In this study, column leaching experiments were carried out to investigate the extraction of Ni and Co from low-grade limonitic laterites from Agios Ioannis mines in central Greece. Tests were carried out in laboratory Plexiglas columns using H₂SO₄ as leaching solution. Parameters determining the efficiency of the process, i.e., acid concentration (0.5 M or 1.5 M) and addition of 20 or 30 g/L of sodium sulfite (Na₂SO₃) in the leaching solution, were also studied. Upflow transport of the leaching solution with the use of peristaltic pumps was carried out, while the pregnant leach solution (PLS) was recycled several times over the entire test duration. The concentration of Ni, Co, Fe, Ca, Al, Mg, and Mn in the PLS was determined by Atomic Absorption Spectroscopy (AAS). The ore and the leaching residues were characterized by different techniques, i.e., X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and differential scanning calorimetry and thermogravimetry (DSC/TG). The experimental results showed that (i) Ni and Co extractions increased with the increase of H₂SO₄ concentration—60.2% Ni and 59.0% Co extractions were obtained after 33 days of leaching with 1.5 M H₂SO₄; (ii) addition of 20 g/L Na₂SO₃ in the leaching solution resulted in higher extraction percentages for both metals (73.5% for Ni and 84.1% for Co, respectively), whereas further increase of Na₂SO₃ concentration to 30 g/L only marginally affected Ni and Co extractions; and (iii) when leaching was carried out with 1.5 M H₂SO₄ and 20 g/L Na₂SO₃, its selectivity was improved, as deduced from the ratios Ni/Mg, Ni/Ca and Ni/Al in the PLS; on the other hand, the ratio Ni/Fe dropped as a result of the higher Fe extraction compared with that of Ni.

This study elucidated the impact of sewage-sludge (SS) and olive-mill-waste (OMW) biochar amendments to soil using tomatoes as a test crop. Four treatments were evaluated: the “control” with no biochar amendment, two SS biochar treatments with the addition of 10 t/ha and 25 t/ha, respectively, and an OMW biochar treatment with the addition of 25 t/ha. Higher yields were observed in both SS biochar treatments, providing evidence that biochar acts as a plant bio-stimulant. Biochar application had positive impacts on carbon sequestration and soil structure. The uptake of heavy metals by all plant parts was very low, indicating that biochar is an appropriate product for land application. Biochar dose and type induced changes in the composition due to the different unique species and biodiversity of microbial communities. Venn diagrams revealed that the majority of the identified taxa were shared among the treatments, and only a small proportion of them were unique in bulk soil between treatments. In the rhizosphere, the OMW-biochar-treated plants showed a higher number of unique taxa. Microbiota structure plays a major role in the stimulation of plant growth; however, further research is needed to understand the impact of these shifts in the functioning of agroecosystems.

The present study focuses on the optimization of alkali activation of ferronickel (FeNi) slag for the production of alkali-activated materials (AAMs). The effect of the main factors including molarity and SiO₂/Na₂O molar ratio in the activating solution, pre-curing and curing time, curing temperature, and aging period on the compressive strength and other properties of the final products is assessed. Emphasis is paid to the study of the effect of low curing temperature to decrease the overall footprint of alkali activation. Several analytical techniques including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and Scanning electron microscopy (SEM) are used for the identification of the mineralogy and the morphology of the precursor and the final products. The experimental results indicate that the produced AAMs achieve a maximum compressive strength of 99 MPa using NaOH molarity 8 mol/L (M), SiO₂/Na₂O molar ratio 1, curing temperature 80 °C, curing time 24 h, and aging period 7 days. SEM/EDS–energy dispersive X-ray spectroscopy analysis indicates that alkali activation results in a homogeneous binding phase, characteristic of the expected iron-rich matrix, while the microstructure of the AAMs is characterized by a glassy and smooth surface without the presence of any visible cracks or defects. The produced AAMs may be used as alternative binders for the replacement of cement in the production of concrete or as construction elements.

This study aimed to assess the impacts of biochar amendments derived from different feedstocks (sewage sludge (SS), olive-mill waste (OMW), compost, and sawdust) in land applications. Tomatoes were used as a test crop in four experiments both under greenhouse and field conditions. SS, OMW, and compost biochar treatments presented 17% to 178.5% higher tomato productivity than control, verifying that biochar behaves as a plant-growth bio-stimulant. This impact is related to the raw material since sawdust biochar did not present results as positive as the other types of biochars. The physicochemical characterization of biochars and their comparison with international and European standards confirmed the safety of their use. A risk-assessment analysis of tomato consumption was conducted in order to explore unfavorable effects on human health. The estimation of cumulative non-carcinogenic risk, found to be between 8.25 × 10⁻³ and 4.23 × 10⁻², and cancer risk for Cr(VI), found to be between 6.56 × 10⁻⁶ and 5.2 × 10⁻⁵, suggested no risk of potential chronic exposure due to tomato consumption cultivated in biochar-amended soils. This study may be used as a recommendation for farmers and agriculturists for maximizing the yield of agricultural crops in the Mediterranean region, improving soil health, and contributing to the sustainable management of agroecosystems.