Development, optimization and use of a reduced-sample, water dispersible clay extraction technique for taxonomic horizon discrimination

Dinwa, Siziphiwe (2018-03)

Thesis (MScAgric)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: Water dispersible clay (WDC) is defined as the colloid fraction which disperses in water without removal of cementing compounds or the use of dispersing agents. It is a commonly determined parameter and is used in many erosion models and is a proxy for aggregate stability and clay dispersivity. There is no standard method for determining WDC, and although modified particle size analysis (PSA) is the most common technique, numerous other methods are also employed to save time, bench space and reduce sample size. These methods have not been tested against the benchmark PSA method and vary in terms of agitation (time and type), extraction, measurement and expression of WDC. This makes comparison between these methods very difficult. This study aims to develop, test and optimise a simple, reduced sample centrifuge method for determining WDC in order to allow analysis of archive samples and assess the use of WDC as a soil classification discriminator on a limited number of soils. A reliable and calibrated, reduced sample size method will be of value for measuring WDC in sample collections, such as the national profile soil collection housed at the Institute for Soil Climate and Water. This would allow for these valuable collections to be included in erosion models. Archived samples of neocutanic B, yellow-brown and red apedal B horizons and borderline neocutanics/red apedal B horizons were selected for this study. Two reduced sample centrifuge methods (using pipetting and decanting to remove the clay suspension) were examined and their efficiency and accuracy was measured with respect to the sedimentation particle size analysis (PSA) method. For both the centrifuging methods the WDC and chemically dispersed clay, a mixture of sodium hexametaphosphate and sodium carbonate, called CDC were determined. This is the chemically dispersed clay without the removal of organic matter or cementing agents. The effect of ultrasonication and shaking time on WDC was assessed for the centrifuge-pipette method by physically agitating the soil with or without prior sonication, and increasing the initial shaking time incrementally from 1 to 30 hours. X-ray diffraction (XRD) analysis was carried out on the WDC and CDC extracts from the benchmark sedimentation method to establish if the mineralogy of these two fractions differed. The WDC and CDC was measured gravimetrically and by turbidity readings. Water dispersible clay correlated poorly with total clay across all samples. The relationship between CDC and total clay was better, but the extraction efficiency of CDC to total clay was only 54%. The extraction efficiency of WDC is highly dependent on the physical agitation energy exerted on the samples. Increasing the headspace in the centrifuge tube increased the WDC extraction efficiency by 32% (absolute). Shaking time has a major influence on WDC extraction efficiency, with a minimum shaking time of 22 hours required to get maximum extraction. This demonstrates the need to standardise the method as numerous extraction techniques use less than 16 hours shaking time for WDC extraction. Sonication prior to shaking for 22 hours results in a WDC extraction efficiency of 94% for the new centrifuge method compared to the traditional PSA method. The centrifuge-pipette method was shown to be effective in selectively separating the < 2 μm phase, thus reducing the need for sedimentation. Turbidity is not a reliable technique to measure clay in a suspension, due to the clay mineralogy affecting turbidity. Model kaolinite and smectite did not give uniform turbidity readings. This means the gravimetric method cannot be replaced, but centrifugation has both a time saving and sample reducing benefit. Neocutanic horizons tended to have WDCh (the WDC fraction expressed as a function of CDC) content higher than the yellow-brown and red apedal horizons, and were distinguishable from red apedal horizons at a 95% confidence level. However, WDC cannot be used to distinguish neocutanic B from yellow-brown apedals horizons. This supported the tacit knowledge that neocutanic horizons have a less stable clay phase than red apedal horizons, but the distinction is not clear in the case of yellow brown apedals. Borderline neocutanic/red apedal horizons and typical neocutanic proved to have similar WDCh content. Given the importance of clay stability in red apedal horizons, it was recommended they are classified as neocutanics rather than red apedals and a tentative threshold of 47% WDCh be used to differentiate between horizons. The new centrifuge technique for the extraction of WDC is a viable alternative to the PSA method and has the benefits of reducing sample size and extraction time and increasing the number of samples that can be analysed at one time. Standardisation of WDC is important due to the effects of agitation type and duration on the extraction efficiency. Furthermore, WDCh shows promise as a classification aid and should be investigated further.

AFRIKAANSE OPSOMMING: Waterverspreibare klei (WVK) word gedefinieer as die kolloïede-fraksie wat versprei in water sonder die verwydering van sementerings-verbindings of met die gebruik van verspreidings-middels. Dit is ‘n parameter wat gereeld bepaal word en word in baie erosie studies gebruik. Dit word ook gebruik as ‘n proksie vir aggregaat-stabiliteit en klei-verspreibaarheid. Daar bestaan geen standaard-metode om WVK te bepaal nie, en al is gewysigde deeltjiegrootte analise (DGA) die mees algemene tegniek, word vele ander metodes ook aangewend om tyd en bankspasie te bespaar, sowel as monstergrootte te verminder. Hierdie metodes is nog nie teen die maatstaf DGA metode getoets nie en varieer vanaf hierdie metode in terme van agitasie (tydsverloop en tipe), ekstraksie, meting, en uitdrukking van WVK. Dit veroorsaak dat vergelyking tussen hierdie metodes baie moeilik is. Hierdie studie beoog om ‘n eenvoudige, verminderde monster sentrifuge metode om WVK te bepaal te ontwikkel, toets en te optimiseer, ten einde die ontleding van argief-monsters toe te laat en die gebruik van WVK as ‘n grondklassifikasie onderskeider, op ‘n beperkte aantal gronde, te assesseer. ‘n Betroubare en gekalibreerde verminderde-monstergrootte metode sal van waarde wees vir die bepaling van WVK in monster versamelings, soos die nasionale grondprofiel versameling gehuisves by die Instituut vir Grond, Klimaat en Water. Dit sal toelaat dat hierdie waardevolle versamelings ook in erosiemodelle ingesluit kan word. Argief-monsters van neokutaniese B, geel-bruin en rooi apedale B horisonne sowel as grensgeval neokutaniese/rooi apedale B horisonne is vir hierdie studie geselekteer. Twee verminderde monster sentrifuge metodes (deur pipettering en afskinking om die kleisuspensie te verwyder) is geëksamineer, en hul doeltreffendheid en akkuraatheid is gemeet met betrekking tot die sedimentasie deeltjiegrootte analise (DGA) metode. Vir albei van die sentrifugerings-metodes is WVK en chemiese verspreibare klei (CVK), deur ‘n mengsel van natrium hexametafosfaat en natrium karbonaat, bepaal. Hierdie is die chemiese verspreibare klei sonder die verwydering van organiese materiaal of sementeringsmiddels. Die effek van ultrasonikasie en skudtyd op WVK is geassesseer vir die sentrifuge-pipet metode deur die grond, met of sonder vorige sonikasie, fisies te agiteer en die aanvanklike skudtyd van 1 tot 30 uur inkrementeel te verhoog. X-straal diffraksie (XSD) analise is uitgevoer op die WVK en CVK ekstrakte van die maatstaf sedimentasie metode om vas te stel of die mineralogie van die twee fraksies verskil. Die WVK en CVK is gravimetries en deur troebelheid lesings gemeet. Waterverspreibare klei korreleer swak met totale klei vir alle monsters. Die verhouding tussen CVK en totale klei is beter, maar die ekstraksie doeltreffendheid van CVK tot totale klei is slegs 54%. Die ekstraksie doeltreffendheid van WVK is hoogs-afhanklik van die fisiese agitasie energie uitgeoefen op die monsters. Verhoging van die hoofruimte in die sentrifugebuis het die WVK ekstraksie doeltreffendheid met 32% (absoluut) verhoog. Skudtyd het ‘n groot invloed op WVK ekstraksie doeltreffendheid, met ‘n minimum skudtyd van 22 uur wat benodig word om maksimum ekstraksie te behaal. Hierdie bevinding demonstreer dat dit nodig is om die metode te standaardiseer want vele ekstraksie tegnieke gebruik minder as 16 uur skudtyd vir WVK ekstraksie. Sonikasie voordat daar geskud word vir 22 uur lei tot WVK ekstraksie doeltreffendheid van 94% vir die nuwe sentrifuge metode in vergelyking met die tradisionele DGA metode. Die sentrifuge-pipet metode is bewys as meer doeltreffend om die < 2μm fase selektief te skei, en verminder dus die behoefte vir sedimentasie. Troebelheid is nie ‘n betroubare tegniek om klei in ‘n suspensie te meet nie, as gevolg van die feit dat klei mineralogie die troebelheid affekteer. Model kaoliniet en montmorilloniet het nie uniforme troebelheid lesings gegee nie. Dit beteken dat die gravimetriese metode nie vervang kan word nie, maar dat sentrifugering albei tydbesparings en monster-verminderings voordele inhou. Neokutaniese horisonne is geneig om ‘n WVKh (Die WVK fraksie uitgedruk as ‘n funksie van CVK) inhoud te hê wat hoër is as dié van die geel-bruin en rooi apedale horisonne, en is onderskeibaar van rooi apedale horisonne by ‘n 95% vertroue vlak, maar WVK kan egter nie gebruik word om neokutaniese horisonne van geel-bruin apedale horisonne te skei nie. Hierdie bevinding ondersteun die implisiete kennis dat neokutaniese horisonne ‘n minder stabiele kleifase besit as rooi apedale horisonne, maar die onderskeiding is nie duidelik in die geval van geel-bruin apedale horisonne nie. Grensgeval neokutaniese/rooi apedale horisonne en tipiese neokutaniese horisonne bevat soortgelyke WVKh inhoud. Gegewe die belangrikheid van klei-stabiliteit in rooi apedale horisonne, is dit aanbeveel dat hulle eerder geklassifiseer word as neokutaniese horisonne in plaas van rooi apedale horisonne, en ‘n proefnemende drempel van 47% WVKh gebruik word om te onderskei tussen horisonne. Die nuwe sentrifuge tegniek vir die ekstraksie van WVK is ‘n lewensvatbare alternatief vir die DGA metode en besit die voordele van monstergrootte en ekstraksietyd vermindering sowel as die aantal monsters te verhoog wat op een slag ontleed kan word. Standaardisering van WVK is belangrik as gevolg van die effekte van agitasie tipe en tydsduur op die ekstraksie doeltreffendheid. Verder toon WVKh groot belofte as ‘n klassifikasie hulpmiddel en moet verder ondersoek word.

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