徐白露, 張 喚, 楊周生
(安徽師范大學(xué) 環(huán)境科學(xué)與工程學(xué)院,安徽 蕪湖 241000)
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納米氧化銅對硝基苯酚的電化學(xué)氧化及降解作用
徐白露,張喚,楊周生
(安徽師范大學(xué) 環(huán)境科學(xué)與工程學(xué)院,安徽 蕪湖241000)
摘要:本文通過電化學(xué)方法探討了納米氧化銅修飾電極對硝基苯酚的電化學(xué)氧化及降解作用.考察了在不同pH值和修飾材料存在不同量度條件下,對硝基苯酚的電化學(xué)行為影響.結(jié)果顯示:納米氧化銅材料能明顯地提高硝基苯酚在電極上氧化還原反應(yīng)的可逆性,ΔE由在裸電極的0.862V降低為0.291V,氧化還原峰電流顯著增加.使用鈦基納米氧化銅材料構(gòu)建的電極,進(jìn)行對硝基苯酚的電化學(xué)氧化,通過紫外可見光譜、高效液相色譜對電解后的溶液檢測,結(jié)果表明:納米氧化銅對硝基苯酚具有很好的催化降解作用.
關(guān)鍵詞:納米氧化銅;對硝基苯酚;催化降解
硝基苯酚類化合物是難降解的環(huán)境污染物質(zhì)之一.研究其電化學(xué)反應(yīng)機理不僅對硝基苯酚類化合物的降解具有意義,而且對于其在人體中的生物轉(zhuǎn)化亦具有參考價值.目前,對于硝基苯酚類化合物通過探討電化學(xué)氧化還原反應(yīng)性質(zhì),研究其在環(huán)境中的轉(zhuǎn)化規(guī)律和尋求解決方法[1].
金屬氧化物納米材料因其應(yīng)用廣泛更是受到人們的關(guān)注.具有尖晶石結(jié)構(gòu)的四氧化三鐵在磁制冷、磁流體和存儲信息等方面有很大的價值[2].氧化銅是一種p型的半導(dǎo)體材料,頻帶間隙為1.2電子伏特[3].納米氧化銅是一種很好的光敏材料,其被廣泛的應(yīng)用于光、電、半導(dǎo)體及電極材料等方面[4].納米級的氧化銅在催化劑方面也有很大的應(yīng)用前景[5].本文探討了對硝基苯酚在納米氧化銅材料修飾電極上的電化學(xué)行為,優(yōu)化了實驗條件,使用該材料對硝基苯酚實施電化學(xué)降解進(jìn)行了探討,取得了較好的降解效果.
1實驗部分
1.1儀器及試劑
CHI660D電化學(xué)工作站,pHS-3C型酸度計,DJS-292B恒電位儀,78-1磁力加熱攪拌器,水熱反應(yīng)釜,DHG-9053型電熱恒溫鼓風(fēng)干燥箱,HC-2062型高速離心機,玻碳電極及修飾電極為工作電極,鉑絲為對電極,Ag/AgCl電極為參比電極,LC20AT型高效液相色譜儀,UV757CRT型紫外分光光度計.氯化銅,硫堇,磷酸二氫鉀,對硝基苯酚均為分析純.
1.2修飾材料及電極的制備
納米氧化銅是根據(jù)文獻(xiàn)[6]的制備方法獲得.修飾電極的制備過程為:將裸玻碳電極在粒徑為0.05um氧化鋁懸浮液中拋光至鏡面,在二次水和無水乙醇中超聲清洗各5分鐘;將玻碳電極置于含5mM硫堇的0.1M磷酸緩沖溶液(pH=6)中,在電位為-0.5~0.5V范圍內(nèi)進(jìn)行循環(huán)伏安掃圖,掃速為50mV/s,掃描40圈,電聚合完成后,將電極用蒸餾水沖洗干凈,室溫晾干,得到聚硫堇修飾電極(PTH/GCE);將5mg納米氧化銅分散于1.0ml的蒸餾水中,超聲分散10分鐘,然后取10uL分散液滴涂在硫堇/玻碳修飾電極上,室溫晾干,即得納米氧化銅/硫堇/玻碳修飾電極(CuO/PTH/GCE).
1.3對硝基苯酚電化學(xué)行為及降解實驗
配制濃度為1.0×10-4mol/L對硝基苯酚溶液,緩沖溶液為0.1M的磷酸緩沖溶液,以鉑絲電極為對電極、Ag/AgCl電極為參比電極,納米氧化銅/硫堇/玻碳電極為工作電極的三電極體系中進(jìn)行循環(huán)伏安實驗.使用鈦基納米氧化銅電極作陽極,恒電流電解濃度為10mg/mL含有0.1M硫酸鈉的對硝基苯酚溶液,在電解經(jīng)過不同時間,取電解液進(jìn)行紫外可見光譜、高效液相色譜檢測.
