2-水化學成分和水化學指標

1、水文地球化學水文地球化學主講：郭清海主講：郭清海中國地質(zhì)大學（武漢）環(huán)境學院中國地質(zhì)大學（武漢）環(huán)境學院一門關于地下水的科學一門關于地下水的科學水化學成分和水化學指標水化學成分和水化學指標授課內(nèi)容授課內(nèi)容v 水的獨特性質(zhì)水的獨特性質(zhì)v 水中溶解組分的水解過程（水中溶解組分的水解過程（Hydrolysis）v 大氣降水的化學特征大氣降水的化學特征v 地表水的化學特征地表水的化學特征v 地下水的化學特征地下水的化學特征v 天然水化學成分的綜合指標天然水化學成分的綜合指標水分子的結(jié)構與性質(zhì)水分子的結(jié)構與性質(zhì)在水分子中，在水分子中，氫、氧原子核氫、氧原子核呈等腰三角形呈等腰三角形排列，氧核位排列，氧核

2、位于兩腰相交的于兩腰相交的頂角上，而兩頂角上，而兩個氫核則位于個氫核則位于等腰三角形的等腰三角形的兩個底角上，兩個底角上，兩 腰 夾 角 為兩 腰 夾 角 為1041044545。 在 水 分 子 中在 水 分 子 中氫、氧原子的氫、氧原子的這種排列，使這種排列，使水分子在結(jié)構水分子在結(jié)構上正負電荷靜上正負電荷靜電引力中心不電引力中心不重合，從而形重合，從而形成水分子的偶成水分子的偶極性質(zhì)。極性質(zhì)。水分子的結(jié)構與性質(zhì)水分子的結(jié)構與性質(zhì)以上圖象為計算機模擬所得的水分子結(jié)構圖。以上圖象為計算機模擬所得的水分子結(jié)構圖。水的獨特性質(zhì)水的獨特性質(zhì) 由于水分子的結(jié)構很特殊，使相鄰水分子之間可以由由于水分子

3、的結(jié)構很特殊，使相鄰水分子之間可以由氫鍵聯(lián)結(jié)，這就導致水在物理化學性質(zhì)方面具有一系氫鍵聯(lián)結(jié)，這就導致水在物理化學性質(zhì)方面具有一系列不同于其他液體的獨特性質(zhì)。列不同于其他液體的獨特性質(zhì)。v 水具有使鹽類離子產(chǎn)生水化作用的能力水具有使鹽類離子產(chǎn)生水化作用的能力v 水具有高的介電效應水具有高的介電效應 v 水具有良好的溶解性能水具有良好的溶解性能天然水的組成天然水的組成v天然水是組成復雜的溶液天然水是組成復雜的溶液 存在于地殼中的存在于地殼中的8787種穩(wěn)定的化學元素中，在天然水中就發(fā)現(xiàn)了種穩(wěn)定的化學元素中，在天然水中就發(fā)現(xiàn)了7070種以上種以上v天然水的化學成分是指天然水的化學成分是指 離子、離子

4、、絡陰離子、復雜絡合物絡陰離子、復雜絡合物 無機分子無機分子( (O O2 2、COCO2 2、H H2 2、CHCH4 4、H H4 4SiOSiO4 4) ) 有機有機分子分子 微生物（微生物（細細菌菌、病毒、真菌寄生蟲、病毒、真菌寄生蟲）（存活時間、吸附、酸性土（存活時間、吸附、酸性土壤）壤） 膠膠體體(10(10-9-9-10-10-7-7m)m)v離子、絡陰離子、復雜絡合物離子、絡陰離子、復雜絡合物單一離子形式：單一離子形式：Ca2+、Mg2+ 、 Na+ 、 K+ 、 Cl- 、 F-絡陰離子形式：絡陰離子形式：SO42- 、 CO32- 、 HCO3- 、 NO3- 、 CrO4

5、2- 、 PO43-復雜絡合物：包括有機和無機絡合物復雜絡合物：包括有機和無機絡合物v地下水中常見的常量組分絡合物有地下水中常見的常量組分絡合物有10種：種：CaSO40、MgSO40、NaSO4- 、KSO4- CaHCO3+ 、MgHCO3+ 、NaHCO30 CaCO30 、MgCO30、NaCO3-天然水的組成天然水的組成 Dissolved substances that can donate a proton are called acids and those that can accept a proton are called bases. The key to unders

