United States

Environmental Protection

Agency

Technology Transfer

Office of Research and

Development

Washington DC 20460

GEPA Capsule Report

EPAl625lR971004

July 1997

Aqueous Mercury Treatment
EPA/625/R-97/004

July 1997

Capsule Report:

Aqueous Mercury Treatment

National Risk Management Research Laboratory

Office of Research and Development

U.S. Environmental Protection Agency

Cincinnati, OH 45268

@ Printed on Recycled Paper
Acknowledgements

This capsule report was prepared under contract number 68-C3-0315 by Eastern Research

Group, inc. (ERG) for the U.S. Environmental Protection Agency’s (USEPA) Office of Research

and Development (ORD). Edwin Barth served as the work assignment manager and provided

technical direction. Linda Stein of ERG directed the editing and production of this report.

James Patterson, of Patterson and Associates, Inc., Chicago, IL was the primary author.

Technical reviewers of this report were:

Richard Osantowski, Radian international Corporation, Milwaukee, WI

Thomas Sorg, National Risk Management Research Laboratory, USEPA, Cincinnati, OH

Donald Sanning, National Risk Management Research Laboratory, USEPA, Cincinnati, OH

Jerry Stober, Environmental Services Division, USEPA, Athens, GA
Notice

This document has been reviewed in accordance with the U.S. Environmental Protection Agency’s

peer and administrative review policies and approved for publication. Mention of trade names or

commercial products does not constitute endorsement or recommendation for use.

. . .

111
Chapter 1 Executive Summary l-l

Chapter 2 Precipitation Treatment Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-l

2.1 Sulfide Precipitation ................................ 2-l

2.2 Coagulation/co-precipitation .......................... 2-2

Chapter 3 Adsorption Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-l

Chapter 4 Ion Exchange Treatment . . . . . . . . . . . . . . . . . . . . , . . . . . , . . . . . . . 4-l

Chapter 5 Other Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-l

Contents

Page

1.1 Purpose ......................................... l-l

1.2 Summary ........................................ l-l

3.1

3.2

3.3

Activated Carbon Adsorption ......................... 3-l

Xanthate Treatment ................................ 3-3

Other Adsorption Processes .......................... 3-4

3.3.1 BPHC Adsorption ............................. 3-5

3.3.2 MHBB Adsorption ............................. 3-5

3.3.3 Coal Fly Ash Adsorption ........................ 3-5

3.3.4 Forager Sponge Adsorption ..................... 3-5

5.1 Chemical Reduction ................................ 5-l

5.2 Membrane Separation .............................. 5-l

5.3 Emerging Technologies ............................. 5-5

5.3.1 Macrocycles Adsorption ........................ 5-5

5.3.2 Biological Detoxification ........................ 5-5

5.3.3 Membrane Extraction .......................... 5-5

iv
Contents (Continued)

Page

Chapter 6 Comparison of Treatment Processes . . . ,. . . . . . . . . . . . , . . . . . . . . 6-l

6.1 Treatment Effectiveness ............................. 6-l

6.2 Residuals Management ............................. 6-l

6.2.1 Mercury Sludges .............................. 6-l

6.2.2 Spent Activated Carbon ........................ 6-l

6.2.3 Concentrated Brine Solutions .................... 6-l

6.3 Economics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2

Chapter 7 Casestudy............................................. 7-l

7.1 Ion Exchange Removal of Mercury from Wastewater at

DOE’s Savannah River Site Effluent Treatment Facility . . . . . 7-l

7.1.1 The ETF ..................................... 7-l

7.1.2 ETF Feed Streams ............................ 7-3

7.1.3 ETF Discharge Limits .......................... 7-3

7.1.4 Ion Exchange Role in the ETF ................... 7-3

7.2 Effluent Treatment Facility Economics , . . . . . . . . . . . . . . . . . 7-5

Chapter 8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-l

V
List of Tables

Table No. Page

2-l

2-2

3-l

3-2

3-3

4-l

4-2

5-l

5-2

6-l

6-2

7-l

7-2

Sulfide Precipitation treatment for mercury .......................... 2-2

Coagulation/co-precipitation treatment results for mercury .............. 2-3

Activated carbon mercury treatment results .......................... 3-3

Starch xanthate treatment for mercury .............................. 3-4

Freundlich isotherm parameters for mercury adsorption ................ 3-5

Summary of mercury-selective chelate resins ........................ 4-2

Ion exchange treatment for mercury in drinking water .................. 4-2

Performance of reduction processes for mercury treatment .............. 5-2

Performance of membrane processes for mercury treatment ............ 5-3

Summary of achievable effluent mercury concentrations ................ 6-1

Operating costs (U.S. $/year 1987) of processes for mercury

removal from chlor-alkali wastewaters .............................. 6-3

Nonradioactive contaminants and discharge limits ..................... 7-4

Radioactive contaminants and discharge limits ....................... 7-4

List of figures

Figure no. Page

2-l

3-l

3-2

5-I

5-2

5-3

5-4

6-l

7-l

7-2

Sulfide precipitation ............................................ 2-l

Types of GAC column design .................................... 3-l

GAC process flowsheet ......................................... 3-2

Principles of the crossflow microfilter ............................... 5-4

Filtrate polishing using microfiltration ............................... 5-4

Schematic representation of mercury extraction with an emulsion

liquid membrane .............................................. 5-6

Schematic representation of mercury ion extraction with an

emulsion liquid membrane ....................................... 5-6

Flow chart of unit operations utilized for technical-economical

analysis ..................................................... 6-2

F/H effluent treatment facility process flow sheet ...................... 7-2

ETF facility O&M cost breakdown ................................. 7-5

Vi
Chapter 1

Executive Summary

1 .I Purpose

This report describes established technologies and

identifies evolving methods for treating aqueous mercury.

The information provided encompasses full-, pilot- and

bench-scale treatment results as presented in the technical

literature. The report describes alternative technologies in

terms of (1) governing physical and chemical principles

(e.g., solubility, oxidation-reduction potential, volatility), (2)

key treatment parameters (e.g., speciation, pH,

precipitating agent type and dosage, or adsorbent type and

dosage), (3) pretreatment requirements, treatment

performance, advantages and disadvantages, design

considerations, and economics when available. This

information can be useful for evaluating mercury treatment

alternatives for industrial wastewater, groundwater, and soil

washing extract.

