System and method for in-situ remediation of nitrate pollution in groundwater containing manganese based on manganese ion threshold effect of permeable reactive barrier

Abstract

A system and method for in-situ remediation of nitrate contamination in manganese-containing groundwater using a permeable reactive barrier (PRB) based on the manganese ion threshold effect, belonging to the field of in-situ groundwater remediation and water treatment technology. The system comprises, sequentially arranged along the groundwater flow direction: a pre-conditioning zone, a manganese concentration control zone, a solid carbon source packing main reaction zone, a post-response support zone, and a monitoring well system. Its advantages lie in overcoming the shortcomings of existing PRB technologies in treating manganese-containing groundwater by failing to fully utilize the promoting effect of manganese ions. By constructing a PRB wall with specific functional zones, it fully utilizes the naturally occurring Mn²⁺ resources in groundwater. Based on the "threshold effect" of Mn²⁺ on the solid-phase antinitrification system and its dual regulation mechanism, it achieves long-term, stable, and highly efficient in-situ removal of nitrates from manganese-containing groundwater.

Description

Technical Field

[0001] This invention belongs to the field of in-situ remediation and water treatment technology for groundwater pollution. Specifically, it relates to a system and method for in-situ remediation of nitrate pollution in manganese-containing groundwater using a permeable reactive barrier (PRB). In particular, it relates to a technical solution for achieving long-term and stable denitrification of groundwater using PRB based on the "threshold effect" of natural Mn²⁺ in groundwater and its dual regulation mechanism. Background Technology

[0002] Nitrate pollution in groundwater has become an increasingly serious environmental problem worldwide. Excessive application of nitrogen fertilizers in agriculture, domestic sewage, and industrial wastewater discharge are the main sources of nitrate pollution in groundwater. Drinking groundwater with high nitrate content can seriously harm human health. Once in the human body, nitrates are reduced to nitrites, which then react with amines to form carcinogenic nitrosamines.

[0003] Permeable reactive barrier (PRB) technology is a pollution treatment system that installs specific reaction media underground. It blocks contaminated zones and transforms pollutants into environmentally acceptable forms without disrupting groundwater flow. Combining biological denitrification with PRB technology is an effective in-situ remediation technique. Solid-state denitrification uses a solid matrix as the carbon source and microbial growth carrier for denitrification, making it particularly suitable for in-situ permeable reactive barrier facilities. Existing technologies include schemes using wheat straw layers, sugarcane flour layers, and polyhydroxyalkanoates (PHA) layers as PRB reaction media, as well as studies using loofah sponges as PRB filling media for nitrate removal from groundwater (with an average NO3⁻-N removal rate of up to 91.58%).

[0004] Meanwhile, high concentrations of divalent manganese ions (Mn²⁺) are commonly found in groundwater, especially in reducing groundwater environments. Existing PRB (permeable bioreactor) technologies for treating manganese-containing groundwater do not consider the potential impact of Mn²⁺ on the denitrification process, and no PRB system has been developed that can actively utilize the Mn²⁺ "threshold effect" to enhance denitrification. Current manganese autotrophic denitrification technologies primarily focus on artificial enhancement systems in wastewater treatment, and do not address in-situ denitrification enhancement technologies utilizing natural Mn²⁺ in PRB.

[0005] Therefore, there is an urgent need to develop a PRB system that can fully utilize natural Mn²⁺ resources in groundwater and achieve in-situ enhanced denitrification based on the Mn²⁺ “threshold effect” and dual regulation mechanism, in order to solve the shortcomings of existing PRB technologies in treating manganese-containing groundwater, which fail to fully utilize the promoting effect of manganese ions. Summary of the Invention

[0006] The purpose of this invention is to provide a system and method for in-situ remediation of nitrate pollution in manganese-containing groundwater using a permeable reactive barrier based on the manganese ion threshold effect. This overcomes the shortcomings of existing PRB technology in failing to fully utilize the promoting effect of manganese ions when treating manganese-containing groundwater. This system fully utilizes the naturally occurring Mn²⁺ resources in groundwater by constructing a PRB wall with specific functional zones. Based on the "threshold effect" of Mn²⁺ on solid-state denitrification systems (exhibiting a concentration-dependent promoting effect in the concentration range of 2~10 mg / L, with a nitrate removal rate of over 99% within 48 hours at 10 mg / L) and its dual regulation mechanism (carbon source supply promotion + manganese autotrophic denitrification synergy), long-term, stable, and efficient in-situ removal of nitrates from manganese-containing groundwater is achieved.

