茶树枝叶制备生物炭负载纳米零价铁净化水中Cr(VI)

茶树废弃物引起的环境破坏和病虫害爆发问题日益突出，对其进行无害化和资源化利用具有重要意义。该研究以修剪的茶树枝叶提取液作为还原剂和封端剂，以提取后的残渣作为炭源，成功制备了一种可高效去除水中六价铬(Cr(Ⅵ))的生物炭负载纳米零价铁复合材料(nanoscale zero-valent iron embedded tea leaves,TLBC-nZVI)。分析了材料用量、溶液初始pH值和温度等对Cr(Ⅵ)去除效果的影响；利用扫描电子显微镜结合能量色散X射线光谱仪(SEMEDS)、傅立叶变换红外光谱仪(FTIR)、X射线粉晶衍射仪(XRD)和X射线光电子能谱仪(XPS)等对材料进行表征，结合吸附动力学、吸附等温线和吸附热力试验探讨了去除机制。结果表明酸性条件、高温、增加材料用量有利于TLBC-nZVI对Cr(Ⅵ)的去除。TLBC-nZVI吸附过程符合准二级动力学模型、颗粒内扩散模型和Freundlich吸附等温模型，该吸附是自发的化学吸热过程。TLBC-nZVI与Cr(Ⅵ)的反应机制为吸附在材料上的Cr(Ⅵ)被零价铁(Fe⁰)和还原性官能团还原为三价铬(Cr(Ⅲ))...

Preparing nano zero-valent iron-embedded biochar with pruned tea wastes for efficient removal of Cr(VI) from aqueous solutions

Water pollution has been one of the most serious environmental issues worldwide, due partly to the excess emissions of heavy metals. Among them, chromium (Cr) is one of the most important raw materials in a variety of industries, including metal mining, tanneries, electroplating, chrome plating, and dye manufacturing. The accumulation of Cr(VI) in the human body cannot be biodegraded, leading to various diseases, such as dermatitis, rhinitis, and even cancer. Thus, the World Health Organization (WHO) has recommended that the permissible limit of Cr for potable water of 0.05 mg/L. The trivalent (III) and hexavalent (VI) Cr forms can often be found in aqueous solutions. Cr(VI) has much higher toxicity, solubility, and mobility than Cr(III). Therefore, it is urgent to remove Cr(VI) from the water environment. Alternatively, the ZVI-embedded biochar can be expected to efficiently remove Cr(VI), due to the synergetic effect of adsorption and reduction. Generally, the ZVI-embedded biochar is produced to load Fe²⁺/Fe³⁺ onto precursor biochar, and then reduce the costly chemical reagents. However, the conventional processes of ZVI-embedded biochar can often be verbose, expensive, and/or release toxic byproducts. Therefore, it is a high demand for a cheap and convenient strategy to produce the ZVI/biochar. Meanwhile, more than one million tons of branches and leaves are pruned from tea trees each year in China, in order to improve branch growth and tea quality. Most residues are discarded or burned, leading to plant diseases and insect pests, or severe air contamination. Hence, it is imperative to develop new applications of pruned tea residues for environmental protection. Pruned tea residues with a rich number of cellulosic and polyphenolic components can be expected to serve as the biomass feedstocks and reducing agents, and then to synthesize the ZVI-embedded biochar. In this work, an inexpensive and convenient approach was developed for the synthesis of nanoscale zero-valent iron-embedded biochar (TLBC-nZVI) using pruned tea residues as biomass feedstocks and reducing agents. A series of batch experiments were carried out to explore the adsorption characteristics of TLBC-nZVI for Cr(VI). Scanning electron microscope with energy dispersive spectrometer (SEM-EDS), Fourier transform infrared spectrometer (FTIR), X-ray diffractometer (XRD), and X-ray photoelectron spectrometer (XPS) were applied to characterize the microscopic morphology and physicochemical properties of TLBC-nZVI before and after reaction with Cr(VI). The results showed that the nZVI was embedded successfully with the TLBC. Batch adsorption experiments demonstrated that the low pH value, high temperature, and a large amount of adsorbent were beneficial to the removal of Cr(VI). Batch adsorption experiments showed that the TLBC-nZVI shared excellent performance in the Cr(VI) removal (164.65 mg/g) from aqueous solutions. The kinetic studies showed that the Cr(VI) removal was also fit better with the pseudo-second-order model and intra-particle diffusion model. Therefore, the process of adsorption was mainly through chemical adsorption, such as surface complexation, electrostatic interactions, and ion exchange processes. The first region line cannot pass the origin in the intra-particle diffusion model, indicating the limited rate by the diffusion of the boundary layer. The second region fitting demonstrated that intraparticle diffusion was the limiting step. The Freundlich model was utilized to better simulate the isothermal adsorption behavior, indicating the adsorption under chemical action. The adsorption thermodynamics showed that the removal process was a chemical, spontaneous and endothermic reaction. The removal mechanisms were as follows: (1) the protonated TLBC-nZVI adsorbed anionic Cr(VI) by electrostatic interaction under acidic conditions; (2) Fe⁰, Fe(II), and some surface functional groups (such as -NH₂ and -OH) reduced Cr(VI) to Cr(III); and (3) Cr(III) were removed through complexation, physical adsorption and coprecipitation. The finding can be served as a potential theoretical reference for the resource utilization of pruned tea wastes and the remediation of heavy metal pollution in water.