Aluminum Passivation Research Articles

Fluorine Rich Borate Salt Anion Based Electrolyte for High Voltage Sodium Metal Battery Development.

This study demonstrates the enhanced performance in high-voltage sodium full cells using a novel electrolyte composition featuring a highly fluorinated borate ester anion (1M Na[B(hfip)4].3DME) in a binary carbonate mixture (EC:EMC), compared to a conventional electrolyte (1M Na[PF6] EC:EMC). The prolonged cycling performance of sodium metal battery employing high voltage cathodes (NVPF@C@CNT and NFMO) is attributed to uniform and dense sodium deposition along with the formation of fluorine and boron-rich solid electrolyte interphase (SEI) on the sodium metal anode. Simultaneously, a robust cathode electrolyte interphase (CEI) is formed on the cathode side due to the improved electrochemical stability window and superior aluminum passivation of the novel electrolyte. The CEIs on high-voltage cathodes are discovered to be abundant in C-F, B-O, and B-F components, which contributes to long-term cycling stability by effectively suppressing undesirable side reactions and mitigating electrolyte decomposition. The participation of DME in the primary solvation shell coupled with the comparatively weaker interaction between Na+ and [B(hfip)4]- in the secondary solvation shell, provides additional confirmation of labile desolvation. This, in turn, supports the active participation of the anion in the formation of fluorine and boron-rich interphases on both the anode and cathode.

Effect of non-metallic silicon content on the microstructure and corrosion behaviour of AlCoCrFeNi high-entropy alloys

AlCoCrFeNi high-entropy alloys (HEAs) have shown excellent mechanical properties and high-temperature oxidation resistance but poor corrosion resistance owing to the high contents of the aluminium passivation film. In this study, Si was added to these alloys with the aim of improving the composition and stability of the passivation films. The microstructure and corrosion behaviour of AlCoCrFeNiSix (molar ratio x = 0, 0.1, 0.2, 0.3, and 0.4) HEAs were investigated. Dendritic and spinodal decomposition regions were observed in all five alloys. In the dendritic region, the dendritic structures transitioned from a petal shape to a dendritic shape with increasing Si content. Phase separation occurred in the spinodal decomposition region, which predominantly consisted of ordered body-centred cubic (B2) and disordered body-centred cubic (A2) phases. The size of the A2 phase decreased, and the amount of the A2 phase increased with increasing Si content. An optimised Si content could enhance the stability of Al and Cr passivation films, whereas excessive Si imposed an adverse effect by forming a Cr passivation film. The corrosion resistance of AlCoCrFeNiSi0.2 HEAs was optimum with a passive current density of 3.2 μA/cm2, which was 0.2 times that of the obtained AlCoCrFeNi HEAs without Si. The type of corrosion changed from local corrosion of AlCoCrFeNiSix (x ≤ 0.3) HEAs to general corrosion of AlCoCrFeNiSi0.4 HEAs. The corrosion mechanisms of the AlCoCrFeNi HEAs with various Si contents were elucidated.

Anodic dissolution of aluminum in non-aqueous electrolyte solutions for sodium-ion batteries

The respective impact of the electrolyte and solvent, during anodic dissolution or passivation of aluminum was investigated in electrolyte solutions developed for sodium-ion batteries.

Highly Fluorinated and Boron Containing Anion Derived Stable Anode and Cathode Interfaces for Long Cycle and High Voltage Sodium Metal Batteries

Sodium metal is a propitious anode for the development of next generation metal batteries owing to its low reduction potential (-2.71 V) and high theoretical capacity (1165 mAh g-1)1. Among various electrolytes, carbonate solvents can be useful to develop high voltage metal batteries with wide temperature range2. However, the aggressive reactivity of sodium metal in carbonate electrolytes lead to the formation of unstable solid electrolyte interface on the metal anode which renders poor long cycling performance to the battery3. In this work, we have fabricated thin and inorganic components (F and B rich) rich solid electrolyte interface (SEI) on Na metal anode and cathode electrolyte interface (CEI) on oxide-based cathode to improve long cycling stability of sodium metal battery. Here, fluorine (F) and boron (B) containing anion is compared with conventional hexafluorophosphate anion (PF6-) in binary carbonate solvent system (EC:EMC). The stable stripping/plating behaviour of both symmetric (Na/Na) and asymmetric cell (Al/Na) in case of F and B containing anion is found compared to the PF6 - anion.The excellent long-term cycling stability of full cell (Na/Na0.67Fe0.5Mn0.5O2) is further attributed to the improved electrochemical stability window and better aluminium passivation on employing F and B containing anion in the electrolyte system as compared to PF6 - anion. The dual role of F and B containing anion to form stable interfaces on both the anode and cathode side provides a pathway in the development of fluorinated electrolytes for high voltage sodium metal batteries.