[0007] The system of the present invention includes, in sequence along the groundwater flow direction: a pre-regulation zone, a manganese concentration control zone, a solid carbon source packing main reaction zone, a post-support zone, and a monitoring well system; The aforementioned pre-conditioning zone is located at the upstream end of the PRB wall and is filled with quartz sand or gravel. It is used to uniformly distribute and initially filter the groundwater entering the PRB, removing suspended solids. The manganese concentration control zone is located downstream of the pre-regulation zone and is filled with manganese salt slow-release material or a dilution water inlet pipe. The manganese salt slow-release material is a porous slow-release particle embedded with MnCl2, which is used to slowly release Mn²⁺ when the Mn²⁺ concentration in the raw water is lower than 2 mg / L, thereby adjusting the Mn²⁺ concentration in the water entering the main reaction zone to 5~10 mg / L. The dilution water inlet pipe is used to introduce low-manganese groundwater or clean water for dilution and adjustment when the Mn²⁺ concentration in the raw water is higher than 10 mg / L. The solid carbon source packing's main reaction zone is located downstream of the manganese concentration control zone, and is filled with natural biomass material as a solid carbon source and biofilm carrier. The surface of the natural biomass material is covered with a complex microbial community containing anaerobic fermenting bacteria, denitrifying bacteria, and manganese oxidation / reduction bacteria. The natural biomass material is selected from one or more of sawdust, rice straw, corn cob, and loofah sponge. The solid carbon source packing's main reaction zone is divided into a carbon source hydrolysis section and a denitrification enhancement section along the groundwater flow direction. The carbon source hydrolysis section is located upstream of the main reaction zone of the solid carbon source packing. It is filled with natural biomass material with a particle size of 2.0~5.0 mm and a packing density of 0.2~0.4 g / cm³. It is used to enrich anaerobic fermentation bacteria and manganese oxidizing bacteria under the drive of Mn²⁺, accelerate the hydrolysis and acidification of the solid carbon source, and release dissolved organic carbon (DOC). The denitrification enhancement section is located downstream of the carbon source hydrolysis section. It is filled with natural biomass material with a small particle size of 0.5~2.0 mm and a filling density of 0.4~0.6 g / cm³. It is used to achieve efficient denitrification nitrogen removal under the "threshold effect" and dual regulation of Mn²⁺. The post-treatment protection zone is located downstream of the main reaction zone of the solid carbon source packing. It is filled with one or more of activated carbon, zeolite, or biochar to adsorb and remove residual nitrate nitrogen and trace organic matter in the effluent, thus ensuring the quality of the effluent.

[0008] The monitoring well system includes several monitoring wells installed upstream, inside and downstream of the PRB wall. Each monitoring well is equipped with an online water quality monitoring probe, including at least a Mn²⁺ concentration sensor, a NO3⁻-N concentration sensor and a dissolved oxygen sensor.

[0009] Preferably, the PRB wall adopts a continuous wall type or a funnel-water gate type structure.

[0010] Preferably, the manganese salt sustained-release material is a cross-linked encapsulated particle of MnCl2, sodium alginate, and polyvinyl alcohol, with a manganese salt mass percentage of 5% to 15% and a sustained-release period of 30 to 90 days.

[0011] Preferably, the natural biomass material is sawdust, rice straw, or loofah sponge that has been pretreated with 1.5% to 3% NaOH solution for 12 to 24 hours.

[0012] Preferably, the volume ratio of the carbon source hydrolysis section to the denitrification enhancement section is 1:2 to 1:4.

