Removal of sulfides from waters and wastewaters by activated carbon

Removal of sulfides from waters and wastewaters by activated carbon

Reactwe Po&mers, 5 (1987) 93-104 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 93 REMOVAL OF SULFIDES FROM WATERS AND W A...

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Reactwe Po&mers, 5 (1987) 93-104 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

93

REMOVAL OF SULFIDES FROM WATERS AND W A S T E W A T E R S BY A C T I V A T E D C A R B O N * ROBERT W. PETERS ** and YOUNG KU

Em#ronmental Engineering, School of Civil Engineering, Purdue University, West Lafctvette, IN 47907 ( U. S. A.) (Received October 1, 1985: accepted in revised form July 15, 1986)

The use of activated carbon treatment for removal of sulfides from wastewaters has been shown to be an effective process. Prefiminary results show dissolved sulfide reductions of 36 to 95% were achieved using Darco $51 powdered activated carbon at a dosage of 1000 m g / l to treat a synthetic wastewater containing 1-50 m g / l of dissolved sulfide. Experiments were performed over a p H range of 6.0 to 12.0," the minimum residual sulfide concentration is achieved at p H ~ 8. 5. The removal of sulfides by activated carbon can be adequately described using either a Langmuir or Freundlich isotherm. Various brands and types of activated carbon were investigated, resulting in different adsorption characteristics. The rate of sulfide removal is fast, generally reaching equilibrium in a ve O, short time (t < 30 rain). Adsorption capacities in the range of 150 to 900 I.tmol / g of activated carbon were observed resulting in Ce being in the range of 6.8 to 10.0 mg S / l for large applied sulfide concentrations (in the absence of heavv metals). The presence of several foreign ions ( N H 4 , CN , etc.) interfered with the removal of dissolved sulfide bv activated carbon adsorption due to a competition for the active sites by the sulfide species and the contaminant ions.

INTRODUCTION Hydrogen sulfide ( H 2 S ) is a colorless gas which has caused considerable public concern because of its strong rotten-egg odor (detectable at levels as low as 0.025 ppm). H2S may become a pollutant of greater concern because of its acute toxicity. The Occupational * Paper presented at the Symposium on Adsorption and Ion Exchange, Devision of Industrial and Engineering Chemistry, American Chemical Society, Chicago, IL, U.S.A., September 9 10, 1985. ** To whom correspondence should be addressed. 0167-6989/87/$03.50

Safety and Health Administration (OSHA) has set a 20 ppm ceiling level and a 50 p p m / 1 0 min period per day exposure limit for H 2 S gas evolution. The primary natural source of H2S is anaerobic microbial action on organic matters containing sulfates. It is also found in natural gases and geothermal exhaust, sewers and sewage treatment plants, and waters of some natural springs. H x S is also a well-known pollutant of kraft paper mills, oil refineries, natural gas plants, chemical production plants, and rubber production plants. Because H2S is soluble in water (ap-

© 1987 Elsevier Science Publishers B.V.

94 proximately 0.1 mol/1 at 25.0°C), it can be transported considerable distances before it is released. Dissolved sulfide may be present in forms of H2S(aq), HS(aq) and S2-(aql, depending on the p H of the particular water, as shown in Fig. 1. Discussions on the solubility of sulfides in solution can be found in the literature [4,11,20]. For low p H conditions ( p H < 5), the predominant species is H2S(aq). For p H > 9.0, S 2 and H S - ions are predominant. At low p H values (pH < 5), part of the H2S(g ) will be lost into the atmosphere due to the HzS(aq)-H2S(g ) equilibrium, causing an HzS(g ) emission problem. As an example, at p H 3.0, 68% of H2S~aq) exists in the gas phase. Thus, the sulfide present in water is poten2 forms tially toxic even in the HS(aq) or S~aq) because of the possible release of H2S gas when the solution p H is changed. Activated carbon has been used to remove gaseous H2S from air, but has not been used for adsorption of dissolved sulfide ions from aqueous solution. To our knowledge, the only previous study involving removal of sulfides from solution was a study by Ku and Peters [10,19]. That study investigated the use of activated carbon treatment as a polishing step for lowering the residual metal concentration following hydroxide precipitation; for example, the residual zinc and cadmium concentrations were reduced in excess of 70% through employment of activated carbon treatment

