CN116655128A

CN116655128A - Low-pressure reverse osmosis defluorination method for high-fluorine underground water

Info

Publication number: CN116655128A
Application number: CN202310454772.1A
Authority: CN
Inventors: 李都望; 史俊; 邓慧萍; 方仁生; 黄有文
Original assignee: Jiangxi Water Group Co ltd; Tongji University
Current assignee: Jiangxi Water Group Co ltd; Tongji University
Priority date: 2023-04-25
Filing date: 2023-04-25
Publication date: 2023-08-29
Anticipated expiration: 2043-04-25
Also published as: CN116655128B

Abstract

The invention discloses a low-pressure reverse osmosis defluorination method for high-fluorine groundwater, which belongs to the field of water treatment, wherein a scale inhibitor and an adsorbent are added into the high-fluorine groundwater to be treated, and the scale inhibitor is polyaspartic acid, polyepoxysuccinic acid and low-pressure reverse osmosis membrane which is polyamide membrane under the conditions of 7.5-8.5 bar pressure, 7.5-8.5 pH value and 15-30℃ temperatureChitosan and AlCl ₃ The adsorbent is Mg-Al-Ce interlayer metal oxide, and the addition amount is 1-6 g/L. The invention takes typical pollutant fluoride in groundwater as a research object, takes a low-pressure reverse osmosis system as a treatment process, explores the defluorination rule of the low-pressure reverse osmosis system, and aims at CaF ₂ The membrane pollution and the high-fluorine concentrated water disposal problem provide targeted countermeasures, and have certain reference value for basic research and practical application of a low-pressure reverse osmosis system.

Description

Low-pressure reverse osmosis defluorination method for high-fluorine underground water

Technical Field

The invention relates to a method for treating groundwater, in particular to a low-pressure reverse osmosis defluorination method for high-fluorine groundwater, and belongs to the field of water treatment.

Background

Fluorine is the most abundant halogen on earth and is widely distributed in air, soil and water environments. Fluorine is the 13 th element in crust, and the mass ratio is 0.059%. Fluorine is the most electronegative, nonmetallic and reactive element, so that in nature fluorine exists only in inorganic fluoride and a few organic fluorides in-1 valence state, and fluorine exists in nature mainly in fluoride salt in minerals such as fluorite (CaF) ₂ ) Cryolite (Na) ₃ AlF ₆ ) Fluorapatite (Ca) ₅ (Cl,F,OH)(PO ₄ ) ₃ ) Etc., only small amounts of natural organofluorides exist, most organofluorides being synthesized artificially. Fluoride produced and manufactured by man is an important source of fluorine in groundwater.

If the water with the excessive fluoride content is drunk for a long time, the water can have adverse effects on various aspects of teeth, bones, nervous system, reproductive system and the like of a human body, and can cause acute diseases and accumulated harm.

According to the different treatment process principles, the drinking water defluorination technology can be divided into a precipitation method, an adsorption method, an electrochemical method, an ion exchange method, a membrane technology and the like, but has the limitation conditions of high operation management requirements, complex pretreatment process, higher energy consumption and cost and the like.

Reverse Osmosis (RO) is the most important and most widely used technology in desalination of sea water and salt water, and a reverse osmosis system consists of four parts: a pretreatment part, a pressurizing part, a membrane element and a post-treatment part. The core of RO systems is a reverse osmosis membrane, which is a homogeneous, non-porous, polymeric composite membrane that acts as a semi-permeable barrier, allowing only water to permeate the membrane and reject solutes. The reverse osmosis membrane transmission mechanism is formed by combining various principles, including size exclusion and charge exclusion or dielectric exclusion, and the low-pressure reverse osmosis technology is widely applied to the fields including semiconductors, food processing, power generation, pharmacy, seawater desalination and the like, so that the problems in the treatment mode can be well solved.

The problems faced by the current low-pressure reverse osmosis technology mainly comprise three points: reducing energy consumption, membrane pollution and concentrated water treatment. With the advent of low-pressure reverse osmosis membranes and ultra-low pressure reverse osmosis membranes and the design and development of various energy recovery devices, the problem of energy consumption in the field of brackish water desalination has been gradually solved. The most limiting of reverse osmosis systems is membrane fouling and scaling. From the location of fouling, membrane fouling can be categorized into surface fouling and internal fouling. Reverse osmosis membranes act as a dense, non-porous membrane, with surface scaling occurring primarily. The types of membrane fouling can be largely classified into biofouling, organic fouling, inorganic fouling and colloidal fouling. Many types of pollution complex effects are most common in practical applications, including calcium salts, silica, organics and microbial scaling, but there is no viable and effective solution to this technical problem in the prior art.

Disclosure of Invention

In view of this, the present invention provides a low pressure reverse osmosis defluorination process for high fluorine groundwater to provide operations for defluorination of high fluorine groundwater in a low pressure reverse osmosis mode while providing a solution to the problem of membrane fouling.

In order to achieve the above purpose, the present invention adopts the following technical scheme:

adding a scale inhibitor and an adsorbent into high-fluorine groundwater to be treated, and passing through a low-pressure reverse osmosis membrane under the conditions of pressure of 7.5-8.5 bar, pH value of 7.5-8.5 and temperature of 15-30 ℃;

the low-pressure reverse osmosis membrane is a polyamide membrane;

the scale inhibitor is polyaspartic acid, polyepoxysuccinic acid, oligomeric chitosan and AlCl ₃ Any one or a mixture of a plurality of them;

the adsorbent is Mg-Al-Ce interlayer metal oxide, and the addition amount is 1-6 g/L.

Based on the technical scheme, the invention can also be improved as follows:

further, the scale inhibitor is 2.5mg/L polyaspartic acid, 2.5mg/L polyepoxysuccinic acid, 0.5mg/L oligomeric chitosan, 10mg/LAlCl ₃ Is a mixture of (a) and (b).