[0013] Preferably, the permeability coefficient of the PRB wall is 2 to 5 times that of the aquifer.

[0014] Preferably, the system also includes a remote monitoring platform, which is connected to the online water quality monitoring probes of each monitoring well via wired or wireless means.

[0015] This invention also provides a method for in-situ remediation of nitrate contamination in manganese-containing groundwater using a permeable reactive wall based on the manganese ion threshold effect, comprising the following steps: (1) Site investigation and system design: Investigate the hydrogeological conditions of the target aquifer, determine the background values ​​and spatial distribution of Mn²⁺ concentration and NO3⁻-N concentration in the groundwater, and determine the location, orientation and size of the PRB wall based on the range of the pollution plume; (2) PRB wall construction: along the groundwater flow direction, a pre-conditioning zone, a manganese concentration control zone, a solid carbon source filler main reaction zone, and a post-protection zone are constructed in sequence; the solid carbon source filler main reaction zone is filled in the order of carbon source hydrolysis section first and denitrification enhancement section last. (3) System startup and microbial biofilm formation: Inoculate the PRB wall with activated sludge or enriched denitrifying bacteria, start up the system under anaerobic conditions, and continue until a stable composite microbial community is formed on the surface of the solid carbon source. (4) In-situ enhanced nitrogen removal operation based on the "threshold effect": Groundwater containing nitrates and Mn²⁺ flows through the PRB wall under the influence of a natural hydraulic gradient: (a) Groundwater first enters the pre-conditioning zone, and after uniform water distribution and preliminary filtration, it enters the manganese concentration control zone; (b) In the manganese concentration control zone, when the raw water Mn²⁺ concentration is below 5 mg / L, the manganese salt slow-release material releases Mn²⁺ to adjust the concentration to 5~10 mg / L; when the raw water Mn²⁺ concentration is above 10 mg / L, dilution water is introduced into the pipeline for dilution and adjustment. (c) After regulation, the manganese-containing groundwater enters the carbon source hydrolysis section of the main reaction zone of the solid carbon source packing. Mn²⁺ drives the enrichment of anaerobic fermentation bacteria and manganese oxidizing bacteria, accelerates the hydrolysis and acidification of natural biomass materials, and releases DOC. (d) DOC-rich groundwater enters the denitrification enhancement section and achieves efficient nitrogen removal through a dual regulation mechanism under the "threshold effect" of Mn²⁺: on the one hand, Mn²⁺ promotes carbon source supply to provide sufficient electron donors for heterotrophic denitrification; on the other hand, Mn²⁺ acts as an electron donor to initiate the synergistic denitrification of Mn(II) autotrophic denitrification and heterotrophic denitrification, and continuously drives electron transfer through the Mn(II)↔Mn(III)↔Mn(IV) redox cycle. (e) The treated groundwater enters the post-containment protection zone, where it is further desorbed by activated carbon or zeolite before flowing out of the PRB wall. (5) Operation effect monitoring and maintenance: The water quality changes upstream and downstream of the PRB wall are monitored regularly through the monitoring well system to evaluate the nitrate removal effect. When the nitrate removal rate drops below 85%, denitrifying bacteria or solid carbon source materials are added through the monitoring well.

[0016] Compared with the prior art, the present invention has the following significant advantages: 1. For the first time, the "threshold effect" of Mn²⁺ is introduced into the field of PRB in-situ remediation technology. Existing PRB technologies do not consider the presence of natural Mn²⁺ in groundwater and its promoting effect on the denitrification process. This invention, for the first time, uses the Mn²⁺ "threshold effect" as the core basis for PRB system design. By setting a manganese concentration control zone, the concentration of Mn²⁺ entering the main reaction zone is actively adjusted to the optimal promoting window (5~10 mg / L), enabling the PRB system to actively utilize natural Mn²⁺ resources in groundwater to enhance denitrification in situ. This technical approach is currently unprecedented in the field of PRB.

