The effects of algae concentration, ferric chloride dose, and pH on the flocculation efficiency of the freshwater algae Chlorella zofingiensis can be understood by considering the nature of the electrostatic charges on the algae and precipitate surfaces. Two critical conditions are identified which, when met, result in flocculation efficiencies in excess of 90% for freshwater algae. First, a minimum concentration of ferric chloride is required to overcome the electrostatic stabilization of the algae and promote bridging of algae cells by hydroxide precipitates. At low algae concentrations, the minimum amount of ferric chloride required increases linearly with algae concentration, characteristic of flocculation primarily through electrostatic bridging by hydroxide precipitates. At higher algae concentrations, the minimum required concentration of ferric chloride for flocculation is independent of algae concentration, suggesting a change in the primary flocculation mechanism from bridging to sweep flocculation. Second, the algae must have a negative surface charge. Experiments and surface complexation modeling show that the surface charge of C. zofingiensis is negative above a pH of 4.0 ± 0.3 which agrees well with the minimum pH required for effective flocculation. These critical flocculation criteria can be extended to other freshwater algae to design effective flocculation systems.

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This study analyzes computer models of the microporous material imogolite. The purpose of this work is to validate computational methods developed for extracting pore size, pore volume, and surface area measurement of molecular level computer models. We accomplished this by comparing model properties with experimental data derived from N2 adsorption isotherms and by simulating CH4 and N2 adsorption and He/SF6 diffusion. Pore volume and pore size are easily determined and fit experimental data within reason for validation purposes. Surface area calculations are at first greater than those determined experimentally. Correcting for the curvature of the pore enables duplication of experimental data within the combined error of both methods. Simulations of adsorption and diffusion fit available experimental results reasonably well and along with previous conclusions allow identification of the most probable model structures. The computer-based method studied should be adequate for analyzing other silicate models such as sol−gel derived microporous membranes, aerogels, and zeolites.