Separation of Mercury With an Ammonium Sulfate Flotation

8/10/2019 Separation of Mercury With an Ammonium Sulfate Flotation

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Separation of Mercury with an Ammonium Sulfate-Ammonium

Thiocyanate-Ethyl Violet Flotation System

Bingyi Chen ( ), Haixin Bai ( ),

Quanmin Li* ( ) and Guoguang Liu ( )

College of Chemistry and Environmental Science, Henan Normal University, Key Laboratory of

Environmental Science and Engineering Education Commission of Henan Province,

Xinxiang, 453002, P. R. China

The separation behavior of mercury by a flotation system consisting of ammonium sulfate, ammonium

thiocyanate and ethyl violet, and the conditions for the separation of Hg(II) with other common metal ions

have been studied. The studies show that in aqueous solutions, Hg(II) combines with NH4SCN and ethyl vio-

let(EV) into dissoluble ternary ion-association complex [Hg(SCN)42-](EV)2. In the presence of ammonium

sulfate, the precipitate is floats well on the surface of the water phase and separates from water thoroughly. It

shows that Hg(II) can be separated completely from Cd(II), Fe(II), Co(II), Ni(II), Mn(II) and Al(III) by flota-

tion at pH1.0. The flotation mechanism of Hg(II) is described in this paper.

Keywords: Flotation separation; Mercury; Ammonium thiocyanate; Ethyl violet.

INTRODUCTION

Mercury is a toxic and harmful element to humans, ani-

mals and the environment, so the work of studying separation

and determination of mercury has assumed great importance.

There are many reports on the determination of mercury. 1-3

The separation behavior of Hg(II) with an ammonium sul-

fate-ammonium thiocyanate-ethyl violet flotation system,

and the conditions for separation of Hg(II) with other metal

ions are described in this paper. The results showed that

Hg(II) can form a dissoluble ternary ion-association complex

[Hg(SCN)42-](EV)2 with NH4SCN and ethyl violet(EV). In

the presence of ammonium sulfate, the precipitate floats well

on the surface of the water phase and separates from water

thoroughly. In the process of phase separation, Hg(II) is

floated quantitatively and separated completely from Cd(II),

Fe(II), Co(II), Ni(II), Mn(II) and Al(III) by flotation at pH

1.0. Compared with the traditional flotation system, this

method does not use harmful reagents and needs no compli-

cated floating gas-flow-path facility, neither does it need a

capillary atomizer or atomic absorption spectrometer that

were used in one reference.3 This method is simple, conve-

nient, rapid, and environmentally friendly and also has the

advantages of using simple instruments, having lower cost,

and homogeneous flotation and separation out of phase.

Therefore, this method has great importance in theory and

practice for determination of m ercury by concentrati on.

EXPERIMENTAL

Reagents and Instruments

Standard solutions of metal ions and buffer solutions of

different pH were prepared as references 4. A stock of stan-

dard solution of 1.0 10-2 M NH4SCN was prepared by dis-

solving 0.7613 g of ammonium thiocyanate (Tianjin Chemi-

cal Plant, Tianjin, China) in distilled water and diluting it to 1

L. The stock solution was standardized by standard AgNO3

solution. Aqueous solutions of ethyl violet (EV) were pre-

par ed by dis sol vin g 0.4 922 g EV in 1 L of disti lled wat er.

Ammonium sulfate (A.R., Peking Chemical Plant, and Pe-

king, China) was used. Other reagents were analytical grade.

A model 722-raster spectrophotometer (Xiamen Ana-

lytical Instruments Plant, Xiamen, China) was employed for

photometry measurements . A Shanghai model pHs-2 pH m e-

ter was used.

Procedure

A given amount of metal ion, NH4SCN solution and

CTMAB were added to a 25 mL graduated color comparison

tube. Acidity was adjusted with buffer solution, and the mix-

ture was diluted to 10 mL. Then 1.0 g of solid ammonium sul-

fate are added and the mixture is shaken thoroughly before

standing for a while. Then the mixture separates into a flota-

tion phase and a salt-water phase. Add 1.00 mL of the aque-

ous salt, 1.00 mL of 1 10-3

M 5-(diethylamino)-2-(5-bro-

Journal of the Chinese Chemical Society,2003,50, 869-873 869

Bingyi Chen, Pingdingshan Engineering College, Pingdingshan, Henan, 467001, P.R. China


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mo-2-pyridylazo)phenol (dissolved in ethanol), 0.50 mL of

10% TritonX-100 (dissolved in distilled water), 3.00 mL of

0.1 M di-sodium tetra-borate into another 25 mL graduated

color comparison tube. After diluting to the mark, the absor-bency is read at 560 nm (560 nm prove d to be the maximum

absorption wavelength of the complex by determining its ab-

sorption at different wavelengths) against the reagent blank.

