The phase transformation of iron minerals induced by aqueous Fe(II) (Fe(II)aq) is a critical geochemical reaction which greatly affects the geochemical behavior of soil elements. How the geochemical ...behavior of rare earth elements (REEs) is affected by the Fe(II)aq-induced phase transformation of iron minerals, however, is still unknown. The present study investigated the adsorption and immobilization of REEs during the Fe(II)aq-induced phase transformation of ferrihydrite. The results show that the heavy REEs of Ho(III) were more efficiently adsorbed and stabilized compared with the light REEs of La(III) by ferrihydrite and its transformation products, which was due to the higher adsorptive affinity and smaller atomic radius of Ho(III). Both La(III) and Ho(III) inhibited the Fe atom exchange between Fe(II)aq and ferrihydrite, and sequentially, the Fe(II)aq-induced phase transformation rates of ferrihydrite, because of the competitive adsorption with Fe(II)aq on the surface of iron (hydr)oxides. Owing to the larger amounts of adsorbed and stabilized Ho(III), the inhibition of the Fe(II)aq-induced phase transformation of ferrihydrite affected by Ho(III) was higher than that by La(III). Our findings suggest an important role for the Fe(II)aq-induced phase transformation of iron (hydr)oxides in assessing the mobility and transfer behavior of REEs, as well as for their occurrence in earth surface environments.
Core Ideas
Recrystallization of Al‐goethites affected mobility of heavy metals in soils.
Co‐existing Cd2+/Cr3+ decreased the Fe atom exchange rates between Fe2+aq and Al‐goethites.
Fe2+aq‐induced ...recrystallization of Al‐goethites stabilized Cd2+ and Cr3+.
More Cr3+ are stabilized due to its similar ionic radius with Fe3+ and Al3+.
Cd2+ ions more active than Cr3+ considering iron mineral activity in soils.
The aqueous Fe2+ (Fe2+aq)–induced recrystallization of iron minerals is an important geochemical process with critical effects on the environmental behavior of metal pollutants in soils. However, the effects of different heavy metals with distinctive physicochemical properties on the Fe2+aq–induced recrystallization of iron minerals and the different stabilization behaviors of these heavy metals remain unclear. This study examined the effects of Cd2+ and Cr3+ on the Fe2+aq–induced recrystallization of Al‐substituted goethites and the simultaneous stabilization of Cd2+/Cr3+ ions by the recrystallized Al‐substituted goethites. Results from stable Fe isotopic tracer experiments and Mössbauer characterization show that Fe atoms were exchanged between Fe2+aq and structural Fe(III) in Al‐substituted goethites with coexisting Cd2+/Cr3+, although both Cd2+ and Cr3+ decreased the exchange rates. During the Fe atom exchange, Al‐substituted goethites were recrystallized and a portion of Cd2+/Cr3+ ions were stabilized by the resulting goethite products. Compared with Cd2+, more Cr3+ ions were immobilized by the recrystallized Al‐substituted goethites due to the lower hydrolysis constant of Cr3+ than Cd2+, as well as the closer ionic radius of Cr3+ with Fe3+ and Al3+. Al‐goethites with higher Al contents further decreased the Fe atom exchange rates while increasing the amounts of immobilized metal ions when in the presence of Cd2+ or Cr3+. The findings of the present study suggest that the physicochemical properties of metal ions play critical roles in affecting their environmental behavior and fates during Fe2+aq–induced recrystallization of iron minerals in soils.
Core Ideas
Recrystallization of Al‐goethites affected mobility of heavy metals in soils.
Co‐existing Cd
2+
/Cr
3+
decreased the Fe atom exchange rates between Fe
2+
aq
and Al‐goethites.
Fe
2+
aq
...‐induced recrystallization of Al‐goethites stabilized Cd
2+
and Cr
3+
.
More Cr
3+
are stabilized due to its similar ionic radius with Fe
3+
and Al
3+
.
Cd
2+
ions more active than Cr
3+
considering iron mineral activity in soils.
