植物根際分泌有機(jī)酸對生物炭吸附Pb(II)的影響

周丹丹,屈芳舟,吳 敏,儲 剛,吳文衛(wèi),2*

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植物根際分泌有機(jī)酸對生物炭吸附Pb(II)的影響

周丹丹1,屈芳舟1,吳 敏1,儲 剛1,吳文衛(wèi)1,2*

(1.昆明理工大學(xué)環(huán)境科學(xué)與工程學(xué)院,云南 昆明 650500；2.云南省環(huán)境科學(xué)研究院,云南 昆明 650034)

本研究采用室內(nèi)模擬實驗的方法,考察了生物炭(熱解溫度200, 300, 400, 500℃)對Pb(II)的吸附行為,并以草酸和檸檬酸為代表,探討有機(jī)酸對生物炭吸附Pb(II)的影響.結(jié)果表明: Langmuir模型較Freundlich模型更適合于對兩類生物炭(花生殼生物炭、松木生物炭)吸附Pb(II)的數(shù)據(jù)進(jìn)行擬合, 200℃制備的花生殼生物炭對Pb(II)的吸附容量最大;生物炭吸附Pb(II)的過程為自發(fā)過程,且花生殼生物炭強(qiáng)于松木生物炭,低溫生物炭強(qiáng)于高溫生物炭;檸檬酸濃度為2.60×10-2mmol/L及草酸濃度為7.65×10-2mmol/L以下時,其在生物炭表面的吸附為Pb(II)提供了更多的吸附位點,從而促進(jìn)了Pb(II)吸附;有機(jī)酸濃度增大以后,占據(jù)生物炭的內(nèi)部孔隙,競爭重金屬吸附位點,從而抑制了Pb(II)在生物炭上的吸附.本研究將為系統(tǒng)認(rèn)識生物炭的環(huán)境效應(yīng)提供重要的基礎(chǔ)信息,有助于全面評估有機(jī)酸影響下生物炭在環(huán)境修復(fù)中的功能.

生物炭；草酸；檸檬酸；Pb(II)；吸附

鉛進(jìn)入環(huán)境中很難去除[1],因而易在環(huán)境中積累[2].它可以通過水和食物鏈進(jìn)入人體,從而造成呼吸、消化、神經(jīng)、血液、泌尿系統(tǒng)和免疫系統(tǒng)的急性或慢性中毒癥狀,甚至死亡[3].目前,吸附法被證實是用于鉛污染環(huán)境中去除鉛離子最簡單且有效的一種方法[4].研究人員已開發(fā)多種材料作為吸附劑應(yīng)用于環(huán)境中鉛離子的去除,這些材料包括活性炭[5-7]、人工合成無機(jī)材料[8]、納米合成材料、生物炭[9]等,其中,生物炭被認(rèn)為是一種環(huán)境友好型且低成本的吸附材料.

生物炭具有多孔、比表面積較大、含氧官能團(tuán)種類較多(如羧基、羥基、酚基等)、礦物質(zhì)較為豐富[10-11]等特性,可以通過絡(luò)合作用、沉淀作用、離子交換作用和陽離子π作用等對重金屬(如鎘、銅、鉛、鉻等)具有強(qiáng)的吸附能力[12-13],因而在重金屬污染治理與修復(fù)方面具有大規(guī)模使用的潛力.目前絕大數(shù)研究關(guān)注的重點是生物炭對污染土壤中重金屬的鈍化效果及機(jī)理,而忽略了土壤(尤其是根際土壤)環(huán)境中植物根際分泌物對重金屬固持的影響.生物炭施入土壤中,植物根系可直接與生物炭顆粒相互作用,如植物細(xì)根或根毛擴(kuò)展到生物炭表面并使其大孔暴露[14],使其對植物根際分泌物(如有機(jī)酸,氨基酸和糖[15])吸附量增加,進(jìn)而降低或促進(jìn)生物炭對重金屬的固持[16-17].然而,有機(jī)酸促進(jìn)或抑制生物炭對重金屬固持的機(jī)制尚未清楚,這還亟待進(jìn)一步研究探明.

