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Acid Pollution of Natural Waters
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Outline of Topics
• Background
– Acids and bases
– pH ranges in the hydrosphere
– Biological effects of acid pollution
• Sources of Acid Pollution
– Acid deposition
– Acid ine drainage
• !esisting Acidification
– pH buffers
– "issol#ed inorganic carbon in natural $aters
– %easuring buffering capacity
– Hoogeneous and heterogeneous buffering
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Acids and Bases
• &ecture 'uestions
– What is an acid( What is a base(
– What is pH(
– )*plain the difference bet$een strong and weak acids+bases,
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pH !anges in the Hydrosphere
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pH of Natural Waters- &akes
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Sources of Acid Pollution
• Questions
– What pollutants cause acidification of natural $aters. and ho$(
– What huan acti#ities generate these pollutants(
• SO/ and NO* eissions
– 0enerated by fossil fuel cobustion 1SO/ especially by coal burning2
– Atospheric o*idation of SO
/
leads to sulfuric acid and sulfate aerosol – Atospheric o*idation of NO leads to nitric acid and nitrate aerosol
– Acidification occurs during $et and dry deposition and due to spring
sno$elt
• Acid ine drainage
– %ining operations generate large 3uantities of ine $aste
– Products of the a3ueous o*idation of e*posed inerals 1ostly 4eS/2 in
the $aste leads to acidification of ground and surface $ater
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)ffects of Acidification on A3uatic )cosystes
• Acute effects – 5nterferes $ith osoregulation – %obili6ation of to*ic etals 1Al. %g. 7n2
• 8hronic effects- reproduction probles – &o$ers calciu le#els in feale fish. hinders ability to produce eggs – 4ertili6ed fish eggs ay de#elop abnorally
– 4rog and salaander eggs can be greatly affected by spring sno$elt
• A3uatic ecosyste effects – 8an result in decline and algal species di#ersity and bioass – %ay effect ecosyste producti#ity – %ay decrease decoposer populations. affecting nutrient cycling and
increasing "O%+PO% le#els
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)ffects of Acid Pollution on Terrestrial )cosystes
• "irect )ffects
– "aage to foliage
• )ffects on Soil
– The nature of soil
– Soil pH
– &oss of nutrients and other etals
– &oss of ability to retain nutrients
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Acid %ine "rainage
• Picture to the left sho$s acid ine
drainage in tar creek. O9 1at a
superfund site2• %ining operations e*pose inerals and
ine $aste to air and $ater
• Acid ine drainage is ostly due to
the biologically-mediated oxidation of
pyrite. 4eS/
• 8olor is due to precipitation of o*idi6ed
iron 1eg as 4e1OH2:2
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%echanis of Pyrite O*idation• Overall process
)(SOH2)(Fe(OH))(O4
15)(OH2
7)(FeS 423222 aq s g l s +→++
++−+−++→++ 16H14Fe2SOOH814FeS
22
42
32
2
O7H14Fe14HO
2
714Fe 2
3
2
2+→++
+++
3
2 3Fe 3H O Fe(OH) (s) 3H+ +
+ → +
• "irect o*idation of S//; by O/ is too slo$< instead it is o*idi6ed by soething else. usually 4e:= that is present naturally in the en#ironent-
• The 4e:= is regenerated by reaction $ith dissol#ed o*ygen
– The Fe3+ is a catalyst for the generation of acid ine drainage
– This is the rate deterining step of the entire process< it is an e*otheric reaction
• %ost acidity in the ine drainage is due to o*idation of disulfide anion to sulfuric acid. but soe
is due to hydrolysis by 4e555. for e*aple-
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5ron 8ycling 8ontrols !ate of Acid 0eneration
• O*idation of disulfide is cataly6ed by 4e:=- regeneration of catalyst is necessary
• )ach cycle generates / protons of acidity• Abiotic o*idation of 4e/= by O/ is fairly slo$ 1τ > ?@@@d2
