Miljøprojekt Nr. 1198 – Oprensning af forureningen på depotet ved Høfde 42 ved hjælp af nul-valent jern

Oprensning af forureningen på depotet ved Høfde 42 ved hjælp af nul-valent jern

Summary and conclusions

In this project the degradation of contaminating compounds by zero-valent iron technologies at the former chemical depot at Høfde 42 was examined. The work was prepared for the Danish EPA and the County of Ringkjøbing and performed in cooperation between Institute of Environment & Resources (E&R), COWI and GeoSyntec Consultants.

The project found that parathion and malathion were rapidly and completely transformed into less toxic compounds. Furthermore many of the degradation products present at the site were seen to be degradable as well. Based on this evidence it has been evaluated that there is basis for initiating a pilot-scale project at the site. In this connection a very promising zero-valent iron technology, which does not require a detailed mapping of the organic phase, has been identified. In the following a summary of the most important activities and results is given.

Background and Purpose

Based on the pesticide contamination at the former chemical depot at Høfde 42, the County of Ringkjøbing and the Danish EPA have decided to initiate a systematic assessment of different remediation technologies.

The main purpose of this assessment is to gain sufficient knowledge about the remediation technologies, which will enable the selection of the technically and economically best solution.

For the last 10 years zero-valent iron has been used for groundwater remediation in permeable reactive barriers, and during the last four to five years injection or admixing of fine iron particles has been used for source remediation. Based on this, E&R has performed experiments with zero-valent iron and the compounds present at Høfde 42. It was found the zero-valent iron has a significant potential for degrading most of the compounds with a significant reduction in mass and toxicity as a result.

Hence the purpose of this report is to evaluate the usability of the zero-valent iron technologies for remediation of the site with emphasis on source remediation. In order to do so the following activities were carried out:

a literature study on zero-valent iron technologies for groundwater- and source remediation with special focus on the compounds at Høfde 42
performance of experiments in the laboratory to support the experiments already performed at E&R
putting the knowledge gained into perspective with regard to utilisation at the site, taking the already planned activities at the site into consideration
a suggestion for a pilot-scale project at the site, which will further document the usability of the technology and enable the final dimensioning of a full-scale remediation project at the site

Activities

The main focus in the project has been the experiments performed at E&R. For these a homogenized sediment sample taken from a hotspot at the site was used. The experiments carried out were as follows:

Batch tests for determination of reactivity using different iron-to-soil ratios (0.0005, 0.005, and 0.05) under ideal mixing conditions
Column recirculation tests for determination of reactivity at a relatively high iron-to-soil ratio (0.05) under more realistic mixing conditions.

Furthermore a literature study was performed, which in combination with the experiments constitutes the basis for the assessment of the usability of the technology at the site; hereunder a suggestion for further experiments in the form of a pilot-scale project before a full-scale remediation is carried out.

Main conclusions

The results obtained from the experiments can be summarised as follows:

an iron-to-soil ratio of 0.05 was necessary in order to gain a significant mass and toxicity reduction during the three months of experiments
microscale iron particle were the most efficient for degrading the compounds in both types of experiments
malathion and methyl and ethyl parathion are all degraded rapidly from high concentrations to below the detection limit (k > 6.0 • 10-2 d-1) (>99 %)
a significant degradation of p-nitrophenol (>90 %), the produced methyl and ethyl amino-parathion (>50 %), the group of analysed triesters (>50 %) and E-sulfotep (30-50 %) was found
a slow degradation of diesters when the mother compounds were gone
a significant degradation of MCPA could not be determined. However the compound has been found to be degradable in earlier experiments.

It is assessed that with the right amount of iron and sufficient time for reaction most of the contaminating compounds can be degraded within five years, during which the most significant degradation will take place during the first year. Furthermore it has been concluded that the “ZVI-clay” technology will be the best suited for testing on the site. One significant reason for this is that the method has been successfully tested abroad, it does not require a precise location of the contaminating organic phase and it is suitable with micro-scale iron.

Results

The water and sediment samples were analysed for 17 relevant compounds by Cheminova A/S.

Batch tests

For the smallest amount of iron at 0.5 g iron/kg sediment a significant reduction of mass or toxicity was not seen after 13 weeks, while a small reduction was seen using the intermediate amount of iron at 5 g iron/kg sediment.

For the largest amount of iron at 50 g iron/kg sediment a significant reduction of mass and toxicity was seen. It was found that a significant degradation or production could be seen for most of the 17 compounds. The most rapidly degradable compounds were methyl and ethyl parathion. These were degraded completely (>99 %) during the first two weeks of the experiment with a production of methyl and ethyl amino-parathion as a result. During the next three weeks of the experiment the produced amino-parathion was shown to be degradable as well (>50 %), though the degradation was slower.

Malathion and p-nitrophenol were also found among the most easily degradable compounds, as these were degraded to the detection limit (>98 %) within five weeks. E-sulfotep and the group of triesters were also degraded (30-60 %) although not as rapidly as the other compounds.

As is the case for amino-parathion the group of diesters are also produced during the first two weeks where after they are degraded (up to 40 %). However, this degradation is rather slow compared to the other compounds. A significant time horizon will therefore be necessary in order to ensure the diesters are degraded.

For MCPA a significant degradation could not be confirmed, while the chlorinated cresol degraded slowly. Since MCPA was found to be degradable in the aqueous phase in earlier experiments, it is believed that with more time for reaction the compound will also be degraded.

With regard to toxicity, this has been reduced 25-50 times within the five week period. How large the reduction was depended upon the type of iron used. The most efficient reduction was seen using microscale iron, which was also the case with regard to mass reduction.

Column tests

The column tests were set up using 50 g iron/kg sediment as in the batch tests with the largest amount of iron. Hence the results of the column tests were seen to resemble the results of these batch tests. The most significant difference was the efficiency of the iron types, along with the certainty of the results. While granular iron was the less efficient type of iron in the batch tests, the nanoscale iron was the less efficient type of iron in the column tests. In fact nanoscale iron was so inefficient that a difference between this column and the control could hardly be seen.

For the microscale and granular iron a degradation of malathion and methyl and ethyl parathion to below the detection limit (>99 %) was seen within 11 weeks. However, not as much amino-parathion was produced, which indicates a higher degree of degradation had taken place.

For the rest of the compounds the degradation was seen to be more efficient using microscale iron than when using granular iron. For both types of iron, p-nitrophenol (40-90 %) and E-sulfotep (~50 %) were degraded during the 11 weeks, while diesters were produced (250-300 %). The main difference was seen for the group of triesters, which were significantly degraded (>50 %) when using microscale iron but not when using granular iron.

The toxicity reduction was also similar to the one found in the batch tests with the same amount of iron, with a reduction of up to 55 times.