FIELD OF INVENTION
The present invention relates to arsenic adsorbent and in particular to a modified
laterite arsenic adsorbent for removing arsenic species in the form of arsenate and
arsenite from an aqueous medium and to its method of manufacture. The invention
is further directed to a method for the removal of arsenic species from aqueous
medium, preferably groundwater. Importantly, the invention by way of the modified
laterite composite with superior surface properties favours high capacity cost-
effective arsenic adsorption leading to enhanced arsenic removal.
BACKGROUND ART
Arsenic is a ubiquitous, nonessential, accumulative element known since 387 B.C.
and is found in water which has flowed through arsenic-rich rocks whereupon it is
introduced into water through the dissolution of minerals and ores. Arsenic is also
used extensively in the commercial production of agricultural pesticides, which
includes herbicides, insecticides, desiccants, wood preservatives and feed additives.
Industrial uses of arsenic include hardening of copper and lead alloys, pigmentation
in paints and fireworks, and the manufacture of glass, cloth, and electrical
semiconductors. Industrial effluents from these uses and the leaching of arsenic from
waste generated from these uses results in increased levels of various forms of
soluble arsenic in water. Combustion of fossil fuel is a source of arsenic in the
environment, through disperse atmospheric deposition. Inorganic arsenic occurs in
the environment in several forms but in natural waters, and thus in drinking-water, it
is mostly found as trivalent arsenite [As (III)] or pentavalent arsenate [As (V)].
Organic arsenic species, abundant in seafood, are very much less harmful to health,
and are readily eliminated by the body.
Drinking-water poses the greatest threat to public health from arsenic since
inorganic arsenic is a documented human carcinogen. Severe health effects have
been observed in populations drinking arsenic-rich water over long periods in
countries world-wide and especially in countries like India and Bangladesh such
arsenic contamination of ground water is thought to be of geological origin and
derives from the geological strata underlying Bangladesh.

The current drinking water quality guideline by WHO for Arsenic is 0.01 ppm.
Drinking water arsenic concentrations greater than 10 ppb pose a significant health
problem throughout the world. Water with arsenic concentrations greater than 10
ppb is common in less developed countries and therefore there is an urgent need for
an inexpensive arsenic removal method to lower its maximum contaminant level.
In order to keep the water supply safe for human consumption and affordable to all,
water utilities are presently examining methods of treating water in order to reduce
levels of arsenic in the water to 5 ppb. One such method is adsorption, which is the
bonding of an aqueous species to the surface of a solid grain. The solid grain is called
the sorbent while the aqueous species is called the sorbate. The nature of the
sorbent, including functional groups and the surface area available for adsorption,
affects the affinity of the sorbent for specific contaminants. Also, the chemical
character, shape, and configuration of the sorbate, its water solubility, its acidity, the
polarity of the molecule, its molecular size and polarizability all affect its ability to
sorb onto the reactive media.
US 5,556,545 deals with a method for removing arsenic species from an aqueous
medium through the use of an activated alumina sorbent, having a particulate size
below 200 micrometers diameter and with sufficient porosity and pore diameters
above 100 A to remove arsenic from water. The spent activated alumina sorbent is
then regenerated and recycled.
S. A. Wasay et. al., in Separation Science and Technology 1996, 31, 1501,
demonstrated the removal of anionic pollutants (like phosphate, silicate, arsenate,
fluoride and selenite) on La(III) and Y(III) impregnated raw alumina. The change in
surface charge due to impregnation was measured and its adsorption capacity of
various anionic pollutants was in the order of fluoride > phosphate > arsenate >
selenite. The adsorption capacity of arsenate on Y (III) and La (III) impregnated
alumina were found to be 14.45 and 12.88 mg/ g in synthetic solution.
However, the use of activated alumina as sorbent contains some inherent limitations
since to make it an economically feasible process, rejuvenation and conditioning of
the sorbent for subsequent use is necessary. The rejuvenation, conditioning and
regeneration of the sorbent to release the adsorbed arsenic create a hazardous

solution which requires further treatment which adds up to its disposal costs. The
lost activated alumina sorbent in the process of regeneration needs to be
continuously replaced which substantially increases the cost of using activated
alumina as a method for removing arsenic from an aqueous medium.
A. Dutta et. al. in Aqua, vol. 40, no. 1 (1991) pp. 25-29, teaches another method for
removing arsenic species from an aqueous medium through the use of activated
carbon. Lime softening and powdered activated carbon alone were found to remove
90% and 15%, respectively of the aqueous arsenic species present. However, the
use of activated carbon has inherent limitations since the activated carbon has a
limited natural capacity for adsorbing arsenic species and has high cost thus making
it less attractive as a chosen method for removing arsenic species from aqueous
medium.
Further, Z. Gu et. al., in Environmental Science and Technology 2005, 39(10), 3833-
3843, demonstrated arsenic removal by activated carbon containing iron oxide.
Nitrogen adsorption-desorption analyses showed the BET specific surface area, total
pore volume, porosity, and average mesoporous diameter all decreased with iron
impregnation and the arsenic adsorption capacity of these synthesized activated
carbons under different process conditions were found in the range of 2.96 to 3.94
mg/g at pH 4.7.
Again, B. E. Reed et. al., in J. Environmental Engineering 2000, 126, 869-873,
showed As (III), As (V), Pb (II) and Hg (II) adsorption on Fe-oxide impregnated
activated carbon. Iron-oxide impregnation increased the pHzpc of the carbon but did
not alter the surface area or the pore volume wherein moderate adsorption for both
arsenic species was noticed (maximum adsorption 4.5 mg/ g for both arsenic
species).
P. B. Bhakat et. al., in Colloids Surfaces A: Physicochemical and Engineering Aspects
2006, 281 (1-3), 237-245, disclosed As (V) adsorption on modified calcined bauxite
where the arsenic adsorption capacity of this adsorbent was found to be a maximum
of 1.57 mg/ g for the As (V) species.

