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污泥基生物炭负载纳米零价铁去除Cr(VI)的性能与机制

曾涛涛, 农海杜, 沙海超, 陈胜兵, 张晓玲, 刘金香

曾涛涛, 农海杜, 沙海超, 等. 污泥基生物炭负载纳米零价铁去除Cr(VI)的性能与机制[J]. 复合材料学报, 2023, 40(2): 1037-1049. DOI: 10.13801/j.cnki.fhclxb.20220324.001
引用本文: 曾涛涛, 农海杜, 沙海超, 等. 污泥基生物炭负载纳米零价铁去除Cr(VI)的性能与机制[J]. 复合材料学报, 2023, 40(2): 1037-1049. DOI: 10.13801/j.cnki.fhclxb.20220324.001
ZENG Taotao, NONG Haidu, SHA Haichao, et al. Performance and mechanism of Cr(VI) removal by sludge-derived biochar loaded with nanoscale zero-valent iron[J]. Acta Materiae Compositae Sinica, 2023, 40(2): 1037-1049. DOI: 10.13801/j.cnki.fhclxb.20220324.001
Citation: ZENG Taotao, NONG Haidu, SHA Haichao, et al. Performance and mechanism of Cr(VI) removal by sludge-derived biochar loaded with nanoscale zero-valent iron[J]. Acta Materiae Compositae Sinica, 2023, 40(2): 1037-1049. DOI: 10.13801/j.cnki.fhclxb.20220324.001

污泥基生物炭负载纳米零价铁去除Cr(VI)的性能与机制

基金项目: 国家自然科学基金(52170164);湖南省教育厅创新平台开放基金项目(19K081)
详细信息
    通讯作者:

    刘金香,博士,教授,硕士生导师,研究方向为水处理理论与技术及污染控制  E-mail:cafardworm@163.com

  • 中图分类号: X703

Performance and mechanism of Cr(VI) removal by sludge-derived biochar loaded with nanoscale zero-valent iron

Funds: National Natural Science Foundation of China (52170164); Opening Funding for Innovation Platform of Education Department in Hunan Province (19K081)
  • 摘要: 针对电镀、冶金、印染等行业产生的含铬废水所导致的环境污染难题,以城市污泥热解获得的污泥基生物炭(SB)为载体,制备了污泥基生物炭负载纳米零价铁(nZVI-SB)材料用于去除水中的Cr(VI),探究了铁炭质量比、初始pH值、投加量、温度等因素对去除Cr(VI)的影响。通过SEM-EDS、XRD和XPS等手段对nZVI-SB去除Cr(VI)的机制进行分析。结果表明:nZVI-SB对Cr(VI)废水具有较好的去除能力。在投加量0.5 g/L、初始pH=2、温度40℃条件下, Fe与SB质量比为1∶1的nZVI-SB(1∶1)对Cr(VI)吸附量最大为150.60 mg/g。Cr(VI)去除过程可通过Langmuir吸附等温式与准二级动力学方程进行拟合。nZVI-SB对Cr(VI)去除机制主要包括吸附、还原和共沉淀。本文表明污泥基生物炭与纳米零价铁可以协同发挥除Cr(VI)作用。
    Abstract: Chromium-containing wastewater was generated in electroplating, metallurgy, printing and dyeing industries, which caused environmental pollution. The sludge-derived biochar (SB) was obtained from the pyrolysis of municipal sludge, and then loaded with nanoscale zero-valent iron (nZVI) to prepare sludge-derived biochar loaded with nanoscale zero-valent iron (nZVI-SB) for the removal of Cr(VI) from water. The effect of the iron to carbon mass ratio, initial pH value, dosage and temperature on the removal of Cr(VI) were explored. SEM-EDS, XRD and XPS were used to characterize the mechanisms of Cr(VI) removal. The results show that nZVI-SB has a desirable removal capacity for Cr(VI). Under the conditions of dosage 0.5 g/L, pH=2 and 40℃, the maximum adsorption capacity of Cr(VI) is 150.60 mg/g by nZVI-SB(1∶1) with a mass ratio of 1∶1 between Fe and SB. The Cr(VI) removal process can be fitted by Langmuir adsorption isotherm and pseudo-second-order kinetic equations. The removal mechanisms of Cr(VI) mainly include adsorption, reduction and co-precipitation. The present study confirms SB and nZVI can synergically remove Cr(VI).
  • 铬(Cr)作为一种重要的生产原材料被广泛应用于制革、冶金、电镀、纺织印刷、电池等行业中[1- 2],也相应产生了含铬废水,如果处理不当,会对生态环境造成危害[3]。水中的铬(Cr)主要以Cr(VI)和Cr(III)两种形式存在,其中Cr(VI)具有溶解度高、迁移能力强、危害大的特点[4]。目前,含Cr(VI)废水的处理方法有沉淀法、光催化法、吸附还原法、离子交换法等[5],其中吸附还原法运行成本低、操作简单、效率高,受到研究者广泛关注。

