DISSOLVED INORGANIC CARBON DIC

Transcription

1 1 Chpter 2 Geochemicl Rections DISSOLVED INORGANIC CARBON Inorgnic or minerlized crbon in its most oxidized stte crries vlence of 4. In minerls C 4 coordintes covlently with oxygen toms s the crbonte nion [CO 2 ] which forms lrgely ionic bond with ctions like C 2. Coordinted with two oxygen toms, it forms the stble liner CO 2 gs molecule. Dissolved in wter s queous crbon dioxide [CO 2(q) ] this redily hydrtes to form crbonic cid [ 2 CO ] nd its two dissocition products bicrbonte [ ], nd crbonte [CO 2 ]. Collectively these four species comprise dissolved inorgnic crbon : m m CO2 m 2CO m m CO 2 cn originte from vriety of rections such s: Dissolution of CO 2 from the tmosphere or from the soil: CO 2(g) CO 2(q) Dissolution of clcite: CCO C 2 Oxidtion of orgnics in wter: C 2 O O 2 2 CO Metmorphic rections: CCO SiO 2 CO 2 CSiO wollstonite is mesured with filtered smple by one of two methods. The smple cn be cidified to convert ll species to CO 2(q) which is then seprted by flushing with n inert gs or under vcuum nd mesured. The concentrtions of the individul species cn then be determined from p. Alterntively, the ionized components of nd CO 2, cn be mesured by n lklinity titrtion (discussed below) nd then used with p to clculte the remining species. Individully, species re importnt contributors the cidity nd buffering cpcity of wter, s well s to the solubilities of crbonte minerls nd complex ions. Their reltive concentrtions re governed by p. p nd the distribution of species For most nturl wters, it is the production of CO 2 from the degrdtion of plnt debris in soils tht contributes much of the. In unsturted nd erobic soils, CO 2 ccumultes in the gs phse, sustining high CO 2 prtil pressures nd high in soil wter. The sequence of rections begins with the dissolution nd hydrtion of CO 2 in wter to produce crbonic cid: CO 2(g) 2 O CO 2(q) 2 O 2 CO Although unhydrted CO 2(q) is n importnt species in solution, the hydrtion step is usully included in the overll rection for CO 2 dissolution. By convention, ll dissolved CO 2, whether hydrted or not, is tken to be in the form 2 CO. Crbonic cid is wek cid, nd redily dissocites to form bicrbionte, which gin dissocites to form crbonte ccording to this sequence of rections: CO 2(g) 2 O 2 CO 2 CO CO 2 The concentrtion of in groundwter minly responds to gs phse control on the uptke of CO 2 from soils or the tmosphere nd minerlogicl control through rock wethering nd precipittion of secondry crbonte minerls. The reltive distribution of species is controlled by p nd the crbonic

2 Environmentl Geochemistry nd Isotopes 2 cid dissocition rections. Vlues for the rection constnts re given in Tble Error! No text of specified style in document.-1, long with their temperture equtions tht re useful for spredsheet clcultions nd modeling. CO 2 dissolution nd hydrtion: CO 2(g) 2 O 2 CO 1 st dissocition of crbonic cid: 2 CO 2 nd dissocition of crbonic cid: 2 CO Precipittion of clcite CCO C 2 2 CO 2CO 1.47 CO 2 PCO CO 2 CO 6.5. CCO C 2 CO Tble Error! No text of specified style in document.-1 Equilibrium constnts for the crbonte system nd temperture equtions (T in C) T C log CO2 log 1 log 2 log CCO Equtions: log CO T T 1.11 T in C log T T 6.58 log T T.62 log CCO 6-5 T T 8.8 From the equtions shown bove, we see tht concentrtions nd distribution of is species is controlled by P CO2 t one end, by crbonte minerl dissolution t the other end, nd by p (i.e. ) throughout (Fig. Error! No text of specified style in document.-1). Shifts in p cuse shift in the reltive concentrtions of these species. A decrese in p leds to greter crbonic cid concentrtion s bicrbonte is converted to crbonic cid [ 2 CO ]. The incresed crbonic cid frction increses the CO 2 prtil pressure, P CO2, of the solution. Below the wter tble, this is n pprent P CO2 (i.e. the CO 2 prtil pressure tht would be exerted if seprte gs phse existed with the wter), wheres t the wter tble or lke surfce CO 2 would equilibrte with the tmosphere [CO 2(q) CO 2(g) ]. An increse in p hs the opposite effect, shifting the reltive distribution of species into the CO 2 field which fvors precipittion of crbonte minerls [CO 2 C 2 CCO ].

