January 13, 2007
Interactions Between Chloride and Sulfate or Silica Removals Using an Advanced Lime-Aluminum Softening Process
By Abdel-Wahab, Ahmed; Batchelor, Bill
ABSTRACT:An advanced softening process called the ultra-high lime with aluminum process (UHLA) was initiated in this research. The UHLA process has the ability to remove sulfate, silica, and chloride from waters such as recycled cooling water and desalination brines. Furthermore, it can remove other scale-forming materials, such as calcium, magnesium, carbonate, and phosphate. The purpose of this paper is to study the interactions among chloride, sulfate, and silica in the UHLA process. Results of equilibrium experiments indicated that sulfate is preferentially removed over chloride. Final chloride concentration increased with increasing initial sulfate concentration. However, initial chloride concentration was found to have negligible effect on final sulfate concentration. Silica was found to have only a small effect on chloride removal. Water Environ. Res., 78, 2474 (2006).KEYWORDS: chloride removal, sulfate removal, silica removal, industrial wastewater, softening, ultra-high lime with aluminum process, cooling water, brine, scale.
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There has been a growing trend toward zero discharge from industrial water systems (Goldblatt et al., 1993), and, in some cases, this has even become a legal requirement (Rossiter and Nath, 1995). Cooling water discharges are major environmental problems and often dominate discharge flows. The environmental effect related to the discharge of cooling water includes the thermal loading and discharge of conditioning chemicals used to prevent biological and physical fouling of the cooling system (e.g., biocides, dispersing agents, and anticorrosives, or their degradation products) (Assink and Deventer, 1995; Bloem and Ropert, 1996). Because cooling water often contributes the highest single water demand in industry, it is the area in which the most significant gains in water recycle can be expected. Increased recycle not only facilitates pollution control efforts by reducing or eliminating a potentially contaminated liquid waste stream, but it also helps industries and utilities benefit economically because of reduced water supply costs. These savings are most significant in areas of water scarcity, where recycle is a necessary conservation measure. However, the recycling of cooling water can be limited also by decreased water quality. Poorer water quality results when dissolved solids are concentrated by evaporation in the cooling tower. Higher nutrient concentrations can increase biofouling. Excessive total dissolved solids and high concentrations of chloride and sulfate can result in increased corrosion. A major problem occurs when increased concentrations of dissolved compounds lead to the formation of scale by chemical precipitation. These compounds are calcium, magnesium, silica, carbonate, sulfate, and phosphate.
An advanced softening process called the ultra-high lime with aluminum process (UHLA) was initiated in this research, and it has excellent potential for improving industrial water use efficiency and achieving zero discharge. It is an innovative modification of high lime softening, in which aluminum is added with lime. The addition of aluminum in the presence of calcium and high pH results in the removal of sulfate, chloride, and silica by precipitation of calcium sulfoaluminate, calcium chloroaluminate, and calcium aluminosilicate solids, respectively. These solids have specific structure and belong to a family of solids known in cement chemistry as aluminoferrite mono (AFm) and aluminoferrite tri (AFt) phases. The AFm and AFt phases are calcium derivatives of a larger family of layered materials called layered double hydroxides (LDHs). This class of compounds contains layers of metal hydroxides that contain two different metallic cations and an interlayer that contains anionic species and water molecules (De Roy et al., 1992; Renaudin, Kubel, Rivera, and Francois, 1999). The metal hydroxide layer contains divalent cation (M^sup 2+^) hydroxide sheets, in which a fraction of the M^sup 2+^ sites have been substituted with trivalent cations (M^sup 3+^). The isomorphous substitution of the divalent cations with the trivalent cations develops permanent positive charge on the hydroxide layers that is counterbalanced by the interlayer onions (Crepaldi et al., 2000; Goswamee et al., 1998). The general chemical composition of LDHs can be represented as follows (Olanrewaju et al., 2000; Rives, 2001; Ulibarri and Hermosin, 2001):
M^sup II^xM^sup III^(OH)^sub 2x+2^(A^sup -n^)^sub 1/n^ . mH^sub 2^O (1)
M^sup II^ = divalent cations (calcium [Ca^sup 2+^], magnesium [Mg^sup 2+^], zinc [Zn^sup 2+^], cobalt [Co^sup 2+^], nickel [Ni^sup 2+^], copper [Cu^sup 2+^], manganese II [Mn^sup 2+^], but also the monovalent cation lithium [Li+]),
M^sup III^ = trivalent cations (aluminum [Al^sup 3+^], chromium [Cr^sup 3+^], ferric iron [Fe^sup 3+^], cobalt III [Co^sup 3+^], manganese III [Mn^sup 3+^]),
A^sup n-^ = interlayer anions with charge (n-) (almost freely selected, organic, and inorganic anions), and
x = the stoichiometric ratios of M^sup II^.
