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Root exudates are crucial for plants returning organic matter to soils, which is assumed to be a major source of carbon for the soil microbial community. This study investigated the influence of root exudates on the fate of arsenic (As) with a lab simulation experiment. Our findings suggested that root exudates had a dose effect on the soil physicochemical properties, As speciation transformation and the microbial community structure at different concentrations. The addition of root exudates increased the soil pH while decreased the soil redox potential (Eh). These changes in the soil pH and Eh increased As and ferrous (Fe(II)) concentrations in soil porewater. Results showed that 40 mg/L exudates addition significantly increased arsenite (As(III)) and arsenate (As(V)) by 541 and 10 times respectively within 30 days in soil porewater. The relative abundance of Fe(III)-reducing bacteria Geobacter and Anaeromyxobacter increased with the addition of root exudates, which enhanced microbial Fe reduction. Together these results suggest that investigating how root exudates affect the mobility and transformation of As in paddy soils is helpful to systematically understand the biogeochemical cycle of As in soil-rice system, which is of great significance for reducing the health risk of soil As contamination.

Arsenic (As) is a priority environmental pollutant in paddy field. The coupling of arsenate (As(V)) reduction with anaerobic methane (CH

Organic matter plays an important role in controlling arsenic(As) release and transformation in soil, however, little is known about the effect of manure application on As behavior in soils with different As contents. In this study, waterlogged incubations using various As-contaminated paddy soils with manure amendment were conducted to investigate how manure application influence As mobilization and methylation in different paddy soils. The results indicated that manure application increased As release in paddy soils with high As (> 30 mg kg−1) contents. Moreover, our findings also showed that manure application increased the relative abundance of arsM-harboring Euryacheota and Planctomycetes at the phylum level and arsM-harbouring Methanocellaceae, Anaerolinea and Bellinea at genus level, thereby promoting As methylation. These results provide important insights for the significant variation in As mobilization and methylation in paddy soils amended with manure. Moreover, our results suggest that serious consideration should be given to the manure application in As-contaminated paddy soil.

Rice is among the most important staple foods worldwide. However, the consumption of rice and rice products poses a potential health risk since rice can hyperaccumulate arsenic (As) and other heavy metals in their grains. In The Bahamas, although rice is heavily consumed, it is not grown locally. Instead, all the consumed rice and its derived products are imported. Recent food surveys in the major rice exporting countries have shown that a significant portion of their market rice products is contaminated with As. However, to date, the prevalence of As in the rice foods available in The Bahamas remains unknown. Therefore, in this study we surveyed the occurrence of As in a selection of rice and rice products that were on sale in the Bahamian market. A total of 21 different rice brands collected. The concentration of As and the potential health risk were estimated by target hazard quotient (THQ), hazard index (HI), and Lifetime Cancer Risk (LCR). Our results showed that only 2% of the samples had an estimated inorganic arsenic (iAs) concentration above the World Health Organization (WHO) safety limits (200 µg/kg) - which is based on global average consumption. However, when we factor for average rice consumption in The Bahamas (486.5 g), 79% of the rice samples had iAs concentration values indicative of carcinogenic risks and 57% had iAs concentration values that suggested non-carcinogen health risks. Based on our results, we recommend urgent follow up studies to further test rice varieties that show the greatest LCR and HI values and to also broaden the study to include more off-brand/generic varieties, cooked rice as well as drinking water.

Anaerobic methane oxidation (AOM) coupled to nitrate, sulfate and iron has been most extensively studied. Recently, AOM coupled with arsenate reduction (AOM-AsR) was demonstrated in laboratory microcosm incubation, however whether AOM-AsR is active in the field conditions remains elusive. Here, we used

To examine the rain-fed Aman rice yield fluctuation due to climatic anomalies overtimes in Bangladesh, we used climate-induced yield index (CIYI), ensemble empirical mode decomposition (EEMD), step-wise multiple regression, isotopic signature, wavelet transform coherence (WTC) and random forest (RF) model. In this work, daily multiple source climatic data which were collected between 1980 and 2017, from 18 weather stations and five atmospheric circulation indices were used for this purpose. The key findings were as follows; by employing principal component analysis (PCA), six temporal variability modes were identified as six corresponding sub-regions with various Aman rice CIYI fluctuations. The Aman rice CIYI in different sub-regions represented a noteworthy 3–4-year quasi-oscillation using the EEMD. The key climate variables (KCVs) including the potential evapotranspiration and cloud cover in September, the minimum temperature in August, and precipitation in July, August, and October were the best rice yield prediction signals in these sub-regions. The results suggest that Aman rice yield could likely decline by 33.59%, and 3.37% in the southwestern and southeastern regions, respectively, if KCV increased by 1 °C or 1%. The RF model suggests that the Indian Ocean Dipole (IOD) significantly influenced the rice yield. Isotopic signatures were employed to confirm the fluctuation and anti-amount effect during the Aman rice-growing period in Bangladesh. The results obtained in this study could be used as a guideline for sustainable mitigation and adaptation measures in managing agro-meteorological hazards in Bangladesh.

