TY - JOUR
T1 - Pb, Cu, and Zn distributions at humic acid-coated metal-oxide surfaces
AU - Wang, Yingge
AU - Michel, F. Marc
AU - Choi, Yongseong
AU - Eng, Peter J.
AU - Levard, Clement
AU - Siebner, Hagar
AU - Gu, Baohua
AU - Bargar, John R.
AU - Brown, Gordon E.
N1 - Funding Information:
This study was supported by U.S. National Science Foundation Grant CHE-0431425 (Stanford Environmental Molecular Science Institute) and by the U.S. National Science Foundation-Center for Environmental Implications for Nanotechnology (based at Duke University) (U.S. National Science Foundation Cooperative Agreement EF-0830093). The LP-XSW-FY data reported in this paper were collected at GeoSoilEnviroCARS (Advanced Photon Source Sector 13) at the Advanced Photon Source, Argonne National Laboratory. GeoSoilEnviroCARS is supported by the U.S. National Science Foundation – Earth Sciences ( EAR-1128799 ) and the U.S. Department of Energy – Geosciences ( DE-FG02-94ER14466 ). The Advanced Photon Source is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. We wish to thank Guangchao Li (Stanford University) for ICP-AES analysis and Prof. Zhenan Bao (Chemical Engineering, Stanford University) for allowing us to use her spin coater. We also wish to thank three reviewers for their valuable suggestions. The STXM data reported in this paper were collected at the Advanced Light Source, Lawrence Berkeley National Laboratory. The Advanced Light Source is supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
Publisher Copyright:
© 2016 Elsevier Ltd
PY - 2016/9/1
Y1 - 2016/9/1
N2 - Mineral surfaces are often coated by natural organic matter (NOM), which has a major influence on metal-ion sorption and sequestration because of the abundance of binding sites in such coatings and the changes they cause in local nanoscale environments. The effects of NOM coatings on mineral surfaces are, however, still poorly understood at the molecular level due to the complexity of these systems. In this study, we have applied long-period X-ray standing wave-fluorescence yield (LP-XSW-FY) spectroscopy to measure the partitioning of naturally present Cu(II) (0.0226%), Zn(II) (0.009%), and Pb(II) (∼0.0004%) between Elliott Soil Humic Acid (ESHA) coatings and three model single-crystal metal-oxide substrates: α-Al2O3 (0 0 0 1), α-Al2O3 (1 −1 0 2), and α-Fe2O3 (0 0 0 1). The competitive sorption effects among these metal ions for binding sites in the ESHA coatings and on the metal-oxide surfaces were investigated as a function of reaction time, calcium content, and solution pH. Pb(II) ions present in the ESHA coatings were found to redistribute to reactive α-Al2O3 (1 −1 0 2) and α-Fe2O3 (0 0 0 1) surfaces after 3 h of reaction (pH = 6.0, [Ca(II)] = 2 mM). Pb(II) partitioning onto these reactive metal-oxide surfaces increased with increasing reaction time (up to 7 d). In addition, the partitioning of Cu(II) and Zn(II) from the ESHA coating to the α-Fe2O3 (0 0 0 1) substrate increased slightly with reaction time (2.4% and 3.7% for Cu(II) and Zn(II), respectively, after 3 h and 6.4% and 7.7% for Cu(II) and Zn(II), respectively, after 72 h of reaction time). However, no changes in the partitioning of Cu(II) and Zn(II) onto the α-Al2O3 (1 −1 0 2) surface were observed with increasing reaction time, suggesting that these ions strongly complex with functional groups in the ESHA coatings. Similar results were obtained for Cu(II) and Zn(II) on the ESHA-coated α-Al2O3 (1 −1 0 2) surfaces in samples without the addition of calcium. However, the amounts of Pb(II) mobilized from the ESHA coatings onto the α-Al2O3 (1 −1 0 2) surfaces increased from 40% (no added Ca) to 58% (with 2 mM Ca) after 72 h of reaction time, possibly due to displacement of Pb(II) by Ca(II) from binding sites in the ESHA coatings. In contrast, Pb(II), Cu(II), and Zn(II) present in the ESHA coatings were found to be unreactive with the α-Al2O3 (0 0 0 1) surface. The observed reactivities of the three ESHA-coated metal-oxide surfaces with respect to metal-ion sorption are consistent with the trend observed for the uncoated metal-oxide surfaces: α-Fe2O3 (0 0 0 1) > α-Al2O3 (1 −1 0 2) > α-Al2O3 (0 0 0 1). In addition, Pb(II) partitioning onto α-Al2O3 (1 −1 0 2) surfaces increased with increasing pH from 4.0 to 9.0 as a result of the increasingly negative surface charge. These results show that intrinsic properties (nature of binding sites, binding affinities, and surface charge) of the ESHA coatings and metal-oxide surfaces, as well as external parameters such as pH and competing ions, are key factors governing the distribution and speciation of metal ions at complex NOM/mineral interfaces.
