Boron separation by adsorption and flotation with Mg-Al-LDHs and SDBS from aqueous solution

Chun Bai,Huifang Zhang,Qinglong Luo,Xiushen Ye,Haining Liu,Quan Li,Jun Li,Zhijian Wu

1 School of Chemical Engineering, Qinghai University, Xining 810016, China

2 Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, China

3 Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province, Xining 810008, China

Keywords:Boron Separation Adsorption Flotation Layered double hydroxide Sodium dodecyl benzene sulfonate

ABSTRACT Layered double hydroxides(LDHs)have been shown to be effective adsorbents for boron.However,solidliquid separation is still a problem when separating boron from industrial radioactive waste liquid.In this research,three types of Mg-Al-LDHs including Mg-Al-LDH(NO3-),Mg-Al-LDH(Cl-)and Mg-Al-LDH(SO42-)were applied to adsorb boron,and moreover sodium dodecylbenzenesulfonate (SDBS) was used to float the LDH particles from aqueous solution after boron adsorption.The results showed that 60 min was sufficient for the equilibrium adsorption of the three LDHs.The boron adsorption capacity of three LDHs was determined as follows: Mg-Al-LDH(NO3-) ＞Mg-Al-LDH(Cl-) ＞Mg-Al-LDH(SO42-),and was 2.0,0.98 and 0.2 mmol·g-1,each ranging from 0 to 80 mmol·L-1 with the initial boron concentration.The efficiency of boron removal by Mg-Al-LDH(NO3-)and SDBS can reach up to 89.7%.Furthermore,the boron flotation mechanism of SDBS and LDHs has been studied,since SDBS as a flotation agent can react with LDHs and penetrate into the interlayer of LDHs in addition to electrostatic attraction.Therefore,LDHs in solution can be floated onto the foam layer to be separated from the solution,and the clarified solution was obtained.The method is simple and promising for boron removal from aqueous solution.

1.Introduction

Boron (10B) has the largest neutron adsorption cross section.In nuclear power plants with pressurized water reactors(PWR),boric acid is usually added to the reactor for chemical compensation control of the reactivity.The boron-containing reactor coolant effluent is treated by a waste liquor treatment system and ultimately discharged to the environment with the plant’s radionuclide waste liquor.Excess boron inhibits cement hardening of radioactive contamination and increases the risk of radionuclide migration.In addition,boron release leads to environmental pollution and is harmful to plants,for example,high boron concentration can reduce phytoplankton biomass and biodiversity.Too much boron in the water would lead to premature ripening,cause massive leaf damage,and reduce fruit yield.In 2011,the World Health Organization (WHO) set the limit for boron in drinking water at 2.4 mg·L-1[1].For agricultural irrigation water,boron limits are kept between 0.3 and 1 mg·L-1due to major concerns about boron toxicity.Therefore,the removal of traces of boron in water is of great importance in some situations.Therefore,it is necessary to pre-treat the process effluent of the nuclear power plant to ensure that the boron concentration in the radioactive waste water discharge system of the power plant meets the requirements of the waste water discharge limit values,and the boron concentration in the receiving water body of the nuclear power plant meets the requirements of the environmental quality standards.

