何 琦,曹鳳梅,盧少勇,陳方鑫,蔡傳倫

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挺水植物生物炭對硫丹的吸附及催化水解作用

何 琦,曹鳳梅,盧少勇*,陳方鑫,蔡傳倫

(中國環(huán)境科學(xué)研究院,國家環(huán)境保護(hù)洞庭湖科學(xué)觀測研究站,湖泊水污染治理與生態(tài)修復(fù)技術(shù)國家工程實驗室,北京 100012)

以美人蕉、菖蒲、蘆葦、茭白、再力花、蘆竹等挺水植物為原料,在限氧升溫(550℃)條件下制備6種不同性質(zhì)的生物炭,研究其組成及結(jié)構(gòu)對硫丹吸附和催化水解作用.結(jié)果表明:550℃下熱解,濕地植物生物炭所含灰分高于一般農(nóng)產(chǎn)品廢棄物,介于10.88%~31.11%間.生物炭表面芳香類官能團(tuán)較多,孔隙部分呈碎片化,以介孔為主.6種生物炭對硫丹都具良好吸附性能.吸附行為主要發(fā)生在含致密有機(jī)質(zhì)的生物炭表面,包括疏水、擴(kuò)散及分配作用等.中/堿性條件下,美人蕉生物炭、菖蒲生物炭及再力花生物炭能有效去除水溶液中的硫丹,去除率為96.77%~98.57%.中性條件下,因生物炭對硫丹的催化,對硫丹的去除率提高約16.57%~72.57%.

生物炭；硫丹；催化水解；芳香類官能團(tuán)

硫丹,屬于有機(jī)氯農(nóng)藥(OCPs),我國1994~ 2004年施用于棉花,煙草和茶樹等農(nóng)作物的硫丹總量約2.57萬t,主要集中在新疆、華東、華中和西南地區(qū)[1].目前,我國已公布的“十三五”生態(tài)環(huán)境保護(hù)規(guī)劃,要求截至2020年基本淘汰硫丹.

硫丹可經(jīng)地表徑流、淋、溶、干/濕沉降等入水,對魚類的生理生化、內(nèi)分泌、遺傳及代謝機(jī)制產(chǎn)生急性或慢性效應(yīng)[2].江蘇省淮河流域的地表水源中α-硫丹達(dá)33.3ng/L, β-硫丹達(dá)6.7ng/L[3].丹江口水庫的間隙水中β-硫丹為22.17ng/L[4].印度恒河流域水體中硫丹總量為166.39ng/L[5].美國南加州地表水、底泥及魚類也檢測出較高的硫丹,且遠(yuǎn)超美國EPA規(guī)定的能引起慢性毒性效應(yīng)的值(56ng/L)[6].

目前,吸附法較經(jīng)濟(jì)高效.去除硫丹的吸附劑有二氧化硅[7]、木炭[7]、粘土[8]、碳納米管[9]和零價鋅[10]等,因其經(jīng)濟(jì)費(fèi)用高或效率低,應(yīng)用受限.生物炭來源廣,價格低,表面吸附位點(diǎn)較多,利于去除有機(jī)污染物.目前生物炭去除疏水性有機(jī)物(HOMs)的研究多集中在吸附或降解污染物[11-12],利用濕地生物炭去除硫丹的研究鮮有報道.

本文利用傅立葉紅外,元素分析及掃描電鏡等分析生物炭表面組成、結(jié)構(gòu)及官能團(tuán),研究其對硫丹的吸附和催化水解,可為現(xiàn)有水處理設(shè)施除硫丹提供高效、低成本且環(huán)境友好的吸附劑,為解決人工濕地植物帶來的二次污染提供可行方案.

1 材料及方法

1.1 生物炭制備

美人蕉、菖蒲、蘆葦、茭白、再力花、蘆竹等挺水植物采集于太湖貢湖灣濕地,瀝干后,干燥粉碎,過200目篩.氮?dú)夥諊?加熱至550℃,熱解2.5h,室溫下冷卻過夜,得到生物炭,分別標(biāo)記CAI-B(美人蕉)、ACC-B(菖蒲)、PHA-B( 蘆葦)、ZIA-B(茭白)、THD-B (再力花)、ARD-B(蘆竹).其中管式爐升溫程序:3.33℃/min升至400℃,再用40min升至550℃,保持2.5h.氮?dú)馑俾? 300mL/min.

