ارام اسماعيل عبد الرحمن

1. Regional government of Kurdistan
Ministry of Higher Education and
Scientific research
University of sulaimani
School of science
Chemistry department
Determination of heavy metals
In wastewaters
Submitted to the chemistry department in the school of Science
-University of Sulaimany- as a partial fulfillment
Of the requirements for the degree of
Bachelor of Science-B.Sc.-
In Chemistry
By
ARAM I. Abdul rahman
Mustafa Hassan
Supervised by
KOSAR HIKMAT HAMA AZIZ
-2012-
1

2. Chapter one
A-Introduction
(A-A) Wastewater
Wastewater is water that has been used and must be treated before it is released into another
body of water, so that it does not cause further pollution of water sources. Wastewater comes
from a variety of sources. Everything that you flush down your toilet or rinse down the drain
is wastewater. Rainwater and runoff, along with various pollutants, go down street gutters
and eventually end up at a wastewater treatment facility. Wastewater can also come from
agricultural and industrial sources. Some wastewaters are more difficult to treat than others;
for example, industrial wastewater can be difficult to treat, whereas domestic wastewater is
relatively easy to treat (though it is increasingly difficult to treat domestic waste, due to
increased amounts of pharmaceuticals and personal care products that are found in domestic
wastewater. For more information about emerging contaminants (1.2.3)
1. Wastewater is simply water that has been used. It usually contains various pollutants,
depending on what it was used for. It is classified into two major categories, by source:
Domestic or sanitary wastewater. This comes from residential sources including
toilets, sinks, bathing, and laundry. It can contain body wastes containing intestinal
disease organisms.
2. Industrial wastewater. This is discharged by manufacturing processes and commercial
enterprises. Process wastewater can contain rinse waters including such things as
residual acids, plating metals, and toxic chemicals.
Wastewater is treated to remove pollutants (contaminants). Wastewater treatment is a
process to improve and purify the water, removing some or all of the contaminants, making
it fit for reuse or discharge back to the environment. Discharge may be to surface water, such
as rivers or the ocean, or to groundwater that lies beneath the land surface of the earth.
Properly treating wastewater assures that acceptable overall water quality is maintained.
In many parts of the world, including in the United States, health problems and diseases
have often been caused by discharging untreated or inadequately treated wastewater. Such
2

3. discharges are called water pollution, and result in the spreading of disease, fish kills, and
destruction of other forms of aquatic life. The pollution of water has a serious impact on all
living creatures, and can negatively affect the use of water for drinking, household needs,
recreation, fishing, transportation, and commerce. (4)
The main reason care should be taken when dealing with waterwaste is to protect other water
sources from becoming contaminated. Thus, most waterwaste is taken by sewer line to a
water treatment facility. There, the solids and most biological organisms are removed from
the water and the clean water is released as effluent, usually into another source of water,
such as a river or lake. If the equipment at the water treatment facility is running properly,
the water will pose no danger to the environment upon being released.
Industrial water waste is often treated slightly differently than standard household
waterwaste. Water that has only been used in the manufacturing process may be able to be
released directly into the environment if a number of conditions are meant. This is often
regulated heavily in many jurisdictions and those industrial facilities can be fined heavily if
found to be abusing the discharge terms. In some cases, even criminal action could be taken
against those responsible. The water will likely contain some solids, especially metals. Those
metals cannot be of types or in concentrations that are considered dangerous.
Further, industrial waterwaste must not be mixed with any other type of water being flushed
from the facility. For example, sewer lines that serve bathrooms and kitchens at such
facilities cannot run into lines serving industrial uses. This protects the effluent from cross
contamination, thus making sure no harmful bacteria is released into an otherwise clean or
safe water supply.
Waterwaste can also be considered runoff from streets and other areas after a rainfall. This
type of water is sometimes not considered that dangerous, and is often funneled directly into
a natural body of water. Some communities and states have begun to change the rules
regarding this and feel oil and other contaminants on the road's surface can have a negative
impact on the environment. To protect the environment, some have mandated special ponds,
called drainage ponds, to collect the water and keep it separate from other water sources.
This waterwaste is not meant to be treated, but may become cleaner over time due to natural
processes.
3

