STOTEN | 中科院土壤所陈梦舫研究员：水热辅助合成生物炭包覆纳米零价铁活化过一硫酸盐高效去除氯苯的性能及机理

图文摘要 | Graphical abstract

导读 | Introduction

纳米零价铁（nZVI）异相活化过一硫酸盐（PMS）高级氧化体系在难降解有机物污染地下水修复领域得到广泛关注，生物炭等碳材料逐渐成为抑制nZVI团聚效应的优质载体。然而，目前大部分nZVI及其碳复合材料的制备方法为基于硼氢化盐的液相还原法，但该方法程序复杂、成本高且易产生有毒物质，所制备的复合材料中nZVI颗粒常暴露于碳载体表面，致使nZVI易发生老化导致稳定性降低。多孔碳材料包覆被认为是同时强化nZVI活性和稳定性的有效改良方法，因此，一种简易、低成本、易规模化和环境友好的包覆型nZVI-碳复合材料（Fe@C）制备思路逐渐成为研究热点，Fe@C对PMS的多位点活化机制及其制备条件的影响规律也需深入探讨。本研究以水稻秸秆为前驱体，采用水热辅助碳还原法成功制备出球状Fe@C，以工业污染场地地下水难降解的典型有机污染物氯苯（MCB）为对象，深入研究了碳热还原温度对Fe@C活化能力的影响规律以及PMS的活化机理。

The advanced oxidation processes (AOPs) based on the heterogeneous activation of nanoscale zero-valent iron (nZVI) towards peroxymonosulfate (PMS) have received increasing attention in the degradation of recalcitrant organic contaminants in groundwater. In order to inhibit the aggregation effects of nZVI, carbon-based materials (e.g., biochar) have been emerged as superior supports. Most of nZVI and its supported composites were synthesized by liquid-phase reduction method using borohydride salt, however, such a method was highly complex, hazardous and cost-intensive. In addition, nZVI particles are generally exposed on the surfaces of supporting materials by this method, resulting in the decreased stability and loss of reactivity due to aging of composites in oxygen and water. Encapsulation of nZVI into porous carbon spheres was proposed to be a promising way to enhance both activity and stability of nZVI. Therefore, it is imperative to develop a facile, cost-efficient, readily scalable and eco-friendly strategy for preparing stable encapsulated nZVI‑carbon composites (Fe@C), and the activation mechanism towards PMS based on multiple active sites as well as the influence of preparation conditions of Fe@C need to be explored in depth. In this study, a hydrothermally assisted carbothermal reduction method was developed to synthesize Fe@C by using rice straw as feedstock, and the catalytic decomposition of PMS was evaluated in the degradation of monochlorobenzene (MCB). MCB was a representative toxic and refractory contaminant in groundwater at industrially contaminated sites. The effects of carbothermal reduction temperatures on the physicochemical properties and catalytic activities of Fe@C and activation mechanism towards PMS were studied.

一、碳包覆nZVI复合材料的制备与晶体和表面官能团表征 

 Synthesis and characterization for crystal structures and surface functional groups of Fe@C

Fe@C复合材料制备过程如图1所示，通过XRD和红外光谱表征（图2）可知，随着碳热温度由400升至800 °C，Fe@HC的Fe₂O₃逐渐被碳还原为FeO并最终生成Fe⁰，同时水热反应生成的无定形碳转化为石墨化碳,且碳结构芳香化增强；而无水热反应过程制备的Fe@C800-im的碳结构石墨化和芳香化程度弱于有水热辅助合成的Fe@C800。

The synthesis procedures of Fe@C composites were shown in Fig. 1. As shown in the XRD and FTIR results (Fig. 2), as the carbothermal temperatures were increased from 400 to 800 °C, the Fe₂O₃ in Fe@HC was gradually reduced by carbon into FeO, and then further partly reduced to Fe^o from simultaneous graphitization and aromatization of amorphous carbons. In addition, the degrees of graphitization and aromatization for Fe@C800-im synthesized in the absence of hydrothermal reactions were weaker than those of Fe@C800 synthesized via hydrothermally assisted carbothermal reactions.

