Overview of Ecological Toxicity of Potassium Chlorate Pollution

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1. Guangdong Engineering Polytechnic, Guangzhou 510520, China; 2. Key Laboratory of Agro-Environment in the Tropics, Ministry of Agriculture/Guangdong Engineering Research Center for Modern Eco-agriculture and Circular Agriculture/Key Laboratory of Agroecology and Rural Environment of Guangzhou Regular Higher Education Institutions, Guangzhou, 510642, China.

1 Introduction

In 1998, Taiwan horticulture scholars occasionally derived inspiration from promoting burning firecrackers nearby longan trees to bloom and successfully took KClO3as plant growth regulator for longan. Later, in longan production areas, such as Fujian, Guangxi, Guangdong, and Hainan in China and Thailand, similar researches and technical extension have been carried out, and chemical regulation of KClO3to longan production period gets constantly mature and widely applied[1-2]. Researches of Manochaietal.[3]indicate that KClO3can promote longan to realize year-round production and multiple planting of three crops two years. Besides, Subhadrabandhuetal.[4]found that KClO3also promotes flowering and fruit bearing for tropical fruit trees such as litchi and mango apart from exerting regulation effect on the longan production period. With extension of technology of KClO3in regulating longan flowering period, secondary pollution of ClO3-toxicity and its residue exerts adverse impact on agricultural ecological environment. This receives wide and wide concern. Combining current domestic and foreign researches about pollution of KClO3to ecological environment, we make overview of progress in researches about KClO3toxicity, to make clear environmental risk of KClO3and provide reference for KClO3application.

2 Overview of ecological toxicity of KClO3 pollution

2.1EffectsofKClO3onlandplantsStrong oxidation of KClO3exerts significant toxic effect on plant growth. According to researches, except the moss, ClO3-is toxic to growth of green plants for 3 to 6 months. Its toxicity exerts excellent control effect on herbs such asPharbitisnil(Linn.) Choisy, Canada thistle, and Sorghum halepense; it can remove leaves of cotton, maize, flax, and soybean; therefore, KClO3has been used as non-selective herbicide, defoliant, or even soil fungicide in the 1930s and 1940s[5]. It is reported that KClO3has obvious concentration gradient toxic effect on rice, wheat seedlings, tomato at normal temperature because after ClO3-enters into soil and gets absorbed by root system of plants to organs, its strong oxidization exerts direct toxic effect on plants[6]. Besides, accumulation of ClO3-and ClO2-within plant body will increase the content of catalase within plant body, leading to falling of leaves ahead of time and early aging of plants[7]. In addition, root system of plants does not have selection for absorption of ClO3-and NO3-, therefore, absorption of ClO3-will inhibit absorption and delivery of plant cells to NO3-, leading to lack of nitrogen and consequently influencing nutrition within plant and growth of plant[8-9].

2.2EffectsofKClO3onlonganTaiwan scholar Yan Changrui[10]successfully promoting langan flowering using KClO3. Later, the production period regulation technology realized breakthrough and rapid development, and Thailand, Malaysia, Guangdong, Fujian, and Guangxi carried out similar experiments, researches and extension[11-13]. At present, Thailand has realized annual batch production and export of off-season longan[14]. Chen Qingxietal[15]studied off-season longan and found that after application of KClO3, parent branches of longan trees have lower starch content, while total sugar, chlorophyll, endogenous ethylene, abscisic acid, trans-Zeatin-riboside, and catalase gradually increase in physiological differentiation period. However, residue of ClO3-, ClO2-, and ClO-in longan leaves and fruit is lower than the detection limit obtained from ordinary ion chromatography, and there is no significant difference in corresponding quality indicators such as fruit color, size, and soluble solids between ordinary longan[13-14].