生物炭的原料來源比較廣泛,包括各種農(nóng)業(yè)[18]、工業(yè)[19]和城市生活垃圾廢棄生物質(zhì)[20].目前,農(nóng)林廢棄物因原料來源豐富、二次污染風(fēng)險低等特點而被廣泛作為生物炭原料,并越來越受到關(guān)注和重視[21].花生是我國主要經(jīng)濟(jì)作物之一,其產(chǎn)品加工過程中所產(chǎn)生的花生殼,除少量被用作粗飼料外絕大部分均被燒掉或扔掉[22]造成了資源浪費.此外,松樹作為一種速生材樹種,是我國三大造林樹種之一,同時作為木材和紙漿材樹種在云南等地區(qū)廣泛種植,但松樹產(chǎn)區(qū)廢棄的大量枝葉亦造成了極大的浪費.因此,本研究選擇農(nóng)林廢棄物(花生殼和松木屑)為原料制備生物炭,通過批量吸附實驗系統(tǒng)研究不同的植物根際分泌有機(jī)酸(草酸(二元酸)和檸檬酸(三元酸))如何影響生物炭吸附Pb(II)的內(nèi)在相應(yīng)機(jī)制.旨在為系統(tǒng)認(rèn)識生物炭的環(huán)境效應(yīng)提供重要的基礎(chǔ)信息,以有助于全面評估生物炭在重金屬污染農(nóng)田土壤修復(fù)中的功能.

1 材料和方法

1.1 材料

采用限氧升溫炭化法[23]將花生殼(PS)和松木屑(PC)于200,300,400,500℃在馬弗爐中熱解4h制得生物炭.稱取各生物炭(10g)于聚乙烯瓶中,用去離子水洗滌至pH值穩(wěn)定后于烘箱中60℃下烘干,研磨過0.25mm(60目)篩,并存于螺口玻璃瓶中備用.原樣、200,300,400,500℃條件下制備的花生殼炭和松木生物炭分別標(biāo)記為PSW、PSW2、PSW3、PSW4、PSW5、PCW、PCW2、PCW3、PCW4、PCW5.

利用元素分析儀(vario MICRO cube,Elementar,德國)測定生物炭中C、H、O和N元素含量;通過全自動物理化學(xué)吸附儀(ASAP2020M,Micromeritics,美國)利用N2測定生物炭比表面積BET;水溶性Ca2+、Mg2+利用火焰原子吸收光譜儀(FAAS)(Z- 2000,Hitachi,Japan)測定[24];生物炭樣品與KBr充分研磨混合并壓片,于傅里葉紅外光譜(Varian640-IR)上測定分析,掃描區(qū)域為4000~400/cm,分辨率4cm;pH值用pH計(AB15,Fisher Scientific,America)測定[25];灰分含量采用灼燒法[26].

1.2 吸附實驗方法

1.2.1 Pb(II)(1000mg/L)儲備液配置 采用Pb (NO3)2在0.01mg/LNaNO3溶液中配制濃度為1000mg/LPb(II)儲備溶液,其中Pb(NO3)2為優(yōu)級純.

1.2.2 生物炭對Pb(II)的吸附等溫線 生物炭對Pb(II)的吸附等溫線:預(yù)實驗結(jié)果顯示,生物炭對Pb(II)的吸附在48h內(nèi)達(dá)到平衡.利用0.01mg/L NaNO3的背景溶液將鉛儲備溶液稀釋至溶液中Pb(II)范圍為1~10mg/L.每個吸附曲線包括8個濃度,每個濃度點設(shè)置2個平行樣.按照1g/L的固液比,稱取(8±0.05)mg生物炭于8mL螺口玻璃樣品瓶中,分別加入(8±0.1)mL 1~10mg/L的Pb(II)溶液(pH值為(4.0±0.05)).螺口玻璃樣品瓶在(25±0.5)℃恒溫振蕩箱中,以120r/min振蕩48h,于2500r/min離心10min,過0.45μm微孔濾膜,利用火焰原子吸收光譜儀(FAAS)(Z-2000,Hitachi,日本)測定濾液中Pb濃度,通過方程(1)計算在不同初始0下,生物炭對Pb(II)的吸附量.

