Recovery of lithium using H4Mn3.5Ti1.5O12/reduced graphene oxide/polyacrylamide composite hydrogel from brine by Ads-ESIX process

Jingsi Cui,Huanxi Xu,Yanfeng Ding,Jingjing Tian,Xu Zhang,Guanping Jin

Anhui Key Lab of Controllable Chemical Reaction&Material Chemical Engineering,School of Chemistry and Chemical Engineering,Hefei University of Technology,Hefei 230009,China

Keywords:Lithium recovery Titanium doped lithium ion sieve Hydrogel Adsorption-electrically switched ion exchange (Ads-ESIX)

ABSTRACT Powdery Li+-imprinted manganese oxides adsorbent was widely used to the recovery of Li+,but there are some difficulties,such as poor stability in acid solution,inconvenience of operation and separation.In this work,a useful hydrogel composite based H4Mn3.5Ti1.5O12/reduced graphene oxide/polyacrylamide(HMTO-rGO/PAM) was fabricated by thermal initiation method with promising stable,conductive and selective properties.The resulting materials were characterized by field emission scanning electron microscope,infrared absorption spectrum,X-ray diffraction,X-ray photoelectron spectroscopy and electrochemical techniques.The recovery of Li+was investigated using HMTO-rGO/PAM from brine by a separated two-stage sorption statically and electrically switched ion exchange desorption process.The adsorption capacity of 51.5 mg?g-1 could be achieved with an initial Li+ concentration of 200 mg?L-1 in pH 10,at 45 °C for 12 h.Li+ ions could be quickly desorbed by cyclic voltammetry (CV) in pH 3,0.1 mol?L-1 HCl/NH4Cl accompanying the exchange of Li+ and H+(NH4+) and the transfer of LMTO-rGO/PAM to HMTO-rGO/PAM.

1.Introduction

In recent decades,the recovery of lithium has attracted increasing attention because of the widespread applications,such as lithium-ion batteries,catalysts,medicine,ceramic glass [1–6].It was reported that annual demand of lithium will achieve 221 billion USD in the year of 2024 [7].Fortunately,over 80% potassium was removed from the residual salt lake brine of Qarham playa of the Qaidam Basin in China,which still contains abundant lithium resource in total with various concentrations (0.01–408.8 3 mg?L-1) [8].Thus,the recovery of Li+from the residual salt lake brine is meaningful in terms of the industry and resource utilization [9,10].Among various uptake methods including precipitation,adsorption,extraction,membrane as well as electrochemistry,adsorption proved to be an effective way with good selectivity [11–14].However,repeated adsorption and desorption of Li+have to use strong acid,which can cause the corrosion of equipment.Electrochemical techniques are environmentfriendly method using electrons as a driving force.The selective electro-separation of Li+with coexisting metal ions is a challenge in brine.Hence,the combination of intermittent adsorption (Ads)and electrically switched ion exchange(ESIX)desorption processes could be a wise choice.Then,selective adsorption of Li+could be performed by agitating the adsorbent in brine,and electrically switched ion exchange desorption could be carried out using the CV method in the recovery tank (Ads-ESIX).In our early study,Ads-ESIX method has been successfully applied to the recovery of Cs+[15,16].Recently,ESIX technique has been developed in Li+ion separation and recovery,for example,H1.6Mn1.6O4/reduced graphene oxide composite film applied to Li+extraction from sea water and reaches 38.78 mg?g-1at the adsorption equilibrium[17].In this case,a kind of adsorbent with good selectivity,conductivity,hydroscopicity and mechanical strength is urgently needed to work as the working electrode for Ads-ESIX technique.

