Chemical Pollutants in Air, Water, Soil, and Solid Wastes Phần 4 pptx

NITR OGEN (AMMONIA) Ammonia (NH 3 ) occurs in v arying concentrations in groundwater, surface water, and waste water. Its occurrence in waters and sludge is primarily attributed to its formation resulting from the reduction of nitrogen-containing organics, deamination of amines and hydrolysis of urea, and also to its use in water treatment plants for dechlorination. Its concentration in groundwaters is relatively low because of its adsorption to soil. Ammonia-nitrogen (NH 3 -N) may be analyzed by the follo wing methods: 1. Colorimetric nesslerization method 2. Colorimetric phenate method 3. Titrimetric method 4. Ammonia-selective electrode method Sample distillation is often required before analysis, especially for w aste waters and sludges where interference effect is significant. Distillation, however, may not be necessary for potable waters or clean and purified samples where the concentration of ammonia is expected to be low. When the titrimetric method is followed, the sample must be distilled. SAMPLE DISTILLATION Distillation of sample is often necessary for the remo val of interfering con- taminants. The sample is buffered at pH 9.5 with borate buffer prior to distillation. This decreases hydrolysis of cyanates (CNO – ) and or ganic nitrogen compounds. Distillation is performed in a 2 L borosilicate glass apparatus or one with aluminum or tin tubes condensing units. Before the sample is distilled, clean the apparatus until it is free from trace ammonia. This is done by distilling 500 mL NH 3 -free distilled w ater containing 20 mL borate buffer and adjusted to pH 9.5 with NaOH. 2.14 © 1997 by CRC Press LLC © 1997 by CRC Press LLC NITR OGEN (NITRATE) Nitrate, which is produced by oxidation of nitrogen, is a mono valent poly- anion having the formula NO 3 – . Most metal nitrates are soluble in water and occur in trace amounts in surface- and groundwaters. Nitrate is toxic to human health and, chronic exposure to high concentrations of nitrate, may cause meth- emoglobinemia. Maximum contaminant limit in potable water imposed by U.S. EPA is 10 mg nitrate as nitrogen/L. Nitrates in water may be analyzed by the following methods: 1. Ion chromatography 2. Nitrate selective electrode method 3. Cadmium reduction method 4. Miscellaneous reduction method 5. Brucine method Methods 3 and 4 are colorimetric procedures based on reduction of nitrate to nitrite, followed by diazotization and then coupling to an azo dye. The analysis may be performed manually or by use of an automated analyzer. Method 2 is applicable in the range 10 –5 to 10 –1 M NO 3 – . The colorimetric method 5 has been found to give inconsistent results. CADMIUM REDUCTION METHOD In the presence of cadmium, nitrate ( NO 3 – ) is reduced to nitrite (NO 2 – ). The nitrite produced is diazotized with sulfanilamide. This is followed by coupling with N-(1-naphthyl)-ethylenediamine to form a highly colored azo dye. The inten- sity of the color developed is measured by a spectrophotometer or a filter photo- meter at 540 nm. The concentration of oxidized N/L (NO 3 – - N plus NO 2 – -N) is read from a standard curve prepared by plotting absorbance (or transmittance) of standard against NO 3 – - N concentrations. The reactions are as follows: 2.15 © 1997 by CRC Press LLC © 1997 by CRC Press LLC NITR OSAMINES Nitrosamines or nitrosoamines are nitroso deri vatives of amines in which a nitroso (NO) group is attached to the nitrogen atom of the amine. These com- pounds have the following general structure: where R and R ′ are alkyl or aryl groups. Nitrosamines are toxic compounds as well as potent animal and human carcinogens (Patnaik, 1992). These substances occur in trace quantities in tobacco smoke, meat products, and salted fish. Some of these compounds are classified by U.S. EPA as priority pollutants in industrial wastewaters, potable waters, and hazardous wastes. These nitrosamines are listed in Table 2.16.1. Such pollutants occurring in environmental samples can be determined by U.S. EPA’s analytical procedures (U.S. EPA 1990, 1992). T able 2.16.1 Nitrosamines Classified as Priority Pollutants by U.S. EPA under the Resource Conservation and Recovery Act CAS no. Compound [62-75-9] N -Nitrosodimethylamine a [10595-95-6] N -Nitrosomethylethylamine [55-18-5] N -Nitrosodiethylamine [621-64-7] N -Nitrosododi- n -propylamine a [924-16-3] N -Nitrosodibutylamine [86-30-6] N -Nitrosodiphenylamine a [100-75-4] N -Nitrosopiperidine [930-55-2] N -Nitrosopyrrolidine [59-89-2] N -Nitrosomorpholine a Also classifi ed as pollutants in the wastewater category. ONN R R′ 2.16 © 1997 by CRC Press LLC © 1997 by CRC Press LLC O XYGEN DEMAND, BIOCHEMICAL Biochemical oxygen demand (BOD) is an empirical test that measures the amount of oxygen required for microbial oxidation of organic compounds in aqueous samples. Such a test measures the amount of oxygen utilized during a specific incubation period (generally, 5 days) for biochemical oxidation of organic materials and oxidizable inorganic ions, such as Fe 2+ and sulfi de. The incubation is performed in the dark at 20 ± 1°C. The results of the BOD analyses are used to calculate waste loadings and to design wastewater treatment plants. Different volumes of sample aliquots are placed in 300 mL incubation bottles and diluted with “seeded” dilution water. The bottles are filled to their full capacity without leaving any headspace, and tightly closed. The BOD bottles are then placed in a thermostatically controlled air incubator or a water bath at 20 ± 1°C in the dark to prevent any photochemical reaction. The dilution water is prepared by adding 1 to 2 mL of phosphate buffer solution to an equal volume of MgSO 4 ⋅ 7 H 2 O (22.5 g/L), FeCl 3 ⋅ 6 H 2 O (0.25 g/L), and CaCl 2 (27.5 g/L) and diluting into desired v olume of reagent grade water. The amounts of components per liter of phosphate buffer solution are KH 2 PO 4 (8.5 g), K 2 HPO 4 (21.75 g), Na 2 HPO 4 ⋅ 7 H 2 O (33.4 g), and NH 4 Cl (1.7 g). The pH of this solution should be 7.2. Because BOD measures the amount of oxygen needed by the microbes to oxidize the organics in the wastewater, this oxygen must, therefore, be supplied initially into the aqueous medium before incubation. The dilution water, therefore, must contain a sufficient quantity of dissolved oxygen (DO). At ambient condi- tions, oxygen is slightly soluble in water. Such an amount of oxygen, however, is often sufficient to oxidize trace organics found in relatively clean samples. To ensure availability of surplus oxygen in the medium, the dilution water should be aerated using an air compressor. This enhances the DO concentration. The concentration of DO before and after incubation is measured. If the microbial population is not sufficiently large in the sample, microorganisms should be added into the dilution water from an outside source. Oxygen consumed by the organics is determined from the difference, and the BOD is calculated as follows: 2.17 © 1997 by CRC Press LLC © 1997 by CRC Press LLC O XYGEN DEMAND, CHEMICAL Chemical oxygen demand (COD) is a measure of the oxygen equi valent of organic matter in the sample that is susceptible to oxidation by a strong oxidizing agent. A boiling mixture of potassium dichromate (K 2 Cr 2 O 7 )–H 2 SO 4 can oxidize most types of organic matter and is generally used in the COD determination. Other strong oxidants, such as KMnO 4 –H 2 SO 4 are also ef fective. Complete oxidation of organic compounds under such strong oxidizing con- ditions produces carbon dioxide and water. Other additional products such as HCl or NO 2 may , however, form if the organic compound contains a Cl or N atom, respectively, in its molecule. COD for any organic compound or any oxidizable inorganic ion (i.e., anions or metal ions in their lower oxidation states) can be theoretically calculated from writing a balanced equation. Some oxidation reac- tions are presented below and how COD may be calculated is shown in the following problem: 2 C H O KCrO acid CO H O (2 mol - pentane reacts with 15 mol O 5 10 2 227 22 2 + →+15 10 10 n ) CHOH O KCrO acid CO H O (1 mol ethanol reacts with 3 mol O 55 2 227 22 2 + →+323 ) 2 Fe O KCrO acid Fe O (Metal ion is oxidized to higher oxidation state. 2+ 2 227 3+ – + →+22 ) 2.18 © 1997 by CRC Press LLC © 1997 by CRC Press LLC . 3 ) occurs in v arying concentrations in groundwater, surface water, and waste water. Its occurrence in waters and sludge is primarily attributed to its formation resulting from the reduction. reduction of nitrogen-containing organics, deamination of amines and hydrolysis of urea, and also to its use in water treatment plants for dechlorination. Its concentration in groundwaters is relatively. terminal oxygen atom is replaced by a nitrogen atom. Triazine is a nitrogen heterocyclic ring containing three N atoms in the ring. NCO O (leaving group) R 1 R 2 1. Carbamate NCO O (leaving group) R 1 R 2 2.