Banca de QUALIFICAÇÃO: GUILHERME ADLER ACIOLE MEDEIROS

USE OF ORGANIC COAGULANTS OF PLANT ORIGIN IN THE MITIGATION OF EUTROPHICATED SEMI-ARID WATER SUPPLY RESERVOIRS 

chlorophyll-a, algal biomass, coagulation, tannins,

Reservoirs in the semi-arid region often face the problem of eutrophication. This phenomenon occurs when there are high concentrations of nutrients, triggering the uncontrolled growth of algal biomass, often with a predominance of cyanobacteria. The uncontrolled growth of algal biomass causes environmental impacts and also poses a threat to public health. To deal with this challenge, the technique known as Floc & Sink (F&S), which involves the coagulation and sedimentation of algal biomass, has been used to mitigate these effects. Polyaluminum chloride (PAC) has been one of the most commonly used coagulants for this purpose. However, excessive use of this product can negatively impact the water's pH and increase the concentration of soluble aluminum, harming aquatic biota. Natural organic coagulants, such as tannins of vegetable origin, emerge as more sustainable alternatives. In this context, this work aimed to verify the efficiency of using tannins of vegetable origin for coagulation and subsequent sedimentation of algal biomass present in reservoirs in the semi-arid region. Three natural coagulants were tested, with tannins extracted from the bark of different trees: Cajueiro, Jurema Preta, and Acácia Negra (Tanfloc SG). The tests were conducted in four reservoirs in the tropical semi-arid region, using three different doses of each coagulant (50, 100, and 200 mg.L-1) applied in Floc & Sink experiments. All coagulants tested reduced turbidity and chlorophyll-a (except the dose of 50 mg/L of cashew tannin). With increasing doses of coagulants, the removal of algal biomass and turbidity was more significant. However, the removal values of these parameters with doses of 100 mg.L-1 and 200 mg.L-1 of tannin from Jurema Preta and Tanfloc SG did not show a significant difference. Zeta potential, pH, and humic substances were also measured after applying coagulants. The approach of the zeta potential to the isoelectric point (0 mV) favored the tannins' ability to neutralize the surface charges of suspended particles. In general, tannins did not cause significant changes in the pH of the water. Cashew tannin increased the concentration of humic substances, while Tanfloc and Jurema Preta tannin did not show statistically significant variations. The tannins tested could mitigate the symptoms of eutrophication in semi-arid reservoirs, as they effectively reduce turbidity and algal biomass, with a low impact on water pH. Among the coagulants tested, Jurema Preta tannin and Tanfloc SG stood out for their efficiency. Thus, these natural coagulants represent more sustainable and effective alternatives for managing water quality in environments affected by eutrophication.