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

Ammonia-selective electrode method Sample distillation is often required before analysis, especially for waste waters and sludges where interference effect is significant.. O XYGEN DEMAN

NITR OGEN (AMMONIA)

Ammonia (NH3) occurs in varying 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 (NH3-N) may be analyzed by the following 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 waste 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 removal 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 organic 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

NH3-free distilled water containing 20 mL borate buffer and adjusted to pH 9.5 with NaOH

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© 1997 by CRC Press LLC

NITR OGEN (NITRATE)

Nitrate, which is produced by oxidation of nitrogen, is a monovalent poly-anion having the formula NO3

– 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 10–1M NO3

– The colorimetric method 5 has been found to give inconsistent results

CADMIUM REDUCTION METHOD

In the presence of cadmium, nitrate (NO3

– ) is reduced to nitrite ( NO2

– ) 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 (NO3

–

-N plus NO2

– -N) is read from a standard curve prepared by plotting absorbance (or transmittance) of standard against NO3

–

-N concentrations

The reactions are as follows:

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© 1997 by CRC Press LLC

NITR OSAMINES

Nitrosamines or nitrosoamines are nitroso derivatives 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)

by U.S EPA under the Resource Conservation and Recovery Act

[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.

R

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© 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 Fe2+ and sulfide 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 MgSO4 ⋅7 H2O (22.5 g/L), FeCl3 ⋅6 H2O (0.25 g/L), and CaCl2 (27.5 g/L) and diluting into desired volume of reagent grade water

The amounts of components per liter of phosphate buffer solution are KH2PO4 (8.5 g), K2HPO4(21.75 g), Na2HPO4 ⋅7 H2O (33.4 g), and NH4Cl (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:

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O XYGEN DEMAND, CHEMICAL

Chemical oxygen demand (COD) is a measure of the oxygen equivalent of organic matter in the sample that is susceptible to oxidation by a strong oxidizing agent A boiling mixture of potassium dichromate (K2Cr2O7)–H2SO4 can oxidize most types of organic matter and is generally used in the COD determination

Other strong oxidants, such as KMnO4–H2SO4 are also effective

Complete oxidation of organic compounds under such strong oxidizing con-ditions produces carbon dioxide and water Other additional products such as HCl

or NO2 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 mol - pentane reacts with 15 mol O

2

(1 mol ethanol reacts with 3 mol O

2

)

(Metal ion is oxidized to higher oxidation state

2+

2

)

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© 1997 by CRC Press LLC

A large number of nitrogen-containing pesticides are characterized by the carbamate or urea functional group or triazine ring in their structures These substances constitute the three most common classes of nitrogen-containing pes-ticides The structural features of these three distinct classes of substances are shown below:

where R1 and R2 are alkyl or aryl groups or hydrogen atoms Compounds con-taining a methyl group attached to the N atom are also known as N-methyl carbamates

Substitution of one or both of the oxygen atoms with sulfur in the above structure gives thiocarbamate

The structure of urea type pesticide is similar to carbamate, except the terminal oxygen atom is replaced by a nitrogen atom

Triazine is a nitrogen heterocyclic ring containing three N atoms in the ring

O

(leaving group)

1 Carbamate

O

(leaving group)

2 Urea Type

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PESTICIDES: ORGANOCHLORINE

Organochlorine pesticides refers to all chlorine-containing organics used for pest control The term, however, is not confined to compounds of any single and specific type of chemical structures or organic functional group(s), but includes

a broad range of substances The grouping together of these compounds is more

or less based on the similarity in their chemical analysis Many chlorinated pesticides that were commonly used in the past are no longer being used now, because of their harmful toxic effect on human and contamination of the envi-ronment Many such pesticides and their residues are still found in the environ-ment in trace quantities in groundwaters, soils, sedienviron-ments, and wastewaters These substances are stable, bioaccumulative, and toxic, and some are also carcinogens (Patnaik, 1992) Table 2.20.1 presents some common chlorinated pesticides, most

of which are listed as priority pollutants by U.S EPA

Chlorinated pesticides in aqueous and nonaqueous matrices may be deter-mined by U.S EPA Methods 608, 625, 505, 508, 8080, and 8270 (U.S EPA 1984–1994) Analysis of these pesticides requires extraction of the aqueous or nonaqueous samples by a suitable organic solvent, concentration, and cleanup of the extracts, and determination of the analytes in the extracts, usually by GC-ECD or GC/MS These steps are outlined below

SAMPLE EXTRACTION

Aqueous samples are extracted with hexane or with methylene chloride by liquid-liquid extraction using a separatory funnel or a mechanical shaker, or by microextraction Aqueous samples can also be extracted by solid phase extraction using a C-18 cartridge Selection of sample volume should be based on the extent

of sample concentration that may be needed to achieve the required detection level in the analysis, as well as the use of packed or capillary column A larger sample concentration is required for packed column than that for capillary column analysis U.S EPA recommends the extraction of 1 L sample to a final volume

of 1 mL for wastewater analysis performed on a packed column For the analysis

of potable water by GC-ECD on a capillary column, concentration of a 35-mL

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© 1997 by CRC Press LLC

PESTICIDES: ORGANOPHOSPHOR US

An important class of pesticides is organophosphorus compounds, which have the following general structure:

where R is an alkyl or aryl group The phosphorus atom in such a compound is bound to one or two oxygen and/or sulfur atom(s) The leaving group may be any organic species that may cleave out from its oxygen or sulfur bond Two typical examples are as follows:

Organic phosphates can cause moderate to severe acute poisoning These substances inhibit the function of the enzyme, acetylcholinesterase by phospho-rylating or binding the enzyme at its esteratic site The symptoms of acute toxicity

P

O (or S)

O ( or S) RO

RO

leaving group

P

O O

C

(Mevinphos)

O

P

S O

(Parathion)

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P h AND Eh

pH is a measure of hydrogen ion [H+] concentration in an aqueous solution

It is defined as

Similarly, hydroxide ion [OH–] concentration may be expressed as pOH, which is

The concentrations [H+] and [OH–] are expressed in molarity (M or mol/L)

In a neutral solution, pH = pOH =7.00 The sum of pH and pOH is 14.0

In an acidic solution, the hydrogen ion concentration is greater than 1.0 ×

10–7M, and thus the pH is less than 7.00 Similarly, in a basic solution, the [H+]

is less than 1.0 × 10–7M and, therefore, the pH is greater than 7.00

The pH of a solution of a strong acid or a strong base can be calculated;

conversely, [H+] and [OH–] can be determined from the measured pH of the solution This is shown in the following examples

Example 1

What is the pH of a solution of 0.005 M HCl?

Because HCl is a strong acid, it will completely dissociate into H+ and Cl– ions Thus, 0.005 M HCl will dissociate into 0.005 M H+ and 0.005 M Cl– ions

= 100 Peak areas of Peak areas of Endri

×

% Breakdown for DDT = 100×

pH = –log H[ ]+

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© 1997 by CRC Press LLC