Humic acid pretreatment for enhancing microbial removal of metals from a synthetic 'wastewater'.
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Date
2004
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Abstract
The presence of heavy metal ions in waste streams is one of the most pervasive
environmental issues of present times. A rotating biological contactor (RBC) was used
to investigate the potential capacity of microbial biofilms in remediation of the metal
ion species from a mixed metal contaminated effluent solution containing Cr+3
, Pb+2
and Cu+2
, each at a concentration of 200 mg r1
• In the first part of this study the
effectiveness of various support materials for the development of microbial biofilms
capable of removing heavy metals from a synthetic effluent was investigated. EDX
analysis showed that none of the support matrices investigated, viz. gravel, polyester
batting and sand, adsorbed metal ions on their surfaces; hence, metal adsorption was
due purely to microbial activities. The biofilms attached more firmly and uniformly to
polyester batting than to gravel and sand. The characteristics of polyester batting which
made it a superior support matrix were its surface roughness and porous hydrophilic
nature, which provided a larger surface area for the adhesion of microorganisms and
attraction of nutrients during the biofilm development process.
The selective accumulation of metal ion specIes by various microbial populations
grown as biofilm using polyester batting as support matrix in separate compartments of
a single-stage RBC bioreactor was examined. Lead ions were readily accumulated by
almost all the microbial biofilms tested. Fungus-dominated biofilms selectively
accumulated chromium ions whereas biofilms comprising mainly bacteria more readily
accumulated copper ions from the mixed metal contaminated effluent solution.
However, where interactions between the bacterial and fungal components were
encouraged the mechanical stability of the biofilms was enhanced so that large amounts
of all three metal ion species were removed by this biofilm.
The combined effect of a series of bench-scale columns containing liquid humic acid
and a three stage RBC bioreactor on the removal of metal ion species from a mixed
metal contaminated effluent was investigated. After seven days of treatment the
combined system had removed approximately 99% of the Cr+3, 98% of the Pb+2 and 90% of the Cu+2 ions from the mixed metal contaminated synthetic effluent.
Complexation of the metal ions with humic acid was the predominant factor accounting
for approximately 68-86% Cr+3
, 70-86% Pb+2 and 53-73% Cu+2 removal levels within
the columns. A large proportion of the remaining Cr+3 and Pb+2, but not of the Cu+2,
was removed in compartment 1 of the RBC. This suggested that the presence of the
former two metals in solution might have reduced the removal of the Cu+2 ions from the
system. The removal of substantially large amounts of the competing ions chromium
and lead during the initial stages of the treatment process meant that copper was
successfully taken up in the second and third RBC compartments. Hence, the economy
of the treatment process was improved as larger quantities of the metal ions were
removed in a shorter period of time than was possible when using the individual
treatments (humic acid-metal complexation and biofilm adsorption) separately. More
than 75%,92% and 86% of the adsorbed Cr+3
, Pb+2 and Cu+2 ions, respectively, were
recovered from the three RBC bioreactor compartments following repeated washing of
the biofilms with 0.1 M HCI. This relatively easy desorption suggested that the metal
ions were simply adsorbed onto the surfaces of the biofilm cells rather than being taken
into the cytoplasm of the cells.
Description
Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2004.
Keywords
Factory and trade waste--Biodegradation., Bioremediation., Hazardous wastes--Environmental aspects., Pollution--Environmental aspects., Sewage--Purification--Heavy metals removal., Humic acid., Sewage--Purification--Chromium removal., Lead abatement., Sewage--Purification--Biological treatment., Microbial biotechnology., Theses--Microbiology.