Avaliação da técnica Flock and Sink para o controle de Raphidiopsis Raciborskii e saxitoxinas em ambientes eutrofizados: um estudo em escala de bancada

Abstract

Eutrophication, characterized by the excessive accumulation of nutrients in water bodies, especially phosphorus and nitrogen, promotes the proliferation of phytoplankton, including cyanobacteria such as Raphidiopsis raciborskii, which can produce saxitoxins (STX), toxic substances that compromise human health and environmental balance. To control blooms and reduce toxin concentrations in eutrophic waters, the Flock and Sink technique was evaluated using Polyaluminum Chloride (PAC) as a coagulant and Luvic soil (LUV) as a ballast material. The process efficiency was analyzed based on the removal of chlorophyll-a, turbidity, true color, and extracellular and total STX. Increasing the PAC dosage had a greater impact on chlorophylla removal than LUV variation. With 101.35 mg.L⁻¹ of LUV, chlorophyll-a removal increased from 37.32% in 2.83 hours to 50% in 9.67 hours, highlighting the influence of time. Turbidity removal increased with 4.05 mg Al.L⁻¹ of PAC (from 49.69% to 57.59%) but dropped to 37.84% with 15.96 mg Al.L⁻¹, suggesting that high dosages compromise floc stability. True color removal was enhanced by PAC, increasing from 55% to over 70% in 2.83 hours, with no significant influence from LUV. Extracellular STX removal varied according to PAC dosage and LUV concentration. The highest removal (~45%) occurred with 398.65 mg.L⁻¹ of LUV + 4.05 mg Al.L⁻¹ of PAC, while 15.96 mg Al.L⁻¹ reduced efficiency to approximately 35%. Total STX exhibited unexpected behavior, with lower removal at higher PAC dosages, contradicting expectations for coagulation and adsorption. Thus, the efficiency of the Flock and Sink technique depends on the balance between PAC, LUV, and sedimentation time. PAC was the main agent in cyanobacteria removal by forming stable flocs, while LUV had limited influence on dissolved material removal. Proper adjustments in concentrations and reaction time are essential to optimize the technique's application, preventing cell lysis and maximizing process efficiency.