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1、水文地球化学,主讲：郭清海,中国地质大学（武汉）环境学院,一门关于地下水的科学,水化学成分和水化学指标,授课内容,水的独特性质 水中溶解组分的水解过程（Hydrolysis）大气降水的化学特征 地表水的化学特征 地下水的化学特征 天然水化学成分的综合指标,水分子的结构与性质,在水分子中，氢、氧原子核呈等腰三角形排列，氧核位于两腰相交的顶角上，而两个氢核则位于等腰三角形的两个底角上，两腰夹角为10445。,在水分子中氢、氧原子的这种排列，使水分子在结构上正负电荷静电引力中心不重合，从而形成水分子的偶极性质。,水分子的结构与性质,以上图象为计算机模拟所得的水分子结构图。,水的独特性质,由于水分子的
2、结构很特殊，使相邻水分子之间可以由氢键联结，这就导致水在物理化学性质方面具有一系列不同于其他液体的独特性质。,水具有使盐类离子产生水化作用的能力水具有高的介电效应 水具有良好的溶解性能,天然水的组成,天然水是组成复杂的溶液存在于地壳中的87种稳定的化学元素中，在天然水中就发现了70种以上天然水的化学成分是指离子、络阴离子、复杂络合物无机分子(O2、CO2、H2、CH4、H4SiO4)有机分子微生物（细菌、病毒、真菌寄生虫）（存活时间、吸附、酸性土壤）胶体(10-9-10-7m),离子、络阴离子、复杂络合物单一离子形式：Ca2+、Mg2+、Na+、K+、Cl-、F-络阴离子形式：SO42-、CO
3、32-、HCO3-、NO3-、CrO42-、PO43-复杂络合物：包括有机和无机络合物地下水中常见的常量组分络合物有10种：CaSO40、MgSO40、NaSO4-、KSO4-CaHCO3+、MgHCO3+、NaHCO30 CaCO30、MgCO30、NaCO3-,天然水的组成,Dissolved substances that can donate a proton are called acids and those that can accept a proton are called bases.The key to understanding acid/base equilibria
4、 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 monovalent cation in solution.
5、,Ion Hydrolysis,Ion 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 hydration sheaths and their ejection i
6、nto 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 hydration sheaths but attract hydrogen i
7、ons 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 added to water.This is because the field
8、 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 the case of AlCl3,it is the Al3+ion that hyd
9、rolyzes 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+,Ion Hydrolysis,In the case of the pH increase associated with the addition
10、 of Na2CO3 to water,it is the CO32 ion that hydrolyzes.For CO32,only two hydrolysis products form:CO32+H+HCO3CO32+2H+H2CO30,Ion Hydrolysis,Thus,when Al3+ions are added to water,amounts of AlOH2+,Al(OH)2+,Al(OH)30 and Al(OH)4 ions are formed.Similarly,whenever CO32 ions are added to water,an equilibr
11、ium 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 describing the hydrolysis reactions.,Ion Hydrolysis,The concept of hydrolysis is really the key to
12、fully understandingwhy 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 are frequently associated with oxidation/reduction reactions.,Ion Hydrolysis,Monovalent ions rar
13、ely 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 in one ionic form in water.,Hydrolysis-Monovalent ions,However,there are some exceptions.F tends to
14、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(晶体学半径，而非水合半径：hydrated radius)and a higher charge density at its surface and so is more likely to attract a hydrogen ion.The hydrolysis
15、 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 hydrolysis product HF0 equal the concentration of F,i.e.,at what pH will aF/aHFo=1.0?From the above equilibrium expre
16、ssion,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 ions,Assuming for simplicity that concentrations equal activities,i.e.,s=1.0,the concentrations of HF0 and F-can be solved
17、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 formally calculated by solving the following two simultaneous equations:,Hydrolysis-Monovalent ions,These results are plotted in the diagram
18、 to the left below at two concentrations of FTotal.,Hydrolysis-Monovalent ions,Two features are noticed from this diagram.First,the hydrolysis product F(the one that has lost a H+with respect to the parent)increases in concentration with increasing pH.Think of this as merely due to the greater likel
19、ihood 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 position of equal concentrations of HF0 and F is independent of the total concentration of fluorine in solution.Such a diagram is called a pH-di
20、stribution 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 of expressing this information.It shows the percentage of the total fluoride that each species comprises as a function of pH.,Hydrolysis-Mo
21、novalent ions,Due to their higher charge,elements of 2+or 2 charge are more likely to hydrolyze than monovalent ions.Examine the accompanying distribution diagrams 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 io
22、ns,Lets 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+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 calc
23、ium and magnesium for most waters?No,not really.,Hydrolysis-Divalent ions,Things begin to look more interesting for trivalent ions like B3+and Fe3+.For 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-Tr
24、ivalent ions,Although Fe3+is exclusively an acid and Fe(OH)4,exclusively a base,the intermediate hydrolysis products FeOH2+,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 sequ
25、ential 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+,Hydrolysis-Trivalent ions,Different 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.