Microplastics (MPs) and sulfamethoxazole (SMX) are emerging contaminants that are ubiquitous in the soil environment. In this study, we investigated MPs polymer type and soil environmental factor effects on SMX adsorption behavior in the soil system. Our results showed that MPs dosage affected the soil particles' SMX adsorption rate and capacity (Qe). Adding 1 % polystyrene (PS) increased the SMX adsorption rate significantly. The value of K

Water contaminated with emerging pollutants has become a serious environmental issue globally. Biochar is a porous and carbon-rich material produced from biomass pyrolysis and has the potential to be used as an integrated adsorptive material. Many studies have shown that biochar is capable to adsorb emerging pollutants from aquatic systems and could be used to solve the water pollution problem. Here, we provided a dual perspective on removing emerging pollutants from aquatic systems using biochar and analyzed the emerging pollutant removal efficiency from the aspects of biochar types, pollutant types and coexistence with heavy metals, as well as the associated mechanisms. The potential risks and future research directions of biochar utilization are also presented. This review aims to assist researchers interested in using biochar for emerging pollutants remediation in aquatic systems and facilitate research on emerging pollutants removal, thereby reducing their environmental risk.

Potentially toxic elements (PTEs) contamination in Wainivesi River, Fiji triggered by gold-mining activities is a major public health concern deserving attention. However, chemometric approaches and pattern recognition of PTEs in surface water and sediment are yet hardly studied in Pacific Island countries like Fijin urban River. In our study, twenty-four sediment and eight water sampling sites from the Wainivesi River, Fiji were explored to evaluate the spatial pattern, eco-environmental pollution, and source apportionment of PTEs. This analysis was done using an integrated approach of self-organizing map (SOM), principle component analysis (PCA), hierarchical cluster analysis (HCA), and indexical approaches. The PTEs average concentration is decreasing order of Fe > Pb > Zn > Ni > Cr > Cu > Mn > Co > Cd for water and Fe > Zn > Pb > Mn > Cr > Ni > Cu > Co > Cd for sediment, respectively. Outcomes of eco-environmental indices including contamination and enrichment factors, and geo-accumulation index differed spatially indicated that majority of the sediment sites were highly polluted by Zn, Cd, and Ni. Cd and Ni contents can cause both ecological and human health risks. According to PCA, two sources such as point and geogenic sources for sediment and also two sources including weathering of rock-water and anthropogenic sources of PTEs for water were identified in the study area. The SOM analysis identified three spatial patterns e.g., Cr-Co-Zn-Mn, Fe-Cd and Ni-Pb-Cu in water and Zn-Cd-Cu-Mn, Cr-Ni and Fe, Co-Pb in sediment. Spatial distribution of entropy water quality index values depicted that northern and northwestern areas possess ‘poor’ to ‘extremely-poor’ quality water. The entropy weights indicated Zn, Cd and Cu as the major pollutants in deteriorating the water quality. Our finding provides a baseline database with eco-environmental and health risk measures for the Wainivesi river contamination.

Arsenic (As) is a toxic metalloid that is ubiquitous in paddy soils, where passivation is the most widely used method for remediating As contamination. Recently, anaerobic methane oxidation coupled with arsenate (As(V)) reduction (AOM-AsR) has been shown to act as a critical driver for As release in paddy fields. However, the effect and mechanism of the passivators on the AOM-AsR process remain unclear. In this study, we incubated arsenate-contaminated paddy soils under anaerobic conditions. Using isotopically labelled methane and different passivators, we found that an iron-based passivator containing calcium sulfate and iron oxide (9:1, m/m) named IBP showed a much better performance than the other passivators. Adding IBP decreased the arsenite (As(III)) concentration in the soil solution by 78% and increased the AOM rate by 55%. Furthermore, we employed high-throughput sequencing and real-time quantitative polymerase chain reaction (qPCR) to investigate the ability of IBP to control As release mediated by AOM-AsR in paddy fields, as well as its underlying mechanism. Our results showed that IBP addition significantly increased anaerobic methanotrophic (ANME) archaea (ANME-2a-c, ANME-2d, and ANME-3) by 91%, and increased the methane-oxidizing bacterium Methylobacter by 262%. Similarly, IBP addition significantly increased the Fe(III) concentration in soil solution by 39% and increased the absolute abundance of Fe(III)-reducing bacteria (Geobacteraceae) by 21 times in soil. Adding IBP may significantly promote AOM coupled with Fe(III) reduction, significantly reducing electron transfer from AOM to As(V) reduction. Hence, IBP may be used as an efficient passivator to remediate As-contaminated soil using an active AOM-AsR process. These results provide a novel insight into controlling soil As release by regulating an active and critical As mobilization pathway in the environment.

