A high-salinity wastewater free silicon targeting adsorbent, a preparation method and application thereof

Abstract

The application relates to a high-salt wastewater free silicon targeted adsorbent as well as a preparation method and application thereof, and belongs to the field of industrial high-salt wastewater functional adsorbent and preparation. The adsorbent is based on an ordered mesoporous-microporous multi-level pore carbon prepared by a molten salt method, boron atoms are in-situ doped in the skeleton, magnesium-aluminum hydroxyl oxides are limitedly grown in the pores, and catechol functional groups are covalently grafted on the surface. The application discards the defects of traditional soft templates and high-melting-point salt processes, utilizes a chloride binary eutectic salt to construct a regular multi-level pore structure in one step, and has the advantages of ordered pores, smooth mass transfer and uniform site distribution. The preparation process has strong repeatability and is suitable for industrial mass production. The product is accurately aimed at the removal scene of more than 20% of high-salt mother liquor neutral molecular monosilicic acid, and solves the problem of deep silicon removal in a high-salt system. The adsorbent has the advantages of stable structure, strong salt interference resistance, high selectivity and excellent regeneration performance, can remove residual free silicon, prevents the fouling and blockage of rear-end equipment, and guarantees the stable operation of a high-salt wastewater zero-discharge system.

Description

Technical Field

[0001] This invention belongs to the technical field of functional adsorbent materials for industrial high-salt wastewater and their preparation technology, specifically relating to a free silicon targeted adsorbent for high-salt wastewater, its preparation method, and its application. Background Technology

[0002] The aforementioned high-salt wastewater free silica targeted adsorbent is a targeted adsorbent for the deep removal of free silica in high-salt systems. It is suitable for the deep polishing treatment of molecular free monosilicic acid in membrane concentrates, evaporation crystallization mother liquors, and reactor residues with a total salt mass fraction of more than 20% in coal chemical, petrochemical, hydrometallurgical, pesticide and pharmaceutical and fine chemical industries.

[0003] The total salt content of high-salt membrane concentrates and evaporation crystallization mother liquors produced in industries such as coal chemical, petrochemical, and hydrometallurgy generally exceeds 20%. The salt components are mainly sodium chloride and sodium sulfate, and also contain large amounts of calcium ions, magnesium ions, fluoride ions, heavy metals, and recalcitrant organic matter and free silicon. Among them, free silicon mainly exists in the neutral molecular state of monosilicic acid (H4SiO4), which is extremely prone to polymerization during the heating and concentration process to form dense silica scale. This scale blocks the pores of the adsorbent material and the heat exchange surface of the evaporator, causing scaling, flux reduction, and salt degradation in the system. This is a key bottleneck for the stable operation of zero-discharge systems for high-salt wastewater.

[0004] For high-salinity wastewater with a salt content of over 20%, existing industrial silicon removal technologies mainly rely on magnesium and aluminum chemical precipitation, which has a large treatment load and low cost, but the effluent still has 50-100 mg / L of free silicon remaining, which cannot meet the requirements for deep and stable operation. Conventional deep silicon removal materials such as activated alumina, ordinary hydrotalcite, and commercial silicon removal resins have poor selectivity, severe competitive adsorption, and rapid decay of adsorption capacity in ultra-high salt and high calcium and magnesium interference systems, and cannot be adapted to the polishing conditions of high-salt mother liquor.

[0005] CN121198223A recommends "A magnesium-aluminum hydrotalcite adsorbent, its preparation method, and its application," which constructs a dual mechanism for silicon removal through layered trapping and interlayer exchange. However, the material is pure hydrotalcite powder, which presents challenges such as difficulty in solid-liquid separation and recovery, inability to be directly used for continuous fixed-bed operation, and competitive adsorption at active sites and decreased adsorption capacity in high-salt, high-calcium, and high-magnesium environments. Typical examples of chemical precipitation for silicon removal include CN115521016A's "A treatment method for removing sulfate and silicon from high-concentration saline wastewater" and CN108751534A's "A resource-based treatment method for wastewater from nano-silica sol production." These methods have high treatment loads, and the residual silicon in the effluent remains at a high level, making it difficult to achieve deep compliance. A typical example of boron-doped porous carbon materials is CN120695779A's "Adsorption-oxidation bifunctional boron-doped porous biochar material and its preparation method and application."

[0006] None of the aforementioned patents provide any technical insights related to the deep removal of free silica from high-salt wastewater, nor do they disclose any insights into the combination scheme of carbon channel-confined loading of magnesium aluminum hydroxy oxides and targeted grafting of catechol.

[0007] In summary, existing modified carbon materials and metal composite adsorbents have significant technical drawbacks: single metal hydroxy oxides are prone to agglomeration, pore blockage, and poor fixed-bed stability; single organic chelate grafted materials have extremely low adsorption capacity; and ordinary carbon supports lack structural confinement and heteroatom regulation, resulting in weak resistance to interference in high-salt environments. In particular, there are currently no composite targeted adsorbents specifically designed for the deep removal of neutral molecular-state monosilicic acid from complex wastewater with a salt content exceeding 20%. Summary of the Invention

[0008] The purpose of this invention is to address the numerous shortcomings of existing silicon removal technologies for high-salinity wastewater, including the high residual silicon content in effluent from conventional chemical precipitation methods, the inability to achieve deep removal to meet standards, the poor selectivity of traditional adsorbent materials in high-salinity, high-calcium, high-magnesium, and especially high-chlorine interference environments, the rapid decay of adsorption capacity, the easy aggregation and deactivation of active nanoparticles, and the weak anti-interference ability and insufficient targeted recognition performance of single modified carbon materials, which make it difficult to meet the industrial pain points of stable and deep removal of neutral molecular monosilicic acid from ultra-high-salinity wastewater with a salt content of over 20%. This invention provides a targeted adsorbent for free silicon in high-salinity wastewater, its preparation method, and its application.

