Effective phosphate removal using Ca-based layered double hydroxide materials

Layered double hydroxides (LDHs), also known as hydrotalcite-like compounds, are a large group of natural and synthetic clay materials with high anion exchange capacity (Goh et al., 2008; Zhang et al., 2008). As shown in Figure 6.1, LDHs are layered materials consisting of positively charged metal hydroxide (brucite-like) sheets with octahedra sharing the edges and the interlayer anions. Their chemical composition can be generally expressed as [MImx2+Mx3+(OH)2] [Anm]x/n m yH20, where M2+ and M3+ are divalent and trivalent cations, and Anm an anion in the interlamellar space to balance the positive charge of the brucite-like layer with associated water molecules (Cavani et al., 1991 ). Moreover, due to the relatively weak inter!ayer bonding, anions such as Clm and NO3m,intercalated in the LDHs, are readily exchanged with various organic and inorganic anions. Because of the anion exchange property, LDHs become a promising reservoir for diverse inorganic, organic, and biomolecular anions, as well as anionic polymers and drugs...