Arsenic (As) pollution in paddy fields is a major threat to rice safety. Existing As remediation techniques are costly, require external chemical addition and degrade soil properties. Here, we report the use of plastic tubes as a recyclable tool to precisely extract As from contaminated soils. Following insertion into flooded paddy soils, polyethylene tube walls were covered by thin but massive Fe coatings of 76.9-367 mg Fe m-2 in 2 weeks, which adsorbed significant amounts of As. The formation of tube-wall Fe oxides was driven by local Fe-oxidizing bacteria with oxygen produced by oxygenic phototrophs (e.g., Cyanobacteria) or diffused from air through the tube wall. The tubes with As-bound Fe oxides can be easily separated from soil and then washed and reused. We tested the As removal efficiency in a pot experiment to remove As from ~ 20 cm depth/40 kg soils in a 2-year experiment and achieved an overall removal efficiency of 152 mg As m-2 soil year-1, comparable to phytoremediation with the As hyperaccumulator Pteris vittata. The cost of Fe hooks was estimated at 8325 RMB ha-1 year-1, and the profit of growing rice (around 16080 RMB ha-1 year-1 can be still maintained. The As accumulated in rice tissues was markedly decreased in the treatment (>11.1%). This work provides a low-cost and sustainable soil remediation method for the targeted removal of As from soils and a useful tool for the study and management of the biogeochemical Fe cycle in paddy soils.

Rice (Oryza sativa L.) consumption is a major route of dietary exposure to arsenic (As) in humans. One main reason for the high accumulation of As in rice grain is the high bioavailability of As in porewater of flooded paddy soil. Recently, it has been shown that the application of soil microbial fuel cell (sMFC) can significantly reduce soil porewater As concentration, however, the effect of sMFC on As accumulation in rice is unknown. Hence, this study was aimed at reducing the As uptake in rice grown in As contaminated soil by sMFCs. A pot experiment was performed to investigate As distribution in rice tissues and the functional microbial communities in soil when the sMFC was installed. The As in the soil porewater and rice plant parts were analyzed. 16S rRNA sequencing and Quantitative PCR were used to examine the microbial community and to quantify the relative abundance of As resistance genes in the rhizosphere, respectively. The results suggest that the sMFC can simultaneously work as an electricity generator and As mitigator. The total As concentrations in the stems, leaves, husks, and rice grains were significantly decreased by 53.4%, 44.7%, 62.6%, and 67.9%, respectively in the plants with sMFC compared to the control. This decrease in As accumulation in the sMFC treatment may be explained by the decrease in the soil porewater dissolve organic matter content and abundance of As reducing gene (arsC). Moreover, known As reducing classes such as Clostridia, Bacilli and Thermoleophilia were significantly enhanced in the control treatment. Integrating the sMFC in rice paddy soil offers a promising way to mitigate As accumulation in rice tissue and reduce dietary As exposure, while simultaneously producing electricity.

River and canal encroachments have become a common problem in Bangladesh. The prevalence of river and canal encroachments has a direct impact on population growth. However, the impacted population appears to be unaware of its negative consequences. To this end, we aimed to investigate the past and present canal scenarios with a focus on canal encroachment as the influencing factor in Ulipur upazila (highly vulnerable) of northern Bangladesh. To examine the impact of this encroachment, the Flood Vulnerability Index (FVI), Adaptive Capacity Index (ACI) and Economic Vulnerability Index (EVI) were used. Our results showed signs of narrowing of the canal structure of Ulipur upazila in 1982 and 1992, while satellite images from 2002 and 2012 showed the presence of encroachment in the middle and lower parts of the canal. The FVI value for Hatia union was 0.703, indicating that this area was highly vulnerable to flooding. According to the ACI, the Pandul union has a high capacity to cope with flood impact, while the Hatia union has less capacity to cope with flood impact. Conversely, Hatia’s union EVI value was 72.8, denoting a high economic vulnerability. Canal encroachments will have negative consequences for these impacted unions. It is critical to reduce the flooding and economic vulnerabilities associated with canal encroachments. It is found that canal excavation is very important for controlling flood water and reducing the damage caused by flooding.

