Xue Cai,Tao Lin,Neng Zhang,Xiaoyao Wei,Xuefeng Yin,

1.College of Bioresources Chemical and Materials Engineering,Shaanxi University of Science &Technology,Xi'an,Shaanxi Province,710021,China

2.Shaanxi Province Key Lab of Papermaking Technology and Specialty Paper,Key Lab of Auxiliary Chemistry & Technology for Chemical Industry,Ministry of Education,Xi'an,Shaanxi Province,710021,China

3.National Demonstration Center for Experimental Light Chemistry Engineering Education,Shaanxi University of Science&Technology,Xi'an,Shaanxi Province,710021,China

Abstract:With the use of acrylic acid(AA)as a monomer,humic acid(HA)and starch as raw materials,potassium persulfate(KPS)as initiator,and N,Ndimethylacrylamide(MBA)as a cross-linking agent,AA/HA/Starch graft copolymer was prepared and characterized by SEM and FT-IR.The effects of temperature,adsorption time,adsorbent dosage,pH value and Cu2+initial concentration of the solution on the adsorption performance of the crosslinked graft copolymer were also investigated.The results showed that the Cu2+adsorption capacity of the AA/HA/Starch graft copolymer increased firstly and then decreased with increasing adsorbent dosage and the initial pH value of Cu2+solution.With the increase of Cu2+initial concentration and the extension of adsorption time,the adsorption amount of Cu2+increased rapidly and then stabilized.And it decreased slightly with the increase of temperature.At pH value of 5.5,temperature of 298 K,adsorbent dosage of 50 mg,adsorption time of 125 min,and 100 mL Cu2+solution with Cu2+initial concentration of 100 mg/L,the Cu2+adsorption capacity of the crosslinked graft copolymer was 238 mg/g.The adsorption of Cu2+by the adsorbent followed the pseudo-second-order kinetic equation and Langmuir isothermal adsorption model,and the adsorption was attached to monolayer chemical adsorption.This study proved that AA/HA/Starch graft copolymer could be used as an efficient adsorbent for the removal of harmful and toxic metal cations such as Cu2+from industrial wastewater.

Keywords:humic acid;starch;acrylic acid;adsorption materials;Cu2+

1 Introduction

Humic acid(HA)is well known to coal and soil chemists.It is a natural macromolecular organic substance[1-3]produced by the decomposition and transformation of animal and plant matter via geochemical reactions or microorganisms.It promotes plant growth,improves soil quality,and increases agricultural production,etc.It mainly contains—OH,—COOH,—SO3H,and other active functional groups,thereby facilitating various chemical reactions[4-9].Not only does it have good physiological activity,absorption,and complexation,but also exhibit good adsorption of heavy metal ions[10].Presently,the rapidly growing green environmental trends have been challenging society's traditional mindsets.Therefore,the development and application of HA have received extensive attention[11].

Starch is a white powder with many hydroxyl groups that exhibit strong hydrophilicity.However,starch particles are insoluble in water,and only small amounts can be dissolved in organic solvents such as dimethyl sulfoxide and N,N-dimethylformamide[12].With the continuous emergence of new products,the need for starch with altered properties is increasing.Some properties of natural starch,such as its insolubility in cold water and its thermal instability does not meet the specific needs for industrial development.Therefore,it requires the modification of its physical and chemical properties.The adsorption capacity of starch[13]can be improved by grafting various functional groups onto the grid structure or framework of crosslinked starch.The new functional groups can increase the surface polarity and hydrophilicity,thus enhancing the adsorption capacity toward polar adsorbates.Ion exchange and complexation play an important role in the adsorption of metal ions by starch-based adsorbents.Because of the persistence,toxicity,and bioaccumulation of heavy metal ions,water pollution caused by these ions has become a serious environmental problem[14-16].The traditional methods for removing heavy metals from wastewater exhibit severe limitations.

