Fluoride Sorption Efficiency of Vermiculite Functionalised with Cationic Surfactant: Isotherm and Kinetics

A critical review on adsorption and recovery of fluoride from wastewater by metal-based adsorbents

Rapid industrialization is deteriorating water quality, and fluoride pollution in water is one of the most serious environmental pollution problems. Adsorption technology is an efficient and selective process for removing fluoride from aqueous solutions using adsorbents. Metal-based adsorbents synergize the advantages of fast adsorption, high adsorption capacity, and excellent selectivity to effectively remove fluoride from water bodies, promising to satisfy environmental sustainability requirements. This paper reviews the metal-based adsorbents: iron-based, aluminum-based, lanthanum-based, cerium-based, titanium-based, zirconium-based, and multi-metal composite adsorbents, primarily focusing on the adsorption conditions and fluoride removal capacities and discusses prospects and challenges in the synthesis and application of metal-based adsorbents. This paper aims to stimulate new thinking and innovation in developing the next generation of sustainable adsorbents.

Sequestration of Hazardous Dyes from Aqueous Solution Using Raw and Modified Agricultural Waste

The continuous degradation of surface water quality by dye materials is of concern globally. Agricultural waste Litchi chinensis (LC) peel in its raw (RL) and modified (CL) forms was used as potential sorbents for sequestration of Congo red (CR) dye from an aqueous solution. The sorbents were characterized before and after sorption with Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Brunauer, Emmett, and Teller (BET) surface area analysis, and X-ray diffraction (XRD). Determination of the point of zero charge (PZC) suggested CR dye sorption from an aqueous solution would be best in acidic pH. Batch experimental drivers such as the effects of time, dosage, initial concentration, pH, and temperature were optimized and used. Results from the study showed that modification with citric acid (CA) reduced the equilibration time from 90 to 15 min. Change in water chemistry did not significantly affect the removal efficiency of the sorbent but rather slightly improved it for both sorbent types. The smaller particle size of <125 μm recorded higher removal efficiency than the larger one (>125 μm). The effect of temperature affects the sorption differently. For the RL system, it decreases with an increase in the temperature, while for the CL system it increases with an increase in temperature. The Langmuir isotherm best described the equilibrium data obtained based on the linearized coefficients with maximum sorption capacities ( $q_{\max }$ ) of 55.56 mg/g (RL) and 58.48 mg/g (CL). The pseudo-second-order model also best described the kinetic data. The thermodynamics study showed that the reaction is both feasible and spontaneous. Both sorbents recorded high removal efficiency for other dyes such as rhodamine B, methylene blue, methyl orange, malachite green, and erythrosin B. The five cycled regeneration/sorption experiments with 0.1 M NaOH as the desorbing agent showed that the regenerated sorbents efficiently removed CR dye from an aqueous solution close to their virgin samples for the first three cycles. This research, therefore, establishes LC peel as a potential eco-friendly, readily available, and effective sorbent for sequestration of hazardous dyes from wastewater.

A Novel and Efficient Metal Oxide Fluoride Absorbent for Drinking Water Safety and Sustainable Development

Inefficient and non-environmentally friendly absorbent production can lead to much resource waste and go against low carbon and sustainable development. A novel and efficient Mg-Fe-Ce (MFC) complex metal oxide absorbent of fluoride ion (F−) removal was proposed for safe, environmentally friendly, and sustainable drinking water management. A series of optimization and preparation processes for the adsorbent and batch experiments (e.g., effects of solution pH, adsorption kinetics, adsorption isotherms, effects of coexisting anions, as well as surface properties tests) were carried out to analyze the characteristics of the adsorbent. The results indicated that optimum removal of F− occurred in a pH range of 4–5.5, and higher adsorption performances also happened under neutral pH conditions. The kinetic data under 10 and 50 mg·g−1 were found to be suitable for the pseudo-second-order adsorption rate model, and the two-site Langmuir model was ideal for adsorption isotherm data as compared to the one-site Langmuir model. According to the two-site Langmuir model, the maximum adsorption capacity calculated at pH 7.0 ± 0.2 was 204 mg·g−1. The adsorption of F− was not affected by the presence of sulfate (SO42−), nitrate (NO3−), and chloride (Cl−), which was suitable for practical applications in drinking water with high F− concentration. The MFC adsorbent has an amorphous structure, and there was an exchange reaction between OH− and F−. The novel MFC adsorbent was proven to have higher efficiency, better economy, and environmental sustainability, and be more environmentally friendly.

Ultra-Low Interfacial Tension Foam System for Enhanced Oil Recovery

The liquid phase of foam systems plays a major role in improving the fluidity of oil, by reducing oil viscosity and stripping oil from rock surfaces during foam-flooding processes. Improving the oil displacement capacity of the foam’s liquid phase could lead to significant improvement in foam-flooding effects. Oil-liquid interfacial tension (IFT) is an important indicator of the oil displacement capacity of a liquid. In this study, several surfactants were used as foaming agents, and polymers were used as foam stabilizers. Foaming was induced using a Waring blender stirring method. Foam with an oil-liquid IFT of less than 10–3 mN/m was prepared after a series of adjustments to the liquid composition. This study verified the possibility of a foam system with both an ultra-low oil-liquid IFT and high foaming properties. Our results provide insight into a means of optimizing foam fluids for enhanced oil recovery.

A review of emerging adsorbents and current demand for defluoridation of water: Bright future in water sustainability

Fluoride contamination of groundwater is a serious problem in several countries of the world because of the intake of excessive fluoride caused by the drinking of the contaminated groundwater. Geological and anthropogenic factors are responsible for the contamination of groundwater with fluoride. Excess amounts of fluoride in potable water may cause irreversible demineralisation of bone and tooth tissues, a condition called fluorosis, and long-term damage to the brain, liver, thyroid, and kidney. There has long been a need for fluoride removal from potable water to make it safe for human use. From among several defluoridation technologies, adsorption is the technology most commonly used due to its cost-effectiveness, ease of operation, and simple physical process. In this paper, the adsorption capacities and fluoride removal efficiencies of different types of adsorbents are compiled from relevant published data available in the literature and represented graphically. The most promising adsorbents tested so far from each category of adsorbents are also highlighted. There is still a need to discover the actual feasibility of usage of adsorbents in the field on a commercial scale and to define the reusability of adsorbents to reduce cost and the waste produced from the adsorption process. The present paper reviews the currently available methods and emerging approaches for defluoridation of water.