JP3976679B2 - Pollutant purification method using reaction wall containing zeolite with nanometer scale iron - Google Patents

Description

  The present invention relates to a method for purifying pollutants using a reaction wall, and more particularly, a reaction medium that is installed in a basement where pollutants exist and is induced by a hydraulic flow of a contamination band of groundwater, and The present invention relates to a pollutant purification method that removes pollutants by chemical reaction with pollutants. In particular, the present invention uses a zeolite with nanometer-scale iron adhering as a reaction medium for the reaction wall, so that the removal of chlorinated organics by granular iron and the removal of heavy metals and nutrients by zeolite are performed in a single manner. This can be done with a reaction wall system.

  Conventionally, granular iron is used as a reaction medium in producing a reaction wall for purifying contaminated groundwater. For example, US Pat. No. 5,575,927 uses iron or ferrous sulfide alone by using as a reaction medium a mixture of iron and ferrous sulfide in relative amounts. In comparison, a method for reducing halogenated hydrocarbons faster is disclosed. Further, US Pat. No. 5,543,059 allows a contaminant containing halogenated hydrocarbons to pass through a stepped iron wall or iron column in which iron particles as a reaction medium are divided into at least three regions according to granular sizes. Thus, a method for purifying this is disclosed.

  In the above prior art, the flow of groundwater mixed with pollutants is allowed to pass through a granular iron reaction wall installed at the main point of the groundwater flow, without adding special additives. It is characterized by removing.

  In the above prior art, it is known that the mechanism for removing contaminants with zero-valent iron is as follows.

Iron existing as zero-valent iron (Fe ⁰ ) undergoes oxidation and forms a redox pair. This is similar to the corrosion reaction that occurs due to the spontaneous oxidation of zerovalent metals that tend to lose electrons and exist in the cationic state. In the case of iron, the redox potential is -0.44V.
Fe ⁰ Ｆｅ Fe ²⁺ + 2e ^- Formula (1)

FIG. 1 is a diagram schematically showing the process of dechlorination of PCE (C ₂ Cl ₄ , tetrachloroethylene) and the standard redox potential. In FIG. 1, the desalting reaction gradually becomes slower from B to A. And point C represents the point with the highest oxidation state, and point D represents the point with the lowest oxidation state. As can be predicted from FIG. 1, the main reducing agents capable of reacting with the chlorinated organic compound are Fe ⁰ , Fe ²⁺ and H ₂ . In the case of the corrosion reaction, most of the direct electron exchange from Fe ⁰ to the alkyl chloride adsorbed on the surface (formula 2) constitutes, but in addition to this, Fe produced by the corrosion reaction ^Examples include dechlorination of ²⁺ (formula 3), dechlorination with H ₂ (formula 4), or action of Fe with H ₂ O. The desalting process of alkyl halide (RX) with these reducing agents can be expressed as the following formula.
^{Fe 0 + RX + H + ⇔} Fe 2+ + RH + X - Formula (2)
^{2Fe 2+ + RX + H + ⇔} 2Fe 3+ + RH + X - Equation (3)
_{H 2 + RX ⇔ RH + H} + + X - Equation (4)

FIG. 2 is a schematic representation of reductive dechlorination of chlorinated organics by electron exchange based on corrosion of zero-valent iron. FIG. 2A schematically shows a reduction reaction of a chlorinated organic compound by zero-valent iron generated directly on the surface of the zero-valent iron, and FIG. 2B shows a reduction reaction of the chlorinated organic compound indirectly caused by ferrous ions. FIG. 2C schematically illustrates the role of zero-valent iron in the reduction reaction of a chlorinated organic compound with H _{2 in the} presence of a catalyst.

  On the other hand, zeolite has been known to be capable of removing nutrient salts such as ammonia nitrogen and heavy metals such as cadmium, lead, copper, and zinc from pollutants by ion exchange. Here, ion exchange means that ions having a charge present in the liquid state are selectively exchanged with other ions having the same type of charge present in the solid state. Such exchange reaction makes it possible to separate and remove specific ions. Since the ion exchange reaction is performed stoichiometrically and does not affect the basic structure of the solid on which the ion exchange is performed, the material can be regenerated.

