Accepted Manuscript Title: Humic substances-enhanced electroremediation of heavy metals contaminated soil Author: Mahdi Bahemmat Mohsen Farahbakhsh Mehran Kianirad PII: DOI: Reference:
S0304-3894(16)30260-6 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.03.038 HAZMAT 17550
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
13-12-2015 17-2-2016 14-3-2016
Please cite this article as: Mahdi Bahemmat, Mohsen Farahbakhsh, Mehran Kianirad, Humic substances-enhanced electroremediation of heavy metals contaminated soil, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.03.038 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Humic substances-enhanced electroremediation of heavy metals contaminated soil
Authors: Mahdi Bahemmata ,*, Mohsen Farahbakhsha, Mehran Kianiradb
Department of Soil Science, Faculty of Agricultural Engineering and Technology,
University of Tehran, Iran b
Department of Biotechnology, Iranian Research Organization for Science and Technology
(IROST), Tehran, Iran
Corresponding author at: Department of Soil Science, Faculty of Agricultural Engineering
and Technology, University of Tehran, Iran, P.O. Box: 31587-77871. Tel.: +98 2632231787. Fax: +98 2632231787 Email addresses: [email protected]
(M. Bahemmat), [email protected]
(M. Farahbakhsh), [email protected]
Using humic acids and fulvic acids as chelating agents to improve EK efficiency
HSs-enhanced EK efficiency was about 2 to 3 times greater than that in unenhanced EK
Fulvic acids provided higher HMs removal than humic acids
Results indicate the suitability of HSs-enhanced EK in real HMs contaminated soil
Abstract The effects of catholyte conditioning and the use of humic acids (HAs) and fulvic acids (FAs) as chelating agents to improve electrokinetic (EK) remediation efficiency were investigated using a real and highly contaminated soil. By applying a constant voltage (2.0 V/cm) to the soil, pH and current changes and heavy metals (HMs) concentration were investigated through a range of durations and positions. The observations demonstrated that both catholyte conditioning with 0.1N HNO3 and using humic substances (HSs) enhance remediation efficiency. After 20 days of EK treatment, the removal efficiency of HMs in HS-enhanced EK remediation was about 2.0 to 3.0 times greater than when unenhanced. The quantity of HMs moving toward the cathode exceeded the anode, from which it could be reasonably inferred that most negatively charged HM-HS complexes were moved by electroosmotic forces. Further, free HM cations and positively charged complexed HMs migrated to the catholyte compartment by electromigration. The results obtained in this study, demonstrate the suitability of HS-enhanced EK remediation in HMs contaminated soil.
Keywords: Electrokinetic remediation, Heavy metals, Humic acids, Fulvic acids
1. Introduction Heavy metals (HMs) are of concern and receive continuous attention due to their toxic properties and the negative health effects they may cause. Both natural and anthropogenic activities result in a significant input of HMs into the environment. These activities include mining and metallurgy, urban and industrial wastes, and applications of sewage sludge and fertilizer [1-7]. Electrokinetic (EK) remediation has proven an effective and low-cost method for treatment of HM contaminated soil, sediment, and groundwater [5, 8, 9]. The effective removal processes on the EK technique are electrolysis of water, electro migration, electroosmosis, and electrophoresis, all processes that govern the mobility and transport of HMs under an electric field in soil. Of these processes, water electrolysis on the electrodes surface causes an increase in pH at the cathode and decrease at the anode. The pH changes have a significant effect on solubility, ionic state and charge, and level of contaminants adsorption. Dissolved HM ions will precipitate at the cathode as oxides, hydroxides, carbonates and other compounds. One of the main obstacles