Further, the preparation method of the Mg-Al-Ce interlayer metal oxide comprises the following steps:

(1) Adding magnesium nitrate hexahydrate, aluminum nitrate nonahydrate, anhydrous cerium chloride and urea into deionized water, stirring at 25 ℃ for 30min, transferring into a high-pressure reaction kettle with a polytetrafluoroethylene lining, heating for reaction, naturally cooling to room temperature and ageing for 12h, centrifugally separating pale yellow precipitate, washing three times with deionized water and absolute ethyl alcohol respectively, drying and grinding to pale yellow powdery solid to obtain Mg-Al-Ce interlayer metal hydroxide;

(2) Transferring the prepared Mg-Al-Ce interlayer metal hydroxide into a 100mL quartz crucible, heating to 400 ℃ by a muffle furnace program, keeping the temperature rising speed at 10 ℃/min and 200 ℃ for 20min, heating to 400 ℃ and roasting for 4h, cooling to room temperature, and grinding to obtain the Mg-Al-Ce interlayer metal oxide

Further, the mol ratio of the magnesium nitrate hexahydrate, the aluminum nitrate nonahydrate and the anhydrous cerium chloride is 10:5:1, and the mol ratio of the urea to the magnesium nitrate hexahydrate is 10:1;

the deionized water was used in an amount of 70mL of deionized water per 6mmol of magnesium nitrate hexahydrate.

Further, the heating reaction is operated by heating to 120 ℃ for reaction for 12 hours;

further, the centrifugation was performed at 6000rpm for 5min.

Further, the drying operation is that the oven is heated to 80 ℃ for 4 hours.

The invention has the beneficial effects that typical pollutant fluoride in groundwater is taken as a research object, a low-pressure reverse osmosis system is taken as a treatment process, the defluorination rule of the low-pressure reverse osmosis system is explored, and CaF is treated ₂ The membrane pollution and the high-fluorine concentrated water disposal problem provide targeted countermeasures, and have certain reference value for basic research and practical application of a low-pressure reverse osmosis system.

Drawings

FIG. 1 is a SEM image of RES-HF-40;

FIG. 2 is a SEM image of RES-HR-40;

FIG. 3 is a SEM image of BW30 XFR;

FIG. 4 is an atomic force microscope image of a RES-HF-40 film;

FIG. 5 is an atomic force microscope image of a RES-HR-40 membrane;

FIG. 6 is an atomic force microscope view of a BW30XFR film;

FIG. 7 is a graph of RES-HF-40, RES-HR-40, BW30XFR film contact angle results;

FIG. 8 is a FTIR spectrum of three reverse osmosis membranes;

FIG. 9 is a graph of three membrane fluoride ion retention performance relationships;

FIG. 10 is a graph of the effect of pressure on RES-HF-40 low pressure reverse osmosis membrane fluoride rejection;

FIG. 11 is a graph of the effect of influent pH on fluoride retention;

FIG. 12 is a graph of the effect of temperature on RES-HF-40 membrane flux;

FIG. 13 is a graph of the effect of temperature on RES-HF-40 fluoride retention;

FIG. 14 is a graph of scale inhibition results for static scaling of three scale inhibitors;

FIG. 15 is a graph showing the effect of scale material content on static scaling of three scale inhibitors;

FIG. 16 is a graph of the effect of temperature on static fouling of three scale inhibitors;

FIG. 17 is a graph showing the scale inhibition effect of polyepoxysuccinic acid in dynamic experiments;

FIG. 18 is a graph showing the results of the scale inhibition effect of polyaspartic acid in dynamic experiments;

FIG. 19 is a graph showing the scale inhibition effect of dynamic experiments with chitosan oligosaccharide;

FIG. 20 is a graph of scale inhibition performance results for a formulated scale inhibitor;

FIG. 21 is a graph showing the effect of the ratio of Mg to Al on the adsorption capacity of the adsorbent;

FIG. 22 is a graph showing the effect of Ce proportion on the adsorption capacity of the adsorbent;

FIG. 23 is a graph showing the effect of urea addition on the adsorption capacity of the adsorbent;

FIG. 24 is a graph showing the effect of hydrothermal temperature on the adsorption capacity of the adsorbent;

FIG. 25 is a graph showing the effect of hydrothermal time on the adsorption capacity of the adsorbent;

FIG. 26 is a graph showing the effect of calcination temperature on the adsorption capacity of the adsorbent;

FIG. 27 is a graph showing the effect of aging time on the adsorption capacity of the adsorbent;

FIG. 28 is a graph showing the effect of initial fluoride concentration on the defluorination performance of an adsorbent material;

FIG. 29 is a graph showing the effect of pH on the defluorination efficacy of an adsorbent material;

FIG. 30 is a statistical graph of Zeta potential of an Mg-Al-Ce interlayer metal oxide;

FIG. 31 is a graph showing the effect of the amount of the additive on the fluoride removal rate and adsorption capacity of the adsorbent;

FIG. 32 is a graph showing the effect of regeneration conditions on the adsorption capacity of Mg-Al-Ce interlayer metal oxides.

Detailed Description

The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

In the examples below, the instruments and reagents involved are shown in tables 1-2 below.

TABLE 1 list of reagent and drug information

Reagent name	Purity of	Manufacturer' s
			Sodium fluoride	Analytical grade	Sigma-Aldrich
Anhydrous sodium carbonate	Analytical grade	Shanghai national pharmaceutical Congress chemical reagent Co., ltd
			Sodium bicarbonate	Analytical grade	Shanghai national pharmaceutical Congress chemical reagent Co., ltd
Anhydrous sodium sulfate	Analytical grade	Shanghai national pharmaceutical Congress chemical reagent Co., ltd
			Sodium chloride	Analytical grade	Shanghai national pharmaceutical Congress chemical reagent Co., ltd
Sodium nitrate	Analytical grade	Shanghai national pharmaceutical Congress chemical reagent Co., ltd
			Disodium hydrogen phosphate	Analytical grade	Shanghai national pharmaceutical Congress chemical reagent Co., ltd
Magnesium nitrate hexahydrate	Analytical grade	Shanghai national pharmaceutical Congress chemical reagent Co., ltd
			Aluminum nitrate nonahydrate	Analytical grade	Shanghai national pharmaceutical Congress chemical reagent Co., ltd
Cerium chloride	Analytical grade	Shanghai national pharmaceutical Congress chemical reagent Co., ltd
			Urea	Analytical grade	Shanghai national pharmaceutical Congress chemical reagent Co., ltd
Hydrochloric acid	Analytical grade	Shanghai national pharmaceutical Congress chemical reagent Co., ltd
			Sodium hydroxide	Analytical grade	Shanghai national pharmaceutical Congress chemical reagent Co., ltd
Absolute ethyl alcohol	Analytical grade	Shanghai national pharmaceutical Congress chemical reagent Co., ltd
			Polyaspartic acid	Analytical grade	SHANGHAI MACKLIN BIOCHEMICAL Co.,Ltd.
Polyepoxysuccinic acid	Analytical grade	Shanghai Ding Feng chemical technology Co., ltd
			Oligomeric chitosan	Analytical grade	Beijing Biotechnology Co., ltd
EDTA disodium salt	Analytical grade	Shanghai national pharmaceutical Congress chemical reagent Co., ltd
			Citric acid	Analytical grade	Shanghai national pharmaceutical Congress chemical reagent Co., ltd

