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Chlorine Cycling in Terrestrial Environments
Linköping University, The Tema Institute, Tema Environmental Change. Linköping University, Faculty of Arts and Sciences.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Klorets kretslopp i terrestra miljöer (Swedish)
Abstract [en]

Chlorinated organic compounds (Clorg) are produced naturally in soil. Formation and degradation of Clorg affect the chlorine (Cl) cycling in terrestrial environments and chlorine can be retained or released from soil. Cl is known to have the same behaviour as radioactive chlorine-36 (36Cl), a long-lived radioisotope with a half-life of 300,000 years. 36Cl attracts interest because of its presence in radioactive waste, making 36Cl a potential risk for humans and animals due to possible biological uptake. This thesis studies the distribution and cycling of chloride (Cl) and Clorg in terrestrial environments by using laboratory controlled soil incubation studies and a forest field study. The results show higher amounts of Cl and Clorg and higher chlorination rates in coniferous forest soils than in pasture and agricultural soils. Tree species is the most important factor regulating Cl and Clorg levels, whereas geographical location, atmospheric deposition, and soil type are less important. The root zone was the most active site of the chlorination process. Moreover, this thesis confirms that bulk Clorg dechlorination rates are similar to, or higher than, chlorination rates and that there are at least two major Clorg pools, one being dechlorinated quickly and one remarkably slower. While chlorination rates were negatively influenced by nitrogen additions, dechlorination rates, seem unaffected by nitrogen. The results implicate that Cl cycling is highly active in soils and Cl and Clorg levels result from a dynamic equilibrium between chlorination and dechlorination. Influence of tree species and the rapid and slow cycling of some Cl pools, are critical to consider in studies of Cl in terrestrial environments. This information can be used to better understand Cl in risk-assessment modelling including inorganic and organic 36Cl.

Abstract [sv]

Klorerade organiska föreningar (Clorg) bildas naturligt i mark och påverkar klorets kretslopp genom att de stannar kvar längre i marken. Detta stabila klor anses ha samma egenskaper som klor-36, som är en långlivad radioisotop med en halveringstid på 300 000 år. Klor-36 förekommer i olika typer av radioaktivt avfall och om klor-36 sprids i naturen finns det en potentiell risk för människor och djur genom biologiskt upptag. Syftet i denna avhandling är att öka kunskapen om fördelningen och cirkulationen av klorid (Cl-) och Clorg i terrestra miljöer med hjälp av studier i laboratoriemiljö samt en fältstudie i skogsmiljö. Resultaten visar att bildningshastigheten av Clorg är högst i barrskogsjord och rotzonen tycks vara en aktiv plats. Det finns också en större mängd Cl- och Clorg i barrskogsjordar än i betesmark och jordbruksmark. Den mest betydande faktorn som styr halterna av Cl- och Clorg är trädsort, medan geografiskt läge, atmosfäriskt nedfall, och jordmån är av mindre betydelse. Bildning och nedbrytning av Clorg sker med liknande hastigheter, men det tycks finnas två förråd av Clorg i jorden varav ett bryts ner snabbt och ett mer långsamt. Bildningshastigheten av Clorg är lägre i jordar med höga halter av kväve medan nedbrytningshastigheterna inte påverkas av kväve.

Slutsatsen från studiernas resultat är att klor i hög grad är aktivt i mark och att Cl- och Clorg halterna bestäms av en dynamisk jämvikt mellan bildning och nedbrytning av Clorg. I studier av klor i terrestra miljöer bör trädsorters inverkan och nedbrytning av olika klorförråd beaktas då det kan ge varierande uppehållstider av Cl- och Clorg i mark. Denna information är viktig vid riskbedömningar av hur radioaktivt klor kan spridas och cirkulera vid en eventuell kärnkraftsolycka.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2016. , 62 p.
Linköping Studies in Arts and Science, ISSN 0282-9800 ; 672
Keyword [en]
Chloride, organic chlorine, chlorination, dechlorination, 36Cl, risk assessment modelling
Keyword [sv]
Klorid, organiskt klor, klorering, deklorering, klor-36, riskmodellering
National Category
Soil Science Environmental Sciences related to Agriculture and Land-use Agricultural Science Forest Science Ecology
URN: urn:nbn:se:liu:diva-125913DOI: 10.3384/diss.diva-125913ISBN: 978-91-7685-813-4 (Print)OAI: diva2:910032
Public defence
2016-04-15, TEMCAS, Hus T, Campus Valla, Linköping, 10:15 (English)
Available from: 2016-03-08 Created: 2016-03-08 Last updated: 2016-03-09Bibliographically approved
List of papers
1. Organic Matter Chlorination Rates in Different Boreal Soils: The Role of Soil Organic Matter Content
Open this publication in new window or tab >>Organic Matter Chlorination Rates in Different Boreal Soils: The Role of Soil Organic Matter Content
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2012 (English)In: Environmental Science and Technology, ISSN 0013-936X, E-ISSN 1520-5851, Vol. 46, no 3, 1504-1510 p.Article in journal (Refereed) Published
Abstract [en]

Transformation of chloride (Cl-) to organic chlorine (Cl-org) occurs naturally in soil but it is poorly understood how and why transformation rates vary among environments. There are still few measurements of chlorination rates in soils, even though formation of Cl-org has been known for two decades. In the present study, we compare organic matter (OM) chlorination rates, measured by Cl-36 tracer experiments, in soils from eleven different locations (coniferous forest soils, pasture soils and agricultural soils) and discuss how various environmental factors effect chlorination. Chlorination rates were highest in the forest soils and strong correlations were seen with environmental variables such as soil OM content and Cl- concentration. Data presented support the hypothesis that OM levels give the framework for the soil chlorine cycling and that chlorination in more organic soils over time leads to a larger Cl-org pool and in turn to a high internal supply of Cl- upon dechlorination. This provides unexpected indications that pore water Cl- levels may be controlled by supply from dechlorination processes and can explain why soil Cl- locally can be more closely related to soil OM content and the amount organically bound chlorine than to Cl- deposition.

