Environ. Sci. Technol. 2009, 43, 7837–7842
Effect of Softening Precipitate Composition and Surface Characteristics on Natural Organic Matter Adsorption CAROLINE G. RUSSELL,* DESMOND F. LAWLER, GERALD E. SPEITEL, JR., AND LYNN E. KATZ The University of Texas at Austin, Department of Civil, Architectural, and Environmental Engineering, C1700, Austin, Texas 78712
Received April 22, 2009. Revised manuscript received July 16, 2009. Accepted August 10, 2009.
Natural organic matter (NOM) removal during water softening is thought to occur through adsorption onto or coprecipitation with calcium and magnesium solids. However, details of precipitate composition and surface chemistry and subsequent interactions with NOM are relatively unknown. In this study, ζ potentiometry analyses of precipitates formed from inorganic solutions under varying conditions (e.g., Ca-only, Mg-only, Ca + Mg, increasing lime or NaOH dose) indicated that both CaCO3 and Mg(OH)2 were positively charged at higher lime (Ca(OH)2) and NaOH doses (associated with pH values above 11.5), potentially yielding a greater affinity for adsorbing negatively charged organic molecules. Environmental scanning electron microscopy (ESEM) images of CaCO3 solids illustrated the rhombohedral shape characteristic of calcite. In the presence of increasing concentrations of magnesium, the CaCO3 rhombs shifted to more elongated crystals. The CaCO3 solids also exhibited increasingly positive surface charge from Mg incorporation into the crystal lattice, potentially creating more favorable conditions for adsorption of organic matter. NOM adsorption experiments using humic substances extracted from Lake Austin and Missouri River water elucidated the role of surface charge and surface area on adsorption.
Introduction Extensive research has been conducted on mechanisms to reduce disinfection byproduct (DBP) formation in water treatment, some of which has focused on optimizing conventional treatment processes (e.g., coagulation, softening) to enhance NOM removal. Earlier work on softening (1-4) demonstrated the ability to remove NOM and advanced the understanding of effects of source water characteristics and operational factors (e.g., lime dose, soda addition, sludge recycle) on the degree of DBP-precursor removal. However, few studies have evaluated the specific mechanisms of NOM removal during softening. NOM removal is thought to occur by some combination of adsorption onto the calcium and magnesium solids formed during softening, and direct precipitation as a calcium or magnesium humate or fulvate (4, 5). The precipitate com* Corresponding author phone: 512-370-1203; e-mail: crussell@ pirnie.com. 10.1021/es900991n CCC: $40.75
Published on Web 09/16/2009
2009 American Chemical Society
position and surface characteristics likely impact the importance of each mechanism and the degree of NOM removal that can be achieved. For example, initial magnesium removal (approximately 10 mg/L as CaCO3) below the point of Mg(OH)2 precipitation, has been correlated to a rapid decline in DOC concentrations (2, 3). That initial magnesium removal is attributed to magnesium incorporation into the CaCO3 solids, impacting the CaCO3 surface characteristics (6-10) and the affinity for NOM adsorption (5). The objective of this research was to evaluate the effect of precipitate composition, especially magnesium incorporation, on the surface characteristics (morphology, charge) and the subsequent effect on NOM adsorption under varying softening conditions.
