Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Alberta, T6G 2E3, Canada.
Department of Earth Sciences, University of Toronto, 22 Russell Street, Toronto, ON, M5S 3B1, Canada.
Sci Rep. 2020 Nov 4;10(1):19007. doi: 10.1038/s41598-020-75924-7.
Biochar (BC) and magnetite (FeO) nanoparticles (MNP) have both received considerable recent attention in part due to their potential use in water treatment. While both are effective independently in the removal of a range of anionic metals from aqueous solution, the efficacy of these materials is reduced considerably at neutral pH due to decreased metal adsorption and MNP aggregation. In addition to synthetic metal oxide-biochar composites for use in treatment and remediation technologies, aggregates may also occur in nature when pyrolytic carbon is deposited in soils. In this study, we tested whether magnetite synthesized in the presence of biochar leads to increased removal efficiency of hexavalent chromium, Cr(VI), at the mildly acidic to neutral pH values characteristic of most natural and contaminated aqueous environments. To do so, magnetite nanoparticles and biochar produced from ground willow were synthesized to form composites (MNP-BC). Batch studies showed that MNP-BC markedly enhanced both adsorption and reduction of Cr(VI) from aqueous solution at acidic to neutral pH as compared to MNP and BC separately, suggesting a strong synergetic effect of hybridizing FeO with BC. Mechanistically, the Cr(VI) removal processes occurred through both adsorption and intraparticle diffusion followed by reduction to Cr(III). Synchrotron-based X-ray absorption spectroscopy analyses confirmed that Cr(VI) was reduced at the surface of MNP-BC, with electrons derived directly from both biochar and magnetite at low pH, while at near-neutral pH, biochar increased Cr(VI) reduction by inhibiting MNP aggregation. Extended X-ray absorption fine structure fitting results confirmed that the Cr(III) precipitates consist of Cr(OH) and chromite (CrFeO) nanoparticles. Our results demonstrate that MNP-BC composites have great potential as a material for the treatment of chromate-containing aqueous solutions across a wide range of pH values, and provide information valuable broadly relevant to soils and sediments that contain biochar.
生物炭（BC）和磁铁矿（FeO）纳米颗粒（MNP）最近都受到了相当多的关注，部分原因是它们在水处理方面有潜在的用途。虽然两者在从水溶液中去除一系列阴离子金属方面都具有独立的效果，但由于金属吸附和 MNP 聚集减少，在中性 pH 值下，这些材料的效果会大大降低。除了用于处理和修复技术的合成金属氧化物-生物炭复合材料外，当热解碳沉积在土壤中时，也可能在自然界中形成聚集体。在这项研究中，我们测试了在生物炭存在下合成的磁铁矿是否会导致在大多数天然和污染的水环境中具有代表性的酸性到中性 pH 值下，六价铬（Cr(VI)）的去除效率增加。为此，合成了磁铁矿纳米颗粒和从柳树中提取的生物炭，形成复合材料（MNP-BC）。批量研究表明，与 MNP 和 BC 单独相比，MNP-BC 在酸性到中性 pH 值下明显增强了 Cr(VI)从水溶液中的吸附和还原，表明 FeO 与 BC 杂交具有很强的协同效应。从机制上讲，Cr(VI)的去除过程通过吸附和颗粒内扩散随后还原到 Cr(III)进行。基于同步加速器的 X 射线吸收光谱分析证实，Cr(VI)在 MNP-BC 的表面被还原，在低 pH 值下，电子直接来自生物炭和磁铁矿，而在近中性 pH 值下，生物炭通过抑制 MNP 聚集来增加 Cr(VI)的还原。扩展 X 射线吸收精细结构拟合结果证实，Cr(III)沉淀物由 Cr(OH)和铬铁矿（CrFeO）纳米颗粒组成。我们的结果表明，MNP-BC 复合材料具有作为处理含铬水溶液的材料的巨大潜力，可在广泛的 pH 值范围内使用，并为含有生物炭的土壤和沉积物提供了广泛相关的有价值信息。
