Electrokinetics-magnetic/alkali Composite-modified Biochar for Remediation of Cd-contaminated soil

Juan ZHAI Anwei WANG Juan LI Chengdong XU Yushan WAN

Abstract [Objectives] This study was conducted to explore the removal performance and mechanism of heavy metal Cd in soil by combining electrokinetic technology and modified biochar. [Methods] The electrokinetics-magnetic/alkali composite-modified biochar method was applied to remediate Cd-contaminated soil. [Results] When remediating the soil for 120 h under electric field strength of 2 V/cm with 0.1 mol/L citric acid as the electrolyte， the current periodically varied with the remediation time. Cd2+ in the soil dissociated and migrated from the soil surface， and the addition of the magnetic/alkali composite-modified biochar in the cathode area affected the hydrolysis balance of heavy metal Cd2+ ions and maintained soil pH in a low range. The exchangeable Cd content at the test sampling points near the cathode area was relatively high， and the organic Cd area was evenly distributed. The average removal rate of Cd in the soil was up to 90.31%. The hexagonal anode device configuration enhanced the electromigration performance and made the soil pH and conductivity change within a certain range， which was beneficial to the migration of Cd2+ and improved the removal rate of soil heavy metal Cd. [Conclusions] This study provides a theoretical basis for the treatment of soil heavy metal pollution and the resource utilization of composite-modified biochar.

Key words Cd; Soil pollution; Electrokinetics; Biochar

Cd is a common heavy metal pollutant. With the rapid economic development， irrational exploitation of resources and improper use of agrochemical products have led to an increasing problem of soil heavy metal pollution in China. The heavy metal Cd pollution in the soil is highly toxic and highly mobile， and has the characteristics of high toxicity， strong mobility， and difficult degradation[1-2]， causing extremely serious potential harm to animals， plants and humans. The persistence and irreversibility of soil heavy metal Cd pollution have become difficult points in the research field. Studies at home and abroad have shown that chemical， physical， and bioremediation methods have been applied to the remediation of soil pollution. Among them， the electrokinetic method has the advantages of wide range of applications， less consumption of chemical reagents， no secondary pollution and less disturbance to the soil[3-5]， and has become a research hotspot of new in-situ remediation technology. Biochar is a solid substance obtained from the pyrolysis of biological materials at high temperature under oxygen-deficient or oxygen-limited conditions. Biochar has the advantages of good adsorption effect， wide sources of raw materials and low economic cost. Biochar modification refers to changing the pore structure and surface chemical properties of biochar to improve its adsorption performance for heavy metals. The commonly used modification methods include acid modification， alkali modification， oxidant modification， negative magnetic modification， nano modification and inorganic material modification[6]. Sun et al.[7] used alkali-modified biochar and found that a large number of oxidative functional groups can be produced on the surface of biochar， which increased the porosity and specific surface area of biochar， thereby improving the adsorption effect of heavy metals. Bashir et al.[8] used magnetically modified straw biochar， and found that the magnetically modified biochar had the ability to fix Cd （II）， and its maximum theoretical adsorption capacity was increased by nearly 2 times.

In this study， electrokinetic technology and magnetic/alkali composite-modified biochar were combined. Bamboo was used to prepare initial biochar， which was subjected to alkali modification and magnetic loading. Through the addition of modified biochar in the outer cylinder of the cathode during the remediation process and the arrangement of multiple anode electrode configuration， the current， pH， conductivity and Cd form changes at each sampling point in the soil during the remediation process were investigated， and the change of Cd removal rate at each sampling point from anodes to cathode was analyzed， so as to explore the removal performance and mechanism of heavy metal Cd in soil by combining electrokinetic technology and modified biochar. This study provides a theoretical basis for soil heavy metal pollution control and composite-modified biochar resource utilization.

Materials and Methods

Test soil

The test soil was taken from the paddy farmland near the Science and Education City of Changzhou University. The surface soil was sampled by the checkerboard sampling method with a sampling depth of 20 cm. The mixture was air-dried and passed through a 100-mesh sieve to test the basic physical and chemical properties of the soil. CdCl2·2.5H2O was dissolved in deionized water to simulate the contaminated soil with a Cd concentration of 50 mg/kg， which was then air-dried， ground and sieved with a 2 mm sieve for use.