1.2 硫丹測定

由氣相色譜電子捕獲檢測器(GC-ECD) (5975C, Aglient,美國)測定硫丹,色譜柱為Hp- 5ms毛細(xì)管柱(35m′0.32mm′0.25μm, Agilent Inc., USA),升溫程序:80℃保持1min,以30℃/min升至180℃,再以3℃/min升至205℃,保持4min,以20℃/min升至290℃,保持2min.檢測器溫度320℃,氮?dú)馕泊祾?0mL/min,進(jìn)樣不分流,進(jìn)樣量1μL.

1.3 吸附實驗

1.3.1 吸附等溫研究 選系列濃度梯度的硫丹混合液(α-硫丹:β-硫丹=1:1,α-硫丹濃度分別為0.50, 1.00, 2.00, 5.00, 8.00, 10.00mg/L).稱15.00mg生物炭于40.00mLEPA小瓶中,加入30.00mL硫丹溶液,置搖床內(nèi),室溫避光,轉(zhuǎn)速220r/min,平衡7h,取上清液3.00mL,加入等量正己烷,充分搖晃、離心,取上層液體加入過量無水硫酸鈉,充分振蕩后測GC-ECD,萃取分液兩次,硫丹萃取率90.00%以上.并監(jiān)控溶液pH值變化.

1.3.2 硫丹水解實驗 在生物炭-水體系中進(jìn)行.分別調(diào)節(jié)1mg/L硫丹溶液(同上),初始pH值為3.20、5.60、7.20、11.00.用磷酸鈉、磷酸氫二鈉及磷酸配置緩沖液.分別稱取15mg生物炭樣品于一系列40mL EPA瓶中,加入上述溶液30mL,添加空白組,設(shè)置3組平行樣.測定平衡濃度及溶液中硫丹二醇濃度,并用水解方程反推發(fā)生水解硫丹的量,根據(jù)質(zhì)量守恒定律計算水解、平衡及吸附各部分占比.

1.4 生物炭特征分析方法

測定生物炭的產(chǎn)率[14]、pH值[14-15]、灰分[14,16]、電位[17]等;用元素分析儀(Elementar VarioEL,德國)測定生物炭的元素組成;并借助掃描電鏡(SEM)(KYKY-2800B,中國)測定表面結(jié)構(gòu);傅立葉紅外光譜儀(Perkin Elmer 1725X,美國)測定生物炭表面官能團(tuán).利用N2吸附等溫線Brnauer-Emmett-Teller法,由比表面積儀(Micromeritics, Norcross, USA)測定比表面積.

1.5 數(shù)據(jù)處理

用Frundlich模型擬合生物炭吸附硫丹的數(shù)據(jù),其線性擬合公式[8]是:

loge= logF+×loge(1)

式中:e表示平衡時吸附量mg/g;e表示平衡時溶液的濃度mg/L;F表示Frundlich吸附系數(shù)mL/g,表示非線性吸附常數(shù).

吸附分配系數(shù)d及有機(jī)碳標(biāo)準(zhǔn)化系數(shù)oc通過公式(2)和(3)算得

式中:oc為有機(jī)碳系數(shù),d和oc(e= 0.001w和0.01w,w為硫丹溶解度)通過以上公式計算.

2 結(jié)果與討論

2.1 不同種類生物炭基本理化特性

表1 生物炭基本特性表

從表1生物炭的理化特性可得:在550 ℃下熱解,生物炭產(chǎn)率約32.00%(ARD-B除外),高于500℃下熱解的秸稈、玉米芯、棉籽殼等廢棄物[18-19],所含灰分10.88%~31.11%,高于玉米芯(4.02%)、柚子皮(9.06%)等[20].呈堿性,為負(fù),與文獻(xiàn)[21]相符,可能與生物炭表面所含酸堿官能團(tuán)及無機(jī)碳成分有關(guān).在550℃下熱解,生物炭孔徑為介孔,比表面積11.02~36.67m2/g,孔徑與比表面積的相關(guān)性弱(=0.1190),說明炭表面部分孔徑可能被灰分覆蓋或堵塞[22],與文獻(xiàn)[14]一致.

2.2 生物炭結(jié)構(gòu)及表面官能團(tuán)特性

2.2.1 表面結(jié)構(gòu)與官能團(tuán)分析 由圖1生物炭SEM圖譜可知,炭表面具多孔和管狀結(jié)構(gòu),后者可能由植物細(xì)胞熱解形成[23].總體上,生物炭孔隙發(fā)育較成熟,部分呈碎片化,孔壁表面有附著物,可能在燒制中降溫過快,部分大孔坍塌,表面粗糙度增加[24],一定程度上減小生物炭比表面積.