100 90 8O HzS(aq

70

HS

60 5O

~ 4o 3O 20 lO I

0

2

4

6

8

10

12

14

pH

Fig. 1. Speciation for the H2S-HS -S 2 equilibria.

following hydroxide precipitation of the heavy metals. Because of the completeness and rapidity of removal of heavy metals by sulfide precipitation, little enhancement in heavy metal removal was achieved with the use of activated carbon as a polishing step following sulfide precipitation. The significant result of the study was that the use of activated carbon adsorption as a polishing step following sulfide precipitation resulted in a significant reduction in the residual sulfide concentration (reductions in excess of 60%), thereby lessening the potential for H2S gas evolution and the concern for sulfide toxicity. Even in the presence of heavy metals, adsorptive capacities of 200-300 ~ m o l / g of powdered activated carbon for removal of sulfide were observed. The presence of C N - gave higher residual sulfide concentrations than that conducted in the presence of N H 4~. This was due to the competition of the negative ions (S 2 , H S - , and C N - ) for the active sites on the carbon possibly hindering the mass transfer for adsorption onto the activated carbon surface. Tables 1 and 2 summarize the significant results obtained with employment of the activated carbon polishing step following hydroxide precipitation and sulfide precipitation, respectively.

TABLE 1 Summary of activated carbon treatment following hydroxide precipitation [10] • Darco $51 and Calgon F-300 remove more than 80% of the zinc following hydroxide precipitation • Darco HD 3000 is not effective for removal of zinc • The adsorptive capacity for Zn(II) by powdered activated carbon (Darco $51) is 10-20 ~mol/g of activated carbon for the pH range of 8 11 • The adsorptive capacity for Cd(II) by powdered activated carbon (Darco $51) is 5-20 /~mol/g of activated carbon for the pH range of 9-11 • Cyanide has more of an adverse effect on zinc removal than does ammonia, particularly for pH < 9

95 TABLE 2 Summary of activated carbon sulfide precipitation [10]

treatment

following

Due to the completeness of metal sulfide precipitations, only a small portion of the residual heavy metal(s) are removed by activated carbon adsorption The residual sulfide concentration is significantly reduced using activated carbon adsorption; - 6 0 % sulfide removal is achieved Adsorptive capacities of 200-300 ~ m o l / g of activated carbon are achieved for removal of sulfide even in the presence of heavy metals The presence of C N caused higher residual sulfide concentration due to a competition by S 2 , HS , and CN for the active sites on the carbon The minimum residual sulfide concentration was obtained at p H - 8.5 with the activated carbon polishing system The rate of sulfide removal is fast reaching equilibrium in a v e ~ short time (t _< 30 min)

Potential applications of the activated carbon polishing step technology include [10]: Removal of heavy metals in the presence of interferences such as ammonia, cyanide, or chelating agents. Removal of metals with very low effluent water quality standards, such as Cd (0.01 mg/1) and Hg (0.002 mg/1). Enhancement of heavy metal removal following conventional hydroxide precipitation. Minimizing potential U 2 S gas evolution. • In facilities where reuse of the treated water is desired or required.

or complexing agents (such as ammonia or cyanide) may also be present. There are more than 13,000 firms in the U.S. engaged in electroplating and metal finishing operations [6]. By far the most widely used process for removal of heavy metals from solution involves chemical precipitation (using hydroxides, carbonates, or sulfides): nearly 75% of the electroplating facilities employ precipitation treatment [12]. With hydroxide treatment, the metals are removed by adding an alkali to adjust the wastewater pH to the point where metals exhibit minimum solubilities. Advantages and limitations associated with the use of hydroxide precipitation are summarized in Table 3. Sulfide precipitation has also been demonstrated to be an effective alternative to hydroxide precipitation [2,3,9,11,13,14,16-18] for removing heavy metals from industrial wastewaters. Attractive features of sulfide

TABLE 3 Advantages and limitations of the hydroxide precipitation process for removal of heavy metals from solution A d~an rages: • Proven effective in industry

• Well suited to automatic control Limitations: Hydroxide precipitates tend to resolubilize if tile solution pH is changed

• The presence of complexing agents may have an adverse effect on metal removal • Cyanide interferes with metal removal by hydroxide precipitation

BACKGROUND

Chromium(VI) is not removed by hydroxide precipitation

Industrial wastewaters generated in the metal finishing and electroplating industries typically contain heavy metals such as Cu, Cd, Ni, Pb, Zn, Cr, etc. Depending on the origin of these wastewaters, chelating agents (such as EDTA, NTA, tartrate, citrate, etc.)

