The Uses of Nanotechnology in Wastewater Treatment


Wastewater is any water that has been contaminated by pollutants such as pathogens, organic and inorganic materials, heavy metals and a wide range of other toxins. Around the world today, more than 2.6 billion people do not have access to clean, potable water forcing them to use contaminated wastewater which often leads to diseases and illness that are potentially life threatening.[1] About 10% of diseases worldwide and 6.3% of deaths (largely in developing countries) could be abolished through improvements to the quality of drinking water.[2] Furthermore, with the limited quantity of available fresh water on earth, the need to reuse and recycle what we have is always increasing.

Nanotechnology has emerged as one of the leading new innovative technologies with a great potential for treating wastewater in a more effective and efficient manner than methods that have been previously used.

Table of Contents

..............1. Benefits
..............2. Potential Issues
.....................2.1 Risks to Human Health
.....................2.2 Availability
..............3. Examples of Different Uses
.....................3.1 Purification by ZnO Particles
..............................3.1.1 Experimental Procedure
..............................3.1.2 Effects on S. Aureus and E. Coli
..............................3.1.3 Applications in Water Treatment
.....................3.2 Polyrhodanine-encapsulated Magnetic Nanoparticles
..............................3.2.1 Production
..............................3.2.2 Properties
.......................................3.2.2.1 Adsorption Capacity
.......................................3.2.2.2 pH Variance
.......................................3.2.2.3 Reusability
..............................3.2.3 Conclusion
.....................3.3 Nanofiltration and Reverse Osmosis
..............................3.3.1 Pre-treatment
..............................3.3.2 Production of Sulfuric Acid
..............................3.3.3 Other Applications
..............4. References


1. Benefits



Conventional water purification techniques involve several technical challenges that increase the cost and reduce the effectiveness of removing certain contaminants from water. The table below provides a basic overview on a few of these techniques.

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Figure 1.1: Conventional Water Purification Techniques

All of the water purification methods mentioned in the table above involve high costs and are not completely effective in purifying the water. Nanotechnology can help overcome this issue. Research states that nanotechnology has the potential to increase the effectiveness of the existing water treatment solutions at a more affordable price.[3] Techniques such as Purification by ZnO particles, Nano filtration by Reverse Osmosis and Polyrhodanine-encapsulated Magnetic Nanoparticles have the ability to effectively remove contaminants even at low concentrations.

Nanomaterials are very effective as a separation medium for water purification as they contain a number of key physio-chemical properties.[4] They are known for their high surface area to mass ratio which occurs as a result of decreasing the size of the adsorbent particle from bulk to nanoscale dimensions. This property of nanomaterials leads to the availability of a high number of atoms or molecules on the surface of contaminants thereby enhancing the adsorption capacities of sorbent materials.[5][6]

Moreover, this large surface area coupled with their size, electronic and catalytic properties provide unparalleled opportunities to develop more efficient water purification catalysts and redox active media. Nanomaterials can also be functionalized with various chemical groups to increase their anity toward a given compound.[4]

Lastly, since more adsorbent atoms/molecules are present per unit mass of the adsorbent, less waste will be generated post treatment as these atoms will be actively utilized for adsorption.[5]


2. Potential Issues


2.1 Risks to Human Health

Although the benefits of using nanotechnology for removing contaminants from water have been widely discussed, more research needs to be conducted in order to assess its potential negative impact on the environment and on human health. A few of the studies conducted on this issue indicate that certain unique properties of nanomaterials such as their shape and size may cause them to be toxic.[7] They may also be responsible for the interaction with biological macromolecules within the human body, leading to the development of diseases and clinical disorders.[7] However till date, water purification from nanotechnology has not led to any ascertained health or environmental problems.[8]

2.2 Availability

Another major challenge is the availability of suppliers of nanomaterials.[8] As mentioned earlier, nanomaterials are known for their high surface area to mass ratio and can therefore perform their function at a more affordable price. However, if suppliers of these materials are not readily available, it could seriously undermine the potential advantages of using this technology.


3. Examples of Different Uses


3.1 Methods of Purification by ZnO Particles[9]


Zinc oxide (ZnO) nano-particles had been shown to enhance activity of an antibacterial agent called ciprofloxacin. Ciprofloxacin is an antibody that counteracts the Staphylococcus aureus and Escherichia coli bacterial strands which are commonly found in 6% of drinking water.