2.3EffectsofKClO3onaquaticorganismsDolfJVWetal.[16-17]found that the toxicity of KClO3on aquatic organisms is closely related with living species. The effects of KClO3on diversity of fishes and large invertebrate aquatic organisms are not high, the acute toxicity concentration is 2442 mg/L and 3815 mg/L respectively, but the effects on algae are greater, especially on brown alga. Generally, acute toxicity concentration of KClO3in sea water to brown alga is lower than 0.1 mg/L, EC50 is lower than 0.105 mg/L, and chronic toxicity concentration is lower than 0.005 mg/L. Also, experiment has proved that the toxic effect of KClO3on alga is restricted by NO3-in water, higher concentration NO3-will weaken the toxicity of KClO3to algae. Researches of Stauber. J.L[18]indicate that when KNO3concentration is lower than 0.005 mg/L, 72h EC50of KClO3to Nitzschia and Dunaliella algae is 1.9 mg/L and 11 mg/L; in water with high concentration of NO3-, 72h EC50of KClO3to Nitzschia and Dunaliella algae exceeds 500 mg/L and 1000 mg/L respectively; in the range of concentration of ClO3-inhibiting division of Nitzschia cell, it exerts no infuence on photosynthesis and ATP content.

2.4EffectsofKClO3onanimalsandhumanbeingsSince KClO3has the taste of salt, animals may take it as salt and get poisoned[18]; once polluted by KClO3, green plants will be stained with salt taste, herbivore may eat and get poisoned[20-21]. Germgard U[22]found that after animal absorbs KClO3, strong oxidization of ClO3-may lead to methemoglobinemia, hemolytic anemia, and kidney poisoning; if birds eat KClO3by mistake, it will reduce ability and quantity of egg production, but there is still no obvious toxic effect on bees; Wang Lietal.[23]found that 800 mg/L ClO3-water solution is negative on mouse bone-marrow micronucleus test, indicating that ClO3-does not have induction effect on mouse bone-marrow micronucleus in this range of concentration. The mechanism of KClO3toxicity to animals is that ClO3-damages their metabolism, leading to abnormal life activities. The lethal concentration (LC) of KClO3for dog is 1200 mg/L, for mouse, it is 1870 mg/L, and for rabbit, it is 2000 mg/L[24].

Phongtapeetal[25]surveyed 40 fruit growers producing off-season longan in Lumphun Province of Thailand (57.5% are male and the average age is 47 years old), and found that in the process of applying KClO3, 80.0% fruit growers do not wear gloves, 82.5% fruit growers do not wear mouth mask, and only 60.0% fruit growers wear long sleeve clothes, and daily average working hours are 2.3 hours. Before exposing to KClO3, the average denatured hemoglobin of fruit growers is 1.16%; after exposing to KClO3, the average denatured hemoglobin of fruit growers rises to 1.36%, and 20.0% fruit growers get methemoglobin, indicating KClO3will induce denature of hemoglobin. Therefore, without labor protection measures, fruit growers exposing to KClO3for a long time will have risk of getting methemoglobin. They also surveyed health condition of local workers engaged in off-season longan production and exposing to KClO3for a long time, and found that 64% workers process off-season longan in three steps, peeling, rinsing, and classification, the average daily working hours are up to 10.6 hours; 68% workers do not wear gloves during working, leading to 66% workers getting allergic skin, 35.2% workers getting methemoglobin, 31.2% getting anemia, 6.5% workers getting leukaemia. Also, they found 22.0% workers have hematuria, 20.5% workers have pyuria, and 7.5% workers have both hematuria and pyuria. No doubt, backward safety and protection measures pose great threat to longan production workers.

