楊培增,岳泓伸,季躍飛,陸雋鶴

土壤銨氮在熱活化過硫酸鹽氧化過程中的轉(zhuǎn)化

楊培增,岳泓伸,季躍飛,陸雋鶴*

(南京農(nóng)業(yè)大學(xué)資源與環(huán)境科學(xué)學(xué)院,江蘇 南京 210095)

采用來自江蘇和河北,具有不同土壤有機(jī)質(zhì)含量和NH4+濃度的土壤樣本,系統(tǒng)地研究了NH4+在熱活化過硫酸鹽(PS)氧化過程中的轉(zhuǎn)化和歸趨,考察了反應(yīng)時(shí)間、PS濃度和外加NH4+對(duì)硝基副產(chǎn)物生成的影響.結(jié)果表明,土壤中的NH4+能夠轉(zhuǎn)化成3-硝基酚、4-硝基酚、2-羥基-5-硝基苯甲酸、4-羥基-3-硝基苯甲酸、2,4-二硝基酚等副產(chǎn)物,它們的生成量隨著反應(yīng)的進(jìn)行先增加后降低.增大PS濃度可促進(jìn)硝基副產(chǎn)物的生成.當(dāng)PS濃度為30mmol/kg,反應(yīng)12h后一硝基酚和一硝基羥基苯甲酸的生成量達(dá)到最大.然而隨著PS濃度進(jìn)一步增大,硝基副產(chǎn)物發(fā)生降解.硫酸根自由基(SO4?-)在硝化過程中起到了關(guān)鍵作用,它能將NH4+氧化生成氨基自由基(?NH2),隨后經(jīng)過一系列自由基鏈?zhǔn)椒磻?yīng)生成二氧化氮自由基(NO2?).同時(shí),SO4?-進(jìn)攻土壤有機(jī)質(zhì)中的酚結(jié)構(gòu)單元,使其氧化生成苯氧自由基,苯氧自由基進(jìn)一步與NO2?結(jié)合生成硝基副產(chǎn)物.天然有機(jī)質(zhì)(NOM)在環(huán)境中無處不在,NH4+在環(huán)境中也普遍存在,PS用于土壤和地下水污染修復(fù)時(shí)生成硝基副產(chǎn)物很可能是一個(gè)普遍現(xiàn)象.

銨氮；過硫酸鹽；土壤有機(jī)質(zhì)；硫酸根自由基；二氧化氮自由基；硝基副產(chǎn)物

過硫酸鹽(PS)高級(jí)氧化技術(shù)是近年來的研究熱點(diǎn)[1-2].PS在常溫下較穩(wěn)定,很難氧化有機(jī)污染物[1].一般情況下,PS可以通過加熱、紫外光照、過渡金屬離子等方式活化產(chǎn)生硫酸根自由基(SO4×-)[1-4]. SO4×-的氧化還原電位(0=2.5～3.1V)和羥基自由基(×OH)相當(dāng)(0=1.8～2.7V),但比×OH穩(wěn)定;且SO4×-降解有機(jī)污染物時(shí)受pH值的影響小,有利于在更廣泛的環(huán)境條件中使用[5-6].此外,SO4×-的壽命比×OH長(zhǎng),這大大提高了有機(jī)污染物與SO4×-的接觸機(jī)會(huì),從而有利于有機(jī)污染物的降解與礦化[5].基于以上優(yōu)點(diǎn),過硫酸鹽高級(jí)氧化技術(shù)在近些年迅速發(fā)展并被廣泛應(yīng)用于有機(jī)污染物的去除[7-14].例如,Waldermer等[11]采用熱活化PS去除地下水中的氯代乙烯,結(jié)果表明,當(dāng)溫度設(shè)定為60℃,PS濃度為0.45mmol/L,反應(yīng)1h后,氯代乙烯的去除率接近100%.劉小寧[12]研究發(fā)現(xiàn),熱活化PS降解對(duì)氯苯酚的反應(yīng)速率隨溫度升高而升高,當(dāng)PS濃度為40mmol/L,60℃反應(yīng)2h后,對(duì)氯苯酚去除率達(dá)到100%.Liu等[13]研究發(fā)現(xiàn),當(dāng)PS為20mmol/kg,溫度從30℃上升至60℃,反應(yīng)1h后,土壤中布洛芬的去除率從10%上升至100%.這些研究均反映出熱活化PS(式1)降解有機(jī)污染物的高效性.與其他活化方法相比,熱活化還有如下優(yōu)點(diǎn):首先,熱活化操作簡(jiǎn)便,可控性強(qiáng);其次,這種方法可以避免引入其他的離子,相對(duì)干凈.此外,熱活化受pH值的影響小,可以在各種pH值條件下使用.

