A paper mill in Minnesota uses 100% chlorine dioxide (ClO₂) to bleach its pulp and the run-off flows into the St. Louis River. ClO₂ is safe when used as a water treatment, but there can be toxic environmental effects over 5 mg/ L.

A Beer’s Law plot of absorbance of ClO₂ versus concentration was developed for the first time in order to continue with this study. Various sites upstream and downstream of the paper mill were sampled to determine an average amount of ClO₂ concentrations in the river at each site.

This study has concluded that the levels of ClO₂ in the St. Louis are under the levels that could be hazardous to the river and its aquatic life. The greatest concentration of ClO₂, 0.3612 mg/L, was found at site 5, which was closest to the paper mill. This value is low enough to indicate there is not a significant effect on the river.

In this project, I monitored the concentration of ClO₂ in the St. Louis River in Cloquet, Minnesota, upstream and downstream from a paper mill plant that lies directly on the river. This plant uses ClO₂ exclusively to bleach the pulp.

My hypothesis was that there would not be any detectable levels of ClO₂ upstream of the paper mill, but there would be significant concentrations of ClO₂ downstream of the mill, perhaps over the maximum amount that is considered safe from a biological viewpoint.

In the past, the pulp and paper industry have used chlorine gas (Cl₂) to bleach pulp. As a result, run-off from these plants contained chlorophenols, chloroform, and chlorinated organic compounds. Traces of these toxic chemicals were detected in the river in the late 1980s and, since then, environmental agencies have motivated the industry to use bleaching techniques that eliminate these chemicals. One new technique to bleach pulp was developed that uses ClO₂. The "Pollution Prevention Act of 1990" was passed to compel the pulp and paper plants to use the new techniques (Elemental Chlorine-Free).

ClO₂ has a greater oxidation potential than Cl₂. During the bleaching process, Cl₂ and ClO₂ turn into chlorinated organics. Cl₂ combines with the lignin (substance that holds wood fibers together), while ClO₂ oxidizes the lignin so it opens up the aromatic structure. Other chlorinated organics formed by ClO₂ bleaching are water-soluble and do not bio-accumulate. Therefore, ClO₂ seemed to be the best alternative that would help the eco-system and still bleach the pulp (Elemental Chlorine-Free).

In addition to use in pulp bleaching, ClO₂ is also used in the treatment of drinking water. The maximum residual concentration of ClO₂ that does not release objectionable taste or odor in treated water is 0.4 to 0.5 mg/L. The maximum amount of ClO₂ that is considered safe from the toxicological viewpoint is up to 5 mg/L (ClO₂: Chemistry and the Environmental Impact of Oxychlorine).

The chemistry of the reaction of ClO₂ in water is not known. There are two possible equlibra for the reaction of ClO₂ with water. It is not known which path the reaction follows.

4 ClO₂ + HCl + 2H₂O <==> 5 HClO₂

or

2 ClO₂ + HClO₃ + HCl + H₂O <==> 4HClO₂

Since there are two possible equations for the reaction, stoichiometry can not be used to determine the concentration of the ClO₂. First, a method had to be developed to determine the concentration and then study of ClO₂ in the St. Louis River was undertaken.

In Cloquet, Minnesota, a pulp and paper plant that lies directly on the St. Louis River started using the new technique of bleaching with ClO₂. In 1994, the mill used 10% of ClO_2, and in 1994, they switched to 100% use of the ClO₂. Determining the reaction of ClO₂ in water would be significant for further study of the environmental impacts of this process of bleaching.

The concentration of ClO₂ was found by creating a Beer’s Law plot of absorbance versus concentration. This was the first time the concentration of ClO₂ in water has been determined.

First, ClO₂ gas was synthesized in water by placing a small piece of sodium chloride (NaClO₂) into a round bottom flask and, using a syringe, 0.5 mL of concentrated hydrochloric acid (HCl) was dropped into the sodium chloride. The synthesis equation is:

NaClO₂ + HCl à HClO₂ (g) + NaCl

A syringe was used to suck the HClO₂ gas out of the flask and inject the HClO₂ into

an erlenmeyer flask filled with 200 mL of distilled water. This produced aqueous ClO_2. The ClO₂ was pipeted in into a cuvette for diode-array analysis, and the absorbance of ClO₂ was determined at 358 nm.

