27 April 2010
NewScientist.com
Magazine issue 2757
Cactus
gum could make clean water cheap for millions
by Helen Knight
FORGET expensive machinery, the best way to purify water could
be hiding in a cactus. It turns out that an extract from the
prickly pear cactus is effective at removing sediment and
bacteria from dirty water.
Many water purification methods introduced into the developing
world are quickly abandoned as people don't know how to use and
maintain them, says Norma Alcantar at the University of South
Florida in Tampa. So she and her colleagues decided to
investigate the prickly pear cactus, Opuntia ficus-indica, which
19th-century Mexican communities used as a water purifier. The
cactus is found across the globe.
The team extracted the cactus's mucilage - the thick gum the
plant uses to store water. They then mixed this with water to
which they had added high levels of either sediment or the
bacterium Bacillus cereus.
Alcantar found that the mucilage acted as a flocculant, causing
the sediment particles to join together and settle to the bottom
of the water samples. The gum also caused the bacteria to
combine and settle, allowing 98 per cent of bacteria to be
filtered from the water (Environmental Science and Technology,
DOI: 10.1021/es9030744). They now intend to test it on natural
water.
Householders in the developing world could boil a slice of
cactus to release the mucilage and add it to water in need of
purification, says Alcantar. "The cactus's prevalence,
affordability and cultural acceptance make it an attractive
natural material for water purification technologies."
But Colin Horwitz of GreenOx Catalysts in Pittsburgh,
Pennsylvania, says many issues remain, including how much land
and water is needed to grow cacti for widespread water
purification, and how households will know all the bacteria have
been removed.
Water
purification method using plant molecules
US7943049
Arsenic is a poisonous metalloid which, because of its
hydroscopic nature, is primarily transported through water. Most
plant species, including the nopal cactus, produce a sticky
substance called mucilage. Mucilage swells in water but is
insoluble and can precipitate ions, bacteria and particles from
aqueous solutions. The invention includes a method of separating
particulates and heavy metals such as arsenic (As) from drinking
water using natural flocculants obtained from cactus mucilage.
The extraction techniques and the methodology for using the
cactus mucilage obtain higher As removal than conventional
methods, like aluminum sulfate precipitation.
FIELD OF
INVENTION
This invention relates to field water purification.
Specifically, a water purification method using plant mucilage.
BACKGROUND
OF THE INVENTION
Arsenic is a metalloid with similar properties to phosphorus.
Arsenic oxidizes to form hygroscopic, colorless, odorless As2O3
and As2O5. The principal means of arsenic dispersion through
nature is via water, and varies from locations based on soil and
arsenic forms.
Arsenic has been attributed to changes in respiratory,
gastrointestinal, hematopoietic, and cardiovascular systems.
Because of the similarities between arsenic and phosphorus,
arsenic can substitute in place of phosphorus in some biological
reactions, making it poisonous. Particularly, consumption of
arsenic-contaminated water may enter the metabolic citric cycle,
inhibiting succinate dehydrogenase and preventing ATP
production. Arsenic poisoning is cumulative and symptoms include
nausea, vomiting, stomach aches, diarrhea, and delirium.
Ingested arsenic is deposited into fingernails and skin Further,
arsenic can remain in hair follicles for years following the
arsenic exposure.
Bangladesh, India, and Nepal have experienced a massive epidemic
from arsenic groundwater contamination. 35 million people are
believed to be consuming water with at least 50 [mu]g/L, and 57
million people drinking water with at least 10 [mu]g/L of
arsenic. Nongovernmental organizations entered the region and
established tube wells to collect groundwater and prevent the
indigenous populations from using bacteria-contaminated surface
water. Over 8 million wells were built since the program began
in the 1970s. Roughly one quarter of Bangladesh's population now
rely on water collected from tube-wells for drinking. However,
testing has revealed one in five of the tube wells are
contaminated by water containing ten to fifty times the arsenic
levels considered safe by the World Health Organization.
Most plant species produce an exopolysaccharide, a polymer of
mono- and polysaccharides and proteins bonded by glycosidic
bonds, referred to as mucilage. Plants secrete the substance to
slow water loss, aid germination, and store food.
