Overlooked role of long capping time and environmental factors in the plateau lake for impairing lanthanum-modified-bentonite's immobilization to phosphate

Abstract

Lanthanum-modified-bentonite(LMB) has been applied for eutrophication management as a phosphate(P)-binding agent in many lakes. However, re-eutrophication took place several years or decades later after the first practice of capping due to dynamic environmental factors in the plateau lake. Here, we investigated the effect of long-term capping and integrated environmental factors in the plateau lake including alkalinity, organic matter, disturbance and photodegradation to the LMB immobilization. Long-term LMB immobilization exhibited C accumulation(82.3%), La depletion(53.5%) and lager size effect in the sediment particle, indicating the breakage of La-O-P bonds and the formation of La-O-C bonds over immobilization time. Additionally, pH(8-10) in the plateau lake could enhance the P desorption and decrease P adsorption through electrostatic repulsion enhancement with the zeta potential reduction(7.2mV). Further disturbance experiment indicated a significant releasing trend of active P and DGT-labile P from the solid phase, pore water to the overlying water after disturbances due to resuspend releasing, particle size and amorphous Fe, Mn and Al's redistribution. Moreover, 31P NMR and EPR results indicated photodegradation after disturbance converted diester phosphate into orthophosphate with long-term LMB immobilization via the oxidation of ·OH in the sediment of the plateau lake. Therefore, management issues for Xingyun Lake may apply to other plateau lakes with low external P input, intermediate depth and intense disturbance.