The aggregation of clay minerals and marine microalgal cells : physicochemical theory and implications for controlling harmful algal blooms

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.