Aluminum Oxide (Alumina), Hydrated

At present, chemical processes are widely used to remove phosphorus and organic matter from water or wastewater either as the only advanced treatment method or as a pre-treatment stage to biological treatment, and aluminium salts have been traditionally used as coagulants. In the present study removal of orthophosphate or dissolved organic matter with the use of two aluminium sources: alum [Al2(SO4)3•18H2O] and aluminium hydroxide [Al(OH)3(s)] is examined for the wastewater treatment process. Amorphous aluminium hydroxide was chosen because it is the hydrolyzed (olated) product of alum and an important adsorbent of inorganic and organic substances in soils since bayerite, gibbsite and boehmite contain a large portion of aluminium hydroxides. Laboratory jar test studies were carried out using either an orthophosphate solution (10 mg l-1) or a tannic acid solution (50 mgC l-1) as simulated wastewater and the effect of parameters such as coagulant dose (up to 15 mg Al l-1 in the case of alum and up to 90 mg Al l-1 in the case of Al(OH)3) and pH (2-12) is investigated. The values of these parameters were based upon measurements on municipal wastewaters from the input of the primary treatment of a wastewater plant in Athens. Orthophosphates have been chosen as a P surrogate as they are the major portion of the total P found in wastewater and tannic acid solution was used as a surrogate for soluble organic matter. It was proven that alum is much more efficient in phosphorus and tannic acid removal than aluminium hydroxide. The optimal pH values are 5-6 in both cases, alum and aluminium hydroxide, although alum is efficient in a wider pH range (4-7) and a mechanism was proposed to interpret these results. In pH values less than 6 the mechanism proposed was chemical bonding between Al species and tannic acid or phosphates creating insoluble complexes while in bigger pH values (6-8) adsorption on solid Al(OH)3. Freundlich isotherm was proven to fit satisfactorily the experimental data for aluminium hydroxide and orthophosphate at 250C suggesting heterogeneous sorption, with KF and N values 49,1 and 0,19 respectively. The findings of this work may not only contribute to a better understanding of the chemistry of chemical wastewater treatment and therefore to an improvement of the process but also on phosphorus and organics fixation in soils that contain a large portion of aluminium hydroxides.

Ye'elimite (Ca 4 Al 6 SO 16 ) is a main mineral in calcium sulfoaluminate cements. Aluminum hydroxide is one of the products formed by hydration of ye'elimite. To characterize this phase, various aluminum hydroxides were synthesized from their chemical constituents using sol‐gel processing and compared with the aluminum hydroxide formed during the hydration of ye'elimite. The nanostructure of aluminum hydroxide formed during the hydration of ye'elimite was investigated in detail using X‐ray diffraction, thermogravimetric analysis, scanning electron microscopy, inductively coupled plasma optical emission spectroscopy, and 27 Al magic angle spinning nuclear magnetic resonance spectroscopy. No evidence was observed that indicated the existence of amorphous aluminum hydroxide in the hydration of ye'elimite. The pH values, thermogravimetric analysis, particle morphology, and Al‐O coordination indicated that the aluminum hydroxide from the hydration of ye'elimite had a crystal‐like structure. The X‐ray diffraction analysis, particle sizes, and ion activity product showed that the aluminum hydroxide from the hydration of ye'elimite had a microcrystalline structure.

An attempt to obtain aluminium hydroxide that could give aluminium oxides of increased thermal stability was made. Aluminium hydroxide was precipitated during a hydrolysis of aluminium chloride in ammonia medium. The influence of preparative conditions, such as a dosing rate of aluminium precursor, pH, duration of the precipitate refluxing and temperature of calcination, on the properties of obtained hydroxides and oxides was investigated. The materials were studied with the following methods: thermal analysis, IR spectroscopy, low-temperature nitrogen adsorption and adsorption–desorption of benzene vapours. Precipitated boehmites had high values of S BET determined from nitrogen adsorption (220–300 m2g–1), good sorption capacity for benzene vapours, developed mesoporous structure and hydrophilic character. It has been proved that a high pH value during the precipitation of aluminium hydroxide favoured better crystallisation of boehmite structure, higher temperature of its dehydroxylation into γ-Al2O3, and delayed transformation of γ phase into α-Al2O3. Aluminium oxides derived from the hydroxides precipitated at a high pH were the most stable at high temperatures, and were characterised with the best surface properties.

