organic arsenic removal from drinking water
TRANSCRIPT
Organic arsenic removal from drinking water
O.S. Thirunavukkarasu a, T. Viraraghavan a,*, K.S. Subramanian b, S. Tanjore c
a Environmental Systems Engineering, Faculty of Engineering, University of Regina, Regina, Sask., Canada S4S 0A2b Product Safety Bureau, Health Canada, Ottawa, Ont., Canada K1A 0K9
c Waterworks Technologies, Calgary, Alta., Canada T2N 1J7
Received 11 May 2001; received in revised form 6 March 2002; accepted 30 April 2002
Abstract
Arsenic occurs in both inorganic and organic forms in water. Although various methods have been adopted to remove inorganic
species of arsenic from drinking water, not much emphasis has been given to the removal of organic species of arsenic. In the present
study column studies were conducted using manganese greensand (MGS), iron oxide-coated sand (IOCS-1 and IOCS-2) and ion
exchange resin in Fe3þ form, to examine the removal of organic arsenic (dimethylarsinate) spiked to required concentrations in tap
water. Batch studies were conducted with IOCS-2, and the results showed that the organic arsenic adsorption capacity was 8 lg/gIOCS-2. Higher bed volumes (585 BV) and high arsenic removal capacity (5.7 lg/cm3) were achieved by the ion exchange resinamong all the media studied. Poor performance was observed with MGS and IOCS-1.
� 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Adsorption; Batch studies; Column studies; Filtration; Ion exchange resin; Iron oxide-coated sand; Manganese greensand; Organic
arsenic species; Water Treatment
1. Introduction
Extensive arsenic contamination of surface and
groundwater has been reported in many parts of the
world. The possible long term effects and the risks
associated with the ingestion of arsenic contaminated
water have compelled the regulatory agencies to pro-
mulgate a lower standard for arsenic in drinking water.
The World Health Organization (WHO) standard for
arsenic is 10 lg/l. The maximum acceptable concentra-tion for arsenic is 25 lg/l in Canada; recently the UnitedStates Environmental Protection Agency (USEPA)
adopted a new arsenic standard for drinking water at 10
lg/l (USEPA, 2001). The existing arsenic standard fordrinking water in Australia is 7 lg/l, whereas EuropeanUnion has stipulated an arsenic standard of 10 lg/l(Yamamura, 2001).
Arsenic, a common toxic element, is present in bothinorganic and organic forms in water. Arsenic contam-
ination of surface water is mainly due to weathering of
rocks, geochemical reactions, contact of arsenic bearing
sediments with aquifer, industrial waste discharges,
volcanic emissions, fertilizers, and mining and smeltingoperations. Though arsenic contamination of ground-
water in Taiwan was reported about three decades ago
(Tseng et al., 1968), the general awareness and attention
of the public has been focussed recently because of
similar reports of contamination of surface and sub
surface waters by As from many other regions in the
world, such as Bangladesh, Mexico, China, Chile, USA,
Canada and India (Burkel & Stoll, 1999; Cebrian,Albores, Aguilar, & Blakely, 1983; Dhar, Biswas, &
Samanta, 1997; Karim, 2000; Koch et al., 1999;
Meranger & Subramanian, 1984).
Arsenic occurs in several different species depending
upon the pH and oxidation potential of the water. The
four arsenic species commonly reported are arsenite
[As(III)], arsenate [As(V)], monomethylarsonate (MMA)
and dimethylarsinate (DMA). Despite the fact that in-organic species are predominant in natural waters, the
presence of MMA and DMA has also been reported
(Anderson & Bruland, 1991; Andreae, 1977; Banerjee,
Helwick, & Gupta, 1999; Braman & Foreback, 1973;
Del Razo, Arellano, & Cebrian, 1990; Chen, Yeh, Yang,
& Lin, 1995).
