Enhancement of phytoextraction efficiency coupling Pteris vittata with low-dose biochar in arsenic-contaminated soil.

Phytoremediation of arsenic (As) by Pteris vittata (P. vittata) is a cost-effective and environmentally friendly method for restoring As-contaminated sites. However, the phytoextraction efficiency is low in some cases, such as clay soil, thus biochar was applied to enhance the efficiency of As extraction. The paper investigated the effect of biochar on soil characteristic, As mobility, and As uptake in P. vittata with a 90-day greenhouse experiment. Biochar derived from rice straw was added at rates of 0.5, 1.5, and 4% (w/w). The results showed that, under biochar amendment, soil pH raised from 5.24 to 6.03 and 4.91 to 5.85, soil dissolved organic carbon (DOC) increased 11.1-46.1% and 2.8-11.2%, respectively, in rhizosphere and bulk soils. Biochar also increased soil catalase (CAT) activity significantly, especially for the rhizosphere soil. Besides, biochar increased the labile As in the soils and transfer coefficient from roots to aboveground, thereby enhancing As accumulation by P. vittata tissues. The accumulation of As in fronds of P. vittata was up to 350 mg kg-1 in 1.5% biochar, which was more than twice the control and far beyond other biochar treatments. The results indicate that biochar addition is favorable to improve phytoremediation of P. vittata in As-contaminated soil and 1.5% (w/w) biochar may be a reasonable application ratio, thus providing an effective solution to enhance the efficiency of As phytoextraction.

Biochar increased soil catalase activity in the rhizosphere of P. vittata.Biochar increased the labile concentration of arsenic in soil and arsenic accumulation in P. vittata significantly.Combining biochar and P. vittata reduced arsenic in soil.Biochar amendment was favorable for phytoremediation of P. vittata in arsenic-contaminated soil.

Enhanced mobilization of Cd from commercial pigments in the rhizosphere of flooded lowland rice.

Although yellow Cd pigments (Cd-YP), widely used in industrial colorants, are considered inert, increasing evidence suggests once released into the environment, photobleaching/weathering mobilizes Cd from these pigments posing a pollution threat. Although general redox conditions and biotic/microbial activity are known to be important factors in determining Cd release, how spatial trends and specific soil processes regulate the Cd-YP behavior are poorly understood. Using plant rhizotrons in controlled environmental conditions, this study investigated the behavior of Cd-YP amendments matched to levels (15 mg kg-1) representative of contaminated soils in Yixing, China. Using high-resolution two-dimensional diffusive-gradient-in-thin-films (HR-2D-DGT), planar-optode (PO) multilayer systems alongside targeted soil and porewater sampling for chemical analysis the biogeochemistry associated with Cd mobilization from Cd-YP rice rhizospheres were determined. The results showed that there was a significant release of Cd into soil porewaters (51.5 µg L-1), but this reduced by 90.9% and stabilized over time (after 6-days). HR-2D-DGT ion-maps revealed pronounced spatial variances. The flux-maxima for Cd, which located within aerobic-rhizosphere zones, was 9 to 19-fold higher than in associated anoxic bulk soil. In general, zones of radial O2 loss (ROL)/higher redox conditions and lower pH were associated with Cd release, with S2- to SO42- transitions marking the boundaries of high-flux areas. Some isolated colocalization of Fe and Cd hotspots were observed in lateral root regions, but on-the-whole Fe/Mn and Cd release were not linked. In addition, microniche development was also an important feature of Cd mobilization due to soil heterogeneity.

Combining Multiple High-Resolution In Situ Techniques to Understand Phosphorous Availability Around Rice Roots.

Resolving chemical/biological drivers of P behavior around lowland/flooded rice roots remains a challenge because of the heterogeneity of the plant-soil interactions, compounded by sampling and analytical constraints. High-spatial-resolution (sub-mm) visualization enables these processes to be isolated, characterized, and deciphered. Here, three advanced soil imaging systems, diffusive gradients in thin-film technique coupled with laser ablation-ICPMS (DGT-LA-ICPMS), O2 planar optode, and soil zymography, were integrated. This trio of approaches was then applied to a rice life cycle study to quantify solute-P supply, through two dimensions, in situ, and low-disturbance high-resolution (HR) chemical imaging. This allowed mechanisms of P release to be delineated by O2, Fe, and phosphatase activity mapping at the same scale. HR-DGT revealed P depletion around both living and dead rice roots but with highly spatially variable Fe/P ratios (∼0.2-12.0) which aligned with changing redox conditions and root activities. Partnering of HR-DGT and soil zymography revealed concurrent P depletion and phosphatase hotspots in the rhizosphere and detritusphere zones (Mantel: 0.610-0.810, p < 0.01). This close affinity between these responses (Pearson correlation: -0.265 to -0.660, p < 0.01) cross-validates the measurements and reaffirms that P depletion stimulates phosphatase activity and Porg mineralization. The µ-scale biogeochemical landscape of rice rhizospheres and detritusphere, as documented here, needs greater consideration when implementing interventions to improve sustainable P nutrition.