In the AFm and AFt phases, octahedral sheets of calcium hydroxide [Ca(OH)^sub 2^] are substituted with Al^sup 3+^ or Fe^sup 3+^, and the charge is neutralized by interlayer anions, such as carbonate (CO^sub 3^^sup 2-^), sulfate (SO^sub 4^^sup 2-^), hydroxide (OH^sup - ^), nitrate (NO^sub 3^^sup -^), chloride (Cl-), bromide (Br-), and iodide (I-) (Birnin-Youri and Glasser, 1998; Francois et al., 1998; Glasser et al., 1999, Rapin et al., 2002; Rapin, Noor, and Francois, 1999; Rapin, Walcarius, Lefevre, and Francois, 1999; Renaudin and Francois, 1999; Renaudin et al., 2000; Renaudin, Francois, and Evrard, 1999). Examples of AFm phases are as follows: calcium monosulfoaluminate [Ca^sub 4^Al^sub 2^SO^sub 4^(OH)^sub 12^] (Glasser et al., 1999), calcium chloroaluminate [Ca^sub 4^Al^sub 2^Cl^sub 2^(OH)^sub 12^] (Birnin-Youri, 1993), and nitrated AFm phase [Ca^sub 4^Al^sub 2^(NO^sub 3^)^sub 2^(OH)^sub 12^] (Renaudin et al., 2000). Ettringite [Ca^sub 6^Al^sub 2^(SO^sub 4^)^sub 3^(OH)^sub 12^] is a well-known example of an AFt phase (Clark and Brown, 2000; Myneni et al., 1998; Perkins and Palmer, 1999).
Interionic exchange can occur between two or more AFm or AFt phases, because the interlayer anions are loosely held by electrostatic forces. This exchange attains equilibrium, thereby producing solid solutions of varying compositions (Bimin-Yauri, 1993; Birnin-Yauri and Classer, 1998; Classer et al., 1999; Pllmann, 1986; Stronach, 1996). Such behavior means that removal of anions in the UHLA process depends on the relative affinity of these anions for the solid phases and that solid solutions may form as a result of anion exchange. Therefore, understanding interactions among components such as chloride, sulfate, and silica in the UHLA process is very important to evaluate multicomponent removal from recycled cooling water and other applications of the UHLA process. The purpose of this paper is to study the interactions between chloride and sulfate and between chloride and silica in the UHLA process.
A series of equilibrium experiments was conducted to evaluate interactions among the processes removing chloride, sulfate, and silica. Initial concentrations of each component were chosen to cover a range of possible applications of the UHLA process, such as cooling water treatment, membrane pretreatment, and brine treatment. First, experiments to study removals of each compound (chloride, sulfate, and silica) by itself were conducted by adding dry lime [Ca(OH)^sub 2^] and dry sodium aluminate (NaAlO^sub 2^) to solutions of sodium chloride (NaCl), sodium sulfate (Na^sub 2^SO^sub 4^), or sodium silicate (Na^sub 2^SiO^sub 3^), Then, similar sets of experiments were conducted to study the interactions between target compounds by adding dry Ca(OH)^sub 2^ and dry NaAlO^sub 2^ to solutions of NaCl + Na^sub 2^SO^sub 4^ or NaCl + Na^sub 2^SiO^sub 3^ The equilibrium experiments were conducted in 250-mL sealed plastic bottles. The bottles were shaken for 2 days at room temperature (23 to 25C) to reach equilibrium. To avoid carbon dioxide contamination, the bottles were put in a closed container with a carbon dioxide absorbent during the reaction time. Samples of the reactors were taken and filtered through 0.45- membrane filters. The filtrates were analyzed for calcium using atomic absorption spectrophotometry, for aluminum with UV spectrophotometry (eriochrome cyanine method; APHA et al., 1995), for chloride and sulfate using ion chromatography, and for silica with UV spectrophotometry (molybdosilicate method, APHA et al., 1995). The pH of a sample was determined before filtration using a pH meter with a combination glass electrode standardized with pH 10.00 and pH 13.00 buffers.
