electrokinetic remediation of fluorine-contaminated soil using approaching cathodes

Ming ZhouShufa ZhuYana LiuHui Wang

Chemical Engineering andPharmaceutics College, HenanUniversity of Science and Technology,Luoyang, P. R. China

Research Articles

Electrokinetic Remediation of Fluorine-Contaminated Soil Using Approaching Cathodes

The objective of this research was to analyze the effects of approaching cathodes (AC)on electrokinetic remediation of fluorine-contaminated soil, compared to one fixedcathode. Important electrokinetic parameters such as soil pH, electrical current, theremediation efficiency of fluorine, and energy consumption were investigated toevaluate electrokinetic remediation with AC (AC-electrokinetic remediation). It wasproved that AC-electrokinetic remediation could increase the removal efficiency offluorine from contaminated soil, because it could increase soil pH and enhanceelectrical current. Experimental results showed that it could improve 5.2% of removalefficiency of fluorine, and save nearly 16% of energy and 20% of time than theconventional electrokinetic remediation. After electrokinetic remediation, the fractionsof fluorine had been changed and fluorine in the soil was mainly found in residualfraction. Experimental results indicated that AC-electrokinetic remediation was aconsiderable technology to eliminate fluorine in contaminated soils.

Keywords: Anionic pollutants; Partitioning; Soil pH; Soil remediation

Received: November 1, 2012; revised: July 6, 2013; accepted: September 18, 2013

DOI: 10.1002/clen.201200599

1 IntroductionNowadays, soil pollution is becoming an increasingly serious threatto public health and the environment in China, because of rapidurbanization, industrial, and agricultural development. These soilcontaminants included not only heavy metal pollutants such ascadmium, lead, chromium, and nickel, but also organic pollutantssuch as benzene, toluene, and trichloroethylene, and other ignoredpollutants, such as fluoride. Apart from natural sources, a largenumber of fluorides in soil may be contributed to numerous humanactivities including phosphate fertilizer production and use,aluminum smelting, glass and brick manufacturing, and so on [1–3]. Groundwater can be contaminated by the leaching of fluoridesfrom fluorine-contaminated soil, and high concentration of fluorineand its compounds are also dangerous to plants and animals [4, 5].Besides being toxic, high concentration of fluorine ions leads to theinhibition of some enzyme reactions, to the linking of biogenous

elements and the disturbance of their balance in the organism. Thesimilar processes of linking together elements by fluorine ions andalkaline fluorides also take place in the soil [6]. So decontaminationof the fluorine-contaminated soil is one of the most importantchallenges. At present, soil remediation technologies can begenerally classified into two groups by ex situ technology andin situ technology. In general, ex situ remediation technology may be

more expensive than in situ remediation technology [7]. So, in situremediation technology for fluorine-contaminated soil is moredesired.Electrokinetic remediation is an in situ and green technology that

was introduced for the remediation of soil, sludge, and sedimentcontaminated with inorganic and organic pollutants. The basis ofelectrokinetic remediation lies in the application of a low intensitydirect electric current to the contaminated environment. On direct-current electrical field, the contaminants can be removed from soilby electroosmosis, electromigration, and other processes. Electro-osmosis (electroosmotic flow) is the motion of liquid induced by anapplied potential through a continuous soil particle network. Andelectromigration is generally considered to be the transport of ionicspecies present in the pore fluid due to the applied electric field andit includes the migration of hydrogen and hydroxide ions towardthe opposite electrode. Its advantages are: the applicability to a widerange of contaminants, in situ remediation, and so on [8]. Becauseof the advantages, electrokinetic remediation has been successfullyapplied to remove a wide range of contaminants, such as a varietyof heavy meals, organic pollutants, and other pollutants such asuranium from contaminated soil, sludge, and sediment [9–17].Our group also tried to use electrokinetic technology to remove

fluorides from contaminated soil. It was proved by experiments thatthe method was feasible, but the removal efficiency of fluorine wasnot satisfactory [18]. Now, during electrokinetic remediation of heavymetals from contaminated soil, a method with approaching anodeswas proved to save remediation energy and enhance the remediationefficiency. The reason is that the nearer to the anode region, thehigher are the oxidation–reduction potential (ORP) and hydrogen ionconcentration. Because both high ORP and low pH value can speed upthe electromigration of heavy metal cations, soil near the anode

region can be rapidly remedied [19]. However, unlike heavy metalpollutants, anionic pollutants such as fluorine can be easily desorbed

Correspondence:Dr. Ming Zhou, Chemical Engineering and PharmaceuticsCollege, Henan University of Science and Technology, Luoyang 471023,P. R. ChinaE-mail: [email protected]