图1 碳包覆nZVI复合材料的制备过程示意图

Fig. 1 Schematic illustration for the synthesis processes of Fe@C composites

图2 不同制备材料的XRD（a）和红外图谱（b）

Fig. 2 XRD patterns (a) and FTIR spectra (b) of different synthetic materials

二、碳包覆nZVI复合材料的表面化学、比表面积和形貌特征

 Surface chemical properties, specific surface areas and morphologies of Fe@C

XPS表征显示随碳热温度由400升至800 °C，Fe@C表面的C含量逐渐增加而Fe、O含量逐渐降低，然而ICP-OES测得的总铁浓度逐渐增加，表明Fe@C800的Fe⁰逐渐被包覆于复合材料内部（表1）。并且，水热反应和高温碳热反应利于Fe@C的碳化、铁负载和孔隙发育（表1）。形貌表征（图3）进一步证实，Fe@C800主要为碳壳包覆nZVI的球状形态，而Fe@C800-im呈现出nZVI颗粒随机分散于不规则块状碳结构的形态。

The C atomic percentages were increased and Fe, O percentages were decreased for Fe@C composites following the increase of calcinating temperatures from 400 to 800 °C, while the total Fe obtained from the ICP-OES measurement were gradually increased. The opposite trend for the total Fe and that on the surfaces of Fe@C composites demonstrated that sufficient iron species were probably encapsulated within Fe@C800 (Table 1). In addition, the hydrothermal procedures and carbothermal reactions at high temperatures benefited carbonization, iron loading and development of porous structures within the Fe@C composites (Table 1). As further confirmed in the morphological results (Fig. 3), spherical particles composed of carbon shells and iron cores were formed for Fe@C800, however, large irregular blocky structures with randomly distributed nZVI particles on the surfaces and in carbon matrix were observed for Fe@C800-im.

表1 包覆型nZVI-碳复合材料的主要理化性质

Table 1 Primary properties of carbon-encapsulated nZVI composites (Fe@C)

图3 Fe@HC（a），Fe@C800（b）和Fe@C800-im（c）的SEM图，Fe@C800的TEM（d）和HRTEM（e）图，Fe@C800-im的TEM图（f）以及Fe@C800的TEM-EDS面扫图（g-j）

Fig. 3 SEM images for Fe@HC (a), Fe@C800 (b) and Fe@C800-im (c), TEM (d) and HRTEM (e) images for Fe@C800, TEM image for Fe@C800-im (f) and TEM-EDS mapping for Fe@C800 (g–j)

三、碳包覆nZVI复合材料活化PMS去除氯苯的性能及机理 

 Removal performance and mechanism of MCB by Fe@C activated PMS

Fe@C800表现出最强活化PMS去除MCB的能力，反应240分钟后，Fe@HC/PMS，Fe@C400/PMS，Fe@C600/PMS，Fe@C800/PMS和Fe@C800-im/PMS体系对MCB的去除率分别为19.2%，26.4%，33.4%，98.5% 和56.0%（图4）。由自由基淬灭实验和EPR表征结果（图5）可知，Fe@C800/PMS体系中同时存在SO₄^•−，•OH，¹O₂和O₂^•−四种活性氧物质，并且•OH起主导作用。根据电化学实验和XPS表征结果（图6），Fe@C800/PMS体系中还存在基于¹O₂和表面电子传递的非自由基机理，碳壳的C=C碳结构、C-OH及C=O等表面官能团和内核nZVI为主要活化位点，反应机理如图7所示。

Fe@C800 exhibited the strongest activation ability towards PMS for removing MCB. After 240 min reaction, the removal efficiencies were increased to 19.2%, 26.4%, 33.4%, 98.5% and 56.0% in Fe@HC/PMS, Fe@C400/PMS, Fe@C600/PMS, Fe@C800/PMS and Fe@C800-im/PMS systems, respectively (Fig. 4). As shown in the results of radical quenching experiments and EPR detection (Fig. 5), all four reactive oxygen species, including SO₄^•−, •OH, ¹O₂ and O₂^•− were present, and •OH was a dominant species in Fe@C800/PMS system contributing to the MCB degradation. Combined with the results of the electrochemical experiments and XPS characterization (Fig. 6), nonradical pathways involving ¹O₂ and surface electron transfer mechanisms were coexisted in the Fe@C800/PMS system and the graphitic sp²-hybridized carbon structures (C=C) and oxygen functional groups (i.e., C-OH and C=O) in the carbon shell and encapsulated inner nZVI particles were the main reactive sites. The activation mechanism of Fe@C800 towards PMS for the removal of MCB was proposed in Fig. 7.