Localized Intensification of Arsenic Release within the Emergent Rice Rhizosphere.

Behavior of trace elements in flooded/lowland rice soils is controlled by root-zone iron oxidation. Insoluble iron species bind/capture toxic elements, i.e., arsenic. However, it was recently observed that within this territory of arsenic immobilization lies a zone of prolific iron release, accompanied by a significant flux of arsenic in close proximity to rice root apices. Questions still remain on how common this phenomenon is and whether the chemical imaging approaches or soils/cultivars used influence this event. Here, three types of ultrathin/high-resolution diffusive gradient in thin films (DGT) substrates were integrated with oxygen planar optodes in a multilayer system, providing two-dimensional mapping of solute fluxes. The three DGT approaches revealed a consistent/overlapping spatial distribution with localized flux maxima for arsenic, which occurred in all experiments, concomitant with iron mobilization. Soil/porewater microsampling within the rhizosphere revealed no significant elevation in the solid phase's total iron and arsenic concentrations between aerobic and anaerobic zones. Contrary to arsenic, phosphorus bioavailability was shown to decrease in the arsenic/iron flux maxima. Rice roots, in addition to their role in nutrient acquisition, also perform a key sensory function. Flux maxima represent a significant departure from the chemical conditions of the bulk/field environment, but our observations of a complete rhizosphere reveal a mixed mode of root-soil interactions.

Arsanilic acid contributes more to total arsenic than roxarsone in chicken meat from Chinese markets.

Organoarsenicals have been used in poultry production for years, however, studies focused on roxarsone (ROX), with little attention to p-arsanilic acid (ASA). We assessed arsenic (As) concentration and speciation in chicken meat collected from 10 cities in China. The geometric mean for total As in 249 paired raw and cooked samples was 4.85 and 7.27 µg kg-1 fw, respectively. Among 81 paired raw and cooked samples, ASA and ROX were detected in >90% samples, suggesting the prevalence of organoarsenical use in China. ASA contributed the most (45% on average) to total As in cooked samples, followed by As(V), DMA, As(III), and ROX (7.2-22%). ASA was found to contribute more to total As in chicken meat compared to ROX for the first time. Arsenic in chicken meat showed considerable geographic variation, with higher inorganic arsenic (iAs) being detected from cities with higher ROX and ASA, indicating that organoarsenical use increased iAs concentration in chicken meat. When health risk was estimated, dietary exposure to iAs would result in an increase of 3.2 bladder and lung cancer cases per 100,000 adults. The result supports the removal of organoarsenicals in poultry production from Chinese market and further supports its removal from the global markets.

Arsenic-resistance mechanisms in bacterium Leclercia adecarboxylata strain As3-1: Biochemical and genomic analyses.

Microbial arsenic transformation is important in As biogeochemical cycles in the environment. In this study, a new As-resistant bacterial strain Leclercia adecarboxylata As3-1 was isolated and its associated mechanisms in As resistance and detoxification were evaluated based on genome sequencing and gene annotations. After subjecting strain As3-1 to medium containing arsenate (AsV), AsV reduction occurred and an AsV-enhanced bacterial growth was observed. Strain As3-1 lacked arsenite (AsIII) oxidation ability and displayed lower AsIII resistance than AsV, probably due to its higher AsIII accumulation. Polymerase chain reaction and phylogenetic analysis showed that strain As3-1 harbored a typical AsV reductase gene (arsC) on the plasmids. Genome sequencing and gene annotations identified four operons phoUpstBACS, arsHRBC, arsCRDABC and ttrRSBCA, with 8 additional genes outside the operons that might have involved in As resistance and detoxification in strain As3-1. These included 5 arsC genes explaining why strain As3-1 tolerated high AsV concentrations. Besides ArsC, TtrB, TtrC and TtrA proteins could also be involved in AsV reduction and consequent energy acquisition for bacterial growth. Our data provided a new example of diverse As-regulating systems and AsV-enhanced growth without ArrA in bacteria. The information helps to understand the role of As in selecting microbial systems that can transform and utilize As.