Materials and Methods Jar tests were conducted with synthetic inorganic water to evaluate the composition, surface charge and morphology of precipitates formed under varying conditions. Then, adsorption experiments were performed on mineral solids representative of softening precipitates to correlate the NOM adsorption capacity of the solids with their surface characteristics. Humic substances were extracts from source waters (Lake Austin and Missouri River) that undergo softening. Precipitation Experiments. Jar tests were conducted in which lime (97.6% Ca(OH)2, 1.4% CaCO3, and trace amounts of several impurities) was added as a slurry or caustic soda (NaOH) was added as a liquid concentrate to induce precipitation in waters containing varying concentrations of Ca2+ and Mg2+ (i.e., Ca-only, Mg-only, or Ca+Mg at varying Ca:Mg ratios). The synthetic inorganic water was prepared with deionized (DI) water, NaHCO3, CaCl2, MgCl2, and KNO3 to mimic the hardness and alkalinity of Lake Austin water (61.6 mg/L Ca2+, 19.2 mg/L Mg2+, and 179 mg/L alkalinity as CaCO3) for the baseline chemical composition. The ionic strength of 0.016 M was based on the composition of Missouri River water. Specific solution compositions and lime/NaOH doses for each experiment are provided in Supporting Information (SI) Table S1. The solution was distributed in 1 L jars fitted with floating tops that prevent CO2 exchange. CO2 exchange was minimized since changes in the dissolved CO2 concentration would impact the pH and, therefore, the precipitate composition and surface characteristics. Lime or NaOH was added to each jar initiating three minutes of rapid mixing (150 rpm, velocity gradient G ) 350 s-1) using a six-place stirrer (Phipps and Bird 7790-400). The solids precipitated during 30 min of slow mixing (45 rpm, G ) 60 s-1). To obtain a representative sample of solids for ESEM imaging and measuring ζ potential, approximately 100 mL of suspension was collected from each jar near the end of the slow mixing period. The remaining suspension was filtered through a 0.45 µm filter. The pH, calcium and magnesium concentrations, alkalinity, and conductivity of the filtered solutions were measured (SI Figures S1-S3). The filtered solids were air-dried and then dissolved in 100 mL DI with three drops of concentrated HNO3 to measure the Ca2+ and Mg2+ composition by flame atomic absorption (FAA). A subset of the solids from the Ca+Mg experiment was analyzed by X-ray diffractometry (XRD). NOM Extraction. To characterize NOM adsorption onto softening precipitates, humic substances were extracted from Lake Austin in Austin, TX (LAHS) and the Missouri River in Omaha, NE (MRHS). Approximately 1800 L of Lake Austin and 130 L of Missouri River water were collected, characterVOL. 43, NO. 20, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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ized (pH, TOC, DOC, UV absorbance at 254 nm [UV254], conductivity, alkalinity, Ca2+, and Mg2+; SI Table S2), and stored at pH 2 and 4 °C in plastic carboys. The humic substances were extracted, following the protocol of Thurman and Malcolm (11), acidified to pH 2 with HCl, and stored at 4 °C in amber bottles. Both extracts were characterized by acidity measurements (12) and their aromaticity was compared via the specific UV absorbance (SUVA, the ratio of UV254 absorbance to DOC) for dilute samples of each extract. Adsorption Experiments. Four representative solids were selected to assess NOM adsorption: CaCO3, Mg(OH)2, MgCO3, and freshly precipitated CaCO3. The CaCO3, Mg(OH)2, and MgCO3 solids were commercially purchased (Sigma). The freshly precipitated CaCO3 was prepared at pH 10.4 by adding lime (150 mg/L CaO) to a CaCl2 and NaHCO3 solution (13). Each solid was characterized by analyzing the surface area (BET), surface charge (ζ-potential), and morphology (ESEM). To differentiate the freshly precipitated CaCO3 from the purchased CaCO3, the latter is referred to as “pre-formed CaCO3” throughout this paper. Equilibrated aqueous solutions for each of the solids were prepared to ensure that neither dissolution nor precipitation occurred