Process for producing zero-valent iron nanoparticles and treating acid mine drainage

Abstract

A process for treating acid mine drainage removes iron ions from the acid mine drainage in the form of zero-valent iron nanoparticles which can be subsequently used for environmental remediation.

Claims

1. Process for producing zero-valent iron and treating acid mine drainage, the process comprising: a) providing aqueous acid mine drainage feedstock including from 50 ppm to 500 ppm of metal ion selected from the group consisting of ferrous iron, ferric iron, and mixtures thereof at a pH of less than 6.9, b) providing an alkali metal borohydride selected from the group consisting of sodium borohydride, potassium borohydride and mixtures thereof, c) mixing the alkali metal borohydride with the acid mine drainage feedstock to form an aqueous suspension of zero-valent iron, wherein the alkali metal borohydride is mixed as a powder with the acid mine drainage feedstock.

2. Process for producing zero-valent iron and treating acid mine drainage, the process comprising: a) providing aqueous acid mine drainage feedstock including from 50 ppm to 500 ppm of metal ion selected from the group consisting of ferrous iron, ferric iron, and mixtures thereof at a pH of less than 6.9, b) providing an alkali metal borohydride selected from the group consisting of sodium borohydride, potassium borohydride and mixtures thereof, c) mixing the alkali metal borohydride with the acid mine drainage feedstock to form an aqueous suspension of zero-valent iron, wherein from about 0.5 to 0.8 g alkali metal hydride per gram of iron ion is mixed with the acid mine drainage feedstock.

3. Process according to claim 1 wherein the amount of alkali metal borohydride mixed with the acid mine drainage feedstock is sufficient to raise the pH of the aqueous suspension to no more than 8.3.

4. Process according to claim 3 wherein the amount of alkali metal borohydride is sufficient to raise the pH of the aqueous suspension to from about 7.90 to about 8.15.

5. Process according to claim 1 wherein the acid mine drainage feedstock includes from about 100 ppm to about 400 ppm of metal ion.

6. Process according to claim 5 wherein the acid mine drainage feedstock includes from about 200 ppm to about 300 ppm of metal ion.

7. Process according to claim 1 wherein the metal ion comprises at least 90 percent by weight ferrous iron.

8. Process according to claim 1 further comprising providing an inert atmosphere, and mixing the alkali metal borohydride with the acid mine drainage feedstock under an inert atmosphere.

9. Process according to claim 1 further comprising separating the zero valent iron from the aqueous suspension to provide separated zero-valent iron and an effluent.

10. Process according to claim 9 wherein the zero-valent iron is separated by filtration.

11. Process according to claim 9 wherein the zero-valent iron is separated magnetically.

12. Process according to claim 9 further comprising storing the separated zero-valent iron in a medium having less than 5 ppm dissolved oxygen.

13. Process according to claim 9 further comprising storing the separated zero-valent iron in ethanol.

14. Process according to claim 1 wherein the alkali metal borohydride powder has a mean particle size of from about 75 nm to about 150 nm.

15. Process according to claim 9 wherein the effluent includes manganese ion, and the process comprises further treating the effluent to precipitate manganese ion from the effluent.

16. Process according to claim 1 wherein the acid mine drainage feedstock is provided at a pH of from about 6.1 to 6.8.

17. Process according to claim 16 wherein the acid mine drainage feedstock is provided at a pH of from about 6.4 to 6.6.

18. Process according to claim 1 wherein the rate of addition of the alkali metal borohydride to the acid mine drainage is controlled to control the particle size of the resulting zero-valent iron.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view of an apparatus for carrying out the process of the present invention.

(2) FIG. 2 is a HAADF-STEM image of a sample of zero-valent iron produced by the process of the present invention.

(3) FIG. 3 is a higher magnification TEM image of the sample of FIG. 2 showing details of the internal structure.

(4) FIG. 4 is a higher magnification HAADF-STEM image showing an oxide shell around the zero-valent iron particles of the sample of FIG. 2.

(5) FIG. 5 is a higher magnification HAADF-STEM/EDS image showing an oxide shell around the zero-valent iron particles of the sample of FIG. 4.

(6) FIG. 6 is a schematic diagram showing a continuous implementation of the process of the present invention.

