1 Introduction and backgroundChlorine (Cl) is a member of the halogen family of elements in group 17 of the periodic table. It is anessential nutrient for all living organisms including humans, animals and plants (Winterton, 2000). Itis rarely found in nature in its elemental form, and typically exists bonded to other elements in theform of chemical compounds (Deborde and Von Gunten, 2008). Its tendency to combine with otherelements and compounds revolutionized the modern world in a way of producing hundreds ofthousands of useful products (Evans, 2005). Chlorine containing compounds are used in manyindustrial processes, such as in the production of paper, plastics, dyes, textiles, medicines,insecticides, fertilizers, solvents, paints and disinfectants (Evans, 2005).
The large-scale use of Cl asdisinfectant in drinking water production not only produces safe drinking water, it also producesdisinfection by-products (DBPs) by reacting with natural organic matter (NOM) dissolved in thesource water (Sobsey et al., 2003). Some chlorinated organic compounds (Clorg) are resistant todegradation and hence persistent (Jones and De Voogt, 1999).Moreover, Clorg are produced naturally in the soils.
Formation and degradation of Clorg affect the Clcycling in terrestrial environments (Montelius et al., 2015). Due to toxic nature of many Cl-containingorganic compounds, (Cantor, 1997; Cemeli et al., 2006; IARC, 1995; IARC, 1999), it is important toexplore the occurrence, formation, distribution and cycling of Cl in both society and nature.The term chlorine (Cl) will be generally used to refer to the collective forms both organic chlorine(Clorg) and inorganic chlorine (Clinorg). Moreover, Chloride (Cl?) will be the same as inorganic chlorineand compounds of organic carbon containing covalently bound chlorine will be referred to generallyas organochlorine compounds (Clorg).
1.1 BackgroundChlorine (Cl) is used as a disinfectant in modern supply of clean drinking water. It is presumed to bemore demanded in future, because of climate changes coupled with the increased temperature andlonger warm seasons. The warm seasons will yield higher amounts of waterborne pathogens (Hunter,2003).Moreover, Cl is an essential micronutrient for plants and participates in several physiologicalprocesses such as osmotic and stomatal regulation, the water-splitting reaction in photosynthesis, anddisease resistance and tolerance (Winterton, 2000). Cl not only improves the yields and quality ofsome crops, excess of Cl is also responsible for salinity stress and toxic to plants (Chen et al.
, 2010).The Cl is so reactive that once it is in the environment, can react with other inorganic material to formchlorides or organic materials to form chlorinated organic compounds (Clorg) (Deborde and VonGunten, 2008). Many Clorg are very stable and highly lipophilic therefore they accumulate in theenvironment and affect humans through the food chain (Henschler, 1994).
Studies have shown that repeated exposure to some Clorg can affect the immune system, the blood, theheart, and the respiratory system of humans and animals (Mandal, 2005). Clorg is so widespread that itwould be difficult to find any human being or plant who does not have detectable levels of Clorg intheir body (WHO, 1997).1.2 Chlorine in drinking water productionDuring the drinking water production from surface water, disinfection is implemented to reduce thewaterborne diseases and improve the quality of water. Usually disinfection is done using chlorinatedcompounds (Sobsey et al., 2003) that can kill harmful pathogens efficiently, but produce undesirablechlorinated organic compounds as a side products known as disinfection by-products (DBPs) byreacting with natural organic matter (NOM), present in the source water. The amount of organicallybound chlorine in water is determined by the sum parameter called adsorbable organic halides (AOX)(Asplund et al.
, 1989). AOX is constituting halogenated chemicals of different structures, from simplechlorinated organic compounds to complex molecules, all possessing different toxicological profiles,from non-toxic to highly toxic, and of which some accumulate in the food chain and pose seriousadverse health and environmental effects (Schowanek et al., 2004). AOX has been reported in sourcewaters (Flodin et al., 1997; Hütteroth et al.
, 2007; Kaczmarczyk and Niemirycz, 2005) as well as inthe drinking water (Peters et al., 1991).1.
2.1 Adsorbable organic halides (AOX) formationThe AOX forming reactions depend on the presence of precursor organic matter, such as humicsubstances, hydrocarbons, chlorophyll, algae, and bacterial metabolites (Adin et al., 1991; Chang etal., 2000; Richardson et al., 1999; Singer, 1994). The doses of oxidants applied in drinking waterproduction do not usually lead to complete oxidation of all organic matter; the formed intermediatesmay constitute AOX or become AOX precursors (Richardson et al., 1999). This precursor complexityleads to a great diversity of AOX compounds.
