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Coagulation behaviors of aluminum salts towards fluoride: Significance of aluminum speciation and transformation
Accepted Manuscript Coagulation behaviors of aluminum salts towards fluoride: Significance of aluminum speciation and transformation Zan He, Huachun Lan, Wenxin Gong, Ruiping Liu, Yuping Gao, Huijuan Liu, Jiuhui Qu PII: DOI: Reference:
S1383-5866(16)30017-X http://dx.doi.org/10.1016/j.seppur.2016.01.017 SEPPUR 12799
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
15 October 2015 13 January 2016 13 January 2016
Please cite this article as: Z. He, H. Lan, W. Gong, R. Liu, Y. Gao, H. Liu, J. Qu, Coagulation behaviors of aluminum salts towards fluoride: Significance of aluminum speciation and transformation, Separation and Purification Technology (2016), doi: http://dx.doi.org/10.1016/j.seppur.2016.01.017
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Coagulation behaviors of aluminum salts towards fluoride: Significance of aluminum speciation and transformation
Zan Hea,b, Huachun Lan a,*, Wenxin Gonga,b, Ruiping Liua, Yuping Gaoa, Huijuan Liua, Jiuhui Qua
a
Key Laboratory of Drinking Water Science and Technology, Research Center For Eco-Environmental Science, Chinese Academy of Sciences, Beijing 100085, China
b
University of Chinese Academy of Sciences, Beijing, 100039, China
*
Corresponding author. Tel.: +86 10 62849160; fax: +86 10 62849160. Email address: [email protected] (H. Lan)
1
Abstract: This study investigates the effects of aluminum (Al) and F interactions on the coagulation behaviors of Al salts, i.e., AlCl3 and polymer aluminum chloride (PACl) at different basicity (B) towards fluoride. These coagulants achieve the optimum fluoride removal over pH 6-7, and PACl exhibits higher removal efficiency than AlCl3 does over pH 4-9. The removal of fluoride is positively correlated with the content of either Al13 or Alb in Al-based coagulants over pH 4-9 and increases in the order: PACl2 > PACl1 > AlCl3. As for PACl, the increased B from 0 to 2.4 improves fluoride removal owing to the elevated ratios of Alb. However, at further increased B to 2.8, the formation of colloidal Al species (Alc) occurs and this effect adversely inhibits fluoride removal thereafter. The removal of fluoride by these coagulants increases with elevated initial fluoride concentrations ([F]0) from 0 to 20 mg/L. At high [F]0 of 80 mg/L, fluoride removal by AlCl3 decreases to as low as 13.1% owing to the formation of soluble Al-F complexes, as indicated from the high residual Al concentrations of 16 mg/L. Interestingly, this adverse effect can hardly be observed for PACl due to the stronger stability of Al13 or Alb. As for AlCl3 and PACl, the introduction of fluoride changes the Al species distribution to some extent owing to the Al-F complxation, as indicated by the analysis of electrospray ionization mass spectrometry. The possible Al-F complexes formed fluoride and different Al species are proposed 2
thereafter. PACl with diverse Al species and high Alb or Al13 content shows higher removal efficiency towards fluoride than AlCl3 does, especially at high [F]0 or under acidic pH and alkaline pH conditions.
Keywords: Fluoride, Al salts, Coagulation, Al species transformation, Al-F complexes, ESI-MS
1. Introduction Fluoride at low concentrations is beneficial to human heath to strengthen bones and to prevent tooth decay [1]. However, the long-term excessive intake of fluoride via drinking water can lead to the wide occurrence of dental and skeletal fluorosis [2], of which over 500 million people are suffering from fluorosis globally [3]. The removal of fluoride from drinking water has received great attention in the past several decades. Defluoridation can be achieved by different processes such as adsorption [4-6], (electro)coagulation [7-9], ion-exchange [10], membrane separation [3], and the Nalgonda process [11]. Among these technologies, coagulation is widely applied due to its simplicity and high efficiency. Aluminum chloride (AlCl3) and polyaluminum chloride (PACl) are commonly used coagulants, and have been proposed and well investigated with respect to fluoride removal [8, 12]. Our previous study indicates that the formation of Al-F complexes 3
involves the removal of fluoride by AlCl3, and this effect benefits fluoride removal to a large extent as compared to Al(OH)3 adsorption [7]. The transformation of different fluoride species, i.e., free fluoride , complexed fluoride (Com-F), and adsorbed fluoride, plays a determining role and is highly dependent on solution pH and the ratio of fluoride to Al (RF:Al) [8,13]. Additionally, Al-F complexation also affects the hydrolysis of Al salts and their distribution and transformation afterwards. On the other hand, the hydrolysis of Al salts tends to form a series of Al species with different structures, charges, and polymerization character [14], and the diversity of Al species possibly impacts the interaction between fluoride and different Al species thereafter. PACl as polymeric species of Al salt shows superiority in removing particulate and organics as compared to traditional monomeric Al such as AlCl3, and PACl exhibits inherent advantages less independence on pH [15], less alkalinity consumption [16], and less sludge production [17]. However, its superiority in terms of fluoride removal as compared to AlCl3 and the mechanisms involved in has rarely been investigated. PACl contains a serial of Al species, of which Al13 ([AlO4Al12(OH)24(H2O)12]7+) is the dominant species [18-19]. Al13 can also be in situ generated in AlCl3 coagulation at pH 5.5, with the in situ Al13 species exhibiting better removal of haloacetic acid precursors than preformed PACl does, as indicated 4
from our previous study [20]. AlCl3 is also observed to be more effective than PACl in removing turbidity and dissolved organic matter in eutrophic water, and this is not only attributed to the generation of in situ Al13 species but to AlCl3 functioning as a pH control agent [21]. These results indicate that Al species play a determining role in the coagulation behaviors of Al salts, and their species distribution and transformation is critically important in evaluating their coagulation behaviors. However, to the best of our knowledge, few studies have focused on the effects of Al species distribution and transformation on fluoride removal by Al coagulation. This study first aims to compare the removal efficiency of different aluminum salts, i.e., AlCl3 and PACl, towards fluoride under different conditions of pH, basicity, and RF:Al. The effects of fluoride on the transformation and distribution of different Al species are also investigated by electrospray ionization mass spectrometry (ESI-MS). Finally, the dominant mechanism involved in the removal of fluoride by different species Al is proposed.
2. Materials and methods 2.1 Chemicals Unless specifically noted, all chemicals used in the tests were of analytical-reagent grade and were used without further purification. 5
The stock solution of sodium fluoride (NaF) was prepared in deionized water. The stock solution of AlCl3, PACl1 and PACl2 (PACl1 and PACl2 stand for different B of PACl), characteristics of which is shown in the Table S1, was freshly prepared prior to use. The fluoride-containing water was obtained by the dilution of NaF stock solution to desired concentrations, and potassium nitrate (KNO3) solution at 0.01 M was added to provide background ionic strengthen. The hydrochloric acid (HCl) or NaOH solution at 1 M were prepared with deionized water to adjust pH in the tests process.
2.2 Coagulants characterization and analytical methods PACl and a commercial AlCl3·6H2O were used for all tests. PACl with different basicity (B, OH/Al molar ratios) were prepared by the method of Zhao et al..[22]. B values were determined by titrimetric methods (Standard method of the chemical industry of China).Total Al concentrations were measured by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) (OPTIMA 2000DV, PerkinElmer, USA).The Al species were analyzed by the ferron colorimetric method [23]. Ala, Alb, and Alc corresponded to monomeric, medium polymer, and large polymer species and/or solid-phase Al(OH)3, respectively. The effect of fluoride on Al species of these coagulants was investigated by electrospray ionization mass spectrometry (ESI-MS). 6
In the absence of fluoride, Al species distribution of coagulants was first measured, and AlCl3 and PACl were dissolved in deionized water to obtain a final solution of 4 mg Al/L. Tetramethylammonium hydroxide pentahydrate ((CH3)4NOH·5H2O) or HCl solution was used to adjust solution pH to desired values. The ESI-MS spectra were recorded with a micromass hybrid quadrupole time-of-flight (TOF) mass spectrometer (2695XE micro, water, US) equipped with an electrospray ion source. Details of the operating procedures and parameters of the apparatus can be referenced in the literature [24]. In the presence of fluoride, AlCl3 and PACl were dissolved in fluoride-containing water to obtain a final mixed solution of 4 mg Al/L and 4 mg F/L, and the Al species measurement of follow procedure was the same as it in the absence of fluoride.
2.3 Jar test The coagulation tests were performed using a Phipps and Bird six-paddle stirrer (MY 3000-6, QianJiangMeiYu Instruments, China) at room temperature (25±2 oC). 300 ml of fluoride-containing water was transferred to a 500 ml beaker and then the stock solution of coagulants was added to make sure that Al dose was the desired value. After addition of the Al-based coagulant, the following procedure consisted of 2 min of mixing at 100 rpm, with pH adjusted to the 7
desired value, 2 min rapid mix at 200 rpm, 15 min slow mix at 40 rpm, and 30 min of settling. After settling for 30 min, supernatants were filtered through 0.45 μm membrane to analyze the concentrations of fluoride and Al in the filtrate. The concentrations of fluoride were measured by the ion selective electrode method (PF-1, Shanghai KangYi Technology), and the concentrations of Com-F were obtained by total fluoride concentration minus free fluoride concentration according to the methods of our previous study [7]. The residual Al concentrations were measured by ICP-OES.
