Pyrogenic Carbon Improves Cd Retention during Microbial Transformation of Ferrihydrite under Varying Redox Conditions

Abstract

Fe(III) (oxyhydr)oxides are ubiquitous in paddy soils and play a key role in Cd retention. Recent studies report that pyrogenic carbon (PC) may largely affect the microbial transformation processes of Fe(III) (oxyhydr)oxides, yet the impact of PC on the fate of Fe(III) (oxyhydr)oxide-associated Cd during redox fluctuations remains unclear. Here, we investigated the effects of PC on Cd retention during microbial (Shewanella oneidensis MR-1) transformation of Cd(II)-bearing ferrihydrite under varying redox conditions. The results showed that in the absence of PC, microbial reduction of ferrihydrite resulted in Cd release under anoxic conditions and Fe(II) oxidation by oxygen resulted in Cd retention under subsequent oxic conditions. The presence of PC facilitated microbial ferrihydrite reductive dissolution under anoxic conditions, promoted Fe(II) oxidative precipitation under oxic conditions, and inhibited Cd release under both anoxic and oxic conditions. The presence of PC and frequent shifts in redox conditions (i.e., redox cycling) inhibited the transformation of ferrihydrite to highly crystalline goethite and magnetite that exhibited less Cd adsorption. As a result, PC enhanced Cd retention by 41−59% and 55−77% after the redox shift and redox cycling, respectively, while in the absence of PC, Cd retention decreased by 5% after the redox shift and increased by 11% after redox cycling. Sequential extraction analysis revealed that 63−78% of Cd was associated with Fe minerals, while 3−12% of Cd was bound to PC, indicating that PC promoted Cd retention mainly through inhibiting ferrihydrite transformation. Our results demonstrate the great impacts of PC on improving Cd retention under dynamic redox conditions, which is essential for applying PC in remediating Cd-contaminated paddy soils.

Mineral-mediated stability of organic carbon in soil and relevant interaction mechanisms

“…Similarly, the redox transformation of Mn can be controlled by the microorganisms for either Mn(II) oxidation [ 132 , 143 , 144 ] or Mn(IV) reduction [ [144] , [145] , [146] , [147] ]. Certain organic matter types, such as pyrogenic carbon, might inhibit the reductive transformation and crystallization of minerals during the microorganism-induced transformation process [ 148 ].…”

“…In short, the binding strength and the site density are the keys to the stability of the mineral-organic carbon association. It is worth noting that the mineral-organic carbon association not only stabilizes organic carbon but is also believed to inhibit mineral crystallization and transformation, leading to a higher overall stability [ 23 , 148 , 250 ].…”

“…Interaction or association with organic carbon might facilitate the formation of short-range-ordered minerals and thus higher organic carbon stabilization [ 27 , 278 , 279 , 289 ]. For instance, the presence of pyrogenic carbon inhibited the biotransformation of ferrihydrite to the crystal goethite and magnetite, thus maintaining a higher sorption capacity [ 148 ]. A loose and porous structure could be found in the mineral growth with the organic carbon due to the inhibited crystallization process, resulting in potent carbon sequestration potential [ 289 ].…”

Enhanced Oxidation of Organic Compounds by the Ferrihydrite–Ferrate System: The Role of Intramolecular Electron Transfer and Intermediate Iron Species

“…Iron oxides, such as ferrihydrite, goethite, and hematite, are ubiquitous in aquatic and most terrestrial environments and participate in various chemical processes. − Ferrihydrite (Fh), characterized by its extensive surface area and unique structure, is particularly reactive and plays a crucial role in regulating the (im)­mobilization contaminants. , On one hand, Fh can contribute to controlling the fate of toxic metals and metalloids by processes such as adsorption/desorption, coprecipitation, and crystallization. − On the other hand, Fh is often utilized as a heterogeneous catalyst to activate H 2 O 2 or persulfate, leading to the degradation of organic contaminants by generating · OH or SO 4 · – . − However, the oxidation of organic contaminants via the combination of Fe­(VI) and Fh remains unexplored.…”

Cadmium Sorption on Alumina Nanoparticles and Mixtures of Alumina and Smectite: An Experimental and Modelling Study

“…Up to date, few works have studied the immobilization of Cd on mixtures of several sorbents. Some of them include the interaction of Cd with the following: clay and microorganisms [29], biochar and MgO composites [30], biochar and Fe/Mg composites, pyrogenic carbon and ferrihydrite [31], mixtures of components in reactive barriers [32], or mixtures of alumina and silica [33]. The development of sorption models to describe Cd sorption in mixtures of sorbents is essential to better understand Cd behavior in the environment.…”