Ferrates Synthesis, Properties, and Applications in Water and

Chapter 29

Enhanced Removal of Cadmium and Lead from Water by Ferrate Preoxidation in the Process of Coagulation *,

Downloaded by UNIV OF OTTAWA on March 10, 2013 | http://pubs.acs.org Publication Date: July 25, 2008 | doi: 10.1021/bk-2008-0985.ch029

Jun Ma Wei Liu, Yingjie Zhang, and Chunjuan Li School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, People's Republic of China Corresponding author: email: [email protected]; fax: 8645182368074

This paper discussed the effect of ferrate preoxidation on enhanced removal of cadmium and lead from water in the process of coagulation. Some factors affecting the removal of heavy metals were discussed such as pH value, the dosage of ferrate and the water quality condition etc. The results showed that ferrate preoxidation could effectively increase the removal efficiency of lead, whilst a little increase of removal efficiency of cadmium; the removal efficiency increased with the increase of pH. The presence of humic acid greatly affected the removal efficiency of lead in the process of coagulation, but hardly affected the removal efficiency of cadmium. The combined effect of adsorption by intermediate iron species formed in the process of ferrate oxidation and the enhanced coagulation of iron colloids co-precipitated with heavy metals might be responsible for the effective removal of heavy metals.

456

© 2008 American Chemical Society

In Ferrates; Sharma, V.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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Introduction Heavy metals are very harmful to the health, they could accumulate in the living bodies with different forms and could not be degraded by some microorganisms, which made the removal of heavy metal important in drinking water treatment. Lead and Cadmium are common heavy metals in raw water, they are harmful to the human body. Lead and its compounds would lead to acute and chronic lead poisoning. The symptom of acute lead poisoning was abdomen angina, hepatitis, high blood pressure, around neuritis, toxic cephalitis and anaemia; the symptom of chronic lead poisoning is neurasthenia. Cadmium in the body may accumulate in stomach, liver, pancreas and hypothyroid as wll as in cholecyst and bones. Cadmium in the bones would replace calcium in bones and make the bones loose and soft, the sufferer would be in pain and finally dead. So it is very important to effectively remove heavy metals in drinking water. Coagulation has been used for heavy metal removal due to the formation of hydrolyzed species of aluminum which have adsorption ability to heavy metals. However, coagulation with aluminum alone has limited effectiveness for heavy metal removal at neutral pH ranges. Although higher pH condition is suitable for the removal of heavy metals but trace amount of heavy metals removal in drinking water treatment might increase the cost of operation by adjusting pH. In recent years, ferrate oxidation has received much attention because of its high oxidizing power, good selective reactivity, and non-toxic decomposition byproducts of ferric ion. Fe(VI) has been known as a green oxidant, coagulant, disinfectant, and antifoulant, therefore a promising multi-purpose water treatment chemical (7-4). Since the newly formed intermediate constituents in the process of ferrate oxidation might have high adsorption ability (5,6), it is the objectives of this research to investigate the effect of ferrate preoxidation on the enhancement of heavy metals removal during aluminum coagulation.

Experimental Methods Materials Potassium ferrate (K Fe0 ) of a high purity (98 % plus) was prepared by the method described by Thompson et al. (7). Potassium ferrate solution was prepared by dissolving potassium ferrate solid in distilled water just before use in order to minimize the loss of ferrate. Aluminum Sulfate (A1 (S0 ) 18H 0, Tianjin Chemical Inc., Tianjin, China) was selected as the coagulant. Humic acid was from England, humic acid solution was prepared by dissolving humic acid in distilled water, then shattered the solution by ultrasonic instrument, then dissolved it in water for 12h at 50°C, followed by filtration 2

4

2

4


3

2

458 through a 0.45 urn filter to separate any particulate matter, finally determined the TOC of the sample. The raw water was obtainedfromRiver Songhua, and the raw water quality is shown in Table I. To this raw water, 250|igL of Pb or SOjigL" of Cd were added respectively to simulate the heavy metal polluted water. _1

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1

2+

Table I. Typical raw water quality


Parameters

Concentrations

Turbidity (NTU)

Colour (CU)

15-80

30-40

COD (mgL') Mn

p

H

8.1-8.2

10-12

Alkalinity (mgCaC0 L') 3

50-60

Experiment Procedures The evaluation of the performance for heavy metal removal was conducted through a jar test apparatus (Model DBJ-621). The raw water was transferred into 6 beakers with 500ml volume. Potassium ferrate was added into the water and stirred for lmin. followed by the addition of alum, and then stirred at 300rpm for lmin. The water was then subject to slow stirring at 60rpm for lOmin. The solution was settled for 20min and the supernatant was withdrawn at lcm below the surface and then the water sample wasfiltratedby a membrane with a pore size of 0.45 |im before analyzing cadmium and lead concentration by atomic absord spectrometer.