Table 2 experimental apparatus and instrument

Example 1

Low pressure reverse osmosis membrane property comparison

Compared with Reverse Osmosis (RO) and Nanofiltration (NF), the low-pressure reverse osmosis technology has different material and structure, fluoride interception efficiency, interception mechanism, model and other aspects, and factors such as pressure, temperature, pH and the like have great influence on the membrane permeation flux and interception rate of the low-pressure reverse osmosis technology, and the low-pressure reverse osmosis technology is compared with the reverse osmosis technology, so that a commercial reverse osmosis membrane is selected: the ceramic BW30XFR and two low pressure reverse osmosis membranes: the Meinares-HF-40 and the Meinares-HR-40 are characterized and analyzed, and the influence of each influencing factor on the efficiency of intercepting fluoride is studied.

1. SEM characterization by scanning electron microscopy

All three commercially available membranes are composite polyamide membranes (TFC), the most widely used class of RO membranes. The polyamide membrane has an asymmetric structure and is composed of an ultrathin separating layer, a porous supporting layer (mainly polysulfone) and a non-woven fabric mechanical supporting layer, wherein the ultrathin separating layer mainly exerts the interception performance of the surface of the membrane, and the property and the structure of the separating layer obviously influence the interception capacity of the reverse osmosis membrane. The polyamide separating layer is formed by IP reaction and can be divided into two layers, the basic polyamide layer is a compact initial layer with high crosslinking degree formed by the reaction, and the top layer is a structure of ridges and valleys formed by nano foaming in the IP reaction.

To investigate the differences in microstructure and performance of low pressure reverse osmosis membranes and reverse osmosis membranes, three commercially available membranes were SEM characterized, the characterization results are shown in figures 1-3.

As can be seen from FIGS. 1-3, RES-HF-40 and RES-HR-40 belong to the same manufacturer's commercial membranes, and have similar membrane surface structures. The size of the ridge valley structure of the RES-HF-40 membrane is about 426nm, the size of the backbone structure of the RES-HR-40 membrane is about 293nm, the thickness of the polyamide membrane at the top is smaller due to the larger backbone structure of the RES-HF-40, the membrane flux is higher than that of the RES-HR-40, the surface of the RES-HR-40 membrane is more compact, and the interception performance is stronger. BW30XFR and RES series have different membrane surface morphology, the 'ridge' and 'valley' structures of the surface are more abundant, the crosslinking degree of the membrane surface is higher, and the membrane interception capability is stronger. Characterization results show that the low-pressure reverse osmosis membrane polyamide film has smaller thickness and the 'ridge valley' structure has larger relative size.

2. Atomic force microscope AFM characterization

Atomic force microscopy AFM was able to characterize film surface roughness, three film AFM characterizations are shown in fig. 4-6.

As can be seen from FIGS. 4-6, RES-HF-40 root mean square roughness R _q An arithmetic average roughness R of 73.9nm _a 55.6nm and a maximum peak-to-trough height of 801nm. RES-HR-40 root mean square roughness R _q At 76nm, arithmetic average roughness R _a 61.4nm and a maximum peak-to-trough height of 553nm. The BW30XFR root mean square roughness is 101nm, the arithmetic average roughness is 75.8nm, and the maximum peak-to-trough height is 1357nm. The AFM data result is consistent with the scanning electron microscope result, and compared with the RES-HR-40, the RES-HF-40 film has larger ridge-valley structure and higher surface roughness. BW30XFR film has high surface cross-linking degree and maximum film roughness.

3. Contact angle characterization

The contact angle can characterize the hydrophilicity and hydrophobicity of the surface of the membrane, and has strong correlation with membrane flux and interception performance. Three commercially available film contact angle characterizations are shown in fig. 7.

As can BE seen from FIG. 7, the RES-HF-40 film had an average contact angle of 18.27, the RES-HR-40 film had an average contact angle of 33.64, and the BE30XFR film had an average contact angle of 35.72. The hydrophilicity was ordered as RES-HF-40> RES-HR-40> BE30XFR. The film roughness order is BE30XFR > RES-HF-40> RES-HR-40. Roughness has a certain influence on the hydrophilicity of a solid surface, and in general, the greater the roughness of the solid surface, the greater the hydrophilicity of the hydrophilic surface. The RES series film meets the above rules, while BW30XFR roughness is the greatest but the lowest hydrophilic, mainly because the surface of BW30 film has a complex structure, resulting in higher roughness, but the polyamide layer thickness is thicker than the RES series film, resulting in reduced water wettability and permeability, larger film contact angle, and weaker relative hydrophilicity. Characterization demonstrated that low pressure reverse osmosis increases membrane flux by increasing surface hydrophilicity.

4. Characterization by Infrared Spectroscopy FTIR

The characterization results are shown in FIG. 8, wherein the upper graph is RES-HR-40, the middle graph is BW30XFR, and the lower graph is RES-HF-40.

As can be seen from FIG. 8, the three reverse osmosis membranes have similar infrared characteristic patterns, and the RES series are found in the infrared fingerprint region (1300-400 cm ^-1 ) The high similarity is maintained, which shows that the two components are very similar, but different from BW30XFR exists in the fingerprint area, which shows that the film composition structure and film making process are different between different manufacturers. FTIR spectra of all three membranes showed characteristic absorption peaks of polysulfone and polyamide, A ₁₅₄₀ /A ₁₂₄₅ The ratio of (2) is as follows: BW30XFR>RES-HF-40>RES-HR-40. This is consistent with AFM characterization results, R _max The value size ordering is: BW30XFR>RES-HF-40>RES-HR-40, demonstrated BW30XFR polyamide separation layer was thickest and RES-HF-40 was thicker than RES-HR-40 due to its larger "ridge valley" structure. The FTIR characterization result and the SEM characterization result both show that the low-pressure reverse osmosis membrane polyamide separation layer is thinner, the membrane permeability is stronger, and the membrane flux is larger.