According to previous reports,Li+-imprinted lithium titanium oxides(LTO,Li2TiO3)[18],Li+-imprinted lithium manganese oxides(LMO,Li1.6Mn1.6O4and Li4Mn5O12) [19,20] are used as the adsorbents for Li+recovery due to their non-toxic and low cost.Spinel structure LMO is unstable because of its dissolution in acidic solution,despite it has higher adsorption capacity and good selectivity for Li+.In compassion with that of LMO,LTO shows excellent stable spinel structure but relatively weak selectivity.Thus,Ti doped LMO(LMTO) was prepared to improve the stability during the ion exchange (desorption) of Li+and H+in acidic solution [21].However,LMO adsorbents could not be used directly in the brine with a recycling difficulty in industrial-scale [13,22,23].Hence,researchers dedicate to immobilizing the LMO powder into spheres,foams,membranes and fibers using various binders,such as chitosan,polyvinyl chloride polysulfone,glutaraldehyde,agar,polyvinyl chloride and polyacrylonitrile [13,22,24–29],respectively.These studies also indicated that the binders could hinder the adsorption because of leakage or water-blocking.Fortunately,polyacrylamide is a promising binding material with better hydrophilia,which hardly affects the adsorption ability [23].It is discovered that,reduced graphene oxide (rGO) could modify the mechanical and thermal properties of polyacrylamide by building a three-dimensional network structure [30–33].Moreover,rGO present excellent conductivity and large surface area,and it could stably work at relative high voltage,which was proved to be an ideal material to assist building working electrode for the extraction of Li+[34].

In this work,lithium manganese titanium composite oxides(LMTO,Li4Mn3.5Ti1.5O12) was synthesized by sol-gel method.After the desorption of Li+from the LMTO,H4Mn3.5Ti1.5O12(HMTO)could be obtained with the formation of Li+imprint.Then,polyacrylamide and reduced graphene oxide composite hydrogel were cross linked with HMTO to form stable,conductive adsorbent(HMTO-rGO/PAM).The recovery of Li+was investigated from brine using HMTO-rGO/PAM by Ads-ESIX method in detail.Under the optimum conditions,HMTO-rGO/PAM was further applied to the recovery of Li+from real brine (which was obtained from Qarham playa of the Qaidam Basin in China).

2.Experimental

2.1.Chemicals and apparatus

Terabutyltitanate (Ti(OC4H9)4) was obtained from Shanghai Lingfeng Chemical Reagent Co.,Ltd.(Shanghai,China).Lithium acetate dehydrate (LiCH3COO?2H2O) was bought from Meryer Chemical Technology Co.,Ltd.(Shanghai,China).Manganese carbonate (MnCO3) andN,N-methylenebisacrylamidebis-acrylamide(BIS) were acquired from Shanghai Macklin Biochemical Co.,Ltd.(Shanghai,China).Natural graphite powder was purchased from Qingdao Jin Lai Graphite Co.,Ltd.(Qingdao,China).Acrylamide(AM),ammonium persulfate (APS),L-ascorbic acid (AA),triethanolamine (TEA),ammonia solution and ammonium chloride were purchased from Sinopharm Chemical Reagent Co.,Ltd.(Shanghai,China).All of these reagents were analytical grade.All chemicals are utilized without further purification.

Cyclic voltammetric(CV)was used to the desorption of Li+from LMTO-rGO/PAM using CHI660B electrochemical workstation(Chenhua,Shanghai,China).The three-electrode system contained LMTO-rGO/PAM as a working electrode,saturated calomel motor as a reference electrode and platinum electrode as an opposite electrode.Infrared spectra (IR) data were measured at IR 200(Nicolet,America).X-ray diffraction (XRD) of the samples is characterized by an X’Pert PRO MPD diffractometer with Cu Kα radiation source (PANalytical,Netherlands) at a working voltage of 40 kV.The morphologies of the samples were detected using scanning electronic microscopy (FE-SEM) on a SU8020 field emission scanning electron microanalyser(Hitachi,Japan).X-ray photoelectron spectroscopy (XPS) data were collected on an ESCALAB250Xi spectrometer (Thermo,America) with Mg K-Alpha radiation as the source of excitation.Li+content is quantified by AA800 atomic absorption spectrophotometer (PerkinEimei,America).

