Simultaneous removal of cadmium from kaolin and catholyte during soil electrokinetic remediation

Desalination 300 (2012) 1–11

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Simultaneous removal of cadmium from kaolin and catholyte during soil electrokinetic remediation Juan Almeira O. a, Chang-Sheng Peng a, b,⁎, Ahmed Abou-Shady b a b

The Key Laboratory of Marine Environmental Science and Ecology, Ministry of Education, Ocean University of China, Qingdao 266100, China College of Environmental Science and Engineering, Ocean University of China, Qingdao 266100, China

a r t i c l e

i n f o

Article history: Received 25 January 2012 Received in revised form 16 May 2012 Accepted 17 May 2012 Available online 23 June 2012 Keywords: Soil electrokinetic remediation Energy saving Cadmium removal Nitric acid Kaolin

a b s t r a c t The influence of the pH variation of a catholyte on the removal of Cd from kaolin and the subsequent precipitation of the Cd in the catholyte was evaluated during soil electrokinetic remediation (EKR). The pH variations of the catholyte were induced by the injection of nitric acid (HNO3) at different concentrations (0.00, 0.01, 0.06, 0.12, 0.18, 0.24, 0.42 M) at a constant rate (0.6 ml/h). The addition of the acid enhanced the removal of Cd from kaolin. When the concentration of the injected HNO3 was higher than 0.06 M, both the electrical energy and acid consumption increased. When the concentration of the injected acid was higher than 0.12 M, the pH in the catholyte markedly increased. The best results were obtained by using 0.06 M HNO3. At this concentration, it was possible to extract 98% of the Cd from kaolin, of which 63% precipitated in the catholyte. Thirteen liters of HNO3 and 30 kW-h of electrical energy were required per cubic meter of treated kaolin. The average extraction rate of Cd was extracted from kaolin reached high values (0.38 mg/h) when less than 90% of the Cd had been removed. For the remaining 10%, the extraction rates noticeably decreased (0.025– 0.059 mg/h). © 2012 Elsevier B.V. All rights reserved.

1. Introduction Electrokinetic remediation (EKR) has become a promising technology for the removal of heavy metals and several organic contaminants from fine-grained soils [1–5]. During EKR, the splitting of the water near the anode produces H + ions and causes the pH to decrease close to the anode. Conversely, water splitting at the cathode generates hydroxides (OH −), which increase the pH close to the cathode. Once formed, the H + and OH − ions migrate to form acid and base fronts. The advance of those fronts through the soil has a significant influence on heavy metal removal. A low pH aids heavy metal desorption from the surface of the soil particles. A high pH causes heavy metals to precipitate and makes them very difficult to remove with an electric field [6]. Scientists around the world have tried to improve EKR performance with diverse approaches. One approach is the addition of chemical reagents into the soil to control the pH at the cathode [7–9]. Another is to add chemical reagents to enhance the removal of precipitated metals from the soil and facilitate their removal at the electrodes [10,11]. Phosphate buffers [12,13], acetic acid [14,15], citric acid [16], and HNO3 [17,18] are among the substances used to adjust the pH. Unfortunately, in many of the studies where acid was used, the total amount of consumed acid was not reported ⁎ Corresponding author at: The Key Laboratory of Marine Environmental Science and Ecology, Ministry of Education, Ocean University of China, Qingdao 266100, China. Tel.: + 86 15853299827; fax: + 86 53266782011. E-mail address: [email protected] (C-S. Peng). 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2012.05.023

and only the concentrations of the acid or the injection rates were specified. Additionally, few of the cited papers report the variation of pH during the process, but they assume a constant pH based on the initial conditions of the process and the periodical addition of acid. Nevertheless, a common conclusion several studies have made is the usefulness of HNO3 for pH reduction and desorption of heavy metals from different soils. These abilities can be attributed to the high strength of this acid. Nitric acid fully dissolves in water and effectively lowers water pH, which in turn facilitates desorption of metals from the soil particles. Even though the use of such reagents has brought good results in terms of pollutant removal, some of these substances are environmentally damaging. For example, the addition of acid to the soil may induce permanent acidity and cause a considerable impact on plant growth [19,20], soil microbiological properties, and enzyme activities [21]. Therefore, it is desirable to reduce the quantity of acid used for soil processing. After heavy metals or other pollutants are removed from the soil using water as a carrier, it is necessary to remove them from the liquid. In this experiment, changes in pH were exploited both to maximize the removal of cadmium from the soil and to precipitate the metal from the catholyte afterward. Thus, it was feasible to simultaneously remove cadmium from kaolin and to separate it from the catholyte using gravity alone, without other chemical additives beside HNO3. The retention time in the cathode chamber and the cathode reservoir was controlled to allow for the evacuation of gases, the continuous recirculation of the electrolyte, the maintenance of a homogeneous pH, and to allow

