Thermal aging of heteroatom substituted Keggin type aluminum oxo polycation solutions: Aggregation behavior and impacts on dissolved organic carbon and turbidity removal
Coagulation processes within water treatment plays an important role in contaminant removal and aluminum-oxo Keggin polycations are proved to be an effective coagulating agents.Previous work demonstrated that heteroatom substitution within the Keggin-type polycation ε-Al 13 to form ε-GaAl 12 and ε-GeAl 12 can enhance removal of bacteria, DOC, and turbidity from wastewater. Additional hydrolysis of the ε-Al 13 species to form larger Al 30 species has also been shown to improve coagulation, but this aspect has not been evaluated for the ε-GaAl 12 and ε-GeAl 12 systems. In the current study, hydrolysis of ε-Al 13, ε-GaAl 12 and ε-GeAl 12 was promoted through hydrothermal aging to evaluate the overall solution stability/behavior and water treatment efficiency. Turbidity measurement of aged solution indicated that Ga substituted aluminum-oxo Keggin polycations remain stable in solution and DLS studies demonstrated greater diversity in particle sizes within the system. Additional thermogravimetric analyses of metal hydroxide precipitates formed from the aging studies indicate that the GaAl 12 system behaves more like an amorphous Al(OH) 3 phase, which has higher solubility than other aluminum hydroxide phases.Hydrothermal aging did not significantly change %DOC removal as all solution showed high efficiency for removal across a range of pH values. GaAl 12 solutions demonstrated good turbidity removal efficiency in all pH range, with enhanced performance at pH 5. The study suggests that larger, relatively stable oligomers do exist within the aged GaAl 12 solutions that may contribute to enhanced contaminant removal in a similar manner to what is observed within the PACl-Al 30 coagulant.
Introduction<!>Experimental<!>Preparing and characterizing MAl 12 solutions<!>Water treatment: DOC removal and Turbidity<!>Characterization of MAl 12 solutions<!>Water treatment: Removal of DOC and Turbidity from Iowa River Water Samples<!>Conclusion<!>Associated Content
<p>Coagulation and flocculation are crucial processes for wastewater treatment and rely on the addition of chemical agents to remove impurities from the water. Wastewater contains dissolved and suspended particles that must be removed before recharge into natural waters. The suspended particles are typically composed of clay, natural organic matter, etc., whereas the dissolved component contains a range of molecular inorganic (i.e. Na + , K + , Ca 2+ , PO 4 3-, CO 3 2-, Cl -) and organic species, including some trace contaminants (i.e. heavy metals herbicides, pesticides, pharmaceuticals) that are harmful to human health and the natural environment. To remove these species, coagulation agents are added to the water to aggregate and/or flocculate the suspended and dissolved particles. These flocculants can physically be separated within the water treatment facility to remove them before recharge.</p><p>Over the past 20 years, efforts to improve coagulation agents have turned away from simple metal salts and towards partially-hydrolyzed polyaluminum nanoclusters. The most widely used coagulation agents are based upon the hydrolysis Al 3+ and Fe 3+ salts, such as alum and ferric chloride. Addition of these metals to water at a circumneutral pH results in hydrolysis of the metal cation to larger nanoscale polyaluminum species and can be used for aggregation of the flocculant. 1,2 Structurally, these polyaluminum cations are based upon the formation of 1 -2 nm Keggin-type clusters, with the exact solution speciation dependent on synthetic conditions. The Keggin-type structural topology consists of a central tetrahedrally coordinated Al 3+ cation surrounded by twelve additional Al 3+ in octahedral coordination to form ~1 nm size [AlO 4 Al 12 (OH) 24 (H 2 O) 12 ] 7+ (Al 13 ) species. 