Indian Journal of Chemical Technology Vol 10, January 2003, pp. 87-95

Articles

Removal of arsenic from water by coagulation treatment using iron and magnesium salt

B Ghosh, M C Das, A K Gangopadhyay, T B Das, K Singh, S Lat, S Mitra, S H Ansari, T K Goswami, S K Chakraborty & N N Banerjee*

Central Fuel Research Institute, Dhanbad 828 1 08, India

Received 1 7 July 2001; revised received 9 September 2002; accepted 10 October 2002

Arsenic in ground water assumes a predicament of global dimension. Among the various methods of removal of arsenic from contaminated water, coagulation-mtration route has been examined in details covering effect of pH, coagulant type and its dose, and initial arsenic load in water. A set of batch experiments were conducted in the laboratory to investigate arsenic removal efficiency from spiked aqueous solution as weD as a few ground water samples collected from the affected zone of West Bengal (India). The removal kinetics of aqueous arsenic spiked samples with iron/magnesium salt as coagulants were studied to assess the feasibility for the use in the arsenic removal in water treatment plant, and a plausible kinetic behaviour in their removal process has been discussed. A comparative performance efficiency for iron salt in contrast to magnesium salt as coagulant has been examined to establish the superiority of iron system. Increase in background ionic species such as cr , N03 - and So2-4 in test solution with iron system have been performed in which cr and N03- ions showed no effect on the efficiency of arsenic removal, while pronounced lowering effect has been observed in case of S02-4 ion. This investigation suggests that for community supply of arsenic free water, the proposed protocol-coagulation with iron salt and subsequent filtration hold promise.

The occurrence of elevated arsenic concentration in ground water as evidenced in vast tract of the Gangetic West Bengal including Bangladesh has attracted global attention I . Argentina, Canada, China, Mongolia, Taiwan as well as Saudi Arabia2-6

reportedly encounter similar episodic predicament. Its potential toxicity has made arsenic one of the regulated inorganic contaminants in drinking water. The current revised standard of arsenic contaminated water is required to meet even stricter limit, i .e., O.O l mg/L as has been promulgated by WH02.

The natural abundance of arsenic in the earth' s crust is 1 .8mg/kg and their presence in many different forms in the minerals is well known8. The natural weathering processes of arsenic bearing minerals and volcanic eruption contribute arsenic in the sea and ground water. Arsenic present in naturally occurring minerals commonly as arsenide, being highly insoluble, pose little threat to the environment, whereas soluble forms of arsenic are of much greater concern due to their ability to leach through soils thereby contaminating the groundwater. Besides the

*For cOiTespondencc

natural sources, human activities, such as smelting of metal ores or use of arsenic compounds notably as pesticide and herbicide, the extensive use of coal for combustion in thermal power station, may cause contamination of water sources9- 1 1 . Comprehensive data base highlighting arsenic profile of Indian Coals was presented elsewhere 12. Chakraborty et al. I I made an extensive survey of groundwater quality in the arsenic affected zones of West Bengal. Their study indicates that the average concentration of arsenic in groundwater varies from 0.02-3.7 mglL. Arsenic may occur in both inorganic and organic forms in natural water. Organic forms of arsenic are seldom found in groundwater. Inorganic arsenic may be present in the formal oxidation states of arsenate (+5) and arsenite (+3). The dominant arsenic species is a function of pH and redox potential. Arsenates (+5) exist in four forms in aqueous solutions such as H3As04, H:!As04• HAsO/- and AsO/-,'t'whereas arsenite (+3) in any of such forms H3As03, H2As03, HAs03 and AsO/, and is favoured under reducing anaerobic condition. It is well established that arsenite, As (+3). the trivalent inorganic species, is more toxic than pentavalent As (+5). The chemistry of arsenic in aqueous

Articles

environment has been reviewed in great detail by Cullen and Reimer'3 and Korte and Femando'4.

