In order to pursue the sodic soil reclamation research. it was necessary to develop a
pretreatment method for sodic soil extracts that would simultaneously remove the
HCO ion and suspended organic matter interferences without adding additional
interferences

Robbins, C.W.

USDA - ARS - NWISRL

A— 9C0. -EmAvotencePot,dN'C 15 20 25 30	 35 4(rir-50mL of 25 moim, J.1,50. 10.050 fneoL -1 IB	6 0 —	 — 0_	5 5 —	 —. 9C 5—Oti-	7 C S —	 —HC07- 	 —7Formate	05' 4 —	 —6 'at-tt3s30— 02 5—5	 .0 15 20 25 30 35 40 45 50mL of 25 moim- 5 4 2 50 4 10.050 meqL - ')Figure la and b Formate results and pH of solutions containing CO and HCO, asaffected incremental additions of 1-1,,S0, to samples that contained no formate andextracts that contained 3.8 mM formateEnvironmentalLaboratory10/92Suspended organic matter removal from aqueousextracts of high pH soilsBy Charles W. RobbinsIn order to pursue the sodic soil reclamation research. it was necessar y to develop apretreatment method ‘for sodic soil extracts that would simultaneousl • remove theHCO ion and suspended organic matter interferences without adding additionalinterferences.Organic matter and clay miner-als in soils have negativel ychar ged surfaces that act much likecation exchan ge resins. When morethan l5 c.r of the exchange sites arcneutralized b y Na' and the finalionic strength of the aqueousextract is less than about 0.04 Al.the clay s and organic matter are dis-persed. Under these conditions. thesolution pH is usually greater than8.4. and carbonate (CO;-) andbicarbonate (HCO.- ) are present inthe extracts. The dispersed or g anicmatter stays in suspension (up to0.2(.7( bv weight) and causes theextracts to he dark colored. 1 Theseunusuall y high-pH. hi g h-sodiumsoils are desi g nated as sodic orsodic soils. Ion chromatograph y(IC) has been adapted for anionanal y sis in soil extracts by Nietoand Frankenber ger and b y Skarie etThe soils used in those studieshad pH values below 5.2. Conse-l)r. Charles 	 it. Robbins i ∎ a .nn 01 .Srtcno n :k:o-frulturtii 11'0\0ot-tit Serrtie. L'.3. 1)(1011-11C111 of	 tkitni)ertv. 1(1(110'.I ile 1(11111's	 (71111 111111 -CA 11.116	 (II 1111A (1171(ivcre Hell rniltitNliet1 in ir(titniv.s.nrettartinon for tletermtntno ion ∎ m dark,\MI(	 nil evtrtict.	 Soil Sei. So,\m. j,	 -21-725 t1959/. Tire intortnotow:0 fill y	 	 U	 w,IA taken from oar! Of Ille∎ Wore ‘1111( Ifquently. the soil extracts probabl ∎did not contain the dispersed organ-ic matter usuall y associated withhigh-sodium soil extracts..A stud y of croppin g and amend-ment effects on sodic soil reclama-tion rates and mechanisms requiredanalysis of a large number of dark-colored. hi g h-or g anic-matterextracts.' Formate and acetate wereproduced in the soil as a result ofsome of the treatments. These twoions may be an important factor inreclaiming sodic soils, even thoughthe y have not previousl y been ana-l y zed. While anal y zing the hi gh pHextracts for formate ion b y IC. itwas found that formate and HCO:ions elute at the same time. Sus-pended organic matter in theextracts plugged and. in some cases.permanently damaged the ICexchange columns. even thou gh theextracts had been filtered throu g h0.2-um nylon filters. Commerciall yavailable filters designed to remove8 :) (1'().2ORGANIC MATTER continuedsuspended matter became pluggedbefore sufficient sample was col-lected for IC analysis, or allowedexcessive organic matter throughthe filters, which in turn pluggedthe IC columns.In order to pursue the sodic soilreclamation research, it was neces-sary to develop a pretreatmentmethod for sodic soil extracts thatwould simultaneously remove theHCO 3-- ion and suspended organicmatter interferences without addingadditional interferences. An addi-tional IC requirement