    纳米零价铁(nZVI)比表面积大、还原性强、表面活性高,对重金属具有良好的去除效果,在环境污染方面受到重视。秦泽敏等[6]对比了还原性铁粉、活性炭及nZVI对Cr(VI)的去除效果,发现nZVI对Cr(VI)的去除率达到了80%以上,远大于还原性铁粉(2%)和活性炭(5%)。然而,由于nZVI粒径小、表面能大及自身磁性,容易产生团聚,导致比表面积和还原能力降低,限制了其在水处理中的应用[7]。为了克服这个缺点,研究人员尝试使用不同的材料作为nZVI载体,以减少颗粒团聚,如膨润土[8]、介孔二氧化硅[9]、活性炭[10]、生物炭[11]等。Shi等[8]采用膨润土作为载体,制备膨润土负载nZVI复合材料,对含Cr(VI)电镀废水的去除率大于90%。Petala等[9]采用介孔二氧化硅负载零价铁纳米,有效地抑制了nZVI的团聚,对Cr(VI)去除能力比nZVI更高。刘剑等[12]通过液相还原法制备了活性炭负载nZVI复合材料,在Cr(VI) 初始浓度50 mg/L、温度40℃、初始pH=2.0、投加量3.0 g/L条件下反应5 min,Cr(VI)去除率为99.4%。生物炭具有较大的比表面积、发达的孔隙结构及丰富的官能团,对重金属具有良好的吸附性能[13-14]。生物炭负载零价纳米铁,能有效降低nZVI的团聚效应[15],而且生物炭良好的导电性,能够增强nZVI的电子转移能力,从而提高nZVI的反应活性[16]。生物炭负载nZVI在重金属废水处理中能发挥协同作用。但目前鲜有通过污泥基生物炭(SB)负载nZVI处理含Cr(VI)废水的报道。

    因此,本文以市政污泥为原料制备SB负载nZVI复合材料(nZVI-SB),探讨其对废水中Cr(VI)的去除性能、Cr(VI)去除的机制,为nZVI-SB应用于含Cr(VI)废水处理提供借鉴。

    脱水污泥取自湖南某污水处理厂,其含水率为50.83%。脱水污泥经研磨、干燥后,通过0.18 μm筛收集过筛粉末污泥,干燥后的粉末污泥挥发分含量为37.94wt%,灰分含量为61.03wt%[14]

    SB制备:称取一定量干燥的粉末污泥填满坩埚并压实,盖紧坩埚盖,制造一个无氧环境。将坩埚置于450℃的马弗炉中热解2 h,得到黑色固体物质即为SB。

    nZVI制备[17]:向装有70 mL超纯水和30 mL乙醇的三颈烧瓶中加入7.4464 g FeSO4·7H2O,在搅拌器上匀速搅拌(150 r/min),直至FeSO4·7H2O完全溶解。然后充入氮气排出空气,持续搅拌,以2.5 mL/min匀速加入适量的硼氢化钠,直至无气泡产生,反应完成。取出沉淀,迅速用超纯水洗涤3次,无水乙醇洗涤2次,得到的黑色固体放入60℃真空干燥箱中干燥8 h,干燥后的黑色固体即为nZVI。

    nZVI-SB制备:将适量的SB加入不同量的FeSO4·7H2O溶液中,使nZVI-SB 中Fe∶C的质量比分别为 1∶4、1∶2、1∶1和2∶1。然后按照上述合成nZVI的方法制备nZVI-SB,分别记为nZVI-SB(1∶4)、nZVI-SB(1∶2)、nZVI-SB(1∶1)和nZVI-SB(2∶1),见表1

    表  1  样品名称缩写
    Table  1.  Abbreviation name of samples
    Sample Fe/wt% SB/wt%
    nZVI-SB(1∶4) 20.0 80.0
    nZVI-SB(1∶2) 33.3 66.7
    nZVI-SB(1∶1) 50.0 50.0
    nZVI-SB(2∶1) 66.7 33.3
    Notes: nZVI—Nanoscale zero-valent iron; SB—Sludge-based biochar.
    下载: 导出CSV 
    | 显示表格

    将1 g污泥和SB分别加入50 mL的逆王水(浓硝酸∶浓盐酸=3∶1)中,在微波消解仪(ETHOS 1,意大利)中消解处理2 h,测定过滤液中重金属浓度。采用电感耦合等离子发射光谱法测定重金属浓度。

    为了研究Fe∶C质量比对Cr(VI)去除效果的影响,将不同质量比(1∶4、1∶2、1∶1和2∶1) nZVI-SB样品0.025 g和50 mL的Cr(VI)溶液(50 mg/L)加入到150 mL锥形瓶中,调节初始pH值为2.0,温度为30℃,在150 r/min的恒温振荡器(IS-RDD3,美国精骐)中反应4 h后取上清液,测量剩余Cr(VI)浓度。

    反应初始pH设置为2、3、4、5、6、7、8、9,Cr(VI)溶液初始浓度为50 mg/L,温度为30℃,准确吸取50 mL该溶液于150 mL锥形瓶中,加入0.025 g nZVI-SB(1∶1),恒温振荡4 h后取上清液,测量Cr(VI)的浓度,以考察初始pH对Cr(VI)去除效果的影响。

    配制Cr(VI)溶液初始浓度为50 mg/L,分别准确吸取50 mL该溶液加入一系列150 mL锥形瓶中,再分别加入0.005 g、0.0125 g、0.025 g、0.035 g、0.05 g、0.065 g nZVI-SB(1∶1),以设置nZVI-SB投加量分别为0.1、0.25、0.5、0.7、1.0和1.3 g/L,在初始pH为2、30℃条件下恒温振荡4 h后取上清液,测量Cr(VI)的浓度。

    采用二苯碳酰二肼分光光度法测定溶液中Cr(VI)的吸光度,重复3次计算剩余Cr(VI)的平均值。Cr(VI)的去除率和吸附量计算如下式所示:

    R=C0CeC0×100\% (1)
    q=(C0Ce)Vm (2)

    式中:R为去除率(%);C0为溶液中初始Cr(VI)质量浓度(mg/L);Ce为反应结束后溶液中Cr(VI)质量浓度(mg/L);q为Cr(VI)的吸附容量(mg/g);V为溶液体积(L);m为nZVI-SB质量(g)。

    将500 mL的Cr(VI)溶液(50 mg/L)加入1000 mL锥形瓶中,调节溶液pH为2,再加入0.5 g/L nZVI-SB(1∶1),在30℃的恒温振荡器中吸附0~720 min,采用准一级动力学模型、准二级动力学模型和颗粒内扩散模型对吸附过程进行拟合,方程式如下式:

    ln(qeqt)=lnqeK1t (3)
    tqt=1K2q2e+tqe (4)
    qt=Kdt12+Ci (5)

