Method for the synthesis of negatively charged manganese feroxyhyte for the selective removal of mercury from water

Abstract

The present invention refers to the method for synthesis an absorbent material consisting of a phase of tetravalent manganese feroxyhyte (δ-Fe.sub.(1-x)Mn.sub.xOOH) with a negatively charged grain surface in which a percentage of iron has been isomorphically substituted by manganese atoms at 0.05-25%. Its' production is carried out in two continuous flow stirred-tank reactors arranged in serial configuration, where mild acidic conditions (pH 5-6) prevail in the first reactor and mild alkaline conditions (pH 9-10) together with high redox potential (600-700 mV) in the second reactor. The material can be used to uptake mercury, as well as other heavy metals from both water and hot gas streams. Specifically, the adsorption capacity is determined by the magnitude of the negative surface charge and the isoelectric point that can be both adjusted by the synthesis process parameters.

Claims

1. A method for preparing tetravalent manganese feroxyhyte (δ-Fe.sub.(1-x)Mn.sub.xOOH, x=0.05-0.25), wherein the method is conducted in two continuous flow stirred-tank reactors arranged in serial configuration, where a first reactor has weakly acidic conditions and wherein a second reactor has weakly alkaline conditions and high redox potential for the production of a material having a high negative charge surface density, the method further comprising the following steps: i) FeSO.sub.4 or FeCl.sub.2 is added to the first reactor as an iron source at a concentration of 1-50 g/L and KMnO.sub.4 is added as a source of manganese at a concentration of 1-20 g/L to perform a reaction for nuclei production of the δ-Fe.sub.(1-x)Mn.sub.xOOH, wherein a pH value of the first reactor is adjusted to a weakly acidic region of (pH between 5-6) by adding one or a combination of more than one of the reagents NaOH, NaHCO.sub.3, Na.sub.2CO.sub.3, KOH, KHCO.sub.3, K.sub.2CO.sub.3, while at the same time a value of the redox potential in the first reactor is adjusted to a range of 450-550 mV with an appropriate adjustment of FeSO.sub.4 or FeCl.sub.2 and KMnO.sub.4 flowrates, then ii) a reaction of negatively charged surface nanocrystals growth proceeds to the second reactor by further addition of FeSO.sub.4 or FeCl.sub.2 and KMnO.sub.4 reagents with a pH value in the second reactor adjusted to an alkaline region (pH between 9-10) by adding one or a combination of more than one of the reagents NaOH, KOH and at the same time a value of the redox potential in the second reactor is adjusted to a range of 600-700 mV, with the appropriate adjustment of the FeSO.sub.4 or FeCl.sub.2 and KMnO.sub.4 flowrates comprises: iii) a residence-reaction time of maximum 20 minutes in the first reactor and at least 2 hours in the second reactor; and iv) a product exiting the second reactor enters a thickening tank, where with mild stirring for a period of 6-36 hours, grains are formed in an irregular shape, having a specific surface area of 100 m.sup.2/g to 300 m.sup.2/g, high negative surface charge (>1.5 mmol H.sup.+/g) and isoelectric point less of than 5.5.

2. The method according to claim 1, wherein the pH of the reaction in the second reactor is 9 and the redox potential is 650 mV.

3. The method according to claim 1, wherein the produced material undergoes mechanical dehydration, and granulation at a granule size of between 250-2500 μm and drying at 100-150° C.

4. The method according to claim 1, wherein material produced after the application of the process is mechanically dehydrated, dried at 100-150° C. and granulated in a granule size of less than 50 μm (5).

5. A single phase adsorbent material of negatively charged tetravalent manganese feroxyhyte (δ-Fe.sub.(1-x)Mn.sub.xOOH) produced according to the method of claim 1, wherein a percentage of 0.05-25% of iron has been isomorphic substituted by manganese atoms, with a specific surface area of 100-300 m.sup.2/g, a negative surface charge density greater than 1.5 mmol H.sup.+/g and an isoelectric point of less than 5.5.

6. A single phase adsorbent material of negatively charged tetravalent manganese feroxyhyte (δ-Fe.sub.(1-x)Mn.sub.xOOH) produced according to the method of claim 2, wherein a percentage of 0.05-25% of iron has been isomorphic substituted by manganese atoms, with a specific surface area of 100-300 m.sup.2/g, a negative surface charge density greater than 1.5 mmol H.sup.+/g and an isoelectric point of less than 5.5.