is that thefinal samples have a low total ionicstrength.' An ideal pretreatmentprocedure would provide clear sam-ples that could be analyzed by thenew IC methods as well as the tra-ditional methods in use b y man ylaboratories.Materials and methodsThe samples used to test thiscleanup method were selected fromaqueous extracts collected during asodic soil reclamation study.' Thesamples were selected such that allexchangeable sodium percentageswere greater than 15. The sodic soilextract pH values were between 8.5and 10.1. and the extract ionicstrengths were less than 0.04 M. Thecombination of high pH and low .ionic strength in these extracts pro-duced high concentrations of sus-pended organic matter in theextracts. Additionall y . saline-sodicsoil extracts that have pH values lessthan 8.2 and ionic strengths greaterthan 0.04 M were selected for com-parison with the sodic soil extracts.The saline-sodic extracts were clear.and free from suspended organicmatter because of the lower pH andhigher ionic strengths than in thesodic soil extracts. All extracts dis-cussed in this paper were preparedby the standard procedures used forpreparing saturation paste extracts. 6The suspended organic mattercontent in the extracts was measuredby adjusting the extracts to pH 3.0 to3.5 with 1.0 M HC1. followed bymaking the solution 2.5 mM Ai"with 50 mM A1CI„. 7 The extractswere then centrifuged for 5 min at4000 rpm and passed through oven-dried (60 °C overnight) preweighedfilter papers. Each filter paper, plusorganic matter. was then oven dried.reweighed. and the suspended organ-ic matter content determined.Two solution series were madeup to verify the steps required toremove the HCO 3-- interference inthe IC formate analyses. The firstseries consisted of 21 100-mLaliquots of 10 mM Na,C0„. To eachwas added a differentvolume of 25mM H,SO, over the ranae 0 to 60mL (Figure la). The solutions weremade up to a final volume of 200mL. The second series consisted of16 50-mL aliquots of a sodic soilextract (pH 9.41 that contained 3.8mM formate. 4.1 mM CO;-. and 29.3mM HCa;. Different volumes of 25mM H,SO over the range 0 to 50mL (Figure 1h) were added to theextracts. and the y were then made upto 100 mL. The CO, produced bythe reactions between the H,SO, andthe CO;- and HCO 3- was removedby placing the samples and extractsin an ultrasonic water bath at roomtemperature while applyin g a vacu-um to the containers. The pH andformate concentrations were mea-sured and the results were plotted(Figures la andOnce it was determined that theorganic matter could be easily removedfrom the dark extracts, that the HC0 -interference could be removed. and thatHCO; and CO- analyses could be per-formed in the same process. sets ofclear and dark extracts were selected todetermine if this method could be usedto prepare sodic and saline-sodic soilextracts for IC analysis. The clearextracts were from saline-sodic soilsand contained very little organic matter.The sodic soil extracts contained 10 to2000 mg/L-' dark red to black organicmaterial that could be removed by acid-ification, A1 .3+ addition, centrifugation,and filtration. The dark extracts selectedwere too dark to be analyzed by colon-metric or turbidimetric methods andcontained sufficient suspended organicmatter to plug the IC columns. Allextracts were titrated with standardized1.0 M HC1 or 0.5 M H,SO, to deter-mine CO:- (pH 8.4) and FCO 3- (pH4.7) using a 2.0-mL microburet. a pHmeter. and combination electrode.' Theextracts were further acidified to pH 3.0to 3.5.Twelve clear soil extracts that were0 to 6 mM acetate and 0 to 4 mM for-mate and titrated to a pH of 3.0 to 3.5with I-LSO, were each divided into twoequal portions. Half of each extract wastreated with Al.,(S0,),. as