Hydrolysis products of water treatment chemical aluminium sulfate octadecahydrate by electrospray ionization mass spectrometry

Polyhedron 26 (2007) 2851–2858 www.elsevier.com/locate/poly

Hydrolysis products of water treatment chemical aluminium sulfate octadecahydrate by electrospray ionization mass spectrometry Arja Sarpola a

a,*

, Heikki Hellman b, Vesa Hietapelto c, Jorma Jalonen b, Jukka Jokela c, Jaakko Ra¨mo¨ a, Jaakko Saukkoriipi b

Water Resources and Environmental Engineering Laboratory, University of Oulu, Linnanmaa, P.O. Box 4000, 90014 University of Oulu, Finland b Department of Chemistry, University of Oulu, Linnanmaa, P.O. Box 3000, 90014 University of Oulu, Finland c Kemira Inc., P.O. Box 171, 90101 Oulu, Finland Received 2 October 2006; accepted 23 January 2007 Available online 3 February 2007

Abstract Hydrolysis and speciation of aluminium sulfate octadecahydrate Al2 ðSO4 Þ3  18H2 O was studied by electrospray time of flight mass spectrometry (ESI TOF MS). Several novel polymeric species were determined. Highly charged polymers, characterized by other methods, such as the Keggin cation [Al13O4(OH)24(H2O)12]7+ and the octameric aluminium hydroxide cluster [Al8(OH)14 (H2O)18](SO4)5 16H2O, were found using ESI-MS as the anions [Al13O4(OH)25(SO4)4]2 and [Al8O(OH)14(SO4)5(H2O)4]2. All the main species identified contained sulfate or hydrogen sulfate. The compositions of the determined ions mimicked those of several stable mineral forms.  2007 Elsevier Ltd. All rights reserved. Keywords: Aluminium; Sulfate; Speciation; Mass spectrometry; Electrospray; Cluster ions

1. Introduction Purified water is arguably the most precious chemical for mankind by far. In the industrialized world, its consumption per person per day can be several hundreds of litres. Meanwhile, billions of people have considerably insufficient drinking water, which in turn is a major reason, for example, for high infant death rates. In this light, there is astonishingly little research that focuses on the aqueous behavior of water treatment chemicals. The present investigation is focused on the by far most used coagulation chemical, aluminium sulfate or alum. The major aim is to figure out complex species emerging when alum is dissolved in water. The current process optimization of coagulating flocculating units is based predominantly on single experiments, not on detailed speciation

*

Corresponding author. Tel.: +358 8 553 4498; fax: +358 8 553 4507. E-mail address: Arja.Sarpola@oulu.fi (A. Sarpola).

0277-5387/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2007.01.035

studies. Variations in composition of raw water cause problems, which can be managed if the most efficiently functioning species and particles are known. Improved the understanding of the impact of process parameters on purification results will provide a key to enhancing the material efficiency of processes. This can be achieved with an increased understanding of the chemistry behind the process. On the other hand, many solid species are crystallized in aqueous aluminium sulfate environments [1–6]. Our secondary aim was to clarify to what extent the emerging aqueous species resemble elementary units of typical crystal aluminium minerals. We have earlier [7–10] used electrospray mass spectrometry in order to investigate speciation of aqueous aluminium chloride solutions. The studies have offered new information on the hydrolysis chemistry of aluminium, because the speciation found by MS differs from that observed using conventional solution phase analyses [11,12].

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219

100%

237

80%

579 561

537

519

457

439 421

339

279

299 317

292

177

137, 139 141

20%

199, 201

359

40% 157, 158, 159

Intensity

60%

0% 100

200

300

400

500

600

m/z Fig. 1. The cationic spectrum of aluminium sulfate octadecahydrate Al2 ðSO4 Þ3  18H2 O, 5 mM at pH 3.76.

