蘭天， 楚穎超， 張玲玲， 趙文， 潘運(yùn)舟， 張家瑋， 朱治強(qiáng)， 吳蔚東*

(1.海南大學(xué)熱帶作物種質(zhì)資源保護(hù)與開發(fā)利用教育部重點(diǎn)實(shí)驗(yàn)室，?？?570228；2.海南省農(nóng)業(yè)科學(xué)院海南省耕地保育重點(diǎn)實(shí)驗(yàn)室，海口 571100)

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椰衣和椰殼生物質(zhì)炭的制備及其對(duì)溶液中Pb2+的吸附

蘭天1,2， 楚穎超1， 張玲玲1， 趙文1， 潘運(yùn)舟1,2， 張家瑋1， 朱治強(qiáng)1， 吳蔚東1*

(1.海南大學(xué)熱帶作物種質(zhì)資源保護(hù)與開發(fā)利用教育部重點(diǎn)實(shí)驗(yàn)室，海口 570228；2.海南省農(nóng)業(yè)科學(xué)院海南省耕地保育重點(diǎn)實(shí)驗(yàn)室，?？?571100)

以椰衣和椰殼作為原材料，在300、500和700 ℃條件下熱解制備生物質(zhì)炭，表征其物理化學(xué)性質(zhì)；同時(shí)，研究所制備的生物質(zhì)炭對(duì)溶液中Pb2+的吸附特征與機(jī)制。結(jié)果表明：隨著熱解溫度升高，所制備的生物質(zhì)炭的含氧官能團(tuán)減少，灰分、pH值、陽(yáng)離子交換量、比表面積和堿性官能團(tuán)的含量隨之升高。熱解溫度升高可促進(jìn)生物質(zhì)炭對(duì)Pb2+的吸附；Langmuir模型可較好地描述所制備的生物質(zhì)炭對(duì)Pb2+的等溫吸附；在供試的6種生物質(zhì)炭中，吸附量最高的是在700 ℃條件下制備的椰衣生物質(zhì)炭，且優(yōu)于大多數(shù)已報(bào)道的用其他材料制備的生物質(zhì)炭。擬合發(fā)現(xiàn)，所制備的生物質(zhì)炭的陽(yáng)離子交換量和灰分含量是影響其吸附Pb2+的重要因子，在初始Pb2+質(zhì)量濃度為200 mg/L條件下，椰衣生物質(zhì)炭對(duì)Pb2+的穩(wěn)定吸附量為9.83～13.91 mg/g，椰殼生物質(zhì)炭為9.68～25.16 mg/g。這表明椰殼生物質(zhì)炭吸附態(tài)Pb2+比椰衣生物質(zhì)炭吸附態(tài)Pb2+更穩(wěn)定。

椰衣； 椰殼； 生物質(zhì)炭； 熱解溫度； 理化性質(zhì)； 吸附； Pb2+

Summary Heavy metal pollution has become more and more serious with the development of industry. Heavy metals can accumulate in plants, and enter into human body through the food chain. Lead (Pb) is one of the most common heavy metal contaminants in the environment, usually discharged with waste water and waste gas. Accumulation of lead in the body can cause physical function disorder, and it is even more harmful to children. Therefore, it is very urgent to control the lead pollution. As a new type of adsorbent for soil heavy metal, biochar has a great advantage compared with other traditional materials. Biomass-derived biochar combines an effective removal of water contaminants with low cost of remediation. Many researchers used waste biomass crops and other materials to make biochar by pyrolysis at high temperature to remove heavy metals. The water movement on the biochar pores is affected by metal adsorption, which is a novel and promising technique to identify the nature of biochar-metal bonds at the solid-liquid interface.

However, few researches were reported using common tropical biomass waste such as coconut shell and coconut fiber to prepare biochars. The coconut fiber become thick after matured, and the loose brown fibrous layers are soft and elastic, mainly are composed of hard stratum corneum. The coconut shell and coconut fiber are abundant, cheap and easy to collect in Hainan Province. Therefore, study on the adsorption of lead by biochars prepared from coconut shell and coconut fiber has a great significance to control heavy metal pollution. The objectives of this study were to: 1) characterize the biochars prepared from coconut shell and coconut fiber under different temperatures; 2) determine the adsorption characteristics and mechanisms of Pb2+on biochars.