[0009] The objective of this invention is achieved as follows: a targeted adsorbent for free silicon in high-salt wastewater, characterized in that the adsorbent uses ordered mesoporous-microporous hierarchical porous carbon prepared by molten salt method as a matrix; the carbon matrix framework is in-situ doped with boron heteroatoms, with a boron doping amount of 1.5 to 4.0 at.%; magnesium aluminum hydroxyl oxide active sites are confined within the pores of the hierarchical porous carbon, with a magnesium-aluminum molar ratio of 2:1 to 3:1 in the magnesium aluminum hydroxyl oxide; and catechol targeted coordination functional groups are anchored on the inner and outer surfaces of the pores of the hierarchical porous carbon through silane covalent grafting.

[0010] In one embodiment, the ordered mesoporous-microporous hierarchical carbon has a mesopore diameter of 2-6 nm, a micropore diameter of 0.4-0.8 nm, and a BET specific surface area of ​​1200-1800 m² / g.

[0011] In one embodiment, the boron heteroatom exists as a framework-substituted boron, which is used to regulate the Lewis acidity of the carbon matrix and suppress the non-specific competitive adsorption of calcium and magnesium ions in high-salt systems.

[0012] In one embodiment, the magnesium aluminum hydroxy oxide is confined within the mesoporous channels, without blocking the microporous adsorption channels, and provides ligand-exchangeable monosilicic acid adsorption active sites; the catechol functional groups are distributed on the channel walls, which are used to form specific chelation recognition of molecular monosilicic acid.

[0013] A method for preparing a free silica-targeted adsorbent for high-salt wastewater includes the following steps: (1) Molten salt-assisted preparation of boron-doped hierarchical porous carbon precursor: A composite eutectic salt formed by potassium chloride and zinc chloride is used as the molten salt system. The mass ratio of potassium chloride to zinc chloride in the eutectic salt is 1:1 to 2:1. Resorcinol-formaldehyde is used as the carbon source and boric acid is used as the boron doping source. After uniform mixing, the mixture is carbonized by high temperature molten salt and cured at constant temperature to obtain boron-doped ordered mesoporous-microporous hierarchical porous carbon. (2) Pore confined loading of magnesium aluminum hydroxy oxides: The boron-doped ordered mesoporous-microporous hierarchical carbon obtained in step (1) is dispersed in a mixed solution of magnesium salt and aluminum salt, and magnesium aluminum hydroxy oxide nano-active phase is generated in the carbon channels by in-situ co-precipitation. (3) Covalent grafting of catechol functional groups into pores: The magnesium aluminum hydroxy oxide nano-active phase obtained in step (2) is dispersed in an organic solvent, and catechol groups are grafted by liquid phase reflux using a silane coupling agent bridging method, so that the catechol groups are fixed on the inner and outer surfaces of the pores. After washing and drying, the finished product high-salt wastewater free silicon targeted adsorbent is obtained.

[0014] In one embodiment, the mass ratio of boric acid to resorcinol in step (1) is 0.15 to 0.4:1, the carbonization temperature is 550 to 650°C, and the nitrogen atmosphere is maintained for 2 to 3 hours.

[0015] In one embodiment, the magnesium salt in step (2) is magnesium nitrate, the aluminum salt is aluminum nitrate, the pH of the mixed solution is controlled at 9.0 to 10.0, the in-situ coprecipitation reaction temperature is 40 to 60°C, and the reaction time is 40 to 80 min.

[0016] In one embodiment, the organic solvent in step (3) is anhydrous ethanol, the silane coupling agent is catechol-based silane coupling agent, the liquid phase reflux temperature is 65-80°C, the reflux time is 4-7 h, and the grafting reaction is carried out in anhydrous ethanol system.

[0017] An application of a high-salt wastewater free silica targeted adsorbent is used for chemical high-salt wastewater with a total salt mass fraction greater than 20%. It is used for deep targeted adsorption and removal of neutral molecular free monosilicic acid H4SiO4 in the effluent of chemical precipitation pretreatment. After adsorption saturation, reversible desorption and regeneration are achieved by using a low-concentration sodium hydroxide solution at room temperature. After rinsing, it can be recycled multiple times.

[0018] In one embodiment, the high-salt chemical wastewater is one or more of the following: membrane concentrate, evaporation crystallization mother liquor, and reactor residue, which contain a total salt mass percentage greater than 20% in the coal chemical, petrochemical, hydrometallurgical, and fine chemical industries.

[0019] The technical effects of the technical solution provided by this invention are as follows: 1. This invention pioneers a composite functional structure using a potassium chloride-zinc chloride binary eutectic salt molten salt method. This structure is coupled with boron doping modification, pore-confined magnesium aluminum hydroxyl oxide loading, and catechol covalent grafting. It overcomes the shortcomings of traditional soft template processes, such as the need for secondary activation and easy pore collapse. It also avoids the shortcomings of disordered pore formation, poor controllability of multi-level pores, and unstable phase transition and poor process repeatability of multi-component mixed salt systems. Relying on the unique advantages of low-temperature eutectic salt, high-temperature stable liquid phase, and Lewis acid catalytic pore formation, this invention simultaneously constructs a well-ordered mesoporous and microporous multi-level pore structure in one step. The structure has high pore order, smooth mass transfer, and uniform distribution of active sites. It overcomes the technical defects of traditional materials, such as strong high-salt interference, poor selectivity, easy aggregation of active sites, and low adsorption capacity. The structure and performance have significant synergistic advantages.