3 Chpter 2 Geochemicl Rections Percent of totl L CO CO p Fig. Error! No text of specified style in document.-1 Reltive distribution of species in pure wter s function of p, t 25 C. Fig. Error! No text of specified style in document.-1 shows tht over the p rnge of most nturl surfce nd groundwters (6.5 to 8.5), is the min species, with minor crbonic cid, s CO 2(q) or 2 CO. Under cid conditions below p 6.4, crbonic cid becomes the dominnt species. CO 2 is present, but t minor to trce concentrtions. Only in highly lkline wters with p greter thn. does it become the mjor species. The reltive concentrtions of ll species cn be clculted from p. The clcultions cn be simplified by considering tht is dominted by one mjor nd one minor crbonte species t ny given p: 2 CO plus below p 8.4, nd plus CO 2 bove p 8.4 (Fig. Error! No text of specified style in document.-1). The following equtions llow clcultion of the reltive concentrtions or frction, f, of these species in either p rnge. Note tht lthough the third component (e.g. CO 2 t low p or crbonic cid t high p) is insignificnt in such mss blnce clcultion, it is importnt in thermodynmic clcultions. CO 2 2 my contribute insignificntly to the totl t neutrl p, but it will hve n ctivity tht cn be redily clculted from the concentrtion of, nd which will ply vitl role in the dissolution nd precipittion of crbonte minerls. Thus, lthough is lmost entirely composed of t neutrl p, it is importnt to clculte the concentrtions of the minor species, 2 CO nd CO 2, for their role in CO 2 degssing nd minerl precipittion rections. The follow derives generic formule for clculting the concentrtions of species from mesured vlue. Note tht these re bsed on the ctivity reltionships for the species, nd therefore include ctivity coefficients to determine species concentrtions. Clculting concentrtions of species from mesured (mol/kg): Acidic below p 8.4 Alkline bove p 8.4 m m 2CO m f f f CO 2 1 m m 2 CO m f CO 2 m m m 2 m 2CO Rerrnging the equtions for the first ( 1 ) nd second ( 2 ) dissocition constnts for species, nd using ctivity coefficients to ccount for ctivity-concentrtion differences from the bove equtions: 2 CO 1 2 2CO m (m m ) (note tht γ 2CO 1) CO (m m 2 CO m 2 CO 2 CO )

4 Environmentl Geochemistry nd Isotopes 4 1 m m γ 1 m 2 m γ m CO 2 γ CO 2 2 m CO 2 γ m 1 1 m CO CO 2 Exmple Error! No text of specified style in document.-1 Clculting 2CO, nd CO 2 from mesured nd p Wht re the concentrtions nd ctivities of 2 CO, nd CO 2 in low ionic strength wter (I 0.01) t 25 C with p of 6.85 nd mesured concentrtion of 64 ppm C? 64 ppm C 5. mol C/kg m 2CO m m CO 2 Considering tht the p is below 8.4, we cn simplify the problem becuse t p 6.85 CO 2 mkes no pprecible contribution to the concentrtion (Fig. Error! No text of specified style in document.-1) nd its ctivity cn be clculted from. This llows the clcultion of the concentrtions of 2 CO nd from, which is mesurement of concentrtion (not ctivity). m m 2CO m mol/kg From the reltionships developed bove, the contributions of bicrbonte nd crbonic cid re estblished from the mesured concentrtion of, using the ctivity coefficients clculted from the Debye-ückel eqution: log γ 0.5 z γ 0.89 m mol/kg 250 ppm m 2CO m m mol/kg 74.4 ppm Thus, the concentrtion of 2 CO is over 20% of the pool. For clcultions of clcite sturtion nd CO 2 degssing, the required ctivities of these species re then determined from the concentrtions of these mjor species: m γ CO 2 2 / / CO m 2CO γ 2CO P CO2 2CO / CO / or 0.04 tmospheres Often, geochemicl nlysis will include mesure of bicrbonte,, rther thn. This is mesured by n lklinity titrtion (below). Clculting the distribution of the remining species is simply mtter of substitution into the pproprite equtions for 1 nd 2. Exmple Error! No text of specified style in document.-2 P CO 2, 2CO nd CO2 from bicrbonte nd p Wht re the ctivities nd concentrtions of 2 CO nd CO 2 nd P CO2 in low ionic strength wter (I 0.01) t 25 C with p of 7.00 nd concentrtion of 122 ppm?

5 5 Chpter 2 Geochemicl Rections ppm m gfw from bove, log γ 0.5 z γ 0.89 nd γ CO m γ p 7.00 for CO 2 : 2 CO 2 2 CO m CO 2 CO 2 / γ CO / mol/kg for 2 CO : CO CO m 2CO 2CO / γ 2CO / mol/kg 2CO For P CO2 : CO P P CO2 CO Thus, for this wter, the concentrtion of CO 2 is n insignificnt component of the totl reservoir. The concentrtion of 2 CO, by contrst is lmost 20% of the pool, nd thus the concentrtion cn be clculted s: m 2CO m m CO mol/kg 28.8 ppm C Further, the P CO2 is greter thn tmospheric (P CO2(tm).45 ) nd so this wter will degs CO 2 to the tmosphere upon contct.