Enhanced electrokinetic remediation removal heavy metal from sludge assisted by the combined biodegradable iminodisuccinic acid and electrolyte circulation technology

ABSTRACT Heavy metal severely restricted the sludge agricultural use. Therefore, it is imperative to seek a novel and proficient method removal heavy metal from sludge. In this study, it proposed that a coupled with biodegradable chelating ligand iminodisuccinic acid and electrolyte circulation technology, which was used to enhance electrokinetic (EK) remediation removing heavy metal from the sludge. Electrical current, electrical conductivity and electrolyte pH, electrical conductivity and sludge pH, the toxic metal concentration with different time and remediation room section were evaluated in this work. Results demonstrated the combined iminodisuccinic acid and electrolyte circulation technology could enhance electrokinetic remediation removal toxic metal from sludge. Also, it can increase the cation and anion dissolution and electric current, and the cathode obtained a higher electrical conductivity of electrolyte and sludge than anode, and maintained a low pH of catholyte and sludge, the enhanced electrokinetic treatments obtained a higher removal efficiency than unenhanced electrokinetic remediation treatment, and the coupled with iminodisuccinic acid and electrolyte circulation enhanced electrokinetic treatment achieved the highest toxic metal extraction efficiency, Cu, Zn, Cr, Pb, Ni and Mn removal efficiencies were 55.0 ± 3.78%, 66.8 ± 2.23%, 64.6 ± 3.87%, 49.3 ± 3.25%, 61.6 ± 4.22% and 57.2 ± 2.67%, respectively, which indicated that iminodisuccinic acid and electrolyte circulation technology can effectively enhance EK remediation removal heavy metals from sludge.


Introduction
A large amount of sludge produced in the wastewater treatment plants (WWTPs) during the wastewater treatment process [1,2].Sludge is composed of nitrogen, phosphorus, potassium and organic matter, which can promote crops growth.On the basis of sludge nutrient substance, the agricultural use was considered as best approach [3].However, many heavy metals (70-90%), such as Cu, Zn, Cr, Pb, Ni, Mn, Cd and As can migrate into the sludge [1,4,5], which can give rise to a seriously environmental risk to biosphere.Also, it can damage the human health by the food chain.Due to heavy metal show the high toxicity and stability, which led to strictly limiting the agricultural use [1,6,7].In the previous studies, many methods were applied to reduce heavy metal, such as leaching techniques, bioleaching, electrokinetic (EK) remediation techniques, ultrasound enhanced citric acid technology and the bioleaching enhanced-EK remediation [2,5,[8][9][10][11].Therefore, it is imperative to seek a novel and feasible method for removing heavy metal from sludge.
Many innovative and novel techniques have been developed to reduce heavy metal from soil, sludge and sediment, such as chelator leaching technology, super critical fluid extraction, chemical immobilisation, oxidation and peroxide remediation [9,10,12,13].One of these remediation approaches are electrokinetic technique which utilised a low level direct current field across contaminated sites, improving charged pollutants mobility by electro-migration, electro-osmotic flow and electrophoresis [2,5,12].Sludge particles surface characteristics, such as cation exchange capacity, adsorption capacity, magnitude of the zeta potential, as well as the speciation and dissolution of contaminants depended on pH [2,8,14].Lower pH produced more H + ions, which