Monitoring of trace metals and metalloids in natural

Monitoring of trace
metals and
metalloids in
natural water
SHUMAIL SAID

Contents
Introduction
Sources of contamination
Sampling equipment
Sample collection & preservation
Limitations
Future developments

Introduction
 Ubiquitous contaminants of aquatic environment
Potential toxicity to aquatic biota and detriment to
human health through ingestion of waters and
contaminated foodstuff
The trace metals of greatest concern include
aluminium, arsenic, cadmium, copper, lead, nickel,
mercury, selenium, silver, and zinc
The maximum tolerable concentrations typically are
in the 1 to 100 g/liter range
Some forms of organometallic compounds (e.g.,
methylmercury and tributyltin) are highly toxic at
quite low concentrations

Source of contamination
Two types of sources
Natural sources
Anthropogenic sources

Natural sources
The waters draining mineralized regions may
contain naturally elevated metal concentrations
 Acid-rock drainage resulting from the natural
weathering of sulfidic ores can be responsible for
elevated dissolved metal concentrations in some
water bodies

Anthropogenic sources
Include point-source inputs such as:
 Industrial discharges
Sewage treatment effluents
 Mining discharges (e.g., tailings, waste rock, mine,
drainage)
 Mineral processing
 Power generation
Vehicles(atmospheric pollution)
Mercury is associated with coal combustion
Shipping could be considered a diffuse source of
metals largely derived from antifouling paints

filterable metals
Filtration is used widely to differentiate between
dissolved and particulate forms of trace elements
The filter pore size and structure will determine the
amount of colloidal material (including bacteria and
other organisms) that will pass through the filters
This will influence the dissolved metal
concentration measured

measurement of total metals
Metals in natural waters can be present in a variety
of forms or species that have different reactivity,
bioavailability, and toxicity to aquatic organisms
 For several metals, bioavailability is better
correlated with the concentration of simple aquated
ion and inorganic complexes
 Metal speciation is influenced by pH, alkalinity,
and the presence of natural organic matter
 Environmental monitoring in systems where
elevated metal concentrations already have been
identified as a concern involving speciation
measurements

DESIGN OF MONITORING
PROGRAMS
Development of the sampling program involves
consideration of :
a.System Heterogeneity
b.Variability
c.Spatial and Temporal Resolution

Monitoring programs for trace
metals
Can be classified broadly into two groups:
1.Measure metal concentrations accurately in a water
body
2.Check compliance against fixed values

Behavior of metals
The behavior of metals will be influenced by:
oPhysical and chemical parameters
oPhysical processes
oSediment resuspension events
oSeasonal and climatic variability
These processes essential in
Designing a meaningful monitoring program
For scoping problems
For establishing the level of replication

Monitoring phases
Monitoring typically comprises three phases:
1.Sample collection
2.Sample pretreatment
3.Sample analysis

SAMPLE COLLECTION
Water sampling involves collecting volumes of
water at precise locations in both space and time
 Depending on the nature of the sampling sit,
involve sampling
By hand from bank locations
By boat
From bridge
From Jetty
From Discharge pipe
From water column
Estuaries body

WATER SAMPLING DEVICES
Sampling devices fall into five basic categories:
1.Grab sampling of surface waters
2. Pumping systems for sampling surface to medium
(6–8 m) depths
3.Depth samplers
4.Flow- or time-activated (automatic) samplers
5. Integrating samplers

Grab water sampling

Many water bodies are shallow(<5m) & well
mixed

Surface water sampling (0-1m)is required

It is by immersion of sample bottle by hand

An alternative is bucket fixed to a plastic rope
Precaution

Contribution from surface is avoided

Contamination from sampler or boat is
avoided

Sampling equipment is kept clean

Pumping systems

Not desireable for ultratrace metals

Applicable where tubing is kept clean

Peristaltic pumps are preferred

Vaccume samplers can also be applied
Limitation

Contamination of tubing if long length is used

Necessary storage & handling attention is
required

Discrete Volume Depth
Samplers
Involves deployment of a bottle via a
wire or plastic line
Filling is triggered by a weight sent
down the line
Most samplers are made up of tough
plastic with some metallic components
 Mercos is commercially available
Precautions
Use of metal cables should be avoided
Contamination checks should be
carried out at regular intervals

Flow or time activated samplers

Consists of pump systems, controller & an array of
bottle samples

Samplers can be preprogrammed to collect samples
on a flow- or time-related basis

Collection can be triggered by water flow or level

Can collect composite samples
Precautions

Sample preservatio needs to be addressed

Sample contamination should be checked regularly

Integrating Samplers
In situations where water quality is highly time-
dependent, information provided by discrete samples
will not be representative of temporal changes
 In these cases, samplers that integrate or time-
average metal loads over a fixed time period or
volume are an alternative
Limitation
The use of mathematical models that simulate the
actions of the samplers is helpful in this respect

