碳化温度对牛粪铜和锌形态及生态毒性的影响

黄 辉,吕雨薇,梁 敏,朱咏莉,梁 文,蒋亚辉,张增强,李荣华*

碳化温度对牛粪铜和锌形态及生态毒性的影响

黄 辉1,2,吕雨薇1,梁 敏3,朱咏莉1,梁 文2,蒋亚辉2,张增强2,李荣华2*

(1.南京林业大学生物与环境学院,江苏 南京 210037；2.西北农林科技大学资源环境学院,陕西 杨凌 712100；3.南京农业大学资源与环境科学学院,江苏 南京 210095)

通过设置不同的热解温度(350,550和750℃)对牛粪废弃物进行碳化处理,并使用光谱技术手段对牛粪炭的微观特点及Cu、Zn赋存形态进行了分析表征,同时结合淋溶和毒性实验探究了热解温度对牛粪炭生态毒性的影响.结果表明,高温碳化明显改善牛粪孔隙结构,使其比表面积从牛粪原料的1.15m2/g提高至牛粪炭的5.51(350℃)～195.90m2/g(750℃).随着热解温度的提高,牛粪炭pH值从8.18(350℃)提高到了10.14(750℃);牛粪炭中Cu、Zn含量则从牛粪原料中的1.22和1.23mg/g分别升高至18.29～35.11和18.58～31.24mg/g.透射电镜-选区衍射以及X射线能谱分析表明,热解处理可使牛粪中Cu、Zn离子分别转化为副黑铜矿(Cu4O3)和红锌矿(ZnO)等金属氧化物,从而明显降低了牛粪炭中水溶态、DTPA提取态以及HNO3-H2SO4提取态的Cu、Zn离子浓度;此外,FTIR分析及混合有机酸浸提实验结果也表明,350℃牛粪炭中酚羟基、烷烃基、羧基、酰胺类等有机官能团通过吸附和络合作用固定未完全转化的Cu离子,而升高热解温度会使得这些官能团显著减少、促进Cu离子的完全转化以及无机物与Cu、Zn离子之间稳定金属氧化物化合键的形成.淋溶和生态毒性实验表明,高于550℃的热解温度能够显著降低牛粪炭中Cu、Zn的溶出率以及生态毒性,是高Cu、Zn含量牛粪废弃物无害化处理的一种推荐优选技术.

热解温度；牛粪；Cu；Zn；环境行为；生态毒性；影响因素

据估算,我国每年的畜禽粪便产生量高达38亿t[1].畜禽粪便因含有丰富的有机质及氮磷钾等养分,是农业种植过程的优质有机肥资源[2-3].然而,当前我国规模化养殖场的畜禽粪便中重金属含量普遍较高[4-7],尤其是Cu和Zn,其含量甚至可高达1622.81和14679.8mg/kg[5-6].较高Cu和Zn含量的畜禽粪便被长期当作有机肥施入农田,存在土壤和农作物重金属污染的环境风险[8-9].

有机固体废物高温热处理技术因具有工艺简单、运行经济、能显著减少有机固体废物的排放体积、促使物质的稳定化和无害化并产生高附加值资源(可燃气、生物油和生物炭)的优点而广受关注[10-13],其中生物炭具有钝化土壤重金属、调控生物多样性及改良土壤盐碱化等多重效果[14-17].例如Méndez等[18]和Yuan等[19]在利用含Cu和Zn污泥制备生物炭时发现,热解处理虽然显著促进了Cu和Zn元素的浓缩,导致总量提高,但却明显抑制了污泥生物炭中Cu和Zn离子的溶出量.Devi等[14]在研究含Cu和Zn纸浆经高温热解后生物炭的性质时指出,经炭化处理后重金属离子的迁移能力会受到抑制.由此表明,热解处理能有效降低有机固体废物中的重金属溶出风险.但有机固体废物经高温热解获得生物炭的环境稳定性受控于热解温度、生物质废物原料类型和化学组成等诸多因素,要推进有机固体废物的热处理技术,仍需进一步开展深入广泛的研究[20].

