Environ Sci Pollut Res DOI 10.1007/s11356-015-4909-5

RESEARCH ARTICLE

Electrokinetic remediation of fluorine-contaminated soil and its impact on soil fertility Ming Zhou 1 & Hui Wang 1 & Shufa Zhu 1 & Yana Liu 1 & Jingming Xu 1

Received: 20 January 2015 / Accepted: 16 June 2015 # Springer-Verlag Berlin Heidelberg 2015

Abstract Compared to soil pollution by heavy metals and organic pollutants, soil pollution by fluorides is usually ignored in China. Actually, fluorine-contaminated soil has an unfavorable influence on human, animals, plants, and surrounding environment. This study reports on electrokinetic remediation of fluorine-contaminated soil and the effects of this remediation technology on soil fertility. Experimental results showed that electrokinetic remediation using NaOH as the anolyte was a considerable choice to eliminate fluorine in contaminated soils. Under the experimental conditions, the removal efficiency of fluorine by the electrokinetic remediation method was 70.35 %. However, the electrokinetic remediation had a significant impact on the distribution and concentrations of soil native compounds. After the electrokinetic experiment, in the treated soil, the average value of available nitrogen was raised from 69.53 to 74.23 mg/kg, the average value of available phosphorus and potassium were reduced from 20.05 to 10.39 mg/kg and from 61.31 to 51.58 mg/kg, respectively. Meanwhile, the contents of soil available nitrogen and phosphorus in the anode regions were higher than those in the cathode regions, but the distribution of soil available potassium was just the opposite. In soil organic matter, there was no significant change. These experiment results suggested that some steps should be taken to offset the impacts, after electrokinetic treatment.

Responsible editor: Zhihong Xu * Ming Zhou [email protected] 1

Chemical Engineering and Pharmaceutics College, Henan University of Science and Technology, Luoyang 471023, China

Keywords Electrokinetic remediation . Fluorine-contaminated soil . Soil fertility

Introduction Now, China, as a large developing country undergoing rapid industrialization and urbanization, is facing an increasingly serious threat of soil pollution. On April 17, 2014, China’s ministry of Environmental Protection and the ministry of Land and Resources jointly issued the first national survey of soil pollution bulletin. Survey results show that the overall national soil environment is not optimistic. The findings show that about 16.1 % of soil in the Chinese mainland suffer pollution, and more seriously, 19.4 % of farming land were polluted. Lots of pollutants caused by human activities entered into the soil, which included heavy metal pollutants such as cadmium, mercury, lead, and chromium, organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) and organic chlorine pesticides (OCPs), and other pollutants, such as fluorides. A large number of fluorides in soil may be due to numerous anthropogenic activities including irrational sewage irrigation, phosphate fertilizer production and use, aluminum smelting, ceramics, glass, and brick manufacturing among others. Especially, soil contamination by fluorine has become an increasingly serious concern for decades due to increased dependence on electrolytic aluminum in China. Compared to soil pollution by heavy metals and organic pollutants, soil contamination with fluorides is usually ignored in China. An important reason is that fluorine is considered by some people as one of the beneficial elements to human being and has no any harm to the body; for example, all sorts of fluoride toothpastes are widely used today in China. Sure, small amounts of fluorides may be beneficial for bone strength and reduce tooth decay. However, high