Next, 4 g of potassium iodide (KI) was dissolved in 10 mL of distilled water. This solution was added to 50 mL of the synthesized ClO₂ solution. The remaining ClO₂ solution was stored in an ice bath in the dark to prevent photo and thermal decomposition between each test.

The solution was titrated with standardized 1M sodium thiosulfate (Na₂S₂O₃) to a yellow end point, then 5 mL of starch solution was added to turn the solution to blue. The solution was titrated until clear. This procedure was repeated two times using the remaining ClO₂ solution to obtain an average concentration of ClO₂.

The synthesis process was repeated ten times to obtain points for the Beer’s Law plot. Stoichiometry was used to determine the concentrations of each solution of ClO₂.

Lastly, a Beer’s Law plot of absorbance versus concentration of ClO₂ in water was determined.

Analysis of St. Louis River Water

The St. Louis River water was analyzed by collecting samples at four sites upstream and four sites downstream of the paper plant. Sampling sites 1 through 4 were upstream of the paper plant, and sampling sites 5 through 8 were downstream of the plant. The depth was measured at each sampling site. Clean, plastic, washed bottles that were taped to prevent light from photo-decomposing the ClO₂ during transportation were used to collect the samples. Before collecting the water samples, the bottles were rinsed with river water, and then the whole plastic container was dipped into the river until the bottle was full. The cap was twisted on while the bottle was submerged, and the bottle was placed into a container of dry ice to slow thermal decomposition during transportation. The temperature of the river water was taken at each site, using an electronic thermometer. The pH of the samples was taken at the laboratory, using an Interface. The absorbance of the ClO₂ was measured using the diode-array, and the concentration was determined by using the Beer’s Law plot developed previously.

Calculated from the absorbance data in Table 1, the average of the upstream absorbencies was —0.0333, and the average of the downstream absorbencies was 0.0069. The standard deviation for upstream was 0.0395, and the standard deviation downstream was 0.00296. The 95% probability that the true average absorbance of ClO₂ upstream of the paper mill is between —0.064 and 0.0342. The downstream probability is between —0.0000288 and 0.00495. The t-test value of upstream and down stream gave a value of 2.944.

Absorbance Up Stream Absorbance Down Stream

The concentration of each sample was determined by using the Beer’s Law Plot.

The calculations for mg/ L of ClO₂ are shown in table 3.

Table 3: ClO₂ levels (mg/ L) at Various Sties Upstream

The results for the absorbance of ClO₂, seen in Table 1, found that the absorbances are non-existent or extremely low upstream of the plant and there are traces of ClO₂ below the plant. The greatest absorbance, 0.0114, is found at site 5, which is exactly at the paper mill. The absorbance lowers after this point, probably from photo and thermal decomposition. The standard deviation of absorbance shows that the range of values for downstream are closer to the downstream average than the upstream points are to their average. The two averages are significantly different because the t-value, 2.944, is greater than the confidence level value at 95%, which is 2.20.

The levels of ClO₂ at each site, seen in Table 3, are all below the toxic level of

5 mg/ L. The lowest amounts are near the beginning of the water source at site 1 and gradually increase, having the greatest amount at site 5. Then the levels decrease and increase again at site 8. The reason for this increase is vague, but the detection levels of ClO₂ in the St. Louis River are still at safe levels.

This study has concluded that there is a significant difference of absorbance between the upstream absorbance of ClO₂ and the downstream absorbance, but the measurements show that the difference is not significant enough to make the river unsafe.

Further studies could be done to obtain more samples taken at lower depths. This test would determine if the ClO₂ absorbance differs, since photo and thermal decomposition has a lesser effect at deeper levels. Taking samples to observe if there are significant amounts of chlorophenols, chloroform, or chlorinated organic compounds in the river is another possibility because even though ClO2 is not found to be directly present, ClO2 by-products could be a significant problem. Testing the absorbance during the winter would also be interesting to see if absorbance changes with drastic change in temperature.

