The bio-flocculants used in this study were synthesised by the Mannich reaction, which includes three reagents: a substrate (tannin extracts of Acacia, Quebracho, and Castanea), formaldehyde, and an amine derivative (ethanolamine, diethanolamine, ammonium chloride). Nine natural flocculants were prepared by combining extracts and amines; these products were evaluated in three different wastewater samples in two experimental phases. In phase I, five physicochemical parameters were analysed. From the data obtained, a multivariate, completely randomised design (CRD-Manava) was used, with a factorial arrangement and mean plots. In phase II, the three bio-flocculants with the most statistically significant responses and their mixtures were examined, evaluating 14 biological and physicochemical parameters. Statistical analysis was guided in this phase by CRD blocks, finding a significant removal in the physicochemical parameters analysed in the different types of wastewater and obtaining removal rates between 50 and 90%, depending on the parameter. At the end of both phases, the bio-flocculants acacia-ammonium chloride and quebracho-diethanolamine were the most efficient in the removal of turbidity (34–99%), true colour (93–100%) and total solids (12–99%). In addition, the natural flocculants showed low mutagenicity index (MI: 0.33–0.93) compared to aluminium sulphate (MI: 4.87–8.81).

The modified tannin by Mannich reaction was investigated for wastewater treatment. The removal of heavy metals, such as copper, chromium and mercury, in industrial wastewater was evaluated through the coagulation–flocculation technique, using modified Acacia tannin (MAT) as a coagulant agent. The successful tannin modification was evaluated by infrared spectopometry (FTIR), nuclear magnetic resonance (NMR); monitoring the removal of heavy metals was performed by atomic absorption (AA) and a direct mercury analyzer (DMA). Additionally, the parameters of water quality, total suspended solids (TSS), turbidity and chemical oxygen demand (COD) were assessed. Different doses of MAT were evaluated (375 ppm, 750 ppm, 1250 ppm and 1625 ppm) and three different levels of pH (4, 7 and 10). The highest percentages of removal obtained were copper 60%, chromium 87%, mercury 50%–80%, COD 88%, TSS 86% and turbidity 94%, which were achieved with the dose of 375 ppm of MAT at pH 10. The coagulation–flocculation process with the modified Acacia tannin is efficient for the removal of conventional parameters and for a significant removal of the metals studied.

The catalytic oxidation of aqueous crystal violet (CV) solutions was investigated using Ni and Fe catalysts supported over Mg–Al oxides synthesized by the autocombustion method. The influence of temperature, loading, and selectivity were studied in the catalytic wet air oxidation (CWAO) of CV. The kind of metal had an important contribution in the redox process as significant differences were observed between Fe, Ni, and their mixtures. The catalysts with only Fe as active phase were more efficient for the oxidation of CV under normal conditions (T = 25 °C and atmospheric pressure) compared to those containing Ni, revealing the influence of the transition metal on catalytic properties. It was found that iron-containing materials displayed enhanced textural properties. The synthesis of Fe/MgAl catalysts by the autocombustion method led to solids with excellent catalytic behavior, 100% CV degradation in eight hours of reaction, 68% selectivity to CO2, and significant reduction of chemical oxygen demand (COD).