Removal of metals by hydroxide precipitation of mixed metal wastes may not be effective because the minimum solubilities occur at different pH levels Hydroxide sludge quantities can be substantial and are generally difficult to dewater due to the amorphous particle structure

96 precipitation and major limitations of the process are summarized in Table 4. Two basic sulfide precipitation processes exist, depending on the form in which the sulfide is fed to the reactor. In Soluble Sulfide Precipitation (SSP), a water-soluble reagent (such as Na2S or NariS) is added to the wastewater. In Insoluble Sulfide Precipitation (ISP), a slightly soluble salt (such as FeS or CaS) is added to the wastewater. Eliminating sulfide reagent overdose in these systems prevents formation of the odor-causing H2S. In currently operated soluble sulfide systems that do not match demand, the process tanks must be enclosed and vacuum evacuated to minimize sulfide odor problems. In the ISP processing, FeS dissolves to maintain the sulfide ion concentration at a level of approximately 0.02 ppb. Most heavy metals have sulfides less soluble than ferrous sulfide enabling the heavy metals to precipitate as metal sulfides. Advantages of ISP processing in-

TABLE 4 Advantages and limitations of the sulfide precipitation process for removal of heavy metals from solution

A duantages: • Attainment of a high degree of metal removal even at low pH ( p H - 2-3) • Low detention time requirements in the reactor due to the high reactivities of sulfides • Metal sulfide precipitation is generally less affected by complexes and interferences than is the corresponding metal hydroxide precipitation • Feasibility of selective metal removal and recovery • Metal sulfide sludge exhibits better thickening and dewatering characteristics than the corresponding metal hydroxide sludge • Metal sulfide sludge is three times less subject to leaching at pH 5 as compared with metal hydroxide sludge making final disposal safer and easier

Limitations: • Potential for HzS gas evolution • Concern for sulfide toxicity

clude the absence of any detectable HzS odor and the ability to reduce hexavalent chromium to the trivalent state, thereby eliminating the need to segregate and pretreat chromium waste streams. Limitations of ISP processing include: larger than stoichiometric sulfide concentrations are required and higher sludge production results (as compared with metal hydroxide precipitation or metal sulfide precipitation using SPP processing). For these various precipitation techniques, little metal hydroxide precipitation occurs for pH < 6, while significant reductions in the residual metal concentration results from sulfide precipitation. The potential does indeed exist for evolution of H2S gas. As an attempt to better control this potential for HzS gas evolution, the use of activated carbon as an adsorbent for sulfide removal was investigated. Due to the historical development and use of activated carbon in water and wastewater treatment, most of the applications and research effort with activated carbon have been oriented towards organics removal [8]. Research efforts on inorganics removal (involving heavy metals and sulfides) have been markedly limited. The authors have provided an interesting literature review on the use of activated carbon for removal of heavy metals, such as cadmium, copper, mercury, chromium, etc., from industrial wastewaters [15]. To date, the only study performed on removal of sulfides from solution was the one by Ku and Peters [10,19]. For a clean aqueous system devoid of heavy metals or complexing agents, the following equilibria relationships exist for waters containing sulfides: H 2 S ~ H ÷ + HS

K 1 = 1.0 × 10 _7

(1)

HS- ~ H + + S

K z = 1.2 × 10 -3

(2)

H 2 0 ~ H + + OH

K w = 1.0 × 10 14

(3)

Total sulfide balance: ST=IS 2 ] +[HS

] +[H2S ]

(4)

97 Charge balance [H+] = [ O H - I + [ H S

l+[S 2 ]

(5)

The equilibrium concentration of all species present can be calculated using the appropriate K, values and solving the above set of simultaneous equations. Using such techniques, Fig. 1 is obtained for a temperature of 25 ° C in a clean aqueous system (ionic strength I=0). Adsorption equilibria

The adsorption of sulfide onto activated carbon was examined by application of two commonly used adsorption models: the Langmuir and Freundlich models. Equations describing these models are listed below:

Freundlich: qe

EXPERIMENTAL

Q°bC 1 + bC

Langmuir: qe=

K N C1/"

where: q~ = amount of solute adsorbed per unit weight of adsorbent: C = measured solute concentration in solution at equilibrium; Q 0 = amount of solute adsorbed per unit weight of adsorbent in forming a complete monolayer on the surface; b = a constant related to the net enthalpy of adsorption: and KN, n = empirical constants. These equations are linearized using the equations listed below: Langmuit .

