2023, Vol. 44
表 1
(Table 1)
高锰酸钾改性椰壳生物炭对水中Cd(Ⅱ)和Ni(Ⅱ)的去除性能及机制
引用本文
张凤智, 王敦球, 曹星沣, 刘桥京, 岳甜甜, 刘立恒. 高锰酸钾改性椰壳生物炭对水中Cd(Ⅱ)和Ni(Ⅱ)的去除性能及机制[J]. 环境科学, 2023, 44(6): 3278-3287.
ZHANG Feng-zhi, WANG Dun-qiu, CAO Xing-feng, LIU Qiao-jing, YUE Tian-tian, LIU Li-heng. Removal Performance and Mechanism of Potassium Permanganate Modified Coconut Shell Biochar for Cd(Ⅱ) and Ni(Ⅱ) in Aquatic Environment[J]. Environmental Science, 2023, 44(6): 3278-3287.
高锰酸钾改性椰壳生物炭对水中Cd(Ⅱ)和Ni(Ⅱ)的去除性能及机制
1. 桂林理工大学环境科学与工程学院, 桂林 541006;
2. 桂林理工大学岩溶地区水污染控制与用水安全保障协同创新中心, 桂林 541006;
3. 桂林理工大学广西环境污染控制理论与技术重点实验室, 桂林 541006
2. 桂林理工大学岩溶地区水污染控制与用水安全保障协同创新中心, 桂林 541006;
3. 桂林理工大学广西环境污染控制理论与技术重点实验室, 桂林 541006
摘要: 以高锰酸钾改性商业椰壳生物炭(MCBC)为吸附剂, 探讨了它对Cd(Ⅱ)和Ni(Ⅱ)的去除性能及机制.当初始pH和MCBC投加量分别为5和3.0 g·L-1时, Cd(Ⅱ)和Ni(Ⅱ)的去除率均高于99%.Cd(Ⅱ)和Ni(Ⅱ)的去除更符合准二级动力学模型, 表明它们的去除以化学吸附为主; Cd(Ⅱ)和Ni(Ⅱ)去除的控速步骤为快速去除阶段, 而该阶段的速率取决于液膜扩散和颗粒内扩散(表面扩散).Cd(Ⅱ)和Ni(Ⅱ)主要通过表面吸附和孔隙填充附着在MCBC上, 表面吸附的贡献更大; MCBC对Cd(Ⅱ)和Ni(Ⅱ)的饱和吸附量分别为57.18 mg·g-1和23.29 mg·g-1, 约为前驱体(椰壳生物炭)的5.74倍和6.97倍.Cd(Ⅱ)和Ni(Ⅱ) 的去除是自发的、吸热的, 具有较为明显的化学吸附热力学特征.Cd(Ⅱ)通过离子交换、共沉淀、络合反应和阳离子-π相互作用附着在MCBC上; 而Ni(Ⅱ)则是通过离子交换、共沉淀、络合反应和氧化还原反应被MCBC去除; 其中, 共沉淀和络合作用是Cd(Ⅱ)和Ni(Ⅱ)表面吸附的主要方式, 且络合产物中无定形的Mn—O—Cd或Mn—O—Ni的占比可能会较高.研究结果将为商业生物炭在重金属废水处理中的实际应用提供重要的技术支持和理论依据.