The dearsenication strategy of contaminated water taken by various investigators includes coagulation­precipitation, adsorption, ion-exchange, reverse osmosis, etc. Arsenic remediation by adsorption /co­precipitation technique in relation to oxidation state of arsenic species [As (+3), and As (+5)] has been studied by Bedner et al. ' 5. It is established that As (+5) is removed more efficiently than As (+3) species. Therefore, transformation of As (+3) to As (+5) has become an essential step for such remediation. They adopted catalytic photo-oxidation followed by adsorption/co-precipitation with oxyhydroxides in their experiments. Various chemical oxidation methods of As (+3) and subsequent removal by ferric sulphate were presented by Yu and Tao'6. The oxidants used are aeration with ozone, hydrogen peroxide, sodium hypochlorite and bleaching powder. It is reported that sodium-hypochlorite is an effective oxidant for removing arsenite from dispersed drinking waler. Coagulation/co-precipitation technique using Fe/AI salt has been described by many workers. Use of alum/ferric chloride, as coagulating agent enhanced by the presence of cationic or anionic polymers was investigated's. The results showed that coagulation with FeCh is more efficient for arsenic removal than coagulation with alum. The use of polymers enhanced arsenic removal. Laboratory and pilot scale trials in UK on adsorption/removal of arsenic with granular ferric media as developed in UK, ' showed good removal efficiency '9. The effectiveness of different Fe3+/Fe2+ modified synthetic zeolites were evaluated for arsenic removal20. Among these, mostly chemical processes are met with success particularly coagulation and precipitation with iron salts. Ion exchange, adsorption and the osmosis methods though proved to be effective in bench-scale, but their application in large-scale treatment operation is rather limited.

This paper examines the comparative effectiveness of two specific coagulants iron and magnesium salts to make the water arsenic free as required for drinking standard. A set of bench labor:ltory experiments was conducted to investigate the arsenic removal from spiked aqueous solution with iron and magnesium salt as coagulants. The effect of pH, coagulant type and its dose, initial arsenic load as well as effect of anionic species like cr, N03' and S02'4 were also examined to optimize arsenic removal efficiency with r�spect to

88

Indian J . Chern. Techno/., January 2003

these variables. The removal kinetics of aqueous arsenic spiked solution with FelMg coagulants were studied for generating optimal technical data for removal process.

The optimized data were extended in laboratory scale treatment to real sample of contaminated ground water obtained from the wells of Utter Kolsur at Deganga, Purandarpur Moth at Baruipur, Gaighata at East Bishnupur, Badekhetera at Govardanga and some other wells of adjoining areas, all belonging to 24 Pargana District of West Bengal and have met with reasonable success.

Experimental Procedure

Reagent and stock solutions Chemicals used were of reagent grade; stock

solutions were stored in plastic vessels, while boro­silicate glasswares were chosen for conducting experiments. For preparing solutions, freshly prepared double distilled water was used. Water samples drawn from the affected zones, were stored in acid washed polypropylene jerrican, in duplicate, of which one was acidified with HN03 and the other was kept without the addition of any reagent required for their quality parameter examination.

Analysis Arsenic concentration was measured by hydride

generation-flame atomic absorption spectrophoto­metric technique22.23. A Philips PU-9360 continuous hydride ·. apour generation system coupled with Shimadzu AA-680 AAS was used for arsenic determination. All quantifications were done on 20mL sample solutions pretreated with KI under acidic condition (5% HCl) and sodium boro-hydride (0.5%) solution was used as reducing agent. For pH measurement, glass electrode combined with reference saturated calomel electrode was used and calibration was done with freshly prepared E-Merk buffer solution of three different pH.

Method Coagulation experiments were carried out at

different pH and at varying coagulant doses with arsenic spiked water samples prepared from l OOmg/L stock solution. The stock solution of As (+3) prepared from sodium meta-arsenite (NaAs0

2), (Riedel-de

Haen, A. G.) and for As (+5) one more oxidation step was followed with As (+3) solution, previously prepared, using arsenic free nitric acid. The stock

Banerjee e/ al.: Removal of arsenic from water by coagulation using iron and magnesium salt AFticles

-c: Q) () "-Q) 0... (U > 0 E Q) .... .� c: Q) !J) .... <t:

100

eo

w

4()

20

0

III

0 2 4 6 pH

8

�As(III) �As(V) -&-Iron -o-Iron

10

•

..