3 Initially the Al 3+ -bearing solutions undergo partial hydrolysis to form the ε-Al 13 isomer and additional aging results in the creation of Al 13 isomers (ẟ-Al 13 , γ-Al 13 ) and larger ~2 nm oligomers ) [  (SO 4 ) 2 ] 16+ (Al 32 )). [3][4][5][6] Polyaluminum coagulants are currently on the market (i.e. PAC, PACI, and ACH) and demonstrate higher performance compared to alum and ferric chloride. Each polyaluminum nanocluster possesses a high positive charge that can more effectively attract the negatively charged contaminants to form neutral flocculants and also physically trap neutral or cationic species in the aggregation process. This means the partially hydrolyzed coagulant is effective at lower dosages, has a wider operational range at different condition (pH, temperature, colloids concentration, etc.), and better floc formation. [7][8][9][10] For example, Mertens et al. 2012 reported PACl coagulants with higher Al 30 content performed best at removing As(V) from natural well water (20-2300 µg/L) to levels that are below the World Health Organization guideline of 10 µg/L. 11 In addition, polyaluminum based coagulants have also shown enhanced performance for treatment of natural organic matter, bacteriophage and clay particles from wastewater. [12][13][14][15][16] Very recently, Wang and co-workers have also shown the use of polyaluminum coagulants in removing dyes, antibiotics and even model microplastics from wastewater. 17,18 Heteroatom substitution of the polyaluminum nanoclusters results in subtle variabilities of the surface chemistry and can give rise to improved coagulation. Both Ga 3+ and Ge 4+ readily substitute into the central tetrahedral site within the ε-Al  19,20 Coagulation behavior of the ε-GaAl 12 and ε-GeAl While the performance of polyaluminum species in PACl coagulation behavior has been evaluated, less is known regarding clusters with heteroatomic substitution. The study by Stewart et al. focused solely on ε-Al 13 and related ε-MAl 12 (M = Ga 3+ and Ge 4+ ) clusters in the removal of suspended and dissolved solids. Additional heteroatom oligomers were also recently identified in thermally aged GaAl 12 and GeAl 12 bearing solutions and resulted in the isolation of several novel species (ẟ-GaAl 12 , Ga 2 Al 18 , Ga 2.5 Al 28.5, Ge 4 Al 48 , δ-Cr n Al 13-n ). [21][22][23][24] These heteroatomic clusters have similarities to the ẟ-Al 13 and Al 30 phase observed in some PACl coagulants, but have not been evaluated for enhanced coagulation performance. Aggregation behavior is also an important piece that contributes to higher performance of coagulants and controls shelf-life for these commercial products, but a detailed understanding of these processes is lacking within heteroatom Al-Keggin systems.</p><p>In the current study, we evaluate solutions of as-synthesized MAl 12 (M= Al, Ga and Ge) clusters that are analogous to commercial coagulate agents. We have hydrothermally aged these solutions for 0 to 7 days to explore their overall aggregation behavior as it relates to formation of oligomeric Keggin-type species. We then evaluated initial assessment of their water treatment efficiency through the comparison of DOC and turbidity removal capacity of different MAl 12 solutions between a pH range 5-9 using humic acid and kaolin spiked Iowa river water. The result of heteroatom substitution on stability and particle size distribution of aged Keggin solution were evaluated using pH, turbidity, and Dynamic Light Scattering (DLS) measurements. The solid precipitates formed during hydrothermal aging of Keggin solutions were characterized using Powder X-ray diffraction (PXRD), Thermogravimetric Analysis (TGA) and Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (SEM-EDS).</p><!><p>All chemicals used for experiments ((AlCl 3 •6H 2 O (Fisher Scientific), NaOH (Fisher Scientific), anhydrous GaCl 3 (Strem Chemicals), GeO 2 (Beantown Chemicals), Humic Acid (Sigma-Aldrich) and Kaolin (Beantown Chemicals)) were used as received. Solution utilized in this study were prepared using ultrapure water (18.2 MΩ.cm, Easypure II) as the solvent.</p><!><p>The Al 13 stock solution was prepared by following Johansson et al. 25 This is the same protocol that is used typically in preparing Keggin-type polyaluminum coagulants (PACl-Al 13 and PACl-Al 30 ). 