[0020] 2. This invention fully discloses the standardized preparation process, precise formulation components, dedicated production equipment and core process parameters. The process logic is rigorous, the interrelationship of the steps is clear, and the repeatability is strong. It is fully adaptable to laboratory preparation and industrial mass production, and solves the industry bottleneck of existing composite adsorption materials having vague processes and being difficult to mass produce.

[0021] 3. This invention is precisely designed for the removal of neutral molecular monosilicic acid from ultra-high salt mother liquor with a concentration of over 20%. It is suitable for the real and complex working conditions of industrial evaporation mother liquor and membrane concentrate, and solves the technical problem of deep silicon removal in high-salt systems in the industry. Through multiple sets of gradient examples and comparative comparisons, the influence of each preparation parameter on the material performance has been clarified, and the technical reliability and creativity are fully supported.

[0022] 4. The adsorbent prepared by this invention has a stable structure, strong resistance to salt interference, high targeting selectivity, excellent regeneration and recycling performance, and is resistant to scaling and organic pollution. It can be stably applied to high-salt mother liquor pretreatment systems for a long time, effectively removing residual free silicon, avoiding the formation of silicon scale from the source, eliminating the problems of blockage and flux decline in downstream equipment, and significantly ensuring the stable and efficient operation of the entire high-salt wastewater zero-discharge system. Detailed Implementation

[0023] Example 1: A method for preparing a free silica targeted adsorbent for high-salt wastewater, comprising the following steps: (1) Preparation of boron-doped ordered hierarchical porous carbon: A potassium chloride-zinc chloride composite eutectic salt system was used (potassium chloride to zinc chloride mass ratio 1:1 to 2:1), resorcinol-formaldehyde was used as the carbon source, and boric acid was used as the boron dopant, with a boric acid / resorcinol mass ratio of 0.25:1; the molten salt was uniformly mixed at 80-120℃ for 2-4 hours, cured at 95℃ for 16 hours, carbonized at 600℃ in a nitrogen atmosphere for 2.5 hours, and after washing and desalting with water, boron-doped hierarchical porous carbon was obtained with a boron doping amount of 2.6 at.%%. (2) Pore-confined loading of magnesium aluminum hydroxy oxides: magnesium aluminum molar ratio 2.5:1, pH=9.5, in-situ precipitation at 50℃ for 60 min, uniform nano magnesium aluminum hydroxy oxides are generated in mesoporous channels; (3) Catechol silane reflux grafting: The anhydrous ethanol system was refluxed at 70°C for 5 hours to complete the covalent grafting modification of the entire pore area.

[0024] The adsorbent parameters prepared in this embodiment are: mesopore diameter 4.3 nm, micropore diameter 0.58 nm, and BET specific surface area 1460 m² / g.

[0025] Performance testing: The total salt content of the wastewater is 22%. The mother liquor contained calcium and magnesium impurities and had an initial silicon concentration of 54 mg / L. After adsorption, the silicon concentration in the effluent was ≤8 mg / L, with a stable removal rate of >85% and a capacity retention rate of >95.8% after 10 regeneration cycles.

[0026] Example 2: A method for preparing a free silica targeted adsorbent for high-salt wastewater, comprising the following steps: (1) Preparation of boron-doped ordered hierarchical porous carbon: A potassium chloride-zinc chloride composite eutectic salt system was used (potassium chloride to zinc chloride mass ratio 1:1 to 2:1), resorcinol-formaldehyde was used as the carbon source, and boric acid was used as the boron dopant, with a boric acid / resorcinol mass ratio of 0.15:1; the molten salt was uniformly mixed at 80-120℃ for 2-4 hours, cured at 85℃ for 20 hours, carbonized at 550℃ in a nitrogen atmosphere for 3 hours, and then washed with water to remove salt to obtain boron-doped hierarchical porous carbon with a boron doping amount of 1.8 at.%; (2) Pore-confined loading of magnesium aluminum hydroxy oxides: magnesium aluminum molar ratio 2:1, pH=9.0, in-situ precipitation at 40℃ for 80 min, uniformly generating nano-magnesium aluminum hydroxy oxide active sites in mesoporous channels; (3) Catechol silane reflux grafting: The anhydrous ethanol system was refluxed at 65°C for 7 hours to complete the whole-pore covalent grafting modification and obtain the finished adsorbent.

[0027] The adsorbent parameters prepared in this embodiment are: mesopore diameter 2.8 nm, micropore diameter 0.48 nm, and BET specific surface area 1350 m² / g.

[0028] Performance testing: The same high-salt mother liquor conditions as in Example 1 (total salt 22%) were used. The sample contains 85% boron doping, calcium and magnesium impurities, and an initial silicon concentration of 54 mg / L. After adsorption, the effluent silicon concentration is ≤10 mg / L, the free silicon removal rate is consistently >81%, and the adsorption capacity retention rate is >95.3% after 10 dilute alkali regeneration cycles. This embodiment adapts to low-load, high-permeability adsorption conditions by reducing boron doping, adjusting the magnesium-aluminum ratio and reaction temperature, and utilizing the transparent and regular multi-level pore structure prepared by the binary eutectic salt molten salt method of this invention. The material has higher pore permeability and is suitable for polishing treatment of high-flow-rate, high-salt mother liquor.