2結(jié)果與討論
2.1納米材料表征分析
獲得的氧化銅納米材料粒子尺寸可用SEM來表征(圖1a).由圖可得,氧化銅納米材料是形狀均一的梭形結(jié)構(gòu).聚硫堇電聚合在玻碳電極表面,形成一個網(wǎng)狀的聚合物結(jié)構(gòu)(圖1b),在這些網(wǎng)狀空隙中,氧化銅納米粒子能夠被固定其中.梭形氧化銅被滴涂到PTH/GCE表面,被固定在網(wǎng)狀結(jié)構(gòu)中,同時聚硫堇分子中的巰基能夠和氧化銅之間可以形成Cu-S化學(xué)鍵[7-8],從而使氧化銅被固定在PTH的網(wǎng)狀中效果更好.
圖1 修飾電極表面的掃描電鏡表征圖Fig.1 SEM images of the as-prepared sample CuO(a), PTH(b)
圖2 不同修飾電極的電化學(xué)阻抗圖Fig.2 EIS of different electrodes in 0.1 M KCl solution containing5.0mM Fe(CN)6]3-/[Fe(CN)6]4-:GCE(a),PTH/GCE(b),CuO/GCE(c), CuO/PTH/GCE(d)
為了考察不同電極的修飾情況,電化學(xué)阻抗曲線被用來表征這種變化.圖2 為不同修飾電極的電化學(xué)阻抗圖.由圖2可知,在含0.1M KCl的[Fe(CN)6]3-/[Fe(CN)6]4-溶液中,GCE(曲線a)表現(xiàn)出的等效電阻值為300±10Ω,PTH/GCE(曲線b)修飾電極的電阻為310±10Ω,CuO/GCE(曲線c)為1250±10Ω,CuO/ PTH/GCE(曲線d)為1720±10Ω.硫堇/玻碳修飾電極與裸電極的電阻數(shù)據(jù)變化不大,這是由于聚硫堇具有很好的導(dǎo)電性所致.而在納米CuO/GCE,由于CuO導(dǎo)電能力比GCE要弱得多,其修飾電極的阻抗變化大.這些變化表明:PTH、CuO被成功修飾到了玻碳電極表面.
2.2對硝基苯酚的電化學(xué)行為
為了考察對硝基苯酚在納米氧化銅上的電化學(xué)性能,本工作分別以不同電極作為工作電極進(jìn)行循環(huán)伏安實驗.圖3為對硝基苯酚在不同的工作電極上的循環(huán)伏安曲線.以GCE作為工作電極時,一對較弱的氧化還原峰電位出現(xiàn)在0.239V和-0.623V,在氧化峰電流是2.25uA,在還原峰電流是8.86uA(曲線a);以氧化銅/硫堇/玻碳修飾電極為工作電極時,在-0.083V出現(xiàn)氧化峰,在-0.214V出現(xiàn)還原峰,氧化峰電流是237.49uA,還原峰的峰電流是597.63uA(曲線c).通過比較發(fā)現(xiàn),氧化還原峰電流明顯增大,電位差值由原來的0.862V縮小為0.291V,反應(yīng)的可逆性增加.這說明納米氧化銅對對硝基苯酚電化學(xué)氧化還原具有明顯的催化效果.而0.083V處的氧化峰和-0.214V處的還原峰對應(yīng)的是酚羥基的氧化還原反應(yīng)[9],-0.579V處的還原峰可能是硝基的還原反應(yīng).
圖3 不同電極對對硝基苯酚電化學(xué)反應(yīng)催化效果圖Fig.3 The catalytic efficiency of electrochemical response of p-nitrophenol on ent modified electrode: bare GCE(a),CuO/GCE(b),CuO/PTH/GCE(c)
圖4 硝基苯酚在CuO/PTH/GCE的電化學(xué)響應(yīng)對pH值變化Fig.4 The effects of solution pH on at electrochemical differ response of p-nitrophenol on CuO/PTH/GCE
2.3影響對硝基苯酚電化學(xué)行為的因素
2.3.1pH值的影響:配制濃度為1×10-4mol/L對硝基苯酚溶液,緩沖溶液為0.1M的磷酸緩沖溶液,設(shè)置pH梯度為:4、5、6、7、8、9,以納米氧化銅/硫堇/玻碳電極為工作電極,獲得不同pH條件下的循環(huán)伏安曲線.如圖4所示,取氧化峰的峰電流進(jìn)行比較發(fā)現(xiàn):在pH=6時,氧化峰電流達(dá)到最大.在酸性條件下,體系中質(zhì)子濃度較大,阻礙了酚羥基氧化反應(yīng)中的失電子過程[10].隨著pH值的增大,氧化峰的峰電流持續(xù)減小,這是由于電極在堿性溶液中不穩(wěn)定[8],造成了納米氧化銅催化能力的降低.