    式中:qe为平衡吸附量(mg/g);qtt时刻的吸附量(mg/g);K1为准一级吸附速率常数(min−1);K2为准二级吸附速率常数(g/(mg·min)−1);Kd为颗粒内扩散速率常数(mg/(m·min1/2));Ci为边界层常数。

    将不同浓度的Cr(VI)溶液(40~110 mg/L)50 mL分别加入150 mL锥形瓶中,调节初始 pH值为2,加入0.5 g/L nZVI-SB(1∶1),分别在20℃、30℃和 40℃的恒温振荡器中吸附4 h,考察温度对Cr(VI)去除效果的影响,并用Langmuir吸附等温线模型和Freundlich吸附等温线模型进行拟合,方程式如下所示:

    Ceqe=Ceqm+1qmKL (6)
    lnqe=(1/n)lnCe+lnKF (7)

    式中:qm为吸附质在单位质量吸附剂的最大吸附容量(mg/g);KL为Langmuir模型的吸附平衡常数;KF为Freundlich模型的吸附平衡常数;1/n为与吸附强度有关的经验参数。

    采用SEM (Inspect F50,美国FEI)观察nZVI-SB微观形貌,对比nZVI-SB去除Cr(VI)前后的形态变化;采用TEM (HT7700,日本日立公司)来观察nZVI在生物炭表面的分布情况及是否生成纳米颗粒;同时采用比表面积分析仪(3Flex 5.02,美国麦克)分析nZVI-SB孔隙结构;通过能量色散X射线光谱(EDS) (X-Max,英国牛津)对nZVI-SB表面的元素组成进行分析;使用XRD (Bruker D8,德国布鲁克)分析样品晶型结构及物象组成,XRD数据通过Jade 6.5软件处理;采用XPS (Escalab 250Xi,美国Thermo Fisher Scientific)分析样品元素组成与价态变化,原始数据采用Avantage软件进行分峰拟合。综合表征结果探讨Cr(VI)去除机制。

    过滤、收集与Cr(VI)反应后的nZVI-SB(1∶1),并将其加入到0.2 mol/L的HCl中振荡脱附2 h。之后将溶液过滤,沉淀用去离子水洗涤至中性,60℃真空干燥12 h后重复Cr(VI)去除试验,计算nZVI-SB再生后对Cr(VI)的去除效果。

    污泥和SB消解液中重金属浓度如表2所示,热解制备成生物炭之后Zn、Pb、Cu、Ba和Cd的浓度有一定程度的富集,这与污泥热解过程中有机质不断分解有关。两者的重金属浓度值都远低于《危险废物鉴别标准浸出毒性鉴别》(GB/T 5085.3—2007)规定值(不超过1/6)[18],说明SB中重金属含量符合规范要求。

    表  2  污泥及污泥基生物炭(SB)消解液中重金属浓度
    Table  2.  Heavy metal concentrations in digestion solution of sludge and sludge-based biochar (SB) (mg·L−1)
    SampleZnPbCuBaCdCr
    Sludge 5.28 0.34 2.37 7.32 0.06 0.14
    SB 7.81 0.51 3.01 8.67 0.09 0.13
    Specified value
    in GB/T
    5085.3—2007[18]
    100.00 5.00 100.00 100.00 1.00 15.00
    下载: 导出CSV 
    | 显示表格

    图1为SB和nZVI-SB(1∶1)处理Cr(VI)前、后的SEM-EDS图像。从图1(a)可看出,SB存在大量的块状结构,表面更加光滑。而nZVI-SB(1∶1)(图1(b))的表面存在大量链状聚集的颗粒物[19],这说明零价铁被负载在SB表面。图1(b)的EDS结果显示有Fe峰,进一步验证nZVI-SB复合材料制备成功。吸附Cr(VI)后,SEM变化不大,而EDS出现了Cr元素,证明nZVI-SB(1∶1)成功固定了铬(图1(c))。EDS分析得出nZVI-SB(1∶4)、nZVI-SB(1∶2)、nZVI-SB(1∶1)和nZVI-SB(2∶1)中Fe的质量百分数分别约为19.87wt%、33.31wt%、49.92wt%和66.63wt%(表3),符合预期比例。

    图2为nZVI-SB(1∶1)处理含Cr(VI)废水前、后的TEM图像。处理含Cr(VI)废水前nZVI颗粒均匀分散并固定在SB表面(图2(a))。结果表明,在SB中引入nZVI颗粒提高了nZVI的分散和稳定性,从而增加了nZVI-SB(1∶1)还原或吸附Cr(VI)的活性位点[20]。吸附Cr(VI)后,nZVI-SB(1∶1)表面纳米颗粒减少,且相应的颗粒物变大(图2(b)),推测这可能是生成了Cr(III)-Fe(III)氢氧化物沉淀;结合图1(c)的EDS图中Cr 峰的出现,表明溶液中的Cr(VI)能够被nZVI-SB(1∶1)去除。

    图  1  SB (a)和污泥基生物炭负载纳米零价铁(nZVI-SB)(1∶1)处理Cr(VI)前(b)、处理Cr(VI)后(c)的SEM-EDS图像
    Figure  1.  SEM-EDS images of SB (a) and sludge-derived biochar loaded with nanoscale zero-valent iron (nZVI-SB)(1∶1) before (b) and after (c) treatment of Cr(VI)
    表  3  SB和nZVI-SB的元素组成
    Table  3.  Elemental composition of SB and nZVI-SB wt%
    SampleCNONaMgAlSiKCaCrFe
    SB40.488.4634.940.190.393.266.430.883.050.33 1.59
    nZVI-SB(1∶4)36.637.5633.050.220.480.630.760.280.300.2219.87
    nZVI-SB(1∶2)30.416.4127.550.180.360.510.620.230.240.1833.31
    nZVI-SB(1∶1)23.923.1121.450.140.270.280.350.190.210.1649.92
    nZVI-SB(2∶1)16.481.7614.450.060.130.110.160.080.080.0666.63
    下载: 导出CSV 
    | 显示表格

