Environ. Sci. Technol. 2002, 36, 3822-3826
Chemical Pretreatment of Formaldehyde-Containing Effluents M. MOUSSAVI,* D. MOWLA, AND H. EDRAKI Shiraz University, Shiraz, Iran
Lime was found in this study to be an efficient reagent to lower the concentration of formaldehyde in highly concentrated effluents down to and below the limits suitable for biological treatment systems. The results show that the reactions leading to formaldehyde elimination can be divided in two steps. In the first step, the reaction is relatively slow. More than two-thirds of the original formaldehyde disappears in the second step in a period as short as one-third of the first step. Such trend is followed in a temperature range of up to 92 °C. Economical considerations suggest maintaining the conditions of the process around the ambient temperature with no heat requirement. It was noticed that the efficiencies of formaldehyde removal better than 99% could be achievable even around room temperature. However, these efficiencies would result in quite a shorter period of time if the temperature was raised. The mathematical representation for the rate of formaldehyde removal was found to appear with an exponential behavior. It will be seen that the rate of formaldehyde removal is strongly dependent on temperature. The present survey proves that the formaldehyde-containing effluents can be treated in a pretreatment step by lime to maintain the formaldehyde concentration in a range that is safe for biological treatment systems.
Introduction The manufacture of formaldehyde by either oxidation or dehydrogenation of methyl alcohol leaves behind a liquid waste that may contain up to 10 000 ppm of this product. These wastes under certain conditions are still more concentrated. The industries that use formaldehyde as raw material generate wastes of similar quality. There are more than 110 000 such firms reported using formaldehyde in the United States in 1995 sources (1). Besides the above sources, formaldehyde may appear in wastes by a number of other sources, among them are the exhaust gas of internal combustion engines (2, 3); stack emissions of the power plants and other stationary sources (4), mainly when the combustion is incomplete; and finally materials commonly used in construction and furnishing (5-7). Formaldehyde is the most stable carbonyl compound appearing in photochemical reactions (8-10) and vegetation emissions (11). Formaldehyde may accumulate in the muscle of certain types of fish in formaldehyde-contaminated water (12). Because of the high toxicity, the permissible exposure limit for formaldehyde is 0.75 mg/L according to regulatory standards (1). The permissible formaldehyde concentration in drinking water is suggested to be 41.1 µg/L (13). There are a number of techniques that may be applied to treat * Corresponding author present address: 155 Buckeye Ave., Oakland, CA 94618; phone: (510)654-0287. 3822
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formaldehyde-containing wastes. The more concentrated wastes are generally disposed of by incineration and decommissioning the underground abandoned storage tanks when applicable. Formaldehyde is recovered from certain concentrated wastes in the form of hexamethylene tetramine by addition of ammonia (14-16), but most industries prefer to use biological systems for treatment of their moderately concentrated wastes. When biological treatment is the choice, the system may turn practically paralyzed if the concentration of formaldehyde reaches certain limits in the influents. This limit depends on a number of elements such as trophic group (17) and the type of microorganisms. A 50% inhibition concentration was found to be about 300 ppm of formaldehyde with glucose and 150 ppm with wood glue (18). A process of adaptation and selection of bacteria increased the oxidizable concentration of formaldehyde in a study up to 1750 ppm (19). It can be seen that a pretreatment step is required to control the concentration of this chemical in effluents if this limit is violated. This limit also depends on the level of expertise and the skill of the operation team. In some plants, the skilled operators have extended this limit. For the effluents to be treated in the plant subject to this study, the maximum allowable limit for formaldehyde in feed was allowed to be 50 ppm. Formaldehyde is chemically the most reactive among the aldehydes. For this reason, it can be readily neutralized by chemical methods. For example, it can be oxidized by ordinary oxidizing agents (20): (i) Chlorine reacts with formaldehyde to yield carbon monoxide:
HCHO + Cl2 f CO + 2HCl
(1)
This reaction takes place at 150 °C and in pressurized vessels if in the aqueous phase. (ii) Hydrogen peroxide:
HCHO + H2O2 f HCOOH + H2O
(2)
HCOOH + H2O2 f CO2 + 2H2O
(3)
It decomposes formaldehyde by a relatively slow rate at 60 °C. (iii) Similar reactions take place in the presence of sodium peroxide:
2HCHO + Na2O2 f 2HCOONa + H2
(4)
Formaldehyde is also oxidized by air oxygen in alkaline solutions:
HCHO + NaOH + 2CuO f HCOONa + H2O + Cu2O (5) Cu2O + air f CuO
(6)
Here, cupric oxide acts as an intermediate for oxygen transfer. It also may be oxidized by manganese oxides and decomposed to carbon dioxide even at room temperature (3). Reducing agents, such as sodium metabisulfite, have been applied to remove formaldehyde at low concentration (21). There are other processes as well that can remove formaldehyde from an aqueous solution. However, they find limited applicability because of the requirement of experienced personnel and high heat costs. Economical and environmental considerations are the main concern for application of the above techniques. 10.1021/es015745z CCC: $22.00
2002 American Chemical Society Published on Web 08/01/2002
TABLE 1. Formaldehyde Removal Efficiency as a Function of Formaldehyde Initial Concentrationa formaldehyde initial concn (ppm)
removal efficiency (%)
formaldehyde initial concn (ppm)
removal efficiency (%)
10 50 100 200 250
nil nil 5.9 6.5 45
300 500 1000 5000 10000
93.8 96.4 98.8 99.1 99.1
a
TABLE 2. Sources of Generation of Formaldehyde-Containing Wastes in Formaldehyde Manufacturing Plant and the Wastes Approximate Concentration source
formaldehyde concn (ppm)a
yard wash water acid wash tower plant wash tower
200 30 000 20 000
a It is assumed that 1 m3 effluent is generated per t of formaldehyde production.