can promote heavy metal dissolution [12,14,15].This process is important for a successful remediation heavy metal, the dissolved cations towards the cathode compartment by the combined electroosmotic and electro-migration flow [15][16][17][18].However, the OH − generated on the surface of cathode during the electrokinetic process, and it migrated towards the anode region in the direct current field, and reacted with the cations in the pore fluid before it reach the cathode compartment, which caused heavy metal and OH − forming metal hydroxide precipitation in the sludge [5,15,19].Heavy metal hydroxide precipitation is a major obstacle for electrokinetic remediation [15].Therefore, it is essential to regulated the cathode electrolyte pH and enhanced heavy metal removal during the EK remediation.In previous studies, many methods have been investigated to improve heavy metal mobility and adjust the cathode electrolyte pH at the cathode compartment [13,[20][21][22], such as circulation systems, that means the anode electrolyte circulation into the cathode chamber to control the catholyte pH, this technology can promote heavy metal desorption during the EK remediation process [2,[23][24][25].
Although several advantages about the electrokinetic technique, the physicochemical properties of heavy metal and their compound interaction with sludge, could significantly affect the remediation efficiency during the electrokinetic process [8,12].As well as, the high organic matter, sulphide and silicate content in the sludge, which could also largely impede heavy metal removal [12,26].Therefore, the various chemical reagents were used to promote heavy metals separating from the contaminated sites [9,10,27,28].For the synthetic chelates, such as diethylene triamine pentaacetic acid (DTPA), ethylenediamine tetraacetic acid (EDTA) and citric acid (CA), which were applied to reduce heavy metal content contaminated sites in the extracting treatments or electrokinetic technique [9,[28][29][30][31][32][33].Although these conventional chelates obtained a high removal efficiency, their physicochemical characteristics determined a non-biodegradable characteristic in the ecosystem.Therefore, the iminodisuccinic acid (IDS) was recommended as easily biodegradable alternatives to DTPA, EDTA and CA.It is water-soluble, non-toxic, and possess superior biodegradable characteristics with more than 80% of IDS degraded after 7 days, which can reduce the side effects of soil washing on soil functions [34,35].Iminodisuccinic acid (IDS) is a novel biodegradable chelant, and it demonstrated the high chelating capacity with metal ions, and outstanding biodegradability during the remediation process, and indicated a high stability, wide pH range, environmental friendliness and low toxicity [34,36].IDS commercial production is mainly relied on green chemistry process involving the fermentation, thermal polymerisation [35], and it contains abundant ligand functional groups such as carboxyl and amide that could be a strong ability to react with metal ions, and it can offer a potential green alternatives to contaminated sites remediation [34,35].
This work aims to evaluate that the coupled with IDS and electrolyte circulation technique were applied to promote metal removal from sludge during enhanced-EK remediation.Specific tasks are (1) electrical conductivity of electrolyte and sludge were evaluated.(2) the pH of electrolyte and sludge were elucidated.(3) heavy metal residual contents in the different time and sludge remediation room sections were investigated.