As seen in Figure 1, the absorbance increases with a greater concentration of ClO₂. The equation for the regression line equation of this graph is:

y = 0.0067476 + 868.89 * x

The Na₂S₂O₃ solution was standardized using titration. First, 9.0544 g of sodium

thiosulfate was dissolved in 500 mL distilled water to make the Na₂S₂O₃ solution. Next,

2 g of KI was dissolved in 20 mL of distilled water, and 0.15 g of potassium iodate (KIO₃) was added into the KI solution. This solution was diluted in 100 mL of distilled water. Then, 20 mL of sulfuric acid solution (1 mL of sulfuric acid to 39 mL distilled water) was mixed with the diluted solution. This solution was titrated with Na₂S₂O₃ until it became a slight yellow color, and then 5 mL of starch was added, changing the solution to blue. Na₂S₂O₃ was then added until the solution turned clear. The procedure to standardize the Na₂S₂O₃ solution was repeated three times, and the average was calculated by stoichiometry. The stoichiometry for this reaction is:

IO₃^- + SI^- + 6H⁺ à 3I₂ + 3H₂0

2S₂O₃^2- + I₂ à S₄O₆^2- + 2I^-

These equations were used to determine the molarity of the Na₂S₂O₃ , by using the formula below.

(g KIO₃) (mol KIO₃ / g KIO₃) (mol I₂/ mol IO₃^-) (mol S₄O₆^2- / mol I₂) = mol S₄O₆^2-

(mol / L S₂O₃^2-) (L S₂O₃^2-) = mol S₂O₃^2-

Calculations for Concentrations of ClO₂

By these equations, the concentrations for each solution of ClO₂ were determined:

2ClO₂ + 2I^- à I₂ + 2ClO₂^-

I₂ + 2S₂O₃^2- à 2I^- + S₄O₆^2-

The following calculations were done:

(mol S₂O₃^2-) (1 mol I₂/ 2 mol S₂O₃^2-) (2 mol ClO₂/ 1 mol I₂) = mol ClO₂

(mol ClO₂/ L ClO₂) = M ClO₂

The average was determined by adding all the upstream absorbencies divided by the number of values. The downstream average was determined the same way but using downstream values.

Upstream: -0.166 / 5 = -0.03325

Downstream: 0.0555 / 8 = -0.0069

The standard deviation was determined by subtracting the average absorbance for upstream from each upstream value and squared. Then these were all added together and divided by the degrees of freedom (number of points minus one). Lastly, this value was square rooted. The standard deviation for downstream was found using the same equation but replacing the upstream values with downstream values.

Upstream: 0.03985

Downstream: 0.002963

The confidence interval was found by multiplying the 95% confidence level at that degree of freedom with the standard deviation. This product was divided by the square root of the number of upstream points, and then added/ subtracted to the average upstream absorbance. The downstream values were found by the same method, except the upstream values were replace by downstream values.

Up stream: [(2.78 * 0.03848) / 2.236] (+ / -) * -0.03325

= -0.06396 < confidence interval < 0.03422

Down Stream: [(2.36 * 0.02960 / 2.828] (+ / -) * 0.006913

= -0.00002881 < confidence interval < 0.0049166895

The t-value was determined by adding the number of points taken downstream minus one. Then the sum was multiplied by the standard deviation for downstream squared; plus, the number of points for upstream minus one, multiplied by the standard deviation of upstream squared. This was divided by the sum of both degrees of freedom, multiplied by the number of points for upstream and downstream. The average downstream absorbance was subtracted from the average upstream and this value was divided by the value above.

T- value: 0.0401625 / 0.0136414607

= 2.944365262

The following equations were used to determine the mg/ L of ClO₂.

(mol ClO₂ / L)(67.4515 g ClO₂/ mol)(1000 mg/ g) = mg ClO₂/ L

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