This document assumes that the reader is already well

versed with the technologies described and is using this

report to better understand each technology’s applicability

for aqueous mercury removal. Thus, the report does not

provide basic descriptions of each technology; such

information can be found elsewhere in the literature. In

addition, the report does not present recommended values

for the common design parameters of technologies.

Values for such parameters as (1) contact time, (2)

volumetric loading rates, (3) dosages, (4) reaction times,

(5) breakthrough times, and (6) mixing requirements can

be determined by conducting treatability studies using the

wastewater to be treated.

1.2 Summary

A broad spectrum of mercury treatment technologies

has been described in the technical literature, ranging from

established full-scale applications to innovative approaches

investigated to date only at bench or pilotscale. The

literature, however, provides only limited information on

actual full-scale treatment technology performance and

almost no full-scale economic data or information on

mercury recovery.

Well-established and widely reported full-scale

technologies are precipitation, coagulation/co-precipitation,

and activated carbon adsorption. Representative data from

aqueous mercury treatment operations using these

methods are provided in this report.

Another technology is ion exchange treatment, which

has historically been limited to the use of anion resins to

process industrial wastewater that contains inorganic

mercury in the complex mercuric chloride form. Chapter 8

provides a case study illustrating the use of an ion

exchange system for mercury removal.

Other, less-established methods for treating aqueous

mercury that are discussed in this report include chemical

reduction, membrane separation, and emerging

technologies involving macrocycles adsorption, biological

treatment, and membrane extraction.

Each of the mercury treatment technologies described

in this report achieves different effluent mercury

concentrations. The effectiveness of treatment provided by

each type of technology depends on the chemical nature

and initial concentration of mercury as well as the presence

of other constituents in the wastewater that may interfere

with the process. As indicated by example data provided,

co-precipitation and ion exchange achieve the lowest

effluent mercury concentrations for many waste streams,

ranging from 0.5 to 5.0 ,ugIL. Membrane technology

typically achieves 80 to 90 percent rejection of mercury.

Other factors, however, such as residuals management

and costs, weigh heavily in selecting the appropriate

treatment approach.

l-l
Chapter 2

Precipitation Treatment Process

This chapter presents information on precipitation and

coagulation/co-precipitation technologies, which are among

the most well-established approaches for removing

mercury from wastewater. The information provided

includes example data from aqueous mercury treatment

operations using these methods.

2.1 Sulfide Precipitation

One of the more commonly reported precipitation

methods for removal of inorganic mercury from wastewater

is sulfide precipitation. In this process, sulfide (e.g., as

sodium sulfide or another sulfide salt) is added to the

wastestream to convert the soluble mercury to the relatively

insoluble mercury sulfide form:

Hg*+ + S*‘ * HgS,,, P-11

As with other precipitation treatment, the process is

usually combined with pH adjustment and flocculation,

followed by solids separation (e.g., gravity settling, filtraFigure 2-1 Sulfide precipitation.

Acid/Base Sulfide precipitant Chemical flocculants

and/or settling aids

lnfluent

tion). A typical process flow diagram for sulfide precipitation is shown in Figure 2-l. The sulfide precipitant is

added to the wastewater in a stirred reaction vessel, where

the soluble mercury is precipitated as mercury sulfide. The

precipitated solids can then be removed by gravity settling

in a clarifier as shown in Figure 2-1. Flocculation, with or

without a chemical coagulant or settling aid, can be used to

enhance the removal of precipitated solids.

Table 2-l presents example sulfide treatment results.

For initial mercury levels in excess of 10 mg/L, sulfide

precipitation can achieve 99.9+% removal. Even with

polishing treatment such as filtration the minimum effluent

mercury achievable appears to be approximately 10 to

100 pg/L. The most effective precipitation, with regard to

minimizing sulfide dosage, is reported to occur in the nearneutral pH range. Precipitation efficiency declines significantly at pH above 9 (Patterson, 1985). Sulfide precipitation appears to be the common practice for mercury control

in many chlor-alkali plants. removal efficiencies of 95 to

99.9 percent are reported for well designed and managed

Sludge

clarifier

2-l
Table 2-1. Sulfide Precipitation Treatment for Mercury (After Patterson, 1985)

Mercury Concentration (pg/L)

Treatment

Chemical

Initial Final

Percent Mercuty Removal Treatment

PH

Additional Treatment

Sodium sulfide NA <3 NA

300-6,000 IO-125 58-99.8

1,000~50,000 10 99999.9

NA

NA

NA

Vacuum filter

Pressure filter

Flocculation + activated carbon

Sodium hydrosulfide

Magnesium sulfide

“Sulfide” salt

131,50

5,000-10,000

300-6,000

NA

NA

NA

20

1 O-50

lo-125

(50 w)

100-300

100

1 O-20

>99.9

99-99.9

58-99.8

NA

NA

NA

3.0

10-11

5.1-8.2

NA

NA

NA

“Filter”

None

Filtration

None

None

Activated carbon

NA = Not available.

treatment Mercury systems (Perry, 1974; U.S. EPA, 1974).

A mercury effluent level of about 65 pg/L has been reported for sodium sulfide treatment of wastewaters from

the chlor-alkali industry; influent mercury concentration was

not reported (U.S. EPA, 1974). Costs of using the sulfide

process for the treatment of chlor-alkali wastewater were

reported to be $0.79/1,000 gal (1987 basis), exclusive of

sludge management. Capital cost (adjusted to 1995 basis)

for a chlor-alkali plant utilizing sodium sulfide addition plus

diatomaceous earth filtration for a lOO-gpm flow was

$2,767.47 /I ,000 gpd capacity (Perry, 1974). One consequence of the application of sulfide precipitation technology

is stockpiles of mercury-laden process sludges, which must

be either disposed of in an environmentally acceptable

manner or processed for mercury recovery. Thus, the

sludge management approach chosen is a key factor in

evaluating the sulfide process for treating such wastewater.

In addition to its inability to reduce mercury below 10 to

100 pg/L, other drawbacks of this method include: (1) the

formation of soluble mercury sulfide species at excess

dosage of sulfide, due to the common ion effect, (2) the

difficulty of real-time monitoring of reactor sulfide levels, (3)

the generation of toxic residual sulfide in the treated

effluent (a potential problem), (4) the difficulty of clarification and sludge processing, and (5) the need to dispose of

sulfide sludges. Investigators have reported that mercury

can resolubilize from sulfide sludges under conditions that

can exist in landfills (Hansen and Stevens, 1992). This

could in mercury contamination of leachate and potential

result ground-water pollution.