In the studies conducted by Del Razo et al. (1990), one
of the water samples collected from the wells in the
*Corresponding author. Tel.: +1-306-585-4094; Fax:+1-306-585-
4855.
E-mail address: [email protected] (T. Viraraghavan).
1462-0758/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.PII: S1462-0758 (02 )00029-8
Urban Water 4 (2002) 415–421
www.elsevier.com/locate/urbwat
Lagunera Region, Northern Mexico had a high DMA
concentration of 20 lg/l. The results of studies by Chenet al. (1995) showed that most of the water samples from
the wells in northeast and northwest Taiwan (endemic
area for blackfoot disease) had a high DMA concen-
tration of 7 lg/l. Anderson and Bruland (1991) reportedthat the methylated species (dominant species was
DMA) comprised on average of 24% of the total dis-solved As in surface waters of the various lakes examined
in California, USA, with the exception of Mono and
Pyramid lakes. In recent studies by Banerjee et al. (1999),
the concentration of organic arsenic species was 10% of
the total arsenic concentration in the groundwater sam-
ples from a superfund site in the northeastern United
States, where the total As concentration was 30 mg/l.
Upon ingestion of inorganic arsenicals, it is methy-lated in the human body and the metabolites of ingested
arsenic eliminated by the kidney and excreted in urine
within 1–3 days (Crecelius, 1977; Vahter, 1994). DMA is
the predominant metabolite excreted in urine and faeces
of animals and humans exposed to inorganic arsenic
(Styblo, Delnomdedieu, Hughes, & Thomas, 1995).
DMA appears to be a potent clastogenic agent causing
gene amplification (Endo, Kuroda, Okamoto, & Horig-uchi, 1992). Yamamoto et al. (1995) showed that chronic
exposure to DMA enhanced tumor development in the
kidney, liver and urinary bladder of F 344 rats. The
toxicity of different arsenic species varies in the order:
arsenite > arsenate� monomethylarsonate ðMMAÞ >dimethylarsinate ðDMAÞ. Inorganic arsenic species areabout 10 to 60 times more toxic than organic arsenic
compounds (MMA and DMA). Methylation of inor-ganic arsenic in the body is a detoxification process,
which reduces the affinity of the compounds for tissues
(Jain & Ali, 2000; Lewis & Tatken, 1978; Penrose, 1974).
The LD50 values for various arsenic species are shown in
Table 1.
Recent research (Kenyon & Hughes, 2001; Petrick,
Jagadish, Mash, & Aposhian, 2001) pointed out, how-
ever, that organic arsenic species are more toxic than
initially thought. Therefore more research is needed for
an estimation of the global arsenic toxicity based onspeciation. It is anticipated that the USEPA may ap-
proach the wide range in arsenic species toxicity as it is
done with mercury, whereby the more toxic species are
the most closely regulated. It is unlikely that organoar-
senic species will be as tightly regulated unless they are
in a high concentration (Miller, Norman, & Frisch,
2000; USEPA (1999, 2000)). Only a few studies have
been conducted to remove organic arsenic from drink-ing water. Ghosh and Yuan (1987) reported that
the adsorption behaviour of both monomethylarsonate
(MMA) and dimethylarsinate (DMA) onto activated
alumina were similar to that of arsenate. Vogels and
Johnson (1998), in their studies with ferrous and ferrate
ions reported that better removal of MMA than DMA
was obtained. In the present study, column studies were
conducted using manganese greensand (MGS) to ex-amine the removal of organic arsenic (DMA) spiked in
tap water. Further, studies with iron oxide-coated sand
(IOCS) prepared by two different methods (IOCS-1 and
IOCS-2) and ion exchange (Amberlite IR-120 resin ac-
tivated with ferric ions) resin were also conducted to
remove organic arsenic (DMA) spiked in tap water, and
the results were compared with the data from the MGS
studies.