6、tanding acid/base equilibria lies in the phenomenon of hydrolysis. Hydrolysis is a reaction that accompanies ion hydration. Hydrogen ions are labile and can transfer from one water molecule to the next in solution. Imagine the hydrogen ions associated with the first neighbor water molecules around a

7、 monovalent cation in solution.Ion HydrolysisIon Hydrolysis Would the cation tend to repel and perhaps eject a hydrogen ion into the bulk solution? Certainly one would think so and the tendency would be greater the higher the charge on the cations. The process of hydrogen ion detachment from hydrati

8、on sheaths and their ejection into the bulk solution is called hydrolysis.Ion Hydrolysis Cation hydrolysis thus results in a decrease in solution pH. Anion hydrolysis operates oppositely. Because of their negative charge, anions not only preserve the hydrogen ions of water molecules in their hydrati

9、on sheaths but attract hydrogen ions from the bulk solution. Anion hydrolysis thus results in an increase in the solution pH.Ion Hydrolysis Monovalent ions are so weakly charged that they rarely hydrolyze. For example, NaCl is considered a neutral salt, i.e., producing no effect on pH when it is add

10、ed to water. This is because the field strengths (charge/surface area ratios) of Na+ and Cl are not sufficiently high to attract or eject hydrogen ions to or from their hydration sheaths.Ion Hydrolysis The addition of AlCl3 or Na2CO3 to water, however, causes a marked change in the pH of water. In t

11、he case of AlCl3, it is the Al3+ ion that hydrolyzes and several hydrolysis products form representing Al3+ ions that have lost one, two, three and four hydrogen ions from their hydration sheaths. Al3+ + H2O AlOH2+ + H Al3+ + 2H2O Al(OH)2+ + 2H+ Al3+ + 3H2O Al(OH)30 + 3H+ Al3+ + 4H2O Al(OH)4 + 4H+Io

12、n Hydrolysis In the case of the pH increase associated with the addition of Na2CO3 to water, it is the CO32 ion that hydrolyzes. For CO32, only two hydrolysis products form: CO32 + H+ HCO3 CO32 + 2H+ H2CO30Ion Hydrolysis Thus, when Al3+ ions are added to water, amounts of AlOH2+, Al(OH)2+, Al(OH)30

13、and Al(OH)4 ions are formed. Similarly, whenever CO32 ions are added to water, an equilibrium concentration of the hydrolysis products, HCO3 and H2CO30 form. The relative amounts of an ion and its hydrolysis products will depend on the initial pH of the solution and the equilibrium constants describ

14、ing the hydrolysis reactions.Ion Hydrolysis The concept of hydrolysis is really the key to fully understanding why elements are found in the forms they are in water; what controls acid/base equilibria and pH buffering in solution; why the solubility of some minerals is pH dependent; why pH changes a

15、re frequently associated with oxidation/reduction reactions.Ion Hydrolysis Monovalent ions rarely hydrolyze in solution because the single positive or negative charge is insufficient to dislodge or attract hydrogen ions to or from the bulk solution. Thus ions like Cl, I, Na+, and K+ are only found i

16、n one ionic form in water. Hydrolysis-Monovalent ions However, there are some exceptions. F tends to attract H+, especially at low pH where an abundance of H+ ions are present in the bulk solution. Why does F hydrolyze and not Cl? F has a smaller crystallographic radius (晶體學半徑，而非水合半徑： hydrated radiu

17、s) and a higher charge density at its surface and so is more likely to attract a hydrogen ion. The hydrolysis reaction (written in reverse) is merely an acid dissociation reaction:Hydrolysis-Monovalent ions Remembering that pH = logaH+ and assuming activities concentrations, at what pH would the hyd

18、rolysis product HF0 equal the concentration of F, i.e., at what pH will aF/aHFo = 1.0? From the above equilibrium expression, it is seen that this would occur at a hydrogen ion activity equal to the value of the hydrolysis reaction constant, i.e., aH+ = 104.0 or at a pH of 4.0.Hydrolysis-Monovalent

