Removal of hexavalent chromium in soil by lignin-based weakly acidic cation exch

State Key Laboratory of Chemical Resource Engineering,Beijing University of Chemical Technology,Beijing 100029,China

Keywords:Lignin Cr(VI)Remediation Soil

ABSTRACT The adsorption of Cr(VI)from soil onto lignin-based weakly acidic cation exchange resin(LBR)has been investigated.Lignin is a three-dimensional amorphous polymer composed of methoxylated phenylpropane units.The unique structure and chemical properties render the lignin suitable for the remediation of hexavalent chromium in the soil.Soil column leaching experiments were conducted to optimize the adsorption conditions.The effects of contact time,pH,adsorbent dosage and temperature on the adsorption of Cr(VI)onto the LBR have been investigated.Experiment data were then correlated with Freundlich and Langmuir isotherms.The Langmuir isotherm model ftis the experimental data better than the Freundlich isotherm.It was found that the LBR has a high adsorption capability for Cr(VI)(3.95 mg·g-1)with a removal rate of 91.9%.Thus,LBR can serve as a good absorbent for the reduction of the concentration of Cr(VI)in soil.

1.Introduction

The pollutants in the soil are easy to diffuse or migrate to the groundwater leading to continuous expansion of the polluted area and great pressure on the ecological environment[1].With continuous development of soil-pollution-control research,people are increasingly aware of the impact of pollutants on soil organisms and the great harm caused by the migration of pollutants to the soil environment.Therefore,many efforts have been made to reduce the soil pollution in recent years,and the purpose of the soil pollution treatment is to prevent pollutants from spreading to nearby water bodies and organisms and to reduce the toxicity of pollutant[2].With fossil energy sources drying up and environmental pollution getting worse,more and more countries and regions begin to develop renewable energy sources,particularly biomass.The cognition of lignin structure and function has been developed continuously[3].Some characterization,such as spectroscopic combined with redox[4-7],thermogravimetry[8]nuclear magnetic resonance spectroscopy[9],and some chemical methods such as ozone oxidation[10],wet chemistry,and photocatalytic degradation[8,11]have been used to study the structure,composition,functional groups and monomer connection modes of lignin.Functional groups,such as aromatic nuclei,ether bonds,phenolic hydroxyl groups,alcohol hydroxyl groups,methoxyl groups and carbonyl groups,in lignin could undergo a variety of chemical reactions under certain conditions,and polymerize to form new functional polymers.

Chromium salt production industry has made considerable progress and caused severe soil pollution.The chromium in soil comes from the waste gas,waste water and waste residue generated during its process[12].Cr(VI)is recognized as a toxic carcinogen worldwide and is a fatal environmental pollutant.There are two main chromium valence states,trivalent and hexavalent.Trivalent chromium is stable,and it is a trace element in the human body,but hexavalent chromium is a serious threat to human health.Hence,there are increasing concerns about soil chromium pollution remediation[13].

Over the last few decades,a lot of researches have tried to deal with soil chromium pollution.In fact,there are mainly two methods for the removal of chromium.On the one hand,reduction,precipitation and complexation of chromium in the soil can decrease its migration and bioavailability.On the other hand,chromium can be separated directly from the soil.Soil remediation technology mainly includes the isolation method,chemical reduction method[14],stabilized curing method[15],leaching method[16],electrochemical remediation method[17],and microbial remediation method[18-20].Wang et al.[21]used nanometer iron powder to repair sediments with different degrees of pollution.Nanometer iron powder can effectively remove Cr(VI)from the sediment,and the removal rate of Cr(VI)is higher than 89.80%.In addition,lignin and lignin-based materials can be used to remove organic and inorganic pollutants.Guo et al.[22]used lignin in black liquor of papermaking to adsorb divalent gold as extractant,and its adsorption order was as follows:Pb2+＞Cu2+＞Cd2+＞Zn2+＞Ni2+;meanwhile,pH and ionic strength were the main factors in the adsorption ability of lignin.The phenolic sites on lignin surface had stronger complexing ability to metal ions than carboxyl sites.Under different conditions,chromium adsorption onto some biomaterials was studied,and lignin as potential adsorbent has been a topic of concern for more people in recent years[23].Wu et al.[24]studied the adsorption properties of sulfonated lignin for Cr3+,and found that the adsorbability of Cr3+was greatly affected by pH value and the amount of adsorbent,and was not affected by ionic strength and other metal ions,while the maximum adsorption capacity for Cr3+was 17.97 mg·g-1.