2.3.2納米材料厚度的影響:在最佳的pH條件下,考察了納米材料厚度對硝基苯酚電化學(xué)行為的影響,獲得的實驗結(jié)果如圖5所示.實驗選擇納米氧化銅材料的濃度為:1mg/ml、2.5mg/ml、5mg/ml、7.5mg/ml、10mg/ml,分別取各濃度的材料10uL滴涂在3mm直徑的玻碳電極上,修飾成不同厚度的電極進(jìn)行試驗比較.為了探討材料厚度與對硝基苯酚電化學(xué)行為之間的關(guān)系,取氧化峰的電流進(jìn)行比較,由圖5可知:該處的峰電流值的隨著材料厚度的增加而增大,在修飾厚度為0.708mg/cm2時,電流值達(dá)到最大,隨著厚度進(jìn)一步增加,峰電流隨之減小.這是因為隨著材料厚度的增加,酚羥基在電極表面催化反應(yīng)的活性位點增加[11-12],納米氧化銅對硝基苯酚的催化能力增強,從而使得相應(yīng)的電流值增大;而當(dāng)材料厚度繼續(xù)增加時,阻礙了電子在電極表面的傳遞,使得氧化峰電流減小[13].
圖5 納米氧化銅材料厚度對硝基苯酚氧化峰電流的影響Fig.5 The effects of thickness of material on peak current at electrochemical degradation in different time
圖6 電解不同時間后溶液的紫外吸收光譜變化Fig.6 Change of spectrum of p-nitrophenol after electrochemical reaction of p-nitrophenol by CuO/PTH/GCE
2.4納米氧化銅對對硝基苯酚催化降解
以鈦板(3.0×1.5×0.1cm)為基電極,根據(jù)文獻(xiàn)[14]制備方法制得CuO/Ti電極.本研究采用二電極隔膜電解槽,以CuO/Ti電極為陽極,Pt電極為陰極,對濃度為10mg/L含有0.1M硫酸鈉的對硝基苯酚溶液(pH=6)進(jìn)行電化學(xué)降解.實驗在DJS-292B恒電位儀上進(jìn)行,對溶液持續(xù)通過恒電流30mA來進(jìn)行降解,實驗中分別取不同時間段的樣品進(jìn)行紫外光譜和液相色譜的測定.
圖7 電解不同時間后對硝基苯酚的高效液相色譜圖的變化Fig.7 Change of HPLC of p-nitrophenol after electrochemicaldegradation in different time
如圖6所示,從UV圖譜中可以觀察到,電解之前的對硝基苯酚存在明顯的3個吸收峰,分別是258nm處的吸收峰(1)、359nm處的吸收峰(2)和560nm處的吸收峰(3),在降解一段時間后,峰1和峰2的吸光度減小,而峰3吸光度增加.這一結(jié)果表明隨著電化學(xué)降解的進(jìn)行,對硝基苯酚的濃度逐漸降低,同時可以觀察到電解溶液由開始的微黃色逐漸轉(zhuǎn)變?yōu)榻瘘S色,這可能是由于醌類物質(zhì)的產(chǎn)生所致[15-16].由此說明在降解實驗中,對硝基苯酚的分子結(jié)構(gòu)發(fā)生變化,在溶液中的含量逐漸降低,能夠被成功降解.
從HPLC圖譜中可以觀察到(如圖7所示),未降解的對硝基苯酚在3.42min和3.92min出現(xiàn)兩個色譜峰,隨著電化學(xué)降解的持續(xù)進(jìn)行,保留時間為3.42min處的色譜峰逐漸增大,而保留時間為3.92min 處的色譜峰逐漸減小.降解前后色譜圖上的這些變化與紫外吸收光譜的變化相一致,進(jìn)一步表明:在電解過程中,對硝基苯酚的分子結(jié)構(gòu)發(fā)生變化,在溶液中的含量逐漸降低,能夠被成功降解.而降解后生成的物質(zhì)有待進(jìn)一步研究確認(rèn).
結(jié)論
本論文通過使用循環(huán)伏安法研究了納米氧化銅對硝基苯酚的電化學(xué)氧化還原的催化作用,同時提高了對硝基苯酚氧化還原反應(yīng)的可逆性;納米氧化銅能夠使得對硝基苯酚被有效的催化降解.