本研究以牛粪为原材料,通过高温缺氧热解技术对牛粪进行热解处理,探讨不同热解温度(350,550和750℃)对牛粪中Cu和Zn环境行为和潜在生态毒性的影响,为高重金属含量畜禽粪便的无害化处理和资源化利用提供理论参考.

1 材料与方法

1.1 实验材料

牛粪样品采自陕西杨凌西北农林科技大学农作三站肉牛养殖场;试验所需试剂包括Cu(NO3)2•3H2O,Zn(NO3)2•6H2O,CaCl2,二乙基三胺五乙酸(DTPA),三乙醇胺(TEA),甲酸,乙酸,乳酸,苹果酸,柠檬酸,浓硝酸,浓盐酸,浓高氯酸,过氧化氢,盐酸羟胺、乙酸铵等,均为购自西陇化工股份有限公司的分析纯试剂.

1.2 牛粪的热解处理

将采集的新鲜牛粪自然风干,使用研钵将风干牛粪研磨并过2mm筛,装入自封袋中备用.将300g牛粪干粉装入1L烧杯,并分别加入800mL Cu(NO3)2和Zn(NO3)2溶液(其中Cu、Zn浓度均为0.5mg/mL),机械混匀搅拌4h后静置2d.此后将上述混合物转入80℃烘箱中进行烘干处理,并将固体残渣充分研磨并过0.15mm筛,之后将固体残渣粉装入事先称重的150mL瓷坩埚,压实并盖上盖子后称重,计算装填物质重量.然后,将坩埚转入马弗炉并于氮气保护条件下以10℃/min速率分别升温至350,550和750℃,并在设定温度下维持2h.此后关闭马弗炉,继续通入氮气2h,待自然降温到室温后,取出坩埚中的热解牛粪残渣(生物炭),小心研磨并过0.15mm筛后,装入自封袋备用.牛粪原料及生物炭样品经HCl-HNO3- HClO4消解后,用Hitachi Z-2000型原子吸收分光光度计测定CuZn总量[21].

1.3 牛粪炭的表征测试

牛粪炭的比表面积(BET)通过N2-吸附-脱附比表面积仪(V-Sorb 2800P,GoldAPP,中国)测定,利用氮气吸附等温线计算牛粪炭吸附累积孔内表面积(csa)以及总孔体积(TPV)和平均孔径(AVP) (AVP=4TPV/csa);用元素分析仪(Vario EL cube CHONS, Elementar,德国)测定C、H、O、N等元素含量;微观形貌及金属矿物晶型采用场发射高分辨透射电镜-选区衍射(TEM-SEAD,JEM-1230, 日本)进行分析;重金属Cu和Zn的化合价态变化通过X射线光电子能谱分析仪(XPS,AXIS Ultra DLD,Shimadzu,日本)分析测定;牛粪炭的表面官能团变化通过傅里叶红外光谱仪(FTIR,Nicolet NEXUS 470,Thermo Nicolet,美国)测定,其中波数范围为4000～400cm-1.

1.4 铜、锌的淋溶能力及牛粪炭的生态毒性测试

在室温条件下,将4份0.5g生物炭样品分别用10mL的去离子水(pH值 6.9)[22]、有机混合酸(总浓度10mmol/L, pH值 5.60,甲酸、乙酸、乳酸、苹果酸和柠檬酸的物质的量比为1:4:2:2:1)、DTPA- TEA-CaCl2(0.005mol/L DTPA+0.01mol/L CaCl2+ 0.1mol/L TEA, pH值 7.30)及HNO3-H2SO4(体积比2:1, pH值 3.18)[23]振荡提取24h,悬液经0.45μm滤膜过滤后,以电感耦合等离子体质谱(ICP-MS)测定滤液中Cu和Zn的含量.每个处理设置3个平行实验.