describedearlier. while the other was not. Acetateand formate concentrations were deter-mined by injecting a 20-ii sample intoa 2-mM potassium benzoate eluantstream (pH 6.0) pumped through aWaters IC-PACTm anion exchange col-umn (Waters Chromatography Div..Millipore Corp.. Milford. Massachu-setts) at 1.5 mUrnin .A Waters model 430 five-elec-trode flow-through IC conductivitydetector was used to measure theion concentrations. The results wereplotted and calculated by a Waters740 data module intearatingrecorder. Under these conditions.acetate and formate eluted at about3.3 and 4.0 min, respectively.Results from the extracts receivingAl 3+ were compared with thosereceiving no Al' (Table 1).The 12 extracts from the acetate-formate IC analyses. plus six addi-tional clear extracts with higher Crconcentrations that had also beenadjusted to pH 3.0 to 3.5. weredivided in half. and one-half of eachextract was treated with Al,(SO4 ),.Those 24 subsamples were ana-l yzed for by IC using the sameprocedures as used for the acetateand formate anal y sis. Chlorideelutes at about 5.6 min b y thatmethod. Those extracts were alsoanalyzed for CI -- by Ag titration!'An additional 18 dark sodic extractswere pH adjusted. A1 3+ added, cen-trifuged. and the supernatant liquidwas filtered through a 0.2-pm filter.These extracts were analyzed by ICand Ag - titration. When present.NO and NO elute at about 7.6and 10.1 min under these IC condi-tions.Ten clear extracts containing 2to 44 mM S0,2 - and ten darkextracts containing 2 to 12 mMSO- were titrated to pH 3.0 to 3.5with 1 M HC1. Each clear extractwas divided into equal portions.One-half was analyzed turbidimetri-cally9 and AlC1, was added to theother, followed by IC analysis. Thedark extracts were treated with 1.010 :10/9 1Table 1	 Comparison of various methods for analyzing anions in high pH soil extractsMethod X Pretreat. X Method Y Pretreat. Y No.samplesConc.rangeRegression* Corr. coeff.(r)Anions (mM )Acetate(cleanIC None IC Al,(SO4 ), 12 0-6 Y=-0.067+1.005X 0.991Formate(clear)IC None C Al 2 (SO4 )_ 12 0-4 Y=0.072+0.998X 0.992CI (clean A g None Ag Al,(S0,)„ 18 1-130 Y=0.042+0.979X 0.989CI (clear) Ag Al2 (S0,), IC Al,(S0,), 18 1-130 Y=0.121 +1.033X 0.986CI (dark) Ag Al,(SO„), IC Al,(SO4 ), 18 1-8 Y=0.130+1.017X 0.988SO, (clean Turbid. None Turbid. AICI, 10 2-44 Y=-0.371 + 0.992X 0.989SO. (clear) Turbid. IC AICI, 10 2-44 Y=-0.373 + 1.007X 0.983SO. (dark) Turbio. AICI, IC AICI, 10 2-12 Y=-0.107 + 1.003X 0.990'X represents the results of the more conventional method or treatment and Y represents the new method or the new treatment.nth 0.08 mill AICI,. centrifu ged. andfiltered through 0.2-urn filters. Theseextracts were anal yzed turbidimetrical-Iv and by IC. The IC eluant foranalysis was 1.0 potassium phtha-late (pH 6.51. pumped at 1.5 mUmin-through the IC-PAC anion exchangecolumn. Chloride. NO, . NO;. andSO:- elute at about 2.5. 3.2. 4.6. and6.9 min. respectivel y , under these con-ditions. The CI added in the pretreat-ment often caused the CI and NO,-peaks to overlap.No comparisons were madebetween AP' treated and untreateddark extracts b y an y of the anionanal ysis methods because of the inter-ferences mentioned.Results (111(1 ch.SCUSSi011Formate anal y sis h\ IC in thepresence of HCO, resulted in anoverestimation of formate. To veri-l y the nature of this overestimation.two sets of solutions were made up.The first contained 10 mM NaCO T;and the second. a sodic soil extract.contained 4.1 mill CO. 29.3 mMHCO,- . and 3.8 mM formate. Vary-ing amounts of 25 mM H,SO, wereadded to each solution sample andthe pH and formate concentrationswere measured and plotted (Fi g uresla and hi. In both cases. the pHdecreased as acid was added. with asharp pH drop. as the