Test biochar

The dried bamboo sample had a length of about 3 cm. N2 was introduced as a protective gas in a vacuum tube furnace， and the sample was heated at a heating rate of 20 ℃/min to 400 ℃ to prepare biochar by pyrolysis. The prepared biochar was taken out， cooled and ground through a 0.25 mm sieve， obtaining the bamboo charcoal carbonized at high temperature and vacuum， which was marked as BC. It was immersed in a NaOH solution and then mixed with iron ion solution in a certain ratio， followed by adjusting pH to 10， heating and stirring. After pyrolysis in a vacuum tube furnace， the magnetic/alkali composite-modified biochar was obtained， and marked as MBC.

Test design

A disc-shaped electric reaction chamber （R=16 cm， L=14 cm） was made of plexiglass. The middle of the device was a cathode chamber. The anodes were arranged in a regular hexagon， and the distance between the two electrodes was 15 cm. The cathode chamber was a cylindrical hollow structure， including an inner tube and an outer tube. The cathode material was a high-purity graphite rod （1 cm in diameter）， and the anodes were stainless steel electrodes. The test device is shown in Fig. 1.

A certain amount of 50 mg/kg Cd-contaminated soil was weighed into the soil sample chamber and compacted. The magnetic/alkali composite-modified biochar （MBC） with a thickness of 2 cm was filled into the outdoor cylinder of the cathode chamber， and the filling height was flush with the contaminated soil. The inner wall of the out cylinder was lined with non-woven cotton cloth， and distilled water was used as a soil saturation liquid. The soil was stood for 24 h to allow saturation under acting force. The electric field intensity was 2 V/cm， and the cathode used 0.1 mol/L citric acid as the electrolyte which was kept at pH<4 （under acidic conditions）. The current was measured every 2 h. Samples were taken at equal distances from the anodes to the cathode， and the soil pH value， electrical conductivity and heavy metal form were measured， respectively.

Analysis methods

The content of Cd in soil was measured by a flame atomic absorption spectrophotometer （AAS）. The form of heavy metals in soil was determined by the Tessier five-step extraction method. The morphology and structure of BC and MBC were measured by scanning electron microscope （SEM）. A fourier transform infrared spectrometer （IS 50， Thermo Fisher Science， USA） was used to determine the changes of functional groups on biochar surface. The crystal structure was determined with an X-ray diffractometer （D/MAX 2500， Rigaku， Japan）. Other indicators were determined by conventional analytical methods.

Results and Discussion

Characterization and analysis of magnetic/alkali composite biochar

SEM-EDS analysis of BC and MBC was carried out to explore the biochar structure. It can be seen from the figure that the surface of BC was compact， and the pore structure was not completely formed， and attached with impurities. After the magnetic/alkali modification， the volatile and unstable structure of the original material disappeared during the carbonization process. Due to the corrosion effect of the alkaline material， the morphology and structure changed， and the carbon pore structure was clear， showing a microporous structure， which further increased its specific surface area， and improved the adsorption to heavy metal Cd2+ ions. The white substance attached to the surface of MBC was FeOx.

Fig. 3（a） shows the X-ray diffraction patterns of BC and MBC， the peaks of which had similar amplitudes. Both BC and MBC had obvious characteristic peaks， at the scanning angles （2θ） of 32.037° and 35.502°， respectively. There was a characteristic peak at 2θ=32.037°， which was corresponding to the crystalline phase of α-Fe3O4， and corresponding to 2θ=35.502°， the crystalline phase was γ-Fe3O4. Meanwhile， MBC had broader and lower peaks than BC， indicating that after modification the crystallinity was reduced， the specific surface area of the crystal grains increased， and the adsorption capacity was stronger[9]. When comparing the crystalline phase between MBC and BC， it could be seen that after modification， Fe3O4 was successfully loaded on the BC matrix， which could improve the adsorption capacity of Cd2+ and had the advantage of recycling.