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Abstract: The composition of volatiles emitted from Newhall nucellar navel oranges were investigated during laboratory-controlled anaerobic storage for a period of 90 days, using preconcentrator coupled with gas chromatography-mass spectrum (GC-MS). Major relative changes occurred for oxygenated volatiles and monoterpene hydrocarbons, while no observable changes were found for sulfides. Before storage, terpenoid hydrocarbons and oxygenated volatiles dominated, with the most abundance of limonene, ethanol, β-myrcene, sabinene, α-pinene, acetaldehyde and Δ-3-carene. During the early 6 days of storage, terpenoid hydrocarbons decreased sharply as much loss of limonene, β-myrcene, sabinene, α-pinene and Δ-3-carene, while oxygenated volatiles increased abruptly and became the first predominant class, with observable growing of methanol, ethanol and ethyl acetate. After 6 days of storage, terpenoid hydrocarbons rose up progressively with storage time, whereas oxygenated volatiles dropped down gradually until the end of the experiment. It is worth noting that twenty-four oxygenated volatiles as artifacts were formed, with predominance of 2-butanol and methyl acetate.
Key words: volatiles; compositions; newhall nucellar navel oranges; anaerobic storage
Clssification No: TS207.3Document Code: APaper No:1001-2443(2016)04-0364-07
Newhall nucellar navel oranges (Citrus sinensis (L.) Osbeck) originating as a limb sport of Washington navel oranges are native to USA and now widely planted in middle part of China. The fruit of Newhall is oval-shaped, 6.48-7.18cm in long and 6.65-7.71cm in diameter, and it weights approximately from 250 to 350 g for each orange, containing 15.5% for soluble solids, 85-105g L-1for sugars, 10-11g L-1for titratable acids and 0.47-0.64g L-1for vitamin C, respectively[1].
Newhall nucellar navel oranges are the most popular navel orange cultivar in China because of their bright color, rich nutrition, sweet flavor and pleasant smelling. The attractive and pleasant flavor of citrus fruit is attributed to a combination of alcohols, aldehydes, ketones, esters, hydrocarbon terpenes, sulfur volatiles and so on in specific proportions[2-4]. For Newhall nucellar navel oranges, forty-two volaitle organic compounds including twenty-seven oxygenated volatiles, thirteen terpene hydrocarbons and three sulfides were identified and quantified in the gases from their fresh fruit in previous studies[5].
During their commercial packing and storage, citrus fruit not excepting Newhall nucellar navel oranges are usually exposed under various anaerobic conditions such as storing in modified or controlled atmospheres, coating with waxes or films, packing in plastic liners and holding in containers or trailers[6,7,4]. These anaerobic or
Received date:2015-10-21
Author’s brief:YANG Geng(1968-),F(xiàn)emale,born in Tongcheng,Anhui Province, senior experimentalist.
引用格式:楊耿,張玉潔,劉書路,等.紐荷爾臍橙(Citrus sinensis (L.) Osbeck)厭氧保存過程中揮發(fā)性風(fēng)味物質(zhì)組成變化[J].安徽師范大學(xué)學(xué)報:自然科學(xué)版,2016,39(4):364-370.
anoxia storage and handing process can inhibit deterioration development of fruit, but anaerobic metabolism is induced, leading to decreases in aroma-active volatiles and accumulation of off-flavor volatiles such as ethanol, acetaldehyde and ethyl acetate[8,9,6,4]. The composition changes of volatiles from citrus fruit could lead to an altered balance of orange aroma away from what is considered “fresh” or desirable and thus becoming “rotten”[2-4]. Thus, measurement of the composition changes of volatiles from citrus fruit based on relative percentage may serve reliable and convenient information for quality evaluation.
Fresh Newhall nucellar navel oranges maintain their external and internal quality in regular atmospheres for only 2-3 weeks after harvest, and thus anaerobic or anoxia storage is often needed when they are to be kept for longer than 3 weeks after harvest. Consequently, in this study Newhall nucellar navel oranges were incubated under N2 atmospheres to stimulate anaerobic respiration in laboratory, and the relative compositional changes of volatiles and the possible artifacts of Newhall nucellar navel oranges were investigated during storage under anaerobic conditions for a period of 90 days.
1Materials and Methods
1.1Materials and chemicals
Fresh ripe Newhall nucellar navel oranges (Citrus sinensis (L.) Osbeck) were obtained from a commercial orchard in the city of Ganzhou in China in November 2012. Fruits at ripe stage were classified as those possessing totally orange-yellow color. Fruits were carefully selected for uniformity size, color, and absence of physical damage, and were randomly divided into three groups for the anaerobic treatment.
All volatiles listed in Table 1 were purchased from Sigma-Aldrich Inc. (Saint Louis, MO, USA) and were of analytical grade.