Electrochemical Passivation of Aluminum As a Current Collector for Solid-State Polymer Batteries : PEO/LiTFSI Polymer Electrolyte

Investigation of the electrochemical properties and passivation mechanism of aluminum as a current collector for solid-state polymer batteries is carried out. Herein, electrochemical stabilities of Polyethylene glycol (PEO) based solid-state electrolyte containing Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt are proved using electrochemical dynamic- and transient-mode polarization with surface scratching. These results indicate that Al metal undergoes passivation with a layer below 4.5 V vs. Li+/Li. And then Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) reveal double layers are formed during the passivation on Al metal surface, with the outer layer composed of metal fluorides (AlF3) and the inner layer composed of metal oxides (Al2O3). The formation of the AlF3 layer on Al metal surface is attributed to the reaction between Al2O3 and HF following as Al2O3 + 6HF → 2AlF3 + 3H2O. Therefore, successful passivation achieved by the formation of AlF3 on the Al2O3 surface of Al metal facilitates to be widely used as a current collector for lithium metal polymer batteries. Therefore, successful passivation by the formation of AlF3 on the Al2O3 surface of Al metal making it a promising choice for the positive electrode in lithium metal polymer batteries.

Highly Oxidative-Resistant Cyano-Functionalized Lithium Borate Salt for Enhanced Cycling Performance of Practical Lithium-Ion Batteries.

Lithium difluoro(oxalato) borate (LiDFOB) has been widely investigated in lithium-ion batteries (LIBs) owing to its advantageous thermal stability and excellent aluminum passivation property. However, LiDFOB tends to suffer from severe decomposition and generate a lot of gas species (e.g., CO2 ). Herein, a novel cyano-functionalized lithium borate salt, namely lithium difluoro(1,2-dihydroxyethane-1,1,2,2-tetracarbonitrile) borate (LiDFTCB), is innovatively synthesized as a highly oxidative-resistant salt to alleviate above dilemma. It is revealed that the LiDFTCB-based electrolyte enables LiCoO2 /graphite cells with superior capacity retention at both room and elevated temperatures (e.g., 80 % after 600 cycles) with barely any CO2 gas evolution. Systematic studies reveal that LiDFTCB tends to form thin and robust interfacial layers at both electrodes. This work emphasizes the crucial role of cyano-functionalized anions in improving cycle lifespan and safety of practical LIBs.

Highly Oxidative‐Resistant Cyano‐Functionalized Lithium Borate Salt for Enhanced Cycling Performance of Practical Lithium‐Ion Batteries

AbstractLithium difluoro(oxalato) borate (LiDFOB) has been widely investigated in lithium‐ion batteries (LIBs) owing to its advantageous thermal stability and excellent aluminum passivation property. However, LiDFOB tends to suffer from severe decomposition and generate a lot of gas species (e.g., CO2). Herein, a novel cyano‐functionalized lithium borate salt, namely lithium difluoro(1,2‐dihydroxyethane‐1,1,2,2‐tetracarbonitrile) borate (LiDFTCB), is innovatively synthesized as a highly oxidative‐resistant salt to alleviate above dilemma. It is revealed that the LiDFTCB‐based electrolyte enables LiCoO2/graphite cells with superior capacity retention at both room and elevated temperatures (e.g., 80 % after 600 cycles) with barely any CO2 gas evolution. Systematic studies reveal that LiDFTCB tends to form thin and robust interfacial layers at both electrodes. This work emphasizes the crucial role of cyano‐functionalized anions in improving cycle lifespan and safety of practical LIBs.