关键词:
高锰酸钾改性
椰壳生物炭(MCBC)
Cd(Ⅱ)和Ni(Ⅱ)
吸附
去除性能及机制
Removal Performance and Mechanism of Potassium Permanganate Modified Coconut Shell Biochar for Cd(Ⅱ) and Ni(Ⅱ) in Aquatic Environment
ZHANG Feng-zhi1
, WANG Dun-qiu1,2,3 , CAO Xing-feng1 , LIU Qiao-jing1 , YUE Tian-tian1 , LIU Li-heng1,2,3
, WANG Dun-qiu1,2,3 , CAO Xing-feng1 , LIU Qiao-jing1 , YUE Tian-tian1 , LIU Li-heng1,2,3
1. College of Environmental Science and Engineering, Guilin University of Technology, Guilin 541006, China;
2. Collaborative Innovation Center for Water Pollution Control and Water Safety in Karst Area, Guilin University of Technology, Guilin 541006, China;
3. Guangxi Key Laboratory of Environmental Pollution Control Theory and Technology, Guilin University of Technology, Guilin 541006, China
2. Collaborative Innovation Center for Water Pollution Control and Water Safety in Karst Area, Guilin University of Technology, Guilin 541006, China;
3. Guangxi Key Laboratory of Environmental Pollution Control Theory and Technology, Guilin University of Technology, Guilin 541006, China
Abstract: In this study, coconut shell biochar modified by KMnO4 (MCBC) was used as the adsorbent, and its removal performance and mechanism for Cd(Ⅱ) and Ni(Ⅱ) were discussed. When the initial pH and MCBC dosage were separately 5 and 3.0 g·L-1, respectively, the removal efficiencies of Cd(Ⅱ) and Ni(Ⅱ) were both higher than 99%. The removal of Cd(Ⅱ) and Ni(Ⅱ) was more in line with the pseudo-second-order kinetic model, indicating that their removal was dominated by chemisorption. The rate-controlling step for Cd(Ⅱ) and Ni(Ⅱ) removal was the fast removal stage, for which the rate depended on the liquid film diffusion and intraparticle diffusion (surface diffusion). Cd(Ⅱ) and Ni(Ⅱ) were mainly attached to the MCBC via surface adsorption and pore filling, in which the contribution of surface adsorption was greater. The maximum adsorption amounts of Cd(Ⅱ) and Ni(Ⅱ) by MCBC were individually 57.18 mg·g-1 and 23.29 mg·g-1, which were approximately 5.74 and 6.97 times that of the precursor (coconut shell biochar), respectively. The removal of Cd(Ⅱ) and Zn(Ⅱ) was spontaneous and endothermic and had obvious thermodynamic characteristics of chemisorption. Cd(Ⅱ) was attached to MCBC through ion exchange, co-precipitation, complexation reaction, and cation-π interaction, whereas Ni(Ⅱ) was removed by MCBC via ion exchange, co-precipitation, complexation reaction, and redox. Among them, co-precipitation and complexation were the main modes of surface adsorption of Cd(Ⅱ) and Ni(Ⅱ). Additionally, the proportion of amorphous Mn—O—Cd or Mn—O—Ni in the complex may have been higher. These research results will provide important technical support and theoretical basis for the practical application of commercial biochar in the treatment of heavy metal wastewater.
Key words:
potassium permanganate modification
coconut shell biochar(MCBC)
Cd(Ⅱ) and Ni(Ⅱ)
adsorption
removal performance and mechanism
随着金属矿开采、冶炼加工和机械制造等行业的快速发展, 大量含镉和镍废水被排入水环境中, 镉和镍的污染已成为一个严重的环境问题[1~4].在自然环境中, 镉和镍具有不可降解性, 并能在生物体内积累, 导致其对生态系统的威胁更为严重.因此, 对含镉和镍的废水进行有效处理是非常必要的.目前, 含镉和镍废水的处理方法主要有化学沉淀法、电化学法、吸附法、离子交换法和膜分离法等[5~8].其中, 吸附法由于操作简单、成本低和处理效率高等优点, 被列为最有前途的重金属废水处理技术[9].
对于吸附法, 污染物的成功去除取决于对目标污染物具有显著亲和力及快速吸附速率的吸附剂的选择[10].有研究证明, 碳基材料(如碳纳米管、石墨烯、生物炭)非常适合作为重金属废水处理的吸附剂[11, 12].其中, 生物炭具有非常明显的成本优势, 使其在重金属废水处理实际应用中有更大的潜力.然而, 生物炭对重金属离子的去除也存在吸附容量相对较低, 吸附选择性较差等不足[13].本课题组前期研究显示, 椰壳生物炭对Cd(Ⅱ)和Ni(Ⅱ)的吸附容量分别仅为9.96 mg·g-1和3.34 mg·g-1.因此, 提高生物炭对重金属的吸附性能是实现其实际应用的重要前提.
有报道指出, 改性是提高生物炭对重金属吸附性能的最为有效的途径之一[14].生物炭的改性方法大体可分为化学改性、物理改性、负载矿物和磁性改性这4类[15].与其他的改性方法相比, KMnO4化学改性不但能够显著地改善生物炭的孔隙性质, 增加含氧官能团的数量, 还能将锰氧化物负载到生物炭上, 从而加强生物炭与重金属离子之间的相互作用, 更显著地提高生物炭对重金属的吸附性能[16].