Two fundamental pathways of AOX formation have been proposed. The first one comprisesenzymatic biosynthesis (Van Pee and Unversucht, 2003). The other pathway is related to chemicaloxidation of organic matter in the presence of reactive halogen compounds (Hua and Reckhow, 2006;Hua and Reckhow, 2007a; Van Pee and Unversucht, 2003). Therefore, during drinking waterproduction, the AOX may be formed as disinfection by-products (DBPs) when various forms ofreactive chlorine is used for disinfection or to prevent growth of pathogens in the distribution system(Ko et al., 2000; Sörensen et al.
, 2001).1.3 Chlorine occurrence and cycling in the environmentCl naturally exists as both inorganic chloride (Cl?) and organic chlorine (Clorg) in soil, vegetation, water as well as in the atmosphere (Öberg, 2002).
Primarily Cl occurs in nature with two stable isotopes,35Cl (ca 76%) and 37Cl (ca 24%). Along with stable isotopes, seven radioactive isotopes of Cl do existof which 36Cl has a very long half-life, 300,000 years. For a longer time, it was widely believed thatall chlorinated organic compounds (Clorg) are anthropogenic, were considered not to participate in biological processes and that chlorine is mainly present in the environment as chloride ions (Cl?) or inorganic chlorine (Clinorg) (Schlesinger, 1997).However, today many studies are documented that chloride participates in a complex biogeochemicalcycle (Asplund and Grimvall, 1991; Van Peè, 2001; Winterton, 2000; Öberg, 1998). These studies reported that large amounts of naturally produced Clorg are present in all environmental compartments.For example, in boreal and temperate soils, 48% to almost 100% of the total Cl has been found asClorg in the upper soil layers (Johansson et al.
, 2003a; Johansson et al., 2003b; Redon et al., 2011).Clorg has also been found in streams, ground water, sea water and surface waters (Grimvall et al.,1991; Manninen and Lauren, 1995; Öberg, 2002). Until recently, more than 5000 natural halogenatedcompounds have been identified so far (Gribble, 2012; Gribble, 2015), of which 2300 are Clorg. Thisindicates the widespread distribution of Clorg in the environment and natural chlorination (transformation of Cl? to Clorg) is considered the main source of Clorg in the environment (Johansson et al.
,2001; Öberg, 2002). Chlorination and de-chlorination processes occur simultaneously in the soils andconsidered the important processes controlling the Cl cycling in the environment (Montelius, 2015).The proposed summary of the natural Cl cycling in the terrestrial ecosystem is presented in Figure 1.Briefly, from the atmosphere, Cl is deposited on land, taken up by the plants and is returned to the soilvia through-fall, stemflow and litter fall. Chlorination (transformation of Cl? to Clorg) anddechlorination (transformation of Clorg to Cl?) processes occur in different soil layers, (For details seeFigure 1).Figure 1: The natural chlorine cycle, showing flowsand transformation processes of chloride (Cl?) andorganic chlorine (Clorg) in a terrestrial ecosystem.
1.Wet and dry deposition of Cl? and Clorg 2. Cl? andClorg from vegetation (e.g. throughfall andstemflow). 3. Uptake of Cl? and Clorg from soil byplant roots.
4. Volatilization of Clorg from the soil tothe atmosphere. 5.
Volatilization of Clorg from plantsto the atmosphere. 6. Transformation of Cl? to Clorg(chlorination) in the humus layer and in the oppositedirection from Clorg to Cl? (dechlorination). 7.Chlorination and dechlorination processes in themineral soil. 8.
Leaching of Cl? and Clorg from thehumus layer to the mineral soil. 9. Weathering ofCl? from the bedrock dispersed to the mineral soiland the humus layer.
So far the two ultimate sources of Cl to soil-plant systems are considered (1) deposition of atmospheric sea spray aerosols (Graedel and Keene, 1996) or (2) weathering from some bedrocks, such ashornblende and apatite (Peters, 1984; Lovett et al., 2005; Mullaney et al., 2009).
However, the atmospheric deposition accounts for the largest source of Cl addition (Graedel and Keene, 1996) and it consists of wet and dry deposition of atmospheric aerosols. The influence of this source is strongly dependent on the distance to the sea and the amount of precipitation (Öberg 1998). In general, Cl deposition is higher in coastal areas than inland areas (Silva et al., 2007; Clarke et al., 2009).
