土壤化学英文精品文稿.ppt

土壤化学英文课件第1页，本讲稿共63页1.Soil pH 第2页，本讲稿共63页The pH of a soil applies to the H+ion concentration in the solution present in soil pores which is in dynamic equilibrium with the predominantly negatively charged surfaces of the soil particles.土壤土壤PH是指土壤孔隙溶液中的氢离子浓度，它与土壤颗粒以负电荷为主的表是指土壤孔隙溶液中的氢离子浓度，它与土壤颗粒以负电荷为主的表面达到动态平衡。面达到动态平衡。2.2.1 Soil pH 土壤土壤PH第3页，本讲稿共63页Hydrogen ions are strongly attracted to the surface negative charges,and they have the power to replace most other cations.氢离子被吸附在表面负电荷氢离子被吸附在表面负电荷,能够置换大多数阳离子能够置换大多数阳离子.The diffuse layer close to a negatively charged surface has a higher concentration of H+ions than the bulk soil solution.接近负电荷层的扩散层具有比土壤溶液更高的氢离子浓度接近负电荷层的扩散层具有比土壤溶液更高的氢离子浓度.When the soil solution is diluted,the diffuse layer expands,causing the pH of the bulk solution to increase.当土壤溶液被稀释时当土壤溶液被稀释时,扩散层会扩大扩散层会扩大,导致土壤溶液的导致土壤溶液的PH升高。升高。第4页，本讲稿共63页This has important implications for the measurement of soil pH in the laboratory.This normally involves mixing dry soil with 2-2.5 times its weight of water,shaking and then measuring the pH in the supernatant solution after 30 minutes.The pH value obtained is about 1 to 1.5 units higher than that of the soil solution near to the solid surfaces where the reactions take place.第5页，本讲稿共63页This dilution effect is usually overcome by measuring the pH in a suspension of soil in a solution of a neutral salt,such as CaCl2 or KCl.Normally,pH is measured in suspensions of soil with distilled water and with a dilute salt.It is usually assumed that the pH value given is for distilled water unless otherwise stated.第6页，本讲稿共63页Soil pH is affected by the changes in redox potential which occur in soils that become waterlogged periodically.Reducing conditions generally cause a pH increase,and oxidation brings about a decrease.Variations of up to 2 units can occur over a year in gley soils prone to waterlogging.Oxidation of pyrite(FeS2)in a soil parent material can cause a marked decrease in pH.第7页，本讲稿共63页Soils have several mechanisms which serve to buffer pH to varying extents,including hydroxy aluminium ions,CO2,carbonatescarbonates and cation exchange reactions.However,even with these buffering mechanisms,soil pH differs significantly due to localized variations within the soil.Diurnal fluctuations（日变化）日变化）of more than 1 unit may occur,as well as variations in different parts of a field.第8页，本讲稿共63页Soil pH usually increases with depth in humid regions where bases are leached down the profile,and can decrease with depth in arid environments where evaporation causes salts to accumulate in the surface horizon.As a result of the variations which can occur,it is not practicable to express soil pH measurements more accurately than to the nearest 0.2 division of a unit.第9页，本讲稿共63页In general,heavy metal cations are most mobile under acid conditions,and increasing the pH by liming usually reduces their bio-availability.However,molybdate anions become more available with increasing pH.Soils generally have pH values within the range 4-8.5,owing to the buffering by Al at the lower end and by CaCO3 at the upper end of the range.第10页，本讲稿共63页Brady states that the normal pH is 5-7 in soils of humid regions,and pH 7-9 in the soils of arid regions.However,the maximum range of pH conditions found in soils is 2-10.5.In a typical temperate environment,such as the UK,soils normally have a pH in the range 4-8.The optimum pH for most arable crops is 6.5 on mineral soils and 5.5 on peaty soils.Soil pH can be raised by liming.