No full-text available
To read the full-text of this research,
you can request a copy directly from the author.
Abstract
The kinetics of the reaction between
manganese dioxide and ferrous ion in acid solution have been investigated by
using the potential of the ferrous-ferric couple as a measure of the extent of
reaction. The experimental conditions were such that the reaction rate was
independent of ferrous, ferric, manganous ions, and acid concentrations and the
agitation was sufficient to prevent bulk diffusion in the solution from being a
rate-determining factor.
The reaction rate of sized samples of
pyrolusite and γ-MnO2 in ferrous
sulphate solution was proportional to the surface area of the solid and was
constant (i.e. " zero-order ") until 50 per cent. of the solid was
consumed. γ-MnO2 reacted about twice
as rapidly as the pyrolusite. The reaction occurred most readily at certain
active sites on the particles and appeared to proceed along crystal boundaries
in such a manner that the active surface area was not significantly changed
during the first half of the reaction. In ferrous perchlorate the reaction rate
of 10 μ diameter pyrolusite was about one-hundredth of that in sulphate
and the reaction appeared to occur at a more even rate over the whole surface
of the particle so that the zero-order law was no longer obeyed. Activation
energies of 7.4 and 5 kcal in sulphate and perchlorate respectively, for the
temperature range 18 to 40 °C, suggest that the difference in rate is a result
of a change in the entropy factor of the Arrhenius equation. It is suggested
that this difference in rate may result from the activation, by sulphate ions,
of a less reactive lower oxide of manganese which is formed on the surface.
... Alternatively, various chemical compounds have been used as reducing agent in the reductive dissolution of MnO 2 . Among them, sulfur dioxide [1][2][3][4][5], ferrous sulfate and chloride [6][7][8][9] as well as iron sulfides [10][11][12] seem to be more attractive due to the availability and cost considerations. Kinetics of the reaction between manganese dioxide and ferrous ion in acid solutions has been studied by Koch [6]. ...
... Among them, sulfur dioxide [1][2][3][4][5], ferrous sulfate and chloride [6][7][8][9] as well as iron sulfides [10][11][12] seem to be more attractive due to the availability and cost considerations. Kinetics of the reaction between manganese dioxide and ferrous ion in acid solutions has been studied by Koch [6]. He concluded that the reaction proceeded on active sites and relatively low values of activation energy (20.9 kJ/mole for ferrous sulfate) derived in the study, might be an indication that the reaction was controlled by the diffusion of Fe 2+ ions. ...
... The value for energy of activation and the associated controlling mechanism obtained in this study are in good agreement with those reported by Koch [6], namely 20.9 kJ/mole for diffusion of ferrous ions. It is also in excellent agreement with the value of 28 kJ/mole reported by Tekin and Bayramoglu [7]; although, they have concluded in their paper that the dissolution reaction was chemical-controlled. ...
In this paper, kinetics of reductive leaching of manganese dioxide ore by ferrous ion in sulfuric acid media has been examined. Experimental results show that increasing temperature from 20 to 60 °C and decreasing ore particle size from 16+20 to 60+100 mesh considerably enhance both the dissolution rate and efficiency. Molar ratios of Fe 2+ /MnO 2 and H 2 SO 4 /MnO 2 in excess to the stoichiometric amounts were needed for successful manganese dissolution. Under the optimum condition (ore particle size of 60+100 mesh, Fe 2+ /MnO 2 molar ratio of 3.0, H 2 SO 4 /MnO 2 molar ratio of 2.0) manganese could be extracted with over 95% efficiency by 20 minutes leaching at room temperature. A kinetic analysis based on dimensionless time method showed that shrinking core – ash diffusion control model fits the experimental results reasonably well. Value of activation energy was found to be 28.1 kJ/mole for the proposed mechanism.
... Manganese oxides participated in the oxidation of ferrous ions to form ferric oxides. The oxidation rates of ferrous ions by pyrolusite and γ-MnO 2 were compared in sulfate solution, and the influence factors of acidity and particle size were considered, indicating that oxidation rate increased about 2 times when pyrolusite was substituted by γ-MnO 2 [18]. Birnessite, formed from microbial oxidation of Mn(II), is often enriched with heavy metals and alkaline and alkali earth metals, and generally exhibits the highest oxidation activity and largest adsorption capacity [2,12,13,16,19]. ...
... As shown in Fig. 10a, when Fe 2+ ion concentration was controlled at 10, 20, and 40 mmol L −1 , after 12 h of reaction, the concentration of consumed Fe 2+ reached about 559, 705, and 990 mg L −1 , which corresponding consumption rates approaching to 100, 87 and 61 %, respectively. The amount of released Mn 2+ can be used to indicate the redox rate [18,20,21,42]. In this work, higher concentration of reactant facilitated the larger capacity for Fe 2+ oxidation. ...
Background:
In soils and sediments, manganese oxides and oxygen usually participate in the oxidation of ferrous ions. There is limited information concerning the interaction process and mechanisms of ferrous ions and manganese oxides. The influence of air (oxygen) on reaction process and kinetics has been seldom studied. Because redox reactions usually occur in open systems, the participation of air needs to be further investigated.