T ) 92 ( 1 °C, lime/formaldehyde molar ratio ) 1.
Through former studies, it was observed that formaldehyde could be easily and economically converted in the presence of lime to nontoxic compounds. Acid- and basecatalyzed Cannizzaro type of reactions convert formaldehyde primarily to methyl alcohol and basic formates at ambient temperature. If these reactions are suppressed by raising the temperature to around 80 °C, then the condensation of formaldehyde to formose sugars will be promoted (22). Butlerov first observed a long time ago that sugar-like products can be produced by formaldehyde condensation (23); however, application of lime for decomposition of formaldehyde in various liquid wastes has recently drawn the attention of a number of investigators. The hospital wastewater containing low concentrations of formaldehyde and synthetic wastes containing higher concentrations of formaldehyde (24) were among those which during the last few decades were reported to be efficiently cleared from formaldehyde by calcium hydroxide. In the present study, this process was considered as a pretreatment step to lower the formaldehyde concentration in the effluents to an accepted range suitable for a biological treatment system.
TABLE 3. Effect of Lime Concentration on Formaldehyde Removal Efficiencya lime/ formaldehyde (molar ratio)
formaldehyde removal efficiency (%)
lime/ formaldehyde (molar ratio)
formaldehyde removal efficiency (%)
1 5
nil nil
10 20
nil nil
a
TABLE 4. Effect of Lime Concentration on Formaldehyde Removal Efficiency lime/ formaldehyde (molar ratio)
formaldehyde removal efficiency (%)
1 0.5 0.1
99.2 99.2 >99.9
a
Experimental Section The experiments were performed with the samples originally containing 10 000 ppm formaldehyde and variable amounts of lime to obtain predetermined ratios of formaldehyde to lime from 20:1 to 1:20 on a molar basis. The samples were made fresh each day by diluting a 37% aqueous solution of commercial formaldehyde manufactured by Merck. Lime was made from an analytical grade stock manufactured by British Drug House. All experiments were conducted at constant temperature. The concentration of formaldehyde in solutions was measured by a colorimetric technique developed by the application of chromotropic acid. The technique was adopted from a method described in American Standards for Testing and Materials (25) based on a modification of the National Institute for Occupation Safety and Health (NIOSH-3500), and the basics have been explained elsewhere (26). The typical range of formaldehyde concentration, which was measured by this technique, is given in Table 1. The products were characterized by applying a gas chromatograph-mass spectrometer. Location of the Sources. There are a number of sources of formaldehyde-containing effluents in formaldehyde manufacturing plants. Table 2 presents the list of these sources and their approximate concentration. It is essential to separate the dilute effluents from the concentrated ones, as will be mentioned later. It is easier and less costly to remove formaldehyde from the highly concentrated effluents. Parameters Affecting the Process. The following four parameters were considered to be the most influential in removal of formaldehyde: Initial Formaldehyde Concentration. Speculations indicate that the efficiency of formaldehyde removal depends on its original concentration. To inspect the dependence, a number of samples were taken in a series of experiments at constant
T ) 92 ( 1 °C, formaldehyde original concentration ) 50 ppm.
lime/ formaldehyde (molar ratio)
formaldehyde removal efficiency (%)
0.07 0.05
99.1 46.0
T ) 92 ( 1 °C, formaldehyde original concentration ) 10 000 ppm.