然而,越來越多的研究發(fā)現(xiàn)過硫酸鹽高級(jí)氧化過程中有害副產(chǎn)物的生成.例如,SO4?-可以與鹵素離子反應(yīng)生成游離鹵以及鹵自由基,這些活性鹵物質(zhì)會(huì)進(jìn)攻酚類底物生成有毒的鹵代副產(chǎn)物,如鹵仿和鹵乙酸等[15-18].近來研究發(fā)現(xiàn),SO4×-也能與土壤或地下水環(huán)境中廣泛存在的亞硝酸鹽(NO2-)發(fā)生反應(yīng),生成二氧化氮自由基(NO2×)(式2)[19-21].NO2×是一種較為溫和的氧化劑,還原電位0為1.03V,在天然有機(jī)質(zhì)(NOM)存在下,NO2×能夠和NOM反應(yīng),生成硝基副產(chǎn)物[19-21].其中,NOM大分子中的酚結(jié)構(gòu)單元為主要的活性位點(diǎn).SO4×-通過電子轉(zhuǎn)移能夠?qū)⒎咏Y(jié)構(gòu)單元氧化成苯氧自由基,苯氧自由基進(jìn)一步與NO2×結(jié)合生成硝基副產(chǎn)物[22-23].

=8.8×108mol/(L×s) (2)

銨(NH4+)是水和土壤中含量最為豐富的無機(jī)氮.銨通常作為肥料被大量釋放到環(huán)境中,是造成地表水的富營(yíng)養(yǎng)化的主要因素之一.土壤中NH4+-N含量從幾mg/kg到幾百mg/kg不等[24].研究表明,NH4+可以被×OH氧化形成氨基自由基(×NH2,0=2.3V),后者可以與氧氣快速結(jié)合生成過氧氨基自由基(NH2OO×)[25-32],NH2OO×經(jīng)過重排后分解生成氮氧自由基(NO×,0=0.39V)[25-26].NO×與×OH或氧氣反應(yīng),生成NO2-或NO2×[33-36](式(3)～(8)).理論上,NH4+也可以被SO4×-氧化為硝酸鹽(NO3-),但尚未有相關(guān)研究的報(bào)道[25].由于SO4×-與×OH氧化能力相近,可以推測(cè)NH4+在SO4×-氧化過程中具有類似的轉(zhuǎn)化途徑.一旦NH4+能夠轉(zhuǎn)化生成NO2×,在有機(jī)質(zhì)存在的條件下,NO2×很可能與有機(jī)質(zhì)反應(yīng)生成硝基副產(chǎn)物.

=1.4×107mol/(L×s) (3)

=3.0×108mol/(L×s) (4)

=1.1×109mol/(L×s) (5)

=1.7×1010mol/(L×s) (6)

=1.6×106mol/(L×s) (7)

=1.0×1010mol/(L×s) (8)

因此,本研究采用熱活化過硫酸鹽處理土壤,探究該過程中NH4+的轉(zhuǎn)化及硝基副產(chǎn)物的生成,考察了反應(yīng)時(shí)間、PS濃度和外加NH4+對(duì)硝基副產(chǎn)物生成的影響,同時(shí)對(duì)NH4+的轉(zhuǎn)化途徑以及硝基副產(chǎn)物的生成機(jī)制進(jìn)行了探討.本研究結(jié)果可為評(píng)估過硫酸鹽高級(jí)氧化技術(shù)的可行性提供依據(jù).