Biochars were prepared by pyrolyzing coconut fiber and coconut shell at the temperatures of 300, 500 and 700 ℃ respectively, and their physical and chemical properties were analyzed.

The results of Fourier transform infrared spectroscopy and element contents indicated that, the carbonation degree of the biochars increased with the pyrolysis temperature, while the amount of oxygen-containing functional groups decreased. In addition, the surface area, ash content, pH, cation exchange capacity (CEC) and basic functional group content enriched with the pyrolysis temperature. The adsorption of Pb2+by biochars derived from coconut fiber and coconut shell was fitted better with Langmuir model. As the pyrolysis temperature increased from 300 to 700 ℃, the amount of Pb2+adsorbed on the biochars gradually increased. Among the six kinds of biochars, the coconut fiber biochar prepared at 700 ℃ had the highest adsorption amount of 180.438 mg/g, which was better than many adsorption materials that reported previously. The CEC and ash content were important factors affecting the adsorption deduced from the fit curve. Under the condition of initial Pb2+concentration with 200 mg/L, the saturated adsorption amounts of coconut fiber and coconut shell biochars were 44.89-96.08 mg/g and 15.82-61.77 mg/g, respectively. The saturated adsorption amount of coconut fiber biochars was higher than that of the coconut shell biochars. However, the stable adsorption amounts of coconut fiber and coconut shell biochars were 9.83-13.91 mg/g and 9.68-25.16 mg/g, respectively.

In conclusion, the different preparation temperatures can directly affect the physical and chemical properties of biochars. Different raw material sources and pyrolysis temperatures have a great influence on the adsorption capacity of Pb2+, and the largest amount of lead adsorption material is YA700(180.438 mg/g). In addition, different raw material sources and pyrolysis temperatures have a great influence on the stable adsorption quantity of lead, and the adsorption of Pb2+on the coconut shell biochar is more stable than coconut fiber biochar.

生物質(zhì)炭是一種在限氧條件下通過(guò)熱解制備的碳材料[1-2]，可以作為土壤改良劑增加土壤碳匯，提高土壤保水保肥能力[3-4]。同時(shí)，生物質(zhì)炭可以有效地鈍化環(huán)境中的重金屬和有機(jī)污染物，降低有害物質(zhì)的生物有效性，提高作物產(chǎn)量[5]。與傳統(tǒng)材料相比，生物質(zhì)炭作為一種重金屬吸附劑有很大的優(yōu)勢(shì)，是一種高效、廉價(jià)的重金屬吸附材料。利用農(nóng)業(yè)廢棄物在高溫條件下制備生物質(zhì)炭，研究其在水溶液中對(duì)重金屬的吸附越來(lái)越受到人們的重視[6-7]。如XU等[8]研究發(fā)現(xiàn)：在200 ℃條件下利用牛糞制備的生物質(zhì)炭(DM200)對(duì)溶液中Cu2+、Zn2+和Cd2+的吸附量分別為48.4、31.6和31.9 mg/g；而DM350對(duì)溶液中Cu2+、Zn2+和Cd2+的吸附量分別為54.4、32.8和51.4 mg/g。生物質(zhì)炭之所以能夠吸附重金屬，是由于生物質(zhì)炭表面具有豐富的極性官能團(tuán)，例如羧基、氨基等，這些官能團(tuán)能夠有效地去除溶液中的重金屬；同時(shí)，生物質(zhì)炭表面具有可以和重金屬發(fā)生沉淀的磷酸根和碳酸根離子[9]。

工農(nóng)業(yè)的快速發(fā)展帶來(lái)越來(lái)越嚴(yán)重的重金屬污染，重金屬在植物體內(nèi)富集，可通過(guò)食物鏈進(jìn)入人體[10]。如Pb通過(guò)工業(yè)廢水和廢氣排放進(jìn)入環(huán)境中，是一種很常見的重金屬污染物。Pb在人體內(nèi)不斷積累可以導(dǎo)致身體功能障礙，對(duì)兒童的生長(zhǎng)發(fā)育有很大的負(fù)面影響。因此，采取有效的手段治理Pb污染變得尤為迫切[6]。生物質(zhì)炭對(duì)Pb的等溫吸附數(shù)據(jù)可以用Langmuir和Freundlich模型很好地?cái)M合[11]。