US 5369072, is directed to an adsorbent media to remove metal contaminants and
natural organic matter from water, prepared by contacting support material such as
sand and olivine with iron containing solutions, followed by drying to coat the
support material with iron.
H. Genc-Fuhrman et. al., in J. colloid and Interface Science 2004, 271(2), 313-320,
demonstrates arsenate adsorption on Bouxsol coated sand and the arsenate
adsorption capacity was found to 3.32 and 1.64 mg/g at pH 4.5 and 7. This prior art
specifically revealed that acid and subsequent heat treatment improved the BET
surface area of Bauxol from 30 to 130 m2/ g.
US 6,821,434 illustrates a system for removing arsenic from water by addition of
inexpensive and commonly available magnesium oxide, magnesium hydroxide,
calcium oxide, or calcium hydroxide to the water that has a strong chemical affinity
for arsenic and rapidly adsorbs arsenic from water in the presence of carbonate to
reduce the concentration of arsenic in water below an acceptable level of 10 ppb.
US 7,309,425 relates to a process for the preparation of arsenic free water by
employing a porous ceramic useful for pressure filtration thereby producing arsenic
free water. This prior art also deals with a process for preparing arsenic free (<10
ppb) water from arsenic contaminated ground water and apparatus therefor.
US 6,042,731 achieves a method for removing arsenic species from an aqueous
medium with iron (II) laden modified zeolite minerals comprising providing an
aqueous medium containing arsenic species in the form of both arsenate and
arsenite, contacting the aqueous medium with the said zeolite mineral so that
arsenic in the form of at least one of arsenate and arsenite contained in the aqueous
medium is adsorbed onto it.
Y.-H. Xu et. al., in J. Hazardous Materials 2002, 92, 275-287, demonstrated
arsenate removal on activated zeolite such as AI-SZP1 that was prepared by treating
a P1 type Shirasu-zeolite (SZP1) with aluminum sulfate solution. An As(V) aqueous
solution with the influent As(V) concentration of 6.7 μM (0.51 mg/ I) was passed
through the column at a constant rate at room temperature to achieve a
breakthrough As(V) concentration of 0.13 μM (0.01 mg/l) in the effluent at pH about

7.3. The adsorption of As(V) was performed in the pH range of 3 to 10 and
maximum adsorption capacity was found as 10.49 mg/ g.
S. Kundu et. al., in Chemical Engineering Journal 2006, 122(1-2), 93-106, utilized
iron oxide coated cement (IOCC) for arsenic removal from aqueous system. The
adsorption of arsenite and arsenate were performed under varying process
conditions of solution pH, adsorbent dose, temperature, etc in synthetic solution. The
Langmuir maximum adsorption were obtained at neutral pH as 0.67 and 6.43 mg/g
for As(III) and As(V) species, respectively.
US 5,603,838 is directed to a process for removing selenium and/or arsenic from
aqueous streams including industrial process waters and drinking water comprising
contacting the stream with a composition bearing lanthanum oxide and alumia
whereby selenium and/ or arsenic are adsorbed.
US 7,368,412 teaches a method of producing an adsorption medium to remove at
least one constituent such as arsenic from a feed stream by passing the feed stream
through the adsorption medium comprising a polyacrylonitrile matrix with one metal
hydroxide incorporated into the polyacrylonitrile matrix.
Apart from the huge number of teachings flowing from the prior arts revealing
various composites towards the treatment of arsenic contaminated water to lower
the maximum contaminant level, the advent of laterite soil to aid such ground water
treatment for arsenic removal is remarkable because of its availability from natural
sources.
Natural laterite has been used for arsenic removal by various researchers (Vithange,
M.; Senevirathna, W.; Chandrajith, R.; Weerasooriya, R., Sci. Total Environ. 379
(2007) 244-248 and often they have used different synonyms of laterite such as red
earth, ferralite, laterite iron concretions, etc. These laterites are products of intense
sub-aerial weathering with large iron and/ or aluminium content. Silicon content is
lower than that in kaolinised parent rocks. Laterite mainly consists of mineral
assemblages of goethite, hematite, gibbsite, kaolinite mineral and quartz
(Schellmann, W. Geology Survey of India, 120, 1986, 1-7) and its composition
varies from one location to another. Arsenic adsorption capacity of different natural
laterite is found to be lesser compared to many commercial adsorbent because of its