in adsorption experiments. Solutions were prepared at pH 10.4, with KNO3 added to achieve 0.016 M ionic strength (SI Table S3). Adsorption experiments were performed on the solids using LAHS concentrations ranging from 1 to 16 mg C/L. An additional set of vials with 8 mg C/L MRHS was prepared for each solid to assess the effect of different NOM characteristics. For each condition (mineral, NOM concentration), three 45 mL vials were prepared: one blank with no solids, and duplicate vials containing 100 mg of the target mineral solid. The freshly precipitated CaCO3 was added as a slurry (1.5 mL of 66 600 mg/L CaCO3). The NOM extract (LAHS or MRHS) was added to 150 mL of the equilibrated aqueous solution to achieve the target NOM concentration, and then the solution was distributed among the three vials to ensure identical initial NOM concentrations. A slightly larger volume (150 rather than 135 mL) of each solution was prepared for characterization (pH, DOC; (13)). The vials were capped headspace free, tumbled at 6 rpm for 24 h, and centrifuged (Beckman Model J2-21 centrifuge) at 20 100g for 15 min to separate the solids and liquids. The 24 h contact time was selected based on a kinetic experiment demonstrating that NOM adsorption onto Mg(OH)2 was 97% complete within 24 h (SI Figure S4). The extent of NOM adsorption was calculated by measuring the DOC concentrations and UV254 in vials with and without solids addition. Conductivity, pH, and calcium and magnesium concentrations were measured to verify that the target conditions were maintained throughout the experiment and that neither dissolution nor precipitation occurred. Analytical Methods. Aqueous measurements included pH, conductivity, calcium, magnesium, alkalinity, TOC, DOC, and UV254. Ionic strength was calculated from the measured conductivity (Radiometer Copenhagen CDM 230), using a correlation similar to Langmuir (14), but established by adding known amounts of KNO3 to an aqueous CaCO3 solution at pH 10.4 (13). Calcium and magnesium were measured by FAA (Perkin-Elmer 1100B) and alkalinity was measured by titration, following Standard Methods (15). A Teckmar-Dohrman Apollo 9000 Combustion Analyzer was used to measure TOC and DOC. UV absorbance was measured using an Agilent 8453E UV-visible Spectroscopy system. XRD spectra of solid samples mounted on aluminum holders were examined with a Siemens X-ray diffractometer and a Cu KR radiation source (λ ) 1.5418 Å). The surface area was assessed through BET isotherms using a Quantachrome Autosorb-1 with nitrogen gas. Air-dried samples (1-2 g) were 7838
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FIGURE 1. Effect of magnesium on CaCO3 morphology (images A and B: 100 mg/L CaO, Images C and D: 189 mg/L CaO). outgassed with helium for 24 h at 240 °C and a five point isotherm was obtained by measuring the volume of absorbed nitrogen at relative pressures (P/Po) from 0.05 to 0.25. BET measurements for Mg(OH)2 solids were not attained since the degassing temperature released water and produced condensed vapor in the glass analysis tube. Images of softening precipitates and the commercial solids were collected, primarily under high vacuum, using a Philips/FEI XL30 environmental scanning electron microscope (ESEM). Surface charge was evaluated by measuring the electrophoretic mobility of the particles using a Zetaphoremeter IV model Z8000 and converted to ζ-potential (mV) using the Smoluchowski equation.
Results and Discussion Precipitate Composition and Surface Characteristics. A softening experiment was conducted in which the initial calcium concentration was constant (61.6 mg/L Ca2+), but three different magnesium concentrations were evaluateds0, 8, 19.2, and 38 mg/L initial Mg2+sat two different lime doses (100 and 189 mg/L CaO). The lower lime dose correlated to typical softening plant conditions, optimizing calcium precipitation but avoiding magnesium hydroxide precipitation. The higher lime dose was selected to ensure Mg(OH)2 precipitation, achieving approximately 95% magnesium removal. ESEM images of particles formed at 0 and 38 mg/L initial Mg2+ and 100 and 189 