1 材料與方法

1.1 材料與試劑

所有試劑均為分析純及以上級(jí)別.過硫酸鈉(Na2S2O8)、硫酸銨、15N標(biāo)記的硫酸銨(純度99%)、3-硝基酚、4-硝基酚、4-羥基-3-硝基苯甲酸、2-羥基-5-硝基苯甲酸和2,4-二硝基酚購(gòu)于阿拉丁(上海).色譜甲醇和甲酸購(gòu)于TEDIA(Fairfield,美國(guó)).硫酸(H2SO4,純度98.3%)、氫氧化鈉、氯化鉀、重鉻酸鉀、氯化鋇和硫酸鎂購(gòu)自國(guó)藥滬試(上海).C18固相萃取小柱(6cc/500mg, D110534)購(gòu)自月旭科技(上海).所有實(shí)驗(yàn)用水均為超純水(18.2MΩ×cm),由Stakpure OmniaTap水凈化系統(tǒng)制備得到(Peculiar Instrument Technology Limited,英國(guó)).

供試土壤分別采集于江蘇南京農(nóng)業(yè)大學(xué)白馬科研基地(119.185521°E,31.614205°N)和河北省邯鄲市曲周縣(114.988701°E,36.807845°N),采樣深度為0～20cm.將土壤樣品磨碎后過10目篩,風(fēng)干老化后再研磨,過20目篩,分裝保存.取10g土壤與一定量純水混合(土水質(zhì)量比為1:2.5),振蕩5min,靜置24h后測(cè)定pH值.土壤有機(jī)質(zhì)(SOM)的含量通過重鉻酸鉀容量法(外加熱法)測(cè)定[37].土壤陽(yáng)離子交換能力(CEC)通過BaCl2-MgSO4交換法測(cè)定[37].土壤NO3--N和NH4+-N用2mol/L氯化鉀溶液浸提后,采用FIAstar 5000流動(dòng)分析儀(Foss Tecator,丹麥)測(cè)定[37].供試土壤的理化性質(zhì)見表1.

表1 供試土壤的理化性質(zhì)

1.2 實(shí)驗(yàn)設(shè)計(jì)

反應(yīng)在50mL離心管內(nèi)進(jìn)行,每組設(shè)置3個(gè)平行.每個(gè)樣品含有10g土壤樣本,然后加入15mL一定濃度的PS溶液,PS濃度設(shè)7.5, 15, 30, 45, 75mmol/kg 5個(gè)水平.在處理江蘇土過程中,額外加入7.5mmol/kg NH4+,其中一組加入物質(zhì)的量濃度1:1的14NH4+和15NH4+,另一組只加入14NH4+.反應(yīng)前將搖床預(yù)熱至60℃,隨后將充分混勻的樣品放入搖床內(nèi)反應(yīng),轉(zhuǎn)速設(shè)為250r/min.分別于0, 3, 6, 9, 12, 15, 18, 21, 24h取出樣品,置于冰水浴冷卻,終止反應(yīng).樣品經(jīng)過3000r/min離心10min,進(jìn)行固液分離.上清液加入H2SO4酸化至pH<2,然后用C18固相萃取(SPE)小柱進(jìn)行富集.在萃取前,SPE小柱依次用5mL甲醇和5mL 酸水(pH值 3)進(jìn)行活化,隨后上樣.樣品富集后,先后用2mL水和2mL 體積分?jǐn)?shù)為3%的甲醇洗去雜質(zhì),加壓吹干.最后將富集在柱內(nèi)的樣品用2mL甲醇洗脫,氮吹濃縮至1mL,過0.45μm濾膜(有機(jī)尼龍66,津隆),轉(zhuǎn)移至液相小瓶中,4℃保存待分析.固相樣品中繼續(xù)加入10mL氫氧化鈉溶液使pH值>13,超聲30min萃取,3000r/min離心10min,取上清液.重復(fù)該過程2次,合并萃取液,加H2SO4酸化后,用SPE富集,步驟同上.

此外,為了研究外加NH4+對(duì)硝基副產(chǎn)物生成的影響,在10g河北土壤樣本中加入15mL含有一定量PS和NH4+的混合溶液,使得PS濃度為30mmol/kg,外加NH4+濃度分別為0, 1.5, 3.0, 4.5, 6.0, 7.5mmol/ kg.其余反應(yīng)條件同上,24h后冰水浴終止反應(yīng).樣品處理過程同上.

