Stability constant of Fe2+ chelates with soluble ligands from incubated soils

Summary The stability constants (log K) of Fe²⁺ chelates were determined on the basis of the shift in peak potential during the reduction of Fe²⁺ by a consortium of soluble ligands from incubated soils. Log K values ranged from 2.6 to 4.5. On average a change in pH of 1 unit induced a change in log K of 0.92 units. Aeration of the anaerobic decomposition products increased log K. The log K for Fe²⁺ chelates was about 0.8 units larger than that for Mn²⁺ chelates. It is considered that the chelation of ferrous iron plays an important role in the mobility and availability of iron to plants.     

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.     

Bonding and oxidation state of iron in humic complexes extracted from some Greek soils

Electron spin resonance and Mössbauer spectroscopy were used to obtain information on the nature of the bonding and of the oxidation state of iron in complexes of iron-humic substances, some prepared in the laboratory and others extracted from samples of Greek soils developed under different vegetative covers. In the synthetic FeHA complexes prepared at pH 2.8, iron was found to be both in trivalent and bivalent states. At pH 4.0, iron was in the trivalent state and only traces of Fe²⁺ were found.The ESR spectra of the natural HA's and FA's indicated that iron forms complexes with both of them. The Mössbauer spectra, however, did not provide verification for existence of Fe³⁺ organically bound to the humic acids. All the Mössbauer spectra of the natural humic acids showed minute quantities of iron in the ferrous state. The respective parameters indicated that the Fe²⁺ is directly bound to HA's. The ferrous iron probably resulted from the reduction of Fe³⁺ during the humification process. The Mössbauer spectrum of an FeFA complex prepared in the laboratory with a fulvic acid: metal ratio of (1:1) at pH 4.0 showed that iron occurred in both trivalent and bivalent states.Differences in the vegetative cover and in the taxonomic category of the soils studied did not have any effect on the Mössbauer parameters of the natural HA's, but differences were reflected in the ESR spectra of the FA's.     

Coupled diffusion and oxidation of ferrous iron in soils. I. Kinetics of oxygenation of ferrous iron in soil suspension

The kinetics of oxidation of iron in an aqueous suspension of a thoroughly reduced low-humus tropical rice paddy soil were followed by measuring the extractable ferrous iron in the whole suspension and in the solution. Three-quarters of the initial ferrous iron was oxidized rapidly (first-order rate constant = 9.2 × 10^?5 s^?1). The subsequent reaction was slow (first-order rate constant = 9.4 × 10^?7 S^?1) and was not studied in detail. The pH fell from 6.6 to 4.9 over the course of the fast reaction. In further experiments the rate of oxidation was followed at constant pH values in the range 6.5 to 4.5. It was concluded that the oxidation of adsorbed iron was much faster than solution iron, and that the adsorbed iron was oxidized at a rate that was nearly independent of the pH. During the reaction some ferrous iron is adsorbed on the ferric hydroxide formed. The proportion of the remaining ferrous iron adsorbed on ferric hydroxide rather than the original exchange surfaces was high at pH > 6.0 and low at pH < 5.0. The rate of oxidation of the ferrous iron was similar whether it was adsorbed on exchange sites or on the ferric hydroxide formed. Since the rate of oxidation of the iron adsorbed on ferric hydroxide was very much slower than that on ferric hydroxide formed in the absence of soil, it is suggested that the rate in soil may be controlled by diffusion of oxygen to the adsorption sites.     