AimsThe biogeochemical cycles of elements from soils to plants are mainly governed by their rhizosphere processes. Understanding these processes is challenging and remains largely unresolved due to the complex interrelationships among different elements and due to a lack of appropriate techniques for simultaneous spatiotemporal monitoring.MethodsThis study employed an In-situ Porewater Iterative (IPI) sampler array (0-22 mm measurement distance every 1.7 mm, with a time interval of 3 to 10 days) to capture the in situ spatiotemporal dynamics of ten elements (Fe, Mn, As, P, S, Cr, Co, Zn, Sb and Cd) in the paddy rhizosphere to examine their covarying changes in time and space dimensions.ResultsThe findings revealed that the solute-phase concentration of most elements, other than Sb and Cd, increased to a peak after 30 days of paddy soil flooding and then decreased. Additionally, Sb and Cd continuously decreased during flooding. Fe (-52%), Mn (-17%), P (-43%), Co (-11%), and As species (-74%) were substantially immobilized within a 10 mm zone around the roots, while Zn (28%) and Cd (41%) increased. The greater immobilization of As and re-mobilization of Cd, in the rhizosphere, are stimulated by biotic oxidation of arsenite to arsenate with root oxygen loss and the pH decrease, respectively.ConclusionsOur study showed most sampled elements covaried with Fe both in time and space in the rhizosphere, but the elements are temporally and spatially determined by multiple biogeochemical processes in soils as well as exudates from plant roots.

Mining activity is a growing environmental concern as it contributes to heavy metals (HMs) pollution in agricultural soils. Microbial communities play an important role in the biogeochemical cycling of HMs and have the potential to be used as bioindicators. Arsenic (As) and lead (Pb) are the most hazardous HMs and are mainly originated from mining activities. However, spatial variation in microbial community in response to As and Pb contamination in paddy soils remains overlooked. In this study, the biological and chemical properties of sixteen soil samples from four sites (N01, N02, N03, and N04) near a Pb-Zn mining site at different As and Pb levels were examined. The results showed that soil pH, total As and Pb, bioavailable As and Pb, nitrate-nitrogen (NO3−-N) and ammonia-nitrogen (NH4+-N) were the most important factors in shaping the bacterial community structure. In addition, significant correlations between various bacterial genera and As and Pb concentrations were observed, indicating their potential roles in As and Pb biogeochemical cycling. These findings provide insights into the variation of paddy soil bacterial community in soils co-contaminated with different levels of As and Pb.

The sharp redox gradient at soil-water interfaces (SWI) plays a key role in controlling arsenic (As) translocation and transformation in paddy soils. When Eh drops, As is released to porewater from solid iron (Fe) and manganese (Mn) minerals and reduced to arsenite. However, the coupling or decoupling processes operating within the redox gradient at the SWI in flooded paddy soils remain poorly constrained due to the lack of direct evidence. In this paper, we reported the mm-scale mapping of Fe, As and other associated elements across the redox gradient in the SWI of five different paddy soils. The results showed a strong positive linear relationship between dissolved Fe, Mn, As, and phosphorus (P) in 4 out of the 5 paddy soils, indicating the general coupling of these elements. However, decoupling of Fe, Mn and As was observed in one of the paddy soils. In this soil, distinct releasing profiles of Mn, As and Fe were observed, and the releasing order followed the redox ladder. Further investigation of As species showed the ratio of arsenite to total As dropped from 100% to 75.5% and then kept stable along depth of the soil profile, which indicates a dynamic equilibrium between arsenite oxidization and arsenate reduction. This study provides direct evidence of multi-elements' interaction along redox gradient of SWI in paddy soils.

Periphyton is ubiquitous in rice paddy fields, however its role in paddy soil arsenic (As) biogeochemistry remains unexplored. In this study, microcosm incubations and extensive field sampling were used to better understand the roles of periphyton on As mobility and transformation at the soil-water interface. Microcosm incubations revealed that periphyton on the paddy soil surface enhanced As release to water and increased methylated As contents at the soil-water interface. Experimental additions of dissolved phosphate did not significantly affect these processes. The presence of periphyton increased the dissolved organic carbon (DOC) content of the surface soil which may have played a role in the increased As mobility. However, the increase in methylated As species at the soil-water interface is indicative of detoxification processes of As by periphyton. The results from the field study revealed a high abundance and diversity of As biotransformation and detoxification genes in periphyton. Genera of Kineosporia, Limisphaera, Ornatilinea, Ktedonosporobacter and Anaerolinea played key roles in shaping arsM harboring microbe communities in field periphyton. These results highlight the importance of periphyton in the behavior of As in paddy soils and can potentially facilitate improved management of As contamination in paddy soils.

