Turbidity is the clarity of water and it has been studied widely in ponds, streams and lakes. Many investigations have looked out how the turbidity of water in these environments is able to affect the aquatic life that lives within. A high degree of turbidity prevents photosynthesis and therefore the production of oxygen leading to the death of aquatic life, however little study has been undertaken to see whether or not particle size can influence turbidity. The purpose of this experiment therefore was to determine whether or not particle size is able to influence turbidity of water.
To determine this silica size particles 5-15nm and 10-20nm were used; 3g of each particle size range was used to make to a suspension using polyethylene glycol 10000 as the suspending agent. Using the stock that had been made up different concentrations of suspensions was made by dilution for each particle size range. The method involved examining the attenuation of light and so the degrees of absorption of each particle size range at different concentrations, using UV-Spectroscopy. The turbidity was determined by observing which particle size range gave the largest absorption reading at different concentrations compared to the other particle size range at the same concentration.
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The results showed smaller particles of silica 5-15nm gave larger absorption readings consistently compared to larger particles of silica 10-20nm. This means that small particles contribute to higher turbidity compared to larger particles. This is because small particles have a larger surface area and scatter more light than larger particles.
The results revealed that smaller particles gave larger absorption readings compared to larger particles due to smaller particles having a larger surface area and the ability to scatter more light. Therefore smaller particles contribute to larger turbidity compared to larger particles. This means that due to small particles having a larger absorption they will absorb more light and therefore preventing it from reaching plants at the bottom of streams, ponds and lakes etc, resulting in a decrease in photosynthesis and so the decrease in the production of oxygen which will lead to more death of aquatic life. It was concluded that smaller particles are more life threatening to aquatic life compared to larger particles.
Turbidity is a "measure of the relative clarity of water, it involves analysing how the presence of suspended material present in water will decrease the amount of light travelling through the sample. Turbidity in water may be caused by suspended particles in the water such as soil, clay, micro-organisms and organic and in-organic matter. There is a direct relationship between turbidity and suspended particles, as the total suspended particles increases the turbidity also increases. The presence of suspended particles in the water causes the water to heat up as the suspended particles absorb the water and temperature increases, As a result of this the concentration of dissolved oxygen is reduced due to warm water holding less dissolved oxygen than colder water. Increased turbidity can also reduce the amount of light that is entering the water, as water is no longer transparent with the presence of suspended particles, this will reduce photosynthesis and production of dissolved oxygen. This can affect life that is present in water such as fish and plant life. In drinking water, the higher the turbidity level, the higher the risk that people may develop gastrointestinal diseases.
Turbidity does not measure the level of suspended particles present in water, but rather the scattering of light due to the presence of suspended particles present in water. In the Samples that contain suspended particles, the way in which the water will interfere with the transmittance of light depends on the shape, size and what those particles consist of when in the water, it is also related to the wavelength that impacts the particles. A particle will absorb the light that impacts it followed by re-radiating the light in all directions.
Turbidity is influenced by a series of factors. The particle size will influence the direction the light is scattered as well as the intensity of scattered light of different wavelengths. Particle shape, refractive index of water and colour of the suspended particles also affects light scattering.
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As the particle concentration of a sample increases the light scattering intensifies. As the scattered light begins to strike more particles, multiple scattering occurs, and more light is absorbed. As the particle concentration exceeds a certain point, the levels of detectable and scattered light decrease showing the highest limit of measurable turbidity. The measurement of turbidity is a key test of water quality. One of the ways to measure turbidity is by attenuation this can be done by using UV-Spectroscopy.
Studies on turbidity
Turbidity has been vastly studied in lakes, rivers, streams and even ponds, investigations have involved looking at how turbidity of water in these environments can affect fish and plant life that lives there. Conclusions have been drawn that turbidity of water in these environments is not necessarily a benefit to fish and plants as it results in death. This is as a result of limitation of light, oxygen, food and poisoning that can result from turbid water.
As mentioned many studies on turbidity have mainly looked out how turbidity can affect fish and plant life and what problem turbidity provides to predators. For example a study from Dr. Jukka Horppila  from Finland entitled Effects of turbidity on feeding and distribution of fish, has looked at how turbidity can effect predators finding their prey and what this affect means for prey by providing cover and protection. However it was found that not all prey benefits from turbid water as too much turbidity can lead to their death and the death of plant life.
