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Imaging the reduction of chromium(VI) on magnetite surfaces using in situ electrochemical AFM
Hexavalent chromium is a highly toxic and readily mobile metal contaminant introduced to the environment through a variety of industrial operations. In the presence of reductants, such as Fe(II), and catalytic mineral surfaces, such as iron oxides surfaces, Cr(VI) may be reduced to a less toxic and...
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| Published in: | Chemical geology 2016-07, Vol.429, p.60-74 |
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| container_title | Chemical geology |
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| creator | Walker, Sarah M. Marcano, Maria C. Bender, Will M. Becker, Udo |
| description | Hexavalent chromium is a highly toxic and readily mobile metal contaminant introduced to the environment through a variety of industrial operations. In the presence of reductants, such as Fe(II), and catalytic mineral surfaces, such as iron oxides surfaces, Cr(VI) may be reduced to a less toxic and relatively insoluble form Cr(III). In this study, we investigate the interaction between Cr(VI) and the surface of the Fe(II)-bearing mineral magnetite, Fe(II)Fe(III)2O4, as an example catalyst, using electrochemical atomic force microscopy (EC-AFM). With this method, the redox potential is controlled by an electrode, and Cr deposition on the magnetite surface is imaged over time as a function of redox potential and pH of the solution. Quantitative analyses of volumetric growth and surface coverage reveal that more precipitation occurs over time at very negative (−500mV at pH3, −750mV at pH7, and −1000mV at pH11) and very positive (+1000mV at pH3 and +500mV at pH11) electrochemical potentials. Up to 70% of the surface is covered with precipitates at pH7, while less coverage is observed at pH11 ( |
| doi_str_mv | 10.1016/j.chemgeo.2016.02.025 |
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Results from growth analyses were confirmed by batch experiments performed on the same samples used in the AFM, where changes in the Cr concentration in solution were monitored using inductively coupled plasma mass spectrometry (ICP-MS). Virtually all of the Cr was removed after 30min at pH7 (starting at 2μM Cr(VI)) at all potentials, ~60% at pH3, and very little removal at pH11. X-ray photoelectron spectroscopy (XPS) analyses of the magnetite surface at pH3 suggest that only Cr(III) phases are present and that Cr and/or Fe–Cr oxide phases are more stable at very reducing conditions (−750mV), while Cr and/or Fe–Cr (oxy)hydroxide phases are present under more moderately-reducing conditions (−250mV). At pH11, any Cr deposited is below the detection limit for XPS analysis (<1% surface coverage); however, Fe(II)/Fe(III) ratios (~0.3) are lower than the expected ratio of magnetite, which indicates that some oxidation of the surface has occurred. This oxidized layer, possibly maghemite (γ-Fe2O3), hematite (α-Fe2O3), or FeOOH, may inhibit the reduction of Cr(VI) on the surface.
•Altering redox potential and pH conditions strongly affects chromium precipitation on magnetite surfaces.•Growth at pH3 occurs at very reducing and very oxidizing conditions and is primarily lateral (i.e., parallel to the surface).•While some growth is observed at pH11, oxidation of the mineral surface impedes Cr sorption at pH11.</description><identifier>ISSN: 0009-2541</identifier><identifier>EISSN: 1872-6836</identifier><identifier>DOI: 10.1016/j.chemgeo.2016.02.025</identifier><language>eng</language><publisher>Elsevier B.V</publisher><subject>Atomic force microscopy ; Catalysts ; Chromium ; Crystallography ; Hexavalent chromium ; Magnetite ; Phases ; Redox ; Toxic ; X-ray photoelectron spectroscopy</subject><ispartof>Chemical geology, 2016-07, Vol.429, p.60-74</ispartof><rights>2016 Elsevier B.V.