目前，利用熱帶生物資源制備生物質(zhì)炭用以去除溶液中重金屬的研究較少。海南省椰殼和椰衣資源豐富，價(jià)格便宜，取材方便。椰子成熟以后，中層果皮椰衣為厚而疏松的棕色纖維層，質(zhì)軟、富有彈性，椰衣纖維屬于硬質(zhì)纖維，彈性和韌性較強(qiáng)；內(nèi)果皮即椰殼，為緊密而堅(jiān)硬的角質(zhì)層[12]。利用椰殼和椰衣制備生物質(zhì)炭，研究其對(duì)溶液中Pb2+的吸附，對(duì)于利用熱帶生物資源修復(fù)重金屬污染具有重要的理論和現(xiàn)實(shí)意義。

1　材料與方法

1.1生物質(zhì)炭的制備

椰子廢棄物取自海南省文昌市。將椰子擦洗干凈，椰殼和椰衣分離，取成熟度一致的椰殼與椰衣足量，切成1 cm × 1 cm × 1 cm大小，在自然條件下風(fēng)干1～2周。分別將一定量風(fēng)干的椰殼和椰衣置于馬弗爐中，在300、500和700 ℃下炭化4 h，冷卻至室溫后磨碎過(guò)2 mm篩，分別記為KA300、KA500、KA700、YA300、YA500、YA700(KA表示椰殼，YA表示椰衣；300、500和700分別表示生物質(zhì)炭的制備溫度)，用密封袋保存，供后續(xù)試驗(yàn)使用[13]。

1.2生物質(zhì)炭理化性質(zhì)表征

利用元素分析儀(EA2400，美國(guó)PE公司)測(cè)定生物質(zhì)炭中的C、H、O、N含量；用Boehm滴定法定量分析生物質(zhì)炭中酸堿官能團(tuán)的含量；用精密酸度計(jì)測(cè)定生物質(zhì)炭的pH值(生物炭的質(zhì)量與水的體積比為1/100)；采用灼燒法測(cè)定生物質(zhì)炭的灰分含量；利用1 mol/L中性乙酸銨法測(cè)定生物質(zhì)炭中的陽(yáng)離子交換量(cation exchange capacity，CEC)[14-15]；采用KBr壓片法制樣，用紅外光譜儀(TENSOR27，德國(guó)Bruker公司)測(cè)定椰殼生物質(zhì)炭吸附劑的紅外圖譜，掃描范圍400～4 000 cm-1，檢測(cè)生物質(zhì)炭中的官能團(tuán)成分及含量；利用掃描電鏡(S-3000N，日本日立公司)觀察生物質(zhì)炭的表面形貌特征。

1.3等溫吸附試驗(yàn)

用Pb(NO3)2配置質(zhì)量濃度為25、50、100、150、300和450 mg/L Pb2+溶液，以0.01 mol/L NaNO3為背景電解質(zhì)，用0.1 mol/L HNO3和NaOH調(diào)節(jié)溶液pH值至5.0，稱取0.050 0 g生物質(zhì)炭于50 mL離心管中，分別加入25 mL已配置的不同質(zhì)量濃度的Pb2+溶液，放入25 ℃恒溫振蕩箱中以200 r/min振蕩24 h，過(guò)濾測(cè)定。每個(gè)處理設(shè)3個(gè)重復(fù)。分別用Freundlich和Langmuir模型擬合生物質(zhì)炭在 25 ℃下對(duì)Pb2+的吸附等溫線。Langmuir模型理論的假設(shè)條件為在均一表面進(jìn)行的單分子層吸附，且被吸附的分子之間無(wú)任何相互作用；Freundlich模型描述的是多層吸附，在高濃度時(shí)吸附容量持續(xù)增加，常用于描述物理吸附。

Langmuir方程：Ce/Q=1/(b·Qm)+Ce/Qm。

(1)

Freundlich方程：lnQ=lnKf+1/n·lnCe。

(2)

其中：Ce為平衡濃度，mg/L；Q為平衡吸附量，mg/g；Qm為最大吸附量，mg/g；b為吸附平衡常數(shù)，L/mg，表示吸附劑的吸附位點(diǎn)對(duì)重金屬離子的親和力大?。籏f為吸附容量，mg/g；n為Freundlich常數(shù)，表示吸附強(qiáng)度大小[16]，g/L。