Therefore,after considering the functional roles of different materials,we selected acrylic acid(AA)as a monomer,potassium persulfate(KPS)as the initiator,and N,N-dimethylbisacrylamide(MBA)as the crosslinking agent.AA/HA/Starch graft copolymers were obtained after graft copolymerization of HA and starch.An orthogonal experiment was carried out under different polymerization conditions,and the optimal preparation conditions for the AA/HA/Starch graft copolymers were obtained with the best adsorption amount of Cu2+to determine the processing parameters and effects of different reaction conditions on adsorption performance.The unreacted monomers were subsequently washed with distilled water to obtain hydrogel adsorbents with a high capacity for the adsorption of Cu2+via this synthetic aqueous solution method[17].

The effects of pH value,temperature,initial concentration of Cu2+,and the dosage of adsorbent on the Cu2+adsorption properties of AA/HA/Starch graft copolymers were evaluated,and the adsorption kinetics and isothermal model of Cu2+adsorption were studied.Hence,an environmentally friendly adsorbent,AA/HA/Starch graft copolymer,is synthesized and shown to be economical,easy to use,and efficient in the removal of heavy metals.

2 Experimental

2.1 Materials and instruments

HA(fulvic acid content greater than 90%)was obtained from Ji'ning Baichuan Chemical Co.,Ltd.and starch(soluble starch)was obtained from Tianjin Kemio Chemical Reagent Development Center.KPS,NaOH,AA,MBA,and copper sulfate were obtained from the Tianjin Damao Chemical Reagent Factory.Hydrochloric acid was obtained from the Sinopharm Group Chemical Reagent Co.,Ltd.All of the above reagents were analytical pure.

Instrumentation used included an XMTD-7000 temperature controller,manufactured by Shanghai Keheng Industrial Development Co.,Ltd.,an Equnix55 Fourier-transfer infrared(FT-IR)spectrometer from Brooke Company(Germany),a JEOL JSM-6700F scanning electron microscope(SEM)from Japan Electronics Co.,Ltd.,a UV-752 spectrophotometer,an MT-5000 pH meter from Shanghai Yukang Science and Education Equipment Co.,Ltd.,and a JA2003 analytical balance from Shanghai Liangping Instrument Co.,Ltd.

2.2 Methods

2.2.1 Synthesis of AA/HA/Starch graft copolymer

A 250 mL three-neck flask was charged with HA(10 g)and a certain amount of starch,based on HA dosage,which were mixed using a numerically controlled ultrasonic scrubber for 30 min.After completion of the ultrasound treatment,the mixture was heated to 55℃and stirred in a water bath.The mixed solid of KSP and MBA(mass fraction of 0.6% and 1.0% of the total monomer content,respectively)was dissolved in distilled water.This solution was placed in a constant pressure drop funnel,and the desired amount of AA neutralized with NaOH was added into another drop funnel.When the temperature was stabilized at 55℃,the chemicals in the two funnels were simultaneously added to the three-neck flask in a dropwise manner within 15 min.Then,the temperature was increased to 65℃until the reaction system became viscous.The solid adsorbent could then be obtained by taking the reaction product out and placing it in an oven for drying.The main factors affecting the preparation of AA/HA/Starch graft copolymers are as follows:(a)the mass ratio of HA to starch(HA:starch),(b)mass ratio of HA to AA(HA∶AA),and(c)the reaction temperature(T).The amounts of initiator and crosslinker were 0.6% and 1.0% of the total amount of monomer,respectively.The influence of the above three factors on the preparation of AA/HA/Starch graft copolymers was discussed by an orthogonal experiment(as shown in Table 1),and the optimum conditions were determined by the amount of Cu2+adsorbed.

2.2.2 Characterization and testing of AA/HA/Starch graft copolymer

(1)FT-IR analysis The samples were ground into a powder of specifiedparticle size,and the infrared spectra of the samples were recorded on an Equinx 55 FT-IR spectrometer using a KBr pellet.The scanning range was 500-4000 cm-1.

Table 1 Orthogonal experiment table

(2)SEM analysis

The sample was fixed on the metal sample stage with conductive glue,and the gold was sprayed to completely cover the surface of the sample,and then the surface morphology of the sample was observed with a SEM.The thickness of the gold plating was approximately 25 nm,and the voltage used was 25 kV.