式中:e為吸附平衡時生物炭對Pb(II)的吸附量, mg/g;0和e分別為初始和吸附平衡時溶液中Pb濃度,mg/L;為溶液體積,mL;為生物炭質(zhì)量, mg.

吸附等溫線以Langmuir(2)和Freundlich(3)模型擬合,其公式如下:

式中:e和m分別為固體平衡吸附量和最大吸附量,mg/g;e為吸附平衡時溶液中Pb濃度,mg/L;L為Langmuir 模型吸附系數(shù),L/mg;F為Freundlich模型吸附系數(shù),(mg/g)?(mg/L);為Freundlich常數(shù).

由于數(shù)據(jù)點的數(shù)量和模型中系數(shù)的數(shù)量是不同的,常用的確定系數(shù)2不能直接比較[27].通過式(4)將2轉(zhuǎn)化為adj2進(jìn)行比較:

式中:是用于擬合的數(shù)據(jù)點數(shù)量;為方程中系數(shù)的數(shù)量.

1.2.3 有機(jī)酸對生物炭吸附Pb(II)的影響實驗 土壤中有機(jī)酸濃度范圍一般在mmol至mmol數(shù)量級之間,但植物根際環(huán)境中有機(jī)酸濃度(如盛花期越橘根系分泌的草酸)可高達(dá)5.67×10-1mmol/L[28].因此,本研究中草酸實驗濃度范圍均設(shè)置為5.53×10-3~ 5.53×10-1mmol/L.檸檬酸的實驗濃度范圍均設(shè)置為2.60×10-3~2.60×10-1mmol/L.

按照1g/L的固液比,稱取(8 ±0.05)mg生物炭放入8mL螺口玻璃樣品瓶中,加入(8 ±0.1) mL濃度為5.53×10-3~5.53×10-1mmol/L草酸或2.60×10-3~2.60× 10-1mmol/L檸檬酸和5mg/LPb(II)的混合溶液(每個吸附曲線包括8個有機(jī)酸濃度,且每個有機(jī)酸濃度下含5mg/L Pb(II)),并用0.1mol/LHNO3或0.1mg/LNaOH溶液調(diào)節(jié)溶液的pH值為(4.00±0.05),于25℃下以120r/min振蕩 48h,懸濁液以2500r/min離心10min,過0.45μm微孔濾膜,利用火焰原子吸收光譜儀(FAAS)(Z-2000,Hitachi,日本)測定濾液中Pb的濃度.每組實驗設(shè)置平行2個,并作空白對照.通過式(5)計算在不同濃度的有機(jī)酸溶液中,生物炭對Pb(II)的吸附量.

式中:Pb為吸附平衡時生物炭對Pb(II)的吸附量, mg/kg;0和e分別為初始和吸附平衡時溶液中Pb濃度,mg/L;為溶液體積,mL;為生物炭質(zhì)量,mg.

2 結(jié)果與討論

2.1 生物質(zhì)及其制備的生物炭特性

生物質(zhì)(PSW和PCW)及其制備的生物炭特性見表1.花生殼生物炭比表面積、水溶性陽離子含量、灰分和pH值均高于松木生物炭(尤其是在高溫?zé)峤鈼l件下),如PSW5的比表面積(BET136.0m2/g)、礦物元素含量(如Ca2+119mg/kg)、灰分含量(8.41%)和pH值(8.17)高于PCW5比表面積(BET114m2/g)、礦物元素含量(如Ca2+26.6mg/kg)、灰分含量(1.85%)和pH值(6.66),這與花生種植過程中大量使用復(fù)合肥有關(guān),且灰分含量高導(dǎo)致生物炭呈堿性[29];而兩類生物炭H/C、O/C和(O+N)/C無明顯差異.

