A method for preparing PRB active media and its application

Abstract

This invention discloses a method for preparing PRB active media and its application. First, silica sand is added to natural pyrite and mixed evenly. Then, a mechanochemical treatment is carried out using an intermittent oxygen-limited high-energy ball milling method. The resulting pyrite powder has a D50 of less than 300 nm, which significantly improves the removal performance of electron-rich organic pollutants such as nitrobenzene and chlorinated hydrocarbons, as well as heavy metals. As a PRB active media, it can efficiently and stably remove pollutants from groundwater, with a pollutant removal rate of over 99%.

Description

Technical Field

[0001] This invention belongs to the field of groundwater pollution remediation materials and technology, specifically relating to a method for preparing a PRB active medium and its application. Background Technology

[0002] For aquifers with high permeability, fast groundwater flow, and large contamination plume areas, one of the most suitable in-situ remediation methods is the construction of in-situ permeable reactive barriers (PRBs) for long-term treatment. PRB is a groundwater remediation technology that uses an in-situ permeable treatment zone as the main remediation component. It utilizes specific reactive media to remove contaminants from groundwater through physical, chemical, and biodegradation methods, transforming them into environmentally acceptable forms. This achieves the goals of blocking contaminants, reducing contamination plumes, and restoring groundwater quality. It offers advantages such as continuous in-situ treatment of multiple contaminants, good treatment effects, high cost-effectiveness, and ease of installation and construction.

[0003] The key to PRB technology lies in the selection of the active medium. Chlorinated hydrocarbons, nitrobenzenes, and heavy metals are common characteristic pollutants in groundwater contaminated by industrial and mining sites, especially chemical plants, in my country. For these pollutants, reducing PRB technology is suitable, and iron-based materials, especially zero-valent iron (ZVFe), are the most widely used active media. ZVFe is a strong reducing agent and has been used to treat chlorinated hydrocarbons, heavy metals, nutrients, and nitroaromatic hydrocarbons in groundwater. However, after reducing pollutants, ZVFe usually transforms into iron oxides and forms a passivation layer on the material surface. This easily leads to the deactivation of ZVFe and loss of PRB permeability, reducing the long-term effectiveness and stability of PRB treatment facilities. This has become one of the bottleneck problems of PRB technology, urgently requiring the development of passivation-resistant and clogging-resistant PRB active media.

[0004] Natural pyrite is mainly found in water bodies, lakes, sediments, and groundwater, and is one of the most abundant natural minerals. The main components of pyrite are FeS and FeS2. Compared with zero-valent iron of the same particle size, it contains more reducing species, thus exhibiting stronger reducing properties, better stability, and easier storage. Furthermore, the aqueous solution obtained after leaching the active components from pyrite is usually slightly acidic, effectively removing the passivation layer. Therefore, it has better anti-clogging properties, longer-lasting effect, and greater stability, making it an important alternative to zero-valent iron as the active medium for reducing PRB.

[0005] Natural pyrite particles are large and have a small specific surface area, resulting in low contact efficiency with pollutants in practical applications, which affects its removal efficiency and reaction rate for target pollutants. High-energy ball milling can process natural pyrite into micro-nano-scale powder, increasing the specific surface area, reducing the crystallinity of the mineral, and increasing the number of suspended uncoordinated bonds and point defects on the mineral surface. This increases the number of active sites and improves the reactivity of the material surface. Moreover, the ball milling process generates no waste liquid or waste gas, making it an important method for the green, large-scale, and low-cost preparation of micro-nano-scale reducing materials.

[0006] However, direct ball milling for the preparation of micro / nano-scale pyrite powder is time-consuming, with many studies employing milling times exceeding 24 hours. This results in high energy consumption, low single-machine production capacity, and the common problems of easy oxidation and agglomeration of micro / nano-scale reducing materials leading to deactivation. Therefore, more effective methods for simultaneous ultrafine milling and stabilization are needed. Screening efficient grinding aids and optimizing ball milling parameters are important approaches for the green, efficient, and low-carbon preparation of micro / nano-scale pyrite powder as an active medium for PRB. While micro / nano-scale powders exhibit high activity, their permeability is correspondingly low. Therefore, methods for filling PRB with micro / nano-scale pyrite active media also need to be developed to simultaneously improve the reactivity and long-lasting effect of PRB. Summary of the Invention

[0007] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0008] In view of the above problems and / or the problems existing in the prior art, the present invention is proposed.

[0009] Therefore, the purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing a PRB active medium.

[0010] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0011] Silica sand was added to natural pyrite, and the mixture was activated by oxygen-limited intermittent high-energy ball milling. After the reaction was completed and cooled to room temperature, the powder was removed to obtain micro-nano-scale pyrite powder with high-efficiency reduction properties.