为了解牛粪炭的生态毒性,试验中将牛粪炭和去离子水以1:10(/)的比例加入到50mL离心管中,于25℃振荡1h后静置0.5h,用0.45μm滤膜过滤上清液,滤液保存备用.之后,在事先灭菌的洁净培养皿中放入一张无菌滤纸,加入上述滤液5mL,并均匀分布播入10颗中国小白菜种子(品种:秦都96-12),盖上培养皿盖后放入25℃培养箱进行培养.每个处理设置4个平行,并设置去离子水处理为对照,培养48h后取出培养皿,计算种子发芽率和平均根长,并按公式(1)进行种子发芽指数(GI)的计算.

GI=处理种子发芽率×处理平均根长/

(对照种子发芽率×对照平均根长) (1)

2 结果与讨论

2.1 热解温度对牛粪炭理化性质的影响

如表1所示,随着热解温度的升高,牛粪炭对应的BET、csa和TPV逐渐增大而AVP逐渐减小.例如,随着热解温度从350℃增加到550和750℃,对应的牛粪炭BET从5.51m2/g提高到81.19和195.90m2/ g,csa从4.55m2/g提高到了36.53和68.35m2/g,TPV从0.03cm3/g增加到0.06和0.12cm3/g,AVP则从20.73nm逐渐减小到6.46～ 6.67nm左右.这表明在缺氧热解过程中牛粪发生了剧烈的物质分解和炭化,促使生物炭的孔隙结构形成[21,24-26].本研究中, 750℃的牛粪炭的AVP为6.67nm,稍大于550℃牛粪炭的6.46nm,说明过高的热解温度会导致炭的孔隙进一步发生崩塌从而产生较大的孔隙[20].与原始牛粪相比,350℃热解使牛粪有机质发生分解并保留了一定量的酸性有机物质,从而导致牛粪炭pH值有所减小;此后,随着温度升高至550和750℃,酸性有机化合物在高温下被分解并生成碱性物质[12],从而导致牛粪炭pH值从8.18升高到9.95和10.14.但不同热解温度下生物炭的EC值不存在显著差异(介于0.95～ 1.21mS/cm),暗示了高温热解条件对原材料组分中可溶性盐成分的影响较小[27].随着裂解温度的提高,灰分产率从350℃的30.08%提高至750℃的49.70%,而炭产率则由46.43%逐渐降低至27.99%,这与Chen等[24]在热解污泥研究中的观察结果相类似;此外,随着热解温度的升高,牛粪炭(350～750℃)的C/N比逐渐增大而H/C比和O/C比降低,表明升高热解温度会使牛粪炭中的芳香环结构含量增加[28-29],且牛粪炭中的碳原子活性降低,渗碳能力受到抑制,暗示了牛粪炭施入土壤后的矿化速度会降低[18,30],这均表明了高温热解能增强生物炭的环境稳定性[27,31].此外,与牛粪原料(Cu 1.22mg/g, Zn 1.23mg/g)相比,热解导致Cu和Zn发生浓缩使其含量分别从18.29和18.58mg/ g(350℃)升高到35.11和31.24mg/g(750℃),这与Jin等[21]和Zhang等[26]对城市污泥的炭化处理结果相类似.

表1 牛粪及牛粪炭的基本理化性质

注:—为未检验;表格同一行中a、b、c、d等小写字母表示不同处理之间数据在<0.05下存在显著差异,数据显著性分析软件为IBM SPSS Statistics (Version 26.0).

2.2 热解温度对牛粪中Cu和Zn赋存形态变化的影响

如图1所示,牛粪中Cu和Zn主要以自由金属离子形式存在.但随着热解过程的进行,350℃热解的牛粪炭中分布着尺寸约为200nm大小的矿物颗粒,对其进一步经SEAD衍射分析后发现,这些矿物组分有副黑铜矿(Cu4O3)、红锌矿(ZnO)、黑铜矿(CuO)等金属氧化物矿物(图1a);此后,当热解温度进一步提高到550和750℃时,由于浓缩效应而使牛粪炭中包裹的金属矿物明显增多[30],且此时牛粪炭中金属矿物种类主要以副黑铜矿和红锌矿为主(图1b和1c).这表明富含高浓度Cu和Zn的牛粪经高温裂解后,Cu和Zn离子会分别相应地转化为Cu4O3及ZnO等金属氧化物,这可能是有机质炭化后强烈固定原物质中金属离子的直接原因[21,26,32].

