title: Acute and chronic toxicity of chlorine dioxide (ClO2) and chlorite (ClO2-) to rainbow trout (Oncorhynchus mykiss).
abstract: OBJECTIVE
  Chlorite (ClO2-) is a primary decomposition product when chlorine dioxide (ClO2) is added during water treatment; therefore the toxic effects of both compounds on aquatic organisms are possible. Limited data are available concerning their toxicity to fish. The aim of this study was to investigate sensitivity of rainbow trout to acute and chronic toxicity of chlorine dioxide and chlorite, and to estimate the Maximum-Acceptable-Toxicant-Concentration (MATC) of those compounds in fish.
  
  
  METHODS
  The acute and chronic toxicity of chlorine dioxide and chlorite to larval and adult rainbow trout was investigated in 96-hour to 20-day laboratory exposures evaluating the wide range spectrum of biological indices under semi-static conditions.
  
  
  CONCLUSIONS
  Median lethal concentration (96-hour LC50) values derived from the tests were: 2.2 mg/l for larvae; 8.3 mg/l for adult fish and 20-day LC50 for larvae was 1.6 mg/l of chlorine dioxide, respectively. Chlorite was found to be from 48 to 18 times less acutely toxic to larvae and adult fish, correspondingly. Both chemical compounds induced similar toxic effects in rainbow trout larvae during chronic tests (they affected cardio-respiratory and growth parameters), but chlorine dioxide had a higher toxic potency than chlorite. A significant decrease in the heart rate and respiration frequency of larvae was established. However, within an increase in exposure duration recovery of cardio-respiratory responses was seen to have occurred in larvae exposed to chlorite. Meanwhile, in larvae exposed to chlorine dioxide, a significant decrease in cardio-respiratory responses remained during all 20-day chronic bioassays. Chlorine dioxide also more strongly affected growth parameters of rainbow trout larvae at much lower test concentrations. Decreased rate of yolk-sack resorption occurred only in the tests with chlorine dioxide.
  
  
  CONCLUSIONS
  Maximum-Acceptable-Toxicant-Concentration (MATC) of 0.21 mg/l for chlorine dioxide and of 3.3 mg/l for chlorite to fish was derived from chronic tests based on the most sensitive parameter of rainbow trout larvae (growth rate). According to substance toxicity classification accepted for Lithuanian inland waters, chlorine dioxide and chlorite can be referred to substances of 'moderate' toxicity to fish.
  
  
  CONCLUSIONS
  Due to its very reactive nature, chlorine dioxide is rapidly (in a few hours) reduced to chlorite, which is persistent also as a biocide but 16 times less toxic to fish, according to MATC. Therefore, it is much more likely that fish will be exposed to chlorite than to chlorine dioxide in natural waters. Presently accepted, the Maximum-Permitted-Concentration of total residual chlorine (TRC) in waste-water discharging into receiving waters is 0.6 mg/l. If this requirement will not be exceeded, it is unlikely that fish would be exposed to lethal or even to sublethal concentrations of chlorine dioxide or chlorite. Furthermore, chlorine dioxide does not generate toxic nitrogenous (chloramines) or carcinogenic organic residuals (trihalomethanes). All these properties make chlorine dioxide a more promising biocide than chlorine.
authors.affiliation: Laboratory of Ecology and Physiology of Hydrobionts, Institute of Ecology of Vilnius University, Akdemijos 2, 08412, Vilnius-21, Lithuania. Gintaras.Svecevicius@takas.lt
authors.forename: Gintaras
authors.initials: G
authors.lastname: Svecevicius
authors.forename: Janina
authors.initials: J
authors.lastname: Syvokiene
authors.forename: Prane
authors.initials: P
authors.lastname: Stasiŭnaite
authors.forename: Liongina
authors.initials: L
authors.lastname: Mickeniene
mesh_terms: D000367:Age Factors; D000818:Animals; D001681:Biological Assay; D002712:Chlorides / Q000633:toxicity*; D017606:Chlorine Compounds / Q000633:toxicity*; D007814:Larva / Q000254:growth & development; D007928:Lethal Dose 50; D017686:Oncorhynchus mykiss; D010087:Oxides / Q000633:toxicity*; D014874:Water Pollutants, Chemical / Q000633:toxicity*
chemical_list: D002712:Chlorides; D017606:Chlorine Compounds; D010087:Oxides; D014874:Water Pollutants, Chemical; C025109:chlorine dioxide; C001599:chlorite