low porosity as well as low specific: surface area. In addition to this, mineral phases
of Fe and Al compound are mainly in the form of oxide that are less effective in
adsorbing the arsenic species compared to Fe/ Al hydroxide or oxyhydroxide (S.
Goldberg et. al.; J. Colloid Interface Sci. 234, 2001, 204-216).
Thus, modification of laterite is very much needed such that the specific surface
area, porosity and conversion of (Fe and Al) oxide to hydroxyl mineral phases
increase drastically.
In connection to raw laterite, M. A. Halim et.al., in the J. of Applied Sciences 2008, 8
(20), 3757-3760, studied few laterite soil samples to monitor their efficiency in
removing arsenic from contaminated water by adsorption filtration method and found
that at room temperature which is the optimum temperature for these soil samples,
three different soil samples adsorbs 1010, 925, 932.5 mg/ Kg of arsenic and their
removal efficiency was found to be 58.74, 65.32 and 65.39 % respectively. Above
room temperature the efficiency of these samples gradually decreased.
F. Partey et. al. in J. of Colloid and Interface Science, 321, (2008), 493-500
investigated arsenate, arsenite absorption onto laterite iron concretions to test its
suitability for use in the low tech treatment of arsenic bearing drinking water and
showed that the sorption of arsenite and arsenate increased with temperature. The
equilibrium sorption capacity for arsenite was larger than that for arsenate over the
temperature range of 25-60 °C and arsenite sorption increases with increasing
solution pH to a maximum at pH 7 and then decreases with further increase in
solution pH whereas arsenate sorption shows little change with increasing solution
pH.
T. Pal et. al. in J. Environ. Sci. Health A Tox Hazard Subst. Environ. Eng. 2007 Mar;
42(4):453-62 is directed to arsenic removal from contaminated ground water by
adsorption on laterite soil. Effects of pH, adsorbent dose, adsorbent size, contact
time, initial arsenic concentration and presence of interfering species on arsenic
removal lead to the finding that 4 hr contact time was sufficient for approximately 98
% and approximately 95 % removal from the contaminated water samples at an
adsorbent dose of 10 g/ L and 20 g/ L for As(III) and As(V) respectively at an initial
concentration level of 0.5 mg/ L at a pH of 5.7 +/- 0.2 and neither did pH of the raw
water change after arsenic removal,nor did iron leach out in water. Although there

was no significant interference from Cl(-), NO3(-), SO4(-2), Ca(2+) and Fe (2+/3+)
on arsenite/ arsenate removal but its removal was little affected due to the presence
of HPO4(-2) and SiO3(-2). Arsenate removal efficiency, however, was decreased to a
large extent in presence of HPO4 (-2) and SiO3 (-2). Both Langmuir and Freundlich
adsorption isotherm models fitted well and the maximum adsorption capacity was
estimated to be 1.384 mg/ g and 0.04 mg/ g for arsenite and arsenate respectively.
A. Maiti et al., in Separation and Purification Technology 2007, 55, 350-359,
demonstrated arsenite adsorption on natural laterite (NL) in neutral pH range. Same
authors in Industrial and Engineering chemistry Research 2008, 47(5), 1620-1629
demonstrated arsenate adsorption on same material and found that the maximum
adsorption of arsenate and arsenite on NL had the values of 0.54 and 0.26 mg/ g
respectively.
It is clearly apparent form the above existing state of the art that such above said
processes for the removal of arsenic with the help of raw laterite may be
summarized to suffer from either one or many of the drawbacks which include:
a) low adsorption capacity due to low surface area, low porosity, and presence of
oxide mineral phase of Fe and All and thus less efficient
b) few of the above said methods on laterite soils are applicable on altering water pH
which is not the pH of the actual contaminated ground water
c) some raw laterites are applicable only on varying the temperature of water other
than room temperaure
d) efficiency of some arsenic adsorbents are effected by the presence of other
interfering ions in ground water
e) some are only effective with longer water contact times
In light of the above said there is a continuous need in the art to develop or modify
the widely available natural or raw laterite such that the specific surface area,
porosity and conversion of oxide to hydroxyl mineral (Fe and Al) phases increase
drastically to promote better arsenic adsorption. In addition to this the method
employed for such modification should be simpler than the preparation of other
standard known adsorbents which would help in the development of a low cost
device for the treatment of Arsenic contaminated water where the arsenic adsorbent

can be used as a low cost column filter for community use. This would in effect
circumvent all the aforementioned difficulties associated with such raw laterites.
More particularly, it is clearly apparent from the above discussions that there is a
strong need in the art to provide for a low cost arsenic adsorbent that would
effectively remove arsenic from water in higher capacity without the need for altering
the pH and temperature of water. The low cost arsenic adsorbent would thus be an
effective, economic and an environment friendly substitute for the costly activated
alumina in use globally for adsorbing arsenic that can be used in household
applications and would help serve community purposes.
OBJECTS OF THE INVENTION
It is thus the basic object of the present invention to provide for a modified laterite
composition that would help in effective removal of arsenic species in the form of
arsenate and arsenite in higher capacity.
Another object of the present invention is to provide for a modified laterite
composition with desired surface properties that would lead to higher adsorption
capacity at lower equilibrium arsenic concentration in aqueous medium than other
well known arsenic adsorbents in the art.
Yet another object of the present invention is to provide for a modified laterite
arsenic adsorbent with desired surface properties that would lead to the abovesaid
desired adsorption characteristics.
Still another object of the present invention is to provide for a clean, economic and
environment friendly process for the manufacture of the said modified laterite
arsenic adsorbent by an optimized acid-alkali treatment method in an industrial
scale.
Another object of the present invention is directed to the manufacture of the
abovesaid modified laterite arsenic adsorbent involving the application of