表1 生物質(zhì)及其制備生物炭的物理化學(xué)性質(zhì) Table 1 Physical and chemical properties of biomass and their produced biochars

此外,隨熱解溫度升高,生物炭C含量和BET增加,而O和H含量降低,說明高溫生物炭的碳化程度更高[30];同時,生物炭的O/C、(O+N)/C和H/C原子比逐漸降低,表明了高溫生物炭含氧官能團(tuán)較少,芳香結(jié)構(gòu)更完備[31];水溶性陽離子(Ca2+和Mg2+)含量逐漸減少,說明熱解溫度越高越不利于生物炭中礦物組分的溶出,這是由高溫生物炭中難溶鈣、鎂晶體礦物(如:焦磷酸、堿式硫酸鎂等)的形成決定的.

利用傅里葉紅外光譜(FTIR)對生物炭表面官能團(tuán)進(jìn)行表征,其結(jié)果見圖1.不同生物質(zhì)來源制備的生物炭所含官能團(tuán)的種類相差不大,但同種官能團(tuán)的吸光度卻略有差別,這表明生物質(zhì)來源將影響生物炭中同種官能團(tuán)的含量.隨著熱解溫度的升高,生物炭中羥基(3800~3200cm)[32]伸縮振動峰減小,這是由于熱解溫度升高使得纖維素等發(fā)生脫水、脫羥基作用而導(dǎo)致.當(dāng)熱解溫度超過300℃后,生物質(zhì)原料中的碳水化合物、脂肪族化合物和脂環(huán)族化合物分解加劇使-CH2的C-H反對稱伸縮振動峰(2942,2904cm-1)減弱并消失.芳香族CH振動峰(640cm)隨熱解溫度的升高變的逐漸明顯,這表明熱解溫度越高,生物炭中所含的非極性脂肪族官能團(tuán)越少,其芳香性越強(qiáng)[33].羧基和酮類中的C=O容易被熱解生成氣體或液體副產(chǎn)物[4],從而使熱解溫度升高生物炭中羧基和酮類中的C=O或芳香環(huán)中C=C (1628cm-1)伸縮振動峰降低.隨熱解溫度的升高, C=C(1516,1512cm-1)和PO43-(1057cm-1)伸縮震動峰消失[32-34],這可能是熱解溫度升高生物炭中磷酸鹽晶體的形成所致.

圖1 生物質(zhì)及其制備生物炭的傅里葉紅外光譜 Fig.1 Stacked FTIR spectra of biomass and their producedbiochars

2.2 生物炭對Pb(II)的吸附等溫線

利用LM和FM對生物炭吸附Pb(II)的等溫線(圖 2)進(jìn)行擬合,其擬合參數(shù)見表2.Langmuir和Freundlich方程都能夠描述生物炭對Pb(II)的吸附等溫線,但LM對PSW、PCW及生物炭吸附Pb(II)的數(shù)據(jù)進(jìn)行擬合,所得校正相關(guān)系數(shù)(adj2)(0.91~0.99)高于FM(0.82~0.97),這表明LM更適合于描述PSW、PCW及生物炭對Pb(II)的吸附;而利用FM擬合所得非線性指數(shù)()值在0.23~0.78之間(<1.0),且隨熱解溫度的升高而降低,說明PSW、PCW及生物炭對Pb(II)的吸附具有很強(qiáng)的異質(zhì)性.

通過計算0.1mg/L和10mg/LPb(II)條件下,生物炭吸附Pb(II)的吸附系數(shù)(d)值(d=Q/C,L/g-,表2)來比較各生物炭對Pb(II)的吸附量.隨Pb(II)濃度的增加,花生殼生物炭和松木生物炭的d均減小,這是因為生物炭對Pb(II)的吸附為非線性吸附.花生殼生物炭對Pb(II)吸附量高于松木生物炭,如PSW2對Pb(II)最大吸附量為(4.09±0.34)mg/g高于PCW2對Pb(II)最大吸附量為(3.66±0.45)mg/g,這與花生殼生物炭較松木生物炭具有更高的比表面積和水溶性陽離子含量(表1)能夠為Pb(II)提供更多的吸附位點有關(guān).此外,隨熱解溫度的升高,兩類生物炭對Pb(II)的吸附量均呈下降趨勢,這可以從兩個方面進(jìn)行解釋.一是,生物炭中含氧官能團(tuán)含量隨熱解溫度的升高而降低(表2和圖2),從而減少了生物炭表面與Pb(II)發(fā)生絡(luò)合作用的位點,進(jìn)而使生物炭對Pb(II)吸附量降低;二是,熱解溫度升高導(dǎo)致生物炭中水溶性陽離子含量降低(表1),從而降低生物炭的可交換性陽離子容量,致使生物炭對Pb(II)的吸附量減少.

