Study On Toxicity Of [C₈mim]PF₆ To Aquatic Organisms And Biodegradability

Ionic Liquids, ILs, are substances composed solely by ion at room temperature. The common ILs consist of an organic cation with delocalized charges and a small inorganic anion, which have broad applied perspective and had been considered as'green product'due to their excellent physical properties of non-volatility, non-flammability and stable characteristic, etc. However, researches in recent years have shown that the friendly nature of ionic liquids on environment and organisms need to be further studied. As a solvent or catalyst used in industry, its release from industrial processes into environment is obvious. Once into aquatic environments, there may be too much of a risk, because its high solubility and stability in water may make them become persistent pollutants in wastewater. Up to now, many articles had reported physical-chemical characteristics and applications of ILs; however, there are not enough for us to study the toxicity and biodegradability to support their applications, some basic data about ILs have been lacking. For such reasons, it is priority to determine its toxic effect to aquatic organisms and biodegradability.Biological toxicity tests are indispensable to estimate the toxicity of one chemical, the same role on aquatic ecosystem and applied risk assessment. The main purpose of acute toxicity test is to determine the chemical LC50, then to make its toxic classification, which is also the basic work in toxicology studies. The aim of the toxicity study is to explore the toxic effects of an ionic liquid based on imidazolium-[C8mim]PF6 (1-octyl-3-methyl-imida- zolium PF6) on Chlorella vulgaris, Daphnia magna and Zebra fish, which were the three typical organisms in aquatic ecosystems.The experimental results showed that [C8mim]PF6 inhibited the growth and chlorophyll a content of C. vulgaris in all concentration groups tested, the more the concentrations, the stronger the inhibiting effect. It also indicated that the toxic effect on chlorophyll a content was stronger than growth of C. vulgaris, the higher concentration groups (100 and 200mg·L-1) even killed some C. vulgaris cell. The changing trend of EC50 with time was exponential, and the formula is y=281.65e-0.1529x, then we can infer the toxic effects of [C8mim]PF6 on C. vulgaris at other times by it. The bioaccumulation ability of algae on [C8mim]PF6 and the ability of [C8mim]PF6 transfer the cell membrane into the active site of C. vulgaris are two major factors to decide its toxic degrees. Because of its hydrophobic characteristic, [C8mim]PF6 easily enter the organic phase, thus, the growth of C. vulgaris have been inhibited entirely in all concentration groups (20~200mg·L-1).There will have various negative impacts on zooplankton such as Daphnia magna when toxic chemicals into the water environment. The behavior and physiology were firstly affected. Since the structure of most ionic liquids are similar to cationic surfactants, they are lipophilic, in addition, there have been proved example that the determinant factor of its toxicity is cation, so we infer that while the cation of ionic liquids entered the body, it has negative effects on its nervous system and enzymatic activity, accordingly reduced its feeding rate, then made it dead. Its 48h LC50=4.47<10mg·L-1, [C8mim]PF6 was high-toxic on Daphnia magna according to aquatic toxicity classification. Moreover, the longer the test time, the stronger its toxicity. The relationship formula between LC50 and time is y=8.2578e-0.0126x, then we can infer the toxic effects of [C8mim]PF6 on Daphnia magna at other times by the formula. [C8mim]PF6 is a class of hydrophobic ionic liquids, which is another major reason for its high-toxic to Daphnia magna. Since the high hydrophobic could enhance the adsorption capacity of adipose tissue on the ion, the substance could easily enter the organic phase, then absorbed and accumulated by organisms.Although the toxic mechanism of [C8mim]PF6 to Zebra fish is similar to Daphnia magna, the accumulation ability of Zebra fish to toxic chemical become stronger due to its larger body. The 96h LC50=126.08>100mg·L-1, [C8mim]PF6 was low-toxic on Zebra fish, in addition, the longer the test time, the stronger its toxicity. The relationship formula of LC50-time is y=304.35e-0.0093x, then inferring the toxic effect of other times using the formula. After 14 days exposure to low concentrations [C8mim]PF6, the accumulation content of every dead Zebra fish was around 0.11mg·g-1, while the more [C8mim]PF6, the stronger the accumulate ability, there was an increase from 0.11 mg·g-1 to 0.28mg·g-1.While [C8mim]PF6 escape into environment, not only did [C8mim]PF6 have negative effects on the quality of water and aquatic organisms survivance, but also on the function of aquatic organisms. Moreover, a potential threat on aquatic ecosystems correspondingly occurred. To ensure the new chemical are'green'and'recyclable', its biodegradability should be considered, which is an key indicator for the evaluation of a new chemical to environmental safety. To some extent, the biodegradability of a chemical can also reflect whether it is acceptable or not and the time accumulated in environment. The ultimate biodegradability was measured using the'Closed Bottle Test'(OECD 301D). In this test, [C8mim]PF6 was added to an aerobic aqueous medium inoculated with original waste water microorganisms and acclimated activated sludge microorganisms with [C8mim]PF6 as the C, N, P sources for microbe, the aim was to explore its biodegradation rate changing under natural conditions and artificial acclimation conditions. Our inspiration for this work came from the development of biodegradable surfactants. We now report our progress toward the further examination of the influence of appending an ester in the side chain of the imidazolium cation on the biodegradability of ILs under the same conditions. The experimental results showed that the biodegradation rate of [C8mim]PF6 after 28 days is 17% when the inoculant was original activated sludge, which was referred as'non-readily biodegradable', moreover, [C8mim]PF6 may stay in environment for a long time. While the inoculant was acclimated activated sludge, the biodegradation reache 60% above, referred as'readily biodegradable'. The biodegradability of ionic liquid could improve by appending an ester in the side chain, which was fundamental for the modification of the highly branched alkyl chain to the linear i to increase its environment-friendly characteristic.The study on the biodegradation of [C8mim]PF6 showed that the aerobic biological method was feasible to degrade it. In order to identify the types of microorganisms playing the role on degrading [C8mim]PF6, know the degradation efficiency of [C8mim]PF6 and its optimal substrate concentration, 3 best from 5 strains high-efficient [C8mim]PF6-degrading bacteria from the acclimated activated sludge have been screened. Through shaking culture experiment, we know the optimal substrate concentration of the 3 strains were 50mg·L-1 t. In a word, the experimental results certainly provide some theoretical basis for using biological method to rapidly remove [C8mim]PF6 and their industrial application in the future, which can greatly reduce their negative effects on environment.