Synergistic adsorption of Cd(II) with sulfate/phosphate on ferrihydrite: An in situ ATR-FTIR/2D-COS study

Abstract

19Elucidation of the co-adsorption characteristics of heavy metal cations and oxyanions on 20 (oxyhydr)oxides can help to better understand their distribution and transformation in many 21 geological settings. In this work, batch adsorption experiments in combination with in situ attenuated 22 total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) were applied to explore the 23 interaction mechanisms of Cd(II) with sulfate or phosphate at the ferrihydrite (Fh)−water interface, 24 and the two-dimensional correlation spectroscopic analysis (2D-COS) was used to enhance the 25 resolution of ATR-FTIR bands and the accuracy of analysis. The batch adsorption experiments 26 showed enhanced adsorption of both sulfate (S) and phosphate (P) on Fh when co-adsorbed with 27 Cd(II); additionally, the desorbed percentages of Cd(II) were much lower in the P+Cd adsorption 28 systems than those in the S+Cd adsorption systems. The spectroscopic results suggested that in the 29 single adsorption systems, sulfate primarily adsorbed as outer-sphere complexes with a small amount 30 of bidentate inner-sphere complexes, while the dominant adsorbed species of phosphate were largely 31 the bidentate nonprotonated inner-sphere complexes, although there was significant pH-dependence. 32 In the co-adsorption systems, the synergistic adsorption of Cd(II) and sulfate was dominantly 33 attributed to the electrostatic interaction, as well as the formation of Fe−Cd−S (i.e., Cd-bridged) 34 ternary complexes. In contrast, Fe−P−Cd (i.e., phosphate-bridged) ternary complexes were found in 35 all of the co-adsorption systems of phosphate and Cd(II); furthermore, electrostatic interaction should 36 also contribute to the co-adsorption process. Our results show that in situ ATR-FTIR in combination 37 with 2D-COS can be an efficient tool in analyzing the co-adsorption mechanisms of anions and heavy 38 metal cations on iron (oxyhydr)oxides in ternary adsorption systems. The co-existence of Cd(II) with 39 sulfate or phosphate can be beneficial for their accumulations on Fh, and phosphate is more efficient 40 than sulfate for the long-term immobilization of Cd(II). 41

Adsorption of phosphate and cadmium on iron (oxyhydr)oxides: A comparative study on ferrihydrite, goethite, and hematite

“…Generally, different mechanisms may contribute simultaneously to the co-adsorption of oxyanions and heavy-metal cations (Elzinga and Kretzschmar, 2013;Rietra et al, 2001;Simanova et al, 2011;Villalobos et al, 2001). Factors such as solution conditions, the nature and density of the adsorbate species, and the surface structure of the adsorbent may comprehensively influence the co-adsorption mechanisms (Collins et al, 1999;Li et al, 2007;Liu et al, 2018;Ostergren et al, 2000). However, understandings on how the surface properties and structures of iron (oxyhydr)oxides affect the synergistic adsorption behaviors and mechanisms of anions and cations are vague.…”

Carbonate Adsorption to Ferrihydrite: Competitive Interaction with Phosphate for Use in Soil Systems

“…Our study has a wide perspective as CO 3 is omnipresent in nature and will interfere in many geochemical processes, as mentioned above. 6,19−30…”

Petrographical and Geochemical Signatures Linked to Fe/Mn Reduction in Subsurface Marine Sediments from the Hydrate-Bearing Area, Dongsha, the South China Sea

“…In this case, the study area is located in the northeastern unstable continental slope of the SCS [84] (Figure 1), and the sediments with abundant Fe/Mn-oxides from the shelf of the SCS might be integrally transported into the Dongsha area due to the structural or gravity instability. The previously deposited Fe/Mn-oxides, in particular, goethite, ferrihydrite, and birnessite with large specific surface areas [28,60,61,85], would spontaneously trigger the onset of AOM in the sediments with high-flux methane seeps. Obviously, the Fe/Mn source, in this case, evidently differs from those previously reported in the northern Okinawa Trough (i.e., Fe 3+ derived from deep-source fluids at methane seeps) [26][27][28].…”

“…In a recent co-culture experiment of ferrihydrite reduction and methane oxidation, ferrihydrite was reduced, and ferrous minerals were formed [87]. In submarine sediments, Fe/Mn-oxides such as ferrihydrite and birnessite are important and ubiquitous geosorbents, which have large specific surface areas of more than 200 m 2 /g [85,88,89] and 63−300 m 2 /g, respectively [90,91]. In addition, the similar biogeochemical processes of Fe and Mn reduction are found in the hydrocarbon-derived Fe-rich nodules from the Dongsha area and carbonate chimneys from the Gulf of Cadiz [34,92,93].…”