ZERO VALENT IRON/IRON OXIDE MINERAL/FERROUS IRON COMPOSITE FOR TREATMENT OF A CONTAMINATE FLUID

Abstract

The present inventors have discovered a novel composition, method of making the composition, system, process for treating a fluid containing a contaminant. The fluid may be aqueous. The contaminated fluid may be in the form of a suspension. The treatment reduces the concentration of the contaminant. The reduction in concentration of a contaminant may be sufficient so as to effect remediation of the fluid with respect to the contaminant. The treatment may reduce the concentration of a plurality of contaminants. The present composition, system, and process are robust and flexible. The composition includes zero valent iron, an iron oxide mineral, and ferrous iron. The ferrous iron promotes maintenance of the iron oxide mineral. The iron oxide mineral promotes the activity of the zero valent iron. The process and system may involve multiple stages. A stage may be optimized for treatment with respect to a particular contaminant. The present composition, system, and process are effective for treating a fluid containing one or more of a variety of contaminants such as toxic metals, metalloids, oxyanions, and dissolved silica. It may be applied to treating various aqueous fluids, such as groundwater, subsurface water, and aqueous industrial waste streams.

Description

BACKGROUND

Wastewater treatment is one of the most important and challenging environmental problems associated with coal-based power generation.

Using wet scrubbers to clean flue gas is becoming more popular worldwide in the electrical power industry. In the coming years, hundreds of wet scrubbers will be installed in the US alone. While wet scrubbers can greatly reduce air pollution, toxic metals in the resulting wastewater present a major environmental problem. The industry prepares to invest billions of dollars in the next decade to meet more the ever more stringent environmental regulations; unfortunately, a cost-effective and reliable technology capable of treating such complicated wastewater is still not available.

The compositions of FGD wastewaters vary greatly, depending not only on the types of coal and limestone used but also on the types of scrubber and processes used. Pretreatment method and management practices also affect wastewater characteristics. According to a recent survey by EPR1 (2006), untreated raw FGD wastewater could have TSS in ˜10,000 mg/L but after settlement, it falls to ˜10 mg/L; the pH typically falls in 5.8-7.3; sulfate is in the range of 1,000-6,000 mg/L; nitrate-N at level of 50 mg/L is not uncommon; chloride, alkalinity and acidity vary from hundreds to thousands ppm; selenium exists in various forms, ranging from dozens of ppb to over 5 ppm, among which, selenate could account for about half of total Se; arsenic ranges from a few ppb to hundreds of ppb; mercury ranges from below 1 ppb to dozens of ppb; and boron can be as high as hundreds of ppm.

Treatment of selanate-Se in wastewater is often considered to be one of the most difficult in toxic metal treatments. Selenium is a naturally occurring chemical element in rocks, soils and natural waters. Although Se is an essential micronutrient for plants and animals, it can be toxic at elevated levels and some of Se species may be carcinogenic. The hexavalent selenium is stable in oxic environments and exists as the selenate (SeO₄²⁻) anion, which is weakly sorbed by mineral materials and generally soluble. Tetravalent Se is the stable valence state under mildly reducing or anoxic condition (0.26 V<Eh<0.55 V at pH 7). It exists as the selenite (SeO₃²⁻) anion, which tends to be bound onto mineral surfaces (e.g., Fe and Mn oxides). Selenate and selenite are more toxic due to their high bioavailability than elemental selenium or metallic selenides.

A biological treatment system, ABMet, has been patented and is being marketed by GE Water.

However, there remains a need for a cost-effective and reliable treatment process for removing toxic pollutants from the wastewater generated by the wet scrubbers operated for flue gas desulfurization in coal-fired power plants.

SUMMARY

The present inventor has developed a chemical treatment process that can cost-effectively treat all major pollutants in the flue gas desulfurization (FGD) wastewater in a single process.

The present inventor developed a fluidized reacting system using a hybrid reactive solid/secondary reagent reactor that can cost-effectively remove many toxic metals from wastewater. The system and process are effective to treat an aqueous suspension. The system uses a reactive solid and a secondary reagent as reactive agents to rapidly reduce selenate to become insoluble selenium species, which are then adsorbed or precipitated along with various of other toxic metals (such as As and Hg, if present) in wastewater onto the iron oxide sludge. The system is particularly effective for removing selenate-Se.

The present process is effective for removing almost all concern toxic metals in an aqueous suspension; in addition, it can remove oxyanion pollutants and metalloids. More particularly, contaminants removable by the present system and process are: most toxic metals such as arsenic, mercury, selenium, cobalt, lead, cadmium, chromium, silver, zinc, nickel, molybdenum, and the like; metalloid pollutants such as boron and the like; many oxyanion pollutants, such as nitrate, bromate, iodate, and periodate, and the like; and the like.

The present system and process use common, non-toxic, and inexpensive chemicals. The present chemical treatment system costs much less to construct and operate than biological treatment systems, which tend to be more complex.