DETAILED DESCRIPTION

(7) The present invention provides a process for preparing zero-valent iron nanoparticles. The process of the present invention can be carried out as a batch process or as a continuous process. The process permits the separation of dissolved iron from acid mine drainage. Dissolved iron is a major constituent of acid mine drainage. In addition to the dissolved iron, acid mine drainage frequently includes significant concentrations of other heavy metals, such as manganese. The present process permits dissolved iron to be separated from the acid mine drainage in the form of nano zero-valent iron, which can be subsequently employed for enviromental remediation purposes. After removal of the zero-valent iron, the acid mine drainage can be subsequently treated to precipitate other heavy metals such as manganese which may be present and remove the heavy metal precipitate(s) from the acid mind drainage.

(8) Preferably, since the chemical properties of acid mine drainage can vary significantly depending on the source of the acid mine drainage, the chemical properties, including the pH and the concentration of ferrous and ferric ions, manganese ion, et al., are measured before initiating the treatment process. Preferably, the concentration of iron ions in the acid mine drainage is at least about 50 ppm, more preferably from about 50 ppm to about 500 ppm, still more preferably from about 100 ppm to about 400 ppm, and still more preferably from about 200 to 300 ppm. While either acid mine drainage containing either ferric or ferrous ions or a mixture of ferric and ferrous ions can be employed in the present process, acid mine drainage including iron in the lower oxidation state is preferred since the amount of reducing agent required to form the zero-valent iron and corresponding operation costs are correspondingly reduced. Sodium borohydride, potassium borohydride, or mixtures of the two can be employed as alkali metal borohydride reducing agents, however, sodium borohydride is presently preferred. The alkali metal borohydride can be in the form of a powder or granular material, or in the form of an aqueous solution, such as an aqueous solution of sodium borohydride and sodium hydroxide available from Montgomery Chemicals, Conshohocken, Pa. When a sodium borohydride powder is employed, the particle size and corresponding surface area of the powder can affect the rate at which the powder is mixed with the acid mine discharge. Similarly, when an aqueous solution of sodium borohydride is employed the concentration of sodium borohydride may affect the rate at which the aqueous solution is mixed with the acid mine discharge.

(9) Zero-valent iron is susceptable to oxidation from dissolved oxygen in the acid mine drainage. To minimize the extent of oxidation, the acid mine drainage is preferrably purged with an inert gas such as nitrogen or argon prior to reducing the iron ions to metallic zero-valent iron with the alkali metal borohydride. Preferably, the reduction reaction takes place in a reactor vessel isolated from the atmosphere and/or continuously purged with an inert gas to minimize the level of dissolved oxygen in the reaction mixture.

(10) The reduction reaction of ferrous/ferric ion with borohydride ion has been found to be sufficiently rapid such that reduction of the iron will take place below a pH of about 8, and faster than the rapid hydrolysis of the borohydride in the acid mine drainage. Thus, zero-valent iron is formed, and the formation of insoluble iron and/or manganese oxide, hydroxide or carbonate is avoided.

(11) The particle size of the zero-valent iron is a function of the rate of addition of sodium borohydride. For example, when sodium borohydride is added at a slow rate, such as over an interval of 10 to 15 minutes, the pH of the acid mine drainage increases slowly, and the particle size of the resulting zero-valent iron is in the range of 50 to 200 nm. In contrast, when the sodium borohydride is added at a faster rate, such as over an interval of 2 to 5 minutes, then the pH increases rapidly, and particle size of the resulting zero-valent iron is in the range of 100 to 300 nm. Thus, the mean particle size of the zero-valent iron can be controlled by controling the rate of addition of the sodium borohydride.

(12) Optionally, the particle size of the zero-valent iron can be controlled by addition of a suitable polymeric thickener material such as carboxymethylcellulose.

(13) It is presently understood that the rate of hydrolysis is given by:
Hydrolysis rate=k.sub.hyd[H.sup.+][BH.sub.4.sup.]

(14) And the reduction rate is given by:
Reduction rate=k.sub.Fe([Fe.sup.2+]/[H.sup.30])[BH.sub.4.sup.]

(15) Thus, there are two effective first-order rate constants (with respect to borohydride), k.sub.hyd [H.sup.+] and k.sub.Fe [Fe.sup.2+]/[H.sup.+], or k.sub.w and k.sub.red respectively. k.sub.w will decrease with pH and k.sub.red will increase with pH. If k.sub.w is 10 times faster than with k.sub.red for the given iron content, even at pH 7.5, then more borohydride is required to speed the reaction with the iron. The reason is simple: if there are two parallel reactions, B and C involving the same reactant, A (i.e. BH.sub.4.sup. in this case), then:
d[B]/dt=k.sub.1[A] d[C]/dt=k.sub.2[A] d[A]/dt=(k.sub.1+k.sub.2)[A]
So
[A]=[A].sub.0 e.sup.(k1+k2)t and [C]=[C].sub.0(k.sub.2/(k.sub.1+k.sub.2))[A].sub.0(1e.sup.(k1+k2)t) Therefore, the slow C reactant (assuming B is the fast one) will only use up a fraction (k.sub.2/(k.sub.1+k.sub.2)) of reactant A. As a result, if reduction is 10 times slower than hydrolysis, 10 times the borohydride is required to reduce all of the iron. Similarly, if reduction is 100 times slower, then we need 100 times higher borohydride concentration to reduce all of the ferrous iron.

