Influences of Humic Acid on Cr(VI) Removal by Zero-Valent Iron From Groundwater with Various Constituents: Implication for Long-Term PRB Performance

A 9-month-long continuous flow column study was carried out to investigate Cr(VI) removal by Fe⁰ with the presence of humic acid. The study focused on the influences of humic acid promoted dissolved iron release and humic acid aggregation in Fe⁰ columns receiving synthetic Cr(VI) contaminated groundwater containing various components such as bicarbonate and Ca. The effects of humic acid varied significantly depending on the presence of Ca. In Ca-free columns, the presence of humic acid promoted the release of dissolved iron in the forms of soluble Fe-humic acid complexes and stabilized fine Fe (hydr)oxide colloids. As a result, the precipitation of iron corrosion products was suppressed and the accumulation of secondary minerals on Fe⁰ surfaces was diminished, and a slight increase in Cr(VI) removal capacity by 18% was record compared with that of humic acid-free column. In contrast, in the presence of Ca, as evidenced by the SEM and FTIR results, humic acid greatly co-aggregated with Fe (hydr)oxides and deposited on Fe⁰ surfaces. This largely inhibited electron transfer from Fe⁰ surfaces to Cr(VI) and reduced the drainable porosity of the Fe⁰ matrix, resulting in a significant decrease in Cr(VI) removal capacity of Fe⁰. The Cr(VI) removal capacity was decreased by 24.4% and 42.7% in humic acid and Ca receiving columns, with and without bicarbonate respectively, compared with that of Ca and humic acid-free column. This study yields new considerations for the performance prediction and design of Fe⁰ PRBs in the environments rich in natural organic matter (NOM).     

Sites and processes of nutrient accumulation in the subsoil through water percolation from the plow layer in a submerged paddy soil microcosm

The leaching of nutrients from the plow layer by water percolation and their accumulation in the subsoil observed in a Japanese paddy field (Katoh et al. 2004: Soil Sci. Plant Nutr., 50, 721-729) were determined semi-quantitatively in a soil column experiment. Ca²⁺, Mg²⁺, K⁺, Mn²⁺, Fe²⁺, and phosphate in percolating water from the plow layer soil column were retained in the subsoil columns that were connected to the plow layer soil column. Fe²⁺, K⁺, and phosphate accumulated in the uppermost part of the subsoil. Accumulation of Fe²⁺ in the uppermost part of the subsoil was presmnably due to the cation exchange process with concomitant desorption of Ca²⁺. In contrast, Ca²⁺ and Mg²⁺ in percolating water from the plow layer soil colmnn accumulated once in the subsoil, and translocated downwards slowly with successive water percolation. Considerable amounts of inorganic carbon (IC) and dissolved organic carbon (DOC) in percolating water from the plow layer soil column were also retained in the subsoil columns. IC did not accumulate a gaseous form.     

Estimating Levels of Micropollutants in Municipal Wastewater

The evolution of microbial communities with increasing carbon tetrachloride concentrations was studied in two anaerobic columns containing sand and two different clay soils, one of which contained high levels of iron. Microbial communities were characterized through analysis of column effluents with denaturing gradient gel electrophoresis and quantitative polymerase chain reaction for archaea and eubacteria as inlet carbon tetrachloride concentrations were increased from 0.8 to 29 μM. Inhibition of microbial activity was observed in both columns, and was associated with the accumulation of chloroform at concentrations of 0.2 to 0.4 μM as inlet CT concentrations were increased to 2.4–3.0 μM in the low-iron clay column and approximately 16 μM in the iron rich clay column. Inhibition was indicated by decreasing rates of methanol and carbon tetrachloride degradation, decreases in effluent levels of DNA, and shifts in microbial communities of the columns. Even with the inhibition observed, in the iron-rich clay column CT degradation continued to the end of the study with inlet CT concentrations of 29 μM, in contrast to the low-iron clay column in which minimal CT degradation occurred once CT inlet concentrations exceeded 3 μM. The greater capacity for CT degradation in the column containing the iron-rich clay was hypothesized to be the result of reaction with biogenic ferrous iron produced by biological dissimilatory iron reduction.     