</rights><lds50>peer_reviewed</lds50><oa>free_for_read</oa><woscitedreferencessubscribed>false</woscitedreferencessubscribed><citedby>FETCH-LOGICAL-a511t-80be5c6204feb9a9027dc90e42cf81fe0d27997db1f16658d2dddb2597480ae73</citedby><cites>FETCH-LOGICAL-a511t-80be5c6204feb9a9027dc90e42cf81fe0d27997db1f16658d2dddb2597480ae73</cites><orcidid>0000-0002-6726-3142</orcidid></display><links><openurl>$$Topenurl_article</openurl><openurlfulltext>$$Topenurlfull_article</openurlfulltext><thumbnail>$$Tsyndetics_thumb_exl</thumbnail><link.rule.ids>314,780,784,27923,27924</link.rule.ids></links><search><creatorcontrib>Walker, Sarah M.</creatorcontrib><creatorcontrib>Marcano, Maria C.</creatorcontrib><creatorcontrib>Bender, Will M.</creatorcontrib><creatorcontrib>Becker, Udo</creatorcontrib><title>Imaging the reduction of chromium(VI) on magnetite surfaces using in situ electrochemical AFM</title><title>Chemical geology</title><description>Hexavalent chromium is a highly toxic and readily mobile metal contaminant introduced to the environment through a variety of industrial operations. In the presence of reductants, such as Fe(II), and catalytic mineral surfaces, such as iron oxides surfaces, Cr(VI) may be reduced to a less toxic and relatively insoluble form Cr(III). In this study, we investigate the interaction between Cr(VI) and the surface of the Fe(II)-bearing mineral magnetite, Fe(II)Fe(III)2O4, as an example catalyst, using electrochemical atomic force microscopy (EC-AFM). With this method, the redox potential is controlled by an electrode, and Cr deposition on the magnetite surface is imaged over time as a function of redox potential and pH of the solution. Quantitative analyses of volumetric growth and surface coverage reveal that more precipitation occurs over time at very negative (−500mV at pH3, −750mV at pH7, and −1000mV at pH11) and very positive (+1000mV at pH3 and +500mV at pH11) electrochemical potentials. Up to 70% of the surface is covered with precipitates at pH7, while less coverage is observed at pH11 (<8%) and pH3 (<2%). Particle growth at pH3 is predominantly lateral in nature with a tendency to form a higher number of smaller adsorbate particles. At pH11, growth is primarily vertical (perpendicular to the surface), and smaller particles tend to aggregate into larger clusters on the surface with increasingly negative redox polarization. These larger clusters (most likely subcrystalline Cr-(oxy)hydroxides) co-exist with rhombic, crystalline particles that likely have preferred crystallographic growth relationships with the magnetite substrate; the composition of this latter type of particle is most likely chromite, FeCr2O4, based on the matching crystallography and stability under the Eh/pH conditions of this experiment.
Results from growth analyses were confirmed by batch experiments performed on the same samples used in the AFM, where changes in the Cr concentration in solution were monitored using inductively coupled plasma mass spectrometry (ICP-MS). Virtually all of the Cr was removed after 30min at pH7 (starting at 2μM Cr(VI)) at all potentials, ~60% at pH3, and very little removal at pH11. X-ray photoelectron spectroscopy (XPS) analyses of the magnetite surface at pH3 suggest that only Cr(III) phases are present and that Cr and/or Fe–Cr oxide phases are more stable at very reducing conditions (−750mV), while Cr and/or Fe–Cr (oxy)hydroxide phases are present under more moderately-reducing conditions (−250mV). At pH11, any Cr deposited is below the detection limit for XPS analysis (<1% surface coverage); however, Fe(II)/Fe(III) ratios (~0.3) are lower than the expected ratio of magnetite, which indicates that some oxidation of the surface has occurred. This oxidized layer, possibly maghemite (γ-Fe2O3), hematite (α-Fe2O3), or FeOOH, may inhibit the reduction of Cr(VI) on the surface.