Results and Discussion Effect of ferrate preoxidation on removal of heavy metal As shown in Figure 1, alum alone could remove some of lead, with the increase of dosage of alum, the removal percentage increases, but when the dosage of alum was more than 40mgL", the removal rate would decrease, which might be due to the bad coagulation with excess amount of alum as a result to lead to lower removal rate of lead. Ferrate preoxidation could improve the removal of lead, at the conditions, of alum dosage 40mgL , ferrate lmgL" , the removal rate was 88.6%, which achieve additional average removal of 28.6% compared with the case of alum alone, also the removal rate of lead increases with the increase of alum dose. The cause of removal of heavy metals by ferrate may due to higher adsorption of the by-products, Fe(OH) , but the higher removal rate would reach only at the conditions of good coagulation. 1

_1

1

3


459 In addition, the results also showed that the removal rate would not increase with the increase of ferrate dosage, which implied that not all of Fe(OH) adsorbed Pb , some of Fe(OH) would adsorb another substance (organic matter), there might be competitive adsorption of many substances in the reactions. 3

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3


100

0 I

i

i

10

20

i

i

i

30

i

i

40

50

I

i

60

1

Dosage (AI (S0 )-18H 0, mgL" ) 2

4

2

Figure 1. Effect offerrate preoxidation on removal of lead. Raw water quality: Turbidity 15-80NTU; Colour 30-40CU; pH8.1-8.2; COD 10-12mgL ; Alkalinity 50-60 (mgCaC0 L' ) ; Pb(II); 250 pgL l

Mn

J

1

3

As indicated in Figure 2, alum alone hardly remove cadmium, even though the dosage of alum was increased, the removal rate was still not increased, which suggested that alum alone could not effectively remove cadmium. Ferrate preoxidation can effectively increase the cadmium removal percentage, for example, the removal rate of cadmium was 20%, 25% at the dosage 0.5, lmgL* Fe0 ', respectively; the removal rate of cadmium increased with the incease of ferrate dosage, but did not increase with the increase of alum dosage, which was differentfromthe .ase of lead. Ii could concluded that the newly formed hydroxide iron might be more prone to adsorb cadmium than hydroxide aluminium. Compared with the data in Figure 1 and Figure 2, it is seen that the removal rate of lead and cadmium is totally different, which might be attributed to the different forms of Pb(II)and Cd(II) in water. pH value could affect the forms of heavy metals, so let us to discuss the effect of pH value on the removal rate of heavy metals. 1

2

4

Effect of ferrate dosage on heavy metals removal Figure 3 showed the percentage removal of lead as a function of ferrate dosage. It indicated that at pH=3, the removal rate of lead increased with the



460

20

30

40

50

60

1

Dosage (AI (S04)-18H O mgL" ) 2

2

l

Figure 2. Effect offerrate preoxidation on the removal of cadmium. Raw water quality: Turbidity 15-80NTU; Colour 30-40CU; pH 8.1-8.2; COD lO-HmgL ; Alkalinity 50-60 (mgCaC0 L ) ; Cd(II) 50 pgL' Mn

1

l

1

3

increase of ferrate dosage, but the removal rate was less than 40%; at pH=7, 9, the removal rate of lead obviously increased, the increase of pH from 3 to 7 caused 60% higher removal of lead. Figure 4 showed the variation of the percentage cadmium removal with respect to ferrate dosages at several selected pH values. It is seen that even though adjusting pH to alkaline condition could increase the removal rate of cadmium by coagulation alone without the dose of ferrate, but the percentage removal of cadmium is limited to a certain extent. However, ferrate preoxidation achieved obvious increase of the removal of cadmium when the water was subjected to ferrate preoxidation. Higher dosage of ferrate achieved higher degree of removal of cadmium but after the dose of ferrate increased beyond 3mgL" the extent of removal of cadmium was decreased. 1

j

Effect of pH on heavy metals removal The condition of pH determine the speciation of heavy metals, it is important to evaluate the effect of pH on the removal of heavy metals during ferrate preoxidation. Figure 5 shows the effect of pH on ferrate preoxidation for lead removal during coagulation with alum. In the case of coagulation with alum alone, the percentage removal of lead was increased as the increase of pH. When pH was below 6, the increase of pH caused sharp increase of the percentage removal of lead. But further increased pH beyond 6, the percentage removal of lead maintained at certain level. However, with ferrate preoxidation, the percentage removal of lead was always higher than the case with alum coagulation alone.