The present system and process are versatile and flexible. The present system and process are more robust and manageable than a biological process when exposed to toxic chemicals or any disturbances and changes in wastewater quality and quantity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustrating a single-stage fluidized bed reactor;

FIG. 2 is a flow chart illustrating a three-stage reaction system;

FIG. 3 is a schematic illustrating a single-stage fluidized bed ZVI/FeOx/Fe(II);

FIGS. 4A, 4B are pictures illustrating a bench scale single-stage reactor;

FIGS. 5A, 5B are pictures illustrating an alternative bench scale single-stage reactor;

FIGS. 6A. 6B, 6C are pictures illustrating a bench scale three-stage ZVI/FeOx/Fe(II) fluidized-bed reactor system; and

FIG. 7 shows three panels of pictures illustrating settling of a mixture of Fe⁰ and magnetite powder rich of surface bound Fe(II); pictures taken after settling for 1 min (left panel), 3 min (middle), and 6 min (right).

DETAILED DESCRIPTION

The present inventors have discovered a novel system for treating wastewater. Experiments have demonstrated the system operable for removal of selenium present as selenate.

According to some embodiments, a reactor system includes zero valent iron. According to some embodiments, ferrous iron is added to a reactor system. The present inventor believes that ferrous iron acts as a passivation reversal agent for zero valent iron. The mechanism is complex. While not wishing to be limited by theory, the present inventor believes that passivation is partially caused by corrosion of iron in a water environment. The present inventor believes that ferrous iron acts to cause conversion of iron corrosion product on the surface of the zero valent iron to magnetite. According to some embodiments, a sufficient amount of magnetite is produced so as to optimize removal of toxic materials by a reaction system including zero valent iron. According to some embodiments, the process produces removable solids. According to some embodiments, the removable solids contain toxic material encapsulated in magnetite. According to some embodiments, the encapsulated toxic material is solid.

Thus, according to some embodiments, the process uses a highly reactive mixture of zerovalent iron (Fe⁰), iron oxide minerals (FeOx), and ferrous iron (Fe^II) to react with, absorb, and precipitate various toxic metals and metalloids from wastewater, forming chemically inert and well crystallized magnetite (Fe₃O₄) particles that can be separated from water and disposed with encapsulated pollutants.

According to some embodiments, the reactive zone is maintained near neutral pH.

The present inventor believes that boron in the wastewater further contributes to passivation and that ferrous iron removes boron form the zero valent iron.

It will be understood that wastewater is illustrative of an aqueous suspension. For example, the present inventor contemplates treating oil refinery waste. Further, the present inventor contemplates treating wetlands.

It will be understood that selenium is illustrative of a toxic material. Other common toxic materials are contemplated. For example, the present inventor contemplates removing arsenic, mercury, cobalt, lead, cadmium, chromium, silver, zinc, nickel, molybdenum, and the like; metalloid pollutants such as boron and the like; many oxyanion pollutants, such as nitrate, bromate, iodate, and periodate, and the like; and the like.

It will be understood that ferrous iron is illustrative of a secondary reagent. The secondary reagent is desirable adapted to act as a passivation reversal agent. Passivation is generally the process of rendering an active material, for example zero valent zinc, inactive. Aluminum ion, Al³⁺, may substitute for (e.g. added as aluminum sulfate) for ferrous iron. It will be understood that iron is illustrative of a reactive solid. The present inventor believes that iron is particularly practical. However, the present inventor contemplates other treatment materials. For example, according to some embodiments, the treatment material is zinc. It will be understood that a reactive system may include the treatment material in zero valent form. According to some embodiments, the reactive system further includes a passivation reversal agent suitable for the zero valent form as may be advantageous.

According to some embodiments, a reactor includes an internal settling zone in communication with a reactive zone. The reactor is illustrated in schematic in FIG. 1. According to some embodiments, the internal settling zone uses gravitational forces to separate solids from liquids. According to some embodiments, mostly liquids remain in the settling zone. According to some embodiments, the internal settling zone is towards the top of the reactor (FIG. 1). According to some embodiments, communication with the reactive zone is via an inlet at the bottom of the internal settling zone. According to some embodiments, effluent is removed from the top region of the internal settling zone. According to some embodiments, the effluent is very clear. Magnetite is known to be black. Settling observed in an experiment over time is illustrated in FIG. 3 of the document “pilot test scale plan” appended hereto, which shows clearer separation of black material and clear fluid over time. The present inventor believes that settling for a separating method is particularly efficient. However, other suitable separating methods are contemplated.

According to some embodiments, a reactive zone includes a central conduit. The central conduit improves mixing. For example, according to some embodiments, the central conduit promotes convective motion.

Thus, according to some embodiments, the reactor system operates as a fluidized bed that employs a motorized stirrer in conjunction with a central flow conduit to create a circular flow within the reactor and provide an adequate mixing between reactive solids and wastewater. An internal settling zone was created to allow solid-liquid separation and return of the solid into the fluidized zone.

FIG. 1 is a schematic illustrating an embodiment of the system and process. A single-stage fluidized-bed system includes a fluidized reactive zone, an internal solid/liquid separating zone, an aerating basin, a final settling basin, and an optional sand filtration bed.

Still referring to FIG. 1, the fluidized zone is the main reactive space where reactive solid, in the form of particles, is completely mixed with wastewater and secondary reagent and where various physical-chemical processes responsible for toxic metal removal occur.

Still referring to FIG. 1, the internal settling zone is to allow particles to separate from water and be retained in the fluidized zone. For high density particles, an internal settling zone with a short hydraulic retention time is sufficient for complete solid/liquid separation. This eliminates the need of a large external clarifier and a sludge recycling system.