Sludge characteristics
Sludge was obtained from WWTPs (Chengdu, China).One part of sludge sample was dried in an oven at 105°C for 24 h, followed by grinding and sieving to a 0.149 mm, then it was analysed sludge characteristics and heavy metal content.Meanwhile, the other sludge sample were air dried at the room temperature, and then followed by grinding and sieving to a 0.149 mm before electrokinetic treatment.
Sludge properties were shown in Table S1, as shown in Table S1, sludge pH and water content were 6.72 ± 0.57 and 81.45 ± 2.12%, respectively.As shown in Table S1, Cu and Zn contents of sludge were 552.45 ± 7.89 and 1142.45 ± 3.56 mg kg −1 , respectively, it indicated that heavy metal content exceed the agricultural use standard in China.

IDS characteristics
Iminodisuccinic acid (IDS) was purchased Bayer, Germany, which demonstrated biodegradable characteristics (biodegradability is more than 80%) and used widely to solve heavy metal pollution in the sludge, soil and sediment, and the DIS of relative molecular mass is 249.17, and solid content is more than 34.00%, and each element is composed of C (38.56%), H (4.45%),N (5.62%), and O (51.37%) [34], and the chemical structure of IDS can be demonstrate in Fig. S1 [34,37].

Electrokinetic treatment and operation condition
As can be seen in Fig. S2, it indicated a schematic diagram of electrokinetic remediation in this work, and which consist of rectangular reactor (length: 200 mm, width: 100 mm, height: 100 mm), the anode and cathode compartments (length: 200 mm, width: 100 mm, height: 100 mm), the anode electrode was IrO 2 and the cathode electrode was stainless steel, both dimensions are length 120 mm, width 90 mm and thickness 10 mm.Meanwhile, the IrO 2 and stainless steel can resist alkali and acid corrosion.One direct current power supply, one multimeter, four peristaltic pumps, and two working solution chambers.To avoid sludge particle entering into working reservoirs, the cellulose filter paper was fixed between working compartments and remediation chamber.Also, the cation-exchange membrane is used to hinder the OH − ion migrating into the sludge porous medium.Therefore, the cation-exchange membrane was fixed between cathode reservoir and sludge remediation chamber.To ensure anolyte and catholyte retaining stable running conditions, two peristaltic pumps circulate the electrolyte into corresponding reservoirs.Also, in order to neutralise the superfluous production H + and OH − , the other two pumps circulate electrolyte into opposite reservoirs.

Electrokinetic experiments
Electrokinetic remediations were carried out in the different operation parameters, which was demonstrated in Table S2.All the EK tests were implemented without pH adjusting, and conducted the fixed voltage for 200 h.In all the EK remediations, to maintain the anode and cathode electrolyte constant, the anode and cathode electrolyte were refreshed interval 24 h to keep constantly working of tests, and ensure regular addition the electrolyte.During the EK remediation treatments, with the electrolyte refreshing interval 24 h.Electrolyte of heavy metal content reduced, which caused sludge heavy metal content decrease, and achieved an objective of heavy metal reduction.Deionised water was employed as anode and cathode electrolyte in the EK1 test, the IDS solution (0.1 mol L −1 ) was used as anode and cathode electrolyte in the EK2 test.The electrolyte has not been circulated during the EK1 and EK2 remediation treatments.The deionised water and IDS solution (0.1 mol L −1 ) coupled with electrolyte circulation for EK3 and EK4 treatments, respectively.The electrolyte was circulated by the peristaltic pump during the EK3 and EK4 remediation treatments.Before each EK tests, 1500 g of air dried sludge sample, which followed by grinding and sieving to a 0.149 mm, and it was homogeneously put into the sludge compartment, and then a saturation process with deionised water were conducted for 24 h in EK1 and EK3 tests, and IDS solution (0.1 mol L −1 ) were conducted for 24 h in the EK2 and EK4 tests.To ensure sludge and IDS solution uniformity, the mixture was stirred several minutes.The sludge was evenly separated into five sections (S1, S2, S3, S4 and S5) from anode to cathode, which were taken at these sections with a plastic tube, after EK treatments for 50, 100, 150 and 200 h, respectively.Each section was analysed for sludge toxic metal content (C) and characteristics.Each toxic metal removal efficiency (R) was calculated as following Equation ( 1): where C 0 is the initial sludge of heavy metal content (mg kg −1 ).

Analytical methods
Using the air ventilation dry sludge sample, then ground and sieved to 0.149 mm, and stored in the desiccator at the room temperature [5,8].Using the weight loss determined sludge water content, the oven heat the sludge at 105°C for 24 h.Sludge pH was analysed in a 2.5:1 (liquid: sludge) suspension using a pH metre (pHS-25, Shanghai Inesa Scientific Instrument Co., Ltd, China), and sludge electrical conductivity (EC) with the conductometer (DDS-307A, Shanghai Inesa Scientific Instrument Co., Ltd, China).Toxic metals content of sludge were analysed by strong acid digestion.Sludge were digested by the mixture strong acid (HNO 3 -HCl-HClO 4 ) and digestive equipment (HD-X60, Changsha, China), which temperature is 180-200°C [38].The Optima5300DV inductively-coupled plasma mass spectrometry (PerkinElmer, USA) was measured heavy metal content.

