Iron filings remove chromium from groundwater

In several parts of the country, soil and groundwater are contaminated with the heavy metal chromium, previously used in large amounts in industry. However, an iron filings filter can now reduce the chromium content of the groundwater to under three microgrammes per litre. But depending on the amount of chromium, the lifetime of the filter can be limited. This is one of the findings of a series of new experiments undertaken under the Danish EPA’s Technology Development Programme for Soil and Groundwater Pollution. The experiments were conducted in connection with the excavation of contaminated soil from a former tannery in Roskilde County. Remediation of the site necessitated pumping up and cleaning large amounts of groundwater contaminated with chromium. Background and objective

Under three microgrammes of chromium per litre

In the past, various industries used the heavy metal chromium for wood impregnation. It was also used by tanneries, and the galvano industries used it for chromium-plating metal parts. Being an element, the chromium did not degrade. On the contrary, it contaminated soil and groundwater in several parts of the country.
One location where this heavy metal has caused serious contamination of the soil and groundwater is Sankt Clara Vej in Roskilde. To excavate and remove the contaminated soil, the groundwater level had to be lowered temporarily and subsequently pumped up, cleaned and diverted into the inlet of Roskilde. Because of the existing chromium content of the inlet, the authorities stipulated that the chromium content in the groundwater to be discharged into the waterway should not exceed three microgrammes per litre.
To satisfy this rigorous requirement, a decision was mad to develop a new filter that could be used to clean the extracted groundwater. The filter had to be designed along the lines of the "reactive walls" theory. Reactive walls represent a sub-soil remediation technology – also called an in-situ technology. With this method, iron filings are placed across the flow path of a contaminant plume, cleaning the groundwater as it passes through.
Chemically, the cleaning process consists of a chemical reaction called a redox reaction between chromium and iron. During this reaction, chromium is converted from the hazardous and mobile form Cr(VI) to the form Cr(III), which is not only immobile but also less harmful – and which is retained in the filter (see box 2).
Box 1: The capacity of the iron
The capacity of the iron depends on how much chromium a given amount of iron filings will be able to retain in the filter. The unit used is milligrammes of chromium retained in the filter per gramme of iron filings used.
The study

Experiments with iron filings

The experiments involving the use of iron filings for cleaning groundwater were conducted at the Institute for Environmental Technology at the Technical University of Denmark. The experiments were all conducted in the form of column experiments, simulating the conditions in an actual filter. To some extent, the experiments were based on findings from similar laboratory experiments previously conducted by the Institute.
The laboratory work involved three column experiments with a varying number of columns: an initial experiment, an in-depth experiment and a final experiment aimed at documenting the effectiveness of the design to be used in the Roskilde filter. The effectiveness was measured based on two parameters: the lifetime of the filter and the chromium concentration in the outlet water.

