土壤和沉积物中总有机碳（TOC）的测定方法.pdf

NCEA-C-1282 EMASC-001 April 2002 METHODS FOR THE DETERMINATION OF TOTAL ORGANIC CARBON(TOC)IN SOILS AND SEDIMENTS Brian A.Schumacher,Ph.D.United States Environmental Protection Agency Environmental Sciences Division National Exposure Research Laboratory P.O.Box 93478 Las Vegas,NV 89193-3478 Ecological Risk Assessment Support Center Office of Research and Development US.Environmental Protection Agency ACKNOWLEDGMENTS This document was prepared by Brian Schumacher of EPA*s National Exposure Research Laboratory,Office of Research and Development(ORD)in response to a request received by ORD*s Ecological Risk Assessment Support Center.Peer review of the document was conducted by Peter Landrum of the National Oceanographic and Atmospheric Administration,and Robert Ozretich of ORD*s National Health and Environmental Effects Research Laboratory.Bruce Duncan,EPA Region 10,and Sharon Thorns,EPA Region 4,provided helpful insights and comments during the preparation of the document.Programmatic review of the document was conducted by the Trichairs of EPA*s Ecological Risk Assessment Forum:Steve Wharton,EPA Region 8;Bethany Grohs,Office of Solid Waste and Emergency Response,Office of Emergency and Remedial Response(OSWER/OERR);and Brenda Jones,EPA Region 5.Finally,we would like to acknowledge the efforts of Mark Sprenger,EPA/OSWER/OERR,in initiating the original request.ii 1.INTRODUCTION Organic matter in soils and sediments is widely distributed over the earth*s surface occurring in almost all terrestrial and aquatic environments(Schnitzer,1978).Soils and sediments contain a large variety of organic materials ranging from simple sugars and carbohydrates to the more complex proteins,fats,waxes,and organic acids.Important characteristics of the organic matter include their ability to:form water-soluble and water insoluble complexes with metal ions and hydrous oxides;interact with clay minerals and bind particles together;sorb and desorb both naturally-occurring and anthropogenically-introduced organic compounds;absorb and release plant nutrients;and hold water in the soil environment.As a result of these characteristics,the determination of total organic carbon(a measure of one of the chemical components of organic matter that is often used as an indicator of its presence in a soil or sediment)is an essential part of any site characterization since its presence or absence can markedly influence how chemicals will react in the soil or sediment.Soil and sediment total organic carbon(TOC)determinations are typically requested with contaminant analyses as part of an ecological risk assessment data package.TOC contents may be used qualitatively to assess the nature of the sampling location(e.g.,was a depositional area)or may be used to normalize portions of the analytical chemistry data set(e.g.,equilibrium partitioning).The purpose of this document,as defined by the Ecological Risk Assessment Forum,is to answer the question of“What is the most appropriate method for soil and/or sediment TOC analyses and what factors should be considered when selecting the method?”2.SOURCES AND FORMS OF CARBON IN SOILS AND SEDIMENTS In soils and sediments,there are three basic forms of carbon that may be present.They are:(1)elemental C,(2)inorganic C,and(3)organic C.The quality of organic matter in sediments is critical to the partitioning and bioavailability of sediment-associated contaminants.For example,Talley et al.,2002,demonstrated that although the majority of PAHs in a dredged sediment were found prefer-entially on coal-derived particles,the PAHs on the clay/silt sediment fraction were more mobile and available,and thus potentially of greater concern.2.1.ELEMENTAL CARBON FORMSElemental carbon forms include charcoal,soot,graphite,and coal.The primary sources for elemental carbon in soils and sediments are as incomplete combustion products of organic matter(i.e.,charcoal,graphite,and soot),from geologic sources(i.e.,graphite and coal),or dispersion of the these carbon forms during mining,processing,or combustion of these materials.1 2.2.INORGANIC CARBON FORMS Inorganic carbon forms are derived from geologic or soil parent material sources.Inorganic carbon forms are present in soils and sediments typically as carbonates.The two most common carbonate minerals found in soils and sediments are calcite(CaCO3)and dolomite CaMg(CO3)2 although other forms may be present(e.g.,siderite,FeCO3)depending on where the soils were formed or where the sediment source was located.It should be noted that calcite and to some extent,dolomite,may also be present in soils and sediments due to agricultural input(i.e.,liming practices).2.3.ORGANIC CARBON FORMS Naturally-occurring organic carbon forms are derived from the decomposition of plants and animals.In soils and sediments,a wide variety of organic carbon forms are present and range from freshly deposited litter(e.g.,leaves,twigs,branches)to highly decomposed forms such as humus.In addition