26、This diagram is shown below.,Hydrolysis-Trivalent ions,One interesting feature of the ferric iron hydrolysis 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
27、 others.,Hydrolysis-Trivalent ions,The diagram for Al3+shows a similar phenomenon.In this case,Al(OH)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 gr
28、oups.,Hydrolysis-Trivalent ions,Using Fe3+and Al3+hydrolysis as comparisons,the hydrolysis reactions 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-.,Hydrol
29、ysis-Trivalent ions,The distribution diagram is shown in the accompanying figure.,Hydrolysis-Trivalent ions,To 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 wate
30、r molecules.Fe3+in solution,for example,also can be expressed as Fe3+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
31、any dissolved ion is really undefined.,Hydrolysis-Trivalent ions,Convention 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
32、and FeO2-.Matching up species of the same charge,it is seen 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 molecu
33、le formula units from an aqueous species without altering the species to which you are referring.,Hydrolysis-Trivalent ions,Now,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 specie
34、s is chosen rather than the latter.Thats fine 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 ions,These missing species should be dominant at low
35、 pH if the iron diagram is a guide,but only 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 o
36、nly the third hydrolysis product,B(OH)30(expressed alternatively as H3BO30),is a dominant species at low pH.,Hydrolysis-Trivalent ions,For 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
37、 table of potential and actual hydrolysis species,and a pH distribution diagram that shows their relative concentrations,are shown below.,Hydrolysis-Higher valent ions,As was found for B3+,the unhydrolyzed cation Si4+and several lower order hydrolysis products just dont appear as major species even
38、at very low pHs.,Hydrolysis-Higher valent ions,For 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
39、species can be expressed 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 ions,The hydrolysis picture for N5+,is as simple as that for a monovalent ion that doesnt
40、 hydrolyze only one ion forms,NO3.,Hydrolysis-Higher valent ions,The 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+oc
41、curs in the form of four hydrolysis species,whereas N5+occurs in only one.Question:Where is the equivalent species to NO3 on the phosphorus diagram?,Hydrolysis-Higher valent ions,S6+,one step up in valence,exhibits a simple speciation behaviour as N5+.One hydrolysis product,SO42,is the dominant spec
42、ies from a 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
43、 onto the first neighbor water molecules of the S6+and the species HSO4 forms and becomes the dominant species at pHs below 2.,Hydrolysis-Higher valent ions,The distribution diagram for S6+is shown in the accompanying figure.,Hydrolysis-Higher valent ions,Hydrolysis of anions is inverse to that of c
44、ations.Because 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 hydr
45、olysis 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.,Hydroly
46、sis-Anions,As 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-Anions,In the above treatment,the impression has been given that only t
47、wo 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,the
48、electronegativity,ionic radii and equilibrium constants for the first hydrolysis products of some divalent cations are recorded.,Electronegativities and degree of hydrolysis,The equilibrium constant values refer to the hydrolysis reaction written in a different form than encountered previously,i.e.M
49、e2+OH MeOH+rather thanMe2+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 in concentration and E,t
50、he electronegativity(polarizability)of the elemental cation.,Electronegativities and degree of hydrolysis,This 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)value provides a much stronger