5. Low pressure reverse osmosis membrane performance comparison

Three relationships between membrane fluoride ion interception efficiency and operating pressure were explored, and experimental conditions were as follows: the initial fluoride concentration was 10mg/L at 25℃and pH=7, and the experimental results are shown in FIG. 9.

As can be seen in fig. 9, the three reverse osmosis membranes differ in fluoride rejection, with BW30XFR rejection being the strongest and RES series low pressure reverse osmosis membranes having lower rejection than BWXFR reverse osmosis membranes. With the increase of pressure, the fluoride retention rate of the three membranes is improved, and the retention rate of BW30XFR fluoride is increased from 95.72% to 97.26%. The RES series low-pressure reverse osmosis membrane has limited fluoride ion removal rate under lower pressure, is lower than 80%, and has the RES-HF-40 fluoride retention rate increased from 68.00% to 92.06% and the RES-HR-40 fluoride retention rate increased from 78.53% to 93.12% along with the increase of the pressure. The BW30XFR reverse osmosis membrane has strong fluoride retention efficiency and is less influenced by process parameters, and the fluoride retention rate can be stably maintained above 95%. The RES series low-pressure reverse osmosis membrane is greatly influenced by technological parameters such as pressure and the like. From the film microstructure characterization analysis, the BW30XFR reverse osmosis membrane polyamide separation layer has large thickness, and the ridge valley structure on the surface of the film has high crosslinking degree, so that fluoride can be effectively trapped. The RES series low-pressure reverse osmosis membrane polyamide separation layer is relatively thin, the surface hydrophilicity of the membrane is stronger, the water flux is higher, and the fluoride interception capability is relatively weak. The low-pressure reverse osmosis membrane has unique interception characteristics and application scenes.

Example 2

The low-pressure reverse osmosis defluorination method for northwest high-fluorine underground water comprises the following steps: adding a scale inhibitor and an adsorbent into high-fluorine underground water to be treated, and passing through a low-pressure reverse osmosis membrane under the conditions of pressure of 7.5-8.5 bar, pH value of 7.5-8.5 and temperature of 15-30 ℃;

further, the low-pressure reverse osmosis membrane is a polyamide membrane, and the model of the low-pressure reverse osmosis membrane is as shown in example 1;

the scale inhibitor is polyaspartic acid, polyepoxysuccinic acid, oligomeric chitosan and AlCl ₃ Any one or a mixture of more of the above, preferably, the scale inhibitor is polyaspartic acid 2.5mg/L, polyepoxysuccinic acid 2.5mg/L, chitosan oligomer 0.5mg/L, LAlCl 10mg/L ₃ Is a mixture of (a) and (b);

the adsorbent is Mg-Al-Ce interlayer metal oxide, the adding amount is 1-6 g/L, a simple hydrothermal synthesis method is adopted to synthesize the Mg-Al-Ce interlayer metal hydroxide precursor, and urea is used as a uniform precipitator.

The preparation method comprises the following steps: 6mmol of magnesium nitrate hexahydrate, 3mmol of aluminum nitrate nonahydrate, 0.6mmol of anhydrous cerium chloride and 60mmol of urea are added into 70mL of deionized water, the obtained clear solution is magnetically stirred for 30min at 25 ℃, transferred into a high-pressure reaction kettle with a 100mL stainless steel polytetrafluoroethylene lining, heated to 120 ℃ in an oven for reaction for 12h, naturally cooled to room temperature and aged for 12h. Centrifuging at 6000rpm for 5min to separate pale yellow precipitate, and washing with deionized water and absolute ethanol three times to remove impurities on the surface of the precipitate. The oven was heated to 80℃and dried for 4h. Grinding the mixture into light yellow powdery solid by a mortar to prepare Mg-Al-Ce interlayer metal hydroxide;

transferring the prepared Mg-Al-Ce interlayer metal hydroxide into a 100mL quartz crucible, heating to 400 ℃ by a muffle furnace program, keeping the temperature at the temperature of 200 ℃ for 20min at the temperature of 10 ℃/min, heating to 400 ℃ for roasting for 4h at the high temperature, cooling to room temperature, and grinding to finally obtain the Mg-Al-Ce interlayer metal oxide.

Example 3

Influence of operating pressure on fluoride removal efficacy

The influence of different pressures on the fluoride retention rate of the RES-HF-40 low-pressure reverse osmosis membrane is explored, and the experimental conditions are as follows: fluoride concentration was 1-10mg/L, temperature was 25deg.C, initial pH was adjusted to 7, and the reverse osmosis membrane was compacted with pure water for 1h before the experiment, as in example 2, and the results were shown in FIG. 10.

As can be seen from FIG. 10, the effect of the RES-HF-40 fluoride retention rate under pressure is remarkable, the pressure is increased from 2.5bar to 10bar, and the fluoride retention rate is increased from 68.00% to 92.06% under the concentration of 10mg/L fluoride in water. At a fluoride feed concentration of 2.5mg/L, the pressure increased from 2.5bar to 10bar and the fluoride retention increased from 58.42% to 70.86%. The greater the concentration of fluoride in the water, the more remarkable the effect of the retention rate under pressure.

Example 4

Effect of pH on fluoride removal efficacy

Under different pH conditions, the change condition of the RES-HF-40 interception rate and the change condition of the pH of the concentrated water fresh water are examined, and the experimental conditions are as follows: the initial concentration of fluoride was 10mg/L, the pressure was 7.5bar, the temperature was 25℃and the reverse osmosis membrane was compacted with pure water for 1 hour before the experiment, and the results were shown in FIG. 11 by referring to example 2.

As can be seen from fig. 11, the RES-HF-40 fluoride retention rate increases from 4 to 10 with the pH of the incoming water, the fluoride retention rate decreases first and then increases, there is a minimum value in the pH range of 5 to 6, the retention rate decreases from 77.66% to 72.05%, then increases rapidly to 95.13%, and at ph=8, the fluoride retention rate reaches 90.37%, and the RES-HF-40 fluoride retention rate is significantly affected by pH. The rejection rate remained at a higher level when pH > 8.