溫度為298K時,Pb(II)在生物炭上的吉布斯自由能變(Δ= –×lnd,kJ/mol)的比較如圖3所示.生物炭對Pb(II)的吸附過程中Δ為負(fù)值,這表明溫度在298K時,Pb(II)在生物炭上的吸附是一種自發(fā)過程,而Δ絕對值越小其吸附的推動力也就越弱.Pb(II)在兩類生物炭上的Δ絕對值大小順序為花生殼生物炭>松木生物炭,說明Pb(II)更容易與花生殼生物炭發(fā)生相互作用,這可能是由于花生殼中水溶性陽離子含量和比表面積較高,促進(jìn)了生物炭對Pb(II)的吸附.隨熱解溫度的升高,Pb(II)在生物炭上的Δ絕對值呈上升趨勢,這與吸附作用的強(qiáng)弱順序一致,說明熱解溫度越高,Pb(II)在生物炭上的吸附自發(fā)程度越小,這與低溫生物炭中水溶性陽離子含量更高且表面含氧官能團(tuán)更為豐富,能夠為Pb(II)提供更多的吸附位點有關(guān).

圖2 生物炭對Pb(II)的吸附等溫線 Fig.2 Sorption isotherms of Pb(II) onbiochars

表2 FM和LM模型對生物炭吸附Pb(II)等溫線擬合參數(shù) Table 2 Fitted parameters of Pb(II) adsorption on sorbents base on FM and LM

注:logF為Freundlich模型吸附系數(shù), (mg/g)/(mg/L);樣品重復(fù)數(shù)=2.

圖3 Pb(II)在花生殼生物炭和松木生物炭的吉布斯自由能變(ΔG)的比較 Fig.3 Gibbs free energy change (ΔG) of Pb(II) in PS biocharsand PC biochars at 298K

2.3 有機(jī)酸對生物炭吸附Pb(II)的影響

檸檬酸和草酸濃度分別在2.60×10-3~2.60× 10-1mmol/L和5.53×10-3~5.53×10-1mmol/L范圍內(nèi),生物炭對Pb(II)的吸附量隨其濃度的增加呈先增加后減少的趨勢(圖4,5).如PSW4對Pb(II)的吸附量約在檸檬酸濃度為1.88×10-2mmol/L和草酸濃度為7.65×10-2mmol/L時達(dá)到最大,當(dāng)其濃度繼續(xù)升高,檸檬酸和草酸均對Pb(II)吸附有抑制作用;PCW4對Pb(II)的吸附量約在檸檬酸濃度為3.60× 10-2mmol/L和草酸濃度為7.65×10-2mmol/L時達(dá)到最大,當(dāng)其濃度繼續(xù)升高,檸檬酸和草酸均對Pb(II)吸附有抑制作用.

