Arsenic uptake and transport of Pteris vittata L.as influenced by phosphate and inorganic arsenic species under sand culture

In order to understand the similarity or difference of inorganic As species uptake and transport related to phosphorus in As-hyperaccumulator,uptake and transport of arsenate(As(V))and arsenite(As(III))were studied using Pteris vittata L.under sandculture.Higher concentrations of phosphate were found to inhibit accumulation of arsenate and arsenite in the fronds of P.vittata.The reduction in As accumulation was greater in old fronds than in young fronds,and relatively weak in root and rhizome.Moderateincreases,from 0.05 to 0.3 mmol/L,in phosphate reduced uptake of As(III)more than As(V),while the reverse was observed at highconcentrations of phosphate(1.0 mmol/L).Phosphate apparently reduced As transport and the proportion of As accumulated in frondsof P.vittata when As was supplied as As(V).It may in part be due to competition between phosphorus and As(V)during transport.Incontrast,phosphate had a much smaller effect on As transport when the As was supplied as As(III).Therefore,the results from presentexperiments indicates that a higher concentration of phosphate suppressed As accumulation and transport in P.vittata,especially inthe fronds,when exposed to As(V);but the suppression of phosphate to As transport may be insignificant when P.vittata exposed toAs(III)under sand culture conditions.The finding will help to understand the interaction of P and As during their uptake process in P.vittata.     

The role of phosphorus in the metabolism of arsenate by a freshwater green alga, Chlorella vulgaris

A freshwater microalga, Chlorella vulgaris, was grown in the presence of varying phosphate concentrations( 10–500 μg/L P) and environmentally realistic concentrations of arsenate(As(Ⅴ))(5–50 μg/L As). Arsenic speciation in the culture medium and total cellular arsenic were measured using AEC-ICP-MS and ICP-DRC-MS, respectively, to determine arsenic biotransformation and uptake in the various phosphorus scenarios. At high phosphate concentration in the culture medium, 100 μg/L P, the uptake and biotransformation of As(Ⅴ) was minimal and dimethylarsonate(DMAs(Ⅴ)) was the dominant metabolite excreted by C. vulgaris, albeit at relatively low concentrations. At common environmental P concentrations, 0–50 μg/L P, the uptake and biotransformation of As(Ⅴ) increased. At these higher As-uptake levels, arsenite(As(Ⅲ)) was the predominant metabolite excreted from the cell. The concentrations of As(Ⅲ) in these low P conditions were much higher than the concentrations of methylated arsenicals observed at the various P concentrations studied. The switchover threshold between the(small) methylation and(large) reduction of As(Ⅴ) occurred around a cellular As concentration of 1 fg/cell. The observed nearly quantitative conversion of As(Ⅴ) to As(Ⅲ) under low phosphate conditions indicates the importance of As(Ⅴ) bio-reduction at common freshwater P concentrations. These findings on the influence of phosphorus on arsenic uptake, accumulation and excretion are discussed in relation to previously published research. The impact that the two scenarios of As(Ⅴ) metabolism, As(Ⅲ) excretion at high As(Ⅴ)-uptake and methylarsenical excretion at low As(Ⅴ)-uptake, have on freshwater arsenic speciation is discussed.     

Effect of external and internal phosphate status on arsenic toxicity and accumulation in rice seedlings

Phosphorus (P) deficiency is thought to exacerbate the arsenic (As) phytotoxicity in paddy rice. The experiments were conducted to investigate the e ects of external phosphate supply on As accumulation in rice and its toxicity under phosphate deficiency conditions. Rice seedlings pretreated with a phosphorus deficient nutrient solution (–P) for 14 d accumulated more As than those pretreated with a normal phosphorus supply nutrient solution (+P). Rice protreated with –P showed As toxicity symptoms after being exposed to 50 mol/L arsenate for 4 h, while +P rice did not show any toxicity symptoms. Arsenic toxicity symptoms can be alleviated by increasing external P concentrations. The arsenate uptake rate and accumulation corresponded with the As toxicity in rice plants. Arsenic concentrations in rice roots decreased with increasing external phosphate concentrations. The lowest As accumulation and the highest P accumulation were found when the external P concentration reached 100 mol/L. In short, P deficiency increased the sensitivity of rice to arsenate and increasing of external phosphate supply could alleviate As toxicity.     

