Size- and surface charge-controlled layered double hydroxides for efficient algal flocculation

We evaluated the effects of particle size and surface charge on the algal flocculation activity of layered double hydroxides (LDHs). LDHs with different lateral sizes of 50, 250, and 2000 nm were prepared by adjusting reaction methods and parameters during co-precipitation of a metal solution under alkaline conditions. LDH particles of 2000 nm readily flocculated ∼100% of Microcystis aeruginosa within 60 min, whereas smaller LDHs did not show significant algal flocculation. We controlled the surface charge of 2000 nm LDHs to be highly positive, almost neutral, or negative using organic moiety coatings of l-serine, succinate, and citrate, respectively. Negatively coated LDHs did not flocculate algae, suggesting the importance of surface charge in the algae-LDH interaction. Taking into account the size and surface charge parameters for algal flocculation, we prepared LDHs with a lateral size of ∼500 nm and a positive surface charge from industrial waste slag, a by-product of alloy manufacturing, from an electric arc furnace. These LDHs showed ∼40% flocculating activity within 60 min, whereas the slag ground into powder showed little flocculating activity after 240 min.