Still referring to FIG. 1, the aeration basin has two purposes: (1) to eliminate residual secondary reagent in the effluent from fluidized zone; and (2) to increase dissolved oxygen level. For a single-stage reactor, effluent from fluidized reactive zone will always contain certain amount of secondary reagent. Oxidation of secondary reagent will consume alkalinity and therefore will lower the pH. To accelerate oxidation of secondary reagent, the aeration basin should maintain a pH of above 7.0. Chemicals such as Ca(OH)₂, NaOH and Na₂CO₃ could be used for pH control.

Still referring to FIG. 1, the final settling tank is to remove flocculent formed in the aeration basin. The floc (fluffy) settled to the bottom can be returned to the fluidized zone and transformed by secondary reagent into dense particulate matter.

Still referring to FIG. 1, upon final settling, a sand filtration bed may be used to further polish the treated water before discharge.

Still referring to FIG. 1, the post-FBR (fluidized bed reactor) stages (aeration-settling-filtration) may not be needed under certain operation conditions.

Referring now to FIG. 2, several fluidized-bed reactors can be combined to form a multi-stage treatment system. It is recommended that each stage maintain its own reactive solid. That is, the solids are separated in each stage. In order to achieve a separate solid system, each stage may have its own internal solid-liquid separation structure.

Still referring to FIG. 2, depending on operating conditions in the FBRs, the wastewater characteristics, and discharge standards, the post FBR treatments (aeration+final clarifier+sand filtration) may not be needed.

Although a multi-stage system is more complex and may result in a higher initial construction cost, a multi-stage fluidized-bed reactor system has several major advantages.

A multi-stage system can achieve higher removal efficiency than a single-stage system under comparable conditions. Further, the FGD wastewater may contain certain chemicals (i.e., phosphate) that may be detrimental to the high reactivity of the reactive solids. A multi-stage system can intercept and transform these harmful chemicals in the first stage and thus reducing the exposure of the subsequent stages to the negative impact of these chemicals. As such, a multi-stage configuration is more stable and robust.

A multi-stage configuration facilitates the control of nitrate reduction, for example in an iron-based system. In a single stage system, because the presence of dissolved oxygen carried in raw wastewater, it tends to be difficult to operate the system in a rigorous anaerobic environment. In a multi-stage system, stage 1 can remove virtually all dissolved oxygen; as a result, the subsequent stages can be operated under rigorous anaerobic environment.

A multi-stage system allows flexible control of different chemical conditions in each individual reacting basin. The chemical conditions in each reactive basin can be controlled by adjusting the pumping rate of supplemental chemicals and turning aeration on or off. A multi-stage system can be operated in a mode of multiple feeding points. Each stage may be operated under different pH and dissolved oxygen condition.

A multi-stage system will lower chemical consumption. In a single-stage complete-mixed system, secondary reagent in the reactor are desirably maintained at a relatively high concentration in order to maintain high reactivity of reactive solids. As a result, the residual secondary reagent in the effluent will be high. This means that more secondary reagent will be wasted and more NaOH (or lime) consumption will be required just to neutralize and precipitate the residual secondary reagent in the effluent. As a result, more solid sludge will be produced and waste disposal cost will increase. In a multi-stage system, residual secondary reagent from stage 1 can still be used in stage 2. In this case, secondary reagent can be added in a way that conforms to its actual consumption rate in each stage. As a result, it is possible to control residual secondary reagent in the effluent in the final stage to be much lower than the one in a single stage system.

Referring to FIG. 3, according to some embodiments, in the system and process illustrated by FIG. 1, the reactive solid includes zero valent iron (ZVI) and iron oxide mineral (FeOx), and the secondary reagent is Fe²⁺. Thus, referring to FIG. 3, a single-stage fluidized-bed ZVI/FeOx/Fe(II) system includes a fluidized reactive zone, an internal solid/liquid separating zone, an aerating basin, a final settling basin, and an optional sand filtration bed.

Still referring to FIG. 3, the fluidized zone is the main reactive space where ZVI and FeOx reactive solids are completely mixed with wastewater and dissolved Fe²⁺ and where various physical-chemical processes responsible for toxic metal removal occur.

Still referring to FIG. 3, the internal settling zone is to allow ZVI and FeOx to separate from water and be retained in the fluidized zone. Because of high density of fully or partially crystallized FeOx particles, an internal settling zone with a short hydraulic retention time would be suffice for complete solid/liquid separation. This eliminates the need of a large external clarifier and a sludge recycling system.

Still referring to FIG. 3, the aeration basin has two purposes: (1) to eliminate residual dissolved Fe²⁺ in the effluent from fluidized zone; and (2) to increase dissolved oxygen level. For a single-stage reactor, effluent from fluidized reactive zone will always contain certain amount of dissolved Fe²⁺. Oxidation of Fe²⁺ will consume alkalinity and therefore will lower the pH. To accelerate oxidation of dissolved Fe²⁺, the aeration basin should maintain a pH of above 7.0. Chemicals such as Ca(OH)₂, NaOH and Na₂CO₃ could be used for pH control.

Still referring to FIG. 3, the final settling tank is to remove iron oxide flocculent formed in the aeration basin. The ferric oxide floc (fluffy) settled to the bottom can be returned to the fluidized zone and transformed by Fe²⁺ into dense particulate matter.

Still referring to FIG. 3, upon final settling, a sand filtration bed may be used to further polish the treated water before discharge.