3. Results and discussion 3.1. Removal efficiency of fluoride by different Al coagulants The removal efficiency of fluoride by AlCl3, PACl1, and PACl2 in a pH range of 4-9 is illustrated in Fig. 1. The optimum pH for fluoride removal was observed to be in pH 6-7 in regardless of coagulant type. With pH increasing from 7 to 9, the removal efficiency of fluoride by PACl1 and PACl2 decreased from 91.4% to 63.2%. Comparatively, fluoride removal by AlCl3 was more significantly dependent on pH and decreased sharply to as low as 17.4%. At pH < 6.0, the removal of fluoride by these three coagulants decreased with lowered pH in the order: AlCl3 > PACl1 > PACl2. For example, the residual fluoride concentrations at pH 5.0 were observed to be 4.3, 2.8, and 1.9 mg/L after coagulation by AlCl3, PACl1, 8
and PACl2, respectively. These results indicated that coagulant type and pH significantly affected fluoride removal performance and PACl1 and PACl2 removed fluoride more efficiently than AlCl3 did, especially under acidic pH and alkaline pH conditions. Chen et al. (2006) reported that Al13 showed high positive charge, stable structure, and strong binding affinity, and is the most active Al species in Al salts responsible for coagulation process [25]. In addition, Al13 also contributed most to the amount of Al species for PACl in basicity range from 1.5 to 2.5 [26, 27]. Our previous study indicated that pH significantly affected the Al species distribution of AlCl3 and preformed PACl, and in acidic or alkaline pH region the ratios of monomeric Al increased whereas polymeric and colloidal Al species decreased as compared to the near-neutral pH region [21]. In pH 6-7, the transformation of monomeric Al3+ within AlCl3 into polymeric Al occurred, and the ratios of medium and large polymeric Al as expressed by Alb were reported to increase from 71% to 82% with elevated pH from 5 to 7 [21]. Consequently, polymeric Al, i.e., Alb or Al13, was inferred to play an important role in the removal of fluoride by Al salts over wide pH range from 4 to 9, and their fluoride removal efficiency increased in the order: PACl2 > PACl1 > AlCl3. Aoudj et al. reported that simultaneous removal of chromium(VI) and fluoride by electrocoagulation-electroflotation: application of a hybrid Fe-Al anode and showed that fluoride removal was mainly ascribed to 9
alumimum plates not to iron plates[28]. Liu et al. reported simultaneous removal of arsenate and fluoride by iron and aluminum binary oxide and experimental result revealed that iron oxyhydroxide (FeOxHy) showed a high removal capability towards As(V) but exhibited little efficacy to remove F in systems where both As(V) and F co-exist or for solutions of fluoride alone and AlOxHy may simultaneously remove As(V) and F over a wide pH range from 4 to 11[29]. The above results indicated that Al-based material showed superiority in removing fluoride as compared to Fe-based material. 3.2 Al species distribution of different Al coagulants over pH 4-9 To investigate the effect of Al speciation on fluoride removal, the Al species distribution of AlCl3, PACl1, and PACl2 after coagulation were analyzed over a wide pH range from 4 to 9, and Al species distribution was observed to be highly dependent on pH for each coagulant (Fig. 2). As for AlCl3, in acidic pH region, the ratios of Ala decreased whereas those of Alb increased with elevated pH, and the Ala reached the minimum ratios of 25.7% and 27.3% at pH 6 and 7 (Fig. 2a). At further elevated pH of 8 and 9, the ratios of Ala increased to 31.4% and 51.5% owing to the formation of aluminates [4]. Alb exhibited the highest ratios of 50.9% and 52.0% in the near-neutral pH region of 6 and 7, and then decreased to 43.6% and 23.1% at elevated pH of 8 and 9. The ratios of 10
Alc varied little and were near to 24.0% at pH 5-9. PACl1 and PACl2 showed similar trends in Al species transformation, which were significantly different from that of AlCl3 over pH 4-9. As for PACl2, the ratios of Alb were determined to be 97.0% and 93.6% at pH 6 and 7, and then slightly decreased to 84.0% and 89.0% at acidic pH of 4 and 5. The lowered Alb ratios were attributed to its dissolving into Ala rather than its transformation to Alc, the ratios of which were near 0 % at pH 4 to 6. Comparatively, the significant decrease of Alb ratios also occurred at elevated pH of 8 and 9; however, this was attributed to the formation of larger polymer and Al(OH)3 precipitates, i.e., Alc, rather than the formation of aluminates as AlCl3. This effect enabled its much higher efficacy to remove fluoride as compared to AlCl3 (Fig. 1). These results indicated that Alb played a determining role in fluoride removal by Al coagulation. The removal efficiency of fluoride was observed to be positively correlated with the Alb content (Fig. S1), and this contributed to the different behaviors of these Al salts towards fluoride as observed in Fig. 1. At neutral pH 6-7, the monomeric Al transformed to medium polymeric Al, and the fluoride removal by AlCl3 was near to that by either PACl1 or PACl2. Under acidic pH conditions, the polymerisation of Al3+ was inhibited in AlCl3 whereas the preformed Alb within PACl1 and PACl2 was stable, and the high Alb ratios enable their higher efficiency to remove fluoride as compared to AlCl3. At 11
alkaline pH 8-9, the formation of aluminates as indicated by Ala was significant in AlCl3 system whereas in the other two systems, the formation of larger polymer and Al(OH)3 precipitates, i.e., Alc, was dominant, and the larger polymer and Al(OH)3 removed fluoride more efficiency than aluminates did. In the absence of fluoride, the Al species distribution and transformation of AlCl3 and PACl over pH 4-9 has been investigated in our previous study [21]. Briefly, the variation trends of Ala and Alb in pH 4-9 were similar to those with fluoride-presenting as observed in this study. As for Alc, however, the introduction of fluoride increased the Alc fractions greatly and Alc contributed most to the total Al at pH 8-9 for both AlCl3 and PACl. It was inferred that fluoride favored the formation of Alc. It was attributed to that the replacement of some hydroxyls by fluoride ions due to their similar chemical property contributed to the transformation of Al ions with less OH- to induce precipitation [7]. 3.3 Removal efficiency of fluoride by PACl with different basicity To further investigate the coagulation behaviors of different Al species towards fluoride, we prepared the PACl with different basicity and compared their fluoride removal efficiency in wide pH ranges. It is noted that basicity governs the polymerization degree of PACl and significantly impacts their Al species distribution [30]. 12
Fig. 3 illustrates the fluoride removal by PACl with basicity ranges of 0-2.8 at pH 6, 7, and 8, and the optimum pH for fluoride removal was observed to be 7. Additionally, the removal of fluoride was observed to be dependent on PACl basicity. At pH 6 and 8, the elevated basicity from 0 to 2.4 contributed to the increased fluoride removal from approximately 69.3% to 80.3%, and the further elevated basicity to 2.8 adversely inhibited fluoride removal to approximately 73.3%. At pH 7, fluoride removal was consistently near to 89.5% in wide basicity range from 0 to 2.4 and then decreased to approximately 81.6% at basicity of 2.8. PACl-2.4 was observed to exhibit the highest efficiency towards fluoride than other PACls at pH 6.0-8.0. Comparatively, PACl basicity at B of below 2.4 showed relatively lower effect on fluoride removal than that of above 2.4 in different pH conditions. PACl basicity plays a determining role in Al speciation and significantly affects fluoride removal thereafter. At high B above 2.5, the amorphous Al(OH)3 is the dominant Al species within PACl coagulant [26]. In this case fluoride removal was mainly attributed to physical adsorption and ion exchange effects [4, 31], and the weakly acidic and acidic pH favored its removal. In B ranges from 1.0 to 1.5, PACl includes different Al species such as Al2, Al4, Al7 and Al13 et al. [27], the ratios of monomeric Al species were observed to increase with lowered B. In this case coagulation dominated in the removal of fluoride, and different 13
mechanisms such as complexed precipitation, charge attraction, and ion exchanges involve in and facilitated fluoride removal accordingly[32-33]. At low B of below 0.5, the monomeric Al species tends to dissolve at low pH and cannot effectively remove fluoride. The residual Al levels were observed to be the lowest at pH 7.0, and this effect provides more particulate Al available for fluoride removal. 3.4 Effect of fluoride on coagulants towards fluoride The formation of Al-F complexes plays an important role in the removal of fluoride by Al salts [7]. Corbillon et al. (2008) reported that initial fluoride concentrations as expressed by [F]0 strongly affected the interactions between aluminum and fluoride [34]. Fig. 4 illustrates the effects of [F]0 on the removal of fluoride by AlCl3, PACl1, and PACl2, the formation of Com-F, and soluble Al at pH 6.5. As shown in Fig. 4a, the removal of fluoride increased with elevated [F]0 in the [F]0 range from 0 to 20 mg/L and little difference was observed among these three coagulants. However, at further elevated [F]0 from 20 to 80 mg/L, these coagulants showed critically different removal behaviors towards fluoride. In AlCl3 coagulation system, the maximum fluoride removal of 17.9 mg/L was observed at 40 mg/L [F]0. At elevated [F]0 of 80 mg/L, the removed fluoride decreased to as low as 10.5 mg/L. Comparatively, the removal of fluoride by PACl1 and PACl2 rarely 14
decreased even at [F]0 of as high as 80 mg/L. It was additionally observed that PACl1 exhibited the highest fluoride removal efficacy among these three coagulants over a wide [F]0 range from 20 to 80 mg/L. The removal of fluoride by Al salts is achieved by the transformation of soluble fluoride to particulate fluoride. The formation of soluble Al-F complexes affected the species distribution and transformation of fluoride and aluminum accordingly [13]. It was observed in Fig. 4b that the ratios of Com-F (RCom-F) increased with elevated [F]0 for three coagulants, and the RCom-F values in AlCl3 coagulation system were much higher than those in the other two systems. The formation of Al-F complexes between fluoride and PACl1 or PACl2 was negligible at [F]0 of below 20 mg/L, and at high [F]0 of 80 mg/L the RCom-F increased slightly to 24% and to 5% for PACl1 and PACl2 systems, respectively. Monomeric Al tends to form Al-F complexes with fluoride, and the ratios of different Al-F complexes such as AlF2-, AlF2−, AlF3, and AlF4− may be quantified by Miniteq software modeling [13]. At pH 6.5, the dissociation of PACl1 and PACl2 into monomeric Al rarely occurred (Fig. 4c), and the formation of Al-F complexes was slight. Additionally, during the preparation of PACl, a certain amount of OH- has been incorporated into them. The attachment of fluoride onto PACl1 and PACl2 maybe favor the formation of insoluble Al-OH-F precipitates rather than soluble Al-F complexes. Al-F complexes formation increased soluble Al concentrations (Fig. 15