1 qe

1 1 1 + --QO bQ ° C

Freundlich: In % = In

K N 4-

I In C n

or

log q~. = log

1 K N +

--

activated carbon treatment: 1. Measure the removal of total sulfide for various operating conditions (pH, activated carbon type and dosage, contact time, temperature, etc.) in the absence of heavy metals. 2. Compare the adsorption of sulfide by the various brands of activated carbon. 3. Develop the Langmuir and Freundlich isotherms for removal of sulfide by activated carbon. 4. Determine the applicability of these isotherms for the adsorptive behavior. 5. Address the effect of the presence of heavy metals on the adsorptive behavior of sulfides.

log C

n

OBJECTIVES

The major objectives are summarized below for the study on removal of sulfides by

PROCEDURE

Batch adsorption experiments were conducted to determine the sulfide-activated carbon adsorption affinity. Synthetic sulfidecontaining waters were used in this study and prepared as stock solutions by dissolving a precalculated amount of reagent grade sodium sulfide (Na2S) in deionized water. Samples for the stock solution were filtered through a 0.45 /~m membrane filter. Various brands of commercially available activated carbon were examined in this study, including Darco $51, Calgon F-300, and Darco HD 3000. The Darco $51 and Calgon F-300 are in the powdered form while the Darco HD 3000 is in the granular form. Approximately 400 ml of the sulfide-containing stock solution was transferred to a 500 ml glass bottle with a stopper (preventing possible evolution of hydrogen sulfide, H2S ). The pH was adjusted with 0.1 N HC1 and NaOH solutions. The initial pH values were varied from 8.0 to -11.0. After pH adjustment and stoppering, the samples were agitated with magnetic stirrers. All experiments were performed at a temperature of 25.0 + 1.0°C. A typical contact time ranged

98

from 80 rain to 12 h. After agitation, an appropriate amount of sample was obtained for sulfide analysis by filtering through a 0.45 ffm membrane filter. The sulfide analyses performed before and after carbon adsorption treatment were analyzed using the titration technique described in Ref. [1], employing an Orion 941600 A g / S electrode.

RESULTS AND DISCUSSION Preliminary experimental studies focused on the batch adsorption of sulfides (referring to the total sulfides concentration, ST) from wastewaters devoid of heavy metals and complexing agents. Three different activated carbons were studied. Darco $51, Darco HD 3000, and Calgon F-300. Figure 2 shows the effect of pH and initial sulfide concentration on the residual sulfide concentration obtained after activated carbon treatment. The carbon dosage was 1000 mg/1 (Darco $51); the contact time was 80.0 min. The adsorptive capacities ranged from 175 ttmol/g of activated carbon (at the point of minimum residual sulfide concentration, i.e., pH - 8 . 5 for an initial sulfide concentration of 5.8 mg/1 resulting in Ce = 0.2 mg $2-/1) to 340 ffmol/g

of activated carbon (at pH - 8 . 5 and an initial sulfide concentration of 12.8 mg/1 resulting in Ce = 1.9 mg $2-/1). Figure 2 shows the minimum residual total sulfide concentration is obtained at pH - 8.5. Figure 3 shows the effect of pH, initial sulfide concentration, and activated carbon type on the efficiency of sulfide removal. The figure shows that the removal efficiency increases with decreasing sulfide concentration for the Darco $51 activated carbon. The use of Darco HD 3000 activated carbon resulted in a higher sulfide removal efficiency as compared with Darco $51 activated carbon. This result was unexpected since the Darco HD 3000 activated carbon was in the granular form, whereas the Darco $51 and Calgon F-300 were in the powdered form. The reason for this behavior is not known at this time. However, this same type of behavior was observed for removal of Hg(II) from dilute solutions by Huang and Blankenship [7]; the Darco HD 3000 typically provided a mercury removal efficiency approximately 5% greater than that by Darco $51 over the pH range of 6 to 10. In the study on Hg(II) removal by Huang and Blankenship [7], Calgon F-300

9O

4.0

80 ~3.0

.g

o

~

70

2.0

9

% m t.o %

~.