Abbreviations: AC, approaching cathode; Ex, exchangeable fraction; Fe/Mn, fraction associated with Fe and Mn oxides; Or, fraction associatedwith organic matter; ORP, oxidation–reduction potential; Res, residualfraction; Ws, water soluble fraction

1

© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.clean-journal.com Clean – Soil, Air, Water 2013, 41 (9999), 1–5

from soilminerals under alkaline conditionmore than under neutraland acidic condition [20]. The nearer to the cathode the higher theOH� concentration and pH value in soil, which can help to desorbfluorine from contaminated soil. So, we tried to use electrokineticmethod with approaching cathodes (AC-electrokinetic remediation)to enhance the remediation process. Feasibility of AC-electrokineticremediation on fluorine-contaminated soil was studied. Meanwhile,in addition to the total fluorine content, it is important todescribe the partitioning of fluorine into different fractions, as thepartitioning information determines the behavior of fluorine, sothe partitioning of fluorine in contaminated soils before and afterelectrokinetic remediation was also compared.

2 Materials and methods

2.1 Sample preparation and its characterization

Experimental soils used in this study were collected from farmlandnear an aluminum smelting plant located in Luoyang City, China’sHenan Province. They were sampled from the surface layer (0–20 cm)and were silty loam. The fluorine concentration, organic mattercontent, and pH of soil samples were 1058mg/kg, 20.51 g/kg, and 8.17,respectively. Under ambient conditions, the soil samples were air-dried and passed through the 2mm nylon sieve prior to their use.

2.2 Electrokinetic experiments

Experimental run A (using one fixed cathode) and run B (using AC)were conducted in the self-made electrokinetic apparatus. Theexperimental conditions are given in Tab. 1. Figure 1 shows thesketch map of the experimental system. The experimental apparatusconsists of the following major parts: (i) the electrode compartment(4 cm� 6 cm� 8 cm); (ii) the soil cell (10 cm� 6 cm� 8 cm); (iii) theelectrolyte reservoir; (iv) the direct-current power supply (GPC6030D,Gwinstek, China); (v) 4-channel peristaltic pump (BT00-300T/DG-4,Longerpump, China). The working electrode was made of graphitesheet in this study (1 cm� 6 cm �8 cm). In every experiment, 200mLdistilled water and 600 g soil sample were mixed and packed intoelectrokinetic cell. NaOH solution was used as the anolyte and

distilled water was used as the catholyte. At a 10mL/min flow rate,the 4-channel peristaltic pump was used to circulate the electrolytein electrode compartment. Anolyte and catholyte were refreshedevery 24h and the cumulative fluoride in anolyte and catholyte wasdetermined. After the experiment, the treated soil was taken out andcut into nine parts (the same size), then every part would be used tomeasure fluorine concentration and soil pH, respectively.Except that three cathodes were inserted in the soil cell as AC, AC-

electrokinetic remediation was carried out in the same experimentalapparatus with the same experimental condition. Approachingcathodes were placed at a distance of 3, 6, and 9 cm from cathodecompartment. They were sequentially switched on 60, 120, and 180hafter the start of the electrokinetic remediation process.

2.3 Analytical methods

An alkali fusion-selective ion electrode technique was used tomeasure total fluoride in soil (fluoride ion selective electrode, SPSIC,China) [21]. Before the measurement, total ionic strength adjustingbuffer (85 g NaNO3 and 58.8 g C6H5Na3O7 · 2H2O in 1 L) was added toliberate fluorine ion, and to adjust the pH of the solution to 5.5.Soil pH was determined in the ratio 1:2.5 of soil/distilled watersuspension by the pH meter (pHS3C, SPSIC, China). The organicmatter content of soil was measured by the PE-2400 elementalanalyzer (Perkin Elmer, USA). Electrical current through the soil cellwas measured using the multimeter (F15B, Fluke, USA) in theelectrokinetic remediation process.Five binding fractions (Ws: water soluble fraction; Ex: exchange-

able fraction; Fe/Mn: fraction associated with Fe and Mn oxides;Or: fraction associated with organic matter; Res: residual fraction)of fluorine were analyzed by the sequential extraction [22]. Thesequential extraction procedure to fractionate different forms offluorine in soil is listed in Tab. 2. To ensure data quality, all chemical

analyses were performed in duplicate.According to the following equation, energy consumption of

electrokinetic remediation is calculated [23].

E ¼Z

ðUIÞdt ð1Þ

where U is voltage drop, I is the current, t is experimental time, andE is energy consumption.