Another study Effects of turbidity on the migration of juvenile banded kokopu (Galaxias fasciatus) in a natural stream , by Jody Richardson, David K. Rowe and Joshua P. Smith. This study looked out how a specific species of fish was sensitive to turbidity levels of 25 > NTU and as a resulted migrate away from such waters of this turbidity.
Allot of studies have been conducted looking at the two principles above but little study has been taken on investigating how particle size of suspended particles can influence turbidity and so affect fish and plant life.
Importance of turbidity
A high degree of turbidity can affect the environment and prevent the growth of fauna and flora. When sunlight becomes blocked and is not able to penetrate through water, which can happen due to the presence of a high particulate concentration interfering with light penetration. If the penetration of light continues to be continuously reduced, photosynthesis will also be reduced resulting in lower daytime release of oxygen in the water. However the decrease in the penetration of light also stops organisms from seeing their food, offspring and predators so it also causes sensory impact. However changes in turbidity are not completely negative for some species of fauna and flora as they have evolved to changes in turbidity and are able to survive the changes that result in decreased sunlight to a degree.
Turbidity can also add to the cost of treatment of surface water supplies which are used for drinking water as the turbidity must be decreased so that effective disinfection (chlorination using chlorine) can occur. It is also important to note that particles present in turbid water are able to provide attachment sites for many heavy metals to add onto these include lead, mercury and cadmium to name a few.
Drinking water should have a turbidity of 5 NTU/JTU or less. Turbidity of more than 5 NTU/JTU would be noticed by users and may cause rejection of the supply. Where water is chlorinated, turbidity should be less than 5 NTU/JTU and preferably less than 1 NTU/JTU for chlorination to be effective.
Factors affecting turbidity
Sources of water contamination include:
Power Plants: Heated water
Feedlots: Organics, solids, nutrients, microorganisms
Industries: Organics, chemicals, color, foam, salts, toxins, heated water
Municipalities: Domestic and industrial wastes; microorganisms, color and foam, nitrogen, phosphorus.
Agricultural Land Drainage: Soil from erosion, fertilizers, pesticides, organics, microorganisms
Mining: Suspended solids, acid mine drainage
Urban Storm Runoff: Industrial dust, dirt, and litter
The flow rate of a body of water is the main factor which can influence turbidity concentration. Water that is running fast is able to carry a greater amount of particles and larger-sized sediment. When it rains heavily more sand, clay, silt and organic particles are carried from land to the surface of water. Changes in flow rate also affect turbidity. If the current, speed or direction of water increases the particulate matter from bottom sediments can be resuspended.
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Disturbing land surface can cause soil erosion. There are many causes of soil erosion such as building and road constructon, mining, logging and forest fires. Storm water is able to carry eroded soil particles to the surface water. This will result in an increase in the turbidity of the water body.
During heavy rain the debris and soil particles from streets, roads, agricultural land and residential areas are washed into streams. Due to the presence of a large amount of pavement for example in urban areas the natural settling places are removed and as a result all the sediment is carried to rivers and creeks by storm drain.
The plants and animals in a body of water will eventually die and decay; as a result the suspended organic particles become released contributing to turbidity. Fish at the bottom of a body of water such as in rivers and lakes can distribute sediment as they move; this can also contribute to turbidity.
The figure below shows how aquatic organisms are generally affected by turbidity.
Turbidity and relation to human health
A high degree of turbidity in drinking water is aesthetically unappealing, and can represent a health concern. Turbidity is able to provide shelter and food for pathogens. If not correctly treated, turbidity will promote the regrowth of pathogens in distribution system, which can lead to waterborne disease outbreaks; this can result in cases of gastroenteritis which has happened in many places in the world. Although a direct relationship between turbidity and health risk cannot be drawn up, there are a number of studies have shown that there is a correlation between removal of turbidity and the removal of protozoa.
