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Vacuum 83 (2009) 142–145

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Vacuum

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Comparison of two methods for removal of arsenic from potable water

Dragan Manojlovic a, Ana Popara a, Biljana P. Dojcinovic b, Aleksandar Nikolic c,Bratislav M. Obradovic d,e, Milorad M. Kuraica d,e, Jagos Puric d,e,*

a Faculty of Chemistry, University of Belgrade, P.O. Box 158, Belgrade 11001, Serbiab Center of Chemistry, Institute of Chemistry, Technology and Metallurgy, Studentski trg 12-16, Belgrade 11000, Serbiac Studen-AGRANA Refinery of Sugar d.o.o. Brcko, Bijeljinska 9, Brcko 76100, Bosnia and Herzegovinad Faculty of Physics, University of Belgrade, P.O. Box 368, Belgrade 11001, Serbiae Center for Science and Technology Development, Obilicev Venac 26, Belgrade 11000, Serbia

Keywords:Potable waterPlasma treatmentDielectric barrier dischargeArsenic

* Corresponding author. Faculty of Physics, UniversBelgrade 11001, Serbia. Fax: þ381 11 3282 582.

E-mail address: epuric@ff.bg.ac.yu (J. Puric).

0042-207X/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.vacuum.2008.03.045

a b s t r a c t

Arsenic, well known of its toxicity, is present in potable water in many areas in the world, as well as inunderground water used for water supply in Vojvodina, a region in Serbia. Its removal from raw water isnecessary before distribution. In this work two methods of arsenic removal from water are compared.First method is water ozonation by introducing ozone in water and then filtration. Second method istreatment of water in plasma reactor and then filtration. High efficiency of the second method wasconfirmed by low concentration of arsenic in filtrate (below detection limit).

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

When arsenic is present in natural water it is a big ecologicalproblem. In many areas in the world potable water contains arsenic.A very small concentration of arsenic is allowed in potable water,due to high toxicity of arsenic compounds, especially arsenites. Inthe last years there are many publications about arsenic toxicityand about the techniques related to the removal of arsenic frompotable water [1–5]. Today, these different techniques are pre-dominantly based on oxidation in the first stage followed by, in thesecond stage, flocculation with microfiltration, adsorption atnatural and synthetic materials, ionic exchange, filtration andmembrane processes which include microfiltration, ultrafiltrationand nanofiltration, reversible osmosis and reversible electrodialy-sis. The efficiency of arsenic removal processes, as well as itstoxicity, depends on the form of arsenic present in the water.Filtration efficiency is larger for As(V) than for As(III). However,adsorption of organic arsenic on oxidation filters is very small.Before the adsorption, it is very important to transform all thespecies of arsenic to As(V). Oxidation efficiency depends on theused oxidation agents. The ozone from the ozonizer reactor is oftenused as an oxidation agent.

In this paper, two commercially available adsorbents, Manga-nese Greensand� and Birm� (Clack Corporation, USA), are used.Manganese Greensand� is formulated from a glauconite greensandwhich is capable of reducing iron, manganese and hydrogen sulfide

ity of Belgrade, P.O. Box 368,

All rights reserved.

from water through oxidation and filtration. Soluble iron andmanganese are oxidized and precipitated by contact with higheroxides of manganese on the greensand granules. Precipitates arethen filtered and removed by backwashing. Advantage is ironreduction over wide pH range. Physical properties are: black color,bulk density 1.35 kg/l, effective size 0.30–0.35 mm, mesh size 16–60. Conditions for operation are: water pH in the range of 6.2–8.5,maximum water temperature 26.7 �C.

Birm� is an efficient and economical media for the reduction ofdissolved iron and manganese compounds from raw watersupplies. Birm�, acting as a catalyst between the oxygen and thesoluble iron compounds, enhances the oxidation reaction of Fe2þ toFe3þ and produces ferric hydroxide which precipitates and may beeasily filtered. The physical characteristics of Birm� provide anexcellent filter media which is easily cleaned by backwashing toremove the precipitant. Its advantage is extremely high ironremoval efficiency. Physical properties are: black color, effectivesize 0.48 mm. Conditions for operation are: water pH in the range6.8–9.0, maximum water temperature 38 �C.

Underground waters are used for 95% of the total water supplyin Vojvodina (region in Serbia). However, about 70% of potablewaters do not fulfill the quality regulations which proscribe themaximum allowed concentration of 10 mg/l. Besides the arsenic, inthese waters high concentrations of humic substances are presentmaking the primary oxidation process difficult.

