Structural Incorporation of Sorbed Molybdate during Iron(II)-Induced Transformation of Ferrihydrite and Goethite under Advective Flow Conditions

Abstract

Aqueous Fe­(II) can induce recrystallization of ferrihydrite and goethite [α-FeOOH] to their more crystalline or molecularly homogeneous counterparts. Despite common association with these and other Fe­(III) (oxyhydr)­oxides, relationships between Fe­(II)-induced transformation and Mo mobility remain poorly constrained. We conducted laboratory column experiments to examine repartitioning of sorbed Mo during Fe­(II)-induced transformation of ferrihydrite and goethite under advective flow conditions. We first pumped (∼0.25 L d–1) artificial groundwater containing 0.1 mM MoO4 2– and buffered to pH 6.5 through columns packed with ferrihydrite- and goethite-coated sand until >90% Mo breakthrough was observed. Extended X-ray absorption fine structure (EXAFS) spectroscopy shows that initial Mo attenuation resulted from inner sphere complexation of MoO4 tetrahedra at ferrihydrite and goethite surfaces. We then pumped Mo-free anoxic artificial groundwater containing 0.2 mM or 2.0 mM Fe­(II) through the columns until effluent Mo concentrations remained <0.005 mM. Raman spectroscopy shows that Fe­(II) introduction induced transformation of both ferrihydrite and goethite to lepidocrocite. Additionally, Fe­(II) introduction mobilized 4–34% of sorbed Mo and total mass release was greater for (i) ferrihydrite compared to goethite columns and (ii) low Fe­(II) compared to high Fe­(II) influent. Effluent pH decreased to ∼5.8 for columns receiving the high Fe­(II) influent and returned to pH 6.5 after 5–10 pore volumes. EXAFS spectroscopy indicates that structural incorporation of MoO6 octahedra into neoformed phases contributes to Mo retention during Fe­(II)-induced transformation. Our results offer new insight into Mo repartitioning during Fe­(II)-induced transformation of Fe­(III) (oxyhydr)­oxides and, more generally, controls on Mo mobility in geohydrologic systems.

Molybdenum(VI) Sequestration Mechanisms During Iron(II)-Induced Ferrihydrite Transformation

“…Progressive Mo(VI) uptake during thermal ferrihydrite transformation was previously reported under alkaline pH conditions, 13 while dual mechanisms were demonstrated in an advective flow experiment, where initial mobilization and subsequent uptake of Mo(VI) were observed at different points over the course of the experiment. 11 There was limited bulk transformation of ferrihydrite in all experiments except the pH 0 6.5 coprecipitated experiments, although nanoscale lepidocrocite and goethite crystallites were consistently detected with TEM-SAED. In addition, several S1 and S2).…”

“…Local coordination environments were determined by nonlinear least squares fitting of the Mo K-edge EXAFS spectra. Initial estimates of path lengths ( R ) for Mo–O, Mo–Mo, and Mo–Fe scattering paths for both phases containing MoO 4 tetrahedra and MoO 6 octahedra were obtained from previous studies. ,, These values were refined during data analysis, which involved shell-by-shell fitting the Fourier transform of the background subtracted k 3 weighted χ­( k ) spectrum of all samples from k = 3 to 11 Å –1 . The amplitude reduction factor of a Na 2 MoO 4 reference material, assuming a first shell Mo–O coordination number (CN) of 4.0, was refined to be 0.82(9) and this value was subsequently fixed for all experimental samples.…”

“…5,8−12 Recent studies suggest that these dynamic metal−mineral interactions strongly influence Mo transport and attenuation in geohydrologic systems. 11,13,14 Consequently, an improved understanding of these interactions could offer important new insights into geochemical controls on Mo mobility and bioavailability in modern soils and sediments, and Mo comportment in sedimentary rocks.…”

“…Iron­(III) (oxyhydr)­oxides often control element mobility and bioavailability in soils, sediments, and associated waters. − These phases limit dissolved concentrations of various nutrient and contaminant elements through adsorption or coprecipitation reactions under oxic conditions. , However, metastable Fe­(III) (oxyhydr)­oxides including ferrihydrite and goethite [α-FeOOH (s) ] are susceptible to dissolution, recrystallization, or transformation under anoxic conditions. , Repartitioning of associated elements can follow different pathways, with specific outcomes dependent upon multiple factors including the availability of electron donors and acceptors, microbial activity, and water chemistry. ,− Recent studies suggest that these dynamic metal–mineral interactions strongly influence Mo transport and attenuation in geohydrologic systems. ,, Consequently, an improved understanding of these interactions could offer important new insights into geochemical controls on Mo mobility and bioavailability in modern soils and sediments, and Mo comportment in sedimentary rocks.…”

“…Ferrihydrite transformation occurs slowly under oxic conditions and at temperatures relevant to most soils and sediments . Aqueous Fe­(II) induces electron transfer and atom exchange (ET-AE), leading to rapid ferrihydrite transformation. − During this reaction, ferrihydrite dissolution occurs concurrently with precipitation of more crystalline phases, including lepidocrocite and goethite, with reaction pathways depending on the Fe­(II) availability, pH, and competing ions. ,, Past studies reveal that Fe­(II)-induced ET-AE is inhibited by adsorbed or coprecipitated elements including Mo­(VI). ,,,, Consistent with thermal ferrihydrite transformation, Fe­(II)-induced transformation promotes structural incorporation accompanied by a tetrahedral to octahedral change in Mo­(VI) coordination. , Arai attributed a similar coordination change to pH-dependent Mo­(VI) polymerization, promoting bamfordite [FeMo 2 O 6 (OH) 3 ·H 2 O] precipitation at goethite surfaces. The bamfordite crystal structure contains MoO 6 octahedra dimers linked via corner sharing to FeO 6 octahedra .…”