Results:
To simulate this process, hexagonal birnessite was prepared and used to oxidize ferrous ions in anoxic and aerobic aqueous systems. The influence of pH, concentration, temperature, and presence of air (oxygen) on the redox rate was studied. The redox reaction of birnessite and ferrous ions was accompanied by the release of Mn(2+) and K(+) ions, a significant decrease in Fe(2+) concentration, and the formation of mixed lepidocrocite and goethite during the initial stage. Lepidocrocite did not completely transform into goethite under anoxic condition with pH about 5.5 within 30 days. Fe(2+) exhibited much higher catalytic activity than Mn(2+) during the transformation from amorphous Fe(III)-hydroxide to lepidocrocite and goethite under anoxic conditions. The release rates of Mn(2+) were compared to estimate the redox rates of birnessite and Fe(2+) under different conditions.
Conclusions:
Redox rate was found to be controlled by chemical reaction, and increased with increasing Fe(2+) concentration, pH, and temperature. The formation of ferric (hydr)oxides precipitate inhibited the further reduction of birnessite. The presence of air accelerated the oxidation of Fe(2+) to ferric oxides and facilitated the chemical stability of birnessite, which was not completely reduced and dissolved after 18 days. As for the oxidation of aqueous ferrous ions by oxygen in air, low and high pHs facilitated the formation of goethite and lepidocrocite, respectively. The experimental results illustrated the single and combined effects of manganese oxide and air on the transformation of Fe(2+) to ferric oxides. Graphical abstract:Lepidocrocite and goethite were formed during the interaction of ferrous ion and birnessite at pH 4-7. Redox rate was controlled by the adsorption of Fe2+ on the surface of birnessite. The presence of air (oxygen) accelerated the oxidation of Fe2+ to ferric oxides and facilitated the chemical stability of birnessite.
... Extensive research works have been performed on the dissolution of MnO 2 in dilute sulfuric or hydrochloric acids in the presence of a variety of reducing agents. Some of the reductants which have been employed are sulfur dioxide (Senanayake, 2004;Nayak et al., 2003;Asai and Konishi, 1986;Grimanelis et al., 1992;Dixit and Raisoni, 1987;Miller and Wan, 1983), aqueous ferrous sulfate or ferrous chloride solutions (Koch, 1957;Tekin and Bayramoglu, 1993;Das et al., 1982), iron sulfides (Kanungo, 1999a,b;Nayak et al., 1999;Vracar and Cerovic, 1984), nitrous acid (Dresler, 1984), organic acids such as EDTA and oxalic acid (Pankratova et al., 2001;Sahoo et al., 2001), hydrogen peroxide (Jiang et al., 2004), methanol (Momade and Momade, 1999) and even some types of carbohydrates like glucose (Trifoni et al., 2001). Among all these reducing agents examined so far, ferrous ion seems to be one of the most satisfactory reductants due to its good reactivity, availability and relatively lower cost. ...
... Among all these reducing agents examined so far, ferrous ion seems to be one of the most satisfactory reductants due to its good reactivity, availability and relatively lower cost. Koch (1957) studied the kinetics of the reaction between manganese dioxide and ferrous ion in acid solutions using the potential of ferrous-ferric couple as a measure of the extent of reaction. It was suggested that the reaction occurred most readily at certain active sites on the manganese dioxide surface and that surface diffusion of ferrous ion was the rate-determining step. ...
The dissolution of a manganese dioxide ore in dilute sulfuric acid facilitated by Fe metal in the form of powdered sponge iron was investigated. Effects of such parameters as molar ratios of sponge iron and sulfuric acid to manganese dioxide, temperature, particle size of sponge iron as well as ore particle size on the dissolution efficiency were studied. The results showed that manganese could be rapidly leached out to a complete degree even at room temperature. Based on both theoretical and experimental facts, it was concluded that the usage of metallic iron for the reductive leaching of manganese dioxide was superior to that of ferrous ion as far as dissolution rate and efficiency were concerned.
... Interestingly, the expected lower MB discoloration in system V relative to system I could not be observed at all tested shaking intensity for 1 d. In other words, the well-documented reductive dissolution of MnO 2 by Fe II [40,41] could not be observed. Therefore, shaking operations definitively significantly influences the mechanism of contaminant removal in Fe 0 /H 2 O systems. ...
... The results indicated that shaking intensities ≥50 min −1 lead to a suspension of in situ generated iron corrosion products and to a delay of the process of MB co-precipitation. More importantly, apart from non-disturbed systems, none of the tested experimental conditions could reproduce the well-documented reductive dissolution of MnO 2 by Fe II species [40,41] which would have been reflected by a delay in MB co-precipitation by Fe 0 in the presence of MnO 2 . Finally, non-disturbed experiments could evidence the welldocumented redox indicator properties of MB. ...
The term mixing (shaking, stirring, agitating) is confusing because it is used to describe mass transfer in systems involving species dissolution, species dispersion and particle suspension. Each of these mechanisms requires different flow characteristics in order to take place with maximum efficiency. This work was performed to characterize the effects of shaking intensity on the process of aqueous discoloration of methylene blue (MB) by metallic iron (Fe(0)). The extent of MB discoloration by three different materials in five different systems and under shaking intensities varying from 0 to 300 min(-1) was directly compared. Investigated materials were scrap iron (Fe(0)), granular activated carbon (GAC), and deep sea manganese nodules (MnO(2)). The experiments were performed in essay tubes containing 22 mL of the MB solution (12 mg/L or 0.037 mM). The essay tubes contained either: (i) no reactive material (blank), (ii) 0-9.0 g/L of each reactive material (systems I, II and III), or (iii) 5g/L Fe(0) and 0 to 9.0g/L GAC or MnO(2) (systems IV and V). The essay tubes were immobilized on a support frame and shaken for 0.8-5 days. Non-shaken experiments lasted for duration up to 50 days. Results show increased MB discoloration with increasing shaking intensities below 50 min(-1), a plateau between 50 and 150 min(-1), and a sharp increase of MB discoloration at shaking intensities >or=200 min(-1). At 300 min(-1), increased MB discoloration was visibly accompanied by suspension of dissolution products of Fe(0)/MnO(2) and suspension of GAC fines. The results suggest that, shaking intensities aiming at facilitating contaminant mass transfer to the Fe(0) surface should not exceed 50 min(-1).