1.4解吸試驗(yàn)

取在初始質(zhì)量濃度200 mg/L下平衡吸附Pb2+的生物質(zhì)炭0.05 g，分別使用25 mL去離子水和1.0 mol/L HCl溶液進(jìn)行連續(xù)提取，然后置于25 ℃恒溫振蕩箱中以200 r/min振蕩24 h，過(guò)濾后用火焰原子吸收法(M6，美國(guó)Termo Elemental公司)測(cè)定溶液中的Pb2+含量[17]。

1.5數(shù)據(jù)分析

試驗(yàn)所得數(shù)據(jù)采用SPSS 17.0進(jìn)行分析，吸附數(shù)據(jù)采用Origin Pro 9.0進(jìn)行擬合。

2　結(jié)果與分析

2.1生物質(zhì)炭的理化性質(zhì)

在不同熱解溫度下椰衣和椰殼生物質(zhì)炭的理化性質(zhì)存在明顯差異(表1)。熱解溫度從300 ℃升到700 ℃，椰衣生物質(zhì)炭的灰分含量從6.11%增加到11.08%，椰殼生物質(zhì)炭的灰分含量從1.90%增加到3.26%，前者的灰分含量總體上高于后者。在不同溫度下制備的生物質(zhì)炭的CEC變化范圍是36.53～92.36 cmol/kg；當(dāng)熱解溫度為500 ℃時(shí)椰衣和椰殼生物質(zhì)炭的CEC值達(dá)到最高，分別為92.36和49.71 cmol/kg；同時(shí)，椰衣生物質(zhì)炭的CEC高于在相同溫度下用椰殼制備的生物質(zhì)炭。隨著制炭溫度的升高，椰衣和椰殼生物質(zhì)炭的堿性官能團(tuán)分別從0.43和0.21 mmol/g上升到0.73和0.45 mmol/g；酸性官能團(tuán)則分別從1.04和0.62 mmol/g下降到0.42和0.19 mmol/g。這主要是由于羧基、酚羥基、內(nèi)酯等含氧官能團(tuán)含量下降導(dǎo)致的。生物質(zhì)炭的pH值趨于中性或偏堿性，用2種原材料制備的生物質(zhì)炭pH值隨溫度的升高分別從7.28和7.15上升到10.79和10.45。

表2是在不同熱解溫度下所獲生物質(zhì)炭的元素組成。當(dāng)制備溫度從300 ℃上升到500 ℃時(shí)，生物質(zhì)炭的含C量顯著上升，隨著溫度繼續(xù)上升，其含量上升不再明顯。椰衣生物質(zhì)炭的含N量隨著熱解溫度的上升而增加，椰殼生物質(zhì)炭的含N量隨著熱解溫度的上升先增加后減少。H元素含量隨著熱解溫度的升高逐漸降低，H/C值隨之下降；椰衣生物質(zhì)炭的H/C值大于椰殼生物質(zhì)炭。熱解溫度升高會(huì)提高生物質(zhì)炭的芳香性；椰衣生物質(zhì)炭的芳香性和極性比椰殼生物質(zhì)炭高。在制備溫度較高的條件下，O/C和(O+N)/C值較低。這反映出在高溫條件下制備的生物質(zhì)炭含氧官能團(tuán)較少。在利用Boehm法的滴定結(jié)果中酸、堿官能團(tuán)的變化規(guī)律也證實(shí)了這一點(diǎn)。

表1　生物質(zhì)炭的物理化學(xué)性質(zhì)

CEC：陽(yáng)離子交換量；SBET：比表面積；(YA300，YA500，YA700)：分別表示在300、500和700 ℃下用椰衣制備的生物質(zhì)炭；(KA300，KA500，KA700)：分別表示在300、500和700 ℃下用椰殼制備的生物質(zhì)炭。

CEC: Cation exchange capacity;SBET: Specific surface area; (YA300, YA500, YA700): Coconut fiber biochars prepared at 300, 500 and 700 ℃, respectively; (KA300, KA500, KA700): Coconut shell biochars prepared at 300, 500 and 700 ℃, respectively.