Potential of Pteris vittata L. for phytoremediation of sites co-contaminated with cadmium and arsenic： The tolerance and accumulation

Field investigation and greenhouse experiments were conducted to study the tolerance of Pteris vittata L. （Chinese brake） to cadmium （Cd） and its feasibility for remediating sites co-contaminated with Cd and arsenic （As）. The results showed that P. vittata could survive in pot soils spiked with 80 mg/kg of Cd and tolerated as great as 301 mg/kg of total Cd and 26.8 mg/kg of diethyltriaminepenta acetic acid （DTPA）-extractable Cd under field conditions. The highest concentration of Cd in fronds was 186 mg/kg under a total soil concentration of 920 mg As/kg and 98.6 mg Cd/kg in the field, whereas just 2.6 mg/kg under greenhouse conditions. Ecotypes of P. vittata were differentiated in tolerance and accumulation of Cd, and some of them could not only tolerate high concentrations of soil Cd, but also accumulated high concentrations of Cd in their fronds. Arsenic uptake and transportation by P. vittata was not inhibited at lower levels （〈20 mg/kg） of Cd addition. Compared to the treatment without addition of Cd, the frond As concentration was increased by 103.8% at 20 mg Cd/kg, with the highest level of 6434 mg/kg. The results suggested that the Cd-tolerant ecotype of P. vittata extracted effectively As and Cd from the site co-contaminated with Cd and As, and might be used to remediate and revegetate this type of site.     

Characterization of arsenate transformation and identification of arsenate reductase in a green alga Chlamydomonas reinhardtii

Arsenic (As) is a pervasive and ubiquitous environmental toxin that has created catastrophic human health problems world-wide. Chlamydomonas reinhardtii is a unicellular green alga, which exists ubiquitously in freshwater aquatic systems. Arsenic metabolism processes of this alga through arsenate reduction and sequent store and efflux were investigated. When supplied with 10 mol/L arsenate, arsenic speciation analysis showed that arsenite concentration increased from 5.7 to 15.7 mg/kg dry weight during a 7-day period, accounting for 18%–24% of the total As in alga. When treated with different levels of arsenate (10, 20, 30, 40, 50 mol/L) for 7 days, the arsenite concentration increased with increasing external arsenate concentrations, the proportion of arsenite was up to 23%–28% of the total As in alga. In efflux experiments, both arsenate and arsenite could be found in the efflux solutions. Additionally, the efflux of arsenate was more than that of arsenite. Furthermore, two arsenate reductase genes of C. reinhardtii (CrACR2s) were cloned and expressed in Escherichia coli strain WC3110 (ΔarsC) for the first time. The abilities of both CrACR2s genes to complement the arsenatesensitive strain were examined. CrACR2.1 restored arsenate resistance at 0.8 mmol/L. However, CrACR2.2 showed much less ability to complement. The gene products were demonstrated to reduce arsenate to arsenite in vivo. In agreement with the complementation results, CrACR2.1 showed higher reduction ability than CrACR2.2, when treated with 0.4 mmol/L arsenate for 16 hr incubation.     

Influence of environmental factors on arsenic accumulation and biotransformation using the aquatic plant species Hydrilla verticillata

Hydrilla verticillata(waterthyme) has been successfully used for phytoremediation in arsenic(As) contaminated water.To evaluate the effects of environmental factors on phytoremediation,this study conducted a series of orthogonal design experiments to determine optimal conditions,including phosphorus(P),nitrogen(N),and arsenate(As(Ⅴ))concentrations and initial pH levels,for As accumulation and biotransformation using this aquatic plant species,while also analyzing As species transformation in culture media after 96-hr exposure.Analysis of variance and the signal-to-noise ratio were used to identify both the effects of these environmental factors and their optimal conditions for this purpose.Results indicated that both N and P significantly impacted accumulation,and N was essential in As species transformation.High N and intermediate P levels were critical to As accumulation and biotransformation by H.verticillata,while high N and low P levels were beneficial to As species transformation in culture media.The highest total arsenic accumulation was(197.2±17.4) μg/g dry weight when As(V) was at level 3(375μg/L),N at level 2(4 mg/L),P at level 1(0.02 mg/L),and pH at level 2(7).Although H.verticillata is highly efficient in removing As(Ⅴ) from aquatic environments,its use could be potentially harmful to both humans and the natural environment due to its release of highly toxic arsenite.For cost-effective and ecofriendly phytoremediation of As-contaminated water,both N and P are helpful in regulating As accumulation and transformation in plants.     