Arsenic Concentrations, Speciation, and Localization in 141 Cultivated Market Mushrooms: Implications for Arsenic Exposure to Humans.

Mushrooms accumulate arsenic (As), yet As concentrations, speciation, and localization in cultivated mushrooms across a large geographic distribution are unknown. We characterized 141 samples of nine species from markets in nine capital cities in China, with samples of Lentinula edodes, Pleurotus ostreatus, and Agaricus bisporus being analyzed for As speciation and localization. Total As concentrations ranged from 0.01 to 8.31 mg kg-1 dw, with A. bisporus (0.27-2.79 mg kg-1) containing the most As followed by P. ostreatus and L. edodes (0.04-8.31 and 0.12-2.58 mg kg-1). However, As in A. bisporus was mostly organic including nontoxic arsenobetaine, while P. ostreatus and L. edodes contained mainly inorganic As (iAs). On the basis of in situ imaging using LA-ICP-MS, As in L. edodes was localized to the surface coat of the cap, while As in P. ostreatus was localized to the junction of the pileus and stipe. When As speciation and daily mushroom consumption (1.37 g d-1 dw) are considered, daily mushroom consumption may result in elevated iAs exposure, with increased bladder and lung cancer rates up to 387 cases per 100000. Our study showed that market mushrooms could be a health risk to the general public so its production should be monitored.

In Situ Selective Measurement of SeIV in Waters and Soils: Diffusive Gradients in Thin-Films with Bi-Functionalized Silica Nanoparticles.

The speciation of selenium (Se) controls its fate and behavior, determining both its biological and environmental activities. However, in situ monitoring of SeIV presents a significant challenge due to its sensitivity to redox change. A novel diffusive gradients in thin films (DGT) technique containing mercapto-, amino-bifunctionalized SBA15 mesoporous silica nanoparticles was developed and evaluated in a series of laboratory and field deployment tests. The SBA-DGT exhibited a linear accumulation of SeIV ( r2 > 0.997) over a 72 h deployment, with negligible accumulation of SeVI(<5%). Consistent prediction of SeIV occurred within ionic strength and pH ranges of 0.1-200 mmol L-1 and 3.6-8, respectively. Limits of detection of the SBA-DGT were 0.03 µg SeIV L-1, which is suitable for natural waters. Moreover, the properties of the bifunctionalized SBA15 enable it to be fabricated within ultrathin (0.05 mm) gel layers for use in conjunction with O2 planar optode imaging. This new sandwich sensor technology with SBA-DGT was validated by mapping the two-dimensional distribution of SeIV and oxygen simultaneously in rice rhizospheres. This study shows that SBA-DGT provides a selective measurement of SeIV in situ, demonstrating its potential for both environmental monitoring and as a research tool for improving our understanding of Se biogeochemical processes.

Field-Scale Heterogeneity and Geochemical Regulation of Arsenic, Iron, Lead, and Sulfur Bioavailability in Paddy Soil.

A method using miniaturized arrayed DGT-probes (PADDI) for high-frequency in situ sampling with LA-ICPMS and CID analysis was developed to measure the field-scale heterogeneity of trace-element bioavailability. Robust calibrations (R2 > 0.99) combined with high-sensitivity (LOD = 0.35 ng cm-2), multielemental detection, and short measurement times were achieved using a new LA-ICPMS microDGT analysis. In the studied paddy-site (size: ∼2500 m2), total element concentrations across the field were approximately uniform (R.S.D. < 10%), but bioavailability was shown to vary significantly as determined from 864 microgel measurements housed within 72 PADDI arrays. Porewater As measurements were unable to differentiate the top/rhizosphere and bulk/deeper-soil layers. However, dynamic sampling with DGT revealed significant differences. Heterogeneity behaviors varied greatly between the different elements. Arsenic bioavailability was stable laterally across the field, but varied with depth, which was in contrast to the trends for Pb. Fe/S(-II) change was bidirectional, differing horizontally and vertically throughout the field. The heterogeneity in Pb bioavailability, due to the high frequency of hotspot maxima that were discretely dispersed across the paddy, proved the most difficult to simulate requiring the greatest number of probe deployments to determine a reliable field-average. The DGT-PADDI system provides a new characterization of infield trends for improved trace-inorganics' management in agricultural wetlands.