mg/L CaO are in Figure 1 (images of particles formed under all eight conditions are in SI Figure S5). CaCO3 precipitates formed in the absence of magnesium (100 mg/L CaO, pH 10.1) exhibited the rhombohedral shape characteristic of calcite (Figure 1, Image A). XRD analysis confirmed the formation of calcite as the dominant precipitate (SI Figure S6). At the higher lime dose (189 mg/L CaO, pH 11.5; Figure 1, Image C), more edges were observed; otherwise, the CaCO3 morphology and size were unaffected by an increase in lime dose or pH. The clustered calcite crystals in Image C may be attributed to CaCO3 precipitation on the surface of undissolved lime, which serves as a “nucleus” for calcite growth. An amorphous solid thought to be undissolved lime was observed in most CaCO3 samples from the research (13), indicating that high surface area solids are available throughout the range of softening conditions for potential adsorption of organic molecules. The absence of the amorphous solid in Ca-only precipitation experiments in which NaOH (rather than lime) was added (SI Figure S7) supports the theory that the solid is undissolved lime, but it was not conclusively identified in our experiments. Figure 1, Images A and B (100 mg/L CaO) reveal a shift in CaCO3 morphology from highly structured calcite rhombs
FIGURE 2. ζ Potential of CaCO3 and Mg(OH)2 under varying softening conditions; the dash/dot and dashed lines represent general data trends for the Ca-only and Mg-only experiments, respectively. in the absence of Mg2+ to more rounded particles in the presence of 38 mg/L Mg2+, consistent with trends hypothesized by Folk (7) and explained by Davis (6). Davis (6) demonstrated through AFM analysis that Mg incorporates into nonequivalent step-types, creating strain at the intersection of the steps, resulting in the observed effects on calcite crystal morphology. The effect of magnesium on CaCO3 morphology was more pronounced at the higher lime dose (Figure 1, Images C and D). The CaCO3 particles in Image C (0 mg/L Mg2+) exhibit a rhombohedral shape, whereas in the presence of magnesium (Image D), the particles became highly elongated. The greater number of edges at the higher pH may have enhanced the capacity for magnesium incorporation into the crystal structure, yielding the more pronounced effect on the morphology. FAA analysis provided an indirect indication of the degree of Mg incorporation in the CaCO3 solids formed in the presence of 38 mg/L Mg2+; at 100 mg/L CaO, the precipitated solids contained 3.8% molar MgCO3. This percent MgCO3 incorporation is within expected range for a mixed Mg-CaCO3 (16). The XRD spectrum (SI Figure S6) revealed a slight shift in the d-spacing for the {104} face, consistent with reported trends for a mixed Mg-CaCO3 (17). At the higher lime dose (189 mg/L CaO), FAA analysis indicated the precipitated solids contained 25.5% molar MgCO3; however, the majority of the precipitated Mg was likely in the form of Mg(OH)2 (SI Figure S8). ESEM images from the Mg-only experiments (SI Figure S9) indicated the Mg(OH)2 precipitates were amorphous at all pH values investigated. The apparent high surface area of the precipitates was anticipated to correlate to a greater NOM adsorption affinity. Precipitate Surface Charge. The ζ-potential of precipitates formed in the Ca-only, Mg-only, and Ca+Mg experiments are shown in Figure 2. Calcium carbonate precipitates formed in Ca-only experiments exhibited a slightly positive ζ-potential at lower pH values (corresponding to the lowest lime dose evaluated, 80 mg/L CaO), negative ζ-potential at pH values ranging from ∼9.0 to 10.5, and positive ζ-potential at pH values ranging from 10.5 to 11.6. The slight scatter in the results is due partially to variable conditions in the experiments (e.g., initial calcium concentrations of 61.6 and 78 mg/L Ca2+, soda ash added to a subset; (13)). ζ potential results could also have been impacted by the solids not reaching thermodynamic equilibrium before the measurements (18); however, the results are considered representative of electrophoretic mobility for precipitates in a softening process.