... The leaching kinetics of pyrolusite and manganese dioxide was widely investigated. Koch [1] used the potential of ferrous-trivalent iron pair to measure the degree of reaction, mainly studing the kinetics of iron ions and manganese dioxide in an acidic solution. The results show that there is a high probability of reaction in some active sites on the surface of manganese dioxide, and the reaction rate is controlled by the surface diffusion of iron ions. ...
In this paper, 3D dual impeller stirring tank was used as a model to simulation combine with experiments to explore the acid leaching kinetics of high-calcium and -silicon manganese ores. The effects of stirring speed, temperature, and aeration rate on the spatial distribution of gas and liquid phases were investigated, and the influence of the gas phase on the flow state was further explored by using CFD (Computational Fluid Dynamics). Meanwhile, the effects of stirring speed, temperature, and aeration rate on the leaching rate of target element manganese were studied by experiments. According to simulations and experiments, stirring speed at 500 rpm perfectly. The aeration rate was 7 m/s, the suitable leaching temperature was 140 °C by experiments. These results also showed that the leaching of high-calcium and -silicon manganese ores follows the control of Interface chemical reaction.The apparent activation energy of the reaction was 43.4 kJ.mol⁻¹.
... Pyrite has been widely used as a low-cost reducing agent for leaching Mn 2+ from LGP (Schippers and Jørgensen, 2001;Vračar and Cerović, 2000). Studies have shown that the surface diffusion of Fe 2+ ions is the controlling step of the reaction between MnO 2 and pyrite (Koch, 1957). Kholmogorov et al. found that the leaching efficiency of Mn 2+ was dependent on the mass ratio of FeS 2 to MnO 2 (Kholmogorov et al., 2000). ...
In this study, electric field and ball milling were used to leach Mn²⁺ from low-grade pyrolusite (LGP). The effects of current density, reaction time, reaction temperature, ball-to-powder weight ratio, and ball milling time on the leaching efficiency of Mn²⁺ from LGP as well as the leaching mechanism were systematically studied. The results showed that the combined use of electric field and ball milling enhanced the leaching of Mn²⁺ from LGP. The leaching efficiency of Mn²⁺ reached 97.79% under the optimum conditions of LGP-to-pyrite mass ratio of 1:0.18, current density of 30 mA/cm², LGP-to-H2SO4 mass ratio of 1:0.4, liquid-to-solid ratio of 5:1, ball-to-powder weight ratio of 1:1, ball milling time of 2 h, temperature of 80 °C, and leaching duration of 120 min. This value was 25.95% higher than that attained without ball milling and 41.45% higher than that attained when neither ball milling nor electric field was employed. Pyrite was fully oxidized to generate additional SO4²⁻ and Fe³⁺, and was further hydrolyzed to form jarosite (KFe3(SO4)2(OH)6) and hydronium jarosite (Fe3(SO4)2(OH)5·2H2O) via ball milling and electric field application. Moreover, the electric field changed the surface charge distribution of the mineral particles and promoted collisions between them as well as the collapse of the crystal lattice, further improving the leaching efficiency of Mn²⁺ from LGP. This study provided a new method for leaching Mn from LGP.
... Methylene blue (MB) is a cationic dye which is preferentially adsorbed onto negatively charged surfaces [18,22]. The suitability of MB for the characterization of processes in dynamic Fe 0 /sand/MnO 2 systems arises from three important facts: (i) MB competes with in situ generated cations (Fe 2+ and Fe 3+ ) for adsorption onto sand [23,24], (ii) MB adsorbs better onto sand than onto (sand coated with) iron oxides [25], and (iii) MnO 2 consumes in situ generated Fe 2+ for its reductive dissolution [21,2627282930313233. Taken together, points (i) through (iii) suggest that the presence of MnO 2 will minimize the extent of disturbance of MB discoloration by sand in the presence of Fe 0 . ...
Significant research has been conducted on the design of filtration systems containing metallic iron for environmental remediation (Fe0 walls) over the past 20 years. However, limited information exists on the rationale for sustainable Fe0 walls. Additionally, very limited research has been conducted on the modification of the efficiency of the system by the addition of solid phases. This research aimed at characterizing the impact of reactive natural manganese oxides (MnO2) in influencing the extent of methylene blue (MB) discoloration by a Fe0/sand system in a gravity driven column experiments. A total of seven columns were used in parallel experiments. Investigated systems are: pure Fe0, pure sand, Fe0/sand mixture and four different Fe0/MnO2/sand mixtures. The volumetric ratio of Fe0 in hybrid systems was 30 %. In three-components-systems, the volumetric ratio of each additive was 35%. Each system was characterized by the time-dependent evolution of the pH value, the iron and MB breakthrough, and the evolution of the hydraulic conductivity (permeability). Results showed enhanced MB discoloration in all Fe0/MnO2/sand systems relative to the Fe0/sand system. The impact of MnO2 on system’s permeability could not be accessed because of the poor mechanic properties of tested minerals. However, the feasibility of sustaining the efficiency of Fe0 walls by admixing granular MnO2 is demonstrated. This tool will help in the development of more sustainable Fe0 walls. Such systems could also lead to substantial improvement of drinking water supply and wastewater treatment in the developing world.
