Apa
ti
te dissolu
tion experimen
ts were conduc
ted using bo
th a fluidized bed
and s
tirred
tank reac
tor over a range of pH,
tempera
ture, solu
tion sa
tura
tion s
ta
te,
and on non-carbona
ted
and carbona
ted apa
ti
te composi
tions: igneous fluorapa
ti
te (FAP)
and sedimen
tary carbona
te fluorapa
ti
te (CFA), respec
tively. From 2 &l
t;pH &l
t;6,
the ra
te of release from dissolu
tion of all apa
ti
te componen
ts [calcium (Ca), phosphorus (P),
and fluoride (F)] increased wi
th decreasing pH for FAP. From 6 &l
t; pH &l
t; 8.5,
the FAP dissolu
tion ra
te is pH independen
t. Measuring apa
ti
te dissolu
tion ra
tes a
t pH &g
t; 8.5 were no
t possible due
to de
tec
tion limi
ts of
the analy
tical
techniques used in
this s
tudy
and the high insolubili
ty of FAP. For
the CFA composi
tions s
tudied,
the dissolu
tion ra
te decreased wi
th increasing pH from 4 &l
t; pH &l
t; 7. During early s
tages of
the dissolu
tion reac
tion for bo
th FAP
and CFA, mineral componen
ts were released in non-s
toichiome
tric ra
tios wi
th reac
ted solu
tion ra
tios of dissolved Ca:P
and Ca:F being grea
ter
than mineral s
toichiome
tric ra
tios, sugges
ting
tha
t Ca was preferen
tially released compared
to P
and F from
the mineral s
truc
ture during
the early s
tages of dissolu
tion. An increase in reac
ted solu
tion pH accompanies
this early eleva
ted release of Ca. As
the dissolu
tion reac
tion proceeded
to s
teady s
ta
te, dissolu
tion became congruen
t. When normalized
to BET measured surface area, FAP dissolved fas
ter from 4 &l
t; pH &l
t; 7 compared
to CFA. The apparen
t Arrhenius ac
tiva
tion energy (E
a) of FAP dissolu
tion over
the
tempera
ture range of 25–55°C a
t pH = 3.0, I = 0.1,
and pCO
2 = 0 is 8.3 ± 0.2 kcal mol
−1. Bo
th
the apparen
t exchange of solu
tion H
+ for solid-bound Ca a
t low pH in
the early s
tage of dissolu
tion
and the E
a of dissolu
tion sugges
t a surface
and no
t a diffusion con
trolled dissolu
tion reac
tion for FAP
and CFA. The degree of undersa
tura
tion of
the solu
tion, ΔG
R, wi
th respec
t to FAP was impor
tan
t in de
termining
the dissolu
tion ra
te. A
t pH = 3.0, I = 0.1,
and pCO
2 = 0,
the dissolu
tion ra
te of FAP was
ter border=0 SRC=/images/glyphs/BQ1.GIF> 5&
times; grea
ter in
the far-from-equilibrium region compared
to
the near-equilibrium slope region.