2.2.Preparation of materials

HMTO (H4Mn5-xTixO12).HMTO was synthesized using sol-gel method [35].LiCH3COO?2H2O was dissolved in 85% ethanol (solution A).Ti(OC4H9)4,MnCO3and 1 ml TEA (chelating agent) were dissolved in absolute ethanol with magnetic stirring of 1 h (solution B),and the molar ratio of Li:Mn:Ti was 4:(5-x):x(x=0,1.5,3,4 and 5).Then,solution A was dropped wisely into solution B under stirring,in which,1 ml of HCl (36%) was added as a catalyst.The resulting sol solution was stirring under 60°C to form dry precursor.After the dry precursor was grounded well,and heated at 600°C in air atmosphere for 5 h,it was further calcinated for 10 h at 800 °C,then,powder-like Li4Mn5-xTixO12could be obtained with a spinel structure (labeled as LMTO).Finally,LMTO was put in 1 mol?L-1HCl for 24 h to finish exchange between H+and Li+.The product underwent vacuum filtration and dry at 80 °C for 15 h,labeled as HMTO.

HMTO-rGO/PAM.Graphite oxide(GO)was prepared by modified Hummer’s method [36].HMTO-rGO/PAM hydrogel was synthesized by thermally initiated polymerization in situ method [30].0.3 g of HMTO was put into 10 ml of ultrasound treated GO solution (concentration:5 mg?ml-1) with stirring of 3 h,in which,1 g of AM (monomer),0.03 g of BIS (cross linker),0.015 g of APS (initiator) were added under magnetic stirring below 5 °C for 0.5 h.The mixture starts free radical polymerization at 60 °C in thermostatic water bath for 4.5 h.Then,polyacrylamide and graphene oxide hydrogel crosslinked HMTO composite could be obtained(HMTO-GO/PAM),which was further treated in 100 ml,2 mg?ml-1of L-ascorbic acid (AA) to conduct mild chemical reduction,the reduced-graphene oxide hydrogel crosslinked HMTO composite present increased stability,conductivity and mechanical strength,which marked as HMTO-rGO/PAM [37].HMTO-rGO/PAM was smashed into granulated ion-sieve,and dried at 65 °C for the following application.

2.3.Batch experiments for the recovery of Li+ using Ads-ESIX method

A pre-weighed (m) HMTO-rGO/PAM was added into LiCl solutions with known volume (V),pH and initial Li+concentration(C0).The pH was adjusted using 0.1 mol?L-1NH4Cl/0.5 mol?L-1NH3?H2O.The uptake of Li+was carried out under stirring (150 r?min-1) for 24 h to reach adsorption equilibrium.The residual Li+concentration (Ce) was measured by AA800 atomic adsorption spectrophotometer.Equilibrium (qe) adsorption capacity of Li+was calculated by Eq.(1) [16,18].The present investigation was performed in LiCl solution from pH 6 to 11.

Adsorption kinetic.Adsorption kinetic experiments were investigated by Eqs.(2) and (3),which related to Laguerre pseudo-first order model and pseudo-second order model.The adsorption (qt)for the samples depending on time (t) was calculated by Eq.(4)before the equilibrium was achieved.

Sorption isotherms.The uptake mechanism and surface properties of HMTO-rGO/PAM towards Li+could be provided by equilibrium isotherms [38].Adsorption isotherms were investigated by Langmuir model of Eq.(5) and Freundlich model of Eq.(6),in whichKLandKFare the Langmuir and Freundlich isotherm parameter,respectively.qeis the uptake amount of Li+at equilibrium,qmis the maximum adsorption capacity of adsorbent,andnis the adsorption intensity at unit concentration [16,18,39].

Electrically switched ion exchange desorption (ESIX).4 slices of HMTO-rGO/PAM(every slice 0.5 cm×1 cm×0.2 cm,0.2 g in total)were put in Li+containing solution(pH 10,100 ml,200 mg?L-1Li+)to uptake Li+(LMTO-rGO/PAM).After adsorption,the LMTO-rGO/PAM was fixed at electrode holder,then,the Li+at LMTO-rGO/PAM could be quickly desorbed by the exchange of H+(NH4+) and Li+using CV method in pH 3,0.1 mol?L-1NH4Cl/HCl (300 cycles,potential range of 0.1–1.2 V and scan rate of 50 mV?s-1).HMTOrGO/PAM could be regenerated with Mn3+oxidize to Mn4+.After the adsorption,the residual Li+in the solution was measured by AAS.The desorption behaviors were characterized by AAS,CV,XPS,SEM,IR and XRD techniques.The desorption capacity of Li+was calculated by Eq.(7).Whereqeis the equilibrium desorption capacity of Li+,Ceis the desorbed Li+concentration,Vis the volume of desorption solution,mis the mass of the material.