Effect of Groundwater Inorganics on the Reductive Dechlorination of TCE by Zero-Valent Iron

Although permeable reactive barriers (PRB) technology appears to be a very suitable and cost effective option, the extent to which remediation results will be realized, greatly depends on the long-term integrity of the system. The formation of mineral precipitates is possibly a major factor in the long-term performance of PRB. Precipitates may passivate reactive surfaces by blocking electron-transfer sites, and thereby reduce the long-term reactivity of the granular iron to degrade groundwater contaminants. To evaluate the potential passivation impacts of inorganic groundwater chemistry, column experiments containing zero-valent iron (Fe⁰) were performed under anoxic conditions to treat two contrasting Danish groundwater types spiked with trichloroethylene (TCE). For most of the experiments using Danish groundwater types, a soft low alkalinity groundwater produced slightly higher TCE dechlorination rate than did a hard high alkalinity groundwater. Compared to a soft low alkalinity baseline groundwater, it was also found the dechlorination of TCE in the column was enhanced in the presence of 1 mM CaCO₃ and 1 mM NaHCO₃. The dechlorination of TCE in the presence of 1 mM KNO₃ and 1 mM Na₂SiO₃ was found to decrease considerably compared with the baseline solution. The results suggest that the composition of field groundwater is likely to strongly affect the ability of Fe⁰ barriers to degrade TCE.     

Adsorption and degradation of 2,4-dichlorophenoxyacetic acid in spiked soil with Fe0 nanoparticles supported by biochar

This work studies the adsorption and degradation of 2,4-dichlorophenoxyacetic (2,4-D) in spiked soil with nanoscale Fe⁰ particles (nFe⁰) and biochar derived from maize straw. When biochar concentration was high, the adsorption capacity of soil was enhanced. Furthermore, 2,4-D degraded completely at loading rates of 0.33 and 0.17 g/L nFe⁰ plus biochar (initial 2,4-D concentration of 10 mg/g) within 40 h, according to equilibrium data. Additionally, the theoretical concentration of chloridion was approximately 84%. Further analysis indicated that the effect of nFe⁰ on 2,4-D degradation was weaker in soil columns than that in soil slurry. By contrast, 2,4-D degradation is positively influenced by biochar application, which prevented the aggregation and corrosion of Fe nanoparticles. Although the enhanced capacity for 2,4-D adsorption on the soil decelerated dechlorination rate, long-term nFe⁰ activity was generated. After 72 h, the efficiency of 2,4-D degradation was approximately 53.2% in the soil columns with biochar support.     

The toxicity of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) to the freshwater green alga Selenastrum capricornutum

RDX was not acutely toxic to the green alga Selenastrum capricornutum when tested at the solubility limit of the compound in algal assay media. A maximum reduction of 38% in cell density occurred after a 96-h exposure to 36.7 mg L^?1; thus, an EC50 could not be determined. The lowest-observed-effect concentration (LOEC) and no-observed-effect concentration (NOEC) for the green alga were 4.8 and 0.5 mg L^?1 (reduction in cell density), respectively, when chronic end points were used to analyze the data. A comparison of the data in this study with RDX toxicity data from the literature for the diatom Navicula pelliculosa, and the blue-green algae (cyanobacteria) Microcystis aeruginosa and Anabeana flos-aquae shows that S. capricornutum is the most sensitive to RDX. A U.S. Environmental Protection Agency (EPA) numerical water quality criterion Final Plant Value based on the green alga data should protect the above algal groups.     

Redoxpotentiale und redoxprozesse von mangan-, eisenund schwefelverbindungen in hydromorphen böden und sedimenten

Results of laboratory experiments with soil material saturated with sea water indicate that, as predicted by thermodynamics, manganese (III, IV)-oxides are first reduced to Mn²⁺-ions (beginning at about +450 mV at pH 6.1.; E₇ ≈ +400 mV), next amorphous iron (III)-oxides are reduced to Fe²⁺-ions (beginning at about +220 mV at pH 6.0; E₇ ≈ +160 mV), and finally sulphates are reduced to sulphides (beginning at about +10 mV at pH 6.0; E₇ ≈ -50 mV). Direct quantitative relations between redox potentials, pH-values and Mn²⁺- (or Fe²⁺-) contents of water-saturated soils and sediments and calculated redox reactions of known manganese and iron systems could not be established.The influence of organic redox systems produced by microbial fermentation processes on the measured potentials and on the reduction of manganese and iron oxides is discussed.A reduction of the oxides by microbially formed sulphides, which themselves are oxidized by this process, seems also to be possible. Therefore, sulphides do not occur as stable sulphur phase in higher amounts before all available Fe-oxides are reduced to Fe²⁺-ions. Then formation of iron monosulphides takes place by precipitation of Fe²⁺- ions by sulphides (H₂S, HS). In a sulphide-stabilized environment redox reactions of sulphur — especially the reaction H₂S_aq = S₀ + 2 H⁺ + 2 e^? — may determined the measured potentials.The results show that the dynamics and morphology of hydromorphic soils and sediments are strongly dependent on microbial processes.     

