The aggregation of clay minerals and marine microalgal cells : physicochemical theory and implications for controlling harmful algal blooms
The aggregation of clay minerals and marine microalgal cells : physicochemical theory and implications for controlling harmful algal blooms
Date
2001-09
Authors
Sengco, Mario R.
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DOI
10.1575/1912/3043
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Keywords
Algal blooms
Clay minerals
Toxic marine algae
Absorption and adsorption
Clay minerals
Toxic marine algae
Absorption and adsorption
Abstract
In recent years, the use of clay minerals has emerged as one of the most promising
strategies for directly controlling harmful algal blooms (HABs). Its principle is based on
the mutual aggregation of algal cells and mineral particles, leading to the formation of
large flocs that rapidly settle to the ocean floor. This work investigated the effectiveness
of various domestic clays against a number of bloom-forming species from the United
States. Twenty-five clays were tested against the dinoflagellate, Karenia brevis (formerly
Gymnodinium breve), and the chrysophyte, Aureococcus anophagefferens. In general,
the highest removal efficiencies (RE > 90% at 0.25 g rl of clay) against K. brevis were
found using montmorillonite, bentonite and phosphatic clays (i.e. a product of phosphate
mining containing large amounts of montmorillonite). The RE of phosphatic clays
remained high (> 80%) even at 0.03 g rl. Kaolinite and zeolite were mostly ineffective
against K. brevis. Removal with clay exceeded those for alum, polyaluminum chloride
(PAC) and several other polymeric flocculants by a factor of two. However, the combination
of phosphatic clay and PAC (at 5 mg rl) decreased the amount of clay needed to
maintain 80% RE by one order of magnitude. Cell viability and recovery remained high
when clay loading stayed below 0.03 g rl with or without resuspension of the sediment.
However, cell mortality approached 100% with 0.50 g rl even with daily resuspension.
Between 0.10 and 0.25 g rl, K. brevis survival and recovery depended on the interplay of
clay loading, the frequency of resuspension, and duration of contact prior to the first
resuspension event. For A. anophagefferens, the RE did not exceed 40% for any clay at
0.25 g rl even in combination with coagulants and flocculants. The highest removal was
achieved by thoroughly mixing the clay slurry (e.g. phosphatic clay) into the cell culture.
The RE by phosphatic clay varied significantly in a survey consisting of 17 different
species from five algal classes. Moreover, the removal trends varied substantially
with increasing cell concentration. For example, cell removal increased with increasing
clay loading and cell concentration for K. brevis. However, RE dropped below 70%
when cell concentration was < 1000 cell ml-1 for clay loadings up to 0.50 g rl. This
suggested that a critical number of organisms should be present for clays to remain effective.
Similarly, enhanced removal with increasing cell concentration was also found in
Akashiwo sanguinea (formerly Gymnodinium sanguineum), Heterosigma akashiwo and
Heterocapsa triquetra. In the six remaining species, RE initially increased then decreased,
or RE remained constant as more cells were treated. The removal pattern among the
species at comparable cell numbers did not correlate with the cross-sectional area (R2 =
0.23), swimming speed (R2 = 0.04), or a type of cell covering (i.e. theca, silica frustule).
However, when the total collision frequency coefficients were calculated (including
collisions due to cell motility) over the interval when clays were < 50 μm, these values
correlated well with the empirical RB's for the flagellated species (R2 = 0.90). These
results suggested that collisions due to cell motility may be important during the early
stages of aggregation when clay sizes are relatively small (i.e. near the surface where the
clay layer is initially added).
The electrophoretic mobility (EPM) of marine microalgae displayed a small range
of negative values. While the values were smaller that those reported from freshwater
species, these results confirmed earlier assumptions that marine species carry a negative
charge like their freshwater counterparts. In addition, these results also revealed that the
stabilities of cell suspensions in seawater are not controlled by charge neutralization.
However, these measurements did not provide direct information on why one species was
more readily removed over another by a given clay mineral (e.g. phosphatic clay).
The EPM of clays in freshwater also exhibited predictable negative values, with
montmorillonites showing the highest stability and phosphatic clays the lowest. Kaolinite
and zeolite displayed a range of intermediate values. These differences vanished when
the clays were suspended in natural seawater (29.6 salinity), reducing the surface charge
to a small range of negative values. This effect occurred even at 1116 of the final salinity
(1.85 salinity). Viewed alone, these results did not provide direct information on why
one clay mineral worked better than another against a given algal species (e.g. K. brevis).
Kinetic and modelling experiments using K. brevis and three minerals revealed
some distinct patterns in aggregation and settling among the clays, including how they
removed the organisms. After dispersing on the surface, phosphatic clays aggregated
quickly by virtue of low stability (low EPM). Cell removal coincided with the onset of
settling. Also, kaolinite aggregated quickly and was controlled by size as well as stability.
However, cell removal followed clay settling over 40 min, after which cell removal
decreased yielding only 46% RE. Bentonite aggregated slowly over 90 min due to its
high stability (high EPM), but produced a number of large voluminous flocs that steadily
removed the algae. The sinking rate of flocs increased as cells became incorporated, but
the onset of settling was delayed when cells were present in phosphatic clay and kaolinite
due to a predicted reduction in aggregate density. The process of kinetics and sedimentation
were modelled using first order equations for all mineral-algae combinations.
Finally, phosphatic clays demonstrated the ability to selectively remove K. brevis
in a mixed culture with the dinoflagellate, Prorocentrum micans, or the diatom, Skeletonema
costatum. While the RE's were generally comparable to individual cultures, the RE
of either species increased in the presence of the other, especially for K. brevis. Similar
results were observed in mesocosm studies using a natural assemblage during a Karenia
bloom. In fact, the RE of K. brevis were higher than would be predicted from single species
laboratory studies given its low initial concentration.
Overall, this research demonstrated the effectiveness of clay treatment against a
number of HAB species in the U.S. This work also provided new insights into the aggregation
phenomenon between minerals and living algal cells by focusing on the physical
(cell size), chemical and behavioral (i.e. motility) properties of both particle types, the
effect of particle concentration, and the aggregation kinetics of the clay-algae system.
Description
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2001
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Citation
Sengco, M. R. (2001). The aggregation of clay minerals and marine microalgal cells : physicochemical theory and implications for controlling harmful algal blooms [Doctoral thesis, Massachusetts Institute of Technology and Woods Hole Oceanographic Institution]. Woods Hole Open Access Server. https://doi.org/10.1575/1912/3043