Pb, Cu, and Zn distributions at humic acid-coated metal-oxide surfaces

Abstract

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 (0001), α-Al2O3 (1−102), and α-Fe2O3 (0001). 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 (1−102) and α-Fe2O3 (0001) surfaces after 3h of reaction (pH=6.0, [Ca(II)]=2mM). Pb(II) partitioning onto these reactive metal-oxide surfaces increased with increasing reaction time (up to 7d). In addition, the partitioning of Cu(II) and Zn(II) from the ESHA coating to the α-Fe2O3 (0001) substrate increased slightly with reaction time (2.4% and 3.7% for Cu(II) and Zn(II), respectively, after 3h and 6.4% and 7.7% for Cu(II) and Zn(II), respectively, after 72h of reaction time). However, no changes in the partitioning of Cu(II) and Zn(II) onto the α-Al2O3 (1−102) 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 (1−102) surfaces in samples without the addition of calcium. However, the amounts of Pb(II) mobilized from the ESHA coatings onto the α-Al2O3 (1−102) surfaces increased from 40% (no added Ca) to 58% (with 2mM Ca) after 72h 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 (0001) 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 (0001)>α-Al2O3 (1−102)>α-Al2O3 (0001). In addition, Pb(II) partitioning onto α-Al2O3 (1−102) 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.