4. Any of the following acts or omissions, whether willful or negligent, shall constitute the waste
of water:
A. Causing or permitting water to discharge, flow or run to waste into any gutter,
sanitary sewer, watercourse or storm drain, or to any adjacent property, from any tap, hose,
faucet, pipe, sprinkler, or nozzle. In the case of irrigation, “discharge,” “flow” or “run to
waste” means that the earth intended to be irrigated has been saturated with water to the
point that excess water flows over the earth to waste. In the case of washing, “discharge,”
“flow” or “run to waste” means that water in excess of that necessary to wash, wet or clean
the dirty or dusty object, such as an automobile, sidewalk, or parking area, flows to waste.
B. Allowing water fixtures or heating or cooling devices to leak or discharge.
C. Maintaining ponds, waterways, decorative basins or swimming pools without
water recirculation devices.
D. Backwashing so as to discharge to waste swimming pools, decorative basins or
ponds in excess of the frequency necessary to ensure the healthful condition of the water or
in excess of that required by standards for professionally administered maintenance or to
address structural considerations, as determined by the director or his or her designee.
E. Operation of an irrigation system that applies water to an impervious surface or
that is in disrepair.
F. Use of a water hose not equipped with a control nozzle capable of completely
shutting off the flow of water except when positive pressure is applied.
G. Irrigation of landscaping during rainfall.
H. Overfilling of any pond, pool ,fountain which results in water discharging to
waste.(5)
4

5. (A-B)Source ofWaste Water
Wastewater can be defined as the flow of used water discharged from homes,
businesses,industries, commercial activities and institutions which is directed to treatment
plants by acarefully designed and engineered network of pipes. This wastewater is further
categorizedand defined according to its sources of origin. The term “domestic wastewater”
refers to flowsdischarged principally from residential sources generated by such activities as
foodpreparation, laundry, cleaning and personal hygiene. Industrial/commercial wastewater
is flowgenerated and discharged from manufacturing and commercial activities such
asPrinting, foodand beverage processing and production to name a few.
InstitutionalWastewater characterizeswastewater generated by large institutions such as
hospitals and educational facilities.Typically 200 to 500 liters of wastewater are generated
for every person connected to thesystem each day. The amount of flow handled by a
treatment plant varies with the time of dayand with the season of the year.
(Major Components)
1. Domestic: food, soap and detergents, bathroom (fecal and urine), and paper.
2. Commercial: bathroom and food from restaurants and other “stores.”
3. Industrial: highly variable, dependent on industry, controlled by pre-treatment
regulations.
4. Runoff from streets: sand and petroleum and tire residues (infiltration, not a direct
discharge). (6)
(A-C)Heavy Metals
Heavy metals are a major concern in the treatment of water due to the toxic and other detrimental
effects these materials can produce. In general, heavy metals are considered to be the following
elements: Copper, Silver, Zinc, Cadmium, Gold, Mercury, Lead, Chromium, Iron, Nickel, Tin,
Arsenic, Selenium, Molybdenum, Cobalt, Manganese, and Aluminum.The term heavy metal
refers to any metallic chemical element that has a relatively high density and is toxic or poisonous
at low concentrations.
Heavy metals can be found in varying concentrations in any natural source of water, but the
main treatment problems exist in the process water of the following industries:
Metal mining and smelting operations
Foundries
Metal planting and finishing
Metal fabricating plants such as automotive
manufacturing, etc.
5

6. While not all heavy metals are involved, research has shown that certain metals such as
mercury, lead and chromium are toxic to aquatic life in relatively low concentrations. For
example, 20 ppm of chromium is fatal to trout after 8 days exposure. If 100 gallons of normal
chromium planting solution is discharged into a waterway it will be toxic to all
microorganisms in the food cycle, even if diluted by 100,000,000 gallons of water.
In larger concentrations these metals may have detrimental health effects on man. Heavy
metals are a cumulative toxin that the body cannot dispose of and they accumulate to
harmful levels with repeated exposure.The presence of heavy metals in a waste stream can
interfere and even destroy the effectiveness or normal waste treatment operations. Activated
sludge secondary treatment plants are especially affected since heavy metals can kill the
necessary bacteria.Heavy metals in water can make it unsuitable for many uses such as
drinking, boiler feed, or process uses where high degree purities are required.(7)
The most commonly encountered toxic heavy metals include:
Arsenic
Lead
Mercury
Cadmium
Iron
Aluminum
Other heavy metals of concern include:
(Antimony,Chromium/chrome,Cobalt,Copper,Manganese,Nickel,Uranium,Vanadium,Zinc)
Most common heavy metals are lead(Pb), mercury(Hg), cadmium(Cd) and arsenic(As)
Heavy metals become toxic when they are not metabolized by the body and accumulate in
the soft tissues. Heavy metals may enter the human body through food, water, air, or
absorption through the skin when they come in contact with humans in agriculture and in
manufacturing, pharmaceutical, industrial, or residential settings.(8)
(A-D)Effect of Heavy Metals OnHealth and Environment
6