According to compound molecular vibration， we applied Fourier transform infrared spectroscopy to characterize the surface functional groups of biochar， so as to judge the chemical structure changes of organic substances and verify the effect of modification on the surface functional groups of biochar. It can be seen from Fig. 3（b） that in the range of 3 300-3 400 cm-1， the hydroxyl stretching vibration peak of MBC changed significantly， showing increased peak intensity amplitude， and the absorption peak at 1 300-1 400 cm-1 was corresponding to the aromatic hydrocarbon C=C skeleton vibration. It indicated that the modified bamboo charcoal was rich in aromatic structure， and the content of oxygen-containing functional groups increased. Meanwhile， it was rich in iron-containing oxides， which improved the exchange capacity of cations and further improved the adsorption capacity of heavy metal ions. After characterizing the BC and MBC series， the magnetic/alkali-modified biochar MBC was selected as the test material for electric remediation of contaminated soil.

Change of current in the electric MBC adsorption process

Fig. 4 shows the change of current with time in the process of electric MBC adsorption of Cd-contaminated soil. It can be seen from Fig. 4 that in the hexagonal anode arrangement structure device， the Cd-contaminated soil was applied with a direct current to form a uniform electric field with the cathode. The magnetic/alkali composite-modified biochar MBC was used as the adsorption material in the outer cylinder of the cathode. When the applied electric field strength was 2 V/cm， the current fluctuated periodically with the remediation time. Parallel tests were adopted， and the current changes of the three groups of tests all rose first， then dropped and was finally stabilized. In the early stage of the test， the current value continued to rise， and reached the peak value at 32 h. Due to the hydrolysis between the electrodes， a large amount of H+ produced by the anodes and the OH- produced by the cathode migrated mutually. The migration of H+ to the cathode made the soil in an acidified state， which promoted the dissociation of Cd2+ in the soil and increased the migration rate[10]. Meanwhile， Cd2+ in the cathode area exchanged with -OH on MBC， and complexed with carbonyl C=O. The Fe3O4 successfully loaded by MBC had a fixed effect on Cd ions in the soil， and promoted the increase of mobile ions in soil micropores， so the current gradually increased.

Juan ZHAI et al. Electrokinetics-magnetic/alkali Composite-modified Biochar for Remediation of Cd-contaminated soil

In the middle stage of soil remediation， the current gradually decreased with the extension of remediation time. At the beginning of the test， there were many mobile ions used for reaction in the soil. When the anodes undergone reaction and electrolysis， the H+ produced by the reaction would quickly migrated to the cathode in the soil[11]， and in an acidic environment， the soluble ions in the soil would be resolved from the surface， releasing more ions， which resulted in a higher current. With the extension of time， soluble metal ions moved at an accelerated speed， exchangeable Cd ions gradually migrated to the cathode， and were adsorbed and removed by MBC partially， so the amount of exchanged Cd ions decreased， as well as the current. In the later stage of remediation， the current fluctuated again. It was because that H+ produced by the anodes and positive ions in the soil were neutralized by OH- in the soil， the number of movable ions decreased， and precipitation occurred， thereby blocking the soil gaps and reducing the current. Repeatedly， the overall trend was that the current decreased as the remediation time increased.

Changes of soil pH and conductivity after electric MBC adsorption

Fig. 5 shows the change in pH at each sampling point， and the initial soil pH was 7.52. The pH value of each sampling point from the anodes to the cathode showed an upward trend. The large amount of H+ generated near the anode end and the OH- continuously generated near the cathode end migrated to the soil， making the pH of the soil near the electrodes at both ends decrease or increase continuously. Adding magnetic/alkali composite-modified biochar in the cathode area increased the percentage base saturation of the polluted soil and enabled its exchange reaction， which reduced the soil hydrogen ion level. MBC was rich in alkaline substances. After the alkaline substances entered the soil， they could be released quickly and neutralize the acidity of the soil[12]， keeping the pH value changing within a low range and controlling the single-direction migration rate of H+. Meanwhile， the pH change trend of each sampling point was relatively gentle. The addition of MBC could increase the pH of the soil， and affect the hydrolysis balance of heavy metal ions， so the heavy metals in the soil were fixed by precipitation and other effects. In the test， the entire soil reaction chamber presented a weakly alkaline environment， reducing the adsorption of Cd by the soil， which was beneficial to accelerate the movement of Cd ions to the cathode and improve the removal efficiency of Cd.