1.3 硝基副產(chǎn)物的定性、定量分析

參考先前報(bào)道的方法對(duì)硝基副產(chǎn)物進(jìn)行定性和定量分析[20-21].富集后的樣品通過高效液相色譜-質(zhì)譜聯(lián)用儀(HPLC-MS/MS)進(jìn)行分析.液相色譜型號(hào)為Agilent 1200,配有Agilent Zorbax Eclipse Plus C18反相柱(150mm × 2.1mm × 3.5 μm),流動(dòng)相為甲醇與水(均加入0.1%體積甲酸酸化),流速0.2mL/min.前10min,甲醇體積分?jǐn)?shù)從20%上升至30%;10～ 20min,甲醇體積分?jǐn)?shù)從30%上升至70%;20～25min,甲醇體積分?jǐn)?shù)從70%上升至100%并保持5min.MS檢測(cè)器型號(hào)為Agilent 6410,配備電噴霧離子化接口(ESI)和三重四極桿質(zhì)量分析器,設(shè)為負(fù)離子模式,其它主要參數(shù)為:毛細(xì)管電壓-4.0kV;碎裂電壓125V;氮?dú)?399.995%)作干燥氣,流速10mL/min;溫度350℃;噴霧器壓力40psi.根據(jù)標(biāo)樣的二級(jí)質(zhì)譜選擇特征的母子離子對(duì)(表2),采用多反應(yīng)監(jiān)測(cè)模式(MRM),通過比較產(chǎn)物與標(biāo)樣的保留時(shí)間及峰面積,確定產(chǎn)物的結(jié)構(gòu)和濃度.

表2 硝基副產(chǎn)物HPLC-MS/MS分析的離子對(duì)選擇和碰撞能量

1.4 硝基副產(chǎn)物的回收率

參考先前報(bào)道的方法對(duì)土樣上清液中硝基副產(chǎn)物的回收率進(jìn)行測(cè)定[21].在50mL離心管內(nèi)加入10g未經(jīng)過處理的江蘇土及15mL水,充分混勻后置于搖床內(nèi),轉(zhuǎn)速設(shè)為250r/min,溫度為室溫.24h后3000r/min離心10min,取上清液.在上清液中加入一定量3-硝基酚、4-硝基酚、4-羥基-3-硝基苯甲酸和2-羥基-5-硝基苯甲酸的混合標(biāo)樣,使得各個(gè)標(biāo)樣的濃度為5μmol/L.隨后加入H2SO4酸化,用SPE富集,并用MS進(jìn)行分析.SPE步驟和MS分析方法同上.各個(gè)物質(zhì)的濃度利用外標(biāo)法進(jìn)行測(cè)定,將測(cè)定濃度除以加標(biāo)值計(jì)算得到回收率.3-硝基酚、4-硝基酚、4-羥基-3-硝基苯甲酸和2-羥基-5-硝基苯甲酸的回收率分別為53.16%±1.41%、39.50%±0.61%、94.19%±2.11%和91.59%±3.05%.樣品中硝基副產(chǎn)物的濃度通過回收率進(jìn)行校正.

2 結(jié)果與討論

2.1 硝基副產(chǎn)物的鑒定

河北土有機(jī)質(zhì)含量為14.90g/kg,NH4+-N含量為26.78mg/kg(相當(dāng)于2mmol/kg).經(jīng)過熱活化PS處理后,土樣上清液的質(zhì)譜如圖1所示.根據(jù)對(duì)譜圖的分析,并結(jié)合已有研究表明,質(zhì)譜峰138、154、182和183可能對(duì)應(yīng)于含氮副產(chǎn)物.根據(jù)二級(jí)質(zhì)譜并結(jié)合分子量,推測(cè)138為硝基酚,進(jìn)一步利用HPLC-MS/MS分離并與標(biāo)樣進(jìn)行比對(duì),確定為3-硝基酚和4-硝基酚,其中4-硝基酚占主導(dǎo);182對(duì)應(yīng)于一硝基羥基苯甲酸,同樣有2種同分異構(gòu)體,分別為2-羥基-5-硝基苯甲酸和4-羥基-3-硝基苯甲酸,其中4-羥基-3-硝基苯甲酸占主導(dǎo);183對(duì)應(yīng)于2,4-二硝基酚.此外,根據(jù)分子量,推測(cè)154為二羥基硝基苯,但由于缺乏標(biāo)準(zhǔn)物質(zhì),無法進(jìn)行進(jìn)一步的結(jié)構(gòu)確認(rèn).以上實(shí)驗(yàn)結(jié)果均由土樣上清液測(cè)定得到,土樣固相中硝基副產(chǎn)物的生成低于檢測(cè)限,故后續(xù)實(shí)驗(yàn)結(jié)果均為土樣上清液.

