In this study, a series of synthetic ferrihydrite samples of Si/Fe molar ratios ranging from zero to 1.5 were heated up to 1000 °C using simultaneous TG–DTA equipment. The XRD, FTIR, SEM–EDS and magnetic susceptibility measurements were carried out prior to and after heating. It has been found that silicate retards ferrihydrite transformation to hematite and affects crystallinity of the product. Low Si admixture in the precursor reduces hematite crystal size severely, but the increase in average crystal dimensions with increasing Si/Fe molar ratio was observed. High Si content results in the formation of hematite which exhibits a wide range of crystal habits. The conversion of pure ferrihydrite to hematite proceeds without any intermediate phase, whereas the increasing silicate content in the pristine oxyhydroxide strongly affects the transformation pathway. During annealing of high-Si ferrihydrites, the presence of two or three intermediate Fe₂O₃ polymorphs (gamma, epsilon and beta) was demonstrated prior to the crystallization of final α-Fe₂O₃. The conditions favoring crystallization of intermediate phases result from progressive silica polymerization which forms separate matrix-type phase and impedes the aggregation of iron oxide nanoparticles.

Activation of natural sepiolite by means of grinding in a planetary mill followed by wet NaOH activation was studied for the purpose of endowing the product with enhanced basicity for potential catalytic/sorptive applications. Synthesized solids were characterized with X-ray powder diffraction (XRD), N₂ adsorption/desorption, scanning electron microscopy (SEM), energy dispersive (EDX), atomic absorption (AAS), Fourier-transform infrared (FTIR) and ²⁹Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopies. Surface basicity was determined by titration with benzoic acid. Grinding changed the pathway of sepiolite phase transformation upon NaOH treatment. The as-received sepiolite evolved to Na-sepiolite (loughlinite) with a micropore system blocked by nanocrystalline Mg(OH)₂, while ground samples yielded magnesium silicate hydrate phase (MSH), with well-developed microporous texture. In unmilled sepiolite desilication involved preferential leaching of Si from the center of the structural ribbons, while in ground samples additional loss of Si from ribbon-ribbon corner linkages was observed. In all cases treatment with NaOH led to enhancement of surface basicity. Synthesized materials were tested as catalysts in a base-catalyzed aldol self-condensation of acetone and oxidation of cyclohexanone to ε-caprolactone, as well as CO₂ sorbents. Catalytic trends depended not only on samples’ basicity, but also on texture and phase composition of the catalysts. Grinding combined with alkali activation proved a simple and effective method for boosting CO₂-sorption capacity of sepiolite to the level comparable to amine-functionalized, acid-activated sepiolite sorbents.

The aim of this research was to determine the effect of thermal conversion temperature and plant material addition to sewage sludge on the PAHs content and the activity of selected γ-radionuclides in biochars, and to conduct an ecotoxicological assessment. The pyrolysis of the mixtures of sewage sludge and plant materials at 300°C and such temperature caused an increase in the contents of 2- and 3-ring hydrocarbons. During the pyrolysis of organic materials at 600°C, the amount of the following compounds was reduced in biochars: benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, indeno[1,2,3c,d]pyrene, dibenzo[a,h]anthracene, and benzo[g,h,i]perylene. Among γ-radioisotopes of the elements, natural radiogenic isotopes were dominant. ¹³⁷Cs was the only artificial radioactive isotope. The pyrolysis of the mixtures of municipal sewage sludge and plant materials revealed that isotope ⁴⁰K had the highest radioactive activity. In the case of other analysed nuclides, activities of ²¹²Pb, ²¹⁴Pb, ²¹⁴Bi, and ¹³⁷Cs were determined after the sample pyrolysis. The extracts from the mixtures of sewage sludge and plant materials were non-toxic to Vibrio fischeri.

The enormous amount of spent catalysts generated worldwide may pose a risk to the environment because of their high load of metals, including vanadium. The latter may be mobilized and released to the environment if managed improperly. Moreover, the catalysts could be considered as secondary resources rather than waste. This study aimed at the efficient extraction of vanadium from spent desulfurization catalyst (SDC) from a sulfuric acid production plant. The raw SDC and the post-extraction residues were characterized in terms of their chemical and phase composition. The metal mobility from the materials was examined with both single-step and multi-step extractions. The environmental risk assessment was performed using sequential extraction. The study revealed that both tested methods (citric acid leaching and bioleaching with Acidithiobacillus thiooxidans) enable the extraction of nearly 96% of V from SDC with a simultaneous reduction of metal mobility. However, the bacterial treatment was found more suitable. The leached residue was mostly (> 90%) composed of SiO₂, which makes it a potential candidate for application in construction (e.g., concrete mixtures) after additional examinations. The study highlights the need to develop a metal extraction process for SDC in a way that metal-free residue could be a final product.