200px-Ciprofloxacin.svg.png
Figure 3.1.1: Chemical Structure of Ciprofloxacin


3.1.1 Experimental Procedure

One relatively straight-forward lab experiment that can be easily carried out to test out this hypothesis involves the following two simple steps:
  1. Synthesis of ZnO nano-particles
  2. Disk Diffusion Assay

The first step is to create the nano-particles catalysts. By employing the concepts of the mechanochemical method, one can react anhydrous (without water) zinc chloride, sodium carbonate, and sodium chloride aqueous solutions to produce an intermediate compound called zinc carbonate in the following reaction:

3ZnCl2 + 3Na2CO3 + 4NaCl → 3ZnCO3 + 10NaCl
This process is carried out usually over the course of 9 hours. Following this, the powder is further calcinated by air for 30 minutes at a temperature of 300oC. The residue is then distilled in water and dried to form ZnO nano-particles by consuming the intermediate zinc carbonate in the following reaction:
ZnCO3 → ZnO + CO2
ZnO_before_and_after.jpg
Figure 3.1.2: Electron micrographs of prepared zinc oxide nanoparticles using a mechanochemical method.


The below video demonstrates the nucleation, growth and functionalization of the zinc oxide nanoparticles through multiscale simulations.




Step 2 involves the preparation of the Disk Diffusion Test. The purpose of this step is to ultimately determine the effect the antibody has on the Staphylococcus aureus and Escherichia coli bacterial strands. A single colony of test strains were grown overnight in the Mueller Hinton liquid medium at 35oC. After 18 hours of exposure, the areas were measured and compared between the sample with and without the ZnO catalyst. The areas of inhibition were measured and compared at a constant time interval after exposure.

3.1.2 Effects on S. Aureus and E. Coli

Over the period of exposure, it was discovered that the ZnO increased the antibacterial activity of ciprofloxacin specifically. With the S. Aureus strand, the area increased by 39-63% whereas the E. Coli area increase varied from 17-93%.

Data_Results.jpg

It should also be noted that the effectiveness of the antimicrobial agents vary with concentration of how much they are applied. An increase in concentration yielded an increase in zone of inhibition.

3.1.3 Applications in Water Treatment

Because of the unique properties ZnO possess, much research had been done in applying this process to water treatment. Bacterial strands such as the S. Aureus and E. Coli can cause severe fever, chills, low blood pressure, cramps and diarrhea. The experiment above suggests one of many effective methods of treating this infection.

Conventional methods involved using nanoparticle suspension (which required filtration in later processes) or UV light (which uses up a lot of energy). Many facilities are now pushing for a greener approach where ZnO nanorods are combined with visible light to fight bacteria contaminated water. The key concept is to incorporate specific defects in ZnO nanorods by creating oxygen vacancies, which then allows visible light absorption. The theory is simple, ZnO dissolves with the absence of light, releasing Zn2+ which have been shown to have antibacterial properties. The cations penetrate the cell membranes of the bacteria, inhibiting growth.

As seen, the implications of these processes are of great commercial value, as photocatalysis of these reactions using solar energy could save millions of dollars by not having to produce artificial UV light to catalyze the reactions.


3.2 Wastewater Treatment Through Polyrhodanine-encapsulated Magnetic Nanoparticles[10]


A heavy metal is defined as one that has at least five times the specific gravity of water. Such metals present themselves naturally in our environment, but also are a product of todays society in industrial processes and runoff from cities. Any exposure to excess amount of these toxic metals will lead to poisoning and in sever cases, death. Other effects include reduced growth and development, organ damage, and nervous system damage.[11] Heavy toxic metals often use water as a mean of transportation and have been proven extremely harmful to ingest.

3.2.1 Production

Polyrhodanine-encapsulated Magnetic Nanoparticles (PR-MNPs) are a new efficient way of effectively removing heavy metal ions from solution. Particles were manufactured from an aqueous solution of rhodanine (7.5mM), iron chloride (6.2mM), and sodium borohydride (26mM), and synthesized by a one-step chemical oxidation polymerization.
Screen_Shot_2011-11-07_at_5.37.03_PM.png
Figure 3.2.1: Manufacturing process of PR-MNPs


Adsorbing will commence due to the metal-binding functional groups provided by the rhodanine monomeric unit. Harvesting of the final product is as simple as subjecting the solution to a magnetic field.

3.2.2 Properties

Various experimental tests have been preformed to determine the adsorption capacity, pH variance, and the recyclability of the PR-MNPs under varying conditions.