Table 1 Compositional changes of volatiles emitted from Newhall nucellar navel oranges during the anaerobic storagea
續(xù)表1
ChemicalsCompositions(%)b0(fresh)c361020304050607080903-heptanone*ndtrtrtrtrtrtrtrtrtrtrtr2,3-butanedionetr0.20.30.60.20.1trtrtrtrtrtr3-hydroxy-2-butanone*ndndndnd0.1ndndndndndndndEstersmethylformate*ndtrtrtrtr0.10.10.10.10.10.1trethylformatetrtr0.1tr0.20.40.30.20.40.20.20.11-methylpropylformate*ndtrtrtrtrtrtrtrtrtrtrtr3-methylbutylformate*ndndndndndtrtrtrtrtrtrtrmethylacetatetr0.70.80.91.21.21.61.81.81.71.31.1ethylacetate0.59.010.810.07.26.45.24.23.94.23.83.4propylacetatetrtrtrtr0.20.20.40.40.50.40.50.4butylacetatetr0.10.1trtrtrtrtrtrtrtrtr1-methylpropylacetate*ndndndtr0.20.81.11.21.31.00.90.72-methylpropylacetate*ndtr0.10.10.10.10.10.10.10.10.10.12-methylbutylacetate*ndtr0.1trtrtrtr0.10.10.10.1tr3-methylbutylacetate*nd0.30.60.30.10.20.10.20.20.10.20.13-methyl-2-butenylacetate*ndndtrtrtrtrtrtrtrtrtrtrmethylpropionate*ndndndndnd0.10.10.20.20.20.20.1ethylpropionate*ndtrtrtr0.30.40.50.50.70.40.40.4propylpropionate*ndndndndndtrtrtrtrtrtrtr1-methylpropylpropoinate*ndndndndndtr0.10.10.20.10.10.1methylbutyratetrtrndndndndndndndtrtrtrethylbutyrate0.10.10.00.00.00.10.10.10.10.10.10.1methylisobutyrate*ndndndndndndndndndtrtrtrethylvalerate*ndndndndndtrtrtrtrtrtrtrmethylisovalerate*ndndndndndndtrndtrtrtrtrmethylhexanionatetrtrtrtrtrtrtrtrtrtrtrtrAcetals1,1’-diethoxy-ethane*ndtrtrtrtrtrtrtrtrtrtrtr2,4,6-trimethyl-1,3,5-trioxane0.2tr0.10.10.10.10.10.10.2trtrtrtotaloxygenatedvolatilesd18.174.580.277.372.371.272.071.270.859.459.649.0Terpenoidhydrocarbonsisoprenetrtrtrtrtrtrtrtrtrtrtrtrα-thujene0.1trtrtr0.10.10.10.10.10.10.20.1camphenetrtrtrtrtrtrtrtrtrtrtrtrβ-pinene0.60.10.10.10.30.30.30.30.30.40.50.4α-terpinenetrtrtr0.10.30.40.40.40.40.40.70.5l-phellandrene0.90.20.10.30.60.70.60.60.70.70.90.7terpinolene0.80.40.30.50.40.70.70.60.70.40.50.6
aTable includes all identified chemicals and percentages add to 100% for each sample.bMean percentage of individual volatile constituents from triplicate experiments.cDays of incubation.dSum of alcohols, aldehydes, ketones, esters and acetals.*Artifact volatiles. nd, not detected. tr, trace (<0.05%).
1.2Anaerobic treatment
For laboratory simulation study, about 2 kg fresh shredded Newhall nucellar navel oranges were weighed and placed in glass reactors with a volume of approximately 8 L. Treatments were tested in triplicate and incubated at room temperature (25±0.5℃) for 90 days. Pure N2gas was maintained between 40-60mL min-1per reactor, which was identified in preliminary work as sufficient to ensure that the O2containers were less than 0.5% for the anaerobic storage. When sampling, 1L Teflon sampling bags (SKC Inc., USA) was used to collect gas from the air outlet of each reactor. Volatile measurements were performed on days 0, 3, 6, 10, 20, 30, 40, 50, 60, 70, 80 and 90 during the incubation.
1.3Volatiles analysis
Volatiles were analyzed by an Entech Model 7100 Preconcentrator (Entech Instruments Inc., CA, USA) coupled with a gas chromatography/mass spectrum (GC/MS, Agilent 5973N). Detailed analysis steps were described elsewhere[10].
1.4Statistical Analysis
Statistical analysis was performed using SPSS 10.0 for Windows. A one-way ANOVA was performed to test the significant variance between the samples. A post hoc examination was conducted to test the significance using the LSD test. The significance level was set as p<0.05.
2Results and Discussion
2.1General
Fig.1 Typical chromatograms showing selected volatiles from fresh Newhall nucellar navel oranges at day 0 when fresh(A) and at day 50(B).