4c), which were positively correlated with the ratios of Com-F (Fig. S2). This effect reduced the amount of insoluble Al available for fluoride removal. Comparatively, the residual Al levels in AlCl3 coagulation system were much higher than those in both PACl1 and PACl2 coagulation systems at [F]0 of above 10 mg/L, and the inhibited fluoride removal by AlCl3 was observed accordingly. The elevated [F]0 may also inhibit the hydrolysis and precipitation of PACl1 and PACl2; however, this effect was relatively slight and the maximum soluble Al concentrations were observed to be as low as below 2.6 mg/L. At high [F]0, however, fluoride inhibited Al3+ precipitation and adversely inhibited fluoride removal by AlCl3 coagulation thereafter. Comparatively, the complexion between fluoride and PACl1/PACl2 with stable polymeric structure may occur; however, the residual Al concentrations were much lower than that observed during AlCl3 coagulation. Moreover, PACl1 showed higher fluoride removal than PACl2 did at [F]0 of above 20 mg/L, this reason would be discussed in detail later. 3.5 Effect of fluoride on Al species transformation Fluoride can affect the hydrolysis process of Al salts as confirmed by Al-Ferron method (Fig. S3), and it was inferred that fluoride may also affect the Al species distribution and transformation of these coagulants. To further illustrate this effect, the Al species distribution of AlCl3 and 16
PACl before and after introducing fluoride was investigated by ESI-MS analysis at pH 4. Fig. 5a illustrates the ESI-MS spectra of AlCl3 solution, and the most intense peaks were assigned to dimeric Al species [Al2O2(OH)(H2O)0-5]+ at mass to charge ratio (m/z) 157 with 100% relative intensity. Other dimeric Al species at m/z 139, 121, and 175 and the monomers [Al(OH)2(H2O)2-3]+ at m/z 97, and Al13 at m/z 213, 219, and 337 were also observed. In addition, the series of polymers containing Al3 cores (at m/z 199, 217, and 235) and Al4 cores (m/z 99, 105, 277, 295, and 313) were also the dominant Al species. It was noted that the multi-charged polymeric series, e.g., Al7, Al14, Al15, and Al16 cores can also be observed in the spectra although their intensity was relatively low. The introduction of fluoride changed the Al species distribution to some extent (Fig 5b). The relative intensity of Al2 (at m/z 121, 139, 157, and 175), Al3 (m/z 199, 217, and 235), and Al13 (m/z 337) diminished whereas that of the ions at m/z 123, 125, 141, 143, 159, 161, 177, 201, 203, 219, 221, 223, 237, 239, and 241 was enhanced. This was attributed to the interactions between different Al species and fluoride. Al1-Al4 species were observed to be the major Al species in regardless of the presence of fluoride or not. On the basis of m/z variation analysis, the main interactions between different Al species and fluoride and the formation of Al-F complexes were proposed in Table 1. 17
As shown in Table 1, in AlCl3 coagulation system, Al2 and Al3 were the dominant Al species to react with fluoride and to form Al-F complexes. Al2-F complexes mainly included Al2F at m/z of 123, 141, 159, and 177, Al2F2 at m/z of 125, 143, 161, and 179, and Al2F3 at m/z of 145 whereas Al3-F complexes included Al3F at m/z of 201, 219, 237, and 255, Al3F2 at m/z of 203, 221, 239, and 257, and Al3F3 at m/z of 205, 223, 241, and 259. It is noted that other Al species such as Al1, Al4, and Al13 also participate in the reaction with fluoride and the proposed Al-F complexes is indicated in Table 1. Fig. 5c and 5d compare the effects of fluoride on the Al species distribution of PACl solution. Al13 at m/z of 219, 225, 237, 328, 337, 355, 364, 373, 400, 418, 427, and 436 were observed to be the dominant Al species therein (Fig. 5c). The introduction of fluoride decreased the relative intensity of Al13 to some extent (Fig. 5d), and the ion peak numbers increased greatly owing to the appearance of monomeric Al1 and small polymers of Al2 and Al3. The dissociation of Al13 into monomer and small polymeric Al is attributed to the replacement of OH- in Al13 with fluoride [35]. Moreover, the main interactions between different Al species and fluoride and the formation of Al-F complexes were listed in Table 2. In PACl coagulation system, Al13 also was the dominant Al species to react with fluoride and to form Al-F complexes as compared to Al2 and Al3 in AlCl3 system, which mainly included Al13F2 at m/z of 339 18
and 357, Al13F4 at m/z of 359 and 377, Al13F6 at m/z of 361, 379, 415, and 433, which mainly contributed to the removal of fluoride. 3.6 Proposed mechanism of fluoride removal by different Al species After being dosed into solution, Al3+ tends to hydrolyze and polymerize immediately, and the formation of Al species such as bimomer, oligimer, Al polymer, sol/gel besides monomer Al as Al(OH)n(3-n)+(n=0-2) occurs [14]. The presence of fluoride complicates the reactions greatly [13]. Additionally, different Al species show critical difference in their structure, charge density, and reactivity towards fluoride. On the basis of the abovementioned interactions between fluoride and Al species, Fig. 6 illustrates the dominant mechanisms involved in the removal of fluoride by different Al species. Monomeric Al is the dominant Al species in AlCl3 solution. In case of acidic pH and high [F]0, the formation of Al-F complexes between soluble Al and fluoride is significant [5, 36, 37], and this effect inhibits Al polymerization and the removal of fluoride therafter. At elevated pH of 6-7, monomeric Al and Al-F complexes tend to precipitate with OH-, and Al-F coprecipitation contribute most to fluoride removal. At further elevated pH to alkaline pH, the OH- concentrations are much higher and the OH--dissolving effect towards Al-F coprecipitation is significant due to the competition between OH- and fluoride. This effect reduces the 19
particulate Al available for fluoride removal, and the formation of soluble aluminates also adversely inhibits fluoride removal. The removal of fluoride is achieved by its attachment onto Al flocs through charge attraction and physical adsorption reactions [38]. PACl contains more Al species as compared to AlCl3 [20], and more complicated mechanisms involve in the removal of fluoride. First, polymeric Al species exhibits higher positive charge than monomeric Al does [14] and shows stronger attraction towards fluoride. On the other hand, the average charge density carried by each Al atom of polymeric Al is lower as compared to monomeric Al [14]. Consequently, Al polymer with high polymerization degree tends to consume relatively small amount of OH- or fluoride ions to achieve charge neutralization of fluoride. This is in accordance with that at low pH condition PACl shows higher fluoride removal efficiency than AlCl3 does. As for PACl2 with higher amount of medium polymeric Al than PACl1, it exhibits higher fluoride removal efficiency accordingly. Additionally, Al13 with stable Keggin-structure achieves charge neutralization and destabilization effect over a wide pH range. At high [F]0, fluoride tends to replace the hydroxyls within Al polymer and show dissolving effect towards medium polymeric Al. The Al polymers as expressed by Alb or Al13 are damaged and transforms into low polymeric Al (Ala) (Fig. 5). At acidic pH condition, the in situ 20
formed Ala can further react with fluoride to form Al-F complexes as AlCl3 does. At neutral pH ranges, the formed complexes between fluoride and low polymers tend to transform into Al-F precipitates with OH- and fluoride ions [7]. In addition, high polymers also achieve effective fluoride removal through ion exchange and coprecipitation. At elevated pH the OH- competition effect also occurs to inhibit fluoride removal and to release the complexed fluoride. In comparison to monomeric Al, polymeric Al shows net structure, and fluoride may attach onto inner- and outer-side of polymeric Al [14]. The outside fluoride is attached by ionic exchange and charge neutralization and is easy to be replaced by OH-. Comparatively, the inside fluoride as being removed by sweep flocculation is more stable than the outside fluoride [39], and it is reluctant to be substituted by OH-. This is in accordance with the release of fluoride from AlCl3 flocs being more significant than that from PACl flocs [40]. This effect enables PACl to exhibit better removal efficiency towards fluoride at high pH than AlCl3. PACl with stable structure show stable fluoride removal over a wide pH range. However, as for one Al atom the polymeric Al13 with Keggin-structure combines less fluoride than medium polymer Al, and this is inhibitive on fluoride removal especially at high [F]0 (Fig. 4a). Generally PACl with diverse Al species shows priority in removing fluoride by coagulation especially at high fluoride concentrations. 21
4. Conclusion Al hydrolysis forms a series of Al species such as bimomer, oligimer, Al polymer, sol/gel besides monomer Al as Al(OH)n(3-n)+ (n=0-2). These Al species show critical difference in molecular structure and charge density, and the different fluoride removal efficiency is observed thereafter. In pH ranges of 4-9 the removal of fluoride by AlCl3, PACl1, and PACl2 is positively correlated with Alb species content and increases in the order: PACl2 > PACl1 > AlCl3. PACl basicity affects Al species distribution and the removal efficiency towards fluoride thereafter. PACl-2.4 shows higher fluoride removal than PACls at other basicity, owing to the highest content of polymeric Al therein. Additionally, the interactions between Al and fluoride affect Al species distribution and transformation and impact fluoride removal thereafter. As for AlCl3, fluoride at high [F]0 forms strong Al-F complexes and impairs fluoride removal. This effect is insignificant for the PACl1 and PACl2 with stable Al13 Keggin-structure and the stable removal of fluoride is observed accordingly. Fluoride tends to dissolve polymeric Al13 into small polymeric Al, and this effect also affect fluoride removal to some extent. Generally PACl with diverse Al species shows priority in removing fluoride especially at high fluoride concentrations. Acknowledgments This work was supported by the National Natural Science Foundation 22
of China (Grant No. 21177143 and 21177144) and the key project of the National “863” High-tech R&D Program of China (2012AA062604). Author Ruiping Liu gratefully acknowledges the support of the Beijing Nova Program (2013054). References: [1] A. Rafique, M.A. Awan, A. Wasti, I.A. Qazi, M. Arshad, Removal of fluoride from drinking water using modified immobilized activated alumina, J. Chem. 2013 (2013) 1-7. [2] J. Fawell, K. Bailey, J. Chilton, E. Dahi, L. Fewtrell, Y. Magara,
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28
Fluoride Removal (%)
100
80
60
40
AlCl3 PACl1 PACl2
20
0 4
5
6
pH
7
8
9
Fig. 1 Removal of fluoride by AlCl3, PACl1, and PACl2 over pH 4-9 (Al doses=40 mg/L, [F]0=8 mg/L)
29
80 Ala
Alb
Alc
(a)
60
40
20
0
(b)
Ratio of different Al species (%)
100 80 60 40 20 0
(c)
100 80 60 40 20 0 4
5
6
7
8
9
pH
Fig. 2 Distribution of Al species in (a) AlCl3, (b) PACl1, (c) PACl2 coagulants after the removal of fluoride over pH 4-9 (Al doses=40 mg/L, [F]0=8 mg/L).