As mentioned earlier the greater the tutrbidity level the greater the risk of people develpoing gastrointestinal diseases. This provides a problem for people who are immune compromised, this is because bacteria and viruses are able to attach to suspened solid. Suspeneded solid also provides a problem with the disinfection of water using chlorine as the suspenede pparticles act as shields for bacteria and viruses. Ultraviolet sterilization is also useless as the bacteria are protected from suspended solids.
Jackson candle method was one of the first and earliest ways to measure turbidity. It involved using a flat bottom glass tube and candle. The measurement was made by way of adding the sample into the tube gently to the point where the visual image of the candle, when viewed from the other end of the tube, diffused to give a uniform flow. This is known as the extinction point. When the intensity of scattered light equalled that of the transmitted light, visual image extinction occurred. And so the depth of sample could be read along the ppm-silica scale, turbidity was referred to as Jackson turbidity units (JTU), one JTU = 1 ppm of silica. The problem with this method is that it is not able to measure turbidity lower than 25 JTU and depends on human judgment. When viewed from straight on the changes that can occur in transmitted light at low concentrations is very slight making it undetectable. However at high concentration this becomes simpler as multiple scattering is able to interfere with direct transmission. This problem can be solved by using the measuring 90 degree scatter and so using a nephelometer. The Jackson candle method is shown below in figure 2.
With the nephelometer the intensity of scattered light is measured using this equipment. This method measure turbidity in Nephelometric Turbidity units (NTU), (Nephele is the Greek work for "cloud", metric means "measure". Nephelometric, therefore, means "measuring cloudiness"). A nephelometer measures the degree of scattered light at a 90 degree angle. As the concenartion of the suspeneded particles begins to increase, they start to interfere with one another. As a result of this increase not all the particles are reached from the light source or the light that is reflected may not be detected from the receiving device. So the 90 degree scattered light method is used for the turbidity measurement can only be used for lower concentrations. This method basically measures the degree of scattered light rather than the absorption of light. This way is able to provide a good correlation with concentration of suspended particles that are able to effect the clarity of water. A nephelometer is shown in figure 3.
A turbidity tube which has a secchi disk attached to the bottom of it is another method used to measure turbidity and is mainly applied to streams and lakes. It requires looking down a black and white disk at a bottom of a tube and then determining how much water is required to make the black and white colour from the disk disappear. This is represented below in figure 4.
The objectives for this project involve:
Formulate a stock suspension of silica 5-15nm and 10-20nm using Polyethylene glycol 1000
Make a number of dilutions for both sized particles.
Measure attenuation of light for each dilution for each size particle using UV-Spectroscopy at 200-600nm.
Determine if there is a correlation between particle size and absorption.
Determine if there is a correlation between concentration and absorption.
MATERIALS AND EQUIPMENT
The materials used are stated below:
Polyethylene glycol 10000(Sigma - Aldrich, UK), Silica powder 5-15nm (Sigma - Aldrich, UK), Silica powder 10-20nm (Sigma - Aldrich, UK), deionized water, weighing balance (up to three decimal places), spatulas, weighing boats, cuvettes, glass stirring rods, beakers (100mL), measuring cylinders (100mL), stopwatch, spectrophotometer (Thermo Electron Corporation UV/VIS), Hot plate (Fisher Scientific), conical flasks, measuring cylinder (100mL), pipettes (1ml), pipettes (2ml), pipettes (5ml), pipettes (10ml), pipette pumps.
Concentration required (% of stock)
Volume required from stock (ml)
Volume required from stock to make required concentration for each sample for silica 5-15nm and 10-20nm
Formulating suspension points to consider
For a suspension to be acceptable it must possess certain qualities, e.g:
The material suspended (silica) must not settle too quickly
If any of the silica does settle to the bottom it must not form a mass (hard), but should be readily dispersed into a uniform mixture when the container is shaken
The suspension formed must not be too viscous, so that it can be poured easily.
ï® = (ï²ï€± - ï²ï€²)d2g
where ï® ï€½ï€ Sedimentation rate of particle
g = acceleration due to gravity
d = particle diameter
ï²ï€±ï€ = Density of particle
ï²ï€²ï€ = Density of medium
ï€ ï¨= Viscosity of medium
- By controlling (d) which is the particle size the rate of sedimentation can be controlled the smaller the particle size the slower the rate of sedimentation.