Characterization and Performance of Granular Iron as Reactive Media for TCE degradation by Permeable Reactive Barriers

The application of Permeable Reactive Barriers (PRBs), an innovative clean-up technology for in-situ groundwater remediation, represents an effective alternative to traditional pump-and-treat systems and has raised strong interest during recent years. From recent statistics of the Italian Water Research Institute (IRSA), trichloroethylene (TCE) from industrial activities is the most widespread contaminant in groundwater. The goal of the research was to test the suitability and performance of a high purity granular iron reactive medium for TCE degradation by PRBs. The suitability was evaluated based on chemical and physical characteristics of the material and the performance of the granular iron, in terms of TCE removal efficiency, was evaluated by column tests.The experimental results showed that the characteristics of the granular iron are suitable for application as a reactive medium, since the hydraulic conductivity values were fully consistent with those reported in the literature, and the leaching tests indicated a reduced release of heavy metals. The overall removal efficiency of TCE was higher than 97% in all the tests performed at the flow rate of 0.25 cm³ min^-1 (corresponding to a groundwater flow velocity of 0.37 m d^-1) both for the 100% iron and the iron-sand columns. Moreover, TCE degradation around 60% was observed even in the first section of the columns fortypical groundwater flow velocity. The TCE reduction in the outlet stream was confirmed by the assessment of chlorine mass balance and by the absence of any reaction intermediate detected by GC-MS.Finally, the concentration profiles in the columns were not in agreement with those expected for a chemistry-controlled kinetic regime. This suggests that TCE degradation rate may have been limited by precipitation phenomena, hindering the contaminant transport to the iron surface.     

Methane production and its fate in paddy fields

Abstract

Oxidation of methane and total water soluble organic carbon (TOC) in the subsoil, which percolated from the plow layer, was investigated in a column experiment. The amounts of both methane and TOC in the leachate decreased by percolation in the subsoil.

Fe²⁺ percolated from the plow layer was nearly completely retained in the subsoil. The decomposition of methane and TOC in the subsoil was considered to result in the coupling with the formation of Fe²⁺. Methane was estimated to contribute ca. 19–21% to the total amount of Fe²⁺ formed in the subsoil by the organic materials in the leachate.     

Fe2+对水稻生长及土壤微生物活性的影响

通过盆栽试验,模拟冷浸田土壤亚铁毒害,研究了土壤-水稻-亚铁-微生物相互作用的体系中,外加Fe2+ 不同处理水平 (0、 100、 200、 400、 800和1600 mg/kg) 对水稻苗期和分蘖期相关生理指标、 土壤微生物活性及其生态特征的影响。结果表明, 在含一定亚铁本底（207.77 mg/kg）的正常稻田土壤中,外源性Fe2+的加入将逐步抑制水稻生长、 降低土壤微生物活性。外源Fe2+浓度达100 mg/kg后,水稻的株高、 干物质积累量显著降低; 水稻叶片生理指标叶绿素含量（SPAD值）、 脯氨酸含量、 抗氧化酶系统活性则显著增加,表明外源Fe2+浓度100 mg/kg 是本研究条件下外源Fe2+ 对水稻生长产生显著毒害影响的临界点; 同时随外源Fe2+浓度的增加, 土壤微生物活性指标土壤微生物量碳、 微生物三大基础菌系总量（细菌、 真菌、 放线菌）、 功能菌系总量（氨化细菌、 固氮菌、 纤维分解菌）、 铁还原菌总量总体是先快速下降,后逐渐平稳降低。 半效应浓度EC50分析表明,外源Fe2+浓度100 mg/kg 为多数土壤微生物活性指标（微生物基础菌系总量、 功能菌系总量、 铁还原菌）EC50变化的临界值; 体系中土壤微生物活性指标和水稻生长指标的变化存在显著的相关性, 表明供试土壤亚铁对水稻生长的影响是亚铁对土壤-植物-土壤微生物系统同步影响的结果。综上结果可知,外源Fe2+浓度100 mg/kg为导致供试土壤中水稻生长及土壤微生物活性受到显著负效应的临界值,进而推知,本研究所用土壤对水稻生长和微生物活性的亚铁毒胁迫临界浓度约为300 mg/kg（含本底）, Fe2+含量超出该浓度时,需采取合理的农艺措施控制其负效应。     