19、ions Assuming for simplicity that concentrations equal activities, i.e., s = 1.0, the concentrations of HF0 and F- can be solved as a function of pH. At a pH of 3.0, aF/aHF0 would equal 0.1 and at a pH of 5.0, aF/aHF0 would equal 10. The concentrations of HFo and F at a variety of pHs can be formall

20、y calculated by solving the following two simultaneous equations:Hydrolysis-Monovalent ions These results are plotted in the diagram to the left below at two concentrations of FTotal.Hydrolysis-Monovalent ionsTwo features are noticed from this diagram. First, the hydrolysis product F (the one that h

21、as lost a H+ with respect to the parent) increases in concentration with increasing pH. Think of this as merely due to the greater likelihood that hydrogen ions will be ejected from the hydration sheath of an ion when hydrogen ions are depleted in the bulk solution (pH increase).Secondly, the positi

22、on of equal concentrations of HF0 and F is independent of the total concentration of fluorine in solution. Such a diagram is called a pH-distribution diagram as it shows the distribution of concentration of an elements aqueous species as a function of pH. The diagram to the right is an alternate way

23、 of expressing this information. It shows the percentage of the total fluoride that each species comprises as a function of pH.Hydrolysis-Monovalent ionsDue to their higher charge, elements of 2+ or 2 charge are more likely to hydrolyze than monovalent ions. Examine the accompanying distribution dia

24、grams for Se2 and Fe2+. Which of these ions behaves like an acid when added to a solution and which behaves like a base?Hydrolysis-Divalent ionsLets now look at the distribution diagrams for Ca2+ and Mg2+. Both ions hydrolyze to form a hydroxide species that becomes dominant at high pH. Why is Mg2+

25、more effective at hydrolysis than Ca2+? Most natural waters have pHs between 4.0 and 9.0, is it important to consider hydrolysis reactions for calcium and magnesium for most waters? No, not really.Hydrolysis-Divalent ionsThings begin to look more interesting for trivalent ions like B3+ and Fe3+. For

26、 ferric iron the hydrolysis reactions can be written like: Fe3+ + H2O FeOH2+ + H Fe3+ + 2H2O Fe(OH)2+ + 2H+ Fe3+ + 3H2O Fe(OH)30 + 3H+ Fe3+ + 4H2O Fe(OH)4 + 4H+Hydrolysis-Trivalent ionsAlthough Fe3+ is exclusively an acid and Fe(OH)4, exclusively a base, the intermediate hydrolysis products FeOH2+,

27、Fe(OH)2+ and Fe(OH)30 can accept or donate protons. Species capable of such dual behavior are termed amphoteric substances.The hydrolysis equilibria can also be expressed as sequential reactions: Fe3+ + H2O FeOH2+ + H FeOH2+ + H2O Fe(OH)2+ + H+ Fe(OH)2+ + H2O Fe(OH)30 + H+ Fe(OH)30 + H2O Fe(OH)4 + H

28、+Hydrolysis-Trivalent ionsDifferent values are obtained for the equilibrium constants depending on which way the hydrolysis reactions are written but the final distribution diagram will look the same. This diagram is shown below.Hydrolysis-Trivalent ionsOne interesting feature of the ferric iron hyd

29、rolysis diagram is that the species Fe(OH)30 never becomes the dominant species in solution at any pH.It often happens that for steric reasons certain hydrolysis products are more likely to form than others.Hydrolysis-Trivalent ionsThe diagram for Al3+ shows a similar phenomenon. In this case, Al(OH

30、)2+ never becomes a dominant species with pH. It is quite common for hydrolysis products with an even number of hydroxide groups to have a greater stability than those with an odd number of hydroxide groups.Hydrolysis-Trivalent ionsUsing Fe3+ and Al3+ hydrolysis as comparisons, the hydrolysis reacti

31、ons for B3+ might be: B3+ + H2O BOH2+ + H BOH2+ + H2O B(OH)2+ + H+ B(OH)2+ + H2O B(OH)30 + H+ B(OH)30 + H2O B(OH)4 + H+If the literature is examined, however, only two aqueous species are listed for boron B(OH)30and H2BO3-.Hydrolysis-Trivalent ionsThe distribution diagram is shown in the accompanyin

