Simultaneous mitigation of Cd and As availability in soil-rice continuum via the addition of an Fe-based desulfurization material.

The simultaneous mitigation of toxic arsenic (As) and cadmium (Cd) in rice grain remains a global challenge. Passivation with natural or artificially modified materials has shown great potential to simultaneously reduce the bioavailability of As and Cd in paddy soils. To date, however, limited materials have are available, with unclear underling mechanisms. Here, a natural iron-based desulfurization material is hypothesized to simultaneously mitigate As and Cd availability in paddy soil-rice continuum, since it is rich in calcium (Ca), iron (Fe), Silicon (Si), manganese (Mn), and sulfur (S). The addition of the proposed material promoted rice growth and reduced soil availability of Cd (extracted with 0.01 mg·L-1 of CaCl2) by 88.0-89.6% and As (extracted with 0.5 mg·L-1 of KH2PO4) by 37.9-69.9%. Grain Cd was reduced by 26.4-51.6%, whereas that of inorganic As (iAs) by 33.3-42.7%. The increased Fe (by 44.2%) and Mn (by 178.6%) in iron plaque on the root surface were conducive to the reduction of grain Cd and iAs after application. Furthermore, the maximum adsorption capacities of the proposed material for Cd and As(III) reached 526.31 and 2.67 mg·g-1, respectively. The coprecipitation with Cd(OH)2 as a product, Fe-As and Ca-As complexation, and ion exchange of Fe2+ released by the material with Cd2+ are involved in the mechanisms underlying the available As and Cd reduction. Combining the safety, low-cost, and high accessibility, Fe-based desulfurization material showed great potential for future safe-utilization of As-Cd contaminated paddy soil via passivation.

Demethylation of arsenic limits its volatilization in fungi.

Arsenic (As) biomethylation is increasingly being regarded as a promising method to volatize As from the environment; however, the As volatilization efficiency of most microorganisms is low. Here, the speciation transformation of dimethylarsinic acid (DMA) as an important methylation intermediate in the cells of Fusarium oxysporum CZ-8F1, Penicillium janthinellum SM-12F4, and Trichoderma asperellum SM-12F1 were investigated. These fungal strains have been certified to volatilize As from As-loaded environment. In situ X-ray absorption near edge structure (XANES) indicated that demethylation of DMA with methylarsonic acid (MMA), arsenate [As(V)], and arsenite [As(III)] as intermediates or products occurred in fungal cells after exposure to DMA for 15 days. 36.7-55.7% of the original DMA could lose one or two methyl groups and be changed into MMA or inorganic As. Chromatographic separation of the cell lysates also supported these findings. Thus it comes that demethylation might be a remarkable internal factor limiting As volatilization efficiency.

Arsenic speciation transformation and arsenite influx and efflux across the cell membrane of fungi investigated using HPLC-HG-AFS and in-situ XANES.

Microorganisms dominated speciation of arsenic (As) play an important role in the biogeochemical cycling of As. In the study, species transformation of arsenite [As(III)] and As(III) influx and efflux across the cell membranes of Trichoderma asperellum SM-12F1, Penicillium janthinellum SM-12F4, and Fusarium oxysporum CZ-8F1 cells were studied using a cellular lysis plus chromatographic separation method and further the in-situ X-ray absorption near edge structure (XANES) analysis. The results indicated that As(III) can enter into fungal cells and that a portion of the As(III) can be exuded out of cells. For both As sequestrated into fungal cytoplasm and As adsorbtion onto cell walls, As(III) was found to be the dominated form of As. XANES analysis showed that As(III) accounted for 58.4%, 59.5%, and 73.0% of the total As in the cells of T. asperellum SM-12F1, P. janthinellum SM-12F4, and F. oxysporum CZ-8F1, respectively. Among these fungal strains, however, there were obvious differences in the relative proportions of arsenate [As(V)], monomethylarsonic acid (MMA), and dimethylarsinic acid (DMA). For T. asperellum SM-12F1, the proportion (%) of MMA was 31.1%, and no As(V) or DMA was detected. For F. oxysporum CZ-8F1, the proportions of As(V) and MMA were 15.8% and 8.8%, respectively, but no DMA was observed. As(V), MMA, and DMA accounted for 4.2%, 29.5%, and 8.1%, respectively, of the P. janthinellum SM-12F4 cells. Some of the intracellular As(III) can be oxidated and methylated by these fungal strains and yield As(V), MMA, and DMA as products.