Defluorination by ion exchange of SO42- on alumina surface: Adsorption mechanism and kinetics

Fluoride removal from coal mining water using novel polymeric aluminum modified activated carbon prepared through mechanochemical process

Defluoridation of coal mining water is of great significance for sustainable development of coal industry in western China. A novel one-step mechanochemical method was developed to prepare polymeric aluminum modified powder activated carbon (PAC) for effective fluoride removal from coal mining water. Aluminum was stably loaded on the PAC through facile solid-phase reaction between polymeric aluminum (polyaluminum chloride (PACl) or polyaluminum ferric chloride (PAFC)) and PAC (1:15 W/W). Fluoride adsorption on PACl and PAFC modified PAC (C-PACl and C-PAFC) all reached equilibrium within 5 min, at rate of 2.56 g mg-1 sec-1 and 1.31 g mg-1 sec-1 respectively. Larger increase of binding energy of Al on C-PACl (AlF bond: 76.64 eV and AlFOH bond: 77.70 eV) relative to that of Al on C-PAFC (AlF bond: 76.52 eV) explained higher fluoride uptake capacity of C-PACl. Less chloride was released from C-PACl than that from C-PAFC due to its higher proportion of covalent chlorine and lower proportion of ionic chlorine. The elements mapping and atomic composition proved the stability of Al loaded on the PAC as well as the enrichment of fluoride on both C-PACl and C-PAFC. The Bader charge, formation energy and bond length obtained from DFT computational results explained the fluoride adsorption mechanism further. The carbon emission was 7.73 kg CO2-eq/kg adsorbent prepared through mechanochemical process, which was as low as 1:82.3 to 1:8.07 × 104 compared with the ones prepared by conventional hydrothermal methods.

Experimental and DFT calculation study on the efficient removal of high fluoride wastewater from metallurgical wastewater by kaolinite

Fluoride pollution and water scarcity are urgent issues. Reducing fluoride concentration in water is crucial. Kaolinite has been used to study adsorption and fluoride removal in water and to characterize material properties. The experimental results showed that the adsorption capacity of kaolinite decreased with increasing pH. The highest adsorption of fluoride occurred at pH 2, with a capacity of 11.1 mg/g. The fluoride removal efficiency remained high after four regeneration cycles. The fitting results with the Freundlich isotherm model and the external diffusion model showed that the non-homogeneous adsorption of kaolinite fit the adsorption behavior better. Finally, the adsorption mechanism was analyzed by FT-IR and XPS. The binding energies of various adsorption sites and the chemical adsorption properties of atomic states were discussed in relation to DFT calculations. The results showed that Al and H sites were the main binding sites, and the bonding stability for different forms of fluoride varies, with the size of Al-F (-7.498 eV) > H-F (-6.04 eV) > H-HF (-3.439 eV) > Al-HF (-3.283 eV). Furthermore, the density of states and Mulliken charge distribution revealed that the 2p orbital of F was found to be active in the adsorption process and was the main orbital for charge transfer.

The model and mechanism of adsorptive technologies for wastewater containing fluoride: A review

With the continuous development of society, industrialization, and human activities have been producing more and more pollutants. Fluoride discharge is one of the main causes of water pollution. This review summarizes various commonly used and effective fluoride removal technologies, including ion exchange technology, electrochemical technology, coagulation technology, membrane treatment, and adsorption technology, and points out the outstanding advantages of adsorption technology. Various commonly used fluoride removal techniques as well as typical adsorbent materials have been discussed in published papers, however, the relationship between different adsorbent materials and adsorption models has rarely been explored, therefore, this paper categorizes and summarizes the various models involved in static adsorption, dynamic adsorption, and electrosorption fluoride removal processes, such as pseudo-first-order and pseudo-second-order kinetic models, Langmuir and Freundlich isotherm models, Thomas and Clark dynamic adsorption models, including the mathematical equations of the corresponding models and the significance of the models are also comprehensively summarized. Furthermore, this comprehensive discussion delves into the fundamental adsorption mechanisms, quantification of maximum adsorption capacity, evaluation of resistance to anion interference, and assessment of adsorption regeneration performance exhibited by diverse adsorption materials. The selection of the best adsorption model not only predicts the adsorption performance of the adsorbent but also provides a better description and understanding of the details of each part of the adsorption process, which facilitates the adjustment of experimental conditions to optimize the adsorption process. This review may provide some guidance for the development of more cost-effective adsorbent materials and adsorption processes in the future.