Bioaugmentation and biostimulation effects on PAH dissipation and soil ecotoxicity under controlled conditions

The degradation of spiked anthracene (ANT), pyrene (PYR) and benzo[a]pyrene (B[a]P) in soil (3000 mg ∑ 3 PAHs kg⁻¹ dry soil) was studied in aerobically incubated microcosms for 120 d. The applied treatments aimed at enhancing PAH removal from the heavily contaminated soils are: (i) bioaugmentation by adding aged PAH-contaminated soil (ACS) containing activated indigenous degraders; and (ii) combined bioaugmentation/biostimulation by incorporating sewage sludge compost (SSC) and decaying rice straw (DRS). The adopted treatments produced higher PAH dissipation rates than those observed in unamended PAH-spiked soils, especially for ANT and PYR in the presence of DRS or ACS (>96%). However, B[a]P was the most recalcitrant hydrocarbon to biodegradation. Extracellular enzyme investigation revealed the existence of ligninolytic activities in all soil treatments, including control but no relationship could be found with PAH dissipation. The ecotoxicological assessment indicated that regardless of applied treatment, PAH-spiked soils were chronically lethal to ostracod Heterocypris incongruens and confirmed the sensitivity of the microcrustacean to the concomitant presence of these three hydrocarbons. Lettuce root elongation inhibition was correlated with PAH level but the presence of SSC conferred a strong phytotoxic capacity to PAH-spiked soils. DRS amendment may constitute a cost-effective alternative for hydrocarbon bioremediation as it has impacted positively on soil microbial activity and enhanced PAH removal with no apparent changes in soil physico-chemical properties.     

The properties, genesis and significance of a man-made iron pan podzol near Castletownbere, Ireland

Beneath a layer of artificially deposited peat debris, an eluvial E horizon and thin iron pan (Bsm) has developed in the upper part of the A_p horizon of a brown podzolic soil (Haplorthod) near Castletownbere, Ireland. The thickness of the E horizon and the depth of the pan are directly related to the thickness of the layer of peat debris. The original soil (Haplorthod) was strongly podzolized with a significant accumulation of organic carbon, iron and aluminium in the spodic Bs. The iron pan of the upper sequum, on the other hand, is rich in iron. This iron pan seems to have developed as a result of reduction of iron in the E horizon, transport of divalent Fe²⁺ cations and precipitation in the Bsm as Fe₂(OH)₃, after the peat layer was deposited. We conclude that podzolization and iron pan development were fundamentally different processes. The study suggests a fundamental change in the iron pan-blanket peat development sequence previously postulated by palaeoenvironmentalists in Ireland.     

Changes of redox and pH conditions in a flooded soil amended with glucose and manganese oxide or iron oxide under laboratory conditions

The changes of Eh and pH in soil suspension (Ah-horizon of a Mollic Gleysol) and Mn²⁺ or Fe²⁺ concentrations in the equilibrium soil solution at different levels of glucose (0%, 0.5% and 1%) and MnO₂ (0%, 0.025%, 0.05% and 0.1%) or Fe₂O₃ (0%, 0.025%, 0.05% and 0.1%) were examined. It was found that the degree of Mn- and Fe-reduction in soil depends mainly on the presence and the amount of an easily decomposable carbon source and to a minor degree on the content of native or added forms of MnO_O2 or Fe₂O₃ in the soil. Theoretical relationships between the water soluble manganese and iron and the Eh and pH values have been verified, when the observed initial drop of Eh was eliminated. It was found that the water soluble manganese content was described best by the Mn₂O₃/Mn²⁺ redox system, and that of iron by the Fe₃ (OH)₃/Fe²⁺ system.     