These methods are easy to cause secondary pollution while removing chromium,and most of the treatment methods are for chromiumcontaining wastewater,and few people are involved in the chemical treatment of chromium in soil.The soil leaching method is a widely used repairing technique,which uses a leaching agent to dissolve heavy metals in the soil to achieve the purpose of repairing the soil.Merdoud and Akretche[25]used electro-dynamic technology to treat heavy metal ions in soil,but the removal effect was not obvious and the cost was too high.However,Torres et al.[26]used the leaching method to treat heavy metal ions in soil on the original basis,which greatly reduced the use of chemicals and energy consumption,and improved the stability and removal efficiency of Cr(VI)in soil.

The current work aims to investigate the adsorption ability of a new kind of lignin-based weakly acidic cation exchange resin(LBR)over Cr(VI)in soil through soil column leaching test,so as to achieve the high removal rate of Cr(VI)and the harmless treatment of the soil.Liang et al.[27]studied the optimization of preparation process and evaluated the adsorption performance of new lignin-based resins by using selfmade weakly acidic cation exchange resin(LBR)as an adsorbent for Cr(VI)adsorption in the simulated wastewater.It has been confirmed that LBR can effectively adsorb Cr(VI)from wastewater,and Cr(VI)in solution showed high adsorption affinity.In order to improve the removal rate of Cr(VI)in the soil and achieve the purpose of harmless soil remediation,it is worthwhile to study the use of LBR as an adsorbent to adsorb Cr(VI)in soil.

2.Materials and Methods

2.1.Preparation of LBR

Lignin-based weakly acidic cation exchange resin(LBR)was prepared by condensation polymerization of sodium lignosulfonate with glucose.Under acidic conditions at high temperature,glucose and other sugars from lignosulfonate could transform into 5-hydroxymethylfurfural(5-HMF)and levulinic acid.Then the polymerization of LBR can be obtained via carbonyl addition of desulfonated lignosulfonate with 5-HMF and levulinic acid.The LBR was prepared by heating glucose(0.5 g)and sodium lignosulfonate(10 g)in dilute sulfuric acid(pH 0.5,85 ml)at 463 K in a batch reactor.Black powdery resin product was obtained after separation by vacuum filtration,washing with distilled water and oven drying at 100°C overnight.

2.2.Preparation of Cr(VI)solution

The stock solution of Cr(VI)was prepared by dissolving 2.5 g K2Cr2O7dried at 120 °C for 2 h in a 250 ml volumetric flask with deionized water.Accordingly,our experimental solution was prepared at 100 mg·L-1by serial dilution from the stock solution of 10.0 g·L-1.

2.3.Preparation of soil samples

The sampling point was located in the campus,which belonged to the north temperate monsoon climate,the altitude of about 20 m-60 m.Soil samples were taken at the surface of the sampling site near the 0 cm-20 cm of soil.The sample was dried and ground into 40-60 mesh powder after removing stones,plant roots,animal residues and other debris.

2.4.Analytical method of Cr(VI)

The amount of Cr(VI)in the filtrate was measured spectrophotometrically by using the diphenylcarbazide method.After the reaction,the supernatant was filtered,and 1 ml of the sample was diluted with 50 ml deionized water,followed by adding 0.5 ml of phosphoric acid and 0.5 ml of sulfuric acid.1.0 ml of diphenylcarbazide solution(2 g·L-1with acetone as solvent)was added.The solution was agitated gently and left for a few minutes,as indicated by a pink-violet color;then the absorbance was measured at 540 nm.

2.5.Adsorption experiment

According to the national standard of the People's Republic of China GB7267-87,the hexavalent chromium analysis method was formed using diphenylcarbazide spectrophotometry.The adsorption capacity of the adsorbent was characterized by the amount of Cr(VI)adsorbed by the LBR adsorbent.The adsorption capacity qeand percentage adsorption E were calculated with Eqs.(1)and(2)as follows:

where Cois the initial concentration of Cr(VI)(mg·L-1),Ceis the equilibrium concentration of Cr(VI)(mg·L-1),m is the mass of adsorbent(g),and V is the volume of the Cr(VI)solution(ml).