Leaching of Waste Pharmaceutical Blister Package Aluminium in Sulphuric Acid Media

In this study, the leaching behaviour of aluminium from waste pharmaceutical blister packages (WPBs) is investigated in sulphuric acid media to build future strategies for aluminium recycling from this non-recycled waste fraction. The results suggest that in hydrometallurgical recycling, passivation of aluminium during leaching can be mitigated in dilute sulphuric acid solutions (0.25 M), at high temperatures (60–80 °C) and specifically with H2O2 addition. With this system, 100% extraction was achieved within five hours under optimized conditions (H2SO4 = 0.25 M, T = 80 °C, H2O2 = 1.25 vol.%). The leaching mechanism is suggested to be based on electrochemical dissolution of metallic aluminium oxidized by H+ or H2O2, followed by fast passivation by Al2O3 and consequent chemical dissolution of Al2O3 at slower kinetics. The calculated activation energy (~69 kJ/mol) suggests that the leaching reaction is controlled by the chemical or electrochemical reaction step rather than diffusion. By WPB leaching, an aluminium sulphate solution could be obtained, suitable for further aluminium sulphate crystallization. This may provide a potential route for the valorisation of aluminium from a currently overlooked waste fraction of pharmaceutical blister packages.

Highly Selective Reduction of Nitrate by Zero-Valent Aluminum (ZVAI) Ball-Milled Materials at Circumneutral pH: Important Role of Microgalvanic Cells for Depassivation of ZVAl and N2-Selectivity.

The passivation of zero-valent aluminum (ZVAl) limits its application in environmental remediation. Herein, a ternary composite material Al-Fe-AC is synthesized via a ball-milling treatment on a mixture of Al0, Fe0, and activated carbon (AC) powders. The results show that the as-prepared micronsized Al-Fe-AC powder could achieve highly efficient nitrate removal and a nitrogen (N2)-selectivity of >75%. The mechanism study reveals that, in the initial stage, numerous Al//AC and Fe//AC microgalvanic cells in the Al-Fe-AC material could lead to a local alkaline environment in the vicinity of the AC cathodes. The local alkalinity depassivated the Al0 component and enabled its continuous dissolution in the subsequent second stage of reaction. The functioning of the AC cathode of the Al//AC microgalvanic cell is revealed as the primary reason accounting for the highly selective reduction of nitrate. The investigation on the mass ratio of raw materials manifested that an Al/Fe/AC mass ratio of 1:1:5 or 1:3:5 was preferable. The test in simulated groundwater suggested that the as-prepared Al-Fe-AC powder could be injected into aquifers to achieve a highly selective reduction of nitrate to nitrogen. This study provides a feasible method to develop high-performance ZVAl-based remedial materials that could work in a wider pH range.

Corrosion of Passive Aluminum Anodes in a Chloroaluminate Deep Eutectic Solvent for Secondary Batteries: The Bad, the Good, and the Ugly.

The passivity of aluminum is detrimental to its performance as an anode in batteries. Soaking of native oxide-covered aluminum in a chloroaluminate deep eutectic solvent gradually activates the electrode surface, which is reflected in a continuously decreasing open circuit potential. The underlying processes were studied by analyzing the 3 to 7 nm thick layer of native oxide after increasing periods of soaking with secondary neutral mass spectrometry, X-ray photoelectron spectroscopy, and energy-dispersive spectroscopy in a transmission electron microscope. They consistently show permeation of electrolyte species into the layer associated with gradual swelling. After extended periods of soaking at open circuit potentials, local deposits of a range of foreign metals have been found in scanning electron microscopy images of the electrode surface. The pitting corrosion is caused by trace metal ion impurities present in the electrolyte and results in highly nonuniform current density distribution during discharge/charge cycling of battery cells as shown by local deposits of aluminum. The processes during soaking at open circuit potentials have been monitored by electrochemical impedance spectroscopy and could be analyzed by fitting an equivalent circuit model for pitting corrosion.

Passive Oxide Film Growth Observed On the Atomic Scale

AbstractDespite the ubiquitous presence of passivation on most metal surfaces, the microscopic‐level picture of how surface passivation occurs has been hitherto unclear. Using the canonical example of the surface passivation of aluminum, here in situ atomistic transmission electron microscopy observations and computational modeling are employed to disentangle entangled microscopic processes and identify the atomic processes leading to the surface passivation. Based on atomic‐scale observations of the layer‐by‐layer expansion of the metal lattice and its subsequent transformation into the amorphous oxide, it is shown that the surface passivation occurs via a two‐stage oxidation process, in which the first stage is dominated by intralayer atomic shuffling whereas the second stage is governed by interlayer atomic disordering upon the progressive oxygen uptake. The first stage can be bypassed by increasing surface defects to promote the interlayer atomic migration that results in direct amorphization of multiple atomic layers of the metal lattice. The identified two‐stage reaction mechanism and the effect of surface defects in promoting interlayer atomic shuffling can find broader applicability in utilizing surface defects to tune the mass transport and passivation kinetics, as well as the composition, structure, and transport properties of the passivation films.

