An ecological remediation method for intercepting and reducing endogenous pollutants released from sediments

Abstract

The application discloses an ecological remediation method for intercepting and reducing endogenous pollutants in sediments, and belongs to the technical field of sewage treatment. The steps of the remediation method comprise the following steps: firstly, mixing active materials with contaminated sediments; secondly, adding a cover layer above the mixture to intercept the pollutants; and finally, planting submerged and / or emergent plants, and harvesting the aquatic plants to reduce the total amount of pollutants. The application proposes the idea of "intercepting and reducing", that is, firstly intercepting the pollutants released from the sediments by using active materials, and then reducing the total amount of pollutants in the sediments by means of ecological remediation. Through the scheme, the concentration of pollutants in the overlying water is lower, and the reduction effect of the pollutants in the sediments is better.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology, specifically relating to an ecological restoration method that first intercepts and then reduces endogenous pollutants released from sediments. Background Technology

[0002] The release of endogenous pollutants from sediments has become a major challenge in the environmental field. Sediment remediation includes in-situ and ex-situ control. Ex-situ control mainly removes contaminated sediment through dredging to reduce pollutant release flux and ecological risks, followed by treatment of the dredged sediment. This technology incurs high subsequent sediment disposal costs and is detrimental to the restoration of aquatic ecosystems and biological resources. In-situ control mainly includes techniques such as in-situ cover, chemical reagent injection, microbial addition enhanced by denitrifying bacteria, ecological restoration, and geochemical engineering. This technology does not require additional treatment sites and has lower remediation costs. However, it is difficult to achieve the goal of in-situ sediment remediation because pollutants in sediments can be released again due to changes in external conditions. In fact, endogenous pollutants can be released again regardless of the sediment's conditions. For example, under anaerobic conditions, heavy metals and phosphorus are released into the overlying water due to the reduction of ferrous minerals; under aerobic conditions, heavy metals are released due to the oxidation of sulfides and heavy metal complexes. Over long periods, phosphorus bound to aluminum is also released again due to the aging of aluminum-containing minerals. Therefore, in-situ passivation technology for single pollutants is not suitable for the final remediation of lake and river sediments.

[0003] Planting aquatic plants can reduce pollutants in sediments, but different plants have different growth cycles. For example, *Potamogeton crispus* is a submerged plant that sprouts in autumn, overwinters, and then withers and dies in summer. This means that in ecological restoration areas such as lakes and rivers where *Potamogeton crispus* is the main submerged plant, endogenous pollutants in the sediment may be released in summer. Furthermore, higher summer temperatures facilitate various biochemical reactions, increasing the likelihood of pollutant release and algal blooms. Aquatic plants primarily grow by sprouting from their roots, and when harvested, the roots are often left in the sediment. Since pollutants mainly accumulate in the roots, root decay can release large amounts of pollutants into the interstitial water. Submerged plants generally have shallow root systems, making it easier for pollutants to be released into the overlying water.

[0004] The most effective method in in-situ sediment remediation is to reduce the total amount of pollutants in the sediment. Only by rationally combining individual methods, optimizing their combination, and overcoming their respective limitations can the remediation of endogenous pollution be ultimately achieved. Summary of the Invention

[0005] The purpose of this invention is to provide an ecological restoration method that first intercepts and then reduces the release of endogenous pollutants from sediments.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] An ecological restoration method for first intercepting and then reducing endogenous pollutants released from sediments includes the following steps:

[0008] First, the active material is mixed with the polluted sediment to obtain a mixed sediment, which achieves the first interception of pollutants. Then, a cover layer is added on the mixed sediment to intercept pollutants a second time. Finally, submerged and / or emergent plants are planted and aquatic plants are harvested to reduce the total amount of pollutants.

[0009] The active material is a clay mineral;

[0010] Specifically, the clay minerals are one or more of the following: iron-bearing minerals, biochar, iron salts, aluminum salts, zeolites, and vermiculite.

[0011] The materials of the cover layer include one or more of the active materials, non-polluting sediments, and soil.

[0012] This invention mixes active materials with contaminated sediments and then covers the mixed sediments with a clean overburden layer to more thoroughly trap pollutants in the sediments. Finally, the total amount of pollutants in the sediments is reduced by planting aquatic plants. The key point is that pollutants released from the sediments due to biochemical processes are refixed inside the sediments, rather than on the sediment surface. This not only inhibits the release of pollutants into the overburden water, but also refixes the pollutants through biochemical reactions inside the sediments. Furthermore, the reduction by aquatic plants achieves in-situ regeneration of the active materials' ability to adsorb pollutants.

[0013] This invention allows for the selection of active materials based on the type of pollutant. For example, if the pollutant is phosphorus, iron salts, aluminum salts, etc., can be added; if the pollutant is ammonia nitrogen, zeolite, vermiculite, biochar, or corresponding modifying materials can be arranged; if the pollutant is heavy metal, iron salts, iron minerals, zeolite, vermiculite, biochar, etc., can be placed; if the pollutant is hydrophobic organic matter, biochar, etc., can be arranged. For complex pollution, the combination of the above-mentioned active materials can achieve the interception of multiple pollutants, fixing them within the sediment.