Still referring to FIG. 3, the reactive solid may initially be zero valent iron, with the iron oxide mineral formed in situ. The iron oxide mineral may coat the zero valent iron.

Still referring to FIG. 3, the system can be operated under various controlled conditions as needed.

According to some embodiments, an iron-based technique employs a mixture of zerovalent iron (ZVI or Fe⁰) and iron oxide minerals (FeOx), and Fe(II) species to react with, adsorb, precipitate, and remove various toxic metals, metalloids and other pollutants from the contaminated wastewater. According to some embodiments, an iron-based physical-chemical treatment process that employs a hybrid Zerovalent Iron/FeOx/Fe(II) Reactor to treat toxic metal-contaminated wastewater. For example, according to some embodiments, the present system and process involve a hybrid Zerovalent Iron/FeOx/Fe(II) reactor for removing toxic metals in wastewater. According to some embodiments, the process employs a fluidized bed system and use a reactive mixture of Fe⁰, Fe^(II) and FeOx to absorb, precipitate, and react with various toxic metals, metalloids and other pollutants for wastewater decontamination. According to some embodiments, toxic metals are encapsulated within iron oxide crystalline (mainly magnetite powder) that are chemically inert and physically dense for easier solid-liquid separation and final disposal.

While not wishing to be limited by theory, the present inventor believes that the following are contributing mechanisms for the present iron based system and process: a) using the reducing power of Fe⁰ and Fe(II) to reduce various contaminants in oxidized forms to become insoluble or non-toxic species; b) using high adsorption capacity of iron oxide surface for metals to remove various dissolved toxic metal species from wastewater; and c) promoting mineralization of iron oxides and growth of certain iron oxide crystalline so that surface-adsorbed or precipitated toxic metals and other pollutants could be incorporated into iron oxide crystalline structure and remain encapsulated in a stabilized form for final disposal.

EXAMPLES

Experimental Results of Using a Hybrid ZVI/FeOx/Fe(II) Reactive System to Treat FGD Wastewater

The present system and process are a result of laboratory research conducted by the present inventor to develop a cost-effective method for removing toxic metals in the flue gas desulfurization wastewater generated from wet scrubbers of coal-fired steam electric power plants. Although developed specifically for treating the FGD wastewater with selenium as the main target contaminant, this chemical reactive system is suitable for general application of removing a wide spectrum of toxic metals in industrial wastewater, tail water of mining operations, and contaminated groundwater.

According to various experimental embodiments, as shown herein, a single stage may achieve 90% selenate removal within 4 hr reaction time. A three-stage system, in comparison, may achieve a 96% removal rate.

The present inventor believes that some exemplary novel aspects are:

• • 1) Discovery of the role of externally-added Fe²⁺ in sustaining the reactivity of Fe⁰ with respect to selenate reduction. Externally-added Fe²⁺ may convert less reactive ferric oxide coating on Fe⁰ particles into a highly reactive mix-valent Fe₃O₄ oxide coating and therefore rejuvenate the passivated Fe⁰ surface.
  • 2) Discovery that surface-bound Fe(II) on magnetite (Fe₃O₄) particles can rapidly reduce selenate to insoluble elemental Se and be removed from the liquid phase.
  • 3) Discovery that the chemical conditions that promote the formation of magnetite (Fe3O4) as a reaction product from the oxidations of Fe0 and surface-bound Fe (coupled with reductions of dissolved oxygen, nitrate, and selenate in the water).
  • 4) Development of a fluidized bed system with an internal settling zone and a central conduit that can (a) retain high concentration of Fe₃O₄ solid particles and therefore offer abundant reactive surface area that can host surface bound Fe(II)-selenate redox reaction; (b) offer an effective mixing condition so that Fe⁰, Fe₃O₄ and s.b.Fe(II) can achieve their respective roles in removing toxic metals; (c) avoid excess diffusion of oxygen from air into the reactive system so that less Fe⁰ and Fe(II) are wasted.
  • 5) Development of a multiple-stage fluidized bed system that will (a) achieve better toxic metal removal efficiency; (b) control nitrate reduction efficiency to a level of desire; (c) reduce consumption of ferrous salt and Fe⁰; (d) reduce or completely eliminate residual dissolved Fe²⁺.

Bench Scale Tests

Single Stage Reactor

Three Bench-Scale Fluidized-Bed Reactors were Fabricated and Operated.

Referring to FIGS. 4A and 4B, Reactor#1 has an internal settling zone (the compartment on the left side) in which it allows reactive solid to separate from the water and be retained within the fluidized zone. Reactor#2 (not shown) is identical to Reactor#1. Reactor#1 and #2 both had an operating capacity of 7.2 L and had an internal settling zone (0.5 L) within the reactors (FIGS. 4A and 4B).

Referring to FIGS. 5A and 5B Reactor#3 is an integral system that has an internal settling zone (far left), an aeration basin (near left), and a second settling basin (right) within the reactor. Reactor#3 had an operating capacity of 10 L. It had a built-in aeration basin (0.6 L) and a built-in final settling basin (FIGS. 5A and 5B). Peristaltic pumps (Masterflex pumps, Cole-Parmer, Illinois) were used to pump in wastewater and the needed chemical reagents. A small aquarium air pump (purchased from Wal-Mart) as used to provide aeration. A motorized stirrer (max. 27 watt, adjustable rpm 100-2000, three-blade propeller stirrer) was used to provided mixing condition.