2. Experimental Aluminium sulfate octadecahydrate Al2 ðSO4 Þ3  18H2 O (Merck, purum) 0.3 g exposed to an open atmosphere was dissolved in 100 mL of deionized, distilled water (10 mM Al3+). The solution was divided into two parts. The first part of the solution was left at its initial pH of 3.76 and the other part of the solution was adjusted to pH 3.95, by a drop-wise addition of concentrated tetramethyl ammonium hydroxide pentahydrate, (CH3)4NOH Æ 5H2O, (TMA, Fluka, reinst). TMA was found to be the only useful base, since alkali metals formed several intense chloride aggregate signals that interfere with the interpretation of the spectra. The pH was determined with a Mettler Toledo MP 220 pH meter using a Mettler Toledo InLab 410 Ag/AgCl-electrode calibrated with FFChemicals’ buffer solutions pH 4.00 and 7.00. The samples were allowed to equilibrate prior to recording for 3 h. We note that more than one day may be needed for aluminium solutions to reach their complete equilibria [11,12], but in our previous studies [7,9] there was little difference between spectra of fresh and 2-week-aged 100 mM AlCl3 solutions. The ESI TOF mass spectra were recorded by a Micromass LCT mass spectrometer equipped with a Z-spray electrospray interface. The solutions were introduced into the spectrometer by a Harvard Apparatus Model 11 syringe pump at flow-rates of 10 lL/min. The operating conditions (Table 1) used were tested by an experimental design calculated by MODDE 3.0 software. Signals from both pH values were recorded for 2 min and combined to a sum spectrum. All the ESI mass spectral data in the positive and negative ion modes were acquired using Masslynx NT software (version 3.4).

Table 1 Operating conditions of a Micromass LCT mass spectrometer for the aluminium sulfate octadecahydrate Al2 ð SO4 Þ3  18H2 O, 5 mM Capillary Ionization Ionization mode Function type Mass range Resolution Duration Scan duration Capillary potential voltage Sample cone voltage RF lens Extraction cone voltage Desolvation gas and capillary temperature Source temperature Cone gas flow (N2) Desolvation gas flow

waters 70 000 0341 Z-spray electrospray ES and ES+ TOF MS 75–1000 5000 1.0 min 1.10 s 4000 V 40.0 V 200.0 V 4.0 V 150.0 C 120.0 C 300 L/h 750 L/h

The interpretation of the spectra was done with the aid of compositions and structures of aluminium sulfate minerals [13] equilibrium calculations [14,15] and computational chemistry [16]. The most probable assignments have been chosen in Table 2 and written as structural formulas. When the structure has no calculated confirmation, the formulas were written according to the protonation order: first oxo, followed by sulfate ligands. Thus, the protonation of the sulfate group could be different than the one suggested: e.g. [Al2(OH)3(SO4)]+ could be [Al2O(OH)2(HSO4)]+. The calculation details are presented in our earlier study [17]. According to normal mass spectrometric practice sodium and potassium impurities were utilized in the interpretation. Sodium originates from the glass equipment used, whereas potassium is a major impurity in reagents.

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Table 2 Cationic and anionic main signals (>2%) and assignments of the sum spectra of aluminium sulfate octadecahydrate Al2 ðSO4 Þ3  18H2 O, 5 mM at pH 3.76 m/z

Cationic species +

141 159 177 139 157 201 219 237 299 317 199 279 158 359 421 439 457 519 537 339 561 579

[Al(OH)(HSO4)] [Al(OH)(HSO4)(H2O)]+ [Al(OH)(HSO4)(H2O)2]+ [Al2(OH)5]+ [Al2(OH)5(H2O)]+ [Al2(OH)3(SO4)]+ [Al2(OH)4(HSO4)]+ [Al2(OH)4(HSO4)(H2O)]+ [Al2(OH)3(HSO4)2]+ [Al2(OH)3(HSO4)2(H2O)]+ [Al3O(OH)6]+ [Al3(OH)6(SO4)]+ [Al3(OH)6(HSO4)(H2O)2]2+ [Al3O(OH)4(HSO4)2]+ [Al3O(OH)(HSO4 )(SO4)2(H2O)]+ [Al3(OH)4(HSO4)2(SO4)]+ [Al3(OH)5(HSO4)3]+ [Al3(OH)3(HSO4)3(SO4)]+ [Al3(OH)4(HSO4)4]+ [Al4O(OH)7(SO4)]+ [Al4(OH)4(HSO4)(SO4)3]+ [Al4(OH)5(HSO4)2(SO4)2]+

m/z 137 292 97 249 209 249 171 512a 212

non-aluminium containing species [H2K(SO4)]+ cationic impurity from the equipment [HSO4] [H3(SO4)2(H2O)3] [H4(SO4)3(H2O)6]2 [H6(SO4)4(H2O)6]2 [K(SO4)(H2O)2] [TMA3(HSO4)2(SO4)] anionic impurity from the equipment

m/z

Anionic species

139 157 193 172 219 139 279 297 179 270 310 441 340 380 420 359 401 441 287 314 431 471 511 428 481 421 448 501 542 562 532 572 612