Electrical current variations during the electrokinetic remediations
Electrical current change in the EK remediations were indicated in Figure 1.Electrical current can directly indicate the electrolyte and sludge conductivity, sludge resistance, electrical power consumption, and anions and cations speciation during the electrokinetic remediation [2,3,5,8].As shown in Figure 1, at the beginning of the EK treatments, electrical current were 8.0, 23.0, 19.0 and 28.0 mA of EK1, EK2, EK3 and EK4 tests, respectively.Due to the H + migrated into sludge, soil and sediment particles, then exchanged and dissolved with the metal ions or salts, and it takes several hours in the EK treatments [3,5,8,20].With the electrokinetic remediation process conducting, the electric current raised a maximum value, which were 79.3, 90.7, 87.6 and 91.5 mA for EK1, EK2, EK3 and EK4 remediation treatments, respectively.Water was oxidised at the anode compartment (2H 2 O→4 H + +4e − +O 2 ), which produced a large amount of H + .Also, the water was reduced at the cathode chamber (2H 2 O+2e − →H 2 + 2OH − ), it produced a large amount of OH − [39][40][41].During the electrokinetic remediation process, oxidation and reduction reaction have occurred simultaneously at the anode and cathode chambers, respectively [42,43], then H + and OH − were migrated into the sludge by electroosmosis, which cause the sludge acidification happen at the low pH zone.It can accelerate metal ions and salts dissolving, and desorbing from the sludge particle.Due to H + transportation velocity is 1.75 times higher than OH − [5,14,43], the H + can migrate at the near cathode chamber of sludge remediation room [5,8,44].At the end of electrokinetic remediation, the electric current rapidly decreased and then maintained stable status, due to the cation and anion (such as metal ions, dissolved salts) transferred into cathode and anode compartments [8,12,45,46], respectively, which led to cations and anions of amount reduced in the sludge.It demonstrated that the IDS, electrolyte circulation and the coupled with IDS and electrolyte circulation can enhance the cation and anion dissolution, and electric current growth.

Sludge electrical conductivity change in EK remediation
Sludge sections electrical conductivity change in EK tests were indicated in Figure 2(b), the EC of untreated sludge is 4800 μS cm −1 , because it adsorbed different ions, such as metal ions, cations and anions in the wastewater treatments.In the EK1 treatment, the S1, S2, S3, S4 and S5 sections of electrical conductivities were 1200, 2000, 2500, 2900 and 3200 μS cm −1 , respectively.In EK2 treatment, the S1, S2, S3, S4 and S5 sections of electrical conductivities were 1100, 2100, 2400, 3000 and 3300 μS cm −1 , respectively.In the EK3 treatment, the S1, S2, S3, S4 and S5 sections of electrical conductivities were 1050, 2300, 2800, 3200 and 3400 μS cm −1 , respectively.In the EK4 treatment, the S1, S2, S3, S4 and S5 sections of electrical conductivities were 1000, 2500, 3000, 3200 and 3300 μS cm −1 , respectively, which shown EC of sludge near the cathode working reservoir (S3, S4 and S5) were higher than near the anode working reservoir (S1 and S2) at the end of electrokinetic remediation, as shown in Figure 2(b), the EC of near the cathode sections in EK3 and EK4 remediation treatments was higher than EK1 and EK2 tests, the EC was closely relied to the cation and anion contents, and the ionic strength of the solution [8,43,47], the metal ions and complexes [Cu(H 2 IDS)] + , [Zn(H 2 IDS)] + , [Cr(H 2 IDS)] + , [Pb(H 2 IDS)] + , [Ni(H 2 IDS)] + , [Mn(H 2 IDS)] + migrated towards the cathode chamber by electro-migration and electro-osmosis, it caused near at the cathode sludge sections ions content was higher than anode sludge sections.As well as, the electrolyte circulation can enhance metal ions and other ions of S1 sludge section migration and accumulation in the near cathode sludge sections, which caused EC of near cathode sludge sections in EK3 and EK4 remediation were higher than EK1 and EK2 remediation.