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enough time for the precipitation of cadmium hydroxide in the catholyte reservoir. The purpose of this work was to explore the effects of injecting different concentrations of HNO3 into the cathode chamber on the removal of cadmium from kaolin and from the catholyte surrounding the cathode. 2. Materials and methods 2.1. The design of the electrokinetic cell An electrokinetic cell was designed and built according to the following specifications. The main body of the cell was made using a Plexiglas tube in which kaolin was packed (Fig. 1). The inner diameter of the cell tube was 19 mm. Two working electrodes were installed, one at each end of the tube. To avoid corrosion, titanium electrodes with iridium and ruthenium oxide coating were used. The electrodes were 18 mm in diameter. For support and to avoid water leakage, the electrodes were fixed in a rubber stopper using a titanium screw, and then, the rubber stopper was fitted into the cell tube. The distance between the working electrodes was 66 mm (Fig. 1b). In addition, the cell had two reference electrodes located at the mid-point between the working electrodes. The reference electrodes were made of titanium and were used to measure the voltage drop through the cell. The kaolin sample was held in position by two filter membranes, one at the anode side and the other at the cathode side (Fig. 1). At the anode

side, a nylon mesh (mesh opening= 0.05 mm) was employed to hold the soil and to facilitate the advance of the acid front. Similarly, at the cathode side, a three-layer membrane was used to slightly retard the advance of the base front (i.e., to inhibit the movement of OH− ions through the soil toward the anode) [22]. The membrane was composed of a mixed cellulose-ester (MCE) layer (pore size= 0.1 μm) covered by polypropylene (PP) layers (pore size= 8 μm) on both sides. The two membranes were fixed to the cell inner wall using a PVC ring that stretched the membranes, similar to a drumhead-like structure (Fig. 1). To build the electrode chambers, a space was left between the membranes and the electrodes. 2.2. Experiment set up Each electrode chamber was provided with an external electrolyte reservoir and an overflow reservoir, both made of PVC (Fig. 2). Each of the reservoirs had a maximum capacity of 20.0 ml. Each electrode chamber was connected to its respective electrolyte reservoir by two polyurethane tubes (inner diameter= 2.50 mm). The first tube connected the bottom of the electrode chamber to the electrolyte reservoir to form an electrolyte inlet. The second tube connected the top of the electrode chamber with the bottom of the electrolyte reservoir as an electrolyte/gas outlet. In addition, to ensure electrolyte circulation between electrode chambers and the electrolyte reservoirs as well as the permanent evacuation of the produced gas, a peristaltic pump (Longer Pump Ltd. Hebei, China. Model BQ-50) with a flow rate of 207 ml/h

Fig. 1. 3-D model (a) and cross sectional view of the electrokinetic cell (b).

J. Almeira O. et al. / Desalination 300 (2012) 1–11

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Acid inlet line Anolyte overflow reservoir Anolyte reservoir

Catholyte reservoir Catholyte supernatant Anolyte/gas Outlet line

Catholyte overflow

Anode Anolyte inlet line

Catholyte precipitated

Cathode Catholyte/ gas outlet line

Pump

Kaolin

Volt-meter

PC

Catholyte inlet line

Current-meter

Acid solution reservoir

Power supply (DC)

Fig. 2. Schematic diagram of the experiment set up (not to scale).

was installed at each of the gas/electrolyte outlet tubes. Finally, an overflow reservoir was connected to each electrolyte reservoir by a polyurethane tube (inner diameter= 2.5 mm) attached to the top (Fig. 2). The estimated retention time in the catholyte reservoir was 0.24 h, a sufficient period of time for metallic flocks to precipitate. In fact, the catholyte reservoir was designed as a sedimentation chamber. The electrolyte reservoirs were covered to avoid excessive water evaporation but permit the release of gases (i.e., oxygen and hydrogen) produced at the electrodes. The electrolyte reservoirs were kept level to avoid any difference in static hydraulic pressure along the sample. To control the pH at the cathode, dilute HNO3 at different concentrations was injected into the catholyte reservoir at a constant rate using a peristaltic pump (Longer Pump Ltd. LEAD 1). The resulting excess catholyte flowed to the catholyte overflow reservoir and was evacuated daily to a glass beaker for further analysis. A DC power supply (PS-303D, Hongkong Longwei Instruments Ltd. Co.) was used. The voltage range used was 0.5–30.0 V, and the current range used was 0.1–1.0 A. 2.3. Sample preparation Commercial kaolin from Shanghai Ludu Chemical Reagent Factory Co. was used for this experiment. The kaolin was extracted from Suzhou deposits in Jiangsu Province, China. Kaolin was used for this experiment because of its homogeneity, relatively low cation exchange capacity, fine grain distribution, and low permeability (see Table 1). A low permeability soil was chosen to confirm the usefulness of electrokinetics for the removal of a pollutant from such media. Kaolin has been widely used in the study of soil electrokinetics because it provides a reproducible, low buffering clayey soil model [26–28]. The elevated concentration of sulfates and associated low pH in kaolin are most probably attributable to the addition of sulfuric acid used during the bleaching process in typical industrial processing of kaolin [29]. The kaolin was first dried at 105 °C for 24 h and then mixed at a 1:1 water/soil ratio with a cadmium sulfate solution prepared with deionized water. The solution contained 250 mg/l of cadmium. After the kaolin sample became liquid, it was mixed manually using a polyethylene spatula until it was homogeneous. It was then was air dried for 4–5 days until the sample reached a 60% water/soil ratio. 2.4. Electrokinetic experiments After the kaolin was packed into the electrokinetic cell, the membranes were installed with a specially built mechanical device

