Scholarly article on topic 'Electrokinetic Gathering and Removal of Heavy Metals from Sewage Sludge by Ethylenediamine Chelation'

Electrokinetic Gathering and Removal of Heavy Metals from Sewage Sludge by Ethylenediamine Chelation Academic research paper on "Chemical engineering"

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Abstract of research paper on Chemical engineering, author of scientific article — Dongdong Pei, Chenxia Xiao, Qinhai Hu, Jianzhao Tang

Abstract Wastewater sludge in Hangzhou Qige Municipal Sewage Plant was used to study the removal rate of heavy metals in sludge using a novel equipment designed in our laboratory. Five groups of sludge samples, mixed with ethylenediamine at the V/V ratio of 0, 0.04, 0.05, 0.06 and 0.07 (C2H8N2 /Sludge sample) were electrified for 120hours at a constant direct voltage of 18V. The effects of heavy metals removal on acidification of sludge generated by anode together with ethylenediamine chelation were studied. The results show that acidification generated by anode has played an important role in removing heavy metals from sludge. The addition of ethylenediamine has no significant effect on pH decrease and electric current density in the electric repair process; however, it effectively improved the removal of heavy metals in sludge. Moreover, it was found that various ethylenediamine dosage leads to different reaction efficiencies during the electrokinetic remediation. When ethylenediamine was added at a ratio of 0.06, the removal rates of Zn, Cu and Pb were 87.51%, 67.64% and 57.79% respectively, the overall removal rate is the best. The addition of ethylenediamine has a significant effect on the change of the migration process of heavy metals present in the experimental sludge. We observed the decreased amount of heavy metals in their stable species such as residual state and organic state as well as the increase in transferable form such as exchangeable state.

Academic research paper on topic "Electrokinetic Gathering and Removal of Heavy Metals from Sewage Sludge by Ethylenediamine Chelation"

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Procedía Environmental Sciences 31 (2016) 725 - 734

The Tenth International Conference on Waste Management and Technology (ICWMT)

Electrokinetic gathering and removal of heavy metals from sewage sludge by ethylenediamine chelation

Dongdong Pei, Chenxia Xiao, Qinhai Hu*, Jianzhao Tang

College of Environmental and Resource Sciences ofZhejiang University, Hangzhou 310058, China

Abstract

Wastewater sludge in Hangzhou Qige Municipal Sewage Plant was used to study the removal rate of heavy metals in sludge using a novel equipment designed in our laboratory. Five groups of sludge samples, mixed with ethylenediamine at the V/V ratio of 0, 0.04, 0.05, 0.06 and 0.07 (C2H8N2 /Sludge sample) were electrified for 120 hours at a constant direct voltage of 18V. The effects of heavy metals removal on acidification of sludge generated by anode together with ethylenediamine chelation were studied. The results show that acidification generated by anode has played an important role in removing heavy metals from sludge. The addition of ethylenediamine has no significant effect on pH decrease and electric current density in the electric repair process; however, it effectively improved the removal of heavy metals in sludge. Moreover, it was found that various ethylenediamine dosage leads to different reaction efficiencies during the electrokinetic remediation. When ethylenediamine was added at a ratio of 0.06, the removal rates of Zn, Cu and Pb were 87.51%, 67.64% and 57.79% respectively, the overall removal rate is the best. The addition of ethylenediamine has a significant effect on the change of the migration process of heavy metals present in the experimental sludge. We observed the decreased amount of heavy metals in their stable species such as residual state and organic state as well as the increase in transferable form such as exchangeable state. © 2016 The Authors.Published by ElsevierB.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of Tsinghua University/ Basel Convention Regional Centre for Asia and the Pacific Keywords: municipal sewage sludge; heavy metals; ethylenediamine; electrokinetic remediation

1. Introduction

With the rapid development of economy and urbanization, sludge outputs multiply as the capacity of municipal wastewater treatment grows. Statistics show that the annual output of municipal dewatering sludge in China is over 30 million tons1. These sludges contain a variety of heavy metals due to the infusion of industrial wastewater and