The change of the conductivity at each sampling point is shown in Fig. 6. It can be seen that due to the arrangement of the regular hexagonal anodes， the conductivity value near the anodes was lower during the electrolysis process， and the addition of MBC in the cathode area increased the soil surface charge and the concentration of Cd2+ ions in the soil. Meanwhile， there was a large amount of OH- near the cathode diffused into the soil sample from the cathode pool， and Cd2+ migrated in a large quantity， so the conductivity near the cathode was higher. The analysis showed that the conductivity of the sampling points was higher than the content of the soil itself， which was due to the accumulation of some anions and Cd2+ ions near the anodes. The soil acidified near the anodes could dissolve precipitates in the soil[13]， causing the pH value to continue to decrease. The difference in the amount of precipitates dissolved at different sampling points resulted in changes in the conductivity value. In the test， the conductivity was about twice that of the tested soil， indicating that the hexagonal electrode arrangement could increase the migration rate of Cd ions. The addition of magnetic/alkali composite-modified biochar in the cathode could maintain the soil pH value， so the conductivity change during the remediation process was relatively gentle.

Distribution and removal rate of heavy metals after electric MBC remediation

Fig. 7（a） showed that the overall exchangeable Cd content in the experimental soil showed an increasing trend， and the exchangeable Cd increased from 43.75% to 71.34%. After adding MBC， which has a complex microporous structure conducive to the internal diffusion of Cd2+ ions in the soil， the unstable state of Cd in the soil was easily transformed into an exchangeable state， and the contents of iron-manganese oxidation state， carbonate-bound state and organic state all had a downward trend， with different decline rates. Due to the hexagonal anode electrode arrangement， part of the charged Cd2+ reacted with OH- produced at the cathode during the migration process， producing a precipitate， which led to a decreased current[14] and reduced the conversion rate of various forms of Cd in contaminated soil. The Cd in the iron-manganese oxidation state was easily converted to an exchangeable state， while the content of the residual Cd had a small fluctuation range. The organic Cd was uniformly distributed in the area. As the remediation time increased， the form of contaminated Cd in the soil would gradually change from an unstable state to a stable state.

It can be seen from Fig. 7（b） that the removal rate of heavy metal Cd at each sampling point gradually decreased from the anodes to the cathode. The Cd concentration at sampling point 1# dropped from 500 to 2.91 mg/m3， and the removal rate of soil Cd reached 90.31 %. The Cd concentration at sampling point 5# decreased from 500 to 170.8 mg/m3， and the Cd removal rate was 65.84%. The Cd removal rates of the remaining sampling points were 89.45%， 82.67% and 79.98%， respectively. During the remediation process， Cd2+ ions in the soil continuously migrated from the anodes to the cathode， and were abundantly enriched in the cathode. The surface of MBC in the cathode area contained a large number of functional groups， and the surface active sites increased[15-16]， which could coordinate with metal ions to form specific metal complexes， which were then fixed by the soil， thereby reducing the migration rate of heavy metal Cd2+ and improving the remediation performance.

Conclusions and Discussion

Electric remediation was combined with magnetic/alkali composite-modified biochar to remediate Cd-contaminated soil. The results showed that the composite-modified biochar had a microporous structure， which showed clear carbon pore structure and increased number of oxygen-containing functional groups， and was successfully loaded with Fe-O， thereby improving the adsorption capacity of Cd2+ ions in the soil.

When remediating the soil with regular hexagonal anode layout structure， the current presented periodic fluctuations with time， and the soil was in an acidified state. The addition of magnetic/alkali composite-modified biochar affected the hydrolysis balance of heavy metal Cd2+ ions， and had good adaptability to soil pH.