3.Results and Discussion

3.1.Characterization of materials

FE-SEM.Fig.1 shows SEM images of relative materials.In Fig.1(A),the morphology of LMTO presents cubic crystals with an average size of (20 ± 5) nm (inset (a)),which was transformed into HMTO to form Li+ionic imprint by HCl treatment(inset(b)),meanwhile,the cubic granules remain stable.To conveniently separate the powder ion sieve,polyacrylamide and reduced-graphene oxide hydrogel were crossed with HMTO to form granulated adsorbent(HMTO-rGO/PAM) with increased conductivity and mechanical strength.In Fig.1(B),the morphology characterization of HMTOrGO/PAM is still stable even after continuous 3 times of Ads-ESIX process.In Fig.1(C),to further investigate the structure,the above HMTO-rGO/PAM was treated by liquid nitrogen,a very well alveolate network structure could be seen with a great deal of nanoparticles (HMTO) at the surface (inset (a)).Moreover,the SEM image of rGO/PAM (inset (b)) also shows alveolate network structure but no particles were observed.Thus,the present HMTO-rGO/PAM is a promising lithium ion sieve,which could be used in the following recovery of Li+.

Fig.1.FE-SEM images of (A) HMTO-rGO/PAM,LMTO (inset(a)),HMTO (inset (b));(B) HMTO-rGO/PAM after continuous 3 times of Li+ recovery by Ads-ESIX process;(C)HMTO-rGO/PAM treated by liquid nitrogen,HMTO-rGO/PAM (inset (a)),rGO/PAM (inset (b)).

XRD.Fig.2(A)shows the XRD patterns of Li4Mn5-xTixO12(LMTO,0 ≤x≤5)compounds.The terminal phases are Li4Ti5O12[40,41]or Li4Mn5O12[42].With the increase of manganese concentration,the color of LMTO changed from white(x=5)to brown and final black(x=0).All the LMTOs show similar spinel structure with a series of typical peaks,and the materials are single-phase with a slight shift in cell parameter fromx=0 tox=5 [41].Reflection peaks of the materials become broadened with the increase of Ti content fromx=0 tox=1.5,then,narrowed with the further increased Ti.This character can be attributed to local strains,which concerned with the different cation size.The phase of LMTO is the strongest at a rate of 70% Mn4+and 30% Ti4+(x=1.5),and in line with the previous reports [41,43].Fig.2(B) shows the XRD patterns of LMTO (a),HMTO (b);HMTO-rGO/PAM (c),after continuous 3 times of Ads-ESIX process including the adsorption of Li+(d) and desorption of Li+(e).The XRD patterns of LMTO (a) and HMTO (b) are very similar,suggesting they have the same spinel structure [44].The 2θ data of LMTO (HMTO) locate at 18.8° (19.1°),36.3° (37.0°) and 44.1°(44.9°),which correspond to the(111),(311)and(400)crystal planes of spinel,respectively.The interplanar distance (d) values of LMTO (HMTO) are 4.7 (4.6),2.5 (2.4) and 2.0 (2.0),respectively.The XRD pattern of HMTO shifts to a larger angle than that of LMTO,indicating the shrinkage of the lattice,which is consistent with the decreased d values [44].All these peaks could be seen in HMTO-rGO/PAM before Li+adsorption(c),and after continuous 3 times of Ads-ESIX process including the adsorption of Li+(d)and desorption of Li+(e).It illustrates that the present materials are reusable.Meanwhile,there is a broad/short peak at about 20°matching to PAM in HMTO-rGO/PAM [30].