The tuna cactus (Opuntia ficus indica) mucilage produced
by the flattened pads of this cactus was of particular interest.
It can easily be recognized by its green, thick long pads, one
linked to the next. The nopal plants are very inexpensive to
cultivate and edible. Nopal pads are formed of complex
carbohydrates that have the ability to store and retain water,
allowing these plants to survive in extremely arid environments.
Nopal mucilage is a neutral mixture of approximately 55
high-molecular weight sugar residues composed basically of
arabinose, galactose, rhamnose, xylose, and galacturonic acid
and has the capacity to interact with metals, cations and
biological substances.
Mucilage is used in producing agar and used as an adhesive
Importantly, mucilage swells in water but is insoluble. As such,
the substance has the potential to precipitate ions, bacteria
and particles from aqueous solutions. Further, the material has
unique surface active characteristics, making it an ideal
candidate for enhancing dispersion properties, creating
emulsifications, and reducing surface tension of high polarity
liquids.
SUMMARY OF
INVENTION
The invention includes a method of separating particulates and
heavy metals such as arsenic (As) from drinking water using
natural flocculants obtained from cactus (i.e. cactus mucilage).
The extraction techniques and the methodology for using the
cactus mucilage obtain higher As removal than previous methods.
The use of low cost flocculants can be implemented in low income
communities or third world countries with drinking water
deficiency.
A gelling extract (GE), a nongelling extract (NE), and a
combined extract (CE) of mucilage from the cactus were collected
and used individually as flocculent to remove contaminants that
reduce water potability.
Cylinder tests using kaolin slurry show mucilage is a better
flocculent of suspended solids than Al2(SO4)3. The same dosage
of mucilage precipitates the same amount of particulate, in one
third the time, as does Al2(SO4)3. Additionally, small doses of
mucilage provided fast settling rates and clear supernatant.
The effective concentration of gelling extract mucilage was
found to be 4 mg/L. This concentration precipitated most of the
slurry within 10 minutes, twice as fast as the next quickest
concentration, 3 mg/l; showing the gelling extract was most
effective at higher concentrations. The non-gelling mucilage
extract was less affected by concentration.
Flocculation studies using the standard jar test and kaolin
slurry solutions were performed on the three extracts. At lower
concentrations, the combined mucilage extract mirrors the
residual turbidity characteristics of aluminum sulfate, where
higher concentrations of aluminum sulfate are more effective at
reducing the residual turbidity of the solution.
The capacity of the gelling mucilage extract to remove arsenic
from contaminated water at low concentration dosing was
determined by adding gelling mucilage extract to a contaminated
water column. The top layer of the water column was removed at
set intervals. The mucilage facilitates removal of arsenic by
transporting arsenic to the water-air interface.
BRIEF
DESCRIPTION OF THE DRAWINGS
For a fuller understanding of the invention, reference should be
made to the following detailed description, taken in connection
with the accompanying drawings, in which:
FIG. 1 is a graph comparing flocculation rates at 3 mg/L
flocculent dosages. A kaolin solution was used at a
concentration of 50 g/L, to mimic contaminated water containing
a high concentration of particles. The flocculation
characteristics of mucilage were tested with a total mucilage
extract (3 ppm), a gelling extract harvested with the mucilage
(3 ppm), or a non-gelling extract harvested with the mucilage (3
ppm). A commercial flocculent (3 ppm Al2(SO4)3) and negative
control without flocculent (control) were used to establish a
baseline and compare the efficiencies of the extracts. The
cylinder was capped and inverted 10 times to mix and the height
of the interface between the supernatant and settling solids was
measured. The gelling extract performed the best.
FIG. 2 is a graph showing increasing efficiency with increased
concentration of gelling extract. A 50 g/L of kaolin clay slurry
were placed into a 100 mL cylinder. Flocculent was added to the
slurry as either 0.01 mg/L of mucilage, 0.1 mg/L of mucilage,
0.5 mg/L of mucilage, 1.0 mg/L of mucilage, 2.0 mg/L of
mucilage, 3.0 mg/L of mucilage, 4.0 mg/L of mucilage, or a
negative control without flocculent. The cylinder was capped and
inverted 10 times to mix and placed on a horizontal surface. The
height of the interface between the supernatant and settling
solids was measured.