[AlO(SO4)] [Al(OH)2(SO4)] [Al(OH)3(HSO4)(H2O)] [Al(OH)3(HSO4)2(H2O)4]2 [Al(SO4)2] [Al2O2(SO4)2]2 [Al2O(OH)(SO4)2] [Al2(OH)3(SO4)2] [Al2O(SO4)3]2 [Al4O3(SO4)4]2 [Al4O2(SO4)5]2 [Al5O5(OH)2(SO4)2] [Al5O3(OH)(SO4)5]2 [Al5O2(OH)(SO4)6]2 [Al5O(OH)(SO4)7]2 [Al6O7(OH)5] [Al6O4(SO4)6]2 [Al6O3(SO4)7]2 [Al7O6(SO4)6]3 [Al7O5(SO4)7]3 [Al7O5(OH)2(SO4)6]2 [Al7O4(OH)(SO4)7]2 [Al8(OH)16(SO4)5(H2O)3]2 [Al9O5(SO4)10]3 [Al10O9(OH)4(SO4)5]2 [Al10O7(OH)(SO4)9]3 [Al10O6(OH)(SO4)10]3 [Al10O4(OH)(SO4)12]3 [Al11O5(SO4)13]3 [Al12O6(OH)(SO4)13]3 [Al13O6(OH)25(SO4)2]2 [Al13O5(OH)25(SO4)3]2 [Al13O4(OH)25(SO4)4]2

The calculated structures are in bold. a Only in raised pH.

Especially the potassium proved to be very informative. Despite many investigations of sulfuric acid [18–23], [SO4]2/[SO3]2 redox reactions have not been observed in mass spectrometric studies and therefore have been ignored. Isotopic patterns and ratios appeared to be the most informative aspect in this study. Aluminium and sodium are monoisotopes (27Al and 23Na) and give only single signals to mass spectra. Potassium, oxygen and sulfur have one minor isotope, in addition to the major isotopes. Hence they show duplets with characteristic relative intensities in mass spectra. All the following isotopes were considered in this study: 39K and 41K (6.73%), 16O and 18O (0.21%), and 32 S and 34S (4.29%). Considering, e.g. the sulfate ion, for instance, the first isotopic signal, 5.11%, was thus a sum signal assigned to both [34S16O4]2 (98u) and [32S16O318O]2 (98u), 4.29% and 4 · 0.21%, respectively, from the main signal [32S16O4]2 (96u). The charges were determined by examining the differences between isotopic signals. Differences of 2u, 1u and 0.66u in an isotopic pattern refer to singly-, doubly- and triply-charged complexes, respectively. The relative intensities

of signal pairs helped to confirm the amount of sulfate ions in the complex. For example signals m/z 400.9 (10.7%) and 401.9 (3.1%, 29% of the m/z 400.9) were assigned to a double charged complex with a mass of 801.8 containing six sulfate ligands as [Al6O4(SO4)6]2, in agreement with theoretical abundance calculations. As an example of assignation we will give here a detailed pathway for identification of the pentameric complex [Al5O3(OH)(SO4)5]2: first, we know all the elements injected in the instrument. The major part of the elements is bound as water, the secondary part as aluminium species and sulfate. Additionally, there are trace quantities of potassium and sodium impurities. The duplet signal at m/z values 340 and 341 (Fig. 3) had an intensity of over 20%, therefore it should be a rational mixture of the major elements Al, O and H as well as SO4 2 . The first isotopic m/z value 340 is even. The mass values of aluminium oxo-hydoxides, -sulfates and -chlorides are also always even (AlO(OH) 60u, Al2(SO4)3 342u, for instance). If the charge of the complex is even so is the m/z value. Since non-charged species cannot be detected by the mass spectrometer, the charge of this complex must be even.