[0012] In a preferred embodiment of the preparation method of the PRB active medium of the present invention, the silica sand accounts for 5% to 30% of the total mass of natural pyrite and silica sand, wherein the particle size range of both natural pyrite and silica sand is 0.5 to 3 mm.

[0013] In a preferred embodiment of the preparation method of the PRB active medium of the present invention, the oxygen-limited environment is formed by evacuating the ball mill jar or filling it with inert gas after feeding the ball mill and before ball milling.

[0014] In a preferred embodiment of the preparation method of the PRB active medium of the present invention, the ball milling activation treatment comprises: the grinding balls being made of zirconium oxide or stainless steel, the ball-to-material mass ratio being 20–40:1, the rotation speed being 200–600 r / min, and the treatment time being 0.5–4 h; the intermittent treatment mode consisting of alternating ball milling for 2–8 s, pausing for 2–8 s, and repeating the process.

[0015] As a preferred embodiment of the preparation method of the PRB active medium of the present invention, wherein: the micro-nano grade pyrite powder D 50 Less than 300nm.

[0016] Another objective of this invention is to overcome the shortcomings of the prior art and provide an application of PRB active media.

[0017] To solve the above-mentioned technical problems, the present invention provides the following technical solution: The application includes uniformly mixing the PRB active medium with a particulate carrier and filling it into a columnar or cylindrical device to form PRB, and pumping groundwater into the device from bottom to top for reducing, adsorbing and removing nitrobenzene, chlorinated hydrocarbons and heavy metals from groundwater.

[0018] As a preferred embodiment of the application of the PRB active medium described in this invention, the carrier has a particle size of 1-2 mm and includes one or more of sand, zeolite, clay, biochar, and activated carbon.

[0019] In a preferred embodiment of the application of the PRB active medium described in this invention, the mass ratio of the PRB active medium to the carrier is 1:0.5 to 4.

[0020] In a preferred embodiment of the application of the PRB active medium described in this invention, the residence time of the groundwater in the PRB is 0.5 to 2 hours.

[0021] As a preferred embodiment of the application of the PRB active medium described in this invention, the application achieves a removal rate of over 99% for nitrobenzene, chlorinated hydrocarbons, and heavy metals; and the effective period of PRB filled with silica sand grinding aid pyrite medium is increased by over 30%.

[0022] Beneficial effects of the present invention:

[0023] (1) This invention uses co-grinding of silica sand and pyrite to prepare micro-nano-grade pyrite powder. The raw materials are widely available, the preparation cost is low, and the preparation process does not generate waste liquid or waste gas. The resulting micro-nano-grade pyrite powder D 50With a wavelength of less than 300nm, it will not leach toxic or harmful substances when applied to wastewater treatment, and has the advantages of being green and clean.

[0024] (2) The present invention uses silica sand grinding method for mechanochemical treatment and optimizes ball milling parameters, which can effectively reduce the crystallinity of pyrite and improve the reaction activity. At the same time, it can promote the formation of a stable layer of silica on the material surface, overcome the problem of easy oxidation and agglomeration of micro and nano materials leading to deactivation, and improve the long-term performance of the material.

[0025] (3) When the micro-nano grade pyrite powder obtained in this invention is applied to treat polluted groundwater, the silica covering the surface also has a certain adsorption capacity, which can improve the contact rate between the active substance and the target pollutant, accelerate the reaction rate, and at the same time have a strong treatment effect on nitrobenzene wastewater with different acidity and alkalinity. Attached Figure Description

[0026] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein:

[0027] Figure 1 This invention illustrates the effect of silica sand ratio on the adsorption of lead and cadmium by ball-milled pyrite in Example 1.

[0028] Figure 2 Example 2 of this invention demonstrates the removal performance of p-nitrobenzene by pyrite powder.

[0029] Figure 3 The removal rate of p-nitrobenzene from pyrite powder was determined in Example 3 of this invention.

[0030] Figure 4 This invention relates to Example 4, which measures the effect of solution pH on the removal of nitrobenzene from pyrite powder.

[0031] Figure 5 The removal performance of p-nitrobenzene by ball milling pyrite five times is shown in Example 5 of this invention.

[0032] Figure 6 Example 6 of this invention illustrates the removal effect of ball-milled pyrite PRB on nitrobenzene in simulated groundwater. Detailed Implementation

[0033] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the examples in the specification.

[0034] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0035] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0036] Example 1

[0037] This embodiment explores the properties of micro-nano-sized pyrite powders with different silica sand contents.

[0038] Natural pyrite with a particle size of 3mm and silica sand were mixed evenly at mass ratios of 10:0, 9:1, 8:2, 7:3, and 6:4, respectively.