32、g figure.Hydrolysis-Trivalent ionsTo explain this difference between the anticipated and the actual hydrolysis pattern for borate species, a digression is necessary.A dissolved ion can be represented with varying number of water molecules. Fe3+ in solution, for example, also can be expressed as Fe3+

33、 H2O, Fe3+ 2H2O or even as H2FeO3+ or H4FeO23+.This is because the choice of how many water molecules are associated with the formula of an ion is arbitrary, reflecting the fact that the actual number of water molecules associated with any dissolved ion is really undefined.Hydrolysis-Trivalent ionsC

34、onvention has it that Fe3+ is chosen and its hydrolysis products expressed as FeOH2+, Fe(OH)2+, Fe(OH)30 and Fe(OH)4-. However, H4FeO23+ could have been chosen instead of Fe3+ and the hydrolysis products expressed as H3FeO22+, H2FeO2+, HFeO20 and FeO2-.Matching up species of the same charge, it is s

35、een that HFeO20 and Fe(OH)30 are the same species. If a water molecule formula unit is subtracted from the latter and rearranged, the former results.Similarly, Fe(OH)4- must be FeO2-. A generalization can be made: You can add or subtract water molecule formula units from an aqueous species without a

36、ltering the species to which you are referring.Hydrolysis-Trivalent ionsNow, lets go back to B3+ hydrolysis in water. H3BO30 can alternatively be expressed as B(OH)30 and H2BO3- as B(OH)4- . Convention has it that the former nomenclature for boron species is chosen rather than the latter.Thats fine

37、but when the distribution diagram for B3+ is compared to that of Fe3+, it is noticed that some species are missing. For example, B3+ itself does not appear nor does BOH2+ or B(OH)2+. Why?Hydrolysis-Trivalent ionsThese missing species should be dominant at low pH if the iron diagram is a guide, but o

38、nly H3BO30 is present at low pH.The reason is that B3+ is so small and effective at hydrolysis that it doesnt exist at any measurable concentration in solution no matter how low the pH.The same is true for BOH2+ or B(OH)2+. B3+ is so effective at hydrolysis that only the third hydrolysis product, B(

39、OH)30 (expressed alternatively as H3BO30), is a dominant species at low pH.Hydrolysis-Trivalent ionsFor elements with valence greater than three, the unhydrolyzed ion and some of the lower hydrolysis products are rarely stable in solution.For example, consider Si4+. A table of potential and actual h

40、ydrolysis species, and a pH distribution diagram that shows their relative concentrations, are shown below.Hydrolysis-Higher valent ionsAs was found for B3+, the unhydrolyzed cation Si4+ and several lower order hydrolysis products just dont appear as major species even at very low pHs.Hydrolysis-Hig

41、her valent ionsFor higher valences, e.g. N5+, the hydrolysis pattern might be expected to get more complicated. However, only one hydrolysis species of N5+ has been detected within the pH range of 0 to 14, NO3.Adding three water molecules to the formula and rearranging, this species can be expressed

42、 as N(OH)6. There is no evidence for the existence of any lower hydrolysis species like N(OH)50 and N(OH)4+ or higher hydrolysis products like N(OH)72 and N(OH)83.Hydrolysis-Higher valent ionsThe hydrolysis picture for N5+, is as simple as that for a monovalent ion that doesnt hydrolyze only one ion

43、 forms, NO3.Hydrolysis-Higher valent ionsThe distribution diagram for phosphorus, another pentavalent cation, is shown alongside the diagram for N5+. Although seemingly different, the two diagrams are equivalent in a topological sense.Its just that over the pH range 0 to 14, P5+ occurs in the form o

44、f four hydrolysis species, whereas N5+ occurs in only one. Question: Where is the equivalent species to NO3 on the phosphorus diagram?Hydrolysis-Higher valent ionsS6+, one step up in valence, exhibits a simple speciation behaviour as N5+. One hydrolysis product, SO42, is the dominant species from a

45、pH of 2.0 to 14.0. Imagine this species as a hydrated S6+ ion with four first neighbor water molecules whose hydrogen ions have all been removed due to its high positive charge.When the pH is less than 3, sufficient hydrogen ions exist in the bulk solution such that one H+ ion can be accepted onto t