2.3 Adsorption of Cu2+by AA/HA/Starch graft copolymer

2.3.1 Construction of the Cu2+standard curve

A 1 mg/mL Cu2+standard solution(10 mL)was accurately transferred into a 100 mL volumetric flask,diluted to the mark with deionized water,shaken,and stored for further use.The concentration of the Cu2+solution was 100 mg/L and the initial pH value of the Cu2+solution was 2.5.And 0.01 g disodium diethyldithiocarbamate trihydrate was accurately weighed,dissolved in deionized water,and transferred to a 100 mL volumetric flask,diluted to the mark with deionized water,shaken,and stored for future use.The concentration of this solution was 100 mg/L.Then,0.4,0.6,0.8,1.2,1.6,2.0,4.0,6.0,8.0,10.0,12.0,14.0,16.0,20.0,and 25.0 mL 100 mg/L Cu2+solution,prepared as above,were added into 100 mL volumetric flasks,respectively,and marked with No.1-15 and then the sodium diethyldithiocarbamate trihydrate solution with the same volume as Cu2+solution was added into the No.1-15 volumetric flasks successively.The mixed solution was adjusted to approximately 9 of the pH value with aqueous ammonia,diluted to the mark with deionized water,and shaken,and stored for use.The absorbance of the mixed solution at 450 nm was measured,and a Cu2+standard curve was constructed.

2.3.2 Determination of the Cu2+adsorption capacity and Cu2+adsorption efficiency of AA/HA/Starch graft copolymer

The absorbance of the Cu2+solutions at 450 nm was measured with a UV spectrophotometer.The equilibrium concentration(Ce,mg/L)of Cu2+was calculated according to the Cu2+standard curve,and the equilibrium adsorption capacity(qe,mg/g)was calculated using the following formula[18]:

whereVis the Cu2+solution volume(L),C0is the initial concentration of the Cu2+solution(mg/L),ηis the adsorption efficiency,andWis the mass of the AA/HA/Starch graft copolymer(g).

2.3.3 Factors affecting Cu2+adsorption by the AA/HA/Starch graft copolymer

(1)Effect of adsorption time on Cu2+adsorption capacity at different temperatures

0.05 g AA/HA/Starch graft copolymer was weighed before adding to 100 mL 100 mg/L Cu2+solution with an initial pH value of 2.5.The solution was oscillated for different periods(25,50,75,100,125,150,175,200,and 225 min,respectively)in a constant-temperature water bath oscillator at 25,30,35,40,and 45℃,respectively.

(2)Effect of initial pH value on Cu2+adsorption capacity 0.05 g AA/HA/Starch graft copolymer was weighed and added to a 250 mL conical flask along with 100 mL 100 mg/L Cu2+solution.The pH value of Cu2+solution was adjusted to 2.5,3.5,4.5,5.5,6.5,7.5,and 8.5,respectively.The solutions were shaken using a constanttemperature water bath oscillator at 25°C for 225 min.(3)Effect of Cu2+initial concentration on Cu2+adsorption capacity

0.05 g AA/HA/Starch graft copolymer was added to a 250 mL conical flask,and 100 mL different concentrations(100,200,300,400,and 500 mg/L)of the Cu2+solutions were added to each flask.The pH value was adjusted to 2.5,and all of the solutions were placed in a constant-temperature water bath oscillator at 25℃for 225 min.

(4)Effect of adsorbent dosage on Cu2+adsorption capacity

Different mass(25,50,75,and 100 mg)of AA/HA/Starch graft copolymers were added to 250 mL conical flasks separately,followed by the addition of 100 mL 100 mg/L Cu2+solution to each flask.The solution pH value was adjusted to 2.5.The samples were all placed in a constant-temperature water bath oscillator at 25°C for 225 min.

2.3.4 Adsorption kinetics

The kinetics of adsorption were studied by determining the rate of solute adsorption.The experimental data were then fitted with a kinetic model to identify the adsorption mechanism.