A comparative study of amorphous and crystalline forms of commercial aluminum hydroxides as inhibitors of alkalisilica reactions in Portland cement mortars has been performed. It was found that at dosages of 1% to 3%, amorphous aluminum hydroxide can efficiently inhibit alkali-silica expansion of Portland cement compositions. High inhibiting activity of amorphous Al(OH)3 additives may be explained by their ability to actively bind Ca(OH)2 formed by the hydration of silicate phases of cement, to form ettringite (with participation of gypsum). Crystalline Al(OH)3 additives that do not possess the ability to interact with Ca(OH)2 even after additional grinding, however, demonstrate week properties to inhibit alkali-silica expansion. This may indicate that the inhibitory effect of Al(OH)3 at least—partly, may be given by its influence on the concentration of Al3+ ions in the pore solution. Some expansion of the samples with admixtures of Al(OH)3 observed during the alkaline expansion accelerated test procedure is not associated with the formation of ettringite and is only due to alkali-silicate reactions.

This study investigates sustainable strategies for large-scale industrial production in the cable industry ( e.g. sheathing applications), focusing on halogen-free flame-retardant (HFFR) additives in polyolefin composites. Three metal hydroxides are evaluated: precipitated aluminium hydroxide (p-ATH), precipitated magnesium hydroxide (p-MDH), and naturally milled magnesium hydroxide (nm-MDH). While p-ATH and p-MDH are chemically processed and carbon-intensive, nm-MDH is derived from untreated minerals, offering a lower environmental impact. All hydroxides were characterised using XRD, TGA, morphological, and spectroscopic analyses. p-ATH and p-MDH show gibbsite and brucite structures, respectively, while nm-MDH features a brucite structure with minor dolomite content, enhancing flame resistance due to higher decomposition temperatures and inert gas release. Flame-retardant polymer composites (FRPCs) were produced by melt-mixing 60 %wt. metal hydroxides into a polyolefin-based ( i.e. EVA/LLDPE/MA-g-LLDPE) matrix. Rheological tests show that p-ATH-based FRPCs maintain liquid-like behaviour, while p-MDH and nm-MDH composites exhibit solid-like properties. Morphological analysis reveals better polymer-filler interaction in p-MDH and nm-MDH composites, supported by reactions with MA-g-LLDPE. All FRPCs demonstrated excellent flame and thermal resistance. Additionally, metal hydroxides reduced oxidative degradation under UV-B exposure, suggesting improved weatherability. • Magnesium and aluminium hydroxides are eco-friendly flame-retardant fillers for polyolefin. • Magnesium hydroxide promotes solid-like behaviour with improved filler–matrix interactions. • Aluminium hydroxide maintains liquid-like characteristics. • All polyolefin-based formulations offer excellent flame and thermal resistance, as well as slightly enhanced UV weathering stability.

The structure and properties of nanoparticles of aluminum hydroxides and oxides obtained by electrochemical, chemical, and combined methods were studied by transmission electron microscopy, X-ray diffraction, thermal analysis, and atomic emission spectroscopy. The influence of synthesis conditions on the structure and morphology of nanoparticles was studied. It was shown that the effect of an electrochemical field allows monophasic systems to be obtained with a narrower range of particle sizes than in the case of chemical deposition.

