School of Environmental Studies & State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, 388 Lumo Rd., Wuhan, Hubei 430074, PR China; Department of Earth and Environmental Sciences, University of Waterloo, 200 University Ave. W., Waterloo, ON N2L 3G1, Canada.
Department of Earth and Environmental Sciences, University of Waterloo, 200 University Ave. W., Waterloo, ON N2L 3G1, Canada.
Environ Int. 2020 Jan;134:105216. doi: 10.1016/j.envint.2019.105216. Epub 2019 Oct 31.
Biochar is an effective, environmentally sustainable material for removing Cr(VI) from water. Potential removal mechanisms include surface reactions or reactions within the biochar structure with direct bonding of Cr(VI) or reduction of Cr(VI) and bonding of the reduced Cr forms. Diffusion process and Cr(VI) and Cr(III) distributions in biochar particles have not been elucidated. Aqueous Cr(VI) removal experiments followed by solid-phase analyses were conducted to evaluate the effectiveness of raw and modified oak wood biochar for removing aqueous Cr(VI) and to further determine removal mechanisms. Results showed that concentrations of Cr(VI) decreased from ~50 to <0.02 mg L at initial pH 2 after 1 d using raw oak wood biochar, with 8.8% of the initial aqueous Cr reduced to Cr(III) in the solution. Similarly, effective removal of Cr(VI) was observed using polysulfide-modified biochar; whereas ~54% of initial Cr(VI) was removed using HNO-treated biochar. Bulk X-ray absorption near-edge structure (XANES) analysis showed Cr is present as Cr(III) within the unmodified biochar, whereas confocal micro-XANES analysis showed the existence of Cr(VI) (0-36%) in selected spots and Cr(0) (43%) in one spot within a biochar sample collected after 30 min. Extended X-ray absorption fine structure (EXAFS) results showed the atomic structure of Cr within the unmodified biochar was similar to Cr(OH), with O and Cr in the first and second shells. Confocal micro-X-ray fluorescence imaging (CMXRFI) results indicated total Cr (tCr) was heterogeneously distributed in the imaged area with a higher intensity close to the particle surface. Redox mapping results indicated no Cr(VI) in the unmodified biochar collected at 30 min; Cr(III) was the primary form and also remained close to the surface at later time. The removal mechanisms likely involve electrostatic attraction and diffusion inside the particle, followed by reduction and ion exchange reactions.
生物炭是一种从水中去除六价铬的有效、环境可持续的材料。潜在的去除机制包括表面反应或生物炭结构内的反应，直接与六价铬键合或还原六价铬并与还原的铬形式键合。扩散过程以及生物炭颗粒中的六价铬和三价铬分布尚未阐明。进行了水相中六价铬去除实验，随后进行固相分析，以评估原始和改性橡木生物炭去除水相中六价铬的效果，并进一步确定去除机制。结果表明，在初始 pH 值为 2 时，使用原始橡木生物炭，在 1 天内，六价铬的浓度从约 50 降至 <0.02 mg/L，溶液中有 8.8%的初始水相铬被还原为三价铬。同样，使用多硫化物改性生物炭也可以有效地去除六价铬；而使用 HNO3 处理的生物炭则去除了约 54%的初始六价铬。体相 X 射线吸收近边结构（XANES）分析表明，未经修饰的生物炭中铬以三价铬形式存在，而共焦微 XANES 分析表明，在 30 min 后收集的生物炭样品中，存在六价铬（0-36%）和零价铬（43%）的选定点。扩展 X 射线吸收精细结构（EXAFS）结果表明，未经修饰的生物炭中铬的原子结构与 Cr(OH)相似，第一壳层和第二壳层中含有 O 和 Cr。共焦微 X 射线荧光成像（CMXRFI）结果表明，总铬（tCr）在成像区域内呈不均匀分布，靠近颗粒表面的强度较高。氧化还原映射结果表明，在 30 min 收集的未经修饰的生物炭中没有六价铬；三价铬是主要形式，在稍后的时间也仍然靠近表面。去除机制可能涉及颗粒内部的静电吸引和扩散，随后是还原和离子交换反应。