For boron removal,the most current research methods are mainly adsorption,reverse osmosis and solvent extraction and so on[2-13].The extraction process is suitable for the high boron situation and often has high separation efficiency.Reverse osmosis is currently mainly used in seawater desalination.The impact of reverse osmosis on boron removal depends on several variables such as: pH,temperature and salinity.The adsorption method is mainly suitable for the low boron concentration situation,and some innovative materials for removing boron from aqueous solutions have made important advances as effective purification and separation techniques.One of these promising materials called layered double hydroxides (LDHs),which structure is similar to Al(OH)3,can be expressed as follows:(An-)x/n·zH2O [2,14,15].Hydrotalcite’s unique layered structure gives it many superior properties,for example,the anions between the laminates can be exchanged with other anions in solution.Therefore,LDHs can be used extensively in wastewater treatment,desalination,and the removal of some other toxic substances of the big ones number of exchangeable anions and their relatively large surface area[16-18].It has been researched for the removal of several hazardous species,such as pesticides,boron,phosphates,fluoride,dichromate,phenols,selenate,dyes,etc.In general,the properties of the interlayer anions of LDHs have a large impact on the anion exchange performance.Anions with higher charge density have greater affinity for LDH,like carbonate,sulfate[17-20].Ca-Al LDHs were synthesized by Zhang and Reardon[21] for removing boron from aqueous solutions.Ferreiraet al.[22] prepared LDHs compounds with different types of metal ions(Mg-Al and Mg-Fe).However,the solid-liquid separation of hydrotalcite after adsorbing boron is a problem worth studying.

In this research,a flotation process,simple in operation,low in cost and promising for large-scale application,was performed to separate LDH adsorbents from aqueous solutions.Three different magnesium-aluminum hydrotalcites,Mg-Al-LDH(),Mg-Al-LDH(Cl-),Mg-Al-LDH(),were synthesized and compared for adsorbing boron from aqueous solution,and then collected by flotation separation with sodium dodecylbenzene sulfonate(SDBS)as the flotation agent.The adsorption and flotation method for boron removal is simple and efficient,and the flotation method for separating LDHs from solution in this paper is original and has the advantages of easy handling,cost saving and promising for large-scale application.In addition,X-ray diffraction (XRD),Fourier transform infrared spectroscopy (FTIR),scanning electron microscopy (SEM) and Raman spectroscopy were applied to explore morphology and microscopic mechanical performance of Mg-Al-LDHs for boron removal.

2.Materials and Methods

2.1.Experimental reagents

Magnesium chloride hexahydrate,aluminum chloride hexahydrate,magnesium sulfate heptahydrate,aluminum sulfate hexadecahydrate,magnesium nitrate hexahydrate,aluminum nitrate nonahydrate,sodium tetraborate decahydrate,sodium dodecylbenzene sulfonate,hydrochloric acid,sulfuric acid,nitric acid,and sodium hydroxide were of analytical grade and all solutions were prepared with deionized water.All the experimental chemicals were purchased from Sinopharm Chemical Reagent Co.,Ltd.,China.

2.2.Preparation of three Mg-Al-LDHs( Cl-, )

The mixture of magnesium and aluminum sources(magnesium nitrate and aluminum nitrate;magnesium chloride and aluminum chloride;magnesium sulfate and aluminum sulfate) was prepared according to 2:1 of the molar ratio of magnesium to aluminum,and the total mixture concentration of magnesium and aluminum is 2 mol·L-1.First,300 ml of the magnesium and aluminum mixed solution was added dropwise into 700 ml of 2 mol·L-1sodium hydroxide solution kept at 40 °C with constant stirring.Then the resulting mixture was heated and aged at 90 °C for 3 h.Finally,the resulting solid products obtained after aging and centrifugal separation was washed by deionized water several times until the pH of the washing water descended to 7-8 before drying in vacuum for 24 h at 60 °C and crushing under 100 μm.

2.3.Sample characterization

The phase and crystallinity of the samples were analyzed by XRD were detected with the XRD diffractometer(6000,Shimadzu,Japan).The change of chemical groups on the sample surface were analyzed by FTIR on an FTS-3500 instrument(Bio-Rad,USA)in the wavenumber range 4000-400 cm-1with a total of 32 scans and a resolution of 2 cm-1under nitrogen flow to remove the moisture.Raman spectra of the samples were utilized by the DXR Raman Microscope (Thermo Scientific,USA).SEM(EVO18,Carl Zeiss,Germany)was used to determine elemental composition and morphological changes.The concentrations of Mg2+and boron in solutions were analyzed by inductively coupled plasma optical emission spectrometry(ICP OES),and that ofCl-,andwere determined by ion chromatography (ICS 1100,Thermo Scientific).