* Qinhai Hu. Tel.: 0571-88982160; fax: 0571-88982160. E-mail address: qhhu@zju.edu.cn

1878-0296 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of Tsinghua University/ Basel Convention Regional Centre for Asia and the Pacific doi:10.1016/j.proenv.2016.02.058

precipitation enrichment of metal ions in urban sewage. Heavy metals in sludge can cause secondary pollution in the process of post-treatment and utilization of sludge. Therefore, it is urgent for us to solve this environmental problem.

Electrokinetic remediation has long been on the spot, for its unique advantages such as high removal efficiency, short processing time and the ability to simultaneously remove various heavy metals in low permeability materials2. Research shows that the existent forms of heavy metals affect the treatment efficiency of electrokinetic remediation. Weng and Yuan (2001)3 have demonstrated that lower pH could improve the removal rate of heavy metals by dissolving and resolving the stable heavy metals to exchangeable metals. Previous research on the use of electrokinetic remediation for the removal of heavy metals in sludge focused mainly on putting sludge in the space between anode chamber and cathode area and decreasing the pH value by adding acid4, or changing the electrolyte composition in cathode chamber5. The potential of decreasing pH through acid generated directly by anode in electrokinetic remediation has rarely been tested. Ethylenediamine is a common solvent and analytical reagent. Research on the use of ethylenediamine as a chelating agent to activate smithsonite6 has been conducted both in China and abroad. However, using ethylenediamine to chelate heavy metals in urban sludge to improve the removal rate has been rarely studied.

In this study, a renovated equipment was used to study the impact of electrokinetic remediation in trapping and removing heavy metals from municipal sludge, meanwhile, the distribution and transformation of heavy metals in different experimental sludge samples have been analyzed. The experimental sludge samples were placed in the anode chamber without adding any acid. Instead, lowering the pH value of sludge was achieved by electrolyzing water in anode. This experiment intends to improve the removal rate of heavy metals in sewage sludge and recycle the heavy metals, meanwhile, explore the optimum operating conditions of the renovated equipment with the addition of ethylenediamine.

2. Materials and Methods

2.1 Apparatus and materials

The experimental samples are dewatered sludge from Qige Sewage Treatment Plant in Hangzhou. The basic characteristics of sludge can be seen in Tablel. The contents of Cu, Pb and Zn in the sludge are relatively high, because Qige Wastewater Treatment Plant receives 30% industrial wastewater.

Table 1 Characteristics of experimental sludge

Physicochemical properties Content

Water content (%) 69.34

pH value 7.53

Total solid (g/L) 53.62

SO42" (mg/Kg) 584.25

Zn (mg/Kg) 1888.19

Cu (mg/Kg) 288.55

Pb (mg/Kg) 233.54

Cd (mg/Kg) 12.54

Cr (mg/Kg) 5.76

The electrokinetic reactor is made of organic glass, which has a cylindric anode region and annular cylindric cathode area. The height of anode region is 12 cm and diameter is 16 cm, the column wall is hollow. The height and the outer diameter of cathode area are 12 cm and 24 cm, respectively. The anode region is wrapped by cathode area. The anode electrode is ruthenium platinum titanium alloy (3.5cm*10cm), and the cathode electrode is stainless steel mesh (75cm* 10cm), which joints at the internal surface of the outer wall of the cathode region. The anode and

cathode chamber are separated by cation exchange membrane, the reactor is also provided symmetrically with water inlet and water outlet for electrolyte recirculation. The electrokinetic treatment system is shown in Figure 1.