3 Results and analysis

3.1 Electrical current through soil cell

Electrical current through soil cell is closely correlated with themobile ion concentration in the soil cell [24]. So, electrical current

Table 1. Electrokinetic experimental conditions

Run

Anolyteconcentration(NaOHmol/L)

Voltage(V)

Treatmenttime (h) Cathode

A 0.1 20 240 One fixed cathodeB 0.1 20 192 Approaching

cathodes

Figure 1. Sketch map of the electrokinetic experi-mental system (the electrode compartment, soil cell,and electrolyte reservoir are made of organic glass).

2 M. Zhou et al.


is a reliable indicator of the amount of ion electromigration. In theelectrokinetic remediation process, the variations of electricalcurrent across the soil cell for runs A and B are shown in Fig. 2.Electrical current in two runs exhibited a same trend. In run A, the

electrical current firstly increased, reached a maximum (60.6mA),then gradually decreased and stayed at the constant value. The reasonis that when voltage gradient was established, firstly the electricalcurrent across the soil cell was low because it took some time forelectrolyte to enter contaminated soil and for soil contaminants andminerals to dissolve and desorb from soil surface. About 35h later,electrical current reached a maximum because of electromigrationof pollutants to electrode and high ion concentrations in pore fluid.Then electrical current began to decrease due to the decrease in themigration of anions and cations in pore fluid. Furthermore,hydroxide ion moving toward anode can be neutralized by hydrogenionmoving toward cathode, hence forming water and decreasing theion concentration in the system.In run B, at the beginning of the experiment, electrical current also

firstly increased, reached themaximum, then decreased for the samereason of run A. But at the 60thh, cathodes began to switch towardanode, so electrical current increased again with the decrease ofelectrode distance. Comparedwith the fixed cathode experiment (runA), run B exhibited higher electrical current at this stage (60–192h).The experimental result suggests that there are more charged ions

during the AC-electrokinetic remediation process, which can in partexplain the probable reason of higher fluorine removal in AC-electrokinetic remediation.

3.2 Soil pH

Variations of soil pH are shown in Fig. 3. Electrokinetic treatment cansignificantly change soil pH. In typical electrokinetic remediation,hydrogen ions generated by electrolysis reaction at anode migratefrom anode to cathode; accordingly, hydroxide ions produced byelectrolysis reaction at cathode migrate from cathode to anode. So,the soil pH in the anode region becomes more acidic and that in the

cathode region becomes more alkaline. In run A, the distribution ofsoil pH in soil cell also was so, the soil pH gradually increased fromanode to cathode.There was a significant difference of soil pH between run A

and run B. Figure 3 shows AC-electrokinetic remediation basicallyalkalized experimental soil, including the soil of the anode regionand the cathode region after 192 h. The reason is that the nearerto the cathode, the higher are the OH� concentration and pH valuein soil. The desorption of fluorine increases with the increaseof soil pH, so the phenomenon is beneficial to remediation offluorine-contaminated soil.

3.3 Removal efficiency of fluorine

Figure 4 shows the concentration of residual fluorine in soil afterelectrokinetic treatments, and the cumulative amount of fluorine inthe anolyte and catholyte are shown in Figs. 5 and 6 during theexperiment.From Fig. 4, compared to the initial fluorine concentration, the

concentration of fluorine in treated soil all decreased (run A and B). Asfluorine has a negative charge, fluorides desorbed from the soil weremoved to the anode by electromigration. Meanwhile, fluorine couldalso be removed as soluble fluoride and fluorine complexes byelectroosmosis. This combination of electromigration and electroos-

mosis eliminated fluorine in contaminated soil, so the concentrationof fluorine in treated soil decreased. Meanwhile, it was apparent thatrun B had higher removal efficiency of fluorine (75%, which wascalculated according to the cumulative amount of fluorine in anolyteand catholyte after the experiment) than that of run A (69.8%).Experimental results showed that AC-electrokinetic remediation wasa more effective method to eliminate fluorine in contaminated soilsbecause of higher soil pH quickly to desorption of fluorine fromcontaminated soil and higher electric current quickly to migratefluorine to the electrode.From Figs. 5 and 6, it could be clearly seen that cumulative fluorine

in anolyte wasmuchmore than that in catholyte in both run A and B.

Table 2. The sequential extraction procedure to fractionate different forms of fluorine in soil

Fluorine species Extract Condition

Ws-F Distilled water Shake 0.5 h at 60°CEx-F 1.0mol/L MgCl2 Shake 1h at 25°CFe/Mn-F 0.04mol/L NHOH ·HCl Shake 1h at 60°COr-F 0.02mol/L HNO3þ 30% H2O2þ 3.2mol/L NH4Ac Shake 0.5 h at 25°CRes-F The total fluorine content minus other forms of fluorine in soil

Figure 2. Changes of current across the soil cell in the two runs. Figure 3. Variations of pH in soil sections in the two runs.