Zerovalent iron powder used in the tests was obtained from Hepure Technology Inc., including I-1200+ and HCl5 (see Batch Test results for more details). Other reagents used in the operation include HCl, FeCl₂, and NaOH.

Start Up

Contrary to what many experts in ZVI technology believed, fresh ZVI does not tend to be effective for chemical reduction of selenate. Batch test results (Appendix B and Appendix C) confirmed that ZVI grains coated with magnetite could achieve a much higher reaction rate than ZVI grains of a relative fresh surface with little or very thin iron rusts. To improve performance of a ZVI system, a unique start-up process is employed to coat the ZVI powder surface with a more reactive and passivation-resistant, chemically-stable magnetite coating. When a reactor was started with using fresh ZVI powder, it took some time under carefully controlled chemical environment to coat ZVI with a magnetite layer.

Several factors are desirably considered in order to have a rapid and successful start-up for a treatment system. First, the physical chemical properties of iron, most important the size distribution of iron particles, are considered. Both reductions of selenate by ZVI and by surface bound Fe(II) (s.b.Fe(II)) on magnetite are surface-mediated heterogeneous reaction; therefore, increasing solid-liquid interfacial area would increase overall reaction rate. Fine ZVI powders could provide larger surface area and therefore achieve higher selenate reduction under comparable conditions. This was confirmed in batch tests. The continuous flow reactor tests were successfully started up five times. It appears that finer iron particles (dominant size: <45 μm in diameter) may be started up faster than larger particles (dominant size: 45-150 μm in diameter). The chemical purity of ZVI powder was found to not a major factor. In batch and continuous-flow tests, various purities and composition of ZVI powder were used. No major differences were observed among the different iron sources with respect to reaction mechanism and rate for selenate reduction. Overtime, the zerovalent iron grains may all be coated with a magnetite coating and in the present of dissolved Fe²⁺, they all achieve high reactivity for selenate reduction.

Generation of a magnetite coating on a ZVI particle is helpful to the success of the system. Appropriate aqueous chemical conditions must be maintained for the purpose. Iron corrosion could produce various iron oxides under different chemical conditions. Our batch and continuous flow reactor tests show that in order to generate magnetite from iron corrosion reaction, three conditions must be met: a pH of 6.5 to 7.5; adequate dissolved Fe²⁺ that can form s.b.Fe(II); and appropriate species and concentration of oxidants. Oxidants can be certain oxyanions such as selenate, nitrate, nitrite, iodate (IO₃⁻) and periodate (IO₄⁻) in the wastewater. Oxidation of ZVI by these oxidants tends to form ferric oxides (most likely lepidocrocite, γ-FeOOH). The small quantity of ferric oxides can be transformed to magnetite in the presence of surface-adsorbed Fe(II). Dissolved oxygen can also serve as an oxidant to generate magnetite (Huang et al. 2006). Low-intensity aeration in the early stage could accelerate the magnetite-coating process. High-intensity aeration should be avoided because it could form large quantity of ferric oxides even in the presence of dissolved Fe²⁺ and moreover, it will waste ZVI. Our experiences from live successful start-ups using simulated FGD wastewater indicates that in general the system will take about one to two weeks for the fresh ZVI to mature; over time, the system will gradually improve before reaching a state of high performance.

As an alternative (and recommended) start-up procedure, we used nitrate solution (add 30 mg/L nitrate-N in tap water, operating HRT=12 hr) instead of simulated FGD wastewater to feed the system. Nitrate would be completely reduced and in the presence of adequate dissolved Fe²⁺, a high quality (better crystallized and less amorphous, containing less ferric oxides or ferrous hydroxides) magnetite coating can be formed on ZVI particles. Start-up with nitrate solution would take only two days.

A general start up procedure and exemplary controlled parameters are:

• • 1) Select ZVI sources. Finer iron powder (<50 μm) is preferred. Low iron purity and rusty surface in general are not a problem.
  • 2) Add 80-100 g/L ZVI powder in the fluidized zone. Turn on mixing equipment.
  • 3) Start-up with FGD wastewater
    • Feed FGD wastewater at a rate equivalent to HRT=12 hrs. The exact compositions of raw FGD wastewater may vary widely, but in general contains high concentration of Cl⁻, sulfate, and relative high concentration of nitrate.
    • Feed FeCl₂ solution (0.1 M FeCl₂ in 0.005 M HCl solution) at a rate equivalent to 1.5 in mole Fe²⁺ per 1 L wastewater
    • Feed NCl at a rate to control the pH in the fluidized zone at 6.8-7.2.
    • If the FGD wastewater contains limited concentration of nitrate (e.g., below 10 mg/L nitrate-N), then a low intensity aeration in the fluidized bed should be provided to accelerate the formation of a magnetite coating.
    • Start-up with nitrate solution
    • Feed nitrate solution (30 mg/L nitrate-N) at a rate equivalent to HRT=12 hrs.
    • Feed FeCl₂ solution (0.1 M FeCl₂ in 0.005 M HCl solution) at a rate equivalent to 1.5 in mole Fe²⁺ per 1 L wastewater
    • Adjust HCl solution (0.1 M HCl) feeding rate to control the pH in the fluidized zone at 7.0-7.5.