to implementing EK remediation as an efficient technique for in situ soil remediation is the undesirable accumulation of HMs close to cathode [10, 11]. However, this obstacle may be overcome by careful planning of the EK process or by use of an enhancing program. For successful removal, it is essential to maintain the HMs in the dissolved phase [1, 12, 13]. Among all the enhanced EK techniques, chelant-enhanced EK has proven effective in both application, and economically. Some synthetic chelants such as EDTA, are toxic, particularly in their free forms , and almost non-biodegradable . However, the efficiency of natural 4
surfactants may be higher than synthetic surfactants, as well as biodegradable, environmental-friendly and less expensive in some cases . Humic substances (HSs) are ubiquitous in nature and their peculiar feature is polyfunctionality, which enables them to interact with both metal ions and organic chemicals and has a significant effect on the environmental fate, mobility, and transport of chemical species especially HMs [17-19]. Based on their solubility as a function of pH, HSs are classified into three main fractions; humic acids (insoluble at acidic pH, but soluble under alkaline conditions), fulvic acids (soluble in aqueous media) and humins (insoluble throughout the whole range of pH-values) . The ability of HSs to form stable complexes with HMs has been well established [17-21]. According to Stevenson , because of the functionality of HSs, such as carboxylic (—COOH), phenolic, alcoholic, and enolic hydroxyl (—OH), amin (—NH2), and S— and P—, containing functional groups are capable of forming stable and mobile complexes with HM ions. Deprotonating of these functional groups occurs at increasing pH and these behave as negatively charged moieties, binding HMs [22, 23]. Therefore, humic acids (HAs) and fulvic acids (FAs) keep the soil contaminants solubilized and mobile. The formation of these complexes facilitates the mobilization and transport of HMs in soils. Hence, the use HAs and FAs as complexing agents has become a promising application in enhanced electroremediation. In contrast with the multitude of studies examining efficiency of different chelating agents in the EK remediation systems [20, 24], the potential for HAs and FAs as chelating agents in the EK remediation of a real and highly contaminated soil has not received much attention.
To date, humic acids-enhanced EK remediation has been employed successfully for the removal of Cu(OX)2  and cadmium  from a spiked contaminated soil. To the best of our knowledge, very few studies have been documented on the application of FAs as chelating agents in the chelate-enhanced EK remediation of a contaminated soil. Based on the aforementioned reports about the interaction of HSs with HMs, we hypothesized that efficiency of EK remediation, in removing HMs from a real contaminated soil, would be enhanced by using HS fractions. Thus, in the present work the objectives were to (1) study the feasibility of EK remediation on real HMs contaminated soil, by using HAs and FAs as enhancing agents and cathode acid conditioning (2) compare the EK removal efficiency of HMs as a function of time and space.
2. Materials and methods 2.1. Soil sampling and characterization An actual HMs contaminated soil was used. A superficial soil sample (0-20 cm), has been classified as a Lithic Xerorthent according to Soil Survey Staff , was collected from the surrounds of a mine in Markazi Province, Iran. The soil was air-dried and ground, to pass through a 2-mm sieve, then homogenized before use. Selected physical-chemical and mineralogical properties of the soil were determined based on the Sparks . Briefly, the sand, silt and clay fractions of the sample were determined by the hydrometer method. Total organic carbon content was determined by the wet oxidation procedure. Cation exchange capacity (CEC) was measured by the 1 M ammonium acetate (pH 7.0) method.