Bioelectrochemical approaches offer a simple, effective, and environmentally friendly solution to pollutant remediation. As a versatile technology, although many studies have shown its potential in soil heavy metal(loid) remediation, the mechanism behind this process is not simple or well-reviewed. Thus, in this review we summarized the impacts of the microbial fuel cells (MFCs) on metal (loids) movement and transformation in the soil environment in terms of changes in soil pH, electromigration, and substrate competition between anode-respiring bacteria and the soil microbial community. Furthermore, the progress of MFCs in the fixation/removal of different elements from the soil environment is described. Hence, this review provides critical insight into the use of the MFC for soil metal(loid) bioremediation.

Global warming severely hinders both rice (Oryza sativa L.) quality and yield by increasing arsenic (As) bioavailability in paddy soils. However, details regarding As biotransformation and migration in the rice-soil system at elevated temperatures remain unclear. This study investigated the effects of increasing temperature on As behavior and translocation in rice grown in As-contaminated paddy soil at two temperature treatments (33 °C warmer temperature and 28 °C as control). The results showed that increasing temperature from 28 °C to 33 °C significantly favored total As, arsenite (As(III)) and arsenate (As(Ⅴ)) release into the soil pore-water. This increase in As bioavailability resulted in significantly higher As(III) accumulation in the whole grains at warmer treatment relative to the control. Moreover, the results suggest that increasing temperature to 33 °C promoted As(III) migration from the roots to the whole grains. Furthermore, the As(V)-reducing Xanthomonadales order and Alcaligenaceae family, and As(V) reductase-encoding arsC gene were enriched in the rhizosphere soils incubated at 33 °C. This suggests that the increase in As bioavailability in that treatment was due to enhanced As(V) reductive dissolution into the soil pore-water. Overall, this study provides new insights on how warmer future temperatures will exacerbate As accumulation in rice grains.

Biochar is widely used as an amendment to optimize the composting process. In this study, we firstly investigated the effects of biochar amendment on methylation and volatilization of arsenic (As), and the microbial communities during manure composting. Biochar amendment was found to increase the concentrations of monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) during mesophilic (days 0–10) and early thermophilic (days 11–15) phases, and promote As volatilization during the maturing phase (days 60–80) of composting. In addition, the abundances of As(V) reductase (arsC) and As(III) S-adenosyl- L -methionine methyltransferase (arsM) genes were higher in the biochar treatment than that in the control. Moreover, biochar amendment influenced the microbial communities by promoting As methylation and volatilization via Ensifer and Sphingobium carrying arsC genes, and Rhodopseudomonas and Pseudomonas carrying arsM genes. This study emphasized the considerable role of biochar on methylation and volatilization of As during manure composting and provided an overall characterization of the community compositions of arsC and arsM genes during manure composting. It will broaden our insights in As biogeochemical cycle during manure composting with biochar amendment, which will facilitate the regulation of As during manure composting and its application in agricultural soil.

A new calcium-modified and starch-stabilized ferromanganese binary oxide (Ca-SFMBO) sorbent was fabricated with different Ca concentrations for the adsorption of arsenic (As) and cadmium (Cd) in water. The maximum As(III) and Cd(II) adsorption capacities of 1% Ca-SFMBO were 156.25 mg/g and 107.53 mg/g respectively in single-adsorption systems. The adsorption of As and Cd by the Ca-SFMBO sorbent was pH-dependent at values from 1 to 7, with an optimal adsorption pH of 6. In the dual-adsorbate system, the presence of Cd(II) at low concentrations enhanced As(III) adsorption by 33.3%, while the adsorption of As(III) was inhibited with the increase of Cd(II) concentration. Moreover, the addition of As(III) increased the adsorption capacity for Cd(II) up to two-fold. Through analysis by X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR), it was inferred that the mechanism for the co-adsorption of Cd(II) and As(III) included both competitive and synergistic effects, which resulted from the formation of ternary complexes. The results indicate that the Ca-SFMBO material developed here could be used for the simultaneous removal of As(III) and Cd(II) from contaminated water.

Soil microbial fuel cells (sMFC) are novel technique that uses organic matters in soils as an alternative energy source. External resistance (ER) is a key factor that influences sMFC performance and also alters the soil biological and chemical reactions. However, little information is available on how the microbial community and soil component changes in sMFC with different ER. Thus, the purpose of this study is to collectively examine the effects of different ER on paddy soil biotic and abiotic components. Eighteen paddy sMFC were constructed and operated at five different ER (2000, 1000, 200, 80, and 50 Ω) in triplicates for 90 days. The effects of the sMFC anodes at different ER were examined by measuring organic matter (OM) removal efficiency, trace elements in porewater, and bacterial community structure in contaminated paddy soil. The results indicate that ER has significant effects on sMFC power production, OM removal efficiency, and bacterial beta diversity. Moreover, ER influences iron, arsenic, and nickel concentration as well in soil porewater. In particular, greater current densities were observed at lower ER (2.6 mA, 50 Ω) as compared to a higher ER (0.3 mA, 2000 Ω). The removal efficiency of OM increased with decreasing ER, whereas it decreased with soil distance away from the anode. Furthermore, principal coordinate analysis (PCoA) revealed that ER may shape the bacterial community that develop in the anode vicinity but have minimal effect on that of the bulk soil. The current study illustrates that lower ER can be used to selectively enhance the relative abundance of electrogenic bacteria and lead to high OM removal.