Coupling bioavailability and stable isotope ratio to discern dietary and non-dietary contribution of metal exposure to residents in mining-impacted areas.

Both dietary and non-dietary pathways contribute to metal exposure in residents living near mining-impacted areas. In this study, bioavailability-based metal intake estimation coupled with stable Pb isotope ratio fingerprinting technique were used to discern dietary (i.e., rice consumption) and non-dietary (i.e., housedust ingestion) contribution to As, Cd, and Pb exposure in residents living near mining-impacted areas. Results showed that not only rice (n = 44; 0.10-0.56, 0.01-1.77, and 0.03-0.88 mg kg-1) but also housedust (n = 44; 2.15-2380, 2.55-329, and 87.0-56,184 mg kg-1) were contaminated with As, Cd, and Pb. Based on in vivo mouse bioassays, bioavailability of As, Cd, and Pb in rice (n = 11; 34 ±â€¯15, 59 ±â€¯13, and 31 ±â€¯15%) were greater than housedust (n = 14; 17 ±â€¯6.7, 46 ±â€¯10, and 25 ±â€¯6.8%). Estimated daily intake of As, Cd, and Pb after incorporating metal bioavailability showed that As intake via rice was 5-fold higher than housedust for adults, whereas As intake via housedust was 3-fold higher than rice for children. For both adults and children, rice was the main source for Cd exposure, while housedust was the predominant Pb contributor. To ascertain the dominant Pb source from housedust ingestion, stable Pb isotope ratios (207Pb/206Pb and 208Pb/206Pb) of hair samples of local residents (n = 27, 0.8481 ±â€¯0.0049 and 2.0904 ±â€¯0.0102) were compared to housedust (n = 27, 0.8485 ±â€¯0.0047 and 2.0885 ±â€¯0.0107) and rice (n = 27, 0.8369 ±â€¯0.0057 and 2.0521 ±â€¯0.0119), showing an overlap between hair and housedust, but not rice, confirming that incidental housedust ingestion was the main source of Pb exposure. This study coupled bioavailability and stable isotope techniques to accurately identify the source of metal exposure as well as their potential health risk.

Novel Method for in Situ Monitoring of Organophosphorus Flame Retardants in Waters.

Widespread use of organophosphorus flame retardants (OPFRs) and their ubiquity in water results in the need for a robust and reliable monitoring technique to better understand their fate and environmental impact. In situ passive sampling using the diffusive gradients in thin-films (DGT) technique provides time-integrated data and is developed for measuring OPFRs here. Ultrasonic extraction of binding gels in methanol provided reliable recoveries for all tested OPFRs. Diffusion coefficients of TCEP, TCPP, TDCPP, TPrP, TBP, and TBEP in the agarose diffusive gel (25 °C) were obtained. The capacity of an HLB binding gel for OPFRs was >115 µg per disc, and the binding performance did not deteriorate with time up to 131 days. DGT performance is independent of typical environmental ranges of pH (3.12-9.71), ionic strength (0.1-500 mmol L-1), and dissolved organic matter (0-20 mg L-1), and also of diffusive layer thickness (0.64-2.14 mm) and deployment time (3-168 h). Negligible competition effects between OPFRs was found. DGT-measured concentrations of OPFRs in a wastewater treatment plant (WWTP) effluent (12-16 days) were comparable to those obtained by grab sampling, further verifying DGT's reliability for measuring OPFRs in waters.

Plant Induced Changes to Rhizosphere Characteristics Affecting Supply of Cd to Noccaea caerulescens and Ni to Thlaspi goesingense.

Changes in soil rhizosphere properties after growing the Cd hyperaccumulator Noccaea caerulescens and the Ni hyperaccumulator Thlaspi goesingense were investigated. Dissolved organic carbon content increased in the rhizosphere, but there were no significant changes in the solution concentrations of Cd and Ni. Concentrations of these metals extracted by NH4Cl and EDTA decreased in the rhizosphere, as did DGT-measured concentrations, indicating a depletion of labile metal in the solid phase. The results could be explained by the increased DOC in the rhizosphere maintaining a higher proportion of the labile metal in solution through complexation, with the overall depletion of metals only manifest in the solid phase. The DGT induced fluxes in soils and sediments (DIFS) model was used to provide key kinetic information on soil processes and labile pool size. These data showed that the more limited metal supply in the rhizosphere after the growth of hyperaccumulators was due to both depletion of the solid phase pool and a lower rate constant of supply from solid phase to solution. The effect on the rate constant, which could be rationalized by the plant sequentially accessing and consuming the more labile pools of metal, was most marked for Cd, which had the highest accumulation factors.

