Chromium exists largely in two oxidation states, namely hexavalent chromium (Cr(VI))
which is carcinogenic, mutagenic to living organisms including humans and trivalent
chromium (Cr(III)) which is known to be 1000 times less toxic than Cr(VI). It is therefore
desirable in most cases to reduce Cr(VI) to Cr(III). Various studies have been conducted on
the Cr(VI) reduction process either in situ or ex situ. However in situ bioremediation using
permeable reactive barrier system appears as a potential and attractive technology compared
to other in situ technologies. This study was conducted to evaluate the reduction of Cr(VI) to
Cr(III) in the short term and regeneration of the biological reactive barrier to achieve
continuous long term operation. It was observed from the study that the chromium hydroxide
Cr(OH)3(s) precipitated and thus affected the porosity and hydraulic conductivity of the
barrier system. It was therefore proposed to implement a regeneration process involving
remobilization of precipitated Cr(OH)3 using a dilute acid (0.1% HCl) and recover Cr(III) by
electrokinetics.
Lowering the pH in the reactor introduced harsh conditions which necessitated the evaluation
of a possible culture shift during the regeneration phase. Microbial culture composition
during bioremediation and after soil washing was evaluated using a 16S rRNA finger printing
method. The microbial barrier was initially inoculated with indigenous bacterial species from
dried sludge. The results presented in the phylogenic tree diagrams confirm that, after
microbial barrier system operation, the well-known Cr(VI) reducers Bacillus mycoides, Lysinibacillus fusiformis and Micrococcus lylae were the predominant species in the
microbial community of the barrier.
The microbial barrier system successfully achieved near complete removal of Cr(VI),
whereby approximately 75% Cr(VI) removal was achieved within 63 days of operation. The
formation of Cr(OH)3(s) was observed in the second week of operation. After 4 weeks of
operating the mesocosm under soil washing with 0.1% HCl and electrokinetics remediation
with a DC voltage of 50-150 V an increase in total chromium (73%) was observed suggesting
that the trapped chromium species in the mesocosm was effectively remobilized with the
assumption that Cr(III) had attached to the cathode forming a white-yellow precipitate layer
around the cathode. Additionally more than 95% Cr(VI) was transformed to lower toxicity
Cr(III) during electrokinetics and soil washing remediation. However, one of the limitations
of electrokinetics is near anode focusing effect whereby a layer of precipitate is formed
around the anode that lead to the reduction of efficiency of the technology. / Dissertation (MSc)--University of Pretoria, 2014. / lk2014 / Chemical Engineering / MSc / Unrestricted
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:up/oai:repository.up.ac.za:2263/43365 |
| Date | January 2014 |
| Creators | Kaimbi, L.A. (Lapaka Albertina) |
| Contributors | Chirwa, Evans M.N., Kaimbi_lapaka@yahoo.com |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Rights | © 2014 University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria. |
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