Fluoride Action Network

Reuse of secondary aluminum ash: Study on removal of fluoride from industrial wastewater by mesoporous alumina modified with citric acid

Source: Environmental Technology & Innovation | August 17th, 2022 | Authors: Li Y, Yang Y, Qu G, Ren Y, et al.
Location: China
Industry type: Aluminum Industry


• Synthesising mesoporous alumina with secondary aluminum ash for the first time.
• Modification with citric acid increased hydroxyl radical on the surface of the material.
• The removal efficiency remains above 98.20% after 5 cycles.
• The maximum adsorption capacity for fluoride ions was 117.98 mg g-1.


Longitudinal loading of citric acid as modifier onto the surface of other supports as highly efficient fluoride removing adsorbents has received extensive attention. But the loading of acid into mesoporous alumina synthesized using industrial solid waste secondary aluminum ash as raw material is rarely reported. In this study, MA was prepared by secondary aluminum ash reuse to industrial solid waste, and the MA was modified with citric acid for fluoride removal. The structural properties of this adsorbent were characterized by N

adsorption desorption isotherm, XRD, FTIR spectroscopy. Meanwhile, the adsorption conditions, such as contact time, initial concentration and pH, were examined to evaluate the effect of fluoride adsorption. The adsorption properties are stable under acidic conditions. Chemisorption is a rate determining step because it complies with the Langmuir model and the pseudo second order model. The adsorption equilibrium was quickly reached in 30 min (117.98 mg g-1). With good cycling performance, 98.20% of the initial adsorption capacity was maintained after 5 cycles. Ion exchange between -OH and fluoride is the main process of fluoride adsorption. The introduction of citric acid greatly increased the amount of -OH in the adsorbent. The extent of the influence of coexisting anions on fluoride adsorption is most pronounced for PO and CO

. The results of high fluoride containing industrial wastewater treatment show that citric acid modified MA can solve the environmental pollution problem of industrial solid waste secondary aluminum ash on one hand, and at the same time provide new ideas for fluoride adsorbents.


Secondary aluminum ash, Resource utilization, Citric acid, MA, Fluoride, Adsorption, Modification

1. Introduction

Secondary aluminum ash (SAA), as a major hazardous waste during aluminum smelting (Anawati and Azimi, 2022), produces about 110 kg of SAA every 1 ton of aluminum and can produce nearly 4 million ton of aluminum ash every year, which, combined with previously untreated SAA, has accumulated million tons of SAA in China (Valeev et al., 2019). Huge amounts of SAA, if not handled properly, can cause serious hazards to the atmosphere, water quality, as well as soil, such as exudation of inorganic salts, heavy metals, etc., in them, which can make land saline alkalinized and heavy metals exceedingly abundant; fluoride dissolution can cause exceedingly high fluoride concentrations in surface water and even groundwater, causing severe consequences such as poisoning or death to aquatic organisms; the slow release of gases such as, NH3 and CH4 is flammable and explosive, creating safety concerns (Li et al., 2022a). In view of the risk of excessive atmospheric, water quality, and soil contamination of SAA, our country issued the national list of hazardous waste in 2016, which listed SAA generated during aluminum processing as a nonmetallic waste, and clearly stipulated that the hazardous characteristics of SAA are toxic and flammable. The tax schedule of environmental protection tax in the environmental protection tax code of the people’s Republic of China, which is in effect on January 1, 2018, provides that untreated SAA contributes an environmental protection tax of 1000 RMBt-1 as hazardous waste (Chen et al., 2022a). Even if SAA is stacked in an industrial solid waste storage facility that is compliant with environmental protection standards, with reference to the regulations on the collection and use regulation of the decontamination fee (State Council Order No. 369), and the industrial “ three waste ” emission trial standard), the manager will still face the problem of paying a huge air pollution, water pollution decontamination fee. Recently, the hazardous properties of SAA were further clarified in our newly revised list of national hazardous waste (2021), and new SAA with reactive hazardous properties were added. Under environmental stress, the innocuous treatment of aluminum ash by aluminum smelters is imminent. How to deal with SAA rationally, reduce the hazard to the environment, and more further how to change to waste treasure and carry out resource recycling and reuse of SAA (Yang et al., 2022), has become the concern of environmental-friendly researchers.

fluoride is one of the trace elements which is necessary for the human body (Biswas et al., 2017). However, excessive intake of fluoride can seriously damage human health, causing osteoporosis, arthritis, brain damage, etc (Zhang et al., 2017). The sanitary requirement of drinking water in our country (GB5749-2006) stipulates that the concentration of fluoride should not exceed 1.0 mg L-1. It has been reported that nearly 100 million people in China have fluoride in their drinking water exceeding this standard (Liu et al., 2014). How to remove the high concentration of fluoride in drinking water has attracted wide attention from researchers.