图1 不同温度下牛粪炭的高分辨透射电镜(TEM)照片及选区衍射(SEAD)光斑图

如图2所示,随着裂解温度从350℃提高到750℃,Zn 2p的谱图并未发生明显变化,说明了Zn元素在热解过程中未发生价态变化(图2a).在结合能1022和1045eV附近出现的Zn 2p 3/2和 Zn 2p 1/2的特征峰也表明了缺氧热解过程中Zn(II)大部分以ZnO形式存在于牛粪生物炭中.然而与Zn不同,牛粪热解过程明显改变了Cu元素的价态.Cu(II)峰的电子结合能与Cu 2p 3/2激发能的差值20.0eV是Cu(II)不同于Cu(I)及Cu(0)的主要区分标志[33],350℃牛粪炭Cu(II)和Cu(0)峰对应电子结合能差值为19.7eV (图2b),这表明了热解过程中Cu(II)→Cu(I)→Cu(0)还原过程的可能发生[34-35],即部分Cu(II)被缺氧还原成了Cu(0),从而促使副黑铜矿(Cu4O3)和黑铜矿(CuO)的形成;当热解温度提高到550和750℃时,电子结合能952和933eV处两个峰仍然存在,但在电子结合能963和943eV处出现两个新峰,表明了热解温度超过550℃能使更低价态的Cu化合物形成,这也证实了高温热解牛粪过程中Cu(II)会被还原为Cu(I)并进一步被还原为Cu(0)[33-34].这些结果证明了,高Cu、Zn含量的牛粪经过不同温度热解处理后,Zn元素的价态未发生改变,主要以ZnO形式存在于牛粪炭中;而Cu元素则以Cu(II)、Cu(I)和Cu(0)等多种价态并存于牛粪炭中,在低温(350℃)环境下形成Cu4O3和CuO,而在高温(550℃及以上)下主要以Cu4O3形式存在于牛粪炭中.

图2 不同热解温度下牛粪炭中Zn (2p)、Cu (2p)的XPS光谱图

2.3 热解温度对牛粪炭Cu和Zn环境淋溶行为的影响

从图3可知,不同热解温度下牛粪炭的Cu和Zn浸提浓度存在显著差异.用去离子水浸提时,350℃牛粪炭中Cu和Zn浸提浓度分别为0.16和0.11mg/g;当炭化裂解温度升高到550℃时,Cu和Zn浸提浓度分别显著降低至7.07和0.40μg/g;当炭化裂解温度进一步升高到750℃时,Cu和Zn浸提浓度分别进一步减少至6.21和0.10μg/g(图3a),这些结果表明牛粪炭中Cu要比Zn更容易溶出,而热解温度高于550℃则对牛粪炭中Cu和Zn的溶出具有抑制作用. DTPA-TEA-CaCl2和HNO3-H2SO4浸提牛粪炭中Cu和Zn元素的淋溶特征与用去离子水浸提基本相似,即随着热解温度从350℃升高到750℃,浸提液中Cu溶出浓度呈降低趋势但仍然处于较高浓度水平(15.3～19.9mg/g),而Zn溶出浓度则从3.2mg/g (350℃)显著减少至低于0.05mg/g(³550℃)(图3b),这表明牛粪炭中具有浓度水平相对较高的螯合态Cu和Zn离子.随着裂解温度升高至750℃,螯合态Cu浓度仍然超过15mg/g,但螯合态Zn离子则显著降低至0.05mg/g以下(图3b),这一结果与XPS分析牛粪炭孔结构中观察到的Cu发生价态变化而Zn的价态不变有关的规律相印证[26,35],即Cu4O3(Cu2O·2CuO)矿物结构容易被螯合剂破坏,而ZnO则较为稳定.HNO3-H2SO4浸提的不同牛粪炭中Cu溶出浓度高达31.1(350℃)、1.1mg/g(550℃)和0.2mg/ g(750℃)(图3c),表明350℃牛粪炭中Cu的生态毒性可能较大,而提高生物炭热解温度则能显著降低Cu的淋溶风险从而减低其生态环境风险,这表明提高热解温度是降低有机固体废弃物中重金属元素淋溶风险和生态毒性的有效手段[36-37].此外,随炭化热解温度的升高,混合有机酸浸提的Zn元素浓度从0.7mg/g(350℃)显著降低到0.05mg/g以下(³550℃) (图3d),这表明高Zn含量的牛粪废弃物经高温(³550℃)炭化后能够有效降低其生态环境风险[21].然而,混合有机酸浸提的Cu浓度呈现先显著升高后显著下降的趋势,这可能与牛粪炭中官能团的种类及含量变化有关[36-37].