Results and Discussion
Effect of Chemical Doses on Chloride Removal. Three initial concentrations of chloride (10, 50, and 100 mM) were investigated at each of three ratios of chemical doses to initial chloride concentrations. Ratios of lime dose to initial chloride concentrations were 1:1, 2:1, and 3:\1, while the aluminum doses were chosen to be equal to one-half of the lime dose for all experiments. This provides a dose of aluminum relative to calcium that is stoichiometric relative to calcium chloroaluminate [Ca^sub 4^Al^sub 2^Cl^sub 2^(OH)^sub 12^]. Figure 1 shows the effect of lime dose on chloride removal and indicates that the removal efficiency for chloride increased with increasing lime and aluminum doses. However, removals of chloride were lower than predictions based on assuming that removal was a result of the formation of calcium chloroaluminate [Ca^sub 4^Al^sub 2^Cl^sub 2^(OH)^sub 12^]. This can be explained by assuming chloride removal was a result of the formation of a solid solution containing calcium chloroaluminate, tricalcium hydroxyaluminate [Ca^sub 3^Al^sub 2^(OH)^sub 12^], and tetracalcium hydroxyaluminate [Ca^sub 4^Al^sub 2^(OH)^sub 14^] solids. Measured final concentrations of calcium, chloride, aluminum, and pH and the results of equilibrium modeling indicated that the fractions of these solids in the solid solution were dependent on
the relative concentrations of chloride, hydroxide, and aluminum hydroxide ion [Al(OH)^sub 4^^sup -^] in the solution (Abdel-Wahab and Batchelor, 2002; Abdel-Wahab et al., 2002).
Effect of Chemical Doses on Sulfate Removal. Figure 2 shows the effect of chemical doses on sulfate removal and shows that removal efficiency of sulfate increased with increasing lime and aluminum doses, which indicates that sulfate precipitates with calcium and aluminum. The hypothesis was made that sulfate removal is controlled by the precipitation of ettringite [Ca^sub 6^Al^sub 2^(SO^sub 4^)^sub 3^(OH)^sub 12^] according to the following reaction:
If ettringite is the only important solid that is precipitated, the ratio of calcium removed to sulfate removed should equal 2.0, for example, at a ratio of lime dose to initial sulfate concentration equal to 2.0, the maximum theoretical removal efficiency for sulfate should be 100%, as shown by the dotted line in Figure 2. The ratios of calcium removed to sulfate removed and the ratios of aluminum removed to sulfate removed are shown in Figure 3. Figures 3a and b indicate that these ratios agreed with the stoichiometric ratios of ettringite precipitation at low chemical doses of lime and sodium aluminate, but they deviated from theoretical stoichiometry at higher doses. Figure 3 shows that the removal ratios increased with increasing lime dose and aluminum dose. This indicates that other solids were formed that are rich in calcium and aluminum with respect to sulfate. One such solid is calcium monosulfoaluminate [Ca^sub 4^Al^sub 2^(SO^sub 4^)(OH)^sub 12^], which is called monosulfate, and has the following formation reaction:
The changes in stoichiometric ratios could also be explained by the formation of other solids, such as tricalcium hydroxyaluminate, tetracalcium hydroxyaluminate, calcium hydroxide, aluminum hydroxide, and calcium sulfate.