• 4e/= o*idation is biologically ediated by bacteria $ho deri#e energy fro the
reaction< they are acidophiles $ho thri#e in lo$ pH en#ironents, !eaction is
?@ ties faster $hen they are present,
3+14 Fe 2+14 Fe
2-
2S
+14 H
+16 H
Thiobacillus Ferrooxidans
o*idation of disulfide
produces acidity
regeneration of 4e:=
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8arbonate 8heistry in Natural Waters• Why is calciu bicarbonate. 8a1H8O:2/1a32. the doinant solute in ost fresh$aters
1so;called calcareousC $aters2(
– 5agine a $ater body 1lake. ri#er. etc2 in e3uilibriu $ith both calcite 1the ost coon
carbonate ineral2 and the atosphere,
2+
32 3 2 ( )CO ( ) CaCO ( ) H O( ) Ca ( ) 2HCO aq g s l aq −+ + + ƒ
2H O
2 2CO ( ) CO ( ) g aqˆ ˆ ̂ †‡ ˆ ̂ ˆ 2H O 2+ 2-
3 3CaCO ( ) Ca CO s +ˆ ˆ ̂ †‡ ˆ ˆ ˆ
2 2 2 3CO ( ) H O H CO ( )aq aq+ →2- -
2 3 3 3H CO ( ) CO ( ) 2HCO ( )aq aq aq+ →
"issolution Processes
Hydration of 8O/ Acid;Base !eaction
Water in e3uilibriu $ith atospheric 8O/ and 8a8O:- pH > D,:.
EH8O:;
F > ?%. E8a/=
F > @,G% 1at /Go
82,
O#erall process- heterogeneous reaction bet$een an acid and a base
"issolution+e#olution of
gaseous carbon dio*ide
"issolution+precipitation of calcite
1or another carbonate ineral2
"issol#ed carbon dio*ide reacts
to gi#e carbonic acid ,
8arbonic acid fro the air reacts $ith
carbonate 1a base2 fro ineral dissolution
to gi#e bicarbonate
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Ocean Acidification
• What is ocean acidificaton(
– pH has decreased fro D,/I to D,?: o#er the last /G@ years,
– Why(
• 8arbonate cheistry Atospheric 8O/ has increased since ?JG@,
• Soe of the 8O/ dissol#es in the oceans and reacts $ith carbonate,
• So $hat(
– !epresents an increase of alost :@K in H= concentration,• !eeber that pH is a logarithic scale,
– 5f it continues unchecked it can greatly affect calcifying
organiss,
• These build shells of 8a8O:
• Their shells can start to dissol#e if the oceans becoe too acidic,
– Soe scientists predict this could happen on a $ide scale in ?@;/@
years if 8O/ eissions go unchecked,
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!esisting Acidification- Buffering Acid Pollution
• What is a buffer( – A 1pH2 buffer is a solution that can absorb a largeC aount of
acid or base $ith a relati#ely sall change in pH
• Ho$ do they $ork( – Basically- added protons or hydro*ide react $ith soething
instead of Lust changing pH
• Buffering in natural systes – Soils and natural $aters are naturally buffered against pH
change 1soe ore so than others2
– 5t is possible to o#er$hel – teporarily or peranentlyC – thebuffering ability of a $ater body
– The carbonate system is the ost iportant bufferingechanis in natural $aters
• 8arbonate syste- all fors of dissolved inorganic carbon 1"582
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Buffering in Natural Waters
• Question
– Ho$ are natural $aters buffered against changes in pH(
– Soe background-
• The ability to resist acidification- pH buffering
• Soils and natural $aters are naturally buffered against pH change
1soe ore so than others2
• 5t is possible to o#er$hel – teporarily or peranentlyC – the
buffering ability of a $ater body
• The carbonate system is the most important buffering
mechanism in natural waters,
– The ability to buffer is 3uantified by the buffer index ,
• "eterined fro titration curves,
• Ne*t slide illustrates by sho$ing the titration cur#e of a carbonate
salt
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ol added acid
@,@ @,G ?,@ ?,G /,@ /,G :,@ :,G
p H
/
I
D
?@
?/
B
u f f e r 5 n t e n s i t y . o l & ; ? p
H ; ?
@,@
@,G
?,@
?,G
/,@
/,G
'uantitati#e Assessent- Buffer 5ntensity
Titration cur#e for ?& of
?% carbonate solution
bicarbonate
buffer region
buffer
intensity
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8arbonate Buffer 5ntensity 18T > ?%2
pH
/ : I G A J D M ?@ ??