Assuming that the ion exchange mechanism is a binary system composed only of specific ions (NH ₄ ⁺ ) in the liquid state and ions (Na ⁺ ) to be exchanged in the solid state (Z), Na in Z ⁺ exchange reaction between NH ₄ ⁺ ions of the ion and the aqueous solution is represented by the following formula (5). Where Z represents a zeolite such as the body of clinoptilolite.
Z · Na ⁺ + NH ₄ ⁺ = Z · NH ₄ ⁺ + Na ⁺ Formula (5)

  Ion exchange occurs at a predetermined location called a pore (see FIG. 3), and in the case of clinoptilolite, the pore size is known to be 4A.

  However, in the conventional reaction wall method, granular iron is used as it is without being separately treated or mixed with other component substances, so that the target pollutant is PCE due to the limit of the redox potential of granular iron. There is a problem that it is limited to substances such as TCE, DCE, VC, and CT, and cannot be applied to pollutants such as PCBs that require a high redox potential, other heavy metals, and nutrient salts. In addition, zeolite has been conventionally used by a method in which it is directly added to an aqueous solution, and there is a problem that applicable substances are limited to heavy metals and nutrient salts.

  In order to solve the above-mentioned problems, the present invention can remove not only organic chloride compounds that can remove the conventional reaction walls containing granular zero-valent iron, but also heavy metals and nutrient salts that can be removed by zeolite. At the same time, the thickness of the reaction wall can be minimized, and the problem of phase separation between iron and zeolite due to the difference in specific gravity can be solved. Purification using a reaction wall filled with a single substance The purpose is to provide a method.

  According to the present invention, a pollutant purification method using a nanometer-scale iron-attached zeolite creates a reaction wall containing a nanometer-scale iron-attached zeolite, and the pollutant passes through the reaction wall. And a step of removing contaminant components by passing contaminants through the reaction wall.

In the zeolite attached with nanometer-scale iron, the zeolite is a substance capable of removing nutrients such as ammonia, nitrogen and phosphorus and heavy metals such as cadmium, lead, copper and zinc by an ion exchange mechanism. Including a kind of clinoptilolite. As shown in Table 1, SiO ₂ , which is the main component of the zeolite, functions as a functional group that binds Fe to the zeolite so that it can take the form of a zeolite with nanometer-scale iron attached thereto.

The zeolite of the present invention comprises
(I) a step of washing the zeolite with distilled water,
(Ii) The zeolite washed in (i) above is placed in a 1.0 M ferric chloride (FeCl ₃ .6H ₂ O) solution and slowly stirred, whereby the ferric chloride (FeCl ₂ ) is added to the internal structure of the zeolite. ₍₃ · 6H ₂ O) solution impregnation;
(Iii) A 1.6 M sodium borohydride (NaBH ₄ ) solution is added to the solution of (ii) with stirring, and the following formula (6)
Fe (H ₂ O) ₆ ³⁻ + 3BH ₄ ⁻ + 3H ₂ O → Fe ⁰ ↓ + 3B (OH) ₃ + 10.5H ₂ Formula (6)
To induce precipitation of Fe ⁰ as follows .

The nanometer-scale iron in the precipitate obtained as described above is stable in the internal structure of the zeolite in the form of Fe ⁰ by strongly bonding with the oxide present in the zeolite, for example, clinoptilolite. Adhere to. FIG. 3 shows the structure of a zeolite to which nanometer-scale iron is attached.

As shown in FIG. 3, the nanometer-scale iron 20 adheres to the zeolite through bonding with SiO ₂ existing outside the zeolite structure.

Contaminants applicable to the method of the present invention include PCE (C ₂ Cl ₄ , tetrachloroethylene), TCE (C ₂ HCl ₃ , trichloroethylene), DCE (C ₂ H ₂ Cl ₂ , dichloroethylene), VC (C ₂ H ₃ Cl, vinyl chloride), CT (CCl ₄ , carbon tetrachloride), trichloromethane (CHCl ₃ ), dichloromethane (CH ₂ Cl ₂ ), chloromethane (CH ₃ Cl) and PCBs (polychlorinated biphenyls) Things are included. These substances become harmless substances such as ethane through a reductive dechlorination reaction in which Cl ⁻ ions are exchanged with H ⁺ ions by electrons generated in the corrosion process of zero-valent iron (Fe ⁰ ). Converted.