The ζ-potential results for the Ca-only experiments generally correlate to the trend in soluble calcium concentrations (SI Figure S1). Calcium concentrations initially decrease with increased lime dose (and pH) to a minimum value (corresponding to carbonate-limited conditions, pH ∼10.0 in these experiments), after which Ca2+ concentrations increase with further lime addition. The increase (to more positive values) in ζ-potential past the point of minimum calcium suggests that, at higher lime doses (corresponding to enhanced softening conditions), CaCO3 particles exhibit increasingly favorable characteristics for adsorbing NOM. If present, undissolved lime could also contribute to the positive ζ potential at higher lime doses (corresponding to the higher pH range in Figure 2). The correlation between ζ-potential and calcium concentrations is consistent with demonstrated calcite surface chemistry. Foxall (19) and others (20, 21) have shown that the electrophoretic mobility of calcite particles is only secondarily affected by pH and that Ca2+ and CO32- (and other carbonate species) are the potential determining ions. While the total carbonate concentration and speciation (HCO3-, CO32-) also varies with lime dose and pH, the stronger influence of Ca2+ on the calcite ζ-potential is explained by selective interaction of the positively charged calcium ions with the crystal surface in systems open to the atmosphere (22). In our experiments, the jars had floating covers to minimize CO2 exchange; however, the synthetic water initially added to the jars had been exposed to the atmosphere and contained NaHCO3 concentrations correlating to pCO2 ≈ 10-2.9. This effect of selective ion adsorption on the ζ-potential of carbonate minerals has been demonstrated by Gence and Ozbay (23). The ζ-potential of Mg(OH)2 particles strongly increased with increasing pH (Figure 2). If the potential-determining sites for Mg(OH)2 were defined by tMgO-, tMgOH°, and tMgOH2+, then the trend in ζ-potential is opposite of expected results. However, an investigation of the MgCO3 saturation quotient (Q ) {Mg2+}{CO3-2}) for the experimental conditions reveals that MgCO3 could also have precipitated. An analysis of the degree of saturation, Ω, for MgCO3 and Mg(OH)2 indicated MgCO3 becomes less important as Mg(OH)2 precipitation becomes more thermodynamically favorable with increasing NaOH dose (SI Figure S10). The pH for the points of 0 charge of MgCO3 and Mg(OH)2 are less than 8 and greater than 12, respectively (24, 25). Therefore, based on the pH values shown in Figure 2, Mg(OH)2 particles are expected to exhibit a positive ζ-potential and MgCO3 particles a negative ζ-potential throughout the pH range (NaOH doses). The ζ-potential trend in the Mg-only experiVOL. 43, NO. 20, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Effects of Increasing Initial Mg2+ Concentration on Precipitate Zeta Potential 100 mg/L CaO 2+
Initial magnesium (mg/L Mg )
pH
8 19.2 38
9.94 10.02 10.05
2+
Ca
2+
(mg/L) Mg
6.4 9.3 12.7
189 mg/L CaO
(mg/L) Zeta-potential (mV)
6.0 15.4 31.7
9.3 32.9 39.7
pH 11.57 11.28 10.61
2+
Ca
(mg/L) Mg2+ (mg/L) Zeta-potential (mV)
50.9 38.3 41.0
0.3 1.0 9.2
39.6 42.5 46.3
TABLE 2. Characteristics of Mineral Solids and Lake Austin and Missouri River HS Adsorption onto the Solids parameter
preformed CaCO3
freshly precipitated CaCO3
MgCO3
Mg(OH)2
+13.26 4.5e 1-2
-22.78 23.55 1-5
+6.08 25f 5-40
ζ potential (mV) Surface Areab(m2/g) Particle Sizec (µm)
-24.36 0.23 2-7
LAHS MRHS
NOM Adsorbedd in mg C/g solid negligible 0.40 negligible 0.36
0.38 0.20
1.60 1.12
LAHS MRHS
NOM Adsorbedd in mg C/m2 negligible 0.089 negligible 0.080
0.016 0.008
0.064 0.045
a
a Measured at pH 10.4 in pre-equilibrated solution (SI Table S3). b Surface area measurements for preformed CaCO3 and MgCO3 were obtained through BET analysis. c Particle diameters estimated from ESEM images (SI Figure S11). d Initial NOM concentration ) 8 mg C/L. e From ref 27. f From ref 28.