AB - Mineral surfaces are often coated by natural organic matter (NOM), which has a major influence on metal-ion sorption and sequestration because of the abundance of binding sites in such coatings and the changes they cause in local nanoscale environments. The effects of NOM coatings on mineral surfaces are, however, still poorly understood at the molecular level due to the complexity of these systems. In this study, we have applied long-period X-ray standing wave-fluorescence yield (LP-XSW-FY) spectroscopy to measure the partitioning of naturally present Cu(II) (0.0226%), Zn(II) (0.009%), and Pb(II) (∼0.0004%) between Elliott Soil Humic Acid (ESHA) coatings and three model single-crystal metal-oxide substrates: α-Al2O3 (0 0 0 1), α-Al2O3 (1 −1 0 2), and α-Fe2O3 (0 0 0 1). The competitive sorption effects among these metal ions for binding sites in the ESHA coatings and on the metal-oxide surfaces were investigated as a function of reaction time, calcium content, and solution pH. Pb(II) ions present in the ESHA coatings were found to redistribute to reactive α-Al2O3 (1 −1 0 2) and α-Fe2O3 (0 0 0 1) surfaces after 3 h of reaction (pH = 6.0, [Ca(II)] = 2 mM). Pb(II) partitioning onto these reactive metal-oxide surfaces increased with increasing reaction time (up to 7 d). In addition, the partitioning of Cu(II) and Zn(II) from the ESHA coating to the α-Fe2O3 (0 0 0 1) substrate increased slightly with reaction time (2.4% and 3.7% for Cu(II) and Zn(II), respectively, after 3 h and 6.4% and 7.7% for Cu(II) and Zn(II), respectively, after 72 h of reaction time). However, no changes in the partitioning of Cu(II) and Zn(II) onto the α-Al2O3 (1 −1 0 2) surface were observed with increasing reaction time, suggesting that these ions strongly complex with functional groups in the ESHA coatings. Similar results were obtained for Cu(II) and Zn(II) on the ESHA-coated α-Al2O3 (1 −1 0 2) surfaces in samples without the addition of calcium. However, the amounts of Pb(II) mobilized from the ESHA coatings onto the α-Al2O3 (1 −1 0 2) surfaces increased from 40% (no added Ca) to 58% (with 2 mM Ca) after 72 h of reaction time, possibly due to displacement of Pb(II) by Ca(II) from binding sites in the ESHA coatings. In contrast, Pb(II), Cu(II), and Zn(II) present in the ESHA coatings were found to be unreactive with the α-Al2O3 (0 0 0 1) surface. The observed reactivities of the three ESHA-coated metal-oxide surfaces with respect to metal-ion sorption are consistent with the trend observed for the uncoated metal-oxide surfaces: α-Fe2O3 (0 0 0 1) > α-Al2O3 (1 −1 0 2) > α-Al2O3 (0 0 0 1). In addition, Pb(II) partitioning onto α-Al2O3 (1 −1 0 2) surfaces increased with increasing pH from 4.0 to 9.0 as a result of the increasingly negative surface charge. These results show that intrinsic properties (nature of binding sites, binding affinities, and surface charge) of the ESHA coatings and metal-oxide surfaces, as well as external parameters such as pH and competing ions, are key factors governing the distribution and speciation of metal ions at complex NOM/mineral interfaces.
KW - Alumina
KW - Ca
KW - Cu
KW - Hematite
KW - Humic acid
KW - LP-XSW-FY
KW - Metal partitioning
KW - Metal-oxide surfaces
KW - Pb
KW - Single crystal
KW - X-ray standing wave
KW - Zn
KW - pH effect
UR - http://www.scopus.com/inward/record.url?scp=84977633649&partnerID=8YFLogxK
U2 - 10.1016/j.gca.2016.05.009
DO - 10.1016/j.gca.2016.05.009
M3 - Article
AN - SCOPUS:84977633649
SN - 0016-7037
VL - 188
SP - 407
EP - 423
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -