Encyclopedia of Inland Waters || Acidification

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    thetic Organics, Radionuclides, Heavy Metals, Acids, and

    r E

    . rising concentrations of dissolved solids, particu-

    are described. The approaches how to mitigate or to

    Definitions and Dimensions

    larly of sulfate in the drainage waters,

    . mobilization of potentially toxic metals, and

    . loss of biologic diversity.The capacity to neutralize additions of acids or basesdepends on the respective concentrations of basedissolved by rain water. The composition of freshwaters, in both soft and hard waters, is differing bythe concentrations of the components of the system(eqns. [5ce]), which together are buffering standardfresh waters to about pH 7.Acidification is the additional input of acids to

    waters from natural and anthropogenic sources(Plate 1) shifting the pH to lower values and usuallyeliminating the carbonate buffering system. The twomost important mineral acids in this respect are sul-furic and nitric acid. Industrial acid emissions havealtered lakes and rivers because of wind transportaway from the source areas and lack of neutralizingmaterials in the geology of the receiving regions, e.g.,in North America and Scandinavia.Accompanying the acidification, further problems

    arise beyond low pH and aciditiy:

    remediate acidifications are presented elsewherewithinthis encyclopedia.

    Chemistry of Acidified Waters andBuffering Mechanisms

    Carbonic Acid and Fresh Water

    Natural fresh waters contain mainly carbonates,HCO3 and CO

    23 , and most fresh waters show

    nearly identical chemical compositions with pH7and the carbonate buffering system dominating.Carbonate-poor geological regions have soft waterswith low mineral content, and carbonate-rich areasshow higher mineralized hard waters.minerals in rock and sediments that can easily beNatural rain is weakly acidic by atmospheric carbondioxide, reacting to carbonic acid in water. Therefore,the chemical characteristic of natural waters is domi-nated by dissolved carbonates, since these are the only

    impacted water for irrigation, fishery and aquacul-tures, and its use as drinking water.Following, the sources, distribution pathways, reac-

    tive modifications, effects on soils and biota, andthe importance of the different kinds of acidificationPOLLUTION AND REMEDIA



    Aquatic Ecosystems and Human Health

    Bioassessment of Aquatic Ecosystems

    Deforestation and Nutrient Loading to Fresh Waters

    Distribution and Abundance of Aqautic Plants Human

    Effects of Climate Change on Lakes




    Invasive Species

    Mercury Pollution in Remote Freshwaters

    Pollution of Aquatic Ecosystems I

    Pollution of Aquatic Ecosystems II: Hydrocarbons, Syn

    Thermal Pollution

    Vector-Borne Diseases of Freshwater Habitats

    AcidificationW Geller and M Schultze, UFZ Helmholtz Center fo

    2009 Elsevier Inc. All rights reserved.

    Introductionnvironmental Research, Magdeburg, Germany

    The adverse factors prohibit the use of thepactsION1

  • l

    2 Pollution and Remediation _ AcidificationPlate 1 Anthropogenic and natural acid waters: Upper left panecations, of strong acid anions, and the concentrationsof two- or three-valent weak acids that function asbuffering systems by stepwise dissociation of theirprotons. The relationship between base cations andstrong acid anions determine whether the water isalkaline, neutral, or acidic.

    Alkalinity and acidity The term alkalinity is equiva-lent to the acid-neutralizing capacity (ANC). Nega-tive values of alkalinity are termed acidity, equivalentto the base-neutralizing capacity (BNC).

    ANC sum of base cation concentrations :Ca2; Mg2; Na; K; NH4 sum of strong acid anion concentrations :SO24 ; NO

    3 ; Cl

    concentrations inmilliequivalents per litre : meq L1 1

    ANCBNC thus.The BNC of acid waters can be measured directly

    by titration, usually with a solution 0.01N sodiumhydroxide till pH 8.2, resulting in a measure of (mmolL1), or (meq L1). The acidity of acid water alsocan be calculated from the pH-value and from themajor weakly acid components of water (Fe, Al, Mn;

    lowering of water level by drought, with aeration of sulfidic sediment

    Report 28/04: http://www.clw.csiro.au/scientific_reports.html). Uppe(Photograph: M. Koschorreck). Lower panels Acid lignite pit lake (p

    also view of acid volcanic Lake Voui http://www.altitude-photo.com

    volcanodiscovery.com/volcano-tours/typo3temp/pics/15e7421124.jpAcid Sulfate Soil: Bottle Bend Lagoon, Australia, acidified aftermetal concentrations given in mg L1) by eqn. [2](if the concentrations of carbon dioxide, dissolvedsilicate, humic acids, and other heavy metals aresmall compared to the sum of iron, aluminum, andmanganese):

    AcidcalcmeqL1 2Fe2

    56 3Fe


    56 3Al

    27 2Mn


    1000 10pH2

    The pH-value is a bulk measure comprising thestrong mineral acids, which are completely disso-ciated into their anions and protons, and the disso-ciated part of the weak acids. The non-dissociatedprotons of the weak acids are set free stepwise duringthe titration process, thereby functioning as bufferingsystems.According to the US standards, acidity is measured

    by the amount of CaCO3 that has to be added toreach neutrality. The dimension (mg CaCO3 L

    1)can be converted to (meq L1) following eqn. [3]:

    AciditymeqL1 mgCaCO3L1=50 3

    Buffering systems: the weak acids of carbon, alumi-num, and iron Acids may enter aquatic systems

    s. (Image by courtesy of CSIRO 2004, Land and water Technical

    r right Acid Rio Tinto draining the Pyrite Belt in Southern SpainH 2.3) in Lusatia, East Germany (Photograph: G. Packroff). See

    /fiche-photo.php?id photo=11994/, http://www.


  • . Buffering by ion exchange (eqn. [9] gives an exam-ple;X active site at the surface of particles or solidmaterial).

    Sites of ion exchange:

    X KH , X H K 9

    . Buffering by protonation of humic substances

    Humic acids:

    R COO H , RCOOH 10

    When acids enter aquatic systems directly, the rele-vant buffering systems are in the liquid phase: thecarbonate system eqns. [5bd], the aluminum buffereqns. [7ac], the iron buffer eqns. [8ac], and reac-tions of further weak acids (e.g., humic/fulvic acids,silicate). The chemical equilibrium between sulfateand hydrogensulfate comes into play if pH dropsbelow 2.5 (eqn. [11]).

    Formation of hydrogensulfate:

    Pollution and Remediation _ Acidification 3directly or after passage of soil anddeeper underground.The contacts of the solid phase of soil and undergroundto the passing acids result in buffering, i.e., proton-consuming reactions. Thereby, the dissolution of miner-als is the main source of metal mobilization among thebuffering processes. The relevant processes are:

    . Buffering by carbonates and carbonic acid eqns.[5ad]

    Carbonic acid system:

    CaCO3 2H , Ca2 CO2 H2O 5a

    CaCO3 H , Ca2 HCO3 5b

    CO23 H , HCO3 5cHCO3 H , H2CO3 5dH2CO3 , CO2 H2O 5e

    . Buffering by silicates, especially aluminosilicatesand aluminum silicates (eqns. [6ad] give exam-ples). The liberated aluminum and the formedgibbsite behave as described in eqns. [7ac].

    Feldspar dissolution:

    KAlSi3O8 7H2OH , AlOH3 3H4SiO4 K6a


    KAlSi3O84H2O4H,Al33H4SiO4K 6c

    Clay dissolution:

    Al2Si2O5OH4 6H , 2Al3 2H4SiO4 H2O 6d

    . Buffering by pedogenic oxides and (oxi)hydroxides(eqns. [7ac] dissolution of gibbsite which formsthe so-called aluminum buffering system and eqns.[8ac] dissolution of ferrihydrite which forms theso-called iron buffering system).

