水化学成分和水化学指标

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- 、 CO32- 、 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 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.,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 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 hydration 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 added 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 the 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+,Ion 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+ 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 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 describing 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 are 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 in 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 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 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 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 formally 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 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 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 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-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 of expressing this information. It shows the percentage of the total fluoride that each species comprises as a function of pH.,Hydrolysis-Monovalent 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 ions,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 calcium 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-Trivalent 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 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+,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. 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 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 groups.,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-.,Hydrolysis-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 water 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 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 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 molecule 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 species 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 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 only 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 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 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 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 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+ occurs 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 species 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 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 cations. 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 hydrolysis 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.,Hydrolysis-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 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, the 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. Me2+ + OH MeOH+ rather than Me2+ + H2O MeOH+ + H+ However, both forms are equivalent because the dissociation reaction of water: H2O H+ + OH can 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, the 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 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 molecules 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 hydrolysis,Some hydrolysis products of metal ions can link up to form bridged polynuclear complexes. Such complexes 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+,Polymerisation,The presumed structures for these polymers are shown below. Waters of hydration around the metal cations are not shown, only the bridging hydroxide groups.,Polymerisation,More 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 elemental 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+ + CrO42 HCrO4, however, polymerizes to form Cr2O72 in aqueous solution. 2 HCrO4 Cr2O72 + H2O,Polymerisation,In 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 hydrolysis products for all metal ions, one can be established for the dissolved metal ion content. Invariably, polymerization is enhanced at high metal concentrations.,Polymerisation,The following figures represent the speciation of hexavalent chromium at two total dissolved metal concentrations and demonstrates both the pH and metal concentration effects on the extent of formation of the polymer.,Polymerisation,In contrast to the formation of mononuclear hydrolysis products, which require the simple ejection of a proton from a water of hydration, the kinetics of polymerization reactions are typically slow. This is because, for polymers to form, two or more monomers must encounter each other in solution and then react. Reaction rates are thus bound to be slow, similar to rates observed for oxidation/reduction reactions that we will discuss later.,Polymerisation,天然水组成的分类,天然水组成可按溶质颗粒大小、化合物类型、相对浓度及相态等分类,按溶质颗粒大小 真溶液 胶体 悬浮液 按化合物类型 无机物 有机物 无机络合物及有机络合物,按相态 固相 液相 气相 按相对浓度 宏量组分 中量组分 微量组分,大气降水的一般特征,大气降水是含杂质较少、矿化度较小的软水，其含盐量一般为20-50 mg/L。 干旱地区的雨水杂质较多，潮湿地区雨水中杂质较少。 滨海地区降水中的Na+ 和Cl-含量较高，而内地降水中的Na+ 和Cl-含量较低。 初降雨水杂质较多，而长期降雨后的雨水杂质较少。,大气降水中的气体组分,大气降水中一般溶有较多的CO2，因此酸性较强，具有强烈破坏岩石的能力。含CO2气体的降水入渗地下后，可发生一系列化学反应，在改变水自身的溶解组分的同时，也强烈地改变着所流经的岩土体的形状，并形成不同类型的次生矿物。 大气降水的溶解氧含量很高。 大气降水中还溶有一定量的惰性气体，它们可随水一直下渗到深处，基本上不改变原来状态，因此可用惰性气体作为判别地下水补给来源与运动途径的指标。,大气降水中的其他化学特征,大气降水的pH值一般为5.5-7.0左右； 大气降水的主要盐类成分为HCO3-，CO32-，SO42-，Cl-，Ca2+，Mg2+，Na+，K+； 大气降水中二氧化硅含量很小，一般不超过0.5 mg/L；,海水的化学特征,海水的无机组分 宏量组分（含量10mg/L）：按含量由大到小的顺序依次为Cl-，Na+，SO42-，Mg2+， Ca2+，K+，HCO3-，Br-； 中量组分（含量在0.1mg/L-10mg/L）：包括Sr，SiO2，B，F，NO3-，Li，Rb等； 微量组分（含量0.1mg/L）：包括I，Ba，Mo，Zn，Ni，As等。 海水的有机组分 海水中有机物的来源为海生生物所释放的碳水化合物、蛋白质等，其有机碳含量一般处于0.1mg/L-2.7mg/L之间。,河水的化学特征,河水的无机组分 主要离子含量的大小顺序与海水相反：即Ca2+ Na+，HCO3- SO42- Cl-； 总含盐量在100-200 mg/L之间，一般不超过500 mg/L； 基本化学组成与河水流经地区的岩土类型直接相关，如石灰岩区的河水中富含Ca2+ 与 HCO3- ，流经含石膏地层的河水SO42-含量较高，等等。 河水的有机组分 河水中的有机组分来源于地面植物腐烂或死亡后的分解物质，总含量一般在10-30 mg/L之间。,湖泊与水库水的化学特征,湖泊水的化学特征 湖泊水的补给源为河流或地下水； 湖泊水的含盐量受补给量与蒸发量大小关系的影响（咸水湖、淡水湖）； 氮、磷营养物质含量很高的污水排入湖泊后，可引起藻类大量繁殖，是湖泊中常见的一种污染，成为富营养化。 水库水的化学特征 水库为人工的淡水湖泊，其水质状态与淡水湖非常相近。,地下水的化学特征,地下水中的Si Si广泛存在于地壳上的各种岩石与矿物中，包括石英、铝硅酸盐、粘土矿物等； 在一般环境条件下，石英的溶解非常缓慢； 硅酸盐和铝硅酸盐（如钾长石、钠长石、钙长石、黑云母、白云母、钠蒙脱石、钙蒙脱石、镁橄榄石等）的不全等溶解可产生硅酸； 不论是石英还是硅酸盐和铝硅酸盐的溶解，水中的Si几乎全部以H4SiO4的形式存在，H3SiO4-相对很少，H2SiO42-则极少。 地下水中SiO2的含量一般在10-30 mg/L之间，一般不超过100 mg/L。,地下水的化学特征,铝硅酸盐与地下水的反应 辉石：CaMg0.7Al0.6Si1.7O6 + 3.4 CO2 + 4H2O = 0.3Al2Si2O5 (OH)4 + Ca2+ + 0.7Mg2+ + 1.1H4SiO4 + 3.4HCO3- 黑云母：KMg3AlSi3O10 (OH)2 + 7CO2+ 7.5H2O = 0.5Al2Si2O5 (OH)4 + K+ + 3Mg2+ + 2H4SiO4 + 7HCO3- 钙长石：CaAl2Si2O8 + 2CO2 + 3H2O = Al2Si2O5 (OH)4 + Ca2+ + 2HCO3- 钠长石：2NaAlSi3O8 + 2CO2 + 3H2O = Al2Si2O5 (OH)4 + 2Na+ + 4SiO2 + 2HCO3-,典型实例：太原盆地孔隙水的硅酸盐矿物体系平衡图,地下水的化学特征,地下水中的Al 地下水中铝的浓度主要受铝的氢氧化物（三水铝石）的溶度积的控制； 2Al3+ + 6 OH- = Al2O33H2O 换言之，地下水中铝的浓度与pH值有关系，酸性条件有利于Al2O3的溶解； 地下水中