2. Materials and methods
Tap water from the City of Regina, Saskatchewan,
Canada was used as the water source for all the exper-
iments. The pH of the tap water was 7.6, and not ad-justed for any of the experiments. In the studies, tap
water was supplemented with the required concentra-
tions of DMA. DMA stock solution (1000 lg/l) was
Nomenclature
C equilibrium concentration (lg/l)Ce effluent adsorbate concentration (lg/l)C0 influent adsorbate concentration (lg/l)k Thomas rate constant (ml/min lg)x mass of solute adsorbed (lg)m mass of adsorbent (g)
K the Freundlich constant indicative of the ad-
sorption capacity of the adsorbent (l/lg)
n experimental constant indicative of the ad-
sorption intensity of the adsorbent
Q volumetric flow rate (ml/min)
q0 maximum solid phase concentration (lg/g)V throughput volume (ml) andr regression coefficient
Table 1
The LD50 values for arsenic species
Arsenic species LD50 (mg/kg) Reference
DMA 1200 Penrose, 1974; Lewis and Tat-
ken, 1978; Jain and Ali, 2000.
As(V) 14
As(III) 4.5
416 O.S. Thirunavukkarasu et al. / Urban Water 4 (2002) 415–421
prepared by dissolving 1.1678 g of cacodylic acid(C2H6AsO2Na, Sigma Chemical, Ontario) in 1 l distilled
water and preserved with 0.5% nitric acid (trace metal
grade). The required working standards were prepared
daily from the stock solution.
MGS manufactured by Inversand company, New
Jersey, USA, was purchased from Watergroup Canada
Ltd., Regina, and used in the column studies. MGS was
conditioned prior to the column studies and the detailsare available elsewhere (Viraraghavan, Subramanian,
& Aruldoss, 1999). Amberlite IR-120 cation exchange
resin manufactured by Rohm and Haas Company,
Pennsylvania, USA, was purchased from Watergroup
Canada Ltd., Regina. The resin was activated with Fe3þ
ions and used in the studies. The activation of the resin
was carried out by mixing 300 g of resin in 0.1M
FeCl3 � 7H2O solution using a paddle mixer for 4 h.The red flint sand used in the preparation of IOCS
was purchased from Watergroup Canada Ltd., Regina.
IOCS-1 was prepared by adding 40 ml of 1M FeCl3 �H2O solution per 500 g of sand and the mixture was
allowed to remain in the column for 12 h. The column
was backwashed for a short duration before starting
the run. In the second method IOCS-2 was prepared as
per Benjamin, Sletten, Bailey, and Bennet (1996) withmodifications. The sand was sieved to a geometric mean
size of 0.6–0.8 mm, acid washed (pH 1; 24 h), rinsed with
distilled water three times and dried at 110 �C for 20 hbefore use. IOCS-2 was prepared in two steps. In step 1,
the solution containing a mixture of 80 ml of 2M
Fe(NO3)3 � 9H2O and 1 ml of 10 M NaOH was pouredover 200 g dried sand placed in a heat resistant dish, and
the mixture was heated at an elevated temperature. Thenthe sand was cooled and washed with distilled water.
In step 2, the solution containing the same mixture of
Fe(NO3)3 � 9H2O and NaOH was poured over 100 g ofthe coated sand from step 1, and heated for a desired
duration. Further the coated sand from the second step
was subjected to five more drying cycles, which consisted
of drying at 110 �C for 4 h followed by 20 h at roomtemperature. The coated sand was stored in cappedbottles.
Batch studies were conducted only with IOCS-2 at
the room temperature of 22� 1 �C using a portablebenchtop C2 classic platform shaker with digital display
on the control panel. In the experiments, the respective
mass of IOCS-2 was transferred to 250 ml conical flasks
containing 100 ml of tap water spiked with DMA. The
flasks were sealed with parafilm and the samples wereshaken at 175 rpm. After completion of the contact
time, the samples from each flask were decanted and
analyzed for residual DMA.