Effect of Chemical Doses on Silica Removal. Two initial concentrations of silica (1.5 and 3.0 mM) were investigated at different ratios of chemical doses to initial silica concentrations. Figure 4 shows the effect of chemical doses on final silica concentrations and shows that silica concentrations decreased with increasing lime and sodium aluminate doses. However, removal efficiency of silica is higher at an initial silica concentration of 3.0 mM than at 1.5 mM. This is because pH values were lower in experiments with an initial silica concentration of 1.5 mM. The hypothesis was made that silica removal could be described as the precipitation of calcium silicate (CaH^sub 2^SiO^sub 4^) (Batchelor and McDevitt, 1984; Batchelor et al., 1991) and/or calcium aluminosilicate [Ca^sub 2^Al^sub 2^Si(OH)^sub 14^] (Glasser et al., 1999). If calcium silicate is the only important precipitated solid, the ratio of calcium removed to silica removed should equal 1.0. Similarly, if calcium aluminosilicate is the only important precipitated solid, the ratio of calcium removed to silica removed should equal 2. Figure 5 shows that, when the ratio of lime dose to initial silica concentration is less than 1.0, the ratio of calcium removed to silica removed is approximately equal to the theoretical ratio of 1.0. However, this ratio increased as the lime dose increased and approached the theoretical stoichiometric ratio of calcium aluminosilicate precipitation (2.0). This behavior indicates that both calcium silicate and calcium aluminosilicate precipitated. Note that the pH values in these experiments were in the range pH 11.6 to 12.2, which are below the pH at which calcium hydroxide would precipitate. Therefore, the only important calcium-containing solids in the system are believed to be calcium silicate and calcium aluminosilicate.
Interactions Between Chloride and Sulfate in UHLA Process. Effect of Chloride Concentration on Sulfate Removal. Three initial concentrations of sulfate (10, 50, and 100 mM) were investigated at three initial concentrations of chloride (10, 50, and 100 mM). Each chloride-sulfate combination was investigated at three molar ratios (100, 200, and 300%) of lime dose to the sum of the initial concentrations of chloride and sulfate (Cl + SO^sub 4^). Aluminum doses were chosen to be 50% of lime dose for all experiments. Final sulfate concentrations are shown in Figure 6 as a function of lime dose at different initial chloride and sulfate concentrations. Chloride was found to have a negligible effect on final sulfate concentrations. The differences among final sulfate concentrations are negligible over a range of O to 100 mM initial chloride concentrations at the same lime dose. This indicates that removal of sulfate does not depend on chloride concentration.
Effect of Sulfate Concentration on Chloride Removal. Figure 7 shows final chloride concentrations as a function of initial sulfate concentrations at an initial chloride concentration of 50 mM. Final chloride concentrations increased with increasing initial sulfate concentrations at all investigated ratios of chemical doses to initial sulfate plus chloride.
Several types of interactions between sulfate and chloride could be observed to explain this behavior. The simplest interaction would be competition for the calcium and aluminum that were added. If doses of calcium and aluminum are below the stoichiometric amount needed to precipitate the solids that contain chloride and sulfate, then the compound that formed the more insoluble sold phases could outcompete the other for the reagents. For example, if sulfate formed more insoluble solid phases, its addition could result in higher concentrations of chloride, because the chloride-containing solid phases would not be able to form. This interaction could explain the behavior shown in Figure 7 for conditions where the doses of calcium and aluminum were not high enough to form both the sulfate-containing solid and the chloride-containing solid. Figure 6 indicates that a sulfate-containing solid with a calcium-to-sulfate ratio near 2.0 was formed, which indicates that the sulfate- containing solid was ettringite . [Ca^sub 6^Al^sub 2^(SO^sub 4^)^sub 3^(OH)^sub 12^]. The most probable chloride-containing solid would be calcium chloroaluminate [Ca^sub 4^Al^sub 2^Cl^sub 2^(OH)^sub 12^]. If these solids are being formed, the critical value of R would be 2.0. Values of R greater than 2.0 would provide an excess of calcium, and values less than 2.0 would provide less calcium than needed to remove all chloride and sulfate. Figure 7 shows that the strongest effect of sulfate on decreasing chloride removal occurred when there was a deficiency of calcium (R = 1 ). However, significant effects occurred when the calcium dose was at the critical value (R = 2) and when calcium was provided in excess (R = 3). Therefore, an additional mechanism is needed to explain the observed results. Increased final chloride concentrations with increasing initial sulfate concentration could also be a result of an increase in the activity of calcium chloroaluminate solid in the presence of sulfate and resulted in increasing the observed solubility product of the solid. Stumm and Morgan (1996) reported that the activity of a constituent in the solid solution formation is increased when it becomes a minor constituent of a solid solution phase. Therefore, and because of the higher affinity of sulfate to form solids with calcium and aluminum in UHLA with respect to chloride and also because of the low solubility of these solids with respect to calcium chloroaluminate solid, the fraction of calcium chloroaluminate solid is small compared with the fraction of sulfoaluminate solids, especially at high initial sulfate concentrations with respect to chloride concentrations. Increased final chloride concentrations with increasing initial sulfate concentration also could be a result of the effect of sulfate on the solid solution structure, which weakens the ability of chloride to be held in the interlayer space of the solid solution formation.
