Currently, the pollution of soils by heavy metals is a problem of paramount relevance and requires the development of proper remediation techniques. In particular, lead is a frequently detected soil contaminant that poses adverse effects to the environment and human health. In this review, we provide an overview of the bioremediation treatments promoted by plants (phytoremediation), fungi, or bacteria that could be applied to areas polluted by lead. These restoration processes have the advantage of being environmentally friendly and cost-effective solutions that exploit plants to immobilize and extract contaminants from soil and water, and fungi and bacteria to degrade them. Phytoremediation is an extensively studied and mature practice, with many in-the-field applications where numerous plant species have been employed. In contrast, bioremediation processes promoted by fungi and bacteria are very promising but, up to now, studies have been mostly performed at a laboratory scale with only a few implementations in real-world situations; therefore, further research is needed.

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In this work, we synthesized and characterized carbon nitride (CN) nanoparticles obtained by the thermal treatment (550 °C) of urea, melamine, dicyandiamine, and dicyandiamine-barbituric acid in an open reactor and evaluated the effect of different precursors on the photocatalytic performance. CNs obtained from melamine, dicyandiamine, and dicyandiamine-barbituric acid were 3D melon-type structures. On the other hand, CN obtained from urea was a 2D microporous, amorphous structure whose melon or graphitic arrangement could not be determined. The presence of structural defective states (mainly C radicals) was corroborated by EPR studies of the solids. The photocatalytic activity of CN powders for methyl orange (MO) discoloration was investigated using 350 nm and simulated solar light (SSL) irradiation. MO removal efficiencies were correlated with the particle’s energy gap, specific surface area, degree of crystallinity, and C radical defects produced upon irradiation. Moreover, the obtained conduction and valence band potentials in the range of −0.60 and + 2.14 V vs NHE (pH = 7), respectively, evidence CN capacity to oxidize water to hydroxyl radicals (HO•) and reduce O2 to superoxide radical anions (O2 •–). The formation of both radicals upon SSL irradiation of CN suspensions was confirmed by EPR experiments. The largest specific surface area, the highest charge carrier density, and the amount of C radical defects observed for CN obtained from urea (CNu) account for their highest photocatalytic performance. However, the estimated CNu photonic efficiency of ca. 8% still indicates a deficient separation/migration efficiency of photoinduced charge carriers. Despite CNu performance being higher than those of other CNs obtained from the thermal treatment of simple precursors, further environmentally friendly strategies are still needed to overcome the intrinsic disadvantages of CN before it may be employed in technological applications. A discussion on probable CN mechanisms forming reactive species and leading to MO decolorization is given.

SiO2@TiO2 core-shell nanoparticles were successfully synthesized via a simple, reproducible, and low-cost method and tested for methylene blue adsorption and UV photodegradation, with a view to their application in wastewater treatment. The monodisperse SiO2 core was obtained by the classical Stöber method and then coated with a thin layer of TiO2, followed by calcination or hydrothermal treatments. The properties of SiO2@TiO2 core-shell NPs resulted from the synergy between the photocatalytic properties of TiO2 and the adsorptive properties of SiO2. The synthesized NPs were characterized using FT-IR spectroscopy, HR-TEM, FE–SEM, and EDS. Zeta potential, specific surface area, and porosity were also determined. The results show that the synthesized SiO2@TiO2 NPs that are hydrothermally treated have similar behaviors and properties regardless of the hydrothermal treatment type and synthesis scale and better performance compared to the SiO2@TiO2 calcined and TiO2 reference samples. The generation of reactive species was determined by EPR, and the photocatalytic activity was evaluated by the methylene blue (MB) removal in aqueous solution under UV light. Hydrothermally treated SiO2@TiO2 showed the highest adsorption capacity and photocatalytic removal of almost 100% of MB after 15 min in UV light, 55 and 89% higher compared to SiO2 and TiO2 reference samples, respectively, while the SiO2@TiO2 calcined sample showed 80%. It was also observed that the SiO2-containing samples showed a considerable adsorption capacity compared to the TiO2 reference sample, which improved the MB removal. These results demonstrate the efficient synergy effect between SiO2 and TiO2, which enhances both the adsorption and photocatalytic properties of the nanomaterial. A possible photocatalytic mechanism was also proposed. Also noteworthy is that the performance of the upscaled HT1 sample was similar to one of the lab-scale synthesized samples, demonstrating the potentiality of this synthesis methodology in producing candidate nanomaterials for the removal of contaminants from wastewater.

In recent years, bioremediation has become very attractive for environmental applications, especially enzyme-based treatments, since they offer high catalytic capacity with milder reaction conditions, production of nontoxic compounds, and environmental...