Binding of Cd by ferrihydrite organo-mineral composites: Implications for Cd mobility and fate in natural and contaminated environments

Abstract

Adsorption and coprecipitation of organic matter with iron (hydr)oxides can alter iron (hydr)oxide surface properties and their reactivity towards nutrient elements and heavy metals. Organo-mineral composites were synthesized using humic acid (HA) and iron oxide, during coprecipitation with ferrihydrite (Fh) and adsorption to pre-formed Fh with two C loadings. The Fh-HA coprecipitated composites have a higher C content and smaller surface area compared to the equivalent adsorbed composites. NanoSIMS shows there is a high degree of spatial correlation between Fe and C for both composites, but C distribution is more uniform in the coprecipitated composites. The C 1s NEXAFS reveals a similar C composition between the Fh-HA coprecipitated and adsorbed composites. However composites at high carbon loading are more enriched in aromatic C, likely due to preferential binding of carboxyl functional groups on aromatic rings in the HA. The amount of Cd sorbed is independent of the composite type, either coprecipitated or adsorbed, but is a function of the C loading. Composites with low C loading show Cd sorption that is almost identical to pure Fh, while composites with high C loading show Cd sorption that is intermediate between pure Fh and pure HA, with sorption significantly enhanced over pure Fh at pH < 6.5. A bidentate edge-sharing binding was identified for Cd on pure Fh and Cd-carboxyl binding on pure HA. These findings have significant implications not only for the sequestration of Cd in contaminated environments but also the coupled biogeochemical cycling of Cd, Fe and C in the critical zone.

Soil organo-mineral associations formed by co-precipitation of Fe, Si and Al in presence of organic ligands

“…In the case of Fe-coprecipitates, SROFe-OM are often described as spherical 2-5 nm ferrihydrite crystallites aggregated in an OM matrix (Chen et al, 2016;Eusterhues et al, 2008;Mikutta et al, 2008;Mikutta et al, 2014;Schwertmann et al, 2005). This model of Fe oxy-hydroxide occlusion in OM is well illustrated in several studies (Du et al, 2018;Eusterhues et al, 2014;Kleber et al, 2015). The density of SROFe-OM aggregates (Eusterhues et al, 2008;Guenet et al, 2017;Mikutta, 2011;Mikutta et al, 2008) together with the size of the ferrihydrite crystallites and their crystallinity (Eusterhues et al, 2008;Mikutta, 2011;Mikutta et al, 2010;Schwertmann et al, 2005) has been shown to vary with the experimental conditions.…”

Natural organic matter decreases uptake of W(VI), and reduces W(VI) to W(V), during adsorption to ferrihydrite

“…For preparation of the Fh-HA binary composite, 0.1 M Fe 3+ salt solution was first reacted with two known amounts of HA solution for 2 h, generating C/Fe molar of 0.47 and 2.33, respectively, then subjected to a base hydrolysis to pH ~7.5. These preparations resulted in Fh-HA composites with approximately 5 wt% and 15 wt% C (Du et al, 2018b), and were abbreviated as FhHA_5%C and FhHA_15%C, respectively. Both pure Fh and Fh-HA composite were rinsed several times until no dissolved organic carbon (DOC) was detected in the supernatant liquid (Du et al, 2018b).…”

“…These preparations resulted in Fh-HA composites with approximately 5 wt% and 15 wt% C (Du et al, 2018b), and were abbreviated as FhHA_5%C and FhHA_15%C, respectively. Both pure Fh and Fh-HA composite were rinsed several times until no dissolved organic carbon (DOC) was detected in the supernatant liquid (Du et al, 2018b).…”

“…This is known to bind strongly with iron (hydr)oxides via surface adsorption and/or (Sposito, 1984;Chen et al, 2014;Kleber, 2015). These processes result in the formation of an organo-mineral composite (Kleber et al, 2015), in which surface properties are quite different from those of pure iron (hydr)oxide, thereby potentially affecting the transport and eventual fate of trace elements (Moon and Peacock, 2012;Liang et al, 2013;Seda et al, 2016;Du et al, 2016;Wang et al, 2016;Flynn and Catalano, 2017;Otero-Fariña et al, 2017;Du et al, 2018a;2018b;Fariña et al, 2018;Woodward et al, 2018;Xue et al, 2019). However, whether and to what extent NOM affects the binding of soluble tungstate oxyanions to mineral surfaces is still unknown.…”

Predicting Kinetics of As(V) Adsorption and Desorption on Mixed Minerals of Ferrihydrite and δ‐MnO2

“…The model parameters derived from the single mineral system of ferrihydrite and δ‐MnO 2 can be applied to the mixed mineral systems. This is different from the mixed systems of minerals and organic matter, in which the interactions between minerals and organic matter may significantly affect the reactivity of the mixture (Christl and Kretzschmar, 2001; Du et al, 2018a, 2018b; Vermeer et al, 1999). However, if ferrihydrite and δ‐MnO 2 were precipitated together during synthesis, the As(V) adsorption and desorption behavior by the co‐precipitates may be different from that based on additivity of these two minerals, which desired further study.…”