Then the flotation yield of Hg(II) is calculated.

Dissolve the precipitate of the flotation phase with eth-

anol and determine the concentration of Hg(II) in the diluted

solution with the aforementioned color development reaction

method, then calculate the flotation yield of Hg(II). The re-

sults agreed well with each other.

RESULTS AND DISCUSSION

Influence of amount of EV on flotation yield of Hg(II)

Using 50g of Hg(II) and 1.50 mL NH4SCN, the influ-

ence of EV amount on flotation yield of Hg(II) was studied

(Fig. 1). Fig. 1 shows that the flotation yield of Hg(II) in-

creases with the increment in EV amount, and Hg(II) cannot

be floated in the absence of EV. Without (NH4)2SO4, Hg(II) is

floated completely when the amount of EV is 0.60 mL. In the

presence of 1.0 g (NH4)2SO4, only 0.50 mL of EV can make

all of Hg(II) float completely. Furthermore, the flotation

yield of Hg(II) remains constant when more EV is used. So,

0.50 mL of EV was selected for all further studies.

Influence of NH4SCN on the flotation yield of Hg(II)

The influence of the amount of NH 4SCN (1.010-2 M)

on the flotation of Hg(II) is showed in Fig. 2. It shows that

Hg(II) cannot be floated without NH4SCN, and the flotation

yield of Hg(II) increases with the increment in amount of

NH4SCN. In the presence of 1.0 g (HH4)2SO4, Hg(II) can be

floated completely when the amount of NH4SCN is 0.40 mL;

otherwise, 1.50 mL NH4SCN is need todo that. Inorder toen-

sure that Hg(II) is floated completely, 1.0 mL of NH4SCN

was selected for subsequent studies.

Reactive mechanism of flotation of Hg(II)

Methyl violet, which exists in acid solutions in cationic

form, can combine with metal complex anion into ion-asso-

ciation complex and be precipitated.5

Ethyl violet(EV) is

similar to methyl violet in those qualities. Therefore it is de-

duced that the precipitate of the flotation phase is ion-asso-

ciation complex associated by cationic ethyl violet(EV +), and

complex anion consists of Hg(II) and SCN-. The calculation

shows that if the ion-association complex is [Hg(SCN)42-]

(EV)2, 50 g Hg(II) cannot be floated completely until the

amount of EV is 0.499mL. It agreed well with theexperimen-

tal results that 0.50 mL EV just made the 50 g Hg(II) float

completely. Therefore the form of the ion-association com-

plex is [Hg(SCN)42-](EV)2.

From the aforementioned results, it is known that

Hg(II) cannot be floated at all without NH4SCN or EV. Only

with the simultaneous presence of NH 4SCN and EV, can

Hg(II) associated with them and form insoluble ternary ion-

association complex [Hg(SCN)42-](EV)2. In the presence of

(NH4)2SO4, the precipitate [Hg(SCN)42-](EV)2float well on

the surface of the water phase and separates thoroughly from

water. In the process of phase separation, Hg(II) is floated

quantitatively. Hence, the reactive mechanism of flotation of

Hg(II ) is as follows,

Hg2+ + 4SCN- [Hg(SCN)4]2-

(water phase) (water phase)

[Hg(SCN)42-] + 2EV+ [Hg(SCN)4

2-](EV+)2

(water phase) (water phase)

Influence of different salts on the flotation yield of

Hg(II)

The influence of different salts on the flotation yield of

870 J. Chin. Chem. Soc., Vol. 50, No. 4, 2003 Chen et al.

0.0 0.1 0.2 0.3 0.4 0.5 0.60

20

40

60

80

100

0g(NH4)