Normal Operation

Once started up successfully, the system requires only low-level maintenance effort. Routine operations and maintenances include one or more of:

• • (a) Monitor the quality of wastewater entering the system. The most important parameters include: pH, alkalinity, acidity, total suspended solid (TSS). Of course, toxic constituents in the raw wastewater should be monitored.
  • (b) Monitor the pH in the fluidized reactive zone. Performance of the system depends mostly on pH. For a single-stage system, pH in the reactive zone should be maintained within 6.5 to 7.5. Both HCl and FeCl₂ can be used to control the system.
  • (c) Monitor the pH in the aeration basin. Dissolved Fe²⁺ can be oxidized more rapidly at pH>7.0. Formation and settling properties of ferric oxide flocculent depends also on pH. Therefore, it is recommended that aeration basin be operated at pH 7.5-8.0.
  • (d) Monitor the performance of settling tank and sand filtration bed. The maintenance requirements are no different from those unit processes in typical water or wastewater treatment plants. Most importantly, the settled sludge should be discharged or returned at an appropriate rate to avoid excessive build-up of the reactor.
  • (e) Excess solid discharge and disposal.

If the raw wastewater contains relative high suspended solids, a pre-settling basin may be needed to reduce TSS in wastewater before entering the system. This can avoid accumulation of inert TSS in the fluidized reactive zone that might dilute the effective ZVI/FeOx solid concentration.

For a single-stage reactor, the concentration of total solid in the fluidized zone could be maintained between 80 and 120 g/L. Assuming that 30 mg Fe²⁺/L be converted to Fe₃O₄ and the reactor is operated at HRT=4 hours (based on test results), we estimate that it will add 0.25 g/L FeOx solid per day and therefore will take 160 days for the reactor to increase its solid from 80 g/L to This estimate conform to the fact that during a three-month continuous flow test (hydraulic retention time varies between 3 to 12 hours), we discharge no solid from the fluidized bed reactor.

ZVI/FeOx reactive solids are considered mature when the surface of ZVI grains is covered with well crystallized magnetite (dark black color after dry) and a significant presence of discrete magnetite crystalline (may be aggregated into a larger particle due to its strong magnetic property). Unlike typical ZVI powder, matured ZVI/FeOx reactive solids will not cement easily when settled at the bottle. Therefore, the reactor could be stopped for weeks with no risk of iron powder cementation. That is, the reactor can be stopped and restarted very flexibly without a need to vacate the ZVI/FeOx mixture from the reactor.

Results

Results of testing are described in Appendix A and Appendix D. The results demonstrate that a single-stage reactive system alone can effectively remove high concentration of selenate within a relatively short reaction time. A multiple-stage system can further improve the overall performance. Since for most FGD wastewater, Se(VI) concentration will be lower than 5 mg/L used in this test (most typically, 1-2 mg/L), the present inventor estimates that an HRT of less than 4 hours would be sufficient for most applications. Moreover, the reactor is operated at near neutral pH.

Multi-Stage Reactor

The start-up procedure and normal operation requirements described for a single-stage system can be similarly applied for a multistage system. Again, it is desirable that nitrate solution be used for rapid start-up. Nitrate solution was also found to be very effective in rejuvenating a fouled system in which the system was accidentally acidified (pH dropped to below 4.0) for a few hours, which might permanently damage iron oxide reactivity and resulted in extremely poor performance even after returning to normal operation conditions.

Referring to FIGS. 6A, 6B, 6C, In this test, Reactor#1, #2, and #3 was combined in sequence to form a three-stage FBRs treatment system. This system was a 24-liter three-stage ZVI/FeOx/Fe(II) fluidized-bed reactor system. Initial testing on the three-stage system is described in Appendix A and Appendix D.

Continuous flow tests were conducted for six months on the bench-scale (24 liter) three state fluidized bed system based on the ZVI/FeOx/Fe(II) technique with high-strength raw FGD wastewater (provided by Southern Company).

The system was demonstrated during a 6 month testing period to be a complete success, as shown in Table 1.

TABLE 1
Major	Concentration in	Concentration after	Removal
Pollutants	FGD wastewater	treatment	Efficiency
Selenium	7.8 mg/L SeO42−—Se	dissolved Se	>98%
		<0.15 mg/L
Mercury	335 μg/L dissolved Hg	dissolved Hg	>99.9%
		<0.2 μg/L
Arsenic*	400 μg/L dissolved	dissolved As	>99.9%
	As(III) and As(V)	<0.2 μg/L
Nitrate	26 mg/L nitrate-N	nitrate-N	>80%
		<5.0 mg/L
Boron**	200~600 mg/L		projected to be
	dissolved B		>70%
Notes:
*The original raw FGD wastewater provided by Southern Co. contains only less than 0.6 μg/L total dissolved As. To evaluate Arsenic treatment effectiveness, 400 μg/L arsenite-As and arsenate-As was added.
**Removal of dissolved boron in the system is still being tested and needs to be further verified.

Laboratory Tests

Extensive laboratory tests have been conducted to understand the treatment conditions and mechanisms.

Referring to FIG. 7, settling of reactive solid (black) from fluid (clear) is illustrated.