The aqueous Fe
2+
(Fe
2+
aq
)–induced recrystallization of iron minerals is an important geochemical process with critical effects on the environmental behavior of metal pollutants in soils. However, the effects of different heavy metals with distinctive physicochemical properties on the Fe
2+
aq
–induced recrystallization of iron minerals and the different stabilization behaviors of these heavy metals remain unclear. This study examined the effects of Cd
2+
and Cr
3+
on the Fe
2+
aq
–induced recrystallization of Al‐substituted goethites and the simultaneous stabilization of Cd
2+
/Cr
3+
ions by the recrystallized Al‐substituted goethites. Results from stable Fe isotopic tracer experiments and Mössbauer characterization show that Fe atoms were exchanged between Fe
2+
aq
and structural Fe(III) in Al‐substituted goethites with coexisting Cd
2+
/Cr
3+
, although both Cd
2+
and Cr
3+
decreased the exchange rates. During the Fe atom exchange, Al‐substituted goethites were recrystallized and a portion of Cd
2+
/Cr
3+
ions were stabilized by the resulting goethite products. Compared with Cd
2+
, more Cr
3+
ions were immobilized by the recrystallized Al‐substituted goethites due to the lower hydrolysis constant of Cr
3+
than Cd
2+
, as well as the closer ionic radius of Cr
3+
with Fe
3+
and Al
3+
. Al‐goethites with higher Al contents further decreased the Fe atom exchange rates while increasing the amounts of immobilized metal ions when in the presence of Cd
2+
or Cr
3+
. The findings of the present study suggest that the physicochemical properties of metal ions play critical roles in affecting their environmental behavior and fates during Fe
2+
aq
–induced recrystallization of iron minerals in soils.
The process of in situ chemical oxidation (ISCO) by persulfate (S2O8 2-) can be accelerated by metal ion activation, so as to more effectively degrade the subsurface pollutants owing to the ...enhancement of sulfate radicals (SO4·-) generation. Mn2+ is a natural soil metal ion usually exists together with organic pollutants in contaminated soils and groundwater. In this study, the oxidative degradation of DDT by Mn2+ activated persulfate at different reaction conditions was systematically studied. The results showed that Mn2+ is a more stable activator for persulfate oxidation than other reported metal ions (e.g. Fe2+ reported in many papers previously). The activation of persulfate by Mn2+ ions gave DDT a long lasting and efficient degradation effect and a high mineralization rate. With fixed DDT and persulfate concentrations, higher Mn2+ concentrations would give DDT higher degradation rates, and higher temperature accelerated DDT degradation with Mn2+ activation. The pH conditions also posed a significant effect on DDT degradation by Mn2+ activated persulfate, and acidic conditions were found to be more favorable for DDT degradation. The results obtained in this study may give a promising indication and new technology with persulfate for in situ remediating soils and groundwater that contaminated by organo- chlorine pesticides.
Ferrihydrite (FHY), a widespread Fe(III) (hydr)oxide, is abundant in earth surface and critical in affecting the environmental behavior of soil elements, such as rare earth elements (henceforth ...referred to REEs). Under anoxic conditions, the coexisting Fe(II) induces FHY conversion to the minerals with high crystallinity, such as lepidocrocite, goethite, or magnetite, where the phase transformation processes were affected by the coexisting ions. The interactions between REE ions (henceforth referred to Ln(III)) and FHY in the transformation system, however, are still not well understood. We therefore investigated Fe(II) aq -induced FHY transformation affected by four kinds of Ln(III) (Ce3+, Nd3+, Tb3+, and Lu3+). The discernible inhibition was found among the different treatments with different Ln(III), and both the FHY transformation and Fe atom exchange ratios decreased with coexistent Ln(III) that have larger atomic number. In this study, the Fe atom exchange ratios with different coexistent Ln(III) were linearly negatively depended on the molar enthalpy values of Ln(III), which are explained by the molar enthalpy considered to be important in affecting the adsorption behaviors of Ln(III). Furthermore, the adsorption behaviors can affect the incorporation efficiency of the Ln(III), so as to affect the Fe atom exchange and FHY phase transformation. The Ln(III) with larger atomic number can be stabilized more, which results from the effects of incompatible REEs in determining the efficiency of immobilizing Ln(III) in the transformed iron (hydr)oxides. Our research suggested the important role of FHY in the distribution of REEs and the key properties of REEs in affecting their different distribution characteristics in earth surface environments.
PDF