[0029] Example 3: A method for preparing a free silica targeted adsorbent for high-salt wastewater, comprising the following steps: (1) Preparation of boron-doped ordered hierarchical porous carbon: A potassium chloride-zinc chloride composite eutectic salt system was used (potassium chloride to zinc chloride mass ratio 1:1 to 2:1), resorcinol-formaldehyde was used as the carbon source, and boric acid was used as the boron dopant, with a boric acid / resorcinol mass ratio of 0.4:1; the molten salt was uniformly mixed at 80-120℃ for 2-4 hours, cured at 105℃ for 12 hours, carbonized at 650℃ in a nitrogen atmosphere for 2 hours, and after washing and desalting with water, boron-doped hierarchical porous carbon was obtained with a boron doping amount of 3.8 at.%%. (2) Pore-confined loading of magnesium aluminum hydroxy oxides: magnesium aluminum molar ratio 3:1, pH=10.0, in-situ precipitation at 60℃ for 40 min, rapidly generating a high-load magnesium aluminum hydroxy oxide nano-active phase in mesoporous channels; (3) Catechol silane reflux grafting: The anhydrous ethanol system was refluxed at 80°C for 4 hours to quickly complete the whole-pore covalent grafting modification and obtain the finished adsorbent.

[0030] The adsorbent parameters prepared in this embodiment are: mesopore diameter 5.2 nm, micropore diameter 0.75 nm, and BET specific surface area 1620 m² / g.

[0031] Performance testing: The same high-salt mother liquor conditions as in Example 1 (total salt 22%) were used. The sample contains 85% calcium and magnesium impurities and an initial silicon concentration of 54 mg / L. After adsorption, the effluent silicon concentration is ≤6 mg / L, the free silicon removal rate is consistently >88%, and the adsorption capacity retention rate is >96.7% after 10 dilute alkali regeneration cycles. This embodiment combines the advantages of the high-order multi-level porous structure unique to the binary eutectic salt molten salt method of this invention with high boron doping, high magnesium-aluminum ratio, and high-temperature rapid modification, which significantly improves the material's resistance to salt interference and saturated adsorption capacity, making it suitable for harsh high-salt mother liquor conditions with high silicon residue and high calcium and magnesium impurities.

[0032] As can be seen from the description of Examples 1 to 3 above, the application of the high-salt wastewater free silica targeted adsorbent of the present invention is for chemical high-salt wastewater with a total salt mass fraction greater than 20%, and for deep targeted adsorption and removal of neutral molecular free monosilicic acid H4SiO4 in the effluent of chemical precipitation pretreatment; after adsorption saturation, reversible desorption and regeneration are achieved by using a low-concentration sodium hydroxide solution at room temperature, such as 0.5-1.5% by mass percentage, and it can be recycled multiple times after rinsing.

[0033] The aforementioned high-salt chemical wastewater refers to one or more of the following: membrane concentrate, evaporation crystallization mother liquor, and reactor residue with a total salt mass percentage greater than 20% in the coal chemical, petrochemical, hydrometallurgical, and fine chemical industries.

[0034] Comparative Example 1 (boron-free multi-level porous carbon support, with all other processes identical to Example 1): This comparative example uses the exact same molten salt-assisted molding, high-temperature carbonization, magnesium aluminum hydroxyl oxide loading, and catechol functional group grafting preparation process as Example 1. The only difference is that boric acid was not added for in-situ boron doping of the carbon framework, resulting in an unmodified pure hierarchical porous carbon-based composite adsorbent material. Parallel adsorption tests were conducted using a real industrial high-salt mother liquor with 54% total salt and high calcium and magnesium impurities. The results show that the surface of the unmodified pure carbon support lacks controllable Lewis acid sites, which cannot suppress the non-specific adsorption behavior of calcium and magnesium ions in the high-salt system. A large number of calcium and magnesium impurity ions preferentially occupy the effective active sites on the material surface and within the pores, severely shielding the monosilicic acid adsorption channels and causing a significant decrease in adsorption capacity. Compared with Example 1 of this invention, the monosilicic acid adsorption capacity of Comparative Example 1 is significantly reduced, with the overall removal rate of free silicon only maintained at 35%–40%, indicating extremely poor deep treatment capability. Meanwhile, during recycling, calcium and magnesium scale easily deposits and accumulates on the material surface. After five regeneration cycles, the adsorption capacity decays by more than 35%, and the salt interference resistance and cycle stability deteriorate significantly. This comparative example fully demonstrates that the in-situ boron doping modification technology of the framework in this invention can precisely control the acid-base properties of the carbon matrix surface, fundamentally suppressing the competitive adsorption of impurity ions in high-salt systems. This is one of the core technologies for adapting the material of this invention to ultra-high salt conditions and achieving efficient and stable silicon removal.