Detailed knowledge of the structural evolution of diverse aluminum (oxy)hydroxides with varying temperatures up to ~300 °C provides insights into the dehydration processes involving transitions among metastable phases on Earth's surface and in the crust. Here, we report the high-resolution solid-state 27Al NMR spectra for three different types of aluminum (oxy)hydroxides (i.e., boehmite, bayerite, and gibbsite) with varying annealing temperatures up to 300 °C, revealing the effect of distinct precursor minerals on the stability of metastable alumina. 27Al MAS NMR results allow us to obtain the quantitative fractions and NMR parameters for each phase during transformation. The results demonstrate that each aluminum (oxy)hydroxide phase follows a unique transformation path to metastable alumina. The 27Al MAS and 3QMAS NMR spectra of boehmite show that a minor but observable [4]Al signal (~2%) is detected at ~50 °C, and the [4]Al fraction gradually increases up to 300 °C (~16%), indicating that the phase transformation from boehmite to γ-/η-Al2O3 occurs at a temperature as low as ~50 °C, significantly lower than earlier estimations based on XRD. Together with the [4]Al fraction, the [5]Al fraction increases from <1% at 50 °C to ~2.3% at 300 °C, whereas the NMR results of bayerite and gibbsite do not show the presence of [5]Al. In addition, the 27Al 3QMAS NMR spectra resolved the [6]Al site in boehmite and that in γ-/η-Al2O3, which could not be uniquely determined from 1D NMR spectra. The population of bayerite abruptly decreases from 100% (at 150 °C), through ~47% (at 200 °C), to 0% (at 250 °C), indicating that the phase transition from bayerite to boehmite + γ-/η-Al2O3 occurs within a narrow temperature range. As for gibbsite, while [4]Al is not observed in the spectra up to 200 °C, the [4]Al fraction of ~2% is observed in the spectra for gibbsite annealed at 250 °C, and the [4]Al fraction increases rapidly to ~15% as the annealing temperature increases to 300 °C, suggesting that the phase transformation into γ-/η-Al2O3 occurs at ~250 °C. The results confirm that the phase transformation paths (gradual vs. dramatic) depend on the type of precursor minerals. Particularly, the onset temperature of the phase transformation from boehmite to metastable alumina (~50 °C) is lower than those from other precursor minerals (>150 °C). Furthermore, the phase transformation from boehmite to γ-/η-Al2O3 occurs gradually within broad temperature ranges from ~50 °C. This is due to their configurational disorder as evidenced by the presence of [5]Al. The observed structural evolution in aluminum (oxy)hydroxides in the low-temperature range facilitates our understanding of the nature of phase transformation and dehydration of oxides and hydroxides in the Earth's surface environments.

Hydrated aluminum oxides (HAO) have been identified as major sinks for contaminants, such as arsenic, in aquatic and soil ecosystems. This research focused on the assessment of As(V)-HAO interactions by combining macroscopic studies with x-ray absorption spectroscopy (XAS). Sorption of As(V) onto HAO was studied as a function of reaction time, sorbate loading, and the presence of phosphate. The short-term isotherm studies indicated that HAO has a high sorption capacity for As(V) (approximately 1.9 x 10 –3 mol As(V) g –1 HAO) and the uptake of As(V) was not significantly affected by equimolar amounts of phosphate at pH 4.5. The results of long-term kinetic studies, where a continuous influx of As(V) was maintained, revealed rapid initial sorption to the external surface of HAO within less than four hours, followed by slow uptake over several weeks, which could be attributed to intraparticle diffusion. Complementary XAS analyses suggested that As(V)-HAO interactions can be described by one average reaction mechanism, resulting in bidentate inner-sphere complexes. This mechanism was independent of time, sorbate loading, and the presence of phosphate under the reaction conditions employed in this study. Overall, the results of this research emphasized that microporous oxyhydroxides of aluminum are much larger sinks for oxyanions, such as As(V), than presently understood.

Influence of different seeds on transformation of aluminum hydroxides and morphology of alumina grains by hot-pressing

A self-powered electrocoagulation system with a single-chamber aluminum–air fuel cell was employed for phosphate removal in this study. Electricity production and aluminum hydroxides in solution were also investigated. When the NaCl concentration increased from 2 mmol/L to 10 mmol/L, the phosphate removal increased from 86.9% to 97.8% in 60 min. An electrolyte composed of 10 mmol/L of NaCl was shown to obtain a maximum power density generation of 265.7 mW/m2. When the initial solution pH ranged from 5.0 to 9.0, 98.5% phosphate removal and a maximum power density of 338.1 mW/m2 were obtained at pH 6.0. Phosphate was mainly removed by aluminum hydroxide adsorption. These results demonstrate that the aluminum–air fuel cell can be applied as electricity-producing electrocoagulation equipment. Aluminum–air fuel cells provide an alternative method to meet the goal of carbon neutrality in wastewater treatment compared with traditional energy-consuming electrocoagulation systems.

Summary Most of the information on soil aggregation and porosity comes from studies of natural soil in which the effects of the different constituents that form the structure overlap. The aim of this research was to study the effects of these constituents separately on well‐characterized artificial aggregates in order to understand them better. To do this, the pore system of model silt aggregates, amended with different amounts of humic acids, iron and aluminium hydroxides or colloidal silica, was investigated at three levels of magnification with water vapour desorption (nanometre sizes), mercury intrusion (micrometre sizes) and microtomography (tens of micrometres). Humic acid and aluminium hydroxide increased aggregate porosities measured by all methods. An increase in porosity with increasing additions of each constituent was indicated only by water desorption. We did not observe any well‐defined trends in the dynamics of average pore radii. The pore surface fractal dimension determined by mercury intrusion was negatively correlated with that measured by water desorption. The pore system in granular media comprises larger voids joined by narrower necks; therefore, we attempted to relate their sizes with a novel approach that combined microtomography with mercury intrusion and extrusion data. We observed a decrease in the size of pore necks that give access to voids of the same sizes with increasing additions of all constituents. With additions of humic acid this effect was the smallest. The mercury intrusion data showed the formation of separate concretions of iron hydroxides and silica in silt aggregates.