... MnO 2 ) or suspension of in-situ generated iron oxides (Fig. SC1 - Interestingly, the expected lower MB discoloration in system V relative to system I could not be observed at all tested shaking intensity for 1 d. In other words, the well-documented reductive dissolution of MnO 2 by Fe II [40,41] could not be observed. Therefore, shaking operations definitively significantly influences the mechanism of contaminant removal in Fe 0 /H 2 O systems. ...
The term mixing (shaking, stirring, agitating) is confusing because it is used to describe mass transfer in systems involving species dissolution, species dispersion and particle suspension. Each of these mechanisms requires different flow characteristics in order to take place with maximum efficiency. This work was performed to characterize the effects of shaking intensity on the process of aqueous discoloration of methylene blue (MB) by metallic iron (Fe0). The extent of MB discoloration by three different materials in five different systems and under shaking intensities varying from 0 to 300 min-1 was directly compared. Investigated materials were scrap iron (Fe0), granular activated carbon (GAC), and deep sea manganese nodules (MnO2). The experiments were performed in essay tubes containing 22 mL of the MB solution (12 mg/L or 0.037 mM). The essay tubes contained either: (i) no reactive material (blank), (ii) 0 to 9.0 g/L of each reactive material (systems I, II and III), or (iii) 5 g/L Fe0 and 0 to 9.0 g/L GAC or MnO2 (systems IV and V). The essay tubes were immobilized on a support frame and shaken for 0.8 to 5 days. Non-shaken experiments lasted for duration up to 50 days. Results show increased MB discoloration with increasing shaking intensities below 50 min-1, a plateau between 50 and 150 min-1, and a sharp increase of MB discoloration at shaking intensities ≥ 200 min-1. At 300 min-1, increased MB discoloration was visibly accompanied by suspension of dissolution products of Fe0/MnO2 and suspension of GAC fines. The results suggest that, shaking intensities aiming at facilitating contaminant mass transfer to the Fe0 surface should not exceed 50 min-1.
... The differentiation of the reactivity of Fe 0 materials in Fe 0 /MB/H 2 O systems is based on the reaction of Fe II with MnO 2 [13,14]. Fe II is generated in-situ from Fe 0 oxidation. ...
A simple method is proposed for testing the reactivity of elemental iron materials (Fe0 materials) using methylene blue (MB) as reagent. The method is based on the oxidative reactivity of FeII for reductive dissolution of MnO2. FeII is produced in-situ by the oxidation of a Fe0 material. The in-situ formed FeII reacted with MnO2 delaying the bulk precipitation of iron corrosion products and thus MB co-precipitation (MB discoloration). For a given MnO2, the extent of MB discoloration delay is a characteristic of individual Fe0 materials under given experimental conditions. The MB discoloration method for testing the reactivity of Fe0 materials is facile, cost-effective and does not involve any stringent reaction conditions.
... A survey of the electrode potentials of the redox couples relevant for the discussion in this study [ [37,40]. By consuming Fe II , MnO 2 accelerates Fe 0 corrosion, producing more adsorption or co-precipitation agents for MB. ...
Methylene blue (MB) was used as a model molecule to characterize the aqueous reactivity of metallic iron in Fe0/H2O systems. Likely discoloration mechanisms under used experimental conditions are: (i) adsorption onto Fe0 and Fe0 corrosion products (CP), (ii) co-precipitation with in-situ generated iron CP, (iii) reduction to colorless leukomethylene blue (LMB). MB mineralization (oxidation to CO2) is not expected. The kinetics of MB discoloration by Fe0, Fe2O3, Fe3O4, MnO2, and granular activated carbon were investigated in assay tubes under mechanically non-disturbed conditions. The evolution of MB discoloration was monitored spectrophotometrically. The effect of availability of CP, Fe0 source, shaking rate, initial pH value, and chemical properties of the solution were studied. The results present evidence supporting co-precipitation of MB with in-situ generated iron CP as main discoloration mechanism. Under high shaking intensities (> 150 min-1), increased CP generation yields a brownish solution which disturbed MB determination, showing that a too high shear stress induced the suspension of in-situ generated corrosion products. The present study clearly demonstrates that comparing results from various sources is difficult even when the results are achieved under seemingly similar conditions. The appeal for an unified experimental procedure for the investigation of processes in Fe0/H2O systems is reiterated.
... The use of natural oxides to sustain Fe 0 reactivity was derived from the well-documented [46,47]. This process was successfully used to demonstrate the importance of corrosion products in the process of contaminant removal by Fe 0 [48][49][50][51][52]. ...