Catalase and ascorbate peroxidase activities are not directly involveded in the silicon‐mediated alleviation of ferrous iron toxicity in rice

Silicon (Si) is the second most abundant element in the soil and can alleviate several abiotic stresses in many plant species. However, the mechanisms involved in alleviating ferrous iron (Fe²⁺) toxicity by Si are still largely unknown, and no study has investigated the role of Si on the Fe²⁺‐induced oxidative stress and antioxidant system in rice. Four cultivars of Asian and African rice (Oryza sativa L. and Oryza glaberrima Steud) were grown for 4 weeks under hydroponic conditions with or without Fe²⁺ (250 mg Fe²⁺ L^?1) and with or without Si (250 mg SiO₂ L^?1). The plants that were treated with Fe²⁺ suffered Fe²⁺ toxicity, and Si helped to alleviate the toxicity symptoms. The bronzing index and the Fe concentration in the foliar tissue increased in the presence of Fe²⁺ but decreased significantly with the application of 250 mg SiO₂ L^?1. The concentration of malonyldialdehyde, that is commonly used as an indicator of oxidative stress, increased in the foliar tissue in the presence of 250 mg Fe²⁺ L^?1 in the nutrient solution. The application of 250 mg SiO₂ L^?1 in the plant nutrient solution treated with Fe²⁺ considerably limited the increase of malonyldialdehyde. However, no significant effect of Si application on the activities of antioxidant enzymes (catalase and ascorbate peroxidase) and non‐enzymatic antioxidants (total ascorbate, reduced ascorbate, oxidized ascorbate, and the ratio of the reduced to oxidized forms) was observed in the rice plants that were grown in the presence of Fe²⁺. These results suggest that Si does not act directly on the antioxidant defense system of rice but reduces the plant Fe²⁺ concentration, which reduces the oxidative stress.     

Stability behavior of soil colloidal suspensions in relation to sequential reduction of soils

The cause of the decrease in the Fe²⁺ concentration of the soil solution in the later period of soil waterlogging was investigated. After 7-d incubation of the soil solutions separated from previously waterlogged soils (PWdS), a greyish precipitate (PPT) was observed in the soil solutions. The color of the PPT became reddish brown after separation from the solutions and freeze-drying. The PPT observed in 14-d-PWdS contained 352.6 g Fe kg^-1, 62.5 g C kg^-1, 22.6 g P kg^-1, 11.3 g Si kg^-1, 9.9 g N kg^-1, 0.7 g Al kg^-1 and a trace amount of Mn. However, Ca, Mg, K, and Na could not be detected. It was concluded that the separated PPT was dominated by amorphous ferric hydroxide based on the chemical analysis, broad IR absorption band at 585 cm^-1 and exothermic peak at 301°C. The data of chemical analysis and the characteristic IR bands of the PPT suggested that organic substances and presumably aluminosilicate anion were adsorbed onto the freshly-formed ferric hydroxide. The dominant phase of the greyish PPT in the reductive soil solution was considered to be ferrous PPT and was assumed to consist mainly of carbonate and/or hydroxide, and concomitantly of phosphate. The formation of the ferrous PPT in the soil solution in the later period of soil waterlogging was considered to (i) cause the decrease of concentration of Fe²⁺ ion and of other divalent cations such as Ca²⁺ due to the re-adsorption of Ca²⁺ on soil clays through the cation exchange reaction with Fe²⁺ ion, and consequently (ii) enhance the dispersion of the soil colloidal suspension.     

Methane and water soluble iron production under controlled soil pH and redox conditions

Abstract

When a soil is flooded, iron (Fe) reduction and methane (CH₄) production occurred in sequence as predicted by thermodynamics. The dissolution and precipitation of Fe reflected both soil pH and soil redox potential (Eh). The objective of our experiment was to determine both CH₄ production and Fe reduction as measured by Fe in solution in a flooded paddy soil over a wide range of closely controlled pH and Eh conditions. The greatest release of CH₄ gas occurred at neutral soil pH in combination with low soil redox potential (‐250 mV). Production of CH₄ decreased when soil pH was lowered in combination with an increase in the soil redox potential above ‐250 mV. Highest concentration of ferrous‐iron (Fe²⁺) under reducing conditions occurred when soil pH was lowered. Thus Fe reduction influenced CH4 formation in the flooded paddy soil. Results indicated that CH₄ production was inhibited by the process of ferric‐iron (Fe³⁺) reduction.     

