Acid Pollution of Natural Waters

8/19/2019 Acid Pollution of Natural Waters

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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


3/23

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,


4/23

pH !anges in the Hydrosphere


5/23

pH of Natural Waters- &akes


6/23

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


8/23

)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


10/23

%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-


11/23

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:=


12/23

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


15/23

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


16/23

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


17/23

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


18/23

• 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− +

+ →


20/23

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/-


22/23

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

Acid Pollution of Natural Waters

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