Angelia L. Seyfferth

Dehalogenation of polybrominated diphenyl ethers and polychlorinated biphenyl by bimetallic, impregnated, and nanoscale zerovalent iron.

Nanoscale zerovalent iron particles (nZVI), bimetallic nanoparticles (nZVI/Pd), and nZVI/Pd impregnated activated carbon (nZVI/Pd-AC) composite particles were synthesized and investigated for their effectiveness to remove polybrominated diphenyl ethers (PBDEs) and/or polychlorinated biphenyls (PCBs). Palladization of nZVI promoted the dehalogenation kinetics for mono- to tri-BDEs and 2,3,4-trichlorobiphenyl (PCB 21). Compared to nZVI, the iron-normalized rate constants for nZVI/Pd were about 2-, 3-, and 4-orders of magnitude greater for tri-, di-, and mono-BDEs, respectively, with diphenyl ether as a main reaction product. The reaction kinetics and pathways suggest an H-atom transfer mechanism. The reaction pathways with nZVI/Pd favor preferential removal of para-halogens on PBDEs and PCBs. X-ray fluorescence mapping of nZVI/Pd-AC showed that Pd mainly deposits on the outer part of particles, while Fe was present throughout the activated carbon particles. While BDE 21 was sorbed onto activated carbon composites quickly, debromination was slower compared to reaction with freely dispersed nZVI/Pd. Our XPS and chemical data suggest about 7% of the total iron within the activated carbon was zerovalent, which shows the difficulty with in-situ synthesis of a significant fraction of zerovalent iron in the microporous material. Related factors that likely hinder the reaction with nZVI/Pd-AC are the heterogeneous distribution of nZVI and Pd on activated carbon and/or immobilization of hydrophobic organic contaminants at the adsorption sites thereby inhibiting contact with nZVI.

Arsenic Localization, Speciation, and Co-Occurrence with Iron on Rice (Oryza Sativa L.) Roots Having Variable Fe Coatings

Arsenic contamination of rice is widespread, but the rhizosphere processes influencing arsenic attenuation remain unresolved. In particular, the formation of Fe plaque around rice roots is thought to be an important barrier to As uptake, but the relative importance of this mechanism is not well characterized. Here we elucidate the colocalization of As species and Fe on rice roots with variable Fe coatings; we used a combination of techniques--X-ray fluorescence imaging, μXANES, transmission X-ray microscopy, and tomography--for this purpose. Two dominant As species were observed in fine roots-inorganic As(V) and As(III) -with minor amounts of dimethylarsinic acid (DMA) and arsenic trisglutathione (AsGlu(3)). Our investigation shows that variable Fe plaque formation affects As entry into rice roots. In roots with Fe plaque, As and Fe were strongly colocated around the root; however, maximal As and Fe were dissociated and did not encapsulate roots that had minimal Fe plaque. Moreover, As was not exclusively associated with Fe plaque in the rice root system; Fe plaque does not coat many of the young roots or the younger portion of mature roots. Young, fine roots, important for solute uptake, have little to no iron plaque. Thus, Fe plaque does not directly intercept (and hence restrict) As supply to and uptake by rice roots but rather serves as a bulk scavenger of As predominantly near the root base.

Silicate Mineral Impacts on the Uptake and Storage of Arsenic and Plant Nutrients in Rice (Oryza sativa L.)

Arsenic-contaminated rice grain may threaten human health globally. Since H₃AsO₃⁰ is the predominant As species found in paddy pore-waters, and H₄SiO₄⁰ and H₃AsO₃⁰ share an uptake pathway, silica amendments have been proposed to decrease As uptake and consequent As concentrations in grains. Here, we evaluated the impact of two silicate mineral additions differing in solubility (+Si(L), diatomaceous earth, 0.29 mM Si; +Si(H), Si-gel, 1.1 mM Si) to soils differing in mineralogy on arsenic concentration in rice. The +Si(L) addition either did not change or decreased As concentration in pore-water but did not change or increased grain-As levels relative to the (+As--Si) control. The +Si(H) addition increased As in pore-water, but it significantly decreased grain-As relative to the (+As--Si) control. Only the +Si(H) addition resulted in significant increases in straw- and husk-Si. Total grain- and straw-As was negatively correlated with pore-water Si, and the relationship differed between two soils exhibiting different mineralogy. These differing results are a consequence of competition between H₄SiO₄⁰ and H₃AsO₃⁰ for adsorption sites on soil solids and subsequent plant-uptake, and illustrate the importance of Si mineralogy on arsenic uptake.

Arsenic concentrations in paddy soil and rice and health implications for major rice-growing regions of Cambodia.

Despite the global importance of As in rice, research has primarily focused on Bangladesh, India, China, and the United States with limited attention given to other countries. Owing to both indigenous As within the soil and the possible increases arising from the onset of irrigation with groundwater, an assessment of As in rice within Cambodia is needed, which offers a base-case comparison against sediments of similar origin that comprise rice paddy soils where As-contaminated water is used for irrigation (e.g., Bangladesh). Here, we evaluated the As content of rice from five provinces (Kandal, Prey Veng, Battambang, Banteay Meanchey, and Kampong Thom) in the rice-growing regions of Cambodia and coupled that data to soil-chemical factors based on extractions of paddy soil collected and processed under anoxic conditions. At total soil As concentrations ranging 0.8 to 18 μg g(-1), total grain As concentrations averaged 0.2 μg g(-1) and ranged from 0.1 to 0.37 with Banteay Meanchey rice having significantly higher values than Prey Veng rice. Overall, soil-extractable concentrations of As, Fe, P, and Si and total As were poor predictors of grain As concentrations. While biogeochemical factors leading to reduction of As(V)-bearing Fe(III) oxides are likely most important for predicting plant-available As, husk and straw As concentrations were the most significant predictors of grain-As levels among our measured parameters.