Example 5

Effect of temperature on fluoride removal efficacy

The change condition of the retention rate of RES-HF-40 and the change condition of the membrane flux under different temperature conditions are examined, and the experimental conditions are as follows: the initial concentration of fluoride was 10mg/L, the pressure was 7.5bar, the temperature was 15-45℃and the pH was adjusted to 7.0, the method was as described in example 2, and the reverse osmosis membrane was compacted with pure water at 10bar for 1 hour before the experiment, and the results were shown in FIGS. 12-13.

As can be seen from fig. 12, as the temperature increases, the RES-HF-40 membrane flux increases slowly, the membrane flux is less affected by temperature under low pressure conditions, the membrane flux is more affected by temperature under high pressure conditions, and the low pressure reverse osmosis membrane flux is more affected by temperature changes than the reverse osmosis membrane.

As shown in FIG. 13, the RES-HF-40 membrane fluoride retention rate was reduced from 83.99% to 78.33% as the temperature was increased from 15℃to 40℃and the increase in temperature was detrimental to fluoride retention by the polyamide membrane.

In conclusion, the higher the surface is, the smaller the influence of the increase of the membrane flux and the decrease of the fluoride retention rate caused by the temperature change is on the reverse osmosis membrane with higher fluoride retention rate, the higher the temperature has a certain negative influence on the RES-HF-40 membrane retention rate due to the limitation of the membrane structure, and the temperature of the system is controlled to be lower than 40 ℃ in practical application.

Example 6

Static scale inhibition experiment of scale inhibitor

To investigate the pair CaF of polyepoxysuccinic acid, polyaspartic acid and oligomeric chitosan ₂ The influence of scaling, a static scale inhibition experiment is carried out, and the operation of the static scale inhibition experiment is set by referring to the method for measuring the scale inhibition performance of a water treatment agent for calcium carbonate deposition (GB/T16632-2019).

250mL of deionized water was added to a 500mL Erlenmeyer flask, and a quantity of calcium chloride solution was added to give a calcium ion content of 200mg. Adding a certain amount of scale inhibitor to prepare a solution, and shaking uniformly. Ph=7 was adjusted. After shaking up, a certain amount of sodium fluoride solution (shaking with addition) was slowly added to make the fluoride ion content 190mg, diluted with water to 500mL, and shaking up. The above procedure was repeated in another 500mL Erlenmeyer flask, except that no scale inhibitor was added, as a blank comparative experiment.

The prepared experimental solution was heated in a constant temperature water bath at 25 ℃ for 10 hours and filtered while hot. And measuring the calcium ion content in the filtrate of the blank control experiment and the scale inhibitor experiment after the filtrate is cooled.

(1) Scale inhibition rate of static scale formation of three scale inhibitors

The addition amount influences the scale inhibition performance of the scale inhibitorThe addition amount of the scale inhibitor for static experiments can provide reference for dynamic scale inhibition experiments. The influence of the addition amount of the scale inhibitor on the scale inhibition rate in the static test is explored. The experimental conditions were as follows: ca (Ca) ²⁺ The content of F is 200mg ^- The content of the scale inhibitor is 190mg, the adding amount of the scale inhibitor is 2-20mg/L, and the result is shown in figure 14.

As can be seen from FIG. 14, in the static experiment, the scale inhibition effect of polyepoxysuccinic acid is best, the scale inhibition rate is increased along with the increase of the addition amount, and when the addition amount is 8mg/L, the scale inhibition rate reaches 90.43%. The polyaspartic acid has good scale inhibition effect, and the scale inhibition rate reaches 88.30% when the addition amount is increased to 8 mg/L. Oligomeric chitosan for CaF ₂ The scale formation also has a certain inhibition effect, the maximum scale inhibition rate under the experimental condition is 75.80 percent, and the scale inhibition rates of the three scale inhibitors are ordered as follows: polyepoxysuccinic acid>Polyaspartic acid>And (3) chitosan oligosaccharide.

(2) Effect of scale forming material content on static scale inhibition of three scale inhibitors

The scale inhibition rate of the static experiment has a certain reference to the dynamic experiment. The influence of the content of scaling substances on static scale inhibition of three scale inhibitors is explored. The experimental conditions were as follows: ca (Ca) ²⁺ Content of 400, 600, 800, 1000, 1200, 1500mg/L, F ^- The addition amount of the scale inhibitor is 15mg/L with the contents of 400, 500, 600, 800, 1000 and 1200mg/L, and the result is shown in figure 15.

As can be seen from FIG. 15, ca ²⁺ And F ^- The content significantly affects the scale inhibition performance of the scale inhibitor, and as the content of scale forming substances increases, the scale inhibition rate of the three scale inhibitors is reduced. When Ca is ²⁺ When the content is increased from 400mg/L to 1500mg/L, the scale inhibition rate of the polyepoxysuccinic acid is reduced from 91.2 percent to 54.2 percent, the scale inhibition rate of the polyaspartic acid is reduced from 88.9 percent to 46.7 percent, and the scale inhibition rate of the oligomeric chitosan is reduced from 79.5 percent to 30.9 percent. When F ^- When the content is increased from 400mg/L to 1200mg/L, the scale inhibition rate of the polyepoxysuccinic acid is reduced from 91.2 percent to 29.1 percent, the scale inhibition rate of the polyaspartic acid is reduced from 88.9 percent to 32.2 percent, and the scale inhibition rate of the oligomeric chitosan is reduced from 79.5 percent to 17.5 percent.

(3) Influence of temperature on static fouling of three scale inhibitors

As can be seen from fig. 16, the static scale inhibition rate of the three scale inhibitors decreases with increasing temperature. When the temperature is increased from 25 ℃ to 45 ℃, the scale inhibition rate of the polyepoxysuccinic acid is reduced from 95.2% to 82.4%, the scale inhibition rate of the polyaspartic acid is reduced from 90.9% to 75.7%, and the scale inhibition rate of the oligomeric chitosan is reduced from 69.5% to 62.5%. With the increase of the solution temperature, the thermal movement of ions and newly formed crystal nuclei is more intense, the collision probability is increased, the capability of continuously growing the newly formed crystalline substances or crystals is stronger, and the scale inhibition effect is weakened. On the other hand, with increasing temperature, caF ₂ K of (2) _sp Reduced, resulting in higher temperatures and CaF at the same level of fouling material ₂ The greater the scaling tendency, the poorer the scale inhibiting effect. At the same time, the temperature rise and the intense thermal movement can influence the scale inhibitor and Ca ²⁺ The chelating effect of the catalyst can reduce the scale inhibition rate.