Catalytic effect of various metal ions on the methylation of mercury in the presence of humic substances

Methylation of Hg²⁺ (Hg(NO₃)₂) in the presence of fulvic acid (FA) and various metal ions has been studied. The concentrations of Hg²⁺ and FA ranged from 5 to 20 mg L^?1 and 171 to 285 mg L^?1 DOC, respectively. The pH range was 3 to 6.5. FA was isolated from an acid brown-water lake by XAD-8 polymeric adsorbent. Methylmercury production in the dark during 2 to 4 days incubation at 30 °C increased with increasing concentrations of Hg²⁺ ion and FA as well as with additions of metal ions (5 to 10 × 10^?5 mole L^?1 The observed catalytic activity of metal ions followed the order Fe³⁺ (Fe²⁺) > Cu²⁺ ≈ Mn²⁺, > Al³⁺. The production of methylmercury had a pH-optimum around 4 to 4.5 at the conditions tested.     

Iron uptake in a bicarbonate-resistant cell line of tobacco

Abstract

In alkaline soils, plant growth is impaired mainly by high pH and high concentration of bicarbonates. The bicarbonate concentration increases the pH value, and causes deficiency of iron. A bicarbonate-resistant cell line (BR line) of tobacco (Nicotiana tabacum L. cv. Burley21) was selected by adding excess bicarbonate ions (20 mmol L^?1) to the culture medium. The pH of the medium was buffered 8.0 to 8.3. Under these conditions, about 80% of iron in the medium became insoluble. However, under such conditions, the BR line grew well. In this report, we examined some characteristics of the growth and iron uptake in the BR line under iron-deficient (i.e. high pH or no-iron) condition. At pH 5.8, the Fe³⁺ reduction activity was not significantly different between the non-selected line and the BR line. At pH 8.0, however, the Fe³⁺ reduction activity of the BR line was higher than that of the non-selected line. In no-iron condition, the growth of the non-selected line was markedly reduced after 2 weeks, while the BR line was not affected. The content of malic acid in both lines increased with the medium pH, and the content in the BR line was higher than that in the non-selected line. The BR line was able to adapt to the conditions, which restricted iron uptake, such as high bicarbonate concentration, high pH, and low iron conditions. The high ability of Fe³⁺ reduction was maintained at even high pH conditions. Further, the BR line may be able to improve the utilization of iron in the cells.     

THE OXIDATION OF IRON SULPHIDES IN SOILS IN RELATION TO THE FORMATION OF ACID SULPHATE SOILS, AND OF OCHRE DEPOSITS IN FIELD DRAINS

Aerating pyritic soils causes acidification and the forrnation of acid sulphate soils, or cat-clay. The Oxidation of pyrite in soils is associated with the deposition in tile drains of a form of ochre quite distinct from that formed by the action of filamentous iron bacteria. Pyrite-derived ochre results from the action of Thiobacillus ferrooxidans, which, below pH 3.5–4.0, catalyses the Oxidation of Fe²⁺ and pyrite. In soils less acid than c. pH 4, pyrite oxidizes relatively slowly by chemical reactions to Fe²⁺ and SO²₄^?. Under these conditions iron enters the drains as Fe²⁺ and is there oxidized by T. ferrooicidans and deposited as hydrated ferric oxide. Once the soil becomes acid enough for T. ferrooxidans to multiply, the rate at which pyrite oxidizes increases several-fold, and at c. pH 3 iron appears in the drainage water in the ferric form. Liming seems to decrease the rate of Oxidation.     