[0035] Comparative Example 2 (grafting without catechol-targeted functional groups, all other processes are completely consistent with Example 1): This comparative example fully retains the boron-doped hierarchical porous carbon matrix structure and the pore-confined magnesium aluminum hydroxyl oxide loading process of Example 1, only omitting the core step of silane liquid-phase reflux grafting of catechol-targeting functional groups. This allows the material to achieve silicon removal solely through ligand exchange of magnesium aluminum hydroxyl oxides. All other preparation parameters, testing conditions, and experimental settings are completely consistent with Example 1. Parallel experimental results show that relying solely on the ligand exchange adsorption mechanism of metal hydroxyl oxides only has limited removal capacity for high-concentration free silicon. For low-concentration residual neutral monosilicic acid after chemical precipitation pretreatment, it lacks specific molecular recognition and targeted capture capabilities, resulting in severely insufficient adsorption selectivity and slow adsorption kinetics. After steady-state adsorption treatment, the residual free silicon in the effluent can only be reduced to 34–36 mg / L, failing to meet the requirements for industrial deep polishing. Furthermore, under harsh conditions of low silicon and high salt, the adsorption equilibrium time is significantly prolonged, and the industrial treatment efficiency is significantly reduced. A precise comparison with the performance data of Example 1 confirms that the catechol-targeting functional group introduced in this invention can form a specific chelation coordination with the cis-hydroxyl group of monosilicic acid, effectively making up for the technical shortcomings of poor selectivity of single metal sites and insufficient deep removal capability of low-concentration silicon. It is a key modification method to achieve precise removal of trace residual silicon in high-salt mother liquor and improve the adsorption selectivity of materials.

[0036] Comparative Example 3 (using ordinary disordered activated carbon as a carrier to replace the ordered hierarchical porous carbon of this invention, with the remaining processes being completely consistent with Example 1): This comparative example uses commercially available, disordered activated carbon as the carrier, completely replacing the ordered mesoporous-microporous hierarchical porous carbon matrix prepared by the potassium chloride-zinc chloride binary eutectic salt molten salt method proprietary in this invention. It strictly follows the complete set of process parameters from Example 1, loading equal amounts of boron doping modification, magnesium aluminum hydroxyl oxide active sites, and catechol targeting functional groups, ensuring that all preparation conditions, raw material ratios, and testing conditions are completely consistent except for the carrier structure. Experimental results show that ordinary activated carbon has inherent defects such as disordered pore structure, chaotic pore size distribution, lack of regular interconnected channels, and lack of spatial confinement effect. The loaded magnesium aluminum hydroxyl oxide nanoparticles cannot be uniformly dispersed and are prone to large-scale aggregation and stacking on the carrier surface and at the pore openings, directly causing problems such as pore blockage, failure of active site embedding, and obstruction of mass transfer channels. The initial adsorption capacity of the comparative material was significantly lower than that of the present invention, and obvious pore blockage and active site deactivation were observed after a single adsorption. After five dilute alkali regeneration cycles, the adsorption capacity decay rate exceeded 45%. It also exhibited problems such as severe powder loss, rapid fixed-bed flux decline, and poor operational stability, making it completely unsuitable for industrial continuous high-salt mother liquor polishing processes. This comparative example fully verifies that the ordered multi-level porous carbon matrix of the binary eutectic salt molten salt method used in the present invention possesses a unique spatial confinement effect and interconnected mass transfer channels, which can effectively inhibit the agglomeration and failure of the metal active phase, ensuring stability throughout the adsorption, mass transfer, and regeneration processes. This is the core structural basis for the excellent cycle stability and industrial application value of the material of the present invention.

[0037] In summary, this invention designs a multi-level porous composite adsorbent with high resistance to salt interference, high adsorption capacity, and high targeting selectivity. Using ordered mesoporous-microporous multi-level porous carbon prepared by the molten salt method as the functional matrix, this invention employs a potassium chloride-zinc chloride binary eutectic salt system, which has unique technical advantages compared to traditional organic soft template methods, high-melting-point carbonate molten salt systems, and multi-component mixed salt systems. The system has a minimum eutectic point of approximately 220°C, extremely low melting point, low high-temperature liquid phase viscosity, and a wide thermal stability window. Within the carbonization temperature range of 550–650°C, it can maintain a stable and homogeneous liquid phase throughout the process, without thermal decomposition or oxidation ablation risks. Furthermore, the system has simple components, controllable proportions, no high-valence impurity ion interference, and excellent process repeatability. Compared to traditional soft template methods, which require secondary KOH activation to form hierarchical pores and are prone to mesopore collapse, high-melting-point carbonate systems at this temperature are only semi-solid, with uneven pore formation and poor orderliness. Furthermore, quaternary mixed salt systems are prone to solid-liquid phase separation and poor pore formation uniformity. This invention utilizes a chloride eutectic salt that can uniformly wet the intermolecular spaces of carbon precursor molecules. Relying on the liquid-phase site-occupying template effect and the dual mechanism of zinc ion Lewis acid catalytic pore expansion, it simultaneously constructs a well-ordered mesopore and micropore hierarchical structure in situ in one step, without the need for secondary strong base activation. This results in excellent pore connectivity, concentrated pore size distribution, and extremely strong structural stability. Simultaneously, the low-temperature stable liquid-phase molten salt environment ensures uniform penetration and bonding of boron heteroatoms to the carbon framework, effectively improving boron doping uniformity and framework stability, and precisely controlling the Lewis acidity of the carbon matrix. This invention achieves in-situ boron doping of the carbon matrix framework through high-temperature carbonization, precisely controlling the Lewis acidity of the carbon material surface and effectively suppressing the non-specific competitive adsorption of calcium and magnesium impurity ions in high-salt systems. It utilizes the ordered interconnected pore structure of hierarchical carbon to achieve a spatial confinement effect, growing magnesium aluminum hydroxyl oxide nano-active sites in situ within the mesoporous channels, serving as the main capacity sites for monosilicic acid adsorption. Simultaneously, it anchors catechol-targeted coordination functional groups on the inner and outer surfaces of the carbon material pores through silane covalent grafting, achieving precise identification and deep removal of low-concentration residual molecular monosilicic acid.

