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1. Indian Journal of Air Pollution Control Vol. V No. II September 2005 pp 66-69
A Method For Recovery Of Metal Mercury From Air Pollution Monitoring
Laboratory Waste
Aparna Gajbhiye, K M Phadke and Animesh Kumar
National Environmental Engineering Research Institute, Nagpur 440 020
Abstract
For determination of Sulphur dioxide (SO2) in ambient air, improved West and Gaeke method is recommended as the
standard method. In this method, the solution after completing the analysis is discarded as laboratory waste. This waste
contains mercury. A method using zinc dust has been optimized for recovering metal mercury from this waste. A
recovery above 98% has been achieved.
Key Words: Mercury recovery, West & Gaeke
Introduction
With the ever-increasing awareness on air and water pollution since the enactment of Water and Air
Pollution Control Acts, people have become more inquisitive about the pollutants’ levels and their effects.
This has resulted in mushroom growth of institutions, consultants and interest groups engaged in monitoring
of various pollutants in complex environment. National Environmental Engineering Research Institute has
been operating an ambient air quality monitoring network in 10 major cities of India with part financial
assistance of Central Pollution Control Board (CPCB, Delhi) under its National Air Quality Monitoring
Programme (NAMP) for monitoring ambient air quality in 90 Indian cities (CPCB notified National Ambient
Air Quality Standards (NAAQS) 11th
April 1994 and 14th
October 1998 for SO2, NO2, SPM, RPM, Pb, CO
and NH3).
For determination of SO2 in ambient air, improved West and Gaeke method has been notified as a
standard method. Ambient air is passed through an absorbing solution of potassium tetrachloro-mercurate
(TCM) at a known flow rate and for a known period of time. Sulphur dioxide in the sampled air gets
absorbed into the absorbing solution and forms dichlorosulphitomercurate complex. This complex is reacted
with pararosaniline and formaldehyde to form colored complex of pararosaniline methyl-sulphonic acid. The
absorbance of this solution is measured at 560 nm to determine SO2 concentration in ambient air. The
solution is then discarded as laboratory waste and discharged down the drain. This waste contains mercury of
weight more than 4–5 g/L in complex form. As compared to this, Indian standard for discharge of mercury
present in industrial and sewage effluents into inland surface water, marine coastal area and public sewers is
only 0.01 mg/L (IS 2490-1982). As mercury in both the organic and inorganic forms is harmful to human
health, it is recommended (WHO, 1976) to remove it from the wastewater before discharging it in the drain.
Efforts were, therefore, made to optimize metal mercury recovery from this wastewater before discharging it
in the drain.
A process to recover metal mercury from this waste, percent recovery and cost effectiveness are
reported in this communication. The technique to recover metal mercury was extended to the waste
generated during the determination of ammonia (NH3) by Nessler’s reagent method. Quantity of metal
mercury that would be recovered from the waste generated during a typical air pollution monitoring and
analysis survey has also been estimated. Settleable mercury could be removed from dental laboratory
wastewater at 99.6 to 99.8% and from absorption media at 92.3 – 94% (NSF, 2002).
Methodology
The waste solution after analysis was collected in a 2 Liter beaker and transferred into a specially designed
recovery bottle (Fig. 1). Zinc powder (1g zinc dust per litre of waste) was added to it and left for reaction for
six to seven days with intermittent stirring once every day. The precipitate (Zn + Hg mud) settled at the
bottom, was removed in another beaker and washed with double distilled water. Dilute hydrochloric acid
(20% HCl) 100 ml was added to the washed precipitate and heated to 50-600
C. While heating, the precipitate
was crushed with the help of a glass rod into granules and heating was continued with the addition of dilute