inexpensive, easily available, acids and alkali on inexpensive and easily available
natural raw laterite.
Yet further object of the present invention is to provide for a method for removing
arsenic species in the form of both arsenate and arsenite from an aqueous medium
by using the abovesaid modified laterite arsenic adsorbent that would effectively
remove arsenic from the aqueous medium without the need for altering the pH and
temperature of the aqueous medium.
Yet further object of the present invention is to provide for a method for removing
the said arsenic species from an aqueous medium by using the abovesaid modified
laterite arsenic adsorbent that would be effective in removal of the arsenic species in
high capacity even in the presence of other interfering ions such as silicate,
phosphate etc. present in ground water.
Yet another object of the present invention is to provide for a method for removing
the said arsenic species from an aqueous medium by the abovesaid modified laterite
arsenic adsorbent which would not require a longer contact time between the
aqueous medium to be purified with the modified laterite.
Yet another object of the present invention is to provide for an efficient method for
removing the said arsenic species from the aqueous medium that would yield pure
drinking water as per USPEA standard.
Still another object of the present invention is to provide for a low cost column filter
for community use.
A further object of the present invention is directed to replace the globally used
costly activated alumina for the removal of arsenic from groundwater by the above
said cost-effective modified laterite arsenic adsorbent.
SUMMARY OF THE INVENTION
Thus according to the basic aspect of the invention there is provided a modified
laterite arsenic adsorbent for removing arsenic species in the form of arsenate and

arsenite from an aqueous medium comprising of surface properties: Surface area,
(m2/g) = 172.9 - 189.8; Total Pore volume (BJH method: ml/g) = 0.345 - 0.362 and
Surface charge (at pH: 7.0 and 0.01 mol/l NaCI) (Coulomb/g) = (+) (2.98 - 3.12)
In another aspect of the present invention there is provided a modified laterite
arsenic adsorbent for removing arsenic species in the form of arsenate and arsenite
from an aqueous medium comprising of further surface properties: Particle size
(spherical), mm = 0.3-0.6; Mesopore volume (ml/g) = 0.213 - 0.220; Micropore
volume (HK method: ml/g) = 0.145 -0.149; Bulk density, (g/ml) = 1.02 - 0.98;
True Density, (g/ml ) = 1.93 - 1.89; Conductivity (1:5, laterite : water mixture),
μS/cm = 29.5; pH (1:5, laterite: water mixture = 7.25 and pHZPC = 7.40.
It is thus a surprising and selective finding of the present invention that natural or
raw laterite when treated with an optimum amount of acid and alkali leads to a
modified laterite composite with enhanced surface properties to act as an effective
arsenic adsorbent with superior adsorption characteristics.
Significantly, there is provided a modified laterite arsenic adsorbent for removing
arsenic species in the form of arsenate and arsenite from an aqueous medium
wherein the adsorption capacity of the said arsenic adsorbent is 21.0 ± 0.03 mg/ g
for arsenate [As(V)] and 8.7 ± 0.03 mg/ g for arsenite [As(III)].
According to a preferred aspect of the invention there is provided a modified laterite
arsenic adsorbent adapted for column filter.
In another aspect of the invention a process for the manufacture of modified laterite
arsenic adsorbent is provided comprising the steps of (a) treating the natural laterite
with a selective amount of acid and alkali; and (b) obtaining therefrom the said
modified laterite of desired adsorption characteristics.
Preferably, the abovesaid step of treating the natural laterite with an optimum
amount of acid and alkali comprises (i) 6M HCI treatment (ii) recovery of ~70-75%
HCl by distillation and (iii) neutralization of acid treated natural laterite using 2M
NaOH to reach a pH value of 6.5 ± 0.02 (iv) filtration to thereby obtain the said
modified laterite of desired adsorption characteristics.

In yet another aspect of the invention there is provided a method for removing
arsenic species in the form of arsenate and arsenite from an aqueous medium
involving the said arsenic adsorbent comprising the steps of:
providing an aqueous medium containing arsenate and arsenite;
contacting the aqueous medium with modified laterite such that arsenate and
arsenite contained in the aqueous medium are adsorbed onto the modified laterite;
and
separating the arsenic adsorbed modified laterite from the aqueous medium.
In accordance with a preferred aspect of the invention there is provided a method for
removing arsenic species in the form of both arsenate and arsenite from an aqueous
medium wherein the aqueous medium has pH ranging from 6.7 to 8.5.
In accordance with yet another preferred aspect of the invention a method for
removing arsenic species in the form of both arsenate and arsenite from an aqueous
medium is provided wherein the said arsenic species are effectively removed from
said aqueous medium in the presence of other interfering ions present in
groundwater such as silicate, phosphate etc.
Significantly, a method for removing arsenic species in the form of both arsenate and
arsenite from an aqueous medium is provided wherein the temperature of the
aqueous medium is preferably room temperature such as 32±2 °C.
More significantly, a method for removing arsenate and arsenite from an aqueous
medium is provided wherein the said arsenic species are effectively removed from
said aqueous medium to achieve an arsenic level of 10 parts per billion and below.
Also, a method for removing arsenate and arsenite from an aqueous medium is
provided wherein the aqueous medium is sourced preferably from underground.
The details of the invention, its objects and advantages are explained hereunder in
greater detail in relation to non-limiting exemplary illustrations as per the following
exemplary illustrations:

BRIEF DESCRIPTION OF THE FIGURES
The foregoing aspects and many of the attendant advantages of this invention will
become more readily appreciated as the same becomes better understood by
reference to the following detailed description, when taken in conjunction with the
accompanying drawings, wherein:
FIG. 1: Graph displaying a breakthrough effluent arsenic concentration of an
aqueous medium sourced from underground after passing over a fixed bed column
packed with modified laterite prepared in accordance with the embodiment of the
present invention where the influent groundwater is highly alkaline and contains
silicate in high amount and phosphate in moderate amounts distributed between
soluble and particulate form.
FIG. 2: FTIR spectra of Natural Laterite and Modified Laterite illustrating the effect of
acid treatment under different HCL concentrations. For the present invention
optimized HCI concentration of 6N is used. (IN HCI = 1 molar HCI)
FIG. 3: TG-DTA curves of natural laterite and modified laterite revealing the
presence of bound water and OH groups in the modified laterite prepared in
accordance with the embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
As already disclosed herein before, the present invention comprises of a low cost
modified/ treated laterite composition with high arsenic adsorption capacity for
removing arsenic contamination from ground water. The modified/ treated laterite
composition effectively adsorbs arsenic even at low concentrations of arsenic in
ground water without the need to alter the pH and temperature of ground water and
remains effective in the presence of other interfering ions in ground water.
Example I: Identification of critical parameters during the process of
modification of raw natural laterite
The low cost high capacity modified laterite arsenic adsorbent is obtained from
natural laterite (NL) by chemical treatment method comprising of selective acid-alkali
treatment.

The above said modification method comprises of three main steps:
a) an appropriate acid (HCI) treatment of natural laterite (NL);
b) a recovery of 70-75% HCI by distillation;
c) neutralization of acid treated NL by using NaOH;
d) obtaining the said modified laterite (ML) by filtration.
Different parameters like acid concentration, acid solid (NL) ratio, acid treatment
duration, final pH of the neutralization etc. are varied to identify the critical
parameters of the modification procedure.
First acid solid (NL) ratio is varied using 4N HCI acid medium in acid treatment step
and liquid phase iron and aluminium concentration per gm of NL is measured against
time for each case. It has been found that 50 g solid to 200 ml acid solution is most
effective solid liquid ratio. Acid concentration was varied from 0.5 to 6.0 molar in the
acid treatment step. The arsenic [As(V) and As(III)] adsorption capacity on ML was
seen to increase almost linearly as acid concentration in the acid treatment step
increased from 0.5 to 6.0 molar during synthesis of ML. This is because with increase
in acid concentration the amount of Fe and Al dissolution in liquid phase increased.
So, more amounts of Fe/AI hydroxide / oxyhydroxide are produced during
neutralization step and these compounds are found to be responsible to adsorp
arsenic ions from water. Further increase in HCI concentration beyond 6N was
avoided due to operational problems since beyond 6N HCI concentration, HCI
evaporates at < 70 °C under atmospheric pressure, where as acid treatment step in
this study is performed at ~70 °C). This acid treatment step is operated under
atmospheric pressure to keep in mind that process should be cost effective and
simple in operation. All other parameters are optimized based on arsenic adsorption
capacity of modified laterite (prepared under particular set of parameters) in batch
mode. The batch mode arsenic adsorption performance of ML (obtained under
varying process parameters) is summarized in Table 1 below.
Besides HCI, other acids like H2SO4 and HNO3 in equal concentrations are
used in this study (other operational parameters are kept in constant). But, ML
obtained using the above-said two other acids other than HCI are found to be less
effective in adsorbing arsenic. On the other hand, boiling point of H2SO4 is very high.
So, recovery of H2SO4 is not possible at atmospheric pressure like HCI.

Table 1: Comparison of arsenic adsorption capacity on ML (obtained under
varying process conditions while identifying the critical parameters)

Example II: Typical modification procedure of raw natural laterite including
the abovesaid identified critical parameters
After identifying the critical parameters as illustrated in Table 1, resulting in high
arsenic adsorption characteristics of modified laterite, a typical modification
procedure of NL was followed wherein, 50±2 gm of 0.3-0.5 mm particle size of
natural laterite and 200 ml of ~6.0 normal HCI solution are taken in a 500 ml glass
beaker and was placed in a temperature controlled water bath at a temperature 70 ±
2 °C. A stirrer assembly made of polypropylene attached with 1 HP motor is used for
stirring at 400 ± 25 rpm for 3.0 hours. The treated mass is placed in a 1 liter round
bottomed flask. The flask is placed on a laboratory mantle heater and connected to a
water-cooled condenser to recover (~160 ml of ~4.5 ± 0.3 normal) excess HCI acid.
The solid liquid mixture is heated during distillation at a temperature range of 104-
110 °C. The distillate of acid-water mixture is collected in a receiving flask. This acid
recovery step makes this process economical in two ways, firstly, consumption of