图3 不同浸提剂提取的不同裂解温度得到的牛粪炭中Cu和Zn含量

图中不同小写字母表示数据在<0.05下存在显著差异

如图4所示, 随着热解温度的升高,牛粪炭中官能团的种类和数量均呈现显著减少的变化.在4000～400cm-1波长范围内,350℃牛粪炭中官能团种类与数量最多,分别存在酚羟基(3391cm-1)、烷烃基(2926和1437cm-1)、羧基(1601cm-1)以及酰胺类(1601, 1096和781cm-1)等官能团化合物[25,38]及600cm-1附近或以下范围内的有机物或无机物与重金属离子之间的配位键等官能团[21].然而,随着碳化裂解温度的提高,牛粪炭中官能团的种类及相对含量均显著减少,与350℃牛粪炭相比,550℃牛粪炭中仅存在少量的磷酸键官能团(1028cm-1)以及明显减少的有机物与重金属离子之间的配位键.当裂解温度提高到750℃时,牛粪炭中仅存在少量的有机物与重金属离子之间配位键,且配位键逐渐发展为无机物与金属离子之间形成的金属氧化物化合键(图4).导致上述官能团变化的原因可能是高温(³550℃)裂解使得牛粪中烃类物质缺氧转化为生物气(例如CO2、CH4或其他有机分子气体)、生物油(芳香结构物质)等附属产品,从而导致有机官能团种类及数量显著减少,并促进金属氧化物化合键的形成[21,26,37].这一结果也与本研究中TEM及SEAD分析的金属氧化物矿物晶型演化结果相印证,即当热解温度从350℃升高到550℃时,大量存在的有机官能团发生缺氧转化,同时结合的Cu离子被未完全转化,使得混合有机酸从牛粪炭中提取的Cu元素浓度较高;而热解温度提高至750℃时,未完全转化的Cu离子进一步形成更加稳定的金属氧化物,从而导致混合有机酸浸提的750℃牛粪炭中Cu浓度显著降低(图3d,图4).

图4 不同热解温度下牛粪炭傅里叶红外光谱图

2.4 牛粪炭的生态毒性

如图5所示,在对照去离子水中,小白菜种子基本能全部发芽(发芽指数为98.7%),其平均根长为19.1mm;与之相比,350℃牛粪炭浸出液中,近70%的小白菜种子受到重金属Cu、Zn毒害而不能发芽(图5a),其平均根长显著受到抑制,仅为12.3mm(图5b),这与低温裂解生物炭中存在大量小分子有机物和较高的活性Cu和Zn有关[36-37].而当热解温度升高至550和750℃时,可使小白菜种子发芽指数明显提高至350℃牛粪炭处理的2.4～3.1倍(图5a),并使小白菜平均根长增长至15.5～16.5mm(图5b),说明提高热解温度至550℃以上,能够减少小分子有机物含量并使生物活性较高的Cu、Zn转化为稳定的低毒性金属氧化物,从而显著降低牛粪炭的生态毒性,并明显缓解这些生物炭对植物生长的抑制效应.

图5 不同热解温度的牛粪炭浸出液中大白菜种子发芽指数及发芽平均根长

生物炭与去离子水比率为1:10(/)

3 结论

3.1 高温热解能将牛粪转化为富含稳定孔隙结构的牛粪生物炭.