30
Fluoride Removal (%)
100
80
60
pH=6 pH=7 pH=8
40 0
1
2
3
B
Fig. 3 Effect of PACl basicity on fluoride removal at different pH values (Al doses = 40 mg/L, [F]0= 8 mg/L ).
31
25 AlCl3
PACl1
PACl2
(a)
Fluoride Removal (mg/L)
20
15
10
5
0
Ratio of Com-F (%)
100
(b)
75
50
25
0
Slouble Al (mg/L)
20
(c)
15
10
5
0 0
20
40 [F]0 (mg/L)
60
80
Fig.4 Effect of fluoride concentration on (a) fluoride removal, (b) Com-F, and (c) soluble Al (Al doses=40 mg/L, pH=6.5).
32
Fig.5 ESI-MS spectra of (a) AlCl3, (b) AlCl3-F, (c) PACl, and (d) PACl-F at pH=4 (Al doses=4 mg/L, [F]0=4 mg/L).
33
Complexed Reaction
xFnF
+
-
Al3+ Ala
-
O ce pla
Ionic-exchange and Coprecipitation
H
+ OH
F
F F
4
Cha
rg e N
F F
e utr
aliz a
5
tion
F F
Sweep Flocculation F F F F F F
-
nF- + Alb
(Al-OH)-F
Al-F-OH
Alm-Fn
+ Alm Re
Adsorption
Coprecipitation
Al-Fx
F
F F F
F
F F
6
7
8
Fig. 6 Proposed Schematic diagram on fluoride removal by different Al species.
34
Table 1 The dominant complexion reactions involved in the interactions between AlCl3 and F and the formation of Al-F complexes m/z
Al1
97 121
139
Species variation [Al(OH)2(H2O)2]+ - OH- + F- → [AlF(OH)(H2O)2]+
99
[Al2O2(OH)(H2O)]+ - OH- + F- → [Al2FO2 (H2O)]+
123
- 2OH- + 2F- → [Al2F2O(OH)]+
125
[Al2O2(OH)(H2O)2]+ - OH- + F- → [Al2FO2 (H2O)2]+
141
- 2OH- +2 F- → [Al2F2O(OH)(H2O)]+ - 3OH- + 3F- → [Al2F3O(H2O)]+
Al2
[Al2O2(OH)(H2O)3]+ - OH- + F- → [Al2FO2 (H2O)3]+ 157
- 2OH- +2 F- → [Al2F2O(OH)(H2O)2]+ [Al2O2(OH)(H2O)4]+ - OH- + F- → [Al2FO2 (H2O)4]+
175
- 2OH- +2 F- → [Al2F2O(OH)(H2O)3]+ [Al3O4(H2O)3]+ - OH- + F- → [Al3FO3(OH)(H2O)2]+
199
Al3
Al13
277
337
161 177 179 201
205 219
- 2OH- +2 F- → [Al3F2O3(H2O)3]+
221
- 3OH- + 3F- → [Al3F3O2(OH)(H2O)2]+
223 237
- 2OH- +2 F- → [Al3F2O3(H2O)4]+
239
- 3OH- + 3F- → [Al3F3O2(OH)(H2O)3]+
241 255
- 2OH- +2 F- → [Al3F2O3(H2O)5]+
257
- 3OH- + 3F- → [Al3F3O2(OH)(H2O)4]+
259
[Al4O5(OH)(H2O)4]+ - OH- + F- → [Al4FO5(H2O)4]+
Al4
159
- 3OH- + 3F- → [Al3F3O2(OH)(H2O)]+
[Al3O4(H2O)6]+ - OH- + F- → [Al3FO3(OH)(H2O)5]+ 253
145
203
[Al3O4(H2O)5]+ - OH- + F- → [Al3FO3(OH)(H2O)4]+ 235
143
- 2OH- +2 F- → [Al3F2O3(H2O)2]+
[Al3O4(H2O)4]+ - OH- + F- → [Al3FO3(OH)(H2O)3]+ 217
m/z
279
- 2OH- +2 F- → [Al4F2O4(OH)(H2O)3]+
281
- 3OH- + 3F- → [Al4F3O4(H2O)3]+
283
[Al13O18(OH)(H2O)]2+- 2OH- + 2F- → [Al13F2O17(OH)]2+
35
339
Table 2 The dominant complexion reactions involved in the interactions between PACl and F and the formation of Al-F complexes m/z
Species variation [Al(OH)2(H2O)2]+ - OH- + F- → [AlF(OH)(H2O)2]+
Al1
97
- 2OH- + 2F- → [AlF2(H2O)2]+ [Al2O2(OH)(H2O)2]+ - OH- + F- → [Al2FO2 (H2O)2]+
139
- 3OH- + 3F- → [Al2F3O (H2O)]+ [Al2O2(OH)(H2O)3]+ - OH- + F- → [Al2FO2 (H2O)3]+
157 Al2
217
141 145 159
- 3OH- + 3F- → [Al2F3O(H2O)2]+
163 177
- 2OH- + 2F- → [Al2F2O(OH)(H2O)3]+
179
- 3OH- + 3F- → [Al2F3O(H2O)3]+
181 201
- 2OH- + 2F- → [Al3F2O3(H2O)2]+
203
- 3OH- + 3F- → [Al3F3O2(OH)(H2O)]+
205
[Al3O4(H2O)4]+ - OH- + F- → [Al3FO3(OH)(H2O)3]+ Al3
101
161
[Al3O4(H2O)3]+ - OH- + F- → [Al3FO3(OH)(H2O)2]+ 199
99
- 2OH- + 2F- → [Al2F2O(OH)(H2O)2]+
[Al2O2(OH)(H2O)4]+ - OH- + F- → [Al2FO2 (H2O)4]+ 175
m/z
219
- 2OH- + 2F- → [Al3F2O3(H2O)3]+
221
- 3OH- + 3F- → [Al3F3O2(OH)(H2O)2]+
223
[Al3O4(H2O)5]+ - OH- + F- → [Al3FO3(OH)(H2O)4]+
237
- 2OH- + 2F- → [Al3F2O3(H2O)4]+
239
- 3OH- + 3F- → [Al3F3O2(OH)(H2O)3]+
241
277
[Al4O5(OH)(H2O)4]+ - 3OH- + 3F- → [Al4F3O4(H2O)3]+
283
295
[Al4O5(OH)(H2O)5]+ - OH- + F- → [Al4FO5(H2O)5]+
297
337
[Al13O18(OH)(H2O)]2+- 2OH- + 2F- → [Al13F2O17(OH)]2+
339
[Al13O18(OH)(H2O)3]2+- 2OH- + 2F- → [Al13F2O17(OH)]2+
357
- 4OH- + 4F- → [Al13F4O16(OH)]2+
359
- 6OH- + 6F- → [Al13F6O15(OH)]2+
361
235
Al4
355 Al13
373
[Al13O18(OH)(H2O)5]2+-4OH-+4F-→[Al13F4O16(OH) (H2O)3]2+
377
-6OH-+6F-→[Al13F6O15(OH) (H2O)2]2+
379
36
409
[Al13O18(OH)(H2O)9]2+-6OH-+6F-→[Al13F6O15(OH)(H2O)6]2+
415
427
[Al13O18(OH)(H2O)11]2+-6OH-+6F-→[Al13F6O15(OH)(H2O)8]2+
433
Highlights
The removal of fluoride follows the order: PACl2 > PACl1 > AlCl3.