Regulated power supply

Buffer pool —

Membrane \ Cathode / Cathode Anode s^ Magnetic rotor \ M Anode )--chamber lembrane / Cathode X Cathode chamber

016cmx12

024cmxl2

Fig. 1. The electrokinetic equipment designed in this study

Sludge was mixed with deionized water (4:1, V/V) and stirred slowly for 24 hours. Then, the sludge sample was continuously stirred by magnetic stirring apparatus after the sample sludge was placed into the anode chamber. Sodium chloride solution (0.05 mol/L) was used as the electrolyte in cathode compartment. We cycled electrolyte in cathode chamber by a peristaltic pump and retrieved the sediment in cathode area.

Five groups of parallel tests were conducted with the first group being a blank control without ethylenediamine (EK-1). The other four treatment groups added with various amounts of ethylenediamine (V/V, C2H8N2 /sludge sample) have ratios of 0.04 (EK-21), 0.05 (EK-22), 0.06 (EK-23) and 0.07 (EK-24), respectively. Five groups of sample sludge were electrified for 120 hours at a constant direct voltage of 18 V. We measured at a 12-hour interval for the total heavy metals in sewage sludge, species distribution as well as several indexes of sludge such as electric current value, pH value and so on.

2.2 Test and analysis methods

The sample analyses were carried out by standard methods. The pH value was measured by pHS-3E Precision meter and the pH measurement sample in anode chamber was the supernatant extracted by oscillating the mixture of sludge and distilled water without CO27. The total amounts of heavy metals in sewage sludge were measured using TAS-990 atomic absorption spectrophotometer following acid digestion using HF, HNO3 and HClO4. The Tessier sequential extraction procedures 8 was used to analyze various species of heavy metals formed in sludge.

3. Results and discussion

3.1 The change ofpH and current density in the course of electrokinetic remediation

As shown in Figure 2, the pH value of sludge declined gradually in the process of electrolysis. As we can see from the figure, the addition of ethylenediamine increased the initial pH value of the sludge, but it has little effect on the acidification of sludge. In the first 12 hours, the pH value of EK-1, EK-21 and EK-22 decreased to less than 3.0; however, EK-23 and EK-24 appear to have a slower decrease to 7.82 and 8.21, respectively. It may be the result of larger amount of added ethylenediamine. On the one hand, increased ethylenediamine will increase the alkalinity of sludge and reduce the effect of acidification in anode. On the other hand, it helps dissolve heavy metal ions and more importantly affects the electric field, thereby changing the transformation of heavy metals and ionic migration. The net effect is the slow decrease of the pH value. At the end of the experiment, the pH values of sludge samples declined to 0.7-1.0. At the same time, the pH value of the electrolyte in the cathode chamber went up to 11.7-12.3.

Fig. 2. Change in pH during electrokinetic treatment of sewage sludge

From Figure 3, we can see the current density of five sludge samples were on the rise in the first 12 hours. This can be explained by the increased migrating ions in sample sludges under the action of sludge acidification and ethylenediamine chelation, The current density of EK-23 and EK-24 still increased while other sludge groups began the decline during 12-24h. It shows that the increased amount of added ethylenediamine can effectively improve the content of migrating ions in sludge. The current density overall is on the decreasing trend with the decrease of ions in the reactor, the reason is the heavy metal ions in anode chamber have been migrated to cathode area to generate the precipitation.

Fig. 3. Change in current density during electrokinetic treatment of sewage sludge

(EK-1, EK-21, EK-22, EK-23 and EK-24 refers to ethylenediamine / sludge samples at a volume ratio (V/V) of

0, 0.04, 0.05,0.06 and 0.07, respectively)

3.2 The removal rate of heavy metals in sludge

As we can see from Table 2, the addition of ethylenediamine has significantly improved the removal rate of heavy metals in sludge and there are only two groups (EK-23, EK-24) reached our experimental objective that the content of heavy metals left in sludge conforms to the discharge standards in China. When ethylenediamine was added at a ratio of 0.07 (EK-24), the removal rates of Zn, Cu and Pb are at the highest. However, compared with the group of EK-23, the increased removal rates are only 0.05% for Zn and Cu, and only 0.07% for Pb. The removal rate tends to reach a constant with the addition of ethylenediamine, suggesting that a ratio of over 0.06 is no longer

needed. So the optimum reaction conditions based on this study were found to be EK-23 (the added ratio of ethylenediamine is 0.06) considering various factors such as cost and overall removal efficiency for Zn, Cu and Pb

(87.51%, 67.64% and 57.79 for Zn, Cu and Pb, respectively).