Local initiative hydrogen production by utilization of aluminum waste materials and natural acidic hot-spring water

Hydrogen is gaining attention as an energy source, but its production through fossil-fuel use is not environmentally friendly. A more sustainable source could be the hydrothermal reaction involving aluminum and water reaction, which owns technical issues on aluminum passivation and material sources for the upscale application. This study analyzes the aluminum-water hydrothermal reaction at laboratory and field scales and includes an environmental assessment of H2 fuel production involving hot-spring water with extremely low pH (~1) and boiling temperature (~373 K). Acidic hot-spring alternates water sources, and its unique feature activates aluminum surface by attacking oxide layer, hence enables H2 generation. Aluminum waste sources include dross and cutting chips, which could replace primary aluminum metal. The highest H2 yield could be obtained of about ~55 mmol H2 gAl−1 for the chip (almost reached theoretical yield) and a smaller amount of ~20 mmol H2 gAl−1 for dross. The environmental assessment comprises carbon dioxide emissions and energy consumption from the overall H2 fuel system resulting in a reduced environmental impact. The proposed hydrogen production method encourages hydrogen energy development from the local scale. In addition, the use of aluminum waste materials is a new and useful waste management strategy than direct disposal, and hot-springs use advances its direct utilization.

Fabrication and Characterization of a-IGZO Thin-Film Transistors With and Without Passivation Layers

The thin-film transistors (TFTs) devices based on amorphous indium gallium zinc oxide (a-InGaZnO, a-IGZO) films were fabricated and characterized in this paper. We have employed the a-IGZO thin film as an active channel layer in the bottom gate structure of transistor, the silicon dioxide (SiO2) as a gate insulator, and the indium tin oxide (ITO) as a gate source and drain electrodes. All of them were grown on ITO glass substrates by radio frequency (RF) magnetron sputtering technique. However, there were still some inherent defects in the a-IGZO materials, it would be affected the performance of the fabricated transistors. Therefore, in this work, it explored to research a-IGZO TFTs with aluminum (Al) passivation layers as sensitive to water and oxygen thereby affect the device stability, and time dependence threshold voltage variation in ambient of passivation layer and reliability. Demonstrably, our experimental results revealed that the W/L (W = wide, L = length) ratio is 10, the on/off ration and the mobility would be better, respectively. Moreover, the results also showed improvement in the performance of a-IGZO TFTs with the Al passivation layers and annealing process treatment.

History of corrosion investigation development in the Institute of Physical Chemistry of the RAS. Part XV. Passivation and chemical oxidation of aluminum and its alloys

History of corrosion investigation development in the Institute of Physical Chemistry of the RAS. Part XV. Passivation and chemical oxidation of aluminum and its alloys

Формирование границ раздела полупроводник-сера-переходный металл

Выполнены термодинамические расчеты при стандартных условиях химических реакций, протекающих при сульфидной пассивации поверхности элементарных полупроводников (Si, Ge), нитридов галлия, индия, алюминия (GaN, InN, AlN). Осуществление сульфидной пассивации нитрида алюминия, кремния и германия термодинамически вероятнее для неокис-ленной поверхности полупроводников, контактирующей с газовой фазой, содержащей серу или сероводород. Существует термодинамическая вероятность сульфидирования окисленной поверхности германия из газовой фазы. Сульфидирование нитридов галлия и индия самопроизвольно возможно из водных растворов сульфидов щелочных металлов. Сравнение кристаллохимических параметров атомов (ковалентные и атомные радиусы), типов симметрии возможных фазовых соединений на границах раздела полупроводник-сульфид-металл и полупроводник-оксид-металл показало, что для сульфидированной поверхности полупроводника имеется большее структурное соответствие, чем для окисленной поверхности полупроводника, контактирующего с переходным металлом. При замещении атомов кислорода на атомы серы в приграничном слое между переходным металлом и полупроводником наблюдается меньший скачок в величинах электроотрицательностей элементов, образующих границу раздела фаз. Формирование с помощью сульфидной пассивации поверхности полупроводника более совершенной и стабильной (по химическим и физическим свойствам) границы раздела фаз способствует созданию выпрямляющих контактов металл-полупроводник с улучшенными электрофизическими параметрами.DOI 10.14258/izvasu(2018)4-02