由此可知,低濃度檸檬酸和草酸均對生物炭吸附Pb(II)起促進(jìn)作用,而高濃度草酸和檸檬酸均對生物炭吸附Pb(II)起抑制作用,這與吳成等[35]研究較為一致,這可以從3個方面進(jìn)行解釋.一是,低濃度檸檬酸(<3.62×10-2mmol/L)和草酸(<7.70×10-2mmol/L)可能使生物炭表面負(fù)電荷[36]和含氧官能團(tuán)[37]含量增加,導(dǎo)致吸附平衡前后體系中的pH值升高0.83,從而為生物炭吸附Pb(II)提供了更多的吸附位點,進(jìn)而增加生物炭對Pb(II)吸附量.二是,在本實驗有機(jī)酸濃度范圍內(nèi),檸檬酸濃度33.62×10-2mmol/L和草酸37.70×10-2mmol/L時,吸附平衡前后體系中的pH值降低0.52,從而增加了體系中有機(jī)酸根陰離子含量.液相中大量的有機(jī)酸根陰離子能夠與生物炭表面含氧官能團(tuán)(如–OH、–COOH等)形成氫鍵[38]而競爭Pb(II)在生物炭上的吸附位點,從而降低Pb(II)生物炭上的吸附量.三是,有機(jī)酸在生物炭上的吸附飽和以后,液相中大量的有機(jī)酸與Pb(II)的絡(luò)合,使Pb(II)在固體顆粒上的吸附降低,從而出現(xiàn)高濃度有機(jī)酸抑制生物炭吸附Pb(II).在實驗濃度范圍內(nèi),大量檸檬酸和草酸在生物炭上的吸附[37]且吸附飽和分別發(fā)生在1.04×10-1mmol/L和2.21×10-1mmol/L以上.與此相反,有機(jī)酸抑制生物炭吸附Pb(II)拐點出現(xiàn)在有機(jī)酸濃度遠(yuǎn)低于這一濃度,如草酸抑制PCW4吸附Pb(II)的拐點出現(xiàn)在其濃度為7.65×10-2附近.因而本實驗體系中可能主要是有機(jī)酸占據(jù)了生物炭內(nèi)部孔隙[39],從而減少生物炭對Pb(II)的物理吸附.此外,檸檬酸濃度為3.62×10-2mmol/L和草酸濃度為7.70×10-2mmol/L時,草酸和檸檬酸對生物炭吸附Pb(II)起促進(jìn)或抑制的程度無明顯差異.根據(jù)草酸鉛為1:1型絡(luò)合物和檸檬酸鉛為2:3型絡(luò)合物,則可以通過計算發(fā)現(xiàn)兩者在該濃度下給出的羧酸根濃度接近,能夠絡(luò)合的鉛離子濃度也接近.

圖4 檸檬酸對生物炭吸附Pb(II)的影響 Fig.4 Effects of citric acid on the adsorption of Pb(II) by biochars

根據(jù)在草酸和檸檬酸存在條件下,生物炭對Pb(Ⅱ)的吸附數(shù)據(jù)分析,繪制出有機(jī)酸影響生物炭吸附Pb(Ⅱ)機(jī)制的示意圖(圖6).有機(jī)酸影響生物炭吸附Pb(Ⅱ)機(jī)制主要包括以下5種:①有機(jī)酸含有的活性基團(tuán)(如羧基)在生物炭上的吸附為Pb(Ⅱ)提供了更多吸附位點,促進(jìn)生物炭對Pb(Ⅱ)的吸附;②有機(jī)酸在生物炭上的吸附能使其生物炭表面負(fù)電荷增加,從而使生物炭對Pb(Ⅱ)吸附量增加;③有機(jī)酸能夠降低溶液中的pH值并增加了有機(jī)酸根陰離子含量,液相中大量的有機(jī)酸根陰離子能夠競爭Pb(Ⅱ)在生物炭上的吸附位點,從而抑制生物炭對Pb(Ⅱ)的吸附;④有機(jī)酸能填塞生物炭中部分孔隙[39-40],降低生物炭對Pb(Ⅱ)吸附量;⑤液相中有機(jī)酸與Pb(Ⅱ)發(fā)生絡(luò)合反應(yīng),從而使生物炭對Pb(Ⅱ)吸附量降低.

圖6 有機(jī)酸影響生物炭吸附Pb(II)機(jī)制示意 Fig.6 Schematic graph of Pb(II)adsorption on biochars as affected by organic acids

3 結(jié)論

3.1 與松木生物炭相比,花生殼生物炭具有更高的比表面積和水溶性陽離子含量,從而使其對Pb(II)的吸附量較高.Langmuir和Freundlich模型都能夠描述生物炭對Pb(II)的吸附等溫線,但LM更適合于描述PSW、PCW及生物炭對Pb(II)的吸附;FM擬合所得非線性指數(shù)()值在0.23~0.78之間,說明兩類生物炭對Pb(II)的吸附均具有很強(qiáng)的異質(zhì)性.熱解溫度升高,生物炭對Pb(II)的吸附量呈下降趨勢,PSW2對Pb(II)的吸附容量最大,這與低溫生物炭中含有更多的含氧官能團(tuán)和可溶性陽離子有關(guān).