(1)First-order model of adsorption kinetics

The first-order model of adsorption kinetics can be described by the quasi-first-order kinetics equation of Lagergren[19]:

whereqt(mg/g)is the adsorption amount of Cu2+on the adsorbent at timet(min),andk1(min-1)is the equation coefficient of quasi-first order kinetics.

(2)Second-order model of adsorption kinetics

The second-order model of adsorption kinetics can be described by the quasi-second-order kinetic equation of Lagergren[20]:

wherek2(g/(mg·min))is the equation coefficient of quasi-second-order kinetics.

2.3.5 Adsorption thermodynamics behavior

In this study,Freundlich and Langmuir[21]models were used to analyze the adsorption thermodynamic behavior.The linear equations are as follows:

(1)Langmuir adsorption isotherm

whereq0is maximum adsorption capacity of adsorption material,Ceis the solution concentration at equilibrium(mg/L),andKLis a constant related to the adsorption energy.

(2)Freundlich isotherm equation:

whereKFis the Freundlich adsorption equation constant,andnis the characteristic constant related to the adsorption strength.Generally speaking,whennis between 1 and 10,adsorption is easy.The larger the value ofn,the stronger the interaction between the adsorption material and the adsorption medium.

3 Results and analysis

3.1 Preparation results and surface characteristics of AA/HA/Starch graft copolymer

3.1.1 Orthogonal testing of AA/HA/Starch graft copolymer synthesis

The results of the orthogonal testing are shown in Table 2.Ij,IIj,andIIIjis the sum of the experimental results at the corresponding level for each factor,respectively.In Table 2,considering the values ofIandR,it can be concluded that when the mass ratio of HA to starch is 1:1 and the mass ratio of HA to AA is 1:5,the Cu2+adsorption capacity of AA/HA/Starch graft copolymer is largest.The temperature has little impact when it ranges from 70℃to 90℃.The optimal reaction temperature was chosen as 70℃in consideration of energy conservation.

3.1.2 FT-IR analysis

FT-IR spectra of AA/HA/Starch graft copolymer,starch,and HA are shown in Fig.1.According to Fig.1,the O—H and N—H stretching vibration peaks of AA/HA/Starch graft copolymer are at 3565 cm-1,the C—H stretching vibration peaks of—CH2—,—CH—,and—CH3—are at 3102 cm-1.The C=O and C—Oabsorption peaks in the ester bond are at 1670 and 1170 cm-1,respectively.It has been proven that an ester bond is formed,which is due to the esterification reaction between the carboxylic group on HA or AA and the hydroxyl group on starch.Due to the influence of the—NH group on the amide,thep-electron of the N atom formsp-πconjugation with the carbonyl group,which causes the carbonyl absorption peak of AA/HA/Starch graft copolymer to shift to 1461 cm-1,a lower wavenumber relative to the carbonyl absorption peak of HA.This is due to the acylation of the—NH2on HA and the—COOH on polyacrylic acid.HA and starch contain many functional groups,including—OH,—COOH,—SO3—,etc.Their active sites are liable to dissociate under the action of heat and KPS.First,the initiator dissolves in water to form a sulfate-free radical,which then forms OH·.OH·attacks the—OH and—COOH groups of HA and abstracts H atoms to form new free radical sites.AA will further attack these networks in the presence of the crosslinking agent to produce a polymer chain,thereby forming an acrylate polymer.

Table 2 Orthogonal expriment analysis

3.1.3 SEM analysis

As shown in Fig.2,AA/HA/Starch graft copolymer has a porous three-dimensional network structure.The inner parts are connected with irregular channels.This is because that a large number of active groups(—OH,—NH2,—COOH)of HA,AA,and starch can lose H atoms and form macromolecular free radicals in the presence of the KPS initiator and the MBA crosslinker.After contacting with vinyl monomers,new free radicals are produced,and the monomers continuously react to form graft polymers.The crosslinking agent containing two vinyl groups also participates in the free radical polymerization to connect the polymer chains.At the same time,macromolecular chains intertwine with each other and hydrogen bonds interact with each other to form a three-dimensional network structure.Thus,there is an increase in the specific surface area of AA/HA/Starth graft copdymer,facilitating the adsorption process.