Cadmium (Cd) contamination in cropland poses a serious threat to food safety and human health. In this study we investigated the effects of grafting on Cd accumulation in the shoots of Solanum melongena (eggplant), Solanum lycopersicum (tomato) and Capsicum annuum L. (pepper) scion plants. The Cd mitigation effects were investigated in the un-grafted, self-grafted and grafted plants in both pot and field experiments. The results showed that grafting onto Solanum torvum rootstock can significantly decreased Cd accumulation in the shoots of eggplant, tomato and pepper scions by up to 85%. Moreover, our results demonstrated that the grafting operation did not significantly contribute to Cd accumulation in the scion plant shoots. However, using S. torvum as rootstock was significantly more effective in reducing Cd accumulation in tomato and pepper leaves as compared to using tomato cultivar Aimiao 2 or pepper cultivar Aimiao 1 as rootstocks. This suggests that the rootstock was responsible for lower Cd accumulation in the three scion plant shoots. Furthermore, there was a significant increase in yield and quality of plants grafted with S. torvum as rootstock. Overall, our finding suggests that grafting could be simultaneously used as a mitigation strategy to reduce Cd accumulation in the Solanaceae plant shoots and promote crop quality and yield. This study provides important insights for the safe use of low Cd-contaminated soils.

In situ immobilization of heavy metals in contaminated soils using industrial by-products is an attractive remediation technique. In this work, titanium gypsum (TG) was applied at two levels (TG-L: 0.15% and TG-H: 0.30%) to simultaneously reduce the uptake of cadmium (Cd), lead (Pb) and arsenic (As) in rice grown in heavy metal contaminated paddy soils. The results showed that the addition of TG significantly decreased the pH and dissolved organic carbon (DOC) in the bulk soil. TG addition significantly improved the rice plants growth and reduced the bioavailability of Cd, Pb and As. Particularly, bioavailable Cd, Pb and As decreased by 35.2%, 38.1% and 38.0% in TG-H treatment during the tillering stage, respectively. Moreover, TG application significantly reduced the accumulation of Cd, Pb and As in brown rice. Real-time PCR analysis demonstrated that the relative abundance of sulfate-reducing bacteria increased with the TG application, but not for the iron-reducing bacteria. In addition, 16S rRNA sequencing analysis revealed that the relative abundances of heavy metal-resistant bacteria such as Bacillus, Sulfuritalea, Clostridium, Sulfuricella, Geobacter, Nocardioides and Sulfuricurvum at the genus level significantly increased with the TG addition. In conclusion, the present study implied that TG is a potential and effective amendment to immobilize metal(loid)s in soil and thereby reduce the exposure risk of metal(loid)s associated with rice consumption.

[Image: see text] In flooded soils, soil–water interface (SWI) is the key zone controlling biogeochemical dynamics. Chemical species and concentrations vary greatly at micro- to cm-scales. Techniques able to track these changing element profiles both in space and over time with appropriate resolution are rare. Here, we report a patent-pending technique, the Integrated Porewater Injection (IPI) sampler, which is designed for soil porewater sampling with minimum disturbance to saturated soil environment. IPI sampler employs a single hollow fiber membrane tube to passively sample porewater surrounding the tube. When working, it can be integrated into the sample introduction system, thus the sample preparation procedure is dramatically simplified. In this study, IPI samplers were coupled to ICP-MS at data-only mode. The limits of detection of IPI-ICP-MS for Ni, As, Cd, Sb, and Pb were 0.12, 0.67, 0.027, 0.029, and 0.074 μg·L(–1), respectively. Furthermore, 25 IPI samplers were assembled into an SWI profiler using 3D printing in a one-dimensional array. The SWI profiler is able to analyze element profiles at high spatial resolution (∼2 mm) every ≥24 h. When deployed in arsenic-contaminated paddy soils, it depicted the distributions and dynamics of multiple elements at anoxic–oxic transition. The results show that the SWI profiler is a powerful and robust technique in monitoring dynamics of element profile in soil porewater at high spatial resolution. The method will greatly facilitate studies of elements behaviors in sediments of wetland, rivers, lakes, and oceans.