表2　生物質(zhì)炭的元素組成與原子比例

各生物質(zhì)炭處理表示的含義詳見表1注。

Please see footnote of Table 1 for details of each biochar treatment.

從圖2可以看出：在掃描電鏡下椰衣生物質(zhì)炭表面呈蜂窩狀，孔道結(jié)構(gòu)完整；椰殼生物質(zhì)炭表面沒(méi)有固定的形態(tài)。隨著熱解溫度的升高，生物質(zhì)炭表面孔隙增多，斷面變得無(wú)序，原有結(jié)構(gòu)出現(xiàn)破壞現(xiàn)象?？梢酝茢?，隨著熱解溫度升高，生物質(zhì)炭有比表面積增大、微孔體積增加的趨勢(shì)；對(duì)比表面積的測(cè)定結(jié)果也證實(shí)了這一點(diǎn)。當(dāng)生物質(zhì)炭的制備溫度在500 ℃以下時(shí)，生物質(zhì)炭的比表面積均小于5 m2/g，當(dāng)制備溫度達(dá)到700 ℃，椰衣和椰殼生物質(zhì)炭的比表面積驟然增加到434.70和398.40 m2/g(表1)。

圖1　椰衣和椰殼生物質(zhì)炭的紅外圖譜Fig.1　Fourier transform infrared spectroscopy of biochars prepared by coconut fiber and coconut shell

2.2等溫吸附線

圖3是用Langmuir和Freundlich模型擬合在不同溫度下制備的椰衣和椰殼生物質(zhì)炭在25 ℃下對(duì)Pb2+的等溫吸附線，表3是各參數(shù)的擬合結(jié)果。比較2種擬合模型的決定系數(shù)(R2)可知，Langmuir可以更好地描述不同熱解溫度制備的椰衣和椰殼生物質(zhì)炭對(duì)Pb2+的吸附：表明其吸附過(guò)程為單分子層吸附[17]。在Langmuir模型中b值與吸附強(qiáng)度有關(guān)，b值越大表明吸附強(qiáng)度越大。椰衣生物質(zhì)炭的吸附強(qiáng)度排序?yàn)閅A700>YA300>YA500，椰殼生物質(zhì)炭的吸附強(qiáng)度隨著熱解溫度的升高而下降。所制備的生物質(zhì)炭對(duì)Pb2+的最大吸附量(Qm)順序?yàn)閅A700>YA500>KA700>YA300>KA500>KA300。隨熱解溫度的上升，用2種原材料制備的生物質(zhì)炭對(duì)Pb2+的最大吸附量均呈現(xiàn)上升趨勢(shì)；在相同熱解溫度條件下，椰衣生物質(zhì)炭比椰殼生物質(zhì)炭具有更大的Qm值。

2.3解吸試驗(yàn)

不同提取劑對(duì)生物質(zhì)炭飽和吸附Pb2+后的解吸量見表4。去離子水解吸附的量代表物理方式的吸附量，其本質(zhì)是靜電吸附，與吸附材料的表面積有關(guān)[18]。HCl解吸出的Pb2+是生物質(zhì)炭通過(guò)陽(yáng)離子-π作用吸附以及和碳酸鹽、磷酸鹽發(fā)生沉淀[8]。生物質(zhì)炭對(duì)Pb2+的穩(wěn)定吸附量=平衡吸附量-HCl解吸附量-去離子水解吸附量。隨著制炭熱解溫度的上升，相同原材料生物質(zhì)炭對(duì)Pb2+的平衡吸附量隨之增加。椰衣生物質(zhì)炭對(duì)Pb2+的平衡吸附量為44.89～96.08 mg/g，椰殼生物質(zhì)炭對(duì)Pb2+的平衡吸附量為15.82～61.77 mg/g。說(shuō)明椰衣生物質(zhì)炭比椰殼生物質(zhì)炭具有更高的平衡吸附量。當(dāng)熱解溫度從300 ℃上升到700 ℃時(shí)，用2種原材料制備的生物質(zhì)炭對(duì)Pb2+的穩(wěn)定吸附量呈現(xiàn)不同的變化趨勢(shì)，椰衣生物質(zhì)炭對(duì)Pb2+的穩(wěn)定吸附量從13.91 mg/g減小到9.83 mg/g，椰殼生物質(zhì)炭則從9.68 mg/g增加到25.16 mg/g。