[0014] The amount of active material added can be determined based on the total amount of pollutants in the sediment layer.

[0015] Planting submerged and emergent aquatic plants can reduce the total amount of sediment pollution. However, different plants have different growth cycles, and the anaerobic environment created when aquatic plants die off leads to the release of endogenous pollutants. This invention proposes an engineering remediation scheme of first intercepting and then reducing pollution. Taking *Potamogeton crispus* as an example, *Potamogeton crispus* cannot survive in the high temperatures of summer, and its death creates an anaerobic environment in the sediment, further leading to the release of heavy metals and phosphorus. By mixing active substances with the sediment, pollutants that may be released into the overlying water are adsorbed and trapped inside the sediment. Then, the growth of *Potamogeton crispus* in autumn and winter reduces the total amount of pollutants in the sediment, while simultaneously regenerating the adsorption capacity of the active substances. Here, the first interception and then reduction does not only occur simultaneously within the same time period. It also includes intercepting pollutants first and then reducing them at different time periods. As mentioned above, in the planting of *Potamogeton crispus*, pollutants are intercepted in summer, and the growth of *Potamogeton crispus* in winter reduces the total amount of pollution in the sediment.

[0016] Preferably, the thickness of the covering layer is 2 to 3 cm.

[0017] Preferably, the root systems of the planted submerged and / or emergent plants are in the mixed sediment.

[0018] To address the issue of pollutant release caused by the decay of aquatic plant roots, a proposed method is to place aquatic plant roots in an area where sediment and active materials are mixed. This not only traps pollutants released from root decay but also promotes further reduction of the total amount of pollutants in the sediment through the growth of aquatic plants.

[0019] Plants primarily transport pollutants through absorption. Therefore, when roots grow within an area covered by active materials, the trapping effect of these materials leads to a higher concentration of pollutants near the roots. During plant growth, this facilitates the migration of more pollutants to the stems and leaves, ultimately resulting in their removal from the sediment during harvesting. If no active materials are placed near the roots, pollutants exist at a spatial distance, meaning they reside in the overlying water or within the active materials covering the surface. Since plant stems and leaves lack an iron film, heavy metals in the overlying water exert a greater toxic effect on the plants, inhibiting the migration of heavy metals from the sediment to aquatic plants. However, placing active materials on the sediment surface presents two risks: firstly, pollutants trapped by the active materials may be released back into the overlying water, exhibiting a stronger toxic effect on plants; secondly, the lower pollutant concentration in the root zone results in lower efficiency for heavy metal migration and conversion by plants.

[0020] Different aquatic plants can be selected for different pollutants. For example, for ammonia nitrogen, cattail, rush, loosestrife, salsa, and hydrangea can be selected. For phosphorus, loosestrife, arrowhead, salsa, and hydrangea can be planted. For heavy metals, canna, umbrella sedge, and salsa can be planted. These aquatic plants have a high enrichment coefficient for heavy metals.

[0021] The beneficial technical effects of the present invention are as follows:

[0022] To address the issue of pollutant release from underwater sediments, this invention proposes a "retention first, reduction later" approach. This involves first using a two-layer active material to retain pollutants released from the sediment, and then reducing the total amount of pollutants in the sediment through ecological restoration. This approach results in lower pollutant concentrations in the overlying water and a more effective reduction of pollutants in the sediment. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the "first intercept, then reduce" scheme described in this invention.

[0024] Figure 2 The effect of different zeolite arrangements on the iron concentration in the overlying water is shown in Example 1, where a is the total iron concentration and b is the ferrous iron concentration.

[0025] Figure 3 The effect of different zeolite arrangements on the concentrations of Zn and Ni in the overlying water is shown in Example 1, where a is the Zn concentration and b is the Ni concentration. Detailed Implementation

[0026] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention. It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the present invention.

[0027] Furthermore, regarding the numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, are also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0028] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar to or equivalent to those described herein may be used in the implementation or testing of this invention.

[0029] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0030] The specific plan for "first interception, then reduction" is as follows:

[0031] Step 1: Select active materials that can function for a long time and are inexpensive. Specifically, these are active substances that can repeatedly adsorb and desorb pollutants or dissolve and precipitate them, and are inexpensive, such as zeolite, vermiculite, biochar, and iron salts.

[0032] Step 2: Mechanically mix the 20-50cm surface sediments from lakes or rivers with active materials; then cover the sediment surface with a thin layer of active material or a layer of uncontaminated sediments or soil, which can be obtained locally, with a thickness of 2-3cm.

[0033] Step 3: Select appropriate submerged or emergent plants based on the depth of the overlying water and the type of pollutants. If the overlying water is shallow, emergent plants can be planted over a large area. In this case, the plant roots can penetrate deeper into the area where the active material is mixed, thus reducing the total amount of pollutants deep within the sediment. If the overlying water is deep, submerged plants should be planted. In this case, the plant roots are shorter, so the active material can be mixed with the top 20cm of sediment.