SAMPLE BOTTLE SELECTION
A number of different plastic and glass bottles have
been used for trace metal sampling
Glass is not favored owing to high concentrations of
trace metals
Polyethylene, Teflon FEP bottles favored for low
metal content and ease of cleaning
Clear plastics are preferred because the pigments
added to colored plastics often contain metals
Fluorocarbon

Cont…
 Polymer bottles are usually used only for collecting
samples for mercury analysis owing to their high
cost
These are preferred for their low mercury content,
ability to withstand very strong acids, and low
permeability toward elemental mercury vapor
(diffusion of mercury from the atmosphere into the
sample on storage is a major potential source of
contamination)

SAMPLE PRETREATMENT
Sample Filtration
Glass & plastic filters units
 plastic units are preferred because they are less
prone to adsorptive losses
Excessive pressure or vacuum should be avoided
because this may cause rupture of algal cells retained
by the filter and release of their intracellular contents
into the filtered sample
The most widely used filters in water analysis are
cellulose-based (depth)membrane filters

 pore sizes
 composition
These can be classified into two broad categories:
Depth filters having a complex system of channels
within the body of the filter
Screen filters with a matrix of very uniform sized
unbranched pores
Glass fiber (depth) filters are not normally used for
trace metals analysis but can be used for the filtration
of samples prior to mercury analysis because they
can be cleaned effectively by heating to 500°C
Filter consideration

Cont…
It is important to minimize the time between sample
collection and filtration
 because adsorption/desorption reactions involving
particulates and bacterial activity can lead to changes
in sample composition
These effects may be minimized:
By storing samples at 4°C in the dark. This often
necessitates storing collected samples in ice-packed
containers or portable battery-powered refrigerators
 Maximum holding times should be specified

Sample Storage and
Preservation
Sample acidification to a pH of below 2 by using
nitric & hydrochloric acid
Typically, between 2 and 10 ml of concentrated
acid is added per liter of sample
The metal content of acids varies between batches,
and the purity of each acid batch should be screened
before use
The addition of an oxidizing agent such as acidified
bromine monochloride (5 ml/liter) has been
recommended for the preservation of mercury
samples
This prevents the formation of volatile elemental
mercury

TRACE METALS ANALYSIS
Selection of Analytical Methods:
The selection of an appropriate method involves
consideration of all or some of the following factors:
 The chemical form of the metal to be analyzed
 The range of analyte concentrations to be
measured
 The lowest concentrations of interest
 The sample matrix and potential interferences
 Required sample throughput
 Cost

Contamination Control
Adequate control of contamination is a critical
factor in obtaining accurate and precise results
This requires stringent cleaning and washing
protocols and a clean laboratory environment
The difficulties encountered in measuring metal
concentrations often vary between laboratories and
depend on the prevailing sources of contamination
Older laboratories with an extensive history of
elemental mercury use can suffer from mercury
contamination problems

Sources of metal
contamination
There are three sources of metal contamination
The reagents used in the analytical procedures
 The surfaces that come into contact with the
samples
The laboratory environment

Cont…
The first contamination source is usually
characterized by a consistent positive bias and may
be reduced by using high-purity reagents
 Alternatively, various procedures are available for
purifying reagents, e.g., distillation, co precipitation,
and recrystallization
A general rule to minimize contamination is:
To keep the number of sample-handling steps to a
minimum Laboratory-derived
 Contamination can be reduced by limiting the
number of metal surfaces in the laboratory

Digestion Procedures
Digestion of water samples is applied prior to total
metals analysis to release metals from particles,
dissolve mineral phases, and oxidize organic matter
 This typically involves addition of an acid or
combination of acids with or without some form of
heating
Additional oxidizing agents such as hydrogen
peroxide also can be added
Care must be taken to compensate for any changes
in sample volume during digestion

Cont…
Microwave heating in sealed vessels with
concentrated acids is the preferred procedure
In some cases, specific oxidizing agents are added
to convert all chemical forms of an element to the
form required for analysis

Analysis of trace metal in saline sample
 Most metals are commonly present at low
nanogram per liter levels and well below the limits
of detection of most instrumental methods
 GFAAS has been the single most important
technique owing to its high sensitivity and low
sample volume requirement
Analysis often involves a matrix separation (to
avoid interferences from the saline matrix)
Reagent purity and clean-room laboratory
techniques are vital to attain accurate results

ANALYTICAL TECHNIQUES
There are following analytical techniques
Atomic Spectrometry
Vapor generation techniques
Electrochemical analysis
Colorimetric methods

FUTURE DEVELOPMENTS
Improvements in water quality monitoring will be
made by the measurement of metal species
concentrations that are more meaningful to
answering the questions posed (e.g., metal
bioavailability)
Increased spatial and temporal coverage and
reduced time between sampling, analysis, and
decision making are desirable
To achieve these goals, portable field analysis, in
situ sensing, and real-time monitoring of metal
concentrations will become increasingly important

Monitoring of trace metals and metalloids in natural

Monitoring of trace metals and metalloids in natural

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