图4碳包覆nZVI复合材料活化PMS对MCB的去除动力学（a）和PMS分解动力学（b）。反应条件：[MCB]₀ =100 μM，[PMS]₀ = 10 mM，[复合材料]₀ = 0.1 g·L⁻¹，pH₀ = 8.0和T = 25 °C

Fig. 4 MCB removal kinetic (a) and PMS consumption (b) in various PMS systems activated by Fe@HC, Fe@C800-im and Fe@C prepared under different carbothermal temperatures. Note: reaction conditions under [MCB]₀ =100 μM, [PMS]₀ = 10 mM, [composite]₀ = 0.1 g·L⁻¹, pH₀ = 8.0, and T = 25 °C

图5 Fe@C800/PMS体系中自由基淬灭结果（a）和不同体系中的EPR捕获谱图（b-d）

Fig. 5 MCB removal by the Fe@C800/PMS system in the presence of MeOH, TBA, β-carotene and p-BQ as radical scavengers (a), EPR spectra of DMPO-SO₄^•−and DMPO-OH adducts (b), TEMP-¹O₂ adduct (c) and DMPO-O₂^•−adduct (d)

图6 不同Fe@C活化PMS体系的循环伏安曲线（a）和计时电流i-t曲线（b），以及Fe@C800反应前后的XPS全扫图（c），C 1s（d）、O 1s（e）和Fe 2p（f）精细图谱

Fig. 6 Cyclic voltammetry curves of different composites obtained between−1.0 – 1.0 V vs. Ag/AgCl at a scan rate of 50 mV/s (a), i-t curves of different Fe@C composites obtained at 0.00 V vs. Ag/AgCl using 50 mM Na₂SO₄ as electrolyte (b), XPS survey spectra (c), C 1s (d), O 1s (e) and Fe 2p (f) of fresh Fe@C800 and Fe@C800 after the reaction (Fe@C800 (AR))

图7 包覆型nZVI-碳复合材料Fe@C800活化PMS降解MCB的机理示意图

Fig. 7 The proposed mechanism for MCB removal in the Fe@C800/PMS system

总结 | Conclusions

本研究通过水热辅助碳热还原法成功制备出碳包覆nZVI复合材料，并表现出优异的活化PMS去除MCB的能力。结果表明，碳热温度为影响还原生成nZVI和碳壳石墨化与芳香化的重要因素，碳壳的石墨化碳结构和表面官能团以及内核nZVI组成活化PMS的多活性位点，形成•OH主导的自由基机理和基于¹O₂和电子传递的非自由基机理，共同参与MCB的去除。本研究不仅提供了一种利用废弃农业生物质合成高活性和稳定nZVI-碳复合材料以活化PMS氧化修复有机物污染地下水的创新方法，并且加深了对铁-碳复合材料活化PMS机理的认识，为工业污染场地原位绿色低碳修复提供了科技支撑。为更好指导实际污染场地地下水修复，后续可围绕该碳包覆nZVI复合材料活化PMS体系在模拟地下水多孔介质中的性能变化、主控因素及作用机制开展进一步研究。

In this study, the nZVI encapsulated carbonaceous catalysts for highly efficient removal of MCB by PMS-based AOPs were successfully synthesized via hydrothermally assisted carbothermal reduction method. It is concluded that the carbothermal temperature imposed important influences on the reductive formation of nZVI and graphitization and aromatization of carbon shells. The graphitized carbon structures and surface functional groups in the carbon shells and nZVI in the inner cores constituted the multiple reactive sites. The •OH-dominated radical pathways and nonradical pathways involving ¹O₂ and surface electron transfer mechanisms participated in the removal of MCB. This study not only developed a novel strategy to synthesize highly-effective and stable nZVI‑carbon composites using the agricultural biomass wastes for PMS-induced oxidative remediation of organic contaminants in groundwater, but also gained thorough understanding into the activation mechanism of iron‑carbon based catalysts towards PMS, thus providing technical support for the green, low-carbon and in-situ remediation technologies for industrially contaminated sites. In order to better guide the groundwater remediation in actual contaminated sites, the future research can be focused on the performance changes, main controlling factors and mechanisms of the nZVI‑carbon composites/PMS system in the simulated groundwater porous media.