12,26 In a typical synthesis, 35 mL of a 0.25M AlCl 3 solution was stirred and heated in a 250 mL glass Erlenmeyer flask on water bath until the temperature was stabilized at 80 o C. Upon thermal equilibration of the solution, 84 mL NaOH (0.25M) was added dropwise under constant stirring. The solution was then cooled to room temperature for use in additional experiments.</p><p>A GaAl 12 solution was prepared by following procedure of Parker et al. 19 with further modification by Shohel et al. 21 The stock solution of containing mixed Al 3+ and Ga 3+ cations in a 6:1 molar ration was formed by combining 30 mL of AlCl 3 (0.25 M) and 5 mL of GaCl 3 (0.25M) in a 250 mL glass Erlenmeyer flask. This solution was heated in a water bath to 80 o C and partial hydrolysis of the heteroatom solution occurred through the addition of 84 mL of NaOH (0.25 M) in a dropwise fashion under constant stirring.</p><p>Similarly, the GeAl 12 stock solution prepared by following Lee et al. 20 with further modification adopted from Shohel et al. 23 Initially a Ge 4+ stock solution was prepared by adding 0.088 g GeO 2 to 96 mL of NaOH (0.25 M) with constant stirring for 20 minutes. This solution was then utilized to partially hydrolyze 40 mL of 0.25M AlCl 3 under constant stirring at 80 o C.</p><p>Thermal aging of the stock solutions occurred by loading 15 ml of the prepared MAl 12 (M = Al 3+ , Ga 3+ , and Ge 4+ ) stock solutions into individual 125 mL Teflon-lined Parr vessels and heated in a gravimetric oven set at 90 o C. As-prepared/unaged solutions are designated as 0 day and then additional hydrothermal aging was completed for 1, 3, and 7 days.</p><p>Solution pH and turbidity of all aged and unaged MAl 12 samples were measured using a VWR SB70P pH probe and Hach 2100N turbidity meter, respectively. Hydrodynamic diameter of MAl 12 species in solution was analyzed by Malvern Zetasizer NanoZS. Both Al 13 and GaAl 12 solutions were filtered through a 0.45 µm membrane filter prior to DLS measurement to analyze the suspended solids. For GeAl 12 , an extra filtration step with a 0.25 µm membrane filter was necessary due to presence of large amount of solid precipitate that formed in this system. During DLS measurement, at least 20 individuals scan were performed to determine particles size distribution within each solution.</p><p>Precipitates were separated from the 7-14 days aged MAl 12 solution by centrifugation and washed with deionized water. The collected solids were dried in oven at 45 °C for 48 hours and then characterized by quantitative gravimetric analysis, Powder X-Ray Diffraction (PXRD), Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (SEM-EDS) and Thermogravimetric Analysis (TGA). Quantitative gravimetric analysis was performed on precipitates separated from MAl 12 aged (14 days) solutions with a Metler Toledo AT20 analytical microbalance. PXRD of samples were performed using Bruker D8 Discover Diffractometer equipped with Cu Kα (λ = 1.5406 Å) radiation and multimode EIGER2R detector. Solid samples were deposited on a zero-background silicon wafer and the data was collected from 10-80° 2θ with a step size of 0.2° and a count time of 5 sec/step. Thermal stability and dehydration of the collected solids were determined by placing 5-10 mg into an aluminum pan and heating on a TA instruments Q500 thermogravimetric analyzer (TGA) at a ramp rate of 10 °C/min. Weight loss data was collected from 20 °C to 600 °C using high purity N 2 as the carrier gas. SEM-EDS was used to image and qualitatively assess elemental composition of carbon-coated precipitates. The samples were placed on a Hitachi S-3400N system equipped with silicon drift detector (XFlash, Bruker AXS) and measurements were carried out at 15 kV beam energy with a 2.5 minute exposure time and 10 mm of working distance.</p><!