This inventor has conducted extensive batch tests (Appendix B, Appendix C, and Appendix D) in addition to the continuous flow tests (Appendix A and Appendix D) to investigate the fundamental chemistry and application issues in the complicated reactive system that comprised of Fe⁰, dissolved Fe²⁺, various FeOx in different forms and compositions, dissolved oxygen, simulated FGD wastewater or real FGD wastewater with a very complex matrix of constituents. Laboratory experiments and their results are described in details and discussed in depth in the appended documents. Findings from these tests are summarized as below:

• • 1) In a rigorous anaerobic condition, selenate (at ppm level concentration) cannot be effective reduced by pure Fe⁰ (with fresh surface that contains negligible iron oxides). Only negligible selenate could be reduced. That is, reactivity of Fe⁰ will be naturally passivated by the presence of selenate. This explains why previous investigators failed to achieve a sustainable removal when using Fe⁰ to reduce selenate.

SeO₄²⁻+2Fe⁰+2H₂O→Se⁰↓+2FeOOH+2OH⁻  (eq. 1)

• • • Lepidocrocite (γ-FeOOH) forms a passive coating on the surface of Fe⁰ particle and therefore inhibits further reaction between Fe⁰ and selenate.
  • 2) In the presence of dissolved oxygen, selenate could be reduced by Fe⁰ at a modest rate; however, to sustain the desired selenate-Fe⁰ reaction, much of Fe⁰ will be wastefully consumed by dissolved oxygen as a result. The implication is: An excessive aerated Fe⁰ system might be able to remove selenate, but the process is economically infeasible due to wasteful consumption of Fe⁰ by oxygen and generation of large quantity of iron oxide sludge.
  • 3) Reduction of selenate could be greatly accelerated in the presence of dissolved Fe²⁺ at circum-neutral pH environment. The reaction rate increases as dissolved Fe²⁺ increase. A presence of 0.3 mM dissolved Fe²⁺ will be adequate. At near neutral pH and anaerobic environment, the reaction will form magnetite as their product.

SeO₄²⁻+2Fe⁰+Fe²⁺→Se(0)↓+Fe₃O₄  (eq. 2)

In this reaction, the direct role of Fe²⁺ might be to facilitating the conversion of passive FeOOH to reactive Fe₃O₄ and therefore, greatly accelerating the reaction.

• • 4) Selenate could be rapidly reduced by s.b.Fe(II) on activated magnetite surface at near neutral or weak acidic pH in the absence of Fe⁰.

SeO₄²⁻+9s.b.Fe^(II)→Se(0)↓+3Fe₃O₄+2OH⁻  (eq. 4)

• • • Unlike Fe²⁺ in the equation 2, Fe(II) here serves as a reductant and directly contributes one electron to the reduction of selenate.
  • 5) Nitrate, which is often present at tens of ppm level in the FGD wastewater, will not inhibit selenate reduction by Fe⁰. Indeed, nitrate was found to slightly accelerate selenate reduction by Fe⁰. In contrast, reduction of nitrate by Fe⁰ will be inhibited by the presence of selenate. In a rigorous anaerobic environment, reduction of nitrate by Fe⁰ can occur only after selenate is completely reduced in the system.
  • 6) Both reductions of nitrate and selenate by Fe⁰ will consume certain amount of Fe²⁺. Nitrate reduction consume 0.75 mM Fe(II)/1.0 mM nitrate; selenate reduction consume approximately 1.0 mM Fe(II)/1.0 mM selenate.
  • 7) The complex matrix of constituents in the FGD wastewater may affect selenate reduction rate in the Fe⁰/FeOx/Fe(II) system. Sulfate will slow down the reaction rate several folds. Chloride at a lower concentration
  • 8) Source of Fe⁰. The mechanisms of Fe⁰-selenate reaction will not be altered by the use of difference Fe⁰ sources. Tests with different purities of Fe⁰ show that Fe⁰ purity has no apparent relationship with the achievable reaction rate. There is no obvious advantage from the use of high pure (>99%), little rusted, electrolytic iron powder (Fisher Scientific) over low-grade (95%), industrial iron filings. The size of iron power however does matters. Fine iron powder will provide more reactive surface than coarse iron powder. Fine iron powder may also mature faster and ease start-up of the system.

Pilot Scale Tests

Prophetic Example

The success of the laboratory-scale prototype has paved the road for constructing a pilot-scale system and conducting extended field demonstrations to further evaluate, develop and refine the technology.

The present inventor contemplates a pilot-scale treatment system based on a proved laboratory-scale prototype and conduct long-term field tests to further develop the technique and finalize its design for commercialization.

The pilot scale test may involve one or more steps, such as: design and construct a pilot treatment system based on the laboratory prototype; conduct on-site long-term demonstrations in conjunction with further laboratory mechanistic study; collaborated closely with industry and other stakeholders to further refine the system to meet the industrial needs and environmental goals. Contemplated pilot scale tests are further described in Appendix D.

The present inventor contemplates an integral treatment system that can treat FGD wastewater at a flow rate of 2 to 5 gallon per minute, which represents about 1% of wastewater expected from a 1,000 megawatt power plant. The pilot system may be mounted on a trailer that is adapted to be hauled to different test sites.

Industrial Operation

Prophetic Example

Based on the bench scale test described above, the present inventor estimates that for treating a 500 gpm FGD waste stream from a 1,000 megawatt, a iron-based system will consume per year: 200 to 400 ton of iron chemical (est. bulk price: $1,000 to $2,000/ton); 200 to 400 tons of concentrated HCl; 50-200 kilowatt electric power consumption. Further, the present inventor estimates that for treating a 500 gpm FGD waste stream from a 1,000 megawatt, a iron-based system will generate per year: 300 to 800 tons of iron oxide (weight in dry mass; laden with toxic metals), to be disposed as a hazardous waste.