FIG. 3 is a graph and data illustrating the settling rates of
gelling extract with increasing dosage concentration. A 50 g/L
of kaolin clay slurry were placed into a 100 mL cylinder.
Flocculent was added to the slurry as either 0.01 mg/L of
gelling extract mucilage, 0.1 mg/L of gelling extract mucilage,
0.5 mg/L of gelling extract mucilage, 1.0 mg/L of gelling
extract mucilage, 2.0 mg/L of gelling extract mucilage, 3.0 mg/L
of gelling extract mucilage, 4.0 mg/L of gelling extract
mucilage, or a negative control without flocculent. The cylinder
was capped and inverted 10 times to mix and placed on a
horizontal surface. The height of the interface between the
supernatant and settling solids was measured and the rate of
sedimentation calculated.
FIG. 4 is a graph showing the increasing settling efficiency of
the non-gelling extract with increasing dosage concentrations. A
50 g/L of kaolin clay slurry were placed into a 100 mL cylinder.
Flocculent was added to the slurry as either 0.01 mg/L of
non-gelling extract mucilage, 0.1 mg/L of non-gelling extract
mucilage, 1.0 mg/L of non-gelling extract mucilage, 2.0 mg/L of
non-gelling extract mucilage, 3.0 mg/L of non-gelling extract
mucilage, 4.0 mg/L of non-gelling extract mucilage, or a
negative control without flocculent. The cylinder was capped and
inverted 10 times to mix and placed on a horizontal surface. The
height of the interface between the supernatant and settling
solids was measured.
FIG. 5 is a graph and data illustrating the settling rates of
non-gelling extract with increasing concentration. A 50 g/L of
kaolin clay slurry were placed into a 100 mL cylinder.
Flocculent was added to the slurry as either 0.01 mg/L of
non-gelling extract mucilage, 0.1 mg/L of non-gelling extract
mucilage, 1.0 mg/L of non-gelling extract mucilage, 2.0 mg/L of
non-gelling extract mucilage, 3.0 mg/L of non-gelling extract
mucilage, 4.0 mg/L of non-gelling extract mucilage, or a
negative control without flocculent. The cylinder was capped and
inverted 10 times to mix and placed on a horizontal surface. The
height of the interface between the supernatant and settling
solids was measured and the rate of sedimentation calculated.
FIG. 6 is a graph showing the efficiency of the combined extract
with increasing dosages. A 50 g/L of kaolin clay slurry were
placed into a 100 mL cylinder. Flocculent was added to the
slurry as either 0.01 ppm of non-gelling extract mucilage, 0.1
ppm of non-gelling extract mucilage, 1.0 ppm of non-gelling
extract mucilage, 2.0 ppm of non-gelling extract mucilage, 3.0
ppm of non-gelling extract mucilage, 4.0 ppm of non-gelling
extract mucilage, 5.0 ppm of non-gelling extract mucilage, or a
negative control without flocculent. The cylinder was capped and
inverted 10 times to mix and placed on a horizontal surface. The
height of the interface between the supernatant and settling
solids was measured and the rate of sedimentation calculated.
FIG. 7 is a graph showing the mucilage efficiency at reducing
residual turbidity at very low doses-comparable with aluminum
sulfate. Standard jar test for flocculent sedimentation. 0.5 g/L
kaolin clay slurry was added to a test jar. The solution was
stirred at 100 rpm and varying amounts of identified flocculent
were added. After 2 minutes, the speed was reduced to 20 rpm for
5 minutes, and mixing was stopped. The solution was allowed to
settle for 30 minutes, and turbidity tests were performed.