The optical properties of black carbon aerosols (BC) are determined by the particles size and the associated non-BC materials, which may be source-related or modified during secondary processing. The one-year long monitoring of BC was first conducted using a Single Particle Soot Photometer (SP2) from December 2013 to November 2014 in Nanjing, a megacity in the Yangtze River Delta region of China. The seasonal variation in the BC size distribution and mixing state were investigated. There was no apparent systematic variation in the mean BC core mass median diameter between seasons, as these values were 226 +/- 12 nm, 217 +/- 13 nm, 211 +/- 15 nm and 221 +/- 12 nm for winter, spring, summer and autumn respectively. The mixing state of BC was quantified as the bulk relative coating thickness (defined as particle size D(p )over core size D-c, D-p/D-c), which ranged from 1.05 to 2.65. The BC was found to be significantly more coated in the winter (D-p/D-c = 1.50 +/- 0.30) than in other seasons (D-p/D-c = 1.27 +/- 0.09, 1.28 +/- 0.10, 1.27 +/- 0.11 in spring, summer and autumn respectively). Higher levels of coating during the winter may due to the contributions of the primary source (with the highest BC mass loadings between seasons) or secondary processes such as low temperature that facilitated the condensation. It was found that the photochemical process may enhance the coatings on BC in summer. At nighttime, the reduced and stabilized planetary boundary layer and the nighttime secondary formation may also lead to BC becoming well mixed with other components. Moreover. BC was shown to be less coated when the NOx concentration was high. However, during all seasons, the BC coating was strongly correlated with other non-BC particulate mass, which suggests that at higher pollution levels BC was more significantly coated with other existing materials through coagulation or condensation by other secondary species.

The increase in toxic heavy metal pollutants in rice paddies threatens food safety. There is an urgent need for lnow-cost remediation technology for immobilizing these trace metals. In this study, we showed that the application of the soil microbial fuel cell (sMFC) can greatly reduce the accumulation of Cd, Cu, Cr, and Ni in the rice plant tissue. In the sMFC treatment, the accumulation of Cd, Cu, Cr, and Ni in rice grains was 35.1%, 32.8%, 56.9% and 21.3% lower than the control, respectively. The reduction of these elements in the rice grain was due to their limited mobility in the soil porewater of soils employing the sMFC. The restriction in Cd, Cu, Cr, and Ni bioavailability was ascribed to the sMFC ability to immobilize trace metals through both biotic and abiotic means. The results suggest that the sMFC may be used as a promising technique to limit toxic trace metal bioavailability and translocation in the rice plants.

Efficient cathode performance is crucial for the power generation of microbial fuel cells (MFCs). Therefore, the development of a cost effective and efficient cathode is urgently needed for the wider application of MFCs. In this study, a novel biochar air cathode with naturally porous structure and high oxygen reduction reaction (ORR) activity prepared from the pyrolysis of Balsa Wood chip at 800 °C was proposed. The biochar chips can be installed directly in the MFC as an air cathode without the need to apply expensive catalysts, binders and a gas diffusion layers. In the single chamber MFC tests, the biochar air cathode achieved a maximum power density of 200 mW/m2 which was comparable to those of the other biochar based air cathodes. Moreover, when they were used in soil MFCs, our results showed that the chip thickness had effect on the performance of the soil MFCs. The 3.5 mm biochar produced the highest power density of 72 mW/m2, which was 45% higher than that of the soil MFC equipped with common carbon felt floating cathode. These findings demonstrate for the first time, that whole biochar chip could be considered as an effective and environmental-friendly material for the production of binder- and catalysis-free air cathode.

Arsenic (As) mobility in paddy soils is mainly controlled by iron (Fe) oxides and iron reducing bacteria (IBR). The Fe reducing bacteria are also considered to be enriched on the anode of soil microbial fuel cells (sMFC). Thus, the sMFC may have an impact on elements' behavior, especially Fe and As, mobilization and immobilization in paddy soils. In this study, we found dissolved organic matter (DOC) abundance was a major determinate for the sMFC impact on Fe and As. In the constructed sMFCs with and without water management, distinctive behaviors of Fe and As in paddy soil were observed, which can be explained by the low or high DOC content under different water management. When the sMFC was deployed without water management, i.e. DOC was abundant, the sMFC promoted Fe and As movement into the soil porewater. The As release into the porewater was associated with the enhanced Fe reduction by the sMFC. This was ascribed to the acidification effect of sMFC anode and the increase of Fe reducing bacteria in the sMFC anode vicinity and associated bulk soil. However, when the sMFC was coupled with alternating dry-wet cycles, i.e. DOC was limited, the Fe and As concentrations in the soil porewater dramatically decreased by up to 2.3 and 1.6 fold, respectively, compared to the controls under the same water management regime. This study implies an environmental risk for the in-situ application of sMFC in organic matter rich wetlands and also points out a new mitigation strategy for As management in paddy soils.