In the column studies with MGS, IOCS-1 and resin, a
column of 10 cm in diameter and 180 cm high was used.
Tap water spiked with the required concentration of
DMA was pumped into the column using a submersible
pump. In the case of studies with MGS and IOCS-1, thepacked volume of the media in the column was 4681 ml
and the empty bed contact time (EBCT) was 5.64 min.
The flow rate to the column was kept at 0.83 l/min (117
m3 m2 d�1). In the studies with ion exchange resins in
Fe3þ form, the flow rate, packed volume, and the EBCT
were 1.23 l/min (176 m3 m2/d), 4182 ml, and 3.4 min
respectively. The column study with IOCS-2 was con-
ducted in a column of size 16 mm diameter and 400 mmlong with suitable stoppers. A mesh was placed inside
the stopper to prevent the escape of adsorbent during
filtration. The flow rate, packed volume, and EBCT
were 17.5 ml/min (96 m3 m2/d), 23 ml, and 2.64 min
respectively. Samples were collected at regular intervals
and analyzed for residual DMA concentration.
The instrument used in this study for the measurement
of arsenic was Varian type SpectrAA––600 ZeemanGFAAS equipped with GTA 100––graphite tube atom-
izer and PSD––100 programmable sample dispenser.
Pyrolytically coated notched partition graphite tubes
(Varian Canada Inc., Toronto) were used in the experi-
ments, and argon gas of ultrahigh purity (99.995%;
Praxair Products Inc., Ontario) was used to sheath the
atomizer and to purge internally. Arsenic hollow cath-
ode lamp (Varian Canada Inc., Toronto) was usedat a wavelength of 193.7 nm with a slit width of 0.5
nm. Nickel nitrate solution at 50 mg/l or palladium-
ð1500 lg=lÞ þmagnesium nitrate (1000 lg/l) solutionwas used as matrix modifiers for calibration.
3. Results and discussion
3.1. Batch studies
Batch kinetic and isotherm studies were conducted
with IOCS-2, in which the initial DMA concentration inthe tap water was 100 lg/l. The results of the kineticstudies showed that the minimum concentration of
DMA achieved was 30.82 lg/l (a removal of 69.18%) ata contact time of 7 h. Ho’s pseudo second order model
(Ho, Wase, & Forster, 1996), shown below was used to
analyze the adsorption data obtained in the kinetic
studies:
tq¼ 1
2kq2eþ tqe
ð1Þ
where k is the rate constant for adsorption (g/lgmin);qe, the amount of adsorbate at equilibrium (lg/g); q, theamount of adsorbate at any time t (lg/g). The aboveequation is rearranged to
q ¼ 2kq2e t1þ 2kqet
ð2Þ
The experimental results from the kinetic studies were
subjected to non-linear analysis using Ho’s model, with
O.S. Thirunavukkarasu et al. / Urban Water 4 (2002) 415–421 417
‘Statistica for Windows’ (release 5.1, ’97 edition) soft-ware using a Quasi-Newton method. The estimated
modelled values obtained with time are shown in Fig. 1.