•Altering redox potential and pH conditions strongly affects chromium precipitation on magnetite surfaces.•Growth at pH3 occurs at very reducing and very oxidizing conditions and is primarily lateral (i.e., parallel to the surface).•While some growth is observed at pH11, oxidation of the mineral surface impedes Cr sorption at pH11.</description><subject>Atomic force microscopy</subject><subject>Catalysts</subject><subject>Chromium</subject><subject>Crystallography</subject><subject>Hexavalent chromium</subject><subject>Magnetite</subject><subject>Phases</subject><subject>Redox</subject><subject>Toxic</subject><subject>X-ray photoelectron spectroscopy</subject><issn>0009-2541</issn><issn>1872-6836</issn><fulltext>true</fulltext><rsrctype>article</rsrctype><creationdate>2016</creationdate><recordtype>article</recordtype><recordid>eNqFUMFKAzEUDKJgrX6CkGM9bE2ym93kJKVYLVS8qDcJafK2Tdnd1GRX8O9Nae-Fgcc8Zob3BqF7SqaU0PJxNzVbaDfgpyzRKWEJ_AKNqKhYVoq8vEQjQojMGC_oNbqJcZcozTkfoe9lqzeu2-B-CziAHUzvfId9jc02-NYN7eRr-YDTKuk66F0POA6h1gYiHuLB6TocXT9gaMD0wR9ucUY3eLZ4u0VXtW4i3J3mGH0unj_mr9nq_WU5n60yzSntM0HWwE3JSFHDWmpJWGWNJFAwUwtaA7GskrKya1rTsuTCMmvtmnFZFYJoqPIxmhxz98H_DBB71bpooGl0B36IigoiSFmynJ6XVoKzXMqCJSk_Sk3wMQao1T64Voc_RYk6NK926tS8OjSvCEvgyfd09EF6-ddBUNE46AxYF1JFynp3JuEf_5OO3g</recordid><startdate>20160701</startdate><enddate>20160701</enddate><creator>Walker, Sarah M.</creator><creator>Marcano, Maria C.</creator><creator>Bender, Will M.</creator><creator>Becker, Udo</creator><general>Elsevier B.V</general><scope>AAYXX</scope><scope>CITATION</scope><scope>7UA</scope><scope>C1K</scope><scope>F1W</scope><scope>H96</scope><scope>L.G</scope><scope>8FD</scope><scope>FR3</scope><scope>KR7</scope><orcidid>https://orcid.org/0000-0002-6726-3142</orcidid></search><sort><creationdate>20160701</creationdate><title>Imaging the reduction of chromium(VI) on magnetite surfaces using in situ electrochemical AFM</title><author>Walker, Sarah M. ; Marcano, Maria C. ; Bender, Will M. ; Becker, Udo</author></sort><facets><frbrtype>5</frbrtype><frbrgroupid>cdi_FETCH-LOGICAL-a511t-80be5c6204feb9a9027dc90e42cf81fe0d27997db1f16658d2dddb2597480ae73</frbrgroupid><rsrctype>articles</rsrctype><prefilter>articles</prefilter><language>eng</language><creationdate>2016</creationdate><topic>Atomic force microscopy</topic><topic>Catalysts</topic><topic>Chromium</topic><topic>Crystallography</topic><topic>Hexavalent chromium</topic><topic>Magnetite</topic><topic>Phases</topic><topic>Redox</topic><topic>Toxic</topic><topic>X-ray photoelectron spectroscopy</topic><toplevel>peer_reviewed</toplevel><toplevel>online_resources</toplevel><creatorcontrib>Walker, Sarah M.</creatorcontrib><creatorcontrib>Marcano, Maria C.</creatorcontrib><creatorcontrib>Bender, Will M.</creatorcontrib><creatorcontrib>Becker, Udo</creatorcontrib><collection>CrossRef</collection><collection>Water Resources Abstracts</collection><collection>Environmental Sciences and Pollution Management</collection><collection>ASFA: Aquatic Sciences and Fisheries Abstracts</collection><collection>Aquatic Science & Fisheries Abstracts (ASFA) 2: Ocean Technology, Policy & Non-Living Resources</collection><collection>Aquatic Science & Fisheries Abstracts (ASFA) Professional</collection><collection>Technology Research Database</collection><collection>Engineering Research Database</collection><collection>Civil Engineering Abstracts</collection><jtitle>Chemical geology</jtitle></facets><delivery><delcategory>Remote Search Resource</delcategory><fulltext>fulltext</fulltext></delivery><addata><au>Walker, Sarah M.</au><au>Marcano, Maria C.</au><au>Bender, Will M.