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concentrations of fluorides are also harmful to human, animals, plants and surrounding environment (Camargo 2003; Domingos et al. 2003). Chronic excess fluoride consumption can lead to skeletal fluorosis, a disease of the bones that affected millions in China. 2013 Health Statistical Bulletin of China showed that 1137 counties were the drinking water type of endemic fluorosis areas in China. The patients with dental fluorosis and skeletal fluorosis were 19.616 million and 1.322 million, respectively. Besides being toxic, high concentration of fluorine ions leads to the inhibition of some enzyme reactions and to the linking of biogenous elements and the disturbance of their balance in the organism. In addition, soil pollution by fluorides has an unfavorable influence on surrounding environment. Lots of fluorides can be leached from contaminated soil and cause groundwater contamination, and high concentration of fluorides can be absorbed by plants which affect growth and development of plants (Evdokimova 2001). Thus, it is significant to study the remediation of fluorine-contaminated soil. Electrokinetic (EK) remediation is an effective method that was used for the treatment of soil, sludge, and sediment contaminated with organic and inorganic pollutants. EK technique is based on the application of a direct electric potential to the contaminated soil by a series of electrodes. On direct current electrical field, there is a variety of reactions and transport processes in the contaminated soil, which result in the mobilization of contaminants toward the electrodes. Among them, the two main transportation mechanisms are called electroosmosis (the net flux of water induced by the electric field through the porous structure of the soil) and electromigration (the movement of ionic species in the electric field toward the electrode of opposite charge) (Acar and Alshawabkeh 1993). The accumulative contaminants in electrodes are extracted by the following methods: precipitation at the electrode, adsorption/electroplating onto the electrode, complexing with ion exchange resins, pumping water near the electrode, etc. (Lu et al. 2012). The advantages of EK technique are as follows: the applicability to a wide range of contaminants, the applicability in low permeability soil, and better removal efficiency (Yang et al. 2014; Wan et al. 2011; Sun and Ottosen 2012; Li et al. 2013). Because of these advantages, EK technique had been successfully applied to remove a wide range of contaminants, such as a variety of heavy meals and organic pollutants from contaminated soil, sludge, sediment, and mine tailings (Maruthamuthu et al. 2011; Pazos et al. 2012; Almeira et al. 2012; Giannis et al. 2012; Castillo et al. 2012; Li et al. 2012; Alcántara et al. 2012; Dong et al. 2013; Rozas and Castellote 2012; Rojo et al. 2014). However, there are few literatures on electrokinetic remediation of fluorine from contaminated soil (Pomes et al. 1999; Kim et al. 2009). Meanwhile, in this technology, an electric field is utilized to promote the movement of contaminants toward the electrode; however, other native compounds that

are present in the soil can also be mobilized. However, there was limited research evaluating the soil fertility after electrokinetic treatment. So, in this work, we carried out the electrokinetic treatment of fluorine-contaminated soil, and its impact on soil fertility was also studied.

Materials and methods Soil sampling Soil was taken from a farmland near an aluminum smelting plant located in Luoyang City, China’s Henan province. They were sampled from the surface layer (0–20 cm) and were silty loam. The fluorine concentration, organic matter content, pH, available nitrogen, available phosphorus, and available potassium of soil samples were 1132 mg/kg, 4.49 %, 8.32, 69.53 mg/kg, 20.05 mg/kg, and 61.31 mg/kg, respectively. Under ambient conditions, the soil was air-dried and sieved in a 2-mm nylon mesh. Electrokinetic setup Electrokinetic experiment was conducted in the self-made electrokinetic reactor. The experimental conditions are tabulated in Table 1. A schematic view of the lab-scale electrokinetic reactor is shown in Fig. 1. The electrokinetic reactor consists of the following major parts: (1) the soil cell (10 cm [L]×6 cm [W]×8 cm [H]), (2) the electrode compartment (4 cm [L]×6 cm [W]×8 cm [H]), (3) the electrolyte reservoir, (4) the direct current power supply (GPC6030D, Gw instek, China), and (5) four-channel peristaltic pump (BT00-300T/DG-4, longerpump, China). The working electrode was made of graphite sheet in this study (1 cm [L]×6 cm [W]×8 cm [H]). In the experiment, 600 g soil sample and 200 ml distilled water were mixed and packed into soil cell. NaOH (0.1 mol/L) was used as the anolyte, and distilled water was used as the catholyte. The addition of NaOH in the anolyte will increase the conductivity in the whole cell. Furthermore, the production of H+ is mediated by the addition of OH− at the anode which can help to the desorption of fluorine from contaminated soil. At a 10-mL/ min flow rate, the four-channel peristaltic pump was utilized to circulate the electrolyte and control the liquid level in electrode compartment. Every 24 h, electrolyte in electrode compartment and electrolyte reservoir was refreshed and the cumulative fluorides in anolyte and catholyte were measured. Table 1