2. HCl to maintain the volume to about 100 ml until mercury was separated. The supernatant was then carefully
decanted into another beaker. The recovered metal mercury was dried with the help of simple filter paper.
The supernatant was also subjected to the above procedure to achieve maximum recovery of metal. The
recovered metal mercury can be used after purification.
Fig. 1: Bottle for Recovery of Mercury
Results and Discussion:
The percent recovery of metal mercury from both the absorbing solution and the waste generated from
improved West and Gaeke method was estimated by adding 1 g zinc dust to different volumes up to 1200 ml
of waste. It was found to be 97- 98% (Table 1).
Table 1: Percentage recovery of metal mercury from West and Gaeke method waste
Sr.
No.
Volume of
absorbing
media in waste
(ml)
Weight of zinc
dust added
(g)
Weight of beaker
with recovered Hg
(g)
Weight of
empty beaker
(g)
Weight of Hg
recovered
(g)
Recovery
(%)
1 250 1.0 87.77 85.792 1.978 98.55
2 250 1.0 112.97 111.038 1.932 96.26
3 500 1.0 90.076 86.142 3.934 97.98
4 1000 1.0 73.142 65.247 7.895 98.31
Average 97.77
Laboratory Waste + Zn
Dust1062
Litre
Bottle
Outlet for collection of Mud
/ precipitate
Tap for collection of Supernatent
Mud / precipitate

3. Percent recovery and cost estimate for recovering metal mercury from the improved West and Gaeke
method waste generated during ambient air quality monitoring programme for different sampling frequencies
has been estimated in Tables 2 and 3 respectively. The annual consumption of absorbing media is estimated
at 37.5 liters. The recovery of metal mercury from the generated waste would be 295 g (Table 2).
Table 2: Recovery of Mercury for different sampling frequencies from West and Gaeke method waste
for one month at 3 sites
No of
sampling
days
Frequency
of
sampling
No of
samples
Volume of
absorbing
media in each
sample
(ml)
No of
blanks
Total Volume
of media
(ml)
Quantity of
Hg in waste
(g)
Metal
recovered
at 98%
recovery
(g)
24 24 hr 24 20 12 720 5.77 5.67
24 8 hr 72 20 12 1680 13.47 13.25
24 4 hr 144 20 12 3120 25.02 24.56
Volume of A.M. required/Year (4 hour sampling frequency) 37440 301.27 295.68
Having succeeded in recovering metal mercury from West and Gaeke method waste, the
investigation was extended to include the waste solution generated in the determination of Ammonia using
Nessler’s reagent method. For this combined waste, the pH of the solution was brought below 2 by adding
HCl. Metal mercury was recovered in four stages with the addition of 1.0 g of zinc dust/l in the first stage
and treating the supernatant with 0.5g zinc dust/l in subsequent three stages. The zinc dust precipitate from
each stage was collected and added to the precipitate of the first stage. The combined precipitate was treated
with dilute HCl as mentioned above and metal mercury was recovered. The total recovery of mercury in four
stages is found to be 98% (Table 2a).
Table 2a: Recovery of Mercury from Ammonia and Composite sample waste for one month at 3 sites
Ammonia sample waste (Theoretical) Composite sample waste
No. of
Samples
Volume of
Nessler’s
Reagent
(ml)
Quantity of
Hg in waste
sample
(g)
Volume of
composite
waste
(ml)
Theoretical
Weight of
Hg
(g)
Actual
weight of
Hg
(g)
Recovery %
144 144 6.336 260 ml SO2
+ 13 ml
Nessler’s
Reagent
2.660 2.617 98.38
Table 3: Cost estimate for recovery of Mercury
INR
Supplier & Grade Cost/L absorbing
solution
Cost recovered/L absorbing
solution
HgCl2 Hg @ 98% recovery
SD Fine Chemicals 'AR' 21.61 16.69 16.42
Qualligens 'SQ' 13.99 11.66 11.47
Merck BDH (India) Pure 22.81 12.54 12.34
Acknowledgement
The authors are grateful to Dr P Nema, Scientist & Head APC Division for the encouragement and Dr. S
Devotta, Director NEERI for kind permission to publish this finding.

4. References
1. CPCB, 1994, 1998:National Ambient Air Quality Standards (NAAQS), notification, Delhi, 11th
April,
1994 and October 14, 1998, Central Pollution Control Board.
2. IS: 2490-1982: Indian Standard Institute, now Bureau of Indian Standards, New Delhi.
3. WHO, 1976: Selected Methods of Measuring Air Pollutants, WHO offset publication No. 24, (1976),
WHO, Geneva.
4. NSF, 2002: Removal of Mercury Amalgam from Dental Office Wastewater, Environmental Technology
Verification Report, NSF 02/01/EPAWQPC-SWP, September 2002, www.drna.com.
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