A：KA300；B：KA500；C：KA700；D：YA300；E：YA500；F：YA700。圖2　椰衣和椰殼生物質(zhì)炭的掃描電鏡表面形態(tài)Fig.2　Scanning electron microscope images of biochars prepared by coconut fiber and coconut shell

A：Langmuir模型擬合曲線；B:Freundlich模型擬合曲線。A: Fit curve by Langmuir model; B: Fit curve by Freundlich model.圖3　椰衣和椰殼生物質(zhì)炭對(duì)Pb2+的等溫吸附曲線Fig.3　Isothermal adsorption curves of biochars from coconut fiber and coconut shell for Pb2+

生物質(zhì)炭Biochars Langmuir Freundlich Qm/(mg/g)b/(L/mg)R2Kf/(mg/g)n/(g/L)R2YA30051.2060.00260.9946.5730.3430.839YA50090.3300.00180.9934.2150.5100.857YA700180.4380.00460.9922.0590.7020.958KA30032.8860.01810.9961.5930.4220.983KA50045.0150.00380.9926.4710.3230.842KA70078.4050.00170.9975.0230.4500.901

各生物質(zhì)炭處理表示的含義詳見表1注。Qm：最大吸附量；b：吸附平衡常數(shù)；Kf：吸附容量；n：吸附強(qiáng)度。

Please see footnote of Table 1 for details of each biochar treatment.Qm: Maximum adsorption capacity;b: Adsorption equilibrium constant;Kf: Adsorption capacity;n: Adsorption intensity.

表4不同提取劑對(duì)Pb2+解吸的影響

Table 4Effects of different extracting agents on desorption of Pb2+mg/g

生物質(zhì)炭BiocharsSAW-DQHCl-DQSAQYA30044.890.1630.8113.91YA50074.724.1058.8811.74YA70096.087.4278.839.83KA30015.820.116.019.68KA50039.490.1417.1622.18KA70061.771.0635.5525.16

各生物質(zhì)炭處理表示的含義詳見表1注。SA:飽和吸附量；W-DQ:去離子水解吸附量；HCl-DQ：HCl解吸附量；SAQ:穩(wěn)定吸附量。

Please see footnote of Table 1 for details of each biochar treatment. SA: Saturated adsorption quantity; W-DQ: Deionized-water desorption quantity; HCl-DQ: HCl desorption quantity; SAQ: Stable adsorption quantity.

3　討論

3.1熱解溫度對(duì)生物質(zhì)炭理化性質(zhì)的影響

椰衣和椰殼生物質(zhì)炭的灰分含量隨著熱解溫度的升高而上升，與AHMAD等[19]利用大豆秸稈、花生皮以及牛糞制備生物質(zhì)炭得出的規(guī)律相同。這是由于隨著熱解溫度的升高，有機(jī)物不斷被燃燒，生物質(zhì)炭本身的礦物質(zhì)相對(duì)不斷增加。隨著熱解溫度的上升，生物質(zhì)炭的pH值呈上升趨勢(shì)，與AL-WABEL等研究得出的結(jié)果[20]類似。這是由于熱解溫度升高導(dǎo)致了酸性官能團(tuán)含量減少，堿性官能團(tuán)含量增加；同時(shí)，灰分含量的升高也是導(dǎo)致pH值上升的原因之一[10]。椰衣生物質(zhì)炭的CEC總體上高于椰殼生物質(zhì)炭。前人的研究表明，CEC和羧基官能團(tuán)的含量以及灰分的礦物學(xué)組成(如K+、Ca2+、Mg2+等)有密切的關(guān)系[21]。通過(guò)擬合發(fā)現(xiàn)，在本研究中CEC和羧基官能團(tuán)的含量沒(méi)有明顯的相關(guān)性，但是灰分含量對(duì)CEC的影響很大。

制炭溫度對(duì)生物質(zhì)炭的元素組成影響很大，隨著熱解的升高，椰衣和椰殼生物質(zhì)炭H/C、O/C、(O+N)/C值呈下降趨勢(shì)，這與AL-WABLE等的研究結(jié)果[20]相一致。H/C和O/C值可分別用來(lái)評(píng)估生物質(zhì)炭的芳香性和極性，O/C值的下降表明生物質(zhì)炭表面含氧官能團(tuán)含量隨熱解溫度的升高而減少；YA700和KA700的H/C值分別為0.42和0.36，低于其他熱解溫度條件下的生物質(zhì)炭。這表明在700 ℃條件下制備的生物質(zhì)炭的炭化強(qiáng)度增加[22]。