7. A single phase adsorbent material of negatively charged tetravalent manganese feroxyhyte (δ-Fe.sub.(1-x)Mn.sub.xOOH) produced according to the method of claim 3, wherein a percentage of 0.05-25% of iron has been isomorphic substituted by manganese atoms, with a specific surface area of 100-300 m.sup.2/g, a negative surface charge density greater than 1.5 mmol H.sup.+/g and an isoelectric point of less than 5.5.

8. A single phase adsorbent material of negatively charged tetravalent manganese feroxyhyte (δ-Fe.sub.(1-x)Mn.sub.xOOH) produced according to the method of claim 4, wherein a percentage of 0.05-25% of iron has been isomorphic substituted by manganese atoms, with a specific surface area of 100-300 m.sup.2/g, a negative surface charge density greater than 1.5 mmol H.sup.+/g and an isoelectric point of less than 5.5.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) The present invention can be fully understood from the detailed description of the synthesis method, figures and examples of application of the method of synthesis of absorbent material that follows.

(2) The enclosed drawings describe:

(3) FIG. 1 is a flow diagram of the synthesis process of the adsorbent material according to the described method.

(4) FIG. 2 is a schematic representation of the grain growth mechanism of the material during the two reaction steps.

(5) FIG. 3 is a correlation diagram of the adsorption capacity, when residual concentration at the limit of 1 μg/L is achieved, of the negatively charged δ-Fe.sub.0.75Mn.sub.0.25OOH (final synthesis pH 9) for mercury in water with different pH values (5-9) compared to the material prepared entirely in acidic environment (pH 4).

(6) FIG. 4 is a correlation diagram of the Hg.sup.0 adsorption capacity of the negatively charged δ-Fe.sub.0.75Mn.sub.0.25OOH (final synthesis pH 9) for a residual concentration equal to the limit of 50 μg/m.sup.3 as a function of the gas stream temperature compared to the material prepared entirely in acidic environment (pH 4).

(7) The correlation diagram of the adsorption capacity, when residual concentration at the limit of 1 μg/L is achieved, of the negatively charged δ-Fe.sub.0.75Mn.sub.0.25OOH (final composition pH 9) for mercury in water with different pH values (5-9) compared to the material prepared entirely in acidic environment (pH 4) is shown in FIG. 3.

(8) The correlation diagram of the Hg.sup.0 adsorption capacity of the negatively charged δ-Fe.sub.0.75Mn.sub.0.25OOH (final composition pH 9) for a residual concentration equal to the limit of 50 μg/m.sup.3 as a function of the gas stream temperature compared to the material prepared entirely in acidic environment (pH 4) is shown in FIG. 4.

DETAILED DESCRIPTION

(9) The purpose of the present invention is to produce a tetravalent manganese feroxyhyte material [Fe.sub.(1-x)Mn.sub.xOOH, where 0.05<x<0.25] with an isoelectric point of less than 5.5 and a high negatively surface charge density (1.5 mmol H.sup.+/g), which results in high mercury adsorption capacity from water, as well as in high adsorption capacity of other heavy metals with valence ≥+2. This objective was achieved by a reaction of Fe(II) and Mn(VII) salts in a two-stage continuous flow reactors where the following reactions take place:

(10) In the first reactor, the Fe (II) salts are precipitated with KMnO.sub.4 in a weakly acidic to neutral environment for the production of the tetravalent manganese feroxyhyte. The pH is adjusted to a constant value of 5-6 by adding a solution of one of the reagents NaOH, NaHCO.sub.3, Na.sub.2CO.sub.3, KOH, KHCO.sub.3, K.sub.2CO.sub.3, while the high redox potential is set to a constant value of 500±50 mV by adding KMnO.sub.4. The reaction time in the first reactor does not exceed 20 min, which allows the formation of nucleated manganese feroxyhyte, but not the full development of the positive surface structure. In the second reactor, the pH is increased with the addition of either NaOH or KOH in the range between 9 and 10, while the redox potential is maintained in the 650±50 mV range with the addition of KMnO.sub.4, so that the increase of the tetravalent manganese feroxyhyte crystals in a high both pH and redox potential environment, will lead to the increase of the negative surface charge, which is necessary for the uptake of the positively charged ions, such as mercury and other heavy metals. It is then followed by the aging process which takes place in stirring tank.