FIG. 8 is a graph showing the mucilage's departure from the
efficiency of aluminum sulfate at higher doses. However,
secondary filtration can be used to reduce the residual
turbidity. Standard jar test for flocculent sedimentation. 0.5
g/L kaolin clay slurry was added to a test jar. The solution was
stirred at 100 rpm and varying amounts of identified flocculent
were added. After 2 minutes, the speed was reduced to 20 rpm for
5 minutes, and then mixing was stopped. The solution was allowed
to settle for 30 minutes, and turbidity tests were performed.
FIG. 9 is a graph showing mucilage efficiency at reducing
residual turbidity at higher dosages. Standard jar test for
flocculent sedimentation. 0.5 g/L kaolin clay slurry was added
to a test jar. The solution was stirred at 100 rpm and varying
amounts of identified flocculent were added. After 2 minutes,
the speed was reduced to 20 rpm for 5 minutes, and then mixing
was stopped. The solution was allowed to settle for 30 minutes,
and turbidity tests were performed.
FIG. 10 is a graph showing water column arsenic levels after
gelling extract mucilage treatment. Arsenic was dissolved in
water at 290 [mu]g/L. 30 ppm of gelling extract was added to the
arsenic solution. After addition of the mucilage the appearance
of solid metallic like particles was observed. After 30 minutes
the particles settled to the bottom, embedded in the mucilage
gel. A sample was analyzed.
FIG. 11 is a graph showing water column arsenic distribution. 86
ppb arsenic was added to a 300 mL water column. The water was
dosed with 5 ppm gelling mucilage extract. 36 hours later, 20 mL
samples were taken from the top, middle, and bottom of the water
column and analyzed for arsenic concentrations. An arsenic
concentration profile was established.
FIG. 12 is a graph showing that the make-up method improves the
mucilage efficiency at reducing As concentration in a water
column. 5 ppm gelling mucilage extract was added to a 300 mL
water column, contaminated with 83.65 ppb of arsenic. The
concentration of the gelling extract was maintained by removing
the top 2% of the water column at 30 minute intervals and
replacing the removed water with a 5 ppm gelling extract/water
solution. Spent mucilage transports arsenic to the water-air
interface, where the arsenic is removed every 30 minutes and
replaced with new, active mucilage.
DETAILED
DESCRIPTION OF THE PREFERRED EMBODIMENT
In the following detailed description of the preferred
embodiments, reference is made to the accompanying drawings,
which form a part hereof, and within which are shown by way of
illustration specific embodiments by which the invention may be
practiced. It is to be understood that other embodiments may be
utilized and structural changes may be made without departing
from the scope of the invention.
The invention includes a process for the removal of suspended
solids and/or arsenic from drinking water using a natural-based
flocculent, such as that derived from Opuntia ficus indica, or
nopal, cactus. Three types of mucilage extract from the cactus
are obtained (a gelling extract (GE), a nongelling extract (NE),
and a combined extract (CE)) and are used individually as a
flocculent for the removal of harmful contaminants that reduce
the potability of water. The process steps are (a) cactus pad
maceration, (b) chosen mucilage fraction extraction, (c) aqueous
dissolution of the solid extract, (d) contaminated water dosing,
(e) flocculation, and (f) supernatant decantation.
Three types of mucilage were extracted: a gelling extract (GE)
and a non-gelling extract (NE) were obtained, and a combined
version (CE) consisting of GE & NE. Cactus plants were
purchased from Living Stones Nursery, Tucson, Ariz. All mucilage
types extracted were stored dry and at room temperature. For the
extraction of NE and GE, cactus pads were cleaned and boiled in
milli Q water until they became tender (approximately 20
minutes). The soft pads were then liquefied in a blender. The pH
of the resulting suspension was then neutralized and the solids
and liquid supernatant were separated in a centrifuge at 4000
rpm. The supernatant was collected, mixed with 1M-NaCl solution
(10:1 ratio), filtered and precipitated with 1:2 ratio of pulp
to acetone to produce the NE extract. The acetone was then
decanted and the precipitate washed with a 1:1 volume ratio of
precipitate to isopropanol. The resulting NE precipitate was air
dried on a watch glass at room temperature. In order to separate
the gelling portion, the centrifuged precipitates were mixed
with 50 mL of 50 mM NaOH. The suspension was stirred for 10 min
and the pH adjusted with HCl to 2. The suspension was
centrifuged and the solids again resuspended in water while the
pH was adjusted to 8 with NaOH. The suspension was then filtered
and the solids were washed following the same procedure as for
the NE extract. For the combined extract, the initial blend was
centrifuged and the supernatant was separated and pH adjusted to
8 with NaOH, washed with acetone and isopropanol as described
above and finally it was air-dried. On average, for each pad
that weighs around 300 g wet weight, a 1.5-2 g dry powder is
obtained.