Electrolyte pH change
Anolyte and catholyte pH change in EK tests were indicated in Figure 3(a).As can be seen in Figure 3(a), the anolyte pH decreased to 2.21 within 5 h of energising, and it maintained high acid condition until at the end of EK1 test.Whereas, the catholyte pH increased to 11.2 within 10 h of energising, and it preserved high alkali condition until at the end of EK1 treatment.Results indicated that anolyte and catholyte pH have a huge different during the EK1 remediation test, due to the deionised water was oxidised at the anode electrode and generated H + .Meanwhile, it released O 2 (2H 2 O→4 H + +4e − +O 2 ).Also, the deionised water was reduced at the cathode electrode and generated OH − with liberation of H 2 (2H 2 O+2e − →H 2 + 2OH − ).As can be seen in Figure 3(a), the anolyte pH decreased from 10.8 to 2.78 within 40 h of energising, and it maintained low pH until at the end of EK2 remediation.However, the catholyte pH raised from 10.8 to 12.5 within 5 h of energising, and it maintained high pH until at the end of EK2 tests.Because the IDS solution was used as electrolyte during the EK2 remediation test, the IDS have a wide pH range and show a high pH in the solution [34].At the middle and end of the EK1 and EK2 remediation treatments, the anolyte maintained low pH range.However, the catholyte maintained high pH range.During the EK3 and EK4 remediation treatments, the anolyte pH decreased slower than EK1.However, in the EK3 and EK4 remediation, the anolyte maintained a low pH after 40 h until at end of the EK remediation, also the catholyte maintained a low pH during the electrokinetic remediation.Due to the electrolyte was circulated into the cathode chamber, and the H + neutralise OH − formed the H 2 O [2,8].

Sludge pH change
Sludge pH variations during the EK remediation process were indicated in Figure 3(b), the initial sludge pH is 6.72.However, the sludge pH produced a huge variation in the EK tests.In the EK tests, the sludge pH raised from anode to the cathode chambers.Results demonstrated that the anode region sludge obtained a lower pH than cathode region.Due to the deionised water was oxidised and generated H + at the anode electrode region, and H + can migrate into the sludge remediation room by electro-migration and electroosmosis effect [2,3,8,48].As well as, the deionised water was reduced at the cathode chamber and produced OH − ions, it also migrated into sludge by electromigration and electroosmosis and caused the high pH during the electrokinetic remediation process.As well as, the sludge pH at the near cathode has a higher than at the near anode, due to the mobility H + (3.63 × 10 −7 m 2 /(v.s) in free solution) is 1.8 times higher than OH − ion [8,12,49].As can be seen in Figure 3(b), the sludge pH of EK3 and EK4 have a low pH, due to the anolyte was circulated into the cathode chamber, the anolyte contained a large amount of H + .Meanwhile, the catholyte contained a large amount of OH − , the former and the latter have occurred neutralisation reaction and formed H 2 O [2,8].Also, due to the mobility rate of H + was more than of OH − and the mobility of OH − from cathode compartment to sludge remediation compartment, which led to the encounter of H + and OH − closer to the cathode chamber, and it produced a sudden variation of pH value, and the high pH value produced in the S3, S4 and S5 sludge sections, and the S5 sludge section obtained a peak value after the EK remediation.

EK1 remediation test
Fig. S3 indicated heavy metal distribution in the different sludge sections with time in the EK1 remediation test.As shown in Fig. S3, compared with heavy metal distribution in the initial sludge, which indicated that heavy metal reduced after EK remediation.It indicated that heavy metal transferred towards cathode electrode from anode compartment by electro-migration and electro-osmosis effect [2,5,8,12,14], and heavy metal removal efficiency raised with increasing remediation time from 50 to 200 h, and transferred into the cathode chamber or sludge sections close to the cathode chamber.The remediation time was 200 h, Cu, Zn, Cr, Pb, Ni and Mn removal efficiencies were 27.8 ± 2.14%, 41.2 ± 1.98%, 40.4 ± 2.12%, 30.2 ± 2.78%, 33.6 ± 2.78% and 34.6 ± 2.78, respectively.As shown in Fig. S3, during EK1 remediation process, heavy metal obtained a higher removal efficiency near the anode section than cathode section, and heavy metal of S3 and S4 sections obtained a higher removal efficiency than other sections, because heavy metal ions reacted with OH − , and then formed M(OH) n precipitate in the cathode sludge sections [5,8,38,50].Also, the water was oxidised in the anode electrode, and produced H + ions, it can lead to acid condition in the anode compartment.In EK remediation test, the low electrolyte pH can lead to heavy metal desorption, charge, fractions, and residual content variation [5,51], and ionic mobility of H + is 1.75 times higher than OH − [8,51], which can hinder heavy metal ions into the cathode compartment.Therefore, heavy metal residual content in the S3 and S4 sections obtained a higher than other sludge sections.During electrokinetic remediation process, water was reduced at the cathode chamber, and produced OH − , which led to high pH at the cathode chamber and prevent heavy metal solubility and transportation, and heavy metal ions adsorbed onto sludge particle, precipitated, carbonates and other compounds in high pH zone, which is favourable increasing the zeta potential.It can promote the colloidal particles through the electroosmosis effect during the electrokinetic remediation [5,8,51].