(see Fig. 3). Then, the electrodes were mounted and the electrolyte reservoirs were attached. The anolyte reservoir was filled with 0.01 M KNO3, and the catholyte reservoir was filled with a 0.01 M KNO3 solution and different concentrations of HNO3. The acid concentrations were matched to the different experiments (see Table 2). The catholyte reservoirs were filled with the acid solutions at the beginning of the experiment and were kept full by the pumps at a constant rate of 0.6 ml/h. Later, the electrodes were connected to a power supply, and direct current was applied at a constant voltage of 1.0 ± 0.1 V/cm for 140 h (i.e., one volt per centimeter of distance between the working electrodes) (see Table 2). 2.5. Analytical methods The electrical current in the cell was measured with a digital ammeter (UT-60E, UNI-T, China) linked to a personal computer (PC). With the computer system, it was possible to read and record current values every hour. In addition, the voltages between the working electrodes and between the working and reference electrodes (i.e., voltage drop) were manually measured daily with a digital voltmeter (UT-60E, UNIT, China). The voltage drop was measured between the anode and reference electrodes (i.e., electrodes A and B in Fig. 1) for the half of the cell next to the anode (i.e., anode side). Likewise, it was measured between

Table 1 Some characteristics of Suzhou kaolin. Parameter

Value

Name Soil classification (ASTM D-2487) [23] Hydraulic conductivity [cm/s] (ASTM D-7100) [24] Particle size distribution Clay [%] Atterberg limits (ASTM D-4318) [25] Plastic limit Liquid limit pH (9045D US-EPA, in water) Conductivity [μS/cm] (in water) Cation exchange capacity [μg/g] (9081 US-EPA) Arsenic [mg/kg] (from fabricant) Lead [mg/kg] (from fabricant) Chlorides [mg/kg]a (from fabricant) Sulfates [mg/kg]a Organic mater (ignition at 400 °C) [%]

Suzhou kaolin Clay 1.1 × 10− 8

a

The values given in mg/kg are calculated for dried soil.

100 32% 78% 3.76 1485 1326 ~2 ~ 10 ~ 370 1705 2

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Cell supports Membrane Screw Sample extraction point

Kaolin Electrokinetic cell Fig. 3. Schematic diagram of the mechanical device designed for membrane installation, soil packing and extrusion.

the cathode and reference electrodes for the half of the cell next to the cathode (i.e., cathode side). After electrokinetic testing, the sample was extruded from the electrokinetic cell with the same mechanical device used for packing the cell (see Fig. 3). Then, the sample was cut into slices of a thickness of 10 ± 1 mm. The pH, conductivity, and cadmium concentration were measured in each slice. Conductivity, red-ox potential, and pH were measured daily in the electrolyte reservoirs. The remaining amount of cadmium in the working electrodes, membranes and anolyte were also determined. The catholyte was separated into three parts for analysis: ‘catholyte overflow’, ‘catholyte supernatant’ and ‘catholyte precipitated’ (see Fig. 2). The supernatant was separated from the flocculated-precipitated cadmium using a syringe. The supernatant was extracted from the top, leaving the precipitated cadmium in the bottom. In order to determine the amount of deposited cadmium, the cathode was soaked in a 1:1 solution of nitric acid to remove the cadmium deposited on the electrode surface [30]. Similarly, the cathode-side membrane was digested with nitric acid following the same procedure as for the kaolin samples. The amount of deposited Cd in the membrane and on the cathode was determined based on the amount of dissolved Cd in the respective acid solutions. To determine the nature of the deposited cadmium on the cathode surface, an x-ray diffraction analysis was performed. The test was performed using a Bruker AXS D8 Advance Diffractometer (USA). The analysis of the spectrum was performed with EVA software provided by Bruker, USA. The conductivity and the pH of kaolin were both measured at a 1:2 soil/water ratio. The pH was measured according to US EPA method 9045D [31]. The pH, conductivity, and red-ox potential of the electrolytes were measured directly in the solutions. The pH and red-ox potential were measured with a PHSJ-4A meter from Shanghai Precision and Scientific Instruments. The conductivity was measured with a DDSJ-308A conductivity meter from Shanghai Precision and Scientific Instruments.

