Study on the Preparation of a Polydiethylene Phenyl Phosphate Resin and its Adsorption of Cu2+ in Wastewater

In view of the problems and imperfections in the synthesis of phosphoric acid resins by existing techniques, we prepared a different phosphoric acid resin and used it to adsorb Cu2+ in wastewater. When phosphoric acid resin is prepared using cross-linked polydiethylbenzene resin as a matrix, the experimental raw material can be more selective and the experimental process is safe and non-toxic. The particle size of the synthesized resin is uniform, most of which are distributed in 100-300 μm, and the functional group content can be as high as 2.4 mmol/g. The static/dynamic adsorption on Cu2+ is excellent, with a static adsorption capacity of 51.2 mg/g. When hydrochloric acid is used as an eluant, the resin has favorable reproducibility, which provides a reference value for the preparation and application of phosphate resins.


Introduction
Since the founding of New China, ion exchange and adsorption technologies in China have developed rapidly.At present, China has developed at least five generations of polymer resins, which have been widely used as materials in various industries such as national defense, military industry, military, health, and medical industry, chemical industry, metallurgy, and environmental protection, and have made great contributions to China's modernization [1][2][3] .According to different standards, the resin has different classification methods.In many resins, the acid resin is polymer solid acid, which takes a cross-linked polymer as the skeleton and connects acidic functional groups.It can selectively react with specific metal ions from the solution containing metal ions to form ionic bonds.And then, the functional resin can release metal ions above the resin under appropriate eluate conditions.Phosphoric acid resin is widely used in the treatment of heavy metal ion pollution [4][5][6][7] .At present, the synthesis of phosphoric acid resin is commonly used as a polychloromethyl polystyrene resin as the parent, which requires chloromethyl ether as the basic raw material in the preparation process.Chloromethyl ether is a strong carcinogenic agent, has a low boiling point, and is volatile, which poses great harm to the operating staff and the surrounding environment [8][9][10] .
Given the problems and deficiencies in the prior art, a different type of phosphorene resin was prepared by using cross-linked polyvinylbenzene resin as a matrix, and the adsorption properties of the resin to Cu 2+ in wastewater were investigated.Experimental instruments include Beaker, 2-methyltetrahydrofuran, Ultrasonic wave cleaner, Thunder magnetic pH meter, Vacuum drying oven, Electronic analytical balance, Ultraviolet spectrophotometer, Ion exchange column, Four mouthed flasks, Measuring cylinder, Scanning electron microscope, etc.

Method
2.2.1.Preparation of polydivinylphenyl phosphate resin.PDVB, phosphorene esters, and solvents are added to the reactor and the reaction is then heated.The resulting reaction liquid is filtered, and the resulting cake is successively washed with ethanol and distilled water to obtain cross-linked polyvinylbenzene phosphate resin.The synthesized phosphate ester resin was then added to the reaction kettle and then concentrated hydrochloric acid was added for hydrolysis.After the reaction, it was washed with distilled water.Then the temperature was controlled at 105℃ and the pressure was 0.1 Mpa for drying.Finally, the cross-linked polydivinylbenzene phosphate resin was obtained.The reaction process is shown in Figure 1.

Scanning electron microscope.
The microstructure of resin was observed by scanning electron microscope (SEM).

Determination of resin functional group content.
The functional group content of phosphate resin was determined concerning GB/T 8144-2008 "Method for determination of exchange capacity of cation exchange resin".

Determination of particle size of resin beads.
Firstly, the synthesized resin was sieved with a standard sieve, and then the size range was calculated through the mesh number-size correspondence table.2+ standard curve.The optimal scanning wavelength was determined by a UV spectrophotometer, and the Cu 2+ standard curve was established under the optimal absorption wavelength.2+ .The static/dynamic adsorption experiments of Cu2+ in water were carried out with the synthesized resin to explore the properties of the resin for Cu 2+ .