Removal of Pentachlorophenol by Adsorption on Magnetite-immobilized Chitin

The application of magnetite-immobilized chitin in pentachlorophenol (PCP) removal was demonstrated in this study. The physicochemical parameters for immobilization of chitin by magnetite, and for PCP adsorption using magnetite-immobilized chitin were optimized. For chitin immobilization, the optimized conditions were: magnetite to chitin (m:c) ratio at 1:2, initial pH 6, 25°C, 200 rpm and 60 min in batch system. The immobilization efficiency (IE) was 99.4% and immobilization capacity (IC) was 2.0 mg chitin mg^?1 magnetite. High initial pH (pH?>?11) and temperature (>30°C) lowered the IE and IC. For PCP (10 mg l^?1) adsorption, the optimized conditions were: 1,500 mg l^?1 immobilized chitin, initial pH 6, 25°C, 200 rpm and 60 min in batch system. The removal efficiency (RE) was 57.9% and removal capacity (RC) was 5.4 mg g^?1. The adsorption ability of immobilized chitin decreased with pH and temperature increased. However, increasing the amount of immobilized chitin (24,000 mg l^?1) can increase the RE up to 92%. Both chitin immobilization and PCP adsorption exhibited Langmuir and Freundlich adsorption isotherms. Results in this study indicated that magnetite-immobilized chitin was a cost-effective and environmental friendly adsorbent to remove environmental pollutants such as PCP.     

Sorption of primisulfuron on soil, and inorganic and organic soil colloids

Inorganic and organic soil colloids are responsible for the sorption of many pesticides. We studied the sorption of the herbicide primisulfuron [methyl 2 N‐[[[[[4,6‐bis(difluoromethoxy)‐2‐pyrimidinyl]amino]carbonyl]amino]sulfonyl]benzoate] on Fe³⁺‐, Al³⁺‐, Ca²⁺‐ and Na⁺‐exchanged montmorillonite, soil organic matter (H⁺‐ and Ca²⁺‐saturated), amorphous iron oxide, and three soils in aqueous media. The sorption on soils was negatively correlated with pH. Ca²⁺‐ and Na⁺‐exchanged montmorillonites are ineffective in the sorption of primisulfuron. The sorption on Fe³⁺‐ and Al³⁺‐exchanged montmorillonite is rapid and follows the Freundlich equation. Fourier transform infrared (FT‐IR) and X‐ray powder diffraction studies of the Fe³⁺‐ and Al³⁺‐montmorillonite samples after the interaction with primisulfuron in chloroform solution suggest that primisulfuron is adsorbed and degraded in the interlayer. Humic acid is more effective in the sorption than is Ca humate, suggesting that the pH of the suspension (3.5 for humic acid and 6.0 for Ca humate) has a strong influence on the sorption of primisulfuron. Experiments on amorphous iron oxide indicate similar pH dependence. Infrared spectra indicate that the protonation of the pyrimidine nitrogen moiety of herbicide and subsequent hydrogen bonding with the surface hydroxyls of Fe oxide is the mechanism acting in the primisulfuron sorption.     

应用穆斯堡尔(Mössbauer)谱学方法研究Fe+++, Fe++与胡敏酸结合的性质

本文应用穆斯堡尔谱学方法,研究了在不同pH条件下Fe⁺⁺⁺,Fe⁺⁺与胡敏酸结合的性质。计算机拟合的pH3.0⁵⁷Fe-胡敏酸络合物泥浆的穆斯堡尔谱和参数表明,三对四极双峰(图1,AA',BB',CC')是合理的。在pH3.0 Fe⁺⁺⁺以高自旋态存在,它和胡敏酸的结合多于一种环境类型。在pH1.0的⁵⁷Fe-胡敏酸络合物中,观察到了Fe⁺⁺⁺的穆斯堡尔谱信号,但没有Fe⁺⁺的信号,在这个样品离心分离出的液体部分检测到了加入量的59.7%的铁,表明在pH1.0时相当数量的Fe⁺⁺⁺被胡敏酸还原成Fe⁺⁺,Fe⁺⁺并不与胡敏酸牢固结合。在30伏电析过的⁵⁷Fe-胡敏酸络合物样品中(pH2.8)出现了Fe⁺⁺⁺,Fe⁺⁺的穆斯堡尔谱,这一结果指示出,在电析过程中由于⁵⁷Fe⁺⁺⁺-胡敏酸络合物悬浮液pH的降低,一部分Fe⁺⁺⁺还原成Fe⁺⁺,在pH2.8相当数量的Fe⁺⁺与胡敏酸牢固结合。根据在80K记录的穆斯堡尔谱,在pH1—3的范围出现了磁有序成份,但在室温记录的穆斯堡尔谱没有磁分裂。     