Chemical Consumptions

For treating 1 m³ FGD wastewater of typical strength, the system will consume:

• • 100-300 g ZVI (Fe⁰) powder
  • 50-120 g iron salt
  • 20-100 g NaOH (or equivalent amount of Ca(OH)₂)
  • <0.2 L concentrated HCl
  • The total chemical cost will be less than $1.0 per 1 m³ FGD wastewater. The system will produce 0.5-1.0 kg waste solid per 1 m³ wastewater treated. A 1,000-megawatt power plant may produce 1,000 to 3,000 m³ FGD wastewater per day (approximately 200-600 gpm) depending the specific operation conditions of the wet scrubbers.

Claims

1. A treatment system for treating an aqueous suspension, wherein the treatment system comprises a chemical reactor system comprising a fluidized bed reactor comprising a reactive zone.

2. The treatment system according to claim 1, wherein the chemical reactor system further comprises an internal settling zone in communication with the reactive zone.

3. The treatment system according to any one of claims 1-2, wherein the internal settling zone is located in the top region of the chemical reactor system.

4. The treatment system according to any one of claims 1-3, wherein the internal settling zone comprises an opening at the bottom of the internal settling zone adapted for the communication with the reactive zone.

5. The treatment system according to any one of claims 1-4, wherein the internal settling zone comprises an outlet adapted for removal of effluent from the internal settling zone.

6. The treatment system according to any one of claims 1-5, wherein the reactive zone comprises a conduit.

7. The treatment system according to claim 6, wherein the conduit is central with respect to the reactive zone.

8. The treatment system according to any one of claims 1-7, wherein the treatment system is a multi-stage system comprising an additional reactor system.

9. The treatment system according to any one of claims 1-8, wherein the reactive zone comprises a reactive solid and a secondary reagent.

10. The treatment system according to claim 9, wherein the reactive solid comprises iron.

11. The treatment system according to any one of claims 9-10, wherein the secondary reagent comprises ferrous iron.

12. The treatment system according to any one of claims 9-11, wherein the reactive solid further comprises an iron oxide mineral.

13. The treatment system according to claim 12, wherein iron mineral comprises magnetite.

14. The treatment system according to any one of claims 1-13, wherein the aqueous suspension comprises a waste influent.

15. The treatment system according to any one of claims 1-14, wherein the aqueous suspension comprises a toxic material.

16. The treatment system according to any one of claims 1-15, wherein the toxic material is selected from the group consisting of selenium, arsenic, mercury, aluminum, antimony, beryllium, thallium, chromium, cobalt, lead, cadmium, silver, zinc, nickel, molybdenum, nitrates, bromates, iodates, periodates, and borates.

17. A process for treating an aqueous suspension, comprising feeding the aqueous suspension to the treatment system according to any one of claims 1-14.

18. The process according to claim 17, wherein the process further comprises removing a toxic material from the aqueous suspension.

19. The process according to claim 18, wherein the removing comprises:

a) at least one of reacting, adsorbing, and precipitating the toxic material from the aqueous suspension so as to form removable solids in treated effluent; and

b) separating the removable solids from the aqueous suspension.

20. The process according to claim 19, wherein the removable solids comprise at least a portion of the toxic material encapsulated in the removable solids.

21. A process for treating wastewater comprising a toxic material, comprising exposing the wastewater to a reactive material system so as to remove toxic material from the wastewater, wherein the reactive material system comprises zero valent iron particles and ferrous iron, wherein the exposing comprises:

a) at least one of reacting, adsorbing, and precipitating the toxic material from the wastewater so as to form removable solids in treated wastewater, wherein the removable solids comprise at least a portion of the toxic material encapsulated in at least a portion of an iron mineral derived from the reactive material system; and

b) separating the removable solids from the treated wastewater.

22. A new and improved fluidized bed apparatus for wastewater treatment comprising a fluidized bed, a fluidized reactive zone, an internal solid/liquid separating zone in fluid communication with said reactive zone, an aerating basin, and a settling basin.

23. The apparatus of claim 22 further comprising control and metering systems for monitoring and manipulating chemical processes within said reactor.

24. The apparatus of claim 23 further comprising a sand filtration bed.

25. The apparatus of claim 22 further comprising a central conduit in the fluidized bed reactor to promote convective fluid flow enhancing mixing.

26. The apparatus of claim 25 further comprising a motorized stirrer in conjunction with said central conduit configured so fluid flow within the conduit is down and flow within the fluidized bed reactor outside the conduit is up.

27. The apparatus of claim 26 further comprising a sand filtration bed.

28. The apparatus of claim 22 further comprising at least one additional fluidized bed apparatus configured as stages in series with said first apparatus.

29. The apparatus of claim 28 further comprising control and metering systems for monitoring and manipulating chemical processes run within said reactors.

30. The apparatus of claim 29 wherein the chemical process conditions within different stages are varied to optimize results.

31. The apparatus of claim 22, wherein the fluidized reactive zone comprises a composition comprising zero valent iron, iron oxide mineral, and ferrous iron.

32. A composition for treating an aqueous suspension comprising zero valent iron, iron oxide mineral, and ferrous iron.