Synthesis and characterization of resin
3.1.1.Determination of phosphite esters.In this experiment, we mainly introduced phosphoric acid groups to polydivinylbenzene microspheres, and the mechanism is as follows.First, the double bond suspended on the polydivinylbenzene microsphere reacts with the phosphite ester, thereby introducing the phosphate ester group into the polymer chain, and then hydrolyzing into phosphate resin in the presence of concentrated hydrochloric acid.Therefore, in this experiment, trimethylphosphite, phosphorene, tropical phosphorene, and triphenylphosphite from phosphorene were used as reactants to introduce functional groups into the polyvinyl benzene resin.In the process of the experiment, additional initial conditions of the experiment are as follows.PDVB, phosphite, and 2-methyltetrahydrofuran are added to the reaction kettle according to the mass ratio of 1:9:30, then the temperature is raised to 80℃ for 48 h, and the filter cake is filtered.The filter cake is washed with ethanol and distilled water in turn to obtain polydivinylbenzene phosphate resin.Then, the synthesized phosphate ester resin was added to the reaction kettle, the concentrated hydrochloric acid with the ratio of 1:8 to phosphate ester resin was added, and the hydrolysis reaction was 48 h at 80℃.After the reaction, the water was washed with distilled water, and then the temperature was controlled at 105℃ and the pressure was 0.1 Mpa.As shown in Table 1, the functional group content on the phosphite resin is barely affected by the type of phosphite ester chosen.All four phosphorene esters selected were able to open carbon-carbon double bonds on the microspheres of polyvinylbenzene, thus introducing phosphorene ester groups, and the subsequent reactions were relatively stable.The slight difference in the final functional group content may be related to the structure of the phosphite ester.Calculations show that the functional group content of the resin can be stabilized at about 2.2 mmol/g.

Determination of Solvent Type.
In the organic reaction, especially in the polymer reaction, the chemical reaction is mostly carried out in the intermolecular.The selection of the appropriate reaction solvent can make the reactants effectively dissolved in it, and can increase the contact area between the reactants.Some solvents also have a catalytic effect, so that the reaction can be better and faster.Therefore, next, we discussed the effects of PDVB and phosphite on the content of final resin functional groups in 2-methyltetrahydrofuran, petroleum ether, and N, N-dimethylamide solvents respectively.Concerning other experimental steps unchanged, the results are shown in Table 2. On the whole, the reaction of PDVB with different phosphite esters in the three different solvents selected can make the reactants dissolve, contact, and react better, so that the final functional group content is about 2.2 mmol/g.

Determination of reaction time and temperature.
The synthetic process of phosphate resin is essentially a chemical reaction, and it is well known that reaction temperature and reaction time are extremely crucial for chemical reactions.In this experiment, there are two important time and temperature requirements.Therefore, we use trimethyl phosphite as the reactant and 2methyltetrahydrofuran as the solvent as an example, with other initial experimental steps remaining unchanged, to explore their effects on the content of functional groups in the resin.
First of all, the reaction temperature and time during the preparation of polydivinylbenzene phosphate resin were discussed.This process is mainly to open the double bond on PDVB and introduce the phosphate group.As shown in Figure 2, under a certain range of reaction temperature and reaction time, other reaction conditions remain unchanged, and with the increase of temperature and the extension of reaction time, the content of the functional group of the final phosphate resin also increases, which may be because the reaction is endothermic.While the reaction time remains constant, appropriately increasing the reaction temperature can promote faster and more open double bonds on the PDVB.Phosphine resins produced at this time are relatively stable.However, when the reaction temperature is 60~120℃ and the reaction time is 24~72 h, the final functional group content of phosphate resin can reach 2.0~2.4 mmol/g, in which trimethyl phosphite is selected from phosphite ester and 2-methyltetrahydrofuran is used as the solvent.After the reaction at 120℃ for 72 h, the functional group of phosphate resin can be as high as 2.4 mmol/g.Second, we discuss the reaction temperature and reaction time during the preparation of phosphorene from polyvinylbenzene phosphate resin.This process is dominated by the hydrolysis of the ester group on the phosphorene to the hydroxyl group in the presence of concentrated hydrochloric acid.
As shown in Figure 3, when the reaction temperature is 60~120℃ and the reaction time is 24~72 h, the reaction temperature and time are positively correlated with the content of the functional group of the phosphate resin, and the final functional group content is also in the range of 2.0~2.4 mmol/g.