圖1 熱活化PS處理河北土土樣上清液質(zhì)譜和色譜分離圖

反應(yīng)條件:河北土10g,反應(yīng)液15mL, PS 30mmol/kg, 60℃, 250r/min, 12h

江蘇土有機(jī)質(zhì)含量為22.94g/kg,NH4+-N含量為13.52mg/kg(相當(dāng)于1mmol/kg).經(jīng)過熱活化PS處理后沒有檢測(cè)到明顯的含氮副產(chǎn)物,這可能與土壤中NH4+含量較低有關(guān).往土壤中加入7.5mmol/kg物質(zhì)的量濃度1:1的14NH4+和15NH4+后,經(jīng)過同樣的處理,并通過MS圖譜中的15N特征同位素指紋來判斷是否生成了含氮副產(chǎn)物.由于反應(yīng)含有物質(zhì)的量濃度1:1的14NH4+和15NH4+,因此,NH4+轉(zhuǎn)化為有機(jī)氮后的產(chǎn)物含有相應(yīng)的同位素特征,即伴隨有(+1)、(+2)…(+)的同位素峰(其中代表分子中含有來自NH4+的N原子數(shù)量),它們的相對(duì)峰度比值符合二項(xiàng)式(1+1)的展開.

圖2 熱活化PS處理江蘇土土樣上清液質(zhì)譜和色譜分離圖

反應(yīng)條件:江蘇土10g,反應(yīng)液15mL, PS 30mmol/kg, NH4+7.5mmol/kg, 60℃, 250r/min, 12h,其中a為加入物質(zhì)的量濃度1:1的14NH4+和15NH4+, b為只加入14NH4+

表3 熱活化PS處理后,土壤中檢測(cè)到的硝基副產(chǎn)物

續(xù)表3

如圖2a所示,反應(yīng)后出現(xiàn)了138和139質(zhì)譜峰,且峰度比接近1:1,而只加入14NH4+實(shí)驗(yàn)組中只有138質(zhì)譜峰(圖2b),說明該物質(zhì)含有一個(gè)氮原子,根據(jù)分子量推測(cè)138為一硝基酚.同樣地,利用HPLC-MS/MS分離并與標(biāo)樣進(jìn)行比對(duì),138確定為3-硝基酚和4-硝基酚,其中4-硝基酚占主導(dǎo)(圖2c).利用上述方法,進(jìn)一步確定182為4-羥基-3-硝基苯甲酸;183為2,4-二硝基酚.此外,根據(jù)15N同位素指紋結(jié)合分子量,推測(cè)198為二羥基硝基苯甲酸,但由于缺乏標(biāo)準(zhǔn)物質(zhì),無法進(jìn)行進(jìn)一步的結(jié)構(gòu)確認(rèn).綜上所述,熱活化PS氧化土壤過程中,NH4+能夠轉(zhuǎn)化生成硝基副產(chǎn)物,它們的分子式與結(jié)構(gòu)見表3.

2.2 河北土中硝基副產(chǎn)物的生成

對(duì)樣品中的硝基副產(chǎn)物濃度進(jìn)行定量分析,如圖3所示,一硝基羥基苯甲酸的濃度高于一硝基酚.當(dāng)PS濃度為30mmol/kg時(shí),硝基副產(chǎn)物的生成量隨著反應(yīng)的進(jìn)行先增加后降低,在反應(yīng)12h達(dá)到最大值,其中一硝基酚和一硝基羥基苯甲酸濃度分別為0.0044和0.0072μmol/kg(圖3a).2,4-二硝基酚生成量較低,故后續(xù)將不對(duì)其進(jìn)行定量分析.此外,二羥基硝基苯(154)由于缺乏標(biāo)準(zhǔn)物質(zhì)無法定量,但其峰面積也隨著反應(yīng)的進(jìn)行先增加后降低.

