Study on the Treatment of Wastewater Containing Mercury by Macromolecular

Containing Mercury by Macromolecular Heavy Metal

Flocculant Mercaptoacetyl Polyethyleneimine

XU Min, CHANG Qing*

School of Environmental and Municipal Engineering

Lanzhou Jiaotong University Lanzhou 730070, China changq47@mail.lzjtu.cn

adsorption and membrane filtration. Thus increases the operation units and the investment and running cost accordingly increase.

To develop a new type of flocculants which can scavenge heavy metal ions in polluted water, some strong ligands for heavy metals, such as dithioic acid or its salt group, were grafted to starch, and phosphonate group was grafted to macromolecular flocculant [3-8].

It is well known that the solubility product values (Ksp) of heavy metal thiolate is usually very small, therefore it can be inferred that sulfhydryl (-SH) is also a strong ligands for heavy metals to form insoluble thiolates. In our recent research, we have incorporated sulfhydryl group to a strong chelating polymer — polyethyleneimine producing a new kind of flocculant which was named as mercaptoacetyl polyethyleneimine (MAPEI)[9]. This new macromolecular

Keywords- macromolecular heavy metal flocculant, heavy metal flocculant, with numerous amino and sulfhydryl

groups, can form strong coordinate bonds with the heavy polyethyleneimine, mercury, wastewater; thioglycolic acid

metal ions. In addition, it can also remove turbidity in

I. INTRODUCTION wastewater. Therefore, it is possible to omit some treatment

units that follow the flocculation to remove heavy metals. With the development of industry, more and more heavy

Thus the wastewater treatment system could be greatly metals are discharged into the environment. Because of the

simplified. persistency, bioaccumulative property and toxicity of these

metals, they damage the environment and human health In this paper, the performance of MAPEI was evaluated by

using the wastewater containing mercury ions and turbidity as seriously. In a report released by the U.S. EPA, heavy metals

target. The mechanism governing the removal of mercury ions such as mercury, lead, cadmium, copper and zinc have been

[1]and turbidity by this flocculant was proposed. Finally, the listed at the top of priority pollutant ranking . Currently,

performance of MAPEI was compared with TGA and PEI many countries have set very strict restrictions for their

respectively. discharges, that is to say, close to zero emissions of these

pollutants would be required. For this reason, in the past years II. EXPERIMENT many people have devoted their efforts to treating wastewater

Materials containing heavy metals.

Flocculation is one of the most important methods of Polyethyleneimine (MW 10000, 16000, 18000, and 20000) wastewater treatment, which is used to remove hydrophobic were purchased from Mid-west Co., Beijing, China. colloids and suspended particles in wastewater. It is nearly Thioglycolic acid (TGA) was provided by Guangfu Chemical impracticable to remove soluble substances directly by Research Institute, Tianjin, China. EDC.HCl was purchased flocculation [2]. from Shanghai Med pep Co. Ltd. Other reagents, unless Therefore, the soluble heavy metals cannot be effectively otherwise specified, were obtained from Beijing Chemical removed by current flocculants. In order to remove the heavy Plant. metals, some operation units should be set prior to / after coagulation, such as chemical precipitation, ion exchange,

Abstract—A novel water-soluble macromolecular heavy metal flocculant mercaptoacetyl polyethyleneimine (MAPEI) was synthesized by reacting polyethyleneimine (PEI) with thioglycolic acid (TGA). Removal of Hg2+ and turbidity from aqueous solutions by induced flocculation of MAPEI was investigated. It was showed that MAPEI is efficient for the removal of Hg2+ and turbidity simultaneously. The highest removal rate of mercury ions can reach above 95%. Factors such as pH, MW of PEI, co-existing ions and so on that could affect the process were studied. In the process of treating wastewater containing both turbidity and mercury ions, mercury ions and turbidity have a synergic removal effect with each other. Compared with TGA, MAPEI shows obvious advantages, including higher removal efficiency, larger flocs and higher floc sedimentation velocity. In addition, compared with PEI, MAPEI can remove both turbidity and mercury ions very well while PEI removes only turbidity efficiently.

CHEN Jinjin

School of Environmental and Municipal Engineering

Lanzhou Jiaotong University Lanzhou 730070, China

Supported by Water Environmental Science and EngineeringInnovation Group of Gansu Province of China (No. 2006-07)

978-1-4244-2902-8/09/$25.00 ©2009 IEEE

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wastewater samples (Hg2+ 50 mg·L-1, turbidity 80 NTU) and Synthesis of MAPEI

the comparison with the results of MAPEI was made.

The MAPEI was prepared by the reaction of

The selectivity of MAPEI was studied by simultaneous

polyethyleneimine with thioglycolic acid, as shown as

removal of mercury and copper ions (CuCl2) from mixed

follows:.

metal systems.

