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.