(11) More specifically, the synthesis method of the tetravalent manganese feroxyhyte adsorbent with the high negative charge in the pH range of natural waters, corresponding to an isoelectric point lower than 5.5, is performed as follows:

(12) Two continuous flow stirred-tank reactors arranged in serial configuration are used. The retention time in the first reactor does not exceed 20 min (FIG. 1), while in the second reactor is at least 2 hours. The amount of product produced is a function of the iron and manganese reagents flow Q.sub.1, Q.sub.2 to the first reactor and Q.sub.3, Q.sub.4 to the second reactor, respectively. The Q.sub.3 and Q.sub.4 flows can be 0 to 5 times greater than Q.sub.1 and Q.sub.2, respectively.

(13) In the reactor (1) an aqueous solution of FeSO.sub.4 or FeCl.sub.2 of 1-50 g/L concentration and a KMnO.sub.4 aqueous solution of 1-20 g/L concentration are simultaneously introduced in continuous flow. The regulation of the flow rate ratio of the Fe (II) and KMnO.sub.4 solutions is controlled by achieving the desired redox potential of 500±50 mV, which is necessary for the nuclei formation of the Fe.sub.(1-x)Mn.sub.xOOH type with the percentage content in Mn(IV) ranging at 12±1 wt %. The reaction product enters the reactor (2), and a new FeSO.sub.4 or FeCl.sub.2 solution of 1-50 g/L concentration and a KMnO.sub.4 solution of 1-20 g/L concentration are added. The regulation of the flow ratio of the Fe (II) and KMnO.sub.4 solutions is controlled by maintaining the desired redox potential of 650±50 mV in the reactor (2). In both reactors (1) and (2), the pH value is adjusted throughout the reaction in the range of 5-6 in the first reactor and in the range of 9-10 in the second one by adding a solution of one or a combination of more than one of the alkaline reagents NaOH, Na.sub.2CO.sub.3, KOH, and K.sub.2CO.sub.3.

(14) The effluent from the reactor (2) is kept in a thickening tank (3) with a gentle stirring for a period of 6-36 hours where maximization and balancing of the negative surface charge is achieved. Under these conditions aging of the granules leads to the formation of amorphous nanocrystals without favoring their agglomeration in secondary morphologies as observed under acidic aging conditions.

(15) The precipitate after thickening is dehydrated by centrifugation (4) and is either molded into 250-2500 μm size granules and dried or first dried and then milled to a grain size of less than 50 μm (5).

(16) The adsorbent material can be used to uptake mercury from water, as well as other heavy metals such as cadmium, lead, nickel, cobalt, vanadium, copper, manganese and iron, preferably in an adsorption bed configuration. Its use is intended for drinking water treatment plants for domestic, industrial and point of use implementation, for the treatment of industrial and municipal waste, as well as for the treatment of flue gases and natural gas.

(17) From the procedure described above the tetravalent manganese feroxyhyte can be formed in the pH range of 3-12. However, at a pH of less than 8, its adsorption capacity for mercury and other heavy metals is dramatically reduced, since the positive surface charge, which repels the homonymous positively charged metal ions, gradually predominates.

(18) In particular, the negatively charged solid tetravalent manganese feroxyhyte has a specific surface area of 200-300 m.sup.2/g and a grain size of 250-2500 μm. Its maximum adsorption capacity (q.sub.max) for water with a pH of 7-8 exceeds 80 μg Hg/mg at a temperature of 20° C. In addition, the elemental mercury removal rate up to saturation from a gas stream exceeds 50 μg Hg/mg.