Example 7

Dynamic scale inhibition experiment of scale inhibitor

The polyepoxysuccinic acid, the polyaspartic acid and the oligomeric chitosan show better CaF in static scale inhibition experiments ₂ The scale inhibition effect needs to be further verified, and the scale inhibition capability is more similar to that of the actual running condition, and the experimental conditions are as follows: initial concentration of fluoride 80mg/L, ca ²⁺ The initial concentration is 400mg/L, the adding amount of the scale inhibitor is 2.5mg/L, the running mode of completely refluxing fresh water and concentrated water is adopted, the pressure is 7.5bar, the temperature is 25 ℃, and the pH is regulated to 7.0. The reverse osmosis membrane was compacted with pure water at 10bar for 1 hour before the experiment, and the results are shown in FIGS. 17 to 20.

As can be seen from fig. 17, in the dynamic scale inhibition experiment, the polyepoxysuccinic acid has a good scale inhibition effect on the low pressure reverse osmosis system. When the adding amount of the polyepoxysuccinic acid is 5mg/L, the membrane flux is stable within 50min, and the membrane flux is 64.08L/(m) during the stable state ² H) the flux reduction was 38.77%. When the adding amount of the polyepoxysuccinic acid is 2.5mg/L, the membrane flux is stable within 60min, and the membrane flux is 44.18L/(m) during the stabilization ² H) the flux reduction was 57.78%. And when the polyepoxysuccinic acid scale inhibitor is added, the membrane flux is kept stable, and the fluctuation is smaller.

As can be seen from fig. 18, in the dynamic resistanceIn the scale experiment, the polyaspartic acid has good scale inhibition effect on a low-pressure reverse osmosis system. When the adding amount of the polyaspartic acid is 5mg/L, the membrane flux is stable within 20min, and the membrane flux is 88.08L/(m) during the stable process ² H) the flux reduction was 20.81%. When the adding amount of the polyaspartic acid is 2.5mg/L, the membrane flux is stable within 30min, and the membrane flux is 56.85L/(m) during the stable process ² H) the flux reduction was 48.95%. And when polyaspartic acid scale inhibitor is added, the membrane flux is kept stable, and the fluctuation is small.

As can be seen from fig. 19, in the dynamic scale inhibition experiment, the oligomeric chitosan has limited scale inhibition effect on the low-pressure reverse osmosis system. When the addition amount of the chitosan oligosaccharide is 5mg/L, the membrane flux is stable within 30min, and the membrane flux is 40.08L/(m) during the stabilization ² H) the flux reduction was 57.84%. When polyaspartic acid scale inhibitor is added, the membrane flux reduction rate at the initial stage of scaling is higher than that of the membrane flux without adding the scale inhibitor, the membrane flux is kept stable in a short time after the membrane flux is rapidly reduced, and the membrane flux has certain fluctuation when the membrane flux is stable. The oligomeric chitosan is used as an environment-friendly scale inhibitor which can be extracted from plants, has a certain gap in scale inhibition performance compared with polyepoxysuccinic acid and polyaspartic acid, and the scale inhibition capacity of the oligomeric chitosan needs to be improved by adding an increased amount or modifying functional groups.

As can be seen from FIG. 20, in the dynamic scale inhibition experiment, three scale inhibitors were simply compounded, and when the compounded scale inhibitor is 2.5mg/L polyaspartic acid+2.5 mg/L polyepoxysuccinic acid+0.5 mg/L oligomeric chitosan+10 mg/LAlCl ₃ The membrane flux was stabilized within 15 minutes at 93.72L/(m) ² H) a 15.64% decrease in membrane flux. When the compound scale inhibitor is 2.5mg/L polyaspartic acid+2.5 mg/L polyepoxysuccinic acid+0.5 mg/L oligomeric chitosan, the membrane flux is stable within 20min, and when stable, the membrane flux is 84.72L/(m) ² H). The compound use of the scale inhibitor can effectively improve the scale inhibition rate.

The polyaspartic acid and the polyepoxysuccinic acid are used as green scale inhibitors, have higher scale inhibition effect, do not contain phosphorus elements and do not cause eutrophication of water body, and are obtained from the dynamic scale experimental data of the three scale inhibitors and the compound scale inhibitor thereofPollution and the like. And (3) compounding a scale inhibitor: 2.5mg/L polyaspartic acid+2.5 mg/L polyepoxysuccinic acid+0.5 mg/L oligomeric chitosan+10 mg/LAlCl ₃ Membrane fouling can be effectively limited.

Example 8

1. Performance characterization analysis is carried out on the Mg-Al-Ce interlayer metal oxide prepared in the example 2, the addition amount of Mg, al, ce and urea in the example 2 is changed, and the influence of the Mg-Al-Ce interlayer metal oxide on the adsorption capacity of the defluorination material is explored by controlling the mixture ratio of different materials.

The adsorption capacity exploration experimental conditions were as follows: the initial concentration of fluoride is 400mg/L, the pH value of the solution is regulated to be=7, the adding amount of the adsorbent is 0.2g, 0.5g, 1g and 2g, the shaking table is kept at the constant temperature of 25 ℃ for 24 hours, the constant rotating speed is 200rpm, and the experimental results are shown in figures 21-23.

The effect of Mg and Al ratios on the adsorption capacity of the adsorbent is shown in fig. 21, mg: at al=2:1, the adsorbent fluoride adsorption capacity is 108.91mg/g at maximum. Mg: at al=1:1 and 3:1, the adsorption capacity was reduced at 95.56mg/g and 90.47mg/g, respectively. Mg: at al=1:2 and 1:3, the adsorption capacities were smaller, 70.09mg/g and 50.26mg/g, respectively. The optimal ratio of Mg to Al is 2:1.