NaOH in the neutralization step decreases by 55-60 % and secondly, recovered acid
was mixed appropriately with concentrated HCI (12 normal available in market) to
produce 6.0 normal HCI solution that was used in different batches. The residual
solid-liquid mixture in the round bottomed flask contains dissolved iron and
aluminium ions (total Fe content: ~0.14 mole and Al content: ~0.03 mole). This
mixture is then placed in a 1 liter glass beaker containing 150 ml water and 200 + 20
ml of 2.0 ± 0.2 normal NaOH (containing 18 ± 1 gm of NaOH) solution is added
slowly with an addition rate of 10 ml per min under high stirring speed of 500 rpm to
precipitate dissolved iron and aluminium ions as hydroxide on solid laterite surface to
increase its activity. The neutralization is carried until a pH range of 6.5 + 0.2 is
reached. The neutralized mass is left overnight to result in a top clear water portion
that is decanted off carefully. The solids are then filtered in a flat ceramic filter and
washed with 500 to 800 ml tap water until no chloride ton is present in modified
laterite. Finally, 50 ± 2 gm of modified laterite is obtained after drying.
Example HI: Method for removing arsenic species in the form of arsenate
and arsenite from an aqueous medium involving the modified laterite
arsenic adsorbent
Fix bed column runs using arsenic contaminated groundwater on ML are presented in
Figure 1. Figure 1 shows that, at 1050 and 5000 bed volume (Column length: 4.0
cm, Diameter of column: 2.54 cm, empty bed contact time (EBCT): ~2 minutes,
flow: 11.0 ml/ min), the effluent water is obtained at breakthrough arsenic
concentration of 10 μg/ I (USEPA standard) and 50 μg/ I (BIS standard), respectively.
The total arsenic concentration in groundwater is ~385 μq/ I [As(V): 175 μg/ I and
As(III): 210 μg/ I]. The collected water is highly alkaline (Total alkalinity: ~575 mg/
I) and highly contaminated with silicate (22 mg/ I). When column length increases to
6.5 cm from 4.0 cm (EBCT increases from 2.0 to 3.3 minutes) ~ 4000 bed volume
(i.e., 32-34 gms of ML has purified almost 120 liters of water), the effluent water is
obtained at breakthrough arsenic concentration of 10 μg/ I using same contaminated
water. So, in spite of high alkalinity, presence of high silicate and moderate
phosphate (0.7 mg/l) concentration in arsenic contaminated groundwater the ML is
capable of removing >99 % arsenic in column mode operation (EBCT: ~ 2 minutes).

The maximum arsenic (III)/ arsenite and arsenic (V)/arsenate adsorption capacity of
the modified laterite of this invention is 8.7 and 21.05 mg/ gm as compared to 0.26
and 0.54 mg/ gm capacity of natural laterite.
The main difference in characteristic features between natural laterite (NL) and
modified laterite (ML) are BET surface area, total pore volume (mesoporous and
microporous volume), bulk density, surface charge (coulomb/g) at pH around 7,
mineral phases, etc. The detailed characteristics of both form of laterite are
presented in Table 2 below.
BET surface area and porosity of any adsorbent are the most important
properties. It is interesting to note that BET surface area and total porosity of ML
(modified laterite) have been drastically improved about 10 folds (from 18.5 tol89.8
m2/ g) and about 16 folds (from 0.02 to 0.35 ml/ g) compared to RL (raw laterite),
respectively. The surface charge at neural pH of ML-water mixture is found to be ~
+3.2 coulomb/ g, whereas this value for RL is only ~ +0.4 coulomb/ g. Higher
positive surface charge on ML compared to RL at neutral pH (~7.0) results in high
adsorption of negative ions like As(V). The BJH pore width of ML is obtained in the
range of 1 to 10 nm (Highest peak at 5.2 nm), which lies in the mesoporous (2 to 20
nm) region. Therefore, ML is more efficient arsenic adsorbent compared to almost
nonporous NL (natural laterite). From the below mentioned Table 2, it is clear that
there is no significant change in elementary metallic composition in ML.


FTIR and TG-DTA studies in Fig. 2 and Fig. 3 reveal that the mineral phases
responsible for arsenic adsorption are drastically different between ML and NL.
FTIR spectra of various form of modified laterite developed using different HCI
concentration in the acid treatment step is shown in Figure 2. The peaks at about
3390-3790 cm"1, are due to the hydroxyl group attachment to Fe and At. From
Figure 2, it is evident that peaks at the above said region are negligible in intensity
for RL compared to ML. It is also noted that intensity of the peaks in above said
region are intensified with increase in acid concentration (HCI) from 2 to 6 molar in
acid treatment step. This reveals high concentration of iron and aluminium
hydroxide/ oxyhydroxide formation in ML. Further increase in HCI concentration