In accordance with US EPA method 7130 [31] (atomic absorption, direct aspiration), the cadmium concentration was determined by flame atomic absorption (FLAA) using a Thermo Scientific iCE 3300AA spectrometer. Kaolin samples were digested as described in US EPA method 3050B [31]. This applied acid digestion is not a complete digestion, but one in which only the elements that are environmentally available are dissolved. The elements bound in silicate structures are not usually mobile in the environment, and therefore, they are referred to as “not environmentally available”. To ensure the quality of the data, the following precautions were taken throughout the experiment: (1) analytic grade reagents from Sinopharm Co. Ltd. China were used; (2) all chemical analyses were performed in duplicate; (3) the current was continuously recorded each hour; (4) the voltage drop was measured twice for each reported value; (5) new electrodes, tubes, reservoirs, and membranes were used for each test; and (6) the electrokinetic cell was soaked overnight in a HNO3 solution (pH = 2.0) before each new test. 3. Results and discussion 3.1. The pH and conductivity of the electrolytes The pH in both the anolyte and the catholyte (Fig. 4) was influenced by the concentration of HNO3 injected into the catholyte. The addition of acid in the catholyte increased the conductivity in the whole cell (Fig. 5). Therefore, the production of H+ at the anode and OH− at the cathode noticeably increased [32]. Furthermore, the production of OH− was mediated by the reduction of NO3 at the cathode (see Eqs. (1)–(4)) [33]. −

−

−

NO3 þ 2e þ H2 O→NO2 þ 2OH −

−

−

−

−

2NO3 þ 16e þ 12H2 O→2NH3 þ 18OH 2NO3 þ 10e þ 6H2 O→N2 þ 2OH −

þ

−

NO3 þ 7H →NH4 þ 3OH

Table 2 Electrokinetic experiments. Run

Cell length [mm]a

HNO3 conc. [M]b

Acid solution flow rate [ml/h]

Test duration [h]

kaolin sample weight [g]

A B C D E F G

66 66 66 66 66 66 66

0.00 0.01 0.06 0.12 0.18 0.24 0.42

0.6 0.6 0.6 0.6 0.6 0.6 0.6

140 140 140 140 140 140 140

11.05 11.14 11.05 10.13 11.57 10.08 10.35

The applied voltage through the cell was 1.0 ± 0.1 V/cm. a Equivalent to the distance between working electrodes. b Is the concentration of the HNO3 solution injected into the catholyte. Both catholyte and anolyte were conditioned with a 0.01 M KNO3 solution.

−

ð1Þ −

ð2Þ ð3Þ ð4Þ

A linear function with negative slope described the relationship between the average pH at the anolyte and the concentration of the injected acid (R2 = 0.9504, see Fig. 4a). As a rule, a higher concentration induced a lower pH. After the acid was injected, the pH of the anolyte remained below 2.0 for most of the experiment (20 h to 140 h). Unlike in the anolyte, the pH did not permanently decrease with the addition of HNO3 in the catholyte. High concentrations of HNO3 (>0.12 M) induced elevations in both conductivity and current (see Fig. 5b). This condition enhanced water splitting at the cathode, which caused a sharp increase in pH that occurred even when the acid concentration was high. Moreover, the addition of acid at a concentration greater than 0.12 M caused a faster increase in pH in the catholyte. The pH curves in Fig. 4b show that the pH remained

J. Almeira O. et al. / Desalination 300 (2012) 1–11

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Linear functions with positive slopes described the relationship between the HNO3 concentration and the average anolyte and catholyte conductivities. The correlation factors (R 2) were 0.9082 for the anolyte (see Fig. 5a) and 0.9756 for the catholyte (see Fig. 5b). However, the effect of the HNO3 concentration on conductivity was greater in the anolyte. Because there was no neutralization of the H + ions, there was a marked reduction in conductivity in the catholyte. The production of OH − ions at the cathode neutralized the H + ions and caused the precipitation of cadmium and other dissolved ions, which reduced the catholyte conductivity. 3.2. Distribution of cadmium in the electrokinetic system A material balance (see Table 3 and Fig. 6a) provides interesting insights on the final distribution of cadmium in the system. First, it is important to reiterate that the digestion performed in this experiment only enabled the detection of environmentally available cadmium. As a result, the detected cadmium ranged from 70% to 99% of the initially added amount. Further calculations were made based on the detectable cadmium rather than on the initially added amount. The transition from an initial low pH to a high pH in later stages of the process allowed for both the removal of cadmium from kaolin and its subsequent precipitation in the catholyte. Fig. 6 and Table 3 show the final distribution of cadmium in each part of the system after 140 h of testing. For clarity, in Fig. 6b, the final concentration of cadmium in kaolin was presented in two graphics: the one on top shows the final concentrations from 0 to 300 ppm (right axis), and the one on the bottom shows the final concentrations ranging from 0 to 25 ppm. The second axis on the left shows the relative concentration C/C0

Fig. 4. Changes of pH at the anode chamber (a) and at the cathode chamber (b) during EKR treatment for different concentrations of nitric acid injected into the catholyte.