(H2C)2N(CH2)2NH+ nHOCCH-H2On2SH (CH2)2NH2O (PEI)(TGA)

(H

2C)2N(CH2)2Nn

(CH2)2NH2CCH2SH

(MAPEI)

OIn this reaction, thioglycolic acid was attached covalently to

polyethyleneimine which was achieved by the formation of

amide bonds between the primary amino groups of PEI and

the carboxylic acid group of the acid[10]. The procedures of

preparation are as follows:

First, PEI (0.03 mole ethyleneimine repeating units) was dissolved in 100 mL distilled water. Thereafter, 1-ethyl-3-3 (dimethylamino-propyl) carbodiimide hydrochloride (EDC.HCl) was added in a final concentration of 60 mM to

activate the carboxylic acid moieties of the subsequently

added TGA. After EDC.HCl was completely dissolved in the PEI solution，TGA (0.06 mole) was added. This optimum

mole mixing ratio was experimentally determined by comparison of coagulation efficiencies of the products that have different mole ratios. The reaction mixtures were incubated for 12h at room temperature under permanent stirring with a magnetic stirrer. Finally, the reaction mixture

was poured into acetone and deposited, then dried at 50 ℃ under a vacuum.

Jar Tests

Jar tests were conducted on a TS 6 jar test instrument with six propellers (Wuhan Hengling Science & Technology Co. Ltd.).

Different amounts of MAPEI were added into 50 mg·L-1

mercury ion solutions (HgCl2) under stirring at a speed of

140r/min for 2 minutes, followed by stirring at 50 r/min for 10

minutes and settling for 10 minutes. The influence of co-existing ions on the removal efficiency of mercury ions

was evaluated by adding 5 mM Na+-, K+, 2.5 mM Ca2+, Mg2+,

5mM Cl, NO3-, SO42- (NaCl, KCl, CaCl2, MgCl2, NaNO3, Na2SO4) respectively. To investigate the influence of

turbidity, 7 mL 1% (m/V) kaolin solution was added into the

sample. The pH of the system was adjusted to about pH 6.0, either with HCl or NaOH as required. The residual mercury and the turbidity of the supernatants were assayed after the settlement.

The scavenging ability of TGA/PEI was also determined by adding different amounts of TGA/PEI solution into Analytical Methods

Turbidity, concentrations of copper ions were determined by a

721 Spectrophotometer (Shanghai Precise Science Co. Ltd. Shanghai, China). Concentrations of mercury ions were analyzed using a SG-921 digital mercury measurement（Jiangfen Electro-analytical Instument Co. Ltd. Jiangsu, China

）

III. RESULTS AND DISCUSSION Effect of MW of PEI on Removal of Mercury ions Different amounts of MAPEI synthesized by PEI with various MW (10000, 16000,18000, 20000) were added to the wastewater samples containing 50 mg·L-1 mercury ions. In the process the pH values were adjusted to 5.0. According to the results, when the dosages of MAPEI varied from 7 to 20 mg·L-1, formation of white flocs that readily settled was observed. After sedimentation, the mercury concentration of the supernatant reduced. The results were shown in Figure 1. 10095)%( +2gH f90o lavoemR85 MW 10000 MW 16000 MW 18000 MW 20000807.510.012.515.017.520.0MAPEI ( mgL-1

)

Fig.1 Mercury removal at different dosage of MAPEI synthesized by PEI

with different MW

Figure 1 shows the removal rate of mercury ions with different dosages. There is an optimal dosage at which the removal rate reached to the maximum. This situation is

similar to that of other heavy metals. The mechanism of the process can be explained as follows:. O2RCCH2+2SH+ HgOORCCH2SHg(II)SCH+

2CR+ 2H2

O4R

CORCO+ R

CCH2

S

S

CH2

CH2

S

Hg(I)S

CH2OCHR

CH2

SH+ Hg

2+the various amino groups (primary,secondary,tertiary) of MAPEI coordinate hydrogen ions in the solution, so that the

OCRval of Hg ( %)2+1009590 85 pH = 3.0 The probable interaction between MAPEI and mercury ions could be coordination, chelating, chemical redox reactions and /or electrostatic attractions. Because of the strong affinity between mercury ions and –S- which forms from ionization of

sulfhydryl group (-SH) of MAPEI, they could easily react to

form a stable insoluble metal-complex with a cross linking

dimensional net structure. Therefore, chelating and coordination are the main interaction in the process. Additionally, there may be redox reaction between MAPEI

and mercury ions. In literature [11] it was reported that sulfhydyl group could be oxidized in the presence of oxidant.