7. Heavy metals are dangerous because they tend to bio accumulate. Bioaccumulation means
an increase in the concentration of a chemical in a biological organism over time, compared
to the chemical's concentration in the environment. Compounds accumulate in living things
any time they are taken up and stored faster than they are broken down (metabolized) or
excreted.
Heavy metals can enter a water supply by industrial and consumer waste, or even from acidic
rain breaking down soils and releasing heavy metals into streams, lakes, rivers, and
groundwater. Now we are going to describe the kinds of heavy metals, their dangerous levels
and the effects of these heavy metals to human health and environment.
The most pollutans heavy metals are Lead, Cadmium, Copper, Chromium, Selenium and
Mercury.(9)
(A-D-A)Lead
 Has a very low melting point of 327 degrees C
 Used as a structural metal in ancient times and for weather proofing buildings
 Romans used it in water ducts and in cooking vessels
 Analysis of ice-core samples from Greenland indicate that atmospheric lead
concentration reached a peak in roman times that was not equaled again until the
renaissance
Sources of lead
 Commonly used in the building industry for roofing and flashing and for
soundproofing
 Used in pipes
 When combined with tin, it forms solder, used in electronics and in other applications
to make connections between solid metals
 Lead is also used in ammunition
 Note: Lead shots have been banned in United States, Canada, Netherlands, Norway
and Denmark
 Lead is used in batteries and sinkers in fishing
Health effects of lead
7

8.  At high levels, inorganic lead is a general metabolic poison
 Lead poisoning effects the neurological and reproductive systems, example: downfall
of roman empire
 Lead breaks the blood-brain barrier and interferes with the normal development of
brain in infants
Environmental effects of lead
Lead accumulates in the bodies of water organisms and soil organisms
Health effects on shellfish can take place even when only very small concentrations of lead
are presentBody functions of phytoplankton can be disturbed when lead interferes.
Phytoplankton is an important source of oxygen production in seas and many larger sea-
animals eat itThat is why we now begin to wonder whether lead pollution can influence
global balances
(A-D-B) Mercury
 Most volatile of all metals
 Highly toxic in vapor form
 Liquid mercury itself is not highly toxic, and most of that ingested is excreted
Sources of Mercury
 Elemental mercury is employed in many applications due to its unusual property of
being a liquid that conducts electricity
 Used in electrical switches, fluorescent light bulbs and mercury lamps
 Emission of mercury vapor from large industrial operations
 Unregulated burning of coal and fuel oil
 Incineration of municipal wastes
 Emissions from mercury containing products :batteries, thermometers, etc.
 Mercury amalgams: dental fillings
Health effects of mercury
8

9.  Disruption of the nervous system
 Damage to brain functions
 DNA damage and chromosomal damage
 Allergic reactions, resulting in skin rashes, tiredness and headaches
 Negative reproductive effects, such as sperm damage, birth defects and miscarriages
Environmental effects of mercury
 Fish are organisms that absorb great amounts of methyl mercury from surface waters
every day (mercury can accumulate in fish and in the food chains)
 The effects that mercury has on animals are:kidneys damage, stomach disruption,
damage to intestines, reproductive failure and DNA alteration
(A-D-C) Cadmium
 Cadmium lies in the same subgroup of the periodic table as zinc and mercury, but is
more similar to zinc
 Coal burning is the main source of environmental cadmium
 Incineration of wastes containing cadmium is an important source of the metal in the
environment
 Cadmium is most toxic in its ionic form unlike mercury
Note: Mercury is most toxic in vapor form and lead, cadmium and arsenic are most
toxic in their ionic forms.
Sources of Cadmium
 Cadmium is used as an electrode in “nicad” batteries
 Cadmium is used as a pigment in paints(yellow color)
 It is also used in photovoltaic devices and in TV screens
 Cigarette smoke
 Fertilizers and pesticides
Note: The greatest proportion of our exposure to cadmium comes from our food
supply- seafood, organ meats, particularly kidneys, and also from potatoes, rice, and
other grains.
9