A combined process coupling phytoremediation and in situ flushing for removal of arsenic in contaminated soil

Phytoremediation and soil washing are both potentially useful for remediating arsenic(As)-contaminated soils.We evaluated the effectiveness of a combined process coupling phytoremediation and in situ soil flushing for removal of As in contaminated soil through a pilot study.The results showed that growing Pteris vittata L.(P.v.) accompanied by soil flushing of phosphate(P.v./Flushing treatment) could significantly decrease the total As concentration of soil over a 37 day flushing period compared with the single flushing(Flushing treatment).The P.v./Flushing treatment removed 54.04% of soil As from contaminated soil compared to 47.16% in Flushing treatment,suggesting that the growth of P.vittata was beneficial for promoting the removal efficiency.We analyzed the As fractionation in soil and As concentration in soil solution to reveal the mechanism behind this combined process.Results showed that comparing with the control treatment,the percent of labile arsenate fraction significantly increased by 17% under P.v./Flushing treatment.As concentration in soil solution remained a high lever during the middle and later periods(51.26–56.22 mg/L),which was significantly higher than the Flushing treatment.Although soil flushing of phosphate for more than a month,P.vittata still had good accumulation and transfer capacity of As of the soil.The results of the research revealed that combination of phytoremediation and in situ soil flushing is available to remediate As-contaminated soils.     

Effects of phosphate on the transport of graphene oxide nanoparticles in saturated clean and iron oxide-coated sand columns

In this study, transport behaviors of graphene oxide (GO) in saturated uncoated (i.e., clean sand) and goethite-coated sand porous media were examined as a function of the phosphate. We found that phosphate enhanced the transport of GO over a wide range of solution chemistry (i.e., pH 5.0–9.0 and the presence of 10 mmol/L Na⁺ or 0.5 mmol/L Ca²⁺). The results were mainly ascribed to the increase of electrostatic repulsion between nanoparticles and porous media. Meanwhile, deposition site competition induced by the retained phosphate was another important mechanism leading to promote GO transport. Interestingly, when the phosphate concentration increased from 0.1 to 1.0 mmol/L, the transport-enhancement effect of phosphate in goethite-coated sand was to a much larger extent than that in clean sand. The observations were primarily related to the difference in the total mass of retained phosphate between the iron oxide-coated sand and clean sand columns, which resulted in different degrees of the electrostatic repulsion and competitive effect of phosphate. When the background solution contained 0.5 mmol/L Ca²⁺, phosphate could be bind to sand/ goethite-coated sand surface by cation bridging; and consequently, promoted competition between phosphate and nanoparticles for deposition sites, which was an important mechanism for the enhanced effect of phosphate. Moreover, the DLVO theory was applicable to describe GO transport behaviors in porous media in the absence or presence of phosphate. Taken together, these findings highlight the important status and role of phosphate on the transport and fate of colloidal graphene oxide in the subsurface environment.     

Effect of ionic strength on adsorption of As（Ⅲ） and As（Ⅴ） on variable charge soils

The study was to investigate the adsorption behavior of arsenite (As(Ⅲ)) and arsenate (As(Ⅴ)) on two variable charge soils, i.e., Haplic Acrisol and Rhodic Ferralsol at different ionic strengths and pH with batch methods. Results indicated that the amount of As(Ⅲ) adsorbed by these two soils increased with increasing solution pH, whereas it decreased with increasing ionic strength under the acidic condition. This suggested that As(Ⅲ) was mainly adsorbed on soil positive charge sites through electrostatic attraction under the acidic condition. Moreover, intersects of As(Ⅴ) adsorption-pH curves at different ionic strengths (a characteristic pH) are obtained for both soils. It was noted that above this pH, the adsorption of As(Ⅴ) was increased with increasing ionic strength, whereas below it the reverse trend was true. Precisely the intersect pH was 3.6 for Haplic Acrisol and 4.5 for Rhodic Ferralsol, which was near the values of PZSE (soil point of zero salt effect) of these soils. The effects of ionic strength and pH on arsenate adsorption by these soils were interpreted by the adsorption model. The results of zeta potential suggested that the potential in adsorption plane becomes less negative with increasing ionic strength above soil PZSE and decreases with increasing ionic strength below soil PZSE. These results further supported the hypothesis of the adsorption model that the potential in the adsorption plane changes with ionic strength with an opposite trend to surface charge of the soils. Therefore, the change of the potential in the adsorption plane was mainly responsible for the change of arsenate adsorption induced by ionic strength on variable charge soils.     