本研究以高锰酸钾改性的椰壳生物炭为吸附剂, 考察了溶液pH和吸附剂投加量对其吸附Cd(Ⅱ)和Ni(Ⅱ)性能的影响; 通过动力学、等温线和热力学分析, 并结合生物炭的表征, 探讨了Cd(Ⅱ)和Ni(Ⅱ)去除的机制, 以期为商业生物炭在重金属废水处理中的实际应用提供技术支持和理论依据.
1 材料与方法 1.1 实验材料本实验使用的主要化学药剂[Cd(NO3)2·4H2O、Ni(NO3)2·6H2O、KMnO4、NaOH和HCl]购自西陇科学股份有限公司, 其纯度均为分析纯.
1.2 KMnO4改性椰壳生物炭的制备KMnO4改性椰壳生物炭(MCBC)的制备流程如下：①将市售椰壳生物炭(20~30目)用去离子水反复洗涤5次, 并在105℃的烘箱中干燥12 h后备用; ②将干燥后的椰壳生物炭与0.9 mol·L-1的KMnO4溶液充分混合(固液比为1 ∶20)后, 在90℃下搅拌2 h; ③将生物炭用去离子水反复洗涤5次后, 并在70℃下干燥12 h. MCBC的比表面积为509.8 m2·g-1.
1.3 吸附实验首先, 称取一定质量的MCBC(0.01~0.3 g), 置于装有50 mL Cd(Ⅱ)或Ni(Ⅱ)溶液(初始浓度：10~200 mg·L-1; 初始pH：2~8)的聚丙烯离心管中.然后, 将离心管放入气浴恒温振荡器(ZD-85, 中国金逸; 转速为220 r·min-1)中, 在不同温度下(298、308和318 K)振荡一定时间(0~24 h).最后, 采用电感耦合等离子体发射光谱仪(7000DV, 美国PE公司)测定离心处理后上清液中Cd(Ⅱ)或Ni(Ⅱ)的浓度, 并通过式(1)和式(2)分别计算它们的吸附量(qt, mg·L-1)和去除率(φ, %).
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(1) |
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(2) |
式中, c0为Cd(Ⅱ)或Ni(Ⅱ)的初始浓度, mg·L-1; ct为t时刻Cd(Ⅱ)或Ni(Ⅱ)的浓度, mg·L-1; m为MCBC的质量, g.
1.4 动力学、等温线和热力学模型 1.4.1 动力学模型采用准一级动力学模型(PFO)、准二级动力学模型(PSO)、液膜扩散模型(LFD)和颗粒内扩散模型(IpD)来分析MCBC去除Cd(Ⅱ)和Ni(Ⅱ)的动力学.上述动力学模型的公式如下[17]：
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(3) |
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(4) |
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(5) |
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(6) |
式中, t为吸附时间h; k1、k2、kLFD和kIpD分别为准一级动力学模型、准二级动力学模型、液膜扩散模型和颗粒内扩散模型的速率常数, 单位分别为h-1、g·(mg·h)-1、h-1和g·(mg·h0.5)-1; AL为液膜扩散模型常数; AI为边界层特征常数.
1.4.2 等温线模型采用Langmuir、Freundlich和Temkin模型对MCBC吸附Cd(Ⅱ)和Ni(Ⅱ)的等温过程进行拟合分析, 其数学表达式分别为[18]：
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(7) |
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(8) |
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(9) |
式中, qe为吸附剂对吸附质平衡时的吸附量, mg·g-1; qmax为饱和吸附量, mg·g-1; KL为Langmuir模型的吸附常数, L·g-1; KF为吸附容量, (mg·g-1)·(mg·L-1)-1/n; n为Freundlich模型吸附常数; R为理想气体常数, 8.314 J·(mol·K)-1; T为热力学温度, K; kT为平衡结合常数, L·mg-1; bT为与吸附热有关的Temkin常数, J·mol-1.
1.4.3 热力学通过吉布斯自由能变(ΔGθ, kJ·mol-1)、焓变(ΔHθ, kJ·mol-1)和熵变[ΔSθ, J·(mol·K)-1]来研究MCBC去除Cd(Ⅱ)和Ni(Ⅱ)的热力学特性.ΔGθ、ΔHθ和ΔSθ可通过以下方程确定[19].
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(10) |
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(11) |
式中, KD为平衡因子(qe/ce).