扫二维码 | 查看原文

https://www.sciencedirect.com/science/article/

abs/pii/S0048969722017387?via%3Dihub

本文内容来自ELSEVIER旗舰期刊Sci Total Environ第829卷发表的论文：

Yang L., Shen J., Zhang W., Wu W., Wei Z., Chen M., Yan J., Qian L., Han L., Li J., Gu M., 2022. Hydrothermally assisted synthesis of nano zero-valent iron encapsulated in biomass-derived carbon for peroxymonosulfate activation: The performance and mechanisms for efficient degradation of monochlorobenzene. Sci Total Environ 829, 154645.

DOI: http://dx.doi.org/10.1016/j.scitotenv.2022.154645

通讯作者：陈梦舫 研究员

中国科学院南京土壤研究所

研究员，博士生导师，污染场地安全修复技术国家工程实验室副主任，江苏省污染场地土壤与地下水修复工程实验室主任，中国土壤学会土壤修复专业委员会顾问，江苏省环境科学学会土壤与地下水专业委员会主任。曾任英国伦敦2012奥运会地下水污染修复顾问、欧盟FP7 NANOREM项目国际顾问、欧美主流工程咨询集团技术总监等职务。主要从事场地土壤与地下水污染防控与修复技术、绿色高效修复纳米功能材料、废弃矿山地下水污染过程与生态治理技术等研究，主持了国家重点研发计划项目，自然科学基金面上、国际合作项目，环保公益性行业专项，863计划子课题，中国科学院知识创新工程及科技网络服务，地方政府技术咨询等二十余项研究课题。

2012年开发了我国首套污染场地健康与环境风险评估HERA软件，已在全国范围内近1000家高等院校、科研院所、环保企业等单位近千余项目得到推广应用，共培养4000多名专业技术人员，已成为我国大型园区、在产企业及开发利用场地土壤风险管控与监测预警的重要工具。已在国际主流刊物上发表SCI论文70多篇（含六篇ESI高被引），出版污染场地风险管控与修复专著三部，建成多个可复制推广的原位地下水示范工程，为我国场地土壤污染风险管控与地下水原位高效修复提供了典型工程范例。

近2年在Sci Total Environ发表的其他论文：

1. Chen, X., Wu, W., Han, L., Gu, M., Li, J., Chen, M. 2022. Carbon stability and mobility of ball milled lignin- and cellulose-rich biochar colloids. Science of the Total Environment, 802: 149759.

2. Ouyang, D., Chen, Y., Chen, R., Zhang, W., Yan, J., Gu, M., Li, J., Zhang, H., Chen, M. 2022. Degradation of 1,4-dioxane by biochar activating peroxymonosulfate under continuous flow conditions. Science of the Total Environment, 809: 151929.

3. Han, L., Nie, X., Wei, J., Gu, M., Wu, W., Chen, M. 2021. Effects of feedstock biopolymer compositions on the physiochemical characteristics of dissolved black carbon from lignocellulose-based biochar. Science of the Total Environment, 751: 141491.

4. Wu, W., Han, L., Nie, X., Gu, M., Li, J., Chen, M. 2021. Effects of multiple injections on the transport of CMC-nZVI in saturated sand columns. Science of the Total Environment, 784: 147160.

第一作者：杨磊 博士研究生

中国科学院南京土壤研究所

博士研究生。主要研究方向为污染场地地下水高级氧化修复技术、环境修复纳米功能材料研发。以第一作者或共同作者在Science of the Total Environment、Chemical Engineering Journal等国际期刊发表论文2篇。

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