圖1 生物炭的SEM掃描圖像

生物炭的傅立葉紅外圖譜(圖2)表明, THD-B, ACC-B及CAI-B在875cm-1附近具小尖峰,是CaCO3特征峰[24].在1218cm-1附近處譜帶為纖維素或半纖維素的官能團(tuán),1521cm-1為羧酸官能團(tuán)的C=O伸縮振動[25].1807cm-1為芳香環(huán)的骨架C=C振動[21],碳碳叁鍵基團(tuán)位于2179cm-1處,氫氧鍵大致位于3700cm-1處.在550℃下熱解,生物炭表面存在大量芳香結(jié)構(gòu),仍存在烯烴和炔烴、纖維素和半纖維素結(jié)構(gòu),部分生物炭表面生成CaCO3沉淀.

圖2 生物炭的FTIR圖譜

表2 生物炭(質(zhì))元素分析

注: CAI、ACC、PHA、ZIA、THD及ARD為相應(yīng)生物炭原料.

2.2.2 元素組成分析 生物炭(質(zhì))的元素組成見表2.可見,各生物炭熱解后,N、C等增加,H、O降低.這可能是熱解中碳鏈發(fā)生脫羧基、脫氧和脫水等反應(yīng),單鍵變雙/叁鍵[23],與圖2紅外譜圖一致.隨熱解溫度升高O/C原子比降低[22],生物炭表面極性官能團(tuán)減少,生物炭表面疏水性越強(qiáng)[26-28],越利于吸附硫丹.

2.3 吸附等溫實驗

圖3 生物炭對硫丹的吸附等溫線

生物炭對硫丹的吸附等溫線擬合的非線性系數(shù)為0.53~0.84,非線性化程度高,與沉積物熱解后生成生物炭的吸附等溫線一致[29].通常,生物炭比表面積越大,其表面微孔越多,提供更多吸附位[24].雖PHA-B、ZIA-B和ARD-B具較大比表面積和較小孔徑,但其非線性系數(shù)較大,非線性程度低,可能與該類生物炭的弱芳香化有關(guān)[30].另外生物炭孔徑與α-硫丹和β-硫丹的吸附常數(shù)logd較明顯正相關(guān)(<0.05,=0.7839和<0.01,=0.8003),且比表面積與其相關(guān)性較弱(= 0.1644,=0.0368),說明生物炭吸附硫丹,除擴(kuò)散作用外[31]還有其它作用力.

除PHA-B、ZIAB及ARD-B外,其余生物炭值小于0.60,說明炭表面的芳香性較高,極性也與紅外光譜結(jié)果一致.硫丹分子與芳香性官能團(tuán)可通過疏水作用結(jié)合[32]而被生物炭吸附.另外,由圖2紅外譜圖可知,生物炭表面也存在類纖維素結(jié)構(gòu),此類非碳化結(jié)構(gòu),在吸附過程可能對硫丹具分配作用[31].

表3 生物炭Freundlich等溫參數(shù)及分配系數(shù)表

注:①:生物炭對α-硫丹的等溫吸附參數(shù);②:生物炭對β-硫丹的等溫吸附參數(shù);表示當(dāng)e=0.001S時,logoc值(mL/g);表示當(dāng)e=0.01w時,logoc值(mL/g);*表示F(mL/g);#表示d(mL/g).

CAI-B、ACC-B及THD-B的N值較小,其生物炭表面可提供較多吸附位點(diǎn),與生物炭具有較小的C/H系數(shù)一致,說明硫丹以表面吸附為主.隨平衡濃度增加(e=0.001, 0.01w), logoc減小,吸附行為發(fā)生在生物炭表面能量較高的致密有機(jī)質(zhì)的吸附位[32].另由表3中l(wèi)ogd可知,生物炭對硫丹的親和力較-硫丹大,可能與-硫丹易在沉積物或植物體富集有關(guān)[33].另外,生物炭表面帶負(fù)電,硫丹為極性非離子型化合物,靜電作用也影響吸附[34].

炭-水體系的pH值為5.67~10.07.炭表面的陽離子在溶液中與H+發(fā)生離子交換,促進(jìn)pH值增大[34].在中/堿性時,硫丹會水解產(chǎn)生硫丹二醇和SO32-[35],增加溶液pH值并減輕硫丹毒性.