{sup 27}Al and {sup 23}Na MAS NMR, powder X-ray diffraction, and infrared spectroscopic investigations of freeze-dried sodium aluminates and aluminum hydroxides formed through acid hydrolysis have been undertaken, with OH/Al hydrolysis ratios between 5.3 and 2.8 being analyzed. Numerous {sup 27}AlNMR resonances were observed, the intensities of which vary as a function of OH/Al ratio, and these have been assigned to four-, five-, and six-coordinate aluminum species constituting a variety of structural moieties. The dominant species at an OH/Al ratio above 4.4 appears to be a Q{sup o}Na[Al(OH);{sub 4}] salt, as indicated by a {sup 27}Al resonance at 86.6 ppm. In addition, a second, broader resonance at 71.3 ppm demonstrates the simultaneous existence of further four-coordinate aluminum species linked thorough oxo bonds to other four-coordinate aluminums (e.g., Q{sup 2} [Al(OH);{sub 2}(OAl){sub 2}];{sup x-}). At an OH/Al ratio between 4.4 and 4.1, a water-soluble phase forms that contains both four- and six-coordinate aluminum. At OH/Al ratios fo 4.0 and below, a water-soluble phase forms that contains both four-and six-coordinate aluminum. AT OH/Al ratios of 4.0 and below, a water-insoluble phase exists possessing four-, five-, and six-coordinate aluminum. At OH/Al{le}3.9 range exhibits {sup 27}Al chemical shifts similar to those reported for transitionalmore » aluminas such as {gamma}-, {eta}-, and 0-Al{sub 2}O{sub 3} and an infrared spectrum similar to pseudo-spinel gels, suggesting that a pseudo-spinel intermediate is the first phase involved in the crystallization of gibbsite. The resonance assigned to five-coordinate aluminum probably results from species involved in the transformation of the pseudo-spinal phase to pseudo-boehmite. The formation of gibbssite on the acid hydrolysis of alkaline sodium aluminate solutions thus appears to follow the pathway pseudo-spinel {r_arrow} pseudo-boehmite {r_arrow} bayerite {r_arrow} gibbsite. 82 refs., 7 figs., 3 tabs.« less

A model simulator NALD-2 has been developed to study the interactions of fluoride and aluminium in the Nalgonda Defluoridation Process, which principally involves the preferential adsorption of fluoride ions onto insoluble aluminium hydroxides that are formed from alum hydrolysis reactions and undergo precipitation. This model represents the defluoridation mechanism taking into account the charged behavior of the amphoteric aluminium hydroxide colloids, charged site densities, and fluoride complexation reactions in order to predict the extent of defluoridation and concentrations of dissolved and colloidal aluminium in the treated water as functions of pH, alum dosage and raw fluoride concentrations. Model validations were carried out using secondary data of Selvapathy & Arjunan (1995) for total residual aluminium in Nalgonda treated water. Slight variations in pH into the acidic range cause substantial increase in colloidal aluminium and hence pH and alum dosages are important control parameters to limit residual aluminium in treated water. The NALD-2 simulator helps in predicting optimum alum dosages for minimum residual aluminium in the treated water. It was concluded that the dosages of alum recommended in the Nalgonda method cannot bring down the residual Al below the permissible limit of 0.2mg/L in the treated water.

Aluminum hydroxides were produced from AlCl3 solution. Their characteristics were studied in terms of corrosion product. When the aluminum hydroxide adheres over the surface of intermetallic compounds exposed on the surface of Al-Fe alloys, corrosion potential of Al-Fe alloys shifts to less noble. 0.1M NaCl solution containing AlCl3 0.01 to 0.1M plays a role of pH buffer. Its maximum pH is about 4.

Studies were made of the dehydroxylation of several aluminium hydroxide modifications and the kinetics ofα-Al2O3 formation. The investigated samples differed in both mineral composition and the level of alkali admixtures. It was found that the rate of formation and the quantity ofα-Al2O3 depend mainly on the purity of the aluminium hydroxides, while the transition forms of alumina depend on the initial type of the aluminium hydroxide.