Carbonate content was determined by using a calcimeter. The mineralogical composition of the soil clay fraction was analyzed with a SIEMENS D-5000 x-ray diffractometer (XRD). The characteristics of the tested soil, prior to EK remediation, are presented in Table 1. 2.2. Extraction of humic substances (HSs) The procedure employed to isolate the HA and FA fractions was that developed by the International Humic Substances Society . HSs were extracted from peat by 0.1M NaOH. Extracted HSs were then separated into HAs and FAs by acidifying the extract to pH 2.0 with 6M HCl. The precipitate (HAs) and the supernatant (FAs) were separated by centrifugation (7000 rpm for 15 min). The precipitated HAs were separated by centrifugation (12000 rpm for 15 min) and redissolved by raising the pH to 7.0 with 0.5M NaOH, after which they were stored in the dark at 4.0 0C as a concentrated stock solution. These resultant solutions were used as peat HA and FA model solutions for pre-treatment of the contaminated soil. 2.3. Elemental analysis, functional groups and FTIR spectroscopy of HAs and FAs used in this study The contents of carbon, hydrogen, oxygen and nitrogen of HAs and FAs samples measured by elemental analysis using a Thermo Finnigan (EA 112) elemental analyzer (Table 2). The total acidity and the properties of phenolic and carboxylic acids of HAs and FAs samples were assayed according to Swift . Briefly, the content of carboxyl groups was determined by ion exchange with calcium acetate and the total acidity by equilibration with Ba(OH)2 . Phenolic hydroxyls were assumed to be equal to the difference between
total acidity and carboxyls (Table 2). FTIR spectra of HAs and FAs samples were recorded on a PERKIN ELMER Spectrum One spectrometer, from potassium bromide pellets. The KBr pellets were obtained by pressing a mixture of 1:20 ratio of HAs and FAs samples and KBr, respectively. 2.4. Preparation of HAs and FAs solutions Prior to performing experiments, stock solutions were prepared for each of the HAs and FAs with a concentration of 15.0 g/L and a pH of approximately 8.2, similar to the initial soil pH. 2.5. Electrokinetic apparatus A laboratory-scale EK reactor was made of Plexiglass and consisted of a central soil cell (30 cm×10 cm×10 cm) and two electrode compartments (10 cm×10 cm×10 cm), with a working volume of 1.0 L. To avoid soil leakage to the electrode compartments, two 5 mm thick pierced Plexiglass plates, separated by a paper filter (Schleicher & Schuell), were used. Electrode compartments were filled with the processing fluid (Table 3). A pair of graphite electrodes was used as an inert electrode (10 cm×5 cm×1 cm). Fig. 1 shows a schematic of the EK test set-up dimensions and soil sampling locations. 2.6. Experimental design Four EK tests were performed under different EK manipulation patterns (Table 3). In all experiments, the anolyte compartment was filled with distilled water, while the catholyte compartment was filled with distilled water in the unenhanced EK (Exp. 1) and
0.1N nitric acid in the enhanced EK (Exp. 2, Exp. 3 and Exp. 4). To keep the electrolyte solutions properties constant, they were refreshed daily during the EK processes. EK tests were performed in a polyethylene container using approximately 5000 g of dry soil, mixed with 2000 mL of distilled water for Exp. 1 and Exp. 2, and with 2000 mL HA and FA solutions at a concentration of 15.0 g/L for Exp. 3 and Exp. 4. To achieve homogeneity, the mixture was stirred manually for several minutes. A constant potential difference (2.0 V/cm) was applied to the saturated soil in all experiments for a treatment time of 20 days, using two graphite electrodes placed inside the cell and a power supply. During the EK processes, the electric current was continuously monitored, and the soil pH was measured daily by inserting a pH electrode directly into the soil and allowing the reading to stabilize at distances of 5, 10, 15, 20 and 25 cm from the anode. Soil samples were taken at these distances with a glass tube, after 5, 10 and 15 days. At the end of each experiment (20 days), the soil sample was immediately sliced every 5 cm along the anode-cathode axis, and each sliced sample section was oven dried at 105 °C for 24 h and analyzed for HMs concentration. 2.7. Determination of heavy metals concentration The content of HMs in the soil before and after EK treatment was determined by acid digestion. Total acid-extractable heavy metals were determined after extraction with 4.0N HNO3 at 80 0C for 16 hours. 2.0 g of samples were weighed and placed in a 125-mL Erlenmeyer flasks and 20 mL of 4.0N HNO3 was added . After 16 hours at 80 0C, samples were filtrated and analyzed for HMs using an atomic absorption spectrophotometer (Shimadzu, Japan). Three replicates were analyzed for each sample and the average value 9
