To evaluate the total mercury accumulation (THg) in the Descoberto river basin environmental protection area (DREPA), nine sediment and water samples were collected from the Descoberto reservoir (lentic environment), and 23 in its tributaries (lotic environment), which are located in a densely urbanized area within the Descoberto river watershed, Brazil. The following physicochemical parameters of water were determined: dissolved oxygen (DO); hydrogen potential (pH); total dissolved solids (TDS); nitrate (NO₃⁻); chloride (Cl⁻); temperature (T); sulfate (SO₄²⁻), and in sediment, the concentration of total mercury (THg) and volatile material (VM) was determined. THg concentrations in sediments showed a significant difference (p = 0.002) between tributaries (0.03 µg g⁻¹ ± 0.02) and reservoir (0.08 µg g⁻¹ ± 0.04), indicating accumulation in the lentic environment. Most of the results evaluated for ecotoxicological risks presented values below the concentration, at which adverse effects would rarely be observed, ERL (effects range low). However, in relation to the enrichment factor (EF), applied to identify the anthropogenic contribution, the results indicate that most of the samples are moderately polluted through atmospheric deposition due to vehicular traffic and agriculture. These results show that the likelihood of methylation in the lentic environment is higher than in the lotic environment.

This work concerns the preparation of Pt/AC catalysts (Pt supported on activated carbon) and their application to the synthesis of hydrocarbon biofuels through the HEFA (hydroprocessing of esters and fatty acids) route. The key motivation for the work was that catalysts based on sulfided Mo supported on γ-Al₂O₃, traditionally employed in the hydroprocessing of petroleum derivatives, (i) are unstable in the HDO (hydrodeoxygenation) of biomass-derived feedstocks and (ii) can contaminate the resulting biofuels with sulfur. In this context, a systematic study on the effects of preparation conditions on the properties of the resulting Pt/AC catalysts and their performance in HEFA was carried out for the first time. Efficient catalysts were obtained, which led to the complete deoxygenation of lauric acid and coconut oil, yielding products composed primarily of n-alkanes. The highest HDO activity was verified for the catalyst prepared using as a support an AC previously subjected to thermal treatment up to 800 °C in a H₂ atmosphere (which removed most of the surface acidic oxygenated groups), depositing Pt over the surface of this support via wet impregnation using a H₂PtCl₆ solution acidified with HCl. The obtained results showed the great potential of the Pt/AC catalysts for the production of hydrocarbon biofuels through the HEFA route.

Artisanal small-scale gold mining (ASGM), an increasingly prevalent activity in South America, generates mercury-contaminated tailings that are often disposed of in the environment, leading to the introduction of mercury into ecosystems and the food web, where it bioaccumulates. Therefore, studying the geochemical processes involved in the desorption and dissolution of mercury in these tailings is essential for critical risk evaluations in the short and long term. For this purpose, sequential extraction procedures (SEPs) can be useful because they help to identify the phases to which Hg is associated, although they also have limitations such as a lack of selectivity and specificity. In this work, we propose a modified four-step SEP: exchangeable mercury (F1), oxidizable mercury (F2), mercury bound to Fe oxides (F3), and strongly bound mercury (F4). To test this adapted sequential extraction method, we evaluated the Hg contamination in mercury-contaminated tailings of the Amazon basin. The results revealed a total mercury concentration of 103 ± 16 mg·kg⁻¹ in the tailings, with a significant portion in F1 (28% of the total), where Hg was bioavailable. The large Hg concentration in F3 (36%) suggested that Fe oxides likely contribute to mercury retention. Together, the SEP results emphasize the urgent need for improved surveillance of gold mining activities and responsible tailings management practices to mitigate environmental contamination and safeguard the health of the Amazon ecosystem.

The production of hydrocarbon-based biofuels has been the target of intense research worldwide. In this context, the core goal of the present work was to investigate the use of mesopore-rich activated carbons (ACs) as support for sulfided Mo-based catalysts intended for the hydroprocessing of lipidic feedstocks. The key motivations for the work were that, in comparison to traditional inorganic supports such as Al₂O₃, ACs are less propense to form coke, due to their lower acidity, and are highly resistant to hydrolysis, which is a very important aspect in the hydroprocessing of lipidic feedstocks because water is abundantly produced during the process. Furthermore, the porosity of ACs can be tailored to give rise to a high mesopore content, which is important for improving the access of bulky triglyceride molecules to metallic active sites located inside the pores network. A systematic study on the effects of the preparation conditions on the properties and performance of the obtained catalysts was carried out for the first time. The highest hydrodeoxygenation (HDO) activity was verified for the catalyst prepared through sequential deposition of Mo and Ni by wet impregnation. The prepared catalyst presented better performance for coconut oil HDO than an industrial sulfided NiMo/Al₂O₃ catalyst. Furthermore, it presented good stability, provided that the sulfidation degree was kept high. The obtained results evidenced that ACs have great potential to replace inorganic support in sulfided Mo-based catalysts.