Electrokinetic experimental conditions

Anolyte

Catholyte

Voltage (V)

Treatment time (h)

0.1 mol/L NaOH

Distilled water

20

240

Environ Sci Pollut Res Fig. 1 A schematic view of the lab-scale electrokinetic reactor (the electrode compartment, soil cell, and electrolyte reservoir are made of organic glass). 1, The DC power supply (GPC6030D, Gw instek, China); 2 and 3, working electrode; 4, the soil cell; 5 and 6, nonwoven fabrics; 7 and 8, the electrode compartment; 9 and 10, the electrolyte reservoir; 11 and 12, simple flow regulator; 13–16, four-channel peristaltic pump (BT00-300T/DG-4, longerpump, China); 17 and 18, gas vent

After the experiment, the treated soil was taken out and cut into ten parts (the same size); then, every part would be used to measure fluorine concentration, soil pH, organic matter content, available nitrogen, available phosphorus, and available potassium, respectively. Analytical methods and the calculation method of electroosmotic flow

Results and discussion Electrokinetic remediation of fluorine-contaminated soil

An alkali fusion-selective ion electrode technique was used to measure total fluorine in soil (fluoride ion selective electrode, SPSIC, China) (Mcquaker and Gurney 1977). Before the measurement, total ionic strength adjusting buffer (TISAB, 58.8 g C6H5Na3O7·2H2O and 85 g NaNO3 in 1 L) was added to liberate fluorine ion and adjust the pH of the solution to 5.5. Soil pH was determined in the ratio 1:2.5 of soil/distilled water suspension by the pH meter (pHS3C, SPSIC, China). Electrical current through the soil cell was measured using the multimeter (F15B, Fluke, USA) in the electrokinetic remediation process. The organic matter content of soil was measured by the PE-2400 elemental analyzer (PerkinElmer, USA). The available nitrogen was determined by the alkaline hydrolysis diffusion; the available phosphorus was extracted by the Olsen method and measured using the molybdenum blue method; the available potassium was extracted with ammonium acetate (1 M at pH 7.0) and further analyzed by atomic absorption spectrophotometry (4510F, SPSIC, China). To ensure data quality, all chemical analyses were performed in duplicate. Electroosmotic flow of electrokinetic remediation can be calculated (Hamed et al. 1990): Qe ¼ K eo PA

electrical potential gradient (V/cm), and A (cm2) is the cross-sectional area of the soil.

Electrical current Electrical current is an important indicator of the electrokinetic remediation process. Figure 2 depicts the variations of electrical current across the soil cell. As shown in Fig. 2, the electrical current firstly increased, reached the maximum (64.1 mA), then gradually began to decrease and remained at the constant value. The reason is that when electric field was established; firstly, the electrical current across the soil cell was low because it took some time for electrolyte to enter contaminated soil and for soil contaminants and

ð1Þ

where Qe (cm3/s) is the electroosmotic flow, Keo (cm2/Vs) is the coefficient of electroosmotic permeability, P is the

Fig. 2 Variations of current across the soil cell

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minerals to dissolve and desorb from soil surface. About 30 h later, electrical current reached the maximum because of electromigration of pollutants to electrode and desorption of ions from the soil surface into the pore water (Reddy et al. 2011). Then electrical current began to decrease due to the decrease in the migration of anions and cations in pore fluid. Moreover, hydrogen ions moving toward cathode can be neutralized by hydroxide ions moving toward anode, hence forming water and decreasing the ion concentration in the system.