    通过N2吸脱附比表面积测量仪测定SB及nZVI-SB(1∶1)比表面积和孔径,结果图3所示。SB和nZVI-SB(1∶1)的比表面积分别为47.07 m2/g和116.32 m2/g(图3(a)),孔径分别为14.34 nm和11.56 nm(图3(b))。nZVI-SB(1∶1)的比表面积远远大于SB,说明SB经过nZVI改性后所得的复合材料具有更大的吸附Cr(VI)潜力。nZVI-SB(1∶1)孔径比SB小,是由于SB具有多孔性结构,nZVI颗粒在制备过程中一部分球状颗粒嵌入生物炭的孔隙中[21],造成孔径减小。

    图  3  SB和nZVI-SB(1∶1)的N2吸附-脱附等温线(a)和孔径分布(b)
    dV/dW—Pore volume
    Figure  3.  N2 adsorption-desorption isotherms (a) and pore size distributions (b) of SB and nZVI-SB(1∶1)
    图  2  nZVI-SB(1∶1)处理Cr(VI)前(a)、 处理Cr(VI)后(b)的TEM图像
    Figure  2.  TEM images of nZVI-SB(1∶1) before (a) and after (b) treatment of Cr(VI)

    SB和nZVI-SB(1∶1)去除Cr(VI)前、去除Cr(VI)后的XRD图谱如图4(a)所示。SB的XRD图谱中,2θ=20.9°、26.79°、35.01°、50.6°、60.01°及68.62°对应的主要物质为SiO2,而2θ=29.37°、39.4°对应的物质为CaCO3[22]。负载nZVI后,nZVI-SB(1∶1)的XRD特征峰发生变化,SiO2和CaCO3的相关峰减弱,甚至消失,检测到与α-Fe0相关的44.7°的特征峰,证明Fe0的存在[23-24],这表明nZVI在SB上负载成功[15]。2θ=35.65°处为Fe2O3[25],可能是nZVI-SB复合材料制备过程中,部分nZVI被氧化。去除Cr(VI)后,nZVI 峰显著减弱,2θ=35.5°处为Cr2FeO4峰,证明nZVI和Cr(VI)之间发生了氧化还原反应,形成了混合铬铁氧化物Cr2FeO4[26]图4(b)为nZVI-SB(1∶1)去除Cr(VI)前、去除Cr(VI)后的XPS全谱,处理前没有Cr(VI)的特征峰,去除Cr(VI)后可观察到Cr2p特征峰,表明复合材料可吸附固定Cr(VI)。

    图  4  SB和nZVI-SB(1∶1)去除Cr(VI)前、去除Cr(VI)后的XRD图谱(a)及nZVI-SB(1∶1)的XPS全谱图(b)
    Figure  4.  XRD patterns of SB and nZVI-SB(1∶1) before and after adsorption of Cr(VI) (a) and XPS full spectrum of nZVI-SB(1∶1) (b)

    图5显示了不同吸附剂类型对Cr(VI)的去除效果,SB、nZVI、nZVI-SB(1∶4、1∶2、1∶1、2∶1)对Cr(VI)的平衡吸附量分别为8.13、88.80、50.00、85.33、99.13和91.33 mg/g。SB的吸附量最低,主要是通过吸附作用固定Cr(VI)[13],而nZVI具有很强的还原性,可通过还原、吸附和共沉淀作用去除水中的Cr(VI)[27]。从图5可知,nZVI-SB中的Fe∶C质量比会对去除效果产生明显的影响。随着Fe/C从0.25增加到1.0,吸附量从50.00 mg/g显著增加到99.13 mg/g,进一步证实nZVI的负载增强了SB对Cr(VI)的去除能力。nZVI的去除性能优于nZVI-SB(1∶4)和nZVI-SB(1∶2),这可能是在nZVI-SB(1∶4)和nZVI-SB(1∶2)中,过量的生物炭占据nZVI的表面活性位点[28],从而影响了nZVI与Cr(VI)的反应。另外,与nZVI-SB(1∶1)相比,nZVI-SB(2∶1)对Cr(VI)的吸附量出现了降低,这可能是过量的nZVI在SB表面出现团聚,从而导致吸附量的下降。结果表明,Fe∶C质量比为1∶1的nZVI-SB去除Cr(VI)的能力最高。因此,选择nZVI-SB(1∶1)进行后续试验。

    图  5  不同Fe∶C质量比对Cr(VI)去除的影响
    Figure  5.  Effect of different Fe∶C mass ratio on Cr(VI) removal

    初始pH值对Cr(VI)去除的影响如图6所示。随着溶液pH从2增加到9,nZVI-SB(1∶1)对Cr(VI)吸附量从99.13 mg/g下降至37.07 mg/g,原因可能是在酸性条件下存在大量的H+,促进nZVI-SB(1∶1)表面上氧化铁的溶解,暴露出更多的反应位点[29]图7是用Visual MINTEQ软件模拟了Cr(VI)在不同pH值(2~12)的存在形态分布曲线图。在高pH值下,复合材料容易去质子化带负电[14, 30],Cr(VI)在溶液中主要以阴离子CrO24形式存在,两者同性相斥,从而造成吸附量下降。当pH>7时,溶液中存在大量的OH,这有利于nZVI-SB(1∶1)表面形成Cr(III)-Fe(III)氢氧化物沉淀,阻碍了nZVI内部的电子向外转移[31],妨碍了Cr(VI)与活性位点的接触,从而抑制反应的进行。