Soil Microbial Fuel Cells (MFCs) are devices that can generate electricity by using the flooded soil's anode respiring microbial consortium . When the MFC starts to work, the microbial community in the anode vicinity rapidly changes. This shift in the microbial community results in many dead cells that may release their DNA (relic DNA) and obscure culture independent estimates of microbial community composition. Although relic DNA is expected to increase in MFCs, the effect of relic DNA has not been investigated in the soil MFCs system. In this study the effect of the MFCs on the soil microbial community composition within the soil profile and the influence of relic DNA were investigated. Microbial community analysis revealed that the MFCs deployment significantly influenced the community composition within the soil profile. The phylum Proteobacteria (34.4% vs 23.6%) and the class Deltaproteobacteria (16.8% vs 5.9%) significantly increased in the MFCs compared to the control, while the phylum Firmicutes (24.0% vs 28.7%) and the class Sphingobacteria (5.3% vs 7.0%) were more abundant in the control. Furthermore, the archaeal phyla Euryarchaeota (40.7% vs 52.3%) and Bathyarchaeota (10.1% vs 17.3%) were significantly lower in the MFCs, whereas the phylum Woesearchaeota (DHVEG6) (24.4% vs 19.4%) was slightly enhanced. Moreover, the results showed that relic DNA can affect the relative abundance of Geobacter and Candidatus Methanoperedens, however, it has no significant effects on the microbial community structure. These results indicate that MFCs can influence the soil microbial community profile, nevertheless the relic DNA generated has minimum effect on the culture independent estimates of microbial community composition.

Soil microbial fuel cells (sMFC) are a novel technique that use organic matters in soils as an alternative energy source. External resistance (ER) is a key factor influencing sMFC performance and, furthermore, alters the soil’s biological and chemical reactions. However, little information is available on how the microbial community and soil component changes in sMFC with different ER. Therefore, the effects of anodes of sMFC at different ER (2000 Ω, 1000 Ω, 200 Ω, 80 Ω and 50 Ω) were examined by measuring organic matter (OM) removal efficiency, trace elements in porewater and bacterial community structure in contaminated paddy soil. The results indicated that ER has significant effects on sMFC power production, OM removal efficiency and bacterial beta diversity. Moreover ER influences iron, arsenic and nickel concentration as well in soil porewater. In particular, greater current densities were observed at lower ER (2.4mA, 50Ω) compared to a higher ER (0.3mA, 2000Ω). The removal efficiency of OM increased with decreasing ER whereas it decreased with soil distance away from the anode. Furthermore, principal coordinate analysis (PCoA) revealed that ER may shape the bacterial communities that develop in the anode vicinity but have minimal effect on that of the bulk soil. The current study illustrates that lower ER can be used to selectively enhance the relative abundance of electrogenic bacteria and lead to high OM removal.

Arsenic (As) behavior in paddy soils couples with the redox process of iron (Fe) minerals. When soil is flooded, Fe oxides are transformed to soluble ferrous ions by accepting the electrons from Fe reducers. This process can significantly affect the fate of As in paddy fields. In this study, we show a novel technique to manipulate the Fe redox processes in paddy soils by deploying soil microbial fuel cells (sMFC). The results showed that the sMFC bioanode can significantly decrease the release of Fe and As into soil porewater. Iron and As contents around sMFC anode were 65.0% and 47.0% of the control respectively at day 50. The observed phenomenon would be explained by a competition for organic substrate between sMFC bioanode and the iron- and arsenic-reducing bacteria in the soils. In the vicinity of bioanode, organic matter removal efficiencies were 10.3% and 14.0% higher than the control for lost on ignition carbon and total organic carbon respectively. Sequencing of the 16S rRNA genes suggested that the influence of bioanodes on bulk soil bacterial community structure was minimal. Moreover, during the experiment a maximum current and power density of 0.31 mA and 12.0 mWm−2 were obtained, respectively. This study shows a novel way to limit the release of Fe and As in soils porewater and simultaneously generate electricity.

The rigidity and low strain of chitosan fibers hindered their broader utility for biomedical applications. In present work, formic acid was employed as an efficient modifier for chitosan to prepare flexible fibers wet-spun from the formic acid modified chitosan solution. The formation of amide linkages between chitosan and formic acid was confirmed by FTIR, (13)C NMR, (1)H NMR and XRD measurements. The degree of formylation evaluated by (1)H NMR spectra was varied from 14.1% to 37.2% as a function of the reaction temperature. The results of the mechanical properties showed that the as-spun fibers exhibited an enhanced ductility with a maximum elongation at break of 21.7% compared with that spun from the chitosan dissolved in diluted acetic acid. The novel flexible chitosan fibers were anticipated to be used as comfortable wound dressing and bandages in biomedical fields.