Reactive atomized zero valent iron enriched with sulfur and carbon to enhance corrosivity and reactivity of the iron and provide desirable reduction products

Abstract

Iron, in the form of particles or iron wool, is used for the remediation of contaminated water. For ensuring that the process generally follows preferred chemical pathways resulting in non-toxic end products, and for providing greater rates of contaminant reduction, the iron is enriched with graphite carbon, at least 4% by weight, and sulfur, at least 0.5% by weight.

Description

BACKGROUND OF THE INVENTION

This invention relates to the use of enriched atomized Zero Valent Iron, typically in the form of particles, for remediation of contaminated water, and particularly to means for controlling the contaminant reduction pathways, which are followed during the remediation process.

It is known to use enriched atomized Zero Valent Iron (ZVI) particles to achieve rapid reduction rates and desirable byproducts for the treatment of large volumes of ground and surface waters contaminated with organic and inorganic pollutants. Nano-scale and micro-scale ZVI particles have been used for insitu and surface treatment of contaminated water, but a means to control contaminant reduction pathways to yield rapid reduction rates and desirable reduction end products has generally not been available.

The treatment of soluble inorganic and organic pollutants using ZVI particles requires the presence of water, which corrodes the ZVI to form electrons and ferrous ions. The corrosion of the ZVI particles is carried out in the presence of dissolved oxygen, which can enhance the corrosion rate according to accepted iron corrosion chemistry.

The reduction of dissolved phase pollutants with ZVI can occur through two mechanisms: 1) direct electron transfer from the corroding iron to the dissolved contaminant compound or 2) indirect reduction by atomic hydrogen formed from the catalytic reduction of water by electrons at catalytic sites that exist on the surface of the ZVI. Direct electron transfer primarily favors the sequential reduction pathways (hydrogenolysis), which generally lead to the formation of undesirable reaction products. For example, direct electron reduction of Trichloroethylene (TCE) first forms 1,2,cis Dichloroethylene (DCE) and then Vinyl Chloride (VC), daughter products both of which are considered more toxic than TCE.

Conversely, the indirect reduction of pollutants involving atomic hydrogen appears to follow elimination and hydrogenation reduction pathways that bypass the hydrogenolysis pathway to yield the more desirable ethylene and ethane. However, as above noted, in any given remediation process, it has not been always possible to control which remediation mechanism is actually followed.

SUMMARY OF THE INVENTION

For better ensuring that a given ZVI contaminant reduction process favor reactions that follow elimination and hydrogenation pathways, as well as possibly providing greater rates of contaminate reduction, the ZVI, typically in the form of particles, but also in the form of an iron wool, is enriched with at least 4% by weight of graphite carbon and 0.5% by weight of sulfur.

DETAILED DESCRIPTION

It has been reported that the percent loadings of ZVI by weight relative to 1) the weight of volume of contaminated water or 2) weight of volume of contaminated geological formation being treated influences the reduction pathways for less reactive pollutants. The use of low loadings of 0.16% by weight of electrolytic type ZVI particles for the reduction of perchloroethylene (PCE), TCE and VC favor elimination and hydrogenation reduction pathways by atomic hydrogen rather than direct electron reduction that favors the hydrogenolysis reduction pathway.

The elimination reaction pathway observed for the reduction of PCE, TCE, and VC for a 0.16% ZVI loading was reported to represent 87%, 97% and 94% of the reactions pathways involved. As an example, the results of the analysis for the distribution of reaction products from the reduction of TCE show the DCE (hydrogenolysis pathway) to be 1.7, mole %, the ethylene to be 77 μmole % (elimination pathway) and the ethane to be 21, mole % (hydrogenation pathway). The 1^st order rate constant for the observed reduction of TCE was reported to be only 0.0020 hr⁻¹ [Arnold, W. et al “Pathways and Kinetics of Chlorinated Ethylene and Acetylene Reaction with Fe(0) Particles” Env. Sci. Tech 2000, 34 (1794-1806)]. We have found that the first order constant for the reduction of TCE can be increased by raising the electrolytic type ZVI to a loading of 5.3% by weight. The observed rate constant was determined to be increased eight times to 0.0163 hr⁻¹. However; at these loadings the hydrogenolysis reaction pathways has a greater influence on the distribution of reaction products by forming more toxic reaction products. The cis DCE is increased from 1.7 μmole % to 24 μmole %, the ethylene is decreased from 77 μmole % to 18 μmole % and the ethane increases slightly from 21 μmole % to 24 μmole %. Apparently, at these loadings the catalytic sites on the electrolytic type ZVI are becoming saturated with electrons, which appear to inhibit atomic hydrogen formations. As a result, greater amounts of electrons are available for direct reduction of the TCE by hydrogenolysis to form cis DCE.