As presented in Fig.1, differences between the chromatogram of the fresh oranges and that of the oranges at day 50 were noticeable. Several new peaks of artifact compounds occurred at day 50. The peak identities and their relative percentages, and the artifacts identified, are listed in Table 1 according to functional classes. In total, sixty-seven volatiles were identified, among which twenty-four volatile chemicals were absent in gases of fresh Newhall nucellar navel oranges and occurred as artifacts in the following storage process, consisting of 3 alcohols(2-butanol, 2-pentanol and 2-methyl-3-buten-2-ol), 3 aldehydes(2-methypropanal, 2-methylbutanal and pentanal), 2 ketones(3-heptanone and 3-hydroxy-2-butanone), 15 esters(methyl formate, 1-methylpropyl formate, 3-methylbutyl formate, 1-methylpropyl acetate, 2-methylpropyl acetate, 2-methylbutyl acetate, 3-methylbutyl acetate, 3-methyl-2-butenyl acetate, methyl propionate, ethyl propionate, propyl propionate, 1-methylpropyl propionate, methyl isobutyrate, ethyl valerate and methyl isovalerate) and 1 acetals (1,1’-diethoxy-ethane) (Table 1 and Fig.1(B)). The concentration of total volatile chemicals gradually rose up to 2729.1μg L-1upon 90 days of storage, approximately 5 times higher than that at day 0(558.8 μg L-1)(Fig.2), and the percentage of total artifact volatiles increased with storage time and attained the peak at day 70, sharing 15.9% of total volatile chemicals released (Table 1). For volatile groups or individual volatile chemicals, their compositions changed significantly during the anaerobic storage of Newhall nucellar navel oranges.
The numbered peaks indicate compounds: 1 acetaldehyde; 2 methanol; 3 ethanol; 4 methyl acetate; 5 2-propanol; 6 2-methyl-propanal; 7 1-propanol; 8 2-butanone; 9 ethyl acetate; 10 2-butanol; 11 2-methyl-1-propanol; 12 2-methyl-butanal; 13 1-butanol; 14 ethyl propionate; 15 propyl acetate;16 3-methyl-1-butanol; 17 2-methyl-1-butanol; 18 1-methylpropyl acetate; 19 2-methylpropyl acetate; 20 2-methyl-3-buten-2-ol; 21 2,4,6-trimethyl-1,3,5-trioxane; 22 ethyl butyrate; 23 butyl acetate; 24 3-hexen-1-ol; 25 1-hexanol; 26 3-methylbutyl acetate; 27 2-methylbutyl acetate; 28 α-thujene; 29 α-pinene; 30 camphene; 31 sabinene; 32 β-pinene; 33 β-myrcene; 34 l-phellandrene; 35 Δ-3-carene; 36 α-terpinene; 37 limonene; 38 γ-terpinene; 39 terpinolene.
Fig.2 Concentrations of three volatile groups and total volatile compounds released from Newhall nucellar navel oranges during the anaerobic storage. Error bars represent the standard deviation
2.2Change in oxygenated volatiles
Fifty-one oxygenated volatiles were determined during the anaerobic storage of Newhall nucellar navel oranges(Table 1), and the concentration of total oxygenated volatiles increased rapidly to attain the maximum(1924.2μg L-1) at day 50, about 18 times higher than that at day 0(101.7 μg L-1)(Fig.2). Methanol, ethanol, 2-butanol, acetaldehyde, 2-butanone and ethyl acetate dominated and were the most important oxygenated volatiles. For the total oxygenated volatiles or major oxygenated compounds except acetaldehyde, their relative percentages showed a significant increasing trend during the anaerobic incubation. The relative percentage of total oxygenated volatiles increase from 18.1% to 49.0% upon 90 days of anaerobic storage, attaining the maximum (80.2%) at day 6. Also, oxygenated compounds were the most predominant function group after 3 days, although they were less than terpenes to some extent again after 90 days. The observably growing oxygenated volatiles were methanol (from 0.8% to 5.9%), ethanol (from 13.9% to 15.6%), 2-butanol (from 0.0% to 11.1%), 2-butanone (from>0.05% to 8.1%) and ethyl acetate (from 0.5% to 3.4%). Particular for ethanol, its ratio reached the peak (59.0%) at day 3 and it became the first abundant volatile chemicals. 2-Butanol and ethyl acetate were the third abundant volatile chemicals at day 40 and at day 3, respectively. For acetaldehyde, it decreased steadily to 0.1% at day 90, but its concentration actually increased and reached a peak at day 50 during the anaerobic storage.