[0038] This invention achieves spatial partitioning of different functional sites and their respective functions through a three-in-one structural synergistic design of matrix heteroatom modification, pore confinement loading, and surface targeted functional group grafting. This overcomes the problems of single function and prominent performance shortcomings of traditional materials. The synergistic effect of each structural feature is achieved by working together to obtain the high salt interference resistance, high capacity, and high selective silicon removal effect that cannot be achieved by a single modified material.

[0039] The boron heteroatoms are doped into carbon in the form of skeletal substitution. The conjugated framework, with boron atoms having 3 valence electrons, is less than the 4 valence electrons of carbon atoms. After boron replaces carbon atoms in the carbon ring, it creates electron-deficient sites in the carbon conjugated π system, endowing the carbon matrix with Lewis acid sites. These Lewis acid sites can accept lone pairs of electrons provided by oxygen atoms in silica molecules, strengthening the coordination interaction between the carbon matrix and free silicon species, improving the affinity of the adsorbent material for silica species in water, and simultaneously regulating the hydrophilicity of the carbon surface, which is beneficial for the mass transfer and diffusion of high-salinity wastewater within the pores.

[0040] Specifically, the synergistic mechanism of the adsorbent of this invention is as follows: the boron-doped hierarchical porous carbon matrix constructs a through-hole structure with high specific surface area and low mass transfer resistance. At the same time, the boron heteroatoms in the framework regulate the acidic sites on the surface, significantly weakening the competitive adsorption of calcium and magnesium ions in high-salt systems, fundamentally improving the material's resistance to high salt and impurity interference. The magnesium aluminum hydroxyl oxide nano-active phase grown in the confined pores specifically captures neutral molecular monosilicic acid through the metal hydroxyl ligand exchange reaction, providing the main adsorption capacity for the adsorption process. Meanwhile, the spatial confinement of the ordered pores completely solves the engineering defects of traditional powdered hydrotalcite, such as easy agglomeration, easy loss, and inapplicability to fixed-bed processes. The catechol functional groups covalently grafted on the pore wall can form specific chelation coordination with the cis-hydroxyl groups of monosilicic acid, achieving precise targeted removal of low-concentration residual free silicon, and greatly improving the adsorption selectivity and deep treatment capability under high-salt and low-silicon conditions.

[0041] This invention provides a standardized and industrially replicable preparation process adapted to this composite adsorbent. The overall process employs an integrated workflow of "molten salt-assisted precursor mixing and curing – high-temperature molten salt carbonization for pore formation and in-situ boron doping – pore-confined in-situ precipitation loading of active sites – low-temperature liquid-phase targeted functional group grafting – washing, drying, and purification." The steps are logically strictly interdependent, constructing a multi-level porous structure, boron-modified sites, metal active sites, and targeted recognition sites step-by-step. Each step has a clear division of labor and works synergistically. The complete preparation steps are as follows: The first step involves the preparation of a boron-doped hierarchical porous carbon matrix using molten salt assistance. Resorcinol and formaldehyde were selected as the carbon source, boric acid as the boron doping modifier, and a potassium chloride-zinc chloride binary eutectic salt as the pore-forming system. The carbon source, boron dopant, and eutectic salt were uniformly mixed in a specific ratio. An ethanol-water mixture and a small amount of ammonia were used to regulate the crosslinking environment. After constant-temperature stirring and curing, the mixture was placed under a nitrogen atmosphere for high-temperature carbonization. Utilizing the eutectic salt liquid-phase template and Lewis acid catalytic effect, a mesoporous-microporous hierarchical structure was constructed in one step, simultaneously achieving in-situ uniform doping of boron heteroatoms into the carbon framework. After carbonization, the matrix was thoroughly washed with water to remove salt and dried to obtain a well-structured and uniformly doped boron-doped hierarchical porous carbon matrix.

[0042] The second step involves pore-confined loading of magnesium aluminum hydroxyl oxides, specifically, pore-confined loading of magnesium aluminum hydroxyl oxide active sites. The prepared boron-doped hierarchical porous carbon is uniformly dispersed in a mixed salt solution of magnesium nitrate and aluminum nitrate. The pH of the system is precisely controlled by adding an alkaline solution dropwise, and an in-situ co-precipitation reaction is carried out under mild water bath conditions. Utilizing the spatial confinement effect of the carbon material's interconnected mesoporous channels, a nanoscale magnesium aluminum hydroxyl oxide active phase is generated in situ within the channels, effectively preventing the aggregation, stacking, and pore blockage of active particles. After settling, filtration, and washing, a supported carbon-based intermediate is obtained.

[0043] The third step involves covalently grafting catechol functional groups onto the pores, a process known as targeted functional group covalent grafting modification. Using a catechol-based silane coupling agent as the targeted modification reagent, a low-temperature liquid-phase reflux reaction is carried out on the carbon-based intermediate loaded with active sites in an anhydrous ethanol inert system. Through the covalent bonding effect of silane hydrolysis and condensation, catechol coordination functional groups are uniformly grafted onto the inner and outer surfaces of the carbon material pores. Finally, after multiple alcohol and water washes to remove unreacted impurities, and constant-temperature drying, the finished high-salt wastewater free silicon targeted adsorbent is obtained.