[0039] The mixture was processed using a planetary ball mill. The grinding balls were made of stainless steel, the ball-to-material mass ratio was 30:1, the rotation speed was 500 r / min, and the intermittent processing mode was started. The grinding was paused for 5 seconds after grinding for 5 seconds, and the process was repeated. The total processing time was 120 min.

[0040] After processing, the powder is cooled to room temperature and removed to obtain micro-nano-sized pyrite powder with different silica content on the surface.

[0041] The prepared pyrite powder was added to 100 ml of simulated wastewater containing 100 mg / L lead and 100 mg / L cadmium, with pyrite powder concentrations of 0.5 g / L and 2 g / L, respectively. The mixture was shaken for 6 hours. The results are shown below. Figure 1 .

[0042] Depend on Figure 1 It can be seen that as the proportion of silica sand increases, the removal rates of lead and cadmium both show an increasing trend, but the increase is limited above 20%, and there is no significant change after exceeding 30%.

[0043] Example 2

[0044] Natural pyrite with a particle size of 3mm and silica sand were mixed evenly at a mass ratio of 10:0 and 8:2.

[0045] The mixture was processed using a planetary ball mill. The grinding balls were made of stainless steel, the ball-to-material mass ratio was 30:1, the rotation speed was 500 r / min, and the intermittent processing mode was started. The grinding was paused for 5 seconds after grinding for 5 seconds, and the process was repeated. The total processing time was 120 min.

[0046] After processing, the powder is cooled to room temperature and removed to obtain micro-nano grade pyrite powder (BMP) and micro-nano grade pyrite powder coated with silica (BMP-Si).

[0047] Pyrite powder was added to 100 ml of simulated wastewater with a nitrobenzene concentration of 10–200 mg / L. The concentration of pyrite powder was 5 g / L, and the mixture was shaken for 6 hours. The results are shown below. Figure 2 .

[0048] Depend on Figure 2 It can be seen that the removal amount of nitrobenzene increases rapidly with the increase of the initial nitrobenzene concentration, and the removal amount of nitrobenzene by a unit mass of pyrite can reach more than 16 mg / g. At the same time, it can be seen by comparison that the removal capacity of pyrite prepared in this invention for nitrobenzene is significantly lower than its adsorption capacity for heavy metals. This is because the removal mechanism of nitrobenzene by pyrite is mainly reduction rather than adsorption.

[0049] Example 3

[0050] The pyrite powders BMP and BMP-Si obtained in Example 2 were added to several 100ml portions of simulated wastewater with nitrobenzene concentrations of 100mg / L. The amount of pyrite powder added was 5g / L. The mixture was shaken for 6 hours, and samples were taken at intervals to detect the nitrobenzene content.

[0051] Figure 3 The removal rate of nitrobenzene by the pyrite powder obtained in this invention is shown. It can be seen that the removal rate of nitrobenzene is very fast within 20 min as the reaction time increases, and then the increase in removal rate decreases rapidly, and the reaction equilibrium is basically reached within 60 min.

[0052] Example 4

[0053] The pyrite powders (BMP and BMP-Si) obtained in Example 2 were added to several 100 ml portions of simulated wastewater with nitrobenzene concentrations of 100 mg / L. The initial pH was adjusted to 3, 5, 7, 9 and 11 using sulfate or sodium hydroxide. The amount of pyrite powder added was 5 g / L. The mixture was shaken for 6 h.

[0054] Figure 4 The effect of initial solution pH on the removal of nitrobenzene by BMP and BMP-Si was shown. It can be seen that the initial solution pH has a relatively small effect on the reduction performance of BMP-Si, indicating that BMP-Si has strong acid and alkali resistance and exhibits strong treatment effects on nitrobenzene wastewater with varying acidity and alkalinity. The removal efficiency of BMP for nitrobenzene, however, decreases rapidly with increasing initial solution pH, indicating that BMP is only suitable for treating acidic wastewater and has poor treatment effects on alkaline wastewater.

[0055] Example 5

[0056] The pyrite powder obtained in Example 2 was added to several 100ml portions of simulated wastewater with a nitrobenzene concentration of 100mg / L. The pyrite powder dosage was 5g / L. The mixture was shaken for 6 hours, and then centrifuged to separate the solid and liquid components. The recovered solid residue was added to another 100ml portion of simulated wastewater with a nitrobenzene concentration of 100mg / L. This operation was repeated 5 times. The nitrobenzene removal effect is shown in the figure. Figure 5 .

[0057] Figure 5 The results showed that in the first two reactions, Ci / CO decreased significantly with increasing reaction time, indicating that the removal effect of nitrobenzene was significant in both reactions, and the difference in removal rate and removal efficiency was small. However, from the third reaction onwards, Ci / CO no longer decreased significantly, and the decrease was even smaller after the fourth reaction, indicating that the removal amount of nitrobenzene by the pyrite powder had reached saturation at this point. Therefore, the micro-nano grade pyrite powder prepared by this invention can be recycled up to 3 times.