YA700和KA700的比表面積遠(yuǎn)遠(yuǎn)高出在其他熱解條件下制備的生物質(zhì)炭。這與AHMAD等的研究結(jié)果[19]相類似，在700 ℃條件下他們制備的2種生物質(zhì)炭的比表面積分別為420和448 m2/g，遠(yuǎn)遠(yuǎn)高于用這2種材料在300 ℃條件下制備的生物質(zhì)炭的比表面積(分別為6和3 m2/g)。這主要是由于高溫條件去除了制炭材料本身的揮發(fā)性物質(zhì)，增加了微孔體積[23]。

3.2生物質(zhì)炭對(duì)Pb2+的吸附特征

椰衣和椰殼生物質(zhì)炭對(duì)Pb2+的等溫吸附特征符合Langmuir模型，屬于單分子層吸附。隨著制炭溫度的升高，生物質(zhì)炭對(duì)Pb2+的最大吸附量不斷增加。這是由于隨著溫度的升高，生物質(zhì)炭具有更大的比表面積和灰分含量，比表面積增大可能提供了更多的吸附位點(diǎn)[24]，灰分的釋放會(huì)加強(qiáng)金屬離子與生物質(zhì)炭表面負(fù)電荷靜電吸引作用，增加對(duì)Pb2+的吸附量[25]；同時(shí)，制備生物質(zhì)炭產(chǎn)生的碳酸鹽、硅酸鹽以及磷酸鹽可以與Pb2+反應(yīng)產(chǎn)生沉淀[26]。CEC也是影響吸附的重要因子，CEC越大，生物質(zhì)炭表面的電荷數(shù)越多，對(duì)陽(yáng)離子的靜電吸附作用越強(qiáng)[27]。與已報(bào)道的生物質(zhì)炭材料對(duì)Pb2+的吸附容量比較(表5)可知，本研究得到的6種生物質(zhì)炭吸附性能較好，其中，椰衣在700 ℃條件下熱解產(chǎn)生的生物質(zhì)炭(YA700)的吸附容量為180.438 mg/g(表3)，大于用多數(shù)生物質(zhì)材料制備的生物質(zhì)炭的吸附容量。

在本研究中離子水解吸的量占吸附總量的比重很小(0.35%～7.72%)。這說(shuō)明生物質(zhì)炭物理吸附不是主要吸附方式，與等溫吸附擬合的結(jié)果一致。HCl解吸量占吸附總量的比例很大，在38.04%～82.05%之間，其中，隨著熱解溫度的升高，吸附了Pb2+的椰衣和椰殼生物質(zhì)炭的HCl解吸量呈上升趨勢(shì)，與生物質(zhì)炭灰分含量的變化規(guī)律相吻合。同時(shí)，相關(guān)性分析結(jié)果顯示，吸附總量和CEC、灰分的相關(guān)系數(shù)分別為0.51和0.84，呈顯著正相關(guān)。這表明CEC和灰分含量是影響生物質(zhì)炭吸附重金屬總量的重要因子。椰衣和椰殼生物質(zhì)炭對(duì)Pb2+的穩(wěn)定吸附量與各制炭溫度顯著相關(guān)，相關(guān)系數(shù)分別為-0.99和0.94；隨著熱解溫度的升高，椰衣生物質(zhì)炭對(duì)Pb2+的穩(wěn)定吸附量呈下降趨勢(shì)，椰殼生物質(zhì)炭對(duì)Pb2+的穩(wěn)定吸附量呈上升趨勢(shì)。此外，椰衣生物質(zhì)炭對(duì)Pb2+的穩(wěn)定吸附量為9.83～13.91 mg/g，椰殼生物質(zhì)炭為9.68～25.16 mg/g。說(shuō)明椰殼生物質(zhì)炭比椰衣生物質(zhì)炭具有更高的Pb2+穩(wěn)定吸附量。生物質(zhì)炭的穩(wěn)定吸附量與各自的C、O含量顯著相關(guān)，相關(guān)系數(shù)分別為0.85和-0.45。這表明隨著熱解溫度的升高，生物質(zhì)炭的炭化程度增加是影響吸附穩(wěn)定性的主要因素。