[0034] Step 4: Harvest the aquatic plants when they have grown to a lush state. This process requires special management to prevent the aquatic plants from rotting in the overlying water. Continue this management and harvesting of the ecosystem over a long period until the total amount of pollution in the sediment reaches the relevant standard values.

[0035] See the diagram for the specific "first intercept, then reduce" plan. Figure 1 .

[0036] Example 1

[0037] The retention effect was verified using laboratory simulation as an example:

[0038] Sediments rich in heavy metals Zn, Ni, Cu, and Cd were selected as the contaminated sediments. These sediments naturally release Zn and Ni into the overlying water. To enhance the experimental results, in this embodiment, 1% Ca(NO3)2 was added to the sediment by weight of the wet sediment to promote the release of heavy metals. Following the principle of selecting active materials in the first step of the "retention first, reduction later" scheme, natural zeolite was chosen as the retention active material to retain the released heavy metals from the sediment. Natural zeolite is inexpensive, structurally stable, and can function continuously for many years. Furthermore, zeolite can repeatedly adsorb and desorb dissolved heavy metals and ammonia nitrogen.

[0039] In addition, 40g of natural zeolite was thoroughly mixed with 190g of polluted sediment, and then 40g of zeolite was evenly placed on the surface of the sediment. The concentration of pollutants in the overlying water was measured at different time periods.

[0040] 40g of zeolite was evenly placed on the surface of the sediment, and the concentration of pollutants in the overlying water was measured at different time points.

[0041] Figure 2 The effect of different zeolite arrangements on the iron concentration in the overlying water is shown, where a is the total iron concentration and b is the ferrous iron concentration. Figure 2 In this context, Ca(NO3)2 indicates that 1% wet weight calcium nitrate is injected into the sediment, while Ca(NO3)2+Zeo-Cap indicates that in addition to Ca(NO3)2, zeolite is also placed on the sediment surface. Ca(NO3)2+Zeo-Mix indicates that in addition to Ca(NO3)2, some zeolite is mixed with the sediment and the remaining zeolite is placed on the sediment surface. No treatment indicates that the sediment is not treated in any way.

[0042] Figure 3 The effect of different zeolite arrangements on the concentrations of Zn and Ni in the overlying water is shown, where a is the Zn concentration and b is the Ni concentration. Figure 3 In this context, Ca(NO3)2 indicates that 1% wet weight calcium nitrate is injected into the sediment, while Ca(NO3)2+Zeo-Cap indicates that in addition to Ca(NO3)2, zeolite is also placed on the sediment surface. Ca(NO3)2+Zeo-Mix indicates that in addition to Ca(NO3)2, some zeolite is mixed with the sediment and the remaining zeolite is placed on the sediment surface. No treatment indicates that the sediment is not treated in any way.

[0043] Example 2

[0044] Sediments contaminated with heavy metals Zn and Cd, with high iron oxide content, were placed in an ecosphere with a sediment layer 10 cm thick. Vallisneria natans of the same shape and size were planted in the ecosphere on July 15th. Some sediments were planted directly with Vallisneria natans without any treatment; some were planted after zeolite was mixed with a 10 cm thick sediment layer, ensuring the roots of the Vallisneria natans were placed within the zeolite-contaminated sediment mixture; and some were planted after zeolite was mixed with a 10 cm thick sediment layer, followed by a 2 cm thick layer of soil, again ensuring the roots were placed within the zeolite-contaminated sediment mixture. The planting density was the same in all three treatment groups. On September 15th, the roots of the Vallisneria natans were harvested, and the Zn and Cd enrichment content in the plants and the heavy metal content in the sediment were measured. The results are shown in Table 1.

[0045] Table 1

[0046]

[0047] Table 1 shows that, compared to directly planting Vallisneria natans, incorporating zeolite into the sediment, and simultaneously adding a topcoat layer while incorporating zeolite, significantly increases the aquatic plants' ability to accumulate heavy metals. The effect of incorporating zeolite and then adding a topcoat layer is particularly significant, demonstrating that the invention's dual interception of pollutants greatly enhances the rate of heavy metal accumulation by plants. Furthermore, the Zn content in the topcoat water of sediments without zeolite is also significantly increased. 2+ The high concentration of heavy metals, compared to no detectable concentrations in other overlying waters, indicates the retention effect of zeolite on heavy metals. From the above data, the dual role of our "retention first, reduction later" approach in the remediation of endogenous pollution in sediments is clearly evident.

[0048] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. An ecological restoration method for first intercepting and then reducing endogenous pollutants released from sediments, characterized in that, Includes the following steps: First, the active material is mixed with the polluted sediment to obtain a mixed sediment. Then, a cover layer is added on top of the mixed sediment to trap pollutants. Finally, submerged and / or emergent plants are planted and the aquatic plants are harvested to reduce the total amount of pollutants. The active material is a clay mineral; The material of the cover layer includes one or more of the active material, non-polluting sediments, and soil. The roots of the planted submerged and / or emergent plants are in the mixed sediment.

2. The ecological restoration method for first intercepting and then reducing endogenous pollutants released from sediments according to claim 1, characterized in that, The thickness of the covering layer is 2-3 cm.

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