3.2 Testing the Cu2+adsorption of AA/HA/Starch graft copolymer

3.2.1 Analysis of factors influencing Cu2+adsorption

(1)The effects of time and temperature on the Cu2+adsorption capacity

The effects of time and temperature on the Cu2+adsorption capacity are shown in Fig.3.As shown in Fig.3,it became clear that the adsorption temperature and time affected the Cu2+adsorption capacity(q)and Cu2+adsorption efficiency(η)of AA/HA/Starch graft copolymer.At a certain temperature and a time within 100 min,the Cu2+adsorption capacity and efficiency of AA/HA/Starch graft copolymer increased rapidly with an increase in time.When the time increased from 100 min to 125 min,the increasing rate of Cu2+adsorption capacity and adsorption efficiency decreased.With the extension of contact time,there were more active adsorption sites on the surface of AA/HA/Starch graft copolymer.Cu2+in a Cu2+solution with a higher mass concentration will rapidly diffuse to AA/HA/Starch graft copolymer surface and pores,resulting in a rapid increase in the Cu2+adsorption capacity and adsorption efficiency.As the adsorption reaction processed,the mass concentration of Cu2+and AA/HA/Starch graft copolymer surface adsorption active sites greatly reduced,and the Cu2+adsorption capacity and adsorption efficiency increased slowly.Finally,the Cu2+adsorption of AA/HA/Starch graft copolymer reached saturation and the adsorption equilibrium time was 125 min.

It can also be seen that at a certain time,when the temperature increased from 298 K to 318 K,the Cu2+adsorption capacity and adsorption efficiency of AA/HA/Starch graft copolymers slightly decreased.At 298 K,the Cu2+adsorption efficiency and adsorption capacity were relatively higher.It may be because that the adsorption process was not only the chemical adsorption but also the physical adsorption;Cu2+,which has been captured by AA/HA/Starch graft copolymer,will fall off from the surface due to the intensified movement,resulting in a decrease in Cu2+adsorption capacity and adsorption efficiency.Low temperatures are favorable for Cu2+adsorption.It can be inferred that the Cu2+adsorption process may be an exothermic reaction and increasing the reaction temperature is not conducive to the adsorption process.

(2)Effect of pH value of the Cu2+solution on Cu2+adsorption capacity

The effect of pH value of the Cu2+solution on Cu2+adsorption capacity is shown in Fig.4.The Cu2+adsorption capacity and adsorption efficiency of AA/HA/Starch graft copolymer increased at first and then decreased with an increase in the initial pH value of the Cu2+solution within a certain range.At a pH value of 5.5,a maximum adsorption capacity of 230 mg/g was observed.It may be because that when the pH value is low,some amino groups in AA/HA/Starch graft copolymer are protonated to form NH3+.The electrostatic repulsion between NH3+and Cu2+results in the reduction of the adsorption capacity for Cu2+.With an increase in pH value,the protonation of amino groups gradually decreases and more amino groups chelate with Cu2+to increase the Cu2+adsorption capacity of AA/HA/Starch graft copolymer.On the other hand,with a decrease in H+concentration in the solution,the—COOH groups on the AA/HA/Starch graft copolymers were deprotonated to form COO-,which enhanced the electrostatic attraction of negatively charged COO-and Cu2+,thus increasing the Cu2+adsorption capacity and adsorption efficiency.However,when the pH value is higher than 5.5,Cu2+is easily hydrolyzable,which is not conducive to adsorption and weakens the Cu2+adsorption performance of AA/HA/Starch graft copolymer.