Passivation behaviour of aluminium current collector in ionic liquid alkyl carbonate (hybrid) electrolytes

The compatibility of current collectors with the electrolyte plays a major role in the overall performance of lithium batteries, critical to obtain high storage capacity as well as excellent capacity retention. In lithium-ion batteries, in particular with cathodes that operate at high voltage such as lithium nickel cobalt manganese oxide, the cathodic current collector is aluminium and it is subjected to high oxidation potentials (>4 V vs. Li/Li+). As a result, the composition of the electrolyte needs to be carefully designed in order to stabilise the battery performance as well as to protect the current collectors against corrosion. This study examines the role of a hybrid electrolyte composed of an ionic liquid (N-methyl-N-propyl pyrrolidinium bis(trifluoromethanesulfonyl)imide or N-methyl-N-propyl pyrrolidinium bis(fluorosulfonyl)imide) and a conventional electrolyte mixture (LiPF6 salt and alkyl carbonate solvents) with correlation to their electrochemical behaviour and corrosion inhibition efficiency. The hybrid electrolyte was tested against battery grade aluminium current collectors electrochemically in a three-electrode cell configuration and the treated aluminium surface was characterised by SEM/EDXS, optical profilometry, FTIR, and XPS analysis. Based on the experimental results, the hybrid electrolytes allow an effective and improved passivation of aluminium and lower the extent of aluminium dissolution in comparison with the conventional lithium battery electrolytes and the neat ionic liquids at high anodic potentials (4.7 V vs. Li/Li+). The mechanism of passivation behaviour is also further investigated. These observations provide a potential direction for developing improved hybrid electrolytes, based on ionic liquids, for higher energy density devices.

Investigation of the Use of an Inorganic Aqueous Solution in Aluminum-Made Phase-Change Heat Transfer Devices

An inorganic aqueous solution, known as IAS, has shown its compatibility with aluminum phase-change heat transfer devices. When using IAS in aluminum devices, aluminum prefers to react with the two oxidizers, permanganate and chromate, rather than water to generate a thin and compact layer of aluminum oxide, which protects the aluminum surface and prevents further reactions. In addition, an electrochemical theory of aluminum passivation is introduced, and the existence of an electrochemical cycle is demonstrated by an aluminum thermosiphon test. The electrochemistry cycle, built up by liquid back flow and tube wall, allows the oxidizers to passivate the aluminum surface inside the device without being directly in contact with it. However, failure was detected while using IAS in thermosiphons with air natural convection cooling. The importance of a continuous liquid back flow to aluminum passivation in phase-change heat transfer devices is pointed out, and a vertical thermosiphon test with natural convection cooling is used to demonstrate that a discontinuous liquid back flow is the main reason of the failures.

Nitrate reduction in water by aluminum alloys particles

Nano zero-valent iron (NZVI) particles have been extensively investigated for nitrate reduction in water. However, the reduction by NZVI requires acidic pH conditions and the final product is exclusively ammonium, leading to secondary contamination. In addition, nanomaterials have potential threats to environment and the transport and storage of nanomaterials are of safety concerns. Aluminum, the most abundant metal element in the earth's crust, is able to reduce nitrate, but the passivation of aluminum limits its application. Here we report Al alloys (85% Al) with Fe, Cu or Si for aqueous nitrate reduction. The Al alloys particles of 0.85–0.08 mm were inactivate under ambient conditions and a simple treatment with warm water (45 °C) quickly activated the alloy particles for rapid reduction of nitrate. The Al-Fe alloy particles at a dosage of 5 g/L rapidly reduced 50 mg-N/L nitrate at a reaction rate constant (k) of 3.2 ± 0.1 (mg-N/L)1.5/min between pH 5–6 and at 4.0 ± 0.1 (mg-N/L)1.5/min between pH 9–11. Dopping Cu in the Al-Fe alloy enhanced the rates of reduction whereas dopping Si reduced the reactivity of the Al-Fe alloy. The Al alloys converted nitrate to 20% nitrogen and 80% ammonium. Al in the alloy particles provided electrons for the reduction and the intermetallic compounds in the alloys were likely to catalyze nitrate reduction to nitrogen.