Effect of biochar and Fe-biochar on Cd and As mobility and transfer in soil-rice system.

In this study, the effects of biochar derived from rice-straw (biochar) and iron-impregnated biochar (Fe-biochar) on Cd and As mobility in rice rhizosphere and transfer from soil to rice were investigated with different application rates. 1-3% biochar reduced porewater Cd in rhizosphere but elevated soluble As, resulting in 49-68% and 26-49% reduction in the root and grain Cd, with a simultaneous increase in root As. Unlike biochar, 0.5% Fe-biochar decreased porewater As throughout rice growth, resulting in reduced root As, which, however, increased Cd uptake by root. Biochar-induced soil As mobilization was probably through competitive desorption and Fe-biochar-induced soil Cd mobilization was probably via soil acidification. The results suggested that biochar and Fe-biochar was effective in reducing Cd and As uptake by rice, respectively, so they may be used as emergency measures to cope with single Cd or As contamination in paddy soils.

Varying effect of biochar on Cd, Pb and As mobility in a multi-metal contaminated paddy soil.

Cd, Pb and As stand as the most prominent contaminants prevailing in Chinese soils. In the present study, biochars derived from water hyacinth (BCW) and rice straw (BCR) were investigated regarding their applicability and durability in soil Cd, Pb, and As immobilization under acid precipitation. Total Cd, Pb, and As in both BCs were below the maximum allowed threshold according to biochar toxicity standard recommended by International Biochar Initiative. To evaluate BCs effect on Cd, Pb, As bioavailability and mobility, CaCl2, KH2PO4 and SPLP extractions were firstly carried out. In neutral extraction with CaCl2 and KH2PO4, significantly reduced Cd/Pb concentrations in CaCl2 extract along with elevated KH2PO4-extractable As were recorded with either BC at 2% or 5%. In SPLP with simulated acid rainwater as extractant, comparable Cd, Pb and As levels were determined in SPLP extract with 2% BCW, while slight to significant increase in SPLP-Cd, Pb or As was recorded with other treatments. Longer-term leaching column test further confirmed the high durability of 2% BCW in Cd immobilization under continuous acid exposure. In parallel, little increase in As concentrations in eluate was determined with 2% BCW compared to no-biochar control, indicating a lowered risk of As mobilization with acid input. However, remarkably higher Pb in leachate from both BCW-only control and 2% BCW-amended soils were noticed at the initial stage of acid leaching, indicating a higher acid-solubility of Pb minerals in BCW (most probably PbO) than in tested soil (PbO2, PbAs2O6). Taken together, BCW exhibited important potential for soil Cd sequestration with little effect on As mobilization under acid precipitation. But it may simultaneously load highly acid-soluble Pb minerals into soils, resulting in elevated Pb mobility upon acid exposure. Therefore, more stringent threshold for Pb content in biochar need to be put forward to secure biochar application in soils subject to anthropogenic acidification.

Efficiency and mechanisms of Cd removal from aqueous solution by biochar derived from water hyacinth (Eichornia crassipes).

This study investigated the efficiency and mechanisms of Cd removal by biochar pyrolyzed from water hyacinth (BC) at 250-550 °C. BC450 out-performed the other BCs at varying Cd concentrations and can remove nearly 100% Cd from aqueous solution within 1 h at initial Cd ≤ 50 mg l(-1). The process of Cd sorption by BC450 followed the pseudo-second order kinetics with the equilibrium being achieved after 24 h with initial Cd ranging from 100 to 500 mg l(-1). The maximum Cd sorption capacity of BC450 was estimated to be 70.3 mg g(-1) based on Langmuir model, which is prominent among a range of low-cost sorbents. Based on the balance analysis between cations released and Cd sorbed onto BC450 in combination with SEM-EDX and XPS data, ion-exchange followed by surface complexation is proposed as the dominant mechanism responsible for Cd immobilization by BC450. In parallel, XRD analysis also suggested the formation of insoluble Cd minerals (CdCO3, Cd3P2, Cd3(PO4)2 and K4CdCl6) from either (co)-precipitation or ion exchange. Results from this study highlighted that the conversion of water hyacinth into biochar is a promising method to achieve effective Cd immobilization and improved management of this highly problematic invasive species.