Decontamination of soils by activation with acids and bases

Hardie, Ailsa Ghillaine (2004-03)

Thesis (MSc)--University of Stellenbosch, 2004.

Thesis

ENGLISH ABSTRACT: New, more receptive surfaces can be generated in soils by a partial dissolution of existing, crystalline solids followed by re-precipitation as poorly crystalline colloids with a larger capacity to adsorb ionic and molecular contaminants. This priming process can be carried out by treating the soil with strong acid or base and then neutral ising it again. The aim of this study was to investigate the effectiveness of acid and base treatments in reducing inorganic contaminant availability in different soil types. The first study involved investigating the change in cation (cadmium[II], copper[II]) and anion (phosphate) sorption of four different soils before and after priming. Hydrochloric acid and KOH were used to adjust the pH of soils to below pH 2 or above pH 12 in the dissolution stage of the priming treatment. After neutralisation it was found that base priming resulted in an increase in metal cation adsorption in all the soils, most notably in the sesquioxidic (increase from 19.5 to 73.5 mmol Cd.kg-1 soil) and kaolinitic soils (from 16.9 to 38.3 mmol Cd.kg-1 soil), whereas acid priming decreased it or had little effect on cation sorption. However, acid priming increased anion sorption in all soil types, to a greater extent than base priming, most notably in the organic soil (from 6.3 to 14.7 mmol P04.kg-1 soil). This can be attributed to the differences in the nature of the precipitate (more aluminous or alumino-siliceous) depending on whether the dissolution was carried out in acid or basic conditions, and the final pH of the soil solution, as the hydroxyaluminium and hydroxyaluminosilicate precipitates which form are known to enhance pH-dependent sorption of metals. In the second study, the soil was suspended in Cd or Cu solutions and then the pH was adjusted to below 2 or above 12 using HCI or KOH. After 5 days of shaking the pH was adjusted to a neutral pH again. The availability of the Cd and Cu was determined at each of the stages in the treatment, and it was found that both the acid and base treatments were effective in removing Cu from solution, whereas only the base treatment was effective in removing Cd. Determinations were also carried out using H2S04 and Ca(OH)2 and it was found that they were equally effective. It can be proposed that this acid-base pair would be of most practical importance as the salt generated is gypsum which is generally considered benign, and can actually help to improve the soil structure. Activation of soils by acid or base conditioning could have some useful applications in decontaminating soils or decreasing the mobility of inorganic contaminants in soils. Primed soils could also be used as cheap absorbents for decontaminating water.

AFRIKAANSE OPSOMMING: DEKONT AMINERING VAN GROND DEUR AKTIVlERING MET SURE EN BASISE Nuwe, meer ontvanklike oppervlakke kan in gronde gegenereer word deur middel van gedeeltelike oplossing van bestaande, kristallyne vaste stowwe gevolg deur herpresipitasie as swak kristallyne kolloïede met 'n groter kapasiteit om ioniese en molekulêre kontaminante te adsorbeer. Hierdie behandelingsproses kan uitgevoer word deur die grond met sterk suur of basis te behandel en dit daarna te neutraliseer. Die doel van hierdie studie was om die effektiwiteit van suur- of basisbehandelings om anorganiese kontaminante te verminder, in verskillende grond tipes te ondersoek. Die eerste studie het die ondersoek na die verandering in katioon- (kadmium[II], koper[lI]) en anioon- sorpsie (fosfaat) van vier verskillende gronde voor en na suur of basis voorbehandeling behels. Soutsuur en KOH was gebruik om die pH van die gronde tot onder pH 2 of bo pH 12 aan te pas gedurende die oplosstadium van die behandelingsproses. Na neutralisasie is dit gevind dat basis voorbehandeling tot 'n toename in metaal katioonadsorpsie in al die gronde gelei het, en dit was veral waarneembaar in die seskwioksied (toename van 19.5 tot 73.5 mmol Cd.kg" grond) en kaolinitiese (van 16.9 tot 38.3 mmol Cd.kg-1 grond) gronde, terwyl suur voorbehandeling dit verlaag het of 'n klein effek op katioonsorpsie gehad het. Suur voorbehandeling, egter, het anioonsorpsie in al die gronde verhoog, tot 'n groter mate as basis voorbehandeling, veral in die organiese grond (van 6.3 tot 14.7 mmol fosfaat.kg' grond). Dit kan toegeskryf word aan die verskille in die aard van die neerslag wat meer alurninium- of alurniniumsilika-ryk kan wees afhangede of die oplossing uitgevoer was in suur of basis kondisies, en ook die finale pH van die grondoplossing omdat die hidroksi-aluminium en hidroksi-aluminiumsilikaat presipitate wat vorm bekend daarvoor is om pH-afhanklike sorpsie van metale te bevorder. In die tweede studie, was die grond in Cd- of Cu-oplossings gesuspendeer en die pH was aangepas tot onder 2 of bo 12 met HCI of KOH. Na vyf dae van skud was die pH weer aangepas tot by neutraal. Die beskikbaarheid van Cd en Cu was bepaal by elke stadium in die behandeling, en dit is gevind dat beide suur- en basisbehandeling meer effektief was in die verwydering van Cu uit oplossing, terwyl slegs die basisbehandeling effektief was in die verwydering van Cd. Bepalings was ook uitgevoer met die gebruik van H2S04 en Ca(OH)2 en dit is gevind dat dit net so doeltreffend was soos HCI en KOH. Dit kan voorgestel word dat hierdie suur-basis paar in die praktyk belangriker sal wees omdat die sout wat gegenereer word, nl. gips, as 'n gunstige sout beskou word. Aktivering van grond deur suur- of basis-kondisionering kan sekere bruikbare toepassings in die dekontaminering van gronde hê of dit kan gebruik word om die mobiliteit van anorganiese kontaminante te verlaag. Suur of basis voorbehandelde gronde kan ook gebruik word as goedkoop absorbante vir die dekontaminering van water.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/49901
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