Optimum pH for fluoride removal by three coagulants is in pH 6-7.
Fluoride removal is positively correlated with Alb or Al13 content. Al-F complexation significantly affects Al species distribution and transformation.
37
S1383-5866(16)30017-X http://dx.doi.org/10.1016/j.seppur.2016.01.017 SEPPUR 12799
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
15 October 2015 13 January 2016 13 January 2016
Please cite this article as: Z. He, H. Lan, W. Gong, R. Liu, Y. Gao, H. Liu, J. Qu, Coagulation behaviors of aluminum salts towards fluoride: Significance of aluminum speciation and transformation, Separation and Purification Technology (2016), doi: http://dx.doi.org/10.1016/j.seppur.2016.01.017
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Coagulation behaviors of aluminum salts towards fluoride: Significance of aluminum speciation and transformation
Zan Hea,b, Huachun Lan a,*, Wenxin Gonga,b, Ruiping Liua, Yuping Gaoa, Huijuan Liua, Jiuhui Qua
a
Key Laboratory of Drinking Water Science and Technology, Research Center For Eco-Environmental Science, Chinese Academy of Sciences, Beijing 100085, China
b
University of Chinese Academy of Sciences, Beijing, 100039, China
*
Corresponding author. Tel.: +86 10 62849160; fax: +86 10 62849160. Email address: [email protected] (H. Lan)
1
Abstract: This study investigates the effects of aluminum (Al) and F interactions on the coagulation behaviors of Al salts, i.e., AlCl3 and polymer aluminum chloride (PACl) at different basicity (B) towards fluoride. These coagulants achieve the optimum fluoride removal over pH 6-7, and PACl exhibits higher removal efficiency than AlCl3 does over pH 4-9. The removal of fluoride is positively correlated with the content of either Al13 or Alb in Al-based coagulants over pH 4-9 and increases in the order: PACl2 > PACl1 > AlCl3. As for PACl, the increased B from 0 to 2.4 improves fluoride removal owing to the elevated ratios of Alb. However, at further increased B to 2.8, the formation of colloidal Al species (Alc) occurs and this effect adversely inhibits fluoride removal thereafter. The removal of fluoride by these coagulants increases with elevated initial fluoride concentrations ([F]0) from 0 to 20 mg/L. At high [F]0 of 80 mg/L, fluoride removal by AlCl3 decreases to as low as 13.1% owing to the formation of soluble Al-F complexes, as indicated from the high residual Al concentrations of 16 mg/L. Interestingly, this adverse effect can hardly be observed for PACl due to the stronger stability of Al13 or Alb. As for AlCl3 and PACl, the introduction of fluoride changes the Al species distribution to some extent owing to the Al-F complxation, as indicated by the analysis of electrospray ionization mass spectrometry. The possible Al-F complexes formed fluoride and different Al species are proposed 2
thereafter. PACl with diverse Al species and high Alb or Al13 content shows higher removal efficiency towards fluoride than AlCl3 does, especially at high [F]0 or under acidic pH and alkaline pH conditions.
Keywords: Fluoride, Al salts, Coagulation, Al species transformation, Al-F complexes, ESI-MS
1. Introduction Fluoride at low concentrations is beneficial to human heath to strengthen bones and to prevent tooth decay [1]. However, the long-term excessive intake of fluoride via drinking water can lead to the wide occurrence of dental and skeletal fluorosis [2], of which over 500 million people are suffering from fluorosis globally [3]. The removal of fluoride from drinking water has received great attention in the past several decades. Defluoridation can be achieved by different processes such as adsorption [4-6], (electro)coagulation [7-9], ion-exchange [10], membrane separation [3], and the Nalgonda process [11]. Among these technologies, coagulation is widely applied due to its simplicity and high efficiency. Aluminum chloride (AlCl3) and polyaluminum chloride (PACl) are commonly used coagulants, and have been proposed and well investigated with respect to fluoride removal [8, 12]. Our previous study indicates that the formation of Al-F complexes 3
involves the removal of fluoride by AlCl3, and this effect benefits fluoride removal to a large extent as compared to Al(OH)3 adsorption [7]. The transformation of different fluoride species, i.e., free fluoride , complexed fluoride (Com-F), and adsorbed fluoride, plays a determining role and is highly dependent on solution pH and the ratio of fluoride to Al (RF:Al) [8,13]. Additionally, Al-F complexation also affects the hydrolysis of Al salts and their distribution and transformation afterwards. On the other hand, the hydrolysis of Al salts tends to form a series of Al species with different structures, charges, and polymerization character [14], and the diversity of Al species possibly impacts the interaction between fluoride and different Al species thereafter. PACl as polymeric species of Al salt shows superiority in removing particulate and organics as compared to traditional monomeric Al such as AlCl3, and PACl exhibits inherent advantages less independence on pH [15], less alkalinity consumption [16], and less sludge production [17]. However, its superiority in terms of fluoride removal as compared to AlCl3 and the mechanisms involved in has rarely been investigated. PACl contains a serial of Al species, of which Al13 ([AlO4Al12(OH)24(H2O)12]7+) is the dominant species [18-19]. Al13 can also be in situ generated in AlCl3 coagulation at pH 5.5, with the in situ Al13 species exhibiting better removal of haloacetic acid precursors than preformed PACl does, as indicated 4
from our previous study [20]. AlCl3 is also observed to be more effective than PACl in removing turbidity and dissolved organic matter in eutrophic water, and this is not only attributed to the generation of in situ Al13 species but to AlCl3 functioning as a pH control agent [21]. These results indicate that Al species play a determining role in the coagulation behaviors of Al salts, and their species distribution and transformation is critically important in evaluating their coagulation behaviors. However, to the best of our knowledge, few studies have focused on the effects of Al species distribution and transformation on fluoride removal by Al coagulation. This study first aims to compare the removal efficiency of different aluminum salts, i.e., AlCl3 and PACl, towards fluoride under different conditions of pH, basicity, and RF:Al. The effects of fluoride on the transformation and distribution of different Al species are also investigated by electrospray ionization mass spectrometry (ESI-MS). Finally, the dominant mechanism involved in the removal of fluoride by different species Al is proposed.
2. Materials and methods 2.1 Chemicals Unless specifically noted, all chemicals used in the tests were of analytical-reagent grade and were used without further purification. 5
The stock solution of sodium fluoride (NaF) was prepared in deionized water. The stock solution of AlCl3, PACl1 and PACl2 (PACl1 and PACl2 stand for different B of PACl), characteristics of which is shown in the Table S1, was freshly prepared prior to use. The fluoride-containing water was obtained by the dilution of NaF stock solution to desired concentrations, and potassium nitrate (KNO3) solution at 0.01 M was added to provide background ionic strengthen. The hydrochloric acid (HCl) or NaOH solution at 1 M were prepared with deionized water to adjust pH in the tests process.
2.2 Coagulants characterization and analytical methods PACl and a commercial AlCl3·6H2O were used for all tests. PACl with different basicity (B, OH/Al molar ratios) were prepared by the method of Zhao et al..[22]. B values were determined by titrimetric methods (Standard method of the chemical industry of China).Total Al concentrations were measured by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) (OPTIMA 2000DV, PerkinElmer, USA).The Al species were analyzed by the ferron colorimetric method [23]. Ala, Alb, and Alc corresponded to monomeric, medium polymer, and large polymer species and/or solid-phase Al(OH)3, respectively. The effect of fluoride on Al species of these coagulants was investigated by electrospray ionization mass spectrometry (ESI-MS). 6
In the absence of fluoride, Al species distribution of coagulants was first measured, and AlCl3 and PACl were dissolved in deionized water to obtain a final solution of 4 mg Al/L. Tetramethylammonium hydroxide pentahydrate ((CH3)4NOH·5H2O) or HCl solution was used to adjust solution pH to desired values. The ESI-MS spectra were recorded with a micromass hybrid quadrupole time-of-flight (TOF) mass spectrometer (2695XE micro, water, US) equipped with an electrospray ion source. Details of the operating procedures and parameters of the apparatus can be referenced in the literature [24]. In the presence of fluoride, AlCl3 and PACl were dissolved in fluoride-containing water to obtain a final mixed solution of 4 mg Al/L and 4 mg F/L, and the Al species measurement of follow procedure was the same as it in the absence of fluoride.
2.3 Jar test The coagulation tests were performed using a Phipps and Bird six-paddle stirrer (MY 3000-6, QianJiangMeiYu Instruments, China) at room temperature (25±2 oC). 300 ml of fluoride-containing water was transferred to a 500 ml beaker and then the stock solution of coagulants was added to make sure that Al dose was the desired value. After addition of the Al-based coagulant, the following procedure consisted of 2 min of mixing at 100 rpm, with pH adjusted to the 7
desired value, 2 min rapid mix at 200 rpm, 15 min slow mix at 40 rpm, and 30 min of settling. After settling for 30 min, supernatants were filtered through 0.45 μm membrane to analyze the concentrations of fluoride and Al in the filtrate. The concentrations of fluoride were measured by the ion selective electrode method (PF-1, Shanghai KangYi Technology), and the concentrations of Com-F were obtained by total fluoride concentration minus free fluoride concentration according to the methods of our previous study [7]. The residual Al concentrations were measured by ICP-OES.