2.4 生物炭對硫丹催化水解

由圖4各pH下生物炭的硫丹去除模式可得:隨pH升高,硫丹自身水解程度增加.且pH為3.20、5.60和7.20時,生物炭對硫丹的催化水解較明顯,可能因酸性(3.20和5.60)時,硫丹自身水解程度弱[36](小于3.00%),忽略生物炭對水解產(chǎn)物硫丹二醇的吸附,生物炭的硫丹水解效率提高7.15~36.05%,-硫丹和-硫丹仍占優(yōu)勢.中性(pH7.20)時,硫丹水解程度約28.05%,仍高于空白,水解率約提高16.57~72.57%,生物炭對硫丹催化水解較明顯.而pH為11.00時, 硫丹自身水解程度大[36](85.65%),硫丹二醇占優(yōu)勢,可與生物炭吸附,因此,發(fā)生水解的硫丹濃度明顯比空白少.

A:發(fā)生吸附硫丹的量(mg/L); B平衡濃度(α-硫丹和β硫丹的總和)(mg/L); C:發(fā)生水解硫丹的量(mg/L)

整體上,中/酸性時CAI-B和PHA-B的硫丹吸附效率較好,分別為52.65%~60.51%, 46.57%~ 47.28%.ARD-B及ZIA-B吸附效果較差(10.86%~ 21.22%).而對ACC-B和THD-B,在酸性時,吸附率約21.22%~44.01%,在中/堿性時,吸附效果較好(33.68%~57.95%).偏堿性時,商務(wù)炭的吸附效果差別不大(43.12%~50.95%).總之,中/堿性時,生物炭的硫丹去除率較高(78.84%~98.57%). CAI- B、ACC-B和THD-B的硫丹去除率(96.77%~ 98.57%)最高.

3 結(jié)論

3.1 6種生物炭對α-硫丹和β-硫丹的去除效果良好, CAI-B(美人蕉), ACC-B(菖蒲)和THD-B(再力花)的硫丹吸附效果(40.17~60.51%)與去除能力(96.77~98.57%)最優(yōu).

3.2 生物炭對硫丹的N為0.53~ 0.84,非線性化程度高.且對β-硫丹的親和力(log)比對-硫丹大.吸附行為以有機(jī)質(zhì)表面吸附為主,除介孔擴(kuò)散作用外,還有靜電、分配及水解催化.

3.3 炭-水體系的pH為5.67~10.07,在中/堿性時,硫丹水解產(chǎn)生硫丹二醇,增加溶液pH并減輕硫丹毒性.當(dāng)體系pH為3.20、5.60和7.20時,生物炭對硫丹的催化水解較明顯.且pH7.20時,對-硫丹和β-硫丹水解效率提高16.57~72.57%.

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Adsorption and catalytic hydrolysis of endosulfan on biochars derived from emergent plants.

HE Qi, CAO Feng-mei, LU Shao-yong*, CHEN Fang-xin, CAI Chun-lun

(State Environmental Protection Scientific Observation and Research Station for Lake Dongtinghu , National Engineering Laboratory for Lake Pollution Control and Ecological Restoration, Chinese Research Academy of Environmental Sciences, Beijing 100012, China). China Environmental Sciences, 2018,38(3)：1126~1132

Six kinds of biochars derived from typically wetland plants were pyrolyzed under 550℃ without oxygen, the adsorption and catalytic hydrolysis behavior of these biochars towards endosulfan were investigated .The results showed that the ash contents were higher than other agricultural wastes, rangeing from 10.88% to 31.11%. The results of characterization suggested that the biochars had a mass of aromatic domains and pores on the surface of biochars. The pores were mainly mesopores and partly fragmented. All the biochars have high adsorption capacity toward endosulfan. The adsorption was mainly occurred on the surface of biochars which contained dense organic matter. Hydrophobic, diffusion and distribution effects played significant role in the process of adsorption. Under the conditions of neutral or alkali solutions, CAI-B, ACC-B and THD-B could efficiently remove endosulfan with uptake rates of 96.77% to 98.57%. Moreover, biochar could catalyze the hydrolysis of endosulfan with an increase of hydrolysis efficiency about 16.57%~72.57% under the condition of neutral solution.

biochar；endosulfan；catalytic hydrolysis；aromatic functional group

X5

A

1000-6923(2018)03-1126-07

何 琦(1993-),女,河南南陽人,碩士,主要從事湖泊有機(jī)污染調(diào)查及控制研究.發(fā)表論文7篇.

2017-08-23

典型湖泊有毒有害化學(xué)品和水環(huán)境調(diào)查(2015FY110900)

* 責(zé)任作者, 研究員, lushy2000@163.com