2SO

4

1.0g(NH4)2SO4

E/%

EV/mL

Fig. 1. Effect of EV amount on flotation yield(E) of

Hg(II). C(EV): 1.0 10-3 M; Hg(II): 50 g;

NH4SCN: (1.0 1 0-2 M): 1.50 mL; total vol-

ume: 10 mL.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40

20

40

60

80

100

0g(NH4)

2SO

4

1.0g(NH4)

2SO

4

E/%

NH4SCN/mL

Fig. 2. Effect of NH4SCN amount on flotation yield(E)

of Hg(II). c(NH4SCN): 1.0 10-2 M; Hg(II): 50

g; Ethyl violet(EV) (1.0 10 -3 M): 0.50 mL;

total volume: 10 mL.


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Hg(II) is shown as Fig. 3. The results show that (NH4)2SO4,

NaNO3 and KBr did not affect the flotation yield of Hg(II)

whereas NaCl decreased the flotation yield of Hg(II). The

reason is as follows.Similar to SCN-, Cl - can also form a complex anion

HgCl42- with Hg(II). The sequence of anionic hydrophobic

ability6 is SCN- Br- Cl -. Therefore, the hydrophobic abil-

ity of HgX42-

(X = SCN-, Br

-, Cl

-) which the X formed with

Hg(II) respectively decreases one by one. As a result, their

abilities to associate with EV+ also decrease. While Hg(II)

forms Hg(SCN)42- with SCN- in the presence of Cl-, a side re-

action of Hg(II) with Cl- generated. However, the larger the

amount of NaCl, the more severe the side reaction and the

greater the decrease in the flotation yield of Hg(II). When

there is KBr or NaCl without NH4SCN, completely floating

50 g Hg(II) needs no less than 0.5 g KBr, and the flotation

yield of Hg(II) is 93% using the same amount of NaCl. This

shows that Hg(SCN)42- has a much stronger association abil-

ity with EV+ than that of HgCl42- or HgBr4

2-. Using more

NaCl will decrease the flotation yield of Hg(II). The reason

may be that some EV+

associating with a greater amount of

Cl - decreases the association ability of HgCl42- and EV+.

Hg(II) can be floated completely again and not be influenced

by NaCl if the amount of NH4SCN is increased to 20 fold of

the former. This also proved that there is a much more stron-

ger association ability between Hg(SCN)42- and EV+ than that

of HgCl42- and EV+. Hg(II) can be floated completely when

KI takes the place of NH4SCN, KBr or NaCl.

From Figs. 1, 2, we know that the flotation yield of

Hg(II) increases with the presence of (NH 4)2SO4. In the ex-

per iment al cours e it is also fou nd that the prese nce of

(NH 4)2SO4 can make Hg(II) float more quickly and com-

pletely, and the phase interface of liqui d-solid is much clearer

than before and less colored by EV which is beneficial for

pho tomet ry mea sur ement s. Hen ce, 1.0 g solid (NH4)2SO 4

was selected for all further studies.

Influence of acidity on the flotation yield of different

metal ions

Using 1.0 g (NH4)2SO4and 50g of each metal ion, we

studied the influence of acidity on the flotation yield of dif-

ferent metal ions at pH 1.0~6.0 (Fig. 4). The experimental re-

sults showed that in the range of pH 1.0~6.0 the flotation

yield of Hg(II) is not affected by acidity and Mn(II), Fe(II),

Co(II) cannot be floated at all. Cd(II) at pH 1.0 and Ni(II),

Al(III) at pH 1.0~4.0 cannot be floated either. At pH 2.0~6.0,

a small portion of Cd(II) is floated and also Co(II), Ni(II),

Al(III) do that over the pH range of 5.0~6.0. The reason is

Cd(II), Co(II), Ni(II)have a smallcomplexabilitywith SCN-.

A small portion flotation of Al(III) may be induced by the hy-

drolysis product Al(OH)3 produced by Al(III). Therefore

Hg(II) can be separated from Cd(II), Fe(II), Co(II), Ni(II),

Mn(II) and Al(III) by flotation at pH 1.0.