><p>Natural water used for water treatment experiments was collected from Iowa river near the University of Iowa campus in February 2020. The physical parameter of collected river water was measured for the overall sample (pH= 7.95, turbidity before settling= 20.0, turbidity after settling 24 hours= 3.60, UV-Vis absorption at 254 nm= 0.0656). Water quality parameters for the collection site of Iowa river is provided by the Iowa Department of Natural resources (Supporting Information Table S2). Settled river water was used for water treatment experiments in accordance with previous studies. 15,27 Humic acid was used to evaluate DOC removal efficiency of aged and unaged MAl 12 solutions at pH= 5, 6, 7, 8, and 9. A stock solution of humic acid with concentration 1.0 g/L was prepared in water and filtered with Whatman® filter paper (Grade: 1, diameter: 100 mm, pore size: 11 µm). Following filtration, 21mL of the stock humic acid solution was then homogeneously mixed with 140 mL of river water. The pH of the Humic Acid spiked River Water (HARW) was then adjusted into desired value using few drops of 1 M or 0.25 M aqueous HCl and NaOH. The absorption spectra of the HARW solution was collected in quartz cuvette on a Cary 5000 UV spectrometer (Agilent technology). The 254 nm wavelength was used to evaluate DOC content. 28 This pH adjusted HARW sample before flocculation experiments had measurable absorption values ~0.80 at 254 nm in the UV/Vis spectrum. For the DOC removal experiments, 10 mL of the HARW solution was placed into separate 20.0 mL glass scintillation vials, followed by addition of 25 µL of the prepared MAl 12 solutions. These solutions were then stirred for 25 minutes with a magnetic stir bar (15.9 x 8 mm) at 200 rpm for the overall flocculation reaction. The generated flocs were initially separated by centrifugation at 6000 rpm, followed by a secondary filtration using 0.45 µm membrane filter. The absorption spectra of treated water and HARW solution was collected and the % DOC removal for all samples as outlined by Weishaar et al. 28 To evaluate turbidity removal, 55 mg of kaolin was added to 660 mL river water in a 1.5 L glass jar and shaken vigorously for five minutes. The pH of the solution was then adjusted to the desired value using 1 M or 0.25 M aqueous HCl and NaOH. Turbidity of stock solution was initially measured using a Hach 2100N turbidity meter to be 90 nephelometric turbidity units (NTU). The impact of the polyaluminum clusters on solution turbidity was evaluated by adding 1.0 mL of each MAl 12 solution into a separate 50 mL aliquot of the prepared kaolin spiked river water in a 60 mL transparent reagent bottle with a magnetic stir bar (15.9 x 8 mm). This solution was initially mixed for one minute at 400 rpm and then stirred for an additional 10 minutes at 100 rpm. Generated flocks were then allowed to separate via gravitational settling over the course of 40 minutes before 30 mL of the solution was carefully removed from bottle using a pipette. The solution from the top 0.5 cm and bottom 2.0 cm were not sampled so that the solution represented the residual turbidity measurement. These solutions were again analyzed using a Hach 2100N turbidity meter.</p><!><p>MAl 12 solutions remained within in a relatively narrow pH range (3.50-4.47), but decreased slightly with hydrothermal aging due to the extended hydrolysis reaction of polyaluminum species (Figure 1A). Continued hydrolysis brought about by aging of the solution leads to the release of H + from deprotonation of surface water groups and formation of bridging hydroxyl groups.</p><p>Previous work by Casey and coworkers demonstrated that the acidity of the ligated water molecules varies as a function of the heteroatom substitution with GaAl 12 <Al 13 <GeAl 12 and this was confirmed with computational results by Bjorklund et al. 29,30 The largest pH differences for all three samples were observed between 0 to 1 day (2.75 -7.20 %) and then from 1 to 3 days (2.59 -5.89 %). The smallest difference in the solution pH was observed between 3 to 7 days (1.03 -2.41 %), suggesting that the forward reaction for the oligomerization process was likely nearing equilibrium.