12

12

10

E 8 0... 0... "' c: OJ 6 ....

(U ::l "'0 4 ·-!J) Q) a:: 2

0

Fig. I-Effect of pH on arsenic removal [As(III)-O.32ppm, As (V)-0.42ppm, Fe-40ppm & T-29°C)

solution was step diluted and mixed with coagulant solution to obtain the desired strength . pH was adjusted by the addition of either dilute NaOH or HCI solutions to reach the solubility product of the respective coagulant resulting in the formation of solid products with higher arsenic concentration.

The experiments were conducted under uniform stirring conditions maintaining constant temperature. The solid products were then separated by filtration and the residual arsenic and iron in the filtrate were determined by AAS technique. Coagulation experi­ments were conducted at different pH, varying between 3 .5-9.5 with fixed iron dose (40 ppm Fe as FeCI3) and in another set of experiments at fixed pH 6.5 as well as 1 1 .5 with varying doses of Fe/Mg as their chloride salts. Similar experiments were also conducted for arsenic removal separately for As (+3) and As (+5) species. The equilibrium concentration of As (+5) and As (+3) in their respective experiment are presented in Figs 1 -3 respectively.

The kinetics of As(+3) and As(+5) removal were studied using FeCh and MgCI2 as coagulant at their respective pH, selected for securing complete precipitation. In all such kinetic experiments, the variation of arsenic concentration with time was determined by isolation of aliquot portion from the reaction vessel kept at temperature (26.5°C), and stirring with magnetic stirrer (300 rpm). The equilibrium concentrations were then determined from the rest of the solution after allowing sufficient time to reach the equilibrium. The time variations of

the arsenic removal and conversion per cent are shown in Figs 4 and 5 respectively.

Results and Discussion The dominant arsenic species as present in aqueous

phase and their removal, being a function of pH is well reflected in Fig. 1 . Both the species, As( + 3) and As( +5), are removed by same iron dose of 40 ppm between the pH interval 6.0-7 .5 to the maximum possible extent. It has also been observed that the removal of As(+3) is less efficient than As(+5) up to pH 7 .5 and follow the similar trend with further increase of pH to attain their maximum value and then start declining, while the removal of As( +5) decl ines more sharply with increasing pH. The corresponding decrease in residual iron concentration in the solution further corroborates the above fact. Therefore, it may be suggested that an efficient removal is attainable at or below pH 7.5 and is suitable for both the species.

Fig. 2 indicates that the removal efficiency of both As( +3) and As( +5) increases with increasing iron dose to certain l imiting value when pH is brought to 6.5. It is possible to surmise that the iron dose with�n the range of 30-40 ppm is good enough to reduce the arsenic load in either of the species as prevalent in groundwater context. A similar trend has been observed with lesser magnesium dose at pH 1 1 .6 but with a lower efficiency as evident from Fig. 3 .

Kinetic of arsenic removal is represented in Fig. 4, steep rise is discernible at the initial stage indicating very fast reaction, presumably ionic in nature. In the

89

Articles

1 00 --C Q) 80 () '-Q) 0.. ro 00 > 0 E Q) 40 '-() C Q) 20 (/) '-

<{ 0

0 10

o

20 00

Indian J. Chern. Techno!., January 2003

•••

• ••

. • 0 .••••. • • • )C , Q

70 80

3

E 0.. 0.. " 2 c o '-

Fig. 2-Effecl of iron dose on arsenic removal [As(I1I) -O.37ppm, As(V)-0.45ppm, pH-6.S & T-27°C]

80�---------------------------------------------------------,

7'0 ...... C Q) oo � Q) 0.. 50 _0... CO > 0 40 E Q) '-

.2 3) C Q) (/) 20 '-

<{ 10

0

--+-As(III).().2ppm -0-As(V).().5ppm

0 2 4 6 8 1 0 Magnesium dose, ppm

Fig. 3--Effecl o f magnesium dose o n arsenic removal

later stage the flat portion almost parallel to time axis, represents adsorption followed by diffusion process that might be operating. It seems two equilibrium processes are simultaneously operating, the first one at the beginning, is obviously the acid base reaction which is almost completed in the initial stage (about