beyond 6M is avoided due to operational problems since beyond 6 molar
concentration, HCI evaporates at <70 °C under atmospheric pressure, whereas the
acid treatment step to manufacture the modified laterite is performed at a
temperature of 70 °C. FTIR peaks at 913 and 687 cm"1 are attributed to the AI-0
bond stretching. These peaks are highly intensified for ML compared to RL. Similar
trends for Fe-0 bond stretching (at 536 and 463 cm"1) are also observed in the FTIR
spectra of ML.
The enrichment of hydroxyl attachment to Al and Fe into ML compared to RL is
further analyzed by TG-DTA analysis (Refer Figure 3). From Figure 3, it is clear that
amount of total weight loss (TG analysis) for ML and NL are ~19 and 9 %,
respectively. The higher magnitude of weight loss for ML is due to higher amount of
water loss. This water loss is associated with removal of bound water as well as
dehydration of hydroxyl group (conversion from oxyhydroxide to oxide). DTA curve
of RL shows two weak endothermic effects (water loss) at 320 and 480 °C
temperature region. Peak at 320 °C seems to be a complex in nature (not sharp),
which indicates existence of both gibbsite and goethite. The peak at 480 °C
attributes to water loss for dehydration of hydroxyl group from Kaolinite. For, ML the
main features on heat effects as determined from the TG-DTA curve is as follows:
(i) ~4.0 % of weight loss up to 220 °C indicates loss of unbound water
molecules,
(ii) A strong endothermic effect within the temperature range 260-320 °C is
observed. This effect attributes to the dehydration of goethite (a-FeOOH).
So, the amount of hydroxyl groups in ML is much more compared to RL.
(iii) Second strong endothermic effect within the temperature range of 450 to
520 °C reveals dehydration of boehamite (Al-oxy/ hydroxide) into Al-
oxide.
It is a well documented fact that arsenic species [As (V) and As (III)] are adsorbed
on OH bonds of hydrous Fe and Al compounds. Therefore, the hydroxyl groups
responsible for arsenic adsorption are much more in ML compared to NL and thus the
capacity of the developed low cost modified/ treated laterite composite material of
the present invention is about 2 to 20 times more effective as compared to many
well known arsenic adsorbents like activated alumina, activated carbon, bauxite,
modified bauxite, granular ferric hydroxide, iron oxide coated sand etc. and 35 to 40
times more as compared to raw natural laterite.

The arsenic adsorption capacity of modified laterite of the present invention in batch
mode is compared with other adsorbents studied by various researchers and the
performance of various such adsorbents alongwith the prior arts are tabulated in
Table 3.
Table 3: Arsenic adsorption capacity of various adsorbents


List of References (prior art) compared in Table 3:
1. S. A. Wasay et. al., Separation Science and Technology 1996, 31, 1501.
2. H. Genc-Fuhrman et. al., in J. colloid and Interface Science 2004, 271(2),
313-320.
3. Z. Gu et. al., in Environmental Science and Technology 2005, 39(10), 3833-
3843.
4. S. Kundu et. al., in Chemical Engineering Journal 2006, 122(1-2), 93-106.
5. Y. -H. Xu et. al., in J. Hazardous Materials 2002, 92, 275-287.
6. B. E. Reed et. al., in J. Environmental Engineering 2000, 126, 869-873.
7. P. B. Bhakat et. al., in Colloids Surfaces A: Physicochemical and Engineering
Aspects 2006, 281(1-3), 237-245.
8. A. Maiti et al., in Separation and Purification Technology 2007, 55, 350-359
and A. Maiti et al., in Industrial and Engineering Chemistry Research 2008,
47(5), 1620-1629.
9. S. K. Maji et. al., in J. of Environ. Sci. and Health Part A, 42 (2007) 453-462
and S. K. Maji et. al., in J. Surface Sci. Technol. 22 (2007) 161-176.
The adsorbents stated in Table 3 include activated carbon, activated alumina and
natural Fe / Al containing materials. With Fe-compounds the modification of base
material (carbon / alumina/ natural Fe / Al containing materials) resulted in the final
adsorbent material comprising of oxide form of iron either in coated or impregnated
form. However, hydroxide / hydrous was found to be more effective towards arsenic
adsorption compared to the oxide form. In most of the prior arts synthetic arsenate
solution was used to test the arsenic adsorption capacity with the fate of arsenite
remaining unexplored that reflects their obvious limitations. On the other hand,
natural Fe / Al containing materials revealed very low adsorption capacity as
compared to any commercial adsorbent which is thought to be due to low surface
area and low porosity of such natural materials. Further, iron coated natural material
also showed low porosity and low surface area (some time even lower than natural
material itself). Therefore, the modification methods of different natural materials of
the stated prior arts could not improve upon the porosity or specific surface area of
such natural materials but could only improve upon the surface functional groups. In
case of the present invention, we could substantially overcome the abovesaid
drawbacks en route modification process of the natural laterite to produce a high

capacity and cost-effective modified laterite arsenic adsorbent to remove the arsenic
species from real and contaminated aqueous medium preferably from underground.
The low cost modified/ treated laterite composition is effective in adsorbing both the
As(V) and As(III) ions and maximum adsorption of the arsenic species is achieved in
the pH range 6.5-8.0 and is also equally effective in the normal pH range of 6.7-8.5
of groundwater and at room temperature.
Thus the low cost high capacity arsenic adsorbent comprising modified/ treated
laterite composition would be effective in easing out the monopoly of all the existing
arsenic adsorbents that are imported for e.g. activated alumina and even costlier
granular ferric hydroxide.
The above examples are given by way of illustration of the present invention and
therefore should not be construed to limit the scope of the present invention. All
percentages are by weight.
The above exemplary illustration of the process of the invention clearly reveals the
composition of the high capacity modified laterite arsenic adsorbent and is directed
to a method for the purification of aqueous medium involving the above said
modified laterite.
The high capacity arsenic adsorbent of the invention is 2 to 20 times more effective
as compared to the well known arsenic adsorbents like activated alumina, activated
carbon, bauxite, modified bauxite, granular ferric hydroxide, iron oxide coated sand
etc. and 30 to 40 times more effective as compared to raw natural laterite.
The process for the manufacture of such modified laterite composition by a selective
acid-alkali treatment method is clean, economic and can be made environment
friendly and industrially viable by installing an absorption unit for the exhaust HCI
gas and a neutralization tank prior to the disposal of the waste water.
Additionally, a method for removing arsenic species in the form of arsenate and
arsenite from an aqueous medium involving the said modified laterite arsenic