was reported. Also, removal efﬁciency of metal ions by electrokinetic is deﬁned as following equation:
Removal efficiency (%) =
C0 - Cf ×100 C0
Where, C0 is initial metals concentration (mg kg-1) of soil, and Cf is the final concentration of metals (mg kg-1) after electrokinetic treatment in soil. 3. Results and Discussion 3.1. FTIR spectra Fourier transform infrared (FTIR) spectra provides valuable information on the functional properties of HAs and FAs, and has thus been widely used for characterizing their spectra . As shown, HAs (Fig. 2a) and FAs (Fig. 2b) FTIR spectra presented a strong band at approximately 3400 cm-1. This band can be attributed to O—H groups of phenols, hydroxyls and carboxyls, or N—H stretch and the peak at about 2927 cm-1 represents aliphatic C—H stretching in C—H2 and C—H3 . Both spectra were approximately similar. Compared to HAs, there are more intensive bands at 1390-1050 cm1
in the FAs sample. The IR absorption band at 1624 cm-1 can be due to C—O stretching of
carboxyl, or the C—N stretching of amide I . There is also a strong band at approximately 1734 cm-1, which is attributed to C—O stretching vibration of carboxylic acid . A very strong and fairly band at 1210 cm-1 indicates the presence of C—O and OH groups of carboxyl acids . The band at approximately 1117 cm-1 was quite intense, and was assigned to the C—O asymmetric stretching vibration of alcohols or carbohydrates 10
moieties . The short length bands in the range 850-400 cm-1 were attributed to the presence of C—H bonds in one- or multi-ring aromatic structures . 3.2. Current density variation during EK treatment The variations in current density depending on the elapsed operating time are shown in Fig. 3. When the catholyte compartment pH was not conditioned (Exp. 1), the electric current was expected to be at a relatively low level, due to formation of metal hydroxide precipitates close to the cathode and forming a high resistance zone close to the cathode side . It can be seen from Fig. 3 that when 0.1N nitric acid conditioning was conducted at the catholyte compartment of the EK system (Exp. 2- 4), that the current intensity immediately increased from 0, reaching a peak value and then decreasing slowly towards 0 again. During first 24 hours of EK process, Exp. 2 reached 25.6 mA/cm2 after 6 hours, Exp. 3 reached 30.3 mA/cm2 after 0.5 hours and Exp. 4 reached 30.1 mA/cm2 after 1 hour. After reaching a peak value, the current intensity curves began to descend in all enhanced EK experiments and approached 0. The current intensity decrease with time, may have been caused by depletion of solutes, precipitation of metal hydroxide and non-conductive solids [5, 8, 36, 37], and electrode polarization . The electrolyte solutions were refreshed daily during the EK experiments. With intermittent replenishment of the solution, the electrical current partially recovered . The use of 0.1N HNO3 as pH conditioning solution at the cathode compartment controlling the catholyte pH, hampered developing a high pH zone and decreased the formation of metal hydroxide precipitates near the cathode side. Furthermore, large amounts of ions were introduced to the catholyte and the soil, resulting in an increased current intensity . This result indicates that pre-treatment of soil using 11
HA and FA solutions causes the electrical conductivity of soil to increase dramatically. These solutions enhanced desorption of ions from the soil surface into the pore water and increased the current intensity under constant voltage conditions. In general, HAs and FAs are ionic chelate, which may provide mobile ions for the soil. Also, in an electric field, the molecules of HAs and FAs could be charged and deprotonated, and adsorbed onto the surface of electrodes and micellar aggregates formation, which could impede the transport of charges throughout the system. 