Emprendedores/as: agentes de cambio

Chlorine and bromine are both members of Group 17 of the periodic table (i.e., the halogens) along with fluorine and iodine, which are also found in aqueous environments. The halogens are characterised by their very high electronegativity, which is due to their outer electron shell being only one electron short of a noble gas configuration. Chlorine has an atomic number of 17, an atomic mass of ~35.5, one main and six minor oxidation states (–1 and 0, +1, +3, +4, +5, +7, respectively) [Haynes, 2015], an estimated upper continental crust concentration of ~370 μg g–1 [Rudnick and

Gao, 2014], an estimated oceanic crust concentration of 20 – 2,800 μg g–1

[Jambon et

al., 1995], and a mean seawater abundance of 19,400 μg g–1 [Millero, 2014]. Bromine, by contrast, has an atomic number of 35, an atomic mass of ~79.9, five main oxidation states (–1, +1, +3, +5, +7) [Haynes, 2015], an estimated upper continental crust concentration of ~1.6 μg g–1 [Rudnick and Gao, 2014], an estimated oceanic crust concentration of 0.1 – 1.3 μg g–1

[Jambon et al., 1995], and a mean seawater abundance of 67 μg g–1

[Millero, 2014]. Chlorine and bromine are enriched in Earth’s hydrosphere relative to their crustal abundance (by factors of up to 970 and 670, respectively) because they are volatile and prefer to exist predominantly as highly soluble halides [Graedel and Keene, 1996].

1.1. Chloride/bromide ratios

The key geochemical differences of Cl–

and Br–

can provide researchers with vital qualitative and quantitative information relating to geochemical reactions in catchments and basins. Consequently, the ratio of dissolved Cl– and Br–, as a molar or mass ratio (from herein mass ratios are adopted; [Cl–

/Br–

]molar = 2.254´[Cl –

/Br–

]mass), has become a

widely quantitative and semi-quantitative parameter in the field of hydrogeochemistry [e.g., Alcalá and Custodio, 2008b; Cartwright et al., 2006; Davis et al., 2004; Davis et

al., 1998; Edmunds, 1996; Herczeg and Edmunds, 1999] and sedimentary (evaporite)

geochemistry [e.g., Cendón et al., 2004; Holser, 1966; Holser, 1970; McCaffrey et al., 1987; Taberner et al., 2000; Valyashko, 1956]. In the context of saline environments, Cl–/Br– ratios have utility for identifying evaporite dissolution/precipitation, quantifying salt additions/losses and delineating salt sources.

Seawater, the primary reservoir for Cl– and Br–, has a relatively uniform Cl–/Br– ratio of approximately 288 [Millero, 2014], and a similar ratio is typically observed in coastal rainfall. However, chemical fractionation at the ocean surface during marine aerosol development causes oceanic aerosols of variable sizes to have Cl–

/Br–

ratios deviating from the mean ocean water ratio, and studies have found that Br–

tends to be enriched in the smaller aerosols [Zhou et al., 1990]. A possible explanation for this is the observation that halide ions have a tendency to cluster near the air-water interface and that this tendency increases with an ion’s polarisability [Jungwirth and Tobias, 2002]. The approximately 30% higher polarisability of Br–

than that of Cl–

means that Br– is likely to be found in greater proportions in smaller oceanic aerosol particles because of their greater surface area to volume ratio [Davis et al., 2004]. As a result, it is hypothesised that the Cl–

/Br–

ratio of atmospheric aerosols will decrease with distance inland and with increasing elevation because of washout effects and the ability of

Tracing terrestrial salt cycling using chlorine and bromine M. A. Short (2017)

17 smaller aerosols to be transported over greater distances and elevations [Davis et al., 2004]. The few available rainwater and groundwater data from continental and countrywide studies tend to find that inland Cl–/Br– ratios are lower than that of seawater, which supports this hypothesis [e.g., Alcalá and Custodio, 2008b; Crosbie et

al., 2012; Davis et al., 2004; Davis et al., 1998].

Halite (NaCl) has a high Cl–

/Br–

ratio because of the exclusion of all but trace quantities of Br– from the crystal lattice [Holser, 1970] and the much higher solubility of Br– [Davis et al., 2004; Davis et al., 1998]. Bulk halite has been observed to have a Cl–

/Br–

ratio range of approximately 2 000 – 10 000 [Davis et al., 2004; Davis et al., 1998], and individual crystals can be found with ratios as high as 100 000 [Davis et al., 1998]. It is only once potassium begins to replace sodium in the crystal lattice of evaporites during brine evaporation (i.e., formation of sylvite and carnallite), that bromine is readily incorporated and Cl–

/Br–

ratios of precipitates begin to fall to 400 [Alcalá and Custodio, 2008b; Taberner et al., 2000]. During the precipitation of evaporite minerals, bromine is enriched in the residual brine, and Cl–/Br– ratios have been found to fall as low as 30 – 80 [Birkle et al., 2009; McCaffrey et al., 1987;

Taberner et al., 2000]. These chemical fractionation characteristics of Cl– and Br– make Cl–

/Br–

ratios an excellent parameter to utilise in environments where evaporite dissolution/precipitation and subsequent brine evolution are expected, making this technique especially attractive in salt lake environments, of either marine or non-marine origin [Eugster and Jones, 1979; Holser, 1970; Macumber, 1991].

Anthropogenic pollution sources have also been found to contain varying Cl–

/Br–

ratios [e.g., Cendón et al., 2015; Davis et al., 2004; Davis et al., 1998; Sollars et al., 1982; Vengosh and Pankratov, 1998]. Sewage effluent typically has an elevated Cl–/Br– ratio (approximately 400 – 900) because of the widespread domestic use of table salt

[Vengosh and Pankratov, 1998]. In contrast, urban runoff can have a wide range of Cl–

/Br–

ratios. In the United Kingdom, Sollars et al. [1982] found that during winter months, urban runoff had a high mean Cl–/Br– ratio (approximately 1200) because of the use of de-icing salt on roadways. However, during the summer months, runoff had a lower mean Cl–/Br– ratio of approximately 20, which was interpreted to be caused by the dissolution of Br–

containing additives in dust expelled from the exhausts of cars fuelled by leaded petrol. The effect of car exhaust aerosols on the Cl–/Br– ratio of urban runoff is likely to have diminished in the decades since the phasing-out of leaded petrol in the 1980s, like reductions in lead pollution on roadways [e.g., MacKinnon et al., 2011; Wang et al., 2006]. However, road de-icing salts are still widely used in countries that experience heavy snow, and Cl–

/Br–

ratios are commonly used to identify where the quality of surface water and groundwater may be impacted by this activity [e.g., Dailey

et al., 2014]. In agricultural soils and water, a potentially large source of anthropogenic

Br can be introduced by Br-containing fertiliser and pesticide, the degradation of which releases Br– into the environment [Flury and Papritz, 1993].