Table 2 Removal efficiency of heavy metals in sewage sludge after electrokinetic treatment

Experiment group

Adding ratio

(v/v, C2H8N2/Sludge sample)

Removal rate (%)

The residual amount of heavy metals

(mg/Kg, Dewatered sludge)

Conform to standard

(mg/Kg, Dewatered sludge, pH=75) 9

Zn Cu Pb Zn Cu Pb Zn Cu Pb

(1000) (500) (1000)

Untreated 0 0 0 0 6160.80 941.13 761.71 No No Yes

EK-1 0 41.63 26.34 35.61 3596.10 693.24 490.50 No No Yes

EK-21 0.04 77.45 51.47 55.51 1389.30 456.73 338.89 No Yes Yes

EK-22 0.05 81.88 64.69 53.55 1116.34 332.31 353.81 No Yes Yes

EK-23 0.06 87.51 67.64 57.79 769.50 304.55 321.52 Yes Yes Yes

EK-24 0.07 87.56 67.69 57.86 766.40 304.08 320.99 Yes Yes Yes

3.3 The residual of heavy metals following the process of electrokinetic remediation

From Figure 4, Zn concentrations decreased as the electrolysis time increases. The addition of ethylenediamine significantly increased the removal efficency of Zn. After 24 hr, the removal rate of Zn remained almost the same in test groups of EK-22, EK-23 and EK-24. However, the residual Zn in EK-22 is slightly lower than that in EK-23. The increase of ethylenediamine dosage didn't show its advantage in increasing the removal rate at this treatment time. That's because, on the one hand, with the increase of ethylenediamine dosage, the increased pH value of sludge weakens the sludge acidification effect, which in turn decreases the removal (Fig 1) of Zn. On the other hand, the primary effect of increasing ethylenediamine dosage is through the dissolution of zinc ions (Fig 3). The electric current density of test group EK-23 and EK-24 are higher than that in EK-22, indicating that the content of mobile ions in sludge is relatively high. The main function of external electric field at this time is on the form transformation of heavy metal Zn. With the extension of reaction time, the removal rate of Zn is promoted by the increased amount of ethylenediamine.

■ EK 1 • EK-21

V . . a EK-22

t EK-23

• ♦ EK-24

Я1 eo -

24 36 46 60 72 B4

Time (h)

96 106 120

Fig. 4. The residual rate of Zn in sewage sludge samples as a function of electrokinetic treatment time

As shown in Figure 5, in the first 36 hr of the reaction, the removal rates of Cu in group EK-23 and EK-24 are lower than that in the control group EK-1 (without ethylenediamine ). The reason may be, on one hand, like Zn, the acid produced in anode neutralized the ethylenediamine, which could weaken the acidification and dissolution of Cu in sludge, leading to a poorer removal rate . On the other hand, it is the competition between the high content of Zn (Fig. 4) affects the electromigration capability of Cu, and weakens the electromigration removal efficency of Cu,. Consequently, as the extension of electrolysis time, the increased dosage of ethylenediamine promoted the removal rate of Cu in sludge.