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There was a second signal of m/z 341 with relative intensity of 27.9% from the major signal of m/z 340. The 1u difference between the signals reveals that the charge of the complex is 2 and the ratio of the signals is near 25% which reveals that there are five sulfate groups (relative intensity of 5.11%). Hence, the assignation so far is [AlxOy(OH)z(SO4)5]2 and the mass of the complex is 2 · 340 = 680u. Since the charge of this preliminary complex is 2, detracting one sulfate from it we have an uncharged species [AlxOy(OH)z(SO4)4]0 with a mass of 584u. Thus the easiest way to figure out the rest of the complex is to divide it to calculatory pieces. We point out, that these pieces are relevant only in terms of matching the mass, they therefore have nothing to do with the structure or protonation order of the final assignation. We therefore next attempt to subtract the sulfates as neutral compounds: Al2(SO4)3 342u and Al2O2(SO4) 182u. The remaining mass is 582u – 342u – 182u = 60u, and the last piece should not contain anything besides aluminium, oxygen or hydroxide and possibly sodium. The neutral species AlO(OH) fulfills this requirement. After finding matching pieces they are combined to one species ¼ Al2 ðSO4 Þ3 342u þ Al2 O2 ðSO4 Þ 182uþ A l OðO HÞ 60u þ SO4 2 96u ¼ ½A l5 O3 ðO HÞðSO4 Þ5 2 m=z 340. Now the simulated intensity of isotopic peak is 4 · 0.21 + 5 · 5.11 = 26.4 which is close enough to the measured 27.9 and there are two signals with distances of 40u and 80u revealing a change of one oxygen to one sulfate and two oxygens to two sulfates. Their isotopic peaks also suit the pattern and therefore confirm the assignation. 3. Results and discussion Aluminium salts precipitate in the neutral pH range [3]. The sulfate systems now studied showed transient opacity beginning at pH 3.95. The turbidity vanished quite rapidly and after 3 h the solution was clear again. Since at this pH value the chloride solution has earlier been perceived to be clear, consisting principally of dimeric and trimeric oxido and hydroxido variants [7,9], the now observed opacity originated probably from TMA sulfate interactions and not from precipitating aluminium hydroxide. The differences in the mass spectra between pHs 3.76 and 3.95 were not significant. No new major signals were observed and no existing signals vanished in the spectra when the pH was raised. In the cationic spectra only minor signals, with intensities less than 1%, increased slightly in the m/z range 600–1000, corresponding to larger complexes, perhaps colloidal TMA-SO4 adducts. In the anionic spectra, the effect of pH was more versatile. The number of minor signals was considerably higher in the whole spectral range, probably due to the exchange of neutral aqua ligands with counter anions during the spraying/nebulation process. The intensity of these signals decreased when the pH was raised from 3.76 to 3.95, indicating that the free sulfate forms diminished. Also, several minor signals in