2.6.Adsorption of Cr(VI)in soil by LBR

The soil column leaching experiment was conducted in a parallel control experiment.We have prepared two glass columns(30 cm×1 cm),where the S column was filled with 30 g of soil,and the S1 column was filled with 30 g of soil and 1 g of LBR(the LBR was completely mixed with the soil).In each glass column,5 g of quartz sand was placed at the upper and lower ends of the column to promote uniform water immersion,and the filtrate was spliced with a lower glass vessel.After the column installed,the soil was fully infiltrated with deionized water[28].Then,a solution of 100 mg·L-1was leached from the upper end of the column for 15 ml,and the flow rate was 5 ml·h-1.All the experiments were repeated three times and the average values were calculated.

3.Results and Discussion

3.1.Effect of extraction time on the adsorption of Cr(VI)in soil by LBR

The effect of contact time on the adsorption behavior was investigated,while the LBR dosage was fixed at 0.1 g and the pH value was fixed at 6 at room temperature.Fig.1 shows the percentage adsorption of Cr(VI)over LBR.The percentage adsorption of Cr(VI)increased gradually along with increased contact times.The adsorption capacity of LBR can reach 1.60 mg·g-1,with a removal rate of 78.9%.It was proposed that the Cr(VI)could be reduced to Cr(III)in the soil by LBR.The adsorption of Cr(VI)by LBR becomes saturated after a certain amount of time,which may be due to the limited adsorption sites of LBR.Therefore,LBR can effectively adsorb Cr(VI),achieving the remediation of Cr(VI)in the soil.

3.2.Effect of pH on the adsorption of Cr(VI)in soil by LBR

Fig.1.Effect of contact time on percentage adsorption of Cr(VI).

The effect of the pH value on the adsorption was investigated,while the LBR dosage was fixed at 0.1 g and the contact time was fixed at 12 h at room temperature.With the increase of pH values,the concentration of Cr(VI)in the leaching solution of soil column increased gradually,and the removal rate of Cr(VI)ranged from 82.5%(pH 2.0)to 56.1%(pH 8.0)as shown in Fig.2.Cr(VI)mainly exists in the form ofwhen the pH is in the range of 2 to 3[29].Therefore,units could be coordinated by positively charged functional groups on the surface of LBR via electrostatic attraction.In addition,under low pH conditions,the surface functional groups of LBR,such asCH2OH,CHO,and CH2CHtend to show a certain degree of reduction potential,which can reduce Cr(VI)into Cr(III):

Accordingly,the optimum pH for the maximum uptake of Cr(VI)was found to be 3.0,it indicated that reducing the value of pH in soil can effectively promote the adsorption of Cr(VI)over LBR.

3.3.Effect of the amount of LBR on the adsorption of Cr(VI)in soil

Fig.2.Effect of the pH value on percentage adsorption of Cr(VI).

The effect of LBR dosage on the adsorption was investigated,while the pH value was fixed at 3 and the contact time was fixed at 12 h at room temperature.The concentration of Cr(VI)in the leachate from the soil column leaching experiment decreased gradually,with the increase in the amount of LBR.The adsorption capacity of Cr(VI)was 3.61 mg·g-1,and the removal rate of Cr(VI)increased from 58.3%to 89.4%as shown in Fig.3.It indicated that the amount of LBR significantly affected the adsorption behavior of Cr(VI)in the soil.

3.4.Effect of temperature on the adsorption of Cr(VI)in soil by LBR

The effect of temperature on the adsorption was investigated,while the LBR dosage was fixed at 1 g and the contact time was fixed at 12 h at the pH value of 3.With the increase in the temperature,the concentration of Cr(VI)in the leaching solution of soil column reduced gradually.The adsorption capacity of Cr(VI)was 3.95 mg·g-1with a removal rate of 91.9% at 40 °C as shown in Fig.4.An explanation is that chemical bond cleavage occurs on the surface of LBR at higher temperatures,resulting in more adsorption active sites[30,31].This suggests that increasing temperature can make LBR effectively adsorb Cr(VI)from soil.

Fig.3.Effect of LBR dosage on percentage adsorption of Cr(VI).

Fig.4.Effect of temperature on the percentage adsorption of Cr(VI).

3.5.Spectroscopic investigations and mechanisms

It can be seen from the XRD spectra of LBR particles,that there is a wide C002 diffraction peak in the range of 10°to 30°,indicating that LBR does not have obvious crystal structure and it may be a kind of lignin-based polymer with a strong random pattern(Fig.5).

