rexresearch.com
rexresearch.com
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
[ PDF ]
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.
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