Filtration systems containing metallic iron as reactive medium (Fe0 beds) have been intensively used for water treatment during the last two decades. The sustainability of Fe0 beds is severely confined by two major factors: (i) reactivity loss as result of the formation of an oxide scale on Fe0, and (ii) permeability loss due to pore filling by generated iron corrosion products. Both factors are inherent to iron corrosion at pH > 4.5 and are common during the lifespan of a Fe0 bed. It is of great practical significance to improve the performance of Fe0 beds by properly addressing these key factors. Recent studies have shown that both reactivity loss and permeability loss could be addressed by mixing Fe0 and inert materials. For a non porous additive like quartz, the threshold value for the Fe0 volumetric proportion is 51 %. Using the Fe0/quartz system as reference, this study theoretically discusses the possibility of (i) replacing Fe0 by bimetallic systems (e.g. Fe0/Cu0), or (ii) partially replacing quartz by a reactive metal oxide (MnO2 or TiO2) to improve the efficiency of Fe0 beds. Results confirmed the suitability of both tools for sustaining Fe0 bed performance. It is shown that using a Fe0:MnO2 system with the volumetric proportion 51:49 will yield a filter with 40 % residual porosity at Fe0 depletion (MnO2 porosity 62 %). This study improves Fe0 bed design and can be considered as a basis for further refinement and detailed research for efficient Fe0 filters.
... 2) are used for the reductive dissolution of MnO 2 (Eq. 3) [26,27]. For the sake of clarity, the oxidation of Fe 0 by water (H + ) and MnO 2 are given by Eq. 4 and 5. ...
The idea that manganese oxide (MnO2) sustains the reactivity of metallic iron (Fe0) is investigated in this study. A multi-elemental aqueous system containing CrVI, CuII, MoVI, SbV, UVI, and ZnII (each about 100 mM) was used as model solution. Non-disturbed batch experiments were performed at initial pH values 4.0 and 6.0 for one month. Three different systems were investigated: (i) MnO2 alone, (ii) “Fe0 + sand”, and (iii) “Fe0 + MnO2”. The experimental vessels contained either: (i) no material (blank), (ii) up to 9.0 g/L of MnO2, or (iii) 5 g/L Fe0 and 0 to 9.0 g/L MnO2 or sand. Results clearly revealed quantitative contaminant removal (> 70 %) confirming the suitability of Fe0 as a highly efficient reactive material for the removal of the 6 tested metallic ions over a pH range applicable to environmental waters. Results also corroborated the suitability of MnO2 to sustain the long-term Fe0 reactivity. Further studies in dynamic systems (column studies) are necessary to fine-tune the use of MnO2 in Fe0 filtration systems.
... If the process of corrosion product precipitation is delayed, the resulting MB discoloration is also delayed. The differentiation of the reactivity of Fe 0 materials in Fe 0 /MB/H 2 O systems is based on the reaction of Fe II with MnO 2 [13,14].Table 1 Main characteristics and iron content of the four tested Fe 0 materials. The material code ( " code " ) is from the author, the given form is as supplied; d (m) is the diameter of the supplied material and the Fe content is given in % mass.). ...
A simple method is proposed for testing the reactivity of elemental iron materials (Fe(0) materials) using methylene blue (MB) as reagent. The method is based on the oxidative reactivity of Fe(II) for reductive dissolution of MnO(2). Fe(II) is produced in-situ by the oxidation of a Fe(0) material. The in-situ formed Fe(II) reacted with MnO(2) delaying the bulk precipitation of iron corrosion products and thus MB co-precipitation (MB discoloration). For a given MnO(2), the extent of MB discoloration delay is a characteristic of individual Fe(0) materials under given experimental conditions. The MB discoloration method for testing the reactivity of Fe(0) materials is facile, cost-effective and does not involve any stringent reaction conditions.
... However, ferrous iron from the Fe III /Fe II redox couple, either in aqueous solution or adsorbed on mineral surfaces, can act as a reductant for organic and inorganic soil components (e.g. MnO 2 )777879 and contaminants [80, 81]. Furthermore, it has been shown that adsorbed or structuralTable 1). ...
A central aspect of the contaminant removal by elemental iron materials (Fe0 or Fe0 materials) is that reduction reactions are mediated by the iron surface (direct reduction). This premise was introduced by the pioneers of the reactive wall technology and is widely accepted by the scientific community. In the meantime enough evidence has been provided to suggest that contaminant reduction through primary corrosion products (secondary reductants) does indeed occur (indirect reduction). It was shown for decades that iron corrosion in the pH range of natural waters (4-9) inevitably yields an obstructive oxide film of corrosion products at the metal surface (oxide film). Therefore, contaminant adsorption on to corrosion products and contaminant co-precipitation with corrosion products inevitably occurs. For adsorbed and coprecipitated contaminants to be directly reduced the oxide film should be electronic conductive. This study argues through a literature review a series of points which ultimately lead to the conclusion that, if any quantitative contaminant reduction occurs in the presence of Fe0 materials, it takes place within the matrix of corrosion products and is not necessarily a direct reduction. It is concluded that Fe0 materials act both as source of corrosion products for contaminant adsorption/coprecipitation and as a generator of FeII and H2 (H) for possible catalytic contaminant reduction.
The necessity of arsenic (As) removal from metallurgical wastewaters is increasing. Despite its wide recognition as a natural oxidant, the utility of Mn oxide for scorodite production is mostly unknown. In acidic solutions containing both As(III) and Fe²⁺, simultaneous oxidation of the two progressed by MnO2 and the resultant As(V) and Fe³⁺ triggered the formation of crystalline scorodite (FeAsO4·2H2O). At 0.5% or 0.25% MnO2, 98% or 91% As was immobilized by day 8. The resultant scorodite was sufficiently stable according to the TCLP test, compared to the regulatory level in US and Chile (5 mg/L): 0.11 ± 0.01 mg/L at 0.5% MnO2, 0.78 ± 0.05 mg/L at 0.25% MnO2. For the oxidation of As(III) and Fe²⁺, 54% (at 0.5% MnO2) or 14% (at 0.25% MnO2) of initially added MnO2 remained undissolved and the rest dissolved in the post As(III) treatment solution. For the Mn recycling purpose, the combination of Mn²⁺-oxidizing bacteria and biogenic birnessite (as homogeneous seed crystal) was used to recover up to 99% of dissolved Mn²⁺ as biogenic birnessite ((Na, Ca)0.5(MnIV, MnIII)2O4·1.5H2O), which can be utilized for the oxidation treatment of more dilute As(III) solutions at neutral pH. Although further optimization is necessary, the overall finding in this study indicated that Mn oxide could be utilized as a recyclable oxidant source for different As(III) treatment systems.