</au><au>Becker, Udo</au><format>journal</format><genre>article</genre><ristype>JOUR</ristype><atitle>Imaging the reduction of chromium(VI) on magnetite surfaces using in situ electrochemical AFM</atitle><jtitle>Chemical geology</jtitle><date>2016-07-01</date><risdate>2016</risdate><volume>429</volume><spage>60</spage><epage>74</epage><pages>60-74</pages><issn>0009-2541</issn><eissn>1872-6836</eissn><abstract>Hexavalent chromium is a highly toxic and readily mobile metal contaminant introduced to the environment through a variety of industrial operations. In the presence of reductants, such as Fe(II), and catalytic mineral surfaces, such as iron oxides surfaces, Cr(VI) may be reduced to a less toxic and relatively insoluble form Cr(III). In this study, we investigate the interaction between Cr(VI) and the surface of the Fe(II)-bearing mineral magnetite, Fe(II)Fe(III)2O4, as an example catalyst, using electrochemical atomic force microscopy (EC-AFM). With this method, the redox potential is controlled by an electrode, and Cr deposition on the magnetite surface is imaged over time as a function of redox potential and pH of the solution. Quantitative analyses of volumetric growth and surface coverage reveal that more precipitation occurs over time at very negative (−500mV at pH3, −750mV at pH7, and −1000mV at pH11) and very positive (+1000mV at pH3 and +500mV at pH11) electrochemical potentials. Up to 70% of the surface is covered with precipitates at pH7, while less coverage is observed at pH11 (<8%) and pH3 (<2%). Particle growth at pH3 is predominantly lateral in nature with a tendency to form a higher number of smaller adsorbate particles. At pH11, growth is primarily vertical (perpendicular to the surface), and smaller particles tend to aggregate into larger clusters on the surface with increasingly negative redox polarization. These larger clusters (most likely subcrystalline Cr-(oxy)hydroxides) co-exist with rhombic, crystalline particles that likely have preferred crystallographic growth relationships with the magnetite substrate; the composition of this latter type of particle is most likely chromite, FeCr2O4, based on the matching crystallography and stability under the Eh/pH conditions of this experiment.
Results from growth analyses were confirmed by batch experiments performed on the same samples used in the AFM, where changes in the Cr concentration in solution were monitored using inductively coupled plasma mass spectrometry (ICP-MS). Virtually all of the Cr was removed after 30min at pH7 (starting at 2μM Cr(VI)) at all potentials, ~60% at pH3, and very little removal at pH11. X-ray photoelectron spectroscopy (XPS) analyses of the magnetite surface at pH3 suggest that only Cr(III) phases are present and that Cr and/or Fe–Cr oxide phases are more stable at very reducing conditions (−750mV), while Cr and/or Fe–Cr (oxy)hydroxide phases are present under more moderately-reducing conditions (−250mV). At pH11, any Cr deposited is below the detection limit for XPS analysis (<1% surface coverage); however, Fe(II)/Fe(III) ratios (~0.3) are lower than the expected ratio of magnetite, which indicates that some oxidation of the surface has occurred. This oxidized layer, possibly maghemite (γ-Fe2O3), hematite (α-Fe2O3), or FeOOH, may inhibit the reduction of Cr(VI) on the surface.
•Altering redox potential and pH conditions strongly affects chromium precipitation on magnetite surfaces.•Growth at pH3 occurs at very reducing and very oxidizing conditions and is primarily lateral (i.e., parallel to the surface).•While some growth is observed at pH11, oxidation of the mineral surface impedes Cr sorption at pH11.</abstract><pub>Elsevier B.V</pub><doi>10.1016/j.chemgeo.2016.02.025</doi><tpages>15</tpages><orcidid>https://orcid.org/0000-0002-6726-3142</orcidid><oa>free_for_read</oa></addata></record> |
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| subjects | Atomic force microscopy Catalysts Chromium Crystallography Hexavalent chromium Magnetite Phases Redox Toxic X-ray photoelectron spectroscopy |
| title | Imaging the reduction of chromium(VI) on magnetite surfaces using in situ electrochemical AFM |
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