Electrokinetic Remediation of Fluorine-Contaminated Soil 3


This indicated that electromigration (from cathode to anode) was thepredominant removal mechanism for electrokinetic remediationof fluorine-contaminated soils instead of electroosmosis. Lots offluorine were attracted to anode and removed by electromigration.It was reported that electroosmosis rate is ten times lower thanelectromigration rate [23].

3.4 Variations of fluorine chemical fractionation

Figure 7a presents the binding forms of fluorine in the soil samplebefore electrokinetic remediation, and Fig. 7b–j presents the bindingforms of fluorine in the nine soil parts (from anode to cathode) afterelectrokinetic remediation (run B).From Fig. 7a, fluorine, mainly bound to the Ws and the Res in

fluorine-contaminated soil before electrokinetic remediation.However, after electrokinetic treatment, partitioning of fluorinein the soil had been changed. From Fig. 7b–j, variations offluorine chemical fractionation have a same change tendency. TheWs-fluorine in soil decreased significantly because of electro-kinetic treatment. The Ex-fluorine and Fe/Mn-fluorine showed adecreasing trend, the Or-fluorine in soil had little change, andfluorine in soil mainly bound to the Res now. The reasons are asfollows.With the increase of soil pH in run B, the Ex-fluorine absorbed

in alkaline soil was exchanged with lots of hydroxide ions,converted to Ws-fluorine. And the Ws-fluorine was removed byelectrokinetic treatment, so the concentration of Ws-fluorine andEx-fluorine decreased. Meanwhile, in an alkaline environment,the Fe/Mn oxides and hydroxides would desorb fluorine ions andabsorb hydroxide ions, so the concentration of Fe/Mn-fluorinealso decreased. With the drop of the Ex-fluorine, Ws-fluorine, andFe/Mn-fluorine, the proportion of Res-fluorine had been heightenedaccordingly.In general, after electrokinetic treatment, fluorine in the soil was

mainly found in relatively stable fraction (Res). Its toxic effects to theorganism and influence on human health were reduced.

Figure 4. Residual fluorine in soil after electrokinetic treatments.

Figure 5. Cumulative mass of fluorine in the electrolyte in run A.

Figure 6. Cumulative mass of fluorine in the electrolyte in run B.

Figure 7. Variation in partitioning of chemical forms for fluorine (Ws: watersoluble fraction; Ex: exchangeable fraction; Fe/Mn: fraction associatedwith Fe and Mn oxides; Or: fraction associated with organic matter;Res: residual fraction).

4 M. Zhou et al.


3.5 Energy consumption

In Tab. 3, the calculation results show that the energy consumption ofrun Bwas lower than that of run A. Compared with run A, run B savedabout 16% of remediation energy to get higher removal efficiency.The reason is that the distance between cathode and anode wasgradually reduced in run B. After working cathode approaches, thetreated soils would not consume energy. Therefore, less remediationenergy is needed to achieve the same fluorine removal efficiency. Thepresent study was conducted in lab-scale apparatus, whose extensionto full-scale applications needs field justification, of course.

4 Concluding remarksIn this paper, experimental results showed that AC-electrokineticremediation was a considerable choice to eliminate fluorine incontaminated soils. In the same intensity of electric field, it couldimprove the removal efficiency of fluorine than the conventionalelectrokinetic remediation, the fluorine removal efficiency of theformer was 75% and the fluorine removal efficiency of the latter was69.8%, because it could continuously increase the soil pH andelectrical current. And AC-electrokinetic remediation could savenearly 16% of energy and 20% of time than the conventional

electrokinetic remediation. Meanwhile after electrokinetic treat-ment, partitioning of fluorine in the soil had been changed andfluorine in the soil was mainly found in Res.

Acknowledgment

The authors acknowledge the financial support for this study byYouth Scientific Funds of Henan University of Science andTechnology (2012QN036).

The authors have declared no conflict of interest.

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Table 3. Fluorine removal and energy consumption of electrokineticexperiment

Run

Fluorineremoval

efficiency (%)

Energyconsumption

(kWh)

Energy consumptionper kilogram dry

fluorine-contaminatedsoil (kWh/kg)

A 69.8 0.223 0.372B 75 0.188 0.313

Electrokinetic Remediation of Fluorine-Contaminated Soil 5


electrokinetic remediation of fluorine-contaminated soil using approaching cathodes

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