FT-IR spectroscopy was performed to observe the sorption of Cr(VI)on LBR in Fig.6.The broad absorption peak around 3405 cm-1was indicated the existence of an OH stretching vibration.The strong absorption peak in the range of 2920 cm-1to 2850 cm-1can be assigned to CH stretching,while the peaks at 1701 cm-1,1460 cm-1,and 1205 cm-1,can be attributed to the stretching vibration of the carbonyl group(CO),the in-plane deformation vibration of the coupling hydroxyl group(OH)and the stretching vibration ofC(O)O of the carboxylic acid group(COOH).The strong absorption peak in the range of 1200 cm-1to 1000 cm-1can be assigned to the CO stretching of alcohols or ethers.

Compared with the FT-IR spectrum of Cr(VI)-loaded LBR,there were significant changes in the FT-IR spectrum of the LBR particles.The four adsorption peaks at 3409 cm-1,1614 cm-1,1209 cm-1and 1105 cm-1shifted to 3405 cm-1,1612 cm-1,1205 cm-1and 1104 cm-1,respectively.Therefore,it can be concluded that the oxygen-containing functional groups on the surface of LBR participated in the Cr(VI)adsorption process and complexed with the generated Cr(III)to form stable structures.

3.6.Adsorption isothermal model analysis

In order to determine the adsorption potential,two common isothermal adsorption models were selected,that is the Langmuir and Freundlich adsorption isotherms.The absorption of Cr(VI)onto LBR was obtained with an initial Cr(VI)concentration of 100 mg·L-1to study the relationship between the equilibrium adsorption capacity of LBR and the equilibrium concentration of Cr(VI)in the soil.

Fig.5.X-ray diffraction pattern of LBR.

Fig.6.FT-IR spectrum of LBR before and after Cr(VI)loading.

The Langmuir adsorption isotherm model is described through Eq.(3)as follows:where qmis the static saturation adsorption amount of heavy metal ions by LBR(mg·g-1),b is the Langmuir constant(L·mg-1),and qmand b can be obtained through Eq.(4)as follows:

The Freundlich adsorption isotherm model is obtained through Eq.(5)as follows:

The logarithmic Eq.(6)is obtained as follows:

where kFis the Freundlich constant of adsorption capacity,is the Freundlich constant that characterizes the adsorption strength,and KFandcan be obtained by Eq.(6).

The isothermal adsorption experimental data were computer fitted and the obtained model parameter values are listed in Table 1.The results have shown that the Langmuir isotherm model fits the experimental data better compared with the Freundlich isotherm(Figs.7 and 8),indicating that the adsorption of Cr(VI)by LBR was monolayer adsorption.According to the results of the Langmuir isotherm adsorption model,the maximum adsorption amount of Cr(VI)on the LBR was 3.95 mg·g-1.According to the results of the Freundlich isotherm adsorption model,the high value of KFindicated that LBR has a higheraffinity for Cr(VI),while the value of n in the range of 1 to 10 indicated that the Cr(VI)adsorption on LBR was favorable in the experimental concentration range.Hence,LBR exhibits a higher adsorption affinity for Cr(VI)in soil.

Table 1 Summary of adsorption isotherm parameters for Cr(VI)

Fig.7.Langmuir isotherm for the adsorption of Cr(VI).

3.7.Thermodynamic analysis

In order to further understand the adsorption mechanism of Cr(VI),the thermodynamic analysis of the adsorption process of Cr(VI)is necessary[32].According to Eqs.(7)and(8),the variation of the Gibbs free energy(ΔG),enthalpy(ΔH)and entropy(ΔS)during the adsorption process of Cr(VI)by LBR may be estimated from Fig.9.The three parameters of thermodynamics can be obtained at different temperatures as follows:

Fig.8.Freundlich isotherm for the adsorption of Cr(VI).

Fig.9.Plot of In b and 1/T.

where b is the Langmuir adsorption constant,R is the ideal gas constant 8.314 J·mo1-1·K-1,and T is the absolute temperature in K.According to the slope and intercept of a straight line,the values of ΔH and ΔS can be obtained from the values of the slope and the intercept.