A series of cylinder tests were performed, shown in FIGS. 1
through 6, to determine the flocculating efficiency of the three
different varieties of mucilage produced the inventors. A kaolin
slurry of 50 g/l was poured into a stoppered 100 ml cylinder, 3
ppm of mucilage flocculent solution or control was added, the
cylinder was capped and inverted completely 10 times for total
mixing of the contents, the cylinder was then placed on a
horizontal surface and the height of the interface between the
supernatant and the settling solids were recorded with time.
The flocculation efficiency was tested, analyzing the three
mucilage extracts against a positive control (Al2(SO4)3) or a
negative control (without flocculent). The flocculants were
added at 3 ppm to the slurry and analyzed as described above.
FIG. 1 shows the mucilage is an excellent flocculent of
suspended solids compared to Al2(SO4)3. Comparing the same
dosage of mucilage and Al2(SO4)3, the mucilage settled the same
amount of particulate matter in 3.6 minutes as Al2(SO4)3 did in
10 minutes. Further, smaller dosages of mucilage provided faster
settling rates and the clearest supernatant. The mucilage was
also found to reduce arsenic concentrations by 50% after 36
hours at low dosages.
The effective concentration and precipitation rates were
determined for gelling extract (GE). The gelling extract was
added to a 50 g/L kaolin slurry, described above, at 0.01 mg/L,
0.1 mg/L, 1 mg/L, 2 mg/L, 3 mg/L, and 4 mg/L. 4 mg/L of gelling
extract mucilage precipitated most of the slurry within 10
minutes, whereas 3 mg/L required 20 minutes to precipitate the
same amount of clay slurry, seen in FIG. 2. However, the
precipitation rates from 0.01 mg/L to 3 mg/L were significantly
closer, the 0.01 mg/L mucilage extract requiring about 5 minutes
more than the 3 mg/L extract, and 15 minutes more than the 4
mg/L extract, to precipitate the same amount of slurry. Thus,
the gelling extract was most effective at a very higher
concentration, but the extract concentration did not drastically
affect the precipitation rates from low to mid level extract
concentrations. The difference between concentrations is more
pronounced from 1 minute to 4 minutes after addition of the
flocculent to a colloid solution, as depicted in FIG. 3. 4 mg/l
gelling mucilage extract precipitated the slurry much quicker
than any other concentration, reducing the level of slurry about
8.5 cm in three minutes. The next most effective concentration,
3 mg/l, reduced the slurry 6 cm in the same time. Lower
concentrations had less effect on the level of the slurry,
reducing the slurry level about 3 cm during the three minute
period.
The non-gelling mucilage extract (NE) was then tested to
determine the effective dose. Nongelling extract was added to a
50 g/L kaolin slurry, described above, at 0.01 mg/L, 0.1 mg/L, 1
mg/L, 2 mg/L, 3 mg/L, and 4 mg/L. Unlike the gelling extract,
the non-gelling extract is less affected by concentration, as
seen in FIG. 4. Between 2 mg/L and 5 mg/L, the nongelling
extract reduces the slurry by approximately 12 cm in 20 minutes.
However, lesser nongelling extract concentrations, between 0.01
mg/L and 2 mg/L, reduce the slurry level by 10 cm in the same
time and require about 30 minutes to reduce the slurry level by
12 cm. Further, the lower concentrations precipitate the slurry
at the same rate as the negative control. The precipitation
rates are seen more dramatically between 2 and 13 minutes, shown
in FIG. 5. The 5 mg/L extract precipitates the slurry most
rapidly, removing about 6 cm in 5 minutes. The nongelling
extract exhibited similar precipitation rates from 2 mg/L to 4
mg/L, removing from 4.25 to 5 cm of slurry in 5 minutes. At
lesser concentrations, from 0.01 mg/L to 1 mg/L, the nongelling
extract precipitates the slurry at the same rate as the negative
control, about 3 cm in 5 minutes.
