Influence of dissolved organic carbon on the efficiency of P sequestration by a lanthanum modified clay. (15th June 2016) - Dithmer, Line
Abstract: A laboratory scale experiment was set up to test the effect of dissolved organic carbon (DOC) as well as ageing of the La–P complex formed during phosphorus (P) sequestration by a La modified clay (Phoslock ® ). Short term (7 days) P adsorption studies revealed a significant negative effect of added DOC on the P sequestration of Phoslock ®, whereas a long-term P adsorption experiment revealed that the negative effect of added DOC was reduced with time. The reduced P binding efficiency is kinetic, as evident from solid-state 31 P magic-angle spinning (MAS) NMR spectroscopy, who showed that the P binding did not change in the presence of DOC. 31 P MAS NMR also reveals that up to 26% of the sequestered phosphate is as loosely bound redox-sensitive P species on the surface of rhabdophane (LaPO4 · n H2 O, n ≤ 3). The ratio between the loosely bound P and lanthanum phosphate did not change with time, however both NMR and La LIII -extended x-ray absorption fine structure (EXAFS) spectroscopy shows a transformation of lanthanum phosphate from the initially formed rhabdophane towards the more stable monazite (LaPO4 ). Furthermore, the effect of natural DOC on the P binding capacity was tested using water and pore water from 16 Danish lakes. Whilst DOC has an immediate negative impact on P binding in the lake water, with time this effect is reduced. Highlights: Dissolved organic carbon interferes with phosphate-removal by lanthanum modified clay. With time the negative impact of DOC is overcome. Tendency for a phase transformation from rhabdophane to monazite over time. High alkalinity tends to counteract negative effect of dissolved organic carbon. Thorough testing prior to real-lake application is necessary.