the m/z range 400–600 vanished, and instead new signals appeared in the m/z range 600–1000. The cationic and anionic spectra at the initial pH are illustrated in Figs. 1 and 3, respectively. In contrast to the chloride systems, few signals assigned to TMA were observed and with very low intensities: the highest intensity, 6%, was m/z 512 [H2TMA3(SO4)3]. This refers to colloidal or uncharged TMA – sulfate compositions. Thus sulfate may play an unexpectedly active role in water purification processes if it is forming colloids with amine containing impurities such as biological decomposition products. This observation could also confirm that the coagulation principle of polyaluminium chlorides is different to that of alum, due to the remarkable role of the sulfate anion. There is also evidence from NMR-studies concerning the alteration of the pathway of aluminium polymerization to form polymeric and solid materials in the presence of sulfate ions [24]. The fact that all the main species identified in this study contained sulfate or hydrogen sulfate, as revealed in Table 2, also refers to this. Only the monomers [Al(H2O)6]3+, [Al(SO4)]+, [Al(SO4)2], [AlOH]2+, [Al(OH)2]+, Al(OH)3(aq), [Al(OH)4], and precipitating minerals are generally considered in equilibrium calculations, because knowledge of thermodynamic stability constants and heats of formation for polymeric aluminium species are not completely available. Due to this fact, these calculations [14,15] have limited relevance to our investigations: at the studied pH values, 3.76 and 3.95, according to the calculations the solution would be supersaturated towards precipitating minerals such as amorphous Al(OH)3, diaspore (AlO(OH)) and jurbanite (AlOHSO4) and the most stable aqueous species would be monomeric the Al3+ hexa aqua ion. Hence, the equilibrium approach was of limited benefit in the case of studying oligomeric aluminium chemistry, due to the absence of the suggested solid phases, (e.g. jurbanite has never been identified in soils and therefore remains a hypothetical equilibrium phase [25]), the slow kinetics of the suggested solid phases, polynuclear ions, as well as the high variability of apparent solubility constants [26–28]. An additional major difference to the chloride system [7–9] was the lack of a series of ions corresponding to the attachment of water to aluminium cores. In chloride systems the Gaussian distribution of signals, with a difference of 18u, suggests how many aqua ligands are attached to one species and helps to confirm the charge of the species [7–9]. Because in sulfate systems those distribution patterns are missing, rapid movement of aqua and hydroxide ligands in outer spheres takes place to a minor extent as compared to the chloride systems. Most of the cationic assignments were of the form [Aln(OH)x(HSO4)y]+ where n = 1–4 and (x + y) = (n/3  1). With the ESI technique multiply charged ions can be obtained depending on the chemical structure of the analyte. Furthermore, the high polarity of water (dielectric constant er 80.4152) stabilizes high charges in the solution [29]. However, there is the possibility of charge

A. Sarpola et al. / Polyhedron 26 (2007) 2851–2858

reduction, which can take place during the evaporation due to protonation or deprotonation [30–32]. The other possible charge reduction process is ion pairing of the aluminium oxo hydroxide species with a counter anion during the evaporation [33–35]. The latter phenomenon explains quite well our findings, where almost all species found contained sulfate. Also the fact that in dilute solutions, where the amount of free counter anions is diminished, more pure oxo hydroxo species were found than in more concentrated solutions [9]. All the phenomena in the ESI process are still under intensive investigations [36,37]. The only cationic monomer with aqua ligands was [Al(OH)(HSO4)]+. The unhydrolyzed octahedral cation [Al(OH2)6]3+ (m/z 45) was not observed due to the low m/z value, outside the measuring range of the instrument used. The trimeric cations were the most abundant species with the largest versatility. In the cations the number of hydroxo groups was on a regular basis equal or higher than that of aluminium (x P n). On the contrary, in the same core the number of aluminium atoms was equal or greater than the number of sulfates (y 6 n), except in two cases: with low intensity signals: m/z 519 and 537 assigned to [Al3(OH)3(HSO4)3(SO4)]+ and [Al3(OH)4(HSO4)4]+, respectively (Table 2). This could indicate that aluminium formes oxo hydroxo cores with sulfate terminal ligands. Also, presence of the hydroxide in all the cationic assignments refers to this. Small size cationic complexes in which sulfates exist as a bridging ligands have been identified by Raman spectroscopy [38]. However, due to their low m/z-ratio, these species were beyond of the scope of this investigation. Due to the lack of complete isotopic distribution for any single species, the signals with m/z values 157, 158 and 159 were probably combinations of three overlapping signals. In addition to the dimeric species [Al2(OH)5(H2O)]+ (Table 2) there could exist a pentameric complex [Al5(OH)11(HSO4)2(H2O)6]2+, but probably at most to a minor extent because of the six aqua ligands. The number of those was two or less in other sulfate containing cations. [Al- (HSO4)(H2SO4)4(H2O)6]2+ was also a possible assignation, but likely also to be minor because in our aluminium studies these kind of large cluster ions with no aluminium oxo hydroxide core structure have not been observed earlier and they could hardly survive in ESI with these measuring parameters. The main cationic signal m/z 219 was assigned to the dimeric [Al2H5O4(SO4)]+, because there is a clear 4% isotope peak with a difference of 2u at m/z 221. Writing the formula otherwise, [Al2(OH)4(HSO4)]+, the composition resembles aluminite [39], Al2 ðSO4 ÞðOHÞ4  7H2 O. The only differences in the observed species as compared to aluminite are a proton in the sulfate and the lack of aqua ligands. Those are only ambiguously defined in solution and may evaporate easily during the measurement. The second highest signal m/z 237 could be assigned to the same complex, but now with one aqua ligand [Al2(OH)4(HSO4)(H2O)]+. We used computational ab initio methods [16] to investigate the minimum energy structure