to the naturally-occurring organic carbon sources are sources that are derived as a result of contamination through anthropogenic activities.The spills or releases of con-taminants into the environment increase the total carbon content present in the soil or sediment.In general,though,the total carbon contribution from contaminants(typically measured in the:g/kg to mg/kg concentration range)to the total organic carbon content(measured in the%range)of the soil or sediment is relatively small to negligible unless a fresh spill has occurred,pure product is present,or a hot spot is sampled.In contrast to spilled contaminants,various sites may contain discrete organic carbon bearing particles such as wood fibers from pulp mill wastes or leather scraps from tannery wastes.At these locations,the total carbon content contribution of these wastes may be a significant to dominant fraction of the TOC determined for the sample.It should be noted that the methods for determining total organic carbon and total carbon contents generally do not distinguish between the sources of the organic carbon forms.Nonetheless,there arc two methods noted below that are capable of qualitatively identifying carbon forms in the soil/sediment,and two methods that analyze specific fractions of the TOC.3.SAMPLE COLLECTION AND HANDLINGPrior to any analyses for TOC,the soil or sediment sample must be collected and properly handled.During the collection and handling of the samples,losses of organic compounds may occur due to:microbial degradation,sample drying,oxidation,volatilization,and sample processing biases(e.g.,selective removal of carbon-bearing components).Soil and sediment samples can be collected by numerous different tools but once collected,the samples are 2 typically stored at 4?C and have a holding time of up to 28 days.While microbial degradation is greatly reduced at 4?C,it is not completely stopped leading to some potential loss of organic materials.Prior to analysis,some methods may require or recommend drying(tither air drying or oven drying)of the sample.Samples that contain volatile organic compounds and those that have been in an anaerobic environment will undergo some loss of organic compounds when exposed to the atmosphere during drying.Volatilization losses may also occur due to poor sealing of the sample container.While these losses are generally small(probably 10%calcium carbonate equivalent),then the H2SO4 strength should be increased to 3N or 4N.Removal of carbonates for samples undergoing dry combustion quantitation is more problematic than for those undergoing wet chemistry techniques(Nelson and Sommers,1996).HCl may be used in a manner similar to that used for the wet chemistry techniques but the samples must be at least air-dried prior to analysis and there is the concern about the loss of organic matter due to its decomposition by the HCI.Alternatively,Bremner(1949)proposed using a mild H2SO3 solution to remove the carbonates but Allison(1965)found it difficult to tell whether the treatment was complete especially if dolomite was present in the sample.It should be noted that the removal of carbonates for samples undergoing dry combustion is not necessary if TOC is going to be determined by difference(see the Analytical Methods for TOC Determination section).Other interferents for the wet chemistry techniques include Fe 2+and Cr which lead to positive errors(i.e.,overestimation)in TOC determinations,and MnO2 which leads to a negative error(i.e.,underestimation)of TOC contents(Schumacher et al.,1995).In a routine sample,both Fe 2+and organic matter are oxidized in the dichromate digestion solution(to be discussed)leading to a positive sample bias in TOC content.Fe 2+may be removed from the sample by oxidation(i.e.,air-drying)at the risk of loss of any volatile organic compounds present in the sample.Excess Cl-in a sample interferes through the formation of chromyl chloride(CrO2Cl2).The consumption of the dichromate ions leads to a positive bias in TOC content.Excessive Cl-may be removed by leaching the sample or through precipitation,as AgCl,by adding AgSO2 to the H2S04 used during the digestion process.In contrast,MnO2 in the sample will actively compete with the dichromate for any oxidizable carbon and thus,lead to negative bias during subsequent quantitation.If large quantities of MnO2 are present,pretreatment of the sample with FeSO4 will remove this interference(Walkley,1947;Jackson,1958).The removal of water from the sample is essential during the determination of TOC by the dry combustion methods.While water passing through the system is removed from the gas flow by a sorbent,this sorbent is generally placed in the system to handle the removal of water created during the combustion process.The free water associated with the sample prior to combustion can be removed by the system sorbent but this will result in the more frequent replacement of the sorbent leading to instrument downtime(for sorbent replacement