1.5 生物炭的表征采用X射线衍射仪(XRD; X'Pert PRO, 荷兰帕纳科公司)、扫描电子显微镜及(SEM)、X射线能量色散光谱仪(EDS, Zeiss meilin compact)、傅里叶变换红外光谱仪(FTIR; Nicolet.iS10, 美国赛默飞世尔科技)和X射线光电子能谱仪(XPS; thermo scientific ESCALAB 250 Xi)对Cd(Ⅱ)和Ni(Ⅱ)去除前后的MCBC的晶形结构、表面形貌及元素分布、表面官能团和元素形态进行表征.
2 结果与讨论 2.1 Cd(Ⅱ)和Ni(Ⅱ)的去除性能 2.1.1 初始pH的影响不同初始pH下, MCBC对Cd(Ⅱ)和Ni(Ⅱ)的去除性能如图 1(a)和图 1(b)所示.在本研究范围内, 随着初始pH的升高, MCBC对Cd(Ⅱ)和Ni(Ⅱ)的吸附量和去除效率均总体呈上升趋势; 而当初始pH大于4或5后, 这种增长趋势变得十分缓慢.一般而言, 初始pH值决定了吸附剂表面电荷和溶液中污染物的形态分布, 进而影响污染物的去除性能[18].根据Cd(Ⅱ)和Ni(Ⅱ)形态分布可知[20], 在本研究的初始pH范围内, Cd(Ⅱ)和Ni(Ⅱ)的形态基本没有改变, 也就意味着MCBC表面性质的变化应是影响Cd(Ⅱ)和Ni(Ⅱ)去除的主要因素.在较低的初始pH情况下(pH为2~3), MCBC表面质子化程度很高, 导致Cd(Ⅱ)和Ni(Ⅱ)与MCBC间的静电斥力, 使它们很难扩散到MCBC表面[17]; 同时, 大量的H+与Cd(Ⅱ)和Ni(Ⅱ)对MCBC表面吸附活性点位的竞争十分激烈[21].当初始pH升高至4~5范围时, MCBC表面的质子化程度降低, 致使它与Cd(Ⅱ)和Ni(Ⅱ)间的静电斥力减弱, 且H+浓度的降低也使它与Cd(Ⅱ)和Ni(Ⅱ)对吸附活性点位的竞争减小, 从而提高了MCBC对Cd(Ⅱ)和Ni(Ⅱ)的去除性能及效率.当初始pH大于4或5时, 液相酸度逐渐接近MCBC的pHPZC[7.42, 图 1(c)], 理论上会使MCBC表面的负电荷增多, 增强MCBC与Cd(Ⅱ)和Ni(Ⅱ)间的引力, 提高Cd(Ⅱ)和Ni(Ⅱ)的去除性能[20].实际上, Cd(Ⅱ)和Ni(Ⅱ)的去除性能并没有明显的提升, 原因可能是MCBC中的Cd(Ⅱ)和Ni(Ⅱ)结合位点是恒定的, 这同时说明在研究范围内静电引力不是Cd(Ⅱ)和Ni(Ⅱ)去除的主要作用[22].此外, Cd(Ⅱ)原溶液(100 mg·L-1)和Ni(Ⅱ)原溶液(50 mg·L-1)的pH分别为5.23和5.42, 这就意味着将MCBC直接投加到原溶液中即可使Cd(Ⅱ)和Ni(Ⅱ)被高效地去除.根据内插法, 此条件下的Cd(Ⅱ)和Ni(Ⅱ)的去除量及去除效率分别为56.58 mg·g-1及58.03%和22.85 mg·g-1及45.69%.
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图 1 不同pH下, Cd(Ⅱ)和Ni(Ⅱ)的去除性能及MCBC的pHPZC Fig. 1 Removal performance of Cd(Ⅱ) and Ni(Ⅱ) at different pH and pHPZC of MCBC |
2.1.2 投加量的影响
MCBC投加量对其去除Cd(Ⅱ)和Ni(Ⅱ)能力的影响如图 2所示.随着投加量从0.2 g·L-1增加至6 g·L-1, Cd(Ⅱ)和Ni(Ⅱ)的吸附量分别从81.90 mg·g-1和39.95 mg·g-1下降至16.60 mg·g-1和8.31 mg·g-1; 而它们的去除率则分别从16.38%和15.98%提高至99.61%和99.72%.这是因为较高的吸附剂投加量能够提供充足的活性点位, 使更多的Cd(Ⅱ)和Ni(Ⅱ)被去除; 但这也会导致活性位点重叠, 并降低固液界面间Cd(Ⅱ)和Ni(Ⅱ)的净通量, 从而降低了MCBC对Cd(Ⅱ)和Ni(Ⅱ)的吸附量[18, 23].由于吸附剂投加量在很大程度上影响污染物去除的成本, 所以适宜的MCBC投加量建议为3 g·L-1.在此条件下, MCBC对Cd(Ⅱ)和Ni(Ⅱ)的吸附量及去除率分别为33.14 mg·g-1及99.43%和16.53 mg·g-1及99.19%.