The relative changes of the oxygenated compounds from Newhall nucellar navel oranges on the anaerobic storage were in accordance with reports on commercial packing and storage of navel oranges[7]. The considerable enhancements of the oxygenated compounds could be related to secondary metabolites of orange substrates from biochemical reaction caused by enzymes[11]or microorganisms[12,13]. The production of alcohols, aldehydes, ketones and esters from fruit under anoxic or anaerobic conditions had been reported[9,6]. For example, methanol, ethanol, acetaldehyde and ethyl acetate as anaerobic metabolites were reported to be strongly accumulated in fruit such as mandarin[14], grapefruit[14]and pear[15]on storage under conditions favoring anaerobiosis. As well known, ethanol as major component of wines is produced from anaerobic fermentation of substrates, and its biosynthesis in fruit such as apples enhanced at greater rate under hypoxic or anoxic storage conditions[9]. 2-Butanol as undirable constituent was found in spirits of grape pomace, which fermented under anaerobic conditions[16]. 2-Butanone had been reported in gases purged and trapped from cherry fruit homogenates after storage under controlled atmosphere (anoxic condition)[8]. The production of esters in fruit could be attributed to esterification of various alcohol moieties and acetyl CoA[9]. Actually, a good correlation were between total alcohols and total esters (r=0.77,p<0.01), particularly between ethanol and ethyl acetate (r=0.90,p<0.01). As also reported by[8], qualitative and quantitative changes in ester production, particularly ethyl acetate, were coincident with the accumulation of ethanol. Acetals were usually produced during the anaerobic fermentation of fruit and grain. For example, 1,1’-diethoxy-ethane was present in grape wine[17]and Chinese ‘Yanghe Daqu’ liquor, which was made from the anaerobic fermentation of grains[18].
2.3Change in isoprene and monoterpenes
As shown in Table 1, isoprene was merely a trivial constituent detected in emitted volatiles and was under 0.05% during the whole anaerobic storage. β-Myrcene, sabinene, α-pinene and Δ-3-carene were the major monoterpenes in addition to limonene during the anaerobic storage of Newhall nucellar navel oranges. During the early 6 days, the concentration of total monoterpenes decreased sharply from 457.1μg L-1to 288.6μg L-1(Fig.2). Also, the relative percentages of total monoterpenes decreased abruptly from 81.9% to 19.8%, as much loss of limonene (from 54.9% to 15.8%), β-myrcene (from 10.8% to 1.8%), sabinene (from 6.6% to 0.8%), α-pinene (from 5.0% to 0.4%) and Δ-3-carene (from 2.0% to 0.3%). Some slight dropping also occurred for α-thujene (from 0.1% to <0.05%), β-pinene (from 0.6% to 0.1%), l-phellandrene (from 0.9% to 0.1%) and terpinolene (from 0.8% to 0.3%). The decrease of monoterpene hydrocarbons from Newhall nucellar navel oranges during the early stage was in accordance with reports on storage of some citrus fruit oil such as Yuzu[19], and Daidai[20]. The monoterpene hydrocarbons can be lost through chemical process and/or physical process. Chemical degradation such as polymerization, oxidation and rearrangement of monoterpene was reported by[19]. For example, limonene could be oxidized to cis- and trans-limonene oxides as artifacts in the citrus fruit oil[12]and also be cyclized to camphene, α-pinene and β-pinene as previously noted[21]. Mycrene could be cyclized to γ-terpinene, α-terpinene, limonene and terpinolene[21]. This process would imply an increase of some monterpenes such as α-pinene and β-pinene and terpinolene, which was inconsistent with the data in this study. For this reason, we assume that this process can be neglible for the loss of monoterpenes. Physical evaporation of inherited constituents could be mainly responsible for the observed relative decrease of monnoterpnes. Oranges as a pool of monoterpenes were shredded before incubation, making these compounds not to be locked in clumps but volatilize rapidly due to increased surface area. For camphene, α-terpinene and γ-terpinene, their percentages were merely trivial and kept steady during the early 6 days of anaerobic incubation, but their concentrations had a decreasing trend.
After the early 6 days of incubation, the concentration of total terpenes increased progressively to reach the peak upon 90 days (1391.9μg L-1), about three times higher than that at day 0 (457.1μg L-1) (Fig.2). The relative percentages of total monoterpenes and all monoterpene hydrocarbons except camphene also grew steadily until the end of the experiment. Monoterpenes shared 51.0% of total volatiles released and became the prevailed class upon 90 day again. The preminently enhanced monoterpenes were limonene (from 15.8% to 36.0%), β-myrcene (from 1.8% to 6.6%), α-pinene (from 0.4% to 2.9%) and Δ-3-carene (from 0.3% to 1.4%). Some slight growing also occurred for α-thujene (from <0.05% to 0.1%), β-pinene (from 0.1% to 0.4%), l-phellandrene (from 0.1 to 0.7%), α-terpinene (from <0.05 to 0.5%), γ-terpinene (from 0.2% to 0.8%), terpinolene (from 0.3% to 0.6%) and sabinene (from 0.8% to 1.0%). For camphene, its percentage still kept trivial and steady. The results indicated that monoterpenes emitted after 6 days were mainly secondary production, most probably the microbial degradation of pectin, which had high contents in oranges and would emit a high rate of monoterpenes when biologically metabolized.
