Reactions Between Fulvic Acid and Aluminum - ACS Publications

25 Reactions Between Fulvic Acid and Aluminum 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.ch025

Effects on the Coagulation Process Brian A. Dempsey Department of Civil Engineering, Pennsylvania State University, University Park, PA 16802

The effects of fulvic acids on the speciation of aluminum are measured by timed colorimetric analyses. Precipitates of Al(OH) (s)(pK = 32.8 for pH 4.5-6.5) form in every region where alum has been shown to be successful for the removal of fulvic acids. Stability functions (average log Κ = 3.39) are reported for the formation of soluble aluminum-fulvic acid complexes. Adsorption functions (fulvic acid on freshly precipitated Al(OH) (s)) are more than 10 times larger than the stability functions for complexation offulvic acids with dissolved aluminum. 3

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F'ULVIC ACID (FA) USUALLY REMOVED

is from raw water by the coagulation process followed by sedimentation and rapid sand filtration. Filter alum (Al (SO ) -14.3H 0 ) is the coagulant most commonly used in the United States. However, ferric chloride, organic polyelectrolytes, and other salts of aluminum(III) or iron are also used. I have previously reported on the removal of F A with salts of aluminum (1-3). The objectives of the work presented here are to determine the effects of F A on the speciation of aluminum (especially when filter alum is used as the coagulant) and to use this information to predict the mechanism of F A removal during water treatment. The experimental time frame in this study (minutes to days) corresponds to the hydraulic residence time of conventional water-treatment and distribution systems. The relatively short time for re2

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0065-2393/89/0219-0409$06.00/0 © 1989 American Chemical Society

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

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AQUATIC HUMIC SUBSTANCES

action and the presence of fulvic acid means that analytically identifiable crystalline aluminum oxides or hydroxides are not expected to be important in these systems.

Removal of Contaminants Using Aluminum Salts Amirtharajah and Mills (4) studied the removal of suspended solids from water with alum as the coagulant. They identified p H values and alum doses that resulted in the formation of a voluminous precipitate of Al(OH) (s). Combinations of p H and coagulant dose that result in a heavy floe that settles are usually identified as the "sweep-floc" zone. The use of these conditions for the coagulation process results in good removal of clays, F A , and other contaminants. Amirtharajah and Mills (4) also identified p H and coagulant dose values at which removal of the contaminants occurs by the charge-neutralization mechanism. In this case, the negative charge of the contaminants is just neutralized by the cationic coagulant species. A n equivalent coagulant-tocontaminant dosage must be used to produce charge neutralization. Over dosing results in restabilization. Lower doses of chemical are required for charge neutralization than for the sweep-floc zone. Although using lower doses has potential advantages, contaminant removal rates are often lower and operational control may be more difficult than for the sweep-floc zone. Amirtharajah (5, 6) explained his experimental results for coagulation with filter alum by assuming that Al(OH) (s) forms whenever coagulation is successful. However, he noted that the results are also consistent with destabilization by soluble polymeric species of aluminum. Edwards and Amir tharajah (7) reported that the removal zones for humic acid involve p H and alum doses similar to those required for the removal of clays, except that the stability zone shifts slightly toward lower p H values (7). Sricharoenchaikit (8) also assumed that the precipitation of Al(OH) (s) precedes the destabilization of contaminant species. Packham and associates (9-11), on the other hand, suggested that F A and the dissolved aluminum from alum can react directly to form a precipitate without the preliminary formation of Al(OH) (s). They stated that F A is precipitated by soluble hy drolyzed species of aluminum when salts of aluminum are used as coagulants (9), whereas they suggested that removal of clays is dependent on the pre liminary precipitation of Al(OH) (s) (JO, II). Rebhun and Narlds (12) sug gested that removal of humic materials by alum near p H 6.7 is due to direct precipitation by the polymeric species A l ( O H ) . Matijevic and co-work ers (13, 14), Hayden and Rubin (15), and Stumm and associates (16,17) also suggested that polymers of hydrolyzed aluminum are active reagents when filter alum is used as a coagulant. Dempsey and co-workers (1-3) used various coagulants that contain aluminum to investigate the conditions required for the coagulative removal