2.2 Coagulation/co-precipitation

Information is available in the literature on the removal

of both inorganic and organic mercury by coagulation/coprecipitation for a variety of mercury-containing wastewaters (Patterson, 1985). Coagulants employed include

aluminum sulfate (alum), iron salts, and lime. For alum

and iron, the dominant mercury removal mechanism is

most likely by adsorptive co-precipitation (Patterson et al.,

1992). Here, one ion is adsorbed into another bulk solid,

formed, for example, by addition of alum and precipitation

of aluminum hydroxide or by addition of an iron (ferrous or

ferric) salt and precipitation of iron hydroxide. The adsorption process is isothermal, and treatment performance can

be enhanced by optimal bulk solids formation and by pH

manipulation to optimize bulk solid surface change and

soluble mercury speciation.

In studies on the treatment of inorganic mercury dosed

to domestic sewage, both iron and alum co-precipitation,

followed by filtration, reduced initial mercury levels of 50 to

60 ,ug/L by 94% to 98%. Lime coagulation treatment,

applied at a higher mercury level of 500 pg/L, achieved 70

percent removal upon filtration (Patterson, 1985). Treatment data for coagulation/co-precipitation are summarized

in Table 2-2. Effluent levels of mercury achieved by alum

treatment range from 1.5 to 102 pg/L, with a typical 5 to 10

pg/L value, and by iron treatment from 0.5 to 12.8 ,ug/L.

2-2
Table 2-2. Coagulation/co-precipitation Treatment Results for Mercury (After Patterson, 1985)

Mercury, pg/L

Coagulant

Salt

Coagulant

Dosage

OWL) Initial Final

Percent Mercut-y Re- Treatment Additional

moval PH Treatment

Alum 1,000 11,300

100 90

100 NA”

21-24 5.9-8.0

NA 50

220 60

20-30 3-8

20-30 3-16”

102 99

11 88

10 NA

5.3-7.4 1 o-34

26.5 47

3.6 94

1.5-6.4 50-81

2.3-21.3 ~23

3

NA

NA

6.7-7.2

7.0

6.4

NA

NA

Filtration

-

-

Filtration

Filtration

Filtration

-

-

Iron 34-72 4.0-5.0

NA 50

40 50

20-30 1-17

20-30 2-17’

2.5 38-50 6.9-7.4

3.5 93 8.0

1.0 98 6.2

0.5-6.8 50-97 NA

1.2-12.8 40-93 NA

Filtration

Filtration

Filtration

-

-

Lime 415

NA

- 500 150 70 11.5 Filtration

0.66 co.2 >69 8.3 -

“Organic mercury.

NA = Not available.

- = None

2-3
Chapter 3

Adsorption Processes

Adsorption processes have the potential to achieve

high efficiencies of mercury removal and/or low effluent

mercury levels. The predominant adsorption process

utilizes activated carbon, but the use of other adsorbents

also are reported in the literature. These include processed vegetable or mineral materials such as

bicarbonate-treated peanut hull carbon (BPHC), modified

Hardwickia bin&a bark (MHBB), coal fly ash, and the

Forager sponge (Namasivayam and Periasamy, 1993; Sen

and De, 1987; Deshkar et al., 1990; U.S. EPA, 1994b).

Metal hydroxides are also used as adsorbents. When

metal hydroxides are employed for adsorptive treatment,,

the process is commonly termed coagulation or co-precipitation. (This process is discussed in Chapter 2.) An

inherent advantage of adsorptive treatment, particularly

when the adsorbent displays isothermal or quasi -isothermal behavior, is that increased treatment efficiency results

Figure 3-1. Types of GAC column design (Calgon Carbon Corp.)

lnfluent

from incremental adsorbent dosage. Isothermal behavior

is observed when, for a fixed initial pollutant concentration,

decreasing residual soluble concentrations are observed

as the dosage of adsorbing treatment material is added.

Unless adsorbent recovery is feasible, these incremental

dosages also result in production of increased wastewater

treatment residuals, requiring ultimate disposal. Variables

other than adsorbent type and’dosage can also affect

adsorption efficiency. Common variables include wastewater pH and pollutant speciation.

4.1 Activated Carbon Adsorption

Granular activated carbon (GAC) is the most commonly

used adsorbent system for treating industrial waste (U.S.

DOE, 1994). This process is used in a variety of confrgurations, as demonstrated in Figures 3-1 and 3-2. GAC

systems may be either pressure or gravity type. They may

Granular Granular

activated lnfluent activated

carbon carbon

1

7-T-l lDownflow in series

(4

Moving-bed

(B)

Effluent

Downflow in parallel Upflow expanded in series

(Cl PI

3-1
Figure 3-2. GAC process flowsheet (after Eckenfelder, 1989)

From storage I

reservoir

----

v-’

Raw water

teed pump

Backwash effluent

Vlr In carbon

ma 9,cup

Slurry

mixing IQ- tank

Re enerated

car %, nretum

Backwash

pump

----_---__-_

Final

effluent

*

Hlgh pressure Eductor

Eductor water

be upflow counter-current type with packed or expanded

carbon beds, or upflow or downflow fixed-bed units with

multiple columns in series (Figure 3-l). Contaminated

water is passed through the columns until the key contaminant is detected at a predetermined level in the effluent.

When multiple columns are placed in series, the first

column can be loaded to a greater capacity, while residual

levels of the contaminant are removed in the downstream

columns. When a column has been loaded to its design

capacity, it may be regenerated or the spent carbon can be

replaced while another column is brought online. An

alternative method of carbon treatment involves use of

powdered activated carbon (PAC). The PAC is typically

added as a slurry into a contact reactor, and the PAC solids

subsequently are removed in a solids separation stage.

The PAC is normally not regenerated for’ reuse due to

unfavorable economics including poor recovery of the PAC.