and gas flow line purging),excessive sorbent chemical loss,and unnecessary expense.Excessive water can be simply removed by air drying or oven-drying the sample at 105?C overnight.One concern about drying the samples prior to analysis is the loss of volatile organic compounds.For the wet oxidation methods,water removal is not essential but the analyst will have to determine the moisture content on a separate subsample to accurately correct the resultant data to an oven-dried basis.9 4.4.2.Wet Chemistry Techniques for the Determination of Total Organic Carbon.Wet chemistry techniques can be divided into two phases,namely,sample extraction and sample quantitation.The extraction technique employed is essentially the same for all methods in the literature with variations existing only in the strength and combination of reagents used during extraction.Quantitation techniques associated with the wet chemistry determination of TOC either rely on titration(manual or automated),calorimetric,gravimetric,or manometric techniques.4.4.2.1.Sample Extraction-The standard wet chemistry technique for the sample extraction involves the rapid dichromate oxidation of organic matter.Perhaps the best known of the rapid dichromate oxidation methods is the Walkley-Black procedure which has been the“reference”method for comparison to other methods in numerous studies.In this procedure,potassium dichromate(K2Cr2O2)and concentrated H2SO4 are added to between 0.5 g and 1.O g(although the range may be up to 1 Og depending on organic carbon content)of soil or sediment.The solution is swirled and allowed to cool(note:the sample must be cooled as a result of the exothermic reaction when the potassium dichromate and sulfuric acids are mixed)prior to adding water to halt the reaction.The addition of H3PO4 to the digestive mix after the sample has cooled has been used to help eliminate interferences from the ferric(Fe 3+)iron that may be present in the sample although in most cases,this step is not necessary(Tiessen and Moir,1993).The chemistry of this extraction procedure is as follows:2Cr2O7 2-+3 C0+16H+=4Cr 3+3CO2+8H20.(3)It should be noted that the concentrations of the reactants and volumes of solutions are not presented in this text due to the numerous variants available in the published literature,all of which are based upon the Walkely-Black procedure.The Walkley-Black procedure is widely used because it is simple,rapid,and has minimal equipment needs(Nelson and Sommers,1996).However,this procedure has been shown to lead to the incomplete oxidation of organic C and is particularly poor for digesting elemental C forms.Studies have shown that the recovery of organic C using the Walkley-Black procedure range from 60 to 86%with a mean recovery being 76%(Walkley and Black,1934).As a result of the incomplete oxidation and in the absence of a site-specific correction factor,a correction factor of 1.33 is commonly applied to the results to adjust the organic C recovery.To overcome the concern of incomplete digestion of the organic matter,the Walkley-Black procedure was modified to include extensive heating of the sample during sample digestion(Mebius,1960).In this variation of the method,the sample and extraction solutions are gently boiled at 150?C for 30 minutes,allowed to cool,and then water is added to halt the reaction.The addition of heat to the system leads to a complete digestion of the organic C in the sample;therefore,no correction factor is needed.The temperature of this method must be 10 strictly controlled because the acid dichromate solution decomposes at temperatures above 150?C(Charles and Simmons,1986).4.4.2.2.Sample Quantitation Upon completion of the sample extraction phase,the quantity of organic carbon present in the soil or sediment can be determined through a variety of different techniques.These techniques include:manual titration,automated titration using potentiometric determination,calorimetry,gravimetric determination,or volumetric/manometric measurement.Each of these techniques will be briefly discussed.Upon examination of Equation 3,the three measurable products of the acid dichromate 2-digestion process are the excess/unused Cr2O7 Cr3+and CO2-Both the Cr2O72-and Cr3+will remain in solution and can be measured titrimetrically or calorimetrically while the evolved CO2,in its gaseous state,can be measured gravimetrically or manometrically.-To perform manual titrimetric quantitation,an indicator solution is added to the digestate.The most common indicators used are ortho-phenanthroline ferrous complex(commercially available as“Ferroin”),barium diphenylamine sulfonate,and N-phenylanlhranilic acid(Nelson and Sommers,1996).The excess Cr2O72-is titrated with ferrous ammonium sulfate Fe(NH4)2(SO4)2*6H2O or ferrous sulfate(FeSO4 until color change occurs in the sample.Color changes associated with these indicators are:(1)green to reddish brown for the orthophenanthroline ferrous complex,(2)purple/blue to green for the barium diph