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图 2 MCBC投加量对Cd(Ⅱ)和Ni(Ⅱ)去除的影响 Fig. 2 Effect of MCBC dosages on Cd(Ⅱ) and Ni(Ⅱ) removals |
2.2 动力学、等温线和热力学 2.2.1 动力学
MCBC去除Cd(Ⅱ)和Ni(Ⅱ)的动力学拟合结果如图 3和表 1所示.由图 3(a)、图 3(b)和表 1可知, Cd(Ⅱ)和Ni(Ⅱ)去除的表观动力学较好地遵循了准一级动力学模型和准二级动力学模型; 但是, 准二级动力学方程的R2值相对更高, 分别为0.996 7和0.985 7; 并且, 由准二级动力学方程得到的Cd(Ⅱ)和Ni(Ⅱ)的平衡吸附量分别为61.06 mg·g-1和25.88 mg·g-1, 与实验结果(qe, exp)的差距更小.这些表明, MCBC对Cd(Ⅱ)和Ni(Ⅱ)的去除应是物理吸附和化学吸附的共同作用, 且该过程受化学吸附控制, 即Cd(Ⅱ)和Ni(Ⅱ)与MCBC之间可能通过共用或交换电子形成化学键[21].
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图 3 MCBC去除Cd(Ⅱ)和Ni(Ⅱ)的动力学拟合 Fig. 3 Fitting plots of kinetics for Cd(Ⅱ) and Ni(Ⅱ) removals by MCBC |
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表 1 MCBC去除Cd(Ⅱ)和Ni(Ⅱ)的动力学拟合参数 Table 1 Fitting parameters of the kinetics for Cd(Ⅱ) and Ni(Ⅱ) removals by MCBC |
从传质过程看[图 3(c)和图 3(d)], Cd(Ⅱ)和Ni(Ⅱ)的去除明显分为3个阶段：快速去除阶段(0~4 h; 0~2 h)、慢速去除阶段(4~18 h; 2~12 h) 和动态平衡阶段或渐近平衡阶段(18~24 h; 12~24 h). 在第1阶段, 由于液相和MCBC表面间较高的浓度差, Cd(Ⅱ)和Ni(Ⅱ)迅速地穿过液膜扩散到MCBC表面, 并与其表面的大量有效吸附点位结合, 发生外表面扩散和表面吸附[17, 24].第2阶段, 由于固液相间浓度差的降低, 扩散到MCBC表面的Cd(Ⅱ)和Ni(Ⅱ)减少; 随着MCBC表面的有效吸附点位减少, 吸附在MCBC表面的Cd(Ⅱ)和Ni(Ⅱ)逐渐向MCBC的孔隙中扩散(即颗粒内扩散)[25], 并进行孔隙填充.第3阶段, 随着固液相间浓度差的进一步降低和有效吸附点位逐渐饱和, 整个体系呈现动态平衡或渐近平衡状态.由表 1可知, 液膜扩散模型和颗粒内扩散模型的R2值均在可接受范围内, 且AL和AI值均不为零, 表明Cd(Ⅱ)和Ni(Ⅱ)的去除速率由液膜扩散和颗粒内扩散共同决定; 同时, 第1阶段的kLFD和kIpD值远高于第2阶段和第3阶段, 说明表面吸附是Cd(Ⅱ)和Ni(Ⅱ) 去除的控速步骤[26].