The results of corrosion studies employing electrolytic type ZVI particles at higher loadings than 0.16% by weight show apparent hydrogen gas production rate of 80 μmoles/KG/days which is indicative that direct electron reduction is occurring [Rearden, E. J. “Zero Valent Iron styles of corrosion and inorganic control of hydrogen pressure buildup” Env. Sci. Tech, 2005, 39 (3311-3317)]. However, we have found that the elimination and hydrogenation reductive pathways for the reduction of TCE can be favored by increasing the number of catalytic sites on the ZVI particles even at loading of 5.3% by weight. The 0.25% of graphite carbon which remains in sponge iron from its manufacturing process apparently adds sufficient sites to the ZVI particle to favor the elimination and hydrogenation pathways.

The catalytic properties of the graphite carbon in sponge iron were identified in U.S. Pat. No. 5,975,798 as producing atomic hydrogen. This was indicated from the absence of increases in pH that results from hydroxyl formation that is produced from iron corrosion reactions according to iron corrosion chemistry. Reduction reactions involving atomic hydrogen results in the production of hydrogen ion, which can neutralize the hydroxyl ions, produced from the corrosion of iron. Increases in the number of catalytic sites on the ZVI can catalyze the electrons to form atomic hydrogen and appears to limit the electrons involvement in direct reduction (hydrogenolysis). The results of corrosion studies employing sponge iron particles containing 0.25% by weight of graphite carbon shows no apparent rate of hydrogen gas production, which is generally associated with the corrosion of iron. The lack of hydrogen gas production is indicative that the electrons are being catalyzed to form atomic hydrogen.

We have observed that the 0.25% by weight of graphite carbon that remains in sponge iron particles from the manufacturing process when used at loadings of 5.3% by weight to treat TCE favors the elimination and hydrogenation pathways. The results of analysis for the reaction product distributions show the cis DCE to be at non-detectable levels, the ethylene content is 16 μmole %, the ethane content is 75, mole % and other ethenes and ethanes amount to 9 μmole %. The first order rate constant for the reduction of TCE was also observed to increase due to the presence of the graphite carbon when compared to that determined for the electrolytic type iron particles, which contained negligible amounts of carbon. The first order rate constant for the reduction of TCE using the sponge iron and electrolytic type iron was determined to be 0.066 hr⁻¹ and 0.016 hr⁻¹, respectively.

Increased loadings of the sponge ZVI particles are also accompanied by an increase in ferrous ion production according to iron corrosion chemistry. This results in an increase of contaminants by reactions with ferrous ions and oxyhydroxides resulting from the corrosion of iron in the presence of ZVI and dissolved oxygen [Satapanajaru, T. et al “Green Rust and Iron Oxide Formation Influences Metolachlor Dechlorination During Zero Valent Iron Treatment” Env. Sci. Tech. 2003, 37 (5219-5227)]. These insoluble ferrous oxyhydroxide compounds can also complex with soluble reduced contaminants and remove them from solution. We have observed atomic hydrogen. However, by the addition of sufficient quantities of catalyst one can create sufficient numbers of catalytic sites on the ZVI surface to favor atomic hydrogen formation and reductions that primarily follow the elimination and hydrogenation pathways rather than the hydrogenolysis pathway.