    图  6  初始pH对nZVI-SB(1∶1)去除Cr(VI)的影响
    Figure  6.  Effect of initial pH on the removal of Cr(VI) by nZVI-SB(1∶1)
    图  7  不同pH值下Cr(VI)形态分布曲线图
    C0—Initial mass concentration of Cr(VI) in solution
    Figure  7.  Cr(VI) form distribution curves under different pH values

    nZVI-SB(1∶1)投加量对Cr(VI)去除效果见图8。当nZVI-SB(1∶1)的投加量从0.1 g/L增加到0.5 g/L时,nZVI-SB(1∶1)对Cr(VI)的去除率从27.48%快速增加到99.13%;当投加量大于0.5 g/L后,去除率基本保持不变。分析原因是,反应初期随着nZVI-SB(1∶1)投加量的增加,活性位点迅速增多[15],导致nZVI-SB(1∶1)对Cr(VI)的去除率迅速增加;但随着投加量的继续增加,而溶液中的Cr(VI)离子有限,对Cr(VI)的去除率增加变缓并趋于稳定。总体上吸附容量随着投加量的增加而减少,从最初的137.33 mg/g减少到38.41 mg/g。原因是随着投加量增加,单位质量nZVI-SB(1∶1)对Cr(VI)的吸附量下降[32]

    图  8  nZVI-SB(1∶1)投加量对nZVI-SB(1∶1)去除Cr(VI)的影响
    Figure  8.  Influence of nZVI-SB(1∶1) dosage on the removal of Cr(VI) by nZVI-SB(1∶1)

    在温度30℃、nZVI-SB(1∶1)投加量为0.5 g/L、Cr(VI)浓度为50 mg/L和溶液初始pH为2的条件下,Cr(VI)在720 min内的去除情况如图9(a)所示。随着反应时间的增加,nZVI-SB(1∶1)对Cr(VI)的吸附容量逐渐增加,前5~180 min吸附量增加较快,之后增加较缓慢,在240 min时基本达到吸附平衡。在初始阶段,吸附容量的迅速增加主要是由于有大量的结合位点可供吸附;随着吸附的继续,吸附位点接近饱和状态,吸附速率减慢,直至达到平衡[33]

    图  9  (a)吸附时间对nZVI-SB(1∶1)去除Cr(VI)的影响;准一级(b)、准二级(c)动力学拟合曲线和颗粒内扩散拟合曲线(d)
    Figure  9.  (a) Effect of adsorption time on the removal of Cr(VI) by nZVI-SB(1∶1); Quasi-first (b), quasi-second (c) kinetic fitting curves and intra-particle diffusion fitting curve (d)

    为了探究nZVI-SB(1∶1)对Cr(VI)的吸附过程和控速步骤,选用准一级动力学模型、准二级动力学模型和颗粒内扩散模型对实验数据进行分析,结果及相应的拟合参数如图9(b)~9(d)表4所示。结果发现,t/qtt呈明显的直线关系,准二级吸附动力学方程的线性相关系数(R2=0.999)大于其准一级动力学方程的线性相关系数(R2=0.855)。而准一级动力学模型拟合的qe(38.57 mg/g)与实际吸附量(99.13 mg/g)相差较大,而准二级动力学模型拟合的qe(103.07 mg/g)与实际值相当,说明准二级动力学方程能够更好地描述nZVI-SB(1∶1)对Cr(VI)的吸附过程。因此,吸附过程以化学吸附为主[34]。颗粒内扩散模型拟合分析发现,吸附分为吸附剂表面吸附和孔道内缓慢扩散两个阶段。直线都不经过原点,说明孔道内扩散不是控制吸附过程的关键步骤[19]。而粒子扩散常数Kd1Kd2,表明表面吸附是Cr(VI)吸附去除的限速步骤[25]

    在pH为2、投加量0.5 g/L、吸附时间4 h条件下,探讨Cr(VI)初始质量浓度(40~110 mg/L)和吸附温度(20℃、30℃及40℃)对Cr(VI)去除的影响,结果如图10(a)所示。随着Cr(VI)初始质量浓度的增加,吸附量呈上升趋势,这可能与传质驱动力有关,较高的Cr(VI)浓度使液相和nZVI-SB(1∶1)表面之间的浓度梯度更大,导致更多的Cr(VI)转移到nZVI-SB(1∶1)表面[35]。在相同Cr(VI)初始质量浓度下,40℃下Cr(VI)吸附量最大,说明升高温度能够促进nZVI-SB(1∶1)对Cr(VI)的去除。原因是较高的温度有利于增加金属离子的迁移率,从而增加Cr(VI)与吸附位点的接触概率[36]

    表  4  nZVI-SB(1∶1)对Cr(VI)的吸附动力学参数
    Table  4.  Adsorption kinetic parameters of Cr(VI) adsorption by nZVI-SB(1∶1)
    Intraparticle diffusion model
    qe/(mg·g−1)K/(min−1)R2Kd/(mg·(m·min0.5)−1)CR2
    Quasi-first order dynamics model 38.570.01130.8555.85822.7310.971
    Quasi-second-stage dynamics model103.070.00050.9990.03898.5510.946
    Notes: qe—Equilibrium adsorption capacity; K—Adsorption rate constant; R2—Linear correlation coefficient; Kd—Particle diffusion constants; C—Constant.
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    图  10  (a)温度对nZVI-SB(1∶1)去除Cr(VI)的影响;(b) Langmuir吸附等温线;(c) Freundlich吸附等温线
    Figure  10.  (a) Influence of temperature on the removal of Cr(VI) by nZVI-SB(1∶1); (b) Langmuir adsorption isotherm; (c) Freundlich adsorption isotherm