Arsenic redox transformation by Pseudomonas sp. HN-2 isolated from arsenic-contaminated soil in Hunan, China

A mesophilic,Gram-negative,arsenite[As(Ⅲ)]-oxidizing and arsenate[As(V)]-reducing bacterial strain,Pseudomonas sp.HN-2,was isolated from an As-contaminated soil.Phylogenetic analysis based on 16 S r RNA gene sequencing indicated that the strain was closely related to Pseudomonas stutzeri.Under aerobic conditions,this strain oxidized 92.0%(61.4 μmol/L) of arsenite to arsenate within 3 hr of incubation.Reduction of As(V) to As(Ⅲ) occurred in anoxic conditions.Pseudomonas sp.HN-2 is among the first soil bacteria shown to be capable of both aerobic As(Ⅲ) oxidation and anoxic As(V) reduction.The strain,as an efficient As(Ⅲ) oxidizer and As(V) reducer in Pseudomonas,has the potential to impact arsenic mobility in both anoxic and aerobic environments,and has potential application in As remediation processes.     

植物超富集砷机制研究的最新进展

砷污染是全球环境热点问题之一,土壤砷污染治理是一个急需解决的难题.随着不同砷超富集植物的发现,植物修复技术因其投资和维护成本低、不易造成二次污染等优点而成为国际学术界研究的热点和前沿领域.深入理解超富集植物体内砷代谢和富集机制是有效利用植物修复技术来治理砷污染土壤的关键.近年来,植物超富集砷机制的研究取得了很大进展.已有的研究结果证明,蜈蚣草(P.vittata)对砷的吸收和转运能力显著高于非超富集植物,转运到地上部的砷主要储存在羽叶细胞的液胞中;蜈蚣草具有很强的抗氧化胁迫能力和五价砷(As(Ⅴ))还原能力;菌根共生有利于蜈蚣草的生长和砷的富集.有关植物超富集砷机制的分子生物学研究也取得了可喜的进展.砷酸盐还原酶基因(PvACR2)和植络素合成酶基因(PvPCS1)都被克隆并表征,cTPI同源基因(PV4-8)也被证明具有将As(Ⅴ)还原成三价砷(As(Ⅲ))的功能.尽管植物超富集砷机制的研究取得了迅速进展,但至今学术界仍没有全面理解为什么这些蕨类植物具有超富集砷的功能,相关功能基因、酶和转运蛋白的研究有待进一步深入.     

砷超富集植物蜈蚣草中磷和钙的亚细胞分布及其与耐砷毒的关系

为了在研究砷超富集植物蜈蚣草中磷和钙的亚细胞分布,并探讨其与蜈蚣草耐砷毒的关系进行了试验研究.结果表明,蜈蚣草吸收的磷主要分布在胞质组分中(平均占各部位总磷含量的44%以上),吸收的钙主要分布在细胞壁组分中(平均占各部位总钙含量的48%以上).与低钙(0.03和2.5 mmol·L-1)处理相比,高钙(5.0 mmol·L-1)处理时,根部的胞质组分和叶柄的细胞器组分中磷含量较高.各器官的亚细胞组分中钙含量随着介质中添加钙浓度的提高而增加.介质钙浓度过高会抑制蜈蚣草的生长.在加砷处理下,根部和叶柄细胞壁组分的磷含量有所减少,但地上部细胞器组分的磷含量及其含磷总量占植株含磷总量的相对比例、根部细胞壁组分的钙含量增加.蜈蚣草自主调节磷和钙的亚细胞水平分布可能是其耐砷毒的机制之一.     