(19) The efficiency (parameter q.sub.1) of the material to uptake mercury from natural waters at pH 6-8 and at the same time to achieve at a residual concentration equal to limit of 1 μg/L is obtained from the diagram of FIG. 3. The measurements refer to manganese feroxyhyte with a 25% substitution rate. In these experiments, the test water was prepared according to NSF protocol and its composition reaches the specifications of the majority of natural waters, since it contains all the parameters that inhibit mercury adsorption. In particular, it contains 88.8 mg/L Na.sup.+, 40 mg/L Ca.sup.2+, 12.7 mg/L Mg.sup.2+, 183 mg/L HCO.sub.3−, 50 mg/L SO.sub.4.sup.2−, 71 mg/L Cl.sup.−, 2 mg/L N—NO.sub.3−, 1 mg/L F.sup.−, 0.04 mg/L P—PO.sub.4.sup.3− and 20 mg/L SiO.sub.2. The value of parameter q.sub.1 for mercury removal from NSF water, at pH 7, at 20° C. was found to be 2.8 μg/mg. Accordingly, the specific adsorption capacity (q.sub.50) for elemental mercury from a gas stream at 120° C. and at an equilibrium concentration of 50 μg/m.sup.3, which is the emission limit, was 13.3 μg/mg for contact time of 0.1 s.

Example of Method Application 1

(20) A solution of 45 g/L FeSO.sub.4 H.sub.2O is fed at a flowrate of Q.sub.1=2 m.sup.3/h and mixed with a 15 g/L KMnO.sub.4 solution of flowrate Q.sub.2=2 m.sup.3/h in the 1 m.sup.3 volume stirred-reactor (1). The flowrate of KMnO.sub.4 is varied so that the redox potential in the reactor (1) is maintained at the 500±50 mV range. The pH of the reaction is adjusted to 5.5±0.1 by the addition of 30% w/w NaOH solution.

(21) The reaction product leaving the reactor (1) enters the 16 m.sup.3 volume stirred-reactor (2), where a solution of 45 g/L FeSO.sub.4 H.sub.2O at a flowrate of Q.sub.3=2 m.sup.3/h and a solution of 15 g/L KMnO.sub.4 at a flowrate of Q.sub.4=2 m.sup.3/h are added. The redox potential in the reactor (2) is regulated in the 650±50 mV range by the addition of KMnO.sub.4 and the pH is regulated at 9±0.1 by the addition of a 30% w/w NaOH solution. The product from the reactor (2) outflow is led to the thickening tank where it is kept under mild stirring for 24 hours, then is mechanically dehydrated by centrifuge or filter press, sized to 250-2500 μm and dried at 110° C. The resulting material has a Fe.sub.0.75Mn.sub.0.25OOH structure and the manganese valence is 4. Its application to an adsorption column with contact time of 3 minutes gives an adsorption capacity of Hg.sup.2+ 2.8 μg/mg in standard NSF water, pH 7 and an equilibrium concentration of 1 μg/L.

Example of Method Application 2

(22) A solution of 15 g/L FeSO.sub.4 H.sub.2O is fed at a flowrate of Q.sub.1=3 m.sup.3/h and mixed with a 15 g/L KMnO.sub.4 solution of flowrate Q.sub.2=1 m.sup.3/h in the 1 m.sup.3 volume stirred-reactor (1). The flowrate of KMnO.sub.4 is varied so that the redox potential in the reactor (1) is maintained in the 500±50 mV range. The pH of the reaction is adjusted to 5.5±0.1 by adding a 5% w/w K.sub.2CO.sub.3 solution.

(23) The reaction product leaving the reactor (1) enters the 32 m.sup.3 volume stirred-reactor (2), where a solution of 15 g/L FeSO.sub.4 H.sub.2O at a flowrate of Q.sub.3=9 m.sup.3/h and a solution of 15 g/L KMnO.sub.4 at a flowrate of Q.sub.4=3 m.sup.3/h are added. The redox potential in the reactor (2) is regulated in the 650±50 mV range by adding KMnO.sub.4 and the pH is regulated at 9±0.1 by adding a 10% w/w KOH solution. The product from the reactor (2) outflow is led to the thickening tank where it is kept under mild stirring for 24 hours, then is mechanically dehydrated by centrifuge or filter press, sized to 250-2500 μm and dried at 110° C. The resulting material has a Fe.sub.0.75Mn.sub.0.25OOH structure and the manganese valence is 4. Its application to an adsorption column with contact time of 3 minutes gives an adsorption capacity of Hg.sup.2+2.8 μg/mg in standard NSF water, pH 7 and an equilibrium concentration of 1 μg/L.