EK2 remediation test
IDS was used to the anolyte and catholyte in the EK2 remediation test.Fig. S4 indicated heavy metal distributions varied with different time and section during the EK2 remediation process.The IDS could increase electrolyte ionic strength, which resulted in a strong increasing the electric current in the EK remediation test.In fact, IDS can enhance the formation of IDS-complexes that can maintain higher electric current intensities in the electric field [5,8,14].The remediation time was 200 h, Cu, Zn, Cr, Pb, Ni and Mn removal efficiencies were 46.4 ± 2.45%, 57.0 ± 2.54%, 57.6 ± 2.78%, 42.2 ± 3.23%, 59.3 ± 2.89% and 48.0 ± 2.14%, respectively.Results demonstrated that EK2 treatment obtained a higher removal efficiency than EK1 treatment, which indicated that IDS can enhance electrokinetic treatment removal heavy metal.IDS was composed of the four carboxyl groups and the nitrogen atom, and the anion of IDS acid can react with heavy metal ion with the ratio 1:1, which is reversible react, and can be interpreted as follows [34,37]: where M m+ is metal ion, IDS n-is the ionic form of iminodisuccinic acid (IDS).As well as, the stability constants of between IDS and metals complexes were indicated, and metal chelating ability orders were: logK[Hg(II)](14.9)>logK[Cu(II)](13.1)>logK[Ni(II)](12.2)>logK [Pb(II)](11.0)>logK[Zn(II)](10.8)>logK[Cr(III)](9.6)>logK[Mn(II)](7.7)[34,37].
In the EK2 remediation, heavy metal residual content (C/C 0 ) obtained a high value in the S3 and S4 sections.Because metal ions can migrate towards to the cathode, and form metal hydroxide precipitation with the OH − at the cathode compartment.However, the S1, S2 and S5 sludge sections have a low residual content during the EK2 remediation test, due to the water was oxidised, it can produce H + ions, and improve heavy metal mobility and dissolution.Also, IDS contains functional groups, which has a strong ability to chelate with metal ions, and can enhance heavy metal obtaining dominant removal efficiency.

EK3 remediation test
Heavy metal distribution varied with the different time and section in the EK3 remediation were demonstrated in Fig. S5, the electrokinetic remediation time was 200 h, Cu, Zn, Cr, Pb, Ni and Mn removal efficiencies were 47.8 ± 4.12%, 54.2 ± 2.31%, 58.2 ± 3.24%, 42.8 ± 2.11%, 59.8 ± 3.21% and 48.6 ± 3.45%, respectively.Results demonstrated that EK3 acquired a higher extraction efficiency than EK1, which indicated that the anolyte and catholyte circulation could enhance EK test removal heavy metal.Due to improvement way of circulating the catholyte and anolyte, and this method could directly neutralise the H + and OH − without the addition of exogenous chelating agents [2,52,53], which can impede a low and high pH at near the anode and cathode chamber, respectively.During EK3 remediation process, the deionised water was used to the electrolyte, it generated H + at the anode compartment, whereas, the cathode compartment can produce the OH − .In the S1 and S2 sections, heavy metal residual content has a lower than other sludge section.Due to the acidic condition can improve heavy metal mobility and transportation.In this study, the anolyte was circulated into the cathode chamber, and it can prevent metal ion and OH − forming the M(OH) n precipitate at near the cathode zone [5,8,52,53].However, heavy metal residual content (C/C 0 ) obtained a high value in the S3 and S4 sections, due to the CO 3 2-and metal ions can form M(CO 3 ) n precipitation, which can prevent heavy metal mobility and dissolution during the EK3 remediation treatment [5,51].