3. Results and discussion 3.1. Removal efficiency of fluoride by different Al coagulants The removal efficiency of fluoride by AlCl3, PACl1, and PACl2 in a pH range of 4-9 is illustrated in Fig. 1. The optimum pH for fluoride removal was observed to be in pH 6-7 in regardless of coagulant type. With pH increasing from 7 to 9, the removal efficiency of fluoride by PACl1 and PACl2 decreased from 91.4% to 63.2%. Comparatively, fluoride removal by AlCl3 was more significantly dependent on pH and decreased sharply to as low as 17.4%. At pH < 6.0, the removal of fluoride by these three coagulants decreased with lowered pH in the order: AlCl3 > PACl1 > PACl2. For example, the residual fluoride concentrations at pH 5.0 were observed to be 4.3, 2.8, and 1.9 mg/L after coagulation by AlCl3, PACl1, 8
and PACl2, respectively. These results indicated that coagulant type and pH significantly affected fluoride removal performance and PACl1 and PACl2 removed fluoride more efficiently than AlCl3 did, especially under acidic pH and alkaline pH conditions. Chen et al. (2006) reported that Al13 showed high positive charge, stable structure, and strong binding affinity, and is the most active Al species in Al salts responsible for coagulation process [25]. In addition, Al13 also contributed most to the amount of Al species for PACl in basicity range from 1.5 to 2.5 [26, 27]. Our previous study indicated that pH significantly affected the Al species distribution of AlCl3 and preformed PACl, and in acidic or alkaline pH region the ratios of monomeric Al increased whereas polymeric and colloidal Al species decreased as compared to the near-neutral pH region [21]. In pH 6-7, the transformation of monomeric Al3+ within AlCl3 into polymeric Al occurred, and the ratios of medium and large polymeric Al as expressed by Alb were reported to increase from 71% to 82% with elevated pH from 5 to 7 [21]. Consequently, polymeric Al, i.e., Alb or Al13, was inferred to play an important role in the removal of fluoride by Al salts over wide pH range from 4 to 9, and their fluoride removal efficiency increased in the order: PACl2 > PACl1 > AlCl3. Aoudj et al. reported that simultaneous removal of chromium(VI) and fluoride by electrocoagulation-electroflotation: application of a hybrid Fe-Al anode and showed that fluoride removal was mainly ascribed to 9
alumimum plates not to iron plates[28]. Liu et al. reported simultaneous removal of arsenate and fluoride by iron and aluminum binary oxide and experimental result revealed that iron oxyhydroxide (FeOxHy) showed a high removal capability towards As(V) but exhibited little efficacy to remove F in systems where both As(V) and F co-exist or for solutions of fluoride alone and AlOxHy may simultaneously remove As(V) and F over a wide pH range from 4 to 11[29]. The above results indicated that Al-based material showed superiority in removing fluoride as compared to Fe-based material. 3.2 Al species distribution of different Al coagulants over pH 4-9 To investigate the effect of Al speciation on fluoride removal, the Al species distribution of AlCl3, PACl1, and PACl2 after coagulation were analyzed over a wide pH range from 4 to 9, and Al species distribution was observed to be highly dependent on pH for each coagulant (Fig. 2). As for AlCl3, in acidic pH region, the ratios of Ala decreased whereas those of Alb increased with elevated pH, and the Ala reached the minimum ratios of 25.7% and 27.3% at pH 6 and 7 (Fig. 2a). At further elevated pH of 8 and 9, the ratios of Ala increased to 31.4% and 51.5% owing to the formation of aluminates [4]. Alb exhibited the highest ratios of 50.9% and 52.0% in the near-neutral pH region of 6 and 7, and then decreased to 43.6% and 23.1% at elevated pH of 8 and 9. The ratios of 10
Alc varied little and were near to 24.0% at pH 5-9. PACl1 and PACl2 showed similar trends in Al species transformation, which were significantly different from that of AlCl3 over pH 4-9. As for PACl2, the ratios of Alb were determined to be 97.0% and 93.6% at pH 6 and 7, and then slightly decreased to 84.0% and 89.0% at acidic pH of 4 and 5. The lowered Alb ratios were attributed to its dissolving into Ala rather than its transformation to Alc, the ratios of which were near 0 % at pH 4 to 6. Comparatively, the significant decrease of Alb ratios also occurred at elevated pH of 8 and 9; however, this was attributed to the formation of larger polymer and Al(OH)3 precipitates, i.e., Alc, rather than the formation of aluminates as AlCl3. This effect enabled its much higher efficacy to remove fluoride as compared to AlCl3 (Fig. 1). These results indicated that Alb played a determining role in fluoride removal by Al coagulation. The removal efficiency of fluoride was observed to be positively correlated with the Alb content (Fig. S1), and this contributed to the different behaviors of these Al salts towards fluoride as observed in Fig. 1. At neutral pH 6-7, the monomeric Al transformed to medium polymeric Al, and the fluoride removal by AlCl3 was near to that by either PACl1 or PACl2. Under acidic pH conditions, the polymerisation of Al3+ was inhibited in AlCl3 whereas the preformed Alb within PACl1 and PACl2 was stable, and the high Alb ratios enable their higher efficiency to remove fluoride as compared to AlCl3. At 11
alkaline pH 8-9, the formation of aluminates as indicated by Ala was significant in AlCl3 system whereas in the other two systems, the formation of larger polymer and Al(OH)3 precipitates, i.e., Alc, was dominant, and the larger polymer and Al(OH)3 removed fluoride more efficiency than aluminates did. In the absence of fluoride, the Al species distribution and transformation of AlCl3 and PACl over pH 4-9 has been investigated in our previous study [21]. Briefly, the variation trends of Ala and Alb in pH 4-9 were similar to those with fluoride-presenting as observed in this study. As for Alc, however, the introduction of fluoride increased the Alc fractions greatly and Alc contributed most to the total Al at pH 8-9 for both AlCl3 and PACl. It was inferred that fluoride favored the formation of Alc. It was attributed to that the replacement of some hydroxyls by fluoride ions due to their similar chemical property contributed to the transformation of Al ions with less OH- to induce precipitation [7]. 3.3 Removal efficiency of fluoride by PACl with different basicity To further investigate the coagulation behaviors of different Al species towards fluoride, we prepared the PACl with different basicity and compared their fluoride removal efficiency in wide pH ranges. It is noted that basicity governs the polymerization degree of PACl and significantly impacts their Al species distribution [30]. 12
Fig. 3 illustrates the fluoride removal by PACl with basicity ranges of 0-2.8 at pH 6, 7, and 8, and the optimum pH for fluoride removal was observed to be 7. Additionally, the removal of fluoride was observed to be dependent on PACl basicity. At pH 6 and 8, the elevated basicity from 0 to 2.4 contributed to the increased fluoride removal from approximately 69.3% to 80.3%, and the further elevated basicity to 2.8 adversely inhibited fluoride removal to approximately 73.3%. At pH 7, fluoride removal was consistently near to 89.5% in wide basicity range from 0 to 2.4 and then decreased to approximately 81.6% at basicity of 2.8. PACl-2.4 was observed to exhibit the highest efficiency towards fluoride than other PACls at pH 6.0-8.0. Comparatively, PACl basicity at B of below 2.4 showed relatively lower effect on fluoride removal than that of above 2.4 in different pH conditions. PACl basicity plays a determining role in Al speciation and significantly affects fluoride removal thereafter. At high B above 2.5, the amorphous Al(OH)3 is the dominant Al species within PACl coagulant [26]. In this case fluoride removal was mainly attributed to physical adsorption and ion exchange effects [4, 31], and the weakly acidic and acidic pH favored its removal. In B ranges from 1.0 to 1.5, PACl includes different Al species such as Al2, Al4, Al7 and Al13 et al. [27], the ratios of monomeric Al species were observed to increase with lowered B. In this case coagulation dominated in the removal of fluoride, and different 13
mechanisms such as complexed precipitation, charge attraction, and ion exchanges involve in and facilitated fluoride removal accordingly[32-33]. At low B of below 0.5, the monomeric Al species tends to dissolve at low pH and cannot effectively remove fluoride. The residual Al levels were observed to be the lowest at pH 7.0, and this effect provides more particulate Al available for fluoride removal. 3.4 Effect of fluoride on coagulants towards fluoride The formation of Al-F complexes plays an important role in the removal of fluoride by Al salts [7]. Corbillon et al. (2008) reported that initial fluoride concentrations as expressed by [F]0 strongly affected the interactions between aluminum and fluoride [34]. Fig. 4 illustrates the effects of [F]0 on the removal of fluoride by AlCl3, PACl1, and PACl2, the formation of Com-F, and soluble Al at pH 6.5. As shown in Fig. 4a, the removal of fluoride increased with elevated [F]0 in the [F]0 range from 0 to 20 mg/L and little difference was observed among these three coagulants. However, at further elevated [F]0 from 20 to 80 mg/L, these coagulants showed critically different removal behaviors towards fluoride. In AlCl3 coagulation system, the maximum fluoride removal of 17.9 mg/L was observed at 40 mg/L [F]0. At elevated [F]0 of 80 mg/L, the removed fluoride decreased to as low as 10.5 mg/L. Comparatively, the removal of fluoride by PACl1 and PACl2 rarely 14
decreased even at [F]0 of as high as 80 mg/L. It was additionally observed that PACl1 exhibited the highest fluoride removal efficacy among these three coagulants over a wide [F]0 range from 20 to 80 mg/L. The removal of fluoride by Al salts is achieved by the transformation of soluble fluoride to particulate fluoride. The formation of soluble Al-F complexes affected the species distribution and transformation of fluoride and aluminum accordingly [13]. It was observed in Fig. 4b that the ratios of Com-F (RCom-F) increased with elevated [F]0 for three coagulants, and the RCom-F values in AlCl3 coagulation system were much higher than those in the other two systems. The formation of Al-F complexes between fluoride and PACl1 or PACl2 was negligible at [F]0 of below 20 mg/L, and at high [F]0 of 80 mg/L the RCom-F increased slightly to 24% and to 5% for PACl1 and PACl2 systems, respectively. Monomeric Al tends to form Al-F complexes with fluoride, and the ratios of different Al-F complexes such as AlF2-, AlF2−, AlF3, and AlF4− may be quantified by Miniteq software modeling [13]. At pH 6.5, the dissociation of PACl1 and PACl2 into monomeric Al rarely occurred (Fig. 4c), and the formation of Al-F complexes was slight. Additionally, during the preparation of PACl, a certain amount of OH- has been incorporated into them. The attachment of fluoride onto PACl1 and PACl2 maybe favor the formation of insoluble Al-OH-F precipitates rather than soluble Al-F complexes. Al-F complexes formation increased soluble Al concentrations (Fig. 15