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for this aluminite analogue ion to confirm the protonation of the sulfate group. The calculated structure of [Al2(OH)4(HSO4)]+ is illustrated in Fig. 2. The aluminium atoms are connected mainly by two hydroxide bridges associated with one bridging sulfate group. Nine major signals were assigned to trimers and the most intensive one, m/z 359, was assigned to [Al3O(OH)4(HSO4)2]+. This structure could be the precursor of precipitating minerals like alunites MAl3(SO4)2(OH)6, where M is an ammonium, oxonium or alkali metal cation [40–43]. The trimeric species are also considered precursors of the Keggin structure [44]. The monomeric aluminium hydroxo sulfate is known as a mineral; jurbanite, AlSO4(OH) Æ 5H2O. An analogous composition was found in this study as the protonated cation m/z 159 [Al(OH)(HSO4)(H2O)02]+ and the deprotonated anion m/z 139 [AlO(SO4)], again missing the aqua ligands [45]. The main anionic signal m/z 219 with 7% isotope signal m/z 221 could be assigned to [Al(SO4)2] as a precursor in solution to MAlðSO4 Þ2  12H2 O, where M is an ammonium or alkali metal cation [46–48], which also is called alum in geological literature. Also mendozite, NaAl(SO4)2 Æ 11H2O and tamarugite, NaAlðSO4 Þ2  6H2 O have similar formulas, differing only in the number of water molecules in the crystal structure [49–51]. There was also a minor 2% signal at m/z 220, indicating that the main signal m/z 219 could be a sum signal of mono charged [Al(SO4)2] and some double charged sulfate containing complex. The best assignation was [Al2(SO4)4]2

Fig. 2. The structure of the aluminite analogue [Al2(OH)4(HSO4)]+ calculated in the gas phase using the BP86 density functional with the polarized valence triple f (TZVP) basis set (Turbomole 5.6 program suite). The two aluminium atoms (light grey) are linked together with two hydroxo bridges (O dark grey and H white) and with one bridging protonated sulfate group (S grey). Both aluminium atoms have hydroxo groups as terminal ligands.

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219

100%

60% 157

Intensity

179

80%

612

562 572

532 542

511 501

481 471

401 420, 421 427.6, 431 441 448

359

380

340

310 314

287

209 195

171, 172

20%

279

249

97

270

212

139

40%

0% 50

150

250

350

450

550

650

m/z Fig. 3. The anionic spectrum of aluminium sulfate octadecahydrate Al2 ðSO4 Þ3  18H2 O, 5 mM at pH 3.76.

since this dimeric four sulfate containing compound had the same composition as, for example, dietrichite, ZnAl2 ð SO4 Þ4  22H2 O, and apjohnite, MnAl2 ðSO4 Þ4  22H2 O [52,53]. The now observed dimeric ion could be the precursor of these minerals. In the anionic spectrum m/z 279 (20%) had a possible isotope peak at m/z 281, with a relative intensity of 12%, indicating mono charged species with one potassium (7%) and one sulfate (4%) or three sulfates. However, no sum formula for those assignations were found and these signals were considered individual signals. They were assigned to [Al2O(OH)(SO4)2] (Table 2) and [Al5O7(OH)2], respectively. The trimeric species were absent in the anionic assignment. Instead, there were several polymeric aluminium complexes with a rising series of sulfate ligands and with only a few oxo or hydroxo ligands. Those anionic complexes had to contain bridging sulfate ligands because the number of oxo bridges was now not adequate to hold the structure together. The origin of these complexes is ambigious. It is possible, that they are cluster anions born during the evaporation due to the competition of aqua ligands and sulfate groups in free coordination places, or on the other hand they could be undissociated alum. In Fig. 4 the structure of the double charged m/z 270 [Al4O3(SO4)4]2 is illustrated. Three of the aluminium atoms (I, II and III) are linked together with unhydrolyzed oxo bridges (A, B and C) forming a stable six-membered ring with one terminal unprotonated sulfate group (a) attached bidentally to aluminium (I). The fourth aluminium atom (IV) is connected to the trimer core via a common oxo bridge (B) and two bridging sulfate groups (b and c). These form two six-membered ring structures

