The influence of soil particle surfaces and soil porosity on the biodegradation of key refuse leachate organic molecules.
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Date
1995
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Abstract
Many studies have been undertaken to determine the effects of soil and soil properties on
migrating metal pollutants. Organic pollutants, however, in addition to their interactions with
soil components , are also susceptible to degradation (catabolism) by microorganisms.
Soil-microorganism-pollutant interactions have, traditionally, been studied in soil columns
(microcosms). One of the shortcomings of column and in situ studies is that the identity and
specific effect(s) of the soil component(s) affecting or influencing attenuation are not known
and cannot readily be determined. Attenuation effects of the soil components are, therefore,
difficult to interpret. ("Attenuation" in this context is the combined effects of both soil
adsorption and microbial catabolism). Attenuation studies often only consider the physical
conditions such as aeration, permeability, flow rate, temperature, etc. This approach assumes
the soil to be a homogeneous matrix with no specific physico-chemical properties attributable
to different components within the matrix. Soil physical factors suspected of influencing
pollutant attenuation could be misleading without consideration of the physico-chemical
interactions between soil components, microorganisms and pollutants. Adhesion of pollutants
and microorganisms seems to be most important in this regard.
The initial phase of this study was undertaken to examine the effects of three different soil
materials on attenuation of key landfill leachate molecules. Examination of the effects of soil
surface type on attenuation focused on adsorption / desorption of the pollutant molecules and
microorganisms. These experiments sought to investigate the physico-chemical effects of soil,
microorganism, pollutant interactions and were done as batch slurry experiments as well as in
soil columns. Two soil horizons from the Inanda soil form (humic A and red apedal B) and
the topsoil (vertic A) from a Rensburg soil form were used. The Inanda topsoil had a high
organic matter content and both the topsoil and subsoil had a kaolinitic clay mineralogy; the
Rensburg topsoil clay mineralogy was predominantly smectitic with a relatively low organic
matter content.
From the batch experiments, the adsorption of a hydrophobic molecule (naphthalene) and a heavy metal (cadmium) were found to be influenced to a significant extent by soil characteristics.
Adsorption of naphthalene was due to the soil organic matter (SOM) content whereas cadmium
adsorption was due to the cation exchange capacity (CEC) of the soil. Soil characteristics did
not seem to have a significant influence on the adsorption of a water soluble compound such
as phenol at the concentrations used. Attenuation of naphthalene was found to be affected by
adsorption of the pollutant molecule (related to SOM) as well as the CEC of the soil. The
attenuation of hydrophobic molecules can possibly be ascribed to the influence of CEC on the
microbial population responsible for attenuation. This would seem to indicate interaction
between the soil surfaces and the catabolizing microbial population. Desorption of the
pollutant (and possibly also of the microbial population) was achieved by the addition of
acetonitrile and methanol both of which reduced the polarity of the water. These solvents were
also found to be toxic to the catabolizing microbial population at high concentrations. The
toxicity thresholds of both solvents for catabolizing microorganisms differed significantly
between soil- (> 15 %, v/v) and soil free (< 5 %, v/v) treatments. This discrepancy cannot
be accounted for by adsorption and is ascribed to physico-chemical interaction between
microorganisms and the soil surfaces. This interaction probably affords protection from,
otherwise, toxic concentrations of solvents or metals. The important effects of soil surfaces
on attenuation processes were thought to be due to the strong adsorption of naphthalene.
Surface attachment of microorganisms was, however, also inferred from results obtained with
phenol. This seemed to indicate that microbial attachment to soil surfaces was an important
aspect in attenuation and did not occur only because of pollutant adsorption.
Soil column experiments were made with both naphthalene and phenol. The naphthalene,
which was adsorbed to the soil, did not leach from the columns to any appreciable extent.