As we know bivalent mercury ions is a weak oxidant, thereby

oxidizing sulfhydyl group to disulfide and the reduced

mercury ions forming an insoluble complex with remaining

sulfhydryl groups. Furthermore, the adsorption between

macromolecular and the soluble mercury ions may also

helpful to the removal of mercury ions. However, upon increasing the dosage of MAPEI beyond the

optimum precipitation condition, a reversal in mercury ion

concentration occurred. This property has also been observed

by other researchers working with macromolecular heavy

metal systems. This phenomenon could be attributed to the net negative charge prevailing in the metal-polymer complex

due to unreacted –S- in MAPEI that hinders particle

aggregation.

Figure 1 also shows that the removal rate of mercury ions increases with the increase in MW of PEI. In this research, four kinds of PEI with MW 10000, 16000, 18000, and 20000 were used in synthesizing MAPEI. The results indicate that MAPEI synthesized by PEI of MW 20000 has a better removal performance. At optimum dosage the removal rate of mercury can reach to 94%. This is probably due to the chain gets longer with the increase of MW of PEI. Thus more sulfhydryl groups could be incorporated to PEI through amidation reaction.

Effect of pH on the Removal of Mercury ions The pH of the wastewater samples containing 50 mg·L-1

mercury ions were adjusted to different values. Jar tests were conducted at different MAPEI doses varied from 7 to 20 mg·L -1

. The results were shown in Figure 2. It was observed that the removal rate of mercury ions increases with the increase in pH. The effect of pH on the mercury removal can be explained as equation (1). When the pH is low, the sulfhydryl groups of macromoleculars exist in the form of –SH. At the same time,

om pH = 4.0e pH = 5.0R80 pH = 6.0 pH = 7.5757.510.012.515.017.520.0MAPEI (mgL-1

)Fig.2 Effect of pH on the mercury removal

H+(H2C)2N(CH2)2NnOH-(CH2)2NH2CCH2S-OH+(H2C)2N(CH2)2NnOH-(CH2)2NH2CCH2SHO(H2C)2NH+(CH2)2NH+n(CH2)2NH3+CCH2SHO (1) macromolecular is electrically positive. Therefore the cationic metal ions can not be removed very well. While the pH is raised, the positive charge of the amino groups decreases, and more sulfhydryl groups ionized to negative –S-, so the coordination and chelating between –S- and mercury ions increased. As a result, a better mercury removal obtained. Effect of Co-existing Metal Ions on Removal of Mercury ions

Four kinds of common metal ions were added into wastewater

samples to investigate the influence of co-existing metal ions on the mercury removal. Among those, Na+, K+ were used as representative alkali metallic elements and Ca2+, Mg2+ as alkaline-earth metallic elements.

The affinity between alkali (or alkaline-earth) metal ions and sulfhydryl group was so weak that they can not compete with the heavy metals for chelating sites [12]. Therefore the presence of these metal ions would not influence the removal 3

of mercury. Indeed, as experimentally observed, these co-existing metals did not consume MAPEI in the process. Furthermore the removal rate of mercury was even increased in the presence of these metal ions. The results were shown in Figure 3.

100% 95/ +2gH fo lavo K+m90e Na+R Ca2+ Mg2+ Control857.510.012.515.017.520.0MAPEI /( mgL-1

)

Fig.3 Effect of co-existing metal ions on the mercury removal

For this case, the increase in mercury removal may be attributed to the neutralization. That is to say, these metal ions can neutralize the net negative charges in the macromolecular . This brings about the decrease of ξ potential and the repulsion along the chain, promoting floc formation and mercury removal.

Effect of Co-existing Anions on the Removal of Mercury ions In these tests three common anions were added into the wastewater samples prior to flocculation, respectively. The results were shown in Figure 4. It was interesting to find that -2- Cl, NO3- can promote the removal of mercury while SO4 suppress.

10095)% ( +2gH 90fo lvo Cl-emR85 NO3- SO42- Control807.510.012.515.017.520.0MAPEI (mgL-1

)

Fig.4 Effect of co-existing anions on the mercury removal

The influence of these anions on the mercury removal is probably brought about by two actions. One is the coordination between these anions and mercury ions. That means in the co-existing system these anions may compete

with the sulfhydyl groups in macromolecular for mercury ions [13]

. The other is the neutralization. The presence of anions would allow for more chelating sites along the macromolecular chain to participate in metal chelation , because anions would be able to neutralize electric repulsive forces among adjacent chelated metal ions [14]. These two reversal actions exist simultaneously. Whether the anions pr omote or suppress the mercury removal depends on the dominant action. For SO42- , coordination with mercury ions is probably the dominant action leading to a decrease in removal rate. However, for the other two anions, since the affinity with mercury ions is not as strong as SO42-, probably neutralization is the main action. This results in an increase in the removal rates.