(16) The process of the present invention can be carried out in a batch mode or as a continuous process. An apparatus for carrying out the process in batch mode is shown in FIG. 1. Untreated acid mine discharge 12 is passed through a prefilter 14 to remove particulate matter (e.g. larger than 20 microns) and deliver to a closed reactor 16 in which the acid mine discharge is continuously mixed by a mixing device 18. Nitrogen is continously delivered to the reactor 16 from a tank 20 to purge oxygen containing air from interior of the reactor 16 and especially from the reaction mixture 22 in the reactor 16. Sodium borohydride is added to the reaction mixture through an injection port 24, and the pH and electode potential of the reaction mixture 22 is monitored using a pH meter 26 as the sodium borohydride is added. When the reaction mixture attains a preselected pH, the reaction mixture 22 is withdrawn from the reactor 16 by suitable means, such as a pump (not shown), and passed through an electromagnetic filter 28 which separates zero-valent iron 32 from the reaction mixture 22 to produce a treated acid mine discharge 30. The zero-valent iron 32 is subsequently removed from the electromagnetic filter 28 and stored in ethanol to prevent oxidation.

Example 1

(17) A sample of acid mine drainage (AMD) was obtained from the Clyde mine site in Fredericktown, Pa. One liter of the AMD sample was filtered using a 20 cartridge filter. The pH of the AMD sample was 6.49, and the sample contained 227.6 ppm iron, 6.83 ppm manganese, and 0.87 ppm dissolved oxygen. The AMD water sample was purged with nitrogen while mixing in the closed system shown schematically in FIG. 1. After about 3 to 5 minutes, 0.01 g aliquots of solid sodium boroyhdride powder reducing agent were added to the AMD sample through a chemical injector port and mixed, and after mixing for about one minute for each aliquot, the pH of the resulting mixture was measured, as reported in Table A below, along with electrochemical potential of the mixture, and a sample of the reaction mixture was withdrawn from the reactor and visually inspected. Addition of the 0.01 aliquots of the sodium borohydride powder continued until the pH of the resulting mixture was measured as 8.05. Hydrogen evolution from the mixture was apparent when the pH was 6.6. The resulting mixture as stirred for an additional 5 to 7 minutes and then passed through a magnetic filter to collect black zero-valent iron particles. After collection of the zero-valent iron particles, the resulting mixture was measured to have a pH of 8.13, 1.80 ppm iron, 4.77 manganese, and 1.34 ppm dissolved oxygen.

(18) The zero-valent iron produced was subjected to chemical and physical analysis at the Penn State Materials Research Institute. Samples were prepared by sonicating the sample to disperse particles in solution. A needle was inserted into a rubber stopper to extract a small amount of the solution. The solution was drop cast on a lacey carbon transmission electron microscope (TEM) support grid and immediately inserted into the TEM under vacuum. The sample was exposed to atmosphere for less than five minutes. TEM and scanning transmission electron microscopy (STEM) were carried out using a Talcs TEM at 200 kV (ThermoFisher Scientific) Energy-dispersive X-ray spectroscopy (EDS) mapping was carried out to investigate the composition of particles. High angle annular dark field STEM (HAADF) providing a better mass contrast than TEM was also carried out, as the contrast was approximately proportional to Z.sup.2, and reversed as compared to TEM.

(19) FIGS. 2-5 are electron micrographs showing results of the analyses of the zero-valent iron produced by the present process. The images show large round elemental Fe particles approximately 100-300 nm in diameter with a thin oxide shell (5 nm thick). Other elements present include Ca, Na, Mg, K, S. The results of EDS quantification of the elemental composition of a sample of the zero-valent iron particles are reported in Table B. With the exception of Fe, O, Ca, most other elements seen in spectrum are at very low levels. Quantification shows four different compositions corresponding most likely to elemental iron, iron oxide, silicon oxide, and calcium oxide.