last COT - wasconverted to HCO ,-- (pH 8.41. andagain as the last HCO, was con-verted to H : CO, (pH 4.7). The for-mate results in the first case (Fi gurela) responded to HCO,- concentra-tion. first sli g htl y increasin g as the_pH decreased and the CO T - wasconverted to HCO . and then. asthe HCO: - was converted to H,CO,and CO 2 . the formate resultsdecreased rapidly to zero just priorto the solution pH havin g reachedthe H 2 S0,—HCO,- titration equiv-alence endpoint at pH 4.7. Figurela shows that the HCO, must heremoved prior to formate analysis.A similar but less ideal set ofcurves is produced with the titrationand formate anal y sis of the sodicsoil extract (Figure 11). Once thepH is below about 5.0. the formateresults remain constant withdecreasing pH.Figure 2a shows an IC trace of asodic soil extract prior to acidifica-tion. and Fi g ure 2b shows the sameextract after adjustin g the pH to 3.3.Lowering the pH and convertingthe COi- and HCO; to H,CO, and,	 -CO, removed the COT- peak anddecreased the area of the formate-HCO - peak to that of formate only.without affecting the acetate orresults.Organic matter coagulation fromaqueous solution. particularly pub-lic water supplies. include shockingthe s y stem by chan g in g the pH(raising or lowering). followed b yadding a cation source. usually analuminum salt. and then filteringthe coa gulated material. The mosteffective procedure in each case isdetermined b y the original pH andthe particular organic materialsbein g removed.' The sodic soilextracts selected to test this pre-treatment method contained from20 to 2100 m g /L ' of suspendedorg anic matter. The y were too darkfor colorimetric and turbiciimetricanal y sis. even when diluted. andthey contained sufficient suspendedmaterials to plug the IC exchan g ecolumns.It was determined that for theseextracts. if the pH were adjusted to3.5 or below. at. a the solution madeabout 2.5 mM Al 3+ with AICI, orAl,(SO 4 ),. the suspended materialflocculated and nearly all of thecolor was removed from solution.The flocculated material could becentrifuged and filtered from solu-tion. Above pH 3.5. the Al"- wasnot consistentl y effective. andbelow 3.0. the IC conductivit ydetector did not recover from thewater peak in time to detectacetate. It was also found that thepH had to be adjusted before theAl" addition: otherwise a muchhigher Al' concentration wasrequired to flocculate the or ganicmatter.Linear regression anal y sis wasmade comparin g pH-adjusted.Al'-pretreated clear extractswith pH-adjusted. clear extractswith no Af" added and analyzedfor acetate and formate by IC(Table I. lines 1 and 2). Theregression slopes were very near-ly 1.0: the absolute intercept val-ues for Y were less than 0.08mM. and the correlation coeffi-cients were g reater than 0.99.12 :10 , ()2a)-0r    0 a)CoC.)                                1	 I0 1	 6	 72 3 4	 5Retention time (min)Column: Waters IC-PAC anion column: detectorconductivity: eluant: 2.0 mM benzoate: flow rate: 1.5sample volume: 20 uL: sample: sodic soilextract: pli=3.3: CO,: 4.8 ma HCO,: 4.3 mM:acetate: 2.1 mkt formate: 3.8 rnAt. CI : 6.1 mM.B    a)0      a)70'at715E0u_              0 1 2 3 4 5 6	 7Retention time (min)Column: Waters IC-PAC anion column: detectorconductivity: eluant: 2.0 mM benzoate: flow rate: 1.5mLrmin': sample volume: 20 4: sample: sodicextract adj. to pH=3.3: CO,: 0.0 mM NCO,: 0.0 mM:acetate: 2.1 mM formate: 3.8 mM: CI: 6.1 mM.Figure 2a and b Saturation pasteextract IC chromatograms before andafter COf- and HCO3 interferenceremoval by lowering the pH and the dis-solved CO2 concentrations.These data show that theA1 3 ' had no effect on the organicanion IC results under these con-ditions. Methyl orange wasorganically used as a pH indicatorto titrate the extracts to 4.7 for theHCO 3- analysis.' but its use wasdiscontinued because it pluggedthe IC columns.Three comparisons are shownfor Cl - analysis (Table 1, lines 3. 