4c), which were positively correlated with the ratios of Com-F (Fig. S2). This effect reduced the amount of insoluble Al available for fluoride removal. Comparatively, the residual Al levels in AlCl3 coagulation system were much higher than those in both PACl1 and PACl2 coagulation systems at [F]0 of above 10 mg/L, and the inhibited fluoride removal by AlCl3 was observed accordingly. The elevated [F]0 may also inhibit the hydrolysis and precipitation of PACl1 and PACl2; however, this effect was relatively slight and the maximum soluble Al concentrations were observed to be as low as below 2.6 mg/L. At high [F]0, however, fluoride inhibited Al3+ precipitation and adversely inhibited fluoride removal by AlCl3 coagulation thereafter. Comparatively, the complexion between fluoride and PACl1/PACl2 with stable polymeric structure may occur; however, the residual Al concentrations were much lower than that observed during AlCl3 coagulation. Moreover, PACl1 showed higher fluoride removal than PACl2 did at [F]0 of above 20 mg/L, this reason would be discussed in detail later. 3.5 Effect of fluoride on Al species transformation Fluoride can affect the hydrolysis process of Al salts as confirmed by Al-Ferron method (Fig. S3), and it was inferred that fluoride may also affect the Al species distribution and transformation of these coagulants. To further illustrate this effect, the Al species distribution of AlCl3 and 16
PACl before and after introducing fluoride was investigated by ESI-MS analysis at pH 4. Fig. 5a illustrates the ESI-MS spectra of AlCl3 solution, and the most intense peaks were assigned to dimeric Al species [Al2O2(OH)(H2O)0-5]+ at mass to charge ratio (m/z) 157 with 100% relative intensity. Other dimeric Al species at m/z 139, 121, and 175 and the monomers [Al(OH)2(H2O)2-3]+ at m/z 97, and Al13 at m/z 213, 219, and 337 were also observed. In addition, the series of polymers containing Al3 cores (at m/z 199, 217, and 235) and Al4 cores (m/z 99, 105, 277, 295, and 313) were also the dominant Al species. It was noted that the multi-charged polymeric series, e.g., Al7, Al14, Al15, and Al16 cores can also be observed in the spectra although their intensity was relatively low. The introduction of fluoride changed the Al species distribution to some extent (Fig 5b). The relative intensity of Al2 (at m/z 121, 139, 157, and 175), Al3 (m/z 199, 217, and 235), and Al13 (m/z 337) diminished whereas that of the ions at m/z 123, 125, 141, 143, 159, 161, 177, 201, 203, 219, 221, 223, 237, 239, and 241 was enhanced. This was attributed to the interactions between different Al species and fluoride. Al1-Al4 species were observed to be the major Al species in regardless of the presence of fluoride or not. On the basis of m/z variation analysis, the main interactions between different Al species and fluoride and the formation of Al-F complexes were proposed in Table 1. 17
As shown in Table 1, in AlCl3 coagulation system, Al2 and Al3 were the dominant Al species to react with fluoride and to form Al-F complexes. Al2-F complexes mainly included Al2F at m/z of 123, 141, 159, and 177, Al2F2 at m/z of 125, 143, 161, and 179, and Al2F3 at m/z of 145 whereas Al3-F complexes included Al3F at m/z of 201, 219, 237, and 255, Al3F2 at m/z of 203, 221, 239, and 257, and Al3F3 at m/z of 205, 223, 241, and 259. It is noted that other Al species such as Al1, Al4, and Al13 also participate in the reaction with fluoride and the proposed Al-F complexes is indicated in Table 1. Fig. 5c and 5d compare the effects of fluoride on the Al species distribution of PACl solution. Al13 at m/z of 219, 225, 237, 328, 337, 355, 364, 373, 400, 418, 427, and 436 were observed to be the dominant Al species therein (Fig. 5c). The introduction of fluoride decreased the relative intensity of Al13 to some extent (Fig. 5d), and the ion peak numbers increased greatly owing to the appearance of monomeric Al1 and small polymers of Al2 and Al3. The dissociation of Al13 into monomer and small polymeric Al is attributed to the replacement of OH- in Al13 with fluoride [35]. Moreover, the main interactions between different Al species and fluoride and the formation of Al-F complexes were listed in Table 2. In PACl coagulation system, Al13 also was the dominant Al species to react with fluoride and to form Al-F complexes as compared to Al2 and Al3 in AlCl3 system, which mainly included Al13F2 at m/z of 339 18
and 357, Al13F4 at m/z of 359 and 377, Al13F6 at m/z of 361, 379, 415, and 433, which mainly contributed to the removal of fluoride. 3.6 Proposed mechanism of fluoride removal by different Al species After being dosed into solution, Al3+ tends to hydrolyze and polymerize immediately, and the formation of Al species such as bimomer, oligimer, Al polymer, sol/gel besides monomer Al as Al(OH)n(3-n)+(n=0-2) occurs [14]. The presence of fluoride complicates the reactions greatly [13]. Additionally, different Al species show critical difference in their structure, charge density, and reactivity towards fluoride. On the basis of the abovementioned interactions between fluoride and Al species, Fig. 6 illustrates the dominant mechanisms involved in the removal of fluoride by different Al species. Monomeric Al is the dominant Al species in AlCl3 solution. In case of acidic pH and high [F]0, the formation of Al-F complexes between soluble Al and fluoride is significant [5, 36, 37], and this effect inhibits Al polymerization and the removal of fluoride therafter. At elevated pH of 6-7, monomeric Al and Al-F complexes tend to precipitate with OH-, and Al-F coprecipitation contribute most to fluoride removal. At further elevated pH to alkaline pH, the OH- concentrations are much higher and the OH--dissolving effect towards Al-F coprecipitation is significant due to the competition between OH- and fluoride. This effect reduces the 19
particulate Al available for fluoride removal, and the formation of soluble aluminates also adversely inhibits fluoride removal. The removal of fluoride is achieved by its attachment onto Al flocs through charge attraction and physical adsorption reactions [38]. PACl contains more Al species as compared to AlCl3 [20], and more complicated mechanisms involve in the removal of fluoride. First, polymeric Al species exhibits higher positive charge than monomeric Al does [14] and shows stronger attraction towards fluoride. On the other hand, the average charge density carried by each Al atom of polymeric Al is lower as compared to monomeric Al [14]. Consequently, Al polymer with high polymerization degree tends to consume relatively small amount of OH- or fluoride ions to achieve charge neutralization of fluoride. This is in accordance with that at low pH condition PACl shows higher fluoride removal efficiency than AlCl3 does. As for PACl2 with higher amount of medium polymeric Al than PACl1, it exhibits higher fluoride removal efficiency accordingly. Additionally, Al13 with stable Keggin-structure achieves charge neutralization and destabilization effect over a wide pH range. At high [F]0, fluoride tends to replace the hydroxyls within Al polymer and show dissolving effect towards medium polymeric Al. The Al polymers as expressed by Alb or Al13 are damaged and transforms into low polymeric Al (Ala) (Fig. 5). At acidic pH condition, the in situ 20
formed Ala can further react with fluoride to form Al-F complexes as AlCl3 does. At neutral pH ranges, the formed complexes between fluoride and low polymers tend to transform into Al-F precipitates with OH- and fluoride ions [7]. In addition, high polymers also achieve effective fluoride removal through ion exchange and coprecipitation. At elevated pH the OH- competition effect also occurs to inhibit fluoride removal and to release the complexed fluoride. In comparison to monomeric Al, polymeric Al shows net structure, and fluoride may attach onto inner- and outer-side of polymeric Al [14]. The outside fluoride is attached by ionic exchange and charge neutralization and is easy to be replaced by OH-. Comparatively, the inside fluoride as being removed by sweep flocculation is more stable than the outside fluoride [39], and it is reluctant to be substituted by OH-. This is in accordance with the release of fluoride from AlCl3 flocs being more significant than that from PACl flocs [40]. This effect enables PACl to exhibit better removal efficiency towards fluoride at high pH than AlCl3. PACl with stable structure show stable fluoride removal over a wide pH range. However, as for one Al atom the polymeric Al13 with Keggin-structure combines less fluoride than medium polymer Al, and this is inhibitive on fluoride removal especially at high [F]0 (Fig. 4a). Generally PACl with diverse Al species shows priority in removing fluoride by coagulation especially at high fluoride concentrations. 21
4. Conclusion Al hydrolysis forms a series of Al species such as bimomer, oligimer, Al polymer, sol/gel besides monomer Al as Al(OH)n(3-n)+ (n=0-2). These Al species show critical difference in molecular structure and charge density, and the different fluoride removal efficiency is observed thereafter. In pH ranges of 4-9 the removal of fluoride by AlCl3, PACl1, and PACl2 is positively correlated with Alb species content and increases in the order: PACl2 > PACl1 > AlCl3. PACl basicity affects Al species distribution and the removal efficiency towards fluoride thereafter. PACl-2.4 shows higher fluoride removal than PACls at other basicity, owing to the highest content of polymeric Al therein. Additionally, the interactions between Al and fluoride affect Al species distribution and transformation and impact fluoride removal thereafter. As for AlCl3, fluoride at high [F]0 forms strong Al-F complexes and impairs fluoride removal. This effect is insignificant for the PACl1 and PACl2 with stable Al13 Keggin-structure and the stable removal of fluoride is observed accordingly. Fluoride tends to dissolve polymeric Al13 into small polymeric Al, and this effect also affect fluoride removal to some extent. Generally PACl with diverse Al species shows priority in removing fluoride especially at high fluoride concentrations. Acknowledgments This work was supported by the National Natural Science Foundation 22
of China (Grant No. 21177143 and 21177144) and the key project of the National “863” High-tech R&D Program of China (2012AA062604). Author Ruiping Liu gratefully acknowledges the support of the Beijing Nova Program (2013054). References: [1] A. Rafique, M.A. Awan, A. Wasti, I.A. Qazi, M. Arshad, Removal of fluoride from drinking water using modified immobilized activated alumina, J. Chem. 2013 (2013) 1-7. [2] J. Fawell, K. Bailey, J. Chilton, E. Dahi, L. Fewtrell, Y. Magara,
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28
Fluoride Removal (%)
100
80
60
40
AlCl3 PACl1 PACl2
20
0 4
5
6
pH
7
8
9
Fig. 1 Removal of fluoride by AlCl3, PACl1, and PACl2 over pH 4-9 (Al doses=40 mg/L, [F]0=8 mg/L)
29
80 Ala
Alb
Alc
(a)
60
40
20
0
(b)
Ratio of different Al species (%)
100 80 60 40 20 0
(c)
100 80 60 40 20 0 4
5
6
7
8
9
pH
Fig. 2 Distribution of Al species in (a) AlCl3, (b) PACl1, (c) PACl2 coagulants after the removal of fluoride over pH 4-9 (Al doses=40 mg/L, [F]0=8 mg/L).