In the preparation of our iron particles for the treatment of large volumes of contaminated water for agriculture use and human activity, enriched graphite carbon was preferred over the use of metal catalyst such as nickel. The release of carbon to the aqueous system being treated due to the corrosion of iron poses no toxic threat. The release of toxic catalytic metal ions during the corrosion of the iron can contaminate the aqueous supply. The carbon is also less expensive than the metal catalyst and more resistant to poisoning by agents encountered in the treatment of contaminated water. We have observed this to occur in the treatment of agriculture wastewater.

The treatment of agriculture waste water for removal of soluble selenium VI for agriculture reuse employing sponge iron containing 0.25% carbon by weight at ZVI loadings of 0.5% by weight in presence of D.O. resulted in a 37% reduction of selenium VI directly to elemental selenium in 50 hours. The use of atomized ZVI containing 2% by weight of nickel resulted in negligible removal of selenium VI over the same period of time. The presence of poisoning agents in the agriculture wastewater is believed to be responsible for the inactivate nickel catalyst in the atomized iron particles.

Justification for Atomized Iron Particles Enriches with Graphite Carbon and Sulfur

The above results indicate that larger quantities of catalytic graphite carbon are required in the ZVI particles to favor reductions of highly reductive pollutants such as 1,1,1, TCA that follow the elimination and Hydrogenation pathways rather than the hydrogenolysis pathways. However, the preparation of sponge ZVI particles with carbon contents above 0.6% by weight as identified in U.S. Pat. No. 5,975,798 is difficult. The graphite carbon exist as an alloy in the sponge ZVI particle and increasing its amount would impact the optimum process conditions used to reduce the iron ore to sponge ZVI. As a result, atomized ZVI particles has been selected and enriched with over 4% by weight of carbon and 0.5% by weight of sulfur. These elements were added to increase corrosivity (rate if corrosion) and reactivity (rate of contaminant reduction) of the iron for treatment of large volumes of agriculture waste water, contaminated ground and surface water in above ground conventional treatment systems and provide desired reduction pathways and products for contaminants that exhibit different reductivities such as TCE and 1,1,1 TCA. The first order rate constants for the direct electron reduction of TCE and 1,1,1 TCA was reported to be 3.9+3.6×10⁻⁴ L m⁻² h⁻¹ and 1.1×10⁻² L m⁻² h⁻¹ [Johnson et al “Kinetics of Halogenated Organic Compound by Iron Metal” Env. Sci. Tech, 1996, 30 (2634-2640)].

The atomized iron is enriched with sulfur during the formation of the atomized iron particles. It is anticipated that the sulfur in the atomize iron will enhance the corrosion of iron to produce additional electrons and ferrous ions in the presence of dissolved oxygen. The sulfur may also be reduced in the formation of the atomized iron to form sulfide which in the presence of ferrous ions forms ferrous sulfide on the surface of the atomize iron particles. The combination of Ferrous Sulfide solutions added to ZVI particles has been reported to provide greater rates of reduction of contaminants than the use of ZVI particles alone [Butler, E. et al “Factors Influencing Rates and Product Transformations of TCE by FeS and Iron Metal” Env. Sci. Tech, 2001, 35 (3884-3891)].

Evaluation of Enriched Atomized Iron

Bench scale kinetic studies were carried out in columns containing 25% by weight of the enriched atomized iron particles in 75% silica sand for the reduction of TCE and VC. These studies also included the use of cast iron particles containing 2.9% by weight of graphite carbon and sponge iron particles containing 0.25% by weight of graphite carbon to evaluate the effect of increasing the graphite carbon on the reduction of TCE and VC. TCE and VC were selected because the TCE is reported to exhibit a first order rate constant that is some 10 times faster than VC when the reduction pathways result from electron reduction (hydrogenolysis).

The results of these studies indicate that the first order rate constants for the reduction of TCE and VC were determined to be 0.768 hr⁻¹ and 4.44 hr⁻¹ using the enriched atomized iron particles, 0.345 hr⁻¹ and 1.15 hr⁻¹ for the cast iron particles and 0.447 hr⁻¹ and 0.390 hr⁻¹ for the sponge iron particles, respectively. These results show that an increase in the quantity of catalytic graphite carbon in the iron particles favor atomic hydrogen reductions even at high loadings of 25% by weight of atomized iron. The greater rate of reduction of VC compared to TCE is indicative that the reductions involve atomic hydrogen rather than direct electron reduction. At these loadings, 89% reduction of TCE and 100% reduction of VC were achieved in less than 3 hours.