The combined extract (CE) exhibited similar precipitation
properties to the nongelling extract. The combined extract was
added to a 50 g/L kaolin slurry, described above, at 0.01 mg/L,
0.1 mg/L, 1 mg/L, 2 mg/L, 3 mg/L, and 4 mg/L. High
concentrations of the combined extracts, between 2 ppm and 5
ppm, precipitate about 11 cm of slurry in 10 minutes and 12 cm
in 20 minutes, seen in FIG. 6. Lower concentrations of the
combined extract required 20 minutes to precipitate the slurry
10 cm, approximately precipitating the slurry at the same rate
as the control.
Flocculation studies were conducted using the standard jar test,
where previously prepared kaolin solutions at 0.5 g/l were
poured into the jars of the jar test apparatus, then stirring at
100 rpm was started and different quantities of the flocculent
solutions were added to each jar always leaving one without
flocculent added to serve as a control. The contents were
stirred for 2 minutes and then the speed was reduced speed to 20
rpm for 5 minutes. After this, agitation was stopped and the
contents were allowed to settle for 30 minutes before collecting
a sample and measuring its turbidity. At lower concentrations,
the combined mucilage extract mirrors the residual turbidity
characteristics of aluminum sulfate, as shown in FIG. 7. Higher
concentrations of aluminum sulfate are more effective at
reducing the residual turbidity of the solution, shown in FIGS.
8 and 9. However, secondary filtration may be used to remove
residual particulates, if desired.
The capacity of the gelling mucilage extract to reduce arsenic
from water was then determined. A 290 ng/L arsenic solution was
dosed with 30 ppm of gelling mucilage extract. After the gelling
mucilage extract was added, solid metallic-like particles were
observed forming in the solution. After 30 minutes the particles
settled to the bottom, embedded in the mucilage gel. A sample of
the solution was analyzed, as seen in FIG. 10. The mucilage
flocculent treatment yielded a reduction of approximately 11% of
the As of the original solution, compared to about 50% for the
control, proving the interaction between the gelling extract and
As.
To determine the action of the mucilage when removing Arsenic,
86 ppb of arsenic was added to a 300 ml water column The water
was dosed with 5 ppm gelling mucilage extract. After 36 hours, a
20 ml sample from the top, middle, and bottom of the water
column were taken and analyzed for arsenic concentration. The
arsenic concentration profile was determined, shown in FIG. 11.
Water taken from the middle of the water column had steady
concentrations of arsenic, whereas the top and bottom of the
water column had fluctuating arsenic concentrations. Arsenic
concentrations in the combined water column were lowest at 1.5
hours, and began to rise again at 2 hours, indicating the
mucilage was saturated and the treatment allowed arsenic to
redissolve. However, arsenic concentrations did go down over
time.
The capacity of the gelling mucilage extract to remove arsenic
from contaminated water at low concentration dosing was
determined using the make-up. A concentration of 5 ppm gelling
mucilage extract was established in a water column The top 2% of
the water column was removed at 30 minute intervals and the
water column volume restored to the original amount by adding a
5 ppm gelling mucilage extract/water solution to the
contaminated water column Spent mucilage transports arsenic to
the water-air interface where it is removed. The mucilage thus
facilitates the removal of arsenic, as seen in FIG. 12.
USE OF
CACTUS MUCILAGE AS A DISPERSANT AND ABSORBANT FOR OIL IN
OIL-WATER MIXTURES
US2013087507
ELECTROSPUN
CACTUS MUCILAGE NANOFIBERS
US2013068692
CACTUS
MUCILAGE AND FERRIC IONS FOR THE REMOVAL OF ARSENATE (AS(V))
FROM WATER
WO2013040389