FT-IR.Fig.3 shows the IR spectra of LMTO(a),HMTO(b),HMTOrGO/PAM before Li+adsorption (c),and after continuous 3 times Ads-ESIX process including the adsorption of Li+(d)and desorption of Li+(e).A typical band at 602 cm-1probably relates to Mn3.5Ti1.5-O12groups (a) [23],which become sharp after the treatment in 1 mol?L-1HCl (b).In curve c,the band at 3018 cm-1relates to-NH2vibration of the amide group.The band at 2927 cm-1matches to the -CH2symmetry and asymmetry stretching vibration of long alkyl chain,suggesting the synthesis of polyacrylamide hydrogel [23].Moreover,the bands at 1650 and 1452 cm-1are concerned with antisymmetry and symmetry stretching vibration of the carbonyl group (-C=O).The band at 1110 cm-1relates to C-C stretching vibration,and the band at 602 cm-1still could be observed,indicating the formation of HMTO-rGO/PAM.After the adsorption of Li+(d) and desorption by CV method (e),the same bands still could be seen.These indicate that the present HMTOrGO/PAM is stable in the continuous Ads-ESIX process.

Fig.3.IR spectra of LMTO(a),HMTO(b),HMTO-rGO/PAM before Li+adsorption(c),after continuous 3 time of Ads-ESIX process on adsorption of Li+(d)and desorption of Li+ (e).

Fig.4.CVs of HMTO-rGO/PAM before(a),after(b)the Li+recovery,obtained LMTOrGO/PAM desorbed by CV for 1 (c),100 (d),200 (e) and 300 (f) cycles.System:pH 3.0,0.1 mol?L-1 NH4Cl/HCl.Scan rate:50 mV?s-1.

Electrically switched ion exchange (ESIX) desorption.Fig.4 shows the CVs of HMTO-rGO/PAM before (a),after the uptake of Li+(b,LMTO-rGO/PAM),the obtained LMTO-rGO/PAM desorbed by CV for 1 (c),100 (d),200 (e) and 300 (f) cycles in pH 3,0.1 mol?L-1NH4Cl/HCl.A pair of peaks (1.08/0.72 V,ΔE=0.36 V) relate the redox of Mn3+/Mn4+in HMTO-rGO/PAM with a quasi-reversible process[45].The distance of redox peaks(1.09/0.42 V,ΔE=0.67 V)is obviously wider compared to that at LMTO-rGO/PAM (c).Since the Li+ions in the LMTO-rGO/PAM could be desorbed by H+(NH4+)due to their similar size using ESIX technic,in the continuous CV treatment within 300 cycles,the CVs are gradually recovered to the original position in curve a,suggesting the regeneration of the adsorbent.Thus,it indicates that the present Ads-ESIX method could be used to the recovery of Li+.

XPS.Fig.5 shows the XPS spectra of relative materials during the recovery process of Li+.In Fig.5(A),the XPS survey of LMTO(a) and HMTO (b) are almost the same with the typical peaks of Mn,Ti,O and C.LMTO can turn to HMTO after stirring in 1 mol?L-1HCl for 24 h,the peaks of O1s and Mn2p became sharper,indicating a higher oxidation degree in HMTO.Although their intensity are decreased due to the mix with rGO/PAM.The XPS survey of HMTO-rGO/PAM (c),LMTO-rGO/PAM recovered Li+(d) and HMTO-rGO/PAM desorbed Li+(e) are basically consist with that of curves a and b except a new N1s peak (come from rGO/PAM).It suggests that the HMTO mixed well with the rGO/PAM in HMTO-rGO/PAM,which shows excellent stability in the recovery process of Li+.In Fig.5(B),LMTO (a) shows Li1s peak at 54.1 eV,which sharply decreases at HMTO after the desorption of Li+(b),and disappears at HMTO-rGO/PAM(c).However,after the recovery of Li+,the Li1s peak reappears at LMTO-rGO/PAM with positive shift of 1 eV (d),after the desorption of Li+,it disappears again at HMTO-rGO/PAM (e),suggesting the regeneration of HMTO-rGO/PAM.In Fig.5(C),the XPS spectra of Mn2p1/2 and Mn2p2/3 at 654.4 and 642.6 eV could be observed at LMTO in curve a and curve d.Mn2p3/2 at 642.6 eV locates between Mn3+and Mn4+at LMTO,which is more close to Mn4+[46].This implies that the LMTO probably contain a large amount of Mn4+and a small amount of Mn3+.The peak of Mn2p3/2 in HMTO (b,c and e) positively shifts to 642.84 eV compared to that in LMTO(a,d),which almost approach Mn4+value (642.9 eV),indicating a dominant position of Mn4+in HMTO [40].The content of Mn3+and Mn4+is difficult to figure out because the XPS spectra are often influenced by many factors[47].In Fig.5(D),the typical peaks of Ti2p3/2 at 458.2 eV and Ti2p1/2 at 464.0 eV relate to TiO2(Ti4+) from curve a to curve e,suggesting a stable oxidation state during the transfer of LMTO and HMTO [35].Therefore,the present HMTO-rGO/PAM (LMTOrGO/PAM) are stable during the Ads-ESIX process,and the materials and method could be used for the recovery of Li+.Fig.6 shows the preparation process of HMTO-rGO/PAM and recovery mechanism of Li+.