Results of pH

pH measurements within the test columns containing the enriches atomized iron at a dosage of 25% by weight with a 4% carbon content and 0.5% by weight sulfur content indicated the reduction of TCE and VC involved atomic hydrogen. pH measurements collected within the column containing the sponge iron at a dosage of 25% with a 0.25% by weight carbon content indicated reductions of TCE and VC through direct electron reduction.

The results of pH measured in the test columns containing the enriched atomized iron over a period of 30 hours show the initial pH to decrease from 7.5 to about 6.8 and remain constant over the duration of the pH measurements. These findings indicate the enrichment of the atomized iron was sufficient to catalyze the quantities of electrons produced at loadings of 25% by weight to form atomic hydrogen as was discussed in U.S. Pat. No. 5,975,798 with the use of sponge iron particles containing much lower weights of graphite carbon of less then 0.6% at ZVI loadings 1.0%.

The result of pH measurements in the columns containing the sponge iron particles show the pH increasing from 7.5 up to 9.0 within 2 hours and remaining constant at 9.5 over the test duration of 26 hours. These results indicate that the reduction of the TCE and VC by the sponge iron is occurring by direct electron reduction at ZVI loadings of 25% by weight. As is expected, the quantities of graphite carbon present in the sponge iron does not provide sufficient sites on the sponge iron particles to catalyze the quantities of electrons produced at the loadings to form atomic hydrogen.

SUMMARY

• • The quantity of graphite carbon and sulfur used to enrich the atomized iron particles during their preparation should be at least 4% by weight of graphic carbon and 0.5% by weight of sulfur.
  • The function of the graphite carbon in the atomized iron is to provide increased rates of degradation of contaminants and provide additional sites on the surface of the iron particles, which catalyze the electrons in the presence of water to form atomic hydrogen. The reductions involving atomic hydrogen appear to favor reactions that follow elimination and hydrogenation pathways rather than direct electron reduction by hydrogenolysis reaction pathways.
  • The function of sulfur is to increase the corrosivity of the atomized iron particles to provide greater rates of production of electrons and ferrous ions. The increased production of ferrous ions can also contribute to greater rates of reduction of contaminant by forming greater amounts of insoluble iron oxyhydroxide flocs, formed during the corrosion of iron in the presence of ZVI particles. The formation of these insoluble iron oxyhydroxide flocs can also complex with soluble reduced contaminants and remove them from solution.
  • The reactivity of the enriched atomized iron particles allows the use of conventional above ground treatment systems for the exsitu treatment of contaminated ground water and surface water for agriculture reuse and human activity. The results of bench scale column studies indicates that silica sand beds containing 25% by weight of enriched atomized iron particles have achieved VC and TCE reductions of 100% and 89%, respectively at flow velocities at 0.06 ft/hr in a bed depth of 0.17 ft. The treatment systems employed would consist of industrial sized large-scale reaction vessels capable of treating thousands of gallons of water per day.
  • The enriched atomized zero valent iron in the form of an iron wool could be used in smaller treatment vessels or cartridges designed for treating smaller volumes of water for residential or commercial use.
  • The increased reactivity of atomized iron wool enriched with carbon and sulfur can provide treatment times of less then 3 hours and achieve desired inorganic and organic contaminant reductions in small volumes of contaminated water at points of residential and commercial use.

Claims

1. A material for use in a water decontamination process comprises zero valent iron enriched with graphite carbon and sulfur.

2. The material of claim 1 wherein said iron is in the form of particles.

3. The material of claim 1 wherein said iron is in the form of iron wool.

4. The material of claim 1 wherein said carbon graphite comprises at least 4% by weight and said sulfur comprises at least 0.5% by weight.