below 3.0 for 80 h in the experiment using 0.12 M HNO3. Then, the pH climbed to approximately 10 and remained elevated until the end of the experiment. A similar tendency was observed for all the other acid concentrations except 0.01 M, for which case the pH reached a peak value of 9.0 and then erratically decreased and increased. Additionally, when HNO3 was not added, the pH started at 6.0, increased beyond 12.0 on the first day, and finally remained constant at approximately11.0. A sustained low pH (b7.0) in the catholyte was attained only with an acid concentration of 0.12 M acid and only for a limited period of time (b90 h). The addition of less acid caused a minor rise in conductivity. Less acid caused low production of OH − at the cathode, and thus, the pH remained low for a longer period. Naturally, when the concentration of the added acid was too low, (e.g., 0.0 M to 0.06 M), the pH increased rapidly. Fig. 4b shows that for concentrations of 0.01 M, 0.06 M, and 0.42 M, the pH of the catholyte rapidly rose beyond neutral values in less than 40 h. Similarly, for concentrations of 0.18 M and 0.24 M, the pH increased above neutral values in less than 60 h. Remarkably, with a concentration of 0.12 M, the pH remained below neutral values for more than 90 h, which was more than half of the duration of the experiment. The equilibrium between the added acid and the produced OH − ions stabilized the pH for a longer period of time. In fact, a relatively low concentration of acid in the catholyte resulted in more effective control of the pH. In other words, for a certain concentration range, it was possible to maintain lower pH values with less HNO3. The occurrence of high pH values even when acid was added to the catholyte was also reported by Giannis et al. [34].

Fig. 5. Conductivity versus time at (a) anode chamber and (b) cathode chamber during EKR testing for different concentrations of nitric acid injected into the catholyte.

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Table 3 Material balance for cadmium. Nitric acid concentration [M]

Initial present [mg] Collected in the anolyte [mg]a Remaining in kaolin [mg] Cathode-side membrane [mg] Cathode [mg] Catholyte supernatant [mg] Precipitated in catholyte [mg] Catholyte overflow [mg] Total detected [mg]b Extracted from kaolin [%]c Precipitated in catholyte [%]d a b c d

0.00

0.01

0.06

0.12

0.18

0.24

0.42

2.76 0.20 1.47 0.20 0.01 0.00 0.52 0.00 2.40 39% 21%

2.79 0.08 0.35 0.23 0.05 0.01 1.34 0.58 2.63 87% 51%

2.76 0.02 0.04 0.01 0.19 0.19 1.58 0.49 2.51 98% 63%

2.53 0.00 0.00 0.00 0.19 0.00 1.08 1.22 2.49 100% 43%

2.89 0.00 0.03 0.00 0.02 0.05 0.91 1.27 2.28 99% 40%

2.52 0.00 0.06 0.00 0.23 0.01 0.38 1.31 2.00 97% 19%

2.59 0.01 0.09 0.00 0.14 0.16 0.71 1.19 2.31 96% 31%

The amount of cadmium precipitated on the anode was negligible. Note that by the performed digestion, only environmentally available cadmium was detectable. Indicates how much cadmium was extracted from kaolin (based on the ‘total detected cadmium’). Indicates how much cadmium was precipitated on the catholyte (based on the ‘total detected cadmium’).

(C= final concentration, mg/kg; C0 = initial concentration mg/kg). In the normalized distance from the anode (x/L), x is the distance of each slice location (i.e., the middle point of each slice) from the anode in mm (see Fig. 1) and L is the total length of the sample (40 mm). High rates of cadmium removal from kaolin were achieved. This corresponded to the relatively low exchange capacity of kaolin [35–37] and the strong soil acidification during the process. Moreover, removal of the cadmium was achieved almost entirely by electromigration because the registered electro-osmosis volume was insignificant for all the tests (≤1.0 ml). The transport of cadmium from kaolin to the catholyte was governed by the pH of the catholyte.

Final concentrations of cadmium in kaolin exceeded 50 mg/kg DW (DW = dry weight) when acid was not added and when 0.01 M HNO3 was added. When HNO3 was not added, cadmium migration was restricted to the kaolin matrix, and only a little amount of cadmium reached the catholyte. Even when the catholyte pH surpassed 7.0 after 2 h, migration inside the kaolin was still possible due to the initial low pH of kaolin and the advance of H + ions toward the cathode. Final cadmium concentrations in kaolin were approximately 10 mg/ kg DW in the experiments using 0.24 and 0.42 M HNO3. Final cadmium concentrations fell below 5 mg/kg DW in the experiments using 0.06 M, 0.12 M, and 0.18 M HNO3. A particularly low cadmium

a 100% 90% 80% 70%

Catholyte precipitated Catholyte supernatant Cathode Cathode side memb. Kaolin Anolyte Catholyte overflow

60% 50% 40% 30% 20% 10% 0%

0.00

0.01

0.06

0.12

0.18

0.24

0.42

Injected nitric acid concentration [M]

b

Fig. 6. Distribution of cadmium over the electrokinetic system (a) and remaining concentration of cadmium in kaolin (b) for different concentrations of nitric acid injected into the cathode chamber.