46、he first neighbor water molecules of the S6+ and the species HSO4 forms and becomes the dominant species at pHs below 2.Hydrolysis-Higher valent ionsThe distribution diagram for S6+ is shown in the accompanying figure.Hydrolysis-Higher valent ionsHydrolysis of anions is inverse to that of cations. B

47、ecause of their negative charge, anions attract hydrogen ions from the bulk solution to the water molecules of their hydration sheaths.This increases the pH of the solution as opposed to cation hydrolysis, which lowers it. However, anion hydrolysis is not as common or as extensive as cation hydrolys

48、is for two reasons: 1) anionic forms of elements occur less frequently than cationic forms in natural waters； and 2) the field strength of an anion is smaller than a cation of the same charge (it has more electrons than protons), and thus its ability to attract protons from solution is weaker.Hydrol

49、ysis-AnionsAs with cations, the propensity for hydrolysis increases with ion charge. For example, where most monovalent anions hydrolyze weakly or not at all, a divalent like S2 hydrolyzes to form two species HS and H2S0.Hydrolysis-AnionsIn the above treatment, the impression has been given that onl

50、y two factors determine the degree of hydrolysis an ion will undergo in water size and charge.Although this is a very useful example to anticipate the hydrolysis pattern for a particular valence form of a given element, the electronegativity of the ion is often more important. In the following table

51、, the electronegativity, ionic radii and equilibrium constants for the first hydrolysis products of some divalent cations are recorded.Electronegativities and degree of hydrolysisThe equilibrium constant values refer to the hydrolysis reaction written in a different form than encountered previously,

52、 i.e. Me2+ + OH MeOH+rather than Me2+ + H2O MeOH+ + H+However, both forms are equivalent because the dissociation reaction of water: H2O H+ + OHcan be added to the former to get the latter. Included in the above table are the pH at which the elemental ion and its first hydrolysis product are equal i

53、n concentration and E, the electronegativity (polarizability) of the elemental cation.Electronegativities and degree of hydrolysisThis table shows that while the ionic radius (Cr) of the elemental cation exhibits a poor correlation with a cations first hydrolysis product, its electronegativity (E) v

54、alue provides a much stronger correlation.The reason for this is that cations with higher electronegativities form a more covalent bond with neighboring water molecules. Covalent bonds are stronger than ionic bonds and thus the internuclear distances between the ion and its neighboring water molecul

55、es are reduced, compared to that expected for ionic bonding.This effectively increases the field strength of the ion and enhances the probability of hydrolysis.Electronegativities and degree of hydrolysisSome hydrolysis products of metal ions can link up to form bridged polynuclear complexes.Such co

56、mplexes are composed of two or more metal ions with OH ions serving as bridges. Some examples are below: a) Be2+ + BeOH+ Be2OH3+ b) 2AlOH2+ Al2(OH)24+ c) 3HgOH+ Hg3(OH)33+PolymerisationThe presumed structures for these polymers are shown below. Waters of hydration around the metal cations are not sh

57、own, only the bridging hydroxide groups.PolymerisationMore complicated structures and formulae such as Al8(OH)204+ and Al13O4(OH)247+ can result by sequential reactions of simpler polymers with monomers or other polymers.Hydroxide groups are not required for the polymerisation of higher valent eleme

58、ntal ions.These ions typically have few remaining protons on their waters of hydration. Cr6+, for example, hydrolyzes to form HCrO4 and CrO42 in aqueous solution. The two species are related through this reaction expression: HCrO4 H+ + CrO42HCrO4, however, polymerizes to form Cr2O72 in aqueous solut

59、ion. 2 HCrO4 Cr2O72 + H2OPolymerisationIn this case, since a decrease in pH favors the formation of HCrO4 relative to CrO42, a decrease in pH also favors the formation of the Cr2O72.Although no general rule can be established to predict the effect of pH changes on the stability of polynuclear hydrol

60、ysis products for all metal ions, one can be established for the dissolved metal ion content.Invariably, polymerization is enhanced at high metal concentrations. PolymerisationThe following figures represent the speciation of hexavalent chromium at two total dissolved metal concentrations and demons