Immobilization of Hg(II) in water with polysulfide-rubber (PSR) polymer-coated activated carbon.

An effective mercury removal method using polymer-coated activated carbon was studied for possible use in water treatment. In order to increase the affinity of activated carbon for mercury, a sulfur-rich compound, polysulfide-rubber (PSR) polymer, was effectively coated onto the activated carbon. The polymer was synthesized by condensation polymerization between sodium tetrasulfide and 1,2-dichloroethane in water. PSR-mercury interactions and Hg-S bonding were elucidated from x-ray photoelectron spectroscopy, and Fourier transform infra-red spectroscopy analyses. The sulfur loading levels were controlled by the polymer dose during the coating process and the total surface area of the activated carbon was maintained for the sulfur loading less than 2 wt%. Sorption kinetic studies showed that PSR-coated activated carbon facilitates fast reaction by providing a greater reactive surface area than PSR alone. High sulfur loading on activated carbon enhanced mercury adsorption contributing to a three orders of magnitude reduction in mercury concentration. μ-X-ray absorption near edge spectroscopic analyses of the mercury bound to activated carbon and to PSR on activated carbon suggests the chemical bond with mercury on the surface is a combination of Hg-Cl and Hg-S interaction. The pH effect on mercury removal and adsorption isotherm results indicate competition between protons and mercury for binding to sulfur at low pH.

A New Approach to Sampling Intact Fe Plaque Reveals Si-Induced Changes in Fe Mineral Composition and Shoot As in Rice

The Fe (oxyhydr)oxide rind, or Fe plaque, that forms on aquatic plant roots is an important sorbent of metal(loid)s and plays a role in the attenuation of metal(loid) uptake into higher plants. However, the mineral composition of Fe plaque and thus its potential to sorb metal(loid)s is affected by solution chemistry. The predominant strategy to characterize Fe plaque using dithionite-citrate-bicarbonate (DCB) extraction and elemental analysis reveals total Fe quantity but misses the mineral structure of the Fe (oxyhydr)oxide. Here, we developed a new technique using gentle sonication to sample intact Fe plaque from the root system and concentrate it for subsequent mineralogical characterization using synchrotron-based X-ray diffraction and X-ray absorption spectroscopy. We then coupled that data with conventional DCB extraction. The sample preparation method was effective at concentrating As-bound Fe plaque minerals in a uniform coating onto membranes that could easily be analyzed with X-ray techniques. Using these methods, we show that the percentage of poorly ordered Fe minerals in Fe plaque increases with increasing pore-water Si in flooded rice paddy soils. These findings have implications for understanding mineral controls on As cycling in the soil-rice nexus, and the sampling approach can be adopted for other aquatic plant systems.

How rice (Oryza sativa L.) responds to elevated As under different Si-rich soil amendments

Several strategies exist to mitigate As impacts on rice and each has its set of trade-offs with respect to yield, inorganic As content in grain, and CH4 emissions. The addition of Si to paddy soil can decrease As uptake by rice but how rice will respond to elevated As when soil is amended with Si-rich materials is unresolved. Here, we evaluated yield impacts and grain As content and speciation in rice exposed to elevated As in response to different Si-rich soil amendments including rice husk, rice husk ash, and CaSiO3 in a pot study. We found that As-induced yield losses were alleviated by Husk amendment, partially alleviated by Ash amendment, and not affected by CaSiO3 amendment. Furthermore, Husk was the only tested Si-amendment to significantly decrease grain As concentrations. Husk amendment was likely effective at decreasing grain As and improving yield because it provided more plant-available Si, particularly during the reproductive and ripening phases. Both Husk and Ash provided K, which also played a role in yield improvement. This study demonstrates that while Si-rich amendments can affect rice uptake of As, the kinetics of Si dissolution and nutrient availability can also affect As uptake and toxicity in rice.

Arsenic, Lead, and Cadmium in U.S. Mushrooms and Substrate in Relation to Dietary Exposure

Wild mushrooms can absorb high quantities of metal(loid)s, yet the concentration, speciation, and localization of As, Pb, and Cd in cultivated mushrooms, particularly in the United States, are unresolved. We collected 40 samples of 12 types of raw mushrooms from 2 geographic locations that produce the majority of marketable U.S. mushrooms and analyzed the total As, Pb, and Cd content, the speciation and localization of As in select samples, and assessed the metal sources and substrate-to-fruit transfer at one representative farm. Cremini mushrooms contained significantly higher total As concentrations than Shiitake and localized the As differently; while As in Cremini was distributed throughout the fruiting body, it was localized to the hymenophore region in Shiitake. Cd was significantly higher in Royal Trumpet than in White Button, Cremini, and Portobello, while no difference was observed in Pb levels among the mushrooms. Concentrations of As, Pb, and Cd were less than 1 μg g(-1) d.w. in all mushroom samples, and the overall risk of As, Cd, and Pb intake from mushroom consumption is low in the U.S. However, higher percentages of tolerable intake levels are observed when calculating risk based on single serving-sizes or when substrate contains elevated levels of metal(loid)s.