Chelation effects on the iron reduction and uptake by low‐iron stress tolerant and non‐tolerant citrus rootstocks

The relative rates of ferric‐iron (Fe³⁺) reduction and uptake by two citrus rootstocks were measured for a series of synthetic Fe³⁺ chelates and microbial siderophores. The rates of Fe³⁺ reduction by the citrus seedlings followed the order: FeHEDTA >> FeDPTA > FeCDTA. No reduction occurred for FeDFOB (ferrioxamine B) and FeTAF (ferric triacetylfusagen). Low rates of Fe³⁺ reduction occurred for Fe2RA3 (ferric rhodotorulic acid). The levels of ⁵⁵Fe taken up the citrus seedlings showed good correlations with the reduction rates. These results indicate the importance of Fe³⁺ reduction in the Fe uptake by citrus rootstocks. The immobility of a large percent of the ⁵⁵Fe taken up by the roots is attributed to the accumulation of Fe in the root apoplasts.     

Iron inefficient and efficient oat cultivars. II. Characterization of phytosiderophore released in response to Fe deficiency stress

Abstract

Iron‐inefficient TAM 0–312 and Fe‐efficient Coker 227 oats (Strategy II plants) differ in their release of phytosiderophore in response to iron‐deficiency stress—the Fe‐efficient Coker 227 releases a phytosiderophore whereas the Fe‐inefficient TAM 0–312 does not. The phytosiderophore released by Coker 227 oats in response to Fe‐deficiency stress does not appear to transport Fe into the plant as Fe phytosiderophore. When the Fe‐inefficient TAM 0–312 and Fe‐efficient Coker 227 oats were subjected to Fe supplied as Fe²⁺(BPDS)₃, Fe³⁺HEDTA, as Fe³⁺EDDHA, Coker 227 utilized the Fe more efficiently than TAM 0–312 in every case. Both cultivars reduced Fe³⁺ as FeCl₃ to form Fe²⁺(BPOS)₃ and responded better to this form of Fe than Fe supplied as the ferric chelate. Reduction of Fe³⁺ at the root appears to be a factor that facilitates iron uptake by Coker 227 oats and the release of a phytosiderophore appears to make more Fe available at the root that can be reduced and transported to plant tops.     

The effect of surface growth of blue-green algae and bryophytes on some microbiological,biochemical, and physical soil properties

Summary The influence of surface growth of inoculated cyanobacteria (blue-green algae) on subsurface properties of a brown earth, silt loam soil was studied in reconstituted flooded soil columns. One blue-green algae species, Nostoc muscorum, become dominant within the first 7 days of inoculation. In light control columns (not inoculated) a bryophyte, Barbula recurvirostra, was dominant although significant growth of indigenous blue-green algae occurred. The blue-green algae counts were in the range of 1×10⁶ g^-1 dry soil in the surface layer (0–0.7 cm) in both columns. Any effect of surface phototrophic growth on soil properties was restricted to the surface layer. In inoculated columns there was a twofold increase in microbial biomass and an eightfold increase in bacterial numbers by week 13. However, bacterial numbers declined so that there was only a 2.8-fold increase by week 21. Dehydrogenase (x2.1), urease (x2.8) and phosphatase (x3.1) activities and polysaccharides (+69%) increased by week 21 as a result of the blue-green algae inoculation along with a significant improvement in soil aggregation. However, similar increases occurred in the light control columns, indicating that given appropriate conditions of light and moisture indigenous species may be ultimately as effective as introduced species in bringing about biochemical and microbiological changes to soil.     

第四纪红粘土发育的红壤在施用石灰和石膏后元素的淋溶特征

本文利用模拟土柱试验研究了施用石灰和石膏对第四纪红粘土发育的红壤中元素淋溶过程的影响。结果表明,施用石灰后１０ｃｍ土层中除Ｃａ＾２＋以外的阳离子元素的淋失量减少,而ＳＯ４＾２－和ＨＣＯ３＾－的淋失量增加;施用石膏后１０ｃｍ土层中所有阳离子元素,特别是ＡＩ＾３＋的淋失量增加。红壤中Ｃａ＾２＋的淋失以自由态为主,施用石膏后与ＳＯ４＾２－结合的比例增加,不同处理中３０ｃｍ土层处铝的淋失以自由态为离,１０     