[0044] This invention employs a step-by-step preparation process with precise and controllable structure. First, a stable multi-level porous carbon framework is formed and boron doping modification is completed to ensure the material's basic resistance to salt interference. Then, the main adsorption active sites are constructed inside the pores to fully utilize the confinement advantages of the pores. Finally, organic targeted functional groups are grafted at low temperature to avoid the destruction of the organic coordination structure by high-temperature processes. This effectively avoids the defects of traditional composite modification processes, such as group failure, pore blockage, aggregation of metal active phase, and unstable performance.

[0045] This invention establishes a standardized process system that is controllable, highly repeatable, and suitable for both laboratory preparation and industrial mass production by precisely defining key process conditions such as raw material ratios, reaction temperatures, reaction times, system pH, and protective gas parameters for each preparation step. The entire process features precise formulations, conventional equipment, and controllable parameters, effectively solving the industry problems of ambiguous processes, poor batch stability, and difficulty in mass production in existing composite adsorbent material preparation methods.

[0046] Compared with existing technologies, the core improvements and corresponding technical effects of the overall technical solution of this invention are as follows: This invention breaks through the performance bottleneck of traditional single silicon removal materials. Through integrated structural design, it maintains high adsorption capacity, high selectivity and excellent recycling performance for neutral molecular monosilicic acid under the harsh conditions of industrial high-salt wastewater with a total salt content of more than 20% and the coexistence of high calcium and magnesium impurities. It can be stably applied to the deep polishing and silicon removal process after chemical precipitation pretreatment, effectively solving the problem of silicon scale deposition in high-salt wastewater zero-discharge systems, and significantly improving the operational stability and effluent quality of high-salt wastewater treatment systems.

[0047] The adsorbent of this invention possesses excellent reversible adsorption and dilute alkali regeneration cycle performance. The complete desorption and regeneration process and mechanism after adsorption saturation are as follows: The adsorption of neutral monosilicic acid by the adsorbent of this invention is a reversible coordination adsorption, without irreversible chemical bonding or permanent site poisoning. Industrial regeneration involves static soaking in a 0.5–1.5 wt% room temperature dilute sodium hydroxide aqueous solution for 20–30 minutes, followed by dynamic countercurrent rinsing. The regeneration time is controlled at 30–60 minutes. After regeneration, the adsorbent is rinsed with clean water until neutral and can then be reused.

[0048] The aforementioned desorption mechanism includes a dual synergistic dissociation effect: firstly, ligand exchange desorption at the active sites of magnesium aluminum hydroxyl oxides, resulting in a large amount of desorption under weakly alkaline conditions. The invention employs two main methods: First, it replaces silicate ions adsorbed on the metal surface, disrupting the metal-silicic acid coordination structure and achieving rapid desorption and regeneration of the main adsorption capacity sites. Second, it regenerates through the chelation and dissociation of catechol-targeted functional groups. A weakly alkaline environment breaks the specific chelation between catechol and the cis-hydroxyl groups of monosilicic acid, completely releasing the targeted, low-concentration residual silicon components. Simultaneously, the boron-doped carbon matrix of this invention provides a stable framework, and the hierarchical porous carbon structure is resistant to alkaline corrosion. Mild, dilute alkaline regeneration conditions do not damage the carbon pore structure, do not cause the removal of boron heteroatoms, and do not deplete the surface organic targeted functional groups, ensuring the stability of the material's structure and performance after multiple cycles. The high-concentration silicon-rich regenerated liquid generated by desorption can be pumped into the sludge settling tank of the upstream chemical precipitation desiliconization process for centralized treatment, achieving a closed-loop process with no secondary pollution, fully meeting the requirements for continuous and stable industrial operation.

[0049] The raw material system, equipment composition, complete process flow, and core control parameters used in this invention are as follows: The raw material formulation system includes a carbon source, a potassium chloride-zinc chloride composite eutectic salt, a doping modifier, a metal precursor, a targeted modifier, and a solvent system. The eutectic salt is composed of potassium chloride and zinc chloride in a mass ratio of 1:1 to 2:1. The proportions of each component are strictly adapted to the material structure forming and performance control requirements: the mass ratio of eutectic salt to carbon source is 3 to 6:1, the molar ratio of formaldehyde to resorcinol is 2.0 to 2.5:1, the volume ratio of ethanol to water is 1:1 to 1.5:1, and the ammonia dosage is... The total system volume is 2%–5%, ensuring the formation of an ordered hierarchical porous structure using the molten salt method; the mass ratio of boric acid to resorcinol is 0.15–0.4:1, precisely controlling the boron doping amount to 1.5–4.0 at.%; the molar ratio of magnesium-aluminum metal precursors is 2:1–3:1, and the mass ratio of metal salt to carbon matrix is ​​0.3–0.6:1, ensuring uniform loading of the nano-active phase within the pores; the mass ratio of catechol-based silane coupling agent to carbon matrix is ​​0.15–0.3:1, ensuring effective grafting of targeted functional groups.

[0050] The preparation equipment includes a constant temperature mixing reaction chamber, a programmable temperature tubular atmosphere furnace, a digital display constant temperature water bath stirrer, a precision pH meter, a vacuum drying oven, a high-speed centrifuge, a standard sieve separator, and a nitrogen supply and purification system. This allows for precise environmental control at each stage, ensuring batch-to-batch stability. The core process parameters are strictly limited as follows: molten salt mixing temperature 80–120℃, mixing time 2–4 h; curing temperature 85–105℃, time 12–20 h; carbonization temperature 550–650℃, heating rate 2–5℃ / min, nitrogen flow rate 80–120 mL / min; metal precipitation pH = 9.0–10.0, reaction temperature 40–60℃, reaction time 40–80 min; silane reflux temperature 65–80℃, reflux time 4–7 h. All parameter ranges perfectly match the scope of the claims.

