Mitigating arsenic accumulation in rice (Oryza sativa L.) from typical arsenic contaminated paddy soil of southern China using nanostructured α-MnO(2): Pot experiment and field application.

Manganese oxides are naturally occurring powerful oxidants and scavengers, which can control the mobility and bioavailability of arsenic (As). However, the effect of synthetic nanostructured manganese oxides on the mobilization and transportation of As at actual paddy soils are poorly understood, especially in soils with low or medium background Mn concentration. In the present study, a novel nano manganese oxide with superior reactivity and surface area has been synthesized. A 90-d soil incubation experiment combined with pot and field rice cultivation trials were designed to evaluate the effectiveness of exogenous α-MnO2 nanorods on the mobilization and transportation of As in soil-rice systems. Our results proved that the addition of α-MnO2 nanorods can effectively control the soil-to-solution partitioning of As under anaerobic conditions. After treatment with different amounts of α-MnO2 nanorods, the content of effective As decreased, offset by an increase in residual As and insoluble binding As (Ca-As and Fe-As). Enhancing the oxidation of As(III) into As(V), the α-MnO2 nanorods increased the adsorption of As onto indigenous iron (hydr)oxides which greatly reduced the soil porewater As content. In addition, pot experiments and field applications revealed that the influx of As into the aerial parts of rice plants (stems, husk and leaves) was strictly prohibited after treatments with different amount of α-MnO2 nanorods; more interestingly, significantly negative correlations have been observed between As and Mn in rice, which indicated that as Mn is increased in soil, As in brown rice decreases. Our results demonstrated that the use of α-MnO2 nanorods in As polluted paddy soil containing low levels of background Mn oxides can be a promising remediation strategy.

Li B ，Zhou S ，Wei D ，Long J ，Peng L ，Tie B ，Williams PN ，Lei M ... -  《-》

Control of arsenic mobilization in paddy soils by manganese and iron oxides.

Reductive mobilization of arsenic (As) in paddy soils under flooded conditions is an important reason for the relatively high accumulation of As in rice, posing a risk to food safety and human health. The extent of As mobilization varies widely among paddy soils, but the reasons are not well understood. In this study, we investigated As mobilization in six As-contaminated paddy soils (total As ranging from 73 to 122 mg kg-1) in flooded incubation and pot experiments. Arsenic speciation in the solution and solid phases were determined. The magnitude of As mobilization into the porewater varied by > 100 times among the six soils. Porewater As concentration correlated closely with the concentration of oxalate-extractable As, suggesting that As associated with amorphous iron (oxyhydr)oxides represents the potentially mobilizable pool of As under flooded conditions. Soil containing a high level of manganese oxides showed the lowest As mobilization, likely because Mn oxides retard As mobilization by slowing down the drop of redox potential upon soil flooding and maintaining a higher arsenate to arsenite ratio in the solid and solution phases. Additions of a synthetic Mn oxide (hausmannite) to two paddy soils increased arsenite oxidation, decreased As mobilization into the porewater and decreased As concentrations in rice grain and straw. Consistent with previous studies using simplified model systems or pure mineral phases, the present study shows that Mn oxides and amorphous Fe (oxyhydr)oxides are important factors controlling reductive As mobilization in As-contaminated paddy soils. In addition, this study also suggests a potential mitigation strategy using exogenous Mn oxides to decrease As uptake by rice in paddy soils containing low levels of indigenous Mn oxides, although further work is needed to verify its efficacy and possible secondary effects under field conditions.

Xu X ，Chen C ，Wang P ，Kretzschmar R ，Zhao FJ ... -  《-》

Reduced arsenic accumulation in indica rice (Oryza sativa L.) cultivar with ferromanganese oxide impregnated biochar composites amendments.