Influence of different surfactants on the flotation yield

of Hg(II)

Fig. 5 shows the influence of different surfactants on

the flotation yield of Hg(II). It shows that cationic surfactant

cetyl-trimethyl ammonium bromide (CTMAB) does not af-

fect the flotation yield of Hg(II). That reason is that the big

cation CTMAB+ is similar to EV+ and can associate with

[Hg(SCN)4]2- as well. When the amount of anionic surfactant

sodium-dodecyl sulfonate(SDS) or neutral surfactant TritonX-

100 was increased, each of them gradually decreased the flo-

tation yield of Hg(II) in different amounts since their pres-

ence increased the solubility of the ternary ion-association

complex [Hg(SCN)42-](EV)2in the water phase. The afore-

said results agreed well with the phenomena that less precipi-

Separation of Mercury by Flotation System J. Chin. Chem. Soc., Vol. 50, No. 4, 2003 871

0.0 0.5 1.0 1.5 2.0 2.5 3.00

20

40

60

80

100

4

3 2

1

E(%

)

salts/g

Fig. 3. The effect of salts on flotation yield(E) of

Hg(II). Hg(II): 50g; NH4SCN (1.010-2 M):

1.00 mL; EV (1.0 10-3 M): 0.50 mL; total vol-

ume: 10 mL; 1. (NH4)2SO4, NaNO3, KBr; 2.

NaCl; 3. KBr (wi tho ut NH4SCN); 4. NaCl

(without NH4SCN).

1 2 3 4 5 60

20

40

60

80

100

Ni(II) Al(III)

Cd(II)

Hg(II)

Mn(II) Fe(II) Co(II)

E(%)

pH

Fig. 4. The effect of pH on flotation yield(E) of

Hg(II). Men+: 50g; NH4SCN (1.0 10-2 M):

1.00 mL; EV: (1.0 1 0-3 M): 0.50 mL; total

volume: 10 mL; (NH4)2SO4: 1.0 g.


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CONCLUSION

The results in Tables 1, 2 show that a system consisting

of ammonium sulfate, ammonium thiocyanate and violet can

separate Hg(II) from Cd(II), Fe(II), Co(II), Ni(II), Mn(II) and

Al(III) by flotation. Furthermore the flotation behaviors of

metal ions were almost the same as each ion existing alone.

According to each metal ions floating behavior, the counter-

part of the ion in the system can be estimated. It is seen from

Table 3 that the results determined by the proposed and the

dithizone methods agreed well with each other and their re-

coveries were 95-106% and 96-110%, respectively, which

proved that the proposed meth od is reliable. It is concluded

that the proposed system is very significant for studying sep-

aration of Hg(II).

Received June 12, 2002.

REFERENCES

1. Wen, X.-R.; Li, Q.-M. Chin. J. Anal. Science 2001,17(3),

224.

2. Li, Q.-M.; Zhang, L.-M.; Liu, Q. Chin. J. Anal. Chem.1999,

27(10), 1208.

3. Dai, J.-X.; Zhang, W.-Q.; Zheng, K.-J.; Guo, X.-W. Chin. J.

Metallurgical Anal. 1998,8(4), 10.4. Chang, W.-B.; Li, K.-A. A Concise Analytical Chemistry

Hand book; Beijing University Press: Beijing, 1981; pp

240-243.

5. Wuhan University; Jilin University; the Chinese Science and

Technology University; Zhongshan University; Edit. Ana-

lytical Chemistry; University Education Press: Beijing,

1994; 518.

6. Mu, S.-F.; Liu, K.-Q. Chin. J. Ions Spectrum; Science Press:

Beijing, 1986; 51.

7. Tang, S.-B. Pollution Monitoring; Chinese Environmental

Science Press: Beijing, 1993; pp 214-217.

Separation of Mercury by Flotation System J. Chin. Chem. Soc., Vol. 50, No. 4, 2003 873

Table 3. Separation and Determination Results of Mercury in Industrial Wastewater (pH = 1.0)

RSD (%) (n = 5)Sample

Hg(II) found

in solid

phase (g/L)

Hg(II) found

in liquid

phase (g/L)

Hg(II) total

content

(g/L)

Flotation

yield of

Hg(II) (E%)

Dithizone

method

(g/L)

Proposed

method

Dithizone

method

1 7.8 0.2 8.0 97.50 8.1 3.9 4.5Industrial

wastewater 2 6.9 0.1 7.0 98.57 7.2 5.1 4.8

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Separation of Mercury With an Ammonium Sulfate Flotation