ments is thus explained by an increase in the molar ratio of positively charged Mg(OH)2 precipitated relative to negatively charged MgCO3, consistent with Gence and Ozbay (23). Precipitates formed when both calcium and magnesium were present exhibited positive ζ-potential values (Figure 2) throughout the range of conditions investigated. These results indicate favorable conditions for removing negatively charged organic matter during softening. The ζ-potential of CaCO3 particles formed in the presence of increasing initial magnesium concentrations and at the two different lime doses are listed in Table 1. ζ-potential values increase dramatically with increasing initial magnesium concentration, consistent with observations from previous researchers (9, 10). The trend is more pronounced for particles formed at the lower lime dose. At 100 mg/L CaO, Mg(OH)2 has not precipitated and the effect of initial Mg concentration on ζ-potential is attributed to Mg incorporation into the calcite crystals. At the higher lime dose, Mg(OH)2 has precipitated; therefore, the ζ-potential for these doses represents an average of the ζ-potential of both CaCO3 (or mixed Mg-CaCO3) and Mg(OH)2 particles. The effect of Mg incorporation into the calcite crystals on the ζ-potential at 189 mg/L CaO cannot be differentiated from the contribution of positively charged Mg(OH)2. Table 1 also lists the precipitation pH and final aqueous Ca2+ and Mg2+ concentrations for the jars at the different lime doses and increasing initial magnesium concentration. The pH is relatively constant (pH ∼ 10) at the lower lime dose. For these jars, the aqueous calcium concentration increases with increasing initial magnesium concentration due to inhibition of CaCO3 precipitation from the magnesium incorporation (26), and the higher calcium concentrations may also contribute to the more positive ζ-potential. At the higher lime dose, pH values decrease substantially with increasing initial magnesium concentration due to the higher degree of Mg(OH)2 precipitation (which lowers the pH). The variation in pH at the higher lime dose could also have some effect on the ζ-potential of Ca and Mg precipitates. The surface chemistry is too complex to attribute the resulting surface charge to any particular parameter. However, clearly a higher initial magnesium concentration results in an increase (to more positive values) in ζ-potential. 7840
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The results of increasingly positive ζ-potential and less structured solids (presumably resulting in increased surface area (8)) with increasing initial magnesium concentration would suggest that higher initial Mg2+ is beneficial for the removal of negatively charged organic matter. This expected trend was observed in softening experiments conducted with LAHS (5). Additionally, the results presented for CaCO3 and Mg(OH)2 precipitation (Figure 2) revealed that both solids were negatively charged at low lime and NaOH doses (respectively), but exhibited positive surface charge at higher doses. The ζ potential values indicate both types of precipitates yield a greater affinity for adsorbing negatively charged organic molecules with increasing lime/NaOH dose. NOM Adsorption onto Preformed Solids. Adsorption experiments were conducted with LAHS and MRHS to directly correlate NOM removal to characteristics of mineral solids representative of precipitates formed in softening. Solid Characteristics. Results from the surface charge and area characterization of the preformed mineral solidss CaCO3, MgCO3, Mg(OH)2sand freshly precipitated CaCO3 are shown in Table 2 (data in the last four rows are discussed later). As these data illustrate, the freshly precipitated CaCO3 and Mg(OH)2 solids exhibited positive ζ-potential; the preformed CaCO3 and MgCO3 held a negative ζ-potential. The ζ-potential of the freshly precipitated CaCO3 (+13.26 mV) was significantly different than the value for the CaCO3 solids precipitated at the same pH for the precipitation experiments (-14 to -6 mV; Figure 2). The difference is explained by a recent publication (18) demonstrating that timing and exposure to CO2 can influence the measurements. Based on our BET measurements for the preformed CaCO3 and MgCO3 and literature values for freshly precipitated CaCO3 and Mg(OH)2 (27, 28), the surface area of freshly precipitated CaCO3 solids is an order of magnitude higher than that of preformed CaCO3 solids, and the surface area of the magnesium solids is approximately 5 times that of freshly precipitated CaCO3. Table 2 lists dimensions for the solids. Particle sizes were estimated from edge-to-edge lengths of individual particles observed in ESEM images (SI Figure S11). The preformed CaCO3 solids ranged from 2 to 7 µm, whereas freshly precipitated CaCO3 solids were 1 to 2 µm. Both the preformed