2.4.Adsorption experiments

A series of boron adsorption experiments were carried out using Mg-Al-LDHs(Cl-,) with a solid-liquid ratio of 10 g·L-1,a boron concentration of 0 to 80 mmol·L-1and an oscillation rate of 500 r·min-1at 25°C.The initial pH of boron solutions was adjusted from 4 to 11 using 0.1 mol·L-1HNO3(HCl or H2SO4)and 0.1 mol·L-1NaOH solutions.Kinetics experiments on LDHs were stirred at 500 r·min-1at different times from 10 to 480 min.Determination of adsorption isotherm:LDHs were placed in a polyethylene bottle and 50 ml of the solution were added.The mixtures were oscillated in a constant temperature oscillator at 25°C for 120 min.The equilibrium adsorption capacity (qe,mmol·g-1) were calculated using Eq.(1):

whereC0(mmol·L-1) is the initial concentration of boron in the solution,Ct(mmol·L-1)is the concentration of boron at a given timet,m(g) is the mass of the Mg-Al-LDHs adsorbent,andV(L) is the total volume of boron solution.

2.5.Flotation experiments

Flotation studies were performed by mixing 0.5 g of SDBS with 500 mL of the mixture solution of boron and Mg-Al-LDHs(Cl-,) obtained after adsorption operation in a flotation cell with a stirring speed of 1700 r·min-1for 5 min before opening the air valve to collect LDHs particles from solution.The boron removal efficiency (E,%) were calculated using Eq.(2):

whereC0(mmol·L-1) is the initial concentration of boron in the solution,Ct(mmol·L-1)is the concentration of boron at a given timet.

3.Results and Discussion

3.1.Characterization of the three produced Mg-Al-LDHs( Cl-,)

XRD pattern shows the sharp and symmetrical peaks at the lower 2θ value,indicating the crystallinity and properties of layered compounds [23-25].The XRD analysis of the product Mg-Al-LDHs(Cl-,) were shown in Fig.1.The absorbed peaks can be represented by the spatial group R3m,which indicates that the hydrotalcite has a rhombohedral crystal structure (JSPDS card 25-0521) consisting with previous studies [26].The peaks (003)and(006)are assigned to the diffraction of the substrate structure,which corresponds to the layered stacking of Mg-Al-LDHs(Cl-,).The remaining peaks are due to the non-basic reflections.d003of Mg-Al-LDHs() is 0.897 nm,d003of Mg-Al-LDHs(Cl-) is 0.772 nm,d003of Mg-Al-LDHs() is 0.879 nm,the layer spacing mainly depends on the size of the anion geometry and the molar ratio of M2+/M3+[26,27].The higher the diffraction peak of XRD is,the larger the number of this crystal plane in the crystal is.(003),(006),(009)are the characteristic peaks of LDHs.The good consistency between the values corresponding to continuous basal plane diffraction(d003=2d006=3d009)indicates that LDH maintains a well regular layered stacking structure.The formation of the layered structure of LDHs can be judged by the appearance of diffraction peaks of the (003),(006),(009) series diffraction peaks.(003)peak which indicates the layer spacing,is affected by the anion properties,and the metal ions have less influence.The order of the layered structure can be characterized by the degree of the peaks (006),(012),(018) and (110) [14,15,28,29].

Fig.1.X-ray diffraction analysis of the different LDHs products.

