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May 20, 2024

A eficácia do cloro

npj Clean Water volume 4, Artigo número: 48 (2021) Citar este artigo 6666 Acessos 7 Citações 8 Detalhes de métricas altmétricas As soluções de cloro são amplamente utilizadas para a produção de biologicamente

npj Água Limpa volume 4, Número do artigo: 48 (2021) Citar este artigo

6666 Acessos

7 citações

8 Altmétrico

Detalhes das métricas

Soluções de cloro são amplamente utilizadas para a produção de água potável biologicamente segura. A capacidade dos sistemas de tratamento de água potável no ponto de uso [POU] ganhou interesse em locais onde sistemas de tratamento centralizados e redes de distribuição não são práticos. Este estudo investigou a atividade antimicrobiana e antibiofilme de três desinfetantes à base de cloro (íons hipoclorito [OCl-], ácido hipocloroso [HOCl] e soluções ativadas eletroquimicamente [ECAS]) para uso em aplicações de água potável POU. A atividade antimicrobiana relativa foi comparada em ensaios de suspensão bactericida (BS EN 1040 e BS EN 1276) utilizando Escherichia coli. A atividade anti-biofilme foi comparada utilizando Pseudomonas aeruginosa séssil estabelecida dentro de um reator de biofilme do Centro de Controle de Doenças [CDC]. HOCl exibiu a maior atividade antimicrobiana contra E. coli planctônica em> 50 mg L-1 de cloro livre, na presença de carga orgânica (albúmen sérico bovino). No entanto, o ECAS exibiu atividade anti-biofilme significativamente maior em comparação com OCl- e HOCl contra biofilmes de P. aeruginosa com ≥50 mg L-1 de cloro livre. Com base nesta evidência, os desinfetantes em que o HOCl é a espécie de cloro dominante (HOCl e ECAS) seriam desinfetantes alternativos adequados à base de cloro para aplicações de água potável em POU.

Uma importante fonte de doenças humanas é o consumo de água biologicamente contaminada1. Isto é especialmente relevante para os países de baixo rendimento (ou seja, o Rendimento Nacional Bruto [RNB] per capita é < 1025 dólares) e para os países menos desenvolvidos (46 países de baixo rendimento que enfrentam graves impedimentos estruturais ao desenvolvimento sustentável), onde cerca de 30% da população, em média, ter acesso a serviços de saneamento básico2. Isto contrasta com os países de rendimento médio-alto (RNB per capita de 4.036 a 12.475 dólares) e de rendimento elevado (RNB per capita > 12.476 dólares), que utilizam predominantemente sistemas centralizados de tratamento de água potável para garantir a produção e o fornecimento de água biologicamente segura3. O papel principal da desinfecção da água potável é controlar microorganismos patogênicos e garantir que a água tratada seja biologicamente segura para beber. O cloro, na forma de hipoclorito de sódio [NaOCl], é o desinfetante mais comum devido ao baixo custo e às propriedades antimicrobianas eficazes4. A presença de cloro residual (0,5–5 mg L-1) nas redes de redistribuição limita o novo crescimento microbiano, ajudando a manter a água biologicamente segura no ponto de entrega3. Organismos indicadores como Escherichia coli, coliformes totais, Enterococos e Clostridium perfingens3,5, que inferem a presença de matéria fecal, são monitorados para garantir a eficácia dos processos de tratamento de desinfecção. O limite recomendado para esses organismos indicadores em água tratada é zero UFC 100 mL−1, devido à sua potencial natureza patogênica3,5. Infelizmente, o uso de desinfetantes à base de cloro dá origem à formação de subprodutos de desinfecção [DBPs]6,7 como trihalometanos8 e ácidos haloacéticos9. Sabe-se que tais subprodutos apresentam propriedades mutagénicas e cancerígenas10 e são, portanto, altamente indesejáveis.