90

two minutes) and in the later stage a slow gradient removal takes place possibly controlled by diffusion process. The study further indicates that once the final equilibrium is reached, no removal is possible due to thermodynamic conditions. Therefore, it i mplies that repeated treatment is required to achieve lower

Banerjee et al.: Removal of arsenic from water by coagulation using iron and magnesium salt Articles

100 . . . _ - - - - - - - - - - . . . _ - - - - - - - - . -() I

� o _ ...

- . - - -

_ _ - - - X - - - - - - - - - -

CU _ . - _ . -

> 00 X - · · · - - · -

E �� . ___ ------------------------------X p.> '"

_fIli 41 f/V� X

-::: � - -0- -As(V)-2.5ppm - Fe I I •• As(1II�.5ppm - Fe 20 X

I

. -X- -As(V)-1 .Oppm - Mg -X-As(V)-O.5ppm - Mg

O � __ .......... � __ ��L_����� __ � __ ���·��-=-�- -���

o 100 200 3:X) Time, minute

4)Q 500 000

Fig. 4-Reduction in arsenic by coagulation - precipitation of Fe(OH)3 [Fe-8S ppm, pH-S.8 & T 26.8°C] and of Mg(OHh [Mg-2.Sppm, pH-l l .8 & T -26.S°C]

3r-----------------------------------------------------�

2.5 E Q. Q.

., 2 C o :;:; () :::J -0 1 .5 �

.2 c ()) �

« 0.5

o -r� a 100

. - .

200

--O--As(V) - 2.5 ppm - Fe -t--As(III) - 0.5 ppm - Fe - - - -As(V) - 1 .0 ppm - Mg

U As(V) - 0.5 ppm - Mg

- - - - . _ - - - - - - - -

3:X) 500 Time, minute

Fig. 5--Kinetics arsenic removal by precipitating of Fe(OHh [Fe-8Sppm, pH-S. 8 & T -26.8°C] and of Mg(OHh [Mg-2.Sppm, pH- 1 1 .8 & T-26.S°C]

9 1

Articles Indian J. Chern. Techno! . . January 20m

100 �------------------------________________________ �

-C Q) 95 � Q) 0..

"' al > o 00 E t]) '-

.� C Q)

cl..4... "T

- NaCI -+- Nlrate -o- Suphate

� 85 « �-

8O+-----------r---------�----------�----------��--------�� a 0.1 0.2 0.3 0.4 0.5

Chloride n itrate and sulphate dose, mol Fig. 6--Effect o f chloride. nitrate and sulphate o n arsenic removal b y FeCI3 treatment [ A s -O.2ppm, pH-6.S. FeCI) - 40ppm & T-27°C J

limiting concentration. For attaining zero concen­tration further polishing of residual arsenic is needed, as it is essential for attaining safe level for drinking purposes. Several possible approaches to control arsenic to a safe level have been attempted by many . . 2 1 26 M h k " I f mvestlgators ' . oreover, t e metic remova 0 arsenic by Mg(OH)2 precipitation as presented in Fi'g. 4 indicates simi lar pattern that is evidenced in the iron system with lower equilibrium conversion. In thi s treatment also the same ionic fast reaction process is followed by adsorption, but the process is deficient due to the fact that precipitation is effectively possible at pH> I I , therefore, further treatment to bring down near pH 7 is required, which is essential for drinking purposes. Again the same acid-base reactions responsible for arsenic removal are valid in both the systems. But in ferric system as high as 90% removal of arsenic can be accomplished, whereas in case of magnesium system it is considerably lower and require longer time to reach the equilibrium.

The effect of the presence of anions like cr, NO]', S042. on the removal efficiency of arsenic were srudied for iron system as shown in Fig. 6. This study indicates cr anci NO]', apparently have no significant effect on the removal process but a pronounced lowering of removal efficiency has been observed for S042 ion.