第11页，本讲稿共63页2 Soil organic matter第12页，本讲稿共63页The main feature which distinguishes soil from regolith(decomposed rock 风化层风化层,土被土被)is the presence of living organisms,organic debris and humus.All soils contain organic matter,although the amount and type may vary considerablyconsiderably.Colloidal soil organic matter has a major influence on the chemical properties of soils,and can be divided into non-humic and humic substances.The non-humic substances comprise unaltered biochemicals such as amino acids,carbohydrates,organic acids,fats and waxes that have not changed from the form in which they were synthesised by living organisms.第13页，本讲稿共63页Humic substances are a series of acidic,yellow to black coloured polyelectrolytes（高分子电解质高分子电解质）of moderately high molecular weight.They are formed by secondary synthesis reactions involving microorganisms and have characteristics which are dissimilar to any compounds in living organisms.They have a wide variety of functional groups,including carboxyl(羧基羧基),phenolic hydroxyl(酚式羟基酚式羟基),carbonyl(羰基羰基),ester and possibly quinone(醌醌)and methoxy groups(甲氧基甲氧基)5,6.While predominantly composed of humic substances,soil humus also contains some biochemicals bound to the humic polymers.The elemental composition of humus is typically(on an ash-free basis):44-53%C,3.6-5.4%H,1.8-3.6%N and 40.2-47%O 7.第14页，本讲稿共63页Traditionally,humus has been separated in the laboratory into three fractions:humin,which is insoluble in alkali,humic acid,which is soluble in alkali and insoluble in acid and fulvic acid,which is soluble in both acid and alkali.These substances cannot be regarded as distinctly different,but merely as part of a continuum of compounds varying in molecular weight,C content,O content,acidity and cation exchange capacity(CEC)in the order:humin humic acid fulvic acid,with N content decreasing through the same sequence 6.第15页，本讲稿共63页Less than half the C in humic acids is aromatic;much of the remainder occurs as unsaturated aliphatic chains containing carboxyl groups which also contribute most of the titratable acidity.Humic acids have molecular weights in the range 20000-100000,and the fulvic acid fraction generally consists of lower-molecular-weight compounds.Apart from containing up to 10%polysaccharides,the general composition of fulvic acid is similar to that of humic acid.The fulvic acid fraction may contain precursors and degradation products of the humic acid fraction.Humins are considered to be humic acid-type compounds adsorbed onto minerals 7.第16页，本讲稿共63页Methods used to determine the organic matter content of soils include（1）either the percentage loss in weight after ignition in a furnace at 375 oC for 16h.(2)or the oxidation of C by acid dichromate followed by the titration of excess dichromate 8.Organic matter contains about 58-60%organic C(%orgC x 1.67=%OM).Within the soil profile,the organic matter content is always highest in the surface horizon,but podzols (灰化土灰化土)and vertisols(变性土变性土)may have some trans-located humic material lower down the profile.2008.10-9第17页，本讲稿共63页3 Clay minerals第18页，本讲稿共63页2.2.3 Clay minerals2.2.3 Clay mineralsClay minerals are products of rock weathering and have marked effects on both the physical and chemical properties of the soils.Their contribution to soil chemical properties results from their comparatively large surface area and permanent surface negative charge.The soil textural class is dependent on the percentages of clay,silt and sand-sized particles.第19页，本讲稿共63页The clay fraction comprises the dispersed minerals of diameter less than 2 um(0.002 mm).In most cases,this specifically applies to the mineralogically distinct group of clay minerals,although it can also include finely ground particles of other minerals.Clay minerals are sheet silicate 硅酸盐硅酸盐(or phyllosilicate页硅酸盐页硅酸盐)structures formed most commonly from combinations of Al-OH octahedra 八八面体面体(gibbsite sheet水铝片水铝片)and Si-O tetrahedra四面四面体体(silica sheet硅片硅片);第20页，本讲稿共63页in addition,a brucite sheet(镁片镁片),comprising Mg and Fe partially substituted for Al in the gibbsite sheet,is found in a lot of minerals.The most common types of clay minerals include:the kaolinites(高岭石高岭石)comprising one silica sheet and one gibbsite sheet(水铝片水铝片)with a Si:Al ratio of 1:1;the illites(伊利石伊利石)comprising two silica sheets and one gibbsite sheet with a Si:Al ratio of 2:1;and the smectites(also called montmorillonites；蒙脱石；蒙脱石)with two silica sheets and one gibbsite sheet,and an Si:Al ratio of 2:1.