Competitive Sorption of Pb(Ⅱ) and Zn(Ⅱ) on Polyacrylic Acid-Coated Hydrated Aluminum-Oxide Surfaces

摘要

Natural organic matter (NOM) often forms coatings on minerals. Such coatings are expected to affect metal-ion sorption due to abundant sorption sites in NOM and potential modifications to mineral surfaces, but such effects are poorly understood in complex multicomponent systems. Using poly(acrylic acid) (PAA), a simplified analog of NOM containing only carboxylic groups, Pb(Ⅱ) and Zn(Ⅱ) partitioning between PAA coatings and α-Al_2O_3 (1 - 102) and (0001) surfaces was investigated using long-period X-ray standing wave-florescence yield spectroscopy. In the single-metal-ion systems, PAA was the dominant sink for Pb(Ⅱ) and Zn(Ⅱ) for α-Al_2O_3(1-102) (63% and 69%, respectively, at 0.5 μM metal ions and pH 6.0). In equi-molar mixed-Pb(Ⅱ)-Zn(Ⅱ) systems, partitioning of both ions onto α-Al_2O_3( 1-102) decreased compared with the single-metal-ion systems; however, Zn(Ⅱ) decreased Pb(Ⅱ) sorption to a greater extent than vice versa, suggesting that Zn(Ⅱ) outcompeted Pb(Ⅱ) for α-Al_2O_3(1 -102) sorption sites. In contrast, ＞99% of both metal ions sorbed to PAA when equi-molar Pb(Ⅱ) and Zn(Ⅱ) were added simultaneously to PAAα-Al_2O_3(0001). PAA outcompeted both α-Al_2O_3 surfaces for metal sorption but did not alter their intrinsic order of reactivity. This study suggests that single-metal-ion sorption results cannot be used to predict multimetal-ion sorption at NOM/metal-oxide interfaces when NOM is dominated by carboxylic groups.