temperature and constant molar lime/formaldehyde ratio. Enough time was given for reactions to reach equilibrium. The concentration of remaining formaldehyde was measured at equilibrium in the reaction vessel. Table 1 shows the results. The observations indicate that the higher initial formaldehyde concentrations are required at lower temperatures for equal removal efficiency if other parameters are held fixed. Lime Concentration. Higher lime concentrations were tried for removal of formaldehyde. Samples were taken at constant temperature and fixed initial formaldehyde concentration. The concentration of the remaining formaldehyde was measured in the reaction vessel at equilibrium when the reaction was complete. Table 3 shows the results. In another series of experiments, the above parameter was investigated for high formaldehyde concentrations. Again, samples were taken at constant temperature and fixed initial formaldehyde concentration. The concentration of remaining formaldehyde was measured in the reaction vessel at equilibrium when the reaction was complete. Table 4 shows the results. Reaction Temperature. Samples with original concentrations of 10 000 ppm formaldehyde were made. Lime was added in order to make a molar concentration ratio of lime/ formaldehyde ) 0.1. The reaction was let to extend to equilibrium in a constant-temperature bath. The concentration of remaining formaldehyde was measured. Table 5 shows the results of formaldehyde removal efficiency.
Results and Discussion The results of this work appearing primarily in Table 5 and other tables indicate that the removal efficiency of formaldehyde depends first on temperature and then on the initial VOL. 36, NO. 17, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 5. Effect of Temperature on Formaldehyde Removal Efficiencya
temp (°C)
required time to reach obsd efficiency (h)
lime/ formaldehyde (molar ratio)
formaldehyde removal efficiency (%)
25 50 60 70 92
21 6.5 3.0 0.75 0.2
0.2 0.1 0.1 0.1 0.1
98.1 97.3 99.7 99.99 >99.99
a
Formaldehyde original concentration ) 10 000 ppm.
TABLE 6. Constants a-c in Equations 8 and 9a a eq 8 eq 9 a
b
3.1 × -8.3 × 105 105
c
-2.6 × 7.1 × 105
105
-3.1 × 105 8.4 × 105
Initial formaldehyde concentration ) 10 000 ppm.
concentration of formaldehyde. The reaction rate of formaldehyde disappearance is also dependent on temperature. Faster reactions are observed to take place at higher temperature. If E represents the efficiency of formaldehyde removal, then with two parameters A and B, which are functions of temperature, a relation may be obtained as
E ) 1/(1 + exp (-(t - A)/B))
FIGURE 1. Rate of formaldehyde disappearance as a function of temperature.
(7)
where t is time. Equation 7 is obtaind for fixed original formaldehyde concentration. A and B are exponentially linear functions of 1/ln T where T is temperature (K). Equations 8 and 9 represent this correlation:
ln A ) a + b(1/ln T) + c exp(-(1/ln T))
(8)
ln B ) a + b(1/ln T) + c exp(-(1/ln T))
(9)
And the constants a-c are tabulated in Table 6. By replacing the values of A and B from eqs 8 and 9 in eq 7 and rearranging this equation, the required residence time to reach a certain efficiency at each temperature may be obtained (Figure 1). The results of the above treatment may be applied for determination of (i) the proper size of the reactor and (ii) the optimum temperature of the reaction. Fate of Formaldehyde. A large number of compounds, generally called formose, may be identified in the reaction products when formaldehyde in aqueous solutions reacts with calcium hydroxide. Figure 2 shows a gas chromatogram of these products. Although small, a mass fraction of these products can be observed in almost all mass numbers from 40 to 144 in a GC-MS chromatogram (Table 7). It is hypothesized that, from C2 to C7, polyols are formed in these reactions (3). A major peak appearing at mass number 43 on the GC-MS chromatogram indicates the formation of a monocarboxylic group when formaldehyde is exposed to lime with the molar concentration ratio of lime/formaldehyde ) 0.1 at 92 °C. The peaks at the same chromatogram at mass numbers around 90 refer to formation of dicarboxylic groups such as oxalic acid. Thermokinetics of the Process. Analysis of the previous data proves that the rate of disappearance of formaldehyde by lime is highly dependent on temperature. The reactions would reach equilibrium faster at higher temperatures, and more complete removal can be expected at higher temperatures and lower residence times. The observations indicate that the reactions can be divided into three steps. The first and last steps are relatively slow. In the second step, which 3824
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FIGURE 2. Gas chromatogram of formaldehyde-lime reactions products (3).