    通过Langmuir和Freundlich吸附等温线模型对Cr(VI)去除过程进行拟合,拟合曲线和拟合参数分别如图10(b)10(c)表5所示。结果表明,Langmuir方程模拟所得的最大吸附量(151.23 mg/g)和实际吸附容量(150.60 mg/g)相当。Langmuir吸附等温线的相关系数均高于Freundlich吸附等温线的相关系数。这表明Langmuir吸附等温模型更适合描述nZVI-SB去除Cr(VI)的过程,即Cr(VI)在nZVI-SB表面的吸附是均匀的单分子层吸附[16]。与其他吸附剂相比,本文中的nZVI-SB(1∶1)对Cr(VI)的去除性能优于绝大部分吸附剂,见表6

    为了更加深入的了解nZVI-SB(1∶1)对Cr(VI)的去除机制,对处理Cr(VI)前、处理Cr(VI)后的nZVI-SB(1∶1)进行XPS分析,结果如图11表7所示。图11(a)为nZVI-SB(1∶1) C1s的高分辨精细图谱,吸附前主要峰有C—C(284.64 eV)、C—O(286.06 eV)和C=O(288.54 eV)。与Cr(VI)反应后,这3个峰的面积存在变化,表明这些基团参与了对Cr(VI)的吸附[16]图11(b)中反应前O1s的主要峰有Fe—O(529.95 eV)、C—O(531.17 eV)和C=O(532.04 eV),吸附后C—O的峰面积增多,C=O的峰面积减少,表明生物炭可以作为电子传递介质,通过表面某些官能团得失电子参与反应[40]。吸附前nZVI-SB(1∶1)的Fe2p光谱中(图11(c)),出现Fe0弱峰(706.70 eV),表明成功合成了nZVI。结合能为724.39 eV 和711.10 eV,对应于Fe(II)的氧化物(FeO);结合能为728.15 eV和714.41 eV对应于Fe(III)的氧化物(Fe2O3),这与XRD的结果一致。吸附后Fe0特征峰消失,Fe(II)峰面积下降,Fe(III)峰面积上升,表明nZVI和Fe(II)被Cr(VI)氧化[41]图11(d)中580.40 eV和590.28 eV处的峰为Cr(VI)特征峰,说明Cr(VI)被nZVI-SB(1∶1)吸附[42]。577.01 eV和586.85 eV处的峰为Cr(III)特征峰,说明部分Cr(VI)能被nZVI-SB(1∶1)还原生成Cr(III)[25]。其中Cr(III)和Cr(VI)分别占元素Cr含量的84.39%和15.61%(表7),大部分Cr以Cr(III)的形式存在。

    表  5  nZVI-SB(1∶1)对Cr(VI)的吸附等温线拟合参数
    Table  5.  Adsorption isotherm fitting parameters of Cr(VI) by nZVI-SB(1∶1)
    Temperature/℃LangmuirFreundlich
    qm/(mg·g−1)KLR2KFnR2
    20141.550.4570.999 98.488.930.821
    30143.840.3380.999104.739.990.744
    40151.230.4330.999106.389.070.766
    Notes: qm—Maximum adsorption capacity; KL—Adsorption equilibrium constant of the Langmuir model; KF—Adsorption equilibrium constant of Freundlich model; n—Constants related to the adsorption intensity.
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    表  6  nZVI-SB(1∶1)和其他吸附剂对Cr(VI)的吸附能力比较
    Table  6.  Comparison of the adsorption capacity of Cr(VI) by nZVI-SB(1∶1) and other adsorbents
    AdsorbentpHTemperature/℃Adsorption
    capacity/(mg·g−1)
    Ref.
    Sludge biochar (500℃) 7 25 7.93 [13]
    Bentonite-supported nanoscale zero-valent iron (B-nZVI) 5 25 39.48 [23]
    Ficus carica biosorbent 3 30 19.68 [36]
    Magnetic nanoparticle-Phosphorene-Titanium nano tubes (MNP-PN-TNT) 9 25 35.00 [2]
    Nanoscale zero-valent iron grafted on acid-activated attapulgite (A-nZVI) 7 27 4.94 [31]
    HNO3 modified quinoa biochar 4 55.85 [37]
    ZnO modified hyacinth biochar 25 43.48 [38]
    Halloysite nanotubes/ploy composites 2 25 855.66 [39]
    nZVI-SB(1∶1) 2 40 150.60 This study
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    图  11  nZVI-SB(1∶1)的XPS图谱:(a) C1s;(b) O1s;(c) Fe2p;(d) Cr2p
    Figure  11.  XPS spectra of nZVI-SB(1∶1): (a) C1s; (b) O1s; (c) Fe2p; (d) Cr2p

    综合2.2节SEM、TEM和XRD分析,推测nZVI-SB(1∶1)去除Cr(VI)的主要机制如图12所示。Cr(VI)首先被nZVI-SB(1∶1)吸附固定,此后部分Cr(VI)与nZVI提供的电子接触后被还原为Cr(III),nZVI被氧化为Fe2+,Fe2+仍具有还原性,可将Cr(VI)还原为Cr(III)。生成的Cr(III)、Fe(III)结合成为Cr(III)-Fe(III)氢氧化物沉淀[19],通过C—C、C—O和C=O等官能团吸附在nZVI-SB(1∶1)上,达到去除Cr(VI)的目的。