Fig. 4 XRD patterns and SEM images before (a, a′) and after (b–d, b′–d′) the scorodite precipitation reaction at different MnO2 doses: 0.15% (b, b′), 0.25% (c, c′), 0.5% (d, d′). XRD peaks: M (ε-MnO2; Akhtenskite, PDF No. 01-089-5171), S (scorodite; JCPDS 37-0468). Fullsize Image
Electrolytic manganese residue (EMR) is a solid waste that generated in electrolytic manganese metal production, which would not only contaminate the environment due it contains soluble manganese, but occupy a large amount of farmlands as well. Hence, recovering manganese from EMR using economical method is the key to achieve the targets of resource recycling and solid waste harmless treatment. In this paper, pure water was used to leach soluble manganese out from EMR. Leaching kinetics model of manganese were established and examined, characteristics of EMR before and after leaching were analyzed by XRF, XRD, EPMA-WDS and SEM. The results revealed that the apparent activated energy (E) during the leaching processes was 11.17 ± 2.02 kJ/mol and the slope in fitting line of Arrhenius equation was -1.343, which indicated that the leaching kinetic model was appropriate and the rate determining step was diffusion process, that is, Mn²⁺ migrate into bulk solution from diffusion layer. The main crystal phases in EMR are quartz (SiO2) and calcium sulfate (CaSO4·nH2O) (n = 0.5 or 2), but the distribution relationship between SiO2 and CaSO4·nH2O was not regular. Mn is in the form of “film” covered in CaSO4 bar in EMR. High temperature and high volume of water could promote Mn²⁺ transport process. The optimal leaching condition was S:L (Solid : Liquid)1:4, temperature was 24 °C and the agitation rate was 300 r/min when combined the cost and efficiency, and 83.35% soluble manganese can be leached out from EMR.
In this paper, the leaching of a Greek manganese ore with sulfate and chloride solutions in various temperatures values is studied and the kinetics of manganese extraction is approximated. A -30+60 Mesh fraction was leached with acidic ferrous sulfate and chloride solutions (0.1M Fe 2+, 0.2M H +) at temperatures ranging from 22 to 80 °C. The other conditions, which were kept constant, were: pulp density 1%, agitation speed 630 rpm. The kinetic results have been analysed by using a diagnostic equation of general use and the shrinking core model. Diffusion was found to be the rate controlling step of the leaching process. From the Arrhenius plot the activation energy has been calculated equal to 1.27 Kcal/mole for chloride solution. The calculation of the activation energy, which was made for the chloride solution by using the initial rates, gave a value of 4.01 Kcal/mole.
Elemental iron (Fe0) has been successfully tested and used for water treatment for decades due to its worldwide availability and inexpensiveness. Fe0-based filtration technology has been used for (i) environmental remediation (e.g. subsurface permeable reactive barriers) (ii) wastewater treatment and (iii) safe drinking provision. The evidence that Fe0 oxidative dissolution and subsequent precipitation at pH > 4.0 is a volumetric expansive process (Voxide > Viron) implies that Fe0 should be amended with non-expansive aggregates such as activated carbon, manganese oxides, pumice or sand. Only such hybrid systems are likely to be sustainable.
The present work focuses on the characterization of the ion selective nature of Fe0-based filters using three azo dyes: methylene blue (cationic), Orange II and Reactive Red 120 (anionic). The dyes are used as indicators for the reactivity of Fe0/H2O system in both batch and column experiments. The idea is to demonstrate that downwards from a Fe0 / sand system; available aggregates are in-situ coated with iron oxide, such that in the medium to long term, the whole system is ion-selective. The selectivity being fixed by positively charged iron oxides.
The characterization of the Fe0/H2O system is realized herein by amending (i) Fe0 with sand and MnO2 in batch experiments and (ii) Fe0 with sand in column experiments. Sand is a pure adsorbent with a negatively charged surface while MnO2 is reactive in nature. MnO2 addition enables the control of the availability of in-situ generated iron corrosion products and thus the role of corrosion product in the process of contaminant removal. The investigated systems in batch mode are (i) pure sand, (ii) pure MnO2, (iii) pure Fe0, (iv) Fe0/sand mixture, (v) Fe0/MnO2 mixt ure and (vi) Fe0/sand/MnO2 mixtures with various amounts of sand and MnO2 loadings. Column experiments were performed with the following systems: (i) pure sand (0 % Fe0), (ii) pure Fe0 (100 % Fe0), and (iii) Fe0/sand (50 % Fe0- vol/vol).