如圖3b所示,硝基副產(chǎn)物的生成量隨著PS濃度的增加先增加后降低.在反應(yīng)12h條件下,當(dāng)PS濃度為30mmol/kg時(shí),一硝基酚和一硝基羥基苯甲酸的生成量達(dá)到最大;當(dāng)PS濃度為60mmol/kg時(shí),二羥基硝基苯(154)的峰面積達(dá)到最大.研究表明,SO4×-通過電子轉(zhuǎn)移能夠?qū)OM大分子中的酚結(jié)構(gòu)單元氧化成苯氧自由基,苯氧自由基進(jìn)一步與NO2×結(jié)合生成硝基副產(chǎn)物[20-23].因此,增大PS濃度有利于SO4×-的生成,一方面促進(jìn)NH4+轉(zhuǎn)化生成NO2×,另一方面氧化土壤有機(jī)質(zhì)生成苯氧自由基,最終促進(jìn)硝基副產(chǎn)物的生成.當(dāng)PS濃度繼續(xù)增大,硝基副產(chǎn)物可能進(jìn)一步被SO4×-氧化.

當(dāng)PS濃度為30mmol/kg,往土壤中加入不同濃度的NH4+后,延長(zhǎng)反應(yīng)時(shí)間至24h.如圖3c所示,硝基副產(chǎn)物的生成量隨著NH4+濃度的增加先增加后降低.當(dāng)NH4+加入量為1.5mmol/kg時(shí),硝基副產(chǎn)物生成量達(dá)到最大,其中一硝基酚和一硝基羥基苯甲酸的濃度分別為0.0056和0.0125μmol/kg.結(jié)果表明,提高NH4+濃度有利于NO2×的生成,并進(jìn)一步轉(zhuǎn)化生成硝基副產(chǎn)物.當(dāng)NH4+濃度繼續(xù)增加時(shí),NH4+可能與土壤有機(jī)質(zhì)競(jìng)爭(zhēng)SO4×-,使得苯氧自由基中間體的生成受到抑制,從而導(dǎo)致硝基副產(chǎn)物生成量下降.

反應(yīng)條件: a.河北土10g,反應(yīng)液15mL, PS 30mmol/kg, 60℃, 250r/min; b.河北土10g,反應(yīng)液15mL, 60℃, 250r/min, 12h; c.河北土10g,反應(yīng)液15mL, PS 30mmol/kg, 60℃, 250r/min, 24h. 柱狀代表可定量的硝基副產(chǎn)物,其濃度對(duì)應(yīng)左縱坐標(biāo)軸;折線代表不可定量的硝基副產(chǎn)物,其峰面積對(duì)應(yīng)右縱坐標(biāo)軸

2.3 江蘇土中硝基副產(chǎn)物的生成

與河北土相似,江蘇土中一硝基羥基苯甲酸的濃度高于一硝基酚.如圖4a所示,在外加NH4+濃度為7.5mmol/kg、PS濃度為30mmol/kg條件下,硝基副產(chǎn)物的生成量隨著反應(yīng)的進(jìn)行先增加后降低,在反應(yīng)12h達(dá)到最大值,其中一硝基酚和一硝基羥基苯甲酸濃度分別為0.0069和0.0186 μmol/kg.2,4-二硝基酚生成量較低,故后續(xù)將不對(duì)其進(jìn)行定量分析.此外,二羥基硝基苯甲酸(198)由于缺乏標(biāo)準(zhǔn)物質(zhì)無法定量,但其峰面積也隨著反應(yīng)的進(jìn)行先增加后降低.

反應(yīng)條件: a.江蘇土10g,反應(yīng)液15mL, PS 30mmol/kg, NH4+7.5mmol/kg,60℃, 250r/min; b.江蘇土10g,反應(yīng)液15mL, NH4+7.5mmol/kg, 60℃, 250r/min, 12h.柱狀代表可定量的硝基副產(chǎn)物,其濃度對(duì)應(yīng)左縱坐標(biāo)軸;折線代表不可定量的硝基副產(chǎn)物,其峰面積對(duì)應(yīng)右縱坐標(biāo)軸