The FTIR spectra of the prepared LDHs(),LDHs(Cl-) and LDHs() samples are shown in Fig.2.The 4000-3000 cm-1range is due to the stretching vibrational peak (νO-H) of O—H groups.The—OH is present in the layered structure of hydrotalcite,as well as in the adsorbed water and interlayer water.The peak at 1630 cm-1is the bending mode of H—O—H (δH-O-H),confirming that water molecules exist in the interlayer region of the LDHs.For Mg-Al-LDHs(): the peak shown at about 1400 cm-1actually consists of two peaks:one sharp peak at 1383 cm-1,the other broader one at about 1360 cm-1,is the ν3asymmetric stretching mode of[18,26,30],and there are two weaker bands at 840 cm-1identified as the ν2mode (out-of-plane bending mode)ofFor Mg-Al-LDHs(Cl-),the peaks shown at about 1370 cm-1and 784 cm-1are the chloride stretches.For Mg-Al-LDHs(),there are two characteristic vibrational adsorption peaks,the strong adsorption peak at around 1120 cm-1and the medium adsorption peak at 616 cm-1[31].Therefore,FTIR results show that there are nitrate,chloride and sulfate reside in the interlayer of the three LDHs.The wavenumbers of 440 and 550 cm-1represents the translational vibration of Mg-OH [32].

The Raman spectra of the prepared LDHs(),LDHs(Cl-) and LDHs() are shown in Fig.3.The wavenumber below 1750 cm-1is mainly the M-OH vibrational peak of the interlayer anion and the laminate metal.The peaks at 480 cm-1and 580 cm-1of LDHs(),LDHs(Cl-)and LDHs()can be assigned to the vibration of M-O [33].The peaks around 1100 cm-1are mainly the vibrational peaks of the interlayer anionsand Cl-.The strong peak of LDHs() at about 980 cm-1is the characteristic vibration of[34].The peaks of Raman spectrum of LDHs() are shifted to a lower wave number,mainly because sulfate is a divalent ion,and the charge on the laminate has changed compared to nitrate and chloride.

Fig.3.Raman spectra of the different LDHs products.

The SEM images of the produced LDHs(),LDHs(Cl-) and LDHs()are shown in Fig.4.From the morphological characterization,three types of LDHs are shown with the typical layered structure morphology of hydrotalcite [35].

Fig.4.SEM images of the different LDHs products: (a) LDHs(),(b) LDHs(Cl-) and (c) LDHs().

3.2.Adsorption and flotation experiments

3.2.1.The effect of pH on the adsorption of boron by LDHs

The effects of pH on the adsorption of boric acid by the three types of Mg-Al-LDHs are shown in Fig.5.The pH of the initial boric acid solution has little effect on the adsorption of boron by three different hydrotalcites.This is due to the good pH buffering capacity of hydrotalcites.If the initial pH of the solution is low,the pH of the solution increases for the dissolution of hydrotalcite compounds [36].If the initial pH value is high,LDHs adsorb some of OH-from the solution,which would lower the pH of the solution[37].No matter the initial pH of the solution is acidic or alkaline,after adding the hydrotalcite,due to the buffering effect of hydrotalcite,the removal of boron will only be performed under pH balance,that is,the pH of the original solution has little influence on the adsorption of boric acid by Mg-Al-LDHs.

Fig.5.Changes of qe-pH of LDHs(),LDHs(Cl-) and LDHs() adsorption of boron.

3.2.2.Adsorption kinetics and adsorption isotherms experiment

The adsorption capacity of three different hydrotalcites for boric acid in solution changes with time as shown in Fig.6.From the kinetic curve it can be seen that the equilibrium adsorption capacity of LDHs(),LDHs(Cl-) and LDHs() on boric acid in solution is obtained after 60 min.The quasi-first-order fit and the kinetic quasi-second-order fit of the adsorption curve are shown in Fig.6.From the regression coefficient it can be seen that the adsorption of boron to LDHs(),LDHs(Cl-) and LDHs() in solution is more in agreement with the quasi-second-order kinetic adsorption model,indicating that the adsorption rate is determined by the square of the number of unoccupied adsorption vacancies on the adsorbent surface,and the adsorption process is controlled by the chemical adsorption mechanism (see Table 1).

