Scavenging of particulate and dissolved lead compounds by coprecipitation with manganese oxyhydroxides
Mn is a geochemically important element that contributes significantly to the cycling of heavy metals. During precipitation, Mn oxyhydroxides scavenge many heavy metals, including Pb, in a variety of natural environments. Because of this phenomenon, the precipitation of Mn oxyhydroxides may provide a remediation technique for removing Pb from contaminated aqueous solutions. Therefore, this study was undertaken to provide a quantitative understanding of the coprecipitation of Pb with Mn oxyhydroxides to demonstrate their capacity to remove Pb permanently from contaminated solutions. To accomplish this, a series of factorial experiments with varying initial Mn and Pb concentrations were run in the presence of a borate buffer or a bicarbonate buffer. All experiments were run in batch reactors, in the presence of a quartz substrate, at 25 degrees celcius, at pH 8.5, and were continuously stirred. Initial Mn and Pb concentrations were varied by half log units from 100 to 0 mg/L and from 3 to 0 mg/L, respectively. Solutions were analyzed for Mn using the formaldioxime colorimetric method and for Pb using AA. Precipitates on quartz surfaces were analyzed by SEM, XPS, and XRD for precipitate identification and morphology. The amount of Mn and Pb associated with the quartz sand was determined by dissolving the precipitates from selected quartz samples using concentrated nitric acid. Finally, a different set of precipitate-coated quartz grains were leached in pH 5 acetic acid solution to assess the metal retention capacity of the precipitated material.
Mn oxyhydroxides precipitated onto the quartz sand in both the borate and bicarbonate buffered experiments. SEM and XPS data revealed tiny crystallites in etch pits on the quartz surfaces that contained predominantly Mn3+. XRD analysis did not produce an X-ray pattern for these Mn oxyhydroxides but did identify the suspended Pb precipitates as hydrocerrusite and Pb(HBO3)2 in the borate buffered experiments and hydrocerrusite in the bicarbonate buffered experiments. Much more Mn and Pb are associated with the quartz surfaces in the borate buffered experiments, but no Pb was associated with quartz surfaces initially (< 6 hrs. of reaction time). Leaching of precipitates resulted in extracted Mn in both experiments but Pb was extracted in only the bicarbonate buffered experiments. The Mn precipitation rate was greater in the borate buffered experiments and higher initial Mn and Pb concentrations appear to increase the precipitation rate in both sets of experiments. These results indicate that Mn oxyhydroxides nucleated onto suspended Pb precipitates. The growing Mn oxyhydroxide particles were attracted to the quartz sand, carrying along the Pb precipitates. Further precipitation of Mn oxyhydroxides on the quartz surfaces trapped the Pb. This process was much more significant in the borate buffered experiments where much more Mn precipitated. The greater amount of Mn oxyhydroxide growth acts as a barrier protecting the Pb from the pH 5 leaching solutions. As a result, Pb was retained by the sand grains from the borate buffered experiments during leaching while significant amounts of Pb (35-100%) was extracted from the sand produced by the bicarbonate experiments. These results strongly suggest that coprecipitation of Pb with Mn oxyhydroxides in the presence of a borate buffer and a quartz substrate may be a remediation tool for Pb contaminated aqueous solutions. Not only will this process remove aqueous Pb2+ from solution but it appears it will also substantially incorporate colloidal Pb particles as well.