3.3. Soil pH variation during EK treatment Fig. 4 depicts the profile of soil pH versus time during the EK experiments. As shown in Fig.4a, the soil pH near the cathode continuously increased with time. The soil pH near the anode declined to about 2.0 or slightly below, due to H+ ion production by electrolysis of water at the anode. At the end of the EK treatment, the soil pH near the cathode was 11.8, and this decreased gradually in a horizontal direction, to reach 3.9 near the anode in Exp. 1, in which catholyte refreshment with 0.1N HNO3 was not performed. The soil pH ranged from 7.7 to 11.8 near the cathode (25 cm from the anode), while soil pH fluctuated from 6.6 to 3.9 near the anode (5 cm from the anode). This pH pattern is typical of unenhanced EK experiments and caused primarily by the electro-migration of H+ and OH-/HCO3- from the anode and cathode compartments, respectively. As seen, Exp. 2, Exp. 3 and Exp. 4 (Table 3) were carried out under different EK manipulation patterns, with a strong acidic solution (0.1N nitric acid) used as catholyte solution to maintain an acidic condition for soil. It can seen in Fig. 4, that the soil pH changes at soil sections close to cathode in these later experiments were significantly different from those in Exp. 1. 12
Regarding the enhanced EK, Fig. 4 indicates that the soil pH in all enhanced EK treatments was lowered from the initial value of 8.2. The proton ions generated at the anode migrated toward the cathode, and the soil pH levels gradually decreased from the anode side. As seen in Fig. 4, the pH of soil changed to below 2.0 from the initial 8.2 near the anode and about 7.0 near the cathode. A similar trend of gradual decrease in soil pH to a constant value in Exp. 2, Exp. 3 and Exp. 4 was recorded, which might be due to the catholyte pH control and the dominance of H+ ions in the electrokinetic cell, because the mobility of H+ ions is 1.75 times greater than that of OH- ion . In these experiments, the soil pH values varied from slightly below 2.0 to 7.0 because of pH controlling by daily catholyte refreshment with 0.1N HNO3. The effect of pre-treatment with HAs and FAs on final soil pH was negligible. However, these results demonstrated that catholyte refreshment with strong acidic solution was effective in controlling soil pH. 3.4. Effect of EK treatment on HMs distribution in the soil cell as a function of time and space 3.4.1 Unenhanced EK The acid-extracted HMs residual in the soil sections as a function of time and space, after the unenhanced EK treatments are shown in Fig. 5. It seems that HMs follow relatively similar behavior in this experiment. HMs generally moved toward the cathode from the anode. As a result, in soils near the anode the metals migrated towards cathode via electromigration and electroosmosis [8, 41]. As expected, increasing remediation time from 5 to 20 days increased the efficiency of removal HMs. From the results for different
treatment times, it may be concluded that higher treatment times increase the HMs removal in soil sections close to the anode. In Exp. 01 where catholyte conditioning was not performed at the end of the experiment (20 d), the percentage of HMs removal close the anode was about 20-40%, while close to the cathode removal percentage was very low because of high pH in regions where HMs were precipitated (Fig. 5). 3.4.2 Acid-enhanced EK The concentration proﬁles of HMs in the soil cell after the acid-enhanced EK experiments (Exp. 2) are presented in Fig. 6. Over time, due to the movement of positive charged HM ions toward the cathode by electromigration, the removal of all HMs was higher at anode region than cathode region. Furthermore, HMs migration was higher with catholyte conditioning using nitric acid, due to easier desorption of HM ions from the soil surface into the pore solution in an acidic condition [7, 42]. For instance, as a result of the low pH close to the anode, lead and cobalt existed as Pb2+ and Co2+, and migrated towards the cathode . By the end of the experiment, 38.1%, 29.1%, 38.3%, 41.7%, 33.4% and 25.4% of Cd, Co, Mn, Ni, Pb and Zn, respectively, had been removed from the soil. The results are comparable with the ﬁndings in various studies in which a similar conditioning solution was introduced [43-45]. 3.4.3 Humic-enhanced EK Fig. 7 shows the acid-extracted HMs concentration in the soil sections as a function of time and space after the humic-enhanced EK treatments. After the pre-treatment soil by HAs, the acid-extracted HMs concentration in soil decreased. In Exp. 3 where the pre-