FIGURE 3. Lake Austin (LA) HS adsorption curves for the mineral solids. and freshly precipitated CaCO3 solids exhibited a rhombohedral shape characteristic of calcite. The MgCO3 also revealed a relatively small particle size, ranging from 1 to 5 µm. Mg(OH)2 “particles” ranged in size from 5 to 40 µm in diameter. ESEM images (SI Figure S11) indicated that all of the solids investigated may exist as aggregates. XRD spectra provided additional information on the crystal structure of the different solids (SI Figure S12). While the Mg(OH)2 solids appeared amorphous in the ESEM images, the XRD spectrum indicates crystalline Mg(OH)2 solids were also present. The solids characterization suggested expected trends for NOM adsorption onto the mineral solids. The positive ζ-potential of Mg(OH)2 and the greater surface area of both magnesium solids were anticipated to enhance NOM adsorption. The negative ζ-potential of the preformed CaCO3 and low surface area of both calcium solids indicated that those solids may have a lower adsorption capacity for organic matter on a mass basis. Adsorption Experiments. The results for LAHS adsorption onto the various solids are shown in Figure 3. Linear adsorption isotherms were fit to the data, according to the following equation: qe ) KCe
(1)
where qe is the mass organic C adsorbed per gram solid at equilibrium, K is the linear adsorption coefficient, and Ce is the aqueous DOC concentration at equilibrium. The linear adsorption coefficients are provided in Figure 3 and decrease in order from Mg(OH)2 > freshly precipitated CaCO3 > MgCO3 > preformed CaCO3. The results indicate Mg(OH)2 solids effectively adsorb NOM. An asymptote was not observed for the data, suggesting that the maximum adsorption capacity of the 100 mg Mg(OH)2 was not reached. The MgCO3 solids also removed NOM, and the DOC data suggest that the maximum adsorption capacity of the MgCO3 solids was also not reached. However, MgCO3 solids adsorbed NOM to a far lesser extent than Mg(OH)2, a result that can be explained by the surface charge of the solids. The positive-charge on Mg(OH)2 particles provides more favorable conditions for NOM adsorption than the negative charge on MgCO3 solids (Table 2). The UV254 results (SI Figure S13) confirm the trends for the DOC data in Figure 3. Organic matter removal by adsorption onto preformed CaCO3 was negligible under the experimental conditions, consistent with results of Liao and Randtke (4). However,
NOM was adsorbed onto freshly precipitated CaCO3. The higher degree of NOM adsorption onto the fresh CaCO3 solids is likely due to the greater surface area on the more finely divided solids and the positive surface charge. Further, if any undissolved lime was present as part of the CaCO3 slurry, that solid could also impact NOM adsorption; under the conditions evaluated, lime would be expected to exhibit favorable surface characteristics for NOM adsorption. The aqueous calcium concentration could also affect NOM adsorption onto the CaCO3 solids. Concentrations of NOMCa ternary complexes increase with increased aqueous Ca2+ concentrations (4, 29) and such complexes may have a role in NOM adsorption to the calcite surface (30). The aqueous Ca2+ (and carbonate) concentrations were equivalent in the vials with preformed and freshly precipitated CaCO3; therefore the difference in NOM adsorption onto these two solids is attributed to the difference in surface characteristics (e.g., charge, area). Table 2 lists the mass of NOM adsorbed onto each of the calcium and magnesium solids on both mass (mg C/g solid) and surface area (mg C/m2 solid) bases at an initial NOM concentration of 8 mg C/L. The values in mg C/g solid reflect trends shown in Figure 3. Normalization of the NOM adsorbed by the surface area of the solids (mg C/m2 solid) provides further insight on the effect of the precipitate surface chemistry on the degree of NOM adsorption. On a surface area basis, NOM adsorption onto the negatively charged MgCO3 was lower than on the positively charged CaCO3 (freshly precipitated) and Mg(OH)2 solids, which adsorbed NOM to a similar extent. These results are consistent with trends observed in controlled softening experiments (5), in which CaCO3 and Mg(OH)2 achieved the same degree of NOM removal on a