Monitoring of trace metals and metalloids in natural water

SHUMAIL SAID

Contents

Introduction Sources of contaminationSampling equipmentSample collection & preservationLimitations Future developments

Introduction

Ubiquitous contaminants of aquatic environmentPotential 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 zincThe maximum tolerable concentrations typically are in the 1 to 100 g/liter rangeSome forms of organometallic compounds (e.g., methylmercury and tributyltin) are highly toxic at quite low concentrations

Source of contamination

Two types of sourcesNatural sourcesAnthropogenic 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 dischargesSewage treatment effluents Mining discharges (e.g., tailings, waste rock, mine, drainage) Mineral processing Power generationVehicles(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 filtersThis 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 parametersoPhysical processes oSediment resuspension events oSeasonal and climatic variability These processes essential inDesigning a meaningful monitoring programFor scoping problems For establishing the level of replication

Monitoring phases

Monitoring typically comprises three phases:1.Sample collection2.Sample pretreatment3.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 locationsBy boatFrom bridgeFrom JettyFrom Discharge pipeFrom water column Estuaries body

WATER SAMPLING DEVICES

Sampling devices fall into five basic categories:1.Grab sampling of surface waters2. Pumping systems for sampling surface to medium (6–8 m) depths3.Depth samplers4.Flow- or time-activated (automatic) samplers5. 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 lineFilling is triggered by a weight sent down the lineMost samplers are made up of tough plastic with some metallic components Mercos is commercially available

PrecautionsUse of metal cables should be avoidedContamination 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

LimitationThe 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 samplingGlass is not favored owing to high concentrations of trace metalsPolyethylene, Teflon FEP bottles favored for low metal content and ease of cleaningClear 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 costThese 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 FiltrationGlass & plastic filters units plastic units are preferred because they are less prone to adsorptive lossesExcessive 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 sampleThe 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 poresGlass 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 PreservationSample acidification to a pH of below 2 by using nitric & hydrochloric acidTypically, 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 useThe 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 environmentThe difficulties encountered in measuring metal concentrations often vary between laboratories and depend on the prevailing sources of contaminationOlder laboratories with an extensive history of elemental mercury use can suffer from mercury contamination problems

Sources of metal contaminationThere are three sources of metal contaminationThe reagents used in the analytical procedures The surfaces that come into contact with the samplesThe 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 recrystallizationA 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 requirementAnalysis 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 techniquesAtomic SpectrometryVapor generation techniquesElectrochemical analysisColorimetric 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 desirableTo achieve these goals, portable field analysis, in situ sensing, and real-time monitoring of metal concentrations will become increasingly important