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紐荷爾臍橙(Citrus sinensis (L.) Osbeck)厭氧保存過程中揮發(fā)性風(fēng)味物質(zhì)組成變化
楊耿,張玉潔,劉書路,于越剛
(安徽師范大學(xué) 環(huán)境科學(xué)與工程學(xué)院,安徽 蕪湖241003)
摘要:采用預(yù)濃縮系統(tǒng)與氣相色譜質(zhì)譜聯(lián)用技術(shù)分析檢測紐荷爾臍橙在實驗室控制厭氧條件下保存90天過程中釋放出來的揮發(fā)性風(fēng)味物質(zhì)組成變化.結(jié)果表明,紐荷爾臍橙厭氧保存過程中含氧化合物和萜烯化合物兩類揮發(fā)性風(fēng)味物質(zhì)組成比例變化明顯,含硫化合物變化不明顯.在尚未保存前,紐荷爾臍橙釋放出來的揮發(fā)性風(fēng)味物質(zhì)主要是含氧化合物和萜烯化合物兩類化合物,其中檸檬烯、乙醇、β-月桂烯、檜烯、α-蒎烯,乙醛和蒈烯是最主要的成分.在保存的前6天,由于檸檬烯、β-月桂烯、檜烯、α-蒎烯和蒈烯5種化合物大量減少導(dǎo)致萜烯類化合物比例隨時間急劇下降,同時由于甲醇、乙醇和乙酸乙酯3種化合物大量增加使得含氧化合物比例隨時間急劇升高,成為最主要的揮發(fā)性風(fēng)味物質(zhì).在保存6天以后到實驗結(jié)束,萜烯類化合物比例隨時間逐漸增高,而含氧化合物比列隨時間逐漸降低.特別值得注意的是紐荷爾臍橙保存過程中有24種含氧化合物是新生成的,其中最主要是2-丁醇和乙酸甲酯.
關(guān)鍵詞:揮發(fā)性風(fēng)味物質(zhì);紐荷爾臍橙;厭氧保存
DOI:10.14182/J.cnki.1001-2443.2016.04.009 10.14182/J.cnki.1001-2443.2016.04.011
收稿日期:2015-11-20
基金項目:國家自然科學(xué)基金(20775002).
作者簡介:徐白露(1991-),女,安徽繁昌縣人,碩士生;通訊作者:楊周生(1963-),安徽桐城人,教授.
中圖分類號:X132
文獻(xiàn)標(biāo)志碼:A
文章編號:1001-2443(2016)04-355-04
Foundation item:Sponsored by National Natural Science Foundation of China(41273095 and 41103067).
Electrochemical Behavior of P-Nitrophenol on Nano Copper Oxide Electrode and Its Degradation
XU Bai-lu,ZHANG Huan,YANG Zhou-sheng
(College of Environmental Science and Engineering, Anhui Normal University, Wuhu 241000, China)
Abstract:Electrochemical behavior of p-nitrophenol on nano copper oxide electrode and its degradation were discussed. The effects of solution pH and thickness of material on electrochemical response of p-nitrophenol were examined. The results show that the difference (ΔE) of peak potential changed from 0.862V on bare GCE to 0.291V on CuO/PTH/GCE and the oxidation peak current increased obviously. Nano copper oxide could improve redox reversibility p-nitrophenol on electrode significantly. Electrochemical oxidation of p-nitrophenol was performed with nano CuO/Ti electrode and electrolyte solution was tested by UV-visible spectra and high performance liquid chromatography, respectively. It illustrated the nano copper oxide could catalyze degradation of p-nitrophenol.
Key words:nano copper oxide; p-nitrophenol; catalyses degradation
Changes in the Composition of Volatiles Emitted from Newhall Nucellar Navel Oranges (Citrus Sinensis (L.) Osbeck)During Anaerobic Storage
YANG Geng,ZHANG Yu-jie,LIU Shu-lu,YU Yue-gang
(College of Environmental Sciences and Engineering, Anhui Normal University, Wuhu 241003, China)
引用格式:徐白露,張喚,楊周生.納米氧化銅對硝基苯酚的電化學(xué)氧化及降解作用[J].安徽師范大學(xué)學(xué)報:自然科學(xué)版,2016,39(4):355-358.