461

Figure 3. Effect of ferrate dosage on the removal of lead during coagulation. Pb(II) spiked; 250 pgL' . Alum dose: WmgL' . Raw water quality: Turbidity 15-80NTU; Colour 30-40CU; pH 8.1-8.2; COD 10-12mgL ; Alkalinity 50-60 (mgCaCOsL ) 1

1

1

Mn

1

0

2 4 6 Dosage (K Fe0 , mgL") 2

4

8

1

Figure 4. Effect offerrate dosage on the removal of cadmium during coagulation by ferrate preoxidation. Cd(II) spiked; 250 pgL' . Alum dose: WmgL' . Raw water quality: Turbidity 15-80NTU; Colour 30-40CU; pH 8.1-8.2; COD 10-12mgU ; Alkalinity 50-60 (mgCaC0 L ) 1

1

l

Mn

l

2



462

2

5

8

11

PH

Figure 5. Effect ofpH on the removal of lead by ferrate preoxidation during coagulation. Pb(II) spiked; 250 pgL' . Alum dose: WmgL' . Raw water quality: Turbidity 15-80NTU; Colour 30-40CU; pH8.1-8.2; COD 10-12mgL Alkalinity 50-60 (mgCaCOjL ) 1

1

Mn

1

pH 5 was the optimum condition for achieving the highest lead removal. In addition, preoxidation with ferrate also widened the optimum range of alum dosages. This results indicated that the intermediate species formed at weakly acidic condition might also have strong adsorption. As seen in Figure 6 that with the increase of pH, there was obvious increase in the percentage removal of cadmium. However, coagulation under neutral condition had limited percentage removal of cadmium when coagulate with alum alone. In contrast, the dose of ferrate caused substantial increase in cadmium percentage removal. At ferrate dose of l^mgL* , the percentage removal of cadmium was increased about one time compared to the case without ferrate preoxidation. From the experiments above, the variation of pH had a great effect on the removal of heavy metals, the higher pH of water, the higher the removal rate. But there was existing great difference between lead and cadmium, even at neutral pH conditions, the removal rate of lead was high, the lead concentration afterfiltrationcould meet with the drinking water standards; but for cadmium, only when the pH value was more than 11, the removal rate was high, so in order to achieve the higher removal rate of cadmium, there are two ways, one is to raise pH value, the other is to increase the dosage of ferrate. 1

Effect of ferrate preoxidation on heavy metals removal in the presence of humic acid Natural organic matter (NOM) in water, such as humic acid, have large numbers of active groups on their surface such as carboxyl, phenolic hydroxyl


463 100

K F e 0 (mgL ) 2

of 80 CO

.

1

4

-•-0

R o 60

•

1

/

E E

4

0

D

E 20 CO

o


10

13

PH Figure 6. Effect ofpH on the removal of cadmium by ferrate preoxidation during coagulation. Cd(II) spiked; 250 pgL' . Alum dose: WmgL' . Raw water quality: Turbidity 15-80NTU; Colour 30-40CU; pH 8.1-8.2; C0D 10-12mgU'; Alkalinity 50-60 (mgCaC0 L' ) 1

1

Mn

J

3

group, amido etc., which make the surface more negative charged. Those active groups could chelate with the heavy metals in water, so the humic acid could affect the forms of heavy metals in water and thus affected the removal rate of heavy metals. As indicated in Figure 7and Figure 5, the removal rate of lead obviously decreased in the presence of humic acid with alum alone, the optimal removal rate wasatpH>10. The removal rate of lead by ferrate preoxidation was still higher than that of alum alone, but the removal rate of lead was lower in the presence of humic acid than that without the presence of humic acid in the case of ferrate preoxidation, the removal rate of lead increased with the increase of ferrate dosages in the presence of humic acid. The presence of humic acid could greatly affect the removal percentage of lead in the process of coagulation, because it could form complex with lead, which was hardly removed by coagulation. Ferrate could oxidize some functional groups of humic acids and could reduce the chelation between humic acid and lead or the newly formed ferric hydroxide could adsorb the complex with lead to raise the removal rate of lead. As illustrated in Figure 8 and Figure 6, the presence of humic acid hardly affected the removal percetage of cadmium, the removal rate at neutral pH and the optimal pH was still the same, which suggested that the presence of humic acid did not affect the removal of cadmium. Humic acid chelated with metal ions according to Irving Williams Serial: Pb > Cu > Ni > Co > Zn > Cd > Fe > Mn > Mg (8). It was obvious that the chelating ability of lead was stronger than that of cadmium, so the presence of humic acid did affect the removal of lead but did not affect the removal of cadmium. 2+