TO 60 -

—■— EK-1

• EK-21

—A— EK-22

- EK-23

♦ EK-24

» ▲

Time (h)

Fig. 5. The residual rate of Cu in sewage sludge samples as a function of electrokinetic treatment time

As shown in Table 2, the electric removal rate of Pb in sludge is lower than that of Zn and Cu. The final removal rates of four test groups with ethylenediamine have no significant difference, with the variation lower than 5%. The increase of ethylenediamine dosage didn't effectively promote the removal rate of Pb. But these test groups' removal effect are still much higher than that of EK-1, indicating that the addition of ethylenediamine still improved the dissolution and removal of Pb, while, there are still 45% of stable Pb cannot be dissolved, converted and removed in the process of electrokinetic remediation. With the increased ethylenediamine dosage, the removal rate of Pb in sludge did not change much. This is because the contents of Cu and Pb in the original sludge are close, and the improved removal rate of Cu will weaken the migration and removal of Pb, which specifically acts on the removal rate of Cu in the sludge goes up but the removal rate of Pb reduced instead (Fig 2) in the test groups of EK-21 and EK-22. compared the residual contents of heavy metals among four test groups with ethylenediamine (Fig. 5 and Fig 6), the electromigration removal of two heavy metals shows are competitive with each other under the same test condition.

Fig. 6. The residual rate of Pb in sewage sludge samples as a function of electrokinetic time

3.4 The form distribution and migration of heavy metals in the process of electrokinetic remediation

From the figures of 2 to 6, we can see the reaction rates in EK-23 and EK-24 are almost the same and it is in the group of EK-23 (the added ratio of ethylenediamine is 0.06) that the optimum result was obtained, therefore, We will just discuss the four test groups (EK-1, EK-21, EK-22 and EK-23) in this section. 3.4.1 The species distribution and migration of Zn

In general, it is very difficult for us to remove Zn out of the sludge. However, when we add the ethylenediamine into sludge and we will change the species distribution of Zn greatly to make it easier to remove. It is mainly reflected in the decrease of Zn in its residual state and increase in Fe-Mn oxidation state by comparing Figure 7 (c) with 7 (a) at the time of 0 h after ethylenediamine pretreatment of 24 hours. As we can see from Figure 7 (b), the addition of ethylenediamine can improve the content of Zn in converted state and make the proportion of converted state of Zn in total amount is on a slow growth trend. Namely, Zn in sludge is dissolved as it is removed, which corresponded to the change of residual Zn (Figure 4).

Fig. 7. Changes of species distribution of Zn in sludge at various electrokinetic treatment times

[::;: Exchangeable state —Carbonate state MFe-Mn oxidation stateB Organic state — Residual state

With the increased ethylenediamine dosage, the content of exchangeable Zn increased markedly. As shown in Figure 7 (c), the proportion of exchangeable Zn declined gradually, meanwhile, the trend of Zn removal is slowed (Figure 4) and the transformation trend of the stable Zn to the transferable state is reduced too. The content of exchangeable Zn has increased to 404 mg/kg before electrolysis, it was increased by 19.47% compared to the raw sludge and improved the effect of the subsequent electrokinetic remediation (Fig 4). In the first 12 hours of

electrokinetic remediation, the applied electric field can just remove the dissolved Zn in sludge, which can be seen from the reduced content of exchangeable Zn following 12 hours (Figure 7 (c)). At the end of the experiment, the content of stable Zn was reduced to 111 mg/kg from 1429 mg/kg, the proportion of exchangeable Zn was increased to 51.95% and the total amount of Zn was decreased to 236 mg/kg from 1888 mg/ kg. Thus, ethylenediamine can chelate the Zn in sludge to promote the transformation of Zn from stable to exchangeable state10 and improved the removal rate.

3.4.2 The species distribution and migration of Cu

As shown in Figure 8, the change of the species distribution of Cu in sludge is similar to Zn. Cu in its organic state was changed to the exchangeable state after ethylenediamine pretreatment of 24 hours. The proportion of Cu in converted state is on a slow growth trend. Namely, Cu in sludge is dissolved as it is removed like Zn and the content of Cu remains in sludge also steadily decline (Fig 5). In the last 12 hours of electrolysis, the exchangeable Cu was declined from 90 mg/kg to 68 mg/kg and the total amount of Cu was decreased by 30 mg/kg. At this point, the rate of electrolysis and the removal of Zn are nearly close, so that Cu in sludge can continue to be removed and the result can be derived from Figure 5.