- Increase viscosity of continuous phase.
Method to prepare Stock
40g of polyethylene glycol 10000 was measured using the weighing scale; this was then transferred to a 100ml beaker. Three grams of silica powder 5-15nm was measured using the weighing scale. The polyethylene glycol 10000 that was measured was melted using the hot plate at 50Â°C while being stirred using the glass rod, when melted the measured silica was added while being gently stirred this was made to 100ml using water. Same method was applied to prepare stock for silica powder 10-20nm. When formulating suspension key points were taken into consideration and are listed below.
Method to prepare samples
Added the required volume for required concentration from stock into a 10ml volumetric flask using a measuring cylinder. Made up to volume using water and a 1ml plastic pipette. This was then added to a curette in preparation for UV-Spectroscopy. Repeated same process for each concentration, preparing duplicates.
Used the UV-spectrophotometer to first record absorbance of blank using water at a wavelength range of 200-600nm. Measured the absorbance of each sample for each particle size range at same wavelength range. Absorption of duplicates was also measured at an interval of 3 minutes. Graph was printed and the absorbance recorded between wavelength range 200-600nm at intervals of 50nm. This was repeated for each concentration for each particle size range.
Table 1 to table 4 show the results that were obtained from the experiment undertaken. The tables show absorbance values for silica particles 5-15nm and 10-20nm at wavelengths 200nm to 600nm at the same concentrations. The results are interesting due to the fact that they give a overall picture that silica particles 5-15nm have larger absorbance values than silica particles 10-20nm. This is apparent in every concentration for example the average absorbance for silica size particles 5-15nm at concentrations 0.65%, 0.55%, 0.25% and 0.16% is higher than that of silica size particles 10-20nm at the same concentration. It is also interesting to note that for all concentrations for both particle sizes that the absorbance starts to decrease as the wavelength increases, there is also a correlation between concentration and absorbance and particle size and absorbance. That as the concentration increases the absorbance also increases and that as particle size decrease the absorbance also increases.
The results from the tables were also used to construct graphs 3-6 to show absorbance against wavelength for both particle sizes at each concentration. These reveal a negative correlation were absorbance decreases with increase in wavelength. However higher concentrations have a consistently higher absorption compared to lower concentrations however smaller particles have higher absorption readings at the same concentrations than larger particles.
The two graphs below show a comparison of absorbance vs. wavelength for both particle size ranges at wavelength 300nm and 600nm which both give a positive correlation. As mentioned there is a correlation between particle size and absorbance and a correlation between concentration and absorbance this will be discussed in the discussion.
The absorbance of silica particles size 5-15nm at wavelength 200nm to 600nm at different concentrations (table 1)
5-15 Wavelength (nm)
Concentration (% Stock)
The absorbance of silica particles size 5-15nm at wavelength 200nm to 600nm at different concentrations (table 2)
Concentration (% Stock)
The absorbance of silica particles size 10-20nm at wavelength 200nm to 600nm at different concentrations (table 3)
Concentration (% Stock)
The absorbance of silica particles size 10-20nmnm at wavelength 200nm to 600nm at different concentrations (table 4)
Concentration (% Stock)
It can be seen from the results that the hypothesis that I made has been supported from the results. It can be clearly seen from the results for each concentration for each particle size that as the concentration is increased the absorption also increases for example the absorption for concentration 0.35% for particle size 5-15nm is greater than the absorption for concentration 0.25% for the same particle size this can be clearly be seen from the tables and the graphs using absorption vs. Concentration. This is also the case with silica particle size 10-20nm for the same concentration were greater the concentration the greater the absorption, therefore the trend is same for both particle sizes. This is clearly because less light is able to reach detector due to absorption from the particles taking place increasing concentration makes the suspension more cloudy hence greater absorption. This also relates back to the beer lambert law where absorption is proportional to the concentration.