[0017] 2. Pioneering a zoned design concept of "carbon source hydrolysis section + denitrification enhancement section". This invention, for the first time, innovatively divides the main reaction zone of the solid carbon source packing into a carbon source hydrolysis section and a denitrification enhancement section based on the target points of the Mn²⁺ dual regulation mechanism. The carbon source hydrolysis section utilizes large-particle-size, low-density packing to enrich anaerobic fermentation bacteria and manganese-oxidizing bacteria, efficiently hydrolyzing the solid carbon source under Mn²⁺ driving force. The denitrification enhancement section utilizes small-particle-size, high-density packing to achieve efficient nitrogen removal under the Mn²⁺ "threshold effect." The two sections synergistically achieve spatiotemporal separation and optimized matching of "carbon source supply" and "nitrogen removal conversion," significantly improving the treatment efficiency and service life of the PRB (Potentially Regenerated Carbon).

[0018] 3. It achieves an integrated design of "in-situ utilization, long-term sustained release, and automatic regulation". This invention achieves in-situ replenishment and long-term sustained release of Mn²⁺ (release period 30-90 days) through the use of manganese salt slow-release materials, solving the engineering problem of insufficient Mn²⁺ concentration in natural groundwater and thus failing to fully utilize the "threshold effect." Simultaneously, through the gradient design of the pre-conditioning zone, manganese concentration control zone, main reaction zone, and post-containment support zone, along with real-time monitoring by the monitoring well system, the long-term, stable, and automated operation of the PRB system is achieved.

[0019] 4. Fully utilize the natural hydraulic gradient of groundwater to achieve zero-energy operation. Unlike existing extraction-treatment remediation technologies, this invention fully utilizes the natural hydraulic gradient of groundwater to drive water flow through the PRB wall, requiring no external power equipment and resulting in extremely low operating costs. Furthermore, this invention uses agricultural and forestry waste (wood chips, rice straw, corn cobs, loofah sponges) as a solid carbon source, with readily available and inexpensive raw materials, realizing the environmentally friendly concept of "treating pollution with waste."

[0020] 5. Achieving synergistic in-situ remediation of nitrates and Mn²⁺ This invention transforms Mn²⁺ pollutants in groundwater, which would otherwise require additional treatment, into denitrification promoting factors, simultaneously achieving efficient nitrate removal and Mn²⁺ resource utilization without the addition of external chemical agents. Groundwater treated by this system achieves a nitrate nitrogen removal rate of over 95% and a total nitrogen removal rate of over 85%, with the effluent quality meeting national groundwater quality standards.

[0021] 6. It has wide applicability and engineering promotion value. This invention is applicable to the remediation of nitrate pollution in manganese-containing groundwater under different hydrogeological conditions. The PRB wall can adopt a continuous wall type or a funnel-gate type structure to adapt to different pollution plume sizes and aquifer conditions. The system has a simple structure, is easy to construct, and has low maintenance costs, and has broad application prospects in fields such as rural groundwater source protection and groundwater pollution remediation in industrial and mining enterprises. Attached Figure Description

[0022] Figure 1This is a schematic diagram of the plan layout of an in-situ remediation system for a permeable reactive wall based on the manganese ion threshold effect. Detailed Implementation Example

[0023] I. Site Overview The groundwater in a certain agricultural area is contaminated with nitrates. The average concentration of NO3⁻-N in the groundwater is 35 mg / L, the average concentration of Mn²⁺ is 3 mg / L, the groundwater flow velocity is 0.3 m / d, the aquifer thickness is 6 m, and the width of the pollution plume is about 20 m.

[0024] II. PRB System Construction This embodiment provides a system for in-situ remediation of nitrates in manganese-containing groundwater using a permeable reactive wall based on the manganese ion threshold effect. The system consists of a pre-conditioning zone, a manganese concentration control zone, a solid carbon source packing main reaction zone, and a post-protection zone arranged sequentially along the groundwater flow direction.

[0025] The width of the pre-conditioning zone is 0.5 m, and it is filled with quartz sand with a particle size of 2~4 mm.