surface area basis (although the mass of CaCO3 precipitated was approximately four times that of Mg(OH)2 precipitated). Effect of NOM Characteristics on Adsorption. Adsorption of the MRHS onto the mineral solids was investigated to evaluate the effect of NOM characteristics on removal in softening. The data in the last four rows of Table 2 indicate that LAHS was adsorbed to a greater extent than MRHS onto the Mg solids, although the difference between LAHS and MRHS adsorption onto the freshly precipitated CaCO3 was not statistically significant. LAHS have a greater acidity (11 meq/g C) than MRHS (7.4 meq/g C), and this difference is the likely explanation for the greater adsorption; i.e., the greater number of functional groups could serve as potential active sites for chemisorption onto the mineral solids (31, 32). Some studies suggest that the aromatic fraction of the organic VOL. 43, NO. 20, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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matter is preferentially removed during softening (1, 3, 33). The SUVA values for LAHS and MRHS were 2.56 and 2.45 L/mg-m, respectively, an insignificant difference in the aromaticity of the two source waters. Results from related research indicated that the maximum amount of Missouri River NOM that could be removed by association with softening precipitates was lower than that for Lake Austin NOM (2). The water chemistry of both source waters is relatively similar (i.e., initial Ca2+ and Mg2+, alkalinity), suggesting that characteristics of the organic matter itself account for the higher degree of removal for Lake Austin NOM. Implications. Magnesium incorporation into CaCO3 precipitates resulted in a more positively charged and lessstructured solid, and the effect on morphology was more pronounced at higher pH. The effect of Mg incorporation on calcite surface characteristics explains results presented previously (5), where a greater degree of NOM removal was observed with higher initial Mg concentration at lime doses before Mg(OH)2 precipitation. Further, the surface charge of all softening precipitates increased with increasing lime/ NaOH dose. The operation of softening plants where NOM removal is desired or mandated represents a difficult balance among several competing goals. Sufficient softening is usually accomplished by removing most of the calcium and little magnesium, i.e., at a pH (lime dose) that avoids Mg(OH)2 precipitation (which is undesirable from a sludge handling perspective). Depending on the alkalinity, most plants operate well short of the dose that would cause that precipitation. The results reported herein suggest that, to improve NOM removal, plants should operate at a slightly higher dose, that is, the maximum lime dose that avoids Mg(OH)2 precipitation.
Acknowledgments This research was funded by an Abel Wolman Fellowship through the American Water Works Association.
Supporting Information Available Additional information including three tables and 13 figures. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) Roalson, S. R.; Kweon, J.; Lawler, D. F.; Speitel, G. E., Jr. Enhanced Softening: Effects of Lime Dose and Chemical Additions. J. Am. Water Works Assoc. 2003, 95 (11), 97–109. (2) Kalscheur, K. N.; Gerwe, C. E.; Kweon, J.; Speitel, G. E., Jr.; Lawler, D. F. Enhanced softening: effects of source water quality on natural organic matter removal and disinfection by-product formation. J. Am. Water Works Assoc. 2006, 98 (11), 93–106. (3) Thompson, J. D.; White, M. C.; Harrington, G. W.; Singer, P. C. Enhanced softening; factors influencing DBP precursor removal. J. Am. Water Works Assoc. 1997, 89 (6), 94–105. (4) Liao, M. Y.; Randtke, S. J. Removing fulvic acid by lime softening. J. Am. Water Works Assoc. 1985, 77 (8), 78–88. (5) Russell, C. G.; Lawler, D. F.; Speitel, G. E., Jr. Natural organic matter coprecipitation with solids formed during softening. J. Am. Water Works Assoc. 2009, 101 (4), 112–124. (6) Davis, K. J.; Dove, P. M.; Wasylenki, L. E.; De Yoreo, J. J. Morphological consequences of differential Mg2+ incorporation at structurally distinct steps on calcite. Am. Mineral. 2004, 89 (5-6), 714–720. (7) Folk, R. L. The natural history of crystalline calcium carbonate: Effect of magnesium content and salinity. J. Sediment. Petrol. 1974, 44 (1), 40–53. (8) Busenberg, E.; Plummer, L. N. Thermodynamics of magnesian calcite solid-solutions at 25°C and 1 atm total pressure. Geochim. Cosmochim. Acta 1989, 53, 1189–1208.
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