treatment soil by HAs and catholyte conditioning was performed, the removal percentages of Cd, Co, Mn, Ni, Pb and Zn were 58.1%, 38.1%, 60.4%, 51.7% 41.5% and 45.5% respectively. The acid-extracted HMs concentrations in humic-enhanced EK treatments were higher than those in Exp. 1 and Exp. 2, which might be attributed to the ability of humic acids to form mobile HM complexes in the soil and the acid conditions produced by EK treatment . 3.4.4 Fulvic-enhanced EK Fulvic-enhanced EK treatment changed the distribution of acid-extracted HMs in the soil, because the electric ﬁeld transported ions toward the oppositely charged electrodes. In Exp. 4 where the pre-treatment soil by FAs and catholyte conditioning was performed, an obvious transport of HMs from the anode to cathode was observed. HMs were moved toward the cathode, although there were some variations in this pattern (Fig. 8). After catholyte pH controlling and soil pre-treatment with FAs, the residual HM concentrations in the soil decreased. The removal percentages of Cd, Co, Mn, Ni, Pb and Zn from the soil column in Exp. 4 after 20 d of treatment were 66.1%, 38.4%, 64.1%, 70.8%, 50.6% and 47.4%, respectively (Fig. 8). As can be seen in Fig. 8, from the analysis of the results for different remediation times, it may be concluded that higher remediation times would increase HMs removal at soil sections close to the anode. 3.4.5 Comparison of HMs distribution after different EK treatments at the end of the experiments
The acid-extracted HMs residual in the soil sections after treatments are presented in Fig. 9. After all treatments, the HMs concentrations were lower than the initial concentration (C/C0 <1). In the case of Cd, in all the experiments, the highest removal content of cadmium was about 100%, 5 cm from the anode (Fig. 9a). When 0.1N HNO3 was used in the catholyte (Exp. 2), more cadmium (36%) was removed, close to the cathode, which was similar with the results in other enhanced EK treatment. However, when HAs and FAs were used as soil pre-treatment, a signiﬁcant improvement compared to the Exp. 2 result was observed (Fig. 9a). The overall cadmium removal eﬃciency percentages were 58.1% and 66.3% for Exp. 3 and Exp. 4, respectively. This may be due to the formation of HA– and FA–cadmium complexes that improved its transport in the electric field. According to Fig. 9b, the removed quantity of cobalt in Exp. 1 was lower than in other treatments, which showed that catholyte conditioning is favorable to the removal of cobalt. In Exp. 2, the removal percentage of cobalt from the soil column was 42.5% and the cobalt concentrations increased from the anode to the cathode in a zigzag shape, which may be attributed to inadequate treatment time. In Exp. 3, the removal percentage of cobalt increased (56.3%), which may be attributed to the mobile complexes Co-HAs formed in the soil and the acid conditions produced by EK . The removal trend of cobalt in Exp.4 was similar with the distribution in Exp. 3 but reduced, which may be attributed to the more soluble metal complexes of FAs because of their lower molecular weights and higher acidic functional groups content, than those of HAs . The highest removal content of cobalt
was 60.3% in Exp. 4, reaching 79%, 5 cm from the anode, which was higher than in other soil sections. Fig. 9c shows the acid-extracted manganese concentration in different soil sections. After all the treatments, the manganese concentrations in the soil decreased. A high concentration of manganese in the cathode side was observed. More than 85% acidextracted manganese was removed, and the manganese concentrations were similar in Exp. 3 and Exp. 4, at 85.7% and 88.8% respectively. The manganese concentrations in HSenhanced EK treatments were higher than those in Exp. 1 and Exp. 2, which may be attributed to the ability of HSs to form mobile complexes in the soil. As can be seen, in Fig 9d, after 20 days of electrokinetic treatments, the percentage removal of Ni was 21.6% in Exp. 1. With catholyte pH controlling, the percent of Ni removed from the soil increased signiﬁcantly. About 45.5%, 66.5% and 77.5% of Ni were extracted from the soil column in Exp. 2, Exp. 3 and Exp. 4 respectively. Clearly the pH control of the catholyte, improved the soil and Ni removal by the electrokinetic treatment. The removal percentages of Pb from the soil column after EK treatment were 21.2% and 33.4% for Exp. 1 and Exp. 2 respectively, based on the acid-extracted lead concentrations (Fig. 9e). By controlling the catholyte pH, and pre-treating the soils with HAs and FAs, residual Pb was decreased. The removal percentages of Pb in Exp. 3 and Exp. 4 were 52.2% and 61.7% respectively (Fig. 9e). When looking at the ﬁnal distribution along the specimen (see Fig. 9e), it is clear that a considerable portion of Pb migrated towards the cathode.