[0026] The manganese concentration control zone is 0.5 m wide and filled with a manganese salt slow-release material. This material consists of cross-linked, embedded particles of MnCl2, sodium alginate, and polyvinyl alcohol (mass ratio MnCl2:sodium alginate:polyvinyl alcohol = 10:5:3), with a particle size of 3-5 mm and a manganese salt mass percentage of 10%. The slow-release period is approximately 60 days. Since the raw water Mn²⁺ concentration is 3 mg / L (below 5 mg / L), the manganese salt slow-release material slowly releases Mn²⁺, adjusting the Mn²⁺ concentration in the water entering the main reaction zone to 10 mg / L.

[0027] The main reaction zone of the solid carbon source packing is 3.0 m wide and is divided into carbon source hydrolysis and denitrification enhancement sections along the groundwater flow direction, with a volume ratio of 1:3.

[0028] Corn cobs (3.0~5.0 mm in diameter, 0.3 g / cm³ in packing density) pretreated with 2% NaOH solution for 18 hours were used to fill the carbon source hydrolysis section to enrich anaerobic fermentation bacteria and methanogenic bacteria under Mn²⁺ driving, thereby accelerating the hydrolysis and acidification of solid-phase carbon source.

[0029] The denitrification enhancement section is filled with poplar wood chips (particle size 0.5~2.0 mm, filling density 0.5 g / cm³) pretreated with 2% NaOH solution for 18 hours. The surface of the wood chips is covered with a complex microbial community containing anaerobic fermentation bacteria, denitrifying bacteria and manganese oxidation / reduction bacteria, which is used to achieve efficient denitrification under the "threshold effect" and dual regulation of Mn²⁺.

[0030] The rear protection zone is 0.5 m wide and filled with granular activated carbon.

[0031] The monitoring well system includes an upstream monitoring well located 5 m upstream of the PRB wall, an intermediate monitoring well located inside the wall, and a downstream monitoring well located 5 m downstream. Each monitoring well is equipped with a Mn²⁺ concentration sensor, a NO3⁻-N concentration sensor, and a dissolved oxygen sensor.

[0032] The PRB wall adopts a continuous wall structure, and the wall permeability coefficient is controlled to be 3 times the permeability coefficient of the aquifer.

[0033] III. In-situ restoration methods S1. Site Investigation and System Design: Investigate the hydrogeological conditions of the target aquifer, measure the concentration of Mn²⁺ in the groundwater to be 3 mg / L and the concentration of NO3⁻-N to be 35 mg / L, and determine the location, orientation, and size of the PRB wall.

[0034] S2, PRB wall construction: Construct each functional area sequentially along the groundwater flow direction according to the above structure.

[0035] S3. System Start-up and Microbial Biofilm Formation: Inoculate the PRB wall with activated sludge from the anaerobic digester of the municipal wastewater treatment plant and start up the system under anaerobic conditions for 20 days until a stable complex microbial community forms on the surface of the solid carbon source.

[0036] S4. In-situ enhanced denitrification operation: Groundwater containing nitrates and Mn²⁺ flows through the PRB wall under the influence of a natural hydraulic gradient: (a) Groundwater first enters the pre-conditioning zone, and after uniform water distribution and preliminary filtration, it enters the manganese concentration control zone; (b) In the manganese concentration control zone, the manganese salt slow-release material releases Mn²⁺, adjusting the Mn²⁺ concentration in the water from 3 mg / L to 10 mg / L; (c) After regulation, the manganese-containing groundwater (Mn²⁺ concentration 10 mg / L) enters the carbon source hydrolysis section. Mn²⁺ drives the enrichment of anaerobic fermentation bacteria and manganese oxidizing bacteria, accelerating the hydrolysis and acidification of corn cobs. The DOC concentration increases from 8.5 mg / L before entering to 25.6 mg / L. (d) DOC-rich groundwater enters the denitrification enhancement section, where it achieves efficient nitrogen removal through a dual regulation mechanism under the "threshold effect" of 10 mg / L Mn²⁺, reducing the NO⁻-N concentration in the effluent to 1.2 mg / L, with a removal rate of 96.6%. (e) The treated groundwater enters the post-treatment protection zone, where it is further desorbed by activated carbon before flowing out of the PRB wall. The NO3⁻-N concentration in the effluent is reduced to 0.8 mg / L, with a removal rate of 97.7%.

