Lead Adsorption\nand Desorption at the Barite (001)\nSurface in the Presence of EDTA

Scaling minerals, such as barite, can cause detrimental\nconsequences\nfor oil/gas pipelines and water systems, but their formation can be\ninhibited by organic chelators such as ethylenediaminetetraacetic\nacid (EDTA). Here, we resolve how EDTA affects sorption and desorption\nof Pb at the barite (001) surface using a combination of X-ray scattering\nand microscopy measurements. In the presence of EDTA, Pb incorporated\nin the topmost part of the barite surface and adsorbed as inner-sphere\ncomplexes on the surface. In barite saturated solutions containing\n[Pb] ≥ 100 μM, overgrowth films grew along step edges.\nThese films were exclusively monolayer thick, indicating that their\ngrowth was a self-limiting process. Approximately half of the Pb was\nremoved after 14.5 h reaction with a Pb-free EDTA solution where most\nof the desorption occurred to adsorbed Pb rather than incorporated\nPb. Dissolution proceeded primarily via step retreat and etch pit\nformation in EDTA, but in deionized water, the secondary phase was\nquickly removed within 3 min. Together these results suggest EDTA\nbinds to both the surface and Pb in solution, which limits Pb sorption.\nHowever, EDTA binding to the surface also inhibits removal of the\nsecondary phase that formed at higher Pb concentrations.