Figure 8 shows a comparison between the removal efficiency of chloride and the removal efficiency of sulfate. Removal efficiencies for both chloride and sulfate increased with increasing lime and aluminum doses. However, sulfate removal efficiency is much higher than chloride removal efficiency at the same chemical doses and at the same initial concentrations of sulfate and chloride. This indicates that sulfate precipitation with calcium, aluminum, and hydroxide is more favorable than chloride precipitation. These results agree with the results of De Roy et al. (2001), Rives (2001), and Ulibarri and Hermosin (2001), who all concluded that sulfate anions have a higher affinity for the interlayer space of the LDHs than chloride anions.
Interactions Between Chloride and Silica Removals in UHLA Process. Figure 9 shows final silica concentrat\ions at different initial chloride concentrations and indicates that chloride has a negligible effect on silica removal. Final silica concentrations approached zero at high lime and aluminum doses, and increased chloride concentrations did not affect silica removal. Effect of silica on chloride removal with UHLA at different initial chloride concentrations and different chemical doses is shown in Figure 10. Silica has a negligible effect on chloride removal with the UHLA process. This could be because the investigated initial silica concentrations (1.5 and 3.0 mM) are low compared with chloride concentrations, although they are typical of concentrations generally found in recycled cooling water. Therefore, the chemical doses that were consumed in precipitating silica were negligible compared with total chemical doses in the system and resulted in small effect on final chloride concentrations.
Interactions between chloride, sulfate, and silica during treatment by the UHLA process have been investigated. Results of these experiments indicated that formation of ettringite and calcium monosulfate is more favorable than formation of calcium chloroaluminate. Therefore, chloride concentration was found to have a negligible effect on sulfate removal with UHLA. On the other hand, increased sulfate concentrations resulted in decreasing the removal efficiency of chloride, even when sufficient calcium and aluminum are present to remove both anions. Silica was found to have a small effect on chloride removal with UHLA. Similarly, chloride was found to have a negligible effect on silica removal with UHLA.
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Credits. This project was funded, in part, with funds from the State of Texas as part of the program of the Texas Hazardous Waste Research Center. The contents do not necessarily reflect the views and policies of the sponsor, nor does mention of trade names or commercial products constitute endorsement or recommendations for use.
Authors. Ahmed Abdel-Wahab is a visiting assistant professor at Texas A&M University at Qatar (Doha, Qatar) and Bill Batchelor is a professor in the Civil Engineering Department, Texas A&M University. Correspondence should be addressed to Ahmed Abdel-Wahab, Texas A&M University at Qatar, P.O. Box B-6, College Station, TX 77844; e- mail: ahmed-abdelwahab@ tamu.edu.
Submitted for publication October 7, 2005; revised manuscript submitted December 22, 2005; accepted for publication January 24, 2006.
The deadline to submit Discussions of this paper is March 15, 2007.
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