[0051] The above description is merely a specific summary of the invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the invention, and these improvements and modifications should also be considered within the scope of protection of the invention.

Claims

1. A targeted adsorbent for free silica in high-salt wastewater, characterized in that: The adsorbent uses ordered mesoporous-microporous hierarchical porous carbon prepared by molten salt method as the matrix; the carbon matrix framework is in-situ doped with boron heteroatoms, with a boron doping amount of 1.5 to 4.0 at.%; magnesium aluminum hydroxy oxide active sites are confined within the channels of the hierarchical porous carbon, and the magnesium-aluminum molar ratio in the magnesium aluminum hydroxy oxide is 2:1 to 3:1; the inner and outer surfaces of the channels of the hierarchical porous carbon are anchored to catechol targeted coordination functional groups through silane covalent grafting.

2. The targeted adsorbent for free silica in high-salt wastewater according to claim 1, characterized in that: The ordered mesoporous-microporous hierarchical carbon has a mesopore diameter of 2–6 nm, a micropore diameter of 0.4–0.8 nm, and a BET specific surface area of ​​1200–1800 m² / g.

3. The targeted adsorbent for free silica in high-salt wastewater according to claim 1, characterized in that: The boron heteroatoms exist as skeletal substituted boron, which are used to regulate the Lewis acidity of the carbon matrix and suppress the non-specific competitive adsorption of calcium and magnesium ions in high-salt systems.

4. The targeted adsorbent for free silica in high-salt wastewater according to claim 1, characterized in that: The magnesium aluminum hydroxy oxide grows confined within the mesoporous channels, without blocking the microporous adsorption channels, and provides ligand-exchangeable monosilicic acid adsorption active sites; the catechol functional groups are distributed on the pore wall surface, which are used to form specific chelation recognition of molecular monosilicic acid.

5. A method for preparing a high-salinity wastewater free silica targeted adsorbent as described in claim 1, characterized in that: Includes the following steps: (1) Molten salt-assisted preparation of boron-doped hierarchical porous carbon precursor: A composite eutectic salt formed by potassium chloride and zinc chloride is used as the molten salt system. The mass ratio of potassium chloride to zinc chloride in the eutectic salt is 1:1 to 2:

1. Resorcinol-formaldehyde is used as the carbon source and boric acid is used as the boron doping source. After uniform mixing, the mixture is carbonized by high temperature molten salt and cured at constant temperature to obtain boron-doped ordered mesoporous-microporous hierarchical porous carbon. (2) Pore confined loading of magnesium aluminum hydroxy oxides: The boron-doped ordered mesoporous-microporous hierarchical carbon obtained in step (1) is dispersed in a mixed solution of magnesium salt and aluminum salt, and magnesium aluminum hydroxy oxide nano-active phase is generated in confined inside the carbon channels by in-situ co-precipitation. (3) Covalent grafting of catechol functional groups into pores: The magnesium aluminum hydroxy oxide nano-active phase obtained in step (2) is dispersed in an organic solvent, and catechol groups are grafted by liquid phase reflux using a silane coupling agent bridging method, so that the catechol groups are fixed on the inner and outer surfaces of the pores. After washing and drying, the finished product high-salt wastewater free silicon targeted adsorbent is obtained.

6. The method for preparing the high-salinity wastewater free silica targeted adsorbent according to claim 5, characterized in that: The mass ratio of boric acid to resorcinol in step (1) is 0.15 to 0.4:1, the carbonization temperature is 550 to 650°C, and the temperature is maintained in a nitrogen atmosphere for 2 to 3 hours.

7. The method for preparing the high-salinity wastewater free silica targeted adsorbent according to claim 5, characterized in that: The magnesium salt mentioned in step (2) is magnesium nitrate, the aluminum salt is aluminum nitrate, the pH of the mixed solution is controlled at 9.0 to 10.0, the in-situ coprecipitation reaction temperature is 40 to 60°C, and the reaction time is 40 to 80 min.

8. The method for preparing the high-salinity wastewater free silica targeted adsorbent according to claim 5, characterized in that: The organic solvent in step (3) is anhydrous ethanol, the silane coupling agent is catechol-based silane coupling agent, the liquid phase reflux temperature is 65-80℃, the reflux time is 4-7h, and the grafting reaction is carried out in anhydrous ethanol system.

9. An application of the free silica targeted adsorbent for high-salinity wastewater as described in claim 1, characterized in that: This product is used for high-salt chemical wastewater with a total salt content greater than 20%. It is designed for deep targeted adsorption and removal of neutral molecular free monosilicic acid (H4SiO4) in the effluent from chemical precipitation pretreatment. After adsorption saturation, it is reversibly desorbed and regenerated using a low-concentration sodium hydroxide solution at room temperature. After rinsing, it can be recycled multiple times.

10. The application of the high-salinity wastewater free silica targeted adsorbent according to claim 9, characterized in that: The high-salt chemical wastewater mentioned refers to one or more of the following: membrane concentrate, evaporation crystallization mother liquor, and reactor residue with a total salt mass percentage greater than 20% in the coal chemical, petrochemical, hydrometallurgical, and fine chemical industries.