3.2.A batch adsorption experiments

pH.The effect of pH on the adsorption capacity of Li+was firstly studied in Fig.7.Because the adsorption properties of HMTO-rGO/PAM much depend on the exchange of H+and Li+.Moreover,HMTO-rGO/PAM will swell a lot under strong base condition,thus,the pH values for the recovery of Li+was investigated between 6 and 11.The adsorption capacity of Li+is increased gradually from pH 6 to pH 10,then,declined after pH 10.The lower Li+sorption appeared not only at low pH due to the competition of H+and Li+,but also at high pH (>10) owing to the competition ofand Li+(arising from pH adjustment with NH4Cl and NH3?H2O).Thus,pH 10 could be selected.

Fig.5.XPS spectra of relative materials during the recovery process of Li+:(A)XPS survey and XPS spectra of(B)Li,(C)Mn,(D)Ti at LMTO(a),HMTO(b),HMTO-rGO/PAM(c),LMTO-rGO/PAM recovered Li+ (d) and HMTO-rGO/PAM desorbed Li+ (e).

Fig.6.Scheme on preparation of HMTO-rGO/PAM and recovery of Li+.

Fig.7.The effect of pH on the adsorption capacity of Li+.

Kinetic model.The adsorption capacity of Li+depending on the time is shown in Fig.8(A),and the relative kinetic parameters are listed in Table 1.R2in the second-order kinetic model is obviously higher than that in the first-order kinetic model.Theqecalculated from the second-order kinetic model is more agree with the experiment results[15,16],which implies that the rate controlling step is chemical adsorption process [48],and this is consist with the previous reports for lithium manganese oxide ion sieve[49,50].Therefore,the second-order kinetic model more fits the adsorption process compared to that of first-order kinetic model.

Adsorption isotherms.The adsorption isotherms at different temperatures are shown in Fig.8(B),and parameters are summarized in Table 2.The Li+adsorption equilibrium at HMTO-rGO/PAM could be explained by Langmuir isotherms with monomolecular adsorption.The maximum adsorption capacities(qm)are 34.98,42.45 and 51.54 mg?g-1at 25,35 and 45°C,respectively.Conditions:adsorption time is 24 h,volume 100 ml,0.2 g dry HMTO-rGO/PAM,pH 10,0.1 mol?L-1NH4Cl/0.5 mol?L-1NH3?H2O.