At present, the commonly used methods of fluoride removal include precipitation, membrane separation, ion exchange, adsorption, electrocoagulation, and electrodialysis, but most of the technologies are still limited in practical engineering applications (Tang et al., 2022). For example, traditional treatment methods such as precipitation and coagulation are difficult to reduce the fluoride concentration to satisfactory levels, and membrane separation and electrochemical methods are costly (Peng et al., 2022). The adsorption method has the advantages of easy operation, low cost, and no secondary by-products (Dao et al., 2021). But the technical advantage of the adsorption method depends on the quality of the adsorbent. The research of fluoride adsorbents is one of the hot spots in recent years, mainly including metal oxides, metal organic framework materials (Zhu et al., 2018) and so on. Therefore, the development of high-efficiency and environmentally-friendly new fluoride-removing materials has become a hot spot in the research and development of fluoride-removing materials (Zhang et al., 2021). Mesoporous materials are of great interest in the fields of catalysts, catalyst supports and absorbents due to their high specific surface area (Kang et al., 2020) and regular pore structure. Currently, the synthesis of materials with large specific surface area mostly uses alkoxides as raw materials and high-carbon organic acids as templating agents, but the preparation cost is high and difficult to produce industrially. Alumina has a high specific surface area, suitable mechanical strength and strong fluoride ion affinity, which makes it one of the most widely used fluoride removal materials (Nallusamy and Vijayakumar, 2021). However, ordinary MA materials still have their own shortcomings. For example, the MA framework is in an amorphous state and has fewer Bronsted acid (proton donor) and Lewis acid (Gao and Blum, 2021) (electron pair acceptor) centers. In some catalytic fields, it is difficult to meet its application requirements, which requires further functional modification of MA materials. Common modified groups can be roughly divided into the following situations: (i) Metal heteroatoms (Fe, Co, Ni, Cu, Zn, Pd, Pt, Zr, etc.); (ii) Metal oxides (TiO 2, NiO, Co3O4, CuO, Cu2O, Re2O7, etc.); (iii) Bimetallic elements (Mo-V, Ni-Mg, Ni-Pd, Ni-MgO, Ce-ZrO, etc.); (iv) Heteropoly acid (Lewis acid, phosphotungstic acid, etc.); (v) Organic Metal complexes (shiff alkali metal ligands, phthalocyanine metal complexes, etc.); (vi) organic groups (amino, methyltriethoxysilane, polydimethylsiloxane, etc.). These groups can be doped into the MA framework or loaded on the surface of the MA by in-situ synthesis (Ren et al., 2021) and impregnation methods (Pan et al., 2021).Some researchers (Mishra et al., 2021) have prepared CuO-doped MA materials by dipping. This material has a stronger adsorption capacity for fluoride in drinking water. Compared with an unmodified MA adsorbent, its adsorption capacity can be increased from 2.232 to 7.770 mg g-1, so that the fluoride ion content in drinking water can reach drinking standards. In addition, some researchers have soaked iron oxide powder in hydrochloric acid for acid modification, and studied the effect of modified iron oxide on fluoride ions. The adsorption has higher fluoride ion adsorption performance and a wider application range of solution pH than the unmodified one. In addition, another researcher (Ternero-Hidalgo et al., 2016) found that after activated carbon is treated with nitric acid, the pore structure of the activated carbon has been greatly changed, and more oxygen-containing functional groups can be obtained, and most of them exist in the form of nitro groups.

However, most of the modification methods are loading hydroxide of the modifier to the surface of the support, while studies of loading citric acid to SAA for the utilization of synthesized MA are very rare (Kapelko-Zeberska et al., 2022). Therefore, in this study, the industrial solid waste SAA was resourced and used to prepare MA, which was simultaneously modified with citric acid, for the experiment of fluoride removal. The effect of added amount of MA, pH on the fluoride removal effect after citric acid modification was investigated separately. The adsorption thermodynamic and kinetic conditions were further explored, and the isothermal adsorption (30 °C, 40 °C, 50 °C) and adsorption kinetics were simulated. The effect of symbiotic ions, such as sulfate ions, acetate ions, bicarbonate ions, chloride ions, carbonate ions, nitrate ions, and phosphate ions, on the adsorption of fluoride was also investigated to explore the adsorption regeneration cycle performance and to conclude the optimal adsorption conditions and effective cycle number. Finally, it should be actually transported to the High Concentration fluoride leaching filtrate of phosphorus gypsum residue field for actual adsorption study of fluoride ions. On the one hand, it alleviates the hazardous effects of SAA on the environment, and on the other hand provides new ideas for fluoride removal.

*To read the full study go to https://www.sciencedirect.com/science/article/pii/S235218642200308X