</p><p>Turbidity of MAl 12 solutions during hydrothermal aging varied significantly, with visible differences observed for the heteroatom substitution. All initial MAl 12 solutions were visibly clear with no observable precipitate at the bottom of the vial prior to the aging process. This was also confirmed by turbidity measurements with values of 0.50-0.85 NTU. Upon hydrothermal aging, increased turbidity was noted for the Al 13 and GeAl 12 solutions, whereas the GaAl 12 aged solutions visibly remained clear (Figure 1C). The NTU values for the MAl 12 solutions reveals that the GeAl 12 solutions are more turbid than Al 13 at the same time points (Figure 1B). We have also noted that although GaAl 12 aged solutions are visibly transparent, turbidity measurements indicated a small increase with aging time (2.00-5.65 NTU). Colloidal particles for all MAl 12 solutions aggregated at the bottom of the vial if left undisturbed for two days. The gravimetric analysis of precipitates formed after 14 days of aging have also shown formation of more insoluble matter for GeAl 12 solution with quantified mass of 115.85 ± 16.85 mg (Figure 1D). Whereas, GaAl 12 solution formed only 2 ± 0.02 mg precipitates and 39.1 ± 2.9 mg were recovered from the note that colloidal species scatter with higher intensity than smaller polyaluminum species is solution; 31,32 thus, we utilize this technique to compare differences in particle size distribution between the pure Al 13 and Ge 4+ -or Ga 3+ -substituted version and not relative amounts of the particles in the solution. For unaged Al 13 solution there are two peaks, with the feature at 1.5 nm corresponding to the hydrodynamic diameter of the Al 13 Keggin-type species and a second one at >100 nm that belongs to Al(OH) 3 colloids. Upon aging, the peak at 1.5 nm disappeared and only the colloidal species (>100 nm) remained in solution. For unaged GaAl 12 solution, we have observed multiple peaks in <100 nm range with center at hydrodynamic diameter 1.5, 8 , and 72 nm. After one and three days of aging, the peak at 1.5 nm remained in the size plot, but the second peak shifted to 11.7 nm. The 1.5 nm peak was not present after 7 days of aging GaAl 12 solution, but peaks at 11.7 nm and 60 nm were present in the size plot. For GeAl 12 , the unaged solution has two features in the <100 nm region: one centered at 1.3 nm and a second, broad peak at approximately 15 nm. After thermal aging the GeAl 12 solution, a feature at 2.8 nm on day 3 appears and becomes more intense after 7 days of treatment.</p><p>Based upon our current understanding of the known Al 3+ polycation species, we can tentatively assign the peaks observed in the DLS. The peak in unaged Al 13 and GaAl 12 solution at approximately 1.5 nm aligns well with the expected hydrodynamic diameter for the known polyaluminum species Al 13 , Al 30 , GaAl 12 or Ga 2.5 Al 28.5 species in solution. 21,[33][34][35] Features at 8 nm and 15 nm for unaged GaAl 12 and GeAl 12 solution may correspond to the aggregation of smaller clusters. For the GeAl 12 solutions, the peak at 1.3 nm can be correlated to a GeAl 12 species and the feature at 2.8 nm have been reported due to the formation of the Ge 4 Al 48 species from condensation of GeAl 12. 23 In addition, all three systems contain particles with sizes greater than 10 nm that contribute to the turbidity of the system and are likely associated with aggregated Keggin-type particles. Characterization of precipitates formed from the MAl 12 system.</p><p>Changes in turbidity within the Al 3+ system is caused by nucleation and precipitation of solid hydroxide phases. To further explore the solid precipitate, we performed additional chemical characterization of the insoluble flocculant to provide a deeper insight into the structure and composition. The PXRD of all precipitates contain a broad feature below 15° 2θ value, suggesting formation of an amorphous material (Figure S1). There are small sharp peaks located at 27.38, 31.72, 45.46, and 56.48° 2θ value for all precipitates due to the presence of NaCl salts that crystalize during sample drying. Morphological analysis of the particles observed with SEM suggest the presence of both micron and submicron particles in all precipitates (Figure S2). EDS analysis identifies the presence of Al, O, Na and Cl in the precipitate obtained from Al 13 solution (Table S1), corroborating our PXRD analysis. EDS also indicated that Ga and Ge were both associated with precipitates collected from GaAl 12 and GeAl 12 solutions, respectively. Given the incorporation of Ga and Ge into the Keggin-type topology, we hypothesized that similar ratios would be observed within the amorphous material. Based upon the semi-quantitative amounts obtained from the SEM-EDS, we can say that the Ga 3+ and Ge 4+ contents are enriched (Al:Ga ratio = 12:3.8 and Al:Ge ratio = 12:2 with Al:M = 12:1 for MAl 12 Keggin) in the precipitate.