Table 1 Parameters of the pseudo-first order and pseudo-second order for the adsorption of Li+①

Table 2 Different model parameters for the adsorption of Li+①

Table 3 Composition of the salt lake brine in Qarham playa of Qaidam basin (pH 5.6)①

Interferes and Reusability.According to the results in Figs.4,5 and 7,the ESIX desorption conditions of Li+was performed by CV method in 30 ml,pH 3.0,0.1 mol?L-1NH4Cl/HCl within 300 cycles.Selective experiment was performed at the conditions of pH 10.0,[Li+]0=200 mg?L-1,25 °C and 12 h.0.2 g dry HMTO-rGO/PAM was putted in 100 ml aqueous solution with a binary mixture of Li+and Na+,K+,Ca2+,Mg2+,respectively.The left concentration of Li+was detected using AA800 spectrophotometer after 12 h adsorption.The adsorption capacity of Li+is 25.92 mg?g-1without interferes.The adsorption capacities of Li+with a binary mixture of Li+and 5 times of Na+,K+and Ca2+,and 10 times of Mg2+are 24.58,23.75,25.50,22.08 mg?g-1,respectively,which still could reach 85.2%of original value(25.92 mg?g-1).This illustrates that the present material shows good selectivity and reusability.The selectivity mechanism of spinel HMTO for lithium can be seen in Fig.6.H+size is almost equivalent to Li+,hence Li+could be exchanged by H+from the LMTO spinel struture in the desorption process.However,other cations like Na+,K+,Ca2+,Mg2+are larger than Li+,therefore these cations are difficult to be adsorped by HMTO.

Fig.8.(A) Effect of time on the adsorption capacity of Li+ by HMTO-rGO/PAM in 100 mg?L-1 (■,a),200 mg?L-1 (□,b) and 300 mg?L-1 (▲,c) Li+ solution at 25 °C.(B)Adsorption isotherms of Li+ at 25 °C (■),35 °C (□) and 45 °C (▲) (Langmuir solid,Fruendlich dash).

Fig.9.(A)Effect of time on the adsorption capacity of Li+by HMTO-rGO/PAM in real brine(initial Li+concentration 134.7 mg?L-1).(B)The desorption quantity of Li+using CVs method depending on the cycles.Insert:The desorption quantity of Li+ using static stripping method depending on time.

3.3.Application

HMTO-rGO/PAM was used to the recovery of Li+from the real brine by Ads-ESIX process,the composite was listed in Table 3.0.2 g dry HMTO-rGO/PAM was immerged in 100 ml brine at 25°C,pH 8.0 was adjusted using NH3?H2O,the initial concentration of Li+is 134.7 mg?L-1.In Fig.9(A),after 24 h adsorption,the absorption equilibrium could reach 22.12 mg?g-1.Then,Li+could be desorbed by CVs in 30 ml,pH 3,0.1 mol?L-1NH4Cl/HCl.In Fig.9(B),the desorption of Li+was performed respectively using CVs or static desorption method in pH 3,0.1 mol?L-1NH4Cl/HCl.It is noticed that the desorption capacity only reaches 13.35 mg?g-1after 16 h static desorption.However,the desorption capacity is 13.12 mg?g-1after 300 cycles desorption (nearly 2 h).Moreover,after continuous 5 times of Ads-ESIX processes,the residual Li+concentration is 27 mg?L-1in brine,over 79% Li+could be recovered from the brine.Thus,the research results confirmed that the present Ads-ESIX method shows more efficiency for the Li+recovery from the real brine.After 5th cycle,the adsorption capacity was found to be 19.76 mg?g-1,which is 89.33% of that detected at the first time(22.12 mg?g-1).And further study is necessary.

4.Conclusions

HMTO-rGO/PAM was prepared with good conductivity,stability and operability,which showed promising selective adsorption towards Li+ions.In the adsorption process,Li+could be selectively adsorbed by HMTO-rGO/PAM from brine solution with pseudosecond order model.In the ESIX processes,Li+could be effectively desorbed by cyclic voltammetry method from the LMTO-rGO/PAM with the regeneration of HMTO in pH 3,0.1 mol?L-1NH4Cl/HCl.Thus,the present investigation could reveal a usefully approach to recover Li+from the brine with increased efficiency.

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 supported by the Ministry of Science and Technology of China (Science and Technology to Boost Economy 2020 Key Project,SQ2020YFF0412719 and SQ2020YFF0404901).The Key Research and Development and Transformation Program Funding in Qinghai Province (2021-GX-105).Anhui Province Key Research and Development Plan (1804e03020316).