. (a) EK-1

0 12 24 36 48 60 72 84 96 108 120 Time (h)

(b) EK-21

100 Ikxx^ Wo^

0 12 24 36 48 60 72 84 96 108 120 Time (%)

(c)EK-:

£ i 20

0 12 24 36 48 60 72 84 96 108 120 Time (h)

(d) EK-23

£ i 20

LJI III

0 12 24 36 48 60 72 84 96 108 120 Time (%)

Fig. 8. Changes of species distribution of Cu in sludge at various electrokinetic treatment times

i:: Exchangeable state —Carbonate state OHFe-Mn oxidation stateB Organic state I— Residual state

As shown in Figure 8, the proportion of exchangeable Cu is increased and the proportion of residual Cu is declined with the increasing dosage of ethylenediamine, the predominant species of Cu in sewage sludge was transformed from its stable state to the exchangeable state. However, the removal rate of heavy metals in sludge was improved slowly at the beginning of electrolysis (Fig 8 vs Fig 5). This is because the addition of ethylenediamine in

the early stage transformed the form of ions (Fig 8 (c) and 8(d)). When the reaction proceeded and the pH decreases, the amount of Cu in its non-steady state and the total amount all began to decline and the removal rate was improved (Fig 2). The addition of ethylenediamine has promoted effectively the dissolution of stable Cu, increased the amount of Cu+ and improved the removal rate of heavy metals in sludge samples. 3.4.3 The species distribution and migration of Pb

As we can see from Figure 9, the residual Pb was transformed into other forms in the process of electrolysis. The migration form of Pb is similar with Cu in the group of EK-21 that the proportion of exchangeable Pb is on an upward trend and the amount of residual Pb is on a downward trend (Fig 6). In the last 12 hours, most of the exchangeable Pb was removed from the sludge. The content of exchangeable Pb decreased from 33 mg/kg to 23 mg/kg and the residual rate is also declined from 49.26% to 44.49%.

ioo .SIEK'.

■ 11 .•■■i i

0 12 24 36 48 60 72 84 96 108 120 Time (h)

, (b) EK-21

0 12 24 36

48 60 72 Time (h)

84 96 108 120

Fig. 9. Changes of species distribution of Pb in sludge at various electrokinetic treatment times i; : Exchangeable state —Carbonate state OHlFe-Mn oxidation state Organic state I..1 Residual state

When more ethylenediamine was added (EK-22), most of the residual Pb was transformed to Pb in its organic state rather than exchangeable state. The effect of added ethylenediamine in EK-22 is the increased organic Pb compared to EK-21, which , however, cannot be removed. The reason why the amount of exchangeable Pb was increased slightly, but cannot be removed in the last 12 hours may be due to the increased ethylenediamine dosage. Which has increased the content of Zn and Cu in the exchangeable state (fig 7, 8), this two kinds of heavy metals are compete with Pb and slow the migrated ability of Pb. When the reaction continued, the content of exchangeable Pb began to increase and the removal rate also began to increase again. For Pb, the increased ethylenediamine dosage can promote effectively the transformation of Pb from residual state to organic state.

4. Conclusions

1. The increase of ethylenediamine dosage has no impact on pH decrease and electric current density in the electric remediation process. Ethylenediamine chelation can improve the removal rate of heavy metals in sewage sludge. The best overall removal rate was obtained when ethylenediamine was added at a ratio of 0.06, under which the removal rates of Zn,Cu and Pb achieved 87.51%, 67.64% and 57.79%, respectively.

2. The species of heavy metals in sludge had changed prior to electrolysis, and ethylenediamine chelation can alter the species distribution effectively leading to the favorable changes of metal species from the stable state to exchangeable state.

3. The increase of the added ratio has changed the form distribution of heavy metals rather than improved the removal rate. For Pb, the increase of ethylenediamine dosag has a little impact on improving the removal rate; however, it reduces the amount of residual Pb by transforming it to other species.

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