Figure 1:
Illustration of experiment arrangement

Figure 2:
Example of how the chromium front progresses up the column as the chromium solution volume passing through the filter increases.
The experiments evaluated the capacity of the iron filings on the basis of various parameters (see box 1), including water type – a distinction was made between groundwater from the chromium-contaminated area and tap water to which contaminants had been added artificially. Other parameters studied included the effect of introducing sand into the filter, adjusting the pH, changing the concentration level and changing the water flow rate through the filter.
The basic test set-up illustrated in figure 1 was employed in all the experiments. The vessel used in the set-up contained either groundwater that had become chromium-contaminated naturally, or tap water that had been artificially chromium-contaminated. The vessel was connected to the inlet at the column base by means of a peristaltic pump. Thus, the water flowed from the bottom upwards through the columns.
The columns were made of plexiglass with an inside diameter of 54 millimetres and a length of either 25 or 50 centimetres. Holes had been drilled longitudinally along the columns and sealed with a Teflon-coated septum, allowing samples to be drawn along the columns by means of a disposable syringe with a hypodermic needle. The third column experiment deviated slightly from this principle in that samples were drawn from the column’s outlet instead of from the sample gates along the column.
A chromium solution was added to the columns in various concentrations and at various flows. The columns were packed with either 100% iron filings or with iron filings mixed with sand. The chemical reaction occurred when the chromium solution entered the columns, and the chromium disappeared from the solution within a few centimetres. This was recorded by measuring the chromium content of samples taken from the sample gates on the column sides. This method was employed to determine a chromium profile which depicted the chromium concentration along the length of the columns.
Depending on the chromium content and the capacity of the iron, yet another chromium profile was determined along the length of the columns after a period of time. This was achieved by again taking samples from the sample gates. The profile showed that the point at which the chromium disappeared from the solution had moved further up in the column. An example of this appears in figure 2. The figure shows that the chromium front for each profile collected has moved further up in the column.
Lastly, the capacity of the column was calculated on the basis of data shown in figure 2. First, the centimetres the chromium front progressed each day were determined. The result was then related to the column’s chromium content. This measurement indicated the iron’s capacity as milligrammes of chromium retained per gramme of iron used.
Main conclusions

Filter paid off

A filter with iron filings makes it possible to achieve even rigorous water treatment requirements for chromium-contaminated water. This is the main conclusion of the project, which benefited extensively from the theoretical knowledge available. The knowledge was employed in a new way. As far as is known, the use of an in-situ filter with iron filings is a new concept in Denmark.
The results indicate that a filter of this type has many applications. It can be used not only in relation to chromium contamination, but also more generally in relation to other contaminant components that are rendered harmless when reduced. These include contamination by chlorinated solvents.
A number of conclusions of more theoretical or technical interest can be drawn from the laboratory work.
Basically, it transpires that the type of filter described above has a limited lifetime. It can only be used until the filter’s total capacity to remove chromium has been exhausted. A similar filter used for removing chlorinated solvents, for example, will in principle have an unlimited – or at any rate considerably longer – lifetime.
Iron filings are costly, and the project therefore sought to identify new methods of enhancing the capacity of iron filings to reduce the costs of the new filter. For example, on the basis of theoretical considerations, tests were conducted in which the iron filings were mixed with quartz sand to determine whether the capacity of the iron filings could be increased by introducing another surface than that of the iron filings on which the precipitate could settle. The experiments showed that this is not directly possible.
Similarly, on the basis of theoretical knowledge, experiments were conducted in which the pH of the contaminated water was adjusted upstream of the filter. The pH was adjusted to 4, even though such a radical adjustment would probably not be feasible in practice. However, the effect was very clear. The experiments did not reveal an upper limit on the amount of chromium that the iron can remove at a pH of 4. There is no doubt, however, that there is an upper limit.
Lastly, tests were carried out to determine whether the rate at which the water flows through the filter impacts on the amount of chromium removed. The water flow normally passes considerably faster through an on site filter than through an in situ installation, and it ran faster than the flow in most of the experiments conducted. The study therefore aimed at determining whether increasing flow rates had any effect. It turned out that there was no simple correlation between the rate of flow and the amount of chromium removed, and no major effects were found.
Project results