Place, publisher, year, edition, pages
American Chemical Society, 2012
National Category
Natural Sciences
urn:nbn:se:liu:diva-75467 (URN)10.1021/es203191r (DOI)000299864400030 ()

Funding Agencies|Swedish Research Council (VR)|2006-5387|

Available from: 2012-03-02 Created: 2012-03-02 Last updated: 2016-03-08
2. Experimental Evidence of Large Changes in Terrestrial Chlorine Cycling Following Altered Tree Species Composition
Open this publication in new window or tab >>Experimental Evidence of Large Changes in Terrestrial Chlorine Cycling Following Altered Tree Species Composition
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2015 (English)In: Environmental Science and Technology, ISSN 0013-936X, E-ISSN 1520-5851, Vol. 49, no 8, 4921-4928 p.Article in journal (Refereed) Published
Abstract [en]

Organochlorine molecules (Cl-org) are surprisingly abundant in soils and frequently exceed chloride (Cl-) levels. Despite the widespread abundance of Cl-org and the common ability of microorganisms to produce Cl-org, we lack fundamental knowledge about how overall chlorine cycling is regulated in forested ecosystems. Here we present data from a long-term reforestation experiment where native forest was cleared and replaced with five different tree species. Our results show that the abundance and residence times of Cl- and Cl-org after 30 years were highly dependent on which tree species were planted on the nearby plots. Average Cl- and Cl-org content in soil humus were higher, at experimental plots with coniferous trees than in those with deciduous trees. Plots with Norway spruce had the highest net accumulation of Cl- and Cl-org over the experiment period, and showed a 10 and 4 times higher Cl- and Cl-org storage (kg ha(-1)) in the biomass, respectively, and 7 and 9 times higher storage of Cl- and Cl-org in the soil humus layer, compared to plots with oak. The results can explain why local soil chlorine levels are frequently independent of atmospheric deposition, and provide opportunities for improved modeling of chlorine distribution and cycling in terrestrial ecosystems.

Place, publisher, year, edition, pages
American Chemical Society, 2015
National Category
Earth and Related Environmental Sciences
urn:nbn:se:liu:diva-118871 (URN)10.1021/acs.est.5b00137 (DOI)000353610300017 ()25811074 (PubMedID)

Funding Agencies|EDF, France; French national radioactive waste management agency (Andra), France; Linkoping University, Sweden; "Fonds National de la Recherche Scientifique" (FNRS) of Belgium

Available from: 2015-06-05 Created: 2015-06-04 Last updated: 2016-04-01
3. Chlorination and dechlorination rates in a forest soil: A combined modelling and experimental approach
Open this publication in new window or tab >>Chlorination and dechlorination rates in a forest soil: A combined modelling and experimental approach
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2016 (English)In: Science of the Total Environment, ISSN 0048-9697, E-ISSN 1879-1026, Vol. 554-555, 203-210 p.Article in journal (Refereed) Published
Abstract [en]

Abstract Much of the total pool of chlorine (Cl) in soil consists of naturally produced organic chlorine (Clorg). The chlorination of bulk organic matter at substantial rates has been experimentally confirmed in various soil types. The subsequent fates of Clorg are important for ecosystem Cl cycling and residence times. As most previous research into dechlorination in soils has examined either single substances or specific groups of compounds, we lack information about overall bulk dechlorination rates. Here we assessed bulk organic matter chlorination and dechlorination rates in coniferous forest soil based on a radiotracer experiment conducted under various environmental conditions (additional water, labile organic matter, and ammonium nitrate). Experiment results were used to develop a model to estimate specific chlorination (i.e., fraction of Cl− transformed to Clorg per time unit) and specific dechlorination (i.e., fraction of Clorg transformed to Cl− per time unit) rates. The results indicate that chlorination and dechlorination occurred simultaneously under all tested environmental conditions. Specific chlorination rates ranged from 0.0005 to 0.01 d− 1 and were hampered by nitrogen fertilization but were otherwise similar among the treatments. Specific dechlorination rates were 0.01–0.03 d− 1 and were similar among all treatments. This study finds that soil Clorg levels result from a dynamic equilibrium between the chlorination and rapid dechlorination of some Clorg compounds, while another Clorg pool is dechlorinated more slowly. Altogether, this study demonstrates a highly active Cl cycling in soils.

Place, publisher, year, edition, pages
Elsevier, 2016
Chlorine cycling, Chloride, Organic chlorine, Radioactive chlorine-36, Modelling
National Category
Soil Science Environmental Sciences related to Agriculture and Land-use Agricultural Sciences Ecology Forest Science
urn:nbn:se:liu:diva-125912 (URN)10.1016/j.scitotenv.2016.02.208 (DOI)000373274700022 ()26950634 (PubMedID)

Funding agencies:  EDF, France; National Radioactive Waste Management Agency (Andra), France; Linkoping University, Sweden

Available from: 2016-03-08 Created: 2016-03-08 Last updated: 2016-05-02Bibliographically approved

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