    表  7  nZVI-SB(1∶1)去除Cr(VI)前、去除Cr(VI)后的C1s、O1s、Fe2p和Cr2p XPS光谱的成分和相应的相对百分比
    Table  7.  Composition and relative percents of C1s, O1s, Fe2p and Cr2p XPS spectra before and after Cr(VI) removal by nZVI-SB(1∶1)
    ComponentsRelative percentage/%Binding energy/eV
    BeforeAfterBeforeAfter
    C1s C—C 58.99 59.78 284.64 284.69
    C—O 26.24 24.00 286.06 286.27
    C=O 14.77 16.22 288.54 288.72
    O1s Fe—O 32.07 30.05 529.95 529.99
    C—O 29.94 57.76 531.17 531.40
    C=O 37.99 12.19 532.04 532.58
    Fe2p Fe0 0.36 0.00 706.70
    Fe(II) 70.31 67.96 711.10/724.39 711.11/724.45
    Fe(III) 29.33 32.04 714.41/728.15 714.44/728.55
    Cr2p Cr(III) 84.39 577.01/586.85
    Cr(VI) 15.61 580.40/590.28
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    nZVI-SB(1∶1)的洗脱再生试验如图13所示。经过3次循环,nZVI-SB(1∶1)对Cr(VI)的去除率逐次降低,第3次能够保持在74.34%。随着洗脱次数的增加,Cr(VI)去除率逐渐下降。这可能是复合材料上的nZVI被洗脱及nZVI-SB(1∶1)表面上Fe0和Fe2+逐渐消耗造成的[32]。但对于SB而言,成本低廉,可以很容易制备,因此后续nZVI-SB再生主要考虑如何保持nZVI活性方面。

    图  12  nZVI-SB(1∶1)对Cr(VI)去除机制[15]
    Figure  12.  Schematic of Cr(VI) removal mechanisms by nZVI-SB(1∶1)[15]
    图  13  nZVI-SB(1∶1)洗脱再生试验
    Figure  13.  nZVI-SB(1∶1) regeneration test

    (1) 以污泥基生物炭(SB)作为载体,通过液相还原法成功制备了纳米零价铁(nZVI)-SB材料,Fe∶C质量比为1∶1时,nZVI-SB(1∶1)对Cr(VI)的去除效果最好。

    (2) 在初始pH值为2、投加量为0.5 g/L、温度为40℃条件下,nZVI-SB(1∶1)对Cr(VI)的最大去除量为150.60 mg/g。Cr(VI)去除过程符合Langmuir吸附等温模型和准二级吸附动力学模型,说明Cr(VI)吸附属于均匀的化学吸附过程。

    (3) nZVI-SB(1∶1)对Cr(VI)的吸附机制主要为Cr(VI)首先被吸附到nZVI-SB(1∶1)表面,随后大部分与nZVI、Fe(II)反生氧化还原反应,生成Cr(III);最后与Fe(III)形成Cr(III)-Fe(III)氢氧化物沉淀。

    (4) 以市政污泥为原料,制备SB负载nZVI,可高效处理含Cr(VI)废水。生物炭有效的解决了nZVI易团聚问题,提高nZVI的活性,生物炭负载nZVI在含Cr(VI)废水处理中能发挥协同作用。

  • 图  1   SB (a)和污泥基生物炭负载纳米零价铁(nZVI-SB)(1∶1)处理Cr(VI)前(b)、处理Cr(VI)后(c)的SEM-EDS图像

    Figure  1.   SEM-EDS images of SB (a) and sludge-derived biochar loaded with nanoscale zero-valent iron (nZVI-SB)(1∶1) before (b) and after (c) treatment of Cr(VI)

    图  3   SB和nZVI-SB(1∶1)的N2吸附-脱附等温线(a)和孔径分布(b)

    dV/dW—Pore volume

    Figure  3.   N2 adsorption-desorption isotherms (a) and pore size distributions (b) of SB and nZVI-SB(1∶1)

    图  2   nZVI-SB(1∶1)处理Cr(VI)前(a)、 处理Cr(VI)后(b)的TEM图像

    Figure  2.   TEM images of nZVI-SB(1∶1) before (a) and after (b) treatment of Cr(VI)

    图  4   SB和nZVI-SB(1∶1)去除Cr(VI)前、去除Cr(VI)后的XRD图谱(a)及nZVI-SB(1∶1)的XPS全谱图(b)

    Figure  4.   XRD patterns of SB and nZVI-SB(1∶1) before and after adsorption of Cr(VI) (a) and XPS full spectrum of nZVI-SB(1∶1) (b)

    图  5   不同Fe∶C质量比对Cr(VI)去除的影响

    Figure  5.   Effect of different Fe∶C mass ratio on Cr(VI) removal

    图  6   初始pH对nZVI-SB(1∶1)去除Cr(VI)的影响

    Figure  6.   Effect of initial pH on the removal of Cr(VI) by nZVI-SB(1∶1)

    图  7   不同pH值下Cr(VI)形态分布曲线图

    C0—Initial mass concentration of Cr(VI) in solution

    Figure  7.   Cr(VI) form distribution curves under different pH values

    图  8   nZVI-SB(1∶1)投加量对nZVI-SB(1∶1)去除Cr(VI)的影响

    Figure  8.   Influence of nZVI-SB(1∶1) dosage on the removal of Cr(VI) by nZVI-SB(1∶1)

    图  9   (a)吸附时间对nZVI-SB(1∶1)去除Cr(VI)的影响;准一级(b)、准二级(c)动力学拟合曲线和颗粒内扩散拟合曲线(d)

    Figure  9.   (a) Effect of adsorption time on the removal of Cr(VI) by nZVI-SB(1∶1); Quasi-first (b), quasi-second (c) kinetic fitting curves and intra-particle diffusion fitting curve (d)

    图  10   (a)温度对nZVI-SB(1∶1)去除Cr(VI)的影响;(b) Langmuir吸附等温线;(c) Freundlich吸附等温线

    Figure  10.   (a) Influence of temperature on the removal of Cr(VI) by nZVI-SB(1∶1); (b) Langmuir adsorption isotherm; (c) Freundlich adsorption isotherm

    图  11   nZVI-SB(1∶1)的XPS图谱:(a) C1s;(b) O1s;(c) Fe2p;(d) Cr2p

    Figure  11.   XPS spectra of nZVI-SB(1∶1): (a) C1s; (b) O1s; (c) Fe2p; (d) Cr2p

    图  12   nZVI-SB(1∶1)对Cr(VI)去除机制[15]