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.ch025

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Reactions Between Fulvic Acid and Aluminum

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of fulvic acids. Some of their results are shown in Figure 1A. The black area designated I in Figure 1A corresponds to conditions in which a sweep-floc can be generated when aluminum is added in the form of filter alum. Removal of fulvic acid apparently occurs by charge neutralization with conditions designated by black area II. Under conditions within the gray dotted region, fulvic acids can be removed by membrane filtration, but not by sedimen tation. Dempsey and co-workers (2) suggested that removal using alum at p H above 6 is due to adsorption of F A on Al(OH) (s), but that some removal by alum at lower p H values is due to the direct precipitation of F A by polymers or even monomers of aluminum hydroxide. Polyaluminum chloride (PAC) is a commercially available coagulant with the formula Al(OH) (Cl) (S0 ) . Typically, χ is 1.2-2.0, and ζ is usually 0.16 or less. Both direct and indirect evidence indicates that P A C contains thermodynamically stable polymers of aluminum, especially Al0 (Al(OH)2)i


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Figure 1. Aluminum doses and pH values that permit removal of fulvic acid from water when (A) alum or (B) polyaluminum chloride is the coagulant.


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(subsequently abbreviated A l ) ; this radical has a +7 charge. P A C achieves good removal of contaminants even at p H less than 4 and A l , less than 1 0 " M), conditions under which neither Al(OH) (s) nor A l are thermodynamically stable (part of the black areas in Figure IB). This coagulative behavior, analogous to that of synthetic organic polyelectrolytes (12, 18), indicates that the polymeric species in P A C are relatively inert. Some of the investigators cited have measured both the removal of contaminants and the speciation of hydrolyzed aluminum. Evidence for the presence or activity of a certain species of hydrolyzed aluminum is usually based on indirect experimental evidence and logical argument (2, 9, 13-15, 19, 20). Hundt (21) characterized the species that form after the addition of alum, aluminum chloride, or P A C to water in the absence of F A or clays. H e showed that when alum was added and the p H was above 4.5, most of the aluminum was retained by a membrane filter. In contrast, filterable species persisted until higher p H values for P A C . Tambo (22), using alum, isolated labile (less than 10-min longevity) species of aluminum with highly positive electrophoretic mobilities. These species have not been determined to be either solid or polymeric. The speciation of the aluminum coagulants, after addition to the raw waters, is uncertain. Critical compilations of thermodynamic data (23, 24) and evaluations of the literature regarding the hydrolysis of aluminum (24, 25) are available. However, reports disagree regarding the best free energy values, and enthalpy values have not been obtained for many of the hydrolysis reactions of aluminum. Additionally, the short time frame of the coagulation process and the presence of contaminants during coagulation make the pre diction of speciation difficult. 13

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Experimental Methods The collection, extraction, cleanup, preservation, and characterization of FAs have been previously described (26). Two FAs, designated FA1 and FA4, were used in these experiments. Both of these materials are derivedfromLake Drummond, VA. Solid-sample C NMR studies indicate that approximately 80% of the organic carbon is aliphatic. FA1 has 11.4 meq and FA4 has 11.6 meq of carboxyl functional groups per g of organic carbon; more than a third of these acidic groups are ionized at pH 3. Aldrich humic acid (HA) was also used in a few experiments. Water was distilled and then treated by a microfilter (Milli-Q) system. All other chemicals were reagent grade or better. Experiments were run in 40-mL glass sample bottles with poly(tetrafluoroethylene) (Teflon)-lined caps at room temperature (23-25 °C). FA solution and stock acetate were added to water to give between 0 and 84 mg/L of FA or HA organic carbon and total acetate of 8 X 10^ M. For comparison, 2.1 to 84 mg/L of FA4 contains 2.4 Χ 10 to 9.7 Χ ΙΟ M of carboxylic functional groups. The pH was adjusted to the desired value by using HCI or NaOH. Then the appropriate amount of alum was injected and the solution was mixed. The stock alum solution contained 0.0018 M total Al(III). Thefinalconcentration of aluminum varied, but it was 1.05 mg/L (3.9 X 10 M) in most cases. Samples were taken at 5 min, 1 h, l3

