The determination of metals in soil through on-site and laboratory analysis
Soil and its uses have been neglected or have been taken for granted for centuries. The human race has been disposing of waste via the soil for as long as we have been on the earth.
Agriculturally based civilisations have used human and animal waste to fertilise the soil and when populations were small this was no problem in fact it was the natural scheme of things but as the human race increased in number the pressure on the soil to produce ever more food be it arable or dairy has put strains on the soil ecosystem. In the 20th century the additions of fertiliser, pesticides and herbicides have stored up problems for the human race and all species.
When the industrialisation began people moved from working the soil to working in factories but unfortunately still applied the same processes to waste disposal thinking that when waste was deposited onto or into the soil then the problem had gone away. Over the last half century or so we have discovered that our forbears were wrong.
Pollution that is added to the soil may adhere to the soil matrix and become immobile but may still be available to plant roots and be taken up into the plant structure from where it can enter the food chain. Other pollutants can slowly permeate through the top soil structure to the sub strata and eventually into the aquifer below. An example of this is our problems with nitrate in drinking water which arise from the intensive use of nitrate fertilisers which have now been washed through the soil and found their way through to the aquifer. This may or may not be the case for heavy metals in soils as their mobility within the soil matrix is dependent upon several factors including the organic content of the soil, the soil type and soil pH.
In report number CSI 015 entitled Progress in Management of Contaminated Sites dated August 2007 the European Environment Agency (EEA) states that member countries had at the time identified 250,000 sites where soil contamination requires clean up. The EEA estimates that there are around 3,000,000 sites across Europe where potentially polluting activities have occurred and where investigation is needed to establish if soil remediation is required. The EEA also predicted that the number of sites requiring remediation is likely to increase by 50% by 2025. This compares to the number of sites where the remediation process has been completed which, over the last 30 years, is estimated to be in the order of 80,000 sites.
The report also states that of the countries and regions within them reporting data to the EEA Heavy Metals were the largest contributor to the list of contaminants, being present at 37.3% of the reported sites with mineral oils next at 33.7% of the reported sites. The figures in the charts below are taken from this EEA report and show the breakdown of the industries and substances/compounds causing the pollution.
The EEA report also mentions briefly that the soil is not being considered as a valuable resource but as a disposable commodity. Where pollution of soil has occurred by whatever process within Europe the two main treatment processes are either excavation of the soil and removal to a waste disposal site or by containment of the area neither of which actually deals with the problem in the long term. There exist processes for the onsite remediation of soils by washing, bio remediation, direct chemical and most recently electro-chemical techniques by either in-situ or ex-situ (off site) systems and these are being taken up more often due in part to the increasingly high costs of landfill. In the UK the Environment Agency applies a fit for purpose approach to soil pollution applying the most stringent standard to agricultural use, lower for residential use and the lowest standard for industrial uses of the land involved.
A hidden source of pollution
The sources of heavy metal pollution tend to be associated with heavy industry, both existing and historical. There is a legacy of pollution residing in the rivers of the developed world.
In a recent paper published by the UK Environment Agency Entitled ‘The Assessment of Metal Mining Contaminated River Sediments in England and Wales’ dated November 2008 states:-
“Although metal discharges were greater during the peak period of active mining in the nineteenth century, significant inputs of dissolved and particulate metals still occur. Past discharges have left a substantial reservoir of highly contaminated sediments in lowland rivers many kilometres downstream of the mines, and these sediments are likely to be causing ecological damage. Re-suspension of these sediments during floods has the potential to cause additional harm to aquatic life, and to contaminate floodplain soils used for agriculture.”
With the predicted increase in the frequency of major flood events due to global warming, the likelihood of the mobilising of heavy metal containing sediments and their deposition onto the fertile land on the margins of rivers is greatly increased. So in the future there is a real risk of some areas of low lying land downstream of mining and industrial areas becoming so contaminated as to be of no use for agricultural purposes where originally these areas were considered the most fertile due to the same flooding events depositing nutrients.
The same is true of herbicides many of which now are present in rivers flowing through or draining from agricultural regions. Some of these compounds are very difficult to remove even using the advanced ozone and activated carbon systems.
Other anthropogenic factors effecting metal pollution of soil are emissions from flue gasses of fossil fuelled power stations, large combustion plants and incinerators. All of these by the act of incineration of the fuel/feedstock have the ability to place large quantities of metals into the atmosphere. An example of which is one of the UK’s largest power stations which has flue gas desulphurisation and is reported on the UK Environment Agency’s pollution register to have discharged 2.25 tonne of Pb, 46kg Cd and 234kg of Ni into the atmosphere in 2006. Again the population at large is unaware that the flue gasses from these combustion plants also contain heavy metals. The introduction of electrostatic precipitators and flue gas scrubbers has reduced the emission to air from these point sources but may well have merely transferred or localised the discharge to water or land.
The use of leaded fuels in the internal combustion engine has also added large quantities of anthropogenic lead into the environment especially near major roads and has arguably caused an amount of learning impairment in children.
One should also not forget that what seem to be innocuous artefacts may also contribute to the heavy metal loading of soils as it is common practice to treat wood for building applications with chemicals many of which contain arsenic and copper which have replaced the phenolic preservatives of yesteryear.