    Aluminum system:

    AlOH3 H , AlOH2 H2O 7a

    AlOH2 H , AlOH2 H2O 7b

    AlOH2 H , Al3 H2O 7c

    Iron system:

    FeOH3 H , FeOH2 H2O 8a

    FeOH2 H , FeOH2 H2O 8b

    FeOH2 H , Fe3 H2O 8cSO24 H , HSO4 11

    The action of the different buffering systems resultsin typical distributions of pH-values obtained fromregional surveys in lakes which are partly acidified.Figure 1 shows the pH-values of German pit lakesresulting from lignite mining forming a three-modaldistribution. Surveys of lakes which are partlyimpacted by atmospheric deposition usually result ina two-modal pH-distribution formed by the alumi-num buffer and by the carbonate buffer.







    ber o

    f lak




    02 3

    Fe3+ HCO3AI3+

    4 5pH

    6 7 8 9

    Figure 1 Frequency distribution of the pH values of 159 mininglakes in Germany (data from Nixdorf et al. (2001) Tagebauseen in

    Deutschland. Umweltbundesamt, Berlin. UBA-Texte 01/35

    http://www.umweltdaten.de/publikationen/fpdf-l/1996.pdf). Thehatched areas indicate the pH ranges of the buffering system

    of ferric iron (Fe3), aluminium (Al3), and bicarbonate (HCO3 ).

  • water-saturated anoxic underground. Once in con-tact with air, the oxidation goes on rapidly and

    waters increase from about 50mg L at pH5 in softwaters, which result from atmospheric deposition, to


    9 Lake Felix

    4 Pollution and Remediation _ AcidificationThe buffering systems govern not only the decreaseof pH as the result of impact of acids. The progressof neutralization is shown by typical titration curvesof water from acid German pit lakes in Figure 2. Theplateaus indicate the action of the iron and the alumi-num buffering systems, respectively. The extent of the




    pH v





    20 2 4

    ML 117ML 110ML 111

    6 8Added alkalinity in meq L1

    10 12 14




    Figure 2 Titration curves of four different mining lakes in theLusatian lignite mining district, Germany (data by courtesy of

    O. Totsche). The shaded areas indicate the pH ranges of the

    buffering systems of ferric iron (Fe3) and aluminum (Al3).Depending on the concentrations of ferric iron and aluminum andthe formed minerals in the particular lakes, the extent of the

    according plateaus differs in the titration curves. Below pH 2.5,

    hydrogensulfate (HSO4 ) may act as buffer. Ion exchange withparticles, silicates, and the carbonate buffering system are

    relevant at pH above 5, depending on the availability of

    suspended particles and (bi-)carbonate.plateaus depends on the concentration of the respec-tive metals in the lake water.

    Pyrite weathering Acids of geogenic origin can beset free by natural weathering or by processes inducedby mining or by agriculture. The most importantprocess is the oxidation of pyrite or similar sulfides(eqns. [1215]). These oxidation processes are accel-erated by sulfur and iron oxidizing bacteria. Theoverall products of the multi-step reaction are sulfuricacid and ferric iron hydroxide.

    FeS2 7=2O2 H2O , Fe2 2SO24 2H 12

    Fe2 1=4O2 H , Fe3 1=2H2O 13

    Fe3 3H2O , FeOH3 3H 14

    FeS2 14Fe3 8H2O, 15Fe2 2SO24 16H 15

    In the case of mining-induced pyrite oxidation,dewatering operations bring pyrite in contactwith air. Before, the pyrite had been stable in the

    The atmospheric pathway distributes acidic gaseous

    SO2, NH3, and NOx over large areas of land. Thesources of SO2 and NOX are combustion of coal, lig-nite, and oil in power plants and in households, wastegases ofmotor cars, and various industrial waste gases,e.g., resulting from roasting of sulfidic ores. NH3 emis-sions often result from stock farming, especially whendone in large-scale units. The acidity ofNH3 emissionsresults from oxidation/nitrification during atmos-pheric transport or after deposition in the top soil.Although the term acid rain is commonly used,

    this kind of acidification usually includes the100g L1 at pH0 in extremely acidic brines of volcanicsprings. The spectrum of dissolved elements originatesfrom the involved mineral acids and from the composi-tion of the thereby dissolved minerals in soils and rocks.We show in Figure 3, the concentration of some

    constituents of acid waters resulting from thedescribed types of acidification. For all types, datafrom ground water, springs, streams and lakes areincluded. However, these data comprise only waterswith pH< 6.The occurring concentrations of sulfate and metals

    result from sulfide oxidation as well as from bufferingprocesses in soil and water (eqns. [59,12,15]). ThepH-values of these acid waters range from 6 to valuesbelow zero under extreme conditions in abandonedmine workings in California. We show in Figure 4,the pH-values of the waters comprised in Figure 3.