Results of batch experiment showed that sand is a good adsorbent for MB and has negligible effect on anionic dyes. MnO2 also favors MB discoloration. Pure Fe0 favors discoloration of both cationic and anionic dyes but shows best discoloration efficiency for Orange II. Among the Fe0 amended systems, the Fe0/sand system is most efficient for dye discoloration. The discoloration efficiency in Fe0-based systems is 75 % for MB and > 95 % for Orange II and RR120. Results confirmed quantitative adsorptive MB discoloration and negligible adsorption of anionic dyes on negatively charged sand. Quantitative discoloration of the anionic dyes (Orange II and RR120) in Fe0-based systems was attributed to high affinities of both species to positively charged iron corrosion products. The ion selective nature of the Fe0/H2O system is elegantly demonstrated.
The kinetics of reductive leaching of manganese from a low-grade manganese oxide ore were studied using cellulose as reductant in dilute sulfuric acid medium. It was found that when the stirring speed was higher than 200 r/min, the effect of gas film diffusion on manganese extraction efficiency could be neglected, and the kinetic behavior was investigated under the condition of elimination of external diffusion influence on the leaching process. Effects of leaching temperature, mass ratio of cellulose and ore, and the sulfuric acid concentration on manganese extraction efficiency were discussed. The kinetic data were analyzed based on the shrinking core model, which indicated that the leaching process was dominated by both ash layer diffusion and chemical reaction at the initial stage, with the progress of leaching reaction, the rate-controlling step switched to the ash layer diffusion. It was also concluded that the sulfuric acid concentration had the most significant influence on the leaching rate, the reaction orders with respect to the sulfuric acid concentration were 2.102 in the first 60 min, and 3.642 in the later 90 min, while the reaction orders for mass ratio of cellulose and ore were 0.660 and 0.724, respectively. An Arrhenius relationship was used to relate the temperature to the rate of leaching, from which apparent activation energies were calculated to be 46.487 kJ/mol and 62.290 kJ/mol at the two stages, respectively. Finally, the overall leaching rate equations for the manganese dissolution reaction with cellulose in sulphuric acid solution were developed. The morphological changes and mineralogical forms of the ore before and after the chemical treatment were discussed with the support of SEM and XRD analyses.
The literature on the determination of mechanisms of mineral leaching reactions is reviewed to identify the sources of uncertainty about the conclusions drawn. Uncertainty about the mechanism of a system arises in the measurement, data processing and decision-making stages. There is uncertainty in the accuracy, validity and adequacy of the data collected. The effects of most of more than 30 variables and phenomena are seldom determined. There is less certainty in tests of model equations against the rate data than is commonly accepted. Approximate calculations of the absolute rate of mass transport in a system could be the basis of a better test of a mechanism. There could be more direct tests of the basic premises and subsidiary assumptions of preferred mechanisms. References are given to illustrate good practices and achievements in reducing the uncertainty.
The kinetics of dissolution and oxygen catalysis of RuO2·xH2O samples, thermally activated at different temperatures in the range ambient to 100°C, are reported. The results of a kinetic study show that increasing the temperature of activation causes an increasingly thick, dissolution-inert and diffuse layer to form on the outside of the particles. The observed decrease in activity towards dissolution with increased annealing temperature is mirrored an by increase in the activity of the samples as catalysts for the oxidation of water to O2 by the BrO−3 ions present. The results of this work re-enforce the crucial link between the water content of a hydrated sample of ruthenium dioxide and its activity as an oxygen catalyst.
The reductive leaching of nickel laterite has attracted the interest of many researchers due to the enhanced kinetics of nickel and cobalt dissolution in the presence of acids and reducing agents during atmospheric, pressure, heap or bio leaching processes. Systematic studies on synthetic oxides and natural ores can shed light on the reaction mechanism and lead to investigations of beneficial reagents for further studies. This paper briefly reviews the literature and describes a comparative study of metal leaching from synthetic goethite spiked with nickel or cobalt and a limonitic laterite ore to rationalise the role of reducing agents in acid media. Results are discussed on the basis of the effect of speciation, surface chemical reactivity of oxides and heterogeneous kinetic models.
The kinetics of the reduction of MnO2 by Fe2+ ions in H2SO4 solution were investigated under well-defined hydrodynamic conditions. The reaction was monitored potentiometrically using a saturated calomel electrode and a Pt electrode. The effect of stirring rate, particle size, temperature, Fe2+ and H+ concentrations were studied. Under high agitation, diffusional resistances are negligible and the rate is controlled by chemical reaction. Evidence supporting this conclusion is given. The kinetic data have been analysed using the “shrinking core model” equation. The rate was found to be proportional to [Fe2+]0.5·[H+]0.5 and the apparent activation energy was determined as 28 kJ/mol. The final rate equation is as follows: .
1.
When pitchblende is dissolved in H2SO4 containing MnO2 or HNO3 as oxidizing agent, the presence of iron and copper minerals greatly accelerates the process and increases the extraction of uranium into the solution.
2.
Of the ferric oxide minerals, limonite has the greatest effect on extraction of uranium by sulfuric acid with MnO2; of the ferrous oxide minerals, siderite, which is the most readily soluble in sulfuric acid, has the most marked effect.
3.
When pitchblende is dissolved in a mixture of nitric and sulfuric acid, ferric oxide passes from the minerals into solution, thereby increasing the extraction of uranium. At acid mixture concentrations less than 100 g/liter, ferrous iron hasan unfavorable effect on the process.