B u f f e
r 5 n t e n s i t y 1 β 2 .
o l & ; ? p
H ; ?
@,@?
@,?
?
?@
H= bufferingcarbonate OH;
buffering
bicarbonate
buffering
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• Buffering of natural $aters is often specified by easureents of its
alkalinity – Alkalinity- the aount of acid that ust be added 1per unit #olue2 to
reach a pH of I,G 1the pH $hen all bicarbonate has been protonated2 – 5n a carbonate;only syste. alkalinity is deterined largely by the
concentration of the dissolved inorganic carbon 1"582
• Typical #alues in natural $aters are ? ol+& – in other $ords. ittakes ? ol of protons to con#ert all the "58 in a ?& saplecopletely to 8O/1a32
• The abo#e e*pression does not consider non;carbonate sources ofalkalinity
• Alkalinity easureents typically only easure homogeneousbuffering ability of the natural $ater
!esisting Acidification- Buffering Acid Pollution
2
3 3
[ ] [HCO ] 2[CO ] [OH ] [H ]alk − − − += + + −
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Hoogeneous Buffering of Natural Waters
• Hoogeneous buffering is the neutrali6ation of added
acid+base by dissolved species
– %ostly due to the bicarbonate buffer syste-
• "58 1alkalinity2 is about ? % in fresh $ater and about / % in
arine $ater,
• Borate species 1JG µ% in sea$ater2 pro#ides appro*iately ?@K of
the buffering ability in arine $aters
• Silicate is $idespread in all natural $aters 1/@ µ% in sea$ater.
?@@ µ% in ri#ers2 and can be an iportant buffer in lo$;carbonate
fresh$ater
3 2 3HCO ( ) H H CO ( )aq aq− +
+ →
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Heterogeneous Buffering of Natural Waters
• Heterogeneous buffering is buffering due to the dissolution or
e#olution+precipitation of a gas or a solid
– Slo$er but ore po$erful than hoogeneous buffering
• 8oon echaniss-
– )#olution+dissolution of gaseous 8O/
3 2 2HCO ( ) H CO ( ) H Oaq g − +
+ → +
– Precipitation+dissolution of calcite ineral
– Acid $eathering of silicates 1proton displaceent2 $ill occur o#er longer tie
periods, 5t is priarily iportant $hen carbonate inerals ha#e been
intensely $eathered 1ie. $hen they start disappearing2,
2
3 3CaCO ( ) H Ca ( ) HCO ( ) s aq aq+ + −+ → +
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Heterogeneous vs Hoogeneous Buffering
2
3 2 2CaCO ( ) 2H CO ( ) Ca + H O s g + +
+ + ƒ
• Hoogeneous is much faster than heterogeneous buffering
• Buffering due to silicate
$eathering is slo$er than dueto calcite and 8O/ dissolution+foration
• Heterogeneous bufferingultiately pro#ides a greatercapacity to assiilate protons
1ie. neutrali6e added acid2• Neutrali6ation of added acid bycarbonate inerals continuesuntil all dissol#e and ultiatelygenerate 8O/-
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Suary- !esisting Acidification
• 8arbonaceous $aters – The carbonate syste 1ie. "58. carbonate inerals. and carbon dio*ide2
pro#ides buffering in ost natural $ater bodies – 8arbonate buffering occurs $hene#er $ater is in contact $ith air and$ith carbonate inerals
– Hoogeneous buffering due to "58 is the iediateC response toacidification
– Heterogeneous buffering pro#ides long;ter buffering
• &ong;ter acidification – Would e#entually o#er$hel the carbonate syste
• No ore hoogeneous+heterogeneous buffering in#ol#ing the carbonatesyste
• All carbonate inerals $ould dissol#e 1$eather2
• Then all carbonate and bicarbonate $ould be transfored to carbonic acid
and 8O/
– Silicate buffering can then occur • But it is slo$. and silicate inerals $eather too
• )#entually transfored to 3uart6. $hich does not $eather 1or buffer2 $ell,
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Buffering of Soils 1(2