The amount of adsorbate at equilibrium qe was esti-mated at 14.61 lg/g. The correlation coefficient (r) forthe plot was 0.99 representing a good correlation to the
observed data, and the model was found to provide a
realistic description of adsorption kinetics of arsenic
(DMA). Isotherm studies were conducted by varying themass of IOCS-2 from 0.1 to 0.9 g, and the equilibrium
time of 7 h from the kinetic studies was kept as the
contact time. A maximum DMA removal of 73.6% (23.4
lg/l) was achieved with 0.9 g of sand. The removal ofDMA (73.6%) obtained in this study was higher than
that (a maximum of 28%) obtained by Vogels and
Johnson (1998) in their studies with ferrous and ferrate
ions.The Freundlich isotherm was used to describe ad-
sorption. The Freundlich isotherm describes equilibrium
on heterogeneous surfaces and is widely recommended
due to its accuracy. It gives more accurate results than
the Langmuir isotherm for a wide variety of heteroge-
neous adsorption systems (Al-Duri, 1996). The isotherm
equation is as follows
xm
¼ KC1=n ð3Þ
where x is the mass of the solute adsorbed at any time t
(lg); m, the mass of adsorbent (g); K, n the experimentalconstant indicative of the adsorption capacity and in-
tensity of the adsorbent, and C, the equilibrium con-
centration (lg/l).The experimental results of isotherm studies were
subjected to non-linear analysis, and the estimated
modelled values were plotted with the concentration of
DMA remaining in solution (Fig. 2). The Freundlich
constants ‘K’ and ‘n’ were estimated at 0.07, and 0.71respectively. The low K value ð0:07 < 1Þ and high 1=nvalue suggested that the arsenic adsorption capacity was
low. A correlation coefficient ‘r’ of 0.94 for the Fre-
undlich isotherm showed that the arsenic adsorption can
be best described by the Freundlich isotherm. The ad-
sorption capacity of IOCS-2 for arsenic (DMA) removalat a residual concentration of 23.4 lg/l was estimated as8 lg/g.
3.2. Column studies
The initial DMA concentration was kept at 100 lg/lfor all the adsorbents used in the column studies. Fixed
bed adsorption columns operated in the downflow modemay perform two functions, namely adsorption and fil-
tration. Breakthrough or exhaustion curves were devel-
oped from the column data by plotting the ratio of
Ce=C0 with the volume of water processed. Variousmathematical models have been developed to predict the
dynamic behavior of the column; the model developed
by Thomas (1948), which is used in the design of fixed
bed adsorption systems is as shown below (Reynolds &Richards, 1996):
CeC0
¼ 1
1þ exp kQ q0m� C0Vð Þ
h i ð4Þ
where Ce is the effluent adsorbate concentration (lg/l);C0, the influent adsorbate concentration (lg/l); k, theThomas rate constant (l/min lg); q0, the maximum solidphase concentration of the solute (lg/g); m, the mass ofthe adsorbent (g); V , the throughput volume (ml); andQ, the volumetric flow rate (ml/min).The results of the column tests were analyzed using
the Thomas model, and the estimated modelled values
of Ce=C0 were compared with the observed values. Theseare shown in Figs. 3 and 4. The results of column studies
are summarized in Table 2. The results showed that
studies with ion exchange resin was the only treatment
method effective in reducing DMA to a level below thebreakthrough concentration of 10 lg/l ðCe=C0 ¼ 0:1Þ.The column filled with the resin continued to remove
DMA below 10 lg/l for a period of six and a half hours.
Fig. 1. Arsenic (DMA) removal in the kinetic studies.Fig. 2. Isotherm plot for arsenic (DMA) removal using IOCS-2.
418 O.S. Thirunavukkarasu et al. / Urban Water 4 (2002) 415–421
In the case of DMA removal with the resin, 585 bed
volumes achieved <25 lg/l residual DMA, and the over-all removal capacity (5.7 lg/cm3) was highest among allother media used. However the maximum solid phase
arsenic concentration q0 (64.8 lg/g) estimated from theThomas model for IOCS-2 was high compared to the
other media used. Increase in the mass of the adsorbent(IOCS-2) or reduction of flow rate to the column might
have enhanced DMA removal in the case of IOCS-2.