Surface modification of quartz sand: A review of its progress and its effect on heavy metal adsorption

Quartz sand (SiO₂) is a prevalent filtration medium, boasting wide accessibility, superior stability, and cost-effectiveness. However, its utility is often curtailed by its sleek surface, limited active sites, and swift saturation of adsorption sites. This review outlines the prevalent strategies and agents for quartz sand surface modification and provides a comprehensive analysis of the various modification reagents and their operative mechanisms. It delves into the mechanism and utility of surface-modified quartz sand for adsorbing heavy metal ions (HMIs). It is found that the reported modifiers usually form connections with the surface of quartz sand through electrostatic forces, van der Waals forces, pore filling, chemical bonding, and/or molecular entanglement. The literature suggests that these modifications effectively address issues inherent to natural quartz sand, such as its low superficial coarseness, rapid adsorption site saturation, and limited adsorption capacity. Regrettably, comprehensive investigations into the particle size, regenerative capabilities, and application costs of surface-modified quartz sand and the critical factors for its wider adoption are lacking in most reports. The adsorption mechanisms indicate that surface-modified quartz sand primarily removes HMIs from aqueous solutions through surface complexation, ion exchange, and electrostatic and gravitational forces. However, these findings were derived under controlled laboratory conditions, and practical applications for treating real wastewater necessitate overcoming further laboratory-scale obstacles. Finally, this review outlines the limitations of partially surface modified quartz sand and suggests potential venues for future developments, providing a valuable reference for the advancement of cost-effective, HMI-absorbing, surface-modified quartz sand filter media.

Efficient preparation of red mud-based geopolymer microspheres (RM@GMs) and adsorption of fluoride ions in wastewater

In this paper, red mud-based geopolymer microspheres (RM@GMs: 75-150 µm) was prepared by dispersion-suspension-solidification method to remove fluoride ions (F^-). It was found that RM@GMs still had good mechanical properties and better F^- removal effect at RM content reached 80 % of the total solid mass. The batch adsorption experiment results showed that the F^- concentration (< 1.5 mg/L) reached the drinking water standard in 45 min at pH = 2 and RM@GMs dosage was 1 g/L. RM@GMs showed maximum adsorption capacity of 76.57 mg/g for F^-, and the adsorption kinetics and isotherm fitted the pseudo-second-order kinetic and Langmuir isotherm model, respectively. RM@GMs exhibited excellent dynamic separation effect at the flow rate of 4 mL/min and column height of 1 cm. In addition, RM@GMs had good selectivity for F^- in the competitive adsorption experiments and followed an order of: PO₄^3- > > SO₄^2- ≈ NO₃^- ≈ Cl^-. In real seawater, natural surface water and tap water, RM@GMs still had excellent F^- removal effect. The adsorption mechanism revealed that RM@GMs removed F^- mainly through the synergistic effect of adsorption and ion exchange. Therefore, this paper provides the potential value for the large-scale utilization of RM in the application of F^--containing wastewater.

The sorption and short-term immobilization of lead and cadmium by nano-hydroxyapatite/biochar in aqueous solution and soil

In this study, the composite materials using different ratios of biochar (BC) to nano-hydroxyapatite (nHAP) were prepared for the remediation of lead (Pb) and cadmium (Cd) contaminated water and soil. The sorption and the immobilization experiments indicated a higher sorption capacity and immobilization efficiency of Pb compared to those of Cd. The characteristics of XRD, FTIR, SEM, and XPS manifested that dissolution-precipitation, cation exchange, complexation, and cation-π interaction were the main four mechanisms for the sorption of Pb²⁺ and Cd²⁺ using composite material PC1 (nHAP/BC = 1/1). From semi-quantitative analysis, the mineral effect accounted for the majority of the immobilization of Pb and Cd. Due to obvious Pb-precipitates in the sorbed material, dissolution-precipitation primarily affected the sorption of Pb using PC1, while the immobilization of Cd was mainly attributable to cation exchange. Such results corresponded to the stable Pb-precipitates and unstable Cd-compounds in soil, among which the latter was prone to be released into the environment. The sorption capacity in aqueous solutions and the immobilization efficiencies in the soil for both Pb and Cd increased with the addition of nHAP, which were linearly correlated to the nHAP proportion in the composite materials. In future practical applications, the percentages of composite materials can be designed according to the specific pollutant concentration. This study sheds light on the explicit immobilization mechanisms for Pb and Cd in aqueous solutions to better understand their behaviors in the soil remediated by relevant materials.

High-efficiency N-doped activated carbon-based defluoridation adsorbent prepared from itaconic acid fermentation waste liquid

Excessive amounts of fluoride in water cause irreversible harm to people and seriously threaten human health.

Nanomaterials in Water Applications: Adsorbing Materials for Fluoride Removal

Fluoride is an important pollutant in many countries, such as China, India, Australia, the United States, Ethiopia, etc [...].