Processos sortivos como etapa de tratamento de águas contaminadas com saxitoxinas: um olhar baseado na literatura e desafios laboratoriais

Abstract

A set of factors and processes result in the entry and accumulation of nutrients in dams and reservoirs, the most relevant being the dumping of untreated or insufficiently treated sewage, as well as runoff from agricultural areas. Such factors, when associated with intense solar radiation and high temperatures - typical in summer - can lead to the phenomenon of eutrophication, with the proliferation of cyanobacteria that produce different toxins. Among these, saxitoxins stand out, which is a group of neurotoxic substances with lethal potential for humans and other mammals. As conventional water treatment plants (WTPs) have low efficiency in removing such dissolved toxins, the presence of such substances in water causes concern. Sorption processes are relatively effective in removing these substances in a final stage of water treatment. With this, a need to consolidate existing knowledge on the topic is identified and therefore this work aims to survey and analyze studies available in recent literature on sorptive treatment of water contaminated with saxitoxins, providing useful information for decision makers and future work. A preliminary study at LABIFI-UERJ is also presented, with treatment experiments using a sorption process to remove saxitoxin from fortified ultrapure water, with detection by LC-MS/MS, using activated carbon, loofah (i.e. Luffa cylindrica) and orange peel biochar. From the literature study carried out, it is revealed that there is a great need for further investigation into the sorption treatment of the most diverse substances that make up this group of toxins, always explaining the experimental conditions applied to allow comparison between treatment options. A more detailed characterization of the adsorbents used in the treatment is identified as essential to identify the most relevant properties to the sorptive process. The use of activated carbons proved to be the most interesting among the materials recently researched, even those originating from natural materials such as coconut shells. Furthermore, it is observed that, in general, the sorption mechanisms for saxitoxins can be either chemisorption or physisorption. From the case study, it is clear the need to use well-established analytical methodology so that the treatability study of these substances can be carried out with reliable results. Finally, both in the Case Study and in the literature investigation, the great relevance of using experimental validation tools is highlighted – such as replicates, control tests, analytical curve, use of certified standard material and blank test –, in order to guarantee the reliability of the treatment carried out. Furthermore, the importance of carrying out treatment studies with adequate and environmentally relevant concentrations of toxins in the experiments is highlighted, so that at least safe levels of these substances are obtained after treatment, to guarantee water security for populations.