Determination of reaction material ratio.
First, the effect of the reaction material ratio on the functional group content was explored by adding different proportions of phosphite ester and solvent, using trimethyl phosphite and 2-methyltetrahydrofuran as examples.As shown in Table 3, the functional group content increases with increasing amounts of phosphite and solvent, when the amounts of solvent and phosphite are fixed respectively.In general, as the material ratio increases, so does the functional basis content.This is because a higher volume of phosphite can facilitate the forward progress of the reaction, and increasing the solvent volume within a certain range can effectively promote dissolution and contact reactions between the reactants.Second, the effect of the concentration of hydrochloric acid on the functional group content was investigated in the synthesis of phosphorene from phosphorene esters.As shown in Table 4, within a certain range, the final functional group content is positively correlated with the amount of concentrated hydrochloric acid added.The type of phosphite may be selected from trimethyl phosphite, triethyl phosphite, tropical phosphite, and triphenyl phosphite; the solvent can choose one of 2-methyltetrahydrofuran, petroleum ether and N, N-dimethylamide; the reaction temperature can be 60~120℃ and the time can be 24~72 h; the mass ratio of polydivinylbenzene, phosphite ester and solvent can be 1:5~10:10~50; the mass ratio of phosphate ester resin to concentrated hydrochloric acid can be 1:3~10; the temperature of the hydrolysis reaction can be 60~100℃, and the time can be 24~72 h.The microstructure of the resin obtained by the experiment is shown in Figure 4.The particle size of the resin is uniform, mainly distributed in the range of 100-300 µm, and the content of functional groups can be as high as 2.4 mmol/g.

Scanning electron microscopy of polydivinylphenyl phosphate resin.
Under the optimum conditions, the synthesized phosphate resin was observed by electron microscope, and its particle size distribution was determined.As shown in Figure 4, the synthesized phosphate resin has a smooth and regular appearance, and it is found through measurement that the resin particle size is uniform, mainly distributed in the range of 100~300 µm, indicating the feasibility of this synthesis path.

Standard curve of Cu 2+
Copper sulfate pentahydrate with different masses was weighed by electronic analytical balance, and copper sulfate solution with different mass fractions (0.01%, 0.025%, 0.05%, 0.075%, and 0.1%) was prepared, and the sample solution was scanned by ultraviolet spectrophotometer.As shown in Figure 5, after analysis and comparison, the wavelength can be set to 800 nm.At a wavelength of 800 nm, the Cu 2+ standard curve is shown in Figure 6.With Cu 2+ mass concentration in the water sample as the horizontal coordinate and absorbance (Abs) as the vertical coordinate, the standard curve  11.464 0.0128, where the correlation coefficient R 2 =0.998, indicating that the linear equation obtained has a good linear correlation, and the test method is reliable and effective.

Determination of static adsorption capacity
Different volumes of polydivinylphenyl phosphate resin (1 ml, 2 ml, 3 ml, and 5 ml) were used as an adsorbent, Cu 2+ was used as an adsorbent, and 500 ml of solutions with different mass fractions (0.1%, 0.05%, and 0.01%) were prepared to investigate the static adsorption of Cu 2+ in the solution.The calculation method of adsorption capacity is shown in Formula (1):      ⁄ (1) where Q 1 represents the static adsorption capacity of phosphate resin, and its unit is mg/g; V represents the volume of the sample liquid containing Cu 2+ , and its unit is L; C 0 represents the initial concentration of Cu 2+ in mg/L.C represents the equilibrium concentration of Cu 2+ , and the unit is mg/L; M represents the mass of the phosphate resin, and its unit is g.The static adsorption capacity of polydivinylphenyl phosphate resin is shown in Table 5, from which the static adsorption capacity of polydivinylphenyl phosphate resin can be calculated to be 51.2 mg/g.

Effect of initial pH value of solution on adsorption of Cu 2+ by polydivinylphenyl phosphate resin
The essence of the adsorption of Cu 2+ by polydivinylphenyl phosphate resin is the adsorption and exchange of hydrogen ions in the functional groups contained in the resin and Cu 2+ in the solution.Therefore, at the same temperature, we take a certain amount of polydivinylphenyl phosphate resin.In addition, the static adsorption experiments were carried out by mixing solutions containing Cu 2+ with different initial pH values (2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0) to explore the influence of solution pH value on the adsorption effect.
As shown in Figure 7, the initial pH of the solution has a great influence on the adsorption effect of the polydivinylphenyl phosphate resin.Under acidic conditions, the static adsorption equilibrium of the polydivinylphenyl phosphate resin increases with the increase of the pH value of the solution, that is, the static adsorption equilibrium of the polydivinylphenyl phosphate resin is negatively correlated with the concentration of hydrogen ions in the solution.This is because the hydrogen ions in the solution will have a competitive reaction with Cu 2+ , which affects the adsorption effect of polydivinylphenyl phosphate resin on Cu 2+ in the solution.