</p><p>We have also observed difference in TGA patterns among precipitates collected from different MAl 12 solutions. All the precipitates continuously lost 27.9-33.6% weight as the temperature increased from 20 to 600 °C (Figure 3A and 3B). The derivative weight loss indicates that the precipitates have two observable two weight loss steps, with the first occurring between 30-150 °C for a total decrease of 6.9, 12.1, and 10.3 wt. % for Al 13 , GeAl 12 , and GaAl 12 , respectively. The second weight loss for these samples occur between 230-520 °C for both Al 13 and GeAl 12 precipitates with a decrease of 14.3% and 13.9%, respectively. This second step occurs over a lower temperature range (185-360 °C) for the GaAl 12 system with weight loss 10.0% . Similar TGA results have also been reported for the Al 3+ system. Two weight loss steps have been reported earlier for different Al(OH) 3 phases, with first one occurring between 30-140°C and corresponding to removal of surface adsorbed water, and the second weight loss (>200°C) correspond to removal of hydroxyl groups to form Al 2 O 3 . 36,37 The second weight loss steps varied for different Al(OH) 3 phases and their synthesis condition. Sato reported that the second weight loss occurred between 290-350°C for amorphous Al(OH) 3 , 400-500°C for pseudoboehmite ((AlO(OH)), 220-350 °C for bayerite (α-Al(OH) 3 ), and 360-520°C for boehmite((γ-AlO(OH)). 36 For gibbsite (γ-Al(OH) 3 ), the removal of the hydroxyl groups occurred in two steps: between 260-330 °C and 460-540 °C. 37 Within our system, it seems that Al 13 and GeAl 13 transition occur in the region more aligned with boehmite or pseudoboehmite phases and the GaAl 12 system transition occurs closer to the transition for amorphous Al(OH) 3 or bayerite. Interestingly, the general order of solubility at room temperature is gibbsite < boehmite < bayerite, which aligns with the relative amount of precipitate that was formed from the MAl 12 system 38 .</p><p>Our TGA results align well with previously literature results regarding the precipitation of solid phases from the Al 13 Keggin system. During aging, the original ε-Al 13 phase can first transform into soluble oligomeric species (e.g. Al 26 , Al 30 ) and then form insoluble Al(OH) 3</p><p>precipitates. 39,40 The exact nature of the solid precipitate that is formed from this pro cess is dependent on the exact solution conditions. When an unhydrolyzed Al 3+ solution is aged at higher temperature (which likely results in the formation of Keggin species, the precipitate that forms is believed to be poorly crystalline pseudoboehmite (AlO(OH)). 5 Similarly alumina sol-gels that contain Al 13 Keggin species have also been shown to create a pseudoboehmite solid phase upon aging 5 . Hsu evaluated the room temperature aging of Al 13 solution created through the addition of carbonate as the hydrolyzing agent and observed that the resulting precipitation in this case was crystalline gibbsite (Al(OH) 3 ). 