3.2 在热解过程中,Cu和Zn会发生浓缩效应而导致总量显著升高;牛粪中的Cu和Zn离子会随热解的温度升高而发生形态转化,缺氧热解使得部分Cu(II)被还原为Cu(I)和Cu(0)并促使副黑铜矿(Cu4O3)和黑铜矿(CuO)的形成,同时会促使Zn(II)转化为稳定的红锌矿(ZnO),从而对Cu和Zn产生强烈的固定作用.

3.3 升高热解温度会使牛粪炭中酚羟基、烷烃基、羧基、酰胺类以及磷酸键等官能团种类及含量均显著减少,同时促使无机物与Cu和Zn离子之间形成更加稳定的金属氧化物化合键,从而明显降低牛粪炭中Cu和Zn的淋溶风险,进而有效降低牛粪炭的生态毒性.

3.4 将温度提升到550℃以上进行安全热解处理,有利于降低高Cu和Zn含量牛粪的环境生态风险.

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Effects of pyrolysis temperatures on occurrence speciation of copper and zinc in cattle manures and their potential ecotoxicity.

HUANG Hui1,2, LÜ Yu-wei1, LIANG Min3, ZHU Yong-li1, LIANG Wen2, JIANG Ya-hui2, ZHANG Zeng-qiang2, LI Rong-hua2*

(1.College of Biology and the Environment, Nanjing Forestry University, Nanjing 210037, China；2.College of Natural Resources and Environment, Northwest A&F University, Yangling 712100, China；3.College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, China)., 2022,42(9)：4240～4247

Pyrolysis to biochar is an efficient technique of disposing organic solid wastes. However, pyrolysis treatment of the cattle manure (CM) containing high concentrations of Cu and Zn is scarcely investigated, and the environmental behaviors of Cu and Zn in the cattle manure biochar (CMB) accompanying with their ecological risk keep unknown. In this study, the occurrence speciation of Cu and Zn in CMB, their leaching characteristics and the ecological risk of CMB from different pyrolysis temperatures (350, 550 and 750oC) were evaluated using spectrum technology (i.e., BET, TEM-SEAD, XPS, and FTIR), the leaching experiments, and the risk tests. Results show that carbonization of CM to CMB with an increasing temperature from 350 to 750oC improves the pore structure of CM and enlarges the specific surface area from 1.15m2/g in raw CM to 5.51～195.90m2/g in CMB. The pH value of the CMB increases from 8.18 (350℃) to pH 10.14 (750℃). Importantly, the Cu concentration increases from 1.22mg/g in raw CM to 18.29～35.11mg/g in CMB while the Zn concentration elevates from 1.23mg/g to 18.58～31.24mg/g. Meanwhile, most of the Cu and Zn are oxidized to paramelaconite (Cu4O3) and zincite (ZnO), which significantly reduces the concentrations of Cu and Zn in forms of extractable ones with deionized water, DTPA-TEA-CaCl2and HNO3-H2SO4as extracting agents, respectively. Furthermore, many functional groups (i.e., phenolic hydroxyl, alkane, carboxyl, amide, etc.) can immobilize labile Cu through adsorption and complexation, but a high pyrolysis temperature (>550oC) tends to arise a significant decrease in species and amounts of functional groups and promote the complete conversion of Cu ions and the formation of stable metal oxide bonds between inorganics and Cu or Zn ions. In summary, the pyrolysis temperature over 550oC could dramatically reduce the leaching rates of Cu and Zn and mitigate the ecotoxicity of CMB, which can be a potential approach to dispose the CM wastes with high concentrations of Cu and Zn.

pyrolysis temperature；cattle manure；copper zinc；environmental behavior；ecological risk；influencing factors

X705,X171

A

1000-6923(2022)09-4240-07

2022-02-23

中央高校基本科研业务费专项资金项目(2452015177);南京林业大学水杉师资科研启动项目(163108167)

*责任作者, 教授, rh.lee@nwsuaf.edu.cn

黄 辉(1993-),男,江苏连云港人,副教授,博士,主要从事固体废弃物资源化与重金属污染控制.发表论文23篇.