Table 3-I summarizes example activated carbon

mercury treatment data. The removal of mercury from

potable water using PAC was studied by Thiem and

colleagues (1976). Treating a spiked water solution

containing IO ,ug/L total mercury, they achieved approximately 80% removal at a pH of 7 and a PAC dosage of

100 mg/L. The study also demonstrated that the addition

of mercury chelating agents, such as ethylene diamine

triacetic acid (EDTA) or tannic acid, prior to contact with the

PAC increased mercury removal efficiency. Concentrations as low as 0.02 mg/L EDTA and 1 mg/L tannic acid

increased mercury removal efficiencies by 10% to 20% .

The mercury removal efficiencies by concentrations of 50

to 200 mg/L also increased mercury removal efficiencies by

10% to 20% over those obtained by PAC alone.

The removal of mercury (II) from synthetic wastes by

11 different brands of commercial activated carbon was

studied by Huang and Blankenship (1984). Among the 11 :

different types of activated carbon, Nuchar SA and Nuchar

SN exhibited a high percent (>99.9) mercury (II) removal

over a wide pH range (2.5 to 11). The other activated

carbons studied displayed maximum total mercury (II)

removal at pH 4 to 5, and the percent mercury (II) removal

dropped markedly at pH values greater than and less than

4 to 5.

Pretreatment or modification of activated carbon with

carbon disulfide solution before use, has been shown to

enhance mercury removal. Humenick and co-investigators

(1974) utilized an activated carbon that was presoaked in

carbon disulfide and then dried and used as PAC. The

pretreated activated carbon removed mercury from an

initial concentration of 10 mg/L down to 0.2 ,ug/L, versus

the 4 mg/L effluent value obtained with the untreated

carbon. The enhanced mercury removal was attributed to

chemisorption reactions. Sulfur atoms have a high affinity

for mercury, as evidenced by the Ksp of HgS (see Table 2-

2). The mercury removal mechanism proposed by Humenick and colleagues (1974) involves transport and diffusion to the carbon disulfide sites and subsequent formation

of a chemical bond between a carbon disulfide molecule

and the mercury ion.

3-2
Table 3-l. Activated carbon mercury treatment results

Mercury Concentration (PglL)

Activated Car- Percent Additional Other

bon Type initial Final Removal Treatment Conditions Reference

PAC 10,000 4,000 60 None SW, BS Humenick et al.,

1974

PAC 10,000 0.2 b99.9

PAC 2,000 NA -100

5 ym filtration, PAC SW, BS Humenick et al.,

presoaked in CS, and dried 1974

Centrifugation or 0.45 grn SW, BS Huang and

filtration Blankenship,

1984

PAC 10

PAC 1.0

GAC O-100

NA -80

0.5 50

Cl.0 >4l

0.45 pm filtration SW, BS Thiem et al., 1976

Settling PW, BS Guarino et al., 1988

None SF, FS EC. Jordan Co.,

1989

GAC 1.7 0.9 47

1.5 0.8 47

Filtration PW, BS Guarino et al., 1988

PAC = Powdered activated carbon.

GAC = Granular activated carbon.

BS = Bench scale.

SW = Synthetic wastewater.

PW = Petrochemical wastewater.

SF = Superfund wastewater.

FS = Full scale.

NA = Not available.

A study was conducted by Guarino and co-investigators (1988) to establish the feasibility of using activated

carbon as an advanced treatment method for petrochemical wastewater. This study investigated petrochemical

wastewater at bench scale, utilizing GAC and PAC. Low

initial mercury levels of 1.5 and 1.7 ,ug/L were reduced to

0.8 and 0.9 /*g/L, respectively, using GAC, while an initial

mercury concentration of ? .O pg/L was reduced to 0.5 pg/L

using PAC. The performance data reported in the literature

suggests that activated carbon treatment can achieve a

residual mercury level of 0.5 to 20 gg/L, dependent in part

on the initial wastewater mercury level (Patterson et al.).

Gates and colleagues (1995) conducted laboratory

work to investigate the feasibility of using inexpensive

sulfur-impregnated activated carbon beads, known as

Mersorb, for mercury removal from aqueous waste. These

studies were conducted to evaluate the treatability of

mercury-containing aqueous and solid mixed wastes stored

at DOE sites, such as the Oak Ridge Y-12 site. The from

aqueous solutions to below 0.2 mg/L. Mersorb worked

under acidic conditions (pH of 2)‘ but its capacity at low pH

was reduced by 50% compared with neutral conditions.

Mersorb beads reportedly had favorable process economics compared with ion exchange.

3.2 Xanthate Treatment

An alternative adsorption material to activated carbon

is starch xanthate, yielding mercury-starch xanthate. One

modification is termed the Metals Extraction by Xanthate

lnsolubilization and Chemical Oxidation (MEXICO) process, also termed the Advanced MEXICO Precipitation

Process (Macchi et al., 1985; Tiravanti et al., 1987). Most

published data on this process appears to be from benchand pilot-scale studies. No published information was

available on full-scale application.

Example data for starch xanthate treatment are

presented in Table 3-2. Campanella and colleagues (1986)

were able to reduce the mercury concentration in a synthetic wastewater at bench scale from 10 to 23 pg/L

3-3
Table 3-2. Starch Xanthate Treatment for Mercury

Mercury Concentration (mg/L)

Initial Final Treatment

PH

Additional Treatment Other

Conditions Reference

10 0.023 1

100 0.001 5

9.5 0.01-0.1 5

9.5 0.005-0.02 5

6.3 -0.2 11

6.3 0.01 11

6.3 0.001 NA

Sedimentation

0.45 pm filtration

Sedimentation

Sedimentation plus 0.45 pm filtration

10 pm filtration

Sodium hypochlorite addition

Activated carbon

SW, ES Campanella et al., 1986

SW, BS Tiravanti et al., 1987

cw, PS Tiravanti et al., 1987

cw, PS Tiravanti et al., 1987

cw, BS Macchi et al., 1985

CW, BS Macchi et al., 1985

CW, BS Macchi et al., 1985

SW = Synthetic wastewater.

BS = Bench scale.

cw = Chlor-alkali wastewater.