40 For this system the author suggested that the mechanism of gibbsite formation began with the breakdown of the Al 13 polycations into monomeric specie, which could then rearrange to form the gibbsite structure. Non-classical mechanisms that results from structural transformation of the oligomeric phases may also be at play in the transition of an amorphous flocculant into a crystalline phase. [41][42][43][44][45] With the inclusion of the heteroatom, we do see differences in the second weight loss step that may suggest subtle differences in the resulting phase and this is supported by previous studies within Fe 3+ systems. Similar to Al 3+ chemistry, Fe 3+ also undergoes hydrolysis reaction in aqueous systems and will rapidly precipitate amorphous or poorly crystalline nanoparticulate phases that can contain a significant fraction of non-stoichiometric dopants 46,47 . If we look specifically at Al 3+ as the dopant, previous studies have shown that Fe 3+ will readily form a solid-solution series with an upper limit of 30% Al 3+ substitution 46,[48][49][50] . Phase transformations for these Al 3+ doped materials vary depending on the presence of the heteroatom in the system. For instance, Al 3+ doped ferrihydrite will favor the formation of hematite over goethite. Adding Al 3+ to goethite will also cause a systematic shift in the temperature region associated with the removal of hydroxyl groups, which is similar to the region that is associated with the GaAl 12 and GeAl 12 systems 47 . The transformational changes observed with Al 3+ doping within Fe 3+ mineral phases suggest that we should expect similar variability to occur within the GaAl 12 and GeAl 12 systems and supports our current TGA results.0</p><!><p>Removal efficiency of DOC from aged and unaged MAl 12 solutions was determined to be pH dependent, where the % removal for all MAl 12 solutions increase with decreasing pH. This result aligns well with previous work reported by Zhang et al. 2008 as they also found that humic acid removal by PACl-Al30 increased as the has the pH of the solutions were lowered from 9 to 4 51 . They suggested that the labile protons on the surface of the Keggin species in the coagulant enables significant charge-neutralization of the humic acid, which destabilizes the macromolecule and allows flocculation to occur. Additionally, the stability of the Keggin species in solution is optimal at a pH of 4-5; thus, higher pH values may have competition between charge neutralization of the DOC and continued metal hydrolysis of the soluble clusters.</p><p>Subtle differences are observed in % DOC removal that can be related to the heteroatom substitution and aging time. For unaged solution, GaAl 12 performed slightly better in higher pH (7-9), followed by Al 13 and GeAl 12 . At pH 5 and 6, unaged GeAl 12 had the highest %DOC removal followed by Al 13 and GeAl 12 . Aging time increases the %DOC removal for Al 13 and GaAl 12 , with an exception of Al 13 solution at pH 9 where we observe a decrease. The GeAl 12 solutions display a different trend, with an initial increase in % removal after one day aging, followed by a systematic decrease for 3 and 7 day aging. We note that the GeAl 12 solutions also displayed the greatest amount of precipitant formed upon aging, so this may also account for the decrease in the overall efficiency observed in this system. Trends related to the turbidity removal efficiency for kaolin spiked Iowa river water using the MAl 12 solutions are less clear than those observed for % DOC. The turbidity removal efficiency of most solutions was lower when the pH was decreased from 9 to 7, but the turbidity removal from pH 7 to 5 does not follow a clear pattern. For example, a turbidity removal efficiency increased between a pH 7 to 6, but decreased when we moved from pH 6 to 5. In general, 1 to 3 days aging increased the turbidity removal efficiency of MAl 12 solutions at different pH, but 7 days aging most cases decreases the efficiency. When comparing effect of heteroatom substitution in unaged solution, Al 13 or GeAl 12 demonstrated better turbidity removal compared to GaAl 12 at higher pH (9 to 8). But at lower pH (7 to 5), the turbidity removal efficiency trends were GaAl 12 >GeAl 12 >Al 13 . For aged solution, GaAl 12 showed superior performance followed by GeAl 12 and Al 13 at most pH ranges. Interestingly pH 9 was the exception because the GeAl 12 solution demonstrated higher efficiency in turbidity removal. The aged GaAl 12 solutions consistently removed turbidity at all pH ranges and this is specifically apparent at pH 5, where the aged GaAl 12 solutions showed remarkably high turbidity removal performance compared to other solutions. Our results suggesting that heteroatom substitutions in Al 13 and subsequent hydrothermal aging have significant impact in their water treatment efficiency with respect to DOC and turbidity removal. We have observed that although at higher pH the water treatment efficiency of GaAl 12 is better than Al 13 and GeAl 12 , this trend does not hold true for lower pH values for DOC removal.