Moreover,atmospheric deposition contains not only Cl?, also Clorg have been found in deposition (Enell &Wennberg, 1991; Grimvall et al., 1991; Laniewski et al., 1995) on the order of 0.07 kg ha–1 y–1(Svensson et al., 2007a).In the regions with low rainfall rate, Cl? expected to be accumulated in the soil due to high evaporation/rainfall ratio. In contrast, in the tropical regions, high rainfall often results in thorough leaching ofCl? from soils and may cause deficiency of Cl? for plants.
Luckily, the internal requirement of Cl? formost plants is small. For example, the Cl requirement for optimal plant growth is in the range 0.2?0.4g kg?1 dry matter, but the average contents of Cl in plants were found in the range 2?20 g kg?1 drymatter (Marschner and Rimmington, 1988), which is about 10-50 times more than the prerequisite.
Cl seems to be in excess of biochemical needs in most locations, for example (Redon et al., 2011)found total chlorine (TX) in forest soils from France in the range (0.45 ?1.0 g kg?1). Therefore, theavailable Cl for the plants could be more than they need via either selective uptake (active) or wateruptake (passive). From the plants, Cl returns to soils, where extensive chlorination of soil organicmatter takes place (Gustavsson et al.
, 2012) and the processes retaining Cl in the soil resulting in accumulation, large soil chlorine storage, allowing extensive Cl recycling by plant transport. The soilsfor which this have been observed are northern soils rich in organic matter (Lovett et al. 2005; Johansson et al. 2003; Bastviken et al. 2006; Clarke et al. 2009; Svensson et al. 2012).
Moreover, the soil organic matter (SOM) levels have also been positively correlated with chlorination and soil Cl storage(Gustavsson et al., 2012).On the other hand many highly productive environments are found far from the coast (McGroddy andSilver, 2011) with little mineral Cl in soils, e.g., with little input from the ultimate Cl sources, and alsolower OM in the soils (Batjes, 2002; De Moraes et al., 1996) which could assume low potential for Claccumulation in the soils, examples of such environments are inland tropical rainforests.
In spite of the very different settings from the Northern ecosystems with high soil organic matter(SOM), the few available data from tropical inland forests in Amazon indicate extensive plant uptakeand release as throughfall (Cornu et al., 1998), i.e., similar extensive Cl cycling as in the Northern forests, but the nature of the Cl cycling and especially the potential for Cl storage in such soils are unclear.
1.3.1 Elimination/degradation of chlorinated compounds in the environmentOccurrence of chlorinated compounds in the environment and the possible toxicity associatedrequired their control and elimination where necessary. Different strategies have been used toeliminate or degrade Clorg in relation with soil and water contamination (Van Pee and Unversucht,2003). There is no single remedy available to degrade or eliminate AOX completely but specificcompounds have been targeted by means of physical, chemical or biochemical processes. Forexample, some specific halogenated organic compounds were decomposed by photochemicalreactions (Dwivedi and Pande, 2012), some by biological treatment (Bhatt et al., 2007) and few byother advanced oxidation processes (AOPs) (Munter, 2001).
1.4 Objective and aim of the thesisThe overall objective of this thesis is to enhance knowledge about chlorine and the role ofchlorination in various systems both in society and nature. Critical knowledge gaps studied includes(i) the consequences of chlorination processes in drinking water (ii) there is a great need for simpleand cost effective ways to characterize the maximum number of chlorinated organic compounds indrinking water, and for this purpose, we developed a new methodology using gas chromatographcoupled with halogen specific detector and (iii) Cl cycling and potential storage in pristine systemsthat is far from the fundamental Cl sources.2 Methodology2.1 Description about drinking waterworks involvedTo determine the sum of organically bound chlorine and individual chlorinated organic compounds inwater, four Swedish waterworks were studied: Berggården in Linköping, Borg´s in Norrköping,Görvälnverket in Stockholm, and Bulltofta in Malmö.
These waterworks were chosen based ondifferent source water characteristics and treatment methodologies. The sources of water, treatmentprocesses and sampling points at each drinking water work are listed in (Figure 2). Berggården,Görvälnverket and Bulltofta use UV irradiation as primary disinfectant and hypochlorite at Borg´s(listed as 1 in disinfection box in Figure 2). Moreover, hypochlorite as secondary disinfectant which isused in the distribution system to prevent regrowth of microorganisms added at Berggården andchloramine on all other waterworks (listed as 2 in disinfection box in Figure 2)