2.2.2 等温线Langmuir、Freundlich和Temkin模型对MCBC去除Cd(Ⅱ)和Ni(Ⅱ)的等温过程的拟合结果如图 4和表 2所示.从R2值看, Cd(Ⅱ)和Ni(Ⅱ)的等温去除更符合Langmuir模型, 表明Cd(Ⅱ)和Ni(Ⅱ)主要是以单层形式附着在MCBC的表面[23].MCBC对Cd(Ⅱ)和Ni(Ⅱ)的最大吸附量分别为57.18 mg·g-1和23.29 mg·g-1(MCBC投加量为1 g·L-1; 初始pH=5), 与实验结果基本是一致的.并且, Cd(Ⅱ)和Ni(Ⅱ)的分离系数RL[RL=1/(1+KL·c0)]的范围分别为0.01~0.10和0.02~0.30, 说明MCBC对Cd(Ⅱ)和Ni(Ⅱ)的去除是有利吸附[27].但是, Cd(Ⅱ)去除的KL值大于Ni(Ⅱ), 说明MCBC对Cd(Ⅱ)的亲和力高于Ni(Ⅱ), 更有利于Cd(Ⅱ)的去除[28].
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图 4 Langmuir、Freundlich和Temkin模型对MCBC去除Cd(Ⅱ)和Ni(Ⅱ)的拟合 Fig. 4 Fitting plots of Langmuir, Freundlich, and Temkin models for Cd(Ⅱ) and Ni(Ⅱ) removal by MCBC |
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表 2 Cd(Ⅱ)和Ni(Ⅱ)等温去除的拟合参数 Table 2 Fitting parameters for isothermal removal of Cd(Ⅱ) and Ni(Ⅱ) |
Temkin模型拟合Cd(Ⅱ)和Ni(Ⅱ)去除实验数据的R2值分别为0.826 3和0.916 5, 尚在可接受的范围内.这说明Cd(Ⅱ)和Ni(Ⅱ)去除过程的吸附热随着MCBC表面覆盖率的增加而降低[29], 且化学吸附在Cd(Ⅱ)和Ni(Ⅱ)的去除中可能起着重要的作用[30].这与动力学分析的结论基本是一致的.
2.2.3 热力学MCBC去除Cd(Ⅱ)和Ni(Ⅱ)的热力学参数如表 3所示.负的ΔGθ值表明在研究范围内MCBC对Cd(Ⅱ)和Ni(Ⅱ)的去除是自发的; 且ΔGθ随着温度的升高而降低, 说明MCBC对Cd(Ⅱ)和Ni(Ⅱ)的去除可能涉及化学反应和温度键合反应[31].此外, Cd(Ⅱ)的ΔGθ均低于Ni(Ⅱ), 表明MCBC对Cd(Ⅱ)的去除需要的能量更少, 且自发性更强.Cd(Ⅱ)和Ni(Ⅱ)去除的ΔHθ值分别为19.92 kJ·mol-1和12.34 kJ·mol-1, 说明MCBC对Cd(Ⅱ)和Ni(Ⅱ)的去除均是吸热的, 且物理吸附和化学吸附在它们的去除中均起着重要的作用[17, 32].正的ΔSθ值表明在Cd(Ⅱ)和Ni(Ⅱ)去除过程中, 固液界面间的随机性和无序性是增加的[33].
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表 3 MCBC去除Cd(Ⅱ)和Ni(Ⅱ)的热力学参数 Table 3 Thermodynamic parameters for removal of Cd(Ⅱ) and Ni(Ⅱ) by MCBC |
2.3 生物炭表征 2.3.1 SEM
图 5展示了Cd(Ⅱ)和Ni(Ⅱ)去除前后MCBC的SEM图和表面元素分布情况.从SEM图看出, 吸附Cd(Ⅱ)和Ni(Ⅱ)后, MCBC表面和孔隙中的颗粒物显著增加, 这可能是Cd(Ⅱ)和Ni(Ⅱ)成功附着在MCBC上形成的无定形沉淀物[22].从表面元素分布情况看, Cd和Ni在MCBC表面的颗粒物上有聚集的趋势, 证明这些颗粒可能是各种含Cd或Ni的化合物; 同时, Cd或Ni的分布与O和Mn的分布具有很好的一致性, 说明Cd或Ni可能在MCBC表面形成CdnMnmOx或NinMnmOx类化合物.同时, Cd和Ni在MCBC的表面和孔隙中均有分布, 也说明颗粒内扩散在Cd(Ⅱ)和Ni(Ⅱ)去除过程中有重要的作用.