第21页，本讲稿共63页In all the minerals except kaolinites,isomorphous substitution within the mineral lattice（晶格）（晶格）gives rise to a net negative charge on the surface of the mineral.This is caused by trivalent Al substituting for tetravalent Si and divalent Mg substituting for Al.In kaolinite the 1:1 units are tightly bonded together by hydrogen bonds between hydrogen and oxygen atoms of adjacent lamellae(薄片薄片).第22页，本讲稿共63页Smectites(蒙脱石蒙脱石)have the largest specific surfaces(700-800 m2/g)owing to relatively weak interlayer(夹层夹层)bonding allowing soil solution to penetrate between the units.The CEC is also quite high(80-120 cmol/kg)as a result of this large specific surface.Vermiculites(蛭石蛭石)have an intermediate specific surface(300-500m2/g)and a high CEC(100-150 cmol/kg)7.第23页，本讲稿共63页The specific surface of these minerals is relatively small(5-100 m2/g)in comparison with the other clay minerals,and their cation exchange capacity is low(3-20 cmol/kg)because very little isomorphous substitution has occurred.Illite minerals have the 2:1 units bonded by K ions,and the specific surface and CEC are larger than those of kaolinite minerals(100 200 m2/g and 10-40 cmol/kg).第24页，本讲稿共63页 Clays rarely exist in pure form in soils;they usually have humic colloids and hydrous oxide precipitate（沉淀物）（沉淀物）linked to them.The combined organo-mineral colloidal complex plays a very important role in controlling the concentrations of ions in the soil solution.第25页，本讲稿共63页4 Oxidation and reduction in soils第26页，本讲稿共63页2.2.4 Oxidation and reduction in soilsSoils are subject to variations in oxidation reduction(redox)status and this mainly affects the elements C,N,O,S,Fe and Mn,although Ag,As,Cr,Cu,Hg and Pb can also be affected 13.Redox equilibria are controlled by the aqueous free-electron activity,which can be expressed as either the pE value(the negative log of the electron activity)or Eh(the milli volt difference in potential between a Pt electrode and the standard H electrode)14.The pE unit has the advantage of allowing electrons to be treated like other reactants or products,allowing both chemical and electrochemical equilibria to be expressed with the single equilibrium constant.The conversion factor for the units is Eh(mV)=59.2 pE 14.第27页，本讲稿共63页Large positive values of pE(or Eh)favor the existence of oxidised species,and low or negative values of pE(or Eh)are associated with reduced species 15.Redox potential(Eh)is measured using a Pt electrode and a calomelelectrode（甘汞）（甘汞）connected to a millivolt meter,but it is difficult to get accurate readings.However,oxydic soil conditions usually give values in the range+300+800mV(pE 5.1-13.5)but mostly from+400+600mv(pE 6.8-10.1).Anaerobic(厌氧的厌氧的)soils have values from+118-414mv(pE+2-7)15,16.第28页，本讲稿共63页Measurements of Eh can be used to determine whether oxidising or reducing conditions exist,but frequently soil colours provide a good indication of redox status.Red and brown colours indicate oxic conditions,and blue-green and grey colours indicate gleyed soils（潜育土）（潜育土）.However,strong-coloured parent materials may mask these colour changes in some soils.Redox reactions in soils are frequently slow but are catalysed by soil microorganisms which are able to live over the full range of pH and pE conditions normally found in soils(pE+12.7 to-6.0 and pH 3-10)15.08/04/1第29页，本讲稿共63页Respiration by microorganisms,mesofauna（土栖（土栖中型动物区系中型动物区系 100um2mm）and plant roots consumes a relatively large amount of oxygen.If the oxygen in a zone of soil becomes exhausted,as happens with waterlogging（渍水）（渍水）or compaction（压实）（压实）,microorganisms with an aerobic respiration predominate and susceptible（活性的）（活性的）elements(Mn,Cr,Hg,Fe,Cu and Mo)are gradually reduced 15,16.