[0037] S5. Operational performance monitoring: Through regular monitoring by the monitoring well system, the nitrate removal rate remained above 92% after 180 days of operation.

Claims

1. A system for in-situ remediation of nitrate contamination in manganese-containing groundwater using a permeable reactive barrier based on the manganese ion threshold effect, characterized in that, The system includes, in sequence along the groundwater flow direction: a pre-regulation zone, a manganese concentration control zone, a solid carbon source and filler main reaction zone, a post-support zone, and a monitoring well system. The aforementioned pre-conditioning zone is located at the upstream end of the PRB wall and is filled with quartz sand or gravel. It is used to uniformly distribute and initially filter the groundwater entering the PRB, removing suspended solids. The manganese concentration control zone is located downstream of the pre-regulation zone and is filled with manganese salt slow-release material or a dilution water inlet pipe. The manganese salt slow-release material is a porous slow-release particle embedded with MnCl2, which is used to slowly release Mn²⁺ when the Mn²⁺ concentration in the raw water is lower than 2 mg / L, thereby adjusting the Mn²⁺ concentration in the water entering the main reaction zone to 5~10 mg / L. The dilution water inlet pipe is used to introduce low-manganese groundwater or clean water for dilution and adjustment when the Mn²⁺ concentration in the raw water is higher than 10 mg / L. The solid carbon source packing's main reaction zone is located downstream of the manganese concentration control zone, and is filled with natural biomass material as a solid carbon source and biofilm carrier. The surface of the natural biomass material is covered with a complex microbial community containing anaerobic fermenting bacteria, denitrifying bacteria, and manganese oxidation / reduction bacteria. The natural biomass material is selected from one or more of sawdust, rice straw, corn cob, and loofah sponge. The solid carbon source packing's main reaction zone is divided into a carbon source hydrolysis section and a denitrification enhancement section along the groundwater flow direction. The carbon source hydrolysis section is located upstream of the main reaction zone of the solid carbon source packing. It is filled with natural biomass material with a particle size of 2.0~5.0 mm and a packing density of 0.2~0.4 g / cm³. It is used to enrich anaerobic fermentation bacteria and methanogenic bacteria under the drive of Mn²⁺, accelerate the hydrolysis and acidification of the solid carbon source, and release dissolved organic carbon (DOC). The denitrification enhancement section is located downstream of the carbon source hydrolysis section. It is filled with natural biomass material with a small particle size of 0.5~2.0 mm and a filling density of 0.4~0.6 g / cm³. It is used to achieve efficient denitrification nitrogen removal under the "threshold effect" and dual regulation of Mn²⁺. The aforementioned post-reaction protection zone is located downstream of the main reaction zone of the solid carbon source packing. It is filled with one or more of activated carbon, zeolite, or biochar to adsorb and remove residual nitrate nitrogen and trace organic matter in the effluent, thus ensuring the quality of the effluent. The monitoring well system includes several monitoring wells installed upstream, inside and downstream of the PRB wall. Each monitoring well is equipped with an online water quality monitoring probe, including at least a Mn²⁺ concentration sensor, a NO3⁻-N concentration sensor and a dissolved oxygen sensor.

2. The system for in-situ remediation of nitrate contamination in manganese-containing groundwater using a permeable reactive barrier based on the manganese ion threshold effect, as described in claim 1, is characterized in that... The PRB wall is constructed using either a continuous wall or a funnel-gate structure.

3. The system for in-situ remediation of nitrate contamination in manganese-containing groundwater using a permeable reactive barrier based on the manganese ion threshold effect, as described in claim 1, is characterized in that... The manganese salt sustained-release material is a cross-linked encapsulated particle of MnCl2, sodium alginate, and polyvinyl alcohol, with a manganese salt mass percentage of 5% to 15% and a sustained-release period of 30 to 90 days.