2+

2+

2+

2+

2+

2+

2+

2+


464

0 I 5

••

7

9

»

11

PH Figure 7. Effect ofpH on lead removal in the presence of humic acid. Raw water quality: Turbidity 15-80NTU; Colour 30-40CU; pH 8.1-8.2; COD 10-12mgL' ; Alkalinity 50-60 (mgCaC0 L'') ; Pb(II); 250 pgL' ; Humic acid :1.75 mgL''(DOC).

Mn

1

1

3

100 0

i

S

CadmiuiTI remc


3

•

f

/K Fe0 (mgL- )

40

1

2

4

—•—0 20

r

n

1

11

6

Figure 8. Effect ofpH on cadmium removal in the presence of humic acid. Raw water quality: Turbidity 15-80NTU; Colour 30-40CU; pH 8.1-8.2; COD 10-12mgV ; Alkalinity 50-60 (mgCaCOsU ) ;Cd(II); 250 pgL' ; Humic acid :1.75 mgL' (DOC) Mn

l

1

1

1

Conclusion 2+

2+

Alum alone hardly removed Pb and Cd in water during the coagulation, especially for Cd . Ferrate preoxidation could effectively increase the removal rate of lead, which made the effluent meet with the drinking water quality standards, but it did not obviously increase the removal rate of cadmium. 2+


465 There were obvious increases in the percentage removal of cadmium with the increase of pH. The water pH was an important factor to affect the removal rate. With the increase of pH, the remove rate increased. Ferrate preoxidation widened the optimum pH ranges for alum to remove heavy metals. At weakly acidic condition, the removal rate was still high. The adsorption ability of hydrolyzed species and the final product Fe(OH) was the main cause to remove heavy metals. The combined effect of adsorption by intermediate iron species formed in ferrate oxidation and the enhanced coagulation of iron colloids co-precipitated with heavy metals might be responsible for the effective removal of heavy metals. The humic acid could greatly affect the removal rate of lead in the process of coagulation, because it could form complex with lead, which was hardly removed by coagulation. Ferrate preoxidation could oxidize the active groups of humic acid and reduce the extent of chelation, so ferrate preoxidation could enhance the removal of lead in the presence of humic acid. The humic acid hardly affected the removal rate of cadmium, for the chelation between humic acid and cadmium was very weak.


3

References 1.

Waite, T.D.; Gray, K.A. Oxidation and coagulation of wastewater effluent utilizing ferrate (VI) ion. Stud. Environ. Sci. 1984, 23, 407-420. 2. Carr, J.D.; Kelter, P.B.; Tabatabai A.; Splichal D.; Erickson J.; McLaugh-lin, C. W. Properties offerrate (VI) in aqueous solution: an alternate oxidant in

wastewater treatment. In: Jolley RL (Ed.), Proceedings of Conference on Water Chlorination Chem Environment Impact Health Effects, Lewis Chelsew.l985, p1285-1298. 3. Sharma, V.K. Potassium ferrate (VI): an environmentally friendly oxidant. Adv. Environ. Res. 2002, 6, 143-156. 4. Jiang, J.Q.; Lloyd, B. Progress in the development and use of ferrate (VI) salt as an oxidant and coagulation for water and wastewater treatment. Water Res. 2002, 36, 1397-1408. 5. Murmann, R.K.; Robinson, P.R. Experiments utilizing ferrate ion for purifying water. Water Res. 1974, 8(8), 543-547. 6. Potts, M.E.; Churchwell, D.R. Removal of redionuclides in wastewater utilizing potassium ferrate (VI). Water Environ. Res. 1994, 66(2), 107-109. 7. Thompson, G.W.; Ockerman, G.W.; Schreyer, J.M. Preparation and purification of potassium ferrate(VI). J. Am. Chem. Soc. 1951, 73, 12791281. 8. Bowe, C.A.; Martin, D.F. Extraction of Heavy Metals by 2-Mercaptoethoxy Groups Attached to Silica Gel. J. Environ. Sci. Heal. A 2004, 39(6), 1479-1485.


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