6o

I

50 4.0

50

1

I

I

I

6.0

70

80

90

I

1

100

]1.0

120

DH

~0.0 5.0 pH

Fig. 2. The effect of pH and initial sulfide concentration on sulfide removal by activated carbon adsorption. Activated carbon: Darco $51: activated carbon dosage: 1000 mg/1, contact time: 80.0 min.

Fig. 3. Effect of pH, initial sulfide concentration, and activated carbon type on the efficiency of sulfide removal by carbon adsorption. Key: ~ : Darco $51, S i = 13.6 mg/1; A: Darco $51, S i = 5.8 mg/1; v,: Darco $51, S i = 1 . 7 mg/1; 15]: Darco H D 3000, S i = 7 . 0 rag/l; Q : Calgon F-300, Si = 7.9 m g / l .

99

containing 8.8, 6.2, and 4.5 mg/l of sulfide initially. The pH during this operation was fixed at pH 9.0. With the granular activated carbon, approach to equilibrium is slower than with powdered activated carbon (see Fig. 4a): equilibrium is achieved after a contact time of approximately 3.0 hours. Figures 5 and 6 show the plots of l/q,, versus 1/C, and log q,. versus log C needed to determine the parameters involved in the Langmuir and Freundtich isotherm models. The figures apply to the sulfide removal by Darco $51 activated carbon. Three different pH conditions were investigated: pH = 8.0, 9.0, and 9.7. The carbon dosage was fixed at 1000 mg/1 while varying the initial sulfide concentration for each pH condition listed. The values of q,. predicted from the Langmuir and Freundlich isotherms are also presented in Figs. 5 and 6 for pH levels of 8.0, 9.0, and 9.7, as well as the values predicted from the overall isotherm (regardless of the pH involved over a pH range of 8.0 to 9.7). As shown by the figures, both models provide a very adequate description of the sulfide adsorbed on the Darco $51 activated carbon. It should be noted, however, that both models provide only marginal improvement over a straight-line fit (linear adsorption). The greatest adsorptive capacity (using the Langmuir model occurs at pH 9.0 for the three pH

gave a poorer mercury removal efficiency than either the Darco $51 or Darco HD 3000 activated carbons. However, in this study, Calgon F-300 gave the best removal of sulfide over the pH range of 5.5 to 11.0 for the three activated carbons studied. Calgon F-300 provided relatively constant removal over a wide pH range; the reason for such behavior is not known at this time. Studies are underway to further explore this phenomenon. Further study of the Calgon F-300 activated carbon is warranted because of its excellent removal of sulfides by adsorption. Figure 3 further shows that the removal efficiency of total sulfide exceeded 66% over the entire pH range studied (regardless of which of the three activated carbons was employed). Figure 4a shows the effect of contact time on the removal of sulfide using the Darco $51 powdered activated carbon at a dosage of 1000 m g / l to treat a synthetic wastewater containing either 12.8 or 6.4 m g / l at a pH of 9.0. Extremely low residual sulfide concentrations are achieved after approximately 30 minutes contact time; the rate of sulfide removal by carbon adsorption is rapid reaching equilibrium after approximately 30 minutes. Figure 4b shows the effect of contact time on the residual sulfide concentration obtained with Darco HD 3000 activated carbon treatment for three different synthetic wastewaters

r

F

I

I

I

I

I

I

• 13 91,T

I

I

I

I

3

4

!

I

I

I

I

J

l "~ !i 12 13

l?

i+ 1o

6~ 4

-L.~. (9 @ @ )0

40

5C
SO ~l+n

(9 ]00

120

'?

t

~'

I

1

l

I

I

~

6

?

8

!C

Contact

Time,

h

Fig. 4. Effect o f c o n t a c t time on sulfide removal at p H 9.0 a n d T = 2 5 . 0 ° C by a d s o r p t i o n on: (a) D a r c o $51 activated c a r b o n ( d o s a g e 1000 mg/1); key: <3, S i = 1 2 . 8 r a g / l : 4. S i = 6.4 m g / l . (b) D a r c o H D 3000 activated c a r b o n ( d o s a g e 1000 nag/I); key: ~ , S i = 8.8 m g / l ; (3, S 1 = 6.2 m g / l : 4, S i = 4.5 rag/1.