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图 5 MCBC去除Cd(Ⅱ)和Ni(Ⅱ)前后的SEM图和表面元素分布 Fig. 5 SEM images and surface element distribution of MCBC before/after removal of Cd(Ⅱ) and Ni(Ⅱ) |
2.3.2 XRD
Cd(Ⅱ)和Ni(Ⅱ)去除前后MCBC的XRD图谱如图 6(a)所示.使用前, MCBC在2θ为35°~40°范围内有一个较宽的特征峰, 该峰指向锰氧化物MnOx(如MnO、MnO2、Mn2O3和Mn3O4等)[34~37].吸附Cd(Ⅱ)和Ni(Ⅱ)后, MCBC并未有新的晶体结构的特征峰出现, 其原因可能是：① Cd和Ni以结晶度低的化合物的形式存在[35]; ② MnOx对重金属离子有很强的亲和力, 能与Cd(Ⅱ)和Ni(Ⅱ)形成内球络合物[37].
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图 6 MCBC去除Cd(Ⅱ)和Ni(Ⅱ)前后的XRD图谱和FTIR图谱 Fig. 6 XRD patterns and FTIR spectra of MCBC before/after removal of Cd(Ⅱ) and Ni(Ⅱ) |
2.3.3 FTIR
图 6(b)为Cd(Ⅱ)和Ni(Ⅱ)去除前后MCBC的FTIR图谱.在吸附Cd(Ⅱ)和Ni(Ⅱ)前, MCBC在3 430、1 630和520 cm-1附近有3个明显的特征峰.它们分别指向O—H伸缩振动[16]、COO—伸缩振动[38]和Mn—O伸缩振动[39].吸附Cd(Ⅱ)和Ni(Ⅱ)后, MCBC在1 380 cm-1附近有新的特征峰形成, 其原因可能是Cd(Ⅱ)和Ni(Ⅱ)在MCBC上形成内球络合物(Cd—O和Ni—O)[40].同时, 吸附Cd(Ⅱ)和Ni(Ⅱ)后, 指向Mn—O的伸缩振动峰出现了不同程度的蓝移, 可能是Cd(Ⅱ)和Ni(Ⅱ)与Mn—O之间发生共沉淀和络合反应[41].
2.3.4 XPS去除Cd(Ⅱ)和Ni(Ⅱ)前后, MCBC的宽频和精细扫描XPS图谱如图 7所示.从MCBC的XPS全谱图可知[图 7(a)], 在吸附Cd(Ⅱ)和Ni(Ⅱ) 后, MCBC主要存在的特征峰为C 1s、O 1s、Mn 2p、Cd 3d和Ni 2p, 这与SEM的结果是一致的.如图 7(b)所示, Mn 2p在640.0、(641.4±0.1)、(642.9±0.3)和(652.7±0.3) eV处的特征峰分别与Mn(Ⅱ)、Mn(Ⅲ)、Mn(Ⅳ)和Mn(Ⅱ)(MnOOH)相关[3, 42]; 吸附Cd(Ⅱ)和Ni(Ⅱ)后, Mn(Ⅱ)的峰消失了; 同时, Mn(Ⅳ)的比例由27.8%分别下降至5.00%和0.00%, 而Mn(Ⅲ)的比例由28.76%上升至45.53%和60.16%, 表明在Cd(Ⅱ)和Ni(Ⅱ)去除过程中还可能存在氧化还原反应; 此外, Mn(Ⅱ)(MnOOH)的比例在吸附Ni(Ⅱ)后显著减少, 说明MnOOH也可能参与了Ni(Ⅱ)的去除[42].如图 7(c)所示, Cd 3d在404.2 eV和411.7 eV处有两个特征峰, 表明Cd可能以CdO、CdOOR、CdCO3、Cd(OH)2、Ar—Cd等方式附着在MCBC表面[17, 43].而由图 7(d)可知, Ni主要以氢氧化镍(854.9 eV和872.4 eV)、络合氧化镍(860.4 eV和879.7 eV)[4, 44]和Ni0(853.6 eV)[45]的形式固定在MCBC表面.
