Search

In this mini-review, the authors comment on selected papers focused on the use of low-cost materials to prevent/remediate environmental pollution (specifically, soil and water pollution). The authors have selected publications corresponding to the years 2021 and 2022, using the searching tools Scopus, Web of Sciences, and Google Scholar to find basic data about the papers, the countries where the researches were carried out, number of citations, and other details indicative of the relevance of the works. Overall, the field of research is receiving growing attention and efforts, providing useful data on classical and new low-cost materials, both raw and modified by means of low-cost procedures, which constitute a clearly interesting alternative to face environmental pollution currently and for the future

In this study, we synthesized MnFe2O4 solid nanospheres (MSN) calcined at different temperatures (200–500 °C) and MSN-based materials mixed with carbon black, for their use as electrocatalysts in the oxygen reduction reaction (ORR) in alkaline medium (0.1 M KOH). It was demonstrated that the calcination temperature of MSN material determined its chemical surface composition and microstructure and it had an important effect on the electrocatalytic properties for ORR, which in turn was reflected in the performance of MSN/CB-based electrocatalysts. The study revealed that the presence of Mn species plays a key role in the ORR activity. Among tested, MSN200/CB and MSN350/CB exhibited the best electrochemical performances together with outstanding stability.

Copper ferrite nanospheres (CFNS) were prepared by a solvothermal method and they were calcined at different temperatures ranging from 200−600 °C, resulting in CFNS-based materials with different both surface chemistry and structural features, which play a crucial role in the electrocatalytic activity of carbon black-containing CFNS catalysts towards the oxygen reduction reaction (ORR). The study was addressed to elucidate which kind of crystalline phases, as well as surface chemistry of CFNS were responsible for enhancing the electrocatalytic activity in ORR. The pristine CFNS and CFNS200 (calcined at 200 °C) exhibited cubic spinel, cuprite and copper as main crystalline phases. In the case of CFNS400, it also displays the cubic phase along with a new phase (tenorite). Apart from the tenorite, CFNS600 exhibited hematite, and the cubic phase of spinel shifted to the tetragonal phase. Regarding the electrocatalytic activity in ORR, the CFNS revealed poor activities compared to those of carbon black-containing CFNS catalysts. The high-performing catalyst was CFNS400/Vulcan in terms of current density and onset potential along with its good stability. This improvement may be assigned to both the chemical composition and structure.

Active carbons of different origins have been oxidized with H2O2 and (NH4)2S2O8 and their oxygen surface complexes have been characterized by TPD, classical titration following Boehm's method and by neutralization calorimetry. The net enthalpies of neutralization, determined by immersion calorimetry into NaOH and HCl 2 N lead to −41.1±1.8 and −52.3±2.0 kJ eq−1 for the acidic and basic sites on the surface. Experiments with NaHCO3 lead to −39.7±1.7 kJ eq−1 for the carboxylic groups alone. These results suggest that the surface groups of active carbons can also be characterized by immersion calorimetry. Results are also given for the variation of the pH of the point of zero charge with the total oxygen content of the surface.

The interaction of methanol and ethanol with active carbons of different origins and containing between 0.81 and 7 mmol g-1 surface oxygen has been examined by immersion calorimetry and by vapor adsorption at 293 K, benzene being the reference. The results obtained by these independent techniques are in excellent agreement. With respect to pure carbons, the surface oxygen leads to excess enthalpies of immersion of 5.32 and 2.64 J mmol-1 oxygen for methanol and ethanol. Alternatively, it appears that the affinity coefficients β(CH3OH) and β(C2H5OH), which appear in the Dubinin−Astakhov equation, are functions of the oxygen content and of the enthalpy of immersion into benzene. The limiting values, as the oxygen content tends to zero, are respectively 0.40 and 0.62, in agreement with the values quoted in the literature. It is also found that the adsorption of CO2 is not affected by the oxygen content of the surface and β(CO2) = 0.40.

It is shown that the adsorption of phenol and 3-chlorophenol from aqueous solutions, by basic active carbons and at an equilibrium solution pH below that of dissociation of the phenols, can be described by an equation of the DRK type with exponent n = 4. The principle of temperature invariance is fulfilled between 283 and 313 K, which means that predictions can be made on the basis of a characteristic energy, Es, and of affinity coefficients, βs, relative to phenol. This approach presents an advantage over the traditional Langmuir isotherm. Moreover, the comparison of the limiting amounts adsorbed by the different carbons suggests that phenol and 3-chlorophenol are adsorbed in a monolayer as observed for carbon blacks, except in the case of activated carbons with a low degree of activation in which molecular-sieve effects can take place. Similar conclusions are obtained from the enthalpies of immersion into the aqueous solutions and their comparison with graphitized carbon blacks.

Active carbons of different origins have been oxidized with H2O2 and (NH4)2S2O8 and their oxygen surface complexes have been characterized by TPD, classical titration following Boehm's method and by neutralization calorimetry. The net enthalpies of neutralization, determined by immersion calorimetry into NaOH and HCl 2 N lead to −41.1±1.8 and −52.3±2.0 kJ eq−1 for the acidic and basic sites on the surface. Experiments with NaHCO3 lead to −39.7±1.7 kJ eq−1 for the carboxylic groups alone. These results suggest that the surface groups of active carbons can also be characterized by immersion calorimetry. Results are also given for the variation of the pH of the point of zero charge with the total oxygen content of the surface.