(20) TABLE-US-00001 TABLE A Sodium borohydride (g) pH mV Notes 0 6.49 7.8 0.01 6.51 1 0.02 6.53 3 0.03 6.6 5.6 H.sub.2 gas evolves 0.04 6.72 9.1 0.05 6.78 13.4 0.06 6.86 16.3 0.07 6.93 19.8 0.08 7.02 25 0.09 7.11 29.8 0.1 7.25 36.3 0.11 7.32 42 0.12 7.5 50 Yellow/green magnetic ppt. 0.13 7.58 55.3 Magnetic ppt. 0.14 7.62 60.5 Dark green magnetic ppt. 0.15 7.75 65.5 Magnetic ppt. 0.16 7.83 71.3 Black, magnetic ppt. 0.17 7.94 77 Magnetic ppt. 0.18 8.05 80 Magnetic ppt.

(21) TABLE-US-00002 TABLE B Error Mass C. Norm. C. Atom C. (3 sigma) Element Series Net (wt. %) (wt. %) (at. %) (wt %) S K series 527 0.42 0.42 0.57 0.14 Fe K series 79988 83.82 83.82 66.05 7.68 O K series 11101 9.25 9.25 25.44 0.95 C K series 0 0 0 0 Na K series 890 0.60 0.60 1.14 0.16 Mg K series 535 0.39 0.39 0.71 0.14 K K series 1078 0.88 0.88 0.99 0.19 Ca K series 5366 4.65 4.65 5.11 0.54

(22) It was found that at low pH-reaction yield was low and some Group I and Group II chemical particle contamination was observed. At pH 8.3, higher iron nano crystals yield was observed as well as Group I and Group II chemical particles contamination was observed. The nano crystals formed in the process were highly magnetic. No radioactive materials were contained in the samples according to lab results. During the reaction nano crystals particle size could be controlled in the range 50-300 nm by controlling the rate of addition of the borohydride.

Example 2

(23) The process of Example 1 was repeated, except that an aqueous solution of 10 percent by weight sodium borohydride; 4 percent by weight sodium hydroxide was prepared from a solution of 12 percent sodium borohydride and 40 percent sodium hydroxide (BoroSpec 1240, Montgomery Chemicals, Conshohocken, Pa.) which was diluted 1:10 with distilled water, to which was added 8.8 g sodium borohydride dissolved in 100 ml water to form a reducing agent. After purging a one liter sample of the AMD with nitrogen for around 3 to 5 minutes, 100 microliter aliquots of the reducing agent were added to the reaction mixture in the reactor and stirred, and samples were withdrawn for visual inspection, until the measured pH of the reaction mixture was 8.15. The reaction mixture was stirred for an additional 5 to 7 minutes and the treated reaction mixture was then passed through a filter to collect black zero-valent iron particles. Results are reported in Table C.

(24) TABLE-US-00003 TABLE C Sodium borohydride (L) pH Notes 0 6.49 100 6.55 200 6.7 300 6.77 400 6.88 500 7.13 600 7.25 700 7.34 M* 800 7.44 M 900 7.48 M 1000 7.56 M 1100 7.72 M 1200 7.8 M 1300 7.88 M 1400 7.95 M 1500 8.05 M 1600 8.19 M *Indicates back magnetic iron precipitate.

Example 3

(25) A continuous implementation 100 of the process of the present invention is shown schematically in FIG. 3. Prefiltered acid mine drainage 110 is supplied at about 1000 gallons per minute. The AMD has a pH of 6.48 and includes about 200 to 250 ppm iron ions, and is supplied through volume control solenoids 1121a, 112b, 112c, to nitrogen purged treament tanks 116a, 116b, 116c. Sodium borohydride is added through injection ports 114a, 114b, 114c and the reaction mixture in each tank 116a, 116b, 116c is stirred to reduce the iron ions to zero-valent iron. The pH in each tank 116a, 116b, 116c is monitored, and when the pH is about 8.1, the contents of the tanks is withdrawn by a pump 120 through pH control valves 118a, 118b, 118c and the reaction mixture is flowed through a magnetic filter unit 122. The treated acid mine waste 134 is stored in treated wastewater storage tank 132 for subsequent treatment or disposal 142. The zero-valent ion 126 is washed from the magnetic filter 122 with ethanol and the resulting effluent is treated in a drying device 128 and the ethanol is recycled while the zero-valent iron is sent to a packaging device 130 to provide a packaged material isolated from atmospheric oxygen.

(26) Various modifications can be made in the details of the various embodiments of the articles of the present invention, all within the scope and spirit of the invention and defined by the appended claims.