Point-of-use [POU] drinking water treatment systems do not require distribution networks and therefore negate the need to maintain residual chlorine levels. The World Health Organization recommends free chlorine concentrations of between 0.2 and 0.5 mg L−1 at point of delivery and use3. The use of conventional chlorine-based disinfectants, such as hypochlorite (OCl-), within POU water disinfection requires the storage and transportation of hazardous chemicals and can also cause the formation of harmful DBPs and the deterioration of taste and odour11. Ultraviolet and ozone are well established as disinfection technologies within both decentralised/POU12,13 and large scale drinking water treatment14,3.3.CO;2-1." href="/articles/s41545-021-00139-w#ref-CR15" id="ref-link-section-d367130989e520"15, but an added benefit of implementing electrochemcially activated solutions [ECAS] is it has capability to be used externally to water treatment systems as part of food production16,17 or in healthcare settings18,19. A limited number of studies have compared ECAS against commonly used chlorine agents for decentralised disinfection applications20,21. Although these preliminary studies were promising, neither study reported the pH of the ECAS studied or their effectiveness against biofilms./p>95%), and dissolved chlorine [Cl2] (<5%)25,26. Additional metastable antimicrobial species including; OH-, O3, H2O2 and O2- are also theorised to be generated although there lifetime and activity within active solutions is debated27,28. The antimicrobial properties of ECAS result from a combination of HOCl and the metastable species that give rise to the observed high ORP values. The mode of action of such solutions is then physical rupture of the inner and outer cell membranes19,29, leading to disruption and failure of microbial functionality, such as energy generation mechanisms23./p>5-log reduction) and there was no significant difference between the three disinfectants, whereby HOCl resulted in a complete log reduction, for OCl- a log reduction of 7.871 ± 0.74 log10 CFU mL−1 was achieved whilst ECAS achieved a 6.806 ± 1.09 log10 CFU mL−1 reduction. At 50 mg L−1 FC, OCl- did not achieve the required 5-log reduction (4.531 ± 0.15 log10 CFU mL−1), resulting in significantly lower antimicrobial activity compared to both HOCl and ECAS (p < 0.0001), whereby there was no significant difference between HOCl and ECAS treatment (p > 0.05). At the lowest FC concentration tested (25 mg L−1) ECAS was the only disinfectant to reduce the bacterial load ≥5 log10 CFU mL−1 (Fig. 2), resulting in a 6.077 ± 1.441 log10 CFU mL−1 log reduction. The log reductions obtained for OCl- and HOCl treatment were both significantly less than ECAS (p < 0.001), whereby HOCl resulted in a 3.207 ± 0.505 log10 CFU mL−1 log reduction, which was significantly greater than the 1.945 ± 0.222 log10 CFU mL−1 log reduction exhibited by OCl- (p = 0.0011). The 5-log reduction CT values for OCl-, HOCl and ECAS with a low organic load demonstrated that NaOCl exhibited the highest CT value (88.96 mg min L−1), followed by HOCl (34.78 mg min L−1) and then ECAS (20.94 mg min L−1)./p> 0.05). However, ECAS resulted in the greatest log reduction (1.606 ± 0.954 log10 CFU mL−1), followed by HOCl (0.978 ± 0.202 log10 CFU mL−1) and OCl- (0.025 ± 0.004 log10 CFU mL−1). The organic loading tested under dirty conditions does not represent concentrations expected within POU drinking water systems. However, results highlight the need to reduce organics present to ensure sufficient antimicrobial activity throughout disinfection stages of drinking water treatment./p> 0.05). In fact, there was no significant reduction in biofilm density between 0 (control) and 5 mg L−1 FC (p > 0.05) for any test disinfectant. Overall, the results demonstrate a dose-response of increasing antimicrobial efficacy with increasing FC concentrations. Interestingly, for ECAS the greatest increase in antimicrobial activity (p = 0.009) occurred at ≥25 mg L−1 FC, whereas the greatest increases for HOCl and OCl- were observed between 0 and 25 mg L−1 (p < 0.0001)./p>25 mg L−1 (i.e. 50, 100, 150 mg L−1). Interestingly, there was no significant difference in the antimicrobial activity exhibited by ECAS at an FC concentration of 25 mg L−1 in either the presence or absence of low organic loading (clean BSA conditions). This shows that low concentrations of organic matter do not unduly interfere with the mechanism of action for ECAS under these experimental conditions. ECAS exhibits very high ORP value (>+1100 mV), due to both reactive chlorine and oxygen species, which in turn drives rapid oxidation reactions. However, the presence of higher concentrations of organic matter will ultimately reduce the ORP through oxidation-reduction reactions50, contributing to a resultant reduction in antimicrobial activity of ECAS, as has been previously observed50,51. Interestingly, previous work by Robinson et al. in 201352 demonstrated that antimicrobial activity of ECAS could be maintained when stored for a 398 day period at 4 °C in the dark, despite showing no detectable FC after 277 days (e.g. < 0.01 mg L−1). This demonstrates the importance of the additional antimicrobial species, other than those that are chlorine derived, contributing to an increased antimicrobial activity. Thus, helping explain the greater antimicrobial activity of ECAS at a FC of 25 mg L−1 in the presence of clean BSA conditions when compared to equivalent HOCl and NaOCl solutions. Further increasing the organic loading of BSA (3.0 g L−1; dirty BSA conditions) within the bactericidal assay greatly reduced the antimicrobial activity of OCl- and ECAS at all FC concentrations tested. In comparison the antimicrobial activity of HOCl was not significantly reduced at FC concentrations >25 mg L−1. Therefore, it is clear that HOCl produced via the dissolution of NaDCC demonstrates a greater antimicrobial activity against planktonic bacteria under dirty BSA conditions. Chemically derived HOCl is more stable than electrochemically generated HOCl solutions, as they do not possess metastable antimicrobial species, that form at the anodic surface53. Chemically derived HOCl also degrades at a slower rate when exposed to sunlight (UV)54, in comparison to electrochemically generated HOCl which degrade at an increased rate55. This highlights the importance of selecting the most appropriate disinfectant for use in POU treatment systems. For example, in instances where filtration or removal of organic matter from bulk water is not standard practice or is difficult, HOCl would provide greater antimicrobial efficacy, compared to NaOCl or ECAS./p>3.3.CO;2-1./p>