10. Health effectsof Cadmium
 Severe pain in joints
 Bone diseases
 Kidney problems
 Its lifetime in the body is several years
 Areas of greatest risk are Japan and central Europe
 In very high levels it poses serious health problems related to bones, liver and kidneys
and can eventually cause death.
Environmental effects of cadmium
 Cadmium can be transported over great distances when it is absorbed by sludge
 This cadmium-rich sludge can pollute surface waters as well as soils
 Cadmium strongly adsorbs to organic matter in soils
 When cadmium is present in soils it can be extremely dangerous, as the uptake
through food will increase
 Soils that are acidified enhance the cadmium uptake by plants
 This is a potential danger to the animals that are dependent upon the plants for
survival – Cadmium can accumulate in their bodies, especially when they eat multiple
plants (10,11)
.
10

11. (A-E)Methods ForRemoving Heavy Metals From Waste Water.
Heavy Metals Removal: Conventional Technologies
A number of techniques for the treatment of heavy metal-containing waste waters have been developedin recent
years, in order to both decrease the amount of metal-containing waste watersproduced by industrial activities and
improve the quality of treated effluents.Various treatments,such aschemical precipitation, coagulation–flocculation,
flotation, ion exchange,electrochemical treatment process by Using Peat, byAdsorption onto Activated Carbon and
membranefiltration, can be utilized to remove heavy metals from contaminated waste waters, each withtheir own
inherent advantages and limitations in application.
(A-E-A)Chemical Precipitation
Chemical precipitation is the most common technology used to remove dissolved (ionic) metalsfrom water solutions,
such as process waste waters containing toxic metals. The ionic metals areconverted to an insoluble form (particle) by
the chemical reaction between the soluble metal compoundsand the precipitating reagent. Typically, the metal
precipitated from the solution is in theform of hydroxide. The conceptual mechanism of heavy metal removal by
chemical precipitationis represented as
where M(OH)nis insoluble metal hydroxide and Mn+and OH- represent dissolved metal ions andprecipitant,
respectively. Therefore, the optimum pH for precipitation of one metal may causeanother metal to solubilize, or start
to go back into solution. Most process waste waters containmixed metals and hence precipitating these different
metals as hydroxides can be a tricky process.Chemical precipitation requires large amounts of chemicals to reduce the
concentration ofmetals to an acceptable level for discharging waste waters into the environment. Other drawbacksof
this method are related to the excessive sludge production that requires further treatments,the cost of sludge
disposal, the slow kinetics of metal precipitation, the poor settling ofmetal hydroxides, the aggregation of metal
precipitates, and the long-term environmental impactof sludge disposal.
11

12. The single component and multi-component hydroxide precipitation andadsorption were studied for different heavy
metals namely Iron (III), Chromium(III), Copper (II), Lead (II), Nickel (II), and Cadmium (II) from aqueous
solutions.By using the jar tester Magnesia (MgO) was used as a precipitator at differentdoses and compared with
other chemicals like lime (CaO) and caustic soda(NaOH). The treatment involves the addition of either magnesia or
lime-waterSuspensions (combined with cationic polyelectrolyte, CPE) in various doses, 1.0 –5.0 g/l for the metal
samples to study the effect of varying doses on the treatmentefficiency. The results show that the percent removal of
metal ions increases toabout 99 % with increasing the MgO dose to some limits. The optimum values ofMgO doses
were found to be 1.5-3.0 g/l. The pH value ranges are 9.5 to 10 withMgO precipitant and pH of 11.5 to 12 with CaO
precipitantwas the most favorable speed of rapid mixing and the slow mixing speed of 15-30rpm, G of (14-35 s-1), for
twenty minutes gave the best results.At the best operatingconditions of the pilot plant, the removal efficiency of metal
ions was more than97% at doses of MgO (1.0-4.0 g/l). (12)
(A-E-B) Ion Exchange
Ion exchange is a reversible chemical reaction wherein an ion present in solution is exchanged with
a similarly charged ion bound to a stationary solid phase (resin). Ion exchange can also be used for
recovering valuable heavy metals from inorganic effluents. After separating the loaded resin, metals
can be recovered in a more concentrated solution by eluting with suitable reagents.
Since the acidic functional groups of the resins consist of sulfonic acid,
it is assumed that thephysico-chemical interactions occurring during metal removal can be expressed as follows:
where (–RSO3-) and Mn+represent the anionic group attached to the ion-exchange resin and themetalcation,
respectively, while n is the coefficient of the reaction component, depending on theoxidation state of metal ions.
Selecting the optimum dosage level depends mainly on the quality offinished water required, considering both
economic and operating factors. Depending on the characteristicsof the ion exchanger, heavy metal removal by ion
12