The testing of the sampling points showed that the exchangeable Cd content near the cathode area was relatively high， and the organic Cd area was evenly distributed. The average removal rate of heavy metal Cd-contaminated soil with a Cd content of 500 mg/kg reached 90.95%. The MBC surface in the cathode area contained a large number of functional groups， which enhanced the electromigration performance of Cd in the soil， which was beneficial to the migration of Cd2+ and improved the removal rate.

References

[1] SUN Z， WU B， GUO P， et al. Enhanced electrokinetic remediation and simulation of cadmium-contaminated soil by superimposed electric field[J]. Chemosphere， 2019， 233（OCT.）： 17-24.

[2] MENA E， VILLASEOR J， CAIZARES P， et al. Effect of a direct electric current on the activity of a hydrocarbon-degrading microorganism culture used as the flushing liquid in soil remediation processes[J]. Separation & Purification Technology， 2014（124）： 217-223.

[3] HABIBUL N， HU Y， SHENG GJJOHM. Microbial fuel cell driving electrokinetic remediation of toxic metal contaminated soils[J]. Journal of Hazardous Materials， 2016（318）： 9-14.

[4] LIU L， LI W， SONG W， et al. Remediation techniques for heavy metal-contaminated soils： Principles and applicability[J]. Science of the Total Environment， 2018（633）： 206-219.

[5] JIANG D， ZENG G， HUANG D， et al. Remediation of contaminated soils by enhanced nanoscale zero valent iron[J]. Environmental Research， 2018（163）： 217-227.

[6] SUN K， TANG J， GONG Y， et al. Characterization of potassium hydroxide （KOH） modified hydrochars from different feedstocks for enhanced removal of heavy metals from water[J]. Environ， Pollut Res Int， 2015， 22（21）： 16640-1651.

[7] CHEN T， ZHOU Z， HAN R， et al. Adsorption of cadmium by biochar derived from municipal sewage sludge： Impact factors and adsorption mechanism[J]. Chemosphere， 2015（134）： 286-293.

[8] WANG Y， LIU RJFPT. Comparison of characteristics of twenty-one types of biochar and their ability to remove multi-heavy metals and methylene blue in solution[J]. Fuel Processing Technology， 2017（160）： 55-63.

[9] TAN X， LIU Y， ZENG G， et al. Application of biochar for the removal of pollutants from aqueous solutions [J]. Chemosphere， 2015（125）： 70-85.

[10] ZHANG H， MA D， QIU R， et al. Non-thermal plasma technology for organic contaminated soil remediation： A review [J]. Chemical Engineering Journal， 2017（313）： 157-170.

[11] ZULFIQAR W， IQBAL MA， BUTT MKJC. Pb2+ ions mobility perturbation by iron particles during electrokinetic remediation of contaminated soil [J]. Chemosphere， 2017（169）： 257-261.

[12] ALI RM， HAMAD H， HUSSEIN M， MALASH GFJEE. Potential of using green adsorbent of heavy metal removal from aqueous solutions： Adsorption kinetics， isotherm， thermodynamic， mechanism and economic analysis [J]. Ecological Engineering， 2016（91）： 317-332.

[13] ZHOU M， XU J， ZHU S， et al. exchange electrode-electrokinetic remediation of Cr-contaminated soil using solar energy[J]. Journal of the Taiwan Institute of Chemical Engineers， 2018（190）： 297-306.

[14] FU R， WEN D， XIA X， et al. Electrokinetic remediation of chromium （Cr）-contaminated soil with citric acid （CA） and polyaspartic acid （PASP） as electrolytes[J]. Chemical Engineering Journal， 2017（316）： 601-608.

[15] NORAINI MN， ABDULLAH EC， OTHMAN R， et al. Single-route synthesis of magnetic biochar from sugarcane bagasse by microwave-assisted pyrolysis[J]. Materials Letters， 2016（184）： 315-319.

[16] LIN L， QIU W， WANG D， et al. Arsenic removal in aqueous solution by a novel Fe-Mn modified biochar composite： Characterization and mechanism[J]. Journal of Environmental Management， 2017（144）： 514-521.