The thermodynamic parameters for the adsorption of Cr(VI)by LBR at different temperatures are listed in Table 2.A negative ΔG value indicates that the Cr(VI)adsorption process on the LBR is spontaneous.The ΔG value increased as temperature increased,it indicated that the LBR showed higher adsorption of Cr(VI)at higher temperatures.The positive ΔH value indicates that the adsorption of Cr(VI)by LBR is an endothermic process,because some chemical reaction changes were involved in the adsorption process of Cr(VI)on LBR,and the Cr(VI)in the adsorbed state was reduced to Cr(III),that is to say,the adsorption process of Cr(VI)by LBR was chemical adsorption.Furthermore,the positive value of ΔS indicates that the randomness of the solid-liquid interface increases during the adsorption process,reflecting that LBR has a good affinity for Cr(VI).Similar trends in the three parameter values of ΔG,ΔH,and ΔS were also found in other biomass-based materials,anion exchange resins and other adsorbents[33,34].

3.8.Dynamic analysis

The mathematical representations of pseudo first-order and secondorder dynamic models are given in Eqs.(9)and(10).Under certain conditions,the adsorption kinetics of Cr(VI)solution with initial concentrations of 100 mg·L-1and 200 mg·L-1for LBR are listed in Table 3;Figs.10 and 11 showed the fitting results of the pseudo-first-order kinetic model and the pseudo-second-order kinetic model.Regardless of the Cr(VI)solution of a medium concentration of 100 mg·L-1or a higher concentration with 200 mg·L-1,the pseudo-second-order kinetic model had a better overall goodness of fit than the pseudo first-order kinetics model.Therefore,the pseudo-second-order kineticmodels can better describe the adsorption kinetics of Cr(VI)in soil by LBR.

Table 2 The values of thermodynamic parameters for Cr(VI)adsorption on LBR

Table 3 Results of Cr(VI)adsorption kinetic fitting

Fig.10.Fitting test of the pseudo-first-order adsorption kinetic of Cr(VI)onto LBR.

From the above thermodynamic analysis results,the adsorption process of Cr(VI)on LBR is dominated by chemical adsorption,which may be due to Cr(VI)being adsorbed on the LBR during the adsorption process and then being reduced to Cr(III),and the Cr(III)may have been complexed with the oxygen-containing polar group on the surface of the LBR.Therefore,we can conclude that the LBR surface film transferring process is a rate controlling step for the entire adsorption process.

Fig.11.Fitting test of the pseudo-second-order adsorption kinetic of Cr(VI)onto LBR.

The pseudo-first-order kinetic model Eq.(9)is obtained as follows:

The pseudo-second-order kinetic model Eq.(10)is obtained as follows:

where qeis a pseudo-first-order rate constant(mg·g-1),qtis the amount of LBR adsorbed at time t(mg·g-1),k1is a pseudo-first-order rate constant(min-1),and k2is a pseudo-second-order rate constant.

3.9.Mechanism of Cr(VI)in soil by LBR

Based on all the above analysis,the mechanism of LBR adsorption of Cr(VI)in soil has been shown in Fig.12,which can be roughly divided into three steps:firstly,at low pH,the surface functional groups of LBR are protonated to form a large number of positively charged active sites.The mutual electrostatic attraction between the positive charge of the active site and the negative charge of the anionicin the soil causes the Cr(VI)to pass through the liquid film layer of the boundary of the LBR,and then adsorbed at the active site;secondly,in the presence of a large amount of hydrogen ions,the adsorbed Cr(VI)reacts with the reducing functional groups on the surface of LBR to form Cr(III);thirdly,the reduced Cr(III)complexes with oxygen-containing functional groups on the surface of the LBR such as hydroxyl,ether,carbonyl,ester and carboxyl groups.Since the bonding strength between Cr(III)and the above functional group is strong,the adsorbed Cr(III)cannot be easily desorbed.

4.Conclusions

Fig.12.Proposed mechanism of Cr(VI)adsorption by LBR.

The purpose of this study was to develop an adsorbent for the adsorption of Cr(VI)in the soil to achieve the purpose of repairing the soil.The experimental results show that the adsorption of Cr(VI)in soil by LBR was affected by contact time,pH,adsorbent dosage and temperature,and the maximum adsorption amount qmis 3.95 mg·g-1with a removal rate of 91.9%.The adsorption mechanism of Cr(VI)on LBR consists of electrostatic attraction,the reduction of Cr(VI)from adsorption state to Cr(III),and the complex coordination between Cr(III)and the oxygen-containing functional groups on the surface of LBR.Therefore,LBR has the potential to be used as an economic and efficient adsorbent material for the removal of Cr(VI)in soil.Future research needs to reveal the use of LBR as an adsorbent to study the adsorption behavior on other heavy metal ions in soil.