Biochar influences soil fertility, N2O emissions, and atmospheric CO2 budgets, and because of its quinone and aromatic structures, it is redox-active. Here we demonstrate that biochar concentrations of 5 and 10 g L–1 stimulate both the rate and the extent of microbial reduction of the Fe(III) oxyhydroxide mineral ferrihydrite (15 mM) by Shewanella oneidensis MR-1, while lower biochar concentrations (0.5 and 1 g L–1) have a negative effect on ferrihydrite reduction. Control experiments showed that biochar particles and not biochar-derived water-soluble organic compounds are responsible for the stimulating and inhibiting effect. We also found that biochar changed the mineral product of ferrihydrite reduction from magnetite (Fe3O4) to siderite (FeCO3). Our study suggests that biochar can influence soil biogeochemistry not only indirectly by changing the soil structure and chemistry but also by directly mediating electron transfer processes, i.e., by functioning as an electron shuttle.

Biochar as an Electron Shuttle between Bacteria and Fe(III) Minerals

This report presents an exhaustive literature review of the effects of engineered nanoparticles on soil microbial communities. The toxic effects on microbial communities are highly dependent on the type of nanoparticles considered. Inorganic nanoparticles (metal and metal oxide) seem to have a greater toxic potential than organic nanoparticles (fullerenes and carbon nanotubes) on soil microorganisms. Detrimental effects of metal and metal oxide nanoparticles on microbial activity, abundance, and diversity have been demonstrated, even for very low concentrations ( 250 mg kg−1), representing a worst case scenario. Considering that most of the available literature has analyzed the impact of an acute contamination of nanoparticles using high concentrations in a single soil, several research needs have been identified, and new directions have been proposed. The effects of realistic concentrations of nanoparticles based on the concentrations predicted in modelization studies and chronic contaminations should be simulated. The influence of soil properties on the nanoparticle toxicity is still unknown and that is why it is crucial to consider the ecotoxicity of nanoparticles in a range of different soils. The identification of soil parameters controlling the bioavailability and toxicity of nanoparticles is fundamental for a better environmental risk assessment.

Impact of engineered nanoparticles on the activity, abundance, and diversity of soil microbial communities: a review.

Formation of ternary complexes between arsenic (As) oxyanions and ferric iron (Fe) complexes of humic substances (HS) is often hypothesized to represent a major mechanism for As-HS interactions under oxic conditions. However, direct evidence for this potentially important binding mechanism is still lacking. To investigate the molecular-scale interaction between arsenate, As(V), and HS in the presence of Fe(III), we reacted fulvic and humic acids with Fe(III) (1 wt %) and equilibrated the Fe(III)-HS complexes formed with As(V) at pH 7 (molar Fe/As ∼10). The local (<5 A) coordination environments of As and Fe were subsequently studied by means of X-ray absorption spectroscopy. Our results show that 4.5–12.5 μmol As(V)/g HS (25–70% of total As) was associated with Fe(III). At least 70% of this As pool was bound to Fe(III)-HS complexes via inner-sphere complexation. Results obtained from shell fits of As K-edge extended X-ray absorption fine structure (EXAFS) spectra were consistent with a monodentate binucle...

Spectroscopic evidence for ternary complex formation between arsenate and ferric iron complexes of humic substances.

The biotransformation of arsenic is highly relevant to the arsenic biogeochemical cycle. Identification of the molecular details of microbial pathways of arsenic biotransformation coupled with analyses of microbial communities by meta-omics can provide insights into detailed aspects of the complexities of this biocycle. Arsenic transformations couple to other biogeochemical cycles, and to the fate of both nutrients and other toxic environmental contaminants. Microbial redox metabolism of iron, carbon, sulfur, and nitrogen affects the redox and bioavailability of arsenic species. In this critical review we illustrate the biogeochemical processes and genes involved in arsenic biotransformations. We discuss how current and future metagenomic-, metatranscriptomic-, metaproteomic-, and metabolomic-based methods will help to decipher individual microbial arsenic transformation processes, and their connections to other biogeochemical cycle. These insights will allow future use of microbial metabolic capabilities...