Therefore, to avoid poisoning and explosion of KClO3, it is required to prevent animals and human beings from eating KClO3by mistake, strictly prohibit grazing in grassland with spraying of KClO3, strictly separate foods and feeds, promptly clean vessels containing KClO3, and strictly forbid pouring KClO3residue, and strictly prohibit storage of phosphorus, sulphur, carbon, organic matters, ammonium compound, cyanide, metal powders together with KClO3. Government organs and legal institutions should formulate and improve relevant laws and regulations, to standardize production, storage, and use of KClO3, implement excellent health and sanitation planning, and take effective labor protection measures, to protect health of relevant people exposing to or applying KClO3in actual work. At present, foreign scholars have carried out extensive researches on migration and conversion and ecological toxicity of ClO2-and ClO-in water environment focusing on ClO2disinfection and bleaching agent. Animal toxicity experiment indicates that there is close relation between ClO3-and ClO2-in high level of in drinking water and anemia of animals, thus these two ions are suspected as factors inducing anemia. Both foreign and domestic scholars pay close attention to disinfection by-products (DBPs) ClO2-and ClO-. In Sanitary Standard for Drinking Water Quality for Drinking Water issued by Ministry of Public Health of China in 2001, it specifies that the maximum mass concentration of ClO2-is 200 μg/L; in the latestGuidelinesforDrinkingWaterQualityissued by the World Health Organization (WHO), it stipulates that the ClO3-concentration of should be as low as possible. Research data of American and Canadian environmental protection departments indicate that too high ClO3-concentration in drinking water or foods will damage thyroid gland and consequently influence normal secretion of hormone. Both ClO3-and ClO2-are included into list of preferred disinfectant and disinfection by-products of drinking water in Code of Federal Regulations, USEPA(2002), and the recommended total residue concentration of ClO2, ClO3-and ClO2-should not exceed 10 mg/L. In the United States, due to emission of chlorate in military industry like fuel for rocket propulsion, pollution of ClO3-to underground water has greatly threatened health of surrounding residents. Government of California State reduces the standard of ClO3-in drinking water from 18 ppb to 4 ppb, but the Environmental Protection Agency detected local foods and found that the average content of ClO3-in milk and lettuce is up to 5.76 ppb, exceeding the specified 4 ppb; Food and Drug Administration (FDA) detected long-leaf lettuce and ice cream of California, Arizona, Florida, Texas and New Jersey states and found that the average content of ClO3-in these foods is as high as 10.49 ppb; NAS reported that in 35 states of America, 93% lettuce and milk and 97% breast milk contain excessive ClO3-; according to estimation of EPA, drinking water of about 15 million people is influenced by ClO3-in the United States[26].

3 Research prospects

Each molecule of KClO3contains 3 oxygen atoms, and structure of ClO3-is SP3 hybrid type, so KClO3has strong oxidation. With wide application of KClO3in industrial and agricultural production, secondary pollution resulted from its strong oxidation and residue will inevitably bring adverse effect on water and soil environment which are essential factor for human beings. Therefore, how to precisely detect and accelerate degradation of ClO3-and its residue will become a leading direction of the research in this field. In the past, ecological toxicity researches of chlorate are restricted by detection technique and means[27]. Now, with improvement and application of Ion Chromatography (IC) and High Performance Liquid Chromatography (HPLC) and other techniques, detection means and methods of toxic substances gradually become mature. For example, the upper limit of detection for ClO3-and ClO2-in water by Ion Chromatography has reached 1.6 mg/L and 3.5 μg/L respectively[28]; Capillary Electrophoresis (CE) is also widely applied in detection of many types of anions including ClO3-[29]. All of these provide assistance for in-depth study of ecological toxicity of chlorate. Besides, with the development of ecological toxicology, broad bean and mouse micronucleus detection methods have been widely applied in chlorate detection[1, 2, 15]. Strengthening the impact evaluation of environmental indication like earthworm and monitoring of soil pollution due to chlorate will be favorable for lifting chlorate monitoring and soil remediation to a new level. In sum, considering that the public are still not aware the ecological toxicity of chlorate, it is required to strengthen science popularization of chlorate, actively carry out pollution detection, and strengthen researches on control over pollution of chlorate to water and soil, so as to completely eliminate and solve the ecological and environmental problem resulted from KClO3and its residue.

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