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图 7 MCBC去除Cd(Ⅱ)和Ni(Ⅱ)前后的XPS分析 Fig. 7 XPS analysis of MCBC before/after Cd(Ⅱ) and Ni(Ⅱ) removal |
2.4 Cd(Ⅱ)和Ni(Ⅱ)去除的潜在机制
根据实验数据分析(动力学、等温线和热力学分析)、Cd(Ⅱ)和Ni(Ⅱ)去除的MCBC表征及相关研究, 提出了MCBC去除Cd(Ⅱ)和Ni(Ⅱ)的潜在途径和机制(图 8).在快速去除阶段, 由于固液相间巨大的含量差(Δc), 大量Cd(Ⅱ)和Ni(Ⅱ)迅速穿过固液界面间的液膜到达MCBC表面(Ⅰ：液膜扩散); 随后, Cd(Ⅱ)和Ni(Ⅱ)又通过表面扩散快速到达吸附点位, 完成表面吸附(Ⅱ：表面扩散和表面吸附); 在此阶段, Cd(Ⅱ)和Ni(Ⅱ)的去除速率主要由液膜扩散和表面扩散控制.由于固液相间浓度差的降低, 扩散至MCBC表面的Cd(Ⅱ)和Ni(Ⅱ)减少, Cd(Ⅱ)和Ni(Ⅱ)的去除进入慢速去除阶段; 同时, 由于MCBC表面的活性点位被大量占据, 通过物理吸附附着在MCBC表面的Cd(Ⅱ)和Ni(Ⅱ)在位势场的作用下向孔隙迁移, 实现孔隙填充(Ⅲ：颗粒内扩散和孔隙填充).当吸附时间超过平衡时间后, Cd(Ⅱ)和Ni(Ⅱ)的去除呈动态平衡状态.
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图 8 MCBC去除Cd(Ⅱ)和Ni(Ⅱ)的潜在机制 Fig. 8 Potential mechanism of Cd(Ⅱ) and Ni(Ⅱ) removal by MCBC |
MCBC对Cd(Ⅱ)和Ni(Ⅱ)的去除主要包括表面吸附和孔隙填充, 其中表面吸附对二者去除的贡献更大.而表面吸附既有物理吸附, 也有化学吸附; 化学吸附是Cd(Ⅱ)和Ni(Ⅱ)去除的主要途径.Cd(Ⅱ)化学吸附去除的方式可能是离子交换、共沉淀、络合反应和阳离子-π相互作用; 而Ni(Ⅱ)则可能是通过离子交换、共沉淀、络合反应和氧化还原反应被去除的[3, 4, 17, 45, 46].其中, 共沉淀和络合作用应是Cd(Ⅱ)和Ni(Ⅱ)去除的主要方式, 而络合产物中无定形的Mn—O—Cd或Mn—O—Ni的占比可能会较高.
3 结论(1) MCBC对Cd(Ⅱ)和Ni(Ⅱ)的去除性能在很大程度上依赖溶液初始pH和MCBC投加量; 当初始pH=5, 投加量为3.0 g·L-1时, Cd(Ⅱ)和Ni(Ⅱ)的去除率均可达99%以上, 相应的吸附量分别为33.14 mg·g-1和16.53 mg·g-1.
(2) Cd(Ⅱ)和Ni(Ⅱ)的去除更符合准二级动力学模型, 化学吸附是Cd(Ⅱ)和Ni(Ⅱ)去除的主要途径; Cd(Ⅱ)和Ni(Ⅱ)的去除过程分为快速去除、慢速去除和动态平衡或渐近平衡这3个阶段, 其中快速去除阶段为该过程的控速步骤, 其速率由液膜扩散和颗粒内扩散共同决定; Cd(Ⅱ)和Ni(Ⅱ)的去除方式主要包括表面吸附和孔隙填充, 其中表面吸附的贡献更大, MCBC对Cd(Ⅱ)和Ni(Ⅱ)的饱和吸附量分别为57.18 mg·g-1和23.29 mg·g-1, 约为椰壳生物炭的5.74倍和6.97倍; Cd(Ⅱ)和Ni(Ⅱ)的去除是自发的、吸热的过程, 具有较为明显的化学吸附热力学特征.
(3) Cd(Ⅱ)通过离子交换、共沉淀、络合反应和阳离子-π相互作用等附着在MCBC上; 而Ni(Ⅱ)则是通过离子交换、共沉淀、络合反应和氧化还原反应被MCBC去除; 其中, 共沉淀和络合作用是Cd(Ⅱ)和Ni(Ⅱ)去除的主要方式, 且络合产物中无定形的Mn—O—Cd或Mn—O—Ni的占比可能会较高.
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