PS = Pilot scale.

following sedimentation. Tiravanti and co-investigators

(1987) were able to reduce mercury at bench scale from

100 to 1 pg/L following 0.45 pm filtration. These researchers also conducted pilot-scale (15 m3/d) experiments on

chlor-alkali wastewater and were able to reduce the

mercury concentration from 9.5 mg/L to a range of 10 to

100 pg/L following sedimentation, and to a range of 5 to 20

pg/L following sedimentation and 0.45 pm laboratory

filtration (to estimate residual soluble mercury). Macchi

and colleagues (1985) conducted bench-scale experiments

on chlor-alkali wastewater and were able to reduce the

mercury concentration from 6.3 to 200 ,ugIL following

10 pm filtration, to 10 PglL following sodium hypochlorite

addition, and to 1 ,ug/L following activated carbon treatment. The process appears able to achieve an effluent

mercury level of 5 to 20 PglL.

Macchi and colleagues (1985) also reported that

mercury can be recovered from the mercury-xanthate

sludges by treating the precipitate with 5 M hydrochloric

acid and sodium hypochlorite. The cost of sodium hypochlorite is relatively insignificant for the chlor-alkali industry,

and the redissolved mercury reportedly could be recycled

to the head of the chlor-alkali plant.

3.3 Other Adsorption Processes

Various other adsorbent alternatives to activated

carbon have been reported to perform in comparable

fashion for mercury treatment. These adsorbents include

BPHC, MHBB, coal fly ash, and the Forager sponge. Each

of these adsorbents is described in the following sections.

Table 4-3 presents mercury adsorption Freundlich

parameter values for these adsorbents, except the Forager

sponge. The Freundlich adsorption equation is:

log 1( = log k + 1 log C, 13-11

m n

Where:

X = the amount of solute (mercury) adsorbed

m = the amount of adsorbent required to adsorb x

k and I= empirical constants (Freundlich parameters)

n

ce = equilibrium concentration (mercury) The Freundlich

parameters k and 1 are equal to the intercept and slope

n

of the line obtained by plotting log x vs. log C,.

M

3-4
Table 3-3. Freundlich Isotherm Parameters for Mercury Adsorption

1

Adsorbent k n Reference

GAC 4.68 3.16 .Namasivayam and Periasamy, 1993

BPHC 42.17 3.50 Namasivayam and Periasamy, 1993

Coal fly ash 1.014 0.053 Sen and De, 1987

(PH 2.2)

Coal fly ash 1.094 0.333 Sen and De, 1987

(PH 3.1)

Coal fly ash 1.230 0.361 Sen and De, 1987

(OH 4.2)

MHBB 1.07 0.324 Deshkar et al., 1990

GAC = Granular activated carbon.

BPHC = Bicarbonate-treated peanut hull carbon.

MHBB = Modified Hardwickia binara bark.

The value of k is roughly an indicator of sorption capacity,

and 1 is an indicator of sorption intensity.

n

3.3.1 BPHC Adsorption

From bench-scale study, using a stock mercury

solution feed of 10 to 20 mg/L, Namasivayam and Periasamy (1993) reported BPHC to be seven times more

effective than GAC for mercury (II) removal. This result

was attributed to the higher porosity plus moderate ion

exchange capacity of BPHC as compared to GAC. The

Freundlich parameters shown in Table 3-3 quantify the

sorption capabilities of BPHC. The desorption capabilities

of BPHC also were reported to be promising. Percent

recoveries of mercury from BPHC and GAC using 0.6 M

HCI were 47% and 13%, respectively, and 87% and 24%,

respectively, using 1.0% KI (potassium iodide). No fullscale data were available on this material.

3.3.2 MHBB Adsorption

A modified Hardwickia Binafa bark was studied at

bench-scale for its adsorption of mercury (II) from water

(Deshkar et al., 1990). Although the media was shown to

be effective in removing mercury (II) from water, it is not as

effective as GAC, as indicated by the Freundlich parameters listed in Table 3-3. No information was reported on the

desorptive properties of the Hardwickia binafa bark.

3.3.3 Coal Fly Ash Adsorption

Coal fly ash, an industrial waste solid, was shown to

adsorb mercury (II) (Sen and De, 1987). Coal fly ash did

not perform as well as GAC, however, as shown by the

Freundlich parameters listed in Table 3-3. Maximum

mercury adsorption by coal fly ash was observed in the pH

range 3.5 to 4.5 (Sen and De, 1987).

3.3.4 Forager Sponge Adsorption

The Forager sponge is an open-celled cellulose sponge

with an amine-containing polymer that reportedly has a

selective affinity for aqueous heavy metals in both cationic

and anionic states. The polymer is reported to form

complexes with ions of transition-group heavy metals,

providing ligand sites that surround the metal and form a

coordination complex. The polymers order of affinity for

metals is reportedly influenced by solution parameters such

as pH, temperature, and total ionic content. Mercury is one

of the metals that is claimed to be removed by the sponge.

In general, the following affinity sequence for representative ions is expected (U.S. EPA, 1994b):

The sponge can be used in columns, fishnet-type

enclosures, or rotating drums. When used in a column,

flow rates of 3 bed volumes per minute are reported to be

obtained at hydrostatic pressure only 2 feet above the bed

and without additional pressurization. Therefore, spongepacked columns are claimed to be suitable for unattended

field use.

Adsorbed ions can be eluted from the sponge using

techniques typically employed to regenerate ion exchange

resins and activated carbons. Following elution, the

sponge can be reused in the next adsorption cycle. The

number of useful cycles is reported to depend on the

nature of the adsorbed ions and the elution technique

3-5
used. Alternatively, the metal-saturated sponge could be sponge may be dried and reduced in volume to facilitate

incinerated. Metals volatilization would be of concern. The disposal (U.S. EPA, 1994b).

3-6
Chapter 4

Ion Exchange Treatment

Resins containing the iminodiacetic acid group will

exchange for cationic mercury selectively over calcium and

magnesium, but copper and cobalt are also readily exchanged. Mercury in the form of anionic complexes, such

as HgCI‘,, can be treated by anion exchange resins. The

thiol resin, Duolite GT-73, is reported to be selective for

mercury in any of its three oxidation states (Ritter and

Bibler, 1992).

Ion exchange processes are typically operated as

packed columns. Usually four operations are carried out in

a complete ion exchange cycle: service,. backwash,

regeneration, and rinse. In the service step, the ion

exchange resin in the packed column is contacted with the

water containing the mercury to be removed. After a target

concentration of mercury in the column effluent is reached,

the resin is said to be spent. A backwash step is then

initiated to expand the bed and to remove fines that may be

clogging the packed bed. The spent resin is then regenerated by exposing it to a concentrated solution of the original

exchange ion, so that a reverse exchange process occurs.

