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

Abstract

Recently, the dispersal of clay minerals has emerged as one of the most promising strategies for 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 research demonstrated that domestic clays can effectively remove a number of bloom-forming species from the United States (>90% removal efficiency), including Karenia brevis and Heterosigma akashiwo , at clay loadings of 0.25 g l[super]-1 or less. Also, clay removal ability exceeded that of conventional coagulants and flocculants up to a factor of two. Algal viability and recovery depended on the interplay between clay loading, the frequency of resuspension, and the duration of contact between mineral particles and cells prior to the first resuspension event. Florida phosphatic clay showed varying removal efficiency (RE) against seventeen algal species from five algal classes. Removal trends also varied with increasing cell concentration. Empirical RE was not correlated with algal size, swimming rate, and the type of outer cell covering (i.e. theca, silica frustule). However, RE was correlated with the predicted collision frequency coefficient, which included the combined effects of cell motility and cell size. Differential removal of Karenia brevis by phosphatic clay was observed in a mixed laboratory culture with two algal species, and in mesocosm tanks containing a natural field assemblage. Not all algal species are removed equally by a given clay loading. Marine algal cells and clay minerals suspended in natural seawater displayed a narrow range of negative electrophoretic mobilities (EPM). In addition, algal suspensions did not show rapid aggregation, despite the low stability of the suspension based on EPM. However, the results of EPM measurements, for several species and clay mineral types, did not correlate with the observed removal patterns. Kinetic studies of the clay-cell system revealed varying rates of aggregation and settling among phosphatic clay, bentonite and kaolinite in the presence of Karenia brevis. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)