The effect of Ce ratio on the adsorption capacity of the adsorbent is shown in fig. 22, where Ce/Mg is calculated when Mg: al=2:1<At 10:1, the adsorption capacity of the adsorbent increases with increasing Ce duty; when Ce/Mg>At 10:1, the adsorbent adsorption capacity decreases with increasing Ce duty due to excess Ce ³⁺ Enters into MgAl interlayer double metal hydroxide crystal lattice to cause lattice distortion, the structure of the adsorption material is changed, and the performance of the adsorbent is reduced. Optimal Mg: al: the Ce ratio was 10:5:1.

As shown in FIG. 23, when the urea addition amount is less than 3g, the adsorption capacity of the prepared adsorption material is continuously increased along with the increase of the urea addition amount, and when the urea addition amount is more than 3g, the adsorption capacity change is not obvious, and the performance of the adsorption material is not influenced by excessive addition. The optimum dosage of urea is 3.6g.

2. The parameters of hydrothermal temperature, hydrothermal time, roasting temperature, roasting time and the like in the embodiment 2 are changed, the influence of hydrothermal conditions and roasting conditions on the adsorption capacity of the material is explored, and the adsorption capacity is explored under the following experimental conditions: the initial concentration of fluoride was 400mg/L, the pH of the solution was adjusted to be 7, the amount of the adsorbent to be added was 1g, and the shaking table was kept at a constant temperature of 25℃for 24 hours at a constant rotation speed of 200rpm, and the results were shown in FIGS. 24 to 26.

The effect of hydrothermal conditions on the adsorbent performance is shown in fig. 24 and 25. The adsorption capacity of the adsorbent is only 20.79mg/g when the hydrothermal temperature is 60 ℃, the adsorption capacity of the adsorbent is increased along with the increase of the hydrothermal temperature, the adsorption capacity of the adsorbent reaches the maximum value of 108.91mg/L when the hydrothermal temperature is 120-180 ℃, and the adsorption capacity of the adsorbent is slightly reduced to 105.36mg/g when the hydrothermal temperature is increased to 200 ℃.

The effect of calcination temperature on the performance of the adsorbent material is shown in figure 26. At 400 ℃ or lower, the adsorption capacity of the adsorbent increases with the increase of the calcination temperature, and the maximum value is 108.91mg/g. With further increase of temperature, the adsorption capacity of the adsorbent is continuously reduced, and the adsorption capacity is only 20.56mg/g when the roasting temperature is 800 ℃.

3. The effect of aging time on the adsorption capacity of the material was examined by changing the aging time in example 2, and the experimental conditions for the adsorption capacity examination were the same as those of example 2, and the results are shown in fig. 27.

As can be seen from FIG. 27, as the aging time increases, the adsorption capacity of the adsorbent increases, and reaches a maximum value of 108.91mg/g under the condition of 12 hours of aging time, and when the aging time continues to extend over 18 hours, the adsorption capacity of the adsorbent decreases slightly to 97.43mg/g.

Example 9

Influencing factors of defluorination efficiency of adsorption material

1. Effect of initial fluoride concentration on the defluorination efficacy of adsorbent materials

In order to explore the influence of the adsorption process of the Mg-Al-Ce interlayer metal oxide and the initial concentration of fluoride on the defluorination performance of the adsorption material, a static adsorption experiment is carried out on the Mg-Al-Ce interlayer metal oxide prepared in the embodiment 2, and the experimental conditions are as follows: fluoride initial concentration 20, 40, 60, 80mg/L, initial pH 7.0, adsorbent dosage 1.0g/L, reaction temperature 25 deg.C, shaking table constant rotation speed 200rpm, adsorption time 12h. The experimental results are shown in FIG. 28.

As shown in FIG. 28, the fluoride adsorption capacity of the Mg-Al-Ce interlayer metal oxide increased with increasing initial fluoride concentration, and the adsorption capacities were 17.89, 36.22, 58.84, 78.12Mg/g at initial fluoride concentrations of 20, 40, 60, 80Mg/L, respectively. As the initial concentration of fluoride increases, the adsorption equilibrium time gradually increases, and when the initial concentration of fluoride is 20, 40, 60 and 80mg/L, the adsorption equilibrium time is about 240, 360, 480 and 540min respectively. The removal rate of the adsorbent was maximum in the first 60 minutes, and the removal rate reached about 50%, after which the rate was gradually gentle and eventually reached adsorption equilibrium. The adsorption capacity of the adsorbent increases with increasing initial fluoride concentration because the adsorbent surface is capable of providing sufficient F ^- Adsorption sites, the higher the initial concentration of fluoride, the solution and the adsorbent surface F ^- The greater the concentration gradient, the greater the F capture by the adsorbent ^- The greater the chance of fluoride adsorption capacity. The preparation material can be obtained through experimental data analysis, and can efficiently adsorb and remove fluorine under the condition of high concentration.

2. Influence of pH on the defluorination efficacy of adsorbent materials

Static adsorption experiments were carried out on the Mg-Al-Ce interlayer metal oxide prepared in example 2 under the following conditions: the initial concentration of fluoride is 20mg/L, the initial pH is adjusted to 4.0-10.0, the adding amount of adsorbent is 1.0g/L, the reaction temperature is constant at 25 ℃, the constant rotation speed of a shaking table is 200rpm, the adsorption time is 12h, and the experimental results are shown in figures 29 and 30.

As can be seen from fig. 29, the pH has a certain influence on the adsorption of fluoride by Mg-Al-Ce interlayer metal oxide, the adsorption capacity at equilibrium is 17.19Mg/g at ph=4, the fluoride removal rate is 85.9%, the adsorption capacity at equilibrium is 17.07Mg/g at ph=7, the fluoride removal rate is 85.4%, and the adsorption capacity at equilibrium is 16.55Mg/g at ph=10, the fluoride removal rate is 82.7%. The isoelectric point of Mg-Al-Ce interlayer metal oxide is ph=7.5 as can be obtained from fig. 30.