Electroosmotic flow during electrokinetic experiment Electroosmotic flow is the motion of liquid induced by an applied potential across a porous soil (Yeung 2011). Some soluble fluorides and fluorine complexes in the contaminated soil could be removed by electroosmotic flow. At a basic pH, the surface charge of soil particles is negative and electroosmotic flow is toward cathode (Kim et al. 2009). During electrokinetic experiment, electroosmotic flow was calculated by measuring the volume change in the catholyte, and the cumulative amount of electroosmotic flow is shown in Fig. 3. From the figure, the cumulative amount of electroosmotic flow in the system increased with the test duration. The flow rate was higher during the initial stages, but it decreased at later stages. Most of the flow occurred within 5 days of test duration. According to Eq. 1, electroosmotic flow is directly proportional to the electrical potential gradient under the assumptions of constant porosity and a constant coefficient of electroosmotic permeability (Jo et al. 2012). Electrical current is related to the voltage gradient (P) applied to the soil cell, so electroosmotic flow rate first increased and then decreased like the variations of electrical current.

Fig. 3 The cumulative amount of electroosmotic flow during electrokinetic treatment

Fig. 4 Residual fluorine in treated soil after electrokinetic treatment

Distribution of fluorine in the electrokinetic system Figure 4 shows the residual concentration of fluorine in soil after electrokinetic treatment, and Fig. 5 shows the cumulative amount of fluorine in the anolyte and catholyte during electrokinetic remediation. Experimental results showed that fluorine could be removed effectively from contaminated soil by electrokinetic treatment using 0.1 mol/L NaOH as the anolyte. From Fig. 4, compared to the initial fluorine concentration, the content of fluorine in treated soil sections all decreased. Because fluorine has a negative charge, fluorides desorbed from the soil were moved to the anode by electromigration. Meanwhile, fluorine could also be removed as soluble fluoride and fluorine complexes by electroosmotic flow. This combination of electromigration and electroosmosis removed fluorine from contaminated soil, so the content of fluorine in treated soil decreased. The removal efficiency of fluorine is 70.35 % which was calculated according to the concentration of residual fluorine in soil after the experiment and 69.19 %

Fig. 5 Cumulative mass of fluorine in the electrolyte

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which was calculated according to the cumulative amount of fluorine in anolyte and catholyte after the experiment. From Fig. 5, it could be clearly seen that cumulative fluorine in the anolyte was much more than that in the catholyte. This indicated that electromigration (from cathode to anode) was the predominant removal mechanism for electrokinetic remediation of fluorine-contaminated soils instead of electroosmosis. More fluorides were attracted to anode and removed by electromigration. It was reported that electromigration rate is ten times higher than electroosmosis rate (Acar and Alshawabkeh 1993).

Effects of EK on some soil properties Fig. 7 Variations of soil organic matter after electrokinetic remediation

Soil pH and soil organic matter Soil available nitrogen, phosphorus, and potassium Soil pH affects the quantity, activity, and types of microorganisms in soils and has a profound influence on plant growth; it thus affects nutrient transformations and the solubility, or plant availability, of many essential plant nutrients. Variations of soil pH are shown in Fig. 6, and electrokinetic treatment can significantly change soil pH through the electrolysis reaction at the electrodes and migration of H+ and OH−. In typical electrokinetic remediation, hydrogen ions generated by electrolysis reaction at anode migrate from anode to cathode; accordingly, hydroxide ions produced by electrolysis 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 this study, the distribution of soil pH in soil cell also was so, the soil pH gradually increased from anode to cathode, although using 0.1 mol/L NaOH as the anolyte. Figure 7 shows the variations of soil organic matter after electrokinetic remediation. As shown in the figure, there was no significant change in soil organic matter. The result indicated that the application of a low-voltage electric field during short periods of time (10 days) did not modify the organic matter content inside the treated soil.

