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clement lee, timothy sng, katie gloe, sydney stayrook

 
 
 

Abstract

Living organisms require trace amount of some heavy metals to survive. However excess exposure to them will lead to many harmful effects. Heavy metals can enter water sources via many ways but cannot be easily removed by normal water-treatment methods.  On the contrary, the high surface area to mass ratios of nanoparticles can greatly enhance the adsorption capacities of sorbent materials and when functionalized with various chemical groups, they increase their affinity towards target compounds. In addition, plants which are classified as mineral accumulators, are able to absorb inorganic materials from soil and retain them in its biomass. Ideally, hyperaccumulators should be plants who have a rapid growth rate and an extensive root system that can accumulate and tolerate the various contaminants. Our project aims to draw a comparison on the effectiveness of removing heavy metal ions between nanoparticles and plants. Our results show that Pistia Stratiotes is indeed a good hyperaccumulator. Through the use of Pisita Stratiotes, an overall decrease in the concentration of copper (II) ions over 7 days, by 81.91% (from 90.70mg/l to 16.43mg/l and a 92.75% decrease in the concentration of copper (II) ions, over a span of 24 hours of treatment, can be observed. Silver nanoparticles were also effective in removing heavy metal at fast rates. There was a 76.88% decrease in the concentration of copper (II) ions, from 498.7mg/l to 115.3mg/l, over a span of 40 minutes and a 55.74% decrease in the concentration of copper (II) ions, from 986.67mg/l to 436.67mg/l, over a span of 40 minutes, can be observed. In addition, there was a decrease in concentration of zinc ions by 58.40%, over a span of 35 minutes, with the introduction of silver nanoparticles. From our results, phytoremediation has proved to be more effective than nanoremediation, at least over a longer period of time. Silver nanoparticles are capable of reducing the heavy metal concentration in water at a faster rate in a short period of time. Nevertheless, it can be observed from our results, that the concentration of heavy metals in water tend to increase after a while (less than a day) and this could be due to the aggregation of the nanoparticles. On the contrary, the decrease in heavy metal concentration is more sustainable when plants are used. Thus, we proposed the combination of the above two methods to remediate contaminated water bodies. Another application for our project is that the valuable heavy-metal minerals and perhaps the silver nanoparticles can both be extracted from the hyperaccumulators that absorbed them in the process of phytoremediation. This method, termed as “phyto-mining” can not only recover heavy-metal minerals, from contaminated water bodies, that can be re-used for other purposes, but also allow us to recycle  the recovered silver nanoparticles or silver compounds, re-synthesized them into silver nanoparticles and then re-used to treat polluted water bodies once again.

 

Background

Heavy metals can enter the water sources via many ways. Murkherjee (2000) provides examples, with agricultural sources including phosphatic fertilizers which contain arsenic, cadmium, manganese, vanadium and zinc, from pig and poultry manures which contain Arsenic and Cadmium, corrosion of galvanized metal objects, from electricity generation etc. These pollutants flow through underground water networks into lakes, ponds and drinking sources. Therefore, the aquatic ecosystem receives the bulk of contaminants from anthropogenic sources (Odjegba, Fasidi, 2004).  As nations around the world develop and progress, industries which dispose lead, zinc, vanadium, copper, cadmium emerge. Fungicides, used by farmers to treat plants, contain copper compounds, organic mercury compounds and organic tin compounds. Excess fungicide seeps into the soil and mixes with ground water (Heitefus, 1989).

Compound

Toxic Levels / ppm

Silver (water)

0.05

Cadmium (water)

0.005

Mercury (water)

0.002

Table 1           An array of different toxic heavy metals that have been found in soil and water.
Adapted from Willams and Adams (2007).

However, certain toxic substances, like heavy metals, cannot be removed by normal water-treatment methods (Timbrell, 1989). Nanoparticles can remove heavy metals and has great potential in improving air, water and soil quality (Biswas and Wu, 2005). In addition, some plants are classified as mineral accumulators as they absorb inorganic materials from soil and retain them in its biomass. An example is Astragalus which accumulates selenium (Manahan, 1990). This is known as phytoremediation whereby plants are used to contain, eliminate or break down the pollutants and restore the environment to its original conditions.

Biswas and Wu (2005) suggest that nanoparticles are a class of materials with nanoparticles of silver has uses that range from burn treatment, arthroplasty to being used in water treatment (Panáček et al., 2006).

Nanomaterials can remove pollutants by binding to the contaminants. Nanoparticles of oxidizing irons can reduce toxic heavy metals, such as lead, nickel, mercury and uranium, to an insoluble form that is locked within the soil (Willams and Adams, 2007). Titanium dioxide nanocrystals, under UV light, are able to convert mercury vapour to mercury oxide. This can also be done by nanocrystals of Fe3O2 under heat. With the introduction of nanoparticles, metals like lead, chromium, arsenic can be immobilized (Booker and Boysen, 2005).

Nanoparticles are commonly synthesized via a reduction reaction, via reducing agents like NaBH4 (Panáček et al., 2006, Tan et al., 2007). However, weaker reductants, like citrate, resulted in a slower reaction, while using a stronger reductant results in small particles that were nondisperse. High pH conditions produce a wide range of nanoparticle sizes when disaccharides were used as reductants (Panáček et al., 2006).

Nanoparticles can be observed using a transmission electron microscope (TEM) as it achieves a resolution of approximately 0.2nm (Booker and Boysen, 2005). In the TEM, a beam of electrons is sent through a sample. Electrons that get through strikes on a phosphor screen and produce images.  When electrons encounter nuclei or other electrons, they scatter, hence are not seen on the phosphor screen. Under ideal conditions, TEM can be used to see individual atoms (Ratner and Ratner, 2003).

 

Hypothesis
  1. Nanoparticles are able to bind to different metal ions with different efficiency.
  2. Nanoparticles with surface modification are able to bind to heavy metal ions more effectively than those without surface modification.
  3. Removal of heavy metal ions is more effective using phytoremediation, as compared to nanoremediation at lower concentrations of heavy metal ions in the water source.