第30页，本讲稿共63页When Fe(III)is reduced to Fe(Il)there is a slight pH increase in acid soils and a slight decrease in alkaline soils.Most waterlogged soils have a pH of 6.7-7.2 16.When the pE begins to rise,Fe(II)is oxidised in preference to Mn(II).Oxides of both Fe and Mn can be dissolved by either decreasing pH or Eh,but Mn oxides are more easily dissolved than Fe oxides.With increasing Eh and pH,Fe oxides precipitate before those of Mn.Small changes in Eh or pH can give rise to either extensive dissolution or precipitation of Fe oxides 18.第31页，本讲稿共63页 Fe and Mn oxides occur as(i)coatings on soil particles where they are intimately mixed with clays,(ii)fillings in voids(空隙空隙)and(iii)concentric nodules.Precipitation of Fe is initially in the form of gelatinous（凝胶（凝胶状的）状的）hydrated Fe oxide(Ferrihydrite,Fe5(O4H3)3水铁矿水铁矿)which gradually dehydrates with ageing to more stable forms,such as goethite针铁矿针铁矿2,18.Ferrihydrite co-precipitates other ions,including Cu,Mn,Mo,Ni,V(钒钒)and Zn,and acts as a scavenger（净化剂）（净化剂）adsorbing both heavy metal cations and anions,such as HPO42+and AsO43-18.第32页，本讲稿共63页These and other heavy metals are affected by the indirect effects of reducing conditions.Sulphate（硫酸盐）（硫酸盐）ions are reduced to sulphide（硫化物）（硫化物）below pE-2.0,which can lead to the precipitation of metal sulphides,such as FeS2,HgS,CdS,CuS,MnS and ZnS 15.When reducing conditions cause the dissolution of hydrous Mn and Fe oxides,their co-precipitated metals are released into the soil solution13.Specialised bacteria,such as Thiobacillus ferrooxidans and Metallo-genium spp.are also involved in the precipitation of Fe and Mn hydrous oxides,respectively 16.April 8,2008第33页，本讲稿共63页Apart from ferrihydrite,the more stable mineral forms of Fe oxide in soils include:goethite(a-FeOOH，针铁矿，针铁矿)which is the most common Fe oxide in temperate,subtropical and tropical soils,haematite(a-Fe2O3，赤铁矿，赤铁矿)which is found mainly in tropical soils第34页，本讲稿共63页The mineral forms of hydrous Mn oxides in soils are:birnessite水钠锰矿水钠锰矿,hollandite锰钡矿锰钡矿,lithiophorite锂硬锰矿锂硬锰矿19.第35页，本讲稿共63页2.3 The adsorption of metal ions by soils and their constituentsThe most important chemical processes affecting the behaviour and bio-availability of metals in soils are those concerned with the adsorption of metals from the liquid phase on to the solid phase.第36页，本讲稿共63页These processes control the concentrations of metal ions and complexes in the soil solution and thus exert a major influence on their uptake by plant roots.Several different mechanisms can be involved in the adsorption of metal ions,including cation exchange(or non-specific adsorption),specific adsorption,organic complexation and co-precipitation.第37页，本讲稿共63页However,although the extent of adsorption can be measured and isotherms（等温线）（等温线）calculated it is frequently difficult to be precise about which particular process is responsible for the retention（保持力）（保持力）of metals in any particular soil.In order to provide an introduction to the adsorption mechanisms referred to in the following chapters,the different adsorption mechanisms will be discussed briefly.第38页，本讲稿共63页2.3.1 Cation exchangeMost heavy metals(with certain exceptions,including the metalloids As,Sb 锑锑and Se and the metals Mo and V钒钒)exist mainly as cations in the soil solutio