EK4 remediation test
Fig. S6 indicated that heavy metal residual content (C/C 0 ) have obviously decreased after the EK4 remediation, it demonstrated that both electrolyte circulation and IDS synergistic action could improve heavy metal mobility.After the electrokinetic remediation treatment, Cu, Zn, Cr, Pb, Ni and Mn removal efficiencies were 55.0 ± 3.78%, 66.8 ± 2.23%, 64.6 ± 3.87%, 49.3 ± 3.25%, 61.6 ± 4.22% and 57.2 ± 2.67%, respectively.In the previous work, the coupled with circulation and EDTA synergistic effect can significantly enhance electrokinetic remediation treatment removing lead from the real field contaminated soil [52], and EDTA can improve EK remediation removing Cu, Cr and Ni from the aged electroplating soil contaminated by using the coupled with circulation technology and dual cation exchange membranes [24].In this work, the IDS played a crucial role during the enhanced electrokinetic remediation.Because the IDS has the four possible equilibrium states, and can be interpreted as follows [34,54]: As can be seen in Fig. S1, IDS chemical structure contained four carboxyl groups (-COOH).
According to the Lewis acids and bases theory, the carboxyl group (-COOH) could deprotonate and bound to metal ion (M m+ ) forming complexes [55,56], and IDS acts as a pentagon ring ligand forming chelates of octahedral structure with heavy metal ions (M 2+ ), and metal complexes with IDS distribution as following: [M(H 2 IDS)], [M(HIDS)] − , [M(IDS)] 2-, [M(OH)(IDS)] 3- [55].In this work, the anolyte pH range maintained 5.0-6.0 during the EK4 remediation treatment, it indicated that metal complexes with IDS mainly existed [M(HIDS)] − in the IDS solution [54].As can be seen in Fig. S6, heavy metal obtained a low residual content in the S1, S2 and S5 sludge sections, due to the IDS can form complexes with metal ions, and the H + of circulation neutralise the production of OH − at the cathode compartment, which can enhance heavy metal mobility and dissolution [34,53].

Comparison of heavy metal removal efficiency
Heavy metal distribution in the sludge section after EK remediation were demonstrated in Fig. S7.As can be seen in Fig. S7, compared with the S2, S3 and S4 sections, the S1 and S5 sections have acquired a high removal efficiency.Cu and Zn obtained the highest removal efficiency, which were 68.00% and 79.00% in the S1 section of EK4 remediation test, respectively, and Cr obtained the highest removal efficiency (71.00%) in S5 section of EK2 remediation, and Pb acquired the highest removal efficiency (59.00%) in the S1 section of EK2 remediation, and Ni acquired the highest removal efficiency (74.00%) in the S5 section of EK2 remediation, and Mn acquired the highest removal efficiency (67.00%) in the S1 section of EK4 remediation.Due to the electrolyte circulation and IDS could enhance EK remediation removing heavy metal from sludge.However, Cu, Zn and Cr obtained the lowest removal efficiency (−1.00%, 7.00% and 30.00%) in the S4 section of EK1 remediation, respectively.Pb obtained the lowest removal efficiency (15.00%) in the S3 section of EK2 remediation.Ni and Mn obtained the lowest removal efficiency (24.00% and 11.00%) in the S3 section of EK1 remediation, respectively.Deionised water was reduced at the cathode chamber and produced OH − , which caused metal ion react with OH − forming M(OH) n precipitation, and it prevent metal ion migrating into cathode compartment by the electro-migration and electro-osmosis effects [2,8].