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in the cathode is described by Eq. (5) [50]. Reduction of the dissolved cadmium (at low pH) in the catholyte occurred at the cathode. In basic media Cd(OH)2 was the predominant insoluble specie. Eq. (6) [50] indicates that reduction of Cd(HO)2 was also likely at the cathode. The x-ray diffraction analysis (Fig. 7) verified that the deposited cadmium on the cathode was mainly composed by zero-valent cadmium. The high intensity peaks accurately matched the metallic cadmium spectrum. Also, cadmium hydroxide was not detectable in the cathode surface. þþ

Cd

−

ðaqÞ þ 2e →CdðsÞEo ¼ −0:4026V −

ð5Þ

−

CdðOHÞ2 ðsÞ þ 2e →CdðsÞ þ 2OH Eo ¼ −0:8240V

ð6Þ

Trapping of cadmium in and on the cathode-side membrane was sharply reduced with the addition of acid. Cadmium at the cathodeside membrane was undetectable for concentrations of HNO3 greater than 0.01 M. 3.3. Cadmium extraction kinetics Even though the duration of each trial was 140 h. the time required for the extraction of cadmium from kaolin and the secondary precipitation in the catholyte was always less than 140 h. We defined ‘extraction time’ as the period of time the pH in the catholyte remained below 7.0. For concentrations of 0.00, 0.01, 0.06, 0.12, 0.18, 0.24, and 0.42 M, the extraction times were 2.5, 114.8, 41.2, 98.9, 68.5, 64.0, and 38.8 h, respectively (see Fig. 4b). Once the pH rose above 7.0, the precipitation of the cadmium in the catholyte took approximately 1.0 h. Therefore, the estimated time required to complete the two operations (i.e., extraction and precipitation) was the ‘extraction time’ plus 1.0 h. For example, Fig. 4b shows that when 0.00 M HNO3 was used, the pH remained below 7.0 for 2.5 h (i.e., the extraction time). An addition of 1.0 h for the precipitation of cadmium in the catholyte means the process took 3.5 h, (i.e., the ‘effective treatment time’). The calculations of the consumption of both the energy and the acid were based on the ‘effective treatment time’. In Fig. 8, the total amount of cadmium extracted from kaolin was plotted against the concentration of the acid. Additionally, the average rate at which cadmium was transported from kaolin to the catholyte was compared with the concentration of acid. The removal was 39% when acid was not added and reached 100% with the addition of 0.12 M acid. Above this concentration, the total removal declined, reaching 96% for 0.42 M HNO3. Even though the differences in total removal for the different acid concentrations were small, the average rate at which the cadmium was extracted ranged widely. For instance, the 700

600

Cd

500

Intensity (Cps)

concentration (near 0.0 mg/kg DW) resulted when the acid concentration was 0.12 M and was caused by the low pH (b3.0) in the catholyte being maintained for more than 90 h. These results are consistent with those of other studies [38–43]. Because cadmium concentrations below 5 mg/kg DW meet UK soil guidelines for residential use [44], working with 0.06 M HNO3 would be preferred because it is more cost-effective. However, the total amount of consumed acid used was difficult to compare the corresponding amount used in other studies because most of the published studies do not provide sufficient information. Only the acid concentrations used are normally specified, and they range from 0.01 M to 0.50 M [45–48], which is consistent with the range used in this study. The constant changes of pH in the catholyte affected the distribution of cadmium within the kaolin sample, the catholyte reservoir, and the catholyte overflow (see Fig. 2). Most importantly, the pH in the catholyte defined how much cadmium precipitated in the catholyte. Using 0.06 M HNO3 maximized both the removal of cadmium from kaolin and the amount of cadmium precipitated in the catholyte. A low final concentration of cadmium was achieved (approximately 2.5 mg/kg DW), and 63% of the detected cadmium precipitated in the cathode chamber (see Fig. 6 and Table 3). At lower HNO3 concentrations, the amount of cadmium removed was lower, whereas for higher concentrations, the amount of cadmium in the catholyte overflow markedly increased. The increase in pH to a value over 7.0 caused the precipitation of nearly all the cadmium present in the catholyte. However, before the pH reached 7.0, some of the cadmium in the catholyte escaped to the catholyte overflow. Thus, the amount of precipitated cadmium in the catholyte exhibited a straightforward relationship with the rate at which the pH increased. Based on the Pourbaix diagram for a cadmium–H2O system [49], from pH 7.0 to pH 11.5 and from an EH of −400 mV (oxidation-reduction potential) to an EH of 1350 mV, cadmium exists as a hydroxide (i.e., Cd(OH)2). In all the trials, the EH in the catholyte had a low limit of 82 mV (at 0.00 M HNO3) and a high limit of 877 mV (at 0.42 M HNO3). Once the pH exceeded 7.0, migration of cadmium into the catholyte was essentially interrupted, and precipitation of the cadmium took place in the catholyte reservoir. Precipitation of cadmium in the catholyte took approximately 1.0 h. In other words, of the 140 h for which the soil samples were treated, only the period of time during which the pH in the catholyte remained below 7.0 was useful in transporting cadmium from kaolin to the catholyte. In order to extract the cadmium that remained in the catholyte overflow the following procedure could be performed. Once the precipitated Cd in the catholyte reservoir is extracted, the Cd that flowed toward the catholyte overflow can be removed by precipitation too. The procedure will be as follows. Re-circulating the catholyte overflow toward the catholyte reservoir and applying electrical current until the pH of the catholyte surpasses 7.0. Then the remaining cadmium can precipitate and be extracted from the bottom of the catholyte reservoir. In addition, the amount of Cd dissolved in the catholyte overflow could be reduced by optimizing the pH control at the catholyte in order to achieve more precipitation of Cd in the catholyte chamber and minimize the flow of Cd toward the catholyte overflow. With regard to the distribution of cadmium in the other parts of the system, we found a significant accumulation of cadmium in the anode chamber when 0.0 M HNO3 was used and a moderate accumulation when 0.01 M and 0.06 M HNO3 were used. For these cases, a low pH in the anode chamber induced the dissolution of cadmium, but the local electrical potential (see Section 3.4) may not have been strong enough for cadmium ions to travel through the kaolin sample. For this reason, some of the ions remained dissolved in the liquid surrounding the anode. Cadmium accumulation on the anode and on the anodeside membrane was negligible. The amount of cadmium accumulated on the cathode varied widely. It was not possible to establish a clear relationship between the accumulation on the cathode and the pH in the catholyte reservoir or the amount of HNO3. The deposition of cadmium