13. exchange is effective in acidic conditionswith pH ranging from 2 to 6. However, ion exchange also has some
limitations in treating wastewaters containing heavy metals. Prior to ion exchange, appropriate pretreatment systems
for secondary
effluent such as the removal of suspended solids from waste water are required. In addition,
Suitable ion-exchange resins are not available for all heavy metals and the capital and operational
Costs are high.
A-E-C) Electrochemical Treatment Techniques
Electro-dialysis (ED) is a membrane separation technique in which ionized species in the solutionare passed through
an ion-exchange membrane by applying an electric potential. The membranesare thin sheets of plastic materials with
either anionic or cationic charge. When a solution containingionic species passes through the cell compartments, the
anions migrate toward the anode and thecations migrate toward the cathode, crossing the anion-exchange and
cation-exchange membranes.Since ED is a membrane process, it requires clean feed, careful operation, and periodic
maintenanceto prevent any damage to the membranes.In conclusion, it is possible to say that physico-chemical
treatments offer various advantages buttheir benefits are counterbalanced by a number of drawbacks such as their
high operational costs dueto the chemicals used, high energy consumption, and handling costs for sludge disposal. (1-
17 )
(A-E-D) Metal Removal from Wastewater Using Peat
Peat has been investigated by several researchers as a sorbent for the capture of dissolved metals from waste streams.
The mechanism of metal ion binding to peat remains a controversial area with ion-exchange, complexation, and
surface adsorption being the prevalent theories. Factors affecting adsorption include pH, loading rates, and the
presence of competing metals. The optimum pH range for metals capture is generally 3.5–6.5. Although the presence
of more than one metal in a solution creates competition for sorption sites and less of a particular ion may be bound,
the total sorption capacity has been found to increase. Studies have also shown that metals removal is most efficient
when the loading rates are low. In addition, recovery of metals and regeneration of the peat is possible using acid
elution with little effect on peat’s sorption capacity.14
13

14. Advantages:
This method is simple, effective and economical means of pollution remediation.
Peat is plentiful and inexpensive
(A-E-E) Removal of Heavy Metals from Industrial Wastewaters by Adsorption onto Activated Carbon
Prepared From an Agricultural Solid Waste
Activated carbon was prepared from coir pith by a chemical activation method and characterized. The adsorption of
toxic heavy metals, Hg(II), Pb(II), Cd(II), Ni(II), and Cu(II) was studied using synthetic solutions and was reported
elsewhere. In the present work the adsorption of toxic heavy metals from industrial wastewaters onto coir pith carbon
was studied. The percent adsorption increased with increase in pH from 2 to 6 and remained constant up to 10. As
coir pith is discarded as waste from coir processing industries, the resulting carbon is expected to be an economical
product for the removal of toxic heavy metals from industrial wastewaters.15
14