Effect of Mercury Ions and Turbidity on Each Other’s Removal Rate

The wastewater samples used in these tests were divided into two groups. One contain the same original concentration of mercury ions (50 mg·L-1) but different original turbidity (80 and 0 NTU respectively). The other contains the same original turbidity (80 NTU) but different concentration of mercury ions (50 and 0 mg·L-1, respectively). The results were shown in Figure 5.

100)%90( etaR la80vomeR2+70 Hg (80NTU) Hg2+ (0 NTU) Turbidity (50mgL-1Hg2+) Turbidity (without Hg2+)607.510.012.515.017.520.0MAPEI ( mgL-

)

Fig.5 Effect of mercury and turbidity on the removal of each other

It can be seen that in the presence of turbidity the removal rate of mercury increased. The maximum removal rate reached above 98% while with no turbidity it was below 95%. At the same time, the range of optimum dosage was widened. The turbidity could promote the removal of mercury ions. This is probably attributed to the floc formed from substances causing turbidity has the “sweep” function for the insoluble colloidal chelate compound of Hg-MAPEI and the weak adsorption for soluble mercury ions.. Therefore the removal rate of mercury increases when the wastewater samples have turbidity.

Figure 5 also indicated that mercury ions in wastewater could greatly promote the removal of turbidity. This is probably because the positive mercury ions can neutralize the negative electric charges of the substance causing turbidity, thus reducing the repulsion between the macromolecular and

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turbidity particles and therefore strengthening the coagulation greatly.

Comparison between MAPEI and TGA / PEI

It was well documented that TGA can be used in wastewater treatment to remove heavy metals [15] . Therefore, a comparison of removing heavy metals between MAPEI and TGA was made first. The wastewater samples in these tests contained both mercury (50 mg·L-1) and turbidity (80 NTU). The removal of mercury ions was determined and compared, as shown in Figure 6.

10095)% ( +2gH f90o lavom MAPEIeR85 TGA807.510.012.515.017.520.0Dosage (mgL1

)

Fig.6 Comparison of mercury removal between MAPEI and TGA

Figure 6 showed that MAPEI was more efficient than TGA for mercury removal. Aside from the tiny floc produced by TGA, the lack of “sweep” and adsorption of large flocs also leads to a decrease in mercury removal.

It is well known that PEI is a kind of cationic flocculant which can be used to reduce the turbidity of wastewater. Generally speaking, PEI has no function of removing heavy metals, because the complex formed is soluble in water.

88 MAPEI PEI86)% (ti84dibrut 82fo lavo80meR78767.510.012.515.017.520.0Dosage ( mgL-1

)

Fig.7 Comparision of turbidity removal between MAPEI and PEI

In this study, a comparison between MAPEI and PEI was also made. PEI and MAPEI were added into the wastewater samples containing both mercury (50 mg·L-1) and turbidity

(80 NTU) respectively. The removal of turbidity were assayed and compared. The results were shown in Figure 7.

It was shown that both PEI and MAPEI are efficient for turbidity removal.

Selectivity of MAPEI

In this section, the wastewater samples contained 50 mg·L-1-1 mercury ions and 50 mg·L copper ions. The results were presented in Figure 8.

10095% /e tra 90lva Hg2+om Cu2+Re858020406080100120MAPEI / (mgL-1

)

Fig.8 Selectivity of MAPEI for two metal ions

At the low dosage of MAPEI, the removal of mercury ions was observed with the formation of white flocs. But no flocs of copper-MAPEI was observed. Upon the addition of more MAPEI, the removal of copper ions became more apparent with the formation of quantities of dark-purple flocs. It can be inferred that a general selectivity order of Hg2+ > Cu2+ may be established. This selectivity trend conforms to the Hard and Soft Acids and Bases (HSAB) theory. As sulfhydryl group is a typical soft base，it prefers to combining with the softer acid . Compared with Cu2+, Hg2+ is a softer acid [16] . This selectivity made it possible to separate these heavy metals respectively by controlling the dosage of MAPEI.

IV. CONCLUSION

A novel procedure for employing a water-soluble, sulfhydryl-containing macromolecular flocculant for heavy metal separation by flocculation was introduced. Detailed analysis revealed that this new flocculant could not only remove turbidity as conventional flocculants, but also remove heavy metals in wastewater. The ability of the new flocculant for mercury scavenging was confirmed. It was also confirmed that pH and MW of PEI have effect on the mercury removal. With the increase in pH or MW of PEI, the removal rate increases. Additionally, it was determined that some co-existing ions (alkali/ alkaline-earth metal ions, Cl-, NO3- ) have positive function of mercury removal while SO42- suppresses the process.

In the process of treating wastewater containing both turbidity and mercury ions, mercury ions and turbidity have synergic

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effect with each other. Compared with TGA, MAPEI showed obvious advantages: higher removal efficiency and higher floc settlement velocity. In addition, MAPEI can not only reduce

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