30
Fluoride Removal (%)
100
80
60
pH=6 pH=7 pH=8
40 0
1
2
3
B
Fig. 3 Effect of PACl basicity on fluoride removal at different pH values (Al doses = 40 mg/L, [F]0= 8 mg/L ).
31
25 AlCl3
PACl1
PACl2
(a)
Fluoride Removal (mg/L)
20
15
10
5
0
Ratio of Com-F (%)
100
(b)
75
50
25
0
Slouble Al (mg/L)
20
(c)
15
10
5
0 0
20
40 [F]0 (mg/L)
60
80
Fig.4 Effect of fluoride concentration on (a) fluoride removal, (b) Com-F, and (c) soluble Al (Al doses=40 mg/L, pH=6.5).
32
Fig.5 ESI-MS spectra of (a) AlCl3, (b) AlCl3-F, (c) PACl, and (d) PACl-F at pH=4 (Al doses=4 mg/L, [F]0=4 mg/L).
33
Complexed Reaction
xFnF
+
-
Al3+ Ala
-
O ce pla
Ionic-exchange and Coprecipitation
H
+ OH
F
F F
4
Cha
rg e N
F F
e utr
aliz a
5
tion
F F
Sweep Flocculation F F F F F F
-
nF- + Alb
(Al-OH)-F
Al-F-OH
Alm-Fn
+ Alm Re
Adsorption
Coprecipitation
Al-Fx
F
F F F
F
F F
6
7
8
Fig. 6 Proposed Schematic diagram on fluoride removal by different Al species.
34
Table 1 The dominant complexion reactions involved in the interactions between AlCl3 and F and the formation of Al-F complexes m/z
Al1
97 121
139
Species variation [Al(OH)2(H2O)2]+ - OH- + F- → [AlF(OH)(H2O)2]+
99
[Al2O2(OH)(H2O)]+ - OH- + F- → [Al2FO2 (H2O)]+
123
- 2OH- + 2F- → [Al2F2O(OH)]+
125
[Al2O2(OH)(H2O)2]+ - OH- + F- → [Al2FO2 (H2O)2]+
141
- 2OH- +2 F- → [Al2F2O(OH)(H2O)]+ - 3OH- + 3F- → [Al2F3O(H2O)]+
Al2
[Al2O2(OH)(H2O)3]+ - OH- + F- → [Al2FO2 (H2O)3]+ 157
- 2OH- +2 F- → [Al2F2O(OH)(H2O)2]+ [Al2O2(OH)(H2O)4]+ - OH- + F- → [Al2FO2 (H2O)4]+
175
- 2OH- +2 F- → [Al2F2O(OH)(H2O)3]+ [Al3O4(H2O)3]+ - OH- + F- → [Al3FO3(OH)(H2O)2]+
199
Al3
Al13
277
337
161 177 179 201
205 219
- 2OH- +2 F- → [Al3F2O3(H2O)3]+
221
- 3OH- + 3F- → [Al3F3O2(OH)(H2O)2]+
223 237
- 2OH- +2 F- → [Al3F2O3(H2O)4]+
239
- 3OH- + 3F- → [Al3F3O2(OH)(H2O)3]+
241 255
- 2OH- +2 F- → [Al3F2O3(H2O)5]+
257
- 3OH- + 3F- → [Al3F3O2(OH)(H2O)4]+
259
[Al4O5(OH)(H2O)4]+ - OH- + F- → [Al4FO5(H2O)4]+
Al4
159
- 3OH- + 3F- → [Al3F3O2(OH)(H2O)]+
[Al3O4(H2O)6]+ - OH- + F- → [Al3FO3(OH)(H2O)5]+ 253
145
203
[Al3O4(H2O)5]+ - OH- + F- → [Al3FO3(OH)(H2O)4]+ 235
143
- 2OH- +2 F- → [Al3F2O3(H2O)2]+
[Al3O4(H2O)4]+ - OH- + F- → [Al3FO3(OH)(H2O)3]+ 217
m/z
279
- 2OH- +2 F- → [Al4F2O4(OH)(H2O)3]+
281
- 3OH- + 3F- → [Al4F3O4(H2O)3]+
283
[Al13O18(OH)(H2O)]2+- 2OH- + 2F- → [Al13F2O17(OH)]2+
35
339
Table 2 The dominant complexion reactions involved in the interactions between PACl and F and the formation of Al-F complexes m/z
Species variation [Al(OH)2(H2O)2]+ - OH- + F- → [AlF(OH)(H2O)2]+
Al1
97
- 2OH- + 2F- → [AlF2(H2O)2]+ [Al2O2(OH)(H2O)2]+ - OH- + F- → [Al2FO2 (H2O)2]+
139
- 3OH- + 3F- → [Al2F3O (H2O)]+ [Al2O2(OH)(H2O)3]+ - OH- + F- → [Al2FO2 (H2O)3]+
157 Al2
217
141 145 159
- 3OH- + 3F- → [Al2F3O(H2O)2]+
163 177
- 2OH- + 2F- → [Al2F2O(OH)(H2O)3]+
179
- 3OH- + 3F- → [Al2F3O(H2O)3]+
181 201
- 2OH- + 2F- → [Al3F2O3(H2O)2]+
203
- 3OH- + 3F- → [Al3F3O2(OH)(H2O)]+
205
[Al3O4(H2O)4]+ - OH- + F- → [Al3FO3(OH)(H2O)3]+ Al3
101
161
[Al3O4(H2O)3]+ - OH- + F- → [Al3FO3(OH)(H2O)2]+ 199
99
- 2OH- + 2F- → [Al2F2O(OH)(H2O)2]+
[Al2O2(OH)(H2O)4]+ - OH- + F- → [Al2FO2 (H2O)4]+ 175
m/z
219
- 2OH- + 2F- → [Al3F2O3(H2O)3]+
221
- 3OH- + 3F- → [Al3F3O2(OH)(H2O)2]+
223
[Al3O4(H2O)5]+ - OH- + F- → [Al3FO3(OH)(H2O)4]+
237
- 2OH- + 2F- → [Al3F2O3(H2O)4]+
239
- 3OH- + 3F- → [Al3F3O2(OH)(H2O)3]+
241
277
[Al4O5(OH)(H2O)4]+ - 3OH- + 3F- → [Al4F3O4(H2O)3]+
283
295
[Al4O5(OH)(H2O)5]+ - OH- + F- → [Al4FO5(H2O)5]+
297
337
[Al13O18(OH)(H2O)]2+- 2OH- + 2F- → [Al13F2O17(OH)]2+
339
[Al13O18(OH)(H2O)3]2+- 2OH- + 2F- → [Al13F2O17(OH)]2+
357
- 4OH- + 4F- → [Al13F4O16(OH)]2+
359
- 6OH- + 6F- → [Al13F6O15(OH)]2+
361
235
Al4
355 Al13
373
[Al13O18(OH)(H2O)5]2+-4OH-+4F-→[Al13F4O16(OH) (H2O)3]2+
377
-6OH-+6F-→[Al13F6O15(OH) (H2O)2]2+
379
36
409
[Al13O18(OH)(H2O)9]2+-6OH-+6F-→[Al13F6O15(OH)(H2O)6]2+
415
427
[Al13O18(OH)(H2O)11]2+-6OH-+6F-→[Al13F6O15(OH)(H2O)8]2+
433
Highlights
The removal of fluoride follows the order: PACl2 > PACl1 > AlCl3.
Optimum pH for fluoride removal by three coagulants is in pH 6-7.
Fluoride removal is positively correlated with Alb or Al13 content. Al-F complexation significantly affects Al species distribution and transformation.
37