Table 1 Kinetic parameters of boron adsorption on LDHs(),LDHs(Cl-) and LDHs()

Table 1 Kinetic parameters of boron adsorption on LDHs(),LDHs(Cl-) and LDHs()

The adsorption isotherms of three hydrotalcites for boric acid are shown in Fig.7 and Table 2.The Langmuir isotherm model and Freundlich isotherm model are used for fitting,and the equation is shown as follows:

Table 2 Parameters of isotherm models for boron on adsorbents Mg-Al-LDHs(),Mg-Al-LDHs(Cl-) and Mg-Al-LDHs()

Table 2 Parameters of isotherm models for boron on adsorbents Mg-Al-LDHs(),Mg-Al-LDHs(Cl-) and Mg-Al-LDHs()

Fig.7.Changes of qe-CB of LDHs(),LDHs(Cl-) and LDHs(): (a) Langmuir model fitting,(b) Freundlich model fitting.

In the formula,qe(mg·g-1) is the equilibrium adsorption amount andCe(mmol·L-1)is the equilibrium concentration of boron in solution.qm(mg·g-1)is the maximum adsorption capacity,andKLis the Langmuir constant.KFand 1/nare Freundlich constants related to adsorption capacity and adsorption strength,respectively.

The adsorptions were more consistent with the Langmuir model,the maximum boron adsorption capacity of the LDHs(),LDHs(Cl-) and LDHs() was 2.0,0.98 and 0.2 mmol·g-1,respectively.The adsorption capacity of three LDHs can be written as follows:LDHs()＞LDHs(Cl-)＞LDHs().This difference in boron adsorption capacity is mainly due to the fact that the boron removal process may involve not only adsorption but also ion exchange,the main factor involved being ion exchange with the different exchange ability of three anions to boron.has a higher charge density and is easily attracted to the positive charge of the laminate and intercalates into the interlayer.Bothand Cl-are univalent negative charges,for the exchange and adsorption of small ions,the size of the interlayer spacing of hydrotalcite has little influence [17].The main reason is that the affinity between interlayer anions and laminates has a greater impact on the ion exchange performance of LHDs.The affinity ofis lower than that of the other two interlayer anions[14].Therefore,is more easily replaced when replaced withthan other two kinds of anionic hydrotalcite.

Through the experimental results and the literature review,it has been found that the power of ion adsorption on hydrotalcite is mainly due to the charge of the laminates.For macromolecules,the layer spacing may have some influence,but this is mainly due to the electrostatic attraction.The layer spacing will automatically realizes macromolecular intercalation by adjusting the size according to the incoming ions.For the exchange and adsorption of small ions,the size of the interlayer spacing of hydrotalcite has little influence.For example,the size of boric acid molecule and borate radical is about 0.35 nm.However,the interlayer spacing of the three synthesized hydrotalcites (Mg-Al-LDHs0.897 nm,Mg-Al-LDHs(Cl-)0.772 nm,Mg-Al-LDHs()0.879 nm)is larger than the size of boron.Therefore,the interlayer spacing has little influence on the adsorption and exchange of small ions.The main reason is that the affinity between interlayer anions and laminates has a greater impact on the ion exchange performance of LHDs.This is consistent with the research results in the literature.which generally obey the following order of stability:＞＞OH-＞-F-＞Cl-＞Br-＞＞I-[14].

3.2.3.Dissolution experiment and blank flotation of LDHs(),LDHs(Cl-)and LDHs()

The results of the dissolution experiment and the blank flotation are shown in Table 3,magnesium ion shows some dissolution,while aluminum ion shows no loss of dissolution in distilled water.Since aluminum is in the framework structure,part of the magnesium replaces the aluminum ion in the framework,which is not very stable [31].And interlayer anions are respectively released in water.After SDBS is added in blank flotation,the concentration of NO3-,Cl-andreleased from LDHs in the solution increases,mainly due to the substitution of dodecylbenzenesulfonic acid ions by other interlayer anions in the LDHs(NO3-,Cl-and).