The effects of biochar (BC) and ferromanganese oxide biochar composites (FMBC1 and FMBC2) on As (Arsenic) accumulation in rice were determined using a pot experiment. Treatments with BC or FMBC improved the dry weights of rice roots, stems, leaves, and grains in soils containing different As contamination levels. Compared to BC treatment, FMBC treatments significantly reduced As accumulation in different parts of the rice plants (P < 0.05), and FMBC2 performed better than FMBC1 did. Furthermore, exposure to 2% FMBC2 decreased the total As concentration in the grain by 68.9-78.3%. The addition of FMBC increased the ratio of essential amino acids in the grain, decreased As availability in the soil, and significantly increased the Fe and Mn plaque contents. The reduced As accumulation in rice can be attributed to As(III) to As(V) oxidation by ferro - manganese binary oxide, which increased the As adsorbed by FMBC. Furthermore, Fe and Mn plaques on the rice root surface decreased the transport of As in rice. Taken together, our results demonstrated the applicability of FMBC as a potential measure for reducing As accumulation in rice, improving the amino acid content of rice grains, and effectively remediating As-polluted soil.

Lin L ，Gao M ，Qiu W ，Wang D ，Huang Q ，Song Z ... -  《-》

Effects of manganese oxide-modified biochar composites on arsenic speciation and accumulation in an indica rice (Oryza sativa L.) cultivar.

A pot experiment was used to investigate arsenic (As) speciation and accumulation in rice, as well as its concentration in both heavily contaminated and moderately contaminated soils amended with manganese oxide-modified biochar composites (MBC) and biochar alone (BC). In heavily As-contaminated soil, application of BC and MBC improved the weight of above-ground part and rice root, whereas in moderately As-contaminated soil, the application of MBC and low rate BC amendment increased rice root, grain weight and the biomass of the plant. Arsenic reduction in different parts of rice grown in MBC-amended soils was greater than that in plants cultivated in BC-amended soils. Such reduction can be attributed to the oxidation of arsenite, As(III), to arsenate, As(V), by Mn-oxides, which also had a strong adsorptive capacity for As(V). MBC amended to As-contaminated soil had a positive effect on amino acids. The Fe and Mn levels in the iron-manganese plaque that formed on the rice root surface differed among the treatments. MBC addition significantly increased Mn content (p < 0.05); the application of 2.0% MBC increased Mn content 36- and 10-fold compared to the control in heavily and moderately As-contaminated soils, respectively. The results indicate that application of Mn oxide-modified biochar to As-contaminated paddy soil could effectively remediate contaminated soil and reduce As accumulation in edible parts of rice.

Yu Z ，Qiu W ，Wang F ，Lei M ，Wang D ，Song Z ... -  《-》

The translocation of antimony in soil-rice system with comparisons to arsenic: Alleviation of their accumulation in rice by simultaneous use of Fe(II) and NO(3)().

Antimony (Sb) accumulation in rice grains is a potential risk to human health. This study aims to develop agronomic practices that can reduce the accumulation of Sb in rice grain in contaminated soil. A pot culture experiment was conducted to investigate the effects of co-application of ferrous iron and nitrate (Fe(II) + NO3-) in paddy soils on Sb uptake by rice. The co-application of Fe(II) and NO3- promoted abiotic/biotic Fe(II) oxidation and mineralization in the rhizosphere soil and formation of Fe plaques, consequently, Sb bioavailability was significantly reduced by enhancing Sb immobilization on the newly formed Fe(III) (hydr)oxides. The results were compared with those for arsenic (As) in the same trial and it was shown that the two metalloids have different translocation behavior in the soil-rice plant system. The adsorption of Sb, especially the Sb(V), on Fe(III) (hydr)oxides was more significantly enhanced by the decreased soil pH after the application of Fe(II) + NO3- than that of As. The uptake of Sb by the roots of rice was much more difficult but it was much easier to be transported from the rice straw to the grains compared to As. The differences might be mainly caused by the different uptake mechanisms of Sb and As by rice plants from paddies. The bioavailable As(III) would be much more efficient in entering into the rice roots than Sb(III) through the aquaporin channel due to its much smaller ionic radius; the bioavailable As(V), entering into the rice roots via phosphate transporters, would also be more efficient in taking up by roots than Sb(V), which pathway from soil to rice roots remains unclear. These findings provide new insights into Sb biogeochemical behavior in soil-rice plant systems and demonstrate that co-application of Fe(II) and NO3- could be a promising strategy for safely-utilizing Sb contaminated sites in the future.

Wang X ，Li F ，Yuan C ，Li B ，Liu T ，Liu C ，Du Y ，Liu C ... -  《-》