As shown in fig. 9f, the soil Zn concentrations decreased, and Zn increased from the anode to cathode, which showed that Zn was removed from the soil cell. In Exp. 1, the removal percentage of Zn was only 15.4% due to high soil pH near the cathode. These results were similar with the results for the other heavy metals. The removal percentages of Zn after 20 d were 25.4%, 45.4% and 47.4% for Exp. 2, Exp. 3 and Exp. 4 respectively, based on the acid-extracted Zn analysis. 3.5 General discussion on HA and FA roles in the EK removal In this study, the removal efficiency of EK remediation was enhanced by conditioning the catholyte pH and using HAs and FAs as natural chelating agents. The migration and removal of the HMs was higher with catholyte conditioning using an acidic solution, because HMs can be easily desorbed into the pore solution from a soil surface in an acidic condition [7, 42]. Hence, these desorbed metal ions can easily be transported by electromigration under an electric ﬁeld [5, 8, 10]. In Exp. 3 and Exp. 4 with pre-treatment of soil by HAs and FAs respectively, and catholyte conditioning, an obvious transport of HMs from the anode to cathode was observed. Two dominant functional groups of HAs and FAs include operationally defined carboxylic acids with pK values less than 7, and phenolic groups that tend to become deprotonated at higher pH. Deprotonating of these functional groups occurs at increasing pH and these behave as negatively charged moieties, binding HMs . The carboxyl (—COOH) group reacts readily with metals [17, 19, 22], and gradually dissociates between pH 2.5 and 7.0 to form the carboxylate (—COO-) group . The phenolic hydroxyl (—OH) group reacts less with metals, and dissociates between pH 8 and 13.5 [22, 47]. During the EK experiments, the soil solution pH values were less 18
than 8.0, leading to the involvement of the deprotonated carboxylic groups in the complexes of HMs and phenolic hydroxyl groups are partially active. Higher EK removal efficiency in Exp. 4 could be due to the content of carboxylic functional groups being higher in FAs than HAs. Therefore, Carboxyl groups are the principal functional groups participating in the reaction chelation with HM ions (M2+), and accordingly, we conclude the mechanism of carboxylic groups chelating M2+ can be expressed as follows: R-COOH + M2+
R-COO-M+ + H+
These complexes would be expected to carry a net positive charge and be transported to the cathode side by electromigration. Moreover, the reaction between HM cations and HS surface can be defined as: M2+ + HAn-
In these conditions, HM-HS complexes have a net negative charge and move in the direction of the anode. Sawada et al. , concluded that electroosmosis is the main driving force for movement of these complexes from soil to the cathode side, even though they are polyvalent anions. Accordingly, overall movement of dissolved complexed HM ions was from anode to cathode side. 4. Conclusions A new EK technique to cleanse real HMs contaminated soil by using of HAs and FAs as chelating agents and controlling catholyte pH was investigated in this study. It was concluded that cadmium, cobalt, manganese, nickel, lead and zinc removal can be enhanced
by HAs and FAs and a controlling catholyte pH. The removal efficiency of heavy metals in HSs-enhanced EK (Exp.3 and Exp.4) was about 2.0 to 3.0 times greater than in unenhanced EK (Exp.1). The enhanced removal of HMs resulted from the characteristics of HSs, included counterion binding, chelating and solubility enhancement in acid media. The quantity of HMs moved to cathode side was more than anode side, from which it may reasonably be inferred that most negatively charged HMs-HSs complexes were moved by electroosmosis force. Further, free HMs cation and positively charged complexed HMs migrated to the catholyte compartment by electromigration. The results obtained in this study, indicate the suitability of HS-enhanced EK remediation in real and highly contaminated soil. However, further investigation should be conducted to achieve a higher removal performance of HMs, such as applying more suitable HSs anolyte conditioning and prolonging treatment duration. Further experiments will be carried out to investigate the suitability of HSs-enhanced EK remediation in situ.