Table 3 Dissolution and blank flotation experiments of LDHs(),LDHs(Cl-) and LDHs() respectively

Table 3 Dissolution and blank flotation experiments of LDHs(),LDHs(Cl-) and LDHs() respectively

Table 3 Dissolution and blank flotation experiments of LDHs(),LDHs(Cl-) and LDHs() respectively

3.2.4.Adsorption of boric acid and flotation experiment after adsorption by Mg-Al-LDHs

Based on our previous flotation experience to date,SDBS is chosen because it is more readily available and relatively inexpensive,which is a commonly used surfactant.SDBS is negatively charged and can be attracted to positively charged LDHs by electrostatic attraction.In the adsorption and flotation process,the boron was first adsorbed by ion exchange of LDHs powder,and after adding of SDBS,SDBS can react with LDHs by electrostatic attraction,which can bring LDHs to the foam layer to separate boron solution.

The results of adsorption of boric acid on Mg-Al-LDHs and flotation of Mg-Al-LDHs by SDBS are shown in Table 4.From the adsorption and flotation experiments,it was shown that magnesium ion has some dissolution loss,while the aluminum ion has no dissolution loss,aluminum as the main framework structure of hydrotalcite has strong binding ability with—OH groups,which is not easy to dissolve in aqueous solution,while magnesium displaces part of aluminum in the synthesis process,which has a certain degree of dissolution loss in aqueous solution,and the dissolution loss of magnesium ion is greatly affected by pH of the solution [22,38].Compared to the dissolution experiment in Table 3,the interlayer anions were displaced by the borate anions in solution,releasing more anions into the aqueous solution [39-41].After flotation through SDBS,the concentration of anions(5.96 mmol·L-1after adsorption and 7.96 mmol·L-1after flotation in solution),Cl-(6.28 mmol·L-1after adsorption and 7.92 mmol·L-1after flotation in solution) and(2.90 mmol·L-1after adsorption and 3.90 mmol·L-1after flotation in solution)increased in the solution,which was due to the replacement of the dodecylbenzenesulfonic acid ions by interlayer anions,resulting in the release of interlayer anions.Compared with the adsorption,the boron concentration in the solution after flotation had some increase (0.71 mmol·L-1after adsorption and 1.03 mmol·L-1after flotation in solution),mainly because the dodecylbenzenesulfonic acid ions in SDBS are part of the binding site between the hydrotalcite layers occupied during flotation [42].

Fig.8 shows the XRD of Mg-Al-LDHs() before and after adsorption/flotation.From the diagram it can be seen that the structure of LDHs hardly changed before and after adsorption/flotation,which shows that the structural stability of LDHs in the flotation of boron.However,we will note that the (003) peak of hydrotalcite after flotation deviates to the right,indicating that the spacing between hydrotalcite layers increases after flotation,consistent with our previous interpretation.Fig.9 shows the cycle times of Mg-Al-LDHs()used in boron adsorption/flotation.The 0.1 mol·L-1HCl and 0.1 mol·L-1NaOH were used to treat the Mg-Al-LDHs(NO3-) after the reaction for parsing and recovering.The initial boron removal rate is 86.7%,after three cycles,the boron removal rate can still reach about 80%.

Fig.8.XRD patterns of Mg-Al-LDHs () before and after adsorption/flotation.

Fig.9.The cycle times of boron removal by Mg-Al-LDHs ().

3.2.5.Mechanism analysis of flotation of boric acid in solution by SDBS/Mg-Al-LDHs()

Fig.10.FTIR spectra (a),Raman spectra (b) and XRD patterns (c) of Mg-Al-LDHs() prepared with SDBS and SDBS/boric acid solution.