Degradation of Para-nitrochlorobenzene by the Combination of Zero-valent Iron Reduction and Persulfate Oxidation in Soil

The oxidation of para-nitrochlorobenzene (pNCB) by persulfate (PS) activated with zero-valent iron (Fe⁰) was investigated through a series of batch experiments. The pNCB reduction ratio increased with the decrease of the initial solution pH. It is found that temperature and Fe⁰ dosage could also influence the pNCB removal. Under the conditions of initial Fe⁰ dosage 0.8 mmol/g, initial pH of 6.6, and 25 °C, 66.3% of pNCB was reduced in 6 h. The pNCB was slightly degraded in the presence of PS alone. When PS was dosed after 2 h of Fe⁰ reduction, significantly higher pNCB removal (94.1%) and mineralization (36.4%) were obtained relative to the case of simultaneous dosing of Fe⁰ and PS (85.3% removal, 22.6% mineralization). This indicates that the reduction product of pNCB was more easily oxidized by PS than pNCB, suggesting that converting the nitro groups of pNCB to amino groups prior to oxidation can enhance their oxidation. These results suggest that a sequential Fe⁰ reduction–PS oxidation process may be an effective strategy to promote pNCB decomposition in contaminated soil.     

Part 1: Vadose-Zone Column Studies of Toluene (Enhanced Bioremediation) in a Shallow Unconfined Aquifer

The objectives of the laboratory study described in this paper were (1) to determine the effectiveness of four nutrient solutions and a control in stimulating the microbial degradation of toluene in the unsaturated zone as an alternative to bioremediation methodologies such as air sparging, in situ vitrification, or others (Part I), and (2) to compare the effectiveness of the addition of the most effective nutrient solution from Part I (modified Hoagland type, nitrate-rich) and hydrogen peroxide (H₂O₂) on microbial degradation of toluene for repeated, simulated spills in the unsaturated zone (Part II). For Part 1, fifteen columns (30-cm diameter by 150-cm height), packed with air-dried, 0.25-mm, medium-fine sand, were prepared to simulate shallow unconfined aquifer conditions. Toluene (10 mL) was added to the surface of each column, and soil solution and soil gas samples were collected from the columns every third day for 21 days. On day 21, a second application of toluene (10 mL) was made, and the experiment was run for another 21 days. Solution 4 was the most effective for microbial degradation in Part I. For Part II, three columns were designated nutrient-rich 3-day toluene columns and received toluene injections every 3 days; three columns were designated as nutrient-rich 7-day columns and received toluene injections every 7 days; and two columns were used as controls to which no nutrient was added. As measured by CO₂ respiration, the initial benefits for aerobic organisms from the O₂ enhancement were sustained by the bacteria for only a short period of time (about 8 days). Degradation benefits from the nutrient solution were sustained throughout the experiment. The O₂ and nutrient-enhanced columns degraded significantly more toluene than the control columns when simulating repeated spills onto the unsaturated zone, and demonstrated a potentially effective in situ bioremediation technology when used immediately or within days after a spill. The combined usage of H₂O₂ and nitrate-rich nutrients served to effectively maximize natural aerobic and anaerobic metabolic processes that biodegrade hydrocarbons in petroleum-contaminated media. Applications of this technology in the field may offer economical advantages to other, more intrusive abatement technologies.     

Artificial permeable redox barriers for purification of soil and ground water: A review of publications

Artificial permeable barriers are used for purification of soil- and groundwater from organic (Cl-containing hydrocarbons and Cl-pesticides) and inorganic (heavy metals and nitrates) pollutants. Most of the created redox barriers are based on introducing electron donors—both inorganic (most often, Fe⁰ and Fe(II)) and organic. By their construction type, they are subdivided into one-time filled barriers arranged in trenches and multiple-time filled barriers that use boreholes for reagent injection. Fe⁰ barriers operate not only as reducers but also as sources of newly formed Fe(III) colloids adsorbing elements with permanent and variable valence. Iron sulfides are also used as a reagent, while Fe(II) and S(-I, -II) serve as reducers. Biochemical redox barriers are also applied, in which the activity of natural anaerobic metal-reducing bacteria is stimulated by adding an available organic substance.