</p><p>Turbidity removal of the GaAl 12 aged and unaged solutions display superior capability in all pH range (9-5) compared to other MAl 12 solutions, with the highest performance observed at pH 5.</p><p>Previous work by Stewart et al. 2009, indicated that the GaAl 12 stock solution performed better in water treatment due to its relative stability, lower acidity, and tendency not to form larger clusters and oligomers. 15 The turbidity and gravimetric experiment from our study also showed relative stability of GaAl 12 with respect to the transformation of polyaluminum species into Al(OH) 3 based precipitate. Our DLS experiment also showed similar particle size distribution for polyaluminum species in unaged to 3 days aged GaAl 12 solution which maybe an indication of stability and less oligomerization behavior. When the article by Stewart et al. was published in 2009, there was no experimental evidence that larger oligomers with the GaAl 12 system existed, but more recent efforts suggested that Ga 2.5 Al 28.5 and Ga 2 Al 18 do occur in aged solutions. 15 These newer species having different charge, size and surface characteristic than the ε-Keggin and may have better capability to neutralize the contaminant and without destabilizing the clusters from solution. For example, solutions containing Al 30 polyaluminum clusters demonstrated higher floc formation capacity than Al 13 bearing solutions and this was linked to a higher number of adsorption sites, more neutralization capacity, and better solution stability (temperature and pH). 12 Owing to those physicochemical changes, Al 30 is a better turbidity removal agent at wider pH range then Al 13 . This suggests that the presence of Ga 2.5 Al 28.5 and Ga 2 Al 18 could also enhance the removal of DOC and turbidity from natural water samples.</p><!><p>In conclusion, our study demonstrated that heteroatom substitution in ε-Al 13 have profound impact on their transformation into other polyaluminum species and insoluble hydroxides. Overall, the GaAl 12 solution was more stable during thermal aging than Al 13 or GeAl 12 and DLS results indicated that this system possessed more diversity within the soluble particle size. In addition, the resulting precipitant form the aging study for GaAl 12 was found to behave in a similar manner upon heating and may contain structural features with similarities to amorphous Al(OH) 3 or bayerite phase, which is known to have higher solubility than other Al 3+ phases. This chemical transformation governs the stability of soluble species and their coagulation performance, including the formation of larger oligomers that may increase % DOC and turbidity removal.</p><p>Results from present work will shed new light on the development of highly effective aluminum-oxo Keggin polycation based coagulants. The difference in water treatment efficiency of different heteroatom-substituted and aged Keggin solution pointing towards importance of further investigations to identify the different possible oligomers that may be present in the system.</p><p>Overall, the relatively coagulation efficiency may depend on a smaller number of more reactive species to achieve the systems level observables that are measured during experimental evaluation of flocculation behavior and contaminant removal. Continued advancements in understanding of the fundamental changes to surface chemistry and stability of both pure and doped Keggin-type polyaluminum coagulants can lead to a targeted approached toward water purification efforts.</p><!><p>The Supporting Information file for this draft is available. The supporting information file contains additional images and results of turbidity measurement, PXRD analysis and SEM-EDS analysis.</p>