I00 225

200

175

150

100

0

0.0

0.2

0.4

0.6 1

0.8

l.O

1.2

1.4 1.6

1 mg

te '

Fig. 5. Plot of 1 / q e versus 1 / C e for removal of sulfide by Darco $51 activated carbon to determine the constants in the Langmuir isotherm. Key: (i): pH 1, Series I; ~: pH 9.0, Series II; ,~: pH 9.7, Series III. T = 25.0°C.

conditions investigated, while the greatest adsorptive capacity occurs at pH 8.0 (using the Freundlich model). These results are consistent with the data presented in Fig. 2 where the minimum sulfide concentration after activated carbon treatment was found for pH 8.5. Figure 7 shows the plot of 1 / q e versus 1 / C for the Langmuir isotherm (for adsorption of sulfide by Darco H D 3000 activated carbon). Two different pH conditions were studied: p H - - 8 . 0 and pH = 9.0. The initial sulfide dosages were fixed at 5.6 mg/1 (for the pH 8.0 series) and 6.2 m g / l (for the pH 9.0 series), while the carbon dosage was varied between 0.1 and 1.0 g/1. Figure 8 presents the data in the form of log qe versus log C to determine the Freundlich model parameters (for sulfide removal by Darco HD 3000 activated carbon). The results of the model

200 180 0.I0 0.08

I

0.06 0.05 0.04

I

I I I I F r e u n d l i c h Model Data:

KN

SerGes I

11 Ill Overal I

0.03 v

I

!

I/n

O. 00754 O. 00645 0.00501 0.00652

I

I

I

I

I

n

O.562 O.593 0.626 0.537

~ i

r

1 e g

r

e

s

pH = 8.0

140

i. 781 i. 686 1.597 1.861

120

s 1iI °

o pH

n

u

100

~

80

~Langmuir

®

~ 0.01

Series

0.00~ 0,006 0.005 0.004

--

--

O.OC

I II

40

0.003

v e r. a. .l.l / / / ~ " oreg

1

(~)

0.02 everal

160

overall

= 9.0

Model Data:

Qo m~ S/m~ C

b l/m~ S.

0.175 0.777 0.354

0.0526 0.0118 0.0258

20

I

I I I

0.2 0.3 0.4

I I

0.6 0.8 1.0

I

2

I

3

I I I

4 5 6

I

8 10

Concentration re, mg/l

0

~

0.0

0.2

0.4

0.6 1

Fig. 6. Plot of log qe versus log C for removal of sulfide by Darco $51 activated carbon to determine the constants in the Freundlich isotherm. Key: Q: pH 8.0, Series I; El: pH 9.0, Series II; ,~: pH 9.7, Series III. T = 25.03C.

Ce '

0.8

1.0

"1.2

1.4

1.6

l mg

Fig. 7. Plot of l / q ~ versus 1 / C e for removal of sulfide by Darco H D 3000 activated carbon to determine the constants in the Langmuir isotherm. Key: (i): pH 8.0, Series I; A: pH 9.0, Series II. T = 25.0°C.

101 0200

I

I

I

I

I

I

I

I

I

I

I

O.ICO 0.080 0.C60

overall isotherm

0040

oH : 9.0

C 033 CqSO

=

pH = 8.0

C 01C

C r e u n d l i c h Model Data:

"

C308 pH

KN

n

~In

C.306 8.0 %0 overall

C.2C,I

0.0087/ C.00908 0.00892

0.876 C.968 0.929

1.142 i 033 1.076

C.003

I

C 002 " 2

I

0.3 3.,1

I

I

I

I

8.6 0.8 1 C

2.0

I

I

3.0 4.0

I 60

I

I

8.0 lO.C

20.0

Ce , mg/1

Fig. 8. Plot of log q~. versus log Q for removal of sulfide by Darco H D 3000 activated c a r b o n to determine the constants in the Freundlich isotherm. Key: •: pH 8.0, Series I: G : pH 9.0, Series II. T = 25.0°C.