7

400 Cd 300

200 Cd

100

Cd

0 10

20

30

40

50

60

70

2-Theta-scale Fig. 7. X-ray diffraction analysis of cathode after electrokinetic testing (step = 0.005°, step time = 1 s, T = 25 °C).

J. Almeira O. et al. / Desalination 300 (2012) 1–11 120%

0.60

110%

0.55

100%

0.50

90%

0.45

80%

0.40

70%

0.35

60%

0.30

50%

0.25

40%

0.20

30%

Cd extracted from kaolin [%]

0.15

20%

Average extraction rate [mg/h]

0.10

10% 0% -0.05

3.4. Energy and HNO3 consumption

Average extraction rate [mg/h]

Cd extracted from kaolin

8

0.05 0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.00 0.45

Injected nitric acid concentration [M] Fig. 8. The removal of Cd from kaolin and the average extraction rate for different concentrations of nitric acid injected into the catholyte.

rate at which cadmium was extracted when acid was not added was not much lower than the rate when acid was added. This suggests that as the Cd content decreases, the extraction rate drastically slows down, and therefore, it takes much more time to extract all the Cd than to effect a partial extraction. In addition, the concentration of the acid played a major role and caused the higher extraction rates attained at concentrations of 0.06 M and 0.42 M.

Fig. 9 shows that for all the experiments, the current density was initially approximately 10 A/m2 and then decreased or increased until it reached a limit value, after which the amplitude of the oscillations became relatively small. Furthermore, a bimodal pattern was clearly established. For HNO3 concentrations from 0.00 M to 0.06 M, the current density decreased with time. For HNO3 concentrations from 0.12 M to 0.42 M, the current density increased with time. The cell resistance rapidly increased when little or no acid was added, but noticeably decreased when the acid concentration was equal to or greater than 0.12 M. Therefore, it is conceivable that there should be an equilibrium point where the current fluctuates very near its initial value (i.e., 10 A/ m 2). Fig. 4 shows that the pH remained low for a long time when the HNO3 concentration was 0.12 M. Most likely, this was directly associated with the steady current density. The addition of acid into the catholyte successfully enhanced the removal of cadmium from kaolin. It also caused a substantial elevation of the electrical current. This result has been reported by several authors [51,52]. According to Eq. (7) [53], (where E is energy in kilowatt h [kW-h], V is the voltage, and t is the time in hours [h], the increase in current implies higher energy consumption. Fig. 9b describes the connection between the concentration of acid and the electrical current and illustrates the relationship between acid concentration and energy consumption (i.e., how many kilowatt hours were needed to treat a cubic meter of kaolin at different acid concentrations). Similarly, a straightforward relationship is presented between the concentration of acid and the acid consumption (i.e., how

a

b

803 Nitric acid comsumption [L/m3]

800

Electric energy comsumption [kWh/m3] 700

[L/m3] or [kWh/m3]

626 600 500 396

378

400 300 200 100 0 0

2

0.00

6

29

0.01

13

30

0.06

60

0.12

63

0.18

79

0.24

84

0.42

Injected nitric acid concentration [M] Fig. 9. Change of current density with time during EKR treatment (a), and estimated consumption of energy and nitric acid (65% purity) for each cubic meter of treated kaolin at different concentrations of nitric acid injected into the catholyte (b).

J. Almeira O. et al. / Desalination 300 (2012) 1–11

many liters of acid per cubic meter of kaolin will be required for treatment at different acid concentrations). The calculations were made for HNO3 of 65% purity and were based on the effective treatment time. Overall, it was evident that reducing the concentration of acid was likely to save both energy and HNO3 while maintaining high cadmium removal rates from both kaolin and the catholyte. t

E ¼ V∫t 0 Adt

9

extracted Cd from kaolin did not significantly increased with the addition of acid; it ranged from 96% to 100%. Based on this, 0.06 M HNO3 was chosen as a more economical acid concentration to treat the sample. At such concentration 13 L of HNO3 and 30 kWh were needed for each square meter of treated kaolin; it will cost $5.5/m 3 and $2.31/m 3 respectively. The prices of nitric acid consumption and electrical energy are given for China.