The rinse step removes excess regeneration solution

before the column is brought back online for the next

service cycle.

Reported advantages and disadvantages of ion exchange include (Clifford et al., 1986):

Advantages

Operates on demand

Is relatively insensitive to variability

Can achieve essentially a zero level of effluent

contaminant

Is available in a large variety of specific resins

Can normally achieve beneficial selectivity reversal

upon regeneration

Disadvantages

n Has potential for chromatographic effluent peaking

n Results in spent regenerant brine that must be

disposed of

q Can yield variable effluent quality

B Cannot typically be used for waters with a high

total dissolved solids content

Ion exchange technology for mercury removal has

historically been limited to the use of anion resins to treat

industrial wastewater that contains inorganic mercury in the

complex mercuric chloride form. For the process to be

effective, the chloride content of the wastewater must be

high, such as that generated by a chlor-alkali plant. This

will yield negatively charged mercury chloride complexes.

If the chloride content of the wastewater is low, either

chlorine or chloride salt could be added to improve removal

process efficiency (Sorg, 1979).

Cation exchange of mercury may be effective if the

anion content of the wastewater is low (Sorg, 1979).

Certain cation exchange resins (Amberlite IR-120 and

Dowex-50W-X8) are reported to be effective for ion

exchange treatment of mercury present in industrial

wastewater (Patterson, 1985). Also, Duolite GT-73, a

cationic resin, contains the thiol (-SH) group and reacts

with ionic mercury. The thiol functional group has a high

selectivity for mercury as well as a strong tendency to bind

certain other metal ions such as copper, silver, cadmium,

and lead.

A .chelate resin is an insoluble polymer to which is

attached a complexing group or groups. This, in turn, can

bond metal cations within the structure so as to form a ring

(or chelate) into which the metal is incorporated. The

reaction involves both ion-exchange and chemical reactions. Table 4-1 lists some chelate resins that are reported

to have a high selectivity for mercury; the table includes the

order of selectivity.

Example ion exchange treatment data for drinking

water are presented in Table 4-2. Mercury removal from

ground water was studied in point-of-entry treatment

(POET) systems installed on private water supply wells

(Sites and Obeholtzer, 1992). Table 4-2 indicates that

lonac SR-4, Purolite S-920, AFP-329, and ASB-2 were

able to remove mercury from the relatively low initial

ground-water concentrations to below 1 pg/L, following

prefiltration.

A full-scale ion exchange process at a defense processes facility has consistently removed mercury via ion

exchange from 0.2 to 70 mg/L down to levels of 1 to 5

,ug/L, following 0.2 pm prefiltration (Ritter and Bibler, 1992).

This system utilizes a macroporous, weakly acidic, polystyreneldivinylbenzene cation resin, with thiol (SH) functional

groups. High levels of mercury in a synthetic wastewater

4-l
Table 4-l. Summary of mercury-selective chelate resins (After Calmon, 1981)

Resin Order of Selectivity

Duolite ES-466 Hg2’~Cu2’7Fe2’7Ni2’rPb2’>Mn2’>C~2’7MgZ’7Na’

Dowex A-l Cu2’7Hg2’7Ni2’>pb2’7ZnZ+>C02’ >Cd2’>Fe2’>Mn2+>Ca2*>Na+

Nisso Alm-525 Hg2’>Cd2+~Zn2~>Pb2*>C~2~>A~~>C~>N~2~

Diaion CR-l 0 Hg2+sCu2‘7pb2+7Ni2+7Cd2+>ZnZ+ZC02c,Mn2’>~~2+~~~z+>~~2+~~g2+~~~2+~~~~~~~~+

Amberlite IRC-718 Hg2’sCu2+:,Pb2*7N~2+7Zn2*>Cd2’>Co2’>Fe2’>Mn2’~Ca2’

Unicellex UR-10 Hg2’7Cu2’7Fe3’~A13’>Fe2’>Ni2’>pb2’~C~~~n2*>Cd2c>Ag2+~~n2’>~~2+~~g2+~~~~~2’

Sirorez-Cu pH>5, Cu2+; pH70, Hg2+

Sumichelate Q-l 0 HgC12>AuCI’,>Ag+%r20,2-

Table 4-2. ion Exchange treatment for mercury in drinking water

Mercury

Concentration

b-a-)

ion Exchange

Resin

Resin Type Initial Final

Additional

Treatment

Other

Conditions Reference

lonac SR-4 Weak acid chelat- 14.88” 0.43” Prefiltration GW, FSb Sites and Oveholtzer, 1992

ing resin

Purolite Hg-specific chelat- 10.67” 0.34” Prefiltration GW, FSb Sites and Obeholtzer, 1992

s-920 ing resin

AFP-329 Weak base anion 12.21” 0.44a Prefiltration GW, FSb Sites and Oveholtzer, 1992

resin

ASB-2

Duolite

GT-73

Amberlite

IRC 718

IRC 718

and

GT 73

Strong base anion 14.31” 0.70” Prefiltration GW, FSb Sites and Oveholtzer, 1992

resin

Weak acid cation 200-70,000 1-5 0.2 pm pretilter DFW, FS Ritter and Bibler, 1992

thiol

Iminodiacetic acid 11,800 15-35 None SW, BS Becker and Eldrich, 1993

resin

(See above) 14,000 15-I ,200 GT 73 used as SMW, BS Becker and Eldrich, 1993

polishing

‘Average value

b3 to 4 gpm

GW = Ground water.

FS = Full scale.

DFW = Defense facility wastewater.

SW = Synthetic wastewater.

BS = Bench scale.

SMW = Smelter wastewater.

4-2
were removed to levels as low as 15 pg/L after 77 bed

volumes of usage, and 3$g/L after 157 bed volumes of

usage (Becker and Eldrich, 1993). This system utilized

Amberlite IRC 718 in bench-scale testing. In further

bench-scale testing, smelter wastewater containing 14.0

mg/L of mercury at a flow rate of 6.7 mUmin was treated

with IRC 718 followed by a polishing ion exchange

column containing GT 73. This system removed mercury to concentrations of 15 to 46 @g/L after 289 bed

volumes, and 1,200 pg/L after 325 bed volumes. This

study further showed that at pH 1.5, the iminodiacetic

acid resin (IRC 718) was highly selective for mercury (II)

over zinc, lead, and cadmium, and that mercury recovery from wastewater on such a resin is feasible provided

strongly complexing anions such as chloride are absent.