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4 h, 1 day, and sometimes 7 days for the analysis of aluminum. The pH was measured for each sampling period. Except for the 5-min samples and whenfiltrationwas used as a separation process, the sample bottles were placed in an ultrasonic bath for 10 min prior to sampling in order to break up any particulates that may have formed. The concentration and speciation of aluminum was determined by the ferron (8-hydroxy-7-iodoquinolinesulfonic acid) method. A 2-mL aliquot of the analyte was placed in a cuvette (path length = 1 cm) and 0.8 mL of the ferron reagent as described by Bersillon (20) was added. The cap was inserted, and macromixing was completed within 5 s of the ferron addition. The pH after addition of ferron must be very consistent (27); addition of acid for digestion of aluminum species should be avoided unless the sample is back-titrated or the molar absorptivity is shown to be unchanged. Absorptivity was 7.78 mM" cm , with a standard deviation of 0.03 at 370 nm. Monomeric aluminum reacts very rapidly, with a reported pseudo-first-order k = 2.3 min (28). This value was confirmed in our experiments for times greater than 15 s, but the reaction was even faster before 15 s, so that 85% of the total aluminum in dilute alum solutions had reacted within 30 s. Reaction rates for most other species of aluminum are considerably slower (19, 27, 28) and the blank-corrected absorbance at 30 s is claimed to represent inorganic monomeric aluminum. Although the rate of nonmonomeric color development increased with increasing concentrations of either FA or total aluminum, the extrapolated contribution of these nonmonomeric species to the 30-s reading was typically less than 10% of the 30-s reading. This contribution was determined on the basis of the slopes (absorbance versus time) at 2 min, when the monomeric aluminum was 99% reacted. Reagent blanks were analyzed by measuring the absorbance of samples (minus aluminum) against distilled-deionized (DDI) water. Ferron reagent was added to both cuvettes. Sample blanks were analyzed by comparing the absorbance of the whole samples (including aluminum and FA) against D D I water; ferron reagent was not added. Solutions of polyaluminum chloride that contain the polymer Al0 (Al(OH) )i2 (abbreviated Ali ) have an initial rate of color development of 0.071 min (our data) to 0.075 min (28); thus, only 3.5% of such aluminum is reacted with ferron at 30 s. The rate of color developmentfromthe PAC that was used in these experiments slows substantially, however, so that only 57% of the A l is reacted after 22 h. On the other hand, the aluminum in suspensions that are predominantly Al(OH) (s) is sometimes totally reacted in less than 1 h, a result indicating greater lability with respect to the ferron reagent than for Ali or other polymeric materials. As a result, definitions of the aluminum that reacts with ferron in 2 h as monomeric plus polymeric (Al plus Al ) and the remaining aluminum (Al ) as Al(OH) (s) cannot be justified for the situations that we have studied. Some plots of absorbance versus time are shown in Figure 2 for diluted alum (very rapid reaction), A l (very slow and monotonie reaction), amorphous Al(OH) (s) (S-shaped curve), and aluminum that is complexed by FA (very rapid reaction for inorganic monomeric aluminum and very slow, monotonie reaction for organically bound aluminum). The S-shaped curve occurred at some sample time (typically at 5 min, 1 h, and 4 h after coagulation) in every case in which Al(OH) (s) was visually observed. The inflection point in the S-shaped curve always occurred within 2 h after the addition of ferron reagent, and the incremental absorbance that occurred after the inflection point could often be removed by membranefiltration.In this work the ferron test is used to determine monomeric (30-s) aluminum and as evidence for the presence of Al(OH) (s). These two uses have been corroborative in every case. Some data forfilterablealuminum are presented in this chapter. Filterable aluminum is defined as thefractionthat passes membrane filters with 0.2-μ m pores. 1

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Reactions Between Fulvic Acid and Aluminum - ACS Publications

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