In this technology, an electric field is utilized to promote the movement of fluorides toward the electrode chambers. However, other native compounds that are present in the soil can also be mobilized, such as nutrients (Mena et al. 2015). Nitrogen (N), phosphorus (P), and potassium (K) are the three essential nutrients for plant growth, and their presence and content are important indicators of the soil fertility and functionality. However, in the soil, N, P, and K are often found in chemical forms that cannot immediately be absorbed and used by plants, and the forms that are not available. For this reason, the available nitrogen, phosphorus, and potassium were measured before and after the electrokinetic treatment; the impact of EK on soil fertility was studied. Figures 8, 9, and 10 show the effect of EK on available soil nutrient contents and distribution. As shown in Fig. 8, the content of soil available N in the soil sections 1–6 increased and that in the soil sections 7–10 decreased. By calculation, the average concentration of available N raised 6.76 % after the electrokinetic treatment. The change of soil available N contents and distribution may be attributed to the hydrolysis and transportation of the nitrogen compound. Under the direct current electric

Fig. 6 Variations of pH in soil sections after electrokinetic remediation

Fig. 8 Variations of soil available nitrogen after electrokinetic remediation

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Compared with available P, soil available K exhibited a reverse trend and the soil available K gradually increased from anode to cathode. Available soil K was accumulated in the soil near cathode. This phenomenon may be attributed to lots of H+ generated by electrolysis reaction and would replace the K+ on the surface or in the gap of potassium-containing clay mineral for the similar radius of H+ and K+ (Cang et al. 2012). Besides, the decrease of soil pH in the anode region would increase the soil mineral dissolution of insoluble potassium in the anode region. Lots of desorbed positive K+ migrate to the cathode.

Fig. 9 Variations of soil available phosphorus after electrokinetic remediation

field, some complex organic nitrogen converted to simple organic nitrogen and inorganic nitrogen by the hydrolysis which increased the content of soil available N (Gosling and Shepherd 2005). In the soil available N, nitrate is one of the important components. Nitrate is an anion, which moved to the anode under the electric field, and lots of nitrates in the soil sections 7–10 were accumulated in the anode region (Jia et al. 2005). From Fig. 9, available P in the treated soil decreased, and the content of available soil P in the anode regions were higher than that in the cathode regions. The lowest content of available soil P was 5.55 mg/kg in the section 10, and the highest content of available soil P was 17.92 mg/kg in the section 1. A lot of available P was removed by the electroosmosis and electromigration after the electrokinetic treatment. Residual available P was accumulated in the soil sections near anode, which was attributed to that the negatively charged phosphorus moved toward the anode under electric field. Meanwhile, after the electrokinetic treatment, soil pH changed, and the lower pH in the anode regions would result in the dissolution of some inorganic phosphorus compounds (aluminum phosphate, calcium phosphorus, and iron phosphate). These desorbed phosphorus with negative charge migrated toward the anode.

Conclusions In this paper, experimental results showed that electrokinetic remediation using 0.1 mol/L NaOH as the anolyte can effectively remove a large proportion of fluorides from contaminated soils. However, on the other hand, the electrokinetic remediation had a significant impact on the distribution and concentrations of soil native compounds. After electrokinetic remediation, in the treated soil, the average value of available nitrogen increased and the average value of available phosphorus and potassium decreased. Experimental results also showed that EK changed the distribution of available N, P, and K. After electrokinetic remediation, from anode to cathode in the soil cell, the soil available N and P gradually decreased and the soil available K gradually increased. In soil organic matter, there was no significant change. This experiment results suggested that some amendments should be performed to return the soil to its initial condition after electrokinetic treatment. For example, based on the changes in soil native compounds’ (available N, P, and K) distribution and concentrations, phosphatic and potash fertilizer needs to be applied in the soil of anode region and nitrogen and phosphatic fertilizer needs to be applied in the soil of cathode region of electric field after electrokinetic remediation. Acknowledgments This research was supported by the National Natural Science Foundation of China (No. 41471256) and Key Scientific Research Project of Henan Province Universities and Colleges (No. 15A610003).

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Fig. 10 Variations of soil available potassium after electrokinetic remediation

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