Assessment And Preventional Method Development On Heavy Metals In Recirculating Aquaculture System

Background: In recent years,with the improvement of living standards,consumers have become more and more strictly to their intake nutrition.The marketing requirement of aquatic productions which is rich in superior quality of proteins is increasingly incremental annually.However,the continually deteriorate eco-environment and decreased farmlands made the traditional aquaculture in a dilemma.Recirculating aquaculture system(RAS)will be one of the most promising modes with its advantages of efficiency and energy conservation as well as reducing sewage.Neverthelss,such system is characterized as closed status with polluted water resource,in which will result high risks of accumulating hazardous substances such as unperceived heavy metals.The effect of heavy metal amplification in biosome can transfer to consumers via food chains,ultimately cause potential food safety hazard.The controls of heavy metal were mainly biological or physicochemical,such as using activated sludge,submergent plant and activated carbon to treat the sewage.Although there are many reports about measures of prevention and control pollutants in RAS and waste water treatment on high concentration of heavy metals,the risk evaluation and control of heavy metals on low level in RAS was rarely reported.This project is aimed to study on typical RAS modes assessing the quality and safety risks of heavy metals.Meanwhile,European seabass was as a typical species for lasting research on accumulation and distribution.After that,optimization approaches of heavy metals prevention methods in recirculating aquaculture system in north areas were performed.Results: Inductively Coupled Plasma(ICP)and Atomic Fluorescence(AFS)were applied to detect heavy metals of five species in RAS.From the results we can conclude that contaminated grade of all elements were unpolluted with chromium ranging between unpolluted and light polluted.Pi values of other elements were under 0.1,which is graded as unpolluted.MPI values of European seabass,Bastard halibut,Giant grouper,Tongue sole and Tilapia were 0.044,0.046,0.080,0.013 and 0.087.According to the grades of heavy metal residue contamination in fishes,all species were unpolluted in this study.Then using the method mentioned above to detect the heavy metals of Dicentrarchus labrax at different growth stages and in different tissues,for exploring the elemental regularities of distribution in Dicentrarchus labrax.In terms of growth stages,the results displayed that all elements have different regularity of distribution with continuous growth.In muscle,content of arsenic increased over time with maximum value of 1.93 mg·kg-1.The results of copper and mercury were opposite,which the maximum value were 0.354 mg·kg-1 and 0.0208 mg·kg-1,respectively.In liver,the content of arsenic,cadmium and copper increased over time with maximum value of 1.47 mg·kg-1,1.09 mg·kg-1 and 143 mg·kg-1,respectively.In gill,content of total arsenic increased over time with maximum value of 0.321 mg·kg-1.Content of chromium,copper,arsenic and mercury offered upgrade firstly than descending latter tendency with the maximum value of 0.356 mg·kg-1,45.8 mg·kg-1,19.2 mg·kg-1 and 0.013 7 mg·kg-1,respectively.In terms of three tissues,the distribution of arsenic followed the order: muscle>liver>gill.Cadmium,copper and zinc followed the order: liver>gill>muscle.Mercury followed the order: muscle>liver>gill.Chromium followed the order: gill>liver>muscle.Moreover,content of all elements in muscle were under national food safety standard(GB 2762-2012)and all detected samples were unpolluted according to the grade of heavy metals residue contamination in fishes.Absorption of modified fly ash and oxidation of ozone were applied to prevent and control heavy matals.Analysis methods such as SEM(scanning electron microscope),XRD(X-ray diffraction),BET(Brunauer,Emmett and Teller)were applied to characterize FA and modified fly ash(MFA).From the results,MFA had a rougher surface and multi-layer structure after treatment.The mainly composition were changed from quartz and mullite to an unnamed zeolite(Na5.44Br0.26[AlSi4]6·4H2O).There was an increase of specific surface area and pores in MFA,which contributed to the substantial improvement of sorption capacity.Square crossing experiment was applied for optimizing heavy metal ions removal,the optimized conditions were as followed: under the normal temperature,initial concentration was 0.2 mg·L-1,absorptive time was 120 minutes and MFA dosage was 5 g·L-1.Square crossing experiment was applied for optimizing heavy metal ions removal using ozone.The flame atomic absorption spectrometer was used to analyze the iron and manganese concentration in treated solution.According to the results,the optimized conditions were as followed: under the normal temperature,MFA dosage was 0 g,initial pH was 7,ozone oxidized time was 15 minutes and ozone oxidized concentration was 15 mg·L-1.Conclusion: Heavy metals in RAS were evaluated and related data provided resource for risk evaluation in RAS.Two methods,absorption of modified fly ash and oxidation of ozone,were developed to prevent and controll low level heavy metal water,which will benefit the subsequent efforts on risk evaluation about RAS and further exploration on prevention and control of heavy metals.