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Figure captions Fig. 1. Schematic diagram of the EK test set-up dimensions and soil sampling locations. Fig. 2. Fourier transform infrared (FTIR) spectra of (a) Humic acids and (b) Fulvic acids. Fig. 3. Time course of current density during EK experiments. Exp. 1: Unenhanced EK; Exp. 2: Acid-enhanced EK, Exp. 3: Humic-enhanced EK and Exp. 4: Fulvic-enhanced EK. Fig.4. pH change in time. Exp. 1: Unenhanced EK; Exp. 2: Acid-enhanced EK, Exp. 3: Humic-enhanced EK and Exp. 4: Fulvic-enhanced EK; S1, S2, S3, S4 and S5: 5, 10, 15, 20 and 25 cm from anode, respectively. Fig. 5. The acid-extracted HMs residual in the soil sections as a function of time and space after the unenhanced EK treatments. (a): cadmium; (b): cobalt; (c): manganese; (d): nickel; (e): lead and (f): zinc. Fig. 6. The acid-extracted HMs residual in the soil sections as a function of time and space after the Acid-enhanced EK treatments. (a): cadmium; (b): cobalt; (c): manganese; (d): nickel; (e): lead and (f): zinc. Fig. 7. The acid-extracted HMs residual in the soil sections as a function of time and space after the Humic-enhanced EK treatments. (a): cadmium; (b): cobalt; (c): manganese; (d): nickel; (e): lead and (f): zinc. Fig. 8. The acid-extracted HMs residual in the soil sections as a function of time and space after the Fulvic-enhanced EK treatments. (a): cadmium; (b): cobalt; (c): manganese; (d): nickel; (e): lead and (f): zinc. Fig. 9. Distribution of the acid-extracted HMs residual in the soil sections after EK treatments. (a): cadmium; (b): cobalt; (c): manganese; (d): nickel; (e): lead and (f): zinc. Exp. 1: Unenhanced EK; Exp. 2: Acid-enhanced EK, Exp. 3: Humic-enhanced EK and Exp. 4: Fulvic-enhanced EK.
Table 1. Selected properties of soil used in this study Soil property
Clay fraction mineralogy Sand (%)
Vermiculite >>> Illite > Smectite > Kaolinite 56.00
Electrical conductivity (dS/m)
Organic carbon (%)
Soil saturation percent (%)
Cation exchange capacity (cmol+ /kg) Carbonate content (%) Concentration of HMs (mg/kg)
Table 2. Elemental composition and contents of functional groups humic and fulvic acids %O Sampl e
(by difference )
Total acidity (meq/g )
Carboxy l (COOH) (meq/g)
Phenoli c OH (meq/g)
Table 3. Experimental condition of the four electrokinetic tests Exp. Anolyte purging Soil saturation Catholyte purging no.
5, 10, 15, 20
5, 10, 15, 20
15 g/L HAs
5, 10, 15, 20
15 g/L FAs
5, 10, 15, 20