15. Chapter Two
Method for the determination of heavy metals such as
1) Voltammeter
2) Chromatography
3) Atomic absorption
4) Atomic emission ICP
Introduction
1) Voltammetry
Is a category of electro analytical methods used in analytical chemistry and various
industrial processes. In voltammetry, information about an analyte is obtained by
measuring the current as the potential is varied (16)
Voltammetry experiments investigate the half cell reactivity of an analyte. Voltammetry is
the study of current as a function of applied potential. These curves I = f (E) are called
voltammograms. The potential is varied arbitrarily either step by step or continuously, or
the actual current value is measured as the dependent variable. The opposite, i.e.,
amperometry, is also possible but not common. The shape of the curves depends on the
speed of potential variation (nature of driving force) and on whether the solution is stirred
or quiescent (mass transfer). Most experiments control the potential (volts) of an electrode
in contact with the analyte while measuring the resulting current (amperes) (17)
To conduct such an experiment requires at least two electrodes. The working electrode,
which makes contact with the analyte, must apply the desired potential in a controlled way
and facilitate the transfer of charge to and from the analyte. A second electrode acts as the
other half of the cell. This second electrode must have a known potential with which to
gauge the potential of the working electrode, furthermore it must balance the charge added
or removed by the working electrode. While this is a viable setup, it has a number of
shortcomings. Most significantly, it is extremely difficult for an electrode to maintain a
constant potential while passing current to counter redox events at the working electrode.
To solve this problem, the role of supplying electrons and referencing potential has been
divided between two separate electrodes. The reference electrode is a half cell with a
known reduction potential. Its only role is to act as reference in measuring and controlling
the working electrodes potential and at no point does it pass any current. The auxiliary
15

16. electrode passes all the current needed to balance the current observed at the working
electrode. To achieve this current, the auxiliary will often swing to extreme potentials at
the edges of the solvent window, where it oxidizes or reduces the solvent or supporting
electrolyte. These electrodes, the working, reference, and auxiliary make up the modern
three electrode system.
There are many systems which have more electrodes, but their design principles are
generally the same as the three electrode system. For example, the rotating ring-disk
electrode has two distinct and separate working electrodes, a disk and a ring, which can be
used to scan or hold potentials independently of each other. Both of these electrodes are
balanced by a single reference and auxiliary combination for an overall four electrode
design. More complicated experiments may add working electrodes as required and at
times reference or auxiliary electrodes.
In practice it can be very important to have a working electrode with known dimensions
and surface characteristics. As a result, it is common to clean and polish working
electrodes regularly. The auxiliary electrode can be almost anything as long as it doesn't
react with the bulk of the analyte solution and conducts well. The reference is the most
complex of the three electrodes; there are a variety of standards used and it is worth
investigating elsewhere. For non-aqueous work, IUPAC recommends the use of the
ferrocene/ferrocenium couple as an internal standard. In most voltammetry experiments,
a bulk electrolyte (also known as a supporting electrolyte) is used to minimize solution
resistance. It is possible to run an experiment without a bulk electrolyte, but the added
resistance greatly reduces the accuracy of the results. With room temperature ionic liquids,
the solvent can act as the electrolyte.(18)
2) Chromatographic analyses
Chromatographic analyses were carried out on a metal-free high-pressure ion
chromatograph, model 2010i (Dionex, Sunnyvale, CA, USA), which is equipped with an
isocratic pump, a post-column pneumatic controller for post-column reagent addition
(equipped with a semi permeable membrane reactor), and a variable-wavelength
absorbance detector (at 520 nm). The ion chromatograph was interfaced to an integrator
unit Spectra-Physics model SP4270 (Spectra-Physics, San Jose, CA USA) for collecting
chromatographic data Three ionic separation column systems were used during the
experimental tests: (1) a cationic separation column (2) an anionic separation column (3) a
bifunctional mixed-bed ion-exchange column
The separation column systems were protected from fouling problems by fixing their
respective guard columns
All the experimental tests were carried out under isocratic eluent flow-rate conditions and
at room temperature. A 100-ml sample loop was used for all measurements. At the
sampling site, groundwater samples were pre-filtered through a filter-membrane (19)
16

17. Chemical reagents and standards
All chemical reagents were analytical grade and contained negligible concentrations of
trace metals Nitric and hydrochloric acids were ultra-pure reagents (Merck, Mexico).
Pyridine-2,6-dicarboxylicacid (PDCA) and PAR monosodium salt were obtained from
Aldrich. Oxalic acid, citric acid, D tartaric acid, lithium hydroxide, sodium hydroxide
ammonium hydroxide (30%), and acetic acid were also analytical reagent grade (Baker,
Mexico).Working standard solutions of metals ranging from0.00195 to 100 mg/ l (1.9 to
1310 mg/ l) were prepared each working day by serial dilution of certified AAS standard
solutions of each metal containing 1000 mg/ l (Merck). Deionized water with conductivity
lower than 0.1 mS was used. Normal precautions for trace analysis were taken, e.g., all
glassware material was carefully cleaned in concentrated nitric acid and vigorously washed
with deionized water.(20)
17