4.and 5). The first comparison wasbetween clear pH-adjusted extracts.with and without Al,(80 4 ), addi-tion, analyzed by Ag - titrationusing chromate as the endpointindicator.' The second comparisonwas between Ag- titration and ICanalysis of pH-adjusted. clearextracts treated with Al,(80 4 ),. Thethird comparison was between A2 --titration and IC anal y sis of darkextracts that had been clarified bypH adjustment. Al 3" addition, cen-trifugation. and filtration. Theregression values for the Cl - datawere not quite as ideal as the previ-ous data, but still showed that thepretreatment did not produce a sig-nificant difference in the results.The two analytical procedures werealso not significantly different. Noattempt was made to compareanalysis of Cl - in the dark extractsby either procedure without firstremoving the organic matter.Three comparisons were madefor SO;- analysis (Table 1. lines 6.7. and 8). The first comparison wasbetween clear, pH-adjusted extracts.with and without A1C1, addition, bythe turbidimetric method. 9 The sec-ond comparison was between tur-bidimetric and IC analysis of clearextracts that had been pH adjustedand AlC1, added. The third compar-ison was between turbidimetric andIC analysis of dark extracts that hadbeen pH adjusted. Al 3+ added, cen-trifuged. and filtered. The regres-sion analysis shows that the Al 3+addition did not affect the results.nor was there a difference betweenthe two procedures (Table 1. lines6. 7. and 8). Data were not com-pared between treated and untreateddark extracts.ConclusionsColloidal organic matter can beremoved from sodic soil saturationpaste extracts by acidifying theextracts. to pH 3.0 to 3.5. addingA1 3 '. centrifuging the extract. andthen filtering it through a 0.2-p.mn y lon filter. By usin g standardizedacid. CO;- and HCO 3 - concentra-tion can be determined durin g thecleanup process by titrating to pH8.4 and then to 4.7. and the HCO 3-interference removed for formateanalysis by IC. Once the extractsare clarified, they can be analyzedfor Cl - and SO2,- by traditional andIC methods used for saline andsodic soil extracts as well asacetate. formate, NC3c - , and NO.;- byIC methods without further samplepreparation. When used as a pHindicator. methyl orange pluggedthe anion column, thus precludingcolorimetric pH titration andremoval of HCO;.References1. ROBBINS. C.W., Soil Sci. Soc. Ant. J. 53.721-725 (1989).2. NIETO. K.F. AND FRANKENBURGER. W.T., JR.,Soil Sci. Soc. Ant. J. 49. 587-592 (1985).3. SKARIE,	 ARNDT. J.L.. AND RICHARDSON.J.L.. Soil Sci. Soc. Ant. J. 51, 901-905( 198131N1.	 87).ROBBI S. C W , Akron. J. 75. 916-920.(1986).5. BASTA. N.T. AND TABAI ABS,	 SodSOC. Ant. J. 49, 84-89 (1985).6. U.S. Salinity Laboratory Staff. "Diagnosisand improvement of saline and alkalisoils. - Act-H .. Hand'. 60 (U.S. Govern-ment Printin g Office, Washington. D.C..1954).7. DEN1PSEY. B.A.. GANHO. R.M.. AND 0 MELIA.C.R.. J. Ant. Water Works Asa/1. 76.141-150 (1984).8. STUMM. N'. AND NIORGAN, 1.1.. AquaticChemistry (John Wiley and Sons, Inc..New York. NY, 19701. pp. 122-124.9 TABATABAL NI..A. AND BREMNER. 1.N1..A Qrolt. J. 62. 805-806 (1970).The use Qt brand names is for the readersconvenience and does not impl y endorsementof these instruments over an y other b y theauthor or the CS. Dept. of Agriculture.AEL 13

Suspended organic matter removal from aqueous extracts of high pH soils

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Suspended organic matter removal from aqueous extracts of high pH soils

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