Example of Method Application 3

(24) A solution of 30 g/L FeSO.sub.4 H.sub.2O is fed at a flowrate of Q.sub.1=4 m.sup.3/h and mixed with a 20 g/L KMnO.sub.4 solution of flowrate Q.sub.2=2 m.sup.3/h in the 2 m.sup.3 volume stirred-reactor (1). The flowrate of KMnO.sub.4 is varied so that the redox potential in the reactor (1) is maintained in the 500±50 mV range. The pH of the reaction is adjusted to 5.5±0.1 by the addition of 30% w/w NaOH solution.

(25) The reaction product leaving the reactor (1) enters the 24 m.sup.3 volume stirred-reactor (2), where a solution of 30 g/L FeSO.sub.4 H.sub.2O at a flowrate of Q.sub.3=4 m.sup.3/h and a solution of 20 g/L KMnO.sub.4 at a flowrate of Q.sub.4=2 m.sup.3/h are added. The redox potential in the reactor (2) is regulated in the 650±50 mV range by the addition of KMnO.sub.4 and the pH is regulated at 9±0.1 by the addition of a 30% w/w NaOH solution. The product from the reactor (2) outflow is led to the thickening tank where it is kept under stirring for 24 hours, then is mechanically dehydrated by centrifuge or filter press, sized to less than 50 μm and dried at 110° C. The resulting material has a Fe.sub.0.75Mn.sub.0.25OOH structure and the manganese valence is 4. Its application to an adsorption column with contact time of 0.01 sec gives an adsorption capacity of Hg.sup.2+ 13.3 μg/mg for an equilibrium concentration of 50 μg/m.sup.3 from a nitrogen gas stream of 120° C.

Example of method Application 4

(26) A 36 g/L FeCl.sub.2 solution is fed at a flowrate of Q.sub.1=12 m.sup.3/h and mixed with a 15 g/L KMnO.sub.4 solution with a flowrate of Q.sub.2=4 m.sup.3/h in the 5 m.sup.3 volume stirred-reactor (1). The flowrate of KMnO.sub.4 is varied so that the redox potential in the reactor (1) is maintained in the 500±50 mV range.

(27) The reaction product leaving the reactor (1) enters the 16 m.sup.3 volume stirred-reactor reactor (2), where a solution of 45 g/L FeSO.sub.4 H.sub.2O at a flowrate of Q.sub.3=6 m.sup.3/h and a solution of 15 g/L KMnO.sub.4 at a flowrate of Q.sub.4=2 m.sup.3/h are added. The redox potential in the reactor (2) is regulated in the 650±50 mV range by the addition of KMnO.sub.4 and the pH is regulated at 9±0.1 by the addition of a 30% w/w NaOH solution. The product from the reactor (2) outflow is led to the thickening tank where it is kept under stirring for 24 hours, then is mechanically dehydrated with a centrifuge or filter press, dried at 110° C. and sized to less than 50 μm. The resulting material has a Fe.sub.0.75Mn.sub.0.25OOH structure and the manganese valence is 4. Its application by injection into a gaseous nitrogen stream containing Hg.sup.0 up to 400 μg/m.sup.3 resulted in the decrease in its concentration below the emission limit of 50 μg/m.sup.3.

Example of Method Application 5

(28) The procedure of Example 3 is followed except that there is no further addition of reactants in the reactor (2). The resulting material has a Fe.sub.0.75Mn.sub.0.25OOH structure and the manganese valence is 4. Its' application to an adsorption column with contact time of 0.01 seconds gives an adsorption capacity of 10.6 μg/mg for elemental mercury Hg.sup.0 for an equilibrium concentration of 50 μg/m.sup.3 from a nitrogen gas stream of 120° C.

(29) The method described in this invention is performed either on laboratory or industrial scale depending on the production capacity of the continuous flow reactor.

(30) The product produced based on the synthesis method of the invention can be used to capture mercury and other heavy metals such as cadmium, lead, nickel, cobalt, vanadium, copper, manganese and iron from aqueous solution or elemental mercury from a gaseous stream, preferably in an adsorption bed or as powder in an spraying system.