Arsenic speciation for the phytoremediation by the Chinese brake fern, Pteris vittata

Arsenic(As)speciation for the phytoremediation by the Chinese brake fern was studied.In particular,the mechanism of how plants induce compounds containing thiol(SH)and proteins by As exposure in terms of the relationship between As and phosphate uptaken into plant cells was examined.Pteris vittata callus could effciently reduce As(V)to As(III)by the rapid introduction of reductase and synthesize thiols leading to phytochelatins production.Furthermore,Pteris vittata could control phosphate concentration in t...     

运用涂铁砂粒进行分散式饮水除砷的效果

采用铁盐处理普通河砂研制成涂铁砂粒(IOCS),通过静态和动态吸附试验观察其分散式饮水除砷效果,并选取地砷病现场进行了验证.结果表明,IOCS性质较为稳定,扫描电镜下可见铁氧化物呈片状分散的覆盖在砂粒表层;IOCS无需活化处理,其对砷的最大吸附发生在30min～60min;pH5～9时,五价砷的去除率随着pH值的升高而下降,三价砷的去除率变化不明显.IOCS对砷的吸附符合朗格缪尔(Langmuir)吸附方程;5次滤柱循环中,75g(50ml)的IOCS分别处理含砷1.0mg·L^-1的水样408～426床(三价砷),390～412床(五价砷),用0.2 mol·L^-1NaOH进行再生处理,砷回收率均在94%以上.充填IOCS 3.0kg的家庭用模拟装置处理含1.0mg·L^-1的五价砷209L和198L,三价砷196L和185L,并在现场实验期间,连续处理含砷0.202～1.733mg·L^-1的水样200L,对水质亦无不良影响.无论是经济上还是技术上,IOCS均是适合于分散式饮水除砷的一种新型除砷剂.     

Phylogenetic analysis and arsenate reduction e ect of the arsenic-reducing bacteria enriched from contaminated soils at an abandoned smelter site

Microbial reduction of As(V) (i.e., arsenate) plays an important role in arsenic (As) mobilization in aqueous environment. In this study, we investigated As(Ⅴ) reduction characteristics of the bacteria enriched from the arsenic-contaminated soil at an abandoned smelter site. It was found that As(Ⅴ) was completely reduced to As(Ⅲ) (i.e., arsenite) in 21 h. After 3-d incubation, a yellow solid was precipitated and the concentration of As(Ⅲ) decreased sharply. After 150 h incubation, ca. 65% of soluble arsenic was removed from the solution. The analysis of the precipitate by scanning electron microscopy and energy dispersive spectrometer (SEM-EDS) and X-ray diffraction (XRD) revealed that the main component was crystalline arsenic sulfide (ASS). Microbial mediated reduction and mobilization of adsorbed As(Ⅴ) on ferric hydroxide was also examined. In the microcosm slurry experiment, ca. 53% of the adsorbed As(V) was reduced to As(Ⅲ) by the bacteria, which resulted in an appreciable release of arsenic into aqueous phase. The released arsenic was present predominantly as As(Ⅲ). The microbial diversity was analyzed by 16S rDNA-dependent molecular phylogeny. A near-full-length 16S rDNA gene clone library was constructed. The 197 clones were analyzed using RFLP (restriction fragment length polymorphism) and 72 OTUs were obtained, which contributed 51% of the content for total clone number in six OTUs. Six bacterial clones in these six OTUs were selected for sequencing and the sequenced clones were found to belong to the group Caloramator, Clostridium, and Bacillus.     

Arsenic speciation for the phytoremediation by the Chinese brake fern, Pteris vittata

Arsenic (As) speciation for the phytoremediation by the Chinese brake fern was studied. In particular, the mechanism of how plants induce compounds containing thiol (SH) and proteins by As exposure in terms of the relationship between As and phosphate uptaken into plant cells was examined. Pteris vittata callus could efficiently reduce As(V) to As(III) by the rapid introduction of reductase and synthesize thiols leading to phytochelatins production. Furthermore, Pteris vittata could control phosphate concentration in the cells corresponding to the concentration of arsenite and arsenate. To our best knowledge, this is the first report to show the mechanisms of such high As tolerance of Pteris vittata using their callus in terms of in vitro approach for the analysis of As speciation and metabolism route.