The results of kinetic studies using IOCS-2 (Vira-
raghavan, Thirunavukkarasu, & Subramanian, 2000) to
remove inorganic arsenic species [As(III) and As(V)] of
arsenic showed that 85–90% arsenic was removed in theinitial phase (1 h) of contact, where the initial concen-
trations of arsenite and arsenate were 100 lg/l; morethan 95% removal was obtained for both As(III) and
As(V) at a pH of 7.6 after six hours of contact. Con-
tinuous column studies were conducted at the University
of Regina, Canada with IOCS-2 for the removal of
inorganic forms of arsenic from drinking water, and
the results were encouraging (Thirunavukkarasu, Vira-raghavan, & Subramanian, 2002). Since the concentra-
tions of organic arsenic present in natural waters are less
compared to inorganic forms (Anderson & Bruland,
1991), it is anticipated that IOCS-2 will be able to re-
move the DMA present in natural waters along with the
inorganic forms. Based on the results of the present
column studies, the performance of IOCS-2 was better
than that of IOCS-1 because in the preparation ofIOCS-2, an effective coating of iron oxide was achieved
on the sand through a high temperature coating process.
Poor performance was observed with MGS and IOCS-1.
4. Practical applicability of treatment systems
A treatment system such as an ion exchange system
was considered by the USEPA (1999, 2000) as one of the
best available technologies for arsenic removal, which
may be applicable to urban water facilities in USA, UK,and Canada. On the other hand, IOCS filtration, an
emerging technology (USEPA, 1999, 2000) may be
suitable for small water facilities, especially in develop-
ing countries such as Bangladesh, Chile, India, and
Mexico because of its simplicity, and ease of operation.
Removal of organic species has limited practical appli-
cability, as they are usually associated with inorganic
species in natural systems. The inorganic species presentin higher concentrations are obviously influencing and
interfering in the ion exchange process. More accessi-
ble and cost effective ion exchangers and/or adsor-
bents should be also studied such as, natural zeolites
and carbonaceous adsorbents as opposed to expensive
synthetic organic ion exchangers (Elizalde-Gonzalez,
Mattusch, Einicke, & Wennrich, 2001). Present studies
Fig. 3. DMA removal in the column studies using MGS, IOCS-1, and
resin.
Fig. 4. DMA removal in the column studies using IOCS-2.
Table 2
Summary of the results from the column studies
Adsorbent MGS IOCS- 1 Resin IOCS- 2
DMA concentration (lg/l) 100 100 100 100
Flow rate (ml/min) 830 830 1230 17.5
Volume of the media (ml) 4681 4681 4182 23
EBCT (min) 5.6 5.6 3.4 2.6
Total throughput volume (l) 572 547 1107 35.7
‘k’ value from Thomas model (ml/minlg) 0.27 0.12 1.23 0.28
Maximum solid phase concentration q0 (lg/g) from Thomas model 0.5 0.5 25 64.8
Bed volumes achieved upto 25 lg/l of As – – 585 480
Overall removal capacity of the media (lg/cm3) 0.7 0.4 5.7 2.6
O.S. Thirunavukkarasu et al. / Urban Water 4 (2002) 415–421 419
clearly showed that both ion exchange resin and IOCSare capable of removing organic species of arsenic from
drinking water. The removal of organic species along
with the inorganic species of arsenic from drinking water
by iron oxide-coated sand clearly showed that IOCS
would be the best choice amongst the treatment systems
for arsenic removal, applicable to small water facilities,
especially in developing countries.
5. Conclusions
1. Batch study results with IOCS-2 showed that the ar-
senic (DMA) adsorption can be best described by the
Freundlich isotherm based on the correlation co-
efficient.
2. The constants estimated from the Freundlich iso-
therm indicated that the arsenic (DMA) adsorption
capacity of IOCS-2 was low.
3. The arsenic (DMA) removal capacities of the mediaused in the column studies decreased in the following
order:
Ion exchange resin > IOCS-2 >MGS > IOCS-1
4. High bed volumes and high bed capacity were
achieved with ion exchange resin in the column stud-ies; these results suggested that ion exchange resin can
be effectively used to remove organic arsenic (DMA)
in drinking water. Poor performance of DMA re-
moval was observed with MGS and IOCS-1.
Acknowledgements
The authors acknowledge major support from Health
Canada for this study.
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