Economics of equal masses of NaOH and Cl 2 being produced

Transcription

1 Chlorine and its Compounds Issue #1: the production of 2 : the chloralkali process 2Na + 2H 2 O > 2NaOH H 2 Economics of equal masses of NaOH and 2 being produced Electrolysis: Anode reaction! (aq) > ½ 2 + e! at a graphite anode (old style cells) or Ti/IrO 2 (newer cells) Cathode reaction: old style mercury cell Na + (aq) + e! > Na/Hg Na/Hg + H 2 O > NaOH + ½ H 2 separate reaction

2 Advantage of the old style cell: H 2 and 2 formed in separate steps Disadvantage: cost of Hg and losses of Hg to the environment > high concentrations of Hg in fish > phase-out of this technology Newer technology: diaphragm or membrane separated cells: use of Nafion as a cation exchange membrane (cations can pass through, but neutrals and anions are repelled) Anode reaction! (aq) > ½ 2 (g) + e! Cathode reaction H + (aq) + e! > ½H 2 (g) NaOH forms because Na + ions cross the ion exchange membrane to restore charge balance due to loss of H + the membrane separates the H 2 (g) and 2 (g) streams no mercury is used

3 Issue #2: Lipophilicity of chlorinated organics Lipophilic compounds are fat-loving and hydrophobic ( waterhating ). Organochlorine compounds are notorious for high lipophilicity. Some lipophilic pesticides O NO 2 Chlordane (insecticide) Chlornitrofen (herbicide) Partition Constant: K part = concentration of solute in solvent 1 concentration of solute in solvent 2 Solvent extraction, organic solvent = solvent 1; water = solvent 2 Octanol as Solvent 1: the octanol-water partition coefficient, K ow Octanol as a model for fat. K ow = concentration of solute in octanol concentration of solute in water

4 Relationship between K ow and bioconcentration factor, BCF BCF = concentration of toxicant in aquatic organism concentration of toxicant in surrounding water BCF = K part % by weight of fat in the organism If octanol is a good model for fat: BCF = K ow % by weight of fat in the organism If fat is ~ 5% of wet weight: BCF ~ 0.05 K ow Environment Canada: K ow > 1000, likely to bioconcentrate Biomagnification: DDT in the Great Lakes back in the 1970s Source [DDT] (ppb) Water (L. Ontario) 0.03 Sediment 27 Plankton 400 Salmon tissue 8,000 Gull tissue CHC 3

5 PCBs in L. Ontario salmon (improvement due to phaseout) conc, ppm year Is biomagnification a real phenomenon? G.E. Leblanc, Environ. Sci. Technol. 1995, 29, 154: higher trophic organisms tend to have higher fat content > possible reason for higher organochlorine concentrations phytoplankton 0.5 % lipid invertebrates 2 ± 1 fish 5 ± 2

6 Kinetics of uptake and depuration Example: 1. c(aq) v c(fish) k 1 2. c(fish) v c(aq) k 2 3. c(fish) v metabolic products k 3 depuration is processes 2+3 rate[1] rate[2] rate[3] = k 1.c(aq) = k 2.c(fish) = k 3.c(fish) (1) dc(fish)/dt = rate[1]! rate[2]! rate[3] = k 1.c(aq) x (k 2 + k 3 ).c(fish) At the steady state: (2) k 1.c(aq) = (k 2 + k 3 ).c(fish, ss) BCF = c(fish, ss)/c(aq) = k 1 /(k 2 + k 3 ) if the steady state has not been reached, eq. (1) must be integrated (see text: will not be tested). then measured BCF does not necessarily represent equilibrium (e.g. organochlorines in Great Lakes fish) bioconcentration is always associated with a low rate of depuration: rate[1] >> {rate[2] + rate[3]}

7 Issue #3: Chlorinated solvents: air and water pollution Chlorinated alkanes: CH 2 2 ; CH 3 ; C 4 ; CH 2 CH 2 ; CH 3 C 3 (MC) Chlorinated alkenes: 2 C=CH (TCE); 2 C=C 2 (PCE or PERC) (Italics = Montreal Protocol substance) Major uses of MC, TCE, PER as degreasing solvents Atmospheric concern is stratospheric ozone depletion: S k OH 10!13 cm 3 molec!1 s!1 or less for the alkanes S k OH 10!11 cm 3 molec!1 s!1 or less for the alkenes Recall that banning MC has already led to a decrease in atmospheric concentration Terrestrial concern is ground water contamination especially by TCE and PER (~70% of all contaminated sites): refer back to Ch. 8: permeable reactive barriers Concern of drinking water contamination, especially by TCE, which is a rodent carcinogen For an industry perspective on TCE (1995), see: For a perspective from the US National Institutes of Environmental Health Sciences (1996), see:

8 Issue #4: Long range transport of organochlorines: Persistent organic pollutants (POPs) Environment Canada's Toxic Substances Management Policy: persistent if t ½ in the atmosphere > 2 days. Ancillary criterion: "evidence of atmospheric transport to remote regions such as the arctic" low chemical and metabolic reactivity often associated with halogenated compounds able to distribute globally if even slightly volatile accumulate in the polar regions if they have low vapour pressure: global distillation Canadian concern: Native Canadians and wildlife in the Arctic.

9 atmospheric persistence low atmospheric reactivity long range transport low volatility high volatility polar deposition global distribution Example DDT CFCs Properties of POPs as a function of vapour pressure Vapour pressure, Pa log p < -4 Mobility very low Properties deposition or retention close to the source log p -4 to -2 low accumulation in mid-latitudes log p -2 to 0 high accumulation in polar regions log p > 0 very high Source: Wania and Mackay, 1996 no deposition Examples PCBs > 8 PAH > 4 rings PCBs ring PAH PCBs ring PAH HCH; DDT chloroaliphatic solvents; CFCs 1-2 ring aromatics

10 Polybrominated diphenyl ethers (PBDEs): an emerging environmental problem widely used as flame retardants flame retardants are used in: S soft furnishings and bedding S childrens sleepware S automobiles and aircraft S TV sets and computer housings current production > 67,000 tonnes per year; toxicity largely unknown long range transport found to bioconcentrate in: S human milk S seal and whale blubber Br Br O O Br n Brn Br Br Left: generic PBDE structure Right: a major component of the penta BDE mixture As of May 2001, the European Union agreed to phase out the use of polybrominated diphenyl ethers as flame retardants because of their increasing environmental contamination

11 Issue #5: Toxic organochlorine aromatic compounds Emphasis on polychlorinated biphenyls (PCBs) and polychlorinated dibenzo-p-dioxins (PCDDs or dioxins ) DDT (structure earlier) a 1940s/1950s insecticide effective against malaria mosquitos; cheap, readily applied synthesis = Friedel-Crafts alkylation of chlorobenzene (2 mol) with chloral hydrate 3 C-CH=O ( 3 C-CH(OH) 2 ) discovered to be persistent in the terrestrial environment, 1960s, and to be ubiquitous in biota, 1960s an emphasis in Rachel Carson s book Silent Spring (reduced fecundity of birds): biomagnification banned in developed countries ; still in use in India for mosquito control: disease control vs environmental impact residues still found in Canadian biota, despite declines; reason is long range atmospheric transport technical product is mainly p,p'-ddt, but also contains o,p'- and o,o'- isomers: o,p'-ddt considered to be weakly estrogenic