Trimethylboroxine Vs. Lithium Tetrafluoroborate As Electrolyte Additives for High Voltage LiCoPO4 Cathode Materials: Key Role of Boron Trifluoride

High voltage cathode materials are promising technologies for increasing the energy density and thus increasing the range of electric vehicles. LiCoPO4 (LCP) is known as a possible high voltage material, which is electrochemically active at 4.8 V vs. Li/Li+ with a theoretical capacity of 167 mAh/g [1], corresponding to a theoretical energy density of ~800 Wh/kg. Unfortunately, this material exhibits poor performance on cycling in commonly used electrolytes containing LiPF6. Recently, Sharabi et al. [2] have used an electrolyte containing LiPF6 with trimethylboroxine (TMB) and fluoroethylene carbonate (FEC) that allows maintaining a capacity of 100 mAh/g after 100 cycles for a LCP electrode. In our previous work [3], we have shown that i) TMB is oxidized to boron trifluoride (BF3), phosphorylfluoride (POF3) and carboanion (CH3 -) and ii) a high amount of additive (>0.5%wt) in a cell results in a very bad stability. However, the role of TMB in the performance of LCP was not clearly identified. Lithium tetrafluoroborate (LiBF4) was investigated as unique lithium salt [4] and as additive in LiPF6 containing electrolyte [5]. It was found that LiBF4 has good Aluminum passivation properties and improves the electrochemical performance of LiNi0.5Co0.2Mn0.3O2. In the presence of LiPF6 salt, LiBF4 is expected to form BF3 and lithium fluoride (LiF) [5]. In the present work, the effect of BF3 gas in the LCP performance is investigated through a comparison between TMB and LiBF4 as additive in LP30. Considering the amount of BF3 produced from the decomposition of both additives, the electrolyte containing 0.2%wt TMB, which presented the best electrochemical behavior [3], and an electrolyte with 0.4%wt LiBF4 are compared in Figure 1. Interestingly, for a theoretical similar amount of BF3 in the cell, LCP exhibits the same performance in both electrolytes. An optimization of the amount of LiBF4 additive in LP30 electrolyte for LCP electrodes will be presented. The best results are obtained with a low content of LiBF4 (<3%wt). Secondly, OEMS [6] and CV experiments on LiBF4-containing electrolyte will confirm the decomposition of LiBF4 and the presence of BF3 during the oxidation scan until 5.2 V. Additionally, impedance experiments (EIS) will be shown and results will be compared to the data obtained in the case of TMB-containing electrolyte. Finally, as LiBF4 is known to passivate the Aluminum well, the effect of TMB and LiBF4 on Aluminum and stainless steel used as current collector in Swagelok cells will be studied by SEM-EDX, XPS and ICP. In order to distinguish between the beneficial effect brought by a better current collector passivation and a possible layer formed on LCP, similar experiments will be performed in coin cells, which utilize a higher area of stainless steel in contact with the electrolyte. Figure 1: Discharge capacity (in mAh/gLCP) vs. Cycle number of LCP electrode (4 mg/cm2) in different electrolytes (120 µL electrolyte, 3 glass fiber separator, Swagelok cell) with the following cycling procedure: 2 cycles at C/15 and subsequent 150 cycles at C/2.

Fluoride Removal from Aluminium Finishing Wastewater by Hydroxyapatite

The hexavalent chromium toxicity forces the aluminium finishing industry to use fluoride-containing chemicals in order to improve aluminium passivation. Disposal of industrial wastewaters containing excess concentration of fluoride to surface and ground waters can cause several health hazards. Effluents containing fluoride need treatment to reduce their concentration to disposable values before being discharged into public sewage. In this study, fluoride-containing wastewater from the aluminium finishing industry was firstly treated by precipitation at pH 12.5, achieving residual fluoride concentrations of 8.69 ± 1.84 mg/L from an initial concentration of 74.5 ± 4.6 mg/L, and then by adsorption of the remaining solution onto hydroxyapatite (HAP), resulting in fluoride effluent concentrations of 4.09 ± 0.16 mg/L. The total fluoride removal efficiency was 94.5 %. The sorption onto HAP data fitted well the Freundlich (r2 = 0.992) and Langmuir (r2 = 0.994) models. The adsorption capacity indicator (KF) was equal to 0.723. The dimensionless constant RL of 0.62 indicates favorable sorption of fluoride onto HAP. It is concluded that the adsorption step using HAP can be used as a post-treatment to remove fluoride from aluminium finishing industry wastewater.