3.2 溫度為298K時,生物炭對Pb(II)的吸附過程中Δ為負(fù)值,這表明Pb(II)在生物炭上的吸附是一種自發(fā)過程,且Δ絕對值大小的順序為花生殼生物炭>松木生物炭.熱解溫度越高,Pb(II)在生物炭上的吸附自發(fā)程度越小,這與低溫生物炭中水溶性陽離子含量更高且表面含氧官能團(tuán)更為豐富,能夠為Pb(II)提供更多的吸附位點有關(guān).

3.3 檸檬酸和草酸濃度分別在2.60×10-3~2.60× 10-1mmol/L和5.53×10-3~5.53×10-1mmol/L時,生物炭對Pb(II)的吸附量均隨有機(jī)酸濃度的增加呈現(xiàn)先增加后減少的趨勢.拐點出現(xiàn)在檸檬酸濃度為2.60× 10-2mmol/L及草酸濃度為7.65×10-2mmol/L附近.

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致謝：本研究的相關(guān)實驗工作由昆明理工大學(xué)的張軍等協(xié)助完成,在此表示感謝.

Effects of organic acids secreted from plant rhizosphereon adsorption of Pb(II) by biochars.

ZHOU Dan-dan1, QU Fang-zhou1, WU Min1, CHU Gang1, WU Wen-wei1,2*

(1.Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650500, China；2.Yunnan Institute of Environmental Science, Kunming 650034, China)., 2019,39(3)：1199~1207

Biochar is a solid product produced by pyrolysis of waste biomass under anaerobic oxygen conditions. Because of its porosity, large specific surface area, rich oxygen-containing functional groups, rich minerals and strong stability, biochar has been widely used in the remediation of heavy metal contaminated soils. The passivation effect of biochar on heavy metals is affected by not only the properties of heavy metals and biochar, but also organic acids secreted from plant rhizosphere in soil. However, the effects of organic acids on biochar-heavy metal interactions have not been studied limitedly. Therefore, in this paper, oxalic acid and citric acid were chosen as organic acids to explore the mechanisim on the adsorption behavior change of Pb(II) on biochars (different pyrolysis temperatures at 200, 300, 400and 500℃). Comparing with the Freundlich model, the Langmuir model was more suitable for fitting the adsorption data of Pb(II) on two types of biocharsi.e peanut shell biocarsand pine biochars. The peanut shell biocars prepared at 200℃ had maximum adsorption capacityon Pb(II). The process of Pb(II) adsorption by biocharswas spontaneous, the adsorption stability of peanut shell biocars was stronger than pine biochars, and the adsorption stability decreased as pyrolysis temperature increased. When the concentration of citric acid and oxalic acid was below 2.60×10-2mmol/L and7.65×10-2mmol/L respectively, the adsorbed organic acids would provide more binding sites for Pb(II), thus promoting the adsorption of Pb(II). When the concentration of organic acids increased, the internal pores of biochars may be occupied by the organic acids, which would compet for the binding sites of heavy metals and thereby inhibite the adsorption of Pb(II) on biochars. This study will provide important basic information for systematically understanding the environmental effects of biochars, and will help to comprehensively evaluate the function of biochars in environmental remediation in the presence of organic acids.

biochar；oxalic acid；citric acid；Pb(II)；adsorption

X131.3

A

1000-6923(2019)03-1199-09

周丹丹(1984-),女,安徽安慶人,昆明理工大學(xué)講師,博士,主要從事污染物環(huán)境化學(xué)行為研究.發(fā)表論文15篇.

2018-07-13

國家自然科學(xué)基金資助項目(41703121);昆明理工大學(xué)人才啟動項目(KKSY201722006);云南省重點研發(fā)計劃資助(2018BC004)

* 責(zé)任作者, 教授級高級工程師, wuwwp@163.com