Regeneration with 3 M NaCl or other complexant for

mercury at near neutral pH yields a solution for which

mercury can reportedly be recovered via reduction to an

insoluble and commercially valuable form (Becker and

Eldrich, 1993).

4-3
Chapter 5

Other Processes

In addition to precipitation, adsorption, and ion exchange treatment technologies, the following processes

are also reported to be applicable to remove mercury from

wastewater: (1) chemical reduction, (2) membrane separation, and (3) various emerging technologies.

5.1 Chemical Reduction

The standard electrode potential of metals determines

their placement in the electromotive series, which is a

series of elements in descending order of their standard

potential. Ionic mercury can be displaced from solution via

reduction by another metal higher in the electromotive

series, and then separated by filtration or other solids

separation technique. Reducing agents include aluminum,

zinc, iron, hydrazine, stannous chloride, and sodium

borohydride. Example data on these reductants are

presented in Table 5-1.

Although the literature includes much discussion of

reduction processes, only limited actual treatment data are

presented. The main advantage claimed for reduction is

that mercury can be recovered in the metallic state (Patterson, 1985). The data in Table 5-1, however, indicate

that most reduction processes cannot effectively achieve

mercury levels below 100 pg/L, and their use would likely

require second-stage polishing.

Experiments were conducted by Gould and colleagues

(1984) at bench scale on Chemical Oxygen Demand

(COD) test wastewater using iron wire (nominal diameter

0.229 mm). Due to the high initial mercury levels (735 to

2,030 mg/L), high recovery efficiencies were observed

(96% to 99%); however, high residual mercury levels were

also observed (22 to 33 mg/L). Experiments were conducted by Grau and Bisang (1995) on synthetic wastewater

with iron felt formed by compressing iron wool. As for other

studies, a high removal efficiency resulted at the high initial

mercury concentration, leaving 68 to 91 ,ug/L residual

mercury.

As noted in Chapter 4, ‘mercury removal from ground

water was studied in POET systems that were installed on

private water supply wells (Sites and Oberholtzer, 1992).

Table 5-l shows that a bimetallic oxidation/ reduction

compound, KDF, which consists of a finely ground alloy of

55% copper and 45% zinc, was able to remove low levels

of mercury down to a range of 0.4 to 1.08 ,ug/L, following

prefiltration. This process may be applicable only for

exceptionally clean solutions, however.

5.2 Membrane Separation

Several membrane processes have been applied for

water and wastewater mercury treatment. These include

ultrafiltration, charged filtration, crossflow microfiltration,

magnetic filtration, and reverse osmosis. Example treatment data for these processes are shown in Table 5-2.

Ultrafiltration systems are pressure-driven membrane

operations that use porous membranes for the removal of

dissolved and colloidal material (Metcalf and Eddy, 1991).

These systems differ from reverse osmosis systems by the

relative driving pressures, usually under 150 psi (1034

kN/m*). Ultrafiltration is normally used to remove colloidal

material and large molecules with molecular weights in

excess of 5,000. Recent studies indicate that effluent from

ultrafiltration using spiral wound elements is suitable as a

feed source for reverse osmosis (Metcalf and Eddy, 1991).

Chelation in combination with ultrafiltration is a process

that has been described for the removal of heavy metals,

including mercury (Kosarek, 1981). This concept is based

on reacting ligands with cationic metallic constituents to

form a metal-containing complex (chelate), and then

removing these metal-containing complexes by ultrafiltration (Kosarek, 1981). The opposite charges of the ionized

ligand and metal attract each other to form a stable chelate :

complex. The properties that facilitate ultrafiltration

membrane rejection of the metal-containing complex

(including mercury complexes) are thought to be (1) the

increased size of the metal chelate complexes, (2) alteration in the ionic shape of the metal, (3) modified solubility,

and (4) reversal of charge from cationic metal to a functionally anionic or electroneutral chelate species (Kosarek,

1981).

Charged membrane ultrafiltration incorporates a

noncellulosic, high flux membrane that is negatively

charged as a result of dissociated subgroups within the

membrane structure. A beneficial aspect of the charged

ultrafiltration membrane is that the negative polarization

minimizes membrane fouling (Kosarek, 1981). Bhattacharyya and colleagues (1979) conducted bench-scale

investigations to determine the feasibility of the simultaneous separation of various heavy metals from scrubber

blowdown wastewater generated in the primary copper

industry. They studied the application of low pressure

ultrafiltration with commercially available, negatively

charged noncellulostic membranes. Typical mercury values

5-1
Table 5-1. Performance of Reduction Processes for Mercury Treatment.

Mercury bg/L)

Reductant

Zinc

Iron

Iron felt

KDF’

Stannous chloride

Sodium borohydride

Initial Final

qooo-10,000 5-10

1,800 140

12,500 830

12,500 750

12,500 470

NA 600

734,000- 22,000

2,030,OOO -33,000

100,000 68-91

6.17-12.11 0.4-l $8

2,800 500

10,000 220

4,000 420

26,000 820

4,700 200

NA Cl0

Treatment pH

NA

11.5

10.0

6.0

2.5

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

Reference

Patterson, 1985

Patterson, 1985

Patterson, 1985

Patterson, 1985

Patterson, 1985

Patterson, 1985

Gould et al., 1984

Grau and Bisang, 1995

Sites and Oberholtzer, 1992

Patterson, 1985

Patterson, 1985

Patterson, 1985

Patterson, 1985

Patterson, 1985

Patterson, 1985

‘Bimetallic copper-zinc oxidation/reduction compound.

NA = Not available.

in the scrubber blowdown were found to be 1.5 to 2.0 mg/L.

Mercury removals for a full-scale system were calculated

by computer simulation based on laboratory-scale data and

were reported to be 91% to 93% (AP = 5.6 x lo5 N/m*,

channel velocity, U = 250 cm/s, and pH = 4.5). For a

3.6 x lo5 kg/d (400 ton /day) copper production plant, the

net operating costs for a closed-loop scrubber blowdown

water recycle system was estimated to be $2,508 /day or

$O.O070/kg Cu production (1995 basis updated assuming

cost components follow change in skilled labor cost index).

