期刊
ENVIRONMENT INTERNATIONAL
卷 134, 期 -, 页码 -出版社
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.envint.2019.105216
关键词
Hexavalent chromium; Biochar; X-ray absorption spectroscopy; Confocal micro-X-ray fluorescence imaging; Redox mapping
资金
- Natural Sciences and Engineering Research Council of Canada (NSERC)
- National Natural Science Foundation of China [41521001, 41877478]
- 111 Program (State Administration of Foreign Experts Affairs) [B18049]
- 111 Program (Ministry of Education of China) [B18049]
- Program of Geological Processes, Resources and Environment in the Yangtze Basin [CUGCJ1702]
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 similar to 50 to< 0.02 mg L-1 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 similar to 54% of initial Cr(VI) was removed using HNO3-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)(3), 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.
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