(3)Effect of the Cu2+initial concentration on Cu2+adsorption capacity

As shown in Fig.5,with the increase in the Cu2+initial concentration in the solution,the Cu2+adsorption capacity of AA/HA/Starch graft copolymer increased rapidly and then gradually stabilized,while the Cu2+adsorption efficiency showed a downward trend.This is because the dosage of AA/HA/Starch graft copolymers in the system was certain,for a low concentration of Cu2+solution,the number of active sites on AA/HA/Starch graft copolymer was sufficient.With the increase in the Cu2+concentration,more Cu2+migrated from the solution to the surface of the AA/HA/Starch graft copolymer.With continuous adsorption,the active sites gradually reach a saturation state and the adsorption capacity of AA/HA/Starch graft copolymer tends to be stable.Therefore,the optimal initial concentration of Cu2+solution is 100 mg/L.

(4)Effect of adsorbent dosage on Cu2+adsorption capacity

The effect of adsorbent dosage on Cu2+adsorption capacity and adsorption efficiency is shown in Fig.6.The results showed that the adsorption capacity of AA/HA/Starch graft copolymer for Cu2+first increased and then decreased with an increase in the AA/HA/Starch graft copolymer dosage,while the Cu2+adsorption efficiency increased fastly at first and then became stable.The main reason is that the number of adsorption active sites increase with an increase in the AA/HA/Starch graft copolymer dosage,and the number of adsorption active sites directly affects the adsorption performance of AA/HA/Starch graft copolymer for Cu2+.As the adsorbent dosage increased,the Cu2+adsorption amount increased.If the adsorbent dosage continues to increase,the Cu2+adsorption amount gradually decreases,and the Cu2+adsorption efficiency gradually increases to a stable state.The main reason is that the increase in the adsorbent dosage will result in more adsorption active sites,but the amount of Cu2+adsorbed by AA/HA/Starch graft copolymer per unit mass will decrease,resulting in a decrease in Cu2+adsorption capacity.Therefore,the dosage of AA/HA/Starch graft copolymer required for the optimal adsorption performance was 50 mg.

3.2.2 Adsorption kinetics

Kinetics studies were carried out under the condition with a solution pH value of 5.5,50 mg AA/HA/Starch graft copolymer,a solution volume of 100 mL,and Cu2+initial concentration of 100 mg/L.Samples were diluted 30-fold,and measurements were taken for 25,50,75,100,125,150,175,200,and 225 min,respectively.The absorbance due to the Cu2+adsorbed by the adsorbent was measured with a UV spectrophotometer at a wavelength of 450 nm,and the adsorption capacity was calculated according to Eq.(1).The adsorption capacity was then analyzed and the experimental data were fit as shown in Figs.7-8.

Table 3 lists the correlation coefficients for linear fitting of the adsorption kinetic models.

From Figs.7-8 and Table 3,it is known that the quasi-second-order adsorption kinetic model better fitsthe experimental data,and its linear correlation coefficientis higher than theof the quasi-firstorder adsorption kinetic model.Besides,the quasisecond-order linear correlation coefficientis closer to 1,and the reaction rate in the adsorption process has a linear relationship with the square of the reactant concentration.The adsorption equilibrium capacity measured at 298 K is 238 mg/g,which is closer to the equilibrium adsorption capacity(265 mg/g)calculated by the quasi-second-order kinetic model.The adsorption equilibrium amount calculated by the quasifirst-order kinetic model is 162 mg/g at 298 K.This shows that the overall adsorption process of AA/HA/Starch graft copolymer for Cu2+can be predicted by the quasi-second-order adsorption kinetic model.This chemical adsorption can be attributed to the ion exchange or shared electron pair between Cu2+and the functional groups on the AA/HA/Starch graft copolymer surface.

Table 3 Parameters of adsorption kinetic models

3.2.3 Isothermal adsorption model

Freundlich and Langmuir adsorption models are often widely used to explain the adsorption mechanism.In these experiments,the initial pH value was 5.5,the dosage of adsorbent was 50 mg,and the volume of the Cu2+solution was 100 mL.The initial concentration of the Cu2+solution was 100,200,300,and 500 mg/L,respectively.The adsorption experiments were carried out at the same temperature,and the oscillation time was 4 h to fully reach the adsorption equilibrium.According to the experimental data,the adsorption isotherms are as shown in Figs.9-11.