表5　多種生物質(zhì)炭對(duì)Pb2+的吸附容量

SA：飽和吸附量；EIC：初始濃度范圍；SD：木屑；RB：米糠；RS：水稻秸稈；CS：玉米秸稈；WS：小麥秸稈；PS：花生殼；CB：銀杉木炭；SB：污泥生物質(zhì)炭。

SA: Saturated adsorption quantity; EIC: Extent of initial concentration; SD: Saw dust; RB: Rice bran; RS: Rice straw; CS: Corn straw; WS: Wheat straw; PS: Peanut shell; CB: Cathaya biochar; SB: Sludge biochar.

4　結(jié)論

4.1熱解溫度影響椰衣和椰殼生物質(zhì)炭的理化性質(zhì)。熱解溫度由300 ℃升至 700 ℃，生物質(zhì)炭H/C、(O+N)/C、O/C值下降，生物質(zhì)炭的炭化程度增加，含氧官能團(tuán)含量減少。生物質(zhì)炭的酸性官能團(tuán)隨熱解溫度的升高而下降，而其比表面積、灰分、pH值、堿性官能團(tuán)的含量隨熱解溫度的升高而增加。

4.2椰衣和椰殼生物質(zhì)炭對(duì)Pb2+的等溫吸附線更符合Langmuir模型，屬于單分子層吸附。隨著熱解溫度的升高，生物質(zhì)炭對(duì)Pb2+的最大吸附量增加，且椰衣生物質(zhì)炭對(duì)Pb2+的最大吸附量比椰殼生物質(zhì)炭高。YA700的最大吸附量為180.438 mg/g，在本文制備的6種生物質(zhì)炭中最大，且大于多數(shù)用其他生物質(zhì)材料制備的生物質(zhì)炭。

4.3生物質(zhì)炭比表面積對(duì)吸附Pb2+總量的影響很小，CEC和灰分含量是影響生物質(zhì)炭吸附Pb2+的重要因子。在Pb2+溶液初始質(zhì)量濃度為200 mg/L時(shí),椰衣生物質(zhì)炭對(duì)Pb2+的穩(wěn)定吸附量為9.83～13.91 mg/g，椰殼生物質(zhì)炭對(duì)Pb2+的穩(wěn)定吸附量為9.68～25.16 mg/g。這說(shuō)明椰殼生物質(zhì)炭所吸附的Pb2+比椰衣生物質(zhì)炭更穩(wěn)定。

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Preparation of biochars from coconut fiber and coconut shell and their adsorption for Pb2+in solution.JournalofZhejiangUniversity(Agric. &LifeSci.), 2016,42(4):469-477

LAN Tian1,2, CHU Yingchao1, ZHANG Lingling1, ZHAO Wen1, PAN Yunzhou1,2, ZHANG Jiawei1, ZHU Zhiqiang1, WU Weidong1*

(1.KeyLaboratoryofProtectionandDevelopmentUtilizationofTropicalCropGermplasmResources,MinistryofEducation,HainanUniversity,Haikou570228,China; 2.HainanKeyLaboratoryofArableLandConservation,HainanAcademyofAgriculturalSciences,Haikou571100,China)

coconut fiber; coconut shell; biochar; pyrolysis temperature; physical and chemical properties; adsorption; Pb2+

國(guó)家自然科學(xué)基金(B070303)；海南省耕地保育重點(diǎn)實(shí)驗(yàn)室(籌建)開放資金(KFZJ20150203).

Corresponding author)：吳蔚東(http://orcid.org/0000-0002-9676-5715)，E-mail:wdwu@hainu.edu.cn

聯(lián)系方式：蘭天(http://orcid.org/0000-0002-4222-6863)，E-mail:lantiancnnm@163.com

2015-12-10；接受日期(Accepted)：2016-04-08；網(wǎng)絡(luò)出版日期(Published online)：2016-07-18

X 131； X 53

A

URL:http://www.cnki.net/kcms/detail/33.1247.S.20160718.2011.002.html