TABLE 7. GC-MS Analysis of Formaldehyde-Lime Reaction Productsa mass
int. (%)
mass
int.(%)
mass
int. (%)
mass
int. (%)
40 44 49 53 57 61 69 73 81 85 89 97 102 114
3.1 34.3 0.1 1.9 8.7 25.7 2.8 8.4 0.4 2.2 0.1 0.4 0.3 0.1
41 45 50 54 58 62 70 74 82 86 90 98 103 115
8.6 48.0 0.1 1.6 4.0 2.3 1.6 8.2 0.1 3.0 0.4 0.2 0.7 0.1
42 46 51 55 59 67 71 75 83 87 91 99 105 144
19.0 27.6 0.5 9.4 1.6 0.7 3.9 2.5 1.2 1.4 8.8 0.1 0.2 0.1
43 47 52 56 60 68 72 77 84 88 92 101 107
100.0 2.2 0.1 8.1 9.4 1.2 4.6 0.4 0.4 0.1 0.1 1.0 0.1
a
Mass range ) 40-144.
is quite fast, almost two-thirds of the formaldehyde disappears. This step takes as long as either the first or the last step. Such a trend is followed in the entire range of
aqueous solutions (28) both indicate that the concentration of unhydrated monomeric formaldehyde in aqueous solutions is lower than what was observed in this study. The dehydration of methylene glycol at equilibrium has given the following constant at 30 °C (28):
[CH2O][H2O]/[CH2(OH)2] ) 5.7 × 10-4 (mol) (14)
FIGURE 3. Thermokinetic behavior of formaldehyde-lime reaction products. temperature in which the experiments were made. The criteria based on the results presented in Table 5 are illustrated in Figure 3. The initial concentration of formaldehyde has prime importance in the extent of reactions leading to its disappearance with calcium hydroxide. The concentration at which formaldehyde begins to react with calcium hydroxide is around 100 ppm at 92 ( 1 °C (Table 1). This implies that up to this concentration formaldehyde is in monomeric nonhydrated form. When gaseous formaldehyde is added to water, it first dissolves as CH2O(aq) to form a saturated solution of formaldehyde. Then it reacts with water to form hydrated formaldehyde. The concentration of hydrated formaldehyde or methylene glycol increases as the formaldehyde concentration is increased:
CH2O(g) + H2O a CH2O(aq)
(10)
CH2O(aq) + H2O a CH2(OH)2
(11)
Monomethylene glycol dimerizes and polymerizes to diand polyoxymethylene glycol according to
HOCH2OH + HOCH2OH a H2O + HO(CH2O)2H HO(CH2O)nH + HOCH2OH a H2O + HO(CH2O)n+1H
Kd (12)
Kp (13)
where Kd applies to the formation of the dimer and Kp applies to the formation of the trimer and all higher polymers remaining in the solution. The polymers of high molecular weight are not reactive with calcium hydroxide. These polymers react and cause the disappearance of formaldehyde mainly when they gain the form of monomethylene glycol. When concentrated aqueous solutions of formaldehyde are diluted, the concentration of polymeric formaldehyde compounds shifts to lower levels through a hydrolyzing step, which is a relatively slow process around ambient temperatures. The reactants demand longer residence time to convert to methylene glycol (Figure 3). The ultraviolet absorption spectrometry (27) and measurement of apparent molecular weight of formaldehyde in
In a more recent source, the above constant has been cited as 20 × 10-4 (mol) (29). Compared to the dimers and trimers, the fraction of monomethylene glycol increases as temperature is increased. It also increases when the formaldehyde concentratrion decreases (30). In another set of experiments, it was observed that formaldehyde did not disappear in the presence of calcium chloride. Neither did it disappear appreciably in the presence of sodium hydroxide. The above experiments gave identical results for both ambient and boiling temperatures. However, when the pH of the formaldehyde solution containing calcium chloride was raised by sodium hydroxide, formaldehyde began to disappear as usual. This implies that it is calcium hydroxide which catalyzes the reactions leading to disappearance of formaldehyde.
Acknowledgments This work was partially supported by Shiraz University Research Council. The GC-MS analyses were done in the Department of Chemistry, Shiraz University.
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(24) Suzuki, O.; Kawai, A. Patent JP52088264, 1977. (25) Annual Book of American Standard Testing and Materials, ASTM Standards, Standard D6007-96. (26) Turoski, V., Ed. Formaldehyde Analytical Chemistry and Toxicology; ACS Advances in Chemistry Series 210; American Chemical Society: Washington, DC, 1985; p 4. (27) Walker, J. F. Formaldehyde; ACS Monograph Series 159; American Chemical Society: Washington, DC, 1964. (28) Walker, J. F. J. Phys. Chem. 1931, 35, 1104-1113.
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(29) Silig, M. I.; Aksel’rod, B. Y. Russ. J. Phys. Chem. 1968, 42, 14791482. (30) Balashov, A. L.; Danov, S. M.; Golovkin, A. Y.; Krasnov, V. L. Russ. J. Appl. Chem. 1996, 69 (2), 190-192.
Received for review October 12, 2001. Revised manuscript received May 25, 2002. Accepted June 7, 2002. ES015745Z