The problems of making measurements in soil
To the layman soil seems to be a homogenous material. This of course is not the case and all manner of factors can have large effects upon the homogeneity of soil.
Building work that has been carried out on the sites can have an effect by the distribution of excavated sub soils onto the surface. The insertion of foundations and hard standing areas can lead to areas of soil compaction which impair the free draining of water.
Substantial flows of water percolating through the soil can lead to finer particles to be flushed away. All of these can affect the physical properties of the soil.
Another problem of soil analysis is that soil is a 3 dimensional matrix, the concentration of any particular measurand can vary across all 3 dimensions.
Point sources of pollution over time lead to ‘plumes’ of pollution across a site where the flow of groundwater and or rainwater has eluted the pollutant from the point source with the structure of the soil acting as the packing in a chromatography column and the flow of ground water eluting the compound through the soil due to partial solubility.
This makes the location of hot spots difficult when using off site laboratory analysis due to the time lag between sampling and result. The most advantageous method to the efficient use of plant and off site laboratory analysis is by the use of an onsite screening process which provides real time data and enables the site operators to select samples for closer examination by the accredited lab.
The sampling regime is crucial to the success of the remediation of a contaminated site and a knowledge of the previous use of the areas can help in locating potential regions of pollution. A grid system of sampling area being used for initial sampling with reference to any site information as to former usage of any particular region within the grid. Once initial sampling has been carried out and the results obtained further sampling of areas with high concentration of pollutants are carried out to locate the source of the pollution.
There are also substantial differences between on-site and laboratory analysis. When on-site analysis is carried out this is generally only using a few grams of soil whereas a laboratory sample will be taken from a sample of a few kilograms which are then homogenised before sub sampling and analysis.
To have confidence in the site test method its correlation with the laboratory approved method must be recognised and established. It is also prudent to achieve confirmatory results by taking sub samples of larger laboratory samples and analysing them with the onsite equipment.
Analytical field tools
These offer solutions to site investigations being able to provide rapid results and guide on site staff to locate ‘Hot Spots’.
They show the extent of the contamination and the extent of the migration of the contaminant and permit a much larger number of samples to be analysed than would be possible if using solely an offsite laboratory due to the cost implications.
This increased data density permits more accurate characterisation of the site by interactive sampling (higher sampling density in suspect regions) and in so doing is highly likely to reduce the costs of soil removal.
Determination of heavy metals
As heavy metals associate themselves with organic compounds within the soil making the metal ion immobile or partially so within the soil structure. To completely release the metals for liquid analysis an extraction process has to be applied to the soil sample. Some methods of determination require the sample to undergo chemical digestion to release the metals so that analysis can be achieved however the XRF method does not require this procedure to be applied.
When analysing soil for heavy metals using off site methods XRF, Atomic absorption and ICP (Inductive Coupled Plasma) are the primary accredited methods with proven high precision but all of the QA systems can be negated by a non representative sample.
On site analysis tools for metals
X-Ray Fluorescence (XRF)
For soil analysis two versions of instrument are used for on-site analysis. Portable unit when the ‘gun’ is placed in close proximity to the soil and a near instantaneous reading is provided or laboratory instruments where samples are presented to the analyser. The hand held units can also be used in the laboratory with the use of an appropriate stand.
The theory of operation:- When high energy X-Rays from the gun strike a metal atom some of the low energy electrons within the atom are literally knocked out of their orbits this leaves a ‘hole’ and higher energy electrons from electron shells further away take up the void but in doing so they have to lose energy to be able to stay in the that orbit this energy is lost as X-rays these X-rays are known as fluorescent X-rays and each metal has a characteristic set of emissions which can be used to identify the metal the amplitude of these provide information regarding the concentration of the element within the target sample.
The advantage of the XRF gun is that it is possible to use safely with the minimum of training and results are almost instantaneous. The disadvantages are that the penetration depth of the X-rays within the soil sample is only a few mm and there are attenuation issues with wet soils or sludges which can be overcome by drying the sample before analysis.
ASV (Anodic Stripping Voltammetry)
ASV is an electrochemical method which requires the metal ions to be in solution therefore the sample has to undergo a wet chemical digestion to release the metal ions and destroy any organics which may be sequestering them. Once the metals are solubilised a sample is added to an electrolyte, a negative charge is applied to the working electrode causing a proportion of the ions to be reduced to the metallic state onto the surface f the electrode. The potential on the electrode is then made positive which then re-oxidises the metal and the ions move back into solution, this oxidation releases an electron the flow of which is measured by another electrode within the solution. The voltage at which the metal re-ionises identifies the metal and the peak height or area is used to calculate its concentration. The advantage of this method is that turbid samples do not interfere with the measurement and once set up analysis can be rapid. Disadvantages are that an extraction process has to be applied to the sample to release the metal ions into solution.
Colorimetric test kits
These are common in water analysis for many cations and anions however the determination of metals in soils posses challenges for colorimetric systems. Due to the requirement to measure bound as well as free metals sample digestion is required and as colorimetric measurements often require quite specific pHs for the colour chemistry to operate properly the technique does not lend itself easily to the determination of metals in soil.
Published: 10th Jun 2009 in AWE International