Effect of temperature on adsorption of Cu 2+ by polydivinylphenyl phosphate resin
Polydivinylphenyl phosphate resin adsorption Cu 2+ is a chemical reaction process, the reaction will be affected by the reaction temperature.Therefore, under the same pH (5.5), we take a certain amount of polydivinylphenyl phosphate resin, respectively, at 20℃, 25℃, 30℃ and 35℃ static adsorption experiments.
As shown in Figure 8, the static adsorption equilibrium of polydivinylphenyl phosphate resin increases with the increase of reaction temperature, that is, increasing temperature can promote the adsorption of polydivinylphenyl phosphate resin to Cu 2+ , which may be because the reaction process is an endothermic reaction.

Dynamic adsorption/desorption test
(1) Dynamic adsorption test: 10 ml pre-treated polydivinylphenyl phosphate resin was loaded into the column by wet method, and a solution (0.01%) containing a certain mass fraction of Cu 2+ was prepared as the adsorption sample liquid.The solution was poured into the column from the upper end and the control flow rate was adjusted to 1 ml/min, and the absorbance of the effluent was measured.As shown in Figure 9, as the outflow continues to increase, its absorbance also continues to rise, that is, the adsorption capacity of the resin continues to decline.When the outflow liquid volume is greater than 2000 ml or so, the absorbance of the outflow reaches the maximum range.Because the functional group content on the polyvinylphenyl phosphate resin is fixed, with the continuous addition of Cu 2+ sample liquid, the residual amount of functional groups that can react with Cu 2+ in the resin decreases with the increase of participating in the reaction, and finally, the adsorption amount of the resin tends to be saturated.Moreover, it can also be seen from the chromatogram column that the color of the resin in the column changes from orange-yellow to blue-green with the continuous addition of the sample liquid, and then becomes dark green and remains unchanged.This is because Cu 2+ first coordinates with the upper layer of resin, and then goes down.With the increase of coordination degree, Cu 2+ enters the interior for coordination, and the color gradually deepens.That is, the maximum exchange capacity of the polydivinylphenyl phosphate resin is reached, and the outflow liquid volume at this time is near 2000 ml.
(2) Dynamic analytical test: It is known that the adsorption nature of polydivinylphenyl phosphate resin on Cu 2+ is the replacement of hydrogen ions on Cu 2+ and phosphate groups, so we selected 5% hydrochloric acid solution as eluent to eluate the saturated polydivinylphenyl phosphate resin.Its analytical curve is shown in Figure 10.At the initial stage, after washing with hydrochloric acid, Cu 2+ on the resin can be resolved at a faster rate.When the effluent reaches about 200 ml, the absorbance approaches 0, indicating that there is no Cu 2+ in the effluent.At the same time, the color of the resin in the chromatographic column also recovers from dark green to the original color.The feasibility of using hydrochloric acid to eluate the resin was verified.We took 5 ml of purified polydivinylphenylphosphate resin, loaded it into a column by the wet method, and used Cu 2+ solution containing 0.1% mass fraction as adsorption sample liquid at a controlled flow rate of 1ml/min.After resin adsorption saturation, elution with 5% hydrochloric acid was performed, and Cu 2+ adsorption was performed on the resolved resin again.The above adsorption/resolution process was repeated and the saturation adsorption capacity of the resin was calculated.

Regeneration test of polydivinylphenyl phosphate resin
As shown in Figure 11, in the process of multiple cyclic adsorption/analysis, the phosphate resin maintained a high adsorption capacity, which fully verified the regeneration ability of the resin and provided a possibility for its wide application.

Figure 2 .
Figure 2. The effect of reaction temperature and reaction time on the content of functional groups during the preparation of crosslinked polydivinylbenzene phosphate resin.

Figure 3 .
Figure 3.The effect of reaction temperature and reaction time on the content of functional group during the preparation of crosslinked polydivinylbenzene phosphate resin.

Figure 7 .
Figure 7.The effect of pH on adsorption of Cu 2+ by phosphoric acid resin.

Figure 8 .
Figure 8.The effect of temperature on adsorption of Cu 2+ by phosphoric acid resin.

Table 1 .
The content of functional groups under different phosphite esters.

Table 2 .
The effect of different solvent types on the content of functional groups.

Table 3 .
The effect of material ratio on functional group content during the preparation of crosslinked polydivinylbenzene phosphate resin.

Table 4 .
The effect of material ratio on functional group content during the preparation of crosslinked polydivinylbenzene phosphate resin.

Table 5 .
Static adsorption of phosphoric acid resin.