L'efficacia del cloro

May 18, 2023

npj Clean Water volume 4, Numero articolo: 48 (2021) Citare questo articolo

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Le soluzioni di cloro sono ampiamente utilizzate per la produzione di acqua potabile biologicamente sicura. La capacità dei sistemi di trattamento dell'acqua potabile al punto di utilizzo [POU] ha guadagnato interesse in luoghi in cui i sistemi di trattamento centralizzati e le reti di distribuzione non sono pratici. Questo studio ha studiato l’attività antimicrobica e anti-biofilm di tre disinfettanti a base di cloro (ioni ipoclorito [OCl-], acido ipocloroso [HOCl] e soluzioni attivate elettrochimicamente [ECAS]) da utilizzare in applicazioni POU per acqua potabile. L'attività antimicrobica relativa è stata confrontata nell'ambito di test in sospensione battericida (BS EN 1040 e BS EN 1276) utilizzando Escherichia coli. L'attività anti-biofilm è stata confrontata utilizzando il noto Pseudomonas aeruginosa sessile all'interno di un reattore a biofilm del Center for Disease Control [CDC]. HOCl ha mostrato la maggiore attività antimicrobica contro l'E. coli planctonico a> 50 mg L−1 di cloro libero, in presenza di carico organico (albume di siero bovino). Tuttavia, l’ECAS ha mostrato un’attività anti-biofilm significativamente maggiore rispetto a OCl e HOCl contro i biofilm di P. aeruginosa a ≥ 50 mg L−1 di cloro libero. Sulla base di queste prove, i disinfettanti in cui HOCl è la specie di cloro dominante (HOCl ed ECAS) sarebbero appropriati disinfettanti alternativi a base di cloro per le applicazioni POU sull'acqua potabile.

Una delle principali fonti di malattie umane è rappresentata dal consumo di acqua biologicamente contaminata1. Ciò è particolarmente rilevante per i paesi a basso reddito (ovvero il reddito nazionale lordo [RNL] pro capite è <1.025 dollari) e per i paesi meno sviluppati (46 paesi a basso reddito che affrontano gravi ostacoli strutturali allo sviluppo sostenibile) dove circa il 30% della popolazione, in media, avere accesso ai servizi igienico-sanitari di base2. Ciò è in contrasto con i paesi a reddito medio-alto (RNL pro capite $ 4.036 – $ 12.475) e ad alto reddito (RNL pro capite > $ 12.476) che utilizzano prevalentemente sistemi centralizzati di trattamento dell’acqua potabile per garantire la produzione e la fornitura di acqua biologicamente sicura3. Il ruolo principale della disinfezione dell’acqua potabile è quello di controllare i microrganismi patogeni e di garantire che l’acqua trattata sia biologicamente sicura da bere. Il cloro, sotto forma di ipoclorito di sodio [NaOCl], è il disinfettante più comune grazie al basso costo e alle efficaci proprietà antimicrobiche4. La presenza di cloro residuo (0,5–5 mg L−1) all'interno delle reti di ridistribuzione limita la ricrescita microbica, contribuendo a mantenere l'acqua biologicamente sicura nel punto di consegna3. Organismi indicatori come Escherichia coli, coliformi totali, Enterococchi e Clostridium perfingens3,5, che suggeriscono la presenza di materiale fecale, vengono monitorati per garantire l'efficacia dei processi di trattamento di disinfezione. Il limite raccomandato per questi organismi indicatori nell'acqua trattata è zero CFU 100 mL−1, a causa della loro potenziale natura patogena3,5. Sfortunatamente, l’uso di disinfettanti a base di cloro dà luogo alla formazione di sottoprodotti della disinfezione [DBP]6,7 come i trialometani8 e gli acidi aloacetici9. È noto che tali sottoprodotti presentano proprietà mutagene e cancerogene10 e sono pertanto altamente indesiderabili.

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-d222113761e520"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>