92

Considering the chemistry of arsenic on coagulation process as studied by Edward24 on Fe-Mn system and the surface complexion as proposed by Dzombak et al.25, a plausible mechanism on the kinetics of arsenic separation has been discussed. In this kinetic study, the relationship between physical and chemical changes that has occurred during co­precipitation is indicated by the difference in attaining their equilibrium in the process of removal. Such removal of 'arsenic from aqueous solution through precipitation-coagulation route may be visualized as :

I . Ionic or acid base reaction II. Followed by adsorption over hydroxide floc

surface.

Addition of Fe or l\1g coagulant to arsenic loaded water ensures conversion of soluble As(+5) and As( +3) species into insoluble reaction products . In fact, the reaction involves in two principal ways: one is the direct precipitation which refers to the insolubilization of the soluble arsenic species subject to specific solubility products being exceeded resulting in the formation of Fe(As04)/Fe(As03) or Mg(HAs04)/Mg(HAs03). The other one is the process in which the species are removed by adsorption onto the surface of the precipitate of Fe(OH)xH20 o r Mg(OHh separated from solution.

Banerjee et al.: Removal of arsenic from water by coagulation using iron and magnesium salt Articles

often called co-precipitation which may be indicated as below:

"' Fe-OH + H2As04 ---7 Fe-H2As04 + H20 '" Mg-OH + H2As04 ---7 Mg-H2As04 + H20

Arsenate Separation "' Fe-OH + H3As03 ---7 Fe-H2As03 + H20 '" Mg-OH + H3As03 ---7 Mg-H2As03 + H20

Arsenite Separation

where "'Fe-OHiMg-OH represents the surface of the solid with the nucleation of precipitation. It is considered to cause random initiation of the reaction sites, each of which expands in space as the hydroxide precipitation starts to grow. This will lead to an aqueous colloidal dispersion containing solid particles of more than one kind, and is l ikely that various solid species will have different surface charge for the given solution condition. In such circumstances it is possible that their charge will be opposite in sign. Under these conditions, mutual coagulation takes place with the formation of flocs. In case of iron system, since zpc (zero point charge) of "'Fe(OHhxH20 is27 near pH 8 wherefrom surface charge changes its sign from positive to negative with increasing pH and the formation of soluble Fe(OHk species starts slowly. The removal process is,

therefore, favoured below pH 7 preferably, at 6.5 where adsorption of arsenic species on Fe(OHh surface acquire positive charge, to facilitate the operating coulumbic force to attract negati ve As( + 3) and As(+5) species formed by dissociation of oxy­arsenic acid. Again in the pH range 7- 1 0 specific interaction seems to occur, since dissociation of arsenic acid is favourable. Whereas Mg (OHh is a pure hydroxide which do not take part in such activity, resulting lower in removal capacity.

It is worthwhile to mention here that effectiveness for removal of As(+3) species is about 88% against As(+3) load 0.5 mg/L, whereas in case of As(+5) it is 96.6% reduction against 2.5 mgIL As(+5). Further removal through adsorption is more sluggish in case of As(+3) than As(+5). In case of Mg system removal of As(+3) species (0.5mgIL load) is less than 60% and that of As(+5) [ 1 .0 mgIL load] is 78%. Of the two coagulants i .e. Fe(OHh and Mg(OHh studied in this investigation, Fe(+3) system distinctly enjoys superiority over Mg(+2) system from the basic kinetic point of view and while formulating strategies for dearsenication of groundwater ostensibly to be used for drinking purpose, this advantage of Fe system should be considered carefully. There are evidences that filtering characteristics of Fe complexes in the form of flocs are rather poor and supposed to provide

Table I--Raw water quality parameter of four well water samples collected from affected zone of West Bcngal

SI. Parameters charactcristic Sampling point with depth of well

No. Utter Koisur Purandarpur Gaighala Badekhatera

Deganga Moth, Baruipur East Bishnupur Govardanga

( 16m) (20m) (2 1 m) (6 I m)