Defektverbindung und fremdionen-dotierung bei der ausfällung von eisen(II)- und eisen(III)- phosphaten

The products obtained by precipitation of iron(III)-phosphates in the pH-range from 2 to 13.35 differ in their composition from the ones reported in the literature, e.g. FePO₄2H₂O or FeH₂(OH)₂PO₄, except at low pH values.The molar ratio of 1 : 1 for P and Fe is disturbed to a significant extent and foreign ions are also incorporated as integral parts of the precipitation products.The change of the P/Fe ratio within the pH-range from 2 to 11 is the result of continuous substitution of OH^? for PO₄^? ions. This exchange may be explained as a transition between phosphate- and hydroxide-“Defektserie” (reference is to sets of compounds which form partial rather than complete isomorphous series), whereby the critical composition and the critical precipitation product are defined by the inflection point of the exchange curve.During the hydrothermal recrystallization of amorphous precipitation products, crystalline iron phosphate (strengite) is formed at low pH values, and crystalline iron hydroxide is formed at high pH values. The precipitation products formed at intermediate pH values have a heterogeneous composition and fail to crystallize under these conditions.     

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.     

Association of soil properties and soil and plant iron to iron deficiency response in rice (Oryza Sativa L.)

Abstract

Problems are invariably encountered when attempts are made to explain the variability in Bray percent yields or plant response in terms of soil or plant iron (Fe). To resolve this inconsistency, the present investigation was initiated to identify a combination of soil extractable Fe, soil properties and form of plant Fe that may be used as a measure of Fe deficiency. The study involved 16 diverse soils, using upland rice (Oryza sativa L.) as the test crop and Fe‐EDDHA [ferric ethylenediamine di (o‐hydroxyl‐phenyl acetic acid)] as source of Fe. The results showed that Bray percent yields were neither related to DTPA (diethylenetriamine pentaacetic acid) or EDTA (ethylenediamine tetraacetic acid) extractable Fe nor with total plant Fe. Even the inclusion of pH, lime, organic carbon and clay data in the regression equations was of no value. However, Bray percent yields were significantly and positively (r = 0.57* ) associated with ferrous Fe (Fe²⁺) in 40‐day‐old rice plants. The explanation concerning variability in Bray percent yields obtained on diverse soils could be increased about one and half 2 times (R²= 0.59*) if the contribution of lime and soil pH was also incorporated in the stepwise regression analysis. The individual contribution to R of lime, pi respectively. Thus, it appears that Fe²⁺ concentration in plants (along with soil pH) may identify Fe deficiency. The critical limit to separate Fe deficient from green rice plants was set at 45 ug Fe²⁺/g in the leaves.     

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。     

Effect of Organic Ligands on Copper(II) Removal from Metal Plating Wastewater by Orange Peel-based Biosorbents

A neutrophilic, autotrophic bacterium that couples iron oxidation to nitrate reduction (iron-oxidizing bacteria [IOB]) under anoxic conditions was isolated from a working bioremediation site in Trail, British Columbia. The site was designed and developed primarily to treat high concentrations of Zn and As that originate from capped industrial landfill sites. The system consisted of two upflow biochemical reactor cells (BCR) followed by three vegetated wetland polishing cells with sub-surface flow and a holding pond. During a 5-year period (2003–2007), the system treated more than 19,100 m³ of contaminated water, removing and sequestering more than 10,700 kg of As, Zn and sulfate at average input water concentrations of: As, 58.6 mg?l^?1 (±39.9 mg?l^?1); Zn, 51.9 mg?l^?1 (±35.4 mg?l^?1) and SO₄ ^2?, 781.5 mg?l^?1 (±287.8 mg?l^?1). The bacterium was isolated in order to better understand the mechanisms underlying the consistent As removal that took place in the system. Analysis using Basic Local Alignment Search Tool (BLAST) database showed that the closest homologies are to Candidatus accumulibacterphosphatis (95 % homology), Dechloromonas aromatica (94 %), and Sideroxydans lithotrophicus ES-1 (92 %) Within the BCR cells, the IOB oxidized Fe²⁺ generated by iron-reducing bacteria (IRB); the source of the iron was most likely biosolids and coatings of iron oxide on locally available sand used in the matrix. We have provisionally designated the novel bacterium as TR1.     