4. The system for in-situ remediation of nitrate contamination in manganese-containing groundwater using a permeable reactive barrier based on the manganese ion threshold effect, as described in claim 1, is characterized in that... The natural biomass material is sawdust, rice straw, or loofah sponge that has been pretreated with 1.5% to 3% NaOH solution for 12 to 24 hours.

5. The system for in-situ remediation of nitrate contamination in manganese-containing groundwater using a permeable reactive barrier based on the manganese ion threshold effect, as described in claim 1, is characterized in that... The volume ratio of the carbon source hydrolysis section to the denitrification enhancement section is 1:2 to 1:

4.

6. The system for in-situ remediation of nitrate contamination in manganese-containing groundwater using a permeable reactive barrier based on the manganese ion threshold effect, as described in claim 1, is characterized in that... The permeability coefficient of the PRB wall is 2 to 5 times that of the aquifer.

7. The system for in-situ remediation of nitrate contamination in manganese-containing groundwater using a permeable reactive barrier based on the manganese ion threshold effect, as described in claim 1, is characterized in that... The system also includes a remote monitoring platform, which communicates with the online water quality monitoring probes of each monitoring well via wired or wireless means.

8. A method for in-situ remediation of nitrate contamination in manganese-containing groundwater using a permeable reactive barrier based on the manganese ion threshold effect, characterized in that, Includes the following steps: (1) Site investigation and system design: Investigate the hydrogeological conditions of the target aquifer, determine the background values ​​and spatial distribution of Mn²⁺ concentration and NO3⁻-N concentration in the groundwater, and determine the location, orientation and size of the PRB wall based on the range of the pollution plume; (2) PRB wall construction: along the groundwater flow direction, a pre-conditioning zone, a manganese concentration control zone, a solid carbon source filler main reaction zone, and a post-protection zone are constructed in sequence; the solid carbon source filler main reaction zone is filled in the order of carbon source hydrolysis section first and denitrification enhancement section last. (3) System startup and microbial biofilm formation: Inoculate the PRB wall with activated sludge or enriched denitrifying bacteria, start up the system under anaerobic conditions, and continue until a stable composite microbial community is formed on the surface of the solid carbon source. (4) In-situ enhanced nitrogen removal operation based on the "threshold effect": Groundwater containing nitrates and Mn²⁺ flows through the PRB wall under the influence of a natural hydraulic gradient: (a) Groundwater first enters the pre-conditioning zone, and after uniform water distribution and preliminary filtration, it enters the manganese concentration control zone; (b) In the manganese concentration control zone, when the raw water Mn²⁺ concentration is below 5 mg / L, the manganese salt slow-release material releases Mn²⁺ to adjust the concentration to 5~10 mg / L; when the raw water Mn²⁺ concentration is above 10 mg / L, dilution water is introduced into the pipeline for dilution and adjustment. (c) After regulation, the manganese-containing groundwater enters the carbon source hydrolysis section of the main reaction zone of the solid carbon source packing. Mn²⁺ drives the enrichment of anaerobic fermentation bacteria and manganese oxidizing bacteria, accelerates the hydrolysis and acidification of natural biomass materials, and releases DOC. (d) DOC-rich groundwater enters the denitrification enhancement section and achieves efficient nitrogen removal through a dual regulation mechanism under the "threshold effect" of Mn²⁺: on the one hand, Mn²⁺ promotes carbon source supply to provide sufficient electron donors for heterotrophic denitrification; on the other hand, Mn²⁺ acts as an electron donor to initiate the synergistic denitrification of Mn(II) autotrophic denitrification and heterotrophic denitrification, and continuously drives electron transfer through the Mn(II)↔Mn(III)↔Mn(IV) redox cycle. (e) The treated groundwater enters the post-containment protection zone, where it is further desorbed by activated carbon or zeolite before flowing out of the PRB wall. (5) Operation effect monitoring and maintenance: The water quality changes upstream and downstream of the PRB wall are monitored regularly through the monitoring well system to evaluate the nitrate removal effect. When the nitrate removal rate drops below 85%, denitrifying bacteria or solid carbon source materials are added through the monitoring well.

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