ð7Þ 3.5. Voltage drop along the electrokinetic cell

An evaluation of the current efficiency was done by comparing the energy consumption with the removed amount of Cd from kaolin. In fact, the injection of acid into the catholyte reduces the current efficiency due to the introduction of non-target ions [54]. Because H + ions travel much faster than other ions, they carry a considerable big share of the total current. That is, the current along with the energy consumption dramatically increases when acid is added. In our experiment when the concentration of the injected nitric acid was between 0.00 M and 0.06 M the amount of extracted Cd from kaolin increased with the consumed energy. A higher extraction rate (98%) was achieved at 0.06 M HNO3. On the other hand, from 0.06 M to 0.42 M HNO3, the energy consumption kept increasing with the addition of acid while the amount of extracted Cd from kaolin ranged between 96% and 100%. There was not considerable increase in Cd removal for these concentrations of acid. Hence, the addition of nitric acid with concentration higher than 0.06 M caused marked reduction in the current efficiency. Increase in the acid concentration not necessarily provided higher Cd removal rates. From 0.00 M to 0.06 M HNO3 the amount of extracted Cd from kaolin increased with the addition of acid, reaching 98% at 0.06 M HNO3. But from 0.06 M to 0.42 M HNO3 the amount of

Resistance and voltage are related by Ohm's law V = IR, where V is voltage in volts, I is current in amperes and R is resistance in ohms. According to this law, local changes in resistance would produce changes in the voltage drop along the anode and cathode sides of the cell. Increasing the resistance produces a higher local voltage drop, while a reduction in the resistance produces a lower voltage drop [55–57]. As a rule, the voltage drop along the anode side was much smaller than on the cathode side (see Fig. 10a). The voltage drop along the cell was not constant (i.e., not uniform). If the voltage drop along the cell was uniform, we should have obtained values close to 3.3 V (i.e., half of the applied voltage on the cell) for both sides. Even though the voltage drop in both the anode and cathode sides started near 3.3 V, it decreased in the anode side and increased in the cathode side throughout the experiment. The decrease of resistance in the anode side was principally linked to the elevated concentration of hydrogen ions near the anode (see Fig. 11). As shown in Figs. 4a and 5a, the presence of hydrogen ions dramatically increased conductivity (i.e., reduced the resistance). A linear function with positive slope (R2 =0.8727) described the relation between HNO3 concentration in the catholyte and the average anolyte

Fig. 10. Voltage drop along the cell for anode side (a) and cathode side (b) for different concentrations of nitric acid injected into the catholyte.

10

J. Almeira O. et al. / Desalination 300 (2012) 1–11

Acknowledgments This work was supported by the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (No. 708060) and the Program for New Century Excellent Talents in University, SEM, China (NCET-08-0508). We are also thankful to Xin-Yu Liu for his support in the graphical design. References

Fig. 11. Kaolin pH profile after electrokinetic testing for different concentrations of nitric acid injected into the catholyte.

conductivity. Furthermore, after 50 h of treatment, the resistance across the anode side remained nearly constant for all HNO3 concentrations. Because the voltage drop was largely controlled by the pH, we suggest that the stabilization of the voltage drop indicated that the pH had reached a nearly constant value. This inference is supported by Fig. 11, which shows a minor longitudinal variation of pH in the anode side. The final pH of kaolin along the anode side was nearly constant, except for 0.00 M HNO3. When acid was not added, the voltage drop through the anode side still had variations larger than 0.5 V after 70 h. In the catholyte, the voltage drop was also influenced by the pH. In the cathode side, the voltage drop continuously increased as hydroxides generated due to the high pH values were deposited near the cathode (see Fig. 10b). A linear function with negative slope (R2 = 0.9541) described the relation between the concentration of HNO3 and the average catholyte conductivity. The voltage drop was lower when more acid was used. 3.6. Kaolin acidification As it was mentioned in the introduction, the low pH will affect the soil ecosystem. In our experiment pH decreased from 3.76 to 2.00 near to the anode. It is important to reiterate that the experiment was designed to find the lowest amount of HNO3 necessary to treat a kaolin sample. The initial low pH of kaolin and its low buffer capacity lead to very low final pH in the sample. However, it is expected that in a natural soil pH will not reach such low values. According to [34] at pH 6.0 the removal of Cd from a natural soil was still achievable. Furthermore, an alternative solution for the stabilization of the soil pH could be reversing the polarity of the electrodes. This will stabilize the pH of the soil by producing OH − ions in the acid zone and H − ions in the base zone. 4. Conclusions Overall, this experiment has shown that using EKR, it is possible to extract cadmium from kaolin (98% recovery) and partially precipitate it at the catholyte (63% recovery). A total of 13 L of HNO3 and 30 kWh were needed for each square meter of treated kaolin; it will cost $5.5/ m 3 and $2.31/m3 respectively. Increasing the precipitation of metal in the catholyte will require a more accurate system for pH control. The pH level in the catholyte was a function of the relationship between the HNO3 concentration, the electrolyte conductivity, and time. It was difficult to achieve a steady pH because the addition of acid brought about both a decrease and an increase of the pH in the catholyte chamber. The pH could be even more unstable in a natural soil because the soil would not be as homogeneous as the kaolin used in this experiment. Finally, it was proven that reducing the quantity of acid used to control the pH substantially reduced energy consumption.

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