(AlII –OB –AlIV –Ob0 –Sb –Ob00 and AlIII –OB –AlIV –Oc0 –Sc – Oc00 ). The fourth sulfate group (d) bridges tridentally the trimeric group and the fourth aluminium forming one six ðAlII –OB –AlIII –Od0 –Sd –Od00 Þ and two eight-membered rings (AlII –Od00 –Sd –Od000 –AlIV –Ob0 –Sb –Ob00 and AlIII –Od0 –Sd –Od000 – AlIV –Oc0 –Sc –Oc00 ). The six-membered rings are almost in the same plane and the fourth sulfate is above the plane. This two layered structure resembles lausenite [54]. Among the most interesting findings were highly charged polymers, which have been synthesized before from aqueous solution as sulfate salts and characterized by X-ray crystallography and 27Al NMR spectroscopy: the Keggin cation [1,2] [Al13O4(OH)24(H2O)12]7+ now found as the anion [Al13O4(OH)25(SO4)4]2 and the recently found octameric cluster [55] ½Al8 ðOHÞ14 ðH2 OÞ18  ðSO4 Þ5  16H2 O, now found as the anion [Al8O(OH)14 (SO4)5(H2O)4]2. When the counter sulfate anions are attached to the aluminium oxo-hydroxo cores in the gas phase during the measurement, they reduce the high positive charges (in the forms of [Al13O4(OH)25]6+ and [Al8O(OH)14]8+) and the cationic species in solution are observed as anions. If this reaction occurs already in bulk solution, the coagulating ability of these species diminish and the active agent of alum could be some other species. In our previous studies concerning exclusively chloride environments [7–9], very stable Al13 structures were clearly observed, but always with an Al/unhydrolyzed oxo ratio 0.8–1.9, which is 3.25 in the solid Keggin system (see above) and 2.2–3.25 in the Al13 species found in this study. Also the Al14 which was always found in pure chloride systems [1] with Al13, is now missing. This could indicate that sulfate forces aluminium to form more organized, compressed, solid like species even in an aqueous environment,

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Fig. 4. The structure of the double charged, four aluminium and four sulfate group containing anion, m/z 270Al4O3(SO4)4]2, calculated in the gas phase using the BP86 density functional with the polarized valence triple f (TZVP) basis set (Turbomole 5.6 program suite). The aluminium atoms (light grey, I–IV) are linked together with unhydrolyzed oxo bridges (dark grey, A, B, C) and bridging sulfate groups (S, grey, a–d). Left, the front view, rights the side view.

whereas the chloride systems prefer more flexible speciation where hydroxo and chlorido ligands compete for free coordination places [56].

observation could also indicate that the coagulation principle of polyaluminium chlorides is different than that of alum due to the remarkable role of the sulfate anion.

4. Conclusions

Acknowledgements

The variety of aqueous species is rich, and many new species were introduced and the presence of sulfate in those was evident. The various polymeric species may play a remarkable role while evaluating functional mechanisms involved in water treatment processes. The polyions may also have biological effects like abilities to increase cell membrane rigidities and cross link biomolecules. In general, resemblance between the observed aqueous polyions and the elementary units of typical natural aluminium compounds proved to be astonishingly high. In addition to water treatment applications this observation may play a role in geochemistry, for evaluating the formation of minerals, for instance. The high ratio of unhydrolyzed oxygen ligands and aluminium and the lack of an Al14 complex in this study could indicate that sulfate forces aluminium to form more organized, compressed, solid like species even in an aqueous environment, whereas the chloride systems prefer more flexible speciation where hydroxo and chlorido ligands competite for free coordination places [56]. Also the lack of TMA-sulfate adducts indicates that sulfate may play an unexpectedly active role in water purification processes if it forms colloids with amine contain impurities like biological decomposition products. This

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