This paper is devoted to the comparison of two procedures forarsenic removal, one using a classical ozonator and the secondusing a coaxial dielectric barrier discharge (DBD) reactor for pri-mary oxidation [6,7]. The water from the well in Sirig (Municipalityof Temerin) was used. The content of total arsenic varied from 50 to

O zonator

Reactor FInputwater

Inputwater

SF A

a

DBD FPotablewater

Potablewater

SF Ab

Fig. 1. Scheme of experiments with classical ozonation (a) and with coaxial DBD plasma reactor (b). F – flocculation, SF – sand filtration, A – adsorption.

D. Manojlovic et al. / Vacuum 83 (2009) 142–145 143

250 mg/l. Also, high concentration of organic carbon, i.e. humicsubstances, is present in this water.

The attempts of primary oxidation using chlorine, potassiumpermanganate, hydrogen peroxide, as well as a classical ozonatordid not give satisfactory results, since arsenic concentration in thefinal water, after flocculation and adsorption, was always higherthan the maximum allowed concentration. Also, ultrafiltration andreversible osmosis processes did not give satisfactory resultsbecause of fast module restrain and sudden efficiency decrease.

2. Experimental setup

For the ozonation classical ozonator OZ-5G (A to Z OzoneSystems Inc., Louisville) as well as a coaxial DBD plasma reactor wasused. Schematic diagrams of the experiments are shown in Fig. 1.

In the case of using the classical ozonator, the produced ozone isinjected into the reactor by the Ventury tube. In this case the ozoneconcentration in treated water was 4 mg/l. Water treatment time inthe reactor was 10 min.

In the coaxial DBD reactor schematically presented in Fig. 2,water flows up through a vertical hollow cylindrical electrode andflows down making a thin dielectric film over the electrode. Fila-mentary dielectric barrier discharge is generated in air within4 mm gap between the dielectric and the water layer by applyingvoltage of 17 kV, see Fig. 2. Ozone is generated in the filamentarydischarge sustained above the flowing water layer as inner barrierelectrode of the coaxial DBD reactor. High voltage at 50 Hz fre-quency is applied between the inner, grounded stainless steelelectrode and outer metal electrode. Ozone and ozonized water

Fig. 2. Schematic picture of coaxial DBD and photograph of the discharge viewed fromthe top.

were generated in the same volume of the discharge. Length of theouter electrode is 40 cm and this is also the length of plasma overthe water layer. To increase the water flow through the reactor(w200 ml/min) three coaxial DBD units are connected parallel.Total electric power used in the three parallel DBD reactors wasw110 W. With a coaxial DBD treatment, dissolved ozone concen-tration in the water that passed through the DBD reactor was also4 mg/l. Under the term concentration of dissolved ozone, concen-tration of oxidants measured by the standard iodometric method ispresumed, see Ref. [6] and references therein.

The arsenic concentration in samples was determined by dif-ferential pulse stripping voltammetry (DPSV) [8,9] with rotationalgold electrode at Metrohm 797 VA Computrace and AAS-HG oninstrument SpectrAA 55 Varian. Electrodes for determination ofarsenic with DPSV were: WE – driving axle Au electrode tip, AE –electrode holter glassy carbon pin, RE – Ag/AgCl reference system.In order to obtain reproducible curves the gold electrode must beelectrochemically conditioned. Deposition potential of arsenic was0 mV and peak potential of As was þ50 mV.

Input water taken from the well in Sirig contained (148� 5) mgof total arsenic As(T), with (113� 3) mg As(III) and (35� 2) mg As(V).In this water As(III) concentration is much higher than As(V), andthis is an additional problem due to the combined flocculation andadsorption technique for arsenic removal from water. For efficientarsenic removal complete oxidation from As(III) to As(V) is neededwith appropriate adsorber.

After oxidation water is lead to flocculator and treated for10 min with 6 mg/l of FeCl3� 6H2O. In the next step sand filtrationof water was made and then water was lead to the adsorptioncolumn, see Fig. 1. The adsorption column was filled with Manga-nese Greensand� and Birm� modified by sorption of 0.01% ofzirconium oxychloride.

Greensand� and Birm� were modified by suspension of 100 g ofsorbents in 700 ml distilled water and with appropriate content of2% zirconium oxychloride addition. After that, bottle with suspen-sion was shaken for 6 h using a rotation shaker. After shaking,sorbents were rinsed with distilled water up to the negativereaction on chlorides. Before using, they were dried at 105 �C.

3. Results and discussion

Compared results of primary ozonation efficiency made byclassical ozonator and DBD reactor in arsenic removal from inputwater are given in Table 1 and Table 2, as well as in Figs. 3 and 4.