如圖4b所示,硝基副產(chǎn)物的生成量隨著PS濃度的增加先增加后降低.在反應(yīng)12h、外加7.5mmol/kg NH4+條件下,當(dāng)PS濃度為30mmol/kg時(shí),一硝基酚、一硝基羥基苯甲酸和二羥基硝基苯甲酸的生成量達(dá)到最大.結(jié)果表明,增大PS濃度有利于SO4×-的生成,既可以促進(jìn)NH4+轉(zhuǎn)化生成NO2×,又能夠氧化土壤有機(jī)質(zhì)生成苯氧自由基,最終促進(jìn)硝基副產(chǎn)物的生成.當(dāng)PS濃度繼續(xù)增大,硝基副產(chǎn)物可能進(jìn)一步被SO4×-氧化.以上實(shí)驗(yàn)結(jié)果與河北土中硝基副產(chǎn)物的生成一致.

2.4 NH4+的轉(zhuǎn)化路徑

以上實(shí)驗(yàn)表明,NH4+在SO4×-氧化過程中的確發(fā)生了轉(zhuǎn)化.參考NH4+在×OH反應(yīng)系統(tǒng)中的轉(zhuǎn)化路徑,推測(cè)NH4+首先與SO4×-反應(yīng)生成×NH2,兩者反應(yīng)速率為3×105mol/(L×s)[25-26].且×OH常常存在于SO4×-氧化過程中,在中性或酸性條件下,×OH濃度較低[38],但也能夠氧化NH4+生成×NH2[25].×NH2在有氧條件下能夠被迅速氧化為NH2O2×,經(jīng)重排分解后生成NO×,繼而被×OH氧化生成NO2-[25-26,33].NO2-能夠被SO4×-或×OH氧化生成NO2×[34,39].此外,NO×也能夠被氧氣直接氧化生成NO2×[33].NO2×自身耦合形成N2O4,后者能進(jìn)一步水解生成NO3-[34].值得注意的是,一旦NH4+的轉(zhuǎn)化過程中能夠生成NO2×,若存在天然有機(jī)質(zhì),NO2×能夠和天然有機(jī)質(zhì)反應(yīng)生成硝基副產(chǎn)物.如圖5所示,對(duì)于額外加入NH4+的江蘇土,反應(yīng)前NH4+-N濃度為8.47mmol/kg,NO3--N僅為0.24mmol/kg,總無機(jī)氮濃度為8.71mmol/kg.當(dāng)PS濃度為30mmol/kg,反應(yīng)12 與24h后,NH4+-N分別減少了1.31和2.27mmol/kg,而NO3--N分別增加了0.64和1.12mmol/kg,沒有檢測(cè)到NO2-,土壤中總無機(jī)氮逐漸降低至8.04和7.56mmol/kg.而此時(shí),江蘇土中檢測(cè)到的硝基副產(chǎn)物中氮含量分別為0.0255和0.009 μmol/kg.由此可見,大部分NH4+-N在熱活化PS處理土壤過程中轉(zhuǎn)化為NO3--N,僅有少量轉(zhuǎn)化成有機(jī)氮.此外,從N質(zhì)量守恒來看,隨著反應(yīng)的進(jìn)行,樣品中的總氮在不斷減少.有研究表明,×NH2自身耦合生成肼(N2H4)或與×OH結(jié)合生成羥胺(NH2OH),兩者均能在PS的作用下轉(zhuǎn)化為N2逸出[40-42].另外,本樣品中還有部分硝基副產(chǎn)物由于缺乏標(biāo)準(zhǔn)物質(zhì)而無法進(jìn)行定量分析.綜上所述,活化過硫酸鹽氧化過程中NH4+轉(zhuǎn)化路徑見圖6.

河北土中NH4+-N濃度為1.91mmol/kg,而NO3--N濃度幾乎為0.經(jīng)過同樣條件處理,土壤中的總無機(jī)氮逐漸降低至1.40和1.16mmol/kg,其中NH4+-N在12h時(shí)轉(zhuǎn)化了1.17mmol/kg,24h時(shí)幾乎完全轉(zhuǎn)化,而NO3--N濃度分別增加至0.66與1.08mmol/kg,同樣沒有檢測(cè)到NO2-.此外,反應(yīng)12與24h后,河北土中硝基副產(chǎn)物的氮含量分別為0.0116和0.0051 μmol/kg.結(jié)果表明,大部分NH4+-N在熱活化PS處理土壤過程中轉(zhuǎn)化為NO3--N,少量能夠轉(zhuǎn)化成有機(jī)氮,仍有一部分N沒有被檢測(cè)到.上述實(shí)驗(yàn)結(jié)果與江蘇土實(shí)驗(yàn)結(jié)果一致.

