Highly stable and nontoxic lanthanum-treated activated palygorskite for the removal of lake water phosphorus

Nutrient pollution of surface water, such as excess phosphate loading on lake surface water, is a significant issue that causes ecological and financial damage. Despite many technologies that can remove available phosphate, such as material-based adsorption of those available phosphate ions, the development of a material that can trap them from the surface water is worth doing, considering other aspects. These aspects are: (i) efficient adsorption by the material while it settles down to the water column, and (ii) the material itself is not toxic to the lake natural microorganism. Considering these aspects, we developed a trace lanthanum-grafted surface-modified palygorskite, a fibrous clay mineral. It adsorbed a realistic amount of phosphate from the lake water (typically 0.13–0.22 mg/L). The raw and modified palygorskite (Pal) includes unmodified Australian Pal, heated (at ~400 °C) Pal, and acid (with 3 M HCl)-treated Pal. Among them, while acid-treated Pal grafted a lower amount of La, it had a higher adsorption capacity (1.243 mg/g) and a quicker adsorption capacity in the time it took to travel to the bottom of the lake (97.6% in 2 h travel time), indicating the adsorption role of both La and clay mineral. The toxicity of these materials was recorded null, and in some period of the incubation of the lake microorganism with the material mixture, La-grafted modified clays increased microbial growth. As a total package, while a high amount of La on the already available material could adsorb a greater amount of phosphate, in this study a trace amount of La on modified clays showed adsorption effectiveness for the realistic amount of phosphate in lake water without posing added toxicity.