Analyzing Fig.9,it can be concluded that as the equilibrium concentration of Cu2+(Ce)increases,the adsorption capacity of AA/HA/Starch graft copolymer for Cu2+increases first and then gradually stabilizes.The Cu2+adsorption capacity of AA/HA/Starch graft copolymer decreases with increasing temperature and this further indicateds that the adsorption process is an exothermic reaction.Peng et al[22]synthesized novel magnetic cellulose-chitosan composite microspheres via combining an emulsification procedure and cellulose-chitosan regeneration from an ionic liquid without further surface modification.The maximum adsorption capacity of magnetic cellulose-chitosan composite microspheres for Cu2+was 65.8 mg/g within 20 h when the initial concentration of Cu2+was 150 mg/L.Therefore,it can be concluded that AA/HA/Starch graft copolymer has a greater adsorption capacity for heavy metal ions compared with the magnetic cellulose-chitosan composite microspheres.

Figs.10-11 are the Langmuir adsorption isotherms and Freundlich adsorption isotherms fitted by experimental data,respectively.Table 4 shows the related parameters of the two isothermal models.

Table 4 Parameters of isothermal adsorption models

The correlation coefficients for the linear fits are listed in Table 4.It is noted that the linear correlation coefficient(RL2=0.9995)of the Langmuir isotherm adsorption curve,is greater than the linear correlation coefficient(RF2=0.9509)of the Freundlich isotherm adsorption fitting curve when the temperature was 298 K.It can be concluded that the Cu2+adsorption process of AA/HA/Starch graft copolymer is better described by the Langmuir adsorption isotherm model.That is to say,the Cu2+adsorption of AA/HA/Starch graft copolymer is mainly monolayer chemical adsorption,which may be combined through ion coordination and ion exchange.The theoretical maximum adsorption capacity(q0)decreases with increasing temperature,which further shows that the Cu2+adsorption process of AA/HA/Starch graft copolymer is an exothermic process.High temperature is not conducive to the progress of the adsorption reaction and theq0was 310.56 mg/g at 298 K.

4 Conclusions

An economic and environmentally friendly adsorbent,exhibiting high efficiency was constructed with the use of acrylic acid(AA)as a monomer,humic acid(HA)and starch as raw materials,potassium persulfate(KPS)as initiator,and N,N-dimethylacrylamide(MBA)as a cross-linking agent.The best conditions for synthesizing AA/HA/Starch graft copolymer were the dosage of crosslinking agent and initiator of 0.6% and 1.0%(based on the total monomer content),respectively,the mass ratio of HA to starch of 1:1 and the mass ratio of HA to AA of 1:5,and the reaction temperature of 70℃.AA/HA/Starch graft copolymer shows good adsorption performance for Cu2+in the aqueous solution.The Cu2+adsorption capacity of AA/HA/Starch graft copolymer first increased and then decreased with increasing adsorbent dosage and the initial pH value of the Cu2+solution.With the increase of the initial concentration of Cu2+and the extension of adsorption time,the adsorption amount of Cu2+increased rapidly and then stabilized.And it decreased slightly with the increase of absorption temperature.When the initial concentration of Cu2+was 100 mg/L,the volume of Cu2+solution was 100 mL,the dosage of AA/HA/Starch graft copolymer was 50 mg,the adsorption time was 125 min,the pH value was 5.5,and the temperature was 298 K,the adsorption capacity of AA/HA/Starch graft copolymer for Cu2+reached 238 mg/g.The adsorption of Cu2+by the copolymer adsorbent was consistent with the quasi-second-order kinetic model and the Langmuir isotherm model,and the adsorption is mainly a monomolecular chemical adsorption.AA/HA/Starch graft copolymer and Cu2+may be combined through ion coordination and ion exchange.Biocompatible materials resulting from environmentally friendly components and preparation methods show promise for the removal of heavy metals from aqueous solutions.