[0058] Example 6

[0059] Since the pyrite powder obtained by this invention is micro-nano grade, with small particle size and strong activity, it is prone to agglomeration and blockage. In order to solve this technical problem, this invention chooses to load the pyrite powder onto millimeter-sized particles to construct a permeable reaction barrier.

[0060] Therefore, this embodiment provides a method for using the micro-nano-scale pyrite powder prepared in this invention as a PRB active medium to construct a permeable reaction barrier.

[0061] Using BMP and BMP-Si pyrite powder obtained in Example 2 as active media, and sand as a carrier, 5 grams and 20 grams of sand were mixed evenly and filled into an organic glass tube with an inner diameter of 2 cm and a height of 8 cm to simulate an in-situ permeable reaction barrier experimental device.

[0062] A 500 mg / L nitrobenzene solution was pumped from bottom to top into an acrylic column using a constant flow pump at a rate of 0.5 mL / min. The waste liquid was collected every 3 hours using an automatic fraction collector, and the concentration of nitrobenzene in the water was measured. Figure 6 .

[0063] The results showed that, compared with PRB filled with only 25 grams of silica sand, the effluent quality of PRB filled with 5 grams of pyrite powder (BMP) without silica sand grinding aid was significantly improved, and the effluent quality of PRB filled with 5 grams of silica sand grinding aid pyrite powder (BMP-Si) was further improved. The nitrobenzene removal rate could be stably reached above 99%, and the stability of the effluent quality was significantly improved.

[0064] It can be seen that the present invention adopts a silica sand-assisted grinding method for pyrite, which not only reduces the cost of raw materials but also improves the processing efficiency of PRB. At the same time, the micro-nano-sized pyrite powder obtained is loaded onto a millimeter-sized carrier, which not only retains the high activity of the pyrite powder but also makes it less prone to agglomeration, further improving the active medium of PRB.

[0065] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing a PRB active medium, characterized in that: include, Silica sand was added to natural pyrite, and the mixture was subjected to oxygen-limited intermittent high-energy ball milling activation treatment. After the reaction was completed and cooled to room temperature, the powder was removed to obtain micro-nano-scale pyrite powder with high-efficiency reduction properties. The silica sand accounts for 30% of the total mass of natural pyrite and silica sand. The ball milling activation treatment takes 120 minutes. The ball milling is carried out intermittently, with the treatment mode being ball milling for 2-8 seconds, pausing for 2-8 seconds, and alternating between the two processes. The micro-nano grade pyrite powder with high reduction performance is used as a PRB active medium for the removal of nitrobenzene. The removal rate is >75% under pH conditions of 3~11, and it can be recycled up to 3 times.

2. The method for preparing the PRB active medium as described in claim 1, characterized in that: The particle size range of both natural pyrite and silica sand is 0.5~3mm.

3. The method for preparing the PRB active medium as described in claim 1, characterized in that: The oxygen-limited environment is created by evacuating the ball mill jar or filling it with inert gas after feeding the ball mill and before ball milling.

4. The method for preparing the PRB active medium as described in claim 1, characterized in that: The ball milling activation treatment involves grinding balls made of zirconium oxide or stainless steel, with a ball-to-material mass ratio of 20-40:1 and a rotation speed of 200-600 r / min.

5. The method for preparing the PRB active medium as described in claim 1, characterized in that: The micro-nano-sized pyrite powder D 50 Less than 300 nm.

6. An application of a PRB active medium prepared by the method described in any one of claims 1 to 5, characterized in that: The application includes uniformly mixing the PRB active medium with a particulate carrier and filling it into a columnar or cylindrical device to form PRB. Groundwater is pumped into the device from bottom to top for the reduction, adsorption and removal of nitrobenzene, chlorinated hydrocarbons and heavy metals from groundwater.

7. The application of the PRB active medium as described in claim 6, characterized in that: The carrier has a particle size of 1-2 mm and includes one or more of the following: sand, zeolite, clay, biochar, and activated carbon.

8. The application of the PRB active medium as described in claim 6, characterized in that: The mass ratio of the PRB active medium to the carrier is 1:0.5~4.

9. The application of the PRB active medium as described in claim 6, characterized in that: The residence time of the groundwater in the PRB is 0.5 to 2 hours.

10. The application of the PRB active medium as described in claim 6, characterized in that: The application can achieve a removal rate of over 99% for nitrobenzene, chlorinated hydrocarbons, and heavy metals; the effective period of PRB filled with silica sand and pyrite grinding media is increased by over 30%.

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