Preconcentration And Determination Of Trace Selenium In Citrus By Carbon Materials Derived From Citrus Pomace And Dietary Assessment Of Selenium In Citrus Fruits

Selenium?Se?is an essential micronutrient and active component of antioxidant glutathione peroxidase,which participates in various physiological and biochemical processes in vivo.The human body cannot synthesize selenium by itself.The main source of selenium in the human body is the intake of selenium in food.However,it can not be consumed excessive,which will cause selenium poisoning,so the intake of selenium must be limited to a reasonable range.Citrus is one of the most important fruit in the world,accounts for a growing proportion of people's daily diet,fruit selenium is in a low range of selenium content in all foods and is difficult to quantify,so there are few reports on the determination of selenium content and its speciation of citrus.Every year,a large number of discarded citrus peel residues are produced because of product processing,which not only causes the waste of biological resources,but also aggravates the environmental pollution to a certain extent.In this study,four materials,porous carbon nanoparticles?NPC?,magnetic porous carbon nanoparticles?Fe₃O₄@NCs?,magnetic carbon nanocomposites?Fe₃O₄@C NPs?and carbon microsphere?CMSs?,were prepared by using waste citrus pomace as carbon source.The characterization results of them were analyzed by scanning electron microscopy?SEM?,transmission electron microscope?TEM?,X-ray diffraction?XRD?,raman spectroscopy?Ramman?,fourier transform infrared spectra?FT-IR?,vibrating sample magnetometer?VSM?and thermal gravimetric analyzer?TG?.Then,the methods for the determination of total selenium,inorganic selenium and its speciation in citrus fruits were established by hydride generation-atomic fluorescence spectrometry?HG-AFS?with the above-mentioned carbon materials as adsorbents.Finally,the established methods were applied to determination the content of selenium in common citrus and selenium-enriched citrus in 10 regions of China.The primary evaluation of dietary nutrition was carried out on the basis of nutrient reference value?NRV?and dietary recommended nutrient intake?RNI?,and the health risk assessment was carried out on the amount of selenium consumed by Chinese residents from citrus fruits by using the upper tolerable limit?UTL?,the upper intake limit?UIL?and the adequate intake?AI?as the reference criteria for adult and lactating women and providing scientific information for consumers.The main results of this paper were as follows:1.NPC was an amorphous porous carbon material with abundant porous structure.Fe₃O₄@NCs were magnetic materials of iron tetroxide coated with porous carbon with an average size of 30 nm.Fe₃O₄@C NPs were magnetic carbon material with spherical structure with an average size of about 20 nm.They had superparamagnetic behavior with a high saturation magnetization.CMSs was a smooth spherical material with an average size of about 20?m.The surface of the four materials existed oxygen containing groups and had good dispersibility in aqueous solution.2.Selenium contents in citrus fruits were determinated by hydride atomic fluorescence spectrometry?HG-AFS?after preconcentrated using Fe₃O₄@NCs.The four materials were used to adsorb Se???,from the comprehensive analysis of pH,time,adsorbrnt amount and temperature,the adsorption efficiency of the four materials for Se??? was Fe₃O₄@NCs>Fe₃O₄@C NPs>NPC>CMSs,Fe₃O₄@NCs was the best adsorbent for the extraction and preconcentration of trace Se??? in citrus and the accurate determination of Se??? content by HG-AFS.It was found that Se??? was quickly adsorbed on Fe₃O₄@NCs at pH 6.This process followed the Langmuir isothermal adsorption model,and the theoretical optimal adsorption capacity was 31.25 mg/g.Then Fe₃O₄@NCs was quantitative eluted with a 3 mL mixture solution containing 3%?m/v? KBH₄ and 1%?m/v? NaOH solution prior to the determination of Se?IV?by hydride generation-atomic fluorescence spectrometry?HG-AFS?.Under the optimized conditions,the good linearity?R²>0.999?,the limit of detection?LOD=1.2 ng/L?,the limit of quantification?LOQ=4 ng/L?of Se???,low RSD?4.1%?,and high enrichment factor?50 fold?were obtained.In order to verify the accuracy of the method,standard reference materials were analyzed,and the determined values were in a good agreement to the certified values.After 5 successive adsorption cycles,recovery of>90%was still maintained for Se???.3.Single factor and response surface method were used to optimize the extraction method of inorganic selenium in selenium-enriched citrus:ultra pure water ultrasonic assisted extraction,material-liquid ratio of 23:1,ultrasonic 30 min,water bath temperature of 71?,water bath of 58 min,water extraction twice,cyclohexane extraction three times.Fe₃O₄@NCS adsorbed Se??? in the solution of inorganic selenium at pH=6.The content of inorganic selenium,organic selenium,Se??? and Se???in Se rich blood orange was determined by this characteristic.It was found that Se??? was mainly in organic state,and the content of Se??? was higher than that of Se???,and organic selenium accounted for 80.64%.4.The selenium contents of 80 samples from 10 typical regions?common regions,selenium-enriched regions and selenium-deficiency regions?were analyzed.The selenium contents of common citrus were 0.26-3.25?g/kg FW with an average content of 1.07?g/kg FW,and the variance coefficient among samples was 58.64%.The selenium contents of selenium-enriched citrus were 0.17-53.60?g/kg FW with an average content of 5.20?g/kg FW,and the coefficient of variation was 226.57%.Only 12% of the samples met the selenium-enriched standard.The highest NRV?%?of selenium were both found in citrus samples from Enshi,and the lowest were in citrus samples from Keshan.RNI?%?of common citrus and selenium-enriched citrus varied in different ages of people.The highest RNI?%?was found in the age group of 0-0.5-year-old,and the lowest was in the age group of 18-50 years old.The results showed that there was a certain selenium intake risk to adults from citrus fruit,the risk index ranged from 0.0059%to107.1097%,which mainly came from the intake of selenium-enriched citrus.The selenium intake risk to lactating women from citrus fruits was safe with a risk index of0.0057% to 32.2776%,which was less than 100%.