Lithium belongs to the critical elements and is used in a variety of high-tech applications. In the context of the circular economy, demand has arisen for technologies that are able to recover high-tech metals from wastes and byproducts. To achieve efficient recovery, apart from assessing metal enrichment, extensive knowledge of metal binding and leaching characteristics is required. The aim of the present study is to investigate the Li contents and mode of occurrence in Greek coal fly ashes. Eight coal fly ashes from different power plants in Greece were collected, and their major constituents were analyzed by X-ray fluorescence spectroscopy (XRF); their mineralogy was studied by X-ray powder diffraction (XRD), and their Li content was determined by ICP–MS. To identify Li binding and leaching characteristics, two sequential extraction methods (Tessier and BCR) were employed. The results showed that the Li content in the samples studied was between 95 and 256 μg/g and could be mainly attributed to the amorphous material encountered in the samples. The sequential extraction experiments revealed that 70–90% of Li is included in the residual fraction, indicating that it is strongly bound to the fly ash matrix.

Drill core samples of two boreholes from the Lofoi lignite-bearing area of Miocene-Pliocene age were analyzed for their trace element contents. The methods used were Instrumental Neutron Activation Analysis (INAA), Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), and Atomic Absorption Spectroscopy (AAS). The results showed that all studied elements occur in concentrations within the reported ones for different coal-bearing basins of Greece. The elements Co, U, Sb, Th, Cs, As, Li, Mo, Y, Cu, Ni, Cr, Pb, Yb, Be, Rb, V, and Lu are enriched in the ashes when compared to their average concentration on the Earth's crust. The enrichment of elements encountered in silicic rocks as well as mafic and ultramafic rocks implies that there are two different types of rocks influencing the formation of the lignite deposit. Very strong (r > 0.8) to intermediate (0.65 < r < 0.8) correlation is observed for the REE as well as for the elements Mo with As, Cu with Sb, Sc with Ni, Co, and V, and Rb with Ga, which is expected for these elements. Intermediate to strong positive correlation is also observed for the elements Th with Zn, Y with Be, Cr with As, Sb with Yb, As with Y, Zn with both Yb and Eu, as well as Cu with Th; this could only be attributed to the hypothesis that both silisic and mafic-ultramafic rocks influenced the formation of the deposit.

Lithium is classified as a critical element and is widely used in a variety of high-tech applications. Within the framework of a circular economy, demand is rising for technologies capable of recovering high-tech metals from waste materials and industrial byproducts. Coal fly ash (CFA) has attracted significant attention as a promising secondary resource for this purpose. Effective recovery requires the assessment of both metal enrichment levels and the underlying binding and leaching characteristics. The present study aims to contribute to advancing lithium recovery technology using coal fly ash as a secondary resource, thereby promoting waste valorization. Fourteen samples of coal fly ash from different power plants were collected and their mineralogy was studied by X-ray powder diffraction (XRD), their major constituents were analyzed by X-ray fluorescence spectroscopy (XRF), and their Li content was determined by inductively coupled plasma mass spectrometry (ICP-MS). Leaching experiments were conducted for selected samples using mineral acids (HCl and HF) and citric acid. Lithium concentrations in the analyzed samples ranged from 80 to 256 mg/kg, indicating enrichment relative to both global ash averages (enrichment factor > 1) and the Earth’s crust (enrichment factor > 2). Li in the samples, could be mainly associated with the amorphous fraction present in the samples. Leaching behavior across the samples follows a consistent trend, with hydrofluoric acid exhibiting the highest lithium extraction efficiency—reaching up to 79.9%.

The adsorption of arsenate from wastewaters was investigated by applying Mg-, Zn- or Ca-modified nut residue biochar activated by nitrogen/steam. The parameters studied were the contact time, adsorbent dose, initial arsenate concentration and solution pH. The adsorption mechanism was investigated. Various analyses of the material before and after arsenate adsorption were carried out, and experimental data were simulated by applying two isotherm models. The results indicated that the maximum removal efficiency of arsenate was 29.4% at an initial concentration of 10 mg/L. The modification of biochar by Mg, Zn or Ca oxides increased the removal rate significantly, from 49.4% at 100 mg/L As⁵⁺ up to 8%, 97% and 97%, respectively. Zn-modified biochar presented an excellent performance for both low and high As⁵⁺ concentrations. All experimental data were accurately fitted by the Freundlich isotherm model (R² = 0.94–0.97), confirming a multilayer adsorption mechanism. For a biochar dose of 2 g/L, the maximum capacity of adsorption was enhanced after Mg-, Zn- or Ca-modification from 12.4 mg/g to 35 mg/g, 50 mg/g and 49 mg/g, respectively. The potential mechanisms of adsorption were ligand exchange, chemical complexation, surface precipitation and electron coordination.