parameters are summarized in Tables 5 and 6. From the parameter values listed in Tables 5 and 6, as well as the correlation coefficients observed, pH is observed to have a very minor effect on the removal of sulfide by activated carbon for the pH range investigated in this study. The maximum observed adsorptive capacities were 800 and 905 ymol S / g of activated

carbon by the Darco $51 and Darco HD 3000 activated carbons, respectively, resulting in C~ values of 6.8 rag/1 (for an applied sulfide concentration of 32.4 rag/l) and 3.3 m g / l (for an applied sulfide concentration of 6.2 mg/l). Note that the Darco HD 3000 likewise resulted in a somewhat larger adsorptive capacity than did the Darco $51 carbon. Figure 9 shows the residual sulfide concentration resulting from activated carbon treatment (with Darco HD 3000) at various pH levels. The synthetic wastewater contained 8.8 mg/1 of total sulfide initially; the contact time with activated carbon was 3.0 hours. The minimum residual sulfide concentration was obtained for pH in the range of 6.0 to 7.2 (for the pH range studied, i.e. 6.0 to 11.2). Referring to Tables 5 and 6, goodness-of-fit tests were performed for each isotherm developed to test the adequacy of these models for removal of sulfide by activated carbon adsorption. The values of X2 calculated were all far below the X2 critical value. Thus both the Langmuir and Freundlich models provide very adequate descriptions for the removal of sulfide by activated carbon. The X2 value obtained for the Freundlich model was lower than the value obtained for the Langmuir model under comparable pH con0.005

TABLE 5 S u m m a r y of the model parameters for the L a n g m u i r isotherm used to describe the removal of sulfide by activated c a r b o n adsorption Activated

pH

carbon used

L a n g m u i r model, qe = Q°bC/( 1 + hC) Q0

b

Correlation

(rag S / r a g C)

( 1 / m g S)

coefficient, r ~

X

2

Degreesof freedom

D a r c o $51

8.0 9.0 9.7 overall

0.0229 0.0308 0.0248 0.0193

0.516 0.248 0.238 0.518

0.963 0.986 0.989 0.877

0.0036 0.0021 0.0044 0.0201

4 7 6 25

Darco HD3000

8.0 9.0 overall

0.166 0.623 0.354

0.0554 0.0148 0.0258

0.985 0.992 0.988

0.00046 0.00052 0.00153

5 6 14

102 TABLE 6 Summary of the model parameters for the Freundlich isotherm used to describe the removal of sulfide by activated carbon Activated carbon used

pH

Darco $51

Darco HD 3000

Freundlich model, qe = Ny

1/n

Correlation coefficient, r 2

X

8.0 9.0 9.7 overall

0.00754 0.00645 0.00501 0.00652

0.562 0.593 0.626 0.537

0.978 0.979 0.990 0.921

0.00088 0.00091 0.00048 0.00827

4 7 6 25

8.0 9.0 overall

0.00871 0.00908 0.00892

0.876 0.968 0.929

0.990 0.992 0.988

0.00032 0.00051 0.00150

5 6 14

ditions; therefore the Freundlich model provides a clearly better description of the sulfide removal by activated carbon in this particular study. However, both models provide only a marginal improvement over a linear adsorption model. Table 6 further shows that favorable isotherms (Freundlich model) result from the use of Darco $51 activated carbon since the values of 1/n are all less than 1.0. Nearly linear isotherms result from the use of Darco HD 3000 (1/n- 1.0) for removal of sulfide from solution. With the Freundlich model, the highest adsorptive capacity is obtained at

2,C

I

I

I

I

I

I

1.6 L

S

1.4

i.c o.~

o

0.6 0.4 0.2

I

o.o 5.0

KNC1/"

I

I

I

I

I

I

6.0

7.0

8.0

9.0

i0.0

II.0

12.0

pH

Fig. 9. Effect of pH on sulfide removal by Darco HD 3000 activated carbon adsorption. Activated carbon dosage: 1000 mg/1; Si = 8.8 mg/1; contact time = 3.0 h; T = 25.0°C.

2

Degrees of freedom

pH 8.0 for both the Darco $51 and the Darco HD 3000 activated carbons (for the pH conditions studied).