Table 1Arsenic sorption at Manganese Greensand� after appropriate primary treatment

Treatment As(T) (mg/l) As(III) (mg/l) As(V) (mg/l)

Input water 148� 5 113� 3 35� 2Without ozonation 131� 3 108� 3 23� 2Classical ozonation 35� 3 32� 2 4� 2DBD reactor 5� 2 3� 1 1� 1

Table 2Arsenic sorption at Birm� after appropriate primary treatment

Treatment As(T) (mg/l) As(III) (mg/l) As(V) (mg/l)

Input water 148� 5 113� 3 35� 2Without ozonation 108� 3 95� 3 8� 2Classical ozonation 10� 2 9� 2 1� 1DBD reactor <1 <1 <1

0

20

40

60

80

100

120

140

160

180

200

DBD tretment(before floculation)

Classical ozonation(before floculation)

Floculation &filtration only

Rawwater

Co

ncen

tratio

n o

f A

s (µg

/l)

As (T)As (III)As (V)

Birm-Zr

Fig. 4. Arsenic treatments with Birm� sorption.

D. Manojlovic et al. / Vacuum 83 (2009) 142–145144

These results are also compared with results obtained duringwater treatment according to mentioned technological procedure,but without primary ozonation.

Used modified sorbents have low impact on arsenic removalfrom water without primary ozonation process, since their effi-ciency was only about 11% for Greensand� and about 27% for Birm�.

With classical ozonator removal efficiency has increased up to76% for Greensand� and up to 93.2% for Birm�, and arsenicconcentration in treated water is at the maximum allowedconcentration limit (10 mg/l).

With coaxial DBD plasma reactor in both sorbents removalefficiency is largely increased for Greensand� 96.6%, i.e. (5� 2) mg/l,and for Birm� over 99.5%, i.e. <1 mg/l. Arsenic concentration in finalwater is far below the detection limit. As expected, removalefficiency for As(V) is higher than for As(III).

Most common arsenic removal techniques involve pre-treatmentof water by oxidation. Coagulation [10], precipitation and otheradsorption [11] techniques have been found to be more efficient forAs(V) than for As(III) removal. Oxidation of arsenic species is moreefficient with DBD due to several reasons. In classical ozonations,ozone was externally produced and then introduced into water.After sorption and homogenization it oxidizes arsenic(III) toarsenic(V). Process of oxidation was more complicated if treatedwater contains high content of humic substances, as was in ourcase, since some of the generated ozone was used for incompleteoxidations of these substances. In the case of the DBD system,ozone was generated directly during water treatment. Also, there isinfluence of UV light on photochemical oxidation of arsenic(III) [12]and organic substances. In the DBD significant amount of hydroxylradical was generated which has more efficient oxidation powerthan ozone (O3 E0¼ 2.07 V; OH�, E0¼ 2.32 V [13]) and their reactionrates are higher in comparison with other oxidation reagents [14].In the DBD two oxidation reactions, one direct and the other afterphotochemical reaction, were predominant.

2OH�D AsðIIIÞ/ 2OHL D AsðVÞ

0

20

40

60

80

100

120

140

160

180

200Greensand-Zr

Co

ncen

tratio

n o

f A

s ( g

/l)

As (T)As (III)As (V)

Rawwater

Floculation &filtration only

Classical ozonation(before floculation)

DBD tretment(before floculation)

Fig. 3. Arsenic treatments with Manganese Greensand� sorption.

AsðIIIÞD hD / AsðIVÞ; AsðIVÞD OH� / AsðVÞ

Since the oxidations of organic substances are, also, moreefficient in the DBD the obtained organic products do not obstructcoagulation and sorption of As(V).

4. Conclusion

The water ozonation efficiency of DBD reactor in process ofprimary ozonation in arsenic removal technological process, withuse of two different commercial sorbents modified by zirconium,was investigated. These results were compared with primaryozonation efficiency of classical ozonator, as well as with arsenicremoval efficiency in the same technological process without pri-mary ozonation. The used modified sorbents have a low impact onarsenic removal from water without the primary ozonation process.

With classical ozonator removal efficiency increase (76% forGreensand� and 93.2% for Birm�), arsenic concentration in treatedwater is at the maximum concentration limit (10 mg/l). With theDBD plasma reactor removal efficiency largely increases, arsenicconcentration in final water is far below the detection limit. Asexpected, removal efficiency for As(V) is higher than As(III) one.

The DBD reactor has significantly larger arsenic oxidation effi-ciency, and the arsenic removal efficiency from input water duringflocculation and adsorption processes is very high. Beside that, usingDBD reactor, time of input water treatment is shorter, and additionalreactor with water detention is not needed. High efficiency of thisplasma reactor is the consequence of its construction that enabledsimultaneous exposition of water layer to ionized gas and UVemission from the electric discharge, with continuous production offree radicals (OH, O, O3) in the treated water. With the DBD reactorboth sorbents, Greensand� and Birm�, can be used. In the case ofa classical ozonator, using Greensand� sorbent poses a risk, becausearsenic concentration in final water is above the allowed limit.

Acknowledgements

This work within the Projects 141043 and 146008 is supportedby the Ministry of Science of the Republic of Serbia.

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