The effect of pH on the defluorination efficacy of the adsorbent is mainly two-way. First, when pH is<7.5, zeta potential of the adsorbent is positive, and the material is shown in TableThe surface is positively charged and can drive F by the action of the charges ^- Near the surface of the adsorbent and bonded to the layers of the material, while the pH>7.5, the Zeta potential of the adsorbent is negative, the surface of the material is negatively charged, and the charge acts to lead F ^- Repulsive forces exist when diffusing to the surface of the adsorbent, and the fluorine removal capacity is rapidly reduced. Second, during the pH rise, OH ^- Ion concentration is increased continuously, OH ^- Occupy adsorption sites of a material by competing effects, leading to F ^- The adsorption rate decreases and the adsorption capacity decreases. At a point of interest, the interlayer metal oxide structure of Mg-Al-Ce is destroyed at too low a pH, resulting in Mg ²⁺ 、Al ³⁺ 、Ce ³⁺ Dissolve out, although Al ³⁺ With complexation F ^- But the effect is limited, and the defluorination performance of the material is severely limited. As can be seen from section 3.2.3, the pH of the concentrated water of the low-pressure reverse osmosis system is slightly acidic and is in the range of strong adsorption capacity of metal oxides between Mg-Al-Ce layers, so that the extra link of pretreatment for pH adjustment in the concentrated water adsorption treatment can be avoided.

3. Influence of the addition amount of the adsorbent on the defluorination efficiency of the adsorbent

Static adsorption experiments were carried out on the Mg-Al-Ce interlayer metal oxide prepared in example 2 under the following conditions: the initial concentration of fluoride is 40mg/L, the initial pH is adjusted to 7.0, the adding amount of adsorbent is 1.0-8.0g/L, the reaction temperature is constant at 25 ℃, the constant rotation speed of a shaking table is 200rpm, the adsorption time is 12h, and the experimental result is shown in figure 31.

As can be seen from fig. 31, as the amount of the adsorbent added increases, the fluoride removal rate gradually increases, and the adsorption capacity of the adsorbent gradually decreases. When the adding amount of the adsorbent is 1.0g/L, the adsorption process reaches equilibrium about 360min, the adsorption capacity is 32.3mg/L, and the fluoride removal rate is 79.42%. When the adding amount of the adsorbent is 6g/L, the adsorption process reaches balance in about 180min, the adsorption capacity is 6.4mg/L, and the fluoride removal rate is 95.01%.

Example 10

The adsorption regeneration experiment was performed on the Mg-Al-Ce interlayer metal oxide in example 2 under the following conditions: after the primary adsorption balance, respectively carrying out alkali washing regeneration by using a mixed solution of 1mmol/L sodium hydroxide and 1mmol/L sodium carbonate, roasting for 2 hours at 400 ℃ of a muffle furnace, and carrying out alkali washing and roasting for 2 hours at 400 ℃ of a mixed solution of 1mmol/L sodium hydroxide and 1mmol/L sodium carbonate, carrying out adsorption experiments after each regeneration, wherein the initial concentration of fluoride is 80mg/L, the initial pH value is regulated to 7.0, the adding amount of the adsorbent is 1.0g/L, the reaction temperature is constant temperature of 25 ℃, the constant rotation speed of a shaking table is 200rpm, the adsorption time is 24 hours, and recording the result is shown in figure 32.

According to FIG. 32, the effect of roasting and regenerating after alkali washing is best, the adsorption capacity can still be kept 80.36% after five times of regeneration, the adsorption capacity is kept 68.71% after five times of regeneration, the adsorption capacity is greatly reduced after the third time of separate roasting and regenerating, and the adsorption capacity is kept 42.59% after five times of regeneration. Alkaline washing is mainly carried out by ion exchange mode ^- The adsorbent is replaced, but the fluoride is adsorbed again by reacting with OH ^- And CO ₃ ^2- Ion exchange enters the adsorbent, and the adsorption process is blocked. After alkali washing, roasting and regenerating can lead OH to be regenerated ^- And CO ₃ ^2- The roasting is lost, a memory effect is formed, a large amount of anions can be adsorbed to enter the plate layer structure when the solution is contacted again, and the adsorption capacity regeneration efficiency is high. The adsorption capacity of the direct roasting regeneration is greatly reduced after three regenerations, because the ion content among the plate layers is less, and the structures such as carbonate cannot be lost during roasting regeneration, so that the plate layer structure is damaged to form the structure such as inverse spinel, and the adsorption capacity of the adsorbent is greatly reduced due to the property change of the adsorbent.

Claims

1. The low-pressure reverse osmosis defluorination method of high-fluorine groundwater is characterized in that a scale inhibitor and an adsorbent are added into the high-fluorine groundwater to be treated, and the high-fluorine groundwater passes through a low-pressure reverse osmosis membrane under the conditions of pressure of 7.5-8.5 bar, pH value of 7.5-8.5 and temperature of 15-30 ℃;

the low-pressure reverse osmosis membrane is a polyamide membrane;

2. The method for low pressure reverse osmosis defluorination of high fluorine groundwater according to claim 1, wherein the scale inhibitor is 2.5mg/L polyaspartic acid, 2.5mg/L polyepoxysuccinic acid, 0.5mg/L oligomeric chitosan, 10mg/LAlCl ₃ Is a mixture of (a) and (b).

3. The method for removing fluorine by low-pressure reverse osmosis of high-fluorine underground water according to claim 1, wherein the preparation method of the Mg-Al-Ce interlayer metal oxide comprises the following steps:

(2) Transferring the prepared Mg-Al-Ce interlayer metal hydroxide into a 100mL quartz crucible, heating to 400 ℃ by a muffle furnace program, keeping the temperature rising speed at 10 ℃/min and 200 ℃ for 20min, heating to 400 ℃ and roasting for 4h, cooling to room temperature, and grinding to obtain the Mg-Al-Ce interlayer metal oxide.

4. The method for low-pressure reverse osmosis defluorination of high-fluorine groundwater according to claim 3, wherein the molar ratio of magnesium nitrate hexahydrate, aluminum nitrate nonahydrate and anhydrous cerium chloride is 10:5:1, and the molar ratio of urea to magnesium nitrate hexahydrate is 10:1;

5. The method according to claim 3, wherein the heating reaction is performed at 120 ℃ for 12 hours.

6. A low pressure reverse osmosis defluorination method for high fluorine groundwater according to claim 3, wherein the centrifugation is performed at 6000rpm for 5min.

7. A low pressure reverse osmosis defluorination process for high fluorine groundwater according to claim 3, wherein the drying operation is oven heating to 80 ℃ for 4h.