  The reaction wall containing the zeolite to which the nanometer-scale iron is attached is dug on site using a backhoe and a clamshell, and installed in the cage. In the zeolite to which nanometer-scale iron is attached, normal soil and excavated soil are mixed at a mixing ratio derived from the measurement of the hydraulic conductivity calculated in the examples described later. In the mixing process, a mixing plant is used to directly mix and put into the basket. A steel sheet pile may be temporarily driven in order to maintain the stability of the soot while the reaction medium is put into the soot.

  In the mixing of the iron-attached zeolite and the mixed soil, the maximum content of nanometer-scale iron is an amount that can prevent an excessive decrease in the hydraulic conductivity due to clogging of the voids. The minimum iron content is an amount that can sufficiently remove contaminating components according to the degree of contamination. The content ratio of iron on the nanometer scale in the reaction wall material containing sandy soil is preferably in the range of 5 to 20% by weight, and most preferably 20% by weight.

  By using the reaction wall containing the zeolite to which the nanometer scale iron is attached, the organic chloride compound that can be removed using the conventional reaction wall and the heavy metal and the nutrient salts that can be removed using the zeolite The reaction wall system can be removed.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the examples are for illustrative purposes and do not limit the invention.

(A. Evaluation of hydraulic conductivity of reaction medium)
10:90 (reactant: soil), 20:80 (reactant: soil), 30:70 (reactant: soil), 50:50 (Reactant: Soil) were mixed at a weight ratio, and the water permeability coefficient was evaluated by a constant water level permeability test method defined by the Korean standard KSF-2322. The test method is as follows.
(1) Prepare a reaction substance whose water permeability is to be measured as a sample, and measure its weight.
(2) Measure the inner diameter of the permeable tube and calculate the cross-sectional area (A).
(3) Place the water permeable tube on the perforated plate and fix it.
(4) Lay a brass wire mesh on the bottom plate of the container.
(5) Fill the water-permeable tube with the sample to a height of 10 cm, and evenly press and fix it with a loading rod to measure the height (L) of the sample.
(6) Subtract the weight of the remaining sample from the weight of the sample measured in (1) above to obtain the weight (wt) of the sample in the permeable tube.
(7) Measure specific gravity and water content of the remaining sample.
(8) Saturate the permeable tube containing the sample with water.
(9) Water is injected through the upper end of the permeable pipe while maintaining the water level constant by overflowing the water through the overflow hole at the upper part of the permeable pipe.
(10) Keep the water level constant and open the drain at the bottom of the permeable pipe to drain the water while maintaining the system until the amount of overflowing water becomes constant.
(11) Measure the amount (Q) and time (t) of flowing water.
(12) Measure the water head difference (h) acting on the top and bottom of the sample.
(13) Measure the water temperature (T).
(14) Measure the water content after the test on the tested sample.
(15) Calculate the hydraulic conductivity using the following formula.
k = Q / iAt = QL / hAt
In the formula, k: hydraulic conductivity (cm / sec), L: length of the sample (cm), A: cross-sectional area of the sample (cm ² ), h: static head difference (cm), t: water permeability time (sec) ), Q: water permeability (cm ³ ).

  The results according to Example 1A are shown in FIG. As shown in FIG. 4, when the content of the zeolite to which nanometer-scale iron adhered was 10 or 20% by weight, there was almost no decrease in the hydraulic conductivity over time. On the other hand, in the case of 30 or 50% by weight, the water permeability decreased remarkably with the passage of time. This is presumably because the pores are clogged by the zeolite to which iron adheres as the content of the zeolite to which nanometer-scale iron adheres increases.

(B. Evaluation of PCE removal effect of the present invention)
In this example, a reaction wall containing a zeolite and sandy soil with nanometer-scale iron adhering in a weight ratio of 20:80 is provided with a width of 1 m, a depth of 0.5 m, and a thickness of 0.01 m. An aqueous solution with a concentration of 100 μM was passed. By measuring the concentration of PCE in the aqueous solution passed through according to time, the purification effect of the pollutant using the nanometer scale iron-attached zeolite of the present invention was evaluated.

