Sorption of Pb(II) by Nanosized Ferrihydrite Organo-Mineral Composites Formed by Adsorption versus Coprecipitation

Nanosized iron hydroxides and organic matter (OM) are important scavengers of nutrient elements and dissolved trace-metals in geologic systems. Despite this, little is known about the sorption characteristics of metals to the iron-OM nanocomposites formed by adsorption (adsorption of OM on presynthesis ferrihydrite) versus coprecipitation (formation of ferrihydrite in the presence of OM). This study investigates lead (Pb) sorption behaviors on ferrihydrite-humic acid (Fh-HA) composites through batch sorption coupled with Pb-L-III, EXAFS. We report that composites formed by coprecipitation have a higher carbon content, smaller specific surface area, and faster Pb sorption rate compared to the equivalent composites formed by adsorption of HA to preformed Fh. Pb sorption on the composites is enhanced at pH < 5 than that on pure Fh, and coprecipitated and adsorbed composites sorb almost equivalent amount of Pb. We identify a bidentate edge-sharing Pb complex on the ferrihydrite surface with a Pb-Fe bond length of 3.33 angstrom and a bidentate inner-sphere Pb complex on the HA fraction of the composite. More Pb ions are associated with the HA fraction of the composites with the increases of pH from 4 to 6.5. More Pb ions are sequestrated by the organic fraction in the adsorbed than in the coprecipitated composites. This research therefore has important implications for predicting the mobility and fate of Pb in iron and organic rich soils and sediments.