硅缓解水稻根系铁毒害

Silicon (Si) can enhance the resistance of plants to many abiotic stresses. To explore whether Si ameliorates Fe2+ toxicity, a hydroponic experiment was performed to investigate whether and how Si detoxifies Fe2+ toxicity in rice (Oryza sativa L.) roots. Results indicated that rice cultivar Tianyou 998 (TY998) showed greater sensitivity to Fe2+ toxicity than rice cultivar Peizataifeng (PZTF). Treatment with 0.1 mmol L-1 Fe2+ inhibited TY998 root elongation and root biomass significantly. Reddish iron plaque was formed on root surface of both cultivars. TY998 had a higher amount of iron plaque than PZTF. Addition of Si to the solution of Fe treatment decreased the amount of iron plaque on root surface by 17.6% to 37.1% and iron uptake in rice roots by 37.0% to 40.3%, and subsequently restored root elongation triggered by Fe2+ toxicity by 13.5% in the TY998. Compared with Fe treatment, the addition of 1 mmol L-1 Si to the solution of Fe treatment increased xylem sap flow by 19.3% to 24.8% and root-shoot Fe transportation by 45.0% to 78.6%. Furthermore, Si addition to the solution of Fe treatment induced root cell wall to thicken. These results suggested that Si could detoxify Fe2+ toxicity and Si-mediated amelioration of Fe2+ toxicity in rice roots was associated with less iron plaque on root surface and more Fe transportation from roots to shoots.     

Stable Incorporation of Co2+ into Ferrite Structure at Ambient Temperature: Effect of Operational Parameters

Removal of heavy metals (HM) from industrial wastewater is of primary environmental importance. The seeded ambient temperature ferrite process, in which heavy metals are removed from solution by their incorporation into a magnetite structure, is an attractive alternative for both complete metals separation and generation of stable magnetic sludge. Despite its potential, the effects of various operational parameters on the composition and stability of the final product are not fully understood to-date. The current paper addresses the combined effect of selected operational parameters on Co²⁺ incorporation efficiency. Co²⁺, which is used in many industries (e.g., plating, alloys manufacturing, catalytic converters, and paint pigments) and hence may pose a significant environment risk, was shown previously to be incorporated successfully into ferrite structures. Co²⁺-bearing ferrites were synthesized from Fe(II) solutions at ambient temperature (20 and 30°C), applying slow oxidation, pH 10.5, 20 g (as Fe) l^?1 ferrite seed, and different Fe²⁺ to Co²⁺ influent ratios. Pure magnetite was used as an initial seed, which was gradually exchanged, as the reaction proceeded, by the Co²⁺-bearing ferrite generated within the reactor (until full exchange at steady state). Under all conditions investigated >99% of the Co²⁺ was removed from solution (yielding Co²⁺ levels below 20 μg/l in the effluent) while the generated precipitates were composed predominately of magnetite/ferrite (based on XRD analysis). Favorable results in terms of Co²⁺ incorporation efficiency and sludge stability were obtained when the following operational conditions were applied: 1:10 Co²⁺:Fe²⁺ molar influent ratio, 30°C, 1,500 mgFe²⁺ l^?1 metal intermediate concentration, and 16 h aging period (pH 10.5).