  The water permeability coefficient of the reaction wall containing the zeolite to which the nanometer-scale iron adhered was 5 cm / hr from the measurement result, and the dynamic gradient was 1/50. The Darcy flow velocity calculated from this was 0.1 cm / hr, and the maximum groundwater flow rate used in the evaluation of the present invention was set to 1 cm / hr corresponding to 10 times the Darcy flow velocity value.

  The concentration of PCE used in this example was analyzed using gas chromatography (6890 series, Hewlett Packard Co., USA). The analysis conditions for gas chromatography are shown in Table 1.

  The result of Example 1B is shown in FIG. FIG. 5 shows the concentration change of 100 μM PCE aqueous solution. As shown in FIG. 5, 80% or more of PCE was removed after 100 hours.

  As can be seen from this result, it was possible to efficiently remove PCE, which is a kind of chlorinated organic substance, from pollutants by using a reaction wall containing zeolite with nanometer-scale iron attached thereto.

In this example, an aqueous solution having an ammonia (NH ₄ ⁺ ) concentration of 40 ppm was used to evaluate the removal efficiency of nutrient salts. Moreover, it carried out on the conditions similar to Example 1 except having analyzed the density | concentration of ammonia using ion chromatography.

The results of this example are shown in FIG. As shown in FIG. 6, ammonia (NH ₄ ⁺⁾ ions are exchanged with Na ⁺ or Ca ^2+, and after lapse of 18 hours to about 87.5% of the ammonia was removed.

  As can be seen from this result, ammonia ions, which are a kind of nutrients, could be efficiently removed by using a reaction wall containing zeolite with nanometer-scale iron attached.

  The present invention can be used in an environmental industry such as the ground environmental industry. The present invention can also be applied to factory complexes and military facilities such as underground oil storage facilities, semiconductor factories, factory dense areas, and petrochemical factories.

It is the figure which showed the process of dechlorination of PCE, and standard oxidation-reduction potential. FIG. 2A is a diagram showing a reduction reaction of a chlorinated organic compound by zero-valent iron generated directly on the surface of the zero-valent iron, and FIG. 2B is a reduction reaction of the chlorinated organic compound indirectly caused by ferrous ions. FIG. 2C is a diagram showing the role of zero-valent iron in the reduction reaction of a chlorinated organic compound by H _{2 in the} presence of a catalyst. It is the figure which showed the structure of the zeolite to which iron of nanometer scale adhered. It is the graph which showed the change of the water permeability coefficient by content of the zeolite to which nanometer scale iron adhered in Example 1A. It is the graph which showed the density | concentration change of PCE in 100 micromol PCE aqueous solution in Example 1B. 6 is a graph showing changes in the concentration of ammonia ions (NH ₄ ⁺ ) in Example 2.

Claims (2)

A method for purifying pollutants using iron-attached zeolite,
(I) a step of washing the zeolite with distilled water,
(Ii) The zeolite washed in (i) is placed in a 1.0 M ferric chloride (FeCl ₃ .6H ₂ O) solution and stirred, whereby the ferric chloride (FeCl ₃ ) is added to the internal structure of the zeolite. • impregnating the 6H ₂ O) solution;
(Iii) A 1.6 M sodium borohydride (NaBH ₄ ) solution is added to the solution of (ii) with stirring, and the following formula (6)
Fe (H ₂ O) ₆ ³⁻ + 3BH ₄ ⁻ + 3H ₂ O → Fe ⁰ ↓ + 3B (OH) ₃ + 10.5H ₂ Formula (6)
The method comprising the steps of inducing precipitation of Fe ⁰ as to produce a reaction wall containing iron is attached zeolite, comprising the steps of providing a place where the reaction wall pollutants passes,
Removing contaminants by passing contaminants through the reaction wall;
Comprising a method. Claim 1 ranges: the reaction wall comprising a zeolite wherein the iron is adhered and comprise a mixture of reactants and soil containing iron is attached zeolite in the mixture ratio by weight 5-20 2. A method for purifying a pollutant using the zeolite to which iron is attached according to 1.

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