CONCLUSIONS The use of activated carbon treatment has been shown effective for removal of sulfides from synthetic wastewaters in the absence of heavy metals or complexing agents. Adsorptive capacities in the range of 150-900 ~ m o l / g of activated carbon were observed for removal of sulfide in the absence of heavy metals; however, maximum capacities were not determined. Even with heavy metals (such as zinc) and complexing agents (NH~ and C N - ) present, adsorptive capacities for sulfide removal by Darco $51 activated carbon of 200-300 ~ m o l / g of activated carbon were achieved [10]. Due to this fact, it is expected that the activated carbon polishing step will be very effective for removal of the excess sulfide sometimes experienced in metal sulfide precipitation. To shed light on the potential for sulfide adsorption, the adsorption of several common sorbates is compared below for which activated carbon is used. The following adsorbabilities in mg adsorbate/g Filtrasorb

103

300 activated carbon of various organics at an equilibrium aqueous concentration of 1.0 g/1 have been reported in the literature [5]: dieldrin anthracene 1,2-dichlorobenzene sulfides (Darco $51, this work) phenol bromoform 1,1,2-trichloroethane chloroform benzene

606 376 129 < 26 21 20 6 3 1

The removal of sulfides by activated carbon can be adequately modeled using either the Langmuir or Freundlich isotherms; the Freundlich model provides a somewhat better fit to the data than does the Langmuir model. The rate of sulfide removal by powdered activated carbon is rapid reaching equilibrium in a very short time (contact time < 30 rain). Granular activated carbon requires greater time to reach equilibrium (contact time > 3.0 h); however, slightly lower residual sulfide concentrations are achieved using Darco HD 3000 activated carbon as compared with Darco $51 activated carbon. Calgon F-300 activated carbon resulted in extremely high removals of sulfide, the use of Calgon F-300 will be further studied. The use of Darco $51 activated carbon resulted in favorable isotherms, while the use of Darco HD 3000 resulted in nearly linear isotherms. In summary, the use of activated carbon treatment shows great promise for removal of low sulfide concentrations in solution. The technology can be applied to industrial wastewaters or for natural groundwaters containing appreciable amounts of sulfide.

ACKNOWLEDGEMENTS The authors wish to acknowledge the support of the School of Civil Engineering at

Purdue University enabling this research to be performed.

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104 13 R.W. Peters, Y. Ku, D. Bhattacharyya and L.-F. Chen, Crystal size distribution of sulfide precipitation of heavy metals, in: S.J. JanEi6 and E.J. De Jong (Eds.), Industrial Crystallization 84, Proceedings 9th Symposium on Industrial Crystallization, The Hague, The Netherlands, September 25-28, 1984, Elsevier, Amsterdam, 1984, pp. 111-123. 14 R.W. Peters, Y. Ku and D. Bhattacharyya, The effect of chelating agents on the removal of heavy metals by sulfide precipitation, Proceedings 16th Mid-Atlantic Industrial Waste Conference, Vol. 16, 1984, pp. 289-317. 15 R.W. Peters, Y. Ku and D. Bhattacharyya, Evaluation of recent treatment techniques for removal of heavy metals from industrial wastewaters, Separation of Heavy Metals and Other Trace Contaminants, AIChE Syrup. Ser., 81 (243) (1985) 165-203. 16 R.W. Peters, and Y. Ku, Batch precipitation studies for heavy metal removal by sulfide precipitation, Separation of Heavy Metals and Other Trace Contaminants, AIChE Symp. Ser., 81 (243) (1985) 9-27.

17 R.W. Peters and Y. Ku, The effect of ammonia on the removal of heavy metals from mixed metal plating wastewaters, Paper presented at the 49th Annual Conference of the Indiana Water Pollution Control Association, Adam's Mark Hotel, Indianapolis, IN, August 19-21, 1985. 18 R.W. Peters and Y. Ku, Removal of heavy metals from industrial plating wastewaters by sulfide precipitation, Proceedings Industrial Wastes Symposia, 57th Water Pollution Control Federation Annual Conference, 1984, pp. 279-311. 19 R.W. Peters and Y. Ku, The use of activated carbon following sulfide precipitation for removal of residual zinc from plating wastewaters, Proceedings 5th International Conference Heavy Metals in the Environment, Athens, Greece, September 10-13, 1985, Vol. 1, pp. 644-648. 20 W. Stumm and J.J. Morgan, Aquatic Chemistry: An Introduction Emphasizing Chemical Equilibria in Natural Waters, 2nd edn., John Wiley & Sons, New York, NY, 1981.