Comparison heavy metal removal efficiency with other EK techniques
As can be seen in Table S3 S4.As can be seen in Table S4, the cadmium and lead obtained a predominantly removal efficiency during the combined electrolyte circulation and anion exchange resin, and electrolyte circulation and EDTA enhanced-EK technique, and both cadmium and lead removal efficiencies exceed 60%.Cu, Cr and Ni have a low removal efficiency in the combined electrolyte circulation and membranes enhanced-EK remediation technique, which were 31%, 16.5% and 34%, respectively.In this study, Cu, Zn, Cr, Ni and Mn have acquired a dominantly removal efficiency in the combined electrolyte circulation and IDS enhanced-EK remediation technique.Whereas, the lead has obtained a lower extraction efficiency than other heavy metal.

Conclusions
(1) IDS and electrolyte circulation technology can promote the cation and anion of sludge dissolution, and electrical current growth in the EK test.(2) The cathode electrolyte EC was higher than anode electrolyte during the electrokinetic remediation, and the EC of sludge near the cathode reservoir was higher than anode reservoir after the EK remediation.
Finally, this work provided the coupled with biodegradable chelating ligand iminodisuccinic acid and electrolyte circulation technology can enhance electrokinetic remediation.In our current research, we have founded the electrokinetic technology has a significant impact on heavy metal fractions.In the future, our work will focus on heavy metals fractions variations during the electrokinetic remediation.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Figure 1 .
Figure 1.Electric current variations during the electrokinetic remediation process.

Figure 2 .
Figure 2. Electrolyte electrical conductivity (a) and sludge electrical conductivity (b) variations during electrokinetic remediation process.

Sichuan
Science and Technology Program (No. 2021YJ0342), Open Project of State Environmental Protection Key Laboratory of Synergetic Control and Joint Remediation for Soil and Water Pollution (No. GHBK-2020-002), Talent Introduction Funds of the Sichuan University of Science and Engineering (No. 2020RC23) and the Key Research and Development Project of Luzhou Science and Technology Planning (2020-SYF-20).
[24], Zn, Cr, Pb, Ni and Mn have acquired the highest removal efficiency in EK4 remediation test, which were 55.0 ± 3.78%, 66.8 ± 2.23%, 64.6 ± 3.87%, 49.3 ± 3.25%, 61.6 ± 4.22% and 57.2 ± 2.67%, respectively.In the electrokinetic remediation field, in order to seek a higher removal efficiency, many researchers used enhanced electrokinetic remediation techniques to reduce heavy metal from contaminated sites, which generally need the multiple procedures.Gao et al.[2]reported that the combined electrolyte circulation and anion exchange resin technique can enhance EK remediation removing cadmium from sludge, and it was effective at obtaining a maximum Cd removal of 60.0% from sludge.Chang et al.[52]adopted the coupled with circulation and EDTA and Na 2 CO 3 working solution enhancing electrokinetic technique to remove Pb from the agricultural land, and can achieve 63.0% of Pb removal efficiency by using the circulation electrokinetic (CEEK) technique and 0.1 mol L −1 EDTA within 6 days treatment.Chang et al.[53]applied a circulationenhanced electrokinetic (CEEK) and phytoremediation to remove Pb from the real lead contaminated site, and results demonstrated that Pb content can be decreased from 5672 mg kg −1 to 2083 mg kg −1 after the enhanced electrokinetic (CEEK) and phytoremediation treatment.Song et al.[24]adopted a new method for EDTA-enhanced EK remediation by coupled with circulation methods and dual cation exchange membranes to remove Cr, Cu, and Ni from aged electroplating soil contaminated, results demonstrated that Cu, Cr and Ni extraction efficiencies were 31.0%,16.5% and 34.0%, respectively.Comparison of this work with other electrolyte circulation enhanced-EK techniques removal heavy metal were demonstrated in Table