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Linking Genes to Microbial Biogeochemical Cycling: Lessons from Arsenic

The mobility, bioavailability, and toxicity of metal(loid)s are influenced by their interactions with phyllosilicates, organic matter, variable charge minerals, and microorganisms. Physicochemical processes influencing the chemistry of metal(loid)s in soil environments include sorption/desorption, solution complexation, oxidation-reduction, and precipitation-dissolution reactions. In particular, the sorption/desorption reactions of metal(loid)s on/from soil sorbents are influenced by pH, nature of soil components, and presence and concentrations of cations and inorganic anions. In recent years, many extraction tests have been used for assessing trace elements mobility and phytoavailability. Chemical speciation of toxic elements may be achieved by spectroscopic analyses (XAS), which provide information about oxidation state, symmetry, and identity of the coordinating ligand environment, and possible solid phases.

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Chemical Processes Affecting the Mobility of Heavy Metals and Metalloids in Soil Environments

Natural organic matter can change As speciation via redox reactions and complexation influencing its mobility and toxicity Here we show that binary and ternary colloids and dissolved complexes of As(V), Fe and organic matter (OM) form at environmentally relevant conditions and analyzed these colloids/complexes using ATR-FTIR- and Mossbauer-spectroscopy Dissolved Fe-OM complexes and ferrihydrite-OM colloids were formed by reacting OM with ferrihydrite (Fe(OH)(3)) Mossbauer-spectroscopy showed that 95% of the Fe in the Fe-OM fraction were present as ferrihydrite-OM colloids while the remaining 5% were in the dissolved fraction In As(V) plus Fe-OM systems (containing both dissolved and colloidal Fe-OM), 35-8 microg As(V)/mg OC was bound to the Fe-OM complexes/colloids compared to <0015 microg As(V)/mg OC in As-OM systems (without Fe) Upon filtration of As-Fe-OM complexes/colloids with a 3 kDa filter, approximately 6% As was found in the dissolved fraction and approximately 94% As in colloidal Fe-OM This suggests that As(V) is associated with Fe-OM mainly via ferrihydrite-OM colloids but to a small extent also in dissolved Fe-OM complexes via Fe-bridging Since As-contaminated soils and aquifers contain Fe(III) minerals and OM, colloids of As with OM-loaded ferrihydrite and complexes of As with dissolved Fe-OM have to be considered when studying As transport

Formation of binary and ternary colloids and dissolved complexes of organic matter, Fe and As.

Natural organic matter (NOM) can affect the behavior of arsenic within surface and subsurface environments. We used batch and column experiments to determine the effect of peat humic acids (PHA), groundwater fulvic acids (GFA), and a soil organic matter (SOM) extract on As sorption/transport in ferrihydrite-coated sand columns. A reactive transport model was used to quantitatively interpret the transport of As in flow-through column (breakthrough) experiments. We found that As(III) breakthrough was faster than As(V) by up to 18% (with OM) and 14% (without OM). The most rapid breakthrough occurred in systems containing SOM and GFA. Dialysis and ultrafiltration of samples from breakthrough experiments showed that in OM-containing systems, As was transported mostly as free (noncomplexed) dissolved As but also as ternary As-Fe-OM colloids and dissolved complexes. In OM-free systems, As was transported in colloidal form or as a free ion. During desorption, more As(III) desorbed (23-37%) than As(V) (10-16%), and SOM resulted in the highest and OM-free systems the lowest amount of desorption. Overall, our experiments reveal that (i) NOM can enhance transport/mobilization of As, (ii) different fractions of NOM are capable of As mobilization, and (iii) freshly extracted SOM (from a forest soil) had greater impact on As transport than purified GFA/PHA.

Prasesh Sharma

Papers

Formation of binary and ternary colloids and dissolved complexes of organic matter, Fe and As.

Influence of Natural Organic Matter on As Transport and Retention

Desorption of arsenic from clay and humic acid-coated clay by dissolved phosphate and silicate

Surface binding site analysis of Ca2+-homoionized clay-humic acid complexes.