and

    other species when the age of the solutions increases.

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

  • 26. BOTTERO & BERSILLON Aluminum and lron(Hl) Chemistry 431

    Table I. Concentration of A l i 3 in Colloids Spectrum Sample Al13a

    1 R = 2.5; = l h 8.8 2 R = 2.5; = 2h 4.7 3 R = 2.5; = 1 day 4.2 4 R = 2.5; = 1 week 3.9 5 R = 2.6; = l h 8.5 6 R = 2.6; = 1 week 6.7 7 R = 2.8; = l h 6.7 8 R = 2.8; = 1 week 6.6 9 R = 3.0; = lh 1-2 (bayerite) 10 R 3.0; = 1 week 0 (bayerite) NOTE: Colloids obtained from 2 7Al MAS NMR patterns and calculated from AF/(AF + A H ratio. The theoretical ratio is 1/13 = 7.7%. FromR = 2.8, the flocculation occurs in the solution.

    /( + A1V1).

    The A l 1 3 content does not change very much (80-100%) from R = 2.2 to 2.8. This behavior implies that the charge of A l 1 3 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 1 3 ) , R = 2.5 (-40% of isolated A l 1 3 and 60% of aggregated A l 1 3 ) from A l = 10"1 to 10"3

    mol /L at p H 6.5, 7.5, and 8.5 led to formation of floes. Solid-state M A S 2 7 A l N M R spectra of these floes precipitated at p H 7.5, [Al] = 10 3 mol /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 1 3 content of the floes at various precipitation p H and aging times. The A l 1 3 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 1 3 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 1 3 ions are not immediately destroyed).

    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)

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

  • 432 AQUATIC HUMIC SUBSTANCES

    Figure 7. 27Al MAS NMR spectra ofPACl floes, R = 2 and 2.5, precipitated at pH

    7.5 after diluting at 1Q-3 mol/L.

    100 50 0 -50-100 (ppm)

    100 50 0 -50*-100...

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