The use of fossil fuels in modern economies has been a success because of the low cost of fossil resources. However, the depletion of fossil reserves, the increase in waste production and global warming concerns have led to increased research on the production of biofuels from renewable resources. Waste production is steadily increasing in quantity and constantly changing in quality, creating enormous risks for the environment and, consequently, for the health of the population. This situation is much more worrying in developing countries, in particular because of the considerable delay in the field of the conversion and recovery of biomaterials, due to their difficulty in approaching the problem in a way that fits their context. The composition of such wastes and residues, rich in organic matter, allows their conversion via biochemical mechanisms, thus constituting an effective solution to address the environmental problems of their disposal. Anaerobic digestion remains a valuable and effective technology for transforming these biomaterials into biogas. The present review focuses on technologies, challenges and areas of application of biogas, especially in China and some African countries, in order to promote the large-scale use of biogas for electricity generation and biofuels. Results point out that China is more used to this technology, while African countries still rely on traditional and less advanced technologies, thus hampering the potential derived from the large availability of biomaterials. Both realities, however, share similar backgrounds about the dimension of the biogas plants and their non-commercial purposes, even if China is recently shifting toward the adoption of a different model. These considerations are used in the article to open an interesting new scenario of political alternatives which may provide a way out from poverty and economic dependence, within the framework of a wider circularity.

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The present work demonstrates the adsorption of hydroquinone (HQ) and resorcinol (R) by activated carbon based on shea residue (Vitellaria paradoxa). The adsorbent was prepared chemically by impregnation with sulfuric acid and coded by the acronym CAK-S. The central composite design (CCD) was used to optimize the main factors that influence the adsorption of HQ or R by activated carbon such as the initial concentration, the pH of the solution, the contact time, and the mass of the carbon on the expected response, which is the adsorbed quantity of the target pollutants. The optimal conditions obtained from the statistical analysis are as follows: concentration of 158 mg/L, pH 3, time of 120 min, and mass of 50 mg for the adsorption of HQ and concentration of 180 mg/L, pH 3, time of 86 min, and mass of 118 mg for the adsorption of R. The maximum quantities of HQ and R adsorbed are 45.02 mg/g and 33.65 mg/g, respectively. The analysis of variance (ANOVA) showed a good relationship between the variables involved with the coefficients of determination R2 = 98.69% for the adsorption of hydroquinone and R2 = 90.55% for that of resorcinol, which means that the model is more suitable to express the adsorbed amount according to the four optimized parameters. The experimental data obtained under these optimal conditions were simulated with two and three parameter nonlinear isotherm models as well as kinetic models. The results show that Elovich kinetic model better describes the adsorption of HQ and R, indicating chemisorption with heterogeneous active sites on the surface of CAK-S. Temkin’s two-parameter model shows that adsorption occurs on heterogeneous surfaces with a nonuniform adsorption energy distribution at the surface and Sips’s three-parameter model confirms the heterogeneity of the surface with a localized adsorption of HQ or R by CAK-S. The thermodynamics study has shown that the adsorption is endothermic ( Δ H 0   > 0 ) and spontaneous ( Δ G 0 < 0 ).

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