    Figure  12.   Schematic of Cr(VI) removal mechanisms by nZVI-SB(1∶1)[15]

    图  13   nZVI-SB(1∶1)洗脱再生试验

    Figure  13.   nZVI-SB(1∶1) regeneration test

    表  1   样品名称缩写

    Table  1   Abbreviation name of samples

    Sample Fe/wt% SB/wt%
    nZVI-SB(1∶4) 20.0 80.0
    nZVI-SB(1∶2) 33.3 66.7
    nZVI-SB(1∶1) 50.0 50.0
    nZVI-SB(2∶1) 66.7 33.3
    Notes: nZVI—Nanoscale zero-valent iron; SB—Sludge-based biochar.
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    表  2   污泥及污泥基生物炭(SB)消解液中重金属浓度

    Table  2   Heavy metal concentrations in digestion solution of sludge and sludge-based biochar (SB) (mg·L−1)

    SampleZnPbCuBaCdCr
    Sludge 5.28 0.34 2.37 7.32 0.06 0.14
    SB 7.81 0.51 3.01 8.67 0.09 0.13
    Specified value
    in GB/T
    5085.3—2007[18]
    100.00 5.00 100.00 100.00 1.00 15.00
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    表  3   SB和nZVI-SB的元素组成

    Table  3   Elemental composition of SB and nZVI-SB wt%

    SampleCNONaMgAlSiKCaCrFe
    SB40.488.4634.940.190.393.266.430.883.050.33 1.59
    nZVI-SB(1∶4)36.637.5633.050.220.480.630.760.280.300.2219.87
    nZVI-SB(1∶2)30.416.4127.550.180.360.510.620.230.240.1833.31
    nZVI-SB(1∶1)23.923.1121.450.140.270.280.350.190.210.1649.92
    nZVI-SB(2∶1)16.481.7614.450.060.130.110.160.080.080.0666.63
    下载: 导出CSV

    表  4   nZVI-SB(1∶1)对Cr(VI)的吸附动力学参数

    Table  4   Adsorption kinetic parameters of Cr(VI) adsorption by nZVI-SB(1∶1)

    Intraparticle diffusion model
    qe/(mg·g−1)K/(min−1)R2Kd/(mg·(m·min0.5)−1)CR2
    Quasi-first order dynamics model 38.570.01130.8555.85822.7310.971
    Quasi-second-stage dynamics model103.070.00050.9990.03898.5510.946
    Notes: qe—Equilibrium adsorption capacity; K—Adsorption rate constant; R2—Linear correlation coefficient; Kd—Particle diffusion constants; C—Constant.
    下载: 导出CSV

    表  5   nZVI-SB(1∶1)对Cr(VI)的吸附等温线拟合参数

    Table  5   Adsorption isotherm fitting parameters of Cr(VI) by nZVI-SB(1∶1)

    Temperature/℃LangmuirFreundlich
    qm/(mg·g−1)KLR2KFnR2
    20141.550.4570.999 98.488.930.821
    30143.840.3380.999104.739.990.744
    40151.230.4330.999106.389.070.766
    Notes: qm—Maximum adsorption capacity; KL—Adsorption equilibrium constant of the Langmuir model; KF—Adsorption equilibrium constant of Freundlich model; n—Constants related to the adsorption intensity.
    下载: 导出CSV

    表  6   nZVI-SB(1∶1)和其他吸附剂对Cr(VI)的吸附能力比较

    Table  6   Comparison of the adsorption capacity of Cr(VI) by nZVI-SB(1∶1) and other adsorbents

    AdsorbentpHTemperature/℃Adsorption
    capacity/(mg·g−1)
    Ref.
    Sludge biochar (500℃) 7 25 7.93 [13]
    Bentonite-supported nanoscale zero-valent iron (B-nZVI) 5 25 39.48 [23]
    Ficus carica biosorbent 3 30 19.68 [36]
    Magnetic nanoparticle-Phosphorene-Titanium nano tubes (MNP-PN-TNT) 9 25 35.00 [2]
    Nanoscale zero-valent iron grafted on acid-activated attapulgite (A-nZVI) 7 27 4.94 [31]
    HNO3 modified quinoa biochar 4 55.85 [37]
    ZnO modified hyacinth biochar 25 43.48 [38]
    Halloysite nanotubes/ploy composites 2 25 855.66 [39]
    nZVI-SB(1∶1) 2 40 150.60 This study
    下载: 导出CSV

    表  7   nZVI-SB(1∶1)去除Cr(VI)前、去除Cr(VI)后的C1s、O1s、Fe2p和Cr2p XPS光谱的成分和相应的相对百分比

    Table  7   Composition and relative percents of C1s, O1s, Fe2p and Cr2p XPS spectra before and after Cr(VI) removal by nZVI-SB(1∶1)

    ComponentsRelative percentage/%Binding energy/eV
    BeforeAfterBeforeAfter
    C1s C—C 58.99 59.78 284.64 284.69
    C—O 26.24 24.00 286.06 286.27
    C=O 14.77 16.22 288.54 288.72
    O1s Fe—O 32.07 30.05 529.95 529.99
    C—O 29.94 57.76 531.17 531.40
    C=O 37.99 12.19 532.04 532.58
    Fe2p Fe0 0.36 0.00 706.70
    Fe(II) 70.31 67.96 711.10/724.39 711.11/724.45
    Fe(III) 29.33 32.04 714.41/728.15 714.44/728.55
    Cr2p Cr(III) 84.39 577.01/586.85
    Cr(VI) 15.61 580.40/590.28
    下载: 导出CSV
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  • 收稿日期:  2022-01-09
  • 修回日期:  2022-03-07
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  • 网络出版日期:  2022-03-24
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