    Types and Extent of Acid Waters

    Atmospheric Deposition and Acid Rainforms acid mine drainage (AMD). Pyrite is not onlyone of the most important iron ores but also occurs asan accompanying mineral in many sulfidic ores ofother metals, in lignite and coal deposits, in shales,or in marine sediments, such as mined clay deposits.Outcrops of natural pyrite deposits provide natural

    conditions for pyrite oxidation. Such natural acidicwaters are known from the Iberian Pyrite Belt insouthern Spain, mountainous regions in New Mex-ico, or the lakes of the Tyrell Basin and the YilgarnBlock in southern Australia.

    Concentrations of dissolved substances in acid watersThe concentrations of total dissolved solids in acid


  • 76

    Acy SO42 Ca Mg Na Fe AI Mn Zn Cu Ni


    nd nd nd nd

    Ore mining

    Acidic sulfate soils

    Coal and lignite mining


    As Cd Pb Co Cr SiK

    Acy SO42 Ca Mg

    Log 1

    0 of a



    ) in m

    eq L

    1 or




    in m

    g L


    Na Fe AI Mn Zn Cu Ni As Cd Pb Co Cr SiK

    Acy(a) SO42 Ca Mg Na Fe AI Mn Zn Cu Ni As Cd Pb Co Cr SiK



































    Figure 3 (Continued)

    Pollution and Remediation _ Acidification 5

  • 6 Pollution and Remediation _ Acidification765deposition of aerosols, of fog, the dry depositionof the mentioned gases, and of dust. In a two-stepprocess, the soils first become acidic after com-plete loss of carbonate minerals, then, acidic

    Acy SO42 Ca Mg Na Fe AI MK

    Acy(b) SO42 Ca Mg Na Fe AI MK








    Log 1

    0 of a



    ) in m

    eq L

    1 or




    in m

    g L


    Figure 3 Characteristics of acid drainage by the concentrations ofbox limits are 25 and 75 percentiles, and whiskers show the 10- and 9or above 90 percentile, respectively (nd not detected).

    Coal and lignite mining(n = 80)

    Ore mining(n = 57)

    Acidic sulfate soils(n = 36)

    Acid volcanic waters(n = 36)


    Waters acidified by atmospheric deposition(by acid rain; n = 36)

    Figure 4 pH values of the waters comprised in Figure 3. Lines witpercentiles, and whiskers show the 10- and 90-percentile values. Sin

    respectively (nd not detected).Waters acidified by atmospheric deposition(by acid rain)waters seeping through the soils dissolve furtherminerals, especially Al-silicates. These waterscause the acidification of ground water and surfacewaters.

    n Zn

    Acid volcanic waters

    Cu Ni As

    nd nd nd

    Cd Pb Co Cr Si

    n Zn Cu Ni As Cd Pb Co Cr Si

    chemical constituents. Lines within the boxes are median values,

    0-percentile values. Single dots indicate data below 10 percentile

    0 1 2 3pH-value

    4 5 6

    hin the boxes are median values, box limits are 25 and 75gle dots indicate data below 10 percentile or above 90 percentile,

  • ate-free, and the affected waters are enriched in

    Belt, the worlds largest deposit of pyrite and other

    Pollution and Remediation _ Acidification 7sulfate and aluminum, with pH-values in a rangebetween 4.5 and 5.5.Under certain conditions, this kind of acidification

    occurs episodically. If the reaction time betweenintroduced acidity and soil is long enough the men-tioned buffering processes are...