3.2.2.1 Adsorption Capacity
Screen_Shot_2011-11-07_at_5.51.19_PM.png
Figure 3.2.2: Adsorption vs. Time
PR-MNPs were mixed with ionic samples of Cd, Mn, Cr, and Hg. The solutions were agitated and resulting metal ion concentration was measured using inductively coupled plasma (ICP) analysis. Experiments were conducted several times and average results were used for graphing and calculations. Results indicated a higher adsorption of mercury over all the other ions, while cadmium followed in second place (Figure 3.2.2). Because of this we will now use mercury for all further testing. This can be predicted using the hard and soft acid and bases model (HSAB), where Hg and Cd are soft acids while sulfur in the polyrhodanine is a soft base. From a kinetics point we do see a high time to reach equilibrium, approximately two hours. In addition, the metal ion radius, interaction energy, and oxidation states of heavy metals have a role to play in adsorption. The particles magnetic properties and large surface area also give it an edge when attracting unwanted metals.

.....................................................

3.2.2.2 pH Variance
Adsorption_vs_pH.png
Figure 3.2.3: Adsorption capacity with Hg vs. pH

After further testing with different values of pH, we find a sharp rise in results from 2.0 - 4.0, and a slower rise from 4.0 - 8.0. This is due to the hydrogen ion occupying the sites where the heavy metal is supposed to protonate. On the other hand an increase in pH decreases the surface charge. This weakens the attraction to the heavy metal ions, and is why we see a smaller slope after 4.0 (Figure 3.2.3), making a pH of 4.0 an ideal choice.






3.2.2.3 Reusability

A major concern of Green Chemistry is the disposal of harmful waste products and the ability of reactants to be reusable rather than depleting. The PR-MNPs are designed for easy separation using an external magnetic source (Figure 3.2.4). Once isolated, the particles are treated with a 0.1 M HCl solution for three hours to prepare for reuse and and to wash away the heavy metal ions. This is followed by neutralization with water, then drying in a vacuum. The adsorption results were above 96% after five consecutive trials (Figure 3.2.5), proving this method to be extremely effective.
Screen_Shot_2011-11-12_at_11.08.09_AM.png
Figure 3.2.4: Separtion of particles with external magnetic field; Figure 3.2.5: Recovery rate of Hg in five consecutive tests
...........................................

3.2.3 Conclusion

When expanding the lab scale experiment to industrial levels we expect to find some dilemmas. First the two hours needed to reach equilibrium and maximum levels of ions adsorbed may prove daunting if mass amounts of water are to be purified. The retrieved particles must also be agitated with HCl for three hours, neutralized and dried. But particles are very easy to separate from solution once the task has been preformed, and can be reprocessed numerous times without major depletion in properties. We find the PR-MNPs are effective in removing heavy metal ions from solution, with the most effective being mercury.


3.3 Purification by Nanofiltration and Reverse Osmosis[13]


The popularity of Nanofiltration has been growing over the past few years. This method is mainly applied to assist in purifying drinking water by softening the water and removing micro pollutants. The process is carried out by applying pressure on the water samples as it is pressed through a separation membrane. Consequently, the larger metal compounds will be filtered out while purified water molecules bypass the membrane.

The key concept nanofiltration employs is solely based on reverse osmosis. This involves having two fluids with different concentrations separated by a semi-permeable membrane. These two fluids will mix until the system reaches a state of equilibrium with uniform concentration, where the fluid with the lower concentration will migrate through the membrane and into the system with a higher concentration. However, if we apply a pressure that exceeds the osmotic pressure on the column of the fluid with higher concentration, we get a reversed effect where the fluid with the higher concentration will begin to flow toward the fluid with lower concentration. During this step, the semi-permeable membrane will allow the water to bypass while filtering out the larger metal ions.

Reverse_Osmosis.png
Figure 3.3.1: Process of Reverse Osmosis


3.3.1 Pre-treatment

Before pressure can be applied, we must treat the water and reduce the organic content. The reason for this is that organic materials get caught along with the heavy metals on the semi-permeable membrane and consequently causes biofouling. In doing so, we increase the production time of the filtration system and the lifespan of the membranes.

Other pre-treatment techniques involve a chemical dosage to reduce the precipitation of salts and the build up of calcium carbonate and barium sulphate. These chemical dosages primarily include concentrated hydrochloric acid and sulfuric acid. In most industries, sulfuric acid is more widely accepted simply due to its relatively easier production process. However, sulfuric acid had been shown to also foul the membranes and as a result, more and more facilities are switching over to using hydrochloric acid.

3.3.2 Production of Sulfuric Acid[14]

Sulfuric acid can be obtained by oxidizing elemental sulfur in the Contact Process. The reaction yields to the synthesis of sulfur dioxide.

