Milica Delić

The utilization of coal fly ash as a raw waste material from thermal power plants represents a promising alternative source for recovering rare earth elements (REEs). The significance of these elements lies in their broad application and uneven and limited global distribution. In this study, a method of extraction of REEs from coal ash was developed and optimized through alkaline pretreatment, followed by acid leaching. The optimization process was conducted using the Plackett-Burman and Box-Behnken designs. The identified optimal conditions involved a pretreatment procedure in an ultrasonic bath set at 95°C for 10 minutes with an ash-to-3M NaOH ratio of 0.5:1 (m/V). Acid leaching was a consecutive step in a microwave oven at 174°C for 30 minutes using an HCl/HNO₃ mixture in a 1:1 volumetric ratio. The method was tested on fly and bottom ash samples from the Kostolac thermal power plant in the Republic of Serbia, achieving 76% and 58% recovery efficiencies, respectively.

The modification of agricultural wastes for improved phosphate removal from wastewater is an innovative approach aimed to address the growing environmental challenges posed by nutrient pollution. As agricultural runoff contributes significantly to the eutrophication of water bodies, the effective removal of phosphorus from wastewater has become a critical necessity for environmental sustainability. This field of study not only explores the repurposing of agricultural residues but also emphasizes their potential as low-cost bio-based sorbents for phosphate remediation, thereby offering a dual benefit of waste reduction and environmental protection. Research has shown that various types of agricultural wastes, when modified through physical and chemical processes, can enhance their ability to capture and retain phosphorus from wastewater. Techniques such as metal loading and biochar production have proven effective, leading to notable advancements in the efficacy of these materials as phosphate adsorbents. Studies have indicated that chemically and thermically modified agricultural wastes can achieve phosphorus removal efficiencies comparable to traditional methods, making them a promising alternative in wastewater treatment applications. Despite the potential benefits, the implementation of modified agricultural wastes also faces several challenges. Issues such as: the economic feasibility of large-scale application, the complexity of modification processes, and concerns about the long-term sustainability of these technologies present significant hurdles. Overall, the modification of agricultural wastes represents a vital intersection of waste management and environmental conservation, promoting a circular bioeconomy while providing practical solutions to the pressing problem of nutrient pollution in aquatic systems.

An ultrasound-assisted dispersive liquid–liquid microextraction (DLLME) method was developed to concentrate and quantify rare earth elements (REEs) (Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) in acidic aqueous solutions. Tetrachloroethylene (PCE) was used as the diluent, di-(2-ethyl hexyl) phosphoric acid (D2EHPA) as the extracting agent, and acetone as the dispersant solvent. The method was optimized at pH = 2.3, T = 25 °C, and V_S = 400 µL of a PCE ÷ D2EHPA mixture (10 ÷ 1) using the response surface methodology (RSM) with a Box–Behnken design. Under optimal conditions, the method proved efficient for the DLLME of most REEs (Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), where the achieved recoveries were in the range of 61–109%, while relative standard deviations were in the range 11–28%. The proposed method was applied to recover REEs from real coal ash leachate samples. A greenness evaluation using the Green Analytical Procedure Index (GAPI), Analytical GREEnness (AGREE), and Analytical Eco-Scale (AES) methodologies revealed acceptable metric scores of 74, 0.61, and 26.6–79.8, respectively.

The increasing pollution from the textile industry, particularly organic azo dyes, presents a significant environmental challenge, necessitating the development of effective and sustainable treatment methods. This study investigates the adsorption potential of hazelnut shells (raw—RHSs; modified—MHSs) for the removal of organic azo dyes from aqueous solutions. As biomass, hazelnut shells are biodegradable and represent a sustainable alternative to synthetic adsorbents, thereby reducing the ecological footprint. Through a series of batch adsorption experiments, the influence of various parameters, including pH, contact time, concentration, and temperature, on adsorption capacity was examined. Characterization of the hazelnut shells was conducted using optical microscopy and ATR-FTIR, XRF, and XRD spectroscopy, confirming its suitability as a biosorbent. The analyzed isotherms showed that adsorption onto RHSs was best fitted by the Freundlich model, while adsorption onto MHSs was best fitted by the Temkin model. Kinetic studies demonstrated that the adsorption process is well described by the pseudo-second-order model, suggesting that chemical adsorption plays a significant role. The maximal adsorption capacity was 62.11 mg/g for RHSs and 80.65 mg/g for MHSs, highlighting the potential of hazelnut shells as an abundant, low-cost, and eco-friendly adsorbent. Furthermore, recycling studies indicated the feasibility of the adsorbent, underscoring its practical applicability in real scenarios.

Considering the rising demand for rare earth elements (REEs), researchers are looking for new sources for their extraction, thereby fostering economic and environmentally justified processing solutions. Among potential industrial sources, coal fly ash emerges as one of the most promising. The recovery of REEs from coal fly and bottom ashes derived from different thermal power plants was the main focus of this study. A dual-step methodology was conducted on ash samples, which involved an ultrasonic roasting process to disintegrate the silica matrix, followed by a microwave-assisted acid leaching step to extract REEs. The roasting procedure was studied using the Plackett–Burman design, and the Box–Behnken design was subsequently implemented to optimize the leaching procedure. The optimized ultrasonic roasting procedure was set up at 95 °C for 10 min with an ash-to-roasting agent (3M NaOH) ratio of 0.5:1 (m/V). For acid leaching, the optimal conditions were obtained at 174 °C for 30 min with an HCl ÷ HNO₃ mixture (1:1 V/V). The standard reference material (NIST 1633c) was used in the conclusive experiments to estimate the average recovery (80%) of REEs. The green aspects of this methodology were evaluated using several metrics (atom economy, E-factor, and energy consumption). The proposed process outperforms high-temperature roasting procedures in terms of greenness; however, the REE recovery rate is lower.