反應(yīng)條件: a.江蘇土10g,反應(yīng)液15mL, PS 30mmol/kg, NH4+7.5mmol/kg, 60℃, 250r/min; b.河北土10g,反應(yīng)液15mL, PS 30mmol/kg, 60℃, 250r/min

圖6 活化過硫酸鹽氧化過程中NH4+的轉(zhuǎn)化路徑

3 結(jié)論

3.1 經(jīng)熱活化PS處理后,河北土和江蘇土中均檢測(cè)出了一硝基酚、一硝基羥基苯甲酸、二硝基酚等副產(chǎn)物.這是由于土壤中的銨氮被SO4×-氧化生成了×NH2,并通過自由基鏈?zhǔn)椒磻?yīng)生成NO2×.NO2×能夠與土壤有機(jī)質(zhì)反應(yīng),生成硝基副產(chǎn)物.

3.2 兩種土壤樣本中硝基副產(chǎn)物的生成量在反應(yīng)12h、PS濃度為30mmol/kg時(shí)達(dá)到最大,且這些硝基副產(chǎn)物大多分布于土樣上清液中,這意味著它們具有較強(qiáng)的遷移性,可隨著地表徑流轉(zhuǎn)移或流入地下,造成更大范圍的污染.

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Transformation of soil ammonium nitrogen in the process of thermally activated persulfate oxidation.

YANG Pei-zeng, YUE Hong-shen, JI Yue-fei, LU Jun-he*

(College of Resource and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, China)., 2022,42(1)：267～275

In order to explore the transformation and fate of soil NH4+in the thermally activated PS oxidation process, this study used soil samples collected from Jiangsu and Hebei provinces with different soil organic matter content and NH4+concentration to conduct experiments, and systematically investigated effects of persulfate (PS) concentration, the addition of NH4+, and reaction time on the formation of nitro by-products. Results show that soil NH4+could be transformed to nitrated byproducts, including 3-nitrophenol, 4-nitrophenol, 2-hydroxy-5-nitrobenzoic acid, 4-hydroxy-3-nitrobenzoic acid, 2,4-dinitrophenol, etc. The formation of nitro by-products increased first and then decreased with reaction time. An increased in PS dose would promote the formation of nitro by-products, and the yields of mono-nitrophenols and hydroxy-mono-nitrobenzoic acids reached the maximum after 12h reaction at 30mmol/kg PS dose. However, nitrated byproducts were degraded at higher PS dose. Note that sulfate radicals (SO4?-) played a key role in the nitration process by oxidizing NH4+to form aminyl radicals (?NH2), and then underwent a series of free radical chain reactions to form nitrogen dioxide radicals (NO2?). Besides, phenol moieties in soil organic matter served as the main reactive sites for SO4?-attack, leading to the formation of phenoxy radicals, which further combined with NO2? to form nitro by-products. NOM is everywhere and NH4+is ubiquitous in the environment. Thus, the formation of nitro by-products will be widespread when PS is applied for contaminatedsoil and groundwaterremediation, which should be taken into consideration when evaluating the feasibility of this technology. This study reveals that the presence of soil NH4+in activated PS oxidation processes could induce the nitration of NOM and the formation of nitrophenolic by-products.

ammonium；persulfate；soil organic matter；sulfate radical；nitrogen dioxide radical；nitrated byproducts

X703.5

A

1000-6923(2022)01-0267-09

楊培增(1994-),女,浙江嘉興人,南京農(nóng)業(yè)大學(xué)博士研究生,主要研究方向?yàn)樗幚砀呒?jí)氧化.發(fā)表論文8篇.

2021-06-04

國(guó)家自然科學(xué)基金資助項(xiàng)目(22076079,22076080);江蘇省研究生科研創(chuàng)新計(jì)劃(030-Z562015603);國(guó)家大學(xué)生實(shí)踐創(chuàng)新訓(xùn)練計(jì)劃項(xiàng)目(201910307043Z)

* 責(zé)任作者, 教授, jhlu@njau.edu.cn