S(s) + O2(g) → SO2(g)
Sulfur dioxide is further oxidized to form sulfur trioxide in a reversible reaction. The extent of yield may be determined by by applying Le Chatalier's principles by either increasing pressure, or concentration of SO2.
2SO2(g) + O2(g) ↔ 2SO3(g)
Sulfur trioxide cannot be directly reacted with water to form sulfuric acid as the reaction is too unstable and exothermic to be deemed safe. Instead, the sulfur trioxide is reacted with concentrated sulfuric acid to form oleum (H2S2O7).
H2SO4(l) + SO3(g) → H2S2O7(l)
With the production of oleum, it is then mixed with water to produce the exact concentration of sulfuric acid desired.
H2S2O7(l) + H2O(l) → 2H2SO4(l)

3.3.3 Other Applications

Despite its primary purpose of purifying drinking water, the nanofiltration technique may also be applied to help promote green chemistry and a cleaner environment by removing pesticides underwater, removal of heavy metals from wastewater, and wastewater recycling in laundries and nitrate removal.

It is also very widely accepted in the food sector as concentrated juice or other beverages can be made easier by this method.

RO_diagram.jpg
Figure 3.3.2: A simple purification process involving reverse osmosis used in kitchen sinks
[15]


The below video further illustrates the process of reverse osmosis and how it can be applied for uses such as water filters under a kitchen sink.






4. References


  1. World Health Organization. Access to safe drinking water improving; sanitation needs greater efforts. N.p., 15 Mar. 2010. Web. 11 Nov. 2011. <http://www.who.int/mediacentre/news/releases/2010/water_20100315/en/index.html>.
  2. Prüss-Üstün, Annette, Robert Bos, Fiona Gore, and Jamie Bartram. Safer water, better health. N.p.: World Health Organization, 2008. 10-11. Web. 11 Nov. 2011. <http://www.who.int/quantifying_ehimpacts/publications/saferwater/en/index.html>.
  3. Berger, Michael. "Water, nanotechnology's promises, and economic reality." 15 Aug. (2007). Web. 11 Nov. 2011. <http://www.nanowerk.com/spotlight/spotid=2372.php>.
  4. Savage, Nora, and Mamadou S. Diallo. Nanomaterials and water purification: Opportunities and challenges. N.p.: Springer, 2005. 337-39. Print.
  5. Pradeep, Anshup T. Noble metal nanoparticles for water purification: A critical review (2009): 6448-49. Web. 14 Nov. 2011. <http://www.dstuns.iitm.ac.in/listpdf/181.pdf>.
  6. Tiwari, Dhermendra K., J. Behari, and Prasenjit Sen. "Application of Nanoparticles in Waste Water Treatment." World Applied Sciences Journal 3 (3) (2008): 417-19. Web. 12 Nov. 2011. <http://www.idosi.org/wasj/wasj3(3)/11.pdf>.
  7. Cloete, Eugene T., Michele Kwaadsteniet, Marelize Botes, and Manuel J. López-Romero. Nanotechnology in Water Treatment Applications. N.p.: Caister Academic Press, 2010. Web. 12 Nov. 2011. <http://www.horizonpress.com/nanotechnology>.
  8. Savage, Nora, and Mamadou S. Diallo. Nanomaterials and water purification: Opportunities and challenges. N.p.: Springer, 2005. 337-39. Print.
  9. Banoee, M.; Seif, S.. ZnO nanoparticles enhanced antibacterial activity of ciprofloxacin against Staphylococcus aureus and Escherichia coli. Journal of Biomedical Materials Research Part B: Applied Biomaterials. 2010. Vol. 93B, 557-561.
  10. Jang, J.; Kong, H.; Song J.. Adsorption of heavy metal ions from aqueous solution by poylrhodanine-encapsulated magnetic nanoparticles. Journal of Colloid and Interface Science. 2011, 505-511
  11. Health Risks Of Heavy Metals. N.p., n.d. Web. 5 Nov. 2011. <http://www.freedrinkingwater.com/water-education/quality-water-heavymeatal.htm>.
  12. Bradford, Traci, and M Nicole Cook. Inductively Coupled Plasma. N.p., n.d. Web. 5 Nov. 2011. <http://www.cee.vt.edu/ewr/environmental/teach/smprimer/icp/icp.html>.
  13. Nanofiltration and Reverse Osmosis. Lenntech BV, 1998. Web. 10 Nov. 2011. <http://www.lenntech.com/nanofiltration-and-rosmosis.htm>.
  14. Sulfuric Acid : Production, Properties and Uses. AUS-e-TUTE, n.d. Web. 10 Nov. 2011. <http://www.ausetute.com.au/sulfacid.html>.
  15. Reverse Osmosis Under Counter Water Filter. N.p., 2011. Web. 10 Nov. 2011. <http://www.neutralexistence.com/shop/reverse-osmosis-under-counter-water-filter/>.