Study On Efficient Production Of Hydroxyl Radicals And Marine Environmental Safety Of Ships’ Ballast Water Treatment

Ocean is a strategic resource base for the sustainable development of human society. The introduction of marine invasive species (MIS) into new environments is increasingly serious with the continuous development of international trade and sharp rise in port throughput. Such MIS introduction has serious consequences, such as breaking the balances of nearshore aquatic species, threatening marine biodiversity and ecological security. Ballast water discharge has been identified as a leading vector for MIS. Guideline G9of the International Maritime Organization (IMO) has identified that the ballast water treatment system (BWTS) should not only kill the aquatic organisms, but also not cause secondary pollution of the marine environment. In this study, mechanism of hydroxyl radicals (-OH) formed from strong ionization discharge (SID) combined with gas-liquid impinging jet is investigated. The key problem of environmentally friendly for-OH-based BWTS are also studied. The following original conclusions are drawn:(1)·OH radicals are the core of AOT, and efficiently production of·OH radicals have been the hotspot and difficulty of academic research. With an alternate discharge mode of micro-streamer and micro-glow, a SID is obtained at atmospheric pressure.O2and H2O are ionized and dissociated by high-energy electrons produced from SID to form highly reactive oxygen species, such as O2+, O3, H2O+and H2O2. Then these highly active species in the gas-phase are injected into the gas-liquid dissolver to form highly active species, especially-OH radicals in the aqueous solution. In order to measure·OH radicals,4-hydroxybenzonic acid (4-HBA) is used as a probe reagent, and the rapid reaction between4-HBA and-OH radicals lead to the formation of3,4-dihydroxybenzoic acid (3,4-DHBA). The concentration of·OH radicals is then determined by measuring the production of3,4-DHBA through high performance liquid chromatography. The existance of initiating agent HO2is also investigated using a catalase method. The maximum concentration of·OH radicals is63.36μmol/L Through comparing the concentration of-OH radicals produced from SID-based system and O3/H2O2processes, it is revealed that the concentration of·OH radicals formed from SID-based system is approximately7times higher than that of O3/H2O processes. Considering high concentration of·OH radicals procuced from SID,a new method for quantitative analysis of·OH radicals is proposed based on the concentration of carbon dioxide generated from the oxidation reaction between4-HBA and·OH radicals in the aqueous solution. The quantitative analysis of the high concentration of·OH radicals has not yet been reported. In addition, the mechanism of the formation of·OH radicals induced in aqueous-phase is clarified, and formation of·OH radicals by SID might occur through the following three pathways:1)-OH radicals primarily originate from reactive oxygen species, such as O2+, O3, H2O+and H2O2in combination with impinging jet.2)·OH radicals are generated upon O2+, O3and O2decomposition by action of hydrodynamic cavitation that causes the formation of cavitation bubbles, which can grow and implode when the pressure of flow field increases.3)-OH radicals are directly generated upon H2O decomposition by action of hydrodynamic cavitation.(2) The relationship between total residucal oxidant (TRO) dose-effect and killing time-effect is investigated. The results realve that biological efficacy is achieved when TRO dose in the range of1.5-2.5mg/L and killing time is6s. The laboratory-scale trials of this·OH radicals technique is an opportunity to assess whether this method could treat ballast water effectively under operational conditions. The biological efficacy test results indicate that viable organisms and bacteria levels in treated water immediately after·OH radicals treatment comply with the performance standard in Regulation D-2of the Ballast Water Convention with the initial TRO level of2.5mg/L, and no re-growth is observed over a period of5days of storage in the treated water tanks. Water qualities of ballast water are improved by·OH radicals treatment.(3) According to the requirement of the IMO G9, relevant chemicals (RCs) such as trihalomethanes (THMs), haloacetic acids (HAAs), haloacetonitriles (HANs) are analyzed to ensure that green oxidant·OH radicals follow the principle of AOT.42kinds of RCs are analyzed in treated ballast water. The results indicate that all their concentrations are lower than World Health Organization drinking water standard.The levels of the measured C-RCs generated from treatment of·OH radicals are respectively2.40and1.04times lower than those of electrolysis and ozonation. Therefore,·OH-based technology for ballast water treatment follows AOT principle, which is environmentally friendly for marine environment.(4) Quantitative risk assessment of marine environment is conducted based on the requirement of the IMO G9.The aquatic toxicity tests of three trophic levels for the treated water are carried out under internationally recognized guidelines.The ecotoxicity tests results indicate that there are no detectable toxic effects on algae, invertebrates, or fish (IrC50,IyC50, NOAEC and NOEC>100%treated ballast water). The MAMPEC model is used to compute predicted environmental concentration (PEC) of RCs in Harbor based on the information of environment, compound and emission. Predicted no effect concentration (PNEC) is estimated from acute and/or chronic toxicity data for aquatic organisms by dividing the lowest available effect concentration by appropriate assessment factors. The ratio between PEC and PNEC of the measured RCs are all less than1, indicating that the probability of ecological risk posed by treated ballast water discharged into harbor will be of little concern. And-OH radicals technology for ballast water treatment follows AOT principle and it is considered to be green, safe and environmentally friendly.In conclusion, high concentration of-OH radicals can be produced from SID in combination with impinging jet. And-OH-based technology for killing invasive alien aquatic organism via ships’ballast water is effective and environmentally friendly. It can serve as a beneficial and useful tool for preventing and controlling the marine biological invasion, protecting the biological diversity of coastal waters,and ensuring the marine ecological security of international transportation.