adsorbent is thus efficient, economic and environment friendly over ail the known
arsenic adsorbents till date.
It is thus possible by way of the present invention to provide for a modified laterite
arsenic adsorbent of high capacity and a method for removing arsenic species in the
form of arsenate and arsenite from an aqueous medium involving the said arsenic
adsorbent from arsenic contaminated water specially sourced from underground.
Also, it is directed to a process for the manufacture of modified laterite arsenic
adsorbent starting from raw natural laterite with the desired adsorption
characteristics as discussed above and in accordance with the embodiment of the
present invention.

We Claim:
1. A modified laterite arsenic adsorbent for removing arsenic species in the form of
arsenate and arsenite from an aqueous medium comprising of surface properties:
Surface area, (m2/g) = 172.9 - 189.8; Total Pore volume (BJH method: ml/g) =
0.345 - 0.362 and Surface charge (at pH: 7.0 and 0.01 mol/l NaCI) (Coulomb/g) =
(+) (2.98- 3.12)
2. A modified laterite arsenic adsorbent for removing arsenic species in the form of
arsenate and arsenite from an aqueous medium as claimed in claim 1 comprising of
surface properties: Particle size (spherical), mm = 0.3-0.6; Mesopore volume (ml/g)
= 0.213 - 0.220; Micropore volume (HK method: ml/g) = 0.145 -0.149; Bulk
density, (g/ml) = 1.02 - 0.98; True Density, (g/ml ) = 1.93 - 1.89; Conductivity
(1:5, laterite : water mixture), μS/cm = 29.5; pH (1:5, laterite: water mixture =
7.25 and pHZPC = 7.40.
3. A modified laterite arsenic adsorbent for removing arsenic species in the form of
arsenate and arsenite from an aqueous medium as claimed in anyone of the
preceding claims wherein the adsorption capacity of the said arsenic adsorbent is
21.0 ± 0.03 mg/ g for arsenate [As(V)] and 8.7 ± 0.03 mg/ g for arsenite [As(III)].
4. A modified laterite arsenic adsorbent as claimed in anyone of the preceding claims
adapted for column filter.
5. A process for the manufacture of modified laterite arsenic adsorbent as claimed in
anyone of the preceding claims comprising the steps of (a) treating the natural
laterite with selective amount of acid and alkali; and (b) obtaining therefrom the said
modified laterite of desired adsorption characteristics.

6. A process as claimed in claim 5 wherein the said step of treating the natural
laterite with a selective amount of acid and alkali comprises (i) 6M HCI treatment (ii)
recovery of ~70-75% HCI by distillation and (iii) neutralization of acid treated natural
laterite using 2M NaOH to reach a pH value of 6.5 ± 0.02 (iv) filtration to thereby
obtain the said modified laterite of desired adsorption characteristics.
7. A method for removing arsenic species in the form of arsenate and arsenite from
an aqueous medium involving the said arsenic adsorbent as claimed in anyone of the
claims 1 to 4 by the steps comprising of:
providing an aqueous medium containing arsenate and arsenite;
contacting the aqueous medium with modified laterite such that arsenate and
arsenite contained in the aqueous medium are adsorbed onto the modified laterite;
and
separating the arsenic adsorbed modified laterite from the aqueous medium.
8. A method for removing arsenic species in the form of both arsenate and arsenite
from an aqueous medium as claimed in claim 7, wherein the aqueous medium has
pH ranging from 6.7 to 8.5.
9. A method for removing arsenic species in the form of both arsenate and arsenite
from an aqueous medium, as claimed in anyone of claims 7 or 8, wherein the said
arsenic species are effectively removed from said aqueous medium in the presence
of other interfering ions present in groundwater such as silicate, phosphate etc.
10. A method for removing arsenic species in the form of both arsenate and arsenite
from an aqueous medium, as claimed in anyone of claims 7 to 9, wherein the
temperature of the aqueous medium is preferably room temperature such as 32±2
°C.

11. A method for removing arsenate and arsenite from an aqueous medium as
claimed in claims 7 to 10, wherein said arsenic species are effectively removed from
said aqueous medium to achieve an arsenic level of 10 parts per billion and below.
12. A method for removing arsenate and arsenite from an aqueous medium as
claimed in claims 7 to 11, wherein the aqueous medium is sourced preferably from
underground.
13. A modified laterite arsenic adsorbent for removing arsenic species in the form of
arsenate and arsenite from an aqueous medium, a process for the manufacture of
such an adsorbent and a method for removing arsenic species in the form of
arsenate and arsenite from an aqueous medium substantially as herein described
and illustrated with reference to the accompanying examples.

Arsenic adsorbent and in particular a modified laterite arsenic adsorbent with
superior surface properties for removing arsenic species in the form of arsenate and
arsenite in high capacity from an aqueous medium, a process for manufacturing the
same and a method for removing the said arsenic species cost-effectively from the
said aqueous medium.