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 l-1 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 l-1. 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 l-1) 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 l-1 with or without resuspension of the sediment. However, cell mortality approached 100% with 0.50 g l-1 even with daily resuspension. Between 0.10 and 0.25 g l-1, 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.

(cont.) For A. anophagefferens, the RE did not exceed 40% for any clay at 0.25 g l-1 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 l-1. 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) ...