From Fig.10 it can be seen that the characteristic vibrational bands of sulfonates are at 1183,1130,1039,and 1011 cm-1respectively,and the FTIR spectrum shows that dodecylbenzenesulfonate ions are present in the Mg-Al-LDHs() treated with SDBS solution and mixed solution with SDBS and boric acid.New peaks at around 1500,2800,and 3000 cm-1are also shown in the Raman spectrum of Mg-Al-LDHs() treated with SDBS solution and mixed solution with SDBS and boric acid.The XRD showed that the(003)peak of Mg-Al-LDHs()treated with SDBS had shifted to the left and the layer spacing widened,the(003)peak of Mg-Al-LDHs()treated with SDBS and boric acid,was shifted further to the left,indicating that the layer spacing was further widened.The above results demonstrate that SDBS as a flotation agent can react with LDHs and penetrate into the hydrotalcite interlayer through electrostatic attraction.Therefore,hydrotalcite in solution can be floated onto the foam layer to be separated from the solution to obtain the clarified solution.The photos before,during and after flotation are shown in Fig.11.The process can be used in the removal of boron in waste water or the preliminary removal of boron-containing solutions in industry,with the advantages of a simple process,promising large-scale application and low environment impact.

Fig.11.The photos before (a),in (b) and after (c) flotation by SDBS.

4.Conclusions

In this paper,three types of Mg-Al-LDHs with different anions including Mg-Al-LDHs(),Mg-Al-LDHs(Cl-) and Mg-Al-LDHs() were prepared and applied to boron removal in aqueous solution.Then a novel method of SDBS flotation was used to collect the LDHs particles after boron adsorption from aqueous solution.The adsorption equilibrium time and the adsorption capacity of the three LDHs were each examined.The adsorption of boron to LDHs in solution is more consistent with the quasi second-order adsorption kinetics model,indicating that the adsorption rate is governed by the square of the number of unoccupied adsorption vacancies on the adsorbent surface,and the adsorption process is controlled by the chemical adsorption mechanism.Examination of the adsorption time shows that 60 min is enough for the equilibrium adsorption capacity of the three LDHs,the maximum adsorption capacity of boron were respectively 2.0,0.98 and 0.2 mmol·g-1for LDHs(),LDHs(Cl-) and LDHs() ranging from 0 to 80 mmol·L-1with the initial boron concentrations,respectively.According to the test results,the adsorption capacity of the LDHs for boron removal in aqueous solution was found as follows:LDHs() ＞LDHs(Cl-) ＞LDHs(),which was mainly because that the boron removal process may not only be adsorbed but also carried out ion exchange,The main reason is that the affinity between interlayer anions and laminates has a greater impact on the ion exchange performance of LHDs.The affinity ofis lower than that of the other two interlayer anions.Therefore,is more readily replaced when exchanged forthan two other types of anionic hydrotalcite.

SDBS proved useful when LDHs were collected in solution,with the boron removal efficiency by SDBS and LDHs() reaching to 89.7%.After flotation through SDBS,the anions concentration in the solution increased,which was due to the replacement of the dodecylbenzenesulfonic acid ions with interlayer anions,which would increase the release of interlayer anions.Compared to adsorption,the boron concentration in the solution increased after flotation,mainly because the dodecylbenzenesulfonic acid ions in SDBS occupied part of the binding site between the hydrotalcite layers during flotation.The mechanism of flotation of boron through SDBS with LDHs was obtained as follows,as a flotation agent,SDBS can penetrate into the interlayer of hydrotalcite through electrostatic attraction.Therefore,hydrotalcite in solution can be floated on the foam layer to be separated from the solution to obtain the clarified solution.The method of separating LDHs particle from the solution after boron adsorption by SDBS is original,simple and promising for boron removal in aqueous solution.This process can also give some inspiration for liquid phase boron removal by flotation,it also has some meaning for flotation separation of other ions in wastewater treatment or other solution.

Data Availability

No data was used for the research described in the article.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (U20A20150),the National Key Research and Development Program of China (2018YFC1903802),the Youth Scientific Research Fund of Qinghai University (2022-QGY-4),and the Kunlun Talent Program of Qinghai Province.