This was despite the addition of acetonitrile to some columns. This was probably due to
greater microbial catabolism caused by desorption and, subsequent, increased soluble
concentrations of the molecule. After extraction from the soil at the end of the experiment it
was clear that the sterile controls held much higher concentrations of naphthalene than the
experimental columns. The soil type and treatments showed little difference in the naphthalen concentration extracted from the soil columns. This did not reflect the differences found
between soil materials in the batch experiments and was probably due to the masking effect
of the soil physical factors on attenuation processes. Unlike naphthalene, phenol, because of
its high solubility, was detected in the column leachates at relatively high concentrations. The
phenol concentrations were much higher for the Inanda subsoil (approximately 4 mM) than the
Inanda topsoil (approximately 2 mM) and Rensburg topsoil (< 1 mM). The Rensburg topsoil
produced the lowest phenol concentrations in the leachate and this can probably be ascribed
to the larger quantity of micropores in this soil. Thus, it seems that the soil physical features
had a pronounced influence on attenuation. Whether this effect was directly on the studied
molecule or indirectly, because of the effects on the microbial population, is not known.
Inoculation of the columns with a phenol catabolizing population had only a slight increased
effect on leachate phenol concentrations from all columns. This increased effect was,
however, only prolonged in the case of the Inanda subsoil. The flow rate through the columns
affected leachate phenol concentration which was lower with a slower flow rate and, thus,
longer retention time.
From the column experiments soil physical parameters were suspected of influencing, and
possibly overriding, the soil surface effects on microbial activity (capacity to catabolize a
organic molecule of interest). Soil porosity, as caused by different soil materials, was
suspected of being the most important soil physical parameter influencing microbial activity.
To investigate the potential effect of soil porosity, relatively homogeneous porous media i.e.
chromatography packing material and acid washed sand were used. These materials had more
defined and distinct porosities and were considered to be suitable for investigating the
fundamental influence of porosity on microbial activity. Saturated continuous flow columns
were used and three types of packing configurations were tested: chromatography packing
(CHROM) material (porous particles); acid washed sand (non-porous) (AWS); and a 1: 1 (w/w)
mixture of chromatography packing and acid washed sand (MIX). Only a single water soluble
molecule, phenol, was used in this phase of the investigation.
Bacterial filtration ("filtration" as a component of "attenuation'') was found to be highest for
the CHROM and lowest for the AWS materials. This difference in microbial retention affected the phenol catabolism in response to increased column dilution rates. The CHROM
and MIX materials had distinctly different porosities than that of the AWS, due to the internal
porosity of the chromatography packing. This greater pore size distribution in the MIX and
CHROM packing materials created pores with different effective pore dilution rates within the
microcosms at similar overall flow rates. The greater pore size distribution in the MIX and
CHROM packing materials facilitated pore colonization since some pores did not participate,
or conduct, mass flow as occurred in macropores. This led to different microcolonization
effects in the macro- vs micropores. Since the MIX and CHROM packing materials had more
micropore colonization sites these packing materials showed a greater range of substrate
affinities (i.e. Ks values) for the phenol substrate.
The extent to which micropore colonization occurred could be detected by the effect it had on
phenol breakthrough curves. In the MIX and CHROM materials, microbial colonization
caused blocking of micropores with a subsequent effect on the phenol breakthrough curves.
The AWS material, however, which had a low inherent microporosity, showed microbially
induced microporosity probably due to biofilm development. The fact that the MIX and
CHROM packing materials facilitated micropore colonization was also responsible for the
greater resistance to, and the recovery from , potentially inhibitory cadmium concentrations.
This effect was also apparent in the presence of acetonitrile, although this effect was not
identical to that observed with cadmium. Finally, column pressure build up as a function of
pore clogging was determined and was found to occur in the order AWS > MIX > CHROM.
This was most likely due to fewer potential liquid flow paths with a higher blocking potential
in the AWS.
Extrapolation of the fundamentals of the above findings led to the conclusion that soil surface- and
soil porosity effects are extremely important factors in determining the behavior of soils
as bioreactors.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1995.
Keywords
Soil absorption and adsorption., Soil microbiology., Soil pollution., Theses--Microbiology.