It is also interesting to note that the absorption values for silica particle sizes 5-15nm was consistently greater than those of silica particle size 10-20nm for every concentration values. The tables and graphs clearly show this for example for silica particle size 5-15nm for concentration 0.65%, 0.60%, 0.25% and 0.17% have greater absorption values than silica particle size 10-20nm for the same concentrations. This is an interesting result as it shows that particle size does influence absorption, the particle size impacts the direction to which the light is scattered as well as intensity of scattered light. Larger particles generally scatter more light than smaller particles also smaller particles have a larger surface area compared to smaller particles and therefore give greater absorption. (figure 5 ).
There were incidences were the absorption values for silica particle size 10-20nm were greater than silica particle size 5-15nm at the same concentration. For example at concentration 0.65% and 0.60% at wavelength 400nm particle of size 10-20nm had higher absorption readings than particles sized 5-15nm. The explanation for this could be as a result of a measuring or weighing error that occurred when making the suspension as a majority of the results show the opposite trend.
There were shortcomings in the experiment in the sense that the powder used silica dioxide was a very dense powder and as a result it was difficult to make a stable suspended suspension, as a result much more Polyethylene glycol 10000 was required 40g to try and accomplish this. It was also difficult to measure an accurate volume of the suspension using the pipettes as not all of it was coming out of the pipettes when measured which meant constantly trying to get all the measured suspension out by constantly tapping the pipette. The experiment conducted assumes that the particles are all spherical however this is not always the case as many particles will be irregular in shape which means that this could have a varied influence on light intensity.
There are a number of methods that can be used to try and improve this experiment and therefore the results. Different types of suspension could be compared with one another with the silica particles suspended in to observe the sedimentation rate, the one that gives the least sedimentation rate could be used for the experiment as this will increase accuracy.
Different size particles could be used for the experiment some smaller and some larger this will give a larger distribution of results that can be discussed and compared. Also other oxides for example titanium oxide and zinc oxide could be used of different sizes. This is interesting because these two materials are used in sun blocks, which means that the absorption could be assessed to look at the effectiveness of the sun blocks as well as the extent of turbidity.
Longer time intervals could be used for example instead of using 3 minute intervals to measure the absorption of each sample use 5 minute intervals. This could indicate suspension stability.
The wavelength range used was 200-600nm this could be increased to get a broader spec of results.
Attenuation of light is just one way to measure the turbidity of a suspension. There many other analytical methods that could be used. A nephelometer could be used where there is more concentration on the property of the particles, which will a beam of light that is focused on them. A nephelometer is used were the detector is setup to side of light beam. The amount of light reaching the detector will be depend on the number of small particles present, the more small particles there are the more light will reach the detector. Heavier particles will not contribute to the turbidity readings as they will settle rapidly. An added advantage is that this type of equipment can measure scattering of light at 90 degree angles.
The experiment that was conducted related absorbance with particle size, were silica particles size 5-15nm and 10-20nm were used, with the smaller particles having larger absorption value due to their size. In this experiment particle size was known before experiment was conducted to look at turbidity. However there may be incidences were the particle size is not known but absorption values are when looking at turbidity by the attenuation method. So particle size must be determined Particle size spectrophotometry is able do this but more common is laser light scattering, were light source is helium-neon laser. The laser beam then beam passes through a cell containing particles to be measured. The scattered light passes through a transform lens, then onto a diode array detector. Result from scatter is a diffraction pattern. The software used then will calculate what particle size distribution would match the pattern seen; 'true' result. The result is an approximation and assumes that all the particles are spherical. However this method does not require any calibration, wide size range possible and gives a very rapid analysis.
The aim of the experiment was to look at how particle size can influence absorption and therefore how the presence of different substance of different particle size can influence turbidity. It was found that smaller particles had more of an influence on absorption than larger particles as they gave larger absorption readings. It was also discovered that as the concentration increased the absorption also increased. From this the link can be drawn that both concentration and particle size influence turbidity. A high concentration along with small particle size will give large absorption results. From these results it can be seen that turbidity can have a large influence on fish and plant life in water.
In conclusion it can be said that turbidity in water can affect plant and marine life. Small particles with a high concentration in water can prevent light entering the water and therefore reduce plant growth by preventing photosynthesis which will result in a decrease in the amount of oxygen present in water leading to the death of marine life. In addition a increase in concentration of turbidity also contributes to this.