The crossflow microfiltration system is based on the

concept of using a dynamic membrane to form a filtration

medium. This process, whose patented form is called

Exxflow, is a solid-liquid separation process in which the

feed suspension sweeps across the face of a filter membrane while pressure differences cause the liquid phase to

pass through the membrane, leaving the solids to be

flushed away in the residual flow. By this means, the solids

are concentrated up in the suspension flow, which is

commonly recycled to the feed end. This contrasts with

“barrier” filtration systems in which the solids build up on

the filtering surface, gradually restricting the flow through

the filter (Squires, 1992). A schematic of the microfiltration

process is shown in Figure 5-l.

Mercury removal via crossflow microfiltration was

reported for a full-scale plant designed to process 200 m3

/day of mixed plating wastewater (Broom et al., 1994). A

process schematic of the plant is shown in Figure 5-2. The

filtrate from the rotary vacuum filter was combined with the

supernatant from a preclarification stage and stored in a

80 m3 balance tank, where the pH was adjusted to 11 to

12, primarily to precipitate cadmium. Sodium hydrogen

sulfide (NaHS) was also added to precipitate any soluble

metals remaining. This conditioned filtrate was then

pumped to the crossflow microfiltration unit, where it was

recycled at an average pressure of 150 kPa. The reject

flow was effectively a concentrate produced by the passage of clean permeate through the filter. With mercury

feed concentrations to the microfiltration plant of 1.27,

0.967, 0.15, and 2.28 mg/L, permeate concentrations of

0.015, 0.015, 0.088, and 0.03 mg/L were achieved, respectively. This represents a removal efficiency of about 95 %.

Removal may have been enhanced by mercury co-precipitation in the balance tank.

5-2










Table 7-1. Nonradioactive Contaminants and Discharge Limits (After Bibler and Wallace, 1987).

Ion

lnfluent to ETF (ppm)

Average Maximum

lnfluent to Ion Exchange (ppm)

Average Maximum

Proposed Limits (ppm)

Average Maximum

NH’, 16 110 4.0 x 1 o-1 2.8 20 -

Hg*+ 0.053 IO 1.3 x IO” 2.4 x IV’ 4.5 x lo-’ 1.75x 10-i

Zn*’ 1.1 100 2.8 x 10’2 2.5 1.48 2.61

C? 0.031 240 7.8 x IO” 6.0 1.71 2.77

cu*+ 0.14 18 3.5 x IO” 5.0 x 10-i 1.30 1.89

Pb” 0.15 38 3.8 x lo” 9.5 x IO“ 2.2 x IO” 4.5 x 10-l

Mn” 0.18 21 4.5 x 10-l 5.3 x IO” - -

NO, 1,015 22,400 25 560 - -

Table 7-2. Radioactive Contaminants and Discharge Limits (After Bibler and Wallace, 1987).

Radionuclide

cs- 134,137

Sr - 89, 90

Co-60

lnfluent to Ion Exchange Release to Streams DOE Guide

mCi/yr mCi/mL mCi/yr mCi/mL mCi/mL

70 1.7 x lo-‘0 0.7 1.7 x lo-‘* 2.9 x 10-5

9 2.2 x IO“’ 0.09 2.2 x lo-” 3.3 x IO4

5 1.2 x lo-‘0 0.5 1.2 x IO.12 3.0 x 10-5

Other p-y 3.0 7.5 x lo-= 3.0 7.5 x lo-‘* -

Total a 3.5 x IO” 8.7 x lO-3 3.5 x IO” 8.7 x lo-” -

indicating that simple storage was a viable option. Similar

resin samples were incorporated in Portland Type II grout

and subjected to structural integrity tests and the Extraction

Procedure (EP) toxicity test. These samples passed the

structural integrity test but not the EP toxicity test. Because

the EP toxicity test involved maintaining the sample at pH

5 with acetic acid and given the high concentration of

calcium ions in grout, exchange of calcium ions for mercuric ions may have taken place. Thus, the storage of spent

resin without incorporation into grout was preferred.

Although regeneration of the spent resin is possible, it

is not deemed desirable at ETF. Mercury can be eluted

from the resin using 3 M HCI or 2 M NaSCN, neither of

which is chemically compatible with materials of construction or processes at SRS. Dissolution of the resin and

reclamation of mercury by chemical means such as

precipitation as the sulfide or reduction to the metal is an

attractive alternative to storage, should recovery and

removal become desirable.

Sulfonic Acid Cation Resins

A macroporous, strong acid cation exchange resin was

chosen for removal of cesium and strontium. Several

commercially available resins have demonstrated cesium

and strontium removal capabilities coupled with ease of

regeneration. Cesium and strontium in the regenerant can

by a relatively small volume of neutral reagent be concentrated further and incorporated in concrete for final disposal. Spent resin can be decontaminated and discarded

in an approved manner.

Several test runs were conducted to determine the

performance of the sulfonic acid resin throughout several

simulated feed, wash, and regeneration cycles. Results

indicated that the effectiveness of the process was less

7-4
than desirable after several cycles had been completed. A

chromatographic effect was observed where concentrated

bands of all metals present were detected in the effluent at

unpredictable times during feed cycles. The frequency and

concentrations of such eluted bands cannot be accurately

predicted in the ETF due to the varying daily concentrations

of influent to the facility. To prevent this behavior the ion

exchange feed was first processed through the Duolite GT73 columns for mercury removal, allowing the sulfonic acid

columns to operate as designed for the removal of cesium

and strontium.

7.2 Effluent Treatment Facility Economics

The treatment plant operates Friday through Sunday of

each week. There are 5 operators during each shift, which

lasts 12 hours, with an additional 0.5 hour turnaround.

During the 1994 fiscal year, 22 million gallons of waste

water were treated at a total cost of $18.8 million. This

results in a unit cost of about $l/gallon. Figure 8-2 presents the ETF cost breakdown components.

Figure 7-2. ETF facility O&M cost breakdown.

A = Health protection - $0.85 M - (4.5%)

B = Materials/chemicals - $0.4 M - (2.1%)

C = Central services - $0.13 M - (0.7%)

7-5

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8-3

*U.S. GOVERNhlENT PRINTING OFFICE: 1YY7 - 650-001180151