ASTM_D_4319_-_93_2001.pdf

Designation:D 4319 93(Reapproved 2001)Standard Test Method forDistribution Ratios by the Short-Term Batch Method1This standard is issued under the fixed designation D 4319;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.Asuperscript epsilon(e)indicates an editorial change since the last revision or reapproval.INTRODUCTIONAs an aqueous fluid migrates through geologic media,certain reactions occur that are dependentupon the chemistry of the fluid itself and upon the chemistry and geochemistry of other fluids and solidphases with which it comes in contact.These geochemical interactions determine the relative rates atwhich chemical species in the migrating fluid(such as ions)travel with respect to the advancing frontof water.Processes of potential importance in retarding the flow of chemical species in the migratingfluid(movement of species at velocities less than the ground-water velocity)include ion exchange,adsorption,complex formation,precipitation(or coprecipitation,for example Ba+and Ra+co-precipitating as the sulfate),oxidation-reduction reactions,and precipitate filtration.This testmethod applies to situations in which only sorptive processes(adsorption and ion exchange)areoperable for the species of interest,however,and is restricted to granular porous media.It is difficult to derive generalized equations to depict ion exchange-adsorption reactions in thegeological environment.Instead,a parameter known as the distribution coeffcient(Kd)has been usedto quantify certain of these sorption reactions for the purpose of modeling(usually,but not solely,applied to ionic species).The distribution coefficient is used to assess the degree to which a chemicalspecies will be removed from solution as the fluid migrates through the geologic media;that is,thedistribution coefficient provides an indication of how rapidly an ion can move relative to the rate ofground-water movement under the geochemical conditions tested.This test method is for the laboratory determination of the distribution ratio(Rd),which may beused by qualified experts for estimating the value of the distribution coefficient for given undergroundgeochemical conditions based on a knowledge and understanding of important site-specific factors.Itis beyond the scope of this test method to define the expert qualifications required,or to justify theapplication of laboratory data for modeling or predictive purposes.Rather,this test method isconsidered as simply a measurement technique for determining the distribution ratio or degree ofpartitioning between liquid and solid,under a certain set of laboratory conditions,for the species ofinterest.Justification for the distribution coefficient concept is generally acknowledged to be based onexpediency in modeling-averaging the effects of attenuation reactions.In reference to partitioning insoils,equilibrium is assumed although it is known that this may not be a valid assumption in manycases.Equilibrium implies that(1)a reaction can be described by an equation and the free energychange of the reaction,within a specific system,is zero,and(2)any change in the equilibriumconditions(T,P,concentration,etc.)will result in immediate reaction toward equilibrium(the conceptis based upon reversibility of reactions).Measured partitioning factors may include adsorption,coprecipitation,and filtration processes that cannot be described easily by equations and,furthermore,these solute removal mechanisms may not instantaneously respond to changes in prevailingconditions.Validity of the distribution coefficient concept for a given set of geochemical conditionsshould not be assumed initially,but rather should be determined for each situation.This is a short-term test and the attainment of equilibrium in this laboratory test is not presumed,although this may be so for certain systems(for example,strictly interlayer ion exchange reactions ofclays).Consistent with general usage,the result of this test could be referred to as“distributioncoefficient”or as“distribution ratio;”in the strictest sense,however,the term“distribution ratio”ispreferable in that the attainment of equilibrium is not implied.1Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959,United States.The distribution ratio(Rd)for a specific chemical species may be defined as the ratio of the masssorbed onto a solid phase to the mass remaining in solution,which can be expressed as:Rd5mass of solute on the solid phase per unit mass of solid phase!mass of solute in solution per unit volume of the liquid phase!(1)The usual units of Rdare mL/g(obtained by dividing g solute/g solid by g solute/mL solution,usingconcentrations obtained in accordance with this test method).Major difficulties exist in the interpretation,application,and meaning of laboratory-determineddistribution ratio