Several critical factors

As already mentioned, the project involved three column experiments. The results of the experiments are relatively comprehensive and the numerical data are therefore not included here, but are available in the project report "Reduction of chromium (VI) in groundwater by means of iron filings". The purpose of the first column experiment was two-fold.
Firstly, to evaluate whether the ability of the iron to remove chromium could be increased by mixing the iron with sand and, secondly, to determine whether the same results could be achieved in the laboratory in tap water artificially contaminated with chromium as in groundwater pumped up from the contaminated site. Water pumping at the site proved a very costly exercise, and tap water used instead would substantially facilitate the laboratory work.
The conclusion of the first column experiment was that it was possible to use tap water instead of contaminated groundwater and that it was worthwhile examining further the effect of mixing the iron filings with sand, although the results were not conclusive.
Thus, one of the purposes of the second column experiment was also to study in more detail the potential of mixing sand in the iron filings (25-100% iron). The experiment also intended to show the effect of varying flow rates through the columns (80-320 millimetres per hour), varying concentration levels (20-300 milligrammes per litre) and, finally, the effect of adjusting the pH of the water to 4 upstream of the columns. One of the results of the second column experiment was that it was not an advantage to introduce sand into the iron filings. It actually looked as if the iron removed less chromium when it was mixed with sand. This is probably because 100% iron filings give a lower chemical potential.
Box 2: Chemical reaction diagrams for the redox process
Fe0 3 e- + Fe3+
CrO42- + 3 e- + 4 H2O Cr(OH)3 + 5 OH-
Netto: Fe0 + CrO42- + 4 H2O Fe(OH)3 +Cr(OH)3 + 2 OH-
The second column experiment also proved that the concentration level did not impact on the amount of chromium removed by the iron filings. At any rate, the data collected showed no clear tendency. Moreover, the second experiment showed that the iron filings generally removed slightly less chromium when the flow rate was increased. Despite this, it was not possible to show any direct correlation between flow rate and capacity. Lastly, the second column experiment concluded that, in the columns in which the pH of the water was adjusted to 4 before the water was added, the chromium front did not move during the course of the experiment. This should of course not be taken to mean that unlimited amounts of chromium can be removed by changing the pH, although, quite clearly, the pH plays a very significant part in the system.
The third column experiment was arranged on the basis of the design it had been decided to use at Sankt Clara Vej in Roskilde. The purpose of the experiment was to record that the filter lived up to the desired requirements: it had to clean the pumped-up water so well that the chromium concentration was less than three microgrammes per litre. As in the case of the real filter, the column was packed with 100% iron filings. In addition, the flow rate corresponded to the rate expected in the filter. However, for technical reasons and because of time constraints, the concentration of chromium had to be scaled up slightly.
The column turned out to clean the water so well that no chromium could be detected. As the column’s capacity was gradually exhausted, chromium began to be detected in the outlet water. Worries had arisen that particulate chromium would pass through the filter before the filter’s capacity was exhausted, but they proved unfounded.
All in all, the experiment showed that a filter with the chosen design could be expected to clean the groundwater so well that it contained less than three microgrammes of chromium per litre, and that, under the given conditions, it would have a lifetime of at least six months. This indicated, in turn, that it would meet the requirements stipulated for the discharge of the groundwater into the inlet of Roskilde.
Summary:

Iron filings remove chromium from groundwater

In several parts of the country, the soil and groundwater are contaminated with the heavy metal chromium, large amounts of which were previously used in industry. However, an iron filings filter can now reduce the chromium content of the groundwater to under three microgrammes per litre. Depending on the amount of chromium, the lifetime of the filter can be limited, though. This is one of the findings of a series of new experiments undertaken under the Danish EPA’s Technology Development Programme for Soil and Groundwater Pollution. The experiments were conducted in connection with the excavation of contaminated soil from a former tannery in Roskilde County. Remediation of the site necessitated pumping up and cleaning large amounts of groundwater contaminated with chromium.
Facts:
Project title:
Reduction of chromium(VI) contamination at Sankt Clara Vej in Roskilde County
Performing organisation(s):
Thomas Locht, Institute for Environmental Technology, Technical University of Denmark
Printed publication:
No printed publication available
Electronic publication:

Financing:
Technology Development Programme for Soil and Groundwater Pollution, Danish EPA
Further information:
Soil Contamination Division, Danish EPA. Phone: +45 3266 0100
The evaluations in this project article are the responsibility of the performing organisation. They do not necessarily reflect the opinion of the Danish EPA.
Printed publications are available from Miljøbutikken, Læderstræde 1-3, DK-1201 Copenhagen K, tel. +45 33 95 40 00, fax +45 33 92 76 90, e-mail:

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