Studies have been carried out for extraction of Cu, Ni, Co, Zn and Mn with simultaneous avoidance of iron dissolution during leaching of manganese nodules in an aqueous SO2–H2SO4–(NH4)2SO4 system. The leaching parameters such as sulphuric acid concentration, ammonium sulphate concentration, time and pulp density were varied to study their effect on extraction of metals and precipitation of iron. It was observed that the presence of both sulphuric acid and SO2 was required for maximum recovery of Cu, Ni, Co, Zn and Mn. The iron in solution decreased with increase in ammonium sulphate concentration. The optimum conditions established for maximum metal extractions were: pulp density 20%, time 3 h, temperature 363 K, SO2 60 g l−1, sulphuric acid 3.0% (v/v) and ammonium sulphate 100 g l−1. Under these conditions the recoveries from a typical nodule sample were: 88.5% Cu, 99.8% Ni, 91.8% Co, 97.8% Zn, 99.6% Mn with iron extraction of 2.4%. Nodules containing different percentage of metal contents were also tested under the established conditions and it was observed that the dissolution of iron varied between 2.4 and 4.9% with no significant variations in recoveries of Cu, Ni, Co, Zn and Mn.
Leaching of manganese nodule for recovering copper, nickel, and cobalt in ammoniacal medium has been extensively studied. During earlier attempts, the ground nodules were subjected to reduction at high temperature 1-~2 prior to leaching, similar to processing of lateritic nickel ores. 13'14 Studies were extended to leach copper, nickel, and cobalt values in ammoniacal solution at high temperature without any prereduction. 15-18 Subsequently it was observed that reductive leaching in ammonia as well as acid medium could be done under aqueous conditions to recover metal values from nodules. Some of the reductants used for such studies in ammoniacal medium include cuprous ion together with carbon monoxide, manganous ion, organic reductants like glucose, formaldehyde, and carbohydrates, 19-29 whereas in acidic medium sulfur dioxide and charcoal have been used. 3~ The use of ferrous ion as a reductant in acid solution for manganese dioxide 33'34'35 is quite well known and can also be used for the nodule materials. Though ferrous iron has not been used as a reductant in ammoniacal solution, its capacity to form a soluble ferrous ammine 36 at particular pH values does render it a good reducing agent. This property of the ferrous ammine has been exploited in the present work to effect aqueous reduction for extraction of copper, nickel, and cobalt values from Indian Ocean nodules. The Indian Ocean manganese nodule samples were provided by the National Institute of Oceanography (CSIR), Goa, India. These were crushed, ground, and sieved to obtain 100 pct < 250/zm. The chemical analysis of the major constituents of the manganese nodule was 0.34 pct Cu, 0.56 pct Ni, 0.12 pct Co, 10.5 pct Fe, 13.0 pct Mn, and 25 pct moisture. 5 g sample was leached in a 250 ml conical flask, fitted with a condenser and a thermometer. The required quantities of ammonia, ammonium salt, and water were added and heated on a heater cum stirrer. Ferrous sulfate was added to the leachant when the desired temperature was attained. Samples were taken at ten-minute intervals. The samples collected from time to time were immediately filtered into a graduated cylinder containing a known amount of hydrochloric acid to avoid any precipitation of iron and manganese on keeping the solution. After required dilution of the sample solutions, the copper, nickel, cobalt, and manganese values were determined by a PerkinElmer atomic absorption spectrophotometer.
The effects of continuous sonication and presonication on the kinetics of oxidative dissolution of ruthenium dioxide hydrate by bromate ions under acidic conditions are reported. Compared with unsonicated and presonicated dispersions the overall rate of dissolution of continuously sonicated dispersions is significantly greater due to a reduction in the average particle size and, hence, an increase in the specific surface area. Powder dispersions subjected to continuous ultrasound and presonication exhibit an initial induction period in their corrosion kinetics; the length of this induction period increases with increasing presonication. This corrosion feature is retained in the dissolution kinetics of powder samples which have been subjected to pre-ultrasound, but which are then stirred during the dissolution process. It is believed that this apparent permanent change in the nature of the powder particles is due to the ultrasound induced formation of a very thin layer of a largely unreactive form of ruthenium dioxide (possibly due to partial dehydration) on the surface of the powder particles. A kinetic scheme, based on this model, is used to account for th e observed kinetics of dissolution of RuO2.xH2O which have been subjected to both continuous sonication and presonication.
Methylene blue (MB) was used as a model molecule to characterize the aqueous reactivity of metallic iron in Fe(0)/H(2)O systems. Likely discoloration mechanisms under used experimental conditions are: (i) adsorption onto Fe(0) and Fe(0) corrosion products (CP), (ii) co-precipitation with in situ generated iron CP, (iii) reduction to colorless leukomethylene blue (LMB). MB mineralization (oxidation to CO(2)) is not expected. The kinetics of MB discoloration by Fe(0), Fe(2)O(3), Fe(3)O(4), MnO(2), and granular activated carbon were investigated in assay tubes under mechanically non-disturbed conditions. The evolution of MB discoloration was monitored spectrophotometrically. The effect of availability of CP, Fe(0) source, shaking rate, initial pH value, and chemical properties of the solution were studied. The results present evidence supporting co-precipitation of MB with in situ generated iron CP as main discoloration mechanism. Under high shaking intensities (>150 min(-1)), increased CP generation yields a brownish solution which disturbed MB determination, showing that a too high shear stress induced the suspension of in situ generated corrosion products. The present study clearly demonstrates that comparing results from various sources is difficult even when the results are achieved under seemingly similar conditions. The appeal for an unified experimental procedure for the investigation of processes in Fe(0)/H(2)O systems is reiterated.
ResearchGate has not been able to resolve any references for this publication.