I pH 7.2 7 7 7.3

2 Total alkalinity (mg/L) 386 394 354 392

3 Total hardness 348 370 348 362

(mg/L as CaCOJ)

4 Chloride (mg/L as CI) 22 27 1 8 28

5 Nitrate (mg/L as N) 0.3 0.4 0.3 0.2

6 Sulphate (mg/L as SO-24) ND NO ND ND 7 Trace metals (mg/L)

Na 1 7 25 20 28 K 1 2 1 3 1 3 i 5

Ca 1 1 5 1 05 95 96

Mg 27 23 2 1 28

Fe 0.28 0.2 1 0.38 0.28 Mn 0.05 0.07 0.08 0. 1 0 A s 0.23 0. 14 0. 1 1 0.09

NO: Not Detectable.

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Articles Indian J. Chern. Techno!., January 2003

Table 2--RemovaI of arsenic from well water samples collected from affected zone of 24-Parganas, West Bengal by FeCI, (Fe-40mgIL) and MgCI2 (Mg-2.5 mgIL) Ueatment

51.. Samples pH Original strength of As removal (%)

No As (mg/L) Fe system Mg system

Utter Kolsur, Deganga ( 16m) 7.2 0.23 93.5 65.0 2

3

4

5

6

7

8

9

Purandarpur Moth, Baruipur, (20m)

Gaighata, East Bishnupur (2 1 m)

Badekhatera Govardanga (61m)

Pond Water Sample, Ramnagar chango village,

Baruipur Block

Ramnagar chango village, (30m ), Baruipur Block

Ramnagar chango village, (7m) Baruipur Block

Ramnagar chango village, (33m), Baruipur B lock"

Ramnagar chango village, (9 1 m), Baruipur Block

ideal condition for bacterial growth. Finally, arsenic associated with these flocs may pose a formidable problem for its disposal.

The study evokes that direct FeCh treatment for removing arsenic to a safe level can be used in water treatment plant with above optimized condition. Since As(+3) is most likely to be present in anaerobic groundwater, therefore, incorporation of pretreatment for oxidizing As(+3) to As(+5) is necessary to enhance the removal efficiency. It is worthwhile to mention that if water sample appears turbid, the dose response optimization with the specific sample is necessary.

In order to measure up the applicability of this process nine well water samples collected from affected zone of the 24-Pargonas district of West Bengal were subjected to arsenic removal test under specified conditions. The raw water quality parameters of the first four samples are depicted

' in

Table 1 which include arsenic concentration along with other trace metals content, pH, alkalinity, hardness, etc. Table 1 shows that the hardness and other quality parameters are within the tolerable limit except the arsenic content ranges from 0.09-0.23ppm. Table 2 shows that the removal percentage of arsenic in the samples with iron system is quite significant to achieve MCL for drinking purpose. Similar experiments with MgCh showed lower removal efficiency.

Conclusion It is concluded from the laboratory coagulation

tests on spiked and real samples with critical variables of pH, coagulation type and dose, contaminant concentration, ionic species CI- , N03 - and S02-4,

94

7.0

7.0

7.3

6.8

7.0

7.0

6.8

7.2

0. 14

0. 1 1

0.09

0.0 1 1

0.053

0.008

0.045

0.01 7

92.9

86.4

88.9

90.9

83.01

75.0

93.3

88.23

50.5

55.0

63 .0

62.5

57 .5 62.0 56.3

60.0

affecting removal of arsenic from contaminated water that chemical treatment with FeCh techniques is more efficient than MgCh and is capable of achieving stringent MCL for arsenic in drinking water as promulgated by WHO. It is established from the kinetics that in ferric system as high as 90% removal can be achieved, which in case of MgCh i s considerably lower and require longer time to reach the equilibrium. Most of the arsenic is removed within 2-3 minutes. Removal of As( +5) is more efficient than As(+3) and is attained at or below pH 7.5. The presence of anionic species like cr and NO)- have no significant hindrance in removal process but S02-4 ions causes considerable depressive effect.

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